PRIMARY BATTERIES – RESERVE SYSTEMS | Thermally Activated Batteries: Lithium

Thermally Activated Batteries: Lithium RA Guidotti, Sierra Nevada Consulting, Minden, NV, USA PJ Masset, Karl Winnacker Institut der Dechema e.V, Frankfurt am Main, Germany & 2009 Elsevier B.V. All rights reserved.

Introduction Thermally activated (thermal) batteries are primary batteries that use molten salts as electrolytes and employ an internal pyrotechnic (heat) source to bring the battery stack to operating temperatures. They are primarily used for military applications, such as missiles and ordnance, and in nuclear weapons. Once activated, they supply the current and the voltage required by a particular load. Thermal batteries are one of the electrical generators that provide the highest power density, which makes them ideal for pulse or short-term applications. Figure 1 compares the energy characteristics of thermal batteries with other types. The specific power increases with increasing energy density for thermal batteries. This is due to the high rate capabilities associated with the much higher electrolyte conductivities and elevated temperatures. The present level of development, however, allows a lifetime of 1 h. The previous article in this encyclopedia describes the older Ca/CaCrO4 technology. This article describes the later, improved technology based on Li and Li-alloy anodes and sulfide cathodes, predominantly FeS2 (pyrite). The earlier Ca/CaCrO4 technology suffered from large intrinsic variability in performance as well as low power and energy densities that are due to parasitic chemical reactions that occurred during discharge. The latter was due to the nature of the separator layer that

formed in situ upon activation. This layer consisting of a mixture of Cr(V) compounds varied in chemistry, depending on the discharge conditions, and was subject to breaching upon overheating of the battery, which could result in a thermal runaway as the battery destroyed itself. It was very difficult to design a battery with this chemistry owing to the poor understanding of the discharge processes and complex chemistry. Subsequently, an empirical approach was mostly used based on earlier experience. The difficulties of engineering a thermal battery based on the Ca/CaCrO4 electrochemistry were obviated with the introduction of Li and Li-alloy/FeS2 couples. These were clean systems with well-characterized and highly predictable and reproducible reactions, in contrast to the Ca/CaCrO4 system. The FeS2 was readily obtained by processing pyrite, which is in abundance and is an inexpensive material when compared with chemically synthesized CaCrO4. It was also ‘greener’ because it did not involve Cr(VI), a known carcinogen. Pyrite is a good semiconductor – both n and p types being reported – with electrical conductivities ranging from 0.03 to 333 S cm1 and an energy band gap of about 0.92 eV at room temperature. This makes it perfect for use in thermal batteries, as it has a higher electrical conductivity at elevated temperatures. This results in improved power and increased lifetime over the Ca/CaCrO4 system.

100 000 100 C Specific power (W kg−1)

10 000 Thermal batteries

1000

10 C

AgO/Zn Li/SO2

1C

Li/SOCl2

100

0.1 C

Zn/air Zn/MnO2

10

Li/MnO2 0.01 C

1 0

100

200 300 400 Specific energy (Wh kg−1)

500

600

Figure 1 Schematic Ragone plot of different electrical generators.

141

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Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

Despite all these benefits, there were several disadvantages inherent to the FeS2-based system. First, a true separator pellet was required, rather than the one formed chemically in situ as in the Ca/CaCrO4 system. This was necessary to prevent direct reaction of the anode with the cathode. Consequently, more piece parts were now required for the battery. The fumed silica used in the catholyte for the Ca/CaCrO4 cell for electrolyte immobilization was not compatible with the Li and Li-alloy anodes because of the greater reactivity of these materials. Magnesium oxide was used because it is thermodynamically compatible with such high-activity anodes. The ultimate separators used a MgO powder with unique pore-size distributions to immobilize the molten electrolyte during discharge; not just any MgO will work for this application. Second, the open-circuit voltage of the Li/FeS2 couple is lower than that of the Ca/CaCrO4 couple – a nominal 2 V versus 3 V. As a result, more cells were required for a given battery voltage and these cells had to be thinner than those for Ca/CaCrO4 batteries for the stack to fit into the same battery size (height).

w/o Li (Li13Si4). Although a material with a higher Li content (Li22Si5) would also work and provide a higher emf, it proved to be too reactive with atmospheric oxygen and moisture during processing in a dry-room environment (o3% relative humidity). The emf of the Li13Si4 phase is B140 mV higher than the LiAl phase at 415 1C (297 mV vs pure Li), which made it more desirable. The Li–Si anode ultimately replaced the Li–Al anode in most thermal batteries. Discharge reactions

The discharge sequence for the Li–Al anode involves the solid solution and LiAl that contains 20 w/o Li. Thus, only a single anode transition is possible: LiAl (b-Al) to Al (a-Al). The discharge reaction for nonstoichiometric LiAl is shown in eqn [I]: Li0:47 Al0:53 -Li0:0578 Al0:53 þ 0:411Liþ þ 0:411e

This corresponds to a capacity of 2259 A s g1. The discharge stages for the Li–Si alloy anodes are shown in eqn [II]: Li22 Si5 -Li13 Si4 -Li7 Si3 -Li12 Si7

Electrochemistry Anodes The bulk of the initial work with the sulfide-based systems used a Li–Al alloy containing 20 w/o Li, essentially LiAl in composition. This became the primary anode for all thermal batteries. A liquid anode (the so-called LAN anode) based on pure Li was later developed at Catalyst Research Corporation (CRC). This consisted of Li immobilized with a special Fe powder, typically 80 w/o, which held the molten Li in place during battery operation by capillary action. (This involves the same principle by which the molten electrolyte is immobilized in the separator layer by use of a special-grade MgO.) The use of pure Li had the advantage of increasing the electromotive force (emf) of a cell by B157 mV at 415 1C over that of the Li–Al anode while providing a much greater discharge current density because the Li is in a liquid form. However, under certain severe environments there was a tendency for the Li to dewet from the Fe. The high activity of the Li can lead to the formation of K metal and its subsequent vaporization owing to a displacement reaction with Kþ in the electrolyte. In addition, there was a need for additional piece parts (e.g., screen and cup) to fabricate the anode. A Li–Si anode was developed in the 1980s at Sandia National Laboratories (SNL) to replace the Li–Al one. This anode had a number of advantages over its Li–Al counterpart. There are a number of compounds that form within the Li–Si system, with the emf increasing with higher Li contents. The one that was used contained 44

½I

½II

The Li13Si4-Li7Si3 transition (44 w/o Li) is preferred for most applications. The discharge reaction for this transition is shown in eqn [III]; this corresponds to a capacity of 1747 A s g1 of alloy: Li13 Si4 -43 Li7 Si3 þ 113 Liþ þ 113 e

½III

This is lower than the capacity of the Li–Al anode material (eqn [I]). However, the alloy has the capability of undergoing multiple transitions and can generally deliver power at a higher rate. Cathodes During the 1970s, a considerable amount of research was done at Argonne National Laboratory (ANL) to study the discharge processes for a number of chalcogenides in molten salts. Both the monosulfides and disulfides were included in this work. This provided the basis for a clear understanding of chemical reactions that occurred during discharge as well as the theoretical possibilities of these materials. Although the work at ANL was focused on rechargeable (secondary) batteries for electric vehicle applications, the work was just as applicable to primary (thermal) batteries. Iron disulfide

To reach the high level of confidence required by thermal batteries, the physicochemical properties of the cathode materials must be well assessed and understood. The main physicochemical properties required for the

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

cathode materials to be used in thermal batteries are highlighted below: redox potential: it should have a discharge potential • compatible with the electrochemical window of the

• •

• • • • • • • • •

electrolyte in order to avoid its oxidation; ability to provide a fixed discharge plateau: it should undergo multiphase discharge and not intercalation; high thermal stability: to minimize thermal decomposition and associated possible chemical reactions caused by the decomposition products (e.g., S2 in the case of FeS2 reacting with the anode or pyrotechnic source in the battery). These products can also result in increased self-discharge; electronically conductive: to minimize the resistance of the cathode; low solubility of the cathode materials in the molten electrolyte: to minimize self-discharge reactions with attendant loss in capacity; low solubility of discharge products in the molten electrolyte: to minimize possible self-discharge reactions; stable toward moisture and/or oxygen: to prevent the production of oxides at the cathode surface (This gives rise to a voltage peak at the beginning of discharge.); ability to be wetted by electrolyte: to minimize the contact resistance at the electrolyte (separator)/electrode interface; low equivalent weight: for higher coulombs per mole; good discharge kinetics (high exchange current density): for high rate capability; reasonable cost; and being environmentally friendly is an additional desirable attribute.

