JP2010015947A - Thermal battery, and electrolyte layer used for the same - Google Patents

Abstract

An electrolyte layer capable of improving battery reliability and battery performance, and a thermal battery using the electrolyte layer are provided.
A thickness change rate after applying a pressure of 10 to ²⁰ N / cm ² at a temperature equal to or higher than a melting point of the molten salt, including a molten salt and a holding body for holding the molten salt is less than 30%. And a thermal battery using the electrolyte layer.
[Selection] Figure 2

Description

  The present invention relates to a thermal battery, and specifically relates to an improvement in an electrolyte layer used in the thermal battery.

  A thermal battery is a type of storage battery, and uses a molten salt that melts at a high temperature as an electrolyte. Since the molten salt does not have ionic conductivity at room temperature, the thermal battery is in an inactive state. On the other hand, when the molten salt is heated to a high temperature, it is in a molten state and becomes a good ion conductor. For this reason, the thermal battery can be activated at a high temperature to supply electricity to the outside.

As described above, since the battery reaction does not proceed unless the molten salt is melted, the thermal battery can exhibit the same battery characteristics as immediately after production even after being stored for a period of 5 to 10 years or longer.
In addition, since an electrode reaction proceeds at a high temperature in a thermal battery, the electrode reaction is much easier to proceed than a battery using an aqueous electrolyte solution or an organic electrolyte solution. Therefore, the thermal battery is excellent in large current discharge characteristics.
Furthermore, the thermal battery can take out electric power in a short time within one second when a start signal is sent to the battery during use. For this reason, thermal batteries are used for power supplies for various defense equipment such as flying objects and induction equipment, emergency power supplies, and the like, taking advantage of the above characteristics.

  Since the molten salt is liquefied at a high temperature, in the thermal battery, the molten salt is usually held in a holding body such as an inorganic material. That is, in the thermal battery, the electrolyte layer disposed between the positive electrode and the negative electrode is generally composed of a molten salt and a holding body.

Conventionally, in order to improve battery characteristics, the electrolyte layer has been improved. For example, Patent Document 1 discloses that the electrolyte layer includes a molten salt electrolyte composed of lithium fluoride, lithium chloride, and lithium bromide, and magnesium oxide as a holding material, and the electrolyte content in the electrolyte layer is 60% by weight. As described above, it has been proposed to be less than 85% by weight.
In Patent Document 2, the electrolyte layer includes a molten salt electrolyte composed of lithium chloride and potassium chloride, and magnesium oxide as a holding material, and the electrolyte content in the electrolyte layer is 55 wt% or more and less than 65 wt%. It has been proposed to do.
Patent Document 3 proposes that the specific surface area of the holding body be 25 to 80 m ² / kg and that the electrolyte content in the electrolyte layer be 50 to 70% by weight.
JP-A-2-61962 (Patent No. 264344) Japanese Patent Laid-Open No. 2-170358 (Japanese Patent No. 2751279) JP 2004-79279 A

  In a thermal battery, the electrolyte layer requires two functions. One function is a function as a separator that prevents a short circuit caused by direct contact between the positive electrode and the negative electrode, and the other function is a function as an ion conductor (functions as an ion path by being liquefied). The same function as the electrolyte of a normal battery.)

  However, even if the above-described conventional electrolyte layer is used, it is difficult to satisfy the reliability and performance of thermal batteries that are recently required.

  Specifically, regarding the reliability of the thermal battery, the above-mentioned patent document does not particularly mention, but the verification and securing of the function as the separator of the electrolyte layer is insufficient, and depending on the configuration conditions or the manufacturing conditions, the electrolyte The layer cannot function sufficiently as a separator. In such a case, the positive electrode and the negative electrode may be short-circuited.

Regarding the performance of the thermal battery, for example, due to the recent high-tech of the equipment, the thermal battery mounted on the equipment has a higher current density (about 0.5 A · cm ⁻² ) than the average current density (about 0.5 A · cm ⁻² ). 1 A · cm ⁻² ) is required. In order to realize such battery performance higher than the conventional one, the conventional electrolyte layer is insufficient.

  Accordingly, an object of the present invention is to provide an electrolyte layer capable of improving battery reliability and battery performance, and a thermal battery using the same, in order to solve the above conventional problems. .

The electrolyte layer for a thermal battery of the present invention includes a molten salt and a holder for holding the molten salt, and has a thickness after applying a pressure of 10 to ²⁰ N / cm ² at a temperature equal to or higher than the melting point of the molten salt. The rate of change is less than 30%. The melting point of the molten salt is 430 ° C. or lower. The temperature above the melting point of the molten salt is preferably 200 ° C. or higher and 600 ° C. or lower. The holding body preferably contains an inorganic powder. More preferably, the inorganic powder includes at least one selected from the group consisting of MgO, SiO ₂ and Al ₂ O ₃ .

  The weight ratio of the molten salt to the holding body is preferably 50:50 to 90:10.

The viscosity of the molten salt at 500 ° C. is preferably less than 2.2 mN · S · m ⁻² . The conductivity of the molten salt at 500 ° C. is preferably 2 S / cm or more.

  The molten salt preferably contains an inorganic cation and an inorganic anion. The inorganic cation is at least one selected from the group consisting of a lithium cation, a sodium cation, a potassium cation, a rubidium cation, and a cesium cation, and the inorganic anion includes a fluorine anion, a chlorine anion, a bromine anion, an iodine anion, and nitric acid. It is preferably at least one selected from the group consisting of ions and carbonate ions.

  The moisture contained in the electrolyte layer is preferably 500 ppm or less.

  The present invention also relates to a thermal battery including a positive electrode, a negative electrode, and the electrolyte layer.

ADVANTAGE OF THE INVENTION According to this invention, the deformation | transformation of the electrolyte layer arrange | positioned in the state pressurized in the thermal battery can be suppressed in the operating temperature of a thermal battery. Therefore, according to the present invention, it is possible to sufficiently suppress a short circuit between the positive electrode and the negative electrode during operation of the thermal battery. That is, the reliability of the thermal battery can be further improved. Moreover, since melting | fusing point of the said molten salt is 430 degrees C or less, the operating temperature range of the thermal battery containing the said molten salt can be taken widely.
Furthermore, when the electrolyte layer contains a molten salt having high ion conductivity in the molten state, both the reliability and discharge characteristics of the thermal battery can be further improved.

FIG. 1 shows a thermal battery according to an embodiment of the present invention.
In the thermal battery of FIG. 1, a power generation unit in which a plurality of unit cells 7 and heat generating agents 5 are alternately stacked is housed in a metal outer case 1. An ignition pad 4 is disposed at the top of the power generation unit, and a spark plug 3 is installed in the vicinity of the top of the ignition pad 4. A heat conduction zone 6 is arranged around the power generation unit. Since the heat generating agent 5 contains iron powder and has conductivity, the unit cells 7 are electrically connected in series via the heat generating agent 5. The exothermic agent 5 is made of, for example, a mixture of Fe and KClO ₄ . When the battery is activated, the Fe powder is sintered as the heat generating agent 5 burns, so that the conductivity of the heat generating agent 5 is maintained from the beginning of discharge (initial combustion) to the end of discharge (end of combustion).

  The outer case 1 is sealed by a battery lid 11 having a pair of ignition terminals 2 and a positive terminal 10a and a negative terminal 10b. The positive electrode terminal 10a is connected to the positive electrode of the unit cell 7 at the top of the power generation unit via a positive electrode lead plate. On the other hand, the negative electrode terminal 10 b is connected to the negative electrode of the unit cell 7 at the bottom of the power generation unit via the negative electrode lead plate 8. A heat insulating material 9a is disposed between the battery lid 11 and the ignition pad 4, and a heat insulating material 9b is disposed between the outer case 1 and the power generation unit.

  As shown in FIG. 2, the unit cell 7 includes a negative electrode 12, a positive electrode 13, and an electrolyte layer 14 disposed between the negative electrode 12 and the positive electrode 13.

  As shown in FIG. 2, the negative electrode 12 includes a negative electrode mixture layer 15 containing a negative electrode active material, and a cup-shaped metal case 16 that houses the negative electrode mixture layer 15. The negative electrode active material is not particularly limited as long as it can be used for a thermal battery. As the negative electrode active material, for example, a lithium alloy such as lithium metal, Li—Al, Li—Si, or Li—B is used. As a constituent material of the metal case 16, for example, iron can be used.

The positive electrode 13 includes a positive electrode active material. The positive electrode active material is not particularly limited as long as it is a material that can be used in a thermal battery. As the positive electrode active material, for example, manganese oxide such as MnO ₂ , vanadium oxide such as V ₂ O ₅ , sulfide such as FeS ₂ , molybdenum oxide, or lithium-containing oxide can be used.

In the present invention, the electrolyte layer 14 is characterized. Specifically, the electrolyte layer includes a molten salt and a holding body that holds the molten salt, and the thickness of the electrolyte layer is increased when a pressure of 10 to ²⁰ N / cm ² is applied at a temperature equal to or higher than the melting point of the molten salt. The rate of change is less than 30%. The melting point of the molten salt is 430 ° C. or lower.
When the rate of change in thickness is 30% or more, the function as a separator that separates the positive electrode and the negative electrode is lowered, which may cause a short circuit.

In a thermal battery, usually, a plurality of unit cells each composed of a negative electrode, a positive electrode, and an electrolyte layer are stacked so as to obtain a predetermined voltage. At this time, for the purpose of reducing the contact resistance between the unit cells, a laminated body composed of a plurality of unit cells is stored in a pressurized state in a metal case. The pressurization to the laminate is performed in a direction perpendicular to the unit cell surface (for example, the surface 17 in FIG. 2) constituting the laminate (in the direction of arrow A in FIG. 2). The applied pressure at this time depends on the structure, such as the size and area of the laminate, but the applied pressure during the production of the thermal battery is about 10 to 20 N per unit cross-sectional area (1 cm ² ) of the unit cell (that is, 10 to 20 N · cm ⁻² ). Here, the cross-sectional area of the unit cell refers to the area of the surface that is perpendicular to the stacking direction of the unit cells and gives the maximum area as the unit cell.

  The operating temperature of the thermal battery is generally as high as about 500 ° C., and at such temperature, the molten salt melts. For this reason, the rigidity of the entire electrolyte layer is reduced at the operating temperature of the thermal battery. In particular, inside the thermal battery, the unit cell is in a pressurized state as described above. For this reason, depending on the configuration of the electrolyte layer and the manufacturing conditions of the electrolyte layer, the electrolyte layer may be deformed during the operation of the thermal battery, and the positive electrode and the negative electrode may be in direct contact with each other to cause a short circuit.

Therefore, in the present invention, as described above, an electrolyte layer having a thickness change of less than 30% when a pressure of 10 to ²⁰ N / cm ² is applied at a temperature equal to or higher than the melting point of the molten salt. Used. Even when such an electrolyte layer is arranged in a pressurized state in the thermal battery, it is suppressed from being greatly deformed at the operating temperature of the thermal battery. Therefore, a short circuit between the positive electrode and the negative electrode can be prevented. That is, according to the present invention, the reliability of the thermal battery can be increased.

  In the present invention, as described above, the melting point of the molten salt is set to 430 ° C. or lower in order to widen the operating temperature range of the thermal battery including the molten salt of the present invention. The melting point of the molten salt is preferably 200 ° C. or higher and 430 ° C. or lower.

  The temperature above the melting point of the molten salt is preferably 200 ° C. or higher and 600 ° C. or lower. Note that the upper limit of the operating temperature of the thermal battery is approximately 600 ° C.

  The change in the thickness of the electrolyte layer can be measured using, for example, an apparatus including a heating unit, a pressurizing unit, a unit for measuring the thickness, and a unit for controlling the unit.

  The heating means may be any means that can heat the electrolyte layer to the operating range of the thermal battery (500 ° C., preferably 600 ° C.) and can control the heating temperature and the like.

  The electrolyte layer usually has a thickness of 0.5 mm (500 μm) to 1.0 mm (1000 μm). Therefore, the means for measuring the thickness is preferably a means capable of measuring a thickness of at least several millimeters, that is, a thickness of about 1.0 mm (1000 μm) to about 3 mm (3000 μm). Furthermore, the measurement accuracy of the measurement means is preferably several μm (2 to 3 μm) or less, and more preferably 1 μm or less.

The pressurizing means may be any means that can pressurize the electrolyte layer to a pressurized state (10 to 20 N · cm ⁻² ) during operation of the thermal battery and can control the pressurization.
When the electrolyte layer is actually pressed by the pressurizing means, is the cross-sectional area of the jig contacting the electrolyte layer close to the cross-sectional area of the electrolyte layer in order to make it as close as possible to the pressurized state of the electrolyte layer in the thermal battery? It is desirable to be approximately equal to that. Here, the cross-sectional area of the electrolyte layer refers to the area of the surface that is perpendicular to the thickness direction of the electrolyte layer and gives the maximum area.

However, it is necessary to pressurize the electrolyte layer with a force of about 10 to 20 N / cm ² . Specifically, the applied pressure needs to be about 10 to 20 N per unit contact area (1 cm ² ) between the electrolyte layer and the pressing jig.
Since the cross-sectional area of the unit cell used in a general thermal battery is 30 mmφ, the maximum required load is as large as about 140 N. Considering the balance between the force required for pressurization and the accuracy (several μm) when measuring the thickness of the electrolyte layer, it is very difficult on the apparatus to pressurize with such a large force. . Therefore, as a result of the examination of the above point by the present inventor, it was found that the cross-sectional area of the pressing jig should be about 0.2 cm ² . In this case, the necessary pressure is about 4N at the maximum.
The cross-sectional area of the electrolyte layer sample may be approximately the same as or larger than the cross-sectional area of the pressing jig.

  For measuring the change in the thickness of the electrolyte layer, an apparatus including the above means can be used without any particular limitation. An example of such an apparatus is a thermomechanical analyzer (TMA (Thermomechanical Analyzer)).

Using the measuring device as described above, for example, as follows, the change in the thickness of the electrolyte layer when a pressure of 10 to ²⁰ N / cm ² is applied at a temperature equal to or higher than the melting point of the molten salt can be measured. it can.
First, the thickness (initial thickness) of the electrolyte layer sample is measured at room temperature and in a state where no pressure is applied. Next, for example, in a state where a pressure of 15 N · cm ⁻² is applied to the electrolyte layer sample, the electrolyte layer sample is heated from room temperature to a temperature equal to or higher than the melting point of the molten salt (a temperature at which the molten salt melts). . In this state, the thickness (thickness after heating and pressing) of the electrolyte layer sample is measured. Using the obtained initial thickness and the thickness after heating and pressing, the formula (1):
([Initial thickness]-[thickness after heating and pressing]) × 100 / [initial thickness] (1)
Thus, the rate of change of the thickness of the electrolyte layer can be obtained.

  In the above measurement, it is considered that the thermal expansion of the jig included in the measuring means and the pressurizing means affects the measurement of the sample thickness and the applied pressure. The former can be canceled by measuring in advance the change in dimension of the jig due to thermal expansion. Regarding the latter, it is desirable to control the pressurizing means in consideration of thermal expansion so that a constant pressure is applied during the measurement.

