Battery pack and method for manufacturing battery pack

Abstract

To provide an assembled battery with improved resistance to lithium deposition while suppressing high-rate deterioration that occurs in a secondary battery, and a method for manufacturing the assembled battery.SOLUTION: An assembled battery includes a plurality of battery cells stacked in a stacking direction, a spacer 30 disposed between adjacent battery cells and formed with a convex portion 32 that protrudes from one battery cell toward the other battery cell and applies pressing force to the battery cell, and constraining means that applies constraining pressure to the plurality of battery cells and the spacer 30 from the stacking direction, and the battery cell contains, in the battery case, an electrode body obtained by winding a positive electrode plate in which a positive electrode mixture layer containing a positive electrode active material is formed on a positive electrode current collector and a negative electrode plate 22 in which a negative electrode mixture layer 222 containing flat graphite particles 40 is formed on a negative electrode current collector 221, and the negative electrode mixture layer 222 includes graphite particles 40 oriented substantially perpendicularly to negative electrode current collector 221 in pressure-receiving portion 223, which is a portion that receives pressure from convex portion 32.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a battery pack and a method for manufacturing a battery pack. [Background technology]

[0002] For example, power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHEVs) require high output voltage and large power, and therefore use assembled batteries in which multiple battery cells are electrically connected. Lithium-ion secondary batteries, which are lightweight and can provide high energy density, are preferably used as the battery cells that make up the assembled batteries. Lithium-ion secondary batteries are secondary batteries that can be charged and discharged by the movement of lithium ions in an electrolyte between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate), which absorb and release lithium ions.

[0003] One example of this type of secondary battery is an electrode assembly formed by stacking positive and negative electrode plates with a separator interposed therebetween, winding them around a winding axis, and compressing them into a flat shape, and then housing the resulting assembly together with an electrolyte in a battery case. In such an electrode assembly, each of the positive and negative electrodes comprises a composite layer containing an active material and the like on a conductive current collector. For example, graphite materials such as natural graphite, artificial graphite, and amorphous forms of natural and artificial graphite are widely used as the active material for the negative electrode because of their high capacity and small irreversible capacity.

[0004] Patent Document 1 discloses a manufacturing method of a battery electrode in which a powdered electrode mixture is supplied onto a current collector foil and pressed to form an electrode, the electrode mixture containing graphite that is elongated in a direction parallel to the graphite layer surface, and after the electrode mixture containing the graphite is supplied onto the current collector foil, a magnetic field is applied to the current collector foil before the pressing, thereby controlling the orientation of the graphite in the electrode mixture to be approximately perpendicular to the current collector foil, and the manufacturing method of a battery electrode and the battery electrode are disclosed.

[0005] Patent Document 2 also discloses a method for manufacturing a battery electrode in which an electrode mixture powder obtained by granulating an electrode mixture is supplied onto a current collector foil and pressed to form an electrode, in which graphite is added to the electrode mixture, and the electrode mixture powder is obtained by granulating the electrode mixture while applying a magnetic field to the electrode mixture containing graphite. The method and battery electrode also disclose a method for manufacturing a battery electrode in which, after the electrode mixture powder is supplied onto the current collector foil, a magnetic field is further applied to the current collector foil before the pressing, thereby controlling the electrode mixture powder to be oriented approximately perpendicular to the current collector foil. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-131379 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-143304 Summary of the Invention [Problem to be solved by the invention]

[0007] It is known that in lithium-ion secondary batteries, the active materials of both the positive and negative electrodes expand and contract as lithium ions are absorbed and released during charging and discharging. When the active materials expand and contract, the electrode assembly (positive and negative electrodes) expands and contracts, making it easier for the electrolyte inside the electrode assembly to be pushed out of the electrode assembly. When the electrolyte leaks out of the electrode assembly, the distribution of salt concentration within the electrode assembly becomes uneven, increasing the internal resistance of the lithium-ion secondary battery. This increase in internal resistance due to uneven salt concentration is particularly noticeable during high-rate charging and discharging, and is known as high-rate degradation.

[0008] Fig. 1 is a diagram illustrating high-rate deterioration of a battery pack. Fig. 2 is an enlarged view of the negative electrode of a battery cell constituting the battery pack shown in Fig. 1. For example, the positive electrode E1 and negative electrode E2 of battery cell B constituting the battery pack shown in S100 in Fig. 1 are stacked so as to face each other with a separator (not shown) sandwiched between them, and an electrolyte solution E3 containing a lithium salt S is held between the positive electrode E1 and negative electrode E2 inside the electrode body.

[0009] In such a battery cell B, as shown in S200 of FIG. 1, during high-rate charging, the movement of lithium ions in the electrolyte solution E3 is slower than the absorption and release by the active material, so the salt concentration of the electrolyte solution E3 in the negative electrode E2 becomes lower than the salt concentration of the electrolyte solution E3 in the positive electrode E1, resulting in a bias in the salt concentration in the stacking direction of the positive electrode E1 and the negative electrode E2.

[0010] In this state, as shown in S300 of Fig. 1, when a driving force is applied from the center of the electrode assembly toward the edge thereof within the plane of the electrode assembly perpendicular to the stacking direction of the positive electrode E1 and the negative electrode E2 due to a change in volume accompanying the expansion of the negative electrode E2, the electrolyte E3 held between the positive electrode E1 and the negative electrode E2 is easily pushed out of the electrode assembly through the voids in the composite layer of the negative electrode E2, as shown in S400 of Fig. 1. When the low-salt-concentration electrolyte E3 leaks from the electrode assembly, the concentration of the electrolyte E3 present outside the electrode assembly decreases.

[0011] Furthermore, during high-rate discharge, the electrode body relaxes, causing electrolyte E3 to flow into the electrode body from outside, but the low-salt-concentration electrolyte E3 accumulates at the ends of the electrode body, resulting in a lower salt concentration at the ends of the electrode body than in the center, as shown in S500 in Figure 1. If repeated high-rate charge and discharge causes a bias in the salt concentration in the in-plane direction of the electrode body, this leads to high-rate degradation, which increases the internal resistance of the battery.

[0012] A battery pack constructed by stacking multiple battery cells B typically has a constrained configuration, so the driving force is large due to the constraining pressure (black arrow in Figure 1), which tends to promote the extrusion of the electrolyte from inside the electrode to the outside.

[0013] Here, Patent Documents 1 and 2 describe that in an electrode containing graphite G oriented substantially perpendicularly to a current collector C, when pressure is applied in the thickness direction of the electrode (the stacking direction of the positive electrode E1 and the negative electrode E2), the pressure is applied preferentially to the oriented graphite G, and therefore the composite layer is less likely to be crushed. Such a technique is expected to suppress deterioration of lithium deposition resistance due to crushing of the composite layer.

