| Among battery systems, lithium-ion battery is considered to be the best candidate for transportation applications. Since the development of the lithium-ion battery by Sony in1991, LiCoO2has been most widely used as a positive electrode material because of its easy synthesis and high in capacity. However, the high cost and toxicity of cobalt compounds for the cathode material of LiCoO2has prompted a search for alternative materials. LiMn2O4-based spinel cathode materials offer a potentially attractive alternative to LiCoO2. Many researches have been studied on finding new synthesis methods and modifying LiMn2O4materials, including coating and doping.The study in this master degree thesis mainly focuses on a novel hydrothermal method to prepare the precursor of LiMn2O4using pre-treated electrolytic manganese dioxide (EMD) and lithium hydroxide (LiOH·H2O) as the main raw materials. Meanwhile, the effects of heat treatment and doping in the host of crystal structure have been investigated.First, the physiochemical properties of the precursor were investigated by thermogravimetry-differential scanning calorimetry (TG-DSC). These results display four weight loss temperature region, including superficial water loss, chemically bonded water loss, the formation of spinel-structured Li1.05Mn1.95O4and the formation of oxygen-lack Li1.05Mn2O4-δ. XRD patterns of the samples heat-treated at500℃show that the pure cubic spinel Li1.05Mn1.95O4formed without any other impurity peaks. When the precursor synthesized by hydrothermal method was heat treated between500and900℃for5h, no impurities, such as Mn2O3or Mn3O4, was detected. The discharge characteristics of Li1.05Mn1.95O4sample heat treated at800℃show the highest initial discharge capacity and the best cycleability, delivering118.2mAh·g-1and retaining112.3mAh·g-1after100cycles.Second, the Co-doped Li1.035CoxMn1.965-xO4and Al-doped Li1.035ALxMn1.965-O4powders were synthesized and characterized by XRD, SEM, TEM, electrochemical test and electrochemical impedance spectra. XRD patterns reveal that both Co and Al substitution do not affect the Fd3m space group of the cathode materials. TEM results demonstrate that the Li1.035Co0.035Mn1.930O4and Li1.035Al0.035Mn1.930O4powders possess a good crystalline state. Due to the stronger bonds of Co-O (662kJ·mol-1) and Al-O (512kJ·mol-1) than that of Mn-O (402kJ·mol-1) as well as fast transport and intercalation kinetics of lithium ions, Co and Al-doped powders demonstrate better cycling and rate performance. The retention rate of samples for Li1.035Co0.035Mn1.930)4and Li1.035Al0.035Mn1.930O4are93.8%and96.4%at0.5C after100cycles, respectively.Then, Co-Ti co-doped Li1.035Co0.025Ti0.02Mn1.920O4and Co-Al co-doped Li1.035Co0.020Al0.025Mn1.920O4were investigated as cathode materials. The co-doped powders display much better rate performance than Co or Al doped powders and enhanced cycling performance to some extent. This is because co-doped cathode materials inherit merits of both two elements. Notably, due to Ti4+ions providing more lattice space for Li+intercalation and deintercalation, Li1.035Co0.025Ti0.02Mn1.920O4shows the excellent rate performance. When the current density increases from0.5Cto1,2,4,8C, the discharge capacity of Li1.035Co0.025Ti0.02Mn1.920O4sample is115.3,112.8,108.1,102.0,95.2mAh-g"1, respectively. Besides, this kind of material shows excellent cycling performance with93.8%of its original value retained after100cycles at0.5C. |
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