Isidoro López

Julien Morey

Jean Bernard Ledeuil

Lénaïc Madec

Hervé Martinez

A critical discussion on the analysis of buried interfaces in Li solidstate batteries. Ex situ and in situ/operando studies

Interfacial electro-chemo-mechanical phenomena determine the performance of Li solid-state batteries (SSBs), thus the study of these processes is key to construct more efficient and stable systems. In this regard, the analysis of interphases, including their evolution during cycling, is probably the most challenging aspect in the field of SSBs, as interfaces in these cells are inherently buried. In this perspective, a critical discussion on the various methodologies currently employed to gain access to buried interfaces and obtain reliable structural and chemical information is provided. Since this aspect is commonly overlooked in the literature, a particular focus has been addressed to rigorous procedures allowing to achieve reproducible and unambiguous results. Simultaneously, the valuable structural, electronic and chemical data collected on interphases by using these robust methods is presented. It is also shown that the development of strategies to probe buried interfaces has paved the way to progress in attractive in situ and operando measurements. On the other hand, new experiments carried out in our laboratory evidence that ion etching for depth profiling analysis, which is widely used to study solid electrolyte interphases (SEIs) and cathode electrolyte interphases (CEIs) in liquid and solid-state Li batteries, is not convenient to elucidate the distribution of chemical components in composite materials present in SSBs due to sensitivity of certain compounds like Li salts to ion bombardment. Finally, our discussion can be extended to other important materials and nanostructures presenting buried interfaces such as layered films with applications in photovoltaics and core-shell nanoparticles.

Introduction.

Lithium solid-state batteries (SSBs) are currently recognized as the most promising technology for the next generation of Li rechargeable batteries. [1][2][3][4][5] In these systems, the liquid electrolyte of common Li ion batteries (LIBs) is replaced by a solid electrolyte, which plays the dual role of Li ion conductor and separator between the electrodes. Thus, the electrochemical properties of SSBs, such as capacity fading and rate capability, are mainly governed by two key aspects: 1) the ionic and electronic conductivity of the solid electrolyte, and 2) the chemical and structural characteristics and stabilities of the diverse solid/solid interfaces into the system. A significant progress has been achieved in the synthesis of solid electrolytes exhibiting ionic conductivities comparable to the one from liquid electrolytes (10 -3 -10 -2 S cm -1 ) and there are numerous reviews addressing the main advantages and drawbacks of oxides, sulfides, polymers and composites working as solid electrolytes (see for instance refs. 3,[6][7][8][9][10][11][12] ). The existence, formation and evolution of interphases in SSBs leads to energetic barriers for Li ion diffusion and electron transfer, thus the improvement of their electrochemical performance can only be achieved through a comprehensive understanding of the electro-chemo-mechanical properties of materials at solid/solid interfaces before and during cycling. Recently, a few reviews have addressed the study of interphases in SSBs from a merely descriptive point of view [13][14][15][16][17] by discussing the main chemical/structural information obtained from diverse analytical techniques. However, a critical discussion on suitable methodologies to gain access to the inherently buried interfaces in SSBs (see below) and reliably probe their morphological, structural and chemical properties has never been considered until now. Indeed, a survey of the literature on SSBs shows plenty of examples on spectroscopic and microscopic characterization of solid/solid interfaces, but almost no report mention how these buried interfaces could be analysed (see for instance refs. [18][19][20][21][22] ) and a few of them describe procedures that depend on the manual skills of a researcher to separate mechanically two connected phases (see for instance refs [23][24][25][26] ). Obviously, these approaches are questionable for sound reasons. Firstly, the probed material could not correspond to the interphase as, commonly, there is no justification or discussion about the region selected for analysis. This situation is worst when the sample preparation procedure is not described in the experimental section because the reader cannot even intend to replicate the measurement, which constitute the basis of the scientific knowledge. 27 A second controversial issue concerns the preservation of the structural and chemical integrity of the inner interphase when two materials are mechanically separated. It seems of common a. Universite de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, 64000 Pau, France. b. Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS 3459, Hub de l'Energie, 80000 Amiens, France.

sense that the mechanical effort exerted to carry out the separation will lead to partial or total destruction of the morphology of the interphase. Chemical degradation effects resulting from this operation have never been assessed in the literature, but they cannot be discarded in the absence of a systematic study. The last source of controversy relates to the homogeneity and thickness of the interphase. When two adjacent materials are mechanically separated, the original interphase is split in two films, each on covering one of the separated materials. The surface of the two coatings can be then probed; however, the spatial interpretation of the results is challenging as the exact depth position of the analysed region in the full thickness of the original interphase is unknown and the chemical composition of the interphase has been showed to be not homogeneous. In this context, the most widespread approach 24,[28][29][30] to tackle this problem consists in performing depth profiling studies in which sequential steps of material sputtering via ionic bombardment and spectroscopy/spectrometric analysis are combined to reconstruct the chemical composition of the interphase through its thickness (see section 5 for more details). In this perspective, we provide experimental results warning about this strategy. Particularly, when SSBs containing polymer or composite solid electrolytes are studied. Another more exotic approach to gain insight on the depth profile composition of the interphase relies on manually etching increasing amounts of the film with a scalpel. 31 This operation was reported to allow for controlling the removing of 5 μm of material. However, the full validity of this approach remains to be proved by replication of the experiments and application of the same methodology to distinct SSBs. The lack of rigorous analysis in most of current contributions dealing with interfacial phenomena in SSBs had led us to write this perspective. We aim at providing a critical discussion of contributions exploiting robust and rigorous methodologies to obtain reliable and valuable information of interphases in SSBs. These methodologies can be categorised into two approaches. The first one relies on the combination of a suitable sample preparation procedure or the use of modified conventional batteries with the subsequent application of an analytical technique, usually a surface analytical technique, to gather structural and/or chemical data. In this perspective, this strategy is called The Uncovering approach. The alternative rigorous approach consists in the direct use of analytical techniques featuring large sampling depth (SD) on SSBs. SD refers to the thickness of the surface of material that can be probed by a particular technique. For certain methods, SDs can achieve values as large as probe a significant volume or the full battery. In this perspective, this strategy is called The Bulk approach. The advantages and limitations of the methodologies used in the two approaches will be critically evaluated. Furthermore, we will discuss relevant examples illustrating the interest in the application of the two approaches to unravel electro-chemomechanical phenomena in SSBs. Additionally, we will see that the proper application of robust methodologies to probe buried interfaces in SSBs paves the way to the implementation of in situ/operando measurements, where the chemical and structural analysis of the interphase during electrochemical cycling is carried out without the necessity of battery dissembling or handling. This kind of experiments avoid sample pollution and deleterious processes associated to relaxation of the battery, both of them found in ex situ analysis [32][33][34][35] where the SSB must be previously dissembled and mounted on a sample holder. Finally, our perspective must not be regarded as a statement of invalidation of previous results obtained from the less rigorous approaches described above. A number of these works have provided fundamental insight on electro-chemo-mechanical phenomena in SSBs. A supplementary objective of our discussion is stimulating the acquisition of new data with the presented more reliable methodologies for allowing comparison with this previously obtained information.

The challenge: buried interfaces.

A significant number of interfaces can be found in SSBs. Considering an electrochemical cell where Li metal is employed as negative electrode (anode), the electro-chemo-mechanical properties of materials in the following interfaces need to be evaluated (Figure 1): A) Negative electrode: grain boundaries, Li/bulk solid electrolyte, Li/impurity. B) Bulk solid electrolyte: grain boundaries, materials interfaces in composite electrolytes (e.g. polymer/inorganic filler). C) Positive electrode: grain boundaries, composite electrode/bulk solid electrolyte, active material/solid electrolyte, active material/conductive additive, solid electrolyte/conductive additive, current collector/active material, current collector/conductive additive and current collector/solid electrolyte. There are some aspects and simplifications in the list and in Figure 1 which deserve comments.

1. The composite electrode/bulk solid electrolyte interface is complex and contains actually diverse kinds of interfaces, but, for analysis purposes, some experimental measurements validate their simplification as a formally single interface in most cases. 36,37 2. When a composite negative electrode is used (e.g. graphite or Si/C blends), the same interfaces listed above for the positive electrode must be considered.

3. Grain boundaries, namely, interfaces between two grains of the same material exhibiting distinct crystallographic orientations, are not depicted in Figure 1 for the sake of simplicity. However, it is important to note that these interfaces play a key role in the performance of the battery as it will discussed in the manuscript. 4. In addition to the existence of interfaces between different materials, SSB are characterized by the presence of a significant amount of voids. In LIBs, these spaces are commonly filled by the liquid electrolyte providing an ionic pathway for Li diffusion. In SSBs, the presence of voids cannot be completely suppressed even after pressing the cathode/solid electrolyte/anode stack.

Please do not adjust margins

Please do not adjust margins Since the formation and evolution of voids are usually associated to interfacial phenomena, we will include them in our discussion in spite of they are not explicitly indicated or mentioned in the list and in Figure 1. The rigorous analysis of all the interfaces in SSBs is not trivial, as they are buried in the sense that two adjacent materials cannot be easily separated to probe the nature and structure of the interphase. Furthermore, separation of the two materials can lead to a deterioration of the chemical and structural characteristics of the interphase and a loss of relevant information. It is surprising that a significant amount of work in the literature on the analysis of interfaces in SSBs does not describe the procedure undertaken to gain access to them and/or the adopted precautions to ensure that the analysed region is representative of the whole studied interphase (see for instance refs. 38,39 ). Additionally, an important number of publications base their spectroscopic surface analysis on the manual ability of the researcher to separate mechanically the interfaces (see for instance refs. 26,31 ). In the interest of the increasing research activity on materials chemistry of interphases in SSBs, we provide a rich overview of rigorous experimental methodologies that can provide, in our own opinion, reliable and valuable information on their electrochemo-mechanical properties. However, this does not mean that all previous conclusions drawn from less rigorous approaches are wrong. As it was stated above, previous less reliable experiments have shed light on interfacial processes in SSBs. The first part of this perspective is devoted to a critical discussion of the two main rigorous approaches explored until now in the field of SBBs to study structural and chemo-electromechanical phenomena in these energy storage devices. Firstly, the Uncovering approach will be considered (section 3). In this strategy, the objective is the study of one or several interphases by a particular analytical technique, usually a surface analytical technique. These originally buried interphases must be previously exposed by using a convenient sample preparation procedure. We will describe the fundamentals, advantages and drawbacks of each procedure, and provide relevant examples of application in SSBs. Similarly, we will discuss the different surface analytical techniques that can be employed in this approach and the remarkable results gathered. Secondly, the Bulk approach will be scrutinised (section 4). This approach tries to circumvent the necessity of sample preparation procedures by the direct application of analytical techniques featuring large SDs to the study of the battery. Again, the principles, advantages and drawbacks of each method will be evaluated.

Several outstanding examples illustrating the interest of this strategy will be also presented. Importantly, it will be evidenced that these two rigorous approached have paved the way to the development of in situ/operando measurements allowing to obtain reliable results on interfacial processes in SSBs. The last part of this perspective addresses the suitability of performing depth profiling experiments based on ion milling for the chemical analysis of interphases in SSBs (section 5). Since this practice is very common in the field and is usually carried out after the mechanical or manual separation of materials, an evaluation seems convenient to avoid misinterpretation of results.

The Uncovering approach: Methodologies based on exposing buried interfaces.

The most obvious strategy to gain access and probe buried interfaces in SSBs consists in a two steps procedure: 1) exposing the surface of the diverse materials contained in the electrochemical cell, and 2) Analyse the treated sample by a surface technique. Therefore, this section is divided into two sub-sections addressing each step

Methodologies to expose buried interfaces.

The simplest procedure relies on the separation of two adjacent materials and the subsequent analysis of the two surfaces which were originally in contact. Despite this methodology has been extensively used, the separation of connected phases can be extremely challenging or impossible. Furthermore, it introduces Please do not adjust margins Please do not adjust margins a source of controversy, as the detachment of the bulk solid electrolyte from Li and composite electrodes is most often carried out manually. Thus, the exact location of the interphase as well as its morphology and chemical composition depend on the manual ability of the researcher and the spatial region chosen for the subsequent analysis. Considering the lack of reliability in this approach, experimental results obtained following this methodology will not be discussed in the perspective.

