JP2015216124A - Alkali metal-sulfur based secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alkali metal-sulfur based secondary battery having a long battery life.SOLUTION: An alkali metal-sulfur based secondary battery comprises: a positive electrode and a negative electrode 2, one of which has a sulfur-based electrode active material; an electrolytic solution including a glyme other than tetraglyme, and an alkali metal salt; and a counter electrode 4 to the positive or negative electrode, which has an alkali metal, an alloy including an alkali metal, carbon. The mixing ratio of the alkali metal salt to the glyme is 0.50-2.0 on a molar basis. With the glyme and the alkali metal salt forms, at least a part thereof forms a complex. In a charge/discharge test (Current density=1/12 C; Charge/discharge voltage=1.5-3.3 V, Discharge condition=1/12 C, and 30°C), the discharge capacity-keeping ratio in the 10-th cycle is 85% or more.

Description

  The present invention relates to a secondary battery using sulfur for a positive electrode or a negative electrode such as a lithium-sulfur battery.

  In recent years, with the widespread use of mobile phone terminals and the research and development of electric vehicles and hybrid electric vehicles that respond to environmental problems, high-capacity secondary batteries have been demanded. As such a secondary battery, a lithium ion secondary battery has already been widely used. However, in order to ensure safety for in-vehicle use, a technique using a flame-retardant glyme as an electrolytic solution has been proposed. (For example, refer nonpatent literature 1). In addition, as an electrolyte for a lithium secondary battery, a lithium salt with a mixing ratio of Li salt to glyme adjusted to 0.70 to 1.25 in terms of mole is used. A technique with improved performance has been proposed (see, for example, Patent Document 1).

On the other hand, lithium-sulfur batteries have attracted attention as secondary batteries having a higher capacity than lithium secondary batteries (see, for example, Patent Documents 2 and 3). Sulfur has a theoretical capacity of about 1670 mAh / g, and has an advantage that the theoretical capacity is about 10 times higher than LiCoO ₂ (about 140 mAh / g), which is a positive electrode active material of a lithium battery, and is low in cost and rich in resources. .
And in the lithium-sulfur battery, the mixing ratio of Li salt (LiCF ₃ SO ₃ ) to tetraglyme was adjusted to about 0.12 to 0.25 (LiCF ₃ SO ₃ is 0.5 to 1 mol / L) in terms of mole. Techniques using an electrolytic solution have been proposed (see, for example, Non-Patent Documents 2 and 3).

JP 2010-73489 A Special table 2008-52762 gazette Japanese Patent No. 3842275

M. Naganaga et al., “Examination of Lithium Secondary Battery Using Grime-LiTFSI Molten Complex”, Proceedings of Battery Conference, Vol.47, p496-497, 2006 Jae-Won Choi et "Rechargeable lithium / sulfur battery with liquid electrolytes containing toluene as additive", Journal of Power Sources, 183, p441-445, 2008 Sang-Eun Cheon et "Rechargeable Lithium Sulfur Battery", Journal of the Electrochemical Society, 150 (6), A796-799, 2003

However, as a result of investigation by the present inventors, in a lithium-sulfur battery, when tetraglyme and Li salt are used as an electrolyte, a side reaction occurs during charge and discharge, and coulomb efficiency (discharge capacity / charge capacity) decreases. At the same time, it has been found that the discharge capacity is significantly reduced by repeated charging and discharging, and the battery life is short. This side reaction is considered to be the elution of lithium polysulfide (Li ₂ S _n ; 2 <n <8) generated during charging and discharging into the electrolyte.
Accordingly, an object of the present invention is to improve the coulomb efficiency by suppressing side reactions during charging / discharging, and to suppress a decrease in discharge capacity due to repeated charging / discharging, thereby providing a long battery life alkali metal-sulfur secondary battery. Is to provide.

