negative electrode

Abstract

The present invention provides a negative electrode that can reduce the rate of increase in area in the planar direction associated with the charging and discharging of a battery. [Solution] A negative electrode comprising a negative electrode current collector and a negative electrode layer on at least one surface of the negative electrode current collector, wherein the negative electrode layer contains a carbon composite material comprising a carbon material having an aspect ratio greater than 1 and Si, the carbon composite material being carbon composite material particles, and the proportion of first carbon composite material particles among all the carbon composite material particles contained in the negative electrode layer, wherein the angle θ between the straight line in the longitudinal axis direction of the carbon composite material particle and the surface of the negative electrode current collector is 50° or more and 90° or less, is greater than 35%.

Description

[Technical Field]

[0001] This disclosure relates to the negative electrode. [Background technology]

[0002] Various technologies have been proposed for negative electrodes, as disclosed in Patent Documents 1 to 3. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-185943 [Patent Document 2] Patent Publication No. 7311169 [Patent Document 3] Japanese Patent Publication No. 2016-103337 [Overview of the project] [Problems that the invention aims to solve]

[0004] In batteries containing silicon (Si) as the negative electrode active material, conventional technology has the problem that the negative electrode containing Si deforms due to the expansion and contraction of Si during charging and discharging of the battery, and consequently, the charge and discharge capacity of the Si-containing battery decreases.

[0005] This disclosure has been made in view of the above circumstances, and its main purpose is to provide a negative electrode that can reduce the rate of increase in area in the planar direction associated with the charging and discharging of a battery. [Means for solving the problem]

[0006] In other words, this disclosure includes the following aspects: <1> A negative electrode comprising a negative electrode current collector and a negative electrode layer on at least one surface of the negative electrode current collector, The negative electrode layer contains a carbon composite material comprising a carbon material with an aspect ratio greater than 1 and Si, The carbon composite material is carbon composite material particles, Among all the carbon composite material particles included in the negative electrode layer, the ratio of the first carbon composite material particles with an angle θ formed between the straight line in the long axis direction of the carbon composite material particles and the surface of the negative electrode current collector being 50° or more and 90° or less is more than 35%, and the negative electrode is characterized by this.

[0007] <2> Among all the carbon composite material particles included in the negative electrode layer, the ratio of the second carbon composite material particles with an angle θ formed between the straight line in the long axis direction of the carbon composite material particles and the surface of the negative electrode current collector being 70° or more and 90° or less is more than 9.2%, and the negative electrode according to <1>.

[0008] <3> The content of Si in the carbon composite material is 1 to 70% by mass, and the negative electrode according to <1> or <2>.

[0009] <4> The carbon material is a bundle-like composite of a plurality of carbon fibers having a plurality of pores, and the negative electrode according to any one of <1> to <3>.

[0010] <5> The Si is supported in the pores, and the negative electrode according to <4>.

Effect of the Invention

[0011] [[ID=3i]] According to the present disclosure, it is possible to provide a negative electrode capable of reducing the area increase rate in the planar direction accompanying charge and discharge of the battery.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a schematic diagram showing an example of the negative electrode of the present disclosure, where (A) is the negative electrode during discharge and (B) is the negative electrode during charging. [Figure 2] FIG. 2 is a schematic diagram showing an example of the method for manufacturing the carbon composite material of the present disclosure. [Figure 3] FIG. 3 is a graph showing the area increase rate in the planar direction of the negative electrode layer in Example 1 and Comparative Example 1. [Figure 4] FIG. 4 is a graph showing the capacity retention rate of the batteries in Example 1 and Comparative Example 1.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of negative electrodes that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships. In the present disclosure, an example of a method for calculating the average particle size of particles is as follows. First, in a scanning electron microscope (hereinafter referred to as SEM) image at an appropriate magnification (for example, 50,000 to 1,000,000 times), for a single particle, the particle size when the particle is regarded as spherical is calculated. Such calculation of the particle size by SEM observation is performed for 2 to 300 particles of the same type, and the average of these particles is taken as the average particle size.

