JP2022128927A - Electrolyte solution for fluorine battery, and fluorine battery - Google Patents

Abstract

To provide a novel electrolyte solution for a fluorine battery, which allows a fluorine ion to react with an electrode, and a fluorine battery.SOLUTION: An electrolyte solution for a fluorine battery comprises: a fluorine-containing supporting electrolyte; and a solvent, dissolving the supporting electrolyte, and containing an asymmetric type fluorine-containing chain ether having a linear first carbon chain group and a linear second carbon chain group different from the first carbon chain group, and including one or more fluorine atoms in the first carbon chain group or the second carbon chain group.SELECTED DRAWING: None

Description

Disclosed herein are fluorine battery electrolytes and fluorine batteries.

Conventionally, as a fluorine battery, for example, at least one of the positive electrode and the negative electrode contains a metal ion-containing fluoride as an active material, and the anion receptor contained in the electrolyte is a fluoride ion and a salt or complex of the metal ion-containing fluoride. It has been proposed that a metal ion-containing fluoride can be dissolved in an electrolytic solution by forming a (see, for example, Patent Document 1). This battery is described as being able to provide a secondary battery that has a small difference between the voltage during discharge and the voltage during charge, has good energy efficiency, and has a long charge/discharge life.

JP 2015-125934 A

However, in the secondary battery described in Patent Document 1 described above, cyclic and chain carbonates used in lithium batteries are used as the solvent for the electrolyte, but metal fluorides are not generated on the positive electrode in the charging reaction. there were. In some fluorine batteries, metal fluorides are generated on the positive electrode during charging, and the metal fluorides on the positive electrode are decomposed during discharging to release fluorine ions into the electrolyte, thereby charging and discharging. Fluorine batteries that perform such charging and discharging are preferable from the viewpoint of energy density, and there has been a demand for members that generate metal fluorides on the electrodes.

The present disclosure has been made in view of such problems, and a main object thereof is to provide a novel fluorine battery electrolyte solution and a fluorine battery in which fluorine ions and electrodes can react.

As a result of intensive research to achieve the above-mentioned object, the present inventors found that in a fluorine battery, when a bilaterally asymmetric fluorine-containing chain ether is used as a solvent for the electrolyte, a fluoride of this metal is formed on the metal surface during charging. and completed the invention disclosed herein.

That is, the fluorine battery electrolyte disclosed herein is
a supporting salt containing fluorine;
Dissolving the supporting salt, having a chain-like first carbon chain group and a chain-like second carbon chain group different from the first chain carbon chain group, the first carbon chain group and the second chain carbon chain group A solvent containing an asymmetric fluorine-containing linear ether containing one or more fluorines in any one of
It has

The fluorine battery disclosed herein is
a positive electrode having a metal that forms a metal fluoride as a positive electrode active material;
a negative electrode;
The fluorine battery electrolyte solution according to any one of claims 1 to 5, interposed between the positive electrode and the negative electrode;
is provided.

INDUSTRIAL APPLICABILITY The present disclosure can provide a novel fluorine battery electrolyte and a fluorine battery in which fluorine ions and electrodes can react. The reason why such an effect is obtained is presumed as follows, for example. When an asymmetric fluorine-containing linear ether is used as a solvent for the electrolyte of a fluorine battery, the interaction between the fluorine ion and the oxygen atom in the ether solvent stabilizes the fluorine ion and promotes the formation of fluorine compounds at the metal interface. , it is presumed that this metal fluoride can be generated on the metal surface during charging.

1 is a schematic diagram showing an example of a fluorine battery 10; FIG. Explanatory drawing which shows an example of a fluorine battery. XRD spectrum of the positive electrode of Experimental Example 1. 3 is a charge/discharge curve of Experimental Example 1. FIG. XRD spectrum of the positive electrode of Experimental Example 2. The charge-discharge curve of Experimental Example 2. XRD spectrum of the positive electrode of Experimental Example 3. XRD spectrum of the positive electrode of Experimental Example 4. XRD spectrum of the positive electrode of Experimental Example 5. XRD spectrum of the positive electrode of Experimental Example 6. XRD spectrum of the positive electrode of Experimental Example 7. XRD spectrum of the positive electrode of Experimental Example 8. The charge-discharge curve of Experimental Example 8. XRD spectrum of the positive electrode of Experimental Example 9. The charge-discharge curve of Experimental Example 9. XRD spectrum of the positive electrode of Experimental Example 10. The charge-discharge curve of Experimental Example 10. XRD spectrum of the positive electrode of Experimental Example 11. A charge-discharge curve of Experimental Example 11. FIG. XRD spectrum of the positive electrode of Experimental Example 12.

