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

AA Cheenady and K Rajan, ACS APPLIED ENERGY MATERIALS, 8, 7310-7321 (2025).

DOI: 10.1021/acsaem.5c00689

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 Sigma 3(111) and Sigma 5(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 Sigma 3(111) STGBs potentially acting as fast diffusion channels and Sigma 5(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 Sigma 5(021) STGBs strongly influenced by stress while diffusion at Sigma 3(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.

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