Study On Preparation And Modification Of Disordered Lithium-rich Cathode Materials For Lithium Ion Batteries

With the increasing demand for lithium-ion batteries with high electrochemical performance,it is urgent to find a new class of cathode materials with high energy density.At present,the successful research on cation disordered lithium-rich cathode materials which subvert the traditional cognition shows that disordered lithium-rich oxides are one of the high energy density cathode materials with great development prospects.However,the further application of this material still faces several challenges,including the generation of irreversible capacity,large polarization,poor cycle stability,etc.In order to speed up the development of cationic disordered cathode materials,researchers have optimized the synthesis process and studied the modification of material defects by synthesizing a new disordered cathode material system.The main purpose of this paper is to provide ideas for the synthesis and modification of cationic disordered lithium-rich cathode materials,so as to improve the comprehensive electrochemical performance of batteries.Therefore,cationic-disordered cathode material 0.4Li₂TiO₃·0.4LiMnO₂(Li_1.2Ti_0.4Mn_0.4O₂)was successfully designed and synthesized by solid-state method.In order to further optimize the materials,the paper carried out a series of researches on process conditions,structural design and material modification.The research contents are as follows:1.Firstly,cationic-disordered cathode materials were synthesized by solid-state method on the basis of nanometer Mn₂O₃.This sample was confirmed to be cationic disordered material by various tests.Subsequently,the process conditions were systematically studied,such as lithium temperature,wet solid-state ball-milled time,lithium content,and carbon mixing during the second solid-state ball-milled.The results show that when the lithium temperature is 950℃,the solid-state ball-milled time is 3 h,the lithium content is 1.2,and the carbon content is 18%,the prepared oxide is completely disordered.The disordered material can be regarded as a solid solution consisting of Li₂TiO₃ and LiMnO₂,in which Mn³⁺participates in the redox reaction to provide capacity.When lithium content reaches or exceeds the percolation threshold in disordered materials,Li⁺can migrate rapidly in 0-TM percolation network.The optimum ratio of Li_1.2Ti_0.4Mn_0.4O₂ in the binary system was confirmed by exploring the technological conditions,and the first discharge capacity up to 165mAh g^-1 at the rate of 0.1 C.In addition,it has relatively good comprehensive cycle stability.2.In order to solve the problem of poor conductivity and structural instability caused by excessive oxygen loss in cationic disordered cathode materials,carbon composite cationic disordered cathode material Li_1.2Ti_0.4Mn_0.4O₂/C was designed and synthesized by one-step solid-state method based on previous research.The results show that the addition of carbon in the precursor mixing process can form an obvious composite carbon layer.The composite carbon layer not only inhibits the growth of the initial particles,but also significantly improves the conductivity of the disordered materials,which greatly reduces the impedance during the electrochemical cycle.Finally,Li⁺diffuse rapidly in the percolation network.The first coulomb efficiency,charge-discharge capacity and cyclic stability of carbon composite disordered material prepared by one-step solid-state method were significantly improved.The optimum carbon composite content is 18%.The first discharge specific capacity of carbon composite disordered materials is up to 178 mAh g^-1 at 0.1 C.Moreover,the capacity retention is 76.1%after 20 cycles.One-step solid-state synthesis of carbon composite materials provides a new direction for the performance optimization of disordered materials.3.In order to further improve the electrochemical performance of the cationic disordered cathode material,the modification of the oxide was studied by F-doping.The results show that F-doping can further inhibit the loss of oxygen and reduce the average valence of anions,so that more transition metal ions participate in the reaction.In addition,F^-is completely doped into the lattice instead of forming the second phase on the surface of the material.At the same time,there is an obvious composite carbon layer to improve the conductivity of the material.It also shows that Li_1.2Ti_0.4Mn_0.4O_1.88F_0.12 is the optimum molecular formula for F-doping content;the first discharge capacity of F-doped disordered material up to 245 mAh^-1 at 0.1 C during 1.5-4.8 V;the capacity retention rate of it is 83.1%for 20^th,and the discharge capacity of 175.2 mAh^-1 at a rate of 2 C.This confirms that F-doping can improve the electrochemical properties of cationic disordered cathode materials.