(Electrolyte for fluorine batteries)
The fluorine battery electrolyte of the present disclosure is an electrolyte used in a fluorine battery using fluorine. This fluorine battery electrolyte contains a supporting salt and a solvent. The supporting salt contains fluorine. Also, the solvent is a liquid that dissolves the supporting salt.

(solvent)
The solvent of the present disclosure has a chain-shaped first carbon chain group and a chain-shaped second carbon chain group different from the first carbon chain group, and the first carbon chain group and the second carbon chain group Any one of them contains an asymmetric fluorine-containing chain ether containing one or more fluorine atoms. The first carbon chain group may have a carbon chain, and examples thereof include alkyl groups. The alkyl group, for example, may be linear or branched. The first carbon chain group may be, for example, an asymmetric fluorine-containing chain ether that is liquid within the temperature range of the fluorine battery, and preferably has 8 or less carbon atoms, more preferably 4 or less carbon atoms. Examples of the first carbon chain group include methyl group, ethyl group, propyl group, butyl group and the like. The first carbon chain group may contain one or more fluorine atoms, or may be fluorine-free when the second carbon chain group contains one or more fluorine atoms. The first carbon chain group has a fluoro group as a substituent. The first carbon chain group may contain any number of fluoro groups at any position.

The second carbon chain group is different from the first carbon chain group. The second carbon chain group may differ from the first carbon chain group in one or more of the positions of fluorine and hydrogen, the number of carbon atoms, and the number of fluorine atoms. The second carbon chain group may have the same number of carbon atoms or the same number of fluorine atoms as the first carbon chain group, but in this case, the substitution position of fluorine may be different. The second carbon chain group may have a carbon chain, such as an alkyl group. The alkyl group, for example, may be linear or branched. The second carbon chain group may be, for example, an asymmetric fluorine-containing chain ether that is liquid within the temperature range of the fluorine battery, and preferably has 8 or less carbon atoms, more preferably 4 or less carbon atoms. Examples of the second carbon chain group include methyl group, ethyl group, propyl group, butyl group and the like. The second carbon chain group may contain one or more fluorines, or may not contain fluorine when the first carbon chain group contains one or more fluorines. The second carbon chain group may contain any number of fluoro groups at any position.

The solvent of the present disclosure may be an asymmetric fluorine-containing chain ether represented by the following formula (1). In this asymmetric fluorine-containing chain ether, the C _n H _2n+1-x F _x group is the first carbon chain group and the C _m H _2m+1-y F _y group is the second carbon chain group. or vice versa. In formula (1), the C _n H _2n+1-x F _x group and the C _m H _2m+1-y F _y group may be different. Also, it may satisfy n≦4 and m≦4. Further, when 1≤x, 0≤y may be set, and when 0≤x, 1≤y may be set. Alternatively, n<m or n>m may be used. Also, when n=m, x and y may be different, and when n=m and x=y, the substitution position of the fluoro group (-F) may be different. Examples of such asymmetric fluorine-containing chain ethers include ether compounds represented by formulas (2) to (4). This ether compound includes, for example, one or more of tetrafluoroethyltrifluoroethyl ether (TFTrFEE), methoxyperfluorobutane (MePFB), and ethyl nonafluorobutyl ether (ENFBE).

This solvent may contain other additive compounds in addition to the asymmetric fluorine-containing chain ether. This solvent may, for example, contain an ether compound as an additive compound. Examples of the ether compound include symmetric hydrocarbon chain ethers, symmetric fluorine-containing chain ethers, cyclic ethers, and fluorine-containing cyclic ethers. This ether compound includes, for example, ether compounds represented by formulas (5) to (8). Examples of the ether compound include bis(trifluoroethylethyl) ether (BTFE), propylene carbonate (PC), fluoroethylene carbonate (FEC), tetraglyme (TGym: G4), triglyme, diglyme, and the like. This solvent may contain, for example, an additive compound in a molar ratio of 1/100 or more to 1/2 or less with respect to the asymmetric fluorine-containing linear ether.