  The measurement of the change in the thickness of the electrolyte layer is preferably performed in a measurement atmosphere in which the moisture content is reduced as much as possible in order to eliminate the influence of moisture as much as possible. This is because the molten salt contained in the electrolyte layer has high hygroscopicity, and the rigidity of the electrolyte layer may change depending on the moisture contained in the measurement atmosphere. As the measurement atmosphere, for example, dry air having a dew point of −50 ° C. or lower can be used.

  The weight ratio between the molten salt contained in the electrolyte layer and the holding body is preferably 50:50 to 90:10. When the amount of the molten salt in the total weight of the molten salt and the holding body is less than 50% by weight, the discharge performance may be significantly lowered. If the amount of the molten salt in the total weight of the molten salt and the holding body is more than 90% by weight, the rate of change in thickness when the electrolyte layer is heated may not be limited to 30% or less.

The holding body included in the electrolyte layer preferably includes an inorganic powder. The inorganic material is preferably at least one selected from the group consisting of MgO, SiO ₂ , and Al ₂ O ₃ .

The specific surface area of the inorganic powder is preferably 10 to 80 m ² · g ⁻¹ . When the specific surface area of the inorganic material is smaller than 10 m ² · g ⁻¹ , the ability to retain the molten salt of the inorganic material is lowered. For this reason, the rate of change in thickness when the electrolyte layer is heated may increase. When the specific surface area of the inorganic substance is larger than 80 m ² · g ⁻¹ , the amount of molten salt retained in the inorganic substance increases, and the amount of molten salt that is relatively effective as an electrolyte decreases. For this reason, the discharge characteristics may deteriorate.

The average particle size of the inorganic powder is preferably 1 to 30 μm. When the average particle size of the inorganic material is smaller than 1 μm, the inorganic material is bulky. As a result, the density of the electrolyte layer is lowered, and thus the amount of molten salt that can be held is lowered, so that the discharge characteristics may be lowered. When the average particle size of the inorganic substance is larger than 30 μm, the moldability of the electrolyte layer is lowered, and thus the strength of the electrolyte layer is lowered. As a result, the reliability of the electrolyte layer and the unit cell including the electrolyte layer may be reduced.
The average particle diameter is a volume-based average particle diameter, and can be measured using a device known in the art.

  The conductivity of the molten salt at 500 ° C. is preferably 2 S / cm or more. The electrolyte layer containing such a molten salt has high ionic conductivity. Therefore, when the electrolyte layer contains such a molten salt, both the reliability and the discharge characteristics of the thermal battery can be further improved.

The molten salt preferably has a viscosity at 500 ° C. of less than 2.2 mN · s · m ⁻² (mPa · s). When the viscosity of the molten salt at 500 ° C. is 2.2 mN · s · m ⁻² or more, the permeability of the molten salt to the holding body is lowered, and the amount of the molten salt held is lowered. As a result, the discharge characteristics may deteriorate.

Examples of the molten salt satisfying the above characteristics (melting point, conductivity and viscosity) include the following.
Specifically, from the viewpoint of controlling the above characteristics, the molten salt constituting the electrolyte layer preferably contains two or more kinds of different salts (that is, mixed salts). That is, the characteristics of the molten salt can be controlled, for example, by appropriately changing the kind of added salt, the molar ratio of the added salt, and the like. Therefore, if it has the above characteristics, the molten salt may be a mixture of a plurality of salts such as a binary system, a ternary system, a quaternary system, and a ternary system.

  For example, the molten salt used in the present invention contains an inorganic cation and an inorganic anion. The inorganic cation is preferably at least one selected from the group consisting of a lithium cation, a sodium cation, a potassium cation, a rubidium cation, and a cesium cation. The inorganic anion is preferably at least one selected from the group consisting of a fluorine anion, a chlorine anion, a bromine anion, an iodine anion, a nitrate ion and a carbonate ion.

  The molten salt used in the present invention can include, for example, a first salt and a second salt. Examples of the first salt include at least one salt selected from the group consisting of LiF, LiCl, and LiBr. Examples of the second salt include at least one salt selected from the group consisting of NaF, NaCl, NaBr, KF, KCl, KBr, RbF, RbCl, RbBr, CsF, CsCl, and CsBr.

As a specific combination of salts contained in the molten salt, for example,
LiF-LiCl-LiBr-NaF,
LiF-LiCl-LiBr-NaF-KCl,
LiF-LiCl-LiBr-NaCl,
LiF-LiCl-LiBr-NaCl-KBr,
LiF-LiCl-LiBr-NaBr,
LiF-LiCl-LiBr-NaBr-KCl,
LiF-LiCl-LiBr-KF,
LiF-LiCl-LiBr-KF-NaF,
LiF-LiCl-LiBr-KCl,
LiF-LiCl-LiBr-KCl-KBr,
LiF-LiCl-LiBr-KBr,
LiF-LiCl-NaBr,
LiF-LiCl-KF,
LiF-LiBr-NaF,
LiF-LiBr-NaF-NaCl,
LiF-LiBr-NaF-KCl,
LiF-LiBr-NaCl,
LiF-LiBr-NaCl-KCl,
LiCl-LiBr-NaF,
LiCl-LiBr-NaF-NaCl,
LiCl-LiBr-NaCl-KF,
LiCl-LiBr-NaBr,
LiCl-LiBr-KF,
LiCl-LiBr-KF-KCl,
LiCl-NaBr-KF,
LiCl-NaBr-CsF,
LiBr-KF-NaCl,
LiBr-KF-NaBr
Etc.

