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