Manufacturing method for secondary battery negative electrode and manufacturing method for secondary battery

Abstract

To provide a manufacturing method for a secondary battery negative electrode that is resistant to swelling even after repeated charging and discharging.SOLUTION: A manufacturing method disclosed herein includes a preparation step for providing particulate graphite as a negative electrode active material, in which the tapped density is 1.10 g / cm3 or more and the sphericity is 0.8 or more, a forming step for forming a negative electrode active material layer containing the negative electrode active material, and a compression step for compressing the negative electrode active material layer. In the compression step, the ratio of the packing density of the negative electrode active material layer after the compression step to the packing density of the negative electrode active material layer before the compression step is 1.35 or higher, and the packing density of the negative electrode active material layer after the compression step is 1.45 g / cm3 or higher.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for producing a negative electrode for a secondary battery and a method for producing a secondary battery. [Background technology]

[0002] Conventionally, graphite has been widely used as the negative electrode active material for the negative electrodes of secondary batteries. Patent Documents 1 to 7 are examples of related art documents. For example, Patent Document 1 discloses that the packing density of a negative electrode active material layer containing graphite is set within a predetermined range. Patent Document 1 also describes that, among graphites, it is preferable to use graphite obtained by spheroidizing natural graphite. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-057465 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-196095 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-201125 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-127913 [Patent Document 5] Japanese Patent Publication No. 2022-190625 [Patent Document 6] Japanese Patent Application Laid-Open No. 2004-095529 [Patent Document 7] Re-tabled publication No. 2017-057123 Summary of the Invention [Problem to be solved by the invention]

[0004] While increasing the density of the negative electrode active material layer is effective for improving the energy density of secondary batteries, negative electrodes with a high-density negative electrode active material layer tend to swell with repeated charge and discharge. In particular, when spherical graphite is used, the loss of spheroidization with repeated charge and discharge cycles can lead to significant swelling of the negative electrode.

[0005] In power sources for driving vehicles, etc., multiple secondary batteries (single cells) are sometimes bound together and used as an energy storage module, but if the negative electrode swells with charge / discharge cycles, there is a risk that the binding pressure of the secondary battery will change. Furthermore, if the strength of the binding member is increased to prevent this, the weight and size of the binding member itself will increase, which may reduce the energy density of the entire energy storage module or, for example, when the energy storage module is installed in a moving object such as a vehicle, the weight will increase and fuel efficiency may deteriorate.

[0006] The present invention has been made in view of the above circumstances, and its main object is to provide a method for producing a negative electrode for a secondary battery that is less likely to swell even after repeated charge and discharge. [Means for solving the problem]

[0007] According to the present invention, a negative electrode active material having a tap density of 1.10 g / cm 3 a forming step of forming a negative electrode active material layer containing the negative electrode active material on a negative electrode current collector, the negative electrode active material layer being formed; and a compressing step of compressing the formed negative electrode active material layer, wherein in the compressing step, a ratio (D2 / D1) of a packing density D1 of the negative electrode active material layer after the compressing step to a packing density D2 of the negative electrode active material layer before the compressing step is set to 1.35 or more, and the packing density D2 of the negative electrode active material layer after the compressing step is set to 1.45 g / cm or less. 3 As described above, a method for producing a negative electrode for a secondary battery is provided.

[0008] The above-described manufacturing method can provide a negative electrode for a secondary battery that is resistant to swelling even after repeated charge and discharge. In addition, the weight and size of the restraining member can be prevented from increasing, thereby preventing deterioration in energy density and fuel efficiency. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view schematically illustrating an electricity storage module according to one embodiment. [Figure 2] FIG. 2 is a perspective view schematically showing the secondary battery of FIG. [Figure 3] FIG. 3 is a schematic vertical cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a graph showing the relationship between the packing density of the negative electrode active material layer and the total swelling amount. [Figure 5] FIG. 5 is a graph showing the relationship between the compression rate in the compression step and the expansion amount in the manufacturing stage. [Figure 6] FIG. 6 is a graph showing the relationship between the compression ratio in the compression step and the amount of expansion during charge-discharge cycles. DETAILED DESCRIPTION OF THE INVENTION

[0010] Preferred embodiments of the technology disclosed herein will be described below with reference to the drawings as appropriate. Matters necessary for implementing the technology disclosed herein other than those specifically mentioned in this specification (e.g., the general configuration and manufacturing process of energy storage modules and secondary batteries that do not characterize the technology disclosed herein) can be understood as design matters for those 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. In the following drawings, components and parts that perform the same function are designated by the same reference numerals, and redundant explanations may be omitted or simplified. Furthermore, in this specification, the notation "A to B" indicating a range means greater than or equal to A and less than or equal to B, and also encompasses the meanings "preferably greater than A" and "preferably smaller than B."

[0011] [Energy storage module] First, a secondary battery and an energy storage module including a negative electrode manufactured by the technology disclosed herein will be described. Fig. 1 is a perspective view that schematically shows an energy storage module 500. Here, the energy storage module 500 includes a plurality of secondary batteries 100 and a restraining mechanism 300. However, the energy storage module 500 may also include other members (for example, spacers) as long as the effects of the technology disclosed herein are not significantly impaired.

