Negative electrode of a secondary battery, method for manufacturing the negative electrode, and secondary battery using the negative electrode

Abstract

To provide a negative electrode containing Si-containing particles and graphite particles and capable of suppressing capacity degradation in repetitive charge and discharge of a secondary battery.SOLUTION: A negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles. A Si content ratio in the first Si-containing particles is higher than a Si content ratio in the second Si-containing particles. The first Si-containing particles each are coated with a first resin binder, and the second Si-containing particles each are coated with a second resin binder. The Tg of the first resin binder is lower than the Tg of the second resin binder. The Tg of the first resin binder is less than 80°C.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a negative electrode of a secondary battery and a method for manufacturing the same. The present invention also relates to a secondary battery using the negative electrode.

Background Art

[0002] In recent years, secondary batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and power sources for vehicle drive such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In the application of power sources for vehicle drive, particularly in the application of power sources for BEV drive, from the viewpoint of extending the cruising range of vehicles, further increase in the capacity of secondary batteries is desired. As a high-capacity negative electrode active material, Si-containing particles are known, and it is known that the use of Si-containing particles can increase the capacity of secondary batteries (see, for example, Patent Document 1). Patent Document 1 discloses a technique of using, as a negative electrode active material, Si-containing particles in combination with graphite particles such as natural graphite.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while Si-containing particles have a high capacity, they exhibit a large volume change due to expansion / contraction during charge and discharge of the secondary battery. Therefore, when Si-containing particles and graphite particles are used in combination, especially when the proportion of Si-containing particles is large, the fillability of these particles decreases when the secondary battery is repeatedly charged and discharged, resulting in disconnection of the conductive path and generation of internal stress. Therefore, when Si-containing particles and graphite particles are used in combination, there is a problem that the cycle characteristics of the secondary battery deteriorate. Specifically, there is a problem that the capacity degradation is large when the secondary battery is repeatedly charged and discharged.

[0006] In view of the above circumstances, an object of the present invention is to provide a negative electrode containing Si-containing particles and graphite particles, which can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

Means for Solving the Problems

[0007] The negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles. The Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles. The first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder. The Tg of the first resin binder is lower than the Tg of the second resin binder. The Tg of the first resin binder is less than 80°C.

[0008] According to such a configuration, it is possible to provide a negative electrode containing Si-containing particles and graphite particles, which can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0009] From another aspect, the method for manufacturing a negative electrode of a secondary battery disclosed herein includes a step of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, where the Si content ratio in the first Si-containing particles is higher than that in the second Si-containing particles, the Tg of the first resin binder is lower than the Tg of the second resin binder, and the Tg of the first resin binder is less than 80°C; a step of mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste; a step of applying the negative electrode paste onto a negative electrode current collector; and a step of drying the applied negative electrode paste.

[0010] According to the negative electrode obtained with such a configuration, excellent capacity degradation resistance can be imparted when the secondary battery is repeatedly charged and discharged.

[0011] From another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the above-described negative electrode.

[0012] According to such a configuration, a secondary battery having excellent capacity degradation resistance when repeatedly charged and discharged can be provided.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that matters that are not mentioned in this specification but are necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the relevant field. In the following drawings, members and parts having the same function are denoted by the same reference numerals and will be described. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, the numerical range expressed as "A to B" includes A and B.

[0015] In this specification, the "secondary battery" refers to a rechargeable power storage device. Also, in this specification, the "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0016] The negative electrode disclosed herein is used in a secondary battery, and preferably used in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0017] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0018] As shown in the figure example, a non-formed portion 62a of the negative electrode active material layer where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the non-formed portion 62a of the negative electrode active material layer, the negative electrode current collector 62 is exposed, and the non-formed portion 62a of the negative electrode active material layer can function as a current collecting portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0019] The shape of the negative electrode current collector 62 is foil-like (or sheet-like) in the figure example, but is not limited thereto. The negative electrode current collector 62 may be in various forms such as rod-shaped, plate-shaped, mesh-shaped, etc. As the material of the negative electrode current collector 62, a metal with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used as in a conventional lithium ion secondary battery, and among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

[0020] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 6 μm or more and 20 μm or less.

