JP4031009B2 - Positive electrode for lithium battery and lithium battery using the same - Google Patents

Description

  The present invention relates to a positive electrode for a lithium battery having a large electrode density, a large charge / discharge capacity per volume, charge / discharge cycle characteristics, large current load characteristics, excellent electrolyte solution permeability, and a lithium battery using the same. In particular, the present invention relates to a high-density positive electrode used for a lithium-based secondary battery and a lithium-based secondary battery using the same. More specifically, in realizing a high energy density lithium-based battery, particularly a high energy density positive electrode for a lithium-based battery to achieve the energy density per volume, which is important for small portable devices, the effect can be achieved with as little addition as possible. The present invention relates to a positive electrode for a lithium battery to which a specific carbon fiber that can be exhibited is added as a conductive additive.

  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, lithium secondary batteries such as lithium ion batteries and lithium polymer batteries using non-aqueous electrolytes are used as secondary batteries for small portable devices such as mobile phones and video cameras. Due to the characteristic of voltage, it is used in many devices. As a positive electrode material used in these lithium secondary batteries, a metal oxide compound represented by lithium cobaltate having a large charge / discharge capacity per unit mass at a high potential is used, and as a negative electrode material, a base material close to lithium is used. A carbon material represented by graphite having a large charge / discharge capacity per unit mass at a low potential 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, new high-capacity positive electrode materials such as iron olivine compounds and metal sulfides, and new high-capacity negative electrode materials such as tin oxide, silicon oxide, lithium alloy, lithium nitride, and composite materials of these and carbon materials have been actively developed. ing.

Moreover, as a secondary battery used for a small portable device, a more compact battery is required, and not only the energy density per mass but also the energy density per volume is required to be high. Accordingly, studies have been started to increase the filling amount in the battery container by increasing the electrode density, and to increase the energy density per volume of the electrode and the battery.
For example, the true density of lithium cobaltate-based oxide mainly used as a positive electrode material is of the order of 5.1 g / cm ^3, as the electrode density is used in less than 3.3 g / cm ^3, 3 Investigations have been made with respect to those of ⁵ g / cm ³ or more.

However, by increasing the electrode density, the number of vacancies in the electrode decreases, which usually leads to a shortage of the electrolyte solution that is important for the electrode reaction existing in the vacancies, and the penetration of the electrolyte solution into the electrode is slowed down. The problem arises. If the electrolyte in the electrode is insufficient, the electrode reaction will be delayed, causing problems such as a decrease in energy density and high-speed charge / discharge performance, and if the electrolyte permeability is slow, the battery manufacturing time will be longer. , Leading to increased manufacturing costs. The problem becomes more prominent when a highly viscous polymer electrolyte such as a lithium polymer battery is used.
In order to solve these problems, attempts have been made to increase the energy density by increasing the amount of active material in the positive electrode by reducing the carbon-based conductive additive added to the positive electrode as much as possible. Further, the electrode density itself can be increased by reducing the amount of bulky carbon-based conductive additive. However, the conductivity of metal oxide compounds such as lithium cobaltate oxide is generally semiconducting. If too little conductive aid is used, the conductivity of the electrode will be reduced, and charging / discharging will be hindered. For this reason, carbon-based conductive additives such as carbon black have been conventionally used in an amount of about 3% by mass or more.

Japanese Patent No. 2695180 JP 2000-208147 A

  An object of the present invention is to improve the electrolyte permeability and electrolyte retention, which are problems in realizing a high-density electrode necessary for achieving a high energy density battery.

  In view of the above-described problems of the lithium-based battery electrode, the present inventors have made extensive studies and as a result, the positive electrode active material, a carbon-based conductive additive containing a specific carbon fiber, and a binder, A lithium battery positive electrode (a positive electrode composed of a positive electrode active material, a carbon-based conductive auxiliary agent and a binder, and does not include a current collector) in which the amount of the auxiliary agent is 0.1 to 2% by mass of the whole positive electrode. It has been found that by using it, a high-performance lithium-based battery having high energy density and good high-speed charge / discharge performance can be obtained without impairing conductivity and electrolyte permeability, and the present invention has been completed.

  It has been studied so far to improve the cycle life of a battery by adding carbon fiber to a lithium-based positive electrode material. For example, Patent Document 1 (Japanese Patent No. 2695180) discloses a lithium secondary battery in which a charge / discharge cycle is improved by adding carbon fiber to an oxide positive electrode. Patent Document 2 (Japanese Patent Laid-Open No. 2000-208147) discloses a positive electrode in which powder carbon such as carbon black, flake carbon such as graphite, and fibrous carbon are mixed. However, in these prior arts, the purpose of adding carbon fibers is to reduce electrode resistance and improve electrode strength, and there is no concept of adding carbon fibers to increase the density of the electrodes, and the addition amount is also generally 5 It is used at about 10% by mass.

