Ultrahigh Power and Energy Density in Partially Ordered Lithium-Ion Cathode Materials

Conventional Li-ion cathodes are stoichiometric intercalation materials (e.g., LiCoO 2 , NCM, LiMn 2 O 4 ). We will present here our efforts to make high capacity, high rate materials based on the earth abundant Mn redox center by partially disordering a cation-excess spinel. In an ordered spinel, the two-phase reaction that occurs at low voltage (~2.7 V) is not only intrinsically slow, but also detrimental to the structural stability because of the large difference in lattice constant between the two end points (cubic and tetragonal phases) 1 . Recently, Li-excess cation disordered rock salt materials have shown remarkable cycling performance-large capacities and high energy densities 2,3 . We applied the Li-excess strategy to the spinel structure to synthesize metal-excess, Li-excess partially-disordered spinels. Disorder is induced to remove the two-phase region, and metal excess and Li-excess is used to create high mobility Li-ions and enablement of O-redox. Two partially disordered spinels with compositions between spinel and rocksalt stoichiometry are presented 4 : Li 1.68 Mn 1.6 O 3.7 F 0.3 (LMOF03) and Li 1.68 Mn 1.6 O 3.7 F 0.3 (LMOF06). A remarkably high specific energy density greater than 1,100 Wh kg –1 (and capacity >360 mA h g –1 ) can be achieved in LMOF03, with nearly half of the capacity coming from oxygen redox. On top of that, facilitated by the beneficial spinel-like cation order and the elimination of two-phase reactions, both LMOF03 and LMOF06 show a very high rate capability with a capacity larger than 100 mA h g -1 at 20 A g -1 . Our strategy of combining Li-excess and cation over-stoichiometry in a spinel structure to enable high energy and power at the same time can also be applied to other materials. 1 Thackeray, M. M., David, W. I. F., Bruce, P. G. & Goodenough, J. B. Lithium insertion into manganese spinels. Materials Research Bulletin 18 , 461-472 (1983). 2 Lee, J. et al. Reversible Mn 2+/Mn 4+ double redox in lithium-excess cathode materials. Nature 556 , 185 (2018). 3 Lee, J. et al. Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries. Science 343 , 519-522 (2014). 4 Ji, H. et al. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nature Energy , doi:10.1038/s41560-020-0573-1 (2020).