[0021] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, at least graphite particles, first Si-containing particles with a high Si content, and second Si-containing particles with a low Si content are used. This will be described in detail with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view showing the particles contained in the negative electrode active material layer 64 shown in FIG. 1. As shown in FIG. 2, the negative electrode active material layer 64 includes graphite particles 12, first Si-containing particles 14 with a high Si content, and second Si-containing particles 16 with a low Si content.

[0022] The graphite constituting the graphite particles 12 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form in which the graphite is coated with an amorphous carbon material.

[0023] The shape of the graphite particles 12 is not particularly limited and may be flaky, spherical, or the like. The graphite particles 12 are preferably spherical graphite particles. When the graphite particles 12 are spherical, the roundness of the graphite particles 12 is preferably from 0.85 to 1, more preferably from 0.88 to 1, and still more preferably from 0.90 to 1.

[0024] In this specification, the "roundness" refers to the ratio of the circumference of a true circle having the same area as the projected area of the particle to the circumference of the particle projection image (that is, roundness = circumference of a true circle having the same area as the projected area of the particle / circumference of the particle projection image). Therefore, the closer the roundness is to 1, the closer the particle projection image is to a true circle, and the closer the particle is to a perfect sphere. The roundness can be determined, for example, by using a commercially available static automatic image analyzer to determine the roundness of 100 or more particles and calculating the average value thereof.

[0025] The average particle diameter (D50) of the graphite particles 12 is not particularly limited. The average particle diameter (D50) of the graphite particles 12 is, for example, from 1 μm to 30 μm, preferably from 5 μm to 25 μm, more preferably from 10 μm to 23 μm, and still more preferably from 12 μm to 20 μm.

[0026] In this specification, the "average particle diameter (D50)" refers to the median diameter (D50), and in the volume-based particle size distribution based on the laser diffraction / scattering method, it refers to the particle diameter corresponding to 50 volume% of the cumulative frequency from the side of fine particles with a small particle diameter. The average particle diameter (D50) can be determined using a commercially available laser diffraction / scattering type particle size distribution measuring device or the like.

[0027] The content ratio of the graphite particles with respect to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably from 40% by mass to 90% by mass, more preferably from 45% by mass to 85% by mass, and still more preferably from 50% by mass to 80% by mass.

[0028] As the first Si-containing particles 14 and the second Si-containing particles 16, for example, particles of a Si-C composite material can be used. The Si-C composite material typically includes a carbon domain and a Si-containing domain. Note that the first Si-containing particles 14 and the second Si-containing particles 16 do not have to be a Si-C composite material, and may be Si particles, Si oxide particles, or the like.

[0029] The carbon domain is, for example, a carbide of a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite or the like. The carbon domain preferably constitutes a carbon matrix. Therefore, the Si-C composite material is preferably a material in which a plurality of Si-containing domains are dispersed in a carbon matrix. In this case, it is advantageous because the carbon matrix can relieve the volume change due to the expansion / contraction of the Si-containing domain.

[0030] The Si-containing domain contains Si and is composed of, for example, Si, Si oxide (SiO x ), Si nitride (SiNx), Si carbide (SiCx), etc. The Si-containing domain is preferably composed of at least one of Si and Si oxide (SiO x ). The Si-containing domain may be fine particles.

[0031] The average particle diameter of the Si-containing domain is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domain" can be obtained as follows. First, the negative electrode active material layer 64 is processed by FIB (focused ion beam) to prepare a sample for scanning transmission electron microscope (STEM) observation. Then, after elemental analysis of the sample by EDX elemental mapping, a BF image (bright field image) and a HAADF image (high angle annular dark field image) are acquired. The diameter of the Si-containing domain can be determined from the contrast and shape obtained from the BF image and the HAADF image. The diameters of 10 or more arbitrarily selected Si-containing domains are determined, and the average value thereof is defined as the "average particle diameter of the Si-containing domain" here.

[0032] The Si-C composite material is, for example, one in which fine particles containing Si are dispersed inside a carbon material; one in which fine particles containing Si have entered into the pores of granulated porous graphite; and the like.

