JP2020155378A - Electrolyte for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To provide an electrolyte for lithium ion batteries, capable of exhibiting good cycle characteristics even at low temperatures.SOLUTION: An electrolyte for lithium ion secondary batteries contains an organic solvent, an electrolyte salt and an additive, the additive containing lithium oxalate borate and fluorine-containing cyclic carbonate.SELECTED DRAWING: None

Description

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

Lithium-ion secondary batteries have the characteristics of higher energy density and electromotive force than lead-acid batteries and nickel-metal hydride batteries. Therefore, in addition to automobiles and houses, mobile phones and notebooks that are required to be smaller and lighter are required. Widely used as a power source for personal computers and the like. Most of these lithium ion secondary batteries use an electrolytic solution for a lithium ion secondary battery in which a lithium salt as an electrolyte salt is dissolved in an organic solvent.

As the organic solvent of the conventional electrolytic solution for a lithium ion secondary battery, ethylene carbonate, propylene carbonate and the like are used. Further, as the lithium salt, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoride (LiBF ₄ ) and the like are used.

The organic solvent as described above is required to have a high dielectric constant enough to ionize a lithium salt and to be usable in a wide temperature range of at least -20 to 60 ° C. In particular, in the case of an in-vehicle power supply for an electric vehicle or the like, high output characteristics are required when starting in a cold region (about −20 ° C.), so improvement of low temperature characteristics is important.

In order to meet this requirement, ethylene carbonate, which is a cyclic carbonate (cyclic carbonate ester), and chain carbonate (chain carbonate), which has a low viscosity and does not have a ring structure, such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, are used. An organic solvent in which the above is mixed is generally widely used.
However, the electrolytic solution mixed with ethylene carbonate has a problem that the viscosity becomes high and the ionic conductivity becomes low at a low temperature.
As a conventional method for solving this problem, a non-aqueous solvent using cyclic carbonates (cyclic carbonate esters) having branched chains such as propylene carbonate and butylene carbonate, and lactones such as γ-butyrolactone and γ-valerolactone. It is known to be used (see, for example, Patent Document 1 and Patent Document 2).

Japanese Patent No. 3369583 Japanese Patent No. 4407205

When a carbon material such as graphite or amorphous carbon is used as the negative electrode active material in the negative electrode of a conventional lithium ion secondary battery, the propylene carbonate or butylene carbonate contained in the electrolytic solution is the carbon during charging. It is co-inserted into the material and further reduced and decomposed. As a result, the layered structure of the carbon material is destroyed, so that a good interface coating (SEI) is not formed on the surface of the negative electrode, and there is a problem that it is difficult to function as a secondary battery.
Further, even when the electrolytic solution contains γ-butyrolactone or γ-valerolactone, there is a problem that it is difficult to function as a secondary battery because these are reduced and decomposed during charging, which hinders the formation of good SEI. .. Despite these problems, when the electrolytic solution is used in a secondary battery and repeatedly charged, the capacity of the battery immediately drops significantly, that is, the cycle life characteristic is poor. There is.

Therefore, an object of the present invention is to provide an electrolytic solution for a lithium ion battery that can exhibit good cycle characteristics even at a low temperature.

As a result of diligent studies, the present inventors have found that the above problems can be solved by using an electrolytic solution for a lithium ion secondary battery containing an additive containing lithium difluorooxalate borate and fluoroethylene carbonate. The following invention has been completed. The gist of the present invention is as follows.

