Negative electrode and zinc secondary battery

Abstract

Provided is a negative electrode that enables a zinc secondary battery to have a prolonged cycle service life. A negative electrode that is used in a zinc secondary battery, the negative electrode containing metal Zn particles, and ZnO particles that have an average grain diameter D50 of 1.3-30.0 µm.

Description

[Technical Field]

[0001] This 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 zinc-air secondary batteries, metallic zinc is known to dendrite from the negative electrode during charging, penetrate the voids in the separator (such as nonwoven fabric) and reach the positive electrode, resulting in a short circuit. Such short circuits caused by zinc dendrites shorten the battery's charge-discharge lifespan.

[0003] To address the above problems, batteries equipped with layered double hydroxide (LDH) separators that selectively allow hydroxide ions to permeate while preventing the penetration of zinc dendrites have been proposed. For example, Patent Document 1 (International Publication No. 2013 / 118561) discloses the provision of an LDH separator between the positive and negative electrodes in a nickel-zinc secondary battery. Patent Document 2 (International Publication No. 2016 / 076047) discloses a separator structure equipped with an LDH separator fitted or bonded to a resin outer frame, and discloses that the LDH separator has high density to the extent that it is impermeable to gas and / or water. This document also discloses that the LDH separator can be composited with a porous substrate. Furthermore, Patent Document 3 (International Publication No. 2016 / 067884) discloses various methods for obtaining a composite material by forming an LDH dense film on the surface of a porous substrate. This method includes the steps of uniformly attaching a starting material that can provide a starting point for LDH crystal growth to a porous substrate, and then subjecting the porous substrate to hydrothermal treatment in an aqueous raw material solution to form a dense LDH film on the surface of the porous substrate.

[0004] Furthermore, LDH-like compounds are known as hydroxides and / or oxides with a layered crystalline structure that are similar to LDH, although they cannot be called LDH. These compounds exhibit hydroxide ion conductivity characteristics so similar to LDH that they can be collectively referred to as hydroxide ion-conducting layered compounds. For example, Patent Document 4 (International Publication No. 2020 / 255856) discloses a hydroxide ion-conducting separator comprising a porous substrate and a layered double hydroxide (LDH)-like compound that seals the pores of the porous substrate, wherein the LDH-like compound is a hydroxide and / or oxide with a layered crystalline structure containing Mg and at least one element including Ti selected from the group consisting of Ti, Y, and Al. Patent Document 5 (International Publication No. 2021 / 229916) discloses an LDH separator using an LDH-like compound comprising (i) Ti, Y, and optionally Al and / or Mg, and (ii) at least one additive element M selected from the group consisting of In, Bi, Ca, Sr, and Ba. Furthermore, Patent Document 6 (International Publication No. 2021 / 229917) discloses an LDH separator containing a mixture of an LDH-like compound and In(OH)3, wherein the LDH-like compound is a layered crystalline hydroxide and / or oxide containing Mg, Ti, Y, and optionally Al and / or In. The separators disclosed in Patent Documents 4 to 6 are said to have superior alkali resistance and to be able to more effectively suppress short circuits caused by zinc dendrites compared to conventional LDH separators.

[0005] Incidentally, another factor that leads to a shortened lifespan of zinc secondary batteries is the change in the morphology of zinc, which is the active material of the negative electrode. That is, as zinc dissolves and precipitates repeatedly due to repeated charging and discharging, the negative electrode changes shape, leading to problems such as increased resistance due to pore blockage and a decrease in the active material due to the accumulation of isolated zinc, resulting in difficulties in charging and discharging. To address this problem, Patent Document 7 (International Publication No. 2020 / 049902) proposes using a combination of ZnO particles and at least two selected from (i) metallic Zn particles of a predetermined particle size, (ii) a predetermined metal element, and (iii) a binder resin having a hydroxyl group as the negative electrode. According to this negative electrode, it is possible to suppress the deterioration of the negative electrode due to repeated charging and discharging in zinc secondary batteries, improve durability, and thereby extend the cycle life.

[0006] Furthermore, Patent Document 8 (Japanese Patent No. 6190101) discloses a negative electrode composite material comprising a negative electrode active material such as metallic Zn or ZnO, a polymer such as an aromatic group-containing polymer, an ether group-containing polymer, or a hydroxyl group-containing polymer, and a conductive additive which is a compound of elements such as B, Ba, Bi, Br, Ca, Cd, Ce, Cl, F, Ga, Hg, In, La, and Mn. It is described that this composite material is suitable for forming a rechargeable battery that exhibits high cycle characteristics, rate characteristics, and Coulomb efficiency while suppressing changes in the shape of the electrode active material, such as dendrites, as well as morphological changes, dissolution, corrosion, and passivation. [Prior art documents] [Patent Documents]

[0007] [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 / 255856 [Patent Document 5] International Publication No. 2021 / 229916 [Patent Document 6] International Publication No. 2021 / 229917 [Patent Document 7] International Publication No. 2020 / 049902 [Patent Document 8] Japanese Patent No. 6190101 Gazette [Summary of the Invention]

[0008] As disclosed in Patent Documents 7 and 8, various attempts have been proposed to address the degradation of cycle characteristics associated with the morphological changes of zinc anodes, but further improvement of cycle characteristics is still required.

