Method for manufacturing non-aqueous electrolyte secondary battery

Abstract

To provide a manufacturing method capable of improving a battery capacitance of a secondary battery.SOLUTION: A manufacturing method disclosed herein contains: a step of constructing a battery assembly in which an electrode body in which a positive electrode and a negative electrode are laminated via a separator, and a non-aqueous electrolyte are housed in a battery case; a step of performing an initial charging of the battery assembly; a high temperature holding step of holding the battery assembly after the initial charging step at a high temperature of 40°C or larger; a normal temperature holding step of holding the battery assembly for three or more hours in a normal temperature after the high temperature holding step; and a gas release step of depressing the battery assembly after the normal temperature holding step along a lamination direction of the electrode body to release it.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are widely used in various fields. For example, secondary batteries are used as power sources for driving vehicles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles. One type of secondary battery of this type is a nonaqueous electrolyte secondary battery that uses a nonaqueous electrolyte solution as an electrolyte. Generally, such a nonaqueous electrolyte secondary battery is manufactured by preparing a battery assembly in which an electrode body and a nonaqueous electrolyte solution are housed inside a battery case, and then subjecting the battery assembly to initial charging, high-temperature aging, and the like.

[0003] Patent Documents 1 and 2 disclose methods for manufacturing the above-mentioned nonaqueous electrolyte secondary battery. For example, Patent Document 1 discloses that an electrode assembly and an electrolyte are housed in a battery case, and that the pressure is reduced during or after preliminary charging, and then the opening of the battery case is sealed. Patent Document 2 also discloses that a battery assembly containing an electrode assembly and a nonaqueous electrolyte is constructed, the battery assembly is restrained, and then initial charging is performed. It also discloses that after initial charging, the restraint on the battery assembly is released, and then a pumping step is performed to apply restraint pressure again. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-90974 [Patent Document 2] Japanese Patent Publication No. 2020-149802 Summary of the Invention [Problem to be solved by the invention]

[0005] When a battery assembly is charged, various reactions occur within the electrode assembly as the battery assembly is charged. For example, in a nonaqueous electrolyte secondary battery, a portion of the nonaqueous electrolyte is reductively decomposed during initial charging, forming a coating called a solid electrolyte interface (SEI) film on the surface of the negative electrode active material. As these reactions occur during charging, gas may be generated within the electrode assembly. If this gas remains within the electrode assembly, the battery capacity may decrease. Patent Document 1 describes a technique for appropriately releasing gas by performing a decompression process after precharging. However, the inventors' research has revealed that simply releasing gas after precharging reduces the capacity of a secondary battery.

[0006] The present invention has been made in view of the above points, and an object of the present invention is to provide a manufacturing method for improving the battery capacity of a secondary battery. [Means for solving the problem]

[0007] The manufacturing method disclosed herein includes the steps of constructing a battery assembly in which an electrode body, in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, and a non-aqueous electrolyte solution are housed in a battery case; initially charging the battery assembly; a high-temperature holding step of holding the battery assembly after the initial charging step at a high temperature of 40°C or higher; a room-temperature holding step of holding the battery assembly at room temperature for more than 3 hours after the high-temperature holding step; and a degassing step of pressing the battery assembly after the room-temperature holding step in the stacking direction of the electrode body and releasing it.

[0008] By initially charging the battery assembly and then maintaining it at a high temperature, various reactions that occur within the electrode assembly during charging can be largely completed, thereby suppressing further gas generation. Therefore, by performing a degassing process after the room-temperature maintaining process, gas remaining within the electrode assembly can be suitably released to the outside of the electrode assembly. By eliminating the gas remaining within the electrode assembly, the battery capacity is improved. Therefore, this configuration makes it possible to manufacture a secondary battery with improved battery capacity. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a flowchart illustrating a manufacturing method according to one embodiment. [Figure 2] FIG. 2 is a perspective view schematically showing a battery according to one embodiment. [Figure 3] FIG. 3 is a vertical cross-sectional view schematically showing the internal structure of a battery according to one embodiment. [Figure 4] FIG. 4 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. [Figure 5] FIG. 5 is a graph showing the relationship between the holding time in the room temperature holding step and the discharge capacity. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein (e.g., the general configuration and manufacturing process of a battery that do not characterize the technology disclosed herein) can be understood as design matters for a person skilled in the art based on prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Furthermore, the notation "A to B" (A and B are arbitrary numbers) indicating a range in this specification means A or more and B or less.

[0011] In this specification, the term "nonaqueous electrolyte secondary battery" refers generally to a battery that uses a nonaqueous electrolyte as an electrolyte and can be repeatedly charged and discharged. A typical example of such a nonaqueous electrolyte secondary battery is a lithium-ion secondary battery. This lithium-ion secondary battery uses lithium (Li) ions as electrolyte ions (charge carriers) and is charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode. In addition, in this specification, the term "active material" refers to a material that reversibly absorbs and releases charge carriers. Note that in the following embodiments, a lithium-ion secondary battery is used as the nonaqueous electrolyte secondary battery, but the technology disclosed herein is not limited to lithium-ion secondary batteries and can also be applied to other nonaqueous electrolyte secondary batteries (e.g., sodium-ion batteries).

[0012] FIG. 1 is a flowchart showing the general steps of the manufacturing method disclosed herein. As shown in FIG. 1, the manufacturing method disclosed herein includes an assembly construction step S10, an initial charging step S20, a high-temperature holding step S30, a room-temperature holding step S40, and a degassing step S50. The manufacturing method disclosed herein is characterized by carrying out the high-temperature holding step S30, in which the battery assembly is held at 60°C or higher, followed by the room-temperature holding step S40, in which the battery assembly is held at room temperature for more than three hours, and then carrying out the degassing step S50, in which the battery assembly is pressed along the stacking direction of the electrode body and released. Therefore, the rest of the manufacturing process may be the same as conventional processes. Furthermore, other steps may be included at any stage.

