Anode and zinc secondary battery

Abstract

Provided is a negative electrode which enables a zinc secondary battery to have a prolonged cycle life. This negative electrode is for use in a zinc secondary battery and contains: ZnO particles, metal Zn particles having an average particle diameter D50 of 85-250 μm; and Bi2O3 particles having an average major axis diameter of 0.3-8.5 μm. In the negative electrode, when the contained amount of the ZnO particles is 100 parts by weight, the contained amount of the metal Zn particles is 1.0-87.5 parts by weight, and the contained amount of the Bi2O3 particles is 0.5-20 parts by weight.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode and a zinc secondary battery. [Background technology]

[0002] In zinc secondary batteries, such as nickel-zinc secondary batteries and air-zinc secondary batteries, metallic zinc precipitates in the form of dendrites from the negative electrode during charging, penetrates the pores of the separator (e.g., nonwoven fabric) and reaches the positive electrode, resulting in a short circuit. Such short circuits caused by zinc dendrites shorten the battery's life after repeated charging and discharging.

[0003] To address the above-mentioned problems, batteries have been proposed that include a layered double hydroxide (LDH) separator that selectively allows hydroxide ions to pass through while preventing zinc dendrites from penetrating. For example, Patent Document 1 (WO 2013 / 118561) discloses a nickel-zinc secondary battery in which an LDH separator is provided between the positive and negative electrodes. Patent Document 2 (WO 2016 / 076047) also discloses a separator structure that includes an LDH separator fitted or bonded to a resin outer frame, and that the LDH separator has such high density that it is gas- and / or water-impermeable. This document also discloses that the LDH separator can be composited with a porous substrate. Patent Document 3 (WO 2016 / 067884) also discloses various methods for forming a dense LDH film on the surface of a porous substrate to obtain a composite material. This method includes the steps of uniformly attaching an initiator substance capable of providing a starting point for LDH crystal growth to a porous substrate, and then subjecting the porous substrate to hydrothermal treatment in a raw material aqueous solution to form a dense LDH membrane on the surface of the porous substrate.

[0004] Another factor that shortens the lifespan of zinc secondary batteries is morphological changes in zinc, the negative electrode active material. Specifically, as zinc dissolves and precipitates during repeated charge and discharge, the negative electrode undergoes morphological changes, resulting in high resistance due to pore blockage and a decrease in the charge active material due to the accumulation of isolated zinc, making charging and discharging difficult. To address this issue, Patent Document 4 (WO 2020 / 049902) proposes the use of a negative electrode in combination with at least two selected from (i) metallic Zn particles of a predetermined particle size, (ii) a predetermined metal element, and (iii) a binder resin containing hydroxyl groups. This negative electrode is said to suppress negative electrode deterioration due to repeated charge and discharge in zinc secondary batteries, improving durability and thereby extending cycle life.

[0005] Furthermore, Patent Document 5 (JP 2021-57339 A) discloses the use of solder as a conductive additive in the negative electrode together with Zn particles and ZnO particles, which is said to suppress deterioration of the negative electrode due to repeated charge and discharge in a zinc secondary battery, improve durability, and extend cycle life. This document also discloses that the solder preferably contains at least one element selected from the group consisting of Sn, Pb, Bi, In, and Zn. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2013 / 118561 [Patent Document 2] International Publication No. 2016 / 076047 [Patent Document 3] International Publication No. 2016 / 067884 [Patent Document 4] International Publication No. 2020 / 049902 [Patent Document 5] Patent Publication No. 2021-57339 Summary of the Invention

[0007] As disclosed in Patent Documents 4 and 5, various attempts have been proposed to address the deterioration of cycle characteristics associated with morphological changes in zinc negative electrodes, but further improvement in cycle characteristics is desired.

[0008] The present inventors have now discovered that the cycle life of a zinc secondary battery can be extended by using a composite material containing a predetermined amount of Bi2O3 particles having a predetermined average major axis diameter in addition to ZnO particles and Zn particles in the negative electrode.

[0009] Therefore, an object of the present invention is to provide a negative electrode that can extend the cycle life of a zinc secondary battery.

