Lithium metal batteries hold promise for pushing cell-level energy densities beyond 300 Wh kg−1 while operating at ultra-low temperatures (below −30 °C). Batteries capable of both charging and discharging at these temperature extremes are highly desirable due to their inherent reduction in the need for external warming. Here we demonstrate that the local solvation structure of the electrolyte defines the charge-transfer behaviour at ultra-low temperature, which is crucial for achieving high Li metal Coulombic efficiency and avoiding dendritic growth. These insights were applied to Li metal full-cells, where a high-loading 3.5 mAh cm−2 sulfurized polyacrylonitrile (SPAN) cathode was paired with a onefold excess Li metal anode. The cell retained 84% and 76% of its room temperature capacity when cycled at −40 and −60 °C, respectively, which presented stable performance over 50 cycles. This work provides design criteria for ultra-low-temperature lithium metal battery electrolytes, and represents a defining step for the performance of low-temperature batteries.