阅读说明：本技术 二次电池用正极和二次电池 (Positive electrode for secondary battery and secondary battery ) 是由 古泽大辅 武泽秀治 盐崎朝树 于 2018-04-27 设计创作，主要内容包括：正极具备正极集电体、形成在正极集电体上且包含有机硅树脂和导电材料的保护层、以及形成在保护层上且包含由含锂的过渡金属氧化物构成的正极活性物质的正极复合材料层。(The positive electrode includes a positive electrode current collector, a protective layer formed on the positive electrode current collector and containing a silicone resin and a conductive material, and a positive electrode composite material layer formed on the protective layer and containing a positive electrode active material composed of a lithium-containing transition metal oxide.)

1. A positive electrode for a secondary battery, comprising:

a positive electrode current collector;

a protective layer formed on the positive electrode collector and containing a silicone resin and a conductive material; and

a positive electrode composite material layer formed on the protective layer and including a positive electrode active material composed of a lithium-containing transition metal oxide.

2. The positive electrode for a secondary battery according to claim 1, wherein the silicone resin is an organopolysiloxane represented by the following compositional formula (1),

R_xSiO_(4-x)/2(1)

in the formula (1), R independently represents a 1-valent hydrocarbon group, the 1-valent hydrocarbon group represented by R is optionally substituted with a halogen atom, and x is a number satisfying 0.1. ltoreq. x.ltoreq.2.

3. The positive electrode for a secondary battery according to claim 2, wherein R in the composition formula (1) represents a substituent selected from the group consisting of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, cyclopentyl, cyclohexyl, phenyl, tolyl, 2-phenylethyl, 2-phenylpropyl, 3-phenylpropyl, vinyl, allyl, chloromethyl, γ -chloropropyl, and 3,3, 3-trifluoropropyl.

4. The positive electrode for a secondary battery according to claim 2 or 3, wherein the organopolysiloxane represented by the composition formula (1) has at least a structural unit containing a silicon atom substituted with a phenyl group.

5. The positive electrode for a secondary battery according to claim 4, wherein in the organopolysiloxane represented by the composition formula (1), the proportion of the phenyl groups bonded to silicon atoms relative to the total amount of the 1-valent hydrocarbon groups R bonded to silicon atoms is 10 mol% or more and 80 mol% or less.

6. The positive electrode for a secondary battery according to any one of claims 1 to 5, wherein the silicone resin contains a hydroxyl group and a hydrolyzable functional group bonded to a silicon atom in a molecule, and a content of the hydroxyl group and the hydrolyzable functional group is 3% by mass or less with respect to a total amount of the silicone resin.

7. The positive electrode for a secondary battery according to any one of claims 1 to 6, wherein the thickness of the protective layer is 1 μm or more and 10 μm or less.

8. The positive electrode for a secondary battery according to any one of claims 1 to 7, wherein the protective layer does not contain inorganic compound particles, a content of the silicone resin is 75% by mass or more and 95% by mass or less with respect to a total amount of the protective layer, and a content of the conductive material is 5% by mass or more and 25% by mass or less with respect to the total amount of the protective layer.

9. The positive electrode for a secondary battery according to any one of claims 1 to 7, wherein the protective layer further contains inorganic compound particles.

10. The positive electrode for a secondary battery according to claim 9, wherein a content of the silicone resin is 15% by mass or more and 55% by mass or less, a content of the conductive material is 2% by mass or more and 20% by mass or less, and a content of the inorganic compound particles is 40% by mass or more and 75% by mass or less, with respect to a total amount of the protective layer.

11. A secondary battery comprising the positive electrode for a secondary battery according to any one of claims 1 to 10, a negative electrode, and an electrolyte.

Technical Field

The present disclosure relates to a positive electrode for a secondary battery and a secondary battery.

Background

A nonaqueous electrolyte secondary battery that performs charge and discharge by transferring lithium ions between a positive electrode and a negative electrode has a high energy density and a high capacity, and therefore, is widely used as a driving power source for mobile information terminals such as mobile phones, notebook computers, and smart phones, or as a power source for power of electric tools, Electric Vehicles (EV), hybrid electric vehicles (HEV and PHEV), and the like.

Patent document 1 discloses an electrode plate for a nonaqueous electrolyte secondary battery in which a primer layer and an electrode active material layer are sequentially laminated on a current collector, wherein the electrode active material layer contains a binder containing electrode active material particles and a metal oxide, and the primer layer contains a silicon element and an oxygen element at a specific ratio. Patent document 1 describes that the presence of a predetermined undercoat layer between the current collector and the electrode active material layer can prevent the electrode active material layer from peeling off and falling off from the current collector, and can provide stable use over a long period of time.

Disclosure of Invention

A positive electrode for a secondary battery is required, which can suppress a temperature rise caused when an abnormality such as an internal short circuit occurs while maintaining a good current collecting property, and can improve the safety of the secondary battery.

