JP2012138350A - Negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery having a high electrode density, and excellent electrolyte retentivity and electrolyte permeability, and offering excellent battery characteristics in a lithium ion secondary battery.SOLUTION: A negative electrode active material layer is made of a material containing an active material composed of a carbon material, carbon fiber being linear and having an average diameter of 100 to 1000 nm, and a binder. The carbon fiber is manufactured by wet spinning or melt spinning.

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery having a negative electrode active material layer containing carbon fiber, and a lithium ion secondary battery having the same.

  Lithium ion secondary batteries have excellent battery characteristics such as operating voltage, battery capacity, cycle life, etc., and have a wide range of fields and applications in place of nickel / cadmium batteries and nickel metal hydride batteries, which have been the mainstream, as a power source with less environmental pollution. It has come to be used in. In recent years, lithium ion batteries have been widely used as power sources for portable terminals and mobile communication devices. Since the lithium ion battery has the highest energy density among the existing secondary batteries, the use of the lithium ion battery as an auxiliary power source for a hybrid vehicle or a fuel cell vehicle or as a large power source for stationary use has been studied. As these electronic devices have higher performance, higher functionality, or expanded applications, further improvements in the performance of lithium ion secondary batteries mounted on them are required.

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. Therefore, studies have been started to increase the energy density per volume of the electrode and battery by enclosing a large amount of electrode in the battery container by increasing the density of the electrode. That is, although the true density of graphite frequently used as the negative electrode active material is about 2.2 g / cm ³ , the current electrode density of the negative electrode using this is about 1.5 g / cm ³ , For the purpose of increasing the energy density, studies have been made to increase the electrode density to 1.7 g / cm ³ or more.

  However, since the number of vacancies in the electrode decreases when the electrode is densified, the electrolyte solution that exists in the vacancies and contributes to the electrode reaction is insufficient, or the permeation rate of the electrolyte solution into the electrode is reduced. When the electrolytic solution is insufficient, the electrode reaction is slowed down, resulting in a problem that the energy density and high-speed charge / discharge performance are lowered and the cycle characteristics are lowered. Moreover, since the battery manufacturing time will become long when the penetration | permeation rate to the electrode of electrolyte solution becomes slow, it leads to an increase in manufacturing cost.

  As a technique for solving these problems, Patent Document 1 discloses that a carbon fiber having a diameter of 1 to 1000 nm is added to an electrode active material to obtain a high-density electrode having a porosity of 25% or less. Yes. However, Patent Document 1 includes, as examples, spherical graphite particles (electrode active material), vapor grown carbon fiber (VGCF) having an average fiber diameter of 20 to 150 nm, acetylene black (conducting aid), and binder. Only a negative electrode for a lithium ion secondary battery having a negative electrode active material layer made of is described.

  Patent Document 2 discloses that a graphite material containing fibrous carbon that forms secondary particles having an average particle diameter of 10 μm or more and 30 μm or less is used as a negative electrode material for a lithium ion secondary battery. In Patent Document 2, there is a description that vapor-grown carbon fiber, PAN-based, or pitch-based carbon fiber may be used as the fibrous carbon. However, the vapor-phase grown is used in the examples. Only carbon fiber.

  In Patent Document 2, a vapor-grown carbon fiber having a fiber diameter of 0.05 μm or more and 0.5 μm or less and a fiber length of 5 μm or more and 50 μm or less is preliminarily kneaded with a binder and a solvent, and then spherical graphite is added. Further, it is described that the kneaded material is layered to form a negative electrode active material layer containing secondary particles of the carbon fiber.

Japanese Patent No. 3930002 JP 2000-133267 A

The negative electrodes for lithium ion secondary batteries described in Patent Documents 1 and 2 have room for improvement in terms of electrolyte retention, electrolyte permeability, and battery characteristics when a lithium ion secondary battery is obtained.
An object of the present invention is to provide a negative electrode for a lithium ion secondary battery having a high electrode density, excellent electrolyte solution retention and electrolyte solution permeability, and excellent battery characteristics when a lithium ion secondary battery is obtained. It is to be.

  In order to solve the above problems, a negative electrode for a lithium ion secondary battery of the present invention is a lithium ion secondary battery having a negative electrode active material layer containing an active material made of a carbon material, carbon fibers, and a binder. The carbon fiber is obtained through a spinning process in which a carbon-containing raw material is spun by a solution method or a melting method, and is linear and has an average diameter in a range of 100 nm to 1000 nm. And

  The negative electrode for a lithium ion secondary battery of the present invention is also a negative electrode for a lithium ion secondary battery having a negative electrode active material layer containing an active material made of a carbon material, carbon fiber, and a binder, A spinning process in which the carbon fibers are spun by a solution method or a melting method to obtain pitch fibers, a pitch fiber infusibilization process (a process in which the pitch fibers are heated in an oxygen-containing atmosphere), and an infusible pitch. It is obtained through a step of heat-treating the fiber in an inert gas atmosphere (carbonization step or carbonization step and graphitization step), and is linear and has an average diameter in the range of 100 nm to 1000 nm.

  The negative electrode for a lithium ion secondary battery according to the present invention contains, in the negative electrode active material layer, linear carbon fibers obtained through the spinning step or the respective steps and having an average diameter in a range of 100 nm to 1000 nm. To do. Thus, the negative electrode for a lithium ion secondary battery of Patent Document 1 containing carbon fibers having a diameter of 1 to 1000 nm obtained by vapor deposition and the carbon fibers obtained by vapor deposition are pre-kneaded. Compared with the negative electrode for lithium ion secondary batteries of Patent Document 2 containing secondary particles having an average particle diameter of 10 μm or more and 30 μm or less, the electrode characteristics (electrolyte retention and electrolyte permeability) and The characteristics of the lithium ion secondary battery used as the negative electrode are improved.

