Secondary battery electrode and method for manufacturing secondary battery electrode

Abstract

To provide an electrode for a secondary battery, achieving improved battery performance by a reduction in ion transfer resistance, and a method of manufacturing an electrode for a secondary battery.SOLUTION: An electrode for a secondary battery includes a current collector foil 10, and a mixture layer 20 formed on the surface of the current collector foil 10, the mixture layer containing graphite particles 30 that allow magnetic field orientation. A hole path ratio that is the ratio of each hole passage length of a hole passage L1 to a hole passage L2 among hole passages L1, L2 formed from one side of the mixture layer 20 toward the other side thereof is 0.8-1.2, the passage L1 passing an inner pore 40 formed inside the graphite particle 30 and the passage L2 passing an outer pore 50 formed between particles of the graphite particles 30.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode for a secondary battery and a method for manufacturing the electrode for a secondary battery. [Background technology]

[0002] Secondary batteries are widely used as so-called portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles. Among secondary batteries, lithium-ion secondary batteries, which are lightweight and have high energy density, are particularly suitable for use as high-output power sources for driving vehicles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles. Lithium-ion secondary batteries are secondary batteries that can be charged and discharged by the movement of lithium ions in an electrolyte between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate), which absorb and release lithium ions.

[0003] In order to achieve even higher output, it is necessary to reduce the internal resistance of such secondary batteries. The internal resistance in the electrodes that make up secondary batteries is composed of multiple resistance components, such as electron transfer resistance at the active material / active material interface and the active material / current collector interface, ion transfer resistance, and interfacial transfer resistance at the active material / electrolyte interface. Among these, ion transfer resistance is a resistance component that is affected by the ease of movement of lithium ions in the electrolyte (ionic conductivity).

[0004] Electrodes used in secondary batteries such as lithium-ion secondary batteries include a conductive current collector foil and a composite layer held on the current collector foil and containing an active material, etc. For example, graphite-based materials such as natural graphite, artificial graphite, and amorphous forms of natural graphite and artificial graphite are widely used as electrode materials for secondary batteries because they have high capacity, small irreversible capacity, and excellent electrical conductivity.

[0005] Patent Document 1 discloses a sealed nonaqueous electrolyte secondary battery having the following characteristics: The nonaqueous electrolyte of the sealed nonaqueous electrolyte secondary battery described in Patent Document 2 contains a gas generating agent that can decompose and generate gas when a predetermined battery voltage is exceeded, and the battery case is equipped with a current interruption mechanism that activates when the pressure inside the battery case increases due to the generation of the gas. Furthermore, the positive electrode of this sealed nonaqueous electrolyte secondary battery includes a positive electrode composite layer that contains at least a positive electrode active material. It is also described that the positive electrode composite layer contains, as a conductive agent, conductive carbon particles and expanded graphite with an average pore diameter of 0.2 μm to 0.5 μm.

[0006] In the technology described in Patent Document 1, the (002) plane (layer plane) of the expanded graphite is oriented by magnetic field orientation so that it stands up relative to the positive electrode current collector, and pores (internal pores) formed between the (002) planes of the expanded graphite are arranged so as to penetrate the expanded graphite in the vertical direction relative to the positive electrode current collector (for example, arranged so that the angle between the central axis of the pore and the surface of the positive electrode current collector is 60° to 90°).

[0007] Patent Document 2 discloses a carbon material for non-aqueous secondary batteries containing graphite capable of absorbing and releasing lithium ions. The carbon material is characterized by having an integrated pore volume of pores with diameters ranging from 0.01 μm to 1 μm of 0.08 mL / g or more, a circularity of 0.88 or more as determined by flow particle image analysis, and a ratio of pore diameter to particle diameter (PD / d50(%)) represented by the following formula (1A) of 1.8 or less. PD / d50 (%) = Mode pore diameter (PD) of pore diameters in the range of 0.01 μm to 1 μm in the pore distribution determined by mercury intrusion porosimetry / Volume-based average particle diameter (d50) × 100 (1A)

[0008] Patent Document 2 describes that a carbon material is provided that enables the production of a non-aqueous secondary battery with high capacity and excellent input / output characteristics, and as a result, a high-performance non-aqueous secondary battery can be provided. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2018-190575 [Patent Document 2] International Publication No. 2016 / 6617 Summary of the Invention [Problem to be solved by the invention]

[0010] Highly crystalline graphite-based materials, which have a well-developed hexagonal network structure of carbon atoms, have a layered structure in which the hexagonal network planes of carbon atoms are stacked to form multiple layers, and the in-plane direction of the layers is represented by the (002) plane in the graphite crystal structure. In lithium-ion secondary batteries, charging and discharging are carried out by the insertion (absorption) of lithium ions between the layers and the desorption (release) of lithium ions from the layers. Highly crystalline graphite-based materials typically have a flat shape, such as a flake or plate, so internal resistance can be reduced by increasing the degree of orientation within the composite layer.

[0011] By using the magnetic field orientation technology described in Patent Document 1 to orient such graphite-based materials perpendicular to the surface of the current collector foil, lithium ions in the electrolyte solution can move in a linear or nearly linear path within the composite layer, thereby reducing the ion migration resistance in the electrode and achieving high input / output characteristics.

[0012] Furthermore, in graphite-based materials with a layered structure, the insertion and desorption of lithium ions occurs from the edge surfaces where the interlayers are exposed. Therefore, it is known that magnetic field orientation improves lithium ion acceptance from the edge surfaces, improving input / output characteristics. However, in graphite-based materials with flat shapes such as flakes or plates, the edge surfaces make up a small proportion of the particle surface. This has led to the problem of lithium ions not moving smoothly at the interface between the electrolyte and the graphite-based material during charge and discharge, resulting in increased interfacial migration resistance.

