Prof Victor L Vanpeene

Julie Villanova

Andrew King

Dr Bernard Lestriez

Eric Maire
email: eric.maire@insa-lyon.fr

Lionel Roué
email: lionel.roue@emt.inrs.ca

Roué

Dynamics of the morphological degradation of Si-based anodes for Li-ion batteries characterized by in-situ synchrotron X-ray tomography

Keywords: lithium-ion battery, silicon anode, X-ray tomography, morphological dynamic

The alloying reaction of silicon with lithium in negative electrodes for lithium-ion batteries causes brutal morphological changes that severely degrade their cyclability. In this study, the dynamics of their expansion and contraction, of their cracking in the bulk and of their debonding at the interface with the current collector are visualized by in-situ synchrotron Xray computed tomography and quantified from appropriate 3D imaging analyses. Two electrodes made with same silicon material having reasonable particle size distribution from applied point of view are compared: one fabricated according to a standard process and the other one prepared with a maturation step, which consists in storing the electrode in a humid atmosphere for a few days before drying and cell assembly. All morphological degradations are significantly restrained for the matured electrode, confirming the great efficiency of this maturation step to produce a more ductile and resilient electrode architecture, which is at the origin of the major improvement in their cyclability.

Introduction

Because increasing the energy density of lithium-ion batteries is still a challenge especially for electric vehicle application, different anode materials with high theoretical capacities have been investigated. [1,2] Among them, silicon has drawn a lot of attention due to its gravimetric capacity ten times higher than graphite (3579 mAh g -1 for Li15Si4 versus 372 mAh g -1 for LiC6). Si is also earth-abundant, low-cost and non-toxic. However, its use in commercial Li-ion batteries is challenging because the capacity retention and coulombic efficiency of Si electrodes are much lower than for graphite electrodes. [3,4] This is related to the important volume change of Si during its lithiation (up to 280% for Li15Si4 versus ~10% for LiC6), which deteriorates the electrode architecture, and destabilizes the solid electrolyte interphase (SEI). The optimization of the mechanical properties of Si-based electrodes is thus a key issue to improve their electrochemical performances. This optimization must be done at the Si particle scale as well as at the composite electrode scale.

The use of nanosized Si materials (nanoparticles, nanowires, nanopillars, nanocomposites) has demonstrated a great efficiency for limiting the pulverization of the Si active material upon cycling as its fracturing resistance is size-dependent. [5] One must however note that the synthesis of such nanosized materials at industrial scale and competitive cost remains an issue, in addition to their low tap density and high surface reactivity. It thus appears more relevant to use micrometric particles, e.g. ball-milled nanocrystalline/amorphous Si [6] or nanostructured Si alloys [1,7] able to prevent the formation of crystalline Li15Si4, which is known to promote particle fracture due to its large phase boundary stress with amorphous LixSi during delithiation.

The mechanical properties (compliance) of the SEI layer must also be optimized to be able to tolerate the large volume variation of the Si particles. To date, the use of appropriate electrolytes or electrolyte additives such as fluoroethylene carbonate (FEC) [8] appears as the most judicious strategy. [9] However, this interfacial issue is still not fully resolved and is probably the most important obstacle for the implementation of Si-rich based anodes in commercial Li-ion batteries.

At a larger spatial scale, the volume change of the electrode is likely to induce its macrocracking, collapsing and/or delamination from the current collector, leading to electrical disconnection of some Si particles. [10] In order to limit these morphological degradations, the cohesion and adhesion strength of the Si electrodes needs to be improved. These have been shown to be highly dependent on the nature of the electrode binder. [7,11] The crucial role of the polymer binder has been brought to light by early studies aiming at comparing the performance of the commonly used polyvinylidene difluoride (PVDF) with those of alternative binders, such as carboxymethylcellulose (CMC). [12,13] It was shown that the chemical interaction of the CMC binder with the Si particles is responsible of its efficiency rather than its physical properties (flexibility). [13] This chemical bonding results from the condensation reaction between -OH groups present at the surface of the Si particles and -COOH groups of CMC, which is favoured using a buffered pH3 slurry media. [14] Since these observations, various functionalized and water-soluble binders have been investigated and some of them have shown promising performances for Si-based electrodes. [15,16] Note that these binders can also act as protective SEI layer, improving the initial coulombic efficiency. [17] It was also shown that the efficiency of acidic aqueous binders can be greatly enhanced through a post-processing treatment (called maturation) consisting of storing the Si-based electrode in a humid atmosphere for a few days before drying and cell assembly. [18] During this maturation step, the binder tends to concentrate at the silicon interparticle contacts, which improves the cohesion strength of the electrode. At the same time, the corrosion of the copper current collector induces the formation of copper carboxylate bonds, improving the adhesion strength the electrode onto the current collector. The result is a significant electrochemical performance gain, up to a factor 10, compared to a standard (not-matured) electrode. This maturation procedure can also restore bonds that are broken during the calendering process, resulting in substantially improved cycling capability of calendered electrodes. [19] A better comprehension of the morphological changes of the Si-based anodes with cycling, which occur at different scales (i.e. from the Si particle to the global electrode architecture as previously argued), requires powerful analytical tools. X-ray computed tomography (XRCT) is relevant for this purpose due to its ability to characterize complex structures in 3D. As listed in Table S1 (Supporting Information), this technique has been used

for in-situ [20][21][22][23][24][25][26][27][28][29][30] and ex-situ [31][32][33] studies on various Si-based electrodes, with a reconstructed volume ranging from ~10 -3 mm 3 [31] to ~10 mm 3 [20] and a voxel size ranging from 0.06 µm [31] to 9.3 µm [20] (the spatial resolution corresponds to about two times the voxel size). Relevant information on the electrode degradation process have been obtained such as its delamination [20] , the Si particle cracking [21,25] and disconnections [23] , the morphological evolution and connectivity of the pores and solid phases. [24,26,28] However, it must be noted that most of these XRCT studies have been performed on thick electrodes made with large Si particles (typically ≥ 20 µm in diam.), which are not very representative of a real electrode.

