Study On Multifunctional Electrolyte Additive And Cathode Material Modification For Lithium-ion Battery

Safety and cycle lifetime of lithium-ion batteries have become the main concerns for their wide applications for electric vehicles and smart grids. The safety characteristics of electrolyte and stability of electrode material directly affects the safety and cycle lifetime of lithium ion batteries. Therefore we can improve the performance of lithium ion battery from both sides of electrolyte and electrode material. In this paper, the author discusses the application of quantum chemistry calculation method in the study of electrolyte additive, and develops a new multifunctional additive. In addition, a coating modification method was synthesized to improve the electrochemical performance of cathode material. The main contents are as following:Firstly, this paper briefly introduces the application of quantum computing calculation method. On the basis of the density functional theory (DFT), the author calculated the frontier orbital energy (HOMO, LUMO), total energy, dipole moment, chemical hardness and Li+ binding energy for solvents and additives using the Gaussian09 programs package, and predicted the electrochemical properties of a new additive.Secondly, the effects of ethylene ethyl phosphate (EEP) as a multifunctional electrolyte additive on safety characteristics and electrochemical performance of lithium-ion batteries are investigated. Based on the flammability test, the self-extinguishing time of the electrolyte with 10% EEP is only less than half of the baseline electrolyte, which indicates that EEP is a highly efficient flame retardant for the electrolyte. During overcharging the LiNi1/3Co1/3Mn1/3O2/Li cells, incorporation of EEP into the electrolyte can postpone the sharp voltage rise. Therefore, EEP is an improver on safety characteristics of lithium-ion batteries, in both terms of flame resistance and overcharge protection. Furthermore, the EEP-containing electrolyte in the half-cells and full-cells both exhibits higher initial Coulombic efficiency and cycling stability than the baseline electrolyte. It is concluded that EEP is a good film-formation additive not only for the graphite anode, but also for the LiNi1/3Co1/3Mn1/3O2 cathode. Therefore, EEP is proposed as a promising multifunctional electrolyte additive for the lithium-ion batteries.Finally, cerium fluoride (CeF3) coated LiNi1/3Co1/3Mn1/3O2 particles are synthesized using a facile chemical deposition method. The results indicate that the cathode particles are uniformly covered with a CeF3 layer (10 nn thick) after 2 wt.% CeF3 surface coating. The coated electrode demonstrates higher initial Coulombic efficiency, better cyclic performance and superior high-rate capacity. LiNii1/3Co1/3Mn1/3O2 coated with 1 wt.% CeF3 showed the best electrochemical performance. The remarkably improved cycling stability and high-rate capacity of the surface-modified cathode material are ascribed to the presence of a stable and thin CeF3 coating layer which effectively reduces the damage of electrode structure and suppresses the increase of impedance during cycling by preventing direct contact of the electrode with the electrolyte.