Stack

Abstract

To provide a stack capable of suppressing reduction of a binding load of a power storage cell.SOLUTION: The technology disclosed herein relates to a stack 500 in which a plurality of rectangular electric storage cells 100 are stacked along the thickness direction of the rectangular electric storage cells 100, and each electric storage cell 100 includes a flat electrode assembly 20 in which a positive electrode 22 and a negative electrode 24 are stacked and wound with a separator 26 interposed therebetween, a nonaqueous electrolyte, and a case 12 that houses the electrode assembly 20 and the nonaqueous electrolyte. Here, when the cell thickness and the spring constant of the electric storage cell are measured for each of the electric storage cells 100, the spring constant of the electric storage cell has a positive correlation with the cell thickness.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to stacks. [Background technology]

[0002] In recent years, stacks including a plurality of storage cells have been suitably used as drive power sources mounted on vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (BEVs). Patent Document 1 is an example of related prior art. Patent Document 1 discloses a battery pack (stack) including a rectangular case having side walls perpendicular to the arrangement direction, a plurality of unit cells (storage cells) arranged in a predetermined direction, and a spacer. The battery pack of Patent Document 1 is characterized in that a surface of the spacer is formed with a plurality of protrusions that contact the side walls and extend along the side walls, and the spring constant of the negative electrode plate of the unit cells is greater than the spring constant of the spacer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-84550 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the stack according to the above-described conventional technology, when the stack is in use, the load reduction of the stack decreases during initial discharge, or under low temperature or low SOC (State of Charge) conditions, and the stack is no longer able to sufficiently restrain the storage cells.

[0005] The technology disclosed herein has been made in view of the above circumstances, and its main purpose is to provide a stack that can suppress a decrease in the restraining load of an energy storage cell. [Means for solving the problem]

[0006] The technology disclosed herein relates to a stack in which a plurality of rectangular energy storage cells are stacked in the thickness direction of the rectangular energy storage cells, and the energy storage cells include a flat electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a non-aqueous electrolyte, and a case that houses the electrode assembly and the non-aqueous electrolyte. Here, each of the energy storage cells satisfies the following requirements: (1) cell thickness (mm), where the cell thickness is obtained by applying a load to the electricity storage cell in a thickness direction at a rate of 0.01 mm / min to 10 mm / min up to a load of 0.2 MPa, and then holding the electricity storage cell for 3 hours under the load of 0.2 MPa, and then measuring the thickness of the electricity storage cell; (2) Spring constant (MPa / mm) of the storage cell, where the spring constant of the storage cell is determined by measuring the thickness X1 (mm) of the storage cell when the load is applied to the storage cell in the thickness direction up to 2 MPa at the same speed as when measuring the thickness of the storage cell, and then reducing the load to 0.01 MPa after the application, as one cycle. This process is repeated five times, and the thickness X1 (mm) of the storage cell when the load is applied to the storage cell up to 0.1 MPa in the fifth cycle and the thickness X2 (mm) of the storage cell when the load is applied to the storage cell up to 0.2 MPa in the fifth cycle are measured, and the spring constant of the storage cell is determined by the following formula (i): Spring constant of the storage cell (MPa / mm) = |(0.2MPa-0.1MPa) / (X2-X1)|···Equation (i); When the spring constant of the storage cell is measured, it has a positive correlation with the cell thickness.

[0007] The inventors' intensive studies have revealed that energy storage cells with small cell thicknesses tend to have large spring constants during initial discharge or under low temperature and low SOC conditions, which reduces the binding load. Therefore, in the energy storage cells constituting the stack according to the present disclosure, the spring constant of the energy storage cells has a positive correlation with the cell thickness. In other words, the smaller the cell thickness of the energy storage cell, the smaller the spring constant of the energy storage cell. This makes it possible to provide a stack that can suppress a reduction in the binding load of the energy storage cells even during initial discharge or under low temperature and low SOC conditions. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view schematically illustrating a stack according to an embodiment. [Figure 2] FIG. 2 is a perspective view of a storage cell according to one embodiment. [Figure 3] FIG. 3 is a schematic vertical cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a perspective view that schematically shows the electrode body attached to the sealing plate. [Figure 5] FIG. 5 is a perspective view schematically showing an electrode assembly. [Figure 6] FIG. 6 is a schematic diagram showing the configuration of the electrode body. [Figure 7] FIG. 7 is a schematic diagram illustrating measurement of the amount of change in cell thickness according to one embodiment. [Figure 8] FIG. 8 is a flow diagram showing a method for manufacturing a stack according to one embodiment. [Figure 9] FIG. 9 is a flow diagram showing a method for manufacturing an energy storage cell according to one embodiment. [Figure 10] FIG. 10 is a graph plotting the spring constants and cell thicknesses of the energy storage cells according to the example and the comparative example, and showing an approximate straight line indicating the correlation between them with a dashed line. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, several preferred embodiments of the stack disclosed herein will be described with reference to the drawings. Matters other than those specifically mentioned in this specification that are necessary for implementing the present disclosure (for example, the general configuration and manufacturing process of a storage cell that do not characterize the present disclosure) can be understood as design matters for those skilled in the art based on conventional technology in the field. The stack disclosed herein can be implemented based on the contents disclosed in this specification and common technical knowledge in the field.

[0010] In the following drawings, the same reference numerals are used to designate components and parts that perform the same function, and redundant explanations may be omitted or simplified. In addition, in this specification, the expression "A to B" indicating a range means not less than A and not more than B, and also encompasses the meanings of "preferably larger than A" and "preferably smaller than B."

[0011] FIG. 1 is a perspective view schematically illustrating a stack 500 according to one embodiment. The stack 500 includes a plurality of rectangular energy storage cells 100 and a restraining mechanism 300. Although not shown here for ease of explanation, the plurality of energy storage cells 100 of the stack 500 are electrically connected by bus bars. 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 (thickness) direction of the energy storage cells 100, the long side (width) direction perpendicular to the short side direction, and the up-down direction, respectively. The short side direction X is also the arrangement direction of the energy storage cells 100. However, these directions are merely provided for ease of explanation and do not limit the installation form of the stack 500 in any way.

[0012] 1, in a stack 500 according to this embodiment, a plurality of rectangular energy storage cells 100 are stacked along a thickness direction X of the energy storage cells 100. In this specification, the "thickness direction of the energy storage cells" refers to a direction perpendicular to the width direction and height direction of the energy storage cells.

[0013] The restraint mechanism 300 is configured to apply a specified restraint pressure to the plurality of energy storage cells 100 in the thickness direction X. Here, the restraint mechanism 300 is configured with a pair of end plates 310, a pair of side plates 320, and a plurality of screws 330. The pair of end plates 310 are aligned in a predetermined thickness direction X. The pair of end plates 310 are arranged at both ends of the stack 500 in the thickness direction X. The plurality of energy storage cells 100 are arranged between the pair of end plates 310 along the thickness direction X. An insulating sheet, an inter-cell separator, or the like may be arranged between the plurality of energy storage cells 100.

[0014] The pair of side plates 320 bridge the pair of end plates 310. The pair of side plates 320 are fixed to the end plates 310 with a plurality of screws 330 so that the restraint load is, for example, approximately 5 to 20 kN. This applies a restraint load to the plurality of energy storage cells 100 in the thickness direction X, and holds the stack 500 together. However, the restraint mechanism is not limited to this. The restraint mechanism 300 may include, for example, a plurality of restraint bands, bind bars, or the like, instead of the side plates 320.

[0015] <Storage cell 100> In this specification, the term "storage cell" refers to a device that can be repeatedly charged and discharged. Storage cells include batteries generally referred to as lithium ion batteries and lithium secondary batteries, as well as lithium polymer batteries and lithium ion capacitors. Secondary batteries generally refer to batteries that can be repeatedly charged and discharged through the movement of charge carriers between the positive and negative electrodes. Here, a lithium ion secondary battery is exemplified as one form of storage cell.

