Balancing vacancy and carrier concentration in halide solid electrolytes for all-solid-state sodium batteries

Abstract Designing halide solid electrolytes (SEs) with high ionic conductivity and good (electro)chemical stability is essential for the advancement of all-solid-state sodium-ion batteries (ASSIBs). Unfortunately, most sodium-based halide SEs experience limited ionic conductivities and ambiguous correlation between their structure features and ion transport properties. Here we report a novel strategy to significantly boost the conductivities of sodium halides by regulating vacancy and charge carrier concentrations through a facile NaCl-poor method. This approach achieves a balanced structure with optimal vacancy and carrier content, rendering several-fold conductivities enhancement of series sodium halides. Furthermore, a fluorination-induced amorphization protocol is employed to enhance (electro)chemical stability and interfacial compatibility without detrimentally influencing conductivities. When paired with an uncoated Na₃V₂(PO₄)₃ cathode and a Na₃PS₄-coated Na₁₅Sn₄ anode, the Na0.5ZrCl₄F0.5 catholyte enables the battery to run for 300 cycles, retaining 94.4% of its initial discharge capacity at room temperature. This study provides a versatile pathway for creating inorganic ion conductors with high conductivity and long-term cyclability, advancing the development of ASSIBs.