JP2017168371A - Electrolyte material, and secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase ion conductivity in a solid or semisolid electrolyte including magnesium ions.SOLUTION: Disclosed is an electrolyte material 1 comprising a magnesium salt 10 and a solvent molecule. A part of the magnesium salt 10 forms a solvate 20 in combination with the solvent molecule. The electrolyte material is solid or semisolid at 25°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrolyte material containing a magnesium salt and a secondary battery using the same.

  Magnesium secondary batteries are attracting attention because they can have a larger charge capacity than lithium secondary batteries and can be formed from inexpensive materials. Electrolyte materials for magnesium secondary batteries have been proposed so far (for example, Patent Documents 1 to 6).

JP 2015-115233 A Japanese Patent Laying-Open No. 2015-074657 Japanese Patent Laid-Open No. 2015-063640 Japanese Patent Laying-Open No. 2015-041572 JP 2014-192128 A JP 2012-181962 A

  From the viewpoints of battery safety, reduction in size and weight, etc., it is often desired that the electrolyte of the secondary battery is solid or semi-solid. However, since magnesium ions are less diffusible in solids than lithium ions, conventionally, there has been a problem that the ionic conductivity of magnesium ions in a solid or semi-solid electrolyte is very low.

  Accordingly, an object of one aspect of the present invention is to improve ion conductivity with respect to a solid or semi-solid electrolyte containing magnesium ions.

  One aspect of the present invention relates to an electrolyte material including a magnesium salt and solvent molecules. A part of the magnesium salt forms a solvate with the solvent molecules, and the electrolyte material is solid or semi-solid at 25 ° C.

  This electrolyte material can exhibit high ionic conductivity of magnesium ions while maintaining a solid or semi-solid state. Among solvates of magnesium salts and solvent molecules, magnesium ions are highly diffusible at a temperature at which the solvate has fluidity. In the electrolyte material, only a part of the magnesium salt forms a solvate, and there is a magnesium salt that does not form a solvate. Therefore, even if the solvate has fluidity, the entire electrolyte material can be maintained as a pseudo-solid in a solid or semi-solid state.

  Another aspect of the present invention relates to a secondary battery including a positive electrode and a negative electrode, and an electrolyte disposed between the positive electrode and the negative electrode. This secondary battery can have high ionic conductivity.

  According to the present invention, it is possible to improve ion conductivity with respect to a solid or semi-solid electrolyte containing magnesium ions.

It is a schematic diagram which shows one Embodiment of electrolyte material and its manufacturing method. It is a graph which shows the relationship between the heat absorption amount of electrolyte material measured by differential scanning calorimetry, and temperature. It is a graph which shows the relationship between the ionic conductivity of electrolyte material (Mg (TFSA) ₂ / monoglyme) and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / diglyme) and temperature. It is a graph which shows the relationship between the ionic conductivity of electrolyte material (Mg (TFSA) ₂ / triglyme), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / CPME), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / MTBE), and temperature. It is a graph which shows the relationship between the ionic conductivity of electrolyte material (Mg (TFSA) ₂ / THF), and temperature. It is a graph which shows the relationship between the ionic conductivity of electrolyte material (Mg (TFSA) ₂ / DEC), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / DMC), and temperature. It is a graph which shows the relationship between the ionic conductivity of electrolyte material (Mg (TFSA) ₂ / EC), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / FEC), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / PC), and temperature. It is a graph which shows the relationship between the ion conductivity of electrolyte material (Mg (TFSA) ₂ / AN), and temperature.

  Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

  FIG. 1 is a schematic view showing an embodiment of an electrolyte material and a method for producing the same. The method shown in FIG. 1 mixes particulate magnesium salt 10 ((a) of FIG. 1) with a solvent, and forms particulate magnesium salt 10 and solvate 20 formed of the magnesium salt and solvent molecules. It includes obtaining the electrolyte material 1 containing ((b) of FIG. 1).

  In the electrolyte material 1 after mixing, the magnesium salt 10 and the solvate 20 are often solid at room temperature. The electrolyte material 1 may be in the form of powder as a whole or a solid molded product. The solid molded product can be obtained by a method such as compression molding and can have an arbitrary shape.