[0033] In the present embodiment, the Si content ratio (S1) in the first Si-containing particles 14 is higher than the Si content ratio (S2) in the second Si-containing particles 16. The Si content ratio (S1) in the first Si-containing particles 14 and the Si content ratio (S2) in the second Si-containing particles 16 are not particularly limited as long as this relationship is satisfied. If these Si content ratios are too low, the effect of improving the cycle characteristics becomes small, and there is a possibility that the effect of increasing the capacity of the secondary battery becomes low. On the other hand, if these Si content ratios are too high, when the secondary battery is repeatedly charged and discharged, the volume change due to the expansion / contraction of the first Si-containing particles 14 and the second Si-containing particles 16 may become too large.

[0034] Therefore, the Si content ratio (S1) in the first Si-containing particles 14 is preferably 45% by mass to 80% by mass, more preferably 55% by mass to 75% by mass. The Si content ratio (S2) in the second Si-containing particles 16 is preferably 20% by mass to 55% by mass, more preferably 25% by mass to 45% by mass.

[0035] Also, the ratio (S1 / S2) of the Si content ratio (S1) in the first Si-containing particles 14 to the Si content ratio (S2) in the second Si-containing particles 16 is preferably 1.2 or more, more preferably 1.5 or more, and still more preferably 1.8 or more. The ratio (S1 / S2) may be 4 or less, 3.5 or less, 3 or less, or 2.5 or less.

[0036] The mass ratio between the first Si-containing particles 14 and the second Si-containing particles 16 is not particularly limited as long as the effects of the present invention can be obtained. In order to obtain a better filling state between the first Si-containing particles 14 and the second Si-containing particles 16, the mass ratio of the first Si-containing particles 14 to the second Si-containing particles 16 is preferably 10:90 to 60:40, more preferably 15:85 to 55:45, and still more preferably 20:80 to 45:55.

[0037] The total content ratio of the first Si-containing particles 14 and the second Si-containing particles 16 to the total of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 is preferably 10% by mass to 60% by mass, more preferably 15% by mass to 55% by mass, and still more preferably 20% by mass to 50% by mass.

[0038] The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 16 is not particularly limited. The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 16 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm, respectively.

[0039] As shown in FIG. 2, the first Si-containing particles 14 are coated with the first resin binder 15. The second Si-containing particles 16 are coated with the second resin binder 17. In the present specification, the "resin binder" refers to a resin component that binds the negative electrode active material particles to each other and the negative electrode active material particles to the negative electrode current collector 62.

[0040] In the illustrated example, the entire first Si-containing particles 14 and the entire second Si-containing particles 16 are each coated with a first resin binder 15 and a second resin binder 17. Thus, the first resin binder 15 and the second resin binder 17 form a coating layer. However, the first Si-containing particles 14 and the second Si-containing particles may each be partially coated with the first resin binder 15 and the second resin binder 17. The coating rate of the first Si-containing particles 14 by the first resin binder 15 is preferably 50% to 100%, more preferably 70% to 100%. The coating rate of the second Si-containing particles 16 by the second resin binder 17 is preferably 50% to 100%, more preferably 70% to 100%. Note that this coating rate is the ratio of the coating area of the resin binder to the surface area of the Si-containing particles. The coating rate can be obtained by acquiring a cross-sectional electron microscope image of the Si-containing particles and calculating the percentage of the total length of the resin binder coating portions on the surface of the Si-containing particles with respect to the outer perimeter length of the Si-containing particles. The average value of the coating rates of five or more arbitrarily selected particles can be adopted as the "coating rate" here.

[0041] In this embodiment, the Tg (glass transition temperature) of the first resin binder 15 is lower than the Tg of the second resin binder 17. In addition, the Tg of the first resin binder 15 is less than 80°C. Therefore, the Tg of the second resin binder 17 is 80°C or higher.

[0042] Thus, in addition to the graphite particles 12, the first Si-containing particles 14 and the second Si-containing particles 16 having different Si contents are used in combination. The first Si-containing particles 14 having a high Si content are coated with the low-Tg first resin binder 15, and the second Si-containing particles 16 having a low Si content are coated with the high-Tg second resin binder 17, thereby suppressing capacity deterioration when the secondary battery is repeatedly charged and discharged. The reason is considered as follows.

[0043] When a secondary battery is repeatedly charged and discharged, the cause of capacity degradation is that when the secondary battery is repeatedly charged and discharged, the volume change due to the expansion / contraction of Si-containing particles is large, resulting in displacement of the Si-containing particles and disconnection of the conductive path. Here, Tg is the temperature at which when the resin is heated (i.e., when thermal energy is applied to the resin), the interaction between molecular chains weakens and the segments of the molecular chains start micro-Brownian motion. Therefore, it can be said that the higher the Tg, the stronger the interaction between molecular chains. For this reason, Tg serves as an index of the ease / difficulty of resin deformation with respect to stress. A resin binder with a Tg of less than 80°C is easily deformed with respect to the stress caused by the expansion / contraction of Si, and a resin binder with a Tg of 80°C or higher is difficult to deform with respect to the stress caused by the expansion / contraction of Si. Therefore, by coating the first Si-containing particles 14 with a high Si content with the first resin binder 15 having a Tg of less than 80°C, the first resin binder 15 can follow the expansion / contraction of Si, and displacement of the first Si-containing particles 14 due to internal stress caused by volume change can be suppressed. Furthermore, by coating the second Si-containing particles 16 with a low Si content with a resin binder having a Tg of 80°C or higher, the expansion / contraction of Si can be suppressed, and the occurrence of disconnection of the conductive path can be suppressed. Thereby, capacity degradation during repeated charge and discharge of the secondary battery can be suppressed.

[0044] Examples of the resin binder having a Tg of less than 80°C used for the first resin binder 15 include polyvinylidene fluoride, polyvinyl alcohol, polyethylene oxide, polylactic acid, etc. From the viewpoint of higher cycle characteristics of the secondary battery using the negative electrode 60, the Tg of the first resin binder 15 is preferably 70°C or lower, more preferably 60°C or lower, and even more preferably 50°C or lower. The Tg of the first resin binder 15 may be -100°C or higher, or 0°C or higher.

[0045] Examples of the resin binder having a Tg of 80°C or higher used for the second resin binder 17 include polyacrylic acid, carboxymethyl cellulose, polyamideimide, polyacrylonitrile, tetrafluoroethylene, polyvinylpyrrolidone, and the like. From the viewpoint of higher cycle characteristics of the secondary battery using the negative electrode 60, the Tg of the second resin binder 17 is preferably 100°C or higher, more preferably 150°C or higher, and still more preferably 200°C or higher. The Tg of the second resin binder 17 may be 400°C or lower, or 350°C or lower.

[0046] Note that the Tg of the first resin binder 15 and the second resin binder 17 can be determined by differential scanning calorimetry (DSC measurement).

[0047] The coating amount of the first Si-containing particles 14 with the first resin binder 15 is not particularly limited. Since the first resin binder 15 is usually insulating, if the coating amount is too large, the battery resistance may increase. On the other hand, if the coating amount is too small, the effects of the present invention may be reduced. Therefore, the coating amount of the first Si-containing particles 14 with the first resin binder 15 is preferably 5% by mass to 75% by mass, more preferably 10% by mass to 50% by mass, and still more preferably 15% by mass to 40% by mass. Similarly, the coating amount of the second Si-containing particles 16 with the second resin binder 17 is not particularly limited, but is preferably 5% by mass to 75% by mass, more preferably 10% by mass to 50% by mass, and still more preferably 15% by mass to 40% by mass. Note that the coating amount is the ratio (%) of the mass of the resin binder to the mass of the Si-containing particles.

[0048] Note that the first Si-containing particles 14 and the second Si-containing particles 16 can be produced according to known methods. Various production methods of particles of the Si-C composite material are known (for example, refer to JP-A-2015-38862, WO 2014 / 046144, and the prior art documents cited in the international publication).

[0049] The negative electrode active material layer 64 may contain components other than the negative electrode active material, and examples thereof include a third resin binder, a conductive material, etc. By using the third resin binder, it is possible to improve the binding property between the negative electrode active material particles and the negative electrode current collector 62, improve the binding property between the resin binders coating the Si-containing particles, etc. As the third resin binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), etc. can be used. CMC also functions as a thickening agent. Examples of the conductive material include carbon black such as acetylene black, carbon fiber, carbon nanotube (CNT), etc. Among them, CNT is preferable. When using CNT as the conductive material, the negative electrode active material layer 64 may contain a dispersant for CNT.

