Potentials of thin film silicon anodes in soft solid-state batteries

Silicon is a promising anode material for solid-state batteries due to its high theoretical capacity, yet severe lithiation-induced volume changes lead to pronounced chemo-mechanical degradation at solid–solid interfaces. This work aims to identify a critical silicon thickness window that enables high reversible capacity while preserving mechanical integrity and interfacial stability in soft solid-state battery systems. By systematically varying the thickness of amorphous silicon thin-film anodes on copper current collectors, distinct thickness-dependent structure–property–performance relationships are revealed. Thin silicon films exhibit excellent cycling stability but limited capacity, whereas thicker films deliver high initial capacities followed by rapid degradation. An intermediate silicon thickness of approximately 500 nm provides the most favourable balance between reversible capacity and cycling stability. Post-cycling analyses link capacity decay at larger thicknesses to stress-driven cracking, delamination and electrical isolation of the silicon layer, consistent with mechanically induced failure during deep lithiation. These results demonstrate that the performance of silicon anodes in polymer-based solid-state batteries is governed by chemo-mechanical coupling and interfacial robustness rather than by active material loading alone. The findings provide practical design guidelines for thin-film silicon anodes in soft solid-state battery architectures.