Revealing Roles of Co and Ni in Mn-Rich Layered Cathodes

Pressing demand for reducing the dependence of the costly Co and Ni elements has recently made Mn-rich layered oxides much more attractive as potential cathodes for rechargeable lithium-ion batteries. Although the Co and Ni effects in Ni-rich cathodes are investigated in detail, there is still a serious lack of fundamental understandings of the influences of the Co and Ni substitution on the structural and electrochemical properties of the Mn-rich layered cathodes. Here, Co-substituted, Co and Ni co-substituted, and Ni-substituted Mn-rich layered cathodes Li(Mn, Ni, Co)O-2 are consciously designed to explore the roles of Co and Ni. These results elucidate that Co4+ is destructive for the structural stability of Mn-rich cathodes due to the increased instability in the lattice oxygen, especially at high potentials, thereby delivering poor cycling stability, while Ni substitution of Co introduces Li/Ni mixing to enhance the lattice oxygen stability and suppress phase segregation and irreversible phase transition, leading to much improved cycling stability. These findings complement the elemental chemistry of the Co-, Ni-, and Mn-based layered oxide cathodes, and benefit the development of Co-free Mn-rich layered cathode materials for future rechargeable lithium-ion batteries.