Microcalorimetry of Li/CFx Cells and Discharge Mechanism

by Jeffrey Read, Donald Foster, Jeffrey Wolfenstine,

and Sheng Zhang

ARL-TR-5043 December 2009

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Army Research Laboratory

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ARL-TR-5043 December 2009

Microcalorimetry of Li/CFx Cells and Discharge Mechanism

Jeffrey Read, Donald Foster, Jeffrey Wolfenstine,

and Sheng Zhang

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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4. TITLE AND SUBTITLE

Microcalorimetry of Li/CFx Cells and Discharge Mechanism

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Jeffrey Read, Donald Foster, Jeffrey Wolfenstine, and Sheng Zhang

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U.S. Army Research Laboratory

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The Li-CFx chemistry has been traditionally used for low rate applications at discharge rates of C/1000 or lower. The Army is 

supporting efforts to engineer Li/CFx batteries for Soldier applications that operate at higher rates, closer to C/10. This effort is 

based on the favorable specific energy and storage characteristics offered by this chemistry. Heat generation under high rate 

discharge is one potential concern in these batteries and, therefore, further study of the mechanism of heat generation and the 

differences in heat generation between various CFx materials is warranted. We demonstrate that the thermal power generated 

when a Li-CFx cell is discharged appears nearly instantaneously in the cell and ceases nearly as quickly when discharge is 

interrupted. Extrapolation of the running potential to zero heat results in a predicted voltage for the Li/CFx cell of 4.25 V, very 

near the theoretical potential calculated by Wood, et al. (1). The microcalorimetry results support the two phase model for 

discharge where a large overpotential, presumably due to the highly disfavored process of breaking covalent C-F bonds, 

dominates the OCV and running potential of the cell.

15. SUBJECT TERMS

CFx, battery, calorimetry

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Jeffrey Read

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iii

Contents

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experiment 1

3. Results and Discussion 2

4. Summary and Conclusions 7

5. References 9

Distribution List 10
iv

List of Figures

Figure 1. Voltage versus specific capacity of Li-CFx cells at four discharge rates. ........................3

Figure 2. Li-CFx cell voltage and thermal power (mW) versus time for 20 mA/g discharge 

rate..............................................................................................................................................4

Figure 3. Overlay of thermal power during each discharge pulse for the 20 mA/g discharge 

rate..............................................................................................................................................4

Figure 4. W/kg of active CFx material at 40, 20, 10, and 5 mA/g. ..................................................5

Figure 5. Correlation between running voltage and heat output in KJ/mol e–.................................6

Figure 6. Voltage versus specific capacity and J/mAh. Comparison of ARC 1000, ARC 2065, 

and ARC 4000 at 40 mA/g.........................................................................................................7

List of Tables

Table 1. Physical and chemical properties of CFx materials tested. ................................................2

Table 2. Summary of values from calorimetry and discharge data. ................................................5
1

1. Introduction

Li-CFx batteries are proposed for use in Soldier applications where a high specific energy 

primary power source is needed. Li-CFx D-cells have demonstrated the rate capability necessary 

for Soldier applications (2, 3, 4, 5). One problem that needs to be addressed is the heat generated 

on discharge, which can be significant at higher discharge rates (3, 4). Microcalorimetry on LiCFx cells (6) has been carried out, but a more detailed examination is warranted.

Li + CF1.0 → C + LiF Eo = 4.57V (1)

The overall discharge reaction of a Li-CFx cell is shown in equation 1. The Eo value is given by 

Wood et. al. (1). for a CF1.0 material. The open circuit voltage (OCV) of a Li-CFx cell is 

normally 3.1–3.6 V, significantly lower than the theoretical value above. This difference between 

theoretical potential and the observed OCV has been ascribed to the formation of ternary phases

(7, 8) such as CLixF or CFLix:Sy where S is a solvent molecule that is co-intercalated with a 

lithium ion. It is proposed that the kinetics for formation of an intercalated phase is more 

favorable than the kinetics of the direct two phase reaction. It is also proposed that the resulting 

intercalated phase decomposes over time to produce the final products carbon and LiF. The 

significant amount of heat generated by a Li-CFx cell would therefore be expected to follow the 

kinetics of this decomposition reaction. One might also assume that this reaction is slow enough 

to observe using microcalorimetry.