The supporting salt is, for example, a compound that dissolves in the above solvent in the temperature range of use of the fluorine battery and contains fluorine ions. For example, an alkylammonium fluoride salt is preferable. Alkyl ammonium ions can be used as carriers by being adsorbed on the negative electrode. This supporting salt may be, for example, an alkylammonium fluoride salt represented by formula (9). In formula (9), R1 to R4 are hydrocarbon groups, and two or more of each may be the same or may be different. This hydrocarbon group may be, for example, a linear or branched chain alkyl group. The hydrocarbon group may have 6 or less carbon atoms, 4 or less carbon atoms, or may have a substituent. Examples of substituents include alkyl groups and fluoro groups. The supporting salts include tetramethylammonium fluoride, tetraethylammonium fluoride, tetrapropylammonium fluoride, tetrabutylammonium fluoride, and the like. Among these, for example, tetraethylammonium fluoride (TEAF) represented by formula (10) is preferable.

(fluorine battery)
The fluorine battery of the present disclosure may include a positive electrode, a negative electrode, and the above-described fluorine battery electrolyte interposed between the positive electrode and the negative electrode. The fluorine battery electrolyte contains the above-described supporting salt and solvent, and detailed description thereof will be omitted.

The positive electrode may have, for example, a metal that forms a metal fluoride as a positive electrode active material. Also, the positive electrode may include a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector. As the positive electrode active material, for example, one or more elements selected from Au, Pt, S, Ag, Fe, Co, Ni, Cu, Cr, Mo, W, V, Sb, Bi, Sn, In, Ce and Pb are used. may be included. This positive electrode active material may be, for example, one or more metals selected from Sn, In, Ce, and Pb, and alloys thereof. This positive electrode active material may contain, for example, a fluoride containing one or more of the above elements. For example, the positive electrode active material may be formed of metal foil or alloy foil, may be a green compact containing metal powder or alloy powder, or may be formed by vapor deposition. The positive electrode current collector is a conductor, and is not particularly limited as long as it has a noble oxidation-reduction potential with respect to the positive electrode active material. One or more of Sb, Bi, Sn, Ni, Pb, Fe, Cr, Zn, In, Ti, Ga, Mn, Al and Zr may be included. Also, the positive electrode current collector may be formed in a layer on the positive electrode active material. The positive electrode current collector and the positive electrode active material may be, for example, metal foils or alloy foils, which may be applied under pressure, or one of the positive electrode active material and the positive electrode current collector may be deposited on the other by a vapor deposition method. It is good also as what is formed.

The negative electrode preferably absorbs and releases carrier ions, and may be one that adsorbs and releases carrier ions like a capacitor, or one that inserts and desorbs carrier ions like an ion secondary battery. . When the negative electrode is configured as a capacitor, it is not particularly limited as long as it can adsorb carrier ions. For example, it may be a noble metal plate. The negative electrode may be made of, for example, one or more of Pt, Au, Ag, and the like. The negative electrode may include a negative electrode current collector and a negative electrode active material formed on the negative electrode current collector. As the negative electrode current collector, the same one as the positive electrode current collector described above can be used.

A fluorine battery of the present disclosure may include a separator between the negative electrode and the positive electrode. This separator is a member that prevents direct contact between the positive electrode and the negative electrode, and allows the fluorine battery electrolyte to pass therethrough. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the fluorine battery. are mentioned. Alternatively, filter paper may be used as a separator. These may be used alone, or may be used in combination.

The shape of the fluorine battery is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, and the like. FIG. 1 is a schematic diagram showing an example of a fluorine battery 10. As shown in FIG. This fluoride ion battery 10 includes a positive electrode 17, a negative electrode 18, and an electrolyte solution 16 for a fluorine battery. The positive electrode 17 has a positive electrode current collector 11 and a positive electrode active material 12 formed on the positive electrode current collector 11 . The negative electrode 18 has a negative electrode current collector 14 and a negative electrode active material 15 formed on the negative electrode current collector 14 . A separator 19 is arranged between the positive electrode 17 and the negative electrode 18 . The fluorine battery electrolyte 16 is interposed between the positive electrode 17 and the negative electrode 18 . The fluorine battery electrolyte 16 contains a supporting salt and a solvent containing the asymmetric fluorine-containing chain ether described above.