[0014] However, as shown in Figure 2, when graphite G, a relatively soft material, is oriented approximately perpendicular to the current collector C, the amount of expansion and contraction of graphite G in the in-plane direction of the electrode body (black arrow in Figure 2) increases, and the amount of electrolyte extruded to the outside of the electrode body increases. As a result, there is a problem that the salt concentration becomes more uneven, making high-rate degradation more likely to progress.

[0015] The present invention has been made to solve such problems, and aims to provide a battery assembly that suppresses high-rate degradation occurring in secondary batteries while improving lithium precipitation resistance, and a method for manufacturing the battery assembly. [Means for solving the problem]

[0016] The battery pack according to one embodiment includes a plurality of battery cells stacked in a stacking direction, spacers disposed between adjacent battery cells in the stacking direction and formed with convex portions that protrude from one battery cell toward the other battery cell and apply a pressing force to the battery cells, and restraining means that applies a restraining pressure to the plurality of battery cells and the spacers in the stacking direction. The battery cells are formed by housing, in a battery case, an electrode assembly in which a positive electrode plate having a positive electrode composite layer containing a positive electrode active material formed on a positive electrode current collector and a negative electrode plate having a negative electrode composite layer containing flat graphite particles formed on a negative electrode current collector are wound together, and the negative electrode composite layer includes graphite particles oriented approximately perpendicular to the negative electrode current collector in a pressure-receiving portion that receives the pressing force from the convex portions.

[0017] Furthermore, a method for manufacturing a battery pack according to one embodiment includes an electrode plate forming step of forming a positive electrode plate in which a positive electrode composite layer containing a positive electrode active material is formed on a positive electrode current collector and a negative electrode plate in which a negative electrode composite layer containing flat graphite particles is formed on a negative electrode current collector; a battery cell constructing step of constructing a plurality of battery cells by accommodating an electrode body in which the positive electrode plate and the negative electrode plate are wound in a battery case; a stacking step of stacking the plurality of battery cells in a stacking direction and arranging spacers between adjacent battery cells in the stacking direction, the spacers having protrusions that protrude from one battery cell toward the other battery cell and apply a pressing force to the battery cells; and a stacking step of applying a confining pressure to the plurality of battery cells and the spacers in the stacking direction. and a restraining step of restraining the negative electrode composite layer in a state in which the pressure is applied, and when the portion that receives the pressing force from the convex portion of the negative electrode composite layer is the pressure-receiving portion, the electrode plate forming step includes: a paste preparation step of preparing a negative electrode composite layer-forming paste containing graphite particles and a solvent; a coating step of coating the negative electrode composite layer-forming paste on the negative electrode current collector to form a coating film; a magnetic field orientation step of applying a magnetic field in a direction approximately perpendicular to the negative electrode current collector to a pressure-receiving portion of the coating film that will be the pressure-receiving portion, thereby orienting the graphite particles contained in the pressure-receiving portion; and a negative electrode composite layer forming step of drying the coating film in which the graphite particles contained in the pressure-receiving portion are oriented so that they are approximately perpendicular to the negative electrode current collector, thereby forming a negative electrode composite layer on the negative electrode current collector. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a battery pack that has improved resistance to lithium precipitation while suppressing high-rate degradation that occurs in secondary batteries, and a method for manufacturing the battery pack. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram illustrating high-rate deterioration of a battery pack. [Figure 2] 2 is an enlarged view of the negative electrode of a battery cell that constitutes the battery pack shown in FIG. 1. [Figure 3] 1 is a partial cross-sectional view showing a battery pack according to a first embodiment. [Figure 4] 4 is a diagram showing the vicinity of the negative electrode plate of a battery cell that constitutes the battery pack shown in FIG. 3. [Figure 5]3 is a flowchart showing a method for manufacturing the battery pack according to the first embodiment. [Figure 6] 10 is a flowchart showing details of an electrode plate forming step. [Figure 7] 10A to 10C are diagrams for explaining an electrode plate forming step. [Figure 8] FIG. 1 is a diagram illustrating current values ​​for each cycle during evaluation of lithium precipitation resistance. [Figure 9] 1 is a graph showing the relationship between the number of cycles and the capacity retention rate. [Figure 10] FIG. 10 is a first diagram illustrating a problem with the battery pack of the comparative example. [Figure 11] FIG. 10 is a diagram illustrating a charge / discharge pattern during evaluation of high-rate cycle characteristics. [Figure 12] 1 is a graph showing the relationship between the number of cycles and the rate of increase in resistance. [Figure 13] FIG. 10 is a second diagram illustrating the problem with the battery pack of the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0020] Embodiment 1 Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. What is shown in the drawings is only a part of the whole, and in reality, many other configurations that are not shown are included. Furthermore, for clarity of explanation, the following description and drawings have been appropriately simplified.

[0021] In the following description, the stacking direction in which multiple battery cells 10 are stacked is referred to as the X direction, a direction perpendicular to the X direction, the width direction of the battery cells 10 is referred to as the Y direction, a direction perpendicular to the X direction and the Y direction, and the height direction of the battery cells 10 is referred to as the Z direction. Also, in the following description, when comprehensively referring to the structures and members on the positive and negative electrode sides of the battery cells 10 that make up the battery pack 1, they may be collectively referred to as the "positive and negative electrodes."

[0022] An overview of the battery pack 1 according to the first embodiment will be described with reference to Fig. 3. Fig. 3 is a partial cross-sectional view showing the battery pack according to the first embodiment. As shown in Fig. 3, the battery pack 1 has a plurality of battery cells 10 stacked in a stacking direction (X direction), spacers 30 arranged between adjacent battery cells 10 in the X direction, and restraining means that applies restraining pressure to the plurality of battery cells 10 and spacers 30 from the X direction. Fig. 3 shows one battery cell 10 that constitutes the battery pack 1 and one spacer 30 adjacent to this battery cell 10, and does not show other components.

[0023] The battery cell 10 is a prismatic battery having, for example, a battery case 11 having a flat rectangular parallelepiped shape and a power generating element housed within the battery case 11. Such a battery cell 10 may be, for example, a rechargeable secondary battery such as a nickel-metal hydride battery, a lithium-ion secondary battery, or an electric double layer capacitor. In this embodiment, a specific example will be described in which the battery cell 10 is configured as a lithium-ion secondary battery.

[0024] The battery case 11 is made of a metal material such as aluminum, an aluminum alloy, or stainless steel. The battery case 11 has a case body in the shape of a rectangular cylinder with an open top and a closed bottom, and a lid in the shape of a rectangular flat plate. The lid is provided to close the opening of the case body and is joined to the case body by welding or the like. The lid seals the inside of the battery case 11 with the power generating element housed inside the case body.

[0025] The lid is provided with a safety valve, an injection port, a sealing member, and a pair of electrode terminals 12. The safety valve opens when the pressure inside the battery case 11 reaches a predetermined level, thereby discharging gas generated inside the battery case 11. The injection port is a hole that penetrates the lid and is used when injecting the electrolyte into the battery case 11. The injection port is airtightly sealed by the sealing member.

[0026] A pair of electrode terminals 12 for external connection are provided near both ends of the lid of the battery case 11 in the Y direction. One of the pair of electrode terminals 12 is a positive electrode terminal 12, and the other is a negative electrode terminal 12. The positive electrode plate 21 is electrically connected to the positive electrode terminal 12 via a positive electrode current collector attached to an exposed portion of the positive electrode plate 21. The negative electrode plate 22 is electrically connected to the negative electrode terminal 12 via a negative electrode current collector attached to an exposed portion of the negative electrode plate 22. The electrode terminals 12 of battery cells 10 adjacent in the X direction are electrically connected to each other by a conductive member such as a bus bar.

[0027] The power generating element includes an electrode assembly 20 and an electrolyte. The electrode assembly 20 is a flat, cylindrical wound electrode assembly formed by stacking long, strip-shaped positive and negative electrode plates 21 and 22 with long, strip-shaped separators 23 interposed therebetween, and then winding the stack around a winding axis and compressing it into a flat shape. The electrode assembly 20 has a pair of curved portions folded back into semicircular shapes at the upper and lower ends in the Z direction, and the curved portions are connected via a pair of flat portions facing each other in the X direction. The electrode assembly 20 is housed in the battery case 11 with one of the pair of curved portions positioned below the battery case 11 and the other positioned above the battery case 11.

[0028] The positive electrode plate 21 includes a long strip-shaped positive electrode current collector and a positive electrode composite layer formed on a part of the positive electrode current collector. The positive electrode current collector is made of a foil of a metal with good conductivity (e.g., aluminum, aluminum alloy, nickel, titanium, stainless steel, etc.).

[0029] The positive electrode mixture layer is provided on at least one surface of the positive electrode current collector, except for a portion along one edge in the width direction. The positive electrode plate 21 also has an exposed portion on the positive electrode side at one edge in the width direction of the positive electrode current collector, where the positive electrode mixture layer is not formed and the positive electrode current collector is exposed. This exposed portion is electrically connected to the electrode terminal 12 of the positive electrode. The positive electrode mixture layer held on the positive electrode current collector contains at least a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode mixture layer may also contain a conductive material, a binder, and other additives.

[0030] The positive electrode active material can be any positive electrode active material that can be used in lithium-ion secondary batteries, without any particular limitation. For example, lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium iron phosphate (LiFePO), nickel cobalt lithium aluminum oxide (NCA), nickel cobalt lithium manganese oxide (NCM), etc. can be used alone or in combination. In addition, other metal elements may be added to the positive electrode active material.

[0031] Suitable conductive materials include carbon materials such as carbon black, such as acetylene black, activated carbon, and graphite. Suitable binders include vinyl halide resins such as polyvinylidene fluoride (PVDF), and polyalkylene oxides such as polyethylene oxide (PEO).

[0032] When explaining the details of the negative electrode plate 22, reference will be made as appropriate to Figure 4. Figure 4 is a diagram showing the vicinity of the negative electrode plate of a battery cell that constitutes the battery pack shown in Figure 3. Note that while Figure 4 shows a portion of the negative electrode plate 22 and the spacer 30, in reality, the wide surface 11a of the battery cell 10 is interposed between the negative electrode plate 22 and the spacer 30.

[0033] The negative electrode plate 22 has a long strip-shaped negative electrode current collector 221 and a negative electrode composite layer 222 formed on a part of the negative electrode current collector 221. The negative electrode current collector 221 is made of a foil of a metal with good conductivity (for example, copper, copper alloy, nickel, titanium, stainless steel, etc.). The negative electrode current collector 221 is made of a foil of, for example, 5 μm. m It has a thickness of about 50 μm.

[0034] The negative electrode mixture layer 222 is provided on at least one surface of the negative electrode current collector 221, except for a portion along one edge in the width direction. The negative electrode plate 22 also has an exposed negative electrode side portion at one edge in the width direction of the negative electrode current collector 221 where the negative electrode mixture layer 222 is not formed and the negative electrode current collector 221 is exposed. The negative electrode mixture layer 222 held by the negative electrode current collector 221 contains at least flat graphite particles 40 as a negative electrode active material capable of absorbing and releasing lithium ions. The negative electrode mixture layer 222 may contain a binder, a thickener, and other additives.

[0035] The density of the negative electrode mixture layer 222 is, for example, 1.0 g / cm 3 ~1.5g / cm 3 is preferred, and in particular 1.10 g / cm 3 ~1.25g / cm 3 The thickness of negative electrode composite material layer 222 is preferably, for example, 10 μm to 100 μm, and particularly preferably 20 μm to 40 μm.

[0036] The graphite particles 40 contained in the negative electrode composite layer 222 preferably have a particle form such as a scale or plate shape, with an average aspect ratio, which is the ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter), of 1.5 or more. In this embodiment, the major axis diameter is the longest diameter in the major axis direction of the graphite particles 40, and the minor axis diameter is the diameter in the minor axis direction perpendicular to the major axis direction of the graphite particles 40. In addition, the major axis diameter of the graphite particles 40 is preferably equal to or less than the thickness of the negative electrode composite layer 222.

[0037] The graphite particles 40 are graphite-based carbon materials that can be magnetically oriented in the longitudinal direction by application of a magnetic field. Examples of such graphite particles 40 include natural graphite, artificial graphite, and amorphous forms thereof (for example, graphite particles 40 whose surfaces are coated with amorphous carbon).

[0038] The average particle size of each graphite particle 40 is preferably, for example, 1 μm to 40 μm, and particularly preferably 5 μm to 12 μm. The average particle size of the graphite particles 40 can be measured using a scanning electron microscope (SEM) or by particle size distribution measurement.

[0039] The proportion of graphite particles 40 in the entire negative electrode mixture layer 222 is preferably, for example, 80% by mass to 99.5% by mass, and particularly preferably 95% by mass to 99.5% by mass, from the viewpoint of achieving high output characteristics and high energy density.

