JP2018170271A - Electrolyte solution for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electrolyte solution for a lithium ion secondary battery, which enables the increase in battery initial capacity, and which is easy to immerse into a positive electrode, a negative electrode and a separator even though it contains an ordinary temperature molten salt; and a lithium ion secondary battery arranged by use of the electrolyte solution.SOLUTION: An electrolyte solution for a lithium ion secondary battery comprises: a lithium imide salt; an ordinary temperature molten salt; and a high-vapor pressure solvent. The high-vapor pressure solvent has a vapor pressure of 1 kPa or more at 20°C. The content of the high-vapor pressure solvent falls in a range of 2 mass% or more and 40 mass% or less to a total amount of the lithium imide salt, the ordinary temperature molten salt and the high-vapor pressure solvent. A lithium ion secondary battery comprises: a positive electrode; a negative electrode; a separator interposed between the positive and negative electrodes; and an electrolyte solution which consists of the above electrolyte solution for a lithium ion secondary battery.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrolytic solution for a lithium ion secondary battery and a lithium ion secondary battery.

  Lithium ion secondary batteries have high energy density and are therefore widely used as power sources for portable electronic devices such as notebook personal computers and communication devices such as cellular phones. This lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolytic solution. As an electrolytic solution for a lithium ion secondary battery, an organic solvent in which a lithium salt is dissolved is conventionally used. In recent years, a room temperature molten salt in which a lithium salt is dissolved has been studied. The room temperature molten salt is also called an ionic liquid, is a salt that contains a cation component and an anion component, is in a molten state at room temperature, and has fluidity. Unlike organic solvents used in conventional lithium ion secondary batteries, room temperature molten salts are non-volatile, have no vapor pressure, and are difficult to ignite. For this reason, a lithium ion secondary battery using a room temperature molten salt as an electrolytic solution is considered to have high safety.

However, the room temperature molten salt is less likely to be impregnated in the positive electrode, the negative electrode, and the separator as compared with a conventional organic solvent. In particular, since a porous polyolefin sheet widely used as a separator has a small pore diameter, it is extremely difficult to impregnate a room temperature molten salt.
For this reason, in the lithium ion secondary battery using room temperature molten salt, using the nonwoven fabric which has a comparatively big void | hole as a separator is examined. For example, Patent Document 1 describes that as a separator, a nonwoven fabric made of one or more kinds of fibers selected from organic fibers and glass fibers and having a porosity of 70% or more is described.

JP 2010-287380 A

  However, a battery using a nonwoven fabric as a separator may be disadvantageous in terms of volumetric energy density because the nonwoven fabric is thicker than a porous polyolefin sheet. Furthermore, since the nonwoven fabric has a higher porosity than a porous polyolefin sheet and the amount of electrolyte used increases, there is a possibility that it may be disadvantageous in terms of weight energy density. In addition, it is necessary to impregnate the positive electrode and the negative electrode in order to increase the energy density of the battery, but room temperature molten salt is difficult to impregnate to the vicinity of the current collector foil of the positive electrode. When the amount is increased, the initial capacity may be reduced.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the initial capacity of a battery, which can easily impregnate a positive electrode, a negative electrode, and a separator while containing a room temperature molten salt. It is providing the electrolyte solution for secondary batteries, and the lithium ion secondary battery using the electrolyte solution.

The inventor can easily impregnate a positive electrode, a negative electrode, and a separator by adding a predetermined high vapor pressure solvent in a predetermined amount to an electrolyte solution containing a lithium imide salt and a room temperature molten salt, and It was found that the initial capacity of the lithium ion secondary battery is improved by using the electrolytic solution to which the high vapor pressure solvent is added.
That is, this invention provides the following means in order to solve the said subject.

(1) The electrolyte for a lithium ion secondary battery according to the first aspect contains a lithium imide salt, a room temperature molten salt, and a high vapor pressure solvent, and the high vapor pressure solvent has a vapor pressure at 20 ° C. Is 1 kPa or more, and the content of the high vapor pressure solvent is in the range of 2 mass% to 40 mass% with respect to the total amount of the lithium imide salt, the room temperature molten salt, and the high vapor pressure solvent. .

(2) In the electrolyte solution for a lithium ion secondary battery according to the above aspect, the high vapor pressure solvent may be a chain ether or a cyclic ether.