[1] An electrolytic solution for a lithium ion secondary battery containing an organic solvent, an electrolyte salt, and an additive.
An electrolytic solution for a lithium ion secondary battery in which the additive contains lithium oxalate borate and a fluorine-containing cyclic carbonate.
[2] The electrolytic solution for a lithium ion secondary battery according to [1], wherein the lithium oxalate borate is lithium difluorooxalate borate (LiDFOB).
[3] The electrolytic solution for a lithium ion secondary battery according to [1] or [2], wherein the fluorine-containing cyclic carbonate is a monofluoroethylene carbonate.
[4] The electrolytic solution for a lithium ion secondary battery according to any one of [1] to [3], wherein the content of the lithium oxalate volate in the electrolytic solution for a lithium ion battery is 0.1 to 2% by mass. ..
[5] The electrolytic solution for a lithium ion secondary battery according to any one of [1] to [4], wherein the content of the fluorine-containing cyclic carbonate with respect to the electrolytic solution for a lithium ion battery is 0.1 to 2% by mass. ..
[6] The mass ratio of the lithium oxalate volate to the fluorine-containing cyclic carbonate (lithium oxalate borate / fluorine-containing cyclic carbonate) is 0.05 to 0.95, according to any one of [1] to [5]. The electrolytic solution for a lithium ion secondary battery described.
[7] The electrolytic solution for a lithium ion secondary battery according to any one of [1] to [6], wherein the organic solvent contains ethylene carbonate.
[8] The electrolytic solution for a lithium ion secondary battery according to any one of [1] to [7], wherein the additive further contains a vinylene carbonate.
[9] The electrolytic solution for a lithium ion secondary battery according to any one of [1] to [8], wherein the positive electrode of the lithium ion secondary battery has a positive electrode active material layer containing lithium iron phosphate.
[10] A lithium ion secondary battery comprising the electrolytic solution for the lithium ion secondary battery according to any one of [1] to [9].
[11] The lithium ion secondary battery according to [10], which includes a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode active material layer containing iron phosphate.

According to the present invention, it is possible to provide an electrolytic solution for a lithium ion battery that can exhibit good cycle characteristics even at a low temperature. The "low temperature" means about −20 ° C.

<Lithium-ion secondary battery electrolyte>
The electrolytic solution for a lithium ion secondary battery of the present invention contains an organic solvent, an electrolyte salt and an additive, and the additive contains lithium oxalate borate and a fluorine-containing cyclic carbonate.
In the present invention, lithium oxadiazole borate and fluorine-containing cyclic carbonate are focused on as additives for forming good SEI even at low temperature and improving cycle characteristics, and by these, not only good capacity characteristics but also low temperature It was found that good cycle characteristics were also exhibited in.
The reason why such an effect is obtained is not clear, but it is presumed as follows. First, among the additives, fluorine, which is a fluorine-containing cyclic carbonate, and boron, which is lithium oxalate borate, contribute to the formation of SEI. It is considered that the formation of SEI prevents direct contact between the electrolytic solution and the active material, thereby improving the cycle characteristics and the like. However, if the SEI becomes too thick, the internal resistance at low temperature will increase. In response to such a phenomenon, the ring structure of the cyclic carbonate of the additive is decomposed into a chain and used for forming a SEI film, thereby preventing the SEI from becoming excessively thick. As a result, it is presumed that an effective SEI is formed and the above-mentioned effect is produced.
Hereinafter, the electrolytic solution for the lithium ion secondary battery will be described in detail.

[Additive]
(Lithium oxalate borate)
Examples of the lithium oxalate borate include lithium difluorooxalate borate (LiDFOB) and lithium bisoxalate borate (LiBOB), and lithium difluorooxalate borate is preferable. Lithium difluorooxalate borate contains fluorine, which promotes the formation of SEI at low temperatures.

The content of lithium oxalate borate in the electrolytic solution is preferably 0.1 to 2% by mass, more preferably 0.1 to 1% by mass. When the content is 0.1 to 2% by mass, the amount of reduction decomposition on the surface of the negative electrode becomes appropriate, and the formed film can be stably maintained.

(Fluorine-containing cyclic carbonate)
The fluorine-containing cyclic carbonate is preferably at least one selected from the group consisting of monofluoroethylene carbonate, 4,4-difluoroethylene carbonate, and 4,5-difluoroethylene carbonate, and is preferably monofluoroethylene carbonate. More preferred.

The content of the fluorine-containing cyclic carbonate in the electrolytic solution is preferably 0.1 to 2% by mass, more preferably 1 to 1.8% by mass. When the content is 0.1 to 2% by mass, the amount of reduction decomposition on the surface of the negative electrode becomes appropriate, and the formed film can be stably maintained.

The mass ratio of lithium oxalate volate to fluorine-containing cyclic carbonate (lithium oxalate borate (preferably LiDFOB) / fluorine-containing cyclic carbonate (preferably FEC)) is preferably 0.05 to 0.95. It is more preferably 1 to 0.7, and even more preferably 0.1 to 0.4. When the mass ratio is 0.05 to 0.95, LiDFOB is bulkier than FEC, the decomposition residue is bulky, the film thickness is increased, and the resistance tends to increase. Therefore, LiDFOB / FEC By setting the value to 0.05 to 0.95, the thickness of the film does not become too large, and SEI with a good resistance value can be formed.

