Performance Evaluation of Commercial Carbon Monofluoride Materials in Lithium Batteries

CF_x particle size affects battery performance.

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

A variety of commercially available CF_x 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% (CF_0.95–CF_1.15). These CF_x 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.

CF_x 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 CF_x, 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% CF_x, 10 wt% carbon black, and 15 wt% PVDF. Cathodes were dried under vacuum at 105 °C for two hours before use.

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.