The Performance Of Ionic Liquid Electrolyte In Lithium-Ion Batteries At Extreme Temperatures

Lithium-ion batteries are widely used in hybrid electric vehicles due to their high energy density, high specificcapacity, high voltage and long cycle life.However,the large-scale practical application of lithium-ion batteries is still faced with crucial challenges on safety, due to the high flammability and high vapor pressure of the organic carbonates which used in commercial lithium-ion batteries. Therefore, it’s very urgent to develop new safety and green electrolyte. Ionic liquid are non-flammable, non-volatile and have good thermal stability, wide electrochemical window, pollution–free which can be applied to improve the safety of the electrolyte in lithium-ion batteries. But the ionic liquid has larger viscosity, which decrease the electric conductivity and affect the battery performance. So the ionic liquids should be modified to achieve the purpose of improving battery performance. In this paper, Pyrrolidinium-based ionic liquid N-propyl-N-methylpyrrolidinium bis(trifluoromethan esulfonyl)imide(PYR13TFSI) and piperidinium-based ionic liquid N-propyl-N-methyl piperidinium bis(trifluoromethan esulfonyl)imide(PP13TFSI) were selected as research object to study the effect of ionic liquids on the battery safety and improve the performance of the battery with organic solvent mixing.Pure ionic liquid has broad electrochemical stability window(> 5.5 V), large viscosity, poor wettability, high thermal stability(400℃ when began to break down).The average capacity of Li Fe PO4 / Li battery with PP13 TFSI and PYR13 TFSI electrolyte were 157.5 m Ah/g and 145.6 m Ah/g; The average capacity of ionic liquid C/Li battery can reach more than 200 m Ah/g, explaining that ionic liquid and C anode can have good compatibility.For the cathode material, special organic additive FEC(fluoro ethylene carbonate) can improve the low temperature and high temperature performance of half cell at the same time, but restraining the room temperature performance. For the anode material, FEC can improve the room and low temperature performance of half cell at the same time, but rejecting rate performance in the high temperature. MP(methyl propionate) can reduce the wettability and freezing point of the electrolyte to improve the battery performance and increase discharge specific capacity of battery in low temperature.We observed that the non-flammability threshold is reached when the organic additive content in the mixtures is lower than 35 vol%, and the addition of IL reduce the kinetics of the organic additive evaporation. Organic additives can improve the compatibility of electrolyte and the pole piece to enhance the cycle performance, cycle coulomb efficiency, and rate performance in extreme temperature for the batteries. At 75℃, the capacity of positive and negative half cell with 85 % ionic liquid composite electrolyte can reach 155 m Ah/g and 375 m Ah/g at room temperature. At-35℃, safety performance of 50 % ionic liquid composite electrolyte is better than that of commercial organic electrolytes. Compound ionic liquid is also applied to the C-Si anode materials.