Titania–Carbon Nanocomposite Anodes for Lithium Ion Batteries Effects of Confined Growth and Phase Synergism

As lithium-ion batteries (LIB) see increasing use in areas beyond consumer electronics, such as the transportation sector, research has been directed at improving LIBs to better suit these applications. Of particular interest are materials and methods to increase Li⁺ capacity at various charge/discharge rates, to improve retention of Li⁺ capacity from cycle-to-cycle, and to enhance various safety aspects of electrode synthesis, cell construction, and end use. This work focuses on the synthesis and testing of three-dimensionally ordered macroporous (3DOM) TiO₂/C LIB anode materials prepared using low toxicity precursors, including ammonium citratoperoxotitanate­(IV) and sucrose, which provide high capacities for reversible Li⁺ insertion/extraction. When the composites are pyrolyzed at 700 °C, the carbon phase restricts sintering of TiO₂ crystallites and keeps the size of these crystallites below 5 nm. Slightly larger crystallites are produced at higher temperatures, alongside a titanium oxycarbide phase. The composites exhibit excellent capacities as LIB anodes at low to moderate charge/discharge rates (in the window from 1 to 3 V vs Li/Li⁺). Composites pyrolyzed at 700 °C retain over 200 mAh/g TiO₂ of capacity after 100 cycles at a C/2 rate (C = 335 mA/g), and do not suffer from extensive cycle-to-cycle capacity fading. A substantial improvement of overall capacities, especially at high rates, is attained by cycling the composite anodes in a wider voltage window (0.05 to 3 V vs Li/Li⁺), which allows for Li⁺ intercalation into carbon. At currents of 1500 mA/g of active material, over 200 mAh/g of capacity is retained. Other structural aspects of the composites are discussed, including how rutile TiO₂ is found in these composites at sizes below the thermodynamic stability limit in the pure phase

Control of TiO₂ Grain Size and Positioning in Three-Dimensionally Ordered Macroporous TiO₂/C Composite Anodes for Lithium Ion Batteries

After several high-profile incidents that raised concerns about the hazards posed by lithium ion batteries, research has accelerated in the development of safer electrodes and electrolytes. One anode material, titanium dioxide (TiO₂), offers a distinct safety advantage in comparison to commercialized graphite anodes, since TiO₂ has a higher potential for lithium intercalation. In this article, we present two routes for the facile, robust synthesis of nanostructured TiO₂/carbon composites for use as lithium ion battery anodes. These materials are made using a combination of colloidal crystal templating and surfactant templating, leading to the first report of a three-dimensionally ordered macroporous TiO₂/C composite with mesoporous walls. Control over the size and location of the TiO₂ crystallites in the composite (an often difficult task) has been achieved by changing the chelating agent in the precursor. Adjustment of the pyrolysis temperature has also allowed us to strike a balance between the size of the TiO₂ crystallites and the degree of carbonization. Using these pathways to optimize electrochemical performance, the primarily macroporous TiO₂/C composites can attain a capacity of 171 mAh/g at a rate of 1 C. Additionally, the carbon in these composites can function as a secondary template for high-surface-area, macroporous TiO₂ with disordered mesoporous voids. Combining the advantages of a nanocrystalline framework and significant open porosity, the macroporous TiO₂ delivers a stable capacity (>170 mAh/g at a rate of C/2) over 100 cycles

Enhanced Oxidation Kinetics in Thermochemical Cycling of CeO₂ through Templated Porosity

Two-step thermochemical cycling was achieved using CeO₂ with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO₂. The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO₂ and nonordered macroporous (NOM) CeO₂. Aggregated CeO₂ nanoparticles with feature sizes similar to those in 3DOM CeO₂ were prepared by fragmenting 3DOM CeO₂ into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO₂ were tested in thermochemical cycles using an infrared furnace. CeO₂ was reduced at ∼1200 °C, and the reduced CeO_2−δ materials were reoxidized under CO₂ at ∼850 °C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO₂. Additionally, 3DOM CeO₂ provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling