Negative electrode and power storage device including the same

Abstract

To suppress capacity degradation after charge-discharge cycles in power storage devices containing silicon-containing graphite particles as a negative electrode active material.SOLUTION: A negative electrode includes a negative electrode active material, which is a silicon-containing graphite particle, a first binder, and a second binder. The silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed within the voids. The first binder is a styrene-butadiene rubber having a glass transition temperature of 1°C or higher. The second binder is a styrene-butadiene rubber having a glass transition temperature of less than 1°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode and an electricity storage device including the negative electrode. [Background technology]

[0002] An example of an electricity storage device is a secondary battery such as a lithium-ion secondary battery. In recent years, this type of secondary battery has been suitably used as a portable power source for personal computers, mobile terminals, etc., and as a power source for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Japanese Patent Application Laid-Open Publication No. 2017-50142 discloses an anode active material for lithium-ion secondary batteries. This anode active material includes a plurality of flake graphite particles and flake silicon particles whose surfaces are coated with carbon. The plurality of flake graphite particles are aggregated to form particles. Silicon particles are present between the plurality of flake graphite particles. The publication states that this configuration can provide an anode active material for lithium-ion secondary batteries that has high capacity and long life.

[0004] Japanese Patent Application Laid-Open No. 2023-156006 discloses a negative electrode for a non-aqueous electrolyte secondary battery. The negative electrode includes an active material layer. The active material layer includes graphite particles, fibrous carbon, silicon-containing particles, and a binder. The BET specific surface area of ​​the graphite particles is 3.5 m. 2 / g or less. The fibrous carbon includes carbon nanotubes. The silicon-containing particles include domains composed of carbon and domains composed of silicon of 50 nm or less. The oxygen content in the silicon-containing particles is 7 wt% or less. The publication states that this configuration makes it possible to provide a negative electrode and a non-aqueous electrolyte secondary battery that suppresses decreases in initial capacity and cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-50142 [Patent Document 2] Japanese Patent Publication No. 2023-156006 Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors wish to suppress the capacity decrease after charge-discharge cycling in an electricity storage device that contains silicon-containing graphite particles as a negative electrode active material. [Means for solving the problem]

[0007] The technology disclosed herein provides a negative electrode. The negative electrode includes a negative electrode active material that is silicon-containing graphite particles, a first binder, and a second binder. The negative electrode includes graphite particles having voids and silicon-containing particles disposed in the voids. The first binder is styrene-butadiene rubber having a glass transition point of 1°C or higher. The second binder is styrene-butadiene rubber having a glass transition point of less than 1°C. This configuration can suppress capacity loss after charge-discharge cycles in an electricity storage device that includes silicon-containing graphite particles as the negative electrode active material. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. As shown in FIG. [Figure 2] FIG. 2 is a schematic diagram of the electrode body 20. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the energy storage device disclosed herein will be described below. The embodiment described herein does not particularly limit the technology disclosed herein. The technology disclosed herein is not limited to the embodiment described herein, unless otherwise specified. The drawings are schematic and do not necessarily reflect the actual product. Furthermore, members and parts that perform the same function are appropriately designated by the same reference numerals, and redundant explanations may be omitted. Furthermore, the notation "A to B" indicating a numerical range means "greater than or equal to A and less than or equal to B" unless otherwise specified, and also encompasses the meaning of "greater than A and less than B."

[0010] In this specification, the term "electricity storage device" refers to a device in which charging and discharging occur by the movement of charge carriers between a pair of electrodes (positive and negative electrodes) via an electrolyte. Electricity storage devices include secondary batteries such as lithium ion secondary batteries; and capacitors such as lithium ion capacitors and electric double layer capacitors. Below, an embodiment in which the electricity storage device is a lithium ion secondary battery will be described.

[0011] Fig. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. Fig. 2 is a schematic diagram of an electrode assembly 20. As shown in Fig. 1, the lithium-ion secondary battery 100 includes the electrode assembly 20, a case 30, and a non-aqueous electrolyte solution 80.

[0012] 1 and 2, the electrode assembly 20 is a wound electrode assembly in which a long, sheet-like positive electrode 50 and a long, sheet-like negative electrode 60 are stacked together with a long, sheet-like separator 70 interposed therebetween and wound in the sheet longitudinal direction (hereinafter simply referred to as the "longitudinal direction"). In the electrode assembly 20, the exposed region 52a of the positive electrode 50 and the exposed region 62a of the negative electrode 60 protrude outward from both ends in the lateral direction perpendicular to the longitudinal direction.

