Electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide an electrode in which an influence from heat generation upon an external impact on a lithium ion secondary battery is suppressed.SOLUTION: An electrode for a lithium ion secondary battery includes a metal foil, a conductive layer formed on at least a part of the metal foil, and an active material layer formed on at least a part of a surface of the conductive layer on the opposite side of the metal foil side. The conductive layer includes a conductive particle and insulating resin. The active material layer includes a first active material layer and a second active material layer. The first active material layer and the second active material layer are stacked so that the first active material layer comes closer to the conductive layer. The second active material layer has larger porosity than the first active material layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrode for a lithium-ion secondary battery and a lithium-ion secondary battery.

Background Art

[0002] Lithium-ion secondary batteries are widely used as power sources for portable electronic devices because they are lightweight and have a high energy density compared to nickel-cadmium batteries, nickel-metal hydride batteries, etc. They are also strong candidates as power sources for hybrid vehicles and electric vehicles. With the recent miniaturization and high functionality of portable electronic devices, further increase in the energy density of these lithium-ion secondary batteries as power sources is expected.

[0003] Although current lithium-ion secondary batteries are at a high level in terms of safety, further improvement in safety is desired due to their high capacity and high output. For example, when a lithium-ion secondary battery is overcharged, it may generate heat. Also, heat generation may occur due to an internal short circuit. Furthermore, since lithium-ion secondary batteries contain a non-aqueous electrolyte containing an organic solvent, problems such as chemical decomposition of the organic solvent with heat generation and gas generation, and an increase in the internal pressure of the battery may occur.

[0004] In response to such problems, Patent Document 1 proposes a technique of providing a conductive layer on the surface of a current collector.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the lithium-ion secondary battery described in Patent Document 1 has a problem in that it is insufficient against locally rapid heat generation due to an external impact. As a result of intensive research by the present inventors, it has been found that in addition to controlling the current generated at the short-circuit portion, the problem can be solved by providing a structure for dissipating the heat generated at the short-circuit portion.

[0007] The present invention has been made in view of such problems, and an object thereof is to provide an electrode that suppresses the influence of heat generation on an external impact on a lithium-ion secondary battery.

Means for Solving the Problems

[0008] In order to achieve the above object, an electrode for a lithium-ion secondary battery according to the present invention includes a metal foil, a conductive layer formed on at least a part of the metal foil, and an active material layer formed on at least a part of a surface of the conductive layer opposite to the side of the metal foil. The conductive layer contains conductive particles and an insulating resin, the active material layer includes a first active material layer and a second active material layer, the first active material layer and the second active material layer are laminated so that the first active material layer is on the side closer to the conductive layer, and the second active material layer is characterized by having a porosity larger than that of the first active material layer.

[0009] When an impact is applied to the lithium-ion secondary battery according to the present invention and an internal short circuit occurs, the insulating resin contained in the conductive layer flows into the short-circuit portion and the short-circuit resistance increases, so that the amount of current generated by the internal short circuit can be suppressed. In addition, since the second active material layer of the electrode has a large porosity, the thermal conductivity decreases. Therefore, the transfer of heat generated at the internal short-circuit portion is less likely to occur between the opposing positive and negative electrodes, and the transfer through the current collector with high heat dissipation is preferentially performed. Therefore, the temperature of the short-circuit portion is less likely to rise, and it is possible to reduce the influence of heat generation.

[0010] Further, when the occupied area per unit area of the conductive particles is A and the occupied area per unit area of the insulating resin is B when the conductive layer is viewed in the thickness direction, it is preferably 0.11 ≦ A / B ≦ 1.0.

[0011] According to this, without reducing the output of the lithium-ion secondary battery, the resistance of the short-circuited portion can be increased, and the effects of the present invention can be further enhanced.

[0012] Further, when the porosity of the second active material layer in the active material layer is C and the porosity of the first active material layer is D, it is preferable that 1.2 ≤ C / D ≤ 3.5.

