JP6919271B2 - Lithium-ion secondary battery electrolyte and lithium-ion secondary battery - Google Patents

Description

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

Due to its high energy density, lithium-ion secondary batteries are widely used as power sources for portable electronic devices such as notebook personal computers and communication devices such as mobile 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.

Conventionally, an organic solvent in which a lithium salt is dissolved has been used as an electrolytic solution for a lithium ion secondary battery. In recent years, lithium imide salt has been used as the lithium salt, and the mixing ratio of the lithium imide salt and the solvent is about 1: 0.8 to 1: 2, and the concentration of the lithium imide salt is significantly higher than before. Many high-salt electrolytes have been reported (for example, Patent Document 1). This lithium imide salt high-concentration electrolytic solution is expected because it suppresses the dissolution of sulfur, which was one of the concerns when using a sulfur-based positive electrode.

International Publication No. 2016/079919

However, it is difficult to impregnate the positive electrode, negative electrode, and separator widely used in lithium ion secondary batteries with the lithium imide salt high-concentration electrolytic solution. In particular, it is extremely difficult to impregnate a microporous polyolefin separator widely used as a separator. Therefore, in a lithium ion secondary battery using a lithium imide salt high-concentration electrolytic solution, it has been studied to use a fibrous non-woven fabric having a high porosity. However, in this case, since the non-woven fabric is usually as thick as 30 to 500 μm, the porosity is high in terms of the volumetric energy density when the non-woven fabric is formed, so that an electrolytic solution is required more than necessary and the weight energy density is high. It is also disadvantageous in terms of points. Further, even if the separator can be impregnated, it is necessary to increase the basis weight of the positive electrode and the negative electrode for high energy density. In such a case, since the film thickness of the positive electrode and the negative electrode is large, it is difficult to impregnate the electrolytic solution up to the vicinity of the current collecting foil of the positive electrode and the negative electrode, and the initial capacity that can be easily obtained cannot be obtained when the basis weight is small.

An object of the present invention has been made in view of the above circumstances, and while containing a high concentration of lithiumimide salt, it is easy to impregnate the positive electrode, the negative electrode, and the separator, and the initial capacity of the battery can be improved. An object of the present invention is to provide an electrolytic solution for a secondary battery and a lithium ion secondary battery using the electrolytic solution.

The present inventor has a positive electrode, a negative electrode, and a separator as an electrolytic solution containing a lithiumimide salt, a predetermined cyclic solvent, and a high vapor pressure solvent having a vapor pressure at 20 ° C. higher than that of the cyclic solvent in a predetermined amount. It has been found that the initial capacity of the lithium ion secondary battery is improved by easily impregnating the solvent with the solvent and using the electrolytic solution thereof.
That is, the present invention provides the following means for solving the above problems.

(1) The electrolytic solution for a lithium ion secondary battery according to the first aspect is a lithium imide salt, at least one cyclic solvent selected from the group consisting of cyclic carbonates, cyclic esters and cyclic ethers, and a vapor pressure at 20 ° C. Contains a higher vapor pressure solvent than the cyclic solvent, and the content ratio of the lithiumimide salt to the cyclic solvent is in the range of 1: 0.8 to 1: 2.0 in terms of molar ratio. The high vapor pressure solvent has a vapor pressure of 1 kPa or more at 20 ° C., and the content of the high vapor pressure solvent is relative to the total amount of the lithiumimide salt, the cyclic solvent, and the high vapor pressure solvent. It is in the range of 0.1% by mass or more and 10% by mass or less.

(2) In the electrolytic 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) The electrolytic solution for a lithium ion secondary battery according to the above embodiment may be at least one chain ether in which the high vapor pressure solvent is selected from the group consisting of dimethyl ether, ethyl methyl ether, and diethyl ether.

(4) The lithium ion secondary battery according to the second aspect is a lithium ion secondary battery including 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 the electrolytic solution for a lithium ion secondary battery according to the first aspect.

(5) In the lithium ion secondary battery according to the above aspect, 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 is , The amount of the grain ^{may be 20 mg / cm 2} or more.

(6) 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 is formed. It may contain at least one of metallic silicon and metallic lithium.

