Engineering a Fast Li+-Diffusion Secondary Phase in Sn Anodes for Low-Pressure All-Solid-State Batteries

Alloys are promising anode candidates for all-solid-state batteries (ASSBs) due to their high specific capacities and regulated interphase with the nonflowing solid-state electrolytes (SSEs). However, selection of alloys is constrained by low initial Coulombic efficiency (ICE), which often excludes candidates with high specific capacity and mechanical deformability─parameters essential for high energy density and low-stack pressure operation. In this work, we reassess and unlock the potential of tin (Sn) anodes in ASSBs. By incorporating bismuth (Bi) as a secondary element, we developed low-cost, diffusion-engineered Sn-Bi binary elemental anodes achieving a high ICE (∼90%) and specific capacity (∼700 mAh g-1). Electrochemical characterizations, first-principles calculations, and cross-sectional morphologies reveal that Li3Bi exhibits a significantly higher Li diffusion coefficient (3.5 × 10-6 cm2 s-1) compared to LiSn and Li13Sn5 (1.5 × 10-14 and 1.3 × 10-11 cm2 s-1, respectively), remains stable in a larger voltage window, and facilitates diffusion throughout the electrode for reversible Sn (de)lithiation. Sn-Bi full cells coupled with a LiNi0.82Co0.11Mn0.07O2 (NCM82) cathode demonstrate high rate capability (1 C) and stable cycling with areal capacities up to 10 mAh cm-2 at 23 °C. Multidimensional engineering strategies, from grain to matrix and interface, are further investigated to improve low-stack pressure (5 MPa) performance. This work advances the potential of alloy anodes for practical application of ASSBs.