JP7710962B2 - 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 so-called portable power sources for personal computers and mobile terminals, 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 the electrodes of a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate), which absorb and release lithium ions.

Electrodes used in secondary batteries such as lithium-ion secondary batteries include a conductive current collector foil (current collector) and a composite layer containing electrode materials such as active material and conductive material held on the current collector foil. In such secondary batteries, the electrode resistance is reduced by increasing the degree of orientation of the electrode material in the composite layer in order to improve the battery characteristics.

Patent Document 1 discloses the following non-aqueous battery and its manufacturing method. The non-aqueous battery and its manufacturing method described in Patent Document 1 include at least a positive electrode and a negative electrode, and at least one of the positive electrode and the negative electrode includes a current collector, an intermediate layer, and an active material layer. The intermediate layer is interposed between the current collector and the active material layer. The intermediate layer includes graphite particles and insulating particles. In a cross section in the thickness direction of the intermediate layer, the graphite particles have a major axis diameter that is at least one time the thickness of the intermediate layer. In an X-ray diffraction measurement of the intermediate layer by the out-of-plane method, the ratio of the intensity of the diffraction line of the graphite crystal to the intensity of the 002 diffraction line of the graphite crystal is 0.0011 or more.

JP 2018-190575 A

In the technology described in Patent Document 1, the graphite particles contained in the intermediate layer are oriented by a magnetic field. Meanwhile, for example, lithium composite oxides, which have high capacity and excellent thermal stability, are preferably used as active material particles for the positive electrode of a lithium ion secondary battery. However, such active material particles are difficult to orient by a magnetic field, so the technology described in Patent Document 1 has the problem of being unable to increase the degree of orientation.

The present invention has been made to solve such problems, and aims to provide an electrode for a secondary battery that can increase the degree of orientation without relying on the magnetism of the active material particles and reduce the internal resistance of the battery, and a method for manufacturing the electrode for a secondary battery.

The electrode for a secondary battery according to one embodiment has a current collector foil and a composite layer formed on the surface of the current collector foil. The composite layer includes a composite material having a magnetic coating formed on at least one surface along the long axis direction of flattened active material particles, and the composite material is oriented so that the long axis direction is approximately perpendicular to the surface of the current collector foil.

In addition, the method for manufacturing a secondary battery electrode according to one embodiment includes a composite material forming step of forming a composite material in which a coating that exhibits magnetism in response to an externally applied magnetic field is formed on at least one surface along the longitudinal direction of flat active material particles; a coating step of coating a surface of a current collector foil with a paste for forming a composite layer that contains at least a composite material and a solvent; a magnetic field orientation step of applying a magnetic field that is substantially perpendicular to the surface of the current collector foil to the composite layer forming coating before the composite layer forming coating formed by the paste for forming a composite layer applied to the surface of the current collector foil dries, thereby orienting the composite material contained in the composite layer forming coating; and a composite layer forming step of drying the composite layer forming coating in which the composite material is oriented to form a composite layer on the surface of the current collector foil.

The present invention provides an electrode for a secondary battery that can increase the degree of orientation regardless of the magnetism of the active material particles and reduce the internal resistance of the battery, and a method for manufacturing the electrode for a secondary battery.

1 is a cross-sectional view showing an electrode for a secondary battery according to a first embodiment. FIG. 2 is a diagram showing a composite material contained in the secondary battery electrode shown in FIG. 1 . 3 is a flowchart showing a method for manufacturing a secondary battery electrode according to the first embodiment. FIG. 1 is a first diagram illustrating a composite material forming process. FIG. 2 is a second diagram illustrating the composite material forming process. FIG. 3 is a third diagram illustrating the composite material forming process. FIG. 2 is a diagram illustrating a coating process. FIG. 13 is a diagram illustrating a magnetic field orientation process.

First embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, the following description and drawings are appropriately simplified for clarity of explanation.

In this embodiment, the "minor axis diameter" is the average of the longest diameters in the minor axis direction of the active material particles 31 and the composite material 30. In this embodiment, the "major axis diameter" is the average of the longest diameters in the major axis direction of the active material particles 31 and the composite material 30. In addition, the minor axis direction and the major axis direction are the same for the active material particles 31 and the composite material 30.

The following describes a preferred embodiment of the secondary battery electrode according to the present embodiment, specifically as an electrode for 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, 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).

