Lithium metal anodes are steadily gaining more attention, as their superior specific capacities and low redox voltage can significantly increase the energy density of rechargeable batteries far beyond those of current Li‐ion batteries. Nonetheless, the relevant technology is still in a premature research stage mainly due to the uncontrolled growth of Li dendrites that ceaselessly cause unwanted side reactions with electrolyte. In order to circumvent this shortcoming, herein, an ionic liquid additive, namely, 1‐dodecyl‐1‐methylpyrrolidinium (Pyr1(12)+) bis(fluorosulfonyl)imide (FSI−), for conventional electrolyte solutions is reported. The Pyr1(12)+ cation with a long aliphatic chain mitigates dendrite growth via the combined effects of electrostatic shielding and lithiophobicity, whereas the FSI− anion can induce the formation of rigid solid–electrolyte interphase layers. The synergy between the cation and anion significantly improves cycling performance in asymmetric and symmetric control cells and a full cell paired with an LiFePO4 cathode. The present study provides a useful insight into the molecular engineering of electrolyte components by manipulating the charge and structures of the involved molecules. An ionic liquid additive of 1‐dodecyl‐1‐methylpyrrolidinium (Pyr1(12)+) bis(fluorosulfonyl)imide (FSI‐) is reported for stable cycling of Li metal anodes. The Pyr1(12)+ cation with a long aliphatic chain engages an electrostatic shielding and lithiophobic effect, whereas the FSI‐ anion induces the formation of rigid solid–electrolyte interphase layers. The synergy between both ions suppresses dendrite growth and improves electrochemical performance significantly.