Surface ultrathin and uniform spinel structure induced by boron and fluorine dual doping for enhanced structure stability of Li-Rich layered oxides

Lithium-rich layered oxides (LLOs) cathode materials possess a highly desirable theoretical specific capacity owing to their special Li-O-Li units with unhybridized O-2p orbitals. However, LLOs suffer from some problems such as low initial coulombic efficiency (ICE), poor rate performance, and fast capacity decay owing to irreversible oxygen loss and structural decay during cycling. To address these issues, NH4BF4 (NBF) treatment is proposed to achieve dual doping with fluorine and boron while inducing a layered@spinel heterostructure on the surface of the LLO materials. The stable surface spinel layer protects the inner layered material from electrolyte corrosion and provide three-dimensional Li+ diffusion channels. The dual-doping of B and F alters the electronic structure, relieves internal charge hysteresis, and eventually inhibits irreversible oxygen release and transition metal migration by decreasing the energy bands of TM 3d-O 2p and non-bonding O-2p to lower energies. Consequently, the treated sample demonstrate an improved electrochemical performance with a high ICE of 90.0 % and a capacity retention of 91.0 % at 0.1 C after 100 cycles.