Moreover, these previous studies are mainly focused on qualitative analyses.

In the present study, in-situ synchrotron XRCT characterization technique is implemented to monitor the morphological evolution with cycling of Si-based electrodes made with micrometric Si particles (d50 = 2.5 µm). The behaviour of two electrodes is compared: one prepared using a standard procedure and the other one prepared with a maturation step. [18] Various morphological parameters are quantified and followed during the 1 st cycle, namely (i) the dimensional change of the electrode, (ii) the volume fraction, size and connectivity of the segmented solid, gas and electrolyte phases, (iii) the volume fraction, size and connectivity of the produced macrocracks, (iv) the surface area of the produced delaminated zones from the current collector and (v) the variation of the Si inter-particle distances. For the first time, the dynamics of the electrode macrocracking and delamination is clearly evidenced from in-situ qualitative and quantitative 3D imaging analyses. These morphological degradations are much less intense for the matured electrode, confirming the efficiency of the maturation procedure to enhance the mechanical strength of the Si-based electrodes.

Results and discussion

Both standard and matured electrodes have the same composition, namely 73.1% wt of ball-milled silicon (d50 = 2.5 m) as active material, 11.0% wt of graphene nanoplatelets (GnP) as conductive additive, 7.3% wt of carboxymethylcellulose (CMC) as binder, and 7.7% wt of citric acid and 0.9% wt of KOH salts originating from the pH3 buffered slurry solution. Their active mass loading is 5.5 mg Si cm -2 and their pristine thickness is 95 µm. Details on the electrode preparation and the maturation procedure are given in the Experimental section.

In-situ synchrotron XRCT measurements were performed during the 1 st cycle on both electrodes using an X-ray compatible electrochemical cell (see Figure S1a, Supporting Information). The total imaged volume for the standard electrode is 13331333210 µm 3 with a voxel size of 650 nm (see Figure S1b, Supporting Information), and 256256210 µm 3 with a voxel size of 200 nm for the matured electrode (see Figure S1c, Supporting Information). This difference is due to the fact that XRCT experiments were performed using two different synchrotron facilities: Soleil synchrotron for the standard electrode and ESRF synchrotron for the matured electrode (see the Experimental section for more detail). This is because the amount of beam time allocated on each beam-line was too short to perform both experiments at the same place. Nevertheless, the resolution change is not affecting our main conclusions, as the morphological changes analysed on both electrodes have dimensions higher than 600 nm. Moreover, both electrodes are compared from an identical reconstructed volume of 190190210 µm 3 . Such a volume can be considered as representative of the whole electrode volume (see Figure S2 and related comments, Supporting Information).

Using appropriate image processing, segmentation and analysis procedures as described in the Experimental section and Supporting Information, various morphological parameters of different identified domains and their evolution during the 1 st cycle have been quantified and compared for both electrodes as presented hereafter. These parameters are listed in Table 1 for the pristine state, after the 1 st discharge and after the 1 st charge. The evolution during the first cycle of some key morphological features for both electrodes, namely their thickness expansion/contraction, the volume fraction of macrocracks, the coating / current collector delamination area and the mean Si interparticle distance variation are collected in Figure 1.

Pristine state

At the pristine state, both electrodes have a similar porosity of about 38-39% (Table 1).

Only 2-3 % of the pores contain gas (i.e. are electrolyte-free), confirming their nearly complete impregnation by the electrolyte. The median diameter of the pores is lower for the matured electrode (d50 = 2.6 µm) than for the standard one (d50 = 5.4 µm). This difference may partly result from the higher XRCT resolution for the matured electrode, permitting a better identification (segmentation) of the smallest pores. Note that the electrode porosities measured here may be underestimated as the reconstructed volumes do not permit the identification (segmentation) of the electrolyte phase located in the submicrometric interstices of the composite solid phase due to resolution limitation. The homogeneous distributions of the electrolyte-filled pores and solid phase in the electrodes are highlighted by their high intra-connectivity (about 98 and 100%, respectively). Here, all the solid components of the electrodes (i.e. Si particles, GnP conductive additive, CMC binder and CA/KOH salts) are considered as a unique domain labelled "solid phase" since they cannot be precisely differentiated due to their similar attenuation coefficient and/or too small size. Actually, only a few large Si particles can be clearly identified in the solid phase (see 3D views of the pristine standard and matured electrodes in Figure S3, Supporting Information). As expected, no macrocrack in the bulk of the electrodes and no significant delaminated zone at the electrode/current collector interface are detected at the pristine state.