  Even if it is a linear carbon fiber obtained through the spinning step or each of the steps, those having an average diameter outside the range of 100 nm to 1000 nm are not used for the following reasons. When the average diameter of the carbon fibers existing in the negative electrode active material layer is less than 100 nm, it is impossible to secure an appropriate gap between the active material particles due to the fine fibers, which is necessary for improving the electrolyte permeability, and exceeds 1000 nm. On the contrary, the gap becomes large and a high-density electrode cannot be obtained.

  Further, in the negative electrode for a lithium ion secondary battery of the present invention, the carbon fiber present in the negative electrode active material layer is a linear carbon fiber obtained through the spinning step or each step, and is the minimum The diameter is preferably 100 nm and the maximum diameter is preferably 1000 nm. If carbon fibers having a diameter of less than 100 nm are present, it is not preferable in terms of securing an appropriate gap between the active material particles by the fine fibers, which is necessary for improving electrolyte permeability. If carbon fibers having a diameter of more than 1000 nm are present, the voids in the portion become large, which is not preferable in terms of obtaining a high-density electrode.

Moreover, the length of the carbon fiber is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less, and further preferably 5 μm or more and 30 μm or less.
Further, the negative electrode active material layer is a linear carbon fiber obtained through the spinning step or each step, and has an average diameter of 100 nm to 1000 nm and an aspect ratio (average length / average diameter). Is preferably obtained through a step of applying a paste-like mixture containing a carbon fiber having a carbon fiber of 50 or less, the active material and the binder to a current collector. Even in the case of linear carbon fibers having an average diameter of 100 nm or more and 1000 nm or less, if the aspect ratio (average length / average diameter) is larger than 50, secondary particles are easily formed in the step of obtaining the paste-like mixture.

  In the negative electrode for a lithium ion secondary battery according to the present invention, the negative electrode active material layer may contain the carbon fiber at a ratio of 0.5 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the active material. preferable. When the content of the carbon fiber is less than 0.5 parts by mass with respect to 100 parts by mass of the active material, the effect of adding the carbon fiber is not substantially obtained, and when it exceeds 20 parts by mass, Since the content of the active material made of a carbon material is relatively reduced, the battery capacity is decreased.

The negative electrode for a lithium ion secondary battery of the present invention can provide good electrode characteristics (electrolytic solution retention and electrolytic solution permeability) even when the electrode density is 1.6 to 2.0 g / cm ³ .
The lithium ion secondary battery having the negative electrode for a lithium ion secondary battery of the present invention has a high energy density and excellent high-speed charge / discharge performance.

  The negative electrode for a lithium ion secondary battery of the present invention has a high electrode density, excellent electrolyte solution retention and electrolyte solution permeability, and excellent battery characteristics can be obtained when a lithium ion secondary battery is obtained.

It is a schematic block diagram which shows an example of the apparatus used when performing a spinning process by a melt | dissolution electrospinning method.

Hereinafter, embodiments for carrying out the present invention will be described.
The negative electrode for a lithium ion secondary battery according to the present invention has a negative electrode active material layer containing an active material made of a carbon material, carbon fiber, and a binder, and the method, shape, and average diameter of the carbon fiber Has been identified. The carbon fiber production method is a method including a spinning step of spinning a raw material containing carbon by a solution method or a melting method. First, this manufacturing method will be described.

As a raw material containing carbon used in the spinning step, a condensed polycyclic aromatic compound having a high carbonization yield is preferable. In particular, pitch is preferable because it contains a large number of condensed polycyclic aromatic compounds. There are the following methods for obtaining the pitch.
First, petroleum tar or coal tar is subjected to atmospheric distillation and vacuum distillation to separate heavy and light fractions, and the resulting heavy fractions are heat-treated as they are and subjected to polycondensation to give the desired softening. It is a method of adjusting to a point. The pitch (anhydrous added pitch) obtained by this method has optical isotropy and is referred to as “General Purpose pitch”. The other is a method in which the obtained heavy fraction is hydrocracked to increase thermal stability and then heat treated to adjust to a predetermined softening point. The pitch (hydrogenated pitch) obtained by this method has optical anisotropy and is called “High Peformance pitch” or “Liquid crystal pitch”, and is used as a raw material for carbon fiber with high strength and high elastic modulus. The

  Carbon fiber using pitch as a raw material is manufactured by performing a spinning process, an infusibilization process, and a heat treatment process in an inert gas atmosphere in this order. In the spinning process, the raw material pitch is spun by a solution method or a melting method to form a fiber. When pitch fibers (fibrous pitch) obtained in the spinning process are heat-treated as they are at a high temperature, the pitch fibers are fused to each other, and therefore an infusibilization process is performed for the purpose of avoiding this.

  As the spinning process, either a solution method in which a raw material containing carbon is spun into a solution or a melting method in which a raw material containing carbon is spun in a molten state may be employed. When the pitch is spun by the melting method, the molecular orientation due to the condensed ring of the condensed polycyclic aromatic compound contained in the pitch advances, and further orientation occurs in the subsequent firing (heat treatment process in an inert gas atmosphere). As the process proceeds, there is an advantage that the obtained carbon fiber has high strength.

