Factors Affecting Rechargeability of Zinc Electrodes Evaluated By in Situ analysis

Abstract

Introduction Zinc is an ideal negative electrode material for batteries with its high specific capacity and high reducing power. However, zinc-based secondary batteries have not been established yet due to the insufficient rechargeability caused by deterioration modes such as dendrite growth and shape changes [1]. Many trials have been made to elucidate how the deterioration proceeds, but they are mostly employed ex situ. Application of in situbattery analytical methods should provide fruitful information on the rechargeability of the zinc electrode system. In this study we examine X-ray diffraction (XRD) mapping to visualize the shape changes and projection-type X-ray fluorescence (XRF) imaging [2] to capture the zinc distribution near the electrode during cycling, and elucidate how the dissolution of the zinc species affects the rechargeability of the zinc electrode. Experimental The shape change phenomena were examined using porous ZnO electrodes formed on the copper current collectors. The ZnO electrode was sandwiched by two NiOOH counter electrodes, inserted with non-woven separators, and was cycled with 50% utilization in aqueous KOH solutions saturated with ZnO. The ZnO and Zn distribution was evaluated in a non-destructive manner by XRD mapping with ca. 900 pixels per electrode in a transmission mode at synchrotron beamline BL28XU, SPring-8 (Hyogo, Japan). The distribution of zinc species near the electrode was evaluated by projection-type XRF imaging in reflection geometry as follows. Zinc was deposited on a copper plate as a working electrode (WE) in ZnO containing 4 M KOH using zinc plate as a counter electrode (CE) and then was oxidized. The images were recorded every 5 s at 9.8 keV near the Zn-K absorption edge using a two-dimensional detector at BL28XU, SPring-8. Results and Discussion The Zn and ZnO formation on reduction and oxidation of the zinc electrode was monitored by XRD mapping. In the course of cycling the ZnO intensity started to decrease at the edge of the electrode. Figure 1(a) depicts the ZnO mapping after the 100threduction cycle in 8 M KOH (ZnO satd.), showing ZnO aggregated in the center of the electrode. This shape change correlated with the loss of the zincate species at the edge and the accumulation in the center leads to the formation of inactive ZnO due to the loss of electrical contact with the current collector and finally to the capacity loss. In contrast, in a less zinc soluble electrolyte of 4 M KOH (ZnO satd.), the shape change was limited as shown in Fig. 1(b), leading to longer cycle life. This implies the importance of fixation of zinc species at the electrode for better rechargeability. The projection-type XRF imaging near the WE clarifies many pillar-like deposits, namely dendrite, formed on the copper substrate (invisible) together with hydrogen bubbles on zinc reduction as shown in Fig. 1(c). A concentrated zincate zone containing such bubbles is seen on oxidation (Fig. 1(d)), indicating the formation of soluble zincate species even in 4 M KOH with ZnO saturation. The zinc rechargeability should also be affected by zinc supersaturation in the alkaline electrolyte and hence the observation of the zinc species in supersaturated solutions by XAFS and NMR would provide useful information. Acknowledgment This work was supported by RISING of NEDO. Reference [1] X.G. Zhang, in Encyclopedia of Electrochemical Power Source, Vol. 5, pp. 454 (2009). [2] A. Nakata et al., Electrochemistry, submitted. Figure 1