H^{δ -}-H^{δ +} Comproportionation Enables Stable Li-N-H-F Solid Electrolyte

Nitride family compounds are among the earliest explored materials for solid electrolytes (SEs). The main challenge lies in effectively enhancing their electrochemical stability without compromising their excellent Li-ion conductivity and Li metal compatibility. Herein, a H^{δ -}-H^{δ +} comproportionation reaction between LiH and NH₄F is employed to synthesize a Li-N-H-F complex, consisting of Li_{2+ x}NHF_x matrix and dispersed LiF nanoparticles. Density functional calculation results show that the incorporated F atoms in Li₂NH lattice lead to structural variation and electron density redistribution, providing a more connected Li-ion network with low migration energy barriers. More importantly, the interfacial side reactions between the Li-N-H-F complex and electrodes are strongly self-limited due to the blocking effect of the in situ formed Li₄NH/LiF-enriched interphases. The newly identified interphase Li₄NH exhibits fast Li-ion migration ability and intrinsic stability toward Li, facilitating stable Li plating/stripping. Based on the superiority in Li-ion conduction and electrode compatibility, the Li-N-H-F solid electrolyte films prepared via cold pressing with 0.5 wt% binder enable stable cycling of Li||Li, Li||TiS₂, and Li||LiCoO₂ all-solid-state batteries.