Decoupling the Effects of Interface Chemical Degradation and Mechanical Cracking in Solid‐State Batteries with Silicon Electrode

Abstract Silicon is a promising negative electrode material for solid‐state batteries (SSBs) due to its high specific capacity and ability to prevent lithium dendrite formation. However, SSBs with silicon electrodes currently suffer from poor cycling stability, despite chemical engineering efforts. This study investigates the cycling failure mechanism of composite Si/Li₆PS₅Cl electrodes by decoupling the effects of interface chemical degradation and mechanical cracking. Chlorine‐rich Li_5.5PS_4.5Cl_1.5 suppresses interface chemical degradation when paired with silicon, while small‐grained Li₆PS₅Cl shows 4.3‐fold increase of interface resistance due to large Si/Li₆PS₅Cl contact area for interface degradation. Despite this, small‐grained Li₆PS₅Cl improves the microstructure homogeneity of the electrode composites, effectively alleviating the stress accumulation caused by the expansion/shrinkage of silicon particles. This minimizes bulk cracks in Li₆PS₅Cl during the lithiation processes and interface delamination during the delithiation processes. Mechanical cracking shows a dominant role in increasing interface resistance than interface chemical degradation. Therefore, electrodes with small‐grained Li₆PS₅Cl show better cycling stability than those with Li_5.5PS_4.5Cl_1.5. This work not only provides an approach to decouple the complex effects for cycling failure analysis but also provides a guideline for better use of silicon in negative electrodes of SSBs.