This article investigates the chemo-mechanical failure mechanisms of silicon (Si) anodes in solid-state batteries (SSBs), focusing on both composite Si/Li6PS5Cl (LPSCl) anodes and solid-electrolyte-free (SE-free) Si anodes. The study combines structural and chemical characterizations with theoretical simulations to reveal the underlying mechanisms that lead to performance degradation in these anodes. The growth of the solid electrolyte interphase (SEI) at the Si/LPSCl interface causes significant resistance increase in composite anodes, leading to fast capacity decay. In contrast, SE-free Si anodes exhibit sufficient ionic and electronic conductivities, enabling high specific capacity. However, microscale void formation during delithiation causes larger mechanical stress at the two-dimensional (2D) interfaces of these anodes compared to composite anodes. Understanding these chemo-mechanical failure mechanisms is crucial for designing improved electrode materials. Solid-state batteries offer high energy density and improved safety compared to conventional batteries with liquid electrolytes. However, chemo-mechanical interactions play a more prominent role in SSBs due to rigid solid/solid contacts and different mechanical properties of the cell components. The SEI formation and particle pulverization in SSBs are influenced by the mechanical rigidity of inorganic SEs and external stack pressure, providing opportunities for better cycling stability. Silicon is a promising anode material due to its high theoretical specific capacity, low lithiation potential, and low lithium dendrite risk. However, the electrochemical performance of Si anodes in SSBs is still poor, with issues such as low actual specific capacity and fast capacity decay. The alloying process at a potential of E=0.3 V (versus Li+/Li) avoids lithium metal nucleation and dendrite growth, achieving higher energy density compared with other alloy anodes. The low cost and good stability of Si in air qualify it for large-scale manufacturing. The large volume changes in Si during lithiation/delithiation pose a challenge from the mechanics perspective. The study identifies three chemo-mechanical issues affecting Si anodes in SSBs: (1) SEI formation at the Si/SE interface due to the instability of Si with sulfide SEs at low lithiation potential; (2) the use of a compact SE-free Si anode leads to a planar Si/SE interface, which causes less SEI degradation per mass of Si and reduces irreversible lithium loss; and (3) contact loss at Si/SE interfaces is less probable during lithiation processes due to the volume expansion of Si, but whether the interfaces remain stable during delithiation processes is an open question. The study reveals that the SEI growth rate at the Si/LPSCl interface is much higher for composite anodes compared to SE-free Si anodes. The SEI components include LPSCl decomposition products and SiOxThis article investigates the chemo-mechanical failure mechanisms of silicon (Si) anodes in solid-state batteries (SSBs), focusing on both composite Si/Li6PS5Cl (LPSCl) anodes and solid-electrolyte-free (SE-free) Si anodes. The study combines structural and chemical characterizations with theoretical simulations to reveal the underlying mechanisms that lead to performance degradation in these anodes. The growth of the solid electrolyte interphase (SEI) at the Si/LPSCl interface causes significant resistance increase in composite anodes, leading to fast capacity decay. In contrast, SE-free Si anodes exhibit sufficient ionic and electronic conductivities, enabling high specific capacity. However, microscale void formation during delithiation causes larger mechanical stress at the two-dimensional (2D) interfaces of these anodes compared to composite anodes. Understanding these chemo-mechanical failure mechanisms is crucial for designing improved electrode materials. Solid-state batteries offer high energy density and improved safety compared to conventional batteries with liquid electrolytes. However, chemo-mechanical interactions play a more prominent role in SSBs due to rigid solid/solid contacts and different mechanical properties of the cell components. The SEI formation and particle pulverization in SSBs are influenced by the mechanical rigidity of inorganic SEs and external stack pressure, providing opportunities for better cycling stability. Silicon is a promising anode material due to its high theoretical specific capacity, low lithiation potential, and low lithium dendrite risk. However, the electrochemical performance of Si anodes in SSBs is still poor, with issues such as low actual specific capacity and fast capacity decay. The alloying process at a potential of E=0.3 V (versus Li+/Li) avoids lithium metal nucleation and dendrite growth, achieving higher energy density compared with other alloy anodes. The low cost and good stability of Si in air qualify it for large-scale manufacturing. The large volume changes in Si during lithiation/delithiation pose a challenge from the mechanics perspective. The study identifies three chemo-mechanical issues affecting Si anodes in SSBs: (1) SEI formation at the Si/SE interface due to the instability of Si with sulfide SEs at low lithiation potential; (2) the use of a compact SE-free Si anode leads to a planar Si/SE interface, which causes less SEI degradation per mass of Si and reduces irreversible lithium loss; and (3) contact loss at Si/SE interfaces is less probable during lithiation processes due to the volume expansion of Si, but whether the interfaces remain stable during delithiation processes is an open question. The study reveals that the SEI growth rate at the Si/LPSCl interface is much higher for composite anodes compared to SE-free Si anodes. The SEI components include LPSCl decomposition products and SiOx