[0012] In the following description, the symbols F, Rr, L, R, U, and D in the drawings represent front, rear, left, right, top, and bottom, and the symbols X, Y, and Z in the drawings represent the short side direction (thickness direction) of the secondary battery 100, the long side direction perpendicular to the short side direction, and the up-down direction perpendicular to the short side direction and the long side direction, respectively. The short side direction X is also the arrangement direction of the multiple secondary batteries 100. However, these directions are merely used for convenience of description and do not limit the installation form of the energy storage module 500 in any way.

[0013] The restraining mechanism 300 is a member that restrains the multiple secondary batteries 100. The restraining mechanism 300 is configured to apply a specified restraining pressure to the multiple secondary batteries 100 in the arrangement direction X. Here, the restraining mechanism 300 includes a pair of end plates 310, a pair of side plates 320, and multiple screws 330. The pair of end plates 310 sandwich the multiple secondary batteries 100 in the arrangement direction X. 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 the multiple screws 330 so that the restraining load is, for example, about 3 to 15 kN, preferably about 5 to 10 kN. As a result, a uniform restraining load is applied to the multiple secondary batteries 100 in the arrangement direction X, and the multiple secondary batteries 100 are held together. However, the configuration of the restraining mechanism is not limited to this. The restraining mechanism 300 may include, for example, a plurality of restraining bands or bind bars instead of the side plate 320 and the plurality of screws 330 .

[0014] Here, the plurality of secondary batteries 100 are arranged between a pair of end plates 310 along the arrangement direction X. The plurality of secondary batteries 100 are preferably restrained by a restraining mechanism 300. Note that Fig. 1 is merely an example, and the shape, size, number, arrangement, etc. of the secondary batteries 100 are not limited to the embodiment disclosed in Fig. 1 and can be changed as appropriate.

[0015] Although not shown here, when the energy storage module 500 is in use, the multiple secondary batteries 100 are electrically connected to one another by conductive members such as bus bars. The connection method is not particularly limited, and may be, for example, series, parallel, or multi-series / multi-parallel. In a preferred embodiment, the multiple secondary batteries 100 are connected in series. This makes it possible to suitably improve the output characteristics to a level suitable for use in a mobile body such as a vehicle.

[0016] FIG. 2 is a perspective view of a secondary battery 100. As can be seen from FIGS. 1 and 2, the multiple secondary batteries 100 are all flat and rectangular, and have the same shape here. The multiple secondary batteries 100 are lined up in an arrangement direction X so that the long sides 12b of the battery cases 10 (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 secondary battery 100 includes a battery case 10, an electrode assembly 20, a positive electrode terminal 30, and a negative electrode terminal 40. Although not shown, the secondary battery 100 further includes a nonaqueous electrolyte. Here, the secondary battery 100 is a nonaqueous electrolyte secondary battery, and is preferably a lithium-ion secondary battery.

[0017] The battery case 10 is a container that accommodates the electrode assembly 20 and the nonaqueous electrolyte. As shown in FIG. 2, the battery case 10 has a flat, bottomed, rectangular parallelepiped (rectangular) outer shape. As shown in FIG. 3, the battery case 10 includes an exterior body 12 having an opening 12h and a sealing plate (lid) 14 that seals the opening 12h. The exterior body 12 has a pair of long sides 12b. The battery 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 battery case 10 is hermetically sealed (sealed). The sealing plate 14 has a liquid injection hole 15 and two terminal extraction holes 18 and 19. The liquid injection hole 15 is for injecting the nonaqueous electrolyte after the sealing plate 14 is assembled to the exterior body 12. The liquid injection hole 15 is sealed with a sealing member 16. The terminal lead-out holes 18 and 19 penetrate the sealing plate 14 in the vertical direction Z.

[0018] As shown in Fig. 3, the positive electrode terminal 30 is electrically connected to the positive electrode tab group 23 of the electrode body 20 inside the battery case 10 via the positive electrode current collector 50. The positive electrode terminal 30 extends from the inside to the outside of the sealing plate 14 through the terminal pull-out hole 18. 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). Here, the positive electrode terminal 30 is crimped to the peripheral portion of the sealing plate 14 surrounding the terminal pull-out hole 18 by crimping.

[0019] As shown in Fig. 3, the negative electrode terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode body 20 inside the battery case 10 via the negative electrode current collecting portion 60. The negative electrode terminal 40 extends from the inside to the outside of the sealing plate 14 through the terminal pull-out hole 19. 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). Here, the negative electrode terminal 40 is crimped to the peripheral portion surrounding the terminal pull-out hole 19 of the sealing plate 14 by crimping.