A specific example of a fluorine battery will be described. FIG. 2 is an explanatory diagram showing an example of a fluorine battery. In this fluorine battery, the positive electrode is Sn, the negative electrode is Pt, the electrolyte for the fluorine battery is TEAF as a supporting salt, and the asymmetric fluorine-containing chain ether is used as a solvent. In this fluorine battery, Sn and F generate SnF2, which is a metal fluoride, and cations are adsorbed on the negative electrode to charge the battery, and as the cations are released into the electrolyte, the metal fluoride is decomposed to discharge. do. This fluorine battery exhibits an electromotive force of -0.1375 V with respect to a standard hydrogen electrode (SHE).

With the fluorine battery electrolyte and the fluorine battery described in detail above, it is possible to provide a novel fluorine battery electrolyte and a fluorine battery in which fluorine ions and electrodes can react. The reason why such an effect is obtained is presumed as follows, for example. For example, even if a symmetric chain ether having the same left and right groups or a cyclic ether is used as the solvent for the electrolytic solution, no metal fluoride is produced at the positive electrode. On the other hand, when an asymmetric fluorine-containing chain ether is used as a solvent for the electrolyte of a fluorine battery, the interaction between fluorine ions and oxygen atoms in the ether solvent stabilizes the solution and promotes the formation of fluorine compounds at the metal interface. It is speculated that this metal fluoride can be generated on the metal surface during charging.

It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

An example in which the electrolytic solution disclosed in the present specification was specifically prepared will be described below as an experimental example. Experimental Examples 1, 3 to 6, and 8 to 10 correspond to Examples, and Experimental Examples 2, 7, 11, and 12 correspond to Comparative Examples.

[Experimental example 1]
Tetraethylammonium fluoride (TEAF, manufactured by Aldrich) and tetrafluoroethyltrifluoroethyl ether (TFTrFEE, Tokyo Kasei) were mixed at a molar ratio of 1:36 to prepare a fluorine battery electrolyte. This electrolytic solution, a 0.1 mm thick tin disc (Sn, 18 mm in diameter, manufactured by Nilaco), a 0.05 mm thick platinum disc (Pt, 14 mm in diameter, manufactured by Nilaco), and a 0.25 mm thick filter paper. (18 mm in diameter, 5C manufactured by Advantech) was produced. Using a Hokuto Denko charge/discharge device, the Sn electrode is the positive electrode and the Pt electrode is the negative electrode, and the battery is charged at a current of 0.01 mA for 2 hours, and then discharged at a current of 0.005 mA to −0.6 V. repeated times. Subsequently, after charging with a current of 0.01 mA for 6 to 10 hours, the Sn electrode was taken out from the coin cell, and after washing, the XRD measurement of the Sn electrode (Rigaku Ultimate, sweep rate 10 ° C./min, X-ray source CuKα) was performed. The presence or absence of SnF ₂ formation was confirmed by comparison with the XRD spectrum of commercially available SnF ₂ powder. 3 is an XRD spectrum of the positive electrode of Experimental Example 1. FIG. 4 is a charge/discharge curve of Experimental Example 1. FIG. The * mark in FIG. 3 is a signal derived from SnF ₂ , and it was confirmed that SnF ₂ was generated on the Sn electrode surface when a bilaterally asymmetric chain ether solvent was used.

(Experimental example 2)
Experimental Example 2 was the same as Experimental Example 1, except that bis(trifluoroethyl)ether (BTFE, manufactured by Aldrich) was used instead of tetrafluoroethyltrifluoroethyl ether in Experimental Example 1. The TEAF:BTFE molar ratio was 1:23. 5 is an XRD spectrum of the positive electrode of Experimental Example 2. FIG. 6 is a charge/discharge curve of Experimental Example 2. FIG. As shown in FIG. 5, in Experimental Example 2, which is a symmetrical linear ether solvent, no signal derived from SnF ₂ was detected.

