JP7499043B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

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

In recent years, the spread of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other applications, which hold the key to the spread of these electric vehicles. As secondary batteries, non-aqueous electrolyte secondary batteries, which are expected to have high energy density and high output, are attracting attention.

Here, Patent Document 1 discloses a technology that aims to provide a suitable electrolyte solution capable of providing a long-life lithium-ion secondary battery, as well as a lithium-ion secondary battery that is excellent in capacity and can be efficiently increased in capacity. Specifically, the technology described in Patent Document 1 aims to solve the above problem by including an electrolyte containing a lithium salt represented by a specific general formula, an organic solvent containing a chain carbonate (having a flexible chemical structure with many freely rotatable bonds) represented by a specific general formula, and an unsaturated cyclic carbonate in an electrolyte solution containing the chain carbonate at a molar ratio of 3 to 6 relative to the lithium salt, or by including the lithium salt at a concentration of 1.1 to 3.8 mol/L.

International Publication No. 2017/179682

However, the inventors' investigations revealed that even in batteries using the technology described in Patent Document 1, sufficient battery characteristics may not be obtained if a highly viscous electrolyte is used, and that there is still room for improvement.

Therefore, the present invention aims to provide a means for further improving the battery characteristics of non-aqueous electrolyte secondary batteries that use highly viscous electrolyte solutions.

The present inventors have conducted intensive research to solve the above problems. In the process, they have found that, depending on the configuration of the electrode active material layer used, a highly viscous electrolyte may not fully penetrate into the electrode active material layer, which causes a decrease in battery performance. Based on this knowledge, the present inventors have focused on the capillary number Ca, which is an index of the permeability of a liquid into a porous body, and have continued their research. As a result, they have found that by controlling the capillary number Ca between the electrolyte and the electrode active material layer at a specified shear rate to a value within a specified range, it is possible to improve the wettability of the electrode active material layer even when a highly viscous electrolyte is used, and have completed the present invention.

That is, according to one embodiment of the present invention, there is provided a nonaqueous electrolyte secondary battery including a power generating element having an electrode including an electrode active material layer containing an electrode active material and an electrolyte solution. Here, in the nonaqueous electrolyte secondary battery, the viscosity of the electrolyte solution is 10 [mPa·s] or more at a shear rate of 10 [s ⁻¹ ] and 300 [mPa·s] or more at a shear rate of 0.1 [s ⁻¹ ]. The battery is characterized in that the capillary number Ca between the electrolyte solution and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] is 1.5×10 ⁻⁷ or more and 2.0×10 ⁻⁶ or less.

According to the present invention, it is possible to further improve the battery characteristics of a non-aqueous electrolyte secondary battery using a highly viscous electrolyte solution.

FIG. 1 is a perspective view showing the appearance of a flat laminated type secondary battery, which is one embodiment of a nonaqueous electrolyte secondary battery according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 3 is a cross-sectional view that illustrates a bipolar secondary battery, which is one embodiment of the nonaqueous electrolyte secondary battery according to the present invention.

[Non-aqueous electrolyte secondary battery]
One aspect of the present invention is a nonaqueous electrolyte secondary battery including a power generating element having an electrode including an electrode active material layer containing an electrode active material and an electrolyte solution, wherein the viscosity of the electrolyte solution is 10 [mPa·s] or more at a shear rate of 10 [s ⁻¹ ] and 300 [mPa·s] or more at a shear rate of 0.1 [s ⁻¹ ], and the capillary number Ca between the electrolyte solution and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] is 1.5×10 ⁻⁷ or more and 2.0×10 ⁻⁶ or less. The nonaqueous electrolyte secondary battery according to this aspect makes it possible to further improve the battery characteristics in a nonaqueous electrolyte secondary battery using an electrolyte solution with high viscosity.

The following describes the above-mentioned embodiment of the present invention with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiment. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios.

Figure 1 is a perspective view showing the appearance of a flat laminated secondary battery, which is one embodiment of the all-solid-state nonaqueous electrolyte secondary battery according to the present invention. Figure 2 is a cross-sectional view taken along line 2-2 shown in Figure 1. By using a laminated type, the battery can be made compact and have a high capacity.

As shown in FIG. 1, the stacked secondary battery 10a has a flat rectangular shape, with a positive electrode current collector 25 and a negative electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked secondary battery 10a, and its periphery is heat-sealed, with the power generating element 21 sealed with the positive electrode current collector 25 and the negative electrode current collector 27 extended to the outside.

The nonaqueous electrolyte secondary battery according to this embodiment is not limited to a laminated flat shape. A wound all-solid-state battery may be cylindrical, or may be a cylindrical shape that has been deformed to have a rectangular flat shape, and is not particularly limited. The cylindrical shape may be a laminate film or a conventional cylindrical can (metal can) as the exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This form can achieve weight reduction.

There are also no particular limitations on the way the current collectors (25, 27) shown in FIG. 1 are removed. The positive and negative current collectors 25 and 27 may be pulled out from the same side, or the positive and negative current collectors 25 and 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in FIG. 1. In addition, in a wound lithium-ion battery, the terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

As shown in Fig. 2, the stacked secondary battery 10a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where the charge/discharge reaction actually proceeds, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a structure in which a positive electrode, an electrolyte layer 17, and a negative electrode are laminated. The positive electrode has a structure in which a positive electrode active material layer 13 containing a positive electrode active material is disposed on both sides of a positive electrode current collector 11'. The negative electrode has a structure in which a negative electrode active material layer 15 containing a negative electrode active material is arranged on both sides of a negative electrode current collector 11". The electrolyte layer 17 has a structure in which an electrolyte solution is held inside the pores of a separator having a porous structure. The electrolyte solution also permeates the inside of the positive electrode active material layer 13 and the negative electrode active material layer 15. The positive electrode, electrolyte layer, and negative electrode are stacked in this order so that one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 face each other through the electrolyte layer 17. As a result, the adjacent positive electrode, electrolyte layer, and negative electrode form one single cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of single cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 2, the outermost positive electrode collectors located on both outermost layers of the power generating element 21 each have a positive electrode active material layer 13 disposed on only one side, but active material layers may be provided on both sides. In other words, instead of using a collector dedicated to the outermost layer with an active material layer provided on only one side, a collector with active material layers on both sides may be used as the outermost layer collector as is. In some cases, it is not necessary to use the positive electrode collector 11' and/or the negative electrode collector 11".

The positive electrode collector 11' and the negative electrode collector 11" are respectively attached with a positive electrode collector plate (tab) 25 and a negative electrode collector plate (tab) 27 that are electrically connected to each electrode (positive electrode and negative electrode), and are structured so as to be sandwiched between the ends of the laminate film 29, which is the battery exterior material, and are led out of the laminate film 29. The positive electrode collector plate 25 and the negative electrode collector plate 27 may be attached to the positive electrode collector 11' and the negative electrode collector 11" of each electrode by ultrasonic welding, resistance welding, or the like, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

The main components of the nonaqueous electrolyte secondary battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the movement of electrons from the electrode active material layer. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition, clad materials of nickel and aluminum, clad materials of copper and aluminum, and the like may be used. Also, a foil in which a metal surface is coated with aluminum may be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

The latter conductive resin includes a resin in which a conductive filler is added as necessary to a non-conductive polymeric material.

Examples of non-conductive polymeric materials include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), or polystyrene (PS). Such non-conductive polymeric materials may have excellent electric potential resistance or solvent resistance.

A conductive filler can be added to the above conductive polymer material or non-conductive polymer material as necessary. In particular, when the resin that serves as the base material of the collector is made only of a non-conductive polymer, a conductive filler is necessarily required to impart conductivity to the resin.