[0013] On the other hand, when a graphite-based material obtained by spheroidizing flake graphite or the like through mechanical and physical treatment is used as the active material for the negative electrode, it is possible to obtain a high energy density and, since the proportion of edge faces occupying the particle surface is higher than that of flake graphite, it has the advantage of reducing interfacial migration resistance.

[0014] Patent Document 2 describes a spherical carbon material in which the orientation and spacing of internal voids (internal pores) are controlled, but does not take into consideration the relative orientation of the internal voids between carbon material particles. Therefore, if the internal voids are randomly oriented within the composite layer, the migration paths of lithium ions become complex, resulting in a problem of increased ion migration resistance in the electrode.

[0015] The present invention has been made to solve such problems, and aims to provide a secondary battery electrode that improves battery performance by reducing ion migration resistance, and a method for manufacturing the secondary battery electrode. [Means for solving the problem]

[0016] An electrode for a secondary battery according to one embodiment includes a current collector foil and a composite layer formed on the surface of the current collector foil and containing magnetically oriented graphite particles, and among the pore paths formed from one surface to the other surface of the composite layer, a pore path ratio, which is the ratio of the pore path lengths of the pore paths passing through external pores formed between the graphite particles to the pore path lengths of the pore paths passing through internal pores formed inside the graphite particles, is 0.8 to 1.2.

[0017] Furthermore, a method for manufacturing an electrode for a secondary battery according to one embodiment includes a graphite particle formation step of forming graphite particles that have a plurality of internal pores and can be magnetically oriented; a coating step of forming a coating film by applying a paste containing at least graphite particles and a solvent to the surface of a current collector foil; a magnetic field orientation step of orienting the graphite particles contained in the coating film by applying a magnetic field to the coating film that is approximately perpendicular to the surface of the current collector foil; and a composite layer formation step of drying the coating film containing the oriented graphite particles to form a composite layer on the surface of the current collector foil, wherein the pore path ratio, which is the ratio of the pore path lengths of the pore paths that pass through the internal pores to the pore paths that pass through the external pores formed between the graphite particles, is set to be 0.8 to 1.2, among the pore paths formed from one surface of the composite layer to the other surface. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide an electrode for a secondary battery that improves battery performance by reducing ion migration resistance, and a method for manufacturing an electrode for a secondary battery. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a cross-sectional view showing an electrode for a secondary battery according to a first embodiment. [Figure 2] 3 is a flowchart showing a method for manufacturing an electrode for a secondary battery according to the first embodiment. [Figure 3] 1 is a table illustrating electrodes of each example and each comparative example. [Figure 4] FIG. 2 is a diagram illustrating a processing mode of a crushing process. [Figure 5] FIG. 10 is a diagram illustrating a method for determining the isotropy of particles. [Figure 6] FIG. 10 is a diagram illustrating a method for determining a hole ratio. [Figure 7] 1 is a graph showing the relationship between the hole ratio and the ion migration resistance in each example and each comparative example. [Figure 8] 1 is a graph showing the DC internal resistance of test battery cells including electrodes of each example and each comparative example. [Figure 9]FIG. 1 illustrates an electrode containing spheroidized graphite particles with randomly oriented internal pores within the particles. [Figure 10] FIG. 2 is a diagram illustrating an electrode containing flake graphite particles. DETAILED DESCRIPTION OF THE INVENTION

[0020] Embodiment 1 Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. In addition, the following description and drawings have been simplified as appropriate for clarity of explanation.

[0021] Hereinafter, as one preferred embodiment of the secondary battery electrode according to this embodiment, a negative electrode of a lithium ion secondary battery will be specifically described. A lithium ion secondary battery is a secondary battery in which charging and discharging are achieved by the conduction of lithium ions 1, which are charge carriers, through an electrolyte between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate) during an electrochemical reaction. Such lithium ion secondary batteries are suitable for use as power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHEVs).

[0022] An outline of a secondary battery electrode (negative electrode plate E1) according to this embodiment will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view showing a secondary battery electrode according to embodiment 1. The cross-sectional view shown in Fig. 1 shows a part of a cross section of the negative electrode plate E1 perpendicular to the surface of the current collector foil 10.

[0023] As shown in Fig. 1, the negative electrode plate E1 has a current collector foil 10 and a composite layer 20 formed on the surface of the current collector foil 10. The current collector foil 10 is formed in the shape of a plate or foil and is made of a metal with good conductivity. Examples of metals that make up the current collector foil 10 include copper, copper alloys, nickel, titanium, and stainless steel. The current collector foil 10 has a thickness of, for example, 5 µm to 50 µm.

[0024] The composite layer 20 is formed on at least one surface of the current collector foil 10, except for an edge along one edge in the width direction. The negative electrode plate E1 also has an exposed portion at that edge of the current collector foil 10 where the composite layer 20 is not formed and the current collector foil 10 is exposed. The exposed portion is a portion that is electrically connected to the external terminal of the negative electrode. The composite layer 20 contains at least graphite particles 30, and is held by the current collector foil 10. The composite layer 20 may further contain a binder and a thickener, and may also contain other additives as necessary.

[0025] The graphite particles 30 function as an active material capable of absorbing and releasing lithium ions 1. From the viewpoint of realizing high output characteristics and high energy density, the proportion of the graphite particles 30 in the entire mixture layer 20 is preferably, for example, 80% by mass to 99.5% by mass, and particularly preferably 95% by mass to 99.5% by mass.

[0026] Examples of binders that can be used include rubbers such as styrene butadiene rubber (SBR) and butyl rubber (BR), and fluorine-based resins such as polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), and polytetrafluoroethylene (PTFE). Examples of thickeners that can be used include celluloses such as carboxymethyl cellulose (CMC) and methyl cellulose (MC). The total proportion of the binder and thickener in the entire mixture layer 20 is preferably 0.5% to 5% by mass, and more preferably 1% to 3% by mass.

[0027] The graphite particles 30 contained in the negative electrode plate E1 according to this embodiment are a graphite-based material that can be oriented in the direction of easy magnetization by application of a magnetic field. The graphite particles 30 preferably have a layered structure in which hexagonal network planes of carbon atoms are stacked to form multiple layers. Furthermore, the graphite particles 30 preferably have a substantially spherical particle shape with an average aspect ratio, which is the ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter), of 1.0 to 1.5. Although such graphite particles 30 have reduced shape anisotropy, application of a magnetic field can orient the layer planes substantially perpendicular to the current collector foil 10. In this embodiment, the major axis diameter is the average of the longest diameters in the major axis direction of the graphite particles 30, and the minor axis diameter is the average of the longest diameters in the minor axis direction perpendicular to the major axis direction of the graphite particles 30.

[0028] The average particle size of each graphite particle 30 is preferably, for example, 1 μm to 40 μm, and particularly preferably 5 μm to 12 μm. The average particle size of the graphite particles 30 can be measured using a scanning electron microscope (SEM) or by particle size distribution measurement.

[0029] The specific surface area of ​​the graphite particles 30 by the BET method is, for example, 2 m 2 / g~10m 2 / g is preferred, and 3m 2 / g~5m 2 It is preferable to set the value to / g.

[0030] The graphite particles 30 having the above properties improve packing compared to, for example, a case where the aspect ratio is 1.5 or more, thereby enabling high energy density and high capacity to be achieved. Furthermore, the surface of the graphite particles 30 can be provided with sufficient edge surfaces for insertion and desorption of lithium ions 1, and adequate voids (external pores 50, described below) are provided within the composite layer 20, allowing the electrolyte to be distributed evenly, resulting in smooth movement of the lithium ions 1. As a result, the high-speed charge / discharge characteristics and input / output characteristics of the battery are improved.

[0031] The graphite particles 30 have a plurality of slit-shaped pores (voids) therein. Hereinafter, the pores formed inside the graphite particles 30 are referred to as internal pores 40. It is preferable that the internal pores 40 are stacked inside each graphite particle 30 and extend in approximately the same direction. Such graphite particles 30 can be said to be particles with a highly isotropic layered structure of graphite, in which the layer planes and internal pores 40 are aligned. The average isotropy of the graphite particles 30 is preferably 75% or more. A method for measuring isotropy in this embodiment will be described later.

[0032] Furthermore, the graphite particles 30 are oriented in the internal pores 40 so that their major axis directions are substantially perpendicular to the surface of the current collector foil 10. That is, the angle formed between the surface of the current collector foil 10 and the major axis directions of the internal pores 40 is 45° to 90°. It is preferable that such internal pores 40 account for 75% or more of the total number of pores formed inside the graphite particles 30.

[0033] In this way, when the internal pores 40 are oriented in approximately the same direction both within and between the graphite particles 30, the migration path of the lithium ions 1 becomes more linear within the composite layer 20. Therefore, in the negative electrode plate E1 when a secondary battery is formed, the ionic conductivity in the electrolyte solution that has permeated the internal pores 40 increases, and the ionic migration resistance decreases.

[0034] In addition, in negative electrode plate E1 when a secondary battery is constructed, the electrolyte permeates into pores (voids) that exist outside graphite particles 30 and are formed between particles in composite layer 20. Hereinafter, the pores between graphite particles 30 in composite layer 20 are referred to as external pores 50.

[0035] One of the features of the negative electrode plate E1 according to this embodiment is the relationship between the pore paths L1 passing through the internal pores 40 and the pore paths L2 passing through the external pores 50. The pore paths L1 and L2 formed from one surface to the other in the thickness direction of the composite layer 20 serve as migration paths for lithium ions 1 in the electrolyte. The lithium ions 1 in the electrolyte preferably migrate from one surface of the composite layer 20 to the other in a straight or nearly straight path. Therefore, as the tortuosity of the pore path approaches 1 (decreases), the migration path of the lithium ions 1 becomes more linear and the resistance decreases. On the other hand, as the tortuosity of the pore path deviates from 1 (increases), the migration path of the lithium ions 1 becomes more curved and the resistance increases. The tortuosity indicates the degree of curvature of the pore path and is proportional to the pore path length and inversely proportional to the thickness of the composite layer 20.

[0036] Analysis by the present inventors has confirmed that increases and decreases in ion migration resistance in an electrode are due not only to external pores present between active material particles but also to internal pores within the particles. Here, factors that cause increases and decreases in ion migration resistance will be explained with reference to Figs. 9 and 10. Fig. 9 is a diagram illustrating an electrode containing spheroidized graphite particles in which the internal pores are randomly oriented. Fig. 10 is a diagram illustrating an electrode containing flake graphite particles.

