JP2022100812A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide a non-aqueous electrolyte secondary battery which is reduced in electric resistance and enhanced in cycle characteristic.SOLUTION: A non-aqueous electrolyte secondary battery disclosed herein comprises a positive electrode, a negative electrode and a non-aqueous electrolyte. The positive and negative electrodes each have a current collector, and a mixture material layer formed on the surface of the current collector. In at least one of the positive and negative electrodes, the mixture material layer contains an active substance, a conductive assistant, a binder and, further, carbon nanotubes as at least one kind of the conductive assistant. And, in a cross-section of the mixture material layer in a lamination direction of the mixture material layer and the current collector, when the cross-section of the mixture material layer is equally divided into six equal parts in the lamination direction, a concentration a of the binder in a layer farthest from the current collector and a concentration b of the binder in a layer closest to the current collector satisfy a>b.SELECTED DRAWING: Figure 3

Description

The present invention relates to a non-aqueous electrolyte secondary battery.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used as portable power supplies for personal computers, mobile terminals, etc., and vehicle drive power supplies for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), etc. It is suitably used for.

Generally, an electrode included in a secondary battery includes a current collector and a mixture layer containing an active material formed on the surface of the current collector. Typically, the mixture layer contains a binder (binder), which contributes to the maintenance of the electrode structure such as the binding between the active material and the current collector and the binding between the active materials. .. Patent Document 1 discloses a configuration in which the positive electrode contains a binder containing a polar functional group and a linear carbon-based conductive material. Further, it is disclosed that the flexibility and the binding force of the positive electrode can be improved at the same time by such a configuration, so that the life characteristic (cycle characteristic) of the lithium ion secondary battery provided with the positive electrode is improved.

Japanese Unexamined Patent Publication No. 2020-35746

By the way, since the active material contained in the mixture layer expands and contracts with charge and discharge, there is a problem that the mixture layer is separated from the current collector due to such expansion and contraction, and the cycle characteristics are deteriorated. Further, since the electric resistance can be increased by changing the existence distribution of the active material in the mixed material layer due to the distribution of the binder contained in the mixed material layer, a technique for reducing the electric resistance is desired.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery having reduced electrical resistance and improved cycle characteristics.

As a result of diligent studies by the present inventor in order to solve the above problems, in the mixture layer containing carbon nanotubes (CNTs), the binder is unevenly distributed on the surface layer of the mixture layer (that is, the region far from the current collector). By doing so, it was found that the electrical resistance is reduced and the cycle characteristics are improved. In general, the uneven distribution of the binder on the surface layer of the mixture layer causes aggregation of the binder, which makes it difficult for the electrolyte to penetrate into the mixture layer, which tends to increase the resistance of the secondary battery. Then, they have found that the presence of carbon nanotubes in them reduces electrical resistance and further improves cycle characteristics, and has completed the present invention.

That is, the non-aqueous electrolyte secondary battery disclosed here is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode and the negative electrode are a current collector and the collector. A mixture layer formed on the surface of the electric body is provided, and in at least one of the positive electrode and the negative electrode, the mixture layer contains an active material, a conductive auxiliary agent, and a binder, and further. Carbon nanotubes are contained as at least one of the above-mentioned conductive aids. Then, in the cross section of the composite material layer in the stacking direction of the mixture layer and the current collector, when the cross section of the mixture layer is divided into six equal parts in the stacking direction, the layer farthest from the current collector. The concentration a of the binder in the above and the concentration b of the binder in the layer closest to the current collector comprises a> b.
According to this configuration, the binders are unevenly distributed on the surface layer of the mixture layer, but the inclusion of carbon nanotubes improves the supply of the electrolyte into the mixture layer and realizes an excellent effect of reducing electrical resistance. In addition, the peel strength of the mixture layer is improved, and the cycle characteristics are improved. Furthermore, when an internal short circuit occurs in the electrode, the current concentrates on the highly conductive carbon nanotubes. As a result, the current collector (metal foil) in contact with the carbon nanotubes becomes locally hot, so that the current collector can be fused and the short-circuit current can be cut off.

Further, in a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed here, 1.2 ≦ a / b ≦ 4 is provided in the above a and the above b.
According to such a configuration, it is possible to realize a more excellent electric resistance reduction effect and an excellent cycle characteristic.

Further, in a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed here, the aspect ratio of the carbon nanotubes is 150 or more. Further, in a preferred embodiment, the aspect ratio of the carbon nanotubes is 200 or more and 5000 or less.
According to such a configuration, further excellent electric resistance reduction effect and cycle characteristics can be realized.

Further, in a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, in the mixture layer containing the carbon nanotubes, the mass of the carbon nanotubes is based on the mass of the entire conductive aid contained in the mixture layer. The ratio is 2% by mass or more. Further, in a preferred embodiment, in the mixture layer containing the carbon nanotubes, the ratio of the mass of the carbon nanotubes to the total mass of the conductive auxiliary agent contained in the mixture layer is 3% by mass or more and 70% by mass or less. be.
According to such a configuration, excellent electric resistance reduction effect and cycle characteristics can be realized at a higher level.

Further, in a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the electrode provided with the mixture layer containing the carbon nanotubes is a positive electrode, and the mixture layer contains fluorine as the binder as a constituent element. Includes polymers contained in.
According to such a configuration, it is possible to realize a positive electrode in which the binder is unevenly distributed on the surface layer portion of the composite material layer, and a non-aqueous electrolyte secondary battery exhibiting an excellent electric resistance reducing effect and cycle characteristics is provided.

It is sectional drawing which shows typically the structure of the lithium ion secondary battery which concerns on one Embodiment. It is sectional drawing for explaining the distribution of the binder contained in the positive electrode mixture layer of the lithium ion secondary battery which concerns on one Embodiment. It is a mapping figure of the fluorine atom when the cross section of the positive electrode mixture layer of Example 7 is analyzed by EPMA.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Matters other than those specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Further, in the following drawings, members / parts having the same function are designated by the same reference numerals, and duplicate explanations may be omitted or simplified. Further, the dimensional relations (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relations. Further, the reference numeral W in the drawing indicates the “width direction”, and the reference numeral T indicates the “thickness direction”. It should be noted that these directions are defined for convenience of explanation, and are not intended to limit the installation mode of the non-aqueous electrolyte secondary battery disclosed herein.
Further, when the numerical range is described as A to B (where A and B are arbitrary numerical values) in the present specification, it is the same as the general interpretation and means A or more and B or less. ..

As used herein, the term "non-aqueous electrolyte secondary battery" refers to a general battery that uses a non-aqueous electrolyte as a charge carrier and can be repeatedly charged and discharged as the charge carrier moves between the positive and negative electrodes. The electrolyte in the non-aqueous electrolyte secondary battery may be, for example, a non-aqueous electrolyte solution, a gel-like electrolyte, or a solid electrolyte. Such a non-aqueous electrolyte secondary battery includes a battery generally called a lithium ion battery, a lithium secondary battery, or the like, a lithium polymer battery, a lithium ion capacitor, or the like. Hereinafter, as an example of the non-aqueous electrolyte secondary battery disclosed herein, a non-aqueous electrolyte liquid lithium ion secondary battery in which a laminated electrode body is housed in an outer body made of a laminated film will be described in detail.

FIG. 1 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery 100 according to an embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 houses an electrode body 20 and a non-aqueous electrolytic solution (not shown) inside the exterior body 10. One end of the positive electrode current collecting terminal 42 is electrically connected to the positive electrode 50 inside the exterior body 10, and one end of the negative electrode current collecting terminal 44 is electrically connected to the negative electrode 60 inside the exterior body 10. .. Further, the other ends of the positive electrode current collecting terminal 42 and the negative electrode current collecting terminal 44 are exposed to the outside of the exterior body 10.

The exterior body 10 is formed in a bag shape by a laminated film. The exterior body 10 has a storage space for accommodating the electrode body 20 and the non-aqueous electrolytic solution inside, and can be sealed by heat welding (heat sealing) the peripheral edge of the accommodation space.
The material constituting the laminated film is not particularly limited, and typically, a material in which a foil-shaped metal and a resin sheet are bonded to each other can be used. For example, a metal layer such as aluminum is provided on the surface of a non-stretched polypropylene film (CPP) for heat welding for the purpose of imparting heat resistance, sealing strength, impact resistance, etc., and polyethylene is further provided on the surface of the metal layer. A laminated film having a three-layer structure provided with an external resin layer made of terephthalate (PET), polyamide (PA), a nylon film, or the like can be used.

The electrode body 20 is configured by laminating a plurality of electrode sheets of the positive electrode 50 and the negative electrode 60 in a state of being insulated by the separator 70. The electrode sheets of the positive electrode 50 and the negative electrode 60 have wide rectangular surfaces, and the wide surfaces are laminated so as to face each other. Here, the stacking direction of the electrode body 20 is the thickness direction T.

The electrode sheet (positive electrode sheet) of the positive electrode 50 includes a sheet-shaped positive electrode current collector 52 having a wide rectangular surface, and a positive electrode mixture layer 54 coated on the surface of the positive electrode current collector 52. ing. A positive electrode current collector exposed portion on which the positive electrode mixture layer 54 is not formed is provided on one edge of the positive electrode current collector 52 in the width direction (W direction in FIG. 1). The positive electrode terminal joining portion 56 is formed by joining the exposed portions of the positive electrode current collectors of each of the laminated positive electrode sheets in a bundle shape from the above-mentioned stacking direction (T direction in FIG. 1).
The electrode sheet (negative electrode sheet) of the negative electrode 60 includes a sheet-shaped negative electrode current collector 62 having a wide rectangular surface, and a negative electrode mixture layer 64 coated on the surface of the negative electrode current collector 62. ing. A negative electrode current collector exposed portion on which the negative electrode mixture layer 64 is not formed is provided on one edge of the negative electrode current collector 62 in the width direction (W direction in FIG. 1). The negative electrode terminal joint portion 66 is formed by joining the exposed negative electrode current collectors of each of the laminated negative electrode sheets in a bundle from the stacking direction (T direction in FIG. 1).

The positive electrode current collecting terminal 42 is a plate-shaped conductive member, one end of which is joined to the positive electrode terminal joining portion 56 inside the exterior body 10, and the other end of which is exposed to the outside of the exterior body 10. At the portion where the positive electrode current collecting terminal 42 penetrates the exterior body 10, two laminated films are laminated so as to sandwich the positive electrode current collecting terminal 42 from the thickness direction T, and the laminated film is formed on the surface of the positive electrode current collecting terminal 42. It is welded. In addition, in order to improve the strength of such welding, a welding film made of resin may be intervened between the positive electrode current collecting terminal 42 and the laminated film.
The negative electrode current collector terminal 44 may have the same configuration as the positive electrode current collector terminal 42 described above, except that one end thereof is joined to the negative electrode terminal joint portion 66.

At least one of the positive electrode 50 and the negative electrode 60 of the lithium ion secondary battery 100 disclosed here has a composite material layer having the configuration disclosed here. Here, the configuration of the mixture layer disclosed here will be described by taking the positive electrode 50 as an example.

The positive electrode 50 includes a positive electrode current collector 52 and a positive electrode mixture layer 54 formed on the surface of the positive electrode current collector 52. As the positive electrode current collector 52, for example, an aluminum foil can be used.

The positive electrode mixture layer 54 includes at least a positive electrode active material, a binder, and a conductive auxiliary agent. Further, the positive electrode mixture layer 54 contains carbon nanotubes (CNTs) as at least one kind of conductive auxiliary agent. Further, the positive electrode mixture layer 54 may contain a dispersant (for example, polyvinyl alcohol or the like) generally used for improving the conductivity of the conductive auxiliary agent.

As the positive electrode active material, a positive electrode active material used for the positive electrode of a general lithium ion secondary battery can be used, and for example, a lithium composite metal oxide having a layered structure or a spinel structure (for example, LiNi _1/3 Co) can be used. _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO ₄ , etc.).

As the binder, the same binder as that used for the positive electrode of a general lithium ion secondary battery can be appropriately adopted. For example, when the positive electrode mixture layer 54 is formed by supplying a paste, a polymer having properties that can be uniformly dissolved or dispersed in the solvent constituting the paste can be used as a binder. When a non-aqueous paste is used, a polymer material that dissolves in an organic solvent such as vinyl halide resin such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC) and polyalkylene oxide such as polyethylene oxide (PEO) is used. Can be used. When a water-based paste is used, a water-soluble polymer material or a water-dispersible polymer material can be preferably adopted. Examples thereof include polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) and the like. Among these, it is preferable to use a polymer containing fluorine as a constituent element as a binder, and for example, PVDF, PTFE or the like is preferably used.
The proportion of the binder in the positive electrode mixture layer 54 is typically 0.5% by mass or more, preferably 1% by mass or more, for example, 1.5% by mass or more, when the entire positive electrode mixture layer 54 is 100% by mass. It is typically 10% by mass or less, preferably 8% by mass or less, for example, 5% by mass or less.

FIG. 2 is a schematic cross-sectional view for explaining the distribution of binders in the cross section of the positive electrode mixture layer 54 in the stacking direction (T direction in the figure) of the positive electrode mixture layer 54 and the positive electrode current collector 52. The positive electrode active material and the conductive auxiliary agent are not shown, and the binder 80 is schematically shown in the form of particles, but it does not reflect the actual shape.
As shown in FIG. 2, the binder 80 included in the positive electrode mixture layer 54 is unevenly distributed on the surface layer portion (that is, the side far from the positive electrode current collector 52) of the positive electrode mixture layer 54. Specifically, when the cross section of the positive electrode mixture layer 54 in the stacking direction (T direction in the figure) of the positive electrode mixture layer 54 and the positive electrode current collector 52 is divided into six equal parts in the stacking direction (that is, T in the figure). When the layers are equally divided into 6 layers having the same thickness in the direction), the concentration a of the binder 80 in the layer farthest from the positive electrode current collector 52 (part A in FIG. 2) and the highest concentration a in the positive electrode current collector 52. The concentration b of the binder 80 in the near layer (part B in FIG. 2) comprises a> b. Since the binders are unevenly distributed on the surface layer in this way, the density of the positive electrode active material in the vicinity of the positive electrode current collector is increased, so that the electron conductivity between the positive electrode active material and the positive electrode current collector is improved and the electrical resistance is reduced. can do. Further, since the binder concentration is relatively low in the vicinity of the positive electrode current collector, the flexibility of the positive electrode mixture layer in the vicinity of the positive electrode current collector is improved, and the positive electrode mixture layer is less likely to be peeled off from the positive electrode current collector. This improves the cycle characteristics.

Conventionally, since the binder is unevenly distributed on the surface layer portion, the efficiency of supplying the electrolyte into the positive electrode mixture layer is lowered, so that the effect of reducing the electric resistance and the effect of improving the cycle characteristics cannot be obtained. However, according to the present invention, even if the binder is unevenly distributed on the surface of the positive electrode mixture layer, the carbon nanotubes are contained in the positive electrode mixture layer, so that the efficiency of supplying the electrolyte into the positive electrode mixture layer is improved. The effect has been found, and excellent battery resistance reduction effect and excellent cycle characteristics have been realized. In addition, when the positive electrode mixture layer contains carbon nanotubes, when an internal short circuit occurs, the current concentrates on the carbon nanotubes with high conductivity, and the temperature near the carbon nanotubes becomes high, so that the positive electrode in contact with the carbon nanotubes. The current collector (eg, aluminum foil) can be fused earlier. As a result, the short-circuit current can be cut off, so that a highly safe non-aqueous electrolyte secondary battery can be realized. Further, since the binders are unevenly distributed on the surface layer portion, the amount of carbon nanotubes existing in the vicinity of the positive electrode current collector increases, so that the positive electrode current collector can be blown out more quickly when an internal short circuit occurs.

The concentration a of the binder 80 in the layer farthest from the positive electrode current collector 52 (part A in FIG. 2) and the concentration b of the binder 80 in the layer closest to the positive electrode current collector 52 (part B in FIG. 2). The ratio (a / b) may be larger than 1, but from the viewpoint of maintaining the shape of the positive electrode mixture layer 54, it is preferable that an appropriate amount of binder is present in the layer closest to the positive electrode current collector, so for example, a. / B is preferably 10 or less. Further, it is more preferable that a / b is 1.2 or more and 4 or less (1.2 ≦ a / b ≦ 4), and according to this range, an excellent electric resistance reduction effect and cycle characteristics can be realized. can.
The concentration of the binder in the cross section of the positive electrode mixture layer 54 can be measured using, for example, an electron probe microanalyzer (EPMA). By detecting the unique constituent elements (for example, fluorine of PVDF) contained in the binder by EPMA and mapping them to the cross section of the positive electrode mixture layer 54, the layer farthest from the positive electrode current collector 52 (part A in FIG. 2). The ratio (a / b) of the concentration a of the binder 80 to the concentration b of the binder 80 in the layer closest to the positive electrode current collector 52 (part B in FIG. 2) can be obtained. The measurement by EPMA for obtaining the above ratio (a / b) may be any of qualitative analysis, semi-quantitative analysis, and quantitative analysis.

Examples of the carbon nanotubes contained in the positive electrode mixture layer 54 include single-walled carbon nanotubes (SWCN), double-walled carbon nanotubes (DWCN), and multi-walled carbon nanotubes (Multi-walled). Carbon Nanotube: MWCN) etc. can be used. These can be used alone or in combination of two or more.
The carbon nanotubes may be manufactured by an arc discharge method, a laser ablation method, a chemical vapor deposition method, or the like.

The average length of the carbon nanotubes is not particularly limited, but is typically 0.1 μm or more, and can be, for example, 1 μm or more or 2 μm or more. Further, if the average length of the carbon nanotubes is long, the carbon nanotubes tend to aggregate and it tends to be difficult to obtain the effect of improving the electron conductivity. Therefore, the average length of the carbon nanotubes is typically 100 μm or less. It can be 75 μm or less or 50 μm or less.
The average outer diameter (diameter) of the carbon nanotube is not particularly limited, but is typically 0.1 nm or more and 100 nm or less, for example, 1 nm or more and 20 nm or less.

The aspect ratio (length / outer diameter) of the carbon nanotubes is not particularly limited, but is preferably 150 or more. With such an aspect ratio, the efficiency of supplying the electrolyte into the positive electrode mixture layer and the electron conductivity can be improved, and the electric resistance can be further reduced. Further, by including the carbon nanotubes having such an aspect ratio, the peel strength at the interface between the positive electrode mixture layer and the positive electrode current collector is improved, so that the cycle characteristics are improved. Further, from the viewpoint of suppressing aggregation of carbon nanotubes, the aspect ratio is preferably 6300 or less, for example. Further, the aspect ratio is more preferably 200 or more and 5000 or less, and further preferably 200 or more and 3030 or less. According to this range, the supply efficiency and electron conductivity of the electrolyte are further improved, and the peel strength at the interface between the positive electrode mixture layer and the positive electrode current collector is further improved, so that a better electric resistance reduction effect and cycle characteristics can be obtained. It can be realized.

In the positive electrode mixture layer 54, one or more conductive aids may be used in combination in addition to the carbon nanotubes as the conductive auxiliary agent. For example, as the conductive auxiliary agent, a carbon material such as carbon black (for example, acetylene black, furnace black, ketjen black, channel black, etc.), graphite, or the like can be used.
The ratio of the entire conductive auxiliary agent in the positive electrode mixture layer 54 is typically 1% by mass or more, preferably 8% by mass or more, for example, 16% by mass or more, when the entire positive electrode mixture layer 54 is 100% by mass. Yes, typically 20% by mass or less, for example 18% by mass or less.

The ratio of carbon nanotubes to the entire conductive auxiliary agent contained in the positive electrode mixture layer 54 is not particularly limited, but is preferably 2% by mass or more and 80% by mass or less, and 3% by mass or more and 70% by mass or less. Is more preferable. At such a ratio, the effect of improving the electron conductivity of the carbon nanotube and other conductive auxiliary agents is suitably exhibited.
The proportion of carbon nanotubes in the positive electrode mixture layer 54 is typically 0.1% by mass or more, preferably 0.32% by mass or more, for example, 0, when the entire positive mixture layer 54 is 100% by mass. It can be .48% by mass or more, typically 16% by mass or less, preferably 12.8% by mass or less, for example 11.2% by mass or less.

As a method for producing the positive electrode mixture layer 54 having the configuration disclosed here, first, a positive electrode active material, a conductive auxiliary agent containing carbon nanotubes, a binder, and a material (dispersant or the like) used as necessary are used. Is dispersed in a suitable solvent (for example, N-methyl-2-pyrrolidone: NMP) to prepare a paste-like (or slurry-like) composition, and an appropriate amount of the composition is applied to the surface of the positive electrode current collector 52. Work. Then, for example, it is dried in a drying oven at 200 ° C. for 1 to 5 minutes while being exposed to dry air, then dried in a drying oven at 150 ° C. for 1 to 10 minutes, and further dried at 110 ° C. for 20 minutes to obtain a positive electrode. The mixed material layer 54 can be manufactured.

The electrodes having the configuration disclosed here have been described above by taking the positive electrode 50 as an example. However, when the negative electrode 60 has the electrodes having the configuration disclosed here, the same configuration as the positive electrode 50 described above may be used. However, various materials are generally changed to those that can be used for the negative electrode of a lithium ion secondary battery.

As the negative electrode current collector 62 included in the negative electrode 60, for example, a copper foil can be used. The negative electrode mixture layer 64 included in the negative electrode 60 contains a negative electrode active material, and for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. Further, the negative electrode mixture layer 64 may further contain a binder, a thickener and the like. As the binder, the same binder as that used for the positive electrode 50 described above can be appropriately adopted, and for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used. When the negative electrode 60 has the electrode configuration disclosed here, it may contain at least carbon nanotubes as a conductive auxiliary agent, and the carbon nanotubes may be the same as the carbon nanotubes used for the positive electrode 50 described above.
In the negative electrode mixture layer 64, a negative electrode active material and a material (binder or the like) used as needed are dispersed in an appropriate solvent (for example, ion-exchanged water) to prepare a paste-like (or slurry-like) composition. , An appropriate amount of the composition can be applied to the surface of the negative electrode current collector 62 and dried.

As the separator 70, various microporous sheets similar to those conventionally used for lithium ion secondary batteries can be used, and for example, a microporous resin made of a resin such as polyethylene (PE) or polypropylene (PP). The sheet is mentioned. The microporous resin sheet may have a single-layer structure or a multi-layer structure having two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Further, the surface of the separator 70 may be provided with a heat resistant layer (HRL).

As the non-aqueous electrolyte solution, the same one as that of the conventional lithium ion secondary battery can be used, and typically, a non-aqueous solvent containing a supporting salt can be used. As the non-aqueous solvent, an aprotic solvent such as carbonates, esters, ethers and the like can be used. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like can be preferably adopted. Alternatively, a fluorinated solvent such as a fluorinated carbonate such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC) and trifluorodimethylcarbonate (TFDMC) is preferably used. be able to. As such a non-aqueous solvent, one kind may be used alone, or two or more kinds may be used in combination as appropriate. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ can be preferably used. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol / L or more and 1.3 mol / L or less.
The non-aqueous electrolyte may contain components other than the above-mentioned non-aqueous solvent and supporting salt as long as the effects of the present invention are not significantly impaired. It may contain various additives such as thickeners.

The lithium ion secondary battery 100 can be used for various purposes. For example, it can be suitably used as a high-output power source (driving power source) for a motor mounted on a vehicle. The type of vehicle is not particularly limited, but typically examples thereof include automobiles, for example, plug-in hybrid vehicles (PHVs), hybrid vehicles (HVs), electric vehicles (EVs), and the like. The lithium ion secondary battery 100 can also be used in the form of an assembled battery in which a plurality of lithium ion secondary batteries 100 are electrically connected.

As described above, a laminated lithium ion secondary battery provided with a laminated electrode body has been described in detail as an example, but this is merely an example and does not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the above-described embodiments. For example, instead of the laminated electrode body, a wound electrode body in which a positive electrode sheet and a negative electrode sheet are wound via a separator sheet may be provided. Further, instead of the exterior body 10, a battery case made of a metal material such as aluminum may be used. Further, an all-solid-state battery using a solid electrolyte as an electrolyte or a polymer battery using a polymer electrolyte may be used.

Hereinafter, examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

[Experiment 1]
<Manufacturing of positive electrode sheet>
(Example 1)
LiFePO ₄ powder as the positive electrode active material, acetylene black (AB) as the conductive auxiliary agent, polyvinylidene fluoride (PVDF) as the binder, polyvinyl alcohol as the dispersant, positive electrode active material: conductive auxiliary agent: binder: Prepare to have a mass ratio of dispersant = 80: 16: 3.8: 0.2, and add to the dispersion medium N-methyl-2-pyrrolidone (NMP) so that the solid content is 46% by mass. , Mixed using a planetary mixer. In this way, a paste for forming a positive electrode mixture layer was prepared.
Next, the paste for forming the positive electrode mixture was applied to both sides of the strip-shaped aluminum foil using a die coater. Then, it was dried in a drying oven at 120 ° C. for 12 hours. A positive electrode sheet was produced by pressing an aluminum foil provided with a dry paste for forming a positive electrode mixture layer.

(Example 2)
Among the methods for producing the positive electrode sheet of Example 1, the drying conditions were changed. The same positive electrode mixture forming paste as in Example 1 was applied to both sides of the strip-shaped aluminum foil using a die coater. Then, it was dried in a drying oven at 200 ° C. for 1 minute to 5 minutes while being exposed to dry air, and then dried in a drying oven at 150 ° C. for 1 minute to 10 minutes. Then, it was further dried at 110 ° C. for 20 minutes. A positive electrode sheet was produced by pressing an aluminum foil provided with a dry paste for forming a positive electrode mixture layer.
(Examples 3-4)
Of the paste for forming the positive electrode mixture layer of Example 1, the conductive auxiliary agent was changed so that the mass ratio was AB: carbon nanotube (CNT) = 50: 50, and other than that, the positive electrode mixture was the same as in Example 1. A layer-forming paste and a positive electrode sheet were prepared. The aspect ratio of the CNT used here is 150.
(Examples 5 to 11)
Among the pastes for forming the positive electrode mixture layer of Example 1, the conductive auxiliary agent was changed so that the mass ratio was AB: carbon nanotube (CNT) = 50: 50, and other than that, the same was applied for forming the positive electrode mixture layer. The paste was prepared. Then, after applying it to both sides of the strip-shaped aluminum foil using a die coater, it is dried in a drying oven at 200 ° C. for 1 to 5 minutes while applying dry air, and then in a drying oven at 150 ° C. It was dried for 1 to 10 minutes. Then, it was further dried at 110 ° C. for 20 minutes. A positive electrode sheet was produced by pressing an aluminum foil provided with a dry paste for forming a positive electrode mixture layer. In Examples 5 to 11, the drying time was set to be different within the above range. The aspect ratio of the CNT used here is 150.

<Manufacturing of lithium-ion secondary battery for evaluation>
Natural graphite (C) is prepared as the negative electrode active material, styrene butadiene rubber (SBR) is prepared as the binder, and carboxymethyl cellulose (CMC) is prepared as the thickener, and the mass ratio is C: SBR: CMC = 94: 4: 4. Weighed in this manner and mixed with ion-exchanged water so that the solid content ratio was 66% by mass. In this way, a paste for forming a negative electrode mixture layer was prepared.
Then, after applying to both sides of the strip-shaped copper foil using a die coater, it was dried in a drying oven at 100 ° C. for 10 hours. A negative electrode sheet was produced by pressing a copper foil provided with a dried paste for forming a negative electrode mixture layer.

Further, as a separator sheet, a porous polyolefin sheet having a three-layer structure of PP / PE / PP was prepared.

The positive electrode sheet and the negative electrode sheet produced above were laminated so as to face each other via the separator sheet to produce a laminated electrode body. A current collector terminal was attached to the laminated electrode body and housed in an aluminum laminated bag. Then, the laminated electrode body was impregnated with a non-aqueous electrolytic solution, and the opening of the aluminum laminated bag was sealed and sealed to prepare a lithium ion secondary battery for evaluation of each example. As the non-aqueous electrolytic solution, LiPF ₆ as a supporting salt was dissolved at a concentration of 1.0 mol / L in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 3. I used the one that was made to.