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. Negative electrode and the following formula
(Here, R 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, and a cyclohexyl group which may be substituted with a halogen atom. And x is 1 to 6). An electrolyte solution containing a glyme excluding tetraglyme and an alkali metal salt, and a counter electrode of the positive electrode or the negative electrode, wherein the alkali metal and the alkali metal are included. An alkali metal-sulfur secondary battery comprising an alloy or a counter electrode having carbon, wherein a mixing ratio of the alkali metal salt to the glyme is 0.50 or more and 2.0 or less in terms of mole, At least a part of the glyme and the alkali metal salt forms a complex, and a charge / discharge test (current density = 1/12 C, charge / discharge voltage = 1.5-3.3 V, discharge condition = 1) 12C, it is 30 ° C.) in the 10th cycle discharge capacity retention ratio of 85% or more.

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 positive electrode or the negative electrode preferably includes the sulfur-based electrode active material, a binder, and a conductive agent.

Further, in the alkali metal-sulfur secondary battery of the present invention, the glyme is triglyme (G3),
The alkali metal salt is preferably a lithium salt.

  According to the present invention, side reactions during charging and discharging are suppressed to improve coulomb efficiency, and a decrease in discharge capacity due to repeated charging and discharging is suppressed, thereby obtaining an alkali metal-sulfur secondary battery having a long battery life. be able to.

3 is a cross-sectional view showing a configuration example of a lithium-sulfur battery 50. FIG. It is a figure which shows the charging / discharging cycle dependence of the charging / discharging curve of the 2nd cycle of the secondary battery which has electrolyte solution which used G4, and made mixing ratio 1.00, and charging / discharging capacity | capacitance. It is a figure which shows the charging / discharging cycle dependence of the charging / discharging curve of the 2nd cycle of the secondary battery which has electrolyte solution which used G4, and made the mixture ratio 0.25, and charging / discharging capacity | capacitance. It is a figure which shows the charge / discharge cycle dependence of the coulomb efficiency of the secondary battery which has the electrolyte solution using G4, and the relationship between the coulomb efficiency of 10th cycle, and a mixture ratio. It is a figure which shows the relationship between the charge capacity / discharge cycle dependence of the discharge capacity maintenance factor of the secondary battery which has electrolyte solution using G4, and the discharge capacity maintenance factor of 10th cycle, and a mixture ratio. It is a figure which shows the relationship between the charging / discharging cycle dependence of the Coulomb efficiency of the secondary battery which has electrolyte solution using G3, and the Coulomb efficiency of 10th cycle, and a mixture ratio. It is a figure which shows the relationship between the charge-and-discharge cycle dependence of the discharge capacity maintenance factor of the secondary battery which has electrolyte solution using G3, and the discharge capacity maintenance factor of 10th cycle, and a mixture ratio. It is a figure which shows the charging / discharging cycle dependence of the coulomb efficiency of the secondary battery which has the electrolyte solution using G4Et. It is a figure which shows the charging / discharging cycle dependence of the discharge capacity maintenance factor of the secondary battery which has electrolyte solution using G4Et. It is a figure which shows the result of the thermogravimetry of the electrolyte solution containing triglyme and an alkali metal salt (LiTFSA). It is a figure which shows the result of the thermogravimetry of the electrolyte solution containing a tetraglyme and an alkali metal salt (LiTFSA). Used G3 as glyme, and solubility when the mixture was Li ₂ S _8, is a graph showing the relationship between the mixing ratio of LiTFSA for glyme (molar basis). Used G4 as glyme, and solubility when the mixture was Li ₂ S _8, is a graph showing the relationship between the mixing ratio of LiTFSA for glyme (molar basis).

