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

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-AlF₃ (denoted as HS-AlF₃), leading to a heterostructure halide architecture, designated as NHxy (where x/y represents the mass ratio of NaCl to HS-AlF₃). The high dispersion of NaCl and HS-AlF₃ 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 AlF₃ 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//Na₃V₂(PO₄)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/FeF₃ 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.