JP7107809B2 - Positive electrode for secondary battery - Google Patents

Description

The present disclosure relates to positive electrodes for secondary batteries.

2. Description of the Related Art In recent years, with the increase in demand for portable devices, there is a demand for higher capacity and higher energy density in secondary batteries as power sources for such devices. Attempts to increase the electrode density in secondary batteries are known to meet these demands.

Patent Document 1 discloses a positive electrode for a lithium battery comprising an active material capable of intercalating and deintercalating lithium ions, a carbon-based conductive agent, and a binder, wherein the amount of the carbon-based conductive agent added is 0.1 to 0.1 in the entire positive electrode. A positive electrode for a lithium-based battery is disclosed, which is characterized by containing 2% by mass of carbon fibers having an average fiber diameter of 1 to 200 nm in a carbon-based conductive additive. The document describes that the inclusion of carbon fibers in the positive electrode for lithium-based batteries suppresses a decrease in the permeability of the electrolytic solution and retains the electrolytic solution better even when the density is increased. .

JP-A-2006-86116

However, the lithium secondary battery using the positive electrode disclosed in Patent Document 1 has a problem of high cell resistance.
In view of the above circumstances, an object of the present disclosure is to provide a positive electrode that reduces the cell resistance of a secondary battery when used in the secondary battery.

A positive electrode for a secondary battery of the present disclosure contains carbon fibers having a fiber diameter of 180 nm or more and 850 nm or less and has a nonwoven fabric structure; and a positive electrode active material that has been converted into a positive electrode active material.

According to the positive electrode for a secondary battery of the present disclosure, a carbon fiber aggregate having a fiber diameter within a specific range and a nonwoven fabric structure, and a positive electrode active material contained in the carbon fiber aggregate and composited with the carbon fiber aggregate When used in a secondary battery, the cell resistance of the secondary battery can be reduced.

1 is an SEM image of a composite of a carbon fiber assembly and a positive electrode active material used for a positive electrode for a secondary battery of Example 1. FIG.

A positive electrode for a secondary battery of the present disclosure contains carbon fibers having a fiber diameter of 180 nm or more and 850 nm or less and has a nonwoven fabric structure; and a positive electrode active material that has been converted into a positive electrode active material.

Normally, inside an electrode manufactured using a mixture of active material particles (particle size of several μm) and carbon-based conductive agent fine particles (particle size of several tens of nm), there are voids due to the difference in size between these dissimilar materials. do. During charging and discharging of a secondary battery including the electrode, ions are conducted through the electrolyte that has permeated the voids. Since the ionic conduction path is formed depending on the location of the gap, the actual ionic conduction path is longer than the straight line distance perpendicular to the electrode cross section, resulting in a decrease in ionic conductivity.
Conventionally, for the purpose of improving electrical conductivity and securing voids between active material particles, there has been known a carbon-based conductive additive that uses carbon fiber as a part (Patent Document 1). However, as mentioned above, there is a problem in terms of cell resistance.
Further, in general, in an electrode material mixture containing carbon fibers, it is difficult to uniformly disperse the fibers during mixing. In the electrode material mixture, aggregation and segregation of the carbon fibers may rather reduce the ionic conductivity.
Furthermore, since the active material particles and the carbon fiber conductive aid are usually only in point contact, it is difficult to improve the electron conductivity by simply mixing them.
In order to solve these problems, the positive electrode for a secondary battery of the present disclosure contains a specific carbon fiber aggregate and a positive electrode active material, and when used in a secondary battery, the secondary battery It is a positive electrode capable of reducing cell resistance and improving rate characteristics of the secondary battery.

