Ionic Liquid-based Supercapacitors and Lithium-ion batteries for HEV application

The interest in ionic liquid-based electrolytes for electrochemical energy storage and conversion systems is growing world wide. The key features that trigger the interest in room temperature ionic liquids (ILs) for different applications, ranging from supercapacitors to lithium ion batteries, are their low vapor pressure and high chemical stability which make them promising electrolytes for the development of safe electrochemical systems. Supercapacitors working with solvent-free ILs require cell configuration and electrode materials tailored in relation to the nature of the IL itself. An asymmetric configuration for electrochemical double-layer carbon supercapacitors (AEDLCs) is a key strategy to take advantage of the wide electrochemical stability windows of IL electrolytes. Indeed, we have already demonstrated that N-methoxyethyl-N- methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR1(2O1)TFSI) IL-based AEDLC might compete with lithium-ion batteries working with conventional organic electrolytes in power-assist HEVs with the added advantage of inherently higher safety. Such PYR1(2O1)TFSI-based AEDLC meets the dynamic power and energy capability targets stated by (U.S. Department of Energy) DOE with Vmax of 3.7 V in the -30 to 60°C temperature range as evaluated by the DOE FreedomCAR benchmark protocols 1,2. Lithium-ion batteries of different chemistry are under study for safe use in automotive application. Given that LiFePO4 is a popular, safe cathode material, great research effort is mainly focussing on anode materials for graphite substitution. Furthermore, the poor performance of graphite in IL- based electrolytes prevents its use in such electrolytes. We have investigated in N-butyl-Nmethylpyrrolidinium (PYR14) bis(trifluoromethansulfonyl)imide (TFSI)-based electrolyte, PYR14TFSI-LiTFSI, the cycling performance of titanium based anode materials, Li4Ti5O12 and TiO2, and of the LiFePO4; for all the three electrode materials it was satisfactory. We have, thus, assembled with this electrolyte the two batteries Li4Ti5O12// LiFePO4 and TiO2//LiFePO4 and have tested them by the DOE protocols for power-assist HEVs. The results of these tests are reported and compared with those performed on the AEDLC supercapacitor working with PYR1(2O1)TFSI.