S‐Doped Carbon Fibers Uniformly Embedded with Ultrasmall TiO2 for Na+/Li+ Storage with High Capacity and Long‐Time Stability

Building a rechargeable battery with high capacity, high energy density, and long lifetime contributes to the development of novel energy storage devices in the future. Although carbon materials are very attractive anode materials for lithium‐ion batteries (LIBs), they present several deficiencies when used in sodium‐ion batteries (SIBs). The choice of an appropriate structural design and heteroatom doping are critical steps to improve the capacity and stability. Here, carbon‐based nanofibers are produced by sulfur doping and via the introduction of ultrasmall TiO2 nanoparticles into the carbon fibers (CNF‐S@TiO2). It is discovered that the introduction of TiO2 into carbon nanofibers can significantly improve the specific surface area and microporous volume for carbon materials. The TiO2 content is controlled to obtain CNF‐S@TiO2‐5 to use as the anode material for SIBs/LIBs with enhanced electrochemical performance in Na+/Li+ storage. During the charge/discharge process, the S‐doping and the incorporation of TiO2 nanoparticles into carbon fibers promote the insertion/extraction of the ions and enhance the capacity and cycle life. The capacity of CNF‐S@TiO2‐5 can be maintained at ≈300 mAh g−1 over 600 cycles at 2 A g−1 in SIBs. Moreover, the capacity retention of such devices is 94%, showing high capacity and good stability. Ultrasmall TiO2 uniformly embedded in S‐doped carbon fibers increases the specific surface area and the microporous volume of the samples. Benefiting from its porous character, this carbon‐based material shows a fast and stable surface Na+/Li+ storage behavior (capacitive effect). In particular, when used as an anode material in Na‐ion full cells, it displays a good rate capacity and reversibility.