According to the present invention, an electrolytic solution for a lithium ion secondary battery, which can easily impregnate the positive electrode, the negative electrode, and the separator and improve the initial capacity of the battery while containing a high concentration of lithiumimide salt, and an electrolytic solution thereof. It becomes possible to provide a lithium ion secondary battery using the above.

It is sectional drawing of the lithium ion secondary battery which concerns on this embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of the respective components may differ from the actual ones. be. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

[Lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view of the lithium ion secondary battery according to the present embodiment. The lithium ion secondary battery 100 shown in FIG. 1 mainly includes a laminate 40, an exterior body 50 that houses the laminate 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 40. Further, although not shown, the electrolytic solution is housed in the exterior body 50 together with the laminated body 40.

In the laminated body 40, the positive electrode 20 and the negative electrode 30 are arranged so as to face each other with the separator 10 interposed therebetween. The positive electrode 20 has a positive electrode active material layer 24 provided on a plate-shaped (film-shaped) positive electrode current collector 22. The negative electrode 30 has a negative electrode active material layer 34 provided on a plate-shaped (film-shaped) 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, respectively. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the exterior body 50. In FIG. 1, a case where one laminated body 40 is provided in the exterior body 50 is illustrated, but a plurality of laminated bodies 40 may be laminated.

(Electrolytic solution)
In the lithium ion secondary battery of the present embodiment, the electrolytic solution is a lithium imide salt, at least one cyclic solvent selected from the group consisting of cyclic carbonates, cyclic esters and cyclic ethers, and a cyclic solvent having a vapor pressure at 20 ° C. Contains a higher vapor pressure solvent. The content ratio of the lithium imide salt to the cyclic solvent is in the range of 1: 0.8 to 1: 2.0 in terms of molar ratio. 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 is in the range of 0.1% by mass or more and 10% by mass or less with respect to the total amount of the lithium imide salt, the cyclic solvent 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 containing the high vapor pressure solvent in the above range, thereby improving the initial capacity of the battery.
The high vapor pressure solvent used in the electrolytic solution of the present embodiment is a solvent that easily volatilizes. Therefore, even if a high vapor pressure solvent is added to the electrolytic solution containing a lithiumimide salt at a high concentration, it is considered that the high vapor pressure solvent volatilizes in a short time and does not easily remain in the electrolytic solution. The reason why the electrolytic solution is easily impregnated into the positive electrode or the like is considered as follows. That is, when the electrolytic solution is injected into the battery in the above range, the viscosity of the electrolytic solution is reduced, and the electrolytic solution is generated by the interaction between the molecules of the high vapor pressure solvent and the cyclic solvent. It is considered that it became easier to penetrate into the electrode (particularly the positive electrode) and the separator. It is also considered that the high vapor pressure solvent repeatedly evaporates and condenses, so that the electrolytic solution in the battery is agitated and fluidized, and the electrolytic solution easily permeates into the positive electrode, the negative electrode, and the separator.
If the content of the high vapor pressure solvent is too small, it may be difficult to obtain the effect of the high vapor pressure solvent. Further, if the content of the high vapor pressure solvent becomes too large, the contents of the lithium imide salt and the cyclic molten salt may decrease relatively, and the initial capacity of the battery may decrease. Therefore, in the present embodiment, the content of the high vapor pressure solvent is set in the range of 0.1% by mass or more and 10% by mass or less.

(Lithium imide salt)
Lithium imide salts 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 (pentafluoroethane sulfonyl) imide), LiN (SO ₂ F) (SO ₂ CF ₃ ), LiN (SO ₂ CF ₃ ) (SO ₂ C ₂ F ₅ ) are used. can do. One of these lithium imide salts may be used alone, or two or more thereof may be used in combination.

(Cyclic solvent)
As the cyclic solvent, at least one selected from the group consisting of cyclic carbonates, cyclic esters and cyclic ethers is used.
Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate. Examples of the cyclic ester include γ-butyrolactone, δ-valerolactone, methyl γ-butyrolactone, ethyl γ-butyrolactone, ethyl δ-valerolactone, and β-propiolactone. Examples of cyclic ethers include oxetane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4-dioxane, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Can be mentioned. These cyclic solvents may be used alone or in combination of two or more.

