JP2018133325A - Positive electrode and secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for secondary cell, capable of expecting an increased capacity and achieving high stability during charging and discharging.SOLUTION: The positive electrode for secondary cell, containing layering double hydroxide (LDH) as a positive electrode active material, can maintain a LDH crystal structure even under the last stage of discharge after repeating a charging and discharging cycle including over-charging and discharging 30 cycles.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a positive electrode and a secondary battery including the positive electrode.

  Although nickel-zinc secondary batteries have been developed and studied for a long time, it is known that there are significant barriers to practical use. This is because the zinc constituting the negative electrode produces dendritic crystals called dendrite during charging, and this dendrite breaks through the separator and causes a short circuit with the positive electrode. On the other hand, nickel cadmium batteries and nickel metal hydride batteries have already been commercialized. However, the nickel-zinc secondary battery has an extremely high theoretical capacity density of about 5 times that of the nickel-cadmium secondary battery, 2.5 times that of the nickel-hydrogen secondary battery, and 1.3 times that of the lithium-ion battery. And the raw material price is low. Therefore, a technique for preventing a short circuit due to zinc dendrite in a nickel zinc secondary battery is strongly desired.

In order to cope with such problems and demands, a layered double hydroxide (LDH) separator having hydroxide ion (OH ⁻ ) conductivity has been proposed. For example, Patent Document 1 (International Publication No. 2013/118561) discloses that in a nickel zinc secondary battery, an LDH separator is provided between a positive electrode and a negative electrode for the purpose of preventing a short circuit due to zinc dendrite. Patent Document 2 (International Publication No. 2016/076047) discloses a separator structure including an LDH separator combined with a porous substrate, and the LDH separator is gas-impermeable and / or It is disclosed to have high density enough to have water impermeability.

On the other hand, Patent Document 3 (Japanese Patent Application Laid-Open No. Sho 61-285661) discloses a general formula [M ²⁺ _(1-x) Fe ³⁺ _x (OH) ₂ ] ^{X +} [(x / n) in an uncharged state in an alkaline storage battery. ) X ⁿ⁻ , yH ₂ O] ^X− (wherein x is 0.05 to 0.4, M ²⁺ represents an oxidizing and reducing cation, and X ⁿ⁻ is at least a complex double hydroxide. It is disclosed to use a positive storage battery electrode that is a double hydroxide of any anion that guarantees the charge of the product cations.

International Publication No. 2013/118561 International Publication No. 2016/076047 JP-A 61-285661

The positive electrode of a nickel zinc battery generally contains nickel hydroxide and / or nickel oxyhydroxide. For example, when a nickel-zinc battery is configured in a discharged state, nickel hydroxide is included in the positive electrode, whereas in a fully charged state, nickel oxyhydroxide is included in the positive electrode. Nickel hydroxide, which is a typical positive electrode active material, is β-Ni (OH) ₂ (theoretical capacity 289 mAh / g, density 4.1 g / cm ³ ) having a stable structure even under strong alkali. It changes reversibly to β-Ni (OH) ₂ ⇔β-NiOOH (average valence 3, density 4.7 g / cm ³ ) by the discharge reaction. By the way, when the nickel zinc battery provided with such a positive electrode is overcharged, a compound of γ-NiOOH (average valence 3.6 to 3.7, density 3.79 g / cm ³ ) is generated on the positive electrode. This γ-NiOOH has a larger number of reaction electrons than the β type, and an increase in capacity can be expected. Specifically, γ-NiOOH can be expected to improve the capacity of nickel hydroxide (β-type), a current material that can only perform one-electron oxidation, by a factor of 1.5. However, α-Ni (OH) ₂ generated by reduction accompanying discharge is unstable and easily returns to β-Ni (OH) ₂ , and volume shrinkage occurs at that time, which causes a problem of deteriorating the electrode. is there. Therefore, there is a demand for a positive electrode active material that can avoid the above problems while having the above-described advantages of γ-NiOOH.

  The present inventors have recently found that certain layered double hydroxides (LDH) can reversibly change the interlayer distance with charge and discharge, and therefore can be employed as a positive electrode active material for secondary batteries. I found out. Further, by adopting a positive electrode (hereinafter also referred to as an LDH positive electrode) containing such LDH as a positive electrode active material, it is expected that an increase in capacity can be expected and a secondary battery having high stability associated with charge / discharge can be provided. Obtained.

  Accordingly, an object of the present invention is to provide a positive electrode for a secondary battery that is expected to increase in capacity and has high stability accompanying charge / discharge.

  According to one aspect of the present invention, there is provided a positive electrode including a layered double hydroxide (LDH) as a positive electrode active material, and LDH even in a final discharge state after 30 charge / discharge cycles including overcharge and discharge are repeated. A positive electrode capable of maintaining a crystal structure is provided.

  According to another aspect of the present invention, there is provided a secondary battery comprising the positive electrode, the negative electrode, and an alkaline electrolyte and / or hydroxide ion conductive solid electrolyte.

It is a figure which shows notionally the nickel zinc secondary battery using the positive electrode by this invention. It is a figure which shows notionally the LDH secondary battery using the positive electrode by this invention. It is a figure which shows the charging / discharging characteristic of the Ni-V-LDH positive electrode (Ni: V = 4: 1) measured in Example A1. It is a figure which shows the charging / discharging characteristic of the Ni-Fe-LDH positive electrode (Ni: Fe = 4: 1) measured in Example A2. Ni-V-LDH (Ni: V = 4: 1) measured in Example A1, before charge / discharge (as pre), full charge state after 10 charge / discharge cycles (after 10 cycle Cha), and charge / discharge The XRD profile in the end-of-discharge state (after 30 cycle Dis) after 30 cycles is shown. Measured in Example A3 (comparative), β-Ni (OH) ₂ before charge / discharge (as pre), full charge state after 10 charge / discharge cycles (after 10 cycle Cha), and 10 charge / discharge cycles The XRD profile in the latter end-of-discharge state (after 10 cycle Dis) is shown. The Fourier transform diagram of EXAFS of Ni-V-LDH (Ni: V = 4: 1) measured in Example A1 before reaction, after charge, and after discharge is shown. The Fourier transform diagram of EXAFS before reaction, after charge, and after discharge of Ni-Fe-LDH (Ni: Fe = 4: 1) measured in Example A2 is shown. The Fourier transform diagram of EXAFS measured in Example A3 (comparative) before the reaction of Ni (OH) ₂ , after charging, and after discharging is shown. It is a SEM image which image | photographed 40000 times the Ni-Fe-LDH powder produced in Example B1. It is a charging / discharging curve measured about the nickel zinc secondary battery which used the Ni-Fe-LDH powder produced in Example B1 for the positive electrode. It is a SEM image which image | photographed 40000 times the Ni-V-LDH powder produced in Example B4. It is the charging / discharging curve measured about the nickel zinc secondary battery which used the Ni-V-LDH powder produced in Example B4 for the positive electrode.

