Preparation Of Ionic Liquid Electrolyte And Its Performance Research Of Lithium-ion Batteries

Lithum- ion batteries have been applied in cell phones, laptops, digital cameras, as well as other potable electric devices and have been considered to be the most promising power sources for the next generation of hybrid electric vehicles(HEV) and electric vehicles(EV), due to high power density and energy density. However, the safety problems such as high flammability, low thermal stability and high vapor pressure of commercial electrolytes, restrict its development. The development of new security electrolyte system to solve the safety of the commercial lithium ion battery is a critical issue. Ionic liquid is considered to replace the conventional organic electrolyte and solve the safety issues by virtue of wide liquid range, wide electrochemical stability window, negligible vo latility, non- flammability, etc. In this work, N- methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide(PP13 TFSI), N-methyl-N-propylpyrrolidinium bis(tri?uoromethanesulfonyl)imide(PYR13 TFSI) and 1,2-dimethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide(DMEI TFSI) were synthesized and mixed with organic solvents, lithium bis(trifluoromethanesulphonyl)imide(Li TFSI) and Lithium difluoro(oxalate)borate(Li DFOB) to prepare composite electrolytes. And the properities of Li Mn1/3Ni1/3Co1/3O2 and graphite electrode in these electrolytes were investigated by cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS) and charge-discharge tests.The results showed that the physical and electrochemical properties of these ionic liquid electrolytes exhibit high thermal stability, low viscosity and a wide electrochemical window(more than 5.5 V). We observed that the non- flammability threshold is reached when the organic additive content in the composite electrolyte is lower than 30 vol%, and the addition of IL reduce the kinetics of the organic additive evaporation. The cyclic performance in Li/Li Mn1/3Ni1/3Co1/3O2 half cells is also encouraging, showing a discharge capacity of ca. 184.9 m Ah/g in the 70 vol% DMEITFSI electrolyte at 0.1 C in the voltage range of 2.75-4.5 V, and the discharge capacity is found to be above 132.6 m Ah/g after 150 cycles at 1 C current rate at 25 ℃. The graphite anode delivers high reversible capacity(332.8 m Ah/g) with coulomb efficiency close to 100% after 30 cycles at 0.1 C in the voltage range of 0.01-2 V. The cycling results for the new electrolytes are comparable to those of commercial electrolyte.Additionally, the new electrolytes can provide enhanced safety and exhibit good compatibility with Li Co1/3Ni1/3Mn1/3O2 cathode at elevated temperatures. Half cells Li/Li Mn1/3Ni1/3Co1/3O2 using the mixed electrolytes with 90 vol% IL display a discharge capacity of 177.2 m Ah/g with 98.1% capacity retention at 1 C current rate at 80 ℃. Meanwhile, half cells Li/C using the mixed electrolytes with 70 vol% IL display a charge capacity of 377.1 m Ah/g after 20 cycles at 1 C current rate at 80 ℃.The appropriate amount of piperidine ionic liquid(PP13TFSI) was added to electrolyte to suppress the pyrrolidine ionic liquid(PYR13TFSI) in the electrode surface decomposition, the electrode material in the two-component electrolyte system is given a good reversible capacity. The initial discharge capacity of the Li Co1/3Ni1/3Mn1/3O2 cathode reaches 154.5 m Ah/g and the coulombic efficiency is 85.5% at 0.1C rate. The CV and scanning electron microscope(SEM) tests illustrate that the presence of Li DFOB in the electrolyte is helpful to form a stable solid electrolyte interphase(SEI) and restrains the co-intercalation and decomposition of DMEI+, PYR13+ on the surface of the electrode.