Ali Darwiche

Fabrizio Murgia

Marcus Fehse

Abdelfattah Mahmoud

Antonella Iadecola

Stéphanie Belin

Camille La Fontaine

Valérie Briois

Raphael P Hermann

Bernard Fraisse

Romain Berthelot

Moulay T Sougrati

Laure Monconduit

Lorenzo Stievano
email: lorenzo.stievano@umontpellier.fr

Operando X-ray absorption spectroscopy applied to battery materials at ICGM: The challenging case of BiSb's sodiation

Keywords: X-ray absorption spectroscopy, Batteries, Energy materials, Electrode materials, Electrochemical mechanism, Operando measurements

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

One of challenges that our society has to face is the sustainable use of energy.

In particular, the demand for advanced energy storage devices has increased significantly, motivated by a variety of different needs of our technologically driven, highly mobile, energy-hungry society. Among the available technologies, 5 electrochemical energy storage devices, i.e., batteries, can provide the solution to the problems connected to the intrinsic intermittency of renewable energy sources, since they can buffer the energy surplus produced by the operating plant in periods of low consumption, and feed it back to the power grid in case of high demand. [1] Moreover, they can fulfill the ever growing demand of energy for portable applications (mobile phones and computers, and more recently electric vehicles). The excellent performance and the well-established technology of lithium-ion batteries (LIB) put them in a key position for supporting this new energy revolution. [2] Several post-LIB technologies, such as lithium-sulfur batteries (LSB) or sodium-ion batteries (NIB), have also been proposed in the last years, as viable performing alternatives to LIB.

Both LIBs and post-LIB systems are based on the so-called "rocking chair" mechanism, where Li + cations are exchanged alternatively between the positive and the negative electrode during cycling. In such a system, the two electrodes can be any sort of material that is able to undergo reversibly a reduction/oxidation process at a specific high or low potential (for the positive or negative electrode, respectively) with the concomitant addition/elimination of Li + cations. For this reason, a large number of different materials able to reversibly form lithiated phases via different reaction mechanisms, i.e., intercalation, [3,4,5] alloying, [6,7,8] or conversion, [9,10,11,12] have been proposed, thus generating a great deal of studies. In addition to the reaction mechanisms at the electrodes, other features concerning the electrolytes and their interaction with the electrodes, including the formation of the solid-electrolyte interphase (SEI), [13] which is of primordial importance for the stability and the cycle life of the battery, have been thoroughly studied.

In such a picture, many characterization techniques such as infrared, [14,15,16,17] Raman, [18,19,17] Mössbauer, [20] X-ray absorption (XAS) [21,22,23,24,[START_REF] Giorgetti | X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation[END_REF][START_REF] Li | [END_REF] and X-ray photoelectron spectroscopy (XPS) [27,28,15] as well as Xray diffraction (XRD) [29,30] have been largely applied, either ex situ, in situ or under operando conditions, for the characterization of the pristine materials and of their reaction mechanisms. Among them, XAS remains a technique of choice for retrieving structural and electronic information, especially when the materials or some of the species formed through the electrochemical reactions are not crystalline and cannot be studied by diffraction techniques. The most important features of XAS are: (i) its element specificity, allowing the study of a single element by concentrating on its K (or in some cases L) absorption edge; (ii) the possibility of tuning it quasi-simultaneously to different sites (for instance Sn and Sb in SnSb), [31] thus providing sources of complementary information on the same compound; (iii) the physico-chemical information contained in the near-edge structure of the XAS signals, which can be used to reveal the formal oxidation state and the local symmetry of the probed atom; (iv) the possibility of performing measurements under operando conditions using specifically developed in situ electrochemical cells. [32] In this case, the physico-chemical properties and the local structure of the studied element can be monitored at all moments during the charge and discharge processes.

The application of XAS to the study of battery materials and their electrochemical reaction mechanisms has started several decades ago: to the best of our knowledge, the first paper on the application of XAS to the field of batteries was published by Mc Breen et al. [33] Many works have been published since that paper, resumed in different reviews which appeared over the years highlighting their principal results and advances. [21,22,23,24,[START_REF] Giorgetti | X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation[END_REF][START_REF] Li | [END_REF] In this paper, after a very short presentation of the technique and of the corresponding experimental methods, its potentialities will be highlighted going through its application to the study of battery materials at Institut Charles Gerhardt Montpellier (ICGM, France) in the last decade. Finally, a challenging example of operando XAS to the study of the sodiation of Bi 0.50 Sb 0.50 simultaneously at both Sb K-edge and Bi L 3 edge will be presented. This example will help us emphasize both the extremely advanced analytic capability of the ROCK beamline of Synchrotron Soleil (France), [34] probably the only beamline allowing continuous alternate measurements of XAS spectra of two elements with absorption edges at such different energies (Sb Kedge and Bi L 3 -edge, 30491 and 13418 eV, respectively), as well as the use of chemometric methods for the analysis of the collected data. These findings perfectly complement the information gathered by operando XRD, allowing a precise description of the electrochemical mechanisms governing both Sb and Bi centres during a charge/discharge cycle.

Advantages and challenges of operando X-ray absorption spectroscopy : the work performed at ICGM

What is XAS?

XAS ( sometimes abbreviated as XAFS for X-ray absorption fine structure) is an element-specific spectroscopic technique providing both chemical and structural information at the local scale (4-5 Å) about the probed element; for this reason, it is well suited for the characterization of both crystalline and amorphous materials. The principles of XAS reside on the resonant absorption of X-rays generated by the photoelectric effect, i.e., the photon-stimulated emission of electrons occurring when the energy of the incident photon equals or exceeds that of the ionisation energy of a specific electron, giving rise to an absorption edge. Depending on the probed element, the energy ranges from soft to hard X-rays regimes (0.2-40 keV). XAS (or XAFS) generally refers to the entire spectrum, which is constituted by the edge region usually abbreviated as XANES (X-ray Absorption Near-Edge Structure), comprising the first 80-100 eV above the absorption edge, and a post-edge region usually abbreviated as EXAFS (Extended X-Ray Absorption Fine Structure), which usually extends to 1000 eV above the absorption edge.

The XANES region, comprising some pre-edge features and the absorption edge itself, is extremely sensitive to the oxidation state and to the coordination of the probed element. On the other hand, the periodic oscillations found in the EXAFS region, generated by the scattering of the emitted photoelectrons with the electrons of the neighbouring atoms, provide quantitative information about the nearest neighbours (bond distances, coordination number and nature of the bonding atoms).

XAS is very versatile and can be applied to all states of matter, from liquid to solids, crystalline or amorphous. It has been used with great success in many research fields, such as catalysis, [35,36,37,38,[START_REF] Van Bokhoven | X-ray absorption and X-ray emission spectroscopy : theory and applications[END_REF] biology, [START_REF] Solomon | [END_REF]41] inorganic metal complexes, [42] nanomaterials, [START_REF] Boscherini | Synchrotron Radiation[END_REF] and electrochemical interfaces. [START_REF] Dewald | [END_REF]15] Several excellent books are also available to obtain further detailed insights on the theory and on the applications of XAS. [START_REF] Teo | EXAFS: Basic Principles and Data Analysis, volume 9 of Inorganic Chemistry Concepts[END_REF][START_REF] Koningsberger | X-ray absorption : principles, applications, techniques of EXAFS, SEXAFS, and XANES[END_REF][START_REF] Bunker | [END_REF][START_REF] Van Bokhoven | X-ray absorption and X-ray emission spectroscopy : theory and applications[END_REF]

Studies carried out at ICGM: anode and cathode insertion materials

In the last 10 years, most of the potentialities of XAS have been applied at ICGM to the study of batteries and battery materials. Having access to different synchrotron sources with complementary energy ranges and resolution, it was possible not only to fully profit of the advantages of this technique in studying several families of electrode materials, but also to extend its capabilities to the study of extremely complicated and challenging systems, improving the strategies for data analysis by applying specific advanced chemometric tools. This advanced approach allowed us, in some specific cases such as the reversible sodiation of antimony, [48] to reach the limits of its sensitivity, demonstrating that XAS is intrinsically unable to observe the formation of some of the intermediates of the electrochemical reaction.

The development of operando XAS was made possible thanks to the previous work of Leriche et al., who designed, in collaboration with the team of beamline SAMBA at Synchrotron Soleil (now working at the more recent beamline ROCK, specialised on operando XAS), a new efficient in situ electrochemical cell. [32] This cell (shown in the Supplementary Material in the section concerning the operando study of BiSb by XAS), based on a previous one developed for operando XRD measurements, [49] allows the investigation of electrode materials cycling under the same experimental conditions as those commonly employed in typical laboratory cells, thus enabling a simplified association of the operando XAS results to the lab-scale electrochemical tests. This cell was later also successfully adapted for operando Mössbauer, [50] infrared [16] and Raman [19] spectroscopy, and even applied for simultaneous XRD-Mössbauer spectroscopy studies. [START_REF] Jumas | Analysis of the features of an electrode material of an electrochemical cell[END_REF] At ICGM, this cell was applied first to the relatively simple study of mixed metal phosphates with olivine structure with general formula LiFe x Mn 1-x O 4 as cathode materials for LIB. [START_REF] Perea | [END_REF]. In this case, the valence state of transition metal cations was deduced from the energy shift and the simultaneous change in the shape of the absorption edge. Quantitatively, both operando Fe and Mn K-edge XANES spectra of a sample with composition LiFe 0.33 Mn 0.67 PO 4 were fitted using a linear combination of the corresponding spectra of the pristine and of the fully lithiated material. The result of this procedure, shown in Fig. 1 allows decoupling the sequential oxidation of Fe and Mn in correspondence to the first and second potential plateau, respectively. Interestingly, an almost linear relationship between the amount of total metal (Mn and Fe together) in the trivalent state and the charge supplied to or released from the battery, is found showing that the main reactions of Li at the working potentials used in this test are indeed the oxidation and reduction of these two metals.

