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

•The highest Na dissolution/deposition capabilities are achieved in a bis(fluorosulfonyl)amide based-ionic liquid among selected 7 kinds of electrolytes.•Na+ ion transport properties of the ionic liquid electrolyte are investigated at different temperatures.•The Na+ coordination structures of the ionic liquid embody a robust SEI, and the suppression of uneven Na metal deposition.•A Na metal full cell with the ionic liquid comprising a low N/P ratio of 1.97 demonstrates outstanding cycleability.