Lithium metal is an ideal anode for high-energy rechargeable batteries at low temperature, yet hindered by the electrochemical instability with the electrolyte. Concentrated electrolytes can improve the oxidative/reductive stability, but encounter high viscosity. Herein, a co-solvent formulation was designed to resolve the dilemma. By adding electrochemically “inert” dichloromethane (DCM) as a diluent in concentrated ethyl acetate (EA)-based electrolyte, the co-solvent electrolyte demonstrated a high ionic conductivity (0.6 mS cm^-1), low viscosity (0.35 Pa s), and wide range of potential window (0 – 4.85 V) at -70^oC. Spectral characterizations and simulations show these unique properties are associated with the co-solvation structure, in which high-concentration clusters of salt-in the EA solvent were surrounded by mobile DCM diluent. Overall, this novel electrolyte enabled rechargeable metallic Li battery with high energy (178 Wh kg^-1) and power (2877 W kg^-1) at -70^oC. https://onlinelibrary.wiley.com/doi/10.1002/anie.201900266.