阅读说明：本技术 全固态电池 (All-solid-state battery ) 是由 菅原启 广瀬英一 佐佐木知也 吉村耕作 鹿岛保伸 于 2019-09-02 设计创作，主要内容包括：本发明涉及一种全固态电池,该全固态电池具备：包含正极活性物质的正极活性物质层；包含负极活性物质的负极活性物质层；以及包含固体电解质和保持上述固体电解质的多孔质聚酰亚胺膜的固体电解质层,上述多孔质聚酰亚胺膜的空隙率为60％以上80％以下、空孔径为0.1μm以上10μm以下。(The present invention relates to an all-solid-state battery including: a positive electrode active material layer containing a positive electrode active material; an anode active material layer containing an anode active material; and a solid electrolyte layer comprising a solid electrolyte and a porous polyimide film that holds the solid electrolyte, wherein the porous polyimide film has a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm.)

1. An all-solid battery, wherein the battery comprises:

a positive electrode active material layer containing a positive electrode active material,

An anode active material layer containing an anode active material, and

a solid electrolyte layer comprising a solid electrolyte and a porous polyimide film that holds the solid electrolyte,

the porous polyimide film has a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm.

2. An all-solid battery, wherein the battery comprises:

a positive electrode active material layer containing a positive electrode active material,

An anode active material layer containing an anode active material, and

a solid electrolyte layer comprising a solid electrolyte and a porous polyimide film that holds the solid electrolyte,

the filling rate of the solid electrolyte filled in the pores of the porous polyimide film is 55% or more.

3. The all-solid battery according to claim 1 or 2, wherein the porous polyimide film has spherical pores, and a ratio of the long diameter to the short diameter, that is, a ratio of the long diameter/the short diameter of pores having a diameter of 1.5 or more is 20% or less.

4. The all-solid battery according to claim 3, wherein a ratio of the major axis to the minor axis, that is, a ratio of major axis/minor axis to pores of 1.5 or more is 15% or less.

5. The all-solid battery according to any one of claims 1 to 4, wherein a circularity of pores of the porous polyimide film is 0.85 or more.

6. The all-solid battery according to claim 5, wherein the roundness is 0.90 or more.

7. The all-solid battery according to any one of claims 1 to 6, wherein the average film thickness of the porous polyimide film is 10 μm or more and 1000 μm or less.

8. The all-solid battery according to claim 7, wherein the average film thickness is 20 μm or more and 500 μm or less.

9. The all-solid battery according to any one of claims 1 to 8, wherein a thickness ratio of the film thickness of the porous polyimide film to the entire thickness of the solid electrolyte layer, that is, a film thickness of the porous polyimide film/the entire thickness of the solid electrolyte layer is 0.5 or more.

10. The all-solid battery according to claim 9, wherein the thickness ratio, i.e., the film thickness of the porous polyimide film/the entire thickness of the solid electrolyte layer, is 0.9 or more.

11. The all-solid battery according to any one of claims 1 to 10, wherein the solid electrolyte contains a sulfide solid electrolyte.

12. The all-solid battery according to claim 11, wherein the sulfide solid electrolyte is a solid electrolyte containing sulfur and at least one of lithium and phosphorus as a constituent element.

Technical Field

The present invention relates to an all-solid-state battery.

Background

In recent years, attention has been paid to all-solid-state batteries in which a liquid electrolyte is replaced with a solid electrolyte.

Japanese patent No. 2018-088306 describes "a solid electrolyte composition containing a sulfide-based inorganic solid electrolyte (a) having conductivity of an ion of a metal belonging to group 1 or group 2 of the periodic table of elements, an antioxidant (B), a dispersion medium (C), and a binder (D), wherein the binder (D) contains polyurethane, polyurea, polyamide, polyimide, and/or polyester. ".

Jp 2013-a 008611 describes "an all-solid-state secondary battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer, wherein at least one of the positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer contains an inorganic solid electrolyte and a binder made of a particulate polymer containing a surfactant having a polyoxyethylene chain. ".

Disclosure of Invention

In all-solid-state batteries including a solid electrolyte layer using a porous film (for example, a porous polyolefin film) as a solid electrolyte retainer, there is a possibility that the battery capacity (hereinafter, sometimes referred to as "cycle characteristics") may decrease when charge and discharge are repeated.

An object of the present invention is to provide an all-solid battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with a case where a porous polyimide film having a porosity of less than 60% or more than 80%, a porous polyimide film having a pore diameter of less than 0.1 μm or more than 10 μm, or a porous polyimide film having a filling rate of the solid electrolyte filled in pores of less than 55% is applied as a holder for holding a solid electrolyte in a solid electrolyte layer included in the all-solid battery.

Specific means for solving the above technical problems include the following aspects.

According to the 1 st aspect of the present invention, there is provided an all-solid battery comprising a positive electrode active material layer containing a positive electrode active material, a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer containing a solid electrolyte and a porous polyimide film holding the solid electrolyte, wherein the porous polyimide film has a porosity of 60% to 80%, and a pore diameter of 0.1 μm to 10 μm.

According to the 2 nd aspect of the present invention, there is provided an all-solid battery comprising a positive electrode active material layer containing a positive electrode active material, a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer containing a solid electrolyte and a porous polyimide film holding the solid electrolyte, wherein the filling rate of the solid electrolyte filled in pores of the porous polyimide film is 55% or more.

According to the 3 rd aspect of the present invention, the porous polyimide film has spherical pores, and the ratio of the major axis to the minor axis (major axis/minor axis) is 1.5 or more, and the proportion of pores is 20% or less.

According to the 4 th aspect of the present invention, the ratio of the aspect ratio (major axis/minor axis) is 1.5 or more, and the proportion of voids is 15% or less.

According to the 5 th aspect of the present invention, the roundness of the pores of the porous polyimide is 0.85 or more.

According to claim 6 of the present invention, the roundness is 0.90 or more.

According to claim 7 of the present invention, in the solid-state battery, the average film thickness is 10 μm or more and 1000 μm or less.

According to the 8 th aspect of the present invention, the average film thickness is 20 μm to 500 μm.

According to the 9 th aspect of the present invention, the thickness ratio of the film thickness of the porous polyimide film to the entire thickness of the solid electrolyte layer (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) is 0.5 or more.

According to the 10 th aspect of the present invention, the thickness ratio (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) is 0.9 or more.

According to the 11 th aspect of the present invention, the solid electrolyte contains a sulfide solid electrolyte.

According to the 12 th aspect of the present invention, the above-mentioned sulfide solid electrolyte is a solid electrolyte containing sulfur and containing at least one of lithium and phosphorus as a constituent element.

(Effect)

According to the above aspect 1, there is provided an all-solid battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with a case where a porous polyimide film having a porosity of less than 60% or more than 80% is used as a holder for holding a solid electrolyte in a solid electrolyte layer provided in the all-solid battery, or a case where a porous polyimide film having a pore diameter of less than 0.1 μm or more than 10 μm is used.

According to the above aspect 2, there is provided an all-solid battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with a case where a porous polyimide film having a filling rate of the solid electrolyte of less than 55% filled in the pores is applied as a holder for holding the solid electrolyte in a solid electrolyte layer provided in the all-solid battery.

According to the above aspects 3 and 4, there is provided an all-solid battery capable of suppressing a decrease in battery capacity due to repeated charge and discharge, as compared with a case where a porous polyimide film having spherical pores and a ratio of pores having a ratio of a major axis to a minor axis (major axis/minor axis) of 1.5 or more is used, the porous polyimide film having a proportion of pores of more than 20%.

According to the above aspects 5 and 6, there is provided an all-solid battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with the case of using a porous polyimide film having a pore circularity of less than 0.85.

According to the above aspects 7 and 8, there is provided an all-solid battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with the case of using a porous polyimide film having a film thickness of less than 10 μm or more than 1000 μm.

According to the above aspects 9 and 10, there is provided an all-solid-state battery in which a decrease in battery capacity due to repeated charge and discharge can be suppressed as compared with a case where the thickness ratio of the film thickness of the porous polyimide film to the entire thickness of the solid electrolyte layer (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) is less than 0.5.

According to the above aspects 11 and 12, there is provided an all-solid-state battery capable of suppressing a decrease in battery capacity due to repeated charge and discharge, as compared with a case where a porous polyimide film having a porosity of less than 60% or more than 80% is applied as a support for holding a solid electrolyte, a case where a porous polyimide film having a pore diameter of less than 0.1 μm or more than 10 μm is applied, or a case where a porous polyimide film having a filling rate of the solid electrolyte filled in pores of less than 55% is applied, the all-solid-state battery containing a sulfide solid electrolyte as the solid electrolyte.

Drawings

Fig. 1 is a schematic view showing an example of the form of the porous polyimide film of the present embodiment.

Fig. 2 is a partially schematic cross-sectional view showing an example of the all-solid-state battery according to the present embodiment.

Detailed Description

An embodiment of an all-solid-state battery as an example of the present invention will be described below.

In the present description, the matters common to embodiment 1 and embodiment 2 are referred to as the present embodiment.

The solid-state battery of embodiment 1 has a positive electrode active material, a negative electrode active material, and a solid electrolyte layer containing a solid electrolyte and a porous polyimide film that holds the solid electrolyte. The porous polyimide film has a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm.

The solid-state battery of embodiment 2 has a positive electrode active material, a negative electrode active material, and a solid electrolyte layer containing a solid electrolyte and a porous polyimide film that holds the solid electrolyte. The filling rate of the solid electrolyte filled in the pores of the porous polyimide film is 55% or more.

In the solid electrolyte layer provided in the all-solid-state battery, for example, a binder resin is used to hold the solid electrolyte. When a binder resin is used for the solid electrolyte layer, a coating film due to the binder resin is formed on the surface of the solid electrolyte, and the lithium ion conductivity is reduced. This reduces the battery capacity (hereinafter, sometimes referred to as "cycle characteristics") during repeated charge and discharge. Therefore, the application of the porous membrane as a support for the solid electrolyte has been studied.

In the formation of the solid electrolyte layer, when the solid electrolyte is held by the porous membrane, the solid electrolyte is filled into the pores of the porous membrane while being pressurized. Examples of the porous film for holding the solid electrolyte include porous polyolefin films. When the porous polyolefin film is used, the strength of the porous polyolefin film is low, and therefore the filling performance of the solid electrolyte filled in the pores of the porous polyolefin film is low. Further, since the porous polyolefin film has irregular pore diameters, the filling performance is low even if the solid electrolyte is filled by pressurization, and further, the amount of the solid electrolyte that can be held is small because of the low porosity. As a result, particles of the solid electrolyte in the solid electrolyte layer are less likely to contact each other, and variation in lithium ion conductivity is likely to occur. Therefore, the battery capacity is likely to be reduced when the charge and discharge are repeated. In addition, porous polyimide films are exemplified as porous film having higher strength than porous polyolefin films. The strength of the porous polyimide film is higher than that of the porous polyolefin film, and the filling property of the solid electrolyte is higher than that of the case of using the porous polyolefin film. Therefore, even when a porous polyimide film is used, it is required to suppress a decrease in battery capacity when charge and discharge are repeated.

Accordingly, the all-solid-state battery according to the present embodiment can suppress a decrease in battery capacity during repeated charge and discharge with the above-described configuration.

In the all-solid battery according to embodiment 1, the porosity of the porous polyimide film holding the solid electrolyte is 60% to 80%, and the pore diameter is 0.1 μm to 10 μm. When the porosity and pore diameter are in this range, the filling property of the solid electrolyte is improved. Therefore, the particles of the solid electrolyte filled in the pores are likely to contact each other, and the uniformity of the lithium ion conductivity is improved, and it is believed that the decrease in the battery capacity during repeated charge and discharge can be suppressed.

In the all-solid battery according to embodiment 2, when the filling ratio of the solid electrolyte is 55% or more, the solid electrolyte filled in the pores is likely to come into contact with each other, and uniformity of lithium ion conductivity is improved. Therefore, it is considered that the decrease in battery capacity during repeated charge and discharge can be suppressed.

< all solid State Battery >

The all-solid-state battery according to the present embodiment will be described with reference to fig. 2.

Fig. 2 is a partially schematic cross-sectional view showing an example of the all-solid-state battery according to the present embodiment. As shown in fig. 2, all-solid battery 100 includes positive electrode active material layer 110, solid electrolyte layer 510, and negative electrode active material layer 310 housed inside an exterior member not shown. The positive electrode active material layer 110 contains a positive electrode active material, and the negative electrode active material layer 310 contains a negative electrode active material. The positive electrode active material layer 110 is provided on the positive electrode collector 130, and the negative electrode active material layer 310 is provided on the negative electrode collector 330. The solid electrolyte layer 510 is disposed between the positive electrode active material layer 110 and the negative electrode active material layer 310 such that the positive electrode active material layer 110 and the negative electrode active material layer 310 face each other. The solid electrolyte layer 510 includes a solid electrolyte 513 and a holder 511 that holds the solid electrolyte 513, and the solid electrolyte 513 is filled in a void of the holder 511. A porous polyimide film is used as the holder 511 for holding the solid electrolyte 513. The positive electrode current collector 130 and the negative electrode current collector 330 are members provided as needed. A positive electrode collector tab and a negative electrode collector tab (both not shown) may be provided at the end of the positive electrode collector 130 and the end of the negative electrode collector 330, respectively.

In the all-solid battery according to embodiment 1, a porous polyimide film having a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm is used as the support 511 provided in the solid electrolyte layer 510.

In the all-solid battery according to embodiment 2, a porous polyimide film is used as the holder 511 provided in the solid electrolyte layer 510. The porous polyimide film has a filling rate of the solid electrolyte 513 filled in the pores of 55% or more.

The all-solid-state battery according to the present embodiment has been described above with reference to fig. 2, but the form of the all-solid-state battery according to the present embodiment is not limited to this. Various forms can be adopted as long as the object disclosed in the all-solid-state battery of the present embodiment can be achieved.

Next, an example of a method for manufacturing the all-solid battery of the present embodiment will be described.

First, a coating liquid for forming the positive electrode active material layer 110 containing a positive electrode active material is applied to the positive electrode current collector 130 and dried, thereby obtaining a positive electrode including the positive electrode active material layer 110 provided on the positive electrode current collector 130.

Similarly, a coating liquid for forming the negative electrode active material layer 310 containing the negative electrode active material is applied to the negative electrode current collector 330 and dried, thereby obtaining a negative electrode including the negative electrode active material layer 310 provided on the negative electrode current collector 330.

The positive electrode and the negative electrode may be respectively subjected to compression processing as needed.

Next, a coating liquid containing the solid electrolyte 513 for forming the solid electrolyte layer 510 is applied to the substrate and dried, thereby obtaining a layered solid electrolyte 513.

Next, the porous polyimide film as the support 511 and the layered solid electrolyte 513 are stacked on the positive electrode active material layer 110 of the positive electrode as the material for forming the solid electrolyte layer 510. Further, the negative electrode is stacked on the solid electrolyte layer 510 forming material so that the negative electrode active material layer of the negative electrode faces the positive electrode active material layer side, to form a laminated structure. In the laminated structure, a positive electrode (positive electrode current collector 130, positive electrode active material layer 110), a solid electrolyte layer 510, and a negative electrode (negative electrode active material layer 310, negative electrode current collector 330) are sequentially laminated.

Next, the laminated structure is subjected to compression processing, and the solid electrolyte 513 is impregnated into the pores of the porous polyimide film as the support 511, thereby holding the solid electrolyte 513 in the porous polyimide film.

Next, the laminated structure is housed in an exterior member (not shown).

Thus, the all-solid battery 100 of the present embodiment was obtained.

The following describes the constituent elements of the all-solid battery of the present embodiment. In the following description, the reference numerals are omitted.

(Positive electrode collector and negative electrode collector)

The positive electrode current collector and the negative electrode current collector are members provided as needed. The material used for the positive electrode current collector and the negative electrode current collector is not particularly limited as long as it is a known conductive material. For example, metals such as aluminum, copper, nickel, and titanium can be used.

(Positive electrode active Material layer)

The positive electrode active material layer is a layer containing a positive electrode active material. If necessary, known additives such as a conductive aid and a binder resin may be included. The positive electrode active material is not particularly limited, and a known positive electrode active material is used. For example, a lithium-containing composite oxide (LiCoO) can be mentioned₂、LiNiO₂、LiMnO₂、LiMn₂O₄、LiFeMnO₄、LiV₂O₅Etc.), lithium-containing phosphates (LiFePO)₄、LiCoPO₄、LiMnPO₄And LiNiPO₄Etc.), conductive polymers (polyacetylene, polyaniline, polypyrrole, polythiophene, etc.), and the like. The positive electrode active material may be used alone in 1 kind, or two or more kinds may be used in combination.

(negative electrode active material layer)

The negative electrode active material layer is a layer containing a negative electrode active material. If necessary, known additives such as a binder resin may be contained. The negative electrode active material is not particularly limited, and a known positive electrode active material is used. Examples thereof include carbon materials (graphite (natural graphite, artificial graphite), carbon nanotubes, graphitized carbon, low-temperature-fired carbon, etc.), metals (aluminum, silicon, zirconium, titanium, etc.), metal oxides (tin dioxide, lithium titanate, etc.), and the like. The negative electrode active material may be used alone in 1 kind, or two or more kinds may be used in combination.

(solid electrolyte layer)

The solid electrolyte layer includes a solid electrolyte and a porous polyimide film as a support for holding the solid electrolyte.

[ solid electrolyte ]

The solid electrolyte is not particularly limited, and known solid electrolytes can be used. Examples thereof include a polymer solid electrolyte, an oxide solid electrolyte, a sulfide solid electrolyte, a halide solid electrolyte, and a nitride solid electrolyte.

The polymer solid electrolyte is not particularly limited. Examples thereof include fluororesins (homopolymers such as polyvinylidene fluoride, polyhexafluoropropylene, and polytetrafluoroethylene, and copolymers having these as a structural unit), polyethylene oxide resins, polyacrylonitrile resins, and polyacrylate resins. The solid electrolyte preferably contains a sulfide in view of excellent lithium ion conductivity. From the same viewpoint, a sulfide solid electrolyte containing sulfur and containing at least one of lithium and phosphorus as a constituent element is preferred.

The oxide solid electrolyte is not particularly limited, and oxide solid electrolyte particles containing lithium may be mentioned. For example, Li may be mentioned₂O-B₂O₃-P₂O₅、Li₂O-SiO₂And the like.

The sulfide solid electrolyte is not particularly limited, and includes a sulfide solid electrolyte containing sulfur and at least one of lithium and phosphorus as a constituent element. For example, 8Li is mentioned₂O·67Li₂S·25P₂S₅、Li₂S、P₂S₅、Li₂S-SiS₂、LiI-Li₂S-SiS₂、LiI-Li₂S-P₂S₅、LiI-Li₃PO₄-P₂S₅、LiI-Li₂S-P₂O₅、LiI-Li₂S-B₂S₃And the like.

The halide solid electrolyte is not particularly limited, and examples thereof include LiI.

The nitride solid electrolyte is not particularly limited, and examples thereof include Li₃N, and the like.

Among these, the solid electrolyte preferably contains a sulfide solid electrolyte, and more preferably a sulfide solid electrolyte. In addition, the sulfide solid electrolyte is preferably a solid electrolyte containing sulfur and containing at least one of lithium and phosphorus as a constituent element.

[ porous polyimide film ]

In the all-solid battery according to embodiment 1, a porous polyimide film having a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm is used.

In the all-solid battery according to embodiment 2, a porous polyimide film having a filling rate of the solid electrolyte filled in the pores of 55% or more is used.

Here, in the present embodiment, "film" is a concept including not only a member generally called "film" but also a member generally called "thin film" and "sheet".

(void fraction)

The porosity of the porous polyimide film applied to the all-solid battery according to embodiment 1 is 60% to 80%. When the porosity is in this range, the cycle characteristics are excellent. The lower limit of the porosity may be 63% or more, or 65% or more. The upper limit of the porosity may be 78% or less, or 77% or less.

The porosity is a value obtained from the apparent density and the true density of the porous polyimide film. The apparent density is the mass (g) of the porous polyimide film divided by the volume (cm) of the entire porous polyimide film including the pores³) And the resulting value. The true density ρ is obtained by polymerizing a porous polyimideMass of polyimide film (g) divided by volume of porous polyimide film excluding empty pores (cm)³) And the resulting value. The porosity of the porous polyimide film was calculated from the following equation.

