JP6004468B2 - Alkali metal-sulfur secondary battery and electrolyte for secondary battery - Google Patents

Description

  The present invention relates to an alkali metal-sulfur secondary battery using sulfur as a positive electrode, such as a sodium-sulfur battery, and an electrolyte for a secondary battery.

In recent years, attention has been focused on power generation using natural energy such as wind energy and solar energy. However, since these natural energies vary greatly, it is necessary to install a large capacity battery for power storage and load leveling. As such large-capacity batteries, lithium ion secondary batteries and sodium-sulfur (Na-S) batteries have been researched and developed, and some are commercialized. In particular, sodium-sulfur batteries are less promising than lithium-ion secondary batteries, and are promising.
By the way, currently commercialized Na-S batteries use β-alumina, which is a solid ion conductor, as an electrolyte, and the operating temperature is as high as about 300 ° C. At this operating temperature, Na metal, which is a negative electrode, is used. The sulfur (S) that is the positive electrode is in a molten state. When such molten Na metal is used, there is a risk of ignition and securing the safety of the battery is an important issue. Therefore, if the Na-S battery can be operated near room temperature, the Na metal electrode becomes a solid state. Therefore, the reactivity is low compared with the molten metal Na, and the safety of the battery can be improved.

  For this reason, a Na-S battery has been reported that achieves room temperature operation using a gel-like organic electrolyte obtained by dissolving polyvinylidene fluoride (PVDF) and glyme in acetone (see Non-Patent Document 1). . In addition, a Na-S battery using tetraethylene glycol dimethyl ether (TEGDME) as an organic electrolyte and realizing room temperature operation has been reported (see Non-Patent Document 2). In addition, a Na—S battery that operates near 90 ° C. using a solid polymer electrolyte based on polyethylene oxide (PEO) has been reported (see Non-Patent Document 3).

Cheol-Wan Park et, Electrochemical and Solid-State Letters, 9, A123-A125 (2006) Hosuk Ryu et, Journal of Power Sources 196 (2011) p5186-5190 Cheol-Wan Park et, Journal of Power Sources 165 (2007) p450-454

However, although the techniques described in Non-Patent Documents 1 and 2 operate at room temperature, there is a problem that the capacity rapidly decreases when the charge / discharge cycle is repeated. This is because sulfur (S) and a reduction product of sulfur are eluted into the electrolytic solution when the organic electrolytic solution is used. Moreover, the organic electrolyte solution used in these documents has low thermal stability. Furthermore, when the present inventor conducted a charge / discharge test of the Na-S battery using the above-mentioned organic electrolyte, the discharge capacity after recharging was extremely smaller than the charge capacity (coulomb efficiency was low). I found it scarce.
In the case of the technique described in Non-Patent Document 3, since the solid polymer electrolyte has low ionic conductivity at room temperature, it is difficult to operate the battery unless the battery is heated to around 90 ° C.
Accordingly, an object of the present invention is to improve the coulomb efficiency, operate at a low temperature, and have a high safety alkali metal-sulfur secondary battery and secondary battery without melting even if the negative electrode is Na metal. It is to provide an electrolytic solution for use.

The alkali metal-sulfur secondary battery of the present invention includes a positive electrode having a sulfur-based electrode active material containing at least one selected from the group consisting of elemental sulfur, metal sulfides, metal polysulfides, and organic sulfur compounds. , The following formula
(Here, R ₁ is any of an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. R ₂ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, a cyclohexyl group which may be substituted with a halogen atom, or ethyl One of the groups),
(Here, R ₃ and R ₄ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups),
(Here, R ₅ and R ₆ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups), or
(Wherein R ₇ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. And at least a part of the glyme and the alkali metal salt forms a complex, and further includes a fluorinated solvent, an ionic liquid, and a hydrocarbon. of one or more selected from the group of the alkali metal and polysulfides alkali metal _{(M 2 S n: 1 ≦} n ≦ 8) and containing a solvent which does not chemically react substantially the grime and the alkali an electrolyte solution comprising a metal salt and the solvent, a said positive electrode the counter electrode, the alkali metal, and a negative electrode having an alloy, or a carbon containing the alkali metal, the gley The mixing ratio of the alkali metal salt, the molar in terms of at least 0.80, and an alkali metal wherein less than or equal to the value determined by the saturation concentration of the alkali metal salt in the glyme against - sulfur-based secondary batteries.

The alkali metal salt is represented by MX, where M is an alkali metal, X is Cl, Br, I, BF ₄ , PF ₆ , CF ₃ SO ₃ , ClO ₄ , CF ₃ CO ₂ , AsF ₆ , SbF. ₆ , AlCl ₄ , N (CF ₃ SO ₂ ) ₂ , N (CF ₃ CF ₂ SO ₂ ) ₂ , PF ₃ (C ₂ F ₅ ) ₃ , N (FSO ₂ ) ₂ , N (FSO ₂ ) (CF _{3 SO 2), N (CF 3} CF 2 SO 2) 2, N (C 2 F 4 S 2 O 4), N (C 3 F 6 S 2 O 4), N (CN) 2, N (CF 3 SO ₂ ) (CF ₃ CO), R ¹ FBF ₃ (where R ¹ F = n-C _m F _{2m + 1} , m = 1 to 4 natural number) and R ² BF ₃ (where R ² = n−C _pH H _2p + 1, that p = is at least one selected from 1-5 of the group consisting of natural numbers) Preferred.

