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

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.