WO2016056495A1 - Electrolyte solution for sodium ion secondary battery, and sodium ion secondary battery - Google Patents

Electrolyte solution for sodium ion secondary battery, and sodium ion secondary battery Download PDF

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WO2016056495A1
WO2016056495A1 PCT/JP2015/078138 JP2015078138W WO2016056495A1 WO 2016056495 A1 WO2016056495 A1 WO 2016056495A1 JP 2015078138 W JP2015078138 W JP 2015078138W WO 2016056495 A1 WO2016056495 A1 WO 2016056495A1
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ion secondary
secondary battery
sodium ion
electrolyte
phosphate
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PCT/JP2015/078138
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French (fr)
Japanese (ja)
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将一郎 酒井
新田　耕司
篤史 福永
瑛子 今▲崎▼
青木　雅裕
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住友電気工業株式会社
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0567—Liquid materials characterised by the additives
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0569—Liquid materials characterised by the solvents
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to an electrolytic solution for a sodium ion secondary battery containing a fluorophosphate ester and a phosphate ester, and a sodium ion secondary battery containing the same.
  • Patent Document 1 proposes using a phosphate ester such as a fluorophosphate ester as a solvent in an electrolyte solution of a lithium ion secondary battery from the viewpoint of ensuring flame retardancy.
  • the sodium ion battery includes a positive electrode, a negative electrode, and a sodium ion conductive non-aqueous electrolyte.
  • Patent Document 1 Although phosphate esters such as fluorinated phosphate esters have high flame retardancy, there is a tendency to reduce the performance of the battery, so that the content in the battery is preferably small. Taught. Actually, when a large amount of fluorinated phosphoric acid ester is used as the solvent in the electrolyte solution of the lithium ion secondary battery, the cycle characteristics and / or the rate characteristics deteriorate. Moreover, charging / discharging itself may be difficult.
  • An object of the present invention is to provide an electrolytic solution that has high flame retardancy and can improve cycle characteristics and rate characteristics of a sodium ion secondary battery, and a sodium ion battery including the same.
  • One aspect of the present invention is an electrolyte solution for a sodium ion secondary battery that includes a sodium salt and a nonaqueous solvent and has sodium ion conductivity, and the nonaqueous solvent includes a fluorophosphate ester and phosphoric acid.
  • the present invention relates to an electrolyte solution for a sodium ion secondary battery containing an ester.
  • Another aspect of the present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the electrolytic solution.
  • the present invention it is possible to improve the cycle characteristics and rate characteristics (large current discharge characteristics) of the sodium ion secondary battery while ensuring high flame retardancy of the electrolytic solution.
  • 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention.
  • 7 is a charge / discharge curve of sodium ion secondary batteries of Examples 12 to 14 and Reference Example 3.
  • An electrolyte for a sodium ion secondary battery includes (1) a sodium salt and a nonaqueous solvent, and has sodium ion conductivity.
  • the non-aqueous solvent includes a fluorophosphate ester and a phosphate ester.
  • the electrolyte contains a phosphate having a relatively low flame retardancy. (As a result, the flame retardancy of the sodium ion secondary battery) can be greatly improved.
  • the phosphate ester since the phosphate ester has a higher solvation energy for lithium ions than the fluorophosphate ester, it is occluded (or inserted) in the negative electrode active material in a state solvated with lithium ions during charging. Similarly, the SEI film is formed and the resistance is increased. The formation of such an SEI film becomes conspicuous as the charge / discharge progresses, and it is considered that the cycle characteristics deteriorate. If the solvation energy of lithium ions and fluorinated phosphoric acid esters is lowered in order to enhance cycle characteristics, the viscosity of the electrolytic solution tends to increase, ion conductivity decreases, and rate characteristics are impaired.
  • the non-aqueous solvent containing the fluorophosphate ester and the phosphate ester is used as the solvent in the electrolyte solution of the sodium ion secondary battery. Since sodium ions have a larger ion radius than lithium ions, the charge density is low, and the solvation energy with fluorophosphates and phosphates is lower than with lithium ions. Therefore, sodium ions can be smoothly inserted into the negative electrode, and the side reaction of the electrolytic solution is suppressed. Therefore, even if charging / discharging is repeated, capacity
  • the solvation energy of phosphate ester and sodium ion is small compared with the solvation energy of phosphate ester and lithium ion. Therefore, even when using phosphate esters, problems such as lithium ion secondary batteries are suppressed, and the merit of lowering the viscosity of the electrolytic solution becomes obvious, and it is easy to ensure high ion conductivity and obtain high rate characteristics. Can do.
  • the non-aqueous solvent preferably contains 20% by mass or more of a fluorophosphate ester.
  • the flame retardancy can be further enhanced, and the effect of suppressing the side reaction of the electrolytic solution in the negative electrode can be further enhanced.
  • the electrolytic solution does not have a flash point or has a flash point of 70 ° C. or higher.
  • the electrolytic solution according to the present embodiment includes a non-aqueous solvent containing a large amount of fluorophosphate as a solvent. Therefore, according to the electrolyte solution which concerns on this embodiment, high flame retardance can be ensured and by extension, the flame retardance of a sodium ion secondary battery can be improved. As a result, according to the electrolytic solution according to the present embodiment, the safety of the sodium ion secondary battery can be improved.
  • the fluorophosphoric acid ester is preferably a polyfluoroalkyl phosphate having 1 to 3 polyfluoroalkyl groups.
  • each of the 1 to 3 polyfluoroalkyl groups is a difluoroalkyl group having 1 to 3 carbon atoms, a trifluoroalkyl group having 1 to 3 carbon atoms, or a tetrafluoroalkyl group having 2 to 3 carbon atoms. is there.
  • Fluorophosphates include tris (2,2,2-trifluoroethyl) phosphate, bis (2,2,2-trifluoroethyl) methyl phosphate and bis (2,2,2-trifluoroethyl) It is preferably at least one selected from the group consisting of ethyl phosphate.
  • a fluorinated phosphoric acid ester tends to impart high flame retardancy. Moreover, it is easy to further improve the cycle characteristics.
  • the phosphate ester may be (6) a trialkyl phosphate having an alkyl group having 1 to 3 carbon atoms, or (7) at least one selected from the group consisting of trimethyl phosphate and triethyl phosphate. .
  • These phosphate esters are easy to reduce the viscosity of the electrolytic solution, and are advantageous in further improving the rate characteristics.
  • the non-aqueous solvent may further include at least one selected from the group consisting of cyclic carbonates and chain carbonates. When such a non-aqueous solvent is used, both cycle characteristics and rate characteristics can be further improved.
  • the non-aqueous solvent preferably further contains propylene carbonate. In this case, the effect of improving the cycle characteristics is further increased.
  • the content of the fluorophosphate in the non-aqueous solvent is preferably more than 50% by mass and 80% by mass or less. When the content of the fluorophosphate in the non-aqueous solvent is within such a range, the cycle characteristics and rate characteristics can be improved in a balanced manner.
  • Another embodiment of the present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the above electrolytic solution. Since such a sodium ion secondary battery contains the electrolyte solution, high cycle characteristics and rate characteristics can be obtained. Moreover, since the sodium ion secondary battery which concerns on this embodiment has high flame retardance, it is excellent also in safety.
  • Electrolytic solution for sodium ion secondary battery contains a sodium salt and a non-aqueous solvent. (Sodium salt) Since the sodium salt dissociates in the electrolytic solution to generate sodium ions (first cation) and anions (first anion), the electrolytic solution has sodium ion conductivity.
  • the kind of the first anion constituting the sodium salt is not particularly limited.
  • the first anion include an anion of a fluorine-containing acid, an anion of a chlorine-containing acid, an anion of an oxygen acid having an oxalate group, an anion of a fluoroalkanesulfonic acid, and a bissulfonylamide anion.
  • These sodium salts may be used alone or in combination of two or more.
  • Examples of the anion of the fluorine-containing acid include a fluorine-containing phosphate anion such as hexafluorophosphate ion (PF 6 ⁇ ); and a fluorine-containing borate anion such as tetrafluoroborate ion (BF 4 ⁇ ). .
  • Examples of the anion of the chlorine-containing acid include perchlorate ion (ClO 4 ⁇ ).
  • anion of the oxygen acid having an oxalate group examples include oxalatoborate ions such as bis (oxalato) borate ion (B (C 2 O 4 ) 2 ⁇ ); tris (oxalato) phosphate ion (P (C 2 O 4 ) Oxalatophosphate ions such as 3 ⁇ ) and the like.
  • oxalatoborate ions such as bis (oxalato) borate ion (B (C 2 O 4 ) 2 ⁇ ); tris (oxalato) phosphate ion (P (C 2 O 4 ) Oxalatophosphate ions such as 3 ⁇ ) and the like.
  • Examples of the anion of the fluoroalkanesulfonic acid include trifluoromethanesulfonic acid ion (CF 3 SO 3 ⁇ ).
  • bis-sulfonyl amide anion for example, bis (fluorosulfonyl) amide anion (FSA: bis (fluorosulfonyl) amide anion)); (FSO 2) (CF 3 SO 2) N - ( fluorosulfonyl such) (perfluoro Bis (trifluoromethylsulfonyl) amide anion (TFSA: bis (trifluoromethylsulfonyl) amide anion, N (SO 2 CF 3 ) 2 ⁇ ), N (SO 2 C 2 F 5 ) 2 ⁇ (Perfluoroalkylsulfonyl) amide anion and the like.
  • FSA and / or TFSA specifically, FSA, TFSA, and a mixture of FSA and TFSA are particularly preferable.
  • the concentration of sodium salt or sodium ion in the electrolytic solution can be appropriately selected from the range of, for example, 0.2 to 10 mol / L, preferably 0.2 to 5 mol / L, more preferably 0.2 to 2.5 mol / L. It is.
  • Non-aqueous solvent A conventional sodium ion secondary battery including an organic electrolyte containing an organic solvent can be operated at a low temperature. However, in the sodium ion secondary battery, it is difficult to stabilize the cycle at a high temperature. When an ionic liquid is used as an electrolyte in an electrolytic solution of a sodium ion secondary battery, the cycle at high temperature can be stabilized, but the utilization factor at low temperature (rate characteristic at low temperature) is low.
  • a nonaqueous solvent containing a fluorophosphate ester (first solvent) and a phosphate ester (second solvent) is used as a solvent in the electrolytic solution. Therefore, according to the electrolytic solution according to the present embodiment, high flame retardancy can be ensured. Thereby, the flame retardance of a sodium ion secondary battery can be improved. In addition, the cycle at high temperature can be stabilized, and the utilization factor at low temperature can be increased.
  • the flash point of the electrolytic solution is preferably 70 ° C. or higher, and preferably has no flash point.
  • the electrolyte is classified as a third petroleum, a fourth petroleum, or the like. Therefore, according to the electrolytic solution according to the present embodiment, high safety can be ensured as compared with the electrolytic solution for lithium ion secondary batteries generally classified as the second petroleum.
  • the fluorinated phosphoric acid ester may be one in which one or two of the three ester forming sites (-OH group) of orthophosphoric acid are esterified, but the following formula ( The compounds represented by I) are preferred.
  • R 1 , R 2 and R 3 each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R 1 , R 2 and R 3 is a fluorinated alkyl group.
  • R 1 to R 3 two or three of them may be the same, all may be the same, or all may be different.
  • the alkyl group represented by R 1 to R 3 include those having 1 to 6 carbon atoms such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, sec-butyl group, and tert-butyl group.
  • An alkyl group can be illustrated.
  • Examples of the fluorinated alkyl group include fluorinated alkyl groups corresponding to these alkyl groups, that is, fluoroalkyl groups having 1 to 6 carbon atoms.
  • Each of the alkyl group and the fluoroalkyl group preferably has 1 to 3 carbon atoms, more preferably 2 to 3 carbon atoms.
  • the number of fluorine atoms in the fluorinated alkyl group is not particularly limited, and can be appropriately selected according to the number of carbon atoms in the fluorinated alkyl group.
  • the number of fluorine atoms in each fluorinated alkyl group can be selected from 1 to 6, for example, and may be 1 to 4.
  • the number of fluorine atoms contained in the fluorinated alkyl group is preferably plural, more preferably 2 to 4, further preferably 2 to 3. That is, the fluorinated phosphate ester is preferably a polyfluoroalkyl phosphate having a polyfluoroalkyl group.
  • the fluorinated alkyl group may have a fluorine atom on any carbon atom constituting the fluorinated alkyl group, but has a fluorine atom on a carbon atom as far as possible from the phosphorus atom of the fluorinated phosphate ester. It is preferable.
  • the fluorinated alkyl group has, for example, a fluorine atom on the carbon atom at the 2-position of the ethyl group in the ethyl fluoride group and on the carbon atom at the 3-position of the n-propyl group in the fluorinated n-propyl group. Is preferred.
  • the number of fluorinated alkyl groups can be selected from 1 to 3. From the viewpoint of ensuring high flame retardancy and excellent charge / discharge characteristics, two or three of R 1 , R 2 and R 3 are fluorinated alkyl groups (such as polyfluoroalkyl groups), and the rest An alkyl group is preferred.
  • Examples of the polyfluoroalkyl group include a difluoroalkyl group having 1 to 3 carbon atoms such as a difluoromethyl group and a 2,2-difluoroethyl group; a trifluoromethyl group, a 2,2,2-trifluoroethyl group, 3, Examples thereof include trifluoroalkyl groups having 1 to 3 carbon atoms such as 3,3-trifluoropropyl group; tetrafluoroalkyl groups having 2 to 3 carbon atoms such as 2,2,3,3-tetrafluoropropyl group, and the like.
  • TFEP tris () is used from the viewpoint of ensuring high flame retardancy and excellent charge / discharge characteristics (cycle characteristics, rate characteristics, etc.).
  • the content of the fluorophosphate ester in the nonaqueous solvent is, for example, 20% by mass or more, preferably 25% by mass or more, and more preferably 30% by mass or more.
  • the content of the fluorophosphate ester in the non-aqueous solvent may be more than 50% by mass, preferably 55% by mass or more, more preferably 60% by mass or more, and further preferably 65% by mass or more. May be.
  • the content of the fluorophosphate ester in the non-aqueous solvent is preferably 90% by mass or less, more preferably 80% by mass or less.
  • the content of the fluorophosphate ester in the non-aqueous solvent may be, for example, 20 to 90% by mass, more than 50% by mass and 90% by mass or less, or more than 50% by mass and 80% by mass or less. 55 to 80% by mass, or 60 to 80% by mass.
  • a lithium ion secondary battery when a non-aqueous solvent containing a fluorophosphate ester is used, it may be difficult to charge and discharge. However, the sodium ion secondary battery can be sufficiently charged and discharged even when such a non-aqueous solvent is used.
  • the phosphate ester as the second solvent is specifically a phosphate ester having no fluorine atom, unlike the fluorinated phosphate ester.
  • phosphate ester examples include trialkyl phosphates such as trimethyl phosphate (TMP) and triethyl phosphate (TEP) (for example, trialkyl phosphate having an alkyl group having 1 to 6 carbon atoms); triphenyl phosphate And triaryl phosphates such as tolyl phosphate (such as triaryl phosphates having an aryl group having 6 to 10 carbon atoms).
  • TMP trimethyl phosphate
  • TEP triethyl phosphate
  • TEP triethyl phosphate
  • triaryl phosphates such as tolyl phosphate (such as triaryl phosphates having an aryl group having 6 to 10 carbon atoms).
  • Phosphoric esters may be used alone or in combination of two or more.
  • trialkyl phosphates having an alkyl group having 1 to 4 carbon atoms such as TMP and TEP are preferable, and trialkyl phosphates having an alkyl group having 1 to 3 carbon atoms are more preferable.
  • the ester portion of the phosphate ester (the alkyl group or aryl group in the above-mentioned phosphate ester) is bulky, the solvation energy with sodium ions is reduced, and thus it is easy to suppress a decrease in capacity. Therefore, from the viewpoint of further improving the cycle characteristics, a trialkyl phosphate having an alkyl group having 2 to 6 carbon atoms (particularly a trialkyl phosphate having an alkyl group having 2 to 4 carbon atoms) such as TEP, or triaryl phosphate is used. It is preferable to use it.
  • the content of the phosphate ester in the non-aqueous solvent is, for example, 80% by mass or less, preferably 70% by mass or less, more preferably 50% by mass or less, and further preferably 40% by mass.
  • the phosphate ester content is preferably 10 to 80% by mass, 10 to 70% by mass, 10 to 50% by mass, 20 to 50% by mass, or 20 to 40% by mass. When the content of the phosphate ester is within such a range, it is easy to balance high flame retardancy with high rate characteristics.
  • a sodium ion secondary battery when charged to a high potential (for example, a charge end voltage of 4.2 V or higher) or repeatedly charged and discharged at a high temperature (for example, 60 ° C. or higher), it is included in the battery.
  • Aluminum such as a positive electrode current collector
  • the capacity may decrease, and the cycle characteristics may deteriorate. From the viewpoint of suppressing such corrosion of the positive electrode current collector, it is effective to increase the content of the fluorophosphate ester and / or to use the phosphate ester.
  • the total content of the fluorophosphate ester and phosphate ester in the non-aqueous solvent is, for example, 60 to 100% by mass.
  • the total content of the fluorophosphate ester and phosphate ester in the non-aqueous solvent is preferably 65 to 100% by mass, more preferably 70 to 100% by mass.
  • the total content of the fluorophosphate ester and the phosphate ester in the non-aqueous solvent may be 60 to 95 mass% or 70 to 90 mass%.
  • the non-aqueous solvent may further include a solvent other than the fluorophosphate ester and the phosphate ester (third solvent).
  • a solvent other than the fluorophosphate ester and the phosphate ester third solvent.
  • a known solvent used as a solvent in an electrolyte solution of a sodium ion secondary battery for example, an organic solvent and / or an ionic liquid, specifically, an organic solvent, an ionic liquid, or an organic solvent and an ionic liquid And a mixture thereof.
  • a 3rd solvent may be used independently and may mix and use 2 or more types.
  • the ionic liquid is synonymous with a salt (molten salt) in a molten state at least at 100 ° C. or less, and is a liquid ionic substance composed of anions and cations.
  • a salt of sodium ion and bissulfonylamide anion may be generally classified as an ionic liquid, but in this specification, for convenience, it is not included in the ionic liquid. Shall.
  • the organic solvent is not particularly limited, and a known organic solvent used for sodium ion secondary batteries can be used.
  • the organic solvent is, for example, ethylene carbonate (EC), fluoroethylene carbonate, difluoroethylene carbonate, vinyl ethylene carbonate, vinylene carbonate, propylene carbonate (PC), butylene carbonate, or the like.
  • Cyclic carbonates; chain carbonates such as dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate; cyclic esters such as ⁇ -butyrolactone, ⁇ -valerolactone, and ⁇ -caprolactone; ethers and the like can be preferably used.
  • ether examples include linear or cyclic ethers such as fluorine-containing ethers; glymes such as tetraglyme; crown ethers and the like.
  • An organic solvent may be used independently and may be used in mixture of 2 or more types.
  • a non-aqueous solvent containing a cyclic carbonate and / or a chain carbonate specifically, a non-aqueous solvent containing a cyclic carbonate, a non-aqueous solvent containing a chain carbonate, or cyclic It is preferable to use a non-aqueous solvent containing a mixture of carbonate and chain carbonate. Among these, when a non-aqueous solvent containing propylene carbonate is used, cycle characteristics and rate characteristics are easily improved.
