JP2011065842A - Electrolyte for lithium secondary battery, and bipolar secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte for a lithium secondary battery contributing to improvement of durability of a collector, through restraint of side reaction between produced HF and a collector. <P>SOLUTION: The electrolyte for the lithium secondary battery contains a fluorine-based solvent, with a viscosity of the electrolyte of 4.0 to 8.0 cP. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrolyte for a lithium secondary battery and a bipolar secondary battery using the same.

    In recent years, reduction of carbon dioxide emissions has been strongly desired for environmental protection. In the automobile industry, there is an expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of motor drive batteries that hold the key to their practical use has been carried out. ing.

  In the development of such a battery, high power density and high capacity density are required, and further, the battery must be lightened.

  As a technique for such a requirement, for example, in a bipolar secondary battery, it has been proposed to use a current collector including a conductive resin layer (hereinafter also referred to as “resin current collector”).

JP 2006-190649 A

  However, in such a bipolar secondary battery, there is a problem that an electrolyte containing a fluorine-based solvent (such as a supporting salt containing fluorine) is hydrolyzed by a trace amount of water remaining in the electrolyte and HF is generated. is there. Compared to metal current collectors such as Al and Cu, resin current collectors have low or no passivity-forming ability, and are therefore easily decomposed by the action of HF (oxidation action), resulting in reduced durability.

  Therefore, an object of the present invention is to provide an electrolyte for a lithium secondary battery that suppresses a side reaction between the generated HF and the current collector and contributes to improving the durability of the current collector.

  The present inventors have intensively studied to solve the above problems. As a result, it has been found that the above problem can be solved by defining the viscosity of the electrolyte containing the fluorine-based solvent to a predetermined value.

  By using an electrolyte containing a fluorinated solvent and having a viscosity of 4.0 to 8.0 cP, the wettability (affinity) at the interface between the current collector and the electrolyte is lowered. For this reason, the side reaction of the generated HF and the current collector can be suppressed, that is, the oxidation reaction on the current collector surface can be suppressed. Therefore, durability of the lithium secondary battery using the electrolyte is improved.

It is a section schematic diagram showing the structure of a bipolar secondary battery. It is a perspective view showing the appearance of a bipolar secondary battery.

  First, a bipolar secondary battery (for example, a bipolar lithium secondary battery) which is a preferred embodiment will be described, but is not limited to the following embodiments. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  When distinguished by the structure and form of the bipolar secondary battery, it is not particularly limited, such as a stacked (flat) battery or a wound (cylindrical) battery, and can be applied to any conventionally known structure.

  FIG. 1 is a schematic cross-sectional view schematically showing the entire structure of the bipolar secondary battery 10. The bipolar secondary battery 10 shown in FIG. 1 has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate film (battery exterior material) 29 that is a battery exterior material. Have.

  As shown in FIG. 1, the power generation element 21 of the bipolar secondary battery 10 has a positive electrode active material layer 13 electrically connected to one surface of a resin current collector 11, and is opposite to the resin current collector 11. It has a plurality of bipolar electrodes 23 formed with a negative electrode active material layer 15 electrically coupled to the side surface. Hereinafter, in the present specification, the “positive electrode active material layer” may be referred to as a “positive electrode layer” and the “negative electrode active material layer” may be referred to as a “negative electrode layer”. Each bipolar electrode 23 is laminated via the electrolyte layer 17 to form the power generation element 21. In other words, the bipolar electrode 23 has the positive electrode active material layer 13 formed on one surface of the resin current collector 11 and the negative electrode active material layer 15 formed on the other surface. In the bipolar secondary battery 10 shown in FIG. 1, the resin current collector 11 is used as the current collector, but not all of the resin current collector 11 is required. Only the current collector may be a resin current collector. In this case, as the current collector other than the resin current collector, a conventionally known material (for example, Al, Cu, SUS, etc.) can be appropriately selected and used. However, preferably, all are resin current collectors.

  In addition, the electrolyte layer 17 has a configuration in which an electrolyte for a lithium secondary battery is held in a central portion in the surface direction of a separator as a base material (having a configuration including a separator and an electrolyte). At this time, the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other through the electrolyte layer 17. The bipolar electrodes 23 and the electrolyte layers 17 are alternately stacked. That is, the electrolyte layer 17 is interposed between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23. ing. In addition, the electrolyte for lithium secondary batteries which comprises the electrolyte layer 17 shown in FIG. 1 contains the electrolyte containing a fluorine-type solvent, The viscosity is a predetermined value, It is characterized by the above-mentioned.

  The adjacent positive electrode active material layer 13, electrolyte layer 17, and negative electrode active material layer 15 constitute one unit cell layer 19. Therefore, it can be said that the bipolar secondary battery 10 has a configuration in which the single battery layers 19 are stacked. In addition, for the purpose of preventing liquid junction due to leakage of the fluorine-based electrolyte from the electrolyte layer 17, a seal portion (insulating layer) 31 is disposed on the outer peripheral portion of the unit cell layer 19. A positive electrode active material layer 13 is formed only on one side of the positive electrode outermost layer current collector 11 a located in the outermost layer of the power generation element 21. The negative electrode active material layer 15 is formed only on one surface of the outermost current collector 11b on the negative electrode side located in the outermost layer of the power generation element 21. However, the positive electrode active material layer 13 may be formed on both surfaces of the outermost layer current collector 11a on the positive electrode side. Similarly, the negative electrode active material layer 15 may be formed on both surfaces of the outermost layer current collector 11b on the negative electrode side.

