Secondary batteries

Abstract

To provide a secondary battery that improves the impregnation of the electrolyte into the wound electrode body by increasing the distance between the positive and negative electrodes, while suppressing the deposition of substances originating from charge carriers that may occur when the distance between electrodes is increased. [Solution] The secondary battery 100 has a wound electrode body 20 in which a positive electrode 30 including a positive electrode active material layer 31, a negative electrode 40 including a negative electrode active material layer 41, and a separator 50 interposed between the positive electrode active material layer 31 and the negative electrode active material layer 41 are stacked and wound around a winding axis WL perpendicular to the longitudinal direction. The negative electrode active material layer 41 has a pair of negative electrode layer ends 41a in the width direction parallel to the winding axis WL that are in contact with adjacent separators 50 and sandwiched between the pair of negative electrode layer ends 41a, and the central part 41b of the negative electrode layer including the center in the width direction of the negative electrode active material layer 41 forms a gap S between it and the adjacent separator 50, and the gap S is located inside both ends in the width direction parallel to the winding axis WL of the positive electrode active material layer 31.

Description

Technical Field

[0001] The present disclosure relates to secondary batteries, and particularly to secondary batteries having a wound electrode body.

Background Art

[0002] Secondary batteries such as lithium-ion secondary batteries have been used in recent years as so-called portable power sources for personal computers, mobile terminals, etc., and power sources for vehicle driving because they are lightweight and have a high energy density. In particular, lithium-ion secondary batteries that are lightweight and have a high energy density are preferably used as high-output power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs).

[0003] In this type of secondary battery, a wound electrode body in which a sheet-like positive electrode including a positive electrode active material layer, a sheet-like negative electrode including a negative electrode active material layer, and a sheet-like separator interposed between the positive electrode active material layer and the negative electrode active material layer are laminated and wound around a winding axis orthogonal to the longitudinal direction may be used as the electrode body.

[0004] For example, Patent Document 1 discloses a battery including a plate group in which a long positive electrode plate and a negative electrode plate having a mixture layer applied on a long current collector are wound in a spiral shape through a separator. In this battery, in the cross section in the width direction of the positive electrode plate, the amount of the mixture applied to at least one surface of the upper and lower ends is larger than the amount of the mixture in the central portion, and in the cross section in the width direction of the negative electrode plate, the amount of the mixture applied to at least one surface of the upper and lower ends is smaller than the amount of the mixture in the central portion. According to the technique described in Patent Document 1, the precipitation of metallic lithium in the central portion of the width direction cross section of the negative electrode plate is suppressed.

[0005] For example, Patent Document 2 discloses a battery using a coated electrode group comprising a positive electrode having a coating layer containing a positive electrode active material formed on a current collector, a negative electrode having a coating layer containing a negative electrode active material formed on a current collector, and a separator that electrically isolates the positive electrode and the negative electrode. In this battery, the coating layer of one of the positive and negative electrodes has a predetermined length in one direction that is longer than the length in one direction of the coating layer of the other electrode, and at least a portion of the end regions at both ends in the one-sided direction is provided with a thickened portion that is thicker than the central portion in the one-sided direction, and the other electrode is located between the thickened portion provided at one end and the thickened portion provided at the other end in the one-sided direction of the coating layer of the other electrode. According to the technology described in Patent Document 2, relative misalignment of the positive electrode, the negative electrode, and the separator is suppressed. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2001-15146 [Patent Document 2] Japanese Patent Publication No. 2012-181922 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, the manufacturing process of secondary batteries sometimes includes a step in which an electrolyte is poured into a battery case containing a wound electrode body, and then the electrolyte poured into the battery case impregnates the wound electrode body. The electrolyte poured into the battery case impregnates the wound electrode body from both ends in the width direction toward the center.

[0008] Here, in the central part of the wound electrode body in the width direction parallel to the winding axis, a power generation section is formed in which the positive electrode active material layer of the positive electrode and the negative electrode active material layer of the negative electrode are wound facing each other with a separator in between. Normally, the power generation section is tightly wound with almost no gaps between the positive electrode active material layer of the positive electrode, the negative electrode active material layer of the negative electrode, and the separator in order to increase the adhesion between each of the positive electrode active material layer and the separator and suppress misalignment and an increase in resistance. In such a wound electrode body, because the distance between the positive electrode and the negative electrode is small, it becomes difficult for the electrolyte to penetrate into the inside of the power generation section, resulting in low impregnation of the electrolyte into the inside of the wound electrode body. As a result, the time required for electrolyte impregnation becomes longer, which leads to the problem of reduced productivity of secondary batteries.

[0009] Therefore, it is conceivable to use a wound electrode body with a large inter-electrode distance, in which the positive electrode active material layer of the positive electrode, the negative electrode active material layer of the negative electrode, and the separator are loosely wound with gaps between them. When such a wound electrode body with a large inter-electrode distance is impregnated with electrolyte, the electrolyte easily penetrates into the interior of the power generation section through the gaps formed between the positive electrode active material layer, the negative electrode active material layer, and the separator, improving the impregnation of the electrolyte into the interior of the wound electrode body. On the other hand, a wound electrode body with a large inter-electrode distance has the problem that, compared to a wound electrode body with a small inter-electrode distance, substances derived from the charge carrier (e.g., metallic lithium) are more likely to precipitate on the surface of the negative electrode active material layer due to repeated charging and discharging.

[0010] This disclosure was made to solve these problems and aims to provide a secondary battery that can improve the impregnation of the electrolyte into the wound electrode body by increasing the inter-electrode distance between the positive electrode and the negative electrode, while suppressing the deposition of substances originating from the charge carrier that may occur when the inter-electrode distance is increased. [Means for solving the problem]

[0011] The secondary battery according to this disclosure has a wound electrode body in which a sheet-shaped positive electrode including a positive electrode active material layer, a sheet-shaped negative electrode including a negative electrode active material layer, and a sheet-shaped separator interposed between the positive electrode active material layer and the negative electrode active material layer are stacked and wound around a winding axis perpendicular to the longitudinal direction. The negative electrode active material layer has a pair of negative electrode layer ends in the width direction parallel to the winding axis that are in contact with adjacent separators, sandwiched between the pair of negative electrode layer ends, and the central part of the negative electrode layer, including the center in the width direction of the negative electrode active material layer, forms a gap between it and adjacent separators, and the gap is located on the inside of both ends in the width direction parallel to the winding axis of the positive electrode active material layer. [Effects of the Invention]

[0012] This disclosure makes it possible to provide a secondary battery that improves the impregnation of the electrolyte into the wound electrode body by increasing the inter-electrode distance between the positive electrode and the negative electrode, while suppressing the deposition of substances originating from the charge carrier that may occur when the inter-electrode distance is increased. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic perspective view showing an example of a secondary battery according to Embodiment 1. [Figure 2] This is a schematic cross-sectional view showing a portion of a secondary battery along the II-II cross-sectional line in Figure 1. [Figure 3] This graph is intended to explain the gaps. [Figure 4] This flowchart shows an example of a method for manufacturing a secondary battery according to Embodiment 1. [Figure 5] This is a schematic cross-sectional view showing a part of the secondary battery according to Embodiment 2. [Modes for carrying out the invention]

[0014] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments. What is shown in the drawings is a part of the whole, and many other configurations not shown are actually included. Also, for clarity of explanation, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

[0015] The width direction shown in the figure is the width direction of the positive electrode 30, the negative electrode 40, and the separator 50, and indicates the width direction of the secondary batteries 100 and 200. The thickness direction shown in the figure is the thickness direction of the positive electrode 30, the negative electrode 40, and the separator 50 included in the wound electrode body 20, and indicates the thickness direction of the secondary batteries 100 and 200.

[0016] Embodiment 1 Referring to FIG. 1, an overview of the secondary battery 100 according to this embodiment will be described. FIG. 1 is a perspective view schematically showing an example of the secondary battery according to Embodiment 1. The secondary battery 100 shown in FIG. 1 is a battery that can be repeatedly charged and discharged, such as a lithium-ion secondary battery or an electric double layer capacitor. Hereinafter, as one preferred embodiment of the secondary battery 100 disclosed herein, the secondary battery 100 will be described by embodying it as a lithium-ion secondary battery.

[0017] A lithium-ion secondary battery uses lithium ions as charge carriers and repeats charging and discharging by the movement of lithium ions in the electrolytic solution between the positive electrode 30 and the negative electrode 40. A lithium-ion secondary battery that is lightweight and has a high energy density is preferably used as a high-output power source for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs).

[0018] As shown in FIG. 1, the secondary battery 100 has a battery case 10 and a wound electrode body 20 housed in the battery case 10. Further, an electrolytic solution impregnated in the wound electrode body 20 is housed in the battery case 10.

[0019] The battery case 10 has, for example, a flat rectangular parallelepiped shape. The battery case 10 is made of, for example, a metal material such as aluminum, an aluminum alloy, and stainless steel. The battery case 10 has a bottomed rectangular tube-shaped case body 11 with an open upper end and a lid 12 in the shape of a rectangular flat plate. The lid 12 is provided to close the opening of the case body 11 and is joined to the case body 11.

[0020] The lid 12 is provided with a liquid injection port 13 and a sealing member 14. The liquid injection port 13 is a hole that penetrates the lid 12 and is used for injecting an electrolyte into the battery case 10. The liquid injection port 13 is hermetically sealed by the sealing member 14. In addition to the liquid injection port 13 and the sealing member 14, the lid 12 is provided with a gas discharge valve that opens when the pressure inside the battery case 10 reaches a predetermined pressure and discharges the gas generated inside the battery case 10.

[0021] Furthermore, the lid 12 is provided with a positive electrode terminal 15 and a negative electrode terminal 16 for external connection so as to protrude outside the battery case 10. For example, the positive electrode terminal 15 and the negative electrode terminal 16 are connected to plate-shaped positive electrode current collector terminals 17 and negative electrode current collector terminals 18 at least partially disposed inside the battery case 10.

[0022] The wound electrode body 20 has, for example, an outer shape formed into a flat shape. The wound electrode body 20 is housed inside the battery case 10 such that the winding axis WL substantially coincides with the width direction of the secondary battery 100.

[0023] The wound electrode body 20 has a positive electrode connection portion 21 formed at one end in the width direction and a negative electrode connection portion 22 formed at the other end in the width direction. The positive electrode connection portion 21 is a portion where the positive electrode current collector 32 described later is exposed without the formation of the positive electrode active material layer 31 described later. The negative electrode connection portion 22 is a portion where the negative electrode current collector 42 described later is exposed without the formation of the negative electrode active material layer 41 described later.

[0024] As the electrolyte, any electrolyte that can be used in lithium-ion secondary batteries can be used without particular restrictions. As the electrolyte, a non-aqueous electrolyte can be used, which is obtained by dissolving a Li salt as a supporting salt (electrolyte) in a non-aqueous solvent (organic solvent).

[0025] As the Li salt, for example, one of LiPF6, LiClO4, LiAsF6, LiBF4, LiSO3CF3, etc., can be used alone or in combination of two or more. As the non-aqueous solvent, for example, one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), etc., can be used alone or in combination of two or more.

[0026] The electrolyte may contain additives in addition to the non-aqueous solvent and supporting salt described above. Examples of additives include gas generators, film-forming agents, dispersants, and thickeners.

[0027] Here, Figure 2 is a schematic cross-sectional view showing a portion of the secondary battery along the II-II cross-sectional line in Figure 1. Note that in Figure 2, components other than the wound electrode body 20 are not shown. The wound electrode body 20 will be described in detail with reference to Figure 2.

[0028] As shown in Figure 2, the wound electrode body 20 is wound around a winding axis WL that is perpendicular to the longitudinal direction, with a sheet-like positive electrode 30 containing a positive electrode active material layer 31, a sheet-like negative electrode 40 containing a negative electrode active material layer 41, and a sheet-like separator 50 interposed between the positive electrode active material layer 31 and the negative electrode active material layer 41 being stacked.

[0029] The wound electrode body 20 is connected to the positive electrode terminal 15 and the negative electrode terminal 16. Specifically, the wound electrode body 20 has a positive electrode 30 electrically connected to the positive electrode terminal 15 via a positive electrode current collector terminal 17 joined to the positive electrode connection part 21, and a negative electrode 40 electrically connected to the negative electrode terminal 16 via a negative electrode current collector terminal 18 joined to the negative electrode connection part 22 (see Figure 1).

[0030] The wound electrode body 20 has a power generation section 23 formed in the center of the width direction parallel to the winding axis WL, in which a positive electrode active material layer 31 and a negative electrode active material layer 41 are wound facing each other via a separator 50. In this power generation section 23, the secondary battery 100 is charged and discharged by the movement of lithium ions between the positive electrode active material layer 31 and the negative electrode active material layer 41.

[0031] The materials constituting the positive electrode 30, the negative electrode 40, and the separator 50 can be any materials that can be used in lithium-ion secondary batteries without any particular limitations.

[0032] The positive electrode 30 is a long, sheet-like component. In addition to the positive electrode active material layer 31, the positive electrode 30 includes a long, sheet-like positive electrode current collector 32. The positive electrode current collector 32 is made of a metal with good conductivity, such as aluminum, aluminum alloy, nickel, titanium, or stainless steel. Aluminum foil can be suitably used as the positive electrode current collector 32. The thickness of the positive electrode current collector 32 is, for example, 5 μm to 20 μm.

[0033] The positive electrode active material layer 31 is formed on the surface (one side or both sides) of the positive electrode current collector 32. In this embodiment, the positive electrode active material layer 31 is formed on both sides of the positive electrode current collector 32, excluding one edge in the width direction of the positive electrode current collector 32.

[0034] The positive electrode active material layer 31 contains a positive electrode active material. The positive electrode active material is a material capable of intercalating and releasing lithium ions, and examples include lithium transition metal composite oxides such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), nickel cobalt nickel cobalt manganese oxide (NCM), and lithium transition metal phosphate compounds such as lithium iron phosphate (LiFePO4). In addition, other metal elements may be added to the positive electrode active material.