[0009] The inventors have now found that by increasing the average particle size D50 of ZnO particles contained in the anode to 1.3 to 30.0 μm, the cycle life of a zinc secondary battery can be extended.

[0010] Therefore, an object of the present invention is to provide an anode capable of extending the cycle life of a zinc secondary battery.

[0011] According to the present invention, the following aspects are provided. [Aspect 1] An anode for use in a zinc secondary battery, comprising metallic Zn particles and ZnO particles having an average particle size D50 of 1.3 to 30.0 μm, wherein the anode contains the above components. [Aspect 2] The anode according to Aspect 1, wherein the average particle size D50 of the ZnO particles is 2.0 to 30.0 μm. [Aspect 3] The anode according to Aspect 1 or 2, wherein the average particle size D50 of the ZnO particles is 2.0 to 10.0 μm. [Aspect 4] The anode according to any one of Aspects 1 to 3, wherein when the particle size distribution of the ZnO particles is measured, at least two peaks are present in the particle size distribution. [Aspect 5] The at least two peaks include a first peak and a second peak corresponding to a particle size smaller than the first peak. The first peak and the second peak are the peaks corresponding to the highest frequency in the particle size distribution or the peaks corresponding to the second highest frequency in the particle size distribution. The negative electrode according to embodiment 4, wherein the first peak particle size D1 corresponding to the first peak is in the range of 2.0 to 10.0 μm, and the second peak particle size D2 corresponding to the second peak is in the range of 0.35 to 4.0 μm. [Aspect 6] The negative electrode further comprises binder fibers composed of a binder resin, The negative electrode according to any one of embodiments 1 to 5, wherein the average fiber diameter of the binder fibers is 0.05 to 0.17 μm. [Aspect 7] The negative electrode according to embodiment 6, wherein the content of the binder fibers is 0.05 to 2 parts by weight when the content of the ZnO particles is 100 parts by weight. [Aspect 8] The negative electrode according to embodiment 6 or 7, wherein the binder resin is at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and cellulose resin. [Aspect 9] The negative electrode according to any one of embodiments 1 to 8, wherein the content of the ZnO particles is 100 parts by weight, and the negative electrode contains 1.0 to 87.5 parts by weight of the metallic Zn particles. [Aspect 10] The negative electrode according to any one of embodiments 1 to 9, further comprising one or more metallic elements selected from In and Bi. [Aspect 11] The negative electrode according to any one of embodiments 1 to 10, wherein the negative electrode is a sheet-shaped press-molded body. [Aspect 12] Positive electrode and, A negative electrode as described in any one of embodiments 1 to 11, A separator that separates the positive electrode and the negative electrode in a manner that allows hydroxide ion conduction, Electrolyte and A zinc secondary battery, including one. [Aspect 13] The zinc secondary battery according to embodiment 12, wherein the separator is a layered double hydroxide (LDH) separator. [Aspect 14] The zinc secondary battery according to embodiment 13, wherein the LDH separator is composited with a porous substrate. [Aspect 15] A zinc secondary battery according to any one of embodiments 12 to 14, wherein the positive electrode contains nickel hydroxide and / or nickel oxyhydroxide, and thereby the zinc secondary battery forms a nickel-zinc secondary battery. [Aspect 16] A zinc secondary battery according to any one of embodiments 12 to 14, wherein the positive electrode is an air electrode, and thereby the zinc secondary battery forms a zinc-air secondary battery. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram illustrating the mechanism by which morphological changes are suppressed in the negative electrode of the present invention. [Figure 2] This figure illustrates the first and second peaks in the particle size distribution of ZnO powder. [Figure 3] This is a cross-sectional image of the negative electrode in the final charged state of Example 14, and a magnified view thereof. [Figure 4] This is a cross-sectional image of the negative electrode in the final charged state of Example 36 (comparison), and a magnified view thereof. [Figure 5] This is a schematic diagram illustrating the mechanism by which morphological changes occur in a negative electrode using conventional ZnO particles with small particle sizes. [Figure 6] This is a schematic diagram illustrating the residual gas inside a negative electrode using conventional ZnO particles with small particle sizes. [Modes for carrying out the invention]

[0013] negative electrode The negative electrode of the present invention is a negative electrode used in a zinc secondary battery. This negative electrode contains metallic Zn particles and ZnO particles. The average particle size D50 of the ZnO particles is 1.3 to 30.0 μm. By increasing the average particle size D50 of the ZnO particles contained in the negative electrode to 1.3 to 30.0 μm, the cycle life of the zinc secondary battery can be extended.