(3) In the electrolyte solution for a lithium ion secondary battery according to the above aspect, the high vapor pressure solvent may be at least one chain ether selected from the group consisting of dimethyl ether, ethyl methyl ether, and diethyl ether.

(4) In the electrolyte solution for a lithium ion secondary battery according to the above aspect, the molar ratio of the lithium imide salt and the room temperature molten salt may be 1: 1 to 1: 5.

(5) A lithium ion secondary battery according to a second aspect is a lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolytic solution, The electrolytic solution is an electrolytic solution for a lithium ion secondary battery according to the first aspect.

(6) In the lithium ion secondary battery according to the above aspect, the positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, and the positive electrode active material layer includes: The basis weight may be 20 mg / cm ² or more.

(7) In the lithium ion secondary battery according to the above aspect, the negative electrode has a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, and the negative electrode active material layer has You may contain at least 1 sort (s) among metallic silicon and metallic lithium.

  According to the present invention, an electrolyte for a lithium ion secondary battery that can be easily impregnated in a positive electrode, a negative electrode, and a separator and can improve the initial capacity of the battery while containing a room temperature molten salt, and the electrolyte are used. A lithium ion secondary battery can be provided.

It is a cross-sectional schematic diagram of the lithium ion secondary battery concerning this embodiment.

  Hereinafter, the present embodiment will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make the characteristics of the present invention easier to understand, there are cases where the characteristic parts are enlarged for the sake of convenience, and the dimensional ratios of the respective components are different from actual ones. is there. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be appropriately modified and implemented without departing from the scope of the invention.

[Lithium ion secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to this embodiment. A lithium ion secondary battery 100 shown in FIG. 1 mainly includes a stacked body 40, an exterior body 50 that accommodates the stacked body 40 in a sealed state, and a pair of leads 60 and 62 connected to the stacked body 40. Although not shown, the electrolyte solution is accommodated in the outer package 50 together with the stacked body 40.

  The stacked body 40 is configured such that the positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32.

  The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10. Leads 62 and 60 are connected to the end portions of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the end portions of the leads 60 and 62 extend to the outside of the exterior body 50. In FIG. 1, the case where there is one laminated body 40 in the exterior body 50 is illustrated, but a plurality of laminated bodies 40 may be laminated.

(Electrolyte)
In the lithium ion secondary battery of this embodiment, the electrolytic solution has a lithium imide salt, a room temperature molten salt, and a high vapor pressure solvent. The high vapor pressure solvent is a solvent having a vapor pressure of 1 kPa or higher at 20 ° C. The content of the high vapor pressure solvent in the electrolytic solution is in the range of 2% by mass to 40% by mass with respect to the total amount of the lithium imide salt, the room temperature molten salt, and the high vapor pressure solvent.

It is considered that the electrolytic solution of the present embodiment easily impregnates the positive electrode, the negative electrode, and the separator by including the high vapor pressure solvent in the above range, thereby improving the initial capacity of the battery. The room temperature molten salt used in the electrolytic solution of the present embodiment is a non-volatile solvent having no vapor pressure because it contains a cation component and an anion component and has a strong electrostatic interaction. On the other hand, high vapor pressure solvents are volatile solvents. For this reason, even if a high vapor pressure solvent is added to an electrolytic solution containing a room temperature molten salt, the high vapor pressure solvent is volatilized in a short time and is unlikely to remain in the electrolytic solution. The reason why the positive electrode is easily impregnated is considered as follows. That is, when an electrolytic solution containing a high vapor pressure solvent in the above range is injected into the battery, the high vapor pressure solvent repeatedly evaporates and condenses in the battery, so that the electrolytic solution in the battery is It is thought that the electrolyte solution was easily immersed in the positive electrode, the negative electrode, and the separator by stirring or fluidizing.
If the content of the high vapor pressure solvent is too large, the contents of the lithium imide salt and the room temperature molten salt are relatively decreased, and the initial capacity of the battery may be decreased. For this reason, in this embodiment, content of the high vapor pressure solvent is set to 40 mass% or less.

