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

Abstract

To provide a negative electrode for lithium-ion secondary batteries capable of improving the charge-discharge cycle characteristics of lithium-ion secondary batteries.SOLUTION: A negative electrode 12 in an embodiment of the disclosure includes a negative electrode current collector 40 and a negative electrode composite material layer 42 formed over the negative electrode current collector 40. The negative electrode composite material layer 42 includes a first layer 44 placed on the negative electrode current collector 40 and a second layer 46 placed on the first layer 44. The second layer 46 includes the internal porosity of the particles, which includes graphite particles A, and the first layer 44 contains graphite particles B. The graphite particles A and B have different internal porosities. The water contact angle of the second layer 46 is equal to or less than 50°. The negative electrode composite material layer 42 contains an alloying material that alloys fibrous carbon with lithium. The fibrous carbon and the alloying material are each contained in a larger amount in the second layer 46 than in the first layer 44. The content of the graphite particles B contained in the first layer 44 is in the range of 50% by mass or more and 90% by mass of the total amount of the graphite particles B in the negative electrode composite material layer 42.SELECTED DRAWING: Figure 2

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 technology]

[0002] 2. Description of the Related Art A widely used secondary battery is, for example, a lithium ion secondary battery which includes a positive electrode, a negative electrode, and an electrolyte solution and performs charging and discharging by transferring lithium ions between the positive electrode and the negative electrode.

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

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-41407 Summary of the Invention [Problem to be solved by the invention]

[0005] However, lithium ion secondary batteries have a problem of deterioration in 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 that can improve the charge / discharge cycle characteristics of the lithium ion secondary battery. [Means for solving the problem]

[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 mixture layer formed on the negative electrode current collector, the negative electrode mixture layer including a first layer disposed on the negative electrode current collector and a second layer disposed on the first layer, the second layer including graphite particles A with an internal particle porosity, the first layer including graphite particles B with an internal particle porosity different from that of the graphite particles A and B, the second layer having a water contact angle of 50° or less, and the negative electrode mixture layer including fibrous carbon Reach and an alloying material that alloys with lithium, wherein the fibrous carbon and the alloying material are contained in the second layer in larger amounts than in the first layer, and the content of the graphite particles B contained in the first layer is 50 mass % or more and in the range of 90 mass % with respect to the total amount of the graphite particles B in the negative electrode mixture layer.

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

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

[0010] [Figure 1] 1 is a cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view of a negative electrode according to an embodiment of the present invention. [Figure 3] FIG. 2 is a schematic diagram showing a cross section of a graphite particle. DETAILED DESCRIPTION OF THE INVENTION

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

[0012] As disclosed herein, by disposing graphite particles A having an internal particle porosity of 10% or less in the second layer, which is the surface side of the negative electrode, and by setting the water contact angle of the second layer to 50° or less, pressure loss when the electrolyte flows into the negative electrode is reduced. As a result, the electrolyte easily penetrates the negative electrode, reducing lithium ion resistance and improving the charge-discharge cycle characteristics of the lithium ion secondary battery. Note that the surface side of the negative electrode refers to the surface facing the separator and positive electrode. Furthermore, graphite particles B having an internal particle porosity of more than 10% are easily crushed during negative electrode production, and therefore have high adhesion between the negative electrode current collector and the graphite particles B. Therefore, by disposing graphite particles B having an internal particle porosity of more than 10% in the first layer disposed on the negative electrode current collector, peeling of the negative electrode active material particles from the negative electrode current collector is suppressed, thereby improving the charge-discharge cycle characteristics of the lithium ion secondary battery.

[0013] Hereinafter, embodiments of a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to the present disclosure will be described in detail with reference to the drawings. In this specification, the expression "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 according to an embodiment. The lithium-ion secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, an electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, an electrode assembly of another shape may be used, such as a laminated electrode assembly formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include a cylindrical, prismatic, coin-shaped, or button-shaped metal case, and a resin case (laminated battery) formed by laminating a resin sheet.

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

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for 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 centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the lithium-ion secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

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

[0018] The positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte that constitute the lithium ion secondary battery 10 will be described in detail below.

[0019] [Positive electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on its surface. The positive electrode composite layer contains, for example, a positive electrode active material, a binder, and a conductive material. The positive electrode composite 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 composite slurry containing a positive electrode active material, a binder, a conductive material, etc., onto the positive electrode current collector, drying and rolling the coating, and forming a positive electrode composite 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 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, and Ge. 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 conductive materials contained in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode composite layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethylcellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0022] [Negative electrode] Fig. 2 is a cross-sectional view of a negative electrode according to one embodiment. Negative electrode 12 shown in Fig. 2 includes a negative electrode current collector 40 and a negative electrode composite layer 42 formed on negative electrode current collector 40. For negative electrode current collector 40, for example, a foil of a metal that is 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 mixture layer 42 formed on the negative electrode current collector 40 includes 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 mixture layer 42 is preferably formed on both sides of the negative electrode current collector 40. Note that the phrase "disposed" of the second layer 46 "on" the first layer 44 means that the second layer 46 may be disposed directly 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 the negative electrode active material, graphite particles B having an internal particle porosity of more than 10%. The second layer 46 contains, as the 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 order to improve 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 be more than 10%, in order to be appropriately crushed during the compression step in negative electrode production, etc., but is preferably 12% to 25%, and more preferably 12% to 23%.

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

[0026] Fig. 3 is a schematic diagram showing the cross section of a graphite particle. As shown in Fig. 3, a graphite particle 30 has, in a cross section of the graphite particle 30, closed voids 34 that do not connect from the interior of the particle to the particle surface, and voids 36 that connect from the interior of the particle to the particle surface. The voids 34 will be referred to hereinafter as internal voids 34. The voids 36 will be referred to hereinafter as external voids 36. In this embodiment, the internal porosity of a graphite particle is a two-dimensional value calculated from the ratio of the area of ​​the internal voids of the graphite particle to the cross-sectional area of ​​the graphite particle, and is specifically calculated by the following procedure.

[0027] <Method for measuring internal porosity> (1) Exposing a cross section of the negative electrode active material. For example, a method for exposing the cross section includes cutting out a part of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode composite layer. (2) A backscattered electron image of the cross section of the exposed negative electrode composite layer is taken using a scanning electron microscope at a magnification of 3,000 to 5,000 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-sections are colored white. (4) From the binarized image, the area of ​​the graphite particle cross section and the area of ​​the internal voids present in the graphite particle cross section are calculated. Here, the area of ​​the graphite particle cross section refers to the area of ​​the region surrounded by the outer periphery of the graphite particle, i.e., the area of ​​the entire cross section of the graphite particle. Furthermore, for voids present in the graphite particle cross section that are 3 μm or less in width, it may be difficult to distinguish between internal and external voids in image analysis, so voids with a width of 3 μm or less may be considered internal voids. Then, from the calculated area of ​​the graphite particle cross section and the area of ​​the internal voids in the graphite particle cross section, the internal porosity of the graphite particle (internal porosity of graphite particle = area of ​​internal voids in graphite particle cross section × 100 / area of ​​graphite particle cross section) is calculated. The internal porosity of the graphite particle is the average value for 10 graphite particles.

[0028] In order to improve charge-discharge cycle characteristics, the amount of graphite particles B contained in first layer 44 is preferably greater than the amount of graphite particles B contained in second layer 46, and is preferably in the range of 50 mass % to 90 mass % of the total amount of graphite particles B in negative electrode mixture layer 42. First layer 44 may contain, as the negative electrode active material, graphite particles A having an internal particle porosity of 10% or less; however, in order to improve charge-discharge cycle characteristics, the content of graphite particles A in first layer 44 is preferably 10 mass % or less of the total amount of graphite particles A in negative electrode mixture layer 42.