[0040] Suitable binders include rubbers such as styrene butadiene rubber (SBR) and butyl rubber (BR). The proportion of the binder in the entire negative electrode mixture layer 222 is preferably 0.05% by mass to 5% by mass, and more preferably 0.1% by mass to 1% by mass.

[0041] Suitable thickeners include celluloses such as carboxymethyl cellulose (CMC), methyl cellulose (MC), etc. The proportion of the thickener in the entire negative electrode mixture layer 222 is preferably 0.05% by mass to 5% by mass, and more preferably 0.1% by mass to 1% by mass.

[0042] Separator 23 is made of a porous insulating resin sheet such as polyethylene (PE) or polypropylene (PP). Such a porous resin sheet may have a single-layer structure or a laminated structure of two or more layers. Furthermore, a porous heat-resistant layer may be provided on a portion of the surface of the resin sheet. Separator 23 is disposed so as to insulate the positive electrode composite layer from negative electrode composite layer 222.

[0043] The positive electrode composite layer of the positive electrode plate 21, the negative electrode composite layer 222 of the negative electrode plate 22, and the separator 23 are impregnated with an electrolyte solution within the battery case 11, and lithium ions, which are charge carriers, are transferred between the active materials of the positive and negative electrodes and the electrolyte as the electrolyte solution permeates minute voids formed in the positive and negative electrode composite layers. Electricity stored in the positive and negative electrode plates is collected by the positive and negative current collector plates, respectively, and output from the positive and negative electrode terminals 12.

[0044] The electrolyte may be, for example, a non-aqueous electrolyte. The non-aqueous electrolyte is a composition in which a lithium salt is dissolved in an organic solvent. Examples of the lithium salt include LiClO4, LiPF6, LiAsF6, LiBF4, and LiSO3CF3. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. These may be used alone or in combination as the electrolyte.

[0045] The battery cells 10 constituting the battery pack 1 are aligned in a row in the X direction with their thicknesses facing each other in the X direction, so that at least one of a pair of wide surfaces 11a extending in the YZ plane faces the opposing surface 31a of the spacer 30.

[0046] The spacer 30 has a flat rectangular parallelepiped base plate 31, and is a plate with a comb-like cross section on at least one of the opposing surfaces of the base plate that faces the battery cell 10, with multiple protrusions 32 formed thereon. ConditionEach of the protrusions 32 of the spacer 30 protrudes from one adjacent battery cell 10 in the X direction toward the other adjacent battery cell 10. The base plate and the protrusions 32 may be formed integrally or may be separate bodies. The multiple protrusions 32 are spaced apart from one another, forming a space between the wide surface 11a of the battery cell 10 and the base plate that can be used as a flow path for cooling air to regulate temperature. The spacer 30 can function as a heat sink for dissipating heat generated inside the battery cells 10.

[0047] The X-direction tip surfaces of the protrusions 32 contact the wide surfaces 11a of the battery cells 10, and apply a pressing force from the X direction to a portion of the battery cell 10 via the tip surfaces. The spacer 30 is made of a resin material such as polypropylene (PP) or polyphenylene sulfide (PPS), or a metal material with good thermal conductivity. The shape, size, and arrangement of the protrusions 32 are not particularly limited and can be designed appropriately depending on the required battery characteristics, etc.

[0048] The distance between the positive and negative electrode plates of the power generating element is prone to change due to factors such as expansion and contraction of the electrode assembly 20 during charging and discharging. If air bubbles or the like are introduced into the electrolyte due to the change in the distance between the positive and negative electrode plates, battery performance may be reduced. Therefore, the battery pack 1 is held together by applying a confining pressure from the X direction to the multiple battery cells 10 and multiple spacers 30 using a confining means attached so as to apply a required confining pressure in the X direction. This keeps the distance between the positive and negative electrode plates constant.

[0049] The restraining means is composed of a pair of end plates arranged at both ends of the battery pack 1, and a restraining member whose both ends are fixed to the pair of end plates so as to bridge the pair of end plates and which adjusts the distance between the end plates.

[0050] A compressive confinement pressure is applied to the battery pack 1 from the X direction. The application of confinement pressure restricts the relative movement of the battery cells 10, spacers 30, and end plates that make up the battery pack 1. The application of confinement pressure also causes the wide surfaces 11a of the battery cells 10 to come into close contact with the protrusions 32 formed on the spacers 30, and a pressing force is applied to the portions of the battery cells 10 that come into close contact with the protrusions 32. The configuration of the constraining means is not limited to this, and for example, the confinement pressure may be applied by adjusting the size of a case that houses the stacked battery cells 10 and spacers 30.

[0051] For example, the restraining pressure applied to the electrode body 20 by the restraining member can be found by dividing the pressing force with which the protrusions 32 press the electrode body 20 via their tip surfaces by the area of ​​the main surface of the flat portion of the electrode body 20 (the surface of the electrode body 20 facing the X direction). The restraining pressure applied by the restraining means is preferably, for example, about 1.5 kgf to 3.0 kgf.

[0052] As shown in Fig. 4, in the negative electrode plate 22 of the battery cell 10 constituting the battery pack 1 according to this embodiment, some of the graphite particles 40 contained in the negative electrode composite layer 222 are oriented approximately perpendicular to the negative electrode current collector 221. The negative electrode composite layer 222 is partitioned into a pressure-receiving portion 223 that receives a high pressing force from the convex portion 32 of the spacer 30, and a non-pressure-receiving portion 224 that receives a low pressing force from the convex portion 32 of the spacer 30. The pressure-receiving portion 223 is a portion of the negative electrode composite layer 222 that faces the convex portion 32 of the spacer 30. The non-pressure-receiving portion 224 is a portion of the negative electrode composite layer 222 that does not face the convex portion 32 of the spacer 30. In Fig. 4, the pressure-receiving portion 223 is indicated by hatching.

[0053] In negative electrode plate 22, graphite particles 40 contained in pressure-receiving portion 223 are oriented approximately perpendicular to negative electrode current collector 221. That is, graphite particles 40 are oriented such that the angle formed between the surface of negative electrode current collector 221 and the major axis direction of graphite particles 40 is 45° to 90°. Graphite particles 40 contained in non-pressure-receiving portion 224, which is the portion of negative electrode mixture layer 222 other than pressure-receiving portion 223, are not orientation-controlled.

[0054] Next, a method for manufacturing the battery pack 1 having the above configuration will be described with reference to Fig. 5. Fig. 5 is a flowchart showing the method for manufacturing the battery pack 1 according to the first embodiment. As shown in Fig. 5, the method for manufacturing the battery pack 1 according to this embodiment includes the following steps S1 to S4.

