WO2017221499A1 - Functional layer including layered double hydroxide, and composite material - Google Patents

Abstract

Provided are: a functional layer including a layered double hydroxide (LDH), said functional layer having significantly improved alkali resistance; and a composite material including said functional layer. This functional layer includes a LDH, and is provided with: a first layer which has a thickness of at least 0.10 µm, and which is formed from LDH microparticles having a diameter of less than 0.05 µm; and a second layer which is provided on the first layer, is the outermost layer, and is formed from large LDH particles having an average particle diameter of at least 0.05 µm.

Description

Functional layer and composite material containing layered double hydroxide

The present invention relates to a functional layer and a composite material containing a layered double hydroxide.

Layered double hydroxide (hereinafter also referred to as LDH) is a substance having exchangeable anions and H ₂ O as an intermediate layer between stacked hydroxide basic layers. It is used as an adsorbent, a dispersant in a polymer for improving heat resistance, and the like.

LDH is also attracting attention as a material that conducts hydroxide ions, and its addition to the electrolyte of alkaline fuel cells and the catalyst layer of zinc-air cells is also being studied. In particular, in recent years, the use of LDH as a solid electrolyte separator for alkaline secondary batteries such as nickel-zinc secondary batteries and zinc-air secondary batteries has been proposed, and an LDH-containing functional layer suitable for such separator applications is provided. Composite materials are known. For example, Patent Document 1 (International Publication No. 2015/098610) discloses a composite including a porous substrate and an LDH-containing functional layer that does not have water permeability and is formed on and / or in the porous substrate. The material is disclosed, and the LDH-containing functional layer has a general formula: M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ⁿ⁻ _{x / n} · mH ₂ O (where M ²⁺ is 2 such as Mg ²⁺⁾ A valent cation, M ³⁺ is a trivalent cation such as Al ³⁺ , A ⁿ⁻ is an n-valent anion such as OH ⁻ , CO ₃ ²⁻ , n is an integer of 1 or more, and x is 0. 1 to 0.4, and m is 0 or more). Since the LDH-containing functional layer disclosed in Patent Document 1 is so dense that it does not have water permeability, when used as a separator, a zinc dendrite that has become a barrier to practical use of alkaline zinc secondary batteries Progress and carbon dioxide intrusion from the air electrode in the zinc-air battery can be prevented.

Furthermore, Patent Document 2 (International Publication No. 2016/076047) discloses a separator structure including an LDH separator combined with a porous substrate, and the LDH separator is gas-impermeable and / or It is disclosed to have high density enough to have water impermeability. This document also describes that LDH separators can have high density, which is evaluated as 10 cm / min · atm or less in terms of He permeability per unit area.

By the way, high hydroxide ion conductivity is required for the electrolyte solution of an alkaline secondary battery (for example, a metal-air battery or a nickel-zinc battery) to which LDH is applied. Therefore, the pH is about 14 and a strong alkaline KOH. It is desirable to use an aqueous solution. For this reason, LDH is desired to have a high alkali resistance that hardly deteriorates even in such a strong alkaline electrolyte. In this regard, in Patent Document 3 (International Publication No. 2016/051934), a metal compound containing a metal element (for example, Al) corresponding to M ²⁺ and / or M ³⁺ of the above general formula is dissolved in an electrolytic solution. There has been proposed an LDH-containing battery that is configured so that erosion of the LDH by the electrolyte is suppressed.

International Publication No. 2015/098610 International Publication No. 2016/076047 International Publication No. 2016/051934

The present inventors have recently found that the alkali resistance of an LDH-containing layer is significantly improved by covering the surface of a dense LDH-containing layer composed of LDH fine particles with large LDH particles having a diameter of 0.05 μm or more. Obtained.

Therefore, an object of the present invention is to provide an LDH-containing functional layer having significantly improved alkali resistance and a composite material including the same.

According to one aspect of the invention, a functional layer comprising a layered double hydroxide (LDH) comprising:
A first layer composed of LDH fine particles having a diameter of less than 0.05 μm and having a thickness of 0.10 μm or more;
A second layer composed of LDH large particles having an average particle diameter of 0.05 μm or more, which is an outermost surface layer provided on the first layer;
A functional layer is provided.

According to another aspect of the invention, a porous substrate;
The functional layer provided on the porous substrate and / or partially incorporated in the porous substrate;
A composite material is provided comprising:

According to another aspect of the present invention, a battery including the functional layer or the composite material as a separator is provided.

It is a schematic cross section which shows one aspect | mode of the LDH containing composite material of this invention. It is a surface SEM image of the functional layer produced in Example 1 (comparison). 4 is a surface SEM image of a functional layer produced in Example 3. 6 is a cross-sectional SEM image of a functional layer produced in Example 3. 6 is a surface SEM image of a functional layer produced in Example 4. 6 is a cross-sectional SEM image of a functional layer produced in Example 4. 6 is a BF-STEM image of the first layer in the functional layer produced in Example 4. FIG. FIG. 6 is a conceptual diagram showing an example of a He permeability measurement system used in Examples 1 to 5. It is a schematic cross section of the sample holder used for the measurement system shown in FIG. 8A and its peripheral configuration.

LDH-containing functional layer and composite material As schematically shown in FIG. 1, the functional layer 14 of the present invention includes a layered double hydroxide (LDH), and includes a first layer 14a, a second layer 14b, And optionally has a third layer 14c. That is, the first layer 14a, the second layer 14b, and optionally the third layer 14c all contain LDH. The first layer 14a is a layer having a thickness of 0.10 μm or more, which is composed of LDH fine particles having a diameter of less than 0.05 μm. The second layer 14b is an outermost surface layer provided on the first layer 14a, and is a layer composed of large LDH particles having an average particle diameter of 0.05 μm or more. Thus, by covering the surface of a dense LDH-containing layer composed of LDH fine particles with LDH large particles having a diameter of 0.05 μm or more, the alkali resistance of the LDH-containing layer can be significantly improved.

