Exceptional electrochemical performance of freestanding electrospun carbon nanofiber anodes containing ultrafine SnO_xparticles

SnOx-carbon nanofiber (CNF) composites are synthesized using the electrospinning technique for use as freestanding electrodes in Li-ion batteries. The electrodes made from the composites carbonized at 750°C (SnOx-CNF-750) with 14.5 wt% SnOxdeliver a remarkable capacity of 674 mA h g-1after 100 cycles when discharged at 0.5 A g-1. This result is considered the highest among those reported in the literature for anodes made from similar electrospun carbon fibers containing SnOxnanoparticles. An increase in carbonization temperature to 950°C (SnOx-CNF-950) results in a significant reduction of the particle content in the fiber due to aggregation of Sn to form nanoparticles external to the fibers, with concomitant degradation of capacities. The presence of amorphous SnOxparticles at the atomic scale embedded in the conductive CNFs is thought to be responsible for the exceptional electrochemical performance of the SnOx-CNF-750 electrodes. These ultrafine particles facilitate the reaction Sn + xLi2O → SnOx+ 2xLi++ 2xe-, making it highly reversible, which is confirmed by the growing peak currents with increasing scan rate indicated by cyclic voltammetry, and the absence of Sn-Sn bonds in the particles revealed by the extended X-ray absorption fine structure spectroscopy (EXAFS). Both the SnOxparticle size and content in the fiber play important roles in controlling the rate and cyclic performance of the SnOx-CNF composite electrodes.