Fig. 1. (Left) CVs of O2-saturated [EMI][TFSI]/0.025 M LiTFSI swept to various cathodic limits. The scan rate is 100 mV/s. (Right) Proposed assignment of peaks. Adapted from Ref. [61]. Copy right 2011, American Chemical Society.

Fig. 2. (a) Polysulfide solubility test in electrolyte solvents. 0.5 M Li2S8 (based on stoichiometric amounts of Li2S and S8) dissolved in a mixture solvent of DOL/DME (1/1, v/v) (left) or [PP13][TFSI]/DOL/DME (2/1/1, v/v) (right). (b) Schematic illustration of the functioning mechanism of polysulfide dissolution and diffusion in ether (left) or [PP13][TFSI]-based (right) solvent. (c) and (e) Cyclic performance presenting specific discharge capacity of Li-S cells for 15 cycles at 0.2 C. (d) and (f) Charge-discharge voltage profiles of the 10th and 11th cycle. The cells were assembled with [PP13][TFSI]/DOL/DME (2/1/1, v/v) - 0.2 M LiNO3-1 M LiTFSI. After an uninterrupted 10 cycles, the cells were rested for 48 h (c and d) or one week (e and f), and then cycled further continuously. Adapted from Ref. [70]. Copy right 2016, The Royal Society of Chemistry.

Fig. 3. Schematic representation: (a) NVP/SE/Na, (b) NVP/IL/SE/Na solid-state batteries. (c) Rate performance of the NVP/IL/SE/Na solid-state battery at room temperature with current rates of 0.2 C, 0.5 C, 1 C, 2 C, 4 C, 6 C, 8 C, 10 C, the inset exhibits the charge/discharge curves of the 1st and 10th cycle at a current rate of 0.2 C for the NVP/IL/SE/Na solid-state battery at room temperature, and (d) cycling performance and Coulombic efficiency of the NVP/IL/SE/Na solid-state battery at room temperature with a current rate of 10 C for 10,000 cycles. Adapted from Ref. [73]. Copy right 2016, John Wiley and Sons.

Fig. 4. (a) The bended battery is able to light up a red LED (～2.0 V), and (b) discharge capacities and Coulombic efficiencies for the battery within 20 cycles in flat and bent (2 cm radius) positions. Adapted from Ref. [98]. Copy right 2016, Elesiver.

Fig. 5. (a) Schematic illustration of charge/discharge processes of the dual-ion battery using Al foil as anode and MCMB as cathode based on an IL electrolyte, (b) Galvanostatic charge-discharge curves of the Al-MCMB DIB at 1 C. Inset is a photograph of a fully charged Al-MCMB DIB which can light up a red and a blue LEDs in series, (c) Long-term cycling performance of the battery at 0.5 C for 300 cycles. (d) Ragone plot of the Al-MCMB DIB compared with conventional energy storage devices. Adapted from Ref. [104]. Copy right 2016, John Wiley and Sons.

Fig. 6. (a) Cycling stability of the electrode in different electrolytes. Adapted from Ref. [118]. Copy right 2013, The Royal Society of Chemistry. (b) Normalized capacitance (C/C20 °C) for the onion-like carbon and VA-CNT electrodes in ([PIP13][FSI])0.5([PYR14][FSI])0.5 IL mixture and PC+1 M [TEA][BF4] electrolytes. Capacitances were calculated at 100 mV s-1, except for the 50 °C (1 mV s-1) and 40 °C (5 mV s-1) experiments. Adapted from Ref. [120]. Copy right 2011, American Chemical Society.

Fig. 7. SEM images of the products by using different amounts of [Bmim][BF4]: (a) 0 μL, (b) 20 μL (precursor of CNTs@TiO2-B NSs), (c) 50 μL. (A color version of this figure can be viewed online). Adapted from Ref. [126]. Copy right 2014, Elesiver. (d) Schematic of the fabrication process for IL-mediated reduced graphene oxide. (e) Specific volumetric, areal, and gravimetric capacitances (60 °C) at varying current densities. Adapted from Ref. [133]. Copy right 2017, American Chemical Society.

Fig. 8. The applications of ILs for electrochemical energy storage devices.