  The electrolyte material 1 can change into a state in which the solvate 20 has fluidity, for example, by heating. As a result, an electrolyte material 1a including a fluid solvate phase 20a containing a solvate and a particulate solid magnesium salt 10 dispersed in the solvate phase 20a is formed (FIG. 1). (C)). Since the solvate often has a relatively low melting point, the electrolyte material 1a including the solvate phase 20a that is fluid in a molten state can be formed at a relatively low temperature. That is, the solid or semi-solid electrolyte material 1a having high ionic conductivity can be formed at a temperature equal to or higher than the melting point of the solvate. The solvate phase is considered to be in the same state as the ionic liquid, and it is considered that magnesium ions acquire high diffusivity in this solvate phase.

  In the present specification, the term “solid or semi-solid” means a state that can be regarded as a solid or semi-solid as a whole material, and also includes a state in which a liquid is partially included (sometimes referred to as “pseudo-solid”). Used. The semi-solid electrolyte material may be in a state substantially free of fluidity or low in fluidity to such an extent that it can maintain its shape alone, for example, in a state referred to by those skilled in the art as a gel.

  By changing the type of the magnesium salt and the solvent, the melting point of the solvate can be adjusted in a wide range. As a result, the temperature at which the electrolyte material exhibits high ionic conductivity can be adjusted in a wide range. The melting point of the solvate can be, for example, −30 ° C. or higher, 10 ° C. or higher, 20 ° C. or higher, or 30 ° C. or higher. If the melting point of the solvate is high to some extent, an electrolyte material having sufficient heat resistance tends to be obtained. The melting point of the solvate can be, for example, 100 ° C. or lower, 90 ° C. or lower, 80 ° C. or lower, or 70 ° C. or lower. When the melting point of the solvate is low, sufficient ionic conductivity is easily obtained at a relatively low temperature.

  In this specification, the melting point is measured by differential scanning calorimetry and is defined as the temperature at the peak top of the endothermic peak accompanying melting of the solvate. The suggested scanning calorimetry is performed under a temperature rising rate of 10 ° C./min. By comparing the melting point of the solvate observed in the differential scanning calorimetry of the electrolyte material with the known melting point of the molecular crystal of the solvate alone, it is confirmed that the solvate is formed in the electrolyte material. Can be confirmed.

  The magnesium salt is formed from a magnesium cation and its counter anion. The counter anion can be selected from those that interact with magnesium ions with such a strength that high diffusibility of magnesium ions can be ensured.

  The counter anion constituting the magnesium salt may be, for example, an anion having a sulfonamide group. Examples of the anion having a sulfonamide group include an anion represented by the following formula (10).

In the formula, R ¹ and R ² each independently represent a fluorine atom or a fluoroalkyl group, or a fluoroalkylene group formed by bonding R ¹ and R ² . R ¹ and R ² may each independently be a trifluoromethyl group or a trifluoroethyl group. The anion represented by the formula (10) may be an anion represented by the following formula (10a), (10b), (10c), or (10d).

Other examples of counter anions include thiocyanate ion (SCN ⁻ ), trifluoromethanesulfonate (CF ₃ SO ₃ ⁻ ), hexafluorophosphate ion (PF ₆ ⁻ ), and tetraborate ion (BF ₄ ⁻ ). Can be mentioned.

  The electrolyte material according to some embodiments may further include other alkali metal salt and / or alkaline earth metal salt in addition to the magnesium salt. For example, the electrolyte material may further contain a sodium salt. The counter anion in this case is not particularly limited, but can be selected, for example, from the anions exemplified for the magnesium salt. When the electrolyte material contains alkali metal ions and / or alkaline earth metal ions other than magnesium ions, the ratio of magnesium ions is based on the total amount of alkali metal ions and alkaline earth metal ions (including magnesium ions). It may be 50 mol% or more.

  Solvent molecules constituting the electrolyte material can be selected from solvents capable of forming solvates with magnesium salts, particularly organic solvents. The boiling point (under 1 atm) of the solvent molecules may be 50 ° C. or higher, 100 ° C. or higher, 150 ° C. or higher, or 300 ° C. or lower. When the boiling point of the solvent molecule is high, an electrolyte material having good heat resistance is easily obtained.

  Solvent molecules include, for example, an ether compound having an oxy group (—O—, a hydrocarbon group bonded to an oxygen atom), a carbonate compound having a carbonate group (—OC (═O) O—), and acetonitrile. It may contain at least one selected from From these, at least one of an ether compound and a carbonate compound may be selected.

  Examples of the ether compound include monoglyme, diglyme, triglyme, cyclopentyl methyl ether (CPME), methyl tert-butyl ether (MTBE), and tetrahydrofuran (THF) represented by the following formula.

  Examples of the carbonate compound include diethyl carbonate (DEC), diethyl carbonate (DMC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC) represented by the following formula.

