Design,Synthesis And Electrochemical Properties Of Electrolyte Additives For Lithium Ion Batteries

Lithium-ion batteries have been recognized as one of the most ideal rechargeable battery because of its advantages of large specific energy,stable discharge voltage,long cycle life,environment friendly,no memory effect,small self discharge and fast charging.It has been widely used in portable electronic devices,such as mobile phone,notebook and camera.And it was also been ultilized as the most promising power supply for electric vehicles.Both of portable electronic devices and electric vehicles require that lithium-ion batteries to have a higher energy density to make them lighter,smaller and have a longer-lasting battery life.Increasing the voltage of lithium-ion batteries is an effective way to improve its energy density and power performance.At present,the main reason limiting the working voltage of lithium-ion batteries is the electrochemical stability of the traditional carbonate electrolyte.When the voltage reaches about 4.5 V,the oxidative decomposition of the electrolyte will occur.It will greatly reduce the cycle performance and safety performance of lithium-ion batteries.In this study,the electrolyte was modified by introducing fluorine-containing additives to improve the electrochemical stability,thereby enhancing the working voltage of lithium-ion batteries.It is difficult to capture the electrons from fluorine-containing molecules because of the strong electronegativity of fluorine element so that can improve the electrochemical stability of these molecules.In this study,we designed a type of fluorine-containing organic additive,3,3-difluoro-2-oxindole,and developed an efficient synthesis method for this additive through a visible-light-promoted direct ortho C-H difluoroacetylation of free anilines with bromodifluoroacetate in the present of tris(2-phenylpyridine)iridium and Na2CO3.This method has a wide substrate tolerance and mild reaction conditions.Using this method,we synthesized a series of target molecules with different functional groups.The theoretical calculations of these additive molecules were performed by using density functional theory(DFT)as implemented in the Gaussian09 software package.The B3 LYP functional and 6-31+G(d)basis set,LanL2 DZ for heavy elements,were used to optimize the ground state geometry of the molecules.The frequency analyses were performed at the same level of theory to confirm each stationary point and make zero-point energy(ZPE)corrections.According to the calculated highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)of the fluorine-containing additives,several representative fluorine-containing additives were screened out and added into 1 mol/L LiPF6/EC?DEC?EMC(1?1?1)electrolyte system in a ratio of 0.2 wt%.Then the electrolyte was added into coin cells with LiNi0.5Mn1.5O4 positive electrode,lithium negative electrode and polyethylene separator.The electrochemical properties of the batteries,such as electrochemical impedance,cyclic voltammogram and rate performance,were measured.The test results show that addition of 3,3-difluoro-5-(methylsulfonyl)indolin-2-one in the lithium ion battery electrolyte can reduce the charge transfer resistance,increase the diffusion rate of lithium ions,and improve the rate capability.