Thermistor layer, battery electrode, battery and thermistor

Abstract

A thermistor layer according to the present invention is provided so as to be disposed in an electrical current path. The thermistor layer includes heat-sensitive particles, a plurality of conductive particles covering the surface of the heat-sensitive particles, and a binder for adhering the plurality of conductive particles. The plurality of conductive particles form a conduction network, at least one surface layer of the heat-sensitive particles is constituted of a thermoplastic resin, and the thermoplastic resin softens at a temperature lower than the temperature at which the binder softens. The thermistor layer is characterized by being provided so as to be highly resistant due to softening and deformation of the thermoplastic resin.

Description

[Technical field]

[0001] The present invention relates to a thermistor layer, an electrode for a battery, a battery, and a thermistor. [Background technology]

[0002] When some kind of failure occurs in an electrical device or battery, the current may continue to flow, causing abnormal heat generation. PTC thermistors are known that increase resistance and reduce the current flow when such abnormal heat generation occurs (see, for example, Patent Document 1). Among PTC thermistors, polymer-based PTC thermistors are known that use a conductive resin layer in which a conductive filler is mixed into the resin layer. In addition to thermistors, there are examples of applications such as battery electrodes, planar heating elements, and thermal sensors. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2001-028301 A Summary of the Invention [Problem to be solved by the invention]

[0004] The temperature at which the electrical resistance of a PTC thermistor rises sharply (switching temperature) is determined almost entirely by the melting point of the crystalline polymer used. This means that there is little freedom in designing the switching temperature. In addition, the switching temperature cannot be set lower than the melting point of the crystalline polymer. The present invention has been made in consideration of the above circumstances, and provides a thermistor layer that allows the temperature at which the resistance increases to be designed and changed with a high degree of freedom and has excellent thermistor characteristics. [Means for solving the problem]

[0005] The present invention provides a thermistor layer disposed in a current path, the thermistor layer including heat-sensitive particles, a plurality of conductive particles covering the surfaces of the heat-sensitive particles, and a binder bonding the plurality of conductive particles, the plurality of conductive particles forming a conductive network, the heat-sensitive particles being particles having at least a surface layer made of a thermoplastic resin, the thermoplastic resin being a resin that softens at a lower temperature than the binder, and the thermistor layer being disposed so as to have a high resistance as the thermoplastic resin softens and deforms. Effect of the Invention

[0006] The thermistor layer of the present invention includes a plurality of conductive particles covering the surface of the heat-sensitive particles, and the conductive particles form a conductive network, which allows the thermistor layer to have excellent conductivity and to function as a current path. The surface layer or the whole of the heat-sensitive particles is made of a thermoplastic resin. Therefore, when the temperature of the thermistor layer becomes higher than the softening point of the thermoplastic resin, the surface layer or the whole of the heat-sensitive particles softens and deforms, and the thermoplastic resin penetrates into the gaps (for example, between adjacent conductive particles) of the thermistor layer, or a part of the heat-sensitive particles becomes recessed. As a result, the conductive network formed by the multiple conductive particles on the surface of the heat-sensitive particles changes or breaks, causing the thermistor layer to become highly resistant. Alternatively, when the surface layer or the whole of the heat-sensitive particles softens and deforms, the contact points at the interface between the thermistor layer and the current path connected to the thermistor layer decrease (for example, peeling occurs at the interface between the conductive particles and the conductive layer of the thermistor layer. Also, for example, the softened heat-sensitive resin penetrates into the interface between the conductive particles and the conductive layer of the thermistor layer). Therefore, the electrical resistance at this interface increases, and the thermistor layer becomes highly resistant.

[0007] In this way, by increasing the resistance of the thermistor layer, the current flowing through the current path can be reduced, and abnormal heat generation can be suppressed. Therefore, the thermistor layer of the present invention can be used as an overheat protection structure. In addition, since the thermistor layer increases in resistance at a temperature at which the thermoplastic resin of the surface layer of the heat-sensitive particles or the entirety of the thermoplastic resin softens and deforms, by sensing that the thermistor layer of the present invention has increased in resistance, it can be detected that the thermistor layer has reached that temperature. Therefore, the thermistor layer of the present invention can be used as a thermal sensor. In addition, by changing the type and molecular weight of the thermoplastic resin contained in the thermistor layer, the temperature at which the thermoplastic resin softens can be changed, making it possible to design and change the temperature at which the thermistor layer becomes highly resistive with a high degree of freedom. In addition, since the temperature at which the thermoplastic resin softens is lower than the melting point of the thermoplastic resin, it becomes possible to set the temperature at which the thermistor layer becomes highly resistive to a relatively low temperature.