The conductive filler can be any material that is conductive, without any particular restrictions. For example, metals and conductive carbon are examples of materials that are excellent in conductivity, potential resistance, or lithium ion blocking properties. There are no particular restrictions on the metal, but it is preferable that the metal contains at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing these metals. There are also no particular restrictions on the conductive carbon. It is preferable that the conductive carbon contains at least one selected from the group consisting of acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene.

There are no particular limitations on the amount of conductive filler added, so long as it is an amount that can impart sufficient conductivity to the current collector, and it is generally 5 to 80% by mass relative to the total mass of the current collector (100% by mass).

The current collector may be a single layer structure made of a single material, or may be a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector contains at least a conductive resin layer made of a resin having electrical conductivity. Also, from the viewpoint of blocking the movement of lithium ions between the layers of the single cells, a metal layer may be provided on part of the current collector.

[Negative electrode active material layer]
The negative electrode active material layer includes a negative electrode active material that can release ions such as lithium ions during discharge and occlude ions such as lithium ions during charge. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb ₂ O ₅ , Li ₄ Ti ₅ O ₁₂ , and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may be used. Here, silicon and tin belong to the 14th group of elements, and are known to be negative electrode active materials that can greatly improve the capacity of non-aqueous electrolyte secondary batteries. These simple substances can occlude and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), making them high-capacity negative electrode active materials. Here, it is preferable to use Si simple substance as the silicon-based negative electrode active material. Similarly, it is also preferable to use silicon oxide such as SiO _x (0.3≦x≦1.6) disproportionated into two phases of Si phase and silicon oxide phase. In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, as the negative electrode active material containing tin element (tin-based negative electrode active material), Sn simple substance, tin alloy (Cu-Sn alloy, Co-Sn alloy), amorphous tin oxide, tin silicon oxide, etc. are mentioned. Among them, SnB _0.4 P _0.6 O _3.1 is exemplified as the amorphous tin oxide. Furthermore, SnSiO ₃ is exemplified as the tin silicon oxide. Furthermore, a metal containing lithium may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and in addition to metallic lithium, lithium-containing alloys are mentioned. Examples of the lithium-containing alloy include an alloy of Li and at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those mentioned above may be used. The present invention is particularly effective when the negative electrode active material expands and contracts greatly during charging and discharging. From this perspective and in terms of high capacity, the negative electrode active material preferably contains a carbon material, metallic lithium, a silicon-based negative electrode active material, or a tin-based negative electrode active material.

The shape of the negative electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, its average particle size (D50) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size (D50) of the active material can be measured by a laser diffraction scattering method.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 40 to 99% by mass, and more preferably within the range of 50 to 90% by mass.

The negative electrode active material layer contains the negative electrode active material as described above, and further contains other additives such as a conductive assistant, a conductive material, and a binder, as necessary.

The conductive assistant is an additive that is blended to improve the conductivity of the active material layer. Examples of the conductive assistant include carbon materials such as carbon black, such as acetylene black. When the negative electrode active material layer contains a conductive assistant, an electronic network is effectively formed inside the negative electrode active material layer, which can contribute to improving the output characteristics of the battery.

The conductive member has the function of forming an electron conduction path in the negative electrode active material layer. In particular, it is preferable that at least a part of the conductive member forms a conductive path that electrically connects from the first main surface that contacts the electrolyte layer side of the negative electrode active material layer to the second main surface that contacts the current collector side. By having such a form, the electron transfer resistance in the thickness direction in the negative electrode active material layer is further reduced. As a result, the output characteristics at high rates of the battery can be further improved. Note that whether or not at least a part of the conductive member forms a conductive path that electrically connects from the first main surface that contacts the electrolyte layer side of the negative electrode active material layer to the second main surface that contacts the current collector side can be confirmed by observing the cross section of the negative electrode active material layer using a SEM or an optical microscope.

The conductive member is preferably a conductive fiber having a fibrous form. Specific examples include carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers, conductive fibers made by uniformly dispersing metals or graphite with good conductivity in synthetic fibers, metal fibers made by fiberizing metals such as stainless steel, conductive fibers in which the surface of organic fibers is coated with a metal, and conductive fibers in which the surface of organic fibers is coated with a resin containing a conductive substance. Of these, carbon fibers are preferred because of their excellent conductivity and light weight.

The optional binder used in the negative electrode active material layer is not particularly limited, but examples include the following materials:

Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Examples of the fluororesin include ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF); vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber); and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

In addition, in this embodiment, the compounding ratio of other additives such as a conductive assistant and a binder that may be contained in the negative electrode active material layer is not particularly limited. The compounding ratio of these additives can be adjusted by appropriately referring to publicly known knowledge about nonaqueous electrolyte secondary batteries.

In the non-aqueous electrolyte secondary battery of this embodiment, the thickness of the negative electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referred to. As an example, the thickness of the negative electrode active material layer is usually about 1 to 1500 μm, preferably 50 to 1200 μm, more preferably 100 to 800 μm, and even more preferably 200 to 650 μm. The thicker the negative electrode active material layer is, the more the negative electrode active material can be retained to exert sufficient capacity (energy density). On the other hand, the thinner the negative electrode active material layer is, the more the discharge rate characteristics can be improved.

[Positive electrode active material layer]
The positive electrode active material layer contains a positive electrode active material.

(Positive Electrode Active Material)
Examples of the positive electrode active material include lithium-transition metal composite oxides such as LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li(Ni-Mn-Co)O ₂ and those in which a part of these transition metals is replaced by other elements, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, a lithium-transition metal composite oxide is used as the positive electrode active material. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably, Li(Ni-Mn-Co)O ₂ and those in which a part of these transition metals is replaced by other elements (hereinafter, also simply referred to as "NMC composite oxide") are used. NMC composite oxide has a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co arranged in an orderly manner) atomic layers are alternately stacked with oxygen atomic layers interposed between them, and contains one Li atom per transition metal M atom. The amount of Li that can be extracted is twice that of spinel-type lithium manganese oxide, i.e., the supply capacity is doubled, allowing for high capacity.

As described above, the NMC composite oxide also includes composite oxides in which a part of the transition metal element is replaced by another metal element. In this case, the other elements include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, and Zn, and preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr, more preferably Ti, Zr, P, Al, Mg, and Cr, and from the viewpoint of improving cycle characteristics, even more preferably Ti, Zr, Al, Mg, and Cr.

Since the NMC composite oxide has a high theoretical discharge capacity, it preferably has a composition represented by the general formula (1): Li _a Ni _b Mn _c Co _d M _x O ₂ (wherein, a, b, c, d, and x satisfy 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, and b+c+d+x=1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. From the viewpoint of cycle characteristics, it is preferable that 0.4≦b≦0.6 in the general formula (1). The composition of each element can be measured, for example, by plasma (ICP) emission spectrometry.

In general, nickel (Ni), cobalt (Co) and manganese (Mn) are known to contribute to capacity and output characteristics from the viewpoint of improving the purity of the material and improving the electronic conductivity. Ti and the like partially replace the transition metals in the crystal lattice. From the viewpoint of cycle characteristics, it is preferable that a part of the transition element is replaced by another metal element, and it is particularly preferable that 0<x≦0.3 in the general formula (1). Since at least one element selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr is dissolved in solid solution, the crystal structure is stabilized, and as a result, it is thought that the capacity of the battery can be prevented from decreasing even when the battery is repeatedly charged and discharged, and excellent cycle characteristics can be realized.