(High vapor pressure solvent)
As the high vapor pressure solvent, carbon dioxide, monohydric alcohol, ketone, nitrile, ester, chain carbonic acid ester, cyclic ether, and chain ether can be used. One type of high vapor pressure solvent may be used alone, or two or more types may be used in combination.

Examples of monohydric alcohols include methanol, ethanol, 1-propanol and 2-propanol. Examples of ketones include acetone, ethyl methyl ketone, and diethyl ketone. Examples of nitriles include acetonitrile.

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

The chain ether and the cyclic ether preferably have 6 or less carbonic acid atoms. The number of oxygen atoms is preferably 1 or more and 3 or less. It is preferable that the carbon atoms of the chain ether and the cyclic ether are saturated bonds. The hydrogen atoms of the chain ether and the cyclic ether may be substituted with fluorine. The cyclic ether is preferably a three-membered ring to a six-membered ring. Examples of chain ethers include 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 oxetane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4-dioxane, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Can be mentioned.

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

(Preparation method of electrolytic solution)
The electrolytic solution of the present embodiment can be prepared, for example, by mixing a lithium imide salt and a cyclic solvent, and then adding a high vapor pressure solvent to the obtained mixture and mixing them. 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 has 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 any 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 has a negative electrode active material and a negative electrode binder, and optionally has a negative electrode conductive material.

(Negative electrode active material)
As the material of the negative electrode active material, various materials used as the negative electrode active material for a known lithium ion secondary battery can be used. Examples of materials for the negative electrode active material include carbon materials, silicon, silicon-containing compounds such as silicon oxide represented by SiOx (0 <x <2), metallic lithium, metals forming alloys with lithium, and these. , An amorphous compound mainly composed of oxides such as tin dioxide, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ) can be mentioned. Examples of the carbon material include graphite (natural graphite, artificial graphite), carbon nanotubes, non-graphitized carbon, easily graphitized carbon, low-temperature calcined carbon and the like. Examples of metals that form alloys 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 nanotubes, carbon material, metal fine powder such as copper, nickel, stainless steel and iron, a mixture of carbon material and metal fine powder, and conductive oxide such as ITO. Be done. Among these, carbon powders such as acetylene black and ethylene black are particularly preferable. When sufficient conductivity can be ensured only by the negative electrode active material 36, the lithium ion secondary battery 100 does not have to contain the conductive material.

(Negative electrode binder)
The binder binds the active materials to each other and also binds the active material to the negative electrode current collector 32. The binder may be any one capable of the above-mentioned bonds, 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), polyvinylidene fluoride (PVF) ) And other fluororesins.

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

Further, an electron conductive conductive polymer or an ionic conductive polymer may be used as the binder. Examples of the electron-conducting conductive polymer include polyacetylene and the like. In this case, since the binder also functions as a conductive material, it is not necessary to add the conductive material. Examples of the ionic conductive polymer include a monomer of a polymer compound (polyether-based polymer compound such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.), a lithium imide salt, LiClO ₄ , LiBF ₄ , and the like. 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 compounding include a photopolymerization initiator or a thermal polymerization initiator compatible with the above-mentioned monomers.

In addition to this, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamide-imide 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 composition 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 in terms of mass ratio. The composition 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 composition ratio of the binder in the negative electrode active material layer 34 is 2.0% by mass ratio. It is preferably 5.0% or more and less than 5.0%.

By setting the contents of the negative electrode active material and the binder within the above range, it is possible to prevent the amount of the binder from being too small to form a strong negative electrode active material layer. In addition, the amount of binder that does not contribute to the electric capacity increases, and the tendency that it becomes difficult to obtain a sufficient volumetric 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 precipitated during discharge may be dissolved as lithium ions. In this case, since the negative electrode active material layer 34 is not required, the volumetric 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 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22. The cathode 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. Therefore, if 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 cells having the same capacity are compared, the energy density of the cells can be improved. can. However, if the amount of grain is too large and the thickness of the positive electrode active material layer 24 becomes too thick, the electrolytic solution may not be impregnated into the positive electrode active material layer 24 and the region acting as the positive electrode active material may be reduced. Therefore, the coating amount is preferably 30 mg / cm ^{2 or less.}

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

(Cathode active material layer)
The positive electrode active material used for the positive electrode active material layer 24 is the storage and release of lithium ions, the desorption and insertion of lithium ions (intercalation), or the doping and desorption of lithium ions and lithium ion counter anions (imide ions). An electrode active material capable of reversibly advancing the doping 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. Composite metal oxide represented by the above elements), lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (however, M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr. more shows one or more elements or VO selected), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) mixed metal oxide, such as Examples include substances, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like.