(Lithium imide salt)
Examples of the lithium imide salt include LiN (SO ₂ F) ₂ (LiFSI: lithium bis (fluorosulfonyl) imide), LiN (SO ₂ CF ₃ ) ₂ (LiTFSI: lithium bis (trifluoromethanesulfonyl) imide), LiN (SO ₂ C ₂ F ₅ ) ₂ (LiBETI: lithium bis (pentafluoroethanesulfonyl) imide), LiN (SO ₂ F) (SO ₂ CF ₃ ), LiN (SO ₂ CF ₃ ) (SO ₂ C ₂ F ₅ ) is used. can do. These lithium imide salts may be used individually by 1 type, and may be used in combination of 2 or more type.

(Normal temperature molten salt)
The room temperature molten salt is a liquid containing a cation component and an anion component. However, in this embodiment, the room temperature molten salt may be a liquid at room temperature in the state of an electrolytic solution containing a lithium imide salt and a high vapor pressure solvent. The normal temperature is a temperature at which a lithium ion secondary battery is normally used, and is 5 ° C. or more and 35 ° C. or less in the Japanese Industrial Standard (JIS Z 8703), but is not limited to this temperature range.

As a cation component, the nitrogen-type cation containing nitrogen, the phosphorus-type cation containing phosphorus, and the sulfur-type cation containing sulfur can be included. These cationic components may contain 1 type independently, and may contain 2 or more types in combination.
Examples of nitrogen-based cations include chain or cyclic ammonium cations such as imidazolium cation, pyrrolidinium cation, piperidinium cation, pyridinium cation, and azonia spiro cation. Examples of phosphorus cations include chain or cyclic phosphonium cations. Examples of the sulfur cation include a chain or cyclic sulfonium cation.

Examples of the anionic _{^{component, AlCl 4 -, NO 2 -}} , NO 3 -, I -, BF 4 -, PF 6 -, AsF 6 -, SbF 6 -, NbF 6 -, TaF 6 -, F (HF) 2. _{^{3 -, p-CH 3 PhSO}} 3 -, CH 3 CO 2 -, CF 3 CO 2 -, CH 3 SO 3 -, CF 3 SO 3 -, (CF 3 SO 2) 3 C -, C 3 F 7 CO ^{_{2, C 4 F 9 SO 3}} -, (FSO 2) 2 N - ( bis (fluorosulfonyl) _{imide), (CF 3 SO 2)} 2 N - ( bis (trifluoromethanesulfonyl) _{imide), (C} 2 _{F 5} SO _{2) 2} N ^- (bis (pentafluoroethanesulfonyl) _{imide), (CF 3 SO 2)} (CF 3 CO) N - (( trifluoromethanesulfonyl) (trifluoromethane carbonylation It includes (Jishianoimido), etc. ^-) imide), _(CN) 2 N. These anionic components may contain 1 type independently, and may contain 2 or more types in combination.

  The anion component preferably contains an anion of an imide. In this case, the affinity with the lithium imide salt is improved. When the anion component includes an anion of an imide, the cation component is preferably a nitrogen cation, and particularly preferably a piperidinium cation, a pyrrolidinium cation, or an azonia spiro cation.

  In this embodiment, it is preferable that content of the lithium imide salt and normal temperature molten salt in electrolyte solution exists in the range of 1: 1-1: 5 by molar ratio. In this case, a high rate characteristic can be suitably realized.

(High vapor pressure solvent)
As the high vapor pressure solvent, monohydric alcohol, ketone, nitrile, ester, chain carbonate, cyclic ether, chain ether can be used. A high vapor pressure solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

  Examples of monohydric alcohols include methanol, ethanol, 1-propanol, and 2-propanol. Examples of ketones include acetone, ethyl methyl ketone, and diethyl ketone. An example of a nitrile is acetonitrile.

  Examples of esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

  The chain ether and cyclic ether preferably have 6 or less carbon atoms. The number of oxygen atoms is preferably 1 or more and 3 or less. The carbon atoms of the chain ether and the cyclic ether are preferably saturated bonds. The hydrogen atom of the chain ether and the cyclic ether may be substituted with fluorine. The cyclic ether is preferably a 3-membered ring to a 6-membered ring. Examples of chain ethers include oxetane, 1,2-dimethoxyethane, dimethyl ether, methyl ethyl ether, diethyl ether, butyl methyl ether, dipropyl ether, cyclopentyl methyl ether, dibutyl ether, and diisopentyl ether. . Examples of cyclic ethers include tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4-dioxane, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Can do.