[0013] In the manufacturing method disclosed herein, as described above, after the initial charging step S20 and the high-temperature holding step S30, the room-temperature holding step S40 is performed, and then the degassing step S50 is performed, thereby effectively eliminating gas retention within the electrode assembly. While not intending to limit the technology disclosed herein, the reason for this effect is presumed to be as follows: By holding the battery assembly at a high temperature after initial charging, various reactions associated with charging within the electrode assembly are promoted. For example, the formation of a coating called an SEI (Solid Electrolyte Interface) film is promoted on the surface of the negative electrode. As reactions associated with charging progress within the electrode assembly, gas may be generated. If such gas remains within the electrode assembly, the battery capacity (discharge capacity) may decrease due to, for example, increased internal resistance. While it was previously thought that the above-mentioned reactions would be promoted by holding the battery assembly at a high temperature, the inventors' investigations have shown that reactions are promoted even after the high-temperature holding step is terminated, for example, due to residual heat. Therefore, by maintaining the electrode body at room temperature for more than 3 hours after the high-temperature maintaining step, the reaction inside the electrode body can be largely completed, and further gas generation can be suppressed. Furthermore, by performing the degassing step at the timing when the room-temperature maintaining step is completed, the accumulation of gas inside the electrode body can be suitably eliminated. This configuration makes it possible to realize a manufacturing method that suitably improves the battery capacity of a nonaqueous electrolyte secondary battery. The manufacturing method disclosed herein will be described in detail below with reference to the drawings.

[0014] FIG. 2 is a perspective view schematically illustrating a battery assembly constructed using the manufacturing method disclosed herein. FIG. 3 is a diagram schematically illustrating the internal structure of a battery assembly constructed using the manufacturing method disclosed herein. FIG. 4 is a diagram schematically illustrating the structure of an electrode body included in a battery assembly constructed using the manufacturing method disclosed herein. In the following description, the symbols L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, top, and bottom, and the symbols X, Y, and Z in the drawings represent the short side direction of the battery assembly, the long side direction perpendicular to the short side direction, and the up-down direction, respectively. However, these directions are merely used for convenience of explanation and do not limit the installation form of the battery assembly in any way.

[0015] In the assembly construction step S10, a battery assembly 100 is constructed in which an electrode body 20 and a non-aqueous electrolyte (not shown) are housed in a battery case 10. The assembly construction step S10 may include, for example, a step of preparing a battery case 10, an electrode body 20, and a non-aqueous electrolyte, a step of housing the prepared electrode body 20 inside the battery case 10, and a step of injecting the non-aqueous electrolyte into the battery case 10 housing the electrode body 20.

[0016] In the assembly construction step S10, first, a battery case 10 is prepared. The battery case 10 includes a case body 12 and a sealing plate 14. As shown in FIGS. 2 and 3, the battery case 10 has a flat rectangular parallelepiped (square) outer shape. The material of the battery case 10 may be the same as that conventionally used, and is not particularly limited. The battery case 10 (case body 12 and sealing plate 14) is made of, for example, aluminum, aluminum alloy, stainless steel, iron, iron alloy, etc.

[0017] The case body 12 is a housing that houses the electrode assembly 20 and the nonaqueous electrolyte. The case body 12 is a bottomed, square-shaped container having an opening 12h on one side (here, the top surface). Here, the opening 12h is approximately rectangular. As shown in FIG. 2, the case body 12 has a rectangular bottom surface 12a having long and short sides, a pair of long side walls 12b that extend upward from the long side of the bottom surface 12a and face each other, and a pair of short side walls 12c that extend upward from the short side of the bottom surface 12a and face each other.

[0018] The sealing plate 14 here is rectangular and is a plate-like member that seals the opening 12h of the case body 12. The sealing plate 14 faces the bottom surface 12a of the case body 12. As shown in FIG. 3 , the sealing plate 14 has two terminal mounting holes 18 and 19 that penetrate the sealing plate 14 in the thickness direction. The terminal mounting holes 18 and 19 are provided at both ends of the sealing plate 14 in the long side direction Y. The terminal mounting hole 18 on one side (left side in FIG. 3 ) is for the positive electrode, and the terminal mounting hole 19 on the other side (right side in FIG. 3 ) is for the negative electrode. The sealing plate 14 also has a liquid injection hole 15 and a gas release valve 17. The liquid injection hole 15 is a through-hole through which electrolyte is injected into the battery case 10 after the sealing plate 14 is assembled to the case body 12. The liquid injection hole 15 is sealed with a sealing member 16 after the electrolyte is injected. The gas release valve 17 is a thin-walled portion that is configured to break when the pressure inside the battery case 10 reaches or exceeds a predetermined value, thereby releasing the gas inside the battery case 10 to the outside.

[0019] The positive electrode terminal 30 and the negative electrode terminal 40 are components that are fixed to the sealing plate 14 in the completed battery. The positive electrode terminal 30 is disposed on one side of the sealing plate 14 in the long side direction Y (the left side in FIGS. 2 and 3). The positive electrode terminal 30 is electrically connected to a plate-shaped positive electrode external conductive member 32 on the outside of the battery case 10. The positive electrode terminal 30 is preferably made of metal, more preferably aluminum or an aluminum alloy, for example. On the other hand, the negative electrode terminal 40 is disposed on the other side of the sealing plate 14 in the long side direction Y (the right side in FIGS. 2 and 3). The negative electrode terminal 40 is electrically connected to a plate-shaped negative electrode external conductive member 42 on the outside of the battery case 10. The negative electrode terminal 40 is preferably made of metal, more preferably copper or a copper alloy, for example. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are components to which bus bars are attached when electrically connecting multiple secondary batteries to each other. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are preferably made of metal, more preferably aluminum or an aluminum alloy, for example, but the positive electrode external conductive member 32 and the negative electrode external conductive member 42 are not essential and may be omitted in other embodiments.