[0037] The negative electrode plate E2 shown in Fig. 9 contains, as an active material, substantially spherical spherical graphite particles 31 in a composite layer 21. For example, by subjecting flake graphite or the like as a raw material to a spheroidizing treatment, spherical graphite having reduced crystallinity and orientation compared to the raw material flake graphite can be obtained. Examples of spheroidizing treatment include a method of spheroidizing by applying mechanical energy and a method of granulating a plurality of fine particles to form spheroids.

[0038] If such spherical graphite particles 31 are used as an active material, the packing property is improved, and therefore a higher capacity can be expected. However, since the lithium ions 1 in the electrolyte solution must move around the particle surface, the migration efficiency of the lithium ions 1 in the electrolyte solution deteriorates, resulting in a problem of an increase in ion migration resistance in the electrode.

[0039] The negative electrode plate E3 shown in FIG. 10 contains flat flake graphite particles 32 as the active material in the composite layer 22. When the composite layer 22 is formed using such flake graphite particles 32, the layer planes of the flake graphite particles 32 tend to be oriented parallel to the surface of the current collector foil 10 due to stresses, etc., that occur when applying a paste containing the flake graphite particles 32 or when consolidating the composite layer 22. Therefore, by using a magnetic field orientation technique to orient the layer planes of the flake graphite particles 32 so that they are approximately perpendicular to the surface of the current collector foil 10, lithium ions 1 in the electrolyte can migrate linearly or nearly linearly between the inside and outside of the flake graphite particles 32. That is, the tortuosity of the pore paths passing through the internal pores 42 in the flake graphite particles 32 and the tortuosity of the pore paths passing through the external pores 52 between the flake graphite particles 32 are both reduced. This reduces the ion migration resistance in the electrode.

[0040] It is also known that such orientation of the flake graphite particles 32 improves lithium ion acceptance from the edge surfaces, resulting in improved input / output characteristics. However, highly crystalline graphite materials having a flat shape, such as a flake or plate shape, have a smaller proportion of edge surfaces per unit volume than low-crystalline spheroidized graphite particles 31, which do not have clear edge surfaces, which poses a problem of increased interfacial migration resistance. As a result, a secondary battery using negative electrode plate E3 containing flake graphite particles 32 as shown in Figure 10 tends to have a higher internal resistance than a secondary battery using negative electrode plate E2 containing spheroidized graphite particles 31 as shown in Figure 9.

[0041] Here, the reason why the effect of reducing ion migration resistance by magnetic field orientation is limited to flat graphite-based materials such as flake graphite particles 32 is thought to be not only the particle shape of the active material (e.g., average particle size, particle form), but also the anisotropy of the layer structure of graphite due to the spheroidization process used to form spheroidized graphite particles 31. In spheroidized graphite particles 31 with a highly anisotropic layer structure of graphite, the orientation of internal pores 41 within the spheroidized graphite particles 31 becomes random, making the pore paths passing through the internal pores 41 more complex and increasing the tortuosity. As a result, the ion migration resistance in the electrode increases.

[0042] Furthermore, it was found that when the difference in the path lengths between the path passing through the internal pores 41 and the path passing through the external pores 51 between the spheroidized graphite particles 31 is large (for example, the path ratio is a value far from 1), lithium ions 1 in the electrolyte concentrate in one of the paths, and the concentration distribution of lithium ions 1 in the electrolyte becomes biased, resulting in an increase in the electrolyte resistance.

[0043] In this embodiment, of the pore paths L1 and L2 passing through the composite layer 20, the pore path ratio, which is the ratio of the pore path length of the pore path L1 passing through the internal pore 40 to the pore path L2 passing through the external pore 50, is a value close to 1. For example, the pore path ratio is preferably 0.8 to 1.2, and more preferably 0.86 to 1.17. This suppresses the concentration of lithium ions 1 in either the internal pore 40 or the external pore 50, thereby reducing the electrolyte resistance. When the pore path lengths of the pore path L1 passing through the internal pore 40 and the pore path L2 passing through the external pore 50 are equal, the pore path ratio is 1, and the electrolyte resistance is minimized. The method for calculating the pore path ratio will be described later.

[0044] Next, a method for manufacturing the negative electrode plate E1 will be described with reference to Fig. 2. Fig. 2 is a flowchart showing the method for manufacturing an electrode for a secondary battery according to the first embodiment.

[0045] As shown in FIG. 2, the method for manufacturing a secondary battery electrode according to this embodiment includes the following steps S1 to S4. In the graphite particle formation step of step S1, graphite particles 30 having a plurality of internal pores 40 therein and capable of being magnetically oriented are formed. In the coating step of step S2, a paste containing at least the graphite particles 30 and a solvent is applied to the surface of the current collector foil 10 to form a coating film. In the magnetic field orientation step of step S3, a magnetic field that is substantially perpendicular to the surface of the current collector foil 10 is applied to the coating film to orient the graphite particles 30 contained in the coating film. In the composite layer formation step of step S4, the coating film containing the oriented graphite particles 30 is dried to form a composite layer 20 on the surface of the current collector foil 10. In the method for manufacturing a secondary battery electrode according to this embodiment, the pore paths L1, L2 formed from one surface of the composite layer 20 to the other surface thereof by the above steps are set so that the pore path ratio, which is the ratio of the pore path length of the pore paths L1 passing through the internal pores 40 to the pore path L2 passing through the external pores 50 formed between the graphite particles 30, is 0.8 to 1.2. Each of these steps will be described in more detail.

[0046] In the graphite particle formation process, the raw graphite-based carbon material (raw graphite) is subjected to a spheroidizing treatment to form approximately spherical graphite particles 30 with an average aspect ratio of 1.0 to 1.5 and an average isotropy of 75% or more.

[0047] Examples of such graphite particles 30 include natural graphite or its amorphous form that has been subjected to a spheroidizing treatment, and are particles formed only from graphite or particles in which graphite accounts for 50% by mass or more, preferably 80% by mass or more, of the entire particle. As the raw graphite, it is preferable to use highly crystalline natural graphite that has a layered structure in which hexagonal net planes of carbon atoms overlap to form multiple layers, and examples include scaly, flake, block, or plate-like natural graphite. In particular, flake graphite is preferred.

[0048] In this embodiment, a pulverization process is applied as a spheroidization process that can control the isotropy of the graphite particles 30. An example of a method for controlling the isotropy will now be described with reference to Fig. 4. Fig. 4 is a diagram illustrating the processing form of the pulverization process.

[0049] As shown in Figure 4, pulverization processes are classified by the type of force applied to the material and the processing form. The forces applied to the material can be broadly divided into four: impact force, compression force, friction force, and shear force. The processing form of pulverization can be broadly divided into two: volume pulverization, which destroys the entire particle using impact force, compression force, and shear force, and surface pulverization, which scrapes away the particle surface using friction force and shear force.

[0050] Among these, it is preferable to select a method that aims at surface pulverization in order to increase the isotropy of the graphite particles 30. For example, a jet mill can be used as a pulverizer for surface pulverization. However, the pulverization method is not limited to this, and any method that ultimately results in a high proportion of surface pulverization may be used, regardless of whether or not volume pulverization is performed. After pulverization, it is preferable to perform a classification process so that the average particle size falls within a desired range.

[0051] For example, substantially spherical graphite particles 30 obtained by surface pulverizing flake graphite have properties in which shape anisotropy is suppressed while maintaining the layered structure of flake graphite, and the proportion of edge surfaces per unit volume is increased. Furthermore, slit-like internal pores 40 are arranged in a stacked state inside such graphite particles 30, so that highly isotropic graphite particles 30 can be obtained in which the major axes of the internal pores 40 are aligned in approximately the same direction. In this way, using a method that aims at surface pulverization as a spheroidizing method can ensure high isotropy of the graphite particles 30. The spheroidizing method is not limited to this method, as long as it can ensure high isotropy in the graphite particles 30.

[0052] Next, in the coating process, first, a solvent is added to a powder containing graphite particles 30, a binder, a thickener, and other additives as needed, and the mixture is kneaded using a kneading machine such as a planetary mixer to prepare a paste for forming a negative electrode composite layer.

[0053] The solvent is appropriately selected depending on the binder used. Examples of the solvent that can be used include non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), dimethylformamide (DMF), and toluene, mixed solvents combining non-aqueous solvents, and aqueous solvents such as water and mixed solvents mainly composed of water.

[0054] Next, the prepared paste is applied to the surface of the current collector foil 10 to form a coating film on the surface of the current collector foil 10. The paste can be applied to the surface of the current collector foil 10 using a coating method such as a die coater, a slit coater, a comma coater, a gravure coater, or a blade coater.

[0055] Next, in the magnetic field orientation step, a magnetic field generator is placed near the current collector foil 10 so that the direction of the magnetic field lines is approximately perpendicular to the surface of the current collector foil 10. Then, a magnetic field that generates magnetic field lines in a direction approximately perpendicular to the surface of the current collector foil 10 is applied to the coating film. The magnetic field generator is not particularly limited as long as it can generate the required magnetic field, and for example, a permanent magnet, an electromagnet, or the like can be used.

[0056] In the magnetic field orientation step, the magnetic flux density of the magnetic field applied to the coating film is, for example, 100 mT to 1 T, and typically 300 mT to 500 mT. The greater the magnetic flux density of the magnetic field, the higher the degree of orientation of the graphite particles 30. The time for applying the magnetic field to the coating film is, for example, about 1 to 30 seconds. The graphite particles 30 contained in the coating film to which the magnetic field is applied are oriented so that the layer plane, which is the direction of easy magnetization, approaches the direction of the magnetic field lines, and therefore the internal pores 40 between the layer planes are oriented substantially perpendicular to the surface of the current collector foil 10.

[0057] Next, in the composite layer formation step, the coating film containing the oriented graphite particles 30 as described above is dried to remove the solvent contained in the coating film. The coating film can be dried using natural air, hot air, low-humidity air, vacuum, infrared, far-infrared, electron beam, or other methods, either alone or in combination. The dried product is then pressed using a pressing method such as a roll press or a flat press to adjust the density and thickness of the composite layer 20. This allows the composite layer 20 to be formed on the surface of the current collector foil 10.

[0058] The above steps can produce the negative electrode plate E1 shown in Fig. 1. In this embodiment, the produced negative electrode plate E1 was evaluated for the isotropy of the graphite particles 30 and the path ratio in the composite layer 20.

[0059] The degree of isotropy of graphite particles 30 was evaluated by performing image analysis on cross-sectional SEM images of graphite particles 30 obtained by FIB-SEM measurement. FIB-SEM refers to processing a sample (e.g., negative electrode plate E1) with a focused ion beam (FIB) and observing the exposed cross section of the sample with an SEM. For example, a sample may be processed by cutting a sample solidified with an appropriate resin at the desired cross section and gradually scraping the cut surface while performing SEM observation.

[0060] A method for determining the isotropy of graphite particles 30 will now be described with reference to Fig. 5. Fig. 5 is a diagram illustrating the method for determining the isotropy of particles. Based on cross-sectional SEM images obtained by FIB-SEM measurement, 60 graphite particles 30 were randomly selected, and the cross section of each selected graphite particle 30 was divided according to the direction of its long axis, and image analysis was performed for each cross section of each graphite slice, to calculate the area and angle of the cross section of each graphite slice. Here, each of the multiple graphite slices included in the cross section of graphite particle 30 is an aggregate in which hexagonal planes of carbon atoms are stacked in approximately the same direction to form multiple layers.

[0061] For example, the graphite particle 30 shown in FIG. 5 contains 14 graphite segments in the cross section of one particle. The angle of the cross section of a graphite segment is the angle between a certain direction and the long axis direction of the cross section. A component vector f was determined for each cross section of the graphite segment, with the area being the magnitude of the vector and the angle being the direction of the vector, and then a resultant vector P was calculated by combining each component vector f. Furthermore, a normal vector n was calculated, with the calculated resultant vector P as the base and the distance to each tip of the component vector f as its magnitude. The ratio of the resultant vector P to the sum of the magnitude of the resultant vector P and the magnitude of each normal vector n was calculated as isotropy (%).

[0062] The pore path ratio was determined by constructing a three-dimensional model from a cross-sectional SEM image obtained by FIB-SEM measurement and performing image analysis on the three-dimensional model. A method for determining the pore path ratio will now be described with reference to FIG. 6. FIG. 6 is a diagram illustrating the method for determining the pore path ratio. The upper part of FIG. 6 shows a three-dimensional model of the negative electrode plate E1. The lower part of FIG. 6 shows an enlarged view of the periphery of region A and region B in the three-dimensional model.

[0063] From the data of the three-dimensional model obtained by FIB-SEM measurement, 10 paths L1 passing through the internal pores 40 and 10 paths L2 passing through the external pores 50 were randomly selected and the average values ​​of the measured path lengths were calculated. Furthermore, the ratio of the path length of the path L1 passing through the internal pores 40 to the path length of the path L2 passing through the external pores 50 (path length of path L1 / path length of path L2) was calculated as the path length ratio.

[0064] As another method for calculating the hole ratio, electrochemical measurement can also be used. When using the electrochemical measurement method, a symmetrical cell for measurement is first constructed, and the ion transfer resistance R is calculated by AC impedance measurement using this cell. ion The ion transfer resistance R ion is the resistance to the movement of lithium ions inside the electrode.

[0065] Thereafter, the total pore path length, which is the sum of all the pore paths (pore paths L1 and pore paths L2) formed in the composite layer 20, is calculated by the following formula (1): In the following formula (1), L is the total pore path length (μm), κ is the conductivity of the electrolyte (S / m), ε is the porosity of the electrode plate (%), and S is the area of ​​the electrode plate (μm 2 ) L=R ion (κεS) Equation (1)

[0066] The path length of hole L2 is then measured by simulation using a three-dimensional model. Furthermore, the path length of hole L1 is calculated by subtracting the path length of hole L2 from the total path length. In this way, the path lengths of hole L2 and hole L1 are obtained using electrochemical measurement, and the path ratio (path length of hole L1 / path length of hole L2) can be calculated.

[0067] The pore path ratio can be controlled by the isotropy of the graphite particles 30, the degree of orientation of the graphite particles 30 relative to the current collector foil 10, and the density of the composite layer 20. The isotropy of the graphite particles 30 can be adjusted by the crystallinity of the raw graphite used in the graphite particle formation process and the method of spheroidization (such as the form of pulverization treatment). For example, the higher the crystallinity of the raw graphite and the higher the proportion of surface pulverization, the higher the isotropy, and therefore the shorter the pore path length of the pore paths L1 passing through the internal pores 40.

[0068] The degree of orientation of the graphite particles 30 relative to the current collector foil 10 can be adjusted by changing the conditions for applying the magnetic field (magnetic flux density of the magnetic field, direction of the magnetic field lines) in the magnetic field orientation step. For example, the closer the angle between the surface of the current collector foil 10 and the major axis direction of the internal pores 40 is to 90°, the shorter the path length of the pore paths L1 passing through the internal pores 40 becomes.

[0069] The density of the composite layer 20 can be adjusted by increasing or decreasing the pressure during pressing performed in the composite layer formation step. For example, increasing the pressure during pressing increases the density of the composite layer 20, crushing the outer pores 50 and increasing the tortuosity, thereby lengthening the path length of the hole paths L2 passing through the outer pores 50.

[0070] Next, examples and comparative examples will be described with reference to FIG. 3. Note that the examples do not limit the present invention. FIG. 3 is a table explaining the electrodes of each example and each comparative example. The average particle size in the table is the volume-based average particle size (D50) measured by laser diffraction. The aspect ratio in the table was measured using cross-sectional SEM images of particles obtained by FIB-SEM measurement.

[0071] Example 1 [Preparation of negative electrode plate] Negative electrode plate E1 was produced according to the flow shown in Fig. 2. In the graphite particle formation step, flake particles (natural graphite) having an average particle size of 14 µm, an aspect ratio of 3.0, and an isotropy of 95% were used as raw graphite, and the flake particles were pulverized using a jet mill to form graphite particles 30. The obtained graphite particles 30 had an average particle size of 7 µm, an aspect ratio of 1.5, and an isotropy of 75%.

