Study of the electrochemical behavior of the "inactive" Li ₂MnO ₃

In this work, we studied the cycling performance of initially inactive Li ₂MnO ₃ electrodes prepared from micron-sized particles, at 30 °C and 60 °C and possible structural transitions that this material can undergo due to de-lithiation. It was found that being activated at elevated temperatures, Li ₂MnO ₃ electrodes demonstrate a steady-state cycling behavior and reasonable capacity retention after aging at 60 °C. The main gases evolved during polarization of the Li ₂MnO ₃ electrodes are O ₂ evolved from the structure and CO ₂ and CO that can be formed due the reaction of oxygen with carbon black. It was found that a transformation of the Li ₂MnO ₃ layered structure into a spinel-like phase occurred during the initial charging of the Li ₂MnO ₃ electrodes, which were characterized as possessing domains of both layered and spinel-like structures. The results of the structural studies of these electrodes obtained by the X-ray diffraction and transmission electron microscopy were found to be in agreement with their Raman spectroscopic responses. We suggest that the mechanism of the charge compensation during the extraction of lithium at 60 °C involves both oxygen removal from the Li ₂MnO ₃ structure and the exchange between Li ⁺ and protons formed during the anodic oxidation of ethylene carbonate or dimethyl carbonate solvents in LiPF ₆ solutions at high potentials (>4.5 V). It is assumed that the proton-containing structure Li _2-xH _x-yMnO _3-0.5y is retained in a discharged state of the electrode and may decompose above 500 °C with the formation of Li ₂O and manganese oxides accompanied by the release of water and CO ₂.