  The first salt preferably accounts for 50 to 95% by weight of the molten salt. The second salt preferably accounts for 5 to 50% by weight of the molten salt.

  The composition of the molten salt is not particularly limited as long as the above characteristics are obtained. For example, since the melting point lowering effect is the greatest, the composition of the molten salt can be a composition that provides a eutectic.

The kind of salt contained in the molten salt and the composition ratio thereof are determined by, for example, fluorescent X-ray analysis and other analysis methods (for example, ICP emission analysis, atomic absorption analysis, ion chromatography, titration, etc.) It can also be obtained by using in combination.
The composition ratio of each salt contained in the molten salt can also be determined from the mixing ratio of each salt.

  The average particle size of the molten salt particles is preferably 5 to 50 μm.

The amount of moisture contained in the electrolyte layer is preferably 500 ppm or less. When the amount of moisture contained in the electrolyte layer exceeds 500 ppm, deterioration of other materials such as lithium metal, which is a negative electrode active material used in a thermal battery, may occur, resulting in a decrease in long-term storage stability.
The moisture contained in the electrolyte layer can be accurately measured by, for example, the Karl Fischer method having a high-temperature vaporizer, EGA-MS (Evolved Gas Analysis by MASS Spectrometry).
The amount of moisture contained in the electrolyte layer is, for example, the amount of moisture contained in the electrolyte layer immediately after fabrication, and the amount of moisture contained in the electrolyte layer after assembly of the thermal battery is controlled by dew point in the work environment. Therefore, there is almost no change from the amount of water contained in the electrolyte layer immediately after fabrication.

The operation of the thermal battery will be described below.
When a high voltage is applied to the ignition terminal 2 by the power source connected to the ignition terminal 2, the spark plug 3 is ignited. Thereby, the ignition pad 4 and the igniting zone 6 are combusted, and then the heat generating agent 5 is combusted, and the unit cell 7 is heated. And the electrolyte layer 14 distribute | arranged to the unit cell 7 will be in a molten state, and the said electrolyte will become an ionic conductor. In this way, the battery is activated and can be discharged.

  At least one of the positive electrode 13 and the negative electrode mixture layer 15 may contain a predetermined molten salt in order to improve ion conductivity. The molten salt contained in the positive electrode and / or negative electrode mixture layer may be the same as the molten salt used in the electrolyte layer 14. Alternatively, the molten salt used in the positive electrode and / or negative electrode mixture layer may be different from the molten salt used in the electrolyte layer 14 as long as the molten salt used in the electrolyte layer 14 has substantially the same melting point.

  In the above description, an internal heating type thermal battery in which an ignition plug is provided inside the battery and the power generation unit is heated from inside the battery to activate the battery has been described. However, the present invention does not include an ignition plug inside the battery. The present invention can also be applied to an external heating type thermal battery in which the power generation unit is heated from outside the battery by a burner or the like to activate the battery.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely, referring an Example, this invention is not limited to these Examples.

Example 1
The unit cell shown in FIG. 2 was produced according to the following procedure. In order to eliminate the influence of moisture as much as possible, the unit cell was basically produced in dry air with a dew point of −50 ° C. or less.
(1) Production of electrolyte Lithium fluoride (LiF), lithium chloride (LiCl), and lithium bromide LiBr) were used as the first salt, and sodium fluoride (NaF) was used as the second salt. These were dried in a vacuum environment at 200 ° C. for 48 hours. After drying, LiF, LiCl, LiBr, and NaF were mixed in a molar ratio of 6: 24: 60: 10 in an argon atmosphere glove box in which the dew point was controlled. An appropriate amount of the obtained mixture was transferred to a crucible made of high-purity alumina, and the crucible was put in a melting furnace. The inside of the melting furnace was an argon atmosphere with a controlled dew point. The mixture was heated and melted in a melting furnace to obtain a mixed salt (molten salt) of LiF, LiCl, LiBr, and NaF.

  The obtained molten salt was naturally cooled. After that, the molten salt was pulverized for about 12 hours by a ball mill using stainless steel balls in an argon atmosphere with a controlled dew point. The obtained molten salt powder was classified with a sieve of 60 mesh (aperture 250 μm). The average particle diameter of the molten salt after classification was about 20 μm (estimated value).

As the support, magnesium oxide (MgO) was used. The specific surface area of MgO used was 20 m ² · g ⁻¹ and the average particle size was 20 μm.

  The molten salt and MgO were mixed at a weight ratio of 60:40. The obtained mixture was heated at 500 ° C. for 12 hours in a melting furnace to fully adjust the molten salt to MgO. After heating, the mixture was naturally cooled.