<Evaluation of binder distribution in positive electrode mixture layer>
The positive electrode sheet of each of the above-mentioned manufactured examples was cut in the stacking direction of the aluminum foil and the positive electrode mixture layer, and the fluorine atom (F) concentration on the cut surface was measured by EPMA. Then, the cut surface of the positive electrode mixture layer is divided into six equal parts in the layering direction of the positive electrode mixture layer and the aluminum foil, and the fluorine atom concentration a in the layer (part A) farthest from the aluminum foil and the layer closest to the aluminum foil. The ratio (a / b) to the fluorine atom concentration b in (part B) was determined. Since the fluorine atom concentration is proportional to the concentration of PVDF which is a binder, the ratio (a / b) of the fluorine atom concentration a and b indicates the ratio of the binder concentration. The results are shown in Table 1. Further, as a representative diagram, FIG. 3 shows a mapping diagram of fluorine atoms by EPMA on the cut surface of the positive electrode mixture layer of Example 7. In FIG. 3, the concentration of fluorine atoms is shown by the shades of white and black, and the darker the white part, the higher the concentration of fluorine atoms. The fluorine atom concentration a indicates the value of the mass% of the fluorine atom with respect to the whole atom existing in the part A, and the fluorine atom concentration b indicates the value of the mass% of the fluorine atom with respect to the whole atom existing in the part B.

<Activation treatment and initial volume measurement>
Each of the prepared lithium ion secondary batteries for evaluation was placed in an environment of 25 ° C. The initial charge power is a constant current-constant voltage method, and each evaluation lithium ion secondary battery is constantly charged to 4.2 V with a current value of 1 / 3C, and then fixed until the current value reaches 1 / 50C. The voltage was charged and the battery was fully charged. Then, each evaluation lithium ion secondary battery was constantly discharged to 3.0 V at a current value of 1 / 3C. The discharge capacity at this time was measured to obtain the initial capacity. Here, "1C" means the magnitude of the current that can charge the SOC (state of charge) from 0% to 100% in one hour.

<Measurement of standardized battery resistance>
Each of the activated lithium ion secondary batteries for evaluation was adjusted to an open circuit voltage of 3.70 V and placed in a temperature environment of −5 ° C. The battery was discharged at a current value of 3C for 8 seconds, and the voltage change amount ΔV at this time was obtained. Moreover, the current value of 20C was obtained. Then, the battery resistance was calculated using the current value of 20C and ΔV. When the resistance of the evaluation lithium ion secondary battery of Example 1 was 1.0, the ratio of the resistance of the evaluation lithium ion secondary battery of each example was determined. The results are shown in Table 1.

<Measurement of capacity retention rate>
Each of the activated lithium ion secondary batteries for evaluation was adjusted to an open circuit voltage of 3.3 V and placed in an environment of 60 ° C. The charge / discharge was a constant current method, and the battery was charged to 4.2 V at a current value of 1 C and then discharged to 3.3 V at a current value of 1 C. This charge / discharge was set as one cycle, and 200 cycles were repeated. Then, the discharge capacity after 200 cycles was measured by the same method as the above initial capacity. Then, the capacity retention rate (%) was obtained by calculating the value of (discharge capacity after 200 cycles / initial capacity) × 100. The results are shown in Table 1.

<Safety evaluation by safety test>
Each of the activated lithium-ion secondary batteries for evaluation is charged with a constant current of 4.2 V at a current value of 1 / 3C, and then charged with a constant voltage until the current value reaches 1 / 10C to be fully charged. I made it into a state. Then, each evaluation lithium ion secondary battery in a fully charged state was placed in an environment of 25 ° C. Next, an iron nail having a diameter of 3 mm was passed through the vicinity of the center of each evaluation lithium ion secondary battery at a speed of 10 mm / sec. At this time, the outer surface temperature of each evaluation lithium ion secondary battery was measured with a thermocouple, and the maximum temperature was measured. Table 1 shows the case where the maximum temperature at this time is less than 250 ° C. as “◯” and the case where the maximum temperature is 250 ° C. or higher as “x”.

Examples 1, 3 and 4 are cases where a positive electrode sheet is prepared by a conventional drying method (eg, 2 hours to 12 hours at 120 ° C.). According to this, as shown in Table 1, the binder contained in the positive electrode mixture layer is not unevenly distributed in the A portion farthest from the aluminum foil, but is more distributed in the B portion closest to the aluminum foil. Recognize. Further, comparing Example 1 with Example 3 and Example 4, it can be seen that the inclusion of carbon nanotubes (CNT) in the positive electrode mixture layer slightly reduces the battery resistance and slightly improves the capacity retention rate. .. However, in Example 1, Example 3 and Example 4, the evaluation of the safety test was not good.
Further, in Example 2, the positive electrode mixture layer did not contain CNTs, and was dried at a higher temperature for a shorter time than the drying conditions of Example 1 to prepare a positive electrode sheet. According to this method, it can be seen that the binder contained in the positive electrode mixture layer is unevenly distributed in the A portion farthest from the aluminum foil. In the battery performance evaluation test, the standardized battery resistance (internal resistance of the battery) increases and the capacity retention rate also decreases compared to Example 1. Therefore, the positive electrode mixture layer does not contain CNTs and the binder is unevenly distributed in the A part. Turns out to be unfavorable.

