Suppressing capacity fading and voltage decay of Li-rich layered cathode material by a surface nano-protective layer of CoF₂ for lithium-ion batteries

Li-rich layered oxides have been regarded as valuable cathode materials for high energy density lithium-ion batteries. However, high initial irreversible capacity, bad rate capability, as well as serious capacity fading and voltage decay hinder their commercial application. In this paper, a nano CoF₂ protective layer is coated on the surface of Li_1.2Ni_0.2Mn_0.6O₂ via a facial wet chemistry method. A high initial discharge capacity of 264.4 mAh g⁻¹ is obtained for 0.5% CoF₂-coated sample and 259.1 mAh g⁻¹ for 1% CoF₂-coated sample owing to the suppression of irreversible release of O₂ and the contribution of electrochemical conversion of CoF₂/LiF. Furthermore, 1% CoF₂-coated sample exhibits the excellent rate capability of 167.5 mAh g⁻¹ at 5 C rate, the superior cycling stability with the capacity retention of 241.0 mAh g⁻¹ and the ameliorative voltage drop of 0.312 V at 0.1C after 100 cycles. The enhanced rate performance as well as stability of capacity and voltage can be attributed to the nano coatings which inhibit the electrolyte-electrode side reaction, enhance the electrochemical kinetics and mitigate structure transition from layered to spinel phase.