Negative electrode for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery, and method for manufacturing negative electrode for non-aqueous electrolyte secondary battery

Abstract

An object of the present application is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can suppress a decrease in high-rate charge-discharge cycle characteristics. The negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present application includes: a negative electrode current collector, a first negative electrode mixture layer provided on the surface of the negative electrode current collector, and a second negative electrode mixture layer provided on the surface of the first negative electrode mixture layer. The first negative electrode mixture layer contains a first negative electrode active material and a first water-soluble polymer material, and the second negative electrode mixture layer contains a second negative electrode active material and a second water-soluble polymer material. The ratio (S1 / V1) of the amount (S1) of the first water-soluble polymer material present on the surface of the first negative electrode active material to the amount (V1) of the first water-soluble polymer material present in the interparticle voids of the first negative electrode active material is larger than the ratio (S2 / V2) of the amount (S2) of the second water-soluble polymer material present on the surface of the second negative electrode active material to the amount (V2) of the second water-soluble polymer material present in the interparticle voids of the second negative electrode active material.

Description

Technical Field

[0001] The present application relates to a negative electrode for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery. Background Art

[0002] The negative electrode constituting a non-aqueous electrolyte secondary battery generally has a negative electrode current collector and a negative electrode mixture layer formed on both surfaces of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material and a binder, and the binder maintains the structure of the negative electrode mixture layer by bonding the particles of the negative electrode active material to each other and the negative electrode active material to the negative electrode current collector.

[0003] Patent Documents 1 and 2 disclose a method in which a slurry is prepared by dry-mixing a negative electrode active material and a binder, so that the binder adheres to the surface of the negative electrode active material. Thus, Patent Document 1 describes that the cycle characteristics are improved because the adhesion force between the negative electrode active materials and between the negative electrode active material layer and the negative electrode current collector can be increased, and Patent Document 2 describes that the charge-discharge efficiency is improved by allowing the binder to hold an electrolytic solution.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-42787

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2005-166446 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] However, when a non-aqueous electrolyte secondary battery is used as a power source for an electric vehicle (EV) or the like, in most cases, charging and discharging are performed at a high rate. Therefore, it is required to suppress a decrease in the high-rate charge-discharge cycle characteristics. However, in a negative electrode mixture layer containing a negative electrode active material having a large amount of binder attached to the surface, the permeability of the electrolytic solution is poor, and thus the high-rate charge-discharge cycle characteristics sometimes decrease.

[0010] Therefore, an object of the present application is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can suppress a decrease in the high-rate charge-discharge cycle characteristics.

[0011] Technical Solution for Solving the Problems

[0012] The negative electrode for a non-aqueous electrolyte secondary battery according to an aspect of the present application is characterized by including: a negative electrode current collector; a first negative electrode mixture layer provided on the surface of the negative electrode current collector; and a second negative electrode mixture layer provided on the surface of the first negative electrode mixture layer. The first negative electrode mixture layer contains a first negative electrode active material and a first water-soluble polymer material, and the second negative electrode mixture layer contains a second negative electrode active material and a second water-soluble polymer material. The ratio (S1 / V1) of the amount (S1) of the first water-soluble polymer material present on the surface of the first negative electrode active material to the amount (V1) of the first water-soluble polymer material present in the interparticle voids of the first negative electrode active material is larger than the ratio (S2 / V2) of the amount (S2) of the second water-soluble polymer material present on the surface of the second negative electrode active material to the amount (V2) of the second water-soluble polymer material present in the interparticle voids of the second negative electrode active material.

[0013] The non-aqueous electrolyte secondary battery according to an aspect of the present application is characterized by including: the negative electrode for a non-aqueous electrolyte secondary battery described above, a positive electrode, and a non-aqueous electrolyte.

[0014] The method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery according to an aspect of the present application is characterized by including: a first negative electrode mixture layer forming step of coating a first negative electrode mixture slurry prepared by kneading a first negative electrode active material and a first water-soluble polymer material on the surface of a negative electrode current collector to form a first negative electrode mixture layer; and a second negative electrode mixture layer forming step of coating a second negative electrode mixture slurry prepared by kneading a second negative electrode active material and a second water-soluble polymer material on the surface of the first negative electrode mixture layer to form a second negative electrode mixture layer, wherein the shearing force during kneading of the first negative electrode mixture slurry is larger than the shearing force during kneading of the second negative electrode mixture slurry.

