JP2008016456A - Negative electrode material for lithium battery and lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material for a battery having low electrode resistance, high electrode strength, excellent electrolyte permeability, high energy density, high quick charge/discharge performance, in the negative electrode material for a lithium battery using carbon fibers as a conducting assistant. <P>SOLUTION: The negative electrode material for the lithium battery contains a carbonaceous negative active material having a specific surface area of 1 m<SP>2</SP>/g or more, a binder comprising styrene butadiene rubber, and carbon fibers having a diameter of 1-1000 nm, and the lithium battery uses the negative electrode material. The negative electrode material contains 0.05-20 mass% carbon fibers, 0.1-6.0 mass% binder comprising styrene butadiene rubber, and 0.3-3 mass% thickener such as carboxymethylcellulose. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a carbon-based negative electrode material for a lithium battery having a large charge / discharge capacity per volume and excellent charge / discharge cycle characteristics and large current load characteristics, a composition for obtaining the negative electrode material, a production method thereof, and a lithium battery. The present invention relates to a lithium battery and a lithium secondary battery using a carbon-based negative electrode material.

  2. Description of the Related Art As mobile devices become smaller and lighter and have higher performance, there is an increasing demand for secondary batteries having high energy density and higher capacities of secondary batteries. Under such circumstances, as secondary batteries for small portable devices such as mobile phones and video cameras, non-aqueous systems such as lithium ion batteries and lithium polymer batteries that use non-aqueous electrolytes due to the characteristics of high energy density and high voltage. Lithium secondary batteries are used in many devices. As a negative electrode material used for these lithium secondary batteries, a carbon material typified by graphite having a large potential charge / discharge capacity per unit mass at a base potential close to lithium (Li) is used. However, these electrode materials are used up to the point where the charge / discharge capacity per mass is close to the theoretical value, and the energy density per mass as a battery is approaching its limit. Therefore, in order to increase the utilization rate as an electrode, attempts have been made to reduce electrode binders and conductive aids that do not contribute to the discharge capacity.

Up to now, polyvinylidene fluoride (abbreviated as PVDF) and fluororesins typified by copolymers thereof have been mainly used as the binder for the negative electrode, but recently it is abbreviated as styrene butadiene rubber (SBR). ) Has been widely used because of its merit such that the addition amount can be reduced and the electrode manufacturing process is simplified because it is used in an aqueous dispersion.
Further, as a conductive auxiliary agent, vapor grown carbon fibers that are highly conductive and effective in electrode strength are used more often than conventional carbon black such as acetylene black. For example, in JP-A-4-155576 (Patent Document 1), JP-A-4-237971 (Patent Document 2) and the like, by adding a vapor-grown carbon fiber to a graphite negative electrode, the electrode resistance is lowered, The load characteristics are improved, the strength of the electrode is increased, the expansion and contraction resistance of the electrode is increased, and the cycle life of the lithium secondary battery is improved.
However, since the vapor-grown carbon fiber is hydrophobic, it is used as a binder in combination with PVDF in an organic solvent dispersion, and is not used in combination with SBR as an aqueous dispersion.

  Further, a secondary battery used in a small portable device is required to be more compact, and not only the energy density per mass but also the energy density per volume is required to be high. Therefore, studies have been started to increase the electrode density of electrode materials that are approaching the theoretical value as described above, to increase the filling amount in the battery container, and to increase the energy density per volume of the electrode and the battery.

For example, graphite, which is most frequently used as a negative electrode material at present, has a true density of about 2.2 g / cm ³ , but so far, a negative electrode density of about 1.5 g / cm ³ has been used. It is considered that the energy density per battery volume can be improved by setting the negative electrode density to 1.7 g / cm ³ or more. However, when the negative electrode density is increased, the number of vacancies in the negative electrode is decreased. There arises a problem that the electrolyte solution that is important for the electrode reaction existing therein is insufficient, and the penetration of the electrolyte solution into the negative electrode is slow. When the electrolyte in the negative electrode is insufficient, the electrode reaction is delayed, the energy density is lowered and the high-speed charge / discharge performance is lowered, and when the permeability of the electrolyte is slow, the battery production time is prolonged, leading to an increase in production cost. . These problems become more prominent when a highly viscous polymer electrolyte solution such as a lithium polymer battery is used.

JP-A-4-155576 JP-A-4-237971

  An object of this invention is to provide the high energy density carbon-type negative electrode material required in order to achieve the high energy density lithium battery excellent in cycling characteristics and high-speed charge / discharge performance.

In light of the problems of the carbon-based negative electrode material, the present inventors have made extensive studies to solve the above problems, and as a result, the carbon-based negative electrode material has a relatively small addition amount and has a binding property. Is used as a binder, and carbon fibers having a fiber diameter of 1 to 1000 nm are uniformly dispersed as a conductive auxiliary agent, so that electrode resistance is low, electrode strength is good, electrolyte solution permeability is high, high energy density and high speed. The inventors have found that a high-performance battery with good charge / discharge performance can be obtained and completed the present invention.
That is, this invention provides the following negative electrode materials for lithium batteries, its manufacturing method, a use, etc.