[0008] The thermoplastic resin is a resin that softens at a lower temperature than the binder, so that the binder can be prevented from softening and the conductive network formed by the conductive particles can be prevented from changing at a temperature lower than the temperature at which the thermoplastic resin softens and deforms. Therefore, the thermistor layer can have excellent conductive properties at a temperature lower than the softening point of the thermoplastic resin, and can prevent power loss from occurring in the thermistor layer. In addition, when the temperature of the thermistor layer becomes higher than the softening point of the thermoplastic resin, the electrical resistance value of the thermistor layer increases rapidly as described above. Thus, the thermistor layer of the present invention has excellent thermistor properties. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1A is a schematic cross-sectional view of a thermistor including a thermistor layer according to one embodiment of the present invention, and FIG. 1B is a schematic top view of a heat-sensitive particle covered with a plurality of conductive particles. [Diagram 2] 1(a) is a schematic cross-sectional view of the thermistor after the thermoplastic resin has softened and deformed, and (b) is a schematic top view of a heat-sensitive particle covered with a plurality of conductive particles. [Diagram 3] 1 is a schematic cross-sectional view of a thermistor including a thermistor layer according to one embodiment of the present invention; [Figure 4] FIG. 1(a) is a schematic plan view of a positive electrode for a battery including a thermistor layer according to one embodiment of the present invention, and FIG. 1(b) is a schematic cross-sectional view of the positive electrode taken along dashed line AA in (a). [Diagram 5] FIG. 1(a) is a schematic plan view of a negative electrode for a battery including a thermistor layer according to one embodiment of the present invention, and FIG. 1(b) is a schematic cross-sectional view of the negative electrode taken along dashed line BB in FIG. [Figure 6] FIG. 1 is a schematic cross-sectional view of a battery including a thermistor layer of one embodiment of the present invention. [Figure 7] 1 is a schematic cross-sectional view of a capacitor including a thermistor layer of one embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The thermistor layer of the present invention is a thermistor layer disposed so as to be disposed in a current path, the thermistor layer including heat-sensitive particles, a plurality of conductive particles covering the surfaces of the heat-sensitive particles, and a binder bonding the plurality of conductive particles, the plurality of conductive particles forming a conductive network, the heat-sensitive particles being particles having at least a surface layer made of a thermoplastic resin, the thermoplastic resin being a resin that softens at a lower temperature than the binder, and the thermistor layer being disposed so as to have a high resistance as the thermoplastic resin softens and deforms.

[0011] The conductive particles preferably form a surface coating layer covering the surface of the heat-sensitive particles, and the surface coating layer is preferably a layer in which a conductive network is formed and has a thickness smaller than the average particle size of the heat-sensitive particles, thereby making it possible to rapidly increase the electrical resistance of the thermistor layer near the temperature at which the thermoplastic resin softens, and thus improving the thermistor characteristics of the thermistor layer. The heat-sensitive particles are preferably an expansion agent coated with the thermoplastic resin, a heat-expandable microcapsule, or a thermoplastic resin particle, which can improve the thermistor characteristics of the thermistor layer. The average particle size of the plurality of conductive particles is preferably 20 nm or more and 100 μm or less, and the average particle size of the heat-sensitive particles is preferably 2 μm or more and 1000 μm or less, so that the thermistor layer can have excellent thermistor characteristics.

[0012] The present invention also provides an electrode for a battery comprising the thermistor layer of the present invention, a current collector sheet, and an electrode active material layer provided on the current collector sheet, the thermistor layer being disposed between the current collector sheet and the electrode active material layer. The present invention also provides a battery comprising the battery electrode of the present invention, an electrolyte, and a container containing the battery electrode and the electrolyte. The present invention also provides a thermistor comprising a thermistor layer of the present invention, a first conductive layer, and a second conductive layer, the thermistor layer being arranged such that a current flows from the first conductive layer through the thermistor layer to the second conductive layer, or from the second conductive layer through the thermistor layer to the first conductive layer.

[0013] The present invention will be described in more detail below with reference to a number of embodiments. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0014] First embodiment FIG. 1(a) is a schematic cross-sectional view of a thermistor including a thermistor layer of the present invention, and FIG. 1(b) is a schematic top view of a heat-sensitive particle covered with a plurality of conductive particles. In this embodiment, a thermistor 10 including a thermistor layer 4 of the present invention will be described. The thermistor 10 of the present embodiment includes a first conductive layer 2, a thermistor layer 4, and a second conductive layer 3, and is configured so that a current flows from the first conductive layer 2 through the thermistor layer 4 to the second conductive layer 3, or so that a current flows from the second conductive layer 3 to the first conductive layer 2 through the thermistor layer 4. In this manner, the thermistor layer 4 can be disposed in a current path. The thermistor layer 4 includes heat-sensitive particles 5, a plurality of conductive particles 6 covering the surfaces of the heat-sensitive particles 5, and a binder that bonds the plurality of conductive particles 6, the plurality of conductive particles 6 forming a conductive network, the heat-sensitive particles 5 being particles having at least a surface layer made of a thermoplastic resin, the thermoplastic resin being a resin that softens at a lower temperature than the binder, and the thermistor layer 4 being provided so as to become highly resistant as the thermoplastic resin softens and deforms.

[0015] The thermistor 10 is a thermistor whose electrical resistance increases as the temperature increases. The thermistor 10 may be a thermistor element, or may be a thermistor incorporated in a battery electrode, a capacitor, a heat sensor, a fire alarm, a sheet heating element, or the like.

[0016] The first conductive layer 2 or the second conductive layer 3 constitutes a current path. The first conductive layer 2 and the second conductive layer 3 can be provided so as not to be in direct contact with each other. The first conductive layer 2 or the second conductive layer 3 may be a metal layer, a semiconductor layer, a carbon layer, or a conductive oxide layer. The first conductive layer 2 or the second conductive layer 3 may be a surface coating layer of a particle or a conductive assistant. The first conductive layer 2 is, for example, a metal foil (thickness: 5 μm to 5 mm) and serves as a base material for the thermistor 10. The second conductive layer 3 may be formed by laminating a conductive layer such as a metal foil on the thermistor layer 4. In this case, the thermistor layer 4 and the conductive layer may be bonded after a paste having the same components as the thermistor layer 4 is applied onto the thermistor layer 4. Alternatively, the second conductive layer 3 may be formed by laminating a conductor on the thermistor layer 4 by a coating method, a vapor deposition method or the like.

[0017] The thermistor layer 4 is a layer whose electrical resistance increases as the temperature increases. The thermistor layer 4 includes heat-sensitive particles 5, a plurality of conductive particles 6 covering the surfaces of the heat-sensitive particles 5, and a binder that bonds the plurality of conductive particles 6. The thermistor layer 4 may also include a foaming agent, a plasticizer, etc. The thermistor layer 4 has a thickness of, for example, 0.1 μm or more and 300 μm or less. The thermistor layer 4 may be a layer having a non-uniform thickness. In this case, the thermistor layer 4 may be provided so that the portions where the thermosensitive particles 5 are present are convex portions and the portions between two adjacent thermosensitive particles 5 are concave portions. The thermistor layer 4 may be provided so that the thermosensitive particles 5 do not substantially overlap. Some of the thermosensitive particles 5 may protrude from the thermistor layer 4. For example, when the thermistor layer 4 is incorporated into a battery electrode (see the second embodiment described later), the thickness of the thermistor layer 4 may be 0.1 μm or more and 10 μm or less. When the thermistor layer 4 is incorporated into a thermistor element, the thickness of the thermistor layer 4 may be 50 μm or more and 300 μm or less. The porosity of the thermistor layer 4 measured by mercury intrusion porosimetry is preferably 5% or more and 65% or less. If the porosity is too low, the conductive network of the conductive particles becomes dense, slowing down the increase in resistance, which is not preferable. On the other hand, if the porosity is too high, the initial resistance of the thermistor layer becomes high, which is not preferable.

[0018] The heat-sensitive particles 5 are particles that soften and deform when the temperature exceeds a certain level, and at least the surface layer of the heat-sensitive particles 5 is made of a thermoplastic resin. The heat-sensitive particles 5 may be particles made of a thermoplastic resin, particles coated with a thermoplastic resin, or capsules made of a thermoplastic resin. For example, the heat-sensitive particles 5 contained in the thermistor layer 4 shown in FIG. 1(a) are particles (single-material particles) made of a thermoplastic resin, and in the heat-sensitive particles 5 contained in the thermistor layer 4 shown in FIG. 3(c), the expansion agent 7 is coated with an outer film 8 of a thermoplastic resin. The shape of the heat-sensitive particles 5 is, for example, granular, fibrous, plate-like, spherical, porous, cotton-like, aggregated, mass, etc. The heat-sensitive particles 5 may be of one type or a combination of two or more types. The average particle size of the heat-sensitive particles 5 contained in the thermistor layer 4 is, for example, 2 μm or more and less than 1000 μm. If the average particle size of the heat-sensitive particles 5 is less than 2 μm, it is necessary to increase the amount (number of parts added) of the heat-sensitive particles 5 contained in the thermistor layer 4 in order to improve the thermistor characteristics, and this increase in the amount of the heat-sensitive particles 5 may increase the electrical resistance value of the thermistor layer 4 at low temperatures. Furthermore, if the average particle size of the heat-sensitive particles 5 is 1000 μm or more, the thermistor layer 4 may not exhibit excellent thermistor characteristics. The average particle size of the heat-sensitive particles 5 may be larger or smaller than the average thickness of the thermistor layer 4. If the average particle size of the heat-sensitive particles 5 is larger than the average thickness of the thermistor layer 4, unevenness will be formed on the surface of the thermistor layer 4, and this unevenness can improve adhesion with the second conductive layer 3.

[0019] Thermoplastic resins are resins that soften and exhibit plasticity without reacting when heated to a temperature higher than a certain temperature (e.g., softening point), and solidify when cooled. Examples of thermoplastic resins include polyolefin, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinylidene chloride, fluororesin, polymethylmethacrylate, polyamide, polyester, polycarbonate, polyphenylene oxide, polyurethane, polyacetal, and acrylic resins. The temperature at which the thermoplastic resin contained in the heat-sensitive particles 5 softens can be adjusted by changing the type and molecular weight of the thermoplastic resin. The softening temperature of the thermoplastic resin is the temperature at which the thermistor layer 4 starts to increase in resistance, so that a suitable thermoplastic resin is selected so that the thermistor layer 4 has the desired thermistor characteristics. The softening point of the thermoplastic resin (the temperature at which the thermoplastic resin softens) is measured according to the measurement method described in JIS·K2235, JIS·K0129, ASTM·D127, or the like. The thermoplastic resin contained in the heat-sensitive particles 5 may be a single type of thermoplastic resin, or may be a mixture of multiple types of thermoplastic resins. The thermoplastic resin contained in the heat-sensitive particles 5 may also act as a binder. Moreover, for the thermoplastic resin contained in the heat-sensitive particles 5, a resin that softens at a temperature lower than the temperature at which the binder contained in the thermistor layer 4 softens is selected.

[0020] The heat-sensitive particles 5 may be particles that expand when the thermoplastic resin of the surface layer thereof softens. For example, the heat-sensitive particles 5 are particles in which a thermally decomposable chemical foaming agent (expansion agent 7) is coated with an outer film 8 of a thermoplastic resin. A thermally decomposable chemical foaming agent is a substance that is chemically decomposed by heat to generate gas. Examples of thermally decomposable chemical foaming agents include dinitrosopentamethylenetetramine (DPT), azodicarbonamide (ADCA), p-p'-oxybisbenzenesulfonylhydrazide (OBSH), inorganic foaming agents (including sodium bicarbonate), and composite foaming agents. The thermoplastic resin and the thermal decomposition type chemical foaming agent in the heat-sensitive particles 5 can be combined so that the temperature at which the thermal decomposition type chemical foaming agent generates gas is lower than the temperature at which the thermoplastic resin softens. When the temperature of such heat-sensitive particles 5 increases and reaches the temperature at which the thermal decomposition type chemical foaming agent generates gas, gas is generated inside the heat-sensitive particles 5. However, since the foaming agent is coated with a thermoplastic resin, the internal pressure of the heat-sensitive particles 5 increases, but the shape of the heat-sensitive particles 5 remains almost unchanged. When the temperature of the heat-sensitive particles 5 increases further and reaches the temperature at which the thermoplastic resin softens, the heat-sensitive particles 5 expand due to the internal pressure. The thickness of the thermoplastic resin outer film 8 of the heat-sensitive particles 5 can be, for example, 0.05 μm or more and 40 μm or less.

