The formation of lithium dendrites is recognized as the worst problem for lithium metal batteries. Herein, a multifunctional additive is proposed for highly stable lithium metal in the carbonate electrolyte of Li-sulfurized polyacrylonitrile batteries with high areal capacity. This multifunctional electrolyte additive (fluoroethylene carbonate and potassium nitrate) forms a stable hybrid solid electrolyte interphase, and the K+ ions provide a self-healing electrostatic shield that can suppress lithium dendrites.