The electrolyte for secondary battery of the present invention has the following formula:
(Here, R ₁ is any of an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. R ₂ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, a cyclohexyl group which may be substituted with a halogen atom, or ethyl One of the groups),
(Here, R ₃ and R ₄ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. is any of the groups),
(Here, R ₅ and R ₆ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups), or
(Wherein R ₇ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. and glyme represented by a is) or the alkali metal salts, substantially becomes a mixing ratio of the alkali metal salt to the glyme, on a molar basis at least 0.80, and wherein the alkali in the glyme It is below a value determined by the saturation concentration of the metal salt, and at least a part of the glyme and the alkali metal salt forms a complex.
In addition, the electrolyte solution for secondary batteries of this invention is applicable not only to the above-mentioned alkali metal-sulfur secondary battery but also to any secondary battery that has a positive electrode and a negative electrode and can be charged and discharged multiple times. is there.

  ADVANTAGE OF THE INVENTION According to this invention, while improving coulomb efficiency, it can operate | move at low temperature, and even if a negative electrode is Na metal, a highly safe alkali metal-sulfur type secondary battery can be obtained without melting.

It is sectional drawing which shows the structural example of a sodium-sulfur battery. It is a figure which shows the result of the thermogravimetry of the electrolyte solution containing a glyme (3MeG4) and an alkali metal salt (NaTFSA). It is a figure which shows the result of the thermogravimetry of the electrolyte solution containing a glyme (6MeG4) and an alkali metal salt (NaTFSA). It is a figure which shows the result of the thermogravimetry of the electrolyte solution containing a glyme (G5) and an alkali metal salt (NaTFSA). It is a figure which shows the charging / discharging cycle dependence (FIG.5 (a)) of the discharge capacity of a secondary battery using a glyme (3MeG4), and the charging / discharging cycle dependence (FIG.5 (b)) of coulomb efficiency. It is a figure which shows the charging / discharging cycle dependence (FIG.6 (a)) of the discharge capacity of a secondary battery using a glyme (6MeG4), and the charging / discharging cycle dependence (FIG.6 (b)) of coulomb efficiency. It is a figure which shows the charging / discharging cycle dependence (FIG.7 (a)) of the discharge capacity of the secondary battery using a glyme (G5), and the charging / discharging cycle dependence (FIG.7 (b)) of coulomb efficiency. It is a figure which shows the charging / discharging cycle dependence (FIG.8 (a)) of the discharge capacity of the secondary battery using a glyme (G4Et), and the charging / discharging cycle dependence (FIG.8 (b)) of coulomb efficiency. It is a figure which shows the charging / discharging cycle dependence (FIG.9 (a)) of the discharge capacity of the secondary battery which added the solvent (HFE), and the charging / discharging cycle dependence (FIG.9 (b)) of coulomb efficiency. The charge / discharge cycle dependency of the discharge capacity of the secondary battery added with the solvent ([P13] [TFSA]) (FIG. 10 (a)) and the charge / discharge cycle dependency of the coulomb efficiency (FIG. 10 (b)) are shown. FIG. It is a figure which shows the charging / discharging cycle dependence (FIG. 11 (a)) of the discharge capacity of the secondary battery which added the solvent (toluene), and the charging / discharging cycle dependence (FIG.11 (b)) of coulomb efficiency. It is a figure which shows the charging / discharging cycle dependence of the discharge capacity of the secondary battery which added the solvent (propylene carbonate).

  Hereinafter, embodiments of the present invention will be described. The alkali metal-sulfur secondary battery according to the present invention includes a positive electrode having a sulfur-based electrode active material, a negative electrode, and the following electrolytic solution.

In the alkali metal-sulfur secondary battery according to the present invention, for example, the positive electrode and the negative electrode described above are arranged with a separator interposed therebetween, an electrolyte is contained in the separator, and a cell is formed. It has a structure in which a plurality of layers are wound or housed in a case. The current collectors of the positive electrode and the negative electrode are each drawn out of the case and electrically connected to a tab (terminal). The electrolytic solution may be a gel electrolyte.
The alkali metal-sulfur secondary battery can be manufactured by a conventionally known method.

<Positive electrode with sulfur-based electrode active material>
The positive electrode has a sulfur-based electrode active material containing at least one selected from the group consisting of elemental sulfur, metal sulfides, metal polysulfides, and organic sulfur compounds. Examples of sulfur-based metal sulfides include lithium polysulfides; Li ₂ S _x (1 ≦ x ≦ 8) and Na ₂ S _x (1 ≦ x ≦ 8). Examples of sulfur-based metal polysulfides include MSx. (M = Ni, Co, Cu, Fe, 1 ≦ x ≦ 2). Examples of organic sulfur compounds include organic disulfide compounds and carbon sulfide compounds.
The positive electrode described above may include the above-described sulfur-based electrode active material, a binder, and a conductive agent. The electrode material slurry (paste) is applied to a conductive carrier (current collector) and dried, whereby the electrode material is supported on the carrier and a positive electrode can be produced. Examples of the current collector include those in which a conductive metal such as aluminum, nickel, copper, and stainless steel is formed on a foil, a mesh, an expanded grid (expanded metal), a punched metal, or the like. Further, a resin having conductivity or a resin containing a conductive filler may be used as the current collector. The thickness of the current collector is, for example, 5 to 30 μm, but is not limited to this range.

