Trace‐amount of Water as An Electrolyte Additive for Sodium Metal Electrode

The high reactivity of water toward Na metal has raised a concern about keeping the electrolytes extra‐dried. In this work, changes in water concentration in electrolytes (with and without fluoroethylene carbonate) show changes in overpotential and the surface chemistry of Na electrodes. In a symmetric cell test, the cell with pristine electrolyte (1M NaClO4 in ethylene carbonate:propylene carbonate) sustained only 22 cycles before reaching the safety limit (5 V) at 1 mA cm‐2. Meanwhile, controlling the water content (40 ppm) extended the cell’s life by 3.5 times. In fluoroethylene‐carbonate‐containing electrolytes, the optimized water concentration (40 ppm) gave the minimum overpotential (12 mV) after 170 cycles. Ex situ X‐ray photoemission spectroscopy showed that water hydrolyzed fluoroethylene carbonate, which changed the Na electrode’s surface chemistry. The appropriate amount of product (NaF) stabilized the electrodes’ surfaces. Electrical impedance spectroscopy showed that the controlled traces amount of water (40 ppm) always gave the minimum values for resistances. For the pristine electrolytes, the resistances attributed to the charge‐transfer process and the solid‐electrolyte interface layer increased 51 times (from 45 Ω to 2290 Ω) after cycling. Meanwhile, for the optimized sample, the resistances remarkably decreased by 93% (from 264 Ω to 19 Ω) after cycling.