[0014] In the present disclosure, a negative electrode having a negative electrode current collector and a negative electrode layer on at least one surface of the negative electrode current collector, wherein the negative electrode layer contains a carbon composite material including a carbon material with an aspect ratio exceeding 1 and Si, the carbon composite material is carbon composite material particles, provided is a negative electrode characterized in that, among all the carbon composite material particles contained in the negative electrode layer, the proportion of the first carbon composite material particles having an angle θ formed between the straight line in the major axis direction of the carbon composite material particles and the surface of the negative electrode current collector of 50° or more and 90° or less exceeds 35%.

[0015] According to this disclosure, a desired number of electron conduction paths can be maintained by compounding a carbon material with Si. Furthermore, the aspect ratio of the carbon material is greater than 1, which restricts the expansion direction of Si. In addition, in the carbon composite material of this disclosure, Si is continuously arranged along the long axis of the particles, making it easier for Si to expand along the long axis during battery charging. By orienting the long axis of the carbon composite material particles vertically such that the angle between the straight line along the long axis of the carbon composite material particles and the surface of the negative electrode current collector is between 50° and 90°, the expansion direction of Si during charging can be controlled vertically, thereby suppressing the expansion of the negative electrode layer in the planar direction compared to conventional Si-based active materials. By suppressing the expansion of the negative electrode layer in the planar direction, the sliding of the carbon composite material particles on the negative electrode current collector is suppressed, which in turn suppresses the sliding of the carbon composite material particles off the negative electrode current collector due to binder peeling and the reduction of electron conduction paths, thereby suppressing a decrease in the battery capacity retention rate and suppressing an increase in battery resistance.

[0016] The negative electrode of this disclosure comprises a negative electrode current collector and a negative electrode layer on at least one surface of the negative electrode current collector.

[0017] The negative electrode layer may be located on at least one surface of the negative electrode current collector, or it may be located on both surfaces of the negative electrode current collector. The negative electrode may have a multilayer structure in which two or more negative electrode layers are formed on at least one surface of the negative electrode current collector. Furthermore, when two or more negative electrode layers are formed, the type of negative electrode active material contained in each negative electrode layer may be the same or different.

[0018] Figure 1 is a schematic diagram showing an example of the negative electrode of this disclosure, where (A) is the negative electrode during discharge and (B) is the negative electrode during charging. The negative electrode 20 shown in Figure 1 has a negative electrode layer 22 containing carbon composite material particles 30 arranged on one surface of the negative electrode current collector 21. In the negative electrode layer 22, the long axes of a predetermined number of carbon composite material particles 30 are oriented vertically, so that when charging, the Si expands vertically, and the expansion of the negative electrode layer in the planar direction can be suppressed.

[0019] The negative electrode layer of this disclosure includes a carbon composite material as the negative electrode active material. The negative electrode layer may also optionally contain at least one of a conductive material and a binder. The negative electrode layer includes at least the carbon composite material of this disclosure as the negative electrode active material, and optionally includes a negative electrode active material other than the carbon composite material. As the negative electrode active material other than the carbon composite material, an active material known as a negative electrode active material for batteries can be used, for example, the carbon material described later can be used. The proportion of carbon composite material in 100% by mass of the negative electrode active material contained in the negative electrode layer may be 10% by mass or more, 20% by mass or more, 50% by mass or more, or 100% by mass or less. The conductive materials and binders used in the negative electrode layer are the same as those described for the positive electrode layer later. The proportion of conductive material in the negative electrode layer may be, for example, 0.1% by mass or more and 5% by mass or less. The proportion of the binder in the negative electrode layer may be, for example, 0.5% by mass or more and 15% by mass or less.

[0020] The negative electrode layer of this disclosure may contain 50% by mass or more of the negative electrode active material, 100% by mass or less, 90% by mass or less, or 74.5% by mass or less.

[0021] Carbon composite materials are carbon composite material particles. Of all the carbon composite material particles contained in the negative electrode layer, the proportion of first carbon composite material particles in which the angle θ between the straight line in the longitudinal axis direction of the carbon composite material particle and the surface of the negative electrode current collector is 50° or more and 90° or less should exceed 35%, and may be 77% or more. Of all the carbon composite material particles contained in the negative electrode layer, the proportion of second carbon composite material particles in which the angle θ between the straight line in the longitudinal axis direction of the carbon composite material particle and the surface of the negative electrode current collector is 70° or more and 90° or less should exceed 9.2%, and may be 45% or more. The degree of orientation of carbon composite material particles in the negative electrode layer, specifically the angle θ between the straight line along the long axis of the carbon composite material particles and the surface of the negative electrode current collector, can be calculated using images such as SEM and X-ray CT of the cross-section of the negative electrode. In this disclosure, 30 to 100 particles are individually extracted from the field of view of the cross-sectional image of the negative electrode, an ellipse approximation is performed to extract the major and minor axes of each particle, the negative electrode current collector is defined as a horizontal plane, the magnitude of the inferior angle (acute angle) at the intersection of the major axis of each particle and the horizontal plane is measured, and this is defined as the particle's inclination angle θ, thereby defining the degree of orientation of each particle. The degree of orientation of carbon composite material particles in the negative electrode layer can be controlled during negative electrode fabrication by applying a predetermined magnetic field to the negative electrode paste for a predetermined time after applying the negative electrode paste to the negative electrode current collector and before drying the negative electrode paste to form the negative electrode layer. The predetermined time may be, for example, 1 second to 1 hour. The predetermined magnetic field may be, for example, 0.1T to 5T or 1T to 3T.

[0022] Carbon composite materials contain carbon material and Si. The Si content in the carbon composite material (Si load in the carbon material) may be, for example, 1% by mass or more, 10% by mass or more, 47% by mass or more, 70% by mass or less, 50% by mass or less, or 49% by mass or less. The Si content in the carbon composite material can be calculated using the following formula. Si content (%) = Mass of Si ÷ (Mass of carbon material + Mass of Si) × 100 The carbon composite material may have its outermost surface coated with a carbon material.

[0023] The carbon material only needs to have an aspect ratio greater than 1, may be 1.5 or greater, may be 2 or greater, may be 2.2 or greater, may be 2.7 or greater, and there is no particular upper limit, but may be 9.2 or less, 7.5 or less, or 4.2 or less. The aspect ratio is determined by acquiring SEM images of the cross-section or surface of an electrode layer containing carbon material or carbon material in powder form, extracting 30 to 100 particles, and calculating the average aspect ratio of these 30 to 100 particles through image analysis. This average value is then used as the aspect ratio of the carbon material. The aspect ratio is expressed as a / b, where a is the length in the long axis direction and b is the length in the short axis direction. If the carbon material has a thickness direction, such as in the form of flakes, the thickness is used as the length b in the short axis direction. The aspect ratio of the carbon composite material may be the same as the aspect ratio of the carbon material, and the aspect ratio of the carbon material may be considered as the aspect ratio of the carbon composite material.

[0024] The length along the long axis of the carbon material may be, for example, 0.1 μm or more, 1 μm or more, 5.6 μm or more, 10.6 μm or more, 100 μm or less, 20 μm or less, or 18.2 μm or less. The length in the longitudinal direction of the carbon composite material may be the same as the length in the longitudinal direction of the carbon material, or the length in the longitudinal direction of the carbon material may be considered to be the length in the longitudinal direction of the carbon composite material.

[0025] Examples of carbon materials include natural graphite, artificial graphite, activated carbon, carbon fiber, mesocarbon microbeads (MCMB), hard carbon, and soft carbon. The shape of the carbon material is not particularly limited, but may be particulate, flake-shaped, plate-shaped, or fibrous. The carbon material may also be carbon particles or carbon fibers. The carbon fibers may be obtained by carbonizing carbon fiber raw materials such as polyacrylonitrile (PAN) resin fibers and cellulose nanofibers. As for the carbonization conditions, for example, the carbon fiber raw materials may be heat-treated at 1000°C to 1500°C in an inert gas atmosphere. Furthermore, the carbon fibers may be carbon materials having a mesoporous structure, such as ordered mesoporous carbon (CMK-3), and carbon nanofibers and carbon nanotubes may also be used. The carbon material may be a composite of bundles of multiple carbon fibers. Being a composite of bundles of multiple carbon fibers further restricts the expansion direction of Si, thereby suppressing deformation associated with the expansion and contraction of Si. The carbon fiber may have at least one pore, or it may have multiple pores. The pores of the carbon fiber may be continuous in the direction of the long axis. The pore size of the pores in the carbon fiber may be, for example, 1 nm to 20 nm. The length (width) of the carbon fiber in the short axis direction may be, for example, 0.1 μm to 3 μm. The length of the carbon fiber along its long axis may be, for example, 0.1 μm or more, 1 μm or more, 5.6 μm or more, 10.6 μm or more, 100 μm or less, 20 μm or less, or 18.2 μm or less. A composite of multiple carbon fibers can be obtained, for example, by utilizing aggregation in a poor solvent environment. For example, by reacting carbon fibers in xylene solvent, which is a poor solvent, while vigorously stirring, it is presumed that the carbon fibers aggregate due to the action of functional groups (such as -COOR groups) present on the surface of the carbon fibers. The resulting dispersion can be passed through a slit and rapidly dried to obtain a composite of multiple carbon fibers. The obtained composite may be cut and pulverized using mechanical milling or the like as needed.

