A new class of 'Solvent-in-Salt' electrolyte for high-energy rechargeable metallic lithium batteries is introduced. This electrolyte has an ultrahigh salt concentration and a high lithium-ion transference number (0.73), making it effective in suppressing lithium dendrite growth and shape change in metallic lithium anodes. It also inhibits lithium polysulphide dissolution, overcoming the 'polysulphide shuttle phenomenon' in lithium-sulphur batteries. This results in high coulombic efficiency and long cycling stability. The electrolyte is based on Li[CF3SO2]2N (LiTFSI) and 1,3-dioxolane (DOL): dimethoxyethane (DME) (1:1 by volume), with salt-to-solvent ratios up to 7 mol per liter. The electrolyte exhibits excellent low-temperature performance and high ionic conductivity. It is applied in Li-S batteries, showing superior electrochemical performance, including high initial specific discharge capacity, high capacity retention, and nearly 100% coulombic efficiency. The electrolyte also demonstrates excellent rate capability and works well in a wide temperature range. The study highlights the potential of this new electrolyte for next-generation high-energy lithium batteries, offering improved safety and performance.A new class of 'Solvent-in-Salt' electrolyte for high-energy rechargeable metallic lithium batteries is introduced. This electrolyte has an ultrahigh salt concentration and a high lithium-ion transference number (0.73), making it effective in suppressing lithium dendrite growth and shape change in metallic lithium anodes. It also inhibits lithium polysulphide dissolution, overcoming the 'polysulphide shuttle phenomenon' in lithium-sulphur batteries. This results in high coulombic efficiency and long cycling stability. The electrolyte is based on Li[CF3SO2]2N (LiTFSI) and 1,3-dioxolane (DOL): dimethoxyethane (DME) (1:1 by volume), with salt-to-solvent ratios up to 7 mol per liter. The electrolyte exhibits excellent low-temperature performance and high ionic conductivity. It is applied in Li-S batteries, showing superior electrochemical performance, including high initial specific discharge capacity, high capacity retention, and nearly 100% coulombic efficiency. The electrolyte also demonstrates excellent rate capability and works well in a wide temperature range. The study highlights the potential of this new electrolyte for next-generation high-energy lithium batteries, offering improved safety and performance.