Regulating Na⁺ vacancies and stabilizing lattice structure via Al³⁺/O^2- doping for low-cost dry-air-stable NaCl-based solid electrolytes

Abstract

Sodium chloride (NaCl) is attractive as a solid electrolyte because of its wide electrochemical stability window, excellent dry-air stability, favorable cathode compatibility, and low cost; however, its intrinsically low ionic conductivity has severely limited practical application. Here, we report an Al³⁺/O^2- co-doping strategy that introduces Na⁺ vacancies and mixed Cl^-/O^2- anion environments into the NaCl lattice, thereby converting NaCl from a poor ionic conductor into a fast-ion-conducting solid electrolyte. The optimized composition, Na_0.4625Al_0.25Cl_0.7875O_0.2125, exhibits an ionic conductivity of 2.7 × 10^-4 S cm^-1 at 30 °C. Through combined X-ray diffraction, solid-state nuclear magnetic resonance, and pair distribution function analysis, Na_0.4625Al_0.25Cl_0.7875O_0.2125 is identified as a single-phase face-centered cubic structure (space group Fm-3m) that hosts a high concentration of Na⁺ vacancies (37.6%) at the 4a sites, creating a continuous 3D transmission network for rapid Na⁺ migration. Electrochemically, Na_0.4625Al_0.25Cl_0.7875O_0.2125 exhibits a wide stability window of 1.05~4.24 V vs. Na₂Sn. In an all-solid-state cell with the configuration NaNi_1/3Mn_1/3Ti_1/3O₂||Na_0.4625Al_0.25Cl_0.7875O_0.2125||Na₃PS₄||Na₂Sn, capacity retentions of 89% and 83% are achieved after 100 cycles over voltage windows of 2.4-4.2 and 2.4-4.4 V, respectively.

Keywords

Halide solid-state electrolytes, solid-state batteries / cubic crystal structure / low cost / dry air stability

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