  Examples of the spinning method capable of obtaining a carbon fiber having a linear shape and an average diameter in the range of 100 nm to 1000 nm include the following solution electrospinning method, melt blown method, and melt electrospinning method. A combination of the meltblown method and the melt electrospinning method can also be performed.

<Solution electrospinning method: Solution method>
In this method, first, pitch is dissolved in a solvent such as carbon disulfide, tetrahydrofuran, pyridine, or quinoline to prepare a pitch solution having a pitch concentration of 1 to 30% by mass, preferably 10 to 20% by mass.
The spinning device to be used includes a container for storing a pitch solution, a nozzle for discharging the pitch solution, a high voltage generator for applying a voltage to charge the pitch solution discharged through the nozzle, and a discharge from the nozzle. And a collector charged with a polarity different from that of the nozzle side for accumulating the spun and spun pitch fibers.

A voltage is applied between the nozzle and the collector and collected by the collector. The method of applying the voltage is arbitrary, and the collector side may be any of positive electrode, negative electrode, and ground. Usually, a method is adopted in which the collector is grounded to make the nozzle a positive voltage. However, in this case, insulation measures for the entire nozzle side are essential.
The voltage applied between the nozzle and the collector is 0.5 to 100 kV. This is appropriately changed depending on the distance between the nozzle and the collector. By setting it as 0.5 kV or more, a pitch solution becomes easy to detach | leave from a nozzle, and the discharge between a nozzle and a collector is suppressed by setting it as 100 kV or less.

  As a form of the collector, a simple static flat plate, a rotary drum, a rotary belt (belt conveyor), and the like are typical, but from the viewpoint of production efficiency, a rotary collection device is preferable to the static flat plate. The distance between the nozzle and the fiber grounding portion at the collector is preferably 10 to 200 mm. The fiber ground contact portion is a portion that is included in the collector and substantially contacts with the fiber first.

By setting this distance to 10 mm or more, it is possible to prevent discharge and the like generated between the nozzle and the collector, and to manufacture fibers while maintaining a stable potential environment. On the other hand, when the distance exceeds 200 mm, the fiber pulling by the electric field becomes weak, and the pitch solution discharged from the nozzle is difficult to be spun into a thin fiber.
When a high boiling point solvent such as pyridine or quinoline is used, it is necessary to warm the atmosphere on the downstream side of the nozzle or reduce the pressure so that the solvent is easily volatilized.

<Melt blown method: Melting method>
In the melt blown method, pitch fiber is spun by discharging a pitch melt obtained by melting pitch together with a heating gas such as air and nitrogen supplied to a gas nozzle to a collector through a spinning nozzle. The heated gas injected from the gas nozzle gives a shearing force to the pitch melt discharged from the spinning nozzle. As a result, fine fibers are obtained. In the melt blown method, no voltage is applied.
As a form of the collector, a simple static flat plate, a rotary drum, a rotary belt (belt conveyor), and the like are typical, but from the viewpoint of production efficiency, a rotary collection device is preferable to the static flat plate.

<Melting electrospinning method: melting method>
In this method, a container for storing the pitch melt, a nozzle for discharging the pitch melt, a high voltage generator for applying a voltage to charge the pitch melt discharged through the nozzle, and the nozzle A spinning device having a collector charged with a polarity different from that of the nozzle side for accumulating pitch fibers spun by spinning is used. The container in which the pitch melt is stored is heated by an electric heater, hot air, a heating medium, or the like.

  A voltage is applied between the nozzle and the collector and collected by the collector. The method of applying the voltage is arbitrary, and the collector side may be any of positive electrode, negative electrode, and ground. Usually, a method is adopted in which the collector is grounded to make the nozzle a positive voltage. However, in this case, insulation measures for the entire nozzle side are essential. The voltage applied between the nozzle and the collector is 0.5 to 100 kV. This is appropriately changed depending on the distance between the nozzle and the collector. By setting it as 0.5 kV or more, a pitch melt becomes easy to detach | leave from a nozzle, and the discharge between a nozzle and a collector is suppressed by setting it as 100 kV or less.

  As a form of the collector, a simple static flat plate, a rotary drum, a rotary belt (belt conveyor), and the like are typical, but from the viewpoint of production efficiency, a rotary collection device is preferable to the static flat plate. The distance between the nozzle and the fiber grounding portion at the collector is preferably 10 to 200 mm. The fiber ground contact portion is a portion that is included in the collector and substantially contacts with the fiber first.

By setting this distance to 10 mm or more, it is possible to prevent discharge and the like generated between the nozzle and the collector, and to manufacture fibers while maintaining a stable potential environment. On the other hand, when the distance exceeds 200 mm, pulling of the fiber by the electric field becomes weak, and the pitch melt discharged from the nozzle is difficult to be spun into a thin fiber.
When the solution method and the melting method are compared, the solution method requires an exhaust gas treatment facility for treating the volatilized solvent, and therefore the melting method is preferable. Further, when the melt electrospinning method, which is a melting method, is compared with the melt brain method, the melt brain method in which no voltage is applied is more preferable than the melt electrospinning method that requires high voltage equipment.

Furthermore, when carrying out the melt electrospinning method, it is preferable to use the spinning device shown in FIG. 1 from the viewpoint of safety. The spinning device includes a raw material container 1, a discharge device 2, a collector 3, and a high voltage power source 4.
The raw material container 1 contains pitch powder material. The discharge device 2 includes a main body 21 that is a container for storing pitch melt, a nozzle 22 formed at the tip of the main body 21, and a pipe 23 that introduces pressurized nitrogen into the main body 21. The raw material container 1 and the main body 21 of the discharge device 2 are connected by a pipe 5. A heater 6 is wound around the main body 21 and nozzle 22 of the discharge device 2 and the outside of the pipe 5. The main body 21 of the discharge device 2 is grounded.