Among the electrode materials described above (total amount of sulfur-based electrode active material and other components, excluding current collector), the content of sulfur-based electrode active material is preferably 50 to 98 mass%, more preferably. Is 80-98 mass%. If the content of the active material is within the above range, it is preferable because the energy density can be increased.
The thickness of the electrode material (thickness on one side of the coating layer) is preferably 20 to 500 μm, more preferably 20 to 300 μm, and still more preferably 20 to 150 μm.

As binders, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA) , Polyalkylene oxides such as polyacrylic acid lithium (PALi), sodium polyacrylate (PANa), ring-opening polymer of ethylene oxide or mono-substituted epoxide, or a mixture thereof.
The conductive agent is an additive blended to improve conductivity, such as carbon powder such as graphite, ketjen black, inverse opal carbon, acetylene black, vapor grown carbon fiber (VGCF), carbon nanotube (CNT). ) And the like. Further, the electrode material may contain a supporting salt (a component contained in the following electrolytic solution).

<Negative electrode>
The negative electrode can have an alkali metal, an alloy containing an alkali metal, a transition metal oxide, or carbon. The negative electrode active material contained in the negative electrode acts to occlude and desorb alkali metal ions. The negative electrode active material is preferably at least one selected from the group consisting of lithium, sodium, carbon, silicon, aluminum, tin, antimony and magnesium. More specifically, TiO ₂ , lithium titanate, lithium metal, sodium metal, lithium aluminum alloy, sodium aluminum alloy, lithium tin alloy, sodium tin alloy, lithium silicon alloy, sodium silicon alloy, lithium antimony alloy, sodium antimony alloy Conventionally known metal materials such as natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, carbon fiber, coke, soft carbon, carbon material such as hard carbon, and carbon material such as amorphous carbon material The negative electrode material can be used. In some cases, two or more negative electrode active materials may be used in combination.
Preferably, the negative electrode is made of metal Na. As will be described later, in the present invention, since the alkali metal-sulfur secondary battery can be operated at a lower temperature (for example, near room temperature) than before, it is used without melting even if the negative electrode is Na metal. It does not require energy for heating and is highly safe.

A separator is disposed between the positive electrode and the negative electrode. Examples of the separator include a glass separator that absorbs and holds an electrolyte solution described later, a porous sheet made of a polymer, and a nonwoven fabric. The porous sheet is composed of, for example, a microporous polymer. Examples of the polymer constituting such a porous sheet include polyolefins such as polyethylene (PE) and polypropylene (PP); laminates having a three-layer structure of PP / PE / PP, polyimide, and aramid. In particular, a polyolefin-based microporous separator is preferable because it has a property of being chemically stable with respect to an organic solvent and can keep the reactivity with the electrolyte solution low. The thickness of the separator made of a porous sheet is not limited, but in the use of a secondary battery for driving a motor of a vehicle, the total thickness is preferably 4 to 60 μm with a single layer or multiple layers. Moreover, it is preferable that the fine pore diameter of the separator made of a porous sheet is 10 μm or less (usually about 10 to 100 nm) and the porosity is 20 to 80%.
As the nonwoven fabric, cotton, rayon, acetate, nylon (registered trademark), polyester; polyolefins such as PP and PE; conventionally known materials such as polyimide and aramid are used alone or in combination. The porosity of the nonwoven fabric separator is preferably 50 to 90%. Furthermore, the thickness of the nonwoven fabric separator is preferably 5 to 200 μm, particularly preferably 10 to 100 μm. When the thickness is less than 5 μm, the electrolyte retainability deteriorates, and when it exceeds 200 μm, the resistance may increase.

<Electrolyte>
The electrolytic solution contains the following glyme and alkali metal salt, and at least a part of the glyme and alkali metal salt forms a complex. The following grime may be used alone or in the form of a mixture of two or more.
The grime has the following formula
(Here, R1 is any one of an alkyl group having 1 to 9 carbon atoms which may be substituted with a fluorine atom, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. R2 is an alkyl group having 1 to 9 carbon atoms which may be substituted with a fluorine atom, a phenyl group which may be substituted with a halogen atom, a cyclohexyl group which may be substituted with a halogen atom, or an ethyl group. Either)
(Here, R3 and R4 are each an alkyl group having 1 to 9 carbon atoms which may be substituted with a fluorine atom, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. Either)
(Here, R5 and R6 are an alkyl group having 1 to 9 carbon atoms which may be substituted with a fluorine atom, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. Either) or
(Here, R7 is any one of an alkyl group having 1 to 9 carbon atoms which may be substituted with a fluorine atom, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. ).

