EMITFSI - an ionic liquid electrolyte for lithium batteries

The ionic liquid, 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)-imide (EMITFSI) was studied as an electrolyte for rechargeable lithium batteries. This work focused on two main topics: cathodic stability and lithium ion transport. Addition of LiTFSI to EMITFSI resulted in a decrease in the conductivity (e.g. from 10.5 to 5.6 mS cm-1 for 0.47 mol dm-3) and the lithium ion diffusion coefficient was found to be 1.2 x 10-7 cm2 s-1 for 0.47 mol dm-3 added Li salt. The transference number for lithium ions in LiTFSI/EMITFSI was found to be proportional to the concentration of the lithium salt. The measured value of 0.04 for 0.47 mol dm-3 is significantly higher than that of LiBF4/EMIBF4 at the same concentration and temperature. This may be explained with two factors; the differences in size and dissociation level of the anions. The charge/discharge rate performance of LiFePO4 carbon composite electrodes with various thicknesses in different concentrations of LiTFSI/EMITFSI electrolytes was studied using 3-electrode cells. At fast charge or discharge rates, discharge capacities were approximately inversely proportional to C-rate. Differences in rate performance were found between charge and discharge and for different concentration of lithium salt in the ionic liquid. Two models are proposed to explain above phenomena; a transmission circuit to represent electrolyte resistance, and a salt depletion model simplified by the assumption of a compact discharge front. An optimised cell was designed and constructed according to the above findings, using a 14 μm LiFePO4 positive electrode, 0.47 mol dm-3 LiTFSI/EMITFSI and a lithium negative electrode. The cell gave a discharge capacity of more than 100 mAh g-1 over 850 cycles.