Titanium and fluorine synergetic modification improves the electrochemical performance of Li(Ni0.8Co0.1Mn0.1)O2

Nickel-rich layered oxides (LiNixCoyMn1-x-yO2) (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(Ni0.8Co0.1Mn0.1)O-2 cathode through a solid phase reaction using the precursors of TiO2 and NH4F. Combining the characterization results of XRD, Ar sputtering assisted XPS, HRTEM, in situ XRD, etc, it is illustrated that Ti4+ and F- co-modification can synergistically modulate the lattice parameter and the Ni2+/Li+ mixing degree for the Li(Ni0.8Co0.1Mn0.1)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. Ti4+ 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 Ti4+ 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 degrees 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 x 10(-11) cm(2) s(-1) and 7.14 x 10(-12) cm(2) s(-1) respectively, larger than those of the pristine NCM (1.37 x 10(-11) cm(2) s(-1) and 4.52 x 10(-12) cm(2) s(-1)). The Ti4+ 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.