Further, as the positive electrode active material, a lithium-free material such as FeF ₃ , a conjugated polymer containing an organic conductive substance, a chevrel phase compound, a transition metal chalcogenide, vanadium oxide, and niobium oxide can be used. These lithium-free materials may be used alone or in combination of two or more. When a lithium-free positive electrode active material that does not contain releaseable lithium is used, metallic lithium or a lithium alloy is used as the negative electrode active material, and lithium is inserted into the positive electrode active material by first discharging and filling the positive electrode active material. It becomes a dischargeable battery. Further, these lithium-free positive electrode active materials may be chemically inserted with lithium using metallic lithium or the like, or may be electrochemically inserted (pre-doped) with lithium.

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

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

The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 80% or more and 96% or less in terms of mass ratio. The composition ratio of the conductive material in the positive electrode active material layer 24 is preferably 2.0% or more and 10% or less in terms of mass ratio, and the composition ratio of the binder in the positive electrode active material layer 24 is 2.0% by mass ratio. It is preferably 10% or more.

"Exterior body"
The exterior body 50 seals the laminate 40 and the electrolytic solution inside the exterior body 50. The exterior body 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and invasion of moisture or the like into the inside of the lithium ion secondary battery 100 from the outside.

For example, as the exterior body 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. As the metal foil 52, for example, an aluminum foil can be used, and as the polymer film 54, a film such as polypropylene can be used. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, for example, polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferable.

"Lead"
The leads 60 and 62 are formed of a conductive material such as aluminum. Then, the leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, 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. With the 10 sandwiched between them, they are inserted into the exterior body 50 together with the electrolytic solution to seal the entrance of the exterior body 50.

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

First, a paint is prepared by mixing a negative electrode active material, a binder and a solvent. If necessary, a conductive material may be further added. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used. The composition 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 terms of mass ratio. These mass ratios are adjusted so as to be 100 wt% as a whole.

The method of mixing these components constituting the paint is not particularly limited, and the mixing order is also not particularly limited. The above paint is applied to the negative electrode current collector 32. The coating method is not particularly limited, and a method usually adopted when producing an electrode can be used. For example, the slit die coat method and the doctor blade method can be mentioned. Similarly, for the positive electrode, a paint for the positive electrode is applied on the positive electrode current collector 22.

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 coated with the paint may be dried in an atmosphere of 80 ° C. to 150 ° C.

Then, the electrodes on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way are pressed by a roll press device or the like, if 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 exterior body 50.

For example, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated, and the positive electrode 20 and the negative electrode 30 are heated and pressed with a press instrument from a direction perpendicular to the stacking direction, and the positive electrode 20, the separator 10, and the negative electrode 30 are heated and pressed. Adhere. Then, for example, the laminated body 40 is put into the bag-shaped exterior body 50 prepared in advance.

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

As described above, the electrolytic solution for a lithium ion secondary battery according to the present embodiment is likely to impregnate the positive electrode, the negative electrode, and the separator even though it contains a room temperature molten salt. Therefore, the initial capacity of the lithium ion secondary battery using the electrolytic solution for the lithium ion secondary battery is improved.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the configurations and combinations thereof in the respective embodiments are examples, and the configurations are added or omitted within the range not deviating from the gist of the present invention. , Replacement, and other changes are possible.

