Secondary battery electrode and method for manufacturing secondary battery electrode

Abstract

To provide a secondary battery electrode which improves durability while suppressing internal resistance of a battery, and a manufacturing method of the secondary battery electrode.SOLUTION: A secondary battery electrode 1 comprises: a collector foil 10 including a reactive functional group on a surface; and a mixture layer 20 formed on the surface of the collector foil 10 and containing an active material, a binder and a carbon nanotube 32 including a surface functional group having reactivity with respect to the reactive functional group. In a thickness direction orthogonal to the surface of the collector foil 10, when an end face of the mixture layer 20 at the side in contact with the collector foil 10 is defined as a rear face and an end face at an opposite side of the rear face is defined as a front face, functional groups derived from the surface functional group exist more in the vicinity of the rear face of the mixture layer 20 in relative to the vicinity of the front face of the mixture layer 20.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

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

[0003] Electrodes used in secondary batteries such as lithium-ion secondary batteries include a conductive current collector foil (current collector) and a composite layer held on the current collector foil and containing electrode materials such as an active material, a conductive material, a binder, etc. Carbon nanotubes are suitable for use as the conductive material contained in the electrode because they can ensure high conductivity with only a small amount.

[0004] Patent Document 1 discloses an electrode including at least a current collector, a conductive material, and an organic compound having a π-electron conjugated cloud as an active material, the conductive material including at least carbon nanotubes, and an energy storage element using the electrode. Patent Document 1 describes that the technology described in the electrode can ensure good current collection between the organic compound active material and the conductive material, thereby significantly reducing the amount of conductive material, thereby enabling electrodes and energy storage devices to be made lighter, with higher capacity and output. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-242386 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the technology described in Patent Document 1 requires the addition of a certain amount of binder to the composite layer due to insufficient bonding strength between the current collecting foil and the composite layer. The binder is added to the composite layer to ensure bonding between the current collecting foil and the composite layer and to stably fix electrode materials such as active materials. However, since the binder itself contributes little to the electrochemical performance of the electrode, it is desirable to minimize the amount of binder used from the perspective of increasing the energy density of the electrode and reducing the internal resistance of the battery. Therefore, the technology described in Patent Document 1 has the problem that the internal resistance of the battery increases as the amount of binder used increases.

[0007] The present invention has been made to solve such problems, and aims to provide an electrode for a secondary battery that has high durability while suppressing the internal resistance of the battery, and a method for manufacturing an electrode for a secondary battery. [Means for solving the problem]

[0008] An electrode for a secondary battery according to one embodiment comprises a current collecting foil having reactive functional groups on its surface, and a composite layer formed on the surface of the current collecting foil and containing an active material, a binder, and carbon nanotubes having surface functional groups that are reactive to the reactive functional groups. In the thickness direction perpendicular to the surface of the current collecting foil, when the end face of the composite layer that comes into contact with the current collecting foil is defined as the back face and the end face opposite the back face is defined as the front face, the functional groups derived from the surface functional groups are more abundant near the back face of the composite layer than near the surface of the composite layer.

[0009] In addition, a manufacturing method of an electrode for a secondary battery according to one embodiment includes a coating step of coating a paste containing an active material, a binder, carbon nanotubes having surface functional groups reactive to the reactive functional groups, and a solvent onto the surface of a current collector foil having reactive functional groups on its surface, and a drying step of heating and drying the coated paste to react the reactive functional groups with the surface functional groups to form a composite layer. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an electrode for a secondary battery that has high durability while suppressing the internal resistance of the battery, and a method for manufacturing the electrode for a secondary battery. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing an electrode for a secondary battery according to a first embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating a bonding state between the carbon nanotubes contained in the secondary battery electrode shown in FIG. 1 and a current collector foil. [Figure 3] 3 is a flowchart showing a method for manufacturing an electrode for a secondary battery according to the first embodiment. [Figure 4] 1 is a graph showing the relationship between the proportion of binder contained in various electrode plates and peel strength. [Figure 5] 1 is a graph showing the relationship between the proportion of binder contained in various electrode plates and the DC internal resistance. [Figure 6] 10 is a table showing the results of investigating the influence of drying conditions on the peel strength between the current collecting foil and the composite layer that constitute the electrode plate. [Figure 7] 10A and 10B are cross-sectional views illustrating a method for quantifying the amount of functional groups present in the vicinity of the front and rear surfaces of a composite layer. [Figure 8] 10 is a table showing the results of examining the amount of functional groups present near the front and back surfaces of a composite layer. DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiment 1 Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. In addition, the following description and drawings have been simplified as appropriate for clarity of explanation.

[0013] As one preferred embodiment of the secondary battery electrode according to this embodiment, the electrode will be specifically described 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).

[0014] The configuration of a secondary battery electrode (electrode plate 1) according to this embodiment will be described with reference to Figs. 1 and 2. Fig. 1 is a cross-sectional view showing a secondary battery electrode according to the first embodiment. Fig. 2 is a diagram schematically showing the bonding state between the carbon nanotubes contained in the secondary battery electrode shown in Fig. 1 and the current collector foil. Note that 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, and Fig. 2 shows an enlarged view of the vicinity of the surface of the current collector foil 10. As shown in Figs. 1 and 2, the electrode plate 1 has a current collector foil 10 and a composite layer 20 formed on the surface of the current collector foil 10.

[0015] The current collector foil 10 is a porous body in the form of a plate, foil, mesh, or the like, and has a thickness of, for example, 5 μm to 20 μm. The current collector foil 10 before the composite layer 20 is formed has reactive functional groups on its surface. The current collector foil 10 is made of a metal or an alloy thereof with good conductivity. There are no particular limitations on the current collector foil 10 as long as it has reactive functional groups on its surface, and it may have functional groups other than reactive functional groups on its surface. Examples of metals that make up the current collector foil 10 include aluminum, copper, nickel, titanium, iron, and stainless steel.

[0016] The reactive functional group is a functional group that can chemically bond with the surface functional group of the carbon nanotube 32 (CNT) described below. The reactive functional group is preferably a functional group such as a hydroxyl group (-OH) that can react with the surface functional group of the CNT 32 to form a covalent bond in a heated environment. Such reactive functional groups may be present on only one side or both sides of the current collector foil 10.

[0017] In the case of a positive electrode plate 1, the metal constituting the current collector foil 10 is preferably aluminum or an aluminum alloy. In the case of a negative electrode plate 1, the metal constituting the current collector foil 10 is preferably copper or a copper alloy.

[0018] The composite material 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. In this embodiment, with respect to both end faces of the composite material layer 20 existing in the thickness direction perpendicular to the surface of the current collector foil 10, the end face on the side in contact with the current collector foil 10 is the back surface of the composite material layer 20, and the end face opposite the back surface is the front surface of the composite material layer 20. Furthermore, the electrode plate 1 has an exposed portion at the edge of the current collector foil 10 where the composite material layer 20 is not formed and the current collector foil 10 is exposed. The exposed portion is a portion that is electrically connected to an external terminal.

[0019] The mixture layer 20 contains at least an active material 31 capable of absorbing and releasing lithium ions, CNTs 32 as a conductive material, and a binder, and is held by the current collector foil 10. In this embodiment, the mixture layer 20 contains CNTs 32 bonded to the surface of the current collector foil 10. This ensures adhesion between the current collector foil 10 and the mixture layer 20. The mixture layer 20 may contain other conductive materials besides CNTs 32 (e.g., acetylene black, graphite, graphene, carbon black) and other additives (e.g., thickeners, dispersants), as needed.

[0020] The density of the composite layer 20 is, for example, 2.0 g / cm 3 in the case of the positive electrode plate 1. 3 ~3.0g / cm 3 Preferably, it is 2.2 g / cm 3~2.8g / cm 3 More preferably, it is 2.4 g / cm to 2.6 g / cm 3 In the case of the negative electrode plate 1, the density of the mixture layer 20 is preferably 1.0 g / cm 3 , for example. 3 ~1.8g / cm 3 and preferably 1.05 g / cm 3 ~1.6g / cm 3 More preferably, it is 1.1 g / cm to 1.4 g / cm 3 It is preferable to set the following.

[0021] In the case of a positive electrode plate 1, the thickness of the mixture layer 20 is preferably, for example, 15 μm to 50 μm, more preferably 18 μm to 40 μm, and particularly preferably 20 μm to 30 μm. In the case of a negative electrode plate 1, the thickness of the mixture layer 20 is preferably, for example, 20 μm to 60 μm, more preferably 25 μm to 50 μm, and particularly preferably 30 μm to 40 μm.

[0022] In the case of a positive electrode plate 1, for example, lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium iron phosphate (LiFePO), nickel cobalt lithium aluminum oxide (NCA), nickel cobalt lithium manganese oxide (NCM), or the like can be used alone or in combination as the active material 31. In addition, other metal elements may be added to the active material 31.

[0023] In the case of the negative electrode plate 1, the active material 31 may be natural graphite, artificial graphite, hard carbon, soft carbon, graphite, tin (Sn), tin oxide (SnO), silicon (Si), silicon oxide (SiO), and lithium titanate (Li4Ti5O 12 ) etc.

[0024] For example, the active material 31 suitable for use in a positive electrode tends to have low conductivity, and therefore the addition of CNTs 32 is highly effective in reducing resistance. Therefore, the structure of the secondary battery electrode according to this embodiment is suitable for the electrode plate 1 of a positive electrode.

[0025] The CNTs 32 used as the conductive material may be, for example, single-walled carbon nanotubes, double-walled carbon nanotubes, or multi-walled carbon nanotubes with three or more walls. The CNTs 32 may be produced by an arc discharge method, a laser ablation method, a chemical vapor deposition method, or the like. These may be used alone or in combination.