[0020] In the assembly construction step S10, an electrode body 20 including a positive electrode 22, a negative electrode 24, and a separator 26 is prepared. As shown in Fig. 4, the electrode body 20 is a wound electrode body in which a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 are stacked in an insulated state via two strip-shaped separators 26, and wound in the longitudinal direction around a winding axis WL. However, the electrode body 20 may also be a laminated electrode body in which a rectangular positive electrode and a rectangular negative electrode are stacked in an insulated state.

[0021] As shown in FIG. 4, the positive electrode 22 (hereinafter also referred to as "positive electrode sheet 22") is a long, strip-shaped member. The configuration of the positive electrode sheet 22 is not particularly limited and may be the same as that used in conventionally known batteries. For example, the positive electrode 22 has a positive electrode current collector 22c, and a positive electrode active material layer 22a and a positive electrode protective layer 22p fixed to at least one surface of the positive electrode current collector 22c. However, the positive electrode protective layer 22p is not essential and may be omitted in other embodiments.

[0022] The positive electrode current collector 22c is strip-shaped. The positive electrode current collector 22c is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. Here, the positive electrode current collector 22c is a metal foil, specifically an aluminum foil. The dimensions of the positive electrode current collector 22c are not particularly limited and may be determined appropriately depending on the battery design. The thickness of the positive electrode current collector 22c is, for example, preferably 2 μm to 30 μm, more preferably 2 μm to 20 μm, and even more preferably 5 μm to 15 μm. A plurality of positive electrode tabs 22t are provided at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 4). The plurality of positive electrode tabs 22t protrude to one side in the long side direction Y (the left side in FIG. 4). The plurality of positive electrode tabs 22t protrude further in the long side direction Y than the separator 26. The positive electrode tab 22t is a part of the positive electrode current collector 22c and is made of metal foil (aluminum foil). At least a part of the positive electrode tab 22t is free from the positive electrode active material layer 22a and the positive electrode protective layer 22p, and the positive electrode current collector 22c is exposed.

[0023] As shown in FIG. 4, the positive electrode active material layer 22a is provided along the longitudinal direction of the strip-shaped positive electrode current collector 22c. The positive electrode active material layer 22a contains a positive electrode active material. The positive electrode active material may be a known positive electrode active material used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, or the like. The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one transition metal element selected from the group consisting of Ni, Co, and Mn. Examples of the lithium composite oxide include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides. These positive electrode active materials may be used alone or in combination.

[0024] The positive electrode active material layer 22a may contain components other than the positive electrode active material, such as a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVDF).

[0025] The content of the positive electrode active material in the positive electrode active material layer 22a (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 22a) is not particularly limited, but is preferably 70 mass% or more, more preferably 80 mass% to 97 mass% or less, and even more preferably 85 mass% to 96 mass% or less. The content of the conductive material in the positive electrode active material layer 22a is not particularly limited, but is preferably 1 mass% to 15 mass% or less, and more preferably 3 mass% to 13 mass% or less. The content of the binder in the positive electrode active material layer 22a is not particularly limited, but is preferably 1 mass% to 15 mass% or less, and more preferably 1.5 mass% to 10 mass% or less. The thickness of the positive electrode active material layer 22a is not particularly limited, and is preferably, for example, 10 μm to 200 μm or less, and may be 50 μm to 100 μm.

[0026] As shown in FIG. 4, the positive electrode protective layer 22p is provided at the boundary between the positive electrode current collector 22c and the positive electrode active material layer 22a in the long side direction Y. Here, the positive electrode protective layer 22p is provided at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 4). However, the positive electrode protective layer 22p may be provided at both end portions in the long side direction Y. The positive electrode protective layer 22p is provided in a strip shape along the positive electrode active material layer 22a. The positive electrode protective layer 22p contains an inorganic filler (e.g., alumina). When the entire solid content of the positive electrode protective layer 22p is taken as 100% by mass, the inorganic filler may account for approximately 50% by mass or more, typically 70% by mass or more, for example, 80% by mass or more. The positive electrode protective layer 22p may contain optional components other than the inorganic filler, such as a conductive material, a binder, and various additive components.

[0027] The positive electrode sheet 22 can be prepared according to a known method, for example, by preparing a positive electrode paste containing a positive electrode active material and optional components, applying the positive electrode paste to the positive electrode current collector 22c, drying it, and pressing it as necessary, thereby preparing the positive electrode sheet 22. Note that in this specification, the term "paste" is used to encompass forms called "slurry" and "ink."

[0028] The negative electrode 24 (hereinafter also referred to as "negative electrode sheet 24") is a long, strip-shaped member, as shown in FIG. 4. The configuration of the negative electrode sheet 24 is not particularly limited and may be the same as that used in conventionally known batteries. For example, the negative electrode 24 has a negative electrode current collector 24c and a negative electrode active material layer 24a fixed onto at least one surface of the negative electrode current collector 24c.

[0029] The negative electrode current collector 24c is strip-shaped. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. Here, the negative electrode current collector 24c is a metal foil, specifically a copper foil. The dimensions of the negative electrode current collector 24c are not particularly limited and may be determined appropriately depending on the battery design. The thickness of the negative electrode current collector 24c is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm. Multiple negative electrode tabs 24t are provided at one end (the right end in FIG. 4) in the long side direction Y of the negative electrode current collector 24c. The multiple negative electrode tabs 24t protrude in the long side direction Y beyond the separator 26. The multiple negative electrode tabs 24t are provided at intervals (intermittently) along the longitudinal direction of the negative electrode 24. The negative electrode tabs 24t protrude to one side (the right side in FIG. 4) in the long side direction Y. The negative electrode tab 24t is a part of the negative electrode current collector 24c and is made of metal foil (copper foil). The negative electrode active material layer 24a is formed on a part of the negative electrode tab 24t. The negative electrode active material layer 24a is not formed on at least a part of the negative electrode tab 24t, and the negative electrode current collector 24c is exposed.

[0030] As shown in FIG. 4, the negative electrode active material layer 24a is provided along the longitudinal direction of a strip-shaped negative electrode current collector 24c. The negative electrode active material layer 24a contains a negative electrode active material. The negative electrode active material is not particularly limited, but carbon materials such as graphite, hard carbon, and soft carbon can be used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0031] The negative electrode active material layer 24a may contain components other than the negative electrode active material, such as a binder, a thickener, etc. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0032] The content of the negative electrode active material in the negative electrode active material layer is preferably 90% by mass or more, more preferably 95% by mass or more and 99.9% by mass or less. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.5% by mass or more and 2% by mass or less. The thickness of the negative electrode active material layer 24a is not particularly limited, but may be, for example, 10 μm or more and 200 μm or less, and preferably 50 μm or more and 100 μm or less.

[0033] The negative electrode 24 can be prepared according to a known method. For example, the negative electrode 24 can be prepared by preparing a negative electrode paste containing a negative electrode active material and optional components, applying the negative electrode paste to the negative electrode current collector 24c, drying the paste, and pressing the paste as necessary.

[0034] The separator 26 is an insulating resin sheet having a plurality of fine through-holes formed therein through which charge carriers can pass. The configuration of the separator 26 is not particularly limited and may be the same as that used in conventional batteries. Examples of the separator 26 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. A heat-resistant layer (HRL) may be provided on the surface of the separator 26.

[0035] The electrode assembly 20 can be fabricated according to a known method. When the electrode assembly 20 is a wound electrode assembly as in the illustrated example, such a wound electrode assembly can be prepared, for example, as follows. First, a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 are stacked so that they are insulated by two strip-shaped separators 26. At this time, as shown in FIG. 4, the positive electrode tab 22t of the positive electrode sheet 22 and the negative electrode tab 24t of the negative electrode sheet 24 are overlapped so that they protrude in opposite directions from the ends of the two separators 26 in the long side direction Y. Next, the prepared laminate is wound in the longitudinal direction around the winding axis WL. At this time, multiple positive electrode tabs 22t are stacked on one side of the long side direction Y to form a positive electrode tab group 23 (see FIG. 3). Multiple negative electrode tabs 24t are stacked on one side of the long side direction Y to form a negative electrode tab group 25 (see FIG. 3). The laminate can be wound according to a known method. The wound laminate is pressed to produce a flat wound electrode body. This pressing can be performed using a known pressing device used in the manufacture of general flat wound electrodes, and is not particularly limited. In this manner, the electrode body 20 can be prepared.

[0036] Furthermore, in the assembly construction step S10, a non-aqueous electrolyte is prepared. The non-aqueous electrolyte is not particularly limited and may be the same as that used in conventionally known batteries. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt (supporting salt). Examples of the non-aqueous solvent that can be used include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Various lithium salts can be used as the supporting salt, and among these, lithium salts such as LiPF6 and LiBF4 are preferred.

[0037] The non-aqueous electrolyte may contain various additives such as a film-forming agent, a gas generating agent, a dispersant, a thickener, etc. Specific examples of the film-forming agent include carbonate compounds such as vinylene carbonate (VC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), chloroethylene carbonate, and methylphenyl carbonate; lithium salts having an oxalato complex as the anion, such as lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), and lithium difluorobis(oxalato)phosphate (LPFO); and the like.

[0038] In the assembly construction step S10, the prepared electrode body 20 is housed in the battery case 10. First, a combined member is prepared in which a positive electrode current collector 50, a negative electrode current collector 60, and a sealing plate 14 are attached to the electrode body 20. As shown in FIG. 3 , the positive electrode current collector 50 is a member including a positive electrode first current collector 51, which is a plate-shaped conductive member extending in the long side direction Y along the inner surface of the sealing plate 14, and a positive electrode second current collector 52, which is a plate-shaped conductive member extending in the up-down direction Z. As shown in FIG. 3 , the negative electrode current collector 60 is a member including a negative electrode first current collector 61, which is a plate-shaped conductive member extending in the long side direction Y along the inner surface of the sealing plate 14, and a negative electrode second current collector 62, which is a plate-shaped conductive member extending in the up-down direction Z.

[0039] Specifically, first, the positive electrode second current collecting portion 52 is joined to the positive electrode tab group 23 of the electrode assembly 20, and the negative electrode second current collecting portion 62 is joined to the negative electrode tab group 25. Next, the sealing plate 14 is placed above the electrode assembly 20, and the positive electrode tab group 23 of the electrode assembly 20 is bent so that the positive electrode second current collecting portion 52 faces one side surface of the electrode assembly 20 (the left side in FIG. 3). This connects the positive electrode first current collecting portion 51 and the positive electrode second current collecting portion 52. Similarly, the negative electrode tab group 25 of the electrode assembly 20 is bent so that each negative electrode second current collecting portion 62 faces the other side surface of the electrode assembly 20 (the right side in FIG. 3). This connects the negative electrode first current collecting portion 61 and the negative electrode second current collecting portion 62. As a result, the electrode body 20 is attached to the sealing plate 14 via the positive electrode current collector 50 and the negative electrode current collector 60 .