  • a nonaqueous solvent containing a cyclic carbonate, a cyclic ester, and / or an ether specifically, a nonaqueous solvent containing a cyclic carbonate, a nonaqueous solvent containing a cyclic ester, A non-aqueous solvent containing ether, a non-aqueous solvent containing a mixture of cyclic carbonate and cyclic ester and ether, a non-aqueous solvent containing a mixture of cyclic carbonate and cyclic ester, a non-aqueous solvent containing a mixture of cyclic carbonate and ether, It is also preferable to use a non-aqueous solvent containing a mixture of a cyclic ester and an ether.
  • the ionic liquid contains a cation (second cation) other than sodium ion and an anion (second anion).
  • the second cation include inorganic cations other than sodium ions and organic cations.
  • the ionic liquid may contain one kind of cation other than sodium ions as the second cation, or may contain a mixture of two or more kinds of cations other than sodium ions.
  • Organic cations include cations derived from aliphatic amines, alicyclic amines or aromatic amines (eg, quaternary ammonium cations), cations having nitrogen-containing heterocycles (ie, cations derived from cyclic amines) Examples thereof include nitrogen-containing onium cations such as: sulfur-containing onium cations; phosphorus-containing onium cations.
  • nitrogen-containing organic onium cations quaternary ammonium cations and cations having a pyrrolidine skeleton, a pyridine skeleton, or an imidazole skeleton as the nitrogen-containing heterocyclic skeleton are particularly preferable.
  • nitrogen-containing organic onium cations include tetraalkylammonium cations (TEA: tetraethylammonium cation), methyltriethylammonium cations (TEMA), tetraalkylammonium cations; 1-methyl-1-propylpyrrolidinium cation ( MPPY or Py13: 1-methyl-1-propylpyrrolidinium cation, 1-butyl-1-methylpyrrolidinium cation (MBPY or Py14: 1-butyl-1-methylpyrrolidinium cation); 1-ethyl-3-methylimidazolium cation (EMI: 1-ethyl-3- ethylimidazolium cation), and / or 1-butyl-3-methylimidazolium cation (BMI: 1-buthyl-3-methylimidazolium cation) and the like.
  • TAA tetraalkylammonium cations
  • inorganic cations include alkali metal ions (such as potassium ions) other than sodium ions, alkaline earth metal ions (such as magnesium ions and calcium ions), and ammonium ions.
  • the second cation preferably contains an organic cation.
  • an ionic liquid containing an organic cation it becomes easy to lower the viscosity of the electrolytic solution, so that the sodium ion conductivity is easily increased and a high capacity is easily secured.
  • the second cation may include an organic cation and an inorganic cation.
  • the bissulfonylamide anion can be appropriately selected from those exemplified for the sodium salt.
  • FSA and / or TFSA specifically, FSA, TFSA, and a mixture of FSA and TFSA are preferred.
  • the ionic liquid examples include a salt of Py13 and FSA (Py13 ⁇ FSA), a salt of Py13 and TFSA (Py13 ⁇ TFSA), a salt of Py14 and FSA (Py14 ⁇ FSA), and a salt of Py14 and TFSA.
  • organic solvents are generally low in flame retardancy and have a low flash point.
  • the flame retardancy of the electrolytic solution can be enhanced by containing a large amount of the fluorophosphate ester.
  • a non-aqueous solvent containing an organic solvent it is preferable to use a non-aqueous solvent containing an organic solvent, and from the viewpoint of suppressing the decomposition of the electrolytic solution as much as possible, it is preferable to use a non-aqueous solvent containing an ionic liquid.
  • Nonaqueous solvents including ionic liquids and organic solvents may be used.
  • the electrolytic solution may contain an additive in addition to the sodium salt and the non-aqueous solvent, if necessary.
  • the total of the sodium salt and the nonaqueous solvent in the electrolytic solution is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more.
  • the total amount of sodium ions and non-aqueous solvent in the electrolyte is within such a range, so that the content of fluorophosphate can be relatively increased, and the effect of improving flame retardancy and charge / discharge characteristics Is easily obtained.
  • a sodium ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, a separator interposed therebetween, and the electrolytic solution. Below, it demonstrates in detail about the components of a battery other than electrolyte solution.
  • the positive electrode includes a positive electrode active material.
  • the positive electrode may include a positive electrode current collector and a positive electrode active material (or a positive electrode mixture) carried on the positive electrode current collector.
  • the positive electrode current collector may be a metal foil or a metal porous body (such as a metal fiber non-woven fabric or a metal porous body sheet).
  • a metal porous body having a three-dimensional network skeleton (particularly, a hollow skeleton) can also be used.
  • As a material for the positive electrode current collector aluminum, an aluminum alloy, or the like is preferable from the viewpoint of stability at the positive electrode potential.
  • a material that occludes and releases (or inserts and desorbs) sodium ions that is, a material that develops capacity by a Faraday reaction
  • Such materials include alkali metal atoms (sodium atoms, potassium atoms, etc.) and transition metal atoms (chromium atoms, manganese atoms, iron atoms, cobalt atoms, nickel atoms, etc., transition metal atoms in the fourth period of the periodic table, etc. ).
  • a part of at least one of an alkali metal atom and a transition metal atom contained in the crystal structure of the compound may be substituted with a typical metal atom such as an aluminum atom.
  • the positive electrode active material preferably contains a transition metal compound such as a sodium-containing transition metal compound.
  • the transition metal compound include known compounds that can be used as a positive electrode active material for a sodium ion secondary battery, such as sulfides, oxides, sodium transition metal oxyacid salts, and sodium-containing transition metal halides.
  • the sulfide include transition metal sulfides such as TiS 2 and FeS 2 ; sodium-containing transition metal sulfides such as NaTiS 2 .
  • the oxide include sodium chromite (NaCrO 2 ), sodium nickel manganate (NaNi 0.5 Mn 0.5 O 2 , Na 2/3 Ti 1/6 Ni 1/3 Mn 1/2 O 2.
  • sodium-containing transition metal oxides such as sodium iron cobaltate (such as NaFe 0.5 Co 0.5 O 2 ), sodium iron manganate (such as Na 2/3 Fe 1/3 Mn 2/3 O 2 ) Etc.
  • sodium-containing transition metal halide examples include Na 3 FeF 6 .
  • sodium chromite and sodium ferromanganate are preferred. A part of at least one of a chromium atom and a sodium atom contained in the crystal structure of sodium chromite may be substituted with another atom. Moreover, at least one part of the iron atom, manganese atom, and sodium atom contained in the crystal structure of sodium iron manganate may be substituted with another atom.
  • the positive electrode mixture can further include a conductive auxiliary and / or a binder, specifically, a conductive auxiliary, a binder, and a mixture of the conductive auxiliary and the binder.
  • the positive electrode is obtained by applying or filling a positive electrode mixture to a positive electrode current collector, drying, and compressing (or rolling) the dried product in the thickness direction as necessary.
  • the positive electrode mixture is usually used in the form of a slurry containing a dispersion medium.
  • Examples of the conductive assistant include carbon black, graphite, and carbon fiber. These conductive assistants may be used alone or in combination of two or more.
  • Examples of the binder include a fluorine resin, a polyolefin resin, a rubber-like polymer, a polyamide resin, a polyimide resin (such as polyamideimide), and / or a cellulose ether. These binders may be used alone or in combination of two or more.
  • As the dispersion medium for example, water or the like is used in addition to an organic solvent such as N-methyl-2-pyrrolidone (NMP).
  • NMP N-methyl-2-pyrrolidone
  • the negative electrode includes a negative electrode active material.
  • the negative electrode may include a negative electrode current collector and a negative electrode active material (or a negative electrode mixture) carried on the negative electrode current collector. Similar to the positive electrode current collector, the negative electrode current collector may be a metal foil or a metal porous body.
  • the material for the negative electrode current collector copper, copper alloy, nickel, nickel alloy, stainless steel, and the like are preferable because they are not alloyed with sodium and stable at the negative electrode potential.
  • Examples of the negative electrode active material include materials that reversibly occlude and release (or insert and desorb) sodium ions, and materials that alloy with sodium. Any of these materials is a material that develops capacity by a Faraday reaction.
  • the negative electrode active material examples include metals or metalloids such as sodium, titanium, zinc, indium, tin, and silicon; alloys obtained from the metal or metalloid; compounds of the metal or metalloid; carbonaceous materials; It can be illustrated.
  • the alloy may further contain other alkali metal, alkaline earth metal, etc. in addition to the metal and metalloid. Examples thereof include metals or alloys thereof, or compounds thereof; and carbonaceous materials.
  • the alloy may further contain other alkali metals, alkaline earth metals and the like in addition to these metals and metalloids.
  • the metal compound examples include lithium-containing titanium oxides such as lithium titanate (such as Li 2 Ti 3 O 7 and / or Li 4 Ti 5 O 12 ), and sodium titanate (Na 2 Ti 3 O 7 and / or Na 4). Examples thereof include sodium-containing titanium oxides such as Ti 5 O 12 .
  • lithium-containing titanium oxides such as lithium titanate (such as Li 2 Ti 3 O 7 and / or Li 4 Ti 5 O 12 ), and sodium titanate (Na 2 Ti 3 O 7 and / or Na 4).
  • sodium-containing titanium oxides such as Ti 5 O 12 .
  • the crystal structure of the lithium-containing titanium oxide at least one of a part of the titanium atoms and a part of the lithium atoms included in the crystal structure may be substituted with another atom.
  • sodium-containing titanium oxide at least one of a titanium atom and a lithium atom included in the crystal structure may be substituted with another atom.
  • carbonaceous material examples include graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon). These carbonaceous materials may be used alone or in combination of two or more.
  • a negative electrode active material can be used individually by 1 type or in combination of 2 or more types. Of these materials, the metal or metalloid compounds (such as sodium-containing titanium oxide) and carbonaceous materials (such as hard carbon) are preferable.
  • the negative electrode is formed by, for example, applying or filling a negative electrode mixture containing a negative electrode active material to a negative electrode current collector according to the case of the positive electrode, drying, and compressing (or rolling) the dried product in the thickness direction. it can.
  • a negative electrode you may use what is obtained by forming the deposit film of a negative electrode active material on the surface of a negative electrode collector by vapor phase methods, such as vapor deposition and sputtering.
  • the negative electrode active material may be pre-doped with sodium ions as necessary.
  • the negative electrode mixture can further include a conductive auxiliary and / or a binder, specifically, a conductive auxiliary, a binder, or a mixture of a conductive auxiliary and a binder.
  • the negative electrode mixture is usually used in the form of a slurry containing a dispersion medium.
  • a conductive support agent, a binder, and a dispersion medium it can respectively select from what was illustrated about the positive electrode suitably.
  • the separator for example, a synthetic resin microporous film, a nonwoven fabric, or the like can be used.
  • the material of the separator can be selected in consideration of the operating temperature of the battery.
  • the synthetic resin constituting the microporous film include polyolefin resin, polyphenylene sulfide resin, polyamide resin (such as aromatic polyamide resin), polyimide resin, and the like.
  • the fiber which forms a nonwoven fabric is comprised with the synthetic resin
  • the said synthetic resin similar to the synthetic resin which comprises a microporous film is mentioned as the said resin.
  • the fibers forming the nonwoven fabric may be inorganic fibers such as glass fibers.
  • the separator may include an inorganic filler such as ceramic particles.
  • Shape of sodium ion secondary battery examples include a square shape, a cylindrical shape, a laminate shape, a coin shape, and a button shape.
  • the sodium ion secondary battery includes, for example, (a) a step of forming an electrode group with a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and (b) an electrode group and an electrolyte solution in the battery case. It can manufacture by passing through the process of accommodating.
  • the sodium ion secondary battery is a coin-type or button-type battery
  • the coin-type or button-type battery may be formed by the following procedure, for example. First, either the positive electrode or the negative electrode is placed in the battery case. Next, a separator is put on the placed electrode. Next, an electrolytic solution is poured into the battery case. Next, the other electrode is placed in the battery case. Thereafter, the battery case is sealed.
  • FIG. 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention.
  • the sodium ion secondary battery includes a stacked electrode group, an electrolytic solution (not shown), and a rectangular aluminum battery case 10 that houses them.
  • the battery case 10 includes a bottomed container body 12 having an upper opening and a lid 13 that closes the upper opening.
  • an electrode group is configured by laminating the positive electrode 2 and the negative electrode 3 with the separator 1 interposed therebetween, and the configured electrode group is a battery case.
  • 10 container bodies 12 are inserted. Thereafter, a step of injecting an electrolytic solution into the container body 12 and impregnating the electrolytic solution into the gaps of the separator 1, the positive electrode 2, and the negative electrode 3 constituting the electrode group is performed.
  • a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the battery case 10 rises.
  • An external positive terminal 14 that penetrates the lid 13 is provided near the one side of the lid 13 with the safety valve 16 in the center, and an external that penetrates the lid 13 is located near the other side of the lid 13.
  • a negative terminal is provided.
  • the stacked electrode group is composed of a plurality of positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed therebetween, all in the form of a rectangular sheet.
  • the separator 1 is formed in a bag shape so as to surround the positive electrode 2, but the form of the separator is not particularly limited.
  • the plurality of positive electrodes 2 and the plurality of negative electrodes 3 are alternately arranged in the stacking direction within the electrode group.
  • a positive electrode lead piece 2 a may be formed at one end of each positive electrode 2.
  • the plurality of positive electrodes 2 are connected in parallel by bundling the positive electrode lead pieces 2 a of the plurality of positive electrodes 2 and connecting them to the external positive terminal 14 provided on the lid 13 of the battery case 10.
  • a negative electrode lead piece 3 a may be formed at one end of each negative electrode 3.
  • the plurality of negative electrodes 3 are connected in parallel by bundling the negative electrode lead pieces 3 a of the plurality of negative electrodes 3 and connecting them to the external negative terminal provided on the lid 13 of the battery case 10.
  • the bundle of the positive electrode lead pieces 2a and the bundle of the negative electrode lead pieces 3a are desirably arranged on the left and right sides of one end face of the electrode group with an interval so as to avoid mutual contact.
  • the external positive electrode terminal 14 and the external negative electrode terminal are both columnar, and at least a portion exposed to the outside has a screw groove.
  • a nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7.
  • a flange 8 is provided in a portion of each terminal accommodated in the battery case 10, and by rotation of the nut 7, the flange 8 attaches an O-ring-shaped gasket 9 to the inner surface of the lid 13. Fixed through.
  • the electrode group is not limited to a laminated type, and may be formed by winding a positive electrode and a negative electrode through a separator. From the viewpoint of preventing metallic sodium from being deposited on the negative electrode, the size of the negative electrode may be made larger than that of the positive electrode.
  • cylindrical and laminated sodium secondary batteries can also be appropriately manufactured according to the same method as described above.
  • Example 1 Production of positive electrode NaCrO 2 (positive electrode active material), acetylene black (conductive aid) and polyvinylidene fluoride (binder) are positive electrode active material / conductive aid / binder (mass ratio) of 90/5 /
  • a positive electrode mixture paste was prepared by dispersing in NMP so as to be 5. The obtained positive electrode mixture paste was applied to both sides of an aluminum foil (length 10 cm ⁇ width 10 cm, thickness 20 ⁇ m), sufficiently dried, rolled, and a positive electrode mixture layer having a thickness of 60 ⁇ m on both sides. 100 positive electrodes having a thickness of 140 ⁇ m were produced.
  • the lead piece for current collection was formed in the one side edge part of the one side of a positive electrode.
  • the electrode group was produced by laminating
  • a negative electrode having a negative electrode mixture layer only on one surface was disposed at one end of the electrode group so that the negative electrode mixture layer was opposed to the positive electrode.
  • the negative electrode which has a negative mix layer only on one side was arrange
  • a bag-like microporous membrane made of polyolefin, thickness: 50 ⁇ m
  • (C) Rate characteristics (low temperature rate characteristics)
  • the sodium ion secondary battery is charged at a temperature of 40 ° C. at a current value of 0.1C rate of time until it reaches 3.4V, and at a current value of 0.1C rate of time until it reaches 1.5V. discharged, the discharge capacity was measured C H at this time.
  • the sodium ion secondary battery was charged at a temperature of 40 ° C. at a current rate of 0.1C rate to 3.4 V, and at a temperature of ⁇ 10 ° C. at a current rate of 0.1C rate.
  • the battery was discharged until 1.5V was reached.
  • Example 2 An electrolyte solution was prepared in the same manner as in Example 1 except that TEP was used instead of TMP. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
  • Comparative Example 1 A positive electrode was produced in the same manner as in Example 1 except that LiCoO 2 was used instead of NaCrO 2 .
  • An electrolyte solution was prepared in the same manner as in Example 1 except that LiFSA (lithium bis (fluorosulfonyl) amide) was used instead of NaFSA.
  • the flash point of the electrolyte was evaluated in the same manner as in Example 1.
  • Example 2 An electrode group was produced in the same manner as in Example 1 except that the obtained positive electrode was used, and a secondary battery was produced in the same manner as in Example 1 except that this electrode group and the above electrolytic solution were used. According to Example 1, cycle characteristics and rate characteristics were evaluated. At this time, the charge end voltage and the discharge end voltage were 4.2 V and 3.0 V, respectively.
  • the secondary battery obtained in Comparative Example 1 is a lithium ion secondary battery.
  • Reference example 1 An electrolyte solution was prepared in the same manner as in Example 1 except that TFEP was not used. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
  • the lithium ion secondary battery B1 of the comparative example could not be charged / discharged, and neither the cycle characteristics nor the rate characteristics could be evaluated.
  • the non-aqueous solvent of the electrolytic solution was the same as that of the comparative example, and charging / discharging could be performed despite containing a large amount of phosphoric acid phosphate.
  • a high rate characteristic of 70% or more and a high cycle characteristic of 90% or more were obtained.
  • the flash point of the electrolyte was not high or high, and the flame retardancy was excellent.
  • the battery C1 of the reference example using only the phosphate ester as the non-aqueous solvent can be charged and discharged.
  • the rate characteristic is high
  • the cycle characteristic is as low as 31%.
  • the rate characteristic has a high value almost equal to that of the battery of the corresponding example.
  • the cycle characteristics of the battery C2 were about 20% lower than in the example. The higher cycle characteristics obtained in the example than in the battery C2 are considered to be due to the formation of a more stable SEI film in the example than in the battery C2.
  • Examples 3-7 An electrolyte solution was prepared in the same manner as in Example 1 except that the mass ratio of TFEP to TMP in the nonaqueous solvent was changed as shown in Table 2, and the third solvent shown in Table 2 was used as necessary. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
  • Examples 8-9 An electrolyte solution was prepared in the same manner as in Example 2 except that the mass ratio of TFEP and TEP in the nonaqueous solvent was changed as shown in Table 2.
  • a sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
  • the results of Examples 3 to 9 are shown in Table 2.
  • Table 2 also shows the results of Examples 1 and 2.
  • Examples 3 to 9 are A3 to A9.
  • Examples 10-11 An electrolytic solution was prepared in the same manner as in Example 1 except that fluorinated phosphate ester shown in Table 3 was used instead of TFEP. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used. The results of Examples 10 to 11 are shown in Table 3. Table 3 also shows the results of Example 1. Examples 10 to 11 are A10 to A11.
  • Examples 12 to 14 and Reference Example 3 (Charging / Discharging Behavior during High Voltage Charging)
  • a positive electrode mixture paste was prepared in the same manner as in Example 1 except that NaNi 1/3 Ti 1/6 Mn 1/2 O 2 was used as the positive electrode active material instead of NaCrO 2 .
