In Situ Optical Microscopy Studies of Spontaneous Oscillatory Growth of Zinc Dendrites

Rechargeable batteries have been an integral part of our society since the invention of lead-acid batteries in mid-1800s.[1] With the recent demand for clean and sustainable energy, recyclable and rechargeable batteries have received renewed attention. Among the rechargeable batteries, metal-air batteries are considered promising because of their high energy density, reliable lifetime, minimal environmental impact, and low cost.[2, 3] Of all the metal-air battery systems, Li-air and Zn-air batteries are extremely attractive because of their high energy densities. While Li is highly reactive, Zn is relatively more stable and desirable because of its mild response to moisture, low cost, abundance, and easy manufacturability. However, the drawbacks associated with Zn-based batteries include relatively low operating potential, poor stability of the air electrode, and poor recyclability.[2] One of the critical factors affecting the rechargeability of Zn-air batteries is the formation of unstable growth morphologies, notably dendrites -- highly anisotropic and porous structures. These dendrites are vulnerable to mechanical vibrations and can cause catastrophic failure by shorting the anode and the cathode. Here, we present results from in situ optical microscopy studies of Zn dendrite growth during electrodeposition of Zn in zinc sulfate (ZnSO 4 ) solutions. All of our experiments are carried out using an electrochemical Hele-Shaw cell. The cell consists of a circular (10 cm in diameter) Zn wire (99.994% purity) anode sandwiched between two Plexiglas plates, separated by a polydimethylsiloxane (PDMS) spacer and filled with 100-μm-thick solution of ZnSO 4 . A vertical Zn electrode (0.25 mm in diameter) placed at the center of the cell serves as the cathode. In order to prevent bubble formation, the electrolyte is introduced slowly into the cell. In our experiments, we observe an interesting phenomenon: oscillatory growth of the dendrites, during electrodeposition in 0.1 M ZnSO 4 with pH = 3.0. Upon applying a constant potential across the electrodes, the dendrites start to grow with a self-similar structure (see Figure 1). From the images of the dendrites obtained as a function of time, we find that the dendrite growth rate changes periodically. Concomitant with this oscillatory growth, we observe a periodic variation in deposition current. Scanning electron microscopy (SEM) images (see Figure 2(a)) of the dendrites reveal that they have self-similar morphology. Tilted (Figure 2(b)) and cross-sectional images (not included here) also show that the dendrites are made of micro-crystallites sticking out of a main arm, and that the dendrite thickness varies periodically along the length of the dendrites. Transmission electron diffraction patterns acquired from the sample show that the growth structures are single-crystalline and with the same crystal orientation. Based upon these initial results, we propose a growth model to explain the oscillatory growth. We gratefully acknowledge funding from the National Science Foundation (NSF-GOALI: DMR-1310639). Figure caption: Figure 1. In situ optical micrograph acquired during electrodeposition of Zn in 0.1 M ZnSO 4 solution with pH = 3 at E we = -2.5 V for 20 minutes. The scale bar is 100 µm in length. Figure 2. (a) Top and (b) 58°-tilt view scanning electron microscopy images of the dendrites extracted from the solution. The layered morphology is characteristic of oscillatory growth. The scale bars are 40 µm in length. Figure 1