This paper introduces a novel class of 'Solvent-in-Salt' (SIS) electrolytes for high-energy rechargeable metallic lithium batteries. Traditional liquid electrolytes typically have a salt concentration below 1.2 mol l−1, but the SIS electrolytes achieve ultrahigh salt concentrations and high lithium-ion transference numbers (0.73). These properties significantly enhance the cyclic and safety performance of next-generation lithium batteries by effectively suppressing lithium dendrite growth and shape change in the metallic lithium anode. Additionally, the SIS electrolytes inhibit lithium polysulfide dissolution in lithium-sulfur batteries, overcoming the 'polysulfide shuttle phenomenon' and achieving near 100% coulombic efficiency and long cycling stability. The study demonstrates the potential of SIS electrolytes to improve both electrochemical performance and safety in metallic lithium batteries.This paper introduces a novel class of 'Solvent-in-Salt' (SIS) electrolytes for high-energy rechargeable metallic lithium batteries. Traditional liquid electrolytes typically have a salt concentration below 1.2 mol l−1, but the SIS electrolytes achieve ultrahigh salt concentrations and high lithium-ion transference numbers (0.73). These properties significantly enhance the cyclic and safety performance of next-generation lithium batteries by effectively suppressing lithium dendrite growth and shape change in the metallic lithium anode. Additionally, the SIS electrolytes inhibit lithium polysulfide dissolution in lithium-sulfur batteries, overcoming the 'polysulfide shuttle phenomenon' and achieving near 100% coulombic efficiency and long cycling stability. The study demonstrates the potential of SIS electrolytes to improve both electrochemical performance and safety in metallic lithium batteries.