JP7247821B2 - A negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery. - Google Patents

Description

TECHNICAL FIELD The invention disclosed in this specification relates to a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery.

Conventionally, as a secondary battery with high energy density, there is known a battery that includes a columnar negative electrode, a separation membrane provided so as to surround the columnar negative electrode, and a positive electrode provided around the separation membrane. (See, for example, Patent Document 1). In this secondary battery, the columnar negative electrode is, for example, carbon fiber with a diameter of 50 μm.

JP 2018-152229 A

However, although the secondary battery of Patent Document 1 has a relatively high energy density, it sometimes has poor input/output characteristics.

The present disclosure has been made in view of such problems, and has a main purpose of improving the energy density and input/output characteristics of secondary batteries.

As a result of intensive research to achieve the above-described object, the present inventors found that the energy density and input/output characteristics of secondary batteries can be improved by devising negative electrodes for secondary batteries. I have completed the invention disclosed in the book.

That is, the negative electrode for secondary batteries disclosed in the present specification is
A fiber bundle in which carbon fibers are bundled,
Graphite particles provided between the carbon fibers;
with
The graphite particles have shape anisotropy and have an average particle size D50 of 1 μm or more and 10 μm or less as measured by a laser diffraction method,
The content of the graphite particles is 5% by weight or more and 50% by weight or less.

In addition, the secondary battery disclosed in this specification is
the negative electrode for the secondary battery described above;
a separation membrane provided to surround the negative electrode for a secondary battery;
a positive electrode including a positive electrode active material and provided around the separation membrane;
is provided.

Further, the method for producing a negative electrode for a secondary battery disclosed in this specification includes:
A method for manufacturing the negative electrode for a secondary battery described above,
(a) filling a nozzle with a slurry containing the graphite particles;
(b) passing the carbon fiber yarn containing the carbon fiber through the nozzle filled with the slurry;
includes.

In the negative electrode for a secondary battery disclosed in this specification, the carbon fibers forming the fiber bundle function as a negative electrode current collector as well as a negative electrode active material. The graphite particles provided between the carbon fibers function as a negative electrode active material with a higher capacity than the carbon fibers. Graphite particles may increase the irreversible capacity of the negative electrode during initial charge/discharge, but if the graphite particle content and average particle size are within a predetermined range, such irreversible capacity is relatively small. Since the irreversible capacity of the negative electrode during the initial charge/discharge can be suppressed while using graphite particles as a high-capacity negative electrode active material, a secondary battery using this negative electrode for a secondary battery has a high energy density. Graphite particles having shape anisotropy function as a conductive material to increase electronic conductivity. A suitable gap is formed to facilitate the movement of carrier ions. Therefore, a secondary battery using this negative electrode for a secondary battery has high input/output characteristics. This negative electrode for a secondary battery can be manufactured by a simple method of filling a nozzle with a slurry containing graphite particles and passing a carbon fiber yarn through the nozzle.

1 is a perspective view showing a schematic configuration of a negative electrode 10; FIG. FIG. 4 is an explanatory view showing an outline of a method for manufacturing the negative electrode 10; FIG. 2 is a perspective view showing a schematic configuration of a pin-type secondary battery 30; FIG. 2 is a perspective view showing a schematic configuration of a prismatic secondary battery 40; FIG. 3 is a side view SEM photograph of negative electrodes for secondary batteries of Comparative Example 1 and Example 1. FIG. 4 is a cross-sectional SEM photograph of the negative electrode for a secondary battery of Comparative Example 1 and Example 1. FIG. Explanatory drawing of the sample for resistance evaluation. Explanatory drawing of the sample for charge/discharge evaluation.