  In the present invention, by adding a specific amount of a specific carbon fiber, for example, even in a high-density electrode having a porosity of 20% or less, the electrolyte permeability is not significantly reduced, and the electrode resistance is low and the electrode strength is as usual. This is based on the knowledge that a good electrode can be obtained.

  The reason why the electrolyte solution permeability of the high-density electrode is improved by adding the carbon fiber is that fine fibers are appropriately dispersed among the highly compressed active material particles, thereby This is probably because fine voids are maintained.

The present invention provides the following positive electrode for a lithium battery and a lithium battery using the positive electrode.
[1] A positive electrode for a lithium battery comprising an active material capable of occluding and releasing lithium ions, a carbon-based conductive aid, and a binder, wherein the carbon-based conductive aid is added in an amount of 0.1 to 2% by weight based on the total amount of the positive electrode In addition, the carbon-based conductive additive contains carbon fibers having an average fiber diameter of 1 to 200 nm, the carbon fibers are graphite-based carbon fibers that have been heat-treated at 2000 ° C. or higher, and lithium ions calculated from true density A positive electrode for a lithium battery, wherein the volume of the active material capable of occlusion and release is 70% or more in the positive electrode volume.
[2] The positive electrode for a lithium battery as described in 1 above, wherein the total amount of metal impurities in the carbon fiber is 30 ppm or less.
[3] The positive electrode for a lithium battery according to 1 or 2, wherein the carbon fiber is a graphite-based carbon fiber containing 0.1 to 100,000 ppm of boron.
[4] The positive electrode for a lithium battery according to any one of 1 to 3, wherein the average aspect ratio of the carbon fiber is 50 to 15000.
[5] The positive electrode for a lithium battery according to any one of 1 to 4 above, wherein the carbon fiber has an average fiber diameter of 10 to 200 nm.
[6] The positive electrode for a lithium battery according to any one of 1 to 5, wherein the carbon fiber has a hollow structure inside.
[7] The positive electrode for a lithium battery according to any one of 1 to 6, wherein the carbon fiber includes a branched carbon fiber.
[8] The positive electrode for a lithium battery according to any one of 1 to 7, wherein the active material is a metal oxide compound capable of occluding and releasing lithium ions.
[9] The positive electrode for a lithium battery as described in 8 above, wherein the average primary particle diameter of the metal oxide compound which is an active material capable of occluding and releasing lithium ions is 0.5 to 30 μm.
[10] In the above 8 or 9, wherein the metal oxide compound which is an active material capable of occluding and releasing lithium ions contains cobalt oxide in an amount of 60% by mass or more, and the electrode density is 3.7 g / cm ³ or more. The positive electrode for lithium-type batteries as described.
[11] In the above 8 or 9, wherein the metal oxide compound, which is an active material capable of occluding and releasing lithium ions, contains manganese oxide in an amount of 60% by mass or more, and the electrode density is 3.2 g / cm ³ or more. The positive electrode for lithium-type batteries as described.
[12] The metal oxide compound which is an active material capable of occluding and releasing lithium ions contains a mixture of cobalt oxide and manganese oxide in an amount of 80% by mass or more, and the electrode density is 3.5 g / cm ³ or more. 10. The positive electrode for a lithium battery as described in 8 or 9 above.
[13] The metal oxide compound which is an active material capable of occluding and releasing lithium ions contains nickel oxide in an amount of 60% by mass or more, and the electrode density is 3.5 g / cm ³ or more. The positive electrode for lithium-type batteries as described.
[14] The metal oxide compound that is an active material capable of occluding and releasing lithium ions contains cobalt-nickel composite oxide in an amount of 60% by mass or more, and the electrode density is 3.6 g / cm ³ or more. Or the positive electrode for a lithium battery according to 9.
[15] The metal oxide compound which is an active material capable of occluding and releasing lithium ions contains 60% by mass or more of a cobalt / manganese composite oxide, and the electrode density is 3.6 g / cm ³ or more. Or the positive electrode for a lithium battery according to 9.
[16] The metal oxide compound, which is an active material capable of occluding and releasing lithium ions, contains 60% by mass or more of cobalt / nickel / manganese composite oxide, and the electrode density is 3.6 g / cm ³ or more. 10. The positive electrode for a lithium battery according to 8 or 9 above.
[17] The positive electrode for a lithium battery as described in 8 or 9 above, wherein the iron oxide compound is contained in an amount of 60% by mass or more in the metal oxide compound which is an active material capable of occluding and releasing lithium ions.
[18] A lithium battery including the positive electrode for a lithium battery according to any one of 1 to 17 as a constituent element.
[19] A lithium secondary battery including the lithium battery positive electrode according to any one of 1 to 17 as a constituent element.
[20] A non-aqueous electrolyte and / or a non-aqueous polymer electrolyte is used, and ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene are used as the non-aqueous solvent used in the non-aqueous electrolyte and / or non-aqueous polymer electrolyte. 20. The lithium secondary battery according to 19, wherein at least one selected from the group consisting of carbonate, butylene carbonate, and vinylene carbonate is included.