[0035] The positive electrode active material layer 31 may further contain a conductive material, a binder, etc. Examples of conductive materials include carbon black such as acetylene black (AB), carbon nanotubes, and graphite. Examples of binders include vinyl halogenated resins such as polyvinylidene fluoride (PVDF) and polyalkylene oxides such as polyethylene oxide (PEO).

[0036] The width of the positive electrode active material layer 31 is, for example, 50 mm or more and 150 mm or less. The width of the positive electrode active material layer 31 is the length in the width direction parallel to the winding axis WL of the positive electrode active material layer 31.

[0037] The thickness of the positive electrode active material layer 31 is, for example, 10 μm or more and 50 μm or less. The thickness of the positive electrode active material layer 31 is the length in the thickness direction perpendicular to the longitudinal and width directions of the positive electrode active material layer 31.

[0038] The negative electrode 40 is a long, sheet-like member. In addition to the negative electrode active material layer 41, the negative electrode 40 includes a long, sheet-like negative electrode current collector 42. The negative electrode current collector 42 is made of a metal with good conductivity, such as copper, copper alloy, nickel, titanium, or stainless steel. Copper foil can be suitably used as the negative electrode current collector 42. The thickness of the negative electrode current collector 42 is, for example, 5 μm to 20 μm.

[0039] The negative electrode active material layer 41 is formed on the surface (one side or both sides) of the negative electrode current collector 42. In this embodiment, the negative electrode active material layer 41 is formed on both sides of the negative electrode current collector 42, except for one edge in the width direction of the negative electrode current collector 42.

[0040] The negative electrode active material layer 41 contains a negative electrode active material. The negative electrode active material is a material capable of intercalating and releasing lithium ions, and examples include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material may also be amorphous coated graphite, in which amorphous carbon is coated on the surface of granular natural graphite.

[0041] The negative electrode active material layer 41 may further contain a binder, a thickener, etc. Examples of binders include rubbers such as butyl rubber (BR) and styrene-butadiene rubber (SBR). Examples of thickeners include celluloses such as methylcellulose (MC) and carboxymethylcellulose (CMC).

[0042] The negative electrode active material layer 41 has a pair of negative electrode layer ends 41a in the width direction that are in contact with adjacent separators 50, and is sandwiched between the pair of negative electrode layer ends 41a. The central portion 41b of the negative electrode active material layer 41, including the center in the width direction, forms a gap S between it and the adjacent separator 50. The gap S is located inside both ends of the positive electrode active material layer 31 in the width direction.

[0043] The gap S has the function of increasing the inter-electrode distance between the positive electrode 30 and the negative electrode 40. This allows the electrolyte to easily penetrate into the power generation unit 23 through the gap S during impregnation. Therefore, compared to the case without the gap S, the impregnation of the electrolyte into the wound electrode body 20 is improved. As a result, the time required for electrolyte impregnation is shortened, and the productivity of the secondary battery 100 is improved. The inter-electrode distance is the average of the shortest distances between adjacent positive electrode 30 and negative electrode 40 via the separator 50.

[0044] Furthermore, because the gap S is located on the inside of both ends in the width direction of the positive electrode active material layer 31, it is possible to suppress the reduction of the gap S and the decrease in the inter-electrode distance by preventing the positive electrode active material layer 31 from entering the gap S together with the separator 50. Therefore, it is possible to suppress the decrease in the impregnation of the electrolyte into the interior of the wound electrode body 20 that may occur when the inter-electrode distance is reduced during electrolyte impregnation.

[0045] Furthermore, the thinned central portion 41b of the negative electrode layer has lower heat dissipation than the pair of negative electrode layer edges 41a, and the ionic conductivity of the electrolyte is higher, so the reactivity of the charge-discharge reaction is improved and it expands and contracts significantly with charging and discharging. The gap S facilitates the expansion of the central portion 41b of the negative electrode layer. As a result, when the negative electrode 40 expands with charging, the gap S shrinks and the distance between electrodes decreases. Therefore, the deposition of metallic lithium that may occur when the distance between electrodes increases can be suppressed by the gap S.

[0046] Therefore, according to the secondary battery 100 of this embodiment, by increasing the inter-electrode distance between the positive electrode 30 and the negative electrode 40, the impregnation of the electrolyte into the wound electrode body 20 can be improved while suppressing the deposition of metallic lithium that may occur when the inter-electrode distance is increased.

[0047] The negative electrode active material layer 41 will be described in more detail. The negative electrode active material layer 41 has grooves 43 formed in the central part 41b of the negative electrode layer, which form a gap S in the wound electrode body 20. The grooves 43 extend in the longitudinal direction of the negative electrode active material layer 41. There may be one groove 43 or multiple grooves 43. From the viewpoint of improving the impregnation of the electrolyte into the interior of the wound electrode body 20, it is preferable that the grooves 43 extend continuously from one end to the other in the longitudinal direction of the negative electrode active material layer 41. The grooves 43 have a depth that allows a gap S to be formed in the wound electrode body 20 when the electrolyte is impregnated.

[0048] The groove 43 may have the same width at the opening and the same width at the bottom, or they may have different widths. The widths of the opening and bottom of the groove 43 are, for example, 1 μm or more and 100 μm or less. In addition, the width of the opening of the groove 43 is shorter than the width of the positive electrode active material layer 31.

[0049] The cross-sectional shape of the groove 43, viewed from the longitudinal direction, is such that a gap S can be formed in the wound electrode body 20 when impregnated with the electrolyte. The cross-sectional shape of the groove 43 may be such that the width of the opening and the width of the bottom of the groove 43 are approximately the same (for example, rectangular), or it may be such that the width of the opening and the width of the bottom of the groove 43 are different (for example, trapezoidal), or it may be such that the width of the bottom of the groove 43 is zero (for example, inverted triangular). The cross-sectional shape does not have to consist only of straight lines; for example, it may be a rounded cross-sectional shape (for example, semicircular).

[0050] Here, Figure 3 is a graph illustrating the gap. The vertical axis of the graph in Figure 3 represents the charging current value (A). The horizontal axis of the graph in Figure 3 represents the gap size (μm) of gap S.

[0051] As shown in Figure 3, it is preferable that the negative electrode active material layer 41 satisfies the following equation (1) when X1 is the gap amount S in the uncharged wound electrode body 20, Y is the maximum expansion amount in the thickness direction of the central part 41b of the negative electrode layer, and X2 is the maximum gap amount S corresponding to the deposition limit current value Z, which is the limiting charging current value at which metallic lithium does not precipitate. X1-Y <X2···(1)

[0052] Here, the gap amount X1 is, for example, the average value of the shortest distance from the central part 41b of the negative electrode layer in the uncharged wound electrode body 20 to the adjacent separator 50. The maximum expansion amount Y is, for example, the average value of the difference between the thickness of the central part 41b of the negative electrode layer in the fully charged wound electrode body 20 and the thickness of the central part 41b of the negative electrode layer in the uncharged wound electrode body 20. The maximum gap amount X2 is, for example, the average value of the shortest distance from the central part 41b of the negative electrode layer that forms a gap S corresponding to the deposition limit current value Z to the adjacent separator 50.

[0053] The gap amount X1, maximum expansion amount Y, and maximum gap amount X2 can be determined, for example, by measuring the thickness of the negative electrode active material layer 41 in the wound electrode body 20 in both the charged and uncharged states using a confocal microscope. The deposition limit current value Z can be determined in advance through experiments or other means.

[0054] By ensuring that the gap amount X1, maximum expansion amount Y, and maximum gap amount X2 satisfy the above equation (1), the deposition of metallic lithium that may occur when the inter-electrode distance increases due to the gap S can be reliably suppressed.

[0055] Furthermore, as shown in Figure 2, it is preferable that the thickness of the central portion 41b of the negative electrode active material layer 41 is thinner than the thickness of each of the pair of negative electrode end portions 41a. The thickness of the negative electrode end portions 41a is, for example, 5 μm or more and 50 μm or less. If it is thinner than the thickness of the negative electrode end portions 41a, the thickness of the central portion 41b of the negative electrode layer is, for example, 10 μm or more and 50 μm or less. The respective thicknesses of the negative electrode end portions 41a and the central portion 41b of the negative electrode layer are the lengths in the thickness direction perpendicular to the longitudinal and width directions, respectively.

[0056] Preferably, the negative electrode active material layer 41 has a width greater than the width of the positive electrode active material layer 31, and both ends in the width direction are located outside the ends in the width direction of the positive electrode active material layer 31. This allows the negative electrode active material layer 41 to sufficiently accept lithium ions released from the positive electrode active material layer 31, thereby suppressing the deposition of metallic lithium. The width of the negative electrode active material layer 41 is, for example, 50 mm or more and 150 mm or less. The width of the negative electrode active material layer 41 is the length in the width direction parallel to the winding axis WL of the negative electrode active material layer 41.

[0057] The separator 50 is a long, sheet-like component. For example, a porous resin sheet made of polyethylene (PE), polypropylene (PP), or other resin can be used as the separator 50. Such a resin sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 50 may also be provided with a heat-resistant layer (HRL).

[0058] Preferably, the separator 50 has a width longer than the respective widths of the positive electrode active material layer 31 and the negative electrode active material layer 41, and both ends in the width direction are located outside the respective ends in the width direction of the positive electrode active material layer 31 and the negative electrode active material layer 41. This ensures insulation between the positive electrode 30 and the negative electrode 40.

[0059] The width of the separator 50 is, for example, 50 mm or more and 150 mm or less. The width of the separator 50 is the length in the width direction parallel to the winding axis WL of the separator 50.

