Construction And Study On Nano-scale Iron Fluoride Electrode As High-rate Lithium Cathode Material

Iron fluoride has attracted a rapidly increasing amount of attention because of its high operating potential, high theoretical capacity of237mAh g1, and relatively low cost. Despite these advantages, the application of iron trifluoride has been limited due to its intrinsic drawbacks, for instance, the slow diffusion of Li+and low electron conductivity. With regards to the features of iron fluoride, our work develops several new strategies to construct four kinds iron fluoride electrode materials with unique nanostructure. X-ray diffraction, Raman spectrum, Scanning Electron Microscopy, Nitrogen adsorption-desorption techniques, Transmission Electron Microscopy, Discharge/charge measurement and Electrochemical impedance spectroscopy are applied to characterize the physicochemical properties and electrochemical behavior of Li+insertion/extraction process of the as-obtained iron trifluoride smaples.A tactful ionic-liquid (IL)-assisted approach on in situ synthesis of iron fluoride/graphene nanosheet (GNS) hybrid nanostructures was developed. To ensure uniform dispersion and tight anchoring of the iron fluoride on graphene, we employed an IL which served not only as a green fluoride source for the crystallization of iron fluoride nanoparticles but also as a dispersant of GNSs. GNSs were chosen as the starting materials instead of graphene oxide (GO), to avoid additional reductive treatment to obtain GNSs from GO, which could easily cause destruction to the structure of iron fluoride. Thus, both the nucleation and crystallization of FeF3·0.33H2O nanoparticles occurred on the surface of GNSs. It is a typical one-pot and in situ method. Owing to the electron transfer highways created between the nanoparticles and the GNSs, the iron fluoride/GNS hybrid cathodes exhibited a remarkable improvement in both high-rate and long-term cycle performance. The iron fluoride/GNS hybrid shows an impressive rate cycling performance of142mAh g1at1C after200cycles and115mAh g1at10C after250cycles.We also reported a facile strategy for the synthesis of mesoporous FeF3·0.33H2O@CMK-3nanocomposite with high electron conductivity and well-developed pore structure by nanocasting technique, in which iron fluoride nanoparticles were confined in mesoporous CMK-3. The intimate conductive contact between the FeF3·0.33H2O nanoparticles and the carbon framework provides an expressway of electron transfer for Li+insertion/extraction. Here, the CMK-3can suppress the growth and agglomeration of FeF3·0.33H2O nanoparticles during the crystallization process. The small FeF3·0.33H2O particles confined in mesoporous carbon matrix exhibited reduced electron and Li+transport resistance, which improved the electrochemical performance of the FeF3·0.33H2O@CMK-3nanocomposite. Additionally, well-defined, continuous channels provide a large specific surface area that increases the electrolyte-electrode contact area, ensuring that the electrolyte could easily penetrate the mesoporous. By combining these outstanding qualities, the FeF3·0.33H2O@CMK-3nanocomposite demonstrated excellent ultra-high rate performance. Remarkably, even under an ultrahigh charge/discharge rate of50C, the confined FeF3·0.33H2O@CMK-3showed a stable high specific capacity of79mAh g1after100cycles.Based on the π-cation interactions between the imidazolium cation of the [Bmim][BF4] and the π-electrons of the carbon nanohorns, the carbon nanohorns (CNHs) carried FeF3·0.33H2O nanocomposites (FeF3·0.33H2O@CNHs) with large specific surface area and well-developed pore structure was presented by a facile solution-based method for the first time. In the FeF3·0.33H2O@CNHs nanocomposites, the FeF3·0.33H2O nanoparticles were mainly located in the interstitial site between horn-shaped carbon nanotubes. The growth and agglomeration of FeF3·0.33H2O nanoparticles were effectively suppressed by the carbon nanohorns. The tiny FeF3·0.33H2O nanoparticles (~5nm) were well dispersed among the conductive network. The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution of the FeF3·0.33H2O@CNHs nanocomposite were measured by nitrogen isothermal adsorption analysis. The BET surface area of the FeF3·0.33H2O@CNHs nanocomposite is as high as268.9m2g1, the BJH average pore size is5.59nm. The high BET specific surface area of the FeF3·0.33H2O@CNHs nanocomposite and the well-developed pore structure provides more reaction sites and is beneficial for electrolyte access. The discharge capacities of the nanocomposite at0.5C,1C,2C,5C,10C and20C are169,157,140,131,120and106mAh g1, respectively. The FeF3·0.33H2O@CNHs composite can delivery a stable high capacity of81mAh g1even under an ultrahigh rate of50C. Meanwhile, the nanocomposite shows a stable cycle performance of153mAh g1at1C after50cycles.At last, we presented a tactful and advanced architecture design of self-supported, binder-free3D hierarchical FeF3·0.33H2O flower-like array directly growing on Ti foil by a solvothermal approach. In order to understand the formation process of the iron fluoride, the morphologies of the samples were studied by SEM at different stages of the reaction. The probable growth mechanism of the3D microflower was explored by the time-dependent analysis. The entire structure of the3D hierarchical architectureis constructed with dozens of nanopetals. These nanopetals are approximately10nm thick and500nm wide, and connect to each other through the center to form3D hierarchical structure approximately1μm in diameter. The products were scraped off from the Ti foil and collected for nitrogen isothermal adsorption measurement to further examine the pore structure of the FeF3·0.33H2O3D microflower. The sample exhibits a unique hierarchical porous structure. The smaller pore of3.5nm is mainly contributed by the pores existing between the FeF3·0.33H2O nanoparticles. The larger pores between10 nm and20nm might stem from the open space between neighboring nanopetals. The excellent electrochemical performance of the FeF3·0.33H2O flower-like array could be definitely ascribed to the synergistic effect of charge transfer expressway, high specific surface area and porous hierarchical structure.