Hereinafter, embodiments of the present invention will be described. The alkali metal-sulfur secondary battery according to the present invention includes a positive electrode or a negative electrode having a sulfur electrode active material, an electrolytic solution containing the following glyme and an alkali metal salt, and a counter electrode of the positive electrode or the negative electrode.
The alkali metal-sulfur secondary battery according to the present invention includes a lithium-sulfur battery, a sodium-sulfur battery in which the positive electrode has a sulfur-based electrode active material, and a battery in which the negative electrode has a sulfur-based electrode active material. Some sulfur-LiCoO ₂ batteries and sulfur-LiMn ₂ O ₄ batteries are exemplified, but not limited thereto.

In the alkali metal-sulfur secondary battery according to the present invention, for example, the above-described positive electrode or negative electrode and a counter electrode are arranged apart from each other via a separator, and an electrolyte is included in the separator to constitute a cell. A plurality of cells are stacked or wound and accommodated in a case. Current collectors of the positive electrode or the negative electrode and the counter 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 or negative electrode having a sulfur-based electrode active material>
The positive electrode or the negative 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 (0 <x ≦ 8), and examples of sulfur-based metal polysulfides include MSx (M = Ni, Cu, Fe, 0 <x ≦ 2). Examples of organic sulfur compounds include organic disulfide compounds and carbon sulfide compounds.
The above-described positive electrode or negative electrode may include the above-described sulfur-based electrode active material, a binder, and a conductive agent. Then, a slurry (paste) of these electrode materials is applied to a conductive carrier (current collector) and dried, whereby the electrode material is supported on the carrier and a positive electrode or a negative 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 (PAALi), ring-opening polymer of ethylene oxide or mono-substituted epoxide, or a mixture thereof.
The conductive agent is an additive blended to improve conductivity, and includes carbon powders such as graphite, ketjen black, inverse opal carbon, and acetylene black, and various carbons such as vapor grown carbon fiber (VGCF). Examples include fibers. Further, the electrode material may contain a supporting salt (a component contained in the following electrolytic solution).

<Counter electrode>
When the positive electrode has the above-described sulfur-based electrode active material, the negative electrode serving as the counter electrode is one or more selected from the group consisting of lithium, sodium, lithium alloys, sodium alloys, and lithium / inert sulfur composites. Negative electrode active material. 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, metals such as 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, etc. Conventionally known negative electrode materials such as crystalline carbon materials such as materials, natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, carbon fiber, coke, soft carbon, hard carbon, and amorphous carbon materials Can be used. Among these, it is desirable to use a carbon material or lithium or a lithium transition metal composite oxide because a battery excellent in capacity and output characteristics can be constituted. In some cases, two or more negative electrode active materials may be used in combination.

When the negative electrode has the above-described sulfur-based electrode active material, as the positive electrode serving as the counter electrode, a material containing a positive electrode active material that occludes and desorbs alkali metal ions can be used. As the positive electrode active material, a lithium transition metal composite oxide is preferable, and examples thereof include a Li—Mn composite oxide such as LiMn ₂ O ₄ and a Li—Ni composite oxide such as LiNiO ₂ . More specifically, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFePO ₄ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ are preferably mentioned. In addition to lithium, any substance that electrochemically inserts and desorbs alkali metal ions can be used without limitation, and examples thereof include sodium. Two or more positive electrode active materials may be used in combination.
The counter electrode may also contain the above-described active material, binder, and conductive agent. These electrode materials can be carried on a conductive carrier (current collector) to produce a counter electrode. A current collector similar to the above can be used.

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. The fine pore diameter of the separator made of a porous sheet is preferably 1 μm or less (usually about 10 nm) and the porosity is preferably 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 electrolyte is the following formula
(Here, R 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, and a cyclohexyl group which may be substituted with a halogen atom. Wherein x is 1 to 6) and an alkali metal salt, and at least a part of the glyme and the alkali metal salt forms a complex.