[0010] According to the present invention, the following aspects are provided. [Aspect 1] A negative electrode for use in a zinc secondary battery, ZnO particles, Metal Zn particles having an average particle size D50 of 85 to 250 μm; Bi2O3 particles having an average major axis diameter of 0.3 to 8.5 μm; Including, The negative electrode has a content of the metal Zn particles of 1.0 to 87.5 parts by weight and a content of the Bi2O3 particles of 0.5 to 20 parts by weight, where the content of the ZnO particles is taken as 100 parts by weight. [Aspect 2] The negative electrode according to aspect 1, wherein the content of the Bi2O3 particles is 1.2 to 13.6 parts by weight relative to 100 parts by weight of the ZnO particles. [Aspect 3] 3. The negative electrode according to aspect 1 or 2, wherein the Bi2O3 particles have an average major axis diameter of 1.2 to 8.5 μm. [Aspect 4] A negative electrode according to any one of aspects 1 to 3, wherein the Bi2O3 particles have a maximum major axis diameter of less than 35 μm. [Aspect 5] A negative electrode according to any one of aspects 1 to 4, wherein the Bi2O3 particles are present on the surfaces of the ZnO particles. [Aspect 6] Aspect 6. The negative electrode of any one of aspects 1 to 5, wherein the Bi2O3 particles are contained in the form of non-aggregated primary particles. [Aspect 7] The negative electrode according to any one of aspects 1 to 6, wherein the negative electrode is a sheet-like pressed product. [Aspect 8] A positive electrode and The negative electrode according to any one of aspects 1 to 7; a separator that separates the positive electrode and the negative electrode in a manner that allows hydroxide ions to be conducted between them; An electrolyte; A zinc secondary battery comprising: [Aspect 9] 9. The zinc secondary battery of claim 8, wherein the separator is a layered double hydroxide (LDH) separator comprising an LDH and / or an LDH-like compound. [Aspect 10] 10. The zinc secondary battery of claim 9, wherein the LDH separator is composited with a porous substrate. [Aspect 11] 11. The zinc secondary battery according to any one of aspects 8 to 10, wherein the positive electrode contains nickel hydroxide and / or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery. [Aspect 12] 11. The zinc secondary battery according to any one of aspects 8 to 10, wherein the positive electrode is an air electrode, thereby forming the zinc secondary battery as a zinc-air secondary battery. [Brief explanation of the drawings]

[0011] [Figure 1A] FIG. 1 is a schematic diagram showing an example in which Bi2O3 particles are present on the surface of metal Zn particles in a negative electrode, and is a diagram showing a case in which the particle diameter of the Bi2O3 particles is small. [Figure 1B] FIG. 10 is a schematic diagram showing another example in which Bi2O3 particles are present on the surface of metal Zn particles in a negative electrode, and is a diagram showing a case in which the particle size of the Bi2O3 particles is within a desirable range. [Figure 1C]FIG. 10 is a schematic diagram showing another example in which Bi2O3 particles are present on the surface of metal Zn particles in a negative electrode, in which the particle diameter of the Bi2O3 particles is large. [Figure 2A] FIG. 1 is an SEM image of the Bi2O3 powder used in Examples 1 to 11, illustrating the major axis diameter. [Figure 2B] FIG. 2B is a diagram for explaining a method for calculating the major axis diameter in the SEM image of FIG. 2A. [Figure 3] 1 is a cross-sectional SEM image of the negative electrode obtained in Example 14. [Figure 4] 1 is a cross-sectional SEM image of the negative electrode obtained in Example 20. DETAILED DESCRIPTION OF THE INVENTION

[0012] negative electrode The negative electrode of the present invention is a negative electrode used in a zinc secondary battery. This negative electrode contains ZnO particles, metal Zn particles, and Bi2O3 particles. The metal Zn particles have an average particle size D50 of 85 to 250 μm. The Bi2O3 particles have an average major axis diameter of 0.3 to 8.5 μm. When the content of ZnO particles is taken as 100 parts by weight, the content of metal Zn particles is 1.0 to 87.5 parts by weight, and the content of Bi2O3 particles is 0.5 to 20 parts by weight. By using a composite material containing a predetermined amount of Bi2O3 particles having a predetermined average major axis diameter together with ZnO particles and Zn particles in the negative electrode, the cycle life of the zinc secondary battery can be extended.

[0013] The mechanism by which the addition of Bi2O3 particles to the negative electrode improves the cycle characteristics of zinc secondary batteries is not entirely clear, but one possible factor is the following. First, when a zinc negative electrode containing Bi2O3 particles is immersed in an electrolyte (for example, a strong alkaline electrolyte), the following local cell reaction occurs, and Bi2O3 is reduced to Bi. Note that when Bi2O3 powder is immersed alone in an electrolyte, the following reaction does not occur because Zn is not present. In other words, the following reaction can be said to be specific to the negative electrode. -Cathode reaction: Bi2O3+3H2O+6e -→2Bi+6OH - -Anode reaction: 3Zn + 12OH - →3Zn(OH)4 2- +6e - 3Zn(OH)4 2- →3ZnO+6OH - +3H2O