  The high vapor pressure solvent is preferably a chain ether or a cyclic ether, and particularly preferably a chain ether such as dimethyl ether, ethyl methyl ether, or diethyl ether. By using these solvents, high rate characteristics can be realized more suitably.

(Method for preparing electrolyte)
The electrolyte solution of this embodiment can be prepared, for example, by mixing a lithium imide salt and a room temperature molten salt, and then adding and mixing a high vapor pressure solvent to the obtained mixture. The high vapor pressure solvent may be added in a liquid state or in a gaseous state. The composition of the obtained electrolytic solution can be measured using gas chromatography.

"Negative electrode"
The negative electrode 30 includes a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32.

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Negative electrode active material layer)
The negative electrode active material layer 34 includes a negative electrode active material and a negative electrode binder, and optionally includes a negative electrode conductive material.

(Negative electrode active material)
As the material for the negative electrode active material, various materials that are used as negative electrode active materials for known lithium ion secondary batteries can be used. Examples of the negative electrode active material include, for example, carbon materials, silicon, silicon-containing compounds such as silicon oxide represented by SiOx (0 <x <2), metal lithium, metal that forms an alloy with lithium, and these And an amorphous compound mainly composed of an oxide such as tin dioxide and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). Examples of the carbon material include graphite (natural graphite, artificial graphite), carbon nanotube, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and the like. Examples of the metal that forms an alloy with metallic lithium include aluminum, silicon, and tin.

(Negative electrode conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotube, carbon material, fine metal powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and fine metal powder, and conductive oxide such as ITO. It is done. Among these, carbon powders such as acetylene black and ethylene black are particularly preferable. In the case where sufficient conductivity can be ensured with only the negative electrode active material 36, the lithium ion secondary battery 100 may not include a conductive material.

(Negative electrode binder)
The binder bonds the active materials to each other and bonds the active material to the negative electrode current collector 32. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And the like.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-) TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber), Vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber The containing rubbers (VDF-CTFE-based fluorine rubber) vinylidene fluoride-based fluorine rubbers such as may be used.

Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also functions as a conductive material, it is not necessary to add a conductive material. Examples of ion-conductive conductive polymers include monomers of polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide, polyphosphazenes, etc.), lithium imide salts, LiClO ₄ , LiBF ₄ , Examples thereof include a composite of a lithium salt such as LiPF ₆ or an alkali metal salt mainly composed of lithium. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  In addition, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like may be used as the binder.

  The contents of the negative electrode active material 36, the conductive material, and the binder in the negative electrode active material layer 34 are not particularly limited. The constituent ratio of the negative electrode active material 36 in the negative electrode active material layer 34 is preferably 90% or more and 98% or less by mass ratio. The constituent ratio of the conductive material in the negative electrode active material layer 34 is preferably 0% or more and 3.0% or less in terms of mass ratio, and the constituent ratio of the binder in the negative electrode active material layer 34 is 2.0% by mass ratio. It is preferably 5.0% or less.

  By making content of a negative electrode active material and a binder into the said range, it can prevent that the quantity of a binder is too small and it becomes impossible to form a strong negative electrode active material layer. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

  Instead of providing the negative electrode active material layer 34, lithium ions may be deposited as metallic lithium on the surface of the negative electrode current collector 32 during charging, and the metallic lithium deposited during discharging may be dissolved as lithium ions. In this case, since the negative electrode active material layer 34 is not necessary, the volume energy density of the battery can be improved. A copper foil can be used as the negative electrode current collector 32.

"Positive electrode"
The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22. The positive electrode active material layer 24 generally has a basis weight of 13 mg / cm ² or more, preferably 20 mg / cm ² or more. The basis weight means the mass of the positive electrode active material layer 24 supported on the surface of the positive electrode current collector 22 per unit area. Accordingly, when the basis weight is large, the amount of the positive electrode active material per unit area increases, so that the capacity of the battery increases, and when the cells having the same capacity are compared with each other, the energy density of the cell can be improved. it can. However, if the weight per unit area becomes too large and the thickness of the positive electrode active material layer 24 becomes too thick, there is a possibility that the region that acts as the positive electrode active material is reduced without the electrolyte solution being impregnated in the positive electrode active material layer 24. Therefore, the basis weight is preferably 30 mg / cm ² or less.