In contrast, two more robust and reproducible strategies allowing for revealing buried interfaces are (i) the preparation of cross-sections and (ii) the utilization of perforated current collectors. In the next sections, a critical discussion of these two sample preparation methods used to reveal buried interfaces in SSBs is provided (Figure 2).

Preparation of cross-sections.

In this approach, a clear cross-section of the SSBs containing well-defined and stacked cathode/solid electrolyte/anode layers is exposed by the action of a particular technique. Thus, the interfaces can be easily visualized by electronic and/or optical microscopy, and subsequently analysed by a wide range of both bulk and surface analytical techniques. The preparation of cross-sections offers a more powerful and rigorous alternative to probe buried interfaces since it is less dependent on the manual skills of the researcher and; thus, it can provide more reliable and reproducible results. Additionally, crosssections potentially enable the evaluation of all the interfaces listed in section 2, while the strategy of separation of adjacent phases only gives access to Li/bulk solid electrolyte and composite electrode/bulk solid electrolyte interfaces. Despite the numerous advantages, the analytical approach based on the preparation of cross-sections is not exempt from some challenges and drawbacks. The first issue is a practical aspect. Two of the possible methodologies to prepare crosssections, mechanical methods and broad ion beam (BIB) polishing, are carried out in an equipment separated from the instrument where the cross-section will be probed. Therefore, the sample containing the cross-section must be correctly mounted in a sample holder suitable for the analysis and transferred to the analytical instrument under air-free conditions, usually by means of a transfer vessel. Placing properly the cross-section on the sample holder is not trivial as the cross-section should lie perfectly flat regarding to the sample holder. This is particularly important for experiments in scanning probe microscopy (SPM) as inclination angles of the cross-section will lead to erroneous interpretations of the results. The situation is ever more challenging when the crosssection must be analysed by more than one technique because sample holders from different instruments are not usually compatible. In order to address this challenge, Atsushi Sakuda et al. have successfully developed a single sample holder allowing for performing BIB polishing and the analysis of the prepared cross-section by SPM and scanning electron microscopy -energy-dispersive X-ray spectroscopy (SEM-EDS) in the three different instruments. 40 Another adventitious problem with cross-sections prepared by mechanical methods or BIB polishing is pollution due to sample handling for changing of sample holder or contact with the atmosphere of the gloves box. This fortuitous degradation can makes difficult, in some cases, probing the chemical composition of cross-sections with surface analytical techniques, particularly with methods characterised by thin sampling depths like time-of-flight secondary ion mass Spectrometry (ToF-SIMS).

The last challenges of cross-section are common to the three methodologies (mechanical methods, focused ion beam (FIB) and BIB polishing) and it is exclusively associated to in situ/operando experiments. As it will detailed in section 3.2, in situ/operando measurements involve the preparation of the cross-section previously to the application of an electrochemical method (e. g. galvanostatic cycling). Since the Please do not adjust margins Please do not adjust margins cross-section to be analysed after or during the electrochemical input lies at one edge of the battery, it can exhibit a different reactivity as compared to the material buried in the bulk of the battery. In other words, the cross-section could suffer from edge effects. Evidences of such a phenomenon have been provided by in situ ToF-SIMS experiments. 41 Finally, external pressure applied to in situ/operando cells based on crosssections is significantly lower and less homogeneous than the one holding more conventional SSBs. As a consequence, electro-chemo-mechanical phenomena observed at interfaces in this kind of experiments can differ from the actual processes leading to the degradation of performance in more conventional SSBs, particularly for batteries containing fully inorganic solid electrolytes. Importantly, non-homogeneous pressure also exacerbates asymmetric deformation of Li by creeping of this soft metal on the side of the battery occupied by the cross-section. This effect, often overlooked in the literature, 42 has been thoroughly studied by SEM. 43 In the next sub-sections; we will present in detail the three different methodologies that have been proposed for the preparation of cross-sections in SSBs so far. The fundamentals and the particular advantages and drawbacks of each procedure will be discussed.

Mechanical methods.

Two procedures have been used to mechanically obtain crosssections of stacked materials in SSBs. The simplest one relies on the employment of manual cutting tools like common or ultrasonic knifes and blades. 20,[44][45][46][47] However, this experimental approach is not very convenient to preserve the morphology of the layered structure in SSBs, as the solid electrolyte is commonly made of very brittle materials like metal oxides or very ductile compounds like polymers. An alternative mechanical technique, which is particularly suitable for electrolytes based on polymers, is cryomicrotomy. 48 This method consists in the controlled sectioning of a material with a glass or diamond knife at extremely low temperatures (-185 °C). [START_REF] Michler | Electron Microscopy of Polymers[END_REF] The procedure can be carried out in commercially available instrumentation and lead to a freestanding thin section (0.5 -5 μm) of the material and a new and flat surface created in the sample block where the thin section was cut out. The two pieces of material can be then probed by diverse spectroscopic, microscopic and spectrometric techniques. At cryogenic temperatures, ductile materials like polymers become brittle, as the temperature is lower than the glass transition temperature, Tg, and the sectioned samples preserve their internal structure. Therefore, interfaces in composite and layered materials are clearly distinguished. Although the method is slightly laborious, it provides well defined cross-sections which better preserve the morphology of the original materials in SSB stacks.

The only example in the literature of the application of cryomicrotomy to a SSB has been reported by researchers at the Canadian company Hydro-Québec. 48 They obtained a crosssection (Figure 3A) where four well-defined layers of the LiNi0.6Mn0.2Co0.2O2 (NMC 622)/PEO:LiTFSI(30:1)/Li battery are discernible (the fourth layer corresponds to the Al current collector in the cathode). Despite this attractive result, cryomicrotomy was unable to reveal clearly the microstructure of the composite cathode (Figure 3C). In contrast, it will be shown that FIB (section 3.1.1.2) and BIB polishing (section 3.1.1.3) are more powerful sample preparation procedures to preserve and delineate the microstructure of composite electrodes (grains, grain boundaries, voids and amorphous material).

Focused ion beam (FIB).

In the context of SSBs, this sample preparation technique consists in milling the stack of materials with a small ion beam (15 -50 nm), usually Ga + , to create a cross-section where the layered structure is clearly revealed. In order to provide a wide cross-section (50 -100 μm) the ion beam must be scanned ("rasterized"). Another attractive application of FIB consists in the extraction of an extremely thin lamellae of the layered material from a solid-state microbattery. This thin section can be subsequently analysed by (Scanning)Transmission Electron Microscopy (S)TEM in combination with other associated and powerful spectroscopic methods (see below) like electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED). A detailed and visual description of this sample preparation methodology in the field of SSBs has been recently published. [START_REF] Au -Lee | [END_REF] The preparation of cross-section by FIB presents a significant advantage regarding to mechanical methods and BIB polishing. The FIB procedure is commonly carried out in the same instrument, under high vacuum conditions, where the analytical measurement will be performed. Thus, there is not necessity for transfer of the prepared cross-section and the sample remains always under high vacuum conditions limiting adventitious chemical deterioration of the exposed surface. Excellent examples of the usefulness of FIB to expose buried interfaces inside composite cathodes have been recently published by the group of Jürgen Janek. [51][52][53] In these studies, cross-sections of cathodes containing NMC 622, thiophosphatebased solid electrolytes and, in some cases, a carbonaceous conductive additive (super C65 or vapor-grown carbon fibers, VGCF) were prepared by FIB directly inside the analysis chamber of a ToF-SIMS instrument. The microstructure of the composite electrode is clearly discernible in the image of the cross-section recorded by secondary electrons (SE) detection (Figure 4A). This well-defined microstructure contrasts with the poorly resolved one obtained after cryo-microtomy (Figure 3C). Despite the potential of FIB to prepare clear cross-sections, a recent work by Y. Shirley Meng et al. warns about the extensive employment of this technique at room temperature to study SSBs, particularly with Li metal anodes. 54 The interaction of the Ga + ions with the diverse surfaces can lead to a local increase of temperature triggering a chemical and structural deterioration of the materials. In the case of Li anodes, this effect results in a severe morphological degradation. As a more reliable alternative, cryo-FIB preserves the structural characteristic of Li and it should be privileged to prepare cross-sections containing temperature-sensitive materials like polymers and associated composites.

A potential drawback of FIB is the small dimensions of the obtained cross-sections. Typical values are 50 μm width x 50 μm depth for a milling time of approximately 3.5 h with a 30 kV Ga + gun. Although these values are indicative as they depend on the sputtering rate of each material. Bigger dimensions could always be achieved at the expanse of increasing the milling time; however, this approach would lead to unpractical sample preparation times and accumulative damage of the exposed cross-section. The small area and particular geometry of crosssections elaborated by FIB thus rise two concerns: 1) the exposed region could be no representative of the chemical composition and structure of the whole material; and 2) the geometrical disposition of the sidewalls of the crater eroded by

ARTICLE

Please do not adjust margins Please do not adjust margins FIB is not suitable for the source-detector angle configuration of certain spectroscopic setups like in conventional XPS instruments.

The emergence of a new FIB gun based on Xe plasma can resolve the limitation associated to the small dimensions of cross-sections created by the ionic Ga + beam. Cross-sections exposed by this new device are significantly bigger as a consequence of an increased sputtering rate. Indeed, it has been reported that materials removal by Xe plasma FIB is 60 times faster than in conventional Ga + FIB. 55 Furthermore, the Xe plasma FIB leads to less surface damage in the exposed crosssection. 56 To the best of our knowledge, this technique has never been applied as sample preparation method to study buried interfaces in SSBs; however, it has been successfully used to acquire large 3D images of composite electrodes by successive steps of Xe plasma FIB and SEM analysis (see below). 57,[START_REF]Plasma FIB -TESCAN AMBER X[END_REF] Areas with dimensions in the order of 500 μm could be sputtered within dozens of minutes. This dimension value is similar to the one achieved by Broad Ion Beam (BIB) polishing (see below).

Broad Ion Beam (BIB) polishing

BIB polishing is a sample preparation procedure which consists in etching one edge of the sample with a parallel and broad beam of energetic ions, usually Ar + . [START_REF] Jiang | [END_REF] To be effective in the preparation of cross-sections, the sample must be covered by a masking plate which lets unprotected only the small volume of material that will be etched. The principle of the technique is the same as in FIB, in the sense that a cross-section with welldefined interfaces is obtained as a result of the sputtering of a surface layer of the stacked material with high-energy ions. However, the broader ion beam makes the surface area sputtered by BIB polishing significantly larger for similar operation times [START_REF] Jiang | [END_REF] (around 700 μm width x 400 μm depth vs 50 x 50 μm for typical FIB) and it is a cheaper and less timeconsuming method. The energy of the Ar + beam can be varied

ARTICLE

Please do not adjust margins

Please do not adjust margins to optimize the rate of polishing or prevent decomposition of the remaining non-sputtered material. The technique is highly convenient to expose the diverse materials contained in heterostructured samples like mesoporous and stacked materials 60,61 or core-shell nanoparticles. 62 Despite the potential of BIB polishing to uncover buried interfaces, there are only a few reports exploiting this sample preparation method for the subsequent analysis of SSBs. 36,41,63,64 Thus, for instance, this procedure allowed us to characterize unambiguously interphases in solid-state microbatteries (section 3.2.6.1) by scanning electron microscopy (SEM), auger electron spectroscopy (AES) and scanning auger microscopy (SAM). 63 The SEM image of the cathode/solid electrolyte interface in the cross-section of a LiCoO2 (LCO)/LiPON/Li microbattery prepared by BIB polishing reveals unambiguously the presence of three distinct phases (Figure 5A). One phase corresponds to the LiPON solid electrolyte and the other two (phase1 and phase2) are ascribed to the LCO active material as it will be detailed in section 3.2.6.1.