[0026] Si may be amorphous or crystalline. When Si is crystalline, the crystallite size measured by X-ray diffraction (XRD) of Si may be between 1 nm and 1 μm. Si may be supported on the surface of the carbon material. If the carbon material is carbon fibers, Si may be supported on the surface of the carbon fibers and in at least one of the pores of the carbon fibers. The surface of the Si after it has been supported on the carbon material may be coated with the carbon material. The method for supporting Si on a carbon material is not particularly limited and includes methods using CVD (chemical vapor deposition), methods of immersing the carbon material in a molten silicon solution, and methods of forming a silicon layer on a carbon material using TEOS (tetraethyl orthosilicate). CVD methods include thermal CVD (chemical vapor deposition using thermal decomposition), PECVD (chemical vapor deposition using plasma), and ALD (atomic layer deposition). An advantage of using thermal CVD is that it allows for the formation of uniform amorphous Si. The Si sources used in CVD are silane (SiH4) and silane halogens (H 4-x SiCl x ) are some examples. The Si component in carbon composite materials can be detected by quantitative analysis methods such as inductively coupled plasma (ICP) spectroscopy and electron beam microanalyzer (EPMA).

[0027] Carbon composite materials may be manufactured by supporting silicon on a substrate made of carbon material.

[0028] Figure 2 is a schematic diagram showing an example of a method for manufacturing the carbon composite material of this disclosure. As shown in Figure 2, carbon fiber raw material 10 may be carbonized to create carbon fibers 11 with multiple pores arranged internally in the longitudinal direction, and then Si may be deposited inside the pores of the carbon fibers 11 to produce a carbon composite material 12.

[0029] Examples of materials for the negative electrode current collector include SUS, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the negative electrode current collector varies depending on its shape, but may be in the range of, for example, 1 μm to 50 μm. The shape of the negative electrode current collector may be foil-like or plate-like. The planar shape of the negative electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The negative electrode current collector may have a buffer layer, an elastic layer, or a PTC thermistor layer on its surface.

[0030] The negative electrode of the present disclosure is used in a battery. The battery has a positive electrode, an electrolyte layer, and a negative electrode. According to the present disclosure, by using the above-described negative electrode in a battery, expansion of the battery can be suppressed.

[0031] [Positive electrode] The positive electrode has a positive electrode layer and may further have a positive electrode current collector as needed. The positive electrode layer may be disposed on one surface of the positive electrode current collector or on both surfaces of the positive electrode current collector. The positive electrode may have a multilayer structure by forming two or more positive electrode layers on at least one surface of the positive electrode current collector. Further, when forming two or more positive electrode layers, the types of positive electrode active materials contained in each positive electrode layer may be the same or different. The positive electrode layer is a layer containing at least a positive electrode active material. Further, the positive electrode layer may contain at least one of a conductive material and a binder as needed.

[0032] Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include LiCoO2, LiMnO2, LiNiO2, LiVO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, and rock salt layer-type active materials such as LiNi 0.8 Mn 0.1 Co 0.1 O2, etc., spinel-type active materials such as LiMn2O4, Li4Ti5O 12 , and olivine-type active materials such as LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4.

[0033] The shape of the positive electrode active material is usually particulate. The positive electrode active material may be primary particles or secondary particles in which primary particles are aggregated. The average particle size of the positive electrode active material is not particularly limited, but for example, it is 0.01 μm or more and 50 μm or less, and may be 0.5 μm or more and 30 μm or less. ​​​

[0034] The proportion of positive electrode active material in the positive electrode layer is, for example, 20% by mass or more, may be 30% by mass or more, or 40% by mass or more. If the proportion of positive electrode active material is too low, a sufficient energy density may not be obtained. On the other hand, the proportion of positive electrode active material in the positive electrode layer is, for example, 95% by mass or less, may be 70% by mass or less, or 60% by mass or less. If the proportion of positive electrode active material is too high, the ionic conductivity and electronic conductivity in the positive electrode layer may relatively decrease.

[0035] The positive electrode layer may contain a conductive material. Adding a conductive material improves the electronic conductivity of the positive electrode layer. Examples of conductive materials include carbon-based conductive materials, metal particles, and conductive polymers. Examples of carbon-based conductive materials include particulate materials such as acetylene black (AB) and Ketjenblack (KB), and fibrous materials such as vapor-processed carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofibers (CNF).

[0036] The proportion of conductive material in the positive electrode layer may be, for example, 0.1% by mass or more. If the proportion of conductive material is too low, there may be insufficient electron conduction paths in the positive electrode layer. On the other hand, the proportion of conductive material in the positive electrode layer may be, for example, 5% by mass or less. If the proportion of conductive material is too high, the proportion of positive electrode active material will be relatively low, which may result in a lower energy density.

[0037] The positive electrode layer may contain a binder. Examples of binders include styrene-butadiene rubber (SBR), polyimide (PI), polyacrylic acid (PAA), acrylonitrile-butadiene rubber (NBR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-isoprene-styrene block copolymer (SIS), and ethylene-propylene-diene copolymer (EPDM).

[0038] The proportion of binder in the positive electrode layer may be, for example, 0.5 mass% or more. If the proportion of binder is too low, the increase in resistance due to charging and discharging may not be sufficiently reduced. On the other hand, the proportion of binder in the positive electrode layer may be, for example, 15 mass% or less. If the proportion of binder is too high, the proportion of positive electrode active material will be relatively low, which may result in a lower energy density.

[0039] The thickness of the positive electrode layer may be, for example, 0.1 μm or more and 1000 μm or less, 1 μm or more and 500 μm or less, or 30 μm or more and 200 μm or less.

[0040] The method for manufacturing the positive electrode layer is not particularly limited, but for example, it may involve mixing the positive electrode active material with a solvent to obtain a positive electrode slurry, coating the positive electrode slurry onto a positive electrode current collector, drying it, and forming a positive electrode layer. When forming the positive electrode layer, a pressing process may be performed to press the positive electrode layer in the thickness direction. Examples of pressing processes include roller pressing and flat plate pressing. Examples of solvents include N-methylpyrrolidone (NMP), tetralin, diisobutyl ketone, butyl butyrate, mesitylene, heptane, dibutyl ether, decane, dodecane, isodecane, and toluene, and may contain two or more of these components.

[0041] Examples of materials for the positive electrode current collector include SUS, Cr, Au, Pt, Zn, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the positive electrode current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the positive electrode current collector may be foil-like or plate-like. The planar shape of the positive electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The positive electrode current collector may have a buffer layer, an elastic layer, or a PTC (Positive Temperature Coefficient) thermistor layer on its surface.

[0042] [Electrolyte layer] The electrolyte layer is a layer formed between the positive electrode layer and the negative electrode layer, and contains at least an electrolyte. The electrolyte may be a liquid electrolyte (electrolyte solution). The electrolyte layer may contain an electrolyte solution or the like. Non-aqueous electrolytes can be used as the electrolyte.

[0043] As a non-aqueous electrolyte, one containing a lithium salt and a non-aqueous solvent is typically used. Examples of lithium salts include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6; and organic lithium salts such as LiCF3SO3, LiN(SO2CF3)2(Li-TFSI), LiN(SO2C2F5)2, and LiC(SO2CF3)3. Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, sulfolane, acetonitrile (AcN), dimethoxymethane, 1,2-dimethoxyethane (DME), 1,3-dimethoxypropane, diethyl ether, tetraethylene glycol dimethyl ether (TEGDME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), and mixtures thereof. From the viewpoint of ensuring high dielectric constant and low viscosity, a mixture of cyclic carbonate compounds such as EC, PC, and BC having high dielectric constant and high viscosity, and chain-like carbonate compounds such as DMC, DEC, and EMC having low dielectric constant and low viscosity may be used, or a mixture of EC and DEC may be used. The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.3 to 5 M.

[0044] The electrolyte layer may be impregnated with the aforementioned electrolyte solution, and a separator may be used to prevent contact between the positive electrode layer and the negative electrode layer. The material for the separator is not particularly limited as long as it is a porous membrane. Examples include polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide resins, with polyethylene and polypropylene being particularly preferred. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a PE / PP two-layer separator, or a PP / PE / PP or PE / PP / PE three-layer separator. The separator may be made of nonwoven fabrics such as resin nonwoven fabric or glass fiber nonwoven fabric.

[0045] The thickness of the electrolyte layer may be, for example, 0.1 μm or more and 100 μm or less, 0.1 μm or more and 500 μm or less, or 0.1 μm or more and 30 μm or less.

[0046] The battery in this disclosure may further include a restraining jig that applies restraining pressure to the positive electrode layer, electrolyte layer, and negative electrode layer along the thickness direction. The restraining pressure may be, for example, 0.1 MPa or more, 1 MPa or more, or 5 MPa or more. On the other hand, the restraining pressure may be, for example, 100 MPa or less, 50 MPa or less, or 20 MPa or less.

[0047] [battery] The type of battery in this disclosure is not particularly limited, but is typically a lithium-ion battery. The battery in this disclosure may also be a liquid battery in which the electrolyte layer contains an electrolyte solution. The liquid battery may also be a non-aqueous liquid battery in which the electrolyte layer contains a non-aqueous electrolyte solution. Furthermore, the battery in this disclosure may be a primary battery or a secondary battery, but is particularly favored as a secondary battery because it can be repeatedly charged and discharged and is useful, for example, as an in-vehicle battery. The shape of the battery is not particularly limited and may be, for example, coin-type, cylindrical, prismatic, sheet-type, button-type, flat-type, or stacked-type. In the case of a battery stack consisting of multiple batteries stacked on top of each other, the battery stack may be monopolar or bipolar.

[0048] Applications of batteries include, for example, powering vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, they may be used as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Batteries may also be used as power sources for mobile devices other than vehicles (e.g., trains, ships, aircraft), and as power sources for electrical products such as information processing devices.

[0049] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure. [Examples]

[0050] (Example 1) [Preparation of carbon fiber] Multiple carbon fibers with multiple pores were prepared by carbonizing PAN resin-based fibers, used as a raw material for carbon fibers, under an inert gas (Ar gas) atmosphere and heat-treated at 1500°C. The aspect ratio of the carbon fiber was 2.7. The longitudinal length of the carbon fiber was 5.6 μm. [Fabrication of composites made of bundles of multiple carbon fibers] Using the above-mentioned carbon fibers, a composite of multiple carbon fiber bundles was prepared by the following method: The carbon fibers were stirred in xylene solvent as a poor solvent, the resulting dispersion was passed through a slit, and the composite of multiple carbon fiber bundles was rapidly dried to obtain the composite. The obtained composite was used as the carbon material. [Fabrication of carbon composite materials] The silicon was supported onto the carbon material using thermal CVD as follows. A carbon material was placed in a chamber, and a gas source for Si was flowed through it while applying the temperature required for thermal decomposition (700°C). This resulted in the support of Si particles on the surface and within the pores of the carbon material, obtaining carbon composite material particles (silicon-carbon composite material particles). The composition of the supported Si was investigated by ICP spectroscopy. The Si content in the carbon composite material was 48%. The Si was amorphous, with crystallite size below the detection limit by XRD.

[0051] [Fabrication of the negative electrode] Carbon composite material particles were used as the negative electrode active material, polyimide as the binder, and carbon nanotubes as the conductive material. These materials were mixed in a mass ratio of 74.5:15:0.5%, and N-methylpyrrolidone was added as the solvent to the resulting mixture. Subsequently, these materials were thoroughly kneaded using a kneader to obtain a negative electrode paste. The negative electrode paste was applied to the copper foil, which served as the negative electrode current collector, using a doctor blade. A magnetic field of 3T was then applied to the negative electrode paste for a predetermined time. After that, the negative electrode paste was dried at 120°C for 15 minutes, and the solvent was completely evaporated to produce a negative electrode with a negative electrode layer on the negative electrode current collector. Furthermore, the obtained negative electrode was placed in a closed muffle furnace, and after several vacuum replacements, the binder was imidized by firing at 450°C for 20 hours in an inert gas (Ar gas) atmosphere.

[0052] [Fabrication of the positive electrode] LiNi 0.8 Mn 0.1 Co 0.1 O2 was used as the positive electrode active material. The positive electrode active material, conductive material (acetylene black), and binder (polyvinylidene fluoride) were mixed in a mass ratio of 95:2.5:2.5%. N-methylpyrrolidone was added as a solvent to the resulting mixture, and these materials were thoroughly kneaded using a kneader to obtain a positive electrode paste. The positive electrode paste was applied to aluminum foil, which served as the positive electrode current collector, using a doctor blade, and dried at 120°C for 15 minutes to produce a positive electrode with a positive electrode layer on the positive electrode current collector. The fabricated positive electrode was pressed using a twin-screw roll press to obtain a positive electrode layer density of 3.2 g / cm³.3 The pressing was performed to achieve the desired result. Furthermore, the capacitance ratio of the positive electrode layer to the negative electrode layer was adjusted so that the positive electrode capacitance / negative electrode capacitance was 1.1.

[0053] [Separator] A polyethylene separator was used as the separator.

[0054] [Electrolyte] As the electrolyte, a carbonate-based solvent was used, and lithium hexafluoride phosphate (LiPF6) was prepared to a concentration of 1 M as the lithium salt. 10% by volume of fluoroethylene carbonate (FEC) and 2% by volume of vinylene carbonate (VC) were added to 100% by volume of the total solvent in the electrolyte. The electrolyte was impregnated into the separator.

[0055] [Battery construction] The fabricated positive and negative electrodes were cut to arbitrary sizes, and a separator impregnated with electrolyte was placed between them. The resulting laminate was covered with a laminate film to construct an electrochemical cell (battery).

[0056] (Comparative Example 1) In the [fabrication of the negative electrode], no magnetic field was applied to the negative electrode paste, and the obtained negative electrode was then subjected to a twin-screw roll press to obtain a density of 1.12 g / cm³ of the negative electrode layer. 3 The battery was manufactured in the same manner as in Example 1, except that the pressing process was performed in the same way as in Example 1.

[0057] [Definition of orientation within the negative electrode] The degree of orientation of carbon composite material particles in the negative electrodes of Example 1 and Comparative Example 1 was calculated using SEM images of the cross-section of the negative electrodes. Of all the carbon composite material particles contained in the negative electrode layer, the proportion of first carbon composite material particles whose angle θ between the straight line in the long axis direction of the carbon composite material particle and the surface of the negative electrode current collector is between 50° and 90° was calculated. Of all the carbon composite material particles contained in the negative electrode layer, the proportion of second carbon composite material particles in which the angle θ between the straight line in the long axis direction of the carbon composite material particle and the surface of the negative electrode current collector is between 70° and 90° was calculated. These results are shown in Table 1.

[0058] [Evaluation of the area increase rate of the negative electrode layer] Each of the negative electrodes prepared in Example 1 and Comparative Example 1 was cut to an arbitrary area, and the prepared negative electrode, the separator impregnated with the prepared electrolyte, and Li metal as the counter electrode were arranged in this order. The resulting laminate was covered with a laminate film to construct an electrochemical cell. [When Li is first inserted (0V vs Li + [Evaluation of the area increase rate of the negative electrode layer of / Li] The resulting electrochemical cell was discharged to 0V (filling the negative electrode layer with Li from the counter electrode Li metal), and the 0V vs Li after the initial Li insertion was performed. + The negative electrode layer area was measured during the Li insertion phase and compared with the negative electrode layer area in the initial state before the first Li insertion. The area increase rate after the initial Li insertion of each negative electrode layer in Example 1 and Comparative Example 1 was calculated using the following formula. These results are shown in Table 1. Anode layer area increase rate (%) = (Anode layer area after initial Li insertion - Anode layer area in the initial state before initial Li insertion) ÷ Anode layer area in the initial state before initial Li insertion × 100 [Li desorption after 20 charge / discharge cycles (1.2V vs Li + [Evaluation of the area increase rate of the negative electrode layer of / Li] Furthermore, the electrochemical cell was subjected to 20 cycles of discharge to 0V and charge to 1.2V (extracting Li from the negative electrode layer). After that, the electrochemical cell was disassembled and the Li desorption rate after 20 charge-discharge cycles (1.2V vs Li) was measured. + The negative electrode layer area of ​​the Li-based system was measured and compared with the negative electrode layer area of ​​the initial state before the first Li insertion. The area increase rates of each negative electrode layer in Example 1 and Comparative Example 1 were calculated using the following formula. These results are shown in Table 1. Negative electrode layer area increase rate (%) = (Negative electrode layer area at the time of Li desorption after 20 charge / discharge cycles - Negative electrode layer area in the initial state before the first Li insertion) ÷ Negative electrode layer area in the initial state before the first Li insertion × 100

