Lithium metal is known as a promising anode material for lithium-based batteries, which possesses an extremely high theoretical capacity.¹ The Li dendrite growth during the cycling of the battery has prevented it from being used as the anode directly.² The origins of dendrite formation may be associated with the mechanisms of Li plating and with the mode of charge transfer during Li reduction or oxidation at the anode-electrolyte interface.³

Herein, density functional theory (DFT) calculations were conducted to analyze the electron transfer between Li (100) and Li cations located in the proximity of the surface in several simulation models. The study includes two common used solvents: ethylene carbonate (EC) and dimethoxyethane (DME), and a LiPF₆ salt, which surround the Li cation over perfect, defect-containing and the Li₂CO₃ passivated Li (100) surfaces. Figure 1 shows the Li deposition on three-Li defect Li (100) surface. Three possible positions for the introduced Li cation were considered (as indicated A, B, and C, respectively in Figure 1a). The calculation results showed that the hollow site is the preferred deposition site (Figure 1b), which is in good agreement with the result of Li deposition on perfect Li (100) surfaces. Figure 1c shows the Bader charges of three defect Li and one introduced Li cation (red circled), located in the large electron-accumulation area. This added cation receives electrons and has a Bader charge of 0. In the other word, it was reduced and plated on the Li metal surface.

Our calculations demonstrate that the Li cation is easily reduced when bonding to DME rather than EC. Additionally, a compact Li₂CO₃ layer inhibits the charge transfer from Li metal to Li cations, thus manipulating the Li plating process. Furthermore, the diffusing and plating of Li cations through other SEI components, including LiF, Li₂O and LiOH, will be studied to illustrate the protective role of coating layers.

References

(1) Kim, H.; Jeong, G.; Kim, Y.-U.; Kim, J.-H.; Park, C.-M.; Sohn, H.-J. Chemical Society Reviews 2013, 42, 9011.

(2) Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J.-G. Energy & Environmental Science 2014, 7, 513.

(3) Love, C. T.; Baturina, O. A.; Swider-Lyons, K. E. ECS Electrochemistry Letters 2015, 4, A24.

Figure 1. (a) side view and top view of three-Li defective slab (three possible deposition sites for Li cations are marked as A-top, B-bridge and C-center, respectively); (b) the most stable optimized deposition structure for the introduced Li cation; (c) charge density distribution for the most stable structure; the corresponding Bader charges (black numbers) of the three defect Li atoms and reduced Li are shown. In the charge density distribution diagrams, yellow represents the electron-accumulation area, while cyan is the electron-depletion area.