Negative electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

This negative electrode is provided with a negative electrode current collector, and a negative electrode mixture layer formed on the negative electrode current collector, wherein: the negative electrode mixture layer comprises a first layer arranged on the negative electrode current collector, and a second layer arranged on the first layer; the second layer includes graphite particles A having a particle internal porosity of at most 10%; the first layer includes graphite particles B having a particle internal porosity of more than 10%; and the second layer has a water contact angle of at most 50°.

Description

Technical Field

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

Background Art

[0002] Secondary batteries, for example, lithium-ion secondary batteries including a positive electrode, a negative electrode, and an electrolytic solution, which perform charge and discharge by moving lithium ions between the positive electrode and the negative electrode, are widely used.

[0003] For example, Patent Document 1 proposes a lithium-ion secondary battery in which a fluorine-containing group is modified on the surface of the negative electrode to improve the affinity with the electrolytic solution.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] By the way, in lithium-ion secondary batteries, a problem is the deterioration of charge-discharge cycle characteristics.

[0006] Therefore, an object of the present disclosure is to provide a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery capable of improving the charge-discharge cycle characteristics of the lithium-ion secondary battery.

[0007] A negative electrode for a lithium-ion secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode composite material layer formed on the negative electrode current collector. The negative electrode composite material layer includes a first layer disposed on the negative electrode current collector and a second layer disposed on the first layer. The second layer includes graphite particles A having an internal particle porosity of 10% or less, the first layer includes graphite particles B having an internal particle porosity of more than 10%, and the water contact angle of the second layer is 50° or less.

[0008] A lithium-ion secondary battery according to one aspect of the present disclosure includes the negative electrode for a lithium-ion secondary battery described above.

[0009] According to the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a lithium-ion secondary battery.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0011] A negative electrode for a lithium-ion secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode composite layer includes a first layer disposed on the negative electrode current collector and a second layer disposed on the first layer. The second layer includes graphite particles A having an internal particle porosity of 10% or less. The first layer includes graphite particles B having an internal particle porosity of more than 10%. The water contact angle of the second layer is 50° or less.

[0012] As in the present disclosure, by disposing graphite particles A with an internal particle porosity of 10% or less in the second layer on the surface side of the negative electrode and making the water contact angle of the second layer 50° or less, the pressure loss when the electrolytic solution flows into the negative electrode is reduced. As a result, the electrolytic solution easily penetrates into the negative electrode, and the lithium ion resistance decreases, so that the charge-discharge cycle characteristics of the lithium ion secondary battery are improved. Note that the surface side of the negative electrode described above is the surface facing the separator or the positive electrode. In addition, since graphite particles B with an internal particle porosity exceeding 10% are easily crushed during the production of the negative electrode, the adhesiveness between the negative electrode current collector and the graphite particles B is high. Therefore, by disposing graphite particles B with an internal particle porosity exceeding 10% in the first layer disposed on the negative electrode current collector, the particles of the negative electrode active material can be prevented from peeling off from the negative electrode current collector, so that the charge-discharge cycle characteristics of the lithium ion secondary battery are improved.

[0013] Hereinafter, embodiments of the negative electrode for a lithium ion secondary battery and the lithium ion secondary battery according to the present disclosure will be described in detail with reference to the drawings. In this specification, the description "numerical value (1) to numerical value (2)" means numerical value (1) or more and numerical value (2) or less.

[0014] FIG. 1 is a cross-sectional view of a lithium ion secondary battery which is an example of an embodiment. The lithium ion secondary battery 10 shown in FIG. 1 includes a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween, an electrolytic solution, insulating plates 18 and 19 disposed above and below the electrode body 14, respectively, and a battery case 15 for housing the above members. The battery case 15 includes a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. Note that instead of the wound electrode body 14, other forms of electrode bodies such as a laminated electrode body in which the positive electrode and the negative electrode are alternately laminated with a separator interposed therebetween may be applied. In addition, examples of the battery case 15 include metal cases such as cylindrical, rectangular, coin-shaped, and button-shaped cases, and resin cases (laminated batteries) formed by laminating resin sheets.

