Process for producing composite material of metal oxide with conductive carbon

Abstract

Provided is a method whereby metal oxide nanoparticles having evenness of size are efficiently and highly dispersedly adhered to conductive carbon powder. This method comprises: a preparation step in which a reaction solution containing water, a compound with a transition metal selected from the group consisting of Mn, Fe, Co, and Ni, and conductive carbon powder and having a pH in the range of 9 to 11 is introduced into a rotatable reactor; a supporting step in which the reactor is rotated to apply shear stress and centrifugal force to the reaction solution, thereby yielding a core of a hydroxide of the transition metal and dispersing the thus-yielded core of a hydroxide of the transition metal and the conductive carbon powder and simultaneously supporting the hydroxide of the transition metal by the conductive carbon powder; and a heat treatment step in which the conductive carbon powder loaded with the hydroxide of the transition metal is heated to thereby convert the hydroxide supported by the conductive carbon powder into an oxide nanoparticle.

Claims

1. A method for producing a composite material of a metal oxide and conductive carbon, comprising: a preparation step to introduce into a rotatable reactor a reaction solution obtained by preparing a base solution comprising water, at least one compound with a transition metal selected from a group consisting of Mn, Fe, Co and Ni which is dissolved in the water, and conductive carbon powder and then adding to the base solution a pH adjuster for adjusting a pH of the reaction solution in a range of 9 to 11; a supporting step to add shear stress and centrifugal force to the reaction solution by rotating the reactor so as to form a core of a hydroxide of the transition metal, disperse the core of a hydroxide of the transition metal obtained and the conductive carbon powder, and simultaneously support a particle grown from the core of the hydroxide of the transition metal by the conductive carbon powder; and a heat treatment step to heat a mixture obtained by mixing the conductive carbon powder supporting the particle grown from the core of the hydroxide of the transition metal with at least one compound with a typical metal selected from a group consisting of elements in groups 1 and 2 of the periodic table so as to react the particle grown from the core of the hydroxide of the transition metal supported by the conductive carbon powder and the compound of a typical metal and transform to a nanoparticle of a compound oxide, wherein the compound of a typical metal is lithium hydroxide, and the nanoparticle of a compound oxide is selected from a group consisting of a nanoparticle of LiMO.sub.2 having a layered rock salt structure, a layered Li.sub.2MnO.sub.3-LiMO.sub.2 solid solution, or a spinel-type LiM.sub.2O.sub.4, wherein M in the formulas is Mn, Fe, Co, Ni or a combination thereof.

2. The method for producing a composite material of a metal oxide and conductive carbon according to claim 1, wherein the reactor comprises concentric cylinders of an external cylinder and an internal cylinder, the internal cylinder having through-holes on a side face, the outer cylinder having a shuttering board on an open end thereof, and in the supporting step, the reaction solution in the internal cylinder is moved to the external cylinder via the through-holes and the core of a hydroxide of the transition metal is formed between an outer wall surface of the internal cylinder and an inner wall surface of the external cylinder by the centrifugal force produced by turning of the internal cylinder.

3. A method for producing a composite material of a metal oxide and conductive carbon, comprising: a preparation step to introduce into a rotatable reactor a reaction solution obtained by preparing a base solution comprising water, at least one compound with a transition metal selected from a group consisting of Mn, Fe, Co and Ni which is dissolved in the water, and conductive carbon powder and then adding to the base solution a pH adjuster for adjusting a pH of the reaction solution in a range of 9 to 11; a supporting step to add shear stress and centrifugal force to the reaction solution by rotating the reactor so as to form a core of a hydroxide of the transition metal, disperse the core of a hydroxide of the transition metal obtained and the conductive carbon powder, and simultaneously support a particle grown from the core of the hydroxide of the transition metal by the conductive carbon powder; and a heat treatment step to heat a mixture obtained by mixing the conductive carbon powder supporting the particle grown from the core of the hydroxide of the transition metal with at least one compound with a typical metal selected from a group consisting of elements in groups 1 and 2 of the periodic table so as to react the particle grown from the core of the hydroxide of the transition metal supported by the conductive carbon powder and the compound of a typical metal and transform to a nanoparticle of a compound oxide, wherein the compound of a typical metal is lithium hydroxide, and the nanoparticle of a compound oxide is selected from a group consisting of a nanoparticle of LiMO.sub.2 having a layered rock salt structure or a layered Li.sub.2MnO.sub.3-LiMO.sub.2 solid solution, wherein M in the formulas is Mn, Fe, Co, Ni or a combination thereof, and hydrothermal treatment is given after heat-treatment in an atmosphere containing oxygen at a temperature of 200 to 300 C. in the heat treatment step.

4. The method for producing a composite material of a metal oxide and conductive carbon according to claim 3, wherein the reactor comprises concentric cylinders of an external cylinder and an internal cylinder, the internal cylinder having through-holes on a side face, the outer cylinder having a shuttering board on an open end thereof, and in the supporting step, the reaction solution in the internal cylinder is moved to the external cylinder via the through-holes and the core of a hydroxide of the transition metal is formed between an outer wall surface of the internal cylinder and an inner wall surface of the external cylinder by the centrifugal force produced by turning of the internal cylinder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows SEM images of the powder in the middle of manufacturing a composite material of LiMn.sub.2O.sub.4 and conductive carbon; (a) is an image of a comparative example and (b) is an image of a working example.