In the above chemical formula, those in which R1 and R2 are methyl groups are abbreviated as pentaglime (G5), and those in which R1 is a methyl group and R2 is an ethyl group are abbreviated as ethylpentalime (G5Et). The compound of Chemical Formula 5 is abbreviated as ethyltetraglyme (G4Et).
In the above chemical formula, those in which R3 and R4 are methyl groups are abbreviated as 3-methyltetraglyme (3MeG4), and those in which R5 and R6 are methyl groups are abbreviated as 6-methyltetraglyme (6MeG4).

  Examples of the alkyl group in the above formula include methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, pentyl group, isopentyl group, hexyl group, heptyl group, octyl group, and nonyl group. These alkyl groups may be substituted at any position with fluorine. When the alkyl group has more than 9 carbon atoms, the polarity of glyme becomes weak, so the solubility of the alkali metal salt tends to decrease. For this reason, the alkyl group preferably has a smaller number of carbon atoms, preferably a methyl group or an ethyl group, and most preferably a methyl group.

  The phenyl group which may be substituted with a halogen atom is not particularly limited, but 2-chlorophenyl group, 3-chlorophenyl group, 4-chlorophenyl group, 2,4-dichlorophenyl group, 2-bromophenyl group, 3- Examples include bromophenyl group, 4-bromophenyl group, 2,4-dibromophenyl group, 2-iodophenyl group, 3-iodophenyl group, 4-iodophenyl group, 2,4-iodophenyl group and the like.

  The cyclohexyl group which may be substituted with a halogen atom is not particularly limited, but 2-chlorocyclohexyl group, 3-chlorocyclohexyl group, 4-chlorocyclohexyl group, 2,4-dichlorocyclohexyl group, 2-bromocyclohexyl group. Group, 3-bromocyclohexyl group, 4-bromocyclohexyl group, 2,4-dibromocyclohexyl group, 2-iodocyclohexyl group, 3-iodocyclohexyl group, 4-iodocyclohexyl group, 2,4-diiodocyclohexyl group and the like. Can be mentioned.

The alkali metal salt described above is represented by MX, M is an alkali metal, and X is a substance that becomes a counter anion. The said alkali metal salt may be used individually by 1 type, and may be used in the form of a mixture of 2 or more types.
There is no restriction | limiting in particular as M, The alkali metal currently used as a supporting salt and an active material in a normal battery can be used. Specific examples include Li, Na, K, Rb, and Cs. More preferred are Li, Na and K, and most preferred is Na.
X is not particularly limited, but Cl, Br, I, BF ₄ , PF ₆ , CF ₃ SO ₃ , ClO ₄ , CF ₃ CO ₂ , AsF ₆ , SbF ₆ , AlCl ₄ , N (CF ₃ SO ₂ ) ₂ , N (CF ₃ CF ₂ SO ₂ ) ₂ , PF ₃ (C ₂ F ₅ ) ₃ , N (FSO ₂ ) ₂ , N (FSO ₂ ) (CF ₃ SO ₂ ), N (CF ₃ CF ₂ SO ₂ ) ₂ , N (C ₂ F ₄ S ₂ O ₄ ), N (C ₃ F ₆ S ₂ O ₄ ), N (CN) ₂ , N (CF ₃ SO ₂ ) (CF ₃ CO), R ¹ FBF ₃ (where R ¹ F = n-C _m F _{2m + 1} , m = 1 to 4 is a natural number, n is normal) and R ² BF ₃ (where R ² = n−C _p H _{2p + 1} , p = 1-5) The natural number of n is preferably at least one selected from the group consisting of n). N (CF ₃ SO ₂ ) ₂ , N (FSO ₂ ) ₂ , and N (CF ₃ CF ₂ SO ₂ ) ₂ are more preferable from the viewpoints of solubility in glyme and ease of forming a complex structure.

Whether at least a part of the above-mentioned glyme and alkali metal salt forms a complex can be determined by thermogravimetric measurement of an electrolytic solution in which these are mixed. That is, complexed glyme is less volatile than non-complexed glyme. For this reason, the electrolyte solution based on the weight reduction by thermogravimetry of only glyme and having a weight reduction by temperature less than this base is considered that at least a part of glyme and alkali metal salt forms a complex.
2 to 4 show the results of thermogravimetric measurement (temperature increase and weight decrease) of the electrolyte using 3MeG4, 6MeG4 and G5 as the glyme and NaTFSA (NaN (CF3SO2) 2) described later as the alkali metal salt, respectively. (Relationship) graph. In addition, we prepared electrolyte solutions with different mixing ratios (molar conversion) of each glyme and NaTFSA, and increased the temperature of the electrolyte solution from room temperature to 550 ° C at a heating rate of 10 ° C min-1 for thermogravimetry. went. In addition, as a measuring device, a suggested thermothermogravimetric simultaneous measuring device (TG / DTA 6200 manufactured by Seiko Instruments Inc.) was used.
For example, NaTFSA / 3MeG4 = 1 in FIG. 2 indicates that the mixing ratio (in terms of mole) of NaTFSA to glyme is 1. Moreover, the curve shown by 3MeG4 in FIG. 2 shows the thermogravimetric measurement of an electrolyte solution made of only 3MeG4. The same applies to FIGS. 3 and 4.

