Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems, страница 4

The effect of gel polymer media was investigated with Py13(FSI) ionic liquid. The gel polymer was prepared by mixing of 5 wt.% of polymer with Py13(FSI) −0.7 M LiFSI and 1,000 ppm of thermal initiator perkadox (Akzo Nobel, USA). The polymer used was made from an ether-based low-molecular-weight cross-linkable polymer precursor (TA210, Daiichi Kogyo Seiyaku, Japan). The chemical formula has tri-function of poly (alkylene oxide) main chain with acrylate chain ends. This polymer has demonstrated a good electrochemical stability and high compatibility with high voltage cathodes like LiCoO₂ [38]. The electrodes were immersed in the mixture (polymer+IL+ initiator) and then heated at 60 °C under vacuum, a step necessary to help the (polymer+IL) electrolyte to penetrate in the pores deeply across the electrode thickness.

Then the cell was heated at 60 °C for 1 h in order to

Table 2 The first electrochemical characteristics of the graphite anode

Electrolyte	First discharge (mAh/g)	CE1 (%)	Reversible capacity (mAh/g)	CE2 (%)
EC-DEC–1 M LiPF6	398	92.7	365	100
EC-DEC–1 M LiFSI	382	93.0	369	100
Py13-FSI+0.7 M LiFSI	468	80	367	98.3
EMI-FSI+0.7 M LiFSI	432	80.5	362	97.6

needed to prepare for the future applications of Li-ion battery technology in HEVs and PHEVs. Since the demonstration of LiFePO₄ by Padhi et al. [39, 40] as a potential cathode material, considerable interest has been generated due to its safety, low cost and environmentally friendly nature [41–47]. Furthermore, side reactions are minimized because of its flat voltage profile at 3.4 V vs. Li/ Li⁺. However, some other parts of the battery can cause problems in the scaled-up configuration. In Li-ion batteries, we found that in general the nature of electrolyte materials can have a great impact on the safety of the battery. In order to improve the safety of the lithium batteries, electrolyte should have lower inflammability and lower reactivity than the conventional electrolytes. Room temperature ionic liquids have suitable properties as safe electrolytes for lithium batteries due to their non-volatility and noninflammability [48]. On the anode side the natural graphite material was used to make lithium-ion cell.

Results and discussion

Summary of data without polymer addition

The viscosity and conductivity values were measured at 20 °C. Table 1 shows the viscosity and the ionic conductivity of the pure “solvents” without salt addition. The conventional organic electrolyte shows the lowest viscosity. The table shows the ionic liquid having lower viscosity shows the higher conductivity.

The conductivity and viscosity (Table 1) of the conventional electrolyte does not follow the expected trend because the organic solvent has covalent bonding and few ions (by auto-ionization) and hence low conductivity. The ionic liquids, on the other hand, are by definition full of ions, and therefore, have higher conductivity than the organic solvents. The conductivity decreased and the viscosity increased when 0.7 M salt is added, possibly due to some ion-pair formation. Thus, in order to not create high ion-pair interaction in the IL, we have limited the salt addition to 0.7 M only. In ionic liquids, since the added salt cannot have the possibility of stabilization of its ionic constituents by solvation, such ion–ion interaction (quasi ion pairs) is not unexpected.