  Further, in the bipolar secondary battery 10 shown in FIG. 1, a positive electrode current collector plate 25 is disposed so as to be adjacent to the outermost layer current collector 11a on the positive electrode side, and this is extended to form a laminate film 29 which is a battery exterior material. Derived. On the other hand, the negative electrode current collector plate 27 is disposed so as to be adjacent to the outermost layer current collector 11b on the negative electrode side, and similarly, this is extended and led out from the laminate film 29 which is an exterior of the battery.

  In the bipolar secondary battery 10 shown in FIG. 1, an insulating part 31 is usually provided around each single battery layer 19. The insulating portion 31 prevents the resin current collectors 11 adjacent to each other in the battery from contacting each other or a short circuit caused by a slight irregularity of the end portion of the unit cell layer 19 in the power generation element 21. It is provided for the purpose. By installing such an insulating part 31, long-term reliability, durability and safety are ensured, and a high-quality bipolar secondary battery 10 can be provided. In addition, the effect is synergistic with the use of an electrolyte containing a fluorinated solvent in the present embodiment.

  Note that the number of stacks of the unit cell layers 19 is adjusted according to a desired voltage. Further, in the bipolar secondary battery 10, the number of stacking of the single battery layers 19 may be reduced if a sufficient output can be secured even if the thickness of the battery is made as thin as possible. Even in the bipolar secondary battery 10, in order to prevent external impact and environmental degradation during use, the power generation element 21 is sealed under reduced pressure in a laminate film 29 that is a battery exterior material, and the positive electrode current collector plate 25 and the negative electrode current collector 25. A structure in which the electric plate 27 is taken out of the laminate film 29 is preferable. Hereinafter, main components of the bipolar secondary battery of this embodiment will be described.

<Electrolyte for lithium secondary battery>
As described above, the electrolyte layer 17 has a configuration in which the electrolyte for the lithium secondary battery is held in the center in the surface direction of the separator as the base material. In addition, there is no restriction | limiting in particular also in the material of a separator, Although a conventionally well-known material can be selected suitably and can be used, The microporous film which consists of polyolefins, such as polyethylene and a polypropylene, is mentioned. Specifically, a polypropylene porous film (PP porous film) is preferable. The thickness is not particularly limited, but is preferably about 25 μm, for example.

  As described above, the electrolyte layer 17 includes an electrolyte for a lithium secondary battery. And this electrolyte for lithium secondary batteries contains the electrolyte containing a fluorine-type solvent, and the viscosity is a predetermined value. Preferably, the electrolyte for a lithium secondary battery is made of an electrolyte containing a fluorinated solvent. More specifically, the electrolyte for a lithium secondary battery includes an electrolyte containing a fluorine-based solvent, and the viscosity of the electrolyte is 4.0 to 8.0 cP (mPa · s).

  Conventionally, a bipolar secondary battery including a resin current collector has a problem that an electrolyte containing a fluorine-based solvent is hydrolyzed by a trace amount of water remaining in the electrolyte and HF is generated. Compared to metal current collectors such as Al and Cu, resin current collectors have low or no passivity-forming ability, and are therefore easily decomposed by the action of HF (oxidation action), resulting in reduced durability.

  By using an electrolyte containing a fluorinated solvent and having a viscosity of 4.0 to 8.0 cP, the wettability (affinity) at the interface between the current collector and the electrolyte is lowered. For this reason, the side reaction of the generated HF and the current collector can be suppressed, that is, the oxidation reaction on the current collector surface can be suppressed. Therefore, durability of the lithium secondary battery using the electrolyte is improved. In addition, when it exceeds 8.0 cP (mPa · s), there is a possibility that output characteristics as a battery may be deteriorated.

  The viscosity of the electrolyte for a lithium secondary battery is 4.0 to 8.0 cP (mPa · s), preferably 4.5 to 6.0 cP (mPa · s), more preferably 4. 5 to 5.5 cP (mPa · s). Within such a range, there is an effect of further suppressing deterioration of the resin current collector without impairing battery performance.

  In order to control the viscosity within a desired range, the solvent or supporting salt constituting the electrolyte containing the fluorine-based solvent is appropriately referred to or combined with the conventionally known knowledge and the examples in the present specification. You can adjust the type and amount.

  Here, “viscosity (viscosity)” used in the present specification means a value measured by a rotational viscometer used in the section of the examples. The temperature condition for measuring the viscosity is preferably 15 to 25 ° C, particularly preferably 20 ° C. In such a temperature range, if the viscosity measured by the rotational viscometer used in the example section is included in the desired viscosity range, at least a part of the technical range is implemented. It should be noted that the viscosity may be slightly changed at a stage before the start of operation of the battery or a stage during the operation. However, if at any of all stages the desired viscosity is achieved, then at least a portion of the technical scope has been implemented. In addition, after the battery is completed, referring to or combining known methods, the battery is disassembled, the electrolyte is extracted, and its viscosity is measured with the rotational viscometer used in the section of Example, and the value is desired. If it is included in the viscosity range, at least a part of the technical range is implemented.