[0050] The content of the negative electrode active material in the negative electrode active material layer 64 (that is, relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the third resin binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0051] The thickness of the negative electrode active material layer 64 is not particularly limited, but for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0052] The density of the negative electrode active material layer 64 is not particularly limited, but for example, it is 0.7 g / cm 3 or more, preferably 1.0 g / cm 3 or more, more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 or less, and may be 2.0 g / cm 3 or less.

[0053] The negative electrode 60 may include members other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the non-formation portion 62a of the negative electrode active material layer. The insulating layer contains, for example, an insulating inorganic filler or the like.

[0054] The method for manufacturing the negative electrode 60 preferably includes a step of preparing first Si-containing particles 14 coated with a first resin binder 15 and second Si-containing particles 16 coated with a second resin binder 17 (hereinafter also referred to as the "coated particle preparation step"), where the Si content ratio in the first Si-containing particles 14 is higher than the Si content ratio in the second Si-containing particles 16, the Tg of the first resin binder 15 is lower than the Tg of the second resin binder 17, and the Tg of the first resin binder 15 is less than 80°C; a step of mixing the first Si-containing particles 14 coated with the first resin binder 15, the second Si-containing particles 16 coated with the second resin binder 17, and the graphite particles 12 in a dispersion medium to prepare a negative electrode paste (hereinafter also referred to as the "paste preparation step"), a step of coating the negative electrode paste on the negative electrode current collector 62 (hereinafter also referred to as the "coating step"), and a step of drying the coated negative electrode paste (hereinafter also referred to as the "drying step").

[0055] In this specification, the term "paste" refers to a mixture in which part or all of the solid content is dispersed in a dispersion medium, and includes so-called "slurry", "ink", etc.

[0056] In the coated particle preparation step, the first Si-containing particles 14 with a high Si content ratio and the second Si-containing particles 16 with a low Si content ratio are prepared. Also, a first resin binder with a Tg less than 80°C and a second resin binder with a Tg of 80°C or higher are prepared.

[0057] For the coating of the first Si-containing particles 14 and the second Si-containing particles 16 with the first resin binder and the second resin binder, known coating methods can be adopted. For example, a solution in which the first resin binder 15 is dissolved in a solvent and a solution in which the second resin binder 17 is dissolved in a solvent are prepared, the first Si-containing particles 14 and the second Si-containing particles 16 are respectively added thereto, and if necessary, the solvent is removed by drying to perform the coating.

[0058] The paste preparation process can be carried out by mixing graphite particles 12, the first Si-containing particles 14 coated with the first resin binder 15, the second Si-containing particles 16 coated with the second resin binder 17, and optional components (e.g., conductive material, third resin binder, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc. according to a known method.

[0059] The coating process can be carried out according to a known method. Specifically, for example, the obtained negative electrode paste is coated on the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, die coater, etc. to perform the coating process.

[0060] The drying process can be carried out according to a known method. Specifically, for example, the dispersion medium is removed from the negative electrode current collector 62 coated with the negative electrode paste using a drying device such as a drying furnace to form the negative electrode active material layer 64. Thereby, the drying process can be carried out. The drying temperature and drying time may be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or longer and 30 minutes or shorter, preferably 30 seconds or longer and 10 minutes or shorter.

[0061] After the drying process, a step of pressing the negative electrode active material layer 64 may be further performed. The pressing step can be carried out according to a known method. Specifically, the pressing step can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. By the pressing step, the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16 contained in the negative electrode active material layer 64 can be densely filled. In this way, the negative electrode 60 can be obtained.

[0062] According to the negative electrode 60 according to the present embodiment, excellent capacity degradation resistance can be imparted when the secondary battery is repeatedly charged and discharged. In addition, since the negative electrode 60 according to the present embodiment uses a negative electrode active material containing Si, the secondary battery can be made to have a high capacity. Therefore, the secondary battery using the negative electrode 60 according to the present embodiment has a high capacity and excellent cycle characteristics.

[0063] Therefore, from another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to FIGS. 3 and 4, taking a lithium ion secondary battery as an example. The following configuration example is a flat rectangular lithium ion secondary battery having a flat wound electrode body and a flat battery case.

[0064] The lithium-ion secondary battery 100 shown in FIG. 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an exterior container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0065] As shown in FIGS. 3 and 4, the wound electrode body 20 has a form in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped via two long separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of a long negative electrode current collector 62. The non-formed portion 52a of the positive electrode active material layer (i.e., the portion where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and the non-formed portion 62a of the negative electrode active material layer (i.e., the portion where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the non-formed portion 52a of the positive electrode active material layer and the non-formed portion 62a of the negative electrode active material layer, respectively.