As shown in FIGS. 2 to 4, it can be seen that the process of weight reduction proceeds in the following three stages (1) to (3).
(1) Weight loss up to 100-200 ° C is due to the evaporation of uncomplexed glyme
(2) The weight loss from 200 to 400 ° C is due to the evaporation of complexed glyme
(3) The weight loss above 400 ° C is derived from the thermal decomposition of alkali metal salt (NaTFSA) .Therefore, when the above process (2) can be confirmed from the results of thermogravimetry, complexing of glyme is observed. Can be considered.
It can be seen that in the system where the mixing ratio of NaTFSA to glyme (molar conversion) is 0.80 or more, all the glyme forms a complex, so there is almost no process of (1), and weight loss starts from 200 ° C or higher. And as will be described later, when the mixing ratio (molar conversion) is 0.80 or more, the concentration of the alkali metal salt (Na salt) is higher than that of the techniques described in Non-Patent Documents 1 and 2, and during charging. Side reactions are suppressed and the coulomb efficiency can be improved.
On the other hand, when the mixing ratio of NaTFSA to glyme (molar conversion) is less than 0.80, the thermal stability of the electrolytic solution is lowered, the concentration of the alkali metal salt (Na salt) is lowered, and the Coulomb efficiency is also lowered. As will be described later, in the case of glyme other than 3MeG4, 6MeG4, G4Et, G5 and G5Et (for example, tetraglyme), the solubility of alkali metal salt (sodium ion) is low, and the mixing ratio (molar conversion) is 0.80 or more. Don't be.

The higher the mixing ratio, the better the coulomb efficiency and the discharge capacity maintenance ratio. However, when the mixing ratio exceeds a value determined by the saturation concentration of the alkali metal salt in the glyme, the alkali metal salt dissolves in the glyme. No longer. Therefore, the upper limit of the mixing ratio is regulated to a value determined by the molar concentration in terms of the saturation concentration of the alkali metal salt in the glyme.
The saturated concentration of the alkali metal salt in glyme is the concentration at which the solid content of the alkali metal salt can be visually confirmed when the alkali metal salt is dissolved in glyme at 30 ° C.
The upper limit of the mixing ratio when the glyme is used is usually 1.25 in terms of mole when the alkali metal salt is a Na salt.

  The electrolytic solution may be a gel gel electrolyte. The gel electrolyte has a configuration in which an electrolytic solution is injected into a matrix polymer made of an ion conductive polymer. As the electrolytic solution, the above-described electrolytic solution of the present invention is used. Examples of the ion conductive polymer used as the matrix polymer include polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), and vinylidene fluoride-hexafluoropropylene (VDF-HEP). And poly (methyl methacrylate (PMMA) and their copolymers, etc. Electrolytic salts such as lithium salts and sodium salts can be well dissolved in polyalkylene oxide polymers.

In the present invention, in addition to the above-mentioned glyme and alkali metal salt, the electrolytic solution has hydrophobicity and is thoroughly mixed with the complex, and alkali metal and alkali metal polysulfide: M _{2 Sn} (M is an alkali metal cation) 1 ≦ n ≦ 8), it is preferable to contain one or more selected from the group of fluorine-based solvents, ionic liquids, and hydrocarbons that do not chemically react.
As a result of investigation by the present inventor, when the above solvent is further added to the glyme and the alkali metal salt as an electrolytic solution of the alkali metal-sulfur secondary battery, the ionic conductivity of the electrolytic solution is increased and the temperature is lowered (room temperature, Alternatively, the coulomb efficiency can be improved even at about 30 ° C. close to room temperature. The reason for this is not clear, but if the electrolyte contains the above solvent, the ionic conductivity increases and current flows more easily, and the viscosity of the electrolyte decreases and the pores inside the sulfur-carbon composite electrode are reduced. This is thought to be because the electrolyte solution easily penetrates and the interface where the electrode and the electrolyte solution can be electrochemically reacted increases. In addition, it is preferable that the solvent is nonflammable because the safety of the obtained battery is improved.

Here, the hydrophobicity of the solvent is determined by visual observation of the presence or absence of phase separation by mixing the solvent and distilled water at a volume ratio of 1: 1. If the phase separation is visible, it is determined to be hydrophobic, and if it is a uniform mixed solution without phase separation, it is determined to be hydrophilic.
Polysulfide alkali metal produced in the battery reaction process: M _{2 Sn} (M is an alkali metal cation, 1 ≦ n ≦ 8) is easily dissolved in a hydrophilic solvent. In the case of a sulfur positive electrode, M _{2 Sn} is dissolved. This indicates that a side reaction occurs during charging, and the Coulomb efficiency and the discharge capacity are reduced by repeated charging and discharging. Therefore, when the solvent is a hydrophobic, in order to suppress the elution of M ₂ S _n generated during charge and discharge, to maintain a high coulomb efficiency.
In addition, the mixing property of the complex and the solvent is determined by visually comparing the presence of phase separation by mixing the complex and the solvent in a volume ratio of 1: 1. If the solvent is not mixed with this complex, it is not suitable as an electrolyte.