  • the positive electrode mixture paste was applied to one side of an aluminum foil having a thickness of 20 ⁇ m, sufficiently dried, and compressed in the thickness direction to produce a positive electrode having a thickness of 60 ⁇ m.
  • the positive electrode was punched into a coin shape having a diameter of 12 mm.
  • a metal sodium disk Aldrich, thickness: 200 ⁇ m
  • the negative electrode was punched into a coin shape having a diameter of 12 mm.
  • the coin-type positive electrode, negative electrode and separator were sufficiently dried.
  • a coin-type negative electrode is placed on a shallow cylindrical Al / SUS clad container, and a coin-type positive electrode is placed on it via a coin-type separator.
  • the liquid was injected.
  • the opening of the container was sealed with a shallow cylindrical Al / SUS clad sealing plate having an insulating gasket on the periphery.
  • a pressure was applied to the electrode group consisting of the negative electrode, the separator, and the positive electrode between the bottom surface of the container and the sealing plate to ensure contact between the members.
  • a coin-type sodium ion secondary battery (half cell) having a design capacity of 1.5 mAh was produced.
  • Electrolytic solution a2 Prepared by dissolving NaFSA in a non-aqueous solvent consisting only of TFEEP.
  • the batteries of the examples using the electrolytic solutions a1, a2, and a3 are designated as batteries A12, A13, and A14, and the battery of the reference example using the electrolytic solution b is designated as the battery C3.
  • the battery was charged at a temperature value of 25 ° C. at a current value of a rate of 0.1 C rate to 4.4 V and discharged at a current value of a rate of 0.2 C rate to 2.4 V.
  • the capacity per unit mass (mAh / g) of the positive electrode active material was measured.
  • the capacity change at the time of charging / discharging is shown in FIG.
  • the electrolyte solution contains a fluorophosphate ester. It can be seen that it is effective to increase the content and / or to use a phosphate ester as a solvent in the electrolytic solution.
  • Examples 15 to 17 and Reference Example 4 charge / discharge behavior at high temperature
  • a coin-type sodium ion secondary battery half cell was produced in the same manner as in Example 12 except that the positive electrode mixture paste was prepared in the same manner as in Example 1.
  • the batteries of the examples using the electrolytic solutions a1, a2, and a3 are designated as batteries A15, A16, and A17, and the battery of the reference example using the electrolytic solution b is designated as the battery C4.
  • the battery was charged at a temperature value of 60 ° C. at a current value of 0.5C hour rate to 3.4V and discharged at a current value of 0.5C time rate to 1.5V. This charge / discharge cycle was repeated 100 times, and the change in capacity during charge / discharge was measured. Assuming that the initial discharge capacity is 100%, the discharge capacity at 100 cycles decreased to 75%. When the same evaluation as described above was performed at a temperature of 90 ° C., when the initial discharge capacity was 100%, the discharge capacity at 100 cycles was reduced to 30%. When the battery after the cycle was disassembled and the positive foil was observed with an SEM, the battery C4 was corroded. Moreover, when the electrolyte solution of the battery after the cycle was taken out and ICP analysis was performed, Al was detected.
  • the content of the fluorinated phosphoric acid ester is increased in the electrolytic solution. And / or using a phosphate ester as a solvent in the electrolyte is effective.
  • the electrolyte solution according to one embodiment of the present invention can improve the cycle characteristics and rate characteristics of a sodium ion secondary battery while ensuring high flame retardancy.
  • a sodium ion secondary battery using such an electrolytic solution is expected to be used as a power source for, for example, a household or industrial large power storage device, an electric vehicle, a hybrid vehicle, and the like.

Abstract

Provided are: an electrolyte solution that is for a sodium ion secondary battery, that comprises a sodium salt and a non-aqueous solvent, that has sodium ion conductivity, and in which the non-aqueous solvent contains a fluorinated phosphoric acid ester and a phosphoric acid ester; and a sodium ion secondary battery containing the electrolyte solution for a sodium ion secondary battery.

Description

ナトリウムイオン二次電池用電解液およびナトリウムイオン二次電池Electrolyte for sodium ion secondary battery and sodium ion secondary battery
　本発明は、フッ化リン酸エステルおよびリン酸エステルを含むナトリウムイオン二次電池用電解液ならびにそれを含むナトリウムイオン二次電池に関する。 The present invention relates to an electrolytic solution for a sodium ion secondary battery containing a fluorophosphate ester and a phosphate ester, and a sodium ion secondary battery containing the same.
　近年、太陽光、風力などの自然エネルギーを電気エネルギーに変換する技術が注目を集めている。また、多くの電気エネルギーを蓄えることができる蓄電デバイスとして、リチウムイオン二次電池、リチウムイオンキャパシタなどの需要が拡大している。 In recent years, technology that converts natural energy such as sunlight and wind power into electrical energy has attracted attention. In addition, demand for lithium-ion secondary batteries, lithium-ion capacitors, and the like as power storage devices that can store a large amount of electrical energy is increasing.
　リチウムイオン二次電池およびリチウムイオンキャパシタでは、引火点が低い有機電解液が使用されており、難燃性の確保も課題の１つである。特許文献１では、難燃性を確保する観点から、リチウムイオン二次電池の電解液における溶媒としてフッ化リン酸エステルなどのリン酸エステルを用いることが提案されている。 In lithium ion secondary batteries and lithium ion capacitors, an organic electrolyte having a low flash point is used, and ensuring flame retardancy is also an issue. Patent Document 1 proposes using a phosphate ester such as a fluorophosphate ester as a solvent in an electrolyte solution of a lithium ion secondary battery from the viewpoint of ensuring flame retardancy.
　一方、蓄電デバイスの市場の拡大に伴い、リチウム資源の価格も上昇しつつある。リチウム資源に比べると、ナトリウム資源は安価である。そこで、ナトリウムイオンがキャリアイオンとして用いられたナトリウムイオン電池が検討されている（例えば、特許文献２）。ナトリウムイオン電池は、正極、負極、およびナトリウムイオン伝導性の非水電解液を含む。 On the other hand, with the expansion of the electricity storage device market, the price of lithium resources is also rising. Compared to lithium resources, sodium resources are cheaper. Therefore, a sodium ion battery in which sodium ions are used as carrier ions has been studied (for example, Patent Document 2). The sodium ion battery includes a positive electrode, a negative electrode, and a sodium ion conductive non-aqueous electrolyte.
特開２０１１－１８７４１０号公報JP 2011-187410 A 特開２０１３－４８０７７号公報JP 2013-48077 A
　特許文献１には、フッ化リン酸エステルなどのリン酸エステルが、高い難燃性を有するものの、電池の性能を低下させる傾向があるため、電池における含有量が少量であることが好ましいことが教示されている。実際に、リチウムイオン二次電池の電解液における溶媒として多量のフッ化リン酸エステルを用いると、サイクル特性および／またはレート特性が低下する。また、充放電自体が困難な場合もある。 In Patent Document 1, although phosphate esters such as fluorinated phosphate esters have high flame retardancy, there is a tendency to reduce the performance of the battery, so that the content in the battery is preferably small. Taught. Actually, when a large amount of fluorinated phosphoric acid ester is used as the solvent in the electrolyte solution of the lithium ion secondary battery, the cycle characteristics and / or the rate characteristics deteriorate. Moreover, charging / discharging itself may be difficult.
　低コスト化が期待されるナトリウムイオン二次電池において、高い難燃性を確保しながらも、サイクル特性とレート特性とを両立できれば極めて有利である。
　本発明の目的は、高い難燃性を有し、ナトリウムイオン二次電池のサイクル特性およびレート特性を向上できる電解液およびそれを含むナトリウムイオン電池を提供することである。 In a sodium ion secondary battery that is expected to be reduced in cost, it is extremely advantageous if both cycle characteristics and rate characteristics can be achieved while ensuring high flame retardancy.
An object of the present invention is to provide an electrolytic solution that has high flame retardancy and can improve cycle characteristics and rate characteristics of a sodium ion secondary battery, and a sodium ion battery including the same.
　本発明の一局面は、ナトリウム塩と非水溶媒とを含み、かつナトリウムイオン伝導性を有するナトリウムイオン二次電池用電解液であって、前記非水溶媒は、フッ化リン酸エステルおよびリン酸エステルを含む、ナトリウムイオン二次電池用電解液に関する。 One aspect of the present invention is an electrolyte solution for a sodium ion secondary battery that includes a sodium salt and a nonaqueous solvent and has sodium ion conductivity, and the nonaqueous solvent includes a fluorophosphate ester and phosphoric acid. The present invention relates to an electrolyte solution for a sodium ion secondary battery containing an ester.
　本発明の他の一局面は、正極と、負極と、前記正極および前記負極の間に介在するセパレータと、上記電解液とを含む、ナトリウムイオン二次電池に関する。 Another aspect of the present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the electrolytic solution.
　本発明によれば、電解液の高い難燃性を確保しながら、ナトリウムイオン二次電池のサイクル特性およびレート特性（大電流放電特性）を向上することができる。 According to the present invention, it is possible to improve the cycle characteristics and rate characteristics (large current discharge characteristics) of the sodium ion secondary battery while ensuring high flame retardancy of the electrolytic solution.
本発明の一実施形態に係るナトリウムイオン二次電池を概略的に示す縦断面図である。1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention. 実施例１２～１４および参考例３のナトリウムイオン二次電池の充放電カーブである。7 is a charge / discharge curve of sodium ion secondary batteries of Examples 12 to 14 and Reference Example 3.
［発明の実施形態の説明］
　最初に、本発明の実施形態の内容を列記して説明する。
　本発明の一実施形態に係るナトリウムイオン二次電池用電解液は、（１）ナトリウム塩と非水溶媒とを含み、かつナトリウムイオン伝導性を有する。ここで、非水溶媒は、フッ化リン酸エステルおよびリン酸エステルを含む。フッ化リン酸エステルを含む非水溶媒をナトリウムイオン二次電池用電解液における溶媒として用いることで、当該電解液が、比較的難燃性が低いリン酸エステルを含むにもかかわらず、電解液（ひいてはナトリウムイオン二次電池）の難燃性を大きく向上できる。 [Description of Embodiment of the Invention]
First, the contents of the embodiment of the present invention will be listed and described.
An electrolyte for a sodium ion secondary battery according to an embodiment of the present invention includes (1) a sodium salt and a nonaqueous solvent, and has sodium ion conductivity. Here, the non-aqueous solvent includes a fluorophosphate ester and a phosphate ester. By using a non-aqueous solvent containing a fluorophosphate as a solvent in an electrolyte for a sodium ion secondary battery, the electrolyte contains a phosphate having a relatively low flame retardancy. (As a result, the flame retardancy of the sodium ion secondary battery) can be greatly improved.
　一方、フッ化リン酸エステルを含む非水溶媒をリチウムイオン二次電池の電解液における溶媒として用いると、リチウムイオン二次電池のサイクル特性および／またはレート特性が損なわれ易く、充放電自体が困難な場合もある。リチウムイオンは、フッ化リン酸エステルとの溶媒和エネルギーが大きいため、充電時には、溶媒和された状態で負極活物質中に吸蔵（または挿入）される。その結果、電解液の分解が起こり、不安定な固体電解質界面（ＳＥＩ：Ｓｏｌｉｄ　Ｅｌｅｃｔｒｏｌｙｔｅ　Ｉｎｔｅｒｆａｃｅ）被膜が形成されて抵抗が大きくなるものと考えられる。また、リン酸エステルは、フッ化リン酸エステルに比べると、リチウムイオンに対する溶媒和エネルギーが大きいため、充電時には、リチウムイオンに溶媒和した状態で負極活物質中に吸蔵（または挿入）され、上記と同様に、ＳＥＩ被膜が形成されて抵抗が大きくなる。このようなＳＥＩ被膜の形成は、充放電が進むにつれて顕著になるため、サイクル特性が低下すると考えられる。サイクル特性を高めるために、リチウムイオンとフッ化リン酸エステルとの溶媒和エネルギーを低下させると、電解液の粘度が高くなり易く、イオン伝導性が低下して、レート特性が損なわれる。 On the other hand, when a non-aqueous solvent containing a fluorophosphate ester is used as a solvent in the electrolyte of a lithium ion secondary battery, the cycle characteristics and / or rate characteristics of the lithium ion secondary battery are likely to be impaired, and charging / discharging itself is difficult. In some cases. Since lithium ions have a large solvation energy with the fluorophosphate, they are occluded (or inserted) into the negative electrode active material in a solvated state during charging. As a result, the electrolytic solution is decomposed, and an unstable solid electrolyte interface (SEI: Solid Electrolyte Interface) film is formed to increase the resistance. In addition, since the phosphate ester has a higher solvation energy for lithium ions than the fluorophosphate ester, it is occluded (or inserted) in the negative electrode active material in a state solvated with lithium ions during charging. Similarly, the SEI film is formed and the resistance is increased. The formation of such an SEI film becomes conspicuous as the charge / discharge progresses, and it is considered that the cycle characteristics deteriorate. If the solvation energy of lithium ions and fluorinated phosphoric acid esters is lowered in order to enhance cycle characteristics, the viscosity of the electrolytic solution tends to increase, ion conductivity decreases, and rate characteristics are impaired.
　本発明の実施形態では、上記のように、フッ化リン酸エステルとリン酸エステルとを含む非水溶媒が、ナトリウムイオン二次電池の電解液における溶媒として用いられている。ナトリウムイオンは、リチウムイオンよりもイオン半径が大きいため、電荷密度が小さく、フッ化リン酸エステルやリン酸エステルとの溶媒和エネルギーがリチウムイオンの場合よりも小さくなる。そのため、負極におけるナトリウムイオンの挿入をスムーズに行うことができ、電解液の副反応が抑制される。よって、充放電を繰り返しても容量低下が抑制され、高いサイクル特性が得られる。また、リン酸エステルとナトリウムイオンとの溶媒和エネルギーは、リン酸エステルとリチウムイオンとの溶媒和エネルギーに比べると小さい。よって、リン酸エステルを用いる場合でも、リチウムイオン二次電池のような不具合は抑制され、電解液の粘度を低下させるメリットが顕在化し、高いイオン伝導性を確保し易く、高いレート特性を得ることができる。 In the embodiment of the present invention, as described above, the non-aqueous solvent containing the fluorophosphate ester and the phosphate ester is used as the solvent in the electrolyte solution of the sodium ion secondary battery. Since sodium ions have a larger ion radius than lithium ions, the charge density is low, and the solvation energy with fluorophosphates and phosphates is lower than with lithium ions. Therefore, sodium ions can be smoothly inserted into the negative electrode, and the side reaction of the electrolytic solution is suppressed. Therefore, even if charging / discharging is repeated, capacity | capacitance fall is suppressed and a high cycle characteristic is acquired. Moreover, the solvation energy of phosphate ester and sodium ion is small compared with the solvation energy of phosphate ester and lithium ion. Therefore, even when using phosphate esters, problems such as lithium ion secondary batteries are suppressed, and the merit of lowering the viscosity of the electrolytic solution becomes obvious, and it is easy to ensure high ion conductivity and obtain high rate characteristics. Can do.
　（２）非水溶媒は、２０質量％以上のフッ化リン酸エステルを含むことが好ましい。この場合、難燃性をさらに高めることができるとともに、負極における電解液の副反応を抑制する効果をさらに高めることができる。 (2) The non-aqueous solvent preferably contains 20% by mass or more of a fluorophosphate ester. In this case, the flame retardancy can be further enhanced, and the effect of suppressing the side reaction of the electrolytic solution in the negative electrode can be further enhanced.
　（３）好ましい実施形態では、電解液は、引火点を有さないか、または引火点が７０℃以上である。本実施形態に係る電解液は、フッ化リン酸エステルを多く含む非水溶媒を溶媒として含む。したがって、本実施形態に係る電解液によれば、高い難燃性を確保することができ、ひいてはナトリウムイオン二次電池の難燃性を高めることができる。その結果、本実施形態に係る電解液によれば、ナトリウムイオン二次電池の安全性を高めることができる。 (3) In a preferred embodiment, the electrolytic solution does not have a flash point or has a flash point of 70 ° C. or higher. The electrolytic solution according to the present embodiment includes a non-aqueous solvent containing a large amount of fluorophosphate as a solvent. Therefore, according to the electrolyte solution which concerns on this embodiment, high flame retardance can be ensured and by extension, the flame retardance of a sodium ion secondary battery can be improved. As a result, according to the electrolytic solution according to the present embodiment, the safety of the sodium ion secondary battery can be improved.
　（４）フッ化リン酸エステルは、１～３個のポリフルオロアルキル基を有するポリフルオロアルキルホスフェートであることが好ましい。ここで、前記１～３個のポリフルオロアルキル基のそれぞれは、炭素数１～３のジフルオロアルキル基、炭素数１～３のトリフルオロアルキル基、または炭素数２～３のテトラフルオロアルキル基である。（５）フッ化リン酸エステルは、トリス（２，２，２－トリフルオロエチル）ホスフェート、ビス（２，２，２－トリフルオロエチル）メチルホスフェートおよびビス（２，２，２－トリフルオロエチル）エチルホスフェートからなる群より選択された少なくとも１種であることが好ましい。このようなフッ化リン酸エステルは、高い難燃性を付与し易い。また、サイクル特性をさらに向上し易い。 (4) The fluorophosphoric acid ester is preferably a polyfluoroalkyl phosphate having 1 to 3 polyfluoroalkyl groups. Here, each of the 1 to 3 polyfluoroalkyl groups is a difluoroalkyl group having 1 to 3 carbon atoms, a trifluoroalkyl group having 1 to 3 carbon atoms, or a tetrafluoroalkyl group having 2 to 3 carbon atoms. is there. (5) Fluorophosphates include tris (2,2,2-trifluoroethyl) phosphate, bis (2,2,2-trifluoroethyl) methyl phosphate and bis (2,2,2-trifluoroethyl) It is preferably at least one selected from the group consisting of ethyl phosphate. Such a fluorinated phosphoric acid ester tends to impart high flame retardancy. Moreover, it is easy to further improve the cycle characteristics.
　リン酸エステルは、（６）炭素数１～３のアルキル基を有するトリアルキルホスフェートであってもよく、（７）トリメチルホスフェートおよびトリエチルホスフェートからなる群より選択された少なくとも１種であってもよい。これらのリン酸エステルは、電解液の粘度を低下させ易く、レート特性をさらに高める上で有利である。 The phosphate ester may be (6) a trialkyl phosphate having an alkyl group having 1 to 3 carbon atoms, or (7) at least one selected from the group consisting of trimethyl phosphate and triethyl phosphate. . These phosphate esters are easy to reduce the viscosity of the electrolytic solution, and are advantageous in further improving the rate characteristics.
　（８）非水溶媒は、環状カーボネートおよび鎖状カーボネートからなる群より選択された少なくとも１種をさらに含んでもよい。このような非水溶媒を用いる場合、サイクル特性およびレート特性の双方をさらに高めることができる。 (8) The non-aqueous solvent may further include at least one selected from the group consisting of cyclic carbonates and chain carbonates. When such a non-aqueous solvent is used, both cycle characteristics and rate characteristics can be further improved.
　（９）非水溶媒は、プロピレンカーボネートをさらに含むことが好ましい。この場合、サイクル特性の向上効果がさらに大きくなる。 (9) The non-aqueous solvent preferably further contains propylene carbonate. In this case, the effect of improving the cycle characteristics is further increased.