[0060] The thickness of the separator 50 is, for example, 10 μm or more and 30 μm or less. The thickness of the separator 50 is the length in the thickness direction perpendicular to the longitudinal and width directions of the separator 50.

[0061] Next, an example of a method for manufacturing a secondary battery 100 according to Embodiment 1 (hereinafter also referred to as "this manufacturing method") will be described with reference to Figure 4. Figure 4 is a flowchart of an example of a method for manufacturing a secondary battery according to Embodiment 1. As shown in Figure 4, the method includes a negative electrode manufacturing step (step S1), an electrode body manufacturing step (step S2), an electrode body connection step (step S3), an electrode body housing step (step S4), and a liquid injection step (step S5).

[0062] <Negative electrode fabrication process (Step S1)> The negative electrode manufacturing process is the process of manufacturing the negative electrode 40 described above. In the negative electrode manufacturing process, first, a slurry or paste-like composition prepared by dispersing the negative electrode active material, and optionally a binder and thickener or other solid components in a solvent, is coated onto the surface of the negative electrode current collector 42 and dried to form a negative electrode active material layer 41 on the surface of the negative electrode current collector 42 in which grooves 43 are not formed.

[0063] The composition can be prepared by mixing a negative electrode active material, and optionally a binder and a thickener or other solid components, with a solvent. Suitable solvents include, for example, water or a water-based mixed solvent. A suitable mixing apparatus can be used for mixing. Examples of mixing apparatus include planetary mixers, ball mills, roll mills, kneaders, and homogenizers.

[0064] Appropriate coating equipment can be used to coat the composition. Examples of coating equipment include die coaters, slit coaters, comma coaters, gravure coaters, blade coaters, and the like.

[0065] During coating, the composition is applied to the surface of the negative electrode current collector 42 along its longitudinal direction, except for one edge in the width direction of the negative electrode current collector 42. Furthermore, during coating, it is preferable to apply the composition such that the thickness of the central portion 41b of the negative electrode layer is thinner than the thickness of each of the pair of negative electrode layer ends 41a. Additionally, during coating, the coating width of the composition is adjusted so that the width of the negative electrode active material layer 41 is greater than the width of the positive electrode active material layer 31. Furthermore, during coating, the coating width of the composition is adjusted so that the width of the negative electrode active material layer 41 is shorter than the width of the separator 50.

[0066] In this embodiment, the composition is applied to both sides of the negative electrode current collector 42, but if necessary, it may be applied to only one side of the negative electrode current collector 42. Also, in this embodiment, the composition is applied such that the thickness of the central portion 41b of the negative electrode layer is thinner than the thickness of each of the pair of negative electrode layer ends 41a, but if necessary, the composition may be applied so that the thickness of the negative electrode active material layer 41 is substantially constant in the width direction.

[0067] For drying the coated composition, an appropriate drying apparatus can be used. Examples of drying apparatus include hot air drying ovens and infrared drying ovens.

[0068] Next, the dried negative electrode active material layer 41 is pressed. For pressing the negative electrode active material layer 41, for example, a roll press machine having a pair of roll-shaped molds with protrusions extending parallel to the axis of rotation along the outer surface can be used. These protrusions correspond to the shape of the grooves 43. Therefore, by passing the negative electrode current collector 42, which has the negative electrode active material layer 41 formed on both sides, between the pair of roll-shaped molds having the protrusions, the thickness and density of the negative electrode active material layer 41 can be adjusted to form the grooves 43.

[0069] Furthermore, instead of a roll press, a flat plate press with a pair of flat plates having protrusions on the molding surface corresponding to the shape of the groove 43 may be used to press the negative electrode active material layer 41. Also, the pressing of the negative electrode active material layer 41 is not limited to the single press described above, but may be a double press. When double pressing the negative electrode active material layer 41, the entire negative electrode active material layer 41 may be pressed with a pair of flat plates or roll-shaped dies without protrusions, and then the central part 41b of the negative electrode layer may be pressed with a pair of flat plates or roll-shaped dies without protrusions.

[0070] In this way, a negative electrode active material layer 41 can be formed in which a groove 43 forming a gap S is formed in the central part 41b of the negative electrode layer.

[0071] <Electrode fabrication process (Step S2)> The electrode fabrication process involves fabricating a wound electrode body 20 using a positive electrode 30, a negative electrode 40, and a separator 50. In the electrode fabrication process, first, the positive electrode 30, the negative electrode 40, and two separators 50 are stacked so that the separators 50 are interposed between the positive electrode active material layer 31 and the negative electrode active material layer 41.

