The driving force for the metallic lithium formation in solid electrolytes, such as the cubic Li7La3Zr2O12 (c-LLZO), is still puzzling, considering the large band gap of c-LLZO and the preference of Li being ionic in oxides. In this paper, density functional theory (DFT) calculations were performed to investigate the ability of the pore surfaces to trap excess electrons, which is essential to Li nucleation tendency. Two interlayer materials were also studied, tetragonal LLZO (t-LLZO) and Li2PO2N (atomic layer deposited LiPON). It was found that excess electrons would be trapped, either around the La atoms on the surface of c-LLZO or dispersed on the surface of t-LLZO. It is also thermodynamically favorable for the excess electrons on La3+ to reduce Li+. In contrast, the excess electrons were located underneath the surface in Li2PO2N, making it hard to reduce Li+. Thus the surfaces in c-LLZO provide a possible electron pathway for metallic Li formation, and the t-LLZO interlayer observed at the Li/c-LLZO interface would not stop Li dendrites due to its similar Li nucleation tendency as c-LLZO. Furthermore, we propose that an interlayer of Li2PO2N at the Li/c-LLZO interface would be efficient and defect tolerant to suppress Li dendrite formation.