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NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11102-2 ARTICLE much lower FSI concentration of only ~0.2 M was needed for the buffered Na–Cl–IL electrolyte, and at the same time reaching better cell performances (power density, CE, cycle life and discharge voltage etc.) than previous room temperature IL electrolytes (Supplementary Table 1). The buffered Na–Cl–IL electrolyte could be a promising candidate for affordable, high-safety energy storage towards real- world applications. In conclusion, we develop a non-flammable and highly con- ductive ionic liquid electrolyte for high-energy/high-voltage Na metal batteries. The ionic liquid electrolyte is comprised of AlCl3, NaCl and [EMIm]Cl and allows reversible Na plating/stripping upon addition of two additives, i.e., ethylaluminum dichloride and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide. The Na metal cells with NVP and NVPF positive electrodes achieve high CE up to 99.9%, and high energy and power density of 420 Wh kg−1 and 1766 W kg−1, respectively. Over 90 % of the ori- ginal capacity can be retained after over 700 galvanostatic charge- discharge cycles. The solid-electrolyte interphase (SEI) probed by XPS and Cryo-TEM shows that the major components included NaCl, Al2O3 and NaF. The non-flammable and highly conductive IL electrolyte can serve as a promising candidate for sodium batteries with high safety and high performance, and can be potentially extended to a broad range of rechargeable battery systems such as Li and K batteries. Methods Preparation of IL electrolytes. IL electrolytes were prepared in an Ar-filled glove box with water and oxygen content below 2 ppm. [EMIm]AlxCly IL was first made by mixing 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) and anhydrous AlCl3 ( ≥ 99.0%, Fluka). [EMIm]Cl was dried at 80 oC under vacuum for 24 h to remove residual water. For a certain molar ratio, e.g., 1.5 of AlCl3/[EMIm]Cl, 1.78 g of [EMIm]Cl and 2.4 g of AlCl3 were weighed in two glass vials, respectively. A small portion of AlCl3 was then slowly added into [EMIm]Cl to avoid dramatic heat generation during the mixing. This process was repeated until all the AlCl3 were introduced, and the mixture was stirred until all the solid was dissolved, followed by adding around 0.3 g of aluminum foil for purification. 1.8 g of the obtained light-yellow, clear liquid was kept at 70 °C for 1 h under vacuum for removal of water, followed by adding 0.172 g NaCl (99.999 %, Sigma-Aldrich) and allowed to stir for 24 h. The supernatant was collected, and stirred with 1 wt% EtAlCl2 (Sigma-Aldrich) for 1 h. The mixture was further added with 4 wt% [EMIm]FSI (dried at 70 °C under vacuum for 12 h before use) and allowed to stir for 6 h to obtain the buffered + EtAlCl2/[EMIm]FSI additive electrolyte. To avoid water absorption of the prepared IL electrolyte, all the agents were stored inside tightly closed and sealed bottles, and placed in Ar-filled glove box. [EMIm]Cl and NaCl were dried via heating under vacuum before use. [EMIm]FSI and N-propyl- N-methylpyrrolidinium bis(fluorosufonyl)imide were dried under vacuum at 70 °C for 12 h before dissolving NaFSI salt. 1 M NaClO4 in EC/DEC (1:1 by vol) with 5 wt % FEC was prepared as conventional organic electrolyte for comparison. Preparation of NVP@rGO and NVPF@rGO. Graphene oxide (GO) was synthe- sized via a modified Hummer’s method with more details described in Supple- mentary Information54. To prepare NVP@rGO, 0.69 g of NH4H2PO4, 0.318 g of Na2CO3 and 0.364 g of V2O5 were dispersed in deionized water, followed by adding 0.72 g of oxalic acid ( ≥ 99.0 %, Sigma-Aldrich) at 70 °C. The mixture was added with 7.3 mL GO aqueous dispersion (11 mg mL−1) under vigorous stirring, and then freeze-dried to obtain the solid NVP@GO precursor. The precursor was grounded using an agate mortar, followed by sintering at 850 oC with a heating rate of 2 oC min−1 in Ar to obtain the NVP@rGO powder. NVPF@rGO was prepared via a one-step hydrothermal method. Briefly, 0.536 g of NaF, 3.51 g of NaH2PO4·2H2O and 1.763 g of VOSO4·xH2O (degree of hydration 3–5, Sigma- Aldrich) were dissolved in 30 mL deionized water, followed by mixing with 7.8 mL of GO aqueous dispersion (11 mg mL−1) for 1 h to obtain a uniform dispersion. The mixture was immediately transferred into a 45 mL Teflon-lined stainless steel autoclave and kept at 120 oC for 10 h. The resulted precipitates were centrifuged at 4,000 rpm using deionized water for 5 times, and the obtained solid was dried at 80 °C for 10 h in a vacuum oven to obtain the NVPF@rGO powder. For bare NVPF, no GO was added with all the other procedures remained the same. Electrochemical measurements. All the electrochemical measurements were con- ducted at room temperature (22 °C) unless otherwise specified. To prepare slurries, 70 wt% NVP@rGO or NVPF@rGO powder was mixed with 20 wt% conductive carbon black (Super C65, TIMICAL) and 10 wt% polyvinylidene fluoride (PVDF, Mw = 180,000, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP, 99.5 %, Sigma-Aldrich). The mixture was stirred for 10 h until a uniform and viscose slurry was obtained, which was coated on a Mitsubishi carbon fibre paper (M30 