High Performance Li₂Ru_1–<i>y</i>Mn_<i>y</i>O₃ (0.2 ≤ <i>y</i> ≤ 0.8) Cathode Materials for Rechargeable Lithium-Ion Batteries: Their Understanding

Abstract

Understanding the origin of the high capacity displayed by Li2MnO3–LiMO2 (M = Ni, Co) composites is essential for improving their cycling and rate capability performances. To address this issue, the Li2Ru1–yMnyO3 series between the iso-structural layered end-members Li2MnO3 and Li2RuO3 was investigated. A complete solid solution was found, with the 0.4 ≤ y ≤ 0.6 members showing sustainable reversible capacities exceeding 220 mAh·g–1 centered around 3.6 V vs Li+/Li. The voltage–composition profiles display a plateau on the first charge as compared to an S-type curve on subsequent discharge which is maintained on the following charges/discharges, with therefore a lowering of the average voltage. We show this profile to evolve upon long cycling due to a structural phase transition as deduced from XRD measurements. Finally we demonstrate, via XPS measurements, the oxidation and reduction of ruthenium (Ru5+/Ru4+) during cycling together with a partial activity of the Mn4+/Mn3+ redox couple. Moreover, we provide direct evidence for the reversibility of the O2– → O– anionic process upon cycling, hence accounting for the high capacity displayed by these materials. This work, by capturing the main redox processes pertaining to these materials, should facilitate their development.