　（１０）非水溶媒中のフッ化リン酸エステルの含有量は、５０質量％を超えて８０質量％以下であることが好ましい。非水溶媒中のフッ化リン酸エステルの含有量がこのような範囲である場合、サイクル特性およびレート特性をバランスよく向上できる。 (10) The content of the fluorophosphate in the non-aqueous solvent is preferably more than 50% by mass and 80% by mass or less. When the content of the fluorophosphate in the non-aqueous solvent is within such a range, the cycle characteristics and rate characteristics can be improved in a balanced manner.
　（１１）本発明の他の一実施形態は、正極と、負極と、正極および負極の間に介在するセパレータと、上記の電解液とを含む、ナトリウムイオン二次電池に関する。このようなナトリウムイオン二次電池は、上記電解液を含むので、高いサイクル特性およびレート特性が得られる。また、本実施形態に係るナトリウムイオン二次電池は、難燃性が高いため、安全性にも優れる。 (11) Another embodiment of the present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the above electrolytic solution. Since such a sodium ion secondary battery contains the electrolyte solution, high cycle characteristics and rate characteristics can be obtained. Moreover, since the sodium ion secondary battery which concerns on this embodiment has high flame retardance, it is excellent also in safety.
［発明の実施形態の詳細］
　本発明の実施形態に係るナトリウムイオン二次電池用電解液およびナトリウムイオン二次電池の具体例を、適宜図面を参照しつつ以下に説明する。なお、本発明はこれらの例示に限定されるものではなく、添付の請求の範囲によって示され、請求の範囲と均等の意味および範囲内での全ての変更が含まれることが意図される。 [Details of the embodiment of the invention]
Specific examples of the electrolyte for sodium ion secondary batteries and the sodium ion secondary battery according to embodiments of the present invention will be described below with reference to the drawings as appropriate. In addition, this invention is not limited to these illustrations, is shown by the attached claim, and it is intended that all the changes within the meaning and range equivalent to a claim are included.
１．ナトリウムイオン二次電池用電解液
　電解液は、ナトリウム塩と、非水溶媒とを含む。
（ナトリウム塩）
　ナトリウム塩は、電解液中で解離してナトリウムイオン（第１カチオン）とアニオン（第１アニオン）とを生成するため、電解液は、ナトリウムイオン伝導性を有する。 1. Electrolytic solution for sodium ion secondary battery The electrolytic solution contains a sodium salt and a non-aqueous solvent.
(Sodium salt)
Since the sodium salt dissociates in the electrolytic solution to generate sodium ions (first cation) and anions (first anion), the electrolytic solution has sodium ion conductivity.
　ナトリウム塩を構成する第１アニオンの種類は、特に限定されない。第１アニオンとしては、例えば、フッ素含有酸のアニオン、塩素含有酸のアニオン、オキサレート基を有する酸素酸のアニオン、フルオロアルカンスルホン酸のアニオン、ビススルホニルアミドアニオンなどが挙げられる。これらのナトリウム塩は、単独で用いてもよく、２種類以上を混合して用いてもよい。 The kind of the first anion constituting the sodium salt is not particularly limited. Examples of the first anion include an anion of a fluorine-containing acid, an anion of a chlorine-containing acid, an anion of an oxygen acid having an oxalate group, an anion of a fluoroalkanesulfonic acid, and a bissulfonylamide anion. These sodium salts may be used alone or in combination of two or more.
　前記フッ素含有酸のアニオンとしては、例えば、ヘキサフルオロリン酸イオン（ＰＦ６ －）などのフッ素含有リン酸アニオン；テトラフルオロホウ酸イオン（ＢＦ４ －）などのフッ素含有ホウ酸アニオンなどが挙げられる。 Examples of the anion of the fluorine-containing acid include a fluorine-containing phosphate anion such as hexafluorophosphate ion (PF 6 − ); and a fluorine-containing borate anion such as tetrafluoroborate ion (BF 4 − ). .
　前記塩素含有酸のアニオンとしては、例えば、過塩素酸イオン（ＣｌＯ４ －）などが挙げられる。 Examples of the anion of the chlorine-containing acid include perchlorate ion (ClO 4 − ).
　前記オキサレート基を有する酸素酸のアニオンとしては、例えば、ビス（オキサラト）ボレートイオン（Ｂ（Ｃ２Ｏ４）２ －）などのオキサラトボレートイオン；トリス（オキサラト）ホスフェートイオン（Ｐ（Ｃ２Ｏ４）３ －）などのオキサラトホスフェートイオンなどが挙げられる。 Examples of the anion of the oxygen acid having an oxalate group include oxalatoborate ions such as bis (oxalato) borate ion (B (C 2 O 4 ) 2 − ); tris (oxalato) phosphate ion (P (C 2 O 4 ) Oxalatophosphate ions such as 3 − ) and the like.
　前記フルオロアルカンスルホン酸のアニオンとしては、例えば、トリフルオロメタンスルホン酸イオン（ＣＦ３ＳＯ３ －）などが挙げられる。 Examples of the anion of the fluoroalkanesulfonic acid include trifluoromethanesulfonic acid ion (CF 3 SO 3 − ).
　前記ビススルホニルアミドアニオンとしては、例えば、ビス（フルオロスルホニル）アミドアニオン（ＦＳＡ：ｂｉｓ（ｆｌｕｏｒｏｓｕｌｆｏｎｙｌ）ａｍｉｄｅ　ａｎｉｏｎ））；（ＦＳＯ２）（ＣＦ３ＳＯ２）Ｎ－などの（フルオロスルホニル）（パーフルオロアルキルスルホニル）アミドアニオン；ビス（トリフルオロメチルスルホニル）アミドアニオン（ＴＦＳＡ：ｂｉｓ（ｔｒｉｆｌｕｏｒｏｍｅｔｈｙｌｓｕｌｆｏｎｙｌ）ａｍｉｄｅ　ａｎｉｏｎ、Ｎ（ＳＯ２ＣＦ３）２ －）、Ｎ（ＳＯ２Ｃ２Ｆ５）２ －などのビス（パーフルオロアルキルスルホニル）アミドアニオンなどが挙げられる。これらのうち、特に、ＦＳＡおよび／またはＴＦＳＡ、具体的には、ＦＳＡ、ＴＦＳＡ、およびＦＳＡとＴＦＳＡとの混合物が好ましい。 As the bis-sulfonyl amide anion, for example, bis (fluorosulfonyl) amide anion (FSA: bis (fluorosulfonyl) amide anion)); (FSO 2) (CF 3 SO 2) N - ( fluorosulfonyl such) (perfluoro Bis (trifluoromethylsulfonyl) amide anion (TFSA: bis (trifluoromethylsulfonyl) amide anion, N (SO 2 CF 3 ) 2 − ), N (SO 2 C 2 F 5 ) 2 − (Perfluoroalkylsulfonyl) amide anion and the like. Among these, FSA and / or TFSA, specifically, FSA, TFSA, and a mixture of FSA and TFSA are particularly preferable.
　電解液におけるナトリウム塩またはナトリウムイオンの濃度は、例えば、０．２～１０ｍｏｌ／Ｌの範囲から適宜選択でき、好ましくは０．２～５ｍｏｌ／Ｌ、より好ましくは０．２～２．５ｍｏｌ／Ｌである。 The concentration of sodium salt or sodium ion in the electrolytic solution can be appropriately selected from the range of, for example, 0.2 to 10 mol / L, preferably 0.2 to 5 mol / L, more preferably 0.2 to 2.5 mol / L. It is.
（非水溶媒）
　有機溶媒を含む有機電解液を含む従来のナトリウムイオン二次電池は、低温で作動させることができる。しかし、前記ナトリウムイオン二次電池においては、高温でのサイクルを安定化させることが難しい。ナトリウムイオン二次電池の電解液における電解質としてイオン液体を用いた場合、高温でのサイクルを安定化させることができるが、低温での利用率（低温でのレート特性）が低い。 (Non-aqueous solvent)
A conventional sodium ion secondary battery including an organic electrolyte containing an organic solvent can be operated at a low temperature. However, in the sodium ion secondary battery, it is difficult to stabilize the cycle at a high temperature. When an ionic liquid is used as an electrolyte in an electrolytic solution of a sodium ion secondary battery, the cycle at high temperature can be stabilized, but the utilization factor at low temperature (rate characteristic at low temperature) is low.
　本発明の実施形態によれば、フッ化リン酸エステル（第１溶媒）と、リン酸エステル（第２溶媒）とを含む非水溶媒が、電解液における溶媒として用いられている。そのため、本実施形態に係る電解液によれば、高い難燃性を確保することができる。これにより、ナトリウムイオン二次電池の難燃性を高めることができる。また、高温でのサイクルを安定化させることができるとともに、低温での利用率も高めることができる。 According to the embodiment of the present invention, a nonaqueous solvent containing a fluorophosphate ester (first solvent) and a phosphate ester (second solvent) is used as a solvent in the electrolytic solution. Therefore, according to the electrolytic solution according to the present embodiment, high flame retardancy can be ensured. Thereby, the flame retardance of a sodium ion secondary battery can be improved. In addition, the cycle at high temperature can be stabilized, and the utilization factor at low temperature can be increased.
　電解液の引火点は、７０℃以上であることが好ましく、引火点を有さない場合も好ましい。引火点が７０℃以上である場合、電解液は、第３石油類、第４石油類などに分類される。そのため、本実施形態に係る電解液によれば、一般に第２石油類に分類されるリチウムイオン二次電池用電解液に比べて、高い安全性を確保することができる。 The flash point of the electrolytic solution is preferably 70 ° C. or higher, and preferably has no flash point. When the flash point is 70 ° C. or higher, the electrolyte is classified as a third petroleum, a fourth petroleum, or the like. Therefore, according to the electrolytic solution according to the present embodiment, high safety can be ensured as compared with the electrolytic solution for lithium ion secondary batteries generally classified as the second petroleum.
（フッ化リン酸エステル）
　フッ化リン酸エステルとしては、オルトリン酸の３つのエステル形成部位（－ＯＨ基）のうち、１つまたは２つがエステル化されたものであってもよいが、全てがエステル化された下記式（Ｉ）で表される化合物が好ましい。 (Fluorophosphate ester)
The fluorinated phosphoric acid ester may be one in which one or two of the three ester forming sites (-OH group) of orthophosphoric acid are esterified, but the following formula ( The compounds represented by I) are preferred.
Figure JPOXMLDOC01-appb-C000001
 （式中、Ｒ１、Ｒ２およびＲ３は、それぞれ独立してアルキル基またはフッ化アルキル基を示し、Ｒ１、Ｒ２およびＲ３のうち少なくとも１つはフッ化アルキル基である。）
Figure JPOXMLDOC01-appb-C000001
 (Wherein R 1 , R 2 and R 3 each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R 1 , R 2 and R 3 is a fluorinated alkyl group.)
　Ｒ１～Ｒ３は、これらのうち２つまたは３つが同じであってもよく、全てが同じであってもよく、または全てが異なっていてもよい。Ｒ１～Ｒ３で表されるアルキル基としては、メチル基、エチル基、ｎ－プロピル基、イソプロピル基、ｎ－ブチル基、ｓｅｃ－ブチル基、ｔｅｒｔ－ブチル基などの炭素数１～６のアルキル基が例示できる。フッ化アルキル基としては、これらのアルキル基に対応するフッ化アルキル基、つまり、炭素数１～６のフルオロアルキル基が例示できる。アルキル基およびフルオロアルキル基の炭素数は、それぞれ、１～３個が好ましく、より好ましくは２～３個である。 Of R 1 to R 3 , two or three of them may be the same, all may be the same, or all may be different. Examples of the alkyl group represented by R 1 to R 3 include those having 1 to 6 carbon atoms such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, sec-butyl group, and tert-butyl group. An alkyl group can be illustrated. Examples of the fluorinated alkyl group include fluorinated alkyl groups corresponding to these alkyl groups, that is, fluoroalkyl groups having 1 to 6 carbon atoms. Each of the alkyl group and the fluoroalkyl group preferably has 1 to 3 carbon atoms, more preferably 2 to 3 carbon atoms.
　フッ化アルキル基が有するフッ素原子の個数は特に制限されず、フッ化アルキル基の炭素数に応じて適宜選択できる。各フッ化アルキル基が有するフッ素原子の個数は、例えば、１～６個から選択でき、１～４個であってもよい。難燃性および充放電特性などの観点からは、フッ化アルキル基が有するフッ素原子の個数は、好ましくは複数個、より好ましくは２～４個、さらに好ましくは２～３個である。つまり、フッ化リン酸エステルは、ポリフルオロアルキル基を有するポリフルオロアルキルホスフェートであることが好ましい。 The number of fluorine atoms in the fluorinated alkyl group is not particularly limited, and can be appropriately selected according to the number of carbon atoms in the fluorinated alkyl group. The number of fluorine atoms in each fluorinated alkyl group can be selected from 1 to 6, for example, and may be 1 to 4. From the viewpoint of flame retardancy and charge / discharge characteristics, the number of fluorine atoms contained in the fluorinated alkyl group is preferably plural, more preferably 2 to 4, further preferably 2 to 3. That is, the fluorinated phosphate ester is preferably a polyfluoroalkyl phosphate having a polyfluoroalkyl group.
　フッ化アルキル基は、フッ化アルキル基を構成するいずれの炭素原子上にフッ素原子を有していてもよいが、フッ化リン酸エステルのリン原子からできるだけ遠い炭素原子上にフッ素原子を有していることが好ましい。フッ化アルキル基は、例えば、フッ化エチル基では、エチル基の２位の炭素原子上、フッ化ｎ－プロピル基では、ｎ－プロピル基の３位の炭素原子上に、フッ素原子を有することが好ましい。 The fluorinated alkyl group may have a fluorine atom on any carbon atom constituting the fluorinated alkyl group, but has a fluorine atom on a carbon atom as far as possible from the phosphorus atom of the fluorinated phosphate ester. It is preferable. The fluorinated alkyl group has, for example, a fluorine atom on the carbon atom at the 2-position of the ethyl group in the ethyl fluoride group and on the carbon atom at the 3-position of the n-propyl group in the fluorinated n-propyl group. Is preferred.
　フッ化アルキル基（ポリフルオロアルキル基など）の個数は１～３個から選択できる。高い難燃性、優れた充放電特性などを確保する観点からは、Ｒ１、Ｒ２およびＲ３のうち、２つまたは３つがフッ化アルキル基（ポリフルオロアルキル基など）であり、残りがアルキル基であることが好ましい。ポリフルオロアルキル基としては、例えば、ジフルオロメチル基、２，２－ジフルオロエチル基などの炭素数１～３のジフルオロアルキル基；トリフルオロメチル基、２，２，２－トリフルオロエチル基、３，３，３－トリフルオロプロピル基などの炭素数１～３のトリフルオロアルキル基；２，２，３，３－テトラフルオロプロピル基などの炭素数２～３のテトラフルオロアルキル基などが挙げられる。 The number of fluorinated alkyl groups (such as polyfluoroalkyl groups) can be selected from 1 to 3. From the viewpoint of ensuring high flame retardancy and excellent charge / discharge characteristics, two or three of R 1 , R 2 and R 3 are fluorinated alkyl groups (such as polyfluoroalkyl groups), and the rest An alkyl group is preferred. Examples of the polyfluoroalkyl group include a difluoroalkyl group having 1 to 3 carbon atoms such as a difluoromethyl group and a 2,2-difluoroethyl group; a trifluoromethyl group, a 2,2,2-trifluoroethyl group, 3, Examples thereof include trifluoroalkyl groups having 1 to 3 carbon atoms such as 3,3-trifluoropropyl group; tetrafluoroalkyl groups having 2 to 3 carbon atoms such as 2,2,3,3-tetrafluoropropyl group, and the like.
　高い難燃性および優れた充放電特性（サイクル特性、レート特性など）を確保する観点から、フッ化リン酸エステルのうち、トリス（２，２，２－トリフルオロエチル）ホスフェート（ＴＦＥＰ：ｔｒｉｓ（２，２，２－ｔｒｉｆｌｕｏｒｏｅｔｈｙｌ）　ｐｈｏｓｐｈａｔｅ）、ビス（２，２，２－トリフルオロエチル）メチルホスフェート（ＴＦＥＭＰ：ｂｉｓ（２，２，２－ｔｒｉｆｌｕｏｒｏｅｔｈｙｌ）　ｍｅｔｈｙｌ　ｐｈｏｓｐｈａｔｅ）およびビス（２，２，２－トリフルオロエチル）エチルホスフェート（ＴＦＥＥＰ：ｂｉｓ（２，２，２－ｔｒｉｆｌｕｏｒｏｅｔｈｙｌ）　ｅｔｈｙｌ　ｐｈｏｓｐｈａｔｅ）からなる群より選択された少なくとも１種が特に好ましい。レート特性をさらに高める観点からは、ＴＦＥＭＰおよび／またはＴＦＥＥＰ、具体的には、ＴＦＥＭＰ、ＴＦＥＥＰ、またはＴＦＥＭＰとＴＦＥＥＰとの混合物を用いることが好ましい。 Among the fluorophosphates, tris (2,2,2-trifluoroethyl) phosphate (TFEP: tris () is used from the viewpoint of ensuring high flame retardancy and excellent charge / discharge characteristics (cycle characteristics, rate characteristics, etc.). 2,2,2-trifluoroethyl) phosphate), bis (2,2,2-trifluoroethyl) methyl phosphate (TFEMP: bis (2,2,2-trifluoroethyl) methyl phosphate) and bis (2,2,2- At least one selected from the group consisting of trifluoroethyl) ethyl phosphate (TFEEP: bis (2,2,2-trifluoroethyl) ethyl phosphate) is particularly preferable. From the viewpoint of further improving the rate characteristics, it is preferable to use TFEEP and / or TFEEP, specifically, TFEEP, TFEEP, or a mixture of TFEEP and TFEEP.
　非水溶媒中のフッ化リン酸エステルの含有量は、例えば、２０質量％以上、好ましくは２５質量％以上、より好ましくは３０質量％以上である。また、非水溶媒中のフッ化リン酸エステルの含有量は、５０質量％より多くてもよく、好ましくは５５質量％以上、より好ましくは６０質量％以上、さらに好ましくは６５質量％以上であってもよい。非水溶媒中のフッ化リン酸エステルの含有量は、好ましくは９０質量％以下、より好ましくは８０質量％以下である。これらの下限値と上限値とは任意に組み合わせることができる。非水溶媒中のフッ化リン酸エステルの含有量は、例えば、２０～９０質量％、５０質量％を超えて９０質量％以下、または５０質量％を超えて８０質量％以下であってもよく、５５～８０質量％、または６０～８０質量％であってもよい。リチウムイオン二次電池では、フッ化リン酸エステルを含む非水溶媒を用いると、充放電を行うことが難しくなる場合がある。しかし、ナトリウムイオン二次電池では、このような非水溶媒を用いても十分に充放電を行うことができる。 The content of the fluorophosphate ester in the nonaqueous solvent is, for example, 20% by mass or more, preferably 25% by mass or more, and more preferably 30% by mass or more. The content of the fluorophosphate ester in the non-aqueous solvent may be more than 50% by mass, preferably 55% by mass or more, more preferably 60% by mass or more, and further preferably 65% by mass or more. May be. The content of the fluorophosphate ester in the non-aqueous solvent is preferably 90% by mass or less, more preferably 80% by mass or less. These lower limit values and upper limit values can be arbitrarily combined. The content of the fluorophosphate ester in the non-aqueous solvent may be, for example, 20 to 90% by mass, more than 50% by mass and 90% by mass or less, or more than 50% by mass and 80% by mass or less. 55 to 80% by mass, or 60 to 80% by mass. In a lithium ion secondary battery, when a non-aqueous solvent containing a fluorophosphate ester is used, it may be difficult to charge and discharge. However, the sodium ion secondary battery can be sufficiently charged and discharged even when such a non-aqueous solvent is used.