[0013] According to this, without reducing the output of the lithium-ion secondary battery, the heat generated at the internal short-circuited portion can be efficiently dissipated through the current collector, and the effects of the present invention can be further enhanced.

Effects of the Invention

[0014] According to the present invention, it is possible to obtain an electrode for a lithium-ion secondary battery that can reduce the influence of heat generation even when an impact is applied to the lithium-ion secondary battery and an internal short circuit occurs, and a lithium-ion secondary battery using the same.

Brief Description of the Drawings

[0015]

Figure 1

Mode for Carrying Out the Invention

[0016] Hereinafter, preferred embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments.

[0017] <Lithium-Ion Secondary Battery> FIG. 1 shows a schematic cross-sectional view of a laminate of the lithium-ion secondary battery of this embodiment.

[0018] A laminated body 10 of a lithium-ion secondary battery can be produced by producing a positive electrode composed of 1, 2, 3, a negative electrode composed of 5, 6, 7, and a separator 4 impregnated with an electrolyte as shown in FIG. 1. Here, the positive electrode can be produced by forming a positive electrode active material layer 1 on a positive electrode current collector 3 or on a conductive layer 2 formed on the positive electrode current collector, and the negative electrode can be produced by forming a negative electrode active material layer 5 on a negative electrode current collector 7 or on a conductive layer 6 formed on the negative electrode current collector. However, in order to exhibit the effects of the present invention, in addition to forming a conductive layer 2 between the positive electrode current collector 3 and the positive electrode active material layer 1 or forming a conductive layer 6 between the negative electrode current collector 7 and the negative electrode active material layer 5, it is necessary to divide the positive electrode active material layer 1 into two layers, i.e., positive electrode active material layers 1a and 1b, or divide the negative electrode active material layer 5 into two layers, i.e., negative electrode active material layers 5a and 5b. In the drawings, 8 and 9 indicate lead-out electrodes of the positive electrode and the negative electrode, respectively.

[0019] <Metal foil having a conductive layer> The metal foil having a conductive layer according to the present embodiment has a metal foil, a conductive layer formed on at least a part of the metal foil, and an active material layer formed on at least a part of the side of the conductive layer opposite to the metal foil, and the conductive layer contains conductive particles and an insulating resin.

[0020] When an external impact is applied to a lithium-ion secondary battery and an internal short circuit occurs, in a lithium-ion secondary battery without such countermeasures, the resistance of the short-circuit portion formed only by the active material layer and the current collector constituting the positive electrode and the negative electrode is low, so a large current may be generated. However, according to the present embodiment, since the insulating resin is contained in the conductive layer of the current collector, when an internal short circuit occurs, the insulating resin flows into the short-circuit portion, the resistance of the short-circuit portion increases, and the generation of a large current can be suppressed.

[0021] The metal foil may be any conductive plate material. For example, a thin metal plate (metal foil) such as copper, nickel, or an alloy thereof, or stainless steel can be used for the negative electrode, and a thin metal plate (metal foil) such as aluminum or an alloy thereof, or stainless steel can be used for the positive electrode.

[0022] The ratio of the conductive particles contained in the conductive layer to the insulating resin can be determined from the areas of both when the metal foil forming the conductive layer is viewed from the thickness direction (i.e., when viewed in plan view from the side opposite to the conductive layer). When the area occupied by the conductive particles in a predetermined area is A and the area of the insulating resin is B, it is preferably 0.11 ≦ A / B ≦ 1.0. Being within this range makes it possible to keep the resistance of the short-circuited portion at a sufficiently high value and also makes it possible to maintain better rate characteristics when the lithium-ion secondary battery is used normally. Since the conductive particles in the conductive layer serve as an electron conduction path between the current collector and the active material layer, if the proportion of the conductive particles is small, the rate characteristics may deteriorate.

[0023] The insulating resin desirably has a resistance value capable of suppressing the generation of a large current when an internal short circuit occurs, and the resistance value is preferably 1.0×10 8 [Ωcm] or more.

[0024] The conductive particles are not particularly limited as long as they are materials with good conductivity, and examples include carbon-based materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. From the perspective of compatibility with resin materials, carbon-based materials are particularly preferred. Examples of carbon-based materials include carbon black, graphene, carbon nanofibers, carbon nanotubes, carbon nanowalls, and graphite.

[0025] <Two-layer active material layer> The active material layer according to this embodiment includes a first active material layer and a second active material layer, and the first active material layer and the second active material layer are laminated such that the first active material layer is on the side closer to the conductive layer, and the second active material layer is characterized by having a porosity larger than that of the first active material layer.

[0026] The active material layer serves to control the conduction of heat generated by internal short circuit. Since the second active material layer has a large porosity, its thermal conductivity is low. Heat generated at the internal short circuit location is less likely to transfer between the opposing positive and negative electrodes, and preferentially transfers through the current collector with high heat dissipation performance. Therefore, it is possible to further suppress the local temperature rise at the short circuit location.

[0027] When the porosity ratio of the active material layer is considered, it is desirable that 1.2 ≤ C / D ≤ 3.5, where C is the porosity of the second active material layer and D is the porosity of the first active material layer. Being within this range can suppress the decrease in the energy density of the lithium-ion secondary battery. By preferentially dissipating the heat generated at the internal short circuit location through the current collector with high heat dissipation performance, it is possible to further suppress the local temperature rise at the short circuit location.

[0028] <Measurement of the porosity of the active material layer> For the porosity of each layer in the first active material layer and the second active material layer, measurement and calculation were performed using cross-sectional SEM. First, the thicknesses of the first active material layer and the second active material layer were measured by cross-sectional SEM, and the density was calculated from the relationship between the areal density and the thickness. Furthermore, the porosity was calculated based on the following calculation formula. Porosity = (1 - density ÷ true density calculated from the material constituting each layer) × 100

[0029] <Formation of the conductive layer on the current collector> A conductive particle and an insulating resin are mixed and dispersed in a solvent such as water or N-methyl-2-pyrrolidone to prepare a paste-like slurry. Next, this slurry is applied, for example, using a comma coater, to one or both sides of a current collector such as an aluminum foil or a copper foil to form a coating film having a predetermined thickness, and then introduced into a drying furnace to evaporate the solvent. When applied to both sides of the current collector, it is desirable that the thickness of the coating film that becomes the conductive layer is the same on both sides. Further, after the solvent evaporation, pressure molding may be performed by a roller press. The thickness of the conductive layer is preferably 1 [μm] or more and less than 10 [μm]. Thereby, when an external impact is applied to the lithium-ion secondary battery and an internal short circuit occurs, it serves to increase the resistance of the short-circuited portion, and at the same time, it does not reduce the output during normal use.

[0030] <Positive electrode> As will be described later, the positive electrode can be manufactured by forming a positive electrode active material layer 1 on a positive electrode current collector 3 or on a conductive layer 2 formed on the positive electrode current collector. When the positive electrode active material layer is divided into a first active material layer and the second active material layer, first, the first active material layer is formed on the conductive layer 2 formed on the positive electrode current collector, and then the second active material layer is further formed thereon.

[0031] (Positive electrode current collector) The positive electrode current collector 3 may be a conductive plate material. For example, a thin metal plate (metal foil) such as aluminum or an alloy thereof, or stainless steel can be used.

[0032] (Positive electrode active material layer) The positive electrode active material layer 1 is mainly composed of a positive electrode active material, a positive electrode binder, and, if necessary, a positive electrode conductive assistant in an appropriate amount.

