Structural Evolution of High Capacity Li-Rich 0.5Li2MnO3*0.5LiMO2 (M=Mn, Ni and Co) Cathode Materials during Electrochemical Cycling
It is observed that the type and amount of transition metal (TM) ions determine the structural integration between the two components of Li-rich materials. For instance, higher Co contents tend to reduce the formation of Li2MnO3-type domains and promote a layered Li(1+x)MO2-type structure, while higher Mn and Ni contents give rise to the formation of a large amount of well-ordered Li2MnO3 domains. During activation, lithium extraction from Li2MnO3 domains occurs with a concurrent removal of oxygen, giving rise to the formation of a layered MnO2-type structure. The observed preferential reduction in the amplitude of the first peak (Mn-O) in Fig.1, representing the first coordination shell of oxygen atoms around the central Mn atom, also confirms the oxygen removal from the material during activation. Lithium reinsertion into a layered MnO2-type structure formed during activation, gives rise to the formation of a structure which is similar to the original Li2MnO3, but Li and O deficient. Consistent structural modifications were observed for the O K-edge NEXAFS spectra of the material during activation (Fig.2). It is observed that oxygen removal occurs only during the first cycle, and presumably, it is the necessary step in activating the Li2MnO3 component of Li-rich materials. During subsequent cycles, lithium extraction and reinsertion in Li2MnO3 domains may involve the unusual participation of oxygen anions into redox processes. The average valence state of Mn in Li2MnO3 domains remains 4+ at all times. Quite contrarily, electrochemical processes in LiMO2 domains involve conventional redox processes of TM ions. Lithium deintercalation from LiMO2domains results into the formation of O1-type stacking faults within the O3 structure, while lithium intercalation reverts these O1-type stacking faults back to the original O3-type.
In order to clarify structural modifications occurring during cycling, we investigated charged and discharged samples in the 2nd, 10th and 25th cycles. It is observed that after the initial oxygen release during activation, no major changes occur in Li2MnO3 domains during subsequent cycling. On the other hand, structural changes in LiMO2 domains are also quite similar to those observed during the 1st cycle. Interestingly, there was no indication of a spinel-like phase being formed upon cycling, as speculated in the literature. As reported previously, both Li2MnO3 and LiMO2 experience structural degradation when used independently as cathode materials in cells and cycled above 4.5 V. In case of Li-rich materials, however, none of these components show degradation after 25th cycle. These results highlight the role of both the components in Li-rich materials. Apparently, the randomly distributed Li2MnO3 domains in Li-rich materials act as a source of excess lithium for the redox-active LiMO2 domains and impart greater overall structural stability above 4.5 V. On the other hand, conventional redox processes of TM ions in the LiMO2 component help to overcome parasitic side reactions in the Li2MnO3 component above 4.5 V involving the electrolyte.
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