Secondary battery and method of manufacturing the same

Abstract

[Project] Provides a highly durable secondary battery. [Solution] A positive electrode and a negative electrode are positioned opposite each other with a separator in between. The negative electrode comprises a negative electrode current collector, a negative electrode composite layer provided on the negative electrode current collector and containing graphite particles and a binder, and a resin layer provided between the surface of the negative electrode composite layer and the separator. The negative electrode composite layer comprises a surface layer in contact with the resin layer and a main layer located between the surface layer and the negative electrode current collector. In the XRD profile of the main layer, the degree of orientation, which is the intensity ratio obtained by dividing the intensity of the (110) plane of the graphite crystal by the intensity of the (002) plane, is 0.03 or higher. The content of the material constituting the resin layer in the surface layer is higher than the content of the binder in the main layer.

Description

Technology Field

[0001] The technology disclosed in this specification relates to a secondary battery and a method for manufacturing a secondary battery. Background Technology

[0002] The negative electrode used in a secondary battery comprises a conductive negative electrode current collector and a negative electrode composite layer provided on the negative electrode current collector. The negative electrode composite layer contains graphite particles having a layered crystal structure. The charging and discharging function is realized by the absorption and release of metal ions between the layers of graphite particles. Accordingly, a technique has been proposed to orient the layer planes of the graphite particles approximately perpendicular to the negative electrode current collector by utilizing the magnetic field orientation of the graphite particles (e.g., Patent Document 1). The closer the layer planes of the graphite particles are to being perpendicular to the negative electrode current collector, the more smoothly the absorption and release of metal ions between the layers of the graphite particles can be performed. Therefore, it is possible to improve the performance of the secondary battery. Prior art literature

[0003] Japanese Patent Publication No. 2018-517267 The problem to be solved

[0004] The surface layer of the negative electrode composite layer and the separator are bonded together. As described above, when the graphite particles included in the negative electrode composite layer are magnetically oriented, the angle formed between the layer plane of the graphite particles and the surface of the negative electrode current collector becomes larger. Consequently, the surface roughness of the surface layer of the negative electrode composite layer increases when magnetically oriented compared to when it is not. As a result, there is a concern that the adhesion between the surface layer of the negative electrode composite layer and the separator may deteriorate. means of solving the problem

[0005] A first aspect of the present invention is a secondary battery in which a positive electrode and a negative electrode face each other with a separator interposed therebetween. The negative electrode comprises a negative electrode current collector, a negative electrode composite layer provided on the negative electrode current collector and comprising graphite particles and a binder, and a resin layer provided between the surface of the negative electrode composite layer and the separator. The negative electrode composite layer comprises a surface layer in contact with the resin layer and a main layer located between the surface layer and the negative electrode current collector. In the XRD profile of the main layer, the degree of orientation, which is the intensity ratio obtained by dividing the intensity of the (110) plane of the graphite crystal by the intensity of the (002) plane, is 0.03 or higher. The content of the material constituting the resin layer included in the surface layer is higher than the content of the binder included in the main layer.

[0006] According to the above structure, the content of the material constituting the resin layer in the surface layer is higher than the content of the binder in the main layer. Additionally, the concave areas present on the surface of the surface layer can be filled by the resin layer. Accordingly, the flatness of the surface layer where the resin layer is formed can be improved compared to the case where the resin layer is not formed. As a result, it becomes possible to improve the adhesion between the surface layer of the negative electrode composite layer and the separator. Brief explanation of the drawing

[0007] Figure 1 is a perspective view of a secondary battery (1). Figure 2 is an exploded plan view of the electrode body (108). Figure 3 is a cross-sectional view of the negative electrode (114). Figure 4 is a flowchart describing the method of manufacturing a secondary battery (1). Figure 5 is a figure illustrating an example of the preparation process of the first paste. Figure 6 is a diagram illustrating steps S20-S55. Figure 7 is a diagram illustrating steps S100 and S110. Figure 8 is a diagram illustrating the heating press process. Figure 9 is a cross-sectional view of the negative electrode of the first comparative example. FIG. 10 is a cross-sectional view showing the negative electrode composite layer (10) formed by step S50. FIG. 11 is a cross-sectional view showing a negative electrode composite layer (10) in which a resin layer (12) is formed by step S55. Figure 12 is a figure illustrating the orientation angle of graphite particles (11). Specific details for implementing the invention

[0008] In the second embodiment of the present technology, in addition to the first embodiment above, when the angle formed between the layer surface of a graphite particle having a layered structure and the surface of a negative electrode current collector is taken as the orientation angle, the average value of the orientation angle in the surface layer may be 45 degrees or more.

