Preparation,Structure And Lithium Storage Properties Of Mg-F Doped LiNi_0.9Mn_0.08Co_0.02O₂

Nickel-rich layered cathode materials,especially LiNi_0.8Mn_0.1Co_0.1O₂ and LiNi_0.8Co_0.15Al_0.05O₂,are the most promising cathode materials for automotive LIBs because of their high discharge capacity(>200 m Ah·g^-1)and high working voltage（≈3.6 V vs.Li⁺/Li）.However,the rapid growth of EVs and HEVs raises concerns over raw material availability for LIBs.The high cost and scarcity of cobalt will become the main obstacle to the wide application of these nickel-rich cathode materials in HEVs and EVs.Therefore,high-Ni and poor-Co layered cathodes have been developed to overcome this obstacle because a high Ni content means an increase in capacity,and a low Co content means a cost reduction.In this paper,a novel high nickel and poor cobalt layered cathode LiNi_0.9Mn_0.08Co_0.02O₂ was synthesized by oxalate precipitation followed by a solid-state reaction.The effects of calcination time and cation/anion doping on the structure,morphology and electrochemical properties of LiNi_0.9Mn_0.08Co_0.02O₂ were investigated by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,Inductively coupled plasma emission spectroscopy（ICP）,galvanostatic charge discharge technique,and electrochemical impedance spectroscopy（EIS）,etc.The stability of the layered structure can be enhanced by optimizing the calcination time.A suitable calcination time（25h）contributes to forming a broader Li slab and resulting in a lower cation mixing,thereby obtaining a more stable layered structure.The discharge capacity of LiNi_0.9Mn_0.08Co_0.02O₂prepared by calcination at 750℃for 25 h in O₂ is 218 m Ah·g^-1 at 0.1 C,and it still maintains 78%of its initial capacity after 100 cycles at 1 C.Even charged/discharged at 5 C,it still delivers a capacity of 140 m Ah·g^-1,exhibiting good cyclic stability and rate performance.F^-and Mg²⁺can enter the lattice of LiNi_0.9Mn_0.08Co_0.02O₂ to form solid solution.F^--Mg²⁺co-doping can effectively improve the cyclic stability.Compared with LiNi_0.9Mn_0.08Co_0.02O₂,Li_0.98Mg_0.02Ni_0.9Mn_0.08Co_0.02O_1.97F_0.03delivers a discharge capacity of the 1C discharge capacity of 191 m Ah·g^-1 with a capacity retention rate was 85.5%after 250 cycles at 1C,exhibiting better cyclic stability.The enhanced performance can be ascribed to the synergistic effect between F^-and Mg²⁺,which further reduces the degree of cation mixing and improves the stability of the layered structure.