Combining density functional theory and ²³Na NMR to characterize Na₂FePO₄F as a potential sodium ion battery cathode

Sodium ion batteries offer an inexpensive alternative to lithium ion batteries, particularly for large-scale applications such as grid storage that do not require fast charging rates and high power output. Moreover, the use of polyanionic structures as cathode materials afford incredibly high structural stability relative to layered transition metal oxides that can undergo a structural collapse upon full removal of the charge carrying ions. Sodium iron fluorophosphate, Na₂FePO₄F, has demonstrated its viability as a potential cathode material for sodium ion batteries, having a robust framework even after multiple charge-discharge cycles. Although solid-state NMR has traditionally been an excellent method for the determination of local structure and dynamic properties of cathode materials during the electrochemical cycling process, reliable assignment of the ²³Na chemical shifts resulting from the paramagnetic hyperfine interaction can be difficult when using only empirical rules. Here we present the use of density functional theory calculations to assign the experimentally observed NMR shifts to the crystallographic sites in Na₂FePO₄F, where it is found that the results do not agree with the previously reported assignment based upon simple geometry arguments. Furthermore, we report the justification of the proposed desodiation mechanism in Na₂FePO₄F on the basis of theoretical arguments, in good agreement with experimental NMR results reported previously.