The Synthesis And Application Of Electrolyte Containing Tetrafluoroborate Ionic Liquid For Lithium Ion Batteries

Due to the high operating voltage, large energy density, long cycle life and less pollution to the environment,Lithium- ion batteries had become mainstream sources of power for mobile devices. Because of the low flash point, volatile, toxic and involved in the thermal decomposition reaction, the traditional organic electrolyte lithium- ion batteries have great security risks. Thus, new types of environmentally friendly and not flammable electrolyte need to be developed. Ionic liquids, are entirely composed of ion compound and can keep the liquid state at room temperature. It is non-volatile and non-flammable; what’s more, these kinds of ionic liquids have stable and wide electrochemical window and high electrical conductivity, and are expected to solve the problem of the safety of the lithium ion battery. In this article, we synthesized two kinds of ionic liquids: 1-Ethyl-3-methylimidazolium tetrafluoroborate(EMIMBF4) and 1-Butyl-3- methylimidazolium tetrafluoroborate(BMIMBF4). We studied the physical and chemical properties and the compatibility with three kinds of the anode material(LiC oO2, LiFePO4, LiNi1/3Co1/3Mn1/3O2).We used N- methylimidazole as a raw material to synthesis target ionic liquids EMIMBF4 and BMIMBF4 by the traditional method and microwave method, we compared the yield about the products. The results showed that when the weight ratio among C4H9 Br, N-methylimidazole and KBF4 was 1.0: 1.0: 1.6; Microwave power was 240 W, reaction temperature was 70℃ and reaction time was 20 minutes, The maximum yield reached to 72%,was higher than the traditional method(60%). The viscosity were 51mm2/s and 123mm2/s, and the conductivity were 14.0×10-3 S/cm and 3.1 ×10-3 S/cm. TGA test results showed that the thermal decomposition temperatures were 323.7℃ and 325.7℃. The electrochemical window about EMIMBF4 was 4.4 V, and 5.3 V about BMIMBF4.We dissolved LiPF6 into EMIMBF4 and BMIMBF4, the concentration were 0.5 mol/L, 0.8 mol/L, 1.0 mol/L. At room temperature, the conductivity of 0.5 mol/L, 0.8 mol/L, 1.0 mol/L LiPF6+EMIMBF4 were 9.2 mS/cm, 5.8 mS/cm, 7.8 mS/cm. And the conductivity of 0.5 mol/L, 0.8 mol/L, 1.0 mol/L LiPF6+EMIMBF4 were 2.52 mS/cm, 1.4 mS/cm, 2.09 mS/cm.LiCoO2 was used as the cathode material, lithium sheet as the negative electrode.We added an equal volume ratio organic electrolyte into the different concentration ionic liquids which contains equal concentration of LiPF6, and EC+DEC(volume ratio=1:1) as the electrolyte to assembly lithium batteries, and tested their electrochemical performance. The results showed that, 0.8 mol/L and 1.0 mol/L LiPF6+EMIMBF4(EC+DEC) electrolyte first charge capacity was 98.4 mAh·g-1 and 95 mAh·g-1, higher than 68.1 mAh·g-1 when the lithium salt concentration is 0.5 mol/L. After 50 cycles, battery capacity was 74.4 mAh·g-1, 84.3 mAh·g-1 and 90.6 mAh·g-1, the capacity retention rate was 110%, 96.1% and 99.8%. 0.8 mol/L LiPF6+EMIMBF4(EC+DEC) electrolyte exhibits excellent first charge and discharge performance, charging specific capacity of 113 mAh·g-1, the discharge capacity of 98.2 mAh·g-1. Coulombic efficiency was 86.9%. After 50 cycles, the capacity was stable. In the experiment, when using 0.8 mol/L LiPF6+EMIMBF4(EC+DEC) ionic liquid as electrolyte, the performance of the battery was best.We used 0.8 mol/L LiPF6+EMIMBF4(EC+DEC) ionic liquid as electrolyte, LiCoO2, LiFePO4, LiN i1/3Co1/3Mn1/3O2 as cathode material. Analyzed each cathode material cycling performance at 0.1C, 0.5C and 1C current density. The results show ed that LiCoO2 was the worst of the three kinds of the anode materials. The specific capacity and cyc le performance of batteries with LiFePO4 and LiN i1/3Co1/3Mn1/3O2 were similar at 0.1C and 0.5C.The first charge and discharge specific capacity maintained at a high value, after 50 cycles, no capacity fade was found, capacity retention rate was 95%.When the current density increased to 1C, LiN i1/3Co1/3Mn1/3O2 material can still maintain a relatively high charge-discharge capacity, capacity retention was 93% after 50 cycles, showing a good performance at cycle characteristics and high charge-discharge rate.