The carbon fiber aggregate in the present disclosure contains carbon fibers with a fiber diameter of 180 nm or more and 850 nm or less, and has a nonwoven fabric structure. The “nonwoven fabric structure” in the present disclosure includes a structure in which carbon fibers are laminated while being entangled with each other.
The carbon fiber aggregate is not particularly limited as long as it has a nonwoven fabric structure and contains carbon fibers having a fiber diameter within the above range. Although the method for producing the carbon fiber aggregate is not particularly limited, it is preferable to use a carbon fiber aggregate produced by an electrospinning method.
Carbon fiber aggregates produced by electrospinning generally form nonwoven structures. Therefore, it is considered that the void structure inside the carbon fiber aggregate has through holes in the cross-sectional direction of the structure, and the ion conduction paths formed along the through holes are more linear. Therefore, in a secondary battery containing such a carbon fiber assembly in the positive electrode, ionic conduction proceeds rapidly during charging and discharging, leading to improvement in ionic conductivity and reduction in cell resistance.
In addition, the carbon fiber assembly produced by the electrospinning method has a better contact state between the positive electrode active material and the carbon fibers than when using a composition in which the positive electrode active material and the carbon fibers are simply mixed. It is considered that the electron conductivity inside the positive electrode is also improved.
Furthermore, in the carbon fiber assembly having a straighter ionic conduction path, the permeation speed of the electrolytic solution also increases. Therefore, when manufacturing a secondary battery containing such a carbon fiber assembly in the positive electrode, the time spent in the process of injecting the electrolyte into the secondary battery, the activation process of the secondary battery containing the electrolyte, etc. can be shortened.

When the carbon fiber assembly is produced by the electrospinning method, the carbon raw material is not particularly limited, and examples thereof include carbon black (CB) and carbon nanotube (CNT).
In this case, the spinning polymer is not particularly limited, but examples include polyvinyl alcohol (PVA), polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), polyacrylonitrile (PAN), and the like. Among these, polyvinyl alcohol (PVA) is preferable because it is suitable for the electrospinning method.

The carbon fiber aggregate may consist of carbon fibers only, or may further contain materials other than carbon fibers. Materials other than carbon fibers in the carbon fiber assembly include, for example, the above spinning polymers and derivatives thereof. However, if the content of materials other than carbon fibers is too high, the electrode reaction may be hindered, so the content of materials other than carbon fibers in the carbon fiber assembly should be 50% by mass or less. is preferably 30% by mass or less, more preferably 10% by mass or less, and it is even more preferable that the carbon fiber assembly does not contain materials other than carbon fibers.

The fiber diameter of the carbon fibers in the carbon fiber aggregate is usually 180 nm or more and 850 nm or less, preferably 190 nm or more and 800 nm or less, more preferably 200 nm or more and 750 nm or less.
When a carbon fiber aggregate is produced by an electrospinning method, the fiber diameter of the obtained carbon fibers is preferably larger than the average particle diameter of the carbon material particles as a raw material. For example, when carbon black (average particle diameter: 110 nm) is used as the carbon raw material, the fiber diameter of the obtained carbon fibers is preferably 110 nm or more. In this case, considering the particle size distribution of the carbon black, the fiber diameter of the obtained carbon fibers is preferably 180 nm or more.
Further, when a carbon fiber aggregate is produced by an electrospinning method, the fiber diameter of the carbon fibers can be increased by increasing the concentration of the spinning polymer in the spinning solution. Spinning of carbon fibers having a large fiber diameter requires a correspondingly high voltage. If the fiber diameter of the carbon fibers is 850 nm or less, the carbon fiber assembly can be produced by the electrospinning method without any problem.
The larger the fiber diameter of the carbon fibers, the higher the strength of the non-woven fabric structure, but the smaller the voids in the carbon fiber aggregate. On the other hand, the finer the fiber diameter of the carbon fiber, the more voids can be secured in the carbon fiber assembly, and a positive electrode having many through-holes in the cross-sectional direction can be obtained, but the strength of the non-woven fabric structure is low.
The fiber diameter of the carbon fibers in the carbon fiber aggregate is calculated, for example, from the SEM image of the carbon fiber aggregate. Examples of SEM observation conditions are as follows.
・Scanning electron microscope: (Nikon Instech Co., Ltd., Quanta200FEG)
・Acceleration voltage: 5 kV
・Magnification: 20000 times

The carbon raw material is preferably a hydrophilic material. The reason for this is that a highly hydrophobic carbon raw material separates from other materials (for example, hydrophilic polymers) during the preparation of a composition to be subjected to the electrospinning method. This is because there is a risk that the process will not progress sufficiently. The carbon raw material may be previously subjected to a hydrophilization treatment.