This work investigates the thermal powder behavior of Li-CFx cells under intermittent discharge 

to try and observe the kinetics of the decomposition reaction. The thermal power behavior was 

also studied as a function of depth of discharge and rate of discharge with three different CFx

materials. The commercially available CFx materials evaluated in this work were prepared by 

high temperature fluorination of coke, carbon black, and carbon fiber precursors. The chemical 

compositions were similar with total fluoride contents in the range of 62.5-64.5 wt% (CF1.05 –

CF1.15). The materials have different physical properties such as true density, particle size, 

surface area, and decomposition temperature.

2. Experiment

CFx materials were obtained from Advanced Research Chemical and were designated ARC 1000 

(coke), ARC 2065 (carbon black), and ARC 4000 (carbon fiber). Table 1 gives the physical and 

chemical properties of the materials. Cathodes were prepared by mixing CFx, PVDF, carbon 

black, dibutylphthalate, and acetone in a stainless steel blender cup. The slurry was cast on glass 

and dried in air. The plasticized films were laminated to treated aluminum grids and then 
2

extracted in methanol. The extracted cathodes were 75% CFx, 10% carbon black, and 15% 

PVDF. Cathodes were dried under vacuum at 105 °C for 2 hr before use. Cells were constructed

from coin cell hardware using 0.020 in lithium, 2 layers of Celgard separator, and 1M LiBF4

propylene carbonate: 1,2-dimethoxyethane electrolyte. Cells were crimp sealed, the impedance 

was measured, and cells were placed in an aluminum cell holder that provides improved heat 

transfer from the cell to the microcalorimeter. The aluminum holder was electrically insulated 

from the metal microcalorimeter chamber using several thin sheets of mica and non-conductive 

heat sink compound. The cell holder was at the bottom of a battery testing insert that fits into the 

microcalorimeter and that has 3 thermal breaks and a thin channel used to get leads from the 

microcalorimeter to the Maccor Series 4000 tester. The microcalorimeter is a Calorimetry 

Sciences Corporation Model 4400 Isothermal Microcalorimeter operating at a nominal 25 °C. 

The practical resolution of this microcalorimeter was ±10 μW and this limited the low end of the 

discharge rate regime chosen. The cells were allowed to equilibrate for 12 hr in the 

microcalorimeter and then they were discharged approximately 10% of their full capacity, at 

rates between 5 and 40 mA/g, and then allowed to rest at open circuit for 3 hr before the next 

discharge. Cells were discharged multiple times until all of the capacity was removed or the 

voltage dropped below 2.0 V.

Table 1. Physical and chemical properties of CFx materials tested.

Property ARC 1000 ARC 2065 ARC 4000

Carbon Source Petroleum Coke Carbon Black Carbon Fiber

Total Fluoride % 62.6 64–65 63.5

Color Light Gray White Light Gray

Median Particle Size, mm 8 < 1 6

BET Surface Area, m2/g 139 350 323

Micropore Surface Area, m2/g 78 82 92

External Surface Area, m2/g 61 268 230

Decomposition Temp., oC 672 660 659

3. Results and Discussion

Figure 1 shows discharge curves for the coke based CFx material at four different rates. The 

spikes in the voltage are the 3 hr rest periods between each 10% discharge. The results show a 

material that has a small but significant change (130 mV) in running potential as a function of a 

large change in discharge rate (5–40 mA/g). The electronic resistance of the cell was calculated 

from the 10 ms voltage change that occurs before and after each rest period. The voltage drops 

were usually 10–20 mV which does not account for the 130 mV shift seen. This indicates that 

although there is some effect on cell polarization due to the strictly electrical resistances of the 
3

cell, another factor is leading to the larger polarization seen between the OCV at 3.1 V and the 

running potential at 2.5–2.6 V.

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

0 100 200 300 400 500 600 700 800

Specific capacity (mAh/g)

Voltage / V

5mA/g

10 mA/g

20mA/g

40mA/g

Figure 1. Voltage versus specific capacity of Li-CFx cells at four discharge rates.

