Comparison of the Different Anode Technologies Used in Thermal Batteries J. Douglass Briscoe*, Emmanuel Durliat**, Florence Salver-Disma**, Ian Stewart***

*ASB Group – Advanced Thermal Batteries

107 Beaver Court, Cockeysville, MD 21030, USA **ASB Group – Aérospatiale Batteries (ASB)

Allée Sainte Hélène, 18021 Bourges cedex, France Fax: +33 2 4848 5601 Phone: +33 2 4848 5639 Email: e.durliat@asb-group.com

*** ASB Group – Missiles and Space Batteries (MSB) Hagmill Road, East Shawhead, Coatbridge, ML5 4UZ, UK

Abstract: This paper presents a comparison of three anode technologies: LAN (lithium anode), lithium aluminum and lithium silicon against iron disulfide and one of a proprietary metal disulfide compound MS2. LAN comes out as the best anode technology in terms of voltage, energy and power densities, internal resistance and polarization. It is also very safe against overheating. Used with our MS2, LAN appears as particularly suitable for high power/high energy applications.

Keywords: Thermal batteries, LAN, Alloy, Comparison

Introduction LAN (Lithium Anode) was compared to lithium aluminum and lithium silicon in battery tests under the same requirements:

- capacity of 3600A-s, - fixed volume for the stack : 84 cubic cm with a

cell diameter of 53mm. In a first phase, the anodes were evaluated with iron disulfide as cathode. Iron disulfide is a standard cathode, which is inexpensive and easily available. In a second phase, the best anode was evaluated versus improved cathodes. This study aimed at complementing existing studies at single cell test level by taking into account the thermal effect (side thermal insulation and end heats). It enables for example to assess and to compare the energy/power volumetric density of the three anodes.

Experimental set-up Battery and cell definition All batteries were made using the can, header, lead assemblies, lateral thermal insulation and end heats of an existing battery. The volume available for the stack was thus a cylinder with a diameter of 53mm and a length of 33.6mm. For each cell type, as many cells as possible were fitted in the battery. Three kinds of anodes were characterized: two alloys: LiAl (with 19% weight of Li) and LiSi

(with 44% weight of Li),

lithium metal mixed in a metallic matrix: LAN supplied by MSB (with 15% weight of Li).

The electrolyte consists of binary salt with magnesia binder. The cathode was either iron disulfide (FeS2) (so called “cathode 1” studied in part 1) or our proprietary metal disulfide compound (MS2) (so called “cathode 3” studied in part 2) mixed with salts. The heat powder for the heat pellets was adapted to the heat sensitivity of each anode. LAN batteries as shown below can be designed with higher heat input. Lithium alloy cells were designed using a 0.1mm thick iron separator. In the case of LiSi anodes, the capacity was determined taking into account the first and second plateaus only, and assuming a capacity of 2000A-s/g. For the study with FeS2 as cathode, all cells had the same adiabatic temperature evaluated using our proprietary model (575°C @ +60°C), the same cathode, the same ratio {thickness of electrolyte over (thickness of anode + thickness of cathode)} and the same coulombic capacity for the anode. Current loads Three different baseline currents were used: 2.2A, 4.4A and 13.2A giving current densities of 0.1A, 0.2A and 0.6A/cm2. The test loads consisted in one of these baseline currents plus pulses of twice the baseline intensity during 100ms every 50s. Discharge Batteries were discharged after conditioning at the test temperature during at least 4 hours. Voltage and current were monitored.

Part 1: Results with standard FeS2 as cathode Results Their overall thickness was 2.98mm with LiAl, 3.19mm with LiSi and 2.40mm with LAN. In the volume available, it was possible to fit stacks with either 11 LiAl cells, 10 LiSi cells or 14 LAN cells. The following figures present the voltage vs. time at -32 (cell @ 490°C) and +60°C (cell @ 575°C) (red and orange curves for LAN, green curves for LiAl and blue curves for LiSi).