The positive electrode active material in the present disclosure is included in the carbon fiber aggregate and composited with the carbon fiber aggregate. Here, the state in which the positive electrode active material is included in the carbon fiber aggregate includes an embodiment in which the positive electrode active material is incorporated inside the carbon fiber aggregate. Further, the state in which the positive electrode active material is combined with the carbon fiber aggregate includes an embodiment in which the positive electrode active material is entangled and integrated with the carbon fiber aggregate.
As described above, in the carbon fiber aggregate, the positive electrode active material is entangled with the carbon fibers, so it is presumed that the contact state between the positive electrode active material and the carbon fiber aggregate is improved.

The positive electrode active material is not particularly limited as long as it is capable of intercalating and deintercalating lithium ions, and examples thereof include _{LiMnxNiyCozO2} ( _x = _y = _z =1/3).
The content ratio of the positive electrode active material and the carbon fiber aggregate in the positive electrode for secondary battery is preferably (positive electrode active material):(carbon fiber aggregate)=96:4 to 80:20. By setting the content ratio within this range, a large amount of the positive electrode active material can be secured and a high energy density can be maintained, and sufficient conductivity can be imparted to the positive electrode for a secondary battery.
When the electrospinning method is adopted in the method for producing a positive electrode for a secondary battery, the mixing ratio of the positive electrode active material and the carbon raw material to be subjected to the electrospinning method realizes the above-described content ratio of the positive electrode active material and the carbon fiber aggregate. It is good if the ratio is possible.

As shown in FIG. 1, which will be described later, in a positive electrode for a secondary battery manufactured using an electrospinning method, a positive electrode active material is incorporated inside a carbon fiber aggregate having a non-woven fabric structure, and is entangled with the carbon fiber aggregate. are unified. As described above, the positive electrode active material is woven into the carbon fiber aggregate during spinning by the electrospinning method, so that the positive electrode active material is incorporated into the nanofiber non-woven fabric structure while being entangled. Therefore, the contact state between the positive electrode active material and the carbon fiber assembly is good, and the conductivity of the positive electrode as a whole is also good, which is considered to contribute to the improvement of the performance.

Evaluation of the positive electrode for a secondary battery of the present disclosure includes electronic conductivity evaluation, liquid permeation rate evaluation, and porosity measurement.
The method for evaluating the electron conductivity of the positive electrode is as follows. A laminate is produced by stacking two dry positive electrodes and applying a predetermined pressure. The resistance value of the obtained laminate is measured, and the electron conductivity of the positive electrode is calculated from the obtained resistance value.

The evaluation method of the liquid permeation rate of the positive electrode is as follows. The positive electrode is processed into a strip of a predetermined size, and one end of the electrode in the longitudinal direction is immersed in propylene carbonate (PC). From the mass permeated by PC (liquid permeation mass) and the time for PC to permeate, the liquid permeation rate is calculated based on the following formula (A).
Formula (A)
v = m/( ^t1 /2)
(In the above formula (A), v represents liquid permeation speed (g/(h ^1/2 )), m represents liquid permeation mass (g), and t represents liquid permeation time (h).)

The method for calculating the porosity of the positive electrode is as follows.
Porosity (%) = (1-m/M) x 100
Density of positive electrode (g/cm ³ ): m=w/L×10
True density of positive electrode (g/cm ³ ): M=(n+c+p)/{(n/N)+(c/C)+(p/P)}
N: Density of positive electrode active material (g/cm ³ )
C: Density of carbon fiber (g/cm ³ )
P: Density of polymer (g/cm ³ )
n: Proportion of the positive electrode active material contained in the positive electrode when the total mass of the positive electrode is 100% by mass (% by mass)
c: Ratio of carbon fiber contained in the positive electrode when the total mass of the positive electrode is 100% by mass (% by mass)
p: Proportion of the polymer contained in the positive electrode when the total mass of the positive electrode is 100% by mass (% by mass)
When the total mass of the positive electrode is 100% by mass, n+c+p=100 (% by mass).
w: weight of positive electrode (mg/cm ² )
L: thickness of positive electrode (μm)
The porosity of the positive electrode may be 25-50%. If the porosity is less than 25%, the porosity becomes too low, and the effect of improving the ionic conductivity by the nonwoven fabric structure is hardly exhibited. If the porosity exceeds 50%, the strength of the obtained carbon fiber aggregate may decrease.

An example of manufacturing a positive electrode for a secondary battery using the electrospinning method is as follows. It should be noted that the positive electrode for a secondary battery of the present disclosure is not limited to those produced by the production examples below.
First, an aqueous polymer solution is prepared by dissolving a spinning polymer in ion-exchanged water. At this time, the fiber diameter of the obtained carbon fibers can be adjusted by the concentration of the aqueous polymer solution. The higher the concentration of the aqueous polymer solution, the thicker the fiber diameter that can be spun. From the viewpoint of speeding up the spinning process, the concentration of the aqueous polymer solution may be 2% by mass to 15% by mass.
The positive electrode active material and the carbon raw material are mixed with this aqueous polymer solution to uniformly prepare the resulting positive electrode material solution (spinning solution).
This positive electrode material solution is fibrillated into a non-woven fabric by an electrospinning method, and a composite of a carbon fiber aggregate having a non-woven fabric structure and a positive electrode active material is deposited on the surface of the collector. Conditions for the electrospinning method are, for example, as follows.
・ Electrospinning device: (eg, MEC Co., Ltd., NANON-03)
・Collector: Aluminum foil (positive electrode current collector)
・Nozzle-collector voltage: 30 kV
・Distance between nozzle and collector: 5 cm
・Spinning time: 5 minutes ・Weight per unit area: 5 mg/cm ²
After that, the laminate (positive electrode precursor) of the composite and the positive electrode current collector is subjected to firing. A part or all of the spinning polymer covering the fiber surfaces in the composite is thermally decomposed and lost by baking, so that a non-woven fabric structure containing carbon fibers with excellent conductivity is formed. Further, by appropriately pressing the baked positive electrode precursor, a positive electrode for a secondary battery containing a carbon fiber aggregate having a non-woven fabric structure and a positive electrode active material entangled and integrated with the carbon fiber aggregate can be obtained. be done.

The negative electrode contains at least a negative electrode active material and, if necessary, may contain a binder, a thickener, and the like.
Examples of negative electrode active materials include carbonaceous materials, metallic lithium, lithium alloys (composite metals), oxides containing no Li atoms, and oxides containing Li atoms.
The binder is not particularly limited, and examples thereof include styrene-butadiene rubber (SBR).
The thickener is not particularly limited, and examples thereof include carboxymethylcellulose (CMC).

A separator is present between the positive and negative electrodes. Ionic conduction occurs between the positive electrode active material and the negative electrode active material through the separator.
Examples of separators include porous films containing polyethylene terephthalate.

A non-aqueous electrolyte may be used in the secondary battery.
Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and mixed solvents thereof can be used as the solvent for the non-aqueous electrolyte.
Examples of the solute (electrolyte) of the non-aqueous electrolyte include lithium salts such as _LiPF6 .

A secondary battery can be manufactured by disposing a positive electrode on one side of a separator and a negative electrode on the other side of the separator, and adding the non-aqueous electrolyte described above to the resulting laminate. The obtained secondary battery may be entirely sealed by using a laminate film as an outer package.

For the secondary battery of the present disclosure, for example, the rate characteristics of the secondary battery can be evaluated by measuring the cell resistance as described below.
First, the conditioning of the secondary battery is repeated several cycles under the following conditions.
・Current density: 0.2 mA/cm ²
・Potential window: 3.0 to 4.1 V
Let the discharge capacity in the final cycle be the cell capacity of the secondary battery. Based on the cell capacity, the secondary battery is charged to an SOC of 60%. After that, discharge is performed at several current densities (for example, 0.2 mA/cm ² , 2.0 mA/cm ² , 4.0 mA/cm ² , 10 mA/cm ² , 20 mA/cm ² , etc.) for 10 seconds each. .
The cell resistance (IV resistance) is calculated from the voltage drop ΔV at each current density and the current value. It can be said that the lower the calculated cell resistance, the better the rate characteristics of the secondary battery.