[0021] The heat-sensitive particles 5 are, for example, heat-expandable microcapsules. Heat-expandable microcapsules are formed by encapsulating a volatile organic solvent (expanding agent 7) such as isobutane, pentane, petroleum ether, hexane, heptane, low-boiling halogenated hydrocarbons, or methylsilane in a capsule (outer membrane 8) made of a thermoplastic resin. The expanding agent is preferably a hydrocarbon-based one. As the temperature of the heat-sensitive particles 5 increases and the volatile organic solvent volatilizes or vaporizes, the internal pressure of the heat-sensitive particles 5 increases. When the temperature of the heat-sensitive particles 5 increases further and reaches a temperature at which the thermoplastic resin softens, the heat-sensitive particles 5 expand due to the internal pressure. The thickness of the capsule shell (outer membrane 8) made of thermoplastic resin can be, for example, 0.05 μm or more and 40 μm or less. The heat-sensitive particles 5 may also be those containing spheres or lumps inside a porous or cotton-like body, or those having fibrous matter attached to the surface (core-shell structure).

[0022] The conductive particles 6 are not particularly limited as long as they are conductive particles, and examples thereof include carbon particles (hard carbon, soft carbon, graphite, carbon black), metal particles, conductive glass particles, and oxide conductor particles (e.g., ITO particles). The average particle size of the conductive particles 6 can be 20 nm or more and 100 μm or less. If the average particle size of the conductive particles 6 is less than 20 nm, the increase in electrical resistance is small even if the heat-sensitive particles 5 soften and change shape, and if it is more than 100 μm, the electrical resistance of the thermistor layer 4 at low temperatures may become high at an added amount that produces a high-temperature effect (excellent thermistor characteristics). The conductive particles 6 are provided so as to cover the surfaces of the heat-sensitive particles 5. The thermistor layer 4 contains an amount of conductive particles 6 that allows the conductive particles 6 to cover the surfaces of the heat-sensitive particles 5. The entire surfaces of the heat-sensitive particles 5 may be covered with the conductive particles 6, or only a portion of the surfaces of the heat-sensitive particles 5 may be covered with the conductive particles 6. When a portion of the heat-sensitive particles 5 protrudes from the thermistor layer 4, the surfaces of the heat-sensitive particles 5 are covered with the conductive particles 6 at least in the thermistor layer 4. The average particle size of the heat-sensitive particles 5 can be set to be 0.1 to 10,000 times the average particle size of the conductive particles 6 .

[0023] The amount of heat-sensitive particles 5 contained in the thermistor layer 4 is preferably 5 parts by mass or more and 1000 parts by mass or less per 100 parts by mass of conductive particles 6 (conductive particles: heat-sensitive particles=1:0.05 to 10). This mixing ratio varies depending on the device in which the thermistor layer 4 is incorporated. When the thermistor layer 4 is incorporated into a battery electrode (see the second embodiment), the amount of heat-sensitive particles 5 contained in the thermistor layer 4 is preferably 5 parts by mass or more and 300 parts by mass or less per 100 parts by mass of conductive particles 6 (conductive particles:heat-sensitive particles=1:0.05-3). When the thermistor layer 4 is incorporated into a planar heating element, the amount of heat-sensitive particles 5 contained in the thermistor layer 4 is preferably 5 parts by mass or more and 1000 parts by mass or less per 100 parts by mass of conductive particles 6 (conductive particles:heat-sensitive particles=1:0.05-10).

[0024] If the amount of heat-sensitive particles 5 contained in the thermistor layer 4 is 5 parts by mass or less per 100 parts by mass of conductive particles 6, the increase in electrical resistance of the thermistor layer 4 when the thermoplastic resin softens and deforms is small, and the thermistor layer 4 may not exhibit excellent thermistor characteristics. On the other hand, if the amount of heat-sensitive particles 5 contained in the thermistor layer 4 is 1000 parts by mass or more per 100 parts by mass of conductive particles 6, the electrical resistance of the thermistor layer 4 may become large, and power loss may occur in the current path in the thermistor layer 4.

[0025] The binder contained in the thermistor layer 4 is a binder that bonds a plurality of conductive particles 6. Examples of the binder include polyolefin, PVP (polyvinylpyrrolidone), acrylic resin, CMC (carboxymethyl cellulose), and SBR (styrene butadiene rubber). The binder may also bond the conductive particles 6 and the heat-sensitive particles 5. The binder may also bond the thermistor layer 4 and the first conductive layer 2. By bonding a plurality of conductive particles 6 with such a binder, a porous body of the conductive particles 6 can be formed in the thermistor layer 4, and a conductive network of the conductive particles 6 can be formed. This allows the thermistor layer 4 to have high conductivity. The amount of binder contained in the thermistor layer 4 is preferably 20 parts by mass or more and 1000 parts by mass or less, relative to 100 parts by mass of the total of the heat-sensitive particles 5 and the conductive particles 6. This allows the conductive particles 6 contained in the thermistor layer 4 to be bonded together to form a conductive network. The amount of binder can be appropriately changed depending on the device in which the thermistor layer 4 is incorporated. Furthermore, if the amount of binder is less than 20 parts by mass, the adhesion between the thermistor layer 4 and the substrate becomes poor, and if the amount of binder is more than 1000 parts by mass, the initial resistance of the thermistor layer 4 becomes too high.