An adventitious side effect of BIB polishing is damage of a thin layer of material surface because, as in FIB, the ion beam leads to local heating triggering the degradation of the morphology and chemical state of temperature-sensitive materials like lithium and polymers. This aspect is particularly relevant in the

ARTICLE

Please do not adjust margins Please do not adjust margins perspective of applications in the field of SSBs, in which lithium, organic (macro)molecules and other temperature-sensitive materials are usually present. Fortunately, technical progress has allowed carrying out BIB polishing under cryogenic conditions. 65 Indeed, we present in this perspective preliminary experimental results from our group illustrating the interest of cryo-BIB polishing to prepare cross-sections of a classical SSB: LiNi0.5Mn0.3Co0.2O2 (NMC 532)/PEO:LiTFSI/Graphite. The image obtained by SEM of the cross-section shows all the layers of the three main components of the battery, including the Al and Cu current collectors (Figure 5B). The interfaces are easily recognizable and the microstructure of the composite cathode is fully revealed. A comparison between this image and the cross-section prepared by cryo-microtomy (Figure 3C) highlights the better performance of BIB polishing to probe in-depth the microstructure of large sections of composite electrodes. Thus, the distribution of active material, polymer containing Li salt and Al current collector could be unambiguously stablished by scanning auger microscopy, SAM (Figure 5D). Importantly, although similar analysis can be undertaken with methodologies exploiting FIB as sample preparation procedure, a comparison between the images in Figure 4A and Figure 5B illustrate the larger dimensions, and thus representability of results, of cross-sections prepared by BIB polishing. On the other hand, a common challenge in the analysis of crosssection prepared by BIB polishing is changing of sample holder after ion etching as BIB polishing and analytical measurement are carried out at different instruments. As discussed in the introduction of section 3.1.1, this operation can ruin the analysis step. A significant advancement to overcome this issue Please do not adjust margins Please do not adjust margins has been recently developed 40 by Atsushi Sakuda et al., which was also discussed in the introduction of section 3.1.1.

Use of perforated current collectors.

The simplest approach to analysis interfaces in a composite cathode (containing active material, conductive additive and binder) without the necessity of disassembling or mechanical cutting would be probing the battery through the face covered by the Al current collector (top of the battery in Figure 1). However, this strategy is not usually feasible. Common current collectors present thickness between 10 -15 μm. All the analytical surface techniques discussed in section 3.2 displays SD in the order of some tens of nanometers, except EDX and RAMAN in which it increases to 1 -6 μm. Therefore, probing the battery through the face covered by the current collector would result in signal produced exclusively from this material precluding the study of the buried interfaces. The easiest solution to overcome this physical problem is assembling the SSB with a current collector exhibiting holes. Holes in the current collector allows passage of radiation or electron beams which can directly interact with the uncovered surface of the electrode. Obviously, the radiation or electron ejection resulting from this interaction can also traverse the holes to be detected. As a consequence, some of the buried interfaces near the current collector side of the battery are exposed and can be probed. The thickness of the probed region depends on the SD of the used microscopic/spectroscopic technique. This approach is highly convenient for the development of several in situ and operando methods in the field of SSBs. Relevant examples of operando XPS studies on SSBs based on perforated current collectors have been reported by the group of Mario El Kazzi. [66][67][68] In these experiments, a special cell design allowed for using a stainless steel mesh as perforated current collector (Figure 6A,B). Despite this approach is highly attractive, it has four drawbacks. Firstly, only interphases near the current collector can be probed as a result of the limited SD of common and synchrotron-based spectroscopic techniques (SDs in the range of 10 -30 nm vs electrodes with a thickness in the range of 40 -100 μm). Thus, only information form this region, which is not representative of the whole electrode, can be collected. The second inconvenience relies on the possible inhomogeneity of electronic conduction pathways within the composite electrode as a consequence of porous structure of the current collector. 34 Thirdly, similarly to in situ/operando experiments performed on cross-sections, pressure in SSBs with perforated current collectors is not as efficient as the one holding batteries with conventional current collectors. Indeed, Mario El Kazzi et al.

have reported recently a slight degradation of the electrochemical performance of their cell for operando XPS measurement regarding to a conventional cell without perforated current collector. 68 This effect was tentatively attributed to the less efficient pressure holding the operando cell and it should be more critical for SSBs made of fully inorganic solid electrolyte, where the applied pressure seems to have a stronger influence to decrease porosity and favour intimate contact between solid electrolyte and active material. 69 Finally, residual analytical signal from the current collector material can partially overlap signals from materials inside the composite cathode disturbing the interpretation of some results. For instance, the Fe 3p peak in the XPS spectrum of a stainless steel mesh, like the one employed by Mario El Kazzi and coworkers, has similar binding energy to the Li 1s peak. 66

Analysis of exposed buried interfaces.

Once the buried interfaces have been uncovered by means of a reproducible and reliable methodology, they can be analysed by a particular technique or a combination of them. At this point of our discussion, it is important to realize that much of the interest of the analysis of interphases in SSBs is placed on elucidating their chemical and structural evolution as a result of the application of an electrochemical input. In other words, the aim of the analysis is usually unveiling chemo-electromechanical phenomena at interfaces triggered by battery charge and discharge. Therefore, we define three distinct possibilities of analysis in the context of the present perspective:

Ex situ analysis: the analysis is carried out on a sample prepared from a battery which has been subjected to an electrochemical input and subsequently disassembled.

In situ analysis: the battery, which has been previously modified or treated with one of the aforementioned sample preparation procedures, is introduced inside the analysis chamber or special cell of the instrument. The electrochemical input can then be applied to the battery in this place, but the analysis of the interphase is only carried out after interruption of the electrochemical input. Switching between electrochemical input and analytical measurement allows for monitoring the evolution of the interphase. Operando analysis: it is similar to the in situ analysis; however, probing of the interphase is performed simultaneously to the application of the electrochemical input. This operation mode enables to follow in real time the evolution of the interphase. The next sections will present a critical discussion about the diverse methods and most relevant associated results that have been applied to the ex situ and in situ/operando study of exposed buried interfaces in SSBs (Figure 7) In view of the similarity between in situ and operando experiments, we have preferred to combine in a single section for the sake of simplicity.

Studies based on optical microscopy (OM).

OM has been mainly adapted to perform operando measurements on SSBs. 45,70,71,[71][72][73][74][75] A notable example of this approach is the study of lithiation of a graphite electrode in a graphite/LPS/InLix battery reported by Akitoshi Hayashi's group. 45 Optical mapping of a cross-section of the cell showed a gradual decrease of the degree of lithiation in the whole electrode after 4 cycles; however, the loss of capacity in Li insertion was more severe at the current collector/solid electrolyte interface. The cross-sections was prepared by mechanical cutting with a tungsten blade.

Please do not adjust margins

Please do not adjust margins

In another interesting study, 72 the cross-section of a S8/ PEO:LiTFSI(20:1)-LLZTO/Li SSB prepared by cutting with a ceramic knife was analysed by operando OM. Importantly, the measurements proved the irreversible dissolution of polysulfides into the solid electrolyte during the first discharge at temperatures in which the ionic conductivity of the electrolyte and the electrode/electrolyte charge transfer resistance are more favourable. Polysulfides dissolution and subsequent shuttling to the anode is a well-known capacityfading process in liquid Li-S batteries. 76 Solid electrolytes have been proposed as a solution to this problem because they can act as physical barriers. 77 However, this work illustrates the incapacity of "solid electrolytes" with low melting temperatures to avoid polysulfide shuttling during cycling at moderate-high temperatures (55 °C -75 °C).

Studies based on scanning electron microscopy (SEM)

and energy-dispersive X-ray spectroscopy (EDS).

Ex situ studies.

SEM, alone or in combination with EDS, is the most popular technique for the ex situ analysis of cross-sections in SSBs. There are numerous examples in the literature on the utilization of this electron microscopy to visualise cross-sections prepared by FIB, [78][79][80] BIB polishing 63,69 or mechanical cut. 47 Since a comprehensive compilation of all these reports is out of the scope of this perspective and the amount of information collected from this method is rather limited, we will focus on the potential of FIB-SEM tomography as a valuable and lessexplored tool to gain access to the mechanical evolution of the morphology of composite electrodes at different stages of their cyclelife. In FIB-SEM tomography, sequential steps of FIB and 2D SEM imaging are repeated to reconstruct a 3D image of a small region of the electrode. [START_REF] Utke | Nanofabrication using focused ion and electron beams: principles and applications[END_REF] Gerbrand Ceder et al. have recently exploited this approach [START_REF] Shi | [END_REF] to assess the evolution with cycling of the microstructure of the composite cathode in a LiNi0.5Mn0.3Co0.2O2 (NMC 532)/75Li2S-25P2S5 (LPS)/InLix battery. The volume occupied by active material, solid electrolyte, conductive additive and voids was identified and calculated from back-scattered electrons (BSE). The experiments evidenced a large increase of the volume of voids from 2.87 vol% (pristine) to 9.50 vol% after 50 cycles leading to a loss of surface contact between active material and other solid state components of the cathode. This phenomenon could explain the observed capacity fading. Indeed, mechanical degradation of electrodes due to the loss of solid-solid interfaces or particle cracking is recognized as a critical challenge in the development of efficient and long cycle life SSBs. [83][84][85] Therefore, FIB-SEM represents a highly attractive method to evaluate the evolution of the microstructure of the diverse components in SSBs with a spatial resolution less than 100 nm. [START_REF] Shi | [END_REF]86 However, the volumes of material probed by this technique are relatively small (60 μm × 40 μm × 30 μm for the experiments carried out by Ceder and coworkers), thus, the analysed region could be no representative of the mechanical behaviour of the whole material. Alternatively, the study of the microstructure of larger volumes can be carried out with X-ray computed tomography, X-CT (section 4.2), although resolutions comparable to FIB-SEM are only attainable with synchrotron Xray sources. 87

In situ/operando studies.

Laboratories at the Canadian company Hydro-Québec have extensively exploited the mechanical preparation of crosssections to carry out in situ SEM analysis of diverse cycling SSBs containing PEO:LiTFSI solid electrolyte. 48,[88][89][90][91] Although most of the reports do not mention the procedure followed to expose Please do not adjust margins Please do not adjust margins cross-sections, the last contribution specifies the use of cryomicrotomy, as it was discussed in section 3.1.1.1. 48 The most attractive application of in situ SEM is the possibility of visualising in real-time the variation in thicknesses of the three stacked materials (composite cathode, solid electrolyte and Li anode) and consequently the position of the associated interfaces. For instance, a gradual thinning of the polymer solid electrolyte due to compositional degradation occurred during galvanostatic cycling of a NMC 622/PEO:LiTFSI(30:1)/Li cell. 48 This phenomenon is marked with an arrow in Figure 3B. The growth of dendrites from the Li/solid electrolyte interface was also observed. Importantly, EDS analysis on these dendrites shows the presence of C, O and Li suggesting that these structures are made of lithium carbide which could originate from polymer reduction.

Studies based on transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM)

and associated techniques.

Ex situ studies.

The preparation of thin cross-sections suitable for robust and reproducible TEM/STEM studies is usually carried out by applying FIB to previously fabricated micro SSBs (vide supra). [START_REF] Au -Lee | [END_REF]92,93 It is a highly complex process which requires a sound expertise. One prominent example of this approach was reported by Y. S. Meng and coworkers. 94 They created a LiCoO2/LIPON/Si nanobattery by the operation of FIB on the corresponding microbattery. Subsequently, the nanobattery was electrochemically charged at different states inside the FIB instrument. A final thinning step by FIB of the nanobattery at different states of charge allowed to perform ex situ scanning TEM in combination with electron energy lose spectroscopy (EELS). The measurements on the overcharged state unravel the existence of three phenomena: the evolution of a detrimental Li accumulation region at the LiCoO2/LIPON interface, the clear formation of a new phase at the LIPON/Si interface containing Li, P and Si, and the deposition of metallic Li at the Si/Cu (current collector) interface. The development of all these new interphases could explain the capacity fading of the nanoelectrochemical cell.

In situ/operando studies.

