JP5843092B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  In this specification, “secondary battery” refers to a general power storage device that can be repeatedly charged. Further, in this specification, the “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the movement of electrons accompanying the lithium ions between the positive and negative electrodes.

  Regarding such a lithium ion secondary battery, for example, Japanese Patent Application Laid-Open No. 2003-151555 discloses non-aqueous electrolysis using a rubber binder (binder) having a glass transition temperature of 0 to 120 ° C. as a binder of a negative electrode active material layer. Liquid secondary batteries have been proposed. In this publication, a rubber-based binder having a glass transition temperature of 0 to 120 ° C. is used for the binder of the negative electrode active material layer, so that it has excellent flexibility and adhesion to the current collector, and at the time of press working the active material layer It is described that a negative electrode active material layer that can be prevented from sticking to a pressing machine and peeling off can be formed. In the publication, styrene butadiene rubber (SBR) is exemplified as the rubber-based binder. In this specification, “SBR” means styrene butadiene rubber as appropriate.

  Japanese Patent Application Laid-Open No. 2003-151556 discloses a non-aqueous solution provided with a negative electrode active material layer containing two or more rubber-based binders selected from rubber-based binders having a glass transition temperature of 0 ° C. to 120 ° C. An electrolyte secondary battery has been proposed. In the publication, styrene butadiene rubber (SBR) is exemplified as the rubber-based binder.

  Japanese Patent Application Laid-Open No. 2003-007304 proposes to use an organic compound containing no fluorine having a glass transition temperature of 7 ° C. to 40 ° C. as a binder for the negative electrode. As such a binder, styrene butadiene rubber is exemplified.

JP 2003-151555 A JP 2003-151556 A JP 2003-007304 A

  By the way, for example, when the electrode active material layer is peeled off from the current collector by charging / discharging, the performance of the battery deteriorates. From the viewpoint of stably maintaining the performance of the battery for a long period of time, it is desirable that the electrode active material layer is difficult to peel from the current collector. In order to prevent the electrode active material layer from being peeled from the current collector, the binding force between the electrode active material layer and the current collector is increased by increasing the amount of the binder contained in the electrode active material layer. However, when a rubber binder such as styrene butadiene rubber is used as the binder for the electrode active material layer, the larger the amount of the binder contained in the electrode active material layer, the more the rubber binder causes the electrode active material layer to be contained in the electrode active material layer. Lithium ion migration is inhibited. For this reason, the reaction resistance of the lithium ion secondary battery tends to increase. Thus, increasing the peel strength of the electrode active material layer and keeping the reaction resistance of the lithium ion secondary battery low conflict with the amount of rubber-based binder used in the electrode active material layer. .

The lithium ion secondary battery according to the present invention includes a current collector and an electrode active material layer that is held by the current collector and includes an electrode active material and a rubber-based binder. Here, the rubber-based binder contains 90% or more by weight of styrene butadiene rubber having a glass transition temperature of −40 ° C. or more and −10 ° C. or less. In the styrene-butadiene rubber, a carboxyl group as a functional group and a hydroxyl group are introduced independently of the carboxyl group. Thereby, the peel strength of the electrode active material layer can be improved, and the reaction resistance at −30 ° C. of the lithium ion secondary battery can be kept low.

In this case, the average particle diameter (D50) of the styrene butadiene rubber may be 130 nm or less, and further 100 nm or less. The amount of the rubber binder contained in the electrode active material layer is preferably 0.7% to 1.2% in weight ratio. The styrene butadiene rubber may have an OH group. Further, the swelling degree of styrene butadiene rubber with respect to ethylene carbonate may be 150% or more. The electrode active material layer may be a negative electrode active material layer.

FIG. 1 is a diagram illustrating an example of the structure of a lithium ion secondary battery. FIG. 2 is a view showing a wound electrode body of a lithium ion secondary battery. 3 is a cross-sectional view showing a III-III cross section in FIG. 2. FIG. 4 is a cross-sectional view showing the structure of the positive electrode active material layer. FIG. 5 is a cross-sectional view showing the structure of the negative electrode active material layer. FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. 1 and is a side view showing a welded portion between the uncoated portion of the wound electrode body and the electrode terminal. FIG. 7 is a diagram schematically illustrating a state of the lithium ion secondary battery during charging. FIG. 8 is a diagram schematically showing a state of the lithium ion secondary battery during discharge. FIG. 9 is a diagram illustrating an example of a cross-sectional SEM image of the negative electrode active material layer. FIG. 10 is a graph showing the relationship between the glass transition temperature of styrene butadiene rubber and the peel strength of the negative electrode active material layer. FIG. 11 is a graph showing the relationship between the average particle diameter (D50) of styrene butadiene rubber and the peel strength of the negative electrode active material layer. FIG. 12 is a graph showing the relationship between the glass transition temperature of styrene butadiene rubber and the reaction resistance at −30 ° C. of a lithium ion secondary battery. FIG. 13 is a graph showing the relationship between the average particle diameter (D50) of styrene butadiene rubber and the reaction resistance at −30 ° C. of the lithium ion secondary battery. FIG. 14 is a diagram illustrating a method for measuring the peel strength of the negative electrode active material layer. FIG. 15 is a graph showing the relationship between the presence or absence of OH groups in styrene butadiene rubber and the peel strength of the negative electrode active material layer. FIG. 16 is a graph showing the relationship between the presence or absence of OH groups in the styrene butadiene rubber and the reaction resistance at −30 ° C. of the lithium ion secondary battery. FIG. 17 is a diagram for explaining the operation of the styrene butadiene rubber when there is an OH group. FIG. 18 is a graph showing the relationship between the degree of swelling of styrene butadiene rubber with respect to ethylene carbonate and the peel strength of the negative electrode active material layer. FIG. 19 is a graph showing the relationship between the degree of swelling of styrene butadiene rubber with respect to ethylene carbonate and the reaction resistance of a lithium ion secondary battery at −30 ° C. FIG. 20 is a diagram illustrating a vehicle equipped with a secondary battery.

  Here, a structural example of a lithium ion secondary battery will be described first. Thereafter, a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to such structural examples as appropriate. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified.

  FIG. 1 shows a lithium ion secondary battery 100. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 200 and a battery case 300. FIG. 2 is a view showing the wound electrode body 200. FIG. 3 shows a III-III cross section in FIG.

  As shown in FIG. 2, the wound electrode body 200 includes a positive electrode sheet 220, a negative electrode sheet 240, and separators 262 and 264. The positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are respectively strip-shaped sheet materials.

≪Positive electrode sheet 220≫
The positive electrode sheet 220 includes a strip-shaped positive electrode current collector 221 and a positive electrode active material layer 223. For the positive electrode current collector 221, a metal foil suitable for the positive electrode can be suitably used. As the positive electrode current collector 221, for example, a strip-shaped aluminum foil having a predetermined width and a thickness of about 15 μm can be used. An uncoated portion 222 is set along the edge on one side in the width direction of the positive electrode current collector 221. In the illustrated example, the positive electrode active material layer 223 is held on both surfaces of the positive electrode current collector 221 except for the uncoated portion 222 set on the positive electrode current collector 221 as shown in FIG. The positive electrode active material layer 223 contains a positive electrode active material. The positive electrode active material layer 223 is formed by applying a positive electrode mixture containing a positive electrode active material to the positive electrode current collector 221.

<< Positive Electrode Active Material Layer 223 and Positive Electrode Active Material Particles 610 >>
Here, FIG. 4 is a cross-sectional view of the positive electrode sheet 220. In FIG. 4, the positive electrode active material particles 610, the conductive material 620, and the binder 630 in the positive electrode active material layer 223 are schematically illustrated so that the structure of the positive electrode active material layer 223 becomes clear. As shown in FIG. 4, the positive electrode active material layer 223 includes positive electrode active material particles 610, a conductive material 620, and a binder 630.

As the positive electrode active material particles 610, a material that can be used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of the positive electrode active material include LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide), LiNiO ₂ (lithium nickelate), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO ₄ ( Lithium transition metal oxides such as lithium iron phosphate). Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ or LiCoO ₂ has a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. LiFePO ₄ having an olivine structure includes, for example, nanometer order particles. Moreover, LiFePO _{4 having} an olivine structure can be further covered with a carbon film.

≪Conductive material 620≫
Examples of the conductive material 620 include carbon materials such as carbon powder and carbon fiber. One kind selected from such conductive materials may be used alone, or two or more kinds may be used in combination. As the carbon powder, various carbon blacks (for example, acetylene black, oil furnace black, graphitized carbon black, carbon black, graphite, ketjen black), graphite powder, and the like can be used.

≪Binder 630≫
Further, the binder 630 binds the positive electrode active material particles 610 and the conductive material 620 included in the positive electrode active material layer 223, or binds these particles and the positive electrode current collector 221. As the binder 630, a polymer that can be dissolved or dispersed in a solvent to be used can be used. For example, in a positive electrode mixture composition using an aqueous solvent, a cellulose polymer (carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), etc.), a fluorine resin (eg, polyvinyl alcohol (PVA), polytetrafluoroethylene, etc.) (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP, etc.), rubbers (vinyl acetate copolymer, styrene butadiene copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), etc.) A water-soluble or water-dispersible polymer such as can be preferably used. In the positive electrode mixture composition using a non-aqueous solvent, a polymer (polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), etc.) can be preferably employed.

≪Thickener, solvent≫
The positive electrode active material layer 223 is prepared, for example, by preparing a positive electrode mixture in which the above-described positive electrode active material particles 610 and the conductive material 620 are mixed in a paste (slurry) with a solvent, applied to the positive electrode current collector 221, and dried. And is formed by rolling. At this time, as the solvent for the positive electrode mixture, either an aqueous solvent or a non-aqueous solvent can be used. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The polymer material exemplified as the binder 630 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder.

  The mass ratio of the positive electrode active material in the total positive electrode mixture is preferably about 50 wt% or more (typically 50 to 95 wt%), and usually about 70 to 95 wt% (for example, 75 to 90 wt%). It is more preferable. Moreover, the ratio of the electrically conductive material to the whole positive electrode mixture can be, for example, about 2 to 20 wt%, and is usually preferably about 2 to 15 wt%. The ratio of the binder to the whole positive electrode mixture can be, for example, about 1 to 10 wt%, and is usually preferably about 2 to 5 wt%.

<< Negative Electrode Sheet 240 >>
As illustrated in FIG. 2, the negative electrode sheet 240 includes a strip-shaped negative electrode current collector 241 and a negative electrode active material layer 243. For the negative electrode current collector 241, a metal foil suitable for the negative electrode can be suitably used. The negative electrode current collector 241 is made of a strip-shaped copper foil having a predetermined width and a thickness of about 10 μm. On one side in the width direction of the negative electrode current collector 241, an uncoated part 242 is set along the edge. The negative electrode active material layer 243 is formed on both surfaces of the negative electrode current collector 241 except for the uncoated portion 242 set on the negative electrode current collector 241. The negative electrode active material layer 243 is held by the negative electrode current collector 241 and contains at least a negative electrode active material. In the negative electrode active material layer 243, a negative electrode mixture containing a negative electrode active material is applied to the negative electrode current collector 241.

<< Negative Electrode Active Material Layer 243 >>
FIG. 5 is a cross-sectional view of the negative electrode sheet 240 of the lithium ion secondary battery 100. As shown in FIG. 5, the negative electrode active material layer 243 includes a negative electrode active material 710, a thickener (not shown), a binder 730, and the like. In FIG. 5, the negative electrode active material 710 and the binder 730 in the negative electrode active material layer 243 are schematically illustrated so that the structure of the negative electrode active material layer 243 becomes clear.

≪Negative electrode active material≫
As the negative electrode active material 710, one kind or two or more kinds of materials conventionally used for lithium ion secondary batteries can be used without any particular limitation. For example, a particulate carbon material (carbon particles) including a graphite structure (layered structure) at least in part. More specifically, the negative electrode active material is, for example, natural graphite, natural graphite coated with an amorphous carbon material, graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon ( Soft carbon) or a carbon material combining these may be used. Note that here, the negative electrode active material 710 is illustrated using so-called flake graphite, but the negative electrode active material 710 is not limited to the illustrated example.

≪Thickener, solvent≫
The negative electrode active material layer 243 is prepared, for example, by preparing a negative electrode mixture in which the negative electrode active material 710 and the binder 730 described above are mixed in a paste (slurry) with a solvent, applied to the negative electrode current collector 241, and dried. It is formed by rolling. At this time, any of an aqueous solvent and a non-aqueous solvent can be used as the solvent for the negative electrode mixture. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). For the binder 730, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 (see FIG. 4) can be used. Further, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder. possible.

<< Separators 262, 264 >>
The separators 262 and 264 are members that separate the positive electrode sheet 220 and the negative electrode sheet 240 as shown in FIG. 1 or FIG. In this example, the separators 262 and 264 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separators 262 and 264, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIGS. 2 and 3, the width b1 of the negative electrode active material layer 243 is slightly wider than the width a1 of the positive electrode active material layer 223. Furthermore, the widths c1 and c2 of the separators 262 and 264 are slightly wider than the width b1 of the negative electrode active material layer 243 (c1, c2>b1> a1).

  In the example shown in FIGS. 1 and 2, the separators 262 and 264 are made of sheet-like members. The separators 262 and 264 may be members that insulate the positive electrode active material layer 223 and the negative electrode active material layer 243 and allow the electrolyte to move. Therefore, it is not limited to a sheet-like member. The separators 262 and 264 may be formed of a layer of insulating particles formed on the surface of the positive electrode active material layer 223 or the negative electrode active material layer 243, for example, instead of the sheet-like member. Here, as the particles having insulating properties, inorganic fillers having insulating properties (for example, fillers such as metal oxides and metal hydroxides) or resin particles having insulating properties (for example, particles such as polyethylene and polypropylene). ).

≪Battery case 300≫
In this example, as shown in FIG. 1, the battery case 300 is a so-called square battery case, and includes a container body 320 and a lid 340. The container main body 320 has a bottomed rectangular tube shape and is a flat box-shaped container having one side surface (upper surface) opened. The lid 340 is a member that is attached to the opening (opening on the upper surface) of the container body 320 and closes the opening.

  In an in-vehicle secondary battery, it is desired to improve the weight energy efficiency (battery capacity per unit weight) in order to improve the fuel efficiency of the vehicle. For this reason, in this embodiment, lightweight metals, such as aluminum and an aluminum alloy, are employ | adopted for the container main body 320 and the cover body 340 which comprise the battery case 300. FIG. Thereby, the weight energy efficiency can be improved.

  The battery case 300 has a flat rectangular internal space as a space for accommodating the wound electrode body 200. Further, as shown in FIG. 1, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200. In this embodiment, the battery case 300 includes a bottomed rectangular tubular container body 320 and a lid 340 that closes the opening of the container body 320. Electrode terminals 420 and 440 are attached to the lid 340 of the battery case 300. The electrode terminals 420 and 440 pass through the battery case 300 (lid 340) and come out of the battery case 300. The lid 340 is provided with a liquid injection hole 350 and a safety valve 360.

  As shown in FIG. 2, the wound electrode body 200 is flatly pushed and bent in one direction orthogonal to the winding axis WL. In the example shown in FIG. 2, the uncoated part 222 of the positive electrode current collector 221 and the uncoated part 242 of the negative electrode current collector 241 are spirally exposed on both sides of the separators 262 and 264, respectively. As shown in FIG. 6, in this embodiment, the intermediate portions 224 and 244 of the uncoated portions 222 and 242 are gathered together and welded to the tip portions 420 a and 440 a of the electrode terminals 420 and 440. At this time, for example, ultrasonic welding is used for welding the electrode terminal 420 and the positive electrode current collector 221 due to the difference in materials. Further, for example, resistance welding is used for welding the electrode terminal 440 and the negative electrode current collector 241. Here, FIG. 6 is a side view showing a welded portion between the intermediate portion 224 (244) of the uncoated portion 222 (242) of the wound electrode body 200 and the electrode terminal 420 (440), and VI in FIG. It is -VI sectional drawing.

  The wound electrode body 200 is attached to the electrode terminals 420 and 440 fixed to the lid body 340 in a state where the wound electrode body 200 is pressed and bent flat. The wound electrode body 200 is accommodated in a flat internal space of the container body 320 as shown in FIG. The container body 320 is closed by the lid 340 after the wound electrode body 200 is accommodated. The joint 322 (see FIG. 1) between the lid 340 and the container main body 320 is welded and sealed, for example, by laser welding. Thus, in this example, the wound electrode body 200 is positioned in the battery case 300 by the electrode terminals 420 and 440 fixed to the lid 340 (battery case 300).

≪Electrolytic solution≫
Thereafter, an electrolytic solution is injected into the battery case 300 from a liquid injection hole 350 provided in the lid 340. As the electrolytic solution, a so-called non-aqueous electrolytic solution that does not use water as a solvent is used. In this example, an electrolytic solution in which LiPF ₆ is contained at a concentration of about 1 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mixed solvent having a volume ratio of about 1: 1) is used. Yes. Thereafter, a metal sealing cap 352 is attached (for example, welded) to the liquid injection hole 350 to seal the battery case 300. The electrolytic solution is not limited to the electrolytic solution exemplified here. For example, non-aqueous electrolytes conventionally used for lithium ion secondary batteries can be used as appropriate.

≪Hole≫
Here, the positive electrode active material layer 223 has minute gaps 225 that should also be referred to as cavities, for example, between the positive electrode active material particles 610 and the conductive material 620 (see FIG. 4). An electrolytic solution (not shown) can penetrate into the minute gaps of the positive electrode active material layer 223. In addition, the negative electrode active material layer 243 has minute gaps 245 that should also be referred to as cavities, for example, between the particles of the negative electrode active material 710 (see FIG. 5). Here, the gaps 225 and 245 (cavities) are appropriately referred to as “holes”. As shown in FIG. 2, the wound electrode body 200 has uncoated portions 222 and 242 spirally wound on both sides along the winding axis WL. On both sides 252 and 254 along the winding axis WL, the electrolytic solution can permeate from the gaps between the uncoated portions 222 and 242. For this reason, in the lithium ion secondary battery 100, the electrolytic solution is immersed in the positive electrode active material layer 223 and the negative electrode active material layer 243.

≪Gas escape route≫
In this example, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200 deformed flat. On both sides of the wound electrode body 200, gaps 310 and 312 are provided between the wound electrode body 200 and the battery case 300. The gaps 310 and 312 serve as a gas escape path. For example, when the temperature of the lithium ion secondary battery 100 becomes abnormally high, for example, when overcharge occurs, the electrolyte may be decomposed and gas may be generated abnormally. In this embodiment, the abnormally generated gas moves toward the safety valve 360 through the gaps 310 and 312 between the wound electrode body 200 and the battery case 300 on both sides of the wound electrode body 200, and from the safety valve 360 to the battery case 300. Exhausted outside.

  In the lithium ion secondary battery 100, the positive electrode current collector 221 and the negative electrode current collector 241 are electrically connected to an external device through electrode terminals 420 and 440 that penetrate the battery case 300. Hereinafter, the operation of the lithium ion secondary battery 100 during charging and discharging will be described.

≪Operation when charging≫
FIG. 7 schematically shows the state of the lithium ion secondary battery 100 during charging. At the time of charging, as shown in FIG. 7, the electrode terminals 420 and 440 (see FIG. 1) of the lithium ion secondary battery 100 are connected to the charger 290. Due to the action of the charger 290, lithium ions (Li) are released from the positive electrode active material in the positive electrode active material layer 223 to the electrolytic solution 280 during charging. In addition, charges are released from the positive electrode active material layer 223. The discharged electric charge is sent to the positive electrode current collector 221 through a conductive material (not shown), and further sent to the negative electrode sheet 240 through the charger 290. In the negative electrode sheet 240, electric charges are stored, and lithium ions (Li) in the electrolytic solution 280 are absorbed and stored in the negative electrode active material in the negative electrode active material layer 243.

<< Operation during discharge >>
FIG. 8 schematically shows a state of the lithium ion secondary battery 100 during discharging. At the time of discharging, as shown in FIG. 8, charges are sent from the negative electrode sheet 240 to the positive electrode sheet 220, and lithium ions stored in the negative electrode active material layer 243 are released to the electrolyte solution 280. In the positive electrode, lithium ions in the electrolytic solution 280 are taken into the positive electrode active material in the positive electrode active material layer 223.

  Thus, in charging / discharging of the lithium ion secondary battery 100, lithium ions travel between the positive electrode active material layer 223 and the negative electrode active material layer 243 through the electrolytic solution 280. At the time of charging, electric charge is sent from the positive electrode active material to the positive electrode current collector 221 through the conductive material. On the other hand, at the time of discharging, the charge is returned from the positive electrode current collector 221 to the positive electrode active material through the conductive material.

  During charging, the smoother the movement of lithium ions and the movement of electrons, the more efficient and rapid charging is considered possible. At the time of discharging, it is considered that the smoother the movement of lithium ions and the movement of electrons, the lower the resistance of the battery, the amount of discharge, and the output of the battery.

≪Other battery types≫
The above shows an example of a lithium ion secondary battery. The lithium ion secondary battery is not limited to the above form. Similarly, an electrode sheet in which an electrode mixture is applied to a metal foil is used in various other battery forms. For example, as another battery type, a cylindrical battery or a laminate battery is known. A cylindrical battery is a battery in which a wound electrode body is accommodated in a cylindrical battery case. A laminate type battery is a battery in which a positive electrode sheet and a negative electrode sheet are stacked with a separator interposed therebetween.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described.

  A lithium ion secondary battery according to an embodiment of the present invention has substantially the same basic structure as the lithium ion secondary battery 100 shown in FIG. For this reason, it demonstrates using the code | symbol attached | subjected to the lithium ion secondary battery 100 suitably, referring the drawing of the lithium ion secondary battery 100 shown in FIG. Here, first, the negative electrode active material layer 243 of the lithium ion secondary battery 100 will be described as an example.

  In the lithium ion secondary battery 100 according to an embodiment of the present invention, as shown in FIGS. 1 and 2, the negative electrode active material layer 243 is held on the negative electrode current collector 241. As shown in FIG. 5, the negative electrode active material layer 243 includes a negative electrode active material 710 and a rubber binder 730.

  In this embodiment, the rubber-based binder 730 is made of styrene butadiene rubber having a glass transition temperature of about −40 ° C. to −10 ° C. and an average particle diameter (D50) of about 100 nm or less by weight. Included.

≪Styrene butadiene rubber≫
Here, the styrene-butadiene rubber is a rubber made of a copolymer of styrene and butadiene, and can be used as a rubber-based binder for the negative electrode active material layer.

≪Glass transition temperature≫
Here, the “glass transition temperature” is a glass transition point (Tg) which is a transition point between a rubber state in which a molecular chain can be deformed or moved to some extent and a glass state in which partial mobility of the molecular chain is lost. Means.

≪Average particle diameter (D50) ≫
Here, the average particle diameter (D50) of the styrene butadiene rubber may be obtained from the particle size distribution of the styrene butadiene rubber measured by the dynamic light scattering method in the emulsion state. The particle size of the styrene butadiene rubber in the negative electrode active material layer 243 may be obtained from the particle size distribution of the styrene butadiene rubber specified based on the SEM image of the cross section of the negative electrode active material layer 243. FIG. 9 is an example of a cross-sectional SEM image of the negative electrode active material layer. Styrene butadiene rubber can be specified based on a cross-sectional SEM image, as shown by SB in FIG. Then, based on the cross-sectional SEM image, the approximate particle diameter of the styrene butadiene rubber may be measured.

  According to the knowledge obtained by the present inventor, when styrene butadiene rubber is used for the negative electrode active material layer 243, the peel strength between the negative electrode current collector 241 and the negative electrode active material layer 243 (hereinafter referred to as "negative electrode" The “peel strength of the active material layer” is correlated with the glass transition temperature and average particle diameter (D50) of the styrene-butadiene rubber. The reaction resistance of the lithium ion secondary battery 100 is also correlated with the glass transition temperature and average particle diameter (D50) of the styrene butadiene rubber.

  FIG. 10 shows the correlation between the glass transition temperature of styrene butadiene rubber and the peel strength of the negative electrode active material layer. FIG. 11 shows the correlation between the average particle diameter (D50) of styrene-butadiene rubber and the peel strength of the negative electrode active material layer. FIG. 12 shows the correlation between the glass transition temperature of the styrene butadiene rubber and the reaction resistance of the lithium ion secondary battery. FIG. 13 shows the correlation between the average particle diameter (D50) of styrene-butadiene rubber and the reaction resistance of the lithium ion secondary battery. FIG. 14 is a diagram illustrating a peel strength test method.

  Here, styrene butadiene rubbers having different glass transition temperatures and average particle diameters (D50) were prepared. Here, the prepared styrene butadiene rubbers have different glass transition temperatures in the range of -40 ° C to 20 ° C. A negative electrode sheet 240 (see FIG. 2) is prepared using styrene butadiene rubbers having different glass transition temperatures and different average particle diameters (D50) as binders of the negative electrode active material layer 243, and the negative electrode current collector 241 and the negative electrode active material layer 243 are produced. The peel strength of was measured. In addition, the reaction resistance of an evaluation cell manufactured using the negative electrode sheet 240 was measured.

≪Sample of styrene butadiene rubber≫
Table 1 shows each sample of the styrene butadiene rubber prepared here. The ratio of butadiene to styrene in the styrene-butadiene rubber was quantified by infrared spectroscopy (IR). For example, as shown in Table 1, the styrene-butadiene rubber prepared here has a butadiene / styrene ratio, a glass transition temperature, and a functional group adjusted.

  In sample 1, the approximate ratio of butadiene to styrene was butadiene: styrene = 51: 49. The glass transition temperature is −30 ° C., the average particle diameter (D50) is 170 nm, and it has a carboxyl group (—COOH) as a functional group.

  In sample 2, the approximate ratio of butadiene to styrene was butadiene: styrene = 44: 56. The glass transition temperature is −10 ° C., the average particle diameter (D50) is 170 nm, and it has a carboxyl group (—COOH) as a functional group.

  In sample 3, the approximate ratio of butadiene to styrene was butadiene: styrene = 25: 75. The glass transition temperature is 25 ° C., the average particle size (D50) is 170 nm, and it has a carboxyl group (—COOH) as a functional group.

  In Sample 4, the approximate ratio of butadiene to styrene was butadiene: styrene = 30: 70. The glass transition temperature is −10 ° C., the average particle diameter (D50) is 90 nm, and it has a carboxyl group (—COOH) as a functional group.

  In Sample 5, the approximate ratio of butadiene to styrene was butadiene: styrene = 22: 78. The glass transition temperature is 25 ° C., the average particle diameter (D50) is 170 nm, and it has a carboxyl group (—COOH) and a hydroxyl group (—OH) as functional groups.

  In sample 6, the approximate ratio of butadiene to styrene was butadiene: styrene = 39: 61. The glass transition temperature is −10 ° C., the average particle diameter (D50) is 80 nm, and it has a carboxyl group (—COOH) and a hydroxyl group (—OH) as functional groups.

  In Sample 7, the approximate ratio of butadiene to styrene was butadiene: styrene = 42: 58. The glass transition temperature is −10 ° C., the average particle diameter (D50) is 130 nm, and it has a carboxyl group (—COOH) and a hydroxyl group (—OH) as functional groups.

  In sample 8, the approximate ratio of butadiene to styrene was butadiene: styrene = 44: 56. The glass transition temperature is −10 ° C., the average particle diameter (D50) is 180 nm, and it has a carboxyl group (—COOH) and a hydroxyl group (—OH) as functional groups.

  Here, in Sample 5, a hydroxyl group (—OH) was added as a functional group while the glass transition temperature, the average particle diameter (D50), and the ratio of butadiene and styrene were substantially the same as in Sample 3.

≪Preparation of negative electrode sheet≫
As described above, a negative electrode sheet was produced under substantially the same conditions unless otherwise specified, except that the styrene butadiene rubber used as the binder of the negative electrode active material layer 243 was different. Moreover, the cell for evaluation was produced using this negative electrode sheet.

Here, a 10 μm thick copper foil was prepared as the negative electrode sheet 240 (see FIG. 2), graphite having an average particle diameter (D50) of 10 μm to 30 μm as the negative electrode active material, and CMC as the thickener. Then, the negative electrode active material, SBR, and CMC are kneaded using water as a solvent so that the weight ratio of the solid content is negative electrode active material: SBR: CMC = 98: 1: 1, and the negative electrode active material layer 243 is formed. A pasty mixture was obtained. Then, the paste mixture was applied to the negative electrode current collector 241, dried, and then rolled. In this embodiment, the negative electrode active material layer 243 is coated on both surfaces of the negative electrode current collector 241, and the basis weight after drying is approximately 10 mg / cm ² by combining both surfaces. Further, the density of the negative electrode active material layer 243 after rolling was set to approximately 2 g / cm ³ .

≪Peel strength≫
Here, the peel strength of the negative electrode active material layer refers to the force required for the negative electrode current collector and the negative electrode active material layer to peel and / or the force required for the negative electrode mixture layer to break in the thickness direction. Means. Here, a rectangular sheet 110 of 120 mm × 15 mm is cut out from the portion where the negative electrode active material layer of the negative electrode sheet produced as described above is formed. Then, as shown in FIG. 14, a rectangular sheet 110 is attached to a negative electrode active material layer of a portion 110 </ b> A of 80 mm × 15 mm from one end and a double-sided tape 112 is fixed to a horizontally installed table 114. The remaining 40 mm portion 110B is grasped with the instrument 116 and pulled up in the vertical direction. At that time, the tool 116 was pulled up, and the force when the negative electrode current collector and the negative electrode active material layer were peeled or the negative electrode mixture layer was broken in the thickness direction was measured as the peel strength of the negative electrode active material layer.

≪Evaluation of peel strength of negative electrode active material layer≫
In the measurement of the peel strength of the negative electrode active material layer, for example, as shown in FIG. 10, when the peel strength of the negative electrode active material layer formed using styrene butadiene rubber having a glass transition temperature of about 25 ° C. is set to 1. The peel strength of the negative electrode active material layer formed using styrene butadiene rubber having a glass transition temperature of about −10 ° C. was about 1.12. Furthermore, the peel strength of the negative electrode active material layer formed using styrene butadiene rubber having a glass transition temperature of about −40 ° C. was about 1.3.

  That is, when a styrene butadiene rubber having a glass transition temperature of about −10 ° C. is used, the peel strength of the negative electrode active material layer is 1 as compared with the case of using a styrene butadiene rubber having a glass transition temperature of about 25 ° C. It can be as strong as 12 times. Further, when a styrene butadiene rubber having a glass transition temperature of about −40 ° C. is used, the peel strength of the negative electrode active material layer is 1.3 times as compared with the case of using a styrene butadiene rubber having a glass transition temperature of 25 ° C. Can be as strong as twice.

  Thus, in the relationship between the glass transition temperature of styrene-butadiene rubber and the peel strength (peel strength between the negative electrode current collector and the negative electrode active material layer), the lower the glass transition temperature, the stronger the peel strength. There was a tendency for the peel strength to become weaker as the temperature was higher.

  In the measurement of the peel strength of the negative electrode active material layer, for example, as shown in FIG. 11, the peel strength of the negative electrode active material layer formed using styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is 1. In this case, the peel strength of the negative electrode active material layer formed using styrene butadiene rubber having an average particle diameter (D50) of about 130 nm was about 1.08. Further, the peel strength of the negative electrode active material layer formed using styrene butadiene rubber having an average particle diameter (D50) of about 80 nm was about 1.2.

  That is, when a styrene butadiene rubber having an average particle diameter (D50) of about 130 nm is used, the peel strength of the negative electrode active material layer is larger than when a styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is used. Can be as strong as 1.08 times. Further, when a styrene butadiene rubber having an average particle diameter (D50) of about 80 nm is used, the peel strength of the negative electrode active material layer is larger than when a styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is used. Can be as strong as 1.2 times.

  Thus, in the relationship between the average particle diameter (D50) of styrene butadiene rubber and the peel strength, the smaller the average particle diameter (D50), the stronger the peel hardness, and the larger the average particle diameter (D50), the weaker the peel strength. The tendency to become was obtained.

≪Test lithium ion secondary battery≫
Next, a plurality of styrene butadiene rubbers having different glass transition temperatures and average particle diameters (D50) were prepared, and an evaluation cell was prepared using a negative electrode sheet in which each styrene butadiene rubber was used as a binder for the negative electrode active material layer. Reaction resistance was evaluated. Here, an 18650 type lithium ion secondary battery was fabricated as an evaluation cell, and the reaction resistance was measured.

≪Evaluation cell positive electrode sheet≫
The positive electrode of the evaluation cell has a 15 μm thick aluminum foil as the positive electrode current collector 221 (see FIG. 2), particles of LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide) as the positive electrode active material, acetylene black as the conductive material, Polyvinylidene fluoride (PVDF) was prepared as a dressing. Then, the positive electrode active material particles and the conductive material using N-methyl-2-pyrrolidone (NMP) as a solvent so that the positive electrode active material particles: conductive material: binder = 87: 10: 3 in the weight ratio of the solid content. And a binder were kneaded to obtain a paste-like mixture forming the positive electrode active material layer 223. Then, the paste mixture was applied to the positive electrode current collector 221, dried, and then rolled. Here, the positive electrode active material layer 223 was applied to both surfaces of the positive electrode current collector 221, and the basis weight after drying was approximately 20 mg / cm ² by combining both surfaces. Further, the density of the positive electrode active material layer 223 after rolling was set to approximately 3 g / cm ³ .

≪Evaluation cell positive electrode≫
For the positive electrode of the evaluation cell, the rolled positive electrode sheet was cut into 50 mm × 700 mm, and an aluminum lead wire was ultrasonically welded to the uncoated portion of the positive electrode current collector.

≪Negative electrode of evaluation cell≫
As the negative electrode of the evaluation cell, the above-described negative electrode sheet was cut out to 50 mm × 850 mm, and a nickel lead wire was spot welded to an uncoated portion of the negative electrode current collector.

≪Evaluator cell separator, electrolyte≫
As the separator for the evaluation cell, a porous separator having a three-layer structure (PP / PE / PP) in which both sides of polyethylene (PE) are sandwiched between polypropylene (PP) is used. The separator has a thickness of 20 μm. In addition, in the electrolyte solution, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a mixed solvent in a volume ratio of EC: DMC: EMC = 3: 4: 3. An electrolytic solution containing LiPF ₆ at a concentration of about 1 mol / liter was used.

  The positive electrode sheet and the negative electrode sheet are stacked in a state where a separator is interposed between the positive electrode sheet and the negative electrode sheet, and wound so as to be accommodated in a predetermined battery case of a so-called 18560 battery. Make up the body. The evaluation cell houses the wound electrode body in a predetermined battery case of an 18560 battery, and connects the positive and negative lead wires to a predetermined wiring configuration. Thereafter, an electrolyte (here, about 5 mL) is poured into the battery case, and a lid is attached to the battery case to seal the battery case.

≪Measurement of reaction resistance≫
Next, the reaction resistance of the evaluation cell is measured.

  Lithium ion secondary batteries tend to have a problem of increased resistance, particularly in a low temperature environment. In consideration of application to a vehicle drive battery mounted on a vehicle, it is required to drive appropriately in a low temperature environment of about -30 ° C. For this reason, the reaction resistance of the evaluation cell in a low temperature environment of about −30 ° C. is evaluated here.

  In the measurement of the reaction resistance, an appropriate conditioning treatment is performed on the evaluation cell constructed as described above, and then the SOC is 60% (charged state of 60% of the rated capacity) in a predetermined temperature environment. AC impedance measurement is performed under the conditions of 001 to 10000 Hz and an amplitude of 5 mV. And the magnitude | size of the diameter of the semicircle in the Nyquist plot obtained by this alternating current impedance measurement is made into the value of reaction resistance. Here, in order to evaluate the reaction resistance of the evaluation cell in a low temperature environment of about −30 ° C., the evaluation cell is left in a temperature environment of −30 ° C. for a certain time (for example, 10 hours), and then −30 ° C. The reaction resistance was measured in the following temperature environment.

≪Evaluation of reaction resistance at -30 ℃
As shown in FIG. 12, in the relationship between the glass transition temperature of styrene-butadiene rubber and the reaction resistance at −30 ° C., the lower the glass transition temperature, the lower the reaction resistance, and the higher the glass transition temperature, the higher the reaction resistance. A trend was obtained.

  For example, as shown in FIG. 12, when the reaction resistance of a lithium ion secondary battery using a styrene butadiene rubber having a glass transition temperature of about 25 ° C. as the binder of the negative electrode active material layer is 1, the glass transition temperature is about 10. The reaction resistance of the lithium ion secondary battery using styrene butadiene rubber at 0 ° C. as the binder of the negative electrode active material layer was approximately 0.94. Further, the peel strength of the negative electrode active material layer formed using the styrene butadiene rubber of the reaction resistance of a lithium ion secondary battery using a styrene butadiene rubber having a glass transition temperature of about −40 ° C. as the binder of the negative electrode active material layer is It was approximately 0.9.

  That is, when a styrene butadiene rubber having a glass transition temperature of about −10 ° C. is used, the reaction resistance at −30 ° C. is 0.94 compared to using a styrene butadiene rubber having a glass transition temperature of about 25 ° C. Can be kept as low as possible. Furthermore, when a styrene butadiene rubber having a glass transition temperature of about −40 ° C. is used, the reaction resistance at −30 ° C. is 0.9% as compared with using a styrene butadiene rubber having a glass transition temperature of about 25 ° C. Can be kept as low as possible.

  Further, regarding the relationship between the average particle diameter (D50) of styrene-butadiene rubber and the reaction resistance at −30 ° C., the smaller the average particle diameter (D50), the lower the reaction resistance at −30 ° C., and the average particle diameter (D50). The tendency that reaction resistance of -30 degreeC became high was acquired, so that was large.

  For example, as shown in FIG. 13, when the reaction resistance at −30 ° C. formed using styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is 1, the average particle diameter (D50) is about 130 nm. The reaction resistance at −30 ° C. formed using styrene butadiene rubber was approximately 0.9. Further, the reaction resistance at −30 ° C. formed using styrene butadiene rubber having an average particle diameter (D50) of about 80 nm was about 0.85.

  That is, when a styrene butadiene rubber having an average particle diameter (D50) of about 130 nm is used, the reaction resistance at −30 ° C. is smaller than that when a styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is used. It can be kept as low as 0.9 times. Further, when a styrene butadiene rubber having an average particle diameter (D50) of about 80 nm is used, the reaction resistance at −30 ° C. is smaller than that when a styrene butadiene rubber having an average particle diameter (D50) of about 180 nm is used. It can be kept as low as 0.85 times.

  Thus, considering the peel strength of the negative electrode active material layer and the reaction resistance of −30 ° C., the styrene butadiene rubber used as the binder of the negative electrode active material layer has a low glass transition temperature and an average particle diameter (D50). There is a tendency that smaller is better.

  In addition, when such styrene butadiene rubber is used, the peel strength of the negative electrode active material layer can be ensured with a smaller amount, so that the amount of rubber-based binder used can be reduced. The rubber-based binder in the negative electrode active material layer also becomes a factor that increases the reaction resistance of the lithium ion secondary battery. By reducing the amount of rubber binder used, the reaction resistance of the lithium ion secondary battery can be kept low.

  As described above, in the lithium ion secondary battery including the negative electrode active material layer including the negative electrode active material and the rubber-based binder, the above-described glass transition temperature is −40 ° C. or higher and −10 ° C. or lower in the rubber-based binder. It is preferable that a certain amount or more of styrene butadiene rubber is included. For example, the rubber-based binder may contain approximately 90% or more of such styrene butadiene rubber by weight. Thereby, for example, when the amount of the rubber-based binder used is comparable, the peel strength of the negative electrode active material layer can be improved, and the reaction resistance of the lithium ion secondary battery at −30 ° C. can be kept low. .

  In this case, the rubber-based binder may be approximately 90% or more, preferably 95% or more, and more preferably 98% or more by weight of the above-mentioned styrene butadiene rubber. That is, the rubber-based binder may include, for example, a styrene-butadiene rubber having a glass transition temperature higher than −10 ° C., or may include a rubber-based binder other than the styrene-butadiene rubber. The rubber-based binder may be, for example, 100% by weight of styrene butadiene rubber.

  Such styrene butadiene rubber preferably has an average particle diameter (D50) of 100 nm or less. The average particle size (D50) of the styrene butadiene rubber is considered to be as small as possible, but considering the availability and workability, the average particle size (D50) of the styrene butadiene rubber is approximately 30 nm or more (more Preferably it is 50 nm or more, more preferably 80 nm or more. Thus, the suitable range of the average particle diameter (D50) of the styrene butadiene rubber is preferably about 30 nm to 100 nm.

  As a result, in a lithium ion secondary battery containing the same amount of rubber-based binder, the lithium ion secondary battery has a relatively high peel strength of the negative electrode active material layer and a relatively low reaction resistance of −30 ° C. A battery can be obtained.

  Further, if the amount of the rubber-based binder contained in the negative electrode active material layer is too large, the resistance is increased, and if it is too small, the required peel strength cannot be obtained. As described above, when the styrene butadiene rubber having a glass transition temperature of −40 ° C. or higher and −10 ° C. or lower is included as a rubber binder of the negative electrode active material layer, the negative electrode active material The amount of the rubber-based binder contained in the layer is preferably about 0.7% to 1.2% in weight ratio. Thereby, it is possible to obtain an appropriate peel strength while keeping the increase in resistance low. More preferably, the amount of the rubber-based binder contained in the negative electrode active material layer is 1.0% or less in weight ratio.

≪Presence of OH group≫
Furthermore, the styrene butadiene rubber as the rubber binder preferably has an OH group as a functional group. Thereby, in particular, the reaction resistance in a low temperature environment of about −30 ° C. can be kept low. Further, when the styrene butadiene rubber has an OH group as a functional group, the peel strength of the negative electrode active material layer is increased. Such an event can be confirmed by measuring the peel strength of the negative electrode active material layer and the reaction resistance of the lithium ion secondary battery at −30 ° C. as described above.

  According to the knowledge of the present inventor, as shown in FIG. 15, when the styrene butadiene rubber as the rubber-based binder of the negative electrode active material layer has no OH group, the number is about 1.2. The peel strength of the negative electrode active material layer can be improved about twice. Further, as shown in FIG. 16, the reaction resistance of the lithium ion secondary battery at −30 ° C. is 1 when the styrene butadiene rubber as the rubber binder of the negative electrode active material layer has no OH group. In some cases, it can be kept as low as about 0.88.

The mechanism of such an event is not specifically known. The inventor considers, for example, as shown in FIG. That is, in the negative electrode active material layer 243A, the styrene butadiene rubber 730A is attached around the negative electrode active material 710A (graphite in the illustrated example). When such a styrene butadiene rubber has OH groups, lithium ions (Li ⁺ ) in the electrolytic solution 280A are separated from the solvent (ethylene carbonate (EC) in the illustrated example) by the action of the OH groups. In this case, in the vicinity of the negative electrode active material, lithium ions (Li ⁺ ) in the electrolytic solution 280A are separated from the solvent (ethylene carbonate (EC) in the illustrated example). For this reason, lithium ions (Li ⁺ ) separated from the solvent are easily taken into the negative electrode active material 710A. In particular, in a temperature environment of −30 ° C., the reaction during charging is rate-determined to a reaction in which lithium ions (Li ⁺ ) are taken into the negative electrode active material layer 243. For this reason, the fact that the styrene butadiene rubber has an OH group contributes particularly to the effect of suppressing the reaction resistance in a temperature environment of -30 ° C. Further, since the binding force is improved when the styrene butadiene rubber has an OH group, it is considered that the function of the styrene butadiene rubber as a binder is improved.

≪Swelling degree≫
Furthermore, according to the knowledge obtained by the present inventors, the styrene butadiene rubber as the rubber-based binder should have a certain degree of swelling with respect to the electrolytic solution. Here, the swelling degree with respect to the electrolytic solution may be evaluated by, for example, the swelling degree of styrene butadiene rubber with respect to ethylene carbonate (EC) which is one of the main solvents of the electrolytic solution. The degree of swelling of styrene butadiene rubber with respect to ethylene carbonate may be obtained by dividing the weight after immersing styrene butadiene rubber in ethylene carbonate by the weight before immersing styrene butadiene rubber in ethylene carbonate. That is, styrene butadiene rubber has a stronger property of absorbing electrolyte as the degree of swelling with respect to ethylene carbonate is higher.

  In this case, the higher the degree of swelling with respect to ethylene carbonate, the stronger the peel strength of the negative electrode active material layer, and the lower the reaction resistance of the lithium ion secondary battery at −30 ° C. can be suppressed. Such an event can be confirmed by measuring the peel strength of the negative electrode active material layer and the reaction resistance of the lithium ion secondary battery at −30 ° C. as described above.

  According to the knowledge of the present inventor, as shown in FIG. 18, assuming that the degree of swelling of styrene butadiene rubber with respect to ethylene carbonate is about 150%, when the degree of swelling is about 200%, about 1. The peel strength of the negative electrode active material layer can be improved by about twice. Further, as shown in FIG. 19, the reaction resistance of the lithium ion secondary battery at −30 ° C. is 1 when the degree of swelling of the styrene butadiene rubber with respect to ethylene carbonate is about 150%, and the degree of swelling is 200%. In the case of the degree, it can be suppressed to about 0.96. Thus, the degree of swelling of styrene butadiene rubber with respect to ethylene carbonate is preferably about 150% or more (more preferably 180% or more, still more preferably 200% or more).

  While various lithium ion secondary batteries according to an embodiment of the present invention have been described above, the lithium ion secondary battery according to the present invention is not limited to the above.

  For example, in the above-described embodiment, the case where styrene butadiene rubber is used as the rubber-based binder of the negative electrode active material layer is exemplified. Such a rubber-based binder can be applied not only as a binder of the negative electrode active material layer 243 (see FIG. 1) but also to an electrode active material layer including an electrode active material and a rubber-based binder. In this case, the electrode active material layer includes a positive electrode active material layer 223 (see FIG. 1).

  That is, the styrene butadiene rubber can also be used as a rubber binder for the positive electrode active material layer 223 (see FIG. 1). In this case, the rubber-based binder of the positive electrode active material layer 223 may contain 90% or more of styrene butadiene rubber having a glass transition temperature of −40 ° C. or more and −10 ° C. or less by weight. In this case, the average particle diameter (D50) of the styrene butadiene rubber may be 100 nm or less. The styrene butadiene rubber may have an OH group. Furthermore, the degree of swelling of styrene butadiene rubber with respect to ethylene carbonate may be 150% or more.

  In addition, as described above, the present invention can improve the peel strength of the electrode active material layer and contribute to the extension of the life of the lithium ion secondary battery. Moreover, this invention contributes to restraining the reaction resistance of -30 degreeC of a lithium ion secondary battery low. For this reason, in particular, vehicle driving such as a hybrid vehicle (for example, a plug-in hybrid vehicle) or a driving battery for an electric vehicle that requires a high level of safety and that requires stable and high output performance even in a low temperature environment. It is suitable for a secondary battery for power supply.

  In this case, the lithium ion secondary battery according to an embodiment of the present invention is, for example, in the form of an assembled battery in which a plurality of secondary batteries are connected and combined as shown in FIG. It can be suitably used as a vehicle drive battery 1000 for driving the motor (electric motor). In particular, the lithium ion secondary battery according to the present invention is a lithium ion having a high capacity (for example, a rated capacity of 3.0 Ah or more) as a driving battery for a hybrid vehicle (particularly, a plug-in hybrid vehicle) or an electric vehicle. Suitable for secondary batteries.

DESCRIPTION OF SYMBOLS 1 Vehicle 100 Lithium ion secondary battery 200 Winding electrode body 220 Positive electrode sheet 221 Positive electrode current collector 222 Uncoated part 223 Positive electrode active material layer 224 Middle part 225 Gap 240 Negative electrode sheet 241 Negative electrode current collector 242 Uncoated part 243 243A Negative electrode active material layer 245 Gap 252, 254 Both sides 262, 264 Separator 280, 280A along the winding axis of the wound electrode body Electrolyte 290 Charger 300 Battery case 310, 312 Gap 320 Container body 322 Lid Container body joint 340 Lid 350 Injection hole 352 Seal cap 360 Safety valve 420, 440 Electrode terminal 420a, 440a Electrode terminal tip 610 Positive electrode active material particle 620 Conductive material 630 Binder 710, 710A Negative electrode active material 730 Binder ( Rubber binder)
730A Styrene butadiene rubber 1000 Vehicle drive battery WL Winding shaft

Claims (6)

A current collector,
An electrode active material layer that is held by the current collector and includes an electrode active material and a rubber-based binder;
Have
The rubber-based binder contains 90% or more by weight of styrene butadiene rubber having a glass transition temperature of −40 ° C. or more and −10 ° C. or less,
The styrene butadiene rubber has an average particle diameter (D50) of 130 nm or less, and the styrene butadiene rubber has a carboxyl group as a functional group and a hydroxyl group independently of the carboxyl group, Lithium ion secondary battery.   The lithium ion secondary battery according to claim 1, wherein an average particle diameter (D50) of the styrene-butadiene rubber is 100 nm or less.   3. The lithium ion secondary battery according to claim 1, wherein an amount of the rubber binder contained in the electrode active material layer is 0.7% to 1.2% in a weight ratio.   The lithium ion secondary battery according to any one of claims 1 to 3, wherein a swelling degree of the styrene butadiene rubber with respect to ethylene carbonate is 150% or more.   The lithium ion secondary battery according to any one of claims 1 to 4, wherein the electrode active material layer is a negative electrode active material layer.   A vehicle driving battery comprising the lithium ion secondary battery according to any one of claims 1 to 5.