As other additives, vinylene carbonate (VC) and ethylene sulfite (ES) may be contained from the viewpoint of stabilizing SEI, and among them, vinylene carbonate is preferably contained. The content of the other additives in the electrolytic solution is preferably 3% by mass or less, and more preferably 0.05 to 2% by mass.

[Organic solvent]
Examples of the organic solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, and the like. Polar solvents such as 1,2-diethoxyethane, tetrohydrafuran, 2-methyltetraethane, dioxolane, and methylacetamide, or mixtures of two or more of these solvents can be mentioned, with ethylene carbonate, ethylmethyl carbonate, and propylene carbonate being preferred. , It is more preferable to contain ethylene carbonate.

Among them, it is preferable to include a combination of ethylene carbonate and ethyl methyl carbonate, or a combination of ethylene carbonate, propylene carbonate and ethyl methyl carbonate.
In this case, the content of ethylene carbonate in the organic solvent is preferably 15 to 60% by volume, more preferably 20 to 50% by volume.
The content of propylene carbonate in the organic solvent is preferably 1 to 15% by volume, more preferably 1 to 10% by volume.
The content of ethyl methyl carbonate in the organic solvent is preferably 40 to 80% by volume, more preferably 50 to 75% by volume.

[Electrolyte salt]
Electrolyte salts include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ CO ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ). Examples thereof include salts containing lithium such as ₂ and LiN (COCF ₂ CF ₃ ) ₂ . In addition, a complex such as an organic acid lithium salt-boron trifluoride complex and a complex hydride such as LiBH ₄ can be mentioned. These salts or complexes may be used alone or as a mixture of two or more. Of these, LiPF ₆ is preferable.
The content of the electrolyte salt in the electrolytic solution is preferably 5 to 15% by mass, more preferably 7 to 13% by mass.

The electrolytic solution of the present invention may be a gel-like electrolyte further containing a polymer compound. Examples of the polymer compound include a fluorine-based polymer such as polyvinylidene fluoride and a polyacrylic polymer such as methyl poly (meth) acrylate. The gel electrolyte may be used as a separator.

The electrolytic solution of the present invention may be arranged between the negative electrode and the positive electrode. For example, the electrolytic solution is filled in the negative electrode and the positive electrode, or in the battery cell in which the negative electrode, the positive electrode, and the separator are housed. Further, the electrolytic solution may be applied on the negative electrode or the positive electrode and arranged between the negative electrode and the positive electrode, for example.

Further, as the lithium ion secondary battery in which the electrolytic solution of the present invention is used, it is preferable that the positive electrode thereof is a battery having a positive electrode active material layer containing lithium iron phosphate. Since the low reduction potential of lithium oxalate volate, which is an additive contained in the electrolytic solution of the present invention, is suitable for the voltage range of the battery, the characteristics of the electrolytic solution of the present invention can be easily brought out.

<Lithium-ion secondary battery>
The lithium ion secondary battery of the present invention is not particularly limited as long as it includes the electrolytic solution for the lithium ion secondary battery of the present invention, and includes, for example, a positive electrode, a separator, a negative electrode, and an electrolytic solution. Further, an insulating layer may be used instead of the separator, or an insulating layer may be provided together with the separator. The insulating layer can be provided between the positive electrode and the separator, or between the separator and the negative electrode.

(Positive electrode)
The positive electrode in the lithium ion secondary battery of the present invention preferably has a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector. The positive electrode active material layer typically includes a positive electrode active material and a binder for the positive electrode.
Examples of the positive electrode active material include lithium metallic acid compounds. Examples of the lithium metal acid compound include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium iron phosphate and the like. Further, as the positive electrode active material, a material using a plurality of metals other than lithium may be used, and an NCM (nickel cobalt manganese) oxide, an NCA (nickel cobalt aluminum) oxide, or the like called a ternary system is used. You may. As the positive electrode active material, one of these substances may be used alone, or two or more of these substances may be used in combination, but lithium iron phosphate is preferable. As described above, since the low reduction potential of lithium oxalate volate is suitable for the voltage range of the battery using lithium iron phosphate as the positive electrode active material, the characteristics of the electrolytic solution of the present invention can be easily brought out. Examples of lithium iron phosphate include LiFePO ₄ , Li (Fe, Mn) PO ₄ , Li (Fe, Co) PO ₄ , Li (Fe, Ni) PO ₄ , and LiFePO ₄ is preferable.

