Background
Cation-disordered rocksalt (DRX) structures is a promising cathode with high capacity and energy density [1]. Unlike conventional well-ordered cathodes, the cations including Li ions and transition-metal (TM) ions share the same sublattice in DRX structures, resulting in a disordered arrangement of Li and TMs (Figure 1). It has been shown that DRX structures offer several advantages over conventional well-ordered cathodes: (1) Li-excess DRX provides excess Li (more than 9%) that is necessary for efficient Li transport with intrinsic high capacity and energy density [2]. Meanwhile, charge compensation is achieved not only by TMs but also partly by nonmetal anions, which allows for excess capacity beyond the theoretical limit of TM redox reactions [3]; (2) the intrinsic disorder in the cation lattice of DRX results in small and isotropic volume changes, helping to remedy the structural degradation of the cathodes [4].
Database
However, the DRX structure presents complexity due to its disordered cations occupation. As a novel type of cathode material over the last decade, a DRX cathode database is essential to give more guidance in understanding and synthesizing DRX cathodes.
In this database, we obtain the fundamental properties of 68 different DRX cathodes through first principle calculations, incorporating d0 transition metal to stabilize each cathode. A Li excess of 20% is set to ensure 0-TM diffusion channels percolation (Figure 2). Each cathode is modeled using a √5 × √5 × 2 supercell (80 atoms), generated with a special quasi-random structure to model the cation disorder [5]. The lattice parameters for each cathode are determined after geometry optimization.
The normalized mixing temperature is employed to be a descriptor to qualitatively describe the synthetic accessibility of 68 cathodes, with a lower mixing temperature indicating better compabtibility (Figure 3). Meanwhile, the GGA+U method is applied to calculate resonable band gap of 68 cathodes, where a lower band gap suggests better electronic conductivity. To evaluate Li-ion diffusion barriers, the bond valence site energy (BVSE) method is utilized, with a lower diffusion barrier indicating better ionic conductivity.
Reference
[1] Zhang, Z.; Liu, J.; Du, P.-H.; Xia, D.; Sun, Q. Screening Na-Excess Cation-Disordered Rocksalt Cathodes with High Performance. ACS Nano 2024, 18 (44), 30584–30592.
[2] Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343 (6170), 519–522.
[3] Seo, D.-H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials. Nature Chem 2016, 8 (7), 692–697.
[4] Clément, R. J.; Lun, Z.; Ceder, G. Cation-Disordered Rocksalt Transition Metal Oxides and Oxyfluorides for High Energy Lithium-Ion Cathodes. Energy Environ. Sci. 2020, 13 (2), 345–373.
[5] Zunger, A.; Wei, S.-H.; Ferreira, L. G.; Bernard, J. E. Special Quasirandom Structures. Phys. Rev. Lett. 1990, 65 (3), 353–356.
[6] Lun, Z.; Ouyang, B.; Kwon, D.-H.; Ha, Y.; Foley, E. E.; Huang, T.-Y.; Cai, Z.; Kim, H.; Balasubramanian, M.; Sun, Y.; Huang, J.; Tian, Y.; Kim, H.; McCloskey, B. D.; Yang, W.; Clément, R. J.; Ji, H.; Ceder, G. Cation-Disordered Rocksalt-Type High-Entropy Cathodes for Li-Ion Batteries. Nat. Mater. 2021, 20 (2), 214–221.