In this way, the positive electrode for a secondary battery of the present disclosure contains the above-described composite of the specific carbon fiber aggregate and the positive electrode active material, so that when used in a secondary battery, the cell of the secondary battery It is possible to provide a secondary battery that can reduce the resistance and has excellent rate characteristics.

EXAMPLES Hereinafter, the present disclosure will be described in more detail with reference to Examples, but the present disclosure is not limited only to these Examples.

[Example 1]
1. Production of Positive Electrode for Secondary Battery First, polyvinyl alcohol (PVA, polymer for spinning, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) was dissolved in ion-exchanged water to prepare a 5% by mass polyvinyl alcohol aqueous solution.
On the other hand, _{LiMnxNiyCozO2} ( _x = _y = _z = 1/3) was used as the positive electrode active material, and hydrophilic carbon black (CB, average particle size: 110 nm) was used as the carbon raw material to form the carbon fiber aggregate. ) were prepared respectively. These positive electrode materials were mixed in a ratio of positive electrode active material:carbon raw material=90% by mass:10% by mass. The resulting mixture was added to the above 5% by mass polyvinyl alcohol aqueous solution, and stirred and kneaded until the entire solution was uniform.
A uniformly prepared positive electrode material solution was made into fibers and non-woven fabrics by an electrospinning method, and a composite of a carbon fiber aggregate having a non-woven fabric structure and a positive electrode active material was deposited on the collector surface. Detailed conditions of the electrospinning method are as follows.
・Electrospinning device: MEC Co., Ltd., NANON-03
- Collector: aluminum foil (positive electrode current collector, thickness: 15 μm)
・Nozzle-collector voltage: 30 kV
・Distance between nozzle and collector: 5 cm
・Spinning time: 5 minutes ・Weight per unit area: 5 mg/cm ²

A laminate (positive electrode precursor) of the composite and the positive electrode current collector was subjected to firing. Detailed conditions of the firing process are as follows.
・Baking temperature: 230°C
・Firing time: 1 hour ・Firing atmosphere: air

The fired laminate was pressed to produce a positive electrode for a secondary battery of Example 1. The detailed conditions of the pressing process are as follows using a roll press machine.
・Press pressure: Line pressure 300 kN/m
・Press time: roll rotation speed 3m/min
The porosity of the positive electrode for secondary battery of Example 1 was 35%.

FIG. 1 is an SEM image of a composite of a carbon fiber assembly and a positive electrode active material used for the positive electrode for a secondary battery of Example 1. FIG. The SEM observation conditions are as follows.
・Scanning electron microscope: Nikon Instech Co., Ltd., Quanta200FEG
・Acceleration voltage: 5 kV
・Magnification: 20000 times The center of FIG. 1 shows the positive electrode active material incorporated into the carbon fiber. As is clear from the drawing direction of the carbon fibers in FIG. 1, the carbon fiber aggregate used in Example 1 had a nonwoven fabric structure. The positive electrode active material was included in the nonwoven fabric structure and composited with the carbon fiber assembly. More specifically, the positive electrode active material was incorporated into the carbon fiber aggregate while being entangled with the carbon fiber aggregate. From FIG. 1, the fiber diameter of the carbon fibers in the carbon fiber assembly was 280 nm.

2. Manufacture of Secondary Battery First, the following negative electrode materials are mixed, and the viscosity is appropriately adjusted with deionized water to prepare a negative electrode mixture paste.
・Negative electrode active material: artificial graphite (C)
・Thickening agent: carboxyl methyl cellulose (CMC)
・Binder: Styrene-butadiene rubber (SBR)
The mixing ratio of the negative electrode active material, the thickener and the binder was (negative electrode active material):(thickener):(binder)=98% by mass:1% by mass:1% by mass.
The prepared negative electrode mixture paste was applied to the surface of a copper foil (thickness: 10 μm) with a comma coater and dried at 160° C. to produce a negative electrode.

As a separator, a porous membrane film (thickness: 25 μm) made of polyethylene terephthalate was prepared.
As the solvent for the non-aqueous electrolyte, a solvent obtained by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of EC:DMC:EMC=1:1:1. was used. This mixed solvent was mixed with LiPF ₆ as a lithium salt electrolyte at a concentration of 1 mol/L to prepare a non-aqueous electrolyte.