Basic properties

The thermodynamic properties of FeS2 are well established. Iron disulfide exists in two forms: pyrite and marcasite. Pyrite is the stable phase of FeS2 above 423 1C. The Fe–S phase diagram has been widely investigated. Iron disulfide has a cubic structure (group Pa3 (Th6)) where the Fe atoms and S2 groups are located on the Cl and Na positions in the NaCl-type structure, respectively. The pyrite can deviate from the ideal stoichiometry of 2.00 by as much as 7.5 a/o, but this does not greatly affect either the lattice perfection or the cube edge of the material. The basic properties of FeS2 vary with the ore’s origins. The bonding in pyrite and related chalcogenides has been the subject of numerous investigations. Oxidation

Sulfates form rather easily on the pyrite surface even at low oxygen pressure. In nature, topochemical pyrite oxidation to FeSO4 occurs readily and depends greatly on

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the ambient moisture content of the air, being greatly accelerated above 20% relative humidity. The thermal decomposition of sulfates leads to the formation of Fe2O3. A large voltage transient (spike) occurs upon activation of a thermal battery if the FeS2 contains impurities such as oxides, sulfates, and elemental sulfur or if the activity of Li is not fixed in the cathode. This interferes with maintaining strict voltage control. This is readily remedied by lithiation, however. Common lithiation agents are Li2O or Li2S, added in small quantities of 1–2 w/o. Thermal stability under inert gases

The thermal stability of pyrite has been widely investigated under inert or corrosive atmosphere by nonisothermal or isothermal analysis. The overall mechanism is described by eqn [IV]: ð1  xÞFeS2 ðsÞ-Fe1x SðsÞ x ¼ 020:2 þ 12 ð1  2xÞS2 ðgÞ

½IV

By isothermal analysis, it was shown that the mass losses during the decomposition process were linear with time and exhibited Arrhenius behavior, with activation energies between 275 and 325 kJ mol1 FeS2. Thermal decomposition becomes significant at temperatures above 550 1C. The pyrite is progressively transformed into pyrrhotite and the FeS2 grains become porous as sulfur gas escapes. In contrast to pyrite, the thermal decomposition of FeS (troilite) does not become significant until 927 1C. Thermal stability in molten salts

When FeS2 thermally decomposes, it forms a nonstoichiometric monosulfide (pyrrhotite) and sulfur vapor that reacts exothermically with the lithium or lithiumalloy anode or dissolved lithium in the molten electrolyte. It can also react with the hot iron in the pyrotechnic used in the battery. This reduces the battery capacity and generates more heat. This, in turn, leads to even further thermal decomposition of FeS2, which can then destroy the battery if thermal runaway occurs. Moreover, it forms a solid insulating Li2S layer in the retained electrolyte. The pyrite and the pyrrhotite dissolve partially in molten salts at high temperatures. As the pyrite thermally decomposes, sulfur gas is able to react with already dissolved sulfur-based species according to the following mechanism to form polysulfides, as shown in eqns [V] and [VI]. These reactions might enhance the dissolution process: 2S2 þ 3S2 ðgÞ34S2 

½V

2S2 þ 5S2 ðgÞ34S3 

½VI

The decomposition kinetics of FeS2 in molten salts has been little investigated by several researchers in halide

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

eutectics. In molten salts, the kinetic constants are at least three orders of magnitude lower than in inert gas atmosphere. The molten salt probably acts as a physical barrier for the ready escape of the sulfur gas and may repress dissociation by increasing the partial pressure of sulfur in the immediate vicinity of the discrete FeS2 particles. Discharge reactions

The discharge reactions that occur when a FeS2 cathode is used in high-temperature batteries have been extensively studied by ANL for rechargeable applications. The four discharge sequences of FeS2 in molten LiCl–KCl are described in eqns [VII]–[X]: Step 1: FeS2 þ 32 Liþ þ 32 e -12 Li3 Fe2 S4 ð‘Zphase’Þ ½VII

The first discharge step consists in the reaction of 1.5 mol of lithium with 1 mol of pyrite. This reaction contrasts to the intercalation process, in which electrochemical insertion of the lithium cation in carbon (intercalation) occurs. It presents the advantage that it ensures a flat discharge plateau before the potential transition. The Li– Si/FeS2 thermal batteries are designed to use only the first cathode transition because of rigid voltage requirements associated with the use of such batteries. This transition is equivalent to 1206 A s g1 of FeS2: Step 2: ð1  xÞLi3 Fe2 S4 3 ð1  2xÞLi2x Fe1x S2 þ Fe1x S

Table 1 Open-circuit potentials (OCPs) for several of the discharge steps involving pyrite at 400 1C

½VIIIa

In eqn [VIIIa], x is close to 0.2. When x ¼ 0, eqn [VIIIb] results for the reduction of Li3Fe2S4: Li3 Fe2 S4 þ Liþ þ e -Li2 FeS2 þ FeS þ Li2 S

½VIIIb

Step 3 : Li2x Fe1x S2 -Li2 FeS2 ð‘Xphase’Þ

½IX

Step 4 : Li2 FeS2 þ 2e -Li2 S þ Fe þ S2

½X

The species actually undergoing reduction in eqn [IX] is the polysulfide, S2 2 , i.e., the oxidation state of Fe in FeS2 is 2þ and not 4þ . This is illustrated in eqn [XI]: S2 2 þ 2e -2S2

These changes are very important for proper thermal management of high-temperature rechargeable batteries. The conductivity of the discharge phases also influences the performance of the cell. Although the conductivity of the pyrite phase is very good, that of the first discharge phase, Li3Fe2S3, is much lower, while that of subsequent discharge phase is intermediate. Both the starting and discharge phases are semiconductors, which is ideal for thermal battery applications. The area-specific impedances of typical anode and cathode for a Li–Si/LiCl–KCl/FeS2 cell discharged at 450 1C and 50 mA cm2 are shown in Figure 2. The change in the anode impedance is insignificant with depth of discharge, while there is a dramatic increase for the cathode. The bulk of the observed increase in impedance is due to the formation of the less-conductive Zphase at the start of discharge. It is also due to KCl precipitation and J-phase formation. At normal discharge rates associated with thermal batteries, the Z-phase begins to discharge to form the X-phase before all of the initial pyrite phase is consumed. The discrete discharge phases are readily evident in the photomicrograph of Figure 3. There is a large volumetric change in the cathode during discharge due to the lower density of the Z-phase.

½XI

Any fugitive sulfur can react very exothermically with the Li-alloy anodes in the battery, which can destroy the battery if thermal runaway occurs. The open-circuit potentials (OCPs) for a number of the major discharge reactions are summarized in Table 1. Note that the first two discharge steps experience entropy changes that result in cell cooling, whereas the discharge of the Li2FeS2 phase results in cell heating.