When the powdery pitch in the raw material container 1 is supplied from the pipe 5 to the main body 21 of the discharge device 2, it is heated by the heater 6 to be in a molten state. Further, since the main body 21 is also heated by the heater 6, the pitch melt state is maintained in the main body 21.
The collector 3 includes a belt conveyor provided with an insulating mesh belt 31 and a metal plate 32 disposed at an upper portion in the belt conveyor. The metal plate 32 is disposed immediately below the nozzle 22 with the insulating mesh belt 31 interposed therebetween. The metal plate 32 is connected to the high voltage power supply 4.

In this spinning device, the high voltage power source 4 is operated to apply a high voltage between the metal plate 23 of the collector 3 and the discharge device 2, and pressurized nitrogen is introduced into the main body 21 of the discharge device 2 from the pipe 23. As a result, the molten pitch in the main body 21 is discharged from the nozzle 22 to the insulating mesh belt 31. Thereby, the spun pitch fiber is collected on the insulating mesh belt 31 and collected.
According to this spinning device, since the nozzle is grounded, an insulation measure for the entire nozzle side becomes unnecessary. In addition, since the pitch fibers are collected not by the metal belt but by the insulating mesh belt, the recovered pitch fibers are in an insulating state, so that the safety is high.

<Infusibilization process and heat treatment process>
The heating temperature in the infusibilization step is set to, for example, 100 to 350 ° C. according to the softening point of the used pitch-based material.
In the case of a carbonization step, the heat treatment step in an inert gas atmosphere is performed at a temperature of 500 ° C. to 1500 ° C., and in the case of a graphitization step, the heat treatment step is performed at a temperature of 2000 ° C. to less than 3200 ° C. When the pitch fiber after the infusibilization process is heated at 500-1500 ° C., a large amount of volatile substances are generated. Therefore, the carbonization process uses a furnace (carbonization furnace) having a structure capable of removing volatile substances. Do it. The graphitization step is performed by transferring the carbon fiber from which the volatile substances have been removed in the carbonization step to a furnace (graphitization furnace) having a structure with high thermal efficiency and no mixing of oxidizing substances.

  In addition, as a heat treatment process after an infusibilization process, it is preferable to perform only a carbonization process and not to perform a graphitization process. By using carbon fibers obtained by performing only the carbonization step as the heat treatment step after the infusibilization step, the carbonization step and the graphitization step are performed as the carbon fiber to be included in the negative electrode active material layer of the lithium ion secondary battery. The cycle characteristics of the battery are improved as compared with the case of using the carbon fiber obtained by performing both of the above.

<Other processing>
If necessary, the fiber length may be adjusted by subjecting the carbon fiber obtained by the above-described method to processing such as pulverization or cutting. The treatment for adjusting the fiber length may be performed before the infusibilization step after the spinning step or before the heat treatment step (carbonization step) after the infusibilization step. Alternatively, it may be performed after the heat treatment step (carbonization step, graphitization step).

Next, the constituent material of the negative electrode active material layer other than carbon fibers and the method for forming the negative electrode active material layer will be described.
<Constituent material of negative electrode active material layer other than carbon fiber>
Natural graphite and artificial graphite can be used as the active material made of a carbon material. Artificial graphite is graphitized by heating the pitch, graphitized mesocarbon spherules obtained by heating the pitch, bulk mesophase, coke, etc., and the surface of these graphitized materials is pitch or phenolic resin. What was baked after coating | cover etc. can be illustrated. Moreover, you may use these carbon materials in mixture. Furthermore, a powdery, spherical, and flake shaped carbon material can be used.

The binder preferably has chemical stability and electrochemical stability with respect to the electrolyte, and is an organic binder that is dissolved and / or dispersed in an organic solvent, and an aqueous bond that is dissolved and / or dispersed in an aqueous solvent. Any of the agents may be used.
Specific examples include fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, resins such as polyethylene and polyvinyl alcohol, and rubbers such as carboxymethyl cellulose and styrene butadiene rubber. Among these, it is preferable to use an aqueous binder such as carboxymethyl cellulose, polyvinyl alcohol, and styrene butadiene rubber. These can also be used together. The content rate of a binder shall be 0.5-20 mass%, for example.

<Method for forming negative electrode active material layer>
A mixture obtained by mixing an active material, carbon fiber, and a binder is dispersed in a solvent to prepare a paste-like negative electrode material, which is applied to the surface of a current collector and the solvent is dried. The negative electrode active material layer can be formed by pressing and further drying so that the density is 1.6 to 2.0 g / cm ³ (preferably 1.8 to 2.0 g / cm ³ ). Examples of the solvent include N-methylpyrrolidone, dimethylformamide, water, alcohol and the like.

Hereinafter, the present invention will be described with reference to specific examples and comparative examples.
[Sample No.1]
First, a pitch fiber was obtained by spinning the pitch by the solution electrospinning method in the following manner.
A pitch having a softening point of 202 ° C. prepared using coal tar as a raw material was prepared and dissolved in carbon disulfide (solvent) to obtain a pitch solution having a concentration of 10% by mass. This pitch solution was sealed in a stainless steel container (capacity 10 mL). A stainless 27G nozzle (inner diameter 0.20 mm, outer diameter 0.42 mm) is attached to the lower end of the container.

