| During recent years, lithium ion battery technology has made great progress in terms of energy density and power capability even higher than primary batteries. Thus, lithium ion batteries have rapidly conquered the consumer market of advanced portable electronics and are now considered as the most promising power sources for future electric vehicles (EVs), hybrid EVs and plug-in hybrid EVs. The purpose of the experiment is to synthesize high voltage materials with high specific capacity, steady cyclability, and good high rate cycle performance. The main testing methods include constant current charge-discharge test, cycle voltammagram (CV) and X-ray Diffraction (XRD) as well as scan electron microscope (SEM).Lithium titanate Li4Ti5O12due to the small volume change during the charging and discharging cycle performance is very execellent, but because of its high potential of about1.5V, as the anode materials, cathode materials for potential requirement as high as possible as well.5V spinel LiNi0.5Mn1.5O4is one of the most promising and attractive cathodes because of its acceptable stability, good cycling performance and high dominant potential plateau at around4.7V. It has been expected that the3V LiNi0.5Mn1.5O4/Li4Ti5O12cells exhibited good cycling performance, flatness in operating voltage and high rate capability.Due to the poor rate performance of Li4Ti5O12materials, and aiming at decreasing calcined temperature and reducing particle size, the wet type and solid state calcination process was developed, the effect of type of raw materials, calcined temperature and alcined time on the structure and properties of materials was studied. The Li4Ti5O12with small particle size and good rate performance was calcined. Aiming at improving the electronic conductivity of Li4Ti5O12, Mg and Ta doping Li4Ti5O12was prepared and studied.The LiOH and anatase TiO2were chosen as the raw material to get the best performance. The influence of sintering temperature and time were studied and the results showed that the optimal performance could be achieved by sintering at750℃for8hours. Its reversible capacity was165mAh/g(0.5C),131mAh/g (20C) and118mAh/g(30C).Mg and Ta were adopted to improve electrochemical performance of Li4Ti5O12. The X-Ray Diffraction test indicated that Mg doping causesd the lattice parameters of Li4Ti5O12to become larger and produce Li2MgTi3O8which has negative effect on material conductivity and Li ion diffusion.The capacity of Li3.95Mg0.05Ti5O12was152.1mAh/g at20C rate and Li398Mgo.o2Ti5O12and Li3.9Mg0.1Ti5O12was138.3mAh/g and120.6mAh/g. Mg and Ta doping can create Ti3+/Ti4+which increase material conductivity.As the cathode material of lithium ion batteries, cubic spinel LiNi0.5Mn1.5O4shows excellent electrochemical performance, such as high discharge plateau at4.7V and high energy density, and it is emerging as an active research topic. Ni0.25Mno.75(OH)2was also synthesized by co-precipitated metal hydroxide by controlling crystallization. The effect of precursor operating conditions on morphology, particle size, tap density and specific surface area was investigated. PH and ammonia content significantly influenced micro-morphology of precursor. With increasing pH, preparing temperature and ammonia content in reaction solution, tap density of precursor was increased and specific surface area was decreased. Meanwhile, influence of sintering temperature and sintering atmosphere of lithiation sintering on material performance was investigated. The optimized sintering conditions were obtained as follows:At the elevating rate of100℃/h, the material was sintered at850℃for24h. The discharge capacity can be reached128mAh/g (0.2C) in the range of3.5V-5.0V and the capacity retention reached98.4%after30cycles.It is found that the Al and F can enter into the lattice of material. The method of Al3+and F co-doping was used to improve the cycling stability of LiNi0.5Mn1.5O4cathode material and the effects of substitution of different transition metal elements by Al3+and Al3+content were investigated. The studies showed that Al3+and F co-doping did not change the structure of material, and significantly improved the cycling stability and rate capability. CV and EIS measurements showed that the enhancement of rate performance was due to the existence of vacancies, which reduced the resistance of lithium ion deintercalation and improved solid diffusion coefficient and the electrochemical activity.The LiNi0.45Mn1.45Al0.1O3.95F0.05/Li3.95Mg0.05Ti5O12full cell system was also investigated. When the mass ratio of the initial discharge capacity under1C rate was141mAh/g, and after200cycles, with the capacity retention rate about90%. |
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