[0033] (Positive electrode active material) As the positive electrode active material, the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or lithium ions and the counter anion of the lithium ions (for example, PF 6 -If it is possible to reversibly proceed with doping and dedoping with [substance name], it is not particularly limited, and known electrode active materials can be used. For example, lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ), lithium manganese spinel (LiMn 2 O 4 ), and a composite metal oxide represented by the general formula: LiNi x Co y Mn z MaO 2 (x + y + z + a = 1, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV 2 O 5 ), olivine-type LiMPO 4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li 4 Ti 5 O 12) , LiNi x Co y Al z O 2 (0.9 < x + y + z < 1.1), etc. Examples of composite metal oxides include these.

[0034] (Binder for positive electrode) The binder for the positive electrode binds the positive electrode active materials together and also binds the positive electrode active material and the current collector. The binder may be any material capable of the above-described binding, and examples thereof include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE). Further, in addition to the above, as the binder, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, etc. may be used. Also, an electronically conductive polymer or an ionically conductive polymer may be used as the binder. Examples of the electronically conductive polymer include polyacetylene. In this case, since the binder also functions as a conductive aid particle, it is not necessary to add a conductive aid. Examples of the ionically conductive polymer include those having conductivity for ions such as lithium ions, and for example, a monomer of a polymer compound (a polyether-based polymer compound such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.) and LiClO 4 、LiBF 4 、LiPF 6 and other lithium salts or alkali metal salts mainly composed of lithium, and those obtained by complexing them, etc. Examples of the polymerization initiator used for the complexing include a photopolymerization initiator or a thermal polymerization initiator compatible with the above monomer.

[0035] (Conductive Aid for Positive Electrode) The conductive aid for the positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer, and known conductive aids can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO.

[0036] (Negative Electrode) The negative electrode can be fabricated by forming a negative electrode active material layer 5 on a current collector 7 for the negative electrode as described below, or on a conductive layer 6 formed on the negative electrode current collector. When the negative electrode active material layer is divided into a first active material layer and the second active material layer, first, the first active material layer is formed on the conductive layer 6 formed on the negative electrode current collector, and then the second active material layer is further formed thereon.

[0037] (Current collector for negative electrode) The current collector 7 for the negative electrode may be any conductive plate material. For example, a thin metal plate (metal foil) such as copper, nickel, an alloy thereof, or stainless steel can be used.

[0038] (Negative electrode active material layer) The negative electrode active material layer 5 is mainly composed of a negative electrode active material, a binder for the negative electrode, and, if necessary, an appropriate amount of a conductive assistant for the negative electrode.

[0039] (Negative electrode active material) Examples of the negative electrode active material include graphite, silicon oxide (SiO x ), metallic silicon (Si), and the like.

[0040] (Binder for negative electrode) There is no particular limitation on the binder for the negative electrode, and the same binder as the binder for the positive electrode described above can be used.

[0041] The content of the binder in the negative electrode active material layer 5 is not particularly limited either, but it is preferably 1 to 20 parts by mass of the entire negative electrode active material layer.

[0042] (Conductive assistant for negative electrode) There is no particular limitation on the conductive assistant for the negative electrode, and the same conductive assistant as the conductive assistant for the positive electrode described above can be used.

[0043] (Electrolyte) Examples of the electrolyte include LiPF 6 , LiClO 4 , LiBF 4 , LiAsF 6 , LiCF 3 SO3 , LiCF 3 , CF 2 SO 3 , LiC(CF 3 SO 2 ) 3 , LiN(CF 3 SO 2 ) 2 , LiN(CF 3 CF 2 SO 2 ) 2 , LiN(CF 3 SO 2 )(C 4 F 9 SO 2 ), LiN(CF 3 CF 2 CO) 2 Salts such as LiBOB can be used. These salts may be used alone or in combination of two or more.

[0044] The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments.

Examples

[0045] Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following examples.

[0046] <Example 1> (Formation of conductive layer on current collector) In Example 1, 1.1 parts by mass of acetylene black as conductive particles, 1.0 part by mass of PVdF as insulating resin, and N-methylpyrrolidone as a solvent were mixed to prepare a slurry for forming a conductive layer. This slurry was applied to both sides of an aluminum foil with a thickness of 12 [μm] and dried at 100 [°C] to obtain a positive current collector on which a conductive layer with a thickness of 0.90 [μm] was formed.

[0047] (Fabrication of positive electrode) LiCoO as the positive electrode active material 296 parts by mass, 2 parts by mass of acetylene black as a conductive assistant, 2 parts by mass of PVdF as a binder, and N-methylpyrrolidone as a solvent were mixed to prepare a slurry for forming an active material layer. This slurry was applied to both sides of the positive electrode current collector on which the conductive layer obtained above was formed, and dried at 100 [°C] to obtain a first active material layer. Further, the slurry was applied to both sides of the first active material layer obtained above, and dried at 100 [°C] to obtain a second active material layer. After that, a positive electrode having a positive electrode active material layer was obtained by pressure molding with a roller press.

[0048] (Fabrication of negative electrode) 83 parts by mass of Si as a negative electrode active material, 2 parts by mass of acetylene black as a conductive assistant, 15 parts by mass of polyamideimide as a binder, and N-methylpyrrolidone as a solvent were mixed to prepare a slurry for forming an active material layer. This slurry was applied to both sides of a copper foil with a thickness of 10 [μm], dried at 100 [°C], and then pressure molded with a roller press and heat treated at 350 [°C] for 3 hours in a vacuum to obtain a negative electrode having a negative electrode active material layer.

[0049] (Fabrication of lithium ion secondary battery for evaluation) The positive electrode and the negative electrode fabricated above were placed in an aluminum laminate pack with a separator made of a polyethylene microporous membrane sandwiched between them, and 1 M LiPF 6 solution (solvent: ethylene carbonate / diethyl carbonate = 3 / 7 (volume ratio)) was injected, and then vacuum sealed to fabricate a lithium ion secondary battery for evaluation.

[0050] (Measurement of rate characteristics) Regarding the evaluation lithium-ion secondary battery fabricated in Example 1, using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Denko Corporation), in a constant temperature bath at 25°C, the voltage range was set from 2.8 [V] to 4.2 [V], and charging and discharging were performed for one cycle at a current value of 0.05C to confirm that the capacity was normal. Similarly, after charging at a current value of 0.05C, discharging was performed at a current value of 0.2C or 2C, the discharge capacity at each rate was determined, and the rate characteristics (100 × 2C discharge capacity / 0.2C discharge capacity) were determined. When the resistance value of the conductive layer formed on the positive electrode current collector is low, the movement of electrons at high rates is not inhibited, so a high maintenance rate is exhibited.

[0051] <Measurement of Battery Surface Temperature> Regarding the evaluation lithium-ion secondary battery fabricated in Example 1, using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Denko Corporation), after charging up to 4.2 [V] in a constant temperature bath at 25 [°C], a nail penetration test was performed. In the nail penetration test, in a constant temperature bath at 25 [°C], the evaluation lithium-ion secondary battery was fixed on a phenolic resin plate with a hole of 10 [mm] in diameter, and an iron nail with a diameter of 3 [mm] and a length of 65 [mm] was vertically pierced into the evaluation lithium-ion secondary battery at a speed of 10 [mm / s], penetrated 10 [mm] from the battery, held for 3 minutes, and then the nail was pulled out. The battery surface temperature 30 seconds after the nail was pierced into the battery was measured.

[0052] <Examples 2 to 11> Lithium-ion secondary batteries of Examples 2 to 11 were obtained in the same manner as in Example 1, except that the ratio of the conductive particles and the insulating resin contained in the conductive layer and the porosity of the second active material layer and the porosity of the first active material layer in the active material layer were changed to those shown in Table 1. Also, using the obtained lithium-ion secondary batteries, the rate characteristics and the measurement of the battery surface temperature of Examples 2 to 11 were carried out in the same manner as in Example 1.