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

As shown in FIG. 1, the electrode plate 1 has a current collector foil 10 and a composite layer 20 formed on the current collector foil 10. The current collector foil 10 is formed in a plate or foil shape and is made of a metal with good electrical conductivity. In the case of a positive electrode, examples of the metal constituting the current collector foil 10 include aluminum and aluminum alloys. In the case of a negative electrode, examples of the metal constituting the current collector foil 10 include copper and copper alloys. 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, except for an edge along one edge in the width direction. The electrode plate 1 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 electrically connected to an external terminal. The composite layer 20 contains at least a composite material 30 mainly composed of active material particles 31, and is held by the current collector foil 10. The composite layer 20 may contain a conductive material, a binder, and other additives (e.g., a thickener, a dispersant) as necessary.

The density of the mixture layer 20 is not particularly limited, but for example, in the case of a positive electrode, it is preferably 1.0 g/cm ³ to 3.8 g/cm ³ , more preferably 1.5 g/cm ³ to 3.0 g/cm ³ , and particularly preferably 1.8 g/cm 3 to 2.7 g/cm ^3. The thickness of the mixture layer 20 is also not particularly limited, but for example, it is preferably 10 μm to 200 μm, more preferably 20 μm to 150 μm, and particularly preferably 30 μm to 100 μm.

The composite material 30 contained in the composite layer 20 has its long axis oriented substantially perpendicular to the surface of the current collector foil 10. In other words, the degree of orientation, which is the average angle between the surface of the current collector foil 10 and the long axis direction of the composite material 30 (active material particles 31), is 45° to 90°, and is preferably 90°.

In the electrode plate 1 in the case of constructing a lithium ion secondary battery, the electrolyte permeates the voids in the composite layer 20. Therefore, when the composite material 30 is oriented in one direction in the composite layer 20, the migration path of the lithium ions in the composite layer 20 becomes linear, so that the conductivity of the ions in the electrolyte increases and the resistance is reduced. It is believed that the migration path of the lithium ions in the composite layer 20 is shortest when the composite material 30 is oriented at 90°.

The conductive material may be, for example, carbon black such as acetylene black (AB), activated carbon, graphite, carbon nanotubes, or other carbon materials. When carbon nanotubes are used, the proportion of the conductive material in the entire composite layer 20 is preferably 0.1% to 5% by mass, more preferably 0.3% to 3% by mass, and particularly preferably 0.5% to 1.5% by mass.

Examples of binders that can be used include polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), styrene butadiene rubber (SBR), and butyl rubber (BR). The proportion of the binder in the entire composite layer 20 is preferably, for example, 0.01% to 10% by mass, more preferably 0.05% to 5% by mass, and particularly preferably 0.1% to 2% by mass.

Next, the composite material 30 will be described in detail with reference to FIG. 2. FIG. 2 is a diagram showing the composite material contained in the secondary battery electrode shown in FIG. 1. As shown in FIG. 2, the composite material 30 has active material particles 31 and a coating 32. The active material particles 31 are a material capable of absorbing and releasing lithium ions. In addition, the active material particles 31 may be a material that is oriented by a magnetic field, or a material that is not oriented by a magnetic field.

In the case of the electrode plate 1 of the positive electrode, for example, various lithium composite oxides can be used as the active material particles 31. Examples of lithium composite oxides having a layered rock salt crystal structure include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium nickel manganese cobalt oxide (LiNi _x Mn _y Co _z O ₂ , x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1). Examples of lithium composite oxides having a spinel crystal structure include lithium manganate (LiMn ₂ O ₄ ), and lithium nickel manganese oxide (LiNi _x Mn _y O ₄ , x+y=2, 0≦x≦0.5). Examples of lithium composite oxides having an olivine crystal structure include lithium iron phosphate (LiFePO ₄ ).

A suitable example of the active material particles 31 is a lithium composite oxide having a layered rock-salt type crystal structure that contains at least one transition metal element selected from the group consisting of nickel, cobalt, and manganese, in addition to lithium. Furthermore, the lithium composite oxide may contain other metal elements.

In the case of the electrode plate 1 of a negative electrode, examples of the active material particles 31 include silicon oxide (SiO), tin oxide (SnO), and lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The active material particles 31 are formed in a flat shape with a predetermined aspect ratio. The aspect ratio is the ratio of the major axis diameter to the minor axis diameter of the active material particles 31 (major axis diameter/minor axis diameter). The aspect ratio of the active material particles 31 is preferably 1.5 or more. If the aspect ratio is less than 1.5, the degree of orientation of the composite material 30 tends to decrease. From the viewpoint of increasing the degree of orientation of the composite material 30, the larger the aspect ratio of the active material particles 31, the more preferable it is.

The active material particles 31 may be primary particles, or may be secondary particles formed by an aggregation of multiple primary particles. The shape of the active material particles 31 can be observed using images obtained by SEM (Scanning Electron Microscope) observation.

When the active material particles 31 are secondary particles, for example, the average particle size of each active material particle 31 is preferably 1 μm to 20 μm, more preferably 2 μm to 15 μm, and particularly preferably 3 μm to 10 μm. The average particle size of the active material particles 31 can be measured using an SEM or by particle size distribution measurement.

The coating 32 is formed on at least one surface along the long axis direction of the active material particles 31. Two types of composite materials 30 are illustrated in FIG. 2. Of the two types of composite materials 30 illustrated in FIG. 2, the composite material 30 illustrated in the upper part of FIG. 2 has a coating 32 on one surface along the long axis direction of the active material particles 31. The composite material 30 illustrated in the lower part of FIG. 2 has a coating 32 on both surfaces along the long axis direction of the active material particles 31. The coating 32 is formed of a magnetic carbon material, a metal, or an alloy thereof. In addition, from the viewpoint of improving the conductivity of the electrode plate 1, it is preferable that the coating 32 itself is conductive.

The composite material 30 is configured so that the ratio of the volume of the coating 32 to the volume of the active material particles 31 is 5% to 20% by volume. If the ratio of the volume of the coating 32 to the volume of the active material particles 31 is less than 5% by volume, the sensitivity of the composite material 30 to the magnetic field becomes insufficient and the degree of orientation tends to decrease. If the ratio of the volume of the coating 32 to the volume of the active material particles 31 is more than 20% by volume, the proportion of the coating 32 in the entire composite layer 20 increases, and the capacity tends to decrease. The coating 32 is preferably formed as a uniform film, but may be a non-uniform film formed to the extent that it does not affect the magnetic field orientation when the composite layer 20 is formed.

Next, the method for manufacturing an electrode for a secondary battery according to this embodiment will be described with reference to FIG. 3. FIG. 3 is a flowchart showing the method for manufacturing an electrode for a secondary battery according to the first embodiment. The method for manufacturing an electrode for a secondary battery according to this embodiment can be applied to both the positive and negative electrode plates 1.

Here, for example, lithium composite oxides with high capacity and excellent thermal stability are preferably used as the active material particles 31 for the positive electrode of a lithium ion secondary battery. The active material particles 31 formed from lithium composite oxides typically have the form of secondary particles consisting of a randomly oriented collection of multiple single crystal primary particles, and are difficult to orient in a magnetic field. Therefore, if such polycrystalline active material particles 31 are used alone, it is difficult to control the orientation of the active material particles 31.

In contrast, in the method for manufacturing an electrode for a secondary battery according to this embodiment, the active material particles 31 can be magnetically oriented regardless of the magnetism of the active material particles 31. Therefore, the method for manufacturing an electrode for a secondary battery according to this embodiment is preferably used for an electrode plate 1 that includes active material particles 31 that are poorly oriented.

As shown in FIG. 3, the method for manufacturing an electrode for a secondary battery according to this embodiment includes the following steps S1 to S4. In the composite material formation step of step S1, a composite material 30 is formed in which a coating 32 that exhibits magnetism in response to an externally applied magnetic field is formed on at least one surface along the longitudinal direction of flat active material particles 31. In the coating step of step S2, a paste for forming a composite layer that contains at least the composite material 30 and a solvent is applied to the surface of the current collector foil 10.

In the magnetic field orientation process of step S3, before the composite layer forming coating film 21 formed by the composite layer forming paste applied to the surface of the current collector foil 10 dries, a magnetic field that is approximately perpendicular to the surface of the current collector foil 10 is applied to the composite layer forming coating film 21 to orient the composite material 30 contained in the composite layer forming coating film 21. In the composite layer forming process of step S4, the composite layer forming coating film 21 in which the composite material 30 has been oriented is dried to form the composite layer 20 on the surface of the current collector foil 10.

Each of the above steps will be described in more detail with reference to Figures 4 to 8. Figure 4 is a first diagram illustrating the composite material forming step. Figure 5 is a second diagram illustrating the composite material forming step. Figure 6 is a third diagram illustrating the composite material forming step. Figure 7 is a diagram illustrating the coating step. Figure 8 is a diagram illustrating the magnetic field orientation step. The cross-sectional views shown in Figures 4 to 8 show a portion of a cross section perpendicular to the surface of the current collector foil 10 or the substrate 40, respectively. Also, the active material particles 31 shown in Figures 4 to 6 are shown as ellipses to simplify the secondary particles, and this ellipses corresponds to one active material particle 31.

First, the composite material forming process includes the first to fourth steps. First, as shown in S1-1 of FIG. 4, in the first step, a composite material forming paste containing active material particles 31 and a solvent is applied to the surface of a flat substrate 40. The solvent used may be the same as that used in the application process described below, or a different type of solvent may be used.

The paste for forming the composite material is prepared by adding a solvent to the active material particles 31 and kneading the mixture using a kneading machine such as a planetary mixer. The paste for forming the composite material is applied to the surface of the substrate 40 using a coating method such as a die coater, slit coater, comma coater, gravure coater, or blade coater.

Figure 4 shows a method using a die coater. The die coater has a die head 50 that ejects the slurry toward the opposing substrate 40 through a predetermined gap G and applies the slurry to the surface of the substrate 40. When using a die coater, a composite material forming paste is ejected from the die head 50 and applied to the surface of the substrate 40 with a gap G between the substrate 40 and the die head 50 that corresponds to the average particle size of each active material particle 31. As a result, a composite material forming coating film 60 having a thickness corresponding to the average particle size of each active material particle 31 is formed on the surface of the substrate 40. The viscosity of the composite material forming paste is appropriately set within a range that can form a composite material forming coating film 60 having the above thickness and does not impair the coatability.

Next, as shown in S1-2 of FIG. 4, in the second step, the composite material forming coating film 60 formed in the first step is dried to remove the solvent contained in the composite material forming coating film 60. As the drying method, natural drying, hot air, low humidity air, vacuum, infrared drying, far infrared drying, electron beam drying, etc. can be used alone or in combination. When the solvent is removed from the composite material forming coating film 60, a plurality of active material particles 31 randomly oriented on the surface of the substrate 40 can be obtained.

Next, as shown in S1-3 of FIG. 5, in the third step, the active material particles 31 arranged on the surface of the substrate 40 are pressed using a pressing method such as roll pressing or plate pressing, thereby orienting the active material particles 31 in a certain direction. FIG. 5 shows an example in which the active material particles 31 are pressed in a direction approximately perpendicular to the surface of the substrate 40 by the roll 70 of a roll pressing machine.

In this way, as shown in S1-4 of FIG. 5, the active material particles 31 can be oriented so that they do not overlap with each other on the surface of the substrate 40 and so that their long axis direction is approximately parallel to the surface of the substrate 40.

Next, as shown in S1-5 of FIG. 6, in the fourth step, a coating 32 is formed on the active material particles 31 that have been oriented in a certain direction in the third step. The coating 32 can be formed by, for example, a chemical vapor deposition (CVD) method, a physical vapor deposition (PVD) method, an ionization deposition method, or the like.

The active material particles 31 are oriented so that their long axis direction is approximately parallel to the surface of the substrate 40, and one side along the long axis direction is exposed facing upward. Therefore, a coating 32 can be formed on at least one side along the long axis direction of the active material particles 31 using various deposition methods.

Then, as shown in S1-6 of FIG. 6, the composite material 30 in which the coating 32 is formed on at least one surface along the longitudinal direction of the active material particles 31 is recovered from the substrate 40. In this manner, the composite material 30 can be formed.

The coating 32 may be formed on multiple surfaces by repeating the above steps 1 to 4. When coatings 32 are formed on multiple surfaces of the active material particles 31, for example, the composite material 30 shown in the lower part of Figure 2 can be formed. By forming coatings 32 on multiple surfaces of the active material particles 31, the rotational force applied to the composite material 30 during magnetic field orientation is applied to the composite material 30 at multiple points, and the rotational force is increased, thereby improving the orientation of the composite material 30.

Next, as shown in FIG. 7, in the coating process, a solvent is added to the composite material 30 and, if necessary, a powder containing a conductive material, a binder, and other additives, and the mixture is kneaded using a kneading machine such as a planetary mixer to prepare a paste for forming a composite layer.

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, water, and mixed solvents mainly composed of water.

The viscosity of the paste for forming the composite layer is preferably 3 Pa·s or less, and more preferably 1 Pa·s or less. From the viewpoint of increasing the degree of orientation of the composite material 30, the lower the viscosity of the paste for forming the composite layer, the more preferable it is. The lower limit of the viscosity of the paste for forming the composite layer is appropriately set within a range that does not impair the coatability.

Next, the prepared composite layer forming paste is applied to the surface of the current collector foil 10 to form a composite layer forming coating film 21 on the surface of the current collector foil 10. The composite layer forming paste can be applied using the various coating methods exemplified in the composite material forming process.

Next, as shown in FIG. 8, in the magnetic field orientation process, 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 composite layer forming coating film 21. There are no particular limitations on the magnetic field generator as long as it can generate the required magnetic field, and 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 composite layer forming coating film 21 is preferably 350 mT or more, and more preferably 500 mT or more. The greater the magnetic flux density of the magnetic field, the greater the degree of orientation of the composite material 30. The time for applying the magnetic field to the composite layer forming coating film 21 is, for example, about 1 to 120 seconds. The composite material 30 contained in the composite layer forming coating film 21 to which the magnetic field is applied is oriented so that its long axis direction is approximately perpendicular to the surface of the current collector foil 10.

Next, in the composite layer forming process, the composite layer forming coating film 21 containing the oriented composite material 30 as described above is dried to remove the solvent contained in the composite layer forming coating film 21. The composite layer forming coating film 21 can be dried using the various drying methods exemplified in the composite material forming process. Furthermore, the dried product is pressed as necessary. This allows the composite layer 20 to be formed on the surface of the current collector foil 10.

The above steps allow the electrode plate 1 shown in FIG. 1 to be manufactured. According to the method for manufacturing an electrode for a secondary battery according to this embodiment, the coating 32 exhibits magnetism, so that the degree of orientation of the active material particles 31 can be increased using a magnetic field, without relying on the magnetism of the active material particles 31 themselves.

Below, examples and comparative examples are described. Note that the examples do not limit the present invention. First, a battery cell for evaluation was constructed according to the following procedure.

(Example)
[Preparation of electrode plate 1]
A positive electrode plate was manufactured according to the flow shown in Fig. 3. The active material particles 31 used were secondary particles of lithium nickel manganese cobalt oxide (NMC particles) having an average composition represented by LiNi _1/3 Co _1/3 Mn _1/3 O _2. The NMC particles had a substantially elliptical shape, with a major axis diameter (average): 5 µm, a minor axis diameter (average): 3 µm, and a volume: 188.5 µm ³ .

In the composite material formation process, a composite material forming paste, which was made by kneading NMC particles and NMP, was applied to the surface of the substrate 40 using a die coater to form a composite material forming coating film 60. The amount of the composite material forming paste applied was adjusted so that the composite material forming coating film 60 had a thickness of 6 μm. The composite material forming coating film 60 was then dried with hot air to obtain a number of NMC particles randomly oriented on the surface of the substrate 40. Thereafter, the NMC particles arranged on the surface of the substrate 40 were roll pressed to orient the active material particles 31 in a fixed direction so that they would not overlap each other.

The active material particles 31 were then placed together with the substrate 40 into a plasma CVD device. The plasma CVD device was used to decompose a gas containing hydrocarbons, and carbon was deposited on one side of the active material particles 31 along the longitudinal direction, forming a coating 32 made of a carbon material having a graphite structure (layered structure), thereby forming a composite material 30. At this time, the amount of coating was adjusted so that the ratio of the volume of the coating 32 to the volume of the active material particles 31 was 10 volume %, and a coating 32 with a thickness of 0.4 μm was formed.