Electrochemical behavior

The potential evolution during the 1 st cycle of the standard and matured electrodes is presented in Figure 1a and 1b, respectively. The potential jumps correspond to XRCT acquisition steps where the cell was let to the open circuit potential. The electrochemical cells were galvanostatically discharged and charged between 1 and 0.005 V vs Li/Li + . The applied current was 180 mA g -1 of Si (C/20) for the standard electrode. A higher current of 600 mA g - 1 Si (C/6) was applied for the matured electrode, as only one day of beam time was allocated for this experiment. A first discharge (charge) capacity of 1311 (734) mAh g -1 of Si, corresponding to an initial coulombic efficiency (CE) of 56.0%, are obtained for the standard electrode. For the matured electrode and despite its higher C-rate inducing a larger polarization, those values are better, respectively 1801 (1088) mAh g -1 and 60.4%. This already highlights positive impact of the maturation on the electrode performance. Note that the observed capacities are much less than the theoretical capacity of Si for both electrodes and are about 2 times lower than those previously observed with a lower areal mass loading of 1-2 mg Si cm -2 . [18] Actually, this can be mainly explained by the fact that the Si particles are not fully lithiated when the present electrodes reach the discharge cut-off potential due to their high polarisation resistance resulting from their high mass loading (5.5 mg Si cm -2 ). Indeed, it is well known that the resistance to the electronic and ionic charge transport through the composite electrode increases with the electrode loading (thickness) as the lengths of the electronic and ionic paths increase with the electrode thickness. 34]

Electrode volume (thickness) variation

The dimensional changes of the electrode mainly occur in the vertical direction (electrode thickness) thanks to the flexibility of the separator whereas the in-plane expansion is constrained by the fixed dimensions of the Cu substrate. This is highlighted in Figure S4 (Supporting Information) which shows the relative dimensional change of both electrodes along the three Cartesian axes.

Figure 1c shows the relative variation of the electrode thickness during the 1 st cycle for the standard and matured electrodes. The theoretical variation in thickness assuming no change in percent porosity is also shown for comparison (dotted lines). This one is higher for the matured electrode due to its higher capacity as shown in Figures 1a-b. Details on the theoretical calculation are presented in Supporting Information. The experimental thickness variation is lower for the matured electrode despite its higher discharge capacity with a maximal expansion of 48% compared to 72% for the standard electrode. Moreover, the observed slope in thickness change is lower than the theoretical slope for the matured electrode, suggesting that its porosity is able to partially buffer the Si expansion in contrast to the standard electrode. This indicates that in the standard electrode the contacts between the Si particles are more rigid and non-slippery. Each particle expands, repels its neighbors and the porosity contained in the interstices between them increases at the same time. On the contrary, for the matured electrode, the contacts between the particles would be slippery, allowing the sliding of the particles and their displacement in the open spaces (porosity). This is consistent with a plastic behaviour in the case of the matured electrode, and a more fragile behaviour in the case of the standard electrode, accentuating its macrocracking upon charge as discussed below. Additionally, an abrupt increase in thickness is observed at a depth-of-discharge (DOD) of 70% for the standard electrode. This can be correlated to the increase of the electrode delamination observed at the same discharge period (Figure 1e). This event also induces a potential drop as seen in Figure 1a, reflecting an increase of the polarisation resistance since this delamination must induce the rupture of the electrical network in the electrode. For both electrodes, a residual thickness expansion is observed at the end of the charge (25 and 21% for the standard and matured electrode, respectively). This indicates irreversible change in the electrode architecture. The incomplete delithiation (deflation) of some Si particles due to their electrical disconnection can contribute to this irreversible deformation.

Electrode macrocracking

The microcracks, likely to occur at the Si particle level 21,25 , is not quantified here due to resolution limitation. Figure 1d shows the evolution of the volume fraction of macrocracks formed in the standard and matured electrodes. For both electrodes, the formation of macrocracks is only observed during the delithiation. This cracking is much more intense for the standard electrode, especially from a SOC of about 75% where an abrupt increase of the volume fraction of cracks can be observed. After the first delithiation, the amount of cracks reaches 11.6% and 3.5% for the standard and matured electrode, respectively.

As shown on the transversal and lateral XRCT images of the electrodes at the end of the 1 st charge (Figure 2a-d), not only the amount but also the size (width) of the macrocracks are larger for the standard electrode. This is confirmed from their respective thickness distribution shown in Figure 4c, which indicates a median crack width of 13.1 m for the standard electrode versus 4.8 m for the matured electrode. The cracks are filled with electrolyte (in dark grey) or gas (in black). The fraction of gas-filled cracks seems to be larger for the standard electrode. Actually, the amount of cracks filled with electrolyte is close to ~2% v in both cases, but the proportion of cracks filled with gas is five times higher for the standard electrode (Figure 4d). This difference can be explained by considering that the wider cracks in the standard electrode should lower the interfacial capillary forces. The density of cracks seems to be higher near the current collector (at z3 ~15 m). It is also possible to observe some large cracks going through the entire electrode thickness on the transversal image of the standard electrode and splitting the electrode along islands of several hundreds of microns in width, as seen in the lateral images of the standard electrode (Figure 2 b-d). These XRCT observations are in accordance with the morphological observations obtained at a larger scale by operando optical microscopy. [18] Complementary to these analyses, it has been possible to follow the crack growth dynamics in the electrodes from 3D imaging acquired along the 1 st charge. For the standard electrode (Figure 3a), the first cracks (0.8% v ) are formed at 30% SOC.

One major crack is formed near the current collector interface and is highly oriented along the electrode thickness (z axis). The other one is smaller and forms at a higher z position with the same orientation. While reaching 43% SOC, the length of these two cracks have increased in the direction perpendicular to the current collector. Also a new crack is formed at a higher z position in the electrode, doubling the total volume fraction of crack at this step (1.6%). From this step and until 71% of SOC, the three main cracks are growing mostly in the x,y directions.