Whether or not the solvent chemically reacts with the alkali metal is determined by immersing an alkali metal foil of 1 cm (length) × 1 cm (width) × 0.02 cm (thickness) in 2 mL of solvent for 1 day. When the gloss is lower than the initial value or the color of the solvent is changed from the initial value, it is determined that the reaction occurs. If the solvent chemically reacts with the alkali metal, a side reaction of the battery occurs, resulting in a decrease in coulomb efficiency and a decrease in discharge capacity maintenance rate (battery life), which is inappropriate.
Whether or not the solvent chemically reacts with alkali metal polysulfide (M _{2 Sn} ) was determined based on whether or not the amount of electricity required for charging the battery was equal to or greater than a threshold value. For example, when the alkali metal is Li, if Li ₂ S _n generated during charge and discharge is solvent and chemical reaction, Li ₂ S _n contributes to the charge and discharge reaction. Therefore, reactions are observed around 2.2 V and 2.0 V when the battery is discharged, and reactions are observed around 2.4 V when charging. On the other hand, if the solvent reacts with Li ₂ S _n, the charge reaction does not occur. Therefore, in the case of Li, if the amount of electricity required for charging was 200 mA hg ⁻¹ or more, it was considered that no chemical reaction occurred. Even when the alkali metal was Na or K, it was considered that no chemical reaction occurred if the amount of electricity required for charging was 200 mAhg ^-1 or more.
Incidentally, when the solvent polysulfide alkali metal M ₂ S _n and chemical reactions, result in reduced degradation of the coulombic efficiency occurs a side reaction of the battery, the discharge capacity retention ratio (battery life), it is unsuitable. For example, as shown by the experiments described below, carbonate-based solvents are known to react chemically with Li ₂ S _n (literature J. Phys. Chem. C, 115 , 25132, (2011)).
The reason why the solvent used in the present invention needs to have the above-described characteristics will be described later (see Table 2 and the description thereof).

As the fluorine-based solvent, hydrofluoroether is preferable. Examples of the hydrofluoroether include HF ₂ CF ₂ CH ₂ C—O—CF ₂ CF ₂ H (1,1,2,2-tetrafluoroethyl (2,2,3,3-tetrafluoropropyl) ether, (Hereinafter referred to as “HFE”).
The ionic liquid used as the solvent of the present invention is an onium salt having a melting point of 100 ° C. or lower. As the ionic liquid, for example, the following formula 4
N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) amide (abbreviated as [P13] [TFSA]).
Moreover, toluene is mentioned as a hydrocarbon used as a solvent of this invention.

In the present invention, the solvent mixing ratio {(solvent) / (alkali metal salt)} to the alkali metal salt is preferably 0.50 to 5.0 in terms of mole.
When the ratio represented by (solvent) / (alkali metal salt) is less than 0.50 in terms of mole, the above-mentioned effect does not occur because the solvent is small, and the input / output density may not be improved. On the other hand, even if the ratio exceeds 5.0, the effect of the solvent is saturated and the cost increases.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples. In addition, unless otherwise indicated,% shows the mass%.

<Experiment A: Experiment with electrolyte containing glyme>
<Preparation of electrolyte>
As the grime, 3MeG4, 6MeG4, G5, G4Et, and G4 (tetraglyme) (manufactured by Nippon Emulsifier Co., Ltd.) were used, respectively. G5Et was synthesized as follows.
Synthesis of G5Et: To a three-necked flask were added 20.53 g (125 mmol) of triethylene glycol monomethyl ether, 15.81 g (156 mmol) of triethylamine, and 50 mL of THF. Then, 19.07 g (100 mmol) of p-toluenesulfonyl chloride was dissolved in 80 mL of THF, and the solution was slowly added dropwise using a dropping funnel and allowed to stir overnight. Thereafter, the precipitate was removed by suction filtration, and the solution was concentrated to remove THF. To the obtained solution, 300 mL of chloroform was added and washed 5 times with 100 mL of pure water using a separatory funnel. The organic phase obtained by washing was dried over MgSO ₄ and filtered, and then the filtrate was concentrated. Thereafter, purification and concentration were performed by column chromatography to obtain [2- [2- (2-methoxyethoxy) ethoxy] ethoxy] p-toluenesulfonate.
Next, 1.65 g (68.8 mmol) of NaH that had been washed with dehydrated hexane was weighed into a three-necked flask, and 50 mL of dry-DMF was added and stirred. Then, 9.2 mL (68.8 mmol) of diethylene glycol monomethyl ether and 50 mL of dry-DMF were added to the dropping funnel, and slowly dropped into the three-necked flask. After stirring for a while, [2- [2- (2-methoxyethoxy) ethoxy] ethoxy] p-toluenesulfonate 14.8 mL (55.0 mmol) and dry-DMF 50 mL were added and slowly dropped into a three-necked flask overnight. Allow to stir. All reactions were performed at room temperature. Thereafter, an appropriate amount of water was added to stop the reaction. After removing DMF as much as possible, 100 mL of pure water was added to the resulting solution, and extraction was performed with Chloroform 50 mL × 4 using a separatory funnel. The organic phase obtained by extraction was dried over MgSO ₄ and filtered, and then the filtrate was concentrated. Thereafter, vacuum distillation (110 ° C.) was performed. And after performing vacuum distillation again using metallic sodium, it was made to heat-vacuum dry overnight and G5Et was obtained.
The product was characterized by ¹ H-NMR.

