EP1829142A1

EP1829142A1 - Heat sources for thermal batteries

Info

Publication number: EP1829142A1
Application number: EP05799416A
Authority: EP
Inventors: Dario R. Dekel
Original assignee: Rafael Advanced Defense Systems Ltd
Current assignee: Rafael Advanced Defense Systems Ltd
Priority date: 2004-10-28
Filing date: 2005-10-27
Publication date: 2007-09-05
Also published as: WO2006046245A1

Abstract

A pyrotechnic heat - source pellet for thermal batteries, consisting of (i) a high surface area metallic powder (ii) an oxidizing agent and (iii) a phase moderating component. The phase moderating component forms in combination with the derivatives of the oxidation reaction of the metal powder and the oxidizing agent a homogeneous mixture having at least one distinct temperature in which a phase transition occurs. The phase moderating component typically consists of halogen salts such as sodium perchlorate, an halide salt and or any mixture thereof.

Description

HEAT SOURCES FOR THERMAL BATTERIES

TECHNICAL FIELD OF THE INVENTION The present invention relates in general to thermal batteries, and more particularly to pyrotechnic heat sources and operational longevity of thermal batteries.

BACKGROUND OF THE INVENTION

Thermal batteries are thermally activated, hermetically sealed power sources generally consisting of series or series-parallel arrays of primary cells. Each cell consists of an anode, a cathode, an electrolyte-separator and a pyrotechnic heat source. Typically, the electrolyte is a mixture of alkali metal halides such as an eutectic mixture of KCI-LiCI which melts at about 350⁰C. Common cathode materials are iron disulfide and cobalt sulphide, and transition metal oxides such as iron oxide can be used as well. Lithium-iron or lithium alloys such as lithium-aluminum and lithium-silicon are generally used as anode materials. The common pyrotechnic heat source consists of a mixture of high surface area metallic powder such as iron powder and an oxidizing agent such as potassium perchlorate. Detailed discussion relating to this pyrotechnic material and oxidizing agent is given for example in : "Pyrotechnic heater compositions for use in thermal batteries", Callaway et al., Proc. Internat. Pyrotechnics Seminar (2001) 28^th 153-168, or in "Development history of Fe/KCIO4 heat powders at Sandia and related aging issues for thermal batteries", R. Guidotti, Sandia Report SA.ND2001-2191 (2001). The main role of the pyrotechnic material when being ignited is to provide the thermal cell with the heat needed to melt the electrolyte.

The mixture used in the pyrotechnic material typically burns at a controlled rate, it must not melt, should not produce gas and must have a caloric output that can be closely controlled. It has to be insensitive enough to be palletized without being ignited and then be able to be reliably lit over a wide temperature range. After ignition the burnt pyrotechnic pellets must be electrically conductive to form inter-cell electrical connectors. For example, burning a Fe/KCIO4 pyrotechnic heat source pellet provides the heat needed to melt the electrolyte, in turn activating the thermal battery. Near stoichiometric ratios of the mixture components such as 63wt% Fe and 37wt% KCIO₄ produce high temperatures that can melt iron. The symbol wt% indicates hereinafter weight percentage. In order to moderate the chemical reaction and control the rate of burning while producing electrically conductive ash, mixtures consisting of iron and up to 17 wt% of potassium perchlorate are typically used. Such a mixture upon burning forms a KCI salt and a thermodynamically stable oxide Feo.94₇O, which is dispersed within the particles of the excessive iron.

The heat is released in a very short period of time; usually between 10msec up to 500msec. Within a short time a thermal battery reaches its maximal temperature in which its electrolyte spacers are molten and warmed. Such a process is named hereinafter as a battery's activation. A battery's temperature continuously decreases following its activation. Thermal batteries provide power as long as their temperature is higher than the respective electrolyte freezing point. In cases in which a long operating time is required, more heat is needed to keep the temperature of the electrolyte above its freezing point. Unfortunately there is a limit to the amount of heat that can be released during the battery's activation due to thermal degradation of cathodic material and electrolyte leakage at high operating temperatures. Therefore, it is practically impossible to increase the amount of heat released in order to extend battery's operational longevity, above a threshold value. This threshold is dependent on the battery's specific composition and structural characteristics

US Patent 3,899,353 discloses thermal batteries maintaining internal temperature above the freezing point of the electrolyte for an extended time by including additional pellets serving as a thermal reservoir. The additional pellets consist of salt mixtures that increase the battery's heat capacity which in turn moderate its cooling rate and therefore increase its discharge duration. However, when thermal reservoir pellets are included between thermal cells a short circuit plate must be added in order to maintain the electrical continuity in the interconnected cell stack. Furthermore, the thermal reservoir pellets utilize additional volume and therefore reduce the volume power density provided.