[0013] As shown in FIGS. 1 and 2, the positive electrode 50 includes a long sheet-like positive electrode collector 52 and a positive electrode active material layer 54. The positive electrode collector 52 is, for example, aluminum foil. In this embodiment, the positive electrode collector 52 has a region where the positive electrode active material layer 54 is provided and an exposed region 52a where the positive electrode active material layer 54 is not provided and the surface of the positive electrode collector 52 is exposed. The positive electrode active material layer 54 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides here) of the positive electrode collector 52. The positive electrode active material layer 54 is not provided at an end (the left end in the drawing) in the short-side direction of the sheet (hereinafter simply referred to as the "short-side direction"). Here, the exposed region 52a is a strip-shaped region at the end (the left end in the drawing) in the short-side direction. As shown in FIG. 1, the current collector plate 42a is attached to the exposed region 52a.

[0014] The positive electrode active material layer 54 contains, for example, a positive electrode active material. The positive electrode active material is not particularly limited as long as the effects of the technology disclosed herein are realized, and any positive electrode active material having a conventionally known composition used for this type of application can be used. The positive electrode active material may be, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like. 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, or the like.

[0015] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Examples of the lithium transition metal composite oxide include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more.

[0016] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of the 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, and Sn. The additional elements may also be metalloid elements, such as B, C, Si, and P; or nonmetallic elements, such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0017] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate. Examples of the positive electrode active material include LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4 and the like can be preferably used.

[0018] The positive electrode active material layer 54 may contain, in addition to the positive electrode active material, a conductive material, a binder, and the like. Examples of conductive materials include carbon black such as acetylene black (AB) and other carbon materials such as graphite. Examples of binders include polyvinylidene fluoride (PVDF). The content of the positive electrode active material relative to the entire positive electrode active material layer 54 is, for example, preferably 70 mass % or more, more preferably 80 mass % to 98 mass %, and even more preferably 85 mass % to 95 mass %. The content of the conductive material relative to the entire positive electrode active material layer 54 is, for example, 0.1 mass % to 20 mass %. The content of the binder relative to the entire positive electrode active material layer 54 is, for example, 0.5 mass % to 15 mass %.

[0019] As shown in FIGS. 1 and 2, the negative electrode 60 includes a long, sheet-like negative electrode current collector 62 and a negative electrode active material layer 64. The negative electrode current collector 62 is, for example, copper foil. In this embodiment, the negative electrode current collector 62 has a region where the negative electrode active material layer 64 is provided and an exposed region 62a where the negative electrode active material layer 64 is not provided and the surface of the negative electrode active material layer 64 is exposed. The negative electrode active material layer 64 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides in this case) of the negative electrode current collector 62. The negative electrode active material layer 64 is not provided at the end in the short side direction (the end on the right side in the figure). Here, the exposed region 62a is a strip-shaped region at the end in the short side direction (the end on the right side in the figure). As shown in FIG. 1, a current collector 44a is attached to the exposed region 62a.

[0020] The negative electrode active material layer 64 includes, for example, a negative electrode active material. In this embodiment, the negative electrode active material includes silicon-containing graphite particles. The silicon-containing graphite particles include, for example, graphite particles having voids and silicon-containing particles disposed in the voids. The voided graphite particles can function as base particles for the silicon-containing particles. Although not particularly limited, from the viewpoint of including a larger number of silicon-containing particles and realizing a high capacity, high energy density, etc. of the lithium-ion secondary battery 100, the voided graphite particles are preferably porous graphite particles. Alternatively, in another embodiment, the voided graphite particles may be aggregates of flake graphite.

[0021] The average particle diameter of the silicon-containing graphite particles is generally 0.1 μm to 20 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter is, for example, 0.3 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 2 μm or more. From the viewpoint of increasing the degree of filling in the negative electrode active material layer 64, the average particle diameter is, for example, 15 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. In this specification, the "average particle diameter" of particles refers to the particle diameter (D 50 The average particle size of the silicon-containing graphite particles may be, for example, the same as the average particle size of the porous graphite particles described above.

[0022] The silicon-containing particles contained in the silicon-containing graphite particles may be, for example, particles containing silicon and capable of functioning as a negative electrode active material. The silicon-containing particles may be, for example, silicon particles or silicon oxide particles. In this embodiment, the particle diameter of the silicon-containing particles is 1 μm or less. Although not particularly limited, from the viewpoint of containing more silicon-containing particles inside the graphite particles (for example, within the voids of the graphite particles), the particle diameter of the silicon-containing particles is generally 500 nm or less, for example, 250 nm or less, preferably 150 nm or less, more preferably 100 nm or less, and even more preferably 50 nm or less. From the same viewpoint, the particle diameter of the silicon-containing particles is, for example, 1 nm or more, preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. The particle diameter of the silicon-containing particles can be determined, for example, by observing the plane of the silicon-containing particles with an electron microscope (SEM) to obtain an SEM image, randomly selecting a plurality of silicon-containing particles (e.g., 10 to 100) from the SEM image, calculating the particle diameter (circle-equivalent diameter) of each, and then calculating the arithmetic mean value thereof. Alternatively, the nominal value provided by the manufacturer or the like may be used.

[0023] Silicon-containing graphite particles can be obtained by known methods. For example, they can be obtained by mixing silicon-containing particles with a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.), followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated graphite substrate and silicon-containing particles in a dispersion medium, drying, and disposing the silicon-containing particles in the pores of the graphite particles as the substrate.

[0024] In this embodiment, the negative electrode active material further contains other graphite particles (hereinafter simply referred to as "other graphite particles"). The other graphite particles are, for example, graphite particles that are substantially free of silicon (preferably, graphite particles that are free of silicon). With respect to the other graphite particles, "substantially free of silicon" means that the silicon content in the other graphite particles is 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.3% by mass or less. The silicon content can be calculated by a conventional method such as ICP analysis. The other graphite particles may be, for example, artificial graphite or natural graphite. The other graphite particles may have an amorphous carbon coating layer on their surfaces. Although not particularly limited, the other graphite particles have, for example, a substantially spherical shape. In this specification, with respect to the other graphite particles, the term "substantially spherical shape" means that the other graphite particles have an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on SEM observation. The average aspect ratio can be determined, for example, by obtaining a planar SEM image of the other graphite particles, randomly selecting a plurality of graphite particles (e.g., 10 to 100) from the SEM image, calculating the aspect ratio of each, and then calculating the arithmetic mean value thereof. The average particle diameter of the other graphite particles may be, for example, 5 μm to 30 μm, or may be 15 μm to 25 μm.

[0025] Although not particularly limited, from the viewpoint of achieving a high capacity and a high energy density of the lithium-ion secondary battery 100, the negative electrode active material layer 64 may contain, for example, 0.3% by mass or more of the silicon-containing graphite particles, preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass. On the other hand, from the viewpoint of suppressing expansion and contraction of the negative electrode 60 during charging and discharging, the negative electrode active material layer 64 may contain, for example, 10% by mass or less of the silicon-containing graphite particles, preferably 7.5% by mass or less, and more preferably 5% by mass or less, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass.

[0026] The negative electrode active material layer 64 may contain a conductive material in addition to the negative electrode active material. Examples of the conductive material that can be used include carbon black such as acetylene black (AB), carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), and carbon fibers.

[0027] The ratio of the negative electrode active material to the entire negative electrode active material layer 64 is, for example, 70 mass % or more, preferably 80 mass % or more, more preferably 85 mass % to 99 mass %, and may be 90 mass % to 95 mass %. The ratio of the conductive material to the entire negative electrode active material layer 64 may be, for example, 0.01 mass % to 3 mass %.

[0028] In this embodiment, the negative electrode active material layer 64 contains a binder. The binder functions, for example, to bond negative electrode active material particles together and to hold the negative electrode active material on the surface of the negative electrode current collector 62. As described above, when silicon-containing graphite particles including graphite particles with voids are used as the negative electrode active material, for example, the binder may enter the voids in the silicon-containing graphite particles, which may suppress the function of the binder. For this reason, ingenuity was required to fully realize the function of the binder. Therefore, the present inventors investigated the type of binder to be used in the negative electrode active material layer 64. In the following description, the term "binder" is used to collectively refer to the entire binder contained in the negative electrode active material layer 64.

[0029] The negative electrode active material layer 64 includes a first binder and a second binder. The first binder is, for example, styrene butadiene rubber (SBR) having a glass transition point of 1°C or higher. The first binder has, for example, a structure in which a plurality of first binder particles are linked in a chain form. This structure can be confirmed, for example, by observing the cross section of the negative electrode active material layer 64 with an SEM. Therefore, the first binder is less likely to penetrate into the voids of the silicon-containing graphite particles, for example, and can link a plurality of negative electrode active material particles over a wider area. From the viewpoint of linking a larger number of negative electrode active material particles over a wider area, the glass transition point of the first binder is preferably 3°C or higher, more preferably 5°C or higher. From the same viewpoint, the glass transition point of the first binder is generally 30°C or lower, for example, 20°C or lower, preferably 15°C or lower, more preferably 12°C or lower, and even more preferably 10°C or lower.

[0030] The second binder is, for example, SBR having a glass transition point of less than 1°C. The second binder is, for example, composed of granular second binder particles. Here, the second binder particles are not interconnected and do not exhibit a structure. The shape and state of the second binder can be confirmed, for example, by observing the cross section of the negative electrode active material layer 64 with an SEM. The second binder has, for example, the function of enhancing adhesion between particles of the negative electrode active material. From the viewpoint of enhancing adhesion between particles of the negative electrode active material, the glass transition point of the second binder is preferably 0.7°C or lower, more preferably 0.5°C or lower. From the same viewpoint, the glass transition point of the second binder is generally −40°C or higher or −30°C or higher, for example, −20°C or higher or −10°C or higher, preferably −7°C or higher, more preferably −5°C or higher, and even more preferably −3°C or higher.

[0031] Although not particularly limited, from the viewpoint of better achieving the effects of the technology disclosed herein, it is preferable that the difference between the glass transition points of the first binder and the second binder be approximately 2°C or more. The difference between the glass transition points of the first binder and the second binder is, for example, 3°C or more, preferably 5°C or more, and more preferably 7°C or more. From the same viewpoint, the difference between the glass transition points of the first binder and the second binder is approximately 40°C or less or 30°C or less, for example, 20°C or less or 17°C or less, preferably 15°C or less, more preferably 13°C or less, and even more preferably 10°C or less.

[0032] The content of the first binder in the negative electrode active material layer 64 may be, for example, the same as, smaller than, or larger than the content of the second binder. Regarding the content ratio (mass ratio) of the first binder to the second binder in the negative electrode active material layer 64, the ratio (first binder:second binder) is, for example, 10:90 to 90:10, and preferably 30:70 to 90:10. While not particularly limited, from the viewpoint of better achieving the effects of the technology disclosed herein, the content of the first binder in the negative electrode active material layer 64 is preferably equal to or greater than the content of the second binder. In this case, the ratio (first binder:second binder) is preferably 50:50 to 90:10.

[0033] In this embodiment, the negative electrode active material layer 64 includes a third binder other than the first binder and the second binder. Examples of the third binder include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), SBR, and polyvinylidene fluoride (PVDF). Among others, carboxymethyl cellulose (CMC) is preferably used as the third binder. Note that the third binder, SBR, is an SBR that does not fall into either the first binder or the second binder.

[0034] When the entire binder in the negative electrode active material layer 64 is taken as 100% by mass, the total content of the first binder and the second binder is approximately 30% by mass to 100% by mass. From the viewpoint of improving the productivity of the negative electrode active material layer 64, and therefore the negative electrode 60, the lithium-ion secondary battery 100, etc., when the entire binder in the negative electrode active material layer 64 is taken as 100% by mass, the total content of the first binder and the second binder is, for example, 40% by mass to 90% by mass, preferably 50% by mass to 80% by mass, more preferably 50% by mass to 70% by mass, and even more preferably 55% by mass to 65% by mass. Note that when the entire negative electrode active material layer 64 is taken as 100% by mass, the total content of the binder may be, for example, 0.5% by mass to 10% by mass.

[0035] The separator 70 may be a porous sheet (film) made of a resin material such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. The 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.

[0036] The case 30 is, for example, an outer container that houses the electrode assembly 20 and the nonaqueous electrolyte 80. Here, the case 30 is a flat, rectangular case. As shown in FIG. 1 , the case 30 has a positive electrode terminal 42, a negative electrode terminal 44, a safety valve 36, and an inlet (not shown). The positive electrode terminal 42 is, for example, a terminal for external connection on the positive electrode side. Here, the positive electrode terminal 42 is electrically connected to the positive electrode 50 of the electrode assembly 20 via a current collector 42a. The negative electrode terminal 44 is, for example, a terminal for external connection on the negative electrode side. Here, the negative electrode terminal 44 is electrically connected to the negative electrode 60 of the electrode assembly 20 via a current collector 44a. The safety valve 36 is, for example, a thin-walled portion that is configured to release internal pressure in the case 30 when the internal pressure rises above a predetermined level. The inlet is, for example, a portion through which the nonaqueous electrolyte 80 is injected into the case 30.

[0037] The nonaqueous electrolyte 80 includes, for example, a nonaqueous solvent and a supporting salt. Examples of the nonaqueous solvent include organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in this type of application. Among these, carbonates are preferred. Examples of carbonates include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC) (preferably monofluoroethylene carbonate), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). As the nonaqueous solvent, one type of nonaqueous solvent may be used alone, or two or more types of nonaqueous solvents may be used in combination. Examples of the supporting salt include lithium salts such as LiPF6, LiBF4, and LiClO4. The concentration of the supporting electrolyte may be, for example, 0.7 mol / L to 1.4 mol / L. The nonaqueous electrolyte 80 may contain additives used in this type of application, as needed. Examples of additives include film-forming agents such as LiB(C2O4)2 (LiBOB) and LiBF2 (C2O4); gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; etc.

[0038] The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can be used, for example, as a storage battery for small-sized power storage devices. The lithium ion secondary battery 100 can also be used, for example, in the form of an assembled battery in which a plurality of batteries are connected in series and / or parallel.

[0039] As described above, the negative electrode 60 includes a negative electrode active material that is silicon-containing graphite particles, a first binder, and a second binder. The silicon-containing graphite particles include graphite particles containing silicon and graphite and having voids, and silicon-containing particles disposed within the voids. The first binder is SBR having a glass transition point of 1°C or higher. The second binder is SBR having a glass transition point of less than 1°C.

[0040] The negative electrode 60 includes silicon-containing graphite particles as the negative electrode active material, thereby achieving high capacity and high energy density. The silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed within the voids. This allows the voids to mitigate expansion and contraction of the silicon-containing particles during charging and discharging, thereby suppressing expansion and contraction of the negative electrode 60. The negative electrode 60 includes a first binder having a glass transition point of 1°C or higher. The first binder, for example, is less likely to penetrate into the voids of the silicon-containing graphite particles. Therefore, in the negative electrode 60, the first binder can, for example, connect multiple negative electrode active material particles over a wider area and prevent the binder from penetrating into the voids, thereby suppressing the binder's function and the obstruction of the conductive paths within the silicon-containing graphite particles. The negative electrode 60 includes a second binder having a glass transition point of less than 1°C. In the negative electrode 60, the second binder can increase the adhesiveness between particles of the negative electrode active material. Furthermore, by including both the first binder and the second binder, which have different glass transition points, the negative electrode 60 can bond more particles of the negative electrode active material over a wider range with higher adhesiveness, and can also increase the adhesiveness between the negative electrode active material layer 64 and the negative electrode current collector 62. This can suppress a decrease in capacity after charge-discharge cycles of an electricity storage device (here, a lithium-ion secondary battery 100) including the negative electrode 60.

[0041] The difference between the glass transition point of the first binder and the glass transition point of the second binder may be 5°C or more. This allows the functions of the first binder and the second binder to be more efficiently realized, respectively. This allows the effects of the technology disclosed herein to be more effectively realized.

[0042] The negative electrode 60 may contain the first binder in an amount equal to or greater than the amount of the second binder. This can effectively reduce the amount of binder that enters the voids of the silicon-containing graphite particles. This can further enhance the effects of the technology disclosed herein.

[0043] The silicon-containing particles may have a particle size of 1 μm or less, which can improve the conductivity of the negative electrode active material and achieve high output.

[0044] The negative electrode 60 may further contain graphite particles that are substantially free of silicon as another negative electrode active material, thereby improving the conductivity of the negative electrode active material layer 64.

[0045] In addition to the above-described negative electrode 60, the technology disclosed herein also provides a lithium ion secondary battery 100. The lithium ion secondary battery 100 includes the negative electrode 60. By including the negative electrode 60, the lithium ion secondary battery 100 is prevented from experiencing a decrease in capacity after charge / discharge cycles.

[0046] Test examples relating to the technology disclosed herein will be described below, but it is not intended that the technology disclosed herein be limited to those shown in the following test examples.

[0047] [Manufacturing test cells] <Examples 1 to 4, Comparative Examples 1 to 3> LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were prepared. These were combined with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of LNCM:AB:PVdF = 92:5:3 and kneaded using a stirring granulator to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of a 15 μm thick aluminum foil and dried. The foil was then pressed to the specified thickness and processed to the specified dimensions to obtain a positive electrode.

[0048] As the negative electrode active material, graphite particles (graphite particles substantially free of silicon) and silicon-containing graphite particles were prepared. The graphite particles had an average particle diameter of 22 μm and a BET specific surface area of ​​1.4 m. 2 The graphite particles were 1 / g. The silicon-containing graphite particles were particles containing voided graphite particles with an average particle diameter of 3 μm and silicon particles disposed within the voids. The particle diameter of the silicon particles was 30 nm. Acetylene black (AB) was prepared as the conductive material. Carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) were prepared as binders. Of these, SBR was prepared as SBRα with a glass transition point of 8°C, SBRβ with a glass transition point of 0°C, and SBRγ with a glass transition point of -10°C. A negative electrode slurry was prepared with a mass ratio of graphite particles / silicon-containing graphite particles / AB / CMC / SBR=93:2:1:1:3. The composition ratio of SBR in each example was as shown in the corresponding column in Table 1. The kneading machine used for the following kneading and mixing was a "Hibismix 2P-1" manufactured by PRIMIX Corporation.

[0049] In preparing the negative electrode slurry, first, graphite particles, silicon-containing graphite particles, AB, and CMC were kneaded at a rotation speed of 30 rpm to obtain a first mixture. Next, water was added to the first mixture and kneaded at a rotation speed of 50 rpm to obtain a second mixture. Next, SBR and water were added to the second mixture and mixed to obtain a negative electrode slurry. The negative electrode slurry was then applied to both sides of a 10 μm thick copper foil and dried, and then pressed to a predetermined thickness and processed to the predetermined dimensions to obtain a negative electrode. The basis weight of the negative electrode slurry on each side was 215 g / m 2 The packing density was 1.6 g / cc. The packing density Z was calculated using the following formula (P): Packing density Z(g / cc)= {Active material layer weight X (g / m 2 )} / {active material layer thickness Y (μm)} (P) It was calculated by using

[0050] A separator was prepared, which consisted of a porous polyolefin sheet (20 μm thick) with a three-layer structure of PP / PE / PP and a heat-resistant layer (4 μm thick). Leads were attached to each of the positive and negative electrodes, and the electrodes were stacked with the separator interposed between them to prepare an electrode assembly. The electrode assembly was inserted into an exterior body of an aluminum laminate sheet, a nonaqueous electrolyte was poured, and the opening of the exterior body was sealed to prepare a test cell for each example. The composition (volume ratio) of the nonaqueous electrolyte was EC / FEC / EMC / DMC = 15:5:40:40. LiPF6 was dissolved in the nonaqueous electrolyte as a supporting electrolyte at a concentration of 1 mol / L.

[0051] [Peel strength test] A 90-degree peel strength test was conducted in accordance with JIS C6481 (1996). Specifically, a peel strength (90-degree peel strength) measurement test method was performed. More specifically, a rectangular test piece was prepared by cutting the prepared negative electrode to a predetermined size. One side of the test piece was fixed to the stand of a tensile tester using an adhesive such as double-sided tape. Tape was attached to the other side of the test piece, and the end of the tape was fixed to a tensile jig. The tensile jig was then pulled upward in a direction perpendicular to the surface of the stand (here, the negative electrode attached to the stand) (a peel angle of 90±5°) at a predetermined speed (e.g., 0.5 mm per second), thereby peeling the negative electrode active material layer and the copper foil stuck to the tape. At this time, the average load during peeling of the negative electrode active material layer from the copper foil was measured, and the average value of this load per unit width was taken as the peel strength (N / mm). The results are shown in the corresponding columns in Table 1. The peel strengths shown in Table 1 are relative values ​​when Example 1 is set to 100. Here, examples with a peel strength of 70 or more are evaluated as having a sufficiently high peel strength.

