JP7763566B2 - Secondary battery electrode and method for manufacturing secondary battery electrode - Google Patents

Description

The present invention relates to an electrode for a secondary battery and a method for manufacturing an electrode for a secondary battery.

Secondary batteries are widely used as portable power sources for devices such as personal computers and mobile terminals, as well as for driving vehicles. Among secondary batteries, lithium-ion secondary batteries, which are lightweight and offer high energy density, are particularly well suited 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 capable of charging and discharging by the movement of lithium ions in an electrolyte between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate), which store and release lithium ions.

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

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

Patent Document 1 discloses a sealed non-aqueous electrolyte secondary battery having the following characteristics: The non-aqueous electrolyte of the sealed non-aqueous electrolyte secondary battery described in Patent Document 2 contains a gas generating agent that decomposes to 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 gas. Furthermore, the positive electrode of this sealed non-aqueous 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.

The technology described in Patent Document 1 uses magnetic field orientation to orient the (002) plane (layer plane) of the expanded graphite so that it stands up relative to the positive electrode current collector. This causes the pores (internal pores) formed between the (002) planes of the expanded graphite to be arranged so that they penetrate the expanded graphite vertically relative to the positive electrode current collector (for example, so that the angle between the central axis of the pore and the surface of the positive electrode current collector is 60° to 90°).

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 0.08 mL/g or more in the pore diameter range of 0.01 μm to 1 μm, 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(%)) expressed 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 obtained by mercury intrusion porosimetry / Volume-based average particle diameter (d50) × 100 (1A)

Patent Document 2 describes the provision of a carbon material that can be used to obtain a non-aqueous secondary battery with high capacity and excellent input/output characteristics, thereby providing a high-performance non-aqueous secondary battery.

Japanese Patent Application Laid-Open No. 2018-190575 International Publication No. 2016/6617

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 overlap 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 between the layers. Highly crystalline graphite-based materials typically have a flat shape, such as a flake or plate shape, so internal resistance can be reduced by increasing the degree of orientation within the composite layer.

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 can move within the composite layer along a linear or nearly linear path. This reduces the ion migration resistance in the electrode, resulting in high input/output characteristics.

In addition, 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. As a result, there is a problem in that lithium ions do not move smoothly at the interface between the electrolyte and the graphite-based material during charge and discharge, resulting in increased interfacial migration resistance.

On the other hand, when graphite-based materials such as flake graphite that have been spheroidized through mechanical and physical processing are used as the active material for the negative electrode, high energy density can be obtained, and there are advantages such as reduced interfacial migration resistance due to a higher proportion of edge faces on the particle surface compared to flake graphite.

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, posing a problem of increased ion migration resistance in the electrode.

The present invention was conceived to solve these problems and aims to provide a secondary battery electrode that improves battery performance by reducing ion migration resistance, as well as a method for manufacturing such an electrode.

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

In one embodiment, a method for manufacturing a secondary battery electrode includes a graphite particle formation process in which magnetically oriented graphite particles having multiple internal pores are formed; a coating process in which a paste containing at least graphite particles and a solvent is applied to the surface of a current collector foil to form a coating film; a magnetic field orientation process in which a magnetic field substantially perpendicular to the surface of the current collector foil is applied to the coating film to orient the graphite particles contained in the coating film; and a composite layer formation process in which the coating film containing the oriented graphite particles is dried to form a composite layer on the surface of the current collector foil. Of the pore paths formed from one surface of the composite layer to the other, the pore path ratio, which is the ratio of the pore path lengths of the paths passing through the internal pores to the pore path lengths of the paths passing through the external pores formed between the graphite particles, is set to be 0.8 to 1.2.

The present invention provides a secondary battery electrode that improves battery performance by reducing ion migration resistance, as well as a method for manufacturing a secondary battery electrode.

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

First Embodiment
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.

The following describes one preferred embodiment of the secondary battery electrode according to this embodiment, specifically as the negative electrode of a lithium-ion secondary battery. 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 vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHEVs).

The secondary battery electrode (negative electrode plate E1) according to this embodiment will be outlined with reference to Figure 1. Figure 1 is a cross-sectional view showing the secondary battery electrode according to embodiment 1. The cross-sectional view shown in Figure 1 shows a portion of the cross section of the negative electrode plate E1 perpendicular to the surface of the current collector foil 10.

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.

The composite layer 20 is formed on at least one surface of the current collector foil 10, excluding an edge along one widthwise edge. The negative electrode plate E1 also has an exposed portion at the 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 the portion electrically connected to the negative electrode's external terminal. 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 needed.

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

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). Thickeners that can be used include celluloses such as carboxymethyl cellulose (CMC) and methyl cellulose (MC). The total proportion of binder and thickener in the composite layer 20 is preferably 0.5% to 5% by mass, and more preferably 1% to 3% by mass.

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 the application of a magnetic field. The graphite particles 30 preferably have a layered structure in which hexagonal planes of carbon atoms overlap 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 suppressed shape anisotropy, the layer planes can be oriented substantially perpendicular to the current collector foil 10 by the application of a magnetic field. 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.

The average particle size of the graphite particles 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 particle size distribution measurement.

The specific surface area of the graphite particles 30 as measured by the BET method is preferably, for example, 2 m ² /g to 10 m ² /g, and more preferably 3 m ² /g to 5 m ² /g.

When the graphite particles 30 have the above properties, packing is improved compared to when the aspect ratio is 1.5 or greater, for example, making it possible to achieve high energy density and high capacity. Furthermore, sufficient edge surfaces for lithium ions 1 to be inserted and removed can be secured on the surface of the graphite particles 30, and adequate voids (external pores 50, described below) are secured 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.

The graphite particles 30 have multiple slit-shaped pores (voids) inside. Hereinafter, the pores formed inside the graphite particles 30 are referred to as internal pores 40. Preferably, multiple internal pores 40 are stacked inside each graphite particle 30, extending in approximately the same direction. Such graphite particles 30 can be said to be particles with a highly isotropic layered structure of graphite, with the layer planes and internal pores 40 oriented in a uniform manner. The average isotropy of the graphite particles 30 is preferably 75% or higher. The method for measuring isotropy in this embodiment will be described later.

Furthermore, the graphite particles 30 are oriented so that the long axis direction of the internal pores 40 is approximately perpendicular to the surface of the current collector foil 10. In other words, the angle between the surface of the current collector foil 10 and the long axis direction 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.

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 within the composite layer 20 becomes more linear. Therefore, in the negative electrode plate E1 used in a secondary battery, the ionic conductivity of the electrolyte that has permeated the internal pores 40 increases, reducing ion migration resistance.

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

One of the characteristics of the negative electrode plate E1 according to this embodiment is the relationship between the pore path L1 passing through the internal pore 40 and the pore path L2 passing through the external pore 50. The pore paths L1 and L2, which are 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. It is preferable for lithium ions 1 in the electrolyte to 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 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 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.

Analysis by the inventors has confirmed that increases and decreases in ion migration resistance in electrodes 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 Figures 9 and 10. Figure 9 is a diagram illustrating an electrode containing spheroidized graphite particles in which the internal pores are randomly oriented. Figure 10 is a diagram illustrating an electrode containing flake graphite particles.

The negative electrode plate E2 shown in Figure 9 contains approximately spherical spheroidized graphite particles 31 as the active material in the composite layer 21. For example, by spheroidizing raw material such as flake graphite, spherical graphite with reduced crystallinity and orientation compared to the raw material flake graphite can be obtained. Spheroidization methods include applying mechanical energy to create spheroids and granulating multiple fine particles to create spheroids.

Using such spherical graphite particles 31 as the active material improves packing and is expected to result in higher capacity. However, because lithium ions 1 in the electrolyte must move around the particle surfaces, the migration efficiency of lithium ions 1 in the electrolyte decreases, resulting in an increase in ion migration resistance in the electrode.

The negative electrode plate E3 shown in Figure 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, stresses such as those generated when applying a paste containing the flake graphite particles 32 or when compacting the composite layer 22 tend to cause the layer surfaces of the flake graphite particles 32 to orient in a direction parallel to the surface of the current collector foil 10. Therefore, by using magnetic field orientation technology to orient the layer surfaces 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 in a linear or nearly linear path within and outside the flake graphite particles 32. In other words, the tortuosity of the pore paths passing through the internal pores 42 within 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.

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-based materials having flat shapes such as flakes or plates have a smaller proportion of edge surfaces per unit volume than low-crystalline spheroidized graphite particles 31, which do not have distinct edge surfaces, resulting in the problem of increased interfacial migration resistance. As a result, secondary batteries using negative electrode plate E3 containing flake graphite particles 32 as shown in Figure 10 tend to have higher internal resistance than secondary batteries using negative electrode plate E2 containing spheroidized graphite particles 31 as shown in Figure 9.

Here, the reason why the effect of reducing ion migration resistance due to magnetic field orientation is limited to flat-shaped 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 morphology), but also the anisotropy of the layered structure of graphite due to the spheroidization process used to form the spheroidized graphite particles 31. In spheroidized graphite particles 31 with a highly anisotropic layered structure of graphite, the orientation of the internal pores 41 within the spheroidized graphite particles 31 becomes random, making the paths through the internal pores 41 more complex and increasing the tortuosity. As a result, the ion migration resistance in the electrode increases.

Furthermore, it was discovered that when there is a large difference in the path lengths between the paths passing through the internal pores 41 and the paths passing through the external pores 51 between the spheroidized graphite particles 31 among the pore paths passing through the composite layer 21 (for example, when the path length ratio is a value far from 1), lithium ions 1 in the electrolyte solution concentrate in one of the pore paths, causing a bias in the concentration distribution of lithium ions 1 in the electrolyte solution, resulting in an increase in electrolyte resistance.

In this embodiment, of the pore paths L1, 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 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 electrolyte resistance is minimized. The method for calculating the pore path ratio will be described later.

Next, a method for manufacturing the negative electrode plate E1 described above will be described with reference to Figure 2. Figure 2 is a flowchart showing a method for manufacturing a secondary battery electrode according to the first embodiment.

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 approximately 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 path ratio, which is the ratio of the pore length of the pore paths L1, L2 formed from one surface of the composite layer 20 to 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 set to 0.8 to 1.2. Each of these steps will be described in more detail below.

In the graphite particle formation process, the raw graphite-based carbon material (raw graphite) is subjected to a spheroidizing process 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.

Examples of such graphite particles 30 include natural graphite or its amorphous form that has been subjected to a spheroidizing process, and are particles formed solely 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 network planes of carbon atoms overlap to form multiple layers, and examples include scaly, flake, block, or plate-like natural graphite. Scaly graphite is particularly preferred.

In this embodiment, a pulverization process is used as a spheroidization process that can control the isotropy of the graphite particles 30. Therefore, an example of a method for controlling isotropy will be described with reference to Figure 4. Figure 4 is a diagram illustrating the processing form of the pulverization process.

As shown in Figure 4, crushing 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 types: impact force, compression force, grinding force, and shear force. Crushing processing forms can be broadly divided into two types: volumetric crushing, which destroys the entire particle using impact force, compression force, and shear force, and surface crushing, which scrapes away the particle surface using grinding force and shear force.

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

For example, the roughly 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, because slit-shaped internal pores 40 are arranged in a stacked state inside such graphite particles 30, highly isotropic graphite particles 30 can be obtained in which the long axes of the internal pores 40 are aligned in roughly the same direction. In this way, using a method that aims for surface pulverization as the spheroidization method makes it possible to ensure high isotropy of the graphite particles 30. The spheroidization method is not limited to this method, as long as it can ensure high isotropy in the graphite particles 30.

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

The solvent is selected appropriately depending on the binder used. Examples of solvents 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 that combine non-aqueous solvents; and aqueous solvents such as water and mixed solvents that mainly consist of water.

The prepared paste is then 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, slit coater, comma coater, gravure coater, or blade coater.

Next, in the magnetic field orientation process, a magnetic field generator is placed near the current collector foil 10 so that the magnetic field lines are oriented approximately perpendicular to the surface of the current collector foil 10. A magnetic field that generates magnetic field lines approximately perpendicular to the surface of the current collector foil 10 is then applied to the coating film. There are no particular limitations on the magnetic field generator as long as it can generate the required magnetic field; for example, a permanent magnet or an electromagnet can be used.

In the magnetic field orientation process, 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 greater the degree of orientation of the graphite particles 30. The magnetic field is applied to the coating film for, for example, 1 to 30 seconds. When a magnetic field is applied, the graphite particles 30 contained in the coating film 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 approximately perpendicular to the surface of the current collector foil 10.

Next, in the composite layer formation process, 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.