Next, the negative electrode 10, the pin-shaped secondary battery 30, and the prismatic secondary battery 40 disclosed in this embodiment will be described with reference to the drawings. 1 is a perspective view showing a schematic configuration of the negative electrode 10, FIG. 2 is an explanatory view showing a schematic of a manufacturing method of the negative electrode 10, FIG. 3 is a perspective view showing a schematic configuration of a pin-shaped secondary battery 30, and FIG. 2 is a perspective view showing a schematic configuration of a secondary battery 40. FIG. Here, for convenience of explanation, a lithium ion secondary battery using lithium ions as a carrier will be described as an example of the secondary battery.

As shown in FIG. 1, the negative electrode 10 is a columnar negative electrode for a lithium ion secondary battery, and includes a fiber bundle 14 in which carbon fibers 12 are bundled and provided between the carbon fibers 12 of the fiber bundle 14. and graphite particles 16.

The fiber bundle 14 is obtained by bundling a large number of straight carbon fibers 12 . The diameter Df of the carbon fibers 12 is preferably 1 μm or more and 200 μm or less. If the diameter Df is 1 μm or more, the strength of the fiber bundle 14 can be ensured and stable charging and discharging can be performed. Further, if the diameter Df is 200 μm or less, the movement distance of ions, which are carriers, does not become too long, and high output performance can be obtained. Also, if the diameter Df is within this range, the energy density per unit volume can be further increased. Alternatively, within this range, the moving distance of ions, which are carriers, can be shortened, and charging and discharging can be performed with a larger current. More preferably, the diameter Df of the carbon fibers 12 is 5 μm or more and 20 μm or less. The longitudinal length Lf of the carbon fibers 12 is preferably 20 mm or more and 200 mm or less. When the length Lf of the carbon fibers 12 is 20 mm or more, the battery capacity can be further increased, and when it is 200 mm or less (especially 150 mm or less), the electrical resistance of the negative electrode 10 can be further reduced, which is preferable. The diameter Db of the fiber bundle 14, that is, the diameter of the negative electrode 10 is preferably 50 μm or more and 1000 μm or less. If the diameter Db is 50 μm or more, the strength of the fiber bundle 14 can be ensured and stable charging and discharging can be performed. Further, if the diameter Db is 1000 μm or less, the movement distance of ions, which are carriers, does not become too long, and input/output characteristics can be improved. The fiber bundle 14 is preferably a bundle of 50 or more and 12000 or less carbon fibers 12 . Within this range, the content of the graphite particles can be 5% by weight or more and 50% by weight, while keeping the diameter Db of the fiber bundle 14 within the range described above, for example.

The fiber bundle 14 may optionally contain a binder. The binder serves to bind the carbon fibers 12 and the graphite particles 16 together to maintain a predetermined shape. Alternatively, thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used singly or as a mixture of two or more. Cellulose-based or styrene-butadiene rubber (SBR) aqueous dispersions, which are water-based binders, can also be used.

The graphite particles 16 are provided between the carbon fibers 12 of the fiber bundle 14 . The graphite particles 16 have shape anisotropy. Graphite particles 16 may have a flattened shape. The graphite particles 16 having shape anisotropy may have, for example, a ratio of short sides to long sides of 1/10 or less. Graphite particles having shape anisotropy include flaky graphite, flaky graphite, exfoliated graphite, expanded graphite, and expanded graphite. Among these, flake graphite is preferred. When the flake graphite is arranged between the carbon fibers 12, it is arranged in such a direction as to be in surface contact with the carbon fibers 12. Therefore, the contact area with the carbon fibers 12 is large, and the electron conductivity of the negative electrode 10 is It is highly effective in increasing The graphite particles 16 have an average particle diameter D50 of 1 μm or more and 10 μm or less as measured by a laser diffraction method. When the average particle size D50 is 1 μm or more, the irreversible capacity of the negative electrode 10 in the initial charge/discharge is small, so the energy density of the secondary battery can be increased. Further, when the average particle size D50 is 10 μm or less, the input/output characteristics of the secondary battery can be improved. The average particle size D50 may be equal to or less than the diameter Df of the carbon fibers 12 . The smaller the average particle size D50, the higher the input/output characteristics. The content Cg of the graphite particles 16 in the negative electrode 10 is 5% by weight or more and 50% by weight or less, preferably 25% by weight or more and 40% by weight or less. Within this range, the input/output characteristics of the negative electrode 10 are high, so the input/output characteristics of the secondary battery can be improved. In addition, when the content Cg is 5% by weight or more, the irreversible capacity of the negative electrode 10 in the initial charge/discharge is small, and when the content Cg is 50% by weight or less, the discharge capacity of the negative electrode 10 is large. can increase