That is, since the first layer 14a is filled with LDH fine particles having a diameter of less than 0.05 μm, it can be said that the first layer 14a is a dense layer, and therefore can function as an LDH separator. However, as described above, the alkali secondary battery LDH is desired to have a high alkali resistance that hardly deteriorates even in a strong alkaline electrolyte. In this respect, in the functional layer 14 of the present invention, the alkali resistance is significantly improved by covering the surface of the dense first layer 14a composed of LDH fine particles with large LDH particles having an average particle diameter of 0.05 μm or more. . This is because the large LDH particles constituting the second layer 14b tend to be chemically stable, and such large LDH particles cover the surface of the first layer 14a, thereby reducing the density of the first layer 14a. This is thought to be because the decrease is suppressed. For example, according to the knowledge of the present inventors, when the average particle diameter of the LDH particles constituting the outermost surface layer is smaller than 0.05 μm, the LDH-containing functional layer is 3 in 70 ° C. in a potassium hydroxide aqueous solution having a predetermined concentration. The density decreases when immersed for a week, but when the average particle size of the LDH particles constituting the outermost surface layer is larger than 0.05 μm, the LDH-containing functional layer is placed in an aqueous potassium hydroxide solution having a predetermined concentration. The denseness does not change even when immersed for 3 weeks at ℃.

The first layer 14a is a layer composed of LDH fine particles. The LDH fine particles are mainly composed of LDH and may contain other trace components such as titania. That is, the first layer 14a is a layer mainly made of LDH, not a composite layer incorporated in another member. Therefore, in the case of the composite material 10 in which a part of the functional layer 14 is incorporated in the porous substrate 12 as shown in FIG. 1, a part of the functional layer incorporated in the porous substrate 12 is the first layer. It is assumed that it is regarded as another third layer 14c, not 14a. That is, the first layer 14a is a layer formed on the outermost surface (which should also be referred to as a main surface) of the porous substrate 12, and is a third layer 14c in which LDH particles are incorporated in the porous substrate 12. Differentiated. In other words, it can be said that the first layer 14 a is a layer that does not include other members such as the porous substrate 12. The diameter of the LDH fine particles constituting the first layer 14a is less than 0.05 μm, and preferably 0.025 μm or less. Since the smaller LDH fine particles are better, the lower limit of the diameter is not particularly limited, but the diameter of the LDH fine particles is typically 0.001 μm or more, more typically 0.003 μm or more. The diameter of the LDH fine particles can be determined by obtaining the cross-sectional polished surface of the functional layer 14 using an ion milling apparatus and then observing the microstructure of the cross-sectional polished surface with a scanning transmission electron microscope (STEM). The thickness of the first layer 14a is 0.10 μm or more, preferably 0.10 to 4.5 μm, more preferably 0.50 to 3.0 μm, and still more preferably 1.0 to 2.5 μm. Thus, the thickness of the first layer 14a is at least twice the diameter of the LDH fine particles. This means that the first layer 14a has a highly dense structure in which a plurality of LDH particles are stacked in the layer thickness direction, and is not a structure in which a plurality of LDH particles are simply connected in a plane. Means. Therefore, it can be said that a large number of LDH fine particles are three-dimensionally connected to each other to constitute one first layer 14a as a whole.

The second layer 14b is a layer composed of LDH large particles. LDH large particles mainly consist of LDH and may contain other trace components such as titania. Since the second layer 14b is the outermost surface layer provided on the first layer 14a, it is a layer mainly composed of LDH that is not incorporated in other members such as the porous substrate 12 like the first layer 14a. is there. The average particle diameter of the large LDH particles constituting the second layer 14b is 0.05 μm or more, preferably 0.05 to 5.0 μm, more preferably 0.1 to 5.0 μm. The average particle diameter of the large LDH particles is measured by observing the surface of the second layer 14b with a scanning electron microscope (SEM) and measuring the longest distance of the particle size based on the obtained image. it can. Specifically, the magnification of the electron microscope (SEM) image is set to 10,000 times or more, and all the obtained particle sizes are arranged in order, and the top 15 points and the bottom 15 points are arranged in order from the average value, and 30 per field in total. An average value for two fields of view can be calculated in terms of points to obtain an average particle size. For length measurement, the length measurement function of SEM software may be used. The thickness of the second layer 14b is not particularly limited, but is preferably 0.1 to 10 μm. Typically, the thickness of the second layer 14b corresponds to the height of one large LDH particle, but two or more large LDH particles may be stacked locally or entirely.

The functional layer 14 (ie, the first layer 14a, the second layer 14b, and optionally the third layer 14c) includes a layered double hydroxide (LDH). As is generally known, LDH is composed of a plurality of hydroxide base layers and an intermediate layer interposed between the plurality of hydroxide base layers. The hydroxide base layer is mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layer of LDH included in the functional layer is composed of an anion and H ₂ O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH comprises OH ^- and / or CO ₃ ^2- . By the way, high hydroxide ion conductivity is required for the electrolyte solution of an alkaline secondary battery (for example, a metal-air battery or a nickel-zinc battery) to which LDH is applied. Therefore, the pH is about 14 and a strong alkaline KOH. It is desirable to use an aqueous solution. For this reason, LDH is desired to have a high alkali resistance that hardly deteriorates even in such a strong alkaline electrolyte. Therefore, the LDH in the present invention is preferably one that does not cause changes in the surface microstructure and crystal structure by the alkali resistance evaluation as described later, and its composition is not particularly limited. Further, as described above, LDH has excellent ionic conductivity due to its inherent properties.