(Positive electrode current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used. Among these, a lightweight aluminum metal thin plate is preferable.

(Positive electrode active material layer)
The positive electrode active material used for the positive electrode active material layer 24 includes lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or counter ions (for example, PF ⁶⁻ ) of lithium ions and lithium ions. An electrode active material capable of reversibly proceeding doping and dedoping can be used.

For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z M} a O ₂ (x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is one type selected from Al, Mg, Nb, Ti, Cu, Zn, Cr Complex metal oxides represented by the above elements), lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) One or more elements or VO selected from the above, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co _y Al _z O ₂ (0.9 <x + y + z < 1.1) and the like, and polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like.

As the positive electrode active material, a lithium-free material such as FeF ₃ , a conjugated polymer containing an organic conductive material, a chevrel phase compound, a transition metal chalcogenide, vanadium oxide, or niobium oxide can be used. These lithium-free materials may be used alone or in combination of two or more. In the case of using a positive electrode active material that does not contain releasable lithium and does not contain lithium, lithium is inserted into the positive electrode active material by first performing discharge using metallic lithium or a lithium alloy as the negative electrode active material. The battery can be discharged. These lithium-free positive electrode active materials may be chemically inserted with lithium using metal lithium or the like, or electrochemically inserted (pre-doped) with lithium.

(Conductive material)
Examples of the conductive material include carbon powder such as carbon black, carbon nanotube, carbon material, fine metal powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and fine metal powder, and conductive oxide such as ITO. . When sufficient conductivity can be ensured with only the positive electrode active material, the lithium ion secondary battery 100 may not include a conductive material.

(Positive electrode binder)
The binder used for the positive electrode can be the same as that for the negative electrode.

"Exterior body"
The exterior body 50 seals the laminated body 40 and the electrolytic solution therein. The outer package 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside.

  For example, as the outer package 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and a separator is provided between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. 10 is inserted into the exterior body 50 together with the electrolyte, and the entrance of the exterior body 50 is sealed.

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and a separator is provided between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. 10 is inserted into the exterior body 50 together with the electrolyte, and the entrance of the exterior body 50 is sealed.

[Method for producing lithium ion secondary battery]
Next, a method for manufacturing the lithium ion secondary battery 100 will be specifically described.

  First, a negative electrode active material, a binder, and a solvent are mixed to prepare a paint. A conductive material may be further added as necessary. As the solvent, for example, water, N-methyl-2-pyrrolidone or the like can be used. The constituent ratio of the negative electrode active material, the conductive material, and the binder is preferably 90 wt% to 98 wt%: 0 wt% to 3.0 wt%: 2.0 wt% to 5.0 wt% in mass ratio. These mass ratios are adjusted so as to be 100 wt% as a whole.

  The mixing method of these components constituting the paint is not particularly limited, and the mixing order is not particularly limited. The paint is applied to the negative electrode current collector 32. There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method. Similarly, the positive electrode paint is applied on the positive electrode current collector 22 for the positive electrode.

  Subsequently, the solvent in the paint applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 to which the paint is applied may be dried in an atmosphere of 80 ° C. to 150 ° C.

  Then, the electrode on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way is subjected to a press treatment by a roll press device or the like as necessary.

  Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the negative electrode active material layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolytic solution are sealed in the outer package 50.

  For example, the positive electrode 20, the negative electrode 30, and the separator 10 are stacked, and the positive electrode 20 and the negative electrode 30 are heated and pressed with a press tool from a direction perpendicular to the stacking direction, and the positive electrode 20, the separator 10, and the negative electrode 30. Adhere. And the laminated body 40 is put into the bag-shaped exterior body 50 produced previously, for example.

  Finally, the lithium ion secondary battery is produced by injecting the electrolytic solution into the outer package 50. Instead of injecting the electrolyte into the exterior body, the laminate 40 may be impregnated with the electrolyte.

