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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11102-2 High-energy rechargeable battery systems have been actively pursued for a wide range of applications from portable electronics to grid energy storage and electric automotive industry1–6. At higher energies, battery safety becomes increas- ingly important, evident from high-profile battery fires/explosion accidents in recent years. Rechargeable batteries using flammable organic electrolytes always risk fire/explosion hazards when short circuit or thermal runaway happens, setting a bottleneck in battery design/engineering and requiring innovations of next-generation battery systems with intrinsically higher safety7,8. For organic electrolytes various strategies have been investigated to mitigate the safety concerns, including the use of voltage or temperature- sensitive separators9–11 and overcharge protection additives12. Developing new electrolyte systems that are intrinsically non- flammable has also been actively pursued13–15. In particular, room temperature ionic liquids (ILs) have been widely explored as promising candidates due to their non-flammable nature16–18. Among them, ILs comprised of AlCl3 and 1-ethyl-3- methylimidazolium chloride ([EMIm]Cl) are a classical chlor- oaluminate based electrolyte system with many desired properties including non-flammability, non-volatility, low viscosity, high conductivity, and high thermal stability and chemical inert- ness17,19. In this electrolyte, AlCl3 complexes with the Cl ion from [EMIm]Cl to produce AlCl4− and EMIm+, and any excess AlCl3 converts a portion of AlCl4− into Al2Cl7–, resulting in the coex- istence of AlCl4− and Al2Cl7−: level i.e., ethylaluminum dichloride (EtAlCl2) and 1-ethyl-3- methylimidazolium bis(fluorosulfonyl) imide ([EMIm]FSI) are key to stabilizing SEI on sodium negative electrode for reversible Na plating/stripping. In a Na/Pt cell containing this IL electrolyte, a CE of ~95% is reached at 0.5 mA cm−2 over ~ 100 reversible Na plating/stripping cycles. With the optimized IL electrolyte, we pair Na negative electrode with sodium vanadium phosphate (NVP) and sodium vanadium phosphate fluoride (NVPF) based positive electrodes to afford high discharge voltage up to ~4 V, high CEs up to 99.9 %, and maximal energy and power density of 420 Wh kg−1 and 1766 W kg−1, respectively based on active material mass of positive electrode. In addition, more than 90% of the original capacity is retained after over 700 cycles. Solid- electrolyte interphase (SEI) analysis reveals SEI compositions including NaCl, Al2O3 and NaF derived from the reactions between Na and the anions in the IL electrolyte. The results shed light on future electrolyte and SEI design towards practical sodium metal batteries with high safety and high energy/power densities. Results Properties of NaCl-buffered AlCl3/[EMIm]Cl ionic liquid. Preparation of IL electrolyte (see “Methods” section) started by mixing anhydrous AlCl3 and [EMIm]Cl at a molar ratio of 1.5:1 to form an acidic room-temperature IL (AlCl3/[EMIm]Cl = 1.5), followed by buffering to neutral with excess NaCl and then adding 1 wt% EtAlCl2 and 4 wt% [EMIm]FSI to afford the final NaCl-buffered chloroaluminate IL electrolyte (referred as ‘buf- fered Na–Cl–IL electrolyte’) (Fig. 1a). Raman spectroscopy was performed to probe the evolution of AlCl4− and Al2Cl7− species in the IL at different stages (Fig. 1b). Both AlCl4– and Al2Cl7− peaks were observed in the starting acidic IL with AlCl3/[EMIm] Cl = 1.5. After NaCl buffering of the electrolyte to neutral, the Al2Cl7− peaks at 309 and 430 cm−1 disappeared while the AlCl4− peak at 350cm−1 strengthened, indicating the conversion of Al2Cl7− to AlCl4− by NaCl on the basis of equation (3). Sub- sequent addition