The same type of electrode material was also used in a second work as a benchmark for our first application of chemometric methods to data analysis, namely Principal Component Analysis (PCA) and Multivariate Curve Resolution -Alternating Least Squares (MCR-ALS). [53] In fact, the combined use of PCA and MCR-ALS offers a unbiased separation method to process large sets of data by expressing them based on their similarities and differences.

In short, PCA is a factor analysis, i.e., a multivariate technique for reducing matrices of data to their lowest dimensionality by the use of an orthogonal factor space: in such methods, the analyzed matrix corresponds to the experimental spectra, and each spectrum is considered as an n-dimensions vector with n corresponding to the number of points within each XAS spectrum. The first principal component has the largest initial variance, i.e., alone it takes into account as much as possible the whole set of data. The following principal components are built orthogonal to the first one, and allow the reproduction of the experimental spectra with the highest possible variance via adapted linear combinations of them. This supposes that the group of analyzed spectra is intrinsically bilinear, which means that all spectra can be expressed as linear combinations of an orthogonal basis set of uncorrelated spectra. This basis set, which has a lower dimensionality than the number of original spectra, is thus obtained from the calculation. In the different linear combinations which are built to reconstruct each experimental spectrum, the multiplication factors of the principal components are usually called scores. The so-obtained principal components are only orthogonal mathematical functions and not real spectra, nevertheless their determination reveals the number of independent spectral components that can be used to describe the whole set of experimental data.

Several methodologies are then available to reconstruct the corresponding real XAS components as well as their evolution such as the MCR-ALS analysis tool.

A detailed description of this method from a theoretical point of view is given by Tauler et al. [54,55]0 who also proposed this method for the analysis of in situ spectroscopic data. For more details about the application of these algorithms to operando XAS for catalysts or batteries, see refs. [56,57,58].

In the case of Fe-Mn mixed-metal olivine cathodes, these methods were applied independently to both whole sets of operando Fe and Mn K-edge EXAFS spectra in order to extract the significant information on the evolution of the local electronic and structural properties of both metal centres during oxidation as a function of the Li content. This innovative approach enabled the detailed reconstruction of the whole reaction mechanism, revealing the simultaneous evolution of the structure of both metal centres even when they were not directly participating to the redox process. In particular, the study of the evolution of the first coordination shell in the Mn K-edge EXAFS spectra, and more specifically its separation into two sub-shells with very different bond distances, allowed the precise description of the Jahn-Teller effect occurring upon oxidation of the manganese centres. This effect is known to be detrimental to the performance of such mixed-metal systems, partially hampering their complete delithiation.

Similar studies were also conducted on TiO 2 -based insertion anode materials as high energy density alternative to commercially applied lithium titanate Li 4 Ti 5 O 12 . Operando XAS was used to follow electronic and local structural changes upon lithiation of anatase and TiO 2 (B) polymorph, which provided the typical spectra shown in Figure 2. Even though both polymorphs are characterised by the same theoretical specific capacity of 336 mAh/g, corresponding to the insertion of 1 Li into the structure and the reduction of Ti 4+ to Ti 3+ , only about 0.6 Li + can be reversibly inserted into anatase (≈200 mAh/g). With TiO 2 (B), conversely, higher reversible capacities exceeding ≈200 mAh/g can be reached. [START_REF] Fehse | Monconduit[END_REF] Moreover, better rate capabilities are reached using TiO 2 (B) using materials with similar particle size distributions. Indeed, these differences in performance are related to their different crystal structure implying distinct electrochemical reaction paths vs. lithium. In fact, the lithiation of anatase phase undergoes successive reaction steps: a first monophasic insertion is followed by a biphasic lattice transformation from a tetragonal to an orthorhombic structure, which is then retained during the following insertion which is accompanied by 4% lattice expansion. [60] XAS coupled to XRD showed that the first monophasic step, occurring up to ≈0.1 Li, does not inflict any local or global structural changes, whereas the following biphasic and monphasic reactions bring along a deformation of the octahedral coordination of Ti (detected, among other, from the modification of the XANES pre-edge features) and a gradual expansion of the coordination shells. [61] On the other hand, the insertion of Li + in TiO 2 (B) follows a monophasic reaction path, with the crystal structure only subject to minor extensions upon lithiation (cf. Figure 2). [62] In the latter case, EXAFS was particularly useful to show for the first time that the structural evolution upon lithiation of TiO 2 (B) follows a two-regime process which matches the characteristic twin-peak of its electrochemical signature (not shown). These insights could hence be utilized to better understand the differences in mechanism and cycling performance of the two polymorphs. [START_REF] Fehse | Monconduit[END_REF]60] In addition to these works, the use of operando XAS was extended to other electrode materials in the framework of collaborations with other institutes. For instance, in the last years a growing activity in the investigation of the electro-chemical mechanism of Prussian Blue analogs was carried out in collaboration with the University of Bologna. [63,64,65,66] Among the main results of these studies, we can highlight a study on copper hexacyanoferrate performed simultaneously by Fe and Cu K-edge XAS, in which it was showed that both metals have a redox activity during lithiation, [66] as well as a parallel work on Prussian Blue analogs where the cyanide groups are replaced by redox-active nitrosyl ligands, in which the redox activity is shared on three different centres, i.e., the nitrosyl group and the two transition metals. [64]

Studies carried out at ICGM: Lithium-sulfur batteries

Simultaneously, a comprehensive work on Li-sulfur systems was also initiated in the framework of the European project EUROLIS. LSB are particularly interesting devices, since the combination of lithium and the inexpensive and naturally abundant sulfur in an electrochemical cell corresponds to the considerable theoretical energy density of 2600 Wh/kg. Even though the maximum practically accessible energy density is predicted to be at best 600 Wh/kg, it remains much higher than that of LIB. [67,68] In spite of these promising properties, LSB also suffer from several drawbacks: the main one is surely the diffusion of polysulfides (Li 2 S n ), produced during the first steps of the reduction of sulfur and highly soluble in the electrolyte, which cause the well-known 'shuttle' phenomenon. [69] The second main drawback is the insulating nature of both solid Li 2 S and elemental sulfur, which preclude the typical preparation of electrodes by casting formulated inks containing sulfur and carbon additive(s) on aluminum current collectors. [70,71] All these disadvantages are strongly impacting both capacity and cyclability of LSB, causing rapid fading and low coulombic efficiency.

Several improvements have been suggested to tackle these drawbacks, including the confinement of sulfur into porous conductive carbon materials to reduce its mobility and improve the electronic conductivity, or the preparation of multifunctional positive electrodes, enhancing sulfur loading and promoting the interaction of polysulfides with the electrode host to prevent their diffu-sion in the electrolyte. [72,73,74,75,76] In many studies, XAS was employed among other techniques for the detailed investigation of the electrochemical mechanism and the diffusion (or retention) of polysulfides, as well as of possible failure paths. [77] In our works, we mostly employed sulfur K-edge XANES to obtain semi-quantitative information about the evolution of the relative amounts of the different sulfur species during the discharge and the charge process, as well as about the modification of the total concentration of sulfur in both cathode and electrolyte, which is connected to the diffusion of the polysulfides in the whole battery. [78,79,80,81,82] This was performed mainly by fitting the XANES spectra, measured either at the electrode surface or on the separator soaked with the electrolyte, to linear combinations of reference spectra of pure sulfur, polysulfides and Li 2 S. Given the low energy of sulfur K-edge (only 2.47 keV), these experiments had to be performed in fluorescence mode on a modified version of our in situ cell, equipped with an extremely thin beryllium window (8 µm) and kept inside a tight container under pure He flow, specifically designed by the beamline team at the XAFS beamline of synchrotron Elettra (Trieste, Italy). [79,80] In a particularly experiment, in addition to the XANES data, also EXAFS spectra could be employed for the study the electrochemical mechanism. [79] Such study, in fact, was carried on using a sulfur-free electrolyte salt based on the LiTDI (Lithium 4,5-dicyano-2-(trifluoromethyl)imidazole) salt dissolved in a 1:1 mixture of tetraethylene glycol dimethyl ether (TEGDME) and dioxolane (DOL), which thus allowed following the EXAFS contribution of the evolving sulfur species during cycling, usually masked by the dominant sulfonate-based electrolyte salt signal. The fitting of the EXAFS spectra during the discharge provided the fraction of Li 2 S in the electrode as well as the average coordination number of the sulfur centres in sulfur and polysulfides (cf. Figure 3). From the average number of neighbours in the sulfur chains it was possible to determine the average length of the polysulfide chains (dashed lines in Figure 3) formed in the electrode during cycling. This results allowed us to thoroughly describe the discharge mechanism in Li-sulfur systems: the pristine S 8 is first converted into

Studies carried out at ICGM: Conversion anode materials for LIB

At the negative electrode side, the application of operando XAS to the study 285 of materials undergoing the conversion reaction such as NiSb 2 and TiSnSb provided valuable insights on the activity and role of the different elements in the complex electrochemical conversion mechanism. In a conversion reaction, [9] lithium reacts with a binary compound containing a transition metal (M = Ti, Mn, Fe, Co, Ni, etc.) and a p-group element (X = O, P, Sb, Sn, etc.), according to:

M a X b + (b * n)Li = M + b Li n X (1)
First verified for transition metal oxides, conversion reactions are rather common also for other chalcogenides, pnictogenides and carbon group semimetals.