Since the positive electrode for a lithium battery of the present invention can be increased in density by increasing the filling amount of the positive electrode active material, a lithium battery having a high energy density, that is, a battery having a large capacity per battery volume, can do.
When the positive electrode is highly filled and densified, the number of vacancies in the positive electrode decreases, leading to a shortage of the electrolyte solution that is important for the electrode reaction existing in the vacancies or the penetration of the electrolyte solution into the electrode. As a result, the delay of electrode reaction, the decrease of energy density, the decrease of high-speed charge / discharge performance, the increase of the manufacturing cost due to the long battery manufacturing time, etc. The positive electrode contains carbon fiber, and even when the density is increased, the decrease in the permeability of the electrolytic solution is suppressed and the retention of the electrolytic solution is improved, so that the above problems can be solved.

1. Carbon fiber Generally, a positive electrode for a lithium-based battery is obtained by forming a positive electrode active material powder of several to several tens of μm at a high pressure using a roll press or the like, so that there are fewer gaps between the powders, and the permeability of the electrolyte is greatly increased. descend. By adding fine tough fibers that are fine and resistant to pressure deformation, fine voids are formed between the electrode active material powders, and the electrolytic solution easily permeates. If the inter-particle conductivity between the electrode active materials is impaired due to voids, the electrode performance will deteriorate, so the added fiber itself is excellent in conductivity, and the fiber length should be as long as possible to increase the conductive path Is preferred. From such a viewpoint, it is necessary to use conductive, tough and fine carbon fibers as the fibers to be added.

(1-1) Fiber diameter of carbon fiber The fiber diameter of the carbon fiber used for the positive electrode for a lithium battery of the present invention is in the range of 1 to 200 nm, preferably 5 to 180 nm, more preferably 10 to 150 nm. It is. Since the average particle diameter of the active material particles generally used is about several to several tens of micrometers, if the fiber diameter of the carbon fiber used for the positive electrode for a lithium battery of the present invention exceeds 200 nm, the voids in the electrode become too large. It is not preferable because the electrode density cannot be increased. Further, if the fiber diameter is too thin (less than 1 nm), it is not preferable because it is buried between the active material particles and void generation in the target electrode becomes impossible.

(1-2) Carbon fiber crystallinity The carbon fiber has a higher crystallinity (graphitization degree). In general, the higher the degree of graphitization of the carbon material, the more the layered structure develops, the harder it becomes, and the higher the conductivity, and it is suitable as an additive for the positive electrode. In order to graphitize the carbon material, high temperature treatment may be performed. In this case, the treatment temperature varies depending on the carbon fiber used, but is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher. In this case, it is effective to add boron, Si, or the like, which is a graphitization co-catalyst that promotes the degree of graphitization, before the heat treatment. The addition amount of the cocatalyst is not particularly limited, but a preferable addition amount is in the range of 10 ppm to 50,000 ppm by mass. If the addition amount is too small, the effect is not obtained, and if it is too large, it remains as an impurity, which is not preferable.

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) Carbon fiber fiber length and aspect ratio The carbon fiber fiber length is not particularly limited. As described above, the longer the fiber length, the better the conductivity in the electrode, the strength of the electrode, and the electrolyte solution retention. In particular, as in the present invention, in order to effectively use a carbon material with a low addition amount, it is preferable that the carbon material is as long as possible as long as the fiber dispersibility in the electrode is not impaired. Although the range of a preferable average fiber length changes also with the kind and fiber diameter of carbon fiber to be used, it is 2-100 micrometers, and the thing of 5-100 micrometers is still more preferable. When the preferable range of this average fiber length is shown by an average aspect ratio (ratio of fiber length to fiber diameter), it is in the range of 20 to 50000, and more preferably in the range of 50 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 with the fiber length. The amount 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 fibers obtained by vapor phase growth, so-called vapor phase growth carbon fibers, have a crystal growth direction substantially parallel to the fiber axis, and the crystallinity in the fiber length direction of the graphite structure is likely to be high. Highly conductive and 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 by, for example, a method in which an organic compound gasified with iron serving as a catalyst is blown into a high temperature atmosphere. As the vapor grown carbon fiber, any of those as manufactured, those heat-treated at about 800 to 1500 ° C., and those graphitized at about 2000 to 3000 ° C. can be used. A material suitable for the electrode active material powder to be used is used, but a heat-treated and graphitized one is preferable because the crystallinity of carbon is advanced and it has high conductivity and high pressure resistance.

