Chemical Characteristics Driving Formability in Li-Excess Disordered Rocksalt Cathodes

Abstract Li-excess disordered rocksalts are promising next-generation Li-ion battery cathode materials that feature a flexible, site-disordered cation sublattice. Understanding how different cations and their respective ratios can be adjusted is crucial to the further development of these materials. Here, we conceive a large, systematic set of potential disordered rocksalt formulations in which the redox-inactive charge compensator site has been substituted with a variety of cation species in varying ratios. From the phase purity of the 69 target formulations, we derive how the features of substituent charge compensator species impact the formability of disordered rocksalts. We find that cation size, cation tendencies to be in a distorted octahedron, and formation enthalpy of competing impurity phases are the primary drivers of amenability to form a locally disordered, single-phase product. We also find correlations of cation species with average structure lattice parameters and short-range order. A subset of rocksalt formulations is tested as Li-ion battery cathodes. We find that structural (crystallinity, short-range order, microstates of redox-active species) and chemical (surface chemistry) differences attributed to charge compensator substitution schemes impact performance metrics, such as capacity and capacity retention, highlighting the robust design space of this exciting class of materials. Insights reported here can inform the development of rocksalt oxides with targeted behavior, possibly including for favorable formation and performance of “δ” phases in Mn-rich rocksalts. In addition, we expect these findings about how cation features affect formability and structure translate to other compositionally complex oxide systems.