Experiments and theoretical study have demonstrated the promise of all-solid-state, high-temperature electrochemical battery cells based on NiF2 as the active cathode material, CaF2 doped with NaF as the electrolyte material, and Ca as the active anode material. These and other all-solid-state cells have been investigated in a continuing effort to develop batteries for instruments that must operate in environments much hotter than can be withstood by ordinary commercially available batteries. Batteries of this type are needed for exploration of Venus (where the mean surface temperature is about 450 °C), and could be used on Earth for such applications as measuring physical and chemical conditions in geothermal wells and oil wells.
All-solid-state high-temperature power cells are sought as alternatives to other high-temperature power cells based, variously, on molten anodes and cathodes or molten eutectic salt electrolytes. Among the all-solid-state predecessors of the present NiF2/NaF:CaF2/Ca cells are those described in “Solid-State High-Temperature Power Cells” (NPO-44396), NASA Tech Briefs, Vol. 32, No. 5 (May 2008), page 40. In those cells, the active cathode material is FeS2, the electrolyte material is a crystalline solid solution of equimolar amounts of Li3PO4 and LiSiO4, and the active anode material is Li contained within an alloy that remains solid in the intended high operational temperature range.
The chemical reactions during discharge of an NiF2/NaF:CaF2/Ca cell are the following:
Overall:
NiF2 + Ca → CaF2 + Ni
At the negative electrode (anode):
Ca + 2F– → CaF2 + 2e–
At the positive electrode (cathode):
NiF2 + 2e– → Ni + 2F–
One of the advantages of the NiF2/NaF:CaF2/Ca material system is that at high temperature, the solid electrolyte material is a conductor of fluoride ions (F–). Homogenous doping of CaF2 with NaF or another aliovalent fluoride salt induces fluoride vacancies and thereby sharply increases ionic conductivity. The electrolyte material can also be heterogeneously doped with ceria, zirconia, or alumina to further enhance fluoride conductivity. By means of a combination of homogenous and heterogeneous doping, the fluoride conductivity can be enhanced several orders of magnitude relative to that of pure CaF2, yielding a fluoride conductivity of 12.6 mS/cm at 440 °C — on a par with conductivities of Li-ion battery electrolytes at room temperature.
Unlike the active electrode materials in Li– anode/FeS2 – cathode cells, the active electrode materials in the present NiF2/NaF:CaF2/Ca cells exhibit negligible solubility in the solid electrolyte material. As a consequence, corrosion of the electrodes and self-discharge of the cell are greatly reduced.
To increase the ionic conductivity of the cathode of an NiF2/NaF:CaF2/Ca cell, in fabricating the cathode, one adds between 20 and 30 weight percent of the electrolyte material to the active cathode material. Similarly, to increase the electronic conductivity, one adds between 10 and 20 weight percent of graphite. The cathode structure as described thus far is then sintered. The cathode discharge reaction produces Ni, which enhances the electronic conductivity of the cathode. The corrosion resistance of Ni in fluorides in the absence of water is excellent. It has been conjectured that CuF2 could be substituted for NiF2 as the active cathode material, in which case the cathode reaction product would be Cu, which would enhance the electronic conductivity of the cathode.
The anode of an NiF2/NaF:CaF2/Ca cell consists of a solid Ca metal layer formed by pressing dendritic Ca into a disk shape and roughening the surface to enhance contact with the cathode/electrolyte/graphite. The conversion of the active anode material (Ca) to the main ingredient (CaF2) of the electrolyte material during discharge is fortuitous in that the accumulation of this material facilitates further discharge, unlike in most other electrochemical power cells, wherein accumulation of discharge products hinders further discharge. Ideally, the anode would be fabricated as a Ca alloy containing approximately 5 mole percent of Na to form the desired NaF dopant for the CaF2 electrolyte as the cell discharges. At 450 °C, this alloy would remain a solid solution.
Several NiF2/NaF:CaF2/Ca cells have been fabricated and tested. The figure presents results from one such test. For testing purposes, these cells have been treated as primary (nonrechargeable) cells, but it is possible that these cells are rechargeable. If further tests confirm that they are rechargeable, then some of the cost and risk associated with manufacture and use of high-temperature batteries could be reduced: Before being installed for use, batteries could be heated to operating temperatures; charged and discharged several times to verify that their voltages, capacities, and discharge-rate capabilities are as expected; then recharged; and finally cooled. In contrast, the voltages, capacities, and discharge-rate capabilities of nonrechargeable batteries cannot be verified prior to final use.
This work was done by William West, Jay Whitacre, and Linda Del Castillo of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-44643
This Brief includes a Technical Support Package (TSP).
NiF2/NaF:CaF2/Ca Solid-State High- Temperature Battery Cells
(reference NPO-44643) is currently available for download from the TSP library.
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Overview
The document discusses the challenges and advancements in battery technology for Venus lander missions, focusing on the development of a novel solid-state fluoride battery. The extreme conditions on Venus, with surface temperatures around 450 °C and high atmospheric pressure, pose significant challenges for power systems. Traditional power generation methods, such as photovoltaic cells, are not viable due to the thick sulfuric acid cloud cover, necessitating alternative solutions.
Several battery technologies have been considered for use in Venus missions, each with limitations. Sodium-sulfur batteries, while designed for high temperatures, have fragile components and specific energies around 170 Wh/kg. Lithium-iron disulfide thermal batteries can operate at Venus temperatures but are limited to short durations and have issues with cathode dissolution. Conventional lithium-sulfur dioxide batteries, although having a flight heritage and specific energies of about 260 Wh/kg, require sub-ambient temperatures for operation.
In contrast, the solid-state fluoride battery proposed in the document offers a promising solution. This battery can operate at Venus surface temperatures without the need for cooling, thanks to its solid electrolyte and electrodes, which enhance safety and robustness. The design eliminates the complexities associated with thermal management systems and conventional thermal battery chemistries, resulting in a more compact, cost-effective, and flexible power source.
The document includes a comparison of the fluoride battery with lithium-sulfur dioxide batteries, highlighting its theoretical open circuit voltage of 2.93V and a theoretical specific capacity of 554 mAh/g, which is superior to the 419 mAh/g of lithium-sulfur dioxide batteries. The projected specific energy for the fluoride battery is estimated to be between 280-400 Wh/kg, making it a competitive option for long-duration missions.
Additionally, the document outlines the potential for the fluoride battery to be rechargeable, which could significantly reduce risks associated with flight battery hardware. This capability would allow for multiple charge and discharge cycles, enabling validation of key performance metrics.
Overall, the solid-state fluoride battery represents a significant advancement in energy storage technology for extreme environments, paving the way for future exploration of Venus and potentially other celestial bodies.
