Solid electrolytes are considered to be able to suppress the Li dendrite growth due to its high shear modulus. However, Li dendrites are still found to grow inside the pores and along grain boundaries, but the driving force for its growth is still unclear. In this paper, garnet-type solid electrolyte Li7La3Zr2O12 (LLZO) is chosen to be studied by Density Functional Theory (DFT), because of its high conductivity and stability again Li metal. Both cubic and tetragonal LLZO structures were investigated for their interfaces with Li, to understand the nucleation of Li metal on the pores surfaces inside LLZO. The partial density of states (PDOS) and the charge transfer was analyzed when the number of electrons and Li atoms fluctuates on the LLZO surface. It was found that external electrons would be trapped on the La atoms at the (110) surfaces, which tends to pass the electrons to the adsorbed Li-ions on the surface and reduce them to Li metal. To suppress the Li dendrite growth, different La substitutions are tested, and their impact on the distribution of external electrons and implication on Li-dendrite formation inside LLZO were analyzed. Similar electronic structure analysis was performed on the crystalline Li2PO2N surface, to compare the Li plating resistance with LLZO.