In situ and operando studies based on TEM and STEM are gaining an increasing interest to assess interfacial processes in SSBs. 13,95,96 The utility of this approach is not limited to the analysis of the chemical composition and structure of interphases by the combination of TEM/STEM and EELS, SAED, or annular bright-field micro (ABF) and high-angle annular darkfield (HAAF) detection. 33,[97][98][99][100][101] The extension and magnitude of the electric double layer or space-charge layer (SCL) at electrode/solid electrolyte interfaces can also be evaluated. 102,103 The SCL arises from the build-up of opposite charges at the electrode/electrolyte interface. It is present in the uncycled SSBs due to differences in chemical and electric potentials between the two solids, eventually triggering the formation of interphases; 104,105 and it is obviously enhanced by the variation of electrochemical potential of the electrode during battery cycling. A paramount example of the relevance of in situ/operando TEM/STEM-based techniques to probe the SCL is the pioneering study of the cathode/solid electrolyte interface in a LiCoO2/Li1-x-yAlyTi2-ySixP3-xO12/Pt solid-state microbattery carried out by K. Yamamoto and coworkers. 102 A combination of FIB thinning, TEM and electron holography (EH), a technique capable of measuring the distribution of the electric potential through a section of the microbattery, revealed the formation of a SCL at the analysed interface of approximately 1.5 μm thickness. This value is much higher than the theoretical Debye length expected for a double-charged layer (in the range of a few Å). However, a more recent ex situ study, 106 mainly based on the combination of TEM with EH and spatially resolved EELS, on an uncycled Cu/Li1+x+yAlx(Ti,Ge)2-xSiyP3-yO12 (LASGTP)/Cu solid state system shows the formation of a significantly thinner SCL (around 10 nm) at the Cu/LASGTP interface. Although these findings suggest that more research efforts should be devoted to clarify the thickness and magnitude of the SCL, it has been proposed that this double layer has a small impact in the charge transfer resistance at the electrode/electrolyte interface. 104,107 Despite in situ measurements based on TEM and associated techniques have showed great promise to unravel fundamental electro-chemo-mechanical phenomena at the buried interfaces in SSBs, this experimental approach presents some limitations. Firstly, it can be only implemented on small electrochemical cells like micro and nano-batteries. These devices consist in a stack of well-defined micrometric layers of cathode, solid electrolyte and anode with the eventual formation of interphases and are of high interest in the framework of powering small electronic devices. However, SSBs for applications in e-mobility and grid storage contain composite cathodes (eventually also composite anodes and solid electrolytes) characterized by the presence of numerous interfaces. Therefore, the chemical and physical events responsible for the electrochemical performance can diverge between the two systems. Additionally, the thickness of crosssections elaborated by FIB for in situ TEM-based measurements is of a few tens of nanometers. At these dimensions, electrochemo-mechanical phenomena and the reactivity of the materials could be radically different, including an enhanced deterioration by electron beam damage. 108 Finally, the problem of reduced and non-homogeneous stack pressure for in situ/operando cells cross-sections discussed in section 3.1.1 is particular relevant in measurements relying on (S)TEM and associated techniques. In these cells, the only stack pressure is exerted by the tip of a micromanipulator.

Studies based on Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM).

Ex situ studies.

As introduced in the ToF-SIMS section (3.2.4.2), the analysis and visualization of spatial distribution of Li compounds with spectroscopic methods based on the excitation by X-rays beams is challenging due to the small cross-section associated to the Li Please do not adjust margins Please do not adjust margins element. AES and SAM offer an attractive solution to this issue as Li produces relatively good signals. Additionally, SAM can provide a lateral spatial resolution in the range of 5-100 nanometers. 109 Despite these advantages, AES and SAM present a challenge which is shared with all the techniques using electron irradiation as source: degradation of electronically non-conductive materials due to thermal degradation triggered by surface charging. 110 This deleterious effect is particularly critical for polymers and organic molecules. Indeed, several solutions have been proposed to mitigate problems associated to surface charging and heating. For instance the use of additional electron or positive ion beams for charge compensation 111 and measurements on thin sections of polymer under cryogenic conditions. 112 A remarkable and rigorous example of the application of AES and SAM for the analysis of interphases in SSBs was recently published by our group. 63 In order to obtain reliable and reproducible results, cross-sections of a LiCO/LiPON/Li microbattery were prepared by BIB polishing, as discussed in section 3.1.1.3, and the exposed cathode/solid electrolyte interface was probed by AES and SAM. The formation of a Lirich interphase was clearly evidenced in the uncycled battery, which is in agreement with previous FIB-STEM/EELS studies. 97 Furthermore, the detection of Li allowed for interrogating on its reversible desertion/insertion in the two phases contained in the cathode after the first charge/discharge (Figure 5C).

In situ/operando studies.

To the best of our knowledge, there is only one example of a truly in situ SAM and AES experiment on SSBs. 43 A LiNi0.80Co0.15Mn0.05O2/Li6PS5Cl/Li battery was analysed, but, unfortunately, the cross-section of the device were prepared by manual cutting. The formation of dendrites was clearly observed during electrochemical cycling. The surface composition of these structures was consistent with the presence of oxidized Li (e.g. Li2O); however, the formation of this species on the surface of dendrites can be explained by the reaction of metallic Li with residual water still present at ultrahigh vacuum conditions.

Studies based on RAMAN spectroscopy and microscopy.

Ex situ studies.

As for XPS and ToF-SIMS experiments, RAMAN measurements on SSBs are relatively popular but they are commonly performed on samples without a reliable and robust preparation procedure. An outstanding exception is the mapping study of the composite positive electrode/solid electrolyte interface in a LiCoO2/LPS/InLix battery published by Masahiro Tatsumisago and co-workers. 37 A cross-section of the sample was prepared by BIB polishing and the exposed flat interface was probed by RAMAN microscopy. The preferential formation of detrimental Co2O3 at the interface, far from the current collector, was evidenced after the first charging process.

In situ/operando studies.

The most relevant work was recently reported by Laurence J. Hardwick and co-workers. 113 They exploit the strategy of perforated current collectors to probe buried interfaces in a LiCoO2/Li6PS5Cl/Li battery. An optically transparent window was partially coated with a thin (50 nm) Au film which played the role of current collector. A central round area of the window was not covered by the metal and it allows the pass of a laser beam to carry out operando RAMAN spectroelectrochemistry. The analysis of interfaces inside the composite LiCoO2 electrode and the Li6PS5Cl/Li interface revealed the formation of ionic isolating Li2S, S and P2Sx containing interphases, in agreement with previous results obtained by diverse ex situ spectroscopic techniques. 16 3.2.6. Studies based on X-ray photoelectron spectroscopy (XPS).

Ex situ studies.

XPS is the most widely used method to study the chemical evolution of interfaces in SSBs. Paradoxically; almost all the reported experiments rely on the analysis of the surface of the materials obtained after the manual separation 23,24,26,[114][115][116][117] or manual etching 31,47 of the electrode/solid electrolyte interface. These operations can be easier for the Li/solid electrolyte interface, but in all cases, the procedures lack reproducibility and raise concerns about if interphases are being actually probed. The scenario is still worst in a remarkable number of papers in which the sample preparation method is not even mentioned. 20,21,25,38,39,118 Therefore, we encourage reviewers to discuss critically the robustness and reliability of the methodologies employed to gain access to the buried interfaces analysed by XPS measurements. Instead of these questionable sample preparation methods, two more reliable approaches have already been used for ex situ XPS studies. The first one consists in the utilization of synchrotron radiation to carry out variable-energy hard XPS and it will be discussed in the section devoted to the Bulk approach as this technique exhibits a relatively large SD which can be exploited to probe buried interfaces in microbatteries without the necessity of sample preparation procedures or the modification of the device. The second strategy consists in the deposition of a very thin layer of Li metal on the solid electrolyte inside the XPS analysis chamber 119 or in a connected preparation chamber. 120 The procedure is ideal to assess the intrinsic chemical reactivity of the electrolyte with a Li electrode, but it is restricted to the study of this interface. Furthermore, it has been recently showed that the reactivity of a Li6.25Al0.25La3Zr2O12 solid electrolyte varies depending on Li is deposited by electron beam evaporation or magnetron sputtering. 121

In situ/operando studies.

The group of Mario El Kazzi has reported [66][67][68] probably the most outstanding examples of how perforated current collectors can be exploited to perform operando monitoring of the electrochemical evolution of interfaces in SSBs. In their first pioneering contribution, 66 they analysed the cathode of a LiCoO2/(Li2S)3-P2S5(LPS)/InLix system employing a stainless steel This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins mesh as perforated current collector. This experiment not only revealed the chemical composition of degradation products at solid electrolyte/active material and solid electrolyte/conductive additive interfaces, but also differences in surface potential between the varied components into the positive electrode, including interphases, offering a complete picture of potential profile inside the composite cathode (Figure 6C,D). As it was already discussed in section 3.1.2, the last experiments carried out by El Kazzi and co-workers suggest a decrease in the electrochemical performance for operando cells with perforated current collectors. 68

Studies based on time-of-flight secondary ion mass

Spectrometry (ToF-SIMS).

Ex situ studies.

A survey of the literature data on the utilization of ToF-SIMS for the analysis of buried interfaces in SSBs reveals the same issue found in XPS measurements: the sample preparation procedure. Most of the publication are based on the analysis of surfaces manually separated or cut. 122 Notable exceptions to this common practice have been recently reported [51][52][53] by Jürgen Janek et al. for NMC 622/Li6PS5Cl/InLix, NMC 622/LPS/Li and NMC 622/Li6PS5Cl/Li4Ti5O12 batteries. Cross-sections of the composite cathodes were prepared by FIB in the ToF-SIMS instrument, as previously discussed in section 3.1.1.2, and the exposed surfaces were subsequently analysed. In one of their contributions, 52 images from composite cathodes containing or lacking VGCF as conductive additive were constructed from characteristic negative ion fragments from NMC 622, VGCF and phosphate degradation products (Figure 4B). The measurements showed unambiguously an increase of the amount of phosphates at the active material/solid electrode interface after electrochemical cycling in both composite electrodes.

In situ studies.

The first in situ study on a SSB has been published by Nobuyuki Ishida and co-workers. 41 They assembled a symmetric LiCoPO4/Co-Li1-xAlxGe2-x(PO4)3/LiCoPO4 cell in which the composite cathode material contained a mixture of LiCoPO4 as active material, Co-Li1-xAlxTi2-x(PO4)3 as solid electrolyte and Pd particles as conductive additive. In order to expose the interfaces, the battery was subjected to BIB polishing. The subsequent elemental distribution mapping of the cross-section by ToF-SIMS evidenced partial reversible delithiation/lithiation of the active material that decreases with increasing cycling. An irreversible delithiation of the solid electrolyte was also observed after the first discharge pointing out the formation of a space charge layer. Interestingly, this study identifies one possible limitation of in situ/operando experiments carried out with cross-sections: exposed surfaces can exhibit a reactivity and electro-chemo-mechanical behaviour radically different to the material beneath. Indeed, ToF-SIMS experiments showed that any change in Li composition could not be detected during electrochemical cycling unless a thin layer (roughly 20 nm) of material was sputtered from the exposed surface previously to the measurement. Recently, Yamagishi et al. have reported operando measurements on a LiNi 0.8Co0.15Al0.05O2/LPS/InLix SSB. 123 A cross-section of the battery was prepared by FIB and the evolution of the chemical composition of the materials, including their spatial distribution, inside the composite cathode was probed by operando ToF-SIMS. The experiments showed the interest of ToF-SIMS as a powerful technique to track the diffusion of Li atoms, which is extremely challenging with most of the methods. Thus, this technique allows for interrogating the degree of reversible (de)lithiation of active material particles. Furthermore, POx -, SOx -and S2 -fragments associated to the oxidation of LPS could be detected. Importantly, the intensity of SO3 -fragment exhibits a variation compatible with previously observed 124 redox activity of the associated interphase, namely, it increases during charging and decreases after discharging.

Studies based on soft X-ray absorption spectroscopy (sXAS).

Ex situ studies.