The positive electrode active material is not particularly limited, but its average particle size is preferably 0.1 to 50 μm, more preferably 0.1 to 30 μm. The average particle size means the particle size (D50) when the volume integration is 50% in the particle size distribution obtained by the laser diffraction / scattering method.
From the viewpoint of lowering the diffusion resistance of lithium ions and improving the cycle characteristics of the battery, the average particle size is even more preferably 0.50 to 2.00 μm.
The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, based on the total amount of the positive electrode active material layer.

Specific examples of the binder for the positive electrode include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), fluorine-containing resin such as polytetrafluoroethylene (PTFE), and polymethyl acrylate (PMA). ), Acrylic resin such as polyvinylidene methacrylate (PMMA), polyvinylidene acetate, polyimide (PI), polyamide (PA), polyvinylidene chloride (PVC), polyethernitrile (PEN), polyethylene (PE), polypropylene (PP) , Polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber (SBR), poly (meth) acrylic acid, carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyvinyl alcohol and the like. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt.
Among these, a fluorine-containing resin is preferable, and polyvinylidene fluoride is more preferable.
The content of the binder in the positive electrode active material layer is preferably 0.5% by mass or more, more preferably 0.5 to 20% by mass, and 1.0 to 10% based on the total amount of the positive electrode active material layer. Mass% is more preferred.

The positive electrode active material layer may further contain a conductive auxiliary agent. As the conductive auxiliary agent, a material having higher conductivity than the positive electrode active material or the negative electrode active material is used. Specifically, carbon black such as Ketjen black and acetylene black (AB), carbon nanotubes, carbon such as rod-shaped carbon, etc. Materials and the like can be mentioned. The conductive auxiliary agent may be used alone or in combination of two or more. When the positive electrode active material layer contains a conductive auxiliary agent, the content of the conductive auxiliary agent is preferably 0.5 to 15% by mass, preferably 1.0 to 9% by mass, based on the total amount of the positive electrode active material layer. More preferably.

The positive electrode active material layer may contain an optional component other than the positive electrode active material, the conductive auxiliary agent, and the binder as long as the effects of the present invention are not impaired. However, the total content of the positive electrode active material, the conductive auxiliary agent, and the binder in the total mass of the positive electrode active material layer is preferably 90% by mass or more, and more preferably 95% by mass or more.
The thickness of the positive electrode active material layer (thickness of each of the plurality of positive electrode active material layers) is not particularly limited, but is preferably 10 to 100 μm, more preferably 20 to 80 μm.

Examples of the material used as the positive electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum, titanium, nickel and stainless steel are preferable, and aluminum is more preferable. The positive electrode current collector is generally made of a metal foil, and the thickness thereof is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm. When the thickness of the electrode current collector is 1 to 50 μm, the handling of the electrode current collector can be facilitated and the decrease in energy density can be suppressed.

(Negative electrode)
The negative electrode in the lithium ion secondary battery of the present invention preferably has a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector. The negative electrode active material layer typically includes a negative electrode active material and a binder for the negative electrode.
Examples of the negative electrode active material include carbon materials such as graphite and hard carbon, a composite of a tin compound, silicon and carbon, and lithium. Among these, a carbon material is preferable, and graphite is more preferable. As the negative electrode active material, these substances may be used alone or in combination of two or more.
The negative electrode active material is not particularly limited, but its average particle size is preferably 0.5 to 50 μm, more preferably 1 to 30 μm.
The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, based on the total amount of the negative electrode active material layer.

Specific examples of the binder for the negative electrode are the same as those of the binder for the positive electrode, and these binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt.
Among these, a fluorine-containing resin is preferable, and polyvinylidene fluoride is more preferable.
The content of the binder in the negative electrode active material layer is preferably 0.5% by mass or more, more preferably 0.5 to 20% by mass, and 1.0 to 10% based on the total amount of the negative electrode active material layer. Mass% is more preferred.

