Tech Briefs

Mixed-Salt/Ester Electrolytes for Low-Temperature Li⁺ Cells

Electrolytes comprising, variously, LiPF6 or LiPF6 plus LiBF4 dissolved at various concentrations in mixtures of alkyl carbonates and alkyl esters have been found to afford improved low-temperature performance in rechargeable lithium-ion electrochemical cells. These and other electrolytes have been investigated in a continuing effort to extend the lower limit of operating temperatures of such cells. This research at earlier stages, and the underlying physical and chemical principles, were reported in numerous previous NASA Tech Briefs articles, the most recent being “Ester-Based Electrolytes for Low- Temperature Li-Ion Cells” (NPO-41097), NASA Tech Briefs, Vol. 29, No. 12 (December 2005), page 59. The ingredients of the solvent mixtures include ethylene carbonate (EC), ethyl methyl carbonate (EMC), methyl butyrate (MB), and methyl propionate (MP). The electrolytes were placed in Li-ion cells containing carbon anodes and LiNi0.8Co0.2O2 cathodes, and the electrical performances of the cells were measured over a range of temperatures down to –60 °C. The electrolytes that yielded the best low-temperature performances were found to consist, variously, of 1.0 M LiPF6 + 0.4 M LiBF4 or 1.4 LiPF6 in 1EC + 1EMC + 8MP or 1EC + 1EMC + 8MB, where the concentrations of the salts are given in molar units and the proportions of the solvents are by relative volume.This work was done by Marshall Smart and Ratnakumar Bugga of Caltech for NASA’s Jet Propulsion Laboratory.

Posted in: Briefs, TSP, Physical Sciences, Battery cell chemistry, Lithium-ion batteries, Electrolytes, Performance tests

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Miniature Bipolar Electrostatic Bipolar Electrostatic

All of the propellant molecules would be ionized. The figure presents a concept of a bipolar miniature electrostatic ion thruster for maneuvering a small spacecraft. The ionization device in the proposed thruster would be a 0.1-micronthick dielectric membrane with metal electrodes on both sides. Small conical holes would be micromachined through the membrane and electrodes. An electric potential of the order of a volt applied between the membrane electrodes would give rise to an electric field of the order of several megavolts per meter in the submicron gap between the electrodes. An electric field of this magnitude would be sufficient to ionize all the molecules that enter the holes.

Posted in: Briefs, TSP, Physical Sciences, Downsizing, Rocket engines, Spacecraft

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Miniature Electrostatic Ion Thruster With Magnet

A miniature electrostatic ion thruster is proposed that, with one exception, would be based on the same principles as those of the device described in the previous article, “Miniature Bipolar Electrostatic Ion Thruster” (NPO-21057). The exceptional feature of this thruster would be that, in addition to using electric fields for linear acceleration of ions and electrons, it would use a magnetic field to rotationally accelerate slow electrons into the ion stream to neutralize the ions. This work was done by Frank T. Hartley of Caltech for NASA’s Jet Propulsion Laboratory.

Posted in: Briefs, TSP, Physical Sciences, Downsizing, Magnetic materials, Rocket engines, Spacecraft

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Miniature Free-Space Electrostatic Ion Thrusters

A miniature electrostatic ion thruster is proposed for maneuvering small spacecraft. In a thruster based on this concept, one or more propellant gases would be introduced into an ionizer based on the same principles as those of the device described in the earlier article, “Miniature Bipolar Electrostatic Ion Thruster” (NPO-21057). On the front side, positive ions leaving an ionizer element would be accelerated to high momentum by an electric field between the ionizer and an accelerator grid around the periphery of the concave laminate structure. On the front side, electrons leaving an ionizer element would be ejected into free space by a smaller accelerating field. The equality of the ion and electron currents would eliminate the need for an additional electron- or ion-emitting device to keep the spacecraft charge-neutral. In a thruster design consisting of multiple membrane ionizers in a thin laminate structure with a peripheral accelerator grid, the direction of thrust could then be controlled (without need for moving parts in the thruster) by regulating the supply of gas to specific ionizer. This work was done by Frank T. Hartley and James B. Stephens of Caltech for NASA’s Jet Propulsion Laboratory.

Posted in: Briefs, TSP, Physical Sciences, Downsizing, Performance upgrades, Gases, Rocket engines, Spacecraft

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Miniature Oxidizer Ionizer for a Fuel Cell

A proposed miniature device for ionizing the oxygen (or other oxidizing gas) in a fuel cell would consist mostly of a membrane ionizer using the same principles as those of the device described in the earlier article, “Miniature Bipolar Electrostatic Ion Thruster” (NPO-21057). The oxidizing gas would be completely ionized upon passage through the holes in the membrane ionizer. The resulting positively charged atoms or molecules of oxidizing gas could then, under the influence of the fringe fields of the ionizer, move toward the fuel-cell cathode that would be part of a membrane/electrode assembly comprising the cathode, a solid-electrolyte membrane, and an anode. The electro- oxidized state of the oxidizer atoms and molecules would enhance transfer of them through the cathode, thereby reducing the partial pressure of the oxidizer gas between the ionizer and the fuelcell cathode, thereby, in turn, causing further inflow of oxidizer gas through the holes in the membrane ionizer. Optionally the ionizer could be maintained at a positive electric potential with respect to the cathode, in which case the resulting electric field would accelerate the ions toward the cathode. This work was done by Frank Hartley of Caltech for NASA’s Jet Propulsion Laboratory.

Posted in: Briefs, TSP, Physical Sciences, Fuel cells, Gases

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Miniature Ion-Mobility Spectrometer

Advantages would include robustness, simplicity, and extreme miniaturization. The figure depicts a proposed miniature ion-mobility spectrometer that would be fabricated by micromachining. Unlike prior ion-mobility spectrometers, the proposed instrument would not be based on a time-of-flight principle and, consequently, would not have some of the disadvantageous characteristics of prior time-of-flight ion-mobility spectrometers. For example, one of these characteristics is the need for a bulky carrier-gas-feeding subsystem that includes a shutter gate to provide short pulses of gas in order to generate short pulses of ions. For another example, there is need for a complex device to generate pulses of ions from the pulses of gas and the device is capable of ionizing only a fraction of the incoming gas molecules; these characteristics preclude miniaturization. In contrast, the proposed instrument would not require a carrier-gas-feeding subsystem and would include a simple, highly compact device that would ionize all the molecules passing through it.

Posted in: Briefs, TSP, Physical Sciences, Downsizing, Microelectromechanical devices, Performance upgrades, Product development

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Miniature Ion-Array Spectrometer

The mode of operation would differ from that described in the preceding article. The figure depicts a proposed miniature ion-mobility spectrometer that would share many features of design and operation of the instrument described in the immediately preceding article. The main differences between that instrument and this one would lie in the configuration and mode of operation of the filter and detector electrodes. A filter electrode and detector electrodes would be located along the sides of a drift tube downstream from the accelerator electrode. These electrodes would apply a combination of (1) a transverse AC electric field that would effect differential transverse dispersal of ions and (2) a transverse DC electric field that would drive the dispersed ions toward the detector electrodes at different distances along the drift tube. The electric current collected by each detector electrode would be a measure of the current, and thus of the abundance of the species of ions impinging on that electrode. The currents collected by all the detector electrodes could be measured simultaneously to obtain continuous readings of abundances of species. The downstream momentum of accelerated ions would be maintained through neutralization on the electrodes; the momentum of the resulting neutral atoms would serve to expel gases from spectrometer, without need for a pump.

Posted in: Briefs, TSP, Physical Sciences, Downsizing, Microelectromechanical devices, Performance upgrades, Product development

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