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Safety and high energy density have long restricted the large-scale practical application of lithium metal batteries because of the unbridled growth of lithium dendrites and the rapid deteriorating cycle performance of the LiNi0.8Co0.1Mn0.1O2(NCM811) cathode. Herein, an additive of RbNO3with multiple functions is proposed for dendrite-free NCM811 lithium metal batteries. Benefiting from the electrostatic shielding effect formed by Rb+during the Li+deposition process and the solvation effect of NO3–to regulate lithium deposition, a high Coulombic efficiency of 95.02% (compared with the low Coulombic efficiency of 89.37% in the blank electrolyte) is acquired in Li//Cu cells, and the uniform growth of the lithium metal deposition with a large strawberry-like morphology is achieved. Moreover, when a cathode of NCM811 matches with a lithium metal anode, an extraordinary capacity retention of 93.67% after 200 cycles with a high Coulombic efficiency of 99.7% in the electrolyte with the RbNO3system (a capacity retention of 80.1% with a Coulombic efficiency of 98.0% for the blank electrolyte) is achieved at 1C. This work provides guidance for the development of high-efficiency additives with dual synergistic regulation effects of anions and cations for lithium metal batteries in the future.