Interfacial Energy Engineering via Fluorine Doping Suppresses Lithium Self-Permeation in Li 3 N Solid Electrolytes

Non-electrochemical lithium (Li) permeation into solid-state electrolytes (SSEs) poses a latent yet critical failure mode for all-solid-state lithium metal batteries (ASSLMBs). This work reveals severe Li self-permeation in the Li3N SSE under pressure and heat, forming mixed conductive regions that undermine interfacial stability. To address this problem, we developed an anion-tuning strategy by doping Li3N with LiF to obtain Li2.9N0.95F0.05. Fluorine substitution elevates the interfacial energy between Li and SSE, effectively suppressing spontaneous Li permeation. Li2.9N0.95F0.05 exhibits high ionic conductivity (5.8 × 10-4 S cm-1) and low activation energy (0.326 eV). Consequently, Li-symmetric cells achieve stable cycling for >1000 h at 0.2 mA cm-2, and the ASSLMB employing Li2.9N0.95F0.05 as SSE interlayers and LiCoO2 cathodes retain 80% capacity after 120 cycles at 0.5 C, demonstrating engineering viability. This study provides an effective pathway to stabilize Li metal interfaces and advance the performance of ASSLMBs.