Investigation On Surface Modification Of LiCoO₂ Cathode Material For Lithium Ion Battery By Two Fast Ion Conductors

Lithium-ion batteries（LIBs）with high energy density,long cycle life,high safety,small size,no memory effect,environmental friendliness and other advantages have been widely used in portable electronic devices for energy conversion and storage as well as new energy electric vehicles.However,with the demand of people for higher energy density storage,LIBs with high-energy density and power density are still a upsurge for us to explore,and the development of high energy density,longevity life cathode materials is one of the key links.Since the LiCoO₂（LCO）cathode materials was first introduced into LIBs by Goodenough in 1980,and it has been attracting the attention of people,and first commercialized by Sony Corporation of Japan in 1991.Due to its high working voltage platform（～ 3.9 V vs Li/Li⁺）,large theoretical specific capacity(274 m Ah g^-1),high volume energy density,tap density and simple preparation process,layered LCO cathode material still in the limelight.In practical application,in order to give full play to its high specific capacity advantage,people get it by increasing the charging cutoff voltage.However,high voltage work environment LCO cathode material will face more serious side reactions.Such as the escape of the surface oxygen,surface interface side reactions,CEI membrane growth,internal resistance increase,Co metal ion dissolution,etc.,lead to the degradation of material surface structure,which is not conducive to deintercalation of Li⁺,and eventually lead to serious electrochemical performance decline（cyclic rate difference,etc.）and even safety hazards surface.In view of the above,LCO cathode materials have been modified by means of bulk doping and surface coating,and some results have been achieved.Among them,bulk doping could enhance the conduction of Li⁺/electrons,inhibiting the phase transition of LCO structure at the depth of Li⁺ removal,the cladding materials of surface coated modification could hinder the direct contact between the LCO active material and the electrolyte,preventing LCO from being corroded by electrolyte,reducing the dissolved amount of Co ions in electrolyte,alleviating CEI growth,so as to achieve the electrochemical performances of LCO materials can be improved comprehensively.In order to further explore the application potential of high-energy LCO,considering all-solid-state batteries development at the same time,the compatible interface issues between active material and electrolyte,based on the advantages of two kinds of modification methods（doping and coating）,in this work,fast ion conductor is used as modified materials,LCO active materials is modified through the path of surface coating-doping integration,so as to realize the electrochemical performances improvement of LCO at 4.5 V.Layered LCO cathode material is first synthesized by molten salt synthesis in this thesis.Subsequently,two fast ion conductor materials LiNbO₃（LNO）,Li₇La₃Zr₂O₁₂（LLZO）are coated on surface of LCO material by sol-gel method,respectively.According to a series of physical and chemical characterizations and analyses,the following results are obtained.We successfully coat LNO,a fast ionic conductor material with excellent electronic conductivity,on the surface of LCO material.With the increase of LNO coating amount,the surface of LCO sample would become rougher and rougher,but its shape and structure do not change significantly.According to the scanning electronic microscope（SEM）,energy disperse spectroscopy（EDS）depth of X ray photoelectron spectroscopy（XPS）and transmission electron microscope（TEM）characterizations results,we find that the modified LCO particles have a certain amount of Nb element doping near surface,at the same time,LNO enrichment on LCO particles surface forms evenly coated layer.And the results of long cycle performance,rate performance,cyclic voltammetry（CV）test at different scan rates and electrochemical impedance spectroscopy（EIS）measurement and high temperature cycle（55 ℃）show that the properties of LCO cathode materials modified by LNO are improved to a large extent,especially improve the rate performance and high temperature cycle performance.Even at the large current density of 20 C(1 C = 274 m A g^-1),discharge capacity of LCO@0.5 LNO materials is still 82 m Ah g^-1（original LCO can not release out capacity）,and high temperature cycle capacity retention is 60.7 % after 500 weeks（only 4.3 % of original LCO）.A thin solid solution buffer layer facilitating the extraction and insertion of Li⁺ is formed by doping and strong NbO bond（stable surface O）can stabilize LCO subsurface structure at 4.5V.LNO coating can avoid direct contact between the surface of LCO active material and electrolyte,which could effectively restrain the surface side reactions（tetravalent cobalt dissolution,surface structure degradation,decomposition of electrolyte,etc.）,thus greatly improving interface Li⁺ transport kinetics,reducing electrode polarization,and achieving the role of stabilizing interface structure and electrochemical cycle performance.Considering that the interface between active material and solid electrolyte in all-solid-state batteries is more compatible,then the LLZO fast ion conductor material with solid electrolyte characteristics is used to modify LCO surface.LCO electrodes modified with different coating amounts LLZO by constant current charge-discharge cycle and rate test etc.,it is found that the optimal coating amount is 1.0 wt.%.We investigate the changes of surface morphology and physical structure of LCO anode material before and after modification by a series of physicochemical characterization methods,the results of which show that the bulk phase structure of LCO does not been changed after modification,but LCO particles have gradient doping of La and Zr elements in the near surface area under high temperature calcination forming a solid solution pinning layer between LLZO and LCO,and uniform LLZO could avoid direct contact between LCO cathode materials and electrolyte,so as to synergistically improve Li⁺ diffusion kinetic behavior,enhance the compatibility between LLZO layer and LCO interface,thus significantly boosting the electrochemical performances of LCO.On the one hand,the thesis research work provides a strategy for expanding its application potential with solving the surface interface problems of LCO materials at 4.5 V.On the other hand,also sheds light on the construction of compatible interfaces in LIBs,which especially has positive enlightening role for the promotion of all-solid-state energy storage and conversion devices.