The study investigates the chemo-mechanical failure mechanisms of composite Si/Li₃PS₄Cl and solid-electrolyte-free silicon anodes in solid-state batteries (SSBs). The research combines structural and chemical characterizations with theoretical simulations to understand the interplay between lithium transport, microstructure evolution, and mechanical stress at interfaces. Key findings include: 1. **SEI Growth and Composition**: The SEI growth rate is significantly higher in composite Si/Li₃PS₄Cl anodes compared to solid-electrolyte-free Si anodes. The SEI components include Li₃PS₄Cl decomposition products (Li₂P, Li₂S, and LiCl) and SiO₂-derived phases (SiO₂, Li₂SiO₃, and Li₂O). 2. **Lithiation/Delithiation Kinetics**: Solid-electrolyte-free Si anodes exhibit high ionic and electronic conductivities, enabling fast lithium transport. The average lithium chemical diffusion coefficient is 1.0 × 10⁻⁸ cm²/s during lithiation, and DFT simulations predict ionic and electronic conductivities of 1.5 × 10⁻⁵ cm⁻¹ and 4.4 × 10⁻⁵ cm⁻¹, respectively. 3. **Chemo-Mechanics of Si Anodes**: The 2D Si/Li₃PS₄Cl interface of solid-electrolyte-free Si anodes shows poor cycling stability due to void formation and increased mechanical stress. Chemo-mechanically coupled phase-field fracture modeling reveals a large stress (0.3 GPa) at the interface during lithiation, leading to 10% plastic strain and void formation. 4. **Full Cell Performance**: Full SSB cells with Si/Li₃PS₄Cl composite and solid-electrolyte-free Si anodes show different cycling performances. The Si/Li₃PS₄Cl composite anode has lower capacity retention (21.9%) after 100 cycles due to continuous SEI growth, while the solid-electrolyte-free Si anode retains 29.3% capacity. Adding a PPC layer to the Si sheet anode improves contact and reduces interface stress, enhancing cycling stability. The study concludes that Si anodes offer a promising alternative to lithium metal anodes in SSBs, with projected specific energy and energy density of 300 Wh kg⁻¹ and 800 Wh L⁻¹, respectively. Further research should focus on improving cycling stability and reducing stack pressure to commercialize Si-based SSBs.The study investigates the chemo-mechanical failure mechanisms of composite Si/Li₃PS₄Cl and solid-electrolyte-free silicon anodes in solid-state batteries (SSBs). The research combines structural and chemical characterizations with theoretical simulations to understand the interplay between lithium transport, microstructure evolution, and mechanical stress at interfaces. Key findings include: 1. **SEI Growth and Composition**: The SEI growth rate is significantly higher in composite Si/Li₃PS₄Cl anodes compared to solid-electrolyte-free Si anodes. The SEI components include Li₃PS₄Cl decomposition products (Li₂P, Li₂S, and LiCl) and SiO₂-derived phases (SiO₂, Li₂SiO₃, and Li₂O). 2. **Lithiation/Delithiation Kinetics**: Solid-electrolyte-free Si anodes exhibit high ionic and electronic conductivities, enabling fast lithium transport. The average lithium chemical diffusion coefficient is 1.0 × 10⁻⁸ cm²/s during lithiation, and DFT simulations predict ionic and electronic conductivities of 1.5 × 10⁻⁵ cm⁻¹ and 4.4 × 10⁻⁵ cm⁻¹, respectively. 3. **Chemo-Mechanics of Si Anodes**: The 2D Si/Li₃PS₄Cl interface of solid-electrolyte-free Si anodes shows poor cycling stability due to void formation and increased mechanical stress. Chemo-mechanically coupled phase-field fracture modeling reveals a large stress (0.3 GPa) at the interface during lithiation, leading to 10% plastic strain and void formation. 4. **Full Cell Performance**: Full SSB cells with Si/Li₃PS₄Cl composite and solid-electrolyte-free Si anodes show different cycling performances. The Si/Li₃PS₄Cl composite anode has lower capacity retention (21.9%) after 100 cycles due to continuous SEI growth, while the solid-electrolyte-free Si anode retains 29.3% capacity. Adding a PPC layer to the Si sheet anode improves contact and reduces interface stress, enhancing cycling stability. The study concludes that Si anodes offer a promising alternative to lithium metal anodes in SSBs, with projected specific energy and energy density of 300 Wh kg⁻¹ and 800 Wh L⁻¹, respectively. Further research should focus on improving cycling stability and reducing stack pressure to commercialize Si-based SSBs.