[0055] In the electrode plate formation process of step S1, a positive electrode plate 21 is formed by forming a positive electrode composite layer containing a positive electrode active material on a positive electrode current collector, and a negative electrode plate 22 is formed by forming a negative electrode composite layer 222 containing flat graphite particles on a negative electrode current collector 221. In the battery cell construction process of step S2, an electrode assembly 20 in which the positive electrode plate 21 and the negative electrode plate 22 are wound is housed in a battery case 11 to construct multiple battery cells 10. In the stacking process of step S3, multiple battery cells 10 are stacked in the stacking direction, and spacers 30 having protrusions 32 that protrude from one battery cell 10 toward the other battery cell 10 and apply a pressing force to the battery cell 10 are placed between adjacent battery cells 10 in the stacking direction. In the constraining process of step S4, the multiple battery cells 10 and the spacers 30 are constrained while applying a constraining pressure in the stacking direction. Each of these processes will be described in more detail.

[0056] Here, one of the features of the battery pack 1 according to this embodiment is the manufacturing process for the negative electrode plates 22 included in the battery cells 10, so the manufacturing method for the battery pack 1 will be described focusing on this point. The electrode plate forming process will be described in detail with reference to Fig. 6. Fig. 6 is a flowchart showing the details of the electrode plate forming process.

[0057] As shown in Fig. 6, the electrode plate formation process includes a manufacturing process for a negative electrode plate 22, which includes a paste preparation process (S1-1), a coating process (S1-2), a magnetic field orientation process (S1-3), and a negative electrode composite layer formation process (S1-4). In describing the manufacturing process for the negative electrode plate 22 in detail, reference will be made as appropriate to Fig. 7, which shows a preferred embodiment of a manufacturing apparatus capable of forming the negative electrode plate 22. Fig. 7 is a diagram for explaining the electrode plate formation process.

[0058] The manufacturing apparatus includes a conveying means 51 for conveying the negative electrode current collector 221, a coating means 52 for applying a paste for forming a negative electrode composite layer to the negative electrode current collector 221, and a magnetic field generating means 53 for orienting the graphite particles 40. The conveying means 51 includes a conveying roller and a driving means for driving the conveying roller. The conveying means 51 conveys the negative electrode current collector 221 from upstream to downstream of the coating means 52 and the magnetic field generating means 53 in that order.

[0059] In the process of manufacturing negative electrode plate 22, in the paste preparation step of step S1-1, a solvent is added to powder containing graphite particles 40, a binder, a thickener, and other additives as needed, and the mixture is kneaded using a kneading machine such as a planetary mixer to prepare a paste for forming a negative electrode composite layer. The viscosity of the paste for forming a negative electrode composite layer is preferably, for example, 400 mPa·s to 3000 mPa·s.

[0060] The solvent is appropriately selected depending on the binder used. Examples of the solvent that can be used include non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), dimethylformamide (DMF), and toluene, mixed solvents combining non-aqueous solvents, and aqueous solvents such as water and mixed solvents mainly composed of water.

[0061] Subsequently, in the coating step of step S1-2, the paste for forming the negative electrode composite layer prepared in the paste preparation step is applied to the surface of negative electrode current collector 221 to form coating film 222p on negative electrode current collector 221. As coating means 52, various coaters such as a die coater, a slit coater, a comma coater, a gravure coater, and a blade coater can be used.

[0062] Next, in the magnetic field orientation step of step S1-3, the magnetic field generating means 53 is placed near the negative electrode current collector 221 so that the direction of the magnetic field lines is approximately perpendicular to the surface of the negative electrode current collector 221. The magnetic field generating means 53 may be, for example, a flat permanent magnet 54 with an iron plate 55 attached to the surface for blocking magnetic force. The iron plate 55 is indicated by hatching in FIG. 7 . The iron plate 55 is placed on the surface of the permanent magnet 54 so as to have a shape corresponding to the flow path formed on the opposing surface of the spacer 30. This allows the magnetic field generating means 53 to be produced, with the permanent magnet 54 exposed and exhibiting a comb-tooth pattern corresponding to the protrusions 32 formed on the opposing surface of the spacer 30. Using this magnetic field generating means 53, a magnetic field that generates magnetic field lines in a direction approximately perpendicular to the surface of the negative electrode current collector 221 is applied to the pressure-receiving portion 223p of the coating film 222p. The pressure-receiving portion 223p is a part of the coating film 222p that will become the pressure-receiving portion 223 of the negative electrode composite material layer 222 when the battery pack 1 is constructed.

[0063] In this embodiment, a magnetic field is applied intermittently in the longitudinal direction of the coating film 222p so that the comb-tooth pattern appears at predetermined intervals on the surface of the coating film 222p. The comb-tooth patterns appearing at predetermined intervals on the coating film 222p are arranged on flat portions when the electrode body 20 is formed.

[0064] In the manufacturing apparatus shown in FIG. 6 , a magnetic field generating means 53 is provided downstream of the coating means 52 and movable up and down below the negative electrode current collector 221, which has the surface of the coating film 222p facing upward. When a magnetic field is to be applied to the coating film 222p, the conveyance of the negative electrode current collector 221 by the conveyance means 51 is temporarily stopped, and the magnetic field generating means 53 is raised from the standby position to the application position, thereby bringing the magnetic field generating means 53 closer to the negative electrode current collector 221. When a magnetic field is not to be applied to the coating film 222p, the magnetic field generating means 53 is lowered from the application position to the standby position, thereby moving the magnetic field generating means 53 away from the negative electrode current collector 221, and conveyance of the negative electrode current collector 221 by the conveyance means 51 is resumed. By repeatedly moving the magnetic field generating means 53 up and down in this manner, a magnetic field can be applied to all of the pressure-receiving portions 223p arranged at predetermined intervals on the coating film 222p.

[0065] In the magnetic field orientation step, the magnetic flux density of the magnetic field applied to the intended pressure-receiving portion 223p of the coating film 222p is, for example, 100 mT to 1 T, and typically 300 mT to 500 mT. The higher the magnetic flux density of the magnetic field, the higher the degree of orientation of the graphite particles 40. The time for applying the magnetic field to the intended pressure-receiving portion 223p of the coating film 222p is, for example, about 1 second to 30 seconds. The graphite particles 40 included in the intended pressure-receiving portion 223p of the coating film 222p to which the magnetic field is applied are oriented so that their major axis direction, which is the direction of easy magnetization, approaches the direction of the magnetic field lines. As a result, the graphite particles 40 are oriented in the intended pressure-receiving portion 223p so as to be approximately perpendicular to the negative electrode current collector 221.