Next, a method for manufacturing this negative electrode 10 will be described with reference to FIG. This manufacturing method includes (a) a step of filling a nozzle 52 with a slurry 50 containing graphite particles 16, and (b) a step of passing a carbon fiber yarn 54 containing carbon fibers 12 through the nozzle 52 filled with the slurry 50. and including.

In step (a), slurry 50 is filled into nozzle 52 . The slurry 50 may be prepared, for example, by mixing the graphite particles 16 and a binder and adding an appropriate solvent to form a slurry. As the graphite particles 16 and the binder, those described above can be used. Examples of the solvent include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide and tetrahydrofuran. . Alternatively, water or water to which a dispersant, a thickener, or the like is added may be used. As the thickening agent, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used singly or as a mixture of two or more. The nozzle 52 filled with the slurry 50 is a cylindrical body tapered toward the outlet 52o, and the outlet is circular. The diameter of the outlet 52o of the nozzle 52 is preferably 60 μm or more and 1200 μm or less. By using such a nozzle 52, the diameter Db of the fiber bundle 14 can be set to 50 μm or more and 1000 μm or less.

In step (b), carbon fiber yarn 54 is passed through nozzle 52 filled with slurry 50 . The carbon fiber yarn 54 is obtained by spinning a large number of filaments that are the carbon fibers 12 . When the carbon fiber yarn 54 enters the slurry 50 , the carbon fibers 12 are separated from each other, the slurry 50 containing the graphite particles 16 enters between the carbon fibers 12 , and the graphite particles 16 are attached to each of the carbon fibers 12 . adheres. After that, when passing through the tapered circular exit 52 o , the carbon fibers 12 are arranged into a rod shape having a circular cross-section with the graphite particles 16 sandwiched between them to form the compact 60 . When this molded body 60 is cut into an appropriate length as necessary and dried, the solvent contained in the slurry 50 is volatilized, and the negative electrode 10 is obtained.

In this manufacturing method, by adjusting the diameter of the outlet 52o of the nozzle 52 and the types and ratios of components contained in the slurry 50, the amount of the graphite particles 16 contained in the negative electrode 10, the amount of the binder, and the diameter of the negative electrode 10 etc. can be adjusted.

Next, a pin-shaped secondary battery 30 using this negative electrode 10 will be described with reference to FIG.

The pin-shaped secondary battery 30 includes a negative electrode 10, a separation membrane (also referred to as partition wall) 18, and a columnar positive electrode 20, as shown in FIG.

The negative electrode 10 is as already described.

The separation film 18 is provided so as to surround the outer peripheral surface (excluding the upper portion) and the lower end surface of the cylindrical negative electrode 10 . Separation film 18 is not provided on the upper end surface of negative electrode 10 . The separation film 18 has conductivity for ions, which are carriers, and insulates the negative electrode 10 and the columnar positive electrode 20 . A polymer having ionic conductivity and insulating properties is suitable for the separation membrane 18 . Examples of the separation membrane 18 include a copolymer of polyvinylidene fluoride (PVdF) and hexafluoropropylene (HFP), polymethyl methacrylate (PMMA), and a copolymer of PMMA and an acrylic polymer. . For example, in the case of a copolymer of PVdF and HFP, part of the electrolytic solution swells and gels the film, forming an ion-conducting film. The thickness of the isolation film 18 is preferably 0.5 μm or more, more preferably 2 μm or more, and may be 5 μm or more in consideration of ensuring insulation, for example. In addition, the thickness of the separation membrane 18 is preferably 20 μm or less, more preferably 10 μm or less, in consideration of suppressing a decrease in ionic conductivity. Therefore, the thickness of the separation membrane 18 is preferably in the range of 0.5 to 20 μm in order to achieve both ionic conductivity and insulation. The separation membrane 18 may be formed, for example, by immersing the negative electrode 10 in a solution containing raw materials and coating the surface thereof.

In the present embodiment, the electrolytic solution is a non-aqueous solvent in which a supporting salt containing lithium ions is dissolved (non-aqueous electrolytic solution). Non-aqueous solvents include, for example, carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, the carbonates include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate, ethyl - chain carbonates such as n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; Chain esters such as methyl formate, methyl acetate, ethyl acetate and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran. sulfolane such as sulfolane and tetramethylsulfolane; and dioxolane such as 1,3-dioxolane and methyldioxolane. Examples of supporting salts include _LiPF6 , _{LiBF4 ,} LiAsF6, _LiCF3SO3 , LiN( _{CF3SO2 ) 2} , LiC( _{CF3SO2 ) 3 ,} LiSbF6, _{LiSiF6 , LiAlF4} , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, LiAlCl ₄ and the like. Among them, 1 selected from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ and LiClO ₄ and organic salts such as LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ and LiC(CF ₃ SO _{2 )} 3 From the viewpoint of electrical properties, it is preferable to use a species or a combination of two or more salts. The concentration of the supporting salt in the electrolytic solution is preferably 0.1 mol/L or more and 5 mol/L or less, more preferably 0.5 mol/L or more and 2 mol/L or less.

The columnar positive electrode 20 is formed in a cylindrical shape having a central hole 20a. In the central hole 20a of the columnar positive electrode 20, the negative electrode 10 whose outer peripheral surface and lower end surface are surrounded by the separation film 18 is arranged. The center hole 20 a is a bottomed cylindrical hole provided along the central axis of the columnar positive electrode 20 from the upper end surface of the columnar positive electrode 20 to just before the lower end surface. Therefore, the outer peripheral surface and lower end surface of the negative electrode 10 are insulated from the inner wall and bottom surface of the central hole 20 a of the columnar positive electrode 20 by the separation film 18 .

The columnar positive electrode 20 contains a positive electrode active material, but if the positive electrode active material does not have conductivity, it may be molded by mixing a conductive material having conductivity. The columnar positive electrode 20 may be formed by mixing, for example, a positive electrode active material, and, if necessary, a conductive material and a binder. Examples of the positive electrode active material include materials capable of intercalating and deintercalating lithium, which is a carrier. Examples of positive electrode active materials include compounds containing lithium and transition metals. Examples of such compounds include oxides containing lithium and a transition metal element, and phosphate compounds containing lithium and a transition metal element. Specifically, a lithium-manganese composite oxide having a basic composition formula of Li _(1-x) MnO ₂ (0≦x≦1, etc., hereinafter the same) or Li _(1-x) Mn ₂ O ₄ , etc., basic composition A lithium cobalt composite oxide having a formula such as Li _{(1-x) CoO2} , a lithium nickel composite oxide having a basic composition formula such as Li _{(1-x) NiO2} , and a basic composition formula of Li _(1-x) CoaNibMncO2 (a _> 0, b>0, c>0, _a + _{b +} c ₌ 1), _{Li (1-x) CoaNibMncO4 (} 0<a<1, 0<b <1, 1 ≤ c < 2, a + b + c = 2), etc., lithium vanadium composite oxides with a basic composition formula of LiV ₂ O ₃ , etc., basic composition formulas of V ₂ O ₅ , etc. A transition metal oxide or the like can be used. Also, a lithium iron phosphate compound having a basic compositional formula of LiFePO ₄ or the like can be used as the positive electrode active material. Among these, lithium-cobalt-nickel-manganese _composite oxides such _as LiCo1 _{/ 3Ni1} / _{3Mn1 / 3O2} and _{LiNi0.4Co0.3Mn0.3O2} are preferable. In addition, the "basic composition formula" means that other elements such as Al and Mg may be included.

