The ionic liquid, l-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. The ionic liquid was synthesised and purified until Br < 40 ,vtppm, H20 < 2 ppm. Effects of water on the cathodic stability limit were studied using a platinum microdisc electrode and a :gold microdisc electrode array. The response of the cathodic current on the water concentration suggests catalytic decomposition of EMr with moisture. The cathodic potential limit shifted negative with addition of lithium salt, especially on a nickel microelectrode, so that deposition and stripping current for lithium was observed. This is attributed to the formation of a solid electrolyte interface (SEI). Evidence for the formation of a SEI was also found from cyclic voltammograms and impedance spectra for lithium metal electrodes as well as open circuit cell potentials. Addition of LiTFSI to EMITFSI resulted in a decrease in the conductivity (e.g., from 10.5 to 5.6 mS cm-l for 0.47 mol dm-3 ) and the lithium ion diffusion coefficient was found to be 1.2 x 10-7 cm2 S·l 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 ofthe anions. The charge / discharge rate performance· of LiFeP04 carbon composite electrodes with various thicknesses in different concentrations of LiTFSI I EMITFSI electrolytes was studied using 3-electrode cells. At fast charge or discharge rates, discharge capacities were approximately inversely proportional to C-rate, suggesting that the capacities were controlled by lithium ion diffusion in the pores of the composite electrode. Differences in rate perfonnance were found between charge and discharge and for different concentrations 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 fmdings, using a 14 LiFeP04 positive electrode, mol dm-3 LiTFSI / EMITFSI and a lithium negative electrode. The cell gave a discharge capacity of more than 100 mAh g-l over 850 cycles.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:484958 |
| Date | January 2007 |
| Creators | Wakizaka, Yasuaki |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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