Specifically, LDH contained in the functional layer 14 is immersed in a 6 mol / L potassium hydroxide aqueous solution containing zinc oxide at a concentration of 0.4 mol / L at 70 ° C. for 3 weeks (ie, 504 hours). In addition, those which do not cause changes in the surface microstructure and crystal structure are preferred in terms of excellent alkali resistance. The presence or absence of changes in the surface microstructure depends on the surface microstructure using an SEM (scanning electron microscope), and the presence or absence of changes in the crystal structure depends on crystal structure analysis using XRD (X-ray diffraction) (for example, (003) derived from LDH) This can be preferably performed depending on whether or not there is a peak shift. Potassium hydroxide is a typical strong alkaline substance, and the composition of the potassium hydroxide aqueous solution corresponds to a typical strong alkaline electrolyte of an alkaline secondary battery. Therefore, it can be said that the above evaluation method of immersing in such a strong alkaline electrolyte at a high temperature of 70 ° C. for a long period of 3 weeks is a severe alkali resistance test. The LDH for alkaline secondary batteries is desired to have high alkali resistance that hardly deteriorates even in a strong alkaline electrolyte. In this respect, the functional layer of this embodiment has excellent alkali resistance that the surface microstructure and crystal structure are not changed even by such severe alkali resistance test. Nevertheless, the functional layer of this embodiment can also exhibit high ionic conductivity suitable for use as a separator for an alkaline secondary battery due to the inherent properties of LDH. That is, according to this aspect, it is possible to provide an LDH-containing functional layer that is excellent not only in ion conductivity but also in alkali resistance.

According to a preferred embodiment of the present invention, the hydroxide base layer of LDH is composed of Ni, Ti, OH groups and possibly inevitable impurities. As described above, the intermediate layer of LDH is composed of an anion and H ₂ O. The alternate layered structure of the hydroxide basic layer and the intermediate layer itself is basically the same as the generally known alternate layered structure of LDH, but the functional layer of this embodiment is mainly composed of the hydroxide basic layer of LDH as Ni. By comprising Ti and OH groups, excellent alkali resistance can be exhibited. Although the reason is not necessarily clear, it is considered that an element (for example, Al) that is considered to be easily eluted in an alkaline solution is not intentionally or actively added to the LDH of this embodiment. Even so, the functional layer of this embodiment can also exhibit high ionic conductivity suitable for use as a separator for an alkaline secondary battery. Ni in LDH can take the form of nickel ions. The nickel ions in LDH are typically considered to be Ni ²⁺ , but are not particularly limited because other valences such as Ni ³⁺ may also exist. Ti in LDH can take the form of titanium ions. The titanium ion in LDH is typically considered to be Ti ⁴⁺ , but is not particularly limited because other valences such as Ti ³⁺ may also exist. Inevitable impurities are optional elements that can be inevitably mixed in the manufacturing process, and can be mixed in LDH, for example, derived from raw materials and base materials. As described above, since the valences of Ni and Ti are not necessarily certain, it is impractical or impossible to specify LDH strictly by a general formula. If it is assumed that the hydroxide base layer is mainly composed of Ni ²⁺ , Ti ⁴⁺ and OH groups, the corresponding LDH has the general formula: Ni ²⁺ _1-x Ti ⁴⁺ _x (OH) ₂ ^{An _{- 2x / n · mH 2 O}} ( ^{wherein, a n-n-valent} anion, n is an integer of 1 or more, preferably 1 or 2, 0 <x <1, preferably 0.01 ≦ x ≦ 0.5, m is 0 or more, typically greater than 0 or 1 or more real number). However, the above general formula should be construed as “basic composition” only, and other elements or ions (other valences of the same element) to the extent that elements such as Ni ²⁺ and Ti ⁴⁺ do not impair the basic characteristics of LDH. It should be understood that it can be replaced by a number of elements or ions, or elements or ions that may be inevitably mixed in the manufacturing process.

According to another preferred embodiment of the present invention, the hydroxide base layer of LDH comprises Ni, Al, Ti and OH groups. As described above, the intermediate layer is composed of an anion and H ₂ O. The alternating layered structure of the hydroxide basic layer and the intermediate layer itself is basically the same as the generally known layered structure of LDH, but the functional layer of this embodiment is configured such that the hydroxide basic layer of LDH is Ni, By comprising a predetermined element or ion containing Al, Ti and OH groups, excellent alkali resistance can be exhibited. The reason for this is not necessarily clear, but the LDH of this embodiment is thought to be because Al, which was previously thought to be easily eluted in an alkaline solution, is less likely to be eluted in an alkaline solution due to some interaction with Ni and Ti. It is done. Even so, the functional layer of this embodiment can also exhibit high ionic conductivity suitable for use as a separator for an alkaline secondary battery. Ni in LDH can take the form of nickel ions. The nickel ions in LDH are typically considered to be Ni ²⁺ , but are not particularly limited because other valences such as Ni ³⁺ may also exist. Al in LDH can take the form of aluminum ions. Aluminum ions in LDH are typically considered to be Al ³⁺ , but are not particularly limited because other valences are possible. Ti in LDH can take the form of titanium ions. The titanium ion in LDH is typically considered to be Ti ⁴⁺ , but is not particularly limited because other valences such as Ti ³⁺ may also exist. The hydroxide base layer may contain other elements or ions as long as it contains Ni, Al, Ti and OH groups. However, it is preferable that the hydroxide base layer contains Ni, Al, Ti, and OH groups as main components. That is, the hydroxide base layer is preferably mainly composed of Ni, Al, Ti and OH groups. Therefore, the hydroxide base layer is typically composed of Ni, Al, Ti, OH groups and possibly inevitable impurities. Inevitable impurities are optional elements that can be inevitably mixed in the manufacturing process, and can be mixed in LDH, for example, derived from raw materials and base materials. As described above, since the valences of Ni, Al, and Ti are not necessarily certain, it is impractical or impossible to specify LDH strictly by a general formula. If it is assumed that the hydroxide base layer is mainly composed of Ni ²⁺ , Al ³⁺ , Ti ⁴⁺ and OH groups, the corresponding LDH has the general formula: Ni ²⁺ _1-xy Al ³⁺ _x Ti ⁴⁺ _y (OH) ₂ A ⁿ⁻ _{(x + 2y) / n} · mH ₂ O (where A ⁿ⁻ is an n-valent anion, n is an integer of 1 or more, preferably 1 or 2, and 0 <x <1, preferably 0.01 ≦ x ≦ 0.5, 0 <y <1, preferably 0.01 ≦ y ≦ 0.5, 0 <x + y <1, m is 0 or more, typically 0. It can be represented by a basic composition that exceeds or is one or more real numbers. However, the above general formula should be construed as “basic composition” only, and other elements or ions (of the same element) such that Ni ²⁺ , Al ³⁺ , Ti ^{4+ and the} like do not impair the basic characteristics of LDH. It should be understood that it can be replaced by other valence elements or ions or elements or ions that may be inevitably mixed in the manufacturing process.

