Structural Design And Lithium-Storage Properties Of One-Dimensional Tin-Based/Carbon Nanocomposites

In recent years, extensive efforts have been devoted to researching the high-capacity and long-lifespan lithium-ion batteries (LIBs), aiming to satisfy the ever-increasing requirements of their applications in large-scale power systems. Tin and its oxides (SnOx, x=0～2) have been considered as one of the most alternative anode materials to replace current commercial graphite, due to their high theoretical capacities and mild voltage plateau. However, the practical applications of SnOx anodes are hampered by their huge volume expansion during lithiation and de-lithiation processes, which can lead to the fracture of SnOx anodes and thereafter the electrical contact loss between electrode material and current collector, thus causing the fast capacity decay of LIBs. To overcome this issue, SnOx/carbon composites have been extensively developed, in which the high-capacity SnOx materials are dispersed or encapsulated in carbon matrix. Here, the carbon matrix can buffer the volume change and thus protect the structure of the overall electrode, while the SnOx materials offer high lithium-storage capacity. Among all the carbon matrixes, one-dimensional (ID) amorphous carbon has attracted much attention owing to its unique properties, such as good lithium-storage capacity, high-effective electron/lithium-ion diffusion pathway, and excellent structural stability. Therefore, rational design of the 1D amorphous carbon matrix for making itself more suitable to resolve the operation issue of SnOx anode is beneficial for obtaining high-performance LIBs anode materials.In this work, by using electrospinning technique and SiOx template method, we design and prepare various 1D amorphous carbon materials, investigate their lithium-ion storage properties, and employ them as the structural components for 1D SnOx/carbon nanocomposites. In order to explore the effect of as-designed structures of these nanocomposites on their lithium-storage capabilities, the morphologies and structures of the 1D carbon matrix, and the content, dispersion as well as crystal species of SnOx component are characterized by many measurements. Thus, the component-structure-performance relationship is established to further prepare the high-performance SnOx/carbon anode.(1) Three 1D polymer-derived amorphous carbons are prepared by electrospinning technique combined with SiOx-template method in this section:polyacrylonitrile (PAN)-derived CNF, polyvinylpyrrolidone (PVP)-derived porous CNF, and Polydopamine (PDA)-derived N-doped carbon tube (N-CT). At first, the effect of N-doping on the electrochemical performance of PAN-derived CNF is investigated. The results show that N element content derceases with the carbonization temperature increasing. The N types in the CNF are divided into tpyridinic N, pyridonic N, and graphitic N, respectively. Among the three N types, graphitic N contributes to the improvement of electron conductivity of CNF, thus enhancing the rate performance of CNF. Then, we evaluate the PVP-derived porous CNF as the anode materials for LIBs. It is found that hollow-structural pore are more beneficial for the penetration of Li+-carrying electrode into the CNF, thus decreasing the Li+-diffusion distance, which leads to improved electrochemical performance, especially the rate performance. At last, based on the aforementioned two works, we employ PDA as the precursor to prepare N-CT with an ultra-thin wall thickness of about 16 nm. The results shows that the graphitic N content in N-CT (carbonization temperature is 750 ℃) is abundant (～3.1 at.%). Benefiting from the synergistic effect of unique hollow structure and abundant N-doping that shorts the Li+-diffusion distance and facilitates the electron conductivity, the optimal N-CT electrode shows excellent rate performance (406 mA h g-1 at2Ag-1).(2) A series of TiO2-SnOx/CNF ternary nanocomposites with different molar ratios of Ti/Sn (=0.05,0.1 and 0.2) are prepared by electrospinning method as well as two-step heat treatments. Among the as-synthesized samples, the optimal electrode (Ti/Sn=0.1) shows the best reversible capacity of 668 mA h g-1 after 50 cycles at 200 mA g-1, increasing by 21% compared with that of original SnOx/CNF electrode. The improved lithium-storage properties of TiO2-SnOx/CNF electrodes stem from the uniform dispersion of ultrafine SnOx nanoparticles in the conductive CNFs and TiO2-adding that work together to increase the reversible capability of the conversion reaction of SnOx. Moreover, we also investigate the effect of carbonization temperature on the morphologies, structures and lithium-storage capabilities of TiO2-SnOx/CNF ternary nanocomposites (Ti/Sn=0.1). It is found that when the carbonization temperature is 700℃, the as-prepared TiO2-SnOx/CNF ternary nanocomposite has the most uniform surface morphology and inner structure, and possesses an excellent CNF matrix with good electron conductivity and lithium-storage capacity, which lead to remarkable rate performance (349 mA h g-1 at the rate of 2 A g-1).(3) We prepare various SnOx/porous CNF nanocomposites by electrospinning and in-situ SiOx template methods. Benefiting from the introduced reasonable porous structures that reduce the Li+-diffusion distance, all SnOx/porous CNF electrodes exhibit improved electrochemical performance as compared with SnOx/CNF. It is found that SnOx/porous CNF electrode with hollow-like pore, among all the samples, has the best electrochemical performance:at the rate of 500 mA g-1, the reversible capacity is 721 mA h g-1 after 100 cycles; when the current rate increases to 4 A g-1, the reversible capacity can retain 253 mA hg-1.(4) We creatively design and synthesize a novel coaxial hierarchical nanostructure of SnO2@porous carbon in carbon tube (SnO2@PC/CT) by filling SnO2@porous carbon core materials into the aforementioned PDA-derived N-CT, in whichthe introduced porous carbon core not only contributes to linking and supporting abundant SnO2 nanoparticles for the fast charge transport, but also inhibits the as-produced Sn nanoparticle aggregation and dissolution during cycling. As a result, the as-prepared SnO2@PC/CT, used as the LIBs electrode, exhibits an outstanding reversible capacity of 1018 mA h g"1 after 400 cycles at 0.5 A g-1, and excellent long-term cyclic performance at high rate (499 mA h g-1 at 2 A g-1 after 1000 cycles).