Polymer-derived carbon nanofiber network supported SnO₂ nanocrystals: A superior lithium secondary battery material

In the present study, new three-dimensional tin dioxide-carbon (SnO ₂-C) composites as anode materials for lithium-ion batteries are achieved via a simple hydrothermal route and subsequent calcination process using polypyrrole-based carbon networks as the support and conductive buffering layer. The structure and morphology of the novel composites are characterized by powder X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction techniques. The resulting carbon networks composed of highly flexible, hollow, and end-opening nanofibers with diameters of approximately 40-70 nm and lengths of several microns are homogenously coated by nano-crystalline SnO₂ (ca. 3-5 nm in size). The electrochemical performance of the mentioned SnO₂-C (15.1% carbon) is investigated by cyclic voltammetry and discharge-charge cycling on half-cells in the potential range of 0.005-2 V at 25 °C. Galvanostatic cycling shows a stable and high charge capacity (598.3 mA h g^-1) at a current density of 100 mA g^-1 over 50 cycles with a low capacity fading of about 0.7% per cycle. By increasing the rate after 5 cycles in steps from 50 to 300 mA g ^-1 up to 30 cycles, a high reversible capacity (657.9 mA h g ^-1) is retained. Much improved lithium ion storage properties in terms of capacity, rate capability, and cycling stability may benefit from both the buffering action of conductive carbon networks and the size effect of SnO₂ nanocrystals.