JP6790882B2 - Manufacturing method of positive electrode for lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a method for manufacturing a positive electrode for a lithium ion secondary battery.

Japanese Patent Application Laid-Open No. 6-196169 (Patent Document 1) describes a positive electrode for a lithium ion secondary battery (hereinafter, may be simply referred to as “positive electrode”) by impregnating aluminum foam (Al) with a slurry. It discloses that it will be manufactured.

Japanese Unexamined Patent Publication No. 6-196169

Al foil is widely used as a current collector for the positive electrode. Typically, the surface of the Al foil is coated with a slurry and dried to produce a positive electrode. The slurry is a particle dispersion liquid in which a positive electrode mixture is dispersed in a solvent. The slurry is prepared by mixing a positive electrode active material, a conductive material, a binder, a solvent and the like.

A porous Al base material such as foamed Al has also been proposed as a current collector (see Patent Document 1). However, the spread of porous Al base materials has not progressed. This is because it is difficult to increase the energy density of the battery. That is, in order to ensure the impregnation property into the porous Al substrate, the viscosity of the slurry needs to be low to some extent. Therefore, the solid content ratio of the slurry is inevitably low. The solid content ratio indicates the mass ratio of components other than the solvent (that is, the positive electrode mixture). Since the solid content ratio of the slurry is low, it is difficult to densely fill the porous Al substrate with the positive electrode mixture.

An object of the present disclosure is to increase the filling amount of the positive electrode mixture in the porous Al substrate.

Hereinafter, the technical configuration and the action and effect of the present disclosure will be described. However, the mechanism of action of the present disclosure involves estimation. The correctness of the mechanism of action should not limit the scope of this disclosure.

The method for producing a positive electrode for a lithium ion secondary battery of the present disclosure includes the following (A) to (C).
(A) Granules are prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent.
(B) The granules are filled in a porous aluminum substrate.
(C) A positive electrode for a lithium ion secondary battery is manufactured by rolling a porous aluminum base material filled with granules.
The granules are prepared to have a solid content ratio of 85% by mass or more and less than 100% by mass and a median diameter smaller than the average open pore diameter of the porous aluminum substrate.

As the solid content ratio of the slurry increases (that is, the amount of solvent decreases), the viscosity of the slurry increases and the impregnation property decreases. Since the slurry does not penetrate into the inside of the porous Al base material, it is considered that the filling amount of the positive electrode mixture is reduced and the filling amount varies widely.

Contrary to such conventional knowledge, the production method of the present disclosure uses significantly less solvent. As a result, the mixture of the positive electrode mixture and the solvent does not become a slurry (particle dispersion) but becomes an aggregate of particles (that is, "granule"). Further, the granules are prepared to have a median diameter smaller than the average open pore diameter of the porous Al substrate. Therefore, the granules can penetrate into the inside of the porous Al base material. As a result, it is considered that the filling amount of the positive electrode mixture increases as compared with the case where the slurry is used.

If the solid content ratio of the granules is less than 85% by mass, the granules tend to grow at the time of mixing, and it is difficult to realize a small median diameter. As a result, the median diameter of the granules becomes larger than the average open pore diameter of the porous Al base material, and the filling amount may rather decrease. If the solid content ratio is further lowered, the mixture becomes a slurry, and granules may not be prepared. When the solid content ratio is 100% by mass (that is, in the absence of any solvent), the aggregation of particles is not promoted and it is difficult to prepare granules.

FIG. 1 is a flowchart showing an outline of a positive electrode manufacturing method according to the embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode manufacturing apparatus. FIG. 3 is a graph showing the relationship between the filling amount of the positive electrode mixture and the solid content ratio of the granules. FIG. 4 is a graph showing the relationship between the standard deviation of the filling amount and the solid content ratio of the granules. FIG. 5 is a graph showing the relationship between the median diameter of the granules and the solid content ratio of the granules.

Hereinafter, embodiments of the present disclosure (hereinafter, also referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of this disclosure.

