Factors affecting capacity and voltage fading in disordered rocksalt cathodes for lithium-ion batteries

Disordered rocksalt cathodes deliver high energy densities, but they suffer from pronounced capacity and voltage fade on cycling. Here, we investigate fade using two disordered rocksalt lithium manganese oxyfluorides: Li₃Mn₂O₃F₂ (Li_1.2Mn_0.8O_1.2F_0.8), which stores charge by Mn²⁺/Mn⁴⁺ redox, and Li₂MnO₂F, where charge storage involves both Mn³⁺/Mn⁴⁺ and oxygen redox (O-redox). Li₃Mn₂O₃F₂ is reported for the first time. We identify the growth of an electronically resistive surface layer with cycling that is present in both Li₂MnO₂F and Li₃Mn₂O₃F₂ but more pronounced in the presence of O-redox. This resistive surface inhibits electronic contact between particles, leading to the observed voltage polarization and capacity loss. By increasing carbon loading in the composite cathode, it is possible to substantially improve the cycling performance. These results help to disentangle O-redox from other leading causes of capacity fading in Mn oxyfluorides and highlight the importance of maintaining electronic conductivity in improving capacity and voltage retention.