(Experimental example 3)
Experimental Example 3 was the same as Experimental Example 1 except that a small amount of fluoroethylene carbonate (FEC, Kishida Chemical Co., Ltd.) was added to the electrolytic solution of Experimental Example 1. The molar ratio of TEAF:FEC:TFTrFEETEAF:FEC:TFTrFEE was 1:1:15. 7 is an XRD spectrum of the positive electrode of Experimental Example 3. FIG. The * mark in the figure is a signal derived from SnF ₂ , and it was confirmed that SnF ₂ was formed on the Sn electrode surface also in Experimental Example 3 containing a bilaterally asymmetric chain ether solvent.

(Experimental example 4)
Experimental Example 4 was the same as Experimental Example 3, except that methoxyperfluorobutane (MePFB, manufactured by Aldrich) was used instead of tetrafluoroethyltrifluoroethyl ether in Experimental Example 3. The molar ratio of TEAF:FEC:MePFB was 1:1:18. 8 is an XRD spectrum of the positive electrode of Experimental Example 4. FIG. The * mark in the figure indicates a signal derived from SnF ₂ , and it was confirmed that SnF ₂ was formed on the Sn electrode surface also in Experimental Example 4 containing a bilaterally asymmetric chain ether solvent.

(Experimental example 5)
Experimental Example 5 was the same as Experimental Example 3, except that 30% of the tetrafluoroethyltrifluoroethyl ether in Experimental Example 3 was replaced with bis(trifluoroethyl)ether (BTFE). The molar ratio of TEAF:FEC:TFTrFEE:BTFE was 1:1:16:8. 9 is an XRD spectrum of the positive electrode of Experimental Example 5. FIG. The * mark in the figure is a signal derived from SnF ₂ , and it was confirmed that SnF ₂ was formed on the Sn electrode surface also in Experimental Example 5 containing a bilaterally asymmetric chain ether solvent.

(Experimental example 6)
Experimental example 6 was the same as experimental example 3, except that ethylnoafluorobutyl ether (ENFBE, Tokyo Chemical Industry Co., Ltd.) was used instead of tetrafluoroethyltrifluoroethyl ether. The molar ratio of TEAF:FEC:ENFBE was 1:1:15. 10 is the XRD spectrum of the positive electrode of Experimental Example 6. FIG. The * mark in the figure indicates a signal derived from SnF ₂ , and the generation of SnF ₂ on the Sn electrode surface was also confirmed in Experimental Example 6 containing a bilaterally asymmetric chain ether solvent.

(Experimental example 7)
Experimental Example 7 was the same as Experimental Example 1 except that fluoroethylene carbonate (FEC) was used in place of tetrafluoroethyltrifluoroethyl ether in Experimental Example 1. The TEAF:FEC molar ratio was 1:24. 11 is the XRD spectrum of the positive electrode of Experimental Example 7. FIG. In Experimental Example 7, no signal derived from SnF ₂ was detected.

(Experimental Example 8: In case of In electrode)
Experimental Example 8 was the same as Experimental Example 3 except that indium In (manufactured by The Nilaco Corporation) was used instead of tin Sn. With the In electrode as the positive electrode and the Pt electrode as the negative electrode, the battery was charged at a current of 0.01 mA for 2 hours, then discharged at a current of 0.005 mA to −0.6 V, and then charged at a current of 0.01 mA for 4 hours. 12 is the XRD spectrum of the positive electrode of Experimental Example 8. FIG. 13 is a charge-discharge curve of Experimental Example 8. FIG. The * mark in the figure is the signal derived from InF ₃ , and the formation of InF ₃ on the In electrode surface was confirmed.

(Experimental Example 9: Ce electrode)
Experimental example 9 was the same as experimental example 3 except that cerium (Ce) (manufactured by Johnson Matthey) was used instead of tin Sn. With the Ce electrode as the positive electrode and the Pt electrode as the negative electrode, charge at a current of 0.005 mA for 1.5 hours, then discharge at a current of 0.005 mA to −1.9 V, and then charge at a current of 0.005 mA for 4 hours. did. 14 is the XRD spectrum of the positive electrode of Experimental Example 9. FIG. 15 is a charge/discharge curve of Experimental Example 9. FIG. The * mark in the figure is a signal derived from CeF ₃ , and the formation of CeF ₃ on the Ce electrode surface was confirmed.