[0009] According to the above configuration, in the surface layer serving as an inlet for metal ions, metal ions migrating from the positive electrode can be made to easily enter between the graphite particles. This makes it possible to reduce the internal resistance of the secondary battery.

[0010] In the third embodiment of the present technology, in addition to the first or second embodiment above, the material constituting the resin layer may be the same as the material constituting the binder.

[0011] According to the above configuration, it is possible to provide adhesive strength to the resin layer.

[0012] In the fourth embodiment of the present technology, in addition to the first to third embodiments above, the material constituting the resin layer may include an ion-permeable resin.

[0013] According to the above configuration, the movement of metal ions is not inhibited by the resin layer.

[0014] In the fifth embodiment of the present technology, in addition to the first to fourth embodiments above, the material constituting the resin layer and the binder may include acrylic resin or polyvinylidene fluoride resin.

[0015] The technology disclosed in this specification is implemented as a method for manufacturing a secondary battery. A sixth aspect of the technology is a method for manufacturing a secondary battery in which a positive electrode and a negative electrode face each other via a separator. The manufacturing method comprises a process of forming an adhesive layer on the surface of a separator. The manufacturing method comprises a process of forming a negative electrode composite layer by coating a paste containing graphite particles and a solvent onto the surface of a negative electrode current collector. The manufacturing method comprises a magnetic field orientation process of applying an external magnetic field to the negative electrode composite layer. The manufacturing method comprises a drying process of drying the negative electrode composite layer after the magnetic field orientation process. The manufacturing method comprises a resin layer forming process of forming a resin layer composed of an ion-permeable resin on the surface of the negative electrode composite layer after the drying process. The manufacturing method comprises a process of heating the adhesive layer and the resin layer while they are in contact with each other.

[0016] According to the above configuration, the adhesive layer and the negative electrode composite layer can be bonded by interposing a highly fluid resin layer. The resin layer can be moved to flow into the concave portion formed by the magnetic field orientation of graphite particles. This makes it possible to improve the adhesion between the negative electrode composite layer and the separator.

[0017] In the seventh embodiment of the present technology, in addition to the sixth embodiment above, the resin layer forming process may include a process of coating an ion-permeable resin onto the surface of the negative electrode composite layer.

[0018] In the eighth embodiment of the present technology, in addition to the sixth or seventh embodiment above, the thickness of the resin layer may be greater than the arithmetic mean roughness of the surface of the negative electrode composite layer.

[0019] According to the above configuration, the resin layer can have a sufficient volume to fill the concave portions existing on the surface of the negative electrode composite layer.

[0020] In the ninth embodiment of the present technology, in addition to the sixth to eighth embodiments above, the resin layer may include an acrylic resin or a polyvinylidene fluoride resin.

[0021] [Example]

[0022] Structure of the secondary battery (1)

[0023] FIG. 1 shows a perspective view of a secondary battery (1) related to the present embodiment. In the present embodiment, as one of the preferred embodiments of the secondary battery (1), it is described by embodying it as a lithium-ion secondary battery. As shown in FIG. 1, the secondary battery (1) comprises a housing (102), a positive electrode terminal (104), a negative electrode terminal (106), an electrode body (108), and a non-aqueous electrolyte (110). The housing (102) accommodates the electrode body (108) and the non-aqueous electrolyte (110). The housing (102) comprises a housing body (102a) and a cover plate (102b). The housing body (102a) is a box body having an opening (102c) at the top. The cover plate (102b) is mounted on the opening (102c) and blocks the opening (102c).

[0024] Each of the positive terminal (104) and the negative terminal (106) is mounted on a cover plate (102b). One end (104a) of the positive terminal (104) is exposed to the outside of the housing (102). The other end (104b) of the positive terminal (104) is connected to the positive connection part (112a) of the electrode body (108) inside the housing (102). One end (106a) of the negative terminal (106) is exposed to the outside of the housing (102). The other end (106b) of the negative terminal (106) is connected to the negative connection part (114a) of the electrode body (108) inside the housing (102).

[0025] FIG. 2 shows an exploded plan view of an electrode body (108). The electrode body (108) is provided with a positive electrode (112), a negative electrode (114), and a separator (116). Each of the positive electrode (112), the negative electrode (114), and the separator (116) is in the shape of a long sheet. The positive electrode (112) and the negative electrode (114) are stacked with the separator (116) interposed. The electrode body (108) is formed by winding the stack of the positive electrode (112) and the negative electrode (114). However, the electrode body (108) does not necessarily have to be a wound electrode body. The electrode body (108) may be, for example, a so-called stacked electrode body in which a plurality of positive electrodes (112) and a plurality of negative electrodes (114) are alternately stacked with the separator (116) interposed.

[0026] The positive electrode (112) is provided with a positive electrode current collector (118) and a positive electrode active material sheet (120). The positive electrode current collector (118) is a conductive sheet. The positive electrode current collector (118) is, for example, an aluminum foil. The positive electrode active material sheet (120) is disposed on the surface of the positive electrode current collector (118). The positive electrode active material sheet (120) is provided with a positive electrode active material. As a positive electrode active material, for example, a lithium composite oxide may be provided. On one side edge in the width direction of the positive electrode (112), a positive electrode exposed portion (118a) is provided in which the positive electrode current collector (118) is exposed. The positive electrode exposed portion (118a) forms a positive electrode connection portion (112a) by being wound and overlapped when forming the electrode body (108).

[0027] The negative electrode (114) is provided with a negative electrode current collector (122) and a negative electrode composite layer (10). The negative electrode current collector (122) is a conductive sheet. The negative electrode current collector (122) is, for example, a copper foil. The negative electrode composite layer (10) is disposed on the surface of the negative electrode current collector (122). Specific details regarding the negative electrode (114) will be described later. On one side edge in the width direction of the negative electrode (114), a negative electrode exposure portion (122a) is provided in which the negative electrode current collector (122) is exposed. When forming the electrode body (108), the negative electrode exposure portion (122a) is wound and overlapped to form a negative electrode connection portion (114a).

[0028] The separator (116) is configured to allow a charge carrier (here, lithium ions) to pass through. Examples of separators (116) include porous polymer membranes such as porous polyethylene membranes, porous polypropylene membranes, porous polyolefin membranes, and porous polyvinyl chloride membranes, as well as lithium ion conductive polymer electrolyte membranes. These separators (116) may be used alone or in combination.

[0029] An adhesive layer (13) is formed on the surface of the separator (116). The specific details of the adhesive layer (13) will be described later.

[0030] The non-aqueous electrolyte (110) penetrates into the interior of the electrode body (108). The non-aqueous electrolyte (110) includes a non-aqueous solvent and a supporting salt. Examples of non-aqueous solvents include carbonates, ethers, esters, nitriles, sulfones, and lactones. Additionally, examples of supporting salts include lithium salts containing fluorine. Each of the non-aqueous solvent and the supporting salt may be composed of a single type of material or a plurality of types of materials.

[0031] Structure of the negative pole (114)

[0032] FIG. 3 shows a cross-sectional view of the negative electrode (114). FIG. 3 also shows a separator (116). In FIG. 3, the direction parallel to the negative electrode current collector (122) is defined as the x and y directions. Also, the direction perpendicular to the negative electrode current collector (122) (the thickness direction of the negative electrode (114)) is defined as the z direction. Additionally, for the purpose of clarity in illustration, only one of the components arranged in multiple places may be labeled. The same applies to subsequent drawings. The negative electrode (114) mainly comprises a negative electrode current collector (122), a negative electrode composite layer (10), a resin layer (12), and an adhesive layer (13).

[0033] The resin layer (12) is provided between the surface (10s) of the negative electrode composite layer (10) and the adhesive layer (13). The resin layer (12) is composed of resin and is a layer that does not contain additive particles (17). The resin material of the resin layer (12) may be an ion-permeable resin. Accordingly, the movement of lithium ions is not inhibited by the resin layer (12). Also, in FIG. 3, the resin layer (12) is similarly represented as a small white circle for clarity of illustration. The composition of the resin layer (12) may vary, and, for example, may be the same as the composition of the binder (14) described later. Accordingly, it is possible to have adhesive strength in the resin layer (12). In this embodiment, the composition of the resin layer (12) is PVdF, which is the same as the binder (14).

[0034] The resin layer (12) has an average thickness (AT2). The average thickness (AT2) of the resin layer (12) is greater than the surface roughness (Ra2) of the surface (10s) of the negative electrode composite layer (10). For example, if the surface roughness (Ra2), defined as the arithmetic mean roughness, is 1.5 μm, the average thickness (AT2) is greater than 1.5 μm. The surface roughness (Ra2) will be described later.

[0035] The adhesive layer (13) is in contact with the separator (116). The adhesive layer (13) is a layer containing resin (16) and additive particles (17). That is, the adhesive layer (13) is a layer containing resin (16) as a base material. Also, in FIG. 3, for clarity of illustration, the resin (16) is similarly represented by a small white circle. Also, the additive particles (17) are similarly represented by a gray circle. The composition of the resin (16) may vary, and, for example, may be the same as the composition of the binder (14) described later. In this embodiment, the composition of the resin is PVdF, which is the same as the binder (14).

