Study Of Modification And Electrochemical Performance Of 4.5V High Voltage LiCoO₂ Cathode Materials

Since lithium-ion batteries with carbon as the nagetive electrode were prepared and commercialized by Sony Company in 1990,they have not only been widely used in small portable mobile device such as notebooks,smartphones,digital cameras,camersas,but also expanded to new fields such aw energy vehicles and energy storage.At the same time,new fields also put forward higher requirements for battery performences.LiCoO₂ has many advantages,such as high theoretical specific capacity,simple synthesis process,high discharge voltage,stable charging and discharging voltage platform,which makes it first used for commercialization.However,LiCoO₂ also has some shortcomings,which limits its application..LiCoO₂ is a layered material with the structure ofα-NaFeO₂.When lithium-ion batteries are charged,the electrostatic repulsion between oxygen atoms is greater than the electrostatic attraction between oxygen and lithium atoms,which results in the expansion of LiCoO₂ along the c axis.However,when the charging voltage reaches 4.2 V,LiCoO₂ begins to change from a hexagonal system with electrochemical activity to a monoclinic system without electrochemical activity.In addition,most of Co³⁺in LiCoO₂ is oxidized to Co⁴⁺with strong oxidizability when LiCoO₂ is highly delithium.Co⁴⁺can easily react with the electrolyte,resulting in corrosion of the electrolyte and dissolution of Co which will reduce the cyclic property of LiCoO₂.It is one of the effective ways to increase the specific capacity of LiCoO₂ material by increasing the charging voltage of LiCoO₂ material.When the charging voltage is increased,more Li⁺is releasing and more charge is stored.However,with the increase of charging voltage,the highly delithium-free Li_1-xCoO₂ is extremely unstable,and its cycle performance and safety performance decrease rapidly.In order to improve its electrochemical performance,the structural stability of the material need be improved by doping modification and surface coating,which can make up for the defects of the material under high voltage.LiCoO₂ cathode materials were prepared by sol-gel method,and then modified by Al-doping and grapheme-coating to improve its electrochemical performance.The main work of this paper is as follows:1.layered structure LiCoO₂ cathode materials were prepared by different preparation methods such as high temperature solid-state method,sol-gel method and hydrothermal method.The physical and electrochemical properties of LiCoO₂ materials were characterized and compared by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The results showed that the structure and surface morphology of LiCoO₂ prepared by sol-gel method with two acetic acid as raw material and acrylic acid as chelating agent had better structure and surface morphology.The electrochemical test results show that the LiCoO₂ prepared by sol-gel method has better cycling and rate performance.Its first charge/discharge capacity of the 0.2 C is 155 mAh·g^-1,and After 50 cycles the retention rate of charge and discharge capacity is 57.4%.2.During the preparation of LiCoO₂ by sol-gel process,Al and Mg were doped in LiCoO₂ separately and the properties of products were compared.The results show that the specific capacity and cycle performance of Al-doped LiCoO₂ are higher than those of Mg-doped LiCoO₂ at 4.5 V high potential.And among the different Al doping amounts,the LiAl_0.15Co_0.85O₂ prepared with Al doping amount of 0.15 has better electrochemical performance.The first discharge specific capacity of the material is 172 mAh·g^-1 at 0.2 C ratio,and the discharge specific capacity of the material remains 74.4%after 50 cycles.3.Graphene-coated lithium cobalt aluminate(LiAl_0.15Co_0.85O₂/GO)was prepared by solvothermal reduction of graphene oxide mixed with lithium cobalt aluminate.The properties of the prepared materials were compared.The results show that the cycling and rate performance of the material are improved after graphene coating.Among them,LiAl_0.15Co_0.85O₂/GO with graphene coating amount of 5wt.%exhibited the best electrochemical performance.The first charge-discharge specific capacity at 0.2 C ratio was178 mAh·g^-1,and the capacity retention rate was 93.8%after 50 cycles of charge-discharge at0.2 C ratio.What’s more,the capacity of LiAl_0.15Co_0.85O₂/GO can be restored to 96.1%after30 cycles at different rates.