The glyme used for the electrolyte may be used alone or in the form of a mixture of two or more. X in the formula represents the number of repeating ethylene oxide units. By using the glyme in which x is 1 to 6, the thermal stability, ion conductivity, and electrochemical stability of the electrolytic solution can be further improved, and the electrolytic solution can withstand high voltage. x is preferably 1 to 5, more preferably 2 to 5, and most preferably 3 or 4.
The oxidation potential of the electrolyte also changes depending on the type of glyme. Therefore, considering application to a secondary battery, it is preferable to adjust the mixing ratio and the like so that the oxidation potential becomes 3.5 to 5.3 V vs. Li / Li ⁺ . The oxidation potential is more preferably 4.0 to 5.3 V vs Li / Li ⁺ .

  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 Li from the viewpoint of versatility.
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.

  The electrolyte solution may contain an optional additive such as an organic solvent in addition to the above-mentioned glyme and alkali metal salt. Examples of the additive include carbonates such as propylene carbonate, ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and onium salts such as fluorides and ionic liquids thereof. However, the additive is preferably 50% by mass or less based on the entire electrolytic solution so as not to prevent complex formation between glyme and the alkali metal salt. Moreover, as a kind of additive, it is preferable to add an additive having a lower donor number than glyme so as not to excessively affect the complex formation.

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. Therefore, based on the weight reduction by thermogravimetric measurement of the electrolyte solution consisting only of glyme, the electrolyte solution whose weight loss by temperature is less than this base is that at least a part of glyme and alkali metal salt forms a complex It is considered.
FIGS. 10 and 11 show the results of thermogravimetric measurement of the electrolyte using triglyme (G3) and tetraglyme (G4) as the glyme and LiTFSA described later as the alkali metal salt (relationship between temperature increase and weight decrease), respectively. The graph of is shown. In addition, we prepared electrolytes with different mixing ratios (molar conversion) of each glyme and LiTFSA, and raised the temperature of the electrolyte from room temperature to 550 ° C at a 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.
Note that LiTFSA / G3 = 1 in FIG. 10 indicates that the mixing ratio of LiTFSA to glyme (in terms of mole) is 1. Further, the curve indicated by G3 in FIG. 10 shows the thermogravimetric measurement of the electrolytic solution composed only of triglyme.

As shown in FIG. 11, it can be seen that the process of weight reduction proceeds in the following three stages (1) to (3).
(1) The weight loss from 100 to 200 ° C is due to the evaporation of uncomplexed glyme
(2) Weight loss from 200 to 400 ° C is due to the evaporation of complexed glyme
(3) The weight loss at 400 ° C or higher is derived from the thermal decomposition of alkali metal salt (LiTFSA) .Therefore, when the process of (2) above can be confirmed from the results of thermogravimetry, glyme is complexed. Can be considered.
In addition, in the system where the mixing ratio of LiTFSA to glyme (molar conversion) is larger than 1, since all glyme forms a complex, there is no process of (1), and weight loss starts from 200 ° C or higher. .

  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 can be well dissolved in the polyalkylene oxide polymer.

In the present invention, it is necessary that the mixing ratio of the alkali metal salt to the glyme is 0.50 or more in terms of mole and not more than a value determined by the saturation concentration of the alkali metal salt in the glyme.
As described in Non-Patent Documents 2 and 3 described above, conventionally, in a lithium-sulfur battery, the mixing ratio of Li salt (LiCF ₃ SO ₃ ) to tetraglyme is 0.25 or less (LiCF _{3 in} terms of mole). It is known to use an electrolytic solution prepared so that SO ₃ is 1 mol / L or less. However, when the present inventor examined, when charging / discharging of such a lithium-sulfur battery was repeated, a side reaction occurred during charging, and the coulomb efficiency (discharge capacity / charge capacity) was lowered. It was found that the discharge capacity was significantly reduced and the battery life was short.

