Modification Of LiCoO₂ Cathode Material And Study Of Its Overcharge Safety

LiCoO₂ is the most widely used cathode material for lithium-ion batteries in the consumer electronics application.However,with the increasing of the upper cut-off voltage of LiCoO₂,its overcharge safety problem has become increasingly prominent which limits its further application.Therefore,the factors affecting the overcharge safety of LiCoO₂/graphite lithium-ion batteries are studied in this thesis,and LiCoO₂is modified by bulk doping,surface coating and dual modification to improve its overcharge safety.The overcharge performance of two types of commercial 493446-type1000 m Ah LiCoO₂（doped with Al and coated with Al₂O₃ respectively）/graphite pouch cells are compared at the rate of 0.05C.The impedance of the pouch cells increases significantly when the SOC exceeds 174%,accompanied by a significant increase in gas production.The increase of the impedance is mainly due to the irreversible phase transition of the LiCoO₂ cathode,with the serious destruction of the morphology,decreasing stability of LiCoO₂ structure and the intensified dissolution of transition metals.The main components of gas are CH₄,C₂H₆,CO₂,H₂ and O₂.The analysis of heat generation shows that there are two main heat generation peaks in LiCoO₂,which are caused by the decomposition of cathode and the side reaction between electrolyte/cathode interface,respectively.In contrast,the Al₂O₃-coated LiCoO₂ cathode shows better overcharge safety.However,both types of pouch cells fail in the 1C/10 V overcharge safety testLiCoO₂ with different content of Al doping,Al₂O₃ coating and Al dual modification are synthesized in kilogram-scale and then fabricated into LiCoO₂/graphite pouch cells.The results show that the initial discharge specific capacity of LiCoO₂/Li coin cell is reduced whether by Al doping,Al₂O₃ coating or Al dual modification,but its cyclic retention is greatly improved.Moreover,the rate discharge performance,high temperature storage performance and cyclic performance at 4.2 V for LiCoO₂/graphite pouch cells have also been enhanced.LiCoO₂ shows the best comprehensive electrochemical performance when the content of Al dual modification reaches 800 ppm Al doping and 1000 ppm Al₂O₃ coating.The overcharge test results of pouch cells with 100%SOC and 0%SOC show that when the content of Al dual modification reaches 800 ppm Al doping and 500 ppm or 1000 ppm Al₂O₃ coating,the pouch cells can pass the overcharge safety test and the maximum temperature during the overcharge process is significantly reduced.The mechanism of improving the overcharge safety of LiCoO₂ by the Al dual modification is proposed.The process of overcharging is divided into four stages:normal charging and discharging stage,initial stage of overcharging,stage of sharply rising temperature and stage from overcharge to thermal runaway.The comparison of overcharge performance shows that the improvement of both bulk and interfacial structure of the materials play an important role for the improvement of the overcharge safety.On the one hand,the release of lattice oxygen in LiCoO₂is reduced when Al is doped into the lattice,which improves the structural stability of LiCoO₂.On the other hand,the contact between LiCoO₂ surface and electrolyte is blocked after Al₂O₃ coating,thus the dissolution of Co is significantly repressed and the interfacial stability of LiCoO₂ is elevated.The synergistic effects of Al doping and Al₂O₃ coating diminish the impedance increase of LiCoO₂ cathode and the thermal stability is promoted,leading to the boosting of overcharge safety of LiCoO₂/graphite pouch cells.The Al₂Ti₇O₁₅-coated LiCoO₂ is firstly proposed to improve its dynamic performance and overcharge safety.Through the first-principle calculation,it is found that the tunnels along the（010）direction of Al₂Ti₇O₁₅ provides the pathway for the rapid diffusion of lithium ions.Meanwhile,Al₂Ti₇O₁₅ is an electronic conductor,which can improve the electronic conductivity of LiCoO₂.Its excellent electronic conductivity mainly comes from the high density of states of Ti-3d orbit near Fermi level.Combined with the experimental test,it is verified that the rate performance,diffusion coefficient of Li⁺and electronic conductivity of the LiCoO₂ have been improved when coated with Al₂Ti₇O₁₅.The calculation results of Al₂Ti₇O₁₅-LiCoO₂ heterojunction show that it is thermodynamically stable and the electrons in Al atoms and Ti atoms tend to migrate to oxygen atoms to form covalent bonds,thus stabilizing the oxygen atoms in LiCoO₂ and improving the overcharge safety.The synthesis studies of Al₂Ti₇O₁₅ show that it can be formed when the molar ratio of Al:Ti:Ti O₂ is adjusted to 1:0.5:5 and 1:0.5:4.5.The Al₂Ti₇O₁₅-coated LiCoO₂ shows best electrochemical performances when the coating content of Al₂Ti₇O₁₅ is 0.3 wt.%and the heat treatment temperature is950℃.The overcharge test of the coin cells and analysis of cathode materials reveal that the voltage rise of the Al₂Ti₇O₁₅-coated LiCoO₂ coin cells is relatively gentle and the dissolution of cobalt decreases.Moreover,the damage of LiCoO₂ particles is obviously retarded and the crystal structure of the LiCoO₂ material is relatively integrated after Al₂Ti₇O₁₅coating.The heat generation analysis shows that the thermal stability of the Al₂Ti₇O₁₅-coated LiCoO₂ is excellent with the increased peak temperature and the reduced amount of heat generation,thereby improving the overcharge safety of the LiCoO₂ cathode.