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LiAl @ 490°C 2.2LiAl @ 575°C 2.2ALiSi @ 490°C 2.2ALiSi @ 575°C 2.2ALAN @ 490°C 2.2ALAN @ 575°C 2.2A

Fig. 1 discharge @ 2.2A, 490 and 575°C

Note that the hot LiSi discharge ended at 1000s, thus explaining the sudden tail end

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LiAl @ 575°C 4.4ALiSi @ 575°C 4.4ALAN @ 575°C 4.4A

Fig. 2 discharge @ 4.4A, 575°C

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LiAl @ 575°C 13.2ALiSi @ 575°C 13.2ALAN @ 590°C 13.2A

Fig. 3 discharge @ 13.2A, 575°C

Remark 1: the hot LiSi batteries showed some overheating after post-mortem (Fig. 2 and 3). That explains why the FeS2 transition from the 1st to the 2nd plateau appears earlier for LiSi batteries than for the other ones, due to increased self-discharge of the cells. Remark 2: Figures 2 and 3 show that the capacity of the LiSi anode is more of the order of 4000A-s than the estimated 3600A-s, if both the 1st and 2nd plateaus are considered. Remark 3: in Fig.1, the cold batteries are cooling down. The performance is thermally limited.

Analysis The LAN anode technology gives by far the best performance in terms of voltage in a given battery volume. This is due to the higher cell electromotive force, the higher weight density and the lower polarization, which is estimated by linear regression of the 2.2A discharge curves in the time frame 80 to 500s and in cold conditions as:

LiAl -0.14 mV/A-s LiSi -0.11 mV/A-s LAN -0.08 mV/A-s

In the 2.2A discharge, the energy and average power (till voltage drop to 75% of the initial maximum voltage) are:

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Energy (kJ)Power (W)

Assuming we could take the same double layer (electrolyte + cathode) for all kinds of anodes, one could integrate then 12 alloy cells (either LiAl or LiSi). In this case LAN still remains the best anode followed by LiSi and then LiAl. In terms of internal resistance, LAN gives also the best results: at 600s, R=9.0mΩ for LAN, 9.3mΩ for LiSi and 11.6mΩ for LiAl. Furthermore, LAN is an anode technology, which is tolerant of a high cell temperature. It is thus a much safer technology than the alloys. The following figure presents two discharges in the same test conditions with a cell temperature of 640°C.

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LAN @ 640°C 4.4A

LAN @ 640°C 4.4A

Fig. 4 LAN cell @ 4.4A, 640°C

Such a hot cell temperature is not possible for alloy based cells, which would fail in thermal runaway. On the example, we can nevertheless see that the LAN cell life is shortened by the thermal decomposition of the cathode. This can be mitigated by improving the cathode.

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Part 2: Results with improved cathodes Battery and cell definition In this part, we used the same battery design as previously. The anode was LAN with a capacity of 3600A-s. We only changed the cathode. Two cathodes were investigated: - One cathode (so called “cathode 2”) made with the

same FeS2 based cathode but with a chemical protector inserted between the cathode and the heat pellet; this weight of FeS2 was similar to the weight of the cathode investigated in the previous part. Thus cathode 2 had the same capacity as cathode 1 but was slightly thicker,

- One cathode (so called “cathode 3”) made with a metal sulfide compound MS2. In this case, we also protected the cathode from the heat pellet by insertion of a chemical protector. To keep things comparable, we designed the cathode so that the total thickness of cathode + protector is equal to the thickness of cathode 1 as evaluated in the first part. Thus cathode 3 had the same thickness as cathode 1 and cathode 2 but 25% less capacity.

