The disclosure of Japanese Patent Application No. 2014-186481 filed on Sep. 12, 2014 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing an electricity storage material.
2. Description of the Related Art
In recent years, lithium ion secondary batteries have been used for hybrid vehicles, electric vehicles, etc. Electrodes of the lithium ion secondary batteries are manufactured by first kneading powder of an active substance etc. and a solution of a thickener to produce slurry of an active material (electricity storage material), and then applying the slurry to a base material such as aluminum foil and drying the slurry. The lithium ion secondary batteries are manufactured by cutting the electrodes into a predetermined size, stacking the resultant electrodes with a separator interposed therebetween, and enclosing the stack and a non-aqueous electrolyte in a packaging material. Japanese Patent Publication No. H08-24043 (JP H08-24043 B) and Japanese Patent No. 2979641 (JP 2979641) describe a method for producing powder of an active substance for a positive electrode by mixing a lithium compound and manganese dioxide and heating the mixture at about 400° C. to 500° C.
Due to poor wettability of powder of an active substance with a solution of a thickener, a mixture of the powder of the active substance and the solution of the thickener tends to be deposited in a manufacturing apparatus. It is therefore difficult to feed the mixture, hindering continuous production of slurry.
It is one object of the present invention to provide an apparatus and a method for manufacturing an electricity storage material which can improve wettability of powder of an active substance with a solution of a thickener.
According to a first aspect of the present invention, an apparatus for manufacturing an electricity storage material includes: a dissolving device that dissolves a thickener in a solvent; a viscosity adjusting device that adjusts viscosity of a solution produced by dissolving the thickener in the solvent by the dissolving device; a stirring device that mixes the solution of the thickener adjusted in viscosity by the viscosity adjusting device and powder of an active substance to produce a first mixture, and stirs the first mixture to produce a second mixture; a heating device that heats the solution of the thickener or the first mixture by the time the stirring device starts stirring the first mixture after the dissolving device produces the solution of the thickener, so that the solution of the thickener contained in the first mixture has already been heated when the stirring device stirs the first mixture; and a kneading device that kneads the solution of the thickener and the powder of the active substance which are contained in the second mixture produced by the stirring device to produce a third mixture.
The solution of the thickener has already been heated when stirred together with the powder of the active substance. Accordingly, the viscosity of the solution of the thickener is reduced, and the wetting rate of the powder of the active substance with the solution of the thickener is increased. Since the powder of the active substance is easily wetted with the solution of the thickener, the mixture of the solution of the thickener and the powder of the active substance is not deposited in the manufacturing apparatus and can therefore be smoothly fed. Due to the improved wettability of the powder of the active substance with the solution of the thickener, the powder of the active substance is quickly dispersed in the solution of the thickener, and damage to the powder of the active substance can be suppressed. Since the powder of the active substance is uniformly dispersed in the solution of the thickener, quality of electrodes can be improved, and battery performance can be enhanced.
The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
An embodiment of the present invention will be described below with reference to the accompanying drawings.
An apparatus and a method for manufacturing an electricity storage material according to the embodiment form, e.g., an apparatus and a method for manufacturing electrodes (positive and negative electrodes) of lithium ion secondary batteries. Electrodes of lithium ion secondary batteries are manufactured by applying slurry of an active material serving as an electricity storage material to a base material such as aluminum foil or copper foil and drying the slurry. The apparatus and the method for manufacturing an electricity storage material according to the present embodiment are an apparatus and a method for manufacturing slurry of an active material.
For positive electrodes, specific examples of the active material include lithium-nickel oxide etc. as an active substance (solid component), N-methylpyrrolidone etc. as a solvent (liquid component), acetylene black etc. as a conductive agent, and polyvinylidene fluoride etc. as a binder. For negative electrodes, specific examples of the active material include graphite etc. as an active substance (solid component), water as a solvent (liquid component), carboxymethyl cellulose etc. as a thickener, and SRB rubber, polyacrylic acid, etc. as a binder. The active material for negative electrodes will be described below.
Poor wettability of powder of the active substance with a solution of the thickener hinders continuous kneading of the powder of the active substance and the solution of the thickener. It is therefore necessary to improve the wettability, namely increase the wetting rate, of the powder of the active substance with the solution of the thickener. The wetting rate can be given by the wetting angle and the viscosity of the solution of the thickener. The wetting angle is given by the surface tension of the powder of the active substance and the surface tension of the solution of the thickener.