Positive Electrode The positive electrode according to the present invention contains layered double hydroxide (LDH) as a positive electrode active material. The positive electrode is characterized as being capable of maintaining the LDH crystal structure even in the final discharge state after 30 charge / discharge cycles including overcharge and discharge have been repeated. Thus, the layered double hydroxide (LDH) can reversibly change the interlayer distance with charge and discharge, and can therefore be employed as a positive electrode active material for a secondary battery. By adopting an LDH positive electrode exhibiting such characteristics, it is possible to provide a positive electrode for a secondary battery that is expected to increase in capacity and has high stability associated with charge / discharge. In particular, the charge / discharge cycle includes overcharge, and it is a surprising finding that even when such overcharge is repeated, the LDH crystal structure can be maintained, and it can be said that extremely high stability against charge / discharge can be expected.

  Typically, the above charge / discharge cycle repeated 30 cycles comprises a positive electrode as a working electrode, a platinum electrode as a counter electrode, a Hg / HgO electrode as a reference electrode, and a 6M KOH aqueous solution as an electrolyte. In the provided electrochemical cell, it is performed under the conditions of a current density of 50 mA / g, a discharge cutoff voltage of 0.2 V, and a charge cutoff voltage of 0.7 V. Further, in the charge / discharge cycle, it is preferable that overcharge is performed up to twice the capacity when the positive electrode active material undergoes one electron reaction, and the discharge is performed at the same current density as the overcharge.

  The crystal structure of LDH contained in the positive electrode can be identified by detecting (003) peak and (006) peak derived from LDH in X-ray diffraction. Then, the (003) peak and the (006) peak derived from LDH are detected at positions within 2θ = ± 1 ° with respect to the peak position before the charge / discharge cycle in the final state of discharge after 30 cycles. Is preferred.

The positive electrode according to the present invention includes layered double hydroxide (LDH) as a positive electrode active material. LDH is known as a compound in which mainly divalent and trivalent (sometimes tetravalent) ions form a layered hydroxide, and an anion such as carbonate ion is inserted between the layers to balance the charge between them. . Here, since divalent and trivalent (sometimes tetravalent) ions can be selected from a wide range of candidates, inexpensive metals such as manganese and iron can be used. This LDH has a structure similar to α-Ni (OH) ₂ and can reversibly change the interlayer distance with charge / discharge. As described above, charging from β-Ni (OH) ₂ to γ-NiOOH can be expected to improve the capacity by 1.5 times compared to charging from β-Ni (OH) ₂ to β-NiOOH. There is a problem that α-Ni (OH) ₂ generated by reduction accompanying discharge is unstable and easily returns to β-Ni (OH) ₂ , and the electrode deteriorates due to volume shrinkage. Therefore, there is a demand for a positive electrode active material that can avoid the above problems while having the above-described advantages of γ-NiOOH. In this regard, according to the positive electrode of the present invention, it can be expected to satisfy these requirements conveniently.

The positive electrode includes LDH as a positive electrode active material. That is, in the positive electrode of the present invention, LDH itself is involved in the charge / discharge reaction of the positive electrode. The LDH contained in the positive electrode preferably contains at least one selected from Ni, Fe and Mn as a constituent element. That is, as is generally known, LDH is composed of a plurality of hydroxide base layers and an intermediate layer interposed between the plurality of hydroxide base layers. The hydroxide base layer is mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layer of LDH is composed of anions and H ₂ O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH comprises OH ⁻ and / or CO ₃ ²⁻ . Therefore, the LDH contained in the positive electrode preferably contains at least one selected from Ni, Fe and Mn as a part of the metal element constituting the hydroxide base layer. Among these, Ni-based LDH containing at least Ni is particularly preferable.

  More preferably, the LDH contained in the positive electrode contains a combination of Ni and V, a combination of Ni and Fe, or a combination of Ni and Mn as constituent elements. That is, the LDH contained in the positive electrode preferably contains Ni and V, or Ni and Fe, or Ni and Mn as a metal element constituting the hydroxide base layer. In other words, preferred examples of LDH contained in the positive electrode include Ni-V-LDH, Ni-Fe-LDH, and Ni-Mn-LDH. Particularly preferred is Ni-V-LDH or Ni-Fe-LDH. In the case of Ni-V-LDH, the molar ratio of V / (Ni + V) is preferably 0.15 to 0.50, more preferably 0.20 to 0.40, still more preferably 0.20. 0.35. On the other hand, in the case of Ni—Fe—LDH, the Fe / (Ni + Fe) molar ratio is preferably 0.15 to 0.50, more preferably 0.20 to 0.40, and still more preferably 0.8. 20-0.35. Within these ranges, the charge / discharge capacity can be further improved when used for the positive electrode.

Secondary Battery As described above, by employing the LDH positive electrode of the present invention, it is possible to provide a secondary battery that is expected to increase in capacity and has high stability associated with charging and discharging. That is, according to a preferred aspect of the present invention, there is provided a secondary battery comprising a positive electrode, a negative electrode, and an alkaline electrolyte and / or hydroxide ion conductive solid electrolyte. The negative electrode is not particularly limited as long as it can perform a charge / discharge reaction with the LDH positive electrode via an alkaline electrolyte and / or a hydroxide ion conductive solid electrolyte. Various negative electrodes such as various negative electrodes can be employed. Examples of preferred negative electrodes include zinc negative electrodes and LDH negative electrodes. A nickel zinc battery can be constructed by employing a zinc negative electrode. In addition, an LDH battery can be configured by employing an LDH negative electrode. Hereinafter, each of these secondary batteries will be described in detail. The alkaline electrolyte may be an aqueous alkali metal hydroxide solution such as an aqueous potassium hydroxide solution. The hydroxide ion conductive solid electrolyte may be LDH such as Mg-Al-LDH.

(1) Nickel Zinc Battery FIG. 1 conceptually shows a nickel zinc battery using the LDH positive electrode of the present invention. As shown in FIG. 1, the nickel zinc battery 10 includes a positive electrode 12, a negative electrode 14, an electrolytic solution 16, and, optionally, a layered double hydroxide (LDH) separator 28. The positive electrode 12 is the LDH positive electrode according to the present invention described above. The negative electrode 14 contains zinc and / or zinc oxide. The electrolyte solution 16 includes an alkali metal hydroxide aqueous solution. The LDH separator 28 separates the positive electrode 12 and the negative electrode 14 so that hydroxide ions can be conducted. According to such a configuration, the capacity of the nickel zinc battery can be improved by using LDH for the positive electrode of the nickel zinc battery.