  In the present specification, the “electrolyte containing a fluorinated solvent” means that even if “fluorine” is contained in the structure of the solvent constituting the electrolyte, “fluorine” is present in the structure of the supporting salt constituting the electrolyte. Whether included or both are included in the concept. In the present specification, the “electrolyte containing a fluorine-based solvent” may be referred to as “fluorine-based electrolyte” or “electrolyte”.

  In other words, the “fluorine-based electrolyte” may or may not contain “fluorine” in the solvent that constitutes the electrolyte as long as “fluorine” is contained in the supporting salt that constitutes the electrolyte. Also good. On the other hand, as long as “fluorine” is contained in the solvent constituting the electrolyte, “fluorine” may or may not be contained in the supporting salt constituting the electrolyte. However, preferably, the fluorine-based electrolyte includes at least “fluorine” in the structure of the supporting salt.

  In addition, although there is no restriction | limiting in particular in the number of the solvent which comprises a fluorine-type electrolyte, From the viewpoint of achieving a desired effect, two to three types are more preferable than one type.

  Here, the HF generation mechanism will be described in more detail. The fluorine-based electrolyte generates hydrogen fluoride (HF) by being thermally decomposed by heat generated by the charge / discharge reaction of the battery or by being hydrolyzed by a small amount of water remaining in the electrolyte. Since the surface of the current collector that is in contact with the active material layer is exposed to an electric potential, the presence of hydrogen fluoride oxidizes the resin contained in the current collector, thereby degrading the current collector. Of course, it goes without saying that the technical scope is not limited to such a generation mechanism.

(solvent)
As described above, the lithium secondary battery electrolyte includes a fluorine-based electrolyte, and the fluorine-based electrolyte preferably includes at least one compound (solvent) represented by the following chemical formula 1:

In the above chemical formula 1, R ^{1 to} R ⁴ are each independently a hydrogen atom, a halogen atom, an alkyl group or a halogenated alkyl group, and X is a carbon atom or an oxygen atom.

  In Chemical Formula 1, the alkyl group may be linear or branched. Moreover, although there is no restriction | limiting in particular in carbon number which comprises an alkyl group, 1-4 are preferable, More preferably, it is 1-2. More specifically, a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, and the like are preferable, but not limited thereto.

  Specifically, the halogen atom is fluorine (F), chlorine (Cl), bromine (Br), or iodine (I), and particularly preferably fluorine (F).

  The halogenated alkyl group is one in which at least one hydrogen atom of the alkyl group is substituted with a halogen atom.

R ^{1 to} R ⁴ are each independently preferably a hydrogen atom, a halogen atom, an alkyl group or a halogenated alkyl group, more preferably at least one of R ^{1 to} R ⁴ is a halogen atom. More preferably, at least one of the at least one halogen atom is a fluorine atom. Particularly preferably, all such at least one halogen atom is a fluorine atom.

X is preferably a carbon atom or an oxygen atom, more preferably an oxygen atom. In more detail, the case where X is a carbon atom is preferably —CH ₂ —.

Specifically, as the compound represented by Chemical Formula 1,
R ^{1 to} R ⁴ are preferably hydrogen atoms and X is preferably an oxygen atom (that is, ethylene carbonate: EC).

Further, it is preferable that R ¹ is a fluorine atom, R ^{2 to} R ⁴ are hydrogen atoms, and X is an oxygen atom (that is, fluoroethylene carbonate: FEC).

Further, it is preferable that R ¹ is a fluorine atom, R ⁴ is a methyl group, R ² and R ³ are hydrogen atoms, and X is an oxygen atom (that is, 4-fluoropropylene carbonate: FPC).

R ¹ and R ⁴ are preferably a fluorine atom, R ² is a hydrogen atom, R ³ is an ethyl group, and X is an oxygen atom (that is, difluorobutylene carbonate: DFBC).

R ¹ and R ⁴ are fluorine atoms, R ² and R ³ are hydrogen atoms, and X is preferably an oxygen atom (that is, difluoroethylene carbonate: DFEC).
It is preferable that it is at least one selected from the group consisting of these. When at least one of these is selected, since it has a particularly good film-forming ability on the negative electrode, the durability of the current collector is improved and the battery performance is improved.

  The content of the compound represented by Chemical Formula 1 is not particularly limited. However, when the amount of the solvent contained in the fluorine-based electrolyte is 100% by volume, the compound represented by Chemical Formula 1 is preferably contained in an amount of 1 to 50% by volume. More preferably, it is contained in an amount of 10 to 40% by volume. In the case of a mixed solvent, it is the amount of each solvent. The same applies to the following.

More preferably, when the amount of the solvent contained in the fluorine-based electrolyte is 100% by volume, the compound represented by Chemical Formula 1 in which at least one of R ^{1 to} R ⁴ is a halogen atom is 1 to 50% by volume. Contained, more preferably 10 to 30% by volume.

  More preferably, the compound represented by Chemical Formula 1 in which at least one of the at least one halogen atom is a fluorine atom is contained in an amount of 1 to 50% by volume, more preferably 10 to 30% by volume.

