Grain Boundary Effects and Stress–Diffusion Coupling in Na₃OCl Antiperovskite for Sodium-Ion Batteries: A Molecular Dynamics Study

While Na-based antiperovskites have emerged as a promising choice as solid-state electrolytes (SSEs) for sodium-ion batteries (SIBs), critical aspects regarding the influence of grain boundaries (GBs), mechanical stress, and their interaction on diffusion in these materials remain uninvestigated. We choose two model systems in cubic Na3OCl (comprising Σ3(111) and Σ5(021) symmetric tilt GBs (STGBs)) and utilize molecular dynamics simulations to document the impact of GBs on Na-ion diffusion and the effects of mechanical stress on GB diffusion. Directional diffusion characteristics along and perpendicular to the GB plane are investigated in the unstressed state and under uniaxial, biaxial, and isotropic states of tensile and compressive loads. Our analysis reveals that Na-ion diffusion at Na3OCl GBs is anisotropic and strongly influenced by the GB structure, with Σ3(111) STGBs potentially acting as fast diffusion channels and Σ5(021) STGBs severely inhibiting diffusion. Further, the effect of mechanical stress on GB diffusion is shown to be a complex function of GB type, stress state, and type of loading, with diffusion at Σ5(021) STGBs strongly influenced by stress while diffusion at Σ3(111) STGBs remains mostly unaffected. Given that fabrication conditions can substantially influence the types of GBs formed in polycrystalline samples, the demonstrated GB effects and stress–diffusion coupling call for careful tailoring of processing conditions for improving ionic conductivity of SSEs for SIBs.