Page 409 - Lindens Handbook of Batteries

14.74 PriMAry BATTerieS

and an energy density of 552 Wh/L, both figures being 67% of the values obtained at 21°C. These
results indicate that the hybrid cathode cells provide superior performance to cells using manganese
dioxide alone, particularly at low temperature.
Mixtures of carbon monofluoride and silver vanadium oxide have also been employed for bio-
medical applications. See Sec. 31.5.6.
Subfluorinated and Semi-Ionic Carbon Fluoride Materials. recent studies have shown that sub-
fluorinated carbon fluorides (SFCFs) provide enhanced performance at low temperatures to -40°C.
49
An initial study, compared partially fluorinated natural graphites with x values of 0.33. 0.48.
50
0.52, 0.63 to a commercial CFx with x = 1.08. Structural studies showed that the SFCF material
consisted of domains of fluorinated carbon intimately mixed with the graphite precursor particles.
The partially fluorinated CFx materials exhibit higher power capability at room temperature and
superior low-temperature performance compared to CF 1.08 . Figure 14.72 shows discharge curves at
room temperature for a Li/CFx coin cell with x = 0.52 at different rates to 2.5C. The cathode mix
contained 80% CFx, 10% acetylene black, and 10% binder and was used with a 1.2 M LiBF in PC/
4
49
DMe (7/3) electrolyte. A ragone plot shows better high-rate power capability above 6.4 kW/kg for
the SFCFs compared to the standard commercial cells. Figure 14.73 shows discharge curves Li/CFx
coin cells with x = 0.65 at -40°C with and without a 3% predischarge at a C/40 rate. These cells used
an electrolyte of 1 M LiBF in PC/DMe (20/80). Predischarged cells with CF 0.65 provided a specific
4
capacity of 610 mAh/g to a 1.5 V cutoff compared to 200 mAh/g for control cells with CF 1.08 tested
under the same conditions.
51
Another study to optimize the electrolyte composition for SFCFs found that 0.5 M LiBF
4
in PC/DMe (20/80) provided improved low-temperature performance compared to higher salt
concentrations and eliminated the need for the predischarge step prior to low-temperature
discharge.
52
The use of CFx materials with semi-ionic character prepared in a two-step fluorination process
and SFCF materials prepared from multiwalled carbon nanotubes (MWCNs) were also found to
have superior properties in terms of rate capability and low-temperature performance compared to
conventional CFx materials.
3.5
a: 2.5C b: 2.0C c: 1.5C
d: 1.0C e: 0.5C f: 0.2C
g: 0.1C h: 0.05C i: 0.01C
3

2.5
Voltage, V 2

1.5
abcd e fg h i

1
0 100 200 300 400 500 600 700
Specific discharge capacity, mAh/g
FIGURE 14.72 Discharge profiles for Li/CF 0.52 coin cells discharged at differ-
ent rates.