[0014] The mechanism by which the cycle life of a zinc secondary battery is extended by keeping the average particle size D50 of ZnO particles within the above range is not entirely clear, but it is thought to be as follows. Here, the average particle size D50 of ZnO particles used as a general industrial material is small, less than 1.0 μm, and in conventional negative electrodes for zinc secondary batteries disclosed in Patent Document 7 (International Publication No. 2020 / 049902), for example, small particle sizes of ZnO particles with an average particle size D50 of about 0.2 μm were used. In this regard, Figure 5 shows the change in the active material during charging and discharging in a conventional negative electrode containing ZnO particles. In Figure 5, the left side shows the negative electrode end, and the right side shows the inside of the negative electrode. First, as shown in Figure 5(i), in the negative electrode in the discharged state before immersion in the electrolyte, ZnO particles are present both at the end and inside. When this negative electrode is immersed in the electrolyte, as shown in Figure 5(ii), a portion of the ZnO is converted to Zn[(OH)4] by an equilibrium reaction. 2- This is the result. At this time, the amount of electrolyte present at the negative electrode end is greater than the amount of electrolyte present inside the negative electrode. Therefore, ZnO particles remain inside the negative electrode, while at the negative electrode end, almost all or all of the ZnO particles are converted to Zn[(OH)4] 2- It changes to this. Subsequently, when this negative electrode is charged, as shown in Figure 5(iii), more metallic Zn is deposited at the negative electrode end compared to the inside of the negative electrode. Subsequently, as shown in Figure 5(iv), when this negative electrode is discharged, metallic Zn is converted to Zn[(OH)4] 2-Although it is converted to ZnO, at this stage, there are almost no ZnO particles remaining at the negative electrode end. Therefore, as shown in Figure 5(v), when ZnO particles precipitate by the equilibrium reaction, ZnO will preferentially precipitate on the ZnO particles remaining inside the negative electrode, resulting in almost no ZnO precipitation at the negative electrode end. Thus, the uneven precipitation of ZnO at the end and inside of the negative electrode causes a change in the morphology of the negative electrode, which is thought to ultimately make charging and discharging difficult.

[0015] In contrast, Figure 1 shows the change in the active material during charging and discharging in the negative electrode of the present invention, which contains ZnO particles with an average particle size D50 within the above predetermined range. As shown in Figure 1(i), in the negative electrode in the discharged state before immersion in the electrolyte, larger particle sizes than conventional ZnO particles are present both at the ends and inside. When this negative electrode is immersed in the electrolyte, as shown in Figure 1(ii), a portion of the ZnO is converted into Zn[(OH)4] by an equilibrium reaction. 2- However, because the ZnO particles are large in size, ZnO nuclei remain even at the negative electrode end. Subsequently, when this negative electrode is charged, metallic Zn is deposited at the negative electrode end and inside, as shown in Figure 1(iii). Then, as shown in Figure 1(iv), when this negative electrode is discharged, metallic Zn is deposited as Zn[(OH)4] 2- Although this changes, ZnO particles remain at the negative electrode end even at this stage. Therefore, as shown in Figure 1(v), when ZnO precipitates due to the equilibrium reaction, ZnO will precipitate not only inside the negative electrode but also on the ZnO particles remaining at the negative electrode end. As a result of this uniform deposition of ZnO at the end and inside of the negative electrode, changes in the negative electrode's morphology are suppressed, and the battery's cycle life is thought to be extended.

[0016] Furthermore, the following are other factors that contribute to the extended cycle life of zinc secondary batteries when using the negative electrode of the present invention. Specifically, as shown in Figure 6, when the negative electrode is immersed in the electrolyte, gas G (e.g., hydrogen gas) may be generated due to side reactions. In this regard, in conventional negative electrodes that use only small-particle ZnO particles, the gaps between the ZnO particles become narrow, making it difficult for gas G to escape from the negative electrode. As a result, gas G remains inside the negative electrode, causing current concentration and making short circuits more likely. In contrast, in the negative electrode of the present invention, since ZnO particles with an average particle size D50 within the above-mentioned predetermined range are used, the larger particle size ensures sufficient gas escape paths. As a result, short circuits are less likely to occur, and the cycle life of the battery is extended.

[0017] Therefore, the average particle size D50 of the ZnO particles contained in the negative electrode is 1.3 to 30.0 μm, preferably 2.0 to 30.0 μm, more preferably 2.0 to 10.0 μm, and even more preferably 2.0 to 7.0 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 accounts for 50% in the particle size distribution obtained by the laser diffraction-scattering method.

[0018] When measuring the particle size distribution of ZnO particles, it is preferable that at least two peaks are present in the particle size distribution. For example, the ZnO particles contained in the negative electrode may be a mixture of large-particle and small-particle ZnO particles. In this way, the small-particle ZnO particles fill the spaces between the large-particle ZnO particles, strengthening the bonds between the ZnO particles. As a result, changes in the morphology of the negative electrode are more effectively suppressed, and the battery cycle life can be extended even further.

[0019] According to a preferred embodiment of the present invention, as shown in Figure 2, the above-mentioned at least two peaks have a first peak and a second peak corresponding to a particle size smaller than the first peak. The first and second peaks are the peaks corresponding to the highest frequency in the particle size distribution or the peaks corresponding to the second highest frequency in the particle size distribution. That is, the first peak may be the peak corresponding to the highest frequency in the particle size distribution and the second peak may be the peak corresponding to the second highest frequency in the particle size distribution, or the first peak may be the peak corresponding to the second highest frequency in the particle size distribution and the second peak may be the peak corresponding to the highest frequency in the particle size distribution. The first peak particle size D1 corresponding to the first peak is preferably in the range of 2.0 to 10.0 μm, and more preferably in the range of 4.0 to 10.0 μm. The second peak particle size D2 corresponding to the second peak is preferably in the range of 0.35 to 4.0 μm, and more preferably in the range of 0.35 to 1.5 μm. When within these ranges, the bonding between ZnO particles becomes extremely strong, and the negative electrode shape is firmly maintained. As a result, changes in the negative electrode's shape are suppressed more effectively, and the battery's cycle life can be extended even further. Furthermore, it becomes possible to achieve a good balance between the moldability and liquid permeability of the negative electrode. Note that the second peak particle size D2 is smaller than the first peak particle size D1 (D2 <D1)ことはいうまでもない。

[0020] The ZnO particles may be commercially available zinc oxide powder that satisfies the above-mentioned predetermined average particle size D50, or zinc oxide powder that has been grown by solid-phase reaction or the like using commercially available zinc oxide powder as a starting material. Alternatively, multiple commercially available products may be mixed and used to satisfy the above-mentioned predetermined average particle size D50.

[0021] The Zn particles contained in the negative electrode are typically metallic Zn particles, but particles of Zn alloys or Zn compounds may also be used. While metallic Zn particles commonly used in zinc secondary batteries can be used, the use of smaller metallic Zn particles is more preferable from the viewpoint of extending the battery's cycle life. Specifically, the average particle size D50 of the metallic Zn particles is preferably 5 to 200 μm, more preferably 50 to 200 μm, and even more preferably 70 to 160 μm. The preferred content of Zn particles in the negative electrode is preferably 1.0 to 87.5 parts by weight, more preferably 3.0 to 70.0 parts by weight, and even more preferably 5.0 to 55.0 parts by weight, based on the ZnO particle content of 100 parts by weight. The metallic Zn particles may be doped with dopants such as In and Bi.

[0022] Preferably, the negative electrode further contains binder fibers composed of a binder resin. The binding force of the binder fibers firmly maintains the shape of the negative electrode, making it possible to further suppress changes in the shape of the negative electrode.

[0023] The average fiber diameter of the binder fibers is preferably 0.05 to 0.17 μm, more preferably 0.08 to 0.17 μm, even more preferably 0.10 to 0.17 μm, and particularly preferably 0.10 to 0.15 μm. This allows for the firm maintenance of the negative electrode shape and effective suppression of changes in the zinc negative electrode's morphology, while suppressing crack formation and difficulty in controlling thickness caused by excessively high bonding strength of the binder fibers. The average fiber diameter can preferably be measured according to the procedure shown in Evaluation 1 of the Examples described later.

[0024] The binder fiber content in the negative electrode is preferably 0.05 to 2 parts by weight, more preferably 0.1 to 2 parts by weight, even more preferably 0.5 to 2 parts by weight, and particularly preferably 1 to 2 parts by weight, based on the ZnO particle content of 100 parts by weight. This makes it possible to more effectively suppress changes in the morphology of the zinc negative electrode.

[0025] The method for forming a negative electrode containing binder fibers is not particularly limited, but for example, the negative electrode can be preferably formed as follows. First, a mixed powder containing ZnO particles, Zn particles, and binder particles (e.g., PTFE particles) is prepared. Next, a predetermined shear pressure is applied to this mixed powder together with a solvent (e.g., propylene glycol or isopropyl alcohol), and the mixture is heated to a predetermined temperature and kneaded. At this time, the shear pressure is preferably 1 to 5 MPa, and more preferably 2 to 5 MPa. The heating temperature is preferably 20 to 60°C, and more preferably 40 to 60°C. By doing so, some or all of the binder particles are converted into fibers, making it easier to obtain binder fibers having the predetermined average fiber diameter. After that, the kneaded material is formed into a sheet and attached to a current collector, and the solvent is removed by drying. In this way, a negative electrode containing binder fibers can be obtained.

[0026] Therefore, the negative electrode may contain binder particles in addition to binder fibers. Various commercially available resin particles for binders can be used as binder particles and are not particularly limited. The average particle size D50 of the binder particles is not particularly limited, but is typically 0.01 to 2 μm, and more typically 0.05 to 1 μm. The binder content in the negative electrode (i.e., the total content of binder particles and binder fibers) is preferably 0.07 to 10 parts by weight, and more preferably 0.3 to 7 parts by weight, when the ZnO particle content is 100 parts by weight.

[0027] The binder resin is preferably at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and cellulose resin (e.g., acetylcellulose resin), and more preferably PTFE, from the viewpoint of fibrosing part or all of it.

[0028] The negative electrode preferably further contains one or more metal elements selected from In and Bi. These metal elements can suppress the generation of undesirable hydrogen gas due to self-discharge of the negative electrode. These metal elements may be included in the negative electrode in any form such as metal, oxide, hydroxide, or other compound, but are preferably included in the form of oxide or hydroxide, and more preferably in the form of oxide particles. Examples of oxides of the above metal elements include In2O3 and Bi2O3. Examples of hydroxides of the above metal elements include In(OH)3 and Bi(OH)3. In any case, when the content of ZnO particles is 100 parts by weight, the In content is preferably 0 to 2 parts by weight in terms of oxide, and the Bi content is preferably 0 to 6 parts by weight in terms of oxide, and more preferably the In content is 0 to 1.5 parts by weight in terms of oxide, and the Bi content is preferably 0 to 4.5 parts by weight in terms of oxide. When In and / or Bi are included in the negative electrode in the form of oxides or hydroxides, not all of the In and / or Bi must be in the form of oxides or hydroxides; some of them may be included in the negative electrode in other forms, such as metals or other compounds. For example, the above metal elements may be doped into metallic Zn particles as trace elements. In this case, the concentration of In in the metallic Zn particles is preferably 50 to 2000 ppm by weight, more preferably 200 to 1500 ppm by weight, and the concentration of Bi in the metallic Zn particles is preferably 50 to 2000 ppm by weight, more preferably 100 to 1300 ppm by weight.

[0029] The negative electrode may further contain a conductive additive. Examples of conductive additives include carbon, metal powders (tin, lead, copper, cobalt, etc.), and precious metal pastes.

[0030] The negative electrode is preferably a sheet-shaped press-molded body. This prevents the detachment of the negative electrode active material and improves electrode density, thereby more effectively suppressing changes in the negative electrode's shape. Such a sheet-shaped press-molded body can be produced by adding a binder to the negative electrode material and kneading it, then press-molding the resulting mixture using a roll press or similar method to form it into a sheet.

[0031] It is preferable to provide a current collector at the negative electrode. Preferred examples of current collectors include copper perforated metal and copper expanded metal. In this case, for example, a negative electrode plate consisting of a negative electrode and a current collector can be preferably manufactured by coating a mixture containing Zn particles, ZnO particles, and a binder onto the copper perforated metal or copper expanded metal. At that time, it is also preferable to press the negative electrode plate (i.e., negative electrode / current collector) after drying to prevent the detachment of the negative electrode active material and to improve the electrode density. Alternatively, a sheet-like press-molded body as described above may be pressed onto a current collector such as copper expanded metal.

[0032] Zinc rechargeable battery The negative electrode of the present invention is preferably applied to a zinc secondary battery. Accordingly, 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 isolates the positive and negative electrodes in a manner that allows hydroxide ion conduction, and an electrolyte. The zinc secondary battery of the present invention is not particularly limited as long as it uses the negative electrode described above and an electrolyte (typically an aqueous alkali metal hydroxide solution). Accordingly, it can be a nickel-zinc secondary battery, a silver-zinc secondary battery, a manganese-zinc secondary battery, a zinc-air secondary battery, or various other alkali-zinc secondary batteries. For example, it is preferable that the positive electrode contains nickel hydroxide and / or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery. Alternatively, the positive electrode may be an air electrode, thereby making the zinc secondary battery a zinc-air secondary battery.

[0033] The separator is preferably a layered double hydroxide (LDH) separator. That is, as mentioned above, LDH separators are known in the fields of nickel-zinc secondary batteries and zinc-air secondary batteries (see Patent Documents 1 to 6), and this LDH separator can also be preferably used in the zinc secondary battery of the present invention. The LDH separator can selectively allow hydroxide ions to pass through while preventing the penetration of zinc dendrites. Combined with the effects of employing the negative electrode of the present invention, the durability of the zinc secondary battery can be further improved. In this specification, an LDH separator is defined as a separator containing layered double hydroxide (LDH) and / or LDH-like compounds (hereinafter collectively referred to as hydroxide ion-conducting layered compounds), which selectively allows hydroxide ions to pass through by exclusively utilizing the hydroxide ion conductivity of the hydroxide ion-conducting layered compounds. In this specification, "LDH-like compounds" are hydroxides and / or oxides with a layered crystalline structure that have hydroxide ion conductivity, although they may not be called LDH, and can be considered equivalents of LDH. However, in a broader sense, "LDH" can also be interpreted as encompassing not only LDH but also LDH-like compounds.

[0034] The LDH separator may be a composite with a porous substrate as disclosed in Patent Documents 1 to 6. The porous substrate may be composed of ceramic materials, metal materials, or polymer materials, but it is particularly preferred to be composed of a polymer material. Polymer porous substrates have the following advantages: 1) they are flexible (therefore they are not easily cracked even when thin), 2) they are easy to make highly porous, 3) they are easy to make highly conductive (because the thickness can be reduced while increasing the porosity), and 4) they are easy to manufacture and handle. Particularly preferred polymer materials are polyolefins such as polypropylene and polyethylene, which have excellent resistance to hot water, acid and alkali, and are also low cost, with polypropylene being the most preferred. When the porous substrate is composed of a polymer material, it is particularly preferred that a hydroxide ion conductive layered compound is incorporated throughout the entire thickness direction of the porous substrate (for example, most or almost all of the pores inside the porous substrate are filled with the hydroxide ion conductive layered compound). In this case, the preferred thickness of the polymer porous substrate is 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 preferably be used.

[0035] 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, but potassium hydroxide is more preferred. Zinc oxide, zinc hydroxide, etc. may be added to the electrolyte to suppress the self-dissolution of zinc-containing materials.