[0015] Effects of the Invention

[0016] According to an aspect of the present application, a non-aqueous electrolyte secondary battery capable of suppressing a decrease in high-rate charge-discharge cycle characteristics can be provided. Description of the Drawings

[0017] Figure 1 is a longitudinal sectional view of a cylindrical secondary battery as an example of an embodiment.

[0018] Figure 2 is a sectional view of a negative electrode as an example of an embodiment.

[0019] Figure 3 (A) thereof is a schematic view showing an example of a cross section of the first negative electrode mixture layer, Figure 3 and (B) thereof is a schematic view showing an example of the second negative electrode mixture layer. Detailed Embodiments

[0020] As described above, there is known a method of preparing a slurry by dry-mixing a negative electrode active material with a binder, thereby attaching the binder to the surface of the negative electrode active material. Since a water-soluble polymer material such as a binder can retain an electrolytic solution, by attaching the water-soluble polymer material to the surface of the negative electrode active material, it is possible to bring the electrolytic solution into contact with the surface of a negative electrode active material such as a carbon material that has poor affinity for the electrolytic solution. However, according to the research by the present inventors, it has been found that in a negative electrode mixture layer containing a negative electrode active material having a water-soluble polymer material attached to its surface, the high-rate charge / discharge cycle characteristics sometimes deteriorate. It is considered that the reason is that since the permeability of the electrolytic solution on the negative electrode surface decreases, the distribution of the electrolytic solution in the negative electrode becomes non-uniform during charge and discharge. Therefore, the present inventors have conducted in-depth research and as a result, have come up with the following negative electrode for a non-aqueous electrolyte secondary battery: the negative electrode mixture layer is provided in two layers, and in the layer on the outer surface side in contact with the electrolytic solution, a large amount of the water-soluble polymer material is present in the interparticle voids of the negative electrode active material to ensure good permeability of the electrolytic solution, and in the layer on the inner side in contact with the negative electrode current collector, a large amount of the water-soluble polymer material is present on the surface of the negative electrode active material to ensure good adhesiveness. According to this negative electrode, a non-aqueous electrolyte secondary battery capable of suppressing deterioration of the high-rate charge / discharge cycle characteristics can be provided.

[0021] Hereinafter, an example of an embodiment of a cylindrical secondary battery according to the present application will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. In addition, the outer package is not limited to a cylindrical shape and can be, for example, a square shape or the like. In addition, in the following description, when multiple embodiments and modification examples are included, it is assumed from the beginning that these characteristic parts are appropriately combined and used.

[0022] Figure 1 It is an axial cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment. Figure 1 In the secondary battery 10 shown, an electrode body 14 and a non-aqueous electrolyte (not shown) are housed in an outer package 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. It should be noted that hereinafter, for the sake of convenience of explanation, the side of the sealing body 16 is referred to as "upper" and the bottom side of the outer package 15 is referred to as "lower" for explanation.

[0023] The opening end of the outer package 15 is blocked by the sealing body 16, whereby the interior of the secondary battery 10 is sealed. Insulating plates 17 and 18 are respectively provided above and below the electrode body 14. The positive electrode lead 19 extends upward through the through-hole of the insulating plate 17 and is welded to the lower surface of the filter 22 which is the bottom plate of the sealing body 16. In the secondary battery 10, the lid 26 which is the top plate of the sealing body 16 electrically connected to the filter 22 becomes the positive terminal. On the other hand, the negative electrode lead 20 extends through the through-hole of the insulating plate 18 and extends toward the bottom side of the outer package 15, and is welded to the inner surface of the bottom of the outer package 15. In the secondary battery 10, the outer package 15 becomes the negative terminal. It should be noted that when the negative electrode lead 20 is provided at the terminal portion, the negative electrode lead 20 extends through the outside of the insulating plate 18 and extends toward the bottom side of the outer package 15, and is welded to the inner surface of the bottom of the outer package 15.