From 71 to 85% SOC, there is coalescence of the earlier cracks and formation of new cracks and thus the volume fraction of cracks and their intra-connectivity are drastically increased respectively from 2.0% to ~7.9% and from 45% to 90%. The initial isolated cracks clearly coalesce into a single fully connected macrocrack. This is particularly visible on the (x,y) lateral images. This might be due to the sudden breakage of the already weakened binding bounds of the composite structure at this moment. During the next charge steps and until the end of the charge, the crack growth continues (up to 11.6% v ) but in a more progressive way and mostly in the x,y lateral directions. Noteworthy, although the higher contraction occurs along the z axis during delithiation (see Figure S4, Supporting Information), the cracks are all formed parallel to this z axis in the standard electrode. This can seem paradoxical from a mechanical point of view. To explain this, we believe that cracking during contraction is due to the constraint imposed by the current collector in the x and y directions. In these two directions, the electrode is strongly contracting but the collector is not. This leads to large deformation incompatibilities in the x and y directions while nothing constrains the electrode in the z direction. This is the reason why the cracks are occurring in planes perpendicular to x and y. This process is commonly known as "mud cracking" occurring when there is a biaxial stress in the film, which fractures into a series of islands similar to that seen in areas of drying mud. [35] In comparison, for the matured electrode (Figure 3b), the crack growth is more progressive and much less important. The cracks are randomly spread in the electrode volume with a low volume fraction and intra-connectivity. At the end of the charge, the volume fraction of macrocracks is only 3.5% with an intra-connectivity of 32%. One big single crack is observed in the diagonal of the observed region. The difference in the cracking pattern is highlighted in Figure 4a-b, which compares 3D views of the cracks formed for the standard (a) and matured (b) electrodes at the end of the 1 st cycle. The largest intra-connected crack cluster is in green and the second one in blue on these 3D images. For the standard electrode, most of the volume of cracks is included in the largest interconnected cluster (74.5%), as the second one is counting for only 1.3%. In comparison, the cracks in the matured electrode form different clusters. The largest one represents 31.8% of the total crack volume. This crack is going through the entire analysed lateral dimension (~190 m) but is limited along the thickness direction of the electrode (~45 m) in contrast to the cracks of the standard electrode which cross all the electrode thickness (~130 m). This could be explained by more compliant anchoring of the matured electrode to the current collector compared to the standard electrode, buffering the contraction incompatibility between the electrode and the collector and thus avoiding cracks initiating from this interface. Moreover, from a basic mechanical point of view, a crack is expected to propagate through a deformed material if the elastic energy released by the crack growth is greater than the energy required to create the fractured surfaces. [36] In other words, the mechanical energy of deformation associated with the contraction of the electrodes can be either released by macrocracking or/and dissipated by a combination of viscoelastic and plastic processes. The four times higher volume fraction of cracks in the standard electrode indicates that the polymeric binding network is more likely to release energy by cracking in this case, which is indicative of a more fragile behaviour of the composite electrode. Therefore, according to basic mechanics, it can be said that maturation changes the fragile mechanical property of the electrode to a more ductile one, which must be related to a significant modification of the binder phase mechanical properties. Here the entire imaged (x,y) lateral section of the electrodes is shown. Note that the black circle observed on some images of the standard electrode (at DOD 100%, SOC 30%, SOC 75%) corresponds to a ring artefact probably enhanced by the presence of the highly absorbent copper collector nearby. [START_REF] Barrett | [END_REF] It appears clearly from these images that the delamination of the Si-based coating from the Cu current collector is very limited for the matured electrode compared to the standard one. This is also visible in the corresponding XRCT movies, highlighting the dynamics of the electrode delamination process (see Video 3

for the standard electrode and Video 4 for the matured electrode, Supporting Information).

The quantification of their relative delaminated area with cycling is presented in Figure 1e.

For the standard electrode, a significant and linear increase of the delaminated area is observed from 25% DOD. An abrupt increase is observed at 70% DOD, which can be correlated to the abrupt increase of the electrode thickness (Figure 1c) and to the potential drop (Figure 1a) as previously discussed. At the end of the 1 st discharge, the delaminated area reaches a maximal value of 17.4%. In comparison, no significant delamination is observed for the matured electrode. During the charge, the delamination area of the standard electrode is firstly reduced until reaching 50% SOC. This can be explained by the deformation of the electrode during shrinkage, restoring some contact with the current collector. This is supported by transversal (x,z) images of the standard electrode where partial restoration of the film/Cu substrate contact is discernible during the charge (see Figure S5a, Supporting Information). From 50% SOC, the delamination area of the matured electrode increases again in accordance with the accentuation of its macrocracking (Figure 1d) which preferentially initiates from the film/current collector interface as previously argued. As

shown in Figure 1e, for the matured electrode, an increase of the delamination area is observed during the charge to reach a maximum value of 1.6%, which is 8 times lower compared to the standard electrode (12.5%), confirming again its stronger adherence to the current collector. This can be explained by the formation of copper-carboxylate coordination bonds between the Cu current collector and the CMC binder, as a result of the atmosphereinduced corrosion of copper activated by the acidic functional groups of the binder during the maturation step. [18] 2.