Discharge reaction

OCP at 400 1C vs Li–Al(V)

Entropy effects

FeS2-Li3Fe2S4 Li3Fe2S4-Li2 þ x Fe1x S2 þ Fe1  y S Li2FeS2-Fe þ Li2S

1.750 1.645

Cooling Cooling

1.261

Heating

2000 Cathode 1500 ASI vs Li−Al (Ω cm2)

144

1000

500 Anode 0 0.0

0.5

1.0

1.5

2.0

Discharge capacity (eq. Li/mol FeS2)(mAh)

Figure 2 Area-specific impedances (ASI) of half cells of a Li–Si (25% electrolyte)/LiCl–KCl/FeS2 cell discharged at 450 1C and 50 mA cm2.

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

Anode

[Fe0, Li2S]

145

Cathode

Li0 Cl−

Li−Si

S22−

Li+

FeS2 Fe2+

Li+

Li+

K+

MgO separator (matrix)

Li3Fe2S4

Load e− Amps

e−

e

Anode: Li → Li0(soln.) quad Cathode: FeS2 → Fe2+ + S22− Overall reaction: 4Li0(soln.) + Fe2+ + S22− → 2Li2S + Fe0

Figure 4 Schematic representation of self-discharge reactions in Li–Si/LiCl–KCl/FeS2 cells.

Figure 3 Photomicrograph of FeS2 phase (white) and Li3Fe2S4 phase (light gray) (650 ) formed during discharge of Li–Si (25% electrolyte)/LiCl–KCl/FeS2 cell at 400 1C and 50 mA cm2.

Problem areas

Although the pyrite cathode does not have all of the technical problems associated with the CaCrO4 cathode, it still has some issues of its own. These include only moderate thermal stability, a voltage transient upon battery activation, and significant solubility in molten salts. Although FeS2 has a finite solubility in molten salts, it is not nearly as great as for CaCrO4. This only becomes an issue if the battery sits on open circuit for a prolonged time or if the battery is discharged under a very light load (e.g., o20 mA cm2). The solution species that arise from FeS2 dissolution can diffuse into the separator and react with anodic species (that originate from some dissolution of the anode) to form elemental Fe and Li2S, as shown in Figure 4. This can cause a loss in the capacity of the battery. Cobalt disulfide Properties

Unlike pyrite, which occurs as a mineral, CoS2 does not exist in any substantial amounts in nature and must be synthesized chemically, which correlatively increases its price. Cobalt disulfide has a lower solubility in molten electrolytes and a much higher electronic conductivity, which permits a higher rate of discharge. Most importantly, it has a much higher thermal stability, starting to decompose only >650 1C, which is B100 1C higher than

that for FeS2. This allows for longer-term applications. During the thermal decomposition of CoS2, sulfur is released according to eqn [XII]: CoS2 -13 Co3 S4 þ 13 S2 ðgÞ

½XII

Thermal batteries that could last for 2 h or more could now be possible. Lifetimes of 41 h have already been demonstrated with several prototype Li–Si/CoS2 thermal batteries. Discharge reactions

The discharge reaction for CoS2 differs from that for FeS2 in that lithiated intermediates are not formed (eqns [XIII]–[XV]): CoS2 þ 43 e -13 Co3 S4 þ 23 S2

½XIII

Co3 S4 þ 83 e -13 Co9 S8 þ 43 S2

½XIV

Co9 S8 þ 16e -9Co0 þ 8S2

½XV

and

There are thus 1.33 equivalents of Li per mole of CoS2 during the first discharge step, compared with 1.50 for FeS2. The capacity for this reaction is 1045 A s g1, which is lower than the first-stage discharge for FeS2 (eqn [VII]). It should be noted that although there is some selfdischarge associated with the use of CoS2 due to solubilization of it or its discharge products in the molten salt, it is not nearly as great as for the case of FeS2. Although direct solubility measurements have not been reported for CoS2 in molten salts, the loss in

146

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium 0.7

20

FeS2

CoS2

1.8

CoS2

40 60 80

FeS2

0.6

1.6

0.5

1.4

0.4

1.2

0.3

1.0

0.2

Total resistance (Ω)

2.0

Cell voltage (V)

Change in capacity (%)

0

0.8 0.1

100

0

50 100 Time on open circuit (min)

150

Figure 5 Loss in capacity of Li–Si (25% electrolyte)/LiCl–LiBr– LiF/MS2 cells as a function of time on open circuit before discharging at 550 1C and 125 mA cm2.

capacity upon standing on open circuit before discharge has been studied at SNL. Figure 5 compares the singlecell response of FeS2 and CoS2 cathodes in the all-Li, LiCl–KBr–LiF electrolyte at 550 1C during discharge at 125 mA cm2. (This is the electrolyte in which the greatest self-discharge is observed for the FeS2 cathode.) The loss of capacity for the cell with the CoS2 cathode was only half of that for the FeS2 cell after 60 min on open circuit. This reduced self-discharge is important for this electrolyte, as one typically would pair CoS2 with the all-Li electrolyte because of its very high ionic conductivity and the high electronic conductivity of CoS2 to maximize the rate capabilities. The Li–Si/CoS2 couple is much better suited for longlife thermal batteries than is Li–Si/FeS2. The major disadvantages of CoS2 over FeS2 are its higher cost – it must be synthesized in the laboratory. However, although the initial potential of an FeS2 cell may be higher than that for the corresponding CoS2 cell, the higher impedance of the discharge phases in the former case begins to dominate the discharge process so that the emf of the CoS2 cell will be higher later in discharge. The performance of the two cathodes is compared in Figure 6 at 400 1C and 125 mA cm2 using the LiBr–KBr–LiCl eutectic electrolyte. The potential of the Li–Si/FeS2 cell was initially higher at the start of discharge, but dropped below that of the Li–Si/CoS2 cell after about 0.5 eq. Li mol1 of sulfide has been extracted. There were two major voltage transitions noted for the FeS2 cathode during discharge, at B0.5 and 1.5 eq. Li mol1 FeS2. The CoS2 cathode exhibited only one voltage transition near 1.75 eq. Li mol1 over the same depth of discharge. Overall, more capacity was extracted from CoS2 than from FeS2 to a 1 V cutoff. Note that there are two peaks in the total polarization for the FeS2 cell at the voltage transitions and that they are greater in magnitude than those for the CoS2 cell. This reflects the higher resistivity of the discharge phases for the Li–Fe–S system as noted above.

0.6 0.0

2.0 2.5 1.0 1.5 Capacity (eq. Li/MS2) (mAh)

0.5

0.0 3.0

Figure 6 Discharge at 400 1C and 125 mA cm2 of Li–Si (25% electrolyte)/FeS2 and Li–Si (25% electrolyte)/CoS2 cells made with LiBr–KBr–LiCl eutectic electrolyte.

Other sulfides Ferrous sulfide

Ferrous sulfide was studied extensively by ANL for use in secondary high-temperature batteries. Although FeS may have applicability for use in secondary batteries, it is of little interest for primary use, despite its extremely high thermal stability (m.p. ¼ 1090 1C) and low solubility in molten salts. Its greatest detriment is its lower emf than FeS2. Ferrous sulfide forms the basis for the lower-voltage plateau during discharge of FeS2 cathodes. This loss of B0.5 V/cell is not acceptable. Nickel sulfides

The thermal stability of NiS2 is intermediate between that of FeS2 and CoS2, as is its emf in a cell. Like CoS2, it also must be chemically synthesized. Like the other disulfides, the product of thermal decomposition is the monosulfide (stoichiometric NiS) and S2 vapor. Discharge reactions In addition to studying FeS2 and CoS2, ANL also examined the electrochemistry of NiS2. The discharge mechanism for this material is shown in eqns [XVI]–[XIX]: NiS2 þ 2e -NiS þ S2