(2) FIG. 2 shows X-ray powder diffraction diagrams of composite materials of LiMn.sub.2O.sub.4 and conductive carbon in working examples of the present invention.

(3) FIG. 3 shows the result of TG analysis of composite materials of LiMn.sub.2O.sub.4 and conductive carbon in working examples of the present invention.

(4) FIG. 4 shows SEM images of a composite material of LiMn.sub.2O.sub.4 and conductive carbon; (a) is an image of a comparative example and (b) is an image of a working example.

(5) FIG. 5 shows a TEM image of a composite material of LiMn.sub.2O.sub.4 and conductive carbon in a working example of the present invention.

(6) FIG. 6 shows the result of the evaluation of the rate characteristics of a half-cell where a composite material of LiMn.sub.2O.sub.4 and conductive carbon is used as a positive electrode active material.

(7) FIG. 7 shows the result of the evaluation of the rate characteristics of a half-cell where a composite material of LiMn.sub.2O.sub.4 and conductive carbon is used as a positive electrode active material.

(8) FIG. 8 shows TEM images of 0.7Li.sub.2MnO.sub.3.0.3LiNi.sub.0.5Mn.sub.0.5O.sub.2 and conductive carbon; (a) is an image of a comparative example and (b) is an image of a working example.

(9) FIG. 9 shows the result of the evaluation of the rate characteristics of a half-cell where a composite material of 0.7Li.sub.2MnO.sub.3.0.3 LiNi.sub.0.5Mn.sub.0.5O.sub.2 and conductive carbon is used as a positive electrode active material.

(10) FIG. 10 shows TG-DTA analysis result of composite materials of Mn.sub.3O.sub.4 and conductive carbon; (a) is a result of a comparative example and (b) is a result of a working example.

(11) FIG. 11 shows an X-ray powder diffraction diagram of a composite material of Mn.sub.3O.sub.4 and conductive carbon in a working example of the present invention.

(12) FIG. 12 shows charging/discharging curves of a half-cell in which a composite material of Mn.sub.3O.sub.4 and conductive carbon in a working example of the present invention is used as a positive electrode active material.

DETAILED DESCRIPTION OF THE INVENTION

(13) A first manufacturing method and a second manufacturing method of the present invention are similar in that they both use the same preparation step and the supporting step; they only differ in terms of the heat treatment step. In the following, the preparation steps and the supporting steps used in the first manufacturing method and the second manufacturing method are explained once and the two different heat treatment steps are explained separately.

(14) (1) Preparation Step

(15) In the preparation step, a reaction solution comprising water, at least one compound with a transition metal selected from a group consisting of Mn, Fe, Co and Ni, and conductive carbon powder and having a pH in a range of 9 to 11 is introduced into a rotatable reactor. In the present invention, water is used as a solvent. The solvent may contain organic solvent to the extent that it does not affect the present invention, but it is preferable that the solvent is water only.

(16) As for the carbon powder, any carbon powder can be used without restriction as long as it has conductivity. Examples are carbon black such as Ketjen Black, acetylene black and channel black, fullerene, carbon nanotube, carbon nanofiber, amorphous carbon, carbon fiber, natural graphite, artificial graphite, graphitized Ketjen Black, activated carbon, and mesoporous carbon. Also, vapor grown carbon fiber can be used. The carbon powder can be used alone, or used as a mixture of two or more kinds. It is preferable that at least a part of the carbon powder is carbon nanotube.

(17) As for the at least one compound with a transition metal selected from a group consisting of Mn, Fe, Co and Ni, any water-soluble compound can be used without restriction. For example, an inorganic metallic salt of the transition metal such as halide, nitrate and sulfate, an organic metallic salt of the transition metal such as formate and acetate, and a mixture thereof can be used. These compounds can be used alone, or used in a mixture of two or more kinds. A compound that contains different transition metal can be mixed at given quantities and used.

(18) It is preferable that adjustment of the pH of the reaction solution is made by an aqueous solution in which a hydroxide of an alkali metal, that is, Li, Na, K, Rb, Cs or Fr, is dissolved. The alkali metal hydroxide can be used alone, or a mixture of two or more kinds can also be used. Besides, a solution of an alkali metal oxide, ammonia or an amine can be used. A sole compound can be used for the adjustment of the pH, or two or more kinds of compounds can be mixed and used.

(19) The reaction solution for the ultracentrifugal treatment is easily prepared by mixing a solution in which the conductive carbon powder and the water-soluble salt of the transition metal are added to water and the water-soluble salt is dissolved, with a solution in which a hydroxide of alkali metal is dissolved in water. Then, the pH of the reaction solution is adjusted in a range of 9 to 11. If the pH is less than 9, the efficiency to form a core of the hydroxide and the efficiency of supporting the hydroxide produced by the conductive carbon powder in the following supporting step are low, and if the pH is more than 11, the speed of insolubilization of the hydroxide in the supporting step is too rapid and it becomes difficult to gain fine hydroxide.