Silicon particle tracking

The tracking of three selected Si particles, sufficiently large to be discernible (segmented) in the solid phase, was performed along the 1 st cycle for the standard and matured electrodes. Figure 6a and 6b shows lateral(x,y) images of 2020 m 2 focused on one of these Si particles (10 µm diam.) along the 1 st cycle for the standard and matured electrodes, respectively. The equivalent images for the particles #2 and #3 are shown in Figure S6a-b (Supporting Information). For both electrodes, the large Si particle, initially well discernible, becomes increasingly greyish as the discharge progresses since its mean attenuation coefficient is reduced proportionally to its density, which decreases as its lithiation level increases. On can note that the surrounding smaller Si particles observed in Figure 6a seem to be fully lithiated earlier as they "vanish" at 70% DOD whereas the large Si particle is still visible. In addition, for the standard electrode at 45% DOD, some pores initially filled with electrolyte become gas-filled. This indicates that the gas, present in the pristine electrode (2.1% v , Table 1) or likely to be produced during the SEI formation [22,28] , can move freely through the global porous network of the electrode. At 100% DOD, the large Si particle is nearly no longer distinguishable from its surrounding matrix (because highly lithiated) for both electrodes. During the charge of the standard electrode, the formation of a gas-filled macrocrack (in black) can be observed, which enlarges as the SOC increases. In comparison, no macrocracks are observed for the matured electrode (see Figure 6b and S6b). Note that for both electrodes, no microcracking of the Si particles is detected here despite their large size. If any, the opening of these microcracks, lower than the present XRCT resolution, may prevent their detection. Furthermore, it should be considered that the poorly crystallized state of the Si particles at the initial state (63% amorphous) is likely to limit their fracturing by favoring a more homogeneous deformation of the Si particles during their lithiation. [39,40] On the other hand, it is noticeable that the silicon particles do not recover their initial bright aspect at the end of the charge, which may indicate that their lithiation is not fully reversible. In addition, they do not recover their initial position, reflecting irreversible change in the electrode architecture.

In order to quantify the distortion of the silicon particle network along cycling, the interparticle distances d1-3 (between particles #1 and #3), d1-2 (between particles #1 and #2), d2-3 (between particles #2 and #3) have been measured along the 1 st cycle for both electrodes.

These distances are typically around 100±50 µm. Their respective relative evolution with cycling can be found in Figure S7 (Supporting Information). The evolution of the Si interparticle distance (expressed in relative variation) averaged from the displacements of the 3 particles is presented in Figure 1f for the standard and matured electrodes. The difference is strikingly visible between both electrodes. In fact, the variation of the inter-particle distance is much larger, less progressive and less reversible for the standard electrode. Maximum variations of 57 and 9% are reached at the end of the discharge and irreversible variations of 19 and 1% are observed at the end of the charge for the standard electrode and matured electrode, respectively. This confirms the higher cohesion strength of the binding matrix of the matured electrode [18] , which limits variation in the electrode thickness associated with the volume expansion/contraction of the Si particles. These results obtained at the Si particle scale are in accordance with the volumetric (thickness) variations measured at the composite electrode scale (Figure 1c). By comparing Figure 1f and1c, one can however note some differences with lower and more reversible variations of the Si particle network compared to the electrode thickness for the matured electrode and more abrupt (less linear) variations at the Si particle network compared to the electrode thickness for the standard electrode. This can be explained by the fact that the relative distance variation between different expanding areas of the matured electrode are lowered compared to the standard electrode, due its more anisotropic expansion. This suggest that the matured electrode has a predisposition for maintaining the Si inter-particle distance while the entire volume of the electrode is expanding, meaning that the different parts of the electrode are expanding at the same rate.

Variation of the electrolyte volume

During the discharge, electrolyte will be drawn into the negative electrode due to its volume expansion and will starve the rest of the cell of electrolyte. Additionally, some electrolyte will be consumed through the SEI formation. An excess of electrolyte is thus required in the cell to accommodate these effects.

The relative variation of the electrolyte volume in the anode during the 1 st discharge is shown in Figure 7. It was established from the acquired XRCT images, in which the electrolyte phase can be segmented and quantified as detailed in Supporting Information. For the standard electrode (black curve), a sudden drop of ~30% of the electrolyte volume is observed during the initial stage of the discharge, which can be explained by the electrolyte consumption due to the SEI formation. From a DOD of 10%, the volume of electrolyte in the electrode is gradually increasing as the electrode volume expands to reach 20% at the end of the 1 st discharge. A similar behavior is observed for the matured electrode (blue curve) but the relative variation of the electrolyte volume is less marked, which is in accordance with the lower thickness expansion of the electrode observed in Figure 1c. Based on these data, one can estimate the excess of electrolyte required in a cell at 200 and 80 µL per cm 3 of anode (with an electrolyte-filled porosity of 36% at the pristine state) for the standard and matured electrode, respectively. These values should differ with calendered (less porous) electrodes.

Conclusion

This study has highlighted the ability of the X-ray computed tomography for imaging and quantifying morphological dynamics of Si-based anodes for Li-ion batteries. From these analyses, it has been demonstrated the great efficiency of a post-processing maturation treatment for (i) limiting the macroscopic deformation (expansion/contraction) of the electrode, (ii) decreasing the formation of macrocracks in the composite electrode; (iii) preventing its delamination from the current collector and (iv) constraining the displacement of the Si particles during the discharge and charge of the electrode. All these observations support the fact that the maturation process brings considerable improvement in the cohesion and adhesion strengths of the Si-based electrode, inducing major gain in their cyclability as demonstrated in our previous work. 18 Moreover, the difference in the cracking pattern analysed for both electrodes indicates that the maturation process strongly modifies the mechanical properties of the binder phase in the composite electrode, allowing thus its network to better accommodate the volume variations. In other words, for the standard electrode, the binder acts as a "cement", and as a "plastic glue" for the matured electrode.