[0014] The discharge reaction at the negative electrode is as follows, and the charge reaction is the opposite: Zn+4OH - →Zn(OH)4 2- +2e - Zn(OH)4 2- →ZnO+2OH - +H2O

[0015] On the other hand, during charging and discharging, in addition to the above reactions, undesirable side reactions can occur in the zinc negative electrode: the negative electrode overcharge reaction shown below, and the self-decomposition reaction of metallic Zn. Note that both of these side reactions occur on the surface of metallic Zn particles. - Overcharge reaction at the negative electrode: 2H2O + 2e - →H2+2OH - - Self-decomposition reaction of metallic Zn: Zn + H2O → H2 + ZnO

[0016] In this regard, Bi present on the surface of metal Zn particles increases the hydrogen generation overvoltage, thereby effectively suppressing overcharge and self-discharge reactions. As a result, it is thought that in zinc secondary batteries, deterioration of the negative electrode due to repeated charge and discharge can be suppressed, improving durability and extending cycle life. Therefore, it is desirable for the negative electrode to have Bi2O3 particles present on the surface of ZnO particles.

[0017] The suppression of hydrogen generation described above is effectively achieved by controlling the average major axis diameter of Bi2O3 particles to 0.3 to 8.5 μm. Here, Figures 1A to 1C show schematic diagrams illustrating the state in which Bi2O3 particles exist on the surface of metal Zn particles. Because the Bi2O3 particles shown in Figure 1A are too small in size (average major axis diameter: less than 0.3 μm), they tend to aggregate easily, resulting in a small amount of Bi2O3 particles present on the surface of the metal Zn particles. In contrast, because the Bi2O3 particles shown in Figure 1C are too large in size (average major axis diameter: more than 8.5 μm), they tend to aggregate less, but the number of Bi2O3 particles present on the surface of the metal Zn particles is small, resulting in a small amount of Bi2O3 particles present on the surface of the metal Zn particles. In contrast, the Bi2O3 particles shown in Figure 1B have an average major axis diameter of 0.3 to 8.5 μm, which allows both particle aggregation and the number of particles present on the surface of the metal Zn particles to be controlled within desirable ranges, resulting in a large amount of Bi2O3 particles present on the surface of the metal Zn particles. Therefore, using a negative electrode containing Bi2O3 particles with such an average major axis diameter in a zinc secondary battery is thought to significantly suppress hydrogen generation and contribute to improved cycle characteristics. Therefore, it is desirable for the negative electrode to contain Bi2O3 particles in the form of non-aggregated primary particles (single particles). The criteria for determining whether aggregation occurs will be described in the Examples section below.

[0018] From the above viewpoints, the average major axis diameter of Bi2O3 particles is 0.3 to 8.5 μm, preferably 1.2 to 8.5 μm, more preferably 2.0 to 7.5 μm, even more preferably 2.7 to 6.5 μm, and particularly preferably 3.5 to 5.0 μm. In the present invention, "major axis diameter" is defined as the length of the long side of the particles. The average major axis diameter can be calculated by observing Bi2O3 powder with a commercially available scanning electron microscope (SEM). A preferred method for calculating the average major axis diameter using SEM is shown in the Examples described below. The reason for evaluating the size of Bi2O3 particles using the average major axis diameter rather than the average particle diameter D50 or the like is as follows. That is, while a particle size distribution analyzer is usually used to calculate the average particle diameter D50, this particle size distribution measurement is affected by the above-mentioned agglomerates. For this reason, in the present invention, the average major axis diameter is used as an index that can more accurately evaluate the size of Bi2O3 primary particles.

[0019] The maximum major axis diameter of the Bi2O3 particles is preferably less than 35 μm, more preferably 2.5 to 30 μm, even more preferably 5.0 to 25 μm, and particularly preferably 10 to 20 μm. However, this maximum major axis diameter must be equal to or greater than the average major axis diameter described above. By narrowing the particle size distribution by controlling the maximum major axis diameter of the Bi2O3 particles in this way, the cycle characteristics of zinc secondary batteries can be further improved. The maximum major axis diameter can be calculated by observing the Bi2O3 powder with a commercially available scanning electron microscope (SEM). A preferred method for calculating the maximum major axis diameter using an SEM is shown in the Examples below.

[0020] The content of Bi2O3 particles in the negative electrode is 0.5 to 20 parts by weight, preferably 1.2 to 13.6 parts by weight, more preferably 1.2 to 10.1 parts by weight, even more preferably 1.2 to 6.6 parts by weight, particularly preferably 1.2 to 4.5 parts by weight, and most preferably 1.2 to 3.3 parts by weight, relative to 100 parts by weight of ZnO particles. This can extend the cycle life of the zinc secondary battery.