[0026] The CNTs 32 form conductive paths between particles of the active material 31 and between the active material 31 and the current collector foil 10, thereby improving the conductivity of the entire electrode. Furthermore, the CNTs 32 can connect the particles of the active material 31 together due to their fiber length, improving the binding strength between the particles of the active material 31. On the other hand, if the fiber length is too long, the CNTs 32 tend to aggregate and reduce dispersibility, making it difficult to achieve the effects of improving the conductivity and binding strength.

[0027] From the viewpoints of electrical conductivity, mechanical properties, and dispersibility, the average fiber length of the CNTs 32 is, for example, preferably 0.1 to 100 μm, more preferably 0.3 μm to 20 μm. From the viewpoints of flexibility and dispersibility, the average outer diameter of the CNTs 32 is, for example, preferably 3.0 nm to 50 nm, more preferably 5.0 nm to 20 nm.

[0028] The CNTs 32 before forming the composite layer 20 have surface functional groups on their surfaces. The surface functional groups are functional groups that can chemically bond with the reactive functional groups of the current collector foil 10. The surface functional groups are functional groups such as hydroxyl groups and carboxyl groups (-COOH) that can undergo a dehydration condensation reaction with the reactive functional groups of the current collector foil 10 to form covalent bonds.

[0029] The amount of surface functional groups is preferably set within a range that does not impair the conductivity, mechanical properties, and dispersibility of the CNTs 32, while taking into consideration good adhesion to the current collector foil 10. The amount of surface functional groups present on the surface of the CNTs 32 is preferably 0.1% by mass to 30% by mass, more preferably 0.5% by mass to 15% by mass, and particularly preferably 1.0% by mass to 5.0% by mass, relative to the mass of the CNTs 32.

[0030] The amount of surface functional groups present on the surface of the CNTs 32 may be increased by previously subjecting the CNTs 32 to a surface treatment to introduce the surface functional groups. Carboxy groups are preferred as the surface functional groups because it is easy to increase the amount introduced.

[0031] The binder binds the electrode materials constituting the composite layer 20 together and also binds the formed composite layer 20 onto the surface of the current collector foil 10. Examples of binders having this function include polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), styrene butadiene rubber (SBR), and butyl rubber (BR). PVdF is preferably used for the positive electrode plate 1. SBR is preferably used for the negative electrode plate 1.

[0032] From the viewpoint of realizing high output characteristics and high energy density, the proportion of active material 31 in the entire composite layer 20 is preferably, for example, 94.0 mass% to 99.8 mass%, more preferably 96.5 mass% to 99.4 mass%, and particularly preferably 97.8 mass% to 99.0 mass%.

[0033] The proportion of CNTs 32 in the entire mixture layer 20 is, for example, preferably 0.1 mass % to 3.0 mass %, more preferably 0.3 mass % to 1.5 mass %, and particularly preferably 0.5 mass % to 1.2 mass %. By using CNTs 32 as the conductive material, the proportion of the conductive material can be reduced, and therefore the proportion of the active material 31 can be relatively increased.

[0034] The proportion of the binder in the entire composite layer 20 is, for example, preferably 0.1 mass % to 3.0 mass %, more preferably 0.3 mass % to 2.0 mass %, and particularly preferably 0.5 mass % to 1.0 mass %. In this embodiment, at least a portion of the CNTs 32 contained in the composite layer 20 are chemically bonded to the surface of the current collector foil 10, thereby improving the binding strength between the current collector foil 10 and the composite layer 20, and therefore the proportion of the binder can be reduced.

[0035] Therefore, the desired peel strength and output characteristics can be obtained without the increase in resistance that occurs when the amount of binder is increased. At the same time, since the amount of conductive material and binder used is small, the amount of active material 31 used can be increased, thereby achieving a high energy density of the electrode plate 1.

[0036] Furthermore, since at least a portion of the CNTs 32 contained in the composite layer 20 are chemically bonded to the surface of the current collector foil 10, the electrode plate 1 contains functional groups derived from the surface functional groups. The functional groups derived from the surface functional groups are at least one of functional groups generated by the mutual bonding of a surface functional group and a reactive functional group, and unreacted surface functional groups. In the electrode plate 1, the functional groups derived from the surface functional groups are present in greater amounts near the back surface of the composite layer 20 than near the surface of the composite layer 20.

[0037] That is, if the vicinity of the surface of the composite layer 20 is defined as the front side portion and the vicinity of the back side of the composite layer 20 is defined as the back side portion, the front side portion contains at least unreacted surface functional groups, and the back side portion contains at least the generated functional groups and unreacted surface functional groups. Thus, the back side portion contains more functional groups derived from the surface functional groups than the front side portion. The front side portion and the back side portion are layers having a thickness of, for example, about 3 μm from the front or back side of the composite layer 20.

[0038] For example, the amount of functional groups contained in the back-side portion (the amount of functional groups derived from surface functional groups) is preferably at least 1.3 times, and more preferably at least 1.6 times, the amount of functional groups contained in the front-side portion. By setting the ratio of the amount of functional groups in the back-side portion relative to the amount of functional groups in the front-side portion as described above, an electrode plate 1 can be obtained that ensures good bonding between the current collecting foil 10 and the composite layer 20 while reducing the amount of binder used. This ratio is particularly effective when the proportion of CNTs 32 in the entire composite layer 20 is 0.1% by mass to 3.0% by mass and the proportion of surface functional groups relative to the mass of CNTs 32 is 0.1% by mass to 30% by mass.

[0039] In the example shown in Fig. 2, at least some of the hydroxyl groups on the surface of the current collector foil 10 and at least some of the carboxyl groups on the surface of the CNTs 32 undergo a dehydration condensation reaction to form ester bonds. This results in a strong bond between the current collector foil 10 and the CNTs 32, making it possible to obtain an electrode plate 1 that has excellent peel strength between the current collector foil 10 and the composite layer 20. Furthermore, as shown in Fig. 2, carboxyl groups and carbonyl groups (>C=O) derived from the surface functional groups are mixed in the area of the electrode plate 1 near the current collector foil 10 and on the back side of the composite layer 20.

[0040] Furthermore, the electrode plate 1 having the above configuration can be combined with an electrolyte containing lithium ions, and if necessary, with an insulating separator through which lithium ions can pass, to form a lithium ion secondary battery.

[0041] The electrolyte may be, for example, a non-aqueous electrolyte. The non-aqueous electrolyte is a composition in which a lithium salt is dissolved in an organic solvent. Examples of the lithium salt include LiClO4, LiPF6, LiAsF6, LiBF4, and LiSO3CF3. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. These may be used alone or in combination as the electrolyte.

[0042] The separator is made of a porous insulating resin sheet such as polyethylene (PE) or polypropylene (PP). Such a porous resin sheet may have a single layer structure or a laminated structure of two or more layers. A porous heat-resistant layer may also be provided on a portion of the surface of the resin sheet.

[0043] Next, a 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 a method for manufacturing an electrode for a secondary battery according to embodiment 1. In the following description, the method for manufacturing an electrode for a secondary battery according to this embodiment can be applied to manufacturing electrode plates 1 for both positive and negative electrodes.

[0044] As shown in FIG. 3 , the method for manufacturing a secondary battery electrode according to this embodiment includes the following steps S1 to S3. Step S1 is a functional group introduction step in which at least one of the CNTs 32 and the current collector foil 10 is subjected to a surface treatment to increase the amount of at least one of the surface functional groups and the reactive functional groups. Step S2 is a coating step in which a paste containing an active material 31, a binder, CNTs 32 having surface functional groups reactive with the reactive functional groups, and a solvent is applied to the surface of the current collector foil 10 having reactive functional groups on its surface. Step S3 is a drying step in which the applied paste is heated and dried to react the reactive functional groups with the surface functional groups, thereby forming a composite layer 20. Each of the above steps will be described in more detail.

[0045] First, in the functional group introduction step, methods for introducing reactive functional groups into the current collector foil 10 include surface modification by various plasma treatments such as atmospheric pressure plasma treatment, vacuum plasma treatment, and corona discharge treatment. For example, hydroxyl groups can be introduced into the surface of the current collector foil 10 by atmospheric pressure plasma treatment, which involves irradiating the surface with plasma using oxygen gas as the plasma generating gas under atmospheric pressure.

[0046] Surface oxidation by acid treatment using a strong acid can be used as a method for introducing surface functional groups into CNTs 32. For example, carboxy groups can be introduced onto the surface of CNTs 32 by immersing the raw CNTs 32 in a mixed acid of sulfuric acid and nitric acid and then heat treating the CNTs.

[0047] The method for introducing the reactive functional groups or surface functional groups is not limited to the above-mentioned method, and other known methods can be used. The amount and structure of the reactive functional groups provided on the surface of the current collector foil 10 and the surface functional groups provided on the surface of the CNTs 32 can be confirmed by, for example, X-ray photoelectron spectroscopy (XPS). Note that if the obtained current collector foil 10 and CNTs 32 each have the desired amount of reactive functional groups or surface functional groups, the functional group introduction step may be omitted.

[0048] Next, in the coating process, a solvent is added to powder containing the active material 31, the CNTs 32, the binder, and other additives as needed, and the mixture is kneaded to prepare a paste for forming the composite layer. A suitable kneading machine such as a planetary mixer can be used to knead these electrode materials.

[0049] The solvent is appropriately selected in consideration of the binder used and the dispersibility of the CNTs 32. Examples of the solvent that can be used include non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), dimethylformamide (DMF), and toluene, mixed solvents combining non-aqueous solvents, and aqueous solvents such as water and mixed solvents mainly composed of water.

[0050] Next, the prepared paste is applied to the surface of the current collector foil 10. The paste can be applied using an appropriate coating device such as a die coater, a comma coater, a knife coater, or a gravure coater.

