Structural Changes in a Li Rich 0.5Li2MnO3 0.5LiMn0.4Ni0.4Co0.2O2 Cathode Material for Li Ion Batteries A Local Perspective

Local structural changes in a Li rich 0.5Li2MnO3 amp; 8727;0.5LiMn0.4Ni0.4Co0.2O2 cathode material are investigated using X ray absorption spectroscopy XAS . The element selective nature of XAS revealed the composite structure of the material, where both Li2MnO3 and LiMn0.4Ni0.4Co0.2O2 components exist as separate domains and also exhibit a distinct electrochemical response. An irreversible oxygen release from Li2MnO3 domains contributes to a large irreversible capacity delivered by the material during activation and gives rise to the formation of a layered MnO2 type structure. Lithium reinsertion into this layered MnO2 type structure during discharge reforms the original Li2MnO3 type structure, which is lithium and oxygen deficient. The average valence state of Mn in Li2MnO3 domains remains unchanged at 4 during charge and discharge, suggesting an unusual participation of oxygen anions of Li2MnO3 domains in redox processes. On the contrary, electrochemical processes in LiMn0.4Ni0.4Co0.2O2 domains involve conventional redox processes of transition metal TM ions. In addition to Ni2 Ni4 and Co3 Co4 redox reactions, a small amount of Mn3 detected in LiMn0.4Ni0.4Co0.2O2 domains also participates in electrochemical processes via a Mn3 Mn4 redox reaction. All structural modifications introduced into the material during activation are recovered upon discharge to 2.5 V, except those caused by the permanent removal of oxygen from Li2MnO3 domain