[0021] The negative electrode may contain metallic Bi particles. As described above, when the negative electrode is immersed in an electrolyte, it is believed that some or all of the Bi2O3 particles are converted to metallic Bi. In this regard, the negative electrode of the present invention also includes an embodiment in which the negative electrode is immersed in an electrolyte for use in a zinc secondary battery and some or all of the Bi2O3 particles are converted to metallic Bi particles. Furthermore, when the negative electrode contains metallic Bi particles (including when the Bi2O3 particles are converted to metallic Bi particles), the content of metallic Bi is converted to Bi2O3 and then included in the content of the Bi2O3 particles.

[0022] The average particle size D50 of the metal Zn particles is 85 to 250 μm, preferably 85 to 200 μm, more preferably 85 to 180 μm, even more preferably 90 to 160 μm, and particularly preferably 90 to 130 μm. In this specification, the average particle size D50 refers to the particle size at which the cumulative volume from the small particle size side in the particle size distribution obtained by laser diffraction / scattering method is 50%. The content of the metal Zn particles in the negative electrode is 1.0 to 87.5 parts by weight, preferably 2.0 to 80 parts by weight, more preferably 3.0 to 70 parts by weight, even more preferably 4.0 to 62.5 parts by weight, and particularly preferably 5.0 to 55 parts by weight, based on 100 parts by weight of the ZnO particles.

[0023] The ZnO particles are not particularly limited and may be a commercially available zinc oxide powder used in zinc secondary batteries or a zinc oxide powder obtained by growing particles from such a starting material through a solid-state reaction, etc. The average particle size D50 of the ZnO particles is preferably 0.1 to 20 μm, more preferably 0.1 to 15 μm, and even more preferably 0.3 to 12 μm.

[0024] The negative electrode may further contain a binder. When the negative electrode contains a binder, the shape of the negative electrode is more easily maintained. Various known binders can be used, but a preferred example is polytetrafluoroethylene (PTFE). It is particularly preferred to use a combination of both PVA and PTFE as the binder.

[0025] The negative electrode may further contain a conductive additive, examples of which include carbon, metal powder (such as tin, lead, copper, or cobalt), and precious metal paste.

[0026] The negative electrode is preferably a sheet-shaped pressed body. This can prevent the electrode active material from falling off, improve the electrode density, and effectively suppress deformation of the negative electrode. Such a sheet-shaped pressed body can be produced by adding a binder to the negative electrode material and kneading the mixture, and then applying press molding such as roll pressing to form the kneaded mixture into a sheet.

[0027] A current collector is preferably provided on the negative electrode. Preferred examples of the current collector include copper punched metal and copper expanded metal. In this case, a negative electrode plate consisting of a negative electrode / negative electrode current collector can be preferably produced by, for example, applying a mixture containing a Zn compound, metallic zinc and zinc oxide powder, and, if desired, a binder (e.g., polytetrafluoroethylene particles) to the copper punched metal or copper expanded metal. In this case, it is also preferable to press the dried negative electrode plate (i.e., the negative electrode / negative electrode current collector) to prevent the electrode active material from falling off and improve the electrode density. Alternatively, the above-mentioned sheet-like pressed compact may be pressure-bonded to a current collector such as copper expanded metal.

[0028] Zinc secondary battery The negative electrode of the present invention is preferably applied to a zinc secondary battery. Therefore, according to a preferred embodiment of the present invention, a zinc secondary battery is provided, comprising a positive electrode, a negative electrode, a separator that separates the positive electrode and the negative electrode in a manner that allows hydroxide ion conductivity, and an electrolyte. The zinc secondary battery of the present invention is not particularly limited as long as it uses the above-described negative electrode and an electrolyte (typically an aqueous alkali metal hydroxide solution). Therefore, it can be a nickel-zinc secondary battery, a silver oxide-zinc secondary battery, a manganese oxide-zinc secondary battery, a zinc-air secondary battery, or any other type of alkaline zinc secondary battery. For example, it is preferable that the positive electrode contains nickel hydroxide and / or nickel oxyhydroxide, thereby forming a nickel-zinc secondary battery. Alternatively, the positive electrode may be an air electrode, thereby forming a zinc-air secondary battery.

[0029] The separator is preferably a layered double hydroxide (LDH) separator. As mentioned above, LDH separators are known in the fields of nickel-zinc secondary batteries and air-zinc secondary batteries (see Patent Documents 1 to 3), and these LDH separators can be preferably used in the zinc secondary battery of the present invention. LDH separators can selectively allow hydroxide ions to pass through while preventing the penetration of zinc dendrites. This, combined with the effects of the negative electrode of the present invention, can further improve the durability of zinc secondary batteries. In this specification, an LDH separator is defined as a separator containing a layered double hydroxide (LDH) and / or an LDH-like compound (hereinafter collectively referred to as a hydroxide ion-conducting layered compound) that selectively passes hydroxide ions solely by utilizing the hydroxide ion conductivity of the hydroxide ion-conducting layered compound. In this specification, an "LDH-like compound" refers to a hydroxide and / or oxide with a layered crystal structure similar to LDH, even if it may not be called an LDH, and can be considered an equivalent of LDH. However, in a broader sense, "LDH" can be interpreted as including not only LDH but also LDH-like compounds.

[0030] The LDH separator may be composited with a porous substrate, as disclosed in Patent Documents 1 to 3. The porous substrate may be composed of any of ceramic, metal, and polymeric materials, but polymeric materials are particularly preferred. Polymeric porous substrates have the following advantages: 1) flexibility (hence, they are less likely to crack even when thin); 2) ease of achieving high porosity; 3) ease of achieving high conductivity (because thickness can be reduced while increasing porosity); and 4) ease of manufacturing and handling. Particularly preferred polymeric materials are polyolefins such as polypropylene and polyethylene, with polypropylene being the most preferred, due to their excellent hot water resistance, acid resistance, and alkali resistance, as well as low cost. When the porous substrate is composed of a polymeric material, it is particularly preferred that the hydroxide ion-conducting layered compound be incorporated throughout the entire thickness of the porous substrate (e.g., most or almost all of the pores within the porous substrate are filled with the hydroxide ion-conducting layered compound). In this case, the polymeric porous substrate preferably has a thickness of 5 to 200 μm, more preferably 5 to 100 μm, and even more preferably 5 to 30 μm. As such a polymeric porous substrate, a microporous membrane such as those commercially available as separators for lithium batteries can be preferably used.

[0031] The electrolyte preferably contains an aqueous solution of an alkali metal hydroxide. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, lithium hydroxide, and ammonium hydroxide, with potassium hydroxide being more preferred. Zinc oxide, zinc hydroxide, or the like may be added to the electrolyte to suppress self-dissolution of the zinc-containing material.

[0032] LDH-like compounds According to a preferred embodiment of the present invention, the LDH separator may contain an LDH-like compound. The definition of the LDH-like compound is as described above. Preferred LDH-like compounds are: (a) a hydroxide and / or oxide having a layered crystal structure containing Mg and one or more elements selected from the group consisting of Ti, Y, and Al, including at least Ti; or (b) a hydroxide and / or oxide having a layered crystal structure containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) an additional element M which is at least one selected from the group consisting of In, Bi, Ca, Sr, and Ba, or (c) A hydroxide and / or oxide having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In, in which the LDH-like compound is present in the form of a mixture with In(OH)3.

[0033] According to a preferred embodiment (a) of the present invention, the LDH-like compound may be a hydroxide and / or oxide having a layered crystal structure containing Mg and one or more elements, including at least Ti, selected from the group consisting of Ti, Y, and Al. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Mg, Ti, optionally Y, and optionally Al. While the above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, it is preferable that the LDH-like compound does not contain Ni. For example, the LDH-like compound may further contain Zn and / or K. This can further improve the ionic conductivity of the LDH separator.

[0034] LDH-like compounds can be identified by X-ray diffraction. Specifically, when X-ray diffraction is performed on the surface of an LDH separator, peaks derived from LDH-like compounds are typically detected in the range of 5°≦2θ≦10°, more typically in the range of 7°≦2θ≦10°. As mentioned above, LDH is a substance with an alternating layer structure in which exchangeable anions and HO exist as intermediate layers between stacked hydroxide base layers. In this regard, when LDH is measured by X-ray diffraction, a peak inherently derived from the crystalline structure of LDH (i.e., the (003) peak of LDH) is detected at 2θ=11-12°. In contrast, when an LDH-like compound is measured by X-ray diffraction, a peak is typically detected in the above-mentioned range, shifted to a lower angle than the peak position of LDH. Furthermore, the interlayer distance of the layered crystalline structure can be determined by the Bragg equation using 2θ corresponding to the peak derived from the LDH-like compound in X-ray diffraction. The interlayer distance of the layered crystal structure constituting the LDH-like compound determined in this manner is typically 0.883 to 1.8 nm, and more typically 0.883 to 1.3 nm.

