Deciphering the Impact of Graphene Oxide Incorporation on Structural, Chemical, and Electrochemical Properties of Na2Fe(SO4)2 Cathode Materials

The performance of polyanionic Na 2 Fe(SO 4 ) 2 (NFS) cathode materials is hindered by their low electronic conductivity and is correlated to their structural and chemical instability. Herein, we incorporated graphene oxide (GO) to modify NFS materials, which can build a carbon layer on the surface and alleviate Na 6 Fe(SO 4 ) 4 phase formation and Fe 2+ oxidation in the lattice. Meanwhile, the performance of NFS materials has been significantly improved, such as the capacity retention showing an increase from 56% to 87% after 200 cycles at 0.5 C with GO incorporation. Soft X-ray absorption spectra (sXAS) reveals that the bulk has higher reversibility of the reaction between Fe 2+ and Fe 3+ than the surface during cycling, which indicates the origin of cycling instability of NFS materials. These findings provide new insights into surface modification for the development of high performance NFS cathodes.