Study On New Electrolyte For Lithium Ion Batteries

To develop high performance lithium ion battery,advanced electrolyte is absolutely indispensable.The current state-of-art electrolytes are highly flammable and volatile because of the presence of organic carbonates.This feature prohibits the development of batteries with safety and wide operating temperature range.The advent of ionic liquids which possess nearly non-volatility,non-flammability and wide stability seem to be a promising breakthrough.Their superior properties show potential to prevent the risk of thermal runaway and to enhance stability at high temperatures.In this paper,N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide(PP13TFSI)was synthesized as cosolvent for organic solvent based electrolyte and vinylene carbonate(VC)and 1,3-propane sultone(PS)were used as additives for ionic liquid(IL)based electrolytes.The high viscosities and low ionic conductivity of IL could lead to capacity-fading and charge/discharge rate limitation.An alternative solution is to add organic solvents EC-DEC-VC as diluents to reduce the high viscosity.So finding the optimum balance between ionic liquid and organic solvents is crucial.Physical properties including conductivity,thermal stability and flammability along with electrochemical performances like cyclic voltammetry and cyclic performance of the mixed electrolyte were discussed.Besides,in order to break restriction on the application of lithium ion battery technology,high temperature performance was tested to detect the high temperature extremes.In mixed electrolyte,safety and stability of the electrolyte were greatly improved by the adding of IL,no flame was observed at the concentration of 80%IL.Meanwhile higher conductivity(10.08 mS/cm)was obtained at the concentration of 40%IL.The mixtures were also utilized as electrolyte in LiFePO4/Li half cells.Cell performances were studied under different charge and discharge rate from room temperature to 80 ?.It appeared that mixed electrolytes displayed outstanding cyclic performance at both room temperature and elevated temperatures.IL can not form steady and efficient solid electrolyte interface(SEI)on electrode,resulting in irreversible capacity loss in the first cycle and poor cyclic performance.Additives intended for the modification of the SEI are expected to solve this problem.Here in this paper,VC and PS were added to the electrolyte system respectively as additives to improve the property of solid electrolyte interface(SEI)and cyclic performance.Cyclic voltammetry and the morphologies of the cycled graphite anode compact SEI film were detected.Charging and discharging performance of LiFePO4/Li half cell has been tested in electrolyte with or without additives.The results showed VC and PS could passivate on graphite electrode before the decomposition of IL electrolyte.The formation of an effective SEI facilitates the insertion and extraction of lithium ion.Cyclic performance of LiFePO4/Li half cells has been greatly improved by using electrolytes containing VC and PS.