½XVI

7NiS þ 2e -Ni7 S6 þ S2

½XVII

3Ni7 S6 þ 8e -7Ni3 S2 þ 4S2

½XVIII

Ni3 S2 þ 4e -3Ni0 þ 2S2

½XIX

The electrochemical performance of NiS2 in LiCl–KCl eutectic electrolyte is compared with that of synthetic FeS2 and CoS2 in Figure 7. The FeS2 catholyte was not lithiated, giving rise to the initial voltage spike at the start of discharge. As expected, the voltage response for the cell with the NiS2 cathode was intermediate between that

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium 0.7 0.6 0.5 1.5

0.4 0.3

1.0

0.2

Total resistance (Ω)

Cell voltage (V)

2.0

0.1 0.5 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 3.5

Capacity (eq. Li/mol MS2) (mAh) NiS2 (lithiated)

FeS2 (unlithiated)

CoS2 (lithiated)

Figure 7 Discharge of Li–Si (25% electrolyte)/MS2 cells at 500 1C and 125 mA cm2 in LiBr–KBr–LiCl eutectic electrolyte. All catholytes are made with synthetic disulfides.

of the CoS2 and FeS2 cells. However, the overall performance of the NiS2 cathode was similar to that of the CoS2 counterpart. Because the costs of the NiS2 precursor agents are much less than those for CoS2, the similar electrochemical performance is an incentive to seriously consider using NiS2 for applications where CoS2 might have been used. However, more data are needed at higher current densities and a wider temperature range – including battery tests – to validate this assumption. The use of NiS as a cathode material would be possible, but as for the case of FeS and FeS2, it has a much lower emf than NiS2 and consequently is of little interest for general thermal battery applications. Other transition metal sulfides

There are a number of transition metal sulfides that have the properties necessary to be considered candidates for use as thermal battery cathodes. Some of these (e.g., CuS and MoS2) have already been used for ambient-temperature systems with nonaqueous electrolytes. However, the use of MoS2 would not be feasible as a thermal battery cathode, as it undergoes intercalation by Li, resulting in a declining voltage during discharge. This would not be acceptable for thermal batteries, as they require strict voltage control during discharge. Other sulfides (e.g., BaNiS2 and Sb2S3) require the use of conductive additives owing to their low electronic conductivities. Or they would form low-melting metal discharge products (e.g., Bi in the case of Bi2S3), which could result in cell shorting. There has been some previous work on the electrochemical study of CuS in LiCl–KCl eutectic. This material does not have the high conductivity of FeS2, however. Although the potential for the CuS cell starts higher than the FeS2 cell, it quickly drops off as the

147

CuS-Cu2S plateau transitions into the lower-voltage Cu2S-Cu0 plateau. (Similar results are obtained with chalcopyrite mineral, CuFeS2.) The use of Cu compounds for cathodes in thermal batteries poses certain risks, as the elemental Cu can form dendrites that can grow through the separator, shorting the cell. The potentials of cells with Ag2S and ‘CrS2’ cathodes were less than those for FeS2 and they showed lower capacities than FeS2, as well. The ohmic losses for Ag2S cells were low, because elemental Ag is formed during discharge, whereas those for ‘CrS2’ cells were quite high. The voltages for VS2 and MnS2 cathodes were considerably higher than that for FeS2. However, the capacities were only about one-fourth as great, which translates into much lower specific energies and energy densities. Consequently, it is unlikely that these can be considered serious competitors to inexpensive, native pyrite as cathode materials for thermal batteries.

Recent developments Thermal sprayed electrode

Today all thermal battery electrodes are formed by pressing powder mixes into pellets that are then stacked to construct the battery. This requires increasing larger – and expensive – presses as the pellet diameters increase. The large inventory of pressing dies that are necessary adds to the equipment costs. Several years ago, the concept of plasma spraying of thermal battery electrodes was evaluated. This approach involves using a thermal spray process to deposit thin-film electrodes onto a graphite paper or stainless-steel current collector. It was demonstrated that both FeS2 and CoS2 cathodes could be formed in this fashion. The main advantages of this type of cathode are as follows: decrease in the cathode thickness (volume gain, be• cause one can deposit only what is necessary and not

• • • •

what must be used because of pellet mechanical issues); reduction in the ohmic losses in the cathode because of better particle–particle contact; better wetting behavior of the cathode particles by the electrolyte; better contact (bonding) with the current collector and; no need for expensive dies.

However, the thermal sprayed cathode process presents several drawbacks: high cost of the process as the equipment is very • expensive; loss of capacity for very thin electrodes due to self• discharge;

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the need to conduct the spraying under an inert at• mosphere cover; a change in composition of the cathode from the • starting feedstock material caused by large FeS par2

• •

ticle bouncing off the substrate, resulting in lower FeS2 content; difficulty in controlling the density of the deposit; and difficulty in commercializing, as it is a batch process.

Composite separator–cathode deposits were also prepared in the same manner by sequential thermal spraying of LiCl–KCl-based separator material onto a pyrite–cathode substrate. Both single cells and batteries were successfully tested using the two-layered, plasmasprayed composites along with plasma-sprayed Li–Si anodes. Currently, work is underway studying the preparation of cathode coatings for thermal batteries using aqueous media. This provides all of the advantages of plasma spraying, but without its disadvantages. The initial results show improved electrochemical performance over pressed powder cathodes. Work is continuing to extend this approach to include the separator and anode (using nonaqueous media), as well, with the goal of producing a three-layered, composite – a complete cell. This will greatly reduce the number of piece parts needed for battery assembly and will reduce the production costs of thermal batteries, if successful. Oxide cathodes Calcium chromate, vanadium pentoxide, and tungsten trioxide

The history, details, and electrochemistry of early thermal batteries that used chromate cathodes as well as those based on WO3 and V2O5 are detailed in a separate article in this encyclopedia. Manganese oxides

The possibility of using MnO2 or other manganese oxides (e.g., LiMn2O4) as cathodes for thermal batteries is appealing because they are seen as ‘green’ in terms of environmental acceptability. However, the use of conductive additives with these materials is necessary. In addition, MnO2 reacts exothermically with Br-containing electrolytes upon melting. Although the manganese oxides exhibit higher potentials than FeS2, the discharge capacities are lower. Unlike bromide-containing melts, both MnO2 and LiMn2O4 are chemically compatible with molten nitrate electrolytes at temperatures well over 300 1C. Discharge rates near 8 mA cm2 are possible at 150 1C and increase to >30 mA cm2 at 300 1C. Self-discharge become important above 300 1C owing to breakdown of the protective Li2O passive film on the Li–alloy anode.

Silver chromate

Silver chromate was studied in the CsBr–LiBr–KBr eutectic in single cells using 10–20% graphite powder in the catholyte as a conductive additive. The Li–Si/CsBr– LiBr–KBr/Ag2CrO4 system could sustain a current density of 32 mA cm2 in single-cell tests at 300 1C. However, in follow-up battery tests, thermal runaway occurred that was traced to reaction of the bromide with the cathode material, thus making this cathode material incompatible with Br-based electrolytes. In a nitratebased electrolyte, however, reasonable performance was observed with Li–Al/Ag2CrO4 cells. They showed lower voltages during discharge than Li–Si/Ag2CrO4 cells because of the lower Li activity of the anode. These cells also showed greater polarization and reduced capacities than cells with Li–Si anodes. The best performance was observed at 200 1C at B7 mA cm2, but with only about half of the capacity of the Li–Si cells under the same conditions. The cell discharge capacity dropped off rapidly above this temperature, much faster than that for cells with Li–Si anodes. The differences in performance of the two anodes may be related to differences in porosity, composition, and morphology of the passive oxide films that form in contact with the molten nitrate electrolyte. Chromium dioxide

Chromium dioxide has been evaluated as a cathode in the CsBr–LiBr–KBr eutectic electrolyte as well. It showed poorer performance at 250 1C and 16 mA cm2 than that for MnO2 and LiMn2O4. The performance was improved at 300 1C but was still inferior to that for the manganese oxides. This material also has a limited thermal stability above 300 1C. Chromium(V) oxides

