Mechanism of electrochemical activity in Li2MnO3

Lithium intercalation compounds based on lithium manganese oxides are of great importance as positive electrodes for rechargeable lithium batteries. It is widely accepted that Li+ may be extracted (deintercalated) from such lithium manganese oxides accompanied by oxidation of Mn up to a maximum oxidation state of +4. However, it has been suggested recently that further Li+ removal may be possible. Among the mechanisms that have been proposed to charge balance the removal of Li+ are Mn oxidation beyond +4 or loss of O-2(-). To investigate this phenomenon we have selected Li2MnO3, a layered compound Li[Li1/3Mn2/3]O-2.(2) with a ready supply of mobile Li+ ions but with all Mn already in the +4 oxidation state. We show that a substantial quantity of Li (at least 1.39 Li) may be removed. At 55 degreesC this occurs exclusively by oxidation of the nonaqueous electrolyte, thus generating H+ which exchange one-for-one with Li+ to form Li2-xHMnO3. The presence of H+ between the oxide layers results in a change of the layer stacking from O3 to P3, the latter being more stable for O-H-O bonding. At 30 degreesC initial Li removal is accompanied by oxygen loss (effective removal of Li2O) but further Li+ removal involves the same proton exchange mechanism as observed at 55 degreesC. The reaction is partially reversible. On extended cycling the material converts to spinel.