Structural Origins of High Performance in Partially Disordered Li- and Mn-Rich Cathodes

The discovery of Li-excess cation-disordered rocksalt (DRX) cathodes lifts the restriction on a specific Li-TM ordering to enable the exploration of cathode materials in a chemical space of earth abundant materials. Recently, phase transformation in disordered Li- and Mn-rich rock-salts (Li 1.05 Mn 0.85 Ti 0.1 O 2 ) have been shown to form high energy density, capacity retention and rate capability cathode materials (named delta) with partial Spinel-like ordering at short coherence lengths. 1 However, atomic scale structural features of the material which determine its high performance such as extent of disordering and Li-transport pathways are not well-understood yet. We first perform symmetry analysis to reveal aspects of the transformation which are central to the nanoscale domain formation in the delta phase and crucial for characterizing its structure. We develop a simple structural order parameter to distinguish the Spinel-like ordering from the disordered rocksalt (DRX) structure, characterizing the extent of disordering, sizes and spatial location of the domains as the transformation proceeds. We perform multi time-scale Kinetic Monte Carlo Simulations for understanding the DRX-to-delta transformation. Our results reveal both thermodynamic and kinetic factors causing the formation of spinel-like nano domains and spatial distribution of Ti and Li-excess in the delta phase of Li 1.05 Mn 0.85 Ti 0.1 O 2 , providing key insights into its high rate capability. Our simulations reveal principles for designing the structure at nanoscale and electrochemical performance through changes in composition. Cai, Z.; Ouyang, B.; Hau, H.-M.; Chen, T.; Giovine, R.; Koirala, K. P.; Li, L.; Ji, H.; Ha, Y.; Sun, Y.; Huang, J.; Chen, Y.; Wu, V.; Yang, W.; Wang, C.; Clément, R. J.; Lun, Z.; Ceder, G. In Situ Formed Partially Disordered Phases as Earth-Abundant Mn-Rich Cathode Materials. Nat. Energy 2023, 1–10. https://doi.org/10.1038/s41560-023-01375-9.