Electrochemical characterization During the discharge of Ca/CaCrO4 cells, Cr(V) compounds are formed that act as separators to limit self-discharge. These materials are electroactive and can themselves be used as cathodes. Specifically, the compositions of these materials are Ca5(CrO4)3Cl (‘531’ compound), which is bright green, and Ca2CrO4Cl (‘211’ compound), which is purple. These materials show promise for use as cathodes in thermal batteries. Catholytes were made with the 531 and 211 compounds with 10% graphite powder and 20% LiCl–KCl eutectic electrolyte and were tested in single cells with Li–Si anodes at 500 1C and 125 mA cm2. The results of preliminary discharge tests are shown in Figure 8 along with comparable data for a Li–Si/FeS2 (lithiated) cell. The potential of the Li–Si/531 couple was substantially greater than that for the Li–Si/FeS2 one, but the overall impedance was higher because of the lower electronic conductivity compared to FeS2. Still, to a 1.6 V cutoff, the

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capacity was comparable for the two cathodes. More work is needed over a wider range of temperatures and current densities with an optimized catholyte composition based on the 531 material. Follow-on battery tests are also needed for validation purposes. Only limited cathode testing has been done with the 211 compound and the initial results indicate that it does not appear to perform as well as the 531 compound.

showed short voltage plateaus. Although the voltage for the CuO cathode was fairly flat, it was lower than that for the FeS2 cathode. In addition, the formation of Cu dendrites during discharge increases the possibility of cell shorting. None of the oxides performed as well as the 531 Cr(V) compound. For almost all of the oxides, the need for incorporation of a conductive additive (e.g., graphite) reduces the ultimate energy density and specific energy that can be realized.

Mixed transition metal oxides

Electrolytes

A comprehensive screening study of almost 100 potential mixed transition metal oxides that could have the properties necessary for use as cathodes in thermal batteries was conducted at SNL. Although some materials had higher potentials than FeS2, they also had reduced capacities. Representative discharge traces are shown in Figure 9. Some materials had a higher initial voltage than FeS2, but then either dropped off quickly with depth of discharge – typical for intercalation reactions – or

The electrolytes suitable for use in thermal batteries require certain properties and are selected according the following criteria: low vapor pressure: the electrolyte should not evap• orate inside the battery; high ionic conductivity: very important for ‘pulse’ • (high-rate) applications; large electrochemical window: i.e., no chemical re• action between the electrode materials and electrolyte

2.4

Cell voltage (V)

2.2 2.0

•

1.8 1.6 1.4

•

1.2 1.0

0

200

400

600

800

1000

1200

•

Specific capacity (A s g−1)

Figure 8 Discharge of cells with Ca5(CrO4)3Cl cathode with graphite and electrolyte (black curve) and natural unfused and lithiated FeS2 cathode blue curve with flooded (25% electrolyte) Li–Si anodes at 500 1C and 125 mA cm2.

• •

3.0

• Cell voltage (V)

2.5 CuVMoO6 2.0

•

Ca5(CrO4)3Cl

SnO2

1.5

Lithium halide electrolytes

CuO FeS2 1.0

constituents – no oxidation of the electrolyte by the cathode materials, and – no reduction of the electrolyte by anode materials; low or no solubility of Li2O: it modifies the electrolyte retention properties of the separator – electrolyte leakage can occur, resulting in ‘soft’ shorts between cells in the battery stack; low solubility of elemental Li and Li-alloy anodes: this decreases the efficiency of the cells by electronic conduction in the molten salt; low solubility of the cathode and anode materials: minimizes self-discharge reactions with attendant loss in capacity; low solubility of discharge products: minimizes possible self-discharge reactions; stable toward moisture and/or oxygen: prevents the production of hydroxides and/or oxides in the molten salt; compatible melting point: lower than the thermal decomposition temperatures of the electrode materials; and ability to wet the binder in the separator and the electrodes: minimizes the contact resistance at the electrolyte (separator)/electrode interface.

0

5

10

15

20

Specific capacity (A min g1)

Figure 9 Discharge of cells of typical mixed transition metal oxides at 500 1C at 125 mA cm2 in LiCl–KCl eutectic electrolyte with flooded (25% electrolyte) Li–Si anodes.

Lithium halide-based mixtures were mainly used for their low melting points, low vapor pressure, and relatively high ionic conductivities. The mainstay electrolyte for thermal batteries has been the LiCl–KCl eutectic that melts at 352 1C. The choice of the electrolyte is usually dictated by the envisioned application. The electrolyte

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used in pyrite-based cells can impact the discharge processes in a number of ways. Concentration gradients of Li þ can occur at the anode–separator interface with corresponding Kþ gradients at the separator–cathode interface under high-rate conditions. High levels of Kþ can lead to increased formation of the J-phase material (K5.5Li0.6Fe24S25.9Cl1.0). However, this phase appears not to be stable above 470 1C. This is most important near the end of life of the battery when its temperature is lowest. The tortuous nature of the separator layer accentuates these gradients. This causes the electrolyte composition to move away from the eutectic composition, resulting in salt precipitation at the electrode interfaces (e.g., LiCl at the anode–separator boundary). This can be avoided, but not entirely suppressed, by the use of all-lithium electrolytes, such as the LiCl– LiBr–LiF low-melting electrolyte. However, this will be at the expense of raising the melting point to 436 1C, which reduces the operating liquidus region for the electrolyte. More recently, other lower-melting eutectic mixtures based on LiBr–KBr–LiF and LiBr–KBr–LiCl eutectics are being used to replace the standard LiCl–KCl eutectic electrolyte. These have higher ionic conductivities and provide a larger liquidus region due to the lower melting points (324.5 and 321 1C, respectively). This results in longer operating life for the battery. The melting points and compositions of a number of alkali halide mixtures are summarized in Table 2.

Table 2

Electrochemical window

The electrochemical window is one of the main features of the electrolytes because it fixes the cathodic and anodic potentials. This decomposition window is very much temperature dependent, which can be quite important because thermal batteries operate at elevated temperature. Figure 10 shows the temperature dependence of the ‘‘decomposition voltage potentials for a number of common alkali metal halides’’ based on thermodynamic data. The cathodic limit is given by the reduction potential of the least stable cation in the mixture, whereas the anodic limit corresponds to the oxidation potential of the least stable anion. The fluorides are the most stable halides: F > Cl > Br > I. In the case of Li-alloy/FeS2 couples, the high redox potential of sulfur-based species means that the electrolyte is stable whatever the halide is in the electrolyte mixture. Ionic conductivity

As ionic media, molten salts exhibit rather high specific ionic conductivities (1–5 S cm1) compared with ionic liquids (1–10  103 S cm1) or solid electrolytes (106– 102 S cm1 at 700 1C). Lithium-based electrolytes exhibit the highest ionic conductivities due to the high mobility of the lithium cation compared with other alkali-based electrolytes, as shown in Figure 11. Electrolytes with higher atomic fractions of Li þ will have higher ionic conductivities.