[0016] FIG. 2 is a perspective view of an energy storage cell 100 according to one embodiment. As shown in FIGS. 1 and 2, a plurality of energy storage cells 100 are arranged in a thickness direction X such that first side walls 12b (described later) face each other. FIG. 3 is a schematic longitudinal cross-sectional view taken along line III-III in FIG. 2. As shown in FIG. 3, the energy storage cell 100 includes a case 10, an electrode assembly 20, and a non-aqueous electrolyte (not shown). In this embodiment, the energy storage cell 100 further includes a positive electrode terminal 30, a negative electrode terminal 40, a positive electrode current collector 50, and a negative electrode current collector 60. Here, the energy storage cell 100 is a lithium-ion secondary battery.

[0017] The case 10 is a housing that houses the electrode assembly 20 and the non-aqueous electrolyte. As shown in FIG. 2, the case 10 has a flat, bottomed, rectangular parallelepiped (rectangular) outer shape. The material of the case 10 may be the same as that conventionally used, and is not particularly limited. The case 10 is preferably made of a metal, and more preferably made of, for example, aluminum, an aluminum alloy, iron, an iron alloy, or the like. As shown in FIG. 2 or 3, the case 10 includes an exterior body 12 having an opening 12h and a sealing plate (lid) 14 that seals the opening 12h. As in this embodiment, the case 10 preferably includes an exterior body 12 having an opening 12h and a sealing plate 14 that seals the opening 12h.

[0018] As shown in FIG. 2, the exterior body 12 includes a bottom 12a, a pair of first side walls 12b extending from the bottom 12a and facing each other, and a pair of second side walls 12c extending from the bottom 12a and facing each other. The bottom 12a is substantially rectangular. The bottom 12a faces the opening 12h. The first side walls 12b are flat. The first side walls 12b extend from the long sides of the bottom 12a. The second side walls 12c extend from the short sides of the bottom 12a. In a plan view, the area of ​​the first side walls 12b is larger than the area of ​​the second side walls 12c.

[0019] The sealing plate 14 is attached to the exterior body 12 so as to close the opening 12h of the exterior body 12. Here, the sealing plate 14 faces the bottom 12a of the exterior body 12. The sealing plate 14 has a substantially rectangular shape in a plan view. The case 10 is integrated by joining (preferably welding) the sealing plate 14 to the periphery of the opening 12h of the exterior body 12. The case 10 is hermetically sealed (sealed).

[0020] As shown in Fig. 3, the sealing plate 14 is provided with a liquid inlet 15, a drain valve 17, and two terminal outlet holes 18 and 19. The liquid inlet 15 is for injecting a non-aqueous electrolyte after the sealing plate 14 is assembled to the exterior body 12. The liquid inlet 15 is sealed with a sealing member 16. The drain valve 17 is configured to break when the pressure inside the case 10 reaches or exceeds a predetermined value, thereby discharging gas inside the case 10 to the outside. The terminal outlet holes 18 and 19 penetrate the sealing plate 14 in the vertical direction Z. The terminal outlet holes 18 and 19 each have an inner diameter large enough to insert the positive electrode terminal 30 and the negative electrode terminal 40 before they are attached to the sealing plate 14 (before being crimped).

[0021] The non-aqueous electrolyte may be the same as conventional ones and is not particularly limited. The non-aqueous electrolyte contains a non-aqueous solvent and a supporting salt (electrolyte salt). The non-aqueous electrolyte may further contain additives as necessary. The non-aqueous solvent includes, for example, carbonates such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent preferably includes a carbonate. In particular, it is preferable that the non-aqueous solvent includes a cyclic carbonate and a chain carbonate. The supporting salt is, for example, a fluorine-containing lithium salt such as lithium hexafluorophosphate (LiPF6). Examples of additives that can be included in the non-aqueous electrolyte include a film-forming agent, a gas generating agent, a dispersant, and a thickener. 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; and 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).

[0022] The positive electrode terminal 30 is disposed at one end of the sealing plate 14 in the long side direction Y (the left end in FIGS. 2 and 3). The negative electrode terminal 40 is disposed at the other end of the sealing plate 14 in the long side direction Y (the right end in FIGS. 2 and 3). As shown in FIG. 3, the positive electrode terminal 30 and the negative electrode terminal 40 extend from the inside to the outside of the sealing plate 14 through the terminal lead-out holes 18 and 19. The positive electrode terminal 30 and the negative electrode terminal 40 are fixed to the sealing plate 14. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are crimped to the peripheral portion surrounding the terminal lead-out holes 18 and 19 of the sealing plate 14 by crimping. Crimped portions 30c and 40c are formed at the ends of the positive electrode terminal 30 and the negative electrode terminal 40 on the exterior body 12 side (the lower end in FIG. 3).

[0023] As shown in Fig. 3, the positive electrode terminal 30 is electrically connected to the positive electrode 22 (see Fig. 6) of the electrode assembly 20 via a positive electrode current collector 50 inside the exterior housing 12. The negative electrode terminal 40 is electrically connected to the negative electrode 24 (see Fig. 6) of the electrode assembly 20 via a negative electrode current collector 60 inside the exterior housing 12. The positive electrode terminal 30 is insulated from the sealing plate 14 by an internal insulating member 80 and a gasket 90. The negative electrode terminal 40 is insulated from the sealing plate 14 by the internal insulating member 80 and a gasket 90.

[0024] A plate-shaped positive electrode external conductive member 32 and a plate-shaped negative electrode external conductive member 42 are attached to the outer surface of the sealing plate 14. The positive electrode external conductive member 32 is electrically connected to the positive electrode terminal 30. The negative electrode external conductive member 42 is electrically connected to the negative electrode terminal 40. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are members to which bus bars or the like are attached that electrically connect the multiple energy storage cells 100 to each other. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are insulated from the sealing plate 14 by an external insulating member 92. Although not shown in FIG. 1 , when the stack 500 is in use, adjacent energy storage cells 100 are electrically connected to each other. For example, of adjacent energy storage cells 100, the positive electrode external conductive member 32 of one energy storage cell 100 and the negative electrode external conductive member 42 of the other energy storage cell 100 are electrically connected by a bus bar or the like. This electrically connects the stack 500. The stack 500 may be connected in series, in parallel, or in a so-called multi-series / multi-parallel connection, which is a combination of series and parallel.

[0025] FIG. 4 is a perspective view schematically showing the electrode assembly 20 attached to the sealing plate 14. Here, the electrode assembly 20 includes three electrode assemblies 20. However, the number of electrode assemblies arranged inside one exterior housing 12 is not particularly limited and may be one or multiple (two or more). Although not particularly limited, it is preferable that multiple electrode assemblies 20 are housed inside the exterior housing 12 (case 10). This can improve the energy density of the energy storage cell 100. Here, the electrode assemblies 20 are electrically connected in parallel. The electrode assemblies 20 are arranged inside the exterior housing 12 with the winding axis WL (see FIG. 6) oriented approximately parallel to the long side direction Y. Here, the electrode assemblies 20 are arranged side by side in a direction (short side direction X) in which the thickness direction of the electrode assemblies 20 coincides with the thickness direction of the energy storage cell 100 (in other words, a direction approximately perpendicular to the first side wall 12b of the case 10). An end face of the electrode body 20 that is perpendicular to the winding axis WL (in other words, the stacking surface where the positive electrode 22 and the negative electrode 24 are stacked) faces the second side wall 12c. Although not shown, an insulating sheet is disposed between the electrode body 20 and the exterior body 12.

[0026] FIG. 5 is a perspective view schematically illustrating the electrode assembly 20. FIG. 6 is a schematic diagram illustrating the configuration of the electrode assembly 20. Note that all of the electrode assemblies 20 housed in the case 10 may have the same configuration. The electrode assembly 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. As shown in FIG. 6, the electrode assembly 20 is a so-called wound electrode assembly in which a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 are stacked with a strip-shaped separator 26 interposed therebetween and wound in the longitudinal direction around a winding axis WL. However, the electrode assembly may also be a laminated electrode assembly in which a rectangular positive electrode and a rectangular negative electrode are stacked while being insulated by a rectangular separator. Alternatively, the electrode assembly may be a laminated electrode assembly in which a rectangular positive electrode and a rectangular negative electrode are stacked while being insulated by a zigzag-folded separator. From the viewpoint of manufacturing costs, the electrode assembly 20 is preferably a wound electrode assembly. The winding axis WL direction is substantially parallel to the long side direction Y.

[0027] The electrode body 20 preferably has a flat outer shape. The electrode body 20 has a pair of curved portions (R portions) 20r and a flat portion 20f connecting the pair of curved portions 20r. One curved portion 20r (upper side in FIG. 5) faces the sealing plate 14, and the other curved portion 20r (lower side in FIG. 5) faces the bottom portion 12a of the exterior body 12. The flat portion 20f faces the first side wall 12b of the exterior body 12. Here, the flat portions 20f of the electrode bodies 20 adjacent to each other in the short side direction X face each other.

[0028] As shown in FIG. 6, the positive electrode 22 includes a positive electrode core 22c, a positive electrode active material layer 22a, and a positive electrode protective layer 22p fixed to at least one surface of the positive electrode core 22c. However, the positive electrode protective layer 22p is not essential and may be omitted in other embodiments. The positive electrode core 22c is strip-shaped. The positive electrode core 22c is preferably made of metal foil, more preferably aluminum foil or aluminum alloy foil. Here, the positive electrode core 22c is aluminum foil. The thickness of the positive electrode core 22c is not particularly limited, but is preferably 5 to 30 μm, more preferably 10 to 25 μm. The number of layers of the positive electrode 22 is preferably 30 or more.

[0029] A plurality of positive electrode tabs 22t are provided at one end of the positive electrode core 22c in the long side direction Y (the left end in FIG. 6). The plurality of positive electrode tabs 22t protrude toward one side in the long side direction Y (the left side in FIG. 6). The plurality of positive electrode tabs 22t protrude further in the long side direction Y than the separator 26. Here, the positive electrode tab 22t is part of the positive electrode core 22c and is made of metal foil (aluminum foil). As shown in FIGS. 3 to 6, the plurality of positive electrode tabs 22t are stacked at one end in the long side direction Y (the left end in FIGS. 3 to 6) to form a positive electrode tab group 23. The positive electrode tab group 23 is electrically connected to the positive electrode terminal 30 via a positive electrode current collector 50.

[0030] As shown in FIG. 6, the positive electrode active material layer 22a is provided in a strip shape along the longitudinal direction of the positive electrode substrate 22c. The positive electrode active material layer 22a contains a positive electrode active material capable of reversibly absorbing and releasing charge carriers. The positive electrode active material is preferably a lithium transition metal composite oxide, more preferably one containing Ni. An example of a lithium transition metal composite oxide containing Ni is a lithium nickel cobalt manganese composite oxide. The positive electrode active material layer 22a may contain optional components other than the positive electrode active material, such as various additive components such as a binder and a conductive material. Preferably, the additive components include a conductive material and a binder. Examples of binders used in the positive electrode active material layer 22a include polyvinylidene fluoride (PVDF). The conductive material used in the positive electrode active material layer 22a is not particularly limited, but a carbon material is preferably used. The density of the positive electrode active material layer 22a is preferably 3.0 g / cc or more, more preferably 3.5 g / cc or more. The greater the density of the positive electrode active material layer 22a, the greater the spring constant of the electrode body 20. On the other hand, the density of the positive electrode active material layer 22a is preferably 3.7 g / cc or less.

[0031] As shown in Fig. 6, the positive electrode protective layer 22p is provided at the boundary between the positive electrode core 22c and the positive electrode active material layer 22a 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). 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.

[0032] As shown in Fig. 6, the negative electrode 24 includes a negative electrode core 24c and a negative electrode active material layer 24a fixed to at least one surface of the negative electrode core 24c. The negative electrode core 24c is strip-shaped. The negative electrode core 24c is preferably made of metal foil, more preferably copper foil or copper alloy foil. Here, the negative electrode core 24c is copper foil.

[0033] A plurality of negative electrode tabs 24t are provided at one end of the negative electrode core 24c in the long side direction Y (the right end in FIG. 6). The plurality of negative electrode tabs 24t protrude toward one side in the long side direction Y (the right side in FIG. 6). The plurality of negative electrode tabs 24t protrude further in the long side direction Y than the separator 26. Here, the negative electrode tab 24t is part of the negative electrode core 24c and is made of metal foil (copper foil). The thickness of the negative electrode core 24c is not particularly limited, but is preferably 5 to 30 μm, more preferably 5 to 15 μm. As shown in FIGS. 3 to 6, the plurality of negative electrode tabs 24t are stacked at one end of the long side direction Y (the right end in FIGS. 3 to 6) to form a negative electrode tab group 25. The negative electrode tab group 25 is provided at a position symmetrical to the positive electrode tab group 23 in the long side direction Y. The negative electrode tab group 25 is electrically connected to the negative electrode terminal 40 via the negative electrode current collecting portion 60 .

[0034] As shown in FIG. 6, the negative electrode active material layer 24a is provided in a strip shape along the longitudinal direction of the negative electrode substrate 24c. The negative electrode active material layer 24a contains a negative electrode active material capable of reversibly absorbing and releasing charge carriers. Examples of suitable negative electrode active materials include graphite, silicon-based materials (Si (silicon)-containing materials), and mixed oxides thereof. The negative electrode active material layer 24a may contain optional components other than the negative electrode active material, such as various additives such as binders, thickeners, and dispersants. Examples of binders used in the negative electrode active material layer 24a include styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC). The conductive material used in the negative electrode active material layer 24a is not particularly limited, but a carbon material is preferably used. The density of the negative electrode active material layer 24a is preferably 1.3 g / cc or higher, and more preferably 1.4 g / cc or higher. The higher the density of the negative electrode active material layer 24a, the greater the spring constant of the electrode body 20. On the other hand, the density of the negative electrode active material layer 24a is preferably 1.7 g / cc or less.

[0035] The separator 26 is a member that insulates the positive electrode active material layer 22a of the positive electrode 22 from the negative electrode active material layer 24a of the negative electrode 24. The separator 26 constitutes the outer surface of the electrode assembly 20. The length Ls of the separator 26 in the long side direction Y is equal to or longer than the length Ln of the negative electrode active material layer 24a in the long side direction Y. The length Ls of the separator 26 in the long side direction Y is equal to or longer than the length La of the positive electrode active material layer 22a in the long side direction Y. The separator 26 may have a heat-resistant layer formed on at least one surface thereof, the heat-resistant layer containing, for example, ceramic particles and a binder. The separator 26 may also have an adhesive layer formed on at least one surface thereof containing a binder. The adhesive layer may have a shape such as dots, stripes, waves, bands (streaks), dashed lines, or a combination thereof in a plan view.

[0036] Separator 26 is preferably a sheet-like member made of a microporous film. Separator 26 is preferably made of a polyolefin resin. Examples of polyolefin resins that can be used include polyethylene (PE), polypropylene (PP), and mixtures thereof.

[0037] There are no particular limitations on the thickness of the separator 26. When a non-aqueous electrolyte is not present in the separator 26, the thickness of the separator 26 is preferably about 10 to 30 μm. In this specification, the "thickness of the separator 26" refers to the thickness including the adhesive layer and heat-resistant layer when the separator 26 has an adhesive layer and heat-resistant layer, and refers to the thickness before press-molding unless otherwise specified.

[0038] As shown in FIG. 3, the positive electrode current collecting part 50 forms a conductive path that electrically connects the positive electrode tab group 23, which is made up of multiple positive electrode tabs 22t, to the positive electrode terminal 30. The positive electrode current collecting part 50 includes a positive electrode first current collecting part 51 and a positive electrode second current collecting part 52. The positive electrode first current collecting part 51 is attached to the inner surface of the sealing plate 14. The positive electrode second current collecting part 52 extends along the second side wall 12c of the exterior body 12. As shown in FIGS. 3 to 5, the positive electrode second current collecting part 52 is attached to the electrode body 20.

