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

Abstract

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 LixTM2-xO2). The reversible capacity and rate capability in these compounds are shown to considerably improve with lithium excess. In particular, Li1.2Ni1/3Ti1/3Mo2/15O2 delivers up to 250 mA h g(-1) and 750 W h kg(-1) (similar to 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 Li1.2Ni1/3Ti1/3Mo2/15O2 to 4.8 V occurs with Ni2+/Ni similar to 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.