Na-ion mobility in layered Na2FePO4F and olivine Na[Fe,Mn]PO4

Abstract

The Na-ion battery is currently the focus of much research interest due to its cost advantages and the relative abundance of sodium as compared to lithium. Olivine NaMPO₄ (M = Fe, Fe_0.5Mn_0.5, Mn) and layered Na₂FePO₄F are interesting materials that have been reported recently as attractive positive electrodes. Here, we report their Na-ion conduction behavior and intrinsic defect properties using atomistic simulation methods. In the olivines, Na ion migration is essentially restricted to the [010] direction along a curved trajectory, similar to that of LiMPO₄, but with a lower migration energy (0.3 eV). However, Na/M antisite defects are also predicted to have a lower formation energy: the higher probability of tunnel occupation with a relatively immobile M²⁺ cation – along with a greater volume change on redox cycling – contributes to the poor electrochemical performance of the Na-olivine. Na⁺ ion conduction in Na₂FePO₄F is predicted to be two-dimensional (2D) in the interlayer plane with a similar low activation energy. The antisite formation energy is slightly higher; furthermore, antisite occupation would not be predicted to impede transport significantly owing to the 2D pathway. This factor, along with the much lower volume change on redox cycling, is undoubtedly responsible for the better electrochemical performance of the layered structure. Where volume change and structural effects do not incur impediments, Na-ion materials may present significant advantages over their Li counterparts.