<Manufacturing method of positive electrode for lithium ion secondary battery>
FIG. 1 is a flowchart showing an outline of a positive electrode manufacturing method according to the embodiment of the present disclosure. The production method of the present embodiment includes "(A) preparation of granules", "(B) filling", "(C) temporary rolling", "(D) drying" and "(E) main rolling". Hereinafter, the manufacturing method of this embodiment will be described step by step.

<< (A) Preparation of granules >>
The production method of the present embodiment includes preparing granules by mixing a positive electrode active material, a conductive material, a binder, and a solvent.

Granules are prepared, for example, by stirring granulation. Here, a general stirring granulator (for example, "High Speed Mixer" manufactured by EarthTechnica Co., Ltd.) can be used. For example, granules are prepared by charging a positive electrode active material, a conductive material, a binder, and a solvent in a predetermined mass ratio into a stirring tank of a stirring and granulating apparatus, stirring and mixing them.

Preferably, the positive electrode active material and the conductive material are mixed in advance. It is expected that the electron conductivity will be improved by adhering the conductive material to the positive electrode active material. Then, the granules can be prepared by mixing the binder and the solvent with the mixture of the positive electrode active material and the conductive material.

The granules of the present embodiment are prepared so as to have a solid content ratio of 85% by mass or more and less than 100% by mass. If the solid content ratio of the granules is less than 85% by mass, the granules tend to grow during mixing (granulation), and it is difficult to realize a small median diameter. As a result, the median diameter of the granules becomes larger than the average open pore diameter of the porous Al base material, and the filling amount may rather decrease. If the solid content ratio is further lowered, the mixture becomes a slurry, and granules may not be prepared. When the solid content ratio is 100% by mass, the aggregation of particles is not promoted, and it is difficult to prepare granules. The granules are preferably prepared to have a solid content ratio of 85% by mass or more and 95% by mass or less.

Further, the granules are prepared to have a median diameter smaller than the average open pore diameter of the porous Al substrate described later. As a result, the granules can penetrate into the inside of the porous Al base material, and it is considered that the filling amount increases. The "median diameter" in the present specification indicates a cumulative 50% particle size from the fine particle side in a volume-based particle size distribution measured by a laser diffraction / scattering method. Hereinafter, the median diameter is also referred to as "D50". The D50 of the granules can be adjusted by the solid content ratio, the rotation speed of the stirring blade, the rotation speed of the crushing blade, the stirring time, and the like. The rotation speed of the stirring blade may be, for example, 3000 to 6000 rpm (typically 4000 to 5000 rpm). The stirring time may be, for example, 20 to 60 seconds.

The granules are preferably less than 1/2 (1/2) of the average open pore diameter of the porous Al substrate, and more preferably 1/3 (1/3) of the average open pore diameter of the porous Al substrate. It is prepared to have a D50 of less than, most preferably less than 1/4 (1/4) of the average open pore size of the porous Al substrate. When the average open pore diameter of the porous Al substrate is, for example, 600 μm, the granules may be prepared to have a D50 of, for example, 97 μm or more and 121 μm or less.

(Positive electrode active material)
The granules may be prepared to contain, for example, 80-98.5% by weight of positive electrode active material. The positive electrode active material should not be particularly limited. The positive electrode active material, for _{example, LiCoO 2, LiNiO 2, LiMnO} 2, LiMn 2 O 4, in _{LiNi x Co y Mn z O 2} ( wherein, x, y, z are, 0 <x <1,0 <y < 1, 0 <z <1, x + y + z = 1), LiFePO _{4 and the} like may be used. One kind of positive electrode active material may be used alone, or two or more kinds of positive electrode active materials may be used in combination. The positive electrode active material may have, for example, a D50 of 1 to 30 μm.

(Conductive material)
The granules may be prepared to contain, for example, 1 to 15% by weight of conductive material. The conductive material should not be particularly limited. The conductive material may be, for example, acetylene black, thermal black, furnace black, vapor-grown carbon fiber (VGCF), graphite or the like. One kind of conductive material may be used alone, or two or more kinds of conductive materials may be used in combination.

(Bundling material)
The granules may be prepared to contain, for example, 0.5-5% by weight of binder. The binder should not be particularly limited. The binder may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), carboxymethyl cellulose (CMC) or the like. One kind of binder may be used alone, or two or more kinds of binders may be used in combination.