  LDH preferably has a particulate form. By being in the form of particles, the surface area is increased and it is possible to cope with a large current discharge. The particulate form of LDH preferably has an average primary particle size of 0.05 to 3 μm, more preferably 0.1 to 3 μm, still more preferably 0.1 to 1 μm, and particularly preferably 0.1 to 0.3 μm. It is 6 μm, most preferably 0.1 to 0.4 μm. Since LDH is generally poor in conductivity, it is considered that the particulate form (particularly, the one having a small primary particle size as described above) is more reactive than the film-like microstructure. In this specification, the primary particle diameter is the longest distance of the diameter of LDH primary particles (plate-like particles), and is observed with a scanning electron microscope (SEM) at a magnification at which primary particles can be observed (for example, 40000 times). It can be measured using the length measurement function of the software. In addition, the average primary particle size is determined by measuring the primary particle size of 10 primary particles per field of view, and by calculating the average value of the primary particle sizes for two fields of view (that is, a total of 20 primary particles). It is desirable to decide.

The method for producing the LDH powder is not particularly limited, but it is preferable to employ a coprecipitation method or a hydrothermal method in that LDH having a small particle diameter as described above can be produced. For example, Ni-M-LDH powder (M is a transition metal such as V or Fe) can be preferably produced by the following procedure.
1) A metal ion aqueous solution containing Ni and transition metal M in a desired ratio is added to a solution obtained by mixing ethylene glycol, aqueous ammonia and sodium carbonate, and stirred.
2) The raw material aqueous solution is put into an autoclave and crystal is grown by hydrothermal synthesis at 150 to 200 ° C. for 3 to 24 hours to obtain an LDH precipitate.
3) The solution containing the LDH precipitate is filtered by suction to collect the LDH precipitate, washed with water, further washed with ethanol, and then dried in vacuum at 80 ° C. to obtain Ni-M-LDH powder.

  The negative electrode 14 contains zinc and / or zinc oxide. Zinc may be contained in any form of zinc metal, zinc compound and zinc alloy as long as it has an electrochemical activity suitable for the negative electrode. Preferable examples of the negative electrode material include zinc oxide, zinc metal, calcium zincate and the like, and a mixture of zinc metal and zinc oxide is more preferable. The negative electrode 14 may be configured in a gel form, or may be mixed with the electrolytic solution 16 to form a negative electrode mixture. The shape of the negative electrode material is not particularly limited, but it is preferably a powder form, which increases the surface area and can cope with a large current discharge.

  The electrolyte solution 16 includes an alkali metal hydroxide aqueous solution. That is, an aqueous solution containing an alkali metal hydroxide is used as the electrolyte solution 16 (for example, the positive electrode electrolyte solution 16a and the negative electrode electrolyte solution 16b). Examples of the alkali metal hydroxide include potassium hydroxide, sodium hydroxide, lithium hydroxide, ammonium hydroxide and the like, and potassium hydroxide is more preferable.

  The electrolyte solution 16 may be separated into a positive electrode electrolyte solution 16a in which the positive electrode 12 is immersed and a negative electrode electrolyte solution 16b in which the negative electrode 14 is immersed. In order to suppress the self-dissolution of zinc and / or zinc oxide, a zinc compound such as zinc oxide or zinc hydroxide is preferably added to the negative electrode electrolyte solution 16b. The electrolyte solution 16 (for example, the positive electrode electrolyte solution 16a and the negative electrode electrolyte solution 16b) may be mixed with the positive electrode 12 and / or the negative electrode 14 to be present in the form of a positive electrode mixture and / or a negative electrode mixture. Further, the electrolytic solution may be gelled in order to prevent leakage of the electrolytic solution.

  When the electrolyte solution 16 is separated into the positive electrode electrolyte solution 16a and the negative electrode electrolyte solution 16b, the positive electrode 12 and the positive electrode electrolyte solution 16a are separated by the negative electrode 14, the negative electrode electrolyte solution 16b, and the layered double hydroxide (LDH) separator 28. It is preferable that the oxide ions are isolated so as to be conductive. Even when the electrolytic solution 16 is not completely separated into the positive electrode electrolytic solution 16a and the negative electrode electrolytic solution 16b (that is, when the common electrolytic solution 16 is in liquid junction in the positive electrode chamber and the negative electrode chamber), the positive electrode 12 is preferably isolated by the negative electrode 14 and the LDH separator 28 so that hydroxide ions can be conducted. In any case, as described above, the LDH separator 28 is known as a dense separator having hydroxide ion conductivity in the field of zinc secondary batteries. Although it can prevent effectively, the suitable aspect is demonstrated below.

  The LDH separator 28 is a ceramic separator containing layered double hydroxide (LDH), and separates the positive electrode 12 and the negative electrode 14 so that hydroxide ions can be conducted. A preferred LDH separator 28 is gas impermeable and / or water impermeable. In other words, the LDH separator 28 is preferably so dense that it has gas impermeability and / or water impermeability. In this specification, “having gas impermeability” means that an object to be measured (that is, LDH separator 28 and / or porous material) in water as described in Patent Document 2 (International Publication No. 2016/076047). This means that even if helium gas is brought into contact with one surface side of the base material 30) with a differential pressure of 0.5 atm, no bubbles are generated due to helium gas from the other surface side. Further, in this specification, “having water impermeability” means a measurement object (for example, an LDH film and / or a porous substrate) as described in Patent Document 2 (International Publication No. 2016/076047). ) Means that water that contacts one side does not permeate the other side. That is, the fact that the LDH separator 28 has gas impermeability and / or water impermeability means that the LDH separator 28 has a high degree of denseness that does not allow gas or water to pass through, and has water permeability. It means not a porous film or other porous material. By doing so, the LDH separator 28 can selectively pass only hydroxide ions due to its hydroxide ion conductivity, and can exhibit a function as a battery separator. For this reason, it has a very effective configuration for physically preventing penetration of the separator by the zinc dendrite that is generated and propagated from the negative electrode 14 during charging to prevent a short circuit between the positive and negative electrodes. Needless to say, the LDH separator 28 may be combined with the porous substrate 30 as shown in FIG. In any case, since the LDH separator 28 has hydroxide ion conductivity, the required hydroxide ions can be efficiently transferred between the positive electrode electrolyte solution 16a and the negative electrode electrolyte solution 16b. 14 can be realized.

