Jeff Sakamoto

Vanadium Oxide-Carbon Nanotube Composite Electrodes for Use in Secondary Lithium Batteries

Abstract : Single-wall carbon nanotubes were used to form the electronically conducting network in lithium intercalation electrodes that incorporated vanadium oxide aerogels as the active material. Sol-gel methods were developed which integrated the nanotubes with V2O5 aerogel synthesis. The similarities in morphology and dimensional scale for the nanotubes and V2O5 ribbons enabled excellent electrical contact to be made between the two phases without seriously affecting the aerogel nanostructure. Intimate contact was established between the two phases at the nanodimensional level while the high pore volume of the aerogel provided electrolyte access throughout the composite material. The electrodes exhibited specific capacities in excess of 400 mAh/g at high discharge rates and retained this level of capacity on cycling.

The Limits of Low‐Temperature Performance of Li‐Ion Cells

The results of electrode and electrolyte studies reveal that the poor low‐temperature (<−30°C) performance of Li‐ion cells is mainly caused by the carbon electrodes and not the organic electrolytes and solid electrolyte interphase, as previously suggested. It is suggested that the main causes for the poor performance in the carbon electrodes are (i) the low value and concentration dependence of the Li diffusivity and (ii) limited Li capacity.

Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

Li7La3Zr2O12 (LLZO) garnet is attracting interest as a promising Li-ion solid electrolyte. LLZO exists in a tetragonal and cubic polymorph where the cubic phase exhibits ∼2 orders of magnitude higher Li-ion conduction. It has been suggested that a critical Li vacancy concentration (0.4–0.5 atoms per formula unit) is required to stabilize the cubic polymorph of Li7La3Zr2O12. This has been confirmed experimentally for Al3+ doping on the Li+ site. Substitution of M5+ (M = Ta, Nb) for Zr4+ is an alternative means to create Li vacancies and should have the same critical Li vacancy concentration, nevertheless, subcritically doped compositions (0.25 moles of Li vacancies per formula unit) have been reported as cubic. Adventitious Al, from alumina crucibles, was likely present in these studies that could have acted as a second dopant to introduce vacancies. In this work, Al-free subcritically doped (Li6.75La3Zr1.75Ta0.25O12) and critically doped (Li6.5La3Zr1.5Ta0.5O12) compositions are investigated. X-ray diffraction indicates that both compositions are cubic. However, upon further materials characterization, including SEM analysis, Raman spectroscopy, Electrochemical Impedance Spectroscopy, and neutron diffraction it is evident that the subcritically doped composition is a mixture of cubic and tetragonal phases. The results of this study confirm that 0.4–0.5 Li vacancies per formula unit are required to stabilize the cubic polymorph of LLZO.

Templated agarose scaffolds for the support of motor axon regeneration into sites of complete spinal cord transection.

Bioengineered scaffolds have the potential to support and guide injured axons after spinal cord injury, contributing to neural repair. In previous studies we have reported that templated agarose scaffolds can be fabricated into precise linear arrays and implanted into the partially injured spinal cord, organizing growth and enhancing the distance over which local spinal cord axons and ascending sensory axons extend into a lesion site. However, most human injuries are severe, sparing only thin rims of spinal cord tissue in the margins of a lesion site. Accordingly, in the present study we examined whether template agarose scaffolds seeded with bone marrow stromal cells secreting Brain-Derived Neurotrophic Factor (BDNF) would support regeneration into severe, complete spinal cord transection sites. Moreover, we tested responses of motor axon populations originating from the brainstem. We find that templated agarose scaffolds support motor axon regeneration into a severe spinal cord injury model and organize axons into fascicles of highly linear configuration. BDNF significantly enhances axonal growth. Collectively, these findings support the feasibility of scaffold implantation for enhancing central regeneration after even severe central nervous system injury.

Enhanced thermoelectric properties of Ba-filled skutterudites by grain size reduction and Ag nanoparticle inclusion

Ba-filled skutterudite compounds, Ba0.3Co4Sb12, with dispersed Ag nanoparticles have been synthesized by ball milling followed by hot pressing. The influence of Ag nanoparticles and their size distribution on electrical and thermal transport properties has been investigated in the temperature range from room temperature to 823 K. It was found that Ag nanoparticles in the Ba0.3Co4Sb12 matrix drastically enhance the electric conductivity and slightly increase the Seebeck coefficient. Surprisingly, Ag nanoparticles do not alter the carrier density of Ag/Ba0.3Co4Sb12 nanocomposites. This large improvement in the electrical conductivity as well as the corresponding power factor is assumed to come primarily from the enhanced mobility in Ag/Ba0.3Co4Sb12 nanocomposites. In addition, a large reduction in thermal conductivity is achieved by grain size reduction resulting from ball milling processing and as a result of the embedded Ag nanoparticles that scatter a wider range of phonon frequencies. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT some 30% higher in comparison to the parent Ba0.3Co4Sb12. Moreover, we found it is advantageous to employ a wider size distribution of Ag nanoparticles to reduce the thermal conductivity to a large degree by enhancing phonon scattering. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites.