FIG. 4 shows the relationship between the mixing ratio of Li salt (LiCF ₃ SO ₃ ) to glyme (G4) and Coulomb efficiency, and FIG. 5 shows the mixing ratio of Li salt (LiCF ₃ SO ₃ ) to glyme (G4). It is an experimental result (after-mentioned) which shows the relationship with discharge capacity maintenance factor.
When the mixing ratio is 0.50 or more, side reactions during charging are suppressed and coulomb efficiency is improved to 95% or more, and a decrease in discharge capacity due to repeated charge and discharge is suppressed, and a discharge capacity maintenance ratio is improved. Battery life will be longer. 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.
From the above, the mixing ratio is specified to be 0.50 or more in terms of mole, and below a value determined by the saturated 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.
When G3 (triethylene glycol dimethyl ether (also referred to as triglyme) is used as a glyme and the alkali metal salt is a Li salt, the mixing ratio determined by the saturation concentration of the Li salt in G3 is 1.67 in terms of mole.
When G4 is used as the glyme and the alkali metal salt is Li salt, the mixing ratio determined by the saturated concentration of Li salt in G4 is 2.00 in terms of mole.

  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%.

<Preparation of electrolyte>
As the glyme, triethylene glycol dimethyl ether (also referred to as “triglyme”, hereinafter referred to as “G3”) (made by Kishida Chemical Co., Ltd.) and tetraglyme (hereinafter referred to as “G4”) (made by Kishida Chemical Co., Ltd.) were used. In addition, G4Et (glyme in which one of the side chains of G4 was changed to an ethyl group) (manufactured by Nippon Emulsifier Co., Ltd.) shown in the following formula 2 was also used in part.

Further, lithium bis (trifluoromethylsulfonyl) amide (hereinafter referred to as “LiTFSA”) represented by the following formula 3 (manufactured by Morita Chemical Co., Ltd.) was used as the alkali metal salt.

  G3 or G4 (including G4Et) and LiTFSA in a glove box under an argon atmosphere in a predetermined mixing ratio (molar conversion; as shown in FIGS. 2 to 9 and Table 1, the mixing ratio (LiTFSA) / (G3 Or G4) = 0.125, 0.250, 0.500, 0.670, 0.830, 1.00, 1.20, 2.00, 2.50, 3.33 (molar conversion) to prepare an electrolytic solution.

<Production of lithium-sulfur battery>
Single sulfur (S ₈ ) is a sulfur-based electrode active material, single sulfur is 60 wt%, ketjen black is 30 wt% as a conductive agent, and PVDF (polyvinylidene fluoride) is mixed as a binder at a rate of 10 wt%. Thus, 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. PVDF was added to this mixture, and an appropriate amount of NMP (N-methylpyrrolidone) was further added and kneaded into a slurry. The obtained slurry was applied to an aluminum foil (current collector) 2b having a thickness of 10 μm, dried at 80 ° C. for 12 hours to evaporate NMP, and then pressed to obtain a positive electrode 2 (see FIG. 1). . A lithium metal plate having a thickness of 200 μm was attached to a stainless disk having a thickness of 7 mm 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 (counter electrode) 4 were laminated through a separator 6 (a glass separator manufactured by Toyo Roshi Kaisha Co., Ltd. having a thickness of 200 μm (trade name: GA-55)), and the electrolyte was further injected. A coin cell case 20 (thickness 7 mm made of SUS304) was enclosed, and a spacer 12 was placed on the negative electrode (counter 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 lithium-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) Charging / discharging test About the secondary battery obtained as mentioned above, the charging / discharging test was done and discharge capacity was calculated | required. In charge / discharge evaluation, the current density is 1/12 C (12 hours rate, the theoretical capacity of the electrode active material is expressed as n (hours), or the current value for discharging or charging is expressed as 1 / n C rate). The charging / discharging voltage was implemented as the range of 1.5-3.3V. Similarly, the discharge condition was 1/12 C. The evaluation was performed in a thermostatic bath maintained at a constant temperature of 30 ° C.
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. The charge / discharge cycle was 20 cycles.

