| In the last20years, lithium ion batteries have been widely used in small electronic devices for their advantages in energy density and cyclic life. However, with the development of electric vehicles and grid, people have higher requirements on the properties of lithium ion batteries, especially on the higher power density, higher density energy and longer cycle life. The focus of research on such power batteries or energy storage batteries is to research the suitable electrode materials. This thesis focuses on the synthesis and improvement of layer ternary cathode material and the synthesis of spinel anode material Li4Ti5O12as the potential next generation electrode materials of lithium ion batteries.Chapter1gives a general introduction about the development and status, structure, working mechanism and applications of lithium ion batteries. A summarization about the cathode and anode materials is conducted. Moreover, the research statuses about the structure, reaction mechanism, synthesis methods, doping and coating of three electrode materials (ternary material LiNixCoyMnzO2, lithium-rich ternary material Li2MnO3·LiMO2and spinel Li4Ti5O12) are mainly reviewed.In Chapter2, we briefly introduce the experimental reagents, processes and equipments used in the project of this thesis. A detailed description on the process of making a2032coin cell is presented, as well as the general characterization methods of materials and electrochemical analysis.In Chapter3, a thermal polymerization route is adopted to synthesize layered LiNi1/3Co1/3Mn1/3O2materials. After annealing the samples at different temperatures from850to1000℃for different time between6and25h, powers of pure α-NaFeO2phase are obtained. By means of optimizing the annealing temperature and annealing time, we have found that the powder annealed at950℃for15h shows the best electrochemical properties with the highest first specific discharge capacity. The initial discharge capacity is188mAh/g at0.1C, and the capacity retention is87.0%after100cycles at0.33C. It exhibits good rate capability with the specific capacity of169mAh/g at1C and110mAh/g at6C.In Chapter4, a series of LiNii/3CO1/3Mn13-xMxO2(M=Mg, Al, Ti and Zr) powders are synthesized by thermal polymerization, the structures and electrochemical properties are investigated to optimize the different doping elements and different content of doping. It is found that the substitution in LiNi1/3Co1/3Mn1/3O2decreases the specific capacity, and the specific capacity decreases with increasing the doping content. The Mg2+and Al3+doping decrease the structure stability and increase the electrode resistance, which lead to the lower cycling and rate performance. Because of the bigger size of Ti4+/Zr4+ions than that of Mn4+ions and the stronger Ti-O/Zr-O binding energy than Mn-O, the substitution for Mn4+would result in higher lithium diffusion coefficient and more stable resistance during the electrochemical cycling. The cyclic stability and rate capability are significantly improved by Ti4+and Zr4+doping with the optimized concentration of x=0.01. The sample LiNi1/3Co1/3Mn1/3-0.01Zr0.0102exhibits the best electrochemical performance with capacity retention of92.7%during100cycles and a capacity of133.9mAh/g at8C rate, corresponding to71.5%of its capacity at0.1C.Compared with the traditional ternary cathode material, the lithium-rich ternary materials Li2MnO3-LiMO2can deliver much higher capacity, and can be considered as a potential high energy density cathode material. In Chapter5, a thermal polymerization route is adopted to synthesize xLi2MnO3·(1-x)LiMO2(M=Ni1/3Co1/3Mn1/3,x=0-1) powders, and the optimized annealing temperature is950℃. Their structure, morphology and electrochemical properties in the voltage range of2.8-4.5V and2.8-4.8V are investigated. The electrochemical activation of Li2MnO3would not fully complete on the initial charge to4.5V. The activating continues during the following cycling. Therefor, the cycling performance of the lithium-rich ternary materials shows a unique variation in the voltage range of2.8-4.5V. Due to the presence of electrochemical inactive Li2MnO3, the rate capability of the lithium-rich materials is worse. In the voltage of2.8-4.8V, the electrochemical activation of Li2MnO3can be fully complete on the initial charge (except for the sample with x=0.8). The capacity fading is serious and the cycling performance is decreased due to the high cutoff voltage. The Zr4+-doped lithium-rich materials are also synthesized. It is found that the Zr4+-doping does not have a pronounced influence on the structure, morphology and electrochemical performance in the voltage of2.8-4.5V.In Chapter6, nanosized Li4T15O12powders are synthesized by a polymerization-based method using furfuryl alcohol (FA) as a polymerizable solvent. It is found that pure spinel phase of Li4Ti5O12is obtained at an annealing temperature of700℃or higher. The electrochemical tests show that the optimized annealing temperature is700℃. The effect of different surfactants (citric acid, polyvinylpyrrolidone and cetyltrimethyl ammonium bromide) on the structure and properties is also investigated. The use of surfactants can improve the powder morphology of nanosized particles with less agglomeration, increase in the BET surface area, which can improve the electrochemical performance. The Li4Ti5O12synthesized with the addition of CTAB (mCTAB:mFA=1:25) exhibits the highest specific capacity and capacity retention, the excellent rate performance with a capacity of156.7mAh/g even at10C, and the favorable low temperature performance with a capacity of141mAh/g at1C even at-20℃.Finally, in Chapter7, the author gives an overview on the originalities and deficiencies of this thesis. Some prospects and suggestions of the possible future research are given. |
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