  Moreover, there exists a branched fiber as a preferable form of vapor grown carbon fiber. The branched portion has a hollow structure in which the entire fiber including the portion is in communication with each other, and the carbon layer constituting the cylindrical portion of the fiber is continuous. 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 stacked 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. The vapor grown carbon fiber, which also serves as a chemical auxiliary, can be kept in close contact with the vapor grown carbon fiber, and the electron conductivity can be maintained and the cycle characteristics can be improved.

  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) Impurities of carbon fibers Since carbon fibers are used for the positive electrode, metals that elute at the potential of the positive electrode are not preferable. Therefore, as impurities, the total sum of metals such as Fe and Cu is preferably 100 ppm or less, and more preferably 30 ppm or less.

(1-6) Addition amount of carbon fiber The content of the carbon fiber is preferably in the range of 0.1 to 2 mass%, preferably 0.5 to 1.5 mass% in the positive electrode. When the content exceeds 2% by mass, the ratio of the electrode active material in the positive electrode becomes small, so that the electric capacity becomes small. If the content is less than 0.1% by mass, the conductivity in the positive electrode is not sufficient, and it becomes difficult to insert and release lithium ions. In order to adjust the content to this range, it can be carried out by adding so as to have the same ratio in the production method.

(1-7) Surface treatment of carbon fiber Carbon fiber that has been surface treated to control 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 oxidation treatment improves the hydrophilicity of the carbon fiber.

2. Examples of an active material that can occlude and release lithium ions used in the positive electrode of the present invention are shown below.

  Cobalt oxides such as lithium cobaltate, manganese oxides such as lithium manganate, nickel oxides such as lithium nickelate, vanadium oxides such as vanadium pentoxide, and complex oxides and mixtures of these It has been used or has been studied as a positive electrode active material for lithium ion batteries. Higher capacity as a battery is being studied by making these positive electrodes into high-density electrodes.

Specifically, the lithium cobalt oxide has a true density of about 5.1 g / cm ³ and is currently used at an electrode density of less than 3.3 g / cm ³ , but by adding carbon fiber to this, Even at an electrode density of 3.7 g / cm ³ , it is possible to suppress a decrease in electrolyte permeability. The true density of lithium manganate is about 4.2 g / cm ³ , and is currently used at an electrode density of less than 2.9 g / cm ^3. By adding carbon fiber to this, an electrode density of 3.2 g Even at / cm ³ , it is possible to suppress a decrease in electrolyte permeability. The true density of lithium nickelate is about 5.0 g / cm ³ , and is currently used at an electrode density of 3.2 g / cm ³ or less, but by adding carbon fiber to this, an electrode density of 3.5 g Even at / cm ³ , it is possible to suppress a decrease in electrolyte permeability. The true density of vanadium pentoxide is about 2.9 g / cm ³ , and is currently used at an electrode density of 2.0 g / cm ³ or less. By adding carbon fiber to this, an electrode density of 2.3 g Even at / cm ³ , it is possible to suppress a decrease in electrolyte permeability.

Further, in the case of a mixture of a cobalt-based oxide such as lithium cobaltate and a manganese-based oxide such as lithium manganate, it is currently used at an electrode density of 3.1 g / cm ³ or less. By adding, it is possible to suppress a decrease in electrolyte permeability even at an electrode density of 3.5 g / cm ³ .

The lithium-containing transition metal oxide used as the positive electrode active material of the present invention is preferably lithium and at least one transition metal element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Mo, and W. In which the molar ratio of lithium to transition metal is 0.3 to 2.2. More preferably, the oxide mainly contains 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. It is 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 preferable to use at least one of materials having a spinel structure represented by at least one of B, P and b = 1 to 0.2 and 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 to 1.2, a = 0.1 to 0.9, b = 0.9 to 0.98, z = 2.01 to 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 lithium battery positive electrode materials, metal sulfides such as titanium sulfide and molybdenum sulfide have been actively studied, and high-density electrodes are being formed. In the present invention, by adding carbon fiber, it is possible to suppress a decrease in electrolyte permeability even at an electrode density of 2.0 g / cm ³ .

An iron olivine compound such as LiFePO ₄ has a high theoretical capacity, uses iron, is excellent in resource, environmental safety, heat resistance, etc., and has been energetically studied as a next-generation lithium ion positive electrode material. Since the true density of LiFePO ₄ is lower than that of a positive electrode material (such as lithium cobaltate) currently used in lithium ion batteries, the need for higher density is even higher. In the present invention, by adding carbon fibers, it is possible to suppress a decrease in electrolytic solution permeability. In addition, LiFePO ₄ has low conductivity, and it can be said that efficient combination with a carbon fiber-based conductive material is essential.

