Oxygen Activity in Li-Rich Disordered Rock-Salt Oxide and the Influence of LiNbO3 Surface Modification on the Electrochemical Performance

摘要

Li-rich disordered rock-salt oxides such as Li1.2Ni1/3Ti1/3Mo2/15O2 are receiving increasing attention as high-capacity cathodes due to their potential as high-energy materials with variable elemental composition. However, the first-cycle oxygen release lowers the cycling performance due to cation densification and structural reconstruction on the surface region. This work explores the influence of lithium excess on the charge compensation mechanism and the effect of surface modification with LiNbO3 on the cycling performance. Moving from a stoichiometric Li1.2Ni1/3Ti1/3Mo2/15O2 composition toward Li-rich Li1.2Ni1/3Ti1/3Mo2/15O2, oxygen redox is accompanied by oxygen release. Thereby, cationic charge compensation is governed by the Ni2+/3+ and Mo3+16+ redox reaction. Contrary to the bulk oxidation state of Mo6+ in the charged state, a mixed Mo valence on the surface is found by XPS. Furthermore, it is observed that smaller particle sizes result in higher specific capacities. Tailoring the surface properties of Li1.2Ni1/3Ti1/3Mo2/15O2 with a solid electrolyte layer of LiNbO3 altered the voltage profile, resulting in a higher average discharge voltage as compared to the unmodified material. The results hint at the interdiffusion of cations from the metal oxide surface coating into the electrode material, leading to bulk composition changes (doping) and a segregated Nb-rich surface. The main finding of this work is the enhanced cycling stability and lower impedance of the surface-modified compound. We argue that surface densification is mitigated by the Nb doping/surface modification.