CN112042037A - Electrolyte for fluoride ion secondary battery and fluoride ion secondary battery using the same - Google Patents

Abstract

The invention provides an electrolyte for a fluoride ion secondary battery and a fluoride ion secondary battery using the same, which can realize a fluoride ion secondary battery capable of fully working even in a low temperature environment. An electrolyte for a fluoride ion secondary battery comprising fluorine hydride anion ([ (FH)_nF]^‑) Or a salt derived from a fluorohydride anion.

Description

Electrolyte for fluoride ion secondary battery and fluoride ion using the same Sub secondary battery

technical field

The present invention relates to an electrolyte for a fluoride ion secondary battery and a fluoride ion secondary battery using the electrolyte.

Background technique

It is known that a fluoride ion secondary battery is a secondary battery using fluoride ion (F ⁻ ) as a carrier, and has a high theoretical energy. Moreover, its battery characteristics are expected to surpass those of lithium-ion secondary batteries.

Here, as solid electrolytes for fluoride ion secondary batteries, PbF ₂ (refer to Non-Patent Documents 1 to 3), PnSnF ₄ (refer to Non-patent Documents 4 and 5), La _1-x Ba _x F _3-x are reported as solid electrolytes for fluoride ion secondary batteries (refer to Non-Patent Documents 6 and 7) and the like. However, since fluoride ion secondary batteries using these solid electrolytes have low ionic conductivity, the operating temperature is high at 150° C. or higher, and the use environment is limited.

In addition, electrolytes with fluoride ion conductivity that operate around room temperature have also been reported (refer to Non-Patent Documents 8 and 9). NH ₄ FHF-PEG polymer is reported in Non-Patent Document 8, and MPPF/TMPA-TFSA ionic liquid is reported in Non-Patent Document 9. However, the ionic conductivity of all electrolytes is not necessarily sufficient, and therefore, development of new electrolytes is required.

[Prior Technology Literature]

(Non-patent literature)

Non-Patent Document 1: J.H.Kennedy et al., J.Electrochem.Soc., 120 (1973) 1441.

Non-patent document 2: J.Schoonman, J. Electrochem. Soc., 123 (1976) 1772.

Non-patent document 3: Y. Danto et al., Thin Solid Films, 55 (1978) 347.

Non-patent document 4: J.-M. Reau et al., Mater. Res. Bull., 13 (1978) 877.

Non-patent document 5: G. Denes et al., J. Solid State Chem., 104 (1993) 239.

Non-patent document 6: J.Schoonman et al., Solid State Ionics, 3-4 (1981) 373.

Non-patent document 7: M.A.Reddy et al., J.Mater.Chem., 21(2011) 17059.

Non-patent document 8: F. Gschwind et al., J. Mater. Chem. A, 3 (2015) 5628.

Non-patent document 9: K. Okazaki et al., ACS Energy Lett., 2(2017) 1460.

SUMMARY OF THE INVENTION

[Problems to be Solved by Invention]

The present invention has been made in view of the above-mentioned background art, and an object of the present invention is to provide an electrolyte for a fluoride ion secondary battery and a fluoride ion secondary battery using the same, which can realize sufficient operation even in a low temperature environment. Fluoride ion secondary battery.

[Technical means to solve the problem]

The inventors noticed that ionic liquids containing fluoride hydride anions ([(FH) _n F] ⁻ ) showed higher ionic conductivity. Furthermore, it was found that if a fluoride hydride anion ([(FH) _n F] ⁻ ) is used as an electrolyte for a fluoride ion secondary battery, a fluoride ion secondary battery having high fluoride ion conductivity can be obtained, thereby completing the this invention.

That is, the present invention provides an electrolyte for a fluoride ion secondary battery, which contains a hydrogen fluoride anion or a salt derived from a hydrogen fluoride anion.

The aforementioned electrolyte for a fluoride ion secondary battery may be an ionic liquid.

The aforementioned ionic liquid may contain cations.