  These may be used alone or in combination of two or more. The solvent molecule may contain at least one compound selected from the group consisting of monoglyme, diglyme, triglyme and cyclopentylmethyl ether. These compounds can contribute to good heat resistance as well as particularly high ionic conductivity.

The ratio of the magnesium salt to the solvent molecule in the electrolyte material is adjusted in such a range that a part of the magnesium salt forms a solvate and a magnesium salt that does not form a solvate remains. The ratio (molar ratio) of the solvent molecule to the magnesium salt in the molecular crystal of the solvate formed by the magnesium salt and the solvent molecule is r ₀ , and the ratio (molar ratio) of the solvent molecule to the magnesium salt in the electrolyte material is r. _{When 1} , when r ₁ is less than r _{0, a} magnesium salt that does not form a solvate may remain. r ₁ / r ₀ may be 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less, for example, 0.1 or more, 0.2 or more, 0.3 or more, or It may be 0.4 or more.

For example, when the magnesium salt is magnesium bistrifluoromethanesulfonimide (Mg (TFSA) ₂ ) and the solvent molecule is monoglyme, in the molecular crystal of the solvate formed by Mg (TFSA) ₂ ) and monoglyme, Mg (TFSA) ₂ ): Monoglyme = 1: 3 (molar ratio, r ₀ = 3). Therefore, by mixing Mg (TFSA) ₂ and monoglyme at such a ratio that r ₁ is smaller than 3, an electrolyte material in which a magnesium salt that does not form a solvate remains can be obtained. In this case, r ₁ may be, for example, 1.2 to 1.8 (r ₁ / r ₀ = 0.4 to 0.6).

Unknown r1 solvent molecules to the magnesium salt in the molecular crystals of the solvate, or there may also vary, usually, by adjusting the r ₁ in the range of 0.2 to 2.0, the magnesium salt one An electrolyte material in which only a part forms a solvate can be formed. When the electrolyte material contains a metal salt other than magnesium salt (alkali metal salt, alkaline earth metal salt), the amount of magnesium salt is a solvent molecule in consideration of the amount of solvate that can be formed with these and solvent molecules. The ratio of solvent molecules is adjusted so as to be excessive with respect to the ratio.

  Table 1 shows some examples of solvates and their molar ratios in the molecular crystals.

  A secondary battery (magnesium secondary battery) according to an embodiment includes a positive electrode and a negative electrode facing each other, and an electrolyte including the electrolyte material according to the above-described embodiment, which is disposed therebetween. The secondary battery according to this embodiment can be used as a secondary battery that can be efficiently charged and discharged at a temperature equal to or higher than the melting point of the solvate contained in the electrolyte material. The electrolyte can be solid or semi-solid at the temperature at which the secondary battery is charged and discharged. Therefore, the secondary battery can be a so-called all-solid battery. The positive electrode, the negative electrode, and other members provided as necessary can be appropriately selected from the configurations usually employed in the secondary battery.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.
1. Material 1-1. Magnesium salt / magnesium bistrifluoromethanesulfonimide (Mg (TFSA) ₂ , manufactured by Kishida Chemical, powder)
1-2. Solvent molecules / monoglyme (manufactured by Kanto Chemical, dehydrated product)
・ Diglime (manufactured by Kanto Chemical, dehydrated product)
・ Triglyme (manufactured by Kanto Chemical, dehydrated product)
・ Cyclopentyl methyl ether (CPME, manufactured by Kanto Chemical, dehydrated product)
・ Methyl tert-butyl ether (MTBE, manufactured by Kanto Chemical, dehydrated product)
・ Tetrahydrofuran (THF, manufactured by Kanto Chemical, dehydrated product)
・ Diethyl carbonate (DEC, manufactured by Kanto Chemical, dehydrated product)
・ Diethyl carbonate (DMC, manufactured by Kanto Chemical, dehydrated product)
・ Ethylene carbonate (EC, manufactured by Kanto Chemical, dehydrated product)
・ Fluoroethylene carbonate (FEC, manufactured by Tokyo Chemical Industry, dried at 100 ° C under reduced pressure for 1 week)
・ Propylene carbonate (PC, manufactured by Kanto Chemical, dehydrated product)
・ Acetonitrile (AN, manufactured by Kanto Chemical, dehydrated product)