When the columnar positive electrode 20 contains a conductive material, the conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect battery performance. Graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.), etc. Use one or a mixture of two or more be able to. When the columnar positive electrode 20 contains a binder, the binder is not particularly limited as long as it serves to bind the active material particles and the conductive material particles and maintain a predetermined shape. Fluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-containing resins such as fluorine rubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) etc. can be used alone or as a mixture of two or more. Cellulose-based or styrene-butadiene rubber (SBR) aqueous dispersions, which are water-based binders, can also be used.

The pin-shaped secondary battery 30 described in detail above has high energy density and high input/output characteristics. The reason why such an effect is obtained is considered as follows. That is, in the negative electrode 10 of the pin-shaped secondary battery 30, the carbon fibers 12 forming the fiber bundle 14 function both as a negative electrode active material and as a negative electrode current collector. The graphite particles 16 provided between the carbon fibers 12 function as a negative electrode active material with a higher capacity than the carbon fibers 12 . Graphite particles 16 may increase the irreversible capacity of the negative electrode during initial charge/discharge, but if the content Cg and average particle diameter D50 of graphite particles 16 are within a predetermined range, such irreversible capacity is relatively small. Since the irreversible capacity of the negative electrode 10 during the initial charge/discharge can be suppressed while using the graphite particles 16 as a high-capacity negative electrode active material, the pin-shaped secondary battery 30 using this negative electrode 10 has a high energy density. In addition, the graphite particles 16 having shape anisotropy function as a conductive material to increase electronic conductivity, and if the content Cg and the average particle size D50 are within a predetermined range, A moderate gap that serves as a passage for carrier ions is formed to smooth the movement of carrier ions. Therefore, the pin-shaped secondary battery 30 using this negative electrode 10 has high input/output characteristics. This negative electrode 10 can be manufactured by a simple method of filling a nozzle 52 with a slurry 50 containing graphite particles 16 and passing a carbon fiber yarn 54 through the nozzle 52 .

Next, a prismatic secondary battery 40 using the negative electrode 10 will be described with reference to FIG.

The prismatic secondary battery 40 includes a battery body 42, a negative electrode current collector 44, and a positive electrode current collector 46, as shown in FIG.

The battery main body 42 includes m pieces (m is an integer of 2 or more) in the left-right direction and n pieces (n is an integer of 2 or more) in the front-rear direction so that the upper end surface of the negative electrode 10 faces upward. ) are arranged side by side and compressed laterally and longitudinally so that the overall shape is a rectangular parallelepiped. Therefore, the columnar positive electrode 20 of the pin-shaped secondary battery 30 forming the battery main body 42 is a rectangular parallelepiped having a central hole 20a. The length of each side of the battery main body 42 is X in the left-right direction, Y in the up-down direction, and Z in the front-rear direction (X<Y<Z).

The negative electrode current collector 44 is a plate-like member made of a conductive material and arranged on the upper surface of the battery main body 42 . The upper end surfaces of all the negative electrodes 10 are connected in parallel to the negative electrode current collector 44 . Examples of the conductive material forming the negative electrode current collector 44 include carbon paper, aluminum, copper, titanium, stainless steel, nickel, iron, platinum, calcined carbon, conductive polymer, conductive glass, and the like. For the purpose of improving properties, electrical conductivity and oxidation resistance (reducing property), aluminum, copper or the like whose surface has been treated with carbon, nickel, titanium, silver, platinum, gold or the like can also be used.

The positive electrode current collector 46 is a plate-like member made of a conductive material, and is arranged on the left side surface of the battery main body 42 . The positive electrode current collector 46 is electrically insulated from the negative electrode 10 and the negative electrode current collector 44 . Examples of the conductive material forming the positive electrode current collector 46 include those exemplified as the conductive material forming the negative electrode current collector 44 .