[0040] Next, the electrode assembly 20 attached to the sealing plate 14 is housed in an electrode assembly holder 29 (see FIG. 3). The electrode assembly holder 29 can be prepared, for example, by folding an insulating resin sheet made of a resin material such as polyethylene (PE) into a bag or box shape. The electrode assembly 20 covered with the electrode assembly holder 29 is inserted into the battery case 10. At this time, it is preferable to insert the electrode assembly 20 so that the winding axis WL is oriented along the bottom surface 12a (i.e., the winding axis WL is oriented parallel to the long side direction Y). This allows gas to be efficiently discharged from inside the electrode assembly 20 in the degassing step S50, which will be described later. Then, the sealing plate 14 is joined to the edge of the opening 12h of the battery case 10 to seal the opening 12h. The battery case 10 and the sealing plate 14 are preferably sealed by, for example, welding. The battery case 10 and the sealing plate 14 can be welded together by, for example, laser welding. This allows the electrode assembly 20 to be placed inside the battery case 10.

[0041] The battery assembly 100 is equipped with various insulating members to prevent electrical conduction between the electrode body 20 and the battery case 10. Specifically, an external insulating member 92 is interposed between the positive electrode external conductive member 32 (negative electrode external conductive member 42) and the outer surface of the sealing plate 14 (see FIGS. 2 and 3). A gasket 90 is attached to each of the terminal mounting holes 18, 19 of the sealing plate 14 (see FIG. 3). An internal insulating member 94 is disposed between a positive electrode first current collecting portion 51 (or a negative electrode first current collecting portion 61) (described later) and the inner surface of the sealing plate 14. The materials for the insulating members described above are not particularly limited as long as they have the required insulating properties. Examples of suitable materials include synthetic resins such as polyolefin resins (e.g., polypropylene (PP) and polyethylene (PE)) and fluorine-based resins (e.g., perfluoroalkoxyalkane (PFA) and polytetrafluoroethylene (PTFE)).

[0042] Then, in the assembly construction step S10, the prepared nonaqueous electrolyte is poured into the battery case 10 containing the electrode assembly 20. The injection of the nonaqueous electrolyte may be carried out in an atmospheric pressure atmosphere or a reduced pressure atmosphere. Preferably, the injection is carried out in a reduced pressure atmosphere. This allows the electrolyte to be injected more quickly. The amount of nonaqueous electrolyte to be injected can be appropriately adjusted so that it spreads throughout the electrode assembly 20. A conventionally known electrolyte injection device can be used as appropriate for the injection of the nonaqueous electrolyte. After the injection of the nonaqueous electrolyte, the injection hole 15 of the sealing plate 14 of the battery assembly is sealed. The injection hole 15 can be sealed by assembling a sealing member 16 having a shape that fits the injection hole 15. In this way, the battery assembly 100 can be constructed.

[0043] In the initial charging step S20, the prepared battery assembly 100 is initially charged. The initial charging is a process of charging the non-aqueous electrolyte secondary battery over the voltage range in which it will be used after manufacture. By performing the initial charging step S20, the battery assembly 100 can be electrochemically activated. The conditions for the initial charging may be the same as those of the conventional battery. Although not particularly limited, the initial charging can be performed by charging at a current of about 0.05 C to 10 C in a room temperature environment (for example, 25°C) until the SOC (State of Charge) reaches about 20% to 90%.

[0044] Although not particularly limited, a pre-charging step may be performed in the assembly construction step S10, at a timing after the injection of the non-aqueous electrolyte solution and before the sealing of the liquid injection hole 15. As described in JP 2000-90974 A, such a pre-charging step may be performed for the purpose of reducing the amount of gas generated after the liquid injection hole 15 is sealed and preventing the gas from remaining inside the electrode body 20 after sealing by discharging gas generated during charging and discharging to the outside of the battery case 10. Pre-charging can be performed by charging at a current of about 0.05 C to 10 C in an environment of 25°C until the capacity is about 10% to 80%.

[0045] When the preliminary charging step is performed, a reaction associated with charging may proceed inside the electrode body 20. When the preliminary charging step is performed, if the period between the completion of the preliminary charging step and the start of the initial charging step is too long, the formation of an SEI film tends to be excessive, which is undesirable, and the battery capacity tends to decrease. From this perspective, the period between the completion of the preliminary charging step and the start of the initial charging step S20 is preferably, for example, within 10 days, and more preferably within 3 days.

[0046] In the high-temperature holding step S30, the battery assembly 100 after the initial charging step S20 is held at a high temperature of 40°C or higher for a predetermined time. By carrying out the high-temperature holding step S30 at a high temperature of 40°C or higher, for example, the formation of an SEI film on the surface of the negative electrode can be favorably promoted. The high-temperature holding step S30 can be carried out by placing the battery assembly 100 in a thermostatic bath or the like set to maintain a predetermined temperature. The high-temperature holding step S30 may be carried out after carrying out the above-described initial charging step S20, in which the battery assembly 100 is held at a high temperature for a predetermined time in the charged state.

[0047] If the holding temperature in the high-temperature holding step S30 is too low, the formation of the SEI film is not sufficiently promoted, which is undesirable. From this perspective, the holding temperature in the high-temperature holding step S30 is, for example, 40°C or higher, and may be 50°C or higher, or 60°C or higher. On the other hand, if the holding temperature in the high-temperature holding step S30 is too high, the formation of the SEI film may proceed too rapidly, resulting in the formation of an excessive amount of SEI film or making it difficult to form a homogeneous SEI film, which is undesirable. From this perspective, the holding temperature in the high-temperature holding step S30 is preferably 85°C or lower, and more preferably 80°C or lower.

