Aqueous Hybrid Supercapacitor Using Protected Pre-Lithiated Carbon Anode

1. Introduction

Pseudocapacitive oxide electrodes generally perform best in aqueous electrolytes. The use of aqueous electrolytes unfortunately limits the maximum cell voltage to no more than 2 V, which is a disadvantage for energy density (E=0.5 CV²). This disadvantage can be overcome by using a water-stable solid electrolyte [1-3]. The new cell configuration is based on the use of a protected Li anode, which consists of a water stable Li-conducting solid electrolyte, a Li-conducting gel electrolyte buffer layer, and Li foil as the negative electrode. The application of the water stable anode allows the implement of aqueous electrolytes at the pseudocapacitive positive electrode. The cell voltage can reach as high as 4.3 V when MnO₂ is used as the positive electrode [1,3]. In this study, we pre-lithiated carbon (Li_xC₆) was adopted as the negative electrode instead of Li.

2. Experimental

Li doping into graphite was conducted with a three-electrode cell at constant current (0.2C) in 1.0 M LiPF₆/EC:DEC. After the pre-doping procedure, the cell was disassembled and reassembled into a pouched cell. The multi-layered anode consists of a LISICON-type solid glass ceramic (Li_1+x+y(Ti,Ge)_2-xAl_xSi_yP_3-yO₁₂ (Ohara Inc., Japan, hereafter denoted as LTAP) as the water-stable solid electrolyte, and a buffer layer consisting of polyethylene oxide with Li(CF₃SO₂)₂N polymer electrolyte (PEO-LiTFSI) between the lithium metal or pre-lithiated carbon and the solid electrolyte. These 3 layers were sealed into a laminated pouched cell leaving a 5 mm x 5 mm window cut out for the solid electrolyte (LTAP) to come into contact with the aqueous electrolyte.

Results and discussion

After Li-doping of graphite in the pre-doping cell, the potential of the negative electrode was 57 mV vs. Li/Li⁺. The color pf graphite changed from black to gold with Li doping. Taking into consideration the irreversible capacity, it can be assumed that pre-doping at 0.2C is sufficient to achieve close to fully doped state. Figure 1 shows the charge/discharge curves using activated carbon positive electrode (LiC₆ | PEO-LiTFSI | LTAP | 1.0 M Li₂SO₄aq. | AC). Maximum cell voltage of 3.7 V was obtained. The AC positive electrode showed capacitive behavior with the potential varying linearly with time between 0.33 and 1.29 V vs RHE. The potential of the negative electrode was -0.24 V vs RHE, and exhibited typical battery-like behavior. The overall cell performance is similar to lithium-ion capacitors. The specific capacitance of the AC positive electrode was 135 F/g.

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) W. Shimizu, S. Makino, K. Takahashi, N. Imanishi, W. Sugimoto, J. Power Sources, 241, 572-577 (2013).

Figure 1. Galvanostatic charge/discharge curves of aqueous hybrid capacitor (LiC₆ | PEO-LiTFSI | LTAP | 1.0 M Li₂SO₄ aq. | AC) at 0.102 mA cm^-2.