type, 30 g m−2). The electrodes were baked in a 120 °C vacuum oven for 2 h for removal of the residual NMP. The electrochemical performances were measured in pouch-type cells. Briefly, carbon tap (Ted Pella) was used to paste the positive electrode (Cu or Pt foil, NVP@rGO or NVPF@rGO electrodes) and negative electrode of Na metal foil onto an aluminum laminated pouch. The Na foil was prepared by rinsing a Na cube (99.9 %, Sigma- Aldrich) in anhydrous dimethyl carbonate ( ≥ 99.0 %, Sigma-Aldrich) for removal of the mineral oil on surface, cutting off the surface oxidation with blades, and pressing a fresh piece into a thin foil. Two nickel tabs (EQ-PLiB-NTA3, MTI) and a piece of glass fibre filter paper (GF/A, Whanman) were served as the current collector and separator, respectively. The obtained pouch was heated in an 80 °C vacuum oven for 8 h, and then transferred into an argon-filled glove box with water and oxygen content below 2 ppm to fill in the electrolyte (200 uL for each cell). The pouch was heat-sealed in the glove box before transferring out for further electrochemical measurement. Cyclic voltam- metry was performed on a CHI760E electrochemical work station. The charge- discharge performances of the cells were measured with a Neware battery testing system (CT-4008-5V50mA-164-U). All the cells were allowed to age for 6 h before charge- discharge measurement. The specific capacity, energy and power density were calcu- lated based on the total mass of NVP@rGO and NVPF@rGO. Characterization. For Raman spectra, IL electrolytes were injected and sealed into transparent plastic pouches in an Ar-filled glove box. The spectra were acquired (250–500 cm−1) using an Ar+ laser (532 nm) with 0.8 cm−1 resolution. The con- ductivity measurement was performed on a conductivity meter (FiveEasy Plus, Mettler Toledo). Prior to characterization, the electrodes were rinsed with anhy- drous dimethyl carbonate for 6 times, and dried under vacuum at room tem- perature. They were further sealed in Ar-filled pouches and quickly transferred into the vacuum chamber to avoid too much exposure to air. The Na ion concentration of the buffered Na–Cl–IL electrolyte was measured using a Thermo Scientific ICAP 6300 Duo View Spectrometer. SEM images were acquired from a Hitachi/S-4800 SEM operated at 15 kV, and EDS analysis was performed on a Horiba/Ex-450 EDS spectroscopy. FIB-SEM was performed on a dual-beam field-emitting focused ion beam microscope (VERSA 3D DualBeam) with an accelerating voltage of 20 kV. TEM image of NVP@rGO was obtained with a JEOL JEM-2100F operated at 200 kV. XRD pattern was measured with a Bruker D8 Advance powder X-ray dif- fractometer with Cu Kα radiation. TGA measurement was performed on a Per- kinElmer/Diamond TG/DTA thermal analyser at a heating rate of 5 °C min−1 in air for NVP@rGO and NVPF@rGO, and in nitrogen for IL and organic electrolyte, respectively. The temperature range used for determining rGO percentage was 180–460 °C, and the weight loss below 180 °C was due to water removal that is also used to determine the water content of products synthesized in aqueous solution55. XPS spectra were collected on a PHI 5000 VersaProbe Scanning XPS Microprobe. All the binding energy values were calibrated with C1s peak (284.6 eV). Depth profile was conducted using Ar ion sputtering at 1 kV and 0.5 μA over a 2 × 2 mm area, corresponding to a SiO2 sputter rate of 2 nm min−1. Glass fibre separators soaked with electrolyte were used to test the flammability of the electrolyte. Cryo- TEM was performed on an FEI Titan Krios cryogenic transmission electron microscope operated at 300 kV. Na was plated on a Cu TEM grid in a 2032 type coin cell at a current density of ~ 0.2 mA cm−2 for 30 min, using 150 uL Na–Cl–IL and one glass fibre as electrolyte and separator, respectively. The coin cell was disassembled in an Ar-filled glove box, followed by removing the residual elec- trolyte on Na-plated Cu TEM grid using anhydrous DMC and drying it under vacuum. The TEM grid was then carefully mounted onto a TEM cryo-holder and transferred into the chamber of Cryo-TEM without exposing to air. Similar pro- cesses were performed for element mapping using a FEI Titan Themis 60-300 transmission electron microscope equipped with a cooling sample holder. Data availability The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Received: 21 March 2019 Accepted: 20 June 2019 References 1. Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652 (2008). 2. Cano, Z. P. et al. Batteries and fuel cells for emerging electric vehicle markets. Nat. Energy 3, 279–289 (2018). 3. Dunn, B., Kamath, H. & Tarascon, J.-M. Electrical energy storage for the grid: a battery of choices. Science 334, 928–935 (2011). 4. Xu, X. et al. A room-temperature sodium–sulfur battery with high capacity and stable cycling performance. Nat. Commun. 9, 3870 (2018). 5. Zhou, D. et al. A stable quasi-solid-state sodium–sulfur battery. Angew. Chem. Int. Ed. 57, 10168–10172 (2018). NATURE COMMUNICATIONS | (2019)10:3302 | https://doi.org/10.1038/s41467-019-11102-2 | www.nature.com/naturecommunications 9PDF Image | sodium metal battery based on an ionic liquid electrolyte
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