A positive electrode for a secondary battery according to one embodiment of the present disclosure includes: a positive electrode current collector; a protective layer formed on the positive electrode collector and containing a silicone resin and a conductive material; and a positive electrode composite material layer formed on the protective layer and containing a positive electrode active material composed of a lithium-containing transition metal oxide.

According to the positive electrode for a secondary battery as one embodiment of the present disclosure, a secondary battery with improved safety can be provided in which a temperature rise caused when an abnormality such as an internal short circuit occurs can be suppressed while maintaining good current collection performance.

Drawings

Fig. 1 is a schematic longitudinal sectional view showing a secondary battery according to an embodiment.

Detailed Description

Patent document 1 discloses the following technique: an undercoat layer containing silicon and oxygen at a specific ratio is provided between the current collector and the electrode active material layer, and more specifically, the undercoat layer is provided by heating a coating film of a coating liquid obtained by dissolving/hydrolyzing a so-called silane coupling agent. However, since a resin obtained by hydrolysis of a silane coupling agent generally has electrical insulation properties, when the undercoat layer is provided on an electrode plate, there is a concern that the current collecting properties may be reduced.

A positive electrode for a secondary battery (hereinafter also referred to as "positive electrode") as one embodiment of the present disclosure includes: a positive electrode current collector; a protective layer formed on the positive electrode collector and containing a silicone resin and a conductive material; and a positive electrode composite material layer formed on the protective layer and containing a positive electrode active material composed of a lithium-containing transition metal oxide.

The inventors of the present invention found that: by providing the protective layer between the positive electrode current collector and the positive electrode composite material layer, it is possible to suppress a temperature rise caused when an abnormality such as an internal short circuit between the positive electrode current collector and the positive electrode composite material layer occurs while maintaining a good current collecting property of the positive electrode, and to improve the safety of a secondary battery (hereinafter also referred to as a "battery"). Further, since the positive electrode having the protective layer containing a silicone resin has excellent flexibility, stress applied to the positive electrode during winding or the like is relaxed, and thus the protective layer and the positive electrode composite material layer are less likely to be broken, and the yield in the battery production process can be reduced. Further, since the weight of the protective layer containing a silicone resin is reduced as compared with a protective layer containing inorganic compound particles as a main component, the total weight of the battery can be reduced while maintaining the function of suppressing temperature rise in the event of an abnormality.

Hereinafter, an example of the embodiment of the present disclosure will be described in detail with reference to the drawings. The drawings referred to in the description of the embodiments are schematic representations, and the dimensional ratios and the like of the components depicted in the drawings may be different from those of the actual components. Specific dimensional ratios and the like should be determined with reference to the following description.

[ Secondary Battery ]

The structure of the battery 10 will be described with reference to fig. 1. Fig. 1 is a sectional view of a battery 10 as an example of the embodiment. The battery 10 includes a positive electrode 30, a negative electrode 40, and an electrolyte. A separator 50 is suitably provided between the cathode 30 and the anode 40. The battery 10 has a structure in which, for example, a wound electrode body 12 in which a cathode 30 and an anode 40 are wound with a separator 50 interposed therebetween and an electrolyte are housed in a battery case. Examples of the battery case for housing the electrode body 12 and the electrolyte include a metal case having a cylindrical shape, a rectangular shape, a coin shape, a button shape, and the like; a resin case (laminate type battery) formed by laminating resin sheets. Instead of the wound electrode assembly 12, an electrode assembly of another form such as a laminated electrode assembly in which positive electrodes and negative electrodes are alternately laminated with separators interposed therebetween may be used. In the example shown in fig. 1, a battery case is constituted by a case main body 15 having a bottomed cylindrical shape and a sealing member 16.

The battery 10 includes insulating plates 17 and 18 disposed above and below the electrode body 12. In the example shown in fig. 1, the positive electrode lead 19 attached to the positive electrode 30 extends to the sealing member 16 side through the through hole of the insulating plate 17, and the negative electrode lead 20 attached to the negative electrode 40 extends to the bottom side of the case main body 15 through the outside of the insulating plate 18. For example, the positive electrode lead 19 is connected to the lower surface of the filter 22 as the bottom plate of the sealing body 16 by welding or the like, and the lid 26 as the top plate of the sealing body 16 electrically connected to the filter 22 serves as a positive electrode terminal. The negative electrode lead 20 is connected to the bottom inner surface of the case main body 15 by welding or the like, and the case main body 15 serves as a negative electrode terminal. In the present embodiment, sealing body 16 is provided with a current interrupt mechanism (CID) and an air release mechanism (safety valve). An exhaust valve (not shown) is preferably provided also at the bottom of the housing main body 15.

The housing main body 15 is a metal container having a bottomed cylindrical shape, for example. A gasket 27 is provided between the case main body 15 and the sealing body 16 to ensure the sealing property inside the battery case. The casing main body 15 preferably has a bulging portion 21 formed by pressing a side surface portion from the outside, for example, for supporting the sealing body 16. The bulging portion 21 is preferably formed annularly along the circumferential direction of the case main body 15, and supports the sealing body 16 on the upper surface thereof.

