Synthesis And Modification Of High-voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Material

Lithium-ion batteries play an essential role in the electronics market,and high energy density and power density lithium-ion batteries for new energy field continue to receive extensive research.High-voltage spinel LiNi_0.5Mn_1.5O₄（LNMO）cathode material is considered as one of the ideal cathode materials for next-generation high-performance Li-ion batteries because of its high energy density(650 Wh kg^-1),high operating voltage（～4.7 V vs.Li/Li⁺）,low manufacturing cost and environmental friendliness.However,the high operating voltage makes LNMO susceptible to side reactions with organic electrolyte during cycling,resulting in severe Mn dissolution and structural degradation leading to rapid capacity decay,which limits its wide application and commercialization.This paper mainly solves the problems of the LNMO cathode material,and use Sr element doping to modulate the crystal surface,in-situ liquid-phase method to simultaneously achieve CeF₃ coating and Cedoping to optimize the structural interface,and piezoelectric material LiTaO₃ coating to modify the LNMO cathode material in order to improve its crystal structure stability and electrochemical performance.The specific work of this thesis is summarized as follows:（1）In situ Sr elemental doping modification was successfully achieved by the sol-gel method.The investigation results show that the Sr doping makes the particle size of the material more uniform,and the original spinel structure becomes a truncated octahedral structure,exposing a more stable（100）crystal plane with enhanced structural stability.The less Mn³⁺content in the Sr-doped material inhibited the disproportionation reaction and dissolution of Mn.The capacity retention of the sample with 0.1 doping was 86.63%after 500 cycles at 1C,and the discharge capacity was up to 110 m Ah g^-1 at a large rate of 20C,which was much higher than that of the unmodified LNMO with excellent electrochemical performance.Sr doping effectively reduced the electrode polarization and charge transfer resistance and enhanced the diffusion of Li⁺.（2）A comprehensive modification of the LNMO cathode material by CeF₃cladding and Cedoping was achieved in one step by in-situ liquid-phase method.the CeF₃ cladding layer alleviated the dissolution of transition metal ions and side reactions between the LNMO surface and electrolyte,enhancing structural stability,while the surface Cedoping improved ionic conductivity.the CeF₃ modification effectively improved the interfacial stability and improved the material’s electrochemical kinetic behavior,fast Li⁺diffusion and ideal pseudocapacitive behaviour resulted in the best electrochemical performance of the 1wt%-CF electrode.The capacity retention was87.01%after 400 cycles at 1C,and still 83.98%after 200 cycles at 55°C.The full cell composed of 1wt%-CeF₃ modified LNMO and Li₄Ti₅O₁₂ as the negative electrode also had good cycling performance.This study provides a solution for the preparation of low-cost high-performance LNMO cathode materials.（3）A coating layer of LiTaO₃ piezoelectric material is introduced to actively power Li-ion transport.A series of characterizations confirm that LiTaO₃ is uniformly coated on the surface of the high-voltage LNMO cathode material,and the coating has almost no effect on its structure and morphology.The capacity retention after 500 cycles at 1C is 86.35%and has excellent multiplicative performance.In addition,the LiTaO₃ coating modification reduces the charge transfer resistance and improves the diffusion coefficient of Li⁺.This can be attributed to the fact that the presence of the coating layer significantly promotes the migration of Li⁺during the discharge process,optimizes the interface of the material,and exhibits more excellent electrochemical properties.