Effect of Thermally Evaporated Silver Nanoparticles on the Surface Morphology and Stability of Electrodeposited Lithium Metal Anode
Authors: Cho Ki-Yeop , Kwon JunHwa , Lee Hwon-gi , Song Hayong , Eom KwangSup 
Published: February 2020

Due to the extra-high capacity (3860 mAhg-1) and the lowest electrochemcial equilibrium potential (-3.040 VSHE), lithium metal has been considered as a promising candidate for high-energy lithium ion battery anode [1]. However, the undesired dendrite growth results in detrimental effects in the cell performance, such as cell short-circuit, low coulombic efficiency, and dead lithium, which have hampered widespread exploits of lithium metal-based batteries. Therefore, in recent year, numerous strategies to prevent or mitigate the problematic dendrite growth, including protective layers on Li metal surface, solid electrolytes, artificial solid electrolyte interphase (SEI) layer and so forth has been reported. Among them, one of the effective methods is to provide heterogeneous lithiophilic nucleation sites on the surface of current collectors as first suggested by Yan et al., [2]. They found that heterogeneous metal seeds, which have solubility of lithium in binary phase diagram, could induce selective deposition of lithium on its surface. Recently, Hou et al., reported lithiophilic Ag nanoparticles as nucleation sites by using electroless deposition for stable electrodeposition of lithium [3]. However, from a heterogeneous nucleation point of view, the morphology of electrodeposited lithium metal will be strongly affected by uniformity of nucleation seed layers, in other words, the more uniform distribution of nucleation seeds, the more stable morphology of lithium.
In this study, we introduce a thermal evaporation method to deposit Ag nanoparticles as well-distributed nucleation sites. The Ag seeds with various evaporation thicknesses (1, 3 and 5 nm) were deposited on the commercial copper foils (thickness ≈ 25 um) to study the effect of evaporation thickness on the morphology of electrodeposited lithium metal. To electrodeposit lithium on Ag-seeded Cu foil and investigate the electrochemical properties of deposited Li metal electrodes, CR 2032-type coin cells were assembled in an Ar-filled glove box (CE: Li strip, WE: Ag-seeded Cu foil, and Celgard separator). We used 1M lithium bis(trifluoromethane)sulfonamide (LiTFSI) dissolved in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume) as electrolyte without any additives.

Figure 1 shows SEM surface morphologies of a pristine copper foil and Ag-seeded Cu foils with different evaporation thicknesses of Ag. After evaporation of Ag 1 nm on the pristine Cu foil (Fig. 1-(a)), it is showed that nano-sized Ag particles uniformly distributed on the surface of Cu foil (Fig. 1-(b)). In addition, from Fig. 1-(b) to Fig. 1-(d) it is clearly confirmed that the sizes of Ag particles increased with increasing evaporation thicknesses. Figure 2 shows the morphologies of electrodeposited lithium on Ag-seeded layers with different evaporation thickness. Figure 2-(a) shows SEM image of lithium metal deposited on a pristine Cu foil, which has well-known dendritic structure. In case of electrodeposited lithium on the Ag 1 nm-seeded Cu foil (Fig. 2-(b)), it also shows dendrites on its surface although heterogeneous nucleation sites are provided. However, the morphology of lithium on the Ag-3nm seeded Cu foil has distinct differences in shape and size of lithium deposits (Fig. 2-(c)). The width size of lithium deposits was smaller than those of Ag 1 nm-seeded Cu foil and pristine Cu foil, and they have interlacing vine-like structure. Further increased evaporation thickness of Ag nucleation layer to 5 nm leads to large morphological changes of electrodeposited lithium metal (Fig. 2-(d)). It shows highly dense structure with less pore and flat surface, meaning that dendrite-free lithium film was deposited.
In summary, we used thermally evaporated Ag nanoparticles as heterogeneous nucleation sites to electroplate uniform lithium metal. From the results, it was concluded that through a facile thermal evaporation controllable and well-distributed heterogeneous nucleation metal seeds can be formed on the commercial Cu current collectors, leading to a dendrite-free deposition of lithium.

References

[1] X.-B Cheng, R. Zhang, C. -Z. Zhao, and Q. Zhang, “Toward Safe Lithium Metal Anode in Rechargeable Batteries : A Review,” Chem. Rev, 117, 10403, 2017.
[2] K. Yan, Z. Lu, H. -W Lee, F. Xiong, P. -C. Hsu, Y. Li, J. Zhao, S. Chu and Y. Cui, “Selective Deposition and Stable Encapsulation of Lithium throuh Heterogeneous Seeded Growth,” Nature Energy, 1, 16010, 2016.
[3] Z. Hou, Y. Yu, W. Wang, X. Zhao, Q. Di, Q. Chen, W. Chen, Y. Liu, and Z. Quan, “Lithiophilic Ag Nanoparticle layer on Cu Current Collector toward Stable Li Metal Anode,” ACS Appl. Mater. Interfaces, 11, 8148, 2019.

Figure 1