In sXAS, highly bright synchrotron radiation is exploited as X-ray source to illuminate the sample at varied wavelengths. In the context of SSBs, most studies rely on the detection in reflection mode of the photoelectrons, total electron yield (TEY) detection, or X-ray fluorescence, total fluorescence yield (TFY) detection, generated as a consequence of the radiation-sample interaction. In these experiments, the absorption spectra of the sample in energy regions associated to low energy transitions of transition metals are commonly recorded, e.g. 840 -890 eV for Ni L-edge, thus, the penetration depth of the technique is relatively low and it can be considered as a surface analytical tool. Notwithstanding, higher energy synchrotron radiation can also be used as bulk methods, e.g. in computed X-ray nanotomography or hard X-ray absorption spectroscopy (hXAS). A brief discussion of the application of these techniques in SSB will be provided in section 4. In SSBs, 34 sXAS is a common method to track the evolution of the oxidation state of transition metals in the cathode and, thus, it gives no information about the chemical composition of interphases. However, the combination of TEY and TFY detection in sXAS experiments can provide valuable information on metal oxidation states at interfaces involving active materials. The penetration depth of these two detection techniques is significantly different. Indeed, the thickness of the probed region in TEY detection is around 10 nm, while it is around 500 nm in TFY detection. 125 Therefore, the combination of the two detection modes can reveal a gradient in metals oxidation states on the surface of the particles, including interfaces. For instance, this strategy has been recently exploited 23 by Xueliang Sun et al. to show the beneficial effect of a protective lithium niobium oxide coating on a LiNi0.8Mn0.1Co0.1O2 (NMC 811) cathode. The SSB contained PEO:LiClO4/Li6.4La3Zr1.4Ta0.6O12 (LLZTO) as solid electrolyte and Li as anode. The combination of Please do not adjust margins Please do not adjust margins TEY/TFY detection suggested that the surface of the coated active material suffers a slight structural degradation while the bulk remains intact after 5 galvanostatic cycles. In contrast, the unprotected material suffers a more severe degradation at both the surface and the bulk. Unfortunately, the samples of the analysed NMC 811 electrodes were obtained after mechanical cutting of the cycled batteries and there is no indication of which precautions were adopted to assure the reproducibility and representability of the XAS spectra.

In situ/operando studies.

Perforated current collectors have been successfully used in in situ and operando sXAS studies of cathodes in SSBs. In a pioneering work, 34 Wanli Yang et al. elaborated an Al current collector containing ordered holes with 50 μm diameter. The modified Al foil was then used as current collector for either composite NMC 111 or LiFePO4 (LFP) cathodes. In the two SSBs, PEO:LiTFSI (10:1) was used as solid electrolyte and Li as anode. In the NMC 111 system, the spectroscopic evolution of the oxidation state of Ni was consistent with the electrochemically derived state of (dis)charge of the battery. contrast, in the LFP cell, the spectroscopically measured oxidation state of Fe evidenced a heterogeneous distribution of the metal oxidation state inside the composite electrode. Indeed, active material near the current collector seems to be more electrochemically active than the same material near the solid electrolyte separator. Interestingly, homogeneous redistribution of Fe oxidation state was reached after a relaxation period ( 4

Studies based on scanning probe microscopy (SPM).

Ex situ studies.

Besides topographic studies carried out by atomic force microscopy (AFM), SPM techniques have been barely implemented for ex situ analysis of interfaces in SSBs. An interesting example of the application of advanced SPM methods to assess interfacial phenomena has been reported by Kaiyang Zeng and co-workers. 127 Electrochemical strain microscopy (ESM) was used to evaluate the diffusion of Li + at the nanoscale level inside a Li1+xAlxGe2-x(PO4)3 (LAGP) solid electrolyte with glass-ceramic structure. As sample preparation method, the roughness of the LAGP pellet was smoothed by polishing with sandpaper and subsequent short thermal etching to remove the damaged surface of the material. The ESM experiments evidenced the emergence of strain at grain boundaries due to Li + accumulation after local negative polarization of the surface. More recently, Atsushi Sakuda et al. have published an outstanding study 40 combining three different SPM techniques and SEM/EDS measurements to assess the electrical properties of a composite cathode, containing only the active material and the solid electrolyte, in a LiNi0.33Mn0.33Co0.33O2 (NMC 111)/LPS/Li battery. In order to prepare a flat and reproducible cross-section of the layered sample, BIB polishing was carried out under cryogenic conditions (-100 °C). Importantly, the prepared cross-section could be transferred between BIB polishing, SPM and SEM/EDS instruments under air-protected conditions in the same sample holder without requiring dismounting. As discussed in section 3.1.1, this new design of sample holder represents a significant advance as it is known that exposed cross-sections are extremely sensitive to manipulation, mainly due to two reasons. First, the clean newly created surface can be rapidly polluted as a consequence of manual operations to change the sample from sample holder. Second, the cross-section should lie perfectly flat regarding to the sample holder. This is particularly important for SPM experiments as inclination angles of the cross-section will lead Please do not adjust margins Please do not adjust margins to erroneous interpretations of the results. In their study, Atsushi Sakuda et al. used SEM/EDS to find unambiguously the spatial distribution of active material and solid electrolyte aggregates and particles inside the cathode. Regarding to SPM techniques, Kelvin probe microscopy (KPM) was exploited to unravel differences in work function of the two materials; scanning spreading resistance microscopy (SSRM) provided differences in electrical resistances; and conductive atomic force microscopy (C-AFM) was employed to record punctual I-V curves on diverse particles or agglomerates. This multi-technique approach led to two main observations. First, overall, the electrical resistance of active material particles decreases after charging except for particles with single-point contact with other NMC 111 particles. Second, there is a slight gradient of work function values (qualitatively comparable to redox potential) for NMC 111 particles through the thickness of the composite electrode after charging the battery. Active material particles near the solid electrolyte separator exhibit a lower work function than particles near the current collector. These results suggest the occurrence of low electronic pathways inside the composite electrode which is detrimental for its performance; however, it is worth to mention that the composite electrodes did not contain carbonaceous electronic conductive additives.

In situ/operando studies.

As stated above, SPM methods can be powerful and unique tools to track in situ or operando the potential drops created in SSBs after electrochemical polarisation. In two separated contributions, 36,64 Hideki Masuda et al. showed that in situ and operando Kelvin probe microscopy (KPM) can be exploited to unravel differences in work function of materials inside the composite cathode and at the composite cathode/solid electrolyte interface in a LiCoPO4/Co-Li1-xAlxTi2-x(PO4)3/Co-Li1-xAlxGe2-x(PO4)3/Co-Li1-xAlxTi2-x(PO4)3/Pd anode free SSB. Importantly, the sample was prepared by BIB polishing. The experiments demonstrated the formation of a sharp potential drop region at the composite cathode/solid electrolyte interface with a rough thickness of 1 μm, suggesting the creation of a space charge layer. Again, this value is significantly larger than the expected one from the Gouy-Chapman model and it is in agreement with the result of a previous in situ TEM-EH study, 102 but significantly larger than the value calculated from ex situ TEM-EH&EELS. 106

The Bulk approach: Methodologies based on techniques featuring large sampling depths.

Conventional microscopy (optical and electron), spectroscopic and spectrometric techniques for materials analysis exhibit a low sampling depth. As a consequence, only the firsts 10 -100 nm of the surface are actually probed with these methods. This constraint explains why exposing buried solid/solid interfaces is the most indicated and reliable procedure to study them. Our previous discussion has illustrated the strength of this approach as well as associated limitations. However, it exits some analytical techniques, suitable for the study of buried interfaces in SSBs, which present a larger penetration depth allowing the investigation of some of the materials in the cell without the need of dissembling, cutting or ion milling procedures. Among these methods (Figure 8), the most common are neutron depth profiling (NDP), micro-and nano-X-ray computed tomography (X-CT) and nuclear magnetic resonance (NMR). Electrochemical impedance spectroscopy (EIS) is another attractive and widespread bulk method allowing for monitoring the formation and evolution of interphases, and their direct impact on the electrochemical performance of the SSB. However, these methods will not be covered in this perspective because we think that they deserve a more comprehensive discussion in a separated manuscript. Readers interested in the application of EIS to the study of interphases in SSBs are invited to consult a recently published review. 128

Neutron depth profiling (NDP).

In NDP, a low energy neutron (n) interacts with a 6 Li atom producing an α ( 4 He) and a triton ( 3 H) particle according to the next equation 6 Li + n → 4 He + 3 H

As a result of this first collision, the 4 He and 3 H particles are released with well-defined energies of 2055 and 2727 keV, respectively. Subsequently, the two ejected particles suffer a number of collisions decreasing their original energies at the moment of their formation. The loss of energy can then be correlated to the initial location of the 6 Li atom inside the SSBs. Therefore, a higher decrease in energy involves a deeper position of the buried lithium atom inside the stacked material. NDP measurements on SSBs has been always carried out with the in situ or operando approach. [129][130][131][132][133] The first study was reported by P. H. L. Notten and coworkers. 129 The subsequent growth of the Li deposits leads to failure of the battery due to short-circuit. Li plating inside the electrolyte is attributed to the residual but no negligible electronic conduction of Li7La3Zr2O12 (σe = 5.5 × 10 -8 S cm -1 at 30 ℃) and Li3PS4 (σe = 2.2 × 10 -9 S cm -1 at 30 ℃). In contrast, LCO/LiPON/Cu microbatteries does not show any sign of Li deposition inside the solid electrolyte, as LiPON has an extremely low ionic conductivity (σe = 10 -15 -10 -12 S cm -1 ). Therefore, this paramount study suggests that electronic conductivity of solid electrolytes is a critical parameter for the cycling life of SSBs, which is rarely reported. Interestingly, a recent operando optical microscopy experiment on a NMC 622/LPS/Li battery suggests that Li plating inside the solid state electrolyte occurs before the growth of dendrites at the Li/LPS interface. 70 Please do not adjust margins

Please do not adjust margins

Despite the great capacity of in situ NDP to monitor Li transport phenomena, particularly at interfaces, this technique shares a fundamental limitation with in situ (S)TEM experiments. The analysed SSBs must contain uniform materials and flat surfaces as found in microbatteries. Furthermore, the sampling depth of the technique is usually lower than 50 μm. Thus, devices comprising composite electrodes or solid electrolytes oriented to e-mobility and grid storage cannot be probed.

X-ray computed tomography (X-CT).

Micro-and nano-X-ray computed tomography (X-CT) is an attractive method enabling the visualization and evaluation of the microstructure of materials in SSBs. Briefly, in X-CT, the assembled SSB is mounted on a sample holder rotating around an axis perpendicular to an incident X-ray beam. 2D transmission X-ray images are then taken at different angles and subsequently combined to generate a 3D image with the aid of digital geometry processing software. Although laboratory-scale instruments exist, the use of synchrotron radiation as X-ray source allows attaining spatial resolutions in the order of 100 nm, which is better than in laboratory instruments (around 1 μm under the best conditions). A comparison of the main features for the two approaches has been recently published for a liquid Li ion battery with a silicon anode. 87 Contrasts in images by X-CT are the result of differences in X-ray attenuation coefficients between chemical elements, thus light elements displaying a low X-ray absorption, like carbon and Li, are hardly distinguished in the rendered images. This aspect limits the information on interfaces obtained by X-CT. Nevertheless, this technique is probably the most powerful one to unravel the electromechanical dynamics of materials and interphases in SSBs with the advantage that batteries comprising composite electrodes and solid electrolytes can be analysed. The detailed description of the microstructure of composite electrodes and its evolution during cycling is receiving a justified increasing attention since it has been less explored than other phenomena in SSBs. 83,134 X-CT can shed light on this overlooked and important aspect for the electrochemical performance and stability of the device. For instance, J. Janek et al. examined the structural changes induced after the first charge in a LCO/Li10GeP2S12/InLix SSBs without external compressive pressure by ex situ X-CT. 135 It was shown that volume expansion of the negative electrode led to bending of the Li10GeP2S12/InLix interface and the whole cell. Furthermore, the formation of cracks was evident at the edges of the battery. Curiously, the stain imposed by volumetric changes produces a beneficial effect: the densification of the solid electrolyte (decrease of porosity from 5.5 % before charging to 2.6 %).

