Templating with colloidal crystals composed of monodisperse spheres is a convenient chemical method to obtain porous materials with well-ordered periodicity and interconnected pore systems. The three-dimensionally ordered macroporous (3DOM) products or inverse opals are of interest for numerous applications, both for the optical properties related to structural color of these photonic crystal materials and because of their bicontinuous nanostructure, i.e., a continuous nanostructured skeleton with large interfacial area and a three-dimensionally interconnected pore system with low tortuosity. This review outlines various synthetic methods used to control the morphology of 3DOM materials with different compositions. It highlights aspects of the choice of colloidal particles, assembly of the colloidal crystal template, infiltration and processing, template removal, and other necessary modifications to enhance the functionality of the materials. It also considers syntheses within the confinement of 3DOM materials and summarizes characterization methods that are particularly useful in the analysis of 3DOM materials. The review then discusses chemical applications of 3DOM materials, namely sorption and controlled release, optical and electrochemical sensors, solar cells, lithium ion batteries, supercapacitors, fuel cells, and environmental and chemical fuel catalysis. A focus is on structural features and materials properties that enable these applications.

Key parameters that influence the specific energy of electrochemical double-layer capacitors (EDLCs) are the double-layer capacitance and the operating potential of the cell. The operating potential of the cell is generally limited by the electrochemical window of the electrolyte solution, that is, the range of applied voltages within which the electrolyte or solvent is not reduced or oxidized. Ionic liquids are of interest as electrolytes for EDLCs because they offer relatively wide potential windows. Here, we provide a systematic study of the influence of the physical properties of ionic liquid electrolytes on the electrochemical stability and electrochemical performance (double-layer capacitance, specific energy) of EDLCs that employ a mesoporous carbon model electrode with uniform, highly interconnected mesopores (3DOm carbon). Several ionic liquids with structurally diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were investigated. We show that the cation size has a significant effect on the electrolyte viscosity and conductivity, as well as the capacitance of EDLCs. Imidazolium- and pyridinium-based ionic liquids provide the highest cell capacitance, and ammonium-based ionic liquids offer potential windows much larger than imidazolium and pyridinium ionic liquids. Increasing the chain length of the alkyl substituents in 1-alkyl-3-methylimidazolium trifluoromethanesulfonimide does not widen the potential window of the ionic liquid. We identified the ionic liquids that maximize the specific energies of EDLCs through the combined effects of their potential windows and the double-layer capacitance. The highest specific energies are obtained with ionic liquid electrolytes that possess moderate electrochemical stability, small ionic volumes, low viscosity, and hence high conductivity, the best performing ionic liquid tested being 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

We study--experimentally and theoretically--the energetics, structural changes, and charge flows during the charging and discharging processes for a new high-capacity cathode material, Li8ZrO6 (LZO), which we study both pure and yttrium-doped. We quantum mechanically calculated the stable delithiated configurations, the delithiation energy, the charge flow during delithiation, and the stability of the delithiated materials. We find that Li atoms are easier to extract from tetrahedral sites than octahedral ones. We calculate a large average voltage of 4.04 eV vs Li/Li(+) for delithiation of the first Li atom in a primitive cell, which is confirmed by galvanostatic charge/discharge cycling data. Energy calculations indicate that topotactic delithiation is kinetically favored over decomposition into Li, ZrO2, and O2 during the charging process, although the thermodynamic energy of the topotactic reaction is less favorable. When one or two lithium atoms are extracted from a primitive cell of LZO, its volume and structure change little, whereas extraction of the third lithium greatly distorts the layered structure. The Li6ZrO6 and Li5ZrO6 delithiation products can be thermodynamically metastable to release of O2. Experimentally, materials with sufficiently small particle size for efficient delithiation and relithiation were achieved within an yttrium-doped LZO/carbon composite cathode that exhibited an initial discharge capacity of at least 200 mAh/g over the first 10 cycles, with 142 mAh/g maintained after 60 cycles. Computations predict that during the charging process, the oxygen ion near the Li vacancy is oxidized for both pure LZO and yttrium-doped LZO, which leads to a small-polaron hole.

