The hybrid approach allows for a reinforcing combination of properties of dissimilar components in synergic combinations. From hybrid materials to hybrid devices the approach offers opportunities to tackle much needed improvements in the performance of energy storage devices. This paper reviews the different approaches and scales of hybrids, materials, electrodes and devices striving to advance along the diagonal of Ragone plots, providing enhanced energy and power densities by combining battery and supercapacitor materials and storage mechanisms. Furthermore, some theoretical aspects are considered regarding the possible hybrid combinations and tactics for the fabrication of optimized final devices. All of it aiming at enhancing the electrochemical performance of energy storage systems.

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Ionic liquid (IL)-based electrolytes containing molecular solvents were shown to be attractive for extreme temperature applications in electric double layer capacitors (EDLCs). In particular, the IL-butyronitrile (BuCN) mixture provides high capacitance (around 125 F g 21 at 500 mA g 21 ) independent of testing temperature, and superior performance at high current rates (reduced current dependence at high rates). Importantly, the IL-BuCN electrolyte can safely operate between 220 and + 80 uC, which overcomes the high temperature limitations of current commercial EDLCs. An additional advantage of IL-solvent mixtures is that the higher concentration of IL ions in the mixtures allows a greater specific capacitance (F g 21 ) to be achieved. The conductivity of the ionic liquid N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide (PYR 14 TFSI) could be increased from 2.48 mS cm 21 up to 45 mS cm 21 by mixing with an appropriate solvent. Importantly, these solvent mixtures also retain a wide electrochemical voltage window, in the range 4-6 V.