[0020] Here, the electrode assembly 20 is a wound electrode assembly formed by stacking a strip-shaped positive electrode and a strip-shaped negative electrode with a strip-shaped separator interposed therebetween and winding them around a winding axis. The electrode assembly 20 has a flat outer shape. Here, the electrode assembly 20 is disposed inside the battery case 10 with the winding axis oriented substantially parallel to the long side direction Y. However, in other embodiments, the electrode assembly 20 may be disposed inside the battery case 10 with the winding axis oriented substantially parallel to the up-down direction Z. The electrode assembly 20 may also be a laminated electrode assembly formed by stacking multiple square-shaped (typically rectangular) positive electrodes and multiple square-shaped (typically rectangular) negative electrodes in an insulated state. The number of electrode assemblies 20 disposed inside one battery case 10 may be one, or two or more (multiple).

[0021] The positive electrode includes a positive electrode current collector and a positive electrode active material layer fixed to at least one surface of the positive electrode current collector. The positive electrode current collector is preferably made of metal, more preferably metal foil. In this example, the positive electrode current collector is aluminum foil. A positive electrode tab group 23 is attached to one end of the positive electrode current collector in the long side direction Y, and is electrically connected to a positive electrode terminal 30 via a positive electrode current collector 50.

[0022] The positive electrode active material layer is provided in a strip shape on one or both sides (preferably both sides) of the positive electrode current collector along the longitudinal direction of the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material capable of reversibly absorbing and releasing charge carriers. Examples of the positive electrode active material include lithium transition metal composite oxides. The positive electrode active material layer may contain optional components other than the positive electrode active material, such as various additive components such as a binder and a conductive material.

[0023] The negative electrode has a negative electrode current collector and a negative electrode active material layer fixed to at least one surface of the negative electrode current collector. The negative electrode is manufactured by a manufacturing method described below. Therefore, as will be described in detail later, the negative electrode current collector is preferably made of metal, more preferably metal foil. In this example, the negative electrode current collector is copper foil. A negative electrode tab group 25 is attached to one end of the negative electrode current collector in the long side direction Y and is electrically connected to the negative electrode terminal 40 via the negative electrode current collector 60.

[0024] The negative electrode active material layer is provided in a strip shape on one or both sides (preferably both sides) of the negative electrode current collector along the longitudinal direction of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material capable of reversibly absorbing and releasing charge carriers. As will be described in detail later, the negative electrode active material essentially contains spheroidized graphite and may contain other negative electrode active materials (e.g., a carbon-containing negative electrode active material or a Si-containing negative electrode active material) as appropriate. The proportion of spherical particles having a sphericity of 0.8 or more in the total graphite is preferably 85% by mass or more, more preferably 92% by mass or more. The proportion of the mass of the negative electrode active material to the total mass of the negative electrode active material layer is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The proportion of the mass of the negative electrode active material is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 98% by mass or less. In such a case, the effects of the technology disclosed herein can be exhibited at a higher level. The negative electrode active material layer may contain any component other than the negative electrode active material, such as various additive components such as a binder, a thickener, a dispersant, etc. Details of the negative electrode will be described in detail in the section on the manufacturing method.

[0025] The separator is a member that insulates the positive electrode from the negative electrode. A suitable separator is a resin porous sheet (microporous membrane) made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator may have a functional layer (e.g., an adhesive layer or a heat resistance layer (HRL)) on the surface of the resin porous sheet.

[0026] The non-aqueous electrolyte may be the same as a conventional one and is not particularly limited. The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt, such as a Li salt or a Na salt). The non-aqueous electrolyte may further contain additives as necessary. The non-aqueous electrolyte is typically liquid, but may also be gel-like. In another embodiment, the secondary battery 100 may include a solid electrolyte instead of the non-aqueous electrolyte. In this case, the separator can be omitted.

[0027] [Method of manufacturing negative electrodes for secondary batteries] Next, a method for manufacturing the negative electrode for the secondary battery 100 and the secondary battery will be described. In this specification, the term "secondary battery" refers to a general electricity storage device that can be repeatedly charged and discharged, and is a concept that encompasses not only so-called secondary batteries such as lithium-ion secondary batteries and nickel-metal hydride secondary batteries, but also capacitors that utilize chemical reactions, such as lithium-ion capacitors and pseudo-capacitor capacitors.

[0028] The negative electrode for the secondary battery 100 disclosed herein is preferably for a non-aqueous electrolyte secondary battery, and particularly preferably for a lithium-ion secondary battery. The negative electrode for the secondary battery 100 disclosed herein can be manufactured by a manufacturing method including, in this order, a preparation step (step 1) of preparing a negative electrode active material, a formation step (step 2) of forming a negative electrode active material layer, and a compression step (step 3) of compressing the formed negative electrode active material layer. Furthermore, other steps may be further included at any stage.

[0029] (Step 1) In the preparation step, a negative electrode active material having a tap density of at least 1.10 g / cm 3 In the following, granular graphite having a tap density of 1.10 g / cm or more and a sphericity of 0.8 or more is prepared. 3 Granular graphite having a sphericity of 0.8 or more is sometimes referred to as "graphite G" to distinguish it from general graphite. Graphite G may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which a core of graphite is coated with an amorphous carbon material.

