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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11102-2 Fig. 17). The key performance parameters of the Na/NVPF@rGO cell in buffered Na–Cl–IL electrolyte, including energy/power density, cycle life, discharge voltage and mass loading out- performed previous cells based on room-temperature IL electro- lytes29,31,41 (Fig. 4d and Supplementary Table 1). The EtAlCl2 additive was found important to enhance the cycling stability of Na batteries with Na–Cl–IL electrolyte, when comparing two Na/NVPF@rGO cells in IL electrolytes with and without 1wt% EtAlCl2 (Supplementary Fig. 18). The presence of EtAlCl2 additive improved cycle life by ~500 cycles at 300 mA g−1, which could be explained by the elimination of trace amounts of residual protons and free chloride ions in the electrolyte via equation (4). Solid-electrolyte interphase chemistry of Na–Cl–IL electrolyte. It is well known that SEI plays a critical role in stabilizing the interface between alkali metal negative electrodes and electro- lytes44–46. Due to the unusual composition of our IL electrolyte, the SEI chemistry could be different from that in conventional organic electrolytes. To this end, we first analyzed the elemental composition and depth profile by X-ray photoelectron spectro- scopy (XPS) of a Na negative electrode from a Na/NVP@rGO cell with the mass loading of NVP@rGO 5.0 mg cm−2. The cell was cycled for 20 cycles at 100 mA g−1 (~0.5 mA cm−2) and stopped at a fully charged state (Na plated on negative electrode). Surface XPS profile identified the presence of Na, O, C, Cl, F, Al and N (Supplementary Fig. 19). XPS profiling by Ar sputtering showed pronounced Na Auger peak at 535.7eV at all sample depths (Fig. 5a). The O 1 s peaks at 531.2, 529.4, 532.2 and 533.6 eV indicated the presence of Na2CO3, Na2O, Na2SO4 and NaOH, respectively (Fig. 5a). The presence of NaOH was only at the surface, as it was generated from the contamination by water when the sample was briefly exposed to air during transfer to XPS. Part of the Na2CO3 could also be from reaction with water and carbon dioxide in air and decreased in intensity after sput- tering. In contrast, the intensity of Na2O and Na2SO4, formed by FSI anion and sodium metal showed no obvious decrease during sputtering, indicating their existence in SEI. As expected, the F 1 s peak at ~685.5 eV confirmed the presence of NaF as the major F- based SEI (Fig. 5b). The FSI anions in [EMIm]FSI were respon- sible for F-based SEI via reactions with the highly reactive Na metal, consistent with previous literature47,48. The Al 2p peaks at 74.5 eV indicated the presence of Al2O3 as a major Al-based SEI component with a small portion of metallic Al observed (Fig. 5c). The two pronounced peaks at ~198.4 and 199.8 eV corresponded to Cl 2p1/2 and Cl 2p3/2 peaks, suggesting NaCl as another major SEI component (Fig. 5d). The weak N 1s peak at ~400 eV indi- cated the presence of N-based species generated from the decomposition of FSI anion (Supplementary Fig. 20), consistent with previous literature based on LiFSI-based organic electro- lytes49,50. Overall, a hybrid SEI formed on sodium metal com- prised of NaF, Na2O, Na2SO4, Al2O3, Al and NaCl contributed to the reversible plating/stripping process of Na in buffered Na–Cl–IL electrolyte. To gain a deeper insight into the Na plating process in buffered Na–Cl–IL electrolyte, cryogenic transmission electron microscope (Cryo-TEM) was used to probe the morphology and elemental composition of plated Na on Cu grids without exposing the sample to air (see “Methods” section). Cryo-TEM was demon- strated recently as a powerful tool in probing the morphological and component information of beam-sensitive battery materials such as Li metal51,52, but not yet used for investigating SEI on sodium thus far. We first investigated the initial Na plating on a Cu grid, which involved Na growth and SEI formation at the initial stage. The plated Na (without exposing to air) demonstrated a spherical morphology (Fig. 5e). High-resolution image showed some clusters