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 Li 2 MnO 3, a layered compound Li[Li 1/3 Mn 2/3 ]O 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 °C this occurs exclusively by oxidation of the nonaqueous electrolyte, thus generating H + which exchange one-for-one with Li + to form Li 2 - x H x MnO 3 . 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 °C initial Li removal is accompanied by oxygen loss (effective removal of Li 2 O) but further Li + removal involves the same proton exchange mechanism as observed at 55 °C. The reaction is partially reversible. On extended cycling the material converts to spinel.