The growth of Li dendrites and the instability of the solid electrolyte interphase (SEI) layer during plating/stripping has hindered the practical application of high-energy-density batteries based on a lithium metal anode. Building a stable interfacial layer is effective in preventing lithium corrosion by the electrolyte and controlling the deposition of lithium metal. Here, we present a robust polydopamine-Cu ion (PDA-Cu²⁺) coating layer formed by the aggregation of nanoparticles and Cu ions, which can be obtained by a subtle immersion strategy. We demonstrate that the PDA-Cu²⁺ protective layer, with a unique structure comprising nanoparticles, can regulate and guide Li metal deposition, and together with Cu ions, forms a lubricating surface to facilitate uniform Li ion diffusion and induce stable SEI layer formation. Li anodes with this PDA-Cu²⁺ layer modification ultimately achieve higher Coulombic efficiencies, which are consistently stable for over 650 cycles at 0.5 mA·cm^-2 without Li dendrites. The introduced PDA-Cu²⁺ coating can adhere to any material of any shape; additionally, the operation can be realized on a large scale because of its simplicity. These merits provide a promising approach for developing stable and safe lithium metal batteries.