[0066] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in a lithium-ion secondary battery may be used. Examples thereof include a sheet or foil made of a metal having good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.). As the positive electrode current collector 52, an aluminum foil is preferred.

[0067] The dimensions of the positive electrode current collector 52 are not particularly limited and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but for example, it is 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0068] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in a lithium-ion secondary battery may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound, etc. can be used. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, etc.

[0069] As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferred. Specific examples thereof include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, etc.

[0070] In this specification, the term "lithium nickel cobalt manganese composite oxide" includes, in addition to oxides composed of Li, Ni, Co, Mn, and O as constituent elements, oxides containing one or more additional elements other than these. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, etc. Further, the additional elements may be semi-metal elements such as B, C, Si, P, etc., or non-metal elements such as S, F, Cl, Br, I, etc. This also applies to the above-mentioned lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, etc.

[0071] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium manganese iron phosphate, etc.

[0072] These cathode active materials may be used alone or in combination of two or more. As the cathode active material, a lithium nickel cobalt manganese composite oxide is particularly preferable because of its excellent various properties such as initial resistance characteristics.

[0073] The average particle diameter (D50) of the cathode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0074] The cathode active material layer 54 may contain components other than the cathode active material, for example, trilithium phosphate, a conductive material, a binder, etc. As the conductive material, for example, carbon black such as acetylene black (AB); carbon fibers such as vapor-grown carbon fiber (VGCF), carbon nanotube (CNT); and other carbon materials (e.g., graphite, etc.) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) etc. can be used.

[0075] The content of the positive electrode active material in the positive electrode active material layer 54 (that is, the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and still more preferably 85% by mass or more and 99% by mass or less. The content of lithium tripolyphosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0076] The thickness per side of the positive electrode active material layer 54 is not particularly limited, but is usually 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, preferably 300 μm or less.

[0077] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0078] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0079] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0080] The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in the electrolytes of general lithium-ion secondary batteries, can be used without particular limitation. Among them, carbonates are preferred, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), etc. Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent consists only of carbonates. As another example, the non-aqueous solvent contains carbonates and esters such as methyl acetate.

[0081] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0082] In addition, the above non-aqueous electrolyte may contain components other than the above-described components, such as film-forming agents such as vinylene carbonate (VC) and oxalato complex; gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and various other additives, as long as the effects of the present invention are not significantly impaired.

[0083] The lithium-ion secondary battery 100 has suppressed capacity degradation when repeatedly charged and discharged, and also has a high capacity. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power sources for driving mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Further, the lithium-ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium-ion secondary battery 100 can typically also be used in the form of a battery pack formed by connecting a plurality of them in series and / or in parallel.

[0084] As described above, as an example, the rectangular lithium-ion secondary battery 100 including the flat wound electrode body 20 has been described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a laminated electrode body (i.e., an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Further, the lithium-ion secondary battery can also be configured as a cylindrical lithium-ion secondary battery, a laminated case type lithium-ion secondary battery, or the like.

[0085] Also, according to a known method, the lithium-ion secondary battery 100 can be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0086] Further, the negative electrode 60 according to the present embodiment is suitable for the negative electrode of a lithium-ion secondary battery, but can be constructed and used as the negative electrode of other secondary batteries, and the other secondary batteries can be configured according to known methods.

[0087] Hereinafter, examples of the present invention will be described in detail, but the present invention is not intended to be limited to those shown in such examples.

[0088] <Fabrication of Negative Electrode> [Example 1] As the negative electrode active material, the following were prepared. The Si content ratios of the first Si-containing particles and the second Si-containing particles were measured using a commercially available ICP analyzer. The average particle diameters (D50) of the first Si-containing particles, the second Si-containing particles, and the graphite particles were measured using a commercially available laser diffraction / scattering particle size distribution analyzer. First Si-containing particles: Si-C composite material, Si content ratio = 65% by mass, average particle diameter (D50) = 6 μm Second Si-containing particles: Si-C composite material, Si content ratio = 35% by mass, average particle diameter (D50) = 7 μm Graphite particles: average particle diameter (D50) = 14 μm

[0089] As the first resin binder, polyvinyl alcohol (Tg = 40°C) was prepared. As the second resin binder, polyamideimide (Tg = 280°C) was prepared. Solutions in which these were dissolved in a solvent were obtained. The Tg values of the first resin binder and the second resin binder were measured using a commercially available differential scanning calorimeter.

