Abstract
Rechargeable sodium metal batteries with high energy density could be important to a wide range of energy applications in modern society. The pursuit of higher energy density should ideally come with high safety, a goal difficult for electrolytes based on organic solvents. Here we report a chloroaluminate ionic liquid electrolyte comprised of aluminium chloride/1-methyl-3-ethylimidazolium chloride/sodium chloride ionic liquid spiked with two important additives, ethylaluminum dichloride and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide. This leads to the first chloroaluminate based ionic liquid electrolyte for rechargeable sodium metal battery. The obtained batteries reached voltages up to ~ 4 V, high Coulombic efficiency up to 99.9%, and high energy and power density of ~ 420 Wh kg−1 and ~ 1766 W kg−1, respectively. The batteries retained over 90% of the original capacity after 700 cycles, suggesting an effective approach to sodium metal batteries with high energy/high power density, long cycle life and high safety.
Highlights
Rechargeable sodium metal batteries with high energy density could be important to a wide range of energy applications in modern society
The high ionic conductivity (~9.2 mS cm−1 at 25 °C) outperforms previously reported ionic liquids (ILs) electrolytes based on bulky cations and anions (e.g., FSI− and TFSI–), allowing for both high-energy density and rate capability/power density of the Na metal cells (Supplementary table 1)[29,30,31,32]
We found that FSI anions was indispensable for a stable Solidelectrolyte interphase (SEI) in our system, FSI alone was not sufficient for long cycle life of Na negative electrode
Summary
Rechargeable sodium metal batteries with high energy density could be important to a wide range of energy applications in modern society. ILs comprised of AlCl3 and 1-ethyl-3methylimidazolium chloride ([EMIm]Cl) are a classical chloroaluminate based electrolyte system with many desired properties including non-flammability, non-volatility, low viscosity, high conductivity, and high thermal stability and chemical inertness[17,19]. 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 coexistence of AlCl4− and Al2Cl7−: AlCl3 þ 1⁄2EMImCl ! In as early as 1990, Melton et al reported the first buffered AlCl3/[EMIm]Cl IL system by adding NaCl, eliminating Al2Cl7– and introducing Na ions into the electrolyte[22] via
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