Methods to increase the wetting rate include increasing the surface tension of the powder of the active substance to increase the wetting angle, reducing the surface tension of the solution of the thickener to increase the wetting angle, and reducing the viscosity of the solution of the thickener. The surface tension of the powder of the active substance can be increased by carrying out surface modification by ultraviolet radiation. The surface tension of the solution of the thickener can be reduced by increasing the temperature of the solution of the thickener or adding a surfactant to the solution of the thickener. The viscosity of the solution of the thickener can be reduced by increasing the temperature of the solution of the thickener or changing the molecular weight of the solution of the thickener. Increasing the temperature of the solution of the thickener is particularly effective in increasing the wetting rate because it can reduce both the surface tension and the viscosity of the solution of the thickener.
Experiments were carried out to see how the viscosity of the solution of the thickener would change with an increase in temperature of the solution of the thickener. The result of the experiments show that the viscosity of the solution of the thickener decreases with an increase in temperature of the solution of the thickener, as shown by a continuous line in
Experiments were carried out to see how the wetting rate of the powder of the active substance with the solution of the thickener would change with an increase in temperature of the solution of the thickener. The wetting rate was evaluated by pouring the powder of the active substance onto the surface of the solution of the thickener and measuring the settling time of the powder of the active substance, namely the time it took for the powder of the active substance to settle to the bottom of the solution of the thickener. The result of the experiments show that the settling time of the powder of the active substance in the solution of the thickener decreases with an increase in temperature of the solution of the thickener, as shown in
The apparatus for manufacturing an electricity storage material according to the present embodiment will be described with reference to
The control device 8 is a device that controls driving etc. of the dissolving device 2, the viscosity adjusting device 3, the heating device 4, the stirring device 5, the cooling device 6, and the kneading device 7. A memory unit 81 of the control device 8 stores data showing the relationship between the viscosity of the solution of the thickener and the temperature of the solution of the thickener (see
The dissolving device 2 is a device that dissolves the thickener in the solvent in a housing. The dissolving device 2 includes a microwave device having a magnetron. The control device 8 drives the microwave device to generate microwaves and applies the microwaves to the solvent supplied into the housing to dissolve the thickener in the solvent.
The viscosity adjusting device 3 is a device that adjusts the viscosity of the solution of the thickener dissolved by the dissolving device 2 in a housing. The viscosity adjusting device 3 includes an ultrasonic device having an ultrasonic wave generating element such as a piezoelectric element. The control device 8 drives the ultrasonic device to generate ultrasonic waves and applies the ultrasonic waves to the solution of the thickener supplied into the housing to adjust the viscosity of the solution of the thickener. That is, the control device 8 decides the viscosity of the solution of the thickener based on the viscosity of the final slurry of the active material and controls viscosity adjustment by applying the ultrasonic waves for a predetermined time so that the solution of the thickener has the decided viscosity.
The heating device 4 is a device that heats the solution of the thickener adjusted in viscosity by the viscosity adjusting device 3 in a housing. The heating device 4 includes an electrical heating wire made of nichrome etc. and a temperature measuring device such as a thermocouple. The control device 8 applies a current to the electrical heating wire to cause the electrical heating wire to generate heat, and heats the solution of the thickener supplied into the housing to temporarily reduce the viscosity of the solution of the thickener. The heating device 4 may include an element other than the electrical heating wire such as a heat pump as long as the element has a heating function.
The stirring device 5 is a device that mixes and stirs the solution of the thickener heated by the heating device 4 and powder of the active substance in a housing. The stirring device 5 includes stirring blades that are rotated by a motor. The control device 8 drives the motor to rotate the stirring blades, so that the stirring device 5 mixes the solution of the thickener and the powder of the active substance introduced into the housing to produce a first mixture, and stirs the first mixture to produce a second mixture.
The cooling device 6 is a device that cools the second mixture produced by the stirring device 5 in a housing. The cooling device 6 includes a heat pump and a temperature measuring device such as a thermocouple. The control device 8 operates the heat pump to cool the second mixture supplied into the housing so as to increase the viscosity of the solution of the thickener contained in the second mixture, namely so as to bring the viscosity of the solution of the thickener contained in the second mixture back to the viscosity adjusted by the viscosity adjusting device 3. The cooling device 6 may include an element other than the heat pump such as a Peltier element as long as the element has a cooling function.