The LDH separator 28 includes a layered double hydroxide (LDH), and is preferably composed of LDH. As described above, the LDH includes a plurality of hydroxide base layers and an intermediate layer interposed between the plurality of hydroxide base layers. The hydroxide base layer is mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layer of LDH is composed of anions and H ₂ O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH comprises OH ⁻ and / or CO ₃ ²⁻ . LDH has excellent ionic conductivity due to its inherent properties. In general, LDH is M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ⁿ⁻ _{x / n} · mH ₂ O (where M ²⁺ is a divalent cation and M ³⁺ is a trivalent cation). is a ^{cation, a n-is} the n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, m is the base composition formula is 0 or more) It is known as a representative. In the above basic composition formula, M ²⁺ may be any divalent cation, and preferred examples include Mg ²⁺ , Ca ²⁺ and Zn ²⁺ , and more preferably Mg ²⁺ . M ³⁺ may be any trivalent cation, but preferred examples include Al ³⁺ or Cr ³⁺ , and more preferred is Al ³⁺ . A ^n- may be any anion, preferred examples OH ^- and _CO ^{3 2-} and the like. Accordingly, in the above basic ^{formula, M 2+} comprises ^{Mg 2+, M 3+} comprises ^{Al 3+, A n-is} OH ^- and / or _CO preferably contains ^{3 2-.} n is an integer of 1 or more, preferably 1 or 2. x is 0.1 to 0.4, preferably 0.2 to 0.35. m is an arbitrary number which means the number of moles of water, and is a real number of 0 or more, typically more than 0 or 1 or more. However, the above basic composition formula is merely a formula of “basic composition” that is typically exemplified with respect to LDH in general, and the constituent ions can be appropriately replaced. For example, it may be replaced with some or all of the M ³⁺ tetravalent or higher valency cations in the basic formula, in which case, the anion A coefficient of ^n-x / n in the general formula May be changed as appropriate.

The LDH separator 28 may be in a plate shape, a film shape, or a layer shape. When the LDH separator 28 is in a film shape or a layer shape, the film or layer LDH separator 28 is combined with the porous substrate 30. For example, it is preferably formed on or in the porous substrate 30. When the plate-like form is used, sufficient hardness can be secured and penetration of zinc dendrites can be more effectively prevented. On the other hand, if the film or layer form is thinner than the plate, there is an advantage that the resistance of the separator can be significantly reduced while ensuring the minimum necessary hardness to prevent the penetration of zinc dendrite. is there. The preferable thickness of the plate-like LDH separator 28 is 0.01 to 0.5 mm, more preferably 0.02 to 0.2 mm, and still more preferably 0.05 to 0.1 mm. Further, the higher the hydroxide ion conductivity of the LDH separator 28, the higher, but it is typically 10 ⁻⁴ to 10 ⁻¹ S / m. On the other hand, in the case of a film-like or layered form, the thickness is preferably 100 μm or less, more preferably 75 μm or less, still more preferably 50 μm or less, particularly preferably 25 μm or less, and most preferably 5 μm or less. Thus, the resistance of the LDH separator 28 can be reduced. The lower limit of the thickness is not particularly limited because it varies depending on the application, but in order to ensure a certain degree of rigidity desired as a separator film or layer, the thickness is preferably 1 μm or more, more preferably 2 μm or more. is there.

  The LDH separator 28 is preferably combined with the porous substrate 30. For example, the porous substrate 30 may be provided on one side or both sides of the LDH separator 28. When the porous substrate 30 is provided on one side of the LDH separator 28, the porous substrate 30 may be provided on the surface of the LDH separator 28 on the negative electrode 14 side or on the surface of the LDH separator 28 on the positive electrode 12 side. Also good. It goes without saying that the porous substrate 30 has water permeability, and therefore the electrolyte solution 16 (for example, the positive electrode electrolyte solution 16a and the negative electrode electrolyte solution 16b) can reach the LDH separator 28. The presence of 30 makes it possible to hold hydroxide ions on the LDH separator 28 more stably. In addition, since the strength can be imparted by the porous substrate 30, the LDH separator 28 can be thinned to reduce the resistance. In addition, a dense film or dense layer of LDH can be formed on or in the porous substrate 30. When providing a porous substrate on one side of the LDH separator 28, a method of preparing a porous substrate and forming an LDH film on the porous substrate can be considered. On the other hand, when providing a porous base material on both surfaces of the LDH separator 28, it is conceivable to perform densification by sandwiching an LDH raw material powder between two porous base materials. In FIG. 1, the porous substrate 30 is provided over the entire surface of one side of the LDH separator 28, but may be provided only on a part of one side of the LDH separator 28 (for example, a region involved in charge / discharge reaction). . For example, when LDH is formed in the form of a film or a layer on or in the porous substrate 30, the porous substrate 30 is provided over the entire surface of one side of the LDH separator 28 due to the manufacturing method. It is typical. On the other hand, when the LDH body is formed in a self-supporting plate shape (which does not require a base material), the porous base material 30 is formed only on a part of one side of the LDH separator 28 (for example, a region involved in the charge / discharge reaction). It may be retrofitted, or the porous substrate 30 may be retrofitted over the entire surface of one side.

  When the porous substrate 30 is provided on one side of the LDH separator 28, the LDH separator 28 may be provided on either the positive electrode 12 side or the negative electrode 14 side of the porous substrate 30. However, the LDH separator 28 is preferably provided on the negative electrode 14 side of the porous substrate 30. By carrying out like this, peeling from the porous base material 30 of the LDH separator 28 can be suppressed more effectively. That is, when zinc dendrite grows from the negative electrode 14 and reaches the LDH separator 28, stress that can be generated as the zinc dendrite grows acts in a direction to press the LDH separator 28 against the porous substrate 30. As a result, the LDH separator 28 becomes difficult to peel off from the porous substrate 30.

  The porous substrate 30 is preferably composed of at least one selected from the group consisting of a ceramic material, a metal material, and a polymer material, more preferably a ceramic material and / or a polymer material, still more preferably. It is a polymer material. More preferably, the porous substrate is made of a ceramic material. In this case, preferable examples of the ceramic material include alumina, zirconia, titania, magnesia, spinel, calcia, cordierite, zeolite, mullite, ferrite, zinc oxide, silicon carbide, and any combination thereof, and more preferable. Is alumina, zirconia, titania, and any combination thereof, particularly preferably alumina and zirconia, most preferably alumina. When these porous ceramics are used, it is easy to form the LDH separator 28 having excellent denseness. Preferable examples of the metal material include aluminum and zinc. Preferable examples of the polymer material include polystyrene, polyether sulfone, polypropylene, epoxy resin, polyphenylene sulfide, hydrofluorinated fluororesin (tetrafluorinated resin: PTFE, etc.), and any combination thereof. It is more preferable to appropriately select a material excellent in alkali resistance as the resistance to the battery electrolyte from the various preferable materials described above.

