Flexible doorway controlled Na+ ion diffusion in NaPSO glassy electrolytes from machine-learning force field simulations

Abstract

All-solid-state sodium batteries (ASSSBs), featuring nonflammable solid-state electrolytes (SSEs) and abundant sodium metal anodes, are attractive candidates for safe, cost-effective grid-scale energy storage. Recent research shows that oxygen doping increases the ionic conductivity, mechanical strength, and formability of Na₃PS_4−xO_x (NaPSO) glassy solid-state electrolytes (GSEs), offering a promising approach for developing durable, energy-dense, and affordable ASSSBs. In the Na₃PS_4−xO_x GSEs, a maximum in the Na⁺ ion conductivity is observed at the x = 0.15 composition. The Na⁺ ion conductivity decreases monotonically with further additions of oxygen. However, the limited understanding of the underlying mechanisms of Na⁺ ion motion, diffusion, and conduction in these GSEs hinders its broader application. Here, we employ machine learning force field (ML-FF) molecular dynamics (MD) simulations to investigate sodium ion (Na⁺) diffusion in NaPSO GSEs. Contrary to prior perspectives, our simulation results indicate that oxygen doping consistently reduces the free volume, which would be expected to inhibit rather than increase the Na⁺ diffusion. Interestingly, we find however, that oxygen doping also enhances the flexibility of the amorphous framework, which paradoxically facilitates Na⁺ diffusion. This dual effect results in an initial rise followed by a fall in diffusion coefficients which is consistent with the measured values of the Na⁺ ion conductivity. Our findings provide atomic-level insights into the impact of oxygen doping on Na⁺ diffusion in NaPSO glassy electrolytes, suggesting improved amorphous framework flexibility as a new strategy to enhance conductivity in solid-state electrolytes.