（リン酸エステル）
　第２溶媒であるリン酸エステルは、具体的には、フッ化リン酸エステルとは異なり、フッ素原子を有さないリン酸エステルである。 (Phosphate ester)
The phosphate ester as the second solvent is specifically a phosphate ester having no fluorine atom, unlike the fluorinated phosphate ester.
　リン酸エステルとしては、トリメチルホスフェート（ＴＭＰ：ｔｒｉｍｅｔｈｙｌ　ｐｈｏｓｐｈａｔｅ）、トリエチルホスフェート（ＴＥＰ：ｔｒｉｅｔｈｙｌ　ｐｈｏｓｐｈａｔｅ）などのトリアルキルホスフェート（例えば、炭素数１～６のアルキル基を有するトリアルキルホスフェートなど）；トリフェニルホスフェート、トリトリルホスフェートなどのトリアリールホスフェート（炭素数６～１０のアリール基を有するトリアリールホスフェートなど）などが例示できる。リン酸エステルは、単独で用いてもよく、２種類以上を混合して用いてもよい。リン酸エステルのうち、ＴＭＰ、ＴＥＰなどの炭素数１～４のアルキル基を有するトリアルキルホスフェートが好ましく、炭素数１～３のアルキル基を有するトリアルキルホスフェートがより好ましい。 Examples of the phosphate ester include trialkyl phosphates such as trimethyl phosphate (TMP) and triethyl phosphate (TEP) (for example, trialkyl phosphate having an alkyl group having 1 to 6 carbon atoms); triphenyl phosphate And triaryl phosphates such as tolyl phosphate (such as triaryl phosphates having an aryl group having 6 to 10 carbon atoms). Phosphoric esters may be used alone or in combination of two or more. Among the phosphate esters, trialkyl phosphates having an alkyl group having 1 to 4 carbon atoms such as TMP and TEP are preferable, and trialkyl phosphates having an alkyl group having 1 to 3 carbon atoms are more preferable.
　リン酸エステルのエステル部位（上記のリン酸エステルにおけるアルキル基やアリール基）が嵩高くなると、ナトリウムイオンとの溶媒和エネルギーが小さくなるため、容量の低下を抑制し易くなる。よって、サイクル特性をさらに高める観点からは、ＴＥＰなどの炭素数２～６のアルキル基を有するトリアルキルホスフェート（特に、炭素数２～４のアルキル基を有するトリアルキルホスフェート）、またはトリアリールホスフェートを用いることが好ましい。 When the ester portion of the phosphate ester (the alkyl group or aryl group in the above-mentioned phosphate ester) is bulky, the solvation energy with sodium ions is reduced, and thus it is easy to suppress a decrease in capacity. Therefore, from the viewpoint of further improving the cycle characteristics, a trialkyl phosphate having an alkyl group having 2 to 6 carbon atoms (particularly a trialkyl phosphate having an alkyl group having 2 to 4 carbon atoms) such as TEP, or triaryl phosphate is used. It is preferable to use it.
　非水溶媒中のリン酸エステルの含有量は、例えば、８０質量％以下、好ましくは７０質量％以下、より好ましくは５０質量％以下、さらに好ましくは４０質量％である。リン酸エステルの含有量は、１０～８０質量％、１０～７０質量％、１０～５０質量％、２０～５０質量％、または２０～４０質量％であることが好ましい。リン酸エステルの含有量がこのような範囲である場合、高い難燃性と、高いレート特性とのバランスを取り易い。 The content of the phosphate ester in the non-aqueous solvent is, for example, 80% by mass or less, preferably 70% by mass or less, more preferably 50% by mass or less, and further preferably 40% by mass. The phosphate ester content is preferably 10 to 80% by mass, 10 to 70% by mass, 10 to 50% by mass, 20 to 50% by mass, or 20 to 40% by mass. When the content of the phosphate ester is within such a range, it is easy to balance high flame retardancy with high rate characteristics.
　なお、ナトリウムイオン二次電池を、高い電位（例えば、４．２Ｖ以上の充電終止電圧）まで充電したり、高温（例えば、６０℃以上）での充放電を繰り返したりすると、電池内に含まれるアルミニウム（正極集電体など）が腐食する場合がある。正極集電体が腐食すると、容量が低下し、サイクル特性が低下する場合がある。このような正極集電体の腐食を抑制する観点からは、フッ化リン酸エステルの含有量を多くしたり、および／またはリン酸エステルを用いたりすることが有効である。 In addition, when a sodium ion secondary battery is charged to a high potential (for example, a charge end voltage of 4.2 V or higher) or repeatedly charged and discharged at a high temperature (for example, 60 ° C. or higher), it is included in the battery. Aluminum (such as a positive electrode current collector) may corrode. When the positive electrode current collector is corroded, the capacity may decrease, and the cycle characteristics may deteriorate. From the viewpoint of suppressing such corrosion of the positive electrode current collector, it is effective to increase the content of the fluorophosphate ester and / or to use the phosphate ester.
　非水溶媒に占めるフッ化リン酸エステルとリン酸エステルとの含有量の合計は、例えば、６０～１００質量％である。非水溶媒に占めるフッ化リン酸エステルとリン酸エステルとの含有量の合計は、６５～１００質量％であることが好ましく、７０～１００質量％であることがより好ましい。非水溶媒に占めるフッ化リン酸エステルとリン酸エステルとの含有量の合計は、６０～９５質量％または７０～９０質量％であってもよい。 The total content of the fluorophosphate ester and phosphate ester in the non-aqueous solvent is, for example, 60 to 100% by mass. The total content of the fluorophosphate ester and phosphate ester in the non-aqueous solvent is preferably 65 to 100% by mass, more preferably 70 to 100% by mass. The total content of the fluorophosphate ester and the phosphate ester in the non-aqueous solvent may be 60 to 95 mass% or 70 to 90 mass%.
（第３溶媒）
　非水溶媒は、フッ化リン酸エステルおよびリン酸エステル以外の他の溶媒（第３溶媒）をさらに含んでもよい。第３溶媒としては、ナトリウムイオン二次電池の電解液における溶媒として用いられる公知の溶媒、例えば、有機溶媒および／またはイオン液体、具体的には、有機溶媒、イオン液体、または有機溶媒とイオン液体との混合物などが挙げられる。第３溶媒は、単独で用いてもよく、２種類以上を混合して用いてもよい。イオン液体は、少なくとも１００℃以下で溶融状態の塩（溶融塩）と同義であり、アニオンとカチオンとで構成される液状イオン性物質である。なお、上記のナトリウム塩のうち、例えば、ナトリウムイオンとビススルホニルアミドアニオンとの塩は、一般にはイオン液体に分類されることもあるが、本明細書中では、便宜上、イオン液体には含まないものとする。 (Third solvent)
The non-aqueous solvent may further include a solvent other than the fluorophosphate ester and the phosphate ester (third solvent). As the third solvent, a known solvent used as a solvent in an electrolyte solution of a sodium ion secondary battery, for example, an organic solvent and / or an ionic liquid, specifically, an organic solvent, an ionic liquid, or an organic solvent and an ionic liquid And a mixture thereof. A 3rd solvent may be used independently and may mix and use 2 or more types. The ionic liquid is synonymous with a salt (molten salt) in a molten state at least at 100 ° C. or less, and is a liquid ionic substance composed of anions and cations. Among the above-mentioned sodium salts, for example, a salt of sodium ion and bissulfonylamide anion may be generally classified as an ionic liquid, but in this specification, for convenience, it is not included in the ionic liquid. Shall.
　有機溶媒としては、特に限定されず、ナトリウムイオン二次電池に使用される公知の有機溶媒が使用できる。有機溶媒は、イオン伝導度の観点から、例えば、エチレンカーボネート（ＥＣ：ｅｔｈｙｌｅｎｅ　ｃａｒｂｏｎａｔｅ）、フルオロエチレンカーボネート、ジフルオロエチレンカーボネート、ビニルエチレンカーボネート、ビニレンカーボネート、プロピレンカーボネート（ＰＣ：ｐｒｏｐｙｌｅｎｅ　ｃａｒｂｏｎａｔｅ）、ブチレンカーボネートなどの環状カーボネート；ジメチルカーボネート、ジエチルカーボネート（ＤＥＣ：ｄｉｅｔｈｙｌ　ｃａｒｂｏｎａｔｅ）、エチルメチルカーボネートなどの鎖状カーボネート；γ－ブチロラクトン、δ－バレロラクトン、ε－カプロラクトンなどの環状エステル；エーテルなどを好ましく用いることができる。エーテルとしては、鎖状または環状のエーテル、例えば、フッ素含有エーテル；テトラグライムなどのグライム；クラウンエーテルなどが挙げられる。有機溶媒は、単独で用いてもよく、２種類以上を混合して用いてもよい。 The organic solvent is not particularly limited, and a known organic solvent used for sodium ion secondary batteries can be used. From the viewpoint of ionic conductivity, the organic solvent is, for example, ethylene carbonate (EC), fluoroethylene carbonate, difluoroethylene carbonate, vinyl ethylene carbonate, vinylene carbonate, propylene carbonate (PC), butylene carbonate, or the like. Cyclic carbonates; chain carbonates such as dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate; cyclic esters such as γ-butyrolactone, δ-valerolactone, and ε-caprolactone; ethers and the like can be preferably used. Examples of the ether include linear or cyclic ethers such as fluorine-containing ethers; glymes such as tetraglyme; crown ethers and the like. An organic solvent may be used independently and may be used in mixture of 2 or more types.
　サイクル特性およびレート特性をさらに高める観点からは、環状カーボネートおよび／または鎖状カーボネートを含む非水溶媒、具体的には、環状カーボネートを含む非水溶媒、鎖状カーボネートを含む非水溶媒、または環状カーボネートと鎖状カーボネートとの混合物を含む非水溶媒を用いることが好ましい。なかでも、プロピレンカーボネートを含む非水溶媒を用いると、サイクル特性およびレート特性を高め易い。また、サイクル特性およびレート特性をさらに高める観点から、環状カーボネート、環状エステル、および／またはエーテルを含む非水溶媒、具体的には、環状カーボネートを含む非水溶媒、環状エステルを含む非水溶媒、エーテルを含む非水溶媒、環状カーボネートと環状エステルとエーテルとの混合物を含む非水溶媒、環状カーボネートと環状エステルとの混合物を含む非水溶媒、環状カーボネートとエーテルとの混合物を含む非水溶媒、環状エステルとエーテルとの混合物を含む非水溶媒を用いる場合も好ましい。 From the viewpoint of further improving cycle characteristics and rate characteristics, a non-aqueous solvent containing a cyclic carbonate and / or a chain carbonate, specifically, a non-aqueous solvent containing a cyclic carbonate, a non-aqueous solvent containing a chain carbonate, or cyclic It is preferable to use a non-aqueous solvent containing a mixture of carbonate and chain carbonate. Among these, when a non-aqueous solvent containing propylene carbonate is used, cycle characteristics and rate characteristics are easily improved. Further, from the viewpoint of further improving cycle characteristics and rate characteristics, a nonaqueous solvent containing a cyclic carbonate, a cyclic ester, and / or an ether, specifically, a nonaqueous solvent containing a cyclic carbonate, a nonaqueous solvent containing a cyclic ester, A non-aqueous solvent containing ether, a non-aqueous solvent containing a mixture of cyclic carbonate and cyclic ester and ether, a non-aqueous solvent containing a mixture of cyclic carbonate and cyclic ester, a non-aqueous solvent containing a mixture of cyclic carbonate and ether, It is also preferable to use a non-aqueous solvent containing a mixture of a cyclic ester and an ether.
　第３溶媒のうち、イオン液体は、ナトリウムイオン以外のカチオン（第２カチオン）と、アニオン（第２アニオン）とを含む。第２カチオンとしては、ナトリウムイオン以外の無機カチオン、有機カチオンなどが例示できる。イオン液体は、第２カチオンとして、ナトリウムイオン以外の１種類のカチオンを含んでもよく、ナトリウムイオン以外の２種類以上のカチオンの混合物を含んでもよい。 Among the third solvents, the ionic liquid contains a cation (second cation) other than sodium ion and an anion (second anion). Examples of the second cation include inorganic cations other than sodium ions and organic cations. The ionic liquid may contain one kind of cation other than sodium ions as the second cation, or may contain a mixture of two or more kinds of cations other than sodium ions.
　有機カチオンとしては、脂肪族アミン、脂環族アミンまたは芳香族アミンに由来するカチオン（例えば、第４級アンモニウムカチオンなど）、窒素含有へテロ環を有するカチオン（つまり、環状アミンに由来するカチオン）などの窒素含有オニウムカチオン；イオウ含有オニウムカチオン；リン含有オニウムカチオンなどが例示できる。
　窒素含有有機オニウムカチオンのうち、特に、第４級アンモニウムカチオン、および窒素含有ヘテロ環骨格としてピロリジン骨格、ピリジン骨格またはイミダゾール骨格を有するカチオンが好ましい。 Organic cations include cations derived from aliphatic amines, alicyclic amines or aromatic amines (eg, quaternary ammonium cations), cations having nitrogen-containing heterocycles (ie, cations derived from cyclic amines) Examples thereof include nitrogen-containing onium cations such as: sulfur-containing onium cations; phosphorus-containing onium cations.
Of the nitrogen-containing organic onium cations, quaternary ammonium cations and cations having a pyrrolidine skeleton, a pyridine skeleton, or an imidazole skeleton as the nitrogen-containing heterocyclic skeleton are particularly preferable.
　窒素含有有機オニウムカチオンの具体例としては、テトラエチルアンモニウムカチオン（ＴＥＡ：ｔｅｔｒａｅｔｈｙｌａｍｍｏｎｉｕｍ　ｃａｔｉｏｎ）、メチルトリエチルアンモニウムカチオン（ＴＥＭＡ：ｍｅｔｈｙｌｔｒｉｅｔｈｙｌａｍｍｏｎｉｕｍ　ｃａｔｉｏｎ）などのテトラアルキルアンモニウムカチオン；１－メチル－１－プロピルピロリジニウムカチオン（ＭＰＰＹまたはＰｙ１３：１－ｍｅｔｈｙｌ－１－ｐｒｏｐｙｌｐｙｒｒｏｌｉｄｉｎｉｕｍ　ｃａｔｉｏｎ）、１－ブチル－１－メチルピロリジニウムカチオン（ＭＢＰＹまたはＰｙ１４：１－ｂｕｔｙｌ－１－ｍｅｔｈｙｌｐｙｒｒｏｌｉｄｉｎｉｕｍ　ｃａｔｉｏｎ）；１－エチル－３－メチルイミダゾリウムカチオン（ＥＭＩ：　１－ｅｔｈｙｌ－３－ｍｅｔｈｙｌｉｍｉｄａｚｏｌｉｕｍ　ｃａｔｉｏｎ）、および／または１－ブチル－３－メチルイミダゾリウムカチオン（ＢＭＩ：１－ｂｕｔｈｙｌ－３－ｍｅｔｈｙｌｉｍｉｄａｚｏｌｉｕｍ　ｃａｔｉｏｎ）などが挙げられる。 Specific examples of nitrogen-containing organic onium cations include tetraalkylammonium cations (TEA: tetraethylammonium cation), methyltriethylammonium cations (TEMA), tetraalkylammonium cations; 1-methyl-1-propylpyrrolidinium cation ( MPPY or Py13: 1-methyl-1-propylpyrrolidinium cation, 1-butyl-1-methylpyrrolidinium cation (MBPY or Py14: 1-butyl-1-methylpyrrolidinium cation); 1-ethyl-3-methylimidazolium cation (EMI: 1-ethyl-3- ethylimidazolium cation), and / or 1-butyl-3-methylimidazolium cation (BMI: 1-buthyl-3-methylimidazolium cation) and the like.
　無機カチオンとしては、例えば、ナトリウムイオン以外のアルカリ金属イオン（カリウムイオンなど）、アルカリ土類金属イオン（マグネシウムイオン、カルシウムイオンなど）、アンモニウムイオンなどが挙げられる。 Examples of inorganic cations include alkali metal ions (such as potassium ions) other than sodium ions, alkaline earth metal ions (such as magnesium ions and calcium ions), and ammonium ions.
　第２カチオンは、有機カチオンを含むことが好ましい。有機カチオンを含むイオン液体を用いることで、電解液の粘度を低下させ易くなるため、ナトリウムイオン伝導性を高め易く、高容量を確保し易くなる。なお、第２カチオンは、有機カチオンと無機カチオンとを含んでもよい。 The second cation preferably contains an organic cation. By using an ionic liquid containing an organic cation, it becomes easy to lower the viscosity of the electrolytic solution, so that the sodium ion conductivity is easily increased and a high capacity is easily secured. The second cation may include an organic cation and an inorganic cation.
　第２アニオンとしては、ビススルホニルアミドアニオンを用いることが好ましい。ビススルホニルアミドアニオンとしては、ナトリウム塩について例示したものから適宜選択できる。ビススルホニルアミドアニオンのうち、特に、ＦＳＡおよび／またはＴＦＳＡ、具体的には、ＦＳＡ、ＴＦＳＡ、およびＦＳＡとＴＦＳＡとの混合物が好ましい。 It is preferable to use a bissulfonylamide anion as the second anion. The bissulfonylamide anion can be appropriately selected from those exemplified for the sodium salt. Of the bissulfonylamide anions, FSA and / or TFSA, specifically, FSA, TFSA, and a mixture of FSA and TFSA are preferred.
　イオン液体の具体例としては、Ｐｙ１３とＦＳＡとの塩（Ｐｙ１３・ＦＳＡ）、Ｐｙ１３とＴＦＳＡとの塩（Ｐｙ１３・ＴＦＳＡ）、Ｐｙ１４とＦＳＡとの塩（Ｐｙ１４・ＦＳＡ）、Ｐｙ１４とＴＦＳＡとの塩（Ｐｙ１４・ＴＦＳＡ）、ＢＭＩとＦＳＡとの塩（ＢＭＩ・ＦＳＡ）、ＢＭＩとＴＦＳＡとの塩（ＢＭＩ・ＴＦＳＡ）、ＥＭＩとＦＳＡとの塩（ＥＭＩ・ＦＳＡ）、ＥＭＩとＴＦＳＡとの塩（ＥＭＩ・ＴＦＳＡ）、ＴＥＭＡとＦＳＡとの塩（ＴＥＭＡ・ＦＳＡ）、ＴＥＭＡとＴＦＳＡとの塩（ＴＥＭＡ・ＴＦＳＡ）、ＴＥＡとＦＳＡとの塩（ＴＥＡ・ＦＳＡ）、およびＴＥＡとＴＦＳＡとの塩（ＴＥＡ・ＴＦＳＡ）などが挙げられる。これらの塩は、単独で用いてもよく、２種類以上を混合して用いてもよい。 Specific examples of the ionic liquid include a salt of Py13 and FSA (Py13 · FSA), a salt of Py13 and TFSA (Py13 · TFSA), a salt of Py14 and FSA (Py14 · FSA), and a salt of Py14 and TFSA. (Py14 · TFSA), salt of BMI and FSA (BMI · FSA), salt of BMI and TFSA (BMI · TFSA), salt of EMI and FSA (EMI · FSA), salt of EMI and TFSA (EMI) TFSA), salt of TEMA and FSA (TEMA · FSA), salt of TEMA and TFSA (TEMA · TFSA), salt of TEA and FSA (TEA · FSA), and salt of TEA and TFSA (TEA · TFSA) and the like. These salts may be used alone or in combination of two or more.
