Facile and fast Na-ion intercalation employing amorphous black TiO_2-x/C composite nanofiber anodes

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>Structural and electronic modification of titanium oxide (TiO<SUB>2</SUB>) nanomaterials induced by the co-introduction 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 TiO<SUB>2-x</SUB> and conductive carbons throughout the NF structure, and evaluate the materials as potential anodes in sodium-ion batteries. The black TiO<SUB>2-x</SUB> 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 TiO<SUB>2</SUB> NFs used as reference, because of (i) an inherent free volume in the glass phase corresponding to the enlarged Na<SUP>+</SUP> sites, (ii) increased electrical conductivity (low bandgap) resulting from the presence of Ti<SUP>3+</SUP>, (iii) introduction of conductive carbon agents around the TiO<SUB>2-x</SUB> domain, and (iv) one-dimensional NF feature allowing numerous Na<SUP>+</SUP> reaction sites at the electrochemical interface. We also elucidate the morphological and structural changes in the nanofibers after discharge and charge by <I>ex-situ</I> characterizations.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We succeed in co-creating amorphous nanofiber structure composed of black TiO<SUB>2-x</SUB> and conductive carbons. </LI> <LI> Black TiO<SUB>2-x</SUB>/carbon nanofibers are prepared by electrospinning followed by a two-step thermal treatment. </LI> <LI> Black TiO<SUB>2-x</SUB>/carbon nanofiber delivers excellent electrochemical performance compared to those of anatase TiO<SUB>2</SUB> nanofiber. </LI> <LI> The benefits and reaction mechanism of the black TiO<SUB>2-x</SUB>/carbon nanofiber anode in the Na-ion cell operation is elucidated. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>