| 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, people have higher requirements on the performance of lithium ion batteries, especially on their energy density and power density. Recently, the focus of research on lithium ion batteries is to improve their energy density. The cathode material is the most important component in a lithium ion cell because it determines to a large extent the cell performances including energy density. This thesis focuses on improving the cycling stability, rate performance and thermal stability of5V spinel cathode material LiNio.5Mn1.5O4.Chapter1gives a brief introduction about the structure, working mechanism and applications of lithium ion batteries. A summarization about several common cathode materials is conducted from the viewpoint of energy density and power density. Moreover, the research statuses about the structure, reaction mechanism, synthesis methods, doping and coating of LiNio.5Mn1.5O4are mainly reviewed. The scope of this thesis is outlined at the end of this chapter.In chapter2, the author introduces the experimental reagents, processes and equipments used in the project of this thesis. A detailed procedure of2032coin cell assembling as well as general characterization methods of materials’structure, morphology and electrochemical properties has been elaborated,In chapter3, powders of LiNi0.5Mn1.5O4, Lio.95Ni0.45Mn1.5Al0.05O4, LiNi0.475Mn1.475Al0.05O4and Li1.05Ni0.5Mn1.45Al0.05O4are synthesized by a thermopolymerization method, to investigate the effects of Al substitution for Ni or (and) Mn in LiNi0.5Mn1.5O4spinel on the structures and electrochemical properties. It is found that Al-doping can significantly improve the cycling stability and rate capability of LiNi0.5Mn1.5O4. The capacity retentions of Al-doped spinels increase to over99%after100cycles at room temperature. However, the effects of Al substitutions for Ni and (or) Mn ions in the LiNi0.5Mn1.5O4are somewhat different:the Lio.95Ni0.45Mn1.5Al0.05O4shows faster capacity fading at an elevated temperature; the Li1.05Ni0.5Mn1.45AI0.05O4has lower capacity but displays higher capacity retention at55℃, its capacity retention after100cycles can reach98%. As a compromise, the Ni/Mn co-substituted sample LiNi0.475Mn1.475Al0.05O4shows the best electrochemical performance with a high specific capacity during cycling at room and elevated temperatures, and excellent rate capability.In chapter4, after a series of Al-doped LiNi0.5-xAl2xMn15-xO4(0<2x<1.0) spinel powders are synthesized. Their structures, electrochemical properties and thermal stabilities are investigated to optimize the content of doped Al. It is found that introductions of Al into LiNi0.5Mn1.5O4decreases the ordering degree of ions in B sites, and finally changes the space group of LiNi0.5Mn1.5O4from ordered P4332to disordered Fd3m gradually. The cycling stability and rate capability are significantly improved by Al-doping in the optimized Al concentration0.05<2x<0.10. The LiNi0.45Al0.10Mn1.45O4gives the best capacity retention (95.4%after500cycles at1C rate) and the best rate capability (119mAh/g at10C, about93.7%of its capacity at0.5C) at room temperature. Moreover, the thermal stability of the spinels is tested on a C80calorimeter and the results show that Al-doping can effectively suppress the exothermic reactions between LiNi0.5Mn1.5O4and electrolyte below220℃and thus improve the safety of this high voltage cathode material.In chapter5, LiNi0.45M0.10Mn1.45O4(M=Fe, Co,Cr) powders are prepared and systematically investigated to compare the effects of Fe, Co, Cr doping. It is found that the Fe-and Cr-doping increase the reversible capacity of LiNi0.5Mn1.5O4, while the Co-doping decreases the capacity slightly. Excellent cycle life is measured for the Fe-, Co-, Cr-doped LiNi0.5Mn1.5O4, about95.9%,93.1%and81.7%of their initial capacities can be retained after500cycles at room temperature. Their capacity retention after200cycles at55℃is94.9%,94.3%and83.6%respectively. Moreover, these three valence transition ions doping also significantly improves the rate performance of LiNi0.5Mn1.5O4. When discharged at10C, LiNi0.45Fe0.10Mn1.45O4and LiNi0.45Cr0.10Mn1.45O4can maintain>90%of their capacity at0.2C. LiNi0.45Co0.10Mn1.45O4performs even better, since it displays a capacity of124mAh/g at10C (97.6%of its capacity at0.2C), with the average discharge voltage of4.50V. Three possible capacity fading mechanisms including structural transformation, the dissolution of the spinel into the electrolyte, and the oxidation of the electrolyte are discussed. The decomposition of the electrolyte is regarded as the most important mechanism.In chapter6, full cells are assembled with the LiNi0.45Co0.10Mn1.45O4optimized in chapter5as the positive electrode, and graphite, zero-strain materials (Li4TisO12and LiCrTiO4) or alloy materials (Sn0.76Co0.24and Sn0.3Co0.3C0.4) as the negative electrodes. The electrochemical properties of these full cells are investigated and their mass energy density and volume energy density are compared. It is found that LiNi0.45Co0.10Mn1.45O4/LiCrTiO4displays excellent rate capability and cycle stability and is a long life battery. But its energy density is only190Wh/kg because of its low working voltage of3.2V. The working voltage of LiNi0.45Co0.10Mn1.45O4/graphite is about4.5V, so it has high mass energy density (350Wh/kg) and volume energy density (520Wh/L). When the alloy (Sn0.76Co0.24and Sn0.3Co0.3C0.4) materials are combined with LiNi0.45Co0.10Mn1.45O4, the discharge voltage of the full cells are about4.3V, and the volume energy density of the full cells can reach700Wh/L, although their cycling performance need to be improved. Moreover, the author has developed a versatile method to fabricate flexible electrodes without current collector. Flexible cells of LiNi0.5Mn1.5O4/Li4Ti5O12, LiNi0.5Mn1.5O4/graphite and LiMn2O4/Li4TisOi2are assemabled and tested. Although their cycling performance is not satisfactory, they may be applied in some special devices and in-situ spectroscopy analysis.In chapter7, Ni0.5Mni.5(C204)2·4H2O particles with high uniformity are synthesized by emulsion methods, nano-, submicron-and micron-size LiNi0.5Mn1.5O4powders are synthesized by using Ni0.5Mn1.5(C2O4)2·4H2O as the precursor. It is found that submicron-LiNi0.5Mn1.5O4heattreated at800℃has the capacity of136mAh/g, and the LiNi0.5Mn1.5O4calcinated at850and900℃exhibit good cycling performance, with a capacity retention over95%after100cycles at room temperature. The electrochemical performance of the LiNi0.5Mn1.5O4at low temperatures is measured. The results show that capacity retention of LiNi0.5Mn1.5O4at low temperatures is heavily influenced by the particle size of active materials. Smaller particles lead to better performance.Finally, in chapter8, the author gives an overview of the originalities and deficiencies of this thesis. Some prospects and suggestions of the possible future research are also given. |
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