Surface Modification Of High Voltage Lithium Cobaltate Cathode Based On Magnetron Sputtering

The theoretical specific capacity of LiCoO₂ is as high as 274mAh/g.However,the actual specific capacity of LiCoO₂ that has not been modified is usually less than145mAh/g at 4.2V charge cut-off voltage.If the charge cut-off voltage is continuously increased on this basis,the specific capacity will be further increased,but it will cause problems such as cobalt dissolution,increased side reaction on the electrode surface,structural deterioration,etc.,which seriously affect the electrochemical performance.In order to solve the above problems,element doping and surface coating are the two most effective means of modification,but it is rare to use both two modification methods.In addition,conventional surface coating is usually carried out with LiCoO₂ powder particles,and the coating materials usually do not have ionic or electronic conductivity,which will hinder the transport of lithium ions and electrons,affecting the improvement of the electrochemical performance of the active cathode material.In this paper,LiCoO₂was doped with Mg by high temperature solid phase method to stabilize LiCoO₂structure and improved its electrochemical performance at 3-4.5V high potential in this paper.Then,the doped LiCoO₂ was graphite coated by ball milling to further improve the electrochemical performance of LiCoO₂ at a high potential.Finally,the magnetron sputtering technique was used to modify the commercial LiCoO₂ electrode sheet by sputtering coating to improve its electrochemical performance at 3-4.5V.The main experimental results and progress was as follows:?1?The Mg doping modification of LiCoO₂ was carried out by high temperature solid phase method.The electrochemical performance of LiCo_?1-x?Mg_xO₂ with different Mg doping concentrations were investigated.The results showed that the doping of Mg can improve the cycle stability of the material,but the initial discharge specific capacity of the material would decrease slightly.When the doping concentration was x=0.02,the cycle performance was optimal.Under the condition of 3-4.5V and 0.2C rate,the specific capacity was 115.15mAh/g after 60 cycles,the capacity retention rate was62.2%;and the specific discharge capacity was 47.8mAh/g at 5C high rate.The discharge specific capacity of undoped LiCoO₂ after 60 cycles was only 17.76mAh/g,and the capacity retention rate was 9.5%.?2?The Mg-doped LiCo_0.98Mg_0.02O₂ and the bare LiCoO₂ were coated by graphite by ball milling,and the electrochemical performance of the materials was tested under a voltage range of 3-4.5V and a charge-discharge rate of 0.2 C.The results showed that the graphite coated LiCo_0.98Mg_0.02O₂ sample had the best performance.The specific capacity after 60 cycles was still 146.15mAh/g,and the capacity retention rate was78.7%.The specific discharge capacity was 41.08mAh/g at 5C high rate.?3?The LiCoO₂ electrode was coated with Li_0.33La_0.56TiO₃?abbreviated LLTO?film by magnetron sputtering.The optimum coating thickness was studied by controlling the sputtering time and the modification mechanism was discussed.The optimum thickness of the LLTO coating layer is about 10 nm.At this time,the specific discharge capacity of the LiCoO₂ electrode in the first cycle was 184.61mAh/g at a voltage range of 3-4.5V and a charge-discharge rate of 0.2C,the specific capacity was100.13mAh/g after 80 cycles and the capacity retention rate was 71.9%.The discharge capacity was 100.13mAh/g at 2C high rate.The reason for the performance improvement was that coating layer can act as a physical barrier to suppress adverse side reactions on the electrode surface at a high potential,prevent the generation of surface passivation layer,and effectively slow the increase of charge transfer impedance.