Melting point and composition of some electrolytes

Electrolytes

Composition (wt.%)

Composition (mol.%)

Melting point ( 1C)

LiCl–KCl LiBr–KBr LiI–KI LiF–LiI LiBr–LiF LiCl–LiI LiF–LiCl

44.8–55.2 52.26–47.74 58.2–41.8 3.7–96.3 91.4–8.6 14.4–85.6 21.2–78.8

58.8–41.2 60–40 63.3–36.7 16.5–83.5 76–24 34.6–65.4 30.5–69.5

354, 352 320 285, 260, 280, 285, 286 410.9 448 368 501

LiF–LiCl–LiBr LiF–LiBr–KBr

LiCl–LiBr–KBr LiF–NaF–KF

9.6–22–68.4 0.67–53.5–45.83 0.81–56–43.18 0.9–48.2–50.9 12.05–36.54–51.41 29.5–10.9–59.6

22–31–47 0.67–53.5–45.83 3–63–34 3.5–54.5–42 25–37–38 46.5–11.5–42

443, 436, 444, 430 324, 323 312 320 321, 310 455

LiCl–KCl–LiF LiCl–KCl–LiBr LiCl–KCl–NaCl

53.2–42.1–4.7 42.1–42.8–15.1 42.63–48.63–8.74

62.7–28.8–9.1 57–33–10 61.2–29.7–9.1

397 416 429

LiCl–KCl–LiI LiCl–KCl–KI LiBr–LiCl–LiI LiF–LiCl–LiI LiCl–LiI–KI

44.2–45.0–10.7 37.6–51.5–10.9 19–24.3–56.7 3.2–13–83.8 2.6–57.3–40.1

57–33–10 54–42–4 16.07–10.04–73.88 11.7–29.1–59.2 8.5–59–32

394 367 368 341 265, 264

LiF–LiCl–LiBr–LiI

4.9–11.2–34.9–49 5.0–19.6–22.6–52.8

15.4–21.7–32.9–30 14.7–35.5–20–29.8

360 318–326

Decomposition voltage (V)

4.5 4.0 3.5 3.0 2.5 2.0

0

200

400

600

800

Specific ionic conductivity (S cm1)

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium 2.5

2.0

1.5

1.0

0.5 350

450 500 550 Temperature (°C)

400

Temperature (°C) LiF

LiCl

KBr

LiI

KCl

151

LiBr

KI

Figure 10 Thermodynamic decomposition potentials for some alkali metal halides.

600

650

LiCl−KCl eutectic 35% MgO (76.0% TD)

LiCl−LiBr−KBr eutectic 30% MgO (69.9% TD)

LiBr−KBr−LiF eutectic 25% MgO (74.0% TD)

LiCl−LiBr−LiF 35% MgO (75.0% TD)

5000 4000 3000 2000 1000 0 300

350

400

450

500

550

600

Temperature (°C) LiCl−KCl eutectic

LiBr−KBr−LiF eutectic

LiBr−KBr−LiCl eutectic

LiCl−LiBr−LiF eutectic

Figure 11 Specific ionic conductivities of several common thermal battery electrolytes.

In thermal batteries, due to the high level of mechanical stresses (acceleration, shock, spin, vibration, etc.), the electrolyte must be firmly immobilized by a binder. It constitutes the so-called ‘separator’. Usually, the binder is made of powders of metallic oxides such as silica, alumina, or magnesia that are electrical insulators. Magnesium oxide is the preferred choice as it is thermodynamically stable in contact with high-activity anodes at elevated temperatures. The conductivity is greatly influenced by the tortuosity of the separator. Conductivities as a function of temperature are presented in Figure 12 for pellets of a number of common thermal battery separator materials. As expected, the all-Li separator has the largest conductivity by far, the reason why it is the preferred choice for high-power application. The tortuosity and porosity (electrolyte volume fraction) of the separator pellets impact the final conductivity. Modeling of the separator conductivity indicates that

Specific ionic resistivity (Ω−cm)

Specific ionic conductivity (mS cm1)

Figure 12 Specific ionic conductivities of some common separator materials used in thermal batteries.

2.5 2.0 1.5 1.0 0.5 0.0 20

30

40 MgO content (wt%)

LiCl−KCl eutectic 400 °C LiBr−KBr−LiF eutectic 450 °C

50

60

LiCl−KCl 500 °C LiCl−LiBr−LiF 550 °C

Figure 13 Specific ionic resistivity of separator pellets as a function of binder (MgO) content B75% TD.

percolation theory of the porous structure with a distribution of porosities best fits the experimental data. The effects of MgO (binder) content on the specific ionic resistivities of representative separators are shown in Figure 13. Thermal properties

Heat management is critical for the proper functioning of a thermal battery. Information regarding the thermal properties of the battery constituents (viz., cathode, anode, separator, and pyrotechnic) is necessary for the design of the battery to determine the amount of insulation required and the amount of heat necessary for a given application. The most important thermal properties are the heat capacity of the electrolyte in the solid and molten states Cp(cr.) and Cp(liq.), respectively, and the

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Table 3 Specific heat capacity Cp (J K1 g1) and specific heat of fusion DHf (J g1) of some electrolytes Salts

CpTf DCp ¼ Cp(liq.)– (J K  1 g  1) Cp(cr.) (J K  1 g  1)

DHf (J g  1)

LiCl–KCl LiI–KI LiF–LiCl–LiBr LiF–LiCl–LiI LiF–LiBr–KBr

0.74 0.55 0.87 1.22 0.505

244, 234.8 71 266, 293.8 157 103, 134

0.26 0.85 0.41 – 0.248

heat of fusion DHf . The heat capacities of mixtures have to be measured because the melting point of the eutectic compositions and single salts are often different. This limits the determination of the heat capacity of the electrolyte over the full temperature range of interest. The heat capacities of a number of electrolytes have already been determined and are summarized in Table 3 along with their heats of fusion. Hygroscopicity of salts

Water must be removed from piece parts and components used in thermal batteries, as any moisture present after the battery is welded shut will ultimately reach the anode during storage or battery discharge. This will reduce the capacity of the anode and increase the fluidity of the electrolyte, which could lead to leakage during operation. Of the three Li halides, LiBr is by far the most hygroscopic. As a result, more care must be taken during processing of electrolytes that contain LiBr (or LiI) to minimize water uptake. Ultimately, the bulk of the moisture is removed by vacuum-drying at elevated temperatures (e.g., typically >100 1C) but o250 1C to avoid the hydrolysis of the salt. In the case of iodide electrolyte, processing must take place under an inert gas to avoid iodide oxidation to elemental iodine. Mechanical properties of the separator

The mechanical properties of the separator are just as important as its electrochemical properties. A high ionic conductivity is of little value if separator pellets cannot be made that are robust and can be handled without cracking or breaking during battery stack assembly. The properties at battery operating temperatures, when the electrolyte is molten, are equally important. Batteries are closed and welded off under a high applied pressure (typically, 1.2–2.5 MPa) to maintain good interfacial contact between the pellets in the battery stack and to prevent movement during any environmental stresses while functioning. Not all MgO materials will function adequately as a binder for the separator. The pore-size distribution and morphology of the MgO particles determine the capillarity of the material for effective electrolyte immobilization. The degree of electrolyte leakage from the

separator and the separator deformation are excellent metrics for binder characterization and qualification. Once the electrolyte melts, a rapid stack relaxation occurs, with the pressure dropping to o0.4 MPa within seconds. This occurs through deformation of the separator pellet, which is evidenced by a decrease in thickness. Empirically, it has been determined that separator pellet deformation in the range of 15–30% is ideal for thermal batteries during operation. Lower values result in reduced interfacial contact between pellets in the battery stack; much higher deformation leads to separator extrusion into the battery wrap, which can result in excessive electrolyte leakage and possible separator breaching. A number of factors influence the deformation process for a given electrolyte, including temperature, porosity, binder content, and applied pressure. The porosity and binder levels have the most effect, followed by applied pressure. If the separator pellets are too dense, there is no place for the electrolyte to go once molten, which leads to electrolyte leakage into the ceramic blanket with which the battery stack is wrapped. This can then lead to parasitic shunting currents. (Porosities of 25–30% are found to be ideal.) There will always be a tradeoff between the electrochemical performance (e.g., resistivity) and mechanical requirements of the separator for severe environments. The latter may require increasing the level of binder, which will lead to an increase in the resistance of the separator and a resulting rise in battery impedance. Solubility of lithium and electronic conductivity