As a more preferred embodiment, in the general formula (1), b, c, and d are 0.44≦b≦0.51, 0.27≦c≦0.31, and 0.19≦d≦0.26, which is preferable from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ has a large capacity per unit weight compared to LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , which are proven in general consumer batteries, and has the advantage of being able to produce a compact and high-capacity battery by improving the energy density, which is also preferable from the viewpoint of range. Note that LiNi _0.8 Co _0.1 Al _0.1 O ₂ is more advantageous in terms of larger capacity. On the other hand, _{LiNi0.5Mn0.3Co0.2O2} has excellent life characteristics comparable to _{LiNi1 / 3Mn1 / 3Co1 / 3O2} .

In some cases, two or more types of positive electrode active materials may be used in combination. From the viewpoint of capacity and output characteristics, a lithium-transition metal composite oxide is preferably used as the positive electrode active material. Of course, positive electrode active materials other than those mentioned above may also be used.

The average particle size of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 30 μm, and more preferably 5 to 20 μm.

In addition, the positive electrode active material layer 15 may further contain other additives such as a conductive assistant, a conductive material, and a binder, as necessary, in the same manner as described above for the negative electrode active material layer 13.

The positive electrode (positive electrode active material layer) can be formed by a method other than the usual method of applying (coating) a slurry, or by any of the following methods: sputtering, vapor deposition, CVD, PVD, ion plating, and thermal spraying.

[Electrolyte layer]
The electrolyte layer of the nonaqueous electrolyte secondary battery according to this embodiment has a structure in which a separator is impregnated with an electrolytic solution.

The electrolyte (liquid electrolyte) functions as a carrier for lithium ions. The electrolyte (liquid electrolyte) that constitutes the electrolyte layer has a form in which a lithium salt is dissolved in an organic solvent. Examples of organic solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and gamma-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Examples of the lithium salt include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , _{and LiCF3SO3} . Among these, the lithium salt is preferably Li( _FSO2 ) _2N ( _LiFSI ) from the viewpoints of battery output and _charge /discharge cycle characteristics.

The electrolyte may further contain additives other than the above-mentioned components. Specific examples of such compounds include, for example, ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, and 1-ethyl-2-vinylethylene carbonate. Examples of the additives include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. In addition, the amount of additive used in the electrolyte can be appropriately adjusted.

One of the features of the nonaqueous electrolyte secondary battery according to the present embodiment is that the viscosity of the electrolyte is specified to be in a relatively large range. That is, the nonaqueous electrolyte secondary battery according to the present embodiment is characterized by using an electrolyte having a high viscosity. Specifically, the viscosity of the electrolyte contained in the nonaqueous electrolyte secondary battery according to the present embodiment is 10 [mPa·s] or more at a shear rate of 10 [s ⁻¹ ] and 300 [mPa·s] or more at a shear rate of 0.1 [s ⁻¹ ].

Here, it is generally known that the viscosity of a solution such as an electrolyte changes depending on the shear rate (the gradient of the flow rate in the thickness direction of the fluid). If the solution has a relatively low viscosity, the viscosity hardly changes even if the shear rate decreases (Newtonian viscosity). In contrast, a solution with a high viscosity shows nonlinearity in response to a decrease in the shear rate, and shows a pattern in which the viscosity increases suddenly in the middle (non-Newtonian viscosity). For this reason, in this embodiment, the viscosity at a shear rate of 10 [s ⁻¹ ] is specified as 10 [mPa·s] or more as a representative value of a region with a relatively high shear rate that has been previously noted as the viscosity of an electrolyte constituting a battery. In addition, in order to take into account the nonlinearity exhibited by the electrolyte with a high viscosity as described above, the viscosity at a shear rate of 0.1 [s ⁻¹ ] is also required to be 300 [mPa·s] or more as a representative value of a region with a relatively low shear rate. Thus, in the technical field of secondary batteries, no study has been conducted at all on the viscosity of the electrolyte in a region where the shear rate is as small as 0.1 [s ⁻¹ ]. In that sense, it can be said that the present invention has been completed based on a technical idea that is completely different from the conventional technology. As shown in Comparative Examples 1 and 2 described later, when a (high-viscosity) electrolyte satisfying the above-mentioned provision is used, if the capillary number Ca between the electrolyte and the electrode active material layer, which will be described in detail later, falls outside the range specified in the present application, the wettability of the electrolyte to the surface of the electrode active material is reduced. On the other hand, as shown in Reference Example 1 described later, when a (low-viscosity) electrolyte not satisfying the above-mentioned provision is used, even if the capillary number Ca falls outside the range specified in the present application, no decrease in the wettability of the electrolyte to the surface of the electrode active material is observed, and it is considered that there is almost no adverse effect on the battery characteristics. In summary, the above-mentioned provision regarding the viscosity of the electrolyte can be positioned as a prerequisite for the occurrence of the problem of the present invention.

Regarding the value of the viscosity of the above-mentioned electrolyte, when the shear rate is 10 [s ⁻¹ ], the viscosity is essentially 10 [mPa·s] or more, preferably 20 [mPa·s] or more, more preferably 30 [mPa·s] or more, particularly preferably 40 [mPa·s] or more, and most preferably 50 [mPa·s] or more. There is no particular limit to the upper limit of the viscosity when the shear rate is 10 [s ⁻¹ ], but it is usually 1×10 ⁵ [mPa·s] or less. In addition, when the shear rate is 0.1 [s ⁻¹ ], the viscosity is essentially 300 [mPa·s] or more, preferably 350 [mPa·s] or more, more preferably 400 [mPa·s] or more, particularly preferably 450 [mPa·s] or more, and most preferably 500 [mPa·s] or more. In addition, there is no particular limit to the upper limit of the viscosity when the shear rate is 0.1 [s ⁻¹ ], but it is usually 1×10 ¹⁰ [mPa·s] or less. The value of the viscosity of the electrolyte solution is the value measured by the method described in the Examples section described later. In addition, the value of the viscosity of the electrolyte solution can be changed by appropriately selecting the type of organic solvent constituting the electrolyte solution and the type or amount of additives added. Furthermore, the viscosity of the electrolyte solution can be increased by increasing the concentration of the lithium salt contained in the electrolyte solution. In this embodiment, from the viewpoint of improving battery characteristics such as the rate characteristics and cycle durability of the battery, the concentration of the lithium salt in the electrolyte solution is preferably 2.5 mol/L or more, more preferably 3.0 mol/L or more, even more preferably 3.5 mol/L or more, more preferably 4.0 mol/L or more, and most preferably 4.5 mol/L or more. On the other hand, there is no particular limit to the upper limit of the concentration of the lithium salt in the electrolyte solution, but it is usually 6.0 mol/L or less.

Another feature of the nonaqueous electrolyte secondary battery according to this embodiment is that the capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ^-1 ] is 1.5 x 10 ^-7 or more and 2.0 x 10 ^-6 or less.

Here, the capillary number Ca is a parameter defined in fluid mechanics and is also called the capillary value. This capillary number Ca is defined as the ratio of the viscous force to the surface tension acting on the boundary between different fluids, and is an index of liquid permeability into a porous body. Specifically, the capillary number Ca is expressed by the following formula:

Ca=μ×V/γ
In the formula, μ is the viscosity coefficient of the liquid (here, the electrolyte), V is the representative flow velocity (= the flow velocity at the center of the pore), and γ is the surface tension of the liquid (electrolyte). Here, the shear rate v _s is expressed as v _s = V/r (r is the pore radius of the pore (here, the pore in the electrode active material layer)) using the above V, so the capillary number Ca is expressed as follows:

Ca=μ× _vs ×r/γ
As described above, according to the studies of the present inventors, it has been found that by controlling the capillary number Ca between the electrolyte and the electrode active material layer when the shear rate (v _s ) is 10 [s ⁻¹ ] to a value within a predetermined range, it is possible to improve the wettability of the electrolyte to the electrode active material layer even when an electrolyte having a high viscosity as defined above is used.

Specifically, in the nonaqueous electrolyte secondary battery according to this embodiment, the capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] is essential to be 1.5×10 ⁻⁷ or more and 2.0×10 ⁻⁶ or less. In addition, from the viewpoint of further improving the wettability between the electrolyte and the electrode active material layer, this capillary number Ca is preferably 1.3×10 ⁻⁶ or less. In another embodiment, the capillary number Ca is preferably 1.7×10 ⁻⁷ or more and 8.5×10 ⁻⁷ or less, more preferably 4.2×10 ⁻⁷ or more and 7.7×10 ⁻⁷ or less, and particularly preferably 4.8×10 ⁻⁷ or more and 5.9×10 ⁻⁷ or less. In addition, in the nonaqueous electrolyte secondary battery according to this embodiment, at least one positive electrode active material layer or at least one negative electrode active material layer may satisfy the above-mentioned predetermined capillary number Ca between the electrolyte. However, from the viewpoint of further improving the effects of the present invention, it is preferable that all of the positive electrode active material layers constituting the battery or all of the negative electrode active material layers constituting the battery satisfy the above-mentioned prescribed capillary number Ca between themselves and the electrolyte, and it is particularly preferable that all of the positive electrode active material layers constituting the battery and all of the negative electrode active material layers constituting the battery satisfy the above-mentioned prescribed capillary number Ca between themselves and the electrolyte.

The mechanism by which the wettability between the electrolyte and the electrode active material layer is improved by controlling the capillary number Ca between the electrolyte and the electrode active material layer to a value within the above-mentioned specified range is not completely clear. However, the following mechanism is presumed. That is, if the electrolyte is a dilute electrolyte of about 1M used in conventional non-aqueous electrolyte secondary batteries, the viscosity of the electrolyte is low, so it was possible to make the electrolyte penetrate deep into the pores without particularly considering the relationship with the pore structure in the electrode active material layer. On the other hand, if the viscosity of the electrolyte increases, such as in the case of a high-concentration electrolyte, it becomes difficult for the electrolyte to penetrate deep into the pores. In particular, it has been found that there is a problem in that the electrolyte does not penetrate when the pore diameter is relatively large. On the other hand, it is presumed that by controlling the capillary number Ca between the electrolyte and the electrode active material layer to a value within the above-mentioned range, a permeation phenomenon due to capillary force occurs, and as a result, even a highly viscous electrolyte can penetrate deep into the pores of the electrode active material layer and wet the wall surface of the pores. If such a highly viscous electrolyte can penetrate deep into the pores, the contact area between the electrode active material and the electrolyte increases, making it easier for the electrode reaction to proceed more efficiently, which is thought to contribute to improved battery performance.

In addition, when calculating the capillary number Ca, the surface tension of the electrolyte, the viscosity at a shear rate of 10 [s ⁻¹ ], and the pore radius (1/2 of the pore diameter (D50)) of the electrode active material layer are required, but the values of these various parameters are measured by the method described in the Examples section below. Here, examples of a method for changing the value of the capillary number Ca include a method for changing the viscosity of the electrolyte and a method for changing the pore radius of the electrode active material layer. As described above, the value of the viscosity of the electrolyte can be changed by appropriately selecting the type of organic solvent constituting the electrolyte and the type or amount of additive. On the other hand, the value of the pore radius of the electrode active material layer can be changed by appropriately selecting the conditions of the press treatment and the number of treatments performed when producing the electrode active material layer.

As the value of the pore diameter of the electrode active material layer, the value of the 50% cumulative diameter (D50) on a volume basis measured in the Examples section described later is adopted here. From the viewpoint of further improving the wettability between the electrolyte and the electrode active material layer, the value of this D50 is preferably 0.2 to 1.0 μm, more preferably 0.5 to 1.0 μm, and even more preferably 0.7 to 0.9 μm. Similarly, the value of the 10% cumulative diameter (D10) on a volume basis measured in the Examples section described later is preferably 0.01 to 0.5 μm, more preferably 0.01 to 0.3 μm, even more preferably 0.01 to 0.25 μm, and particularly preferably 0.01 to 0.2 μm. The porosity of the electrode active material layer (this value is also measured by the method described in the Examples section) is preferably 20 to 30%, more preferably 22 to 27%, and even more preferably 24 to 26%.

In the nonaqueous electrolyte secondary battery according to this embodiment, a separator is used in the electrolyte layer. The separator has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and the function of acting as a partition between the positive electrode and the negative electrode.

Examples of the separator's form include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

For example, a microporous (microporous membrane) separator made of a porous sheet made of a polymer or fiber can be used. Specific forms of the porous sheet made of the polymer or fiber include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); laminates of multiple layers of these (for example, a laminate with a three-layer structure of PP/PE/PP), polyimide, aramid, hydrocarbon resins such as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fibers, and microporous (microporous membrane) separators.

The thickness of the microporous (microporous membrane) separator cannot be unequivocally defined, as it varies depending on the intended use. As an example, in applications such as secondary batteries for driving motors in electric vehicles (EVs), hybrid electric vehicles (HEVs), fuel cell vehicles (FCVs), etc., it is desirable for the single layer or multilayer to be 4 to 60 μm. It is desirable for the micropore diameter of the microporous (microporous membrane) separator to be a maximum of 1 μm or less (usually a pore diameter of about several tens of nm).

As the nonwoven separator, conventional materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide, aramid, etc., may be used alone or in combination. The bulk density of the nonwoven fabric is not particularly limited as long as sufficient battery characteristics can be obtained with the impregnated polymer gel electrolyte. Furthermore, the thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

The separator is preferably a separator in which a heat-resistant insulating layer is laminated on a porous substrate (separator with heat-resistant insulating layer). The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. The separator with heat-resistant insulating layer is a highly heat-resistant one with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. By having a heat-resistant insulating layer, the internal stress of the separator that increases when the temperature rises is alleviated, so that a heat shrinkage suppression effect can be obtained. As a result, the induction of short circuits between the electrodes of the battery can be prevented, resulting in a battery configuration in which performance deterioration due to temperature rise is unlikely to occur. In addition, by having a heat-resistant insulating layer, the mechanical strength of the separator with heat-resistant insulating layer is improved, and the separator is less likely to break. Furthermore, due to the heat shrinkage suppression effect and high mechanical strength, the separator is less likely to curl during the battery manufacturing process.

The inorganic particles in the heat-resistant insulating layer contribute to the mechanical strength and thermal contraction suppression effect of the heat-resistant insulating layer. The material used as the inorganic particles is not particularly limited. For example, oxides (SiO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ ), hydroxides, and nitrides of silicon, aluminum, zirconium, and titanium, as well as composites thereof, can be mentioned. These inorganic particles may be derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or may be artificially produced. In addition, only one type of these inorganic particles may be used alone, or two or more types may be used in combination. Among these, from the viewpoint of cost, it is preferable to use silica (SiO ₂ ) or alumina (Al ₂ O ₃ ), and it is more preferable to use alumina (Al ₂ O ₃ ).

The basis weight of the heat-resistant particles is not particularly limited, but is preferably 5 to 15 g/m ^2. Within this range, sufficient ion conductivity is obtained, and it is preferable in terms of maintaining heat resistant strength.

The binder in the heat-resistant insulating layer serves to bond the inorganic particles together and between the inorganic particles and the resin porous substrate layer. The binder ensures that the heat-resistant insulating layer is formed stably and prevents peeling between the porous substrate layer and the heat-resistant insulating layer.

The binder used in the heat-resistant insulating layer is not particularly limited, and compounds such as carboxymethylcellulose (CMC), polyacrylonitrile, cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), and methyl acrylate can be used as the binder. Of these, it is preferable to use carboxymethylcellulose (CMC), methyl acrylate, or polyvinylidene fluoride (PVDF). These compounds may be used alone or in combination of two or more.

The binder content in the heat-resistant insulating layer is preferably 2 to 20% by weight, based on 100% by weight of the heat-resistant insulating layer. If the binder content is 2% by weight or more, the peel strength between the heat-resistant insulating layer and the porous substrate layer can be increased, and the vibration resistance of the separator can be improved. On the other hand, if the binder content is 20% by weight or less, the gaps between the inorganic particles are appropriately maintained, ensuring sufficient lithium ion conductivity.

The heat shrinkage rate of the separator with heat-resistant insulation layer is preferably 10% or less in both MD and TD after being held for 1 hour under conditions of 150°C and ² gf/cm2. By using such a highly heat-resistant material, it is possible to effectively prevent the separator from shrinking even if the heat generation amount increases and the internal temperature of the battery reaches 150°C. As a result, it is possible to prevent the induction of short circuits between the electrodes of the battery, resulting in a battery configuration in which performance degradation due to temperature rise is unlikely to occur.

[Positive and negative current collector plates]
The material constituting the current collector plate (25, 27) is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a lithium ion secondary battery can be used. As the material constituting the current collector plate, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof is preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. In addition, the same material may be used for the positive electrode current collector plate 25 and the negative electrode current collector plate 27, or different materials may be used.

[Positive and negative electrode leads]
Although not shown in the figures, the current collectors (11', 11") and the current collector plates (25, 27) may be electrically connected via positive and negative electrode leads. As the constituent materials of the positive and negative electrode leads, materials used in known lithium ion secondary batteries may be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant insulating heat-shrink tube or the like so as to prevent contact with peripheral devices or wiring, causing electric leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, and a bag-shaped case using a laminate film 29 containing aluminum, which can cover the power generating element as shown in Figures 1 and 2, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of being excellent in high output and cooling performance and being suitable for use in batteries for large equipment for EVs and HEVs. In addition, since the group pressure applied from the outside to the power generating element can be easily adjusted, a laminate film containing aluminum is more preferable as the exterior body.

The stacked secondary battery according to the present embodiment has a configuration in which multiple single cell layers are connected in parallel, and thus has high capacity and excellent cycle durability. Therefore, the stacked secondary battery according to the present embodiment is suitable for use as a power source for driving EVs and HEVs.

[Method of manufacturing non-aqueous electrolyte secondary battery]
The method for producing the non-aqueous electrolyte secondary battery according to the present embodiment is not particularly limited, but as an example, first, electrodes (positive and negative electrodes) are produced. As a method for producing the electrodes, an electrode active material layer is produced by preparing an electrode active material slurry, applying the electrode active material slurry to the surface of a current collector, drying the electrode active material slurry, and then performing a pressing process. Then, the obtained positive and negative electrodes are laminated with a separator interposed therebetween, and placed inside a pack made of an aluminum laminate film or the like, which is an exterior body. Then, an electrolyte is poured in and the pack is sealed in a vacuum to produce a battery.

Here, in order to produce the nonaqueous electrolyte secondary battery according to the above-mentioned embodiment, it is first necessary to control the viscosity of the electrolyte solution to a value within a predetermined range. To control the viscosity of the electrolyte solution, as described above, the type of organic solvent constituting the electrolyte solution and the type or amount of additives added may be appropriately selected.

On the other hand, in order to prepare the non-aqueous electrolyte secondary battery according to the above-mentioned embodiment, it is necessary to control the capillary number Ca between the electrolyte and the electrode active material layer to a value within a predetermined range. To control the capillary number Ca, for example, the conditions of the press treatment and the number of times of treatment when preparing the electrode active material layer may be appropriately selected. The press means for performing the press treatment when preparing the electrode active material layer is not particularly limited, and for example, a calendar roll, a flat plate press, etc. may be used. The press pressure is also not particularly limited, but a press pressure of about 10 to 30 kN may usually be adopted. Here, if the press pressure is increased or the number of times of pressing is increased, moderate cracks will occur in the dried coating film (precursor of the electrode active material layer), and a large number of pores with a relatively small pore diameter will be generated. As a result, the term of the pore radius (r) in the formula for the capillary number Ca will become smaller, and the value of the capillary number Ca will also become smaller. In this case, although a large number of pores with a relatively small pore diameter are generated, the value of the porosity in the electrode active material layer will not decrease so much. Therefore, the control of the capillary number Ca in the present invention is based on a technical idea that is completely different from the simple control of the porosity in the conventional technology. That is, according to another aspect of the present invention, a method for producing a nonaqueous electrolyte secondary battery according to the above-mentioned aspect of the present invention is also provided. The production method includes a step of applying an electrode active material slurry containing an electrode active material to a surface of a current collector when preparing the electrode active material layer in the electrode, drying the electrode active material slurry, and then performing a press treatment. The press pressure or the number of press treatments is selected so that the capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] is 1.5×10 ⁻⁷ or more and 2.0×10 ⁻⁶ or less.

The number of times the pressing process is performed when making the electrode is difficult to define because it can vary depending on the pressing pressure, but it is preferably 2 to 7 times, and more preferably 3 to 5 times.

The positive and negative tabs are joined to the power generating element by welding. If necessary, it is desirable to remove excess tabs by trimming after welding. There are no particular limitations on the joining method, but using an ultrasonic welding machine is advantageous in that it does not generate heat (heat) during joining and can be joined in an extremely short time, preventing deterioration of the electrode active material layers (negative and positive active material layers) due to heat. In this case, the positive and negative tabs can be arranged so that they face each other on the same side (same removal side). This allows the positive electrode tabs of each positive electrode to be bundled together and removed from the exterior as a single positive electrode current collector. Similarly, the negative electrode tabs of each negative electrode can be bundled together and removed from the exterior as a single negative electrode current collector. The positive electrode current collector (positive electrode current collector tab) and the negative electrode current collector (negative electrode current collector tab) may also be arranged so that they are on opposite sides (different removal sides).

There are no limitations on the method for covering the power generating element with the exterior body, and conventionally known knowledge may be referenced as appropriate. As one example, the power generating element is sandwiched between a laminate film used for the battery exterior body, with the positive electrode current collector plate (positive electrode current collector tab) and the negative electrode current collector plate (negative electrode current collector tab) sandwiched from above and below so that they can be removed from the battery exterior body.

Next, three sides of the outer periphery (sealed portion) of the upper and lower laminate films are sealed by thermocompression. By thermocompression sealing three sides of the outer periphery, a three-sided sealed body is obtained. At this time, it is preferable to seal the heat-sealed portion of the side from which the positive electrode current collector plate (positive electrode current collector tab) and negative electrode current collector plate (negative electrode current collector tab) are taken out. This is because if these positive electrode current collector plate and negative electrode current collector plate are in the opening during the subsequent electrolyte injection, there is a risk that the electrolyte will splash out during the electrolyte injection.

Although the above describes a battery with a stacked structure, the battery is not limited to a stacked structure, and various shapes such as a rectangular, paper-shaped, cylindrical, or coin-shaped battery can be used.

