Dual Electrolyte Hybrid Supercapacitor Using Liquid-Liquid Interface
Authors: Sakata Hiyori , Makino Sho , Ayato Yusuke , Mochizuki Dai , Tatsumi Hirosuke , Sugimoto Wataru 
Published: February 2020

Hybrid supercapacitors utilzing a supercapacitive electrode and a battery-type electrode exhibit enhanced capacitor performance compared to conventional symmetric supercapacitors. The authors have denonstrated high energy density (>700 Wh kg-1) and high cell voltage (4.3 V) by combining metal oxides positive electrode that have high capacitance in aqeuous electrolytes and a Li negative electrode with low standard electrode potential.1,2 This hybrid supercapacitor ( Li | polymer electrolyte | solid electrolyte | 1.0 M Li2SO4 | RuO2×nH2O) utilizes a LISICON-type glass ceramic as a water-stable solid electrolyte to separate the aqueous and non-aqueous electrolytes and prevent penetration of water to Li. The conductivity of the Li-conducting glass ceramic used (~10-4 S cm-2 at 25°C) is not sufficent enough to afford the power performance characteristic of supercapacitors. In this study, a dual-electrolyte hybrid supercapacitor without solid electrolyte was investigated by utilizing a liquid/liquid interface was developed with aqueous electrolyte/ionic liquid-based electrolyte and its capacitive behavior was investigated.
For the aqueous electrolyte, 1.0 M Li2SO4 was used and a 0.5 M LiTFSI/N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) were used as the ionic liquid-based electrolyte. RuO2×nH2O deposited on a glassy carbon surface was used as the positive electrodes (204 µg cm-2). Li foil negative electrode (5 mm x 5 mm) was placed into the 0.5 M LiTFSI/PP13TFSI electrolyte. RuO2×nH2O positive electrode was placed into 1.0 M Li2SO4. Charge/discharge test was carried out in two-electrode configuration with RuO2×nH2O positive electrode and Li negative electrode at 25 °C. The higher and lower cut-off voltages were 3.8 and 2.8 V, respectively.
The resistance of Li | 0.5 M LiTFSI/PP13TFSI | 1.0 M Li2SO4 | Pt cell with liquid-liquid interface was 491 W cm2 based on EIS measurement, which is ~1/3 of our best data of 1360 W cm2 when a solid electrolye is used (Li | alginate gel-0.5 M LiTFSI/PP13TFSI | LISICON | Li2SO4 | Pt ). Charge/discharge curves of the dual-electrolyte hybrid supercapacitor without solid electrolyte consisting of Li | 0.5 M LiTFSI/PP13TFSI | 1.0 M Li2SO4 | RuO2×nH2O at 0.141 mA cm-2 is shown in Fig. 1. The cell voltage profile exhibited capacitive behavior between 2.8 and 3.8 V, similar to our previously research.4 The potential of the Li negative electrode was nearly constant around -2.45 V vs RHE during charge/discharge. The specific capacity and specific capacitance calculated from discharge at 0.141 mA cm-2 was 51.1 mAh (g-RuO2)-1 and 188 F (g-RuO2)-1, respectively. The IR drop of the negative electrode was 0.194 V (Fig. 2a). An equivalent series resistance (ESR) was 535 W cm2, smaller than that of the Li | alginate gel impregnated with 0.5 M LiTFSI/PP13TFSI | LISICON | Li2SO4 | activated carbon (AC) cell of 1350 W cm2)at 0.255 mA cm-2 (Fig. 2b), and agrees well with the results estimated from EIS. Dual-electrolyte hybrid supercapacitor using liquid-liquid interface (without solid electrolyte) may be considered as a post Li-ion capacitor device.
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3. S. Chou, Y. Wang, J. Xu, J. Wang, H. Liu and S. Dou, Electrochem. Commun., 31, 35 (2013).
4. M. Yamagata, K. Soeda, S. Yamazaki, and M. Ishikawa, Electrochem. Solid-State Lett., 14, A165 (2011).

Figure 1