  Particularly preferably, the compound represented by Chemical Formula 1 in which all of at least one halogen atom is a fluorine atom is contained in an amount of 1 to 50% by volume, more preferably 10 to 30% by volume.

  When the compound (solvent) represented by Chemical Formula 1 is used and the content thereof is in the above desired range, side reaction between the generated HF and the current collector is suppressed, that is, the current collector surface is oxidized. The reaction can be suppressed. In addition, due to the effect of the compound (solvent) represented by Chemical Formula 1, a conductive film containing fluorine is formed on the negative electrode, thereby improving the durability of the current collector and improving the battery performance.

  Moreover, it is also preferable that the electrolyte for lithium secondary batteries contains other than the compound (solvent) shown by Chemical formula 1, for example, when ethyl methyl carbonate (EMC) is contained, it is especially preferable.

  The electrolyte for a lithium secondary battery is preferably one kind as a solvent, but it is also preferred that two or more kinds or three or more kinds are used. For example, in the case of two types of combinations, a combination of EC / EMC is preferable. If there are three types of combinations, an EC / EMC / FPC combination, an EC / EMC / FEC combination, an EC / EMC / DFBC combination, and the like are preferable.

  Among these, particularly good combinations are a combination of EC / EMC and a combination of EC / EMC / FEC.

First, in the case of the combination of EC / EMC, in the electrolyte for a lithium secondary battery, it is particularly good that LiPF ₆ and LiBF ₄ are included at the same concentration as the supporting salt described later. Specifically, it is preferably contained at a concentration ratio of 0.4 to 0.6 M / L: 0.6 to 0.4 M / L, more preferably 0.5 M / L: 0.5 M / L. In the case of a combination of EC and EMC, these solvents do not contain fluorine in the structure. In this case, the use of a supporting salt containing fluorine in the structure is essential.

In the case of the combination of EC / EMC / FEC, in the electrolyte for a lithium secondary battery, it is particularly good that LiPF ₆ is contained in an amount of 0.9 M / L or more as a supporting salt described later. Furthermore, the ratio of EC and EMC in the total mass of the solvent is preferably 10 to 40% by mass and 10 to 60% by mass, respectively.

  Although there is no restriction | limiting in particular also in the mass ratio in 2 types of combinations, In the case of the combination of EC and EMC, Preferably it is 30-50 mass%: 70-50 mass%, More preferably, 40-50 mass%: 60- 50% by mass, particularly preferably 40% by mass: 60% by mass.

  In addition, when the fluorine-based solvent (for example, FPC, FEC, DFBC) is included in the three combinations, the content thereof is preferably 30% by mass or less, more preferably 10 to 30% by mass. %.

  Of course, the electrolyte for a lithium secondary battery may contain a solvent other than the solvent described above.

(Supporting salt)
The fluorine-based electrolyte preferably contains at least one supporting salt. As long as “fluorine” is contained in the supporting salt constituting the electrolyte, fluorine may not be contained in the constitution of the supporting salt, but preferably fluorine is contained.

Although there is no particular limitation as a supporting salt constituting the fluorine-based _{electrolyte, LiPF 6, LiBF 4, Li} (CF 3 SO 2) 2 N, Li (CF 3 CF 2 SO 2) 2 N and Li _{(FSO 2)} 2 N etc. are mentioned. The electrolyte containing at least one of these supporting salts has the effect of suppressing the amount of HF generated in the battery, and also increases the viscosity of the entire fluorine-based electrolyte, thereby suppressing side reactions with the resin current collector. There is an effect to. These may be one type or two or more types. That is, preferably two or more F-type supporting salts may be contained.

These contents (concentrations) are not particularly limited, but when the total concentration of the supporting salt contained in the fluorine-based electrolyte is 1.0 M / L, at least one of these is preferably 0.01 to 0.5M / L (1-50 mol%) is contained. Specifically, in Example 1, the concentration of the supporting salt contained in the fluorine-based electrolyte is 1.0 M / L, but LiPF ₆ is contained at 0.9 M / L (90 mol%), and LiBF ₄ is contained. 0.1 M / L (10 mol%) is contained. By including two or more kinds of such fluorinated supporting salts (that is, by setting the preferable concentration within the above range), there is an effect of suppressing the amount of HF generated in the battery, and the fluorinated electrolyte as a whole Since the viscosity also increases, there is an effect of suppressing side reactions with the resin current collector.

  The bipolar secondary battery includes a predetermined fluorine-based electrolyte, and an electrolyte for a lithium secondary battery in which the viscosity of the electrolyte is in a predetermined range, and a current collector including a conductive resin layer. .

  Since the bipolar secondary battery contains a desired electrolyte for a lithium secondary battery, the wettability (affinity) at the interface between the current collector and the electrolyte is lowered. For this reason, the side reaction of the generated HF and the current collector can be suppressed, that is, the oxidation reaction on the current collector surface can be suppressed. Therefore, the durability of the lithium secondary battery using the electrolyte can be improved.

