Facile and fast Na-ion intercalation employing amorphous black TiO2-x/C composite nanofiber anodes

Abstract

Structural and electronic modification of titanium oxide (TiO2) nanomaterials induced by the cointroduction of fully disordered glass phase and oxygen vacancies can lead to remarkable advances in the electrode performance in emerging energy storage systems. We report on the effective co-creation of fully amorphous nanofibers (NFs) composed of black TiO2-x and conductive carbons throughout the NF structure, and evaluate the materials as potential anodes in sodium-ion batteries. The black TiO2-x nanofiber is successfully fabricated by electrospinning a precursor solution followed by a two-step sequential thermal treatment in an air and reducing atmosphere. The NF electrode could deliver approximately two-fold higher 2nd discharge capacity and an excellent kinetic performance even under high rates compared to that delivered by anatase-structured white TiO2 NFs used as reference, because of (i) an inherent free volume in the glass phase corresponding to the enlarged Nathorn sites, (ii) increased electrical conductivity (low bandgap) resulting from the presence of Ti3+, (iii) introduction of conductive carbon agents around the TiO2-x domain, and (iv) one-dimensional NF feature allowing numerous Nathorn reaction sites at the electrochemical interface. We also elucidate the morphological and structural changes in the nanofibers after discharge and charge by ex-situ characterizations. (c) 2018 Elsevier Ltd. All rights reserved.