The functional layer 14 (particularly LDH contained in the functional layer 14) preferably has hydroxide ion conductivity. In particular, the functional layer preferably has an ionic conductivity of 0.1 mS / cm or more, more preferably 0.5 mS / cm or more, and more preferably 1.0 mS / cm or more. The higher the ionic conductivity, the better. The upper limit is not particularly limited, but is, for example, 10 mS / cm. Such high ionic conductivity is particularly suitable for battery applications. For example, for the practical application of LDH, it is desired to reduce the resistance by thinning the film. However, according to this embodiment, an LDH-containing functional layer having a low resistance can be provided. It is particularly advantageous in the application of LDH as a solid electrolyte separator for secondary batteries.

Preferably, the functional layer 14 is provided on the porous substrate 12 and / or a part thereof is incorporated into the porous substrate 12. That is, according to a preferred aspect of the present invention, the porous substrate 12 and the functional layer 14 provided on the porous substrate 12 and / or partially incorporated in the porous substrate 12 are included. A composite material 10 is provided. For example, like the composite material 10 shown in FIG. 1, a part of the functional layer 14 may be incorporated in the porous substrate 12 and the remaining part may be provided on the porous substrate 12. At this time, portions of the functional layer 14 on the porous substrate 12 are the first layer 14a and the second layer 14b, and a portion (composite layer) of the functional layer 14 incorporated into the porous substrate 12 is the third layer. Layer 14c. The third layer 14c typically has a form in which the pores of the porous substrate 12 are filled with LDH.

The porous substrate 12 in the composite material 10 of the present invention is preferably capable of forming an LDH-containing functional layer on and / or in it, and the material and porous structure are not particularly limited. Typically, the LDH-containing functional layer is formed on and / or in the porous substrate, but the LDH-containing functional layer is formed on the nonporous substrate, and then nonporous by various known methods. The porous substrate may be made porous. In any case, it is preferable that the porous base material has a porous structure having water permeability in that the electrolyte solution can reach the functional layer when incorporated in the battery as a battery separator.

The porous substrate 12 is preferably composed of at least one selected from the group consisting of ceramic materials, metal materials, and polymer materials, and more preferably selected from the group consisting of ceramic materials and polymer materials. And at least one kind. More preferably, the porous substrate is made of a ceramic material. In this case, preferable examples of the ceramic material include alumina, zirconia, titania, magnesia, spinel, calcia, cordierite, zeolite, mullite, ferrite, zinc oxide, silicon carbide, and any combination thereof, and more preferable. Is alumina, zirconia, titania, and any combination thereof, particularly preferably alumina, zirconia (eg, yttria stabilized zirconia (YSZ)), and combinations thereof. When these porous ceramics are used, it is easy to form an LDH-containing functional layer having excellent denseness. Preferred examples of the metal material include aluminum, zinc, and nickel. Preferred examples of the polymer material include polystyrene, polyethersulfone, polypropylene, epoxy resin, polyphenylene sulfide, hydrophilic fluororesin (tetrafluorinated resin: PTFE, etc.), cellulose, nylon, polyethylene, and any combination thereof. Is mentioned. Any of the various preferred materials described above has alkali resistance as resistance to the electrolyte of the battery.

The porous substrate 12 preferably has an average pore size of at most 100 μm, more preferably at most 50 μm, for example, typically 0.001 to 1.5 μm, more typically 0.00. 001 to 1.25 μm, more typically 0.001 to 1.0 μm, particularly typically 0.001 to 0.75 μm, and most typically 0.001 to 0.5 μm. By being within these ranges, it is possible to form an LDH-containing functional layer that is so dense that it does not have water permeability, while ensuring the desired water permeability and strength as a support for the porous substrate. In the present invention, the average pore diameter can be measured by measuring the longest distance of the pores based on the electron microscope image of the surface of the porous substrate. The magnification of the electron microscope image used for this measurement is 20000 times or more. All the obtained pore diameters are arranged in order of size, and the top 15 points and the bottom 15 points are arranged in order from the average value, and 30 points per visual field are combined. The average pore diameter can be obtained by calculating an average value for two visual fields. For the length measurement, a length measurement function of SEM software, image analysis software (for example, Photoshop, manufactured by Adobe) or the like can be used.

The porous substrate 12 preferably has a porosity of 10 to 60%, more preferably 15 to 55%, still more preferably 20 to 50%. By being within these ranges, it is possible to form an LDH-containing functional layer that is so dense that it does not have water permeability, while ensuring the desired water permeability and strength as a support for the porous substrate. The porosity of the porous substrate can be preferably measured by the Archimedes method.

The functional layer 14 preferably does not have air permeability. That is, the functional layer is preferably densified with LDH to such an extent that it does not have air permeability. In this specification, “not breathable” means one surface of a measurement object (that is, a functional layer or a composite material) in water as described in Patent Document 2 (International Publication No. 2016/076047). This means that even when helium gas is brought into contact with the side at a differential pressure of 0.5 atm, no bubbles are generated due to helium gas from the other side. By doing so, the functional layer or the composite material as a whole can selectively pass only hydroxide ions due to its hydroxide ion conductivity, and can exhibit a function as a battery separator. . When considering application of LDH as a solid electrolyte separator for a battery, there was a problem that a bulk LDH dense body had high resistance, but in a preferred embodiment of the present invention, strength can be imparted by a porous substrate. Therefore, the LDH-containing functional layer can be thinned to reduce the resistance. Moreover, since the porous substrate can have water permeability and air permeability, the electrolyte can reach the LDH-containing functional layer when used as a battery solid electrolyte separator. That is, the LDH-containing functional layer and composite material of the present invention are used as solid electrolyte separators applicable to various battery applications such as metal-air batteries (for example, zinc-air batteries) and other various zinc secondary batteries (for example, nickel-zinc batteries). It can be a very useful material.