(20) As the rotatable reactor, any reactor that can add ultracentrifugal force to the reaction solution can be used without restriction, and the reactor described in FIG. 1 of Patent Document 3, which comprises concentric cylinders of an external cylinder and an internal cylinder, where through-holes are placed on the side face of the rotatable internal cylinder, and where a shuttering board is placed on the open end of the external cylinder, is suitably used. The use of this suitable reactor is explained in the following.

(21) The reaction solution for the ultracentrifugal treatment is introduced into the internal cylinder of the reactor. Reaction solution that has been prepared beforehand can be introduced into the internal cylinder, or reaction solution can be introduced by preparing it in the internal cylinder. It is preferable to put water, the conductive carbon powder and the water-soluble salt of the transition metal in the internal cylinder, turn the internal cylinder to dissolve the water-soluble salt of the transition metal in water and at the same time disperse the conductive carbon powder in the solution, after which the turning of the internal cylinder is suspended, and then, a solution in which the alkali metal hydroxide is dissolved in water is put into the internal cylinder to adjust the pH, and then the internal cylinder is turned again. This is because dispersion of the conductive carbon powder becomes excellent by the first turning so that the dispersibility of the nanoparticle of the metal oxide supported by the conductive carbon powder becomes excellent.

(22) (2) Supporting Step

(23) In the supporting step, shear stress and centrifugal force are added to the reaction solution by rotating the reactor so as to form a core of the hydroxide of the transition metal, disperse the core of a hydroxide of the transition metal obtained and the conductive carbon powder, and simultaneously support the hydroxide of the transition metal by the conductive carbon powder.

(24) It is considered that the formation of the core of the hydroxide is realized by the mechanical energy of shear stress and centrifugal force that is applied to the reaction solution. The shear stress and centrifugal force are produced by centrifugal force added to the reaction solution by the turning of the reactor. The centrifugal force added to the reaction solution in the reactor is the centrifugal force in a category generally referred to as ultracentrifugal force, which is generally 1500 kgms.sup.2 or more, preferably 70000 kgms.sup.2 or more, and especially preferably 270000 kgms.sup.2 or more.

(25) An embodiment to use the suitable reactor with an external cylinder and an internal cylinder can be explained as follows. When the internal cylinder of the reactor in which the reaction solution is introduced is turned, the reaction solution in the internal cylinder is moved to the external cylinder via the through-holes, the reaction solution slides up between the outer wall surface of the internal cylinder and the inner wall surface of the external cylinder to the upper part of the inner wall surface of the external cylinder, by the centrifugal force produced by the turning of the internal cylinder. As a result, shear stress and centrifugal force are added to the reaction solution, and by the mechanical energy, the core of the hydroxide of the transition metal is formed between the outer wall surface of the internal cylinder and the inner wall surface of the external cylinder. Then this core grows while being dispersed in the reactor, and becomes supported on the conductive carbon powder.

(26) In the reaction, it is preferable that a gap between the outer wall surface of the internal cylinder and the inner wall surface of the external cylinder is narrower because greater mechanical energy can be added to the reaction solution. The gap between the outer wall surface of the internal cylinder and the inner wall surface of the external cylinder is preferably 5 mm or less, more preferably 2.5 mm or less, especially preferably 1.0 mm or less. The gap between the outer wall surface of the internal cylinder and the inner wall surface of the external cylinder can be set up by the width of the shuttering board of the reactor and the quantity of the reaction solution that is introduced into the reactor.

(27) There is no strict restriction on the turning time of the internal cylinder; the time can change according to the quantity of reaction solution or turning speed (the value of centrifugal force) of the internal cylinder, but is generally within the range of 0.5 to 10 minutes. By applying the ultracentrifugal treatment, most of the transition metal contained in the reaction solution is supported as a hydroxide by the conductive carbon powder in a short period of time.

(28) After the reaction is finished, the turning of the internal cylinder is stopped and the conductive carbon powder that supports an even-size fine particle of the hydroxide of the transition metal is retrieved. In the recovered product, the conductive carbon powder supporting a fine particle of the hydroxide generally forms an aggregation that has a small diameter of 1000 nm or less and a comparatively even size.

(29) (3) Heat Treatment Step

(30) a. Heat Treatment Step in the First Manufacturing Method

(31) In the first manufacturing method, the retrieved conductive carbon powder supporting the fine particle of the hydroxide of the transition metal is washed as needed and then heat-treated so that the hydroxide is transformed into an oxide nanoparticle on the conductive carbon powder. In the first manufacturing method, because the composite material where the hydroxide is supported on the conductive carbon powder as an even-size fine particle is used, the oxidation reaction of the hydroxide of the transition metal progresses rapidly and evenly, and thus the nanoparticle of the oxide obtained is also fine and has an even size.

