Molecular dynamics simulations of amorphous NaFePO₄ as an Na-ion battery cathode material

Abstract

While LiFePO 4 found wide applications as a high-performance Li-ion battery cathode material, its sodium analog, crystalline NaFePO 4 , cannot deliver its attractive theoretical capacity of 154 mAh ⋅ g[Formula: see text] at practical (dis)charge rates due to the low ionic conductivity of the stable Maricite phase of NaFePO 4 . Recently, it was found that amorphization greatly enhances the rate capability of NaFePO 4 turning it into an attractive Na-ion battery cathode material. Here, we study the effect of amorphization on the rate-limiting ionic conductivity. To this end, structure models of amorphous NaFePO 4 are produced by simulated melt-quenching of Maricite. Ion transport pathways in the resulting glass structure are then compared to those in crystalline Maricite to provide a more in-depth understanding of the mechanism behind the significantly enhanced rate performance. Static bond valence site energy landscape analyses reveal a considerable reduction of the sodium migration energy for crystalline Maricite from about 1.6 eV to 0.65(11) eV for 1D paths and 0.77(15) eV for 2D paths in amorphous NaFePO 4 . Detailed molecular dynamics simulations then reveal that the first local Na[Formula: see text] redistributions can even occur with the extremely low migration energy of 0.12 eV.