[0024] The outer package 15 is, for example, a metal outer can having a bottomed cylindrical shape. A gasket 27 is provided between the outer package 15 and the sealing body 16 to ensure the airtightness of the interior of the secondary battery 10. The outer package 15 has, for example, a groove portion 21 that supports the sealing body 16 by pressing the side surface portion from the outside. The groove portion 21 is preferably formed in a ring shape along the circumferential direction of the outer package 15, and the sealing body 16 is supported thereon by means of the gasket 27.

[0025] The sealing body 16 has, laminated in order from the side of the electrode body 14, a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a lid 26. Each member constituting the sealing body 16 has, for example, a disc shape or a ring shape, and the members other than the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective central portions, and the insulating member 24 is interposed between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 breaks, whereby the upper valve body 25 expands toward the lid 26 side and moves away from the lower valve body 23, thereby cutting off their electrical connection. If the internal pressure further rises, the upper valve body 25 breaks, and the gas is discharged from the opening portion 26a of the lid 26.

[0026] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte constituting the secondary battery 10 will be described in detail, particularly the negative electrode active material contained in the negative electrode mixture layer 32 constituting the negative electrode 12.

[0027] [Negative Electrode]

[0028] Figure 2 This is a cross-sectional view of the negative electrode 12 which is an example of an embodiment. The negative electrode 12 includes: a negative electrode current collector 30, a first negative electrode mixture layer 32a provided on the surface of the negative electrode current collector 30, and a second negative electrode mixture layer 32b provided on the surface of the first negative electrode mixture layer 32a. The thicknesses of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may be the same or different from each other.

[0029] The negative electrode current collector 30 is, for example, a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film having such a metal disposed on the surface layer. The thickness of the negative electrode current collector 30 is, for example, 5 μm to 30 μm. The first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b (hereinafter, sometimes the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are collectively referred to as the negative electrode mixture layer 32) contain a negative electrode active material and a water-soluble polymer material. In addition, the negative electrode mixture layer 32 may contain an adhesive. Examples of the adhesive include fluororesins, PAN, polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), and the like. They may be used alone or in combination of two or more.

[0030] The negative electrode active material is not particularly limited as long as it can reversibly store and release lithium ions. Examples thereof include graphite particles, Si-based materials, metals alloyed with lithium such as tin (Sn), or alloys, oxides, etc. containing metal elements such as Sn. The negative electrode active material preferably contains graphite particles. The content of the graphite particles in the negative electrode active material can be set, for example, to 90 mass% to 100 mass%.

[0031] The graphite particles are natural graphite, artificial graphite, etc., and are not particularly limited, and artificial graphite is preferred. The interplanar spacing (d 002 ) of the (002) plane of the graphite particles used in the present embodiment based on the wide-angle X-ray diffraction method is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more. In addition, it is preferably less than 0.340 nm, and more preferably 0.338 mm or less. In addition, the crystallite size (Lc(002)) of the graphite particles used in the present embodiment obtained by the X-ray diffraction method is, for example, preferably 5 nm or more, more preferably 10 nm or more. In addition, it is preferably 300 nm or less, and more preferably 200 nm or less. When the interplanar spacing (d 002 ) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity of the secondary battery 10 tends to be larger than when the above ranges are not satisfied.

[0032] Graphite particles can be made, for example, as follows. The coke (precursor) as the main raw material is crushed into a specified size, aggregated with an aggregating agent, and then calcined at a temperature of 2600°C or above in a state of being pressed into a block shape to graphitize it. The block-shaped molded body after graphitization is crushed and sieved to obtain graphite particles of the desired size. Here, the internal porosity of the graphite particles can be adjusted by the particle size of the precursor after crushing, the particle size of the precursor in the aggregated state, etc. For example, the average particle size of the precursor after crushing (the median particle size D50 converted by volume, the same below) is preferably in the range of 12μm to 20μm. In addition, the internal porosity of the graphite particles can also be adjusted by the amount of volatile components added to the block-shaped molded body. When a part of the aggregating agent added to the coke (precursor) volatilizes during firing, the aggregating agent can be used as a volatile component. As such an aggregating agent, asphalt can be exemplified.

[0033] In addition, graphite particles can be produced, for example, as follows. The coke (precursor) as the main raw material is crushed into a specified size, and they are agglomerated with an agglomerator such as asphalt, and then calcined at a temperature above 2600°C to graphitize them, and then sieved, thereby obtaining graphite particles of the desired size. Here, the internal porosity of the graphite particles can be adjusted by the particle size of the precursor after crushing, the particle size of the precursor in the aggregated state, etc. For example, the average particle size of the precursor after crushing is preferably in the range of 12μm to 20μm.

[0034] The water-soluble polymer material is preferably a material that functions as a thickener of the slurry. The water-soluble polymer material can function as an adhesive. As the water-soluble polymer material, for example, carboxymethyl cellulose (CMC) or its salt, polyacrylic acid (PAA) or its salt (PAA-Na, PAA-K, etc., and partially neutralized salt), polyvinyl alcohol (PVA), etc. can be cited. They can be used alone or in combination of two or more.

[0035] Next, refer to Figure 3 , the negative electrode active material and the water-soluble polymer material in the negative electrode mixture layer 32 are described. Figure 3 (A) is a schematic diagram showing an example of a cross section of the first negative electrode mixture layer, Figure 3 (B) is a schematic diagram showing an example of the second negative electrode mixture layer. Figure 3 As shown in (A), the first negative electrode mixture layer 32a includes a first negative electrode active material 34a and a first water-soluble polymer material 36a. Figure 3As shown in (B) of FIG. 0, the second negative electrode mixture layer 32b contains a second negative electrode active material 34b and a second water-soluble polymer material 36b. The ratio (S1 / V1) of the amount (S1) of the first water-soluble polymer material 36a present on the surface of the first negative electrode active material 34a to the amount (V1) of the first water-soluble polymer material 36a present in the interparticle voids of the first negative electrode active material 34a is larger than the ratio (S2 / V2) of the amount (S2) of the second water-soluble polymer material 36b present on the surface of the second negative electrode active material 34b to the amount (V2) of the second water-soluble polymer material 36b present in the interparticle voids of the second negative electrode active material 34b. That is, in the first negative electrode mixture layer 32a, most of the first water-soluble polymer material 36a is present on the surface of the first negative electrode active material 34a, and in the second negative electrode mixture layer 32b, most of the second water-soluble polymer material 36b is present in the interparticle voids of the second negative electrode active material 34b. With this configuration, the adhesion between the negative electrode current collector 30 and the negative electrode mixture layer 32 can be improved, and the permeability of the electrolyte in the negative electrode mixture layer 32 can be improved, so that a decrease in the high-rate charge-discharge cycle characteristics of the battery can be suppressed. In addition, a decrease in the low-rate charge cycle characteristics can also be suppressed. Here, the amount of the water-soluble polymer material present in the interparticle voids or on the surface of the negative electrode active material refers to a two-dimensional value obtained by measuring the cross-section of the negative electrode mixture layer 32. For the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the water-soluble polymer material present on the surface of the negative electrode active material and the water-soluble polymer material present in the interparticle voids of the negative electrode active material are visualized according to the following steps, respectively, whereby S1 / V1 and S2 / V2 can be compared.

[0036] <Measurement Method of S1 / V1 and S2 / V2>

[0037] (1) Expose the cross-section of the negative electrode mixture layer. As a method for exposing the cross-section, for example, the following method can be cited: Cut a part of the negative electrode and process it with an ion milling device (for example, IM4000PLUS manufactured by Hitachi High-Tech Corporation) to expose the cross-section of the negative electrode mixture layer.

[0038] (2) Using SEM-EDX (for example, Flat QUAD manufactured by Bruker Corporation), map the elements derived from the water-soluble polymer material in the exposed cross-section of the negative electrode mixture layer, and take images of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, respectively. Here, the element derived from the water-soluble polymer material refers to a characteristic element contained in the water-soluble polymer material. For example, when the water-soluble polymer material is the Na salt of CMC, the Na element can be mapped. The measurement conditions for the cross-section of the negative electrode mixture layer are as follows, for example.

[0039] Magnification of the cross-section: 800 times

[0040] Accelerating voltage of electrons: 5 kV

[0041] Emission current: 10 μA

[0042] Probe current: High

[0043] Condensing lens: 1.0

[0044] Sampling time: 180 sec

[0045] (3) According to the images obtained from the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b respectively, if possible, the comparison between S1 / V1 and S2 / V2 can be made visually, or the binary processing can be performed on each image using image analysis software (for example, ImageJ manufactured by the National Institutes of Health, USA), and S1 / V1 and S2 / V2 can be numerically converted and compared.

[0046] The first negative electrode active material 34a and the second negative electrode active material 34b can be the same. In addition, the first water-soluble polymer material 36a and the second water-soluble polymer material 36b can be the same. By making the materials included in the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b common, the cost can be reduced. In addition, even if the materials included in the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are made common, by making the manufacturing methods of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b different as described later, the relationship of S1 / V1 > S2 / V2 can also be satisfied.

[0047] At least one of the first negative electrode active material 34a and the second negative electrode active material 34b may contain a Si-based material. The Si-based material is a material capable of reversibly storing and releasing lithium ions and functions as a negative electrode active material. As the Si-based material, for example, Si, an alloy containing Si, silicon oxide such as SiO x (X is 0.8 to 1.6), etc. The Si-based material is a negative electrode material that can increase the battery capacity compared to the negative electrode active material. From the aspects of increasing the battery capacity and suppressing the decrease in the high-rate charge-discharge cycle characteristics, etc., the content of the Si-based material in the first negative electrode active material 34a or the second negative electrode active material 34b is preferably 0.5% by mass to 10% by mass, more preferably 3% by mass to 7% by mass.

[0048] Next, a method for manufacturing the negative electrode 12 will be described. The method for manufacturing the negative electrode 12 includes: a first negative electrode binder layer forming step of coating a first negative electrode binder paste prepared by kneading a first negative electrode active material 34a and a first water-soluble polymer material 36a on the surface of the negative electrode current collector 30 to form a first negative electrode binder layer 32a; and a second negative electrode binder layer forming step of coating a second negative electrode binder paste prepared by kneading a second negative electrode active material 34b and a second water-soluble polymer material 36b on the surface of the first negative electrode binder layer 32a to form a second negative electrode binder layer 32b.

[0049] The first negative electrode binder paste can be prepared, for example, as follows.

[0050] (1) Mix the first negative electrode active material 34a and the first water-soluble polymer material 36a to prepare a first mixture.

[0051] (2) Appropriately add a solvent to the first mixture and knead. The solvent is, for example, water. In addition, the amount of the added solvent is, for example, 10% by mass to 30% by mass relative to the total amount of the first negative electrode active material 34a and the first water-soluble polymer material 36a.

[0052] (3) Add an adhesive such as styrene-butadiene copolymer rubber (SBR) to the first mixture. Further, stir the first mixture to prepare the first negative electrode binder paste.

[0053] The second negative electrode binder paste can be prepared, for example, as follows.

[0054] (1) Mix the second water-soluble polymer material 36b and a solvent to prepare a second mixture. The solvent is, for example, water. In addition, the amount of the solvent is, for example, 40% by mass to 60% by mass relative to the total amount of the second water-soluble polymer material 36b and the second negative electrode active material 34b to be added next.

[0055] (2) Add the second negative electrode active material 34b to the second mixture.

[0056] (3) Knead the second mixture. An appropriate solvent can be additionally added during kneading.

[0057] (4) Add an adhesive to the second mixture. Further, stir the second mixture to prepare the second negative electrode binder paste.

[0058] The shearing force during kneading of the first negative electrode mixture paste is greater than that during kneading of the second negative electrode mixture paste. Thus, in the second negative electrode mixture layer 32b, most of the second water-soluble polymer material 36b is disposed in the interparticle voids of the second negative electrode active material 34b, and in the first negative electrode mixture layer 32a, most of the first water-soluble polymer material 36a is disposed on the surface of the first negative electrode active material 34a. According to this configuration, the electrolyte permeability of the second negative electrode mixture layer on the outer surface side in contact with the electrolyte and the adhesion between the first negative electrode mixture layer and the negative electrode current collector 30 are improved, and thus a decrease in the high-rate charge-discharge cycle characteristics of the secondary battery 10 can be suppressed. When at least one of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b contains a Si-based material with large expansion and contraction during charge and discharge, the above effects are significantly exerted. It should be noted that in the present application, "kneading" means mixing a mixture containing a negative electrode active material, a water-soluble polymer, and a solvent by applying a shearing force.

[0059] Then, the first negative electrode mixture paste is coated on both sides of the negative electrode current collector 30 and dried (first negative electrode mixture layer forming step), and then the second negative electrode mixture paste is coated on both sides of the first negative electrode mixture layer 32a and dried (second negative electrode mixture layer forming step). Further, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are calendered using a calender roll, whereby the negative electrode mixture layer 32 can be formed. The second negative electrode mixture paste can also be coated on the first negative electrode mixture layer 32a before drying.

[0060] [Positive electrode]

[0061] The positive electrode 11 is composed of, for example, a positive electrode current collector such as a metal foil, and a positive electrode mixture layer formed on the positive electrode current collector. As the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum, a film having the metal disposed on the surface layer, etc. can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, etc.

[0062] The positive electrode 11 can be produced, for example, as follows: The positive electrode mixture paste containing a positive electrode active material, a binder, a conductive agent, etc. is coated on the positive electrode current collector, dried to form the positive electrode mixture layer, and then the positive electrode mixture layer is calendered.

[0063] As the positive electrode active material, a lithium transition metal oxide containing transition metal elements such as Co, Mn, Ni, etc. can be exemplified. The lithium transition metal oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Coy M 1-y O z 、 Li x Ni 1- y M y O z 、 Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). They can be used alone or in combination. From the aspect of achieving high capacity of non-aqueous electrolyte secondary batteries, the positive electrode active material preferably contains Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3) and other lithium nickel composite oxides.

[0064] Examples of the conductive agent include carbon-based particles such as carbon black (CB), acetylene black (AB), Ketjen black, and graphite. They can be used alone or in combination of two or more.

[0065] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, and polyolefin-based resins. They can be used alone or in combination of two or more.

[0066] [Separator]

[0067] The separator 13 is, for example, a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous membranes, woven fabrics, and non-woven fabrics. As the material of the separator, olefin-based resins such as polyethylene and polypropylene, and cellulose are suitable. The separator 13 can be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. In addition, it can be a multi-layer separator including a polyethylene layer and a polypropylene layer, and those obtained by coating the surface of the separator 13 with materials such as aromatic polyamide-based resins and ceramics can be used.

[0068] [Non-aqueous electrolyte]

[0069] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolyte solution), and may also be a solid electrolyte using a gelled polymer or the like. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and a mixed solvent of two or more of them can be used. The non-aqueous solvent may contain a halogenated substituent obtained by substituting at least a part of the hydrogen of these solvents with a halogen atom such as fluorine.

[0070] Examples of the above esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone, butyl pentanoate, and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0071] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl 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, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether.

[0072] As the above halogenated substituent, preferably, fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as fluoromethyl propionate (FMP) are used.

[0073] 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 n F 2n+1 ) x(1 < x < 6, n is 1 or 2), LiB 10 Cl 10 、LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 21+1 SO2)(C m F 2m+1 SO2){1, m is an integer of 1 or more} and other imide salts, etc. The lithium salt can be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., LiPF6 is preferably used. The concentration of the lithium salt is preferably set to 0.8 to 1.8 mol per 1 L of the solvent.

[0074] Examples

[0075] Hereinafter, the present application will be further described by way of examples, but the present application is not limited to these examples.

[0076] <Examples>

[0077] [Fabrication of positive electrode]

[0078] As the positive electrode active material, lithium nickel cobalt aluminate containing aluminum (LiNi 0.88 Co 0.09 Al 0.03 O2) is used. Mix in such a way that 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as the conductive agent, and 0.9 part by mass of polyvinylidene fluoride powder as the binder, and then add an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture paste. Coat this paste on both sides of a positive electrode current collector formed of aluminum foil (thickness 15 μm) by the doctor blade method. After drying the coating film, roll the coating film with a calender roll to fabricate a positive electrode having a positive electrode mixture layer formed on both sides of the positive electrode current collector.

[0079] [Fabrication of negative electrode]

[0080] First, prepare the first negative electrode mixture paste. Mix in such a way that graphite particles are 95 parts by mass and SiO is 5 parts by mass, and use this as the negative electrode active material. Mix 100 parts by mass of the negative electrode active material and 1 part by mass of carboxymethyl cellulose (CMC), add 20 parts by mass of water to this mixture and knead. Then add 1 part by mass of styrene-butadiene copolymer rubber (SBR) to this mixture and stir to prepare the first negative electrode mixture paste.

[0081] Next, prepare the second negative electrode mixture paste. First, mix 50 parts by mass of water and 1 part by mass of CMC, and put 100 parts by mass of negative electrode active material into this mixture for kneading. Then, add 1 part by mass of SBR to this mixture and stir to prepare the second negative electrode mixture paste. The shear force during the kneading of the first negative electrode mixture paste is greater than that during the kneading of the second negative electrode mixture paste.

[0082] Apply the first negative electrode mixture paste to both sides of the negative electrode current collector formed of copper foil by the doctor blade method, and dry it to form the first negative electrode mixture layer. Further, apply the above-mentioned second negative electrode mixture paste on the first negative electrode mixture layer and dry it to form the second negative electrode mixture layer. At this time, the coating mass ratio per unit area of the first negative electrode mixture paste and the second negative electrode mixture paste is set to 5:5. Roll the first negative electrode mixture layer and the second negative electrode mixture layer using a rolling roller to produce a negative electrode. Observe the cross-section of the negative electrode, and the result is S1 / V1 > S2 / V2.

[0083] [Fabrication of Non-aqueous Electrolyte]

[0084] Add 5 parts by mass of vinylene carbonate (VC) to 100 parts by mass of a non-aqueous solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3, dissolve LiPF6 at a concentration of 1.5 mol / L, and use this as the non-aqueous electrolyte.

[0085] [Fabrication of Non-aqueous Electrolyte Secondary Battery]

[0086] (1) Install the positive electrode lead on the positive electrode current collector, install the negative electrode lead on the negative electrode current collector, and then wind the positive electrode and the negative electrode with a spacer formed of a microporous membrane made of polyethylene to produce a wound electrode body.

[0087] (2) Arrange insulating plates above and below the electrode body, weld the negative electrode lead to the outer package, weld the positive electrode lead to the sealing body, and accommodate the electrode body in the outer package.

[0088] (3) Inject the non-aqueous electrolyte into the outer package by a vacuum method, and then seal the opening end of the outer package with a sealing body using a gasket to obtain a non-aqueous electrolyte secondary battery.

[0089] [Comparative Example 1]

[0090] Use the first negative electrode mixture paste to form the second negative electrode mixture layer, and perform the same operations as in the example except for this.

[0091] [Comparative Example 2]

[0092] The first negative electrode mixture layer was formed using the second negative electrode mixture paste, and the operation was the same as in the example except for this.

[0093] <Comparative Example 3>

[0094] The first negative electrode mixture layer was formed using the second negative electrode mixture paste, and the second negative electrode mixture layer was formed using the first negative electrode mixture paste. The operation was the same as in the example except for this.

[0095] [Measurement of Capacity Retention Rate of High Rate Cycling]

[0096] At an ambient temperature of 25 °C, the non-aqueous electrolyte secondary battery of each example and each comparative example was charged at a constant current of 1C (4600 mA) until 4.2 V, and then charged at a constant voltage of 4.2 V until 1 / 50C. Then, it was discharged at a constant current of 0.5C until 2.5 V. This charge and discharge was set as one cycle, and 100 cycles were performed. The capacity retention rate of the high rate cycling of the non-aqueous electrolyte secondary battery of each example and each comparative example was obtained by the following formula.

[0097] Capacity retention rate = (Discharge capacity of the 100th cycle / Discharge capacity of the 1st cycle) × 100

[0098] [Measurement of Capacity Retention Rate of Initial Low Rate Cycling]

[0099] At an ambient temperature of 25 °C, the non-aqueous electrolyte secondary battery of each example and each comparative example was charged at a constant current of 0.3C (1380 mA) until 4.2 V, and then charged at a constant voltage of 4.2 V until 1 / 50C. Then, it was discharged at a constant current of 0.5C until 2.5 V. This charge and discharge was set as one cycle, and 50 cycles were performed. The capacity retention rate of the high rate charge and discharge cycling of the non-aqueous electrolyte secondary battery of each example and each comparative example was obtained by the following formula.

