Synthesis And Modification Of Single-crystal NCM811 Cathode Material

As consumer demands for higher energy density continue to rise,high-nickel layered cathode materials such as Li Ni_xM_1-xO₂（0.6≤x<1,M=Co,Mn,Al,etc.）have been considered as the most promising candidates for electric vehicle power batteries,due to their large reversible capacity（≥200 m Ah g-1）,high voltage（≥4.3 V）,and cost-effectiveness.However,as the nickel content increases,the defects of high-nickel ternary materials,such as microcracks and surface/interface reactions,gradually become apparent,hindering their further large-scale production.To address the aforementioned issues,this paper uses high-temperature molten salt Li Cl-KCl to synthesize highly-dispersed,single-crystal high-nickel ternary cathode materials with improved performance,by adjusting the sintering temperature and flux ratio.To provide a reliable solution for microcracks and surface/interface reactions,surface modifications such as aluminum fluoride coating and tetra-n-butyl titanate pre-treatment were employed.The specific research contents are as follows:（1）Through thermal gravimetric analysis of the molten salt system,the crystal growth mechanism is approximately revealed.By adjusting the sintering temperature and the amount of Li Cl-KCl high-temperature molten salt,single-crystal Li Ni_0.83Co_0.11Mn_0.06O₂ materials with different sizes and dispersions are synthesized.The results show that the ternary single crystal material prepared with a calcination temperature of 850℃and 25 wt%of 0.59Li Cl-0.41KCl has the best electrochemical performance,with a first discharge specific capacity of 178.34m Ah g^-1 at 1 C and a cycle retention of 85.69%after 200 cycles.Even at a high current density of 10 C,it still has a discharge specific capacity of 150 m Ah g^-1.（2）Through in-situ aluminum fluoride liquid-phase coating,the problem of uneven and discontinuous coating caused by traditional mechanical ball milling coating is overcome.The results show that the material has the best electrochemical performance when the aluminum fluoride coating amount is 2 mol%,with a first discharge specific capacity of 177.7 m Ah g^-1at 1 C and a cycle retention of 86.02%after 200 cycles.Even at a high current density of 10 C,it still has a discharge specific capacity of 151 m Ah g^-1.（3）Through pretreatment with tetrabutyl titanate and subsequent annealing,SC-NCM@2%TBOT composite positive electrode material exhibiting excellent electrochemical performance was achieved.A series of characterizations revealed that the tetrabutyl titanate pretreatment enabled synergistic modification through Ti⁴⁺doping and Li Ti₂O₄ encapsulation.Ti⁴⁺doping effectively broadened the lattice spacing,suppressed the H2-H3 phase transition,stabilized the layered structure,and improved the lithium ion diffusion rate while reducing electrochemical polarization.The uniform Li Ti₂O₄ coating layer effectively eliminated excess lithium salt,isolated surface/interface side reactions,improved the stability of surface/interface structure,enhanced the structural stability,and significantly improved the electrochemical performance of single-crystal Li Ni_0.83Co_0.11Mn_0.06O₂.Under 1C current density,the initial discharge specific capacity of the Ti4+-doped and Li Ti₂O₄-encapsulated single-crystal NCM811 positive electrode material reached 177 m Ah g^-1,with a capacity retention rate of 89%after 200 cycles,while the original single-crystal NCM811 positive electrode material exhibited specific capacities of 175 m Ah g^-1 and 84%,respectively.Clearly,the cycling performance of the modified single-crystal NCM811positive electrode material has been significantly improved.