[0072] In the coating process, graphite particles 30, SBR as a binder, and CMC as a thickener were mixed in a mixing ratio of 98:1:1, and ion-exchanged water was added as a solvent and kneaded to prepare a paste. The prepared paste was applied to both sides of copper foil, which was the negative electrode current collector foil 10. The negative electrode paste had a basis weight of 4.7 mg / cm. 2 The coating amount was adjusted so that a coating film was formed on the surface of the copper foil.

[0073] In the magnetic field orientation step, a pair of permanent magnets was placed above and below the copper foil with the coating film formed on its surface, and a magnetic field was applied to the coating film. The magnetic flux density of the magnetic field was 500 mT, and the magnetic field application time was 10 seconds. The direction of the magnetic field lines when applying the magnetic field was approximately perpendicular to the surface of the copper foil. This oriented the graphite particles 30 so that the long axis direction of the internal pores 40 was approximately perpendicular to the surface of the copper foil.

[0074] The coating film containing the magnetically oriented graphite particles 30 is dried with hot air and then cut to a predetermined size. 3The negative electrode plate E1 of Example 1 thus obtained had a path ratio of 0.860.

[0075] Example 2 The density of the composite layer 20 is 1.15 g / cm 3 A negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the hole path ratio of the negative electrode plate E1 of Example 2 obtained in this manner was 0.940.

[0076] Example 3 The density of the composite layer 20 is 1.20 g / cm 3 A negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the hole path ratio of the negative electrode plate E1 of Example 3 obtained in this manner was 1.01.

[0077] Example 4 The density of the composite layer 20 is 1.30 g / cm 3 A negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the hole path ratio of the negative electrode plate E1 of Example 4 obtained in this manner was 1.17.

[0078] (Comparative Example 1) A negative electrode plate was produced in the same manner as in Example 1, except that the graphite particle formation step was omitted and the graphite particles 30 used in Example 1 were replaced with flake particles of raw graphite.

[0079] (Comparative Example 2) Scalloped particles of raw graphite were crushed in a ball mill, a volumetric crushing mill, to form spherical particles. The resulting spherical particles had an average particle size of 7 μm, an aspect ratio of 1.5, and an isotropy of 55%. A negative electrode plate was manufactured using the spherical particles through the same coating, magnetic field orientation, and composite layer formation steps as in Example 1. The negative electrode plate of Comparative Example 2 thus obtained had a hole path ratio of 1.69.

[0080] (Comparative Example 3) Except for omitting the magnetic field orientation step, a negative electrode plate was manufactured in the same manner as in Example 1. The negative electrode plate of Comparative Example 3 thus obtained had a hole path ratio of 2.11.

[0081] Comparative Example 4 The density of the composite layer is 1.15g / cm 3 A negative electrode plate was produced in the same manner as in Example 1, except that roll pressing was carried out so that the hole path ratio of the negative electrode plate of Example 2 obtained in this manner was 1.87.

[0082] [Preparation of positive electrode plate] The positive electrode plate was fabricated as follows. The positive electrode active material was LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Nickel manganese cobalt oxide (NMC) with an average composition represented by O2 was used. Acetylene black (AB) was used as the conductive material. PVDF was used as the binder. NMP was used as the solvent.

[0083] The positive electrode active material, conductive material, and binder were weighed out so that the mass ratio was NMC:AB:PVDF = 96:3:1, and the required amount of solvent was added and kneaded to prepare a paste. The prepared paste was applied to both sides of aluminum foil, which served as the positive electrode current collector foil. The positive electrode paste had a basis weight of 5.55 mg / cm. 2 The applied paste was then dried with hot air and cut to a predetermined size, and the density of the composite layer was adjusted to 2.6 g / cm. 3 A positive electrode plate was fabricated by roll pressing so that the thickness became as follows:

[0084] [Preparation of electrolyte] The electrolyte was prepared by dissolving the supporting salt LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[0085] [Construction of evaluation battery cell] A battery cell for evaluation was constructed by placing a negative electrode plate (Examples 1 to 4, Comparative Examples 1 to 4) and a positive electrode plate facing each other with a porous separator made of polyethylene (PE) between them inside a battery case, adding an electrolyte solution, and sealing the case.

[0086] [Evaluation of ion transfer resistance] The ion migration resistance was measured by measuring the AC impedance of a symmetric cell in which a non-Faraday reaction was performed. The results are shown in Figure 7. Figure 7 is a graph showing the relationship between the hole ratio and the ion migration resistance in each example and each comparative example.

[0087] As shown in FIG. 7, although no evaluation was performed on Comparative Example 1, the resistance values ​​of the ion migration resistance in all of Comparative Examples 2 to 4 were 6.0 mΩ or more, whereas the resistance values ​​of the ion migration resistance in all of Examples 1 to 4 were 6.0 mΩ or less, confirming that the resistance was reduced.

[0088] As can be seen from these results, it was confirmed that even if the particle shape of the active material contained in the negative electrode plate is approximately spherical with the same aspect ratio, the closer the hole path ratio is to 1, the lower the ion migration resistance becomes.

[0089] [Evaluation of DC internal resistance (DCIR)] The direct current internal resistance (DCIR) of each test battery cell was measured to determine the DC internal resistance. The results are shown in Figure 8. Figure 8 is a graph showing the DC internal resistance of test battery cells including electrodes from each example and comparative example. DCIR measurements were performed by measuring the voltage drop for 10 seconds from the start of discharge when discharging from a 50% SOC (State of Charge) at a current of 15 C in a temperature environment of -30°C, and then dividing this voltage drop by the corresponding current value to calculate the resistance (mΩ).

