Double sodium fluorides as candidates for solid electrolytes and cathode protective coatings: Computational study and experimental verification

Daniil M. Chernyshov, Anatoly Volkov, Sergey N. Marshenya, Alexander A. Golubnichiy, Konstantin A. Lyssenko, Stanislav S. Fedotov, Dmitry A. Aksyonov

https://doi.org/10.1016/j.jallcom.2026.188312 

The current study by Daniil Chernyshov, Dr. Dmitry Aksyonov and colleagues is dedicated to the computational and experimental investigation of double sodium rare-earth fluorides (β-NaYF₄ and β-NaLaF₄) as candidates for solid electrolytes and cathode protective coatings in all-solid-state sodium batteries. DFT calculations were combined with solid-state synthesis, single-crystal growth, and electrochemical impedance spectroscopy. The results showed that β-NaLaF₄ exhibits a wide electrochemical stability window (up to ~5.4 V vs. Na⁺/Na), strong electronic insulation, low Na⁺ migration barriers (~0.3–0.4 eV), and stable, well-adhered interfaces with sodium metal. Experimental measurements confirmed a room-temperature ionic conductivity of ~10^-5 mS cm^-1, sufficient for thin cathode coatings. Aliovalent Ca doping in powder samples suppressed conductivity due to simultaneous substitution at Na and La sites, while single crystals grown from a eutectic melt successfully introduced Na vacancies, demonstrating a viable route to enhance ionic transport. The work highlights β-NaLaF₄ as a promising protective coating material and outlines defect-engineering strategies for future solid-electrolyte applications.