Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

The discovery of facile Li transport in disordered, Li-excess rocksalt materials has opened a vast new chemical space for the development of high energy density, low cost Li-ion cathodes. We develop a strategy for obtaining optimized compositions within this class of materials, exhibiting high capacity and energy density as well as good reversibility, by using a combination of low-valence transition metal redox and a high-valence redox active charge compensator, as well as fluorine substitution for oxygen. Furthermore, we identify a new constraint on high-performance compositions by demonstrating the necessity of excess Li capacity as a means of counteracting high-voltage tetrahedral Li formation, Li-binding by fluorine and the associated irreversibility. Specifically, we demonstrate that 10–12% of Li capacity is lost due to tetrahedral Li formation, and 0.4–0.8 Li per F dopant is made inaccessible at moderate voltages due to Li–F binding. We demonstrate the success of this strategy by realizing a series of high-performance disordered oxyfluoride cathode materials based on Mn²+/⁴+ and V⁴+/⁵+ redox.Vehicle Technologies Program (U.S.) (Contract No. DE-AC02-05CH11231)United States. Department of Energy. Office of Energy Efficiency and Renewable Energy. Advanced Battery Materials Research Program (Subcontract No. 7056411)National Science Foundation (U.S.) (Reward No. OCI-1147503)National Science Foundation (U.S.) (grant number ACI- 105357)National Science Foundation (U.S.) (NSF DMR 172025)United States. Department of Energy (Contract No. DE-AC02-06C H11357)United States. Department of Energy. Office of Science (contract no. DE-AC02-05CH11231