The kneading device 7 is a device that kneads the solution of the thickener and the powder of the active substance which are contained in the second mixture cooled by the cooling device 6 in a housing to produce a third mixture. The kneading device 7 includes stirring blades that are rotated by a motor. The control device 8 drives the motor to rotate the stirring blades, so that the kneading device 7 stirs and kneads the mixture supplied into the housing to produce the slurry of the active material. As described in detail later, a kneading index is set based on kinetic energy of particles of the active material, the mean free path of the particles of the active material, and a kneading time for the active material. The control device 8 sets kneading conditions so that the set kneading index is equal to or lower than a target value, and controls kneading of the active material according to the set kneading conditions.
As used herein, “mixing in the stirring device 5” means adding the powder of the active substance to the solution of the thickener, “stirring in the stirring device 5” means stirring the first mixture (preliminary kneading) and refers to the state before cooling the solution of the thickener, and “kneading in the kneading device 7” means kneading the second mixture (main kneading) and refers to the state after cooling the solution of the thickener.
Processing that is performed by the control device 8 will be described below with reference to
Specifically, the control device 8 reads from the memory unit 81 the data showing the relationship between the viscosity of the solution of the thickener and the dissolution rate to solubility of the thickener in the solvent and the data showing the relationship between the viscosity of the solution of the thickener and the dissolution time of the thickener, and supplies a predetermined amount of thickener and a predetermined amount of solvent into the housing of the dissolving device 2. The control device 8 drives the microwave device of the dissolving device 2 to apply microwaves to the solvent in the housing, thereby dissolving the thickener in the solvent. The control device 8 stops driving the microwave device after it drives the microwave device for such a time that the dissolution rate to solubility of the thickener in the solvent reaches the predetermined value.
Referring to
Dissolution using microwaves is performed by vibrating the solvent by microwave radiation and thus causing the solvent to penetrate the thickener. A desirable frequency band of the microwaves is a frequency band in which the solvent tends to absorb energy of the microwaves. For example, a frequency band from 0.9 GHz to 400 GHz is used in the case of using water as the solvent.
The thickener may be dissolved in the solvent by stirring as in conventional examples. In the present embodiment, however, the thickener is dissolved in the solvent by vibrating the solvent with microwaves. This is because the thickener can be more efficiently dissolved in the solvent by using microwave vibrations than by using a stirring force or by heating such as heating of the solvent to, e.g., a high temperature, as shown in
That is, the time T required to adjust the viscosity μ of the solution of the thickener to target viscosity μs is T12 in the case of using a stirring force, and T13 (>T12) in the case of heating. However, the use of microwaves can reduce the time T to T11 (<T12<T13). Dissolution using microwaves therefore requires less electric power than dissolution using a stirring force.
The control device 8 then reads data relating to viscosity adjustment (step S6 in
Specifically, the control device 8 drives the ultrasonic device of the viscosity adjusting device 3 to apply ultrasonic waves to the solution of the thickener in the housing for the predetermined viscosity adjustment time so as to adjust the viscosity of the solution of the thickener. The control device 8 stops driving the ultrasonic device after it drives the ultrasonic device for such a time that the viscosity of the solution of the thickener reaches a predetermined value.
Viscosity adjustment of the solution of the thickener will be described. As shown in
The viscosity μ of the solution of the thickener is adjusted to the predetermined viscosity range of μa to μb shown in
The viscosity of the solution of the thickener may be adjusted by cutting the molecular chains of the thickener with shear energy generated by a stirring force as in conventional examples. In the present embodiment, however, the viscosity of the solution of the thickener is adjusted by cutting the molecular chains of the thickener with collision energy and shear energy which are generated by ultrasonic waves. This is because the viscosity of the solution of the thickener can be more efficiently adjusted by using ultrasonic waves than by using a stirring force, as shown in
That is, the time T required to adjust the viscosity μ of the solution of the thickener to target viscosity μp is T2 in the case of using a stirring force. However, the use of ultrasonic waves can reduce the time T to T1 (<T2). The viscosity adjustment using ultrasonic waves therefore requires less electric power than the viscosity adjustment using a stirring force. The viscosity μ of the solution of the thickener decreases with an increase in viscosity adjustment time T and eventually becomes equal to the viscosity of water.
