Elucidating lithium-ion diffusion kinetics in cation-disordered rocksalt cathodes†

Abstract

Disordered rocksalt (DRX) cathodes have emerged as a promising alternative to conventional nickel/cobalt-based layered oxides owing to their higher specific capacities using earth-abundant elements. However, the poor rate capability of DRX is a critical bottleneck in practical battery operations, which is often attributed to sluggish lithium and/or electronic conduction. In this study, we elucidated the lithium diffusion mechanism in DRX, exploiting a ‘diffusion cluster’ model within a machine-learning scheme, thus effectively addressing the complexity of randomly distributed cations in the structure. Our findings revealed that DRXs intrinsically possess various diffusion paths with activation barriers that widely range from 200 meV to 1.3 eV owing to diverse lithium hopping environments created by disordered cations. Notably, we discovered that migration bottlenecks along lithium percolation paths are primarily caused by large energy differences between 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 attributed to the distortion of 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 the other 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 in flattening the landscape of lithium site energies, thus balancing the degree of cation disorder in DRX.