As the Na salt, sodium bis (trifluoromethanesulfonyl) amide (hereinafter referred to as “Na [TFSA]”) represented by the following formula 13 (made by Kishida Chemical Co., Ltd.) was used.
The above-mentioned glyme and Na [TFSA] were mixed in a glove box under an argon atmosphere to prepare an electrolytic solution having Na [TFSA] at a predetermined molar concentration.

<Production of sodium-sulfur battery>
Using elemental sulfur (S8) as a sulfur-based electrode active material, mixing elemental sulfur at 60 wt%, ketjen black as conductive agent at 30 wt%, and PVA (polyvinyl alcohol) as binder at a ratio of 10 wt%, A positive electrode material 2a (see FIG. 1) was prepared. First, simple sulfur and ketjen black were mixed, and then heated at 155 ° C. to compound simple sulfur and ketjen black. An appropriate amount of NMP (N-methyl-2-pyrrolidone) in which PVA was dissolved was further added to this mixture and kneaded into a slurry. The obtained slurry was applied to an aluminum foil (current collector) 2b having a thickness of 20 μm, dried at 80 ° C. for 12 hours to evaporate NMP, and then pressed to obtain a positive electrode 2 (see FIG. 1). . A sodium metal plate having a thickness of 200 μm was attached to a stainless steel disk having a thickness of 500 μm to produce a negative electrode.

  An appropriate amount of the electrolyte solution was added to the positive electrode 2 in a glove box under an argon atmosphere, and the electrolyte solution was immersed in the positive electrode 2 at 60 ° C. for 60 minutes. The positive electrode 2 and the negative electrode 4 are laminated via a separator 6 (glass separator made by Toyo Roshi Kaisha with a thickness of 200 μm (trade name GA-55)), and the electrolyte is injected, and then a 2032 type coin cell The case 20 (SUS304 thickness 3.2 mm) was sealed, and the spacer 12 was placed on the negative electrode (negative electrode) 4. A spring 14 is disposed on the spacer 12. The coin cell case 20 was sealed from above the spring 14 with a lid 22 to produce a sodium-sulfur battery 50 having the structure shown in FIG. A gasket 10 is interposed on the side wall of the coin cell case 20.

<Evaluation>
(1) Constant Current Charge / Discharge Test The secondary battery obtained as described above was discharged at a constant current with a current density of 139 mA g ⁻¹ (normalized by the weight of sulfur contained in the positive electrode). The voltage was in the range of 1.0 to 3.0 V, and the test was carried out in a thermostatic bath maintained at 60 ° C. The discharge current density was 1 / 12C (12 hours rate, the theoretical capacity of the electrode active material was expressed as n (hours), and the current value expressed as 1 / n C rate).
In addition, since the positive electrode (sulfur electrode) is manufactured in a charged state, the first cycle of the charge / discharge cycle proceeds only in the discharge process, and the charge and discharge processes proceed in the second and subsequent cycles. Therefore, the order of charge / discharge is as follows: first cycle discharge → second cycle charge → second cycle discharge → third cycle charge → third cycle discharge. In this way, 20 charge / discharge cycles were performed, and the constant current charge / discharge was performed in each cycle.

(2) Coulomb efficiency, charge / discharge capacity From the charge capacity and discharge capacity obtained by the above constant current charge / discharge (mAh / g: g is per mass of elemental sulfur), the Coulomb efficiency (% ) = Discharge capacity / charge capacity. The coulomb efficiency is a value indicating how much the charged amount of electricity can be taken out by discharging, and the closer the value is to 100 (%), the better.
The obtained results are shown in FIGS.

When 3MEG4, 6MEG4, G5, G4Et or G5Et is used as the glyme and the mixing ratio of Na [TFSA] to glyme is 0.80 or more in terms of mole (see Examples 1 to 14 in Table 1), the Coulomb efficiency is It can be seen that it can be operated at a low temperature (60 ° C.) while improving.
On the other hand, in the case of Comparative Example 1 using G4 as the glyme and in the case of Comparative Example 2 using G5 as the glyme and the mixing ratio of Na [TFSA] to glyme being less than 0.80 in terms of mole, the discharge capacity and coulomb Efficiency has dropped significantly.

<Experiment B: Experiment in which a solvent was added to the electrolyte>
Since the electrolyte used in Experiment A does not have high ionic conductivity (less than 1.0 mS / cm) at room temperature (about 30 ° C.), the battery was operated at 60 ° C. Therefore, in order to increase the ionic conductivity of the electrolytic solution at room temperature (about 30 ° C.), a solvent was added to the electrolytic solution.
First, as a solvent suitable for the present invention, a solvent having hydrophobicity, thoroughly mixed with a complex of glyme and Na salt, and not chemically reacting with Na metal and alkali metal polysulfide was found experimentally, and as a result. Are summarized in Table 2.
In Table 2, [C2mim] [TFSA] is 1-ethyl-3-methylimidazolium-bis (trifluoromethanesulfonyl) amide, and PFPM represents methyl pentafluoropropionate.