Conversion materials, i.e., materials undergoing the conversion reaction, allow reversible capacities as high as 1500 mAh/g, far exceeding that of graphite Data analysis was performed using a chemometric approach, which enabled us to visualize for the first time that the lithiation and delithiation reactions of Sn and Sb do not occur simultaneously, but are slightly shifted. In fact, as depicted in Figure 4, during charge, when the lithiated species are oxidized one can observe that the onset of Sn EOC component formation (red curve) precedes the Sb one (blue curve), thus confirming that the two metals do not react at the same time, in spite of a certain overlap between their respective reaction. In this way, it was possible to attribute the presence of two plateaus in the electrochemical signature, represented by the two peaks in the inverse of the derivative of the potential with the charge at ≈ 0.5 and ≈ 0.9 V in Figure 4, to this shift in the reaction of the two p-group metals. This result highlights the advantage of using a chemometric approach, with respect to state-of-the-art conventional ones, in solving a complex mixture reaction processes and reliably reconstructing them.[57]

(

Studies carried out at ICGM: Sb-based anode materials for NIB

In one of our first studies on NIB in 2012, we demonstrated for the first time that pure antimony can be successfully used as anode material, sustaining a capacity close to 600 mAh/g with excellent cycle life and rate capability. [89] This result showed that, contrary to LIB, antimony has a very specific affinity for sodium, and gave a strong impulse to our research which was thus extended to many other Sb-based negative electrode materials such as SnSb [START_REF] Darwiche | Monconduit[END_REF] and FeSb 2 . [91] The specific affinity of Sb for Na in all these compounds is strongly dependent upon the electrochemical reaction mechanism, which does not simply go through the formation of the intermediate species expected from the phase diagram as in the case of LIB, but rather through unexpected amorphous phases not visible by XRD, and which could be identified, in the case of pure Sb, only by a complex approach involving operando Pair Distribution Function (PDF) analysis, ex situ nuclear magnetic resonance (NMR) spectroscopy and the support of Density Functional Theory (DFT) calculations. [92] In the framework of this large research project, XAS was also applied with alternate success for the comprehension of the electrochemical reaction mechanisms of the different materials. In the case of pure Sb, for instance, a study by operando Sb K-edge XAS assisted by a chemometric approach, 121 Sb Mössbauer In the case of FeSb 2 , the combination of operando Sb K-edge XAS, magnetic measurements and 57 Fe Synchrotron Mössbauer Spectroscopy, the latter used for the first time in the field of energy storage materials, allowed us to clarify the role of iron and antimony in the reversible sodiation mechanism. [93] Our findings show that the reaction of Na with FeSb Along with these studies, other Sb-containing electrode materials were investigated and showed interesting performance. Among them, one has to consider the case of Bi-Sb system, which has recently shown interesting properties as anode material in K-ion batteries (KIB) and Mg-ion batteries (MIB). [94,95] In the following section, the study of its electrochemical mechanism by several batteries. [105] In this system, the best cycling properties over 100 cycles were obtained with Bi 0.88 Sb 0.12 and Bi, while Sb failed to show appreciable capacity.

More recently, Murgia et al. showed that in Mg-ion system Sb alone is not active, and that only the association of Sb with Bi enables the electrochemical magnesiation of Sb, leading to the formation of monophasic Mg 3 (Sb 1 -x Bi x ) 2 alloy, identified by solid state NMR, at the end of the discharge. [94] This last paper is the only one, to the best of our knowledge, where some specific insights concerning the cycling mechanism of Bi-Sb alloys vs. alkali and alkaline-earth metals could be proposed with the support of diffraction and/or spectroscopy techniques.

In this section, the electrochemical sodiation mechanism of Bi 1 -x Sb x alloys is investigated by a combination of XRD, XAS and 121 Sb Mössbauer spectroscopy.

Several polycrystalline Bi 1 -x Sb x compositions (with x = 0.5, 0.56, 0.78 and 0.9)

were synthesized by high energy ball milling, and characterised by XRD. Their electrochemical properties were studied in half cells vs. Na metal using a 1 M NaClO 4 in PC electrolyte containing 5 % fluoroethylene carbonate as additive.

The detailed results of this study are reported in the Supplementary Material.

In short, a strong dependence of the capacity on composition is observed, with a decrease of the capacity retention with the decrease of the Sb content. The full operando characterisation by XRD as well as by simultaneous Sb K-edge and Bi L 3 -edge XAS is presented for the composition Bi 0.5 Sb 0.5 . More details about the synthesis of the materials and of the electrodes, as well as about the different characterisation techniques, are given in the Supplementary Material.

calculation of the a lattice parameter using Berger's formula: [106] a = (0.454469 ± 0.00044) -(0.02398 ± 0.00072)x (2)

As shown in Figure 7, the observed linear variation of the lattice parameters a and c with Sb concentration for Bi 1-x Sb x samples clearly reveals that they follow the Vegard's law. The sample used for the operando XRD and XAS study is Bi 0.50 Sb 0.50 , characterised by a 0.50/0.50 Bi/Sb atomic ratio as confirmed by XRD (cf. Fig. 7), and hereafter simply labelled BiSb.

Additional analyses by 121 Sb Mössbauer spectroscopy (cf. Supplementary Material) did not provide further information, this technique not being able to discriminate the different compositions.

The reversible sodiation of BiSb followed by operando XRD

The operando XRD patterns collected through the whole first discharge/charge cycle and the following second discharge of a BiSb electrode are shown in Figure 8. The whole series of data can be divided in three parts: the first discharge, the first charge and the second discharge. The details of the different phases identified during cycling are resumed in the Supplementary Materials. to a new set of peaks which can be attributed to a phase close to NaBi [PDF 00-004-0701], but with slightly lower a and c cell parameters. This difference in cell parameters could be due to the the substitution of a portion of Bi by Sb, leading to a ternary compound with composition NaBi 1-x Sb x . It is worth noticing that it coincides with the appearance of a peak at about 0.64 V in the derivative of the galvanostatic curve (cf. Supplementary Materials). At the end of the charge, the ternary intermediate NaBi 1-x Sb x has completely disappeared in favor of a Bi-Sb alloy isostructural to both Bi and Sb (R3m) with refined cell parameters indicating a composition close to Bi 0.9 Sb 0.1 according to Vegard's law (cf. Fig. 7). Considering the Sb-poor stoichiometry of this binary phase, amorphous antimony is also expected to be simultaneously formed, as previously observed for the desodiation of sodiated Sb electrodes. [89,92,48] During the second discharge, the Bi-Sb alloy formed at the end of the charge disappears progressively in favor of the formation of the ternary NaBi ing on the cut-off energy, and two sets of three ionization chamber detectors rapidly exchangeable via a pneumatic system. This set-up allows the use of a wide range of sample environments which can be mounted on the motorized sample stage, and in particular the setting up of several in situ electrochemical cells which can be studied in parallel via a completely automatised system (cf.

Supplemetanry Material). The flux available on the sample permits the measurement of XAS spectra with an acceptable signal-to-noise ratio if the studied sample is prepared with an adapted thickness (e.g., a sample with a loading of 19 mg/cm 2 with a composition Bi 0.50 Sb 0.50 will have an acceptable transmission at both edges, providing an absorbance step of 0.3 and 1.0 for the Sb K-edge and Bi L 3 -edge, respectively). The sample stage of the experimental set-up is shown in the Supplementary Material.

Bi L 3 -edge XAS results

The operando Bi L 3 -edge XAS spectra (XANES and EXAFS portions) collected during the first cycle (sodiation/desodiation) are shown in Figure 9 (cf.

Supplementary Material for the corresponding electrochemical cycling curve.)

The absorption edge is characterised by large and broad features, and its overall shape and position varies only slightly during cycling. This result is somehow expected, the pristine and the formed compounds being all intermetallics. Consequently, the XANES part of the spectrum bears only negligible information and is hence of little use for the identification of the different species formed during the electrochemical process. On the other hand, the oscillation of the EXAFS part are rather intense and visible up to k = 10 Å-1 , and can thus be used to describe the average coordination around the Bi centres.

A PCA analysis was performed on the whole set of the XAS data in order to extract the maximum amount of useful information contained in the operando spectra. Since the XANES part is varying very little during the process, the differences between the principal components are concentrated in the EXAFS region. The variance plot obtained (cf. Supplementary Material) shows rather clearly that more than 98% of the variance of the XAS spectra can be described by three principal components, the residual part can be attributed to the experimental noise. The examination of the shape of the principal components and of the respective scores (see details in the Supplementary Material) support this hypothesis, with both shape and evolution of the first three components varying gradually through the different processes, and those of the following ones changing in a totally hectic way.

Given that the so-obtained principal components are orthogonal mathematical functions and not real EXAFS spectra, it is possible, based on their number, to reconstruct corresponding real EXAFS components as well as their evolution using the MCR-ALS method, recently reviewed in Ref. [57]. In this case, the MCR-ALS analysis produces the three EXAFS spectra shown in Fig. 10 together with their evolution along the electrochemical cycle. These components correspond perfectly to specific spectra measured along the in situ measurements, in line with their observed evolution (cf. Supplementary Material).

The obtained MCR-ALS components can thus be treated as normal spectra, and fitted using the conventional approach (cf. Supplementary Material for the

Sb K-edge XAS results

The same data analysis approach was used also to study the operando Sb Kedge XAS spectra collected during the first cycle (cf. Supplementary Material).

Differently from the Bi L 3 -edge spectra, PCA analysis suggests the presence of four independent components, which where reconstructed using MCR-ALS, and that are shown in Fig. 11 together with their evolution along the electrochemical cycle. Only some of these components correspond to specific spectra measured along the in situ measurements, in line with their observed evolution (cf. Supplementary Material).

The fit of MCR-ALS components 1 and 3 is relatively straightforward, the two of them representing pristine (Bi 0.50 Sb 0.50 ) and the material obtained at the end of discharge, Na 3 (Bi 0.50 Sb 0.50 ). Indeed, component 1 can be fitted with the same shells and virtually the same parameters used for fitting the corresponding Bi L 3 -edge spectrum, whereas only the first Na shell of component 3 is visible in the spectrum, thus hampering the identification of its composition as in the case of the Bi measurement.

The other two components are more interesting, since they do not correspond to any experimental spectrum, and never reach a 100% intensity during cycling. Upon cycling, a gradual decrease of the capacity is observed, strongly dependent on the amount of bismuth. The observed increase of the segregation between the first discharge and the following charge suggests a gradual segregation of the two metals upon cycling. The poor performance of Bi, probably due to stronger electrolyte degradation than for Sb, might be then the cause of rapid capacity fading.

Conclusion

In the last decade, a strong activity on the application of XAS to the operando characterisation of battery and battery materials has been developed at ICGM. Different families of electrode materials and battery systems, going from insertion to conversion materials for LIB, to LIS systems, to Sb-based alloy materials for NIB. In all these systems, XAS has played a major role for the determination of the cycling mechanism, in particular when the electrode materials went through extensive nanostructuring and/or amorphisation.