[0036] LDH-like compounds According to a preferred embodiment of the present invention, the LDH separator may contain an LDH-like compound. The definition of an LDH-like compound is as described above. Preferred LDH-like compounds are: (a) A hydroxide and / or oxide with a layered crystalline structure comprising Mg and one or more elements selected from the group consisting of Ti, Y, and Al, including at least Ti, or (b) A layered crystalline hydroxide and / or oxide comprising (i) Ti, Y, and optionally Al and / or Mg, and (ii) an additive element M selected from the group consisting of In, Bi, Ca, Sr and Ba, or (c) Hydroxides and / or oxides of a layered crystalline structure comprising Mg, Ti, Y, and optionally Al and / or In, wherein the LDH-like compound exists in the form of a mixture with In(OH)3.

[0037] According to a preferred embodiment (a) of the present invention, the LDH-like compound may be a hydroxide and / or oxide with a layered crystalline structure containing Mg and at least one element selected from the group consisting of Ti, Y, and Al, including at least Ti. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Mg, Ti, optionally Y, and optionally Al. 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, but 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.

[0038] 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 originating from LDH-like compounds are typically detected in the range of 5°≦2θ≦10°, and more typically in the range of 7°≦2θ≦10°. As mentioned above, LDH is a material with an alternating layered structure in which exchangeable anions and H2O exist as intermediate layers between stacked hydroxide base layers. In this respect, when LDH is measured by X-ray diffraction, a peak originating from the crystal structure of LDH (i.e., the (003) peak of LDH) is originally detected at 2θ=11~12°. In contrast, when LDH-like compounds are measured by X-ray diffraction, peaks are typically detected in the aforementioned range, shifted to a lower angle than the peak position of LDH. Furthermore, the interlayer distance of the layered crystal structure can be determined by Bragg's equation using the 2θ corresponding to the peak originating 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 way is typically 0.883–1.8 nm, and more typically 0.883–1.3 nm.

[0039] The LDH separator according to embodiment (a) above preferably has an atomic ratio of Mg / (Mg+Ti+Y+Al) in the LDH-like compound determined by energy-dispersive X-ray analysis (EDS) of 0.03 to 0.25, more preferably 0.05 to 0.2. Furthermore, 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. Additionally, 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. Finally, 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, alkali resistance is further improved, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively achieved. By the way, the conventionally known LDH for LDH separators is: General formula: M 2+ 1-x M3+ x (OH)2A n- x / n ·mH2O (where 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) and can be represented by the basic composition. In contrast, the atomic ratios in the LDH-like compound generally deviate from those of the LDH in the above general formula. Therefore, it can be said that the LDH-like compound in this embodiment generally has a different composition ratio (atomic ratio) from that of conventional LDH. Note that EDS analysis is performed by using an EDS analyzer (for example, X-act, manufactured by Oxford Instruments) to 1) capture an image at an acceleration voltage of 20 kV and a magnification of 5,000 times, 2) perform point analysis at intervals of about 5 μm in the point analysis mode, perform three-point analysis, 3) repeat the above 1) and 2) one more time, and 4) calculate the average value of a total of six points.

[0040] According to another preferred embodiment (b) of the present invention, the LDH-like compound can be a hydroxide and / or oxide having a layered crystal structure containing (i) Ti, Y, and optionally Al and / or Mg, and (ii) the additive element M. Therefore, a typical LDH-like compound is a composite hydroxide and / or composite oxide of Ti, Y, the additive element M, optionally Al, and optionally Mg. The additive element M is In, Bi, Ca, Sr, Ba, or a combination thereof. The above elements may be replaced with other elements or ions to such an extent that the basic characteristics of the LDH-like compound are not impaired, but it is preferable that the LDH-like compound does not contain Ni.

[0041] The LDH separator according to embodiment (b) above preferably has an atomic ratio of Ti / (Mg+Al+Ti+Y+M) in the LDH-like compound determined by energy-dispersive X-ray analysis (EDS) of 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 preferably has an atomic ratio of 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 preferably has an atomic ratio of 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 preferably has an atomic ratio of 0 to 0.10, more preferably 0 to 0.02. Furthermore, the atomic ratio of Al / (Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.05, and more preferably 0 to 0.04. Within this range, alkali resistance is further improved, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively achieved. Incidentally, conventionally known LDH separators have the general formula: M 2+ 1-x M 3+ x (OH)2A n- x / n ·mH2O (in the formula, M 2+ M is a divalent cation. 3+ A is a trivalent cation, n- LDH can be represented by the basic composition (where is an n-valent anion, n is an integer greater than or equal to 1, x is between 0.1 and 0.4, and m is greater than or equal to 0). In contrast, the atomic ratios in LDH-like compounds generally deviate from the above general formula for LDH. For this reason, LDH-like compounds in this embodiment generally have a different composition ratio (atomic ratio) than conventional LDH. EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) acquiring an image with an acceleration voltage of 20 kV and a magnification of 5,000x, 2) performing 3-point analysis with an interval of about 5 μm in point analysis mode, 3) repeating 1) and 2) once more, and 4) calculating the average value of the total of 6 points.