The above steps allow the negative electrode plate E1 shown in Figure 1 to be manufactured. In this embodiment, the manufactured negative electrode plate E1 was evaluated for the isotropy of the graphite particles 30 and the pore ratio within the composite layer 20.

The degree of isotropy of the graphite particles 30 was evaluated by performing image analysis on cross-sectional SEM images of the graphite particles 30 obtained by FIB-SEM measurement. Note that 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 can 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.

A method for determining the isotropy of graphite particles 30 will now be described with reference to Figure 5. Figure 5 is a diagram illustrating the method for determining particle isotropy. 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. Image analysis was then performed on each cross section of each graphite slice, and the cross-sectional area and angle of each graphite slice were calculated. Here, each of the multiple graphite slices included in the cross section of graphite particle 30 is an aggregate in which the hexagonal planes of carbon atoms are stacked in approximately the same direction to form multiple layers.

For example, the graphite particle 30 shown in Figure 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 vector's magnitude and the angle being the vector's direction, 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 of the tips 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 then calculated as isotropy (%).

The hole path ratio was determined by constructing a three-dimensional model from cross-sectional SEM images obtained by FIB-SEM measurement and performing image analysis on the three-dimensional model. Therefore, a method for determining the hole path ratio will be explained with reference to Figure 6. Figure 6 is a diagram illustrating the method for determining the hole path ratio. The upper part of Figure 6 shows a three-dimensional model of negative electrode plate E1. The lower part of Figure 6 shows an enlarged view of the area around region A and the area around region B in the three-dimensional model.

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

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

Then, the total pore length of all pores (pores L1 and L2) formed in the composite layer 20 is calculated using the following formula (1): L in the formula (1) is the total pore 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) ...Formula (1)

Then, the path length of hole L2 is measured through a 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 determined using electrochemical measurement, and the path ratio (path length of hole L1 / path length of hole L2) can be calculated.

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 the crushing process). For example, the higher the crystallinity of the raw graphite and the higher the proportion of surface crushing, the higher the isotropy, and therefore the shorter the pore path length L1 passing through the internal pores 40.

The degree of orientation of the graphite particles 30 relative to the current collector foil 10 can be adjusted by changing the magnetic field application conditions (magnetic flux density of the magnetic field, direction of the magnetic field lines) in the magnetic field orientation process. 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 path L1 passing through the internal pores 40 becomes.

The density of the composite layer 20 can be adjusted by increasing or decreasing the pressure applied during the pressing process in the composite layer formation process. For example, increasing the pressure applied during pressing increases the density of the composite layer 20, causing the outer pores 50 to collapse and increase the tortuosity, thereby lengthening the length of the hole paths L2 passing through the outer pores 50.

Next, examples and comparative examples will be described with reference to Figure 3. Note that the examples do not limit the present invention. Figure 3 is a table explaining the electrodes of each example and 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.

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%.

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 served as a negative electrode current collector foil 10. The coating amount of the negative electrode paste was adjusted so that the coating weight was 4.7 mg/ ^cm2 . This formed a coating film on the surface of the copper foil.

In the magnetic field orientation process, a pair of permanent magnets were 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.

The coating film containing the magnetically oriented graphite particles 30 was dried with hot air, cut to a predetermined size, and roll-pressed to produce negative electrode plate E1 so that the density of composite layer 20 became 1.12 g/ ^cm3 . The path ratio of negative electrode plate E1 of Example 1 obtained in this manner was 0.860.

Example 2
Negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the density of composite layer 20 became 1.15 g/cm ^3. The negative electrode plate E1 of Example 2 thus obtained had a path ratio of 0.940.

Example 3
Negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the density of composite layer 20 was 1.20 g/cm ^3. The negative electrode plate E1 of Example 3 thus obtained had a path ratio of 1.01.

Example 4
Negative electrode plate E1 was produced in the same manner as in Example 1, except that roll pressing was performed so that the density of composite layer 20 became 1.30 g/cm ^3. The negative electrode plate E1 of Example 4 thus obtained had a path ratio of 1.17.

(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 scaly particles of raw graphite were used instead of the graphite particles 30 used in Example 1.

(Comparative Example 2)
Spherical particles were formed by crushing raw graphite flake particles in a ball mill, a volumetric crushing mill. 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 processes as in Example 1. The negative electrode plate of Comparative Example 2 thus obtained had a hole path ratio of 1.69.