  Supplementally, since the electrolyte for a lithium secondary battery in a bipolar secondary battery includes an electrolyte containing a fluorine-based solvent, the wettability (affinity) at the interface between the current collector and the electrolyte is lowered. When the wettability between the resin current collector and the fluorine-based electrolyte decreases, side reactions in the vicinity of the resin current collector are suppressed. As a result, the bipolar secondary battery can obtain a high charge / discharge capacity. Bipolar secondary batteries can provide excellent high-temperature storage characteristics.

  Further, the electrolyte containing at least one of the above supporting salts has an effect of suppressing the amount of HF generated in the battery, and the viscosity of the entire fluorine-based electrolyte is also increased, so that a side reaction with the resin current collector There is an effect to suppress.

  The bipolar lithium ion battery described above is characterized by the structure of the electrolyte. Hereinafter, other main components will be described.

<Resin current collector>
The resin current collector includes a conductive resin layer. Preferably, the resin current collector is composed of a resin layer having conductivity. A resin layer has electroconductivity, contains resin essentially, and plays the role of a collector. In order for the resin layer to have conductivity, as a specific form,
1) A form in which the polymer material constituting the resin is a conductive polymer,
2) A form in which the resin layer contains a resin (polymer) and a conductive agent (conductive filler),
Is mentioned.

  In the above 1), the conductive polymer is selected from materials that are conductive and have no conductivity with respect to ions used as a charge transfer medium. These conductive polymers are considered to be conductive because the conjugated polyene system forms an energy band. As a typical example, a polyene-based conductive polymer that has been put into practical use in an electrolytic capacitor or the like can be used. Specifically, polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, polyoxadiazole, or a mixture thereof is preferable. Polyaniline, polypyrrole, polythiophene, and polyacetylene are more preferable from the viewpoints of electron conductivity and stable use in the battery.

  The conductive agent used in the above form 2) is selected from conductive materials. Preferably, from the viewpoint of suppressing ion permeation in the resin layer having conductivity, it is desirable to use a material that does not have conductivity with respect to ions used as the charge transfer medium. Specific examples include aluminum materials, stainless steel (SUS) materials, carbon materials such as graphite and carbon black (Ketjen black), silver materials, gold materials, copper materials, and titanium materials. Among these, carbon black (Ketjen black) is preferable. These conductive agents may be used alone or in combination of two or more. Moreover, you may use the alloy material which used 2 or more types of silver materials, gold | metal | money materials, aluminum materials, stainless steel materials, carbon materials, etc.

  The resin used in the form 2) is preferably polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoro. Ethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), or a mixture thereof Is mentioned. Among these, polypropylene (PP) is preferable. These materials have a very wide potential window and are stable to both positive and negative electrode potentials. In addition, since it is lightweight, it is possible to increase the output density of the battery.

  The ratio of the conductive agent in the resin layer is not particularly limited. However, preferably, it is preferably 1 to 30% by mass, more preferably 1 to 15% by mass, and even more preferably 10% by mass based on the total of the polymer material (eg polypropylene) and the conductive agent (eg ketjen black). % Conductive agent is present. By allowing a sufficient amount of the conductive agent to be present, sufficient conductivity in the resin layer can be secured.

  The shape (form) of the conductive agent is not limited to the particle form, and may be a form other than the particle form put into practical use as a so-called filler-based conductive resin composition such as a carbon nanotube.

  Although the average particle diameter of a electrically conductive agent is not specifically limited, It is desirable that it is about 0.01-10 micrometers. In the present specification, the “particle diameter” means the maximum distance L among the distances between any two points on the contour line of the conductive agent. As the value of “average particle diameter”, the average particle diameter of particles observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The calculated value shall be adopted. Particle diameters and average particle diameters of active material particles to be described later can be defined similarly.

  The resin layer can be manufactured by a conventionally known method. For example, it can be manufactured by using a spray method or a coating method. Specifically, there is a technique in which a slurry containing a polymer material is prepared, applied and cured. A conductive agent is mentioned as another component contained in the said slurry. As another method, a polymer material and a conductive agent are heated and mixed in an extrusion kneader to produce pellets of a desired size (for example, about 1 to 3 mm), and the pellets are extruded and formed. Can also be produced.

  The thickness of the resin current collector is not particularly limited, but it is preferably as thin as possible to increase the output density of the battery. In a bipolar secondary battery, the resin current collector that exists between the positive electrode and the negative electrode may have a high electrical resistance in the direction parallel to the stacking direction, so the thickness of the resin current collector can be reduced. Is possible. Specifically, the thickness of the resin current collector is preferably 0.1 to 150 μm, more preferably 10 to 100 μm, and more preferably 10 to 50 μm.

  The shape of the resin current collector is not particularly limited, but is preferably a sheet.

  Since the bipolar secondary battery includes a specific electrolyte for a lithium secondary battery, the wettability (affinity) at the interface between the resin current collector and the electrolyte is lowered. For this reason, the side reaction of the generated HF and the current collector can be suppressed, that is, the oxidation reaction on the current collector surface can be suppressed. As a result, a bipolar secondary battery with improved durability can be provided.

  Supplementally, since the viscosity of the electrolyte is within a predetermined range, the wettability between the resin current collector and the fluorine-based electrolyte is reduced, and side reactions in the vicinity of the resin current collector are suppressed. As a result, the bipolar secondary battery has a high charge / discharge capacity.