  A ground electrode (conveyor-type collector) was placed at a position 120 mm directly below the nozzle, and a voltage of 25 kV generated by a high voltage generator was applied between the collector and the nozzle. In this state, 0.01 MPa of pressurized nitrogen was introduced into the container, and the pitch solution in the container was discharged from the nozzle to the collector for spinning. As a result, spinning proceeded satisfactorily and an extremely fine pitch fiber was obtained.

Next, an infusibilization step was performed by heating the obtained pitch fiber in the air stepwise at 200 ° C. for 24 hours, then at 250 ° C. for 12 hours, and further at 300 ° C. for 5 hours. .
Next, after introducing nitrogen gas into the furnace in which the infusibilization step has been performed to form a nitrogen gas atmosphere, the temperature is raised to 1000 ° C. at a rate of 3 ° C./min, and then heat treatment (carbon is maintained at 1000 ° C. for 0.5 hours. Process).

Next, the carbon fiber subjected to the heat treatment is placed in a graphitization furnace, and argon gas is introduced into the furnace to form an argon gas atmosphere. After the temperature is increased to 2400 ° C. at a rate of 3 ° C./min, the temperature is increased to 2400 ° C. A heat treatment (graphitization step) was performed for 10 hours.
As a result, a carbon fiber having a minimum diameter of 180 nm and a maximum diameter of 220 nm was obtained. The average diameter of the obtained carbon fiber was 200 nm. These values are average values calculated by measuring 200 carbon fibers in the SEM image.

  The carbon fibers thus obtained were pulverized with an agate mortar so that the average fiber length was 8 μm. That is, the aspect ratio (average value) of this carbon fiber is 40 (8000/200). When this carbon fiber was applied to an X-ray diffractometer and the (002) plane distance was measured, it was 0.3465 nm. The plane spacing was calculated from the θ angle of the peak top of the (002) plane.

  The carbon fibers produced as described above were mixed so as to be 3 parts by mass with respect to 100 parts by mass of the mesophase microsphere graphitized product (active material). This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A portion of this pasty carbon material was taken, and the contained carbon fiber portion was separated and recovered, and SEM image analysis was performed. As a result, the carbon fibers were present in a substantially linear form without agglomerating to form secondary particles, and the average length of the carbon fibers was 5 μm.
This paste-like carbon material was applied to one side of a copper foil as a current collector using a doctor blade applicator with a clearance set to 200 μm to produce an electrode plate. The electrode plate was dried at 100 ° C. for 12 minutes and then pressed so that the electrode density was 1.80 to 2.00 g / cm ³ . This was vacuum-dried at 130 ° C. all day and night to obtain a test electrode in which a negative electrode active material layer was formed on the surface of the current collector.

Among the obtained test electrodes, those having an electrode density of 1.90 g / cm ³ were measured for porosity and penetration rate by the following methods.
The porosity K is calculated by the following equation (1) using the bulk density A, the true density B, the mass M, and the volume V including the voids. K = M (1 / A-1 / B) / V (1)
Therefore, the bulk density A of the electrode was calculated from the dimensions and mass of the electrode, and the true density B of the electrode was calculated by proportionally calculating the mixing ratio from the true densities of the active material, carbon fiber, and binder. The porosity K was calculated by substituting V calculated from A and B calculated in this way and the dimensions of the electrode into the equation (1). As a result, the porosity K of this test electrode was 0.127 (12.7%).

In order to measure the permeation rate of the electrode, a test piece having a density of 1.9 g / cm ³ among the obtained test electrodes was cut into a disk shape having a diameter of 18 mm. Moreover, the low-volatile propylene carbonate (PC) which has a viscosity substantially the same as various electrolyte solutions was prepared. Then, in the atmosphere at 25 ° C., a drop (3 μl) of PC is dropped at the center of the circular surface of the test piece, and the time taken for the PC to permeate the entire test piece is examined. It was. As a result, the penetration speed of this test electrode was 260 seconds.

In addition, by interposing a porous separator between the obtained test electrode having an electrode density of 1.90 g / cm ³ and the lithium foil used as a counter electrode, and immersing in a non-aqueous electrolyte, A test battery was prepared. As a non-aqueous electrolytic solution, LiPF ₆ is adjusted to a concentration of 1 mol / kg in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of EC: EMC = 1: 2. What was dissolved in was used.

When the battery characteristics of the test battery were examined, the discharge capacity was 345 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.5%. The charge acceptance was 35.1% at a charge of 1.0 C, and the discharge rate was 91.6% at a discharge of 2.0 C.
“C” is a coefficient indicating the time ratio of the current value during constant current discharge (charging), and “1.0 C charging (discharging)” is the current at which charging (discharging) is completed in one hour. It means that constant current charging (discharging) is performed by value. “Discharging (charging) at 2.0 C” means that constant current discharging (charging) is performed at a current value at which discharging is completed in 0.5 hours.

[Sample No.2]
First, the pitch was spun by the melt blown method (melting method) by the following method to obtain pitch fibers.
A pitch having the same softening point as No. 1 at 202 ° C. was prepared and sealed in a stainless steel container (capacity 10 mL). A stainless steel 27G (inner diameter 0.20 mm, outer diameter 0.42 mm) nozzle is attached to the lower end of the container, and an outer cylinder is attached to the outside of the nozzle so that preheated gas can flow. Electric heaters that can control the temperature of different systems were wound around the outside of the container and the outside of the outer cylinder. A temperature controller was connected to each heater, and the pitch temperature in the container and the pitch temperature in the nozzle were controlled to be 350 ° C.

