Elucidating Lithium Ion Diffusion Kinetics in Cation-Disordered Rocksalt Cathodes

Disordered rocksalt (DRX) cathodes have emerged as a promising alternative to conventional nickel/cobalt-based layered oxides due to their potential for higher specific capacities using earth-abundant elements. However, the poor rate capability of DRX has been a critical bottleneck in practical battery operations, often attributed to sluggish lithium and/or electronic conduction. In this study, we elucidate the lithium diffusion mechanism in DRX exploiting a ‘diffusion cluster’ model, effectively addressing the complexity of randomly distributed cations in the structure. Our findings reveal that DRXs intrinsically possess various diffusion paths with activation barriers widely ranging from 200 meV to 1.3 eV, owing to the diverse lithium hopping environments created by disordered cations. Importantly, we discovered that migration bottlenecks along lithium percolation paths are primarily caused by the large energy differences among lithium sites (as high as ~ 1 eV), rather than the transition state energy during lithium hopping, contrary to the conventional diffusion mechanism in ordered structures. The significantly broad distribution of lithium site energies is found to be inherited by the distortion in the shape and size of lithium sites in oxides caused by disordered cations in DRX, e.g., Li_1.2Mn_0.4Ti_0.4O₂. Consequently, the large energy step from one site to another acts as a de facto barrier for lithium hopping, impeding the overall lithium diffusion process. This new finding suggests that the key to improve the rate performance of DRX lies at flattening the landscape of lithium site energies, balancing with the degree of cation disorderness in DRX.