Preparation And Properities Study Of Lithium-rich Cathode Material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂

Lithium-rich layered material is ideal candidate for electric vehicle power battery cathode material since it has many advantages.In this study,layered Li-rich material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was synthsised by combustion method and sol-gel method.X-ray diffraction?XRD?and scanning electron microscopy?SEM?tests were uesd to investigate the crystal structure and physical morphology respectively.The electrochemical properties of the material was investigated by galvanostatic chargedischarge test,cyclic voltammetry?CV?test and electrochemical impedance spectroscopy?EIS?test.The source for combustion,calcination temperature and calcination time in the combustion method process were optimized.The optimum preparation conditions were as follows: the source of combustion is polyvinylpyrrolidone?PVP?,the calcination temperature is 800 °C,the calcination time is 12 h.The first discharge capacity of the material is 246.2 m Ah·g-1 at 0.1C?1C = 250 m Ah·g-1?,the coulombic efficiency at the first charge-discharge process is 79.33%,and the discharge capacity is 121.9 m Ah·g-1 after 100 cycles at the current of 1C,retaining 78.35%.In order to improve the cyclic properties of the material,Fe-doped and Cu Ocoated were used.The best Fe element doping amount x = 0.03,the capacity retention rate increased from 74.49% to 83.7% at the 0.2C current circulation 50 times.Fe element doping into the material lattice,inhibit the phase transition of the material during the cycle process,which coule improve the cycle performance of the material.After the Cu O coating was carried out,the capacity retention rate was 91.4% after cycled 50 times at the current of 0.2C.Cu O existed in the coating layer for the pristine material,decreasingthe electrolyte/electrode surface area and further inhibiting undesirable reactions,thus achieving an excellent cycling performance.Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was prepared by sol-gel method,the effects of chelating agents including three different kinds of chelating agents?citric acid,glycolic acid,and polyvinyl pyrrolidone?on its performance were studied.Electrochemical tests show the sample prepared with polyvinyl pyrrolidonethat has the highest initial discharge capacity?230.8 m Ah·g-1?and initial charge/discharge efficiency?70.9%?.And with glycolic acid the material has a good cycle performance,the retention rate was 91.7% at 1C current after 100 times circulation.The use of PVP could short the particle size with short embedded path,which can improve the large current discharge performance.Nb-doped,V-doped,V and Mo-doped and LiVO₃-coated modification were carried out on the material.The best Nb element doping amount is x = 0.015,the first discharge capacity of the material decreased,but the cycle performance and rate performance have been improved.The capacity retention rate increased from 91.7% to 99.1% at 1C current afrer cycle 100 times.It was found that with doping x = 0.02 and x = 0.03 V-element,the material had a excellent cycle performance,and the capacity retention rate of the material was 99.8% after 50 times circulation of 0.2C current.Doped Mo element in the V-doped material,found that the material cycle performance decreased but still greatly improved compared to undoped material.With doped x = 0.03 V and y = 0.03 Mo,the material discharged 106 m Ah·g-1 in the 5C current.Ion doping could stabilize the crystal structure of the material,improve the cycle performance.While the large ion radius element doping can extend the path of Li+ de-embedding which could improve the rate performance.LiVO₃ coating was carried out used to modify the material.The results showed that the specific capacity of the first discharge was increased from 211.7 m Ah·g-1 to 221.6 m Ah·g-1,The discharge capacity of 5C was increased from 70 m Ah·g-1 to 100 m Ah·g-1.LiVO₃ has a spinel structure with good ion conductivity,which can improve the rate performance of raw material.Finally,the effect of different morphologies on the properties of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was studied.The discharge capacity,cycle performance and magnification properties of the hollow microspheres prepared by high temperature solid phase method were better than the material prepared by sol-gel method because the hollow structure improves the contact area between the material and the electrolyte,increase the utilization of the material and migration rate of Li-ions.The hollow microspheres was modified by V element doping,the doping amount was x = 0.03.It was found that the capacity retention rate increased from 90.3% to 93.3% after cycling 100 times at 1C,but the discharge capacity of the material has decreased.