in SEI with clear lattice fringes showing a d-spacing of 0.347 nm indexed to the (012) planes of α-Al2O3, which was also confirmed by diffraction pattern (Fig. 5f). In addition, the compact stacking of many nanocubes with an average size of ~10 nm was observed on the edge of SEI, with lattice fringes at a d-spacing of 0.284 nm indexed to (200) planes of NaCl and corroborated by diffraction pattern (Fig. 5g). Element mapping analysis on these regions was performed using scanning transmission electron microscopy (STEM), indicating the presence of Na, O, Cl, Al, F and N that was in accordance with the XPS results, confirming the hybrid SEI composition of this novel IL electrolyte (Fig. 5h). The overlapped Na and Cl mapping indicated the presence of NaCl, which was consistent with the stacking cubes and diffraction pattern of NaCl detected in Cryo-TEM (Fig. 5f). The F mapping mainly distributed in the region near the surface, and showed a good overlap with Na mapping, which was in accordance with the XPS results that indicated the presence of NaF layer. The merged Na and Al mapping showed the aggregation of Al with the formation of some Al clusters, rather than distribute uniformly with Na in the SEI matrix (Fig. 5h). It can be explained by the fact that Al and Na cannot form an alloy; thus, Al might prefer to plate on Al rather than Na, which could account for the interconnected structure of Al observed in the mapping image. Discussion Compared with previous IL electrolytes for Na cells, the Na–Cl–IL electrolyte system is interesting in several ways. First, the high ionic conductivity (~9.2mS cm−1 at 25°C) outperforms previously reported IL electrolytes based on bulky cations (e.g., benzyldi- methylethylammonium and N-butyl-N-methylpyrrolidinium) and anions (e.g., FSI− and TFSI–), allowing for both high-energy density and rate capability/power density of the Na metal cells (Supple- mentary table 1)29–32. The EMIm cation is unique among other cations since it provides delocalized positive charge around the imidazolium ring, effectively increasing the cation-anion distance and affording lower viscosities than ILs with other cations, owing to reduced Coulomb (electrostatic) interactions between ion pairs53. Second, the SEI components are unique with the inclusion of AlOx and NaCl due to Na reaction/passivation by chloroaluminate species, which facilitates the stabilization of Na plating/stripping cycling. This led to a cycle life of over 700 cycles, the longest among all the reported IL-based Na cells29–31,41 (Fig. 4d). We found that although FSI anions was indispensable for a stable SEI in our system, FSI alone was not sufficient for long cycle life of Na negative electrode. This was based on inferior cycling stability of Na/NVP@rGO cell in a non-chloroaluminate based electrolyte 1 M NaFSI in [EMIm]FSI IL electrolyte, dis- playing low and fluctuating CEs of only ~90%, despite the fact that it had a much higher FSI anion concentration of ~6M compared with only ~0.2 M in the buffered Na–Cl–IL electrolyte (Supplementary Fig. 21). Similarly, the Na/NVP@rGO cell using NaFSI in N-propyl-N-methylpyrrolidinium bis(fluorosufonyl) imide IL electrolyte (molar ratio of 2:8) showed fluctuating CEs after ~ 65 cycles when cycling at 150 mA g−1 (Supplementary Fig. 22). In addition, an inferior rate performance was demon- strated using NaFSI/N-propyl-N-methylpyrrolidinium bis(fluor- osufonyl)imide IL electrolyte compared with that based on buffered Na–Cl–IL electrolyte (Supplementary Fig. 23). Another important aspect was that previous IL electrolytes with highly concentrated F-based species (e.g., over 5M of FSI anion concentration in NaFSI-[N-propyl-N-methylpyrrolidinium]FSI elec- trolyte with a molar ratio of 2:8)32 were much higher in cost than conventional organic electrolytes due to expensive FSI species. A 8 NATURE COMMUNICATIONS | (2019)10:3302 | https://doi.org/10.1038/s41467-019-11102-2 | www.nature.com/naturecommunicationsPDF Image | sodium metal battery based on an ionic liquid electrolyte
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