<Active material layer>
[Positive electrode (positive electrode active material layer) and negative electrode (negative electrode active material layer)]
The active material layer 13 or 15 includes an active material, and further includes other additives (for example, a binder and a conductive auxiliary agent) as necessary.

(Positive electrode active material layer)
The positive electrode active material layer 13 includes a positive electrode active material. The positive electrode active material preferably contains lithium, a transition metal, and a composite oxide. As the positive electrode active material, for example, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni—Co—Mn) O _2, and lithium-such as those in which a part of these transition metals are substituted with other elements Examples include transition metal composite oxides, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. Preferably, a lithium-transition metal composite oxide is used as the positive electrode active material from the viewpoint of capacity and output characteristics. Specifically, LiMn ₂ O ₄ is particularly preferable. Of course, positive electrode active materials other than those described above may be used.

(Negative electrode active material layer)
The negative electrode active material layer 15 includes a negative electrode active material. The negative electrode active material preferably contains at least one of metallic lithium, a lithium-containing alloy, a carbon material that can be doped / undoped with lithium ions, or lithium titanate. Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, lithium-transition metal composite oxides (for example, Li ₄ Ti ₅ O ₁₂ ), metal materials, lithium alloy negative electrode materials, and the like. . Specifically, hard carbon is particularly preferable. In some cases, two or more negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, a carbon material or a lithium-transition metal composite oxide is used as the negative electrode active material. Of course, negative electrode active materials other than those described above may be used.

  The average particle diameter of each active material contained in each active material layer 13, 15 is not particularly limited, but is preferably 1 to 25 μm from the viewpoint of increasing the output. Particularly preferably, the positive electrode active material is about 20 μm and the negative electrode active material is about 15 μm.

(Binder)
The positive electrode active material layer 13 and the negative electrode active material layer 15 preferably contain a binder.

  Although it does not specifically limit as a binder used for each active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile (PEN), polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC), ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR) ), Isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and hydrogenated product thereof, styrene / isoprene / styrene block copolymer and hydrogenated product thereof Thermoplastic polymers such as products, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolymer (FE) ), Tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE) , Fluororesin such as polyvinyl fluoride (PVF), 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 Fluoro rubber (VDF-PFP-TFE fluoro rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber Examples thereof include vinylidene fluoride-based fluororubber such as (VDF-CTFE-based fluororubber), an epoxy resin, and the like. Among these, polyvinylidene fluoride, polyimide, styrene / butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable. Specifically, polyvinylidene fluoride (PVdF) is particularly preferable. These suitable binders are excellent in heat resistance, have a very wide potential window, are stable at both the positive electrode potential and the negative electrode potential, and can be used for the active material layer. These binders may be used alone or in combination of two.

  The amount of the binder contained in each active material layer is not particularly limited as long as it is an amount capable of binding each active material, but preferably 0.5 to 15 mass with respect to the active material layer. %, More preferably 1 to 10% by mass.

(Additives (conducting aids, etc.))
Examples of other additives that can be included in the active material layer include, but are not limited to, conductive assistants and ion conductive polymers, and conventionally known knowledge can be referred to and applied as appropriate.

  The conductive assistant refers to an additive that is blended in order to improve the conductivity of the positive electrode active material layer or the negative electrode active material layer. Examples of the conductive assistant include carbon materials such as carbon black such as acetylene black, graphite, and vapor grown carbon fiber. Of these, acetylene black is preferable. When the active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

  Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

  The compounding ratio of the components contained in the positive electrode active material layer and the negative electrode active material layer is not particularly limited. The blending ratio can be adjusted by appropriately referring to known knowledge about lithium secondary batteries. For example, the conductive auxiliary is preferably 0.5 to 15% by mass, more preferably 1 to 10% by mass with respect to the active material layer.

  The thickness of each active material layer is not particularly limited, and conventionally known knowledge about the battery can be appropriately referred to. For example, the thickness of each active material layer is about 2 to 100 μm, more preferably about 2 to 50 μm, and the thickness of the positive electrode active material layer is about 25 μm. The thickness is about 20 μm.

  The method for producing the active material layer is also not particularly limited, and conventionally known knowledge can be referred to as appropriate or applied in combination. For example, as a solid content, an active material, a binder, a conductive auxiliary agent, and the like are mixed, and an appropriate amount of a viscosity adjusting solvent (for example, NMP) is added to prepare a slurry. It can be produced by applying it to a resin current collector and applying pressure, heating, and drying as necessary.

  In this embodiment, by using such a positive electrode active material and a negative electrode active material, there is an effect of improving energy density as a bipolar secondary battery.

<Outermost layer current collector>
As the material of the outermost layer current collector, for example, a metal or a conductive polymer can be adopted. From the viewpoint of ease of taking out electricity, a metal material is preferably used. Specifically, metal materials, such as aluminum, nickel, iron, stainless steel, titanium, copper, are mentioned, for example. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum and copper are preferable from the viewpoints of electron conductivity and battery operating potential.

<Tab and lead>
A tab may be used for the purpose of taking out the current outside the battery. The tab is electrically connected to the outermost layer current collector or current collector plate, and is taken out of the laminate sheet which is a battery exterior material.

