Preparation And First Principle Study Of Lithium Ion Battery Anode Material Li₄Ti₅O₁₂

The spinel-type lithium titanate(Li₄Ti₅O₁₂)has "zero strain" performance,that is,the cell volume changes very small after lithium insertion.Therefore,it has extremely high cycle life and stability,and is considered to be one of the most promising anode materials for lithium ion batteries.For the defects of low electronic conductivity and ionic conductivity,the performance is usually improved by carbon coating and ion doping.In this paper,Li₄Ti₅O₁₂ anode material was prepared by high temperature mechanochemical method and its modification was studied.The effects of ball milling process parameters,carbon coating and Cu²⁺doping on the performance of Li₄Ti₅O₁₂ were discussed by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,constant current charge and discharge,cycle and rate performance test,cyclic voltammetry（CV）and AC impedance test（EIS）,and the phenomena in the experiment were explained by first-principles calculation.The effects of ball milling temperature,milling time,ball-to-powder weight ratio and lithium-titanium molar ratio n（Li）/n（Ti）on the phase composition,morphology and electrochemical properties of Li₄Ti₅O₁₂ were investigated.The results show that Li₄Ti₅O₁₂ has the best performance when the ball milling temperature is 750℃,the milling time is 6 h,the ball-to-powder weight ratio is 20:1,and n（Li）/n（Ti）=0.84.The size of the particles is basically between 0.1 μm and 1μm,the first discharge specific capacity is 157.8 mAh·g-1 at 0.1C,and the first discharge specific capacity is 148 mAh·g-1 at 0.5C,after 20 cycles,the specific capacity is still stable at 140 mAh·g-1.Based on the optimal experimental conditions,the effects of carbon coating and Cu²⁺doping on the composition,morphology and electrochemical properties of Li₄Ti₅O₁₂ were investigated.The results show that carbon coating does not change the spinel structure of lithium titanate and can reduce its agglomeration at high temperatures.Among the three carbon sources of citric acid,glucose and sucrose,the coating effect of glucose is the best.After the coating,the transfer resistance of Li₄Ti₅O₁₂ is reduced by 2/3,and the electrochemical performance is obviously improved.After 50 cycles at 1C,the capacity remains at 145 mAh·g-1.The low concentration（x=0.1）of Cu²⁺doping does not change the phase composition of Li₄Ti₅O₁₂.When the doping amount x=0.15 and x=0.2,the weak diffraction peak of rutile phase TiO2 appears in the product,and the particles of the product doped with Cu²⁺are fine and uniform.The transfer resistance is significantly reduced,and the cycle performance is significantly improved.The performance of the sample is best when the doping amount x=0.15,the first specific discharge capacity at 1,2,5,10C is 145.9,132.5,111.1,89.3 mAh·g-1,respectively.The AC impedance test shows that the lithium ion diffusion coefficient is increased by two orders of magnitude.The first principle study of Li₄Ti₅O₁₂ materials from a microscopic perspective shows that the volume of Li₄Ti₅O₁₂ shrinks by 1.04%after lithium insertion;the density of states shows that Li₄Ti₅O₁₂ is a material with insulator property,while Li₇Ti₅O₁₂ is with metal property;the charge density map and the charge differential density map confirmed that there is a strong covalent bond between Ti and O,which is beneficial to the stability of Li₄Ti₅O₁₂ structure;Cu²⁺doping is to form an impurity band near the Fermi level to make d orbital electrons jumping between the 16d positions of the octahedron to redistribute the charge,reduce the band gap,and improve the electronic conductivity of Li₄Ti₅O₁₂.