[1] A negative electrode material for a lithium battery, comprising a carbon-based negative electrode active material having a specific surface area of 1 m ² / g or more, a binder made of styrene butadiene rubber, and carbon fibers having a fiber diameter of 1 to 1000 nm.
[2] The negative electrode material for a lithium battery as described in 1 above, wherein the styrene-butadiene rubber is fine particles having an average particle diameter of 10 to 500 nm.
[3] The carbon fiber content is 0.05 to 20% by mass with respect to the total amount of the carbon-based negative electrode active material, the binder and the carbon fiber, and the content of the binder made of styrene butadiene rubber is 3. The negative electrode material for a lithium battery as described in 1 or 2 above, which is 0.1 to 6.0% by mass.
[4] The negative electrode material for a lithium battery according to any one of 1 to 3, further comprising a thickener.
[5] The lithium battery as described in 4 above, wherein the content of the thickener is 0.1 to 4% by mass with respect to the total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the thickener. Negative electrode material.
[6] The negative electrode material for a lithium battery as described in 4 above, wherein the thickener is carboxymethylcellulose.
[7] The negative electrode material for a lithium battery as described in any one of 1 to 6 above, wherein the specific resistance of the negative electrode at 25 ° C. is 0.5 Ωcm or less.
[8] The negative electrode material for a lithium battery according to any one of 1 to 7, wherein the carbon fiber is a graphite-based carbon fiber that has been heat-treated at 2000 ° C. or higher.
[9] The negative electrode material for a lithium battery as described in any one of 1 to 7 above, wherein the carbon fiber is a graphite-based carbon fiber having an oxygen-containing functional group introduced on the surface by oxidation treatment.
[10] The negative electrode material for a lithium battery according to any one of 1 to 7, wherein the carbon fiber is a graphite-based carbon fiber containing 0.1 to 100000 ppm of boron.
[11] The average spacing d ₀₀₂ of (002) plane measured by X-ray diffraction of the graphite carbon fiber, a negative for a lithium battery according to the 8 or less 0.344nm electrode material.
[12] The negative electrode material for a lithium battery as described in any one of 1 to 11 above, wherein the carbon fiber has a hollow structure inside.
[13] The negative electrode material for a lithium battery according to any one of 1 to 12, wherein the carbon fiber includes a branched carbon fiber.
[14] The negative electrode material for a lithium battery as described in any one of 1 to 13, wherein the carbon-based negative electrode active material contains Si.
[15] The above 1 to 14, wherein the carbon-based negative electrode active material is a non-graphite carbon material, and the density of the mixture layer comprising the negative electrode active material, the binder, and the conductive additive is 1.5 g / cm ³ or more. The negative electrode material for lithium batteries according to any one of the above.
[16] The negative electrode material for a lithium battery as described in any one of 1 to 15 above, wherein the carbon-based negative electrode active material before forming the electrode is carbonaceous particles satisfying the following requirements:
(1) The average circularity measured by a flow type particle image analyzer is 0.70 to 0.99,
(2) The average particle diameter by laser diffraction method is 1-50 μm.
[17] The negative electrode material for a lithium battery according to any one of 1 to 16, wherein the carbon-based negative electrode active material contains a graphite carbon-based material of 50% by mass or more.
[18] The negative electrode material for a lithium battery as described in 17 above, wherein the graphite material contains boron.
[19] The lithium battery according to any one of 1 to 18 above, wherein the carbon-based negative electrode active material before forming the electrode of the electrode active material is a carbonaceous particle containing 50% by mass or more of graphite particles satisfying the following requirements: Anode material:
(1) The average circularity measured by a flow type particle image analyzer is 0.70 to 0.99,
(2) The average particle diameter by laser diffraction method is 1-50 μm.
[20] The negative electrode material for a lithium battery as described in 17 above, wherein the graphite-based carbon material is a carbonaceous particle containing 50% by mass or more of graphite particles satisfying the following requirements:
(1) C ₀ of (002) plane in X-ray diffraction measurement is 0.6900 nm or less, La (crystallite size in the a-axis direction)> 100 nm, Lc (crystallite size in the c-axis direction)> 100 nm,
(2) A specific surface area of 1.0 to 10 m ² / g,
(3) True density is 2.20 g / cm ³ or more,
(4) Laser Raman R value (peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 by laser Raman spectrum) 0.01 to 0.9.
[21] The negative electrode material for lithium batteries as described in 15 above, wherein the density of the mixture layer comprising the negative electrode active material, the binder, and the conductive additive is 1.7 g / cm ³ or more.
[22] A styrene-butadiene rubber aqueous dispersion in a carbon fiber / active material dispersion obtained by dispersing carbon fibers having a fiber diameter of 1 to 1000 nm and a carbon negative electrode active material having a specific surface area of 1 m ² / g or more in a thickener aqueous solution. A method for producing a composition for a negative electrode material for a lithium battery, comprising adding a liquid and stirring and mixing.
[23] A carbon-based negative electrode having a specific surface area of 1 m ² / g or more after a carbon fiber / active material dispersion is added to a thickener aqueous solution to stir the carbon fiber having a fiber diameter of 1-1000 nm and stirred to disperse the carbon fiber. 23. The method for producing a composition for a negative electrode material for a lithium battery as described in 22 above, which is prepared by adding an active material and stirring and mixing.
[24] A carbon-based negative electrode having a specific surface area of 1 m ² / g or more after a carbon fiber / active material dispersion is added to a thickener aqueous solution and a carbon fiber having a fiber diameter of 1 to 1000 nm is added and stirred to disperse the carbon fiber. 23. The method for producing a composition for a negative electrode material for a lithium battery as described in 22 above, wherein the active material is added, stirred and mixed, and further prepared with an aqueous thickener solution.
[25] A carbon fiber / active material dispersion is added to a thickener aqueous solution with a carbon-based negative electrode active material having a specific surface area of 1 m ² / g or more, and after stirring and mixing, carbon fibers having a fiber diameter of 1 to 1000 nm are added and stirred. The method for producing a composition for a negative electrode material for a lithium battery as described in 22 above, wherein the composition is prepared by dispersing carbon fibers.
[26] After the carbon fiber / active material dispersion is dry-stirred with a carbon-based negative electrode active material having a specific surface area of 1 m ² / g or more and a carbon fiber having a fiber diameter of 1 to 1000 nm to disperse the carbon fiber, a thickener aqueous solution The method for producing a composition for a negative electrode material for a lithium battery as described in 22 above, wherein the composition is prepared by adding and stirring and mixing.
[27] The above 22 to 26, wherein the concentration of the thickener in the aqueous solution of the thickener is 0.3 to 5% by mass, and the concentration of the styrene butadiene rubber in the aqueous dispersion of styrene butadiene rubber is 10 to 60% by mass. The manufacturing method of the composition for negative electrode materials for lithium batteries in any one of these.
[28] The method for producing a composition for a negative electrode material for a lithium battery as described in any one of 22 to 27 above, wherein the thickener is carboxymethylcellulose.
[29] A composition for a negative electrode material for a lithium battery obtained by the method according to any one of 22 to 28 above.
[30] A negative electrode material composition for a lithium battery in which carbon fibers having a fiber diameter of 1 to 1000 nm are dispersed in a thickener aqueous solution.
[31] The lithium as described in 30 above, wherein the concentration of the thickener in the aqueous solution of the thickener is 0.3 to 5% by mass, and the proportion of the carbon fiber in the entire composition is 0.1 to 10% by mass. A negative electrode material composition for a battery.
[32] The negative electrode material composition for a lithium battery as described in 30 or 31, wherein the thickener is carboxymethylcellulose.
[33] The negative electrode for a lithium battery as described in any one of 1 to 21 above, wherein the composition for a negative electrode material for a lithium battery as described in 29 above is applied onto a metal current collector foil, dried and then pressure-molded. Wood.
[34] The negative electrode material for a lithium battery as described in 33 above, wherein the metal current collector foil is a copper foil or a copper alloy foil having a thickness of 1 to 50 μm.
[35] A lithium battery comprising the negative electrode material for a lithium battery according to any one of 1 to 21, 33 and 34 as a constituent element.
[36] A lithium secondary battery comprising the negative electrode material for a lithium battery according to any one of 1 to 21, 33, and 34 as a constituent element.
[37] At least one selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate as a nonaqueous solvent for the nonaqueous electrolyte using a nonaqueous electrolyte. 37. The lithium secondary battery as described in 36 above, wherein
Hereinafter, the present invention will be described in detail.

1. Carbon fiber Generally, a carbon-based negative electrode material is obtained by mixing a carbon-based negative electrode active material powder having an average particle diameter of several to several tens of μm with a binder and a conductive additive in a wet manner, and uniformly mixing it on a metal current collector foil. It can be obtained by pressure molding after coating and drying.
Conventionally, carbon black powders such as acetylene black have been mainly used as the conductive auxiliary, but a conductive auxiliary having a higher aspect ratio such as carbon fiber can form a network on the entire electrode. From the viewpoint of the conductivity of the entire electrode, the conductivity can be increased with a small addition amount. In general, carbon black requires about 5% by mass based on the total amount of the carbon-based negative electrode active material, the binder, and the conductive additive, whereas conductive fibers having a large aspect ratio such as vapor-grown carbon fibers are used. Even if it is 3% by mass or less, for example, 1% by mass, the effect is sufficiently exhibited. In addition, as described above, the energy density of carbon-based electrode active materials has recently approached its limit, and attempts have been made to increase the density of electrodes in order to increase the energy density per volume. The system conductive auxiliary agent is deformed by molding (high-pressure press) at the time of densification, and the conductive path and the electrolyte penetration path in the electrode are hindered. On the other hand, carbon fibers are not easily deformed by pressure, and even in high-density electrodes, the conductive path network and electrolyte solution penetration path are maintained, and even if expansion and contraction occur during the electrode reaction, the strength of the electrodes is increased by the fiber network. Thus, a carbon-based negative electrode material having a high capacity and good high-speed charge / discharge performance can be obtained even in a high-density electrode.
In order to realize such a function, it is preferable that the added carbon fiber itself is excellent in conductivity, and that the fiber diameter is as thin as possible and the fiber length is long in order to increase the conductive path. From such a viewpoint, it is necessary to use conductive, tough and fine carbon fibers as the conductive fibers to be added.

1-1. Fiber diameter of carbon fiber If the fiber diameter of the carbon fiber used in the negative electrode material for a lithium battery of the present invention is too thick, voids in the electrode become too large to increase the electrode density, which is not preferable. Since the average particle diameter of carbon-based electrode active material particles currently used for Li ion batteries and Li polymer batteries is about several to several tens of micrometers, the fiber diameter of carbon fibers is about 1 μm at the maximum. Also, if the fiber diameter is too thin, it is buried between the active material particles and cannot form a network in the electrode, and void formation between the active materials becomes impossible. is necessary. For the above reasons, the fiber diameter of the carbon fiber that can be used for the negative electrode material for a lithium battery of the present invention is in the range of 1 to 1000 nm, preferably in the range of 10 to 500 nm. In terms of the average fiber diameter, a range of 5 to 500 nm is preferable, and a range of 10 to 200 nm is more preferable.

1-2. The degree of crystallinity of carbon fiber It is desirable that the degree of crystallinity of carbon fiber (so-called graphitization degree) is higher. In general, the higher the degree of graphitization of the carbon material, the more the layered structure is developed, the harder the carbon material is, and the more the conductivity is improved. As described above, the carbon-based negative electrode material for lithium batteries is suitable for use. In order to graphitize the carbon material, it is generally sufficient to treat it at a high temperature. In this case, the treatment temperature varies depending on the carbon fiber used, but is preferably 2000 ° C. or higher, and more preferably 2500 ° C. or higher. In this case, it is effective to add boron or Si, which is a graphitization cocatalyst that works to promote the degree of graphitization, before the heat treatment. The addition amount of the cocatalyst is not particularly limited, but if the addition amount is too small, the effect is not obtained, and if it is too much, it remains as an impurity, which is not preferable. A preferable addition amount is 0.1 to 100,000 ppm, and more preferably 10 to 50,000 ppm.

Although crystallinity of the carbon fibers is not particularly limited, preferably the average spacing d ₀₀₂ is 0.344nm less by X-ray diffraction method, even more preferably less 0.339 nm, C-axis direction of the thickness of the crystal Lc is 40 nm or less.

