Aluminum Chloride-based Catholytes for Stable High-voltage Solidstate Sodium Batteries

Abstract

Stable high-voltage cycling and low-cost solid electrolytes are imperative for sodium-based all-solid-state batteries to compete with traditional lithium-ion batteries in terms of energy density and cost. Herein, we present the fundamental highvoltage interactions between low-cost NaAlCl4-based solid-state catholytes and layered-oxide cathode active materials and fluorination strategies to enhance the stability. The usual strategy of fluorine incorporation into the bulk electrolyte structure to extend the effective oxidative stability proves uniquely ineffective for NaAlCl4, but notably enhances the ionic conductivity to 0.1 mS cm -1 . Conversely, a NaF coating of the cathode active material can improve the high-voltage cycling stability. Through X-ray photoelectron spectroscopy analysis, interfacial interactions at high-voltages between NaAlCl4 and NaNi0.5Mn0.5O2 cathode are elucidated and can be effectively mitigated by employing a NaF coating, which proves to be essential for improving high-voltage cycling stability. This work serves as a foundation for understanding halide-catholytecathode high-voltage interactions and strategies to improve sodium-all-solid-state battery energy density with low-cost materials.