Nanostructured Lithium-Nickel-Manganese Oxides Cathode Material And Its Electrochemical Properties Research For Lithium-Ion Batteries

Lithium-ion batteries are widely applied in electronic system, space technique, electric vehicle and military field. The positive electrode material is one of the most important technologies for lithium-ion battery. LiCoO₂ is being used as the cathode material in the majority of commercial lithium-ion batteries with good capacity, reversibility and rate capability, but it suffers from relatively high cost and the toxicity of cobalt. Lots of efforts have been made to develop possible alternatives as cathode.Nanomaterials exhibit various new kinds of physical properties. Nanostructured electrodes have showed special properties in interior structure, morphology and electrochemical performance, which attract much attention by researchers all over the world. Based on purposes of realizing and analyzing the possible physical phenomena about nanostructured cathode of lithium-ion batteries, this thesis focus on LiNi_0.5Mn_0.5O₂ system and adopt citric acid assisted sol-gel method for synthesizing. The best synthesis techniques are optimized, the structure and properties of obtained sample is systematic studied by several characterization instruments. And the influence of nanostructured electrode toward the electrochemical performance is deeply explored.Hence, this thesis involves the contents as follows: Formation method and technique about nanostructured materials; Influence of components variety toward structure and property. We attempt to synthesize nanostructured cathode which have fine capacity and cycle performance initially, then improve its high rate performance through both shorten diffusion path of Li⁺ in grains and enhance specific surface area to decrease effective current density. The experiment contrast synthesis factors such as initial addition amount of Li, sintering temperature and time. The experiment results show that the sample of LiNi_0.5Mn_0.5O₂ which was sintered for 10h at 900℃from Li/(Ni+Mn)=1.05 in start material exhibits the best interior structure and electrochemical performance. The particles are uniform with size of about 80 nm and exhibit fine capacity and cycling retention in the voltage range of 2.5-4.5V at a specific current of 20mA/g. Comparing with micro-sized cathode material, this nanostructured LiNi_0.5Mn_0.5O₂ has improved performance at high rate charge-discharge.The apparent diffusion coefficient of Li⁺ in synthesized nanostructured LiNi_0.5Mn_0.5O₂ cathode was measured by potential step method. The promoting effect of decreased size of nano-crystallite toward the improvement of Li⁺ diffusion coefficient is theoretically confirmed. Because the sample which has the highest apparent diffusion coefficient is not the minimum-sized one, this thesis correct the influence from size, which is geometrical factor, toward the apparent diffusion coefficient, then further discuss how decreased crystallite size influence intrinsic diffusion coefficient. Analysis shows that the apparent diffusion of materials is determined by both crystallite size factor and intrinsic diffusion factor. Its change law is the addition of those two factors. As a result, it is not optimum to unilaterally emphasize small crystallite size. The influence from small size toward ion diffusion and capacity of materials should also be considered during the synthesizing process. The improved practical performance of electrode would balance both crystallite size and property.