Electrochemical Utilization of Iron IV in the Li1.3Fe0.4Nb0.3O2 Disordered Rocksalt Cathode

Abstract Interest in alkali‐rich oxide cathodes has grown in an effort to identify systems that provide high energy densities through reversible oxygen redox. However, some of the most promising compositions such as those based solely on earth abundant elements, e. g., iron and manganese, suffer from poor capacity retention and large hysteresis. Here, we use the disordered rocksalt cathode, Li 1.3 Fe 0.4 Nb 0.3 O 2 , as a model system to identify the underlying origin for the poor performance of Li‐rich iron‐based cathodes. Using elementally specific spectroscopic probes, we find the first charge is primarily accounted for by iron oxidation to 4+ below 4.25 V and O 2 gas release beyond 4.25 V with no evidence of bulk oxygen redox. Although the Li 1.3 Fe 0.4 Nb 0.3 O 2 is not a viable oxygen redox cathode, the iron 3+/4+ redox couple can be used reversibly during cycling.