Figure 2 shows the discharge voltage and microcalorimeter response as a function of time for a 

cell discharged at 20 mA/g. In this figure, the 3 hr rest periods are obvious. The figure shows that 

the thermal power generated by discharge mirrors the electrical power delivered by the cell 

almost exactly. The data in figure 2 also allows for the calculation of the I2R heat from the 10 ms

voltage drops. The I2

R heat averages 28 μW, only about 1% of the total thermal power output 

from the cell. Figure 3 shows an overlay of the thermal power curves for each successive pulse 

from the data in figure 2, with the first and last pulse left out for clarity. The data shows that heat 

generation begins immediately upon discharge and reaches 98% of its steady state value within 

20 min. The heat generation ceases immediately when discharge is stopped, with 98% of the 

steady state value being reached within 25 min. This indicates that if a ternary intercalation 

phase forms, it decomposes extremely rapidly. These 20 min lag times are consistent with the 

measured time constant of the microcalorimeter itself and, therefore, are not an indication that an 

intercalated phase persists. The time for the cell potential to reach a steady state value is 5 min

on discharge. After discharge ceases, the cell does not reach a steady state open circuit voltage

even after 3 hr. It appears that the open circuit potential and the heat generation are not 

correlated.
4

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

0 20 40 60 80

Time (Hours)

Voltage/V

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

Thermal Power (mW)

Figure 2. Li-CFx cell voltage and thermal power (mW) versus time for 20 mA/g discharge rate.

0.0

0.5

1.0

1.5

2.0

2.5

0 50 100 150 200 250 300

Time (minutes)

Thermal Power (mW)

Figure 3. Overlay of thermal power during each discharge pulse for the 20 mA/g discharge rate.
5

One interesting aspect of the data is how the thermal power is nearly independent of discharge 

state. Figure 4 shows the thermal power for each discharge step normalized to the weight of CFx

in the cathode. One can see small changes in the thermal power output at the beginning and the 

end of discharge that are reflected in the cell polarization, but otherwise it is constant. This 

constancy points to a 2 phase reaction mechanism for discharge where no significant amount of 

intermediate phase forms. 

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10

Step #

W/Kg of Active CFx

40 mA/g

20 mA/g

10 mA/g

5 mA/g

Figure 4. W/kg of active CFx material at 40, 20, 10, and 5 mA/g.

Table 2 summarizes the discharge data and also gives values for quantities calculated from the 

data. The running voltage is taken from the curves in figure 1 and represents the highest point on 

the discharge curve. The heat in J/mAh is calculated by integrating the thermal power curves 

over time. The theoretical OCV is obtained by first calculating ∆G from the heat in KJ/mol e

–

and T∆S, where ∆S is taken from Wood, et al. (1) for the reaction of Li and CF1.0. This value is 

converted into a potential using ∆G = nFE and added to the running potential and the 10 ms IR 

drop to get a theoretical OCV. These OCV values agree well with the values of Wood et.al. (1).

Table 2. Summary of values from calorimetry and discharge data.

Rate (mA/g) 40 20 10 10 5

Average J/mAh 7.08 6.89 6.68 6.74 6.55

Heat (KJ/mol e–) 186.7 178.7 178.7 179.1 174.8

Running Voltage 2.479 2.574 2.574 2.566 2.608

Theoretical OCV 4.470 4.466 4.466 4.467 4.463
6

As stated before, the change in cell polarization with discharge rate is not related purely to the 

electrical resistances in the cell, as this can account for only 20 mV of polarization even at 

40 mA/g. Most of the cell polarization is related to the overpotential associated with the 

discharge reaction. When the change in heat output between different discharge rates is 

converted to a voltage using ∆G = nFE, the voltage drop seen between cells at different rates can 

be accurately predicted. In addition, when the heat in KJ/mol e

– is plotted against running 

potential, one obtains figure 5. The data falls on a straight line that, when extrapolated back to 

zero heat, results in the theoretical OCV for the Li/CFx system of 4.2 V. This result would mean 

that both the running potential and the OCV are the result of a large activation barrier for 

discharge, and that the polarization seen as a function of discharge rate is simply an extension of 

this.

CFX ARC1000 

y = -0.0091x + 4.2037

R2

 = 0.9953

2.50

2.52

2.54

2.56

2.58

2.60

2.62

2.64

174 176 178 180 182 184 186 188

Heat (KJ/mol e-)

IR Corrected Running Voltage

The data is an average from all steps

• Running voltage is IR corrected

• Heat is corrected for I2

R. 

Figure 5. Correlation between running voltage and heat output in KJ/mol e

–

.

The other CFx materials, ARC 2065 and ARC 4000 were tested under conditions identical to the 

ARC 1000 material with the results for the 40 mA/g rate shown in figure 6. The ARC 2065 

material shows less polarization on average than the ARC 1000 material with lower Joules/mAh. 

