High Coulombic Efficiency Na–O₂ Batteries Enabled by a Bilayer Ionogel/Ionic Liquid

Abstract

Sodium–oxygen (Na–O2) cells are a promising high energy density storage technology with a theoretical specific energy of 1605 Wh kg–1. However, this technology faces certain challenges in order to achieve both a high practical energy density as well as long-term cycling capability. In this Letter, a superior Coulombic cyclic efficiency, close to 100%, has been demonstrated by the use of a bilayer electrolyte composed of an ionogel and an ionic liquid electrolyte, reported herein for the first time. The presence of the ionogel plays a major role in the prevention of side reactions originating at the anode, providing a promising route to extend cell cycling, whereas the ionic liquid is essential to support high reaction rates at the cathode.

Functional Binders Based on Polymeric Ionic Liquids for Sodium Oxygen Batteries Using Ionic Liquid Electrolytes

“…The larger diameter cathode is employed in these cells due to previous studies which showed enhanced performance for this configuration when employing an IL electrolyte. This cell feature underlines the origins of this study which focuses on the ORR-OER cathode due to its limiting performance when paired with the excellent Na metal cycling performance of several IL systems. , Before testing, all cells were flushed with pure oxygen (Ultrahigh purity oxygen grade >99.999% oxygen, <5 ppm moisture, and <3 ppm hydrocarbon gas, Supagas) and rested for 3 h at 60 °C. The cells were tested galvanostatically by using a Biologic VMP 300 multichannel potentiostat, operated between 1.6 and 3.6 V vs Na/Na + at a constant current density of 0.1 mA cm –2 and with a capacity limitation of 0.30 mAh cm –2 .…”

“…Carbon nanofiber matts (CNF) were fabricated by electrospinning, as previously described. 34 In brief, the precursor solution, 9 wt % polyacrylonitrile (PAN), was prepared in dimethylformamide (DMF), which was stirred to obtain a homogeneous, clear solution. The solution was then loaded into a 5.5 mL plastic syringe for electrospinning.…”

“…Ionic liquids (ILs), on the other hand, offer low vapor pressure, nonflammability, good chemical resistance, and a wide electrochemical window and, as such, are considered to be promising alternative electrolytes for Na–O 2 batteries. Among ILs, N -butyl- N -methyl­pyrrolidinium bis­(trifluoromethyl­sulfonyl)­imide, [C 4 mpyr]­[TFSI], has been reported to be stable against superoxide species generated during ORR, as determined by in situ Raman spectroscopy and electrochemistry analysis. − Ha et al used a 16.6 mol % NaTFSI/[C 4 mpyr]­[TFSI] electrolyte and a carbon nanofiber mat (CNF) as the air cathode for sodium–oxygen batteries. The discharge products included NaO 2 and irreversible products formed during the ORR which resulted in low Coulombic efficiency (CE), determined by the ratio charge capacity/discharge capacity.…”

Single- Versus Dual-Ion UV-Cross-Linked Gel Polymer Electrolytes for Li–O2 Batteries

“…propylene carbonate, PC); ethers (e.g. dimethoxyethane, DME); others such as dimethyl sulfoxide (DMSO) 7 or ionic liquids 9,10 . Among all, tetraethylene glycol dimethyl ether (TEGDME) is the most used liquid electrolyte due its low volatility, wide electrochemical window (beyond 4.5 V versus Li 0 /Li + ) good solubility of metal alkali salts and relatively low cost [11][12][13][14][15] .…”

“…Aprotic liquid electrolyte cells, also called “nonaqueous,” are the most popular because of their highest theoretical capacity . Common aprotic solvents are organic carbonates (e.g., propylene carbonate), ethers (e.g., dimethoxyethane), and others such as dimethyl sulfoxide (DMSO) or ionic liquids. , Among all, tetraethylene glycol dimethyl ether (TEGDME) is the most used liquid electrolyte because of its low volatility, wide electrochemical window (beyond 4.5 V vs Li 0 /Li + ), good solubility of metal alkali salts, and relatively low cost. − However, its liquid nature prompts some drawbacks related to safety issues such as the potential leaking of the toxic and flammable organic electrolyte in the cell …”