Straightforward strategy toward a shape-deformable carbon-free cathode for flexible Li–air batteries in ambient air

Abstract

Flexible lithium–air batteries (F-LABs) operating under ambient air have attracted much interest as small-sized energy storage devices in routine household settings. Although the instability of the commonly used carbon cathode has prompted the search for a carbon-free cathode, there has not been much success in their development thus far. Here, we report a novel strategic approach to fabricate a large-area carbon-free cathode for shape-deformable F-LABs in ambient air. An electronically conductive and catalytic layer consisting of metallic iridium (Ir) and outermost IrOx was deposited on porous, flexible polyimide nanofibers (PI@IrOx NFs) by radio frequency (RF) sputtering, which was employed as the flexible cathode. Their properties were evaluated under dynamic deformation modes such as bending, folding, twisting, and crumpling. The F-LAB cells with PI@IrOx NFs were stable for 150 cycles using pure O2, and in a dry-air atmosphere (O2/CO2/N2), they remain stable for 300 cycles (=600 h) with low discharge/charge overpotentials by the redox mediator and solid-state electrolyte (L1.3Al0.3Ti1.7(PO4)3, LATP). We elucidate the reaction mechanism at the IrOx surface using ex-situ analyses and density functional theory (DFT) calculations. When applied to pouch-/cable-type F-LAB cells using two different electrolytes (gel-polymer and poly(ethylene oxide)(PEO)/LATP composite electrolytes), the PI@IrOx NFs show superior electrochemical characteristics during repetitive 300 bending cycles for over 60 h in ambient air. Furthermore, the F-LAB cells could be assembled using 3D-printed shape-deformable polyurethane (PU) as the packaging material, demonstrating their practical operation in small devices for the first time.