[Example 1]
(Preparation of electrolytic solution)
EC (ethylene carbonate) and PC (propylene carbonate) were mixed at a volume ratio of 1: 1 to prepare EC / PC. LiFSI (lithium bis (fluorosulfonyl) imide) and the above EC / PC mixed solvent were mixed at a molar ratio of 1: 1.3. 1.0 part by mass of ethanol was added as a high vapor pressure solvent to 99.0 parts by mass of the obtained mixture and mixed to prepare an electrolytic solution having an ethanol content of 1.0% by mass.

(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. Parts 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 amount of the positive electrode active material had a grain size of 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 pressure-treated by a roll press. In this way, a positive electrode was produced.

(Preparation of negative electrode)
97 parts by mass of graphite as the negative electrode active material, 1 part by mass of acetylene black as the conductive auxiliary agent, 2 parts by mass of polyvinylidene fluoride (PVdF) as the binder, and 70 parts by mass of N-methyl-2-pyrrolidone are mixed to prepare the negative electrode. A paint for forming an active material layer was prepared. The prepared negative electrode mixture coating material was applied to a copper foil (thickness: 8 μm) so that the coating amount of the negative electrode active material 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, the negative electrode was produced.

(Manufacturing of lithium ion secondary battery)
The above positive electrode and negative electrode were laminated via a separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. In the negative electrode of the laminate, a nickel negative electrode lead is attached to the protruding end of the copper foil without the negative electrode active material layer, while in the positive electrode of the laminate, the aluminum foil without the positive electrode active material layer is provided. An aluminum positive electrode lead was attached to the end of the protrusion by an ultrasonic welding machine. This laminated body was inserted into the outer body of the aluminum laminated film and heat-sealed except for one peripheral portion to form a closed portion. Finally, after injecting the electrolytic solution into the exterior body, the remaining one location was sealed with a heat seal while reducing the pressure with a vacuum sealer to prepare a lithium ion secondary battery. Two lithium ion secondary batteries were manufactured, one for analyzing the composition of the electrolytic solution and the other for evaluating the charge / discharge characteristics. The electrolytic solution was collected from a lithium ion secondary battery for analyzing the electrolytic solution composition. Next, as a result of analyzing the composition of the collected electrolytic solution using gas chromatography, it was confirmed that the content of ethanol in the electrolytic solution was the same as the content (1.0% by mass) at the time of preparing the electrolytic solution. rice field.

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

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

[Examples 2-28, Comparative Examples 1-9]
The material of the lithium imide salt of the electrolytic solution, the mixing ratio of the lithium imide salt and the cyclic solvent, the material of the high vapor pressure solvent, and the content of the high vapor pressure solvent are as shown in Table 1 below, respectively, and the positive electrode activity of the positive electrode. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the amount of the solvent was as shown in Table 2 below. Since carbon dioxide (Example 4), dimethyl ether (Example 26) and ethyl methyl ether (Example 27) used as the high vapor pressure solvent are gases at room temperature, the mixture of the lithiumimide salt and the cyclic solvent is stirred. At the same time, an electrolytic solution was prepared by bubbling the mixture with a gas of a high vapor pressure solvent.

In Table 1, the lithium imide salt "LiTFSI" is a lithium bis (trifluorosulfonyl) imide, and "LiBETI" is a lithium bis (pentalfluoroethanesulfonyl) imide.

For the produced lithium ion secondary battery, an electrolytic solution was collected in the same manner as in Example 1, and the composition of the collected electrolytic solution was analyzed. As a result, 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 in all the lithium ion secondary batteries.
Further, the 0.2C-initial discharge capacity of the produced lithium ion secondary battery was measured in the same manner as in Example 1. The results are shown in Table 2.

From the results in Table 2, the lithium ion secondary batteries of Examples 1 to 28 using the electrolytic solution containing the high vapor pressure solvent within the range of the present invention all show high initial discharge capacities at 0.2. You can see that. In particular, Examples 26 to 28 having a positive electrode active material with a grain size of 20 mg / cm ² or more have a thicker positive electrode active material layer than Examples 1 to 25, and have an initial discharge capacity at 0.2 C. Shows a high value, indicating that the energy density of the cell is improved.

