Structural Changes in Li2MnO3 Cathode Material for Li-Ion Batteries

Structural changes in Li2MnO3 cathode material for rechargeable Li-ion batteries are investigated during the first and 33 rd cycles. It is found that both the participation of oxygen anions in redox processes and Li+-H+ exchange play an important role in the electrochemistry of Li2MnO3. During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO2-type structure, while the presence of protons in the interslab region, as a result of electrolyte oxidation and Li+-H+ exchange, alters the stacking sequence of oxygen layers. Li re-insertion by exchanging already present protons reverts the stacking sequence of oxygen layers. The re-lithiated structure closely resembles the parent Li2MnO3, except that it contains less Li and O. Mn4+ ions remain electrochemically inactive at all times. Irreversible oxygen release occurs only during activation of the material in the first cycle. During subsequent cycles, electrochemical processes seem to involve unusual redox processes of oxygen anions of active material along with the repetitive, irreversible oxidation of electrolyte species. The deteriorating electrochemical performance of Li2MnO3 upon cycling is attributed to the structural degradation caused by repetitive shearing of oxygen layers. Structural changes in Li2MnO3 provide evidence for the unusual electrochemical processes such as Li+-H+ exchange and the participation of oxygen anions in redox reactions. The observed decline in the electrochemical performance of the material upon cycling, mainly attributed to the Li+-H+ exchange and associated structural changes, also provides a global explanation for the reported cycling behavior of Li2MnO3-containing Li-rich cathode materials. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.