[0066] Next, in the negative electrode composite material layer forming process of step S1-4, the coating film 222p containing the oriented graphite particles 40 as described above is dried to remove the solvent contained in the coating film 222p. Drying methods for drying the coating film 222p include natural drying, hot air drying, low-humidity air drying, vacuum drying, infrared drying, far-infrared drying, and electron beam drying, and these methods can be used alone or in combination. The dried film is then pressed using a pressing means such as a roll press to adjust the density and thickness of the negative electrode composite material layer 222. This allows the negative electrode composite material layer 222 to be formed on the negative electrode current collector 221. Note that such drying means and pressing means may be provided downstream of the magnetic field generating means 53 and on the conveying path of the conveying means 51. Through the above processes, the negative electrode plate 22 with a magnetic field applied to the intended pressure-receiving portion 223p can be formed.

[0067] The electrode plate forming step also includes a manufacturing step of the positive electrode plate 21. The manufacturing step of the positive electrode plate 21 is not particularly limited, but for example, a method can be adopted in which the electrode materials (positive electrode current collector, positive electrode active material, conductive material, binder, solvent, and other additives) of the positive electrode plate 21 are used, and the magnetic field orientation step is omitted from the manufacturing step of the negative electrode plate 22. This makes it possible to form the positive electrode plate 21 in which a positive electrode composite layer is formed on the positive electrode current collector.

[0068] Next, in the battery cell construction process of step S2, first, the electrode body 20 is fabricated using the negative electrode plate 22, positive electrode plate 21, and separator 23 obtained in the electrode plate formation process. When fabricating the electrode body 20, the positive electrode plate 21, negative electrode plate 22, and two separators 23 are stacked so that the positive electrode composite layer and the negative electrode composite layer 222 face each other with the separator 23 interposed therebetween. The stacked positive electrode plate 21, negative electrode plate 22, and two separators 23 are wound around a winding axis and compressed into a flat shape having a predetermined thickness to form the electrode body 20 in a flat cylindrical shape.

[0069] It is preferable that the lengths of the positive electrode composite layer of positive electrode plate 21, the negative electrode composite layer 222 of negative electrode plate 22, and separator 23 in the width direction are configured such that the length of negative electrode composite layer 222 is greater than the length of the positive electrode composite layer, and the length of separator 23 is greater than the length of negative electrode composite layer 222. It is also preferable that positive electrode plate 21, negative electrode plate 22, and two separators 23 are arranged with a shift in the width direction so that negative electrode composite layer 222 covers the positive electrode composite layer at both ends in the width direction, and separator 23 covers negative electrode composite layer 222 at both ends in the width direction.

[0070] The electrode assembly 20 thus fabricated is housed inside the case body through an opening in the case body. The opening of the case body housing the electrode assembly 20 is closed with a lid, and the two are joined together to form a battery case 11. Furthermore, an electrolyte is poured into the battery case 11 through an inlet in the lid, and the inlet is airtightly sealed using a sealing member. This completes the construction of the battery cell 10.

[0071] Next, in the stacking process of step S3, a plurality of battery cells 10 obtained in the battery cell construction process are prepared, and the plurality of battery cells 10 are stacked in the X direction so that the thickness direction of each battery cell 10 is the X direction. Next, spacers 30 are placed between battery cells 10 adjacent in the X direction so that at least one of a pair of wide surfaces 11a of each battery cell 10 faces the opposing surface of the spacer 30 in the X direction. In this way, the plurality of battery cells 10 and the plurality of spacers 30 are stacked alternately in the X direction.

[0072] Next, in the restraining process of step S4, the multiple battery cells 10 and multiple spacers 30 stacked in the stacking process are sandwiched between a pair of end plates from both sides in the X direction. Then, by tightening the end plates with restraining members fixed to the end plates, a restraining pressure is applied to the multiple battery cells 10 and multiple spacers 30 in the X direction, and bus bars are attached to each battery cell 10. The battery pack 1 can be manufactured through these processes.

[0073] Next, the present invention will be described more specifically based on examples, but the examples do not limit the present invention.

[0074] (Example) [Formation of negative electrode plate] A negative electrode plate 22 was produced according to the flow shown in FIG. 6. For the graphite particles 40, flake natural graphite (C) having an average particle size of 7 μm and an aspect ratio of 1.5 was used. For the binder, SBR was used. For the thickener, CMC was used. Furthermore, ion-exchanged water was used as the solvent.

[0075] First, in the paste preparation process, graphite particles 40, binder, and thickener were weighed out so that the mass ratio was C:SBR:CMC=98.8:0.5:0.7, and a required amount of solvent was added and kneaded to prepare a paste for forming a negative electrode composite layer with a viscosity of 1000 mPa·s.

[0076] Next, in the coating step, the paste for forming a negative electrode composite layer was applied by coating means 52 to both sides of copper foil (thickness: 8 μm) serving as negative electrode current collector 221 transported by transport means 51. The paste for forming a negative electrode composite layer had a coating weight of 4.7 mg / cm 2 The coating amount was adjusted so that the coating film 222p was formed on the surface of the copper foil.

[0077] In the magnetic field orientation step, a magnetic field generating means 53 was placed below the copper foil having the coating film 222p formed on its surface, and a magnetic field was applied to the pressure-receiving portion 223p of the coating film 222p. The magnetic flux density of the magnetic field during application was 500 mT, and the magnetic field application time was 1 second. The direction of the magnetic field lines during application was approximately perpendicular to the surface of the copper foil. This caused the graphite particles 40 contained in the pressure-receiving portion 223p to be vertically oriented so that their long axis direction was approximately perpendicular to the surface of the copper foil.

[0078] In the negative electrode composite material layer forming step, coating film 222p containing graphite particles 40 magnetically oriented in pressure-receiving portion 223p is dried by a drying means, and then cut to a predetermined size. 3 The negative electrode plate 22 was thus obtained.

[0079] [Forming the positive electrode plate] The positive electrode plate 21 was fabricated as follows. The positive electrode active material was LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Nickel manganese cobalt lithium oxide (NMC) with an average composition represented by O2 was used. AB was used as the conductive material. PVDF was used as the binder. NMP was used as the solvent.