(32) There is no strict restriction on the atmosphere of the heat treatment. Heat treatment can be done in a vacuum, in an inert atmosphere such as nitrogen and argon, or in an atmosphere containing oxygen such as oxygen and air. Also, there is no restriction on the temperature and duration of the heat treatment; this can change according to the composition of the target oxide and the quantity of preparation, but is generally within the range of 10 minutes to 10 hours at a temperature between 200 to 300 C. in the case of heat treatment in an atmosphere containing oxygen, within the range of 10 minutes to 10 hours at a temperature between 250 to 600 C. in the case of heat treatment in an inert atmosphere, and within the range of 10 minutes to 10 hours at a temperature between room temperature to 200 C. in the case of heat treatment in a vacuum atmosphere.

(33) It is preferable to perform the heat treatment at a temperature of 200 to 300 C. in an atmosphere containing oxygen. This is because the conductive carbon powder is not destroyed by burning even in an atmosphere containing oxygen if the temperature is 300 C. or less and a metal oxide can be obtained with good crystalline structure. If the heat treatment is given in an atmosphere that does not contain oxygen, the oxide may be reduced and the target oxide may not be obtained.

(34) The composite material of a metal oxide and conductive carbon obtained by the first manufacturing method of the present invention is suitable as an electrode material of a battery and an electrochemical capacitor; especially, a composite material of Fe.sub.2O.sub.3, MnO, MnO.sub.2, Mn.sub.2O.sub.3, Mn.sub.3O.sub.4, CoO, Co.sub.3O.sub.4, NiO, or Ni.sub.2O.sub.3 and conductive carbon is suitable as a negative electrode active material in a lithium ion secondary battery.

(35) b. Heat Treatment Step in the Second Manufacturing Method

(36) In the second manufacturing method, the retrieved conductive carbon powder supporting a fine particle of the hydroxide of the transition metal is washed as needed, mixed with at least one compound with a typical metal selected from a group consisting of elements in groups 1 and 2 in the periodic table, and heat-treated so that the hydroxide of the transition metal supported by the conductive carbon powder and the compound of the typical metal are made to react and transformed into a nanoparticle of a compound oxide. In this manufacturing method, because the conductive carbon powder supporting the hydroxide as an even-size fine particle is used, the reaction between the hydroxide of the transition metal and the compound of the typical metal progresses in a rapid and even manner, and the nanoparticle of the compound oxide obtained is also fine and has an even size.

(37) As the compound of the typical metal belonging to group 1 in the periodic table, that is, Li, Na, K, Rb, Cs or Fr, or the compound of the typical metal belonging to group 2 in the periodic table, that is, Be, Mg, Ca, Sr, Ba and Ra, a compound containing the typical metal can be used without any restriction; for example, an inorganic metallic salt of the metal such as hydroxide, carbonate, halide, nitrate and sulfate, an organic metallic salt of the metal such as formate, acetate, oxalate and lactate, or a mixture of these can be used. These compounds can be used alone or used as a mixture of two or more kinds. A compound that contains different typical metal can be mixed at given quantities and used. It is preferable to use hydroxide because impurities such as a sulfur compound or a nitrogen compound do not remain behind and a compound oxide can be obtained rapidly.

(38) A kneaded material is obtained by combining the conductive carbon powder supporting a fine particle of the hydroxide of the transition metal obtained by the supporting step and the compound of the typical metal with an adequate quantity of dispersion medium as needed, and kneading while vaporizing the dispersion medium as needed. As the dispersion medium for kneading, a medium that does not adversely affect the composite material can be used without any restriction; for example, water, methanol, ethanol or isopropyl alcohol can be suitably used, and water can be used especially suitably.

(39) There is no strict restriction on the atmosphere of the heat treatment. Heat treatment can be done in a vacuum, in an inert atmosphere such as nitrogen and argon, or in an atmosphere containing oxygen such as oxygen and air. Also, there is no restriction on the temperature and duration of the heat treatment; this can change according to the composition of the target oxide and the quantity of preparation, but is generally within the range of 10 minutes to 10 hours at a temperature between 200 to 300 C. in the case of heat treatment in an atmosphere containing oxygen, within the range of 10 minutes to 10 hours at a temperature between approximately 250 to 600 C. in the case of heat treatment in an inert atmosphere, and within the range of 10 minutes to 10 hours at a temperature between room temperature to approximately 200 C. in the case of heat treatment in a vacuum atmosphere.

(40) It is preferable to perform the heat treatment at a temperature of 200 to 300 C. in an atmosphere containing oxygen. This is because the conductive carbon powder is not destroyed by burning even in an atmosphere containing oxygen if the temperature is 300 C. or less and a compound oxide can be obtained with good crystalline structure. If the heat treatment is given in an atmosphere that does not contain oxygen, the compound oxide may be reduced and the target compound oxide may not be obtained.

(41) The composite material of a compound oxide and conductive carbon that is obtained by the second manufacturing method of the present invention is suitable as an electrode material of a battery and an electrochemical capacitor. Especially, the composite material having the conductive carbon and the nanoparticle of LiMO.sub.2 that has a layered rock salt structure, a layered Li.sub.2MnO.sub.3-LiMO.sub.2 solid solution, or a spinel-type LiM.sub.2O.sub.4 (M in the formula is Mn, Fe, Co, Ni or a combination thereof), which is obtained by using lithium hydroxide as the hydroxide of the typical metal in the heat-treatment step, is suitable as a positive electrode active material of a lithium ion secondary battery.

(42) Examples of LiMO.sub.2 that has a layered rock salt structure, a layered Li.sub.2MnO.sub.3-LiMO.sub.2 solid solution, or a spinel-type LiM.sub.2O.sub.4 are, LiCoO.sub.2, LiNiO.sub.2, LiNi.sub.4/5Co.sub.1/5O.sub.2, LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, LiNi.sub.1/2Mn.sub.1/2O.sub.2, LiFeO.sub.2, LiMnO.sub.2, Li.sub.2MnO.sub.3LiCoO.sub.2, Li.sub.2MnO.sub.3LiNiO.sub.2, Li.sub.2MnO.sub.3LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, Li.sub.2MnO.sub.3LiNi.sub.1/2Mn.sub.1/2O.sub.2, Li.sub.2MnO.sub.3LiNi.sub.1/2Mn.sub.1/2O.sub.2LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, LiMn.sub.2O.sub.4 and LiMn.sub.3/2Ni.sub.1/2O.sub.4. By the second manufacturing method of the present invention, a composite material that contains a nanoparticle of the compound oxide and conductive carbon powder with good dispersion can be obtained. Especially, a composite material that contains a nanoparticle with a primary particle of 10 to 40 nm is particularly suitable as a positive electrode active material of a lithium ion secondary battery, and produces a lithium ion secondary battery with excellent rate characteristics. Particularly, a composite material using carbon nanotube as at least a part of the conductive carbon powder is a positive electrode active material that has high conductivity and that leads to a lithium ion secondary battery having especially excellent rate characteristics.

(43) In the heat treatment step, if a LiMO.sub.2 having a layered rock salt structure or a layered Li.sub.2MnO.sub.3LiMO.sub.2 solid solution is intended to be obtained, a spinel may be simultaneously formed in some cases. In such cases, it is preferable to give hydrothermal treatment after the aforementioned heat treatment, preferably heat treatment at 200 to 300 C. in an atmosphere containing oxygen. Then, the spinel is denatured into a layered structure by the hydrothermal treatment and a layered structure with high purity can be obtained. The hydrothermal treatment can be carried out under high temperature hot water at a temperature of 100 C. or more and at an atmospheric pressure of 1 bar or more, after the powder after the heat treatment and water, preferably a lithium hydroxide aqueous solution, are introduced into an autoclave.

EXAMPLES

(44) The examples of the present invention are shown as follows, but the present invention is not limited to the following examples.

(45) (1) Composite Material of LiMn.sub.2O.sub.4 (Spinel) and Conductive Carbon

(46) a. Manufacture of a Composite Material

Example 1

(47) A reactor shown in FIG. 1 of Patent Document 2 (JP 2007-160151 A), which comprises concentric cylinders of an external cylinder and an internal cylinder, has through-holes on the side face of the internal cylinder, and has a shuttering board on the open end of the external cylinder, is used. A solution in which 2.45 g of Mn(CH.sub.3COO.sub.2).sub.2.4H.sub.2O and 0.225 g of Ketjen Black (diameter: approximately 40 nm) are added to 75 mL of water was introduced into the internal cylinder, and the internal cylinder was turned for 300 seconds to add centrifugal force of 70000 kgms.sup.2 to the reaction solution so that Mn(CH.sub.3COO).sub.2.4H.sub.2O was dissolved and Ketjen Black was dispersed. Then the turning of the internal cylinder was suspended and a solution in which 0.6 g of LiOH.H.sub.2O was dissolved into water was added into the internal cylinder. The pH of the solution was 10. Then, the internal cylinder was turned again for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. In the meantime, a core of Mn hydroxide was formed between the inner wall of the external cylinder and the outer wall of the internal cylinder; this core grew and was supported on the surface of Ketjen Black. After the turning of the internal cylinder was stopped, Ketjen Black was filtered and retrieved, and dried in air at 100 C. for 12 hours. When the filtrate was inspected by ICP spectrometry, it was found that 95% or more of Mn contained in the raw material Mn(CH.sub.3COO).sub.2.4H.sub.2O was supported. Then, the powder after drying and an aqueous solution that contained LiOH.H.sub.2O in an amount that made the ratio of Mn:Li=2:1 were mixed and kneaded, and after drying, the kneaded material was given heat treatment for 1 hour in air at the temperature of 280 C. so that a composite material was obtained.

Example 2

(48) The procedure of Example 1 was repeated except that heat treatment was given in air at 300 C. for 1 hour instead of heat treatment in air at 280 C. for 1 hour.

Example 3

(49) The procedure of Example 1 was repeated except that heat treatment was given in air at 350 C. for 1 hour instead of heat treatment in air at 280 C. for 1 hour.

