Though Te has excellent figure of merit (ZT), the severe element diffusion and reaction at the Te/metallic-electrodes interface render high contact resistivity (ρ_c) and low device conversion efficiency (η). Therefore, it is critical to develop suitable barrier layers for optimizing the bonding between Te and metallic electrodes. In this work, an appropriate barrier layer, NiTe_2-m (Ni_xTe (x=0.500~0.908)), was screened based on gradient structure. No reaction layers and defects at the interface of Ni_0.5Te/Te_0.985Sb_0.015/Ni_0.5Te were detected before and after aging at 473 K. Low ρ_c (less than 10 μΩ·cm²) and high η (about 75% of the theoretical value under a temperature difference of 180 K (hot end: 473 K)) were achieved and maintained stable during aging, showing excellent thermal stability of the interface. When x>0.500, the thickness of the interface reaction layer decreased with x increasing, showing the retarding effect dominating the growth behavior of interface reaction layer not from the usual thermodynamic factors, such as interface reaction energy and composition gradient, but from the “atom vacancy” on formation of the reaction layer.