[0036] The material of the additive particle (17) is preferably heat-resistant and insulating, and also highly durable against the electrolyte. Examples of the material of the additive particle (17) include metal compounds or polymer compounds such as polyimide. Suitable examples of metal compounds include α-alumina (Al2O3), boehmite (Al2O3·H2O), magnesium hydroxide (Mg(OH)2), and magnesium carbonate (MgCO3). Additionally, one or more of these materials may be used for the additive particle (17).

[0037] The additive particles (17) included in the adhesive layer (13) are bonded together by the resin (16). Accordingly, since a heat-resistant porous layer can be formed, it is possible to prevent an internal short circuit of the secondary battery (1).

[0038] The negative electrode composite layer (10) is provided on the negative electrode current collector (122). The negative electrode composite layer (10) comprises graphite particles (11), a binder (14), and a conductive agent not shown. For the graphite particles (11), a graphite-based carbon material such as natural graphite, artificial graphite, or an amorphous form thereof may be used. For example, the graphite particles (11) may be flat, flaky graphite, or spheroidized graphite formed by spheroidizing flaky graphite. In this embodiment, flaky graphite is used as the graphite particles (11). This is because flaky graphite is flat and can be stably oriented by applying a magnetic field.

[0039] In FIG. 3, graphite particles (11) are shown as hatching. The median diameter of the graphite particles (11) is, for example, within the range of 15 to 40 micrometers. Furthermore, the median diameter referred to here means the particle size (D50) at 50% of the cumulative value in the volume-based particle size distribution measured by the laser diffraction and scattering method. In this embodiment, the median diameter of the graphite particles (11) is set to 20 micrometers.

[0040] The binder (14) is a component for binding graphite particles (11) together. The binder (14) is mixed into the negative electrode composite layer (10). The binder (14) contains a resin in the form of particles. In FIG. 3, the binder (14) is similarly represented by a white circle. Examples of binders (14) include polyvinylidene fluoride (PVdF), various acrylic resins, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), etc.

[0041] As an acrylic resin, a homopolymer obtained by polymerizing monomers such as acrylic acid, methacrylic acid, acrylamide, methacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, methacrylate, methyl methacrylate, ethylhexyl acrylate, and butyl acrylate into one type may be used. Additionally, the acrylic resin may be a copolymer obtained by polymerizing two or more of the above monomers.

[0042] The binder (14) may be composed of a single type of material or a plurality of types of materials. In this embodiment, the binder (14) is made of PVdF.

[0043] The negative electrode composite layer (10) is provided with a surface layer (10A) and a main layer (10B). The surface layer (10A) is a layer in contact with the resin layer (12). The main layer (10B) is a layer located between the surface layer (10A) and the negative electrode current collector (122). The roughness of the surface (10s) of the surface layer (10A) is the surface roughness (Ra2). In addition, various indicators can be used for the surface roughness (Ra2). In this embodiment, the arithmetic mean roughness is used as the surface roughness. The arithmetic mean roughness can be measured according to the JIS B 0601:2001 standard. The same applies to the surface roughness (Ra1) described later.

[0044] In the surface (10s), there are a number of concave portions between the graphite particles (11). These concave portions are filled with a resin layer (12). In other words, the surface layer (10A) contains a resin layer (12). Here, the content of the resin layer (12) contained in the surface layer (10A) is called the resin layer content. Also, the content of the binder (14) contained in the main layer (10B) is called the binder content. The resin layer content is higher than the binder content. For example, the resin layer content may be 50% or more, and the binder content may be 5% or less.

[0045] Based on the above, it can be seen that the surface layer (10A) and the main layer (10B) can be distinguished depending on whether the resin layer content is higher than the binder content. That is, it is possible to define the boundary between the surface layer (10A) and the main layer (10B) based on the resin layer content.

[0046] Orientation of graphite particles

[0047] As described above, the graphite particle (11) is flat, flake-like graphite. The flake-like graphite is composed of a number of stacked plate-shaped (graphene) structures having hexagonal crystals. Therefore, the graphite particle (11) has a roughly plate-like shape. The graphite particle (11) has a (002) plane and a (110) plane. The (002) plane is a layer plane (a plane horizontal to the graphite layer) of the layered graphite particle. The (002) plane is a plane horizontal to the carbon network of the graphene sheet constituting the graphite particle (11). The (110) plane is a cross-sectional (plane vertical to the graphite layer) of the layered graphite particle. The (110) plane is a plane vertical to the (002) plane of the graphite particle. That is, as shown in FIG. 3, the graphite particles (11) have a long, slender shape such that the length in the direction parallel to the layer plane is longer than the length in the direction perpendicular to the layer plane.