[0052] [Initial capacity measurement] The test cell was subjected to one charge-discharge cycle in an environment of 25°C, in which constant current charging (CC charging) was performed at a charge rate of 0.05 C until the voltage between the positive and negative electrodes reached 4.2 V, and then constant current discharging (CC discharging) was performed at a discharge rate of 0.05 C until the voltage between the positive and negative electrodes reached 2.5 V. The discharge capacity at this time was measured and used as the initial capacity.

[0053] [Cycle characteristic evaluation] The test cell was placed in a thermostatic chamber at 25°C and subjected to CC charging at 0.33 C up to 4.14 V, followed by CV charging until the current reached 0.1 C, and then fully charged. It was then CC discharged at 0.33 C down to 3 V. This cycle constitutes one charge-discharge cycle, and 200 charge-discharge cycles were performed. The following formula (X): Capacity retention rate (%) = (discharge capacity after 200 cycles / initial capacity) x 100 (X) The capacity retention rate after 200 cycles was calculated for each test cell based on the above. The results are shown in the corresponding column in Table 1. Here, examples with a capacity retention rate of 65% or more are evaluated as having suppressed capacity loss after charge-discharge cycling.

[0054] [Table 1]

[0055] As shown in Table 1, the negative electrodes of Examples 1 to 4 had higher peel strengths of the negative electrode active material layer from the negative electrode current collector than those of Comparative Examples 1 to 3. In the test cells of Examples 1 to 4, the decrease in capacity retention rate after charge-discharge cycling was suppressed. The negative electrodes of the test cells of Examples 1 to 4 contained a negative electrode active material that was silicon-containing graphite particles containing graphite particles having voids and silicon-containing particles disposed in the voids, a first binder that was styrene-butadiene rubber having a glass transition point of 1°C or higher, and a second binder that was styrene-butadiene rubber having a glass transition point of less than 1°C. The results shown in Table 1 demonstrate that this configuration can suppress the decrease in capacity after charge-discharge cycling.

[0056] The techniques disclosed herein may include the techniques described in the following sections. Section 1: A negative electrode, a negative electrode active material that is a silicon-containing graphite particle including graphite particles having voids and silicon-containing particles disposed in the voids; a first binder that is a styrene butadiene rubber having a glass transition point of 1°C or higher; a second binder that is a styrene butadiene rubber having a glass transition temperature of less than 1°C; a negative electrode. Section 2: Item 2. The negative electrode according to item 1, wherein the difference between the glass transition point of the first binder and the glass transition point of the second binder is 5° C. or more. Section 3: Item 3. The negative electrode according to item 1 or 2, comprising the first binder in an amount equal to or greater than the amount of the second binder. Section 4: 4. The negative electrode according to any one of items 1 to 3, wherein the silicon-containing particles have a particle size of 1 μm or less. Section 5: Item 5. The negative electrode according to any one of items 1 to 4, further comprising graphite particles that are substantially free of silicon as another negative electrode active material. Item 6: Item 6. An electricity storage device comprising the negative electrode according to any one of Items 1 to 5.

[0057] Although the embodiments of the technology disclosed herein have been described above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0058] 20 Electrode body 30 cases 42 Positive terminal 44 Negative terminal 50 positive electrode 60 negative electrode 70 Separator 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A negative electrode, a negative electrode active material that is a silicon-containing graphite particle including graphite particles having voids and silicon-containing particles disposed in the voids; a first binder that is a styrene-butadiene rubber having a glass transition point of 1°C or higher; a second binder that is a styrene butadiene rubber having a glass transition temperature of less than 1°C; a negative electrode.

2. The negative electrode according to claim 1 , wherein a difference between a glass transition point of the first binder and a glass transition point of the second binder is 5° C. or more.

3. The negative electrode according to claim 1 , comprising the first binder in an amount equal to or greater than the amount of the second binder.

4. 3. The negative electrode according to claim 1, wherein the silicon-containing particles have a particle size of 1 μm or less.

5. 3. The negative electrode according to claim 1, further comprising graphite particles substantially free of silicon as another negative electrode active material.

6. An electricity storage device comprising the negative electrode according to claim 1 or 2.

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