Subsequently, the control device 8 reads data relating to the temperature of the solution of the thickener (step S10 in
Specifically, the control device 8 reads from the memory unit 81 the data showing the relationship between the viscosity of the solution of the thickener and the temperature of the solution of the thickener, and applies a current to the electrical heating wire of the heating device 4 to cause the electrical heating wire to generate heat. The control device 8 stops applying a current to the electrical heating wire of the heating device 4 when the temperature of the solution of the thickener in the housing has reached the predetermined value and the viscosity of the solution of the thickener has been temporarily reduced.
The control device 8 then drives the stirring device 5 (step S14 in
Specifically, the control device 8 drives the motor of the stirring device 5 to rotate the stirring blades for the predetermined stirring time so that the stirring device 5 mixes the solution of the thickener and the powder of the active substance introduced into the housing to produce the first mixture, and stirs the first mixture to produce the second mixture. The control device 8 stops driving the motor when the stirring blades have been rotated for the predetermined stirring time.
The control device 8 then operates the cooling device 6 based on the data that has been read earlier, i.e., the data relating to the temperature of the solution of the thickener (step S17 in
Specifically, the control device 8 operates the heat pump of the cooling device 6 based on the data showing the relationship between the viscosity of the solution of the thickener and the temperature of the solution of the thickener. The control device 8 stops operating the heat pump of the cooling device 6 when the temperature of the solution of the thickener in the housing has reached the predetermined value and the viscosity of the solution of the thickener has been brought back to the viscosity adjusted by the viscosity adjusting device 3.
The control device 8 then reads data relating to kneading of the mixture of the solution of the thickener and the powder of the active substance etc. (step S20 in
Specifically, the control device 8 reads from the memory unit 81 data on the kneading time, and drives the motor to rotate the stirring blades for a predetermined kneading time so that the kneading device 7 kneads the second mixture supplied into the housing. The control device 8 stops driving the motor when the stirring blades have been rotated for the predetermined kneading time.
Setting of the kneading index and the kneading conditions will be described. As shown by the experimental result of
As the kneading circumferential speed v of the stirring blades increases, the particles of the active material collide with the stirring blades more frequently during kneading, and therefore have a higher probability of being damaged. If the particles of the active material are damaged and broken into smaller particles, the overall surface area of the particles is increased, and decomposition of the electrolyte is facilitated. The capacity retention rate P of the battery is thus significantly associated with damage to the particles of the active material.
Factors in the damage to the particles of the active material include the kneading time t for the active material and the solid content rate (solid content/(solid content+liquid content)) η of the active material in addition to the kneading circumferential speed v of the stirring blades. Accordingly, the number of collisions of the particles of the active material is obtained based on a known mean free path by using a model of the particles of the active material which move freely in a predetermined space. As given by the following formula (1), cumulative collision energy D of the particles of the active material serving as the kneading index can be obtained by multiplying the kinetic energy mv2/2 of the particles of the active material, the number of collisions √(2)·η·σ·v of the particles of the active material, and the kneading time t for the active material. The damage state of the particles of the active material due to kneading can thus be predicted before kneading is performed.
where “D” represents the cumulative collision energy of the particles of the active material, “m” represents the weight of a single particle of the active material, “v” represents the kneading circumferential speed of the stirring blades, “η” represents the solid content rate of the active material, “σ” represents the mean particle size of the particles of the active material, and “t” represents the kneading time for the active material.
The relationship between the capacity retention rate P of the battery and the cumulative collision energy D of the particles of the active material is obtained as shown in
As described above, the number of collisions of the particles of the active material is obtained based on the mean free path of the particles of the active material by using the model of the particles of the active material which move freely in a predetermined space. The cumulative collision energy of the particles of the active material can be obtained by multiplying the number of collisions of the particles of the active material, the kinetic energy of the active material, and the kneading time for the active material, and the cumulative collision energy thus obtained can be used as an index of durability of the battery. Since the damage state of the particles of the active material due to kneading can be predicted before kneading is performed, kneading can be performed such that the particles of the active material are less likely to be damaged. A durable battery can therefore be manufactured.
