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CFx particle size affects battery performance.

The performance of six different commercially available carbon monofluoride (CFx) materials was evaluated at four different discharge rates. The physical and chemical properties of the CFx materials were correlated to cell performance in lithium coin cells. This study was undertaken to determine how the physical and chemical properties of commercial CFx materials affect discharge capacity, discharge voltage, rate capability, and voltage delay in Li-CFx cells.

A variety of commercially available CFx materials was evaluated in this work. The chemical compositions of the various materials are similar, with total fluoride contents in the range of 60–65 wt% (CF0.95–CF1.15). These CFx materials are prepared from different starting materials such as cokes, graphites, carbon fibers, and carbon blacks, and have different physical properties such as particle size, surface area, and decomposition temperature.

CFx powders from Advanced Re search Chemical (Catoosa, OK) and Lodestar (Howell, NJ) were used as received. The decomposition temperature for each material was measured on a Perkin Elmer TGA 7 at a heating rate of 5 °C/min under N2 atmosphere. Surface area measurements using N2 absorption were carried out on a Micromeritics ASAP 2010 after degassing samples at 250 °C. Particle morphology was characterized using optical microscopy.

Cathodes were prepared by mixing CFx, PVDF, carbon black, dibutylphthalate (DBP), and acetone in a stainless steel blender cup, and then this slurry was cast on glass and dried in the air. The plasticized films were laminated to treated aluminum grids and extracted in methanol to remove the DBP. The cathode composition after extraction was 75 wt% CFx, 10 wt% carbon black, and 15 wt% PVDF. Cathodes were dried under vacuum at 105 °C for two hours before use.

Optical microscope images of the various Carbon Monofluoride Powdersconfirm the particle size and morphology for the materials.
Cells were constructed from 2035 coin cell hardware using 0.020" thick lithium foil, two layers of Celgard separator, and 1 M LiBF4 propylene carbonate: 1,2–dimethoxy ethane (1:1 wt/wt) electrolyte. Cell impedance and OCV were measured for each cell before being placed on test. The cells were allowed to equilibrate for three hours at 20 °C before being discharged at rates of 5, 10, 20, and 40 mAh/g. The discharge was tailored to the cathode weight of each cell to insure that any difference in cathode density or thickness was minimized. Cells were discharged until all of the capacity was removed, or the voltage dropped below 2.0V.

In this study, it was found that most of the six different CFx materials give a discharge profile that can be interpreted as a two-phase discharge. The materials have similar specific capacities, except for ARC 3000, which has a discharge capacity that is 20% lower. This can be accounted for at least partially by the lower fluoride content. The main physical property that can be tied to performance is the particle size, with smaller particle sizes giving less over-potential and higher running voltage. The ability to produce a nano-sized CFx might be one approach to increase the running potential of Li/CFx cells.

This work was done by Jeffrey Read, Michelle Marx, Jeffrey Wolfenstine, Sheng Zhang, and Don Foster of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. ARL-0089

This Brief includes a Technical Support Package (TSP).

Performance Evaluation of Commercial Carbon Monofluoride Materials in Lithium Batteries (reference ARL-0089) is currently available for download from the TSP library.

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