The metallic lithium solubility in the electrolyte may induce self-discharge due to the native electronic conductivity in the molten salt electrolyte. The measured lithium solubility in the LiCl–KCl eutectic is between 1 and 2 mol%. The electronic conductivity increases monotonically with temperature and activity of the Lialloy anode. Workers at ANL studied the self-discharge rates of a Li-alloy anode in LiCl–KCl and LiBr–KBr– CsBr eutectics and reported self-discharge rates in the former of 1.0, 1.4, and 1.9 mA cm2 at temperatures of 395, 415, and 436 1C, respectively. The self-discharge rate for the LiBr–KBr–CsBr eutectic was much lower, 0.18 mA cm2 at 415 1C, which shows that the electrolyte composition and the Li activity of the anode have a dramatic influence on the self-discharge process. The self-discharge rates for Li–Al/FeS2 cells on open circuit were almost 80 times less for Li–Si/FeS2 cells. Solubility of sulfur-based species

By recombination of the dissolved sulfur in the molten salt with dissolved lithium arising from the Li-based alloy anode, the formation of a solid insulating layer of Li2S is observed in the retained electrolyte along with elemental

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

Fe. The nature of the sulfur-containing species arising from dissolution of FeS2 in molten salts has not been totally ascertained but is believed to involve polysulfide species, as shown in eqn [XX]:

153

was used, loss of capacity dropped to 0.05% per day at a rate of 1–5 mA cm2 at 200 1C.

Lower-melting electrolytes FeS2 ðsÞ-Fe

2þ

2

þ S2 ðpolysulfideÞ

½XX

(The sulfur is originally present in FeS2 in the form of polysulfide.) The polysulfide may undergo dissociation to form elemental sulfur that can be lost from the melt by evaporation: S2 2 -0:5S2 ðgÞ þ S2

½XXI

Or, it can undergo oxidation: S2 2 -0:5S2 ðgÞ þ S þ e

½XXII

The chemistry and electrochemistry of S and Fe–S species are complex and not yet completely understood. In addition, there is the possibility in a conventional thermal cell of direct chemical reaction of the Fe–S solution species with dissolved Li that results from the Lialloy anode: 4Li0 ðsolÞ þ Fe2þ þ S2 2 -Fe0 ðsÞ þ 2Li2 SðsÞ

½XXIII

This would explain the observation of clusters of particles of Fe and Li2S found in the separators of deeply discharged Li–Si/FeS2 cells (see Figure 4). The composition of the electrolyte has a significant effect upon self-discharge, owing to differences in the solubility of the various sulfur-containing species arising from the FeS2 in contact with the melt. The discharge current density and temperature also impact the process, with greater losses occurring at lower current densities and higher temperatures. For example, Li-alloy/FeS2 cells with an all-Li electrolyte (3.21LiF–13.04LiCl– 83.75LiI mol.%) lost capacity of the upper-voltage plateau at the rate of 0.172% per day at 350 1C at discharge rates of 20–60 mA cm2. This increased to 0.217% per day at 450 1C. When a lower-melting electrolyte (0.95LiCl–5.14LiBr–45.09LiI–16.75KI–32.07CsI mol.%) Table 4

There has been increased interest in the use of lowermelting electrolytes in thermal batteries. This would reduce the amount of heat required for operation and would reduce the skin temperature of the batteries. A lighter heat pellet would translate into a shorter and lighter battery, which would increase the specific energy and energy density. The incorporation of iodide ions and Rb and Cs cations results in a number of promising candidates with melting points o300 1C; these are listed in Table 4. The performance of the Li–Si/FeS2 couple with the LiCl–LiBr–LiI–KI–CsI pentanary eutectic has shown reasonable performance in battery stacks at temperatures as low as 200 1C. However, an intrinsic problem at these lower temperatures is the reduced rate capability due to kinetic effects and higher battery impedance because of the increased resistivity of the separator. As a result, sustainable current densities of o5 mA cm2 are typical, which is more than two orders of magnitude less than what is typical for conventional thermal batteries operating at 400 1C or higher. One way to engineer around this is to electrically parallel several battery stacks to share the load. The use of iodides would result in a thermal battery that would be somewhat heavier than one based on the LiCl–KCl eutectic due to the higher density of the salt – especially for an all-iodide system. These salts are also more expensive than the bromides and chlorides. (Cs and Rb halides are even more expensive.) In addition, iodide electrolytes suffer from sensitivity toward oxidation by oxygen to form elemental iodine. Melting of these salts during electrolyte preparation must be done under an inert atmosphere and the materials stored under similar conditions. This is not practical for the commercial manufacturing of thermal batteries because of the constraints it imposes. In addition, there is evidence of increased self-discharge for batteries using such salts.

Halide eutectics with melting points o300 1C

Electrolyte

Composition (mol.%)

Melting point (1C)

Conductivity (S cm  1)

LiBr–RbBr LiBr–CsCl LiI–KI LiBr–CsBr LiCl–KCl–CsCl LiCl–KCl–RbCl–CsCl LiBr–KBr–CsBr LiBr–LiI–KI–CsI LiCl–LiBr–LiI–KI–CsI

42–58 42–58 40–60 59–41 57.5–13.3–20.2 55.5–18.7–1.4–24.3 56.1–18.1–25.3 9.6–54.3–16.2–19.9 3.5–9.2–52.4–15.7–19.2

271 262 260 259 265 258 236 189 184, 151

1.33 2.68 2.22 – 0.28 – – – –

at 567 1C at 800 1C at 607 1C at 280 1C

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Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

The separator resistivity is the largest contributor to the overvoltage, Ztot, which can be defined as follows: Ztot ¼ ZactA þ ZactC þ ZirA þ ZirS þ ZirC þ ZconcA þ ZconcC

½XXIV

where the ZactA and ZactC refer to activation polarization, ZirA, ZirC, and ZirS refer to ohmic polarization, and ZconcA and ZconcC refer to concentration polarization. (‘A’ refers to the anode, ‘C’ to the cathode, and ‘S’ to the separator.) The types and extent of overvoltage will depend primarily upon the electrochemical system, temperature, current density (which depends on the load and electrode surface area), electrolyte composition, and physical configuration of the cell. Other factors may also be involved in certain situations. Generally, the IR losses associated with the pyrotechnic are not significant. Nitrate-based systems

Some nitrate-based systems offer the potential of use at much lower temperatures than the best alkali halide systems because of the much lower melting points. The prominent nitrate system is the LiNO3–KNO3 eutectic with the composition 42–58 mol%. This electrolyte has a melting point of 124.5 1C with an ionic conductivity of 0.875 S cm1 at 287 1C. The conventional sulfide electrode materials are not compatible with molten nitrates, so that oxides must be used instead. Because these are typically insulators, conductive additives such as graphite must be added to the catholyte mixes – typically at a level of 10–15%. This automatically reduces the maximum energy density and specific energy that is possible. Cathodes that have been examined with molten nitrates include Ag2CrO4, MnO2, LiMn2O4, LiCoO2, and CrO2. The compatibility of various anode materials with the molten nitrate electrolyte has also been studied. The only reason that high-activity Li anodes can be used at all with the highly oxidizing molten nitrates is the protective passive film of Li2O that forms on melting of the electrolyte. However, this film is only stable up to a certain temperature depending on the anode material; Li–Al is more stable than Li–Si and shows an exotherm starting near 284 1C, with a major exotherm at 315 1C. In the case of the Li–Si anode, an exotherm starts near 200 1C, with a major exotherm at 260 1C as the passive film breaks down. Although single cells and heated battery stacks have been built using the nitrate electrolyte, the use of this technology with an internally heated thermal battery does not appear practical due to the exothermic reaction of the anode with the molten electrolyte if the passive film fails. The thermal impulse that the battery will experience at the anode–separator interface during activation will be well above the initiation temperature for exothermic reaction. When the passive film on the anode

breaks down, the resulting chemical reaction is extremely violent and would pose unacceptable hazards to nearby equipment and personnel and can result in complete meltdown of the battery. Chlorates and perchlorates

