Reversible Tetrahedral-Site Migration Inducing an Additional Charge Compensation Reaction in Li-Rich Layered Oxide 0.4Li₂MnO₃–0.6LiNi0.5Mn0.5O₂

The electronic and crystal structures of a Li-rich layered oxide (LLO), 0.4Li2MnO3–0.6LiNi0.5Mn0.5O2, are investigated during charging and discharging. The LLO band consists of hybridized transition-metal 3d and O 2p orbitals. Electron-induced X-ray emission spectroscopy (EXES) independently observes the energy distribution of each orbital, revealing the specific roles in the charge compensation accompanying electrochemical charging and discharging. In the initial irreversible charging, Ni is oxidized in the voltage-changing region but is reduced in the voltage-plateau region. Additionally, the local structure of Mn changes as it is oxidized, while that of O is continuously oxidized. Crystal pair distribution function (PDF) analysis reveals that the pristine oxide is a composite consisting of hexagonal LiMeO2 (where Me = Mn, Ni) and monoclinic Li2MnO3 phases. The constituent transition metals migrate substantially due to delithiation in the initial irreversible charging and eventually merge into a single LiMeO2 phase. The merged LiMeO2 phase shows two types of cation migrations: Ni migration to the octahedral Li site in the Li layer and Mn migration to the tetrahedral site in the Li layer. Mn(IV) migrating to the tetrahedral site during charging should form a partially occupied Mn t2 orbital due to orbital hybridization with the surrounding 2p orbitals. This should yield electrons, which can be extracted for further charging. Hence, the large capacity of the LLO electrode is attributed to the structural arrangements, which generate the available electrons for highly charged states.