  In addition, nitrogen gas preheated to 350 to 360 ° C. was flowed into the outer cylinder so that the linear velocity was 100 m / s through the gap at the tip of the nozzle. Furthermore, a collector was installed at a position 120 mm below the tip of the nozzle. In this state, 0.2 to 0.3 MPa of pressurized nitrogen was introduced into the sealed container, and the molten pitch in the container was discharged from the nozzle to the collector for spinning. As a result, spinning proceeded satisfactorily and an extremely fine pitch fiber was obtained.

Next, an infusibilization step, a carbonization step, and a graphitization step for the obtained pitch fiber were performed in the same manner as in No. 1. As a result, a carbon fiber having a minimum diameter of 800 nm and a maximum diameter of 1000 nm was obtained. The average diameter of the obtained carbon fiber was 900 nm. These values were measured by the same method as No. 1.
The carbon fibers thus obtained were pulverized in an agate mortar so that the average fiber length was 8 μm. That is, the aspect ratio (average value) of this carbon fiber is 9 (8000/900 = 8.8). When this carbon fiber was applied to an X-ray diffractometer and the (002) plane distance was measured, it was 0.3462 nm.

  This carbon fiber was mixed so as to be 3 parts by mass with respect to 100 parts by mass of the graphitized product of the same mesophase spherules used in No. 1. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A portion of this pasty carbon material was taken, and the contained carbon fiber portion was separated and recovered, and SEM image analysis was performed. As a result, the carbon fibers were present in a substantially linear form without agglomerating to form secondary particles, and the average length of the carbon fibers was 6 μm.
Using this pasty carbon material, a test electrode was obtained in the same manner as in No. 1. And when the porosity and penetration rate were measured by the same method as No. 1 using the obtained test electrode whose electrode density is 1.91 g / cm ³ , the porosity K is 0.122 (12 2%) and the permeation rate was 240 seconds.

In addition, by interposing a porous separator between the obtained test electrode having an electrode density of 1.91 g / cm ³ and the lithium foil used as a counter electrode, and immersing in a non-aqueous electrolyte solution, A test battery was prepared. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 344 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.5%. The charge acceptance was 35.3% at 1.0 C, and the discharge rate was 91.7% at 2.0 C discharge.

[Sample No.3]
First, pitch was spun by the melt electrospinning method (melting method) by the following method to obtain pitch fibers.
A pitch having the same softening point as No. 1 at 202 ° C. was prepared and sealed in a stainless steel container (capacity 10 mL). A stainless steel 24G (inner diameter: 0.31 mm, outer diameter: 0.57 mm) nozzle is attached to the lower end of the container. Electric heaters that can control the temperature of different systems were wound around the outside of the container and the outside of the nozzle. A temperature controller was connected to each heater, and the pitch temperature in the container and the pitch temperature in the nozzle were controlled to be 350 ° C.

A ground electrode (conveyor type collector) was placed at a position of 60 mm directly below the nozzle, and a voltage of 25 kV generated by a high voltage generator was applied between the collector and the nozzle. In this state, 0.1 MPa of pressurized nitrogen was introduced into the container, and the melt pitch in the container was discharged from the nozzle to the collector for spinning. As a result, spinning proceeded satisfactorily and an extremely fine pitch fiber was obtained.
Next, an infusibilization step, a carbonization step, and a graphitization step for the obtained pitch fiber were performed in the same manner as in No. 1. As a result, a carbon fiber having a minimum diameter of 500 nm and a maximum diameter of 600 nm was obtained. The obtained carbon fiber had an average diameter of 550 nm. These values were measured by the same method as No. 1.

The carbon fibers thus obtained were pulverized in an agate mortar so that the average fiber length was 8 μm. That is, the aspect ratio (average value) of this carbon fiber is 15 (8000/550 = 14.5). When this carbon fiber was applied to an X-ray diffractometer and the (002) plane distance was measured, it was 0.3462 nm.
This carbon fiber was mixed so as to be 3 parts by mass with respect to 100 parts by mass of the same mesophase spherulite graphitized product used in No. 1. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A portion of this pasty carbon material was taken, and the contained carbon fiber portion was separated and recovered, and SEM image analysis was performed. As a result, the carbon fibers were present in a substantially linear form without agglomerating to form secondary particles, and the average length of the carbon fibers was 6 μm.
Using this pasty carbon material, a test electrode was obtained in the same manner as in No. 1. And when the porosity and penetration rate were measured by the same method as No. 1 using the obtained test electrode whose electrode density is 1.91 g / cm ³ , the porosity K is 0.122 (12 2%) and the permeation rate was 240 seconds.

In addition, by interposing a porous separator between the obtained test electrode having an electrode density of 1.91 g / cm ³ and the lithium foil used as a counter electrode, and immersing in a non-aqueous electrolyte solution, A test battery was prepared. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of the test battery were examined, the discharge capacity was 345 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.5%. The charge acceptance was 35.4% at 1.0 C, and the discharge rate was 92.0% at 2.0 C discharge.

[Sample No.4]
As carbon fiber (VGCF) produced by the vapor phase growth method, one having a diameter of 140 nm to 160 nm and an average fiber length of 3 μm was obtained. The carbon nanofibers were subjected to an X-ray diffractometer and the (002) plane spacing was measured to be 0.3383 nm.
The average diameter of this carbon fiber is 150 nm, and the aspect ratio (average value) of this carbon fiber is 20 (3000/150).
This carbon fiber was mixed so as to be 3 parts by mass with respect to 100 parts by mass of the same mesophase spherulite graphitized product used in No. 1. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A portion of this pasty carbon material was taken, and the contained carbon fiber portion was separated and recovered, and SEM image analysis was performed. As a result, the carbon fibers agglomerated to form secondary particles having an average particle diameter of 18 μm and existed in a pill-like form.
Using this pasty carbon material, a test electrode was obtained in the same manner as in No. 1. And when the porosity and penetration rate were measured by the same method as No. 1 using the obtained test electrode with an electrode density of 1.92 g / cm ³ , the porosity K was 0.117 (11 0.7%) and the penetration rate was 280 seconds.