of 1 wt% EtAlCl2 resulted in a noticeable further enhancement of the AlCl4− peak. This was attributed to reactions of EtAlCl2 with trace amounts of protons and undissolved NaCl in the buffered AlCl3/[EMIm]Cl = 1.5 IL with the generation of AlCl4−, C2H6 and Na+ via 28: EtAlCl2 þ Hþ þ 2NaCl ! C2H6ðgÞ þ AlCl4 þ 2Naþ ð4Þ No obvious change in the Raman spectrum of chloroaluminate species was observed after the addition of 4wt% [EMIm]FSI (Fig. 1b). The final buffered electrolyte (named buffered Na–Cl–IL hereon) was comprised of Na+, AlCl4−, EMIm+ and FSI− with Na+ and FSI− molar concentration of ~ 1.76 M and ~0.2 M, respectively. An important property of the buffered Na–Cl–IL was its high ionic conductivity of ~9.2 mS cm−1 at 25 °C, which was 2–20 times higher than those of previously reported IL electrolytes based on bulky cations (e.g., N-butyl-N-methylpyrrolidinium and N-propyl-N-methylpyrrolidinium) for Na batteries29–32 (Fig. 1c). The ionic conductivity was comparable to conventional organic electrolytes, for example, ~6.5 mS cm−1 of 1 M NaClO4 in propylene carbonate (PC), and 6.35 mS cm−1 of 1 M NaClO4 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 by weight)33. The thermal stability of our buffered Na–Cl–IL electrolyte was compared with a conventional organic electrolyte 1 M NaClO4 in EC/DEC (1:1 by vol) with 5 wt% FEC additive by thermogravi- metric analysis (TGA) (Fig. 1d). The organic electrolyte showed a rapid weight loss above 132°C, and lost ~85% of the original weight at 230 oC due to decomposition of the carbonate solvents AlCl3 þ 1⁄2EMImCl ! AlCl4 þ 1⁄2EMImþ AlCl4 þ AlCl3 ! Al2Cl7 ð1Þ ð2Þ The AlCl3/[EMIm]Cl-based ILs have been used as electrolytes for rechargeable metal batteries20,21. An example was recharge- able aluminum-graphite battery developed by our group and others with fast and highly reversible AlCl4− intercalation/de- intercalation into graphite positive electrode, and Al2Cl7− plating and stripping on Al negative electrode20. Nevertheless, it is desirable to develop higher voltage and higher energy density battery systems utilizing chloroaluminate IL electrolytes. A pro- mising strategy is replacing Al by more reactive metal negative electrodes with lower standard electrode potentials such as sodium and lithium, which could raise the battery voltage and allow the use of well-established positive electrode materials with higher energy densities. Indeed, researchers have been pursuing this direction since almost 30 years ago. In as early as 1990, Melton et al. reported the first buffered AlCl3/[EMIm]Cl IL sys- tem by adding NaCl, eliminating Al2Cl7– and introducing Na ions into the electrolyte22 via Al Cl þ NaCl ! 2AlCl þ Naþ 274 ð3Þ Thus far, however, reversible and stable deposition and strip- ping/oxidation of Na metal in buffered AlCl3/[EMIm]Cl ILs towards rechargeable Na batteries have been hindered, with or without the use of a variety of electrolyte additives such as HCl23, [EMIm]HCl224,25, triethanolamine hydrochloride26 and thionyl chloride27. These additives can stabilize Na redox to limited degrees, affording Coulombic efficiencies (CEs) of 65–94% for Na plating/stripping23–27. For instance, the CE record of reversible Na redox was 94% achieved with ~ 6 Torr HCl added to NaCl- buffered AlCl3/[EMIm]Cl = ~ 1.7 IL at 6.4 mA cm−2, but it rapidly decayed at a lower current density23. None of the chlor- oaluminate ILs could afford multicycle Na plating/stripping with sufficiently high CE to pair with sodium positive electrode for Na battery cells17. Here we present an ionic liquid electrolyte based on NaCl-buffered AlCl3/[EMIm]Cl for safe and high energy Na batteries. Two electrolyte additives at the 1 to 4% by mass 2 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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