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

Abstract

Conventional Li-ion cathodes are stoichiometric intercalation materials (e.g., LiCoO2, NCM, LiMn2O4). 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 densities2,3. 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 presented4: Li1.68Mn1.6O3.7F0.3 (LMOF03) and Li1.68Mn1.6O3.7F0.3 (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).