[0051] Next, in the composite layer forming process, the applied paste is heated and dried under predetermined drying conditions to remove the solvent contained in the paste. In this process, a dehydration condensation reaction between the reactive functional groups and the surface functional groups proceeds. For example, when the solvent is NMP (boiling point 202°C), the drying conditions during heat drying are preferably 150°C or less for 100 seconds or more. The drying conditions can be changed as appropriate depending on the type and amount of solvent used, but are preferably drying conditions that allow the solvent to volatilize over time.

[0052] The heating and drying method can be a method using a hot air dryer, an infrared heater, a far-infrared heater, or the like. The dried product is then pressed as necessary to form a composite layer 20 on the surface of the current collector foil 10. The electrode plate 1 shown in FIG. 1 can be manufactured by the above steps.

[0053] Next, the present invention will be described in more detail based on examples, but the examples do not limit the present invention.

[0054] [Preparation of positive electrode plate and evaluation of peel strength] First, a positive electrode plate 1 was manufactured according to the flow shown in FIG. 3. The positive electrode active material (active material 31) was LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Nickel manganese cobalt lithium oxide (NMC) with an average composition represented by O2 was used. CNT32 was either CNT32 that had been subjected to acid treatment in the functional group introduction step (acid-treated CNT), or CNT32 that had been omitted from the functional group introduction step and not subjected to acid treatment (non-acid-treated CNT). CNT32 contained carboxyl groups as surface functional groups. PVdF was used as the binder. NMP was used as the solvent.

[0055] NCM, CNT32, and PVdF were weighed out to various mass ratios, and the required amount of solvent was added and kneaded to prepare pastes for forming positive electrode composite layers. The mass ratios of NCM, CNT32, and PVdF were adjusted so that the total was 100% by mass for both the acid-treated and non-acid-treated CNTs, with the CNT32 ratio set to 1.0% by mass, the PVdF ratio varied between 0.5% and 2.0% by mass, and the remainder was NCM.

[0056] Next, each of the prepared pastes for forming the positive electrode composite layer was applied to one side of an aluminum foil serving as a positive electrode current collector foil 10. The aluminum foil contained a hydroxyl group as a reactive functional group. Each paste for forming the positive electrode composite layer had a basis weight of 6 mg / cm. 2 The coating amount was adjusted so that the density of each positive electrode plate was 2.5 g / cm. Then, each coated paste was dried with hot air at 150°C for 120 seconds, and after drying, pressed to prepare each positive electrode plate. 3 The thickness was 35 μm. In this way, nine types of positive electrode plates were obtained, each having a mixture layer 20 containing the binder at a ratio of 0.5 mass % to 2.0 mass %.

[0057] The peel strength of the positive electrode plates obtained in this manner was evaluated. Figure 4 is a graph showing the relationship between the proportion of binder contained in various electrode plates and the peel strength. The horizontal axis of Figure 4 represents the proportion (mass%) of binder contained in the composite layer 20, and the vertical axis represents the peel strength (N / m). In addition, in the graph shown in Figure 4, the circular plots represent positive electrode plates using acid-treated CNTs, and the diamond plots represent positive electrode plates using non-acid-treated CNTs.

[0058] The peel strength was measured in accordance with JIS Z0237:2009 and evaluated by a 180-degree peel test using a tensile tester. Specifically, a double-sided tape of a predetermined size was attached to a steel plate, and a composite layer 20 cut to a width of 10 mm and a length of 150 mm was attached to the opposite side of the double-sided tape. The adhesive was then pulled in a 180-degree direction at a speed of 40 mm / min and peeled off. The average stress at this time was taken as the peel strength (N / m).

[0059] The results of measuring the peel strength in this manner showed that the peel strength of the positive electrode plate using untreated CNTs decreased as the proportion of binder decreased, and that the peel strength decreased significantly especially when the proportion of binder was less than 1.0 mass%.

[0060] On the other hand, the positive electrode plate using acid-treated CNTs exhibited a higher level of peel strength than the positive electrode plate using non-acid-treated CNTs, although the peel strength decreased as the binder content decreased. For example, the positive electrode plate using acid-treated CNTs exhibited a peel strength of 1.7 N / m when the binder content was 0.5 mass%, which is less than 1.0 mass%, and thus a peel strength equal to or greater than the peel strength required for manufacturing was ensured. Here, in this embodiment, the peel strength required for manufacturing is a peel strength of 1.5 N / m or greater, as indicated by the dashed line in the graph of FIG. 4.

[0061] The reason for this result is thought to be that the acid treatment increased the number of surface functional groups on the CNTs 32, which in turn increased the amount of surface functional groups on the CNTs 32 that bonded with the reactive functional groups on the current collector foil 10, thereby improving the adhesion between the current collector foil 10 and the composite layer 20.

[0062] Next, the effect of the amount of binder on battery performance was examined with reference to Figure 5. Figure 5 is a graph showing the relationship between the proportion of binder contained in various electrode plates and the DC internal resistance. Figure 5 also shows the results of evaluating the DC internal resistance of three types of test battery cells constructed to evaluate battery performance.

[0063] [Construction of evaluation battery cells and battery performance evaluation] The three types of test battery cells are lithium-ion secondary batteries constructed using three types of positive electrode plates, each of which uses acid-treated CNTs from the positive electrode plates described in Fig. 4 and has a composite layer 20 containing binder at a ratio of 0.5 mass %, 1.0 mass %, and 1.5 mass %, respectively. The test battery cells were constructed using these three types of positive electrode plates according to the following procedure.

[0064] First, a negative electrode plate was prepared as follows: Natural graphite (C) was used as the negative electrode active material; Carboxymethyl cellulose (CMC) was used as the thickener; SBR was used as the binder; and Ion-exchanged water was used as the solvent.

[0065] The negative electrode active material, thickener, and binder were weighed out so that the mass ratio of C:CMC:SBR was 98:1:1, and the required amount of solvent was added and kneaded to prepare a paste for forming a negative electrode composite layer. Next, the prepared paste for forming a negative electrode composite layer was applied to one side of copper foil, which was the current collector foil of the negative electrode. The paste for forming a negative electrode composite layer had a basis weight of 4 mg / cm. 2 The coated amount was adjusted so that the density of the negative electrode plate was 1.2 g / cm. The coated paste was then dried with hot air at 150°C for 120 seconds, and pressed after drying to produce a negative electrode plate. 3 and the thickness was 45 μm.

[0066] In addition, an electrolyte solution was prepared by dissolving the supporting salt LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[0067] Then, the composite layer 20 of the positive electrode plate and the composite layer of the negative electrode plate that had been fabricated were placed opposite each other, and the stacked electrode body with a PE separator interposed therebetween was placed inside an aluminum laminate exterior material, and an electrolyte solution was added and sealed to construct a laminated evaluation battery cell.

[0068] The direct current internal resistance (DCIR) of the three types of test battery cells constructed was measured, and the evaluation results are shown in Figure 5. Figure 5 is a graph showing the relationship between the proportion of binder contained in various electrode plates and the direct current internal resistance. The vertical axis of Figure 5 shows the direct current internal resistance as a relative value, with the direct current internal resistance (Ω) of the test battery cell including a positive electrode plate with a binder proportion of 1.5 mass % set as 100%. It is believed that the lower the percentage value showing the relative value of the direct current internal resistance, the more suppressed the increase in resistance.

[0069] The DC internal resistance was measured by adjusting the SOC (State Of Charge) of various test battery cells after initial charging and discharging to 60%, discharging them at 5C for 10 seconds in a temperature environment of 25°C, and measuring the voltage change before and after using a charge / discharge device. The DC internal resistance was then calculated as the average value of the voltage change divided by the current value.

[0070] 5, the DC internal resistance was reduced by 1.9% for the test battery cell including a positive electrode plate with a binder proportion of 1.0% by mass, and by 5.6% for the test battery cell including a positive electrode plate with a binder proportion of 0.5% by mass, compared to the test battery cell including a positive electrode plate with a binder proportion of 1.5% by mass. As can be seen from these results, it was confirmed that the binder proportion can be reduced by using acid-treated CNT32, and that the internal resistance of the battery decreases as the binder proportion decreases.

[0071] Next, the influence of the drying conditions in the drying step on the peel strength of the composite layer 20 was examined with reference to Fig. 6. Fig. 6 is a table showing the results of examining the influence of the drying conditions on the peel strength between the current collecting foil and the composite layer that constitute the electrode plate.

[0072] Fig. 6 shows the results of an investigation into the drying process of a positive electrode plate using acid-treated CNTs, among the positive electrode plates described in Fig. 4, in which composite layer 20 containing 0.75 mass % of binder is formed on the surface of current collector foil 10. In the drying process, hot air drying was performed on the paste applied to current collector foil 10 under various drying conditions (drying temperature and drying time) shown in Fig. 6. The peel strength of 12 types of positive electrode plates obtained in this manner was measured using the same peel strength measurement method as above.

[0073] As a result of measuring the peel strength, it was found that a positive electrode plate with an excellent peel strength of 2.5 N / m or more could be obtained when drying conditions were applied in which the drying temperature was 150°C or less and the drying time was 120 seconds or more.

[0074] Therefore, two types of positive electrode plates obtained under different drying conditions were extracted from the positive electrode plates obtained under the drying conditions shown in Fig. 6, and the chemical bonding state of elements present near the front and back surfaces (front and back portions) of composite layer 20 was examined. The results are described below.

[0075] The two types of positive electrode plates used to examine the bonding state were a positive electrode plate with a peel strength of 2.2 N / m and a positive electrode plate with a peel strength of 2.8 N / m, as shown in Fig. 6. The positive electrode plate with a peel strength of 2.2 N / m was designated Sample No. 1, and the positive electrode plate with a peel strength of 2.8 N / m was designated Sample No. 2, and the front and back surfaces of the composite layer 20 included in each sample were observed by XPS.

[0076] Sample No. 1 is a positive electrode plate obtained under drying conditions of a drying temperature of 180°C and a drying time of 60 seconds, and Sample No. 2 is a positive electrode plate obtained under drying conditions of a drying temperature of 120°C and a drying time of 120 seconds.

[0077] The bonding state was evaluated by quantifying the amount of functional groups present near the front and back surfaces of the composite layer 20 in each sample, i.e., the total amount of carboxyl groups and carbonyl groups derived from the surface functional groups. The method for quantifying the amount of functional groups will be described with reference to Fig. 7. Fig. 7 is a cross-sectional view illustrating the method for quantifying the amount of functional groups present near the front and back surfaces of the composite layer.

[0078] In quantifying the amount of functional groups, a layer having a thickness of 3 μm from the surface of composite layer 20 in each sample was defined as front portion 20a, and composite layer 20 was observed from the front side by XPS. For back portion 20b, which was a layer having a thickness of 3 μm from the back surface of each sample, a part of current collecting foil 10 was peeled off from composite layer 20, and then the exposed part of back portion 20b from the back surface of composite layer 20 was observed by XPS.

[0079] Furthermore, for the front side portion 20a and the back side portion 20b of the sample, the amounts of functional groups were calculated as relative values based on the amounts of functional groups quantified by XPS, with the front side portion 20a of Sample No. 1 being taken as 100%. The results are shown in Figure 8. Figure 8 is a table showing the results of examining the amounts of functional groups present near the front and back sides of the composite layer.

[0080] As shown in FIG. 8, in each sample, the amount of functional groups was greater in the back side portion 20b than in the front side portion 20a. Specifically, in sample No. 1, the amount of functional groups in the back side portion 20b was 1.38 times that in the front side portion 20a. In addition, in sample No. 2, the amount of functional groups in the back side portion 20b was 1.67 times that in the front side portion 20a. In other words, it was confirmed that functional groups derived from the surface functional groups in the positive electrode plate were present in greater amounts in the back side portion 20b of the composite layer 20 than in the front side portion 20a of the composite layer 20.

[0081] 6 and 8, when the paste coated on the surface of the current collector foil 10 is heated and dried using a thermal history that passes through a temperature range below the boiling point of the solvent, it takes time for the solvent to volatilize, ensuring sufficient time for the reactive functional groups and surface functional groups to react with each other. This is thought to promote the reaction, increasing the amount of functional groups generated by bonding between the reactive functional groups and surface functional groups.

[0082] As described above, the secondary battery electrode according to this embodiment includes current collector foil 10 having reactive functional groups on its surface, and composite layer 20 formed on the surface of current collector foil 10 and containing active material 31, a binder, and CNTs 32 having surface functional groups reactive to the reactive functional groups. In addition, in the thickness direction orthogonal to the surface of current collector foil 10, when the end face of composite layer 20 that contacts current collector foil 10 is defined as the back face and the end face opposite the back face is defined as the front face, the secondary battery electrode has more functional groups derived from the surface functional groups near the back face of composite layer 20 than near the surface of current collector foil 10.

[0083] With this configuration, the reactive functional groups react with the surface functional groups to form chemical bonds. In the electrode plate 1 in which the CNTs 32 are chemically bonded to the current collector foil 10 through this reaction, high peel strength can be achieved even with a reduced amount of binder. Furthermore, in a secondary battery including this electrode plate 1, the internal resistance is suppressed by reducing the amount of binder used, thereby achieving high output characteristics.

[0084] Furthermore, it is preferable that the reactive functional group contains a hydroxyl group and the surface functional group contains a carboxyl group. With this configuration, a covalent bond is formed by the reaction between the hydroxyl group and the carboxyl group. In an electrode plate 1 in which the current collector foil 10 and the CNTs 32 are firmly bonded via a covalent bond, the peel strength between the current collector foil 10 and the composite layer 20 is improved. Furthermore, the hydroxyl groups of the current collector foil 10 and the carboxyl groups of the CNTs 32 can be easily increased using various methods, making it possible to increase the number of surface functional groups of the CNTs 32 that bond to the reactive functional groups of the current collector foil 10. This improves the bonding strength between the current collector foil 10 and the CNTs 32.

[0085] Furthermore, the amount of functional groups contained near the back surface of the composite layer 20 is preferably 1.3 times or more, and more preferably 1.6 times or more, the amount of functional groups contained near the front surface of the composite layer 20. As the amount of functional groups contained near the back surface of the composite layer 20 increases, the effect of improving the binding strength resulting from the bonding between the current collecting foil 10 and the CNTs 23 becomes even stronger.

[0086] Furthermore, the amount of the surface functional groups is preferably 0.1% by mass to 30% by mass, and particularly preferably 1.0% by mass to 5.0% by mass, relative to the mass of the CNTs 32. With this configuration, the binding strength between the current collector foil 10 and the CNTs 32 can be improved without impairing the electrical conductivity, mechanical properties, and dispersibility of the CNTs 32.

