7 research outputs found
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<sup>+</sup> capacity at various charge/discharge rates, to improve retention
of Li<sup>+</sup> 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) TiO<sub>2</sub>/C LIB anode materials prepared
using low toxicity precursors, including ammonium citratoperoxotitanateÂ(IV)
and sucrose, which provide high capacities for reversible Li<sup>+</sup> insertion/extraction. When the composites are pyrolyzed at 700 °C,
the carbon phase restricts sintering of TiO<sub>2</sub> 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<sup>+</sup>). Composites pyrolyzed at 700 °C
retain over 200 mAh/g TiO<sub>2</sub> 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<sup>+</sup>), which
allows for Li<sup>+</sup> 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 TiO<sub>2</sub> is found in these composites at sizes below
the thermodynamic stability limit in the pure phase
Control of TiO<sub>2</sub> Grain Size and Positioning in Three-Dimensionally Ordered Macroporous TiO<sub>2</sub>/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 (TiO<sub>2</sub>), offers a distinct safety
advantage in comparison to commercialized graphite anodes, since TiO<sub>2</sub> has a higher potential for lithium intercalation. In this
article, we present two routes for the facile, robust synthesis of
nanostructured TiO<sub>2</sub>/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 TiO<sub>2</sub>/C composite with mesoporous walls. Control over the size
and location of the TiO<sub>2</sub> 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 TiO<sub>2</sub> crystallites and the degree of carbonization. Using these
pathways to optimize electrochemical performance, the primarily macroporous
TiO<sub>2</sub>/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 TiO<sub>2</sub> with disordered mesoporous voids. Combining the advantages
of a nanocrystalline framework and significant open porosity, the
macroporous TiO<sub>2</sub> delivers a stable capacity (>170 mAh/g
at a rate of C/2) over 100 cycles
Enhanced Oxidation Kinetics in Thermochemical Cycling of CeO<sub>2</sub> through Templated Porosity
Two-step thermochemical cycling was achieved using CeO<sub>2</sub> with sub-micrometer sized macropores, allowing for substantially
improved CO production at fast cycle rates when compared to nonporous
CeO<sub>2</sub>. 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) CeO<sub>2</sub> and nonordered macroporous (NOM) CeO<sub>2</sub>. Aggregated
CeO<sub>2</sub> nanoparticles with feature sizes similar to those
in 3DOM CeO<sub>2</sub> were prepared by fragmenting 3DOM CeO<sub>2</sub> into its building blocks using ultrasonication. The three
templated materials and nonporous, commercial CeO<sub>2</sub> were
tested in thermochemical cycles using an infrared furnace. CeO<sub>2</sub> was reduced at ∼1200 °C, and the reduced CeO<sub>2−δ</sub> materials were reoxidized under CO<sub>2</sub> 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 CeO<sub>2</sub>. Additionally, 3DOM CeO<sub>2</sub> 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