A >200 Wh kg^-1 anode-free Na pouch battery at -40°C enabled by manipulating electrolyte equilibrium

Cryogenic energy-dense batteries are essential for cold-climate applications. Anode-free configuration raises great opportunities for maximizing the cell-level energy density of batteries. However, high energy density of low-temperature anode-free batteries is still plagued by the lack of competent electrolytes with combined high compatibility towards metal anode and stability at high voltages. Herein, we report a high-energy-density anode-free Na battery at low temperatures via electrolyte association-dissociation equilibrium regulation. The optimized equilibrium facilitates the desolvation process and the formation of inorganic-rich anode/cathode-electrolyte interphases, thus simultaneously promoting low-temperature kinetics and high-voltage stability of the battery. Consequently, a high Coulombic efficiency (99.90%) of Na plating/stripping and a wide electrochemical stability window of electrolyte (>4.3 V) at -40°C are achieved. The anode-free Al@C||NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cell delivers a record-high energy density of 204 Wh kg^-1 _{entire cell} at -40°C among the reported low-temperature rechargeable batteries. This work represents a defining step for energy-dense batteries which can operate at low temperatures.