Practically, only a few thermal reservoir pellets can be added between the cell stack assembly and at both its ends. Otherwise, the assembly structure of the battery becomes considerably complicated.

US Patent 5,770,329 discloses thermal batteries in which the cathodes of each thermal cell are substituted with cathode precursors made of ignitable materials. These cathode precursors when ignited provide the heat required for melting the solid electrolyte. The burnt products of such cathode precursors are electrically conducting and therefore serve as a cathode in each and every thermal cell. Such batteries are considerably more efficient in terms of weight, volume and power densities, which make room for enhanced thermal insulation, thus promoting batteries' operational longevity. However, employing a cathode consisting of the burnt product of the pyrotechnic source imposes limitations on the cell voltage and therefore on the power provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the exothermic peaks of differential thermal analyses (DTA) thermograms of a typical pyrotechnic heat source pellet (PHSP) according to the present invention;

FIG. 2 is a graph showing a discharge curve of a typical thermal battery of PRIOR ART;

FIG. 3 is a graph a discharge curve of a thermal battery consisting of a PHSP of the present invention;

FIG. 4 is a graph comparing internal resistances of the battery in Fig. 2 and the battery in Fig. 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Generally, a thermal battery includes at least one cell containing an anode, a cathode, an electrolyte-separator, and a pyrotechnic heat source pellet. A pyrotechnic heat source pellet (PHSP)₁ according to the present invention consist of (i) a high surface area metallic powder (ii) an oxidizing agent and (iii) a phase moderating component. The selection of the metal powder and the oxidizing agent is made as in the prior art, considering the quantity of heat provided by the oxidation and the operating temperature of the cell and the battery. Typically, metals such as iron, cobalt, nickel, titanium, copper, vanadium, or a mixture of some or all of such metals, are used in a cell of the present invention at a weight ratio of 50% - 90%. Oxidizing agents typically used are perchlorate salts and chromate salts, but any such agent is also applicable in accordance with the present invention. The oxidizing agent is added at a weight ratio of up to 35%.

The phase moderating component (PMC) incorporated in the PHSP of the invention is introduced in order to form in combination with the derivatives of the oxidation reaction of the metal powder and the oxidizing agent, a homogeneous mixture having at least one distinct temperature in which a phase transition occurs. Such a property is achieved by using a suitable combination of materials in proper quantitative relationships. The PMC consists of halogen salts such as lithium perchlorate, sodium perchlorate,

magnesium perchlorate, an halide salt and or any mixture thereof. A

PMC typically makes up to 30% in weight of the PHSP. The total heat released by oxidising the same amount of metal in a prior art a PHSP is substantially the same as the heat released in the oxidation of a PHSP of the invention. However, only a portion of the heat evolved by burning the PHSP of the invention is consumed by the electrolyte, in turn maintaining its above freezing point temperature. Molten and warmed products of the burning reaction consume the other portion of this released heat. Some of the heat is made available to the electrolyte at a significantly later time in the form of the heat of fusion associated with at least one phase transition of the mixture of the products of the burnt PHSP. Therefore, a substantially higher amount of oxidisable metal can be oxidised according to the present invention, providing more of the heat to be released over a prolonged period of time.

Mixtures of compounds in which a few exothermic phase transitions associated with solidification are known. Studies of systems having an eutectic and one or two peritectic phase transitions occurring at different temperatures are reported for example by: (i) (Corin E. and Soifer L. Journal of Thermal Analysis, Vol. 50 (1997) 347-354; (ii) Dibirov M.A. et al., Zhurnal Neorganicheskoi Khimii (1997), 42(8), 1390-1391, and (iii) Li Ruiqing et al., Journal of Beijing University of Iron and Steel Technology (1987), 9(3), 114- 22.

As known in the art an appropriate ratio of selected components of the PMC provides for the occurrence of at least one peritectic phase transition in which a transition from uniformly liquid phase to a mixture of solid and liquid occurs. Another phase transition occurs at the eutectic temperature when the products of the burning are completely frozen. The specific reactant combination of a PHSP of the present invention, uniquely determines its eutectic temperature, which is preferably chosen closely above the freezing point of the electrolyte. However, a given combination of reactants with different concentrations can bring about at least one additional phase transition at a temperature higher than that of the eutectic temperature. The temperature of this at least one additional phase transition can be adjusted by varying concentration of selected components of the PMC. It is also possible, according to the invention, to determine the ratio between the quantities of heat released spent at melting and heating the electrolyte during battery activation, and the quantity of heat of fusion released at the phase transition points. To this end, an appropriate choice of metals, oxidizing agents and PMC constituting the PHSP of the invention at an effective quantitative consideration is implemented.

