Na-Ion Mobility in Layered Na2FePO4F and Olivine NaFePO4

Materials for sodium-ion batteries are attracting renewed interest. Olivine NaFePO₄ and layered Na₂FePO₄F are interesting materials that have been reported recently as possible positive electrodes. Here, we report their Na-ion conduction behaviour and intrinsic defect properties using atomistic simulation methods.[1,2] In the NaFePO₄ olivine, Na ion migration is essentially restricted to the [010] direction along a curved trajectory, similar to that of LiFePO₄, but with a lower migration energy. However, Na/Fe antisite defects are also predicted to have a lower formation energy: the higher probability of tunnel occupation with a relatively immobile Fe²⁺ cation – along with a greater volume change (17%) 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 impede transport significantly owing to the 2D pathway. This factor, along with the much lower volume change (3.7%) on redox cycling, is undoubtedly responsible for the better electrochemical performance of the layered structure.

[1] R. Tripathi, S.M. Wood, M.S. Islam and L.F. Nazar, Energy and Envrion. Sci., 2013, 6, 2257-2264

[2] M.S. Islam and C.A.J. Fisher, Chem. Soc. Rev., 2014, 43, 185-204