Abstract
Introduction Aqueous rechargeable alkaline zinc batteries have attracted considerable interest as potential candidates for energy storage systems because of their high theoretical capacity for metallic zinc (820 mAh g−1) and corrosion resistance1. In particular, alkaline MnO2/Zn batteries are considered candidates for energy storage because of their low cost, availability of raw materials, and high theoretical capacity (308 mAh g-1) based on one-electron reduction. However, concerns regarding rechargeability remain an obstacle in the utilization of large-scale applications. In the typical discharge-charge mechanism of an alkaline MnO2/Zn battery, several phenomena deteriorate both electrodes. At the MnO2 positive electrode, spinel ZnMn2O4 is known to be formed in discharging, and at the Zn negative electrode, dendrite growth and shape change induce short circuits. To enhance the electrochemical performance, we investigated the use of polymer hydrogel electrolytes (HEs) because they have high ionic conductivity, good electrolyte/electrode formation, and lack of leakage issues2. In this study, HEs based on low-cost biocompatible xanthan gum, one of polysaccharide, and KOH solutions were prepared for use in quasi-solid-state MnO2/Zn rechargeable batteries. Experimental XGn (n = n × 10-1 g mL-1, n 0.1, 0.2, and 0.3 g) denoted (XG1, XG2, and XG3) hydrogel electrolytes were obtained by mixing n × 10-1 g mL-1 XG with (4 M KOH + 0.3 M ZnO) with stirring for 30 min. The obtained HEs were allowed to rest for 48 h to absorb the KOH. The ionic conductivity, water retention, symmetrical Zn/Zn cell, and interfacial resistance of the HEs were characterized. A full cell consisting of γ-MnO2 positive electrode and zinc negative electrode was assembled in a pouch-cell configuration, and the HE was sandwiched between two electrodes. X-ray diffractometry, Raman spectroscopy, and X-ray photon spectroscopy were used to understand the charge-discharge reaction mechanism of MnO2 in the HEs. Results and Discussion XG1 exhibited a high ionic conductivity of 0.46 S cm−1 at room temperature. However, the ionic conductivity decreased with increasing the content of XG. Figure 1a shows the Arrhenius plots of the inverse of the charge-transfer resistance (R ct) of the Zn electrode in the two electrolyte environments, as evaluated by electrochemical impedance spectroscopy. The activation energies of Zn deposition/dissolution in both electrolytes were almost the same. These results suggest that the rate of the interfacial process in XG2 was comparable to that in an alkaline solution.Figure 1b shows the initial four cycles of charge-discharge curves of the MnO2/XG2/Zn cell at a current density of 0.2 C-rate. The maximum discharge capacity reached 298 mAh g-1 in the 1st cycle, which is close to the theoretical capacity of the one-electron reduction (308 mAh g-1) of MnO2 to MnOOH. Moreover, X-ray diffractometry, Raman spectroscopy, and X-ray photoelectron spectroscopy have shown that inactive ZnMn2O4 is formed during the 1st discharging, which degrades the electrode during long-term cycling. The current study highlights the potential of XG-based HEs in rechargeable alkaline MnO2/Zn batteries.This study was based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References M. R. Bailey, S. W.Donne, Electrochimica acta, 56, 5037 (2011).C. Iwakura, H. Murakami, S. Nohara, N. Furukawa, H. Inoue, J. Power Sources, 152, 291 (2005). Figure 1
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