Variants of the PHSP according to the present invention consist of iron, perchlorate salt and a PMC. Other transition metals such as cobalt, nickel, copper, titanium, vanadium, and combinations thereof are also used in addition to iron. The PMC consists of lithium perchlorate, sodium perchlorate, magnesium perchlorate, a halide salt, and/or a mixture thereof. In such cases the weight percentage of the perchlorate salts can be higher than 30% whenever the heat of formation of the reaction is smaller.

In a preferred embodiment of the invention, the PHSP is prepared by mixing iron powder and perchlorate salt with a gradual addition of the PMC powders, typically by the use of a mixer such as a turbuls mixer for about one hour. The components of the PMC are sodium perchlorate, lithium perchlorate fine powder (Fluka-Germany), lithium fluoride fine powder (Merck-Germany) or a mixture thereof. The homogeneous blend is then pressed at 0.5 to 3 ton/cm².

To form a battery according to the invention, electrolyte- binder pellets are made as known in the art such as by mixing LiCI and KCI powders with a binder and by pressing the mixture at 0.5 to 3 ton/cm². The anodes are prepared by mixing lithium-aluminum alloy powder and eutectic LiCI-KCI electrolyte at a proper weight ratio, followed by pressing the mixture at 1.5-

2.5 ton/cm² for example. Cathodes are prepared as known in the art by mixing iron disulfide powder and fused all-lithium electrolyte-MgO mixture at a proper weight ratio, followed by pressing the mix at 1.5-2.5 ton/cm² for example. All anode, cathode, electrolyte and PHSP pellets are prepared in a dry environment having a relative humidity of less than 1%. A cell is formed as known in the art by an anode and a cathode separated by an electrolyte- binder pellet, and in which a PHSP is placed adjacent to the cathode.

Further features and properties provided by the present invention are described by reference to the examples and drawings below.

EXAMPLE 1

Reference is made to Fig. 1 showing two differential thermal analysis (DTA) thermograms of a PHSP according to the present invention. A sample of a PHSP of the invention is heated and then cooled afterwards, in an inert atmosphere. Curve 10 represents the temperature profile measured over time. Curve 12 represents the differences between the temperature of the sample of a PHSP of the invention and the ambient temperature measured over time while heated. Curve 13 represents the differences between the temperature of the same PHSP and the ambient temperature measured over time while sample is cooled. (The scale of the temperature axis shown relates only to curve 10, the temperature scale corresponding to curves 12 and 13 is not shown.) Exothermic reactions or exothermic phase transitions produce heat and therefore exhibit an increase in the temperature differences. Exothermic peak 14 reflects the heat released by the burning PHSP during battery's activation. Additional exothermic peaks 16 and 18 are due to the heat evolved in association with phase transitions of the products of the oxidation of the PHSP. PHSPs of the prior art produce upon oxidation a similar first exothermic peak such as 14. However, PHSPs in prior art do not produce additional exothermic peaks such as 16 and/or 18.

EXAMPLE 2

The electric discharge of a conventional battery consisting of 47 cells in series, each measuring 60 millimetres in diameter is measured. Each cell consists of 7gr PHSP. The composition of the PHSP is Fe/KCIO₄ providing about 2450 cal/pellet at a burning rate exceeding 100 mm/sec without any significant gas formation. The battery is conditioned at -54°C and then discharged at a constant load of 120W with pulses of 1250W each. Reference is made to Fig. 2 in which a discharge curve of this battery is shown. Curve 50 represents the battery's output voltage - time profile, measured at its terminals. Battery's discharge pulses 52 are induced by alternately switching on the constant load to connect the battery's terminals, employing discharge pulses as mentioned above. The terminal voltage values corresponding to discharge pulses 52 decrease in time, due to an associated increase in the battery's internal resistance, which is correlated with its decreasing temperature.

EXAMPLE 3

A thermal battery according to the present invention is constructed similarly to that of the battery of EXAMPLE 2, except for the 7 gr PHSP consisting of Fe - KCIO₄ - LiCIO₄ at a weight percent ratio of about 84:10:6 respectively. This PHSP provides by burning about 2450 cal/pellet at about same burning rate as the PHSP of EXAMPLE 2.

Reference is made to Fig. 3 showing a discharge curve of this exemplary battery. The battery's voltage 54 is measured at its terminals over time. Switching on the same load at the same switching pulses as in EXAMPLE 2, induces discharge pulses 56. The battery's voltage - time profile maintains an even level for a significantly longer time and the electrolyte's freezing point is reached later than in the case of a battery of prior art.

