Research by the Peter Bruce Group and collaborators at Japan Synchrotron Radiation Research Institute (JASRI) and Uppsala University as reported in Energy & Environmental Science explains that the greatest barrier to improving the specific energy of the Li-ion battery is the cathode. While Si offers a route to increase the capacity at the anode this needs to be balanced by higher energy storage cathodes. The capacity of the cathode is limited by storing electrons on the transition metal ions alone, such as in LiMn2O4, where electrons are stored on the Mn 3+/4+ redox couple. There is a great deal of interest in increasing charge storage in transition metal oxide cathodes beyond the limit of transition metal redox activity. While redox reactions on sulfur in transition metal sulfides are well known only recently has O redox activity in transition metal oxides been recognised. They report a new intercalation cathode material, a lithium manganese oxyfluoride based on Li2MnO2F, with a high capacity to store charge by invoking redox activity on the Mn cations and O anions. It has a disordered rocksalt structure, which avoids the structural changes and consequent severe changes in voltage observed for O-redox layered transition metal oxide cathodes during the 1st cycle.