Study On The Reaction-type Surface Modification Of High-voltage LiCoO₂

Since the commercialization of lithium-ion batteries,they have been widely used in 3C electronic devices because of their excellent performance characteristics,and have strongly contributed to the development of the global timely communication field,and nowadays they are also making a splash in electric vehicles,energy storage and aerospace.With the advent of the 5G era and the development of new energy vehicles,people have put forward higher requirements for the energy density of lithium-ion batteries.At present,the cathode material is the short board for the development of high energy density lithium-ion batteries,and increasing the de-embedded lithium level of the cathode material of lithium-ion batteries is one of the feasible strategies to achieve higher energy density lithium-ion batteries.Lithium cobalt oxide（LiCoO₂,LCO for short）is the earliest commercialized cathode material and still occupies most of the lithium-ion battery cathode material market for 3C products by virtue of its ultra-high volumetric energy density and excellent electrochemical performance.Increasing the application cut-off voltage of LCO to further improve its actual specific capacity is an effective measure to maintain the competitiveness of LCO in the market.However,the high application cut-off voltage greatly reduces the service life of LCO,partly because the crystal structure and volume of LCO change more drastically during the charging and discharging process,and partly because more drastic interfacial side reactions will occur between LCO and electrolyte.In this thesis,reaction-type surface coating is used as a means of modification,to investigate the modification mechanism and modification effect of different reaction-type coating materials on high voltage LiCoO₂,the main work is as follows:Firstly,the effect of surface modification of high-voltage LCO by electronic conductors was investigated.The nano-Sn（OH）₄was coated on the surface of LCO using the liquid phase method,and then sintered at high temperature.During sintering process,the nano-Sn（OH）₄ is easily converted to nano-SnO₂.Then a part of SnO₂ and LCO substrate generated a stable new phase Li₂Co₃SnO₈ by chemical reaction at high temperature,which effectively protected the substrate material,and part of SnO₂ produced oxygen defects under the action of high temperature to improve its own electronic conductivity and also greatly improve the electronic conductivity of LCO.The coated modified LCO exhibited excellent cycling and multiplicative properties.Secondly,the modification effect of ionic conductor coating on high voltage LCO was investigated.The nano-TiO₂,Li₂CO₃ and LCO were ground in a certain ratio and then heat treated at high temperature.At high temperature,TiO₂,Li₂CO₃ and LCO interact with each other,and part of the lithium in LCO is precipitated,forming lithium defects and also generating Li-Ti-O ionic conductor layer and stable spinel phase（Co3O4）on the surface of LCO by in situ reaction,and significantly changing the surface morphology of LCO.The coating layer can effectively protect the substrate material and improve the ionic conductivity of LCO,and improve the cycling stability and rate performance of high-votage LCO to a certain extent.Finally,NH₄F,Mg（NO₃）₂and LCO were made into a mixed aqueous solution in a certain ratio and heated with stirring.In this process F^-and Mg²⁺combine on the surface of LCO to form MgF₂,and the subsequent sintering process makes the F and Mg elements diffuse into the surface layer of LCO to form a Li-Co-Mg-O-F solid solution layer,which effectively stabilizes the surface structure of LCO and improves its cyclic stability under high-votage.However,MgF₂ is an insulator,and too much coating will increase the interfacial impedance of LCO and reduce its multiplicative performance.