Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems, страница 6

Thus, the LiFSI salt has good performance on both anode and cathode side. Also, we have shown that ionic liquid based on FSI as counter anion can be a good candidate for Li-ion battery [36]. However, for large configurations of Li-ion batteries for transportation, the safety aspect must be the first priority in the battery composition. Then, graphite/Py13(FSI)-LiFSI/LiFePO₄ seems to be the best choice in terms of safety and reversible capacity of the anode and the cathode. However, the power performance was found limited to 4 C rate which indicates that further improvement is thus needed. The polymer addition to the ionic liquid imparts the passivation layer; however with 5 wt.% of polymer the interface impedance is higher than with IL alone. The present work was undertaken to understand more fully the effect of polymer addition in the IL on the interface properties and the electrochemical performance for vapor pressure-free battery. For further tests we have selected the Py13(FSI) IL, based on its higher safety level as reported by Dahn et al. [37]. Using accelerated rate calorimetry the reactivity between some ILs and charged electrode materials (Li₁Si,

Li₇Ti₅O₁₂ and Li_0.45CoO₂) was examined. In spite of the fact that most of the ILs alone show thermal stability higher than 250 °C–450 °C [37], when they are used in the cell at charged state, the thermal behavior is completely different. Some ILs have shown worse stability than conventional organic electrolytes. The ILs with EMI cations are worse in safety than those with BMMI, Py13, PP14, and TMBA. In other work Sakaebe et al. [49] have shown much improvement of the thermal stability of some ILs-TFSI achieved with LiTFSI additive, by using DSC. Mitsumoto et al. [50] have demonstrated by the charge/discharge cycling the stability of Li/Py13-FSI/LiCoO₂ cell, as compared to EMI and PP13 cations and to TFSI and FSI anions. They have found better cycling performance with Py13-FSI IL. Calorimetric study of Py13-FSI-based IL with LiTFSI salt additive, with charged and discharged graphite anodes has shown stable thermal behavior [51].

In-situ impedance spectroscopy of Li/IL/LiFePO₄ cells with and without polymer

1.    Ionic liquid vs. conventional organic electrolyte

The in situ stepwise impedance spectroscopy was used to evaluate the Li/IL/LiFePO₄ cells at different states of discharge. The effect of polymer on the stability of the electrode interface was investigated with same cells by adding 1 or 5% by weight of the polymer. Figure 2 shows the first charge–discharge at C/24 curves and related impedance spectra of Li/LiFePO₄ of: (a) standard electrolyte (ECDEC-LiPF₆), (b) IL (Py13(FSI)-LiFSI); the first-coulombic efficiency was found 97% and 100%, respectively. The reversible capacity was close to159 mAh/g for both cells.

The impedance spectroscopy measurements were taken at different states of charge of the cathode. A typical Nyquist plot of a Li/LiFePO₄ half cell is shown in Fig. 2. At high frequencies, the plot starts as a semicircle and, as the frequency decreases, it changes to a straight line. The semicircle reflects the impedance of the electrochemical reaction of the cell, while the straight line indicates diffusion of the electroactive species. The charge-transfer or the interface resistance (Ri) of the electrodes is determined by the two intersection points of the semicircle with the real axis. The diffusion resistance (Rd) is obtained from the difference between the total resistance (Rt) and the (ohmic+Ri) resistance.