(formula) porosity (%) {1- (d/ρ) } × 100 [1- { (w/t)/ρ) } ] × 100

d: apparent Density (g/cm) of porous polyimide film³)

ρ: true Density (g/cm) of porous polyimide film³)

w: weight (g/m) of porous polyimide film²)

t: thickness (μm) of porous polyimide film

The pore diameter (average value of pore diameters) is in the range of 0.1 to 10 μm. The void pore diameter is a value expressed in the form of an average value. By making the pore diameter of the voids 0.10 μm or more, the cycle characteristics are excellent. Further, by setting the pore diameter to 10 μm or less, deterioration of battery characteristics such as short circuit can be suppressed. From the viewpoint of more excellent cycle characteristics, the pore diameter may be 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, or 0.5 μm or more. In addition, from the viewpoint of suppressing the deterioration of the battery characteristics such as short circuit, the thickness may be 7 μm or less, preferably 5 μm or less, and may be 3 μm or less, or 2 μm or less. The pore diameter of the pores was observed and measured by a Scanning Electron Microscope (SEM) as described below.

From the viewpoint of excellent cycle characteristics, the porous polyimide film applied to the all-solid battery according to embodiment 1 preferably has a porosity of 63% to 80%, a pore diameter of 0.3 μm to 7 μm, more preferably a porosity of 65% to 78%, a pore diameter of 0.4 μm to 5 μm, even more preferably a porosity of 65% to 77%, and a pore diameter of 0.5 μm to 3 μm.

The porous polyimide film having a porosity of 60% to 80% and a pore diameter of 0.1 μm to 10 μm is preferable in that the filling rate of the solid electrolyte filled in the pores is easily 55% or more.

In the porous polyimide film applied to the all-solid battery according to embodiment 2, the filling rate of the solid electrolyte filled in the pores is 55% or more. When the filling rate of the solid electrolyte filled in the pores is within this range, the cycle characteristics are excellent. The filling rate of the solid electrolyte filled in the pores of the porous polyimide film is preferably high, and may be 60% or more, 70% or more, 80% or more, or 85% or more.

The filling ratio of the solid electrolyte filled in the pores is determined by the ratio of the density before and after impregnation with the solid electrolyte. Specifically, the calculation is as follows. First, the apparent density of the porous polyimide film before filling the solid electrolyte was determined. Next, the density of the porous polyimide film after the solid electrolyte was impregnated into the pores of the porous polyimide film by applying pressure was determined. The filling ratio of the solid electrolyte layer was then calculated by the following formula.

(formula) filling ratio (%) (density after solid electrolyte filling/density before solid electrolyte filling) × 100

In order to make the filling ratio of the solid electrolyte filled in the pores of the porous polyimide film 55% or more, for example, it is easy to realize when a porous polyimide film having a porosity of 60% or more and 80% or less and a pore diameter of 0.1 μm or more and 10 μm or less is used.

In the all-solid-state battery according to the present embodiment, the shape of the pores of the porous polyimide film is preferably spherical (approximately spherical). The "spherical shape" in the hollow includes both a spherical shape and a substantially spherical shape (a nearly spherical shape). Specifically, the spherical shape means that the ratio of the major axis to the minor axis (major axis/minor axis) is 1 or more and less than 1.5, and the proportion of particles is more than 80%. The proportion of particles having a ratio of the major axis to the minor axis (major axis/minor axis) of 1 or more and less than 1.5 is preferably 85% or more, more preferably 90% or more. That is, the porous polyimide film preferably has spherical pores, and the ratio of the major axis to the minor axis (major axis/minor axis) is 1.5 or more and 20% or less. The proportion of particles having a ratio of the major axis to the minor axis (major axis/minor axis) of more than 1.5 is more preferably 15% or less, and still more preferably 10% or less. The shape is closer to a regular sphere as the ratio of the major axis to the minor axis is closer to 1.

The holes are preferably shaped such that the holes are connected to each other (see fig. 1). The pore diameter of the portion where the pores are connected to each other may be, for example, 1/100 or more and 1/2 or less, preferably 1/50 or more and 1/3 or less, and more preferably 1/20 or more and 1/4 or less, which is the maximum diameter of the pores. Specifically, the average pore diameter of the portion where pores are connected to each other may be 5nm to 1500 nm.

The ratio of the maximum diameter to the minimum diameter of the pores (the ratio of the maximum value to the minimum value of the pore diameter) is not particularly limited, and may be, for example, 1 to 2. Preferably 1 to 1.9, more preferably 1 to 1.8. In this range, more preferably, it is close to 1. The "ratio of the maximum diameter to the minimum diameter of the pores" is a ratio represented by a value obtained by dividing the maximum diameter of the pores by the minimum diameter (i.e., the maximum value/minimum value of the pore diameter).

The roundness of the hole is not particularly limited, and may be, for example, 0.85 or more. Preferably 0.9 or more, more preferably 0.92 or more, and further preferably 0.95 or more.

The roundness of the hole is defined by the following equation, where the opening area of the hole is a and the length of the outer shape of the hole is L. Roundness is (4 pi A) 0.5/L. The roundness is 1 when the circle is perfect, and the roundness is reduced when the circumferential length L is longer than the sectional area a.

The average pore diameter (average pore diameter) of the interconnected portions of the pores, the ratio of the maximum diameter to the minimum diameter of the pores, and the circularity of the pores are values observed and measured by a Scanning Electron Microscope (SEM). Specifically, first, a sample for measurement is prepared by cutting the porous polyimide film. Then, the measurement sample was observed and measured by using VE SEM manufactured by keyence (keyence) corporation using image processing software of standard equipment. The observation and measurement were performed for 100 holes in the cross section of the measurement sample, and the average value (arithmetic mean diameter) of the hole diameters, the minimum diameter and the maximum diameter of the holes were determined. Further, with respect to the pore diameters of the portions where the pores are connected to each other, the average (arithmetic mean diameter) of the pore diameters of the portions where the pores are connected to each other was also obtained for 100 pore portions. When the shape of the hole (the shape of the hole in the measurement cross section) is not circular, the longest portion is defined as the diameter. The roundness of the hole was calculated based on the above definition by obtaining the average value (arithmetic mean circumference) of 100 measurement circumferences of the hole portion.

When the ratio of the maximum diameter to the minimum diameter of the pores and the circularity of the pores are within the above ranges, the dispersion of the pore diameter is preferably suppressed. In addition, since variation in the conductivity of lithium ions passing through the porous polyimide film of the present embodiment can be suppressed, the cycle characteristics are excellent, and an all-solid-state battery can be easily obtained.

The thickness (average value) of the porous polyimide film is not particularly limited, and may be, for example, 10 μm or more and 1000 μm or less. The film thickness may be 15 μm or more, or 20 μm or more. The thickness may be 500 μm or less, 400 μm or less, or 300 μm or less.

In the all-solid battery of the present embodiment, the solid electrolyte layer may be provided by a solid electrolyte and a porous polyimide film filling the pores with the solid electrolyte. The thickness ratio of the film thickness of the porous polyimide film to the entire thickness of the solid electrolyte layer (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) in the solid electrolyte layer holding the solid electrolyte may be 0.5 or more. When the thickness ratio (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) is 0.5 or more, the solid electrolyte is easily held in the porous polyimide film. The thickness ratio may be 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more. The thickness ratio (porous polyimide film/solid electrolyte layer overall thickness) may be 1.0 (i.e., the film thickness of the porous polyimide film in the solid electrolyte layer may be the same as the thickness of the solid electrolyte layer).

The thickness ratio (film thickness of the porous polyimide film/entire thickness of the solid electrolyte layer) was calculated as follows. A sample was collected from an all-solid-state battery as a measurement target so as to include a solid electrolyte layer. Next, the collected sample is embedded in an epoxy resin, and then the epoxy resin is cured. The cured product was sliced with a Microtome equipped with a diamond blade to obtain an observation sample in which a cross section of the measurement sample was exposed. The SEM image of the cross section was observed with a Scanning Electron Microscope (SEM) at a magnification that enables observation of the entire thickness of the solid electrolyte layer. Then, the thickness ratio of the film thickness of the porous polyimide film to the entire thickness of the solid electrolyte layer was calculated.

(method for producing porous polyimide film)

Next, an example of a preferable production method of the porous polyimide film applied to the all-solid-state battery according to the present embodiment will be described.

The method for producing a porous polyimide film according to the present embodiment includes, for example, the following steps.

A step 1 of forming a coating film by applying a polyimide precursor solution containing particles and a polyimide precursor, and then drying the coating film to form a coating film containing the polyimide precursor and the resin particles.

And a2 nd step of heating the coating film to imidize the polyimide precursor to form a polyimide film, the step including a treatment of removing the particles.

In the description of the manufacturing method, the same components are denoted by the same reference numerals in fig. 1. In the reference numerals in fig. 1, 31 denotes a substrate, 51 denotes a release layer, 10A denotes a hole, and 10 denotes a porous polyimide film.

(step 1)

In step 1, a polyimide precursor solution containing particles and a polyimide precursor (particle-dispersed polyimide precursor solution) is first prepared. Next, a particle-dispersed polyimide precursor solution is applied on the substrate to form a coating film containing the polyimide precursor solution and the particles. Thereafter, the coating film formed on the substrate is dried to form a coating film containing the polyimide precursor and the particles.

The substrate as the coating particle-dispersed polyimide precursor solution is not particularly limited. Examples thereof include: resin substrates such as polystyrene and polyethylene terephthalate; a glass substrate; a ceramic substrate; metal substrates such as iron and stainless steel (SUS); a composite substrate obtained by combining these materials, and the like. Further, a release layer may be provided on the substrate as needed, and the release layer may be subjected to a release treatment using, for example, a silicone-based or fluorine-based release agent.

The method for coating the particle-dispersed polyimide precursor solution on the substrate is not particularly limited. Examples of the method include various methods such as a spray coating method, a spin coating method, a roll coating method, a bar coating method, a slit extrusion coating method, and an inkjet coating method.

The amount of the polyimide precursor solution to be applied for obtaining a coating film containing the polyimide precursor solution and the particles may be set to an amount that can provide a predetermined film thickness.