A comprehensive review of all the X-CT experiments conducted on SSBs is out of the scope of this perspective and they would deserve a detailed discussion in a separated manuscript. Thus, only some representative and relevant X-CT experiments will be discussed in this section. A significant number of X-CT studies has been devoted to research on symmetric Li/solid electrolyte/Li cells. In this context, the group of Nitash Balsara has carried out extensive efforts aimed at elucidating and mastering the growing of Li dendrites and protrusions in symmetric cells containing a rigid polystyrene-block-poly(ethyleneoxide) (PS-b-PEO) block copolymer blended with LiTFSI salt. [136][137][138][139][140] Probably, the most fascinating finding extracted from their synchrotron-based X-CT experiments is the clear observation of the formation of Li protrusions from crystalline impurities inside the Li metal electrode. The formation of dendrites has been traditionally attributed to uneven Li deposition at the Li electrode/solid electrolyte interfaces. However, the results obtained by Balsara et al. suggest that Li protrusions and dendrites start in crystalline impurities, probably electronically isolators like Li2O and Li3N, initially buried in the bulk of the Li electrode, beneath the electrode/solid electrolyte interface. This observation is extremely relevant as efficient suppression of dendritic structures could be only possible with Li electrodes featuring high purity. This aspect is commonly overlooked in the literature on SSBs. Furthermore, Balsara et al. examined the effect of Li salt concentration, temperature and current intensity on the geometry and morphology of dendrites and protrusions. All the experiments described above were performed ex situ. Notwithstanding, numerous in situ and operando X-CT measurements have also been reported. For instance, M. T.

McDowell and coworkers exploited the in situ approach to assess the evolution of the microstructure of the solid electrolyte in a Li/Li1+xAlxGe2-x(PO4)3(LAGP)/Li symmetric cell. 141 Despite the noticeable formation and growing of an interphase layer between the electrodes and the solid electrolyte, it was shown that the formation, propagation and widening of microcracks was responsible for the failure of the cell. The evolution during cycling of the microstructure of composite electrodes has also been successfully probed by in situ and operando X-CT. 142, 143 An interesting example has been published by Yuta Kimura and coworkers. They combined synchrotron X-CT and X-ray absorption near edge structure (XANES) spectroscopy to evaluate operando the 3D spatial distribution of the state of (dis)charge (SOC) of LCO particles in a composite electrodes containing Li2.2C0.8B0.2O3 (LCBO) as solid electrolyte without the presence of conductive additives. 144,145 The analysis of a volume comprising the full thickness of the composite electrode (ca 50 μm and ca 500 x 500 μm edges) revealed a heterogeneous distribution of SOCs. The SOC maps after charge and discharge points out a less efficient delithiation or lithiation, respectively, of LCO particles embedded in the centre of agglomerates of active material particles. The authors propose that this phenomenon is due to poor ionic conductivity at LCO/LCO particles interfaces. A comparison of the volume probed by the study of Kimura et al. and the typical one analysed by common Ga + FIB-SEM tomography (see section 3.2.2.1) evidences one of the advantages of X-CT regarding FIB-SEM tomography: the volume of probed material is significantly larger, thus the obtained results should be more representative. Additionally, X-CT is a non-destructive sample method and, consequently, it can be implemented with operando and in situ spectroelectrochemical measurements. Another advantage consists in the time This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins required for the analysis of the sample. FIB-SEM tomography involves sequential steps of FIB sectioning and microspic/spectroscopic analysis and, thus, it can be a very time-consuming methodology. In contrast, a full CT-XANES measurement at diverse wavelengths reported by Kimura et al. took approximately 35 minutes (one single image at a fixed wavelength took only 26 seconds). The main limitation of CT-XANES stem from the reduced chemical information accessible for this technique. Conversely, the surfaces sequentially exposed in FIB-SEM tomography can be probed with a wide range of microscopic and spectroscopic techniques providing comprehensive chemical information like ToF-SIMS. 146,147 A second drawback of the technique relies on the difficulty to distinguish light elements and voids as it was stated above.

Nuclear magnetic resonance (NMR).

Li has two stable isotopes with non-zero nuclear spin number (I): 6 Li (I = 1, 7.42 % abundance) and 7 Li (I = 3/2, 92.58 % abundance). 148 This property has made NMR experiments on 6 Li and 7 Li nucleus a valuable tool to assess the diffusion and distribution of Li + and, in some cases, Li metal at interfaces. In the context of interfacial studies in SSBs, NMR experiments are obviously carried out in the solid state. A description of the particularities of solid state NMR (ssNMR) and the main techniques used in this spectroscopic approach is out of the scope of the present perspective. We will only provide a short explanation of the information that can be extracted from a particular technique when pertinent. Readers interested in getting a deeper knowledge on the principles and main methodologies in ssNMR 149,150 and/or their application to energy storage devices 151 are invited to consult the cited reviews. In the context of interfacial studies on SSBs, NMR measurements have only been performed ex situ to the best of our knowledge.

The rate of spontaneous exchange of Li + at solid/solid interfaces is a critical parameter for the performance of the battery. However, quantification of this process cannot be carried out with the techniques discussed so far. Alternatively, NMR appears as a valuable tool to extract kinetic parameters associated to Li + exchange at interfaces. This information can be gathered from experiments based on the transfer of magnetisation between two Li sites featuring distinguishable chemical shifts (δ). In the absence of strong dipolar interactions between the two nearby Li nuclei, this transfer arises from the spatial diffusion of Li + between the two sites. Since the amount of transferred magnetisation is integrated at different times and the experiments can be performed at several temperatures, the self-diffusion coefficient and the activation energy for the spontaneous Li + exchange can be calculated under the assumptions of a model derived from the Fick's law. This approach is commonly known in the literature as 2D-EXSY measurements.

The group of Marnix Wagemaker has exploited 2D-EXSY measurements to study spontaneous Li + exchange at Li6PS5X (X = Cl, Br)/Li2S 152,153 interfaces. Interestingly, the activation energy for the process at uncycled batteries was found to be low (0.12 and 0.13 eV for interfaces involving Li6PS5Cl and Li6PS5Br respectively), meaning that Li transport at these interfaces is relatively facile. However, diffusion of Li + at the interface is slow as the Li + self-diffusion coefficient (~ 10 -11 cm 2 s -1 ) is lower than the same parameter in the bulk of the solid electrolyte (~ 10 -9 cm 2 s -1 ). Thus, limitation of Li + transport over the solid electrolyte/active material interface was attributed to a small contact area between the two phases as Li + crossing is not energetically hampered. The same group also analysed the spontaneous Li + exchange at the Li6PS5Br/Li2S interface after galvanostatic cycling. 153 The application of only two full charge/discharge cycles led to a significant increase of activation energy from 0.13 to 0.39 eV. It was suggested that contact loss between the two phases due to repeated Li2S volumetric changes and/or formation of interphases could account for this increment.

2D-EXSY has also been exploited to assess the impact of the space charge layer on the Li diffusion resistance at the solid electrolyte/active material interface. To this end, the NASICON-type Li1.5Al0.5Ge1.5(PO3)4 (LAGP) 154 solid electrolyte was blended, in three different experiments, with three distinct LixV2O5 samples (a common cathode active material) exhibiting diverse Li chemical potential as a consequence of the different degree of lithiation. Hence, LiV2O5 presented the same Li chemical potential than LAGP and it did not result in the formation of a space charge layer, while Li2V2O5 and Li0.2V2O5 had lower and higher, respectively, Li chemical potential than LAGP and led to the growing of a space charge layer. The Li exchange NMR measurements revealed the detrimental effect of the space charge layer in the Li transport resistance. The measured activation energy increased from 0.32 eV for the space charge layer free LAGP/LiV2O5 interface to 0.52 eV for the LAGP/Li2V2O5 interface Therefore, this work suggests that the formation of the space charge layer could have a strong negative impact on the power capability of SSBs.

Li + diffusion over grain boundaries inside solid electrolyte is another critical property in SSBs which has been tackled with 2D-EXSY measurements. Wagemaker et al. have estimated the kinetics of this process by mixing two similar argyrodites solid electrolytes: Li6PS5Cl and Li6PS5Br. 155 Since the Li environments in the two materials gave signals with well-separated chemical shifts, Li exchange NMR measurements could be made to quantify its diffusion across the Li6PS5Cl/Li6PS5Br interface. Although this experiment cannot be properly considered as an evaluation of Li + diffusion through grain boundaries of a single material, it is a clever and good approximation. The calculated activation energy of the spontaneous Li + exchange was 0.27 eV, which is similar to the value obtained with other NMR analysis based on spin-lattice relaxation.

Li exchange experiments are excellent tools to study composite solid electrolytes containing Li + conductive inorganic filler as the promise of improvement of ionic conductivity in these systems versus the fully polymer electrolyte relies on the fast transfer of Li + between the polymer and the inorganic phases. Indeed, the efficiency of this exchange is nowadays a source of controversy. 156-158 A remarkable example of the application of 2D EXSY techniques to evaluate Li diffusion across the organic/inorganic interface in composite electrolytes has been recently published by López del Amo and coworkers. 159 The analysed electrolyte consisted in a mixture of PEO:LiTFSI(20:1) and 10 vol% LLZO. The experiments confirmed the spontaneous Li + exchange between polymer, LLZO and a layer of LiOH covering the surface of LLZO which is formed due to the Please do not adjust margins Please do not adjust margins sensitivity to moisture of this material. However, the calculated rate for Li diffusion are low compared to the value roughly estimated from electrochemical impedance measurements. Thus, Li + conduction for this solid electrolyte must occur only through the polymer phase.

Although Li 2D-EXSY is probably the most powerful technique to quantify the kinetics of Li diffusion at interfaces, it is not exempt of some limitations. Firstly, the two Li environments in the two phases involved in the exchange should result in chemical shifts sufficiently separated 155 or marked difference in spin-lattice relaxation times. 152 López del Amo and coworkers have shown that discrimination between two close Li signals can be notably improved by application of a 1 H-7 or 6 Li cross polarization step previous to the 2D-EXSY sequence. The second limitation concerns the spin-lattice relaxation time as this parameter determines the longer exchange time that can be probed. The spin-lattice relaxation time decreases in the presence of paramagnetic ions; thus it can make challenging the study of Li exchange at interfaces involving common active materials like LCO or NMC. Finally, experiments are carried out ex-situ. This can introduce a source of misinterpretation when the spontaneous Li exchange of a solid electrolyte with an active material at different state of charges is studied. The sample preparation requires disassembling of the battery and recovery of the composite electrode or chemical lithiation of the active material followed by blending and gridding with the solid electrolyte.

Another interesting application of solid-state NMR to the study of interfaces in SSBs is the tracking of formation and growing of Li dendrites by 7 Li magnetic resonance imaging (MRI). For instance, Clare P. Grey et al have used this technique to detect and visualise the onset and evolution of this Li microstructures in a Li/Li6.5La3Zr1.5Ta0.5O12(LLZTO)/Li symmetric cell. 160 The stark difference in chemical shift between 7 Li environments in the bulk metal and LLZTO allowed the unambiguous localisation of each phase, including dendritic Li, in the device. Importantly, the experiments revealed the growing of Li microstructures on the surface of the Li metal electrodes, probably corresponding to dendrites, before any sign of degradation in the voltage-time curves during galvanostatic cycling of the system. Furthermore, it was shown that the growing of dendrites occurred simultaneously on the plated and stripped lithium electrodes. The unexpected formation of Li microstructures on the stripped electrode was ascribed to roughing of the surface due to uneven Li releasing and subsequent formation of hot spots exhibiting enhanced current densities.

Despite 7 Li MRI is an attractive tool to monitor and visualise the growing of detrimental Li microstructures such as dendrites, the technique presents a poor spatial resolution (300 μm in above the experiments). This is the reason why the system studied by Clare P. Grey et al comprised a solid electrolyte with a thickness of 2.5 mm, which is unpractical from the point of view of energy density.

Other techniques.

