JP7180466B2 - lithium ion battery - Google Patents

Description

The present disclosure relates to lithium ion batteries with good volumetric energy density.

Batteries have been actively developed in recent years. For example, the automotive industry is developing batteries for use in electric or hybrid vehicles. Among various batteries, lithium-ion batteries have the advantage of high energy density. For example, Patent Document 1 discloses an electrode cell in which the surface of a carbon material thin film having voids is coated with an electrolyte, and the voids are filled with a positive electrode active material.

JP 2010-244911 A

The main object of the present disclosure is to provide a lithium-ion battery with good volumetric energy density (energy per unit volume).

In order to solve the above problems, the present disclosure provides a negative electrode that is a carbon fiber assembly in which a plurality of carbon fibers are three-dimensionally connected, an electrolyte membrane covering the plurality of carbon fibers, and the carbon fiber assembly. a positive electrode filled in the voids of the body and containing a positive electrode active material, a conductive material, and an electrolytic solution, wherein the carbon fiber has a higher degree of graphitization than the carbon fiber in a cross section perpendicular to the axial direction. Provided is a lithium ion battery containing a high particle portion.

According to the present disclosure, since it has a structure in which the positive electrode is filled in the voids of the carbon fiber assembly (negative electrode), it is possible to obtain a lithium ion battery with a favorable volumetric energy density. Furthermore, according to the present disclosure, since the carbon fibers contain particles with a high degree of graphitization, a lithium-ion battery with a good volumetric energy density can be obtained.

In the above disclosure, the particle portion may be at least one of graphite and carbon nanotubes.

In the above disclosure, the ratio of the particle portion to the total of the carbon fiber and the particle portion may be 18.2% by mass or more.

In the above disclosure, the ratio of the particle portion to the total of the carbon fiber and the particle portion may be 26.1% by mass or more and 57.1% by mass or less.

In the above disclosure, an intermediate portion having a higher degree of graphitization than the carbon fiber and a lower degree of graphitization than the particle portion may be formed around the particle portion.

In the above disclosure, the carbon fiber includes a core portion positioned at the center in a cross section in a direction orthogonal to the axial direction, and a sheath portion formed on the outer periphery of the core portion and having a lower degree of graphitization than the core portion. and the particle portion may be formed in both the core portion and the sheath portion.

In the above disclosure, an intermediate portion having a higher degree of graphitization than the sheath portion and a lower degree of graphitization than the particle portion may be formed around the particle portion in the sheath portion.

In the above disclosure, the carbon fibers may be pitch-based carbon fibers.

In the above disclosure, the electrolyte membrane may contain the polymer and the electrolytic solution.

In the above disclosure, the polymer may be a fluoropolymer.

The lithium-ion battery according to the present disclosure has the effect of having good volumetric energy density.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating an example of a carbon fiber aggregate and an electrolyte membrane in the present disclosure; 1 is a schematic diagram illustrating an example of a lithium ion battery in the present disclosure; FIG. It is a schematic diagram explaining an example of the carbon fiber in this indication. It is a schematic diagram explaining an example of the carbon fiber in this disclosure. 1 is an SEM image of carbon fibers obtained in Example 1. FIG. 4 is an SEM image of carbon fibers obtained in Comparative Example 1. FIG. 1 shows the results of a charge/discharge test on evaluation batteries obtained in Examples 1 to 4 and Comparative Example 1. FIG. FIG. 3 is a schematic diagram illustrating the relationship between the graphite addition rate and the degree of graphitization of carbon fibers.

The lithium-ion battery according to the present disclosure will be described in detail below.

FIG. 1 is a schematic diagram illustrating an example of a carbon fiber aggregate and an electrolyte membrane in the present disclosure. As shown in FIG. 1, in the present disclosure, a carbon fiber assembly 10 in which a plurality of carbon fibers 1 are three-dimensionally connected is used as a negative electrode. Furthermore, an electrolyte membrane 20 is formed so as to cover the plurality of carbon fibers 1 . Further, the carbon fiber assembly 10 has voids 2 inside. The void 2 is filled with a positive electrode containing a positive electrode active material, a conductive material, and an electrolytic solution.

FIG. 2 is a schematic diagram illustrating an example of a lithium ion battery in the present disclosure. As shown in FIG. 2, the lithium ion battery 100 includes a negative electrode that is a carbon fiber assembly 10 in which a plurality of carbon fibers 1 are three-dimensionally connected, and a positive electrode active material that is filled in the gaps 2 of the carbon fiber assembly. 3a, a positive electrode 30 containing a conductive material 3b and an electrolytic solution 3c.

FIG. 3 is a schematic diagram illustrating an example of carbon fibers in the present disclosure. FIG. 3(a) is a schematic perspective view of the carbon fiber, and FIG. 3(b) is a schematic cross-sectional view of the carbon fiber in the axial direction Z. FIG. As shown in FIG. 3( a ), the carbon fiber 1 includes the particle portion 3 having a higher degree of graphitization than the carbon fiber 1 in the cross section in the direction perpendicular to the axial direction Z. As shown in FIG. As shown in FIG. 3(b), the particle portion 3 may be wholly included in the carbon fiber 1, or may be partially included in the carbon fiber 1. As shown in FIG. In the latter case, the particle portion 3 may be partially exposed from the carbon fiber 1 .

According to the present disclosure, since it has a structure in which the positive electrode is filled in the voids of the carbon fiber assembly (negative electrode), it is possible to obtain a lithium ion battery with a favorable volumetric energy density. Here, conventional batteries have a structure in which, for example, a positive electrode current collector, a positive electrode active material layer, an electrolyte layer (e.g., a separator layer containing an electrolytic solution), a negative electrode active material layer, and a negative electrode current collector are laminated. Lithium ion batteries are known. In contrast, in the present disclosure, the carbon fiber aggregate functions both as the negative electrode active material layer and as the negative electrode current collector, so that the volumetric energy density can be improved. In addition, since the positive electrode is filled in the voids of the carbon fiber assembly (negative electrode), the negative electrode and the positive electrode are arranged in close proximity via the insulating film. As a result, the moving distance of Li ions, which are carriers, is shortened. As a result, the ionic conduction resistance can be reduced and the output characteristics are improved.

Furthermore, according to the present disclosure, since the carbon fibers contain particles with a high degree of graphitization, a lithium-ion battery with a good volumetric energy density can be obtained. Specifically, since the particle portion has a high degree of graphitization, the capacity can be increased. In addition, since the particle portion is enclosed in the carbon fiber, the edge (end surface) of the particle portion is less exposed, and deterioration of the particle portion due to the electrolytic solution, for example, can be suppressed. In addition, since the carbon fibers are interspersed with particle portions having a high degree of graphitization, the expansion and contraction of the carbon fibers can be alleviated throughout the fibers, and the occurrence of cracks in the carbon fibers can be suppressed. Therefore, cracks are less likely to occur even if the diameter of the carbon fiber is increased. Furthermore, by increasing the diameter of the carbon fiber, the volume ratio of the electrolyte membrane can be relatively decreased. In addition, as will be described later, when a material for forming the particle portion (e.g., graphite) is added to the raw material mixture to form the precursor fiber, and heat treatment is performed, the crystallinity of the carbon fiber around the particle portion is also improved. . Therefore, the capacity can be increased. From these points, it is possible to improve the volumetric energy density.

Patent Document 1 describes a method of coating the surface of a carbon material thin film having voids with an electrolyte, for example, a method of immersing a carbon material thin film having voids in a solution in which an electrolyte is dissolved, followed by drying. . In the carbon fiber thin film after drying, voids corresponding to the volume of the solvent are formed. The presence of voids reduces the volumetric energy density. In addition, Patent Document 1 does not describe any working examples, and it is unknown whether or not it actually functions as a battery. In particular, the electronic and ionic conduction paths of the filled positive electrode are likely to be inadequate.

1. Negative Electrode The negative electrode in the present disclosure is a carbon fiber assembly in which a plurality of carbon fibers are three-dimensionally connected. “Three-dimensionally connected” means that a plurality of carbon fibers are bonded to each other and form a three-dimensional network structure.

In addition, the carbon fiber contains particles having a higher degree of graphitization than the carbon fiber in the cross section in the direction orthogonal to the axial direction. In a cross section perpendicular to the axial direction, the carbon fiber usually includes a plurality of particle portions. In particular, it is preferable to form a sea-island structure in which the carbon fiber is the sea and the plurality of particle portions are islands.

