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

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Chlorides and fluorides solid-state electrolytes (SEs) exhibitcontrasting 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 duringmechanochemical 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 promoteNa-ion transport in NHxy, especially along the NaCl-based heterostructures with AlF3 and NaF. This class of SEs achieves high ionic conductivity approaching 10-4 S/cm 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 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/cm2. 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.