The aforementioned electrolyte for a fluoride ion secondary battery may be a soft viscous ionic crystal.

The aforementioned soft sticky ionic crystals may be salts of hydrogen fluoride anions and cations.

The aforementioned cation may be a cyclic cation.

The aforementioned cyclic cation may be a cation derived from an imidazolium-based compound.

The aforementioned cation derived from the imidazolium-based compound may be 1-ethyl-3-methylimidazolium.

The aforementioned cyclic cation may be a cation derived from a pyrrolidinium-based compound.

The aforementioned cation derived from the pyrrolidinium-based compound may be dimethylpyrrolidinium or N-ethyl-N-propylpyrrolidinium.

In addition, the present invention provides a fluoride ion secondary battery including the above-described electrolyte for a fluoride ion secondary battery, a positive electrode, and a negative electrode.

(effect of invention)

According to the electrolyte for fluoride ion secondary batteries of this invention, the fluoride ion conductivity in a fluoride ion secondary battery can be improved. As a result, it is possible to realize a fluoride ion secondary battery whose temperature characteristics of charge and discharge capacity are improved and which can sufficiently operate even in a low temperature environment.

Description of drawings

Figure 1 shows a state diagram of _EMPyr (FH)nF.

Figure 2 shows a state diagram of _DMPyr (FH)nF.

FIG. 3 shows a schematic diagram of the three-pole evaluation unit used in the examples.

FIG. 4 shows a charge-discharge curve of a fluoride ion secondary battery using the electrolyte of Example 1. FIG.

FIG. 5 shows a charge-discharge curve of a fluoride ion secondary battery using the electrolyte of Example 2. FIG.

FIG. 6 shows a charge-discharge curve of a fluoride ion secondary battery using the electrolyte of Example 3. FIG.

FIG. 7 shows a charge-discharge curve of a fluoride ion secondary battery using the electrolyte of Example 4. FIG.

Detailed ways

Hereinafter, embodiments of the present invention will be described.

<Electrolyte for Fluoride Ion Secondary Battery>

The electrolyte for a fluoride ion secondary battery of the present invention contains a hydrogen fluoride anion or a salt derived from a hydrogen fluoride anion.

[Hydrofluoride ([(FH) _n F] ^- )]

The hydrogen fluoride anion, which is a constituent material of the electrolyte for a fluoride ion secondary battery of the present invention, is represented by the structural formula of [(FH) _nF ] ^- . n does not necessarily have to be an integer. The fluorine hydride anions constituting the electrolyte for a fluoride ion secondary battery of the present invention may be one type alone or two or more types may be mixed.

The fluoride hydride anion constituting the electrolyte for a fluoride ion secondary battery of the present invention is not particularly limited, but preferably has a structure in which n is an integer of 1 to 3 as represented by the following chemical formulae (1) to (3). In the case of the fluorine hydride anions represented by chemical formulae (1) to (3), the dissociation pressure of HF is sufficiently low, and it can be handled safely.

[hua 1]

[cation]

In the electrolyte for a fluoride ion secondary battery of the present invention, the cation used in combination with the hydrogen fluoride anion ([(FH) _n F] ^- ) is not particularly limited, and can be appropriately selected so as to express the desired fluoride ion secondary battery battery characteristics.

The structure of the cation that can constitute the electrolyte for a fluoride ion secondary battery of the present invention may be a chain structure or a cyclic structure, and is not particularly limited. As a chain cation, the cation represented by the following chemical formula (4) is mentioned, for example.

[hua 2]

In the above chemical formula (4), R ¹ to R ⁴ are each independently hydrogen, alkyl, fluoroalkyl or alkoxyalkyl. When R ¹ to R ⁴ are an alkyl group, a fluoroalkyl group or an alkoxyalkyl group, the number of carbon atoms thereof is, for example, 10 or less, preferably 6 or less, more preferably 4 or less, and still more preferably 2 or less. In particular, R ¹ to R ⁴ are hydrogen or an alkyl group having 4 or less carbon atoms, preferably an alkyl group having 2 or less carbon atoms, a fluoroalkyl group or an alkoxyalkyl group. In the chemical formula (4), R ¹ and R ² or R ³ and R ⁴ may be connected to form a ring structure.