The capabilities of XAS where evidently demonstrated in the example of the study of Bi-Sb alloy materials, where its contribution for the understanding of the mechanism is strongly complementary to that of XRD. The importance of XAS in this particular experiment is even more important considering the highly challenging experimental conditions that had to be set for collecting simultaneously operando Sb K-edge and Bi L 3 -edge XAS spectra on the same cycling cell.

Figure 1 :

 1 Figure 1: (a) Quantitative evaluation (in % of each metal) of the Fe 3+ and Mn 3+ content during the first two charge/discharge cycles obtained from the linear combination fitting of the operando Mn (b) and Fe (c) K-edge XANES spectra of LiFe 0.33 Mn 0.67 PO 4 recorded upon cell operation at C/10 rate (a). Some spectra between the two cycles could not be recorded due to beam loss during cycling.

Figure 2 :

 2 Figure 2: XAS operando Ti K-edge spectra depicting the evolution of (a) XANES pre-edge and main edge feature of TiO 2 anatase and (b) phase-uncorrected EXAFS local structure of TiO 2 (B) upon lithitation. A1, A2, A3 and B features of the XANES pre-edge of anatase and their evolution are highlighted in (a).

Figure 3 :

 3 Figure 3: Average coordination number of S in sulfur and polysulfides (red circles) during the first discharge determined from the EXAFS spectra measured in the 1 M LiTDI TEGDME:DOL electrolyte, and corresponding amplitude attenuation factor S 2 0 of the Li-S and S-S contributions, indicating the growth in intensity of Li2S and the decrease of polysulfide contribution towards the end of the discharge. The average coordination of the most important polysulfides is represented by the horizontal dashed lines. The corresponding electrochemical curve measured during the discharge is shown on top.

S 82 -

 82 along the plateau at high potential; S 82 -is then transformed into shorter 280 polysulfides during the potential slope, and finally Li 2 S gradually precipitates during the low potential plateau. This mechanism could be later confirmed by oter techniques, such as for instance Raman spectroscopy.[19]

  372 mAh/g), the negative electrode material commonly used in commercial Liion batteries. During a conversion reactions, the pristine electrode material loses its crystallinity, and composite materials including nanosized species are formed, making it impossible to follow the reaction by long-range order techniques such as XRD. Moreover, the species formed at the end of the discharge process are particularly reactive and unstable, and therefore the use of operando techniques becomes essential for the study of reaction mechanisms.Among conversion materials, pnictogenides with general formula M x Sb y are expected to react with lithium by forming a matrix of Li 3 Sb in which nanoparticles of the transition metal M are embedded. The actual reaction mechanisms, however, is usually more complex and dependent on the specific compound. For instance, FeSb 2 ,[START_REF] Villevieille | Monconduit[END_REF] is known to form intermediate lithiated insertion phases before starting the veritable conversion reaction, and additional phases might also form throughout the whole electrochemical cycle. Also NiSb 2 undergoes a relatively complex lithiation process to form nickel metal and Li 3 Sb for a theoretical capacity of 532 mAh/g.[84] However, while XRD suggested the possible formation of an intermediate ternary insertion solid solution during the first discharge, it did not allow following the whole mechanism due to the complete amorphisation of the system. With the help of operando Ni K-edge XAS, on the other hand, it was possible to follow the chem-ical state of Ni during the first two lithiation/delithiation cycles, showing that a reversible conversion takes place, with the formation of Ni metal nanoparticles at the end of lithiation and the reversible formation of a Ni-Sb phase close to pristine NiSb 2 at the end of the delithiation process.[85] Moreover, strong differences were observed when comparing operando results with ex situ measurements on cycled NiSb 2 at the same state of charge prepared two weeks before the XAS measurement campaign. Indeed, these systems were found to produce metastable species during the discharge which undergo relaxation under OCV conditions, and which are thus detrimental for a possible effective application of such materials in commercial systems. In short, this comparison highlighted the importance of real-time investigation tools for the study of battery materials, and particularly for those undergoing a conversion reactions or containing particularly reactive species.More recently, operando XAS was applied to the study of the electrochemical cycling mechanism of the ternary intermetallic TiSnSb,[86] a promising conversion-type negative electrode material for lithium batteries.[87,88] However, differently from the case of NiSb 2 , this study was carried out at three different absorption edges, namely Ti, Sn, and Sb K-edge, and data analysis was performed by using chemometric tools such as PCA and MCR-ALS in order to extract the maximum information contained in the whole set of operando data. Given the very large difference in energy between the K-edges of Ti and of the other two p-group elements, implying very different sample thicknesses, as well as beamline settings (monochromator, detectors, etc.), two different experiment had to be performed: a first experiment at the Ti K-edge revealed that the direct conversion of TiSnSb upon lithiation leads to the formation of Ti metal nanoparticles, which were identified from the modification of the XANES spectra, which become very similar to that of Ti metal, and especially by the loss of Sb and Sn neighbours around the Ti centres, and the simultaneous increase of the number of the Ti ones. EXAFS also confirmed that this reaction is reversible upon the following delithiation, with Ti binding back to both Sn and Sb, finally confirming that TiSnSb reacts with lithium through an authen-tic conversion reaction.[86] The second experiment carried out simultaneously at both Sn and Sb K-edges confirmed that both elements are electrochemically active upon lithiation. In fact, EXAFS detected a drastic modification in nature, bond distance and number of neighbours around both Sn and Sb centres, substantiating the formation of Li 7 Sn 2 and Li 3 Sb previously detected by 119 Sn Mössbauer spectroscopy and XRD.[87] Globally, small but significant differences between the EXAFS spectrum of the material obtained after one complete electrochemical cycle and that of pristine crystalline TiSnSb suggest the formation of a ternary "Ti-Sn-Sb-like" species with distinct structural properties from pristine TiSnSb.

Figure 4 :

 4 Figure 4: Derivatives of the MCR-ALS concentration profiles of the Sn (red) and Sb (blue) EOC components and overall dQ/dV profile measured in the operando cell (green) during charge.

  spectroscopy and DFT calculations confirmed that Sb reacts with Na to form disordered Na 3 Sb via the formation of amorphous Sb as an intermediate and that, upon charge, Na 3 Sb is gradually desodiated to amorphous Sb.[48] However, it was possible to demonstrate that XAS (as well as Mössbauer spectroscopy) are intrinsically unable to spot the other intermediate Na x Sb phases previously identified by NMR and PDF.[92] Differently from the example of Sb, XAS was very useful in the study of the cycling mechanisms of both SnSb and FeSb 2 , which also go through extensive amorphisation during cycling. In the case of SnSb, its reversible sodiation-desodiation reaction was investigated by simultaneous operando Sn and Sb K-edge XAS and 119 Sn Mössbauer spectroscopy assisted chemometric data analysis.[31] During the sodiation, which goes through a two-step process, it was not only possible to disclose the formation of Na 3 Sb and Na 15 Sn 4 , a rarely observed phase in pure Sn-based electrodes, at the end of the discharge, but also to identify for the first time an amorphous form of α-Sn, thermodynamically unstable at ambient conditions, as a reaction intermediate, see Figure 5. These insights on the mechanism of SnSb vs. Na provide a basis for understanding its excellent cycle life and capacity retention, which lies in the gradual formation of amorphous, nano-confined intermediate phases and correlated elastic softening of highly sodiated tin and antimony phases which have enhanced ability to absorb and mitigate the strong volume changes occurring upon sodiation and desodiation.

2 during discharge leads to the formation of Na 3

 3 Sb along with superparamagnetic Fe nanoparticles containing small fractions of Sb, as previously detected by low temperature ex situ57 Fe Mössbauer spectroscopy.[91] Pristine FeSb 2 is never recovered by desodiation, while iron nanoparticles grow in size, and continue growing also along the following discharge, as testified by the increase of the Lamb-Mössbauer factor of the spectal component representing Fe nanoparticles. Even though such

Figure 5 :

 5 Figure 5: Evolution of the concentration of the MCR-ALS spectral components for Sn and Sb K-edges during subsequent sodiation, desodiation and sodiation of SnSb, and corresponding experimental electrochemical signature. EOD and EOC mean the end of discharge and end of charge, respectively.

430 3 .

 3 complementary techniques including XAS will be presented and discussed in detail. This work will highlight both the ability of XAS in providing useful information on electrochemical mechanisms, and the advanced technology of the ROCK beamline at Synchrotron Soleil allowing the simultaneous operando investigation of several elements presenting absorption edges at very different energies in battery materials. An experimentally challenging case study: the sodiation of Bi 0.50 Sb 0.50 simultaneously followed by Sb K-edge and Bi L 3 -edge XAS3.1. IntroductionBoth antimony and bismuth, elements of the group V, crystallise in the rhombohedral space group [PDF 01-085-1329 and PDF 01-085-1322, respectively],[96,97] and form a solid solution over the whole composition range.[98] Bi-Sb alloys have been studied in the past for their peculiar properties. While pure bismuth and antimony show a relatively good electric conductivity, their alloys show complex semiconductor properties strongly dependent upon the composition, and some of them, e.g. Bi 0.9 Sb 0.1 , were the first experimentally observed 3D topological insulators, i.e., materials with conducting surface states and an insulating interior.[99] Bi-Sb alloys also showed superconductivity properties: for instance Bi 0.4 Sb 0.6 is a superconductor with a critical temperature T c , of approximately 2 K.[100] Lin et al., on the other hand, reported the thermoelectric properties of Bi 1-x Sb x alloy nanowires.[101] In the last decade, several examples of application of Bi-Sb alloys as anode materials in battery systems were published, including LIB, NIB and MIB. For instance, Li et al. recently reviewed the works concerning the application of Bi and Sb in NIB, underlining in particular the potential of Bi-based materials in batteries owing to their multiple features such as narrow band gap, ion conductivity, and environmental friendliness, and indicating a few examples of Bi-Sb alloy systems.[102] Zhao et al. studied Bi-Sb alloys prepared by high energy ball milling, finding that such alloys exhibit electrochemical performances in NIB superior to those of their counterparts Bi and Sb.[103] The same materials also show elevated capacities in LIB at high current rates. Xie et al. proposed a ternary Sn-Bi-Sb system for NIB, describing a clear relationship between the cycling properties and the composition.[104] The formation of ternary Na-Bi-Sb alloys has been suggested as the basis for the good cycling stability. Arthur et al. used electrodeposited Bi 1-x Sb x as anodes for Mg-ion

Figure 7 :Figure 8 :

 78 Figure 7: The variation of lattice parameters a and c (top) and that of the unit cell volume (bottom) versus the molar fraction x of antimony in Bi 1-x Sbx alloys. The dashed lines should be considered as guides to the eye.

