A critical challenge in the development of safe, long-lasting secondary batteries with lithium metal anodes is protecting the surface of the anode to prevent dendrite formation during charging¹ or to minimize interactions between the anode and active species in the electrolyte.^2,3 One approach is to place a protective layer in the cell, such as a coating on the anode itself or as a free-standing membrane material, which can inhibit many of the mechanisms that lead to failure of the lithium metal anode. These layers must be mechanically robust such that they do not fail over the course of many cycles. However, these materials experience stress during cycling, which can lead to mechanical failure at high current densities. While such phenomena have been examined in certain cathode materials, such as LiCoO₂,⁴ it has yet to be investigated in depth for anode protective layers. The stress arises from gradients in the lithium concentration that lead to the formation of eigenstrains. Here, we present a continuum-scale model to examine the effect of anisotropy in mechanical and transport properties on eigenstrains in protective layer materials. The model solves for the mechanical equilibrium of the layer, with the anisotropic linear elasticity tensor as well as the concentration-dependent anisotropic eigenstrain in the material based on the steady-state concentration profile. The von Mises stress is calculated as a function of the anisotropic properties and applied current. This stress can be compared to the yield stress of the protective layer material to determine whether it is sufficiently robust for many charge/discharge cycles of the lithium metal anode. The presented work will cover a range of cases from single-crystal domains to polycrystalline films with varying levels of anisotropy. By examining the peak von Mises stress as a function of the applied current density, the model will predict design guidelines for the relevant material parameters.

References

1. Y. Cao, X. Meng, and J. W. Elam, ChemElectroChem, 3, 858–863 (2016).

2. R. Cao, W. Xu, D. Lv, J. Xiao, and J.-G. Zhang, Adv. Energy Mater., 5, 1402273 (2015).

3. S. E. Doris, A. L. Ward, P. D. Frischmann, L. Li, and B. A. Helms, J. Mater. Chem. A, 4, 16946–16952 (2016).

4. H. Wang, Y.-I. Jang, B. Huang, D. R. Sadoway, and Y.-M. Chiang, J. Electrochem. Soc., 146, 473–480 (1999).