[0026] The binder contained in the thermistor layer 4 is selected to be one that does not soften at a temperature lower than the softening temperature of the thermoplastic resin contained in the heat-sensitive particles 5. This makes it possible to prevent the conductive network of the conductive particles 6 from breaking down at a temperature lower than the softening temperature of the thermoplastic resin, and allows the thermistor layer 4 to have high conductivity. Therefore, it is possible to reduce power loss in the current path passing through the thermistor layer 4 between the first conductive layer 2 and the second conductive layer 3. In addition, because the temperature at which the thermoplastic resin softens is lower than the temperature at which the binder softens, the thermoplastic resin (heat-sensitive particles 5) begins to soften before the binder, and the conductive network formed by the contact of the conductive particles 6 in the thermistor layer 4 is destroyed by the deformation of the heat-sensitive particles 5 accompanying the softening of the thermoplastic resin. This allows the electrical resistance of the thermistor layer 4 to be suddenly increased near the temperature at which the thermoplastic resin softens.

[0027] The combination of the thermoplastic resin (heat-sensitive particles 5) and the binder contained in the thermistor layer 4 can be appropriately selected so that the thermistor layer 4 operates (has high resistance) at a desired temperature. The softening temperature of the thermoplastic resin is preferably at least 50° C. lower than the softening temperature of the binder. For example, when the thermistor layer 4 is incorporated in a battery electrode for a lithium ion battery (see the second embodiment), since the temperature at which the separator's shutdown function operates is approximately 180° C., the thermoplastic resin contained in the heat-sensitive particles 5 can be selected so that the softening temperature of the thermoplastic resin is 130° C. to 150° C. In addition, the binder contained in the thermistor layer 4 can be selected to be one that softens at 200° C. to 220° C.

[0028] When the thermistor layer 4 is incorporated into a planar heating element or a heat sensor for a fire alarm, the desired operating temperature of the thermistor layer 4 is 70° C. to 150° C., so a thermoplastic resin can be selected so that the thermoplastic resin contained in the heat-sensitive particles 5 softens at a temperature of 50° C. to 130° C. In addition, a binder that softens at 100° C. to 200° C. can be selected as the binder contained in the thermistor layer 4.

[0029] When the thermistor layer 4 is at a temperature lower than the temperature at which the thermoplastic resin of the heat-sensitive particles 5 softens, the surfaces of the heat-sensitive particles 5 are covered with conductive particles 6, and the conductive particles 6 form a conductive network, as shown in Fig. 1(a). Also, on the surfaces of the heat-sensitive particles 5, the conductive particles 6 form a conductive network, as shown in Fig. 1(b).

[0030] The conductive particles 6 contained in the thermistor layer 4 can form a surface coating layer 11 that covers the surfaces of the heat-sensitive particles 5. The surface coating layer 11 is a layer formed from a plurality of conductive particles 6 that form a conductive network, and can have a thickness thinner than the average particle size of the heat-sensitive particles 5. The surface coating layer 11 can have a shape that fits the surface of the heat-sensitive particles 5, and the outer surface of the surface coating layer 11 can be provided so as to contact the second conductive layer 3. By the conductive particles 6 forming such a surface coating layer 11, the conductive network of the conductive particles 6 can be easily broken when the heat-sensitive particles 5 soften and deform, and the thermistor layer 4 can have a high resistance.

[0031] When the temperature of the thermistor layer 4 becomes higher than the temperature at which the thermoplastic resin of the heat-sensitive particles 5 softens, the heat-sensitive particles 5 soften and deform, as shown in Fig. 2(a), causing the thermoplastic resin to enter the gaps and voids between the particles, or the heat-sensitive particles 5 to become recessed and create voids 9. Furthermore, when the heat-sensitive particles 5 deform, the conductive network of the conductive particles 6 on the surface of the heat-sensitive particles 5 breaks, as shown in Fig. 2(b). As a result, the resistance of the thermistor layer 4 increases, and the current flowing through the current path between the first conductive layer 2 and the second conductive layer 3 via the thermistor layer 4 decreases.

[0032] As in the thermistor 10 shown in FIG. 3, when the thermosensitive particles 5 included in the thermistor layer 4 are particles that expand when the thermoplastic resin (outer film 8) of the surface layer thereof softens, when the temperature of the thermistor layer 4 becomes higher than the temperature at which the thermoplastic resin of the thermosensitive particles 5 softens, the thermoplastic resin outer film 8 expands due to the internal pressure of the thermosensitive particles 5 and penetrates into the gaps and voids between the particles. The thermoplastic resin that penetrates between adjacent conductive particles 6 cuts off the conductive contact between the conductive particles 6 and destroys the conductive network of the conductive particles 6. The conductive network of the conductive particles 6 on the surface of the thermosensitive particles 5 is also destroyed. As a result, the thermistor layer 4 becomes highly resistive, and the current flowing through the current path passing through the thermistor layer 4 between the first conductive layer 2 and the second conductive layer 3 decreases. The thermistor layer 4 may contain a thermally decomposable chemical foaming agent in addition to the heat-sensitive particles 5. In this case, as the temperature of the thermistor layer 4 increases and reaches a temperature at which gas is generated from the thermally decomposable chemical foaming agent, the gas generated from the foaming agent causes the thermistor layer 4 to expand, generating voids inside the thermistor layer 4. When the temperature of thermistor layer 4 increases further and reaches a temperature at which the thermoplastic resin softens, the softened thermoplastic resin of the heat-sensitive particles 5 flows into the voids and the conductive network is destroyed. This allows the thermistor layer 4 to have a high resistance.

[0033] The thermistor layer 4 can be formed, for example, as follows. After dispersing the conductive particles 6, binder, and dispersant in a solvent, the heat-sensitive particles 5 are added to this slurry. The viscosity and solid content of the slurry are then adjusted to prepare a coating slurry. The coating slurry is applied onto the first conductive layer 2, and the coating film is dried to form the thermistor layer 4.

[0034] Second embodiment The second embodiment relates to a battery electrode (battery positive electrode or battery negative electrode) incorporating the thermistor layer 4 of the present invention, and a battery incorporating this battery electrode. FIG. 4(a) is a schematic plan view of a battery positive electrode, and FIG. 4(b) is a schematic cross-sectional view of the battery positive electrode taken along dashed line AA in FIG. 4(a). FIG. 5(a) is a schematic plan view of a battery negative electrode, and FIG. 5(b) is a schematic cross-sectional view of the battery negative electrode taken along dashed line BB in FIG. 5(a). FIG. 6 is a schematic cross-sectional view of a battery incorporating the battery positive electrode shown in FIG. 4(a)(b) and the battery negative electrode shown in FIG. 5(a)(b). Note that the thermistor layer 4 is omitted in FIG. 6. The battery 30 is not particularly limited as long as it is a battery that may undergo thermal runaway, and is, for example, a non-aqueous electrolyte secondary battery (such as a lithium ion secondary battery).

[0035] The thermistor layer 4 is similar to that in the first embodiment. The thermoplastic resin contained in the thermistor layer 4 is selected so that the thermistor layer 4 can prevent thermal runaway of the battery 30 without interfering with normal charging and discharging of the battery 30. For example, in a lithium ion secondary battery, a thermoplastic resin can be selected so that the thermistor layer 4 has high resistance at 130°C to 150°C.

[0036] In the battery positive electrode 18, the thermistor layer 4 is disposed between the positive electrode current collector sheet 13 and the positive electrode active material layer 15. The thermistor layer 4 can be provided so as to coat the positive electrode current collector sheet 13. The thermistor layer 4 can also be provided so as not to come into contact with the positive electrode current collector sheet 13 and the positive electrode active material layer 15. This allows the thermistor layer 4 to have high resistance when the battery 30 abnormally heats up due to overcharging, internal short circuit, etc., and this high-resistance thermistor layer 4 can reduce the current flowing between the positive electrode current collector sheet 13 and the positive electrode active material layer 15. This makes it possible to prevent the battery 30 from experiencing thermal runaway. The positive electrode current collecting sheet 13 corresponds to the first conductive layer 2 in the first embodiment, and the positive electrode active material layer 15 (or the conductive coating or conductive assistant contained in the positive electrode active material layer 15) corresponds to the second conductive layer 3 in the first embodiment.

[0037] In the battery negative electrode 19, the thermistor layer 4 is disposed between the negative electrode current collector sheet 14 and the negative electrode active material layer 16. The thermistor layer 4 can be provided so as to coat the negative electrode current collector sheet 14. The thermistor layer 4 can also be provided so as not to come into contact with the negative electrode current collector sheet 14 and the negative electrode active material layer 16. This allows the thermistor layer 4 to have high resistance when the battery 30 generates abnormal heat due to overcharging, internal short circuit, etc., and this high-resistance thermistor layer 4 can reduce the current flowing between the negative electrode current collector sheet 14 and the negative electrode active material layer 16. This makes it possible to prevent the battery 30 from experiencing thermal runaway. The negative electrode current collector sheet 14 corresponds to the first conductive layer 2 in the first embodiment, and the negative electrode active material layer 16 corresponds to the second conductive layer 3 in the first embodiment.

[0038] In the battery 30 shown in FIG. 6, the thermistor layer 4 is provided on both the battery positive electrode 18 and the battery negative electrode 19, but the thermistor layer 4 may be provided on only one of the battery positive electrode 18 and the battery negative electrode 19.

[0039] Third embodiment The third embodiment relates to an aluminum electrolytic capacitor incorporating the thermistor layer 4 of the present invention. Figure 7 is a schematic cross-sectional view of a capacitor 50 of this embodiment. Aluminum electrolytic capacitor 50 uses oxide film 43 formed on the surface of anode aluminum electrode 41 as a dielectric layer. This oxide film 43 is very thin, so aluminum electrolytic capacitor 50 has a large capacitance. However, if oxide film 43 is destroyed by an electrochemical reaction, the conventional aluminum electrolytic capacitor becomes unusable and may explode or emit smoke.

[0040] In capacitor 50 of the present embodiment, a thermistor layer 4 is provided between anode aluminum electrode 41 and anode lead 48. In addition, a thermistor layer 4 is provided between cathode aluminum electrode 42 and cathode lead 49. The thermistor layer 4 is similar to that in the first embodiment. The thermoplastic resin contained in the thermistor layer 4 is selected so that thermistor layer 4 does not interfere with the normal operation of capacitor 50 and prevents capacitor 50 from exploding or emitting smoke.

[0041] If oxide film 43 of capacitor 50 is destroyed by an electrochemical reaction, capacitor 50 may generate abnormal heat. This abnormal heat can increase the resistance of thermistor layer 4, and this high-resistance thermistor layer 4 can reduce the current flowing between anode aluminum electrode 41 and anode lead 48 and the current flowing between cathode aluminum electrode 42 and cathode lead 49. This can prevent capacitor 50 from exploding or emitting smoke. Anode lead 48 or cathode lead 49 corresponds to first conductive layer 2 in the first embodiment, and anode aluminum electrode 41 or cathode aluminum electrode 42 corresponds to second conductive layer 3 in the first embodiment.

