Novel organic molecule enabling a highly-stable and reversible sodium metal anode for room-temperature sodium-metal batteries

The cycle life of sodium metal batteries is hampered primarily by the unwarranted growth of sodium dendrites and their low Coulombic efficiency. Electrolyte additives can extend the cycle life by modifying the local interfacial electrochemistry of the metal anodes. Nevertheless, a high overpotential for deposition impeded the stripping/plating stability. We present 9-Fluorenone (9F), a novel electrolyte additive for highly stable and reversible sodium metal batteries. Even at a high current density of 20 mA cm-2, sodium deposition occurs seamlessly with this electrolyte additive. According to thermochemistry calculations, 9F molecules modulate the breakdown barrier and shield sodium-ion solvation shells from further decomposition, limiting the loss of metal anode and electrolyte inventory. As a result, a reversible stripping/plating of sodium can be accomplished for over 1200 h with a minimal overpotential of 25 mV. By combining metal sulfide cathodes, sodium metal anodes with ether-based electrolyte, and 9F additive molecules, we demonstrate the feasibility of developing a stable and long-lasting sodium-sulfur battery that operates at room temperature and last >300 cycles.