Next, the electrolyte is injected into the three-side sealed body through the opening of the remaining side of the three-side sealed body using a liquid injection device. This forms an electrolyte layer in which the separator is impregnated with the electrolyte. At this time, it is preferable to store the three-side sealed body in a vacuum box connected to a vacuum pump so that the electrolyte can be impregnated into the laminate inside the three-side sealed body, particularly the separator and the electrode active material layer, as quickly as possible. Furthermore, it is preferable to perform the injection while reducing the pressure and creating a high vacuum inside. After the injection, it is preferable to remove the three-side sealed body from the vacuum box and release the reduced pressure once, and more preferably to return it to atmospheric pressure. According to the inventors' investigations, it has been found that by performing the steps of reducing pressure and releasing the reduced pressure in this manner, even a highly viscous electrolyte can be sufficiently penetrated into the pores of the electrode active material layer.

Finally, the inside of the three-side sealed body is again placed in a high vacuum state, and the remaining side is sealed. In this way, a laminate-type (laminated structure) non-aqueous electrolyte secondary battery can be obtained, in which single cell layers are stacked such that the positive electrode and the negative electrode face each other via a separator. However, if there is no particular problem in sealing the remaining side while maintaining the initial high vacuum state, it is not essential to carry out the above-mentioned depressurization-release process.

Although one embodiment of a nonaqueous electrolyte secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

For example, the type of battery to which the nonaqueous electrolyte secondary battery of the present invention can be applied includes a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector.

Figure 3 is a cross-sectional view that shows a bipolar secondary battery (hereinafter, simply referred to as a "bipolar secondary battery") that is one embodiment of the all-solid-state lithium-ion secondary battery according to the present invention. The bipolar secondary battery 10b shown in Figure 3 has a structure in which a roughly rectangular power generating element 21, where the actual charge and discharge reaction proceeds, is sealed inside a laminate film 29 that is the battery exterior body.

As shown in FIG. 3, the power generating element 21 of the bipolar secondary battery 10b of this embodiment has a plurality of bipolar electrodes 23 in which a positive electrode active material layer 13 electrically connected to one surface of the current collector 11 is formed, and a negative electrode active material layer 15 electrically connected to the opposite surface of the current collector 11 is formed. Each bipolar electrode 23 is stacked via a solid electrolyte layer 17 to form the power generating element 21. The solid electrolyte layer 17 has a structure in which a solid electrolyte is formed into a layer. As shown in FIG. 3, the solid electrolyte layer 17 is sandwiched and arranged between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23.

Adjacent positive electrode active material layer 13, solid electrolyte layer 17, and negative electrode active material layer 15 constitute one single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a configuration in which the single cell layers 19 are stacked. The positive electrode active material layer 13 is formed only on one side of the outermost current collector 11a on the positive electrode side, which is located in the outermost layer of the power generation element 21. The negative electrode active material layer 15 is formed only on one side of the outermost current collector 11b on the negative electrode side, which is located in the outermost layer of the power generation element 21. A seal portion (insulating layer) 31 is disposed on the outer periphery of the single cell layer 19. This prevents liquid junctions due to leakage of electrolyte from the electrolyte layer 17, and prevents contact between adjacent current collectors 11 in the battery and short circuits caused by slight misalignment of the ends of the single cell layers 19 in the power generation element 21.

The sealing portion is a component specific to bipolar secondary batteries (series-stacked batteries) and has the function of preventing leakage of the electrolyte from the electrolyte layer. In addition, it can also prevent contact between adjacent current collectors in the battery and short circuits caused by slight irregularities at the ends of the stacked electrodes. There are no particular limitations on the material that can be used to make the sealing portion, but polyolefin resins such as polyethylene and polypropylene, epoxy resins, rubber, polyimide, etc. can be used. Of these, it is preferable to use polyolefin resins from the standpoints of corrosion resistance, chemical resistance, film-forming properties, and economy.

Furthermore, in the bipolar secondary battery 10b shown in FIG. 3, a positive electrode current collector (positive electrode tab) 25 is disposed adjacent to the outermost current collector 11a on the positive electrode side, and is extended to lead out from the laminate film 29, which is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is disposed adjacent to the outermost current collector 11b on the negative electrode side, and is similarly extended to lead out from the laminate film 29.

The number of times the cell layers 19 are stacked is adjusted according to the desired voltage. In addition, in the bipolar secondary battery 10b, the number of times the cell layers 19 are stacked may be reduced if sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In order to prevent external impacts and environmental deterioration during use, the bipolar secondary battery 10b is also preferably structured such that the power generating element 21 is vacuum sealed in the laminate film 29, which is the battery exterior, and the positive electrode current collector 25 and the negative electrode current collector 27 are exposed to the outside of the laminate film 29.

[Battery pack]
A battery pack is a device that consists of multiple batteries connected together. More specifically, it is a device that uses at least two batteries and is configured in series or parallel or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

A small, removable battery pack can be formed by connecting multiple batteries in series or in parallel. Then, multiple such small, removable battery packs can be further connected in series or in parallel to form a large-capacity, high-output battery pack (battery module, battery pack, etc.) suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. How many batteries are connected to form a battery pack, and how many stages of small battery packs are stacked to form a large-capacity battery pack, can be determined according to the battery capacity and output of the vehicle (electric vehicle) in which it is to be installed.

[vehicle]
The battery or a battery pack formed by combining a plurality of batteries can be mounted on a vehicle. In the present invention, a battery with excellent long-term reliability and long life can be configured, so that by mounting such a battery, a plug-in hybrid electric vehicle with a long EV driving distance or an electric vehicle with a long driving distance per charge can be configured. For example, if the battery or a battery pack formed by combining a plurality of batteries is used in a hybrid vehicle, a fuel cell vehicle, or an electric vehicle (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles, etc.), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles), it becomes a vehicle with a long life and high reliability. However, the use is not limited to automobiles, and it can be applied to various power sources of other vehicles, for example, mobile bodies such as trains, and can also be used as a mounted power source for an uninterruptible power supply device, etc.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[Example 1]
<Preparation of Positive Electrode>
A solid content consisting of 95% by mass of LiNi _0.5 Mn _0.3 Co _0.2 O (average particle size: 15 μm) as a positive electrode active material, 2% by mass of carbon nanotubes as a conductive assistant, and 3% by mass of polyvinylidene fluoride as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent was added to this solid content and mixed to prepare a positive electrode active material slurry. Next, the obtained positive electrode active material slurry was applied to one side of an aluminum foil (thickness 20 μm) as a current collector and dried. Thereafter, the obtained laminate was compressed five times using a roll press of 20 kN to prepare the positive electrode of this example.

The cross section of the positive electrode active material layer constituting the positive electrode thus obtained was observed using a scanning electron microscope (SEM), and the pores were fitted to a circle by image analysis using software Image-Pro Premier (manufactured by Media Cybernetics). From the obtained pore distribution, the 50% cumulative diameter (D50) and 10% cumulative diameter (D10) of the pore diameter on a volume basis were calculated. As a result, the D50 of the positive electrode active material layer in this example was 0.7 μm, and the D10 was 0.2 μm. In addition, the porosity of the positive electrode active material layer calculated by the above image analysis was 24%.

<Preparation of negative electrode>
A solid content consisting of 96% by mass of graphite (average particle size: 20 μm) as a negative electrode active material, 1% by mass of acetylene black as a conductive assistant, and 2% by mass of styrene-butadiene rubber (SBR) and 1% by mass of carboxymethyl cellulose (CMC) as binders was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent was added to this solid content and mixed to prepare a negative electrode active material slurry. Next, the obtained negative electrode active material slurry was applied to one side of a copper foil (thickness 20 μm) as a current collector and dried. In this way, the negative electrode of this example was prepared.