[0072] When stacking, the positive electrode 30 and negative electrode 40 are stacked with a slight offset in the width direction such that the exposed portions of the positive electrode current collector 32, which forms the positive electrode connection portion 21 and the negative electrode connection portion 22, and the exposed portion of the negative electrode current collector 42 protrude outward from both ends in the width direction of the separator 50. Furthermore, when stacking, the central portion 41b of the negative electrode layer forms a gap S between it and the adjacent separator 50, and the stacking is carried out so that the gap S is located inside both ends in the width direction of the positive electrode active material layer 31. Furthermore, when stacking, it is preferable to stack the materials so that both ends in the width direction of the negative electrode active material layer 41 are located further out than both ends in the width direction of the positive electrode active material layer 31 and the negative electrode active material layer 41, respectively.

[0073] A wound electrode body 20 is obtained by winding the stacked positive electrode 30, negative electrode 40, and two separators 50 around a winding axis WL. Furthermore, the obtained wound electrode body 20 can be molded into a flat shape by pressing it. A flat wound electrode body 20 may also be made by winding the positive electrode 30, negative electrode 40, and separators 50 in a flat shape.

[0074] <Electrode connection process (step S3)> The electrode connection process involves connecting the wound electrode body 20 to the positive electrode terminal 15 and the negative electrode terminal 16. In the electrode connection process, first, a cover unit is formed by attaching the positive electrode terminal 15, to which the positive electrode current collector terminal 17 is connected, and the negative electrode terminal 16, to which the negative electrode current collector terminal 18 is connected, to the cover 12. Next, the positive electrode current collector terminal 17 of the cover unit is joined to the positive electrode connection portion 21 of the wound electrode body 20, and the negative electrode current collector terminal 18 of the cover unit is joined to the negative electrode connection portion 22 of the wound electrode body 20. For joining, for example, ultrasonic welding, resistance welding, or other joining methods can be used.

[0075] This manufacturing method may include an electrode drying step for drying the wound electrode body 20 connected to the positive electrode terminal 15 and the negative electrode terminal 16. In the electrode drying step, the wound electrode body 20 is heated to remove moisture from within the wound electrode body 20. Such an electrode drying step may be performed after the electrode connection step and before the liquid injection step, for example, after the electrode connection step and before the electrode housing step.

[0076] <Electrode housing process (step S4)> The electrode housing process involves housing the wound electrode body 20, to which the positive electrode terminal 15 and the negative electrode terminal 16 are connected, inside the battery case 10. In the electrode housing process, first, the wound electrode body 20, to which the positive electrode terminal 15 and the negative electrode terminal 16 are connected, is housed inside the case body 11. Next, the opening of the case body 11 is closed with the lid 12, and the case body 11 and the lid 12 are joined together. For joining, a joining method such as laser welding can be used.

[0077] <Injection process (Step S5)> The electrolyte injection process involves injecting electrolyte into the battery case 10, which houses the wound electrode body 20. In the electrolyte injection process, first, electrolyte is injected into the battery case 10 through the injection port 13. Then, the injected electrolyte is impregnated into the wound electrode body 20. After the impregnation of the wound electrode body 20 with electrolyte is complete, a sealing member 14 is welded to the injection port 13 to seal it, thereby obtaining the secondary battery 100.

[0078] In the electrolyte injection process, the injected electrolyte impregnates the wound electrode body 20 from both end faces in the width direction toward the center. Because the wound electrode body 20 has a large inter-electrode distance due to the formation of gaps S, when the electrolyte is impregnated, the electrolyte easily penetrates into the inside of the power generation unit 23 through the gaps S. In order to improve the impregnation of the electrolyte into the inside of the wound electrode body 20, it is preferable to perform the electrolyte injection process under reduced pressure inside the battery case 10.

[0079] Furthermore, after the liquid injection process, the secondary battery 100 is subjected to an initial charging process and an aging process under predetermined conditions to obtain a secondary battery 100 that is ready for use.

[0080] In the secondary battery 100 manufactured in this manner, the negative electrode 40 has a thinned negative electrode layer center 41b. As the negative electrode 40 expands during charging, the gap S shrinks due to the expansion of the negative electrode layer center 41b, reducing the distance between the electrodes. Therefore, the deposition of metallic lithium, which may occur when the distance between the electrodes increases due to the gap S, can be suppressed.

[0081] Therefore, according to this manufacturing method, it is possible to manufacture a secondary battery 100 that can improve the impregnation of the electrolyte into the wound electrode body 20 by increasing the inter-electrode distance between the positive electrode 30 and the negative electrode 40, while suppressing the deposition of metallic lithium that may occur when the inter-electrode distance is increased.

[0082] Embodiment 2 The secondary battery 200 according to Embodiment 2 will be described with reference to Figure 5. Figure 5 is a schematic cross-sectional view showing a part of the secondary battery according to Embodiment 2. Figure 5 is a cross-sectional view corresponding to Figure 2, and the illustration of components other than the wound electrode body 20 is omitted.

