A new class of high capacity cation-disordered oxides for rechargeable lithium batteries: Li-Ni-Ti-Mo oxides

Recent successes with disordered Li-excess materials and applications of percolation theory have highlighted cation-disordered oxides as high capacity and energy density cathode materials. In this work, we present a new class of high capacity cation-disordered oxides, lithium-excess nickel titanium molybdenum oxides, which deliver capacities up to 250 mA h g^-1. These materials were designed from percolation theory which predicts lithium diffusion to become facile in cation-disordered oxides as the lithium-excess level increases (x > 1.09 in Li_xTM_2-xO₂). The reversible capacity and rate capability in these compounds are shown to considerably improve with lithium excess. In particular, Li_1.2Ni_1/3Ti_1/3Mo_2/15O₂ delivers up to 250 mA h g^-1 and 750 W h kg^-1 (∼3080 W h l^-1) at 10 mA g^-1. Combining in situ X-ray diffraction, X-ray absorption near edge spectroscopy, electron energy loss spectroscopy, and electrochemistry, we propose that first charging Li_1.2Ni_1/3Ti_1/3Mo_2/15O₂ to 4.8 V occurs with Ni²⁺/Ni^∼3+ oxidation, oxygen loss, and oxygen oxidation in this sequence, after which Mo⁶⁺ and Ti⁴⁺ can be reduced upon discharge. Furthermore, we discuss how oxygen loss with lattice densification can affect lithium diffusion in the material by decreasing the Li-excess level. From this understanding, strategies for further improvements are proposed, setting new guidelines for the design of high performance cation-disordered oxides for rechargeable lithium batteries.