Practical level of low-N/P ratio sodium metal batteries: On the basis of deposition/dissolution efficiency in the aspects of electrolytes and temperature

Rechargeable sodium metal batteries have drawn immense interest as next-generation batteries, owing to their high theoretical energy density and the natural abundance of Na resources. However, Na metal batteries with excess amounts of Na metal which typically employ traditional organic electrolytes, are plagued by a myriad of electrochemical deficiencies that cause low Coulombic efficiencies, severe dendrite formation, thermal runaways, and fire hazards during operations. To establish a pathway for high energy density Na metal batteries, this work focuses on the practicability of the Na lean-metal battery and presents an in-depth perspective into the electrochemical mechanisms of utilizing a bis(fluorosulfonyl)amide ionic liquid electrolyte at both room- and intermediate temperatures. Here, the ionic liquid electrolyte is confirmed to yield a higher Na metal deposition/dissolution efficiency than common organic electrolytes. Electrochemical and computational investigations entailing the ionic liquid performance reveal that elevating the operating temperature to 90 °C increases the number of anions coordinated with Na+ and leads to the formation of a robust anion-based interfacial layer that facilitates effective and smooth Na metal deposition/dissolution. Furthermore, full cells comprising a Na metal negative with restricted loading mass and a Na₃V₂(PO₄)₃ positive electrode (low negative/positive electrode capacity ratio of 1.97, weight ratio of 0.19) demonstrates outstanding cycleability with high Coulombic efficiency (∼100%) and practical energy density (280 Wh kg⁻¹).