[0048] Here, the “orientation degree” of the graphite particle (11) is defined. The peak intensity originating from the (110) plane of the graphite particle (11) by powder X-ray diffraction measurement is denoted as PI1. Also, the peak intensity originating from the (002) plane is denoted as PI2. The peak intensity (PI1) originates from the ab-axis direction of the graphite crystal. The peak intensity (PI2) originates from the c-axis direction of the graphite crystal. Furthermore, the intensity ratio of the peak intensity (PI1) divided by the peak intensity (PI2) is defined as the orientation degree. That is, it is defined as “orientation degree = PI1 / PI2”.

[0049] The greater the degree of orientation, the more graphite particles (11) are oriented in the negative electrode composite layer (10) such that the c-axis is parallel to the negative electrode current collector (122) (the xy plane direction of Fig. 3). That is, the greater the degree of orientation, the higher the proportion of graphite particles (11) that are standing upright at an angle with respect to the surface of the negative electrode current collector (122).

[0050] And according to the inventors' research, it was found that when the magnetic field orientation described below is not performed, the degree of orientation of the main layer (10B) becomes less than 0.03. And it was found that by performing magnetic field orientation, the degree of orientation of the main layer (10B) can be made greater than 0.03. That is, it is possible to determine whether magnetic field orientation is being performed depending on whether the degree of orientation of the main layer (10B) is greater than 0.03.

[0051] <Method for manufacturing secondary battery (1)>

[0052] Using the flowchart of FIG. 4 and the schematic diagrams of FIG. 5 to 8, a method for manufacturing the above-described secondary battery (1) will be explained.

[0053] In step S2, an adhesive layer (13) is formed on the surface of the separator (116) (process for forming the adhesive layer). The paint for forming the adhesive layer (13) is a paste-like paint in which a resin (16), additive particles (17), and a solvent are mixed and dispersed. This paint is applied to the surface of the separator (116). Conventional general coating devices can be used for the coating process. Examples of coating devices include gravure coaters, slit coaters, die coaters, comma coaters, dip coaters, etc. After coating, the solvent is removed from the coated material using a suitable drying means. Accordingly, the adhesive layer (13) is completed.

[0054] In step S10, a first paste is prepared (paste preparation process). The first paste is a paste containing graphite particles (11). The paste may be in the form of a slurry or an ink.

[0055] Using FIG. 5, an example of the preparation process for the first paste is described. Graphite particles (11), a binder (14), and a conductive agent (15) are each supplied to a mixing machine (210) in predetermined amounts. A solvent is added to the materials supplied to the mixing machine (210), and the mixture is mixed. Accordingly, the first paste is completed. The mixing machine (210) may be of various types, and, for example, a planetary mixer may be used.

[0056] In steps S20-S55, a process of forming a negative electrode composite layer (10) on the first surface (122s1) of the negative electrode current collector (122) is performed. This is explained in detail using FIG. 6. In step S20, a first coating process is performed. In the first coating process, a first paste containing graphite particles (11) is coated onto the surface of the negative electrode current collector (122). Specifically, the negative electrode current collector (122) is sent out at a constant speed in the conveying direction (T1) from a roll (122R) on which the negative electrode current collector (122) is wound. Then, the first paste is applied onto the negative electrode current collector (122) using a first coater (211). Accordingly, a first layer (L1) is formed. Various types of coaters can be used for the first coater (211). For example, the first coater (211) may be a slot die coater.

[0057] In step S40, an external magnetic field is applied to the first layer (L1) (magnetic field orientation process). Specifically, a magnetic field generating means (221) is used to apply a magnetic field such that the direction of the magnetic field lines (MF) is approximately perpendicular to the surface of the negative current collector (122). The magnetic field generating means (221) may be a permanent magnet or an electromagnet. Accordingly, the (002) plane of the graphite particles (11) can be oriented so that it is approximately parallel to the direction of the magnetic field lines.

[0058] In step S50, the first layer (L1) is dried (drying process). Specifically, it is dried by irradiating it with hot air (HA). Furthermore, the means used in the drying process are not limited to hot air, and drying by low humidity air, vacuum, infrared rays, far infrared rays, electron beams, etc., can be used alone or in combination. Through the drying process, the solvent can be removed from the first layer (L1). Accordingly, a negative electrode composite layer (10) can be formed on the first surface (122s1) of the negative electrode current collector (122).

