Influence of Lithium Diffusion into Copper Current Collectors on Lithium Electrodeposition in Anode-Free Lithium-Metal Batteries

Abstract

With increasing demands for lithium-based batteries that have higher energy densities, access to better negative electrode materials becomes crucial. Lithium metal which has a very high theoretical specific capacity, a very low standard potential and a very low density can be considered one of the best negative electrode materials for next-generation lithium-based batteries. In order to fully exploit the merits of lithium metal, a cell configuration in which lithium is directly electrodeposited (and then stripped) on (from) the negative electrode current collector (i.e., copper foil) is essential. Such “anode-material-free” (or ”anode-free”) lithium-metal batteries can have significantly higher gravimetric and volumetric energy densities than conventional graphite-based lithium-ion batteries. However, poor control of the lithium electrodeposition directly on the copper current collector, especially in conventional carbonate electrolytes, limits the development of such batteries. It is therefore essential to improve the understandings of the lithium electrodeposition, especially its nucleation process, and the interactions between lithium and the copper substrate.According to the classical electrodeposition theory, a larger overpotential will lead to a decrease in both the critical free energy for the nucleation and the critical radius of the nuclei, which should facilitate the nucleation process since more clusters can reach the critical radius required to form stable nuclei with a lower energy barrier. The concept was utilized via the application of a potentiostatic nucleation pulse to attain two-dimensional instantaneous lithium nucleation on lithium-metal electrodes in a previous study by Rehnlund et al.[1] Using such a strategy should also provide some insights into the lithium nucleation on copper substrates. In addition, it is also important to consider interactions between lithium and the copper substrate. A study, in which Lv et al.[2] used operando neutron diffraction to track the spatial distribution of lithium during lithium electrodeposition and stripping on copper, revealed that some lithium was actually taken up by the copper substrates most likely via the grain boundaries during the electrodeposition. In other studies by Rehnlund et al., the results also showed that lithium can diffuse into copper and that the effect of this diffusion can be readily seen after electrodepositing a small amount of lithium (as would be the case during the lithium nucleation stage).[3,4] However, the relation between such a lithium diffusion behavior and the lithium nucleation on copper substrates is not clearly studied.With a series of electrodeposition experiments, we demonstrate that it is highly possible that the lithium diffusion into the copper substrate can influence the nucleation process. Due to the presence of such diffusion, small lithium clusters and nuclei may be lost during the nucleation process, which makes it difficult to obtain a larger number of lithium nuclei with a homogeneous distribution on the copper surface. This then leads to inhomogeneous lithium electrodeposits with poor morphologies. It is, however, demonstrated that the nucleation of lithium on copper can be significantly improved if an initial chemical prelithiation of the copper surface is performed. This prelithiation saturates the copper surface with lithium and hence decreases the influence of lithium diffusion via the grain boundaries. In this way, the lithium nucleation can be made to take place more homogenously on the copper surface, especially when a short potentiostatic nucleation pulse that can generate a large number of nuclei is used.[1] D. Rehnlund, C. Ihrfors, J. Maibach, L. Nyholm, Mater. Today 21 (2018) 1010–1018.[2] S. Lv, T. Verhallen, A. Vasileiadis, F. Ooms, Y. Xu, Z. Li, Z. Li, M. Wagemaker, Nat. Commun. 9 (2018) 1–12.[3] D. Rehnlund, F. Lindgren, S. Böhme, T. Nordh, Y. Zou, J. Pettersson, U. Bexell, M. Boman, K. Edström, L. Nyholm, Energy Environ. Sci. 10 (2017) 1350–1357.[4] D. Rehnlund, J. Pettersson, K. Edström, L. Nyholm, ChemistrySelect 3 (2018) 2311–2314.