Nitrogen Harmonized Fluoride Solid‐State Electrolytes With Modulus‐Mismatch‐Induced Interfacial Defects for Fast Li + Transport

ABSTRACT Fluoride‐based solid electrolytes possess excellent air stability, strong oxidation stability and wide electrochemical windows, however their application potential is hindered by intrinsic poor ionic conductivity. Herein, a fused‐salt‐etched nitrogen‐modulated fluoride electrolyte (LAFN) is designed to improve conductivity performance via synergistic interfacial construction and lattice coordination regulation. The chloride softened grain boundaries and constructed K(Na)Cl/LAFN heterostructures optimize solid‐solid contact, while the Young modulus mismatch between K(Na)Cl and LAFN domains generates massive lattice distortion and defect regions, which can percolate to form continuous pathways for rapid Li + diffusion. Nitrogen doping mediates the generation of self‐limiting stable Li 3 N‐LiF composite layer on Li metal, effectively inhibiting the anode side reactions. It also enables the weakening of rigid Li‐F ionic bonding with the enrichment of lithium vacancies, and the moderate Li‐N coordination balances the activation of Li + migration and structural stability. As a result, the room‐temperature ionic conductivity of LAFN is elevated to 2.04 × 10 −4 S cm −1 , delivering outstanding rate capability and long‐cycle stability in both symmetric and full cells. The LiFePO 4 ‐based solid‐state full cells retain the 81.9% capacity retention after 200 cycles at 0.5 C. When paired with conversion‐type FeF 3 , the cells deliver a high reversible capacity of 233.24 mAh g −1 after 100 cycles.