The average primary particle size of the positive electrode active material is not particularly limited, but is usually preferably 0.1 to 50 μm, and the volume of particles of 0.5 to 30 μm is preferably 95% or more. More preferably, the volume occupied by particle groups having a particle size of 3 μm or less is 18% or less of the total volume, and the volume occupied by particle groups of 25 to 30 μm 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.

3. Electrode production The method for producing a positive electrode for a lithium battery according to the present invention is not particularly limited. Generally, after mixing the above-described positive electrode active material, carbon fiber, and binder material, it is applied onto a supporting substrate such as a metal current collector. Thereafter, it can be produced by drying and pressing.
As a method of mixing each material, (1) a method of mixing a positive electrode active material (including a conductive auxiliary such as carbon black in some cases; the same applies hereinafter), carbon fiber and a binder material at a time, and (2) a positive electrode active material. A method of mixing the material and carbon fiber and then mixing the binder material, (3) A method of mixing the positive electrode active material and the binder, and then mixing the carbon fiber, (4) After mixing the carbon fiber and the binder material, Examples include a method of mixing substance materials.

  The dispersion state in the electrode differs depending on the material type, composition ratio, combination, etc., and affects electrode resistance, liquid absorbency, etc., so it is necessary to select an optimal mixing method depending on conditions.

  The method of mixing the positive electrode active material and the carbon fiber may be agitated with, for example, a mixer. Although the stirring method is not particularly limited, for example, apparatuses such as a ribbon mixer, a screw type kneader, a spartan luzer, a redige mixer, a planetary mixer, and a universal mixer can be used.

  The method of mixing the binder material into the positive electrode active material, carbon fiber or a mixture thereof is not particularly limited, but the method of kneading with a solvent after mixing in a dry method, or the method of mixing the binder material with a solvent and diluting the binder material with a solvent, Or the method of knead | mixing with these negative electrode materials is mentioned. These solvent-containing mixtures are coated on a current collector (base material) and formed into a sheet. In order to adjust the viscosity of the solvent-containing mixture, a thickening agent such as a polymer such as CMC (sodium carboxymethyl cellulose) or polyethylene glycol is used. Materials may be added.

  As the binder material, known materials such as fluorine-based polymers such as polyvinylidene fluoride and polytetrafluoroethylene, and rubber-based materials such as styrene butadiene rubber (SBR) can be used. As the solvent, known solvents suitable for each binder, for example, toluene, N-methylpyrrolidone, acetone and the like in the case of a fluorine-based polymer, and water and the like in the case of SBR can be used.

When the positive electrode active material is 100 parts by mass, the amount of the binder used is suitably 0.5 to 20 parts by mass, particularly preferably about 1 to 15 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 type kneader, a spartan luzer, a redige mixer, a planetary mixer, a universal mixer can be used.

The positive electrode sheet of the present invention can be produced by applying the kneaded mixture to a current collector.
Application | coating to the electrical power collector after kneading can be implemented by a well-known method. For example, after applying with a doctor blade or a bar coater, a method of forming with a roll press or the like can be mentioned.
As the current collector, known materials such as aluminum, stainless steel, nickel, titanium and alloys thereof, platinum, and a carbon sheet can be used.

After drying these coated electrode sheets by a known method, a desired thickness while adjusting the volume ratio of the positive electrode active material in the positive electrode to 80% or more by a known method such as a roll press or a pressure press, Mold to density.
The press pressure may be determined within a range in which the volume ratio of the positive electrode active material in the positive electrode can be adjusted to 70% or more. Since it depends on the positive electrode active material used, it cannot be generally stated, but usually a pressure of 1 ton / cm ² or more is applied. The thickness of the electrode sheet varies depending on the shape of the target battery and is not particularly limited, but is usually 0.5 to 1000 μm, preferably 5 to 500 μm.

4). Battery Production The battery of the present invention uses the positive electrode containing the carbon fiber of the present invention, and can be produced by a known method. In particular, a high-density positive electrode containing carbon fiber can be preferably used as a positive electrode of a high energy density non-aqueous secondary battery such as a lithium ion battery or a lithium polymer battery. A typical method for producing a lithium ion battery and / or a lithium polymer battery will be described below.

  The positive electrode sheet prepared above is processed into a desired shape, combined with a graphite negative electrode sheet or a lithium metal negative electrode sheet, and laminated on the positive electrode sheet / separator / negative electrode sheet so that the positive electrode and the negative electrode are not in contact with each other. Store in a container such as a mold, cylinder, or sheet mold. 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. Subsequently, a lithium ion battery or a lithium polymer battery can be produced by injecting an electrolytic solution and sealing the container.