According to the apparatus 1 for manufacturing an electricity storage material, the solution of the thickener has already been heated when stirred together with the powder of the active substance. Accordingly, the viscosity of the solution of the thickener is reduced, and the wetting rate of the powder of the active substance with the solution of the thickener is increased. Since the powder of the active substance is easily wetted with the solution of the thickener, the mixture of the solution of the thickener and the powder of the active substance is not deposited in the manufacturing apparatus 1 and can therefore be smoothly fed. Due to the improved wettability of the powder of the active substance with the solution of the thickener, the powder of the active substance is quickly dispersed in the solution of the thickener, and damage to the powder of the active substance can be suppressed. Since the powder of the active substance is uniformly dispersed in the solution of the thickener, quality of the electrodes can be improved, and battery performance can be enhanced.
In the above embodiment, the heating device 4 is placed between the viscosity adjusting device 3 and the stirring device 5. However, the heating device 4 may be placed between the dissolving device 2 and the viscosity adjusting device 3. In this case, the viscosity of the solution of the thickener has already been reduced when viscosity adjustment of the solution of the thickener is performed. Accuracy of viscosity adjustment by the viscosity adjusting device 3 can thus be improved.
The heating device 4 may be configured as follows. The stirring device 5 may include an electrical heating wire etc. so as to have a heating function. In this case, since the stirring device 5 stirs the solution of the thickener together with the powder of the active substance and heats the solution of the thickener, manufacturing efficiency can be improved. Moreover, since no separate heating device is needed, an increase in apparatus cost can be suppressed.
The viscosity adjusting device 3 may include a high-power ultrasonic device so as to have a heating function. In this case, since the viscosity adjusting device 3 adjusts the viscosity of the solution of the thickener and heats the solution of the thickener, manufacturing efficiency can be improved. Moreover, since no separate heating device is needed, an increase in apparatus cost can be suppressed. The viscosity adjusting device 3 may include stirring blades instead of the ultrasonic device. In this case, the viscosity adjusting device 3 includes an electrical heating wire etc. so as to have a heating function.
The dissolving device 2 may have a heating function that is carried out by the microwave device. In this case, since the dissolving device 2 dissolves the thickener in the solvent and heats the solution of the thickener, manufacturing efficiency can be improved. Moreover, since no separate heating device is needed, an increase in apparatus cost can be suppressed. The dissolving device 2 may include stirring blades instead of the microwave device. In this case, the dissolving device 2 includes an electrical heating wire etc. so as to have a heating function.
In the above embodiment, the cooling device 6 is placed between the stirring device 5 and the kneading device 7. However, the kneading device 7 may include a heat pump so as to have a cooling function. In this case, since the viscosity of the solution of the thickener is brought back to the viscosity adjusted by the viscosity adjusting device 3, dispersibility of the powder of the active substance in the solution of the thickener can be improved. Since the powder of the active substance is uniformly dispersed in the solution of the thickener by kneading in the kneading device 7, quality of the electrodes can be improved, and battery performance can be enhanced. The cooling device 6 may be placed after the kneading device 7. The apparatus 1 for manufacturing an electricity storage material may not include the cooling device 6 and the solution of the thickener may be naturally cooled.
The above embodiment is described with respect to the case of manufacturing the active material for negative electrodes of lithium ion secondary batteries. However, the present invention is also applicable to the case of manufacturing an active material for positive electrodes of lithium ion secondary batteries. In this case, microwaves are applied when a binder such as polyvinylidene fluoride is dissolved in a solvent such as N-methylpyrrolidone. However, no ultrasonic waves are applied in the case where a conductive agent such as acetylene black is mixed with the solution. This is because the viscosity of the solution can be adjusted according to the amount of conductive agent such as acetylene black to be mixed.
The electricity storage material to which the invention is applied is not limited to the active material for electrodes of lithium ion secondary batteries. The present invention is also applicable to any electricity storage materials such as, e.g., materials for capacitors.
In order to further improve the wetting rate, a surfactant is added to the solution of the thickener to reduce the surface tension of the solution of the thickener and thus increase the wetting angle. A fluorosurfactant can be used as a surfactant that improves wettability of the powder of the active substance with the solution of the thickener, because the fluorosurfactant is chemically stable and is not decomposed during charging and discharging. The use of the fluorosurfactant can thus enhance battery performance.
Number | Date | Country | Kind |
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2014-186481 | Sep 2014 | JP | national |