Since the prismatic secondary battery 40 described in detail above also uses the negative electrode 10, the energy density is high and the input/output characteristics are also high.

It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

For example, in the above-described embodiment, the carrier of the secondary battery is lithium ions, but it is not particularly limited to this, and alkali metal ions such as sodium ions and potassium ions, Group 2 element ions such as calcium ions and magnesium ions good too. Moreover, the positive electrode active material may be a material capable of intercalating and deintercalating carrier ions. Moreover, although the electrolytic solution is a non-aqueous electrolytic solution, it may be an aqueous electrolytic solution.

In the above-described embodiments, the secondary battery using an electrolyte was exemplified, but an all-solid battery that does not use an electrolyte may be used.

In the above-described embodiment, the fiber bundle 14 constituting the negative electrode 10 is a bundle of a large number of straight carbon fibers 12. However, it is not limited to this. It may be

Although the negative electrode 10 has a columnar shape in the above-described embodiment, it is not limited to this, and may be, for example, an elliptical column or a polygonal column (for example, a triangular column, a square column, a hexagonal column, etc.).

In the above-described embodiment, the columnar positive electrode 20 of the pin-shaped secondary battery 30 has a cylindrical shape, but it is not particularly limited to this. prism, etc.). Note that "space filling" means filling a space with three-dimensional objects without gaps.

In the above-described embodiment, the negative electrode current collector 44 and the positive electrode current collector 46 of the prismatic secondary battery 40 are plate-shaped, but are not particularly limited to a plate-like shape. It may be shaped like a net, a punched sheet, a glass body, a porous body, a foam, a fiber group formed body, or the like. Also, the negative and positive current collectors 44 and 46 may have the same shape or may have different shapes.

In the above-described embodiment, the battery body 42 of the prismatic secondary battery 40 is described as an assembly of the pin-shaped secondary batteries 30 having the columnar positive electrodes 20. However, the battery body 42 is not particularly limited to this. Alternatively, the negative electrode 10 with the separation membrane 18 may be independently erected without being in contact with each other in the rectangular parallelepiped positive electrode having substantially the same size as the battery main body 42 .

In the above-described embodiment, the positive electrode current collector 46 is provided on the left side of the prismatic secondary battery 40, but it is not particularly limited to this, and may be provided on the right side, the front surface, or the rear surface. , or may be provided on the bottom surface. However, since the length X in the left-right direction of the prismatic secondary battery 40 is shorter than the length Y in the vertical direction and the length Z in the front-rear direction, the positive electrode current collector 46 is provided on the left side or the right side of the prismatic secondary battery 40 . is preferably provided. This is because the electrical resistance of the positive electrode is reduced by doing so.

An example in which the negative electrode 10 was produced will be described below as an example. It goes without saying that the present disclosure is not limited to the following examples, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[Example 1]
1. Preparation of Negative Electrode 10 Scale-like natural graphite (manufactured by Fuji Graphite Industries, RCG-2Q, average particle size D50 = 2.2 μm) as graphite particles 16 and polyvinylidene fluoride (PVdF) are added to N-methylpyrrolidone (NMP). A slurry 50 was prepared by dispersing. The slurry 50 was filled in a nozzle 52 having an outlet 52o constricted to φ300 μm (step (a)). Subsequently, the carbon fiber yarn 54 (manufactured by Nippon Graphite Fiber, YS-90A-10S (spun of 1000 single yarns with a diameter of 7 μm)) is cut to an appropriate length, passed through a nozzle 52, and molded. A body 60 was obtained (step (b)). Finally, the compact 60 was dried to remove the NMP solvent. In this way, the negative electrode 10 in which the graphite particles 16 were arranged between the carbon fibers 12 was obtained.

2. Calculation of content of graphite particles 16 in negative electrode 10 Weight x [g] of carbon fiber yarn 54, weight y [g] of negative electrode 10, weight of graphite particles 16 in solid content (PVdF and graphite particles 16) in slurry 50 Taking the ratio as z [%], the content Cg [%] of the graphite particles 16 in the negative electrode 10 was calculated by the following formula (1).
Graphite content Cg [%]=(y−x)×z/y (1)
For example, in Example 1, x=5, y=7, z=90 and Cg=25.7.

3. Observation of Cross Section of Negative Electrode 10 The side surface and cross section of the negative electrode 10 were observed with a secondary electron microscope. 5 shows a side SEM photograph of the negative electrode 10 of Example 1, and FIG. 6 shows a cross-sectional SEM photograph of the negative electrode 10 of Example 1. As shown in FIG. 7 and 6 also show SEM photographs of the negative electrode of Comparative Example 1 for comparison. 5 and 6, the graphite particles 16 are present between the carbon fibers 12, and in the portion where the graphite particles 16 are present between the carbon fibers 12, the carbon fibers 12 are in surface contact via the graphite particles 16. I found out that

4. Evaluation of Electronic Resistance of Negative Electrode 10 The negative electrode 10 was cut to a length of 60 mm, and each 5 mm on each end was connected to a Ni tab via a conductive paste, and the connecting portion was covered with a Cu foil to prepare a sample for resistance evaluation. This sample was placed on a protective plate made of polypropylene (see FIG. 7), a resistance measuring instrument was connected to the Ni tab, and the DC resistance was measured by the two-terminal method. Table 1 shows the results.

5. Half-cell charge-discharge evaluation of negative electrode 10 A sample similar to the resistance evaluation sample described above is sandwiched between polyethylene single-layer low-resistance separators, and further sandwiched between Li foils with a length of 40 mm that serve as counter electrodes, and a Ni tab is connected to the Li foil. , to prepare a charge-discharge evaluation sample (see FIG. 8). This was placed in an Al-laminated bag, filled with an electrolytic solution, and sealed to produce a half cell. The electrolyte used was a solvent obtained by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, to which 1M _LiPF6 was added. . Using this half cell, after performing conditioning charge and discharge in the potential range of 0.005 to 1.5 V (vs. Li ⁺ /Li), a constant current and constant voltage charge and discharge test was performed to determine the initial charge and discharge efficiency and the negative electrode active material. (Carbon fiber 12 and graphite particles 16) The discharge capacity per weight was determined. In the constant current constant voltage charge/discharge, the potential range was 0.005 to 1.5 V (vs. Li ⁺ /Li), and the current value at constant current was 0.1C. After that, constant current discharge at 0.2 C and constant current discharge at 5 C were performed in a potential range of 0.005 to 1.5 V (vs. Li ⁺ /Li), and the discharge capacity at 5 C with respect to the discharge capacity at 0.2 C was performed. A ratio of discharge capacity was obtained. Note that in the charge/discharge evaluation using a half cell, the case where the negative electrode 10 absorbs lithium is called charging, and the case where lithium is released from the negative electrode is called discharging. Table 1 shows the results.

[Examples 2 to 4]
Example 2 was the same as Example 1, except that the content of graphite particles was 38.3% and the nozzle 52 having an outlet 52o constricted to φ350 μm was used. Example 3 was the same as Example 1, except that the content of graphite particles was 50.9% and the nozzle 52 having an outlet 52o constricted to φ400 μm was used. Example 4 was the same as Example 1, except that the content of graphite particles was 5.1% and the nozzle 52 whose outlet 52o was constricted to φ270 μm was used. Table 1 shows the results.

[Example 5]
In Example 5, scale-like natural graphite (Fuji Graphite Industry Co., Ltd., BF-10A, average particle size D50 = 10 µm) was used as graphite particles, the content of graphite particles was 26.6%, and the outlet 52o was narrowed to φ320 µm. Example 1 was the same as Example 1, except that the nozzle 52 with a different shape was used. Table 1 shows the results.

