Unraveling the incompatibility mechanism of ethylene carbonate-based electrolytes in sodium metal anodes

Abstract

Ethylene carbonate (EC) is widely used in lithium-ion batteries due to its optimal overall performance with satisfactory conductivity, relatively stable solid electrolyte interphase (SEI), and wide electrochemical window. EC is also the most widely used electrolyte solvent in sodium ion batteries. However, compared to lithium metal, sodium metal (Na) shows higher activity and reacts violently with EC-based electrolyte (NaPF6 as solute), which leads to the failure of sodium metal batteries (SMBs). Herein, we reveal the electrochemical instability mechanism of EC on sodium metal battery, and find that the combination of EC and NaPF6 is electrically reduced in sodium metal anode during charging, resulting in the reduction of the first coulombic efficiency, and the continuous consumption of electrolyte leads to the cell failure. To address the above issues, an additive modified linear carbonate-based electrolyte is provided as a substitute for EC based electrolytes. Specifically, ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) as solvents and fluoroethylene carbonate (FEC) as SEI-forming additive have been identified as the optimal solvent for NaFP6 based electrolyte and used in Na4Fe3(PO4)2(P2O7)||Na batteries. The batteries exhibit excellent capacity retention rate of about 80% over 1000 cycles at a cut-off voltage of 4.3 V.