Effect of KOH Concentration on Viscoelastic and Conformational Changes of Xanthan Gum-Based Hydrogel Electrolytes
Authors: Morisaki Tomoko , Chiku Masanobu , Higuchi Eiji , Inoue Hiroshi 
Published: December 2024

Introduction

Lithium-ion batteries (LIBs) are currently the most practical rechargeable batteries, but their energy density is insufficient for applications such as electric vehicles and power storage. Moreover, the safety issues for LIBs have not yet been completely resolved. Therefore, research on the development of next-generation batteries with high safety and high energy density is being promoted. Recently, aqueous rechargeable batteries using zinc as the negative electrode has attracted considerable attention because zinc has many attractive features: low standard potential (−0.76 V vs. SHE), high hydrogen overpotential, high gravimetric and volumetric theoretical capacities (820 mAh g-1, 5855 mAh cm-3), and low cost. However, for the implementation of aqueous rechargeable zinc batteries, there are some serious issues such as zinc dendrite formation, shape change and side reactions (hydrogen evolution, corrosion, passivation, etc.) at the negative electrode. We have demonstrated the effectiveness of crosslinked polyacrylate-based hydrogel electrolytes (HEs) as an alternative to aqueous alkaline solutions to improve these issues (1). We have recently become interested in physical gels in which the host polymer is cross-linked by intermolecular interactions such as hydrogen and ionic bonding, and prepared new HEs by mixing xanthan gum (XG), one of polysaccharides, as host polymer and aqueous KOH solutions. The XG-based HEs showed ionic conductivity comparable to the corresponding alkaline aqueous solutions. However, the details of the influence of the gel structure on the diffusion of zincate in the gel and the deposition and dissolution of zinc at the electrode/hydrogel electrolyte interface remain unclear. In this study, the viscoelasticity of XG-based HEs with different KOH concentrations and XG contents were evaluated to investigate how the gel structure, and in particular the conformation of XG, changes with different KOH concentrations and XG contents.

Experimental

HEs were prepared by mixing various concentrations of aqueous KOH solutions and XG powders with stirring. A rheometer was used to measure the viscoelasticity of the prepared HEs. In addition, structural changes in XG in the HEs were investigated by Infra-red (IR) spectroscopy and gel permeation chromatography (GPC).

Results and Discussion

Change in viscosity, which was measured with a rheometer, with the concentration of KOH for the HEs with 0.05, 0.1 and 0.3 g mL-1 XG is shown in Fig. 1. The XG structure consists of a β-glucan main chain with long side chains (molecular weight about 600) consisting of three molecules of monosaccharide attached in an orderly fashion. In water, XG molecules exist in a state where the individual main chains are elongated due to electrostatic repulsion between COO- groups in the side chains. In aqueous KOH solutions, as the KOH concentration increases, the negative charges of COO- groups on the side chains are shielded by the interaction with K+, reducing the electrostatic repulsion. As a result, the XG molecules become entangled, increasing viscosity. In Fig. 1, as expected, viscosity increased as KOH concentration increased up to 1 M. However, it decreased significantly above 1 M and was a minimum at 4 M. For the HE prepared by using 4 M KOH solution, GPC analysis suggested that some of the side chains were cleaved. This may cause the decrease in viscosity. When the KOH concentration was higher than 4 M, the viscosity increased again, as can be seen from Fig. 1. The analysis of IR spectra for HEs exhibited that the structure of XG molecules scarcely changed as the KOH concentration increased from 4 M to 7 M. Therefore, the increase in viscosity at KOH concentrations above 4 M is probably due to the salting out.
This work is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Reference

C. Iwakura, H. Murakami, S. Nohara, N. Furukawa, H. Inoue, J. Power Sources, 152, 291 (2005).

Figure 1