[0035] In the LDH separator according to aspect (a), the atomic ratio of Mg / (Mg + Ti + Y + Al) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.03 to 0.25, more preferably 0.05 to 0.2. The atomic ratio of Ti / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0.40 to 0.97, more preferably 0.47 to 0.94. The atomic ratio of Y / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0 to 0.45, more preferably 0 to 0.37. The atomic ratio of Al / (Mg + Ti + Y + Al) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.03. Within these ranges, the LDH separator exhibits superior alkali resistance and more effectively suppresses short circuits caused by zinc dendrites (i.e., dendrite resistance). Incidentally, conventionally known LDH separators have the general formula: M 2+ 1-x M3+ x (OH)2A n- x / n ·mH2O (in the formula, M 2+ is a divalent cation, M 3+ is a trivalent cation, and A n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more. In contrast, the atomic ratios in the LDH-like compounds generally deviate from the general formula of LDH. Therefore, it can be said that the LDH-like compounds in this embodiment generally have compositional ratios (atomic ratios) different from those of conventional LDH. Note that EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing images at an accelerating voltage of 20 kV and a magnification of 5,000x, 2) analyzing three points spaced about 5 μm apart in point analysis mode, 3) repeating the above steps 1) and 2) once more, and 4) calculating the average value of the six points in total.

[0036] According to another preferred embodiment (b) of the present invention, the LDH-like compound may be a hydroxide and / or oxide having a layered crystal structure containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) an additional element M. Thus, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Ti, Y, the additional element M, optionally Al, and optionally Mg. The additional element M is In, Bi, Ca, Sr, Ba, or a combination thereof. While the above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, it is preferred that the LDH-like compound does not contain Ni.

[0037] In the LDH separator according to the above aspect (b), the atomic ratio of Ti / (Mg+Al+Ti+Y+M) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.50 to 0.85, more preferably 0.56 to 0.81. The atomic ratio of Y / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.20, more preferably 0.07 to 0.15. The atomic ratio of M / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.35, more preferably 0.03 to 0.32. The atomic ratio of Mg / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.10, more preferably 0 to 0.02. The atomic ratio of Al / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.04. Within the above range, the alkali resistance is more excellent, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively realized. Incidentally, LDHs conventionally known for LDH separators are represented by the general formula: M 2+ 1-x M 3+ x (OH)2A n- x / n ·mH2O (in the formula, M 2+ is a divalent cation, M 3+ is a trivalent cation, and A n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more. In contrast, the atomic ratios in the LDH-like compounds generally deviate from the general formula of LDH. Therefore, it can be said that the LDH-like compounds in this embodiment generally have compositional ratios (atomic ratios) different from those of conventional LDH. Note that EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing images at an accelerating voltage of 20 kV and a magnification of 5,000x, 2) analyzing three points spaced about 5 μm apart in point analysis mode, 3) repeating the above steps 1) and 2) once more, and 4) calculating the average value of the six points in total.

[0038] According to yet another preferred embodiment (c) of the present invention, the LDH-like compound is a hydroxide and / or oxide having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In, and the LDH-like compound may exist in the form of a mixture with In(OH)3. The LDH-like compound of this embodiment is a hydroxide and / or oxide having a layered crystal structure containing Mg, Ti, Y, and optionally Al and / or In. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Mg, Ti, Y, optionally Al, and optionally In. The In contained in the LDH-like compound may not only be intentionally added to the LDH-like compound, but may also be unavoidably mixed into the LDH-like compound due to the formation of In(OH)3, etc. While the above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, it is preferable that the LDH-like compound does not contain Ni. Conventionally known LDHs for LDH separators are represented by the general formula: M 2+ 1-x M 3+ x (OH)2A n- x / n ·mH2O (in the formula, M 2+ is a divalent cation, M 3+ is a trivalent cation, and A n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more. In contrast, the atomic ratios in LDH-like compounds generally deviate from the above general formula of LDH. Therefore, it can be said that the LDH-like compounds in this embodiment generally have compositional ratios (atomic ratios) different from those of conventional LDHs.

[0039] The mixture according to the above aspect (c) contains not only an LDH-like compound but also In(OH)3 (typically composed of an LDH-like compound and In(OH)3). The inclusion of In(OH)3 can effectively improve the alkali resistance and dendrite resistance of the LDH separator. The content of In(OH)3 in the mixture is preferably an amount that can improve the alkali resistance and dendrite resistance without substantially impairing the hydroxide ion conductivity of the LDH separator, and is not particularly limited. In(OH)3 may have a cubic crystal structure, or may have a structure in which In(OH)3 crystals are surrounded by the LDH-like compound. In(OH)3 can be identified by X-ray diffraction. [Example]

[0040] The present invention is further illustrated by the following examples.

[0041] Examples 1 to 11 (1) Preparation of the positive electrode Paste-type nickel hydroxide positive electrode (capacity density: approx. 700 mAh / cm 3 ) was prepared.

