Interfacial Tuning of Sulfohalide Electrolytes by LiBF₄ for Stable Lithium Metal Batteries

Lithium metal batteries (LMBs) incorporating solid-state electrolytes (SSEs) promise high energy density and safety, yet their practical deployment is hindered by poor interfacial stability between SSEs and lithium metal anodes. Here we show that a simple incorporation of LiBF₄ into the sulfohalide (Li₃SCl) framework forms a mixture Li₃SCl@LiBF₄ (LSC@BF) SSE via a two-step solid-state synthesis, preserving a high room-temperature ionic conductivity of 4.32 × 10^-4 S cm^-1 with a low activation energy of 0.22 eV while fundamentally altering the interface. X-ray photoelectron spectroscopy and electron microscopy reveal that LiBF₄ promotes the in situ formation of a mechanically robust, LiF-rich solid-electrolyte interphase at the SSE|Li interface. This LiF-rich layer effectively suppresses lithium dendrite growth and stabilizes the interface, enabling symmetric Li|LSC@BF|Li cells to achieve stable lithium plating/stripping for over 800 h at 0.2 mA cm^-2. Cross-sectional post-mortem imaging confirms a dense, void-free interface without dendrite penetration. Our work demonstrates that LiBF₄ incorporation offers a simple, scalable strategy to simultaneously maintain high ionic conductivity and resolve interfacial instability in sulfohalide SSEs for high-performance LMBs.