Preparation And Performance Study Of High Voltage And High Specific Capacity Lithium Cobaltate Cathode Materials

As a clean energy storage carrier,lithium-ion batteries have attracted widespread attention due to their environmental friendliness and low cost.High energy density lithium-ion batteries are an important development direction in the field of energy storage.At present,the energy density of lithium-ion batteries is mainly constrained by the cathode material,so it is very important to develop high-voltage,high-capacity cathode materials.Lithium cobaltate has high compaction density and theoretical specific capacity.However,the charging voltage in practical application is only 4.2 V.Increasing the charging voltage bring higher energy density to lithium cobaltate cathode.However,a high cut-off voltage（>4.45 V）will lead to problems such as irreversible phase transition and collapse of the layered structure,which will affect the cycle stability and safety of lithium-ion batteries.Therefore,in order to further improve the energy density and electrochemical performance of batteries using lithium cobaltate as the cathode material,optimizing the electrode material through modification strategies has become the only way.In this work,aiming at the failure mechanism of lithium cobaltate under high pressure,element doping,nanometerization,and surface coating are used for modification.At the same time,different strategies are studied for the modification mechanism and effect of high-voltage lithium cobaltate.The specific work content is as follows:First of all,single-element doping modification of lithium cobaltate was investigated.The results show that Mg doping can effectively improve the electronic conductivity of the material,reduce the Warburg impedance and promote the diffusion of Li⁺.The sample with a doping amount of 0.02 maintains 93%of the capacity after 50 cycles at 3-4.6 V,1C.And the discharge specific capacity at 10 C reaches 126.7 m Ah·g^-1.Al doping can significantly enhance the stability of the main structure of lithium cobaltate,and has the least effect on the stress and strain inside the material crystal.Although Ti doping is hard to improve the cycle performance of the material,the first-turn cycling efficiency and specific capacity of the modified sample were increased from 93.9%,205.3 m Ah·g^-1 to97.1%,209 m Ah·g^-1.Secondly,after mastering the modification effect of different element doping,Mg-Al-Ti co-doping strategy is used to synergistically improve the electrochemical performance of lithium cobaltate.Mg doping to Li sites broadens the lithium layer spacing and increases the lithium ion diffusion coefficient from 4.31×10^-16 to 1.19×10^-14cm²·s^-1.And Mg acts as a pillar in the deeply delithiated state to effectively stabilize the layered structure.The doping of Al and Ti into the Co sites can synergistically enhance the structural stability of lithium cobaltate and simultaneously promote the release of the specific capacity of the cathode material.In addition,the size control of the co-doping lithium cobaltate by the nano-cobalt source results in a more uniform distribution of doping elements in the nano-modified sample and a shorter lithium ion diffusion path.The modified sample showed excellent long-term cycle stability and rate performance at high current density.The capacity maintained at 91%after 600 cycles at 5 C,and the specific discharge capacity at 10 C reached 132 m Ah·g^-1.Finally,although element doping and nanosizing modification strategies have achieved good results,they have not been able to effectively solve the problem of side reactions on the surface of electrode materials.The surface of co-doping lithium cobaltate is coated with different amounts of ionic conductor materials.An appropriate amount of coating effectively improves the contact state between the electrode and the electrolyte,promotes the diffusion of lithium ions at the interface,and the discharge specific capacity at 5 C and 10 C reaches 169.4 and 139.8 m Ah·g^-1.However,excessive coating will increase the electrochemical polarization of the material,hinder electron and ion conduction,and thus reducing the electrochemical performance.