[0059] FIG. 10 shows a negative electrode composite layer (10) formed by step S50. FIG. 10 is a cross-sectional view similar to FIG. 3. In this step, the surface (10s) of the negative electrode composite layer (10) is exposed. And, because the graphite particles (11) are oriented in a magnetic field, the surface roughness (Ra2) of the surface (10s) is large.

[0060] In step S55, a resin layer (12) is formed on the surface of the negative electrode composite layer (10) after the drying process (resin layer formation process). Specifically, a paint in which a resin and a solvent are mixed and dispersed is prepared. This paint is applied to the surface of the negative electrode composite layer (10). If necessary, a drying process is performed after application. Furthermore, since conventional general devices can be used for the application and drying processes in step S55, a detailed description is omitted.

[0061] FIG. 11 shows a negative electrode composite layer (10) in which a resin layer (12) is formed by step S55. FIG. 11 is a cross-sectional view similar to FIG. 10. Since the concave portions present on the surface (10s) are filled by the resin layer (12), the surface roughness of the surface (10s) is reduced.

[0062] In steps S60-S95, a process of forming a negative electrode composite layer (10) on the second surface (122s2) of the negative electrode current collector (122) is performed. Additionally, since the contents of each of steps S60, S80, S90, and S95 are the same as the contents of steps S20, S40, S50, and S55 described above, the explanation is omitted. Accordingly, a negative electrode (114) is formed. The negative electrode (114) has a configuration in which a negative electrode composite layer (10) is disposed on both sides of the negative electrode current collector (122).

[0063] In step S100, the negative electrode (114) is pressed (press process). As shown in FIG. 7, for example, a roll press device (230) is used in the press process. The roll press device (230) is configured to roll the negative electrode (114) passing between a pair of rollers (231). Accordingly, the density and thickness of the negative electrode composite layer (10) can be adjusted.

[0064] In step S110, slit processing is performed. As shown in FIG. 7, in slit processing, the negative electrode (114) being sent in the feed direction (T1) is cut by a cutter (240). Accordingly, the width of the negative electrode (114) in the short length direction can be made to a width corresponding to the size of the secondary battery (1).

[0065] In step S120, an electrode body (108) is manufactured (electrode body manufacturing process). As shown in FIG. 2, in the electrode body manufacturing process, a positive electrode (112), a negative electrode (114), and a separator (116) are stacked so that the positive electrode (112) and the negative electrode (114) face each other with a separator (116) interposed. Specifically, the stack of the positive electrode (112) and the negative electrode (114) is wound.

[0066] In step S130, the electrode body (108) is pressed while being heated (heating press process). As shown in FIG. 8, for example, a flat plate press device (250) is used in the heating press process. Accordingly, the electrode body (108) can be compressed into a flat shape having a predetermined thickness. In addition, in the heating press process, as shown in FIG. 3, the adhesive layer (13) and the resin layer (12) can be heated while in contact with each other. Thus, the separator (116) and the negative electrode composite layer (10) can be bonded together with the resin layer (12) interposed. Furthermore, this heating press process may be performed after the electrolyte injection process (S150) described later.

[0067] In step S140, an electrode housing process is performed. Specifically, as shown in FIG. 1, a positive electrode terminal (104) and a negative electrode terminal (106) mounted on a cover plate (102b) are welded to each end of the electrode body (108). The electrode body (108) is housed inside the housing body (102a) through an opening (102c). After that, the housing body (102a) and the cover plate (102b) are joined together using laser welding or the like.

[0068] In step S150, a non-aqueous electrolyte is injected into the housing (102) through an injection port not shown (electrolyte injection process). Then, the injection port is hermetically sealed using a sealing member. By the above, the secondary battery (1) is completed.

[0069] Assignment

[0070] The problem is explained using the first comparative example of FIG. 9 and the present embodiment of FIG. 3. FIG. 9 is a cross-sectional view similar to FIG. 3. The first comparative example ( FIG. 9) is different from the present embodiment ( FIG. 3) in that magnetic field orientation is not performed on the negative electrode composite layer (310).

[0071] In the negative electrode composite layer (310) of the first comparative example (Fig. 9), magnetic field orientation is not performed. Therefore, the layer surface (longer side) of the graphite particles (11) is not oriented in a direction (z-direction) that is approximately perpendicular to the negative electrode current collector (122). Because of this, the path length (PL1) of the pores reaching from the separator (116) to the negative electrode current collector (122) increases. As a result, the curvature of the negative electrode composite layer (310) increases. Here, the curvature is the ratio of the path length (PL1) to the straight distance (SD) connecting the separator (116) to the negative electrode current collector (122). As the curvature increases, the absorption and release of Li ions between the layers of graphite particles (11) are not performed smoothly. Therefore, as the degree of curvature increases, the internal resistance of the secondary battery (1) increases, and the performance of the secondary battery (1) deteriorates.

