JP2018125240A - Method for manufacturing positive electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the filling rate of filling a positive electrode mixture into a porous aluminum base material.SOLUTION: A method for manufacturing a positive electrode for a lithium ion secondary battery comprises the following (A) to (C): (A) the step of mixing a positive electrode active material, a conductive material, a binding material and solvent, thereby preparing granules; (B) the step of filling the granules into a porous aluminum base material; and (C) the step of rolling the porous aluminum base material with the granules filled therein, thereby manufacturing the positive electrode for a lithium ion secondary battery. The granules are prepared to have a solid content rate of 85 mass% or more and less than 100 mass%, and a median diameter smaller than an average open pore diameter of the porous aluminum base material.SELECTED DRAWING: Figure 1

Description

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

  JP-A-6-196169 (Patent Document 1) discloses a positive electrode for a lithium ion secondary battery (hereinafter sometimes simply referred to as “positive electrode”) by impregnating foamed aluminum (Al) with a slurry. The manufacturing is disclosed.

JP-A-6-196169

  Al foil is widely used as a positive electrode current collector. Typically, the positive electrode is manufactured by applying a slurry to the surface of the Al foil and drying it. The slurry is a particle dispersion in which the 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 substrate 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 to the porous Al base material, the viscosity of the slurry needs to be low to some extent. Therefore, the solid content ratio of the slurry is inevitably low. Solid content ratio shows the mass ratio of components (namely, positive electrode compound material) other than a solvent. Since the solid content ratio of the slurry is low, it is difficult to densely fill the porous Al base material with the positive electrode mixture.

  An object of the present disclosure is to increase the filling amount of the positive electrode mixture into the porous Al base material.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of the present disclosure should not be limited by the correctness of the mechanism of action.

The manufacturing method of the positive electrode for lithium ion secondary batteries of this indication contains the following (A)-(C).
(A) A granule is prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent.
(B) The granules are filled into a porous aluminum substrate.
(C) The positive electrode for lithium ion secondary batteries is manufactured by rolling the porous aluminum base material with which the granule was filled.
The granule is prepared so as 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 size of the porous aluminum base material.

  When the solid content ratio of the slurry increases (that is, the 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 decreases and the variation in the filling amount also increases.

  Contrary to such conventional knowledge, in the production method of the present disclosure, the solvent is greatly reduced. As a result, the mixture of the positive electrode mixture and the solvent does not become a slurry (particle dispersion) but an aggregate of particles (ie, “granule”). Further, the granules are prepared so as to have a median diameter smaller than the average open pore diameter of the porous Al substrate. Therefore, the granule can penetrate into 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.

  When the solid content ratio of the granules is less than 85% by mass, it is difficult to achieve a small median diameter because the grain growth of the granules tends to proceed during mixing. 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 decrease instead. Further, when the solid content ratio is lowered, the mixture is slurried, so that there is a possibility that granules cannot be prepared. When the solid content ratio is 100% by mass (that is, no solvent is present), aggregation of particles is not promoted, and it is difficult to prepare granules.

FIG. 1 is a flowchart illustrating an outline of a method for manufacturing a positive electrode according to an 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, an embodiment 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 the present disclosure.

<Method for producing positive electrode for lithium ion secondary battery>
FIG. 1 is a flowchart illustrating an outline of a method for manufacturing a positive electrode according to an embodiment of the present disclosure. The manufacturing method of this 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 in order.

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

  The granules are prepared, for example, by stirring granulation. Here, a general stirring granulator (for example, “High Speed Mixer” manufactured by Earth Technica Co., Ltd.) can be used. For example, a positive electrode active material, a conductive material, a binder and a solvent are charged in a predetermined mass ratio into a stirring tank of a stirring granulator, and a granule is prepared by stirring and mixing.