[0039] 3, the negative electrode current collecting part 60 forms a conductive path that electrically connects the negative electrode tab group 25 consisting of the multiple negative electrode tabs 24t to the negative electrode terminal 40. The negative electrode current collecting part 60 includes a negative electrode first current collecting part 61 and a negative electrode second current collecting part 62. The configurations of the negative electrode first current collecting part 61 and the negative electrode second current collecting part 62 may be the same as those of the positive electrode first current collecting part 51 and the positive electrode second current collecting part 52 of the positive electrode current collecting part 50.

[0040] Here, the storage cells 100 constituting the stack 500 disclosed herein satisfy the following requirements for each of the storage cells 100: (1) Cell thickness (mm), where the cell thickness is obtained by applying a load to the electricity storage cell 100 in the thickness direction at a rate of 0.01 mm / min to 10 mm / min up to a load of 0.2 MPa, and then holding the electricity storage cell 100 for 3 hours in a state where the load remains at 0.2 MPa, and then measuring the thickness of the electricity storage cell 100; (2) Spring constant (MPa / mm) of the energy storage cell, where the spring constant of the energy storage cell is determined by applying a load to the energy storage cell 100 in the thickness direction of the energy storage cell 100 up to 2 MPa at the same speed as when measuring the thickness of the energy storage cell 100 described above, and then reducing the load to 0.01 MPa after the application, as one cycle. This process is repeated five times, and the thickness X1 (mm) of the energy storage cell 100 when the load is applied to 0.1 MPa in the fifth cycle and the thickness X2 (mm) of the energy storage cell 100 when the load is applied to 0.2 MPa in the fifth cycle are measured, and the spring constant of the energy storage cell is determined by the following formula (i): Spring constant of the storage cell (MPa / mm) = |(0.2MPa-0.1MPa) / (X2-X1)|···Equation (i); When the spring constant of the energy storage cell 100 is measured, the spring constant has a positive correlation with the cell thickness. With this configuration, it is possible to suppress a decrease in the binding load of the stack 500 when the stack 500 is in use. In this specification, the term "positive correlation" refers to a relationship in which, when one value (here, the cell thickness) increases, the other value (here, the spring constant) also increases.

[0041] Conventional stacks have had the problem that the stack's restraint load decreases during initial discharge or under low temperature or low SOC (State of Charge) conditions during use, making it impossible to sufficiently restrain the energy storage cells. In light of this problem, the present inventors conducted extensive research and found that energy storage cells with small cell thicknesses tend to have large spring constants during initial discharge or under low temperature or low SOC conditions, which reduces the restraint load. Therefore, the energy storage cells 100 constituting the stack 500 according to the present disclosure are characterized in that the spring constant of the energy storage cells has a positive correlation with the cell thickness. In other words, the energy storage cells 100 constituting the stack 500 according to the present disclosure have smaller spring constants as the energy storage cells become thinner. This configuration can suppress a decrease in the restraint load during initial discharge or under low temperature or low SOC conditions.

[0042] The "cell thickness" in this specification can be measured using a universal testing machine (for example, an Autograph (manufactured by Shimadzu Corporation)) specifically by the following method. FIG. 7 is a schematic diagram illustrating measurement of the amount of change in cell thickness according to one embodiment. In FIG. 7, the direction in which the universal testing machine 600 applies a load in the thickness direction of the energy storage cell 100 is indicated by a hollow arrow.

[0043] As shown in Fig. 7, first, the energy storage cell 100 is set in the universal testing machine 600 so that a load is applied in the thickness direction (direction of the white arrow). More specifically, the energy storage cell 100 is set so that the pair of first side walls 12b of the energy storage cell 100 abuts against the universal testing machine 600. When the energy storage cell 100 is viewed horizontally, the universal testing machine 600 is set so that the load is applied to the flat portion 20f of the electrode assembly 20. Here, the energy storage cell 100 is set in the universal testing machine 600 so that the load is applied to the location indicated by the imaginary line in Fig. 3.

[0044] Then, a load is applied to the energy storage cell 100 in the thickness direction of the energy storage cell 100 (shown by the hollow arrow) at a pressurizing rate of 0.01 mm / min to 10 mm / min. The pressurizing rate at this time is designated as a first pressurizing rate S1. When the load reaches 0.2 MPa, the energy storage cell 100 is maintained at the 0.2 MPa load for three hours. After three hours, the thickness of the energy storage cell 100 is measured using a displacement meter (not shown), and this thickness can be designated as the "cell thickness." The thickness of the energy storage cell 100 may be measured at one location or multiple locations (two or more locations). When the thickness displacement of the energy storage cell 100 is measured at multiple locations, the cell thickness can be obtained by calculating an average value from the measurements at the multiple locations. The means for measuring the thickness of the energy storage cell 100 is not particularly limited, and for example, a laser displacement meter (e.g., Keyence Corporation, product name IL-S025) or the like can be used.

[0045] The "spring constant" in this specification can be measured using the universal testing machine described above, specifically, by the following method. First, the energy storage cell 100 is set in the universal testing machine 600 in the same manner as when measuring the cell thickness. Then, a load is applied to the energy storage cell 100 in the thickness direction of the energy storage cell 100 (indicated by the outline arrow) at a pressurizing rate of 0.01 mm / min to 10 mm / min. If the pressurizing rate at this time is designated as a second pressurizing rate S2, the first pressurizing rate S1 and the second pressurizing rate S2 are set to be the same rate. Then, after applying a load up to 2 MPa, the load on the energy storage cell 100 is reduced to 0.01 MPa. This cycle is defined as one cycle, and this process is repeated for five cycles. Then, the thickness of the energy storage cell 100 when the load is applied up to 0.1 MPa in the fifth cycle is defined as X1 (mm), and the thickness of the energy storage cell 100 when the load is applied up to 0.2 MPa in the fifth cycle is defined as X2 (mm). The absolute value of the cell thickness change can then be calculated using the following formula (i), and this value can be used as the "spring constant of the energy storage cell." The thicknesses X1 and X2 can be measured using the same method as the "cell thickness." The thicknesses X1 and X2 may be measured when a predetermined load is reached, or may be obtained by continuously measuring the thickness change while a load is being applied to or reduced from the cell, and plotting the thicknesses X1 and X2 from the obtained thickness change. Spring constant of the storage cell (MPa / mm) = |(0.2MPa - 0.1MPa) / (X2 - X1)| Equation (i)

[0046] The "spring constant" described above was measured under conditions in which a large load of 2 MPa was applied to the energy storage cell 100 five times, causing fluctuations in the spring constant to settle and become constant. The "spring constant" measured under such conditions is an estimated spring constant assuming an actual use of the stack 500 according to this embodiment. Furthermore, although the thickness of each energy storage cell 100 may vary with continued use of the stack 500, the relative cell thickness order among the multiple energy storage cells 100 constituting the stack 500 does not change. Therefore, it is believed that the requirement that the spring constant of the energy storage cell 100 has a positive correlation with the cell thickness is maintained even when the stack 500 according to this embodiment is continued to be used.

[0047] The spring constant of the energy storage cell 100 can vary depending on, for example, the amount of non-aqueous electrolyte present in the electrode assembly 20. More specifically, the non-aqueous electrolyte in the electrode assembly 20 can be present, for example, inside the separator 26. Furthermore, the non-aqueous electrolyte can also be present in the form of a layer as a non-aqueous electrolyte layer between the separator 26 and the positive electrode active material layer 22a, or between the separator 26 and the negative electrode active material layer 24a. The spring constant of the electrode assembly 20 tends to decrease as the amount of non-aqueous electrolyte present in the electrode assembly 20 increases, and tends to increase as the amount of non-aqueous electrolyte decreases.

[0048] The spring constant of the energy storage cell 100 can also vary depending on the hardness of the electrode assembly 20. For example, typically, the spring constant tends to increase as the thickness of the positive electrode 22, negative electrode 24, separator 26, etc. that make up the electrode assembly 20 decreases. Furthermore, the spring constant also tends to increase as the density of the positive electrode active material layer 22a and the negative electrode active material layer 24a increases. In other words, when comparing two cases where the thickness of the electrode assembly 20 is constant, the spring constant tends to increase as the density of the positive electrode active material layer 22a and the negative electrode active material layer 24a increases. In other words, in energy storage cells that make up a conventional stack, the spring constant of the cell tends to increase as the cell thickness decreases. In other words, there is a negative correlation between the spring constant and the thickness.