[0072] Meanwhile, in the negative electrode composite layer (10) of the present embodiment (Fig. 3), magnetic field orientation is performed. Accordingly, the layer surface (longer length surface) of the graphite particles (11) is oriented in a direction (z-direction) that is approximately perpendicular to the negative electrode current collector (122). Because of this, the path length (PL2) of the pores in the present embodiment can be made shorter than the path length (PL1) of the pores in the first comparative example. Therefore, in the present embodiment, the curvature is smaller than in the first comparative example, so the internal resistance can be reduced. However, when the graphite particles (11) are magnetically oriented, the angle formed between the layer surface of the graphite particles (11) and the surface of the negative electrode current collector (122) becomes larger. Accordingly, the pointed tip surface ((110) surface) of the graphite particle (11) ends up protruding upward from the surface of the negative electrode composite layer (10) (see region (A2)). As a result, the surface roughness (Ra2) of the negative electrode composite layer (10) of this embodiment becomes larger than the surface roughness (Ra1) of the negative electrode composite layer (310) of the first comparative example. There is a risk that the adhesion between the surface of the negative electrode composite layer that is magnetically oriented and the separator (116) will deteriorate compared to the adhesion between the surface of the negative electrode composite layer that is magnetically oriented and the separator (116).

[0073] <Effect>

[0074] In the negative electrode composite layer (10) of the present embodiment (Fig. 3), the degree of orientation is 0.03 or higher. That is, the negative electrode composite layer (10) is magnetically oriented. Therefore, since the curvature of the negative electrode composite layer (10) is reduced, the internal resistance can be reduced. In addition, the resin layer content ratio in which the resin layer (12) is included in the surface layer (10A) is higher than the binder content ratio in which the binder (14) is included in the main layer (10B). Accordingly, a sufficient amount of resin layer (12) can be placed on the surface layer (10A) to fill the concave portions present on the surface (10s). Thus, the fine concave portions of the surface (10s) can be sufficiently filled by the resin layer (12). That is, since the surface roughness of the surface (10s) can be reduced by the resin layer (12), the adhesion between the surface layer (10A) and the adhesive layer (13) can be increased. Therefore, it is possible to improve the adhesion between the negative electrode composite layer (10) and the separator (116).

[0075] The average thickness (AT2) of the resin layer (12) is greater than the surface roughness (Ra2) of the surface (10s) of the negative electrode composite layer (10). Accordingly, the resin layer (12) can have a sufficient volume to fill the concave portions present on the surface (10s). Thus, a state can be created in which the resin layer (12) is sufficiently filled into the gap formed by the magnetic field orientation of the graphite particles (11) (refer to region (A0)). Since the flatness of the surface (10s) of the negative electrode composite layer (10) can be increased by the resin layer (12), it becomes possible to improve the adhesion between the negative electrode composite layer (10) and the separator (116).

[0076] Orientation angle of graphite particles

[0077] Using FIG. 12, the “orientation angle” of the graphite particle (11) is explained. FIG. 12 is a cross-sectional view similar to FIG. 3. FIG. 12 describes a representative graphite particle (11). The graphite particle (11) has a layer surface (11p). As described above, the layer surface (11p) is a (002) plane that is horizontal to the graphite layer. Here, the direction in which the layer surface (11p) extends is indicated by a virtual line (IL). In addition, the angle formed by the virtual line (IL) and the surface of the negative electrode current collector (122) is called the orientation angle (OA).

[0078] The orientation angle (OA) can be measured using a cross-sectional SEM image of the negative electrode composite layer (10). For example, a plurality (e.g., 50) of graphite particles (11) may be extracted from the surface layer (10A) in a cross-section cut at a random location. Then, the orientation angle (OA) may be measured for each of the extracted plurality of graphite particles (11), and the arithmetic mean of the measurement results may be calculated.

[0079] And according to the research of the inventors, it was found that when the resin layer forming process (steps S55, S95) described above is not performed, the average value of the orientation angle of the surface layer (10A) in the secondary battery (1) after completion is less than 45 degrees. On the other hand, it was found that when the resin layer forming process is performed, the average value of the orientation angle of the surface layer (10A) in the secondary battery (1) after completion can be made 45 degrees or more. Accordingly, in the surface layer (10A) which serves as the inlet for lithium ions, lithium ions that have moved from the positive electrode (112) can easily enter between the graphite particles (11). It becomes possible to reduce the internal resistance of the secondary battery (1).

