2, Saarland University, Saarbruecken, Saarland, Germany
The development of new electrode materials for Li-ion batteries and supercapacitors remains at the focal point of ongoing research activities. Electrospinning is a highly promising process to produce nanofiber as electrode materials; however, most published works on electrospinning crushed the fiber mats and introduced polymer binder and conductive additive to obtain electrodes. Yet, the direct use of continuous fiber mats offers great advantages: the completely free-standing electrode can be produced by a one-pot synthesis, no binder and no conductive additive is needed, and the electrodes present a higher capacity and superior rate handling.
In this study, we present for the first time the synthesis and electrochemical characterization of hybrid Nb2O5/C and Ti2Nb10O29/C continuous nanofiber mats as battery electrodes. Starting from a one-pot synthesis process, followed by thermal treatment between 850-1000 °C, we demonstrate a strategy to tailor the crystalline structure of the metal oxide, while maintaining the highly graphitic nature of the carbon phase. Depending on the present crystal phase (orthorhombic or tetragonal Nb2O5, and monoclinic Ti2Nb10O29) and crystal coherence length (15-40 nm), differences in the electrochemical performance are observed. Tested as Li-ion battery electrodes, tetragonal Nb2O5/C and monoclinic Ti2Nb10O29/C hybrid fiber mats present high capacity values of 243 mAh/g and 267 mAh/g, normalized to the full electrode mass. Nb2O5/C presented the highest rate performance, maintaining over 78 % of the initial capacity at a high rate of 5 A/g. The higher rate performance and stability of orthorhombic and tetragonal Nb2O5, compared to monoclinic Ti2Nb10O29, relates to the low energy barriers for Li+ transport in this crystal structures, with no phase transformation as corroborated by in situ XRD.
This approach for electrodes completely free of polymer binder and added conductive additives presents several advantages compared to conventional polymer-bound electrodes. A significant amount of energy and cost can be avoided by using only one synthesis process for the free-standing electrode preparation. Also, the hybrid fibers present an improved high rate performance and conductivity, resulting from the excellent charge propagation in the continuous nanofiber network.
 B. Scrosati, J. Hassoun, Y.-K. Sun, Energy & Environmental Science 2011, 4, 3287-3295.
 S. Cavaliere, S. Subianto, I. Savych, D. J. Jones, J. Roziere, Energy & Environmental Science 2011, 4, 4761-4785.
 a) A. Tolosa, B. Krüner, N. Jäckel, M. Aslan, C. Vakifahmetoglu, V. Presser, Journal of Power Sources 2016, 313, 178-188; b) A. Tolosa, B. Krüner, S. Fleischmann, N. Jäckel, M. Zeiger, M. Aslan, I. Grobelsek, V. Presser, Journal of Materials Chemistry A 2016, 4, 16003-16016.