  The material which comprises a tab is not restrict | limited in particular, The well-known highly electroconductive material conventionally used as a tab for lithium secondary batteries can be used. As a constituent material of the tab, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable, and aluminum, copper, and the like are more preferable from the viewpoint of light weight, corrosion resistance, and high conductivity. Is preferred. Note that the same material may be used for the positive electrode tab and the negative electrode tab, or different materials may be used.

  The positive terminal lead and the negative terminal lead are also used as necessary. As the material of the positive terminal lead and the negative terminal lead, a terminal lead used in a known lithium secondary battery can be used. It should be noted that the part taken out from the battery outer packaging material 29 has a heat insulating property so as not to affect the product (for example, automobile parts, particularly electronic devices) by contacting with peripheral devices or wiring and causing leakage. It is preferable to coat with a heat shrinkable tube or the like.

<Battery exterior material>
As the battery exterior material 29, a known metal can case can be used, and a bag-like case using a laminate film containing aluminum that can cover a power generation element (battery element) can be used. For example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used as the laminate film, but the laminate film is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high output and cooling performance, and can be suitably used for a battery for large equipment for EV and HEV.

<Insulation part>
The insulating portion 31 prevents liquid junction due to leakage of the electrolyte from the electrolyte layer 17. Further, the insulating portion 31 prevents the resin collectors adjacent to each other in the battery from coming into contact with each other or a short circuit caused by a slight irregularity of the end portion of the unit cell layer 19 in the power generation element 21. It is provided for the purpose.

  Any material may be used for the insulating portion 31 as long as it has insulating properties, sealing properties, heat resistance, and the like. For example, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, rubber and the like can be used. Among these, polyethylene resin and polypropylene resin are preferably used as the constituent material of the insulating portion 31 from the viewpoints of corrosion resistance, chemical resistance, ease of production (film forming property), economy, and the like.

  In addition, said bipolar secondary battery can be manufactured by a conventionally well-known manufacturing method.

<Appearance structure of bipolar secondary battery>
FIG. 2 is a perspective view showing the appearance of a stacked flat bipolar lithium secondary battery, which is a typical embodiment of a bipolar secondary battery.

  As shown in FIG. 2, the laminated flat bipolar secondary battery 50 has a rectangular flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for taking out electric power from both sides thereof. Has been pulled out. The power generation element (battery element) 57 is encased in the battery exterior material 52 of the bipolar secondary battery 50, and the periphery thereof is heat-sealed. The power generation element (battery element) 57 includes a positive electrode tab 58 and a negative electrode tab 59. It is sealed in a state where it is pulled out to the outside. Here, the power generation element (battery element) 57 corresponds to the power generation element (battery element) 21 of the bipolar secondary battery 10 shown in FIG. 1 described above, and the positive electrode (positive electrode active material layer) 13, A plurality of single battery layers (single cells) 19 constituted of an electrolyte layer 17 and a negative electrode (negative electrode active material layer) 15 are laminated.

  The lithium ion battery is not limited to a laminated flat shape, and a wound lithium ion battery may have a cylindrical shape, or such a cylindrical shape. There is no particular limitation such that the object may be deformed into a rectangular flat shape. In the said cylindrical shape thing, a laminate film may be used for the exterior material, and the conventional cylindrical can (metal can) may be used, for example, It does not restrict | limit. Preferably, the power generation element (battery element) is covered with an aluminum laminate film. With this configuration, weight reduction can be achieved.

  The tabs 58 and 59 shown in FIG. 2 are not particularly limited, and the positive electrode tab 58 and the negative electrode tab 59 may be drawn from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be pulled out. However, the present invention is not limited to the one shown in FIG. 2, for example. Further, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

  In addition to the above effects, the lithium ion battery is a vehicle driving power source that requires high volume energy density and high volume output density as a large capacity power source for electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles. And can be suitably used for an auxiliary power source.

  This will be described in more detail using the following examples and comparative examples. However, the technical scope is not limited only to the following examples.

<Example 1>
1. Production of Resin Current Collector Polypropylene (90% by mass) and ketjen black (10% by mass) (Lion EC600JD) as a conductive agent were heated and mixed in an extrusion kneader to produce about 2 mm pellets. The pellet was heated and extruded to form a resin current collector with a thickness of 30 μm.

2. Step of forming positive electrode layer As a positive electrode active material, a solid content of 85% by mass of LiMn ₂ O ₄ (average particle diameter: 20 μm), 5% by mass of acetylene black as a conductive additive and 10% by mass of PVdF as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content to prepare a positive electrode slurry.

  The positive electrode slurry is mixed with the 1. The positive electrode layer was formed by applying and drying on one side of the resin current collector prepared in (1). Pressing was performed so that the thickness of the positive electrode layer was 25 μm on one side.

3. Formation of negative electrode layer and completion process of bipolar electrode As a negative electrode active material, a solid content consisting of 85% by mass of hard carbon (average particle size: 15 μm), 5% by mass of acetylene black as a conductive additive and 10% by mass of PVdF as a binder On the other hand, an appropriate amount of NMP, which is a slurry viscosity adjusting solvent, was added to prepare a negative electrode slurry.

