New Sodium Halide Solid Electrolytes with High Ionic Conductivity for All-Solid-State Batteries

Sodium halide solid state electrolytes (SSEs) suffer from low ionic conductivity (< 10-4 S/cm), which limits their applications in sodium all solid-state batteries (ASSBs). Here, a new LaCl3-based Na+ superionic conductor (Na1-xZrxLa1-xCl4) with high ionic conductivity of 2.9 × 10-4 S cm-1 (30 ℃) and low activation energy of 0.33 eV was developed. Structure characterizations by X-ray diffraction reveal a hexagonal structure (P63/m) of Na1-xZrxLa1-xCl4, with Na+ ions forming a one-dimensional diffusion channel along the c-axis. First-principles calculations and fine structure characterizations reveal that the ionic conductivity of Na1-xZrxLa1-xCl4 is mainly determined by the size of Na+-channels and the Na+/La3+ mixing in the 1D diffusion channels. The expanding of Na+ ion diffusion channel enhances the ionic conductivity, while Na+/La3+ mixing in the diffusion channel hinders the Na+ diffusion. Further structural adjustments can achieve mS/cm level ionic conductivity. The Na2Sn|Na2.9PS3.9Cl0.1|Na0.7Zr0.3La0.7Cl4|NaCrO2 all-solid-state batteries demonstrate an initial capacity of 119 mA h/g and high columbic efficiency of 98% at 0.1C, highlighting the excellent oxidation stability of Na0.7Zr0.3La0.7Cl4. This work demonstrates the possibility of high ionic conductivity sodium ion halide SSEs, which would promote the development of high voltage sodium ASSBs.