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

  The granule of this embodiment is prepared so as to have a solid content ratio of 85% by mass or more and less than 100% by mass. When the solid content ratio of the granules is less than 85% by mass, it is difficult to realize a small median diameter because the grain growth of the granules tends to proceed during mixing (granulation). 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 decrease instead. Further, when the solid content ratio is lowered, the mixture is slurried, so that there is a possibility that granules cannot 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 granule is 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 so as to have a median diameter smaller than the average open pore diameter of the porous Al base material described later. Thereby, it is considered that the granules can penetrate into the porous Al base material, and the filling amount increases. The “median diameter” in the present specification indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method. Hereinafter, the median diameter is also referred to as “D50”. D50 of a granule can be adjusted with solid content ratio, the rotation speed of a stirring blade, the rotation speed of a crushing blade, stirring time, etc. The rotation speed of the stirring blade may be 3000 to 6000 rpm (typically 4000 to 5000 rpm), for example. 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 size of the porous Al substrate, more preferably 1/3 (1/3) of the average open pore size of the porous Al substrate. Less than, most preferably having a D50 of less than 1/4 (1/4) of the average open pore size of the porous Al substrate. When the average open pore size 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 granule may be prepared so as to contain, for example, 80 to 98.5% by mass of the positive electrode active material. The positive electrode active material should not be particularly limited. Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _x Co _y Mn _z O ₂ (where x, y, and z are 0 <x <1, 0 <y < 1, 0 <z <1, x + y + z = 1), LiFePO ₄ or the like. One type of positive electrode active material may be used alone, or two or more types of positive electrode active materials may be used in combination. The positive electrode active material may have a D50 of 1 to 30 μm, for example.

(Conductive material)
The granules may be prepared to contain, for example, 1 to 15% by mass of a 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 type of conductive material may be used alone, or two or more types of conductive materials may be used in combination.

(Binder)
Granules may be prepared to contain, for example, 0.5-5% by weight 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), and the like. One kind of binder may be used alone, or two or more kinds of binders may be used in combination.

(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 manufacturing method of this embodiment includes filling a granule into a porous Al substrate.

(Porous aluminum substrate)
The porous Al substrate may be in the form of a sheet. The porous Al substrate may have a thickness of 0.1 to 1 mm (typically 0.2 to 0.5 mm), for example. The porous Al base material can be produced, for example, by foaming and then solidifying molten Al. Alternatively, the porous Al base material can 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 can have a three-dimensional network structure. By using the porous Al base material as the current collector, it is expected that the electron conductivity in the thickness direction of the positive electrode is improved as compared with the case where the Al foil is the current collector.

  The porous Al substrate includes a plurality of open pores. The average open pore size (sometimes referred to as “nominal pore size”) of the porous Al substrate is measured as follows. That is, the surface of the porous Al substrate is observed with an electron microscope (SEM) or the like. Randomly 20 open pores are extracted. The ferret diameter of each open pore is measured. The arithmetic average of the 20 ferret diameters is taken as the average open pore diameter. The porous Al base material may have an average open pore diameter of 150 to 1000 μm, for example. From the viewpoints of electron conductivity and strength, the porous Al substrate preferably has an average open pore size 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 size of 550 μm or more and 650 μm or less.

From the viewpoint of energy density, the porous Al base material desirably has a high porosity.
The porosity is given 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 substrate is calculated by dividing the mass of the porous Al substrate by the apparent volume (= area × thickness) of the porous Al substrate. The porous Al substrate preferably has a porosity of 75% or more, more preferably has a porosity of 80% or more, and most preferably has a porosity of 85% or more. From the viewpoint of mechanical strength, the porous Al base material may have a porosity of 90% or less, for example.

  The porous Al base material may have a connection portion with a current collecting lead. For example, it can be considered that a portion to be a connection portion in the porous Al base material is previously crushed by rolling. Thereby, since a granule cannot penetrate | invade into the said part, a current collection 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 disposed at the outlet of the filling tank 10. Further thereafter, a drying furnace 40 and a main rolling roll 50 are arranged. The curved arrows drawn on each roll indicate the rotation direction of each roll.

  The filling tank 10 is filled with the 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 2. That is, the granule 1 is filled in the porous Al base material 2.

<< (C) Temporary rolling >>
The manufacturing method of this embodiment includes rolling the porous Al base material 2 filled with the granules 1. By rolling, the granules 1 are fixed in the porous Al base material 2 to complete the positive electrode (electrode). The rolling may be performed only once or may be performed in two or more times. For example, the granule 1 is temporarily fixed in the porous Al substrate 2 by performing temporary rolling immediately after the filling of the granule 1. Drying is performed after temporary rolling. After drying, the positive electrode can be adjusted to the final target thickness by main rolling.