In the coating process, the composite material 30, AB as a conductive material, and PVdF as a binder were mixed in a predetermined mixing ratio, and NMP was added as a solvent and kneaded to prepare a composite layer forming paste (viscosity 1.2 Pa·s). The prepared composite layer forming paste was applied to both sides of the aluminum foil (thickness 15 μm) which is the current collector foil 10. The coating amount of the composite layer forming paste was adjusted so that the basis weight was 10 mg/cm ^2. As a result, a composite layer forming coating film 21 was formed on the surface of the aluminum foil.

In the magnetic field orientation process, a pair of permanent magnets were arranged to sandwich from above and below the aluminum foil on which the composite layer forming coating film 21 was formed, and a magnetic field was applied to the composite layer forming coating film 21. The magnetic flux density of the magnetic field was 500 mT, and the magnetic field application time was 2.0 seconds. The direction of the magnetic field lines when the magnetic field was applied was approximately perpendicular to the surface of the aluminum foil. This caused the composite material 30 to be oriented so that its long axis direction was approximately perpendicular to the surface of the aluminum foil.

The composite layer-forming coating film 21 containing the magnetically oriented composite material 30 was dried with hot air at 110° C., cut to a predetermined size, and pressed to produce a positive electrode plate. The pressed composite layer 20 had a density of 2.7 g/cm ³ and a thickness of 45 μm.

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

[Construction of evaluation battery cell]
The composite layer 20 sides of the manufactured positive electrode plates were placed opposite each other, and the electrode assembly with a three-layered (PE/PP/PE) separator having a laminated polyethylene (PE) layer and a polypropylene (PP) layer was placed between them was housed inside an aluminum laminate exterior material, and an electrolyte was added and sealed to construct a laminated evaluation battery cell (positive electrode facing cell).

(Comparative Example)
A positive plate was manufactured in the same manner as in the Example, except that the magnetic field orientation process was omitted in the manufacturing process of the positive plate. Then, this positive plate was used to construct an evaluation battery cell of the Comparative Example in the same manner as in the Example.

[evaluation]
Next, the battery performance of each evaluation battery cell of the examples and comparative examples was evaluated. In evaluating the battery performance, each evaluation battery cell was subjected to a conditioning process at 3.0 V to 4.2 V, and then adjusted to a SOC (State Of Charge) of 50% in an environment of 25° C., and the resistance value (Ω) was measured by an AC impedance method. The measurement conditions were an input voltage of 100 mV and a frequency range of 0.1 Hz to 100,000 Hz.

The diameter of the semicircle was read from the Nyquist plot of the impedance obtained by the measurement, and the resistance value of each evaluation battery cell was derived. As a result, when the resistance value of the evaluation battery cell of the comparative example was set to 100%, the relative value of the resistance value of the evaluation battery cell of the example was 96.2%.

In addition, the average value of the tortuosity of each evaluation battery cell was calculated using the value of the ionic resistance R _ion obtained by the AC impedance method. The ionic resistance R _ion is the resistance related to the movement of lithium ions inside the electrode. The tortuosity is a value indicating the degree of bending of the movement path of lithium ions in the composite layer, and can be calculated by the following formula (1).
τ={R _ion (κεS)/L}...Formula (1)

Here, τ is the tortuosity, R _ion is the ionic resistance (/Ω·cm ² ), L is the thickness of the composite layer (μm), κ is the conductivity of the electrolyte (S/m), ε is the porosity of the composite layer (%), and S is the area of the reaction interface (μm ² ). The closer the tortuosity is to 1, the more linear the migration path is, so that inside the electrode, the conductivity of the ions in the electrolyte is increased and the resistance is reduced.

The tortuosity of the evaluation battery cell of the embodiment was 2.0, and the tortuosity of the evaluation battery cell of the comparative example was 2.8. As can be seen from these results, the tortuosity of the evaluation battery cell of the embodiment was lower than that of the evaluation battery cell of the comparative example, and it was confirmed that the conductivity of the ions in the electrolyte inside the electrode was improved and the electrical resistance was reduced.

In this way, the presence of the highly oriented composite material 30 in the composite layer 20 makes the migration path of the lithium ions in the composite layer 20 linear (the tortuosity approaches 1), and the migration distance of the lithium ions from the main surface of the composite layer 20 to the current collecting foil 10 can be shortened.

As described above, in the electrode for a secondary battery according to this embodiment, the composite layer 20 formed on the surface of the current collector foil 10 contains a composite material 30 in which a magnetic coating 32 is formed on at least one surface along the long axis direction of flat active material particles 31. The composite material 30 is oriented so that the long axis direction is approximately perpendicular to the surface of the current collector foil 10.