  1-δ Sb δ intermediate, already identified during the first charge, before leading to the final ternary Na 3 (Bi 0.5 Sb 0.5 ) analogous to the end of the first discharge, which demonstrates a complete and quite reversible sodiation/desodiation mechanism.It is worth noting that the combination of Sb and Bi helps to this reversibility compared to pure Bi. On the other hand, the presence of Bi enables the formation of crystalline intermediate phases, differently from the case of pure Sb where the intermediate sodiated species were fully amorphous.3.4. The reversible sodiation of BiSb followed by operando Sb K-edge and Bi L 3 -edge XASThe operando Sb K-edge and Bi L 3 -edge XAS study was performed on the same sample used for the XRD study, labelled BiSb, at the beamline ROCK of Synchrotron Soleil (Gif-sur-Yvette, France). This measurement implies continuous alternate measurements of the XAS spectra of two elements with absorption edges at very different energies (Sb K-edge and Bi L 3 -edge are at 30491 and 13418 eV, respectively), and ROCK is, to the best of our knowledge the only beamline allowing such type of experiment.[34] The beamline is equipped with a first focusing mirror, the set of two rapidly interchangeable Quick-XAS Si(111) and Si(200) channel-cut monochromators covering a range of energies from 4 to 40 keV, each one coupled to an adapted set of mirrors used to eliminate the high order harmonics with a rejection coefficient ranging from 10 4 to 10 8 depend-

Figure 9 :

 9 Figure 9: (a) XANES and (b) EXAFS operando Bi L 3 -edge XAS spectra measured during the first electrochemical cycles of BiSb electrode vs. Na. The horizontal line labelled EOD represents the end of the first discharge.

605Figure 10 :Na 3 (

 103 Figure 10: EXAFS spectral components (top) obtained by MCR-ALS analysis of the operando Bi L 3 -edge XAS spectra, and variation of their intensity (b) during electrochemical cycling. The vertical black line represent the end of the first discharge (EOD).

Component 2

 2 is very similar to component 2 found in the Bi L 3 -edge data, and must be fitted including the contributions of Bi, Na and Sb neighbours. All other fitting attempts with fewer components were unsuccessful. As in the previous case, this component fits well with the composition of the ternary Na(Bi 1 -x Sb x ) alloy, having the same crystal structure of NaBi. Component 4, instead, must be fitted with the two shells representing Sb and Na, whereas it does not contain any contribution from bismuth. Given the relative intensity of the two Sb and Na contributions, it fits well with the stoichiometry of NaSb, even though in a previous work we have demonstrated

Figure 11 :

 11 Figure 11: EXAFS spectral components (top) obtained by MCR-ALS analysis of the operando Sb K-edge XAS spectra, and variation of their intensity (b) during electrochemical cycling. The vertical black line represent the end of the first discharge (EOD).

3. 5 .• 1 st charge Na 3

 53 DiscussionThe electrochemical study of the different compositions (cf. Supplementary Material) shows a gradual modification of the cycling properties and of the mechanism on going from Sb to Bi, with a gradual decrease of the capacity retention and a corresponding gradual modification of the mechanism with the Bi content. In particular, the modification of the potential of the different plateaus with respect to those of pure Bi and Sb suggests the formation of different sodiated compounds, i.e., the formation of ternary Na-Bi-Sb alloys both as intermediates during the discharge and as the final product obtained at the end of the discharge. This hypothesis is supported by operando XRD, which rather clearly confirms the formation of a compound with a composition close to Na 3 (Bi 0.50 Sb 0.50 ) at the end of the first discharge. EXAFS, on the other hand, is rather neutral to this conclusion, confirming the complete sodiation of both Bi and Sb but not providing any information about the second neighbours, i.e., about the composition of the sodiated alloy. Unfortunately, also ex situ 121 Sb Mössbauer spectroscopy is not sensitive enough to contribute to the identification of these phases obtained at the end of the discharge and charge processes (cf. Supplementary Material). EXAFS, on the other hand, becomes much more helpful when one looks at the intermediate phases. Indeed, during the first discharge, two different intermediate components appear simultaneously with different intensities. The first one, the only intermediate clearly visible from the Bi side, is represented by a ternary Na-Bi-Sb compound, which is likely the crystalline intermediate also detected by XRD. The second one, on the other hand, is visible only from the Sb side and is clearly a Bi-free Sb-rich sodiated intermediate either represented by a composition similar to BiSb, or to a mixture of pure Sb and of Na 3 Sb. Similar Na-Sb intermediate species, amorphous in nature, could be observed by PDF but are not detectable in XRD.[92] The same two components appear again, even though with reversed intensities, during the following charge from the decomposition of Na 3 (Bi 0.50 Sb 0.50 ), suggesting an increased segregation of Sb and Bi along cycling. Interestingly, the segregation is probably not extended to the formation of long-range ordered Sb-Na species, since the ternary Na 3 (Bi 0.50 Sb 0.50 ) is reformed again at the end of the second discharge, as detected by XRD. It is unfortunate that the XAS experiment had to be stopped in the middle of the charge due to technical problems with the beamline, but the collected spectra are sufficient to provide a clear modification of the spectra in the middle of the charge compared to the middle of the discharge. On the basis of these observation, and taking into account the compositions of the different intermediates identified by EXAFS and XRD, one can propose a overall cycling mechanism for BiSb (the prefix (a) stands for amorphous): • 1 st discharge Bi 0.5 Sb 0.5 → (a)Na y Bi 1-x Sb x + (a)Na z Sb → Na 3 Bi 0.50 Sb 0.50 (3) Bi 0.50 Sb 0.50 → Na y Bi 1-x Sb x + (a)Na z Sb → Bi 0.5 Sb 0.5 + 3Na (4)

Table 1 :

 1 Lattice parameters resulting from the refinement of the XRD patterns of the Bi 1-x Sbx samples. Unit cell parameters of Sb and Bi are given for comparison, standard deviations are given in brackets.

	Compound	a, Å	c, Å	α, β, γ, Å	V , Å3
	Sb	4.3070(1) 11.2730(2) 90, 90, 120 181.10(1)
	Bi 0.10 Sb 0.90	4.328(1)	11.357(1) 90, 90, 120 184.27(1)
	Bi 0.22 Sb 0.78	4.355(1)	11.434(1) 90, 90, 120 187.81(1)
	Bi 0.44 Sb 0.56	4.410(1)	11.576(1) 90, 90, 120 194.98(1)
	Bi 0.50 Sb 0.50

Acknowledgements

Thanks to all concerned beamlines and scientists. Thanks to Alistore for the post-doc grant of MF. The research on Li-sulfur batteries received funding from the European Union Seventh Framework Programme under the grant agreement 314515 (EUROLIS). Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. Synchrotron SOLEIL (France) is gratefully acknowledged for providing beamtime at the ROCK beamline (financed by the French National Research Agency (ANR) as part of the Investissements d'Avenir programme, reference ANR-10-EQPX-45).

The research group Catalyse, Réactivité de Surface et Rayonnement Synchrotron (GDR CNRS 3590) is gratefully acknowledged for its information and supporting activity for the application of the chemometric approach in the analysis of spectroscopic data.

Characterisation of pristine Bi-Sb alloys

The Bi 1-x Sb x alloys with x = 0.5, 0.55, 0.80 and 0.9 prepared by ball milling were analysed by XRD as shown in Figure 6. All Bi 1-x Sb x samples crystallize as expected in the same R3m space group. The diffraction patterns clearly show the shift of the (012) diffraction peak to higher angles with the increase of Sb ratio in the alloys. The same trend is observed for the other diffraction peaks (cf. Supplementary Materials). The results of profile matching of the diffraction patterns of the Bi 1-x Sb x samples are reported on Table 1, the rhombohedral Bi-Sb structure being expressed in terms of equivalent hexagonal unit cell. The corresponding refined a and c hexagonal cell parameters and volume decrease with increasing Sb content, in line with the size of the atomic radii of Sb and Bi (1.45 and 1.60 Å, respectively). The composition of Bi


Bouchra Asbani

Camille Douard

Thierry Brousse

Jean Le Bideau
email: jean.lebideau@cnrs-imn.fr

High temperature solid-state supercapacitor designed with ionogel electrolyte

Keywords: Solid-state supercapacitor, Ionogel, Electrolyte, High temperature, Electrochemical double layer capacitor, Energy storage Solid-state supercapacitor, Ionogel, Electrolyte, High temperature, Electrochemical double layer capacitor, Energy storage

To date, the design of high temperature electrochemical double layer capacitors (EDLCs) remains a challenge more specifically related to the electrolyte. The electrochemical performance of EDLC can be tuned by the temperature, and a better performance can be achieved at higher temperature if all the components are well suited to this purpose. Subsequently, an all solid-state EDLC operated at high temperature using porous carbon electrodes and a solid-like electrolyte was fabricated. Such an approach opens the route towards safer and flexible devices. An ionogel was used as the solid electrolyte and concomitantly as separator in this all-solid EDLC. The synthetized ionogel exhibits a high ionic conductivity over a wide temperature range, from 4 mS.cm -1 at 20 °C up to 26 mS.cm -1 cm at 100 °C. The all solid state EDLC thus prepared was able to withstand operating temperature as high as 100 °C, under a 2.5 V cell voltage. Long term galvanostatic charge-discharge cycling over 25 000 cycles were achieved at 100 °C, thus demonstrating very good capacitance retention. The ionogel-based EDLCs therefore appear to be suitable for high temperature environments.

Introduction

Electrochemical double-layer capacitors (EDLCs) also called supercapacitors (SCs) promise to play an important role in meeting the demands of electronic devices and integrated systems [START_REF] Frackowiak | Supercapacitors: Materials, Systems and Applications[END_REF]. The use of such supercapacitor potentially enables far-reaching advances in backup energy storage and high pulse power applications like portable consumer electronics, memory back-up systems, military devices, space equipment, hybrid vehicles, next-generation all-electric cars, and large industrial scale power and energy management. EDLCs store energy using accumulation of ions at the interface between a highly porous carbon-based electrode and an electrolyte [START_REF] Conway | Electrochemical Supercapacitors[END_REF]. The charge storage mechanism is mainly capacitive with no chemical modification of the electrode involved during the charge/discharge process. As a result, EDLC can sustain millions of cycles, providing fast charge/discharge rates and subsequently high power density, while keeping a reasonable energy density.

The electrode materials of EDLCs must have a high specific surface area, which is of utmost importance to achieve high energy densities. Highly porous carbons, such as activated carbons, are commonly used as supercapacitor electrodes due to their high specific surface area and fair electronic conductivity [START_REF] Frackowiak | Carbon materials for the electrochemical storage of energy in capacitors[END_REF][START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF][START_REF] Simon | Materials for electrochemical capacitors[END_REF][START_REF] Davies | Material advancements in supercapacitors: From activated carbon to carbon nanotube and graphene[END_REF][START_REF] Borenstein | Carbon-based composite materials for supercapacitor electrodes: a review[END_REF]. Additionally, activated carbons can be used with a wide variety of electrolytes, including acidic and basic aqueous electrolytes, organic electrolytes and ionic liquid (ILs) [START_REF] Frackowiak | Carbon materials for the electrochemical storage of energy in capacitors[END_REF].

Indeed, the electrolyte is the other core component of EDLCs, and must meet the requirements of high ionic conductivity and wide electrochemical stability potential window [START_REF] Zhong | Review of electrolyte materials and compositions for electrochemical supercapacitors[END_REF]. EDLCs employing activated carbon high surface area electrodes and conventional electrolytes such as N(Et)4BF4 in acetonitrile or propylene carbonate, represent the state of the art technologies currently on the market [START_REF] Frackowiak | Supercapacitors: Materials, Systems and Applications[END_REF][START_REF] Conway | Electrochemical Supercapacitors[END_REF][START_REF] Frackowiak | Carbon materials for the electrochemical storage of energy in capacitors[END_REF][START_REF] Frackowiak | Carbon materials for supercapacitor application[END_REF][START_REF] Simon | Materials for electrochemical capacitors[END_REF]. However, the use of such electrolyte leads to a number of safety issues and environmental drawbacks.

ILs are room temperature molten salts, entirely composed of cations and anions, the nature of which influences the chemical/electrochemical and physical properties [START_REF] Armand | Ionic-liquid materials for the electrochemical challenges of the future[END_REF]. For instance, the nature of the cation and the anion determines the potential window as well as the melting point or the viscosity, which in turn affects the ionic conductivity. Moreover, ILs typically show very low vapor pressure and high thermal stability, which allows high operating temperature [START_REF] Brandt | Ionic liquids in supercapacitors[END_REF]. Nevertheless, leakage still remains an issue with respect to their use in EDLCs since they are in a liquid state at room temperature. Additionally, most of EDLCs designed with ILs as electrolyte have been operated not higher than 80°C [START_REF] Brandt | Ionic liquids in supercapacitors[END_REF][START_REF] Balducci | High temperature carbon-carbon supercapacitor using ionic liquid as electrolyte[END_REF][START_REF] Ruiz | Ionic liquid-solvent mixtures as supercapacitor electrolytes for extreme temperature operation[END_REF][START_REF] Thangavel | High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device[END_REF],

although with impressive cycling ability up to 150 000 cycles at 60°C with activated carbon electrodes coupled with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) electrolyte [START_REF] Thangavel | High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device[END_REF]. Only few reports using various carbon electrodes including carbide derived carbons, bucky papers, or activated carbons demonstrate the possibility to increase the temperature up to 100°C [START_REF] Sato | Electrochemical properties of novel ionic liquids for electric double layer capacitor applications[END_REF][START_REF] Kurig | Electrochemical Characteristics of Carbide-Derived Carbon 1-Ethyl-3methylimidazolium Tetrafluoroborate Supercapacitor Cells[END_REF][START_REF] Largeot | Microporous Carbon-Based Electrical Double Layer Capacitor Operating at High Temperature in Ionic Liquid Electrolyte[END_REF][START_REF] Dagousset | Electrochemical characterisations and ageing of ionic liquid/gbutyrolactone mixtures as electrolytes for supercapacitor applications over a wide temperature range[END_REF][START_REF] Varela | Piperidinium ionic liquids as electrolyte solvents for sustained high temperature supercapacitor operation[END_REF] or above (up to 140°C) [START_REF] Jiang | High Temperature Monolithic Biochar Supercapacitor Using Ionic Liquid Electrolyte[END_REF][START_REF] Haque | Thermal influence on the electrochemical behavior of a supercapacitor containing an ionic liquid electrolyte[END_REF], but only few hundreds of cycles are shown. Moreover, the mass loading of the electrodes if either very low or unknown which makes the comparison with standard EDLC difficult and which gives some doubts about extrapolated power and energy densities as suggested in the literature [START_REF] Gogotsi | True Performance Metrics in Electrochemical Energy Storage[END_REF][START_REF] Balducci | A Guideline for Reporting Performance Metrics with Electrochemical Capacitors: From Electrode Materials to Full Devices[END_REF].

In comparison to liquid electrolytes, designing solid electrolytes confining ILs, namely ionogels [START_REF] Guyomard-Lack | Destructuring ionic liquids in ionogels: enhanced ionic liquid properties for solid devices[END_REF][START_REF] Guyomard-Lack | Hybrid Silica-Polymer Ionogel Solid Electrolyte with Tunable Properties[END_REF], is of great interest. ILs host networks can be silica based, polymer based, or hybrid organic-inorganic based. This approach enables to obtain solid like electrolyte with high ionic liquid content simultaneously with obtaining the separator. It avoids also leakage thus limiting packaging constraints and increasing the safety (no flammability nor vapor tension), while keeping good mechanical and thermal properties. It is worth noting that the ionogel can be also easily shaped for the desired application in supercapacitors [START_REF] Wang | High-performance all solid-state microsupercapacitor based on patterned photoresist-derived porous carbon electrodes and an ionogel electrolyte[END_REF][START_REF] Brachet | All Solid-State Symmetrical Activated Carbon Electrochemical Double Layer Capacitors Designed with Ionogel Electrolyte[END_REF][START_REF] Negre | Ionogel-based solid-state supercapacitor operating over a wide range of temperature[END_REF][START_REF] Brachet | Solder-reflow resistant solid-state micro-supercapacitors based on ionogels[END_REF][START_REF] Soeda | Outstanding features of alginate-based gel electrolyte with ionic liquid for electric double layer capacitors[END_REF][START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF].

Extending the temperature range is of great importance since this could solve the problem of operating EDLCs in severe conditions required for specific applications such as oil drilling, space hardware, high temperature manufacturing processes, etc. Thus, EDLCs with high energy and power densities that can withstand harsh temperature environments are extremely desirable. Moreover, to the best of our knowledge there is no systematic work on high temperature EDLCs, since the studies are usually limited to safe temperature operating window which does not exceed 60°C as already mentioned above [START_REF] Brandt | Ionic liquids in supercapacitors[END_REF][START_REF] Balducci | High temperature carbon-carbon supercapacitor using ionic liquid as electrolyte[END_REF][START_REF] Ruiz | Ionic liquid-solvent mixtures as supercapacitor electrolytes for extreme temperature operation[END_REF][START_REF] Thangavel | High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device[END_REF]. Indeed, it seems that an increase of temperature above 100°C severely limits the safe operating voltage window which shrinks down to 0.5V at 200°C for EDLC operated with butyltrimethylammonium bis-(trifluoromethylsulfonyl)imide [START_REF] Haque | Thermal influence on the electrochemical behavior of a supercapacitor containing an ionic liquid electrolyte[END_REF][START_REF] Fletcher | Ternary Mixtures of Sulfolanes and Ionic Liquids for Use in High-Temperature Supercapacitors[END_REF].To date, thermally stable ionogels have been used as electrolytes in EDLCs and other devices requiring high temperatures [START_REF] Wang | High-performance all solid-state microsupercapacitor based on patterned photoresist-derived porous carbon electrodes and an ionogel electrolyte[END_REF].

Nevertheless, the operating temperature of ionogel electrolytes has been most often limited below 60 °C [START_REF] Negre | Ionogel-based solid-state supercapacitor operating over a wide range of temperature[END_REF] even if some devices using an ionogel have demonstrated impressive cycling stability after being submitted to a solder reflow process at 260°C for few minutes [START_REF] Brachet | Solder-reflow resistant solid-state micro-supercapacitors based on ionogels[END_REF]. To the best of our knowledge, only one paper examines the use of physical gels using ILs (mixture of EMIm TFSI and SiO2 particles) in an EDLC based on multi-walled carbon nanotubes electrodes operated at 100°C [START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF]. However, the related device is also built with very low mass loading and only operated over a 2V cell voltage, with a limited cycling ability of 2 000 cycles.

In the present paper, we depict the concept of all solid state EDLCs that can sustain temperature as high as 100 °C over a large number of charge/discharge cycles. It combines two activated carbon electrodes assembled with an ionogel. This ionogel is obtained from a sol made of EMImTFSI, tetramethoxysilane (TMOS), dimethyldimethoxysilane (DMDMS) [START_REF] Brachet | Solder-reflow resistant solid-state micro-supercapacitors based on ionogels[END_REF]. It is a key point that DMDMS affords flexibility to the silica matrix and enables the fabrication of a crack-free ionogel directly onto the carbon electrodes. This ionogel exhibits high ionic conductivity and good mechanical properties, and is thus suitable to be implemented in high temperatures all-solid-state EDLCs. These ionogel based EDLCs can be operated up to 3 V, demonstrating a high cycling stability (25 000 cycles) at 100 °C as well as a high specific capacitance. Such ionogel based EDLCs are intended for use at a high temperature.