From Table 2, HFE (HF ₂ CF ₂ CH ₂ C—O—CF ₂ CF ₂ H) (made by Daikin Industries), which is a hydrofluoroether, [P13] [TFSA] (made by Kanto Chemical Co., Inc.) which is an ionic liquid Toluene, which is a hydrocarbon, was used as a solvent. For comparison, propylene carbonate was used as a solvent.
<Preparation of electrolyte and production of battery>
G5 was used as the glyme, Na [TFSA] was used as the Na salt, and the mixture was mixed at a ratio of Na [TFSA] /G5=1.0 (mol conversion). Moreover, each solvent was mixed and the electrolyte solution was prepared so that it might become the ratio of (solvent) / Na [TFSA] = 4.0 (molar conversion).
Using this electrolytic solution, a sodium-sulfur battery was produced in the same manner as in Experiment A.
<Evaluation>
In the same manner as in Experiment A, a constant current charge / discharge test was performed to determine the Coulomb efficiency and the charge / discharge capacity. The operating temperature of the battery during the test was 30 ° C.
The obtained results are shown in FIGS.

In Examples 21 to 23 using HFE, [P13] [TFSA], and toluene as the solvent, it can be seen that the coulombic efficiency was improved and the operation was possible at room temperature (30 ° C.). In particular, when HFE and [P13] [TFSA] were used, the coulomb efficiency was increased. In the case of Examples 21 to 23, the ionic conductivity of the electrolyte at 30 ° C. was 3.0 mS / cm or more.
In any solvent, when the solvent is added so that (solvent) / Na [TFSA] = 1.0 or more (molar conversion), the ionic conductivity of the electrolyte at 30 ° C. should be 1.0 mS / cm or more. It was confirmed. In addition, in the case of a glyme complex (Na [TFSA] /G5=1.0 (mol conversion)) to which no solvent was added, the ionic conductivity of the electrolyte at 30 ° C. was 0.61 mS / cm.
On the other hand, in the case of Comparative Example 21 using propylene carbonate as a solvent, the discharge capacity and the Coulomb efficiency were greatly reduced.

2 Positive electrode 4 Negative electrode 50 Sulfur-based secondary battery

Claims (3)

A positive electrode having a sulfur-based electrode active material comprising at least one selected from the group consisting of elemental sulfur, metal sulfides, metal polysulfides, and organic sulfur compounds;
Following formula
(Here, R ₁ is any of an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. R ₂ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, a cyclohexyl group which may be substituted with a halogen atom, or ethyl One of the groups),
(Here, R ₃ and R ₄ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups),
(Here, R ₅ and R ₆ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups), or
(Wherein R ₇ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. And at least a part of the glyme and the alkali metal salt forms a complex, and further includes a fluorinated solvent, an ionic liquid, and a hydrocarbon. of one or more selected from the group of the alkali metal and polysulfides alkali metal _{(M 2 S n: 1 ≦} n ≦ 8) and containing a solvent which does not chemically react substantially the grime and the alkali An electrolytic solution comprising a metal salt and the solvent ;
A counter electrode of the positive electrode, the negative electrode having the alkali metal, the alloy containing the alkali metal, or carbon;
With
An alkali metal-sulfur secondary battery in which a mixing ratio of the alkali metal salt to the glyme is 0.80 or more in terms of mole and not more than a value determined by a saturation concentration of the alkali metal salt in the glyme. The alkali metal salt is represented by MX, where M is an alkali metal, X is Cl, Br, I, BF ₄ , PF ₆ , CF ₃ SO ₃ , ClO ₄ , CF ₃ CO ₂ , AsF ₆ , SbF. ₆ , AlCl ₄ , N (CF ₃ SO ₂ ) ₂ , N (CF ₃ CF ₂ SO ₂ ) ₂ , PF ₃ (C ₂ F ₅ ) ₃ , N (FSO ₂ ) ₂ , N (FSO ₂ ) (CF _{3 SO 2), N (CF 3} CF 2 SO 2) 2, N (C 2 F 4 S 2 O 4), N (C 3 F 6 S 2 O 4), N (CN) 2, N (CF 3 SO ₂ ) (CF ₃ CO), R ¹ FBF ₃ (where R ¹ F = n-C _m F _{2m + 1} , m = 1 to 4 natural number) and R ² BF ₃ (where R ² = n−C _pH H _2p + 1, p = is at least one selected from 1-5 of the group consisting of natural numbers) according Sulfur-based secondary battery - alkali metal according to 1. Following formula
(Here, R ₁ is any of an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. R ₂ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, a cyclohexyl group which may be substituted with a halogen atom, or ethyl One of the groups),
(Here, R ₃ and R ₄ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. is any of the groups),
(Here, R ₅ and R ₆ are each an alkyl group optionally substituted with 1 or 2 carbon atoms, a phenyl group optionally substituted with a halogen atom, or a cyclohexyl optionally substituted with a halogen atom. One of the groups), or
(Wherein R ₇ is an alkyl group having 1 or 2 carbon atoms which may be substituted with fluorine, a phenyl group which may be substituted with a halogen atom, or a cyclohexyl group which may be substituted with a halogen atom. and glyme represented by a is) or,
And alkali metal salts, substantially becomes a mixing ratio of the alkali metal salt to the glyme, at a molar basis 0.80 or more, and be less than or equal to a value determined by the saturation concentration of the alkali metal salt in the glyme the glyme and at least partially electrolytic solution for a secondary battery which form a complex with the alkali metal salt.