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

Abstract

amount of lithium alloyed with silicon (up to Li 3.75 Si) compared to lithium intercalated in graphite (up to LiC 6 ). However, the silicon volume expansion induced by its lithiation is much higher, up to 280% versus 10% for graphite. [1] This translates to electrode morphology instability during cycling at different scales: i) at the micrometer scale, upon binder bridge breakage, either between particles or between particles and the current collector, which lead to the appearance of cracks and delamination, respectively and ii) at the nanometric scale with the destabilization of the solid electrolyte interphase (SEI), which must reform, causing excess accumulation. [2] Together with the intrinsic silicon material limitations (particle fracture and nanoporosification), [3][4][5] these constitute the main electrode failure mechanisms. To increase the SEI stability, improvements have been made on the electrolyte formulation. 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. [9] Specifically, coordination bonds offer significant strength, flexibility, and reversibility. They may also have self-healing properties, providing a stronger electrode resiliency to silicon volume variation. [10] Coordination binders are also advantageous in other ways. They are easy to process with simple addition of metallic salts to conventional formulation (no advanced polymer chemistry required). Moreover, they are easy to probe at the bulk scale with standard spectroscopic techniques (e.g., infrared spectroscopy). Such a strategy has been applied mainly to crosslinked alginate binders, [11][12][13][14][15][16] though also to some other binder systems. [17][18][19][20][21] The reticulation by coordination leads to clear improvement of the electrochemical performances, particularly of the capacity retention. This is attributed to better adhesion and cohesion of the electrodes upon cycling asThe physical crosslinking of polymeric binders through coordination chemistry significantly improves the electrochemical performance of silicon-based negative electrodes. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy is used to probe the nanoscale morphology of such electrodes. This technique reveals the homogeneous coordination of carboxylated binder with Zn cations and its layering on the silicon surface. The solid electrolyte interphase (SEI) formed after the first cycle is denser with Zn-coordinated binder an...

Coordinatively Cross-Linked Binders for Silicon-Based Electrodes for Li-Ion Batteries: Beneficial Impact on Mechanical Properties and Electrochemical Performance

“…In fact, the main reduction peak(s) on the d Q /d V curves at 0.9–1.1 V (vs Li + /Li 0 ) seen for both formulations is associated with the reduction of FEC. ,, Interestingly, this phenomenon is more intense for formulations without Zn, which tends to confirm that the coordination with Zn increases the ability of the binder to act as an artificial passivation (SEI) layer. This result is in complete agreement with our previous STEM-EDX study on ZnSO 4 -containing electrodes after one cycle, which evidenced that the surface of the silicon particles covered by the binder (including Zn) are exempt of standard inorganic SEI components (containing F and P) …”

“…We recently demonstrated that such a coordinated CMC/CA/Zn(II) binder acts, at least partially, as an artificial SEI. 19 We here show that such bonds have also a beneficial impact on the electrode's mechanical properties as evidenced by scratch tests and nanoindentation experiments. The invariability of the mechanical properties (hardness and elastic modulus) of the cross-linked binder in the presence of the electrolyte, unlike the uncross-linked binder, is also demonstrated.…”

“…This result is in complete agreement with our previous STEM-EDX study on ZnSO 4 -containing electrodes after one cycle, which evidenced that the surface of the silicon particles covered by the binder (including Zn) are exempt of standard inorganic SEI components (containing F and P). 19 Like with ZnSO 4 , the ZnO formulation capacity retention is higher than its reference formulation with a discharge capacity of about 2530 vs 1930 mAh•g Si −1 at the 60th cycle. The capacity loss by cycle is also higher for the latter, which reaches approximately 8 vs 5.6 mAh•g Si −1 for ZnO-containing electrodes.…”

“…). On the other hand, from our recent 7 Li NMR quantifications, 19 the amount of Li in the SEI (in the forms of LiF, Li x PO y F z , and other species) corresponds to an extra 345 and 373 mAh•g Si −1…”

“…, for the ZnSO 4 (0.22) and its reference formulation, respectively (see eq S2 in the SI). 19 Therefore, both in the presence and absence of Zn(II), the electrolyte degradation is the major source of extra capacity. Interestingly, the latter is decreased for Zn-containing electrodes, in agreement with previous work which indicates that coordinated binder acts, at least partially, as an artificial SEI.…”