Experimental section 4.1. Materials

The active material was Si powder (99.999%, 20 mesh, Materion) ball-milled under argon atmosphere for 20 h using a SPEX 8000 mixer with a ball-to-powder mass ratio of 5:1. The Si powder (4.5 g) was introduced along with 3 stainless-steel balls (one of 14.3 mm diam. and two of 11.1 mm diam., total weight of 22.3 g) into a stainless-steel vial (55 mL). The fraction of amorphous Si in the milled powder was estimated at 63% from Raman analysis. [30] The obtained Si powder consists of micrometric agglomerates with a median size ~6 m (determined by laser scattering method using a Malvern Mastersizer 2000) made of submicrometric particles more or less welded together. After the slurry mixing step (see below), the Si agglomerates are broken and the median size is reduced to 2.5 m (Figure S8, Supporting Information). Graphene nanoplatelets (GnP) (M grade from XGSciences, average diameter = 15 µm, average thickness = 6-8 nm, surface area = 120-150 m² g -1 according to the supplier's data) was used as conductive additive. Carboxymethyl cellulose (CMC) (DS = 0.7, Mw = 90 000 g mol -1 , Sigma-Aldrich) was used as binder. Citric acid (99.5+%, Alfa Aesar)

and KOH salts (85+%, Alfa Aesar) were used to prepare a pH3 buffer solution (0.17 M citric acid + 0.07 M KOH) as slurry medium.

Electrode preparation and cell assembly

A slurry was prepared by mixing 200 mg of powder (80 % wt Si, 8 % wt CMC and 12 % wt GnP) in 0.5 mL of pH 3 buffer solution. Mixing was performed using a magnetic stirrer at 500 rpm for 1 h in presence of 3 silicon nitride balls (9.5 mm diam.). Flat copper current collectors were prepared by surface polishing of copper discs (3 mm diam., 1 mm thick). This step is mandatory to have a perfectly planar surface of copper and to resolve more clearly the current collector/electrode interface imaged by XRCT. The copper discs were coated with the slurry in order to reach an active mass loading of ~5.5 mg Si cm -2 . The electrodes were not calendered. After the coating step, the electrodes were dried at room temperature in air for 12 h. The labelled "standard electrode" was then dried at 100°C under vacuum. Before this final drying step, the labelled "matured electrode" was stored 2 days at room temperature in an environmental chamber at ~80% relative humidity (RH). The maturation conditions have been previously optimized as described elsewhere. [18] Note that the maturation accentuates the surface oxidation of the Si powder but this one has no significant impact on the electrode performance. [18] The electrodes were mounted in a two-electrode Swagelok ® cell compatible to the XRCT measurements. A schematic view of the electrochemical cell is presented in Figure S1a (Supporting Information). The cell housing was made of perfluoroalkoxy alkane (PFA) polymer with a wall thickness reduced to 2.5 mm near the electrode to ensure a lower X-ray attenuation and better image quality. The Si-based electrode was placed towards a lithium metal electrode (1 mm thick) deposited on a copper current collector, acting as a counter and reference electrode. The electrodes were separated with a borosilicate glass-fiber (Whatman GF/D) membrane soaked with an electrolytic solution of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1) with 10 wt. % fluoroethylene carbonate (FEC).

The good contact between the different components of the cell was ensured by a spring placed on the counter electrode side, which was slightly compressing the cell (load ~ 6 N).

In situ synchrotron XRCT measurements

In situ synchrotron XRCT analyses were performed at the Soleil synchrotron (Gif-sur-Yvette, France) for the standard electrode and at the ESRF (European Synchrotron in Grenoble, France) for the matured electrode. The electrochemical cell was placed perpendicularly to the propagation path of the X-ray beam. The Si electrodes were cycled at room temperature in galvanostatic mode at full capacity between 1 V and 5 mV vs. Li/Li + at a current density of 180 mA g -1 of Si (C/20) for the standard electrode and 600 mA g -1 of Si (C/6) for the matured electrode both in discharge (lithiation) and charge (delithiation) using a OrigaFlex OGF500 potentiostat/galvanostat. A higher C rate was applied for the matured electrode due to a limited amount of beam time allocated for this experiment. The cell was left at the open circuit potential during the XRCT acquisition periods performed at regular intervals (typically, every hour).

For the XRCT experiments on the standard electrode performed at the Psiché beam-line of the Soleil facility, a beam monochromatized to 25±1 keV and a flux of 10 13 ph s -1 were used. 1500 2D projections were recorded along a 180° cell rotation with 45 ms of exposure per projection on a CCD camera detector for a total acquisition time of ~1 min per scan. 3D tomographic volumes of 13331333210 μm 3 with an isotropic voxel size of 650 nm were reconstructed in 32 bit floating-point representation from 2D projections using the PyHST2 software. [41] A volume of 190190210 μm 3 was extracted from the total acquired volume, in order to compare identical volumes for both standard and matured electrodes. The greyscale value in the final reconstructed volume represents the attenuation coefficient, so more attenuating (usually denser) regions appear brighter. The Fiji software was used for the image processing and analyses of the reconstructed volumes. [42] Image greyscale alignment was performed by taking the grey level of the separator as a reference.

