Study of dendrite growth via Zn plating behavior via liquid phase in-situ TEM

Abstract text (incl. figure legends and references)

Nowadays, daily life without batteries is hard to be imagined. Recently, the scale of batteries becomes quite large to use in electronic cars. This scale growth develops the safety and durability of batteries as the most critical issue in the battery field. The most common reason for battery breakage is circuit shortage due to dendrite growth on electrodes. Li ions were usually absorbed in the anode during the charging of the Li-ion battery. However, Li ions could be plated on the anode under abnormal conditions such as overcharging. Additionally, when charging the metal-air battery, discussed as a future battery type, metal ions are always plated on the anode. In both cases, the way of ion plating should occur as smoothly coating the anode, not as drastic dendritic growth on the anode.

Understanding the plating mechanism on charging can shine to avoid dendrite growth. In principle, plating is simply an electrochemical reaction between electrons and ions, so plating should be studied at the nanoscale and correlated to bulk. Liquid phase in-situ transmission electron microscopy (TEM) is the best method to investigate the plating phenomena at the fundamental scale [1].

In this study, we investigated the Zn plating behavior via liquid phase in-situ TEM as a model study because Zn-air battery uses an aqueous solution and has an advantage in studying additive effects. We controlled the electrolyte flow to observe mass transport's effect during charging.

In-situ experiments were performed using a Thermofisher Scientific Titan HOLO at 200 kV. The TEM holder was a DENSSolution Stream holder designed for liquid phase electrochemical in-situ TEM experiments. The 0.5 M ZnSO4 aqueous solution was prepared as the electrolyte solution. The electrolyte solution flowed at a few µm/min., which is enough to conserve the ion concentration against electrochemical reaction around the electrode.

In the case of the flowing electrolyte solution, the Zn deposition occurred around the electrode surface. The shape of deposited Zn looked like coating the electrode surface. In the case of the non-flowing electrolyte solution, the Zn deposition occurred outward from the electrode. The shape of deposited Zn looked like dendrite grown from the electrode. Without flowing the electrolyte, the ion concentration near the anode was too low to continue the plating at the anode surface, so plating occurred at the surface of previously plated Zn where the ion concentration was still high enough to be plated and so on. The high ion concentration gradient at no flowing condition may lead to dendritic growth [2].

Avoiding dendrite growth while charging the batteries is the key to securing the safety and durability of batteries. The Zn plating behavior via liquid phase in-situ TEM was studied to reveal the dendrite growth mechanism. Depending on electrolyte flowing, the plating behavior was changed due to the ion concentration gradient. The effect of ion diffusion rate and additives on plating behavior will be studied in the future.

Acknowledgment

J.P. acknowledges the project iNEW FKZ 03F0589A from BMBF. S.B. acknowledges "Electroscopy" (grant no. 892916) from the Marie Sklodowska-Curie action.

References

[1] Y. Liu et al. Acc. Chem. Res. 2021, 54, 2088

[2] J. Tan et al. J. Electrochem. Soc. 2016, 163, A318

Figure 1. TEM images of Zn plating with flowing (a) and without flowing (b) electrolyte solution. The scale bar in (a, b) are 2 µm.