  As described above, the electrolyte for a lithium ion secondary battery according to the present embodiment is easily impregnated in the positive electrode, the negative electrode, and the separator while containing the room temperature molten salt. For this reason, the initial capacity of the lithium ion secondary battery using the electrolytic solution for a lithium ion secondary battery is improved.

  The embodiments of the present invention have been described in detail with reference to the drawings. However, the configurations and combinations of the embodiments in the embodiments are examples, and the addition and the omission of the configurations are within the scope not departing from the gist of the present invention. , Substitutions, and other changes are possible.

[Example 1]
(Preparation of electrolyte)
Normal temperature containing LiFSI (lithium bis (fluorosulfonyl) imide) as a lithium imide salt, P13 (N-methyl-N-propylpyrrolidinium) as a cation component, and FSI (bis (fluorosulfonyl) imide) as an anion component The molten salt was weighed at a molar ratio of 1: 4 and mixed. To 90 parts by mass of the obtained mixture, 10 parts by mass of ethanol as a high vapor pressure solvent was added and mixed to prepare an electrolyte solution having an ethanol content of 10% by mass. The composition of the electrolytic solution is shown in Table 1 below.

(Preparation of positive electrode)
97 parts by mass of NCA (composition: LiNi _0.80 Co _0.15 Al _0.05 O ₂ ) as the positive electrode active material, 1 part by mass of acetylene black as the conductive auxiliary agent, and 2 parts by mass of polyvinylidene fluoride (PVDF) as the binder Part and 70 parts by mass of N-methyl-2-pyrrolidone were mixed to prepare a coating material for forming a positive electrode active material layer. The prepared positive electrode mixture paint was applied to an aluminum foil (thickness: 12 μm) so that the basis weight of the positive electrode active material was 15 mg / cm ^2, and then dried at 100 ° C. to form a positive electrode active material layer. . The formed positive electrode active material was subjected to pressure treatment by a roll press. In this way, a positive electrode was produced.

(Preparation of negative electrode)
97 parts by mass of graphite as a negative electrode active material, 1 part by mass of acetylene black as a conductive auxiliary agent, 2 parts by mass of polyvinylidene fluoride (PVdF) as a binder, and 70 parts by mass of N-methyl-2-pyrrolidone are mixed together. A paint for forming an active material layer was prepared. The prepared negative electrode mixture paint was applied to a copper foil (thickness: 8 μm) so that the negative electrode active material weight was 9 mg / cm ^2, and then dried at 100 ° C. to form a negative electrode active material layer. . The formed negative electrode active material layer was pressure-molded by a roll press. In this way, a negative electrode was produced.

(Production of lithium ion secondary battery)
The above positive electrode and negative electrode were laminated through a separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer were opposed to each other to obtain a laminate. In the negative electrode of the laminate, a negative electrode lead made of nickel is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer. On the other hand, in the positive electrode of the laminate, the aluminum foil without the positive electrode active material layer is provided. A positive electrode lead made of aluminum was attached to the end of the protrusion with an ultrasonic welding machine. The laminated body was inserted into the outer body of the aluminum laminate film and heat sealed except for one peripheral portion to form a closed portion. And finally, after inject | pouring the said electrolyte solution into an exterior body, the remaining one place was sealed with the heat seal, reducing pressure with a vacuum sealer, and the lithium ion secondary battery was produced. Two lithium ion secondary batteries were prepared for analyzing the electrolyte composition and evaluating the charge / discharge characteristics. An electrolyte solution was collected from a lithium ion secondary battery for analyzing the electrolyte composition. Next, as a result of analyzing the composition of the collected electrolyte using gas chromatography, it was confirmed that the content of ethanol in the electrolyte was the same as the content (10% by mass) at the time of preparing the electrolyte.

  Table 1 shows the composition of the electrolyte solution of the produced lithium ion secondary battery. Table 2 shows materials and basis weights of the positive electrode and negative electrode active materials.

(0.2C-initial discharge capacity)
The manufactured lithium ion secondary battery was charged at a constant current of up to 4.4 V at a constant current density of 0.2 C (current density that becomes fully charged in 5 hours = 0.6 mA per 1 cm ^{2 of the} positive electrode active material). Then, it discharged to 3.0V with the constant current density of 0.2C, and let the amount of discharge be the initial stage discharge capacity. The obtained initial discharge capacity (mAh) was converted into a discharge capacity (mAh / g) per mass of the positive electrode active material. The results are shown in Table 2.