The functional layer 14 or the composite material 10 including the functional layer 14 preferably has a He permeability per unit area of 10 cm / min · atm or less, more preferably 5.0 cm / min · atm or less, and still more preferably 1. 0 cm / min · atm or less. It can be said that the functional layer having the He transmittance within such a range has extremely high density. Therefore, the functional layer having a He permeability of 10 cm / min · atm or less can prevent a high level of passage of substances other than hydroxide ions when applied as a separator in an alkaline secondary battery. For example, in the case of a zinc secondary battery, permeation of zinc ions or zincate ions in the electrolytic solution can be extremely effectively suppressed. In this way, it is considered in principle that Zn permeation is remarkably suppressed, so that growth of zinc dendrites can be effectively suppressed when used in a zinc secondary battery. The He permeability is measured through a process of supplying He gas to one surface of the functional layer and allowing the He gas to pass through the functional layer, and a process of calculating the He permeability and evaluating the density of the functional layer. The The He permeability is expressed by the following formula: F / (P × S), using the He gas permeation amount F per unit time, the differential pressure P applied to the functional layer when He gas permeates, and the membrane area S through which He gas permeates. Calculated by Thus, by evaluating the gas permeability using He gas, it is possible to evaluate the presence or absence of denseness at a very high level, and as a result, substances other than hydroxide ions (especially zinc dendrite growth can be performed). It is possible to effectively evaluate a high degree of compactness such that Zn that is caused is not transmitted as much as possible (only a very small amount is transmitted). This is because the He gas has the smallest structural unit among a wide variety of atoms or molecules that can constitute the gas, and the reactivity is extremely low. That is, He forms He gas by a single He atom without forming a molecule. In this respect, since hydrogen gas is composed of H ₂ molecules, a single He atom is smaller as a gas constituent unit. In the first place, H ₂ gas is dangerous because it is a combustible gas. Then, by adopting the He gas permeability index defined by the above-described formula, objective evaluation regarding the denseness can be easily performed regardless of differences in various sample sizes and measurement conditions. In this way, it is possible to simply, safely and effectively evaluate whether or not the functional layer has a sufficiently high density suitable for a zinc secondary battery separator. The measurement of the He permeability can be preferably performed according to the procedure shown in Evaluation 4 of Examples described later.

The LDH that constitutes the second layer 14b includes an aggregate of a plurality of plate-like particles (that is, LDH plate-like particles), and the plurality of plate-like particles have a functional layer layer surface (the fine irregularities of the functional layer). It is preferably oriented in a direction that intersects perpendicularly or obliquely with the layer surface (when viewed macroscopically to a negligible level). Although LDH crystals are known to have the form of plate-like particles having a layered structure, the above vertical or oblique orientation is a very advantageous characteristic for an LDH-containing functional layer (for example, an LDH dense film). This is because an oriented LDH-containing functional layer (eg, an oriented LDH dense film) has a hydroxide ion conductivity perpendicular to this in the direction in which the LDH plate-like particles are oriented (ie, in a direction parallel to the LDH layer). This is because there is a conductivity anisotropy that is much higher than the conductivity in the direction. In fact, it is already known that in an oriented bulk body of LDH, the conductivity (S / cm) in the orientation direction is one digit higher than the conductivity (S / cm) in the direction perpendicular to the orientation direction. That is, the vertical or oblique orientation in the LDH-containing functional layer maximizes the conductivity anisotropy that the LDH oriented body can have in the layer thickness direction (that is, the direction perpendicular to the surface of the functional layer or porous substrate). Alternatively, it can be significantly extracted, and as a result, the conductivity in the layer thickness direction can be maximized or significantly increased. In addition, since the LDH-containing functional layer has a layer form, lower resistance than that of the bulk form LDH can be realized. The LDH-containing functional layer having such an orientation is easy to conduct hydroxide ions in the layer thickness direction. In addition, since it is densified, it is extremely suitable for use in functional membranes such as battery separators (eg, hydroxide ion conductive separators for zinc-air batteries) where high conductivity in the layer thickness direction and denseness are desired. Suitable.

The functional layer 14 preferably has a thickness of 100 μm or less, more preferably 75 μm or less, still more preferably 50 μm or less, particularly preferably 25 μm or less, and most preferably 15 μm or less. Such thinness can reduce the resistance of the functional layer. When the functional layer 14 is formed as an LDH film on the porous substrate 12, the thickness of the functional layer 14 corresponds to the total thickness of the first layer 14a and the second layer 14b. Further, when the functional layer 14 is formed over and in the porous substrate 12, this corresponds to the total thickness of the first layer 14a, the second layer 14b, and the third layer 14c. In any case, when the thickness is as described above, a desired low resistance suitable for practical use in battery applications and the like can be realized. The lower limit of the thickness of the LDH alignment film is not particularly limited because it varies depending on the application, but in order to ensure a certain degree of hardness desired as a functional film such as a separator, the thickness is preferably 1 μm or more. Preferably it is 2 micrometers or more.

The manufacturing method of the LDH-containing functional layer and the composite material is not particularly limited, and the LDH-containing functional layer and the composite material are manufactured by appropriately changing various conditions of the LDH-containing functional layer and the composite material manufacturing method (see, for example, Patent Documents 1 to 3). be able to. For example, (1) a porous substrate is prepared, (2) a titanium oxide sol or a mixed sol of alumina and titania is applied to the porous substrate and heat-treated to form a titanium oxide layer or an alumina / titania layer, (3) The porous base material is immersed in a raw material aqueous solution containing nickel ions (Ni ²⁺ ) and urea, and (4) the porous base material is hydrothermally treated in the raw material aqueous solution to form the LDH-containing functional layer as the porous base material. By forming the upper layer and / or in the porous substrate, the LDH-containing functional layer and the composite material can be produced. In particular, by forming a titanium oxide layer or an alumina / titania layer on the porous substrate in the above step (2), not only can the raw material of LDH be provided, but it can also function as a starting point for LDH crystal growth. The LDH-containing functional layer highly densified on the surface can be uniformly formed without unevenness. In addition, the presence of urea in the above step (3) raises the pH value due to the generation of ammonia in the solution utilizing the hydrolysis of urea, and the coexisting metal ions form hydroxides. LDH can be obtained. Further, since carbon dioxide is generated in the hydrolysis, LDH in which the anion is carbonate ion type can be obtained.