(solvent)
As the solvent, an appropriate solvent should be selected in consideration of the dispersibility or solubility of the binder. For example, when the binder is PVdF, an organic solvent such as N-methyl-2-pyrrolidone (NMP) can be used. For example, when the binder is PAA, CMC or the like, water (pure water) can be used as the solvent.

<< (B) filling >>
The production method of the present embodiment includes filling the granules in a porous Al substrate.

(Porous aluminum base material)
The porous Al base material may be in the form of a sheet. The porous Al substrate may have a thickness of, for example, 0.1 to 1 mm (typically 0.2 to 0.5 mm). The porous Al substrate can be produced, for example, by foaming molten Al and then coagulating it. Alternatively, the porous Al base material can also be produced by subjecting the foamed resin to Al plating and then removing the foamed resin by heat treatment. The porous Al base material may be made of pure aluminum or may be made of an aluminum alloy.

The porous Al substrate may have a three-dimensional network structure. By using the porous Al base material as a current collector, it is expected that the electron conductivity in the thickness direction of the positive electrode will be improved as compared with the case where the Al foil is a current collector.

The porous Al substrate contains a plurality of open pores. The average open pore diameter (sometimes referred to as "nominal pore diameter") of the porous Al substrate is measured as follows. That is, the surface of the porous Al substrate is observed by an electron microscope (SEM) or the like. Twenty open pores are randomly extracted. The ferret diameter of each open pore is measured. The arithmetic mean of the 20 ferret diameters is taken as the average open pore diameter. The porous Al substrate may have, for example, an average open pore diameter of 150 to 1000 μm. From the viewpoint of electron conductivity and strength, the porous Al substrate preferably has an average open pore diameter of 300 μm or more and 900 μm or less, more preferably 450 μm or more and 750 μm or less, and most preferably. Has an average open pore diameter of 550 μm or more and 650 μm or less.

From the viewpoint of energy density, it is desirable that the porous Al substrate has a high porosity.
The porosity is calculated by the following formula:
Porosity (%) = (1-apparent specific gravity of porous Al) ÷ true specific gravity of Al x 100%
Is calculated by. The apparent specific gravity of the porous Al base material is calculated by dividing the mass of the porous Al base material by the apparent volume (= area x thickness) of the porous Al base material. The porous Al substrate preferably has a porosity of 75% or more, more preferably a porosity of 80% or more, and most preferably a porosity of 85% or more. From the viewpoint of mechanical strength, the porous Al substrate may have, for example, a porosity of 90% or less.

The porous Al base material may have a connection portion with a current collector lead. For example, it is conceivable to crush the portion of the porous Al base material that should be the connecting portion by rolling in advance. As a result, the granules cannot penetrate into the portion, so that the current collecting lead can be welded.

(manufacturing device)
FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode manufacturing apparatus. The electrode manufacturing apparatus 100 includes a filling tank 10, a filling roll 20, a temporary rolling roll 30, a drying furnace 40, and a main rolling roll 50. The filling tank 10 is provided with an inlet and an outlet. A plurality of filling rolls 20 are arranged in the filling tank 10. A temporary rolling roll 30 is arranged at the outlet of the filling tank 10. After that, a drying oven 40 and a main rolling roll 50 are arranged. The curved arrows drawn on each roll indicate the direction of rotation of each roll.

The filling tank 10 is filled with granules 1. The porous Al base material 2 is supplied to the filling tank 10 from the inlet. As the filling roll 20 rotates, the granules 1 flow, and the granules 1 enter the porous Al base material 2. That is, the granules 1 are filled in the porous Al base material 2.

<< (C) Temporary rolling >>
The production method of the present embodiment includes rolling a porous Al base material 2 filled with granules 1. By rolling, the granules 1 are fixed in the porous Al base material 2, and the positive electrode (electrode) is completed. The rolling may be carried out only once, or may be carried out in two or more times. For example, the granules 1 are temporarily fixed in the porous Al base material 2 by performing temporary rolling immediately after filling the granules 1. After tentative rolling, drying is carried out. After drying, the positive electrode can be adjusted to the final target thickness by main rolling.