[0049] The method for controlling the numerical range of the spring constant of the energy storage cell 100 (i.e., making the spring constant and thickness positively correlated) is not particularly limited, and can be controlled by various methods. For example, the method is not limited to this, and can be controlled by changing the cell restraint load of the energy storage cell 100 in the activation step S108 during the manufacture of the energy storage cell 100. As will be described in detail later, by reducing the restraint load on the energy storage cell 100 during the activation step, the amount of non-aqueous electrolyte inside the electrode assembly 20 can be increased. This can reduce the spring constant of the energy storage cell 100. On the other hand, by increasing the restraint load on the energy storage cell 100 during the activation step, the amount of non-aqueous electrolyte inside the electrode assembly 20 can be reduced. This can increase the spring constant of the energy storage cell 100. By using this method, the spring constant of the energy storage cell 100 can be reduced by increasing the amount of non-aqueous electrolyte in an energy storage cell 100 with a small cell thickness. On the other hand, by reducing the amount of non-aqueous electrolyte in an energy storage cell 100 with a large cell thickness, the spring constant of the energy storage cell 100 can be increased. This allows the spring constant and thickness to have a positive correlation.

[0050] <Stack 500 manufacturing method> The present disclosure has been described above. An example of a method for manufacturing the stack 500 according to this embodiment will now be described. Note that the method for manufacturing the stack 500 is not limited to the following method. FIG. 8 is a flow diagram showing a method for manufacturing the stack 500 according to one embodiment. The stack 500 disclosed herein can be manufactured by a manufacturing method including a preparation step S10 and a stacking step S20. Furthermore, the method for manufacturing the stack 500 disclosed herein may include other steps at any stage in addition to the above steps, and the remaining manufacturing process may be the same as conventional methods.

[0051] (Preparation step S10) In the preparation step S10, an energy storage cell 100 including the electrode assembly 20, the non-aqueous electrolyte, and the case 10 as described above is prepared. Details of the energy storage cell 100 prepared in the preparation step S10 have already been explained above, so a duplicate explanation will be omitted here. In the preparation step S10, the energy storage cell 100 may be purchased and prepared, or the energy storage cell 100 may be manufactured and prepared.

[0052] (Lamination step S20) In the stacking step S20, the plurality of energy storage cells 100 are stacked along the thickness direction X of the energy storage cells 100. The stacking step S20 may be the same as a conventionally known stack manufacturing method. Here, the plurality of energy storage cells 100 are sandwiched between a pair of end plates 310 in the arrangement direction X, and are constrained by side plates 320 and a plurality of screws 330. In this manner, the stack 500 according to this embodiment is manufactured.

[0053] <Method of manufacturing the energy storage cell 100> Although not particularly limited, the energy storage cells 100 stacked in the stack disclosed herein (in other words, the energy storage cells 100 prepared in the preparation step S10) can be prepared, for example, by the manufacturing method described below. The manufacturing method of the energy storage cell 100 can also be incorporated as a sub-step of the preparation step S10. Note that the manufacturing method of the energy storage cell 100 described below is not limited to the following method. FIG. 9 is a flow chart showing a manufacturing method of the energy storage cell 100 according to one embodiment. As shown in FIG. 9, although not particularly limited, the energy storage cell 100 can be manufactured, for example, by a manufacturing method including an electrode body preparation step S101, an assembly step S102, a drying step S103, a liquid injection step S104, a degassing and charging step S105, a pressure reduction step S106, a liquid injection hole sealing step S107, an activation step S108, and an aging step S109, typically in this order. Furthermore, the method for manufacturing the energy storage cell 100 disclosed herein may include other steps at any stage in addition to the steps described above, and the rest of the manufacturing process may be the same as conventional ones.

[0054] (Electrode body preparation step S101) In the electrode body preparation step S101, a flat electrode body 20 is prepared in which a positive electrode 22 and a negative electrode 24 are stacked with a separator 26 interposed therebetween. In the electrode body preparation step S101, the electrode body 20 may be purchased and prepared, or the electrode body 20 may be manufactured and prepared.

[0055] Although not limited thereto, when manufacturing and preparing the electrode body 20, which is a wound electrode body, it can be prepared, for example, by the following method. First, a strip-shaped positive electrode 22, a negative electrode 24, and a separator 26 are prepared. When manufacturing and preparing the positive electrode 22 and the negative electrode 24, a positive electrode active material forming slurry is first applied to a positive electrode substrate 22c and dried. As a result, a positive electrode active material layer 22a is disposed on the positive electrode substrate 22c. After drying, the positive electrode substrate 22c and the positive electrode active material layer 22a are rolled. As a result, the positive electrode 22 can be obtained. This rolling method can be a conventionally known method such as a rolling roller. Furthermore, the density and thickness of the positive electrode active material layer 22a can be adjusted by adjusting the rolling conditions (rolling strength, etc.). Similarly, a negative electrode active material forming slurry is applied to a negative electrode substrate 24c and dried. As a result, a negative electrode active material layer 24a is disposed on the negative electrode substrate 24c. After drying, the negative electrode substrate 24c and the negative electrode active material layer 24a are rolled together to obtain the negative electrode 24. The rolling method can be the same as the method for producing the positive electrode 22 described above.

[0056] Then, a cylindrical wound body (cylindrical body) including the prepared strip-shaped positive electrode 22, strip-shaped negative electrode 24, and strip-shaped separator 26 is produced. Specifically, for example, a winding device including a winding unit is prepared. Next, the positive electrode 22, the negative electrode 24, and two separators 26 are each wound into a reel shape and set on the winding device. Next, the leading ends of the two separators 26 are fixed to the winding core of the winding unit. That is, the two separators 26 are sandwiched between the winding core. Next, the strip-shaped positive electrode 22 and the strip-shaped negative electrode 24 are stacked with the two separators 26 interposed therebetween. At this time, if the separator 26 has an adhesive layer, the adhesive layer of the separator 26 is oriented opposite the negative electrode 24. Then, the positive electrode 22, the negative electrode 24, and the separator 26 are wound by rotating the winding core while supplying the strip-shaped positive electrode 22 and the strip-shaped negative electrode 24. After the winding is completed, a stop tape (not shown) is attached to the end of the separator 26. In this manner, a cylindrical body can be produced.

[0057] Next, the cylindrical body produced above is press-molded. This makes it possible to prepare a flat electrode body 20 as shown in Fig. 6. It is preferable to adjust the press-molding conditions (e.g., pressure, holding time, etc.) appropriately depending on, for example, the number of windings and the properties of the adhesive layer of the separator 26. Press-molding may be performed at room temperature or while heating (at a high temperature).

[0058] Here, in some embodiments, it is preferable to calculate the thickness of the electrode assembly 20 before the activation step S108. As will be described in detail later, this configuration allows the restraint load to be adjusted according to the thickness of the electrode assembly 20 in the activation step S108 described later. This allows the spring constant of the manufactured energy storage cell 100 to be suitably adjusted. The timing for calculating the thickness of the electrode assembly 20 is not particularly limited as long as it is after the positive electrode 22 and the negative electrode 24 are rolled and before the activation step S108 described later. The thickness of the electrode assembly 20 can be calculated by directly measuring the thickness of the electrode assembly 20 after pressing. In addition, the thicknesses of the positive electrode 22 after rolling, the negative electrode 24 after rolling, and the separator 26 after rolling are each measured. The thickness of the electrode assembly 20 before the activation step S108 can be calculated from the obtained thickness and the number of windings and stackings of the electrode assembly 20.

[0059] (Assembly process S102) In the assembly step S102, the electrode body 20 is placed in the case 10 (exterior body 12) to prepare an assembly. In this specification, the term "assembly" refers to an energy storage cell that has been assembled to a state prior to the liquid injection hole sealing step described below.

