Preparation And Modification Of Spinel Lithium Manganate By Hydrothermal Method

As an electrochemical energy storage technology,lithium ion battery plays an important role in the sustainable development of society and economy.Compared with traditional secondary batteries(lead-acid,nickel-metal hydride,and nickel-cadmium),lithium-ion batteries have higher energy density,longer service life,and higher operating voltages(in the 4 V range).In recent years,lithium-ion batteries have been widely used in hybrid vehicles,electric vehicles,and new portable devices.Traditional cathode materials for lithium-ion batteries mainly include lamellar Li Co O₂,spinel LiMn₂O₄and olivine Li Fe PO₄.Among them,spinel LiMn₂O₄with three-dimensional lithium ion migration channels has been considered as one of the most promising cathode materials due to its advantages of good safety,abundant raw materials,low cost and environmental friendliness.However,the rapid capacity decay of spinel LiMn₂O₄(especially at high temperatures)and poor performance at high rates limit its application.Aiming at the above problems,this paper continuously optimized the synthesis process of LiMn₂O₄material,and modified it by means of ion doping,aiming to obtain spinel LiMn₂O₄cathode material with excellent electrochemical performance.The specific research contents are as follows:(1)Study on the preparation of spinel lithium manganate by hydrothermal method.A simple and controllable hydrothermal synthesis route was used to investigate the effects of different process conditions,such as Li/Mn ratio,hydrothermal reaction temperature,the amount of reducing agent added and calcination temperature,on the crystal structure and morphology of LiMn₂O₄products,and to analyze the effects of different experimental conditions on the electrochemical properties of LiMn₂O₄.Under the conditions of lithium manganese molar ratio of 1,aniline/KMn O₄molar ratio of 0.2,hydrothermal temperature of 200?and calcination temperature of 650?,spinel LiMn₂O₄material with complete crystallization,uniform particle size and excellent electrochemical performance was obtained.The first specific discharge capacity reaches 139.9 m Ah·g^-1at 0.2 C,and the capacity retention rate is 63.76%after 100 cycles at room temperature.In the following research,the samples obtained under these conditions were doped to improve their cycling stability and rate performance.(2)Ti⁴⁺was used to doping LiMn₂O₄cathode material.LiMn_2-xTi_xO₄with different Ti doping amounts was prepared by hydrothermal method(x=0,0.01,0.02,0.03,0.04,0.05).It was found that the particle size of the sample material doped with Ti was slightly reduced and the particle dispersion was improved,which shortened the transport path of lithium ions and was conducive to the rapid insertion and removal of lithium ions.The electrochemical performance of the LiMn_1.97Ti_0.03O₄sample is the best when the doping amount of Ti is 0.03.At room temperature(25?),the first specific discharge capacity of LiMn_1.97Ti_0.03O₄at 0.2 C is 136 m Ah·g^-1,after 100cycles,the specific discharge capacity is 120.9 m Ah·g^-1,the capacity retention rate is88.9%,and the specific discharge capacity at 5 C is 96.5 m Ah·g-1.The specific capacity is up to 73.4 m Ah·g-1 at 10 C,and the electrochemical reversibility is excellent.At high temperature(55?),the T3 sample also had excellent circulability,and the capacity retention rate was 75.44%after 100 cycles.The cyclic voltammetry test and electrochemical impedance spectroscopy show that the diffusion coefficient of lithium ions is improved,and the doping of Ti⁴⁺increases the diffusion rate of lithium ions,and reduces the impedance of the electrode in the process of charge and discharge.(3)The doping modification of LiMn₂O₄cathode material was studied by Mg²⁺.LiMn_2-xMg_xO₄(x=0,0.01,0.02,0.03,0.04,0.05)with different Mg doping were prepared by hydrothermal method.The morphology of the synthesized sample is hexahedron or polyhedron,and the particle shape of the doped material is more regular.Among them,LiMn_1.97Mg_0.03O₄has excellent cycling performance at both room temperature and high temperature.At room temperature(25?),the first specific discharge capacity at 0.2 C is 124.3 m Ah·g^-1,and the capacity retention rate is94.29%after 100 cycles.The specific discharge capacity at 5 C is 94.8 m Ah·g^-1.The specific discharge capacity can reach 74.0 m Ah·g^-1at 10 C.At high temperature(55?),the initial discharge specific capacity is 125.6 m Ah·g^-1,and the capacity retention rate is 85.43%after 1 and 100 cycles.The results show that the Mg²⁺ion replaces part of Mn³⁺ion,alleviates the lattice distortion and stabilizes the crystal structure,thus greatly improving the cyclic stability.