　第３溶媒のうち、有機溶媒は、一般に難燃性が低く、低い引火点を有する。本実施形態に係る電解液では、非水溶媒がこのような有機溶媒を含む場合でも、フッ化リン酸エステルを多量に含むことで、電解液の難燃性を高めることができる。また、低温特性などの観点からは、有機溶媒を含む非水溶媒を用いることが好ましく、電解液の分解をできるだけ抑制する観点からは、イオン液体を含む非水溶媒を用いることが好ましい。イオン液体および有機溶媒を含む非水溶媒を用いてもよい。 Among the third solvents, organic solvents are generally low in flame retardancy and have a low flash point. In the electrolytic solution according to the present embodiment, even when the non-aqueous solvent contains such an organic solvent, the flame retardancy of the electrolytic solution can be enhanced by containing a large amount of the fluorophosphate ester. Further, from the viewpoint of low temperature characteristics and the like, it is preferable to use a non-aqueous solvent containing an organic solvent, and from the viewpoint of suppressing the decomposition of the electrolytic solution as much as possible, it is preferable to use a non-aqueous solvent containing an ionic liquid. Nonaqueous solvents including ionic liquids and organic solvents may be used.
　電解液は、必要に応じて、ナトリウム塩および非水溶媒に加え、添加剤を含んでもよい。電解液中に占めるナトリウム塩および非水溶媒の合計は、好ましくは７０質量％以上、より好ましくは８０質量％以上、さらに好ましくは９０質量％以上である。電解液中に占めるナトリウムイオンおよび非水溶媒の合計がこのような範囲であることで、フッ化リン酸エステルの含有量を相対的に高めることができ、難燃性および充放電特性の向上効果が得られ易くなる。 The electrolytic solution may contain an additive in addition to the sodium salt and the non-aqueous solvent, if necessary. The total of the sodium salt and the nonaqueous solvent in the electrolytic solution is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. The total amount of sodium ions and non-aqueous solvent in the electrolyte is within such a range, so that the content of fluorophosphate can be relatively increased, and the effect of improving flame retardancy and charge / discharge characteristics Is easily obtained.
２．ナトリウムイオン二次電池
　本発明の実施形態に係るナトリウムイオン二次電池は、正極と、負極と、これらの間に介在するセパレータと、上記の電解液とを含む。
　以下に、電解液以外の電池の構成要素についてより詳細に説明する。 2. Sodium Ion Secondary Battery A sodium ion secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, a separator interposed therebetween, and the electrolytic solution.
Below, it demonstrates in detail about the components of a battery other than electrolyte solution.
（正極）
　正極は、正極活物質を含む。正極は、正極集電体と、正極集電体に担持された正極活物質（または正極合剤）とを含んでもよい。
　正極集電体は、金属箔でもよく、金属多孔体（金属繊維の不織布、金属多孔体シートなど）であってもよい。金属多孔体としては、三次元網目状の骨格（特に、中空の骨格）を有する金属多孔体も使用できる。正極集電体の材料としては、正極電位での安定性の観点から、アルミニウム、アルミニウム合金などが好ましい。 (Positive electrode)
The positive electrode includes a positive electrode active material. The positive electrode may include a positive electrode current collector and a positive electrode active material (or a positive electrode mixture) carried on the positive electrode current collector.
The positive electrode current collector may be a metal foil or a metal porous body (such as a metal fiber non-woven fabric or a metal porous body sheet). As the metal porous body, a metal porous body having a three-dimensional network skeleton (particularly, a hollow skeleton) can also be used. As a material for the positive electrode current collector, aluminum, an aluminum alloy, or the like is preferable from the viewpoint of stability at the positive electrode potential.
　正極活物質としては、ナトリウムイオンを吸蔵および放出（または挿入および脱離）する材料（すなわち、ファラデー反応により容量を発現する材料）が使用できる。このような材料としては、アルカリ金属原子（ナトリウム原子、カリウム原子など）と遷移金属原子（クロム原子、マンガン原子、鉄原子、コバルト原子、ニッケル原子などの周期表の第４周期の遷移金属原子など）とを含む化合物が挙げられる。このような化合物では、当該化合物の結晶構造中に含まれるアルカリ金属原子および遷移金属原子の少なくともいずれか一方の一部が、アルミニウム原子などの典型金属原子で置換されていてもよい。 As the positive electrode active material, a material that occludes and releases (or inserts and desorbs) sodium ions (that is, a material that develops capacity by a Faraday reaction) can be used. Such materials include alkali metal atoms (sodium atoms, potassium atoms, etc.) and transition metal atoms (chromium atoms, manganese atoms, iron atoms, cobalt atoms, nickel atoms, etc., transition metal atoms in the fourth period of the periodic table, etc. ). In such a compound, a part of at least one of an alkali metal atom and a transition metal atom contained in the crystal structure of the compound may be substituted with a typical metal atom such as an aluminum atom.
　正極活物質は、ナトリウム含有遷移金属化合物などの遷移金属化合物を含むことが好ましい。遷移金属化合物としては、ナトリウムイオン二次電池の正極活物質として使用できる公知のもの、例えば、硫化物、酸化物、ナトリウム遷移金属酸素酸塩、ナトリウム含有遷移金属ハロゲン化物などが挙げられる。硫化物としては、例えば、ＴｉＳ２、ＦｅＳ２などの遷移金属硫化物；ＮａＴｉＳ２などのナトリウム含有遷移金属硫化物などが挙げられる。酸化物としては、例えば、亜クロム酸ナトリウム（ＮａＣｒＯ２）、ニッケルマンガン酸ナトリウム（ＮａＮｉ０．５Ｍｎ０．５Ｏ２、Ｎａ２／３Ｔｉ１／６Ｎｉ１／３Ｍｎ１／２Ｏ２など）、鉄コバルト酸ナトリウム（ＮａＦｅ０．５Ｃｏ０．５Ｏ２など）、鉄マンガン酸ナトリウム（Ｎａ２／３Ｆｅ１／３Ｍｎ２／３Ｏ２など）などのナトリウム含有遷移金属酸化物などが挙げられる。ナトリウム含有遷移金属ハロゲン化物としては、例えば、Ｎａ３ＦｅＦ６などが挙げられる。これらのうち、亜クロム酸ナトリウム、鉄マンガン酸ナトリウムなどが好ましい。亜クロム酸ナトリウムの結晶構造中に含まれるクロム原子およびナトリウム原子の少なくとも一方の一部を他の原子で置換してもよい。また、鉄マンガン酸ナトリウムの結晶構造中に含まれる鉄原子、マンガン原子およびナトリウム原子の少なくとも１つの一部が他の原子で置換されていてもよい。 The positive electrode active material preferably contains a transition metal compound such as a sodium-containing transition metal compound. Examples of the transition metal compound include known compounds that can be used as a positive electrode active material for a sodium ion secondary battery, such as sulfides, oxides, sodium transition metal oxyacid salts, and sodium-containing transition metal halides. Examples of the sulfide include transition metal sulfides such as TiS 2 and FeS 2 ; sodium-containing transition metal sulfides such as NaTiS 2 . Examples of the oxide include sodium chromite (NaCrO 2 ), sodium nickel manganate (NaNi 0.5 Mn 0.5 O 2 , Na 2/3 Ti 1/6 Ni 1/3 Mn 1/2 O 2. Etc.), sodium-containing transition metal oxides such as sodium iron cobaltate (such as NaFe 0.5 Co 0.5 O 2 ), sodium iron manganate (such as Na 2/3 Fe 1/3 Mn 2/3 O 2 ) Etc. Examples of the sodium-containing transition metal halide include Na 3 FeF 6 . Of these, sodium chromite and sodium ferromanganate are preferred. A part of at least one of a chromium atom and a sodium atom contained in the crystal structure of sodium chromite may be substituted with another atom. Moreover, at least one part of the iron atom, manganese atom, and sodium atom contained in the crystal structure of sodium iron manganate may be substituted with another atom.
　正極合剤は、正極活物質に加え、導電助剤および／またはバインダ、具体的には、導電助剤、バインダ、導電助剤とバインダとの混合物をさらに含むことができる。正極は、正極集電体に正極合剤を塗布または充填し、乾燥し、必要に応じて、乾燥物を厚み方向に圧縮（または圧延）することにより得られる。正極合剤は、通常、分散媒を含むスラリーの形態で使用される。 In addition to the positive electrode active material, the positive electrode mixture can further include a conductive auxiliary and / or a binder, specifically, a conductive auxiliary, a binder, and a mixture of the conductive auxiliary and the binder. The positive electrode is obtained by applying or filling a positive electrode mixture to a positive electrode current collector, drying, and compressing (or rolling) the dried product in the thickness direction as necessary. The positive electrode mixture is usually used in the form of a slurry containing a dispersion medium.
　導電助剤としては、例えば、カーボンブラック、黒鉛、炭素繊維などが挙げられる。これらの導電助剤は、単独で用いてもよく、２種類以上を混合して用いてもよい。
　バインダとしては、例えば、フッ素樹脂、ポリオレフィン樹脂、ゴム状重合体、ポリアミド樹脂、ポリイミド樹脂（ポリアミドイミドなど）、および／またはセルロースエーテルなどが挙げられる。これらのバインダは、単独で用いてもよく、２種類以上を混合して用いてもよい。
　分散媒としては、例えば、Ｎ－メチル－２－ピロリドン（ＮＭＰ：Ｎ－ｍｅｔｈｙｌ－２－ｐｙｒｒｏｌｉｄｏｎｅ）などの有機溶媒の他、水などが用いられる。 Examples of the conductive assistant include carbon black, graphite, and carbon fiber. These conductive assistants may be used alone or in combination of two or more.
Examples of the binder include a fluorine resin, a polyolefin resin, a rubber-like polymer, a polyamide resin, a polyimide resin (such as polyamideimide), and / or a cellulose ether. These binders may be used alone or in combination of two or more.
As the dispersion medium, for example, water or the like is used in addition to an organic solvent such as N-methyl-2-pyrrolidone (NMP).
（負極）
　負極は、負極活物質を含む。負極は、負極集電体と、負極集電体に担持された負極活物質（または負極合剤）とを含んでもよい。
　負極集電体は、正極集電体と同様に、金属箔または金属多孔体であってもよい。負極集電体の材料としては、ナトリウムと合金化せず、負極電位で安定であることから、銅、銅合金、ニッケル、ニッケル合金、ステンレス鋼などが好ましい。 (Negative electrode)
The negative electrode includes a negative electrode active material. The negative electrode may include a negative electrode current collector and a negative electrode active material (or a negative electrode mixture) carried on the negative electrode current collector.
Similar to the positive electrode current collector, the negative electrode current collector may be a metal foil or a metal porous body. As the material for the negative electrode current collector, copper, copper alloy, nickel, nickel alloy, stainless steel, and the like are preferable because they are not alloyed with sodium and stable at the negative electrode potential.
　負極活物質としては、例えば、ナトリウムイオンを可逆的に吸蔵および放出（もしくは挿入および脱離）する材料、ナトリウムと合金化する材料などが挙げられる。いずれの材料も、ファラデー反応により容量を発現する材料である。 Examples of the negative electrode active material include materials that reversibly occlude and release (or insert and desorb) sodium ions, and materials that alloy with sodium. Any of these materials is a material that develops capacity by a Faraday reaction.
　このような負極活物質としては、ナトリウム、チタン、亜鉛、インジウム、スズ、ケイ素などの金属もしくは半金属；前記金属もしくは半金属から得られる合金；前記金属もしくは半金属の化合物；炭素質材料などが例示できる。なお、合金は、前記金属および半金属以外に、さらに他のアルカリ金属、アルカリ土類金属などを含んでもよい。金属またはその合金、もしくはその化合物；および炭素質材料などが例示できる。なお、合金は、これらの金属および半金属以外に、他のアルカリ金属、アルカリ土類金属などをさらに含んでもよい。 Examples of the negative electrode active material include metals or metalloids such as sodium, titanium, zinc, indium, tin, and silicon; alloys obtained from the metal or metalloid; compounds of the metal or metalloid; carbonaceous materials; It can be illustrated. The alloy may further contain other alkali metal, alkaline earth metal, etc. in addition to the metal and metalloid. Examples thereof include metals or alloys thereof, or compounds thereof; and carbonaceous materials. The alloy may further contain other alkali metals, alkaline earth metals and the like in addition to these metals and metalloids.
　金属化合物としては、チタン酸リチウム（Ｌｉ２Ｔｉ３Ｏ７および／またはＬｉ４Ｔｉ５Ｏ１２など）などのリチウム含有チタン酸化物、およびチタン酸ナトリウム（Ｎａ２Ｔｉ３Ｏ７および／またはＮａ４Ｔｉ５Ｏ１２など）などのナトリウム含有チタン酸化物が例示できる。リチウム含有チタン酸化物の結晶構造においては、当該結晶構造に含まれるチタン原子の一部およびリチウム原子の一部の少なくとも一方が他の原子で置換されていてもよい。また、ナトリウム含有チタン酸化物の結晶構造において、当該結晶構造に含まれるチタン原子およびリチウム原子の少なくとも一方の一部が他の原子で置換されていてもよい。 Examples of the metal compound include lithium-containing titanium oxides such as lithium titanate (such as Li 2 Ti 3 O 7 and / or Li 4 Ti 5 O 12 ), and sodium titanate (Na 2 Ti 3 O 7 and / or Na 4). Examples thereof include sodium-containing titanium oxides such as Ti 5 O 12 . In the crystal structure of the lithium-containing titanium oxide, at least one of a part of the titanium atoms and a part of the lithium atoms included in the crystal structure may be substituted with another atom. In the crystal structure of the sodium-containing titanium oxide, at least one of a titanium atom and a lithium atom included in the crystal structure may be substituted with another atom.
　炭素質材料としては、易黒鉛化性炭素（ソフトカーボン）、難黒鉛化性炭素（ハードカーボン）などが例示できる。これらの炭素質材料は、単独で用いてもよく、２種類以上を混合して用いてもよい。
　負極活物質は、一種を単独でまたは二種以上を組み合わせて使用できる。
　これらの材料のうち、前記金属もしくは半金属の化合物（ナトリウム含有チタン酸化物など）、炭素質材料（ハードカーボンなど）などが好ましい。 Examples of the carbonaceous material include graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon). These carbonaceous materials may be used alone or in combination of two or more.
A negative electrode active material can be used individually by 1 type or in combination of 2 or more types.
Of these materials, the metal or metalloid compounds (such as sodium-containing titanium oxide) and carbonaceous materials (such as hard carbon) are preferable.
　負極は、例えば、正極の場合に準じて、負極集電体に、負極活物質を含む負極合剤を塗布または充填し、乾燥し、乾燥物を厚み方向に圧縮（または圧延）することにより形成できる。また、負極としては、負極集電体の表面に、蒸着、スパッタリングなどの気相法で負極活物質の堆積膜を形成することにより得られるものを用いてもよい。負極活物質には、必要に応じて、ナトリウムイオンをプレドープしてもよい。 The negative electrode is formed by, for example, applying or filling a negative electrode mixture containing a negative electrode active material to a negative electrode current collector according to the case of the positive electrode, drying, and compressing (or rolling) the dried product in the thickness direction. it can. Moreover, as a negative electrode, you may use what is obtained by forming the deposit film of a negative electrode active material on the surface of a negative electrode collector by vapor phase methods, such as vapor deposition and sputtering. The negative electrode active material may be pre-doped with sodium ions as necessary.
　負極合剤は、負極活物質に加え、導電助剤および／またはバインダ、具体的には、導電助剤、バインダ、または導電助剤とバインダとの混合物をさらに含むことができる。負極合剤は、通常、分散媒を含むスラリーの形態で使用される。導電助剤、バインダ、および分散媒としては、それぞれ、正極について例示したものから適宜選択できる。 In addition to the negative electrode active material, the negative electrode mixture can further include a conductive auxiliary and / or a binder, specifically, a conductive auxiliary, a binder, or a mixture of a conductive auxiliary and a binder. The negative electrode mixture is usually used in the form of a slurry containing a dispersion medium. As a conductive support agent, a binder, and a dispersion medium, it can respectively select from what was illustrated about the positive electrode suitably.
　（セパレータ）
　セパレータとしては、例えば、合成樹脂製の微多孔膜、不織布などが使用できる。
　セパレータの材料は、電池の使用温度を考慮して選択できる。微多孔膜を構成する合成樹脂としては、例えば、ポリオレフィン樹脂、ポリフェニレンサルファイド樹脂、ポリアミド樹脂（芳香族ポリアミド樹脂など）、ポリイミド樹脂などが例示できる。また、不織布を形成する繊維が合成樹脂によって構成されている場合、当該樹脂としては、微多孔膜を構成する合成樹脂と同様の合成樹脂が挙げられる。不織布を形成する繊維は、ガラス繊維などの無機繊維であってもよい。セパレータは、セラミックス粒子などの無機フィラーを含んでもよい。 (Separator)
As the separator, for example, a synthetic resin microporous film, a nonwoven fabric, or the like can be used.
The material of the separator can be selected in consideration of the operating temperature of the battery. Examples of the synthetic resin constituting the microporous film include polyolefin resin, polyphenylene sulfide resin, polyamide resin (such as aromatic polyamide resin), polyimide resin, and the like. Moreover, when the fiber which forms a nonwoven fabric is comprised with the synthetic resin, the said synthetic resin similar to the synthetic resin which comprises a microporous film is mentioned as the said resin. The fibers forming the nonwoven fabric may be inorganic fibers such as glass fibers. The separator may include an inorganic filler such as ceramic particles.
（ナトリウムイオン二次電池の形状）
　ナトリウムイオン二次電池の形状としては、角型、円筒型、ラミネート型、コイン型、ボタン型などが挙げられる。 (Shape of sodium ion secondary battery)
Examples of the shape of the sodium ion secondary battery include a square shape, a cylindrical shape, a laminate shape, a coin shape, and a button shape.
（ナトリウムイオン二次電池の製造方法）
　ナトリウムイオン二次電池は、例えば、（ａ）正極と、負極と、正極および負極の間に介在するセパレータとで電極群を形成する工程、ならびに（ｂ）電極群および電解液を電池ケース内に収容する工程を経ることにより製造できる。ナトリウムイオン二次電池がコイン型またはボタン型の電池である場合、コイン型またはボタン型の電池は、例えば、以下の手順で形成してもよい。まず、電池ケース内に、正極および負極のいずれか一方の電極を載置する。つぎに、載置された電極にセパレータを被せる。つぎに、電池ケース内に電解液を注液する。つぎに、他方の電極を電池ケース内に載置する。その後、電池ケースを密閉する。 (Method for manufacturing sodium ion secondary battery)
The sodium ion secondary battery includes, for example, (a) a step of forming an electrode group with a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and (b) an electrode group and an electrolyte solution in the battery case. It can manufacture by passing through the process of accommodating. When the sodium ion secondary battery is a coin-type or button-type battery, the coin-type or button-type battery may be formed by the following procedure, for example. First, either the positive electrode or the negative electrode is placed in the battery case. Next, a separator is put on the placed electrode. Next, an electrolytic solution is poured into the battery case. Next, the other electrode is placed in the battery case. Thereafter, the battery case is sealed.