1-3. Fiber length and aspect ratio of carbon fiber The fiber length of carbon fiber is not particularly limited. As described above, the longer the fiber length, the better the electrical conductivity in the electrode, the strength of the electrode, and the electrolyte solution retention, but this is not preferred because the fiber dispersibility in the electrode is impaired. Although the range of a preferable average fiber length changes with kinds and fiber diameter of the carbon fiber to be used, it is 0.5-100 micrometers, and the thing of 1-50 micrometers is more preferable. When the preferable range of this average fiber length is shown by the average aspect ratio (ratio of the fiber length to the fiber diameter), it is in the range of 5 to 50000, and more preferably in the range of 10 to 15000.

  It is preferable that the carbon fiber is branched (branched) because the conductivity of the entire electrode, the strength of the electrode, and the electrolyte solution retention are further increased. However, if there are too many branched fibers, the dispersibility in the electrode is impaired as in the fiber length, so it is preferable that they are contained in an appropriate ratio. The proportion of these branched fibers can be controlled to some extent by the production method and the subsequent pulverization treatment.

1-4. Carbon Fiber Manufacturing Method The carbon fiber manufacturing method used in the present invention is not particularly limited. Examples thereof include a method in which a polymer is formed into a fiber by a spinning method and heat-treated in an inert atmosphere, and a vapor phase growth method in which an organic compound is reacted at a high temperature in the presence of a catalyst. Carbon fiber obtained by vapor phase growth method (vapor phase growth carbon fiber) has a crystal growth direction parallel to the fiber axis, and the crystallinity in the fiber length direction of the graphite structure tends to be high. Conductive, high-strength carbon fibers can be obtained.

  In order to achieve the object of the present invention, vapor grown carbon fibers in which crystals grow in the fiber axis direction and the fibers branch are suitable. The vapor grown carbon fiber can be produced, for example, by blowing an organic compound gasified with iron serving as a catalyst in a high temperature atmosphere. Vapor-grown carbon fibers can be used as manufactured, those heat-treated at 800-1500 ° C., or graphitized at 2000-3000 ° C., and suitable for the electrode active material powder to be used. A material subjected to heat treatment or graphitization is preferable because it has advanced carbon crystallinity and high conductivity and high pressure resistance.

  Moreover, there exists a branched fiber as a preferable form of vapor grown carbon fiber. The branched portion may have a portion having a hollow structure in which the entire fiber including the portion is in communication with each other. Therefore, the carbon layer which comprises the cylindrical part of a fiber is continuing. The hollow structure is a structure in which the carbon layer is wound in a cylindrical shape, and includes a structure that is not a complete cylinder, a structure having a partial cut portion, and a structure in which two laminated carbon layers are combined into one layer. . Further, the cross section of the cylinder is not limited to a perfect circle but includes an ellipse or a polygon.

Vapor-grown carbon fibers often have irregularities and disturbances on the fiber surface, and thus have the advantage of improving the adhesion with the electrode active material. In particular, when carbonaceous powder particles are used as the electrode active material and used as the negative electrode of a secondary battery, the adhesion with the carbonaceous material serving as the nucleus is improved, so that the carbonaceous material and the conductive material are electrically conductive even after repeated charge and discharge. It is possible to maintain a close contact state without dissociating with the vapor grown carbon fiber which also serves as a conductivity aid, and to maintain electronic conductivity and to improve cycle characteristics.
When the vapor grown carbon fiber contains a lot of branched fibers, a network can be formed efficiently, and high electronic conductivity and thermal conductivity are easily obtained. In addition, the active material can be dispersed so as to wrap, increasing the strength of the electrode and maintaining good contact between the particles.

1-5. Addition amount of carbon fiber The carbon fiber content is preferably in the range of 0.05 to 20% by mass with respect to the total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the thickener optionally blended. The content is preferably 0.1 to 15% by mass, more preferably 0.5 to 10% by mass. If the content exceeds 20% by mass, the active material ratio in the electrode becomes small, and the electric capacity becomes small. When the addition amount is less than 0.05% by mass, effects such as reduction in electric resistance and improvement in electrolyte permeability with respect to the carbon-based negative electrode material for lithium batteries of the present invention do not appear. In order to adjust the content to this range, it can be carried out by adding the same ratio in the production method.

1-6. Surface treatment of carbon fiber Carbon fiber having a surface treatment for controlling the dispersion state in the electrode can be used. The surface treatment method is not particularly limited, and examples thereof include those made hydrophilic by introducing an oxygen-containing functional group by oxidation treatment, and those made hydrophobic by fluorination treatment or silicon treatment. In addition, a phenol resin coating or a mechanochemical treatment may be used. If the surface treatment is excessively performed, the conductivity and strength of the carbon fiber are remarkably impaired, and therefore an appropriate treatment is required. The oxidation treatment can be performed, for example, by heating the carbon fiber in air at 500 ° C. for about 1 hour. This treatment improves the hydrophilicity of the carbon fiber.
In the present invention, styrene butadiene rubber as a binder is often used as an aqueous dispersion, and therefore, oxidized carbon fibers having a hydrophilic surface are preferable.

2. Styrene butadiene rubber (SBR) (binder)
SBR is used for the electrode binder of the carbon-based negative electrode material for lithium batteries of the present invention. SBR can be used in a smaller amount than fluororesin-based binders such as PVDF that have been mainly used so far, and is mixed with an electrode active material because an aqueous dispersion is used. Compared to the organic solvent system, there is an advantage that the electrode manufacturing process is simplified such that explosion-proof equipment is unnecessary. An electrode using SBR as a binder is excellent in low-temperature characteristics and high-speed charge / discharge characteristics because the glass transition point (Tg) of SBR is generally low.

  There are two types of SBR: emulsion polymerization SBR and solution polymerization SBR. The emulsion polymerization SBR is obtained in a latex form, but may be dried and used as a dry rubber. Among the solution polymerization SBRs, there are a random type, a block type, a symmetrical block type, and the like depending on a copolymerization mode of styrene and butadiene. There is also a high styrene rubber having a high styrene composition ratio and a high glass transition point (Tg). Furthermore, there is a modified SBR obtained by copolymerizing an unsaturated carboxylic acid or an unsaturated nitrile compound. These have slightly different physical properties such as adhesiveness, strength and thermophysical properties depending on the polymerization mode and styrene / butadiene copolymer ratio. These SBRs used as the binder for the carbon-based negative electrode material for lithium batteries of the present invention can be selected and used in accordance with the type of the negative electrode active material to be used.

  Among them, a latex type aqueous dispersion in which SBR obtained by emulsion polymerization or solution polymerization is dispersed in water is easy to mix with the carbon-based negative electrode active material, and therefore it is preferably used for the carbon-based negative electrode material for lithium batteries of the present invention. Can do. Further, in order to improve the compatibility of the obtained electrode with the electrolyte and the low temperature characteristics, it is preferable that the copolymerization ratio of styrene is 50% by mass or less and the glass transition point is 0 ° C. or less.

  Further, in order to uniformly disperse SBR in the carbon-based negative electrode active material and to adhere the carbon-based negative electrode active material efficiently, it is difficult to use whether the SBR particles in the dispersion are too large or too small. Become. The number average particle size of SBR suitable for use in the carbon-based negative electrode material for lithium batteries of the present invention is in the range of 10 to 500 nm.

  The content of SBR in the negative electrode material varies depending on the carbon fiber and the carbon-based negative electrode active material to be mixed, and cannot be generally limited. And the resistance also increases. Moreover, the negative electrode reaction site is reduced, and the capacity is further reduced. If the amount is too small, the effect as a binder is reduced, and the negative electrode collapses during battery assembly or charge / discharge, which is not preferable because the charge / discharge cycle life is reduced. The amount of SBR added is preferably 0.1 to 6.0% by mass, more preferably 0%, based on the total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the thickener optionally blended. It is 0.3-5.0 mass%.

3. Carbon-based negative electrode active material A carbon-based negative electrode active material contains carbon and can electrochemically occlude and release ions.
In the present invention, since an aqueous dispersion of SBR, preferably SBR, is used as the electrode binder, the higher the specific surface area of the carbon-based negative electrode active material, the better the wettability, the easier to handle, the electrode strength and the electrolyte solution It is also advantageous for retention. Specifically, a specific surface area (BET specific surface area) measured by the BET method is 1 m ² / g or more. However, if the specific surface area is too high, side reactions with the electrolytic solution are likely to occur. A preferable range of the BET specific surface area is 1.0 to 7.0 m ² / g, and more preferably 1.5 to 6.0 m ² / g.

