Study On Preparation And Modification Of Lithium-rich Cathode Material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ For Lithium Ion Batterys

Lithium ion battery lithium-rich cathode material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ have received wide attention,because of its low cost,high energy density,good safety capability and cycle stability.But lithium-rich material has some disadvantages such as its high irreversible capacity and low rate performance.So my thesis selected Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ as research object,then studied the structure and electrochemical properties of lithium-rich cathode material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂,which was prepared by combustion method,and explored the effect of cation doping on the properties.Then,the lithium-rich material was prepared by sol-gel method,and the effect of fluoride coating on its electrochemical properties was studied.The specific work and results are as follows:1.The lithium-rich three-element cathode material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂powders were prepared by combustion method at different calcination temperature and calcination time.This paper investigated the effects of different calcination temperature and calcination time on the crystal structure,morphology and electrochemical properties of the cathode materials.Finally,the best sintering conditions were obtained,that is maintaining 16 hours at the temperature 800 ℃.The synthesized sample under this condition were tested at current density of 20 m A/g and voltage of 2.0⁴.7 V.The first discharge capacity of this material was 228.9 m Ah/g,and the coulomb efficiency was72.4 %.After 50 cycles,the capacity retention rate was 84.6 %.The results show that the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ material prepared by this method has good electrochemical performance.2.The Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ and Li[Li_0.2Mn0.54-x Ni0.13Co0.13Mx]O2（M=Ce,La）cathode materials were synthesized by combustion method.The test results showed that all the samples had a typical layered structure of the a-Na Fe O2.The layeredstructure of the lithium-rich cathode materials was not changed after doping with Ce and La.The sample of x=0.034 in all samples,which include undoped and doped Ce samples,had the highest initial discharge capacity--245.9 m Ah/g at 2.0⁴.7 V and 0.1 C,and the coulomb efficiency was 74.8%.After 50 cycles,the capacity retention rate was87.2 % at 0.1 C.The capacity retention rate remained was 82.5 % at 0.5C rate.The sample of x=0.03 in all samples,which include undoped and doped La samples,had the highest initial discharge capacity--248.7m Ah/g at 2.0⁴.7V and 20 m A/g,and the coulomb efficiency was 74.9%.After 50 cycles,the capacity retention rate was 86.9%.Even at 0.5C rate,the capacity retention rate remained was 82.2%.The test results show that Ce and La doping improve the electrochemical properties of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂.3.The cathode materials Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ was successfully prepared by sol-gel method,and then coated with Ca F2 and Li F.The test results showed that all the samples had a typical layered structure of the a-Na Fe O2.The layered structure of the lithium-rich cathode materials was not changed after coating with Ca F2 or Li F.When the current density was at 0.1 C and cutoff voltage was between 2.0 V and 4.7 V,the sample of 3wt.% Ca F2 coating in all samples,which include uncoated and coated Ca F2 samples,had the highest initial discharge capacity--240.7 m Ah/g,and the coulomb efficiency was 76.5 %.After 50 cycles,the capacity retention rate was 91.9 %.Even at1 C rate,the capacity retention rate remained at 79.6 %.Compared with uncoated samples,the Li F coated samples have a high initial discharge capacity--238.7m Ah/g at2.0⁴.7 V and 20 m A/g,and the coulomb efficiency was 77.8 %.After 50 cycles,the capacity retention rate was 89.3%,the capacity retention rate remained at 77.4 % at 0.5C rate.The test results show that Ca F2 and Li F coating improve the electrochemical properties of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂.