Method for manufacturing a positive electrode of a lithium ion secondary battery and method for manufacturing a lithium ion secondary battery

Abstract

To improve the input / output characteristics of a lithium-ion secondary battery.SOLUTION: A method for manufacturing a positive electrode for a lithium-ion secondary battery including a current collector, a positive electrode active material particle held by the current collector, a conductive material, and a positive electrode mixture layer containing a binder includes a kneading step (S12), which is an infiltration step of aggregating primary particles to form a hollow shell, and arranging the fibrous conductive material inside and outside the secondary particles of a porous positive electrode active material having a shell portion with a through hole, together with the binder, and a pressing step (S15) of crushing at least some of the secondary particles. Due to increase in the resistance of a positive electrode plate, the input / output characteristics of the lithium ion secondary battery are improved.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a positive electrode of a lithium-ion secondary battery and a method for manufacturing a lithium-ion secondary battery. More specifically, the present invention relates to a method for manufacturing a positive electrode of a lithium-ion secondary battery with low reaction resistance and a method for manufacturing a lithium-ion secondary battery.

Background Art

[0002] In electric vehicles, such as electric cars, or hybrid vehicles having a motor and an engine as a driving source of the vehicle, lithium-ion secondary batteries have been increasingly used as a power source in recent years. In such a lithium-ion secondary battery, electrodes are provided with materials (active materials) capable of reversibly occluding and releasing lithium ions (Li ions). Charging and discharging are performed by the movement of Li ions between the positive and negative electrodes. However, since such active materials themselves have low electron conductivity, usually, in the positive electrode, a conductive material such as carbon powder is mixed to form a composite material to form the positive electrode. In this case, to improve the output of the positive electrode, it is desirable to increase the reaction area and reduce the resistance.

[0003] The positive electrode plate of a lithium-ion secondary battery includes a current collector and an active material layer held by the current collector and containing active material particles and a conductive material. The active material particles include primary particles generated by firing and secondary particles formed by the aggregation of these primary particles. The secondary particles have a shell portion and a hollow portion formed inside the shell portion. Generally, the smaller the particle size, the larger the specific surface area contributing to the reaction. Therefore, it is conceivable to pulverize the secondary particles into primary particles to increase the specific surface area. However, if the secondary particles are pulverized into primary particles in advance, the active material particles are likely to aggregate. Aggregation causes problems such as an increase in viscosity, a decrease in solid content, and poor drying, resulting in extremely poor handling in the composite material preparation process. Therefore, there has been a problem that the target positive electrode plate cannot be easily formed when the particles are made into primary particles.

[0004] Therefore, in the invention described in Patent Document 1, the conductive material contained in the active material layer is characterized in that the secondary particles are arranged as they are in both the hollow part of the secondary particles of the active material particles and between the active material particles outside the secondary particles.

[0005] In addition, in the power storage element of the invention described in Patent Document 2, the first electrode plate includes a current collector and a composite layer laminated on the current collector. The composite layer contains at least one of a binder and a conductive aid, primary particles of an active material, and secondary particles formed by aggregating a plurality of the primary particles to have a hollow region inside. A part of at least one of the binder and the conductive aid is arranged in the hollow region.

[0006] As described above, secondary particles of the active material having a hollow part are generated from a plurality of primary particles of the active material. Such secondary particles are suitable for the occlusion and release of lithium due to the crystal structure of the active material. Then, through holes are provided in the secondary particles, and a conductive material is arranged inside to reduce the resistance of the positive electrode.

[0007] In the inventions described in Patent Document 1 and Patent Document 2, the resistance is reduced by arranging a conductive material inside the secondary particles of the active material.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the inventions described in Patent Document 1 and Patent Document 2, the resistance is reduced by arranging a conductive material inside the secondary particles of the active material, but it is desirable to further reduce the resistance. The problem to be solved by the method for manufacturing a positive electrode of a lithium-ion secondary battery and the method for manufacturing a lithium-ion secondary battery according to the present invention is to further reduce the resistance of the positive electrode.

Means for Solving the Problems

[0010] To solve the above problems, in the method for manufacturing a positive electrode of a lithium-ion secondary battery according to the present invention, a positive electrode of a lithium-ion secondary battery including a current collector, positive electrode active material particles held by the current collector, a conductive material, and a positive electrode mixture layer containing a binder is provided. The manufacturing method includes an impregnation step of disposing the fibrous conductive material together with the binder inside and outside secondary particles of a porous positive electrode active material having a shell portion formed in a hollow shell shape by aggregation of primary particles and having through holes, and a pressing step of crushing at least a part of the secondary particles.

[0011] It is preferable that the average diameter of the fibrous conductive material is 60% or less of the average diameter of the through holes of the secondary particles. In the pressing step, it is preferable that 50% or more of the positive electrode active material particles are primary particles.

[0012] In the impregnation step, it is preferable to impregnate a binder containing the fibrous conductive material into 85% or more of the quantity of the secondary particles together with the fibrous conductive material. The through holes of the secondary particles preferably have a diameter of 50 nm or more.

[0013] The diameter of the fibrous conductive material is preferably 1 nm to 100 nm. The length of the fibrous conductive material is preferably 100 nm to 1000 nm. The positive electrode active material is preferably composed of a lithium transition metal oxide.

[0014] The fibrous conductive material preferably consists of carbon nanotubes. The method for manufacturing a lithium-ion secondary battery can include the method for manufacturing a positive electrode of a lithium-ion secondary battery described above.

Advantages of the Invention

[0015] In the method for manufacturing the positive electrode of the lithium-ion secondary battery of the present invention, the reaction resistance of the positive electrode can be suppressed, and the input / output characteristics of the lithium-ion secondary battery can be improved.

Brief Description of the Drawings

[0016]

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Mode for Carrying Out the Invention

[0017] (Principle and Outline of the Embodiment) The present invention will be described with reference to the drawings according to an embodiment of a method for manufacturing a lithium ion secondary battery. In the method for manufacturing a lithium ion secondary battery of the present embodiment, in the positive electrode manufacturing process, the purpose is to manufacture a positive electrode plate 2 that increases the reaction area and reduces the resistance in order to improve the output of the positive electrode. Therefore, not only the surface area of the particles of the positive electrode active material itself is increased, but in addition, the conductive material is appropriately arranged. Hereinafter, the specific configuration will be described.

[0018] <Lithium Ion Secondary Battery> In the lithium ion secondary battery of the present embodiment, carbon or graphite capable of inserting and extracting lithium is used for the negative electrode, and various lithium composite oxides are used for the positive electrode.

[0019] During charging, lithium atoms in the crystal of the positive electrode are released into the electrolyte as lithium ions. At the same time, lithium ions in the electrolyte penetrate into the crystal of the negative electrode, and during discharging, the lithium atoms in the negative electrode return to the positive electrode to operate.

[0020] <Material of the positive electrode active material particles 3> The synthesis method of the positive electrode active material generally involves mixing lithium salt powder (such as LiOH, Li2Co3, etc.) and transition metal oxide powder, and firing this mixture for synthesis.

[0021] Specific examples of the positive electrode active material include lithium cobaltate (LiCoO2). Further, lithium nickelate (LiNiO2) can be mentioned. Also, lithium manganate (LiMn2O4) can be mentioned. Also, lithium nickel cobalt manganate (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2) can be mentioned. Further, lithium iron phosphate (LiFePO4), etc. can also be mentioned. By firing, first, primary particles with a small diameter are generated, and further, a plurality of primary particles are aggregated to generate secondary particles.

[0022] <Manufacturing process of the positive electrode plate 2> Generally, in the manufacturing process of the positive electrode plate 2 (see Fig. 18), a conductive material 33 and a binder 34 whose viscosity is adjusted by a solvent are mixed with granular positive electrode active material particles 3 that are the raw materials of the positive electrode plate 2 to create a paste-like positive electrode mixture. The paste-like positive electrode mixture is applied in layers to a current collector (positive electrode substrate) 5 made of, for example, Al foil. The positive electrode mixture layer 4 formed by the application is dried in a drying furnace and shaped to a predetermined thickness in a pressing process. Then, it is cut into a predetermined shape to complete the positive electrode plate 2.

[0023] <Configuration of the secondary particles 32> FIG. 1 is a schematic diagram showing the appearance of the secondary particles 32 of the positive electrode active material particles 3. FIG. 2 is an end view showing the inside of the secondary particles 32 of the positive electrode active material. The positive electrode active material particles 3 synthesized by the method as described above are such that the primary particles 31 grow reflecting the crystal form of the active material. The average diameter thereof is, for example, 0.5 to 3 μm. These primary particles 31 aggregate to form secondary particles 32, and the particle size becomes larger. The size of the secondary particles 32 is preferably, for example, an average diameter of 10 to 100 μm. The average diameters of the primary particles 31 and the secondary particles 32 are not necessarily regular in shape, but can be the 50% integrated value measured by a laser diffraction particle size distribution measurement method using the Mie scattering theory.

[0024] The fired secondary particles 32 may have a shell portion 32a, a hollow portion 32b formed inside the shell portion 32a, and a through hole 32c penetrating the shell portion 32a. There are those with many through holes 32c and those with few through holes 32c, but in the present embodiment, those with many through holes 32c, which are porous, are preferred.

[0025] <Average diameter W of the through hole 32c> The average diameter W of the through hole 32c is the 50% average particle diameter by SEM (scanning electron micrograph) observation, and varies from large to small. For the purpose of the present embodiment, the fibrous conductive material 33 (see FIG. 3) is disposed inside the secondary particles 32 of the positive electrode active material through the through holes 32c together with the resin of the binder 34. That is, a diameter of the through hole 32c for receiving the fibrous conductive material 33 having an average diameter D is required. As a condition for the average diameter W of the through hole 32c, it is that it can be disposed inside through the through hole 32c of the secondary particles 32 of the positive electrode active material. For this reason, it is desired that the average diameter D of the fibrous conductive material 33 is 60% or less of the average diameter W of the through hole 32c of the secondary particles 32. In other words, it is desired that the average diameter W of the through hole 32c of the secondary particles 32 is 167% or more of the average diameter D of the fibrous conductive material 33.

[0026] <Conductive material 33> FIG. 3 is a micrograph of the conductive material 33 made of carbon nanotubes. The conductive material 33 of the positive electrode plate 2 in the present embodiment is composed of carbon nanotubes. A carbon nanotube is a substance in which a six-membered ring network (graphene sheet) made of carbon is in a single-layer or multi-layer coaxial tubular shape, and is a molecule in which layered carbon is in a tubular shape. It is characterized by having very high electrical conductivity, heat conductivity, and heat resistance. In the present embodiment, shapes such as single layer / multi layer, open / closed ends are not restricted. According to the conductive material 33 made of carbon nanotubes, even if the resin used for the binder 34 is usually a material that does not conduct electricity, high electrical conductivity can be added. With this conductive material 33, the electrical connection between the current collector 5, the positive electrode active material particles 3, and the electrolytic solution becomes good, and the resistance of the positive electrode plate 2 can be kept low.

[0027] <Average length L of the conductive material 33> For that purpose, it is desirable that the carbon tubes contact each other to construct a conductive network. Carbon nanotubes exhibit electrical conductivity and heat conductivity even in small amounts when they are long, and their strength also increases. Therefore, from this viewpoint, it is desirable that the average length L of the fibrous conductive material 33 is long.

[0028] On the other hand, in the present embodiment, the carbon nanotubes are arranged inside the secondary particles 32 of the positive electrode active material through the through holes 32c of the secondary particles 32 of the positive electrode active material together with the resin of the binder 34. Therefore, it is not necessarily the case that the longer the better. If the average length L of the fibrous conductive material 33 is too long, aggregation occurs due to the intermolecular force and hydrogen bond between the carbon nanotubes, and it becomes difficult to penetrate into the secondary particles 32.

[0029] For the purpose of uniformly arranging the conductive material 33 in the positive electrode mixture layer 4 of the present embodiment, it is not necessary to make the lengths uniform, and it is also preferable that carbon nanotubes of various lengths are mixed. Therefore, in the present embodiment, the average length L of the fibrous conductive material 33 is, for example, 100 to 1000 nm.

[0030] <Average diameter D of the conductive material 33> The average diameter D of the conductive material 33 should be such that it can be disposed inside through the through holes 32c of the secondary particles 32 of the positive electrode active material as a condition. Therefore, it is desirable that the average diameter D of the fibrous conductive material 33 is 60% or less of the average diameter W of the through holes 32c of the secondary particles 32.

[0031] On the other hand, if the material is too thin, the contact area with the active material is small, and sufficient conductivity may not be ensured. Note that since the purpose of the average diameter D is also to be disposed inside through the through holes 32c of the secondary particles 32 of the positive electrode active material, a uniform thickness is not necessarily required. It is desirable to actually conduct tests and set it in relation to the through holes 32c of the secondary particles 32 of the positive electrode active material.

[0032] Therefore, in this embodiment, an example of the average diameter D of the fibrous conductive material 33 is, for example, 1 nm to 100 nm. <Binder 34> The binder 34 integrates the positive electrode active material particles 3 and the conductive material 33 in the kneading process (Fig. 9: S12), coats the positive electrode composite layer 4 on the current collector 5 (S13), and fixes it in the drying process (S14). In this embodiment, when referring to the binder, it is a concept including a solvent for adjusting viscosity and additives for adjusting other properties. Then, it is shaped in the pressing process (S15). Examples of the material of the binder 34 include PVdF (polyvinylidene fluoride). Note that in this embodiment, the material of the binder is not particularly limited. In the experimental examples described later, for ease of handling, etc., the cold embedding resin "Specifics - 20 (registered trademark)" made of an epoxy resin from Stoeras is used for testing. It has been confirmed that even if the type of resin is changed in this way, there is no difference in characteristics between the product and the experimental results. Thus, the binder 34 itself is not limited in this embodiment.

[0033] Note that the objective of this embodiment is to dispose the fibrous conductive material 33, together with the resin of the binder 34, inside the secondary particles 32 of the positive electrode active material through the through holes 32c of the secondary particles 32. Therefore, the binder 34 before curing is required to have fluidity such that it can infiltrate into the secondary particles 32. The degree is appropriately adjusted according to the characteristics of the secondary particles 32 and the characteristics of the conductive material 33.

[0034] <Infiltration rate [%]> Here, in this embodiment, the "infiltration rate [%]" means that in the micrograph after the kneading step (S12) which is the infiltration step, when the binder 34 containing the fibrous conductive material 33 enters the inside of the secondary particles 32, the density of the image of the secondary particles 32 becomes darker. Therefore, the infiltration rate [%] was calculated by visually observing the respective shades. That is, the infiltration rate [%] is the ratio of the number of secondary particles 32 into which the binder 34 containing the conductive material 33 has entered the hollow portion 32b to the total number of secondary particles. In this embodiment, the infiltration rate [%] is set to be 85% or more.

[0035] <Positive electrode composite material> The positive electrode composite material is composed of positive electrode active material particles 3, a conductive material 33, a binder 34 containing a solvent, and necessary additives. An example of the distribution is that the positive electrode active material particles 3 (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2) is 75 to 99%, the conductive material 33 (carbon nanotube) is 0.3 to 15%, and the binder 34 (PVdF) is 0.3 to 15%. This material is kneaded to produce a paste of the positive electrode composite material. The viscosity in this embodiment is generally 1000 to 5000 Pa·s.

[0036] <Positive electrode composite material layer 4> The obtained positive electrode composite material is applied to a current collector 5 made of Al foil (S13), and through a drying step (S14), the thickness is adjusted and the surface is flattened by a pressing step (S15), thereby completing the positive electrode composite material layer 4. At this time, a part of the secondary particles 32 is crushed to become primary particles 31, which will be described in detail later.

[0037] <Features of the method for manufacturing a lithium-ion secondary battery according to this embodiment> The feature of the method for manufacturing a lithium-ion secondary battery according to this embodiment is that it includes a kneading step (S12) as an impregnation step and a pressing step (S15) as a crushing step.

[0038] The "impregnation step" is a step of disposing a fibrous conductive material 33 together with a binder 34 in a hollow portion 32b of secondary particles 32 of a positive electrode active material having a shell portion 32a having a hollow or porous through-hole 32c formed in a spherical shell shape by aggregation of primary particles 31.

[0039] The "crushing step" is a step of crushing secondary particles 32 into primary particles by pressing the positive electrode plate 2 from both sides with a pressing roll (not shown). <Kneading step (impregnation step) (S12)> FIG. 4 is a schematic diagram showing a positive electrode active material before the kneading step. In the figure, only an end view of the secondary particles 32 is shown for the sake of explanation. Actually, it also includes primary particles that did not grow into secondary particles 32 and primary particles 31 in which the secondary particles 32 are cracked, but they are omitted. Also, it is shown schematically and does not reflect the actual size. The same applies to FIGS. 5 to 7. FIG. 4 is the state before kneading, that is, before mixing with the binder 34 containing the conductive material 33.