After a coating film containing a polyimide precursor solution and particles is formed, the coating film is dried to form a coating film containing a polyimide precursor and particles. Specifically, the coating film containing the polyimide precursor solution and the particles is dried by a method such as heat drying, natural drying, or vacuum drying, to form a coating film. More specifically, the coating film is dried so that the solvent remaining in the coating film is 50% or less, preferably 30% or less, relative to the solid content of the coating film, thereby forming a coating film.

(step 2)

The 2 nd step is a step of heating the coating film containing the polyimide precursor and the particles obtained in the 1 st step to imidize the polyimide precursor to form a polyimide film. The step 2 includes a treatment for removing particles. The porous polyimide film can be obtained by removing particles.

In the step 2, as for the step of forming a polyimide film, specifically, the coating film containing the polyimide precursor and the particles obtained in the step 1 is heated to imidize the coating film, and further heated to form an imidized polyimide film. The imidization rate increases as the imidization proceeds, and the resulting polymer becomes difficult to dissolve in an organic solvent.

In the 2 nd step, a treatment for removing particles is performed. The particle removal may be performed during the imidization of the polyimide precursor by heating the coating film, or may be performed from the polyimide film after the imidization (after the imidization).

In the present embodiment, the process of imidizing the polyimide precursor is represented by the following process: the coating film containing the polyimide precursor and the particles obtained in the step 1 is heated to imidize the coating film, and the polyimide film is formed after the completion of imidization.

In terms of particle removability, the treatment for removing particles is preferably performed when the imidization ratio of the polyimide precursor in the polyimide film is 10% or more in the process of imidizing the polyimide precursor. When the imidization ratio is 10% or more, the form is easily maintained.

The following describes the particle removal process.

First, the treatment for removing the resin particles will be described.

Examples of the treatment for removing the resin particles include a method of removing the resin particles by heating, a method of removing the resin particles by an organic solvent that dissolves the resin particles, and a method of removing the resin particles by decomposition with a laser or the like. Among these, a method of removing the resin particles by heating or a method of removing the resin particles by an organic solvent dissolving the resin particles is preferable.

As a method of removing by heating, for example, in the process of imidizing the polyimide precursor, resin particles may be decomposed by heating for imidization to remove the polyimide precursor. In this case, it is not necessary to perform an operation of removing the resin particles with a solvent, which is advantageous in reducing the number of steps.

As a method for removing the resin particles by the organic solvent, for example, a method of dissolving and removing the resin particles by contacting the resin particles with the organic solvent (for example, immersing the resin particles in the solvent) in which the resin particles are dissolved may be mentioned. In this state, it is preferable that the resin particles are immersed in the solvent, because the dissolution efficiency of the resin particles is improved.

The organic solvent for removing the resin particles, which dissolves the resin particles, is not particularly limited as long as the organic solvent does not dissolve the polyimide film before the completion of imidization and the polyimide film after the completion of imidization and can dissolve the resin particles. Examples thereof include ethers such as Tetrahydrofuran (THF); aromatic compounds such as toluene; ketones such as acetone; and esters such as ethyl acetate.

When the resin particles are removed by dissolution and removal to form a porous body, the resin particles are preferably dissolved in a general-purpose solvent such as tetrahydrofuran, acetone, toluene, or ethyl acetate. Depending on the resin particles and the polyimide precursor used, water may be used.

In addition, when the resin particles are removed by heating to make them porous, the polyimide precursor is thermally decomposed at a temperature at which the film is imidized, without being decomposed at a drying temperature after coating. From this point of view, the thermal decomposition starting temperature of the resin particles may be 150 ℃ to 320 ℃, preferably 180 ℃ to 300 ℃, and more preferably 200 ℃ to 280 ℃.

Next, the treatment for removing the inorganic particles will be described.

As the treatment for removing the inorganic particles, there is a method of removing the inorganic particles by using a liquid that can dissolve the inorganic particles but does not dissolve the polyimide precursor or the polyimide (hereinafter, may be referred to as "particle-removing liquid"). The particle-removing liquid is selected according to the inorganic particles used. Examples thereof include: aqueous solutions of acids such as hydrofluoric acid, hydrochloric acid, hydrobromic acid, boric acid, perchloric acid, phosphoric acid, sulfuric acid, nitric acid, acetic acid, trifluoroacetic acid, citric acid, and the like; an aqueous solution of an alkali such as sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, sodium carbonate, potassium carbonate, ammonia, or the above organic amine. Depending on the inorganic particles and the polyimide precursor used, water may be used alone.

In the 2 nd step, a heating method for heating the coating film obtained in the 1 st step to imidize the coating film to obtain a polyimide film is not particularly limited. For example, a method of heating in two stages is mentioned. When the heating is performed in two stages, specific examples thereof include the following heating conditions.

The heating conditions in stage 1 are preferably temperatures at which the shape of the particles can be maintained. Specifically, it may be, for example, 50 ℃ to 150 ℃ inclusive, and preferably 60 ℃ to 140 ℃ inclusive. The heating time may be in the range of 10 minutes to 60 minutes. The heating time can be shortened as the heating temperature is higher.

The heating conditions in the 2 nd stage include, for example, heating at 150 ℃ to 450 ℃ inclusive (preferably 200 ℃ to 430 ℃) and 20 minutes to 120 minutes inclusive. By heating in this range, the imidization reaction further proceeds, and a polyimide film can be formed. The heating may be performed in a stepwise manner or at a constant rate until the temperature reaches the final temperature of heating during the heating reaction.

The heating conditions are not limited to the two-stage heating method described above, and for example, a method of heating in one stage may be employed. In the case of the method of heating in one stage, for example, the imidization can be completed only by the heating conditions shown in the above-mentioned 2 nd stage.

In the step 2, it is preferable to expose the particles by performing a treatment for exposing the particles in order to increase the aperture ratio. In the step 2, the treatment for exposing the particles is preferably performed during the imidization of the polyimide precursor or after the imidization and before the treatment for removing the particles.

In this case, for example, when a coating film is formed on a substrate using a particle-dispersed polyimide precursor solution, the particle-dispersed polyimide precursor solution is applied to the substrate to form a coating film in which particles are embedded. Next, the coating film is dried to form a coating film containing the polyimide precursor and the particles. The coating film formed by this method exhibits a state in which particles are embedded. Before the particle removal treatment by heating the coating film, the particle may be exposed from the polyimide film during the imidization of the polyimide precursor or after the imidization (after the imidization).

In the step 2, the treatment for exposing the particles is performed, for example, when the polyimide film is in the following state.

When the treatment for exposing the particles is performed when the imidization ratio of the polyimide precursor in the polyimide film is less than 10% (that is, when the polyimide film is in a state of being soluble in a solvent), the treatment for exposing the particles embedded in the polyimide film may be a wiping treatment, a treatment of immersing in a solvent, or the like. The solvent used in this case may be the same as or different from the solvent used in the particle-dispersed polyimide precursor solution of the present embodiment.

When the treatment for exposing the particles is performed when the imidization ratio of the polyimide precursor in the polyimide film is 10% or more (i.e., in a state of being hardly soluble in water or an organic solvent) or when the polyimide film is in a state of being imidized, there may be mentioned a method of exposing the particles by mechanical cutting with a tool such as sandpaper, or a method of exposing the resin particles by decomposition with a laser or the like when the particles are resin particles.

For example, in the case of mechanical cutting, a part of particles existing in an upper region of particles embedded in a polyimide film (i.e., a region of the particles on a side away from a substrate) is cut together with the polyimide film existing on the upper portion of the particles, and the cut particles are exposed from the surface of the polyimide film.

Thereafter, the polyimide film exposed from the particles was subjected to the above-described particle removal treatment to remove the particles. Thus, a porous polyimide film from which particles were removed was obtained (see fig. 1).

In the above, although the step of producing the porous polyimide film in which the treatment for exposing the particles is performed in the 2 nd step is shown, the treatment for exposing the particles may be performed in the 1 st step in order to increase the aperture ratio. In this case, in the step 1, after the coating film is obtained, a treatment for exposing the particles is performed in the process of drying the coating film to form the exposed particles. By performing the treatment for exposing the particles, the porosity of the porous polyimide film is increased.

For example, in the process of forming a coating film containing a polyimide precursor and particles by drying the coating film after obtaining the coating film containing the polyimide precursor solution and particles, the coating film is in a state in which the polyimide precursor can be dissolved in a solvent as described above. In this state of the coating, the particles can be exposed by, for example, wiping treatment or treatment of immersing in a solvent. Specifically, the polyimide precursor solution present in the region of the particle layer having a thickness equal to or greater than the thickness of the particle layer can be removed by performing a treatment of exposing the particle layer by wiping with a solvent, for example. Particles present in an upper region of the particle layer (i.e., a region of the particle layer on the side separated from the substrate) are exposed from the surface of the coating film.

In the step 2, the substrate used in the step 1 for forming a coating film may be peeled off when a dried coating film is formed, may be peeled off when the polyimide precursor in the polyimide film is in a state of being hardly soluble in an organic solvent, or may be peeled off when the polyimide precursor is in a film state after imidization.

The porous polyimide film was obtained through the above steps. The porous polyimide film may be subjected to post-processing.

Here, the imidization rate of the polyimide precursor will be described.

Examples of the polyimide precursor partially imidized include precursors having a structure of a repeating unit represented by the following general formula (V-1), the following general formula (V-2) and the following general formula (V-3).

[ solution 1]

In the general formula (V-1), the general formula (V-2) and the general formula (V-3), A, B has the same meaning as A, B in the formula (I). l represents an integer of 1 or more, and m and n each independently represent 0 or an integer of 1 or more.

The imidization ratio of the polyimide precursor is represented by the ratio of the number of imide ring-closed bonding portions (2n + m) to the number of total bonding portions (2l +2m +2n) in the bonding portion of the polyimide precursor (the reaction portion of the tetracarboxylic dianhydride and the diamine compound). That is, the imidization ratio of the polyimide precursor is represented by "(2 n + m)/(2l +2m +2 n)".

The imidization ratio of the polyimide precursor ("(2 n + m)/(2l +2m +2 n)") was measured by the following method.

Determination of the imidization ratio of the polyimide precursor-

Preparation of polyimide precursor sample

(i) A polyimide precursor composition to be measured is coated on a silicone wafer in a thickness range of 1 μm to 10 μm to prepare a coating film sample.

