Anionic Redox in Li-Rich Rocksalt Oxides Studied Via X-Ray Photoelectron Spectroscopy

Abstract

Layered LiMO2 compounds can be considered as a progressive substitution of excess Li+ for M3+ in the [MO2] layers, with a 1:1 Li:M ratio. A derivative from this type of compound corresponds to a material with the general formula of Li(Li1/3M2/3)O2. This has been alternatively described with the formula Li2MO3 (where M has been reported as Mn, Ru, Ti, Zr, Sn, Pt), and often referred to as a “Li-rich” rocksalt oxide (with Li/M > 1 and space group C2/m). Li-rich rocksalt oxides are of technological importance as cathode materials in rechargeable Li-ion batteries. Recently, the possibility of using oxygen as a redox centre in this kind of oxides has been established in the design of cathode materials with enhanced capacities (> 200 mAh/g). [1-3] Among the numerous compounds with Li-rich Li2MO3 structure type, Li2MnO3 is the most studied so far; the combination with other transition metals such as Co and Ni can help to improve the capacity, rate capability and reduce the capacity fading. In this work, a study of a series of Li-rich compounds with rock salt structure was synthesised with a general formula of Li1+δ(Ni1-x-yCoxMny)1-δO2 and electrochemically cycled versus lithium metal. An increase in the specific capacity compared with materials of the same family such as layered LiNi1/3Co1/3Mn1/3O2 was observed, in line with numerous reports by research groups worldwide. In this presentation, X-ray photoemission spectroscopy of O1s taken at different stages of the load curve of the Li1+δ(Ni1-x-yCoxMny)1-δO2 electrode material will be presented. The experimental evidence pertaining to the oxidation of O2- driving to the formation of peroxo-like species (O2)n− in the material and charge compensating the extraction of Li+ ions by the removal of electrons from lattice oxygen; that allows this material to have more capacity than expected will be critically discussed.