<Preparation of Electrolyte Solution>
The electrolyte solution of this example was prepared by dissolving lithium salt Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI) at a concentration of 4 mol/L (4 M) in dimethyl carbonate (DMC) which is a non-aqueous solvent.

The shear rate-viscosity characteristics of the electrolyte thus obtained were measured using a rheometer: MCR302 (manufactured by Anton Paar). At this time, the measurement range of the shear rate was set to 0.01 to 1000 [s ⁻¹ ]. Then, from the graph showing the obtained shear rate-viscosity characteristics, the viscosity was read at shear rates of 0.1 [s ⁻¹ ] and 10 [s ⁻¹ ], and was 500 [mPa·s] and 51 [mPa·s], respectively. In addition, the surface tension of the electrolyte obtained above was measured by the ring method (ring method) specified in JIS K2241:2017, and was 30 [mN/m].

The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated from the pore radius (½ of the above D50) of the positive electrode active material layer constituting the positive electrode produced above and the viscosity and surface tension of the electrolyte prepared above, and was found to be 6.0 × 10-6 ^.

<Preparation of test cell>
The positive electrode and the negative electrode obtained above were cut to a size of 12.4 cm ² and 13.8 cm ² , respectively. An aluminum foil with an aluminum terminal was laminated on the current collector (aluminum foil) of the positive electrode. Meanwhile, a copper foil with a nickel terminal was laminated on the current collector (copper foil) of the negative electrode.

Next, a separator (Celgard (registered trademark) 3501, made of polypropylene (PP) by Celgard) was inserted on the electrode active material layer side of the positive and negative electrodes to form a laminate. This laminate was sandwiched between an exterior body made of a heat-sealed aluminum laminate film (thickness 150 μm), and the electrolyte prepared above was injected. The inside of the exterior body was then depressurized to a vacuum, the depressurization was released once and returned to atmospheric pressure, and then the pressure was reduced to a vacuum again and sealed, thereby producing a pouch-type lithium-ion battery (test cell) having a power generation element in which the positive electrode active material layer and the negative electrode active material layer are stacked so that they face each other with the separator interposed therebetween.

<Evaluation of Test Cell (Measurement of Electric Double Layer Capacity (C _dl ))>
For the test cell prepared above, before charging and discharging, the electric double layer capacity (C _dl ) per mass of the electrode active material layer was measured by electrochemical impedance (EIS). In addition, when EIS measurement is performed before charging and discharging, it is not necessary to consider the influence of the formation of an SEI film on the surface of the electrode active material. Therefore, the value of the measured electric double layer capacity (C _dl ) reflects the size of the contact area between the electrolyte and the surface of the electrode active material. In addition, the impedance Z obtained from the equivalent circuit when the charge transfer process is rate-limiting in the electrochemical reaction is expressed by the following formula. Therefore, here, C _dl was measured based on the impedance Z obtained by EIS measurement based on the following formula. The results are shown in Table 1 below. In addition, the value of the electric double layer capacity (C _dl ) shown in Table 1 is a relative value when the value of the electric double layer capacity (C _dl ) in Reference Example 1 described later is used as a reference.

Z = _Rs + _Rc / (1 + _{jωRcCdl )}
In the formula, _Rs is the solution resistance, _Rc is the charge transfer resistance, _Cdl is the electric double layer capacitance, j is the imaginary unit, and ω is the angular frequency (=2πf, where f is frequency).

[Example 2]
In this example, an electrolyte solution was prepared in the same manner as in Example 1, except that the concentration of the lithium salt (LiFSI) in the electrolyte solution was 3.5 mol/L (3.5M). The shear rate-viscosity characteristics of the electrolyte solution were measured in the same manner as above, and the viscosity was 400 [mPa·s] and 26 [mPa·s] at shear rates of 0.1 [s ⁻¹ ] and 10 [s ⁻¹ ], respectively. The surface tension of the electrolyte solution was 29 [mN/m].

A pouch-type lithium ion battery (test cell) of this example was produced in the same manner as in Example 1 described above, except that the above electrolyte was used. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 3.1 x 10 ^-6 . The electric double layer capacity (Cdl) of the obtained test cell was then measured in the same manner as above. The results are shown in Table 1 below.

[Example 3]
In this example, the positive electrode was produced by the same method as in Example 1 described above, except that the number of roll press treatments when producing the positive electrode was three. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this way, the 50% cumulative diameter (D50) and 10% cumulative diameter (D10) of the pore diameter on a volume basis were calculated, and the D50 of the pore diameter of the positive electrode active material layer of this example was 0.9 μm, and the D10 was 0.25 μm. The porosity of the positive electrode active material layer was 26%.

A pouch-type lithium ion battery (test cell) of this example was produced in the same manner as in Example 1 described above, except that the above positive electrode was used. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 7.7 x 10 ^-6 . The electric double layer capacity (Cdl) of the obtained test cell was then measured in the same manner as above. The results are shown in Table 1 below.

[Comparative Example 1]
In this comparative example, the positive electrode was produced by the same method as in Example 1 described above, except that the number of roll press treatments when producing the positive electrode was two. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this way, the 50% cumulative diameter (D50) and 10% cumulative diameter (D10) of the pore diameter on a volume basis were calculated, and the D50 of the pore diameter of the positive electrode active material layer of this comparative example was 1.5 μm, and the D10 was 0.35 μm. The porosity of the positive electrode active material layer was 28%.

A pouch-type lithium ion battery (test cell) of this comparative example was produced in the same manner as in Example 1 described above, except that the above positive electrode was used. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 1.3 x 10 ^-5 . The electric double layer capacity (Cdl) of the obtained test cell was then measured in the same manner as above. The results are shown in Table 1 below.

[Comparative Example 2]
In this comparative example, the positive electrode was produced by the same method as in Example 1 described above, except that the number of roll press treatments when producing the positive electrode was one. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this way, the 50% cumulative diameter (D50) and 10% cumulative diameter (D10) of the pore diameter on a volume basis were calculated, and the D50 of the pore diameter of the positive electrode active material layer of this comparative example was 4 μm, and the D10 was 1.3 μm. And the porosity of the positive electrode active material layer was 40%.

A pouch-type lithium ion battery (test cell) of this comparative example was produced in the same manner as in Example 1 described above, except that the above positive electrode was used. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ⁻¹ ] was calculated to be 3.4×10 ⁻⁵ . The electric double layer capacity (Cdl) of the obtained test cell was then measured in the same manner as above. The results are shown in Table 1 below.

[Reference Example 1]
In this reference example, an electrolyte solution was prepared in the same manner as in Example 1, except that the concentration of the lithium salt (LiFSI) in the electrolyte solution was 2 mol/L (2M). When the shear rate-viscosity characteristics of the electrolyte solution were measured in the same manner as above, the viscosity was 50 [mPa·s] and 12 [mPa·s] when the shear rate was 0.1 [s ⁻¹ ] and 10 [s ⁻¹ ], respectively. The surface tension of the electrolyte solution was 30 [mN/m].

A pouch-type lithium ion battery (test cell) of this reference example was produced in the same manner as in Example 1 described above, except that the above electrolyte was used. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ⁻¹ ] was calculated to be 1.4×10 ⁻⁶ . The electric double layer capacity (Cdl) of the obtained test cell was then measured in the same manner as above. The results are shown in Table 1 below. As described above, the electric double layer capacity (Cdl) values shown in Table 1 are relative values based on the electric double layer capacity (Cdl) value in this reference example.