  Preferably, the LDH separator 28 is composed of an aggregate of a plurality of LDH plate-like particles, and the plurality of LDH plate-like particles have their plate surfaces on the surface of the porous substrate 30 (ignoring fine irregularities caused by the porous structure). The orientation is such that it intersects perpendicularly or diagonally with the main surface of the porous substrate when observed macroscopically as much as possible. Note that the LDH separator 28 may be at least partially incorporated in the pores of the porous substrate 30, and in that case, LDH plate-like particles may also exist in the pores of the porous substrate 30.

  The manufacturing method of the LDH separator 28, for example, the LDH separator 28 combined with the porous substrate 30, is not particularly limited, and is manufactured by referring to a known manufacturing method of the LDH separator (for example, Patent Documents 1 and 2). be able to. Further, as disclosed in Patent Document 2, the outer periphery of the LDH separator 28 and / or the porous substrate 30 may be covered with a resin outer frame.

(2) LDH Secondary Battery FIG. 2 conceptually shows an LDH secondary battery using the LDH positive electrode of the present invention. As shown in FIG. 2, the secondary battery 20 includes a positive electrode 22, a negative electrode 24, and an electrolyte solution and / or a solid electrolyte 26. The positive electrode 22 is the LDH positive electrode of the present invention, and preferably includes, as a positive electrode active material, a layered double hydroxide (LDH) containing at least one selected from the group consisting of Ni, Fe and Mn as a constituent element. . The negative electrode 24 includes a layered double hydroxide (LDH) containing at least one selected from the group consisting of Cu, Al, and Zn as a constituent element as a negative electrode active material. The electrolytic solution and / or the solid electrolyte 26 (hereinafter, both may be collectively referred to as the electrolyte 26) are an alkaline electrolytic solution and / or a hydroxide ion conductive solid electrolyte. Thus, by employing the LDH positive electrode 22 of the present invention, the interlayer distance of the LDH can be reversibly changed with charge / discharge. Further, by employing a layered double hydroxide (LDH) containing a predetermined constituent element such as Cu or Al as a negative electrode active material for the secondary battery 20, the interlayer distance is reversibly changed with charge and discharge. be able to. Therefore, by adopting the LDH positive electrode and the LDH negative electrode, it is possible to provide a secondary battery that is expected to increase in capacity, has high stability accompanying charge / discharge, and does not cause a short circuit due to zinc dendrite.

The negative electrode 14 includes LDH as a negative electrode active material. That is, in the secondary battery 20 of the present invention, LDH itself participates in the charge / discharge reaction of the negative electrode 14. LDH contained in the negative electrode 14 contains at least one selected from Cu, Al, and Zn as a constituent element. Again, as is generally known, LDH is composed of a plurality of hydroxide base layers and an intermediate layer interposed between the plurality of hydroxide base layers. The hydroxide base layer is mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layer of LDH is composed of anions and H ₂ O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH comprises OH ⁻ and / or CO ₃ ²⁻ . Therefore, the LDH contained in the negative electrode 24 contains at least one selected from Cu, Al, and Zn as a part of the metal element constituting the hydroxide base layer. Among these, Al-based LDH containing Al is preferable.

  More preferably, the LDH included in the negative electrode 24 contains a combination of Cu and Al or a combination of Zn and Al as constituent elements. That is, the LDH contained in the positive electrode 22 preferably contains Cu and Al, or Zn and Al as the metal elements constituting the hydroxide base layer. In other words, preferable examples of LDH contained in the negative electrode 24 include Cu—Al—LDH and Zn—Al—LDH.

  The electrolyte 26 is an alkaline electrolyte and / or a hydroxide ion conductive solid electrolyte. That is, the secondary battery 20 of the present invention may be configured as a liquid battery, may be configured as an all-solid battery, or may be configured as a battery having both elements. A typical alkaline electrolyte is an aqueous alkali metal hydroxide solution. Examples of the alkali metal hydroxide include potassium hydroxide, sodium hydroxide, lithium hydroxide, ammonium hydroxide and the like, and potassium hydroxide is more preferable. On the other hand, the hydroxide ion conductive solid electrolyte is in the form of a film-like, layered, or plate-like member that isolates the positive electrode 22 and the negative electrode 24 so as to allow hydroxide ion conduction and not allow electronic conduction. (Ie, in the form of a separator). The solid electrolyte may be dense or porous, and the structure is not limited as long as it does not contain an electron conductive substance. For example, it may be a green compact layer in which hydroxide ion conductive solid electrolyte particles are deposited in a thin layer and pressed, or integrated by a technique such as heating or hydrothermal treatment. Also good. In particular, since the secondary battery 20 of the present invention does not require the use of an electrolytic solution, no particular problem (for example, deterioration or collapse due to penetration of the electrolytic solution) occurs even when the green compact layer is used. Alternatively, a hydroxide ion conductive solid electrolyte formed into a film may be disposed as a separator. The hydroxide ion conductive solid electrolyte is not particularly limited as long as it is a solid electrolyte having hydroxide ion conductivity, but an inorganic solid electrolyte is preferable. Examples of the hydroxide ion conductive inorganic solid electrolyte include layered double hydroxide (LDH), layered perovskite oxide, and the like. LDH is most preferable because it is inexpensive and exhibits high hydroxide ion conductivity. In particular, as described above, LDH separators are known in the fields of nickel zinc secondary batteries and air zinc secondary batteries (see Patent Documents 1 and 2), and this LDH separator is used as the secondary battery 20 of the present invention. Also preferably used. This LDH separator may be a composite with a porous substrate as disclosed in Patent Documents 1 and 2, but in that case, pores are formed over the entire region in the thickness direction of the porous substrate. It is desirable that the inside be filled with LDH. By doing so, it is possible to smoothly exchange hydroxide ions with the positive electrode 22 and the negative electrode 24 in contact with the LDH separator. Therefore, when there is a portion in the porous base material where the pores are not filled with LDH, it is desired that such a portion be removed by cutting, polishing or the like to be used as a separator.

  According to a preferred embodiment of the present invention, the secondary battery 20 includes an alkaline electrolyte. That is, the secondary battery 20 can be configured as a liquid battery. A preferable alkaline electrolyte is an aqueous alkali metal hydroxide solution as described above, and more preferably an aqueous potassium hydroxide solution. The secondary battery 20 preferably further includes a separator that separates the positive electrode 22 and the negative electrode 24 so that hydroxide ions can be conducted. As the separator, a known separator material such as a nonwoven fabric that allows passage of an electrolytic solution or hydroxide ions may be used. Advantages of the secondary battery 20 in the case of a liquid battery include (i) that zinc dissolution precipitation and shape deterioration that are problematic in the zinc negative electrode do not occur, and (ii) that the volume change of the positive electrode and the negative electrode is small. And (iii) a short circuit due to zinc dendrite does not occur. The advantages (i) and (ii) above lead to good cycle characteristics.

According to another preferred embodiment of the present invention, the secondary battery 20 includes a hydroxide ion conductive solid electrolyte, thereby forming an all-solid secondary battery. A preferred hydroxide ion conductive solid electrolyte in this embodiment is a layered double hydroxide (Mg—Al—LDH) containing Mg and Al as constituent elements. Again, as is generally known, LDH is composed of a plurality of hydroxide base layers and an intermediate layer interposed between the plurality of hydroxide base layers. The hydroxide base layer is mainly composed of metal elements (typically metal ions) and OH groups. The intermediate layer of LDH is composed of anions and H ₂ O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anion in LDH comprises OH ⁻ and / or CO ₃ ²⁻ . Therefore, Mg-Al-LDH contains Mg and Al as the metal elements constituting the hydroxide base layer. Particularly preferably, the hydroxide ion conductive solid electrolyte is a green compact of Mg—Al—LDH. Advantages of the secondary battery 20 when it is an all-solid-state battery include (i) high energy density, (ii) no liquid leakage, and (iii) no gas generation that causes deterioration of the liquid battery. It is safer than water batteries.

  According to still another preferred embodiment of the present invention, the secondary battery 20 includes both an alkaline electrolyte and a hydroxide ion conductive solid electrolyte. That is, the secondary battery 20 may be a battery having both elements of a liquid battery and an all-solid battery. A preferred hydroxide ion conductive solid electrolyte in this embodiment is a layered double hydroxide (Mg—Al—LDH) containing Mg and Al as constituents, and Mg—Al—LDH is as described above. . Specifically, it is preferable that the hydroxide ion conductive solid electrolyte is a green compact of Mg—Al—LDH, and the green compact is impregnated with an alkaline electrolyte. Advantages of the secondary battery 20 in the case of a battery having both a liquid battery and an all-solid-state battery include (i) that the energy density can be increased, (ii) that liquid leakage is less likely to occur, and (iii) liquid Gas generation that causes deterioration of the system battery is unlikely to occur, and it is safer than the aqueous battery, and (iv) the interface resistance is lower than that of the all-solid-state battery.

  The present invention is more specifically described by the following examples.

Example A1
(1) Preparation of positive electrode and XRD analysis Ni-V-LDH (molar ratio Ni: V = 4: 1), acetylene black, N-methyl-2-pyrrolidone (NMP), and fluorine-based binder ( KFL- # 9130 manufactured by Kuraha Co., Ltd.) was mixed at a weight ratio of 7: 3: 35: 4. After apply | coating the obtained mixture to carbon paper (Torayca TGP-H-120, Toray Industries, Inc.), it vacuum-dried at 80 degreeC and the solvent was scattered and it was set as the LDH positive electrode. An XRD profile was obtained by XRD analysis of the crystal structure of the positive electrode in the initial state. This XRD analysis confirmed the LDH crystal phase of the positive electrode active material under the conditions of a voltage of 40 kV, a current of 40 mA, and a measurement range (2θ): 5-90 ° using an XRD apparatus (Ultima IV, manufactured by Rigaku Corporation). .

(2) Production of electrochemical cell The obtained LDH positive electrode (working electrode), platinum electrode (counter electrode), and Hg / HgO electrode (reference electrode) were incorporated into a beaker, and a 6M KOH aqueous solution was injected as an electrolyte. An electrochemical cell (beaker cell) was prepared.

(3) Charge / Discharge Test With respect to the obtained electrochemical cell, the capacity when the active material undergoes one electron reaction is double (strictly, the capacity when Ni in LDH undergoes one electron reaction is 2. The battery was overcharged at a current density of 50 mA / g up to twice the capacity (500 mAh / g), and discharged to 0.2 V at the same current density, and this charge / discharge test was repeated 10 or 30 cycles. These charge / discharge cycles were performed at a discharge cut-off voltage of 0.2 V and a charge cut-off voltage of 0.7 V. The positive electrode was taken out from the beaker cell, and XRD analysis was performed in the same manner as in the above (1). XRD profiles in the end-of-charge state and in the end-of-discharge state after 30 cycles were obtained, respectively, (003) peak and (006) peak (each 2 (detected at around θ = 11 ° and 22 °) was confirmed at the end of 30-cycle discharge, and when (003) peak and (006) peak remained, It was confirmed whether or not the peak returned to a position within 2θ = ± 1 ° from the peak position.

Example A2
An electrochemical cell was prepared and a charge / discharge test was conducted in the same manner as in Example A1, except that Ni-Fe-LDH (molar ratio Ni: Fe = 4: 1) was used instead of Ni-V-LDH.

Example A3 (comparison)
An electrochemical cell was prepared and a charge / discharge test was conducted in the same manner as in Example A1 except that Ni (OH) ₂ was used instead of Ni-V-LDH. In the charge / discharge test, the overcharge up to twice the capacity when the active material reacts with one electron is strictly up to 1.7 times the theoretical capacity of 289 mAh / g (actually 500 mAh / g). went.

Results The results of the charge / discharge test in Examples A1 to A3 were as follows.