  The mixture was pulverized by a ball mill using stainless steel balls in an argon atmosphere with a controlled dew point for about 12 hours. The obtained powder was classified with a sieve of 60 mesh (aperture 250 μm). The average particle diameter of the mixture powder after classification was about 25 μm (estimated value).

The powder of the obtained mixture was molded at a pressure of 3 ton / cm ² to obtain a disc-shaped electrolyte layer having a diameter of 13 mm and a thickness of about 0.5 mm.

(2) Production of positive electrode FeS ₂ powder (average particle size 12 μm) was used as the positive electrode active material. The positive electrode active material, the molten salt, and silica powder (average particle size of about 0.2 μm) were mixed at a weight ratio of 70:20:10. The obtained mixture was molded at a pressure of 3 ton / cm ² to obtain a disc-shaped positive electrode having a diameter of 13 mm and a thickness of about 0.4 mm.

(3) Production of negative electrode Li-Al alloy powder (lithium content: 20% by weight) was used as the negative electrode active material. The negative electrode active material and the molten salt were mixed at a weight ratio of 65:35. The obtained mixture was molded at a pressure of 3 ton / cm ² to obtain a negative electrode mixture layer having a diameter of 11 mm and a thickness of about 0.4 mm.
Next, the obtained negative electrode mixture layer is put into a cup-shaped metal case made of stainless steel (SUS304), the opening end portion of the metal case is bent inward, and the peripheral portion of the negative electrode mixture layer is caulked. The mixture layer was fixed in the metal case. In this way, a disc-shaped negative electrode having a diameter of 13 mm and a thickness of about 0.5 mm was obtained.

(4) Production of unit cell A positive electrode and a negative electrode were laminated via an electrolyte layer to obtain a unit cell shown in FIG. The obtained unit cell was designated as unit cell 1.

Example 2
In the production of the electrolyte layer, a unit cell 2 was obtained in the same manner as in Example 1 except that the mixing ratio of the molten salt and the holding body (MgO) was 50:50 (weight ratio).

Example 3
In the production of the electrolyte layer, a unit cell 3 was obtained in the same manner as in Example 1, except that the mixing ratio of the molten salt and the holding body was 70:30 (weight ratio).

Example 4
In the production of the electrolyte layer, a unit cell 4 was obtained in the same manner as in Example 1 except that the mixing ratio of the molten salt and the support was 80:20 (weight ratio).

Example 5
Example 1 except that LiF and LiCl were used as the first salt, NaBr was used as the second salt, and the molar ratio of LiF, LiCl, and NaBr was 7:64:30 in the preparation of the electrolyte layer. A unit cell 5 was obtained by the same method as described above.

Example 6
Example 1 except that LiF and LiBr were used as the first salt, NaCl was used as the second salt, and the molar ratio of LiF, LiBr, and NaCl was 14:71:16 in the preparation of the electrolyte layer. A unit cell 5 was obtained by the same method as described above.

Example 7
Example 1 except that LiCl and LiBr were used as the first salt, NaBr was used as the second salt, and the molar ratio of LiCl, LiBr, and NaBr was set to 44:25:31 in the preparation of the electrolyte layer. A unit cell 7 was obtained in the same manner as above.

<< Comparative Example 1 >>
A unit cell A was obtained in the same manner as in Example 1 except that a molten salt consisting only of LiCl and KCl was used in the production of the electrolyte layer, and the molar ratio of LiCl to KCl was 59:41.

<< Comparative Example 2 >>
In the production of the electrolyte layer, the same procedure as in Example 1 was applied except that a molten salt composed of LiF, LiCl, and LiBr was used and the molar ratio of LiF, LiCl, and LiBr was set to 21:23:56. Battery B was obtained.

[Evaluation]
(1) Evaluation method of electrolyte layer As explained above, a pressure of 15 N · cm ⁻² was applied to the electrolyte layer, and the rate of change in thickness of the electrolyte layer when heated from room temperature to 500 ° C. was determined.
Specifically, first, an electrolyte layer sample having a diameter of 26 mm and a thickness of 1.2 mm (initial thickness) was prepared.
Using a predetermined thermomechanical analyzer, a pressure of 15 N · cm ⁻² was applied, and the thickness of the electrolyte layer sample (thickness after heating and pressing) when heated from room temperature to 500 ° C. was measured.
Using the above formula (1), the rate of change in the thickness of the electrolyte layer was determined from the initial thickness and the thickness after heating and pressing.

The viscosity at 500 ° C., the conductivity at 500 ° C., and the melting point of the molten salt used in each example and each comparative example were evaluated as follows.
Furthermore, the battery characteristics of the unit cell were measured as follows.

(2) Evaluation method of viscosity of molten salt The evaluation of the viscosity of the electrolyte was performed as follows.
The molten salt sample was heated to 500 ° C. and melted. The molten sample was placed in a sample container and the sample temperature was maintained at 500 ° C. A probe suspended from the top of the sample was immersed in the molten sample in the sample container. With the probe immersed in the sample, the sample container was rotated at 20 or 200 revolutions per minute. The torsional stress generated in the probe was measured by rotation, and the obtained value was defined as the viscosity of the molten salt at 500 ° C.
In this measurement, LiCl-KCl molten salt, for which the experimental value of viscosity has already been reported, was used as a standard sample, and was used for calibration of the measuring apparatus.

