First-principles calculations are performed to investigate interface properties, Li defects formation and migration mechanism across the interface between negative metal electrode and solid electrolyte (Li/γ-Li3PO4). We have analyzed the band alignment between Li and Li3PO4, interfacial charge distribution and electronic properties to elucidate the properties of the model interface. Our results show that the high Li-ion (Li+) defect formation energy is determined by the Li metal Fermi level leading to low ionic conductivity of Li metal/electrolyte interface. The electronic structure study of this Li metal/Li3PO4 interface provides information on the Li defect formation and migration, which will help us to improve the ionic conductivity for future Li-ion battery.