Moreover, by interposing a porous separator between the obtained test electrode having an electrode density of 1.92 g / cm ³ and the lithium foil used as a counter electrode, A test battery was prepared. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 343 mAh / g, and the load reverse capacity was 22 mAh / g. The charge / discharge efficiency calculated from these values was 94.0%. The charge acceptance was 29.2% at 1.0 C, and the discharge rate was 89.0% at 2.0 C discharge.

[Sample No.5]
Graphitized mesophase spherules (no carbon fiber mixed) used in No. 1 and polyvinylidene fluoride (PVdF), a binder, are graphitized mesophase spherules by mass ratio: PVdF = 90: Mixing was performed so that the ratio was 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.
A test electrode was obtained by the same method as No. 1 except that this pasty carbon material was used. And when the porosity and penetration rate were measured by the same method as No. 1 using the obtained test electrode with an electrode density of 1.95 g / cm ³ , the porosity K was 0.104 (10 4%) and the permeation rate was 1500 seconds.

In addition, by interposing a porous separator between the obtained test electrode having an electrode density of 1.95 g / cm ³ and the lithium foil used as a counter electrode, and immersing in a non-aqueous electrolyte, A test battery was prepared. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 344 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.5%. The charge acceptance was 25.1% at 1.0C and the discharge rate was 87.4% at 2.0C discharge.

[Sample Nos. 6-10]
First, in the same method as No. 2, pitch was spun by a melt blown method (melting method) to obtain pitch fibers. At that time, the pitch temperature in the container and the pitch temperature in the nozzle were controlled to be 320 ° C. for Nos. 6 and 7 and 300 ° C. for Nos. 8-10. Also, nitrogen gas preheated to 350 to 360 ° C. is flown into the outer cylinder at a gap between the nozzle tips so that the linear velocity is 1200 m / s for No. 6 and 7 and 700 m / s for No. 8 to 10. did.

Next, the infusibilization step and the carbonization step for the obtained pitch fiber were performed in the same manner as in No. 1. No graphitization step was performed. Thereby, in No. 6 and No. 7, carbon fibers having a minimum diameter of 300 nm and a maximum diameter of 600 nm were obtained. In Nos. 8 to 10, carbon fibers having a minimum diameter of 400 nm and a maximum diameter of 1000 nm were obtained.
The carbon fibers thus obtained were pulverized by a jet mill so that the average fiber lengths of Nos. 6 to 10 were 10 μm, 20 μm, 10 μm, 30 μm, and 150 μm, respectively. That is, the aspect ratios of the obtained carbon fibers were Nos. 6 to 10, and were 22, 44, 14, 43, and 214, respectively. Moreover, when this carbon fiber was applied to the X-ray-diffraction apparatus and the (002) plane space | interval was measured, it was 0.3462 nm.

  Each carbon fiber was mixed so that it might be 3 mass parts with respect to 100 mass parts of graphitized products of the same mesophase spherules used in No. 1. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. PVdF was dissolved by adding N-methylpyrrolidone to each mixture to obtain a pasty carbon material.

A portion of each pasty carbon material was taken, and the contained carbon fiber portion was separated and collected, and SEM image analysis was performed. As a result, for Nos. 6-9, the carbon fibers are present in a substantially linear form without agglomerating to form secondary particles, and the average length of the carbon fibers is No. 6-9. , 8 μm, 18 μm, 8 μm and 25 μm, respectively.
Regarding No. 10, the carbon fiber was in a linear form and the carbon fiber was agglomerated to form secondary particles (diameter: 12 to 13 μm). The linear carbon fiber had an average length of 120 μm.

Using each pasty carbon material, a test electrode was obtained in the same manner as in No. 1. Then, the electrode density of the resulting test electrodes Nanba6～10, respectively ^{1.80g / cm 3, 1.60g / cm} 3, 1.70g / cm 3, 1.60g / cm 3 1.70g When the porosity and the permeation rate were measured in the same manner as in No. 1 using a sample having a / cm ³ , the porosity K was No. 6 to 10 and 0.169 (16.9%) and 0. 0, respectively. 261 (26.1%), 0.215 (21.5%), 0.259 (25.9%), 0.220 (22.0%). The permeation rates were Nos. 6 to 10 and were 200 seconds, 150 seconds, 160 seconds, 150 seconds, and 190 seconds, respectively.

In addition, among each of the test electrodes obtained in Nos. 6 to 10, a porous separator is interposed between the electrode density measured as described above and the lithium foil used as a counter electrode, and non-aqueous Each test battery No. 6 to 10 was produced by immersing in an electrolytic solution. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of these test batteries were examined, in No. 6, the discharge capacity was 345 mAh / g and the load reverse capacity was 21 mAh / g. The charge / discharge efficiency calculated from these values was 94.3%. The charge acceptance was 35.2% at 1.0 C, and the discharge rate was 91.0% at 2.0 C discharge. In No. 7, the discharge capacity was 344 mAh / g and the reverse load capacity was 19 mAh / g. The charge / discharge efficiency calculated from these values was 94.8%. The charge acceptance was 35.3% at 1.0 C and the discharge rate was 91.4% at 2.0 C discharge.