Example 4

(50) The procedure of Example 2 was repeated except that 0.225 g of carbon mixture in which Ketjen Black (diameter: approximately 40 nm) and carbon nanotube (diameter: approximately 20 nm, length: several hundred nm) were mixed at the mass ratio of 3:1 was used instead of 0.225 g of Ketjen Black.

Example 5

(51) The procedure of Example 2 was repeated except that 0.225 g of carbon mixture in which Ketjen Black (diameter: approximately 40 nm) and carbon nanotube (diameter: approximately 20 nm, length: several hundred nm) were mixed at the mass ratio of 1:1 was used instead of 0.225 g of Ketjen Black.

Example 6

(52) Acetylene black as a conductive agent was mixed with the composite material of Example 2 in a quantity of 5% by mass of the composite material.

Comparative Example 1

(53) A solution in which 2.45 g of Mn(CH.sub.3COO).sub.2.4H.sub.2O, 0.33 g of CH.sub.3COOLi (Mn:Li=2:1) and 0.225 g of carbon mixture in which Ketjen Black (diameter: approximately 40 nm) and carbon nanotube (diameter: approximately 20 nm, length: several hundred nm) were mixed at the mass ratio of 1:1 was added to 75 mL of water was introduced into the internal cylinder of the reactor used in Example 1, and the internal cylinder was turned for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. After the turning of the internal cylinder was stopped, the liquid part was collected and inspected by ICP spectrometry, and it was found that only approximately 30% of Mn that was contained in Mn(CH.sub.3COO).sub.2.4H.sub.2O as a raw material was supported by the carbon mixture. Therefore, all the contents in the reactor were retrieved, evaporated and dried in air at 100 C. Then, they were heat-treated in air at 300 C. for 1 hour, and a composite material was obtained.

(54) FIG. 1 shows SEM images of the material after the supporting step and before the heat treatment step; (a) is a SEM image of the material in Comparative Example 1 and (b) is a SEM image of the material in Example 5. From the SEM image (b), it is found that, in Example 5, the carbon mixture supporting a fine particle of hydroxide forms an aggregation of a comparatively even size that has a diameter of 1000 nm or less. On the other hand, from the SEM image (a), it is found that, in Comparative Example 1, most of the compounds are amorphous, though partial aggregation is found, and this amorphous compound covers the carbon mixture.

(55) FIG. 2 shows X-ray powder diffraction diagrams of Examples 1 to 3. At every temperature, crystallization of LiMn.sub.2O.sub.4 was found. Especially LiMn.sub.2O.sub.4 in the composite materials of Examples 2 and 3, which was heat-treated at 300 C. or more, showed high crystallization. FIG. 3 shows the result of TG analysis of the composite materials of Examples 1 to 3 in an air atmosphere where the temperature raising rate was 1 C./minute and the weight reduction amount was evaluated as carbon. In the composite material of Example 3, which was given heat treatment at 350 C., weight loss was hardly observed, and it was concluded that Ketjen Black was burnt in the course of the heat treatment. Therefore, it was found that heat treatment in air at 300 C. was particularly preferable.

(56) FIG. 4 shows SEM images of the composite materials after the heat treatment step; (a) is a SEM image of the composite material of Comparative Example 1 and (b) is a SEM image of the composite material of Example 5. From the SEM image (b), it is found that an even-size particle is formed in Example 5. FIG. 5 is a TEM image of the composite material of Example 5, and it is found that an initial particle of LiMn.sub.2O.sub.4 with a diameter of 10 to 40 nm is formed with good dispersibility. On the other hand, from the SEM image (a) of FIG. 4, it is found that the composite material of Comparative Example 1 contains grains in various sizes, including a large aggregation, and it is found that the dispersibility of LiMn.sub.2O.sub.4 is insufficient. This difference is considered to reflect the difference in the form of the compound on the conductive carbon powder in the material after the supporting step and before the heat treatment step.

(57) b. Evaluation as a Half-Cell

(58) Polyvinylidene fluoride in an amount of 10% by mass of the total was added to the composite material of each of Examples 2, 4 to 6 and Comparative Example 1 and the mixture obtained was formed to produce a positive electrode. A half-cell including the positive electrode, 1M LiPF.sub.6 ethylene carbonate/diethyl carbonate (1:1) solution as an electrolyte, and lithium as a counter electrode was produced. For the half-cell obtained, the charge/discharge characteristics were evaluated under a wide range of conditions of electric current density. Note that while this evaluation is an evaluation of a half-cell, a similar effect can also be expected in a whole-cell using a negative electrode.

(59) FIG. 6 shows the relationship between the rate and discharge capacity of the half-cells using the composite materials of Examples 2 and 6 and Comparative Example 1, while FIG. 7 shows the relationship between the rate and discharge capacity of the half-cells using the composite materials of Examples 2, 4 and 5, and Comparative Example 1.

(60) As can be seen from FIG. 6, by using the composite material of Example 2, compared with using the composite material of Comparative Example 1 that had inadequate dispersibility of LiMn.sub.2O.sub.4, the discharge capacity and rate characteristics of the half-cell was improved. Also, by mixing the conductive agent with the composite material of Example 2 (Example 6), the discharge capacity of the half-cell was improved, and a half-cell with excellent rate characteristics, which showed a gradual decrease in capacity as the rate increased, was obtained. Also, as can be seen from FIG. 7, by replacing a part of Ketjen Black of the composite material in Example 2 with carbon nanotube (Examples 4 and 5), the discharge capacity of the half-cell was improved significantly without introducing a conductive agent in the composite material. This is considered to result from the high conductivity of carbon nanotube. On the other hand, in the half-cell that employed the composite material of Comparative Example 1 with inadequate dispersibility of LiMn.sub.2O.sub.4, the capacity was remarkably low, even though carbon nanotube was contained in the composite material, and the capacity rapidly declined as the rate increased. Example 5 and Comparative Example 1 both employ a carbon mixture of Ketjen Black and carbon nanotube at the ratio of 1:1 in manufacturing composite materials, and the difference between the discharge capacity and rate characteristics of the half-cell using the composite material in Example 5 and the discharge capacity and rate characteristics of the half-cell using the composite material in Comparative Example 1 is considered to reflect the difference in the dispersibility of LiMn.sub.2O.sub.4 in the composite materials shown in FIGS. 4 and 5.

(61) (2) Composite Material of 0.7Li.sub.2MnO.sub.3.0.3LiNi.sub.0.5Mn.sub.0.5O.sub.2 and Conductive Carbon

(62) a. Manufacture of a Composite Material

Example 7

(63) A solution in which 1.54 g of Mn(CH.sub.3COO).sub.2.4H.sub.2O, 0.274 g of Ni(CH.sub.3COO).sub.2, 0.21 g of the carbon mixture of Ketjen Black (diameter: approximately 40 nm) and carbon nanotube (diameter: approximately 20 nm, length: several hundred nm) in the mass ratio of 1:1 were added to 75 mL of water was introduced into the internal cylinder of the reactor used in Example 1, and the internal cylinder was turned for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution so that Mn(CH.sub.3COO).sub.2.4H.sub.2O and Ni(CH.sub.3COO).sub.2 were dissolved and the carbon mixture was dispersed. The turning of the internal cylinder was suspended, and a solution in which 0.6 g of LiOH. H.sub.2O was dissolved into water was added into the internal cylinder. The pH of the solution was 10. Then, the internal cylinder was turned again for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. In the meantime, cores of Mn hydroxide and Ni hydroxide were formed between the inner wall of the external cylinder and the outer wall of the internal cylinder; these cores grew and were supported on the surface of the carbon mixture. After the turning of the internal cylinder was stopped, the carbon mixture was filtered and retrieved, and dried in air at 100 C. for 12 hours. When the filtrate was inspected by ICP spectrometry, it was found that 95% or more of Mn and Ni contained in the raw materials Mn(CH.sub.3COO).sub.2.4H.sub.2O and Ni(CH.sub.3COO).sub.2 was supported. Then, the powder after drying and an aqueous solution of LiOH.H.sub.2O in an amount in the ratio of Mn:Li=1:2 were mixed and kneaded, and after drying, the kneaded material was given heat treatment in air at the temperature of 250 C. for 1 hour. Further, a composite material was obtained by introducing the powder after heat treatment and 2 mol/L of LiOH aqueous solution into the autoclave and giving hydrothermal treatment in saturated vapor at 200 C. for 12 hours.

Comparative Example 2

(64) A solution in which 1.54 g of Mn(CH.sub.3COO).sub.2.4H.sub.2O, 0.274 g of Ni(CH.sub.3COO), 0.78 g of CH.sub.3COOLi (Mn:Li=1:2) and 0.21 g of carbon mixture in which Ketjen Black (diameter: approximately 40 nm) and carbon nanotube (diameter: approximately 20 nm, length: several hundred nm) were mixed at the mass ratio of 1:1 was added to 75 mL of water was introduced into the internal cylinder of the reactor used in Example 1, and the internal cylinder was turned for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. After the turning of the internal cylinder was stopped, the liquid part was collected and inspected by ICP spectrometry, and it was found that only approximately 30% of Mn and Ni that were contained in the raw materials Mn(CH.sub.3COO).4H.sub.2O and Ni(CH.sub.3COO).sub.2 was supported by the carbon mixture. Therefore, all the contents in the reactor were retrieved, evaporated and dried in air at 100 C., and heat-treated at 250 C. for 1 hour and a composite material was obtained.

