Zn is a promising anode material for next-generation large-scale energy storage devices. However, irregular shape evolution on its surface during cycling causes electrode degradation. The shapes and crystal structures of the deposits naturally originate from the initial behaviors of the depositions. At the initial stage of deposition, a micro-protrusion initiates on the Zn electrode, leading to an irregular shape evolution. This study focuses on the initial steps of Zn deposition using a multiscale simulation comprising density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations. This simulation allows analyses of phenomena from the picometer to the nanometer scale to yield mechanistic insight into the shape evolution of the deposits with respect to the electronic state of a particular species. The DFT calculations indicate that the Zn adatom exhibits specific behavior during surface diffusion: faster flat surface diffusion on the (0001) surface and slower interlayer diffusion. The KMC simulations show an irregular shape evolution based on the surface diffusion behavior of Zn as follows: (i) a two-dimensional (2D) hexagonal nucleation of the (0001) surface occurs on the substrate; (ii) the adatoms accumulate on the first layer to form layer-by-layer structures; (iii) the layer-by-layer structure forms the mountain structure, where the top layer exhibits a small area; and (iv) the top layer results in the protrusion. Therefore, the (0001) surface and interlayer diffusion rates are significant in the irregular shape evolution.

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Zn is one of the promising candidates as negative electrode materials for next-generation secondary batteries, especially those for large capacity applications such as grid-scale energy storage. While the Zn has numbers of attracting features such as crustal abundance, applicability of aqueous system, etc., which enable to realize the secondary batteries with high performance and safety with lower cost, the major drawback is evolution of irregular morphology called mossy structure with highly filamentous features at the electrode surface during charge-discharge cycles. Unlike the conventional dendritic growth which proceeds under diffusion-limited conditions, formation of the mossy structure takes place without such a condition. At the same time, it has been reported that the growth of these irregular structures could be suppressed by applying metallic species as additives to form uniform electrodeposition [1]. We have investigated formation process of the mossy structure on the Zn anode surface, focusing on its initial stage in combination with the additive effects such as Pb and Sn [2]. In this presentation, we will introduce the results of our research on the nucleation and growth process of the mossy structure through experimental analysis and computational modeling. For the theoretical calculation, we attempted to develop multi-scale simulation model by employing density functional theory (DFT) and kinetic Monte Carlo (KMC) approaches [3], and the results will be described focusing on the effects of the metallic additives. This work is financially supported in part by MEXT/JSPS Grant-in-Aid for Scientific Research No. 21H01642. [1] For example, F. Mansfeld, S. Gilman, J. Electrochem. Soc. 117, 1328 (1970); Y. Ito, M. Nyce, R. Plivelich, M. Klein, D. Steingart, S. Banerjee, J. Power Sources, 196, 2340 (2011). [2] For example, T. Otani, M. Nagata, Y. Fukunaka, T. Homma, Electrochim. Acta, 206, 366 (2016); T. Otani, Y. Fukunaka, T. Homma, Electrochim. Acta, 242, 364 (2017). [3] Y. Onabuta, M. Kunimoto, S. Wang, Y. Fukunaka, H. Nakai, T. Homma, J. Phys. Chem.C, submitted.

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