[0030] Graphite G is so-called spheroidized graphite, with a sphericity of 0.8 or more. As described in Patent Document 1, for example, the use of spheroidized graphite is useful from the viewpoint of reducing the specific surface area and improving battery characteristics (e.g., increasing capacity and reducing resistance). However, according to the findings of the present inventors, spheroidization is lost with charge / discharge cycles, making spheroidized graphite relatively more susceptible to swelling than other negative electrode active materials (e.g., graphite with low sphericity). The sphericity of graphite G may be, for example, 0.85 or more, or 0.9 or more. In such cases, applying the technology disclosed herein is particularly effective. The sphericity of graphite G may be approximately 0.99 or less, 0.95 or less, or 0.94 or less, from the viewpoint of ease of production, etc.

[0031] In this specification, "sphericity" refers to the arithmetic mean value of 4π × (particle area) / (perimeter)^2 obtained by acquiring multiple particle projection images using a commercially available particle imaging analyzer (e.g., FPIA-3000 manufactured by Sysmex Corporation) and calculating the arithmetic mean for each particle. The closer the sphericity value is to 1, the closer the particle is to a perfect sphere.

[0032] Graphite G has a tap density of 1.10 g / cm 3 This makes it possible to suppress the amount of swelling caused by the active material, and to stably form a negative electrode active material layer that satisfies the desired packing density in the compression step (step 3) described below. From this perspective, the tap density of graphite G is 1.12 g / cm 3 More than 1.15 g / cm is preferable. 3 The tap density of graphite G is 1.30 g / cm 3 Preferably less than 1.20 g / cm 3 The following is more preferred:

[0033] In this specification, the term "tap density" refers to the density (mass of sample / volume after tapping) calculated from the volume after tapping and the mass of the sample when a commercially available tapping-type density measuring device is used, a predetermined mass of sample is allowed to fall naturally into the attached tapping cell, and then the sample is mechanically tapped 800 times.

[0034] Although not particularly limited, the average particle size of graphite G is preferably 10 to 20 μm, more preferably 13 to 17 μm. Here, the "average particle size" refers to the particle size at 50% of the cumulative value (D50) in the volume-based particle size distribution measured by a particle size distribution measuring device based on a laser diffraction / scattering method.

[0035] In this step, in addition to graphite G having the above-described properties, conventionally known negative electrode active material may also be prepared as the negative electrode active material, as long as the effects of the technology disclosed herein are not significantly impaired. For example, graphite that does not satisfy the above-described properties (graphite other than graphite G, for example, scaly graphite or flake graphite), carbon-containing negative electrode active materials such as hard carbon and soft carbon, Si-containing negative electrode active materials such as Si, silicon oxide, silicon carbide, and silicon nitride, and the like.

[0036] In this case, the negative electrode active material preferably contains graphite G having the above-described properties as a main component (a component accounting for 50% by mass or more), more preferably 80% by mass or more of graphite G, even more preferably 95% by mass or more of graphite G, and particularly preferably consists essentially of graphite G (98% by mass or more of graphite G).

[0037] In the (Step 2) forming step, a negative electrode active material layer containing the negative electrode active material prepared in the (Step 1) preparing step is formed on a negative electrode current collector. This step includes, for example, a current collector preparing step (Step 2-A) of preparing a negative electrode current collector, a slurry preparing step (Step 2-B) of preparing a negative electrode composite slurry, and an applying step (Step 2-C) of applying the negative electrode composite slurry to the negative electrode current collector. The order of the current collector preparing step (Step 2-A) and the slurry preparing step (Step 2-B) is not particularly limited. Furthermore, other steps may be included at any stage.

[0038] In the current collector preparation step (step 2-A), a negative electrode current collector is prepared. The negative electrode current collector is preferably made of metal, and more preferably made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The negative electrode current collector is preferably made of metal foil, and more preferably made of copper foil or a copper alloy foil. Although not particularly limited, the thickness of the metal foil is, for example, 5 to 35 μm, and preferably 6 to 20 μm.

[0039] In the slurry preparation step (step 2-B), a negative electrode composite slurry containing the negative electrode active material (at least the graphite G having the above-described properties) prepared in the preparation step (step 1) is prepared. Specifically, solid materials for the negative electrode active material layer (e.g., negative electrode active material, binder, thickener, etc.) are mixed with a predetermined solvent. Note that in this specification, the term "slurry" refers to a mixture in which some or all of the solids are dispersed in a solvent, and is a term that encompasses paste, ink, etc.

[0040] Examples of binders that can be used include styrene butadiene rubber (SBR) and its modified products, acrylonitrile butadiene rubber and its modified products, and polyvinylidene fluoride (PVdF). Examples of thickeners that can be used include cellulose-based polymers such as carboxymethyl cellulose (CMC). Examples of solvents that can be used include aqueous solvents containing water and non-aqueous solvents such as N-methyl-2-pyrrolidone.

[0041] Although not particularly limited, the proportion of the negative electrode active material (e.g., graphite G having the above-described properties) in the total solid content of the negative electrode mixture slurry is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, from the viewpoint of achieving high energy density, etc. The proportion of the negative electrode active material is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 98% by mass or less, from the viewpoint of achieving the effects of the technology disclosed herein at a higher level.

