Mesoporous Enhanced Heterostructured Halide Solid Electrolytes with High Air Stability and High Abundance for Sustainable Sodium Metal Batteries

Abstract Chlorides and fluorides solid‐state electrolytes (SEs) exhibit contrasting extremes in ionic conductivity and moisture sensitivity. In light of these conflicting factors, we introduce a NaCl‐based SE reinforced by mesoporous α‐AlF 3 (denoted as HS‐AlF 3 ), leading to a heterostructure halide architecture, designated as NHxy (where x/y represents the mass ratio of NaCl to HS‐AlF 3 ). The high dispersion of NaCl and HS‐AlF 3 during mechanochemical process enables the generation of defective and amorphous structures and nanodomains in NHxy along with F‐Cl anion substitution effect at grain boundaries. These factors collectively promote Na ion transport in NHxy, especially along the NaCl based heterostructures with AlF 3 and NaF. This class of SEs achieves the high ionic conductivity approaching 10 −4 S cm −1 at 30 °C. Specifically, NH54 exhibits excellent long‐term air stability at 35% relative humidity, maintaining high ion conductivity without degradation. The raw material cost of this Na‐based electrolyte is less than $10 USD kg −1 when considering the production in a large scale. The corresponding Na//Na symmetric cells demonstrate the stable cycling for at least 1000 h at 0.1 mA cm − 2 . The Na//Na 3 V 2 (PO 4 ) 3 cells assembled with the NH54 after air exposure exhibit the remarkable longevity, sustaining over 400 cycles at 60 °C. The conversion‐type Na/NH54/FeF 3 cells deliver a high capacity of 500 mAh g −1 . This work opens the new pathways for inorganic SEs with enhanced economic value, ion conductivity, air stability and suitability for energy‐dense conversion reaction batteries.