[Examples 2 to 30, Comparative Examples 1 to 6]
Table 1 shows the lithium imide salt material of the electrolyte, the cation or anion component of the room temperature molten salt, the mixing ratio of the lithium imide salt and the room temperature molten salt, the material of the high vapor pressure solvent, and the concentration of the high vapor pressure solvent, respectively. A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the amount of the positive electrode active material of the positive electrode and the weight of the negative electrode active material of the negative electrode were as shown in Table 2 below. Since dimethyl ether used as a high vapor pressure solvent in Examples 23 and 28 and ethyl methyl ether used as a high vapor pressure solvent in Examples 24 and 29 are gases at room temperature, a mixture of a lithium imide salt and a normal temperature molten salt is used. The electrolyte solution was prepared by bubbling a gas of a high vapor pressure solvent into the mixture while stirring.

  In Table 1, “LiTFSI” of the lithium imide salt is lithium bis (trifluorosulfonyl) imide, and “LiBETI” is lithium bis (pentalfluoroethanesulfonyl) imide. “P14” of the cation component of the ambient temperature molten salt is N-butyl-N-methylpyrrolidinium, “PP13” is N-methyl-N-propylpiperidinium, “TMPA” is trimethylpropylammonium, “P222 (201) "Is triethylmethoxyethylphosphonium. “TFSI” of the anion component of the ambient temperature molten salt is bis (trifluoromethanesulfonyl) imide, and “BETI” is bis (pentafluoroethanesulfonyl) imide.

About the produced lithium ion secondary battery, the electrolyte solution was extract | collected similarly to Example 1, and the composition of the extract | collected electrolyte solution was analyzed. As a result, in any lithium ion secondary battery, it was confirmed that the content of the high vapor pressure solvent in the electrolytic solution was the same as that at the time of preparing the electrolytic solution.
Moreover, about the produced lithium ion secondary battery, it carried out similarly to Example 1, and measured 0.2C- initial stage discharge capacity.

  From the results shown in Table 1, the lithium ion secondary batteries of Examples 1 to 30 using an electrolytic solution containing a high vapor pressure solvent having a vapor pressure of 1 kPa or higher at 20 ° C. within the scope of the present invention are the initial values at 0.2C. It can be seen that the discharge capacities are all high. On the other hand, the lithium ion secondary battery of Comparative Example 1 using an electrolytic solution not containing a high vapor pressure solvent could not be charged / discharged at 0.2C. The lithium ion secondary batteries of Comparative Examples 2 to 4 and 6 using an electrolytic solution containing an organic solvent having a vapor pressure of less than 1 kPa at 20 ° C. all had low initial discharge capacities at 0.2C. Also, the lithium ion secondary battery of Comparative Example 5 using an electrolytic solution containing a high vapor pressure solvent having a vapor pressure of 1 kPa or higher at 20 ° C. exceeding the range of the present invention also has an initial discharge capacity at 0.2 C. It became low.

In Examples 28 to 30, the basis weight of the positive electrode active material is as large as 22 mg / cm ² (that is, the thickness of the positive electrode active material layer is thick), but the initial discharge capacity at 0.2 C shows a high value. . This is because the electrolytic solution easily penetrates into the positive electrode active material layer. In contrast, Comparative Example 6 is different from Comparative Example 3 in that the weight of the positive electrode active material is as large as 22 mg / cm ² , but the initial discharge capacity at 0.2 C is significantly reduced. I understand that. This is because the electrolytic solution hardly penetrates into the positive electrode active material layer, so that the region that acts as the positive electrode active material decreases when the positive electrode active material layer becomes thick.

[Examples 31 to 50]
Table 3 shows the lithium imide salt material of the electrolyte, the cation or anion component of the room temperature molten salt, the mixing ratio of the lithium imide salt and the room temperature molten salt, the material of the high vapor pressure solvent, and the concentration of the high vapor pressure solvent, respectively. A lithium ion secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode active material weight of the positive electrode and the material of the negative electrode active material were as shown in Table 4 below. . In Examples 48 to 50, a mixture in which LiFSI and LiTFSI were mixed at a molar ratio of 0.6: 0.4 was used as the lithium imide salt.