[0100] Capacity retention rate = (Discharge capacity of the 50th cycle / Discharge capacity of the 1st cycle) × 100

[0101] [Measurement of Capacity Retention Rate of Low Rate Charge and Discharge Cycling]

[0102] At an ambient temperature of 25 °C, the non-aqueous electrolyte secondary battery of each example and each comparative example was charged at a constant current of 0.3C (1380 mA) until 4.2 V, and then charged at a constant voltage of 4.2 V until 1 / 50C. Then, it was discharged at a constant current of 0.5C until 2.5 V. This charge and discharge was set as one cycle, and 500 cycles were performed. The capacity retention rate of the high rate charge and discharge cycling of the non-aqueous electrolyte secondary battery of each example and each comparative example was obtained by the following formula.

[0103] Capacity retention rate = (discharge capacity at the 500th cycle / discharge capacity at the 1st cycle) × 100

[0104] Table 1 summarizes the results of the capacity retention rates of various charge-discharge cycles of the non-aqueous electrolyte secondary batteries of the examples and comparative examples. It should be noted that the higher the value of the capacity retention rate of the charge-discharge cycle, the more the reduction of the charge-discharge cycle characteristics is suppressed.

[0105] [Table 1]

[0106]

[0107] Layer 1: First negative electrode mixture layer, Layer 2: Second negative electrode mixture layer

[0108] The secondary battery of the example has a higher capacity retention rate than the secondary battery of the comparative example in any cycle test, and in particular, the high-rate charge-discharge cycle characteristics are better than those of the comparative example. It is considered that this is because: in the negative electrode mixture layer of the example, the permeability of the electrolyte in the layer on the outer surface side in contact with the electrolyte is improved, and the adhesion between the layer on the inner side in contact with the negative electrode current collector and the negative electrode current collector is improved.

[0109] Description of reference numerals

[0110] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Spacer, 14 Electrode body, 15 Outer package, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Groove portion, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cover, 26a Opening portion, 27 Gasket, 30 Negative electrode current collector, 32 Negative electrode mixture layer, 32a First negative electrode mixture layer, 32b Second negative electrode mixture layer, 34a First negative electrode active material, 34b Second negative electrode active material, 36a First water-soluble polymer material, 36b Second water-soluble polymer material.

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a negative electrode current collector, a first negative electrode mixture layer provided on the surface of the negative electrode current collector, and a second negative electrode mixture layer provided on the surface of the first negative electrode mixture layer. The first negative electrode mixture layer contains a first negative electrode active material and a first water-soluble polymer material. The second negative electrode mixture layer contains a second negative electrode active material and a second water-soluble polymer material. The first negative electrode active material and the second negative electrode active material are the same. The ratio (S1 / V1) of the amount (S1) of the first water-soluble polymer material present on the surface of the first negative electrode active material to the amount (V1) of the first water-soluble polymer material present in the interparticle voids of the first negative electrode active material is greater than the ratio (S2 / V2) of the amount (S2) of the second water-soluble polymer material present on the surface of the second negative electrode active material to the amount (V2) of the second water-soluble polymer material present in the interparticle voids of the second negative electrode active material.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein, The first negative electrode active material and the second negative electrode active material contain a Si-based material, and the content of the Si-based material is 0.5% by mass to 10% by mass relative to the mass of the first negative electrode active material or the second negative electrode active material containing the Si-based material.

3. A non-aqueous electrolyte secondary battery, comprising: The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, a positive electrode, and a non-aqueous electrolyte.

4. A method for manufacturing the negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, comprising: a first negative electrode mixture layer forming step of coating a first negative electrode mixture slurry prepared by kneading a first negative electrode active material and a first water-soluble polymer material on the surface of the negative electrode current collector to form a first negative electrode mixture layer; and a second negative electrode mixture layer forming step of coating a second negative electrode mixture slurry prepared by kneading a second negative electrode active material and a second water-soluble polymer material on the surface of the first negative electrode mixture layer to form a second negative electrode mixture layer, wherein the shear force during kneading of the first negative electrode mixture slurry is greater than the shear force during kneading of the second negative electrode mixture slurry, and the first negative electrode active material and the second negative electrode active material are the same.

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