[0040] FIG. 5 is a schematic diagram showing the state of a paste-like positive electrode composite material in which the binder 34 containing the conductive material 33 and the secondary particles 32 which are positive electrode active material particles 3 are mixed but not yet kneaded. In this state, the binder 34 containing the conductive material 33 exists around the secondary particles 32, but the binder 34 containing the conductive material 33 has not yet penetrated into the hollow portion 32b of the secondary particles 32.

[0041] FIG. 6 is a schematic diagram showing the state of a paste-like positive electrode composite material after kneading the binder 34 containing the conductive material 33 and the secondary particles 32 which are the positive electrode active material particles 3. The binder 34 containing the conductive material 33 has its viscosity adjusted by an appropriate solvent and has a viscosity such that it can penetrate into the through holes 32c of the secondary particles 32. Further, since the conductive material 33 is composed of fibrous and flexible carbon nanotubes, it enters the hollow portion 32b from the through holes 32c of the secondary particles 32 along with the penetration of the PVdF (polyvinylidene fluoride) of the fluid with adjusted viscosity. In this case, the average length L of the conductive material 33 has variations even if it is constant. The conductive material 33 with a short length can entirely enter the hollow portion 32b of the secondary particles 32. On the other hand, the conductive material 33 with a long length may have one end entering the hollow portion 32b and the other end protruding outside the secondary particles 32. In such a case, it can be said to be preferable as it serves to conduct between the inside and outside of the secondary particles 32. Thus, the variations in the length of the conductive material 33 play their respective roles and construct a conductive material network by the conductive material 33 in the positive electrode composite material layer 4.

[0042] <positive electrode composite material layer 4> FIG. 7 is a schematic diagram showing the positive electrode composite material layer 4 after the pressing step (S15). In the kneading step (S12), the conductive material 33 is arranged in the hollow portion 32b of the secondary particles 32 so that the impregnation rate [%] is 85% or more. In this way, the conductive material 33 is sufficiently arranged inside and outside the secondary particles 32 together with the binder 34. In such a state, in the pressing step (S15), the positive electrode plate 2 is pressed from both sides by a pressing roll (not shown). The general purpose of the pressing step (S15) is to set the thickness of the positive electrode composite material layer 4 of the positive electrode plate 2 to a set value and to form the surface of the positive electrode plate 2 flat. In the present embodiment, in addition to these purposes, it is aimed to crush the secondary particles 32 among the positive electrode active material particles 3 to form primary particles.

[0043] Specifically, in the pressing step (S15), more than 50% of the positive electrode active material particles 3 of the secondary particles 32 are crushed to "primary particleize". Note that the "primary particleization" in the present embodiment does not necessarily mean decomposing to the primary particles 31 of a single positive electrode active material, but includes crushing such that the hollow portions 32b of the secondary particles 32 are exposed and decomposed into agglomerates of a plurality of primary particles 31.

[0044] <Manufacturing Process of Lithium-Ion Secondary Battery> FIG. 8 is a flowchart showing the manufacturing process of the lithium-ion secondary battery of the present embodiment. With reference to FIG. 8, the outline of the manufacturing process of the lithium-ion secondary battery of the present embodiment will be described.

[0045] <Source Step (S1)> In the present embodiment, first, the source step (S1) is performed. Here, the source step is a step of creating elements of the lithium-ion secondary battery. Specifically, it is a step of creating a positive electrode plate 2, a negative electrode plate (not shown), and a separator that constitute the battery elements of the lithium-ion secondary battery.

[0046] The manufacturing process of the lithium-ion secondary battery of the present embodiment is characterized by the creation of the positive electrode plate 2. Therefore, with reference to FIG. 9, the "positive electrode plate manufacturing process", which is a part of the source step (S1), will be described.

[0047] <Positive Electrode Plate Manufacturing Process> In the positive electrode plate manufacturing process, a secondary particle manufacturing step (S11), a kneading step (S12), a coating step (S13), a drying step (S14), a pressing step (S15), and a cutting step (S16) are performed.

[0048] <Secondary Particle Manufacturing Step (S11)> In the secondary particle manufacturing process (S11), raw materials are mixed and fired to generate secondary particles 32 via primary particles 31. However, since well-known techniques can be used, detailed procedures are omitted. In the secondary particle manufacturing process (S11), particles are created that include a shell portion 32a formed in a spherical shell shape by the aggregation of primary particles 31 and a hollow portion 32b inside the shell portion 32a, and that have many through-holes 32c. The average diameter is 10 to 100 μm, and the average diameter W of the predetermined through-holes 32c is 50 nm or more.

[0049] <Kneading process (including impregnation process) (S12)> In the kneading process (S12) of the present embodiment, a paste-like positive electrode mixture containing positive electrode active material particles 3 including secondary particles 32, fibrous conductive material 33, and a binder 34 with adjusted viscosity is kneaded by a kneader. In the present embodiment, by this kneading, the binder 34 accompanied by the fibrous conductive material 33 penetrates into the hollow portion 32b through the through-holes 32c of the secondary particles 32. At this time, carbon nanotubes, which are the fibrous conductive material 33, are disposed inside and outside the secondary particles 32 together with the binder 34. Conditions such as the viscosity of the positive electrode mixture, the kneading speed, and the time are finally optimized by measuring the reaction resistance of the positive electrode plate 2.

[0050] <Coating process (S13)> The paste-like positive electrode mixture in which carbon nanotubes, which are the fibrous conductive material 33, are disposed inside and outside the secondary particles 32 is coated on a current collector 5 made of Al foil that serves as a positive electrode substrate. The current collector 5 is cut into a rectangle here. A coating device (not shown) includes a nozzle corresponding to the positive electrode mixture layer 4 along the width direction (direction orthogonal to the longitudinal direction) of the current collector 5. The nozzle applies the paste-like positive electrode mixture by relatively moving the positive electrode mixture from one end to the other end in the longitudinal direction of the current collector 5 for scanning. The relative movement may move either the stage on which the current collector 5 is placed or the nozzle itself. The coating device levels the applied paste-like positive electrode mixture to a predetermined thickness using a roller or a doctor blade.

[0051] <Drying process (S14)> When the coating process (S13) is completed, for example, the solvent of the binder 34 is volatilized by an infrared drying device or the like to cure the binder 34 to a certain hardness.

[0052] <Pressing process (crushing process) (S15)> The pressing process is performed using a pressing device (not shown). The pressing device includes at least one pressing roll, and presses in such a way as to relatively move and scan from one longitudinal end to the other end of the positive electrode composite layer 4 formed on the current collector 5. The pressing roll contacts the positive electrode composite layer 4 at a position closer to the current collector 5 than the surface of the positive electrode composite layer 4 formed in the coating process (S13). That is, the gap (the distance between a pair of pressing rolls, or the distance between the pressing roll and the stage facing the pressing roll) is shorter than the thickness of the positive electrode plate 2 by a predetermined length. That is, the pressing device acts to crush the positive electrode composite layer 4.

[0053] The purpose of the pressing process (S15) is the same as in the prior art in that the thickness of the positive electrode composite layer 4 is set to a specified value and the surface of the positive electrode composite layer 4 is shaped flat. In addition to those, the pressing process (S15) of the present embodiment has the following action as a crushing process. The binder 34 of the positive electrode composite layer 4 has some plasticity even after drying and thus undergoes plastic deformation, while the secondary particles 32 have little plasticity and are crushed and divided with respect to the compressing force.

[0054] From the state as shown in the diagram schematically showing the positive electrode composite layer 4 before the pressing process (S15) in FIG. 6, it becomes the state as shown in FIG. 7. In this way, the secondary particles 32 are crushed, resulting in primary particle formation in which they are decomposed into small particles. Note that this “primary particle formation” does not mean that they completely become primary particles, but rather means that large secondary particles 32 become small fragments 32d and the particle size approaches the size of the primary particles 31.

[0055] That is, when the secondary particles 32 are crushed and divided, the surface area of the positive electrode active material of the secondary particles 32 themselves increases, thereby increasing the reaction area of the positive electrode active material. Also, the wall surface of the hollow portion 32b that has not been exposed to the outside until now is also exposed.

[0056] Furthermore, many fibrous conductive materials 33 exist in the vicinity of the wall surface of the hollow portion 32b. These fibrous conductive materials 33 will also be exposed due to the crushing of the secondary particles 32. In particular, since the pressing step (S15) is performed after the drying step (S14), the conductive material 33 is difficult to flow. Therefore, even if the secondary particles 32 are granulated into primary particles, they will not aggregate. Furthermore, even after passing through the pressing step (S15), the conductive material 33 remains present in the vicinity of the wall surface of the hollow portion 32b. Therefore, these factors combine to ensure a conductive network from the hollow portion 32b, so that the effective specific surface area contributing to the reaction of the secondary particles 32 increases.

[0057] <Cutting step (S16)> When the pressing step (S15) is completed, the size of the positive electrode plate 2 is cut according to the specifications of the battery to be assembled as needed, and the positive electrode plate 2 is completed.

[0058] <Assembly of lithium-ion secondary battery> In the source step (S1), although the description is omitted, the negative electrode plate manufacturing step and the separator manufacturing step are performed simultaneously with the positive electrode plate manufacturing step shown in FIG. 9.

[0059] When these procedures are completed in the source process (S1), the process of assembling the cell battery is carried out. Here, first, the winding process (S2) is performed. In the winding process (S2), the positive electrode plate 2 and the negative electrode plate created in the source process (S1) are laminated with a separator sandwiched therebetween, and are wound in that state. In order to accommodate the wound positive electrode plate 2, negative electrode plate, and separator in the battery case, shaping is performed, and a flat pressing process (S3) is carried out to bring the electrode plates into close contact with each other. The electrode plate group composed of the positive electrode plate 2, negative electrode plate, and separator shaped in the flat pressing process has an Al current collector 5 without the positive electrode composite material layer 4 of the positive electrode at one end, and a Cu current collector without the negative electrode composite material layer of the negative electrode protrudes at the other end (not shown). These current collectors are pressure-welded, and in the terminal welding (S4), the positive electrode internal terminal and the negative electrode internal terminal are welded respectively. External terminals are attached to these internal terminals via the lid of the battery case. Then, the wound and flattened electrode plate group and the positive electrode terminal and negative electrode terminal welded thereto are inserted into the battery case in the case insertion (S5) process. In the can sealing welding (S6) process, the battery case and the lid are sealed by laser welding or the like. At this stage, the liquid injection port of the lid is open. Thereafter, in the cell drying (S7) process, after sufficiently drying the moisture remaining in the battery case, non-aqueous electrolyte is injected from the liquid injection port in the liquid injection and sealing (S8) process. When the liquid injection is completed, the liquid injection port is sealed. Thus, the assembly of the cell battery is completed. When the assembly of the cell battery is completed, a process of activation (S9) is carried out for the purpose of forming an SEI (Solid Electrolyte Interphase) film and the like. Here, initial charging, aging, etc. are performed to chemically activate the cell battery. And in the inspection (S10) process, inspections such as cell voltage and battery resistance are carried out, and those that exhibit predetermined performance become products. In the case of a vehicle-mounted lithium ion secondary battery, about 6 to 12 cell batteries are stacked to form a battery module, and further, a plurality of battery modules are housed in a container, and a control device, various sensors, etc. are mounted to form a vehicle battery pack.

[0060] (Operation of the Embodiment) Next, the operation of this embodiment will be described with experimental examples. <Experiment 1> Figure 10 is a comparison table showing the comparison between the positive electrode plate 2 of Example 1 of this embodiment in Experiment 1 and the positive electrode plate 2 of Comparative Example 1 which is a prior art. In Experiment 1, the input and output of the positive electrode plate 2 were compared by changing the conditions between the conventional positive electrode plate 2 and the positive electrode plate 2 of this embodiment. The binder used in the experiment is the cold embedding resin "Specifics-20 (registered trademark)" made of epoxy resin from Struers for testing because of its ease of handling and the like. It has been confirmed that this resin can derive results equivalent to those of PVdF (polyvinylidene fluoride) used in products.

[0061] <Particle shape> Figure 11 is a micrograph showing the appearance of the secondary particles 32 of the active material of Comparative Example 1. Figure 12 is a micrograph showing the cross-section of the secondary particles of the active material of Comparative Example 1.

[0062] Comparative Example 1 of the prior art shows the prior art. The particle shape of the target secondary particles 32 is porous, but the average diameter W of the through holes 32c is as small as 25 nm, which is less than 50 nm.

[0063] Figure 13 is a micrograph showing the appearance of the secondary particles 32 of the active material of Example 1 of this embodiment. Figure 14 is a micrograph showing the cross-section of the secondary particles of the active material of Example 1 of this embodiment.

[0064] On the other hand, in Example 1 of this embodiment, the particle shape of the secondary particles 32 is porous, and the average diameter W of the through holes 32c is as large as 200 nm, which is 50 nm or more. When kneaded with the binder 34, in Comparative Example 1, the impregnation rate was 54.8% which is less than 85%, but in Example 1, the impregnation rate was 90% which is 85% or more.

[0065] Although the description is omitted, as a result of conducting a number of experiments by changing the conditions, it was confirmed that the relationship between the average diameter W of the through holes 32c and the impregnation rate [%] is that when the average diameter W of the through holes 32c is 50 nm or more, the impregnation rate [%] is generally 85% or more.

[0066] <Composite material state> In Comparative Example 1 of the prior art, the average diameter D of the granular acetylene black, which is the conductive material 33, exceeds 200 nm and 30 nm, and the average length L is also less than 200 nm and less than 500 nm. And the conductive material 33 was present only outside the secondary particles 32.

[0067] On the other hand, in Example 1 of the present embodiment, the average diameter D of the fibrous carbon nanotube, which is the conductive material 33, is 20 nm (10% of the through-hole 32c) and 30 nm or less, and the average length L is 700 nm and 500 nm or more and 1000 nm or less. And the conductive material 33 has entered into the hollow portion 32b from the through-hole 32c of the secondary particle 32, and exists not only outside but also inside the secondary particle 32.

[0068] <Positive electrode plate 2 after pressing> FIG. 15 is a micrograph showing a state in which the secondary particles 32 of the active material of Comparative Example 1 are not crushed. If the secondary particles 32 are not crushed, the hollow portion 32b cannot contact the conductive material 33, so there will be a region that cannot contribute to the reaction.

[0069] Even if the secondary particles 32 are crushed in the pressing step (S15), the conductive material 33 does not exist near the wall surface of the hollow portion 32b of the crushed secondary particles 32. Therefore, even if the wall surface of the hollow portion 32b is exposed, that region ultimately cannot contribute to the reaction.

[0070] FIG. 16 is a micrograph showing a state in which the secondary particles of the active material of Example 1 of the present embodiment are crushed. In Example 1 of the present embodiment, the secondary particles 32 are crushed in the pressing step (S15). Furthermore, the conductive material 33 has penetrated and exists in the hollow portion 32b originally. Therefore, the conductive material 33 also exists near the wall surface of the hollow portion 32b of the crushed secondary particles 32. Therefore, if the wall surface of the hollow portion 32b is exposed, that region can contribute to the reaction. Also, the conductive material 33 arranged inside and outside the secondary particle 32 effectively forms a conductive network.

[0071] <Results> In Comparative Example 1, in the positive electrode active material particles 3, there is a region that cannot contact the conductive material 33 and cannot be connected to the conductive network, and thus cannot contribute to the reaction. For this reason, the input / output of the positive electrode plate 2 is insufficient.

[0072] On the other hand, in Example 1 of the present embodiment, the secondary particles 32 are primary particleized, increasing the surface area of the positive electrode active material. At the same time, the conductive material 33 also exists near the wall surface of the hollow portion 32b of the crushed secondary particles 32. For this reason, the conductive material 33 disposed inside and outside the secondary particles 32 effectively forms a conductive network. As a result, the input / output of the positive electrode plate 2 is improved.

[0073] <Experiment 2> FIG. 17 is a table showing the conditions of Example 2, Comparative Example 2, and Comparative Example 3 in Experiment 2. The input / output characteristics were compared by changing the electrode plate state, the average diameter W of the through holes of the secondary particles 32 of the active material, the impregnation rate [%], and the average diameter D of the conductive material 33. Example 1 of Experiment 1 and Example 2 of Experiment 2 are common.

[0074] Here, FIG. 18 is a micrograph of the cross section of the positive electrode plate 2 in Example 2. Example 2 shown in FIG. 18 is an example in which fibrous conductive materials 33 are arranged inside and outside the secondary particles 32 and pressed to be primary particleized. FIG. 19 is a micrograph of the cross section of the positive electrode plate 2 in Comparative Example 2. Comparative Example 2 shown in FIG. 19 is an example in which fibrous conductive materials 33 are arranged inside and outside the secondary particles 32 but not pressed to be primary particleized. FIG. 20 is a micrograph of the cross section of the positive electrode plate 2 in Comparative Example 3. As shown in FIG. 20, Comparative Example 3 is an example in which some of the secondary particles 32 are primary particleized, but the conductive material 33 is granular acetylene black instead of fibrous carbon nanotubes.

[0075] <Electrode plate state> FIG. 21 is a magnified micrograph of a part of the cross section of the positive electrode plate 2 in Example 2. As shown in FIG. 21, it can be seen that in Example 2, the fibrous conductive material 33 exists inside and outside the secondary particles 32 that are partially primary particleized.

[0076] FIG. 22 is a magnified micrograph of a part of the cross-section of the positive electrode plate 2 in Comparative Example 2. In Comparative Example 2, as shown in FIG. 22, it can be seen that the fibrous conductive material 33 exists inside and outside the secondary particles 32.

[0077] FIG. 23 is a magnified micrograph of a part of the cross-section of the positive electrode plate 2 in Comparative Example 3. In Comparative Example 3, the fibrous conductive material 33 does not exist inside and outside the secondary particles 32 in which part of the secondary particles has been granulated. <Average diameter W of through-holes in secondary particles 32 of the active material> The average diameter W of the through-holes in the secondary particles 32 of the active material in Example 2 is as large as 200 nm, the average diameter W of the through-holes in the secondary particles 32 of the active material in Comparative Example 2 is slightly smaller at 100 nm, and the average diameter W of the through-holes in the secondary particles 32 of the active material in Example 3 is small at 25 nm.

[0078] <Average diameter D of the conductive material 33> The average diameter D of the fibrous conductive material 33 in Example 2 is as thin as 10 nm (5% of the through-hole 32c), and the average diameter D of the fibrous conductive material 33 in Comparative Example 2 is similarly as thin as 10 nm. On the other hand, the conductive material 33 in Comparative Example 3 is granular and has an average diameter D of 200 nm or more.

[0079] <Infiltration rate [%]> The infiltration rate [%] in Example 2 is 90.0%. On the other hand, in Comparative Example 2, it is 63.3%, and in Comparative Example 3, it is as low as 54.8%. Especially in Comparative Example 3, even if the binder 34 could infiltrate into the secondary particles 32, there was almost no accompanying entry of the conductive material 33 because of its large average diameter D.

[0080] <Results> FIG. 24 is a graph comparing the input-output characteristics in Experimental Example 2. Here, the input-output characteristics of Comparative Example 2, which is the prior art, were set to 100%. At this time, in Example 2, it showed high input-output characteristics of approximately 135%.

[0081] In Comparative Example 2, the input / output characteristics were low, generally around 90%. The reasons include that the secondary particles 32 were not primary particleized. Also, the average diameter W of the through-holes 32c of the secondary particles 32 of the positive electrode active material was small and the impregnation rate [%] was low.

[0082] In Comparative Example 3, in the positive electrode active material particles 3, even though some of the secondary particles 32 were primary particleized, since the conductive material 33 was granular rather than fibrous, it can be presumed that it was difficult to enter the hollow part 32b and it was difficult to form a conductive network.

[0083] In Comparative Example 2, since the fibrous conductive material 33 was used, it can be said that it was easy to form a conductive network between the conductive materials 33. However, it was difficult for the fibrous conductive material 33 to enter the secondary particles 32. Furthermore, since the secondary particles 32 were not primary particleized, the surface area was small, and it can be presumed that the wall surface of the hollow part 32b of the secondary particles 32 became a region that contributed little to the reaction, which was the cause of the decrease in the input / output characteristics.

[0084] On the other hand, in Example 2 of the present embodiment, the fibrous conductive material 33 easily enters the secondary particles 32. Furthermore, the surface area of the positive electrode active material increases due to the primary particleization of the secondary particles 32. At the same time, the fibrous conductive material 33 also exists near the wall surface of the hollow part 32b of the crushed secondary particles 32. Therefore, the conductive materials 33 arranged inside and outside the secondary particles 32 easily form an effective conductive network. As a result, the input / output characteristics of the positive electrode plate 2 are improved.

[0085] (Effects of the present embodiment) (1) In the method for manufacturing a lithium-ion secondary battery of the present embodiment, the resistance of the positive electrode plate 2 can be reduced and the input / output characteristics of the positive electrode can be improved.

[0086] (2) In this embodiment, primary particles 31 aggregate to form a hollow shell shape, and secondary particles 32 of a porous positive electrode active material having a shell portion 32a with a through-hole 32c are provided. And a kneading step (including an impregnation step) (S12) of arranging a fibrous conductive material 33 together with a binder 34 inside and outside thereof is provided. Therefore, the fibrous conductive material 33 can also be arranged inside the secondary particles 32.

[0087] (3) The secondary particles 32 are crushed by a pressing step (S15) which is a crushing step, so as to be primary particles. Therefore, the active material primary-particleized in a dispersed state can be arranged in the positive electrode mixture layer 4 without causing aggregation of the primary-particleized active material particles.

[0088] (4) Also, the pressing step (S15) is no different from the conventional pressing step for adjusting the thickness of the positive electrode mixture layer 4 and shaping the surface. Therefore, even in the conventional production method, by adjusting the pressing gap or the like, the manufacturing method of the lithium-ion secondary battery of this embodiment can be implemented without introducing a particularly new device or the like.

[0089] (5) Since the average diameter D of the fibrous conductive material 33 is 60% or less of the average diameter W of the through-holes of the secondary particles 32, the fibrous conductive material 33 easily enters the hollow portion 32b of the secondary particles 32 together with the binder 34.

[0090] (6) In the pressing step (S15), since 50% or more of the secondary particles 32 of the positive electrode active material particles 3 are made into primary particles, the surface area contributing to the reaction is increased. (7) The kneading step (S12) which is an impregnation step impregnates a binder 34 containing a fibrous conductive material 33 into 85% or more of the quantity of the secondary particles 32 together with the fibrous conductive material 33. Therefore, since many fibrous conductive materials 33 can be arranged near the hollow portion 32b, an effective conductive network can be constructed.

[0091] (8) The through-hole 32c of the secondary particle 32 has a diameter of 50 nm or more. Therefore, the binder 34 containing the fibrous conductive material 33 can be smoothly introduced into the hollow portion 32b of the secondary particle 32.

[0092] (9) Since the diameter of the fibrous conductive material 33 is set to be as thin as 1 nm to 100 nm, the binder 34 containing the fibrous conductive material 33 can be smoothly introduced into the hollow portion 32b of the secondary particle 32.

[0093] (10) The length of the fibrous conductive material 33 is set to be 100 nm to 1000 nm. Therefore, a conductive network that connects the hollow portion 32b of the secondary particle 32 to the outside can be constructed. Moreover, it will not be too long to cause aggregation.

[0094] (11) Since the positive electrode active material is composed of a lithium transition metal oxide, a high-performance lithium ion secondary battery can be obtained. (12) The fibrous conductive material 33 is composed of carbon nanotubes. Therefore, an extremely high-conductivity conductive network with excellent input / output characteristics can be constructed.

[0095] (13) When creating the paste-like positive electrode composite material, since the positive electrode active material is in the state of the secondary particle 32, the active material particles are likely to aggregate. When aggregating, the viscosity increases or the solid content decreases, making it difficult to cause drying failure. Therefore, good handling can be achieved in the composite material creation process.

[0096] (14) By making the positive electrode with excellent input / output characteristics, a high-performance lithium ion secondary battery can be manufactured. (Modification example) The above embodiment can also be implemented as follows.

[0097] ○ The particle sizes of the primary particles 31 and secondary particles 32, the average diameter W of the through holes 32c, the average diameter D of the conductive material 33, the average length L of the conductive material, the impregnation rate, etc. exemplified in this embodiment, and the viscosity of the binder 34 are examples. Therefore, it goes without saying that those skilled in the art can optimize them, and they are not limited to these numerical ranges.

[0098] ○ The positive electrode active material is selected as an optimal material according to the purpose of the lithium-ion secondary battery, etc., and the manufacturing method such as its firing is also optimized by those skilled in the art. ○ The configuration of the binder 34 is not limited to that exemplified in the embodiment, and the material, viscosity, kneading conditions, etc. are appropriately optimized by those skilled in the art according to the material, characteristics, shape of the positive electrode active material, and the material and shape of the conductive material 33.

[0099] 〇 The flowchart of the embodiment is an example and is not limited to its order and content. ○ In the embodiment, a vehicle-mounted lithium-ion secondary battery is exemplified, but it is not limited by its purpose, size, etc.

[0100] ○ The present invention should not be construed as being limited by the above embodiment, and it goes without saying that those skilled in the art can implement it by adding, deleting, or replacing its configuration without departing from the scope of the claims.

Explanation of Reference Numerals

[0101] 2... Positive electrode plate 3... Positive electrode active material particles 31... Primary particles 32... Secondary particles 32a... Shell part 32b... Hollow part 32c... Through hole 32d... Fragment W... Average diameter (of the through hole) 33... Conductive material D... Average diameter (of the conductive material) L... Average length (of the conductive material) 34... Binder 4... Positive electrode mixture layer 5... Current collector S1... Source process S11…Secondary particle manufacturing process S12…Mixing process (including impregnation process) S13…Coating process S14…Drying process S15…Pressing process (crushing process) S16…Cutting process

Claims

1. A current collector, a positive electrode composite layer including positive electrode active material particles held on the current collector, a conductive material, and a binder, A method for manufacturing a positive electrode of a lithium ion secondary battery, comprising: An impregnation step of disposing the fibrous conductive material together with the binder inside and outside of secondary particles of a porous positive electrode active material in which primary particles are aggregated to form a hollow shell shape and having a shell portion provided with through holes; A pressing step of crushing at least a part of the secondary particles, characterized in that: The through holes of the secondary particles have a diameter of 50 nm or more; The fibrous conductive material is composed of carbon nanotubes having an average diameter of 60% or less of the average diameter of the through holes of the secondary particles; In the impregnation step, a binder containing the fibrous conductive material is impregnated into 85% or more of the quantity of the secondary particles together with the fibrous conductive material. A method for manufacturing a positive electrode of a lithium ion secondary battery.

2. The method for manufacturing a positive electrode of a lithium ion secondary battery according to claim 1, wherein in the pressing step, 50% or more of the positive electrode active material particles are primary particles.

3. The method for manufacturing a positive electrode of a lithium ion secondary battery according to claim 1 or 2, wherein the fibrous conductive material has a diameter of 1 nm to 100 nm.

4. The method for manufacturing a positive electrode of a lithium ion secondary battery according to any one of claims 1 to 3, wherein the fibrous conductive material has a length of 100 nm to 1000 nm.

5. The method for manufacturing a positive electrode of a lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material is composed of a lithium transition metal oxide.

6. A method for manufacturing a lithium ion secondary battery, including the method for manufacturing a positive electrode of a lithium ion secondary battery according to any one of claims 1 to 5.

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