Si/C/TiO₂ Composite Nanofibers Prepared Using a Sulfur-Templating Method for Lithium Ion Batteries

Presently, lithium Ion Batteries (LIBs) are widely viewed as an optimal candidate for green energy storage and all-electric vehicles. To meet the challenging goals demanded by the above mentioned applications, several obstacles have to be overcome, including lowering the cost, increasing the capacity, enhancing the rate and cycling performance and improving the safety. Nowadays, nanotechnology is playing a critically important role in the advancement of high performance LIBs due to its excellent mechanical properties and huge specific surface area. In this work, silicon nanoparticles (SiNP) of high specific capacity are confined within the structurally stable titanium dioxide nanofiber (TiO ₂ NF) matrix to improve its electrochemical performance. Si/C/TiO ₂ composite nanofibers were fabricated using an electrospinning method, followed by a carbonization in helium gas at 800 ^o C for 4 hrs. Sulfur was employed as a template to facilitate the formation of void structure, thus providing space to the large volume expansion of silicon during lithiation process and mitigate pulverization of silicon particles. Carbonized nanofibers containing TiO ₂ and Si nanoparticles were also prepared for comparison. The structure, morphology, phase and composition of these nanofibers were characterized using Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, powder X-ray diffraction, and thermogravimetric analyzer. Carbonized nanofibers containing TiO ₂ showed a low but stable specific capacity of 105 mAh g-1 after 100 cycles at a current density of 0.09 A g ^-1 . Carbonized nanofibers containing silicon nanoparticles demonstrated an initially high but fast degrading capacity, which is only 73 mAh g ^-1 after 100 cycles. In contrast, the capacity of silicon in SiNP/C/TiO ₂ nanofibers prepared using a sulfur-templating method is ~ 3459 mAh g ^-1 at the fourth cycle, 52 % of which can be maintained after 80 cycles. It is believed that sulfur is functioning as a template to help the formation of void between silicon nanoparticles and TiO ₂ , thus allowing for the ~300% volume change during Si lithiation and delithiation process.