Cathode 1 Cathode 2 Cathode 3 Cathode FeS2 FeS2 +

Protector MS2 + Protector

Thickness t t + 0.13mm

t

Capacity C C 0.75 C

Table 1: Summary of cathode definitions

“Cathode 3” was tested at 4 different cell temperatures: 575, 590, 600 and 640°C. As a comparison, “cathode 1” and “cathode 2” were tested at 575°C. Batteries were built with these cathodes and with 14 cells each. Considering “cathode 2”, fitting 14 cells was only possible by removing 2mm of packing. Current loads Two different baseline currents were used: 2.2A and 4.4A giving current densities of 0.1A/cm2 and 0.2A/cm2. The test loads consisted in one of these baseline currents plus pulses of twice the baseline intensity during 100ms every 50s. Discharge Batteries were discharged after conditioning at the test temperature for at least 4 hours. Voltage and current were monitored. Results Figure 5 presents results of MS2 and FeS2 (with or without chemical protector) in a 2.2A discharge.

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Fig. 5 LAN vs. MS2 and FeS2 @ 2.2A

Figure 6 presents results obtained at 4.4A.

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Fig. 6 LAN vs. MS2 and FeS2 @ 4.4A

Figure 7 presents results obtained with MS2 at high cell temperatures.

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MS2 @ 640°CMS2 @ 600°CMS2 @ 590°C

Fig. 7 LAN / MS2 at various cell temperatures

Analysis The chemical protection of a FeS2 cathode against the heat pellet increases the performance of the battery by a factor of about 2 in a very low discharge rate (See Fig. 5). The slope of the discharge curve increases drastically at 400s for the cathode without protector (red curve) whereas the slope of the discharge curve of the cathode with protection (purple curve) remains unchanged till about 800s. For

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shorter discharges (Fig. 6), the protection of the cathode does not bring any advantage. The use of MS2 also enables the internal resistance per cell to be reduced by about 25%, as shown on the following table.

Internal Resistance per cell

(mΩ) @ 575°C

time (s) FeS2 +

Protector MS2 +

Protector 200 4.5 3.6 400 5.5 4.1 500 7.5 5.0 600 8.9 5.9

The use of LAN/MS2 is thus of high interest for high power density applications. The polarization is also much smaller with LAN/MS2 (about -0.038mV/A-s) than with LAN/FeS2+Protector (about -0.090mV/A-s). Furthermore, figure 6 shows that as the current density increases (from 0.1A/cm2 on Fig. 5 to 0.2A/cm2 on Fig. 6), MS2 shows increasing cell efficiency compared to FeS2. Indeed, figure 6 shows that the performance of both FeS2+protector and MS2 are quite the same although the MS2 “cathode 3” had about 25% less capacity than the protected FeS2 “cathode 2”. The use of MS2 thus increases the efficiency of LAN by about 30%. Figures 8 and 9 highlight the higher efficiency of MS2 by presenting the voltage as a function of depth of discharge (DoD) in the cathode. They clearly show that MS2 has an advantage over FeS2 in terms of efficiency.

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Fig. 8 voltage vs. DoD in the cathode (2.2A discharge

@ +575°C)

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Fig. 9 voltage vs. DoD in the cathode (4.4A discharge

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The use of LAN/MS2 is thus of high interest for high energy density applications. The results with a cell temperature of 640°C show that the performances of LAN/MS2 are very stable on a wide range of cell temperature. The use of LAN/MS2 enables to design much safer batteries than the use of alloys, and especially LiSi. It is also particularly suitable for the design of high duration/high energy batteries. Nevertheless in a wide range of applications, iron disulfide remains a good compromise between cost and performance; additionally, its performance can be further improved by protecting it from the heat pellet for life duration above 400s.

Conclusion We have compared the LAN technology to lithium aluminum and lithium silicon. We have shown that: - LAN is the best technology in terms of voltage per

cell, internal resistance and polarization. It is furthermore a very safe technology due to its robustness to overheating.

- Its performance can be optimized by coupling LAN to an iron disulfide cathode with protection from the heat pellet (life duration > 400s), or to our proprietary metal disulfide compound (life duration > 800s).

FeS2 as shown in figures 5 and 6 gives very good performance up to 600 to 800s duration, and is a very good compromise between cost and performance. MS2 is of interest for very long life batteries or special very high power applications. Using all technologies and specially LAN, the ASB Group (ASB, MSB, ATB) is able to cope with the very wide range of customer requirements by designing reliable and safe thermal batteries.

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