Enhancement of discharge performance of Li/CFx cell by thermal treatment of CFx cathode material

In this work we demonstrate that the thermal treatment of CFx cathode material just below the decomposition temperature can enhance discharge performance of Li/CFx cells. The performance enhancement becomes more effective when heating a mixture of CFx and citric acid (CA) since CA serves as an extra carbon source. Discharge experiments show that the thermal treatment not only reduces initial voltage delay, but also raises discharge voltage. Whereas the measurement of powder impedance indicates the thermal treatment does not increase electronic conductivity of CFx material. Based on these facts, we propose that the thermal treatment results in a limited decomposition of CFx, which yields a subfluorinated carbon (CFx−δ), instead of a highly conductive carbon. In the case of CFx/AC mixture, the AC provides extra carbon that reacts with F2 and fluorocarbon radicals generated by the thermal decomposition of CFx to form subfluorinated carbon. The process of thermal treatment is studied by thermogravimetric analysis and X-ray diffraction, and the effect of treatment conditions such as heating temperature, heating time and CFx/CA ratio on the discharge performance of CFx cathode is discussed. As an example, a Li/CFx cell using CFx treated with CA at 500 °C under nitrogen for 2 h achieved theretical specific capacity when being discharged at C/5. Impedance analysis indicates that the enhanced performance is attributed to a significant reduction in the cell reaction resistance.