(65) FIG. 8 is TEM images of the composite materials of Example 7 and Comparative Example 2. FIG. 8 shows that the composite material of Example 7 contains an even crystal with a diameter of approximately 20 nm. On the other hand, the composite material of Comparative Example 2 contains a crystal with a diameter of 5 nm or less and a length of approximately 100 nm, and the crystal size is not even. This is considered to reflect the fact that, in the supporting step, a fine particle of hydroxide is supported by the carbon mixture with good dispersibility in Example 7, while in Comparative Example 2, only a material in which an aggregation of uneven size and an amorphous compound cover the carbon mixture is obtained. That is, in Example 7, even reaction proceeds and an even-size nanoparticle of a compound oxide is formed in high disparsibility in the heat-treatment and hydrothermal treatment, while in Comparative Example 2, uneven reaction proceeds and uneven-size compound oxide is formed in the heat treatment step.

(66) b. Evaluation as a Half-Cell

(67) Polyvinylidene fluoride in an amount of 10% by mass of the total was added to the composite material of each of Example 7 and Comparative Example 2 and the mixture obtained was formed to produce a positive electrode. A half-cell including the positive electrode, 1M LiPF.sub.6 ethylene carbonate/diethyl carbonate (1:1) solution as an electrolyte, and lithium as a counter electrode was produced. For the half-cell obtained, the charge/discharge characteristics were evaluated under a wide range of conditions of electric current density. Note that while this evaluation is an evaluation of a half-cell, a similar effect can also be expected in a whole-cell using a negative electrode.

(68) FIG. 9 shows the relationship of the rate and the discharge capacity of half-cells using the composite materials of Example 7 and Comparative Example 2. The half-cell using the composite material of Comparative Example 2 showed a remarkably small capacity compared with the half-cell using the composite material of Example 7; and the capacity significantly decreased as the rate increased, and little capacity was shown at a rate over 30 C. On the other hand, the half-cell using the composite material of Example 7 showed remarkably excellent rate characteristics and had a capacity over 50 mAhg.sup.1 even at the rate of 100 C.

(69) (3) Composite Material of Mn.sub.3O.sub.4 and Conductive Carbon

Example 8

(70) A solution in which 2.41 g of Mn(CH.sub.3COO).sub.2.4H.sub.2O and 0.5 g of Ketjen Black (diameter approximately 40 nm) were added to 75 mL of water was introduced into the internal cylinder of the reactor used in Example 1, and the internal cylinder was turned for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution so that Mn(CH.sub.3COO).sub.2.4H.sub.2O was dissolved and Ketjen Black was dispersed. Then, the turning of the internal cylinder was suspended, and 0.3N NaOH aqueous solution was added into the internal cylinder. The pH of the solution was 10.5. Then, the internal cylinder was turned again for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. In the meantime, a core of Mn hydroxide was formed between the inner wall of the external cylinder and the outer wall of the internal cylinder, this core grew and was supported on the surface of Ketjen Black. After the turning of the internal cylinder was stopped, Ketjen Black was filtered and retrieved, and dried in air at 100 C. for 12 hours. Then it was further heat-treated in air at 130 C. for 16 hours and a composite material was obtained.

Comparative Example 3

(71) A solution in which 2.41 g of Mn(CH.sub.3COO).sub.2.4H2O and 0.5 g of Ketjen Black (diameter: approximately 40 nm) were added to 75 mL of water was introduced into the internal cylinder of the reactor used in Example 1, and the internal cylinder was turned for 300 seconds to add 70000 kgms.sup.2 of centrifugal force to the reaction solution. Ketjen Black was filtered and retrieved, dried in air at 130 C. for 16 hours, and a composite material was obtained.

(72) FIG. 10 shows the result of TG-DTA analysis of the composite materials in Example 8 and Comparative Example 3 at the temperature raising rate of 1 C./minute in air atmosphere. Also, FIG. 11 shows an X-ray powder diffraction diagram of the composite material of Example 8. As can be seen from FIG. 11, Mn.sub.3O.sub.4 was formed in the composite material of Example 8. FIG. 10 shows that a weight loss of approximately 90% occurred in the composite material of Comparative Example 3, while a weight loss of approximately 40% occurred in the composite material of Example 8. This weight loss is due to the burning of Ketjen Black. In Comparative Example 3, most of the Mn was not supported by Ketjen Black, even though the same amount of Mn(CH.sub.3COO).sub.2.4H.sub.2O was used as in Example 8. On the other hand, in Example 8, most of the Mn(CH.sub.3COO).sub.2.4H.sub.2O was supported by Ketjen Black.

(73) Polyvinylidene fluoride in an amount of 10% by mass of the total was added to the composite material of Example 8 and the mixture obtained was formed to produce a positive electrode. A half-cell including the positive electrode, 1M LiPF.sub.6 ethylene carbonate/diethyl carbonate (1:1) solution as an electrolyte, and lithium as a counter electrode was produced. For the half-cell obtained, the charge/discharge characteristics were evaluated. The result is shown in FIG. 12. A capacity of approximately 800 mAhg.sup.1 was observed in the range of 0 to 2.5 V against Li/Li.sup.+, and the composite material was found to be suitable for a negative electrode in a lithium ion secondary battery.

INDUSTRIAL APPLICABILITY

(74) By the present invention, a composite material of a metal oxide and conductive carbon, which is suitable in the field such as a fuel battery, a secondary battery, an electrochemical capacitor, or an antistatic material, can be obtained.