Structural Changes in a Li-Rich 0.5Li2MnO3*0.5LiMn0.4Ni0.4Co0.2O2Cathode Material for Li-Ion Batteries: A Local Perspective

Local structural changes in a Li-rich 0.5Li 2 MnO 3 * 0.5LiMn 0.4 Ni 0.4 Co 0.2 O 2 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 Li 2 MnO 3 and LiMn 0.4 Ni 0.4 Co 0.2 O 2 components exist as separate domains and also exhibit a distinct electrochemical response. An irreversible oxygen release from Li 2 MnO 3 domains contributes to a large irreversible capacity delivered by the material during activation and gives rise to the formation of a layered MnO 2 -type structure. Lithium reinsertion into this layered MnO 2 -type structure during discharge reforms the original Li 2 MnO 3 -type structure, which is lithium and oxygen deficient. The average valence state of Mn in Li 2 MnO 3 domains remains unchanged at 4+ during charge and discharge, suggesting an unusual participation of oxygen anions of Li 2 MnO 3 domains in redox processes. On the contrary, electrochemical processes in LiMn 0.4 Ni 0.4 Co 0.2 O 2 domains involve conventional redox processes of transition-metal (TM) ions. In addition to Ni 2+ /Ni 4+ and Co 3+ /Co 4+ redox reactions, a small amount of Mn 3+ detected in LiMn 0.4 Ni 0.4 Co 0.2 O 2 domains also participates in electrochemical processes via a Mn 3+ /Mn 4+ 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 Li 2 MnO 3 domains.