An amorphous trimetallic (Ni-Co-Fe) hydroxide-sheathed 3D bifunctional electrode for superior oxygen evolution and high-performance cable-type flexible zinc-air batteries

The emerging flexible/wearable electronics have greatly stimulated research on portable batteries with high specific energy and excellent mechanical properties. State-of-the-art zinc-air batteries (ZABs) are potential candidates for flexible energy supply; however, their development is hindered by the sluggish kinetics of the oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) of an air cathode. Herein, we demonstrate a 3D integrated bifunctional oxygen electrode of NiCo2O4@NiCoFe-hydroxide nanoarrays for flexible all-solid-state ZABs. Owing to the intact mesoporous nanoarrays synergized with the amorphous trimetallic hydroxide sheath, the free-standing NiCo2O4@NiCoFe-hydroxide electrode exhibited excellent bifunctional activities with an ultralow potential difference of 695 mV between OER and ORR. The NiCo2O4@NiCoFe-hydroxide-based planar aqueous ZAB achieved high discharge capacity (723 mA h g(zinc)(-1) at 10 mA cm(-2)), high energy density (864.2 W h kg(zinc)(-1) at 5 mA cm(-2)) and long cycle life of up to 250 h. More significantly, cable-type all-solid-state ZABs fabricated with the 3D oxygen electrode demonstrated an impressive volumetric energy density of 38.1 mW h cm(-3) and high mechanical flexibility even after 2000 bending cycles, highlighting their enormous potential for flexible/wearable energy applications.