[0060] In the assembly process S102, the positive electrode second current collecting portion 52 is attached to the positive electrode tab group 23 of the electrode assembly 20, and the negative electrode second current collecting portion 62 is attached to the negative electrode tab group 25. Next, the positive electrode terminal 30 and the negative electrode terminal 40 are attached to the sealing plate 14. Next, the positive electrode terminal 30 and the positive electrode first current collecting portion 51, and the negative electrode terminal 40 and the negative electrode first current collecting portion 61 are respectively joined by a conventionally known method (e.g., ultrasonic welding, resistance welding, laser welding, etc.). Next, the electrode assembly 20 is housed in an insulating sheet. The insulating sheet 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. Then, it is preferable to house (insert) the electrode assembly 20 covered with the insulating sheet into the internal space of the exterior body 12. Then, the exterior body 12 of the case 10 and the sealing plate 14 are joined to produce an assembly. This joining can be performed by welding, for example, laser welding. In the assembly step S102, the liquid injection hole 15 is not sealed.

[0061] In the assembly step S102, the electrode body 20 is preferably arranged in a direction (short side direction X) in which the thickness direction of the energy storage cell 100 (in other words, a direction substantially perpendicular to the first side wall 12b of the case 10) coincides with the thickness direction of the electrode body 20. In other words, the electrode body 20 is preferably arranged inside the exterior body 12 so that the winding axis WL is parallel to the bottom 12a of the exterior body 12.

[0062] (Drying process S103) In the drying step S103, the assembly is dried to remove moisture contained in the assembly (for example, the inside of the electrode assembly 20, etc.). Such drying can be performed by a known method. For example, the drying step S103 can be performed by transporting the assembly (the case 10 containing the electrode assembly 20) to a drying furnace (not shown) and heating it.

[0063] The drying temperature and drying time in the drying step S103 can be adjusted as appropriate depending on the amount of moisture contained in the electrode assembly 20, etc. The drying temperature is not particularly limited as long as it is within a range that can remove moisture, but it is desirable to dry at a temperature that does not damage the separator 26 of the electrode assembly 20. Furthermore, the drying step S103 is preferably performed in a reduced pressure atmosphere. This allows the drying time in the drying step S103 to be shortened. However, this is not a limitation, and the drying step S103 may also be performed in an atmospheric pressure atmosphere. Note that the drying step S103 is optional, and in some embodiments, the drying step S103 can be omitted.

[0064] (Liquid injection process S104) In the liquid injection step S104, a nonaqueous electrolyte is injected into the case 10 housing the electrode assembly 20 through an injection hole 15 provided in the sealing plate 14. The liquid injection step S104 may be performed in an atmospheric pressure atmosphere or a reduced-pressure atmosphere, but is preferably performed in a reduced-pressure atmosphere. This improves the impregnation of the nonaqueous electrolyte into the electrode assembly 20, allowing the liquid injection step S104 to be completed in a shorter time. In the liquid injection process, the nonaqueous electrolyte is injected in an amount that will permeate the entire electrode assembly 20. A conventionally known nonaqueous electrolyte injection device can be used as appropriate for the liquid injection step. Note that, as with conventional methods, examples of pressurized gases that can be used to pressure-feed the nonaqueous electrolyte include inert gases such as nitrogen (N2), dry air, and the like. After the liquid injection step S104 is completed, it is preferable to appropriately pressurize or depressurize the inside of the case 10.

[0065] (Gas removal and charging process S105) In the degassing and charging step S105, the assembly is charged. This allows a coating to be formed on the surface of the negative electrode active material layer 24a. Furthermore, by discharging gas generated during charging and discharging to the outside of the case 10, the amount of gas generated inside the case 10 after the liquid inlet 15 is sealed is reduced, and gas retention within the electrode body 20 after the liquid inlet 15 is sealed can be suppressed. The charging conditions in this step are not particularly limited and may be the same as those in conventional manufacturing methods. Although not particularly limited, for example, the degassing and charging step S105 can be performed by charging at a current of about 0.05 C to 1 C in a room temperature environment (e.g., 25°C) until the SOC reaches about 5% to 20%.

[0066] In the degassing and charging step S105, it is preferable to constrain the assembly in the thickness direction X of the case 10 and charge the assembly while applying a load. This makes it easier to push out gas from the case 10, thereby suppressing gas buildup within the electrode body. When constraining the case 10 in the degassing and charging step S105, it is preferable to apply a load to the first side wall 12b of the case 10. Furthermore, when constraining the case 10 in this manner, it is preferable that the center CP of the electrode body 20 (see FIG. 3) and the center of the constrained area are approximately aligned when viewed from the front from the first side wall 12b of the case 10. Furthermore, although there are no particular limitations on the constrained area, it is preferable to constrain an area on the first side wall 12b of the case 10 that is 280 to 295 mm in the long side direction Y and 85 to 95 mm in the up-down direction Z, centered on the center CP of the electrode body 20. The load applied to the case 10 is not particularly limited, but is preferably 0.25 to 0.80 MPa. In this specification, the "center CP of the electrode body 20" refers to the point where the center MY in the long side direction Y and the center MZ in the vertical direction Z intersect perpendicularly on the surface of the electrode body 20 having the flat portion 20f (the surface facing the first side wall 12b of the case 10), as shown in Figure 3.

[0067] (Decompression step S106) In the depressurization step S106, the pressure inside the case 10 is reduced, thereby further discharging gas present inside the case 10 (for example, air or gas generated in the degassing / charging step S105) to the outside of the case 10. The depressurization step S106 may be performed by the same means as those used in depressurization steps in conventional manufacturing methods of this type, and is not a particular feature of the technology disclosed herein, so further detailed description will be omitted. Note that the implementation of the depressurization step S106 is optional, and in some embodiments, the depressurization step S106 can be omitted.

[0068] (Injection hole sealing process S107) In the liquid inlet hole sealing step S107, the liquid inlet 15 of the assembly is sealed. The liquid inlet hole sealing step S107 may be the same as a conventionally known method for manufacturing the energy storage cell 100, and is not particularly limited. Here, the liquid inlet 15 is sealed with a metallic sealing member 16, and the sealing is performed by welding the metal portion of the sealing member 16 to the sealing plate 14 (the peripheral edge of the liquid inlet 15). However, the method for sealing the liquid inlet 15 is not limited to this. Although not shown in the drawings, the liquid inlet 15 may also be sealed with a rivet such as a blind rivet.

[0069] (Activation step S108) In the activation step S108, the storage cell 100 whose liquid injection hole 15 has been sealed through the liquid injection hole sealing step S107 is charged while a load is applied in the thickness direction of the storage cell 100.

[0070] The activation step S108 allows a coating to be formed on the surface of the negative electrode active material layer 24a. Furthermore, the activation step S108 allows the spring constant of the energy storage cell 100 to be controlled. As described above, the spring constant of the electrode assembly 20 tends to decrease as the amount of nonaqueous electrolyte present in the electrode assembly 20 increases, and the spring constant of the electrode assembly 20 tends to increase as the amount of nonaqueous electrolyte present in the electrode assembly 20 decreases. Here, when the restraining load during the activation step S108 is small, the amount of nonaqueous electrolyte present inside the electrode assembly 20 of the energy storage cell 100 can be increased. This allows the spring constant of the energy storage cell 100 to be reduced. On the other hand, when the restraining load during the activation step S108 is large, the amount of nonaqueous electrolyte present inside the electrode assembly 20 of the energy storage cell 100 can be reduced. This allows the spring constant of the energy storage cell 100 to be increased. In other words, the spring constant of the energy storage cell 100 can be controlled by adjusting the restraining load on the energy storage cell 100 during the activation step S108. In the activation step S108, the load applied to the energy storage cell 100 is preferably 0.5 to 0.7 MPa. With this configuration, the spring constant of the energy storage cell 100 can be set within a suitable range.

