| With the wide applications of lithium-ion batteries (LIBs) in electric vehicles, energy storage power plants and small electronic appliances, higher and higher demands are put forward on their cycle stability, rate performance, safety, and particularly energy density. The highest energy density of today’s LIBs is about 210 Wh kg-1 and 650 Wh L-1, being less than the values of a quarter of oil, which seriously restricts the uses of LIBs in electric vehicles. The energy density of the commercial cathode materials (such as LiCoO2, LiFePO4, LiNixCoyMn1-x-y etc.) is limited by their theoretical specific capacities. Therefore, searching for a new high-energy density cathode material to replace the existing commodity electrode material is very important to the development of the next generation LIBs. For all cathode materials studied today, lithium-rich cathode materiasare the most promising candidates. In this thesis, to overcome the disadvantages of lithium-rich materials, the compositions and various cation-dopings are used to optimize their electrochemical performances. Its sensitivity of electrochemical properties to the temperature has also been studied.In Chapter 1, the author gives a brief introduce to the working principles and the common electrode materials of lithium-ion batteries. The history and current status of the layered lithium-rich material are discussed comprehensively.In Chapter 2, the author introduces the main reagents and instruments used in the experiments. A brief description on the preparation process and related characterization methods for lithium-ion batteries is given.In Chapter 3, the author synthesized a serious of lithium-rich materials xLi2MnO3·(1-x)LiNio.5Mno.502 by an acrylic gel polymerization process, and the preparations were optimized. By comparing the electrochemical performances of xLi2Mn03·(1-x)LiNio.5Mno.502 with different x values, the author found the highest energy density material 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2. It was determined that the optimal sintering conditionsare 950℃ and 10h.In Chapter 4, the author synthesized lithium-free cathode material Cr2O5 and then combined it with Li-rich cathode to form composite materials with different Cr2O5 contents. After comparing the electrochemical performance of the composite materials, the author found that the first coulombic efficiency of the lithium-rich materials can be regulated by adjusting the content of C12O5material. The composite materials maintained the characteristics of high specific capacity. The optimal material C34 (Cr2O5 mass ratio 17%)sample shows a coulombic efficiency of 102% for the first cycle and a specific capacity of >230 mAh g-1.In Chapter 5, the author synthesized lithium-rich materials Mo-doped Li1.2Nio.2Mno.6-xMoxO2. According to the structural analyses, it can be concluded that: 1) owing to the liquid phase sintering, the particle size of materials increases with increasing the Mo content; 2) the Mo-doped samples show improved ion/electronic conductivities so that the rate performance is improved. The optimal sample is Li1.2Nio.2Mno.59Moo.01O2, which exhibits a discharge capacity of 110 mAh g’at 5C rate and a capacity of 229 mAh g-1 at 0.1 C rate after 204 cycles. The phenomenon of voltage decay is also limited.In Chapter 6, the author synthesized Li1.2Ni0.2Mn0.6-xVxO2 and Li1.2Nio.2Mno.6-xNbxO2. The V-doped samples show improved ionic/electronic conductivities, and hence the rate performance is improved. The optimal sample Li1.2Nio.2Mno.59Vo.01O2 exhibits a discharge capacity of 115 mAh g’at IOC rate and a capacity of 234 mAh g-1 at 0.1C rate after 188 cycles with a capacity retention of 95.5%. The phenomenon of voltage decay with cycling is also limited.The optimalNb-doped samples Li1.2Nio.2Mno.55Nbo.05O2 exhibits a discharge capacity of 75 mAh g-1 at 5C rate.In Chapter 7, the author synthesized lithium-rich materials doped with trivalent metal ionsLi(Lio.2Nio.i5Mo.iMno.55)02 (M=Al, Fe, Co, Cr), tetravalent metal ions Li(Lio.2Nio.2Mno.55Mo.o5)02 (M=Sn, Si, Zr, Ti) and Co-doped samples Li(Lio.2Nio.2-0.5xCoxMno.6-o.5x)O2 (x=0.05,0.1,0.133,0.4). The Co doping can improve the discharge capacity of the lithium-rich materials, but the cycling stability is worsenedwith the increase of Co content. The Li(Lio.2Nio.i33Coo.i33Mno.534)02sample shows the highest discharge capacity of 276 mAh g-1.In Chapter 8, the author compared the temperature effects on the properties of a lithium-rich material with commercial cathode materials:LiNii/3Co1/3Mn1/3O2 (NCM111), LiNio.5Coo.2Mno.3O2 (NCM523) and LiNi0.815Co0.15Al0.03502(NCA). The lithium-rich material is more sensitive to the temperature than the commercial materials. The characteristic of temperature-sensitivity for lithium-rich materials is mainly caused by the thicker surface films on the electrode particle surface formed during cycling. The characteristic of lithium-rich materials becomes more sensitive with increasing the Li2MnO3 content.In Chapter 9, the author give an overview of the innovation and deficiencies of this thesis. Some prospects and suggestions for the future work are presented as well. |
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