Improved Li/CFx Cells with Partial Reduction of CFx

Li/CFx batteries offer a very high specific capacity of 864 mAh/g and long shelf life. The high specific energy of the Li/CFx cell has led to its use in low-to-medium discharge applications; however, use at high rates is limited due to the presence of an initial voltage delay during discharge. This delay limits power delivery during pulse discharge, particularly at low operation temperatures. A number of potential engineering, and chemical remedies have been proposed to improve power delivery, including cell design and structural materials, low and high-temperature fluorination, carbon precursors, particle size and shape, electrolyte, and electrolyte additives. A key limitation of the Li/CFx battery is that the CFx is electrically insulating causing high internal impedance upon initial discharge. As the cell discharges, loss of fluorine leads to carbonized material. Addition of 10 wt% or more of conductive carbon (e.g. acetylene black, graphite, or carbon fiber) to the cathode reduces the initial cell resistance at the cost of lowered cell capacity due to less active material and formation of additional interstitial cavities between the added carbon particles. An alternate strategy is to produce a conductive carbon coating around the CFx particle using chemical vapor deposition.  More recent work has shown that use of sub-fluorinated (CFx)n – 0.33 < x < 0.63 – performs better than commercial CFx at high-rate discharges due to the higher conductivity of the sub-fluorinated material. The trade-off is a loss of capacity with lower fluorine content.

To improve discharge rates, we have developed a partial reduction pretreatment to produce a thin amorphous carbon coating on the CFx particle. CFx (x = 1.07) (Fluorstar PC-10) was partially reduced by reacting with a solvated electron solution in liquid ammonia resulting in rapid formation of a carbon film only on the surface of the CFx particle (Figure 1). The amount of reduction was determined from F^-released during reduction. The reduction extent was controllable to achieve an equivalent of 0.5 to 5 wt% of C to CFx (i.e. the % C considering both the reduced C and CFx). Upon reduction, CFx changes appearance and texture from a white, translucent, and free flowing powder to a black, slightly sticky material. Raman spectroscopy indicates that the carbon in the reduced CFx material is in an amorphous (i.e. non-graphitic) phase while IR indicates the loss of surface C-F bonding after reduction. Dry, bulk-powder resistance measurements indicates that this surface-reduced material is significantly more conductive than an equal C wt% mixture of unreduced CFx and graphite powder. For example, the resistivity of the 2.15 wt% surface-reduced material is comparable to that of a mixture composed of 7-10 wt% graphite mixed with unreduced CFx. These resistance measurements suggest that the reduction process produces a low resistance carbon shell around the CFx material.

During discharge testing, cathodes consisting solely of the reduced CFx (no binder or carbon additives) supported discharge rates that were equivalent to cells containing initially unreduced CFx but having 7× the amount of carbon added as graphite. An example discharge curve is shown in Figure 2 for a reduced CFx cathode (2.15 % reduced C content) in a cell with 1.0 M LiBF₄ electrolyte in 1:1 EC/DME. The 15.9 mg of cathode material (12.6 mg/cm²) supported a discharge rate of 173 mA/g (C/3.64) with a capacity of 639 mAh/g (to 2.0 V cutoff).  The effect of % reduction on the discharge rate and capacity was examined by galvanostatic discharge and impedance spectroscopy.  Pretreatment by solvated electron reduction produces a Li/CFx cathode material that eliminates or minimizes the need for added carbon or inert binders while producing superior discharge rates with high capacity.