Preparation And Bulk Doping Modification Of Micron-sized Single Crystal LiNi_0.5Mn_1.5O₄

With the vigorous development of lithium-ion battery energy storage technology,the research interested in high energy density cathode materials is also increasing.Among them,the spinel LiNi_0.5Mn_1.5O₄ has attracted more and more attention due to its advantages such as high energy density and power density,high working voltage and three-dimensional lithium-ion diffusion channels.However,the high working voltage of spinel LiNi_0.5Mn_1.5O₄ will accelerate the oxidation and decomposition of the electrolyte,destroy the stability of the interface between the material and the electrolyte,and cause rapid deterioration of battery performance.Besides,the poor structural stability of the traditional secondary particles is easy to rupture during the process of repeated charging and discharging,as a result,the increased contact area between the material and the electrolyte intensifies the corrosion effect of the electrolyte on the material,which greatly shortens the service life of the battery.Therefore,this thesis uses the alcohol gel solvent method to prepare the LiNi_0.5Mn_1.5O₄cathode material.The electrochemical performance test results prove that the cycle stability of the micron-sized single crystal LiNi_0.5Mn_1.5O₄material has been improved significantly.In order to further improve the specific capacity and rate performance of the material,the thesis uses Mg to replace the Ni and Mn elements in the single crystal LiNi_0.5Mn_1.5O₄ respectively to investigate the influence of the two doping methods on the electrochemical performance of the material.The main research contents and results are as follows:（1）Micron-sized single crystal LiNi_0.5Mn_1.5O₄ materials with different particle sizes were synthesized by alcohol gel solvent,and the influence of single crystal particle size on battery performance was explored.The electrochemical test results show that the single crystal LiNi_0.5Mn_1.5O₄ material with an average particle size of 4.429μm possesses superior electrochemical performance.It has a capacity retention rate of 87.3%after 500 cycles of 1 C at25℃.Even at a high temperature of 55℃,the capacity retention rate after 200 cycles at a rate of 1C can reach 76.65%.The cyclic performance of micron-sized single crystal material is better than the secondary particle LiNi_0.5Mn_1.5O₄ material prepared by conventional methods.The results of X-ray photoelectron spectroscopy characterization and in-situ electrochemical impedance spectroscopy test indicate that the micron-level single crystal material effectively inhibits the interface side reaction between the material and the electrolyte and forms thinner CEI film.The research results demonstrate that single crystallization of large particles is a feasible strategy to improve the cycle stability of high-pressure materials,and reveals the relationship between preparation conditions,multi-scale microstructure and electrochemical performance.The established alcohol gel solvent method is simple and easy to implement,which is conducive to industrial production.（2）Because of the larger particle size of micron-level single crystal material,the discharge specific capacity and rate performance of the material are reduced to a certain extent.Therefore,in order to improve these shortcomings,the micron-level single crystal LiNi_0.5Mn_1.5O₄ material was further doped with Mg to form LiNi_0.47Mg_0.03Mn_1.5O₄ and LiNi_0.5Mn_1.47Mg_0.03O₄ by replacing Ni and Mn respectively.The effects of two doping methods on the morphology,particle size,crystal type and electrochemical properties of the material were studied.According to the electrochemical test results,when Mg replaces Mn,the prepared LiNi_0.5Mn_1.47Mg_0.03O₄ material has higher electrochemical performance,which is higher than that of the LiNi_0.47Mg_0.03Mn_1.5O₄material.According to analysis,the incorporation of Mg²⁺reduces the overall average valence of the elements in the material.The Jahn-Teller effect is suppressed,and the cycle stability of the material is improved.Moreover,the doping of Mg also significantly increases the lattice volume of the material,broadens the transmission space of Li⁺,and improves the rate performance of the material.In summary,as a high-voltage cathode material,spinel LiNi_0.5Mn_1.5O₄ has a wide range of application prospects.Aiming at the essential shortcomings of LiNi_0.5Mn_1.5O₄ material,this thesis carried out micron-level single crystallization and Mg selective doping modification,which reduces the side reaction between the material/electrolyte and the dissolution of the transition metal in the material,thereby significantly improving the rate performance of the single crystal material.It effectively enhances the electrochemical behavior of the spinel LiNi_0.5Mn_1.5O₄ material,and provides guidance for its industrial application.