[0042] According to yet another preferred embodiment (c) of the present invention, the LDH-like compound is a layered crystalline hydroxide and / or oxide 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 in this embodiment is a layered crystalline hydroxide and / or oxide 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 that may be contained in the LDH-like compound may not only be intentionally added to the LDH-like compound, but may also be inevitably mixed into the LDH-like compound due to the formation of In(OH)3, etc. 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, but it is preferable that the LDH-like compound does not contain Ni. Incidentally, the conventionally known LDH as an LDH separator has the general formula: M 2+ 1-x M 3+ x (OH)2A n- x / n ·mH2O (in the formula, M 2+ M is a divalent cation. 3+ A is a trivalent cation, n- LDH can be represented by the basic composition (where is an n-valent anion, n is an integer greater than or equal to 1, x is between 0.1 and 0.4, and m is greater than or equal to 0). In contrast, the atomic ratios in LDH-like compounds generally deviate from the above general formula for LDH. For this reason, LDH-like compounds in this embodiment generally have a different composition ratio (atomic ratio) than conventional LDH.

[0043] The mixture according to embodiment (c) above 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 effectively improves the alkali resistance and dendrite resistance of the LDH separator. The amount of In(OH)3 in the mixture is preferably such that it improves alkali resistance and dendrite resistance without significantly impairing the hydroxide ion conductivity of the LDH separator, and is not particularly limited. In(OH)3 may have a cube-like crystalline structure, or the In(OH)3 crystal may be surrounded by the LDH-like compound. In(OH)3 can be identified by X-ray diffraction. [Examples]

[0044] The present invention will be further explained by the following examples.

[0045] Examples 1-44 (1) Preparation of the positive electrode Paste-type nickel hydroxide cathode (capacitance density: approximately 700 mAh / cm³) 3 I prepared ).

[0046] (2) Fabrication of the negative electrode The following various raw material powders were prepared. • Metallic zinc powder (manufactured by DOWA Electronics Co., Ltd., doped with Bi and In; Bi: 70 ppm by weight, In: 200 ppm by weight, average particle size D50: 120 μm) • PTFE particles (manufactured by Daikin Industries, Ltd., product number: D-210C, average particle size D50: 0.25 μm)

[0047] Furthermore, eleven types of ZnO powder with different average particle sizes D50 were prepared. Each ZnO powder was a commercially available product, with average particle sizes D50 of 0.35 μm, 0.7 μm, 1.0 μm, 1.3 μm, 1.5 μm, 2.0 μm, 4.0 μm, 5.0 μm, 10.0 μm, 40.0 μm, and 70.0 μm, respectively. These ZnO powders were used either as is, or mixed in predetermined proportions to achieve the average particle sizes D50 shown in Tables 1 and 2.

[0048] The average particle size D50 of the ZnO powder was calculated by dispersing the sample in water and placing it in a laser diffraction / scattering particle size distribution analyzer (Horiba, Ltd., LA-950V2) to measure the particle size distribution. The refractive index of the particles was set to 2.00, and the refractive index of the dispersion medium (water) was set to 1.33 for the measurement. The first peak particle size D1 and the second peak particle size D2 were calculated from the obtained particle size distribution. The average particle size D50, first peak particle size D1, and second peak particle size D2 for each example of ZnO powder are shown in Tables 1 and 2.

[0049] 100 parts by weight of ZnO powder was mixed with 5.7 parts by weight of metallic Zn powder and 1 part by weight of PTFE particles as a binder resin. This mixture was heated and kneaded together with propylene glycol to obtain a compound in which the PTFE particles were fibrous. At this time, the fiber diameter of the binder fibers was adjusted as shown in Tables 1 and 2 by appropriately changing the shear pressure from 0.5 to 7.0 MPa and the heating temperature from 15°C to 70°C. The resulting compound was rolled in a roll press to obtain a negative electrode active material sheet. The negative electrode active material sheet was pressed onto tin-plated copper expanded metal to obtain the negative electrode.

[0050] (3) Preparation of electrolyte A 48% potassium hydroxide aqueous solution (manufactured by Kanto Chemical Co., Ltd., special grade) was mixed with deionized water to adjust the KOH concentration to 5.4 mol%, and then zinc oxide was dissolved in it at a concentration of 0.42 mol / L by heating and stirring to obtain the electrolyte.

[0051] (4) Creation of evaluation cells Each of the positive and negative electrodes was wrapped in nonwoven fabric, and current extraction terminals were welded to them. The prepared positive and negative electrodes were then placed opposite each other via an LDH separator and sandwiched between laminate films with current extraction ports, and three sides of the laminate film were heat-sealed. Electrolyte was added to the resulting open-topped cell container, and the electrolyte was thoroughly permeated into the positive and negative electrodes by vacuuming or other means. After that, the remaining side of the laminate film was also heat-sealed to create a simple sealed cell.

[0052] (5) Evaluation Rating 1 : Measurement of binder fiber diameter The negative electrodes of Examples 1-44 were immersed in the electrolyte prepared in (3) above to dissolve the ZnO and Zn particles. Then, the PTFE extracted in a 3 μm × 4 μm field of view was observed using a field emission scanning electron microscope (FE-SEM, Hitachi High-Tech Corporation, S-4800) at a magnification of 30,000x. The acquired SEM images were imported into image processing software (Adobe Illustrator, Adobe). The fiber diameter of the binder fibers was measured at 10 points, and the average value was taken as the average fiber diameter. The results are shown in Tables 1 and 2.

