Decreasing Li/Ni Disorder and Improving the Electrochemical Performances of Ni-Rich LiNi_0.8Co_0.1Mn_0.1O₂ by Ca Doping

Abstract

Decreasing Li/Ni disorder has been a challenging problem for layered oxide materials, where disorder seriously restricts their electrochemical performances for lithium-ion batteries (LIBs). Element doping is a great strategy that has been widely used to stabilize the structure of the cathode material of an LIB and improve its electrochemical performance. On the basis of the results of previous studies, we hypothesized that the element of Ca, which has a lower valence state and larger radius compared to Ni²⁺, would be an ideal doping element to decrease the Li/Ni disorder of LiMO₂ materials and enhance their electrochemical performances. A Ni-rich LiNi_0.8Mn_0.1Co_0.1O₂ cathode material was selected as the bare material, which usually shows severe Li/Ni disorder and serious capacity attenuation at a high cutoff voltage. So, a series of Ca-doped LiNi_0.8(1-x)Co_0.1Mn_0.1Ca_0.8xO₂ (x = 0-8%) samples were synthesized by a traditional solid-state method. As hypothesized, neutron diffraction showed that Ca-doped LiNi_0.8Co_0.1Mn_0.1O₂ possessed a lower degree of Li/Ni disorder, and potentiostatic intermittent titration results showed a faster diffusion coefficient of Li⁺ compared with that of LiNi_0.8Mn_0.1Co_0.1O₂. The Ca-doped LiNi_0.8Mn_0.1Co_0.1O₂ samples exhibited higher discharge capacities and better cycle stabilities and rate capabilities, especially under a high cutoff voltage with 4.5 V. In addition, the problems of polarization and voltage reduction of LiNi_0.8Mn_0.1Co_0.1O₂ were also alleviated by doping with Ca. More importantly, we infer that it is crucial to choose an appropriate doping element and our findings will help in the research of other layered oxide materials.