High Conductivity Zr-Rich Sodium Fluorochloride Catholytes for All Solid-State Sodium Batteries

All-solid-state sodium batteries (ASSBs) are a potential cost-effective alternative to their lithium-ion analogues. However, their broad adoption requires catholytes that combine high ionic conductivity, good electrochemical stability, and mechanical deformability. Addressing these challenges, we report a new family of amorphous Zr-rich fluorochlorides, Na0.8Zr0.8Ta0.2Cl4.2‑x F0.8+x (x = 0, 1) synthesized via a one-step mechanochemical route. These catholytes exhibit sodium-ion conductivities of ∼1 mS·cm–1 and low activation energies (∼322 meV), along with high oxidative stability up to 4.5 V vs. Na+/Na due to their dual-anion (Cl–/F–) oligomeric framework. ASSBs employing these halides coupled with Na3V2(PO4)3 cathodes deliver capacities of ∼100 mAh·g–1 with 90% retention over more than 300 cycles in a 2.5–4.1 V window due to suppressed interfacial side reactions. Pairing the optimal catholytes with a Na3V2(PO4)2O2F cathode enables cycling up to 4.4 V vs. Na+/Na, enhancing energy density. This work presents a general strategy to enhance the ionic conductivity and anodic stability of halide-based solid electrolytes.