[0090] As shown in FIG. 8, the DC internal resistance values ​​were 1.4 mΩ or more in all of Comparative Examples 1 to 4, whereas the DC internal resistance values ​​were 1.4 mΩ or less in all of Examples 1 to 4, confirming that the resistance was reduced.

[0091] As can be seen from these results, it was confirmed that DC internal resistance was reduced when the particle shape of the active material contained in the negative electrode plate was approximately spherical, but DC internal resistance deteriorated when the particles were scaly and had a large aspect ratio, even if they were highly isotropic. Furthermore, when the particle shape of the active material was approximately spherical, the higher the isotropy, the better the electrode orientation due to magnetic field orientation, and the better the DC internal resistance. Furthermore, it was found that the closer the hole ratio was to 1, the greater the effect of reducing DC internal resistance.

[0092] As described above, the secondary battery electrode according to this embodiment includes current collector foil 10 and composite layer 20 formed on the surface of current collector foil 10 and containing magnetically oriented graphite particles 30. Of the pore paths L1, L2 formed from one surface to the other of composite layer 20, the pore path ratio, which is the ratio of the pore length of pore paths L1 passing through internal pores 40 formed inside graphite particles 30 to the pore path L2 passing through external pores 50 formed between graphite particles 30, is 0.8 to 1.2.

[0093] With this configuration, multiple internal pores 40 are oriented in approximately the same direction both within and between graphite particles 30, making the pore paths L1 passing through the internal pores 40 more linear and allowing smoother movement of lithium ions 1 in the electrolyte. As a result, ion migration resistance in the electrode is reduced, and the high-speed charge / discharge characteristics and input / output characteristics of the battery are improved.

[0094] Furthermore, by setting the hole ratio to 0.86 to 1.17, the effect of reducing the ion migration resistance is further enhanced.

[0095] The graphite particles 30 preferably have an average aspect ratio of 1.0 to 1.5. This increases the ratio of edge faces per unit volume of the particles, thereby reducing interfacial migration resistance. Furthermore, by forming the graphite particles 30 into a substantially spherical shape with an average aspect ratio of 1.0 to 1.5, the packing of the graphite particles 30 is improved, allowing the density of the composite layer 20 to be increased. As a result, high energy density and high capacity can be achieved.

[0096] Graphite particle 30 has a layered structure in which hexagonal net planes of carbon atoms overlap to form multiple layers, and has an average isotropy of 75% or more. Isotropy is calculated by calculating a resultant vector P obtained by combining component vectors f obtained for each graphite segment contained in graphite particle 30, with the cross-sectional area of ​​the graphite segment, which is an aggregate of layers, as the size and the angle between the cross-section and a certain direction as the direction, and each normal vector n, whose magnitude is the distance from the tip of component vector f to the resultant vector P as the base, and then calculating the ratio of the magnitude of resultant vector P to the sum of the magnitude of resultant vector P and the magnitude of each normal vector n.

[0097] The more isotropic the layer structure of the graphite particles 30 thus obtained is, the more the orientation of the electrode can be improved by magnetic field orientation, and as a result, the more the internal resistance can be reduced.

[0098] According to the method for manufacturing a secondary battery electrode according to this embodiment, a secondary battery electrode that exhibits the above-mentioned effects can be manufactured. [Explanation of symbols]

[0099] 1. Lithium-ion 10 Current collecting foil 20, 21, 22 Composite layer 30 graphite particles 31 Spheroidized graphite particles 32 Flake graphite particles 40, 41, 42 Internal pores 50, 51, 52 External pores E1, E2, E3 negative electrode plate L1, L2 channel

Claims

1. a graphite particle forming step of forming graphite particles having a plurality of internal pores therein and capable of being magnetically oriented; a coating step of coating a paste containing at least the graphite particles and a solvent on a surface of a current collector foil to form a coating film; a magnetic field orientation step of applying a magnetic field to the coating film in a direction substantially perpendicular to the surface of the current collecting foil to orient the graphite particles contained in the coating film; a composite layer forming step of drying the coating film containing the oriented graphite particles to form a composite layer on the surface of the current collecting foil, Among the pore paths formed from one surface to the other surface in the thickness direction of the composite layer, a pore path ratio, which is a ratio of the pore path length of the shortest pore path passing through the internal pores to the pore path length of the shortest pore path passing through the external pores formed between the graphite particles, is set to be 0.8 to 1.2, The graphite particles are The average aspect ratio is 1.0 to 1.5, It has a layered structure in which hexagonal planes of carbon atoms are stacked to form multiple layers, the long axis direction of the internal pores between the layer surfaces is oriented substantially perpendicular to the current collecting foil, In a cross-sectional SEM image of the composite layer in the thickness direction, a composite vector is calculated by combining component vectors obtained for each of the graphite segments contained in the graphite particle, with the area of ​​the cross section of the graphite segments that is an aggregate of the layers as a size and the angle formed by the long axis direction of the cross section and a certain direction as a direction, and each normal vector is calculated with the composite vector as a reference and the distance to each of the tips of the component vectors as a size, the average isotropy calculated as a ratio of the magnitude of the resultant vector to the sum of the magnitude of the resultant vector and the magnitudes of each normal vector is 75% or more; In the graphite particle forming step, A method for manufacturing an electrode for a secondary battery, comprising: subjecting natural graphite to at least surface pulverization to form spheroidized graphite particles so that the average isotropy is 75% or more.

2. A method for manufacturing an electrode for a secondary battery as described in claim 1, wherein the hole ratio is 0.86 to 1.17.

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