As the carbon-based negative electrode active material, both a material mainly composed of a non-graphite carbon material and a material mainly composed of a graphite-based carbon material can be used.
Non-graphitic carbon materials are carbon materials that do not have the three-dimensional crystal regularity of graphite, and include turbulent structure carbon materials and amorphous carbon materials, such as glassy carbon and heat treatment temperature. A carbon material that has not been crystallized because it is low is also included.
The graphite-based carbon material is a carbon material having three-dimensional crystal regularity of graphite, and includes natural graphite produced naturally, artificial graphite obtained by heat-treating a graphitizable carbon material, and iron. Kish graphite obtained by reprecipitation of graphite from the melt is also included.
Further, the “main component” means that the material occupies 50% by mass or more of the whole, preferably 60% by mass or more, more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

Non-graphite carbon materials as main components include carbon materials obtained by heat-treating non-graphite polymers such as phenol resins, pitch and coke heat-treated at about 1000 ° C., and conjugated systems such as conductive polymers. Examples include heat-treated polymers and CVD carbon deposited on a substrate by a thermal CVD method. In addition, a material in which Si is mixed during heat treatment of these materials to increase the electric capacity as a negative electrode can be mentioned.
Of these non-graphitic carbon materials, spherical ones having as high a degree of circularity as possible are preferable because they can suppress side reactions with the electrolytic solution when used when producing electrode sheets and when used in batteries.
A preferable circularity is 0.70 to 0.99 as an average circularity measured by a flow type particle image analyzer.
The average particle diameter of these non-graphitic carbon materials varies depending on the target electrode sheet shape, and is not particularly limited, but is generally used in the range of an average particle diameter by laser diffraction of 1 to 50 μm.
The bulk density of the negative electrode material using these non-graphitic carbon materials varies depending on the true density of the carbon-based active material and is not particularly limited. Generally, the true density of the non-graphitic carbon material is 1.9 g / cm ^3. The density of the mixture layer composed of the negative electrode active material, the binder, and the conductive additive (electrode bulk density) is 1.5 g / cm ³ or more, more preferably 1.7 g / cm ³ or more. .

As the main component of the graphite-based carbon material, a graphite-based carbon material that is generally used as a carbon-based active material of a Li ion battery can also be used in the present invention. The graphite-based active material has the characteristics that the crystallinity develops, the insertion and release of Li ions occur uniformly, and the diffusion is fast, so that the change in the discharge potential of the battery is small and the high load characteristics are excellent. . These are used at a true density as high as about 2.2 g / cm ³ and an electrode bulk density of 1.5 g / cm ³ . In the present invention, the porosity can be further reduced to an electrode bulk density of 1.7 g / cm ³ or more.
This graphite-based active material is also preferably as high as possible in the circularity, the average circularity measured by a flow type particle image analyzer is 0.70 to 0.99, and the average particle diameter by laser diffraction is about 1 to 50 μm. Is used.
The graphite material preferably has as high crystallinity as possible, and the C ₀ of the 002 plane in X-ray diffraction measurement is 0.6900 nm (d ₀₀₂ = 0.3450 nm) or less, and La (crystallite size in the a-axis direction) Is preferably larger than 100 nm, and Lc (crystallite size in the c-axis direction) is preferably larger than 100 nm. The laser Raman R value is 0.01 to 0.9 (R value: peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 by laser Raman spectrum) are preferred, true density 2.20 g / cm ³ or more Is preferred.

  Heat treatment by adding boron to the graphite-based active material is preferable because crystallinity is improved and compatibility with the electrolyte and stability are improved. The addition amount of boron is not particularly limited, but if the addition amount is too small, the effect is not obtained, and if it is too much, it remains as an impurity, which is not preferable. A preferable addition amount is in the range of 0.1 mass ppm to 100,000 mass ppm, more preferably 10 mass ppm to 50,000 mass ppm.

4). Negative electrode for lithium battery 4-1. Carbon-based negative electrode active material / electrode binder / composition for negative electrode material containing carbon fiber Generally, a carbon-based negative electrode material for a lithium battery is a carbon-based negative electrode active material and an electrode binder, and in some cases, carbon black or graphite. It is manufactured by adding other conductive aids such as fine powder, mixing in a predetermined ratio in a wet or dry manner, applying on a metal current collector such as Cu, and drying and pressing. However, in the carbon-based negative electrode material for a lithium battery according to the present invention, carbon fibers having a large aspect ratio are added, so that carbon fibers cannot be uniformly dispersed by conventional methods. In addition, since the SBR aqueous dispersion is used as the electrode binder, hydrophobic carbon fibers, particularly graphitized carbon fibers cannot be sufficiently dispersed.

The carbon-based negative electrode material for a lithium battery of the present invention is obtained by applying and molding on a current collector a composition for a negative electrode material in which a binder composed of a carbon-based negative electrode active material, carbon fiber, and SBR is mixed. However, the preparation of this negative electrode material composition is not conventionally known, that is, carbon fibers having a fiber diameter of 1 to 1000 nm, carbon-based negative electrode active materials having a BET specific surface area of 1 m ² / g or more, and other carbon-based materials if desired. The powder conductive assistant is sufficiently dispersed in a thickener aqueous solution (for example, a carboxymethyl cellulose compound aqueous solution), and then an aqueous dispersion of styrene butadiene rubber is added and stirred and mixed in a relatively short time.
This method is a method intended to easily disperse the carbon fiber in an aqueous solution in advance and then add an SBR aqueous dispersion and stir and mix at that time. When it takes time to stir and mix after the addition of the SBR binder, the carbon fibers once dispersed may agglomerate again. As a result, the electrode specific resistance increases or the electrolyte permeability decreases. Degrading performance. The reason why the carbon fibers agglomerate again is that the affinity between the binder added later and the aqueous solution of the thickener is high, so that the hydrophobic carbon fibers dispersed in the thickener are gradually removed from the thickener. This is thought to be due to separation. Specific methods include, for example, the following methods (A) to (D).

(A) After adding carbon fiber to the thickener aqueous solution and stirring to sufficiently disperse the carbon fiber, adding the carbon-based negative electrode active material and optionally other carbon-based powder conductive aid, stirring and mixing, A method of adding an SBR aqueous dispersion and stirring and mixing in a relatively short time.
(B) After adding carbon fiber to the thickener aqueous solution and stirring to sufficiently disperse the carbon fiber, adding the carbon-based negative electrode active material and optionally other carbon-based powder conductive assistant, stirring and mixing, A method of adjusting the viscosity by adding a thickener aqueous solution, adding an SBR aqueous dispersion, and stirring and mixing in a relatively short time.
(C) After adding a carbon-based negative electrode active material and optionally another carbon-based powder conductive additive to the thickener aqueous solution, stirring and mixing, and then adding and stirring the carbon fiber to sufficiently disperse the carbon fiber A method of adding an SBR aqueous dispersion and stirring and mixing in a relatively short time.
(D) Dry-stir the carbon-based negative electrode active material powder, carbon fiber, and optionally other carbon-based powder conductive auxiliary agent to sufficiently disperse the carbon fiber, and then add the thickener aqueous solution and stir and mix. Then, a method of adding an SBR aqueous dispersion and stirring and mixing in a relatively short time.

The standard of the stirring time after the addition of the SBR aqueous dispersion depends on the type of carbon fiber, the stirring method, the amount, etc., and thus cannot be limited in general, but is determined as needed depending on the state of the composition when allowed to stand after stirring. Generally, it is within 120 minutes, preferably 10 to 90 minutes.
Among the above methods, (A) and (B) are particularly preferable because the dispersion of carbon fibers is relatively easy. In this case, the addition of the carbon fiber is preferably performed so that the proportion of the carbon fiber is 0.1 to 10% by mass with respect to the total amount of the carbon-based negative electrode active material, the binder, and the conductive additive. .

In the above method, the thickener is a nonionic polymer used to control the viscosity of the negative electrode material composition so that it can be applied to the current collector.
Examples of the thickener include polyethylene glycols, celluloses, polyacrylamides, poly N-vinyl amides, poly N-vinyl pyrrolidones, etc. Among them, polyethylene glycols, carboxymethyl cellulose (CMC), etc. Celluloses and the like are preferable, and carboxymethyl cellulose (CMC) is particularly preferable. The blending amount of the thickener depends on the type of the thickener and cannot be generally specified, but is usually 0.1 to the total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the thickener. 4 mass% is preferable and 0.3-3 mass% is more preferable. The thickener is preferably used as an aqueous solution. The concentration of the thickener in the aqueous solution of the thickener is a concentration that allows the viscosity to be 50 to 5000 mPa · s, preferably 100 to 3000 mPa · s at room temperature, and is usually in the range of 0.3 to 5% by mass.
Hereinafter, the carboxymethyl cellulose (CMC) compound which is a preferable thickener will be described as an example.
The physical properties such as the molecular weight of the CMC compound are not particularly limited and vary depending on the type of carbon fiber and negative electrode active material used. However, the CMC compound does not participate in the battery reaction, and if it is too much, the proportion of the negative electrode active material in the electrode decreases. Therefore, the addition amount should be as small as possible. The CMC compound concentration in the CMC compound aqueous solution is preferably as low as possible within the range in which the effect of the thickener can be exhibited. A preferable blending amount of the CMC compound is 0.1 to 4.0% by mass, and more preferably 0.3 to 3% by mass with respect to a total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the CMC compound. %. The CMC compound is used as an aqueous solution of 0.3 to 5% by mass, preferably about 1% by mass, and the viscosity in that case is 50 to 5000 mPa · s at room temperature, preferably 100 to 3000 mPa · s.

