Electrochemical Anode Behavior of α-FeSi₂ co-Sintered with Solid Electrolyte (Supporting Information)

It is essential in the production of the next generation of rechargeable batteries to elucidate the reaction phases formed after co-sintering in all-solid-state batteries with oxide solid electrolytes and silicon materials and their electrochemical behavior. Herein, we explore the presence or absence of reaction phases and the charge/discharge behavior after co-sintering with the solid electrolyte Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) and various silicon active anode materials. In the co-sintering investigation of LAGP and silicon active anode materials, α-FeSi₂ do not react with LAGP and the electronic conductivity of α-FeSi₂ is maintained even after sintering. In addition, using a mixed sintered sheet consisting of α-FeSi₂, solid electrolyte LAGP, and carbon additive vapor-grown carbon fiber, a charge/discharge test is performed at 105 °C for the cell with a Li metal counter electrode and polymer electrolyte. The results of X-ray diffraction measurements of the disassembled test cell after charging/discharging confirm that the reduction decomposition reaction proceeding and disappearing of LAGP phase completely when Li is inserted into the active anode material for the first time. However, when Li is further inserted, Li alloy phases, Li₂₂Si₅, and Li₂₂Ge₅ appear, and they show reversible charge/discharge behavior of 400 mA h g⁻¹. This is presumed to be owing to the partial reduction of α-FeSi₂ to Si and the reduction decomposition of LAGP to produce Ge during the Li insertion process, followed by the formation of Li alloy phases with Li and Si or Ge. Furthermore, while the solid electrolyte LAGP disappear during the first Li insertion, the formation of a Li₄SiO₄ phase with a broad peak is observed, suggesting that the Li₄SiO₄ phase might function as a Li conductor.