Preparation And Modification Of Li[Li_0.2Ni_0.2Mn_0.6]O₂Li-rich Cathode Material With High Capacity

With the development of the society and economy, the requirements of energyand environmental have deepened. The problem of energy crisis especially oil crisisforces people to develop electric vehicles and new energy to replace thenon-renewable energy. Lithium-ion batteries have been considered as ideal powersource electric vehicles and large-scale energy storage application. Although thetraditional lithium-ion batteries have shown an important role in small devices, theystill can not meet the demand for high-power, high-capacity storage. the actualcapacity of commercialized LiCoO2、LiNi1/3Co1/3Mn1/3O2、LiNi0.5Co0.2Mn0.3O2、Li2MnO4、LiFePO4is less than200mAh·g-1, which limits its further development.Recently, Lithium-rich manganese-based solid solution lithium-ion batteries havearoused much attention for its high capacity.In this thesis, the lithium-rich manganese-based cathode materials of thelithium ion secondary battery xLi2MnO3·(1-x)LiNi0.5Mn0.5O2have been prepared byco-precipitation-calcination, and Mg2+/Zn2+doping and ZnO/ZrO2coatingmodification were done. X-ray diffraction, scanning electron microscopy,transmission electron microscopy, cyclic voltammetry, impedance test, andcharge/discharge test of the synthetic material has been investigated. The work is asfollows:The precursor MnaNib(OH)2was synthesized by co-precipitation, mixed withLi2CO3, and calcined to obtain final Li-rich cathode material. The influences of theamount of lithium, calcination temperature, holding time on the structure andproperties of materials have been investigated. The results show that the longer theheat treatment time, the higher the temperature, the more completed polymorph canbe obtained, and the layered structure is more obvious, but this not means a goodelectrochemical performance. Considering the cost, efficiency, and theelectrochemical properties, the optimal condition was identified as8%excesslithium, x=0.5, calcining at900oC and holding12h.The effects of Mg2+，Zn2+doping on the structure and performance ofLi[Li0.2Ni0.2Mn0.6]O2cathode material have been studied. Doping during theprecursor preparation process and during ball-milling process was compared.Different doping amount was discussed. The result shows that both Mg2+and Zn2+ doping are all benefit for electrochemical performance. From the XRD and SEMresults before and after50cycles, it can be seen that the sample doping with0.01Mg by ball-milling shows significantly improved cycle performance and the layeredstructure is well kept. CV tests shows that doping materials has improvedreversibility and EIS shows the impedence is decreased after doping.In addition, the effects of ZnO, ZrO2coating on the structure and performanceof Li[Li0.2Ni0.2Mn0.6]O2cathode material have been studied. The result shows thatboth ZnO and ZrO2can distributed on the particle surface of Li[Li0.2Ni0.2Mn0.6]O2.The cycle performance and rate performance are improved by optimum amountcoating. The improved electrochemical performance can be attributed to that thecoating layer reduced the direct contact of the positive electrode active material withthe electrolytic solution and inhibited the side reactions.