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

While LiFePO4 found wide applications as a high-performance Li-ion battery cathode material, its sodium analog, crystalline NaFePO4, cannot deliver its attractive theoretical capacity of 154 mAh . g(-1) at practical (dis)charge rates due to the low ionic conductivity of the stable Maricite phase of NaFePO4. Recently, it was found that amorphization greatly enhances the rate capability of NaFePO4 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 NaFePO4 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 NaFePO4. Detailed molecular dynamics simulations then reveal that the first local Na+ redistributions can even occur with the extremely low migration energy of 0.12 eV.