[Reference Example 2]
In this reference example, except that the concentration of the lithium salt (LiFSI) in the electrolyte was 2 mol/L (2M), a pouch-type lithium ion battery (test cell) of this reference example was produced by the same method as in Comparative Example 1 described above. In this reference example, the capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ⁻¹ ] was calculated to be 3.0×10 ⁻⁶ . Then, for the obtained test cell, the electric double layer capacity (Cdl) was measured by the same method as above. However, the electric double layer capacity (Cdl) could not be measured in the test cell of this reference example. This was thought to be because the electrolyte did not penetrate well into the positive electrode active material layer.

[Example 4]
In this example, a positive electrode was produced in the same manner as in Example 1 described above, except that the pressure and number of times of the roll press treatment in producing the positive electrode were changed as shown in the following Table 2. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this manner, the 50% cumulative diameter (D50) of the pore diameter on a volume basis was calculated, and the D50 of the pore diameter of the positive electrode active material layer of this example was 1.0 μm.

An observation sample was prepared by dropping an appropriate amount of the electrolyte solution prepared in Example 1 onto the positive electrode active material layer constituting the positive electrode prepared above, allowing it to permeate, and freezing it in that state. The capillary number Ca between the electrolyte solution and the positive electrode active material layer at a shear rate of 10 [ ^s ] was calculated to be 8.5 × 10 ^.

The observation samples thus prepared were observed by cryo-electron microscopy using a transmission electron microscope (TEM) equipped with equipment (such as a cryo-holder) that allows observation at low temperatures (-160 to -270°C) to confirm the state of electrolyte penetration into the pores of the positive electrode active material layer. The results are shown in Table 2 below. Note that if the electrolyte had penetrated into the pores, it was marked with an ◯, and if it had not, it was marked with an ×.

[Example 5]
In this example, a positive electrode was produced in the same manner as in Example 4 described above, except that the pressure and number of times of the roll press treatment in producing the positive electrode were changed as shown in the following Table 2. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this manner, the 50% cumulative diameter (D50) of the pore diameter on a volume basis was calculated, and the D50 of the pore diameter of the positive electrode active material layer in this example was 0.5 μm.

Using the positive electrode thus produced, an observation sample was produced in the same manner as above, and observed by cryo-electron microscopy. The results are shown in Table 2 below. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 4.3 x 10-6 ^.

[Example 6]
In this example, a positive electrode was produced in the same manner as in Example 4 described above, except that the pressure and number of times of the roll press treatment in producing the positive electrode were changed as shown in the following Table 2. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this manner, the 50% cumulative diameter (D50) of the pore diameter on a volume basis was calculated, and the D50 of the pore diameter of the positive electrode active material layer of this example was 0.2 μm.

Using the positive electrode thus produced, an observation sample was produced in the same manner as above, and observed by cryo-electron microscopy. The results are shown in Table 2 below. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 1.7 x 10-6 ^.

[Comparative Example 3]
In this comparative example, a positive electrode was produced in the same manner as in Example 4 described above, except that the pressure and number of times of the roll press treatment in producing the positive electrode were changed as shown in the following Table 2. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this manner, the 50% cumulative diameter (D50) of the pore diameter on a volume basis was calculated, and the D50 of the pore diameter of the positive electrode active material layer of this example was 5.0 μm.

Using the positive electrode thus produced, an observation sample was produced in the same manner as above, and observed by cryo-electron microscopy. The results are shown in Table 2 below. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 4.3 x 10-5 ^.

[Comparative Example 4]
In this comparative example, a positive electrode was produced in the same manner as in Example 4 described above, except that the pressure and number of times of roll press treatment in producing the positive electrode were changed as shown in the following Table 2. From the pore distribution of the positive electrode active material layer constituting the positive electrode produced in this manner, the 50% cumulative diameter (D50) of the pore diameter on a volume basis was calculated, and the D50 of the pore diameter of the positive electrode active material layer of this example was 0.05 μm.

Using the positive electrode thus produced, an observation sample was produced in the same manner as above, and observed by cryo-electron microscopy. The results are shown in Table 2 below. The capillary number Ca between the electrolyte and the positive electrode active material layer at a shear rate of 10 [s ^-1 ] was calculated to be 4.3 x 10-7 ^.

From the results shown in Table 1, in Examples 1 to 3 in which the capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] is in the range of 1.5×10 ⁻⁷ or more and 2.0×10 ⁻⁶ or less, it can be seen that even when a high-viscosity electrolyte is used, the decrease in the value of the electric double layer capacity (C _dl ) is suppressed. In contrast, in Comparative Examples 1 and 2 in which the capillary number Ca is outside the above range despite the use of a high-viscosity electrolyte, a decrease in the value of the electric double layer capacity (C _dl ) was confirmed. Therefore, it was shown that the configuration of the present invention can suppress the decrease in the electric double layer capacity (C _dl ). The value of the electric double layer capacity (C _dl ) is an index of the wettability between the electrode active material layer and the electrolyte (more specifically, the permeability of the electrolyte into the inside of the pores constituting the electrode active material layer). Therefore, according to the configuration of the present invention in which the decrease in the electric double layer capacity (C _dl ) can be suppressed, further improvement in the battery characteristics can also be expected. This fact is also supported by the fact that in Table 2, it was confirmed that the electrolyte solution penetrated into the pores of the positive electrode active material layer in the examples of the present invention.

In addition, in Reference Example 1, which does not use a highly viscous electrolyte, even when the capillary number Ca is outside the range specified in the present application, no decrease in the electric double layer capacity (C _dl ) was observed. This supports the idea that the problem of a decrease in the electric double layer capacity (C _dl ) occurs specifically when a highly viscous electrolyte is used.

10a: stacked secondary battery;
10b bipolar secondary battery;
11 current collector,
11' positive electrode current collector,
11″ negative electrode current collector,
11a: outermost current collector on the positive electrode side;
11b: outermost current collector on the negative electrode side;
13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generating element,
23 bipolar electrodes,
25 Positive electrode current collector (positive electrode tab),
27 negative electrode current collector (negative electrode tab),
29 Laminating film 31 Sealing portion.

Claims (5)

A non-aqueous electrolyte secondary battery comprising a power generating element having an electrode including an electrode active material layer containing an electrode active material and an electrolyte solution,
The viscosity of the electrolyte is 10 [mPa·s] or more at a shear rate of 10 [s ⁻¹ ] and 300 [mPa·s] or more at a shear rate of 0.1 [s ⁻¹ ];
The nonaqueous electrolyte secondary battery has a capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ⁻¹ ] of 3.1×10 ⁻⁶ or more and 6.0×10 ⁻⁶ or less . 2. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the electrode active material layer has a pore diameter having a volume-based 50% cumulative diameter (D50) of 0.2 to 1 μm. 3. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the electrolyte concentration in the electrolytic solution is 3.0 mol/L or more. 4. The nonaqueous electrolyte secondary battery according to claim 1 , wherein the 10% cumulative pore diameter (D10) of the pores in the electrode active material layer is 0.01 to 0.2 μm. A method for producing a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, comprising the steps of:
When preparing the electrode active material layer in the electrode, the method includes a step of applying an electrode active material slurry containing an electrode active material to a surface of a current collector, drying the slurry, and then performing a press treatment;
a pressing pressure or a number of times of pressing is selected so that a capillary number Ca between the electrolyte and the electrode active material layer at a shear rate of 10 [s ^-1 ] is 3.1 x 10-6 ^or more and 6.0 x 10-6 ^or less .