The sealing member 16 includes: a filter 22 having a filter opening 22a formed therein, and a valve body disposed on the filter 22. The valve body closes the filter opening 22a of the filter 22, and breaks when the internal pressure of the battery 10 rises due to heat generation caused by an internal short circuit or the like. In the present embodiment, the lower valve body 23 and the upper valve body 25 are provided as the valve bodies, and the insulating member 24 and the cover 26 having the cover opening 26a are further provided between the lower valve body 23 and the upper valve body 25. Each member constituting sealing body 16 has, for example, a disk shape or a ring shape, and members other than insulating member 24 are electrically connected to each other. Specifically, the filter 22 and the lower valve body 23 are engaged with each other at respective edge portions, and the upper valve body 25 and the cover 26 are also engaged with each other at respective edge portions. The lower valve body 23 and the upper valve body 25 are connected to each other at their central portions, and an insulating member 24 is interposed between the edge portions. When the internal pressure rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 23 is broken at the thin portion, and the upper valve body 25 expands toward the lid 26 and separates from the lower valve body 23, thereby cutting off the electrical connection between the two.

[ Positive electrode ]

The positive electrode 30 includes: the positive electrode includes a positive electrode current collector, a protective layer formed on the positive electrode current collector, and a positive electrode composite material layer formed on the protective layer.

The positive electrode current collector contains aluminum, and is composed of, for example, a metal foil containing aluminum simple substance or an aluminum alloy. The aluminum content in the positive electrode current collector is 50 mass% or more, preferably 70 mass% or more, and more preferably 80 mass% or more, with respect to the total amount of the positive electrode current collector. The thickness of the positive electrode current collector is not particularly limited, and is, for example, about 10 μm to 100 μm.

The positive electrode composite material layer contains a positive electrode active material composed of a lithium transition metal oxide. Examples of the lithium transition metal oxide include lithium transition metal oxides containing lithium (Li) and transition metal elements such as cobalt (Co), manganese (Mn), and nickel (Ni). The lithium transition metal oxide may contain additional elements other than Co, Mn, and Ni, and examples thereof include aluminum (Al), zirconium (Zr), boron (B), magnesium (Mg), scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), lead (Pb), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), and silicon (Si).

Specific examples of the lithium transition metal oxide include, for example, Li_xCoO₂、Li_xNiO₂、Li_xMnO₂、Li_xCo_yNi_1-yO₂、Li_xCo_yM_1-yO_z、Li_xNi_1-yM_yO_z、Li_xMn₂O₄、Li_xMn_2-yM_yO₄、LiMPO₄、Li₂MPO₄F (in each chemical formula, M is at least 1 of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B, 0<x≤1.2、0<y is less than or equal to 0.9, and z is less than or equal to 2.0 and less than or equal to 2.3). These may be used alone in 1 kind or in combination of two or more kinds.

The positive electrode composite layer suitably further includes a conductive material and a binder material. The conductive material contained in the positive electrode composite material layer is used to improve the conductivity of the positive electrode composite material layer. Examples of the conductive material include carbon materials such as Carbon Black (CB), Acetylene Black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more.

The binder contained in the positive electrode composite material layer is used for maintaining a good contact state between the positive electrode active material and the conductive material and improving the adhesion of the positive electrode active material or the like to the surface of the positive electrode collector. Examples of the binder include fluorine-based resins such as Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), Polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. In addition, these resins may be used in combination with a carboxymethyl celluloseVitamin (CMC) or its salt (CMC-Na, CMC-K, CMC-NH)₄And the like, or partially neutralized salts), polyethylene oxide (PEO), and the like. These may be used alone or in combination of two or more.

The positive electrode 30 includes a protective layer formed on a positive electrode current collector, and a positive electrode composite material layer is formed on the protective layer. The protective layer contains at least a silicone resin and a conductive material. The silicone resin contained in the protective layer has a main chain composed of Si — O bonds having a very large bond energy, and therefore has excellent heat resistance. Further, silicon dioxide (SiO) is generated by thermal decomposition of the silicone resin₂) Therefore, the protective layer according to the present embodiment also functions as a separator for separating the positive electrode current collector from the positive electrode composite material layer after thermal decomposition of the silicone resin due to internal short-circuiting or the like. By providing such a protective layer between the positive electrode current collector and the positive electrode composite material layer, even when an abnormality such as an internal short circuit occurs, the positive electrode current collector and the positive electrode composite material layer can be isolated from each other, and a redox reaction between aluminum contained in the positive electrode current collector and a lithium transition metal oxide contained as a positive electrode active material in the positive electrode composite material layer can be suppressed, thereby suppressing a temperature increase of the battery 10.

The silicone resin contained in the protective layer is an organopolysiloxane having a three-dimensional network structure, represented by the following compositional formula (1).

