Multiscale Simulation of Irregular Shape Evolution during the Initial Stage of Zn Electrodeposition on a Negative Electrode Surface

Abstract

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.

Effect of Li+ Addition during Initial Stage of Electrodeposition Process on Nucleation and Growth of Zn

“…41 A unit cell is sampled with Monkhorst-Pack k-point grids 42 (1 × 1 × 1) to sample the first Brillouin zone for comparison with the results of our previous study. 32 Geometrical optimizations were performed with convergence thresholds of 10 −4 Ry and 2.0 × 10 −3 Ry/ Bohr for energy and forces, respectively. Four atomic layer slab models were constructed based on the Zn hexagonal close-packed (hcp) structure: the Zn(0001) surface, Zn(0−110) surface, and step-terrace model of the Zn(0001) surface (Fig.…”

“…All these conditions were the same as those in our previous study. 32 The RISM part is implemented from a previous study by Nishihara et al 35 The solvent system in the RISM region contained H 2 O (1.00 g cm −3 ) and 6.0 mol l −1 LiOH. The temperature was set at 300 K. The Laue-RISM calculation was conducted with the closure of the model proposed by Kovalenko and Hirata.…”

“…Lennard-Jones parameters for Zn atoms were set as ε = 0.79 kcal mol −1 , σ = 3.4 Å. 32 Our previous study assumed four specific surface diffusions: surface diffusion on the (0001) surface, (0−110) surface, "interlayer diffusion," and the diffusion along the step-terrace, called "edge diffusion" (Fig. 1).…”

“…It suggested that the surface diffusion rates on the Zn surface are important for the initiation of the mossy structures. 32 The evolution of the nano scale protrusion, which is the initial structure of the mossy structures, is caused by the rapid surface diffusion on the flat (0001) surface and the slow diffusion from the deposited (0001) layer to the lower (0001) layer, referred to as interlayer diffusion. The suggested mechanism is as follows: (i) the rapid surface diffusion on the flat (0001) surface causes 2D nucleation on the flat surface, (ii) the Zn atoms deposit on the flat surface as layerby-layer structures due to the slow interlayer diffusion, (iii) the Zn adatoms accumulate on the top of the layer-by-layer structures, and (iv) the layer-by-layer structures form the nano-protrusion, which leads to the initiation of mossy structures.…”

“…Furthermore, a comparison of the activation energy difference between at the solid-liquid and solid-gas interfaces indicates that the surface diffusion rates are affected by the solvation of the water molecules surrounding the surface. 32 The solvent molecules interact with the Zn adatom and the Zn surface and vary the Zn adatom behavior of the surface diffusion. Alkali metal cations, such as Li + , could change the solvent molecule distributions in the interfacial area, which could have a non-negligible effect on the shape evolution of Zn.…”