significantly overtake graphite, thanks to its higher specific capacity (3579 mAh g −1 vs 372 mAh g −1 for graphite). Other assets of this material are its low delithiation potential (0.4 V vs Li + /Li), abundance and low cost. However, its use in commercial batteries still remains in practice very limited due to its strong cyclical variation in volume, which can reach 280% in a completely lithiated state (Li 3.75 Si). This results in an irreversible change in the morphology of the Si particles (fragmentation, nanoporosification) as well as severe mechanical damage to the electrode film (cracking, delamination of the current collector). In addition, the expansion/contraction cycles of silicon lead to a degradation of the solid electrolyte interphase (SEI), which must reform with each cycle. [2] Many partial solutions have been proposed to increase the cyclability of negative silicon-based electrodes, such as the nanostructuring of silicon, the combination with carbon or other elements, in the form of composite or alloy, the mixture with graphite, designing the electrodes architecture, the use of electrolytic additives. [3][4][5][6][7][8][9][10][11][12][13] Numerous studies have underlined the key role of the binder on the cyclability of silicon-based electrodes. This one is essential for maintaining the cohesion of the electrode film and its adhesion to the current collector during cycling. The binder also acts more or less effectively as an artificial passivation layer. [14][15][16] The literature is abundant on this subject but it remains difficult to take advantage of it because few studies have been devoted to electrodes of loading and/or density high enough to be rationalizable for the application. A strong limitation in all studies relating to the binder is the difficulty of visualizing it. This one is in the form of nanometric deposits, on the surface or at the point of contact between the particles, which requires very high resolution techniques and/or sensitive to the presence of light elements. [17][18][19][20] Finally, the precipitation of the degradation products of the electrolyte masks the binder in all post-mortem studies. Although it is a key constituent of the formulation of silicon-based electrodes, the binder remains in most cases invisible to observation and its effects must be deduced from the more global measurements made on the electrode.
In the present work, an alternative to the standard ex-situ and destructive focused ion beam scanning electron microscopy (FIB/SEM) analysis procedure is demonstrated for monitoring the morphological degradation of a single Si/graphite (1/1 mass ratio) blended electrode for Li-ion batteries. For this purpose, a FIB milled microcavity is created in the pristine electrode, which is observed in FIB-polished cross section by SEM at different cycling periods (pristine, 1 st , 9 th and 50 th cycles). This allows studying the same cycled electrode as for an in-situ method. Its cyclinginduced morphological change is characterized at the electrode and particle scales by monitoring the evolution of the electrode thickness, mass and porosity, the Si particle morphology, Si interparticle distance, surface fraction and twisting of the graphite flakes. This is correlated to the evolution of the electrode discharge capacity and impedance. As a result, a more comprehensive view of the degradation phenomena of the Si/graphite blended electrode is established.
The role of the physico-chemical properties of the water soluble PAA binder on the lithium electrochemical performance of highly loaded silicon/graphite 50/50 wt% negative electrodes has been examined as a function of the neutralization degree x in PAAH1-xLix at initial cycle in an electrolyte not-containing ethylene carbonate. Electrode processing in acidic PAAH binder at pH 2.5 leads to a deep copper corrosion resulting in a significant electrode cohesion and adhesion to the current collector surface, but the strong binder rigidity may explain the big cracks occurring at the electrode surface at first cycle. The non-uniform binder coating on the materials surface leads to an important degradation of the electrolyte explaining the lowest initial coulombic efficiency and the lowest reversible capacity among the studied electrodes. When processed in neutral pH, the PAAH0.22Li0.78 binder forms a conformal artificial SEI layer on the materials surface, which minimizes the electrolyte reduction at first cycle and then maximizes the initial coulombic efficiency. However, the low mechanical resistance of the electrode and its strong cracking explain its low reversible capacity. Electrodes prepared at intermediate pH 4 combine the positive assets of electrodes prepared at acidic and neutral pH. They lead to the best initial performance with a notable areal capacity of 7.2 mAh cm -2 and the highest initial coulombic efficiency at around 90%, a value much larger than the usual range reported for silicon/graphite anodes. All data obtained with complementary characterization techniques were discussed as a function of the PAA polymeric chain molecular conformation, microstructure, and surface 3 adsorption or grafting, emphasizing the tremendous role of the binder on the electrode initial performance.
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