[0071] The charging conditions in the activation step S108 are not particularly limited and may be the same as those in conventional manufacturing methods. For example, the activation step S108 can be performed by charging at a current of about 0.05 C to 10 C in a room temperature environment (e.g., 25°C) until the SOC reaches about 20% to 100%. Furthermore, the method of restraining the energy storage cell 100 is not particularly limited and may be the same as that in the degassing and charging step S105. Therefore, a redundant description will be omitted here.

[0072] When the thickness of the electrode assembly 20 before the activation step S108 is calculated, it is preferable to adjust the load applied to the energy storage cell 100 according to the thickness of the electrode assembly 20 before the activation step S108. More specifically, among the multiple energy storage cells 100 to be assembled into the stack 500 (in other words, prepared in the preparation step S10), it is preferable to reduce the restraining load for the energy storage cell 100 having a relatively small thickness of the electrode assembly 20 and to increase the restraining load for the energy storage cell 100 having a relatively large thickness of the electrode assembly 20. Among the multiple energy storage cells 100, the spring constants of the energy storage cells come to have a positive correlation with the cell thickness. By subjecting these energy storage cells 100 to the preparation step S10 during the production of the energy storage cells 500, it is possible to make the spring constants of the energy storage cells 100 constituting the stack 500 after production have a positive correlation with the cell thickness. The numerical range of the restraint load adjusted among the plurality of energy storage cells 100 is preferably adjusted within the range of 0.5 to 0.7 MPa.

[0073] (Aging process S109) In the aging step S109, the energy storage cell 100 that has undergone the activation step S108 is stored in a predetermined temperature environment for a predetermined time while maintaining the charged state. This allows a coating to be more suitably formed on the surface of the negative electrode active material layer 24a. Note that the aging step S109 is not an essential step in the technology disclosed herein. In some preferred embodiments, the aging step S109 can be omitted.

[0074] The conditions for the aging step S109 can be adjusted appropriately depending on the desired coating formation mode and are not particularly limited. For example, the cell temperature (aging temperature) in the aging step can be set to 30°C or higher, preferably 40°C or higher, for example 50°C or higher, or even 60°C or higher. The upper limit of the aging temperature is not particularly limited, but can be set to, for example, approximately 80°C or lower. Temperature control in the aging step can be performed using, for example, a thermostatic bath. The duration of the aging step (aging time) can be changed appropriately depending on, for example, the aging temperature and is not particularly limited. For example, when the aging temperature is set to approximately 45 to 70°C, the aging time is preferably set to approximately 5 to 20 hours. Furthermore, when the aging temperature is set to approximately 70 to 75°C, the aging time is preferably set to approximately 5 to 15 hours.

[0075] Furthermore, in the aging step S109, aging is preferably performed while applying a load in the thickness direction X of the energy storage cell 100. The amount of load applied to the energy storage cell in the aging step S109 is not particularly limited, but is preferably 0.5 to 0.7 MPa. Furthermore, the process may proceed to the aging step S109 while maintaining the load restrained (applied) in the activation step S108.

[0076] The above describes one example of a manufacturing method for the stack 500. The stack 500 can be used for various purposes, and can be suitably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. The type of vehicle is not particularly limited, and examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

[0077] Hereinafter, examples of the technology disclosed herein will be described, but the technology disclosed herein is not intended to be limited to these examples.

[0078] [Evaluation 1: Evaluation of the relationship between cell thickness and spring constant] <Preparing the electrode body> Here, an electrode body was prepared for use in the evaluation storage cells according to the examples and comparative examples. Specifically, a positive electrode active material-forming slurry was applied to an aluminum foil serving as a positive electrode core and dried to form a positive electrode active material layer on the aluminum foil. After drying, the aluminum foil and the positive electrode active material layer were rolled together to form a positive electrode. Similarly, a negative electrode active material-forming slurry was applied to a copper foil serving as a negative electrode core and dried to form a negative electrode active material layer on the copper foil. After drying, the copper foil and the negative electrode active material layer were rolled together to form a negative electrode. Then, the thicknesses of the positive electrode, negative electrode, and separator after rolling were measured, and the positive electrode, negative electrode, and separator were stacked and wound together. In this way, in this example, a wound electrode body having a pair of curved portions as shown in FIG. 5 was prepared as the electrode body, in which a strip-shaped positive electrode and a strip-shaped negative electrode were stacked and wound together with a strip-shaped separator interposed therebetween. A total of 18 such electrode bodies were produced, six of each type: small electrode bodies (small), large electrode bodies (large), and medium electrode bodies (medium).

[0079] <Measurement of electrode thickness> The thickness (mm) of the 18 electrode bodies prepared above was calculated from the thicknesses of the positive electrode, negative electrode, and separator measured above and the number of windings (stacking) of the electrode body. After calculation, the electrode bodies were divided into three groups each of small electrode bodies (small), medium electrode bodies (medium), and large electrode bodies (large), for a total of nine electrode bodies, and these were used in the Examples and Comparative Examples.

[0080] <Creating an assembly> Three assemblies were prepared using three of each of the nine electrode bodies according to the example prepared above. Specifically, one assembly consisting of three electrode bodies (small) with small electrode body thicknesses, one assembly consisting of three electrode bodies (medium) with medium electrode body thicknesses, and one assembly consisting of three electrode bodies (large) with large electrode body thicknesses were prepared. Using the same procedure, three assemblies were prepared using three of each of the nine electrode bodies according to the comparative example. At this time, based on the electrode body thickness (mm) measured above, the total thickness of the three electrode bodies constituting each assembly is shown in the "Electrode body thickness before activation (mm)" column in Table 1.

[0081] (Example) <Construction of energy storage cells> Next, the three assemblies were subjected to an activation process. At this time, the restraint load during the activation process was set to 0.5 MPa for the electrode body (small) with a small electrode body thickness, 0.6 MPa for the electrode body (medium) with an intermediate electrode body thickness, and 0.7 MPa for the electrode body (large) with a large electrode body thickness. Three storage cells according to the example were prepared under the same conditions except for this.

[0082] The cell thickness of each of the storage cells prepared above was measured. A universal testing machine (Autograph AGX-V, manufactured by Shimadzu Corporation) and a laser displacement meter (LK-G157, manufactured by Keyence Corporation) were used for the measurements. First, the storage cell was set in the universal testing machine as shown in Figure 7. A load was applied to the storage cell in the thickness direction (indicated by the white arrow) at a rate of 10 mm / min. When the load reached 0.2 MPa, the storage cell was maintained at 0.2 MPa for 3 hours. After 3 hours, the thickness of the storage cell was measured at two points (two locations) using a laser displacement meter, and the average value was calculated from the obtained measurements. This gave the cell thickness (mm) of the storage cell. The results are shown in Table 1.

[0083] Next, the spring constant of each of the energy storage cells prepared above was measured. The same equipment was used for this measurement as for the cell thickness measurement. First, the energy storage cell was set in a universal testing machine as shown in FIG. 7 . A load was applied to the energy storage cell in the thickness direction (indicated by the white arrow) at a rate of 10 mm / min. After applying a load up to 2 MPa, the load on the energy storage cell was reduced to 0.01 MPa. This cycle constitutes one cycle, and this process was repeated for five cycles. The thickness X1 (mm) of the energy storage cell when the load was increased to 0.1 MPa in the fifth cycle, and the thickness X2 (mm) of the energy storage cell when the load was increased to 0.2 MPa in the fifth cycle were measured. The thicknesses X1 and X2 of the energy storage cell were measured at two points on the energy storage cell, and average values ​​were calculated from the measured values. The absolute value of the cell thickness change was then calculated using the thicknesses X1 and X2 of the obtained energy storage cell according to the following formula (i), and this value was designated as the "spring constant (MPa / mm) of the energy storage cell." The results are shown in Table 1. Furthermore, the spring constants of the energy storage cells according to the examples are plotted on the vertical axis and the cell thicknesses on the horizontal axis as open circles, and the approximate straight line showing the correlation between these is shown as a dashed line in Figure 10. Spring constant of the storage cell (MPa / mm) = |(0.2MPa - 0.1MPa) / (X2 - X1)| Equation (i)

[0084] (Comparative Example) Three energy storage cells were prepared in the same manner as in the Example, except that the load in the activation step was uniform (0.6 MPa) for all assemblies (small, medium, large). Then, as in the Example, the "electrode body thickness before activation (mm)," "cell thickness (mm)," and "spring constant of the energy storage cell (MPa / mm)" were measured. The results are shown in Table 1. Furthermore, the spring constant of the energy storage cell according to the Comparative Example is plotted on the vertical axis and the cell thickness on the horizontal axis with black circles, and an approximate straight line showing the correlation between these is shown in Figure 10.