In No. 8, the discharge capacity was 345 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.5%. The charge acceptance was 35.3% at 1.0 C and the discharge rate was 91.4% at 2.0 C discharge. In No. 9, the discharge capacity was 343 mAh / g, and the load reverse capacity was 19 mAh / g. The charge / discharge efficiency calculated from these values was 94.8%. Further, the charge acceptability was 35.3% at 1.0 C, and the discharge rate was 91.8% at 2.0 C discharge.
In No. 10, the discharge capacity was 345 mAh / g, and the reverse load capacity was 19 mAh / g. The charge / discharge efficiency calculated from these values was 94.8%. Further, the charge acceptance was 33.0% at 1.0C, and the discharge rate was 90.1% at 2.0C discharge.

[Sample No. 11]
As carbon fiber (VGCF) produced by the vapor phase growth method, those having a diameter of 140 nm to 160 nm and an average fiber length of 10 μm were obtained. The carbon nanofibers were subjected to an X-ray diffractometer and the (002) plane spacing was measured to be 0.3383 nm.
This carbon fiber has an average diameter of 150 nm and an aspect ratio of 67 (10000/150 = 66.6).
This carbon fiber was mixed so as to be 3 parts by mass with respect to 100 parts by mass of the same mesophase spherulite graphitized product used in No. 1. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A portion of this pasty carbon material was taken, and the contained carbon fiber portion was separated and recovered, and SEM image analysis was performed. As a result, the carbon fibers agglomerated to form secondary particles having an average particle diameter of 25 μm and existed in a pill-like form.
Using this pasty carbon material, a test electrode was obtained in the same manner as in No. 1. And when the porosity and penetration rate were measured by the same method as No. 1 using the obtained test electrode whose electrode density is 1.70 g / cm ³ , the porosity K is 0.219 (21 9%) and the penetration rate was 200 seconds.

In addition, by interposing a porous separator between the obtained test electrode having an electrode density of 1.70 g / cm ³ and the lithium foil used as a counter electrode, and immersing in a non-aqueous electrolyte, A test battery was prepared. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 344 mAh / g and the load reverse capacity was 25 mAh / g. The charge / discharge efficiency calculated from these values was 93.2%. Further, the charge acceptance was 29.4% at 1.0 C, and the discharge rate was 89.5% at 2.0 C discharge.

  The composition of the obtained test electrodes No. 1 to No. 11 (the aspect ratio of the carbon fibers present in the active material layer and the carbon fibers before being mixed with the binder), electrode characteristics and battery characteristics are shown in the table below. 1 is shown collectively. Further, as the cycle characteristics of the test batteries No. 1 to No. 11, the capacity retention rate after 50 cycles was examined. The results are also shown in Table 1 below.

From these test results, the following can be understood.
Test electrodes No. 1 to No. 4 and 6 to No. 11 containing carbon fibers in the negative electrode active material layer are compared with No. 5 test electrodes not containing carbon fibers in the negative electrode active material layer. In addition, the porosity is high and the penetration rate is fast. The test batteries No. 1 to No. 4 and No. 6 to No. 11 having test electrodes containing carbon fibers in the negative electrode active material layer as negative electrodes contain carbon fibers in the negative electrode active material layer. Compared with the No. 5 test battery having a test electrode that was not used as the negative electrode, excellent battery characteristics were obtained.

  Of the test batteries No. 1 to No. 4 and No. 6 to No. 11, the test batteries No. 1 to No. 3 and No. 6 to No. 9 are added to the negative electrode active material layer. Particularly excellent battery characteristics are obtained when the carbon fibers are linear and have an average diameter in the range of 100 nm to 1000 nm. In addition, since the aspect ratio of the carbon fiber before mixing with the binder is 214, the No. 10 test battery is mixed with the binder to form a secondary particle after mixing with the binder. By being included, battery characteristics (particularly capacity retention rate) were low.

DESCRIPTION OF SYMBOLS 1 Raw material container 2 Discharge device 21 Main body 22 Nozzle 23 Piping 3 Collector 31 Insulation mesh belt 32 Metal plate 4 High voltage power supply 5 Piping 6 Heater

Claims (6)

A negative electrode for a lithium ion secondary battery having a negative electrode active material layer containing an active material made of a carbon material, carbon fiber, and a binder,
The carbon fiber is obtained through a spinning process in which a raw material containing carbon is spun by a solution method or a melting method, and is linear and has an average diameter in the range of 100 nm to 1000 nm. Battery negative electrode. A negative electrode for a lithium ion secondary battery having a negative electrode active material layer containing an active material made of a carbon material, carbon fiber, and a binder,
The carbon fiber is obtained through a spinning process of spinning pitch by a solution method or a melting method to obtain pitch fiber, an infusible process of pitch fiber, and a process of heat-treating the infusible pitch fiber in an inert gas atmosphere. The negative electrode for a lithium ion secondary battery is linear and has an average diameter in the range of 100 nm to 1000 nm.   The said negative electrode active material layer was obtained through the process of apply | coating the paste-form mixture containing the carbon fiber whose aspect ratio is 50 or less, the said active material, and a binder to a collector. The negative electrode for lithium ion secondary batteries as described.   4. The lithium according to claim 1, wherein the negative electrode active material layer contains the carbon fiber at a ratio of 0.5 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the active material. Negative electrode for ion secondary battery. Electrode density is 1.6-2.0 g / cm < ³ >, The negative electrode for lithium ion secondary batteries of any one of Claims 1-4.   The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of any one of Claims 1-5.

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