(Comparative Example 3)
A negative electrode plate was manufactured in the same manner as in Example 1, except that the magnetic field orientation step was omitted. The negative electrode plate of Comparative Example 3 thus obtained had a hole path ratio of 2.11.

(Comparative Example 4)
A negative electrode plate was produced in the same manner as in Example 1, except that roll pressing was performed so that the density of the composite layer became 1.15 g/cm. The negative electrode plate of Example 2 thus obtained had a path ratio of ^1.87 .

[Preparation of positive electrode plate]
The positive electrode plate was fabricated as follows. The positive electrode active material was lithium nickel manganese cobalt oxide (NMC) with an average composition represented by LiNi _1/3 Co _1/3 Mn _1/3 O _2. The conductive material was acetylene black (AB). The binder was PVDF. The solvent was NMP.

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 a positive electrode current collector foil. The coating amount of the positive electrode paste was adjusted so that the basis weight was 5.55 mg/ ^cm2 . The applied paste was then dried with hot air, cut to a predetermined size, and roll pressed to produce a positive electrode plate with a composite layer density of 2.6 g/ ^cm3 .

[Preparation of Electrolyte Solution]
An electrolyte solution was prepared by dissolving a supporting salt LiPF ₆ at a concentration of 1 mol/L in a mixed solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[Construction of evaluation battery cell]
A negative electrode plate (Examples 1 to 4, Comparative Examples 1 to 4) and a positive electrode plate were placed facing each other with a porous separator made of polyethylene (PE) interposed between them. The electrode assembly was then housed inside a battery case, and an electrolyte solution was added and the case was sealed to construct a battery cell for evaluation.

[Evaluation of ion migration 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.

As shown in Figure 7, although no evaluation was performed on Comparative Example 1, the ion migration resistance values for Comparative Examples 2 to 4 were all 6.0 mΩ or higher, whereas the ion migration resistance values for Examples 1 to 4 were all 6.0 mΩ or lower, confirming that resistance was reduced.

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

[Evaluation of DC Internal Resistance (DCIR)]
Direct current internal resistance (DCIR) measurements were performed on each test battery cell to determine the DC internal resistance. The results are shown in FIG. 8 . FIG. 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 value 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 value (mΩ).

As shown in Figure 8, the DC internal resistance values for all of Comparative Examples 1 to 4 were 1.4 mΩ or higher, whereas the DC internal resistance values for all of Examples 1 to 4 were 1.4 mΩ or lower, confirming that resistance was reduced.

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.

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

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

Furthermore, a hole ratio of 0.86 to 1.17 further enhances the effect of reducing ion migration resistance.

It is also preferable that the graphite particles 30 have an average aspect ratio of 1.0 to 1.5. This increases the proportion of edge faces per unit volume of the particle, reducing interfacial migration resistance. Furthermore, by forming the graphite particles 30 into a roughly spherical shape with an average aspect ratio of 1.0 to 1.5, the packing of the graphite particles 30 is improved, allowing the composite layer 20 to be densified. As a result, high energy density and high capacity can be achieved.

The graphite particles 30 have a layered structure in which hexagonal planes of carbon atoms overlap to form multiple layers, and the average isotropy is 75% or more. The isotropy is calculated by calculating a resultant vector P, which is a composite of component vectors f obtained for each graphite segment contained in the 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, which has the magnitude of the distance from the tip of each component vector f to the resultant vector P as the base, as the ratio of the magnitude of the resultant vector P to the sum of the magnitudes of the resultant vector P and the normal vector n.

The more isotropic the layered structure of the graphite particles 30 obtained in this way is, the more the orientation of the electrode can be improved by magnetic field orientation, and as a result, the internal resistance can be reduced.

Furthermore, the method for manufacturing a secondary battery electrode according to this embodiment makes it possible to manufacture a secondary battery electrode that achieves the above-mentioned effects.

1 Lithium ion 10 Current collector foil 20, 21, 22 Composite layer 30 Graphite particles 31 Spheroidized graphite particles 32 Scaly graphite particles 40, 41, 42 Internal pores 50, 51, 52 External pores E1, E2, E3 Negative electrode plates L1, L2 Hole path

Claims (2)

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. The method for producing an electrode for a secondary battery according to claim 1, wherein the hole ratio is 0.86 to 1.17.