From the obtained charge capacity and discharge capacity (mAh / g: g is per mass of elemental sulfur), Coulomb efficiency (%) = discharge capacity / charge capacity was determined in each charge / discharge cycle. 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.
Further, discharge capacity retention ratio (%) = discharge capacity at the nth cycle / discharge capacity at the second cycle was determined. The discharge capacity retention ratio is a value indicating the stability of repeated charge and discharge, and the value is preferably closer to 100 (%).

(2) Dissolution test of LiTFSA in G3 or G4 Prepare an electrolyte solution with mixing ratios of 1.43, 1.67, and 2.00 (molar conversion) for G3, and mixing ratios of 2.00, 2.50, and 3.33 (molar conversion) for G3. The solubility at 0 ° C. was visually observed. When the solid content of LiTFSA was visually observed, it was determined that LiTFSA was insoluble and not suitable for practical use.

  The obtained results are shown in Tables 1 and 2 and FIGS.

  As shown in Tables 1 and 2, in the case of G3, when the mixing ratio of the electrolytic solution exceeds 1.67, and in the case of G4, the mixing ratio of the electrolytic solution exceeds 2.00, LiTFSA becomes insoluble in glyme and a battery is manufactured. I couldn't.

FIG. 2 shows a charge / discharge curve (relationship between charge / discharge capacity and cell voltage) for the second cycle of a secondary battery having an electrolyte solution with a mixing ratio of 1.00 using G4 (FIG. 2 (a)), and The charge / discharge cycle dependency of the discharge capacity (FIG. 2B) is shown. 3 is a charge / discharge curve (FIG. 3 (a)) of the second cycle of a secondary battery having an electrolyte solution using G4 and a mixing ratio of 0.25, and the charge / discharge cycle dependency of the charge / discharge capacity (FIG. 3). 3 (b)).
2 and 3, it can be seen that when the mixing ratio is less than 0.50, side reactions occur during charging, and the discharge capacity decreases due to repeated charge and discharge.

FIG. 4 shows the charge / discharge cycle dependency of the Coulomb efficiency of a secondary battery having an electrolyte solution using G4 (FIG. 4A), and the relationship between the Coulomb efficiency of the 10th cycle and the mixing ratio (FIG. 4B). )). FIG. 4 shows that when the mixing ratio is less than 0.50, the Coulomb efficiency is reduced to less than 95%, and further, the Coulomb efficiency is greatly reduced by increasing the number of cycles.
On the other hand, when the mixing ratio is 0.50 or more, the side reaction during charging is suppressed and the coulomb efficiency is improved.

FIG. 5 shows the charge / discharge cycle dependency of the discharge capacity maintenance rate of the secondary battery having an electrolyte solution using G4 (FIG. 5 (a)), and the relationship between the discharge capacity maintenance rate of the 10th cycle and the mixing ratio ( FIG. 5 (b)) is shown. FIG. 5 (a) shows that when the mixing ratio is less than 0.50, the discharge capacity retention ratio decreases, and further, the discharge capacity retention ratio significantly decreases as the number of cycles increases.
On the other hand, it was found that when the mixing ratio was 0.50 or more, the reduction in discharge capacity due to repeated charge and discharge was suppressed, the discharge capacity retention rate was improved, and the battery life was prolonged. In particular, when the mixing ratio was 0.50 or more, the discharge capacity retention rate at the 10th cycle was 85% or more.
4 and 5, the higher the mixing ratio, the more the Coulomb efficiency and the discharge capacity retention rate are improved. Therefore, the upper limit of the mixing ratio is determined by the solubility shown in Tables 1 and 2 above.

FIG. 6 shows the charge / discharge cycle dependency of the Coulomb efficiency of a secondary battery having an electrolyte using G3 (FIG. 6A), and the relationship between the Coulomb efficiency of the 10th cycle and the mixing ratio (FIG. 6B). )). From FIG. 6, it can be seen that when the mixing ratio is less than 0.50, the Coulomb efficiency is reduced to less than 70%, and further, the Coulomb efficiency is greatly reduced by increasing the number of cycles.
On the other hand, when the mixing ratio is 0.50 or more, side reactions during charging are suppressed, and the coulomb efficiency is improved to 70% or more.