2. Preparation of Electrolyte Material An electrolyte sample containing Mg (TFSA) ₂ and a solvent in the combinations and ratios shown in Table 2 and solid at room temperature (25 ° C.) was prepared by the following procedure.
In a glove box (MBraun, UniLab2000) filled with argon gas, Mg (TFSA) ₂ was weighed into a sealed glass reaction vessel. Mg (TFSA) ₂ was previously dried at 100 ° C. under reduced pressure. Next, a predetermined amount of solvent was added to the reaction vessel using a microsyringe under an argon atmosphere. Mg (TFSA) ₂ and the solvent were stirred with a magnetic stirrer at room temperature to obtain a powdery mixture of Mg (TFSA) ₂ and solvent molecules.
The obtained powdery mixture was pulverized in a glove box using an agate pestle and a mortar. The powder after pulverization was compressed using a hand press to form a disk shape having a diameter of 13 mm and a thickness of about 1 mm to obtain a solid sample of the electrolyte material.

3. Evaluation 3-1. Differential Scanning Calorimetry (DSC) Measurement With respect to the electrolyte material sample, differential scanning calorimetry was performed under the following conditions.
・ Measuring device: DSC-60 (manufactured by Shimadzu Corporation)
・ Temperature increase rate: 10 ° C / min

FIG. 2 is a graph showing the results of differential scanning calorimetry for Mg (TFSA) ₂ / monoglyme. FIG. 2 also shows the measurement results of molecular crystals of Mg (TFSA) ₂ and Mg (TFSA) ₂ / monoglyme. The electrolyte material of Mg (TFSA) ₂ / monoglyme showed an endothermic peak, and the melting point (35.8 ° C.) obtained from the endothermic peak almost coincided with the melting point of the molecular crystal of Mg (TFSA) ₂ / monoglyme. From this measurement result, it was confirmed that the electrolyte material was a mixture containing Mg (TFSA) ₂ / monoglyme molecular crystals and solid Mg (TFSA) ₂ not forming a solvate. Regarding other electrolyte materials, the observed melting point was confirmed to be a mixture containing molecular crystals as well.

3-2. Ionic conductivity A solid sample of the electrolyte material was introduced into a closed electrochemical evaluation cell and heat-treated at 80 ° C. for 2 days. Thereafter, ion conductivity was measured by an alternating current impedance method using a multi-channel potentiostat / galvanostat (Biologic, VSP3). The measurement was performed at each temperature in increments of 10 ° C. ranging from −30 ° C. to 80 ° C. The sample was allowed to stand at each temperature for 3 hours until the temperature of the sample became constant, and then the ion conductivity was measured. The temperature of the sample was controlled using a small environmental tester (Espec Corp., SU-241).

  3 to 14 are graphs showing the relationship between the ionic conductivity and temperature when the solvent molecule is monoglyme, diglyme, triglyme, CPME, THF, DEC, DMC, EC, FEC, PC or AN, respectively. The horizontal axis of the graph is a temperature indicated by a value of (1 / absolute temperature) × 1000. As shown in these measurement results, all the electrolyte materials showed high ionic conductivity even at a relatively low temperature such as 80 ° C. (2.83 in the graph). Among them, an electrolyte containing monoglyme, diglyme, triglyme, or CPME as a solvent (FIGS. 3 to 6) showed particularly high ionic conductivity.

  DESCRIPTION OF SYMBOLS 1,1a ... Electrolyte material, 10 ... Magnesium salt, 20 ... Solvate (solid), 20a ... Solvate phase (solvate in a molten state).

Claims (6)

An electrolyte material comprising a magnesium salt and solvent molecules,
A part of the magnesium salt forms a solvate with the solvent molecule,
The electrolyte material is solid or semi-solid at 25 ° C.,
Electrolyte material. The solvate has a melting point;
The electrolyte material is solid or semi-solid at a temperature above the melting point of the solvate,
The electrolyte material according to claim 1.   The electrolyte material according to claim 1, wherein the solvent molecule includes an ether compound having an oxy group.   The electrolyte material according to any one of claims 1 to 3, wherein the solvent molecule includes at least one compound selected from the group consisting of monoglyme, diglyme, triglyme, and cyclopentylmethyl ether. The electrolyte material according to any one of claims 1 to 4, wherein the magnesium salt is a salt of a magnesium cation and a counter anion represented by the following formula (10).

[Wherein, R ¹ and R ² each independently represent a fluorine atom or a fluoroalkyl group, or a fluoroalkylene group formed by combining R ¹ and R ² . ]   A secondary battery comprising: a positive electrode and a negative electrode; and an electrolyte that is disposed between the positive electrode and the negative electrode and includes the electrolyte material according to claim 1.

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