[0029] In order to improve charge-discharge cycle characteristics, the amount of graphite particles A contained in second layer 46 is preferably greater than the amount of graphite particles A contained in first layer 44, and is preferably in the range of 40 mass % to 100 mass % of the total amount of graphite particles A in negative electrode mixture layer 42. Second layer 46 may contain graphite particles B as the negative electrode active material, but in order to improve charge-discharge cycle characteristics, the content of graphite particles B in second layer 46 is preferably 50 mass % or less of the total amount of graphite particles B in negative electrode mixture layer 42.

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

[0031] <Graphite particles A with internal porosity of 10% or less> For example, coke (precursor), which is the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then fired at a temperature of 2600°C or higher to graphitize the particles, followed by sieving to obtain graphite particles A of a desired size. Here, the internal porosity can be adjusted to 10% or less by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the average particle size (median diameter D50) of the crushed precursor is preferably in the range of 12 μm to 20 μm.

[0032] <Graphite particles B with internal porosity exceeding 10%> For example, the coke (precursor) that is the main raw material is crushed to a predetermined size, agglomerated with a binder, and then pressed into a block shape. This block is then fired at a temperature of 2600°C or higher to be graphitized. The graphitized block is then crushed and sieved to obtain graphite particles B of the desired size. The internal porosity can be adjusted to more than 10% by adjusting the amount of volatile components added to the block.

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

[0034] The graphite particles A and B used in this embodiment may be natural graphite, artificial graphite, or the like, but are not particularly limited thereto. However, artificial graphite is preferred in terms of ease of adjusting the internal porosity. The interplanar spacing (d 002) is, for example, preferably 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. The crystallite size (Lc(002)) of the graphite particles A and B used in this embodiment, determined by X-ray diffraction, is, for example, preferably 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 refers to the volume average particle size (Dv50) at which the volume integrated value becomes 50% in the particle size distribution measured by laser diffraction scattering.

[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 order to facilitate penetration of the electrolyte into the negative electrode 12 and improve the charge-discharge cycle characteristics.

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

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

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

[0039] The packing density of the negative electrode mixture layer 42 (or the packing density of the second layer 46) is, for example, 1.50 g / cm 3 in order to make the water contact angle of the second layer 46 50° or less. 3 ~1.65g / cm 3 The range of 1.4 g / cm is preferred.3 ~1.5g / cm 3 The range is more preferable.

[0040] The negative electrode mixture layer 42 may contain an alloying material as the negative electrode active material. The inclusion of an alloying material enables the lithium-ion secondary battery to have a high capacity. The alloying material may be contained in the same amount in the first layer 44 and the second layer 46, or in a greater amount in either layer. However, in order to prevent a decrease in the charge-discharge cycle characteristics of the lithium-ion secondary battery, it is preferable that the alloying material be contained in a greater amount 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% to 100% by mass with respect to the total amount of the alloying material in the negative electrode mixture layer 42. Note that, if the proportion of the alloying material in the negative electrode mixture layer 42 is high, the effect of improving the charge-discharge cycle characteristics is reduced. 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 mixture layer 42. The lower limit of the content of the alloying material is preferably 5 mass % or more, and more preferably 8 mass % or more, of the total amount of the negative electrode active material in the negative electrode composite layer 42, in order to increase the capacity of the lithium ion secondary battery.

[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 elements that alloy with lithium and can be used as the negative electrode active material include Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, and Bi. Among these, Si and Sn are preferred, with Si being particularly preferred, from the viewpoint of achieving high capacity.

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

[0043] Furthermore, a conductive layer made of a highly conductive material may be formed on the surface of SiO and LSX particles. An example of a suitable conductive layer is a carbon coating made of a carbon material. The carbon coating may be made of, for example, carbon black, acetylene black, ketjen black, graphite, or a mixture of two or more of these. Examples of methods for carbon-coating the surfaces of SiO and LSX particles include CVD using acetylene, methane, or the like, and methods in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with SiO or LSX particles and heat-treated. Alternatively, a carbon coating may be formed by adhering carbon powder such as carbon black to the particle surface using a binder.

[0044] The preferred SiO has a sea-island structure in which fine Si particles are dispersed uniformly in an amorphous silicon oxide phase, and is represented by the general formula SiO x It is expressed as (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 mass % with respect to the total mass of SiO.

[0045] The average particle size of the Si particles dispersed in the silicon oxide phase is generally 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less, before charge / discharge. After charge / discharge, the average particle size is preferably 400 nm or less, and more preferably 100 nm or less. By miniaturizing the Si particles, the volume change during charge / discharge is reduced, improving cycle characteristics. The average particle size 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 is specifically determined as the average of the longest diameters of 100 Si particles. The silicon oxide phase is composed of, for example, an aggregate of particles finer than Si particles.

[0046] Suitable LSXs have 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 based on the total mass of LSX, similar to the case of SiO. Also, the average particle size of the 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 the 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, which 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 the 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, for example, by a ball mill to make fine particles. (3) Heat-treat the pulverized mixture at 600 to  1000 °C in an inert atmosphere, for example.

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

[0050] Negative electrode mixture 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, PAA) or a salt thereof, styrene-butadiene rubber, and carboxymethyl cellulose (hereinafter, CMC) or a salt thereof.

[0051] The content of the binder in negative electrode mixture layer 42 is, for example, preferably 0.5 mass % to 10 mass % relative to the total amount of negative electrode mixture layer 42, and more preferably 1 mass % to 5 mass %.

[0052] Styrene-butadiene rubber is a substance that affects the water contact angle of the second layer 46. Therefore, when styrene-butadiene rubber is contained in the negative electrode mixture layer 42, it is preferable that the amount of styrene-butadiene rubber be greater in the first layer 44 than in the second layer 46, in order to make the water contact angle of the second layer 46 50° or less.

[0053] The negative electrode mixture layer 42 preferably contains fibrous carbon. The inclusion of fibrous carbon forms a good conductive path in the negative electrode mixture layer 42, and the charge / discharge cycle characteristics can be more effectively improved. The first layer 44 and the second layer 46 may contain the same amount of fibrous carbon, or the first layer 44 and the second layer 46 may contain more fibrous carbon than the second layer 46. However, when the second layer 46 contains more alloying material than the first layer 44, it is preferable that the second layer 46 contain more fibrous carbon than the first layer 44 in order to maintain a conductive path to the alloying material.

[0054] Examples of fibrous carbon include carbon nanotubes (CNTs) and carbon nanofibers. The CNTs may be not only single-walled CNTs but also double-walled CNTs, multi-walled CNTs, and mixtures thereof. The CNTs may also be vapor-grown carbon fibers. The fibrous carbon has a diameter of 2 nm to 20 μm and a total length of 0.03 μm to 500 μm, for example. The content of the fibrous carbon in negative electrode mixture layer 42 is preferably 0.01 mass % to 5 mass %, and more preferably 0.5 mass % to 3 mass %, relative to the total amount of negative electrode mixture layer 42.

[0055] The thickness of the negative electrode mixture 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, but from the viewpoint of easily obtaining an effect of improving the charge-discharge cycle characteristics, the thickness of the second layer 46 is preferably at least 1 / 3 of the thickness of the negative electrode mixture 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, or alloying material, or may contain other conventionally known negative electrode active materials. In any case, the intermediate layer may be designed within a range that does not impair the effects of the present disclosure.

