From the Direct Observation of a PAA‐Based Binder Using STEM‐VEELS to the Ageing Mechanism of Silicon/Graphite Anode with High Areal Capacity Cycled in an FEC‐Rich and EC‐Free Electrolyte

Abstract

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.

Nanoscale Morphological Characterization of Coordinated Binder and Solid Electrolyte Interphase in Silicon‐Based Electrodes for Li‐Ion Batteries

“…Although some SEI could decrease the likelihood of identifying the binder in between particles, it is also possible that this evolution is a consequence of the binder deformation and/or partial rupture under the stress arising with silicon particle expansion and contraction. [7] Phosphorus and fluorine (arising from LiF, Li x PO y F z , etc.) are used as inorganic SEI markers and their respective positions are identified as highly correlated in both formulations.…”

“…Moreover, the visualization of the binder conformation on the active material particles has rarely been achieved. [7,23] In this context, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDX) can be invaluable in the characterization of pristine and cycled electrode samples. This allows for direct imaging of the binder and SEI distribution on the nanometric scale.…”

“…This allows for direct imaging of the binder and SEI distribution on the nanometric scale. A STEM-valence electron-energy-loss spectroscopy analysis could potentially be applied to retrieve those phases as it was done in the study of Xiong et al [7] However, the EDX allows the discretization of their constitutive elements along with their relative quantification. This specificity is necessary to analyze the coordination through particular cations.…”

“…This has led to enhanced silicon electrode cycle life, also recently in full-cell configuration. [6,7] In parallel, it is relevant to push the research on binders, especially to improve their mechanical properties as it appears to be a straightforward way to tackle the electrode cracking and delamination issues.Studies have shown that binder reticulation through covalent or coordination bonds increases silicon electrode performance by helping to mitigate stress during cycling as long as the silicon/binder interface is not the breaking point. [8] However, these types of bonds are not of the same value because of differences in their strength and reversibility.…”

“…This has led to enhanced silicon electrode cycle life, also recently in full-cell configuration. [6,7] In parallel, it is relevant to push the research on binders, especially to improve their mechanical properties as it appears to be a straightforward way to tackle the electrode cracking and delamination issues.…”