Na-Rich Disordered Rock Salt Oxyfluoride Cathode Materials for Sodium Ion Batteries

The existing classes of Na-based cathode materials and their chemistries are still limited, mainly with respect to the increasing demand for alternative post-Li technologies. In this letter, a newly synthesized Na-rich disordered rock salt (DRS) oxyfluoride with the nominal composition Na2MnO2F is reported as a cathode candidate for Na-ion batteries (SIBs). Rietveld refinement analysis confirmed that the synthesized compound has a DRS structure with larger lattice compared to Li-rich homologues. During the first cycle, up to 1.7 Na+/f.u. can be extracted at a slow rate, while a better capacity retention and cycling stability are obtained at high rate, reminiscent of electrode-electrolyte interaction. Further, X-ray absorption fine structure (operando and ex situ) confirmed the Mn oxidation state evolution upon cycling in agreement with the cyclic voltammetry redox profile, emphasizing the reversible Na+ (de)insertion and change of the Mn local ordering. This work is an additional input to the limited series of cathode candidates for SIBs.