On the other hand, in Examples 5 to 11 including CNTs and the binders are unevenly distributed in the A part (that is, the binder concentration is higher in the A part than in the B part), the standardized battery resistance is lower and the capacity is lower than in the example 1. The retention rate has improved. Furthermore, the safety test also gave good results. Therefore, a lithium ion secondary battery containing a CNT and having a positive electrode mixture layer in which the binder is unevenly distributed in the A portion (that is, a> b) has an excellent battery resistance reducing effect and excellent cycle characteristics, and further. It can be seen that a highly safe non-aqueous electrolyte secondary battery can be realized. Among them, it can be seen that in Examples 6 to 9 having 1.2 ≦ a / b ≦ 4, a particularly excellent electric resistance reducing effect and cycle characteristic improving effect can be realized.

[Experiment 2]
(Examples 12 to 17)
The CNTs having an aspect ratio of 150 contained in the paste for forming the positive electrode mixture layer of Example 7 were changed to CNTs having an aspect ratio of 200 to 6300. Except for this, a lithium ion secondary battery for evaluation was manufactured in the same manner as in Example 7. In addition, the above-mentioned evaluation of the binder distribution of the positive electrode mixture layer, the activation treatment and the initial capacity measurement, the measurement of the standardized battery resistance, the measurement of the capacity retention rate, and the safety evaluation by the safety test were carried out in the same manner. The results of Examples 1 to 2, Examples 7, and Examples 12 to 17 are shown in Table 2.

As shown in Table 2, from the results of Examples 7 and 12 to 17, CNTs are contained in the positive electrode mixture layer regardless of the aspect ratio of CNTs, and the binder is unevenly distributed in the A portion farthest from the current collector foil. By doing so, it can be seen that an excellent electric resistance reduction effect and capacity retention ratio are realized, and further, high safety is realized. Further, comparing Examples 7 and 12 to 17, it can be seen that when the aspect ratio of CNT is 200 or more and 5000 or less, a particularly excellent electric resistance reduction effect and an excellent capacity retention rate can be realized.

[Experiment 3]
(Examples 18-23)
The mass ratio of AB and CNT (aspect ratio 150) contained in the paste for forming the positive electrode mixture layer of Example 7 was changed to AB: CNT = 98: 2 to 20:80. Except for this, a lithium ion secondary battery for evaluation was manufactured in the same manner as in Example 7. In addition, the above-mentioned evaluation of the binder distribution of the positive electrode mixture layer, the activation treatment and the initial capacity measurement, the measurement of the standardized battery resistance, the measurement of the capacity retention rate, and the safety evaluation by the safety test were carried out in the same manner. The results of Examples 1 to 2, Examples 7, and Examples 18 to 23 are shown in Table 3.

As shown in Table 3, when Example 2 is compared with Examples 7 and 18 to 23, excellent electric resistance reduction effect and capacity retention rate are obtained regardless of the mass ratio (mass%) of CNTs to the entire conductive auxiliary agent. It can be seen that high safety is realized. Further, comparing Examples 7 and 18 to 23, when the mass ratio of CNT to the entire conductive auxiliary agent is 3% by mass or more and 70% by mass or less, a particularly excellent electric resistance reduction effect and capacity retention rate can be realized. I understand.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above.

10 Exterior body 20 Electrode body 42 Positive electrode current collector terminal 44 Negative electrode current collector terminal 50 Positive electrode 52 Positive electrode current collector 54 Positive electrode mixture layer 60 Negative electrode 62 Negative electrode collector 64 Negative electrode mixture layer 70 Separator 80 Binder 100 Lithium ion secondary battery

Claims (7)

A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.
The positive electrode and the negative electrode include a current collector and a mixture layer formed on the surface of the current collector.
In at least one of the positive electrode and the negative electrode,
The mixture layer is
Contains active material, conductive aid, and binder,
Carbon nanotubes are contained as at least one of the conductive aids, and
In the cross section of the mixture layer in the stacking direction of the mixture layer and the current collector.
When the cross section of the mixture layer is divided into six equal parts in the stacking direction, the concentration a of the binder in the layer farthest from the current collector and the concentration b of the binder in the layer closest to the current collector are obtained. , A> b, a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to claim 1, wherein the a and the b have 1.2 ≦ a / b ≦ 4. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the carbon nanotube has an aspect ratio of 150 or more. The non-aqueous electrolyte secondary battery according to claim 3, wherein the carbon nanotube has an aspect ratio of 200 or more and 5000 or less. One of claims 1 to 4, wherein in the mixture layer containing the carbon nanotubes, the ratio of the mass of the carbon nanotubes to the total mass of the conductive auxiliary agent contained in the mixture layer is 2% by mass or more. The non-aqueous electrolyte secondary battery described in the section. The fifth aspect of claim 5, wherein in the mixture layer containing the carbon nanotubes, the ratio of the mass of the carbon nanotubes to the total mass of the conductive auxiliary agent contained in the mixture layer is 3% by mass or more and 70% by mass or less. Non-water electrolyte secondary battery. The invention according to any one of claims 1 to 6, wherein the electrode provided with the mixture layer containing the carbon nanotubes is a positive electrode, and the mixture layer contains a polymer containing fluorine as a constituent element as the binder. Non-aqueous electrolyte secondary battery.

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