If the SBR concentration in the SBR aqueous dispersion is too high, mixing in a short time is difficult, and if it is too low, the viscosity of the composition thickened with the CMC aqueous solution is lowered, which is not preferable. Therefore, the SBR concentration in the SBR aqueous dispersion is preferably 10 to 60% by mass.
Since the dispersion state in the electrode differs depending on the type, composition ratio, combination, and the like of each material and affects electrode resistance, liquid absorbency, etc., it is necessary to select optimum composition and concentration conditions.

  A variety of stirrers can be used. For example, apparatuses such as a ribbon mixer, a screw-type kneader, a Spartan-Luzer, a Redige mixer, a planetary mixer, a defoaming kneader, a universal mixer with stirring blades, and a paint shaker can be used. Among these, the one that can be stirred relatively easily in the four methods described above includes a planetary mixer, a defoaming kneader, and a universal mixer with stirring blades, and a defoaming kneader and a universal mixer with stirring blades are preferable. A universal mixer with stirring blades is particularly preferred.

4-2. Preparation of sheet-like negative electrode material By applying the negative electrode material composition obtained by the above-described method to the current collector foil, the sheet-like negative electrode material for the lithium battery of the present invention can be produced.
Application of the composition to the current collector foil can be carried out by a known method, and examples thereof include a method of applying the composition with a doctor blade, a bar coater, or the like and then forming it with a roll press or the like.
As the current collector, a copper foil is used in the current Li ion battery, but a known material such as a copper foil, an aluminum foil, a stainless steel foil, a nickel foil, a titanium foil and an alloy foil thereof, and a carbon sheet can be used. . Among these, copper foil and copper alloy foil are preferable from the viewpoints of strength, electrochemical stability, cost, and the like.
Although there is no restriction | limiting in particular in the thickness of the collector foil used by this invention, when too thin, intensity | strength will fall and the problem will arise in the intensity | strength of a sheet-like negative electrode material, and the handleability at the time of application | coating. On the other hand, if the thickness is too thick, the ratio of the mass and volume of the current collector foil in the battery structure is increased, the energy density of the battery is decreased, and the sheet-like electrode at the time of battery production becomes stiff, which hinders winding. Come on. Therefore, the thickness of the current collector foil is preferably from 0.5 to 100 μm, particularly preferably from 1 to 50 μm.
These coated electrode sheets are dried by a known method, and then formed into a desired thickness and density by a known method such as a roll press or a pressure press.

The pressing pressure varies depending on the negative electrode active material used, and is not particularly limited because it depends on the target electrode density. Usually, the pressing pressure is 1 ton / cm ² or more. The thickness of the electrode sheet varies depending on the shape of the intended battery and is not particularly limited, but is usually formed to 0.5 to 2000 μm, preferably 5 to 1000 μm.

4-3. Characteristics of Negative Electrode Material The carbon-based negative electrode material for a lithium battery of the present invention obtained by the method described above has a low specific resistance as an electrode because carbon fibers are well dispersed. The lower the electrode specific resistance, the higher the current density in charging / discharging the battery, enabling high-speed charging / discharging. Also, the low electrode specific resistance means that the carbon fiber network is sufficiently widened, the electrode strength is increased, and the charge / discharge cycle life of the battery is prolonged.
The electrode specific resistance of the negative electrode material of the present invention is 0.5 Ωcm or less and further 0.3 Ωcm or less at 25 ° C.

  In addition, when it takes time to stir and mix after adding SBR in the production methods (A) to (D), the carbon fibers once dispersed easily aggregate as described above, and as a result, a negative electrode material obtained as a result. The specific resistance increases. Specifically, in the methods (A) to (D) above, the specific resistance of the negative electrode material composed of a composition that takes time for stirring after the addition of SBR is the carbon fiber in the methods (A) to (D). Compared to the specific resistance of a negative electrode material made of a composition that does not use, it improves only about 10% at most. On the other hand, the specific resistance of the negative electrode material of the present invention by the methods (A) to (D) is the specific resistance of the negative electrode material made of a composition not using carbon fiber in the methods (A) to (D). Compared to at least 20% improvement, many improve more than 40%. In other words, the negative electrode material of the present invention can reduce the electrode specific resistance to 80% or less, and further to 60% or less, compared to an electrode obtained by removing carbon fibers having a fiber diameter of 1 to 1000 nm from the negative electrode material. is there.

The same can be said for the permeability of the electrolyte. That is, if it takes time to stir and mix after adding SBR in the methods (A) to (D), the electrolyte permeability of the obtained negative electrode material cannot be sufficiently improved. This is remarkable when the density of the electrode is increased. Specifically, for example, when the electrode density of a graphite-based negative electrode is 1.7 g / cm ³ or more, the method is composed of a composition that takes time for stirring after the addition of SBR in the methods (A) to (D). The electrolyte solution permeation rate of the negative electrode material is improved only by about 30% at the maximum compared to the electrolyte solution permeation rate of the negative electrode material made of a composition not using carbon fiber in the methods (A) to (D). On the other hand, the electrolyte solution permeation rate of the negative electrode material of the present invention by the methods (A) to (D) is that of the negative electrode material made of a composition not using carbon fiber in the methods (A) to (D). Compared to the electrolyte penetration rate, it is improved by at least 35%, and many are improved by 60% or more.

5. Lithium battery A high-performance lithium battery can be produced by combining the negative electrode material for a lithium battery of the present invention with various positive electrode materials such as lithium cobalt oxide. In particular, the negative electrode material for lithium batteries of the present invention has a great need for high energy density non-aqueous secondary batteries such as Li ion batteries and Li polymer batteries that are currently growing in the market.
Hereinafter, although the manufacturing method of Li ion battery and Li polymer battery using this invention negative electrode material is demonstrated, the manufacturing method of a battery is not limited to these.

5-1. Cathode active material materials Cobalt-based oxides such as lithium cobaltate, manganese-based oxides such as lithium manganate, nickel-based oxides such as lithium nickelate, and composite oxides and mixtures thereof are currently positive electrodes for Li-ion batteries. Used as an active material.

As a positive electrode active material of a lithium battery using the carbon-based negative electrode material for a lithium battery of the present invention, various materials other than the metal oxide material can be used. The lithium-containing transition metal oxide used as the positive electrode active material of the lithium battery of the present invention is preferably at least one transition metal element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W And an oxide mainly containing lithium and a compound having a molar ratio of lithium to transition metal of 0.3 to 2.2. More preferably, it is an oxide mainly containing at least one transition metal element selected from V, Cr, Mn, Fe, Co and Ni and lithium, and the molar ratio of lithium to transition metal is 0.3 to 0.3. The compound of 2.2. In addition, Al, Ga, In, Ge, Sn, Pb, Sb, Bi, Si, P, B, or the like may be contained in a range of less than 30 mole percent with respect to the transition metal present mainly. Among the above positive electrode active materials, the general formula Li _x MO ₂ (M is at least one of Co, Ni, Fe, and Mn, x = 0 to 1.2), or Li _y N ₂ O ₄ (N is at least It is preferable to use at least one of materials having a spinel structure represented by y = 0 to 2).

Further, the positive electrode active material _{Li y M a D 1-a} O 2 (M is Co, Ni, Fe, at least one of Mn, D is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag , W, Ga, In, Sn, Pb, Sb, Sr, B, P, at least one material other than M, y = 0 to 1.2, a = 0.5 to 1), or Li _{z (N b E 1-b} ) 2 O 4 (N is Mn, E is Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag, W, Ga, in, Sn, Pb, Sb, Sr It is particularly preferred to use at least one material having a spinel structure represented by at least one of B, P, b = 1 to 0.2, z = 0 to 2).

_{Specifically, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1-b _{O 2, Li x Mn 2 O} 4, Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2- _c O ₄ (where x = 0.02 to 1.2, a = 0.1 to 0.9, b = 0.8 to 0.98, c = 1.6 to 1.96, z = 2. 01-2.3). The most preferred lithium-containing transition metal _{oxides, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1 _-b O _z (x = 0.02-1.2, a = 0.1-0.9, b = 0.9-0.98, z = 2.01-2.3). In addition, the value of x is a value before the start of charging / discharging, and increases / decreases by charging / discharging.

As other next-generation Li battery positive electrode materials, metal sulfides such as titanium sulfide and molybdenum sulfide, and iron olivine compounds such as LiFePO ₄ can also be used. In particular, iron olivine compounds such as LiFePO ₄ have a high theoretical capacity, use iron, and are excellent in resource, environmental safety, heat resistance, and the like.

The average particle size of the positive electrode active material is not particularly limited, but is preferably 0.1 to 50 μm. It is preferable that the volume of particles of 0.5 to 30 μm is 95% or more. More preferably, the volume occupied by a particle group having a particle diameter of 3 μm or less is 18% or less of the total volume, and the volume occupied by a particle group of 15 μm or more and 25 μm or less is 18% or less of the total volume. Although the specific surface area is not particularly limited, but is preferably 0.01 to 50 m ² / g by the BET method, in particular 0.2 to 10 m ² / g are preferred.

5-2. Production of positive electrode sheet The production method of the positive electrode sheet of the lithium battery of the present invention is not particularly limited. In general, a positive electrode active material such as lithium cobaltate and an electrode binder, and in some cases, a conductive additive such as carbon black or graphite fine powder, or a carbon fiber used for the carbon-based negative electrode material for a lithium battery of the present invention are used. In addition, it can be produced by mixing it at a predetermined ratio in a wet or dry manner, applying it on a current collector such as Al, drying and pressing.

  For example, lithium cobaltate powder and acetylene black (abbreviated as AB) are dry-mixed at a predetermined composition ratio and mixed with a high-speed small-sized mixer with a blade (IK mixer), and then the polyvinylidene fluoride as an electrode binder is mixed with the positive electrode mixture. An N-methylpyrrolidone (NMP) solution containing a ride (PVDF) is added so as to have a predetermined mass ratio, and is kneaded with a planetary mixer to obtain a composition for a positive electrode material.

NMP is further added to this composition for a positive electrode material, the viscosity is adjusted, and then applied to a predetermined thickness on a rolled Al foil (25 μm) for a positive electrode by using a doctor blade. Mold to desired thickness and density. The press pressure is not particularly limited, but is generally about 1 × 10 ^{3 to} 3 × 10 ³ kg / cm ² . Then, it can further heat-dry under reduced pressure and a positive electrode sheet can be produced.

As the electrode binder of the positive electrode material, in addition to the above-described PVDF, fluorine-based polymers such as polytetrafluoroethylene, and rubbers such as SBR and acrylate polymers used for the carbon-based negative electrode material can be used. As the solvent, a known solvent suitable for each electrode binder, for example, the above-mentioned N-methylpyrrolidone in the case of a fluorine-based polymer, toluene, acetone or the like, or water in the case of SBR can be used. .
The amount of the electrode binder used in the positive electrode material is suitably 0.5 to 20 parts by mass, particularly preferably about 1 to 15 parts by mass, when the positive electrode active material is 100 parts by mass.

  The kneading method after the addition of the solvent is not particularly limited. For example, a known apparatus such as a ribbon mixer, a screw kneader, a Spartan rewinder, a Redige mixer, a planetary mixer, a universal mixer with stirring and splashing can be used.

  The negative electrode material sheet and the positive electrode material sheet of the present invention described above are processed into a desired shape and laminated on the positive electrode material sheet / separator / negative electrode material sheet so that the positive electrode and the negative electrode are not in contact with each other. Store in containers such as molds, cylinders, and sheet molds. If there is a possibility that moisture or oxygen has been adsorbed during stacking and storage, it is dried again in an inert atmosphere under reduced pressure and / or a low dew point (-50 ° C. or lower), and then transferred to an inert atmosphere with a low dew point. Next, by injecting an electrolytic solution and sealing the container, a Li ion battery and / or a Li polymer battery can be produced.

  Although a well-known separator can be used, polyethylene and polypropylene porous microporous films are particularly preferable from the viewpoint of being thin and having high strength. The porosity is preferably higher from the viewpoint of ion conduction. However, if the porosity is too high, it causes a decrease in strength and a short circuit between the positive electrode and the negative electrode, and is usually used at 30 to 90%, preferably 50 to 80%. The thickness is preferably thin from the viewpoints of ionic conduction and battery capacity, but if it is too thin, it causes a decrease in strength and a short circuit between the positive electrode and the negative electrode, so that the thickness is usually 5-100 μm, preferably 5-50 μm. These microporous films can be used in combination of two or more kinds or other separators such as a nonwoven fabric.

As the electrolyte in the non-aqueous secondary battery of the present invention, particularly a lithium ion battery and / or a Li polymer battery, a known non-aqueous electrolyte can be used.
As the non-aqueous electrolyte of the present invention, known ones such as a non-aqueous electrolyte obtained by dissolving an electrolyte salt in a non-aqueous solvent and a non-aqueous polymer electrolyte obtained by swelling a polymer solid electrolyte with a non-aqueous solvent can be used.

  Non-aqueous solvents include diethyl ether, dibutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol dimethyl ether, ethylene glycol phenyl ether, etc. Ether; formamide, N-methylformamide, N, N-dimethylformamide, N-ethylformamide, N, N-diethylformamide, N-methylacetamide, N, N-dimethylacetamide, N-ethylacetamide, N, N-diethyl Acetamide, N, N-dimethylpropionamide, hexamethylphosphorylamide Amides such as dimethyl sulfoxide, sulfolane, etc .; dialkyl ketones such as methyl ethyl ketone, methyl isobutyl ketone; ethylene oxide, propylene oxide, tetrahydrofuran, 2-methoxytetrahydrofuran, 1,2-dimethoxyethane, 1,3-dioxolane, etc. Cyclic ethers; carbonates such as ethylene carbonate and propylene carbonate; γ-butyrolactone; N-methylpyrrolidone; solutions of organic solvents such as acetonitrile and nitromethane are preferred. Furthermore, preferably ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, vinylene carbonate, esters such as γ-butyrolactone, ethers such as dioxolane, diethyl ether, diethoxyethane, dimethyl sulfoxide, acetonitrile, tetrahydrofuran, etc. Particularly preferred are carbonate-based non-aqueous solvents such as ethylene carbonate and propylene carbonate. These solvents can be used alone or in admixture of two or more.

A lithium salt is used as the electrolyte salt. Commonly known lithium salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCl, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN (CF ₃ SO ₂ ) _{2 and the} like. is there.

Examples of the polymer solid electrolyte include polyethylene oxide derivatives and polymers containing the derivatives, polypropylene oxide derivatives and polymers containing the derivatives, phosphate ester polymers, polycarbonate derivatives and polymers containing the derivatives, and the electrolyte salts described above. Can be mentioned.
There are no restrictions on the selection of members necessary for battery configuration other than those described above.

  The negative electrode material for a lithium battery according to the present invention uses SBR as a binder, and carbon fibers having a fiber diameter of 1 to 1000 nm are highly dispersed as a conductive auxiliary agent, and has low electrode resistance and electrode strength. It is possible to obtain a high-performance battery that is good, excellent in electrolyte permeability, high energy density, and high-speed charge / discharge performance.

  The present invention will be described in more detail below with typical examples. Note that these are merely illustrative examples, and the present invention is not limited thereto.

[1] Average circularity:
The average circularity of the carbon material was measured as follows using a flow particle image analyzer FPIA-2100 (manufactured by Sysmex Corporation).
The sample for measurement was purified by removing fine dust through a 106 μm filter. A sample dispersion for measurement was prepared by adding 0.1 g of the sample into 20 ml of ion-exchanged water and uniformly dispersing 0.1 to 0.5% by mass of an anionic / nonionic surfactant. Dispersion was performed by treating for 5 minutes using an ultrasonic cleaner UT-105S (manufactured by Sharp Manufacturing System Co., Ltd.).
The outline of the measurement principle and the like is described in “Powder and Industry”, VOL.32, No. 2,2000, JP-A-8-136439 (US Pat. No. 5,721,433), etc. Specifically, it is as follows.
When the dispersion liquid of the measurement sample passes through the flow path of a flat and transparent flow cell (thickness: about 200 μm), strobe light is irradiated at 1/30 second intervals and imaged with a CCD camera. A certain number of the still images were taken and analyzed, and calculated according to the following formula.
Circularity = (circle circumference obtained from equivalent circle diameter) / (perimeter of particle projection image)
The equivalent circle diameter is the diameter of a true circle having the same projected area as the circumference of the actually imaged particle, and the circumference of the circle obtained from this equivalent circle diameter is divided by the circumference of the actually imaged particle. Value. For example, the value is 1 for a perfect circle, and the value becomes smaller as the shape becomes more complicated. The average circularity is an average value of circularity of each measured particle.

[2] Average particle size:
It measured using the laser diffraction scattering type particle size distribution measuring apparatus Microtrac HRA (made by Nikkiso Co., Ltd.).

[3] Specific surface area:
Using a specific surface area measuring device NOVA-1200 (manufactured by Yuasa Ionics Co., Ltd.), the measurement was performed by the BET method, which is a general method for measuring the specific surface area.

[4] Method A for preparing a composition for a carbon-based negative electrode material:
At room temperature, carbon fiber is added to a 1% by mass CMC aqueous solution (manufactured by Daicel Chemical Industries, Ltd., Daicel 2200) in a universal mixer with stirring blades (TK Hibismix, manufactured by Tokushu Kika Kogyo Co., Ltd.). After adding at a ratio and stirring and dispersing for 30 minutes at a rotational speed of 25 rpm, a predetermined amount of carbon-based negative electrode active material powder was added and mixed by stirring for 30 minutes. Thereafter, 40% by mass SBR aqueous dispersion (manufactured by Zeon Corporation, BM400B) was added and mixed with stirring for 15 minutes to obtain a composition for a carbon-based negative electrode material.

Method A-2:
After adding a 40 mass% SBR aqueous dispersion (manufactured by Nippon Zeon Co., Ltd., BM400B), a carbon-based negative electrode material composition was obtained in the same manner as in Method A except that the mixture was stirred and mixed for 100 minutes.

Method A-3:
A carbon-based negative electrode material composition was obtained in the same manner as in Method A, except that 40% by mass SBR aqueous dispersion (manufactured by Nippon Zeon Co., Ltd., BM400B) was added and stirred for 150 minutes.

Method B:
Predetermined carbon fiber in a 1% by mass CMC aqueous solution (manufactured by Daicel Chemical Industries, Ltd., Daicel 2200) in a universal mixer with stirring blades (TK Hibismix, manufactured by Tokushu Kika Kogyo Co., Ltd.) at room temperature After stirring for 30 minutes at a rotational speed of 25 rpm and dispersing, a predetermined amount of carbon-based negative electrode active material powder was added and mixed with stirring for 30 minutes. Furthermore, a predetermined amount of the same 1% by mass CMC aqueous solution is added for viscosity adjustment, and then 40% by mass SBR aqueous dispersion (manufactured by Nippon Zeon Co., Ltd., BM400B) is added and mixed by stirring for 15 minutes, and the composition for carbon-based negative electrode material I got a thing.

Method C:
At room temperature, in a universal mixer with a stirring blade (TK Hibismix, manufactured by Tokushu Kika Kogyo Co., Ltd.), a 1% by mass CMC aqueous solution (manufactured by Daicel Chemical Industries, Ltd., Daicel 2200) is used as a carbon-based negative electrode active. After adding the substance powder at a predetermined ratio and stirring and mixing at a rotational speed of 25 rpm for 30 minutes, adding a predetermined amount of carbon fiber, stirring and dispersing for 30 minutes, and then dispersing a 40% by mass SBR aqueous dispersion (Nippon Zeon Corporation) Manufactured, BM400B) was added and mixed with stirring for 15 minutes to obtain a composition for a carbon-based negative electrode material.

Method D:
The carbon-based negative electrode active material powder and the carbon fiber were dry-stirred and mixed twice at a predetermined ratio at 10000 rpm for 2 minutes in a desktop high-speed mixer with blades (IKA mixer). This mixture was transferred to a universal mixer with stirring blades (TK Hibismix, manufactured by Tokushu Kika Kogyo Co., Ltd.), and a predetermined amount of 1% by mass CMC aqueous solution (manufactured by Daicel Chemical Industries, Ltd., Daicel 2200) was added. Then, after stirring and mixing at room temperature for 30 minutes, 40% by mass SBR aqueous dispersion (manufactured by Nippon Zeon Co., Ltd., BM400B) was added and mixed by stirring for 15 minutes to obtain a composition for a carbon-based negative electrode material.

Method Ref (when no carbon fiber is added):
At room temperature, in a universal mixer with stirring blades (TK Hibismix, manufactured by Tokushu Kika Kogyo Co., Ltd.), a 1% by mass CMC aqueous solution (manufactured by Daicel Chemical Industries, Ltd., Daicel 2200) is charged with a predetermined amount of carbon. The negative electrode active material powder was added and mixed with stirring for 30 minutes. Thereafter, 40% by mass SBR aqueous dispersion (manufactured by Zeon Corporation, BM400B) was added and mixed with stirring for 15 minutes to obtain a composition for a carbon-based negative electrode material.

[5] Manufacture and evaluation of electrodes and batteries (1) Manufacture of carbon-based negative electrode sheet The composition for carbon-based negative electrode material prepared by the above method is doctor blade on a rolled copper foil (18 μm) manufactured by Nippon Foil Co., Ltd. Was applied to a predetermined thickness. This was vacuum-dried at 120 ° C. for 1 hour and punched out to 18 mmφ. Further, the punched electrode is sandwiched between super steel press plates and pressed so that the press pressure is 1 × 10 ^{3 to} 3 × 10 ³ kg / cm ² with respect to the electrode, and a desired electrode density (about 100 μm) 1.6 g / cm ³ or 1.8 g / cm ³ ).
Then, it dried at 120 degreeC for 12 hours with the vacuum dryer, and was set as the carbon type negative electrode sheet for battery evaluation.

(2) Preparation of positive electrode material composition Cathode active material LiCoO ₂ , acetylene black manufactured by Denki Kagaku Kogyo Co., Ltd., and vapor-phase-processed graphite fiber manufactured by Showa Denko Co., Ltd. were dry-blown at 93: 1: 2 (mass ratio). N-methylpyrrolidone containing 12% by mass of KF polymer L1320 (polyvinylidene fluoride (PVDF)) manufactured by Kureha Chemical Industry Co., Ltd. in a cathode mixture mixed at 10000 rpm for 1 minute × 2 times with a desktop high speed mixer (IK mixer). NMP) solution) was added so that the mass ratio of the positive electrode mixture to PVDF was 96: 4, and kneaded with a planetary mixer to obtain a positive electrode material composition.

(3) Manufacture of positive electrode sheet NMP was further added to the positive electrode material composition to adjust the viscosity, and then applied to a predetermined thickness on a rolled Al foil (25 μm) manufactured by Showa Denko KK using a doctor blade. This was vacuum-dried at 120 ° C. for 1 hour and punched out to 18 mmφ. Further, the punched electrode was sandwiched between super steel press plates and pressed so that the pressing pressure was 1 × 10 ³ kg / cm ² with respect to the electrode, and the electrode density was 3.3 g / cm ³ at about 100 μm. .
Then, it dried at 120 degreeC for 12 hours with the vacuum dryer, and was set as the electrode for evaluation.

(4) Evaluation of electrolyte solution permeation rate Electrolysis of propylene carbonate (abbreviated as PC) having a viscosity almost equal to that of various electrolyte solutions and low volatility on various negative electrode sheets (18 mmφ) in air at 25 ° C. As a solution, 3 μl was appropriately reduced with a microsyringe, and the time required for the PC to penetrate into the electrode was compared (average value of three times at each level).

(5) Measurement of electrode specific resistance The volume specific resistance value (25 degreeC) was measured by the 4-probe method about various negative electrode sheets.

(6) Production of Li-ion battery test cell A triode cell was produced as follows under a dry argon atmosphere with a dew point of -80 ° C or lower.
In a polypropylene screw-attached lid cell (inner diameter: about 18 mm), the negative electrode sheet with copper foil prepared in (1) above and the positive electrode sheet with Al foil prepared in (3) above were separated by separators (polypropylene microporous). A film (Selgard 2400), 25 μm) was sandwiched and laminated. Further, a reference metal lithium foil (50 μm) was laminated in the same manner. An electrolytic solution was added thereto to obtain a test cell.

(7) Electrolytic solution EC system: A mixed product of 8 parts by mass of EC (ethylene carbonate) and 12 parts by mass of DEC (diethyl carbonate), and 1 mol / liter of LiPF ₆ was dissolved as an electrolyte.

(8) Charging / discharging cycle test A constant current low voltage charging / discharging test was conducted at a current density of 0.6 mA / cm ² (equivalent to 0.3 C).
CC (constant current: constant current) charging was performed at 0.6 mA / cm ² from the charging rest potential to 4.2 V. Next, it switched to CV (constant volt: constant voltage) charge at 4.2 V, and stopped when the current value dropped to 25.4 μA.
As the discharge, CC discharge was performed at 0.6 mA / cm ² (equivalent to 0.3 C), and cut off at a voltage of 2.7 V.