Some work has been done studying the use of molten chlorates and perchlorates as possible battery electrolytes. For example, LiClO3 melts at 128–129 1C and has been studied with Li–Al anodes at 140 1C. Both chlorates and perchlorates depend on a protective passive film to prevent catastrophic reaction with highly reducing anode materials. Thus, they possess the same hazards as the molten nitrates and are not considered viable electrolyte candidates for low-temperature thermal battery use. Tetrachloroaluminates

Another category of low-melting molten salts is the tetrachloroaluminate. The melting point of NaAlCl4, for example, is 154 1C, whereas that for LiAlCl4 is 143.5 1C. This NaAlCl4 electrolyte has been used in the Na-S cells but requires a ceramic Na þ conductor separator (e.g., b00 – alumina). However, if the molten salt is placed in contact with a high-activity anode material, such as Li–Si alloy, the Al3 þ is reduced to Al0. The tetrachloroaluminates also have a significant vapor pressure of AlCl3 at temperatures >200 1C. In addition, they suffer from poor conductivity – only 250–500 mS cm1 at 200 1C. Consequently, tetrachloroaluminates are not considered viable for possible thermal battery applications. Organic salts

A number of organic salts with low melting points have been examined for possible high-temperature battery use (m.p. of 100–350 1C). They include acetamides, acetates, formates, urea, and mixtures thereof. They have limited thermal stability and all react with Li alloys when molten. In addition, they possess very low conductivities, o20 mS cm1 at 150 1C for some urea mixtures. Although NaSCN–KSCN mixtures have been proposed as possible low-temperature electrolytes, similar compatibility and conductivity issues arise with their use, making them impractical for thermal battery use. Other organic salts that have been screened include lithium trifluoromethanesulfonimide (LiN(CF3SO2)2 or ‘lithium imide’; m.p. ¼ 229.5 1C), lithium trifluoromethanesulfonate (Li(CF3SO3) or ‘lithium triflate’, m.p. ¼ 160.8 1C), and dimethylsulfone ((CH3)2SO2, m.p. ¼ 108.5 1C). When molten, all suffered to some degree from similar incompatibilities with high-activity anodes. There are a number of tetraalkylammonium salts that have reasonable low melting points that are potential candidates for low-temperature thermal batteries. Some of these show great promise. The tetramethylammonium imide salt, for example, is stable to 300 1C in the presence of Li–Si alloy. More work in this area is currently

Primary Batteries – Reserve Systems | Thermally Activated Batteries: Lithium

underway to explore the full potential of this category of salts. The advantage of these types of salts is that the organic cation can be easily modified by changing the functional groups to modify the salts’ properties to be better suited for certain low- to medium-temperature battery applications. This is not an option with conventional inorganic salts. Ionic liquids (the so-called room-temperature molten salts) have also been examined but are quite expensive and some are also moisture and oxygen sensitive. In addition, they tend to react with high-activity anodes at elevated temperatures.

Conclusions Thermal batteries based on Li and Li-alloy anodes possess unique properties such as high reliability, high rates, and long storage lives. This makes them ideal candidates for many military and nuclear weapon applications. The bulk of the chemistry is based on sulfide cathodes. There is a vast improvement in the understanding of the chemistry and electrochemistry of couples based on such anodes and cathodes over the complex chemistries and reactions associated with the older Ca/CaCrO4 system. This greatly simplifies battery design and production. Research for next generation of batteries will be more focused on developing quicker, simpler, and cheaper ways of manufacture that do not depend on pressed powder technologies. Incorporation of alternative electrolytes will also be part of this strategy. The end result will be thermal batteries with higher energy densities and specific energies.

Nomenclature Symbols and Units 211 531 Cp Hf gact gconc gir gtot

Ca2CrO4Cl Ca5(CrO4)3Cl heat capacity heat of fusion activation polarization concentration polarization ohmic polarization total polarization

Abbreviations and Acronyms ANL ASI CRC emf LAN

Argonne National Laboratory area-specific impedance Catalyst Research Corporation electromotive force liquid anode

m.p. SNL

155

melting point Sandia National Laboratories

See Also: Primary Batteries – Reserve Systems: Seawater Activated Batteries: Magnesium; Thermally Activated Batteries: Calcium; Thermally Activated Batteries: Overview.

Further Reading Guidotti RA (1988) Methods of Achieving the Equilibrium Number of Phases in Mixtures Suitable for Use in Battery Electrodes, e.g., for Lithiating FeS2. US Patent 4,731,307. Guidotti RA and Masset PJ (2006) Thermally activated (‘thermal’) battery technology. Part I: An overview. Journal of Power Sources 161(2): 1443. Guidotti RA, Odinek J, and Reinhardt FW (2006) Characterization of Fe/ KClO4 heat powders and pellets. Journal of Energetic Material 24(4): 271. Guidotti RA and Reinhardt FW (1996) Characterization of MgO powders for use in thermal batteries. Internal Publication of Sandia National Laboratories Report SAND90-2104. Guidotti RA and Reinhardt FW (1996) Screening study of mixed transition-metal oxides for use as cathodes in thermal batteries. Proceedings of the 37th Power Sources Conference, p. 251. Guidotti RA and Reinhardt FW (1998) A study of the ignition process in a center-hole-fired thermal battery. Proceedings of the 38th Power Sources Conference, p. 223. Guidotti RA, Reinhardt FW, Dai J, and Reisner DE (2006) Performance of thermal cells and batteries made with plasma-sprayed cathodes and anodes. Journal of Power Sources 160: 1456. Guidotti RA, Reinhardt FW, and Odinek J (2004) Overview of hightemperature batteries for geothermal and oil/gas borehole power sources. Journal of Power Sources 136(2): 257. Guidotti RA, Reinhardt FW and Smaga JA (1990) Self-discharge study of Li-alloy/FeS2 thermal cells. Proceedings of the 34th International Power Sources Symposium, p. 132. Klasons V and Lamb CM (2002) Thermal batteries. In: Linden D and Reddy T (eds.) Handbook of Batteries, 3rd edn., p. 21.1. New York: McGraw-Hill. Kupfer WA (1994) Brief history of thermal batteries. Proceedings of the 36th Power Sources Conference, p. 300. Masset PJ (2006) Iodide-based electrolytes: A promising alternative for thermal batteries. Journal of Power Sources 160: 188. Masset PJ and Guidotti RA (2006) Thermal activated (‘thermal’) battery technology. Part II. Molten salt electrolytes. Journal of Power Sources 164(1): 397. Masset PJ and Guidotti RA (2008) Thermal activated (‘thermal’) battery technology. Part IIIa. FeS2. Journal of Power Sources 177(2): 595. Masset PJ and Guidotti RA (2008) Thermal activated (‘thermal’) battery technology. Part IIIb. Sulfur- and oxide-based cathode materials. Journal of Power Sources 178(1): 456. Masset PJ, Poinso JY, Schoeffert S and Poignet JC (2002) Decomposition kinetics of FeS2 pyrite in molten salts. Proceedings of the 40th Power Sources Conference, p. 242. Masset PJ, Schoeffert S, Poinso JY, and Poignet JC (2005) LiF–LiCl–LiI vs. LiF–LiBr–KBr as molten salt electrolyte in thermal batteries. Journal of the Electrochemical Society 152(2): A405. Masset PJ, Schoeffert S, Poinso JY, and Poignet JC (2005) Retained molten salt electrolytes in thermal batteries. Journal of Power Sources 139: 356. Schoeffert S (2005) Thermal batteries modeling, self-discharge, and self-heating. Journal of Power Sources 143: 361.