Li metal anode is the most attractive material for rechargeable batteries due to its high capacity of 3860 mAh/g and lowest electrochemical potential among metals. However, short-circuiting owing to non-uniform deposition (dendrites) has plagued Li metal anodes with short cycle life and associated safety issues. To address these issues, a number of methods such as additives in electrolyte and artificial protection layer have been proposed, but sufficient control of Li morphology has not been achieved. The morphology of electrodeposit metal is known to be highly dependent on the crystal orientation of the electrodes [1]. Recently, Mitsuhashi et al. have also reported that the morphology of electrodeposited Zn depends on the crystal orientation of the substrates and a substrate with (0001) orientation is likely to initiate non-uniform deposition [2]. Such morphological variations are attributed to the orientation dependence of charge transfer and the surface diffusion kinetics of the depositing metal. In an analogous way, the initiation of Li dendrites can also be influenced by the crystal orientation of current collectors. However, few studies have approached the dendrite problem from this perspective. In the present study, we investigated the morphology of Li electrodeposition on single crystal Cu collectors compared to on a polycrystalline Cu collector. In addition, a numerical analysis of the initial stage of Li electrodeposition under galvanostatic conditions was conducted to elucidate the mechanism for the morphological variation.

The electrodeposition of Li for different Cu current collectors was evaluated by using a three-pole cell. The working electrode is polycrystalline Cu (poly-Cu) and single crystal Cu (single-Cu) of (111) orientation. Prior to experiments, crystal orientation of Cu collectors was analyzed by using electron backscatter diffraction (EBSD). An electrolyte used for experiments is consisted of 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) with a 5:5 weight ratio. Li was deposited at a current density of 0.5 mA/cm² and a charge capacity of 0.1 mA/cm².

Figure 1(a) is a SEM image of Li precipitates on a poly-Cu current collector. The Li precipitates have a spherical shape, and their size vary widely with the depositing positions. Figure 1(b) shows the orientation map of poly-Cu by EBSD overlapped with Figure 1(a). The shape variation of Li precipitates corresponds with the crystal grain of Cu. Comparing with the orientation map, we find that the most uniform Li precipitates are observed on a (111)-oriented grain. Figure 1(c) and 1(d) show SEM images of Li precipitates on a single-Cu(111) current collector. Unlike Li on poly-Cu, precipitates on single-Cu(111) are small and uniform in size over the entire area. This result suggests that the single-Cu(111) collector is effective in suppression of non-uniform deposition of Li. Numerical analysis indicates that the initial stage of Li electrodeposition plays an important role in the morphological variation. This is due to the crystal orientation dependence of the Li adatom concentration at equilibrium Γ₀. Considering the chemical potential upon adsorption of Li adatom, we evaluate the order of magnitude of Γ₀ for different crystal orientation as Γ₀(111) < Γ₀(001) < Γ₀(101) < Γ₀(high-index planes). Our calculation show that a small Γ₀ results in a small radius and narrow size distribution of Li nucleus. This crystal orientation dependence leads to non-uniformity of Li precipitates on poly-Cu.

Reference

[1] S. Itoh et al., Surface Technol., 5, 27 (1977).

[2] T. Mitsuhashi et al., Thin Solid Films, 590, 207 (2015).