Multiscale Simulation for Effects of the Li+ Addition at the Initial Stage of Zn Electrodeposition Processes
Authors: Onabuta Yusuke , Wang Songyi , Kunimoto Masahiro , Nakai Hiromi , Homma Takayuki 
Published: December 2020

A Zn negative electrode is a promising material for large-scale energy storage devices because of its safety and high energy density. The problem for its application is the irregular deposition such as dendritic and mossy structural growths on the electrode during charge-discharge cycles. Application of additives is an effective and practical solution to control the surface morphology. Recently, it is reported that cationic surfactants lead to the smooth surface morphology of the Zn deposits [1]. Our past study also proposed that the Li+ addition particularly could control the morphology effectively [2]. To consider the effect of Li+, which has not been well understood, it is important to systematically analyze the cationic additive effects. Besides, it should be understood at the atomic scale. In this study, a multiscale simulation that combined a kinetic Monte Carlo (KMC) simulation and a first-principle calculation based on density functional theory (DFT) was performed to investigate the Li+ additive effects for the Zn electrodeposition from the molecular viewpoint.

Among several parameters, the surface diffusion of electrodeposited adatoms is known to be one of the most critical factors to determine the morphology of deposits. This indicates that profound understanding of the connection between the surface diffusion behavior and the morphology in the presence of additives leads us to elucidate its effect at the atomic scale. So we examined the surface diffusion of the Zn adatom by DFT as a significant step for deposition. The calculated activation barriers of the surface diffusions were implemented in the KMC simulations. All DFT calculations were performed by Quantum-ESPRESSO code with the effective screening medium + the reference interaction site model (ESM-RISM) [3]. The surface diffusion and deposition were taken into account as possible events in the KMC simulation code. There are three types of the surface diffusion included in the simulations: (a) surface diffusion on (0001), (b) on (0-110), and (c) interlayer diffusion on (0001) called Ehrlich-Schwoebel barrier.

The surface diffusion of the Zn adatom on (0001) in the presence of Li+ or K+, as common cationic additives, were simulated by DFT. The activation energy in the presence of Li+ was 13.7 kJ/mol, while that in the presence of K+ was 14.5 kJ/mol; the surface diffusion in the presence of Li+ is faster. The detailed analysis of the solvation structure at the solid-liquid interface showed that the interaction between the Zn adatom and the water molecule was weakened in the presence of Li+. This is because Li+ forms strong solvation structures and water molecules are more likely to interact with Li+ than the Zn adatom. On the other hand, the water molecules near the surface were relatively relaxed in the presence of K+. In this case, the Zn adatom is solvated strongly. The difference in such solvation structures causes the difference in diffusion tendencies (Fig. 1). The KMC simulations revealed that the fast surface diffusion on (0001) by the effect of Li+ enhanced the growth of (0001) on the surface. This effect is critical to determine the surface morphology of the deposits.

[1] M. Shimizu, K. Hirahara, S. Arai, PCCP, 21, 7045-7052 (2019).

[2] T. Otani, T. Yasuda, M. Kunimoto, M. Yanagisawa, Y. Fukunaka, T. Homma, Electrochim. Acta, 305, 90-100 (2019).

[3] S. Nishihara, M. Otani, Phys.
Rev. B, 96, 115429-115433 (2017).

Figure 1