[0048] The retention time of the high-temperature retention step S30 cannot be generalized because it depends on the set temperature, battery size, etc., but it is preferably about 6 to 36 hours, and can be set to about 12 to 24 hours, for example. As an example, the high-temperature retention step S30 can be preferably carried out by setting the retention temperature at 50°C to 80°C and the retention time at 6 to 24 hours.

[0049] In the room-temperature holding step S40, the battery assembly 100 after the high-temperature holding step S30 is held in a room-temperature environment for more than three hours. The room-temperature holding step S40 can be performed, for example, by placing the battery assembly 100 in a thermostatic chamber set at room temperature (25°C ± 10°C). As described above, by performing the room-temperature holding step S40 after the high-temperature holding step S30, the reaction associated with charging can be largely completed. This makes it possible to suppress further gas generation inside the electrode body 20. This makes it possible to more effectively exert the effects of performing the degassing step S50, which will be described later.

[0050] The room temperature holding step S40 is performed at room temperature, specifically, at 15°C or higher and 35°C or lower (preferably, 20°C or higher and 30°C or lower). The holding time in the room temperature holding step S40 is performed for more than 3 hours from the viewpoint of increasing the capacity of the secondary battery. The holding time is not particularly limited as long as it is more than 3 hours, and is preferably, for example, 6 hours or higher, more preferably 12 hours or higher, and may be 24 hours or higher, or may be 48 hours or higher. From the viewpoint of improving the efficiency of the manufacturing process, the upper limit of the holding time in the room temperature holding step S40 is, for example, preferably 72 hours or lower, and may be 60 hours or lower. The room temperature holding step S40 is preferably performed for 24 hours or higher and 72 hours or lower, for example, in a room temperature environment (15°C or higher and 35°C or lower).

[0051] Although not particularly limited, at least one of the high-temperature holding step S30 and the room-temperature holding step S40 may include a pressing process in which the battery assembly 100 is pressed. That is, at least one of the high-temperature holding step S30 and the room-temperature holding step S40 may be performed while the battery assembly 100 is pressed (constrained). In this case, the battery assembly 100 may be pressed (constrained) along the stacking direction of the electrode assembly 20. This reduces the inter-electrode distance between the positive electrode 22 and the negative electrode 24 of the electrode assembly 20, thereby reducing the amount of gas generated during the high-temperature holding step S30 and the room-temperature holding step S40 that remains within the electrode assembly 20. The pressing (constraining) method is not particularly limited, and a general procedure employed in the manufacture of conventional nonaqueous electrolyte secondary batteries can be used. For example, the battery assembly 100 can be pressed (constrained) by sandwiching the pair of long side walls 12b of the battery assembly 100 between constraining plates and connecting the constraining plates with a bridging member.

[0052] When the battery assembly 100 is pressed in at least one of the high-temperature holding step S30 and the room-temperature holding step S40, the pressure applied thereto is not particularly limited and may be adjusted appropriately depending on the size of the battery assembly 100, the winding of the electrode assembly 20, etc. The pressure applied when pressing the battery assembly 100 in at least one of the high-temperature holding step S30 and the room-temperature holding step S40 is preferably set to a pressure lower than the pressure applied in the degassing step S50, which will be described later. For example, in at least one of the high-temperature holding step S30 and the room-temperature holding step S40, when the length L1 in the stacking direction of the battery assembly 100 before pressing (see FIG. 2) is taken as 100%, pressing is preferably performed so that the length L1 in the stacking direction of the battery assembly 100 is shortened by 7.5% to 10%, and more preferably by 7.5% to 8.6%. Alternatively, in at least one of the high-temperature holding step S30 and the room-temperature holding step S40, the battery assembly 100 may be pressed along the stacking direction of the electrode body 20 with a pressure of 0.05 kN or more and 4 kN or less (more preferably 0.05 kN or more and less than 0.15 kN).

[0053] In the degassing step S50, the battery assembly 100 after the room temperature holding step S40 is pressed at least once along the stacking direction of the electrode body 20 and then released. By performing this degassing step after the room temperature holding step S40, gas can be discharged from the inside to the outside of the electrode body 20, and gas accumulation inside the electrode body is suitably eliminated.

[0054] In the manufacturing method disclosed herein, by performing the degassing step S50 after the room temperature holding step, it is possible to effectively eliminate the accumulation of gas inside the electrode body even when pressing is performed with a relatively small pressure. This prevents excessive pressure from being applied to the electrode body 20, thereby preventing the active material layer from becoming thin. Furthermore, the equipment required for pressing can be simplified. Therefore, this manufacturing method not only enables the manufacture of a nonaqueous electrolyte secondary battery with improved battery capacity, but also simplifies the equipment required. Although not particularly limited, in the degassing step S50, when the length L1 in the stacking direction of the battery assembly 100 before pressing is taken as 100%, pressing is preferably performed so that the length L1 in the stacking direction of the battery assembly 100 is shortened by 7.5% to 10%, more preferably by 8.6% to 10%, and even more preferably by 8.8% to 10%. Alternatively, in the degassing step S50, the battery assembly 100 may be pressed in the stacking direction of the electrode body 20 with a pressure of 0.1 kN or more and 5 kN or less (more preferably 0.15 kN or more and 1 kN or less).

[0055] When a pressing process is performed in at least one of the high-temperature holding step S30 and the room-temperature holding step S40, the pressing pressure in the degassing step S50 is preferably set to a pressure higher than the pressures used in the pressing processes in the high-temperature holding step S30 and the room-temperature holding step S40. This allows gas remaining inside the electrode body 20 to be more efficiently discharged to the outside of the electrode body 20.

