Investigating Structural Evolution and Oxygen Redox in Disordered Rock Salt Cathodes for Lithium-Ion Batteries

Disordered rock salt (DRX) structures present promising opportunities as cathode materials for lithium-ion batteries, owing to their potential for high energy density (>1000 Wh/kg) and specific capacity (>300 mAh/g).[1] DRX cathodes feature mixed occupancy of Li and transition metals (TM) on the same crystallographic site while the anionic site is occupied by O (and/or F). Li is taken in excess to facilitate a percolating network for Li transport.[2] Oxygen redox (O-redox) plays a significant role in the capacity of these cathodes, with Li excess enabling greater O-redox activity, commonly observed in Mn, Ni, and Fe-based systems.[3] However, the oxidation of lattice oxygen (O^2-) can lead to O^n- (0 < n < 2) resulting in different simultaneous mechanisms. The less stable Oⁿ⁻ may disproportionate to O²⁻ and O₂ gas that leaves the lattice. The Oⁿ⁻ may also get reduced back to O²⁻ or O^m− (n < m < 2). Thus, O redox during high charging states can trigger complex processes, including oxygen release, TM migration, and disproportionation of Oⁿ⁻ species.[4]
This study focuses on understanding the fate of oxidized oxygen in DRX systems by investigating the Li1.2Ti0.6Ni0.2O2 (LTNO) cathode material. The LTNO has been found to show structural evolution upon charge-discharge cycles.[5] The presence of a greater number of TMs mixed with Li in the considered system might also affect the stability of delithiated structures through entropy contributions. Using ab initio molecular dynamics (AIMD) simulations, we explore the structural evolution and redox behaviour during delithiation at high charge states. Key questions include the relationship between TM migration and O-O dimerization, the stability of delithiated structures, and the onset of O-O dimerization during cycling. By comparing short-range ordered and fully random structures, we aim to elucidate the role of cation disorder in redox mechanisms. This work provides critical insights into improving the reversibility and stability of DRX cathodes, supporting the development of high-performance lithium-ion batteries.

1. Assat, G., & Tarascon, J. M. (2018). Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries. Nature Energy, 3(5), 373-386.
2. Squires, A. G., & Scanlon, D. O. (2023). Understanding the limits to short-range order suppression in many-component disordered rock salt lithium-ion cathode materials. Journal of Materials Chemistry A, 11(25), 13765-13773.
3. Chen, D., Ahn, J., & Chen, G. (2021). An overview of cation-disordered lithium-excess rocksalt cathodes. ACS Energy Letters, 6(4), 1358-1376.
4. McColl, K., House, R. A., Rees, G. J., Squires, A. G., Coles, S. W., Bruce, P. G., ... & Islam, M. S. (2022). Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes. Nature communications, 13(1), 5275.
5. Li, B., Kumar, K., Roy, I., Morozov, A. V., Emelyanova, O. V., Zhang, L., ... & Tarascon, J. M. (2022). Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox. Nature Materials, 21(10), 1165-1174.