Growth Behavior of ZnO during Discharge of Zinc Negative Electrode
Authors: Otani Tomohiro , Nagata Masato , Fukunaka Yasuhiro , Homma Takayuki 
Published: February 2020

Zinc is promising negative electrode for secondary batteries due to high energy density and low cost. However, morphological changes of the electrode during charge-discharge cycles result in poor cyclability. Especially, formation of ZnO during discharge (anodic dissolution) is important process because precipitation of ZnO strongly influences cyclability and discharge capacity of the electrode [1]. In addition, ZnO formation mechanism is characteristic since ZnO is not directly formed by electron transfer but by dynamic concentration change of ions near electrode surface as described in Eq. (1) and (2).

Zn + 4OH− = Zn(OH)4
2− + 2e− (1)

Zn(OH)4
2− = ZnO + 2OH− + H2O (2)

After formation of zincate ion by oxidation (Eq. (1)), ZnO precipitates when local concentration of zincate ion exceeds solubility limit (Eq. (2)).

In previous study, ZnO morphologies were qualitatively classified based on dissolution conditions [2]. However, nucleation and growth behavior of ZnO, which is coupled with dynamic concentration change near electrode surface, is still unclear.

In this study, in order to shed light on formation process of ZnO accompanying anodic dissolution, morphological and structural evolution of ZnO were analyzed in detail. Anodic dissolution was performed in galvanostatic conditions (+20 and +40 mA cm−2) in electrolyte containing 2.0 – 6.0 M KOH and 0.050 M ZnO. Effects of Li+addition were investigated by adding 0.50 M LiOH to electrolyte. Zinc working electrode was horizontally upward faced to counter electrode in order to avoid natural convection effect [3]. Counter electrode was zinc wire and reference electrode was Hg/HgO. Crystal structure of precipitated ZnO was analyzed by X-ray diffraction (XRD) and Raman spectroscopy. Scanning electron microscopy (SEM) was used to analyze morphology of the precipitates.

First, to understand the formation process of ZnO, morphology and structure of precipitates were analyzed in time-resolved manner; dissolution was stopped at several oxidation time (200, 400, 600, 750 s) in 2.0 M KOH at +20 mA cm−2 and precipitates were characterized. At 750 s, electrode potential drastically changed to positive value. This potential increase was considered as passivation of electrode by precipitated ZnO [1]. From XRD measurements, ZnO peaks first appeared at 600 s. Simultaneously, SEM observations showed ZnO crystals with a few μm length. On the other hand, Raman scattering showed peak of ZnO E1 mode, which was assigned to defective ZnO (Zn excess ZnO), even at 200 s and 400 s where no ZnO peaks appeared in XRD. At 600 s, ZnO E2 mode (crystalline ZnO) also appeared and became dominant over E1 mode at passivated condition (750 s). These results suggested that the formation process of ZnO transiently changed; low crystalline ZnO formed at initial stages of dissolution and crystal growth of ZnO became dominant as dissolution proceeded.

We hypothesized that this transient behavior was originated from dynamic change in nucleation and growth behavior of ZnO as follows. First, at initial stages of dissolution, nucleation by zincate condensation in supersaturated solution was favored. Then, crystal growth became dominant by decomposition of zincate species on precipitated ZnO surface. This assumption was testified by Li+ addition; in homogeneous precipitation of ZnO from zincate solutions, Li+ was known to prevent decomposition of zincate on ZnO surface and following crystal growth of ZnO particles [4]. Accordingly, the crystal growth observed at the latter stages of dissolution would be influenced by Li+ when this process was originated from zincate decomposition on the surface. As a result, acicular morphology of ZnO was changed to particulate by Li+ addition. Intensity of ZnO peaks decreased in XRD profiles. Thus, crystal growth was mitigated by Li+addition and this results supported the hypothesis above.

From these results, ZnO growth behavior accompanying anodic dissolution of zinc dynamically changed from nucleation to crystal growth. Dominant growth behavior observed at the latter stages of dissolution was possibly caused by decomposition of zincate species on ZnO surface.

This study was supported by JSPS KAKENHI Grant Number 25249104. T. O. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan.

[1] A. Nakata, H. Arai, T. Yamane, T. Hirai, Z. Ogumi, J. Electrochem. Soc., 163, A50 (2016).

[2] R. W. Powers, M. W. Breiter, J. Electrochem. Soc., 116, 719 (1969).

[3] I. Arise, Y. Fukunaka, F. R. McLarnon, J. Electrochem. Soc., 153, A69 (2006).

[4] N. Uekawa, R. Yamashita, Y. J. Wu, K. Kakegawa, Phys. Chem. Chem. Phys., 6, 442 (2004).