Lithium-Compatible Halide Superionic Conductors

Lithium-metal-halide (Li-M-X) compounds, featuring high Li-ion conductivity and excellent cathode compatibility, are among the most promising solid-state electrolyte (SSE) candidates for all-solid-state Li batteries (ASSLBs), which are widely regarded as next-generation energy storage systems. However, their susceptibility to reduction reactions severely limits the use of Li metal anodes in halide-based ASSLBs, constraining the achievable energy density. In this work, we smash through this limit by designing a new class of Li-M-X superionic conductors through first-principles studies based on electronic structure modification of the M-site element. Utilizing Yb²⁺ instead of traditional M^3+/4+/5+, we identify the Li₄YbCl₆ phase exhibiting outstanding stability against Li anode and high voltage cathodes (e.g., LiCoO₂, LiMn₂O₄, LiFePO₄, LiNiPO₄). Li₄YbCl₆ possesses an ultrawide band gap exceeding 7 eV and a broad electrochemical stability window of ∼4.25 V. The Li/Li₄YbCl₆ interface is kinetically stabilized, with limited charge transfer and strong Yb-Cl covalency underpinning its superior stability. Li₄YbCl₆ also exhibits a high Li-ionic conductivity of 0.15 mS/cm at 300 K with an activation energy of 0.35 eV, which is further enhanced to 1 mS/cm through anion substitution (Li₄YbCl₃Br₃). This breakthrough in designing Yb²⁺-based halide SSEs provides critical insights for the development of lithium-stable solid electrolytes for high-energy-density ASSLBs.