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Synchrotron X-ray total scattering and pair distribution function analysis are used to investigate the structural changes during solution synthesis of the Li₇P₃S₁₁solid electrolyte. 

Silicon is considered an important anode material for solid-state batteries (SSBs) because of its unique properties in addressing key challenges associated with Li metal anodes such as dendrite formation and morphological instability. Despite many exciting results from previous reports on solid-state Si anodes, the initial Coulombic efficiency (ICE), a critical parameter that characterizes the electrochemical reversibility for the first cycle and directly influences the energy density of the battery, has not been well considered. Here we study the electrochemical stability between Si and three representative solid electrolytes (SEs), including a typical sulfide (75Li 2 S-25P 2 S 5 , LPS), an iodide-substituted sulfide (70(0.75Li 2 S-0.25P 2 S 5 )-60LiI, LPSI), and a hydride-based SE (3LiBH 4 -LiI, LBHI), to improve the ICE of solid-state Si anodes. Combining first-principles computations, electrochemical measurements, ex situ XPS characterizations, and mechanical measurements, we report that LBHI demonstrates superior electrochemical and chemical stability with Si anodes compared with sulfide-based SEs, enabling a high-performance solid-state Si anode with a record high ICE of 96.2% among all Si anodes reported to date. The excellent stability of LBHI with Si anode was also demonstrated in solid-state full cells with nickel-rich layered oxide cathodes. The research provides novel insights into developing high-performance Si anodes for practical applications.