The negative electrode active material layer may contain a conductive auxiliary agent. Specific examples of the conductive auxiliary agent include the same as in the case of the positive electrode active material layer. The conductive auxiliary agent may be used alone or in combination of two or more.
When the negative electrode active material layer contains a conductive auxiliary agent, the content of the conductive auxiliary agent is preferably 1 to 30% by mass, preferably 2 to 25% by mass, based on the total amount of the negative electrode active material layer. Is more preferable.
It should be noted that the negative electrode active material layer may contain any components other than the negative electrode active material, the conductive auxiliary agent, and the binder as long as the effects of the present invention are not impaired, as in the case of the positive electrode active material layer. And its content is also the same.
The thickness of the negative electrode active material layer (thickness of each of the plurality of negative electrode active material layers) is not particularly limited, but is preferably 10 to 100 μm, more preferably 20 to 80 μm.

Examples of the material constituting the negative electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, copper, titanium, nickel and stainless steel are preferable, and copper is more preferable. The negative electrode current collector is also generally made of a metal foil like the positive electrode current collector, and its thickness is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm. When the thickness of the electrode current collector is 1 to 50 μm, the handling of the electrode current collector can be facilitated and the decrease in energy density can be suppressed.

(Insulation layer)
It is preferable to have an insulating layer between the positive electrode and the separator or between the separator and the negative electrode. The insulating layer effectively prevents short circuits between the positive and negative electrodes. The insulating layer is preferably a layer having a porous structure containing insulating fine particles and a binder for an insulating layer, and the insulating fine particles are bound by a binder for an insulating layer.

The insulating fine particles are not particularly limited as long as they are insulating, and may be either organic particles or inorganic particles. Specific organic particles include, for example, crosslinked polymethyl methacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamide-2-methylpropanesulfonate), and the like. Examples thereof include particles composed of organic compounds such as polyacetal resin, epoxy resin, polyester resin, phenol resin, and melamine resin. Inorganic particles include silicon dioxide, silicon nitride, alumina, boehmite, titania, zirconia, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb ₂ O ₅ ), tantalum oxide (Ta ₂ O ₅ ), potassium fluoride, and foot. Examples thereof include particles composed of inorganic compounds such as lithium pentoxide, clay, zeolite, and calcium carbonate. Further, the inorganic particles may be particles composed of known composite oxides such as niobium-tantalum composite oxide and magnesium-tantalum composite oxide. One type of insulating fine particles may be used alone, or a plurality of types may be used in combination.
The average particle size of the insulating fine particles is not particularly limited as long as it is smaller than the thickness of the insulating layer, and is, for example, 0.001 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.1 to 0.6 μm. is there.
The content of the insulating fine particles contained in the insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, and further preferably 60 to 85% by mass based on the total amount of the insulating layer. When the content of the insulating fine particles is within the above range, the insulating layer can form a uniform porous structure and is provided with appropriate insulating properties.

As the binder for the insulating layer, the same type as the binder for the positive electrode described above can be used. The content of the binder for the insulating layer in the insulating layer is preferably 5 to 50% by mass, more preferably 10 to 45% by mass, still more preferably 15 to 40% by mass, based on the total amount of the insulating layer.
The thickness of the insulating layer is preferably 1 to 10 μm, more preferably 2 to 8 μm, and even more preferably 3 to 7 μm.

(Electrolytes)
As the electrolyte used in the lithium ion secondary battery of the present invention, as described above, the electrolyte solution for the lithium ion secondary battery of the present invention is used.

The lithium ion secondary battery of the present invention may be any of a wound type and a laminated type, but the laminated type is preferable. In this case, the negative electrode and the positive electrode may be provided alternately along the stacking direction. Further, the separator may be arranged between each negative electrode and each positive electrode, and when an insulating layer is provided, it may be provided between the negative electrode and the separator or between the positive electrode and the separator.
A plurality of positive electrode current collectors constituting each positive electrode are collectively attached to a positive electrode tab or the like, and connected to a positive electrode terminal via a positive electrode tab or the like. Further, the plurality of negative electrode current collectors constituting each negative electrode are collectively attached to the negative electrode tab or the like, and connected to the negative electrode terminal via the negative electrode tab or the like.

The lithium ion secondary battery usually has a casing, and the positive electrode and the negative electrode described above may be housed in the casing. The casing is not particularly limited, but may be an outer can or the like, or may be an outer film. As the exterior film, it is preferable that the negative electrode, the separator and the positive electrode are arranged between the two exterior films or one exterior film is folded in half, for example.