Materials used:
<Negative electrode active material>
SCMG-1: Spherical graphite particles manufactured by Showa Denko KK
Average particle size: 24.5 μm, Average circularity: 0.93
X-ray C ₀ : 0.6716 nm, Lc: 459 nm,
Raman R value: 0.05,
Specific surface area: 1.2 m ² / g, True density: 2.17 g / cm ³

SCMG-2: Spheroidal graphite particles manufactured by Showa Denko KK
Average particle size: 19.0 μm, Average circularity: 0.91
X-ray C ₀ : 0.6716 nm, Lc: 489 nm,
Raman R value: 0.06
Specific surface area: 2.5 m ² / g, True density: 2.17 g / cm ³

MAG: Graphite particles manufactured by Hitachi Chemical Co., Ltd.
Average particle size: 20.1 μm, Average circularity: 0.85,
X-ray C ₀ : 0.6716 nm, Lc: 420 nm,
Raman R value: 0.10
Specific surface area: 3.2 m ² / g, True density: 2.20 g / cm ³

Shanghai MC: Mesophase graphite particles made by Shanghai Sugisugi,
Average particle size 17.4 μm, average circularity 0.88,
X-ray C ₀ : 0.6732 nm, Lc: 82.0 nm,
Raman R value: 0.15
Specific surface area: 1.3 m ² / g, True density: 2.15 g / cm ³

MCMB: Osaka Gas Chemical Co., Ltd. mesophase spherical graphite particles,
Average particle size 16.6 μm, average circularity 0.94,
X-ray C ₀ : 0.6729 nm, Lc: 84.4 nm,
Raman R value: 0.12,
Specific surface area: 1.1 m ² / g, True density: 2.19 g / cm ³

LBCG: Spherical natural graphite manufactured by Nippon Graphite Industry Co., Ltd.
Average particle size 24.0 μm, average circularity 0.85,
X-ray C ₀ : 0.6717 nm, Lc: 283.5 nm,
Raman R value: 0.23
Specific surface area: 4.6 m ² / g, True density: 2.27 g / cm ³

<Positive electrode active material>
LiCoO ₂ : manufactured by Nippon Chemical Industry Co., Ltd., average particle size 28.9 μm, average circularity 0.96

<Carbon fiber>
VG: Vapor growth graphite fiber,
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 8 μm,
Average aspect ratio: 60,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6767 nm, Lc: 48.0 nm

VG-A: Vapor growth carbon fiber (non-graphitized product, fired at 1200 ° C.),
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 8 μm,
Average aspect ratio: 65,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6922 nm, Lc: 3.0 nm

VG-B: Vapor growth graphite fiber (2% boron added during graphitization),
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 8 μm,
Average aspect ratio: 60,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6757 nm, Lc: 72.0 nm

VG-H: Vapor growth graphite fiber (jet mill pulverization),
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 5 μm,
Average aspect ratio: 37,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6769 nm, Lc: 47.0 nm

VG-O: Vapor-grown graphite fiber (500 ° C. oxidized product),
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 8 μm,
Average aspect ratio: 55,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6769 nm, Lc: 42.0 nm

VG-F: Vapor growth graphite fiber,
Average fiber diameter (from SEM image analysis): 80 nm,
Average fiber length (from SEM image analysis): 6 μm,
Average aspect ratio: 90,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6801 nm, Lc: 35.0 nm

VG-T: Vapor growth graphite fiber,
Average fiber diameter (from SEM image analysis): 20 nm
Average fiber length (from SEM image analysis): 6 μm,
Average aspect ratio: 150,
Branch degree (from SEM image analysis): 0.1%
X-ray C ₀ : 0.6898 nm, Lc: 30.0 nm

<SBR electrode binder>
BM-400B: 40 mass% aqueous dispersion manufactured by Nippon Zeon Co., Ltd., glass transition point-5 ° C. (from DSC), average particle size 120 nm.

Example: Production and Evaluation of Negative Electrode and Battery According to the composition and method shown in Table 1 (Table 1-1: Negative electrode material density 1.6 g / cm ³ , Table 1-2: Negative electrode material density 1.8 g / cm ³ ) A carbon-based negative electrode material was prepared, and the electrode specific resistance and the electrolyte penetration rate were measured. Next, a Li ion battery test cell was prepared in combination with the above positive electrode material, and the negative electrode capacity density and (charge / discharge) cycle characteristics were measured and evaluated. The results are also shown in Table 1.
As is clear from Table 1, the negative electrode produced from the negative electrode material composition prepared by the methods (A) to (D) and (A-2) to (A-3) was compared with a carbon fiber-free product. Thus, the electrolyte permeability is improved, and the cycle characteristics of the battery are improved. In particular, when the methods (A) to (D) are used, all of the electrode specific resistance, electrolyte permeability and capacity density are improved, and the cycle characteristics when a battery is obtained are also greatly improved. I understand. Further, the mixing method was effective in the order of (A) = (B)>(C)> (D).

Claims (24)

Contains a carbon-based negative electrode active material having a specific surface area of 1 m ² / g or more, a binder made of styrene butadiene rubber comprising fine particles having an average particle diameter of 10 to 500 nm, and carbon fibers having a fiber diameter of 1 to 1000 nm and an average fiber diameter of 80 nm or more. And a density of a mixture layer composed of a negative electrode active material, a binder, and a conductive additive is 1.7 g / cm 3 or more.   The negative electrode material for a lithium battery according to claim 1, wherein a copolymerization ratio of styrene in the styrene-butadiene rubber is 50% by mass or less.   The carbon fiber content is 0.05 to 20% by mass with respect to the total amount of the carbon-based negative electrode active material, the binder and the carbon fiber, and the content of the binder made of styrene butadiene rubber is 0.1. The negative electrode material for a lithium battery according to claim 1, wherein the content is ˜6.0% by mass.   Furthermore, the negative electrode material for lithium batteries in any one of Claims 1-3 containing a thickener.   5. The negative electrode for a lithium battery according to claim 4, wherein the content of the thickener is 0.1 to 4 mass% with respect to the total amount of the carbon-based negative electrode active material, the binder, the carbon fiber, and the thickener. Wood.   The lithium battery negative electrode material according to claim 4, wherein the thickener is carboxymethylcellulose.   The negative electrode material for a lithium battery according to any one of claims 1 to 6, wherein the specific resistance of the negative electrode at 25 ° C is 0.5 Ωcm or less.   The negative electrode material for a lithium battery according to any one of claims 1 to 7, wherein the carbon fiber is a graphite-based carbon fiber heat-treated at 2000 ° C or higher.   The negative electrode material for a lithium battery according to any one of claims 1 to 7, wherein the carbon fiber is a graphite-based carbon fiber having an oxygen-containing functional group introduced to the surface by oxidation treatment.   The negative electrode material for a lithium battery according to any one of claims 1 to 7, wherein the carbon fiber is a graphite-based carbon fiber containing 0.1 to 100,000 ppm of boron. 9. The negative electrode material for a lithium battery according to claim 8, wherein an average interplanar spacing d ₀₀₂ of the (002) plane of graphite-based carbon fiber by X-ray diffraction is 0.344 nm or less.   The negative electrode material for a lithium battery according to any one of claims 1 to 11, wherein the carbon fiber has a hollow structure therein.   The negative electrode material for a lithium battery according to any one of claims 1 to 12, wherein the carbon fiber includes a branched carbon fiber.   The negative electrode material for a lithium battery according to any one of claims 1 to 13, wherein the carbon-based negative electrode active material contains Si. The negative electrode material for a lithium battery according to any one of claims 1 to 14, wherein the carbon-based negative electrode active material before forming the electrode is a carbonaceous particle satisfying the following requirements:
(1) The average circularity measured by a flow type particle image analyzer is 0.70 to 0.99,
(2) The average particle diameter by laser diffraction method is 1-50 μm.   The negative electrode material for a lithium battery according to any one of claims 1 to 15, wherein the carbon-based negative electrode active material contains a graphite carbon-based material of 50% by mass or more.   The negative electrode material for a lithium battery according to claim 16, wherein the graphite material contains boron. The negative electrode material for a lithium battery according to any one of claims 1 to 17, wherein the carbon-based negative electrode active material of the electrode active material before forming the electrode is a carbonaceous particle containing 50% by mass or more of graphite particles satisfying the following requirements. :
(1) The average circularity measured by a flow type particle image analyzer is 0.70 to 0.99,
(2) The average particle diameter by laser diffraction method is 1-50 μm. The negative electrode material for a lithium battery according to claim 16, wherein the graphite-based carbon material is a carbonaceous particle containing 50% by mass or more of graphite particles satisfying the following requirements:
(1) C ₀ of (002) plane in X-ray diffraction measurement is 0.6900 nm or less, La (crystallite size in the a-axis direction)> 100 nm, Lc (crystallite size in the c-axis direction)> 100 nm,
(2) A specific surface area of 1.0 to 10 m ² / g,
(3) True density is 2.20 g / cm ³ or more,
(4) Laser Raman R value (peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 by laser Raman spectrum) 0.01 to 0.9.   A composition for a negative electrode material for a lithium battery in which carbon fibers having a fiber diameter of 1 to 1000 nm are dispersed in a thickener aqueous solution, wherein the concentration of the thickener in the thickener aqueous solution is 0.3 to 5% by mass, The composition for a negative electrode material for a lithium battery, wherein the proportion of carbon fibers in the entire composition is 0.1 to 10% by mass, is applied onto a metal current collector foil, dried and then pressure-molded. The negative electrode material for lithium batteries in any one of -19.   The negative electrode material for a lithium battery according to claim 20, wherein the metal current collector foil is a copper foil or a copper alloy foil having a thickness of 1 to 50 µm.   The lithium battery which contains the negative electrode material for lithium batteries in any one of Claims 1-21 as a component.   The lithium secondary battery which contains the negative electrode material for lithium batteries in any one of Claims 1-21 as a component.   A non-aqueous electrolyte is used, and as the non-aqueous solvent of the non-aqueous electrolyte, at least one selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate is used. Item 24. The lithium secondary battery according to Item 23.