Moreover, as another chain cation, the cation represented by the following chemical formula (5) is mentioned, for example.

[hua 3]

In the above chemical formula (5), R ¹ to R ⁴ are each independently hydrogen, alkyl, fluoroalkyl or alkoxyalkyl. When R ¹ to R ⁴ are an alkyl group, a fluoroalkyl group or an alkoxyalkyl group, the number of carbon atoms thereof is, for example, 10 or less, preferably 6 or less, more preferably 4 or less, and even more preferably 2 or less. In particular, R ¹ to R ⁴ are hydrogen or an alkyl group having 4 or less carbon atoms, preferably an alkyl group having 2 or less carbon atoms, a fluoroalkyl group or an alkoxyalkyl group. In chemical formula (5), R ¹ and R ² or R ³ and R ⁴ may be connected to form a ring structure.

Among the chain cations represented by the chemical formula (5), tetraethylammonium (N2222) or 5-azanium spiro[4.4]nonane (AS[4.4]) is preferable. In the case of tetraethylammonium (N2222) or 5-azonium spiro[4.4]nonane (AS[4.4]), a soft viscous ionic crystal phase with high ionic conductivity near normal temperature can be formed.

Moreover, as another chain cation, the cation represented by the following chemical formula (6) is mentioned, for example.

[hua 4]

In the above chemical formula (6), R ¹ to R ³ are each independently hydrogen, alkyl, fluoroalkyl or alkoxyalkyl. When R ¹ to R ³ are an alkyl group, a fluoroalkyl group or an alkoxyalkyl group, the number of carbon atoms thereof is, for example, 10 or less, preferably 6 or less, more preferably 4 or less, and still more preferably 2 or less. In particular, R ¹ to R ³ are hydrogen or an alkyl group having 4 or less carbon atoms, preferably an alkyl group having 2 or less carbon atoms, a fluoroalkyl group or an alkoxyalkyl group. In addition, a part or all of hydrogen in each structure may be replaced with fluorine. In the chemical formula (6), R ¹ and R ² or R ³ and R ⁴ may be connected to form a ring structure.

Among the chain cations represented by the chemical formula (6), Tetraethylphosphonium (P2222) is preferable. In the case of tetraethylphosphonium (P2222), a soft viscous ionic crystal phase with high ionic conductivity near normal temperature can be formed.

Moreover, as another cyclic cation, the cation represented by the following chemical formula (7) is mentioned, for example.

[hua 5]

In the above chemical formula (7), R ¹ or R ² are independently hydrogen, alkyl, fluoroalkyl or alkoxyalkyl, and R ³ is a functional group for forming a cyclic structure and contains at least carbon. When R ¹ or R ² is an alkyl group, a fluoroalkyl group or an alkoxyalkyl group, the number of carbon atoms thereof is, for example, 10 or less, preferably 6 or less, more preferably 4 or less, and still more preferably 2 or less. In particular, R ¹ or R 2 is hydrogen or an alkyl group having 4 or less carbon atoms, preferably an alkyl group having 2 or less carbon atoms, a fluoroalkyl group or an alkoxyalkyl group. The ring structure composed of N and R ³ may be a five-membered ring structure, a six-membered ring structure, or a seven-membered ring structure. In addition, the cyclic structure composed of N and R ³ may be aromatic or non-aromatic. Further, the cyclic structure composed of N and R ³ is preferably, for example, a pyrrolidine structure, a pyrrole structure, a piperidine structure, or a pyridine structure. In addition, in the chemical formula (7), R ¹ and R ² may be connected to form a cyclic structure.

Among the cations that can constitute the electrolyte for a fluoride ion secondary battery of the present invention, a cyclic cation is preferable. When cations having a cyclic structure are used, the cations are regularly arranged on the interface of the active material, so that a structure in which fluoride ions are easily diffused can be formed. As a result, the reaction rate of at least one of the fluorination reaction and the defluorination reaction of the active material can be increased.

