(Invited) Aqueous Hybrid Supercapacitor Using Water-Stable Multilayered Li+-Doped Carbon Negative Electrode with Alginate Gel As a Buffer Layer
Authors: Makino Sho , Sugimoto Shigeyuki , Sugimoto Wataru 
Published: February 2020

The energy performance of electrochemical capacitors can be increased by employing an asymmetric configuration combining a porous carbon positive electrode with a Li+-doped carbon (Li
x
C6) negative electrode. The advantage of such devices, known as lithium-ion capacitors, is the higher cell voltage of ~3.8 V as a result of the low standard electrode potential of -2.95 V vs SHE for Li
x
C6, leading to increase of energy density up to ~15 Wh kg-1. We have developed advanced aqueous hybrid supercapacitors consisting of metal oxide positive electrodes such as RuO2 nanosheet and water-stable multilayered Li negative electrode with high energy density of ~720 Wh (kg-positive electrode)-1 and high cell voltage of 4.2 V at 60˚C.1,2 Room temperature operation was made possible by employing a multilayered Li
x
C6 negative electrode using polyethylene oxide (PEO) with the addition of an ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI) as the buffer layer.3 In this study, alginate gel containing ionic liquid-based electrolyte was used as buffer layer instead of PEO-based polymer electrolyte to further lower the ESR of multilayered Li
x
C6 negative electrode. Capacitive behaviors of aqueous hybrid supercapacitor using multilayered Li
x
C6negative electrode with alginate gel as buffer layer was investigated by charge/discharge test at 25˚C.

Alginate gel impregnated with 0.5 M LiTFSI/PP13TFSI was prepared by following a literature procedure.4 Li+ pre-doping into graphite electrode was conducted with a three-electrode cell at constant current (0.05C) in 1.0 M LiPF6/EC-DEC (1:1). After the pre-doping procedure, the cell was disassembled and Li
x
C6 was taken out. The multilayered negative electrode consists of a LISICON-type solid glass ceramic (Li1+x+y
(Ti,Ge)2-x
Al
x
Si
y
P3-y
O12, LTAP (Ohara Inc., Japan)) as the water-stable solid electrolyte, and the alginate gel with 0.5 M LiTFSI/PP13TFSI as the buffer layer and Li
x
C6 was fabricated by following previously described methods.1,2 Electrochemical impedance spectroscopy (EIS) was carried out to estimate the resistance of multilayered Li
x
C6 negative electrode in 1.0 M Li2SO4 at 25 ˚C. Activated carbon (MSP-20, AC) was used as the positive electrode. Constant current charge/discharge tests of the aqueous hybrid supercapacitor consisting of AC positive electrode and multilayered Li
x
C6negative electrode were conducted in a beaker-type flooded cell with a Ag/AgCl/KCl(sat.) reference electrode at 25 ˚C. The lower and higher cut-off voltage during charge/discharge tests was 2.7 and 3.7 V, respectively.

The total resistance of a cell consisting of Li
x
C6 | alginate gel with 0.5 M LiTFSI/PP13TFSI | LTAP | 1.0 M Li2SO4 | Pt measured by EIS measurement was 302 Ω cm2, which is 1/3 of the multilayered Li
x
C6 negative electrode with PEO-LiTFSI-PP13TFSI.3 Comparison of charge/discharge behaviors for aqueous hybrid supercapacitors consisting of Li
x
C6 | alginate gel with 0.5 M LiTFSI/PP13TFSI or PEO-LiTFSI-PP13TFSI | LTAP | 1.0 M Li2SO4 | AC at 25˚C were shown in Fig. 1. Alginate gel with 0.5 M LiTFSI/PP13TFSI exhibited ideally capacitive voltage profile between 2.7 and 3.7 V while the IR drop was observed at 0.510 mA cm-2. The electrode potential of AC positive electrode showed capacitive behavior and fluctuated between 0.30 and 1.02 V vs RHE without IR drop. The IR drop mainly originated from the multilayered Li
x
C6 negative electrode. The IR drop of negative electrode for the alginate gel with 0.5 M LiTFSI/PP13TFSI and PEO-LiTFSI-PP13TFSI was 0.237 and 0.641 V, respectively. The ESR of the multilayered Li
x
C6 negative electrode for the alginate gel with 0.5 M LiTFSI/PP13TFSI and PEO-LiTFSI-PP13TFSI was 296 and 801 Ω cm2, respectively. The specific discharge capacity for the alginate gel with 0.5 M LiTFSI/PP13TFSI and PEO-LiTFSI-PP13TFSI was 19.4 and 6.5 mAh (g-AC)-1 at 0.510 mA cm-2, respectively. The alginate gel with 0.5 M LiTFSI/PP13TFSI contributed to a substantial decrease in the ESR for multilayered Li
x
C6negative electrode and improvement of capacitive properties for the advanced aqueous hybrid supercapacitor at room temperature.

Acknowledgements

This work was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) and Chubu Electric Power Co., Inc.

1. S. Makino, Y. Shinohara, T. Ban, W. Shimizu, K. Takahashi, N. Imanishi, and W. Sugimoto, RSC Adv., 2, 12144 (2012).

2. S. Makino, T. Ban, and W. Sugimoto, Electrochemistry, 81, 795 (2013).

3. S. Makino, R. Yamamoto, S. Sugimoto, W. Sugimoto, J. Power Sources,accepted, DOI: 10.1016/j.jpowsour.2016.04.058.

4. M. Yamagata, K. Soeda, S. Yamazaki, and M. Ishikawa, Electrochem. Solid-State Lett., 14, A165 (2011).

Figure 1