A strategy to improve the electrochemical performance of Ni-rich positive electrodes: Na/F-co-doped LiNi0.6Mn0.2Co0.2O2 *

Abstract

Ni-rich layered lithium transition metal oxides LiNi x Mn y Co z O2 (1 – y – z ≥ 0.6) are promising candidates for cathode materials, but their practical applications are hindered by high-voltage instability and fast capacity fading. Using density functional theory calculations, we demonstrate that Na-, F-doping, and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi0.6Mn0.2Co0.2O2 (NMC622) during the charging process, which may attain greater discharge capacity. F doping may inhibit the diffusion of Li ions at the beginning and end of charging; Na doping may improve Li ion diffusion due to the increase in Li layer spacing, consistent with prior experiments. Na/F-co-doping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons: (i) a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and (ii) Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus, stabilizes the structure. This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification, which is a promising strategy for designing efficient Li ion batteries.