　図１は、本発明の一実施形態に係るナトリウムイオン二次電池を概略的に示す縦断面図である。ナトリウムイオン二次電池は、積層型の電極群、電解液（図示せず）およびこれらを収容する角型のアルミニウム製の電池ケース１０を具備する。電池ケース１０は、上部が開口した有底の容器本体１２と、上部開口を塞ぐ蓋体１３とで構成されている。 FIG. 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention. The sodium ion secondary battery includes a stacked electrode group, an electrolytic solution (not shown), and a rectangular aluminum battery case 10 that houses them. The battery case 10 includes a bottomed container body 12 having an upper opening and a lid 13 that closes the upper opening.
　ナトリウムイオン二次電池を組み立てる際には、まず、正極２と負極３とをこれらの間にセパレータ１を介在させた状態で積層することにより電極群が構成され、構成された電極群が電池ケース１０の容器本体１２に挿入される。その後、容器本体１２に電解液を注液し、電極群を構成するセパレータ１、正極２および負極３の空隙に電解液を含浸させる工程が行われる。 When assembling a sodium ion secondary battery, first, an electrode group is configured by laminating the positive electrode 2 and the negative electrode 3 with the separator 1 interposed therebetween, and the configured electrode group is a battery case. 10 container bodies 12 are inserted. Thereafter, a step of injecting an electrolytic solution into the container body 12 and impregnating the electrolytic solution into the gaps of the separator 1, the positive electrode 2, and the negative electrode 3 constituting the electrode group is performed.
　蓋体１３の中央には、電池ケース１０の内圧が上昇したときに内部で発生したガスを放出するための安全弁１６が設けられている。安全弁１６を中央にして、蓋体１３の一方側寄りには、蓋体１３を貫通する外部正極端子１４が設けられ、蓋体１３の他方側寄りの位置には、蓋体１３を貫通する外部負極端子が設けられる。 In the center of the lid 13, a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the battery case 10 rises. An external positive terminal 14 that penetrates the lid 13 is provided near the one side of the lid 13 with the safety valve 16 in the center, and an external that penetrates the lid 13 is located near the other side of the lid 13. A negative terminal is provided.
　積層型の電極群は、いずれも矩形のシート状である、複数の正極２と複数の負極３およびこれらの間に介在する複数のセパレータ１により構成されている。図１では、セパレータ１は、正極２を包囲するように袋状に形成されているが、セパレータの形態は特に限定されない。複数の正極２と複数の負極３は、電極群内で積層方向に交互に配置される。 The stacked electrode group is composed of a plurality of positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed therebetween, all in the form of a rectangular sheet. In FIG. 1, the separator 1 is formed in a bag shape so as to surround the positive electrode 2, but the form of the separator is not particularly limited. The plurality of positive electrodes 2 and the plurality of negative electrodes 3 are alternately arranged in the stacking direction within the electrode group.
　各正極２の一端部には、正極リード片２ａを形成してもよい。複数の正極２の正極リード片２ａを束ねるとともに、電池ケース１０の蓋体１３に設けられた外部正極端子１４に接続することにより、複数の正極２が並列に接続される。同様に、各負極３の一端部には、負極リード片３ａを形成してもよい。複数の負極３の負極リード片３ａを束ねるとともに、電池ケース１０の蓋体１３に設けられた外部負極端子に接続することにより、複数の負極３が並列に接続される。正極リード片２ａの束と負極リード片３ａの束は、互いの接触を避けるように、電極群の一端面の左右に、間隔を空けて配置することが望ましい。 A positive electrode lead piece 2 a may be formed at one end of each positive electrode 2. The plurality of positive electrodes 2 are connected in parallel by bundling the positive electrode lead pieces 2 a of the plurality of positive electrodes 2 and connecting them to the external positive terminal 14 provided on the lid 13 of the battery case 10. Similarly, a negative electrode lead piece 3 a may be formed at one end of each negative electrode 3. The plurality of negative electrodes 3 are connected in parallel by bundling the negative electrode lead pieces 3 a of the plurality of negative electrodes 3 and connecting them to the external negative terminal provided on the lid 13 of the battery case 10. The bundle of the positive electrode lead pieces 2a and the bundle of the negative electrode lead pieces 3a are desirably arranged on the left and right sides of one end face of the electrode group with an interval so as to avoid mutual contact.
　外部正極端子１４および外部負極端子は、いずれも柱状であり、少なくとも外部に露出する部分が螺子溝を有する。各端子の螺子溝にはナット７が嵌められ、ナット７を回転することにより蓋体１３に対してナット７が固定される。各端子の電池ケース１０内部に収容される部分には、鍔部８が設けられており、ナット７の回転により、鍔部８が、蓋体１３の内面に、Ｏ－リング状のガスケット９を介して固定される。 The external positive electrode terminal 14 and the external negative electrode terminal are both columnar, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A flange 8 is provided in a portion of each terminal accommodated in the battery case 10, and by rotation of the nut 7, the flange 8 attaches an O-ring-shaped gasket 9 to the inner surface of the lid 13. Fixed through.
　電極群は、積層タイプに限らず、正極と負極とをセパレータを介して捲回することにより形成したものであってもよい。負極に金属ナトリウムが析出するのを防止する観点から、正極よりも負極の寸法を大きくしてもよい。 The electrode group is not limited to a laminated type, and may be formed by winding a positive electrode and a negative electrode through a separator. From the viewpoint of preventing metallic sodium from being deposited on the negative electrode, the size of the negative electrode may be made larger than that of the positive electrode.
　なお、円筒型およびラミネート型のナトリウム二次電池も、前記と同様の手法に準じて適宜製造することができる。 It should be noted that cylindrical and laminated sodium secondary batteries can also be appropriately manufactured according to the same method as described above.
　以下、本発明を実施例および比較例に基づいて具体的に説明するが、本発明は以下の実施例に限定されるものではない。 Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.
実施例１
（１）正極の作製
　ＮａＣｒＯ２（正極活物質）とアセチレンブラック（導電助剤）とポリフッ化ビニリデン（結着剤）とを正極活物質／導電助剤／バインダ（質量比）が９０／５／５となるようにＮＭＰに分散させて、正極合剤ペーストを調製した。得られた正極合剤ペーストを、アルミニウム箔（縦１０ｃｍ×横１０ｃｍ、厚さ２０μｍ）の両面に塗布し、十分に乾燥させ、圧延して、両面に厚さ６０μｍの正極合剤層を有する総厚１４０μｍの正極を１００枚作製した。なお、正極の一辺の一方側端部には、集電用のリード片を形成した。 Example 1
(1) Production of positive electrode NaCrO 2 (positive electrode active material), acetylene black (conductive aid) and polyvinylidene fluoride (binder) are positive electrode active material / conductive aid / binder (mass ratio) of 90/5 / A positive electrode mixture paste was prepared by dispersing in NMP so as to be 5. The obtained positive electrode mixture paste was applied to both sides of an aluminum foil (length 10 cm × width 10 cm, thickness 20 μm), sufficiently dried, rolled, and a positive electrode mixture layer having a thickness of 60 μm on both sides. 100 positive electrodes having a thickness of 140 μm were produced. In addition, the lead piece for current collection was formed in the one side edge part of the one side of a positive electrode.
（２）負極の作製
　ハードカーボン（負極活物質）とポリアミドイミド（バインダ）とを負極活物質／バインダ（質量比）が９５／５となるようにＮＭＰに分散させて、負極合剤ペーストを調製した。得られた負極合剤ペーストを、負極集電体としての銅箔（縦１０ｃｍ×横１０ｃｍ、厚さ２０μｍ）の両面に塗布し、十分に乾燥させ、圧延して、両面に厚さ６５μｍの負極合剤層を有する総厚１５０μｍの負極（または負極前駆体）を９９枚作製した。また、負極集電体の片面のみに負極合剤層を形成する以外は、上記と同様にして、２枚の負極（または負極前駆体）を作製した。負極の一辺の一方側端部には、集電用のリード片を形成した。 (2) Production of negative electrode Hard carbon (negative electrode active material) and polyamideimide (binder) are dispersed in NMP so that the negative electrode active material / binder (mass ratio) is 95/5 to prepare a negative electrode mixture paste. did. The obtained negative electrode mixture paste was applied to both sides of a copper foil (vertical 10 cm × width 10 cm, thickness 20 μm) as a negative electrode current collector, sufficiently dried, rolled, and negative electrode having a thickness of 65 μm on both sides. 99 negative electrodes (or negative electrode precursors) having a mixture layer and a total thickness of 150 μm were prepared. Further, two negative electrodes (or negative electrode precursors) were produced in the same manner as described above except that the negative electrode mixture layer was formed only on one surface of the negative electrode current collector. A lead piece for current collection was formed on one end of one side of the negative electrode.
（３）電極群の組み立て
　正極と、負極との間に、セパレータを介在させて積層することにより、電極群を作製した。このとき、電極群の一方の端部には、片面のみに負極合剤層を有する負極を、その負極合剤層が正極と対向するように配置した。また、電極群の他方の端部には、片面のみに負極合剤層を有する負極を、その負極合剤層が正極と対向するように配置した。セパレータとしては、袋状の微多孔膜（ポリオレフィン製、厚さ５０μｍ）を用い、内部に正極を収容した状態で、負極と積層した。 (3) Assembly of electrode group The electrode group was produced by laminating | stacking a separator between the positive electrode and the negative electrode. At this time, a negative electrode having a negative electrode mixture layer only on one surface was disposed at one end of the electrode group so that the negative electrode mixture layer was opposed to the positive electrode. Moreover, the negative electrode which has a negative mix layer only on one side was arrange | positioned in the other edge part of an electrode group so that the negative mix layer might oppose a positive electrode. As the separator, a bag-like microporous membrane (made of polyolefin, thickness: 50 μm) was used, and laminated with the negative electrode in a state where the positive electrode was accommodated inside.
（４）電解液の調製
　ＴＦＥＰ（第１溶媒）とＴＭＰ（第２溶媒）とを含む非水溶媒〔第１溶媒／第２溶媒（質量比）＝７０／３０〕に、ＮａＦＳＡを溶解させて電解液を調製した。このとき、電解液中のＮａＦＳＡの濃度は１ｍｏｌ／Ｌとした。 (4) Preparation of electrolyte solution NaFSA was dissolved in a nonaqueous solvent [first solvent / second solvent (mass ratio) = 70/30] containing TFEP (first solvent) and TMP (second solvent). An electrolyte solution was prepared. At this time, the concentration of NaFSA in the electrolyte was 1 mol / L.
（５）ナトリウムイオン二次電池の組み立て
　上記（３）で得られた電極群と、上記（４）で得られた電解液とを、アルミニウム製の容器本体に収容した。電極群の正極に接続されたリードを、アルミニウム製の蓋体に設けられた外部正極端子に接続し、負極に接続されたリードを蓋体に設けられた外部負極端子に接続した。次いで、容器本体の開口部を、蓋体で密閉して、公称容量２６Ａｈの図１に示すナトリウムイオン二次電池を完成させた。 (5) Assembly of sodium ion secondary battery The electrode group obtained in the above (3) and the electrolytic solution obtained in the above (4) were accommodated in an aluminum container body. The lead connected to the positive electrode of the electrode group was connected to the external positive terminal provided on the aluminum lid, and the lead connected to the negative electrode was connected to the external negative terminal provided on the lid. Next, the opening of the container main body was sealed with a lid to complete the sodium ion secondary battery shown in FIG. 1 having a nominal capacity of 26 Ah.
（６）評価
　上記（４）で得られた電解液、および上記（５）で得られたナトリウムイオン二次電池を用いて、下記の評価を行った。
　（ａ）電解液の引火点
　ＪＩＳ　Ｋ　２２６５－２に準拠し、セタ密閉式引火点測定器を用いて、電解液の引火点を測定した。 (6) Evaluation The following evaluation was performed using the electrolytic solution obtained in (4) above and the sodium ion secondary battery obtained in (5) above.
(A) Flash point of electrolyte solution Based on JIS K 2265-2, the flash point of the electrolyte solution was measured using a setter hermetic flash point measuring device.
　（ｂ）サイクル特性
　ナトリウムイオン二次電池を、２５℃の温度で、時間率０．５Ｃレートの電流値で、３．４Ｖになるまで充電し、時間率０．５Ｃレートの電流値で、１．５Ｖになるまで放電し、このときの放電容量（初期放電容量）を測定した。上記と同様の条件での充電および放電のサイクルを繰り返し、２００サイクル目の放電容量を測定し、初期放電容量を１００％としたときの比率（容量維持率）を算出した。 (B) Cycle characteristics A sodium ion secondary battery was charged at a temperature value of 25C at a current value of a rate of 0.5C rate until reaching 3.4V, and a current value of a rate of 0.5C rate was 1 It discharged until it became 0.5V, and the discharge capacity (initial discharge capacity) at this time was measured. The cycle of charging and discharging under the same conditions as described above was repeated, the discharge capacity at the 200th cycle was measured, and the ratio (capacity maintenance ratio) when the initial discharge capacity was 100% was calculated.
　（ｃ）レート特性（低温レート特性）
　ナトリウムイオン二次電池を、４０℃の温度で、時間率０．１Ｃレートの電流値で、３．４Ｖになるまで充電し、時間率０．１Ｃレートの電流値で、１．５Ｖになるまで放電し、このときの放電容量ＣＨを測定した。
　ナトリウムイオン二次電池を、４０℃の温度で、時間率０．１Ｃレートの電流値で、３．４Ｖになるまで充電し、－１０℃の温度で、時間率０．１Ｃレートの電流値で、１．５Ｖになるまで放電した。このときの放電容量ＣＬを求め、放電容量ＣＨに対するＣＬの比率（％）を算出して、レート特性の指標とした。 (C) Rate characteristics (low temperature rate characteristics)
The sodium ion secondary battery is charged at a temperature of 40 ° C. at a current value of 0.1C rate of time until it reaches 3.4V, and at a current value of 0.1C rate of time until it reaches 1.5V. discharged, the discharge capacity was measured C H at this time.
The sodium ion secondary battery was charged at a temperature of 40 ° C. at a current rate of 0.1C rate to 3.4 V, and at a temperature of −10 ° C. at a current rate of 0.1C rate. The battery was discharged until 1.5V was reached. Calculated discharge capacity C L at this time, and calculates the ratio of C L to the discharge capacity C H (%), was used as an index of the rate characteristic.
実施例２
　ＴＭＰに代えて、ＴＥＰを用いる以外は実施例１と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。 Example 2
An electrolyte solution was prepared in the same manner as in Example 1 except that TEP was used instead of TMP. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
比較例１
　ＮａＣｒＯ２に代えてＬｉＣｏＯ２を用いる以外は、実施例１と同様にして正極を作製した。
　ＮａＦＳＡに代えて、ＬｉＦＳＡ（リチウムビス（フルオロスルホニル）アミド）を用いる以外は実施例１と同様に電解液を調製した。電解液の引火点を実施例１と同様にして評価した。 Comparative Example 1
A positive electrode was produced in the same manner as in Example 1 except that LiCoO 2 was used instead of NaCrO 2 .
An electrolyte solution was prepared in the same manner as in Example 1 except that LiFSA (lithium bis (fluorosulfonyl) amide) was used instead of NaFSA. The flash point of the electrolyte was evaluated in the same manner as in Example 1.
　得られた正極を用いる以外は、実施例１と同様にして電極群を作製し、この電極群と上記の電解液とを用いる以外は、実施例１と同様にして二次電池を作製した。実施例１に準じて、サイクル特性およびレート特性の評価を行った。このとき、充電終止電圧および放電終止電圧は、それぞれ４．２Ｖおよび３．０Ｖとした。比較例１で得られた二次電池は、リチウムイオン二次電池である。 An electrode group was produced in the same manner as in Example 1 except that the obtained positive electrode was used, and a secondary battery was produced in the same manner as in Example 1 except that this electrode group and the above electrolytic solution were used. According to Example 1, cycle characteristics and rate characteristics were evaluated. At this time, the charge end voltage and the discharge end voltage were 4.2 V and 3.0 V, respectively. The secondary battery obtained in Comparative Example 1 is a lithium ion secondary battery.
参考例１
　ＴＦＥＰを用いない以外は実施例１と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。 Reference example 1
An electrolyte solution was prepared in the same manner as in Example 1 except that TFEP was not used. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
参考例２
　ＴＭＰに代えて、ＥＣおよびＤＥＣを含む混合溶媒（ＥＣ：ＤＥＣ（体積比）＝１：１）を用いる以外は、実施例１と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。
　実施例１～２、比較例１、および参考例１～２の結果を表１に示す。実施例１～２はＡ１～Ａ２であり、比較例１はＢ１であり、参考例１～２はＣ１～Ｃ２である。 Reference example 2
An electrolytic solution was prepared in the same manner as in Example 1 except that a mixed solvent containing EC and DEC (EC: DEC (volume ratio) = 1: 1) was used instead of TMP. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
Table 1 shows the results of Examples 1 and 2, Comparative Example 1, and Reference Examples 1 and 2. Examples 1 and 2 are A1 and A2, Comparative Example 1 is B1, and Reference Examples 1 and 2 are C1 and C2.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　表１に示されるように、比較例のリチウムイオン二次電池Ｂ１では、充放電を行うことができず、サイクル特性およびレート特性ともに評価できなかった。これに対し、実施例では、電解液の非水溶媒は比較例と同じで、フッ化リン酸エステルを多量に含むにもかかわらず、充放電を行うことができた。実施例では、７０％以上の高いレート特性と、９０％以上の高いサイクル特性が得られた。また、実施例では、電解液の引火点がないかまたは高く、難燃性に優れていた。 As shown in Table 1, the lithium ion secondary battery B1 of the comparative example could not be charged / discharged, and neither the cycle characteristics nor the rate characteristics could be evaluated. On the other hand, in the examples, the non-aqueous solvent of the electrolytic solution was the same as that of the comparative example, and charging / discharging could be performed despite containing a large amount of phosphoric acid phosphate. In the example, a high rate characteristic of 70% or more and a high cycle characteristic of 90% or more were obtained. Further, in the examples, the flash point of the electrolyte was not high or high, and the flame retardancy was excellent.
　リン酸エステルのみを非水溶媒として用いた参考例の電池Ｃ１でも、充放電が可能である。電池Ｃ１では、レート特性は高いものの、サイクル特性は３１％と低い。
　リン酸エステルに代えてＥＣ／ＤＥＣを用いた参考例の電池Ｃ２では、レート特性は、対応する実施例の電池とほぼ同等の高い値が得られる。しかし、電池Ｃ２のサイクル特性は、実施例と比べると２０％ほど低くなった。電池Ｃ２よりも実施例で高いサイクル特性が得られたのは、実施例では電池Ｃ２に比べてより安定なＳＥＩ被膜が形成されたことによるものと考えられる。 The battery C1 of the reference example using only the phosphate ester as the non-aqueous solvent can be charged and discharged. In the battery C1, although the rate characteristic is high, the cycle characteristic is as low as 31%.
In the battery C2 of the reference example using EC / DEC instead of the phosphate ester, the rate characteristic has a high value almost equal to that of the battery of the corresponding example. However, the cycle characteristics of the battery C2 were about 20% lower than in the example. The higher cycle characteristics obtained in the example than in the battery C2 are considered to be due to the formation of a more stable SEI film in the example than in the battery C2.