[0042] Electrical Resistivity Measurement The following samples 1 to 9 were prepared. (Sample 1) A uniform slurry was prepared by adding 10 parts by mass of high-purity graphite particles (UP-5α (average particle size: approximately 5.5 μm) manufactured by Nippon Graphite Industries Co., Ltd.) (conductive particles) and 5 parts by mass of water-soluble acrylic resin (binder) to 100 parts by mass of ion-exchanged water and mixing them. A coating paste was prepared by adding 5 parts by mass of spherical polyolefin A (average particle size: approximately 7 μm, softening point: approximately 135°C, commercially available product) (thermosensitive particles) to this slurry and dispersing it uniformly. The softening point of the water-soluble acrylic resin was approximately 205°C. The prepared coating paste was applied using a gravure coater and dried to form a thermistor layer with a thickness of about 3 μm on an aluminum foil (thickness: about 15 μm) (first conductive layer).

[0043] A paste for a positive electrode active material layer was prepared containing carbon-coated lithium iron phosphate particles (90 parts by mass), acetylene black (conductive agent) (3 parts by mass), and PVDF (polyvinylidene fluoride) (7 parts by mass). This paste for a positive electrode active material layer was applied onto the thermistor layer and dried to form a positive electrode active material layer (thickness: about 50 μm) (second conductive layer).

[0044] A laminate (thermistor 10) of aluminum foil, thermistor layer, and second conductive layer was compressed to a thickness of about 50 μm using a roller press. The compressed thermistor 10 was then left in a dryer at 80° C. for 5 minutes, and then left in a desiccator at room temperature for 10 minutes. Sample 1 was then prepared by punching a test piece having a diameter of 15 mm from thermistor 10.

[0045] (Sample 2) Sample 2 was prepared in the same manner as Sample 1, except that the heat-sensitive particles used were 3 parts by mass of spherical polyolefin A (average particle size: approximately 7 μm, softening point: approximately 135°C, commercially available product) and 2 parts by mass of thermally expandable microcapsules (average particle size: 20 μm, foaming initiation temperature: 135°C, commercially available product).

[0046] (Sample 3) Sample 3 was prepared in the same manner as Sample 1, except that 5 parts by mass of spherical polyolefin B (average particle size: 4.5 μm, softening point: 115° C., commercially available product) was used as the heat-sensitive particles.

[0047] (Sample 4) Sample 4 was prepared in the same manner as Sample 2, except that CMC (carboxymethyl cellulose) was used as the binder.

[0048] (Sample 5) Sample 5 was prepared in the same manner as Sample 1, except that no heat-sensitive particles were added.

[0049] (Sample 6) Sample 6 was prepared in the same manner as Sample 1, except that 35 parts by mass of spherical polyolefin A (average particle size: 7 μm, softening point: 135° C.) was used as the heat-sensitive particles.

[0050] (Sample 7) Sample 7 was prepared in the same manner as Sample 1, except that 5 parts by mass of fibrous polyolefin (average length 1000 μm, commercially available product) was used as the heat-sensitive particles.

[0051] (Sample 8) Sample 8 was prepared in the same manner as sample 1, except that a 10 μm thick stainless steel foil was used for the second conductive layer 3 of thermistor 10, and the laminate of aluminum foil, thermistor layer, and second conductive layer was made to a thickness of 30 μm using a roll press.

[0052] (Sample 9) Sample 9 was produced in the same manner as Sample 1, except that a paste containing silica particles (85 parts by mass), high-purity graphite particles, acetylene black (conductive agent) (5 parts by mass), and SBR (styrene butadiene rubber) (10 parts by mass) was prepared, and this paste was applied onto the thermistor layer and dried to form a second conductive layer.

[0053] (Heat treatment) After completing the resistivity measurement for Samples 1 to 9 described below, Samples 1 to 9 were left in a dryer at 180° C. for 2 hours.

[0054] (Resistivity measurement) Samples 1, 2, 3, 4, 5, 6, 7, 8, or 9 before heat treatment or Samples 1, 2, 3, 4, 5, 6, 7, 8, or 9 after heat treatment were sandwiched between two gold-plated copper cylinders (diameter: 11.3 mm) and a pressure of 0.98 MPa was applied to measure the electrical resistivity of Samples 1 to 9 before heat treatment or Samples 1 to 9 after heat treatment. The measurement results are shown in Table 1.

[0055] [Table 1]

[0056] In the samples with added heat-sensitive particles (samples 1-4, 6-9), the electrical resistivity of the samples after heat treatment was higher than that of the samples before heat treatment. This confirmed that the heat-sensitive particles soften and deform, causing the thermistor layer to have a high resistance. It was also confirmed that the electrical resistivity of the samples after heat treatment was 300 Ω·cm or more higher than that of the samples before heat treatment in samples 1, 2, 3, 8, and 9.

[0057] Battery nail piercing experiment (Making lithium-ion batteries) A uniform slurry was prepared by adding 10 parts by mass of high-purity graphite particles (UP-5α (average particle size: approximately 5.5 μm) manufactured by Nippon Graphite Industries Co., Ltd.) (conductive particles) and 5 parts by mass of water-soluble acrylic resin (binder) to 100 parts by mass of ion-exchanged water and mixing them. A coating paste was prepared by adding 10 parts by mass of spherical polyolefin particles (average particle size: approximately 7 μm, softening point: approximately 135°C, commercially available product) (thermosensitive particles) to this slurry and dispersing them uniformly. The softening point of the water-soluble acrylic resin was approximately 205°C. The prepared coating paste was applied using a gravure coater and dried to form a thermistor layer with a thickness of about 3 μm on each side of an aluminum foil (thickness: about 15 μm).