(Experimental Example 10: Pb electrode)
Experimental example 10 was the same as experimental example 3 except that lead Pb (manufactured by The Nilaco Corporation) was used instead of tin Sn. With the Pb electrode as the positive electrode and the Pt electrode as the negative electrode, the battery was charged at a current of 0.01 mA for 2 hours, and then discharged to −0.6 V at a current of 0.005 mA, which was repeated twice. Subsequently, the battery was charged with a current of 0.01 mA for 5 hours. 16 is the XRD spectrum of the positive electrode of Experimental Example 10. FIG. 17 shows charge-discharge curves of Experimental Example 10. FIG. As shown in FIG. 16, the * mark in the figure indicates the signal derived from PbF ₂ , and the formation of PbF ₂ on the Pb electrode surface was confirmed.

(Experimental example 11)
Experimental Example 11 was the same as Experimental Example 7 except that propylene carbonate PC (manufactured by Kishida Chemical Co., Ltd.) was used in place of fluoroethylene carbonate (FEC) in Experimental Example 7. The TEAF:PC molar ratio was 1:24. 18 is the XRD spectrum of the positive electrode of Experimental Example 11. FIG. 19 is a charge-discharge curve of Experimental Example 11. FIG. In Experimental Example 11, no signal derived from SnF ₂ was detected.

(Experimental example 12)
Experimental Example 12 was the same as Experimental Example 7 except that Tetraglyme TGym (G4, manufactured by Kishida Chemical Co., Ltd.) was used instead of fluoroethylene carbonate (FEC) in Experimental Example 7. The TEAF:TGym molar ratio was 1:24. 20 is the XRD spectrum of the positive electrode of Experimental Example 12. FIG. As shown in FIG. 20, in Experimental Example 12, no signal derived from SnF ₂ was detected.

Table 1 summarizes the solvents used in the experimental examples, the presence or absence of fluoride formation on the electrode, the chemical structures of the solvents, and the like. As shown in Table 1, cyclic ethers and symmetric chain ethers alone did not produce fluorides on metals. On the other hand, it has been found that in the case of compounds containing bilaterally asymmetric fluorine-containing chain ethers, for example, even if they contain cyclic ethers or bilaterally symmetric chain ethers, fluorides are formed on metals.

It goes without saying that the present disclosure is by no means limited to the above-described experimental examples, and experiments can be performed in various modes as long as they fall within the technical scope of the present disclosure.

The fluorine battery electrolyte and fluorine battery disclosed herein can be used in the energy industry, such as the battery industry.

10 fluorine battery, 11 current collector, 12 positive electrode active material layer, 14 current collector, 15 negative electrode active material layer, 16 fluorine battery electrolyte, 17 positive electrode, 18 negative electrode, 19 separator.

Claims (7)

a supporting salt containing fluorine;
Dissolving the supporting salt, having a chain-like first carbon chain group and a chain-like second carbon chain group different from the first chain carbon chain group, the first carbon chain group and the second chain carbon chain group A solvent containing an asymmetric fluorine-containing linear ether containing one or more fluorines in any one of
A fluorine battery electrolyte having 2. The fluorine battery according to claim 1, wherein the second carbon chain group differs from the first carbon chain group in one or more of the positions of fluorine and hydrogen, the number of carbon atoms, and the number of fluorine atoms. Electrolyte. 3. The electrolyte solution for a fluorine battery according to claim 1, wherein said solvent is an asymmetric fluorine-containing chain ether represented by the following formula (1).

4. The fluorine battery electrolyte solution according to claim 1, wherein said solvent contains one or more of tetrafluoroethyltrifluoroethyl ether, methoxyperfluorobutane and ethyl nonafluorobutyl ether. The fluorine battery electrolyte solution according to any one of claims 1 to 4, wherein the supporting salt contains an alkylammonium fluoride salt. a positive electrode having a metal that forms a metal fluoride as a positive electrode active material;
a negative electrode;
The fluorine battery electrolyte solution according to any one of claims 1 to 5, interposed between the positive electrode and the negative electrode;
Fluorine battery with 7. The fluorine battery according to claim 6, wherein said positive electrode has said positive electrode active material which is one or more metals selected from Sn, In, Ce and Pb and alloys thereof.

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