[0090] The first Si-containing particles, the first resin binder solution, and the solvent were mixed using a disperser at 3000 rpm. Thus, particles in which the surface of the first Si-containing particles was coated with the first resin binder were obtained. Similarly, the second Si-containing particles, the second resin binder solution, and the solvent were mixed using a disperser at 3000 rpm. Thus, particles in which the surface of the second Si-containing particles was coated with the second resin binder were obtained.

[0091] As the conductive material, single-walled carbon nanotubes (SWCNTs) were prepared. The SWCNTs were prepared in the form of a dispersion. As the binders, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared.

[0092] A negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, first resin binder, second resin binder, CMC, PAA, SBR, and SWCNTs in a mass ratio of 60:12:28:3:7:1:1:1.5:0.1 was prepared by the following procedure.

[0093] First, graphite particles, CMC, and PAA were dry-blended using a planetary mixer. An SWCNT dispersion and a dispersion medium were added to the obtained mixture and kneaded using a planetary mixer. To the obtained kneaded product, first Si-containing particles coated with a first resin binder, second Si-containing particles coated with a second resin binder, and a dispersion medium were added and mixed using a planetary mixer. Further, SBR and a dispersion medium were charged into a planetary mixer and diluted and mixed to obtain a negative electrode paste.

[0094] The prepared negative electrode paste was applied to the surface of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. After roll-pressing the negative electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a negative electrode sheet.

[0095] [Example 2] A negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polylactic acid (Tg = 60°C) and the second resin binder was changed to polyacrylic acid (Tg = 110°C).

[0096] [Comparative Example 1] A negative electrode sheet of Comparative Example 1 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polyamideimide (Tg = 280°C) and the second resin binder was changed to polyvinyl alcohol (Tg = 40°C).

[0097] [Comparative Example 2] A negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1, except that the second resin binder was changed to polyvinyl alcohol (Tg = 40°C).

[0098] [Comparative Example 3] A negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that the first resin binder was changed to polyamideimide (Tg = 280°C).

[0099] [Comparative Example 4] A negative electrode sheet of Comparative Example 4 was obtained in the same manner as in Example 1, except that the mass ratio of the solid content of the negative electrode paste was changed to graphite particles: first Si-containing particles: second Si-containing particles: first resin binder: second resin binder: CMC: PAA: SBR: SWCNT = 60:40:0:10:0:1:1:1.5:0.1.

[0100] [Comparative Example 5] A negative electrode sheet of Comparative Example 5 was obtained in the same manner as in Example 1, except that the mass ratio of the solid content of the negative electrode paste was changed to graphite particles: first Si-containing particles: second Si-containing particles: first resin binder: second resin binder: CMC: PAA: SBR: SWCNT = 60:0:40:0:10:1:1:1.5:0.1.

[0101] <Fabrication of Lithium-Ion Secondary Battery for Evaluation> LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM: AB: PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll-pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0102] A separator made of porous polyolefin was prepared. Leads were attached to each of the above-prepared negative electrode sheet and positive electrode sheet, and they were laminated via the separator to fabricate an electrode assembly. This was housed together with a non-aqueous electrolyte in a case made of an aluminum laminate film. As the non-aqueous electrolyte, a solution in which LiPF6 as a supporting salt was dissolved at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40 was used. Then, the case was sealed to obtain a lithium-ion secondary battery for evaluation.

[0103] <Cycle characteristic evaluation> Each of the fabricated evaluation lithium ion secondary batteries was placed in an environment at 25°C. Each evaluation lithium ion secondary battery was subjected to constant current charging up to 4.2 V at a current value of 0.4 C, and then constant voltage charging was performed until the current value reached 0.1 C. Next, each evaluation lithium ion secondary battery was discharged at a constant current of 0.4 C until 2.5 V. And the discharge capacity at this time was measured to obtain the initial capacity.

