Cycling Stability Based on the Morphological Characteristics of Si-Based Anode Materials for Sulfide-Based All Solid-State Batteries

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Sulfide-based all-solid-state batteries (ASSBs) have emerged as promising next-generation energy storage devices due to their high safety and energy density. Si, with its high theoretical capacity (3579 mAh g-1 for Li3.75Si), is a promising anode material for ASSBs. However, its large volume change (~300%) during cycling remains a significant challenge. Recent studies have shown excellent performance of micron-sized pure Si in sulfide-based ASSBs, but inconsistent cycle life characteristics complicate understanding of Si anode degradation mechanisms. To address this, nanolayer Si-coated graphite (Si/Gr) composites have been proposed, but their performance and degradation mechanisms in sulfide-based ASSBs remain unclear.This study investigates the impact of Si morphological characteristics on the cycle stability of sulfide-based ASSBs and compares the degradation mechanisms of Si/Gr and pure Si composite anodes. Half-cells were fabricated using Si/Gr composites (~10 μm spherical particles, ~50 nm Si coating) and pure Si (1-5 μm). Cycle performance was evaluated for 50 cycles. Microstructural and chemical changes were analyzed using m-XRD, SEM/EDS, XPS, and TEM/EDS/EELS.The initial capacity of Si/Gr was about 820 mAh g-1, which was higher than that of pure Gr(340 mAh g-1), but after 50 cycles, the cycle retention was 82%, which was faster than that of pure Gr(97%). Pure Si, on the other hand, showed a sharp decrease in capacity to nearly 0% in 10 cycles. The faster degradation of Si/Gr compared to pure Gr was attributed to interactions between Si and Gr within the composite, which had a greater impact than interfacial reactions with the solid electrolyte. Specifically, the Si coating gradually reduced Gr crystallinity during cycling, diminishing lithium storage capacity. For pure Si, crack formation at the Si/SE interface and excessive SEI formation during delithiation led to rapid capacity loss. In conclusion, Si/Gr offered higher capacity than Gr but degraded faster, while the thin Si coating had local volume changes better than micron-sized Si, resulting in higher capacity and improved cycle life.This research elucidates the degradation mechanisms of Si-C composites in sulfide-based ASSBs and provides insights for optimizing Si-based anodes. These findings offer valuable guidance for developing high-performance sulfide-based ASSBs with enhanced long-term reliability.