[0059] [Evaluation of battery capacity retention rate] For each of the batteries prepared in Example 1 and Comparative Example 1, the change in capacity retention rate during the cycle period was measured by charge-discharge cycle testing. For charging, a constant current of 0.3C (where C is the time rate) was applied, and after the battery voltage reached 4.2V, a constant voltage was applied and each battery was charged until the current value reached 0.01C. For the discharge process, a constant current of 0.3C was applied to each battery until the battery voltage reached 2.5V. A 10-minute pause was included between charging and discharging cycles. The battery capacity was measured after 20 consecutive charge and discharge cycles, and the capacity retention rate was calculated by dividing the discharge capacity after 20 cycles by the discharge capacity after 1 cycle. These results are shown in Table 1.

[0060] [Table 1]

[0061] Figure 3 is a graph showing the rate of increase in the area of ​​the negative electrode layer in the planar direction in Example 1 and Comparative Example 1. In the negative electrode of Comparative Example 1, the planar area of ​​the negative electrode layer after the initial Li insertion increased by more than 8% compared to before the initial Li insertion. However, with the negative electrode of Example 1, by limiting expansion in the planar direction, it is possible to suppress the increase in the planar area of ​​the negative electrode layer after the initial Li insertion compared to before the initial Li insertion to 1.5% or less. In the negative electrode of Comparative Example 1, the planar area of ​​the negative electrode layer after Li desorption following 20 charge-discharge cycles increased by more than 3.2% compared to before the initial Li insertion. However, with the negative electrode of Example 1, by limiting expansion in the planar direction, it is possible to suppress the increase in the planar area of ​​the negative electrode layer after Li desorption following 20 charge-discharge cycles compared to before the initial Li insertion to 0.2% or less.

[0062] Figure 4 is a graph showing the battery capacity retention rate in Example 1 and Comparative Example 1. In Comparative Example 1, the capacity retention rate decreased to 89% after 20 cycles, but in Example 1, the capacity retention rate after 20 cycles was 95%, demonstrating that the decrease in capacity retention rate can be suppressed. [Explanation of Symbols]

[0063] 10 ...Carbon fiber raw materials 11 ... Carbon fiber 12 ... Carbon composite materials 20...Negative electrode 21...Negative electrode current collector 22 ... Negative electrode layer 30 ...Carbon composite material particles

Claims

1. A negative electrode comprising a negative electrode current collector and a negative electrode layer on at least one surface of the negative electrode current collector, The negative electrode layer contains a carbon composite material comprising a carbon material with an aspect ratio greater than 1 and Si, The aforementioned carbon composite material is a carbon composite material particle, A negative electrode characterized in that, of all the carbon composite material particles contained in the negative electrode layer, the proportion of first carbon composite material particles in which the angle θ between the straight line in the longitudinal direction of the carbon composite material particle and the surface of the negative electrode current collector is 50° or more and 90° or less exceeds 35%.

2. The negative electrode according to claim 1, wherein, of all the carbon composite material particles contained in the negative electrode layer, the proportion of second carbon composite material particles in which the angle θ between the straight line in the longitudinal direction of the carbon composite material particle and the surface of the negative electrode current collector is 70° or more and 90° or less exceeds 9.2%.

3. The negative electrode according to claim 1, wherein the Si content in the carbon composite material is 1 to 70% by mass.

4. The negative electrode according to claim 1, wherein the carbon material is a composite of bundles of carbon fibers having a plurality of pores.

5. The negative electrode according to claim 4, wherein the Si is supported within the pores.

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