Particularly preferred LDH-containing functional layers and methods for producing composite materials have the following characteristics, and these characteristics are considered to contribute to the realization of various characteristics of the functional layer of the present invention.
a) As a mixed sol of alumina and titania used in the above step (2), some kind of mixed sol (for example, amorphous alumina solution (Al-ML15, manufactured by Taki Chemical Co., Ltd.)) and titanium oxide sol solution (M6, Taki Chemical) Using a mixed sol) including
b) In the step (2), the heat treatment temperature of the sol applied to the porous substrate is relatively low, preferably 70 to 300 ° C. (eg 150 ° C.),
c) In the above step (2), the sol is applied by spin coating a plurality of times to thicken the spin coat layer (for example, the spin coat layer thickness is 5 μm),
d) In the above step (3), nickel ions (Ni ²⁺ ) are supplied in the form of nickel nitrate, and at this time, the urea / NO ₃ ^— molar ratio is preferably 8 to 32 (for example, 16) adding urea so that
e) Hydrothermal treatment in the above step (4) is at a relatively low temperature, preferably 70 to 150 ° C. (eg 110 ° C.), and the hydrothermal treatment time is relatively long, preferably 6 hours or more, more preferably 8 to 45) and / or f) after the step (4), the functional layer is washed with ion-exchanged water, and then the functional layer is dried at a relatively low temperature, preferably from room temperature to 70 ° C. (for example, room temperature). To do.

The present invention will be described more specifically with reference to the following examples.

Example 1 (Comparison)
Various functional layers and composite materials containing Ni, Al and Ti-containing LDH were prepared and evaluated by the following procedures.

(1) Preparation of porous substrate 70 parts by weight of a dispersion medium (xylene: butanol = 1: 1) and binder (polyvinyl butyral: Sekisui Chemical Co., Ltd.) with respect to 100 parts by weight of zirconia powder (manufactured by Tosoh Corporation, TZ-8YS) 11.1 parts by weight of BM-2 manufactured by Co., Ltd., 5.5 parts by weight of a plasticizer (DOP: manufactured by Kurokin Kasei Co., Ltd.), and 2.9 parts by weight of a dispersant (Rheodor SP-O30 manufactured by Kao Corporation) The slurry was obtained by mixing and defoaming the mixture by stirring under reduced pressure. The slurry was molded into a sheet shape on a PET film using a tape molding machine so that the film thickness after drying was 220 μm to obtain a sheet molded body. The obtained molded body was cut out to have a size of 2.0 cm × 2.0 cm × thickness 0.022 cm and baked at 1100 ° C. for 2 hours to obtain a zirconia porous substrate.

For the obtained porous substrate, the porosity of the porous substrate was measured by the Archimedes method and found to be 40%.

Further, when the average pore diameter of the porous substrate was measured, it was 0.2 μm. The average pore diameter was measured by measuring the longest distance of the pores based on an electron microscope (SEM) image of the surface of the porous substrate. The magnification of the electron microscope (SEM) image used for this measurement is 20000 times. All obtained pore diameters are arranged in order of size, and the top 15 points and the bottom 15 points are arranged in order from the average value. An average value for two visual fields was calculated at 30 points to obtain an average pore diameter. For length measurement, the length measurement function of SEM software was used.

(2) Alumina / titania sol coat on porous substrate Amorphous alumina solution (Al-ML15, manufactured by Taki Chemical Co., Ltd.) and titanium oxide sol solution (M6, manufactured by Taki Chemical Co., Ltd.) have a weight ratio of 1 : 1 to prepare a mixed sol. 0.2 ml of the mixed sol was applied onto the zirconia porous substrate obtained in (1) above by spin coating. In spin coating, the mixed sol was dropped onto the substrate rotated at 8000 rpm, and the rotation was stopped 5 seconds later. The substrate was allowed to stand on a hot plate heated to 100 ° C. and dried for 1 minute. Thereafter, heat treatment was performed at 150 ° C. in an electric furnace. The thickness of the layer thus formed was about 1 μm.

(3) Preparation of raw material aqueous solution As raw materials, nickel nitrate hexahydrate (Ni (NO ₃ ) ₂ · 6H ₂ O, manufactured by Kanto Chemical Co., Inc., and urea ((NH ₂ ) ₂ CO, manufactured by Sigma-Aldrich) are prepared. Nickel nitrate hexahydrate was weighed so as to be 0.015 mol / L, put into a beaker, and ion-exchanged water was added thereto to make a total volume of 75 ml. Urea weighed in a ratio of urea / NO ₃ ⁻ (molar ratio) = 16 was added thereto, and further stirred to obtain a raw material aqueous solution.

(4) Film formation by hydrothermal treatment A Teflon (registered trademark) sealed container (autoclave container, content of 100 ml, outer side is a stainless steel jacket) and the raw material aqueous solution prepared in (3) above and the substrate prepared in (2) above Was enclosed together. At this time, the base material was fixed by being floated from the bottom of a Teflon (registered trademark) sealed container, and placed horizontally so that the solution was in contact with both surfaces of the base material. Thereafter, hydrothermal treatment was performed at a hydrothermal temperature of 110 ° C. for 3 hours to form LDH on the substrate surface and inside. After a predetermined time has elapsed, the substrate is taken out of the sealed container, washed with ion-exchanged water, allowed to stand at room temperature for 12 hours, dried, and a part of the functional layer containing LDH is incorporated into the porous substrate. Obtained in the form. The thickness of the obtained functional layer was about 5 μm (including the thickness of the portion incorporated in the porous substrate).

Example 2
A functional layer and a composite material were produced in the same procedure as in Example 1 except that the hydrothermal treatment time in the film forming step by hydrothermal treatment was 8 hours.

Example 3
A functional layer and a composite material were produced in the same procedure as in Example 1 except that the hydrothermal treatment time of the film forming step by hydrothermal treatment was 16 hours.

