Multiscale Simulation for the Initial Stage of the Zn Electrodeposition with Kinetic Monte Carlo Based on First-Principle Calculation
Authors: Onabuta Yusuke , Onuki Kosuke , Wang Songyi , Kunimoto Masahiro , Nakai Hiromi , Homma Takayuki 
Published: May 2020

Zn negative electrodes for the secondary battery are attractive for the grid-scale energy storage. One of the main issues for its application is a morphological change of Zn during the electrodeposition on the cathode. In particular, formation mechanism of the mossy structure has not been clarified yet. Our previous study observed the layer-like Zn deposits before the evolution of the mossy structure[1]. However, cause of the layer-like structure is still unclear. To reveal further detailed mechanism, molecular level understanding is significant. Kinetic Monte Carlo (KMC) simulation with first-principle calculation is a powerful tool to analyze the morphological formation from the molecular level. This study analyses the initial stage of the Zn electrodeposition by KMC with the parameters derived from first-principle calculations to provide insights into the cause of the unique morphological changes of Zn.
In the KMC simulation code, the surface diffusion and deposition were taken into account. (a) surface diffusion on pure facets, (b) edge diffusion along islands, (c) interlayer diffusion were included as the surface diffusion events. All surface diffusion rates were estimated by the transition state theory from first principle calculations. The deposition rate corresponds to current density set in the actual electrochemical measurements. First-principle calculations with the solvation were performed by Vienna ab initio simulation package (VASP) with the polarized continuum model (PCM) and Quantum-ESPRESSO code with the effective screening medium + the reference interaction site model (ESM-RISM)[2]. ESM-RISM can describe the electrochemical interface with the applied potential and the surrounding electrolyte. Nudged elastic band was used to calculate the transition state. Cu(111) and Zn(0001) were utilized as a typical substrate species.
The transition state of the surface diffusion of the Zn adatom on Cu(111) and Zn(0001) were calculated in the gas phase initially. The activation energy on Cu was much higher than on Zn, which were 4.38 and 0.58 kJ/mol, respectively. This tendency showed a good agreement with the general discussion of the surface diffusion[3]. To realize the solvation effects on each surface, transition state of the surface diffusion with PCM were calculated. The activation energy on Cu(111) was 4.79 kJ/mol, which was 0.41 kJ/mol higher than the gas phase. On the other hand, the activation energy on Zn(0001) in solvent was only 0.19 kJ/mol higher than the gas phase. The Zn adatom behavior was affected by not only the simple solvation but the interfacial structure among the adatom, solvents and surfaces, indicating that the interaction between the surfaces and solvent species is a significant factor for the surface diffusion behavior. These differences could derive from the difference of the surface electronic state. Since the surface electronic state depends on the facets, the interfacial interaction would affect and change the balance of the surface diffusion among the facets. This effect will cause the change of the initial deposition behavior. Combining these activation energies with KMC as the parameters, we could find the relationship between the morphological features of metals and the electronic state of the surface.
Fig. The transition state of the surface diffusion of the Zn adatom in gas and solvent phases
[1] T. Otani, Y. Fukunaka, T. Homma, Electrochim. Acta, 242, 364-372 (2017).
[2] S. Nishihara, M. Otani, Phys.
Rev. B, 96, 115429-115433 (2017).
[3] K. Iokibe, K. Azumi, H. Tachikawa, J. Phys. Chem. C, 111, 13510-13516 (2007).

Figure 1