Study On Synthesis And Characteristics Of Nano Composite Particles For Lithium-ion Battery

Comparing with LiFePO4 and LiMnO2 cathode materials, Nickel rich cathode material has attracted more attention with its high energy density in the electromobile field for the improvement of vehicle range. But LiNi0.6Co0.2Mn0.2O2(NCM-622) materials commonly undergo structure degradation and thermal instability due to their phase transformation in high voltage. Now the doping and surface modification such as surface coating have been the effective way to solve the structure and thermal instability of NCM-622 materials. The thesis focuses on the synthesis and modification of NCM-622 materials, a series of research works are as follows:(1) In the early research work, to study the technological conditions of coating on the surface of NCM-622 powders by ultrasonic eletrodeposition(UED), we first chose SiO2 spheres with good monodispersity as substrates to prepare Ag@SiO2 and Au@SiO2 particles through adjusting different reaction parameters of UED method. The experimental results showed that the Ag nanoparticles on the SiO2 surface delivered uniform particle size distribution and excellent monodispersity when the current density is 15mA/cm2, the electrolyte concentration is 10-4 mol/L, the ultrasonic power is 50w and the reaction time is 1h. And the Au nanoparticles on the SiO2 surface exhibited more metal loadings and good monodispersity when the current density is 35mA/cm2, the electrolyte concentration is 10-3 mol/L, the ultrasonic power is 100w and the reaction time is 1.5h.(2) We tried to coat Au particles on the surface of NCM-622 powders by UED method. Firstly we prepared NCM-622 cathode materials by raw materials including NiSO4·6H2O、CoSO4·7H2O、MnSO4·H2O and LiOH·H2O via co-precipitation method. Then we successfully coated the Au particles on the surface of NCM-622 materials to obtained Au@NCM-622 cathode materials through UED method. Afterwards we assembled the NCM-622 and Au@NCM-622 powders into button cells and carried out their electrochemical performance. The results of test delivered that the initial discharge capacity of NCM-622 electrode is 175.6mAh/g at 0.1C rate while Au@NCM-622 electrode is only 147.5mAh/g, and NCM-622 electrode retained 70.4% of the original 175.6mAh/g at 0.1C while Au@NCM-622 electrode retained only 39.2%. However, the capacity loss of Au@NCM-622 electrode is 17.5% after 100 cycles at 1C. While the NCM-622 electrode showed a gradual decrease in discharge capacity, leading to the capacity loss of 30% after 100 cycles at 1C. These suggested that the Au coating on the surface of NCM-622 powders by UED could improve the cycling performance but decrease the discharge capacity and rate capability of cathode materials.(3) We deposited ultrathin(5nm) and highly conformal TiO2 coating onto NCM-622 powders by ALD technique to obtained TiO2@NCM-622 cathode materials. The electrochemical performance of NCM-622 and TiO2@NCM-622 samples had been measured. The initial discharge capacity of TiO2@NCM-622 electrode is 187.7mAh/g at 0.1 C and the capacity retention of TiO2@NCM-622 is 85.9% after 100 cycles at 1C. The results showed that TiO2@NCM-622 samples exhibited the significantly improved discharge capacity and cycleability compared with NCM-622 samples.