Sodium metal

Electrodes were mainly consisting either of 11 mm diameter disks punch out of Al supported calendared NVPF/Csp/PVDF tapes with a 92/4/4 in weight ratio and a loading of 12 mg cm^–2. Powdered composite mixtures (NVPF/Csp with a 90/10 in weight ratio) were occasionally used. A unit of 1 M NaPF₆ (Stella, Japan) dissolved in PC (BASF, Germany) was used as the electrolyte, glass fiber (GF/D, Whatman) was used as separator, and sodium metal (Sigma-Aldrich) and hard carbon were used as negative electrode for half cells and full cells respectively, throughout the paper unless otherwise specified. In the initial stages of the exploration of the third plateau of NVPF (Supplementary Figure 1), the electrolyte 1 M NaPF₆ dissolved in PC/EC/DMC (1/1/1 in volume ratio) was used. The Swagelok and coin-type cell were assembled in the glovebox (MBRAUN, Germany) either in half cells or full cells, in which the current density of C/10 (1C= 128 mA g^–1) was applied by a MPG-2 or VMP-3 potentiostat (Bio-Logic, France). The potentiostatic mode by limiting the current to less than C/100 at 4.8 V (vs. Na⁺/Na⁰) was only employed in the first formation charge process. The GITT (every Δx = 0.1) was performed in the second cycle of NVPF-2.75 sample after the first activation cycle, and the relaxing process was controlled either by dV/dt ≤ 0.1 mV s^–1 or 4 h.

Size-tailored NCM622 cathode material was received from BASF SE, dried in a vacuum for 12 h and stored in an argon glovebox (MBraun, [O₂] and [H₂O] < 0.1 ppm)^{19 (link)}. 1 M lithium ethoxide solution was prepared by reacting absolute ethanol (6 ppm H₂O, 99%, Merck) with lithium metal (Albemarle Germany GmbH). Ethanol was dried over sodium metal and diethyl phthalate (99%, Merck), refluxed for 2 h, distilled and stored over 0.3 nm molecular sieve (Merck). For 0.5 M niobium ethoxide solution, Nb(OCH₂CH₃)₅ (99.95%, Sigma-Aldrich) was dissolved in absolute ethanol. All of the synthesis steps were performed under an argon atmosphere. To prepare Li₂CO₃/LiNbO₃-coated NCM622 cathode material, an amount of 5.94 g of NCM622 powder was added to 812 µL of niobium ethoxide solution. Next, the relevant amount of 1 M lithium ethoxide solution was added and the mixture was sonicated for 30 min at room temperature, followed by drying in a vacuum. The resultant powder was ground using mortar and pestle and then heated in a quartz tube furnace for 2 h at 300 °C in air (5 °C/min heating rate). Sample S6 was prepared following the same procedure, except that the final heating step was performed under oxygen flow (99.998%, Air Liquide). The uncoated (pristine) and coated NCM622 cathode materials were stored under an argon atmosphere.

The graphite powder (46304 Graphite powder, synthetic, APS 7‐11 micron, 99 %) was purchased from Alfa Aesar. Sodium metal, 1‐methyl‐2‐pyrrolidinone solution (NMP, anhydrous, 99.5 %), and poly(vinylidene fluoride) binder (PVDF, average M_w≈534000 by gel permeation chromatography, powder) were purchased from Merck. NaPF₆ (99.0 %), EMC (99.99 %), PC (99.99 %), and FEC (99.9 %) solvents were purchased from Guangdong Canrd New Energy Technology Ltd. CR2032 coin cells and parts were purchased from both Pikem and Guangdong Canrd New Energy Technology Ltd.