[0087] According to the method for manufacturing a secondary battery electrode according to this embodiment, a secondary battery electrode that exhibits the above-mentioned effects can be manufactured.

[0088] The method for manufacturing an electrode for a secondary battery according to this embodiment includes a coating step of coating a paste containing an active material 31, a binder, CNTs 32 having surface functional groups reactive to the reactive functional groups, and a solvent onto the surface of a current collector foil 10 having reactive functional groups on its surface, and a drying step of heating and drying the coated paste to react the reactive functional groups with the surface functional groups to form a composite layer 20.

[0089] Furthermore, the method for manufacturing an electrode for a secondary battery according to this embodiment includes a functional group introduction step in which, prior to the coating step, the amount of at least one of the surface functional groups and reactive functional groups is increased by performing a surface treatment on at least one of the CNTs 32 and the current collector foil 10.

[0090] It is also preferred that the reactive functional group contains a hydroxyl group and the surface functional group contains a carboxyl group.

[0091] Furthermore, in the drying step, it is preferable to heat-dry the applied paste at a temperature of 150°C or lower for 100 seconds or longer. In this drying step, the paste applied to the surface of the current collector foil 10 is heated and dried with a thermal history that passes through a temperature range below the boiling point of the solvent, thereby promoting the reaction between the reactive functional groups and the surface functional groups and efficiently increasing the amount of CNTs 32 bonded to the current collector foil 10. This further improves the bonding strength between the current collector foil 10 and the CNTs 32.

[0092] Therefore, according to this embodiment, it is possible to provide an electrode for a secondary battery that has high durability while suppressing the internal resistance of the battery, and a method for manufacturing the electrode for a secondary battery. [Explanation of symbols]

[0093] 1 Electrode plate 10 Current collecting foil 20 Composite layer 20a Front part 20b Back part 31 Active material 32 Carbon nanotubes

Claims

1. a current collecting foil having a reactive functional group on its surface; The active material, the binder, and the reactive functional group are formed on the surface of the current collector foil. a composite layer including carbon nanotubes having reactive surface functional groups; and In a thickness direction perpendicular to the surface of the current collecting foil, When the end face is the back face and the end face opposite to the back face is the front face, the surface functional group The functional groups are present in greater amounts near the back surface of the composite layer than near the surface of the composite layer. Sushi, When the amount of functional groups derived from the surface functional groups is defined as the functional group amount, The amount of the functional group contained in the vicinity of the back surface of the composite layer is The amount of the functional group contained in the electrode for a secondary battery is 1.3 times or more.

2. 2. The method of claim 1, wherein the reactive functional groups include hydroxyl groups and the surface functional groups include carboxyl groups. Electrode for secondary battery.

3. When the amount of functional groups derived from the surface functional groups is defined as the functional group amount, The amount of the functional group contained in the vicinity of the back surface of the composite layer is 3. The electrode for a secondary battery according to claim 1, wherein the amount of the functional group contained in the electrode is 1.6 times or more.

4. The amount of the surface functional group is 0.1% by mass to 30% by mass of the carbon nanotube.

4. The electrode for a secondary battery according to claim 1, wherein the content of the polymer in the electrode is 1% by mass.

5. The amount of the surface functional groups is 1.0% by mass to 5.0% by mass of the carbon nanotubes.

4. The electrode for a secondary battery according to claim 1, wherein the content of the cations in ...

6. A current collector foil having reactive functional groups on its surface is provided with an active material, a binder, and the reactive functional groups on the surface of the current collector foil. Carbon nanotubes having surface functional groups reactive to hydroxybenzoates and a solvent containing a coating step of coating the sheet; The applied paste is heated and dried to bond the reactive functional groups and the surface functional groups. a drying step of reacting the above to form a composite layer; and In a thickness direction perpendicular to the surface of the current collecting foil, when an end surface of the composite layer that is in contact with the current collecting foil is defined as a back surface and an end surface opposite to the back surface is defined as a front surface, the functional groups derived from the surface functional groups are present in greater amounts near the back surface of the composite layer than near the surface of the composite layer; When the amount of functional groups derived from the surface functional groups is defined as the functional group amount, A method for manufacturing an electrode for a secondary battery, wherein the amount of functional groups contained in the vicinity of the back surface of the composite layer is 1.3 times or more the amount of functional groups contained in the vicinity of the front surface of the composite layer.

7. Before the coating step, By performing a surface treatment on at least one of the carbon nanotubes and the current collecting foil, a functional group introduction process for increasing the amount of at least one of the surface functional groups and the reactive functional groups. The method for producing an electrode for a secondary battery according to claim 6, comprising the steps of:

8. 8. The method for producing an electrode for a secondary battery according to claim 6, wherein the reactive functional group includes a hydroxyl group and the surface functional group includes a carboxyl group.

9. In the drying step, the applied paste is heated at a temperature of 150°C or less for 100 seconds or more. The method for producing an electrode for a secondary battery according to claim 8, wherein the electrode is dried by heating.

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