Experimental

Carbon electrodes fabrication

Carbon electrodes were prepared using YP-50 (Kuraray) activated carbon powder (75 wt%) as the active material, carbon black Super-P (15 wt%) as a conductive additive and PVDF-HFP polymer as a binder (10 wt%). The PVDF was in suspension in N-methyl-2-pyrrolidone at 10 wt%. The mixture was blended under magnetic stirring during several hours. The obtained black paste was then casted on aluminum current collectors. Carbon electrodes were then dried out in a vacuum oven during 18 hours at 100 °C.

The mass loading was 9 mg.cm -2 , determined by weighting the aluminum current collector before and after carbon casting.

Ionogel synthesis and casting

Ionogels were obtained from a sol made of tetramethoxysilane (TMOS ≥ 99%, Fulka), dimethyldimethoxysilane (DMDMS), formic acid (FA, ≥ 98%, Sigma Aldrich) and the IL 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI 99.9%, Solvionic). The sol was stirred at room temperature for 5 min. The [TMOS/DMDMS]/IL:FA molar ratio was [70/30]/50:1. This composition allowed getting crack free solid electrolyte. The mixture was then casted on the carbon electrode and was aged for 2 days at room temperature. Concomitantly, a 2 mm thick self-standing ionogel is prepared with the same experimental conditions, for ionic conductivity measurements. This thick ionogel is left for aging 5 days at ambient temperature.

The obtained ionogel film on the carbon electrode was homogeneous and transparent. Then, the electrode underwent further drying at 0.5 mbar and 25 °C for 18 h. Electrodes assembling was performed in a glove box (H2O<0.1ppm, O2<0.1ppm) using Swagelok cells with two stainless steel electrodes, without the need of a standard paper or polymer separator, nor extra step since with 50% of IL in these ionogels the surface is wetted by a nanometric layer of IL., and since these ionogels are soft and compliable.

Characterization

For the DSC measurements (Q20 calorimeter, TA Instrument), the samples were dried at 60°C under vacuum (0.5 mbar) for 18h and sealed in hermetic aluminum pans. Samples were then quenched to -150°C at 40 °C.min -1 and heated from -150 to 50°C at 5 °C.min -1 once the thermal equilibrium has been reached. TGA measurements were conducted from room temperature to 600 °C at 0.5 °C.min -1 under pure oxygen gas, after 30 minutes of stabilization at 40 °C. SEM observations were performed on cross sections of electrodes using a FEG-SEM Merlin Zeiss microscope.

Porosimetry measurements were performed using adsorption and desorption of gaseous nitrogen on the silica matrices obtained after IL extraction from the ionogel, by mean of Soxhlet apparatus with acetonitrile. The specific surface was obtained by the Brunauer-Emmett-Teller (BET) method. The pore size distribution and the pore volume were obtained by the Barett-Joyner-Halenda (BJH) method.

Potential Electrochemical Impedance Spectroscopy (PEIS) was used to determine the ionic conductivity in the temperature range -40 °C to +100 °C. Ionogel pellets were sandwiched between two stainless steel electrodes under argon atmosphere in a Swagelok-type cell. In case of IL alone, the measurement of the ionic conductivity requires the use of an additional Teflon ring. PEIS experiments were carried out in a frequency range from 200 kHz to 10 Hz with an amplitude of 50 mV and an acquisition rate of 6 points per decade. The ionic conductivity was calculated using the equation σ = e/(RS), where e was the thickness (cm) of ionogel pellet, S was the geometric surface of ionogel and R (Ω) was the resistance, taken at the intersection of the Nyquist plot with the Zreal X-axis.

The electrochimical characterizations of solid state EDLCs were performed at 25°C and at 100 °C according to sequence shown in Fig. 1, using a Swagelok cell with two stainless steel electrodes, using a VMP3 (Bio-Logic SA, France) potentiostat/galvanostat in order to evaluate the capacitance and the electrochemical stability window of the EDLCs. The electrochemical properties of EDLCs have been performed at 25°C and 100°C on the two symmetric electrodes (1.13cm 2 ), both with the ionogel electrolytes facing each other and assembled in a Swaglock cell, without any additional separator. PEIS, Cyclic Voltammetry (CV) and Galvanostatic Cycling with Potential Limitation (GCPL) were evaluated on a Biologic VMP3 operated under EC-Lab software with a two electrodes configuration. PEIS experiments on EDLCs were carried out in a frequency range from 200 kHz to 10 mHz with an amplitude of 50 mV. Cyclic voltammograms were measured between 0 and 3V using different scan rates of 5, 10, 20, 50, 100, 150 and 200 mV/s. Galvanostatic cycling GCPL has been performed on the EDLCs between 0 and 2.5 V with a constant charge discharge current ∼1 A.g -1 .

Fig 1 . Experimental process for the electrochimical characterizations of all solid state EDLCs.

Results and discussion

Fig. 2 (a-c) show ionogel pellet and film after 5 days aging at room temperature. A flexible material is obtained (Fig. 2 b-c). No leakage of liquid was observed even after several months. The TGA thermograms of the IL and of the ionogel exhibit single step decomposition as depicted from Fig. 3a. For both, the mass are almost unchanged until 350 °C. Above 350 °C, monotonic decrease in weight loss is observed, similar to that of pure EMIm TFSI. As expected, the ionogel exhibits favorable thermal stability. The inception of thermal decomposition is detected at about 350 °C and complete degradation occurs at about 470 °C for EMIm TFSI and at 440 °C for the ionogel. In this last case the remaining weight corresponds to the silica scaffold which is not degraded upon heating the ionogel.

Nevertheless the TGA results indicate that the ionogel possesses quite similar thermal stability than that of the corresponding IL, and thus it appears suitable for high temperature energy storage applications which is not possible for aqueous (< 100 °C) or organic (< 70 °C) based electrolytes [START_REF] Kühnel | Mixtures of ionic liquid and organic carbonate as electrolyte with improved safety and performance for rechargeable lithium batteries[END_REF][START_REF] Mcewen | Nonaqueous Electrolytes for Electrochemical Capacitors: Imidazolium Cations and Inorganic Fluorides with Organic Carbonates[END_REF], thus also avoiding leakage issue. Fig. 3b displays the conductivity plot of the ionogel and of the non-confined IL, determined with PEIS. The ionic conductivity of pristine EMIm TFSI is 9 mS.cm -1 at 20 °C, which is consistent with reported literature data [START_REF] Brachet | All Solid-State Symmetrical Activated Carbon Electrochemical Double Layer Capacitors Designed with Ionogel Electrolyte[END_REF][START_REF] Brachet | Solder-reflow resistant solid-state micro-supercapacitors based on ionogels[END_REF]. For the ionogel with the composition [70/30]/50, the ionic conductivity is close to that of the corresponding IL between 20 °C and 100 °C. A value of 4 mS.cm -1 was obtained at 20 °C. Noteworthy, this value increases up to 26 mS.cm -1 at 100°C. The ionic conductivity of the EMIm TFSI shows a break in the conductivity slope at -10 °C, corresponding to the phase transition from the liquid state to the solid state. The ionogel does not show such a break in the ionic conductivity which fades steadily as the temperature decreases down to -40°C. This rather high ionic conductivity of the ionogel at low temperature is due to confinement effect, which quenches part, but not all, the liquid-tosolid transition [START_REF] Guyomard-Lack | Destructuring ionic liquids in ionogels: enhanced ionic liquid properties for solid devices[END_REF]. The DSC measurements (inset in Fig. 3c) shows the glass transition temperature (Tg) of EMIm TFSI measured at -70 °C. The value of Tg decreases only slightly, likely within experimental error, with silica content, from -88 °C to -90 °C. N2-sorption measurements have been used to determine the surface area, pore volume and pore size distribution of the samples (Fig. 3d). The BET surface area is 800±10 m 2 .g -1 . From Fig. 3d it can be seen that the isotherm is of type IV with a typical hysteresis loop in a relative pressure range of 0.5 to 1.0, corresponding to a mesoporous structure [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF]. Moreover, the inset in Fig. 3d shows the BJH pore size distribution plot with a maximum centered at ~ 8 nm. The configuration of the all-solid-state EDLC is schematically shown in Fig. 4a. The ionogel acts as the electrolyte and also provides a physical barrier between electrodes to prevent short circuits. Fig. 4 b-c are the SEM cross sections of the carbon electrode before and after ionogel casting. The ionogel forms a 20 µm thick film on the surface of the carbon electrode. Fig. 5a andb show CVs plots obtained with the IL and with the ionogel electrolyte at 25°C and 100 °C over a 0 to 3 V voltage range at scan rates from 5 mV.s -1 to 200 mV.s -1 . At room temperature, the CVs (Fig. 5a) obtained with the IL exhibit a capacitive behavior as can be seen from the rectangular shapes of the CV. The CV shape of the all-solid-state EDLC with the ionogel is very similar (Fig. 5c). At 100 °C the CVs remain quasi-rectangular whatever the scan rate. Moreover, the improved ionic conductivity of the IL and of the ionogel at this temperature compared to that obtained at 25°C provides a less distorted profile (Fig. 5b-d) thus indicating faster kinetic at high temperature, which was expected for such device. The gravimetric and areal capacitances at 25 °C and 100 °C for the all-solid-state EDLC are shown in Fig. 5e and 5f respectively. At low scan rates (5 mV.s -1 ), the gravimetric capacitance (20 F.g -1 , total mass of both electrodes) and areal capacitance (177 mF.cm -2 , total area of the device) measured at 25°C for the EDLC based on the IL are in very good agreement with previously reported values using reasonable mass loadings (> 2 mg.cm -2 ) [START_REF] Balducci | High temperature carbon-carbon supercapacitor using ionic liquid as electrolyte[END_REF][START_REF] Ruiz | Ionic liquid-solvent mixtures as supercapacitor electrolytes for extreme temperature operation[END_REF][START_REF] Thangavel | High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device[END_REF][START_REF] Sato | Electrochemical properties of novel ionic liquids for electric double layer capacitor applications[END_REF][START_REF] Kurig | Electrochemical Characteristics of Carbide-Derived Carbon 1-Ethyl-3methylimidazolium Tetrafluoroborate Supercapacitor Cells[END_REF][START_REF] Largeot | Microporous Carbon-Based Electrical Double Layer Capacitor Operating at High Temperature in Ionic Liquid Electrolyte[END_REF]. It can be noted that the EDLCs based on the ionogel shows only a 10% difference at 25°C with the EDLCs with the non-confined IL. Due to the low ionic conductivity of the electrolytes at room temperature, both devices (IL and ionogel based) exhibit a strong rate dependence of the capacitance which fades by 50% at 200 mV.s -1 . When the cycling temperature is raised to 100°C, the gravimetric and areal capacitances are very close for both devices and are increased by ca 5% compared to those at 25°C. The capacitance decay upon increased cycling rate is also less drastic than at 25°C, with almost 75% of the capacitance retained at 200 mV.s -1 instead of 50% at 25°C