[Comparative Example 1]
Comparative Example 1 was the same as Example 1, except that no graphite particles were used and a nozzle 52 having an outlet 52o narrowed to φ250 μm was used. Table 1 shows the results.

[Comparative Examples 2 and 3]
Comparative Example 2 was the same as Example 1, except that the content of graphite particles was 1.7% and the nozzle 52 having an outlet 52o narrowed to φ250 μm was used. Comparative Example 3 was the same as Example 1, except that the content of graphite particles was 60.4% and the nozzle 52 having an outlet 52o constricted to φ500 μm was used. Table 1 shows the results.

[Comparative Example 4]
In Comparative Example 4, flake natural graphite (Fuji Graphite Industries, BF-20A, average particle size D50 = 22 µm) was used as the graphite particles, the content of the graphite particles was 26.6%, and the outlet 52o was narrowed to φ400 µm. Example 1 was the same as Example 1, except that the nozzle 52 with a different shape was used. Table 1 shows the results.

[Comparative Example 5]
In Comparative Example 5, spherical graphite (manufactured by Nippon Graphite Co., Ltd., CGB-10, average particle diameter D50 = 9.8 μm) was used as graphite particles, the content of graphite particles was 24.8%, and the outlet 52o was narrowed to φ320 μm. Example 1 was the same as Example 1, except that the nozzle 52 with a different shape was used. Table 1 shows the results.

As shown in Table 1, the negative electrode 10 containing 5 to 50% by weight of the scale-like graphite particles 16 having an average particle diameter D50 of 1 to 10 μm between the carbon fibers 12 has a high initial charge-discharge efficiency (that is, the first The irreversible capacity during charging and discharging was small), and the discharge capacity was large. Therefore, it was found that the secondary battery using this negative electrode 10 has a high energy density. In addition, this negative electrode 10 had a high rate characteristic represented by the ratio of the discharge capacity at 5C to the discharge capacity at 0.2C. Therefore, it was found that the secondary battery using this negative electrode 10 has high input/output characteristics. Among these, it was found that the energy density and input/output characteristics of the secondary battery were particularly high in the negative electrode 10 in which the content of the graphite particles 16 was 25% by weight to 40% by weight. In addition, it was found that the negative electrode 10 has high electron conductivity and can be made relatively long, so that the degree of freedom in the shape of the secondary battery is improved.

10 negative electrode, 12 carbon fiber, 14 fiber bundle, 16 graphite particles, 18 separation membrane, 20 columnar positive electrode, 20a center hole, 30 pin secondary battery, 40 prismatic secondary battery, 42 battery body, 44 negative electrode current collector, 46 positive current collector, 50 slurry, 52 nozzle, 52o outlet, 54 carbon fiber yarn, 60 compact.

Claims (5)

A fiber bundle in which carbon fibers are bundled,
Graphite particles provided between the carbon fibers;
with
The graphite particles have shape anisotropy and have an average particle size D50 of 1 μm or more and 10 μm or less as measured by a laser diffraction method,
The content of the graphite particles is 5% by weight or more and 50% by weight or less,
Negative electrode for secondary batteries. 2. The negative electrode for a secondary battery according to claim 1, wherein said graphite particles are flake graphite. The content of the graphite particles is 25% by weight or more and 40% by weight or less,
The negative electrode for a secondary battery according to claim 1 or 2. The negative electrode for a secondary battery according to any one of claims 1 to 3;
a separation membrane provided to surround the negative electrode for a secondary battery;
a positive electrode including a positive electrode active material and provided around the separation membrane;
A secondary battery with A method for producing a negative electrode for a secondary battery according to any one of claims 1 to 3,
(a) filling a nozzle with a slurry containing the graphite particles;
(b) passing the carbon fiber yarn containing the carbon fiber through the nozzle filled with the slurry;
A method for producing a negative electrode for a secondary battery, comprising:

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