Research On 4.5V High Voltage LiCoO₂ Cathode Material For High Power Density Lithium Ion Battery

As a high degree industrialized cathode material,LiCoO₂ has the advantage of simple synthesizing process,stable charge and discharge voltage platform,and high theoretical capacity,etc.However,the material has enough strong oxidation performance for the decomposition of electrolyte when the deintercalation of lithium is too much.So,in the actual application only 140mAh/g of discharge specific capacity is allowed and the voltage limit is only 4.2V.Meanwhile,ordinary LiCoO₂,at a cut-off voltage of 4.2V,has been difficult to meet demand for high energy density such as '3C' electronic products.In order to get the cathode materials with high specific capacity,excellent cycling performance under high voltage,high temperature solid phase method was used for doping modification of LiCoO₂ in this paper.Precursor reaction temperature interval was studyed by TGA,and physical characterization methods such as SEM,XRD were studyed on the material of the surface morphology,crystal structure,etc.All kinds of electrochemical testing methods were used to get the characterization of samples' comprehensive electrochemical properties.Innovative work of this paper is as follows:?1?Doping elements of Al and Mn were selected respectively,which were used for single-doped modification of LiCoO₂.Raw material mixing process,different sintering temperature,sintering time and the doping ratio were studyed.Results showed that doping Al could improve the cycling performance of the material,however the initial discharge specific capacity was reduced.Sintering conditions of presintering 750 ?,6 h,eventually burned 900 ?,16 h was the best conditions for composite materials.LiCo_0.98Al_0.02O₂ had the best performance of all samples,after 50 cycles capacity remained at a rate of 84.7%.Mn single doped can inhibit the cathode materials phase transformation from hexagonal to monoclinic above 4V voltage,thereby enhancing the cycling performance.The ball mill mixing method was involved in the experiments,the sintering condition is presintering 750 ?,10 h,eventually burning 920 ?,10 h.Under this condition,cathode materials LiCo_0.97Mn_0.03O₂ had good cycling performance,after 50 cycles it still retained 90.6% of the initial capacity.However according to SEM,materials become agglomeration caused by particles decreases,so the capacity is decreased.?2?In order to solve the problem that single doping can not meet simultaneously the requirements of high capacity,high potential and excellent cycling performance,Mg,V and F were used to improve the electrochemical performance of LiCoO₂ in the range of 2.7-4.5V voltage.The synthesized cathode material LiCo_0.97Mg_0.02V_0.01O_1.98F_0.02 had good cycling performance and rate performance.The initial discharge specific capacity was 169mAh/g at the rate of 0.5C,and the capacity was still 125.6mAh/g under 5C.In addition,the different doping ratio of synthesis of LiCo_0.97-xMg_xV_0.03O_2-xF_x?x=0.01,0.02?was studyed,research results when x=0.02 the material had the best discharge capacity and excellent cycling performance of all samples,doping vanadium could decrease the polarization and enhance the discharge capacity of the material.Mg and F entered into the crystal lattice of stable crystal structure of materials,which would reduced material cation disorder degree.?3?Finally,cathode material LiCo_0.97Mg_0.01V_0.02O_1.98F_0.02 were synthesised in different time under the final sintering,which used another sources of vanadium?NH₄VO₃?.In the process of synthesising,Mg and F were not equimolar doped.The result showed that synthesised LiCo_0.97Mg_0.01V_0.02O_1.98F_0.02 also had good cycling performance,the sintering time growth would generate agglomeration of materials.