High-energy-density primary (nonrechargeable) electrochemical cells capable of relatively high discharge currents at temperatures as low as –40 °C have been developed through modification of the chemistry of commercial Li/CFx cells and batteries. The commercial Li/CFx units are not suitable for high-current and low-temperature applications because they are current limited and their maximum discharge rates decrease with decreasing temperature.
The term “Li/CFx” refers to an anode made of lithium and a cathode made of a fluorinated carbonaceous material (typically graphite). In commercial cells, x typically ranges from 1.05 to 1.1. This cell composition makes it possible to attain specific energies up to 800 Wh/kg, but in order to prevent cell polarization and the consequent large loss of cell capacity, it is typically necessary to keep discharge currents below C/50 (where C is numerically equal to the current that, flowing during a charge or discharge time of one hour, would integrate to the nominal charge or discharge capacity of a cell). This limitation has been attributed to the low electronic conductivity of CFx for x ≈ 1. To some extent, the limitation might be overcome by making cathodes thinner, and some battery manufacturers have obtained promising results using thin cathode structures in spiral configurations.
The present approach includes not only making cathodes relatively thin [≈2 mils (≈0.051 mm)] but also using subfluorinated CFx cathode materials (x <1) in conjunction with electrolytes formulated for use at low temperatures. The reason for choosing sub-fluorinated CFx cathode materials is that their electronic conductivities are high, relative to those for which x >1. It was known from recent prior research that cells containing subfluorinated CFx cathodes (x between 0.33 and 0.66) are capable of retaining substantial portions of their nominal low-current specific energies when discharged at rates as high as 5C at room temperature. However, until experimental cells were fabricated following the present approach and tested, it was not known whether or to what extent lowtemperature performance would be improved.
For the experimental cells, cathodes were fabricated by spray deposition of multiple layers of cathode mixtures onto roughened 1-mil (≈0.025-mm)-thick aluminum- foil current collectors. Each cathode mixture consisted of a CFx powder and carbon black suspended in a binder/solvent solution of poly(vinylidene fluoride) in N-methyl-2-pyrrolidinone. For some of the cells, the CFx was sub-fluorinated by various amounts (x = 0.53 or x = 0.65). For other cells, used as controls, a fully fluorinated industrial CFx (x = 1.08) was used.
Each resulting cathode structure, 1 to 3 mils (about 0.025 to 0.076 mm) thick, was vacuum furnace dried, then incorporated into a standard coin cell case along with a separator, lithium foil anode, and an electrolyte consisting of LiBF4 dissolved at a concentration if 0.5 M in an 80/20 DME/PC (dimethoxy ethane/propylene carbonate) solvent mixture. The cells were tested in galvanostatic discharges at room temperature and –40 °C at currents from 2C to C/40. The fully fluorinated and sub-fluorinated cells performed comparably at rates as high as 2C at room temperature. At –40 °C, the sub-fluorinated cells exhibited approximately 3 times the specific capacities of the fully fluorinated cells when discharged at C/10 and C/5 discharge rates (see figure).
This work was done by Jay Whitacre, Ratnakumar Bugga, Marshall Smart, G. Prakash, and Rachid Yazami 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 Electronics/ Computers category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-43219, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
High-Energy-Density, Low-Temperature Li/CFx Primary Cells
(reference NPO-43219) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically focused on High-Energy-Density, Low-Temperature Lithium/Carbon Fluoride (Li/CFx) primary cells, referenced as NPO-43219 in NASA Tech Briefs. It aims to disseminate findings from aerospace-related developments that have broader technological, scientific, or commercial implications.
The primary focus of the document is on the advancements in battery technology, particularly the performance of Li/CFx cells under extreme conditions. These batteries are designed to operate efficiently at low temperatures, which is crucial for aerospace applications where environmental conditions can be harsh. The document highlights the potential of sub-fluorinated CFx materials, which have shown promising results in maintaining high energy density and capacity even at temperatures as low as -40˚C.
The Technical Support Package outlines the benefits of these batteries, including their ability to support high current outputs while retaining significant capacity. This characteristic is particularly important for applications that require reliable power sources in extreme environments, such as space missions or remote operations.
Additionally, the document provides contact information for further inquiries, directing interested parties to the Innovative Technology Assets Management at JPL (Jet Propulsion Laboratory) for additional research and technology information related to this area. It emphasizes that the information is provided under the Commercial Technology Program of NASA, aiming to foster innovation and partnerships in technology development.
The document also includes various figures (Figures 1-14) that likely illustrate key data, experimental results, or design schematics related to the Li/CFx cells, although specific details about these figures are not provided in the text.
In summary, this Technical Support Package serves as a resource for understanding the advancements in Li/CFx battery technology, particularly its application in low-temperature environments, and encourages collaboration and further exploration in this innovative field. It underscores NASA's commitment to sharing technological advancements that can have wider applications beyond aerospace, contributing to the development of more efficient and reliable energy storage solutions.