  As described above, the molten salt has a high hygroscopic property, and there is a concern that the molten salt may change in weight and properties due to moisture in the measurement atmosphere. For this reason, the handling of the molten salt sample and the measurement of the viscosity were performed in dry air having a dew point of −50 ° C. or less in order to eliminate the influence of moisture as much as possible.

(3) Method for evaluating conductivity of molten salt The measurement of conductivity is performed by measuring Sato et al. (For example, “Density and Electrical Conductivity of Molten NaF—AlF ₃ System”, Joe Sato et al. No. 12 (1977)) and the like. Specifically, a conductivity cell composed of a quartz body and a platinum electrode was fabricated. A KCl aqueous solution or LiCl-KCl molten salt having a known electrical conductivity value was measured in advance by the AC impedance method, and the cell constant of the conductivity cell was determined. Using the conductivity cell for which the cell constant was determined, the resistance value of the molten salt at 500 ° C. was measured. Using the obtained resistance value, the conductivity of the molten salt was obtained. In addition, the electrical conductivity of the molten salt at an arbitrary temperature can be accurately measured by this method.

(4) Evaluation of Melting Point of Molten Salt The melting point of the molten salt used in the electrolyte was determined by differential scanning calorimetry (DSC). The molten salt sample was heated to a predetermined temperature at a predetermined temperature increase rate, and then cooled at a predetermined temperature decrease rate. The melting point of the molten salt was determined from the start temperature of the endothermic peak when the sample was heated and the start temperature of the exothermic peak when the sample was cooled.

(5) Evaluation method of unit cell The battery characteristics of a unit cell, which is one unit of a battery, were evaluated as follows.
The unit cell was sandwiched between two plates capable of controlling the temperature with such a force that the positive electrode and the negative electrode were not damaged, and heated to a predetermined temperature. The unit cell was discharged at the predetermined temperature until the battery voltage reached 1.2 V, and the discharge capacity was determined.
The heating temperature of the unit cell was set to 500 ° C., which is the operating temperature of a conventional thermal cell including an electrolyte layer containing LiCl—KCl molten salt, and 425 ° C. in order to investigate the influence on the low temperature side.
The current value when discharging the unit cell was 1.0 A / higher than the average current value (0.5 A / cm ² ) at 500 ° C. of the conventional thermal battery including the electrolyte layer containing the LiCl—KCl molten salt. cm ² . The current value is a current value per unit cross-sectional area of the unit cell.

The discharge capacity at each temperature and each discharge current value of each unit cell is the discharge when each unit cell is discharged at a current of 0.5 A / cm ² at 500 ° C. until the battery voltage drops to 1.2 V. It was determined as a capacity ratio to capacity (that is, a relative ratio of each unit cell with a discharge capacity value at 500 ° C. and a discharge current of 0.5 A / cm ² as 100%).
In the above examples and comparative examples, the discharge capacity was determined for five unit cells for each temperature, and the average value of the obtained ratios was taken as the capacity ratio.

  Table 1 shows the types and composition ratios of the salts constituting each molten salt. Table 2 shows the viscosity of the molten salt at 500 ° C., the conductivity and melting point at 500 ° C., the weight ratio of the molten salt to the support (MgO) in the electrolyte layer, and the change in thickness when the electrolyte layer is pressurized and heated. And the capacity ratio at 425 ° C and the capacity ratio at 500 ° C.

As described above, the electrolyte layer is required to have a function as a separator that prevents a short circuit due to contact between the positive electrode and the negative electrode, and a function as an ion conductor.
First, from the viewpoint of the function of the electrolyte layer as a separator, when comparing the rate of change of the thickness of the electrolyte layer, as shown in Table 2, in the unit cells 1 to 7, compared to the unit cells A to C, It can be seen that the deformation rate is less than 30% and the separator function is sufficient.

Next, when the function of the electrolyte layer as an ion conductor is compared from the viewpoint of battery characteristics, as shown in Table 2, in the unit cells 1 to 7, compared with the unit cells A and B, 425 ° C. and 500 ° C. At both degrees Celsius, it has a high discharge performance of over 90% despite a high discharge current value of 1.0 A / cm ² . That is, it can be seen that the electrolyte layer used in the present invention can sufficiently function as an ionic conductor.

This is considered as follows.
As shown in Table 2, the molten salt used in the examples has a lower viscosity than the molten salt used in the comparative example. Since the viscosity is low, the molten salt used in the examples tends to penetrate into the holding body as compared with the molten salt used in the comparative example. If the amount of molten salt absorbed inside the holding body is large, the amount of molten salt existing outside the holding body is relatively lowered. For this reason, even when the electrolyte layer is heated to a temperature equal to or higher than the melting point of the molten salt, the degree of decrease in rigidity of the electrolyte layer is small. Therefore, a highly reliable thermal battery can be provided by making the rate of change of the thickness of the electrolyte layer in the operating state of the thermal battery less than 30%.