When the cation constituting the electrolyte for a fluoride ion secondary battery of the present invention is a cyclic cation, it is preferably a heterocyclic structure containing a cation central element (N element, P element). In addition, the ring structure may be aromatic or non-aromatic.

The cation that can constitute the electrolyte for a fluoride ion secondary battery of the present invention is preferably a cation derived from an imidazolium-based compound. If it is a cation derived from an imidazolium-based compound, an ionic liquid phase having high ionic conductivity around normal temperature can be formed.

Further, among cations derived from imidazolium-based compounds, 1-ethyl-3-methylimidazolium (EMIm) is more preferable. In the case of 1-ethyl-3-methylimidazolium (EMIm), the melting point is low and the ionic conductivity is high.

The cation that can constitute the electrolyte for a fluoride ion secondary battery of the present invention is preferably a cation derived from a pyrrolidinium-based compound. A cation derived from a pyrrolidinium-based compound has a low melting point and high ionic conductivity.

Further, among the cations derived from pyrrolidinium-based compounds, dimethylpyrrolidinium (DMPyr) or N-ethyl-N-propylpyrrolidinium (EMPyr) is more preferable. Dimethylpyrrolidinium (DMPyr) or N-ethyl-N-propylpyrrolidinium (EMPyr) has a low melting point and high ionic conductivity.

[form]

The form of the electrolyte for a fluoride ion secondary battery of the present invention is not particularly limited, and may be any of liquid, gel, and solid. The form of the electrolyte for a fluoride ion secondary battery of the present invention depends on the type of cation used in combination with the fluoride hydride anion ([(FH) _n F] ^- ), and the n of the fluoride hydride anion ([(FH) _n F] ^- ). number changes. Therefore, the preferred form of the electrolyte for the fluoride ion secondary battery can be appropriately selected.

Further, in the present invention, the form of the electrolyte for a fluoride ion secondary battery is preferably an ionic liquid or a soft viscous ionic crystal.

FIG. 1 shows a state diagram of _EMPyr (FH)nF, which is an example of the electrolyte for a fluoride ion secondary battery of the present invention. The melting point of EMPyr(FH) _2.0 F where n is 2.0 is 30°C, and a soft viscous ionic crystal (Ionic Plastic Crystal (IPC)) is formed around room temperature (25°C). In addition, the conductivity of EMPyr(FH) _2.0 F at 25°C was 19.0 mScm ^-1 .

In addition, the state diagram of DMPyr(FH) _n F is shown in FIG. 2 . The melting point of DMPyr(FH) _2.0 F in which n is 2.0 is 52°C, and a soft viscous ionic crystal (Ionic Plastic Crystal (IPC)) is formed around room temperature (25°C). In addition, the conductivity of DMPyr(FH) _2.0 F at 25°C was 10.3 mScm ^-1 , and the conductivity at 40°C was 14.4 mScm ^-1 .

<Fluoride ion secondary battery>

The battery for fluoride ion secondary batteries of the present invention includes the electrolyte for fluoride ion secondary batteries of the present invention, a positive electrode, and a negative electrode. If the battery for fluoride ion secondary batteries of the present invention uses the electrolyte for fluoride ion secondary batteries of the present invention, other structures are not particularly limited.

In the present invention, for the standard electrode potential of the negative electrode for a fluoride ion secondary battery, a positive electrode material that provides a sufficiently high standard electrode potential is selected, and the electrolyte for a fluoride ion secondary battery of the present invention is arranged between them, and is composed of Therefore, the characteristics as a fluoride ion secondary battery are high, and a desired battery voltage can be realized.

