Performance of the LiFePO4 in supercapatteries

Significant efforts have been devoted to finding new electrochemical storage devices capable of satisfying the high energy/power demands of electric cars.1 In this context, it is desirable to find a device capable of combining the advantageous properties of batteries and supercapacitors in so-called supercapatteries (SCT).2 SCT are devices that accumulate charge by means of different mechanisms: faradaic reactions in one electrode and electric double layer in the other. In this work, a SCT was assembled with a battery cathode (LiFePO4), a mesoporous carbon as anode, and a mixture of Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt and the ionic liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) as electrolyte. Here, the objective is to evaluate the performance of the LiFePO4 in this SCT compared to a standard ion-lithium cell (LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (electrolyte); lithium metal (anode)).3 The capacities are summarized in Table 1. At the lowest current densities, LiFePO4 present similar capacities in both cells, and the SCT achieves a high energy density (43.3 W h kg-1). However, the capacity is reduced at 2C by 63% in SCT and 74% in the battery. This effect shows the difficulty of LiFePO4 in equalizing the kinetics of the electric double layer of the anode, reducing the energy density to 15.3 W h kg-1 at 102 W kg-1. Based on these results, our group is dedicated to the synthesis of LiFePO4 with morphologies more suitable for fast ionic insertion and to meet the demands of a SCT.