R_xSiO_(4-x)/2(1)

(wherein R independently represents a 1-valent hydrocarbon group, the 1-valent hydrocarbon group represented by R is optionally substituted with a halogen atom, and x is a number satisfying 0.1. ltoreq. x.ltoreq.2). X in the composition formula (1) represents: the degree of substitution per 1 silicon atom, i.e., per 1 hydrocarbon group having a valence of 1 represented by R in the structural unit constituting the organopolysiloxane. x is preferably a number satisfying 0.8. ltoreq. x.ltoreq.1.9, more preferably a number satisfying 1.2. ltoreq. x.ltoreq.1.8.

Examples of the structural unit constituting the organopolysiloxane represented by the above composition formula (1) include R₃SiO_1/2M units, R shown₂SiO_2/2D cell, RSiO shown_3/2T units and SiO_4/2The Q unit shown. Group formula (I)(1) X in (b) can be determined from the presence ratio of these structural units constituting the organopolysiloxane. The silicone resin has a T unit and/or a Q unit as a structural unit, thereby forming a three-dimensional network structure having a branched structure.

The 1-valent hydrocarbon group (hereinafter also referred to as "hydrocarbon group R") optionally substituted with a halogen atom, represented by R, has, for example, 1 or more and 10 or less carbon atoms, preferably 1 or more and 6 or less carbon atoms. The halogen atom of the optionally substituted hydrocarbon group R is, for example, a fluorine atom, a chlorine atom or the like. Specific examples of the hydrocarbon group R include alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, and octyl; cycloalkyl groups such as cyclopentyl and cyclohexyl; aryl groups such as phenyl and tolyl; aralkyl groups such as 2-phenylethyl, 2-phenylpropyl and 3-phenylpropyl; alkenyl groups such as vinyl and allyl; and halogen-substituted hydrocarbon groups such as chloromethyl, γ -chloropropyl, and 3,3, 3-trifluoropropyl, but are not limited thereto. The hydrocarbyl group R is preferably an alkyl group having 1 to 4 carbon atoms or a phenyl group, and particularly preferably a methyl group or a phenyl group, from the viewpoint of easy synthesis or acquisition.

From the viewpoint of improving heat resistance, the silicone resin preferably has at least a structural unit containing a silicon atom substituted with a phenyl group. For example, the silicone resin is an organopolysiloxane represented by the above compositional formula (1), and the ratio of the phenyl groups bonded to the silicon atoms to the total amount of the 1-valent hydrocarbon groups R bonded to the silicon atoms is preferably 10 mol% or more and 80 mol% or less, and more preferably 20 mol% or more and 60 mol% or less. In the silicone resin, when the ratio of the phenyl groups bonded to the silicon atoms to the total amount of the hydrocarbon groups R is in the above range, the heat resistance of the protective layer is further improved.

The silicone resin preferably contains a hydroxyl group (silanol group) bonded to a silicon atom in the molecule. As described later, when the coating film containing the silicone resin and the conductive material is heated to form the protective layer, the silanol group contained in the silicone resin is subjected to dehydration condensation with another silanol group or a hydroxyl group on the surface of the current collector. In addition, the hydrolyzable functional group bonded to a silicon atom in the silicone resin also has a function equivalent to a silanol group. The hydrolyzable functional group is not limited as long as it is a substituent which undergoes dehydration condensation with a silanol group or the like by heating, and examples thereof include alkoxy groups such as methoxy group and ethoxy group, acetoxy group, and amino group. The content ratio of the hydroxyl group and the hydrolyzable functional group bonded to the silicon atom in the silicone resin is, for example, preferably 3 mass% or less, more preferably 0.1 mass% or more and 2 mass% or less, with respect to the total amount of the silicone resin. The proportion of the structural unit containing a silanol group or a hydrolyzable functional group to the total structural unit constituting the silicone resin is preferably about 20 mol% or less, and more preferably 1 mol% or more and 10 mol% or less.

The silicone resin preferably has a polystyrene-equivalent weight average molecular weight in a Gel Permeation Chromatography (GPC) range of 1000 to 5000000, more preferably 4000 to 3000000.

Such a silicone resin can be produced by a conventionally known method. For example, the target product can be obtained by co-hydrolyzing the corresponding organochlorosilanes in the presence of an alcohol having 1 to 4 carbon atoms depending on the proportion of the structural unit contained in the structure of the target silicone resin, and removing hydrochloric acid and low boiling point components by-produced. Further, alkoxysilanes, silicone oils, and cyclic siloxanes may be used as starting materials, and in this case, the target silicone resin can be obtained by using an acid catalyst such as hydrochloric acid, sulfuric acid, or methanesulfonic acid, adding water for hydrolysis as the case may be, to advance the polymerization reaction, and then removing the acid catalyst and low-boiling components used in the same manner.

Specific examples of the starting material for synthesizing the silicone resin include chlorosilanes such as methyltrichlorosilane, ethyltrichlorosilane, phenyltrichlorosilane, dimethyldichlorosilane, and diphenyltrichlorosilane; alkoxysilanes such as methoxysilanes corresponding to the individual chlorosilanes, but are not limited thereto. The silicone resin may be used alone, or two or more types of silicone resins having different ratios of the hydrocarbon group and the silanol group substituted on the silicon atom may be used in combination.

As the silicone resin contained in the protective layer, an organic resin-modified silicone resin may be used, and for example, an epoxy resin-modified silicone resin, an alkyd resin-modified silicone resin, a polyester resin-modified silicone resin, or the like may be used. However, from the viewpoint of heat resistance, the silicone resin contained in the protective layer is preferably a so-called linear silicone resin substantially composed of the organopolysiloxane represented by the above compositional formula (1). The silicone resin is preferably, for example, the following organopolysiloxane: the 1-valent hydrocarbon group represented by the above composition formula (1) is selected from the group consisting of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, cyclopentyl, cyclohexyl, phenyl, tolyl, 2-phenylethyl, 2-phenylpropyl, 3-phenylpropyl, vinyl, allyl, chloromethyl, γ -chloropropyl, and 3,3, 3-trifluoropropyl groups, and more preferably from the group consisting of methyl and phenyl groups, x is a number satisfying 1.2. ltoreq. x.ltoreq.1.8, the content ratio of the hydroxyl group bonded to the silicon atom and the hydrolyzable functional group is 3 mass% or less, more preferably 0.1 mass% or more and 2 mass% or less, relative to the total amount of the silicone resin, and the polystyrene-equivalent weight average molecular weight based on GPC falls within the range of 4000 to 3000000.

The content of the silicone resin contained in the protective layer may be, for example, 10 mass% or more and 99.9 mass% or less, and preferably 15 mass% or more and 99 mass% or less, with respect to the total amount of the protective layer. In the case where the protective layer does not contain inorganic compound particles (hereinafter, also referred to as "inorganic particles"), the content of the silicone resin is, for example, preferably 60 mass% or more and 99 mass% or less, and more preferably 75 mass% or more and 95 mass% or less, with respect to the total amount of the protective layer. In the case where the protective layer contains inorganic particles, the content of the silicone resin is, for example, preferably 10 mass% or more and 60 mass% or less, and more preferably 15 mass% or more and 55 mass% or less, with respect to the total amount of the protective layer.

The content of the silicone resin may be, for example, 0.01 mass% or more and 3.0 mass% or less, and preferably 0.02 mass% or more and 2.0 mass% or less, with respect to the total amount of the positive electrode. When the protective layer does not contain inorganic particles, the content of the silicone resin is, for example, preferably 0.05 mass% or more and 2.0 mass% or less, and more preferably 0.09 mass% or more and 1.52 mass% or less, with respect to the total amount of the positive electrode. When the protective layer contains inorganic particles, the content of the silicone resin is, for example, preferably 0.02 mass% or more and 1.5 mass% or less, and more preferably 0.04 mass% or more and 1.21 mass% or less, with respect to the total amount of the positive electrode.

The protective layer contains both silicone resin and a conductive material. By including the conductive material in the protective layer provided between the positive electrode current collector and the positive electrode composite material layer, good current collection performance of the positive electrode 30 is ensured. The conductive material may be, for example, the same as the type of conductive material used in the positive electrode composite material layer, and specific examples thereof include carbon materials such as Carbon Black (CB), Acetylene Black (AB), ketjen black, and graphite, but are not limited thereto. These may be used alone or in combination of two or more.

Further, the present inventors found that: by using the carbon material as the conductive material in the positive electrode 30 according to the present embodiment, the effect of suppressing the temperature rise at the time of occurrence of an abnormality is further improved as compared with the case of not containing a conductive material. The reason why the effect of suppressing the temperature rise is improved by the conductive material containing the silicone resin and the carbon material is not yet established, and it is presumed that the effect is as follows. It can be considered that: for example, the radicals on the surface of the carbon material supplement active species generated by thermal decomposition of the binder, the electrolyte solution, and the like during abnormal heat release, and can suppress temperature rise. Further, it can be considered that: a compound having an Si — C bond is generated between the organic silicon resin thermally decomposed by the temperature increase and the carbon material to form an oxygen barrier layer, whereby the oxidation reaction of aluminum of the positive electrode current collector can be suppressed. The conductive material is preferably the above carbon material, and more preferably an amorphous substance containing a large amount of radical species, such as acetylene black or ketjen black.

The content of the conductive material contained in the protective layer may be, for example, 1 mass% or more and 40 mass% or less, and preferably 2 mass% or more and 25 mass% or less, with respect to the total amount of the protective layer. In the case where the protective layer does not contain inorganic particles, the content of the conductive material is, for example, preferably 1 mass% or more and 40 mass% or less, and more preferably 5 mass% or more and 25 mass% or less, with respect to the total amount of the protective layer. In the case where the protective layer contains inorganic particles, the content of the conductive material is, for example, preferably 1 mass% or more and 30 mass% or less, more preferably 2 mass% or more and 20 mass% or less, with respect to the total amount of the protective layer. From the viewpoint of ensuring the current collection property, the content of the conductive material in the protective layer is preferably higher than the content of the conductive material in the positive electrode composite material layer.

The content of the conductive material may be, for example, 1 mass% or more and 40 mass% or less, and preferably 2 mass% or more and 25 mass% or less, with respect to the total amount of the positive electrode. In the case where the protective layer does not contain inorganic particles, the content of the conductive material is, for example, preferably 0.01 mass% or more and 0.6 mass% or less, and more preferably 0.01 mass% or more and 0.31 mass% or less, with respect to the total amount of the positive electrode. In the case where the protective layer contains inorganic particles, the content of the conductive material is, for example, preferably 0.01 mass% or more and 0.5 mass% or less, and more preferably 0.01 mass% or more and 0.28 mass% or less, with respect to the total amount of the positive electrode.

The protective layer may contain inorganic particles. As a positive electrode for a nonaqueous electrolyte secondary battery having a protective layer containing inorganic particles, japanese patent application laid-open No. 2016-127000 discloses a positive electrode for a nonaqueous electrolyte secondary battery having a protective layer containing an inorganic compound having a thickness of 1 to 5 μm and a lower oxidizing power than a lithium transition metal oxide and a conductive material between a positive electrode current collector containing aluminum as a main component and a positive electrode composite material layer containing a lithium transition metal oxide. The inorganic particles contained in the protective layer have an effect of suppressing a temperature rise in the case of abnormality of the battery 10, similarly to the silicone resin, but the protective layer containing the inorganic particles as a main component has high rigidity. In contrast to the positive electrode having the protective layer containing inorganic particles as a main component as disclosed in japanese patent application laid-open No. 2016-127000, the positive electrode 30 according to the present embodiment contains a silicone resin instead of a part or all of the inorganic particles, and thus the stress applied to the positive electrode 30 when the electrode body 12 is wound or the like is relaxed, and therefore, the protective layer and the positive electrode composite material layer formed on the current collector are less likely to crack, and the yield in the production process of the battery 10 can be reduced. Since the silicone resin has a smaller density and a lighter weight than the inorganic particles, the use of the silicone resin instead of part or all of the inorganic particles can reduce the weight of the protective layer and thus the total weight of the battery 10 while maintaining the function of suppressing the temperature rise during the occurrence of an abnormality. Further, while it is desirable to use a binder for the protective layer mainly composed of inorganic particles in order to ensure mechanical strength and adhesion to the current collector or the composite material layer, the positive electrode 30 according to the present embodiment can ensure mechanical strength of the protective layer and adhesion to the current collector or the composite material layer by using a silicone resin without using a binder.

The inorganic compound constituting the inorganic particles is not particularly limited, and from the viewpoint of suppressing the redox reaction, it is preferable that the oxidizing power is lower than that of the lithium transition metal oxide contained in the positive electrode composite material layer. Examples of such inorganic compounds include inorganic oxides such as manganese oxide, silica, titania and alumina, and alumina (Al) is preferable because of its excellent high thermal conductivity₂O₃). The inorganic particles may have, for example, a center particle diameter (volume average particle diameter measured by a light scattering method) of 1 μm or less, preferably 0.2 μm or more and 0.9 μm or less.

The content of the inorganic particles contained in the protective layer may be, for example, 20 mass% or more and 85 mass% or less, preferably 40 mass% or more and 75 mass% or less, and more preferably 55 mass% or more and 70 mass% or less, with respect to the total amount of the protective layer. The inorganic particle content may be, for example, 0.01 mass% or more and 8 mass% or less, preferably 0.03 mass% or more and 5 mass% or less, and more preferably 0.06 mass% or more and 2.7 mass% or less, relative to the total amount of the positive electrode.

In the present embodiment, a binder may be used for the protective layer for the purpose of securing the mechanical strength of the protective layer, improving the bondability of the protective layer to the positive electrode current collector or the bondability of the protective layer to the positive electrode composite material layer, but the binder may not be contained. When a binder is used, for example, the same binder as the type of the binder used in the positive electrode composite material layer can be used, and specific examples thereof include fluorine-based resins such as PTFE and PVdF; PAN, polyimide-based resin, acrylic resin, polyolefin-based resin, and the like, but are not limited thereto. These may be used alone or in combination of two or more. When the binder is used, the protective layer may contain the binder in an amount of 0.1 mass% or more and 20 mass% or less with respect to the total amount of the protective layer, and preferably does not contain the binder.

In the case where the protective layer does not contain inorganic particles and is substantially composed of only the silicone resin and the conductive material, the content ratio (mass ratio) of the silicone resin to the conductive material is preferably 60: 40-99: 1. more preferably 75: 25-95: 5. in the present specification, "substantially consisting of only" means that the content of elements other than the constituent elements is a trace amount, and is, for example, 0.1 mass% or less.

In the case where the protective layer is substantially composed of only the silicone resin, the conductive material, and the inorganic particles, the content ratio (mass ratio) of the total amount of the silicone resin and the conductive material to the inorganic particles is preferably 60: 40-25: 75. more preferably 45: 55-30: 70. further, in the case where the protective layer is substantially composed of only the silicone resin, the conductive material, and the inorganic particles, the content ratio (mass ratio) of the total amount of the silicone resin and the inorganic particles to the conductive material is preferably 99: 1-70: 30. more preferably 98: 2-80: 20. alternatively, in the case where the protective layer is substantially composed of only the silicone resin, the conductive material, and the inorganic particles, it is preferable that: the content of the silicone resin is 15 mass% or more and 55 mass% or less, the content of the inorganic particles is 40 mass% or more and 75 mass% or less, the content of the conductive material is 2 mass% or more and 20 mass% or less, and the silicone resin, the conductive material, and the inorganic particles are included in an amount of 100 mass% in total, with respect to the total amount of the protective layer.

The thickness of the protective layer is, for example, 1 μm or more and 10 μm or less, preferably 1 μm or more and 5 μm or less. If the protective layer is too thin, the effect of suppressing the temperature rise at the time of occurrence of an abnormality may be reduced, and if the protective layer is too thick, the energy density of the positive electrode 30 may be reduced.

Examples of the method for analyzing the components contained in the protective layer include the following methods.

(1) The battery 10 is disassembled and the electrode body 12 is taken out, and further separated into the positive electrode 30, the negative electrode 40, and the separator 50.

(2) A sample including the positive electrode current collector, the protective layer, and the positive electrode composite material layer was obtained by cutting out a predetermined range from the positive electrode 30 obtained in (1).

(3) The binder is dissolved by using an organic solvent that dissolves the binder contained in the positive electrode composite layer and does not dissolve the silicone resin, and the positive electrode composite layer is removed from the positive electrode 30.

(4) The protective layer is cut from the sample obtained in (3) using a cutting tool or the like.

(5) The constituent components of the protective layer containing the silicone resin, the conductive material, and the like obtained in (4) are qualitatively and quantitatively determined using a known analysis device such as a Nuclear Magnetic Resonance (NMR) device or a fourier transform infrared spectrophotometer (FT-IR). The structure of the monomer unit constituting the silicone resin can be analyzed by subjecting the silicone resin to a pretreatment for cleaving siloxane bonds of the silicone resin with Tetraethoxysilane (TEOS) under an alkali condition, and measuring the resulting ethoxylate with a gas chromatography-mass spectrometer (GC-MS). The molecular weight of the silicone resin can be measured as a weight average molecular weight in terms of polystyrene, for example, by using a Gel Permeation Chromatography (GPC) apparatus.

The organic solvent used in the above (3) is known, and for example, when a fluorine resin such as PVdF is used as a binder contained in the positive electrode composite layer, acetonitrile is used as the organic solvent, whereby only the positive electrode composite layer can be removed from the positive electrode 30. Alternatively, instead of the step (3), the thicknesses of the positive electrode composite material layer and the protective layer may be measured in advance, and only the positive electrode composite material layer may be cut out using a cutting tool or the like based on the measured thicknesses. The thicknesses of the positive electrode composite material layer and the protective layer can be measured by, for example, subjecting the positive electrode 30 obtained in (1) above to cross-sectional processing by the cross-sectional polishing (CP) method, observing the ground surface by a Scanning Electron Microscope (SEM), and performing image processing on the obtained SEM image.

An example of the method for manufacturing the positive electrode 30 according to the present embodiment is shown. First, a solution is prepared by adding a silicone resin to an organic solvent in which the silicone resin is soluble, and a dispersion is prepared by adding an electrically conductive material and, if necessary, additives such as inorganic particles to the solution. The obtained dispersion liquid is applied to the surface of the positive electrode current collector, and the coating layer is dried, whereby the protective layer can be formed on the positive electrode current collector. When the positive electrode composite material layer is provided on both surfaces of the positive electrode current collector, the protective layer is also provided on both surfaces of the positive electrode current collector.

The organic solvent used for preparing the dispersion is not particularly limited as long as it can dissolve or disperse the silicone resin, and examples thereof include saturated aliphatic hydrocarbons such as n-pentane and hexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as benzene, toluene, xylene, and mesitylene; cyclic ethers such as Tetrahydrofuran (THF) and dioxane; ketones such as methyl isobutyl ketone (MIBK); halogenated alkanes such as trichloroethane; a halogenated aromatic hydrocarbon such as chlorobenzene, and a mixture of two or more of them may be used.

Next, a positive electrode active material, a conductive material, a binder, and a dispersion medium such as N-methyl-2-pyrrolidone (NMP) are mixed to prepare a positive electrode composite slurry. The obtained positive electrode composite material slurry was applied to the surface of the protective layer formed on the positive electrode current collector. After the coating layer is dried, the coating layer is rolled by rolling means such as a rolling roll to form a positive electrode composite material layer on the protective layer, whereby the positive electrode 30 according to the present embodiment can be produced. By rolling with the rolling means, the positive electrode active material particles on the surface of the positive electrode composite material layer on the side of the protective film are trapped in the protective layer, and irregularities are formed at the interface between the protective layer and the positive electrode composite material layer. The positive electrode composite material layer and the protective layer can be bonded to each other with an anchor effect due to the formed irregularities. The method for applying the dispersion liquid of the protective layer or the positive electrode composite material slurry is not particularly limited, and may be performed using a known application device such as a gravure coater, a slit coater, or a die coater.

