Modification Research Of Phosphate Coated LiNi_0.4Co_0.2Mn_0.4O₂ As The Cathode Material Of Lithium Ion Battery

As a cathodic material of lithium-ion battery,ternary material is considered to be one of the most potential cathode materials in research and application for lithium-ion batteries because it has high specific capacity,discharge ratio,good safety and low cost.But the side reactions taking place between ternary material and electrolyte will make the structure of the ternary material be changed during the charging-discharging process,the resistance of the electrode reaction will increase,thus material cycle performance becomes poor.Surface coating is a good method to solve the problem,because the coated layer can separate the active substance and the electrolyte.In this thesis,ternary material LiNi_0.4Co_0.2Mn_0.4O₂ was chosen used due to its lower cobalt content and higher nickel,manganese content,make it more price advantageous and higher specific capacity.On the other hand,the stability of the materials will be declined because of the low cobalt content,of course,material performance and cycle performance needs to be further improved.Coprecipitation method is adopted to carry out the surface coating on FePO₄ and LaPO₄.Coprecipitation coating was divided into two steps,the first step of reaction in liquid phase and the second is solid phase reaction.The effect of the coating amount?roasting time and roasting temperature on the electrochemical performances of the coated LiNi_0.4Co_0.2Mn_0.4O₂ material were investigated by means of X-ray diffraction,scanning electron microscope?SEM?,energy spectrum testing?EDS?,tap density test and electrochemical tests.The physical characterization results show that the crystal structure of the coated product did not change;and the different amount of phosphate material is coated on the particle surface.It has been found that the LiNi_0.4Co_0.2Mn_0.4O₂ sample coated by FePO₄ has the largest initial specific capacity of 164.57 mAh/g at 0.1 C discharge when the coating quantity was2%,the roasting time was 10 h and the calcination temperature was 600?,and the discharge specific capacity of 128.98 mAh/g and the capacity retention is 87.64%after 100 cycles at charge of 0.5 C and discharge of 1 C,which is higher than that of pristine materials and the other roasting conditions materials.The influence of LaPO₄ contents(contents of LaPO₄ were1%,2%,3%,4%and 5%)on the properties of LiNi_0.4Co_0.2Mn_0.4O₂ was studied.Electrochemical performance test results show that LiNi_0.4Co_0.2Mn_0.4O₂ material coated by3%LaPO₄ has the best performance,which the discharge capacity was 139.96 mAh/g higher than that of none-coated material?119.85 mAh/g?and the capacity retention?50 times charging at 0.5 C and discharge at 1 C?was increased from 81.70%to 94.16%.Researching different calcination temperature that coated LiNi_0.4Co_0.2Mn_0.4O₂ samples get the best sintering temperature is 600?under in the best coated quantity,thus get the best coating conditions.The pristine material and best coating conditions synthetic material do the cycle test,the cut-off voltage from 2.7 V to 4.5 V,the discharge capacity was 150.26 mAh/g and the retention was 91.13%after 50 cycle,higher than 59.71%of the pristine material.Comparing the synthesized of two kinds of coated materials performance under the optimal synthetic conditions,we draw that LaPO₄ coated LiNi_0.4Co_0.2Mn_0.4O₂ sample showed better electrochemical performance.