Carbon composite materials

Abstract

This invention provides a carbon composite material that can suppress the decrease in battery charge and discharge capacity. [Solution] A carbon composite material characterized by containing a carbon material having an aspect ratio of 2.7 or more and 7.5 or less, and Si.

Description

[Technical Field]

[0001] This disclosure relates to carbon composite materials. [Background technology]

[0002] Various technologies have been proposed for carbon composite materials, such as those disclosed in Patent Documents 1 to 4. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent Publication No. 7311169 [Patent Document 2] Japanese Patent Publication No. 2002-260658 [Patent Document 3] Japanese Patent Publication No. 2024-115207 [Patent Document 4] International Publication No. 2020 / 218021 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In batteries containing silicon (Si) as the negative electrode active material, conventional technology has a problem in that the shape of the Si becomes irregular due to the expansion and contraction of Si during charging and discharging of the battery, which in turn reduces the charge and discharge capacity of the Si-containing battery.

[0005] This disclosure is made in view of the above circumstances and primarily aims to provide a carbon composite material that can suppress the decrease in battery charge and discharge capacity. [Means for solving the problem]

[0006] In other words, this disclosure includes the following aspects: <1> A carbon composite material characterized by containing a carbon material having an aspect ratio of 2.7 or more and 7.5 or less, and Si.

[0007] <2> The crystallite size of the Si measured by X-ray diffraction (XRD) is 1 nm to 1 μm, and the carbon composite material according to <1>.

[0008] <3> The Si is amorphous, and the carbon composite material according to <1>.

[0009] <4> The content of the Si in the carbon composite material is 1% by mass to 70% by mass, and the carbon composite material according to any one of <1> to <3>.

[0010] <5> The content of the Si in the carbon composite material is 10% by mass to 50% by mass, and the carbon composite material according to <4>.

[0011] <6> The outermost surface is coated with the carbon material, and the carbon composite material according to any one of <1> to <5>.

[0012] <7> The carbon material is a bundle-like composite of a plurality of carbon fibers having a plurality of pores, and the carbon composite material according to any one of <1> to <6>.

[0013] <8> The length in the short-axis direction of the carbon fiber is 0.1 μm to 3 μm, and the length in the long-axis direction of the carbon fiber is 1 μm to 20 μm, and the carbon composite material according to <7>.

[0014] <9> The Si is supported in the pores, and the carbon composite material according to <7> or <8>.

Advantages of the Invention

[0015] According to the present disclosure, it is possible to provide a carbon composite material capable of suppressing a decrease in the charge and discharge capacity of a battery

Brief Description of the Drawings

[0016] [Figure 1] Figure 1 is a schematic diagram showing an example of a method for manufacturing the carbon composite material of this disclosure. [Figure 2] Figure 2 is a graph showing the relationship between the aspect ratio of the carbon material and the battery capacity retention rate in the examples and comparative examples. [Modes for carrying out the invention]

[0017] Embodiments of this disclosure are described below. Matters other than those specifically mentioned herein but necessary for the implementation of this disclosure (e.g., general composition and manufacturing processes of carbon composite materials not characterizing this disclosure) can be understood as design matters for those skilled in the art based on prior art. This disclosure can be implemented based on the content disclosed herein and common technical knowledge in the art. Furthermore, the dimensions (length, width, thickness, etc.) shown in the diagram do not necessarily reflect the actual dimensions. In this disclosure, an example of a method for calculating the average particle size is as follows: First, the particle size is calculated for a single particle, assuming it is spherical, using a scanning electron microscope (SEM) image at an appropriate magnification (e.g., 50,000 to 1,000,000 times). This calculation of particle size by SEM observation is performed for 200 to 300 particles of the same type, and the average of these particles is taken as the average particle size.

[0018] This disclosure provides a carbon composite material characterized by comprising a carbon (C) material having an aspect ratio of 2.7 to 7.5 and Si (silicon).

[0019] According to this disclosure, by compounding a carbon material with Si, a desired number of electron conduction paths can be maintained. Furthermore, by having an aspect ratio of 2.7 or higher for the carbon material, the expansion direction of Si is restricted, and deformation of Si due to expansion and contraction can be suppressed. As a result, when a carbon composite material is used as an active material, side reactions on the surface of the active material can be suppressed, and the decrease in charge and discharge capacity associated with charging and discharging of a battery using the active material can be suppressed.

[0020] The carbon composite material of this disclosure comprises a carbon material and Si. Carbon composite materials are carbon composite material particles and carbon composite material fibers, etc. 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.

[0021] The carbon material only needs to have an aspect ratio between 2.7 and 7.5, and the upper limit may be 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.

[0022] 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.

[0023] 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.

[0024] 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).

