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.