[0083] The secondary battery 200 according to this embodiment has the same configuration as the secondary battery 100 according to Embodiment 1, except that a part of the negative electrode current collector 42 is thinned and the thickness of the negative electrode active material layer 41 is substantially constant in the width direction. Therefore, in the description of Embodiment 2, the same reference numerals as in Embodiment 1 are used for the same components as in Embodiment 1 and their descriptions are omitted.

[0084] As shown in Figure 5, the secondary battery 200 has a negative electrode 40 in which a negative electrode active material layer 41 is formed on the surface of a negative electrode current collector 42. The thickness of the central part 42a of the negative electrode current collector 42, which corresponds to the central part 41b of the negative electrode layer, is thinner than the thickness of each of the pair of current collector ends 42b that sandwich the central part 42a. Furthermore, the thickness of the negative electrode active material layer 41 is substantially constant in the width direction. That is, the thickness of the central part 41b of the negative electrode layer is substantially the same as the thickness of each of the pair of negative electrode end ends 41a. In this way, because a negative electrode active material layer 41 with substantially constant thickness in the width direction is formed on the surface of the negative electrode current collector 42 in which the central part 42a of the current collector is thinned, a groove 43 forming a gap S is formed in the central part 41b of the negative electrode layer.

[0085] In a secondary battery 200 having such a negative electrode 40, the negative electrode active material layer 41 has a pair of negative electrode layer ends 41a in contact with adjacent separators 50, and the central portion 41b of the negative electrode layer forms a gap S between itself and the adjacent separator 50. The gap S is located on the inside of both ends in the width direction of the positive electrode active material layer 31.

[0086] Therefore, the secondary battery 200 according to this embodiment can obtain the same effects as the secondary battery 100 according to Embodiment 1. Furthermore, according to the secondary battery 200 according to this embodiment, the proportion of the negative electrode current collector 42 to the negative electrode 40 is reduced because the central part 42a of the current collector is thinned, so the energy density can be increased.

[0087] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its spirit. [Explanation of symbols]

[0088] 10 Battery case 11 Case body 12 Lid 13 Liquid injection port 14 Sealing material 15 Positive terminal 16 Negative terminal 17 Positive current collector terminal 18 Negative current collector terminal 20 Winding electrode body 21 Positive electrode connection part 22 Negative electrode connection part 23 Power generation part 30 positive electrode 31 Positive electrode active material layer 32 Positive electrode current collector 40 negative electrode 41 Negative electrode active material layer 41a Negative electrode layer end 41b Negative electrode layer center 42 Negative electrode current collector 42a Center of current collector 42b End of current collector 43 Groove 50 Separators 100, 200 secondary battery S Gap WL Winding shaft

Claims

1. The device has a wound electrode body in which a sheet-like positive electrode containing a positive electrode active material layer, a sheet-like negative electrode containing a negative electrode active material layer, and a sheet-like separator interposed between the positive electrode active material layer and the negative electrode active material layer are stacked and wound around a winding axis perpendicular to the longitudinal direction. The aforementioned negative electrode active material layer is A pair of negative electrode layer ends in the width direction parallel to the winding axis are in contact with the adjacent separator. The negative electrode layer is sandwiched between the ends of the pair of negative electrode layers, and the central portion of the negative electrode layer, including the center in the width direction of the negative electrode active material layer, forms a gap between it and the adjacent separator. The aforementioned gap is A secondary battery located on the inside of both ends in the width direction parallel to the winding axis of the positive electrode active material layer.

2. The aforementioned negative electrode active material layer is A secondary battery according to claim 1, wherein when X1 is the amount of gap in the uncharged wound electrode body, Y is the maximum expansion amount in the center of the negative electrode layer, and X2 is the maximum amount of gap corresponding to the deposition limit current value, which is the limit charging current value at which no material originating from the charge carrier is deposited, the following formula (1) is satisfied. X1-Y<X2...(1)

3. The aforementioned negative electrode active material layer is The secondary battery according to claim 1, wherein the thickness of the central portion of the negative electrode layer is thinner than the thickness of each of the ends of the pair of negative electrode layers.

4. The aforementioned negative electrode active material layer is Formed on the surface of the negative electrode current collector, The aforementioned negative electrode current collector is The secondary battery according to claim 1, wherein the thickness of the central part of the current collector corresponding to the central part of the negative electrode layer is thinner than the thickness of each of the pair of end parts of the current collector that sandwich the central part of the current collector.

5. The aforementioned negative electrode active material layer is The width is longer than the width of the positive electrode active material layer, The secondary battery according to claim 1, wherein both ends in the width direction are located outside the ends in the width direction of the positive electrode active material layer.

6. The aforementioned separator is, The width is longer than the width of the positive electrode active material layer and the negative electrode active material layer, The secondary battery according to claim 1, wherein both ends in the width direction are located outside the width directions of the positive electrode active material layer and the negative electrode active material layer, respectively.

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