[0080] The positive electrode active material, conductive material, and binder were weighed out so that the mass ratio was NMC:AB:PVDF = 96:3:1, and the required amount of solvent was added and kneaded to prepare a paste for forming a positive electrode composite layer. The prepared paste for forming a positive electrode composite layer was applied to both sides of aluminum foil (thickness 15 μm) serving as a positive electrode current collector. The paste for forming a positive electrode composite layer had a basis weight of 5.55 mg / cm. 2 The coated paste for forming a positive electrode composite layer was dried and then cut to a predetermined size to obtain a positive electrode composite layer having a density of 2.6 g / cm. 3 In this way, the positive electrode plate 21 was obtained.

[0081] [Preparation of electrolyte] The electrolyte was prepared by dissolving the supporting salt LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[0082] [Construction of evaluation battery cell] A negative electrode plate 22 and a positive electrode plate 21 were wound facing each other with a polyethylene (PE) separator 23 interposed therebetween, and this was compressed into a flattened shape to form an electrode assembly 20, which was then housed inside a battery case 11, and an electrolyte solution was added and sealed, thereby constructing an evaluation battery cell for the example. In the negative electrode plate 22 of the evaluation battery cell for the example, the graphite particles 40 contained in the negative electrode mixture layer 222 are oriented approximately perpendicular to the negative electrode current collector 221 in the pressure-receiving portion 223, and are oriented randomly to the negative electrode current collector 221 in the non-pressure-receiving portion 224.

[0083] (Comparative Example 1) A negative electrode plate 22a was formed in the same manner as in the example, except that the magnetic field orientation step was omitted. Furthermore, an evaluation battery cell of Comparative Example 1 was constructed using the negative electrode plate 22a in the same manner as in the example. In the negative electrode plate 22a of the evaluation battery cell of Comparative Example 1, the graphite particles 40 contained in the negative electrode composite layer 222a were randomly oriented relative to the negative electrode current collector 221 (see FIG. 10 ).

[0084] (Comparative Example 2) A negative electrode plate 22b was formed in the same manner as in the Example, except that a magnetic field was applied to the entire surface of the coating film 222p in the magnetic field orientation step. Furthermore, a test battery cell for Comparative Example 2 was constructed using the negative electrode plate 22b in the same manner as in the Example. In the negative electrode plate 22b of the test battery cell for Comparative Example 2, the graphite particles 40 contained in the negative electrode composite layer 222b were oriented approximately perpendicular to the negative electrode current collector 221 (see FIG. 13).

[0085] [Evaluation of battery characteristics] Next, the battery characteristics were evaluated for each test battery cell (Example, Comparative Examples 1 and 2). To evaluate the battery characteristics, each test battery cell was sandwiched between two spacers 30 and constrained in the X direction with a constraining pressure of 2 kgf.

[0086] [Measurement of initial characteristics] The restrained test battery cell was charged at a constant current of 0.2C in a 25°C environment until the battery voltage reached 4.1V, and then discharged at a constant current of 0.2C to 3V, and the initial capacity was measured. The restrained test battery cell was also discharged at a current of 15C from 50% SOC (State of Charge) in a 25°C environment, and the voltage drop was measured for 10 seconds from the start of discharge. The initial IV resistance was calculated by dividing the measured voltage drop by the corresponding current value. Note that "C" is the unit of current value, and "1C" means the current value at which the battery's rated capacity is fully discharged in 1 hour.

[0087] [Lithium deposition resistance] By measuring the capacity retention rate, it is possible to evaluate whether or not lithium deposition occurs (whether the lithium deposition resistance is good or bad). The test battery cell, whose initial characteristics had been measured, was subjected to 450 cycles of pulse charge / discharge, in which the charge / discharge voltage was increased stepwise every 50 cycles from 4.1 V to 3.0 V, at a temperature of -6.7°C, as shown in Figure 9. Figure 8 is a diagram illustrating the current value per cycle during the evaluation of lithium deposition resistance. The battery capacity was measured every 50 charge / discharge cycles, and the capacity retention rate (%) after each cycle was calculated using the following formula (1): Capacity retention rate (%) = (capacity of test battery cell after cycling / initial capacity of test battery) × 100 Formula (1)

[0088] The results are shown in Figure 9. Figure 9 is a graph showing the relationship between the number of cycles and the capacity retention rate. The vertical axis of the graph shown in Figure 9 represents the capacity retention rate (%), and the horizontal axis represents the number of cycles (times). The larger the value of the capacity retention rate, the smaller the capacity deterioration of the battery after high-rate charge-discharge cycles, indicating excellent resistance to lithium precipitation.

[0089] 9, in the test battery cell of Comparative Example 1, capacity degradation increased with each cycle, and after 450 cycles, the capacity retention rate was 90%. In contrast, in the test battery cell of the Example, capacity degradation was suppressed to the same extent as in the test battery cell of Comparative Example 2. Specifically, the capacity retention rate after 450 cycles was 92% for both the test battery cells of the Example and Comparative Example 2.

[0090] The reason for this result will be explained with reference to Fig. 10. Fig. 10 is the first diagram illustrating the problem with the battery pack of the comparative example. Fig. 10 shows an example of a partial cross-sectional view of an evaluation battery cell of Comparative Example 1 in a restrained state.

[0091] 10 , in the negative electrode plate 22a of the test battery cell of Comparative Example 1, the graphite particles 40 contained in the negative electrode composite layer 222a are randomly oriented relative to the negative electrode current collector 221, and therefore the negative electrode composite layer 222a is easily crushed by pressure from the stacking direction of the positive electrode plate 21 and the negative electrode plate 22a, which is parallel to the X direction. Therefore, it is thought that the high pressing force applied by the constraining pressure from the X direction to the portion of the negative electrode composite layer 222a facing the protrusion 32 crushes that portion, resulting in a deterioration in lithium deposition resistance.

[0092] On the other hand, in the case of the evaluation battery cell of the example, the graphite particles 40 included in the pressure-receiving portion 223 of the negative electrode composite layer 222 were oriented so as to be approximately perpendicular to the negative electrode current collector 221, thereby achieving the effect of suppressing the crushing of the pressure-receiving portion 223, to which a high pressing force is applied. In this way, by controlling the orientation of the graphite particles 40 included in the negative electrode composite layer 222, in the portion that is subjected to a high pressing force by the confining pressure, so as to be approximately perpendicular to the negative electrode current collector 221, it was possible to achieve a level of lithium deposition resistance equivalent to that achieved when the graphite particles 40 were oriented overall.