In FIG. 2, a temporary rolling roll 30 is disposed at the outlet of the filling tank 10. The porous Al base material 2 filled with the granules 1 is desirably temporarily rolled immediately after leaving the filling tank 10. Thereby, dropping of the granules 1 is suppressed. The compression ratio at the time of temporary rolling is preferably 10% or more and 50% or less. The compression ratio is the following formula:
Compression rate (%) = (Thickness before compression−Thickness after compression) ÷ Thickness before compression × 100%
Is calculated by When the compression rate at the time of temporary rolling is 10% or more, dropping of the granules 1 tends to be suppressed. If the compression ratio during pre-rolling (before drying) exceeds 50%, the solvent may ooze out. This may require cleaning of the transport path, rolls, and the like. However, solvent leaching is considered to have little effect on the performance and quality of the finished electrode. Therefore, as long as productivity is ignored, the compression rate may exceed 50%. The compression rate is more preferably 10% or more and 40% or less, and most preferably 10% or more and 30% or less.

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

<< (E) Main rolling >>
The manufacturing method of this embodiment may include further rolling the positive electrode after drying. In the main 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 to 150 μm.

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

<Experiment 1>
<Comparative example>
The following materials were prepared:
Positive electrode active material: Nickel manganese lithium cobaltate Conductive material: Acetylene black Binder solution: NMP solution of PVdF 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 by a dry method. As a result, a powder mixture was obtained. Next, the binder solution and the solvent were charged into the stirring tank. A positive electrode active material, a conductive material, a binder, and a solvent were mixed. As a result, a slurry was prepared. 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 into a porous Al substrate. The slurry was dried. The porous Al substrate was rolled. This produced a positive electrode.

<Comparative Examples 2-4>
As shown in Table 1 below, a positive electrode was produced by the same production 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 by a dry method. As a result, a powder mixture was obtained. 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. A positive electrode active material, a conductive material, a binder, and a solvent were mixed. The rotation speed of the stirring blade was 4500 rpm, and the stirring time was 20 seconds. Thereby, granule 1 was prepared. The solid content was “positive electrode active material: conductive material: binder = 91: 8: 1 (mass ratio)”. The solid content ratio was 85% by mass.

  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 Microtrack Bell). The measurement results are shown in Table 1 below.

<< (B) Filling, (C) Temporary rolling, (D) Drying, (E) Main rolling >>
An electrode manufacturing apparatus 100 shown in FIG. 2 was prepared. Granules 1 and porous Al base material 2 were supplied to electrode manufacturing apparatus 100. Thereby, the granule 1 was filled in the porous Al base material 2. Furthermore, the positive electrode was manufactured by rolling the porous Al base material 2 filled with the granules 1. In addition, the compression rate at the time of temporary rolling was set to 20%.

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

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

  Similarly, 20 disc samples were punched 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 value 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 example, the filling amount of the positive electrode mixture was increased as compared with the comparative example. This is probably because the granules had a solid content ratio of 85% by mass or more and less than 100% by mass and had a D50 smaller than the average open pore size of the porous Al base material. In the embodiment, not only the filling amount is large, but also the variation (standard deviation) in 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 granule. 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 granule, 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 granule 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, D50 of the granules tends to be remarkably reduced.

<Experiment 2>
In Examples 1 to 3, the compression ratio during temporary rolling was examined. 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 ooze out.
“+” Indicates that the granule has fallen off slightly or the solvent has oozed out slightly.
“±” indicates that the solvent oozes out.
“-” Indicates that the granules dropped out.

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

  The above embodiments and examples are to be considered in all respects as illustrative and not restrictive. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 granule, 2 base material, 10 filling tank, 20 filling roll, 30 temporary rolling roll, 40 drying furnace, 50 this rolling roll, 100 electrode manufacturing apparatus.

Claims (1)

Preparing a granule by mixing a positive electrode active material, a conductive material, a binder, and a solvent;
Filling a porous aluminum substrate with the granules, and rolling the porous aluminum substrate filled with the granules to produce a positive electrode for a lithium ion secondary battery,
The granules are
Having a solid content ratio of 85% by mass or more and less than 100% by mass, and having a median diameter smaller than the average open pore size of the porous aluminum base material,
A method for producing a positive electrode for a lithium ion secondary battery.