Titanium and fluorine synergetic modification improves the electrochemical performance of Li(NiCoMn)O

Nickel-rich layered oxides (LiNi x Co y Mn 1− x − y O 2 ) ( x ≥ 0.8, NCM) are intensively developed cathode materials for lithium-ion batteries owing to their high energy and low price, however, their application is impeded by poor cycle stability. Herein we explored a Ti and F co-doped Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 cathode through a solid phase reaction using the precursors of TiO 2 and NH 4 F. Combining the characterization results of XRD, Ar sputtering assisted XPS, HRTEM, in situ XRD, etc , it is illustrated that Ti 4+ and F − co-modification can synergistically modulate the lattice parameter and the Ni 2+ /Li + mixing degree for the Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 cathode material. Particularly, density functional theory (DFT) calculations demonstrate that Ti and F co-doping is beneficial to form stable crystal structures with a layered phase and rock-salt phase. Ti 4+ and F − co-dopants induce the formation of an ultra-thin rock-salt phase on the cathode surface, which provides a protective layer on the nickel-rich cathode surface, so as to enhance the electrochemical performance. The optimal Ti 4+ and F − co-doped sample 0.5Ti@0.5F-NCM shows a superior discharge capacity of 202.2 mA h g −1 at 1C and 45 °C, and a capacity retention of 88.1% after 200 cycles, much higher than the retention of 45.2% for NCM. For 0.5Ti@0.5F-NCM, the lithium-ion diffusion coefficients after the 1st and 100th cycles are 2.67 × 10 −11 cm 2 s −1 and 7.14 × 10 −12 cm 2 s −1 respectively, larger than those of the pristine NCM (1.37 × 10 −11 cm 2 s −1 and 4.52 × 10 −12 cm 2 s −1 ). The Ti 4+ and F − co-doping can suppress the H2-H3 phase change of the cathode during the charge and discharge process and reduce the charge transfer resistance. The results provide a simple and feasible design strategy via cation@anion dopants to boost the electrochemical performance of nickel-rich cathodes for lithium-ion batteries. Ti 4+ and F − co-dopants expand the lattice spacing of Ni-rich cathode materials and form ultra-thin rock salt phases on the surface of the cathode, thereby improving the electrochemical performance of lithium-ion batteries.