  A known separator can be used, but from the viewpoint of being thin and having high strength, a polyethylene or polypropylene porous microporous film is preferable. The porosity is preferably as high as possible from the viewpoint of ionic conduction, but if it is too high, it will cause a decrease in strength and a short circuit between the positive electrode and the negative electrode, so it 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, and therefore it 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 an electrolyte and an electrolyte in a non-aqueous secondary battery, particularly a lithium ion battery and / or a lithium polymer battery, known organic electrolytes, inorganic solid electrolytes, and polymer solid electrolytes can be used.

  Examples of organic electrolytes 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, and ethylene glycol phenyl ether. 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, methyl ethyl carbonate, propylene carbonate, vinylene carbonate, esters such as γ-butyrolactone, ethers such as dioxolane, diethyl ether, diethoxyethane, dimethyl sulfoxide, Acetonitrile, tetrahydrofuran, etc. are mentioned, Especially preferably, carbonate type nonaqueous solvents, such as ethylene carbonate and propylene carbonate, can be used. These solvents can be used alone or in admixture of two or more.

Lithium salts are used as solutes (electrolytes) for these solvents. 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 a polyethylene oxide derivative and a polymer containing the derivative, a polypropylene oxide derivative and a polymer containing the derivative, a phosphate ester polymer, a polycarbonate derivative and a polymer containing the derivative.
There are no restrictions on the selection of members necessary for battery configuration other than those described above.

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.
The physical properties used in the following examples were measured by the following methods.

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

2. 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.

3. Battery evaluation method:
(1) Production of carbon fiber-containing positive electrode A positive electrode active material, acetylene black (hereinafter sometimes abbreviated as AB) manufactured by Denki Kagaku Kogyo Co., Ltd., and carbon fiber are dried at a predetermined composition ratio, high-speed small bladed A positive electrode material mixture was prepared by mixing at 10000 rpm for 30 seconds × 2 times with a mixer (IK mixer). This may be abbreviated as N-methylpyrrolidone (hereinafter, abbreviated as NMP) containing 12% by mass of KF polymer L # 1320 (polyvinylidene fluoride (hereinafter abbreviated as PVDF)) manufactured by Kureha Chemical Industry Co., Ltd. ) Solution) was added so that the mass ratio of the positive electrode material mixture and PVDF was 97: 3, and kneaded with a planetary mixer to obtain a positive electrode kneaded paste.

Next, NMP was further added to the positive electrode kneaded paste to adjust the viscosity, and then applied to a rolled aluminum foil (thickness 25 μm) manufactured by Showa Denko KK to a predetermined thickness using a doctor blade. 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 the press pressure is about 1 × 10 ^{2 to} 3 × 10 ² N / mm ² (1 × 10 ^{3 to} 3 × 10 ³ kg / cm ² ) with respect to the electrode. To a desired electrode density with a thickness of about 100 μm. Then, it dried for 120 hours and 120 degreeC with the vacuum dryer, and it was for evaluation.

(2) Mesocarbon microbeads (hereinafter sometimes abbreviated as MCMB) Production of graphite negative electrode A negative electrode combined with the carbon fiber-containing positive electrode produced in (1) above was produced by the following method. Osaka Gas Co., Ltd. Mesocarbon Microbead Graphite (average particle size 17 μm, specific surface area 1.8 m ² / g), Denki Kagaku Kogyo Co., Ltd. acetylene black (AB) dry at a mass ratio of 95: 5, with blades The mixture was mixed at 10000 rpm for 30 seconds × 2 times with a high-speed small mixer (IK mixer) to prepare a negative electrode material mixture. To this, KF polymer L # 9210 (N-methylpyrrolidone (NMP) solution containing 10% by weight of polyvinylidene fluoride (PVDF)) manufactured by Kureha Chemical Industry Co., Ltd. has a mass ratio of the negative electrode material mixture to PVDF of 95: 5. In addition, the mixture was kneaded with a planetary mixer to obtain a negative electrode kneaded paste.

NMP was further added to the negative electrode kneaded paste to adjust the viscosity, and then applied to a rolled copper foil (thickness 20 μm) manufactured by Nippon Foil Co., Ltd. to a predetermined thickness using a doctor blade. 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 the press pressure is about 1 × 10 ^{2 to} 3 × 10 ² N / mm ² (1 × 10 ^{3 to} 3 × 10 ³ kg / cm ² ) with respect to the electrode. It pressed so that it might become. Then, it dried for 120 hours and 120 degreeC with the vacuum dryer, and it was for evaluation. The thickness was about 100 μm, and the electrode density was about 1.5 g / cm ³ .