[0053] Rating 2 Cycle characteristics Using a charge / discharge device (TOSCAT3100, manufactured by Toyo System Co., Ltd.), chemical conversion was performed on a simple sealed cell with 0.1C charging and 0.2C discharging. Subsequently, a 1C charge / discharge cycle was performed. The charge / discharge cycle was repeatedly 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. The number of charge / discharge cycles for each example was calculated as a relative value with the number of charge / discharge cycles in Example 1 set to 1.00, and this was adopted as an index indicating the cycle characteristics (cycle characteristic ratio). The results are shown in Tables 1 and 2, and it was confirmed that the cycle characteristics were improved by including ZnO powder with a predetermined average particle size D50 in the negative electrode.

[0054] Rating 3 : Observation of the negative electrode cross-section For Examples 14 and 36 (comparative), chemical conversion was performed on a simple sealed cell using a charge / discharge device (TOSCAT3100, manufactured by Toyo System Co., Ltd.) with 0.1C charging and 0.2C discharging. Furthermore, after 1.2C charging, the negative electrode in the fully charged state was removed and its cross-section was polished. Composition analysis was performed on the edges of the polished negative electrode cross-section using an EDS analyzer (device name: X-act, manufactured by Oxford Instruments). This analysis was performed by 1) acquiring images with an acceleration voltage of 15.0kV and a magnification of 300x, and 2) using point analysis mode. For reference, the negative electrode cross-sectional image and its magnified view for Example 14 are shown in Figure 3, and the negative electrode cross-sectional image and its magnified view for Example 36 (comparative) are shown in Figure 4. EDS analysis of the negative electrode end of Example 14 revealed that the oxygen concentration (1st point: 43.47 atm%, 2nd point: 9.27 atm%) was higher than the Zn concentration (1st point: 10.12 atm%, 2nd point: 5.59 atm%) at each point, confirming that ZnO remained at the negative electrode end in the final charged state (see Figure 1(iii)). On the other hand, EDS analysis of the negative electrode end of Example 36 revealed that the oxygen concentration (1st point: 19.44 atm%, 2nd point: 11.48 atm%, 3rd point: 5.42 atm%) was significantly lower than the Zn concentration (1st point: 39.97 atm%, 2nd point: 42.03 atm%, 3rd point: 46.93 atm%) at each point, confirming that metallic Zn was dominant at the negative electrode end in the final charged state (see Figure 5(iii)).

[0055] [Table 1]

[0056] [Table 2]

Claims

1. The negative electrode used in a zinc secondary battery, Metallic Zn particles, ZnO particles with an average particle size D50 of 1.3 to 30.0 μm, The negative electrode, including the negative electrode.

2. The negative electrode according to claim 1, wherein the average particle size D50 of the ZnO particles is 2.0 to 30.0 μm.

3. The negative electrode according to claim 1, wherein the average particle size D50 of the ZnO particles is 2.0 to 10.0 μm.

4. The negative electrode according to any one of claims 1 to 3, wherein when the particle size distribution of the ZnO particles is measured, at least two peaks are present in the particle size distribution.

5. The at least two peaks include a first peak and a second peak corresponding to a particle size smaller than the first peak. The first peak and the second peak are the peaks corresponding to the highest frequency in the particle size distribution or the peaks corresponding to the second highest frequency in the particle size distribution. The first peak particle size D corresponding to the first peak １ The second peak particle size D is in the range of 2.0 to 10.0 μm, and corresponds to the second peak. ２ The negative electrode according to claim 4, wherein the thickness is in the range of 0.35 to 4.0 μm.

6. The negative electrode further comprises binder fibers composed of a binder resin, The negative electrode according to any one of claims 1 to 3, wherein the average fiber diameter of the binder fibers is 0.05 to 0.17 μm.

7. The negative electrode according to claim 6, wherein the content of the binder fibers is 0.05 to 2 parts by weight when the content of the ZnO particles is 100 parts by weight.

8. The negative electrode according to claim 6, wherein the binder resin is at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and cellulose resin.

9. The negative electrode according to any one of claims 1 to 3, wherein the content of the ZnO particles is 100 parts by weight, and the negative electrode contains 1.0 to 87.5 parts by weight of the metallic Zn particles.

10. The negative electrode according to any one of claims 1 to 3, further comprising one or more metallic elements selected from In and Bi.

11. The negative electrode according to any one of claims 1 to 3, wherein the negative electrode is a sheet-shaped press-molded body.

12. Positive electrode and, A negative electrode according to any one of claims 1 to 3, A separator that separates the positive electrode and the negative electrode in a manner that allows hydroxide ion conduction, Electrolyte and A zinc secondary battery, including one.

13. The zinc secondary battery according to claim 12, wherein the separator is a layered double hydroxide (LDH) separator.

14. The zinc secondary battery according to claim 13, wherein the LDH separator is composited with a porous substrate.

15. The zinc secondary battery according to claim 12, wherein the positive electrode contains nickel hydroxide and / or nickel oxyhydroxide, thereby the zinc secondary battery forms a nickel-zinc secondary battery.

16. The zinc secondary battery according to claim 12, wherein the positive electrode is an air electrode, and thereby the zinc secondary battery forms a zinc-air secondary battery.

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