Synthesis And Characterization Electrochemical Properties Of Ultrafine LiCoO₂ Powders

Lithium cobalt oxide (LiCoO₂) possesses high specific capacity and good cyclic stability, and is currently one of the most widely used cathode materials for Li-ion batteries. They have been conventionally synthesized by solid-state reaction at temperatures over 850°C, resulting in relatively large particle size and difficulty in controlling stoichiometry because of high activation energy for the diffusion of reacting species in solid state and volatilization of lithium at high temperatures. The larger particle sizes not only have smaller surface areas that limit electrochemical reactivity, but also lead to slower charging and discharging rates and lower power density. To overcome these difficulties, many efforts have been devoted to developing LiCoO₂ with smaller particle size, and solution based sol-gel synthesis have emerged as an attractive alternative. In such process, reacting species can be mixed thoroughly in solution, making it possible to complete the synthesis with lower temperature and shorter reaction time.In this dissertation, the LiCoO₂ electrode material have been systematically investigated. Lithium cobalt oxide powders were synthesized by sol-gel process using four different organic acids as chelating agents, such as citric acid, maleic acid, propionic acid and acrylic acid, and metal acetates as the source of metal ions as well as oxidants. Effect of four different organic acids on the synthesis of LiCoO₂ powders was investigated through X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Among them, pure LiCoO₂ powders could be prepared by using acrylic acid as chelating agents. Therefore, acrylic acid was chose to synthesize the lithium cobalt oxide powder by sol-gel and electrospinning, respectively.X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterization the microstructure and morphology of sol-gel and electrospinning powders. Coin cells (CR2025 size) were assembled in an argon-filled dry-boxthe using as-synthesized LiCoO₂ powders as positive electrode material. Metallic lithium was used as anode. The electrochemical behavior of the system was evaluated in ethylene carbonate/dimethyl carbonate solution, using 1 M LiPF6 as supporting electrolyte. Subsequently, the cyclic voltammogram curves and galvanostatic charge-discharge properties were tested using electrochemical workstation and multi-channel battery test system. Then compared the performance of nanofibers derived powders with sol-gel powders.Sol-gel based electrospinning has been developed to synthesize phase pure LiCoO₂ powders at relatively low temperature with excellent crystallinity and ultrafine particle size. Compared to LiCoO₂ powders synthesized from regular sol-gel processes, the nanofiber derived powders possess high initial discharge capacity and good cyclic stability. With additional surface modification of La2O3 coating, the retention of initial capacity is increased to 91% at 30th cycle and 83% at 50th cycle without decreasing its initial capacity.