[0042] (2) Preparation of the negative electrode The following raw material powders were prepared. ZnO powder (manufactured by Seido Chemical Industry Co., Ltd., JIS standard type 1 grade, average particle size D50: 0.2 μm) Metallic Zn powder (manufactured by DOWA Electronics Co., Ltd., average particle size D50: 120 μm) ·Bi2O3 powder (manufactured by Vital Materials, average major axis diameter: 0.5μm)

[0043] The average major axis diameter of the Bi2O3 powder was calculated as follows. First, using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, SU-3500), the Bi2O3 powder was observed at a magnification that allowed at least 1,000 Bi2O3 particles (primary particles) to be included in any given observation field. The acquired SEM images are shown in Figures 2A and 2B. The magnification in Figures 2A and 2B was 10,000x, and the observation field was 12.5 × 8.5 μm. As shown in Figure 2A, the length of the long side of the Bi2O3 particles was defined as the "major axis diameter." Next, as shown in Figure 2B, the acquired SEM image was imported into image processing software (Adobe Illustrator, Adobe), and nine division lines were drawn horizontally to divide the observation field into eight equal parts. Particles that touched or crossed one or more of the second to eighth division lines, excluding the two at both ends, were extracted. The major axis diameter of each extracted particle was measured, and the average value was designated as the "average major axis diameter." Furthermore, the largest major axis diameter among the major axis diameters of each particle measured by the above method was designated as the "maximum major axis diameter" described below.

[0044] According to the blending ratios shown in Table 1, ZnO powder was mixed with propylene glycol, Zn metal powder, polytetrafluoroethylene (PTFE), and optionally Bi2O3 powder. The amount of PTFE added was 1.7 parts by weight per 100 parts by weight of ZnO particles. The resulting mixture was rolled using a roll press to obtain a negative electrode active material sheet. The negative electrode active material sheet was then pressure-bonded to a tin-plated copper expand metal to obtain a negative electrode.

[0045] (3) Preparation of electrolyte Ion-exchanged water was added to a 48% aqueous potassium hydroxide solution (special grade, manufactured by Kanto Chemical Co., Ltd.) to adjust the KOH concentration to 5.4 mol%, and then 0.42 mol / L of zinc oxide was dissolved therein by heating and stirring to obtain an electrolyte solution.

[0046] (4) Preparation of evaluation cell Each of the positive electrode and the negative electrode was wrapped with a non-woven fabric, and the current extraction terminals were welded. The prepared positive electrode and negative electrode were opposed to each other through an LDH separator, sandwiched between a laminated film provided with a current extraction port, and three sides of the laminated film were heat-sealed. An electrolytic solution was added to the thus obtained cell container with an open top, and the electrolytic solution was sufficiently permeated into the positive electrode and the negative electrode by evacuation or the like. Thereafter, the remaining one side of the laminated film was also heat-sealed to form a simple sealed cell.

[0047] (5) Evaluation <Cycle characteristics> Using a charge-discharge device (manufactured by Toyo System Co., Ltd., TOSCAT3100), chemical conversion was performed on the simple sealed cell at 0.1C charging and 0.2C discharging. Thereafter, 1C charge-discharge cycles were performed. Repeated charge-discharge cycles were performed under the same conditions, and the number of charge-discharge cycles until the discharge capacity decreased to 70% of the discharge capacity of the first cycle of the prototype battery was recorded, and this was adopted as an index indicating the cycle characteristics. The results are as shown in Table 1, and it was confirmed that the cycle characteristics were improved by adding Bi2O3 particles in the range of 0.5 to 20 parts by weight with respect to 100 parts by weight of ZnO particles for the negative electrodes of various compositions.

[0048]

Table 1

[0049] Examples 12-22 For Examples 12 and 14, in exactly the same manner as Examples 1 and 3 respectively, and for Examples 13 and 15 to 22, evaluation cells were fabricated in the same manner as Example 3 except that they were changed to the commercially available Bi2O3 powders shown below. <Bi2O3 powder> - Examples 13 and 19: Manufactured by Vital Materials Co., Ltd., average major axis diameter: as shown in Table 2 - Examples 15 to 18 and 22: Manufactured by 5N Plus Co., Ltd., average major axis diameter: as shown in Table 2 - Examples 20 and 21: Manufactured by High-Purity Chemical Research Institute Co., Ltd., average major axis diameter: as shown in Table 2

[0050] The cycle characteristics of the obtained evaluation cells were evaluated in the same manner as in Examples 1 to 11. The results are shown in Table 2, and it was confirmed that the cycle characteristics were improved by adjusting the average major axis diameter of the Bi2O3 particles to within the range of 0.3 to 8.5 μm. Furthermore, the presence or absence of aggregation was confirmed for the negative electrodes in Examples 13 to 22 before evaluating the cycle characteristics, as shown below.