<Method for producing electrolyte for fluoride ion secondary battery>

The electrolyte for a fluoride ion secondary battery of the present invention can be obtained by reacting a salt composed of a target cation and a halide ion with hydrogen fluoride. The reaction method is not particularly limited, and for example, those described in R. Taniki, K. Matsumoto, R. Hagiwara, K. Hachiya, T. Morinaga, T. Sato, J. Phys. Chem. B, 117 (2013) 955. method of manufacture.

Example

Next, examples of the present invention will be described, but the present invention is not limited to these examples.

<Examples 1 to 4>

[Manufacture of Electrolytes 1 to 4]

EMIm(FH) _2.3 was produced using the method described in R. Taniki, K. Matsumoto, R. Hagiwara, K. Hachiya, T. Morinaga, T. Sato, J. Phys. Chem. B, 117 (2013) 955. F as electrolyte 1, EMPyr(FH) _2.3 F as electrolyte 2, EMPyr(FH) _2.0 F as electrolyte 3, and DMPyr(FH) _2.0 F as electrolyte 4.

In addition, the melting point of EMIm(FH) _2.3 F of Electrolyte 1 is -65°C, and the melting point of EMPyr(FH) _2.3 F of Electrolyte 2 is -37°C. Therefore, the forms of Electrolyte 1 and Electrolyte 2 at 25°C are ionic liquids . On the other hand, the melting point of EMPyr(FH) _2.0 F of Electrolyte 3 is 30°C, the melting point of DMPyr(FH) _2.0 F of Electrolyte 4 is 52°C, and the morphology of Electrolyte 3 and Electrolyte 4 at 25°C are soft viscous ionic crystals .

[Production of fluoride ion secondary battery]

Using the following materials, a fluoride ion secondary battery was produced by the following method.

(electrolyte)

As the electrolytes, the ionic liquids obtained above, namely, Electrolytes 1 to 2, and soft viscous ionic crystals, namely, Electrolytes 3 to 4, were used.

(positive electrode mixture film)

As the positive electrode, a CuF ₂ mixed electrode was used. CuF ₂ particles (manufactured by AlfaAesar), acetylene black (manufactured by Strem chemicals) for imparting electron conduction paths, and PTFE (manufactured by Strem chemicals) for maintaining adhesion between particles were weighed in a mass ratio of 85:10:5, respectively. (manufactured by Aldrich), and formed into a film form after being sufficiently mixed to form a positive electrode mixture film.

(negative electrode mixture film)

As the negative electrode, a CuF ₂ /Cu hybrid electrode was used. As with the positive electrode mixture film, CuF particles (manufactured by Alfa Aesar), Cu particles (manufactured by Aldrich), acetylene black (manufactured by Stremchemicals), and PTFE (manufactured by Stremchemicals ₎ were weighed in a mass ratio of 50:35:10:5, respectively. Aldrich), mixed well and then formed into a film to form a negative electrode mixture film.

(Fluoride ion secondary battery)

The positive electrode mixture film (2.5 mg) and the negative electrode mixture film (12.5 mg) were respectively crimped on a platinum mesh under a pressure of 20 MPa for 10 minutes. In order to provide an ion conduction path, ionic liquid, that is, an electrolyte 1 to 1, was immersed in the gap inside the mixture film. 2 or soft sticky ionic crystals which are electrolytes 3 to 4, resulting in positive and negative electrodes. In addition, two sheets of PTFE membranes (manufactured by Merck, thickness: 65 μm) were impregnated with ionic liquids, namely, electrolytes 1 to 2 or soft viscous ionic crystals, namely, electrolytes 3 to 4, and used as separators. In addition, a copper wire (manufactured by Niraco, 1 mm in diameter) was used as a dummy reference electrode. The copper wire is pre-insulated with Teflon (registered trademark) heat-shrinkable tube, and only both ends are exposed to maintain continuity.

The obtained positive electrode, separator, and negative electrode were stacked in a dedicated three-pole evaluation cell (manufactured by ECFrontier) shown in FIG. 3, and a pseudo-reference electrode was inserted from the upper part of the cell so that one end was in contact with the separator only, forming a Fluoride ion secondary battery.