Electrochemical properties of vanadium oxide aerogels

Abstract Aerogels are well-known mesoporous materials whose low density and high surface area result from synthesis methods that enable the pore solvent to be removed without collapsing the solid network phase. The interconnected porosity provides both molecular accessibility and rapid mass transport via diffusion, and for these reasons transition metal oxide aerogels are gaining increased interest as intercalation electrode materials for lithium-ion batteries. The present paper reviews recent research on vanadium oxide aerogels that has been directed at establishing both their fundamental properties and unique ways of incorporating these materials into electrode structures. The experiments used to determine the fundamental electrochemical properties of vanadium oxide aerogels involve the use of a sticky-carbon electrode that is designed to both hold the material and serve as the current collector. The results show that these materials combine elements of both capacitor and battery behavior as the materials possess both high specific capacitance (>2000 F/g) and high capacity for lithium incorporation (>450 mAh/g). The unique morphology of the vanadium oxide aerogel has led us to consider alternate electrode structures because traditional methods may compromise the mesoporous, high surface area morphology. One approach has involved the incorporation of single wall carbon nanotubes as the electronically conducting network. The resulting nanocomposites effectively retain the aerogel morphology and exhibit excellent electrochemical properties. These electrodes are especially effective at high discharge rates. Another approach has involved the preparation of aerogel electrodes that possess an inverted opal structure. The fabrication route is based on combining the templating of polystyrene spheres with sol—gel synthesis and leads to materials that possess a hierarchical pore structure. This architecture is also effective at retaining high lithium capacity at high current densities.

Conversion efficiency of skutterudite-based thermoelectric modules

Presently, the only commercially available power generating thermoelectric (TE) modules are based on bismuth telluride (Bi2Te3) alloys and are limited to a hot side temperature of 250 °C due to the melting point of the solder interconnects and/or generally poor power generation performance above this point. For the purposes of demonstrating a TE generator or TEG with higher temperature capability, we selected skutterudite based materials to carry forward with module fabrication because these materials have adequate TE performance and are mechanically robust. We have previously reported the electrical power output for a 32 couple skutterudite TE module, a module that is type identical to ones used in a high temperature capable TEG prototype. The purpose of this previous work was to establish the expected power output of the modules as a function of varying hot and cold side temperatures. Recent upgrades to the TE module measurement system built at the Fraunhofer Institute for Physical Measurement Techniques allow for the assessment of not only the power output, as previously described, but also the thermal to electrical energy conversion efficiency. Here we report the power output and conversion efficiency of a 32 couple, high temperature skutterudite module at varying applied loading pressures and with different interface materials between the module and the heat source and sink of the test system. We demonstrate a 7% conversion efficiency at the module level when a temperature difference of 460 °C is established. Extrapolated values indicate that 7.5% is achievable when proper thermal interfaces and loading pressures are used.

Electron microscopy characterization of hot-pressed Al substituted Li7La3Zr2O12

Hot-pressing was used to prepare a dense (97% relative density) cubic Al substituted Li7La3Zr2O12 material at temperatures lower than typically used for solid-state and/or liquid phase sintering. Electron microscopy analysis revealed equiaxed grains, grain boundaries, and triple junctions free of amorphous and second phases and no Al segregation at grain boundaries. These results suggest that Al2O3 and/or Al cannot act as a sintering aid by reducing grain boundary mobility. If Al2O3 acts as a sintering aid its main function is to enter the lattice as Al to increase the point defect concentration of the slowest moving species.

Excellent stability of a lithium-ion-conducting solid electrolyte upon reversible Li+/H+ exchange in aqueous solutions

Batteries with an aqueous catholyte and a Li metal anode have attracted interest owing to their exceptional energy density and high charge/discharge rate. The long-term operation of such batteries requires that the solid electrolyte separator between the anode and aqueous solutions must be compatible with Li and stable over a wide pH range. Unfortunately, no such compound has yet been reported. In this study, an excellent stability in neutral and strongly basic solutions was observed when using the cubic Li7 La3 Zr2 O12 garnet as a Li-stable solid electrolyte. The material underwent a Li(+) /H(+) exchange in aqueous solutions. Nevertheless, its structure remained unchanged even under a high exchange rate of 63.6 %. When treated with a 2 M LiOH solution, the Li(+) /H(+) exchange was reversed without any structural change. These observations suggest that cubic Li7 La3 Zr2 O12 is a promising candidate for the separator in aqueous lithium batteries.

Electrochemical Properties of Vanadium Oxide Aerogels and Aerogel Nanocomposites

The electrochemical properties of high surface area transition metal oxide aerogels are extremely interesting because aerogels serve to amplify surface effects. As a result, the electrochemical properties are dominated by surfaces rather than by bulk behavior. In the case of vanadium oxide aerogels this leads to extraordinary electrochemical properties, including an extremely high capacity for lithium and electrochemical responses that are both battery-like and capacitor-like. By exploiting sol-gel synthesis, it is possible to synthesize nanocomposite electrodes in which aerogels are in intimate contact with carbon nanotubes. The resulting nanocomposites exhibit superior electrochemical properties, especially at high discharge.