In this section, several techniques with large SDs and less explored for the analysis of interfaces in SSBs will be discussed. Bun Tsuchiya and coworkers have recently developed 161 a new method based on the bombardment of thin SSBs with a beam of O 4+ ions and high-energy elastic recoil ion-beam detection (ERD) to track in situ the evolution of the concentration of Li, and eventually H, at the diverse interfaces in an LCO/Li1.4Al0.4Ge0.7Ti0.9P3O12 (LATP)/Pt microbattery. It was evidenced that charging leads to Li depletion at the LiCoO2/LATP interface and formation of a space charge layer with a 120 nm thickness in the solid electrolyte. The limitations of this approach are similar to that found in NDP: batteries with small dimensions, i.e. microbatteries, and well-defined and flat surfaces can only be analysed. Therefore, experiments cannot be conducted on composite and thick solid electrolytes and electrodes. In variable energy hard X-ray photoelectron spectroscopy (VE-HAXPES), the energy of a synchrotron light source can be tuned (3 -8 keV) to increase the inelastic mean free path (IMFP) of photoelectrons. 162 As a consequence, the detection depth of HAXPES is significantly larger than in XPS experiments carried out in common laboratory XPS instruments, which usually use Al Kα = (1486.7 eV) as X-ray source. Xueliang Sun et al. used VE-HAXPES to evaluate the formation and thickness of interphases in three different layered materials relevant in the SSBs research field: LATP/LCO, LATP/Al2O3/LCO and LATP/Li3PO4/LCO. In the two last configurations, Al2O3 and Li3PO4 played the role of a buffer material aimed at preventing interfusion of elements between the LATP and LCO layers. The combination of several techniques including VE-HAXPES indicated that Li3PO4 was the most effective buffer material. In situ and operando HAXPES measurements on a cycling LCO/ Li1+x+yAlx(Ti, Ge)2-xSiyP3-yO12 (LASGTP)/Li microbattery have been also carried out recently. 163 Hard X-ray enabled recording the core-level XPS spectra for several elements in a thickness region comprising the Al (current collector)/LCO/LASGTP interfaces. Similarly to the operando XPS experiments with a conventional laboratory instrument reported by the group of Mario El Kazzi, [66][67][68] a shift in the binding energy of peaks associated to the solid electrolyte was observed while the position of the Al 1s peaks of the current collector (grounded to the analyser) remained unchanged. Surprisingly, the O 1s peak corresponding to the lattice oxygen from LCO splitted into two peaks after charging. Hence, the authors suggested a participation of oxygen anions in the redox activity of the active material. Although Mario El Kazzi at al. studied also LCO as active material in their first contribution, 66 they did not provide unfortunately O 1s core-level spectra to allow for comparison with this stunning finding. Despite the increased depth detection provided by HAXPES vs laboratory XPS, the value is insufficient to be considered as a bulk technique for conventional SSBs. Sampling depths in HAXPES can attain 20-30 nm, which is extremely thin as compared with the thickness of common composite electrodes (40 -100 μm) for applications in e-mobility or grid storage. However, we have preferred to discuss HAXPES in the framework of the bulk approach as most applications of this technique in the field of SSBs aim at developing a nondestructive method which avoids also the modification of the current collectors. Please do not adjust margins Please do not adjust margins

Another challenge in HAXPES concerns the decrease of the Xray cross-sections of elements with an increase of the wavelength of the X-ray source. 164 Indeed, this is one of the main reasons why HAXPES is commonly carried out with synchrotron X-ray sources featuring a high brightness. A quick insight of the effect of X-ray energy on the quality of XPS data can be gathered from the aforementioned experiments reported by Xueliang Sun et al. using VE-HAXPES. 162 Finally, since a highly bright source is mandatory to obtain spectra with good quality in HAXPES, it remains to be proved if these X-ray sources do not lead to degradation of sensitive materials contained in SSBs like Li metal, organic polymers and Li salts.

A note of warning: depth profiling based on the use of ion etching.

Currently, the most popular approach to analyse buried interphases by XPS and TOF-SIMS is ion etching in the analysis chamber. Briefly, the procedure relies on the sequential repetition of sputtering a layer of surface material by an ion beam and subsequent analysis of the newly created surface by one of the two techniques. This approach enables the construction of a depth profile in which the composition of a hetero-layered material can be resolved. Unlike BIB polishing (section 3.1.1.3), the ion gun for depth profiling impinges directly on the sample with a higher angle of incidence (typically between 45° -60°) creating a crater larger than the analysis spot. The most common ion beam employed until now is based on energetic Ar + plasma. Nowadays, depth profiling is widely in the analysis of interphases in batteries to gain access to the spatially-resolved chemical composition of solid electrolyte interphases (SEIs) and cathode electrolyte interphases (CEIs). 165 The first documented study of the implementation of Ar + ion etching to study buried interfaces in the context of SSBs dates back to 2001 and it was carried by Masayoshi Watanabe and coworkers. 28 In their contribution, the SEI formed on a Li foil after prolonged contact (>1 month at 60°C) with a polymer solid electrolyte, made of cross-linked mono-acrylated and tri-acrylated copolymers of ethylene oxide and propylene oxide blended with LiTFSI, was analysed by XPS depth profiling. The spectra taken at increasing etching times unravelled a tri-layered SEI in which the outermost layer consists in a mixture of organic components (polymer residues and decomposition products from Li salts) and a minor amount of LiF, the intermediate layer is mainly made of Li2CO3 or LiOH and LiF, and the inner layer contains a mixture of Li2O and LiF.

In this perspective, we provide experimental results assessing common interpretations of depth profiling studies based on ion etching to define spatially the chemical composition of interphases in Li ion and solid-state batteries. In our study, the stability under the ion beam of one highly popular solid electrolyte polymer film (PEO + LiTFSI) was evaluated. The surface of the film was initially irradiated for 10 s with an Ar + beam at room temperature (298 K) similar to the one used previously by the group of Watanabe and in most of literature reports (Figure 9, upper). Despite the short duration of the ion etching step, the chemical state of the components of the film is severely affected. As it could be expected, the C 1s core XPS spectrum evidence the formation of degradation products containing an increasing amount of C-C(H) (≈ 285 eV) and probably a mixture of C(H)x-Fy and C-Ox bonds (small signals between 287.5 -291.5 eV). Importantly, the F 1s core spectrum shows the formation of an important amount of LiF (684.5 eV) as decomposition product. The origin of LiF is probably an ion induced degradation reaction of the LiTFSI salt. Therefore, depth profiling by Ar + etching on buried interphases in SSBs can lead to the misinterpretation of a hetero-layered material comprising an outermost layer made of an organic-rich phase, actually corresponding to the no sputtered surface which has not been degraded, and an inner and more inorganic layer rich in LiF, actually corresponding to the degradation of the Li salt after sputtering of the material. Furthermore, certain inorganic compounds also suffer from chemical instability when exposed to Ar + beams. This is the case for Li2CO3 and LiOH, which are commonly found in SEIs and CEIs, and are known to partially convert into Li2O. 166,167 Thus, the presence of Li2O as an inherent and inner component of interphases can also be critically discussed if this information was gathered from Ar + ion etching.

In order to prevent the deleterious reactions triggered by Ar + etching, we envisaged two different potential solutions. Firstly, the temperature of the process was reduced (143 K). This approach had a negligible beneficial effect suggesting that the degradation reaction is induced by the formation of free radicals intermediates and it is not only a result of the low electronic and thermal conductivities of organic materials (Figure 9, bottom). 168 In a second strategy, the etching was carried out under more gentle conditions by using an argon gas cluster ion beam (GCIB). This strategy has been successfully applied to the surface analysis of sensitive organic layered materials in the field of photovoltaics. 169 Indeed, the use of GCIB at both room (Figure 9, upper) and low (Figure 9, bottom) temperatures reduces the degradation of the PEO:LiTFSI film as the intensity of the peaks associated to decomposition products in the C 1s core spectra are significantly lower and the intensity of the peak associated to PEO is almost constant. Notwithstanding, inspection of the F 1s core spectra reveals the unambiguous formation of LiF after sputtering. Therefore, LiTFSI is also degraded under these conditions, although in a lesser extent when compared to Ar + experiments.

Since etching rates (nm s -1 ) are significantly slower with GCIBs than with Ar + beams and inorganic materials are hardly sputtered with GCIBs, there is thus no clear advantage in using GCIBs for depth profiling in SSBs containing LiTFSI salt or a mixture of inorganic and organic materials. Moreover, considering the degradation phenomenon under Ar + , depth profiling using Ar + ion etching should not be performed without caution for such inorganic/organic composites. 28,29,116,170 Please do not adjust margins

Please do not adjust margins

The performance of current SSBs is dictated by the electrochemo-mechanical phenomena occurring at the interfaces of materials. Hence, the comprehensive understanding of these processes is vital to advance toward the development of commercially available SSBs. However, the buried nature of the interfaces makes difficult the acquisition of reliable results by common spectroscopic and microscopic techniques. This critical aspect is usually circumvented in the increasing number of reports on interfacial studies in SSBs. Indeed, from the perspective of interfacial studies, the buried nature of interfaces in SSBs is the critical difference with the same kind of analysis in traditional Li-ion batteries (LIBs) containing liquid electrolytes. In LIBs, the cathode/separator/anode assembly can be relatively easily separated without compromise the structural integrity and chemical composition of interphases. Although, even in this devices, there can be controversy about certain manipulations, e.g. the suitability of rinsing the electrodes after disassembling of the cell. 171 Performing the same separation strategy in SSBs is not convenient for the reasons explained in section 1. Furthermore, if the cathode/solid electrolyte/anode assembly can be easily separated in a SSB, it means that interfacial contact in the system is poor and the observed reactivity could be actually mitigated regarding a battery with better interfacial contacts which is more representative of the intrinsic reactivity of the materials. Despite the more challenging character of analysis of interfaces in SSBs, the solid state of all the components of these systems makes easier the development of in situ and operando methods under ultra-high vacuum conditions. In LIBs, the application of this kind of techniques is obviously more difficult due to the evaporation and stability of the liquid electrolyte. For instance, in situ XPS experiments have been carried out on LIBs, but the batteries comprised an ionic liquid electrolyte stable under high vacuum conditions and a specially conceived Li permeable amorphous carbon membrane. [172][173][174] Therefore, SSBs present the advantage of simple in situ and operando compatibility with very powerful characterization techniques (TEM, SEM, XPS, ToF-SIMS among others).However, as thoroughly discussed in this perspective, careful sample preparation procedures must be adopted to ensure the reliability and reproducibility of the results.

In this perspective, we have evaluated the two main approaches employed nowadays to tackle the challenge of achieving reliable, reproducible and representative information from buried interfaces in SSBs. Likewise, excellent examples illustrating the strength of these approaches to extract valuable electro-chemo-mechanical interfacial data have been discussed. Furthermore, it has been shown that the implementation of robust procedures to get access and analyse buried interfaces has paved the way to the development of in situ and operando measurements.