First, slit grooves were formed in each of the positive electrode for a secondary battery of Example 1 and the negative electrode. Next, the electrode compound material at the ends of both electrodes was peeled off from the collector foil by a physical method, and then the collector lead was joined by ultrasonic welding. The obtained positive electrode and negative electrode were opposed to each other with a separator interposed therebetween to produce a laminate. The resulting laminate was impregnated with a non-aqueous electrolyte and sealed with a laminate film as an outer packaging to produce a secondary battery of Example 1.

[Example 2]
In "1. Production of positive electrode for secondary battery" in Example 1, the concentration of the polyvinyl alcohol aqueous solution was changed from 5% by mass to 12% by mass, and the fiber diameter of the carbon fibers in the carbon fiber aggregate was changed to 280 nm. A positive electrode for a secondary battery and a secondary battery of Example 2 were manufactured in the same manner as in Example 1, except that the thickness was changed from to 850 nm.

[Example 3]
In "1. Production of positive electrode for secondary battery" in Example 1, the concentration of the polyvinyl alcohol aqueous solution was changed from 5% by mass to 2% by mass, and the fiber diameter of the carbon fibers in the carbon fiber aggregate was 280 nm. A positive electrode for a secondary battery and a secondary battery of Example 3 were produced in the same manner as in Example 1, except that the thickness was changed from 180 nm to 180 nm.

[Example 4]
In "1. Production of positive electrode for secondary battery" in Example 1, _{LiMnxNiyCozO2} ( _x = _y = _z = 1/3) was used as the positive electrode active material, and hydrophilic carbon black ( CB, average particle diameter: 110 nm) and carbon nanotubes (CNT, fiber diameter: 10 to 15 nm, length: 10 μm, manufactured by Cnano) are prepared, and these positive electrode materials are used as positive electrode active material: CB: CNT A positive electrode for a secondary battery and a secondary battery of Example 4 were manufactured in the same manner as in Example 1, except that they were mixed in a ratio of =90% by mass:5% by mass:5% by mass.

[Comparative Example 1]
Production of positive electrode for secondary battery” in Example 1, except that the positive electrode material solution was applied to the surface of aluminum foil (positive electrode current collector, thickness: 15 μm) with a comma coater. 1, a positive electrode for a secondary battery and a secondary battery of Comparative Example 1 were manufactured.

[Comparative Example 2]
In "1. Production of positive electrode for secondary battery" in Example 1, the concentration of the polyvinyl alcohol aqueous solution was changed from 5% by mass to 1.5% by mass, and the fiber diameter of the carbon fibers in the carbon fiber aggregate A positive electrode for a secondary battery and a secondary battery of Comparative Example 2 were manufactured in the same manner as in Example 1, except that the thickness was changed from 280 nm to 110 nm.

[Comparative Example 3]
In "1. Production of positive electrode for secondary battery" in Example 1, instead of hydrophilic carbon black (CB, average particle diameter: 110 nm) as a carbon raw material, carbon nanofibers (VGCF, fiber diameter: 150 nm, length: 6 μm, manufactured by Showa Denko), and the positive electrode material solution was applied to the surface of the aluminum foil (positive electrode current collector, thickness: 15 μm) with a comma coater. A positive electrode for a secondary battery and a secondary battery of Comparative Example 3 were manufactured.

[Comparative Example 4]
In "1. Production of positive electrode for secondary battery" of Example 1, instead of hydrophilic carbon black (CB, average particle diameter: 110 nm), mesophase pitch carbon fiber (MCF, fiber diameter: 8.0 nm) was used as the carbon raw material. 5 μm, length: 20 μm, Petka Materials Co., Ltd.), and the positive electrode material solution was applied to the surface of aluminum foil (positive electrode current collector, thickness: 15 μm) with a comma coater. In the same manner as in Example 1, a positive electrode for a secondary battery and a secondary battery of Comparative Example 4 were produced.

3. Evaluation of Positive Electrode for Secondary Battery The positive electrodes for secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to the following measurements.