[0056] In the degassing step S50, the battery assembly 100 is pressed along the stacking direction of the electrode body 20. In the example shown in FIG. 2, the pair of long side walls 12b of the battery assembly 100 may be pressed from both sides along the short side direction X. At this time, when the area of ​​the region of the electrode body 20 where the positive electrode active material layers 22a are present is taken as 100%, it is preferable to press at least 50% of the region. This makes it possible to effectively push gas generated by the reaction that has progressed inside the electrode body 20 to the outside of the electrode body 20. The pressing performed in the degassing step S50 is performed so that, when the area of ​​the region of the electrode body 20 where the positive electrode active material layers 22a are present is taken as 100%, for example, 55% or more of the region is pressed, but it may also be 75% or more, or 90% or more, or may be performed so that, for example, 100% (i.e., the entire region where the positive electrode active material layers 22a are present) is pressed.

[0057] In the degassing step S50, the battery assembly 100 needs to be pressed at least once, and the number of times is not particularly limited. For example, the pressing may be performed once or multiple times (i.e., two or more times). Furthermore, the duration of time for which the pressing is maintained in the degassing step S50 is not particularly limited. According to the findings of the present inventors, for example, pressing for about one second at the pressure described above can sufficiently eliminate stagnation of gas. The duration of time for which the pressing is maintained in the degassing step S50 is preferably at least one second, and may be 10 seconds or more, or even one minute or more. From the viewpoint of manufacturing efficiency, the duration of time for which the pressing is maintained is preferably 24 hours or less, and more preferably 12 hours or less. In the degassing step S50, the battery assembly 100 is preferably pressed for one second or more so that the length L1 in the stacking direction of the battery assembly 100 is shortened by 8.6% to 10% when the length L1 in the stacking direction of the battery assembly 100 before pressing is taken as 100%.

[0058] In the degassing step S50, the battery assembly 100 is pressed and then released. Here, "releasing the battery assembly" not only means completely removing the battery assembly 100 from the restraining members, etc., but also includes reducing the pressing pressure. Specifically, in the degassing step S50, the battery assembly 100 is pressed with a pressure of 0.1 to 5 kN, and then released so that the pressure becomes less than the pressing pressure applied in the degassing step S50.

[0059] The nonaqueous electrolyte secondary battery obtained by the manufacturing method disclosed herein can be used for various purposes, and can be suitably used, for example, as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, and examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). The nonaqueous electrolyte secondary battery obtained by the manufacturing method disclosed herein can also be suitably used in the construction of assembled batteries.

[0060] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to these examples.

[0061] First, lithium nickel cobalt manganese composite oxide (NCM) was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and PVdF as the binder. These were mixed in N-methylpyrrolidone (NMP) as the solvent so that the mass ratio of NCM:AB:PVdF was 87:10:3 to prepare a slurry for forming the positive electrode active material layer. This slurry was applied in strips to both sides of a long piece of aluminum foil and dried to produce a positive electrode sheet. Next, graphite (C) was prepared as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the dispersant. These were mixed in ion-exchanged water as the solvent so that the mass ratio of C:SBR:CMC was 98:1:1 to prepare a slurry for forming a negative electrode active material layer. This slurry was applied in strips to both sides of a long copper foil and dried to prepare a negative electrode sheet.

[0062] Two separator sheets were prepared, each with a heat-resistant layer containing alumina and PVdF on the surface of a PE substrate. The prepared positive electrode sheet and negative electrode sheet were stacked facing each other with the separator sheet in between and wound to prepare an electrode assembly. The nonaqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent of EC, EMC, and DMC in a volume ratio of 1:1:1. The wound electrode assembly and nonaqueous electrolyte were then housed in a battery case to construct a rectangular parallelepiped battery assembly.

[0063] The battery assembly of Example 1 was subjected to a preliminary charge in a temperature environment of 25°C, where it was charged at 1.0 C to an SOC of 5%. After the preliminary charge, it was left at room temperature for 7 days. Next, it was charged at a current value of 1.0 C to an SOC of 50%, and then an initial charge was performed by maintaining that SOC. The battery assembly after the initial charge was subjected to a high-temperature holding step at a holding temperature of 65°C for a holding time of 13 hours. After the high-temperature holding step, a room-temperature holding step was performed in which it was held at room temperature (25°C) for 3 hours. The high-temperature holding step and the room-temperature holding step were performed in a state where the battery assembly was pressed along the stacking direction of the electrode body. The high-temperature holding step and the room-temperature holding step were performed in a state where the battery assembly was pressed so that the length in the stacking direction of the battery assembly before pressing was shortened by 8.6% when the length of the battery assembly in the stacking direction before pressing was taken as 100%. After the room-temperature holding step, a degassing step was performed. In the degassing process, the battery assembly was pressed in the stacking direction of the electrode body so that the length of the battery assembly in the stacking direction was shortened by 8.8% when the length of the battery assembly in the stacking direction before pressing was taken as 100%, and then released. The pressing was maintained for 1 second. In this manner, the evaluation secondary battery of Example 1 was produced.

[0064] <Examples 2 to 5> The evaluation secondary battery of Example 2 was fabricated in the same manner as Example 1, except that the holding time of the room temperature holding step was changed to 6 hours. The evaluation secondary battery of Example 3 was fabricated in the same manner as Example 1, except that the holding time of the room temperature holding step was changed to 12 hours. The evaluation secondary battery of Example 4 was fabricated in the same manner as Example 1, except that the holding time of the room temperature holding step was changed to 24 hours. The evaluation secondary battery of Example 5 was fabricated in the same manner as Example 1, except that the holding time of the room temperature holding step was changed to 48 hours.

[0065] <Comparative Example 1> The evaluation secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the room temperature holding step and the degassing step were not carried out after the high temperature holding step.

