Na_2.5Cr_0.5Zr_0.5Cl₆: A New Halide-Based Fast Sodium-Ion Conductor

All-solid-state batteries employing solid electrolytes (SEs) have received widespread attention due to their high safety. Recently, lithium halides are intensively investigated as promising SEs while their sodium counterparts are less studied. Herein, a new sodium-ion conductor with a chemical formula of Na_2.5Cr_0.5Zr_0.5C_l6 is reported, which exhibits high room temperature ionic conductivity of 0.1 mS cm^-1 with low migration energy barrier of ≈0.41 eV. Na_2.5Cr_0.5Zr_0.5C_l6 has a Fm-3m structure with 41.67 mol.% of cationic vacancies owing to the occupation of Cr (8.33 mol.%) and Zr (8.33 mol.%) ions at Na sites. Supercell calculations show that the lowest columbic energy configuration has Cr/Zr/V (where V is the vacancy) clusters in the structure. Nonetheless, the clusters have mixed effects on the sodium ion conduction pathway, based on the Bond Valence Energy Landscape calculation. A global 3D Na-ion transport percolation network can be revealed in the lowest energy supercell. Effective pathways are connected through the NaCl₆ and VCl₆ nodes. Besides, Raman spectroscopy and ²³Na solid-state nuclear magnetic resonance spectroscopy further prove the tunable structure of the SEs with different Cr to Zr ratios. The optimization between the concentration of Na⁺ and vacancies is crucial to create an improved network of Na⁺ diffusion channels.