  The negative electrode slurry was applied to the surface of the resin current collector on which one side of the positive electrode layer was formed, on which the positive electrode layer was not formed, and dried to form a negative electrode layer. Pressing was performed so that the thickness of the negative electrode layer was 30 μm on one side. This formed a bipolar electrode in which a positive electrode layer was formed on one side of the resin current collector and a negative electrode layer was formed on the other side. A heating roll press was applied to the bipolar electrode, and the heating press was performed to such an extent that the electrode did not break through the film.

  The obtained bipolar electrode was cut into 140 mm × 90 mm, and the peripheral portion of the electrode 10 mm had a portion where no electrode layer (both positive electrode layer and negative electrode layer) was previously applied (one side was 20 mm for degassing) Was provided). As a result, a bipolar electrode having a 120 mm × 60 mm electrode portion and a seal margin of 10 mm on one side of the peripheral portion and a seal margin of 20 mm on one side was produced.

  Next, a sealing sheet (PP) was placed on the electrode-uncoated portion around the positive electrode of the bipolar electrode. Next, a separator (PP-made porous film having a thickness of 25 μm) was installed so as to cover all of the resin current collector on the positive electrode side. Then, the sheet | seat for sealing was installed in the electrode non-application part (the same part as the part which installed the said sheet | seat for sealing).

4). Preparation of Fluorine Electrolyte Next, the following materials were mixed at a predetermined ratio to prepare a fluorine electrolyte so that the viscosity was 4.4 cP. That is, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at 40:60 (volume ratio) to prepare a solvent. To this solvent, as a supporting salt, LiPF ₆ was added at a concentration of 0.9 mol / L, and LiBF ₄ was added at a concentration of 0.1 mol / L, respectively. Was made. The concentration of the fluorine-based electrolyte thus obtained was measured by the following method and confirmed to be 4.4 cP.

  The viscosity was a cone / plate type rotary viscometer at a temperature of 20 ° C. (BROOKFIFLD LVDV-II + Pro CP, rotor number CP40).

5. Lamination process After laminating three layers of such bipolar electrodes, the sealing material was pressed and heated from above and below (heat and pressure), each layer was sealed, three sides were sealed, and a seal portion was formed.

  Fluorine electrolyte was injected from one side and vacuum sealed.

  A part (electrode tab: electrode tab: part of a 130 μm × 80 mm 100 μm thick Al plate (electrode current collector plate) that can cover the entire projection surface of the obtained battery element. An electrode current collector (high voltage terminal) having a width of 20 mm was produced. A bipolar secondary battery element is sandwiched between these terminals and covered with a laminate film containing aluminum as a battery exterior material so as to cover them, and the entire bipolar secondary battery element is pressurized by pressing both sides at atmospheric pressure. A bipolar secondary battery with improved contact between the high voltage terminal and the battery element was completed.

6). High temperature storage test A battery charged to SOC 100% at 25 ° C was stored in a 45 ° C constant temperature bath, and the voltage drop after 7 days was determined.

<Examples 2 to 15>
A bipolar secondary battery was produced in the same manner as in Example 1 except that the fluorine-based electrolyte had the composition and viscosity shown in Table 1, and a high-temperature storage test was performed.

<Comparative Examples 1-3>
A bipolar secondary battery was produced in the same manner as in Example 1 except that the fluorine-based electrolyte had the composition and viscosity shown in Table 1, and a high-temperature storage test was performed.

11 Resin current collector,
10, 50 Bipolar secondary battery,
11a The outermost layer current collector on the positive electrode side,
11b The outermost layer current collector on the negative electrode side,
13 positive electrode active material layer (positive electrode layer),
15 negative electrode active material layer (negative electrode layer),
17 electrolyte layer,
19 cell layer,
21, 57 power generation element,
23 Bipolar electrode,
25 positive current collector,
27 negative current collector,
29 Laminate film, battery exterior material 31 Seal part,
58 positive electrode tab,
59 Negative electrode tab.

Claims (5)

  An electrolyte for a lithium secondary battery, comprising an electrolyte containing a fluorinated solvent, wherein the electrolyte has a viscosity of 4.0 to 8.0 cP. The electrolyte containing the fluorinated solvent has the following chemical formula 1:

In the above chemical formula 1, R ^{1 to} R ⁴ are each independently a hydrogen atom, a halogen atom, an alkyl group or a halogenated alkyl group, and X is a carbon atom or an oxygen atom.
Containing at least one compound represented by
2. The electrolyte for a lithium secondary battery according to claim 1, wherein when the amount of the solvent contained in the electrolyte containing the fluorinated solvent is 100% by volume, 1 to 50% by volume of the compound represented by Chemical Formula 1 is contained.   The lithium secondary according to claim 1 or 2, wherein the compound represented by Chemical Formula 1 is at least one selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, 4-fluoropropylene carbonate, and difluorobutylene carbonate. Battery electrolyte. The electrolyte containing the fluorine-based solvent is selected from the group consisting of LiPF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ CF ₂ SO ₂ ) ₂ N, and Li (FSO ₂ ) ₂ N. The electrolyte for lithium secondary batteries according to any one of claims 1 to 3, comprising at least one selected from the group consisting of: The electrolyte for a lithium secondary battery according to any one of claims 1 to 4,
A current collector comprising a conductive resin layer;
A bipolar secondary battery.

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