(ii) The coating film sample was immersed in Tetrahydrofuran (THF) for 20 minutes, and the solvent in the coating film sample was replaced with Tetrahydrofuran (THF). The solvent to be impregnated is not limited to THF, and may be selected from solvents that do not dissolve the polyimide precursor and that are miscible with the solvent components contained in the polyimide precursor composition. Specifically, alcohol solvents such as methanol and ethanol, and ether compounds such as dioxane can be used.

(iii) Taking out the coating sample from THF, and adding N₂The gas was blown to THF adhered to the surface of the coating film sample to remove the THF. The polyimide precursor sample is prepared by drying a coating sample by performing a treatment for 12 hours or more at a reduced pressure of 10mmHg or less and at a temperature of 5 ℃ to 25 ℃.

Preparation of 100% imidized Standard sample

(iv) A polyimide precursor composition to be measured was applied to a silicone wafer in the same manner as in the above (i), to prepare a coating film sample.

(v) The coated sample was heated at 380 ℃ for 60 minutes to effect imidization, thereby preparing a 100% imidization standard sample.

Determination and analysis

(vi) The infrared absorption spectra of the 100% imidization standard sample and the polyimide precursor sample were measured using a Fourier transform infrared spectrophotometer (FT-730 manufactured by horiba Seisakusho). Determine 1780cm^-1Near absorbance peak (Ab' (1780 cm) due to imide bond^-1) 1500cm relative to a 100% imidized standard sample^-1Near light absorption peak (Ab' (1500 cm) attributed to aromatic ring^-1) I' (100).

(vii) Similarly, the measurement was carried out on a polyimide precursor sample to obtain 1780cm^-1Near light absorption peak attributed to imide bond (Ab (1780 cm)^-1) Relative to 1500 cm)^-1Near light absorption peak (Ab (1500 cm) due to aromatic ring^-1) A ratio of (i), (x).

Then, the imidization ratio of the polyimide precursor was calculated based on the following formula using the measured absorption peaks I' (100), I (x).

Formula (la): imidization rate of polyimide precursor I (x)/I' (100)

Formula (la): i '(100) ═ Ab' (1780 cm)^-1))/(Ab’(1500cm^-1))

Formula (la): i (x) ═ Ab (1780 cm)^-1))/(Ab(1500cm^-1))

The measurement of the imidization rate of the polyimide precursor is applied to the measurement of the imidization rate of the aromatic polyimide precursor. When the imidization rate of the aliphatic polyimide precursor is measured, a peak derived from a structure which does not change before and after the imidization reaction is used as an internal standard peak without using an absorption peak of an aromatic ring.

Next, each component in the particle-dispersed polyimide precursor for producing the porous polyimide film will be described.

(polyimide precursor)

The polyimide precursor is obtained by polymerizing a tetracarboxylic dianhydride with a diamine compound. Specifically, it is a resin (polyamic acid) having a repeating unit represented by the general formula (I) of a polyimide precursor.

[ solution 2]

(in the general formula (I), A represents a 4-valent organic group, and B represents a 2-valent organic group.)

In the general formula (I), the 4-valent organic group represented by a is a residue obtained by removing 4 carboxyl groups from a tetracarboxylic dianhydride as a raw material.

On the other hand, the 2-valent organic group represented by B is a residue obtained by removing 2 amino groups from a diamine compound as a raw material.

That is, the polyimide precursor having the repeating unit represented by the general formula (I) is a polymer of tetracarboxylic dianhydride and a diamine compound.

The tetracarboxylic dianhydride may be any of aromatic and aliphatic tetracarboxylic dianhydride compounds, but an aromatic compound is preferable. That is, in the general formula (I), the 4-valent organic group represented by a is preferably an aromatic organic group.

Examples of the aromatic tetracarboxylic acid dianhydride include pyromellitic dianhydride, 3 ', 4,4 ' -benzophenonetetracarboxylic acid dianhydride, 3 ', 4,4 ' -biphenylsulfone tetracarboxylic acid dianhydride, 1,4,5, 8-naphthalene tetracarboxylic acid dianhydride, 2,3,6, 7-naphthalene tetracarboxylic acid dianhydride, 3 ', 4,4 ' -diphenyl ether tetracarboxylic acid dianhydride, 3 ', 4,4 ' -dimethyldiphenylsilanetetracarboxylic acid dianhydride, 3 ', 4,4 ' -tetraphenylsilanetetracarboxylic acid dianhydride, 1,2,3, 4-furantetracarboxylic acid dianhydride, 4,4 ' -bis (3, 4-dicarboxyphenoxy) diphenyl sulfide dianhydride, 4,4 ' -bis (3, 4-dicarboxyphenoxy) diphenyl sulfone dianhydride, 4,4 ' -bis (3, 4-dicarboxyphenoxy) diphenylpropane dianhydride, 3,3 ', 4, 4' -perfluoroisopropylidenediphthalic anhydride, 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride, 2,3,3 ', 4' -biphenyltetracarboxylic dianhydride, bis (phthalic acid) phenylphosphine oxide dianhydride, p-phenylene-bis (triphenylphthalic acid) dianhydride, m-phenylene-bis (triphenylphthalic acid) dianhydride, bis (triphenylphthalic acid) -4,4 '-diphenyl ether dianhydride, bis (triphenylphthalic acid) -4, 4' -diphenylmethane dianhydride, etc.

Examples of the aliphatic tetracarboxylic acid dianhydride include butanetetracarboxylic acid dianhydride, 1,2,3, 4-cyclobutanetetracarboxylic acid dianhydride, 1, 3-dimethyl-1, 2,3, 4-cyclobutanetetracarboxylic acid dianhydride, 1,2,3, 4-cyclopentanetetracarboxylic acid dianhydride, 2,3, 5-tricarboxycyclopentylacetic acid dianhydride, aliphatic or alicyclic tetracarboxylic acid dianhydrides such as 3,5, 6-tricarboxynorbornane-2-acetic acid dianhydride, 2,3,4, 5-tetrahydrofurantetracarboxylic acid dianhydride, 5- (2, 5-dioxotetrahydrofuryl) -3-methyl-3-cyclohexene-1, 2-dicarboxylic acid dianhydride, bicyclo [2,2,2] -oct-7-ene-2, 3,5, 6-tetracarboxylic acid dianhydride, and the like; 1,3,3a,4,5,9 b-hexahydro-2, 5-dioxo-3-furyl) -naphthol [1,2-c ] furan-1, 3-dione, 1,3,3a,4,5,9 b-hexahydro-5-methyl-5- (tetrahydro-2, 5-dioxo-3-furyl) -naphthol [1,2-c ] furan-1, 3-dione, 1,3,3a, aliphatic tetracarboxylic acid dianhydrides having an aromatic ring such as 4,5,9 b-hexahydro-8-methyl-5- (tetrahydro-2, 5-dioxo-3-furyl) -naphthol [1,2-c ] furan-1, 3-dione.

Among these tetracarboxylic acid dianhydrides, aromatic tetracarboxylic acid dianhydrides are preferable, and specifically, pyromellitic acid dianhydride, 3,3 ', 4, 4' -biphenyltetracarboxylic acid dianhydride, 2,3,3 ', 4' -biphenyltetracarboxylic acid dianhydride, 3,3 ', 4, 4' -diphenyl ether tetracarboxylic acid dianhydride, and 3,3 ', 4, 4' -benzophenonetetracarboxylic acid dianhydride are preferable, pyromellitic acid dianhydride, 3,3 ', 4, 4' -biphenyltetracarboxylic acid dianhydride, and 3,3 ', 4, 4' -benzophenonetetracarboxylic acid dianhydride are more preferable, and 3,3 ', 4, 4' -biphenyltetracarboxylic acid dianhydride is particularly preferable.

The tetracarboxylic dianhydride may be used alone in 1 kind, or 2 or more kinds may be used in combination.

When 2 or more kinds are used in combination, an aromatic tetracarboxylic dianhydride or an aliphatic tetracarboxylic dianhydride may be used in combination, or an aromatic tetracarboxylic dianhydride and an aliphatic tetracarboxylic dianhydride may be used in combination.

On the other hand, the diamine compound is a diamine compound having 2 amino groups in the molecular structure. The diamine compound may be any of aromatic and aliphatic compounds, but is preferably an aromatic compound. That is, in the general formula (I), the 2-valent organic group represented by B is preferably an aromatic organic group.

Examples of the diamine compound include p-phenylenediamine, m-phenylenediamine, 4 '-diaminodiphenylmethane, 4' -diaminodiphenylethane, 4 '-diaminodiphenyl ether, 4' -diaminodiphenyl sulfide, 4 '-diaminodiphenylsulfone, 1, 5-diaminonaphthalene, 3, 3-dimethyl-4, 4' -diaminobiphenyl, 5-amino-1- (4 '-aminophenyl) -1,3, 3-trimethylindane, 6-amino-1- (4' -aminophenyl) -1,3, 3-trimethylindane, 4 '-diaminobenzimide, 3, 5-diamino-3' -trifluoromethylbenzimide, and mixtures thereof, 3, 5-diamino-4 ' -trifluoromethylbenzanilide, 3,4 ' -diaminodiphenyl ether, 2, 7-diaminofluorene, 2-bis (4-aminophenyl) hexafluoropropane, 4 ' -methylene-bis (2-chloroaniline), 2 ', 5,5 ' -tetrachloro-4, 4 ' -diaminobiphenyl, 2 ' -dichloro-4, 4 ' -diamino-5, 5 ' -dimethoxybiphenyl, 3 ' -dimethoxy-4, 4 ' -diaminobiphenyl, 4 ' -diamino-2, 2 ' -bis (trifluoromethyl) biphenyl, 2-bis [4- (4-aminophenoxy) phenylbiphenyl]Propane, 2-bis [4- (4-aminophenoxy) phenyl]Hexafluoropropane, 1, 4-bis (4-aminophenoxy) benzene, 4 ' -bis (4-aminophenoxy) -biphenyl, 1,3 ' -bis (4-aminophenoxy) benzene, 9-bis (4-aminophenyl) fluorene, 4 ' - (p-phenyleneisopropyl) dianiline, 4 ' - (m-phenyleneisopropyl) dianiline, 2 ' -bis [4- (4-amino-2-trifluoromethylphenoxy) phenyl ] aniline]Hexafluoropropane, 4' -bis [4- (4-amino-2-trifluoromethyl) phenoxy]Aromatic diamines such as octafluorobiphenyl; aromatic diamines having 2 amino groups bonded to an aromatic ring and hetero atoms other than the nitrogen atom of the amino group, such as diaminotetraphenylthiophene; 1, 1-m-xylylenediamine, 1, 3-propylenediamine, tetramethylenediamine, pentamethylenediamine, 1, 8-octanediamine, 1, 9-nonanediamine, 4-diaminoheptamethylenediamine, 1, 4-cyclohexanediamine, isophoronediamine, tetrahydrodicyclopentadienyldiamine, hexahydro-4, 7-methyleneindanyldimethylenediamine, tricyclo [6,2,1,0 ]^2.7]Aliphatic diamines and alicyclic diamines such as undecene dimethyl diamine and 4, 4' -methylenebis (cyclohexylamine).