FIG. 7 shows the charge / discharge cycle dependency of the discharge capacity retention rate of the secondary battery having an electrolyte solution using G3 (FIG. 7A), and the relationship between the discharge capacity retention rate and the mixing ratio at the 10th cycle ( FIG. 7 (b)) is shown. As shown in FIG. 7A, when the mixing ratio is less than 0.50, the discharge capacity retention rate decreases, and further, the discharge capacity retention rate decreases significantly as the number of cycles increases (the discharge capacity retention rate is 50% or less before 10 cycles). )
On the other hand, it was found that when the mixing ratio was 0.50 or more, the reduction in discharge capacity due to repeated charge and discharge was suppressed, the discharge capacity retention rate was improved, and the battery life was prolonged. In particular, when the mixing ratio was 0.50 or more, the discharge capacity retention rate at the 10th cycle was 80% or more.

  FIG. 8 shows the charge / discharge cycle dependency of the Coulomb efficiency of a secondary battery having an electrolyte solution (mixing ratio is 1.00) using G4Et. From FIG. 8, even if G4Et is used, the side reaction at the time of charge is suppressed and the coulomb efficiency is improved to 90% or more.

  FIG. 9 shows the charge / discharge cycle dependency of the discharge capacity retention rate of a secondary battery having an electrolyte solution (mixing ratio is 1.00) using G4Et. From FIG. 9, it was found that even when G4Et was used, the discharge capacity retention rate was improved by suppressing the decrease in discharge capacity due to repeated charge and discharge, and the battery life was prolonged. In particular, the discharge capacity retention rate at the 10th cycle was 90% or more.

<Evaluation test of side reaction (elution of lithium polysulfide into electrolyte)>
Then, it produced as a by-reactions during charging and discharging, lithium polysulphides; were evaluated elution into the electrolytic solution of _{(Li 2 S n 2 <n} <8). In the discharge reaction using a sulfur-based electrode active material, S ₈ is reduced to Li ₂ S, and the reduction reaction process is a multi-step process. Lithium polysulfide (Li ₂ S _n ; 2 <n <8, It passes through the chemical species of Li ₂ S ₈ , Li ₂ S ₄ , and Li ₂ S ₂ , where n is a natural number. Of these species, the high solubility of any species of the electrolyte, Li ₂ S _n is eluted from the electrode, the reduction of the discharge capacity to Li ₂ S _n can not be involved in the cell reaction Connected. This is because the battery reaction only occurs on the electrode.
Therefore, to prepare a Li ₂ S _n artificially added an excess amount of the electrolytic solution, was calculated solubility of Li ₂ S _n after a predetermined time. Specifically, when a mixture of S ₈ and Li ₂ S (both powders) in various molar ratios is added to the electrolyte, the following formula:
Li ₂ S + (n-1) / 8 S ₈ = Li ₂ S _n
Li ₂ S _n is formed by the reaction of