XRCT acquisitions on the matured electrode were recorded at the nano-analysis endstation ID16B of the ESRF using holotomography. [43,44] Phase contrast imaging was acquired using a conic and monochromatic beam with an energy of 17.5 keV and a flux of 10 12 ph s -1 .

Four tomographies were acquired for four different distances between the detector and the sample along the beam propagation way. A set of 1499 projections were recorded on a PCO edge camera (2560×2160 pixels) along a 360° rotation with an exposure time of 150 ms per step. The pixels of the detector were binned (1280×1080 pixels) in order to have four times the flux per pixel and thus reducing the total acquisition time down to ~24 min per scan. A reconstructed volume of 256256210 μm 3 with an isotropic voxel size of 200 nm was obtained following a two-steps procedure: (i) phase retrieval calculation using an in-house developed octave script based on a Paganin-like approach using a delta/beta~25 and (ii) filtered back-projection reconstruction using PyHST2 software. [41] Final volumes of 190190210 μm 3 (after reforming) in a 32 bit floating point were obtained.

Details on the image segmentation procedure and on the quantitative analyse methods can be found in Supporting Information.

  Figure 3a-b shows 3D views of the standard and matured electrodes at different state-of-charges (SOC) where newly formed cracks at a given SOC are in red and those previously formed are in grey. For clarity considerations, lateral (x,y) images corresponding to the dashed blue plane in the 3D views are also displayed. The dynamics of the electrode macrocracking is also highlighted from the XRCT movies (see video 1 for the standard electrode and video 2 for the matured electrode, Supporting Information).

[37] 2 . 4 .

 24 Figure 5a-b presents (x,y) lateral images acquired at the electrode/current collector

Figure 1 .

 1 Figure 1. Evolution during the first cycle of the potential for the (a) standard and (b) matured electrode, (c) thickness change (the theoretical variation in thickness assuming no change in percent porosity is shown for comparison (dotted lines)), (d) crack volume fraction, (e) delaminated area and (f) Si interparticle distance variation for the standard (black curves) and matured (blue curves) electrodes. The analyzed volume is 190 × 190 × 210 µm 3 for both electrodes.

Figure 2 .

 2 Figure 2. XRCT slice images in the (a) transversal x-z plan and (b-d) lateral x-y plans at different z positions, namely (b) near the separator (z190 µm), (c) in the middle zone (z250

Figure 3 .

 3 Figure 3. 3D rendering of the crack formation with increasing state-of-charge (SOC) for the (a) standard and (b) matured electrodes. The crack formation is also shown in 2D lateral (x,y) views corresponding to the dashed blue area displayed in the 3D views. The cracks formed at each charge step are highlighted in red and those formed during the previous steps are in grey in 3D views or white in 2D views.

Figure 4 .

 4 Figure 4. 3D rendering of the cracks at the end of the 1 st cycle for the (a) standard and (b) matured electrodes. The green islet is the largest intra-connected volume of cracks and the blue is the second largest one; (c) distribution of the crack width and (d) histograms of proportion of cracks filled with gas or electrolyte for the standard and matured electrodes at the end of 1 st cycle.

Figure 5 .

 5 Figure 5. XRCT lateral images of the Cu/electrode interface showing the formation of delaminated zones (highlighted in red) during the 1 st discharge/charge for (a) the standard and (b) matured electrodes.

Figure 6 .

 6 Figure 6. Evolution during the 1 st cycle of XRCT lateral images focused on a large Si particle for the (c) standard and (d) matured electrodes.

Figure 7 .

 7 Figure 7. Relative variation of the electrolyte volume for the standard (black curve) and matured (blue curve) electrodes during the 1 st discharge. The analyzed volume is 190 × 190 × 210 µm 3 for both electrodes.

Figure S1 .

 S1 Figure S1. (a) Schematic representation of the electrochemical cell used for in-situ XRCT experiments. XRCT images in the lateral (x,y) plan of the (b) standard electrode and (c) matured electrode acquired at Soleil and ESRF synchrotron, respectively. The red circle is the indicative size of the ESRF imaged volume. The blue square is the indicative size of the compared volume (190 × 190 × 210 µm 3 ) for both electrodes.

Figure S2 .

 S2 Figure S2. Evolution of the mean volume fraction of solid phase in the standard electrode (a) at the pristine state and (b) after the 1 st cycle as a function of the analyzed domain size.

Figure S3 .

 S3 Figure S3. 3D views of the segmented solid phase (in red with large Si particles in yellow), electrolyte-filled pores (in cyan) and gas-filled pores (in grey/white) for the (a) standard and (b) matured electrodes at the pristine state.

Figure S4 .

 S4 Figure S4. Relative variation of the lateral (x, y) and transversal (z) dimensions of the (a) standard electrode and (b) matured electrode during the 1 st cycle.

Figure S5 .

 S5 Figure S5. XRCT transversal images and lateral images in the electrode/current interface zone acquired at different DOD and SOC levels for the (a) standard electrode and (b) the matured electrode.

Figure S6 .

 S6 Figure S6. Evolution during the 1 st cycle of XRCT lateral images focused on Si particles #2 and #3 for the (a) standard and (b) matured electrodes.

Figure S7 . 1 )

 S71 Figure S7. Evolution of the Si inter-particle distances d1-2, d1-3 and d2-3 (expressed in relative variation) during the 1 st cycle for the standard and matured electrodes

Table 1 .