(1) Electron Conductivity of Positive Electrode Two dried positive electrodes were placed one on top of the other, and a predetermined pressure was applied to produce a laminate. The resistance value of the obtained laminate was measured, and the electronic conductivity was calculated from the obtained resistance value.

(2) Evaluation of Liquid Permeation Rate of Positive Electrode Propylene carbonate (PC), which has almost the same viscosity as the non-aqueous electrolyte used and is a low-volatility electrolyte solvent, was prepared. The positive electrode was processed into a strip of a predetermined size, and one end of the electrode in the longitudinal direction was immersed in PC. The liquid permeation rate was calculated based on the following formula (A) from the permeation mass of PC (liquid permeation mass) and the permeation time of PC.
Formula (A)
v = m/( ^t1 /2)
(In the above formula (A), v represents liquid permeation speed (g/(h ^1/2 )), m represents liquid permeation mass (g), and t represents liquid permeation time (h).)

4. Evaluation of Secondary Battery The secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to the following measurements.
First, the conditioning of the secondary battery was repeated for 3 cycles under the following conditions.
・Current density: 0.2 mA/cm ²
・Potential window: 3.0 to 4.1 V
The discharge capacity at the third cycle was taken as the cell capacity of the secondary battery. Based on the cell capacity, the secondary battery was charged to an SOC of 60%. After that, the batteries were sequentially discharged at the following current densities for 10 seconds each.
・Current density: 0.2 mA/cm ² , 2.0 mA/cm ² , 4.0 mA/cm ² , 10 mA/cm ² , 20 mA/cm ²
The cell resistance (IV resistance) was calculated from the voltage drop ΔV at each current density and the current value.

5. Discussion Table 1 below summarizes the data of the positive electrodes for secondary batteries and the secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 4. In Table 1 below, the values of liquid permeation rate, electronic conductivity and cell resistance are all normalized values with Comparative Example 1 as the standard (100%).
In Table 1 below, the details of the types of carbon raw materials are as follows.
・ CB: hydrophilic carbon black (average particle size: 110 nm)
・ CNT: carbon nanotube (fiber diameter: 10 to 15 nm, length: 10 μm, manufactured by Cnano)
・VGCF: carbon nanofiber (fiber diameter: 150 nm, length: 6 μm, manufactured by Showa Denko)
・MCF: Mesophase pitch carbon fiber (fiber diameter: 8.5 μm, length: 20 μm, manufactured by Petka Materials)

First, Comparative Example 2 will be examined. Comparative Example 2 had a slow liquid permeation rate of 107%, a low electronic conductivity of 107%, and a high cell resistance of 88%. These physical properties of Comparative Example 2 were better than those of Comparative Example 1, but inferior to those of Examples 1 to 4, which will be described later. The reason for this is thought to be that as a result of the fiber diameter of the carbon fibers being too thin at 110 nm, the carbon raw material having a particle size larger than 110 nm (approximately half of the carbon black used) could not be incorporated into the carbon fiber aggregate. . This is considered to mean that when a carbon fiber aggregate having too small a fiber diameter is used, the effects of the electrospinning method cannot be fully enjoyed.

Next, Comparative Example 3 will be examined. Comparative Example 3 had a slow liquid permeation rate of 105%, a low electronic conductivity of 105%, and a high cell resistance of 90%. All of these physical properties of Comparative Example 3 were inferior to those of Comparative Example 2 above. The reason assumed for this is as follows.
When conventional carbon fibers such as CB and VGCF are mixed with a positive electrode active material, coated by a normal method, and pressed, it is difficult to randomly integrate these carbon fibers. The reason why Comparative Example 3 is inferior to Comparative Example 2 in the above three physical properties is that the use of CB and VGCF in the production of the positive electrode by the coating method did not cause such random accumulation, and in addition, carbon This is probably because the fibers agglomerate and segregate to deteriorate the ionic conductivity inside the obtained positive electrode.

