Fluorinated amorphous halides with improved ionic conduction and stability for all-solid-state sodium-ion batteries

Designing halide solid electrolytes with high ionic conductivity and good (electro)chemical stability is essential for the advancement of all-solid-state sodium-ion batteries. Unfortunately, most sodium-based halide solid electrolytes experience limited ionic conductivities and ambiguous correlations between their structure features and ion transport properties. Here we report a design strategy to boost the conductivities of sodium halides by regulating vacancy and charge carrier concentrations through a facile Na- and Cl-deficient compositions 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. The promoted conductivities of the fluorinated sample are primarily due to increased local structural disorder and enhanced prismatic Na coordination. When paired with an uncoated Na₃V₂(PO₄)₃ positive electrode and a Na₃PS₄-coated Na₁₅Sn₄ negative electrode, the Na_0.5ZrCl₄F_0.5 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 all-solid-state sodium-ion batteries.