 1 Main morphological characteristics (volume fraction v, intra-connectivity c, median diameter d50) of the different domains identified from XRCT analyses for the standard and matured electrodes at the pristine state, after the 1 st discharge and after the 1 st charge

				Pristine		1 st discharge		1 st charge
			v	c	d50	v	c	d50	v	c	d50
			(%)	(%)	(µm)	(%)	(%)	(µm)	(%)	(%)	(µm)
		Solid phase	60.7 99.9	-	77.5 99.9	-	65.5	98.2	-
	Standard electrode	Gas-filled pores Electrolyte-filled pores Total pores	2.1 37.2 98.1 20.4 39.3 95.4	6.6 5.2 5.4	2.7 19.8 93.1 11.6 22.5 93.3	9.5 2.6 7.7	10.1* 24.4* 34.5*	42.9* 11.2* 95.3* 2.8* 92.7* 10.3*
		Cracks	-	-	-	-	-	-	11.6	74.5	13.1
		Delaminated area**	0.1	-	-	17.4	-	-	12.5	-	-
		Solid phase	61.6 99.5	-	81.3 99.9	-	66.7	99.9	-
	Matured electrode	Gas-filled pores Electrolyte-filled pores Total pores Cracks	3.1 35.3 97.6 23.4 38.4 97.9 --	3.8 1.9 2.6 -	0.7 18.0 91.6 46.5 18.7 91.8 --	4.6 2.0 2.9 -	2.9* 30.4* 33.3* 3.5	12.7* 6.4* 98.1* 2.3* 98.4* 3.2* 31.8 4.8
		Delaminated area**	0.2	-	-	0.2	-	-	1.6	-	-

* including cracks ** expressed in area fraction s (%)

Acknowledgements

The authors thank the Natural Sciences and Engineering Research Council (NSERC) of Canada (Grant No. RGPIN-2016-04524) for supporting this work. We thank also the Soleil synchrotron's reviewing committee for the shifts allocation. A particular thank to J. Villanova and J.P. Suuronen also for the time generously allocated for the measurements on the ID16B end-station at the ESRF. Finally a special thought for the precious help of Q. Lemarié, G. Meyruey, J. Adrien during the in-situ XRCT measurements.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

Dynamics of the morphological degradation of Si-based anodes for Li-ion batteries characterized by in-situ synchrotron X-ray tomography Victor Table S1. Summary of the literature data on XRCT studies of Si-based anodes. They are classified along their spatial resolution. The reference numbers correspond to those labelled in the full paper.

Details on the quantitative analyses of the XRCT images

Dimensional change. The variation of the electrode thickness was determined from the displacement of the interface between the electrode and the separator. This interface was determined from lateral images, where the separator composed of interconnected borosilicate fibers appears clearly in bright on the greyscale images. As the electrode surface is not perfectly flat, the separator/electrode interface is set by considering the mean z position where the electrode is not predominantly present. The lateral dimensional changes were estimated by evaluating the variation of geometrical distances between specific features of the electrodes, Intra-connectivity. The intra-connectivity of an identified phase was estimated by using a "labelling" method allowing identifying the independent 3D objects of a volume. Each object, defined as a cluster of connected voxels, is labelled with a different grey value. The intra-connectivity of a phase is then calculated as the ratio of the volume of the largest cluster of the phase over the entire phase volume.

Size distribution. The size (diameter) of the pores and cracks was estimated thanks to a dedicated mathematical morphology operation. This process involves successive erosion and dilation steps of the image for structural elements of size n. The difference between each dilated and eroded image allows retrieving the amount of elements of size 2n in the image.

The structural element chosen here is a sphere.

Crack growth. The cracks formed in the electrode volume are newly formed elements with the greyscale value of the electrolyte phase or gas phase. The fraction of cracks formed at each acquisition step was estimated by subtracting the binary images of the porosity at the current step from that of the previous one. The operation is repeated for all acquisition steps by considering separately the electrolyte and gas filled porosities. Noise reduction by image erosion and manual corrections were added in order to avoid parasitic effects of the volume contraction. These corrections are crucial for the electrolyte-filled porosity, which is noisier than the gas-filled porosity. Then both crack images are summed. 3D rendering were obtained from an in-house developed plug-in implemented in the Fiji software [2].

Delaminated surface area. The delaminated surface area was estimated from the porosity at the electrode/current collector interface. Since this porosity is less attenuating the beam, it appears darker and thus can be isolated by thresholding on the low greyscale value of the histogram at each acquisition step. Manual corrections were mandatory in order to avoid the parasitic effect of the star-burst artefacts present at the electrode/Cu interface.

Details on the calculation of the theoretical variation in thickness presented in Fig. 1c.

Assuming no change in the percent porosity (i.e. the pores expands by the same amount that the Si particles) and that the variation of the electrode thickness is equivalent to its volume variation (i.e. the electrode expands only in the out-of-plane direction) and that all the discharge capacity is associated with the formation of SiLix (i.e. SEI formation is not considered), the theoretical variation in electrode thickness can calculated according to the following equation:

where m,Si is the Si mass fraction in the pristine electrode (0.732), ρtheo is the true density of the pristine electrode (2.20 g cm -3 ) [3], ρSi is the true density of Si (2.33 g cm -3 ), VSi is the intrinsic volume expansion of SiLix (0.0782% per mAh/g Si since its expansion is 280% for x