Example 4
A functional layer and a composite material were produced in the same procedure as in Example 1 except that the hydrothermal treatment time of the film-forming process by hydrothermal treatment was 30 hours.

Example 5
In the alumina / titania sol coating process, spin coating was performed at a rotational speed of 4000 rpm, the same spin coating was repeated three times to increase the thickness of the spin coating layer, and the hydrothermal treatment time of the film formation process by hydrothermal treatment was set to 45 hours. Except for the above, a functional layer and a composite material were produced in the same procedure as in Example 1. The spin coat layer had a thickness of 5 μm, and the obtained functional layer had a thickness of about 10 μm.

<Evaluation>
The following various evaluations were performed on the obtained functional layer or composite material.

Evaluation 1 : Observation of surface microstructure and measurement of average particle diameter of large particles The surface microstructure of the functional layer was observed with a scanning electron microscope (SEM, JSM-6610LV, manufactured by JEOL) at an acceleration voltage of 10 to 20 kV. did. As a result, for the functional layers of Examples 2 to 5, a second layer composed of large particles having a diameter of 0.05 μm or more was confirmed, whereas for the functional layer of Example 1 (comparative), such a layer was confirmed. Was not. For reference, FIGS. 2, 3 and 5 show surface SEM images of the functional layers of Example 1 (comparative), Example 3 and Example 4, respectively. Therefore, the average particle size of the functional layers of Examples 2 to 5 was measured by the following procedure. The average particle size of the large particles constituting the second layer was measured by measuring the longest distance of the particle size based on the surface electron microscope (SEM) image of the functional layer. The magnification of the electron microscope (SEM) image used for this measurement is 10,000 times or more. All the obtained particle sizes are arranged in order, and the top 15 points and the bottom 15 points are arranged in order from the average value. An average value for two fields of view was calculated at 30 points to obtain an average particle size. For length measurement, the length measurement function of SEM software was used. The results were as shown in Table 1.

Evaluation 2 : Cross-sectional microstructure observation and thickness measurement Ion milling device (manufactured by Hitachi High-Technologies Corporation, IM4000) After obtaining the cross-sectional polished surface of the functional layer, the microstructure of the cross-sectional polished surface is observed with the surface microstructure. As a result, the functional layers of Examples 2 to 5 were composed of fine particles and were not incorporated into the porous substrate (that is, formed on the outermost surface of the porous substrate). ) While a first layer, a second layer composed of large particles, and a third layer in which the particles are incorporated into the porous substrate (ie formed inside the porous substrate) are identified, examples For the functional layer 1 (comparative), only the first layer composed of fine particles and the third layer in which the particles were incorporated into the porous substrate were observed.For reference, Examples 3 and 4 are shown in FIGS. Functional layer 14 (ie, first layer 14a, second layer 4b and the cross-sectional SEM images of the third layer 14c), and the thickness of the first layer and the thickness of the second layer were measured based on the obtained cross-sectional SEM images. The thickness was 1.0 μm (Example 1), 0.8 μm (Example 2), 0.7 μm (Example 3), 0.8 μm (Example 4), and 0.8 μm (Example 5). The thickness of each was as shown in Table 1.

Evaluation 3 : Fine particle observation by STEM and measurement of fine particle diameter In order to measure the particle diameter of the fine particles contained in the first layer of the functional layer, measurement was performed by an atomic resolution transmission scanning electron microscope (STEM). Specifically, after obtaining a cross-sectional polished surface of a functional layer using an ion milling apparatus (manufactured by GATAN, Dual Mill 600 type), the microstructure of the cross-sectional polished surface is analyzed by an atomic resolution transmission scanning electron microscope (STEM, JEOL). Observation was performed at an acceleration voltage of 200 kV using a JEM-ARM200F). From the obtained BF-STEM images, it was confirmed that in any of Examples 1 to 5, the diameter of each fine particle contained in the first layer was within a range of 0.025 μm or less. FIG. 7 shows a STEM image of the cross-sectional polished surface of the first layer in the functional layer obtained in Example 4.

Evaluation 4 : Alkali resistance evaluation I
Zinc oxide was dissolved in a 6 mol / L potassium hydroxide aqueous solution to obtain a 6 mol / L potassium hydroxide aqueous solution containing zinc oxide at a concentration of 0.4 mol / L. 15 ml of the aqueous potassium hydroxide solution thus obtained was placed in a Teflon (registered trademark) sealed container. A 2 cm × 2 cm square composite material was placed on the bottom of the sealed container with the functional layer facing upward, and the lid was closed. Thereafter, after holding at 70 ° C. for 1 week (ie, 168 hours) or 3 weeks (ie, 504 hours), the composite material was removed from the sealed container. The removed composite material was dried overnight at room temperature.

A He permeation test was performed on the obtained sample in order to evaluate the denseness of the functional layer from the viewpoint of He permeation. First, the He transmittance measurement system 310 shown in FIGS. 8A and 8B was constructed. The He permeability measurement system 310 is a function in which He gas from a gas cylinder filled with He gas is supplied to the sample holder 316 via the pressure gauge 312 and the flow meter 314 (digital flow meter), and is held in the sample holder 316. The layer 318 was configured to be transmitted from one surface to the other surface and discharged.

The sample holder 316 has a structure including a gas supply port 316a, a sealed space 316b, and a gas discharge port 316c, and was assembled as follows. First, an adhesive 322 was applied along the outer periphery of the functional layer 318 and attached to a jig 324 (made of ABS resin) having an opening at the center. Support members 328a and 328b (made of PTFE) provided with gaskets made of butyl rubber as sealing members 326a and 326b at the upper and lower ends of the jig 324 and further provided with openings formed from flanges from the outside of the sealing members 326a and 326b. ). Thus, the sealed space 316b was partitioned by the functional layer 318, the jig 324, the sealing member 326a, and the support member 328a. The functional layer 318 is in the form of a composite material formed on the porous substrate 320, but the functional layer 318 is disposed so that the functional layer 318 side faces the gas supply port 316a. The support members 328a and 328b were firmly fastened to each other by fastening means 330 using screws so that He gas leakage did not occur from a portion other than the gas discharge port 316c. The gas supply pipe 34 was connected to the gas supply port 316a of the sample holder 316 assembled in this way via a joint 332.