The ARC 4000 material shows more polarization than the ARC 1000 material, but interestingly 

lower Joules/mAh as well. Plots of running voltage against heat in KJ/mol e

– give results similar 

to figure 5 with linear fits for both the ARC 2065 and ARC 4000 materials. The theoretical OCV 

by extrapolation for the ARC 2065 material was 4.32 V and for the ARC 4000 material was 4.20

V. The difference in thermal behavior between the three materials is relatively small, about 1–
7

2% depending on how it is calculated. A graph incorporating all of the data shows that the three 

materials occupy a range of heat/capacity values between 6.3 and 7.0 J/mAh depending on 

discharge rate with only one exception, the 5 mA/g ARC 2065 cell, falling outside this range. 

This result would be surprising if one assumes that the discharge reaction takes place through the 

formation of an intercalate, since it is expected that physical properties such as surface area and 

particle size should influence the intercalation kinetics. The result is more understandable if one 

assumes that discharge takes place through a two phase reaction, as this will depend mainly on 

the chemical properties of the materials.

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

0 200 400 600 800

Specific Capacity (mAh/g)

Voltage / V

6.5

7.0

7.5

8.0

8.5

Joules/mAh

CFx 1000

CFx 2065

CFx 4000

CFx 1000

CFx 2065

CFx 4000

Figure 6. Voltage versus specific capacity and J/mAh. Comparison of ARC 1000, ARC 2065, and ARC 

4000 at 40 mA/g. 

4. Summary and Conclusions

This study shows that the thermal power of Li-CFx cells is highly rate dependent, but nearly 

independent of the state of discharge. Intermittent discharge data shows how heat generation 

starts immediately when discharge begins and ceases almost as quickly when discharge is 

stopped. There is no significant tail in the thermal power curve that could be interpreted as the 

decomposition of an intercalation compound. Extrapolation of the running potential versus heat 

to a zero value results in an OCV of 4.25 V, very near the theoretical potential calculated by 

Wood et al. (1). The calorimetry results on three physically different CFx materials turn out to be 
8

nearly identical. This shows that physical properties, such as surface area and particle size, have 

almost no effect on the heat produced and that chemical composition is probably more important. 

The results of this study suggest that the discharge mechanism is simply a two phase reaction 

where the large activation energy needed to break C-F bonds results in significant overpotential 

on discharge.
9

5. References

1. Wood, J. L.; Valerga, A. J.; Badachhape, R. B.; Parks, G. D.; Kamarchik, P.; Margrave, J. L. 

Thermodynamic and Kinetic Data of Carbon-Fluorine Compounds; TR-ECOM-0056-F;

(1974); Wood, J. L.; Badachhape, R. B.; Lagow, R. J.; Margrave, J. L. Heat of Formation of 

Poly (Carbon Monofluoride). Journal of Physical Chemistry 1969, 73 (9), 3139.

2. Tan, T.; Tsukamoto, H. 200% Capacity Improvement of ARMY BA5590 Pack with Using 

Newly Developed D-size Li/CFx Batteries. Workshop on Carbon Fluorides for Lithium 

Batteries, Pasadena, CA, Feb. 2006.

3. Suszko, A.; Thompson, R.; Stevens, Diane. Performance of Lithium Carbon Monofluoride 

for Soldier Portable Power Sources. Workshop on Carbon Fluorides for Lithium Batteries, 

Pasadena, CA, Feb. 2006.

4. Sazhin, S. V.; Ramaswami, K.; Gurrie, T. J.; Niendorf, C. R.; Suszko, A. Development of 

Lithium Fluorinated Carbon System for Military Battery Packs, p.61, Proc. of the 42nd

Power Sources Conf., Philadelphia, PA, June 2006.

5. Tan, T.; Lam, P.; Tsukamoto, H.; Hendrickson, M.; Plichta, E. Li/(CF)n Battery for Low 

Temperature and High Power Application, p.73, Proc. of the 42nd Power Sources Conf., 

Philadelphia, PA, June 2006.

6. Rohde, D.; Root, M. J.; Foster, D. Li/CFx Cell and Material Development for High Rate 

Applications, p.350, Proc. of the 37th Power Sources Conf., Cherry Hill, NJ, June 1996.

7. Whittingham, M. S. J. Electrochem. Soc. 1975, 122 (4), 526.

8. Watanabe, N.; Hagiwara, R.; Nakajima, T.; Touhara, H.; Ueno, K. Electrochemica Acta

1982, 27 (11), 1615.
10

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