Synthesis and Application of Multifunctional Ionic Liquids for Lithium Ion Batteries

Abstract

Ionic liquids have been considered as attractive additives in lithium ion batteries, due to their unique advantages. In this study, two new types of ionic liquids 3-(2-amino-2-oxoethyl)-1-vinylimidazolium bis(trifluoromethylsulfonyl) amide ([VAIM][TFSI]) and 1-vinyl-3-propionamide imidazolium bis(trifluoromethylsulfonyl) imide ([VPIM][TFSI]) were synthesized using two-step methods. The purities of the two ionic liquids were determined to be more than 99 % by 1H NMR spectra. The water content of the two ionic liquids were measured to be below 300 ppm by Karl Fischer titration. No chloride ions were detected by adding AgNO3. The physicochemical, thermodynamic, and electrochemical properties of pure ionic liquids were measured and described. The densities of the two ionic liquids are high and decrease linearly with the increase of temperature. Their viscosities are also high and decrease with increasing temperature. Both the two ionic liquids are non-Newtonian fluids at low temperature. [VPIM][TFSI] shows Newtonian fluid behavior at temperatures higher than 55°C. The two ionic liquids also display high thermal stabilities, with a range of [VAIM][TFSI] from -30 ± 5°C to 300 °C, and [VPIM][TFSI] from -50 ± 5°C to 200 °C. Four binary mixtures, including [VAIM][TFSI] and acetonitrile, [VAIM][TFSI] and ethyl acetate, [VPIM][TFSI] and dimethyl carbonate (DMC), and [VPIM][TFSI] and ethyl methyl carbonate (EMC), were used to evaluate the physicochemical and thermodynamic properties. In mixtures with [VAIM][TFSI] , both the density and the viscosity increase with increasing mole fraction of ionic liquid. The densities decrease linearly with increasing temperature from 25 °C to 60 °C at all mole fractions. The excess molar volume and viscosity deviations were calculated from experimental values. Conductivities were determined using a three-electrode cell. A maximum conductivity is obtained at about 0.07 mole fraction of [VAIM][TFSI] at all temperatures. In the [VPIM][TFSI] system, density and viscosity were also measured with different mole fractions of [VPIM][TFSI] at different temperatures from 20 °C to 60 °C. Both density and viscosity increase with the increasing of mole fraction of ionic liquid, and the densities decrease linearly with increase in temperature at all mole fractions from 20 °C to 60 °C. The excess molar volumes calculated from the experimental data are negative in [VPIM][TFSI]-based mixtures. Viscosity deviations in [VPIM][TFSI] and DMC mixtures are negative at all mole fraction of [VPIM][TFSI], however, the values in the [VPIM][TFSI] and EMC mixtures are negative before 0.6 mole fraction and then are positive. Three types of electrolytes, 1M (M: mol/L) LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (w/w=3:7), 1 M (M= mol/L) LiPF6 in EC/DMC/EMC (v/v/v=1/1/1), and 1 M (M= mol/L) LiPF6 in EC/DMC/EMC (w/w/w=1/1/1) were applied in four half-cells, including SiC/Li, LTO/Li, NCM622/Li, and LFP/Li. Both the two multifunctional ionic liquids have two functional groups, the basic group can control the water and HF at a low level in the three electrolytes. The unsaturated group can be decomposed on the surface of the four electrodes to form a positive protected film to improve the capacity, cycling, and rate performances of lithium ion batteries. In the SiC/Li half-cell system, the ionic liquid [VPIM][TFSI] additive can improve the cycling performance by the formation of a protective film. The cell with 0.5 wt % [VPIM][TFSI] delivers a maximum capacity and a higher capacity retention than a cell without [VPIM][TFSI]. In the LTO/Li half-cell system, the ionic liquid [VAIM][TFSI] restrains the gas generation to protect the battery from expansion and rapid breakdown. The LTO battery with 0.5 wt % ionic liquid [VAIM][TFSI] displays good cycle and rate performances. A small layer on the surface of the electrode was observed after charge-discharge cycles. The layer was formed by the decomposition of ionic liquid. In NCM622/Li half-cells, the addition of [VPIM][TFSI] improves the electrochemical window. [VPIM][TFSI] at the concentration of 1.0 wt % enhances the cyclic and current rate performance of the cells. A suitable surface film was found on the surface of NCM622 electrode using the electrolyte with 1.0 wt % [VPIM][TFSI]. In the LFP/Li half-cell system, the two ionic liquids can also improve the cyclic and rate performances. The cells with 0.5 wt % [VAIM][TFSI] or [VPIM][TFSI] have the best cyclic performances, and the cells with ionic liquids have better rate performances. CV, EIS, SEM and XPS are used to analyze the performance and mechanism of the cells.