[0053] The evaluation results of Examples 1 to 11 are shown in Table 1. By forming a conductive layer on the positive electrode current collector as in Examples 1 to 11 and making the porosity of the second active material layer in the active material layer larger than the porosity of the first active material layer, a low battery surface temperature was shown. Also, by setting the ratio C / D, which is the ratio of the porosity of the second active material layer to the porosity of the first active material layer, within a suitable range, a tendency to show an even lower battery surface temperature was confirmed. Further, by setting the ratio A / B, which is the ratio of the conductive particles to the insulating resin contained in the conductive layer, within a suitable range, a tendency to show a low battery surface temperature while maintaining high rate characteristics was confirmed.

[0054] <Comparative Examples 1 to 3> Lithium-ion secondary batteries of Comparative Examples 1 to 3 were obtained in the same manner as in Example 1, except that the presence or absence of the conductive layer, the ratio of the conductive particles to the insulating resin contained in the conductive layer, the porosity of the second active material layer in the active material layer, and the porosity of the first active material layer were changed to those shown in Table 1. Also, using the obtained lithium-ion secondary batteries, the rate characteristics and the battery surface temperature of Comparative Examples 1 to 3 were measured in the same manner as in Example 1.

[0055] The evaluation results of Comparative Examples 1 to 3 are shown in Table 1. In Comparative Example 1, the conductive layer was absent and a relatively high battery surface temperature was shown. Also, in Comparative Example 2, although the conductive layer was present with A / B in a suitable range, the porosity of the second active material layer was smaller than the porosity of the first active material layer, so a relatively high battery surface temperature was shown. Further, in Comparative Example 3, in addition to the absence of the conductive layer, the porosity of the second active material layer was smaller than the porosity of the first active material layer, so the highest battery surface temperature was shown.

[0056]

Table 1

Industrial Applicability

[0057] By providing a conductive layer on the current collector and forming the active material layer into two layers, namely a second active material layer with a large porosity and a first active material layer with a small porosity, a lithium ion secondary battery with suppressed heat generation effects can be provided.

Explanation of Signs

[0058] 1... positive electrode active material, 1a... first positive electrode active material layer, 1b... second positive electrode active material layer, 2... conductive layer provided on the positive electrode, 3... positive electrode current collector, 4... separator, 5... negative electrode active material, 5a... first negative electrode active material layer, 5b... second negative electrode active material layer, 6... conductive layer provided on the negative electrode, 7... negative electrode current collector, 8, 9... leads, 10... laminate of the lithium ion secondary battery.

Claims

1. A metal foil, a conductive layer formed on at least a part of the metal foil, and an active material layer formed on at least a part of a surface of the conductive layer opposite to the side of the metal foil, wherein the conductive layer contains conductive particles and an insulating resin, the active material layer contains a first active material layer and a second active material layer, the first active material layer and the second active material layer are laminated such that the first active material layer is on the side closer to the conductive layer, the second active material layer has a porosity larger than that of the first active material layer, and when the occupied area per unit area of the conductive particles when the conductive layer is viewed in the thickness direction is A and the occupied area per unit area of the insulating resin is B, A / B≤0.

5. An electrode for a lithium ion secondary battery characterized by this.

2. When the occupied area per unit area of the conductive particles when the conductive layer is viewed in the thickness direction is A and the occupied area per unit area of the insulating resin is B, 0.11≤A / B≤0.

5. The electrode for a lithium ion secondary battery according to Claim 1, characterized by this.

3. When the porosity of the second active material layer in the active material layer is C and the porosity of the first active material layer is D, 1.2≤C / D≤3.

5. The electrode for a lithium ion secondary battery according to any one of Claims 1 or 2, characterized by this.

4. A lithium ion secondary battery using the electrode for a lithium ion secondary battery according to any one of Claims 1 to 3.

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