Understanding the structural and electronic properties of the cathode material NaFeF in a Na-ion battery.

Na-ion battery cathode material NaFeF and the corresponding desodiated products were investigated by using first-principle density functional theory calculations within the generalized gradient approximation (GGA) + U framework. Our results show that NaFeF is the only energetically stable intermediate phase among the cases considered in the present work ( x = 0.75, 0.5, 0.25), leading to theoretically two voltage plateaus, i.e., 2.63 V (1 ≥ x ≥ 0.5) and 2.82 V (0.5 ≥ x ≥ 0) in NaFeF, respectively. The mean charge voltage of the extraction of the first 0.5 Na ions agrees well with the experimental value, but the one of the extraction of the second 0.5 Na ions exhibits a relatively low value comparing to the experimental results, indicating that another end phase exists, which is consistent with the recent experiments. Furthermore, the electronic structures upon the desodiation are also discussed, and charge localization into distinct Fe and Fe is found in the intermediate-phase NaFeF. [ABSTRACT FROM AUTHOR]