Lithium-ion capacitor (LICs), a novel electrochemical energy storage class combined the advantages of lithium-ion batteries and supercapacitor, are attractive for high energy densities, high power densities, and long cycle life. A typical LIC is made up of a Li-ion battery (LIB) anode, an electrochemical double-layer capacitor (EDLC) cathode, and the common Li-ion electrolyte. Graphene-based nanomaterials have been extensively studied as the electrodes for Li-ion or charge storage in LIBs and EDLCs, respectively. Recently, we have developed reduced graphene oxide (rGO) in composite with MnOx with reversible lithium storage capacity of 900 mA/h. Meanwhile, we have obtained charge storage capacitance up to 150F/g using rGO electrode with an ionic liquid (IL) electrolyte. In this work, we will report our studies on the conducting characteristics of the IL/Li-ion hybrid electrolyte and their compositional impacts on both Li-ion and charge storage in the graphene-based nanocomposites towards high-performance LICs operable in a broad temperature range.

The novel electrolyte series are a mixture of representative IL, e.g. EmimBF₄, C8minBF₄, C8mimPF₆, and the common Li-ion electrolyte, e.g. 1M LiPF₆-EC-DEC or 1M LiBF₄-EC-DEC, with various preset volume ratios. The ionic conductivities of the hybrid electrolyte series were determined at temperatures from 25^oC to 80^oC, from which conduction activation energies were calculated. Further, the influences of the hybrid electrolyte composition on the ionic conductivity, activation energy, stability window, Li-ion and charge storage in the graphene-based nanocomposites were systematically studied.       

Results showed that ionic conductivities of the hybrid electrolytes increased in general with the addition of Li-ion electrolyte and the activation energies of the ionic conductance increased with the addition of IL. It is interesting to observe a secondary maxima point at around 1:1 volume ratio (IL: Li-ion electrolyte) and higher ionic conductivities of such hybrid electrolyte than that of the neat Li-ion electrolyte above 65^oC. The hybrid electrolyte remains relatively stable up to 6V starting from the second cycle after the occurrence of the irreversible reactions corresponding to IL partial decomposition and SEI formation in the first charge/discharge cycle. The novel LIC consisting of the hybrid Li-conducting/IL electrolyte, rGO-MnOx anode, rGO cathode, is anticipated to have a high deliverable energy densities both gravimetrically and volumetricallyand operable in a broad temperature range.