[0015] The case body 16 is, for example, a metal container having a bottomed cylindrical shape. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure the airtightness inside the battery. The case body 16 has, for example, an overhanging portion 22 that supports the sealing body 17, with a part of the side surface portion protruding inward. The overhanging portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and the insulating member 25 is interposed between the peripheral edges of each. When the internal pressure of the lithium-ion secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is interrupted. When the internal pressure further rises, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] In the lithium-ion secondary battery 10 shown in FIG. 1, the positive electrode lead 20 attached to the positive electrode 11 extends toward the sealing body 17 side through the through hole of the insulating plate 18, and the negative electrode lead 21 attached to the negative electrode 12 extends toward the bottom side of the case body 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0018] Hereinafter, the positive electrode 11, negative electrode 12, separator 13, and electrolytic solution constituting the lithium-ion secondary battery 10 will be described in detail.

[0019] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector 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 or an aluminum alloy, or a film having such a metal disposed on its surface layer can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, and a conductive material. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, etc. onto the positive electrode current collector, drying and rolling the coating film, and forming the positive electrode mixture layer on both sides of the positive electrode current collector.

[0020] The positive electrode active material is mainly composed of a lithium-containing metal composite oxide. Examples of the metal elements contained in the lithium-containing metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, Ge, etc. An example of a suitable lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0021] Examples of the conductive material contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins may be used in combination with carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0022] [Negative electrode] FIG. 2 is a cross-sectional view of a negative electrode which is an example of the embodiment. The negative electrode 12 shown in FIG. 2 includes a negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the negative electrode current collector 40. As the negative electrode current collector 40, for example, a foil of a metal stable within the potential range of the negative electrode, such as copper or a copper alloy, or a film having such a metal disposed on its surface layer can be used.

[0023] The negative electrode composite material layer 42 formed on the negative electrode current collector 40 is composed of a first layer 44 and a second layer 46. The first layer 44 is disposed on the negative electrode current collector 40, and the second layer 46 is disposed on the first layer 44. The negative electrode composite material layer 42 is preferably formed on both surfaces of the negative electrode current collector 40. Note that the phrase "the second layer 46 is 'disposed' 'on' the first layer 44" means that the second layer 46 may be directly disposed on the first layer 44, or an intermediate layer may be interposed between the second layer 46 and the first layer 44.

[0024] The first layer 44 contains, as a negative electrode active material, graphite particles B having an internal particle porosity exceeding 10%. The second layer 46 contains, as a negative electrode active material, graphite particles A having an internal particle porosity of 10% or less. The internal porosity of the graphite particles A may be 10% or less in terms of improving charge-discharge cycle characteristics, etc., but is preferably 1% to 5%, and more preferably 3% to 5%. The internal porosity of the graphite particles B may exceed 10% in terms of being appropriately crushed by a compression process in negative electrode manufacturing, etc., but is preferably 12% to 25%, and more preferably 12% to 23%.

[0025] The graphite particles A having an internal particle porosity of 10% or less are particles with a small BET specific surface area. For example, they are in the range of 1.0 m 2 / g to 1.6 m 2 / g. The graphite particles B having an internal particle porosity exceeding 10% are particles with a large BET specific surface area. For example, they are 3.0 m 2 / g to 20 m 2 / g. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0026] Figure 3 is a schematic diagram showing a cross-section of the graphite particles. As shown in Figure 3, the graphite particles 30 have a closed void 34 that does not connect from the inside of the particle to the particle surface and a void 36 that connects from the inside of the particle to the particle surface in the cross-sectional view of the graphite particles 30. The void 34 is hereinafter referred to as the internal void 34. The void 36 is hereinafter referred to as the external void 36. In the present embodiment, the internal porosity of the graphite particles is a two-dimensional value obtained from the ratio of the area of the internal voids of the graphite particles to the cross-sectional area of the graphite particles, and specifically, it is obtained by the following procedure.

[0027] <Measurement Method of Internal Porosity> (1) Expose the cross-section of the negative electrode active material. As a method for exposing the cross-section, for example, a method of cutting a part of the negative electrode, processing it with an ion milling apparatus (for example, IM4000PLUS manufactured by Hitachi High-Tech Corporation), and exposing the cross-section of the negative electrode composite layer can be mentioned. (2) Using a scanning electron microscope, take a backscattered electron image of the exposed cross-section of the negative electrode composite layer. The magnification when taking the backscattered electron image is from 3,000 times to 5,000 times. (3) Import the cross-sectional image obtained as described above into a computer, perform binarization processing using image analysis software (for example, ImageJ manufactured by the National Institutes of Health, USA), and obtain a binarized processed image in which the particle cross-section in the cross-sectional image is colored black and the voids existing in the particle cross-section are colored white. (4) Calculate the area of the graphite particle cross-section and the area of the internal voids existing in the graphite particle cross-section from the binarized processed image. Here, the area of the graphite particle cross-section refers to the area of the region surrounded by the outer periphery of the graphite particles, that is, the total area of all the cross-sectional parts of the graphite particles. Also, for voids with a width of 3 μm or less among the voids existing in the graphite particle cross-section, it may be difficult to distinguish whether they are internal voids or external voids in image analysis, so voids with a width of 3 μm or less may be regarded as internal voids. Then, from the calculated area of the graphite particle cross-section and the area of the internal voids of the graphite particle cross-section, calculate the internal porosity of the graphite particles (internal porosity of graphite particles = area of internal voids of graphite particle cross-section × 100 / area of graphite particle cross-section). The internal porosity of the graphite particles is the average value of 10 graphite particles.

[0028] The graphite particles B contained in the first layer 44 are preferably more than the graphite particles B contained in the second layer 46 in terms of improving the charge-discharge cycle characteristics, and are preferably in the range of 50% to 90% by mass with respect to the total amount of the graphite particles B in the negative electrode composite material layer 42. The first layer 44 may contain, as a negative electrode active material, graphite particles A having an internal porosity of 10% or less, but in terms of improving the charge-discharge cycle characteristics, the content of the graphite particles A in the first layer 44 is preferably 10% or less by mass with respect to the total amount of the graphite particles A in the negative electrode composite material layer 42.

[0029] The graphite particles A contained in the second layer 46 are preferably more than the graphite particles A contained in the first layer 44 in terms of improving the charge-discharge cycle characteristics, and are preferably in the range of 40% to 100% by mass with respect to the total amount of the graphite particles A in the negative electrode composite material layer 42. The second layer 46 may contain, as a negative electrode active material, graphite particles B, but in terms of improving the charge-discharge cycle characteristics, the content of the graphite particles B in the second layer 46 is preferably 50% or less by mass with respect to the total amount of the graphite particles B in the negative electrode composite material layer 42. [[ID=�]]

[0030] The graphite particles A and B are produced, for example, as follows.

[0031] <Graphite particles A with an internal porosity of 10% or less> For example, coke (precursor) as the main raw material is pulverized to a predetermined size, and after being aggregated with a binder and fired at a temperature of 2600 °C or higher to be graphitized, it is sieved to obtain graphite particles A of a desired size. Here, the internal porosity can be adjusted to 10% or less depending on the particle size of the precursor after pulverization, the particle size of the precursor in the aggregated state, etc. For example, the average particle size (median diameter D50) of the precursor after pulverization is preferably in the range of 12 μm to 20 μm.

[0032] <Graphite particles B with an internal porosity of more than 10%> For example, coke (precursor) as the main raw material is crushed into a predetermined size, aggregated with a binder, and then further pressure-molded into a block shape, and fired at a temperature of 2600 °C or higher to be graphitized. The block-shaped molded body after graphitization is crushed and sieved to obtain graphite particles B of a desired size. The internal porosity can be adjusted to more than 10% depending on the amount of volatile components added to the block-shaped molded body.

[0033] When a part of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as a volatile component. Pitch is exemplified as such a binder.

[0034] The graphite particles A and B used in this embodiment are not particularly limited, such as natural graphite and artificial graphite, but artificial graphite is preferable in terms of ease of adjusting the internal porosity. The interplanar spacing (d 002 ) of the (002) plane of the graphite particles A and B used in this embodiment by wide-angle X-ray diffraction is preferably, for example, 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. Also, the crystallite size (Lc(002)) of the graphite particles A and B used in this embodiment determined by X-ray diffraction is preferably, for example, 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. The average particle size of the graphite particles A and B is not particularly limited, but is, for example, 1 μm to 30 μm. The average particle size means the volume average particle size (Dv50) at which the volume integration value is 50% in the particle size distribution measured by the laser diffraction scattering method.

[0035] The water contact angle of the second layer 46 may be 50° or less, preferably 40° or less, and more preferably 35° or less in terms of facilitating the infiltration of the electrolytic solution into the negative electrode 12 and improving the charge-discharge cycle characteristics.

[0036] The water contact angle is determined by dropping 2.2 μL of water droplets onto the sample surface (the surface of the second layer) using a contact angle meter (Kyowa Interface Science, DM-501), photographing the shape of the water droplets immediately after dropping, and measuring from the obtained image using the θ / 2 method.

[0037] The water contact angle of the second layer 46 varies depending on, for example, the volume ratio of the graphite particles A in the second layer 46, the packing density of the negative electrode composite layer 42 (or the packing density of the second layer 46), and the like.

[0038] The volume ratio of the graphite particles A in the second layer 46 to the total volume of the second layer 46 is preferably 29% by volume or more, more preferably 50% by volume or more, in terms of making the water contact angle of the second layer 46 50° or less.

[0039] The packing density of the negative electrode composite layer 42 (or the packing density of the second layer 46) is, for example, 1.50 g / cm 3 ~1.65 g / cm 3 in the range is preferable, and 1.4 g / cm 3 ~1.5 g / cm 3 in the range is more preferable.

[0040] The negative electrode composite layer 42 may contain an alloying material as the negative electrode active material. By including the alloying material, it becomes possible to increase the capacity of the lithium-ion secondary battery. The alloying material may be contained in the same amount in the first layer 44 and the second layer 46, or may be contained more in either one. However, from the viewpoint of suppressing a decrease in the charge-discharge cycle characteristics of the lithium-ion secondary battery, it is preferably contained more in the second layer 46 than in the first layer 44. The content of the alloying material in the second layer 46 is preferably in the range of 75% by mass to 100% by mass with respect to the total amount of the alloying material in the negative electrode composite layer 42. Note that when the ratio of the alloying material in the negative electrode composite layer 42 increases, the effect of improving the charge-discharge cycle characteristics decreases. Therefore, the content of the alloying material is preferably 15% by mass or less with respect to the total amount of the negative electrode active material in the negative electrode composite layer 42. The lower limit value of the content of the alloying material is preferably 5% by mass or more, more preferably 8% by mass or more, with respect to the total amount of the negative electrode active material in the negative electrode composite layer 42 from the viewpoint of increasing the capacity of the lithium-ion secondary battery and the like.

[0041] The alloying material is composed of an element that alloys with lithium, a compound containing an element that alloys with lithium, or both. Examples of the element that alloys with lithium applicable to the negative electrode active material include Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, Bi, and the like. Among them, from the viewpoint of increasing the capacity, Si and Sn are preferable, and Si is particularly preferable.

[0042] Examples of the compound containing Si include a silicon oxide phase and a compound containing Si dispersed in the silicon oxide phase, a lithium silicate phase and a compound containing Si dispersed in the lithium silicate phase, and the like. The silicon oxide phase and the compound containing Si dispersed in the silicon oxide phase are hereinafter referred to as "SiO". The lithium silicate phase and the compound containing Si dispersed in the lithium silicate phase are hereinafter referred to as "LSX".

[0043] In addition, a conductive layer composed of a material with high conductivity may be formed on the particle surfaces of SiO and LSX. An example of a suitable conductive layer is a carbon film composed of a carbon material. The carbon film is composed of, for example, carbon black, acetylene black, ketjen black, graphite, and a mixture of two or more of these. As a method for carbon-coating the particle surfaces of SiO and LSX, examples include a CVD method using acetylene, methane, etc., and a method in which coal pitch, petroleum pitch, phenol resin, etc. are mixed with the particles of SiO and LSX and heat treatment is performed. Also, a carbon film may be formed by fixing carbon powder such as carbon black to the particle surface using a binder.

[0044] Suitable SiO has a sea-island structure in which fine Si particles are dispersed substantially uniformly in an amorphous silicon oxide phase, and is represented by the general formula SiO x (0.5 ≦ x ≦ 1.6). From the viewpoint of achieving both battery capacity and cycle characteristics, the content of Si particles is preferably 35 to 75% by mass based on the total mass of SiO.

[0045] The average particle diameter of the Si particles dispersed in the silicon oxide phase is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 50 nm or less. After charge and discharge, it is preferably 400 nm or less, and more preferably 100 nm or less. By miniaturizing the Si particles, the volume change during charge and discharge becomes smaller and the cycle characteristics are improved. The average particle diameter of the Si particles is measured by observing the cross-section of SiO using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and specifically, it is obtained as the average value of the longest diameters of 100 Si particles. The silicon oxide phase is composed of, for example, an aggregate of particles finer than the Si particles.

[0046] Suitable LSX has the general formula Li 2z SiO (2+z)It has a sea-island structure in which fine Si particles are dispersed substantially uniformly in a lithium silicate phase represented by (0 < z < 2). The content of Si particles is preferably 35 to 75% by mass with respect to the total mass of LSX, similar to the case of SiO. Also, the average particle size of Si particles is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 50 nm or less. The lithium silicate phase is composed of, for example, an aggregate of particles finer than Si particles.

[0047] As described above, the lithium silicate phase is Li 2z SiO (2+z) It is preferably composed of a compound represented by (0 < z < 2). That is, Li4SiO4 (Z = 2) is not included in the lithium silicate phase. Li4SiO4 is an unstable compound and reacts with water to show alkalinity, so it may alter Si and cause a decrease in charge and discharge capacity. From the viewpoints of stability, ease of production, lithium ion conductivity, etc., it is preferable that the lithium silicate phase has Li2SiO3 (Z = 1) or Li2Si2O5 (Z = 1 / 2) as a main component.

[0048] SiO can be produced by the following steps. (1) Mix Si and silicon oxide in a weight ratio of, for example, 20:80 to 95:5 to prepare a mixture. (2) At least before or after the preparation of the above mixture, pulverize Si and silicon oxide by, for example, a ball mill to make fine particles. (3) Heat-treat the pulverized mixture at 600 to 1000 °C in, for example, an inert atmosphere.

[0049] In addition, in the above steps, LSX can be produced by using lithium silicate instead of silicon oxide.

[0050] The negative electrode composite material layer 42 preferably contains a binder. Examples of the binder include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, polyacrylic acid (hereinafter referred to as PAA) or its salt, styrene-butadiene rubber, carboxymethyl cellulose (hereinafter referred to as CMC) or its salt, and the like.

[0051] The content of the binder in the negative electrode composite material layer 42 is preferably, for example, 0.5% by mass to 10% by mass, more preferably 1% by mass to 5% by mass, based on the total amount of the negative electrode composite material layer 42.

[0052] Since styrene-butadiene rubber is a substance that affects the water contact angle of the second layer 46, when the negative electrode composite material layer 42 contains styrene-butadiene rubber, it is preferably contained more in the first layer 44 than in the second layer 46 in terms of making the water contact angle of the second layer 46 50° or less.

[0053] The negative electrode composite material layer 42 preferably contains fibrous carbon. By including fibrous carbon, a good conductive path is formed in the negative electrode composite material layer 42, and the charge-discharge cycle characteristics can be more effectively improved. The fibrous carbon may be contained in the same amount in the first layer 44 and the second layer 46, or may be contained more in either one. However, when the alloying material is contained more in the second layer 46 than in the first layer 44, the fibrous carbon is preferably contained more in the second layer 46 than in the first layer 44 in terms of maintaining the conductive path to the alloying material.

[0054] Examples of the fibrous carbon include carbon nanotubes (CNT), carbon nanofibers, etc. The CNT may be not only single-layer CNT but also two-layer CNT, multi-layer CNT, and mixtures thereof. Also, the CNT may be vapor-grown carbon fiber. The fibrous carbon has, for example, a diameter of 2 nm to 20 μm and a total length of 0.03 μm to 500 μm. The content of the fibrous carbon in the negative electrode composite material layer 42 is preferably, for example, 0.01% by mass to 5% by mass, more preferably 0.5% by mass to 3% by mass, based on the total amount of the negative electrode composite material layer 42.

[0055] The thickness of the negative electrode composite material layer 42 is, for example, 30 μm to 100 μm, or 50 μm to 80 μm on one side of the negative electrode current collector 40. The thicknesses of the first layer 44 and the second layer 46 may be the same as or different from each other. However, in terms of easily obtaining an effect of improving charge and discharge cycle characteristics, etc., the thickness of the second layer 46 is preferably 1 / 3 or more of the thickness of the negative electrode composite material layer 42, and more preferably in the range of 1 / 3 to 1 / 2.

[0056] As described above, an intermediate layer may be provided between the first layer 44 and the second layer 46. The intermediate layer may contain the above-described graphite particles A, graphite particles B, and alloying material, or may contain other conventionally known negative electrode active materials or the like. In any case, the intermediate layer may be designed as long as the effects of the present disclosure are not impaired.

[0057] The negative electrode 12 is manufactured, for example, by the following method. A first negative electrode composite material slurry for the first layer 44 containing graphite particles B, a binder, etc. is prepared. Also, a second negative electrode composite material slurry for the second layer 46 containing graphite particles A, a binder, etc. is prepared. Then, the first negative electrode composite material slurry is applied onto the negative electrode current collector 40, and the coating film is dried to form the first layer 44 on the negative electrode current collector 40. Next, the second negative electrode composite material slurry is applied onto the first layer 44, and the coating film is dried to form the second layer 46 on the first layer 44, and then the first layer 44 and the second layer 46 are compressed. In this way, a negative electrode 12 in which a negative electrode composite material layer 42 including the first layer 44 and the second layer 46 is formed on the negative electrode current collector 40 is obtained.

[0058] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, an olefin-based resin such as polyethylene, polypropylene, or a copolymer containing at least one of ethylene and propylene, cellulose, etc. is suitable. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0059] [Electrolyte solution] The electrolyte solution contains a solvent and an electrolyte salt. As the electrolyte salt, for example, lithium salts such as LiBF4 and LiPF6 are used. As the solvent, for example, esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP), ethers, nitriles, amides, and mixed solvents of two or more of these are used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine.

[0060] Examples of the halogen-substituted product include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

Example

[0061] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0062] <Example 1> [Positive electrode] 90 parts by mass of lithium cobaltate as a positive electrode active material, 5 parts by mass of graphite as a conductive material, and 5 parts by mass of polyvinylidene fluoride powder as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a current collector made of aluminum foil (thickness 15 μm) by the doctor blade method, and after the coating film was dried, the coating film was compressed by a rolling roller to produce a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector.

[0063] [Preparation of graphite particles A] The coke was pulverized until the average particle size (median diameter D50) reached 15 μm, pitch as a binder was added to the pulverized coke, and the coke was aggregated until the average particle size (median diameter D50) reached 17 μm. After this aggregate was calcined at a temperature of 2800 °C to be graphitized, sieving was performed using a 250-mesh sieve to obtain graphite particles A with an average particle size (median diameter D50) of 26 μm.

[0064] [Production of graphite particles B] The coke was pulverized until the average particle size (median diameter D50) reached 15 μm, pitch as a binder was added to the pulverized coke and aggregated, and then, under an isotropic pressure of 1.6 g / cm 3 ~1.9 g / cm 3 a block-shaped compact having a density of was formed. After this block-shaped compact was calcined at a temperature of 2800 °C to be graphitized, the block-shaped compact was pulverized, sieving was performed using a 250-mesh sieve to obtain graphite particles B with an average particle size (median diameter D50) of 19 μm.

[0065] [Production of the negative electrode] Using graphite particles B as the negative electrode active material, these were mixed such that the mass ratio of graphite particles B:CMC:styrene-butadiene rubber was 100:1:1, an appropriate amount of water was added, and a first negative electrode composite material slurry for the first layer was prepared. Also, using a mixture obtained by mixing such that graphite particles A were 86 parts by mass and the Si compound (SiO) was 14 parts by mass as the negative electrode active material, these were mixed such that the mass ratio of negative electrode active material:CMC:styrene-butadiene rubber was 100:1:1, an appropriate amount of water was added, and a second negative electrode composite material slurry for the second layer was prepared.

[0066] The first negative electrode composite material slurry was applied to both sides of a negative electrode current collector made of copper foil, the coating film was dried to form a first layer on both sides of the negative electrode current collector. Next, the second negative electrode composite material slurry was applied on the first layer formed on both sides of the negative electrode current collector, the coating film was dried to form a second layer. Then, the coating film was rolled using a roller to produce a negative electrode in which a negative electrode composite material layer including the first layer and the second layer was formed on both sides of the negative electrode current collector. The density of the negative electrode composite material layer was 1.6 g / cc, and the thickness ratio of the second layer to the first layer was 1:1.

[0067] When the water contact angle of the second layer in the fabricated negative electrode was measured, it was 31°. Since the measurement method is as described above, it is omitted.

[0068] When the internal porosity of graphite particles A and B in the fabricated negative electrode was measured, it was 5% and 22% respectively. The same internal porosity of the particles was also found in the following Examples and Comparative Examples. Since the measurement method is as described above, it is omitted.

[0069] With respect to the total volume of the second layer, the volume ratio of graphite particles A in the second layer was 86% by volume, and the volume ratio of the Si compound in the second layer was 14% by volume. Since the graphite particles and the Si compound used were equivalent, the masses of the graphite particles and the Si compound materials charged into the negative electrode composite slurry corresponded directly to the volumes of the graphite particles and the Si compound materials. That is, the above volume % is synonymous with mass %.

[0070] [Electrolyte] Vinylene carbonate (VC) was added at 1% by mass to a mixed solvent in which ethylene carbonate (EC), fluoroethylene carbonate (FEC), and diethyl carbonate (DEC) were mixed at a volume ratio of 27:3:70, and LiPF6 was dissolved at a ratio of 1.2 mol / L to prepare an electrolyte.

[0071] [Test cell] The positive electrode and the negative electrode were laminated so as to face each other with a separator interposed therebetween, and this was wound to produce an electrode body. Next, the electrode body and the above electrolyte were housed in a battery case body having a bottomed cylindrical shape, and after injecting the above electrolyte, the opening of the battery case body was sealed with a gasket and a sealing body to produce a test cell.

[0072] [Example 2] A test cell was produced in the same manner as in Example 1, except that in the preparation of the second negative electrode composite slurry, a mixture in which 29 parts by mass of graphite particles A, 57 parts by mass of graphite particles B, and 14 parts by mass of the Si compound were mixed was used as the negative electrode active material.

[0073] The water contact angle of the second layer in the fabricated negative electrode was 50°. Also, with respect to the total volume of the second layer, the volume ratio of graphite particles A in the second layer was 29% by volume, the volume ratio of graphite particles B in the second layer was 57% by volume, and the volume ratio of the Si compound in the second layer was 14% by volume.

[0074] <Example 3> In the preparation of the second negative electrode composite slurry, a test cell was fabricated in the same manner as in Example 1, except that these were mixed so that the mass ratio of the negative electrode active material to CMC was 100:1 (that is, styrene-butadiene rubber was not added). The water contact angle of the second layer in the fabricated negative electrode was 28°.

[0075] <Example 4> A test cell was fabricated in the same manner as in Example 1, except that the thickness ratio of the second layer to the first layer was 1:2. The water contact angle of the second layer in the fabricated negative electrode was 31°.

[0076] <Example 5> In the preparation of the second negative electrode composite slurry, a test cell was fabricated in the same manner as in Example 1, except that these were mixed so that the mass ratio of the negative electrode active material to CMC to styrene-butadiene rubber to CNT was 100:1:1:1. The water contact angle of the second layer in the fabricated negative electrode was 31°.

[0077] <Comparative Example 1> A mixture obtained by mixing 93 parts by mass of graphite particles B and 7 parts by mass of an Si compound (SiO) was used as the negative electrode active material, and these were mixed so that the mass ratio of the negative electrode active material to CMC to styrene-butadiene rubber was 100:1:1, and an appropriate amount of water was added to adjust the negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil, and after drying the coating film, the coating film was rolled using a roller to fabricate a negative electrode in which negative electrode composite layers were formed on both sides of the negative electrode current collector. Using this negative electrode, a test cell was fabricated in the same manner as in Example 1. The water contact angle of the negative electrode composite layer in the fabricated negative electrode was 119°. With respect to the total volume of the negative electrode composite layer, the volume ratio of the Si compound in the negative electrode composite layer was 7% by volume.

[0078] <Comparative Example 2> In the preparation of the first negative electrode composite slurry, a mixture obtained by mixing 86 parts by mass of graphite particles A and 14 parts by mass of Si compound (SiO) was used as the negative electrode active material. A test cell was fabricated in the same manner as in Example 1, except that in the preparation of the second negative electrode composite slurry, graphite particles B were used as the negative electrode active material.

[0079] The water contact angle of the second layer in the fabricated negative electrode was 119°. With respect to the total volume of the first layer, the volume ratio of graphite particles A in the first layer was 86% by volume, and the volume ratio of the Si compound in the first layer was 14% by volume.

[0080] <Comparative Example 3> In the preparation of the second negative electrode composite slurry, a mixture obtained by mixing 86 parts by mass of graphite particles B and 14 parts by mass of Si compound (SiO) was used as the negative electrode active material. A test cell was fabricated in the same manner as in Example 1, except for this.