Among these, as the diamine compound, an aromatic diamine compound is preferable, and specifically, for example, p-phenylenediamine, m-phenylenediamine, 4 '-diaminodiphenylmethane, 4' -diaminodiphenyl ether, 3,4 '-diaminodiphenyl ether, 4' -diaminodiphenyl sulfide, 4 '-diaminodiphenyl sulfone are preferable, and 4, 4' -diaminodiphenyl ether and p-phenylenediamine are particularly preferable.

The diamine compound may be used alone in 1 kind, or 2 or more kinds may be used in combination. When 2 or more kinds are used in combination, the aromatic diamine compound or the aliphatic diamine compound may be used in combination, or the aromatic diamine compound and the aliphatic diamine compound may be used in combination.

The number average molecular weight of the polyimide precursor may be 1000 or more and 150000 or less, more preferably 5000 or more and 130000 or less, and still more preferably 10000 or more and 100000 or less.

When the number average molecular weight of the polyimide precursor is in the above range, the reduction in solubility of the polyimide precursor in a solvent can be suppressed, and the film-forming property can be easily ensured.

The number average molecular weight of the polyimide precursor was measured by a Gel Permeation Chromatography (GPC) method under the following measurement conditions.

Column: dongcao TSKgel alpha-M (7.8mm I.D. 30cm)

Eluent: DMF (dimethylformamide)/30 mM LiBr/60mM phosphoric acid

Flow rate: 0.6mL/min

Injection amount: 60 μ L

The detector: RI (differential refractive index detector)

The content (concentration) of the polyimide precursor may be 0.1 to 40 mass%, preferably 0.5 to 25 mass%, and more preferably 1 to 20 mass% with respect to the entire polyimide precursor solution.

(particle)

The particles are not particularly limited as long as the particles are dispersed without being dissolved in the particle-dispersed polyimide precursor solution of the present embodiment, and can be removed in a particle removal step described later when a porous polyimide film is produced. The particles are roughly classified into resin particles and inorganic particles described later.

In the present specification, the term "undissolved particles" includes a case where particles are not dissolved in a target liquid at 25 ℃ and are dissolved in a range of 3 mass% or less.

The volume average particle diameter D50v of the particles is not particularly limited. The volume average particle diameter D50v of the particles may be, for example, 0.1 to 10 μm. The volume average particle diameter of the particles may be 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, or 0.5 μm or more. The volume average particle diameter of the particles may be 7 μm or less, 5 μm or less, 3 μm or less, or 2 μm or less. The volume particle size distribution index (GSDv) of the particles is preferably 1.30 or less, more preferably 1.25 or less, and most preferably 1.20 or less. Volume particle size distribution index of particles the particle size distribution index of particles in the particle-dispersed polyimide precursor solution was determined from (D84v/D16v)^1/2Is calculated.

The particle size distribution of the particles in the particle-dispersed polyimide precursor solution of the present embodiment is measured as follows. The solution to be measured was diluted, and the particle size distribution of the particles in the solution was measured using a Coulter counter LS13 (manufactured by Beckman Coulter). Based on the measured particle size distribution, a volume cumulative distribution is plotted from the small diameter side with respect to the divided particle size range (particle size segment), and the particle size distribution is measured.

In the volume cumulative distribution drawn from the small diameter side, the particle diameter at 16% cumulative is the volume particle diameter D16v, the particle diameter at 50% cumulative is the volume average particle diameter D50v, and the particle diameter at 84% cumulative is the volume particle diameter D84 v.

When the volume particle size distribution of the particles in the particle-dispersed polyimide precursor solution of the present embodiment is difficult to measure by the above-described method, the volume particle size distribution is measured by a method such as a dynamic light scattering method.

The shape of the particles is preferably spherical. When a porous polyimide film is produced using spherical particles, a porous polyimide film having spherical pores can be obtained.

In the present specification, "spherical" in the particles includes both spherical and substantially spherical (a shape close to spherical). Specifically, the ratio of the major axis to the minor axis (major axis/minor axis) is 1 or more and less than 1.5, and the proportion of the particles is more than 80%. The proportion of particles having a ratio of the major axis to the minor axis (major axis/minor axis) of 1 or more and less than 1.5 is preferably 90% or more. The closer the ratio of the major diameter to the minor diameter is to 1, the more spherical the particle is.

As the particles, any of resin particles and inorganic particles can be used, and resin particles are preferably used.

As described later, the resin particles can be easily produced into nearly spherical particles by a known production method such as emulsion polymerization. Further, since the resin particles and the polyimide precursor are organic materials, the particle dispersibility in the coating film and the interfacial adhesion with the polyimide precursor are easily improved as compared with the case of using inorganic particles. In addition, when a porous polyimide film is produced, a porous polyimide film having pores and pore diameters closer to uniformity can be easily obtained. For these reasons, resin particles are preferably used.

Examples of the inorganic particles include silica particles. Silica particles are suitable inorganic particles in that nearly spherical particles can be obtained. For example, a porous polyimide film having pores in a nearly spherical state can be obtained by using a particle-dispersed polyimide precursor solution using silica particles in a nearly spherical state. However, when silica particles are used as the particles, the particles are less likely to absorb volume shrinkage in the imidization step, and thus the polyimide film after imidization tends to be prone to have fine cracks. From this viewpoint, it is also preferable to use resin particles as the particles.

The content of the particles in the particle-dispersed polyimide precursor solution of the present embodiment may be in the range of 20 mass% to 90 mass% (preferably 25 mass% to 87 mass%, more preferably 30 mass% to 85 mass%) with respect to 100 parts by mass of the solid content in the solution.

Specific materials of the resin particles and the inorganic particles are explained below.

Resin particles-

Specific examples of the resin particles include vinyl polymers typified by polystyrenes, poly (meth) acrylics, polyvinyl acetates, polyvinyl alcohols, polyvinyl butyrals, polyvinyl ethers, and the like; a condensed polymer represented by polyester, polyurethane, polyamide, or the like; hydrocarbon polymers typified by polyethylene, polypropylene, polybutadiene, and the like; fluorine-based polymers represented by polytetrafluoroethylene, polyvinyl fluoride, and the like; and the like.

Here, "(meth) acrylic acid" is meant to include either "acrylic acid" or "methacrylic acid". In addition, the (meth) acrylic acid group includes (meth) acrylic acid, (meth) acrylate, and (meth) acrylamide.

The resin particles may or may not be crosslinked. In the case of crosslinking, a difunctional monomer such as divinylbenzene, ethylene glycol dimethacrylate, nonanediol diacrylate or decanediol diacrylate, or a polyfunctional monomer such as trimethylolpropane triacrylate or trimethylolpropane trimethacrylate may be used in combination.

When the resin particles are vinyl resin particles, the monomer is polymerized. Examples of the vinyl resin monomer include the following monomers. Examples thereof include vinyl resin units obtained by polymerizing the following monomers: styrenes having a styrene skeleton such as styrene, alkyl-substituted styrenes (e.g., α -methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene, 4-ethylstyrene, etc.), halogen-substituted styrenes (e.g., 2-chlorostyrene, 3-chlorostyrene, 4-chlorostyrene, etc.), vinylnaphthalene, etc.; (meth) acrylic acid esters such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, lauryl (meth) acrylate, and 2-ethylhexyl (meth) acrylate; vinyl nitriles such as acrylonitrile and methacrylonitrile; vinyl ethers such as vinyl methyl ether and vinyl isobutyl ether; vinyl ketones such as vinyl methyl ketone, vinyl ethyl ketone, and vinyl isopropenyl ketone; acids such as (meth) acrylic acid, maleic acid, cinnamic acid, fumaric acid, and vinylsulfonic acid; bases such as aziridine, vinylpyridine, and vinylamine; and so on.

As other monomers, monofunctional monomers such as vinyl acetate may be used in combination.

The vinyl resin may be a resin obtained by using these monomers alone or a copolymer of 2 or more monomers.

The resin pellets are preferably polystyrene or poly (meth) acrylic resin pellets from the viewpoints of productivity and suitability for a pellet removal step described later. Specifically, polystyrene, styrene- (meth) acrylic copolymer, and poly (meth) acrylic resin particles are more preferable, and polystyrene and poly (meth) acrylate resin particles are most preferable. These resin particles may be used alone in 1 kind, or two or more kinds may be used in combination.

The resin particles preferably retain their shapes during the process of producing the polyimide precursor solution of the present embodiment, and during the process of applying the polyimide precursor solution and drying the coating film (before removing the resin particles) when producing the porous polyimide film. From this point of view, the glass transition temperature of the resin particles may be 60 ℃ or higher, preferably 70 ℃ or higher, and more preferably 80 ℃ or higher.

The glass transition temperature is determined from a DSC curve obtained by Differential Scanning Calorimetry (DSC), more specifically, the "extrapolated glass transition onset temperature" described in the method for determining the glass transition temperature of JIS K7121-.

Inorganic particles-

Specific examples of the inorganic particles include inorganic particles such as silica (silica) particles, magnesium oxide particles, aluminum oxide particles, zirconium oxide particles, calcium carbonate particles, calcium oxide particles, titanium oxide particles, zinc oxide particles, and cerium oxide particles. As described above, the shape of the particles may be spherical. From this viewpoint, the inorganic particles are preferably inorganic particles such as silica particles, magnesium oxide particles, calcium carbonate particles, magnesium oxide particles, and aluminum oxide particles, more preferably inorganic particles such as silica particles, titanium oxide particles, and aluminum oxide particles, and still more preferably silica particles. These inorganic particles may be used alone in 1 kind, or two or more kinds may be used in combination.