for the same cycling rate. Such behavior was not reported on previous study on ionogel-based EDLC [START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF] where capacitance still fade from 50% between 10 mV.s -1 and 80 mV.s -1 , when increasing the temperature from 25°C to 100°C. For this last case, this might be due to the physical gel (mixture of EMIm TFSI and SiO2 particles) instead of the chemical gel (ionogel). With the ionogel, a better homogeneity is reached due to the impregnation of the sol on the carbon electrode, a better wetting of carbon particles is obtained, and a bicontinuous IL / host network interface is formed.

However, in our cell design, the thickness of the ionogel as a separator is 2 × 20 µm. At 100°C, the resistance of such separator should be 0.14 Ω as determined from the ionic conductivity of the ionogel at 100°C. However, from EIS analysis (Fig. 6), the resistance is 2.6 Ω. Thus it seems that the resistance of the cell is mainly due to ion diffusion in the porosity of the carbon of the electrodes. Moreover, when the temperature is raised from 25°C up to 100°C, the ionic conductivity of the ionogel increases by a factor of 6.5, but practically the cell resistance is only decreased by a factor of 4, thus pointing out again the role of ions diffusion in the carbon electrodes. The same remark applies for ionic liquid based cells. Indeed, Kuraray YP50 carbon is a microporous carbon with most of the pores below 2 nm and a cumulative pore volume close to 0.7 mL.g -1 [START_REF] Porada | Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation[END_REF]. At medium frequencies region a small semi-circle is observed. The presence of a semi-circle has been reported by different authors using carbon based electrodes with IL or ionogel electrolyte [START_REF] Negre | Ionogel-based solid-state supercapacitor operating over a wide range of temperature[END_REF][START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF].

Various explanations have been proposed including the carbon electrode/aluminum current collector interface [START_REF] Porada | Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation[END_REF][START_REF] Portet | Modification of Al current collector surface by sol-gel deposit for carbon-carbon supercapacitor applications[END_REF] or the resistive-capacitive behavior of carbon electrodes at high frequencies for which the shape and width of these semicircles depend on the conductivity of the electrolyte at the electrode/separator interface and inside the separator [START_REF] Kurig | Electrical Double Layer Capacitors Based on Two 1-Ethyl-3-Methylimidazolium Ionic Liquids with Different Anions[END_REF]. Both explanations are provided by Soeda et al and can be proposed in the case of our cells due to similarities with this last study [START_REF] Soeda | Outstanding features of alginate-based gel electrolyte with ionic liquid for electric double layer capacitors[END_REF].

For both EDLCs, all plots obtained at 25 °C and 100 °C show a diffusive behavior in the low frequency region. The slope of the 45° portion of the curve (Warburg impedance) is a result of the frequency dependence of ion diffusion/transport in the electrolyte confined in the porous carbon electrodes [START_REF] Conway | Electrochemical Supercapacitors[END_REF]. More interestingly is the decrease of the equivalent series resistance (ESR) from 5.5 to 2.5 Ω for the IL and from 10 to 2.6 Ω in for the ionogel based EDLC when the temperature increases from 25 °C to 100 °C. This clearly demonstrates that despite a higher ESR at 25°C which will penalize the power density at room temperature compared to the device using IL impregnated in a standard separator, the ionogel electrolyte cancels this drawback at 100°C with a similar ESR for both devices. Such interesting behavior for the solid state EDLC at 100°C can be due to an improvement of the ionogel / electrodes interfaces, allowing fast ions transports from ionogel bulk electrolyte to the electrode surface at high temperature, as previously evidenced by our team [START_REF] Brachet | Solder-reflow resistant solid-state micro-supercapacitors based on ionogels[END_REF]. As reported by some authors, at higher voltages, the deviations from ideal capacitive behavior are observed in CVs and PEIS data, and can be due to the specific adsorption of ions from the IL and/or to faradaic processes occurring on/inside carbon based electrodes based on the presence of undesirable traces of residual species (water, halogens) in the IL [START_REF] Kurig | Electrical Double Layer Capacitors Based on Two 1-Ethyl-3-Methylimidazolium Ionic Liquids with Different Anions[END_REF]. This observation is consistent with previously reported studies using ILs in EDLCs at high temperature, and concluding that the operating voltage window must be limited even to less than 1.2V, in order to assume long term cycling when the operating temperature was raised up to 100°C and above [START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF][START_REF] Fletcher | Ternary Mixtures of Sulfolanes and Ionic Liquids for Use in High-Temperature Supercapacitors[END_REF]. The insets of Fig. 7 present the CVs at 20 mV.s -1 before and after every 1000 GCPL cycles. The solid state EDLC retains 92 % of its initial capacitance after 25 000 cycles, which reveals stable behavior for the electrode material and the ionogel electrolyte. These long term cycling abilities could be attributed to the limitation of the cell voltage at a maximum of 2.5 V, but it also evidences the good interface between the electrodes and the ionogel. It is the first time to our knowledge that such long term operation of EDLC using ILs and ionogels at 100°C is reported in the literature. Former reports on ionogel-based EDLCs [START_REF] Ortega | Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid[END_REF] have shown a capacitance retention of 88% after 2000 cycles.

Conclusion

A high-performance all-solid-state EDLC was designed with two activated carbon electrodes and an ionogel as the electrolyte. The all-solid-state supercapacitor has demonstrated very good performance at 25°C and 100°C compared to an EDLC using IL as electrolyte. The fabrication of such all-solid-state supercapacitor strongly depends on the ionogel with exhibit a wide electrochemical window (0-3 V) and a high ionic conductivity. Moreover, these performances are not degraded at 100°C even with long term cycling tests thus demonstrating high thermal stability. In addition, the solid state EDLC exhibits very good long-term stability over 25 000 charge/discharge cycles which suggests a potential use in applications requiring mechanically stable devices that can be operated up to 100 °C. Further studies are in progress in order to evaluate the temperature stability of solid state EDLCs above 100°C.

Fig. 2 .

 2 Fig. 2. (a) Bulk ionogel pellet and (b-c) film after aging, with molar ratio [70/30]/50.

Fig. 3 .

 3 Fig. 3. (a) TGA showing thermal the thermal stability of the pristine IL and of the ionogel. (b) Conductivity versus temperature (c) DSC profiles of the IL and of the ionogel (d) N2 adsorption/desorption isotherm; inset: BJH pore size distribution for the ionogel after IL extraction.

Fig. 4 .

 4 Fig. 4. (a) Schematic drawing of the fabrication process of an all-solid-state EDLC; SEM micrographs of the cross section of carbon electrode (b) before and (c) after ionogel casting, insets: carbon paste on 12 mm diameter aluminum current collector (125 µm thick) (b) before and (c) after ionogel casting.

Fig. 5 .

 5 Fig. 5. Cyclic voltammetry data at different scan rates. (a) CVs at 25 °C and (b) CVs at 100 °C for the IL, (c) CVs at 25 °C and (d) CVs at 100 °C for the ionogel, (e) Gravimetric capacitance (F.g -1 ) and (f) Areal capacitance (mF.cm -2 ) vs scan rate at 25 °C and 100 °C for EDLCs with the IL and the ionogel.

Fig. 6 shows

 6 Fig.6shows Nyquist plots of carbon EDLCs with the IL and with the ionogel electrolytes at 25 °C and 100 °C, obtained before and after every CV and 1000 GCPL cycles. The proposed all solid state device is designed to be operated at high temperature; PEIS data below 25°C are not shown here since they are of poor practical interest.

Fig. 6 .

 6 Fig. 6. Nyquist plot recorded after every 1000 GCPL cycles at room temperature (a, c) and 100 °C (b, d) for the IL (a, b) and for the ionogel (c, d) EDLCs.

Fig. 7 .

 7 Fig. 7. shows the cycling stability of the EDLCs at 25 °C and 100 °C for both IL and ionogel. The cell voltage was limited to 2.5 V in order to avoid the capacitance fade observed upon long term cycling tests using 3V as upper cut-off voltage (not shown here). Such fade in capacitance when using a [0-3V] cell voltage window can be due to the degradation of the electrolyte during cycling, or to the oxidation of

Fig. 7 .

 7 Fig. 7. GCPL data at room temperature (a, c) and 100 °C (b, d) for EDLCs using IL (a, b) and ionogel electrolytes (c, d); insets: CV at 20 mV.s -1 recorded after every 1000 GCPL cycles according to the program presented Fig.1; at the 25,000 th cycle, the charge efficiency is of 78, 85, 73 and 83 % for a, b, c, d resp..

Acknowledgment

The authors thank the French network on electrochemical energy storage (RS2E) for the financial support and gratefully acknowledge Kuraray Company for providing YP50 activated carbon. The authors also thank Fanch Guillou for providing special cell for testing EDLCs.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.