[0025] Carbon composite materials may be manufactured by supporting silicon on a substrate made of carbon material.

[0026] Figure 1 is a schematic diagram showing an example of a method for manufacturing the carbon composite material of this disclosure. As shown in Figure 1, 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.

[0027] The carbon composite material of this disclosure is used as a negative electrode active material for batteries. A battery has a positive electrode layer, a negative electrode layer, and an electrolyte layer, and typically has a positive electrode including a positive electrode layer and a negative electrode including a negative electrode layer. According to this disclosure, by using the above-mentioned carbon composite material as the negative electrode active material of a battery, it is possible to suppress the decrease in the battery's charge and discharge capacity.

[0028] [Positive electrode] The positive electrode has a positive electrode layer and, if necessary, further has a positive electrode current collector. The positive electrode layer may be arranged 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 in which two or more positive electrode layers are formed on at least one surface of the positive electrode current collector. Furthermore, when two or more positive electrode layers are formed, the type of positive electrode active material 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. The positive electrode layer may also optionally contain at least one of a conductive material and a binder.

[0029] Examples of the positive electrode active material include, for example, oxide active materials. Examples of the oxide active materials include, for example, LiCoO2, LiMnO2, LiNiO2, LiVO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, and LiNi 0.8 Mn 0.1 Co 0.1 O2 and other rock salt layer type active materials such as LiMn2O4, Li4Ti5O 12 , and spinel type active materials such as Li(Ni 0.5 Mn 1.5 )O4, olivine type active materials such as LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4.

[0030] The shape of the positive electrode active material is usually particulate. The positive electrode active material may be primary particles or secondary particles formed by aggregation of primary particles. 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.

[0031] The proportion of the positive electrode active material in the positive electrode layer is, for example, 20% by mass or more, and may be 30% by mass or more, or 40% by mass or more. If the proportion of the positive electrode active material is too small, sufficient energy density may not be obtained. On the other hand, the proportion of the positive electrode active material in the positive electrode layer is, for example, 95% by mass or less, and may be 70% by mass or less, or 60% by mass or less. If the proportion of the positive electrode active material is too large, the ionic conductivity and electron conductivity in the positive electrode layer may relatively decrease.

[0032] The positive electrode layer may contain a conductive material. By adding a conductive material, the electron conductivity of the positive electrode layer is improved. Examples of the conductive material include, for example, carbon-based conductive materials, metal particles, and conductive polymers. Examples of the carbon-based conductive materials include particulate materials such as acetylene black (AB) and ketjen black (KB), and fibrous materials such as vapor grown carbon fiber (VGCF), carbon nanotube (CNT), and carbon nanofiber (CNF).

[0033] 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.

[0034] 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).

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] [Negative electrode] The negative electrode has a negative electrode layer and, if necessary, further has a negative electrode current collector. The negative electrode layer may be arranged on one surface of the negative electrode current collector, or 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. The negative electrode layer is a layer containing at least a 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 above. 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 negative electrode layer may contain 10% by mass or more, 20% by mass or more, 50% by mass or more, 100% by mass or less, 90% by mass or less, or 74.5% by mass or less of the negative electrode active material. The conductive material and binder used in the negative electrode layer are the same as those described for the positive electrode layer above. 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.

[0040] 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.

[0041] [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.

[0042] 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.

[0043] 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.

[0044] 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 50 μm or less, or 0.1 μm or more and 30 μm or less.

[0045] 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.

[0046] [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.

[0047] 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.

[0048] 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]

[0049] (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.

[0050] (Example 2) In preparing the carbon fibers, a carbon composite material was fabricated in the same manner as in Example 1, except that multiple carbon fibers with multiple pores, an aspect ratio of 4.2, and a longitudinal length of 10.6 μm were used. The Si content in the carbon composite material was 49%. The Si was amorphous, with a crystallite size below the detection limit by XRD.

[0051] (Example 3) In preparing the carbon fibers, a carbon composite material was fabricated in the same manner as in Example 1, except that multiple carbon fibers with multiple pores, having an aspect ratio of 7.5 and a longitudinal length of 18.2 μm were used. The Si content in the carbon composite material was 47%. The Si was amorphous, with a crystallite size below the detection limit by XRD.

[0052] (Comparative Example 1) In the preparation of the carbon fibers, instead of preparing the carbon fibers and fabricating the composite material, spherical Si particles, which are crystalline, have an aspect ratio of 1.1, and a longitudinal length of 8.2 μm, were used as a substitute for the carbon composite material.

[0053] (Comparative Example 2) In preparing the carbon fibers, instead of using multiple carbon fibers each having multiple pores, particulate activated carbon with multiple pores, having an aspect ratio of 1.5 and a longitudinal length of 8.6 μm, was prepared. The carbon composite material was then prepared in the same manner as in Example 1, except that the prepared activated carbon was used directly as the carbon material without creating the composite material described above. The Si content in the carbon composite material was 48%. The Si was amorphous, with a crystallite size below the detection limit by XRD.

[0054] (Comparative Example 3) In preparing the carbon fibers, instead of using multiple carbon fibers each having multiple pores, particulate activated carbon with multiple pores, having an aspect ratio of 2.2 and a longitudinal length of 9.9 μm, was prepared. The carbon composite material was then prepared in the same manner as in Example 1, except that the prepared activated carbon was used directly as the carbon material without creating the composite material described above. The Si content in the carbon composite material was 49%. The Si was amorphous, with a crystallite size below the detection limit by XRD.

[0055] (Comparative Example 4) In preparing the carbon fibers, instead of using multiple carbon fibers each having multiple pores, a fibrous activated carbon with multiple pores and an aspect ratio of 9.2 and a longitudinal length of 40.1 μm was prepared. The carbon composite material was then prepared in the same manner as in Example 1, except that the prepared activated carbon was used directly as the carbon material without creating the composite material. The Si content in the carbon composite material was 47%. The Si was amorphous, with a crystallite size below the detection limit by XRD.

[0056] [Fabrication of the negative electrode] As the first negative electrode active material, artificial graphite was used, as the second negative electrode active material, the carbon composite materials prepared in Examples 1-3 and Comparative Examples 1-4 were used, as the second negative electrode active material, SBR was used as the first binder, polyacrylic acid as the second binder, and carbon nanotubes were used as the conductive material. These materials were mixed in a mass ratio of 70:27.5:1:1:0.05%, and distilled water was added 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 a copper foil negative electrode current collector using a doctor blade, dried at 100°C for 15 minutes, and the water was completely evaporated to produce a negative electrode with a negative electrode layer on the negative electrode current collector. The fabricated negative electrode was pressed using a twin-screw roll press to obtain a negative electrode layer density of 1.2 g / cm³. 3 Pressing was performed to achieve this result.

[0057] [Fabrication of the positive electrode] LiNi 0.8 Mn 0.1 Co 0.1O2 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 80°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.

[0058] [Separator] As the separator, a separator was used that had polyethylene as the base material and an alumina (Al2O3) modified layer supported on the base material.

[0059] [Electrolyte] As the electrolyte, an ethylene carbonate-based solvent was used, and lithium hexafluoride phosphate (LiPF6) was prepared to a concentration of 1.2 M as the lithium salt. The electrolyte was impregnated into the separator.

[0060] [Battery construction] A coin-shaped battery was fabricated by stacking the fabricated positive electrode, an electrolyte-impregnated separator, and a negative electrode.

[0061] [Electrochemical evaluation] Each carbon composite material in Examples 1-3 and Comparative Examples 1-4 was evaluated using each electrochemical cell (battery) that utilized each carbon composite material. The capacity retention rate of each manufactured battery was measured over the cycle period through 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 capacity of each battery was measured after 20 consecutive cycles of this procedure. The capacity retention rate of each battery was calculated by dividing the discharge capacity after 20 cycles by the discharge capacity after 1 cycle. The results are shown in Table 1.

[0062] [Table 1]

[0063] [result] Figure 2 is a graph showing the relationship between the aspect ratio of the carbon material and the battery capacity retention rate in the examples and comparative examples. As shown in Figure 2 and Table 1, it was demonstrated that batteries using carbon composite materials containing carbon materials with an aspect ratio greater than 1 and Si exhibited a higher capacity retention rate compared to batteries using Si particles. [Explanation of Symbols]

[0064] 10 ...Carbon fiber raw materials 11 ... Carbon fiber 12 ... Carbon composite materials

Claims

1. A carbon composite material characterized by containing a carbon material having an aspect ratio of 2.7 or more and 7.5 or less, and Si.

2. The carbon composite material according to claim 1, wherein the crystallite size measured by X-ray diffraction of the Si is 1 nm to 1 μm.

3. The carbon composite material according to claim 1, wherein the Si is amorphous.

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

5. The carbon composite material according to claim 4, wherein the Si content in the carbon composite material is 10% by mass to 50% by mass.

6. The carbon composite material according to claim 1, wherein the outermost surface is coated with the carbon material.

7. The carbon composite material according to claim 1, wherein the carbon material is a composite in the form of a bundle of a plurality of carbon fibers having a plurality of pores, and the pore diameter of the pores is 1 nm to 20 nm.

8. The carbon composite material according to claim 7, wherein the length of the carbon fiber in the short axis direction is 0.1 μm to 3 μm, and the length of the carbon fiber in the long axis direction is 1 μm to 20 μm.

9. The carbon composite material according to claim 7, wherein the Si is supported within the pores.

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