The inherent high resistance of electrolyte/electrode interface in all-solid-state-lithium-secondary batteries (SSLB) poses a significant hurdle for the SSLB development. The interfacial resistivity between Li7La3Zr2O12 (LLZ) and LiCoO2 is decreased by introducing a thin Nb layer (∼10 nm) at this interface. The interface modification approach using a Nb interlayer dramatically improves the discharge capacity and rate capability of a SSLB.