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Enabling high‐performing alkali metal anodes at low temperature and in lean electrolyte conditions is critical for the advancement of next‐generation batteries with high energy density and improved safety. We present an ether–ionic liquid composite electrolyte to tackle the problem of dendrite growth of metallic sodium anode at low temperatures ranging from 0 to −40 °C. This composite electrolyte enables a stable sodium metal anode to be deeply cycled at 2 mA cm−2 with an ultrahigh reversible capacity of 50 mAh cm−2 for 500 hours at −20 °C in lean electrolyte (1.0 μL mAh−1) conditions. Using the composite electrolyte, full cells with Na3V2(PO4)3 as cathode and sodium metal as anode present a high capacity retention of 90.7 % after 1,000 cycles at 2C at −20 °C. The sodium–carbon dioxide batteries also exhibit a reversible capacity of 1,000 mAh g−1 over 50 cycles across a range of temperatures from −20 to 25 °C. [ABSTRACT FROM AUTHOR]

Abstract: Sodium metal is an attractive anode for next‐generation energy storage systems owing to its high specific capacity, low cost, and high abundance. Nevertheless, uncontrolled Na dendrite growth caused by the formation of unstable solid electrolyte interphase (SEI) leads to poor cycling performance and severe safety concerns. Sodium polysulfide (Na2S6) alone is revealed to serve as a positive additive or pre‐passivation agent in ether electrolyte to improve the long‐term stability and reversibility of the Na anode, while Na2S6‐NaNO3 as co‐additive has an adverse effect, contrary to the prior findings in the lithium anode system. A superior cycling behavior of Na anode is first demonstrated at a current density up to 10 mA cm−2 and a capacity up to 5 mAh cm−2 over 100 cycles. As a proof of concept, a high‐capacity Na‐S battery was prepared by pre‐passivating the Na anode with Na2S6. This study gives insights into understanding the differences between Li and Na systems. [ABSTRACT FROM AUTHOR]