[0085] [Table 1]

[0086] <Result> As shown in FIG. 10, it was confirmed that the spring constant of the energy storage cell according to the example had a positive correlation with the cell thickness, whereas the spring constant of the energy storage cell according to the comparative example had a negative correlation.

[0087] [Evaluation 2: Restraint load evaluation under low temperature and low SOC conditions] <Preparation of electrode body ~ assembly> Here, a restraint load evaluation was performed under low temperature and low SOC conditions. First, assemblies according to examples and comparative examples were prepared for this evaluation. Specifically, an electrode body (small) with a small electrode body thickness was fabricated using the same method as in "Evaluation 1" above, and then a cell assembly consisting of three such electrode bodies (small) was prepared. Similarly, an electrode body (medium) with a small electrode body thickness was fabricated using the same method as in "Evaluation 1" above, and then a cell assembly consisting of three such electrode bodies (medium) was prepared.

[0088] <Fabrication of energy storage cells> (Example) The assembly prepared above was then subjected to an activation process to prepare a storage cell according to the example. At this time, the restraint load during the activation process was set to 0.5 MPa for the assembly (small) consisting of an electrode body with a small electrode body thickness, and 0.6 MPa for the assembly (medium) consisting of an electrode body with a medium electrode body thickness. The storage cell according to the example obtained in this manner was fabricated.

[0089] (Comparative Example) Energy storage cells were produced in the same manner as in the example, except that the load in the activation step was set to a uniform value (0.6 MPa) for all assemblies (small and medium).

[0090] <Building the stack> (Example) Of the storage cells according to the example fabricated above, three small storage cells made of electrode bodies with small electrode body thicknesses and three medium storage cells made of electrode bodies with medium electrode body thicknesses (i.e., a total of six storage cells) were bound together to construct one stack. The cell thicknesses (mm) and average cell thicknesses of the storage cells constituting the stack according to the example are shown in Table 2. Table 2 also shows the spring constant (MPa / mm) of each storage cell measured by the same method as described above.

[0091] (Comparative Example) Of the energy storage cells according to the comparative example fabricated above, three small energy storage cells having an electrode body with a small electrode body thickness and three medium energy storage cells having an electrode body with a medium electrode body thickness (i.e., a total of six energy storage cells) were bound together to construct one stack. At this time, the average cell thickness after activation of the energy storage cells constituting the stack according to the example was adjusted to be approximately the same as the average cell thickness after activation of the energy storage cells constituting the stack according to the comparative example. The cell thicknesses (mm) and average cell thicknesses of the energy storage cells constituting the stack according to the comparative example are shown in Table 2. Table 2 also shows the spring constant (MPa / mm) of each energy storage cell measured by a method similar to that described above.

[0092] <Load evaluation under low temperature and low SOC conditions> Load evaluation was performed on the stacks according to the examples and comparative examples under low temperature and low SOC conditions. Specifically, the stacks according to the examples and comparative examples were placed in a -30°C environment with an SOC of 10%. The load on the stack at this time was measured using the universal testing machine described above. The results are shown in the "Load (MPa) under low temperature and low SOC conditions" column in Table 2.

[0093] [Table 2]

[0094] <Result> As can be seen from Table 2, the stack according to the example had a higher load at -30°C and SCO 10% than the comparative example. In other words, the stack according to the example was able to suppress a load decrease at low temperatures and low SOC. From the above, it can be seen that a stack constituted by energy storage cells in which the spring constant of the energy storage cell has a positive correlation with the cell thickness can suppress a load decrease of the stack (small loss of restraint load) even under low temperature and low SOC conditions.

[0095] Although preferred embodiments of the present disclosure have been described above, the above embodiments are merely examples. The present disclosure can be implemented in various other forms. The present disclosure 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 modifications, and it is also possible to add other modifications to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it can be deleted as appropriate.

[0096] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A stack in which a plurality of rectangular storage cells are stacked along the thickness direction of the rectangular storage cells, The storage cell is a flat electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a non-aqueous electrolyte, and a case that accommodates the electrode assembly and the non-aqueous electrolyte; For each of the above storage cells, the following requirements: (1) cell thickness (mm), where the cell thickness is obtained by applying a load to the electricity storage cell in a thickness direction at a rate of 0.01 mm / min to 10 mm / min up to a load of 0.2 MPa, and then holding the electricity storage cell for 3 hours under the load of 0.2 MPa, and then measuring the thickness of the electricity storage cell; (2) Spring constant (MPa / mm) of the storage cell, where the spring constant of the storage cell is determined by measuring the thickness X1 (mm) of the storage cell when the load is applied to the storage cell in the thickness direction up to 2 MPa at the same speed as when measuring the thickness of the storage cell, and then reducing the load to 0.01 MPa after the application, as one cycle. This process is repeated five times, and the thickness X1 (mm) of the storage cell when the load is applied to the storage cell up to 0.1 MPa in the fifth cycle and the thickness X2 (mm) of the storage cell when the load is applied to the storage cell up to 0.2 MPa in the fifth cycle are measured, and the spring constant of the storage cell is determined by the following formula (i): Spring constant of the storage cell (MPa / mm) = |(0.2MPa-0.1MPa) / (X2-X1)|···Equation (i); When the spring constant of the storage cell is measured, the spring constant has a positive correlation with the cell thickness. Item 2: The stack according to Item 1, wherein the electrode body is a wound electrode body. Item 3: The stack according to item 2, wherein a plurality of the wound electrode bodies are housed in the case. [Explanation of symbols]

[0097] 10 cases 12 Exterior body 14 Sealing plate 20 Electrode body 22 Positive electrode 24 Negative electrode 26 Separator 100 storage cells 300 Restraint mechanism 500 stacks 600 Universal Testing Machine S10 Preparation process S20 Lamination Process S101 Electrode body preparation process S102 Assembly process S103 Drying process S104 Liquid injection process S105 Gas removal and charging process S106 Decompression process S107 Liquid injection hole sealing process S108 Activation process S109 Aging process

Claims

1. A stack in which a plurality of rectangular energy storage cells are stacked along a thickness direction of the rectangular energy storage cells, The storage cell is a flat electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a non-aqueous electrolyte, and a case that accommodates the electrode assembly and the non-aqueous electrolyte; For each of the storage cells: (1) Cell thickness (mm), where the cell thickness is obtained by applying a load to the storage cell in the thickness direction at a rate of 0.01 mm / min to 10 mm / min up to a load of 0.2 MPa, and then holding the storage cell for 3 hours while maintaining the load at 0.2 MPa, and then measuring the thickness of the storage cell; (2) Spring constant (MPa / mm) of the storage cell, where the spring constant of the storage cell is determined by applying a load to the storage cell in the thickness direction of the storage cell up to 2 MPa at the same speed as when measuring the thickness of the storage cell, and then reducing the load to 0.01 MPa after the load application. This process is repeated for five cycles. The thickness X1 (mm) of the storage cell when the load is applied to 0.1 MPa in the fifth cycle and the thickness X2 (mm) of the storage cell when the load is applied to 0.2 MPa in the fifth cycle are measured, and the spring constant of the storage cell is determined by the following formula (i): Spring constant of the storage cell (MPa / mm)=|(0.2 MPa−0.1 MPa) / (X2−X1)|...Equation (i); When the spring constant of the storage cell is measured, the spring constant has a positive correlation with the cell thickness.

2. The stack of claim 1 , wherein the electrode body is a wound electrode body.

3. The stack according to claim 2 , wherein a plurality of the wound electrode bodies are housed in the case.

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