<Manufacturing method of lithium ion secondary battery>
In the lithium ion secondary battery of the present invention, for example, an electrode structure produced by laminating a positive electrode, a separator, and a negative electrode by a crimping process is housed in an exterior body, and an electrolytic solution is sealed, and then the battery is sealed. It can be manufactured by sealing as follows.
When the insulating layer is provided, for example, the insulating layer may be formed on at least one surface of the positive electrode active material layer and the negative electrode active material layer.

[Preparation of positive electrode]
(Formation of positive electrode active material layer)
In the formation of the positive electrode active material layer, first, a composition for the positive electrode active material layer containing the positive electrode active material, the binder for the positive electrode, and the solvent is prepared. The composition for the positive electrode active material layer may contain other components such as a conductive additive to be blended if necessary. The positive electrode active material, the binder for the positive electrode, the conductive auxiliary agent and the like are as described above. The composition for the positive electrode active material layer is a slurry.

Water or an organic solvent is used as the solvent in the positive electrode active material layer composition. Specific examples of the organic solvent include one or more selected from N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, and dimethylformamide. Of these, N-methylpyrrolidone is preferred.
The solid content concentration of the composition for the positive electrode active material layer is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

The positive electrode active material layer may be formed by a known method using the above-mentioned composition for the positive electrode active material layer. For example, the above-mentioned composition for the positive electrode active material layer is applied onto the positive electrode current collector and dried. Can be formed by
Further, the positive electrode active material layer may be formed by applying the composition for the positive electrode active material layer on a base material other than the positive electrode current collector and drying it. Examples of the base material other than the positive electrode current collector include known release sheets. As for the positive electrode active material layer formed on the base material, preferably, after forming an insulating layer on the positive electrode active material layer, the positive electrode active material layer may be peeled off from the base material and transferred onto the positive electrode current collector.
The positive electrode active material layer formed on the positive electrode current collector or the base material is preferably pressure-pressed. By pressurizing, it becomes possible to increase the electrode density. The pressure press may be performed by a roll press or the like.

[Preparation of negative electrode]
(Formation of negative electrode active material layer)
In forming the negative electrode active material layer, first, a composition for the negative electrode active material layer containing the negative electrode active material, the negative electrode binder, and the solvent is prepared. The composition for the negative electrode active material layer may contain other components such as a conductive auxiliary agent to be blended as needed. The negative electrode active material, the binder for the negative electrode, the conductive auxiliary agent and the like are as described above. The composition for the negative electrode active material layer is a slurry.

As the solvent in the negative electrode active material layer composition, the same solvent as in the positive electrode active material layer composition can be used, and the solid content concentration thereof is also the same.

The negative electrode active material layer may be formed by a known method using the negative electrode active material layer composition. For example, the negative electrode active material layer composition is applied onto the negative electrode current collector and dried. Can be formed by
Further, the negative electrode active material layer may be formed by applying the composition for the negative electrode active material layer on a base material other than the negative electrode current collector and drying it. Examples of the base material other than the negative electrode current collector include known release sheets. For the negative electrode active material layer formed on the base material, preferably, after the insulating layer is formed on the negative electrode active material layer, the negative electrode active material layer may be peeled off from the base material and transferred onto the negative electrode current collector.
The negative electrode active material layer formed on the negative electrode current collector or the base material is preferably pressure-pressed. By pressurizing, it becomes possible to increase the electrode density. The pressure press may be performed by a roll press or the like.

(Formation of insulating layer)
The composition for an insulating layer used when forming the insulating layer contains inorganic particles, a binder for the insulating layer, and a solvent. The composition for the insulating layer may contain other optional components to be blended as needed. Details of the inorganic particles, the binder for the insulating layer, and the like are as described above. The composition for the insulating layer is a slurry. As the solvent, water or an organic solvent may be used, and the details of the organic solvent include those similar to those of the organic solvent in the positive electrode active material layer composition. The solid content concentration of the composition for the insulating layer is preferably 5 to 75% by mass, more preferably 15 to 50% by mass.

The insulating layer can be formed by applying the composition for an insulating layer on the positive electrode or negative electrode active material layer and drying it. The method of applying the composition for the insulating layer to the surface of the positive electrode or negative electrode active material layer is not particularly limited, and for example, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a bar coating method, a gravure coating method, etc. Screen printing method and the like can be mentioned.
The drying temperature is not particularly limited as long as the solvent can be removed, but is, for example, 40 to 120 ° C, preferably 50 to 90 ° C. The drying time is not particularly limited, but is, for example, 30 seconds to 20 minutes.