実施例３～７
　非水溶媒におけるＴＦＥＰとＴＭＰとの質量比を表２に示すように変更し、必要に応じて表２に示す第３溶媒を用いる以外は、実施例１と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。 Examples 3-7
An electrolyte solution was prepared in the same manner as in Example 1 except that the mass ratio of TFEP to TMP in the nonaqueous solvent was changed as shown in Table 2, and the third solvent shown in Table 2 was used as necessary. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
実施例８～９
　非水溶媒におけるＴＦＥＰとＴＥＰとの質量比を表２に示すように変更する以外は、実施例２と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。
　実施例３～９の結果を表２に示す。表２には実施例１および２の結果も合わせて示した。実施例３～９は、Ａ３～Ａ９である。 Examples 8-9
An electrolyte solution was prepared in the same manner as in Example 2 except that the mass ratio of TFEP and TEP in the nonaqueous solvent was changed as shown in Table 2. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
The results of Examples 3 to 9 are shown in Table 2. Table 2 also shows the results of Examples 1 and 2. Examples 3 to 9 are A3 to A9.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表２に示されるように、実施例３～６でも、実施例１または実施例２と同様に、高いサイクル特性およびレート特性が得られた。 As shown in Table 2, also in Examples 3 to 6, high cycle characteristics and rate characteristics were obtained as in Example 1 or Example 2.
実施例１０～１１
　ＴＦＥＰに代えて、表３に示すフッ化リン酸エステルを用いる以外は実施例１と同様に電解液を調製した。得られた電解液を用いる以外は、実施例１と同様に、ナトリウムイオン二次電池を作製し、評価を行った。
　実施例１０～１１の結果を表３に示す。表３には実施例１の結果も合わせて示した。実施例１０～１１はＡ１０～Ａ１１である。 Examples 10-11
An electrolytic solution was prepared in the same manner as in Example 1 except that fluorinated phosphate ester shown in Table 3 was used instead of TFEP. A sodium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the obtained electrolytic solution was used.
The results of Examples 10 to 11 are shown in Table 3. Table 3 also shows the results of Example 1. Examples 10 to 11 are A10 to A11.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　実施例のナトリウムイオン二次電池Ａ１０およびＡ１１でも、実施例の電池Ａ１に匹敵するサイクル特性が得られた。電池Ａ１０およびＡ１１のレート特性は、電池Ａ１に比べて大幅に向上した。 Also in the sodium ion secondary batteries A10 and A11 of the example, cycle characteristics comparable to the battery A1 of the example were obtained. The rate characteristics of the batteries A10 and A11 were greatly improved compared to the battery A1.
実施例１２～１４および参考例３（高電圧充電時の充放電挙動）
　ＮａＣｒＯ２に代えて、ＮａＮｉ１／３Ｔｉ１／６Ｍｎ１／２Ｏ２を正極活物質として用いる以外は実施例１と同様にして正極合剤ペーストを調製した。正極合剤ペーストを、厚さ２０μｍのアルミニウム箔の片面に塗布し、十分に乾燥させ、厚み方向に圧縮して、厚さ６０μｍの正極を作製した。正極は、直径１２ｍｍのコイン型に打ち抜いた。
　金属ナトリウムディスク（アルドリッチ社製、厚さ２００μｍ）をニッケル集電体に圧着して、総厚７００μｍの負極を作製した。負極は、直径１２ｍｍのコイン型に打ち抜いた。 Examples 12 to 14 and Reference Example 3 (Charging / Discharging Behavior during High Voltage Charging)
A positive electrode mixture paste was prepared in the same manner as in Example 1 except that NaNi 1/3 Ti 1/6 Mn 1/2 O 2 was used as the positive electrode active material instead of NaCrO 2 . The positive electrode mixture paste was applied to one side of an aluminum foil having a thickness of 20 μm, sufficiently dried, and compressed in the thickness direction to produce a positive electrode having a thickness of 60 μm. The positive electrode was punched into a coin shape having a diameter of 12 mm.
A metal sodium disk (Aldrich, thickness: 200 μm) was pressure-bonded to a nickel current collector to produce a negative electrode having a total thickness of 700 μm. The negative electrode was punched into a coin shape having a diameter of 12 mm.
　コイン型の正極、負極およびセパレータを十分に乾燥させた。浅底の円筒型のＡｌ／ＳＵＳクラッド製容器に、コイン型の負極を載置し、その上にコイン型のセパレータを介してコイン型の正極を載置し、所定量の電解液を容器内に注液した。次いで、周縁に絶縁ガスケットを具備する浅底の円筒型のＡｌ／ＳＵＳクラッド製封口板で、容器の開口を封口した。これにより、容器底面と封口板との間で、負極、セパレータおよび正極からなる電極群に圧力を印加し、部材間の接触を確保した。こうして、設計容量１．５ｍＡｈのコイン型のナトリウムイオン二次電池（ハーフセル）を作製した。 The coin-type positive electrode, negative electrode and separator were sufficiently dried. A coin-type negative electrode is placed on a shallow cylindrical Al / SUS clad container, and a coin-type positive electrode is placed on it via a coin-type separator. The liquid was injected. Subsequently, the opening of the container was sealed with a shallow cylindrical Al / SUS clad sealing plate having an insulating gasket on the periphery. Thereby, a pressure was applied to the electrode group consisting of the negative electrode, the separator, and the positive electrode between the bottom surface of the container and the sealing plate to ensure contact between the members. Thus, a coin-type sodium ion secondary battery (half cell) having a design capacity of 1.5 mAh was produced.
　セパレータとしては、ガラスマイクロファイバー（ワットマン社製、グレードＧＦ／Ａ、厚さ２６０μｍ）製のセパレータを用いた。
　電解液としては、下記の電解液ａ１、ａ２、ａ３またはｂを用いた。電解液中のＮａＦＳＡの濃度は１ｍｏｌ／Ｌとした。
　電解液ａ１：ＴＦＥＥＰとＴＥＰとを含む非水溶媒〔ＴＦＥＥＰ／ＴＥＰ（質量比）＝７０／３０〕に、ＮａＦＳＡを溶解させて調製した。
　電解液ａ２：ＴＦＥＥＰのみからなる非水溶媒に、ＮａＦＳＡを溶解させて調製した。
　電解液ａ３：ＴＦＥＰとＴＥＰとを含む非水溶媒〔ＴＦＥＰ／ＴＥＰ（質量比）＝７０／３０〕に、ＮａＦＳＡを溶解させて調製した。
　電解液ｂ：ＴＦＥＰとＰＣとを含む非水溶媒〔ＴＦＥＰ／ＰＣ（質量比）＝７０／３０〕に、ＮａＦＳＡを溶解させて調製した。
　電解液ａ１、ａ２およびａ３のそれぞれを用いた実施例の電池を電池Ａ１２、Ａ１３およびＡ１４とし、電解液ｂを用いた参考例の電池を電池Ｃ３とする。 As the separator, a separator made of glass microfiber (manufactured by Whatman, grade GF / A, thickness 260 μm) was used.
As the electrolytic solution, the following electrolytic solution a1, a2, a3 or b was used. The concentration of NaFSA in the electrolytic solution was 1 mol / L.
Electrolytic solution a1: Prepared by dissolving NaFSA in a non-aqueous solvent [TFEEP / TEP (mass ratio) = 70/30] containing TFEEP and TEP.
Electrolytic solution a2: Prepared by dissolving NaFSA in a non-aqueous solvent consisting only of TFEEP.
Electrolytic solution a3: Prepared by dissolving NaFSA in a non-aqueous solvent [TFEP / TEP (mass ratio) = 70/30] containing TFEP and TEP.
Electrolyte solution b: prepared by dissolving NaFSA in a non-aqueous solvent [TFEP / PC (mass ratio) = 70/30] containing TFEP and PC.
The batteries of the examples using the electrolytic solutions a1, a2, and a3 are designated as batteries A12, A13, and A14, and the battery of the reference example using the electrolytic solution b is designated as the battery C3.
　電池を、２５℃の温度で、時間率０．１Ｃレートの電流値で、４．４Ｖになるまで充電し、時間率０．２Ｃレートの電流値で２．４Ｖになるまで放電した。このとき、正極活物質の単位質量当たりの容量（ｍＡｈ／ｇ）を測定した。充放電時の容量変化を図２に示す。 The battery was charged at a temperature value of 25 ° C. at a current value of a rate of 0.1 C rate to 4.4 V and discharged at a current value of a rate of 0.2 C rate to 2.4 V. At this time, the capacity per unit mass (mAh / g) of the positive electrode active material was measured. The capacity change at the time of charging / discharging is shown in FIG.
　図２に示されるように、電池Ａ１２～Ａ１４では、正極電位が４Ｖを超える高電位まで充電するにも拘わらず、安定して充放電を行うことができた。一方、電池Ｃ３では、４．２Ｖ以上に充電した場合に充電カーブに乱れが生じ、十分な放電容量が得られなかった。
充放電後の電池分解して正極箔を、走査型電子顕微鏡（ＳＥＭ：ｓｃａｎｎｉｎｇ　ｅｌｅｃｔｒｏｎ　ｍｉｃｒｏｓｃｏｐｅ）で確認したところ、電池Ｃ３では腐食が見られた。また、充放電後の電池Ｃ３の電解液を取り出して、誘導プラズマ結合（ＩＣＰ：Ｉｎｄｕｃｔｉｖｅｌｙ　ｃｏｕｐｌｅｄ　ｐｌａｓｍａ）分析を行ったところ、Ａｌが検出された。 As shown in FIG. 2, in the batteries A12 to A14, charging / discharging could be performed stably even though the positive electrode potential was charged to a high potential exceeding 4V. On the other hand, when the battery C3 was charged to 4.2 V or more, the charging curve was disturbed and a sufficient discharge capacity could not be obtained.
When the battery after charging / discharging was disassembled and the positive electrode foil was confirmed with a scanning electron microscope (SEM), corrosion was observed in battery C3. Moreover, when the electrolytic solution of the battery C3 after charging / discharging was taken out and subjected to induction plasma coupled (ICP) analysis, Al was detected.
　このような結果から、正極電位が４．２Ｖ以上となる高電位まで充電する場合には、アルミニウムの腐食を抑制し、容量の低下を抑制する観点から、電解液において、フッ化リン酸エステルの含有量を多くすること、および／または電解液における溶媒としてリン酸エステルを用いることが有効であることが分かる。 From these results, in the case of charging to a high potential where the positive electrode potential is 4.2 V or higher, from the viewpoint of suppressing the corrosion of aluminum and suppressing the decrease in capacity, the electrolyte solution contains a fluorophosphate ester. It can be seen that it is effective to increase the content and / or to use a phosphate ester as a solvent in the electrolytic solution.
　実施例１５～１７および参考例４（高温時の充放電挙動）
　正極合剤ペーストとして、実施例１と同様にして調製したものを用いる以外は、実施例１２と同様にして、コイン型のナトリウムイオン二次電池（ハーフセル）を作製した。
　電解液ａ１、ａ２およびａ３のそれぞれを用いた実施例の電池を電池Ａ１５、Ａ１６およびＡ１７とし、電解液ｂを用いた参考例の電池を電池Ｃ４とする。 Examples 15 to 17 and Reference Example 4 (charge / discharge behavior at high temperature)
A coin-type sodium ion secondary battery (half cell) was produced in the same manner as in Example 12 except that the positive electrode mixture paste was prepared in the same manner as in Example 1.
The batteries of the examples using the electrolytic solutions a1, a2, and a3 are designated as batteries A15, A16, and A17, and the battery of the reference example using the electrolytic solution b is designated as the battery C4.
　電池を、６０℃の温度で、時間率０．５Ｃレートの電流値で、３．４Ｖになるまで充電し、時間率０．５Ｃレートの電流値で、１．５Ｖになるまで放電した。この充放電サイクルを、１００回繰り返し、充放電時の容量変化を測定した。初期の放電容量を１００％としたとき、１００サイクル時の放電容量は、７５％にまで低下した。
　上記と同様の評価を、９０℃の温度で行ったところ、初期の放電容量を１００％としたとき、１００サイクル時の放電容量は３０％にまで低下した。サイクル後の電池を分解して正極箔をＳＥＭで観察したところ、電池Ｃ４では腐食が見られた。また、サイクル後の電池の電解液を取り出して、ＩＣＰ分析を行ったところ、Ａｌが検出された。 The battery was charged at a temperature value of 60 ° C. at a current value of 0.5C hour rate to 3.4V and discharged at a current value of 0.5C time rate to 1.5V. This charge / discharge cycle was repeated 100 times, and the change in capacity during charge / discharge was measured. Assuming that the initial discharge capacity is 100%, the discharge capacity at 100 cycles decreased to 75%.
When the same evaluation as described above was performed at a temperature of 90 ° C., when the initial discharge capacity was 100%, the discharge capacity at 100 cycles was reduced to 30%. When the battery after the cycle was disassembled and the positive foil was observed with an SEM, the battery C4 was corroded. Moreover, when the electrolyte solution of the battery after the cycle was taken out and ICP analysis was performed, Al was detected.
　このような結果から、高温下（例えば、６０℃以上の温度）でのアルミニウムの腐食を抑制し、容量の低下を抑制する観点からは、電解液において、フッ化リン酸エステルの含有量を多くすること、および／または電解液における溶媒としてリン酸エステルを用いることが有効であることが分かる。 From such a result, from the viewpoint of suppressing corrosion of aluminum at a high temperature (for example, a temperature of 60 ° C. or higher) and suppressing a decrease in capacity, the content of the fluorinated phosphoric acid ester is increased in the electrolytic solution. And / or using a phosphate ester as a solvent in the electrolyte is effective.
　本発明の一実施形態に係る電解液では、高い難燃性を確保しながら、ナトリウムイオン二次電池のサイクル特性およびレート特性を向上できる。このような電解液を用いるナトリウムイオン二次電池は、例えば、家庭用または工業用の大型電力貯蔵装置、電気自動車、ハイブリッド自動車などの電源としての利用が期待される。 The electrolyte solution according to one embodiment of the present invention can improve the cycle characteristics and rate characteristics of a sodium ion secondary battery while ensuring high flame retardancy. A sodium ion secondary battery using such an electrolytic solution is expected to be used as a power source for, for example, a household or industrial large power storage device, an electric vehicle, a hybrid vehicle, and the like.
　１：セパレータ
　２：正極
　２ａ：正極リード片
　３：負極
　３ａ：負極リード片
　７：ナット
　８：鍔部
　９：ガスケット
　１０：電池ケース
　１２：容器本体
　１３：蓋体
　１４：外部正極端子
　１６：安全弁 1: Separator 2: Positive electrode 2a: Positive electrode lead piece 3: Negative electrode 3a: Negative electrode lead piece 7: Nut 8: Hook 9: Gasket 10: Battery case 12: Container body 13: Lid 14: External positive terminal 16: Safety valve

Claims (11)

  1. 　ナトリウム塩と非水溶媒とを含み、かつナトリウムイオン伝導性を有するナトリウムイオン二次電池用電解液であって、前記非水溶媒は、フッ化リン酸エステルおよびリン酸エステルを含む、ナトリウムイオン二次電池用電解液。 An electrolyte for a sodium ion secondary battery comprising a sodium salt and a non-aqueous solvent and having sodium ion conductivity, wherein the non-aqueous solvent contains a fluorinated phosphate and a phosphate. Secondary battery electrolyte.
  2. 　前記非水溶媒中の前記フッ化リン酸エステルの含有量は、２０質量％以上である、請求項１に記載のナトリウムイオン二次電池用電解液。 The electrolyte solution for a sodium ion secondary battery according to claim 1, wherein the content of the fluorophosphate in the non-aqueous solvent is 20% by mass or more.
  3. 　引火点を有さないか、または引火点が７０℃以上である、請求項１または請求項２に記載のナトリウムイオン二次電池用電解液。 The electrolyte for sodium ion secondary batteries according to claim 1 or 2, wherein the electrolyte does not have a flash point or has a flash point of 70 ° C or higher.
  4. 　前記フッ化リン酸エステルは、１～３個のポリフルオロアルキル基を有するポリフルオロアルキルホスフェートであり、
    　前記ポリフルオロアルキル基のそれぞれは、炭素数１～３のジフルオロアルキル基、炭素数１～３のトリフルオロアルキル基、または炭素数２～３のテトラフルオロアルキル基である、請求項１～請求項３のいずれか１項に記載のナトリウムイオン二次電池用電解液。     The fluorophosphate ester is a polyfluoroalkyl phosphate having 1 to 3 polyfluoroalkyl groups,
    Each of the polyfluoroalkyl groups is a difluoroalkyl group having 1 to 3 carbon atoms, a trifluoroalkyl group having 1 to 3 carbon atoms, or a tetrafluoroalkyl group having 2 to 3 carbon atoms. 4. The electrolyte for a sodium ion secondary battery according to any one of 3.
  5. 　前記フッ化リン酸エステルは、トリス（２，２，２－トリフルオロエチル）ホスフェート、ビス（２，２，２－トリフルオロエチル）メチルホスフェートおよびビス（２，２，２－トリフルオロエチル）エチルホスフェートからなる群より選択される少なくとも一種である、請求項１～請求項４のいずれか１項に記載のナトリウムイオン二次電池用電解液。 The fluorinated phosphoric acid ester includes tris (2,2,2-trifluoroethyl) phosphate, bis (2,2,2-trifluoroethyl) methyl phosphate and bis (2,2,2-trifluoroethyl) ethyl. The electrolyte for a sodium ion secondary battery according to any one of claims 1 to 4, wherein the electrolyte is at least one selected from the group consisting of phosphates.
  6. 　前記リン酸エステルは、炭素数１～３のアルキル基を有するトリアルキルホスフェートである、請求項１～請求項５のいずれか１項に記載のナトリウムイオン二次電池用電解液。 6. The electrolyte for a sodium ion secondary battery according to claim 1, wherein the phosphate ester is a trialkyl phosphate having an alkyl group having 1 to 3 carbon atoms.
  7. 　前記リン酸エステルは、トリメチルホスフェートおよびトリエチルホスフェートからなる群より選択される少なくとも一種である、請求項１～請求項６のいずれか１項に記載のナトリウムイオン二次電池用電解液。 7. The electrolyte for a sodium ion secondary battery according to claim 1, wherein the phosphate ester is at least one selected from the group consisting of trimethyl phosphate and triethyl phosphate.
  8. 　前記非水溶媒は、環状カーボネートおよび鎖状カーボネートからなる群より選択される少なくとも一種をさらに含む請求項１～請求項７のいずれか１項に記載のナトリウムイオン二次電池用電解液。 The electrolyte solution for a sodium ion secondary battery according to any one of claims 1 to 7, wherein the non-aqueous solvent further includes at least one selected from the group consisting of cyclic carbonates and chain carbonates.
  9. 　前記非水溶媒は、プロピレンカーボネートをさらに含む請求項１～請求項７のいずれか１項に記載のナトリウムイオン二次電池用電解液。 The electrolyte solution for a sodium ion secondary battery according to any one of claims 1 to 7, wherein the non-aqueous solvent further contains propylene carbonate.
  10. 　前記非水溶媒中の前記フッ化リン酸エステルの含有量は、５０質量％を超えて８０質量％以下である、請求項１～請求項９のいずれか１項に記載のナトリウムイオン二次電池用電解液。 The sodium ion secondary battery according to any one of claims 1 to 9, wherein a content of the fluorophosphate in the non-aqueous solvent is more than 50% by mass and 80% by mass or less. Electrolyte.
  11. 　正極と、負極と、前記正極および前記負極の間に介在するセパレータと、請求項１～請求項９のいずれか１項に記載の電解液とを含む、ナトリウムイオン二次電池。
          A sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the electrolyte solution according to any one of claims 1 to 9.
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