Anion Rotation, Cation Disorder and Ionic Conductivity in Sodium Solid Electrolytes

Solid electrolytes (SEs) with high ionic conductivity and good interfacial stability are critical enablers for safer, more energy dense solid-state sodium-ion batteries (SSSBs). In this talk, I will discuss the in-depth study of two novel SEs for SSSBs using large-scale simulations with machine learning interatomic potentials. The first SE is the Na_3-xY_1-xZr_xCl₆ (NYZC) that is both electrochemically stable and chemically compatible with oxide cathodes. We show that the high ionic conductivity of NYZC can be attributed to abundant Na vacancies and cooperative MCl₆ rotation. We also discuss the effect of different synthesis methods to control polymorphism and cation disorder in NYZC and the corresponding effect on Na-ion conduction. The second SE is a mixed anion Na borohydride, which has a significantly higher conductivity than its constituent end members. We probe the effect of different anion rotations on the overall conductivity and identify the underlying mechanisms for Na conductivity changes. These studies provide new insights into the compositional and synthetic design of sodium SEs with improved conductivity and electrochemical stability.