The positive electrode and the negative electrode obtained as described above are pressure-bonded via a separator to form an electrode component. The specific method of crimping the positive electrode and the negative electrode is to press the positive electrode, the separator, and the negative electrode on top of each other (if there are multiple layers, they are alternately arranged and superposed) with a press or the like. It is good to do it by doing. The pressing conditions may be such that the positive electrode active material layer and the negative electrode active material layer are not compressed more than necessary. Specifically, the press temperature is 50 to 130 ° C., preferably 60 to 100 ° C., and the press pressure is, for example, 0.2 to 3 MPa, preferably 0.4 to 1.5 MPa. The press time is, for example, 15 seconds to 15 minutes, preferably 30 seconds to 10 minutes.

In the electrode structure obtained as described above, for example, the positive electrode current collector is connected to the positive electrode terminal, the negative electrode current collector is connected to the negative electrode terminal, and the electrode structure is housed in the exterior body, and the electrolytic solution of the present invention is sealed therein. After that, the lithium ion secondary battery of the present invention can be manufactured by sealing it so as to be in a sealed state.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The obtained lithium ion secondary battery was evaluated by the following evaluation method.
(Charging / discharging characteristics)
(1) 25 ° C cycle maintenance rate:
Each of the batteries obtained in Examples and Comparative Examples was cycled at 25 ° C. at charge / discharge rates of 1C and 1C, and the capacity retention rate in 500 cycles was measured.
(2) -20 ° C cycle maintenance rate:
Each of the batteries obtained in Examples and Comparative Examples was cycled at −20 ° C. at charge / discharge rates of 0.18C and 0.18C, and the capacity retention rate in 1000 cycles was measured.

[Example 1]
(Preparation of positive electrode)
LiFePO ₄ (average particle size 18 μm) was 92.8 parts by mass as a positive electrode active material, and carbon black (SUPER P Li manufactured by Imeris GC Japan, average particle size 0.040 μm) was 5 parts by mass as a conductive auxiliary agent. , 2.2 parts by mass of polyvinylidene fluoride (KF polymer L # 9305 manufactured by Kureha) as an electrode binder and N-methylpyrrolidone as a solvent were mixed to adjust the solid content concentration to 60% by mass. Slurry was obtained. This slurry for the positive electrode active material layer was applied to both sides of an aluminum foil having a thickness of 15 μm as a positive electrode current collector, pre-dried, and then vacuum dried at 120 ° C. After that, the positive electrode current collector coated with the slurry for the positive electrode active material layer on both sides is pressure-pressed by a roller at a linear pressure of 400 kN / m, and further punched into an electrode size of 467 mm × 175 mm square, and the positive electrode active material is punched on both sides. A positive electrode having a layer was used. Of the dimensions, the area to which the positive electrode active material was applied was 432 mm × 175 mm. The thickness of the positive electrode active material layer formed on both sides was 64.25 μm per side.

(Preparation of negative electrode)
100 parts by mass of graphite as the negative electrode active material, 1.5 parts by mass of styrene butadiene rubber (SBR) (BM-451B manufactured by Nippon Zeon) as a binder for electrodes, and carboxymethyl cellulose (CMC) (manufactured by Daicel Fine Chem Ltd.) as a thickener. 1.5 parts by mass of CMC Daicel 2200) and water as a solvent were mixed to obtain a slurry for a negative electrode active material layer adjusted to have a solid content of 50% by mass. This slurry for the negative electrode active material layer was applied to both sides of an electrolytic copper foil (NC-WS-10 manufactured by Furukawa Electric Co., Ltd., thickness 10 μm) having a thickness of 12 μm as a negative electrode current collector, and vacuum dried at 100 ° C. After that, the negative electrode current collector coated with the slurry for the negative electrode active material layer on both sides is pressure-pressed by a roller at a linear pressure of 500 kN / m, and further punched into an electrode size of 468 mm × 181 mm square, and the negative electrode active material is punched on both sides. It was a negative electrode having a layer. Of the dimensions, the area to which the negative electrode active material was applied was 439 mm × 181 mm. The thickness of the negative electrode active material layer formed on both sides was 43.5 μm per side.

