Zinc Electrodes in Water-Organics Electrolytes: In Situ Analysis of Suppressed Shape Change

Abstract

Introduction Zinc-based secondary batteries are promising power sources as inexpensive and high energy density systems, however, they have not fully been established due to the insufficient lifetime of the zinc electrode, mainly caused by the dendrite growth and shape change [1]. To elucidate how the deterioration proceeds, many trials have been made. We have employed operando X-ray diffraction (XRD) [2] and X-ray fluorescence (XRF) imaging [3] to find that the zinc species formed during discharging is dissolved in the electrolyte at least for several minutes before it is precipitated as ZnO even if the electrolyte is preliminarily saturated with ZnO, implying supersaturation of the zinc species. Such dissolved zinc species can induce undesirable morphology change during cycling to cause deterioration. To suppress the dissolution of the zinc species, we have proposed water activity control by using hydrophilic organic compounds dissolved in the alkaline electrolyte [4]. Namely, the formation of the soluble zincate anions Zn(OH)4 2- formed by the electrochemical reaction (1) can be suppressed by moving the equilibrium in the chemical reaction (2) to the right hand side if the water activity is decreased. It has been shown that the addition of the organic compounds decrease the vapor pressure (water activity) and ZnO solubility, leading to long lifetime of the zinc electrode without dendrite growth.Zn + 4OH- = Zn(OH)4 2- + 2e- (1) Zn(OH)4 2- = ZnO + H2O + 2OH- (2) In this study we examine in situ X-ray diffraction (XRD) mapping to show how the addition of the organic compound affects the shape change behavior of the zinc electrode. Experimental Porous ZnO composite electrodes were formed on the copper current collectors and were sandwiched by two NiOOH counter electrodes, inserted with non-woven separators in the experimental cells. They were cycled with 50% utilization in aqueous KOH solutions saturated with ZnO. Organic compounds were occasionally added to the electrolyte. Attention was paid to undesirable oxidation of the organic compound at the counter electrode not to occur. The ZnO and metallic zinc distribution in the zinc electrode of a rectangle shape 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). Such non-destructive analysis can prevent undesirable composition changes (metallic zinc oxidation) that can occur when the electrode is exposed to air. Results and Discussion The addition of the organic compound in the alkaline electrolyte was proved by enhance cycle performance. The porosity of the ZnO composite electrode was maintained when the organic compound was added, suggesting that ZnO oxidation and reduction take place without morphology changes. The formation of metallic zinc and ZnO on reduction and oxidation of the zinc electrode, respectively, was monitored by the XRD mapping. On reduction, it was found that metallic zinc was preferentially formed at the periphery of the rectangle. This is probably because of good supply of ions and electrons there. On the other hand, ZnO growth was observed mainly in the center of the electrode, typical of shape change. If metallic zinc formed at the periphery of the electrode is oxidized on the spot, such shape change is unlikely to occur. This implies that zinc transportation through the electrolyte results in the shape change. The shape change can lead to the formation of inactive ZnO in the center of the electrode with insufficient electrical contact with the current collector and finally to the capacity loss.The shape change was suppressed with the addition of the organic compound in the electrolyte; more uniform formation of metallic zinc and ZnO was observed. It is thus expected that the organic compound reducing the water activity contributes to suppression of the zinc dissolution and fixation of the zinc species at the electrode for better rechargeability. Acknowledgment This work was supported by RISING of NEDO. Reference [1] F.R. McLarnon and E.J. Cairns, J. Electrochem. Soc., 138, 645-664 (1991).[2] A. Nakata et al., Electrochim. Acta, 166, 82-87 (2015).[3] A. Nakata et al., Electrochemistry, 83, 849-851 (2015).[4] A. Nakata et al., J. Electrochem. Soc., 163, A50-A56 (2016).