High Performance Li₂Ru1–yMnyO₃ (0.2 ≤ y ≤ 0.8) Cathode Materials for Rechargeable Lithium-Ion Batteries: Their Understanding

Understanding the origin of the high capacity displayed by Li 2 MnO 3 –LiMO 2 (M = Ni, Co) composites is essential for improving their cycling and rate capability performances. To address this issue, the Li 2 Ru 1– y Mn y O 3 series between the iso-structural layered end-members Li 2 MnO 3 and Li 2 RuO 3 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 (Ru 5+ /Ru 4+ ) during cycling together with a partial activity of the Mn 4+ /Mn 3+ redox couple. Moreover, we provide direct evidence for the reversibility of the O 2– → 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.