[0066] <Evaluation of discharge capacity> The evaluation secondary batteries prepared above for each example were fully charged and fully discharged in an environment of 25°C, and the discharge capacity (mAh) measured during discharge was taken as the discharge capacity (mAh) of the evaluation secondary battery for each example. The results are shown in Figure 5. Figure 5 also shows the relationship between the discharge capacity ratio of each example, assuming the discharge capacity of Comparative Example 1 to be 1, and the implementation time of the room temperature holding step. It can be said that the larger the value of this discharge capacity ratio, the larger the battery capacity.

[0067] As shown in Fig. 5, the test secondary batteries of Examples 1 to 5, which underwent the room-temperature holding step and the degassing step, have improved discharge capacities compared to the test secondary battery of Comparative Example 1. This is because the room-temperature holding step is performed after the high-temperature holding step, allowing the reaction inside the electrode assembly to proceed sufficiently, suppressing further gas generation thereafter. Furthermore, it is presumed that the degassing step performed after the room-temperature holding step allows the gas generated during the room-temperature holding step to be suitably expelled from inside the electrode assembly, thereby increasing the discharge capacity of the test secondary batteries.

[0068] Furthermore, comparing Examples 1 to 5, the discharge capacity of the test secondary batteries is significantly increased in Examples 2 to 5, in which the room temperature holding step was performed for 6 hours or more. This is presumably because, by performing the room temperature holding step for 6 hours or more, the reaction accompanying charging is almost complete, and the degassing step can be performed at a timing when further gas generation is suppressed. In other words, by setting the holding time of the room temperature holding step to 6 hours or more, the amount of gas remaining inside the electrode body is likely to be reduced, and the discharge capacity of the test secondary battery is presumably improved. Therefore, it can be seen that the room temperature holding step is preferably performed at room temperature for more than 3 hours, and more preferably at room temperature for 6 hours or more.

[0069] Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0070] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A method for manufacturing a non-aqueous electrolyte secondary battery, comprising: a step of constructing a battery assembly in which an electrode body, in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, and a non-aqueous electrolyte are housed in a battery case; a step of initially charging the battery assembly; a high-temperature holding step of holding the battery assembly after the initial charging step at a high temperature of 40°C or higher; a room-temperature holding step of holding the battery assembly at room temperature for more than 3 hours after the high-temperature holding step; and a degassing step of pressing the battery assembly after the room-temperature holding step in the stacking direction of the electrode body and releasing it. Item 2: The manufacturing method according to Item 1, wherein in the degassing step, the battery assembly is pressed along the stacking direction of the electrode body with a pressure of 0.1 kN or more and 5 kN or less. Item 3: The manufacturing method according to Item 1 or 2, wherein in the degassing step, the battery assembly is pressed so that the length in the stacking direction of the battery assembly is shortened by 7.5% to 10% when the length in the stacking direction of the battery assembly before pressing is taken as 100%. Item 4: The manufacturing method according to any one of Items 1 to 3, wherein in the room temperature holding step, the battery assembly is held at room temperature for 6 hours or more. Item 5: The manufacturing method according to any one of Items 1 to 4, wherein at least one of the high-temperature holding step and the room-temperature holding step further includes a pressing process of pressing the battery assembly along the stacking direction of the electrode body, and the pressing pressure in the degassing step is higher than the pressing pressure in the pressing process. Item 6: The manufacturing method according to any one of Items 1 to 5, wherein the positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the electrode body is stacked such that the positive electrode active material layer and the negative electrode active material layer face each other with the separator interposed therebetween, and in the degassing step, when an area of ​​a region of the electrode body where the positive electrode active material layer is present is taken as 100%, at least 50% of the area is pressed along the stacking direction. [Explanation of symbols]

[0071] 10 Battery case 12 Case body 12a Bottom 12b Long side wall 12c short side wall 12h opening 14 Sealing plate 20 Electrode body 22 Positive electrode (positive electrode sheet) 24 Negative electrode (negative electrode sheet) 26 Separator 30 Positive terminal 40 Negative terminal 50 Positive electrode current collector 60 Negative electrode current collector 100 battery assembly

Claims

1. constructing a battery assembly in which an electrode body, in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, and a non-aqueous electrolyte solution are housed in a battery case; initial charging of the battery assembly; a high-temperature holding step of holding the battery assembly after the initial charging step at a high temperature of 40°C or higher; a room temperature holding step of holding the battery assembly at room temperature for more than three hours after the high temperature holding step; a degassing step of pressing the battery assembly after the room temperature holding step in a stacking direction of the electrode bodies and releasing the pressed battery assembly; Including, at least one of the high-temperature holding step and the room-temperature holding step further includes a pressing process of pressing the battery assembly along a stacking direction of the electrode body; The method for producing a nonaqueous electrolyte secondary battery, wherein the pressure applied in the degassing step is higher than the pressure applied in the pressing treatment.

2. The manufacturing method according to claim 1 , wherein the degassing step comprises pressing the battery assembly along the stacking direction of the electrode bodies at a pressure of 0.1 kN or more and 5 kN or less.

3. 2. The manufacturing method according to claim 1, wherein in the degassing step, the battery assembly is pressed so that the length of the battery assembly in the stacking direction is shortened by 7.5% to 10% when the length of the battery assembly in the stacking direction before pressing is 100%.

4. The manufacturing method according to claim 1 , wherein the room temperature holding step holds the battery assembly at room temperature for six hours or more.

5. the positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; the negative electrode comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the electrode body is laminated such that the positive electrode active material layer and the negative electrode active material layer face each other with the separator interposed therebetween, 2. The manufacturing method according to claim 1, wherein in the degassing step, when an area of ​​a region in which the positive electrode active material layer is present in the electrode body is taken as 100%, at least 50% of the area is pressed along the stacking direction.

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