[0104] The charge and discharge described above was repeated 250 cycles with one cycle of charge and discharge. The discharge capacity after 250 cycles was determined in the same manner as the initial capacity. As an index of the cycle characteristics, the capacity retention rate (%) was determined from (discharge capacity after 250 cycles of charge and discharge / initial capacity) × 100. The results are shown in Table 1.

[0105]

Table 1

[0106] From the results in Table 1, it can be seen that when the Si content ratio in the first Si-containing particles is higher than that in the second Si-containing particles, the first Si-containing particles are coated with a resin binder having a Tg of less than 80°C, and the second Si-containing particles are coated with a resin binder having a Tg of 80°C or higher, the capacity retention rate after 250 cycles of charge and discharge is significantly high. Therefore, according to the negative electrode disclosed herein, it can be seen that capacity degradation when the secondary battery is repeatedly charged and discharged can be suppressed.

[0107] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

[0108] That is, the negative electrode of the secondary battery, its manufacturing method, and the secondary battery disclosed herein are as follows in items [1] to [7]. [1] A negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles, The Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, The first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder, The Tg of the first resin binder is lower than the Tg of the second resin binder, The Tg of the first resin binder is less than 80°C, Negative electrode. [2] The negative electrode according to item [1], wherein the Tg of the second resin binder is more than 100°C. [3] The negative electrode according to item [1], wherein the Tg of the first resin binder is 50°C or lower and the Tg of the second resin binder is 200°C or higher. [4] The content ratio of the graphite particles with respect to the total of the graphite particles, the first Si-containing particles, and the second Si-containing particles is 40% by mass to 90% by mass, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:40. The negative electrode according to any one of items [1] to [3]. [5] The negative electrode according to any one of items [1] to [4], wherein the first Si-containing particles and the second Si-containing particles are each particles of an Si-C composite material. [6] A step of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, where the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, the Tg of the first resin binder is lower than the Tg of the second resin binder, and the Tg of the first resin binder is less than 80°C, A step of mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste, A step of coating the negative electrode paste on a negative electrode current collector, and The step of drying the applied negative electrode paste A method for manufacturing a negative electrode including the above [7] A positive electrode, a negative electrode, an electrolyte, A secondary battery comprising: The secondary battery, wherein the negative electrode is the negative electrode according to any one of items [1] to [5].

Explanation of symbols

[0109] 12 Graphite particles 14 First Si-containing particles 15 First resin binder 16 Second Si-containing particles 17 Second resin binder 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where no positive electrode active material layer is formed 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Portion where no negative electrode active material layer is formed 64 Negative electrode active material layer 70 Separator sheet (separator) 100 Lithium ion secondary battery

Claims

1. A negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles, the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, the first Si-containing particles are coated with a first resin binder, and the second Si-containing particles are coated with a second resin binder, the Tg of the first resin binder is lower than the Tg of the second resin binder, the Tg of the first resin binder is less than 80°C, a negative electrode.

2. The negative electrode according to claim 1, wherein the Tg of the second resin binder is more than 100°C.

3. The negative electrode according to claim 1, wherein the Tg of the first resin binder is 50°C or lower, and the Tg of the second resin binder is 200°C or higher.

4. The negative electrode according to claim 1, wherein the content ratio of the graphite particles to the total of the graphite particles, the first Si-containing particles, and the second Si-containing particles is 40% by mass to 90% by mass, and the mass ratio of the first Si-containing particles to the second Si-containing particles is 10:90 to 60:

40.

5. The negative electrode according to claim 1, wherein the first Si-containing particles and the second Si-containing particles are each particles of an Si-C composite material.

6. A step of preparing first Si-containing particles coated with a first resin binder and second Si-containing particles coated with a second resin binder, wherein the Si content ratio in the first Si-containing particles is higher than the Si content ratio in the second Si-containing particles, the Tg of the first resin binder is lower than the Tg of the second resin binder, and the Tg of the first resin binder is less than 80°C, a step of mixing the first Si-containing particles coated with the first resin binder, the second Si-containing particles coated with the second resin binder, and graphite particles in a dispersion medium to prepare a negative electrode paste, a step of coating the negative electrode paste on a negative electrode current collector, and a step of drying the coated negative electrode paste, a method for manufacturing a negative electrode including these steps.

7. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to claim 1. ​

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