When the wettability and dispersibility of the inorganic particles in the solvent of the polyimide precursor solution are insufficient, the surfaces of the inorganic particles may be modified as necessary. Examples of the method of surface modification include: a method of treating with an alkoxysilane having an organic group represented by a silane coupling agent; a method of coating with an organic acid such as oxalic acid, citric acid, or lactic acid; and so on.

(solvent)

The solvent is not particularly limited as long as the polyimide precursor is dissolved in the particle-dispersed polyimide precursor solution and the particles are dispersed without being dissolved. The solvent may be any of an organic solvent and an aqueous solvent. The solvent may be selected depending on the state in which the polyimide precursor is dissolved or the particles are dispersed without being dissolved.

Examples of the solvent include organic solvents and aqueous solvents described below. When an aqueous solvent is used as the solvent, an organic amine compound described later is preferably added to dissolve the polyimide precursor.

As the solvent, an aqueous solvent is preferable from the viewpoint of environment and cost. In particular, when resin particles are used as the particles, an aqueous solvent is preferably used in order to obtain a state in which the polyimide precursor is dissolved and the resin particles are further dissolved and dispersed.

Organic solvent-

The organic solvent is selected so that the polyimide precursor in the particle-dispersed polyimide precursor solution of the present embodiment is dissolved, and the particles are dispersed without dissolving. When an organic solvent is selected, a mixed solvent of a good solvent (S1) for the polyimide precursor and a solvent (S2) other than the good solvent (S1) is preferable.

The good solvent (S1) for the polyimide precursor is used in the preparation of the polyimide precursor solution. The good solvent in the present embodiment means a solvent having a solubility of the polyimide precursor of 5 mass% or more. Specific examples thereof include aprotic polar solvents such as N, N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, N-ethylpyrrolidone, tetramethylurea, 1, 3-dimethyl-2-imidazolidinone, dimethylpropyleneurea, dimethyl sulfoxide, γ -butyrolactone, β -propiolactone, γ -valerolactone, and γ -caprolactone.

Among these, N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea, 1, 3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, and γ -butyrolactone are preferable, N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea, dimethyl sulfoxide, and γ -butyrolactone are more preferable, and N, N-dimethylacetamide, N-methylpyrrolidone, and γ -butyrolactone are further preferable.

As the solvent other than the good solvent for the polyimide precursor (S2), a solvent having low solubility of the particles to be used is selected. Examples of the method of selecting the solvent include the following methods: particles are added to the target solvent, and the solvent is selected so that the dissolution amount is 3 mass% or less.

Examples of the solvent other than the good solvent for the polyimide precursor (S2) include hydrocarbon solvents such as n-decane and toluene; alcohol solvents such as isopropyl alcohol, 1-propanol, 1-butanol, 1-pentanol, and phenethyl alcohol; glycol solvents such as ethylene glycol, propylene glycol, ethylene glycol monomethyl ether, 1-methoxy-2-propanol, diethylene glycol monomethyl ether, and triethylene glycol monomethyl ether; ether solvents such as diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and methyl cellosolve acetate; phenol solvents such as phenol and cresol; and so on.

In the case of using the above resin particles as the particles, the polarity of the solvent (S1) is very high, and thus the solvent (S1) alone may dissolve not only the polyimide precursor but also the resin particles. Therefore, the mixing ratio of the solvent (S1) and the solvent (S2) may be determined so that the polyimide precursor is dissolved and the resin particles are not dissolved. In addition, in order to prevent dissolution of the resin particles, for example, disturbance of the shape of pores, during heating of the coating film of the particle-dispersed polyimide precursor solution, the boiling point of the solvent (S2) is preferably 10 ℃ or higher, more preferably 20 ℃ or higher than that of the solvent (S1).

Aqueous solvent-

The aqueous solvent in the present specification is specifically an aqueous solvent as shown below.

The aqueous solvent is an aqueous solvent comprising water. Specifically, the aqueous solvent may be a solvent containing 50% by mass or more of water with respect to the entire aqueous solvent. Examples of the water include distilled water, ion-exchanged water, ultrafiltration water, and pure water.

The content of water is preferably 50 mass% to 100 mass%, more preferably 70 mass% to 100 mass%, and still more preferably 80 mass% to 100 mass% of the total aqueous solvent.

When the aqueous solvent contains a solvent other than water, examples of the solvent other than water include a water-soluble organic solvent and an aprotic polar solvent. As the solvent other than water, a water-soluble organic solvent is preferable from the viewpoint of mechanical strength of the porous polyimide film. Here, water solubility means that the target substance is dissolved in water at 25 ℃ by 1 mass% or more.

In particular, the aqueous solvent may contain an aprotic polar solvent in order to improve various properties (for example, transparency, mechanical strength, heat resistance, electrical properties, solvent resistance, and the like) of the porous polyimide film. In this case, in order to suppress dissolution and swelling of the particles in the particle-dispersed polyimide precursor solution, the amount thereof may be 40% by mass or less, preferably 30% by mass or less, based on the total amount of the aqueous solvent. In order to suppress dissolution and swelling of the resin particles at the time of drying and film formation of the polyimide precursor solution, the polyimide precursor solution may be used so that the content (solid content) of the particles and the polyimide precursor in the particle-dispersed polyimide precursor solution is 3 mass% to 200 mass%, preferably 3 mass% to 100 mass%, more preferably 3 mass% to 50 mass%, and still more preferably 5 mass% to 50 mass%.

The water-soluble organic solvent may be used alone in 1 kind, or may be used in combination in 2 or more kinds.

When resin particles are used as the particles, the water-soluble organic solvent is preferably an organic solvent in which the resin particles are insoluble. The reason for this is that, for example, even when the resin particles are not dissolved in the resin particle dispersion liquid in the case of preparing an aqueous solvent containing water and a water-soluble organic solvent, the resin particles may be dissolved in the process of obtaining a coating film of the particle-dispersed polyimide precursor solution.

When resin particles are used as the particles and a water-soluble organic solvent that dissolves the resin particles is used as the aqueous solvent, the amount of the water-soluble organic solvent may be 40 mass% or less, preferably 30 mass% or less, based on the total aqueous solvent, in order to prevent the particles in the particle-dispersed polyimide precursor solution from dissolving and swelling. In order to prevent the resin particles from dissolving and swelling when the coating film of the particle-dispersed polyimide precursor solution is dried and formed into a film, the amount of the water-soluble organic solvent may be 3 to 50 mass%, preferably 5 to 40 mass%, and more preferably 5 to 35 mass% with respect to the total amount of the particles and the polyimide precursor in the particle-dispersed polyimide precursor solution.

Examples of the water-soluble organic solvent include water-soluble ether solvents, water-soluble ketone solvents, and water-soluble alcohol solvents described below.

The water-soluble ether solvent is a water-soluble solvent having an ether bond in one molecule. Examples of the water-soluble ether solvent include Tetrahydrofuran (THF), dioxane, trioxane, 1, 2-dimethoxyethane, diethylene glycol dimethyl ether, and diethylene glycol diethyl ether. Among these, tetrahydrofuran and dioxane are preferred as the water-soluble ether solvent.

The water-soluble ketone solvent is a water-soluble solvent having a ketone group in one molecule. Examples of the water-soluble ketone solvent include acetone, methyl ethyl ketone, and cyclohexanone. Among these, acetone is preferred as the water-soluble ketone solvent.

The water-soluble alcohol solvent is a water-soluble solvent having an alcoholic hydroxyl group in one molecule. Examples of the water-soluble alcohol solvent include methanol, ethanol, 1-propanol, 2-propanol, t-butanol, ethylene glycol, monoalkyl ether of ethylene glycol, propylene glycol, monoalkyl ether of propylene glycol, diethylene glycol, monoalkyl ether of diethylene glycol, 1, 2-propanediol, 1, 3-butanediol, 1, 4-butanediol, 2, 3-butanediol, 1, 5-pentanediol, 2-butene-1, 4-diol, 2-methyl-2, 4-pentanediol, glycerol, 2-ethyl-2-hydroxymethyl-1, 3-propanediol, and 1,2, 6-hexanetriol. Among these, methanol, ethanol, 2-propanol, ethylene glycol, monoalkyl ether of ethylene glycol, propylene glycol, monoalkyl ether of propylene glycol, diethylene glycol, and monoalkyl ether of diethylene glycol are preferable as the water-soluble alcohol solvent.

When the aqueous solvent contains an aprotic polar solvent other than water, the aprotic polar solvent used in combination is a solvent having a boiling point of 150 ℃ to 300 ℃ and a dipole moment of 3.0D to 5.0D. Specific examples of the aprotic polar solvent include N-methyl-2-pyrrolidone (NMP), N-Dimethylformamide (DMF), N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), Hexamethylenephosphoramide (HMPA), N-methylcaprolactam, N-acetyl-2-pyrrolidone, 1, 3-dimethyl-2-imidazolidinone (DMI), N' -dimethylpropyleneurea, tetramethylurea, trimethyl phosphate, triethyl phosphate, and the like.

When a solvent other than water is contained as the aqueous solvent, the boiling point of the solvent to be used is preferably 270 ℃ or lower, preferably 60 ℃ to 250 ℃ or lower, and more preferably 80 ℃ to 230 ℃ or lower. When the boiling point of the solvent used is in the above range, the solvent other than water is less likely to remain in the polyimide film, and the polyimide film having high mechanical strength can be easily obtained.

When an aqueous solvent is used as the solvent, the solvent may contain an organic amine compound described later and water in a proportion of 50 mass% or more of the total solvent. The aqueous solvent may contain an aprotic polar solvent in an amount of 3 to 50 mass% based on the total amount of the particles and the polyimide precursor.

(organic amine Compound)

When the solvent is an aqueous solvent, the polyimide precursor is dissolved in water by adding an organic amine compound. The organic amine compound is preferably the following compound: this compound ammonium-converts a polyimide precursor (carboxyl group thereof) to an ammonium salt, improves the solubility thereof in an aqueous solvent, and also functions as an imidization accelerator. Specifically, the organic amine compound is preferably an amine compound having a molecular weight of 170 or less. The organic amine compound may be a compound other than the diamine compound which is a raw material of the polyimide precursor.

The organic amine compound may be a water-soluble compound. Water solubility means that the target substance is dissolved in water at 25 ℃ by 1 mass% or more.

Examples of the organic amine compound include a primary amine compound, a secondary amine compound, and a tertiary amine compound.

