Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems, страница 5

Behavior of anodes and cathodes in ionic liquid electrolytes

Recently [36], we have evaluated the graphite anodes in various ionic liquid electrolytes without polymer and the first electrochemical characterization is recapitulated in Table 2. The solid electrolyte interphase (SEI) with IL is well formed on the graphite anode, with 80% of coulombic efficiency in the first cycle. The reversible capacity was stable at 1 C cycling and the highest value was obtained with Py13(FSI) at 360 mAh/g. When LiPF₆ is replaced by LiFSI salt, the anode shows excellent performance with reversible capacity close to the theoretical capacity of 369 mAh/g and 93% coulombic efficiency in the first cycle. The LiFSI salt has a positive effect on the formation of coherent passive layer on the graphite. Further, the cell with FSI salt in EC/DEC has shown a stable SEI with good reversible capacity at 1 C rate, close to the theoretical value of 369 mAh/g.

The cell with IL based on EMI-FSI has shown 362 mAh/g as reversible capacity but only 80.5% coulombic efficiency. However, for the ionic liquid Py13, the reversible capacity was found close to the theoretical value of 367 mAh/g, while the first-coulombic efficiency was 80%. All these data explain well that LiFSI salt in FSI-based ionic liquid is suitable for use with the anode graphite, without any secondary reactions.

Similarly, the salient features of LiFePO₄ cathode in these electrolytes are encapsulated in Table 3 here. The impedance characteristics of these interfaces have been described in our recent paper [36]. The reversible capacity with EC/DEC-LiPF₆ was 158 mAh/g with 97.5% as coulombic efficiency in the first cycle (CE1). In curves of charge–discharge with LiFSI salt, the reversible capacity was quite comparable to EC-DEC-LiPF₆ electrolyte at 156.5 mAh/g, and 98% in the first cycle coulombic efficiency. With the ionic liquid Py13-FSI, a lower reversible capacity of 143 mAh/g was obtained with only 93% coulombic efficiency. However, with ionic liquid EMI (FSI), higher reversible capacity and coulombic efficiency, 160 mAh/g and 95%, respectively, were obtained.

Table 3 First electrochemical characteristics of the LiFePO₄ cathode

During our study we noticed that the viscosity probably affects the performance of LiFePO₄ material due to the carbon coating on the surface of LiFePO₄ particles. When electrolyte has high viscosity as in an ionic liquid, the wettability of the carbon layer is more difficult due to its large surface area. Then the lithium-ions cannot migrate easily across this layer particularly in the first cycles. Moreover the viscosity can also prohibit the wettability of all the electrodes in depth, both anodes and cathodes because of the quasi tree-dimensional fractal nature of the electrodes, which contains not only meso-pores (easy accessible by the electrolyte) but also micro-pores which limit the electrolyte accessibility.

For impregnation and good contact of the electrolyte with the electrodes in high-viscosity systems such as the ionic liquids, a pretreatment in vacuum at 60 °C is necessary. On the cathode side, the reversible capacity was thus improved to 140 mAh/g at 1 C for Py13(FSI) by using vacuum treatment. The EMI(FSI) IL shows the highest value at 148 mAh/g, even higher than the reference electrolyte. Also, the rate capability has shown a small increase in the power performance until 2 C rate; however, above 2 C rate, high jump in the capacity was delivered from 40 to 80 mAh/g.