Examples of materials for the particle portion include carbon materials. The carbon material is preferably a material having a high degree of graphitization (crystallinity). Examples include graphite such as natural graphite, artificial graphite, and expanded graphite; , and fullerenes. The carbon materials may be used singly or in combination of two or more.

The particle part has a higher degree of graphitization than the carbon fiber. The degree of graphitization can be evaluated, for example, by the D/G ratio. Here, in the Raman spectrum, a G band peak derived from the graphite structure appears at 1590 cm ⁻¹ , and a D band peak derived from defects appears at 1350 cm ⁻¹ . Let these ratios be D/G ratios. A smaller D/G ratio means a higher degree of graphitization. The D/G ratio is, for example, 1.2 or less, and may be 0.8 or less.

Examples of the shape of the particle portion include granular, scale-like, and fibrous. The average size of the particle portion is, for example, 5 μm or less, and may be 0.5 μm or less. The average size of the particle portion can be obtained by, for example, SEM observation. The size of the particle part refers to the length of the longest part in the cross section. The number of samples is preferably large, for example 100 or more.

The ratio of the particle part to the total of the carbon fiber and the particle part is, for example, 18.2% by mass or more, and may be 26.1% by mass or more. If the proportion of the particle portion is too small, the volume energy density may not be sufficiently improved. On the other hand, the ratio of the particle part to the total of the carbon fiber and the particle part is, for example, 65% by mass or less, and may be 57.1% by mass or less. If the proportion of the particle portion is too high, fiberization may become difficult.

For example, when graphite and carbon nanotubes are used, the ratio of carbon nanotubes to the total of graphite and carbon nanotubes is, for example, 1% by mass or more, may be 3 % by mass or more, or may be 5% by mass or more. . On the other hand, the ratio of carbon nanotubes to the total of graphite and carbon nanotubes is, for example, 10% by mass or less.

Preferably, an intermediate portion having a higher degree of graphitization than the carbon fiber and a lower degree of graphitization than the particle portion is formed around the particle portion. When a material for forming the particle part (e.g., graphite) is added to the raw material mixture to form a precursor fiber and heat treated, the crystallinity of the carbon fiber around the particle part is also improved, and the intermediate part is formed. .

The carbon fiber may or may not have a core-sheath structure. As shown in FIG. 3( a ), the core-sheath structure is composed of a core portion 1 a positioned at the center and a sheath portion 1 b formed on the outer periphery of the core portion in a cross section perpendicular to the axial direction Z of the carbon fiber 1 . It is a structure having The sheath preferably has a lower degree of graphitization than the core. This is because the core portion has high rigidity and the core portion has low rigidity, so that the flexibility is higher than that of carbon fibers in which all the fibers are highly rigid. In addition, since the degree of graphitization of the sheath is low, there are other advantages such as the speed of the intercalation reaction on the surface of the carbon fiber being easily improved and side reactions at high temperatures being suppressed.

When the carbon fiber has a core-sheath structure, a particle portion is formed in at least one of the core portion and the sheath portion. The particle portion may be formed in the core portion and the sheath portion may not be formed, the particle portion may be formed in the sheath portion and the particle portion may not be formed in the core portion, and the core portion and the sheath portion may not be formed. Particle portions may be formed on both sides. The average size of the grains formed in the sheath may be smaller than the average size of the grains formed in the core.

Similarly, when the carbon fiber has a core-sheath structure, an intermediate portion may be formed in at least one of the core portion and the sheath portion. The intermediate portion may be formed in the core portion and the sheath portion may not be formed, the intermediate portion may be formed in the sheath portion and the intermediate portion may not be formed in the core portion, or the core portion and the sheath portion may be formed without the intermediate portion. An intermediate portion may be formed in both. For example, in FIG. 4, an intermediate portion 1c having a higher degree of graphitization than the core portion 1a and a lower degree of graphitization than the particle portion 3 is formed in the core portion 1a. Similarly, an intermediate portion 1c having a higher degree of graphitization than the sheath portion 1b and a lower degree of graphitization than the particle portion 3 is formed in the sheath portion 1b.

The average diameter of the carbon fibers is, for example, 5 μm or more, may be 10 μm or more, may be 30 μm or more, or may be 50 μm or more. As described above, in the present disclosure, since the carbon fiber is dotted with particles having a high degree of graphitization, the expansion and contraction of the carbon fiber can be alleviated throughout the fiber, and the occurrence of cracks in the carbon fiber can be suppressed. . Therefore, cracks are less likely to occur even if the diameter of the carbon fiber is increased. Also, by increasing the diameter of the carbon fiber, the volume ratio of the electrolyte membrane can be relatively reduced. Therefore, the volume energy density can be improved. On the other hand, the average diameter of carbon fibers is, for example, 100 μm or less.

Also, the average fiber length of the carbon fibers is, for example, 1 mm or more and 50 mm or less. The average diameter and average fiber length of carbon fibers can be determined, for example, by SEM observation. The number of samples is preferably large, preferably 100 or more for each.

A carbon fiber aggregate has voids surrounded by a plurality of carbon fibers. The porosity of the carbon fiber aggregate is, for example, 50% or more, and may be 60% or more. On the other hand, the porosity of the carbon fiber aggregate is, for example, 95% or less, and may be 90% or less. If the porosity is too high, the amount of the positive electrode to be introduced into the voids to adjust the positive/negative electrode capacity ratio is reduced, possibly resulting in a decrease in the volumetric energy density. The porosity can be determined with a common mercury porosimeter.

Examples of the shape of the carbon fiber aggregate include a sheet shape. Also, the thickness of the carbon fiber assembly is, for example, 5 μm or more, may be 10 μm or more, or may be 50 μm or more. On the other hand, the thickness of the carbon fiber aggregate is, for example, 500 μm or less, and may be 200 μm or less.

The method of forming the carbon fiber aggregate is not particularly limited, but an example thereof includes a method of sequentially performing a fiberization step, an infusibilization step, a carbonization step and a graphitization step.

A fiberization process is a process of obtaining a precursor fiber by melting and spinning a raw material mixture. The raw material mixture contains a carbon fiber forming material and a particle portion forming material. Materials for forming carbon fibers include, for example, pitch. Pitch includes, for example, isotropic pitch and anisotropic pitch. Isotropic pitch is a non-graphitizable material, and it is difficult to obtain carbon fibers with a high degree of graphitization. On the other hand, in the present disclosure, carbon fibers with a high degree of graphitization can be obtained by adding a material for forming the particle portion (for example, graphite). On the other hand, the higher the mesophase ratio of the anisotropic pitch, the higher the viscosity of the melt, so the fiberization must be performed at a high temperature. Therefore, it is preferable that the mesophase ratio of the anisotropic pitch is, for example, 20% by mass or less. Also, the softening point of the pitch is, for example, 280° C. or less.

The material for forming the particle portion is the same as the material for the particle portion described above. The ratio of the material for forming the particle portion in the raw material mixture is, for example, 40% by mass or less. If the proportion of the material for forming the particle part is too high, the fluidity of the melt tends to decrease, and the spinnability tends to decrease. On the other hand, the ratio of the material for forming the particle part in the raw material mixture is, for example, 5% by mass or more, may be 10% by mass or more, or may be 15% by mass or more.

As a method of melting the raw material mixture, for example, a method of heating a carbon fiber forming material (for example, pitch) at a temperature equal to or higher than its softening point can be mentioned. The melt can be used as such for spinning. Alternatively, the melt can be cooled, processed into pellets and the pellets used for spinning.

The spinning method includes, for example, a melt spinning method such as a meltblown method and a spunbond method. Moreover, it is preferable to obtain a non-woven fabric-like precursor fiber (precursor fiber assembly) by spinning.

The infusibilizing step is a step of infusibilizing the precursor fibers so that they do not melt during the carbonization step or the graphitization step. In the infusibilizing step, it is preferable to heat the precursor fiber while raising the temperature to oxidize the inside and outside of the precursor fiber. It is preferable to raise the temperature from room temperature to 300° C., for example. The heating atmosphere is preferably an atmosphere in which the precursor fibers are oxidized.

The carbonization step is a step of carbonizing the infusibilized precursor fibers. The heat treatment temperature is, for example, 1000° C. or higher and 1500° C. or lower. Moreover, the heat treatment atmosphere includes, for example, an inert gas atmosphere such as argon and nitrogen.

The graphitization step is a step of graphitizing the carbonized precursor fibers. Moreover, when the carbonization step is not performed, the graphitization step is a step of graphitizing the infusibilized precursor fibers. The heat treatment temperature is, for example, 2000° C. or higher and 3000° C. or lower. Moreover, the heat treatment atmosphere includes, for example, an inert gas atmosphere such as argon and nitrogen.

2. Electrolyte Membrane The electrolyte membrane in the present disclosure coats a plurality of carbon fibers. The electrolyte membrane separates the negative electrode (carbon fiber aggregate) from the positive electrode.

The electrolyte membrane preferably contains a polymer and an electrolytic solution. Also, the polymer is preferably electronically insulating. For example, an electronically insulating and ionically conductive electrolyte membrane can be obtained by swelling an electronically insulating polymer with an electrolytic solution.

Examples of the polymer include fluorine-based polymers such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE), and fluororubber. Other examples of polymers include polyethylene oxide (PEO), polyacrylic acid (PAA), polymethyl methacrylate (PMMA), and polyvinyl alcohol (PVA).

The electrolyte membrane may contain an inorganic filler. Examples of inorganic fillers include alumina, silica, and zirconia.

The thickness of the electrolyte membrane is, for example, 0.5 μm or more, and may be 1 μm or more. On the other hand, the thickness of the electrolyte membrane is, for example, 50 μm or less, and may be 30 μm or less. The coverage of the electrolyte membrane with respect to the carbon fibers is preferably high, for example 95% or more. In particular, it is preferable that all portions of the carbon fiber assembly other than the current collecting portion are covered with the electrolyte membrane.

A method of forming the electrolyte membrane includes a method of immersing a carbon fiber assembly coated with a polymer membrane in an electrolytic solution. Moreover, as a method of coating the carbon fiber aggregate with a polymer film, for example, a method of electrodeposition by immersing the carbon fiber aggregate in a solution containing a polymer can be mentioned.

3. Positive Electrode The positive electrode in the present disclosure is filled in the voids of the carbon fiber assembly and contains a positive electrode active material, a conductive material, and an electrolytic solution. The positive electrode has a positive electrode active material and a conductive material dispersed in an electrolytic solution and has fluidity. Moreover, the positive electrode only needs to be filled in the voids of the carbon fiber assembly to the extent that it functions as a battery. That is, the positive electrode may be completely filled in the voids of the carbon fiber aggregate, or may be filled so as to partially leave the voids in the carbon fiber aggregate.

The solid content ratio in the positive electrode is, for example, 30% by mass or more, and may be 40% by mass or more. If the solid content ratio in the positive electrode is too low, the ratio of the positive electrode active material also becomes relatively low, possibly failing to improve the volumetric energy density. On the other hand, the solid content ratio in the positive electrode is, for example, 90% by mass or less, and may be 80% by mass or less. If the solid content ratio in the positive electrode is too high, the viscosity becomes high, and it may become difficult to fill the voids of the carbon fiber aggregate.

Examples of positive electrode active materials include oxide active materials. Examples of oxide active materials include rock salt layered active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn ₂ O ₄ and Li ₄ . Spinel-type active materials such as Ti ₅ O ₁₂ and Li(Ni _0.5 Mn _1.5 )O ₄ and olivine-type active materials such as LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ and LiCoPO ₄ can be used.

Examples of the shape of the positive electrode active material include particulate. The average particle size (D ₅₀ ) of the positive electrode active material is, for example, 0.5 μm or more, and may be 1 μm or more. On the other hand, the average particle size (D ₅₀ ) of the positive electrode active material is, for example, 50 μm or less, and may be 30 μm or less. The average particle size ( _D50 ) can be obtained from the results of particle size distribution measurement by a laser diffraction scattering method.

The conductive material imparts electronic conductivity to the positive electrode. Examples of conductive materials include carbon materials. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and ketjen black (KB), and fibrous carbon materials such as carbon fibers, carbon nanotubes (CNT), and carbon nanofibers (CNF). . In the present disclosure, since the positive electrode active material is dispersed in the electrolytic solution, the conductive material is preferably a material capable of forming a long electron conduction path. In that respect, it is preferable to use a fibrous carbon material.

The ratio of the conductive material in the positive electrode is, for example, 0.5 parts by mass or more, and may be 1 part by mass or more, when the positive electrode active material is 100 parts by mass. On the other hand, the ratio of the conductive material is, for example, 20 parts by mass or less, and may be 10 parts by mass or less, when the positive electrode active material is 100 parts by mass.

The electrolyte imparts ionic conductivity to the positive electrode. The electrolytic solution preferably contains a Li salt and a solvent. Examples of Li salts include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiC( Organic lithium salts such as CF ₃ SO ₂ ) ₃ are included. Also, the Li salt is preferably a lithium imide salt. Lithium imide salts include, for example, R ¹ SO ₂ N(Li)SO ₂ R ² (R ¹ and R ² are each independently a fluorinated hydrocarbon having 1 to 12 carbon atoms or a substituted product thereof). The compound represented by is mentioned. Specific examples of lithium imide salts include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI).

The concentration of Li salt in the electrolytic solution is, for example, 0.5 mol % or more, and may be 1 mol % or more. On the other hand, the concentration of Li salt in the electrolytic solution is, for example, 5 mol % or less, and may be 3 mol % or less.

The solvent may be water or a non-aqueous solvent. Non-aqueous solvents include, for example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), butylene carbonate (BC), γ-butyrolactone, sulfolane , acetonitrile, and any mixture thereof. Also, an ionic liquid may be used as the solvent.

Also, the solvent may be an ether-based solvent. Examples of ether-based solvents include cyclic ethers such as 1,4-dioxane (DX), tetrahydrofuran (THF), diglyme, triglyme, and tetraglyme, chains such as 1,2-dimethoxyethane (DME), dimethyl ether, and diethyl ether. ethers. As the ether-based solvent, the fluorine-substituted cyclic ether and the chain ether may be used.

As a method of forming the positive electrode, for example, there is a method of filling the voids of the carbon fiber assembly with a positive electrode slurry containing a positive electrode active material, a conductive material, and an electrolytic solution.

4. Lithium Ion Battery The lithium ion battery in the present disclosure may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because they can be repeatedly charged and discharged, and are useful, for example, as batteries for vehicles. The secondary battery also includes the use of a secondary battery as a primary battery (use for the purpose of initial charging only).

Note that the present disclosure is not limited to the above embodiments. The above embodiment is an example, and any device that has substantially the same configuration as the technical idea described in the claims of the present disclosure and produces the same effect is the present invention. It is included in the technical scope of the disclosure.

[Example 1]
(Preparation of negative electrode)
Isotropic pitch and graphite particles were mixed. The mixing ratio was isotropic pitch:graphite=90:10 by mass . The carbonization yield of the isotropic pitch is 50%, and the ratio of the carbon fiber finally obtained and the graphite included in the carbon fiber is a mass ratio of carbon fiber: graphite = 45: 10. The ratio of graphite to the total of carbon fiber and graphite is 18.2% by mass .

The obtained raw material mixture was melted and extruded to obtain precursor fibers in the form of a non-woven fabric (fiberization step). Next, the non-woven fabric-like precursor fiber was heat-treated at 350° C. in an air atmosphere (infusibilization step). Next, the infusible precursor fiber was subjected to stepwise heat treatment from 2300° C. to 3000° C. in an N ₂ atmosphere (carbonization step and graphitization step). Thus, a carbon fiber aggregate (negative electrode) was obtained.

(Preparation of electrolyte membrane)
The obtained carbon fiber aggregate was coated with a polymer. PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) was used as the polymer. The carbon fiber assembly was immersed in a solution of PVDF-HFP dissolved in acetone solvent, and the surface of the carbon fiber was coated with a polymer film by electrodeposition. As a result, a carbon fiber aggregate coated with a polymer film was obtained.

(Preparation of positive electrode slurry)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was prepared as a positive electrode active material, and acetylene black and vapor grown carbon fiber (VGCF) were prepared as a conductive material. As an electrolytic solution, a solution was prepared by dissolving LiPF ₆ in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) to a concentration of 1M. Next, 50% by mass of the positive electrode active material, 5.6 % by mass of the conductive material, and 44.4% by mass of the electrolytic solution were weighed and sufficiently mixed to obtain a positive electrode slurry.

(Preparation of battery for evaluation)
A carbon fiber assembly coated with a polymer film was prepared, a part of the polymer film was removed, and Au vapor deposition was performed to form a current collector. After that, a Ni tab (negative electrode tab) was arranged so as to be in contact with the current collector. The resulting structure was placed in a SUS case of the same volume, and then filled with positive electrode slurry to obtain a positive electrode. In addition, current collection of the positive electrode was performed from the SUS case. The SUS case was hermetically sealed while taking care that the current collectors of the positive and negative electrodes were not in contact with each other. Thus, an evaluation battery was obtained.

[Example 2]
A carbon fiber aggregate and an evaluation battery were obtained in the same manner as in Example 1, except that the mixing ratio of isotropic pitch and graphite particles was set to a mass ratio of isotropic pitch:graphite=85:15. The ratio of graphite to the total of carbon fiber and graphite is 26.1% by mass .

[Example 3]
A carbon fiber aggregate and an evaluation battery were obtained in the same manner as in Example 1, except that the mixing ratio of isotropic pitch and graphite particles was set to a mass ratio of isotropic pitch:graphite=60:40. The ratio of graphite to the total of carbon fiber and graphite is 57.1% by mass .

[Example 4]
Carbon nanotubes (CNT) were further used, and the mixing ratio of isotropic pitch, graphite particles and CNT was set to a mass ratio of isotropic pitch: graphite: CNT = 84: 15: 1. A carbon fiber assembly and an evaluation battery were obtained in the same manner. The ratio of graphite is 25.9% by mass and the ratio of CNT is 1.7 % by mass with respect to the total of carbon fiber, graphite and CNT. Also, the ratio of the particle portion (graphite and CNT) to the total of the carbon fiber and particle portion (graphite and CNT) is 27.6% by mass .

[Comparative Example 1]
A carbon fiber assembly and an evaluation battery were obtained in the same manner as in Example 1, except that graphite particles were not used.

[evaluation]
(SEM observation)
The carbon fiber aggregates obtained in Example 1 and Comparative Example 1 were observed with a scanning electron microscope (SEM). The results are shown in FIGS. 5 and 6. FIG. As shown in FIG. 5A, in Example 1, it was confirmed that unevenness derived from graphite was formed on the surface of the carbon fiber. In addition, as shown in FIG. 5(b), in Example 1, it was confirmed that fine graphite particles were included in the cross section of the carbon fiber. Also, the graphite was oriented along the axial direction. Therefore, it was suggested that the electron conductivity of the carbon fiber is improved. In contrast, as shown in FIGS. 6A and 6B, in Comparative Example 1, the surface and cross section of the carbon fibers were smooth.

(Charging and discharging test)
The evaluation batteries obtained in Examples 1 to 4 and Comparative Example 1 were subjected to a charge/discharge test. The measurement conditions were a voltage range of 3.0V to 4.2V and a rate of 0.2C. Several cycles of charging and discharging were performed for conditioning, and then the discharge capacity was measured. Table 1 shows the results. The discharge capacity in Table 1 is a relative value when the result of Comparative Example 1 is set to 100.

Also, a half cell was produced in order to evaluate the properties of the carbon fiber aggregate. Specifically, in the same manner as in Examples 1 to 4 and Comparative Example 1, except that the same electrolytic solution as in Example 1 was used instead of the positive electrode slurry, and the Li foil was used as the counter electrode. A half cell was produced for each. A charging/discharging test was performed on the obtained half-cell in the same manner as described above. The results are shown in FIG. Since this half-cell is a cell for evaluating the characteristics of the negative electrode (carbon fiber aggregate), a decrease in the cell voltage is defined as charge, and an increase in the cell voltage is defined as discharge.

As shown in Table 1 and FIG. 7, it was confirmed that Examples 1 to 4 had higher discharge capacity and negative electrode capacity than Comparative Example 1. In particular, in Examples 2 to 4, it was confirmed that the discharge capacity and the negative electrode capacity were significantly higher than those in Comparative Example 1.

(Raman spectral mapping measurement)
Raman spectral mapping measurement was performed on the carbon fiber aggregates obtained in Examples 1-4. Specifically, Raman spectroscopy mapping measurement was performed on the cross section of the carbon fiber to obtain the distribution of the degree of graphitization. As a result, the carbon fibers of Examples 1 to 4 all have a core portion with a relatively high degree of graphitization in the center of the cross section, and a sheath portion with a relatively low degree of graphitization in the outer edge of the cross section. It was confirmed that Furthermore, the particle portions (flake-like minute graphite particles) were present in the form of islands in both the core portion and the sheath portion. In addition, the total area of the particles in the sheath was smaller than the total area of the particles in the core.

FIG. 8 is a schematic diagram illustrating the relationship between the graphite addition rate and the degree of graphitization of carbon fibers. As shown in FIG. 8, the reference for Comparative Example 1 (no graphite added) is _L1 . On the other hand, L ₃ is the carbon fiber obtained in Example 1 (graphite addition rate of 10 mass %), Example 2 (graphite addition rate of 15 mass %), and Example 3 (graphite addition rate of 40 mass %). It shows the trend of the degree of graphitization of In Examples 1 to 3, it was confirmed that the degree of graphitization of the carbon fibers increased as the graphite addition rate increased (L ₃ ). On the other hand, _L2 indicates the degree of graphitization when it is assumed that the added graphite does not contribute to the graphitization of carbon fibers at all. Specifically, L ₂ is the result of adding the graphitization degree of added graphite to the graphitization degree of L ₁ .

As shown in FIG. 8, since _L3 is higher than _L2 , the particle part (graphite particle) included in the carbon fiber is exposed to the surroundings during heat treatment (carbonization process and graphitization process) It was suggested that it has the function of improving the degree of graphitization of carbon fibers. That is, it was suggested that the obtained carbon fiber had the above-described intermediate portion formed around the particle portion.

Further, the carbon fibers obtained in Examples 1 to 4 had a core-sheath structure, and the particle portion was formed in the sheath portion. In heat treatment, the temperature on the outside of the carbon fiber rises first. Therefore, it was suggested that an intermediate portion having a higher degree of graphitization than the sheath portion and a lower degree of graphitization than the particle portion was formed around the particle portion in the sheath portion. By using particles such as graphite and CNT, which have hexagonal carbon planes (basal planes) on the surface, a crystallization template effect occurs during heat treatment, and the degree of graphitization (crystallinity) of carbon fibers also increases. presumed to have improved.

DESCRIPTION OF SYMBOLS 1... Carbon fiber 1a... Core part 1b... Sheath part 2... Void 3a... Positive electrode active material 3b... Conductive material 3c... Electrolytic solution 10... Carbon fiber assembly 20... Electrolyte membrane 30... Positive electrode 100... Lithium ion battery

Claims (9)

a negative electrode that is a carbon fiber assembly in which a plurality of carbon fibers are three-dimensionally connected;
an electrolyte membrane covering the plurality of carbon fibers;
a positive electrode filled in the voids of the carbon fiber assembly and containing a positive electrode active material, a conductive material, and an electrolytic solution;
has
The carbon fiber includes a particle portion having a higher degree of graphitization than the carbon fiber in a cross section in a direction orthogonal to the axial direction ,
A lithium ion battery , wherein the ratio of the particle portion to the total of the carbon fiber and the particle portion is 18.2% by mass or more and 65% by mass or less. 2. The lithium ion battery according to claim 1, wherein said particle portion is at least one of graphite and carbon nanotubes. 3. The lithium ion battery according to claim 1, wherein the ratio of said particle portion to the total of said carbon fiber and said particle portion is 26.1% by mass or more and 57.1% by mass or less. 4. An intermediate portion having a higher degree of graphitization than the carbon fiber and a lower degree of graphitization than the particle portion is formed around the particle portion. The lithium ion battery according to . A core-sheath structure in which the carbon fiber has a core positioned at the center and a sheath formed on the outer periphery of the core and having a lower degree of graphitization than the core in a cross section in a direction orthogonal to the axial direction. with
5. The lithium ion battery according to any one of claims 1 to 4 , wherein said particle portion is formed on both said core portion and said sheath portion. 6. The lithium ion battery according to claim 5 , wherein an intermediate portion having a higher degree of graphitization than said sheath portion and a lower degree of graphitization than said particle portion is formed around said particle portion in said sheath portion. 7. The lithium ion battery according to any one of claims 1 to 6 , wherein said carbon fibers are pitch-based carbon fibers. 8. The lithium ion battery according to any one of claims 1 to 7 , wherein said electrolyte membrane contains a polymer and said electrolyte. 9. The lithium ion battery of claim 8 , wherein said polymer is a fluoropolymer.

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