| With the rapid depletion of fossil fuels and growing environmental pollution, environmentally benign electric vehicles are bound to enter a great number of families. Due to their advantages of large energy density and long cycle life, lithium ion battery has been recognized as ideal auxiliary power sources for electric vehicles(EVs) and hybrid electric vehicles(HEVs). However, the commercialized LiCoO2 cathode material is not suitable for power sources for EVs and HEVs, due to its high cost and poor safety. Therefore, exploration of inexpensive electrode materials with high electrochemical and high safety has become the research focus in the area. Owing to its advantages of high redox potential(4 V vs. Li/Li+), low cost, abundant Mn source in nature, environmental benignity, and good safety, spinel LiMn2O4 cathode material is believed to be one of the most competitive materials. However, it still suffers from severe capacity fading during cycling, especially at elevated temperatures.In this dissertation, porous nanoparticles-constructed granule LiMn2O4 was prepared by a solid-state reaction, using a highly reactive Mn3O4 as Mn source. The granule LiMn2O4 obtained was further modified by a coating technology, in order to achieve a high-performance cathode material with high discharge capacity, high rate capability, excellent cycling performance, and high tap density. The relation between the preparation conditions, chemical/phase compositions, microstructures and electrochemical performance was investigated in detail. The main results were summarized as follows:(1) LiMn2O4 nanoparticles were synthesized by a solid-state reaction route, using a highly reactive Mn3O4 as Mn source. The optimum synthesis conditions were investigated. The results show that pure spinel LiMn2O4 can be achieved in the synthesis temperature range of 650-900?C. Among them, the 650?C-obtained one exhibits the best electrochemical performance. At the current rates of 0.2 and 5 C(1 C=148 mA·g-1), the discharge capacity of the 650 ?C-obtained material was measured to be 126 and 102 mAh·g-1, respectively, showing a capacity retention of 80.0 %. After cycled at 25 ?C and 1 C for 200 cycles, it delivers a discharge capacity of 119 mAh·g-1, retaining 92.2 % of its initial capacity. After cycled at 55 ?C and 1 C for 100 cycles, it preserves a discharge capacity of 92 mAh·g-1, maintaining 77.4 % of its initial capacity. The higher electrochemical performance of the 650 ?C-obtained LiMn2O4 may be related to its ultrafine partice size, high crystallinity and fewer oxygen defects.(2) Porous nanoparticles-constructed granule LiMn2O4 was prepared, and its electrochemical performance was characterized. A well mixed mixture of Li2CO3 and Mn3O4 was preheated at 400 ?C, and the obtained powder was treated by a pelletizing process, followed by calcining at 650 ?C, yielding LiMn2O4 aggolomerates with a size of 40-60 μm. The results show that the granule LiMn2O4 is constructed by LiMn2O4 nanoparticles partially fused together, presenting an irregular 3-dimension porous morphology with a pore diameter less than 50 nm, showing a tap density of 2.05 g · cm-3. Its rate capability and cycling performance were much improved without its discharge capacity compromised, the diffusion coefficient of lithium ion(DLi) of the material which is 9.33×10-11 cm2·s-1 increases significantly. At the current rates of 0.2 and 10 C, the discharge capacity of the granule LiMn2O4 was measured to be 125 and 109 mAh·g-1, respectively, showing a capacity retention of 87.2 % %. After cycled at 25 ?C and 1 C for 200 cycles, it delivers a discharge capacity of 120 mAh·g-1, retaining 97.6 % of its initial capacity. After cycled at 55 ?C and 1 C for 100 cycles, it preserves a discharge capacity of 96 mAh·g-1, preserving 80.0 % of its initial capacity. The greatly improved rate capability and cycling performance, as well as its large discharge capacity, can be ascribed to its unique microstructure.(3) The Co-Al coating technology was used to further improve cycling performance of the porous nanoparticles-constructed granule LiMn2O4. The mixed mixture of Li2CO3 and Mn3O4 was calcined at 400?C, The powder was once more heated at 650 ?C, followed by pelletizing. And the obtained aggolomerates were coated by Co and Al ions with a citric acid sol-gel method and calcining at 650 ?C, yielding a Co-Al coated porous LiMn2O4 aggolomerates. The results show that the Co-Al coated granule LiMn2O4 consists of pure spinel phase, suggesting that the Co and Al ions are incorporated into the spinel lattice of LiMn2O4, and replace a part of Mn cations. The electrochemical performance of all the Co-Al coated granule LiMn2O4 samples was improved to a certain degree. Among them, the one contained 3.5 wt.% of Co and Al ions shows the highest electrochemical performance, the diffusion coefficient of lithium ion(DLi) of the material is 1.44×10-10 cm2·s-1. At the current rates of 0.2 and 10 C, the discharge capacity of the granule LiMn2O4 was measured to be 120 and 110 mAh·g-1, respectively, showing a capacity retention of 91.7 %. After cycled at 25 ?C and 1 C for 200 cycles, it delivers a discharge capacity of 117 mAh·g-1, retaining 95.1 % of its initial capacity. After cycled at 55 ?C and 1 C for 100 cycles, it preserves a discharge capacity of 103 mAh·g-1, preserving 85.8 % of its initial capacity. The greatly improved rate capability and cycling performance, as well as its large discharge capacity, can be ascribed to its unique microstructure. The porous granule LiMn2O4 coated with a proper amount of Co and Al ions exhibits remarkably improved rate capability and cycling performance especially at elevated temperatures. The incorporated Co ions may result in the enhanced electronic conductivity, while the addition of Al ions may lead to the improved lattice stability, and may restrain the John-Teller distortion duing charge/discharge process. As a result, the electrochemical performance of the Co-Al coated granule LiMn2O4 is further improved. |
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