Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems, страница 8

The diffusion resistance is plotted in Fig. 5b as a function of %DoD with different amounts of polymer in the IL. The same behavior of Rd may be noted at different states of charge with or without polymer additions. The highest value of Rd is obtained with the monophase iron phosphate; at 0% DoD with LiFeO₄ and at 100% DoD with FePO₄ phase. The diffusion process is easier when the diphase LiFePO₄–FePO₄ coexists, and particularly in the range of 50–70% DoD.

In order to investigate whether there is further stabilization after cycling, cells were cycled at C/24 for three cycles. The impedance measurements were taken at fully delithiated state (100% DoD). Figure 6 shows the impedance curves of the cells: (a) standard electrolyte (EC-DEC-1M LiPF₆), (b) IL (Py13-FSI), (c) (IL+1% polymer) and (d) (IL+5% polymer). The stabilization can be reached for the standard electrolyte (Fig. 6a) after the first cycle at 100 Ω of the total resistance (Rt) compared to the IL cell, in which (Fig. 6b) the stabilization can be achieved only after three cycles with Rt=194 Ω its Ri=156 Ω (Fig. 6b) compared to the standard electrolyte (Fig. 6a) with Rt=105 Ω and Ri=37 Ω. When a small amount of polymer is added (1%), stabilization still reached at the third cycle but at lower Rt=163 Ω and Ri=124 Ω (Fig. 6c). The addition of higher polymer content (5%; Fig. 6d) shows that the interface is not well stabilized and the Rt is 200 Ω with 166 Ω associated with interface resistance. Also to be noticed is the fact that the initial ohmic resistance in Fig. 6 was found higher in the standard organic electrolyte with 23 Ω. This resistance may be attributed to the electrolyte and/or to the organic reaction film formed by interaction of organic electrolyte with the lithium metal electrode; in the presence of IL, however, this ohmic resistance becomes vanishingly small, as would be expected for these high-conducting media, which also have no tendency to form resistive films at the surface which, if anything, would be expected to be covered by a conducting ionic salt layer.

The cells were cycled at C/4 rate for a long cycle life aging (Fig. 7), the reversible capacity was found stable in all cases at 149, 152, and 148 mAh/g respectively for (a) IL, (b) (IL+1 wt.% polymer) and (c) (IL+5 wt.% polymer). The coulombic efficiency remains constant during the cycling life with 99.6% for all cells. However, we noted an abnormal behavior with 5% polymer cell. In the first 20 cycles, the capacity increases and the coulombic efficiency fluctuates before the cell reaches the stable condition. Moreover, this result confirms the stabilization of both electrode interfaces of LiFePO₄ and lithium electrodes with IL which probably suppresses the dendrite formation on the lithium metal. The rate capability of the cell, Li/(IL+5 wt.% polymer)/LiFePO₄, is shown in Fig. 8. The cell delivers the full capacity until around C/2 rate, and then the discharged capacity starts decreasing to reach 82% at 2 C. At 4 C rate, the capacity dropped sharply to 47% and continues dropping as the discharge rate is increased. This drop in

Fig. 7 Cycling behavior of Li/LiFePO₄ cells in different electrolytes

the capacity above 2 C rate perhaps indicates that the limiting process lies in the ionic liquid: it is well-known that a very high concentration of ions in the molten salt can lead to “over-population” of ions that can cause phenomena such as ion-pair formation and “salting out”, in the sense that ion–ion distance is short thus disabling the “free” ion to make full contribution to the conduction [52, 53]. Such a situation, of course, does not exist in the standard organic electrolyte [36] in which the solvent molecules are abundantly available to solvate the lithium ions which then contribute to conduction.

Li-ion cell with IL and polymer gel