Among these, at least one selected from the group consisting of secondary amine compounds and tertiary amine compounds (particularly tertiary amine compounds) is preferable as the organic amine compound. When a tertiary amine compound or a secondary amine compound (particularly a tertiary amine compound) is used as the organic amine compound, the solubility of the polyimide precursor in a solvent is easily increased, the film-forming property is easily improved, and the storage stability of the polyimide precursor solution is easily improved.

In addition, the organic amine compound may include a polyvalent amine compound having 2 or more members in addition to the 1-membered amine compound. When a polyamine compound having 2 or more members is used, pseudo-crosslinked structures (pseudo-structure of ) are easily formed between molecules of the polyimide precursor, and the storage stability of the polyimide precursor solution is easily improved.

Examples of the primary amine compound include methylamine, ethylamine, n-propylamine, isopropylamine, 2-ethanolamine, 2-amino-2-methyl-1-propanol, and the like.

Examples of the secondary amine compound include dimethylamine, 2- (methylamino) ethanol, 2- (ethylamino) ethanol, and morpholine.

Examples of the tertiary amine compound include 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, pyridine, triethylamine, picoline, N-methylmorpholine, N-ethylmorpholine, 1, 2-dimethylimidazole, and 2-ethyl-4-methylimidazole.

The tertiary amine compound is preferable in terms of pot life of the polyimide precursor solution and uniformity of film thickness of the film. From this viewpoint, at least one selected from the group consisting of 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, pyridine, triethylamine, picoline, N-methylmorpholine, N-ethylmorpholine, 1, 2-dimethylimidazole, 2-ethyl-4-methylimidazole, N-methylpiperidine and N-ethylpiperidine is more preferable. More preferably, the solvent is at least one selected from the group consisting of 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, triethylamine, N-methylmorpholine, N-ethylmorpholine, 1, 2-dimethylimidazole, 2-ethyl-4-methylimidazole, N-methylpiperidine and N-ethylpiperidine.

Here, as the organic amine compound, an amine compound having a nitrogen-containing heterocyclic structure (particularly, a tertiary amine compound) is also preferable in view of film-forming properties. Examples of the amine compound having a nitrogen-containing heterocyclic structure (hereinafter referred to as "nitrogen-containing heterocyclic amine compound") include isoquinolines (amine compounds having an isoquinoline skeleton), pyridines (amine compounds having a pyridine skeleton), pyrimidines (amine compounds having a pyrimidine skeleton), pyrazines (amine compounds having a pyrazine skeleton), piperazines (amine compounds having a piperazine skeleton), triazines (amine compounds having a triazine skeleton), imidazoles (amine compounds having an imidazole skeleton), morpholines (amine compounds having a morpholine skeleton), polyanilines, polypyridines, and polyamines.

The nitrogen-containing heterocyclic amine compound is preferably at least one selected from the group consisting of morpholines, pyridines, piperidines, and imidazoles, and more preferably a morpholine compound (amine compound having a morpholine skeleton) in view of film-forming properties. Among these, at least one selected from the group consisting of N-methylmorpholine, N-methylpiperidine, pyridine, 1, 2-dimethylimidazole, 2-ethyl-4-methylimidazole and picoline is more preferable, and N-methylmorpholine is further more preferable.

Among these, the organic amine compound may be a compound having a boiling point of 60 ℃ or higher (preferably 60 ℃ or higher and 200 ℃ or lower, and more preferably 70 ℃ or higher and 150 ℃ or lower). When the boiling point of the organic amine compound is 60 ℃ or higher, volatilization of the organic amine compound from the polyimide precursor solution during storage can be suppressed, and a decrease in solubility of the polyimide precursor in the solvent can be easily suppressed.

The organic amine compound may be contained in an amount of 50 mol% or more and 500 mol% or less, preferably 80 mol% or more and 250 mol% or less, and more preferably 90 mol% or more and 200 mol% or less, relative to the carboxyl group (-COOH) of the polyimide precursor in the polyimide precursor solution.

When the content of the organic amine compound is in the above range, the solubility of the polyimide precursor in the aqueous solvent is easily improved, and the film-forming property is easily improved. In addition, the storage stability of the polyimide precursor solution is also easily improved.

The organic amine compound may be used alone in 1 kind, or two or more kinds may be used in combination.

(other additives)

The particle-dispersed polyimide precursor solution of the present embodiment may contain a catalyst for promoting imidization, a leveling agent for improving film formation quality, and the like.

As the catalyst for promoting the imidization reaction, a dehydrating agent such as an acid anhydride, an acid catalyst such as a phenol derivative, a sulfonic acid derivative, or a benzoic acid derivative, and the like can be used.

Further, for example, a conductive material (conductivity (for example, volume resistivity of less than 107 Ω · cm) or semiconductivity (for example, volume resistivity of 107 Ω · cm to 1013 Ω · cm)) added for imparting conductivity may be contained.

Examples of the conductive agent include carbon black (for example, acidic carbon black having a ph of 5.0 or less); metals (e.g., aluminum, nickel, etc.); metal oxides (e.g., yttrium oxide, tin oxide, etc.); ion conductive materials (for example, potassium titanate, LiCl, and the like); and so on. These conductive materials may be used alone in 1 kind, or two or more kinds may be used in combination.

Method for producing particle-dispersed polyimide precursor solution

The method for producing the particle-dispersed polyimide precursor solution of the present embodiment includes the following methods (i) and (ii).

(i) Method for mixing and dispersing particles after preparing polyimide precursor solution

(ii) Method for preparing particle dispersion liquid, and method for synthesizing polyimide precursor in the dispersion liquid

(i) method of mixing and dispersing particles after production of polyimide precursor solution

First, the following methods can be mentioned for the polyimide precursor solution before the particles are dispersed: a tetracarboxylic dianhydride and a diamine compound are polymerized in a solvent by a known method to produce a resin (polyimide precursor), and a polyimide precursor solution is obtained.

In the case of an aqueous solvent, the polyimide precursor solution is obtained by polymerization in the presence of an organic amine using the aqueous solvent. As another example, the following method may be mentioned: a tetracarboxylic dianhydride and a diamine compound are polymerized in an organic solvent such as an aprotic polar solvent (e.g., N-methylpyrrolidone (NMP)) to produce a resin (polyimide precursor), and then the resin (polyimide precursor) is precipitated by being put into an aqueous solvent such as water or alcohol. Then, the polyimide precursor and the organic amine compound are dissolved in an aqueous solvent to obtain a polyimide precursor solution.

Next, the obtained polyimide precursor solution was mixed with particles and dispersed.

In the case of producing the resin particles, for example, in the case of vinyl resin particles, the resin particles can be produced in an aqueous solvent by a known polymerization method (radical polymerization method such as emulsion polymerization, soap-free emulsion polymerization, suspension polymerization, miniemulsion polymerization, and microemulsion polymerization).

For example, when the emulsion polymerization method is applied to the production of vinyl resin particles, monomers such as styrene and (meth) acrylic acid are added to an aqueous solvent in which a water-soluble polymerization initiator such as potassium persulfate and ammonium persulfate is dissolved. Then, a surfactant such as sodium lauryl sulfate or diphenyl oxide disulfonate is further added as necessary, and polymerization is carried out by heating while stirring, thereby obtaining vinyl resin particles.

When the particle-dispersed polyimide precursor solution of the present embodiment is prepared in an aqueous solvent, the particle-dispersed polyimide precursor solution is obtained by mixing and stirring the aqueous solvent dispersion of the resin particles obtained by the above-described method and the polyimide precursor solution obtained as described above.

When the particle-dispersed polyimide precursor solution of the present embodiment is prepared in an organic solvent, the resin particles are taken out as a powder from the aqueous solvent dispersion of the resin particles by a known method such as reprecipitation or freeze drying, and mixed and stirred with the polyimide precursor solution obtained as described above. Alternatively, the resin particle powder taken out may be redispersed in an organic solvent (may be a single solvent or a mixed solvent) in which the resin particles are not dissolved, and mixed and stirred with the polyimide precursor solution.

The method of mixing, stirring and dispersing is not particularly limited. In addition, a known nonionic or ionic surfactant may be added to improve dispersibility of the particles.

When commercially available particles (resin particles or inorganic particles) are used, when the particles are available in the form of a powder, the particles can be mixed and dispersed at a desired concentration regardless of whether the solvent of the particle-dispersed polyimide precursor solution is an organic solvent or an aqueous solvent. When the particles are obtained as a dispersion of particles, the dispersion of particles and the polyimide precursor solution obtained as described above are mixed and dispersed by the same method as in the case of producing the particles, and a particle-dispersed polyimide precursor solution is produced.

(ii) method of preparing a particle dispersion, and preparing a polyimide precursor in the dispersion

When the particle-dispersed polyimide precursor solution of the present embodiment is prepared in an organic solvent, first, a solution is prepared in which particles are dispersed in an organic solvent in which the particles are not dissolved but the polyimide precursor is dissolved. Next, in this solution, tetracarboxylic dianhydride and a diamine compound are polymerized to produce a resin (polyimide precursor), and a particle-dispersed polyimide precursor solution is obtained.

When the particle-dispersed polyimide precursor solution of the present embodiment is prepared in an aqueous solvent (aqueous solvent), an aqueous solvent dispersion of particles is first prepared. Next, in this solution and in the presence of an organic amine, a tetracarboxylic dianhydride and a diamine compound are polymerized to produce a resin (polyimide precursor), and a particle-dispersed polyimide precursor solution is obtained.

When resin particles are used as the particles, the surfaces of the resin particles may be coated with a resin having a chemical structure different from that of the original resin in order to improve the dispersibility thereof in the particle-dispersed polyimide precursor solution of the present embodiment. The resin to be coated may be changed depending on the chemical structure of the solvent or polyimide precursor used. Examples of the resin to be coated include resins having an acidic group or a basic group. As a method for coating the resin particles with the resin, for example, in the case of producing vinyl resin particles by emulsion polymerization, the following method can be mentioned: after the polymerization of the monomer derived from the original resin particles is completed, a small amount of a monomer having an acidic group or a basic group such as methacrylic acid or 2-dimethylaminoethyl methacrylate is further added to continue the polymerization.

Among these, the method (ii) is preferable as a method for producing the particle-dispersed polyimide precursor solution of the present embodiment, because the particle dispersibility can be further improved.