(3) Evaluation of electrolyte penetration time In the atmosphere at 25 ° C., the positive electrode (18 mmΦ) prepared in (1) above has a viscosity almost equal to that of various electrolytes in the atmosphere at 25 ° C. and is volatile. 3 μl of low propylene carbonate (hereinafter sometimes abbreviated as “PC”) was dropped as an electrolyte with a microsyringe, and the time required for the PC to penetrate into the electrode was measured. The measurement was performed three times, and the average value was used as the evaluation value.

(4) Production of lithium ion battery test cell A triode cell was produced as follows. The following operation was carried out in 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 positive electrode with aluminum foil prepared in (2) above and the negative electrode with copper foil prepared in (3) above were separated by a separator (polypropylene microporous film (cell guard). 2400) and 25 μm). 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.

(5) the electrolyte of ethylene carbonate (hereinafter sometimes abbreviated as EC.) 8 parts by mass and methyl ethyl carbonate (hereinafter, may be abbreviated to MEC.) With a mixed product of 12 parts by weight, the LiPF ₆ 1 as an electrolyte .2 mol / liter dissolved.

(6) Charge / Discharge Cycle Test A constant current / constant voltage charge / discharge test was conducted at a current density of 0.6 mA / cm ² (equivalent to 0.3 C).
Charging was performed by CC (constant current: constant current) at 0.6 mA / cm ² from the rest potential to 4.2 V. Next, it switched to CV (constant volt | bolt: constant voltage) charge by 4.2V, and stopped when the electric current value fell to 25.4 microamperes.
As the discharge, CC discharge was performed at 0.6 mA / cm ² (corresponding to 0.3 C), and cut off at a voltage of 2.7 V.

電極体積は、電極の寸法より算出した。電極中の正極活物質質量は、正極活物質、炭素材料、バインダー材料の混合比から求めた正極活物質質量％を実測した電極質量に乗じて算出した。
4). Volume ratio (%) of positive electrode active material in positive electrode:
It calculated based on the following formula.

The electrode volume was calculated from the dimensions of the electrode. The mass of the positive electrode active material in the electrode was calculated by multiplying the measured electrode mass by the mass% of the positive electrode active material obtained from the mixing ratio of the positive electrode active material, the carbon material, and the binder material.

Example 1 Electrolyte Permeability Evaluation of Various Electrodes Using the positive electrode active material and carbon fiber shown below, the 12 types of LiCoO ₂ positive electrodes shown in Table 1 and 6 by the above methods 3 (1) to (2) and 6 A seed Li ₂ Mn ₂ O ₄ positive electrode was prepared, and the PC permeation time was measured by the method 3 (3) above. The composition, density and results of the electrodes are shown in Table 1.

<Positive electrode active material>
LiCoO ₂ : manufactured by Nippon Chemical Industry Co., Ltd., average particle size: 20 μm,
Li ₂ Mn ₂ O ₄ : Mitsui Metal Mining Co., Ltd. average particle size: 17 μm.
<Carbon fiber>
CF: vapor-grown graphite fiber,
Average fiber diameter (from SEM image analysis): 150 nm,
Average fiber length (from SEM image analysis): 8 μm,
Average aspect ratio: 53,
Branching degree (calculated from SEM image analysis, the number of branches per 1 μm fiber length; the same applies hereinafter)
: About 0.1 / μm,
X-ray _{C 0: 0.6767nm, Lc: 48.0nm} .
CF-S: Vapor growth graphite fiber,
Average fiber diameter (from SEM image analysis): 120 nm,
Average fiber length (from SEM image analysis): 12 μm,
Average aspect ratio: 100,
Branching degree: about 0.02 piece / μm,
X-ray _{C 0: 0.6767nm, Lc: 48.0nm} .
NF: Vapor growth graphite nanofiber,
Average fiber diameter (from SEM image analysis): 80 nm,
Average fiber length (from SEM image analysis): 6 μm,
Average aspect ratio: 75,
Branching degree: 0.1 / μm,
X-ray _{C 0: 0.6801nm, Lc: 35.0nm} .
NT: Vapor growth graphite nanotube,
Average fiber diameter (from SEM image analysis): 25 nm,
Average fiber length (from SEM image analysis): 5 μm,
Average aspect ratio: 200,
Branching degree: 0.1 / μm,
X-ray _{C 0: 0.6898nm, Lc: 30.0nm} .

As is apparent from Table 1, by adding carbon fiber, the electrolyte solution permeation time of the positive electrode material is significantly shortened as compared with the product without carbon fiber. Specifically, when the degree of reduction of the penetration time based on the non-added carbon fiber in the LiCoO ₂ positive electrode is compared, the penetration time is reduced to 20 to 31%. The penetration time is also shortened when LiMn ₂ O ₄ is used.

Example 2: Charging / discharging cycle characteristics of a lithium ion battery test cell A positive electrode prepared in the same manner as in Example 1 was combined with the negative electrode, and the cycle characteristics were evaluated according to the battery evaluation method. The results are shown in Table 2.