(Preparation of electrolyte)
FEC, LiDFOB, vinylene carbonate (VC) as additives, and electrolyte salt as additives in a solvent obtained by mixing ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) in the volume ratio shown in Table 1. An electrolytic solution was prepared by dissolving LiPF ₆ at the ratio shown in Table 1.

(Battery manufacturing)
The 18 negative electrodes, 17 positive electrodes, and 34 separators obtained above were laminated to obtain an electrode laminate. Here, the negative electrode and the positive electrode were arranged alternately, and a separator was arranged between each negative electrode and the positive electrode. Further, as the separator, a polyethylene porous film was used.
The exposed ends of the positive electrode current collectors of each positive electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined. Similarly, the exposed ends of the negative electrode current collectors of each negative electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined.
Next, the laminate was sandwiched between aluminum laminate films, the terminal tabs were projected to the outside, and the three sides were sealed by laminating. A laminated cell was manufactured by injecting the electrolytic solution obtained above from one side left unsealed and vacuum-sealing.

[Examples 2 and 3]
An electrolytic solution was prepared in the same manner as in Example 1 except that the composition of the organic solvent, additives, and electrolyte salt in the electrolytic solution was as shown in Table 1, and further, a battery (laminated cell) was prepared and evaluated. went. The results are shown in Table 1.

[Example 4]
In the positive electrode active material, the average 92 parts by mass 92.8 parts by mass of LiFePO ₄ and to its amount of particle diameter The average particle diameter of 1.07μm to LiFePO ₄ of 18 [mu] m, 6 parts by weight of a conductive auxiliary agent of 5 parts by mass, the electrode A positive electrode was prepared by setting the amount of the binder for use from 2.2 parts by mass to 2 parts by mass, and an electrolytic solution was prepared in the same manner as in Example 1 except that the blending of the organic solvent, the additive and the electrolyte salt was shown in Table 1. Further, a battery (laminated cell) was produced in the same manner as in Example 1 and evaluated. The results are shown in Table 1.

[Comparative Example 1]
A positive electrode was prepared by adjusting the positive electrode active material from 92.8 parts by mass to 92 parts by mass, the conductive auxiliary agent from 5 parts by mass to 6 parts by mass, and the electrode binder from 2.2 parts by mass to 2 parts by mass, and using the electrolytic solution. An electrolytic solution was prepared in the same manner as in Example 1 except that the composition of the organic solvent, the additive, and the electrolyte salt was as shown in Table 1, and a battery (laminated cell) was further prepared and evaluated. .. The results are shown in Table 1.

[Comparative Examples 2 and 3]
An electrolytic solution was prepared in the same manner as in Example 1 except that the composition of the organic solvent, additives, and electrolyte salt in the electrolytic solution was as shown in Table 1, and further, a battery (laminated cell) was prepared and evaluated. went. The results are shown in Table 1.

Claims (11)

An electrolytic solution for a lithium ion secondary battery containing an organic solvent, an electrolyte salt, and an additive.
An electrolytic solution for a lithium ion secondary battery in which the additive contains lithium oxalate borate and a fluorine-containing cyclic carbonate. The electrolytic solution for a lithium ion secondary battery according to claim 1, wherein the lithium oxalate borate is lithium difluorooxalate borate (LiDFOB). The electrolytic solution for a lithium ion secondary battery according to claim 1 or 2, wherein the fluorine-containing cyclic carbonate is a monofluoroethylene carbonate. The electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the content of the lithium oxalate volate with respect to the electrolytic solution for a lithium ion battery is 0.1 to 2% by mass. The electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the content of the fluorine-containing cyclic carbonate with respect to the electrolytic solution for a lithium ion battery is 0.1 to 2% by mass. The lithium according to any one of claims 1 to 5, wherein the mass ratio of the lithium oxalate volate to the fluorine-containing cyclic carbonate (lithium oxalate borate / fluorine-containing cyclic carbonate) is 0.05 to 0.95. Electrolyte for ion secondary batteries. The electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the organic solvent contains ethylene carbonate. The electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the additive further contains a vinylene carbonate. The electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the positive electrode of the lithium ion secondary battery has a positive electrode active material layer containing lithium iron phosphate. A lithium ion secondary battery comprising the electrolytic solution for a lithium ion secondary battery according to any one of claims 1 to 9. The lithium ion secondary battery according to claim 10, further comprising a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode active material layer containing iron phosphate.