Revealing the Mechanism of Sodium Diffusion in Na_<i>x</i>FePO₄ Using an Improved Force Field

Olivine NaFePO₄ is a promising cathode material for Na-ion batteries. Intermediate phases such as Na_0.66FePO₄ govern phase stability during intercalation-deintercalation processes, yet little is known about Na⁺ diffusion in Na_<i>x</i>FePO₄ (0 < <i>x</i> < 1). Here we use an advanced simulation technique, Randomized Shell Mass Generalized Shadow Hybrid Monte Carlo Method (RSM-GSHMC) in combination with a specifically developed force field for describing Na_<i>x</i>FePO₄ over the whole range of sodium compositions, to thoroughly examine Na⁺ diffusion in this material. We reveal a novel mechanism through which Na⁺/Fe²⁺ antisite defect formation halts transport of Na⁺ in the main diffusion direction [010], while simultaneously activating diffusion in the [001] channels. A similar mechanism was reported for Li⁺ in LiFePO₄, suggesting that a transition from one- to two-dimensional diffusion prompted by antisite defect formation is common to olivine structures, in general