[ negative electrode ]

The negative electrode 40 is composed of a negative electrode current collector made of, for example, a metal foil, and a negative electrode composite material layer formed on the surface of the current collector. As the negative electrode current collector, a foil of a metal such as copper that is stable in the potential range of the negative electrode, a film in which the metal is disposed on the surface layer, or the like can be used. In the negative electrode composite layer, it is preferable that a binder is contained in addition to the negative electrode active material. The negative electrode 40 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like to a negative electrode current collector, drying the coating layer, and then rolling the coating layer to form negative electrode mixture layers on both surfaces of the current collector.

The negative electrode active material is not particularly limited as long as it can reversibly store and release lithium ions, and for example, carbon materials such as natural graphite and artificial graphite; metals such as silicon (Si) and tin (Sn) that are alloyed with lithium; or an alloy or a composite oxide containing a metal element such as Si or Sn. The negative electrode active material may be used alone, or two or more of them may be used in combination.

As the binder contained in the negative electrode mixture layer, a fluorine-based resin, PAN, a polyimide-based resin, an acrylic resin, a polyolefin-based resin, or the like can be used as in the case of the positive electrode 30. When the negative electrode composite slurry is prepared using an aqueous solvent, it is preferable to use styrene-butadiene rubber (SBR), CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof (which may be PAA-Na, PAA-K, or the like or a partially neutralized salt), polyvinyl alcohol (PVA), or the like.

[ separator ]

A porous sheet having ion permeability and insulation properties may be used as the separator 50. Specific examples of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. As a material of the separator 50, olefin resin such as polyethylene and polypropylene, cellulose, and the like are suitable. The separator 50 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Further, a multilayer separator including a polyethylene layer and a polypropylene layer may be used, or a separator in which an aramid resin or the like is coated on the surface of the separator 50 may be used.

A filler layer containing an inorganic filler may be formed at an interface of the separator 50 and at least one of the cathode 30 and the anode 40. Examples of the inorganic filler include oxides and phosphoric acid compounds containing at least 1 of titanium (Ti), aluminum (Al), silicon (Si), and magnesium (Mg). The filler layer can be formed by, for example, applying a slurry containing the filler to the surface of the positive electrode 30, the negative electrode 40, or the separator 50.

[ electrolyte ]

The electrolyte includes a solvent and an electrolyte salt dissolved in the solvent. As the electrolyte, a solid electrolyte using a gel polymer or the like may be used, but from the viewpoint of ease of filling into the voids of the protective layer and suppression of temperature rise at the time of occurrence of an abnormality, the electrolyte is preferably a liquid electrolyte. Examples of the solvent include a nonaqueous solvent such as an ester, an ether, a nitrile such as acetonitrile, an amide such as dimethylformamide, and a mixed solvent of two or more of these solvents, and water. The nonaqueous solvent may contain a halogen-substituted compound obtained by substituting at least a part of the hydrogen atoms of these solvents with a halogen atom such as fluorine.

Examples of the esters include cyclic carbonates such as Ethylene Carbonate (EC), Propylene Carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ -butyrolactone and γ -valerolactone; and chain carboxylates such as methyl acetate, ethyl acetate, propyl acetate, Methyl Propionate (MP), and ethyl propionate.

Examples of the ethers include cyclic ethers such as 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1, 2-butylene oxide, 1, 3-dioxane, 1, 4-dioxane, 1,3, 5-trioxane, furan, 2-methylfuran, 1, 8-cineole and crown ethers; chain ethers such as 1, 2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzylethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1, 2-diethoxyethane, 1, 2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1-dimethoxymethane, 1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

As the halogen substituent, a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC); and fluorinated chain carboxylic acid esters such as fluorinated chain carbonates and Fluorinated Methyl Propionate (FMP).

The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF₄、LiClO₄、LiPF₆、LiAsF₆、LiSbF₆、LiAlCl₄、LiSCN、LiCF₃SO₃、LiCF₃CO₂、Li(P(C₂O₄)F₄)、LiPF_6-x(C_nF_2n+1)_x(1<x<6. n is 1 or 2), LiB₁₀Cl₁₀LiCl, LiBr, LiI, chloroborane lithium, lower aliphatic carboxylic acid lithium, Li₂B₄O₇、Li(B(C₂O₄)F₂) And salts of boric acid; LiN (SO)₂CF₃)₂、LiN(C₁F_2l+1SO₂)(C_mF_2m+1SO₂) And { l and m are integers of 1 or more }, and the like. The lithium salt may be used alone or in combination of two or more. Among these, LiPF is preferably used from the viewpoint of ion conductivity, electrochemical stability, and the like₆. The concentration of the lithium salt is preferably 0.8 to 1.8mol based on 1L of the solvent.