Next, He gas was supplied to the He permeability measurement system 310 via the gas supply pipe 334 and permeated through the functional layer 318 held in the sample holder 316. At this time, the gas supply pressure and the flow rate were monitored by the pressure gauge 312 and the flow meter 314. After permeation of He gas for 1 to 30 minutes, the He permeability was calculated. The calculation of the He permeability is based on the permeation amount of He gas per unit time F (cm ³ / min), the differential pressure P (atm) applied to the functional layer during He gas permeation, and the membrane area S (cm ² ) and calculated by the formula of F / (P × S). The permeation amount F (cm ³ / min) of He gas was directly read from the flow meter 314. Further, as the differential pressure P, the gauge pressure read from the pressure gauge 312 was used. The He gas was supplied so that the differential pressure P was in the range of 0.05 to 0.90 atm. The results were as shown in Table 1.

Evaluation 5 : Identification of functional layer The crystal phase of the functional layer was measured with an X-ray diffractometer (RINT TTR III manufactured by Rigaku Corporation) under the measurement conditions of voltage: 50 kV, current value: 300 mA, measurement range: 10 to 70 °. As a result, an XRD profile was obtained. About the obtained XRD profile, JCPDS card NO. Identification was performed using a diffraction peak of LDH (hydrotalcite compound) described in 35-0964. As a result, the functional layers obtained in Examples 1 to 5 were all identified as LDH (hydrotalcite compound).

Evaluation 6 : Alkali resistance evaluation II
Zinc oxide was dissolved in a 6 mol / L potassium hydroxide aqueous solution to obtain a 6 mol / L potassium hydroxide aqueous solution containing zinc oxide at a concentration of 0.4 mol / L. 15 ml of the aqueous potassium hydroxide solution thus obtained was placed in a Teflon (registered trademark) sealed container. A composite material having a size of 1 cm × 0.6 cm was placed on the bottom of the sealed container so that the functional layer faced upward, and the lid was closed. Thereafter, after holding at 70 ° C. for 3 weeks (ie, 504 hours), the composite material was taken out from the sealed container. The removed composite material was dried overnight at room temperature. The obtained sample was observed for microstructure by SEM and crystal structure by XRD. At this time, the change in the crystal structure was determined by the presence or absence of a shift in the (003) peak derived from LDH in the XRD profile. As a result, in any of Examples 1 to 5, no change was observed in the surface microstructure and crystal structure.

Evaluation 7 : Elemental analysis evaluation (EDS)
The cross section of the functional layer was polished by a cross section polisher (CP). Using FE-SEM (ULTRA55, manufactured by Carl Zeiss), a cross-sectional image of the functional layer was acquired with one field of view at a magnification of 10,000 times. The elemental analysis of the LDH film on the substrate surface of this cross-sectional image and the LDH portion (point analysis) inside the substrate was performed with an EDS analyzer (NORAN System SIX, manufactured by Thermo Fisher Scientific) under the condition of an acceleration voltage of 15 kV. went. As a result, LDH constituent elements C, Al, Ti and Ni were detected from the LDH contained in the functional layers obtained in Examples 1 to 5. That is, Al, Ti and Ni are constituent elements of the hydroxide basic layer, while C corresponds to CO ₃ ²⁻ which is an anion constituting the intermediate layer of LDH.

Claims (14)

A functional layer containing layered double hydroxide (LDH),
A first layer composed of LDH fine particles having a diameter of less than 0.05 μm and having a thickness of 0.10 μm or more;
A second layer composed of LDH large particles having an average particle diameter of 0.05 μm or more, which is an outermost surface layer provided on the first layer;
Having a functional layer. 2. The functional layer according to claim 1, wherein the diameter of the LDH fine particles is 0.025 μm or less, and the thickness of the first layer is 0.50 to 3.0 μm. 3. The functional layer according to claim 1, wherein the average particle diameter of the large LDH particles is 0.1 to 5.0 μm, and the thickness of the second layer is 0.1 to 10 μm. When the layered double hydroxide is immersed in a 6 mol / L potassium hydroxide aqueous solution containing zinc oxide at a concentration of 0.4 mol / L for 3 weeks at 70 ° C., the surface microstructure and crystal structure change. The functional layer according to any one of claims 1 to 3, which does not occur. The layered double hydroxide is
A plurality of hydroxide base layers composed of Ni, Ti and OH groups, or composed of Ni, Ti, OH groups and inevitable impurities, and anions and H interposed between the plurality of hydroxide base layers Composed of an intermediate layer composed of ₂ O, or
It is composed of a plurality of hydroxide base layers containing Ni, Al, Ti and OH groups, and an intermediate layer composed of anions and H ₂ O interposed between the plurality of hydroxide base layers. Item 4. The functional layer according to any one of Items 1 to 3. The layered double hydroxide constituting the second layer includes an aggregate of a plurality of plate-like particles, and the plurality of plate-like particles intersect each other perpendicularly or obliquely to the layer surface of the functional layer. The functional layer according to any one of claims 1 to 5, which is oriented in such a direction. The functional layer according to any one of claims 1 to 6, wherein the functional layer has a He permeability per unit area of 10 cm / min · atm or less. The functional layer according to any one of claims 1 to 7, wherein the functional layer has a thickness of 100 μm or less. The functional layer according to any one of claims 1 to 7, wherein the functional layer has a thickness of 50 µm or less. The functional layer according to any one of claims 1 to 7, wherein the functional layer has a thickness of 15 μm or less. A porous substrate;
The functional layer according to any one of claims 1 to 10, which is provided on the porous substrate and / or a part thereof is incorporated into the porous substrate;
Including composite materials. The composite material according to claim 11, wherein the porous base material is composed of at least one selected from the group consisting of a ceramic material, a metal material, and a polymer material. The composite material according to claim 11 or 12, wherein the composite material has a He permeability per unit area of 10 cm / min · atm or less. A battery comprising as a separator the functional layer according to any one of claims 1 to 10 or the composite material according to any one of claims 11 to 13.

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