[0093] [Evaluation of high-rate cycle characteristics] The test battery cell for which the initial characteristics had been measured was subjected to 600 charge / discharge cycles in a temperature environment of 25°C, with the charge / discharge pattern shown in Figure 11 being one cycle. Figure 11 is a diagram illustrating the charge / discharge pattern used in evaluating high-rate cycle characteristics. After every 100 charge / discharge cycles, the amount of voltage drop after each cycle was measured in the same way as in measuring the initial characteristics, and the IV resistance after each cycle was calculated by dividing the measured voltage drop by the corresponding current value. The resistance increase rate (%) after each cycle was then calculated using the following formula (2): Resistance increase rate (%) = (IV resistance after 2000 cycles - initial IV resistance) / initial IV resistance × 100 Equation (2)

[0094] The results are shown in Figure 12. Figure 12 is a graph showing the relationship between the number of cycles and the rate of increase in resistance. The vertical axis of the graph shown in Figure 12 represents the rate of increase in resistance (%), and the horizontal axis represents the number of cycles (times). A lower rate of increase in resistance indicates better high-rate cycle characteristics.

[0095] 12, in the test battery cell of Comparative Example 2, the resistance increased with each cycle, and after 600 cycles, the resistance increase rate was 240%. In contrast, in the test battery cell of the Example, the resistance increase was suppressed to the same extent as in the test battery cell of Comparative Example 1. Specifically, the resistance increase rate after 600 cycles was 200% for both the test battery cells of the Example and Comparative Example 2.

[0096] The reason for this result will be explained with reference to FIG. 13. FIG. 13 is a second diagram illustrating the problem with the battery pack of the comparative example. FIG. 13 shows an example of a partial cross-sectional view of an evaluation battery cell of Comparative Example 2 in a constrained state. Note that in FIG. 13, the entire negative electrode composite layer 222b is hatched to indicate that a magnetic field was applied to the entire surface of coating film 222p.

[0097] 13, in the negative electrode plate 22b of the test battery cell of Comparative Example 2, the graphite particles 40 contained in the negative electrode composite layer 222b are generally oriented substantially perpendicular to the negative electrode current collector 221. This increases the amount of expansion and contraction of the graphite particles 40 along the in-plane direction of the electrode assembly 20, which is perpendicular to the stacking direction of the positive electrode plate 21 and the negative electrode plate 22b. This increases the change in volume of the negative electrode plate 22b, and increases the amount of electrolyte extruded to the outside of the electrode assembly 20. This is thought to be the cause of the progression of high-rate degradation.

[0098] On the other hand, in the case of the evaluation battery cell of the example, the graphite particles 40 contained in the pressure-receiving portion 223 of the negative electrode composite layer 222 are oriented substantially perpendicular to the negative electrode current collector 221, but the graphite particles 40 contained in the non-pressure-receiving portion 224 are not oriented, which is thought to have reduced the amount of expansion and contraction of the graphite particles 40 along the in-plane direction. In this way, by controlling the orientation of only a portion of the graphite particles 40 contained in the negative electrode composite layer 222, it was possible to suppress an increase in resistance to a level equivalent to that when the orientation of the graphite particles 40 is not controlled.

[0099] As described above, according to the present embodiment, it is possible to provide a battery pack that suppresses high-rate degradation occurring in a secondary battery while improving resistance to lithium precipitation, and a method for manufacturing the battery pack. [Explanation of symbols]

[0100] 1 battery pack 10 battery cells 11 Battery case 11a wide surface 12 electrode terminal 20 Electrode body 21 Positive electrode plate 22, 22a, 22b negative electrode plate 23 Separator 30 spacer 31 Base Plate 31a Opposite surface 32 Convex part 40 graphite particles 51 Transportation 52 Coating method 53 Magnetic field generating means 54 Permanent Magnets 55 Iron Plate 221 Negative electrode current collector 222, 222a, 222b Negative electrode composite layer 222p coating 223 Pressure receiving part 223p Pressure receiving section 224 Non-pressure receiving part B Battery cell C Current collector E1 positive electrode E2 negative electrode E3 electrolyte G Graphite S lithium salt

Claims

1. a plurality of battery cells stacked in a stacking direction; a spacer disposed between the battery cells adjacent to each other in the stacking direction, the spacer having a protrusion formed thereon that protrudes from one of the battery cells toward the other of the battery cells and applies a pressing force to the battery cells; a restraining means for applying a restraining pressure to the plurality of battery cells and the spacers in the stacking direction, The battery cell is configured by winding a positive electrode plate, in which a positive electrode composite layer containing a positive electrode active material is formed on a positive electrode current collector, and a negative electrode plate, in which a negative electrode composite layer containing flat graphite particles is formed on a negative electrode current collector, into a battery case, and the wound electrode assembly is accommodated in the battery case. the negative electrode composite layer includes the graphite particles oriented approximately perpendicular to the negative electrode current collector in a pressure-receiving portion that is a portion that receives the pressing force from the convex portion, and the negative electrode composite layer includes the graphite particles whose orientation is not controlled in a non-pressure-receiving portion that is a portion other than the pressure-receiving portion.

2. an electrode plate forming step of forming a positive electrode plate in which a positive electrode composite layer containing a positive electrode active material is formed on a positive electrode current collector and a negative electrode plate in which a negative electrode composite layer containing flat graphite particles is formed on a negative electrode current collector; a battery cell construction step of constructing a plurality of battery cells by housing an electrode assembly formed by winding the positive electrode plate and the negative electrode plate in a battery case; a stacking step of stacking the plurality of battery cells in a stacking direction and disposing a spacer between adjacent battery cells in the stacking direction, the spacer having a protrusion formed thereon, the protrusion protruding from one battery cell toward the other battery cell and applying a pressing force to the battery cell; a restraining step of restraining the plurality of battery cells and the spacers while applying a restraining pressure in the stacking direction, When a portion of the negative electrode mixture layer that receives the pressing force from the protrusion is defined as a pressure-receiving portion, The electrode plate forming step includes: a paste preparation step of preparing a paste for forming a negative electrode composite layer, the paste including the graphite particles and a solvent; a coating step of coating the negative electrode composite layer forming paste onto the negative electrode current collector to form a coating film; a magnetic field orientation step of applying a magnetic field in a direction substantially perpendicular to the negative electrode current collector to a pressure-receiving portion of the coating film that will serve as the pressure-receiving portion, thereby orienting the graphite particles included in the pressure-receiving portion; a negative electrode composite layer forming step of drying the coating film, in which the graphite particles contained in the pressure-receiving portion are oriented so as to be substantially perpendicular to the negative electrode current collector, to form the negative electrode composite layer on the negative electrode current collector; Including, a negative electrode composite layer including the graphite particles oriented substantially perpendicular to the negative electrode current collector in the pressure-receiving portion, and the graphite particles not having controlled orientation in a non-pressure-receiving portion other than the pressure-receiving portion.

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