[0081] The water contact angle of the second layer in the fabricated negative electrode was 110°. With respect to the total volume of the second layer, the volume ratio of the Si compound in the second layer was 14% by volume.

[0082] <Comparative Example 4> In the preparation of the second negative electrode composite slurry, a mixture obtained by mixing 21.5 parts by mass of graphite particles A, 64.5 parts by mass of graphite particles B, and 14 parts by mass of Si compound (SiO) was used as the negative electrode active material. A test cell was fabricated in the same manner as in Example 1, except for this.

[0083] The water contact angle of the second layer in the fabricated negative electrode was 103°. With respect to the total volume of the second layer, the volume ratio of graphite particles A in the second layer was 21.5%, and the volume ratio of the Si compound in the second layer was 14% by volume.

[0084] [Evaluation of capacity retention rate at 200 cycles] The test cell was charged at a constant current of 0.5C until the battery voltage reached 4.2V under a temperature environment of 25°C, and then charged at a constant voltage until the current value reached 1 / 50C at 4.2V. Subsequently, constant current discharge was performed at a constant current of 1.0C until the battery voltage reached 2.5V. Also, this charge and discharge were performed 200 cycles, and based on the following formula, the capacity retention rate in the charge and discharge cycles was obtained.

[0085] Capacity retention rate = (discharge capacity at the 200th cycle / discharge capacity at the 4th cycle) × 100 Table 1 shows the evaluation results (capacity retention rate at 200 cycles) for the test cells of Examples 1 to 5 and Comparative Examples 1 to 4.

[0086]

Table 1

[0087] The test cells of Examples 1 to 5 had higher capacity retention rates in the charge and discharge cycles than the test cells of Comparative Examples 1 to 4, and the charge and discharge cycle characteristics were improved.

[0088] From these results, it can be said that by using a negative electrode in which the second layer contains graphite particles A with an internal particle porosity of 10% or less, the first layer contains graphite particles B with an internal particle porosity exceeding 10%, and the water contact angle of the second layer is 50° or less, the charge and discharge cycle characteristics of the lithium-ion secondary battery are improved.

Explanation of symbols

[0089] 10 Lithium-ion secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 Case body 17 Sealing body 18 Insulating plate 18,19 Insulating plate 20 Positive electrode lead 21 Negative electrode lead 22 Protrusion 23 Filter 24 Lower valve body 25 Insulating member 26 Upper valve body 27 Cap 28 Gasket 30 Graphite particles 34 Internal void 36 External void 40 Negative electrode current collector 42 Negative electrode composite layer 44 First layer 46 Second layer

Claims

1. A negative electrode current collector and a negative electrode composite material layer formed on the negative electrode current collector, The negative electrode composite material layer includes a first layer disposed on the negative electrode current collector and a second layer disposed on the first layer, The second layer contains graphite particles A with an internal particle porosity of 10% or less, and the first layer contains graphite particles B with an internal particle porosity of more than 10%, The water contact angle of the second layer is 50° or less, The negative electrode composite material layer contains fibrous carbon and an alloying material that alloys with lithium, The fibrous carbon and the alloying material are each contained more in the second layer than in the first layer, The content of the graphite particles B contained in the first layer is in the range of 50% by mass or more and 90% by mass with respect to the total amount of the graphite particles B in the negative electrode composite material layer. A negative electrode for a lithium-ion secondary battery.

2. The volume ratio of the graphite particles A in the second layer to the total volume of the second layer is 29% by volume or more. The negative electrode for a lithium-ion secondary battery according to Claim 1.

3. The negative electrode composite material layer contains styrene-butadiene rubber, The styrene-butadiene rubber is contained more in the first layer than in the second layer. The negative electrode for a lithium-ion secondary battery according to any one of Claims 1 to 2.

4. The styrene-butadiene rubber contains 90% by mass or more and 100% by mass of all the styrene-butadiene rubber contained in the negative electrode composite material layer in the region on the negative electrode current collector side half. The negative electrode for a lithium-ion secondary battery according to Claim 3.

5. The thickness of the second layer is 1 / 3 or more of the thickness of the negative electrode composite material layer. The negative electrode for a lithium-ion secondary battery according to any one of Claims 1 to 4.

6. A lithium-ion secondary battery including the negative electrode for a lithium-ion secondary battery according to any one of Claims 1 to 5.

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