In FIG. 2, the temporary rolling roll 30 is arranged at the outlet of the filling tank 10. It is desirable that the porous Al base material 2 filled with the granules 1 is tentatively rolled immediately after being discharged from the filling tank 10. As a result, the shedding of the granule 1 is suppressed. The compressibility during tentative rolling is preferably 10% or more and 50% or less. The compression ratio is the following formula:
Compressibility (%) = (Thickness before compression-Thickness after compression) ÷ Thickness before compression x 100%
Is calculated by. When the compressibility during tentative rolling is 10% or more, the granules 1 tend to be suppressed from falling off. If the compressibility during tentative rolling (before drying) exceeds 50%, the solvent may seep out. This may require cleaning of the transport path, rolls, etc. However, the exudation of the solvent is considered to have a small effect on the performance and quality of the finished electrode. Therefore, as long as productivity is ignored, the compression ratio may exceed 50%. The compression ratio is more preferably 10% or more and 40% or less, and most preferably 10% or more and 30% or less.

<< (D) Drying >>
The production method of the present embodiment may include drying the positive electrode. The drying oven 40 may be, for example, a hot air type drying oven, an infrared type drying oven, or the like. In the case of the hot air type, the hot air temperature may be, for example, about 100 to 160 ° C. In the present embodiment, since the solid content ratio of the granules 1 is very high, the furnace length of the drying furnace 40 can be shortened. This is expected to reduce the drying cost. Further, in the present embodiment, there may be cases where natural drying is sufficient, or drying is substantially unnecessary.

<< (E) Main rolling >>
The production method of the present embodiment may further include rolling the dried positive electrode. In this rolling, the positive electrode is adjusted to the final thickness. The positive electrode may be finally rolled to have a thickness of, for example, 50-150 μm.

Examples will be described below. However, the following examples do not limit the scope of this disclosure.

<Experiment 1>
<Comparison example>
The following materials were prepared.
Positive Electrode Active Material: Nickel Manganese Lithium Cobalt Acid Conductive Material: Acetylene Black Binder Solution: PVdF NMP Solution Solvent: NMP
Porous Al base material: Foamed Al (average open pore diameter 600 μm, thickness 0.3 mm, porosity 85%)

The positive electrode active material and the conductive material were put into the stirring tank of the stirring granulator. The positive electrode active material and the conductive material were mixed in a dry manner. This gave a powder mixture. Then, the binder solution and the solvent were charged into the stirring tank. The positive electrode active material, the conductive material, the binder and the solvent were mixed. As a result, a slurry was prepared. The composition of the solid content was "positive electrode active material: conductive material: binder = 89.5: 8: 2.5 (mass ratio)". The solid content ratio was 45% by mass.

The slurry was filled in a porous Al substrate. The slurry was dried. The porous Al substrate was rolled. As a result, a positive electrode was manufactured.

<Comparative Examples 2 to 4>
As shown in Table 1 below, the positive electrode was manufactured by the same manufacturing method as in Comparative Example 1 except that the solid content ratio of the slurry was changed.

<Example 1>
<< (A) Preparation of granules >>
The positive electrode active material and the conductive material were put into the stirring tank of the stirring granulator. The positive electrode active material and the conductive material were mixed in a dry manner. This gave a powder mixture. The rotation speed of the stirring blade was 4500 rpm, and the stirring time was 15 seconds. Next, the binder solution was charged into the stirring tank. The positive electrode active material, the conductive material, the binder and the solvent were mixed. The rotation speed of the stirring blade was 4500 rpm, and the stirring time was 20 seconds. As a result, granule 1 was prepared. The composition of the solid content was "positive electrode active material: conductive material: binder = 91: 8: 1 (mass ratio)". The solid content ratio was 85% by mass.

The D50 of the granule 1 was measured by a laser diffraction / scattering type particle size distribution measuring device "Microtrack MT3000II" and a dry sample feeder (both manufactured by Microtrac Bell). The measurement results are shown in Table 1 below.

