Structural Evolution of High Capacity Li-Rich 0.5Li2MnO3*0.5LiMO2 (M=Mn, Ni and Co) Cathode Materials during Electrochemical Cycling

Li-rich cathode materials with a general chemical formula (x)Li₂MnO₃*(1-x)LiMO₂ (M= Mn, Ni and Co) are in spotlights since last few years. They offer reversible capacities in excess of ~250 mAh/g when charged above 4.5 V, which is significantly higher than that delivered by the commercially available LiCoO₂ (~140 mAh/g) cathode material, and exhibit high rate capabilities[1]. However despite their attractive properties, no clear consensus exists on how the two components, namely Li₂MnO₃ and LiMO₂ of Li-rich materials coexist and how the structure evolves upon cycling. Herein, we report bulk and surface structural modifications in Li-rich cathode materials during electrochemical cycling using X-ray absorption spectroscopy (XAS). In-operando XAS measurements in the transmission mode at the Mn, Ni and Co K-edges of the material provided information about bulk structural changes, while the O K-edge Near-Edge X-ray absorption fine structure (NEXAFS) measurements in the total-electron yield (TEY) mode provided information about the near-surface structural modifications.

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[2][3]. 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[4]. 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[4]. 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 2^nd, 10^th and 25^th 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 1^st 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[4][5]. In case of Li-rich materials, however, none of these components show degradation after 25^th 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[4].

References:
[1] J. Li, R. Klöpsch, M.C. Stan, S. Nowak, M. Kunze, M. Winter, S. Passerini, J. Power Sources 2011, 196, 4821.
[2] J. Rana, R. Kloepsch, J. Li, T. Scherb, G. Schumacher, M. Winter, J. Banhart, J. Mater. Chem. A 2014, 2, 9099.
[3] J. Rana, R. Kloepsch, J. Li, M. Stan, G. Schumacher, M. Winter, J. Banhart, 2015, In preparation.
[4] J. Rana, M. Stan, R. Kloepsch, J. Li, G. Schumacher, E. Welter, I. Zizak, J. Banhart, M. Winter, Adv. Energy Mater. 2014, 4, 1.
[5] S.-C. Yin, Y.-H. Rho, I. Swainson, L.F. Nazar, Chem. Mater. 2006, 18, 1901.