[0080] The reason why the average value of the orientation angle of the surface layer (10A) can be made 45 degrees or more by forming a resin layer (12) is explained. When the graphite particles (11) of the surface layer (10A) are oriented in a magnetic field, the average value of the orientation angle of the graphite particles (11) becomes 45 degrees or more. And, if the resin layer (12) is not formed, pressure is applied to the surface layer (10A) by a press process (step S100) or a heat press process (step S130), causing the graphite particles (11) to collapse. Therefore, the average value of the orientation angle of the graphite particles (11) becomes less than 45 degrees. On the other hand, when the resin layer (12) is formed, a state can be created in which the resin layer (12) is sufficiently filled in the gap formed by the magnetic field orientation of the graphite particles (11) (refer to FIG. 11, region (A3)). That is, the graphite particles (11) can be supported by the resin layer (12). Accordingly, even when pressure is applied to the surface layer (10A), it is possible to make it difficult to knock over the graphite particles (11).

[0081] That is, depending on whether the average value of the orientation angle of the surface layer (10A) is 45 degrees or more, it is possible to determine whether the resin layer (12) is formed and whether the resin layer formation process is performed.

[0082] Although embodiments have been described in detail above, they are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific embodiments exemplified above. The technical elements described in this specification or drawings exhibit technical utility either individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings achieves multiple objectives simultaneously, and possesses technical utility in itself by achieving one of those objectives.

[0083] <Variation Example>

[0084] The timing for performing the resin layer formation process (steps S55, S95) is not limited to the timing of the present embodiment and may vary. For example, the resin layer formation process may be performed before the drying process (step S50) or after the pressing process (step S100). Explanation of the symbols

[0085] 1: Secondary battery 10: Poison composite layer 10A: Surface layer 10B: Main floor 11: Graphite particles 12: Resin layer 13: Adhesive layer 14: Binder 17: Added particles 112: Straight Drama 114: Buguk 116: Separator 122: Buguk entire house

Claims

Claim 1 A secondary battery having a positive electrode and a negative electrode facing each other via a separator, wherein the negative electrode comprises a negative electrode current collector, a negative electrode composite layer provided on the negative electrode current collector and comprising graphite particles and a binder, and a resin layer provided between the surface of the negative electrode composite layer and the separator, wherein the negative electrode composite layer comprises a surface layer in contact with the resin layer and a main layer located between the surface layer and the negative electrode current collector, wherein in the XRD profile of the main layer, the degree of orientation, which is the intensity ratio of the (110) plane of the graphite crystal divided by the intensity of the (002) plane, is 0.03 or higher, and the content of the material constituting the resin layer included in the surface layer is higher than the content of the binder included in the main layer. Claim 2 A secondary battery according to claim 1, wherein, when the angle formed between the layer surface of the graphite particles having a layered structure and the surface of the negative electrode current collector is defined as the orientation angle, the average value of the orientation angle in the surface layer is 45 degrees or more. Claim 3 In claim 1, the material constituting the resin layer is the same as the material constituting the binder, in a secondary battery. Claim 4 In claim 3, the material constituting the resin layer comprises an ion-permeable resin, in a secondary battery. Claim 5 In claim 3, the material constituting the resin layer and the binder comprises an acrylic resin or a polyvinylidene fluoride resin, in a secondary battery. Claim 6 A method for manufacturing a secondary battery in which a positive electrode and a negative electrode face each other via a separator, comprising: a process of forming an adhesive layer on the surface of the separator; a process of forming a negative electrode composite layer by coating a paste containing graphite particles and a solvent on the surface of a negative electrode current collector; a magnetic field alignment process of applying an external magnetic field to the negative electrode composite layer; a drying process of drying the negative electrode composite layer after the magnetic field alignment process; a resin layer forming process of forming a resin layer composed of an ion-permeable resin on the surface of the negative electrode composite layer after the drying process; and a process of heating the adhesive layer and the resin layer while in contact with each other. Claim 7 A method for manufacturing a secondary battery according to claim 6, wherein the resin layer forming process comprises a process of coating the surface of the negative electrode composite layer with the ion-permeable resin. Claim 8 A method for manufacturing a secondary battery according to claim 6, wherein the thickness of the resin layer is greater than the arithmetic mean roughness of the surface of the negative electrode composite layer. Claim 9 A method for manufacturing a secondary battery according to claim 6, wherein the resin layer comprises an acrylic resin or a polyvinylidene fluoride resin.

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