<< (B) filling, (C) temporary rolling, (D) drying, (E) main rolling >>
The electrode manufacturing apparatus 100 shown in FIG. 2 was prepared. Granule 1 and porous Al base material 2 were supplied to the electrode manufacturing apparatus 100. As a result, the granules 1 were filled in the porous Al base material 2. Further, the porous Al base material 2 filled with the granules 1 was rolled to produce a positive electrode. The compressibility during temporary rolling was set to 20%.

<Examples 2 and 3, Comparative Examples 5 and 6>
As shown in Table 1 below, the positive electrode was manufactured by the same manufacturing method as in Example 1 except that the solid content ratio was changed.

<Evaluation>
Twenty disk samples (2 cm in diameter) were punched from the porous Al substrate by the punching punch. The mass of 20 disc samples was measured by a precision balance. Twenty arithmetic mean values were calculated. Hereinafter, this arithmetic mean value is referred to as a blank value.

Similarly, 20 disc samples were punched out from the positive electrode. The mass of the disk sample was measured with a precision balance. The filling amount of the positive electrode mixture was calculated by subtracting the blank value from the mass of the disk sample. The arithmetic mean and standard deviation of the filling amount were calculated. The results are shown in Table 1 below.

<Result>
As shown in Table 1 above, in the examples, the filling amount of the positive electrode mixture was increased as compared with the comparative example. It is considered that the granules were prepared so as to have a solid content ratio of 85% by mass or more and less than 100% by mass and a D50 smaller than the average open pore size of the porous Al substrate. In the embodiment, not only the filling amount is large, but also the variation (standard deviation) of the filling amount is small.

FIG. 3 is a graph showing the relationship between the filling amount of the positive electrode mixture and the solid content ratio of the granules. FIG. 4 is a graph showing the relationship between the standard deviation of the filling amount and the solid content ratio of the granules. As shown in FIGS. 3 and 4, the behavior of the filling amount with respect to the change in the solid content ratio is completely different between the slurry and the granules. That is, in the slurry, when the solid content ratio exceeds 55% by mass, the higher the solid content ratio, the smaller the filling amount and the larger the standard deviation of the filling amount. On the other hand, in the granules, the higher the solid content ratio, the larger the filling amount and the smaller the standard deviation of the filling amount. When the solid content ratio of the granules is 85% by mass or more, the filling amount is remarkably increased, and the variation in the filling amount is remarkably suppressed.

FIG. 5 is a graph showing the relationship between the median diameter of the granules and the solid content ratio of the granules. In the region where the solid content ratio is 85% by mass or more, the D50 of the granules tends to be remarkably small.

<Experiment 2>
The compressibility at the time of temporary rolling was examined in Examples 1 to 3. The results are shown in Table 2 below. In Table 2 below, "++", "+", "±" and "-" indicate the following contents, respectively.
“++” indicates that the granules did not fall off and the solvent did not exude.
A "+" indicates that the granules were slightly shed or the solvent was slightly exuded.
“±” indicates that the solvent has exuded.
"-" Indicates that the granules have fallen off.

As shown in Table 2 above, it was confirmed that the exudation of the solvent tends to be suppressed at a compression rate of 50% or less. It was confirmed that the shedding of granules tended to be suppressed at a compression rate of 10% or more. Therefore, the compressibility at the time of temporary rolling (before drying) is preferably 10% or more and 50% or less.

It should be considered that the above embodiments and examples are exemplary in all respects and are not restrictive. The scope of the present disclosure is not described above, but is indicated by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 granule, 2 base material, 10 filling tank, 20 filling roll, 30 temporary rolling roll, 40 drying oven, 50 rolling roll, 100 electrode manufacturing equipment.

Claims (1)

To prepare granules by mixing a positive electrode active material, a conductive material, a binder, and a solvent.
The present invention comprises the production of a positive electrode for a lithium ion secondary battery by filling the porous aluminum base material with the granules and rolling the porous aluminum base material filled with the granules.
Drying the positive electrode for the lithium ion secondary battery, and
Rolling the dried positive electrode for a lithium ion secondary battery,
Including
The granules
It is prepared to have a solid content ratio of 85% by mass or more and less than 100% by mass, and a median diameter smaller than the average open pore diameter of the porous aluminum substrate.
A method for manufacturing a positive electrode for a lithium ion secondary battery.