About the produced lithium ion secondary battery, the electrolyte solution was extract | collected similarly to Example 1, and the composition of the extract | collected electrolyte solution was analyzed. As a result, in any lithium ion secondary battery, it was confirmed that the content of the high vapor pressure solvent in the electrolytic solution was the same as that at the time of preparing the electrolytic solution.
Moreover, about the produced lithium ion secondary battery, it carried out similarly to Example 1, and measured 0.2C- initial stage discharge capacity. The results are shown in Table 4.

In Table 4, metal silicon powder was used as the “metal silicon” of the negative electrode active material. The negative electrode was produced as follows.
83 parts by mass of metal silicon powder, 2 parts by mass of acetylene black, 15 parts by mass of polyamideimide, and 82 parts by mass of N-methyl-2-pyrrolidone were mixed to prepare a slurry for forming a negative electrode active material layer. This slurry was applied to a copper foil (thickness: 8 μm) so that the basis weight of the negative electrode active material was 2 mg / cm ² and dried at 100 ° C. to form an active material layer. Thereafter, the negative electrode was pressure-molded by a roll press and then heat-treated at 350 ° C. for 3 hours in a vacuum to obtain a negative electrode having a negative electrode active material layer thickness of 19 μm.

In Table 4, a metal lithium foil (thickness: 20 μm, basis weight: 1 mg / cm ² ) was used as the “metal lithium” of the negative electrode active material. The negative electrode was produced by pressure-pressing a metal lithium foil on a copper foil (thickness: 8 μm).

  In Table 4, “-” of the negative electrode active material did not use the negative electrode active material. The lithium ion secondary battery was the same as in Example 1 except that the positive electrode active material layer of the positive electrode and the copper foil (thickness: 8 μm) as the negative electrode current collector were laminated via a separator. It was made. In this lithium ion secondary battery, lithium ions are deposited as metallic lithium on the surface of the negative electrode current collector during charging, and the metallic lithium deposited during discharging is dissolved as lithium ions.

  From the results of Table 4, Examples 31 to 33 and Example 48 using metallic silicon as the negative electrode active material, Examples 34 to 36 and Example 49 using metallic lithium, Examples in which lithium was deposited on the current collector It can be seen that in Examples 37 to 47 and Example 50 in which lithium is deposited, the initial discharge capacity at 0.2 C is further increased as compared with Examples 1 to 30 using graphite as the negative electrode active material. .

  DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode active material layer, 26 ... Positive electrode active material, 28 ... Conductive material, 30 ... Negative electrode, 32 ... Negative electrode collector, 34 ... Negative electrode active material layer 40 ... Laminated body, 50 ... Exterior body, 60,62 ... Lead, 100 ... Lithium ion secondary battery

Claims (7)

A lithium imide salt, a room temperature molten salt, and a high vapor pressure solvent,
The high vapor pressure solvent has a vapor pressure of 1 kPa or more at 20 ° C .;
For a lithium ion secondary battery in which the content of the high vapor pressure solvent is in the range of 2% by mass to 40% by mass with respect to the total amount of the lithium imide salt, the room temperature molten salt, and the high vapor pressure solvent. Electrolytic solution.   The electrolyte solution for a lithium ion secondary battery according to claim 1, wherein the high vapor pressure solvent is a chain ether or a cyclic ether.   The electrolytic solution for a lithium ion secondary battery according to claim 1 or 2, wherein the high vapor pressure solvent is at least one chain ether selected from the group consisting of dimethyl ether, ethyl methyl ether, and diethyl ether.   Content of the said lithium imide salt and the said normal temperature molten salt is 1: 1-1: 5 by molar ratio, The electrolyte solution for lithium ion secondary batteries as described in any one of Claims 1-3. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution,
The lithium ion secondary battery whose said electrolyte solution is the electrolyte solution for lithium ion secondary batteries as described in any one of Claims 1-4. The positive electrode has a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, and the positive electrode active material layer has a basis weight of 20 mg / cm ² or more. The lithium ion secondary battery described in 1.   The negative electrode has a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, and the negative electrode active material layer contains at least one of metal silicon and metal lithium. Item 7. The lithium ion secondary battery according to Item 5 or 6.

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