Effective surface protection of lithium metal anode is the enabling factor for next-generation high-energy batteries. However, the exacting requirements on the stability, mechanical properties and homogeneities of the protection layer hinder the realization of an ideal artificial interface. Among all the material choices, diamond with its renowned mechanical strength and exceptional electrochemical inertness is a prime candidate for lithium metal stabilization. Herein, by special synthetic techniques and rational design, we successfully rendered this desirable material compatible as lithium metal interface, which could strictly satisfy all the critical requirements. By fabricating high-quality nanodiamond film with long-range homogeneity but weak adhesion to the current collector, lithium can be electroplated solely underneath the film and effectively protected from parasitic reactions with liquid electrolyte. Notably, the nanodiamond interface possessed the highest modulus of all the lithium metal interfaces reported so far (>200 GPa), which can effectively arrest dendrite propagation to afford a dense deposition morphology. And the good flexibility of the thin film can accommodate the volume change of electrode during cycling. Importantly, since pinholes and mechanical damages during cycling are the main failure mechanisms of the artificial coatings developed so far, a unique double-layer nanodiamond design was proposed for the first time to enhance the defect tolerance of the nanodiamond interface, which enabled more uniform ion flux and mechanical properties as confirmed by both simulation and experimental results. Thanks to the multifold advantages of the double-layer nanodiamond interface, high Coulombic efficiency of over 99.4% can be obtained at a current density of 1 mA cm-2 and an areal capacity of 2 mAh cm-2. Moreover, with ~250% excess Li, more than 400 stable cycles can be realized in prototypical lithium-sulfur cells at a current density of 1.25 mA cm-2, corresponding to an average anode Coulombic efficiency of above 99%.