  Furthermore, in the electrolyte layer, if the amount of the molten salt existing outside the holding body is reduced, it is considered that the function of the electrolyte layer as an ionic conductor is lowered. However, by using a molten salt having higher conductivity than the conventional molten salt as in the examples, the electrolyte layer can be used as an ionic conductor even when the amount of the molten salt existing outside the holding body is smaller than the conventional one. Can function as well. Therefore, in this case, it is possible to provide a thermal battery having high reliability and further improved battery characteristics.

On the other hand, the molten salt used in Comparative Example 1 has a high viscosity and is considered to be inferior in the permeability of the molten salt into the holding body. For this reason, in the electrolyte layer used in Comparative Example 1, the amount of molten salt absorbed inside the holding body is small, and the amount of molten salt present around the holding body is relatively large. When the amount of the molten salt present around the holding body (that is, outside the holding body) increases, the rigidity of the electrolyte layer decreases when the electrolyte layer is heated to the melting point of the molten salt or higher. In such a case, the electrolyte layer cannot sufficiently function as a separator.
Furthermore, the molten salt used in Comparative Example 1 has a lower conductivity than the molten salt used in the Examples, and cannot exhibit sufficient ionic conductivity. For this reason, the capacity ratio at 425 ° C. of the unit cell of Comparative Example 1 is lower than the capacity ratio at 425 ° C. of Examples 1-7.

  Regarding the electrolyte layer, the melting point of the molten salt contained therein is also an important factor. The molten salt used in Comparative Example 2 has a melting point of 438 ° C. For this reason, the capacity ratio at 425 ° C. is significantly reduced. Although it is preferable that the operating temperature range of the thermal battery is wide, when the unit cell produced in Comparative Example 2 is used, it is difficult to take a wide operating temperature range of the thermal battery.

  A thermal battery often requires a high voltage in addition to a discharge at a high current. For this reason, as shown in FIG. 1, a general thermal battery includes a power generation unit in which a plurality of unit cells and heating agents are alternately stacked. The performance of the thermal battery depends greatly on the performance of the unit cell. In the present invention, the performance of the electrolyte layer contained in the unit cell is improved, and thus the unit cell performance is improved. Therefore, according to the present invention, a high-performance thermal battery can be provided regardless of the number of unit cells to be stacked.

  In addition, in the said Example, although the shape of the electrolyte layer was made into the disk shape of diameter 13mm, the magnitude | size and shape of an electrolyte layer are not specifically limited. The shape of the electrolyte layer may be, for example, a donut shape with a hole in the center, a semicircular shape, or a square shape.

  According to the present invention, there are provided an electrolyte layer having a sufficient separator function and ion conduction function and capable of exhibiting sufficient performance in terms of reliability and performance, and a thermal battery using the electrolyte layer. be able to.

It is the perspective view made into a section by notching a part of the thermal battery according to an embodiment of the present invention. It is a longitudinal cross-sectional view of the unit cell used for the thermal battery of one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exterior case 2 Ignition terminal 3 Spark plug 4 Ignition pad 5 Heating agent 6 Heating zone 7 Unit cell 8 Negative electrode lead plate 9a, 9b Thermal insulation material 10a Positive electrode terminal 10b Negative electrode terminal 11 Battery cover 12 Negative electrode 13 Positive electrode 14 Electrolyte 15 Negative electrode mixture Layer 16 Current collector

Claims (11)

An electrolyte layer for a thermal battery,
The electrolyte layer includes a molten salt and a holding body for holding the molten salt, and a rate of change in thickness of the electrolyte layer after applying a pressure of 10 to ²⁰ N / cm ² at a temperature equal to or higher than the melting point of the molten salt. Is less than 30%,
The electrolyte layer whose melting point of the said molten salt is 430 degrees C or less.   The electrolyte layer according to claim 1, wherein the temperature of the molten salt is equal to or higher than the melting point and is 200 ° C or higher and 600 ° C or lower.   The electrolyte layer according to claim 1 or 2, wherein a weight ratio of the molten salt to the holding body is 50:50 to 90:10.   The electrolyte layer according to claim 1, wherein the holding body includes an inorganic powder. The electrolyte layer thermal battery according to claim 4, wherein the inorganic powder includes at least one selected from the group consisting of MgO, SiO _2, and Al ₂ O ₃ . The electrolyte layer according to claim 1, wherein the molten salt has a viscosity at 500 ° C. of less than 2.2 mN · S · m ⁻² .   The electrolyte layer according to any one of claims 1 to 6, wherein the electric conductivity of the molten salt at 500 ° C is 2 S / cm or more.   The electrolyte layer according to claim 1, wherein the molten salt contains an inorganic cation and an inorganic anion.   The inorganic cation is at least one selected from the group consisting of a lithium cation, a sodium cation, a potassium cation, a rubidium cation, and a cesium cation. The inorganic anion includes a fluorine anion, a chlorine anion, a bromine anion, an iodine anion, and nitric acid. The electrolyte layer according to claim 8, which is at least one selected from the group consisting of ions and carbonate ions.   The electrolyte layer in any one of Claims 1-9 whose water | moisture content contained in the said electrolyte layer is 500 ppm or less.   A thermal battery comprising the positive electrode, the negative electrode, and the electrolyte layer according to claim 1 disposed between the positive electrode and the negative electrode.

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