[0042] The proportion of the thickener (e.g., CMC) in the total solid content of the negative electrode mixture slurry is preferably 0.1 to 2 mass %, more preferably 0.5 to 1 mass %. According to the inventors' investigations, by setting the proportion of the thickener to a predetermined value or less, it becomes easier to form a negative electrode active material layer that meets the desired packing density in the compression step (step 3) described below. Furthermore, residual stress can be reduced, and the effects of the technology disclosed herein can be exerted at a higher level.

[0043] By adjusting the mixing conditions of the negative electrode composite slurry, it becomes easier to adjust the negative electrode active material layer to a desired thickness in the application step (step 2-C) described below. Furthermore, by increasing the amount of thickener or decreasing the amount of solvent used, the final viscosity (viscosity in the application step described below) can be increased. This allows the packing density D1 described below to be kept small, and the thickness of the negative electrode active material layer after drying to be increased.

[0044] In the application step (step 2-C), the prepared negative electrode mixture slurry is applied to the surface of the negative electrode current collector using a conventionally known coating device and dried. This causes the negative electrode active material layer to adhere to the surface of the negative electrode current collector. The coating device is not particularly limited, and examples that can be used include a gravure coater, a comma coater, a slit coater, and a die coater. Drying can be performed according to a known method.

[0045] When forming a negative electrode active material layer on both sides of the negative electrode current collector, the negative electrode composite slurry is applied to both sides of the negative electrode current collector. In this case, although not particularly limited, the thickness of the negative electrode active material layer after drying (total thickness of both sides) may typically be 100 to 400 μm, for example, 110 to 350 μm. By setting the thickness of the negative electrode active material layer to the above range, it becomes easier to suitably adjust the value of the ratio (D2 / D1) in the compression step (Step 2) described later.

[0046] The coating density of the negative electrode composite slurry is 0.95 g / cm from the viewpoint of high energy density. 3 More than 1g / cm is preferable. 3The above is more preferable. From the viewpoint of achieving the effects of the technology disclosed herein at a higher level, the coating density of the negative electrode mixture slurry is 1.1 g / cm. 3 Preferably less than 1.095 g / cm 3 The following is more preferred:

[0047] (Step 2) In the compression step, the negative electrode active material layer formed on the negative electrode current collector is compressed. In this step, the packing density D1 (g / cm 3 ) of the negative electrode active material layer before the compression step is 3 ) to the packing density D2 (g / cm 3 ) (compressibility, D2 / D1) is 1.35 or more, and the packing density D2 of the negative electrode active material layer after the compression step is 1.45 g / cm 3 Compression treatment is carried out so that the above conditions are met. The conditions for the compression treatment (for example, the press pressure and line speed) are not particularly limited, but as an example, a roll press may be used, the line speed may be set to a range of 5 to 120 m / min, and the linear pressure may be set to a range of 1.0 to 3.0 t / cm.

[0048] In order to adjust the compressibility of the negative electrode active material layer, it is effective to (a) increase the thickness of the negative electrode active material layer (coating thickness before compression) in the previous application step (step 2-C) and (b) increase the pressing pressure in this step (reducing the thickness of the negative electrode active material layer after compression), and it is more preferable to use both of the above (a) and (b). In addition, generally, the thicker the negative electrode active material layer (coating thickness before compression), the stronger the pressing pressure required to compress it to the target thickness.

[0049] By setting the ratio (D2 / D1) to a predetermined value or greater, the negative electrode (negative electrode active material layer) can be expanded in advance during the manufacturing process, for example, during the activation step (step 30) described below. This prevents the negative electrode from expanding even after repeated charging and discharging when the secondary battery 100 is actually used by a user after distribution on the market. From this perspective, the ratio (D2 / D1) is preferably 1.36 or greater, and more preferably 1.38 or greater. Furthermore, from the perspective of achieving the effects of the technology disclosed herein at a higher level, and from the perspectives of workability and ease of fabrication, the ratio (D2 / D1) is preferably approximately 1.45 or less, and more preferably 1.42 or less.

[0050] Furthermore, by setting the packing density D2 of the negative electrode active material layer after the compression step to a predetermined value or more, a high energy density can be achieved. Furthermore, since a high packing density D2 makes the negative electrode (negative electrode active material layer) prone to swelling, applying the technology disclosed herein is particularly effective. Therefore, a high energy density and suppression of swelling can be balanced at a high level. From this perspective, the packing density D2 is set to 1.47 g / cm. 3 It is preferable that the packing density D2 is equal to or greater than 1.51. From the viewpoint of achieving a higher level of the effects of the technology disclosed herein, the packing density D2 is preferably equal to or less than approximately 1.51. This allows voids to be formed inside the negative electrode active material layer, providing a place for the swollen negative electrode active material to escape. This prevents the swollen amount from being directly reflected in changes in thickness.

[0051] Although not particularly limited, the thickness of the negative electrode active material layer after the compression step (total thickness of both sides) is usually thinner than that before the compression step, and may be typically 110 to 350 μm, for example 120 to 300 μm. In this manner, the negative electrode disclosed herein can be produced.

[0052] [Secondary battery manufacturing method] A secondary battery can be manufactured by a manufacturing method including a fabrication step of fabricating an electrode assembly using the negative electrode manufactured by the above manufacturing method. The secondary battery can be manufactured by a manufacturing method including, for example, (step 10) an electrode assembly fabrication step of fabricating an electrode assembly using the above negative electrode, (step 20) an assembly step of housing the fabricated electrode assembly in a battery case to construct a battery assembly, and (step 30) an activation step of activating the battery assembly, in this order. Furthermore, other steps may be further included at any stage.

[0053] (Step 10) In the electrode assembly fabrication step, a positive electrode and a separator are separately prepared, and the negative electrode fabricated by the above-described fabrication method, for example, is wound up opposite the positive electrode with the separator interposed between them, thereby fabricating the electrode assembly 20 for the secondary battery 100.

[0054] In the construction step (step 20), the electrode assembly 20 fabricated in the electrode assembly fabrication step (step 10) and a separately prepared non-aqueous electrolyte are housed in a battery case 10. This step includes, for example, a sealing step (step 21) and a liquid injection step (step 22). First, in the sealing step (step 21), the positive electrode tab group 23 of the electrode assembly 20 is joined to the positive electrode current collector 50, and the negative electrode tab group 25 of the electrode assembly 20 is joined to the negative electrode current collector 60. This integrates the sealing plate 14 and the electrode assembly 20. Next, the sealing plate 14 is placed over the opening 12h of the exterior body 12, and the electrode assembly 20 is placed inside the exterior body 12. Next, the sealing plate 14 is welded to the periphery of the opening 12h of the exterior body 12, thereby integrating the exterior body 12 and the sealing plate 14. Next, in the liquid injection step (step 22), a non-aqueous electrolyte is prepared and injected into the battery case 10 through the liquid injection hole 15 of the sealing plate 14. In this way, a battery assembly for the secondary battery 100 is constructed.

[0055] In the activation step (Step 30), the constructed battery assembly is charged at least once. Preferably, the constructed battery assembly is charged to a predetermined state of charge (SOC), for example, SOC 50% or higher, for example, SOC 50%, and then stored at room temperature (for example, 25±5°C) for a predetermined time (for example, 12 to 48 hours, for example, 24 hours). After that, it is charged to SOC 100% and then discharged to the discharge voltage at the time of shipment (for example, SOC 30% or lower, for example, SOC 20%). The battery assembly can be charged and discharged in the same manner as in the past. Typically, an external power source is connected between the positive electrode terminal 30 and the negative electrode terminal 40, and charging or discharging is performed until the terminals reach a predetermined state of charge. The liquid inlet 15 is then sealed with a sealing member 16, and the battery case 10 is hermetically sealed. In this manner, the secondary battery 100 can be prepared.

[0056] According to the technology disclosed herein, the negative electrode (negative electrode active material layer) can be expanded in advance during the manufacturing process, for example, during the activation process (step 30) described below. This allows the negative electrode to be prevented from expanding even during repeated charge and discharge cycles when the battery is actually used by users after distribution to the market. Therefore, in a preferred embodiment, the "amount of expansion during the manufacturing process," defined as the difference (T2 - T1) between the thickness T1 (mm) of the battery assembly upon completion of the sealing process (step 21) (before the electrolyte injection process) and the thickness T2 (mm) of the secondary battery 100 upon completion of the activation process (step 30), is typically greater than 1, preferably 1.2 or greater, and more preferably 1.5 or greater. The difference (T2 - T1) may be approximately 2.5 or less, or even 2 or less.

[0057] [Uses of secondary batteries and energy storage modules] The secondary battery 100 and the power storage module 500 can be used for a variety of purposes, but are suitable for applications that require a high level of balance between high energy density and suppressed swelling, such 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). By mounting the power storage module 500 on a moving object such as a vehicle, the fuel efficiency (electricity cost) of the moving object can be improved.

[0058] Several test examples relating to the present invention will be described below, but it is not intended to limit the present invention to these test examples. In these test examples, secondary batteries were constructed using different methods for manufacturing the negative electrode (negative electrode active material layer), and the amount of swelling was evaluated at the manufacturing stage and the subsequent evaluation stage (during charge-discharge cycles simulating actual use).

[0059] [Preparation of negative electrode] First, a negative electrode active material (graphite) having the sphericity and tap density shown in Table 1 was prepared (preparation step). Next, graphite as the negative electrode active material, SBR as the binder, and CMC as the thickener were mixed in a mass ratio of negative electrode active material:binder:thickener = 98:1:1, and an appropriate amount of ion-exchanged water was added as a solvent to prepare a negative electrode composite slurry. Next, the prepared negative electrode composite slurry was applied to both sides of a negative electrode current collector (copper foil) at the application density shown in Table 1 and dried to form a negative electrode active material layer (formation step). In this test example, the thickness (total of both sides) of each negative electrode active material layer was varied within the range of 110 to 350 μm to vary the packing density ratio (D2 / D1) in the compression step described below.