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

[0058] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include olefin-based resins such as polyethylene, polypropylene, and copolymers containing at least one of ethylene and propylene, and cellulose. 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] The electrolyte solution contains a solvent and an electrolyte salt. Examples of the electrolyte salt include lithium salts such as LiBF4 and LiPF6. Examples of the solvent include 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. The non-aqueous solvent may contain a halogen-substituted solvent in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0060] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylates such as methyl fluoropropionate (FMP). [Example]

[0061] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0062] Example 1 [Positive electrode] A positive electrode mixture slurry was prepared by mixing 90 parts by weight of lithium cobalt oxide as the positive electrode active material, 5 parts by weight of graphite as the conductive material, and 5 parts by weight of polyvinylidene fluoride powder as the binder, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). This slurry was applied to both sides of a current collector made of aluminum foil (thickness 15 μm) by doctor blade coating. After drying, the coating was compressed with a rolling roller to produce a positive electrode with positive electrode active material layers formed on both sides of the positive electrode current collector.

[0063] [Preparation of graphite particles A] Coke was pulverized to an average particle size (median diameter D50) of 15 μm, and pitch was added as a binder to the pulverized coke, followed by agglomeration to an average particle size (median diameter D50) of 17 μm. The agglomerates were graphitized by firing at a temperature of 2800°C, and then sieved using a 250-mesh sieve to obtain graphite particles A with an average particle size (median diameter D50) of 26 μm.

[0064] [Preparation of graphite particles B] The coke was crushed to an average particle size (median diameter D50) of 15 μm, and pitch was added to the crushed coke as a binder to agglomerate it. Then, the coke was further agglomerated under an isotropic pressure of 1.6 g / cm. 3 ~1.9g / cm 3 This block-shaped compact was graphitized by firing at a temperature of 2800°C, and then pulverized and sieved using a 250 mesh sieve to obtain graphite particles B with an average particle size (median diameter D50) of 19 µm.

[0065] [Preparation of negative electrode] Graphite particles B were used as the negative electrode active material, and the graphite particles B:CMC:styrene butadiene rubber mass ratio was 100:1:1. An appropriate amount of water was added to prepare a first negative electrode composite slurry for the first layer. A mixture of 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, and the mixture was mixed so that the negative electrode active material:CMC:styrene butadiene rubber mass ratio was 100:1:1. An appropriate amount of water was added to prepare a second negative electrode composite slurry for the second layer.

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

[0067] The water contact angle of the second layer of the produced negative electrode was measured and found to be 31.degree.. The measurement method was as described above and will not be described again.

[0068] In the produced negative electrode, the intra-particle porosity of graphite particles A and B was measured and found to be 5% and 22%, respectively. The following examples and comparative examples also had the same intra-particle porosity. The measurement method is the same as described above, so it will not be repeated here.

[0069] The volume ratio of graphite particles A in the second layer was 86 volume % and the volume ratio of Si compound in the second layer was 14 volume % relative to the total volume of the second layer. Because the graphite particles and Si compound used were equivalent, the mass of the graphite particles and Si compound material added to the negative electrode composite slurry directly corresponds to the volume of the graphite particles and Si compound material. In other words, the above volume % is synonymous with mass %.

[0070] [Electrolyte] An electrolyte solution was prepared by adding 1 mass% of vinylene carbonate (VC) to a mixed solvent of ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), and diethyl carbonate (DEC) in a volume ratio of 27:3:70, and dissolving LiPF6 at a ratio of 1.2 mol / L.

[0071] [Test cell] The positive electrode and the negative electrode were stacked facing each other with a separator interposed therebetween and wound up to prepare an electrode assembly. The electrode assembly and the above-mentioned electrolyte solution were then housed in a cylindrical battery case body with a bottom, and after the above-mentioned electrolyte solution was poured into the battery case body, the opening of the battery case body was sealed with a gasket and a sealing member to prepare a test cell.

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

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

[0074] Example 3 A test cell was fabricated in the same manner as in Example 1, except that in preparing the second negative electrode composite slurry, the negative electrode active material and CMC were mixed so that the mass ratio was 100:1 (i.e., 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 produced in the same manner as in Example 1, except that the thickness ratio of the second layer:first layer was set to 1:2. The water contact angle of the second layer in the produced negative electrode was 31°.