<Evaluation of Fluoride Ion Secondary Battery>

[Constant current charge and discharge test]

Using a potentiostat device (Hokuto Electric Co., Ltd., HZ-7000 or HZ-Pro), a constant-current charge-discharge test was carried out under the following measurement conditions.

(measurement conditions)

Working temperature: 25℃

RATE:

Examples 1 to 3: 52.8 mA (g-CuF ₂ ) ^-1 (=C/10rate)

Example 4: 10.6mA(g- _CuF2 ) ^-1 (=C/50rate)

Working Potential Area (Copper Pseudo-Reference Electrode Standard):

Example 1: -0.40V～0.60V

Example 2: -0.35V～0.65V

Embodiments 3 to 4: -0.30V to 0.70V

FIG. 4 shows a charge-discharge curve of a fluoride ion secondary battery using EMIm(FH) _2.3 F of Example 1, and FIG. 5 shows a fluoride ion using EMPyr(FH) _2.3 F of Example 2. The charge-discharge curve of the secondary battery, the charge-discharge curve of the fluoride ion secondary battery using EMPyr(FH) _2.0 F of Example 3 is shown in Fig. 6(a), and the charge-discharge curve of the fluoride ion secondary battery using Example 4 is shown in Fig. 7 Charge-discharge curves of DMPyr(FH) _2.0 F fluoride ion secondary batteries. In addition, FIG. 6( b ) shows the relationship between the number of cycles and the capacity of the fluoride ion secondary battery using the EMPyr(FH) _2.0 F of Example 3.

In Examples 1 to 4, the reversible reaction of CuF ₂ can be confirmed

In addition, since the theoretical capacity of CuF ₂ is 528 mAh(g-CuF ₂ ) ^-1 , in the fluoride ion secondary battery using the electrolyte of Example 1, an initial capacity corresponding to 90% of the theoretical capacity was obtained. .

In the fluoride ion secondary battery using the electrolyte of Example 3, a charge-discharge test of 50 cycles was possible at 25° C. and a C/10 rate.

In the fluoride ion secondary battery using the electrolyte of Example 4, a charge-discharge test can be performed at 25° C. and a C/50 rate.

Claims (11)

What is claimed is: 1. An electrolyte for a fluoride ion secondary battery, comprising a hydrogen fluoride anion, or a salt derived from a hydrogen fluoride anion. 2 . The electrolyte for a fluoride ion secondary battery according to claim 1 , wherein the electrolyte for a fluoride ion secondary battery is an ionic liquid. 3 . 3 . The electrolyte for a fluoride ion secondary battery according to claim 2 , wherein the ionic liquid contains cations. 4 . 4 . The electrolyte for a fluoride ion secondary battery according to claim 1 , wherein the electrolyte for a fluoride ion secondary battery is a soft viscous ionic crystal. 5 . 5 . The electrolyte for a fluoride ion secondary battery according to claim 4 , wherein the soft sticky ionic crystal is a salt of a hydrogen fluoride anion and a cation. 6 . 6. The electrolyte for a fluoride ion secondary battery according to claim 3 or 5, wherein the cation is a cyclic cation. 7. The electrolyte for a fluoride ion secondary battery according to claim 6, wherein the cyclic cation is a cation derived from an imidazolium-based compound. 8. The electrolyte for a fluoride ion secondary battery according to claim 7, wherein the cation derived from the imidazolium-based compound is 1-ethyl-3-methylimidazolium. 9 . The electrolyte for a fluoride ion secondary battery according to claim 6 , wherein the cyclic cation is a cation derived from a pyrrolidinium-based compound. 10 . 10. The electrolyte for a fluoride ion secondary battery according to claim 9, wherein the cation derived from the pyrrolidinium-based compound is dimethylpyrrolidinium or N-ethyl-N-propylpyrrolidinium . 11 . A fluoride ion secondary battery comprising the electrolyte for a fluoride ion secondary battery according to claim 1 , a positive electrode, and a negative electrode. 12 .

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