Study On Synthesis Process And Optimized Electrochemical Properties Of LiNi_0.5Mn_0.5O₂ Cathode As Lithium-ion Batteries

With the development of the technology and electric vehicles, lithium-ion battery is ushering in a broad space for development. The main cathode material which has widely used in lithium-ion battery is lithium cobalt oxide. But it has some problems, just like its high cost, environmental unfriendliness. So people need to seek the alternative materials. In recent years, the researchers found that the lithium cobalt oxide could be instead of Ni, Mn Co solid solution, LiMnO2, LiNiO2, and LiNi0.5Mn0.5O2 material. So the new material system just like LiNi0.5Mn0.5O2 has been widespread concern and research.But the LiNi0.5Mn0.5O2 material has its own problem. LiNi0.5Mn0.5O2 adopts the layered R-NaFeO2 structure (space group R3m), but a considerable amount of Li and Ni is exchanged between the Li(3a) and Ni/Mn (3b)layers due to the similar size of the Li+ and Ni2+ cations. Two compositional strategies exist for reducing the Ni content in the Li layer and increasing the layered character of the structure. The disorder of the ions makes the capacity and cycle performance poor. As an entry point for the study, in order to reduce the disorder of the Li-ions and Ni-ions we choose the optimized preparation process to prepare the material. This paper uses the lithium carbonate as the solvent of the molten salt for the molten salt method, bicarbonate as the precipitant solvent to aqueous for co-precipitation method and NaNi0.5Mn0.5O2 as the precursor for ion-exchange method. We discuss the advantages of these three methods and the relationship of the material structure and electrochemical properties.The molten salt method that the carbonate as the solvent, compared to the conventional solid-phase method, it can reduce the sintering temperature and time. Due to the massive existence of Li-ions, the Ni-ions could not inset into the lithium layer. So it can reduce the Li+/Ni2+ ions disorder. We can get the best structure and performance when the sample synthesized o 850 degrees for 5 hours. The initial cycle capacity is about 100mAh/g, and after 50 cycles it's up to 130mAh/g stable.In the co-precipitation method, we discussed the process of synthesis of precursor. We want to synthesize the layered structure in the precursor. Because this method can reduce the Li-ions inset into the Ni-ions layers. Firstly we synthesize the precursor at 50 degrees under the protection of nitrogen deposition. Then the precursor was sintered at 900℃for 20 hours. The sample's structure is excellent. At 0.2C rate and the range of 2.5 to 4.5V, the charge and discharge efficiency is as high as 97.1%. The initial capacity is about 140.2mAh/g, after 50 times of the cycle, it remains 130mAh/g.Finally, we used ion-exchange hydrothermal method to synthesize LiNi0.5Mn0.5O2 materials. We first synthesized NaNi0.5Mn0.5O2 precursor. Because of the difference between the ions radius of the Na-ion and Ni-ion, it can form the layer structure without ions disorder. Then we use Li-ions to make ion-exchange of Na-ions. It can effective reduce the disorder of the Li-ions and Ni-ions. We get the optimal synthesis conditions to synthesize NaNi0.5Mn0.5O2 at 900℃for 5 hours. Then we can get the finally sample by at 240℃, in the hydrotherm of 3mol/L of LiOH. The reaction lasts 16 hours,3 times total of 48 hours. Finally we get the LiNi0.5Mn0.5O2 material with excellent structure. The initial discharge capacity of the material reaches 170mAh/g. After 50 cycles it remains 150mAh/g.These three methods can solve the LiNi0.5Mn0.5O2 material's problem which is reduction of Li-ions and Ni-ions disorder in the transition metal (TM) layers. We prepare the implementation of these optimizations to improve the electrochemical properties of the material.This means that our work played a role to the optimized structure and the electrochemical performance in this material.