Electrochemical Properties of Graphite Positive Electrode in Zinc Halide-Based Aqueous Solutions
Authors: Sugimoto Yohta , Tsujimoto Shota , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Since fossil fuels cause global warming and environmental issues, renewable energies such as wind and solar power are rapidly becoming popular as alternative energy sources. On the other hand, large-scale energy storage devices are necessary to store the generated power because of the intermittent nature of renewable energies. While lithium-ion batteries, which are currently the most widely used type of battery, have high energy density, they have disadvantages such as the use of flammable organic solvents, the high cost, and the uneven distribution of lithium. In addition, there are also concerns about their further large-scale application because of the scarcity of the rare elements. Therefore, rechargeable batteries using aqueous electrolytes, which are expected to be safe and low-cost, have recently attracted much attention. One of these batteries is the aqueous rechargeable zinc battery, which uses zinc metal for the negative electrode. While Mn or V-based oxides have been reported for the positive electrode, alternative positive electrodes with high capacity are expected to be developed[1]. Recently, it has been reported that BrCl intercalation into graphite positive electrode mixed with LiCl and LiBr results in large capacity[2].

Objective

In order to use graphite as a positive electrode in aqueous rechargeable zinc batteries, in this study, we investigated electrochemical properties of graphite positive electrode containing ZnCl2-ZnBr2 in concentrated aqueous ZnCl2-ZnBr2 solutions.

Experiments

Natural graphite was mixed with ZnCl2 and ZnBr2 at a molar ratio of 14:1:1 by ball milling for 15 min under argon atmosphere. Then, 5 wt% polytetrafluoroethylene was added and hand-mixed. The mixture was punched into a 5 mm diameter and pressed onto a titanium mesh at 0.2 kN to fabricate a composite electrode (ZBC-G). In the three-electrode cell, the ZBC-G electrode was used as the working electrode, zinc plate was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. Several electrolytes were prepared by dissolving different concentrations of ZnBr2 in 17 mol kg−
1 (m) ZnCl2 solution, followed by an addition of 4 or 2 wt% of polyethylene oxide.

Cyclic voltammetry (CV) and constant-current charge-discharge measurements were performed. Regarding the CV measurement, the sweeping rate and range were set at 1 mV s−
1 and from 0 to 1.5 V, respectively. Regarding the charge-discharge measurements, the current density and cutoff potential were set at 50.5 mA g−
1 and from 0 to 1.5 V, respectively. After the CV measurements, the electrodes held at 2.0 V for 15 h were investigated by Raman spectroscopy and X-ray diffraction (XRD).

Results and Discussions

It was implied from the shape of the resultant CVs that Br2 was severely evolved in 17 m ZnCl2 + 11 m ZnBr2 electrolyte. The resultant Raman spectrum and XRD pattern suggested that Br2 was surely intercalated into graphite. Since it was clear that reaction of Br2 intercalation competes with Br2 evolution reaction, CV and charge-discharge measurements were performed in 17 m ZnCl2 electrolyte (Fig. 1). A clear redox pair was confirmed from the CV, and the charge-discharge curve showed two-step potential plateaus area for both charge and discharge processes. It was speculated that the charge plateau at about 1.06 and 1.4 V were Br2 and Cl2 evolution, respectively, while the discharge plateau at about 1.1 and 0.9 V were Cl2 and Br2 reduction, respectively. The split G-band in the Raman spectrum and the broadened XRD peaks of the graphite electrode after the potential holding at 2.0 V indicated that Br2 was surely intercalated into graphite although the amount of the intercalated Br2 was small (Fig. 2).

It was indicated that the Br2 was intercalated into graphite even without containing Br− in the electrolyte, suggesting that Br- in the composite electrode was oxidized before the Br2 intercalation. However, compared to the ZnCl2-ZnBr2 electrolyte, the Br2 intercalation reaction into graphite was less pronounced, suggesting that it is necessary to consider suitable electrode/electrolyte design that allows efficient intercalation of the generated Br2 into graphite. In the conference, further discussion regarding BrCl intercalation and electrode/electrolyte optimization will be presented.

References

[1] G. Li, L. Sun, S. Zhang, C. Zhang, H. Jin, K. Davey, G. Liang, S. Liu, J. Mao, Z. Guo, Adv. Funct. Mater.,
34 (2023) 2301291.

[2] Yang, C., Chen, J., Ji, X., Pollard, T. P., Lü,X., Sun, C. J., Hou, S., Liu, Q., Liu, C., Qing, T., Wang, Y., Borodin, O., Ren, Y., Xu, K., and Wang, C., Nature,
569 (2019), 245-250.

Figure 1