Cation-disordered oxides for rechargeable lithium battery cathodes

Abstract

The demands for high-energy density cathode materials for rechargeable lithium batteries are ever increasing. This is because such cathode materials will enable smaller and lighter rechargeable lithium batteries for complex applications such as in electric vehicles or in grid energy storage. Nearly all high-energy density cathode materials for rechargeable lithium batteries have been sought from well-ordered close-packed oxides in which lithium and transition metal ions occupy distinct sites. In contrast, cation-disordered oxides, whose cation distribution is fully or partially random, have received only a limited attention, because lithium diffusion tends to be limited by their cation-disordered structure. In the first part of this thesis, from the study of Li₁.₂₁₁Mo₀.₄₆₇Cr₀.₃O₂, it is demonstrated that cation-disordered oxides can be promising cathode materials if they contain enough lithium excess (x >/=1.09 in LixTM₂-xO₂). Li₁.₂₁₁Mo₀.₄₆₇Cr₀.₃O₂ forms into a layered structure, but transforms almost completely to a cation-disordered structure during cycling. While common wisdoms would expect poor cyclability of this material due to limited lithium diffusion in its structure, the reversible capacity of this material is remarkably high (~265 mAh/g), which demonstrates that lithium diffusion can be facile in the cation-disordered structure. Using ab initio calculations, we show that this counterintuitive behavior is due to percolation of a certain type of active diffusion channels (0-TM channels) in disordered Li-excess materials, which becomes more extensive as the lithium-excess level increases. In the second part of this thesis, a new class of high capacity cation-disordered oxides (Li-Ni-Ti-Mo oxides) is designed based on the 0-TM percolation theory. As the theory predicts, the reversible capacity and rate capability in these materials considerably improve with lithium excess. In particular, Li₁.₂Ni₁/₃Ti₁/₃Mo₂/₁₅O₂ delivers high capacity and energy density up to 250 mAh/g and 750 Wh/kg at 10 mA/g. Combining in situ X-ray diffraction, electron energy loss spectroscopy, and X-ray absorption near edge spectroscopy, we investigate the redox mechanism of the new materials and discuss how oxygen loss with lattice densification can affect lithium diffusion in the materials by decreasing the lithium-excess level. From these understandings, strategies for further improvements are proposed, setting new guidelines for the design of high performance cation-disordered oxides for rechargeable lithium batteries.