(1) Measurement of charging / discharging characteristics When the electrochemical cell was subjected to a charging / discharging test in Examples A1 and A2, a capacity exceeding nickel hydroxide was obtained. FIG. 3 shows the charge / discharge characteristics of Ni-V-LDH (Example A1), and FIG. 4 shows the charge / discharge characteristics of Ni-Fe-LDH (Example A2). When the valence change of Ni in the Ni-V-LDH positive electrode and Ni-Fe-LDH positive electrode charged by using X-ray absorption fine structure analysis (XAFS) was examined, it changed from divalent to 3.5 valence by charging. It was confirmed that

(2) XRD analysis of structural change accompanying charging / discharging In Example A1, in order to investigate the detailed mechanism of the charging / discharging reaction in the Ni-V-LDH positive electrode (Ni: V = 4: 1), ex-situ XRD analysis Went. FIG. 5 shows the state of Ni-V-LDH before charge / discharge (as pre), the full charge state after 10 charge / discharge cycles (after 10 cycle Cha), and the end-of-discharge state after 30 charge / discharge cycles (after 30). (cycle Dis) shows an XRD profile. As can be seen from FIG. 5, the peaks caused by the (003) plane and the (006) plane of Ni-V-LDH are shifted to the high angle side by charging, which means that the interlayer distance is narrowed by charging. . Further, in the discharge after 30 charge / discharge cycles, the peaks resulting from the (003) plane and the (006) plane have returned to the same positions as before the charge / discharge, that is, returned to the original structure. This indicates that Ni-V-LDH exhibits very high reversibility in the charge / discharge cycle. That is, in Example A1, the (003) peak and (006) peak, which are crystal phase peaks derived from LDH, remain even after the end of the discharge after 30 cycles, and these peak positions are 2θ from the corresponding peak positions in the initial state. = It was confirmed that the range was within ± 1 °.

On the other hand, when the same measurement was performed for β-Ni (OH) ₂ of Example A3 (comparative), the XRD profile shown in FIG. 6 was obtained. As a result, as shown in FIG. 6, a part of β-Ni (OH) ₂ undergoes a large structural change to γ-NiOOH due to overcharge (due to the (001) plane and (002) plane of γ-NiOOH. It was found that the structure did not return to the original structure even when discharged, and the structure collapsed. Moreover, it turned out that a crystal | crystallization becomes amorphous by repeated charging / discharging. Note that an α-Ni (OH) ₂ phase was not observed. On the other hand, the Ni-V-LDH positive electrode of Example A1 maintains a structure even after discharge, in contrast to β-Ni (OH) _2, and thus can be said to be a stable positive electrode material even under strong alkali. .

(3) Estimation of structural change during charge / discharge from EXAFS spectrum In Examples A1 and A2, Ni-V-LDH (Ni: V = 4: 1) or Ni-Fe-LDH (molar ratio Ni: Fe = 4) In order to investigate the detailed mechanism of the charge / discharge reaction in 1), wide-area X-ray absorption fine structure (EXAFS) measurement was performed. Specifically, measurements were made on the BL11S2 line of the Aichi Synchrotron Light Center, Science and Technology Exchange Foundation. FIG. 7 shows the Fourier transform diagram of EXAFS of Ni-V-LDH (Example A1) before the reaction, after charging and after discharging, and FIG. 8 shows the Ni-Fe-LDH (Example A2) before the reaction. The Fourier transform diagram of EXAFS after charge and after discharge is shown. Further, as Example A3 (comparison), FIG. 9 shows a Fourier transform diagram of EXAFS before the reaction of Ni (OH) ₂ , after charging, and after discharging. The peak in the EXAFS Fourier transform diagram indicates ions present at a distance on the horizontal axis from the nickel ion present at the center. In this regard, in the Ni-V-LDH and Ni-Fe-LDH shown in FIGS. 7 and 8, the distance between the oxygen ions present in the first coordination sphere and the adjacent nickel ions present in the second coordination sphere Is also shrunk and returned to its original position after discharge. On the other hand, in Ni (OH) ₂ shown in FIG. 9, it can be seen that the position of the adjacent nickel ions has changed greatly due to charging and has not returned to the original position after discharging. Therefore, it was found that LDH reversibly repeats expansion and contraction both in the interlayer and in the plane in accordance with charge / discharge, in contrast to Ni (OH) ₂ .

(4) Consideration of reaction mechanism associated with charge / discharge Nickel hydroxide is one in which protons are desorbed with charge and protons recombine with discharge, but LDH and nickel hydroxide are greatly different from a structural change. Therefore, it is considered that the reaction mechanism of LDH is different from proton desorption / recombination, which is the main mechanism of nickel hydroxide. In particular, the volume change during charge and discharge of LDH is about 9%, which is greatly different from 66% of nickel hydroxide, and is rather close to 4% of LiCoO ₂ which is a typical positive electrode material of a lithium ion battery. From this, it is considered that the charge / discharge mechanism of LDH is not proton desorption / recombination but ion insertion / desorption as in a lithium ion battery. More specifically, an anion is inserted between the layers in order to balance the charge accompanying charging (oxidation), and the anion inserted accompanying discharging (reduction) seems to be detached. Based on this knowledge, a lithium ion battery type secondary battery can be designed. For example, a nickel-based LDH is used for the positive electrode, and a material from which anions are eliminated by reduction (for example, Cu—Al—LDH) is used for the negative electrode. The electrode reaction is
- the positive electrode: Ni-V-LDH + A -
⇔ Ni-V-LDH · A + e ⁻
・ Negative electrode: Negative electrode ・ A + e ⁻負極 Negative electrode + A ⁻
(A ^- is an anion, Ni-V-LDH · A is a Ni-V-LDH has an anion inserted)
Etc. are considered. It is mentioned ^- OH Examples of ^- typical A. For the negative electrode, LDH (for example, Cu—Al—LDH) can be used as a material from which OH ⁻ is eliminated by reduction. If an LDH sheet (for example, Mg-Al-LDH sheet) known as a hydroxide ion conductive solid electrolyte is used as the solid electrolyte, an all-solid LDH battery can be constructed. An all-solid-state LDH battery can be easily stacked, so that the voltage can be increased, and the volume of the electrolyte and the package material can be omitted, so that the energy density is also increased. Therefore, this type of battery is expected to be optimal for battery applications that require high energy density, such as a stationary type and a vehicle-mounted type.

Example B1
(1) Preparation of Ni—Fe—LDH powder Ni—Fe—LDH (Ni ₈ Fe ₂ (OH) ₂₀ (CO ₃ ) · mH ₂ O with a molar ratio of Ni: Fe of 4: 1, where m is 0 or more ) Was prepared by the following procedure. 20 ml of ethylene glycol, 15 ml of aqueous ammonia and 5 ml of sodium carbonate were mixed. Nickel (II) nitrate hexahydrate (Ni (NO ₃ ) ₂ · 6H ₂ O, manufactured by Kishida Chemical Co., Ltd., special grade) and iron nitrate (III) 9 hydrate (Fe (NO _{3) 3} · _9H 2 O, Kishida chemical Co., Ltd., special grade) and Ni: Fe (molar ratio) = 4: stirred with aqueous metal ion solution 10ml in a proportion of 1. The obtained aqueous raw material solution was put into an autoclave and crystal was grown by hydrothermal synthesis at 170 ° C. for 18 hours to precipitate an LDH precipitate. The precipitate-containing solution thus obtained was suction filtered to collect the precipitate. The collected LDH precipitate was washed with water, further washed with ethanol, and then dried at 80 ° C. in vacuum to obtain Ni—Fe—LDH powder.