[0076] <Example 5> A test cell was produced in the same manner as in Example 1, except that in preparing the second negative electrode mixture slurry, the negative electrode active material:CMC:styrene butadiene rubber:CNT were mixed so that the mass ratio thereof was 100:1:1:1. The water contact angle of the second layer in the produced negative electrode was 31°.

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

[0078] <Comparative Example 2> A test cell was produced in the same manner as in Example 1, except that in the preparation of the first negative electrode composite slurry, a mixture of 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, and in the preparation of the second negative electrode composite slurry, graphite particles B was used as the negative electrode active material.

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

[0080] <Comparative Example 3> A test cell was produced in the same manner as in Example 1, except that in preparing the second negative electrode composite slurry, a mixture of 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.

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

[0082] <Comparative Example 4> A test cell was produced in the same manner as in Example 1, except that in preparing the second negative electrode composite slurry, a mixture of 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.

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

[0084] [Evaluation of capacity retention rate after 200 cycles] The test cell was charged at a constant current of 0.5 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 1 / 50 C at 4.2 V. It was then discharged at a constant current of 1.0 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 200 times, and the capacity retention rate during the charge / discharge cycle was calculated using the following formula.

[0085] Capacity retention rate = (discharge capacity at 200th cycle / discharge capacity at 4th cycle) x 100 Table 1 shows the evaluation results (capacity maintenance 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 charge-discharge cycles than the test cells of Comparative Examples 1 to 4, and thus had improved charge-discharge cycle characteristics.

[0088] From these results, it can be said that the charge-discharge cycle characteristics of a lithium-ion secondary battery were improved by using a negative electrode in which the second layer contained graphite particles A with an internal particle porosity of 10% or less, the first layer contained graphite particles B with an internal particle porosity of more than 10%, and the second layer had a water contact angle of 50° or less. [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 plates, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 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 mixture layer formed on the negative electrode current collector, the negative electrode mixture 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 and has an internal porosity of the first layer, the graphite particles A and the graphite particles B have different internal porosities; the second layer has a water contact angle of 50° or less; the negative electrode mixture layer includes fibrous carbon and an alloying material that forms an alloy with lithium, the fibrous carbon and the alloying material are contained in the second layer in larger amounts than in the first layer, a content of the graphite particles B in the first layer is in the range of 50 mass % or more and 90 mass % with respect to a total amount of the graphite particles B in the negative electrode mixture layer.

2. the negative electrode mixture layer contains styrene-butadiene rubber, 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the styrene-butadiene rubber is contained in a larger amount in the first layer than in the second layer.

3. 3. The negative electrode for a lithium ion secondary battery according to claim 2, wherein the styrene-butadiene rubber contained in the negative electrode mixture layer accounts for 90% by mass or more and 100% by mass or less of all the styrene-butadiene rubber contained in the negative electrode mixture layer in a half region on the negative electrode current collector side.

4. 4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the thickness of the second layer is at least one-third of the thickness of the negative electrode mixture layer.

5. 5. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the graphite particles A have an internal porosity lower than that of the graphite particles B.

6. 6. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the graphite particles A have an internal porosity of 1% or more and 5% or less.

7. 7. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the graphite particles B have an internal porosity of 12% or more and 25% or less.

8. 6. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the graphite particles A have an internal porosity of 1% or more and 5% or less, and the graphite particles B have an internal porosity of 12% or more and 25% or less.

9. The BET specific surface area of ​​the graphite particles A is 1.0 m ２ / g or more, 1.6m ２ / g or less, and the BET specific surface area of ​​the graphite particles B is 3.0 m ２ / g or more, 20m ２ The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the SiO2 content is 0.15g / g or less.

10. The content of the alloying material is 5% by mass or more and 15% by mass or less with respect to the total amount of the negative electrode active material in the negative electrode mixture layer. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 9.

11. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to any one of claims 1 to 10.

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