The electrochemical window is the potential range in which an electrolyte/solvent system does not get reduced or oxidized. Usually, voltammograms are measured, and the potentials at which specific current densities are reached are identified as the electrochemical limits. We measured electrochemical limits of several electrolytes-including ionic liquids-and show that this approach has disadvantages that can be overcome by an alternate approach of defining electrochemical limits. The choice of the cutoff current density, J cut-off , is arbitrary and strongly affects the determined electrochemical windows, which are strongly influenced by electrolyte mass transport. Moreover, the J cut-off method does not provide an accurate estimate of the electrochemical window at electrodes with high surface areas, where the capacitive currents are large. We propose a method that requires no definition of J cut-off . This method minimizes electrolyte mass transport effects, gives realistic electrochemical stability limits at high surface area electrodes, and is less affected by experimental parameters such as the scan rate. The method is based on linear fits of the current-voltage curve at potentials below and above the onset of electrolyte decomposition. The potential at which the two linear fits intersect is defined as the electrolyte electrochemical limit. Electrolytes and solvents are essential for the proper function of a variety of electrochemical devices, such as batteries and supercapacitors. The voltage polarization in these devices can cause electrochemical degradation of the electrolyte and solvent, resulting in device malfunction. This can be avoided by constraining the working range of the device to the electrochemical window limited by the electrolyte and solvent, i.e., the potential range in which they are chemically stable and do not get reduced or oxidized.1,2 Due to their localized charges, the inherently ionic electrolytes often have a lower electrochemical stability than neutral solvent molecules, and therefore frequently limit the device working range.2 There has been extensive research on modifying the structure of electrolytes to improve their electrochemical stability, and thus expanding their working range. [3][4][5][6][7] The determination of the electrochemical window of electrolytes is not well defined. Generally a current-voltage polarization curve is measured, starting at a voltage at which the electrolyte is electrochemically stable, followed by increasing or decreasing the potential to observe the anodic or cathodic decompositions, respectively. The potential at which a specific current density, J, is reached is defined as the cathodic or anodic limit of the electrolyte. This approach has several disadvantages, as noted by several researchers.3-5 Firstly, since the choice of the cut-off current density is arbitrary, different standards have been applied in the literature. Cut-off current densities, J cut-off , in the range of 0.01 to 5.0 mA/cm 2 were reported, 3,7-20 resulting in electrochemical ...

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.

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.

Doped Li8ZrO6 (LZO) is a pseudolayered material under consideration for lithium-ion battery cathodes and solid electrolyte coatings. The effects of doping LZO with Ce, Ti, Mg, Nb, and Y on structure, band gaps, conductivity, and activation energy for ion migration are investigated both experimentally and by quantum mechanical calculations. Optical band gaps decrease for all doped materials compared to undoped LZO. While all dopants reduce the electronic conductivity at room temperature slightly, doping with Mg or Nb increases ionic conductivity by an order of magnitude. Introducing a high loading of Nb into LZO decreases the activation energy for Li-ion diffusion in the 22–120 °C range. Calculations on lithium-ion diffusion in LZO show that it occurs by a polaron–vacancy complex mechanism. The energy barrier is lowest for the lithium hopping in a zigzag fashion between tetrahedral voids within adjacent layers. The diffusion barrier is reduced as the number of Li vacancies increases during battery charging. We calculated surface energies for 10 surfaces, and we find that the most stable surface is the (001) surface with the tetrahedral Li layer being exposed. The delithiation energy on the (001) surface was found to be slightly higher than that in the bulk. The Li-ion diffusion barriers from the surface to the bulk were also calculated on the (001) surface, and the diffusion energy barrier across the (001) surface was found to be smaller than the energy barrier along the (001) direction in the bulk, and also lower than the barrier for the lowest-energy path in the bulk (which is a hop between tetrahedral voids in adjacent layers as shown in the related graphic). These characterizations of surface and doping effects will assist future materials design.

Lithium-ion batteries (LIBs) are promising devices for high capacity, rechargeable electrical energy storage; however, LIBs are currently limited by the low specific capacity of the cathode compared to the anode. In previous work, our group demonstrated the viability of a novel cathode material, Li8ZrO6 (LZO), through computational and experimental results. Here we report a general synthesis for transition-metal-doped LZO, and we study the effects of doping on electrochemical delithiation and relithiation. A synthesis using transition-metal (M) doped ZrO2 nanoparticle/carbon black composites as precursors produces doped M-LZO with grain sizes between 35 and 67 nm. The materials were tested as electrode materials. Specific capacities of the doped materials depend on the transition metal and on the Li:Zr ratio used in the synthesis, but they are generally higher than in similarly prepared undoped LZO. In this set of cathode materials, Fe3+-doped LZO/C composites showed the highest specific capacities, with an initial discharge capacity higher than two Li ions per formula unit, a specific capacity of 175 mAh/g maintained after 140 cycles, and a specific capacity greater than 80 mAh/g at a rate of 5C. The effects of doping were also investigated by density functional calculations of dopant locations, band gaps, and delithiation energies. We found that all of the dopants that we studied are more favorably located in Li ion sites than in Zr ion sites. The calculated doping effects on structural parameters agree well with experiments. We also found that doping with any of these ions leads to smaller band gaps. Electronic structure calculations with the HSE06 exchange-correlation functional show that deintercalation after doping with Ce3+, Cu2+, or Co2+ at a Li site decreases the attainable cell voltage, whereas Fe3+ doping at a Li site increases it. Because of the large polarization and high carbon content of the M-LZO/C composite electrodes, further materials optimization will be needed before they become practical for LIBs.