Next, Comparative Example 4 will be examined. Comparative Example 4 had a slow liquid permeation rate of 97%, a low electronic conductivity of 97%, and a high cell resistance of 106%. These physical properties of Comparative Example 3 were inferior to those of Comparative Example 1. The reason assumed for this is as follows.
When carbon fibers having a fiber diameter that is too large as in Comparative Example 4 are used, ion conduction is inhibited by the carbon fibers themselves, so it is considered difficult to reduce the cell resistance.

On the other hand, in Examples 1 to 4, the liquid permeation rate was 111% or more, which was faster than the conventional one, the electronic conductivity was 115% or more, which was higher than the conventional one, and the cell resistance was 81% or less, which was lower than the conventional one. . The reason assumed for this is as follows.
First, in Comparative Example 1, which is a comparison target, a simple mixture of the positive electrode active material and the carbon raw material was used for the production of the positive electrode, so the actual ion conduction path is longer than the linear distance of the cross section of the positive electrode as described above. As a result, the ion conductivity of the positive electrode for a secondary battery of Comparative Example 1 is low.

On the other hand, as in Examples 1 to 4, a carbon fiber aggregate having a non-woven structure, preferably a carbon fiber aggregate having a non-woven structure formed by making a carbon raw material into long fibers by an electrospinning method and accumulating them. The body is considered to be in a state in which the void structure inside the carbon fiber assembly penetrates along the cross-sectional direction. Therefore, it is thought that ion conduction progresses rapidly during charging and discharging of a secondary battery that includes the carbon fiber aggregate in the positive electrode, resulting in higher ion conductivity and lower cell resistance than before. be done. In Examples 1 to 4, the reason why the liquid permeation rate is higher than that in Comparative Example 1 is considered to be due to the void structure inside the carbon fiber assembly as described above. By having such a void structure, it is considered that the time required for both the step of injecting the electrolytic solution and the step of activating the battery can be shortened in the secondary battery manufacturing process.

In the manufacturing process of the positive electrode of Examples 1 to 4, a mixed solution of the carbon raw material and the positive electrode active material was used to spin these different materials at the same time. Therefore, the positive electrode active material was entangled in the non-woven fabric structure of the resulting carbon fiber assembly. Therefore, it is considered that the contact state between the positive electrode active material and the carbon fiber aggregate is improved, and the electron conductivity inside the obtained positive electrode is improved as compared with the conventional one.

In particular, in the positive electrodes of Examples 1 to 3, the contact area between the carbon fiber assembly and the positive electrode active material is wider than before, and the positive electrode active materials are connected to each other by the carbon fibers at the shortest distance. A lower cell resistance could be realized in the secondary battery that was developed. At the same time, since the fiber diameter of the carbon fibers in the carbon fiber aggregate is in the range of 180 nm or more and 850 nm or less, sufficient voids can be secured in the obtained positive electrode for secondary batteries, and the electrolyte solution etc. can be more easily absorbed in the voids. As a result of being able to store a large amount, it was possible to increase the ionic conductivity and the like of the entire positive electrode.
In particular, in the positive electrode of Example 4, the carbon fiber assembly obtained was reinforced by combining carbon black and carbon nanotubes. As a result, it is believed that the non-woven fabric structure of the carbon fiber aggregate can be maintained for a long period of time.

In addition, in the positive electrodes of Examples 1 to 4, since the carbon fiber aggregate and the positive electrode active material are composited, the carbon fiber aggregate is more effective than the case of using a simple mixture of the carbon fiber and the positive electrode active material. and the positive electrode active material are in better contact with each other.

When the positive electrodes of Examples 1 to 3 are compared, the positive electrode of Example 2, which includes a carbon fiber assembly having a smaller fiber diameter, has the fastest liquid permeation rate, the highest electronic conductivity, and the highest cell resistance. found to be the lowest. The reason for this is thought to be that when the fiber diameter is 180 nm or more, the finer the fiber diameter, the more voids can be secured inside the carbon fiber aggregate.

Claims (1)

A positive electrode for a secondary battery,
a carbon fiber aggregate containing carbon fibers having a fiber diameter of 180 nm or more and 850 nm or less and having a nonwoven fabric structure;
A positive electrode for a secondary battery, comprising a positive electrode active material that is included in the carbon fiber aggregate and composited with the carbon fiber aggregate.

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