Nitrogen-doped carbon fibers embedding CoO_x nanoframes towards wearable energy storage

As continuous consumption of the world's lithium reserves is causing concern, alternative energy storage solutions based on earth-abundant elements, such as sodium-ion batteries and zinc-air batteries, have been attracting increasing attention. Herein, nanoframes of CoOx are encapsulated into carbonized microporous fibers by electrospinning zeolitic imidazolate frameworks to impart both a sodium-hosting capability and catalytic activities for reversible oxygen conversion. The ultrahigh rate performance of sodium-ion batteries up to 20 A g-1 and ultrastable cycling over 6000 cycles are attributed to a dual-buffering effect from the framework structure of CoOx and the confinement of carbon fibers that effectively accommodates cyclic volume fluctuation. Both in situ Raman and ex situ microscopic analyses unveil the reversible conversion of CoOx during the sodiation/desodiation process. The excellent ORR activity, superior to that of commercial Pt/C, is mainly ascribed to the abundant Co-N-C species and the full exposure of active sites on the microporous framework structure. Flexible and rechargeable sodium-ion full batteries and zinc-air batteries are further demonstrated with great energy efficiency and cycling stability, as well as mechanical deformability.