With this configuration, the degree of orientation of the active material particles 31 can be increased using a magnetic field, regardless of the magnetism of the active material particles 31. In addition, the flat active material particles 31 are oriented perpendicular to the surface of the current collector foil 10, shortening the distance that lithium ions travel in the composite layer 20, thereby improving the conductivity of ions in the electrolyte inside the electrode. This makes it possible to obtain a lithium ion secondary battery with reduced internal resistance.

Furthermore, the ratio of the volume of the coating 32 to the volume of the active material particles 31 is preferably 5% to 20% by volume. With this configuration, a composite material 30 having sufficient sensitivity to magnetic fields can be formed, and the degree of orientation of the composite material 30 containing the active material particles 31 can be further increased.

Furthermore, it is preferable that the active material particles 31 have an aspect ratio of 1.5 or more. With such a configuration, the degree of orientation of the composite material 30 containing the active material particles 31 can be further increased.

Furthermore, it is preferable that the coating 32 is conductive. With this configuration, a conductive coating 32 is formed on the surface of the active material particles 31, improving the conductivity of the electrode plate 1. As a result, the output performance of the battery is improved.

Furthermore, it is preferable that the active material particles 31 are secondary particles formed by aggregating multiple primary particles of a lithium composite oxide containing at least one of nickel, cobalt, and manganese. With this configuration, even polycrystalline active material particles 31 with poor orientation can be increased in degree of orientation using a magnetic field.

Therefore, the secondary battery electrode according to this embodiment can increase the degree of orientation without relying on the magnetism of the active material particles 31, thereby reducing the internal resistance of the battery.

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.

Reference Signs List 1 Electrode plate 10 Current collector foil 20 Composite layer 21 Composite layer forming coating 30 Composite material 31 Active material particles 32 Coating 40 Substrate 50 Die head 60 Composite material forming coating 70 Roll G Gap

Claims (10)

A current collecting foil;
a composite layer formed on a surface of the current collecting foil,
The mixture layer is
The composite material includes a flat active material particle and a magnetic coating formed on at least one surface along the major axis of the flat active material particle,
The composite material comprises:
The electrode for a secondary battery is oriented so that the long axis direction is substantially perpendicular to the surface of the current collecting foil. The electrode for a secondary battery according to claim 1, wherein the ratio of the volume of the coating to the volume of the active material particles is 5% by volume to 20% by volume. The electrode for a secondary battery according to claim 1 or 2, wherein the active material particles have an aspect ratio of 1.5 or more. The electrode for a secondary battery according to any one of claims 1 to 3, wherein the coating has electrical conductivity. The electrode for a secondary battery according to any one of claims 1 to 4, wherein the active material particles are secondary particles formed by aggregating multiple primary particles of a lithium composite oxide containing at least one of nickel, cobalt, and manganese. a composite material forming step of forming a composite material in which a coating film that exhibits magnetism in response to an externally applied magnetic field is formed on at least one surface along the major axis of a flat active material particle;
a coating step of coating a paste for forming a composite layer containing at least the composite material and a solvent on a surface of a current collecting foil;
a magnetic field orientation step of applying a magnetic field in a direction substantially perpendicular to a surface of the current collecting foil to the composite layer forming coating film formed from the composite layer forming paste before the composite layer forming coating film is dried, thereby orienting the composite material contained in the composite layer forming coating film;
a composite layer forming step of drying the composite layer forming coating film in which the composite material is oriented to form a composite layer on the surface of the current collecting foil;
The present invention relates to a method for producing an electrode for a secondary battery. The method for manufacturing a secondary battery electrode according to claim 6, wherein the ratio of the volume of the coating to the volume of the active material particles is 5% to 20% by volume. 8. The method for producing an electrode for a secondary battery according to claim 6 , wherein the active material particles have an aspect ratio of 1.5 or more. The method for manufacturing a secondary battery electrode according to any one of claims 6 to 8, wherein the coating has electrical conductivity. The method for manufacturing a secondary battery electrode according to any one of claims 6 to 9, wherein the active material particles are secondary particles formed by aggregating multiple primary particles of a lithium composite oxide containing at least one of nickel, cobalt, and manganese.