Charging Mechanism of Li₂MnO₃

Abstract

Operando mass spectroscopy\ndemonstrates quantitatively that lithium\nextraction from Li₂MnO₃ is charge compensated\nby oxygen loss (O-loss) not oxidation of oxide ions that are retained\nwithin the structural framework (O-redox). This fact is confirmed\nby X-ray absorption and emission spectroscopy. Li NMR shows that the\ntwo-phase core–shell structure, which forms on charging, is\ncomposed of an intact Li₂MnO₃ core and a highly\ndisordered shell containing no Li, with a composition close to MnO₂. Discharge involves Li insertion into the disordered shell.\nCO₂ and O₂ are detected on charging at 15 mA\ng^–1, whereas charging by galvanostatic intermittent\ntitration technique (GITT) forms only CO₂; an observation\nin agreement with the previously described model of oxygen evolution\nfrom high-voltage cathodes producing singlet O₂ that reacts\nwith the electrolyte forming CO₂. The dominance of oxygen\nevolution over O-redox is in accordance with the model of O-loss occurring\nwhen the oxide ions are undercoordinated; O in the shell devoid of\nLi is coordinated by only 2 Mn.