Interfacial Electronic Properties Dictate Li Dendrite Growth in Solid Electrolytes

The experimental observation of Li dendrite growth inside mechanically hard solid electrolytes (SEs) raised an important question: can hard SEs mechanically stop Li-dendrite growth? Here, we report a multiscale model coupling density functional theory calculations with the phase-field method to address the question. In particular, we investigate the roles of internal defects, such as pores and crack surfaces, inside a number of SEs including cubic Li₇La₃Zr₂O₁₂ (c-LLZO), β-Li₃PS₄, Li_1.17Al_0.17Ti_1.83(PO₄)₃ (LATP), and Li₂PO₂N. It is shown that LLZO surfaces have a much smaller band gap than the corresponding bulk and thus could trap significant excess electrons, whereas the other three systems do not exhibit significant differences in the surface and bulk band gaps. A fully coupled phase-field model was then developed to further examine the impact of excess surface electrons on the Li dendrite growth morphology in polycrystalline LLZO. This model successfully explained the experimentally observed dendrite intergranular growth and revealed that the trapped electrons may produce isolated Li-metal nucleation, leading to a sudden increase of Li-dendrite penetration depth. Finally, we compared the basic material properties and found that the ranked Li dendrite resistance in these SEs, based on the surface electronic properties instead of mechanical properties, is consistent with a broad range of experimental observations. Therefore, surface band gap and its alignment with Li-metal, as well as the excess electron distribution, can be used as key material properties to determine Li dendrite resistance of SEs.