[0060] Next, the formed negative electrode active material layer was compressed. This compression treatment was carried out to obtain a packing density D1 (g / cm 3 ) of the negative electrode active material layer before the compression step. 3 ) to the packing density D2 (g / cm3 The compression step was carried out so that the ratio (compressibility, D2 / D1) of the packing densities D2 of the negative electrode active material layer after the compression step would be the values ​​shown in Table 1 (compression step). Specifically, the line speed was set to 20 m / min, and the roll press was performed with varying linear pressures in the range of 1.0 to 1.5 t / cm to vary the packing density ratio (D2 / D1). The thickness of the negative electrode active material layer after the compression step (total of both sides) was 100 to 300 μm. In this manner, a negative electrode was obtained.

[0061] [Secondary battery production] First, the negative electrode prepared above was placed opposite a positive electrode with a separator interposed therebetween to prepare an electrode assembly (electrode assembly preparation step). Next, the electrode assembly was placed in a battery case, and the opening of the exterior body was sealed with a sealing plate (sealing step). Next, two metal plates, each 10 mm smaller than the outer edge of the long sides of the battery case, were prepared, and the pair of long sides were sandwiched between the metal plates. A load (1.5 MPa as surface pressure) was applied, and the thickness T1 after 30 seconds was measured. Next, a nonaqueous electrolyte was injected into the battery case, and a battery assembly was constructed (injection step, construction step).

[0062] Next, the battery assembly was charged to an SOC of 50% in a temperature environment of 25°C, and then stored for 24 hours. After 24 hours, the battery assembly was charged to an SOC of 100%, and then discharged to an SOC of 20% (activation process). This resulted in the production of a lithium-ion secondary battery. Next, the thickness T2 of the lithium-ion secondary battery after the activation process was measured in the same manner as for the thickness T1 described above. The thickness T1 of the battery assembly after the construction process was then subtracted from the thickness T2 of the lithium-ion secondary battery after the activation process to calculate the "amount of swelling during the manufacturing stage." The results are shown in Table 1.

[0063] Next, a charge-discharge cycle was performed simulating actual use, and the amount of swelling was measured. Specifically, the secondary battery was first sandwiched between metal plates of the same size as above, and the secondary battery was adjusted to a state of charge (SOC) of 20%. A load (1.5 MPa as surface pressure) was applied in the same manner as above, and the thickness T3 was measured. Next, the secondary battery was held in a fixed-size constrained state, and a constant current charge was performed at a charge rate of 0.5 C until the SOC reached 95%, followed by a constant current discharge at a discharge rate of 0.5 C until the SOC reached 5%. This cycle was repeated 200 times, with a one-hour rest period between each cycle. After one hour had elapsed since the charge-discharge cycle, the voltage change had settled and reached a steady state. The secondary battery was then adjusted again to a state of 20% SOC, and a load (1.5 MPa as surface pressure) was applied in the same manner as above, and the thickness T4 was measured. The "amount of swelling during charge-discharge cycling" was calculated by subtracting the thickness T3 before the charge-discharge cycling from the thickness T4 after the charge-discharge cycling. The results are shown in Table 1. Table 1 also shows the "total swelling amount" which is the sum of the "swelling amount at the manufacturing stage" and the "swelling amount during charge / discharge cycles."

[0064] [Table 1]

[0065] FIG. 4 is a graph showing the relationship between the packing density D2 of the negative electrode active material layer and the total swelling amount. As shown in FIG. 4, the total swelling amount tended to increase as the packing density D2 of the negative electrode active material layer (and thus the tap density of the negative electrode active material) increased. While not intended to be particularly restrictive, this is thought to be because, when the packing density D2 of the negative electrode active material layer is low, it means that there are voids inside, so the swelling amount moves inward and acts to fill the voids, thereby reducing the impact on thickness change. Conversely, when the packing density D2 of the negative electrode active material layer is high, the swelling does not move inward, and the swelling amount is likely to be reflected in thickness change.

[0066] FIG. 5 is a graph showing the relationship between the compression ratio in the compression process, i.e., the ratio (D2 / D1), and the amount of swelling during the manufacturing stage. As shown in FIG. 5 and Table 1, in all examples except Comparative Example 4, the amount of swelling during the manufacturing stage tended to increase as the compression ratio during the compression process increased. While not intended to be particularly restrictive, the reason for this is thought to be that applying a large force during compression without causing cracks in the negative electrode active material makes it more likely that residual stress will remain. This residual stress is likely to be released by wetting after injection, resulting in swelling of the negative electrode and an increase in the amount of swelling of the battery. Note that Comparative Example 4 showed a different trend, but this is thought to be because the tap density of the negative electrode active material was extremely low, making it difficult for the packing density D2 of the negative electrode active material layer to increase during compression, resulting in cracks in the negative electrode active material.