  As is apparent from Table 2, the cycle characteristics of the carbon fiber-free product are significantly reduced by increasing the electrode density. On the other hand, the electrode to which various carbon fibers are added increases the capacity per volume (volume capacity density) by increasing the electrode density, and the cycle characteristics do not deteriorate much.

Claims (20)

A positive electrode for a lithium battery comprising an active material capable of occluding and releasing lithium ions, a carbon-based conductive aid, and a binder, wherein the amount of carbon-based conductive aid added is 0.1 to 2 mass% of the total positive electrode, and Carbon-based conductive aid contains carbon fibers with an average fiber diameter of 1 to 200 nm, and the carbon fibers are graphite-based carbon fibers that have been heat-treated at 2000 ° C. or higher, and can occlude and release lithium ions calculated from true density. A positive electrode for a lithium-based battery, wherein the volume of the active material is 70% or more in the positive electrode volume .   The positive electrode for a lithium-based battery according to claim 1, wherein the total amount of metal impurities in the carbon fiber is 30 ppm or less. The positive electrode for a lithium-based battery according to claim 1 or 2 , wherein the carbon fiber is a graphite-based carbon fiber containing 0.1 to 100,000 ppm of boron. The positive electrode for a lithium battery according to any one of claims 1 to 3 , wherein the carbon fiber has an average aspect ratio of 50 to 15000. The positive electrode for a lithium battery according to any one of claims 1 to 4 , wherein the carbon fiber has an average fiber diameter of 10 to 200 nm. The positive electrode for a lithium battery according to any one of claims 1 to 5 , wherein the carbon fiber has a hollow structure therein. The positive electrode for a lithium battery according to any one of claims 1 to 6 , wherein the carbon fiber includes a branched carbon fiber. The positive electrode for a lithium battery according to any one of claims 1 to 7 , wherein the active material is a metal oxide compound capable of occluding and releasing lithium ions. The positive electrode for a lithium battery according to claim 8 , wherein an average primary particle diameter of the metal oxide compound which is an active material capable of occluding and releasing lithium ions is 0.5 to 30 µm. Metal oxide-based compounds cobalt oxide in the lithium ion is capable of absorbing and releasing active material contained more than 60 wt%, the electrode density of claim 8 or 9 is 3.7 g / cm ³ or more Positive electrode for lithium batteries. Metal oxide-based compounds, manganese-based oxide in a lithium ion is capable of absorbing and releasing active material contained more than 60 wt%, the electrode density of claim 8 or 9 is 3.2 g / cm ³ or more Positive electrode for lithium batteries. A metal oxide compound which is an active material capable of occluding and releasing lithium ions contains a mixture of cobalt oxide and manganese oxide in an amount of 80% by mass or more, and the electrode density is 3.5 g / cm ³ or more. Item 10. The positive electrode for a lithium battery according to Item 8 or 9 . Nickel oxide contains 60 mass% or more of lithium ion in the metal oxide-based compound is capable of absorbing and releasing active material, the electrode density of claim 8 or 9 is 3.5 g / cm ³ or more Positive electrode for lithium batteries. Metal oxide-based compounds cobalt-nickel-based composite oxide in a lithium ion is capable of absorbing and releasing active material contained 60% by mass or more, according to claim 8 or 9 electrode density of 3.6 g / cm ³ or more A positive electrode for a lithium-based battery as described in 1. Metal oxide-based compounds cobalt-manganese-based composite oxide in a lithium ion is capable of absorbing and releasing active material contained 60% by mass or more, according to claim 8 or 9 electrode density of 3.6 g / cm ³ or more A positive electrode for a lithium-based battery as described in 1. Metal oxide-based compounds of cobalt-nickel-manganese-based composite oxide in a lithium ion is capable of absorbing and releasing active material contained 60% by mass or more, according to claim 8 electrode density of 3.6 g / cm ³ or more Or the positive electrode for a lithium battery according to 9 . Positive electrode for lithium batteries according to the lithium ion in the metal oxide-based compound according to claim 8 or 9 iron olivine acid compound is Ru contains more than 60% by mass is capable of absorbing and releasing active material. Lithium battery comprising as a component a positive electrode for lithium battery according to any one of claims 1 to 17. Lithium secondary battery comprising as a component a positive electrode for lithium battery according to any one of claims 1 to 17. A non-aqueous electrolyte and / or a non-aqueous polymer electrolyte is used, and ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene are used as the non-aqueous solvent used in the non-aqueous electrolyte and / or non-aqueous polymer electrolyte. The lithium secondary battery according to claim 19 , comprising at least one selected from the group consisting of carbonate and vinylene carbonate.