In Table 1, we compile the advantages and drawbacks of all the methodologies used in each approach following criteria from a ARTICLE Please do not adjust margins Please do not adjust margins sample preparation outlook, namely the level of sample destruction, the compatibility with certain materials and SSBs, the risk of degradation of sensitive materials, the adaptability to in situ/operando experiments and the dimensions of the analysable area for procedures involving exposing crosssections. In Table 2, a different perspective is considered. Thus, the advantages and drawbacks for the diverse analytical techniques more frequently used in the Uncovering approach and the methods of the bulk approach are assessed from analytical criteria such as lateral resolution, level of extracted chemical information or possible degradation of materials due to analytical beam-sample interaction. Please do not adjust margins Please do not adjust margins Finally, we have examined with new experiments carried out in our laboratory the suitability of depth profiling studies to unveil and spatially resolve the chemical composition of interphases in SSBs containing a blend of polymer and Li salt as electrolyte. Our results suggest that extreme precaution must be adopted to interpret the data as polymers and Li salts are sensitive to the ion milling beam even with soft GCIBs and low operation temperatures (143 K). Since organic molecules and residual amounts of Li salt are common components of the SEI and CEI in conventional LIBs, similar precautions should be considered in the depth profiling analysis of these samples. The growing interest in SSBs at both the academic and industrial level is expected to attract increasingly the focus on interfacial phenomena as a key issue to improve the capabilities of the devices. Moreover, buried interfaces are commonly found in materials science. For instance, hetero-layered films in photovoltaics, core-shell nanostructures and anti-corrosion surface coatings. Thus, all the discussed strategies also concern the analysis of a wide range of materials and devices. Similarly, the field of analysis of interfaces in SSBs could adopt techniques and methodologies exploited in LIBs and devices facing the challenge of buried interfaces. For instance, vibrational sum frequency generation (vSFG) spectroscopy is a powerful method that has been exploited in LIBs 175 and other electrochemical devices [176][177][178] for probing molecular interactions at interfaces. VSFG spectroscopy detects resonant vibrational transitions which are both IR and RAMAN actives. The recorded sum-frequency photon can only be emitted by molecules localised at the interface of two centrosymmetric environments. This particularity makes vSFG spectroscopy a surface-sensitive technique. A more detailed description on the physical principles behind this spectroscopy can be found in the references cited above. In LIBs, vSFG has provided relevant information about the varied orientation that can adopt absorbed solvent molecules on the surface of several active materials like Si anodes 179 and LCO cathodes. 180,181 The application of this analytical tool to the study of interfaces in SSBs could reveal unnoticed molecular interactions, particularly in systems containing organic polymers or composites as electrolytes. For instance, vSFG spectroscopy could be employed to unravel the orientation and interaction of functional groups at the polymer/inorganic filler interface. It is worth nothing that this technique is surface-sensitive, thus it will require rigorous sample preparation procedures. Neutron computed tomography could be also an attractive technique to track the distribution of Li at interfaces of SSBs, as scattering cross-sections of light elements are enhanced as compared to analogous experiments based on x-ray sources.

Please do not adjust margins Please do not adjust margins Furthermore, neutron computed tomography is sensitive to isotopes, with 6 Li and 7 Li exhibiting extremely different scattering cross-sections. 182 Preliminary experiments 183 seem validate the application of this analytical tool to delineate a Li profile at the cathode/solid electrolyte interface in a TiS2/ 6 Li3PS4/In- 6 Li battery. To conclude, we expect that our perspective makes researchers scrutinizing materials with buried interfaces aware of the suitability of the chosen methodology to uncover them besides the attainable structural and/or chemical information that can be extracted from a particular analytical technique. Similarly, we encourage researchers interested in the analysis of interfacial phenomena in SSBs to combine the discussed robust methodologies with less explored analytical techniques, like vSFG, to gain deeper insight on electro-chemo-mechanical processes at interfaces.

Figure 1 . 4 |

 14 Figure 1. Schematic illustration of buried interfaces existing in Li-metal SSBs.

Figure 2 .

 2 Figure 2. Current rigorous methodologies used to expose interfaces in SSBs allowing their subsequent reliable analysis.

Figure 3 .

 3 Figure 3. (A) Image recorded by SEM of a cross-section from a NMC 622/PEO:LiTFSI(30:1)/Li battery prepared via cryo-microtomy. (B) Images recorded of the same cross-section at the beginning and the end of a galvanostatic cycling experiment monitored in situ by SEM. A dramatic thinning of the polymer solid electrolyte is clearly observed at the end of cycling. (C) Image recorded by SEM of the same cross-section showing the formation of Li dendrites after galvanostatic cycling. Note the poor resolution of the microstructure of the NMC cathode achieved by cryomicrotomy. Figures adapted with permissions from reference 48. Copyright (2020) American Chemical Society.

Figure 4 .

 4 Figure 4. (A) Image recorded by SE detection of a cross-section from the cathode of a NMC 622/β-Li3PS4 /Li battery prepared by FIB inside the analysis chamber of a ToF-SIMS instrument. Note the good resolution of the microstructure of the composite electrode. (B) Images recorded by ToF-SIMS of four different FIB-prepared cross-sections of two composite cathodes, differing in the presence or absence of the conductive additive VGCF, before and after galvanostatic cycling. Selected fragments were exploited to construct the images. Thus, C2 -is representative from VGCFs, NiO2 -from NMC 622 and PO2 -and PO3 -from interphases containing phosphate or phosphite. Figure adapted with permissions from reference 52. Copyright (2020) American Chemical Society.

Figure 5 .

 5 Figure 5. (A) Image recorded by SEM of a cross-section from a LiCO/LiPON/Li microbattery prepared by BIB polishing at RT. (B) Image recorded by SEM of a cross-section from a NMC 532/PEO:LiTFSI/Graphite battery prepared by BIB polishing at 133 K. Note the excellent resolution of the microstructure of the cathode compared to cryo-microtomy and the large dimensions of the cross-section compared to FIB. (C) Images recorded by SEM and SAM of two different cross-sections from the microbattery in (A) after fist charge and discharge. (D) Images recorded by SEM and SAM of the cross-section in (B) focused on a region near the Al CC/cathode interface. Note the coherent distribution extracted from SAM for NMC (CoLMN signal), polymer-LiTFSI blend infiltrated in the cathode (SLVV signal) and CC (AlKLL signal). Figures (A) and (C) adapted with permissions from reference 63. Copyright (2017) American Chemical Society.

Figure 6 . 10 |

 610 Figure 6. (A) Schematic drawing of the spectroelectrochemical cell developed by the group of Mario El kazzi for operando XPS experiments on SSBs. (B)Top-view image recorded by SEM of a LiCoO2/(Li2S)3-P2S5(LPS)/InLix battery showing the surface of the composite cathode including the network of the stainless steel mesh (SSM) which plays the role of perforated current collector. (C) Operando monitoring of shifting in the core level S 2p and P 2p spectra of peaks associated to the solid electrolyte during charge of the aforementioned battery. In this experiment, the cathode was grounded. (B) Plot of shifting in BE for diverse peaks associated to materials in the cathode during charge when the cathode is grounded. Since the solid electrolyte is an electric isolator, its associated peaks shift at lower BEs with increasing electrochemical potentials. Since LCO and carbon electronic additive are electric conductors, their associated peaks do not shift because they follow the polarisation of the battery. Figures adapted from Ref. 66 with permission from the PCCP Owner Societies.

Figure 7 .

 7 Figure 7. Overview of main analytical techniques currently used for the study of exposed buried interfaces in SSBs based on the interaction of radiation, electrons or ions (A) and atomic probes (B) with the uncovered surfaces of the diverse materials. The figure represents experiments carried out on well-defined prepared cross-sections; however, a number of the techniques depicted in A have also been applied to surfaces exposed following the strategy of perforated current collector (see text for more details). Sampling depth (SD) refers to the thickness of the surface of material than can be probed with a particular techniques and Lateral resolution (LR) refers to the ability of a particular technique for distinguish two distinct points on the surface of the material.

  -40 hours) at open circuit voltage. The authors explained the different behaviours for NMC 111 and LFP cathodes as the result of their distinct charge conductivities, (de)lithiation mechanisms and microstructures. Adopting the strategy of perforated current collectors, Carlos A. F. Vaz et al. have recently showed 126 the possibility to study the chemical composition of interphases by in situ and operando sXAS. The experiments evidenced the formation of Li2CO3 and Li2O in the graphitic areas of a composite graphite electrode after Li insertion in a graphite/LiI doped LPS/InLix battery.

Figure 8 .

 8 Figure 8. Overview of main analytical techniques used in the bulk approach to probe buried interfaces in SSBs.

20 |

 20 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Figure 9 .

 9 Figure 9. Depth profiling study on a PEO:LiTSFI (10:1) film using Ar + at 4 keV or 𝐴𝐴𝐴𝐴 2000 + at 4 keV (2 eV per Ar atom) as sputtering beam. The experimental data is represented by dots and the fitting line results from convolution of the proposed components.

Table 1 .

 1 Resume of advantages and drawbacks for the uncovering and bulk approaches considering aspects associated to the preparation of samples from SSBs and the key features of the prepared samples. Induction of a nonhomogeneous distribution of the electrical field This journal is © The Royal Society of Chemistry 20xxPlease do not adjust margins Please do not adjust margins

	Approach	Methodology/technique		Advantages	Drawbacks
	Uncovering buried interfaces	(Cryo)Microtomy	• Fast			• Significantly destructive
			• Creation of large cross-	• No assured preservation
			sections		of microstructure even under
			• Simple procedure to	cryogenic conditions
			prepare samples for in situ	• Only used for SSBs with
			and operando experiments	polymer solid electrolyte
						until now
		FIB	• Less destructive than	• Creation of small cross-
			(cryo)microtomy and BIB	sections (50 μm × 50 μm).
			polishing		Although enlarged with Xe
			• Best	procedure	for	plasma beam
			preparation of extremely thin	• Produce damage in the
			cross-sections for (S)TEM-	created surface (minimized if
			EELS analysis		carried out under cryogenic
			• Combined with a number	conditions and/or combined
			of	microscopic	and	with a final low-energy
			spectroscopic	techniques:	polishing step)
			(S)TEM-EELS, SEM, ToF-SIMS.	• Preservation	of	the
			• Only method allowing the	microstructure of certain
			preparation of nanobatteries	materials only possible under
			from microbatteries for in	cryogenic conditions, e. g. Li
			situ	and	operando	electrodes
			experiments		• Complex procedure for
						preparation of nanobatteries
						for in situ and operando
						experiments
						• Impossible application on
						composites electrodes and
						solid electrolytes for in situ
						and operando experiments
		BIB polishing	• Preparation	of	large	• More destructive than
			cross-sections (ca 700 μm	FIB,	but	less	than
			width x 400 μm depth)	(cryo)microtomy
			• Combined with a number	• Produce damage in the
			of	microscopic	and	created surface due to
			spectroscopic	techniques:	heating effects on thermally
			SEM, AES-SAM, ToF-SIMS,	insulating	materials	like
			cAFM, SSRM, KPM, RAMAN.	polymers.	(minimised	if
			• Allowing	for	the	process carried out under
			preparation of samples for a	cryogenic conditions)
			great variety of in situ and	• Exposed	cross-section
			operando experiments (in	less reactive than the bulk in
			situ less complex than for FIB)	in	situ	and	operando
			• Possible application on	experiments (edge effect)
			composites electrodes and
			solid electrolytes for in situ
			and operando experiments
		Perforated current collectors • Non-destructive	• More difficult composite
			• Excellent	for	the	electrode processing due to
			preparation of samples for in	casting or pressing of
			situ and operando XPS and	materials on a perforated
			sXAS experiments, where	current collector
			spatial resolution or sensitive	•
			are an issue
			• Possible application on
			composites electrodes and

Table 2 .

 2 Resume of advantages and drawbacks for the uncovering and bulk approaches considering aspects associated to the analytical measurement of samples. No chemical or structural information on interphases This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

	Approach	Technique		Advantages		Drawbacks
	Uncovering buried interfaces	SEM + (EDS)	• Good lateral resolution	• Limited	chemical
			• Information		on	information on interphases
			microstructure of materials in	• Obtained	chemical
			2D an even 3D when	information by EDS no
			combined with FIB (FIB	representative	of	the
			tomography)			chemical state of cross-
							section surface (large SD)
							• E-beam	damage	of
							organic materials and Li
		(S)TEM + (EELS, EH)	• Excellent		lateral	• Limited	chemical
			resolution			information on interphases
			• Li detection possible with	• E-beam	damage	of
			EELS				organic materials and Li
			• Probing of electric fields	• Limited to the study of
			possible with EH		extremely thin samples (<
							100 nm)
							• In-situ and operando
							experiments restricted to
							nanobatteries
		XPS	• Rich chemical information	• Poor lateral resolution
			on interphases			• Challenging	for	Li
			• Possibility of probing	detection
			differences	in	electric	• Ambiguous
			potentials, particularly in in	interpretations	when
			situ	and	operando	combined	with	depth
			experiments			profiling

Journal of Materials Chemistry A

Acknowledgements

This perspective was carried out as part of the HUB RAISE2024 project. This project has received funding from Excellence Initiative of Université de Pau et des Pays de l'Adour -I-Site E2S UPPA, a French "Investissements d'Avenir" programme.

Conflicts of interest

There are no conflicts to declare.