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

Abstract

Nickel-rich layered oxides (LiNi_xCo_yMn_1−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(Ni_0.8Co_0.1Mn_0.1)O₂ 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(Ni_0.8Co_0.1Mn_0.1)O₂ 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 °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 cm² s⁻¹ and 7.14 × 10⁻¹² cm² s⁻¹ respectively, larger than those of the pristine NCM (1.37 × 10⁻¹¹ cm² s⁻¹ and 4.52 × 10⁻¹² cm² 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.