Reference is made to FIG. 4 in which the internal resistance over time profile of the battery of EXAMPLE 2 is compared with a respective profile of this exemplary battery of the invention, measured at different points in time. Curve 60 represents a monotonically increasing internal resistance time profile of the prior art battery. The internal resistance considerably increases after 550 seconds measured from activation, as the electrolyte approaches its freezing point. Curve 62 represents the internal resistance - time profile of the battery of the invention. After 560 seconds measured from the battery's activation in which its temperature is still above the electrolyte freezing point; the internal resistance reaches a local maximum 64 followed by a local minimum 66 associated with the additional exothermic peaks as described in EXAMPLE 1. The decrease in the internal resistance and the associated local minimum are caused by a temporary temperature increase resulting from additional heat conducted to the electrolyte.

Two exothermic peaks 16 and 18 are present in the DTA curve of a sample of a PHSP of the invention described in Fig. 1 to which reference is again made. Such exothermic peaks, which are associated with phase transitions of the products of the PHSP burning, are in fact related to the heat of fusion emitted by portions of the mixture of these products. A portion of this heat when conducted into each thermal cell separately moderates its cooling rate and substantially prolongs its operational life.

Claims

1. A method for prolonging the operational life of a cell of a thermal battery heated by burning a pyrotechnic heat source, said method comprising heating said cell by the heat of fusion emitted by products of said burning.

2. A method as in claim 1 , further comprising selecting a combination of constituents of the pyrotechnic heat source such that at least a portion of said products of said burning solidifies.

3. A method as in any of claims 1 or 2, wherein said pyrotechnic heat source consists of

• 50% - 90 % in weight of a high surface area metal powder comprising at least iron; • up to 35 % in weight of at least a perchlorate salt for oxidising said metal powder, and

• up to 30 % in weight of a phase moderating component (PMC) comprising at least a halogen salt.

4. A pyrotechnic heat source pellet (PHSP) of a thermal battery having an operating temperature range, said PHSP comprising:

• 50% - 90 % in weight of a high surface area metal powder consisting of at least iron; • up to 35 % in weight of a perchlorate salt for oxidising said metal powder, and

• up to 30 % in weight of a phase moderating component (PMC) consisting of at least a halogen salt, wherein the oxidation products of said PHSP combined with said PMC of said PHSP have at least one phase transition in which at least a portion of said oxidation products solidifies, and wherein said at least one phase transition is associated with a temperature 5 within said operating temperature range.

5. A PHSP as in claim 4, wherein said metal powder further comprises a metal selected from a group of metals including: cobalt, nickel, copper, vanadium, titanium and any combination

[0 thereof.

6. A PHSP as in claim 4, wherein said PMC consists of a salt selected from a group of salts including: lithium perchlorate, sodium perchlorate, magnesium perchlorate, an halide salt and any mixture

L 5 thereof.

7. A thermal battery of the type heated to an operating temperature range consisting of at least one cell, said at least one cell comprising:

ZO • a metallic anode selected from a group comprising lithium-iron, lithium-aluminum and lithium-silicon alloys;

• a cathode consisting of at least a compound selected from a group comprising iron sulfide, iron 5 disulfide, cobalt disulfide and cobalt oxide;

• a pyrotechnic heat source pellet (PHSP) adjacent to said cathode, wherein said PHSP comprises:

50% - 90% in weight of a high surface area metallic powder consisting of at least iron; up to 35 % in weight of perchlorate salt for oxidising said metal powder; up to 30 % in weight of a phase moderating component (PMC) consisting at least of a halogen salt, and • an electrolyte-binder pellet consisting of at I east one lithium compound separating between said anode and said cathode, and wherein the oxidation products of said PHSP combined with said PMC of said PHSP has at least one phase transition in which at least a portion of said oxidation products solidifies, and wherein said at least one phase transition is associated with a temperature within said operating temperature range.

8. A thermal battery as in claim 7, in which said electrolyte includes potassium chloride and lithium chloride.

9. A thermal battery as in claim 7, in which said el&ctrolyte is an all- lithium electrolyte.

10. A thermal battery as in claim 7, in which said metallic powder further comprising a metal selected from a group including: cobalt, nickel, titanium, vanadium, copper and any combination thereof.

11. A thermal battery of claim 7, in which said PMC is a salt selected from a group of salts including: lithium perchlorate, sodium perchlorate, magnesium perchlorate, a halide salt and any combination thereof.