[0058] A paste for a positive electrode active material layer was prepared, which contained lithium iron phosphate particles (90 parts by mass) coated with a carbon film, acetylene black (conductive agent) (3 parts by mass), and PVDF (polyvinylidene fluoride) (7 parts by mass). The paste for a positive electrode active material layer was applied to the thermistor layers on both sides of an aluminum foil and dried to form a positive electrode active material layer (thickness: about 20 μm). In this way, the positive electrode of the example was produced. A comparative positive electrode was also prepared in the same manner except that no thermistor layer was provided.

[0059] A paste for the negative electrode active material layer containing amorphous carbon as the negative electrode active material and PVDF as the binder was prepared, and the paste for the negative electrode active material layer was applied to both sides of the copper foil and dried to form a negative electrode active material layer. In this way, the negative electrode was produced.

[0060] A plurality of sheets of the positive electrodes of the embodiment and a plurality of sheets of the negative electrodes were alternately stacked with a separator therebetween, and fixed with tape to prepare an electrode laminate. This electrode laminate was inserted into a rectangular aluminum case, and the positive and negative electrodes were electrically connected, and an electrolyte was poured into the case, and the case was covered to prepare a lithium-ion battery of the embodiment. The electrolyte used was a mixture of 70 parts by mass of ethylene carbonate and 30 parts by mass of diethyl carbonate, in which lithium hexafluorophosphate was dissolved as a supporting electrolyte. The cover was also provided with a safety valve. In addition, a comparative lithium ion battery was produced in the same manner as the lithium ion battery of the example, except that the comparative positive electrode was used.

[0061] (Nail penetration test) The lithium ion battery of the Example or the lithium ion battery of the Comparative Example was charged for 4 hours at a current of 30A and a voltage of 3.60V (CCCV charging), after which a thermocouple was attached to the square aluminum case, and a nail with a diameter of 5mm was inserted into the side of the square aluminum case to penetrate the battery, forcing an internal short circuit to occur. The temperature rise of the aluminum case due to heat generated by this internal short circuit was monitored using a thermocouple. In addition, the opening and closing of the safety valve on the lid and the presence or absence of smoke were observed. The measurement results are shown in Table 2.

[0062] [Table 2]

[0063] In the lithium ion battery of the embodiment, the maximum temperature was 94.7°C, the safety valve did not open, and no smoke was observed. In contrast, in the lithium ion battery of the comparative example, the maximum temperature reached 147.7°C, the safety valve opened, and smoke was observed. From these results, it was confirmed that the thermistor layer of the positive electrode becomes highly resistant due to heat generation accompanying an internal short circuit, thereby suppressing heat generation due to the short circuit current. [Explanation of symbols]

[0064] 2: First conductive layer 3: Second conductive layer 4: Thermistor layer 5: Heat-sensitive particles 6: Conductive particles 7: Expanding agent 8: Outer film 9: Gap 10: Thermistor 11: Surface coating layer 13: Positive electrode current collector sheet 14: Negative electrode current collector sheet 15: Positive electrode active material layer 16: Negative electrode active material layer 18: Positive electrode for battery 19: Negative electrode for battery 22: Separator 23: Electrolyte 24: Container 26: Positive electrode terminal 27: Negative electrode terminal 30: Battery 41: Anode aluminum electrode 42: Cathode aluminum electrode 43: Oxide film 45: Electrolyte 46: Electrolytic paper 48: Anode lead 49: Cathode lead 50: Aluminum electrolytic capacitor

Claims

1. A thermistor layer disposed in a current path, the thermistor layer includes heat-sensitive particles, a plurality of conductive particles covering the surfaces of the heat-sensitive particles, and a binder that bonds the plurality of conductive particles; The conductive particles form a conductive network, The heat-sensitive particles are particles having at least a surface layer made of a thermoplastic resin, the thermoplastic resin is a resin that softens at a lower temperature than the binder, The thermistor layer is characterized in that the thermistor layer is provided so as to have high resistance as the thermoplastic resin softens and deforms.

2. a plurality of conductive particles form a surface coating layer covering the surfaces of the heat-sensitive particles; The thermistor layer according to claim 1 , wherein the surface coating layer is a layer in which the conductive network is formed, and has a thickness smaller than an average grain size of the heat-sensitive particles.

3. 3. The thermistor layer according to claim 1, wherein the heat-sensitive particles are an expansion agent coated with the thermoplastic resin, a heat-expandable microcapsule, or a thermoplastic resin particle.

4. The average particle size of the plurality of conductive particles is 20 nm or more and 100 μm or less, 4. The thermistor layer according to claim 1, wherein the heat-sensitive particles have an average particle size of 2 μm or more and 1000 μm or less.

5. A thermistor layer according to any one of claims 1 to 4, a current collecting sheet, and an electrode active material layer provided on the current collecting sheet, The thermistor layer is disposed between the current collecting sheet and the electrode active material layer.

6. A battery comprising the battery electrode of claim 5, an electrolyte, and a container for containing the battery electrode and the electrolyte.

7. A thermistor layer according to any one of claims 1 to 4, a first conductive layer, and a second conductive layer, The thermistor layer is arranged so that a current flows from a first conductive layer through the thermistor layer to a second conductive layer, or from a second conductive layer through the thermistor layer to a first conductive layer.

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