[0067] Fig. 6 is a graph showing the relationship between the compression ratio in the compression step, i.e., the ratio (D2 / D1), and the amount of expansion during charge-discharge cycling. As shown in Fig. 6 and Table 1, in Comparative Example 3, the total amount of expansion was kept small. This was thought to be due to the low packing density D2 of the negative electrode active material layer and the low energy density. Therefore, when the packing density D2 of the negative electrode active material layer is low, the problems of the technology disclosed herein do not arise in the first place, and the effects of the technology disclosed herein are also limited.

[0068] On the other hand, in the other comparative examples (Comparative Examples 1, 2, 4, and 5), the swelling amount during charge-discharge cycling generally exceeded 3.0. The reason for this is that, for Comparative Examples 1 and 2, the total swelling amount was not significantly different from that of each example, as shown in Table 1. Therefore, it is believed that the low compression rate in the compression process resulted in a relatively small swelling amount during the manufacturing stage, and the amount that did not swell during the manufacturing stage swelled during the charge-discharge cycling. Furthermore, for Comparative Example 4, as described above, the low tap density of the negative electrode active material likely led to cracking of the negative electrode active material, increasing the contact surface with the electrolyte and promoting the growth of an SEI (Solid Electrolyte Interphase) film. Furthermore, for Comparative Example 5, the negative electrode active material had low sphericity and a relatively flattened shape, which likely led to a large swelling amount associated with the release of sphericity.

[0069] In contrast to these comparative examples, in Examples 1-7, the packing density D2 of the negative electrode active material layer was 1.45 g / cm 3 The results are significant, demonstrating a high energy density, while at the same time suppressing the amount of swelling during charge-discharge cycles to a relatively small value (less than 3.0). While not intended to be a particularly restrictive interpretation, the reason for this is thought to be that by setting the compression ratio in the compression process to 1.35 or more and intentionally leaving a large amount of residual stress during compression, the amount of swelling during the manufacturing stage (specifically, the activation process) increases, and the timing of swelling can be controlled, thereby suppressing swelling during charge-discharge cycles. From the above, the effects of the technology disclosed herein are also supported by experimental results.

[0070] Although the preferred embodiment of the present invention has been described above, the above embodiment is merely an example. 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 technical common sense in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiment. For example, it is possible to replace part of the above-described embodiment with other modifications, or to add other modifications to the above-described embodiment. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0071] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: As a negative electrode active material, the tap density is 1.10 g / cm 3 a forming step of forming a negative electrode active material layer containing the negative electrode active material on a negative electrode current collector, the negative electrode active material layer being formed; and a compressing step of compressing the formed negative electrode active material layer, wherein in the compressing step, a ratio (D2 / D1) of a packing density D1 of the negative electrode active material layer after the compressing step to a packing density D2 of the negative electrode active material layer before the compressing step is set to 1.35 or more, and the packing density D2 of the negative electrode active material layer after the compressing step is set to 1.45 g / cm or less.3 The above is a method for producing a negative electrode for a secondary battery. Item 2: The manufacturing method according to Item 1, wherein in the forming step, the proportion of the mass of the negative electrode active material to the total mass of the negative electrode active material layer is 98.5 mass % or less. Item 3: In the preparation step, the tap density is 1.20 g / cm 3 Item 1 or Item 2. The manufacturing method according to Item 1 or 2, wherein the graphite is prepared as follows: Item 4: The manufacturing method according to any one of Items 1 to 3, wherein in the compression step, the ratio (D2 / D1) is set to 1.42 or less. Item 5: The packing density of the negative electrode active material layer after the compression step is 1.51 g / cm 3 Item 5. The manufacturing method according to any one of Items 1 to 4, wherein: Item 6: A method for producing a secondary battery, comprising a step of producing an electrode assembly using a negative electrode produced by the method according to any one of items 1 to 5. [Explanation of symbols]

[0072] 10 Battery case 20 Electrode body 100 Secondary battery 300 Restraint mechanism 500 Energy Storage Module

Claims

1. As a negative electrode active material, a tap density of 1.10 g / cm ３ a preparing step of preparing granular graphite having a sphericity of 0.8 or more; a forming step of forming a negative electrode active material layer containing the negative electrode active material on a negative electrode current collector; a compressing step of compressing the formed negative electrode active material layer; Including, In the compression step, the ratio (D2 / D1) of the packing density D1 of the negative electrode active material layer after the compression step to the packing density D2 of the negative electrode active material layer before the compression step is set to 1.35 or more, and the packing density D2 of the negative electrode active material layer after the compression step is set to 1.45 g / cm ３ That is all. A method for producing a negative electrode for a secondary battery.

2. In the forming step, the proportion of the mass of the negative electrode active material to the total mass of the negative electrode active material layer is 98.5 mass% or less. The method of claim 1.

3. In the preparation step, the tap density is 1.20 g / cm ３ The graphite is prepared as follows: The method according to claim 1 or 2.

4. In the compression step, the ratio (D2 / D1) is set to 1.42 or less. The method according to claim 1 or 2.

5. The packing density of the negative electrode active material layer after the compression step is 1.51 g / cm ３ The following shall apply: The method according to claim 1 or 2.

6. A method for producing a secondary battery, comprising the step of producing an electrode assembly using the negative electrode produced by the method according to claim 1 or 2.

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