The experimental method is as follows.
First, S ₈ (powder made by Wako Pure Chemical Industries, Ltd.) and Li ₂ S (powder made by Aldrich) were mixed in a mortar at a molar ratio shown in Table 3, and the excess amount was added to the electrolytic solution. The electrolytic solutions used G3 and G4 as the glyme, LiTFSA as the alkali metal salt, and the mixing ratio (molar conversion) of glyme and alkali metal salt was as shown in Table 3.
Then, the mixture electrolytic solution 60 ° C. was added, and stirred for 100 hours, further centrifuged after standing at room temperature for 48 hours, was collected supernatant solution as the saturated Li ₂ S _n solution. The saturated Li ₂ S _n solution electrochemically was all oxidized to S ₈ were calculated from the ultraviolet-visible absorption spectrum of S _8, the solubility as total S concentration.
The electrochemical oxidation of the saturated Li _{2 Sn} solution uses a two-chamber H-type cell in which the working electrode chamber and the counter electrode chamber are separated by a glass filter, using carbon felt for the working electrode and lithium for the reference / counter electrode. Metal was used. The measurement solution was used plus the appropriate amount of glyme and LiTFSA saturated Li ₂ S _n solution. Then, while stirring the working electrode chamber, a constant voltage of 3.0 V was maintained for 12 hours to oxidize Li _{2 Sn} to S ₈ .
Tables 3 and 4 show the results obtained for G3 and G4, respectively. For example, it is assumed that a mixture of S ₈ and Li ₂ S in a ratio of 3: 8 mainly produces Li ₂ S ₄ , but in addition to this, Li ₂ S ₈ and Li ₂ S ₂ etc. It is thought to include. However, in this experiment, for the (LiTFSA / glyme) mixing ratio, are intended to grasp the relative behavior of solubility of various Li ₂ S _n, the detailed data of each chemical species single lithium polysulphides Since it is not required, the chemical species mainly generated is assumed to be “assumed lithium polysulfide”.

As is apparent from Tables 3 and 4, regardless of whether G3 or G4 is used as the glyme, the solubility of Li ₂ S ₈ is about 1 as the mixing ratio (in mol) of LiTFSA to glyme increases from 0.25 to 0.5. It can be seen that when the mixing ratio is 1, it is further reduced significantly. Therefore, when the mixing ratio of LiTFSA to glyme (molar conversion) is 0.5 or more, dissolution of lithium polysulfide, which is a side reaction at the time of charging, is suppressed, and a decrease in coulomb efficiency and discharge capacity due to repeated charging and discharging is prevented. It has been found.
Of the lithium polysulfides, the solubility values of S ₈ , Li ₂ S ₄ , Li ₂ S ₂ and Li ₂ S are small in the first place, and the influence of the change in the mixing ratio is small. That is, it is considered that the influence of dissolution of Li ₂ S ₈ is extremely dominant as a side reaction during charging, and it is most important to reduce the solubility of Li ₂ S ₈ .

FIG. 12 and FIG. 13 show the relationship between the solubility when G3 and G4 are used as the glyme and the mixture is Li ₂ S ₈ and the mixing ratio of LiTFSA to glyme (in terms of mole). It can be seen that the solubility of Li ₂ S ₈ decreases significantly as the mixing ratio increases beyond 0.5.

2 Positive electrode 4 Negative electrode (counter electrode)
50 Lithium-sulfur battery

Claims (4)

A positive electrode or a negative 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 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, and a cyclohexyl group which may be substituted with a halogen atom. And x is 1 to 6), an electrolyte solution containing glyme excluding tetraglyme and an alkali metal salt,
A counter electrode of the positive electrode or the negative electrode, the alkali metal, an alloy containing the alkali metal, or a counter electrode having carbon, and an alkali metal-sulfur secondary battery,
The mixing ratio of the alkali metal salt to the glyme is 0.50 or more and 2.0 or less in terms of mole, and at least a part of the glyme and the alkali metal salt forms a complex, and a charge / discharge test The discharge capacity retention rate at the 10th cycle at (current density = 1 / 12C, charge / discharge voltage = 1.5-3.3V, discharge condition = 1 / 12C, 30 ° C.) is 85% or more. Alkali metal-sulfur secondary battery. 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) Sulfur-based secondary battery - alkali metal according to claim 1, wherein.   The alkali metal-sulfur secondary battery according to claim 1, wherein the positive electrode or the negative electrode contains the sulfur electrode active material, a binder, and a conductive agent. The glyme is triglyme (G3),
The alkali metal-sulfur secondary battery according to any one of claims 1 to 3, wherein the alkali metal salt is a lithium salt.