Cells assembly and testing

The EC:DMC-FEC electrolyte used in the experiment contains 1.0 mol/L NaPF₆ (99% purity, Capchem Co. Ltd., China) dissolved in a solution made of fluoroethylene carbonate (FEC) (99% purity, Capchem Co. Ltd., China):ethylene carbonate (EC) (99% purity, Capchem Co. Ltd., China):dimethyl carbonate (DMC) (99% purity, Kermel, China) with a 10:20:70 volume ratio. In contrast, the FEC-free electrolyte contains only EC and DMC with a 30:70 volume ratio. The water content of the electrolyte is ~7 ppm measured by the Karl Fischer Titrator equipment (Aquatest 2010, USA). Using coin cell type 2025, we assemble the cells with 600 μm-thick and 16 mm-diameter Na-metal foil (99.99% purity, TED PELLA, INC., USA) as the anode. We use 16 mm copper foil (99.99% purity, 10 μm thickness, Kermel, China) as the counter electrode. During the assembly, we place the TEM copper mesh grid (TED PELLA, INC., USA) in the center of the copper foil to collect the deposited Na-metal dendrites for TEM observations. The cells (Na metal|Cu foil) are charged and discharged at a current density of 0.5 or 1.0 mA cm⁻² and a capacity of 0.5, 1.0, 2.0 mAh cm⁻²_, respectively. The charge voltage for the cells is set to 1.0 V vs. Na⁺/Na. The impedance spectroscopy of all cells is carried out using a Solartron analytical electrochemical workstation (mode 1470E) at a frequency range of 1 MHz–0.01 Hz.

The 2025-type symmetric Na||Na coin cells were assembled for electrochemical cycling testing. To be specific, Na foils with a thickness about 200 μm were used as both anode and cathode, nickel foam as spacer, and glass fiber (Whatman) as separator. The electrolyte is 1 M NaPF₆ in EC:DMC (volume ratio of 30:70) in the FEC-free cell, while the EC:DMC-FEC cell contains 1 M NaPF₆ in FEC/EC/DMC (volume ratio of 10:20:70). The amount of electrolyte for each cell was controlled to be 60 μL. Galvanostatic cell tests were conducted on LANHE battery testing systems. The discharge time is 1 h, the cut-off voltage is 1.0 V vs. Na⁺/Na, and the value of current is set according to target capacity loading.

The full cell test utilizes a cathode containing NVOPF (160 mg), Super P (20 mg), and PVDF (20 mg) (all with 99.9% purity purchased from Capchem Co. Ltd., China), which are added to 1 mL N-Methyl pyrrolidone (99.95%, Capchem Co. Ltd., China) under continuous stirring (400 r min⁻¹) conditions for 12 h. The as-obtained slurry is then homogeneously coated on an Al foil with a blade coater (Laurell, USA). The final mass loading of the NVOPF cathode after drying was ~5.0 mg cm⁻². The voltage windows for the full cells (Na-metal|NVOPF) were set to 2.8–4.2 V.