Uncontrolled lithium growth and the consequent interfacial reaction are obstacles to the practical application of lithium metal anode. Here, a multifunctional subassembly of carbon nanotube paper grafted with a thin layer of LiNO3 (LNO@CP) is proposed. According to the results of experiments and theory calculation, the nucleation sites are enriched and the nuclei are tailored into spherical shape on LNO@CP. Consequently, the Li plating gradually fills the void of LNO@CP and then grows outward uniformly, resulting in an increased Li reversibility and a stable solid electrolyte interphase layer. The LNO@CP covering the Li backup source works as a protective layer to defense the electrolyte corrosion effectively. The lithium metal battery with this multifunctional subassembly delivers a remarkable capacity retention of 81.2% after 500 cycles. This work emphasizes that the a protective layer such as LNO@CP is crucial to the high performance lithium metal battery.