On the other hand, the lithium ion secondary battery of Comparative Example 1 using the electrolytic solution containing no high vapor pressure solvent could not be charged / discharged at 0.2C. The lithium ion secondary batteries of Comparative Examples 2 to 4 using an electrolytic solution containing an organic solvent having a vapor pressure of less than 1 kPa at 20 ° C. had a low initial discharge capacity at 0.2 C. Further, a comparative example using the lithium ion secondary battery of Comparative Example 5 using an electrolytic solution containing a cyclic solvent in an amount smaller than the range of the present invention and an electrolytic solution containing a cyclic solvent in an amount larger than the range of the present invention. The lithium ion secondary battery of Comparative Example 8 and the high vapor pressure solvent of Comparative Example 8 using an electrolytic solution containing 6 and 7 lithium ion secondary batteries and a high vapor pressure solvent in an amount smaller than the range of the present invention are included in the range of the present invention. The initial discharge capacity at 0.2 C was low for all of the lithium ion secondary batteries of Comparative Example 9 using the electrolytic solution contained in a large amount.

[Examples 29 to 46]
The lithiumimide salt material and the cyclic solvent material of the electrolytic solution are shown in Table 3 below, the amount of the positive electrode active material of the positive electrode and the material of the negative electrode active material are shown in Table 4 below, respectively. A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

For the produced lithium ion secondary battery, an electrolytic solution was collected in the same manner as in Example 1, and the composition of the collected electrolytic solution was analyzed. As a result, 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 in all the lithium ion secondary batteries.
Further, the initial discharge capacity of the produced lithium ion secondary battery was measured in the same manner as in Example 1, and the obtained initial discharge capacity (mAh) was calculated as the discharge capacity per mass of the positive electrode active material (mAh / g). Converted to. The results are shown in Table 4.

In Table 3, as the negative electrode active material "metallic silicon", metallic silicon powder was used. The negative electrode was prepared as follows.
83 parts by mass of metallic silicon powder, 2 parts by mass of acetylene black, 15 parts by mass of polyamide-imide, 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 2.0 mg / cm ^2, and dried at 100 ° C. to form an active material layer. Then, the negative electrode was pressure-molded by a roll press and then heat-treated at 350 ° C. for 3 hours in vacuum to obtain a negative electrode having a thickness of the negative electrode active material layer of 19 μm.

In Table 3, a metallic lithium foil (thickness: 20 μm) was used as the negative electrode active material “metallic lithium”. The negative electrode was produced by pressure-bonding a metallic lithium foil to a copper foil (thickness: 8 μm).

In Table 3, the negative electrode active material “−” did not use the negative electrode active material. The lithium ion secondary battery is 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) which is the negative electrode current collector are laminated with the separator so as to face each other. I made it. 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 precipitated during discharge is dissolved as lithium ions.

From the results in Table 4, the negative electrodes were found in all of Examples 29 to 31 in which metallic silicon was used as the negative electrode active material, 32 to 34 in which metallic lithium was used, and Examples 35 to 46 in which lithium was precipitated in the current collector. It can be seen that the initial discharge capacity of the positive electrode active material at 0.2 C is further higher than that of Examples 1 to 28 in which graphite is used as the active material.

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

Claims (4)

A lithium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolytic solution.
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 grain size of 20 mg / cm ² or more.
The electrolytic solution contains a lithium imide salt, at least one cyclic solvent selected from the group consisting of cyclic carbonates, cyclic esters and cyclic ethers, and a high vapor pressure solvent having a vapor pressure at 20 ° C. higher than that of the cyclic solvent. Contains and
The content ratio of the lithium imide salt to the cyclic solvent is in the range of 1: 0.8 to 1: 2.0 in terms of molar ratio.
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 is in the range of 0.1% by mass or more and 10% by mass or less with respect to the total amount of the lithiumimide salt, the cyclic solvent, and the high vapor pressure solvent. pond. The high vapor pressure solvent, lithium ion secondary batteries according to claim 1 which is a linear ether or cyclic ether. The high vapor pressure solvent, dimethyl ether, ethyl methyl ether, lithium ion secondary batteries according to claim 1 or 2 is at least one chain ether selected from the group consisting of diethyl ether. 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 metallic silicon and metallic lithium. Item 2. The lithium ion secondary battery according to any one of Items 1 to 3.

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