[0051] <Presence or absence of aggregation> After embedding the negative electrode in resin and polishing the cross section, the polished negative electrode cross section was observed at 1000x magnification using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, SU-3500). The negative electrode observation range was 10 mm, and cross-sectional SEM images were obtained over a 125 μm × 85 μm field of view at 1 mm intervals (a total of 11 fields of view). If aggregation of Bi2O3 particles was confirmed in two or more fields of view among the obtained cross-sectional SEM images, it was determined that aggregation existed. As a result, it was determined that aggregation existed in Examples 13 to 18 and 21, and that aggregation did not exist in Examples 19, 20, and 22.

[0052] For reference, cross-sectional SEM images of the negative electrodes obtained in Examples 14 and 20 are shown in Figures 3 and 4, respectively. As shown in Figure 3, it was confirmed that at least a portion of the Bi2O3 particles existed in the form of aggregated particles in the negative electrode produced in Example 14. On the other hand, as shown in Figure 4, it was confirmed that the Bi2O3 particles existed in the form of single particles in the negative electrode produced in Example 20. Here, as shown in Table 2, the addition of Bi2O3 particles improved the cycle characteristics in both Examples 14 and 20, but it is believed that the cycle characteristics of Example 20 were even more improved than those of Example 14 because a large amount of Bi2O3 particles existed in the negative electrode in the form of primary particles.

[0053] [Table 2]

[0054] Examples 23-27 For Examples 23, 24, and 26, evaluation cells were fabricated in exactly the same manner as Examples 1, 17, and 20, respectively. For Examples 25 and 27, evaluation cells were fabricated in the same manner as Example 3, except that the commercially available Bi2O3 powders shown below were used. <Bi2O3 powder> - Example 25: Manufactured by Taiyo Kogyo Co., Ltd., average major axis diameter and maximum major axis diameter: as shown in Table 3 - Example 27: Manufactured by 5N Plus, average major axis diameter and maximum major axis diameter: as shown in Table 3

[0055] For the obtained evaluation cells, the cycle characteristics were evaluated in the same manner as in Examples 1 to 11. The results are as shown in Table 3. It was confirmed that when the average major axis diameter of the Bi2O3 particles was the same, the cycle characteristics improved as the maximum major axis diameter became smaller (i.e., the particle size distribution became narrower).

[0056]

Table 3

Claims

1. A negative electrode for use in a zinc secondary battery, ZnO particles; Metal Zn particles having an average particle size D50 of 85 to 250 μm; Bi having an average major axis diameter of 0.3 to 8.5 μm ２ O ３ Particles and Including, The content of the metal Zn particles is 1.0 to 87.5 parts by weight when the content of the ZnO particles is 100 parts by weight, and the Bi ２ O ３ The negative electrode has a particle content of 0.5 to 20 parts by weight.

2. When the content of the ZnO particles is 100 parts by weight, the Bi ２ O ３ 2. The negative electrode according to claim 1, wherein the content of the particles is 1.2 to 13.6 parts by weight.

3. The Bi ２ O ３ 3. The negative electrode according to claim 1, wherein the particles have an average major axis diameter of 1.2 to 8.5 μm.

4. The Bi ２ O ３ 3. The negative electrode according to claim 1, wherein the maximum major axis diameter of the particles is less than 35 μm.

5. The Bi is deposited on the surface of the ZnO particles. ２ O ３ 3. The negative electrode of claim 1, wherein particles are present.

6. The Bi ２ O ３ 3. The negative electrode according to claim 1, wherein the particles are contained in the form of non-agglomerated primary particles.

7. The negative electrode according to claim 1 or 2, wherein the negative electrode is a sheet-like pressed product.

8. A positive electrode and The negative electrode according to claim 1 or 2, a separator that separates the positive electrode and the negative electrode in a manner that allows hydroxide ions to be conducted between them; An electrolyte; A zinc secondary battery comprising:

9. 9. The zinc secondary battery according to claim 8, wherein the separator is an LDH separator containing a layered double hydroxide (LDH) and / or an LDH-like compound.

10. The zinc secondary battery according to claim 9 , wherein the LDH separator is composited with a porous substrate.

11. 9. The zinc secondary battery according to claim 8, wherein the positive electrode comprises nickel hydroxide and / or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery.

12. 9. The zinc secondary battery according to claim 8, wherein the positive electrode is an air electrode, thereby forming the zinc secondary battery into a zinc-air secondary battery.

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