(2) SEM observation and measurement of particle diameter The obtained Ni-Fe-LDH powder was observed with a scanning electron microscope (SEM) (manufactured by JEOL, JSM-6610LV). FIG. 10 shows an SEM image of the LDH powder taken at a magnification suitable for primary particle observation (40000 times). As shown in FIG. 10, the primary particle size of the Ni—Fe—LDH powder was 0.1 to 0.6 μm. The average primary particle size of the Ni—Fe—LDH powder was 0.24 μm. The average primary particle size was measured by measuring the longest distance of the diameter of the plate-like particle in the SEM image. The magnification of the SEM image used for this measurement is 40000 times, and the primary particle size is measured for 10 primary particles per field of view. An average value was calculated to obtain an average primary particle size. The length measurement function of SEM software was used for length measurement.

(3) Evaluation by X-ray diffraction X-ray diffraction measurement was performed on the obtained Ni-Fe-LDH powder. Specifically, using an X-ray diffractometer (manufactured by Rigaku Corporation, RINT TTR III), an XRD profile of the powder is obtained under measurement conditions of voltage: 50 kV, current value: 300 mA, measurement range: 10 to 70 °. It was. From the obtained XRD profile, it was confirmed that the powder was LDH.

(4) Production and evaluation of nickel zinc secondary battery (4a) Electrode production Ni-Fe-LDH powder and acetylene black (AB) are mixed at a weight ratio of 20:80, and polytetrafluoroethylene (PTFE) is added. After kneading, it was formed into a sheet. The obtained sheet was pressed into foamed nickel to produce an LDH electrode.

(4b) Production of nickel zinc secondary battery LDH electrode as positive electrode, Zn-ZnO electrode (including a mixture of zinc metal and zinc oxide) as negative electrode, and 6M KOH aqueous solution as electrolyte solution with saturated dissolution of zinc oxide A nickel-zinc secondary battery was constructed.

(4c) Charging / discharging test The charging / discharging test was done with respect to the obtained nickel zinc secondary battery. The charge / discharge test was conducted using a 580-type Battery Test System manufactured by Toyo Technica Co., Ltd. at a 10-hour rate. The obtained charging / discharging curve is shown in FIG. Further, the measured discharge capacity is shown in Table 1 as a relative value with respect to the case where the Ni (OH) ₂ positive electrode is used (example B7 described later).

Examples B2 and B3
Except for changing the Ni-Fe-LDH composition so that the molar ratio of Ni: Fe shown in Table 1 is 3: 1 (Example B2) or 2: 1 (Example B3), the same as Example B1, A nickel zinc secondary battery was produced and evaluated. The results were as shown in Table 1. The average primary particle size of the Ni—Fe—LDH powder was in the range of 0.1 to 3 μm.

Example B4
(1) Ni-V-LDH powders prepared Ni: molar ratio of V is 4: 1 _{Ni-V-LDH (Ni 4} V-LDH (Ni 8 V 2 (OH) 20 (CO 3) · mH 2 O (Where m is 0 or more) was prepared by the following procedure: 20 ml of ethylene glycol, 15 ml of aqueous ammonia and 5 ml of sodium carbonate were mixed, and the resulting mixture was mixed with nickel (II) chloride hexahydrate (NiCl _2. 6H ₂ O, manufactured by Kishida Chemical Co., Ltd., special grade) and vanadium chloride (III) (VCl ₃ , manufactured by Kishida Chemical Co., Ltd., special grade) at a ratio of Ni: V (molar ratio) = 4: 1 The resulting aqueous raw material solution was placed in an autoclave, and crystal growth was carried out by hydrothermal synthesis at 170 ° C. for 18 hours to precipitate LDH precipitates. Argument filtered the precipitate was collected. The collected LDH precipitate was washed with water and further washed with ethanol to give Ni-V-LDH powder dried at 80 ° C. in vacuo.

(2) SEM observation and measurement of particle diameter The obtained Ni-V-LDH powder was observed by SEM in the same manner as in Example B1. FIG. 12 shows an SEM image of the LDH powder taken at a magnification suitable for primary particle observation (40000 times). As shown in FIG. 12, the primary particle size of the Ni-V-LDH powder was 0.05 to 0.5 μm. The average primary particle size of the Ni-V-LDH powder was 0.15 μm.

(3) Evaluation by X-ray diffraction X-ray diffraction measurement was performed on the obtained Ni-V-LDH powder in the same manner as in Example B1 to obtain an XRD profile of the powder. From the obtained XRD profile, it was confirmed that the powder was LDH.

(4) Production and Evaluation of Nickel Zinc Secondary Battery A nickel zinc secondary battery was produced and evaluated in the same manner as in Example B1, except that the obtained Ni-V-LDH powder was used. The obtained charge / discharge curve is shown in FIG. Further, the measured discharge capacity is shown in Table 1 as a relative value with respect to the case where the Ni (OH) ₂ positive electrode is used (example B7 described later).

Examples B5 and B6
Except that the Ni-V-LDH composition was changed so that the molar ratio of Ni: V shown in Table 1 was 3: 1 (Example B5) or 2: 1 (Example B6), A nickel zinc secondary battery was produced and evaluated. The results were as shown in Table 1. The average primary particle size of the Ni-V-LDH powder was in the range of 0.05 to 3 μm.

Example B7
A nickel zinc secondary battery was produced and evaluated in the same manner as in Example 1 except that Ni (OH) ₂ powder was used instead of Ni-Fe-LDH powder. The results were as shown in Table 1.

Claims (10)

  A positive electrode comprising a layered double hydroxide (LDH) as a positive electrode active material, wherein the positive electrode can maintain an LDH crystal structure even in a final discharge state after 30 charge / discharge cycles including overcharge and discharge.   In an electrochemical cell in which the charge / discharge cycle comprises the positive electrode as a working electrode, a platinum electrode as a counter electrode, a Hg / HgO electrode as a reference electrode, and a 6M KOH aqueous solution as an electrolyte, The positive electrode according to claim 1, which is performed under conditions of a density of 50 mA / g, a discharge cutoff voltage of 0.2 V, and a charge cutoff voltage of 0.7 V.   The positive electrode according to claim 1 or 2, wherein the overcharge is performed up to twice the capacity when the positive electrode active material has undergone one-electron reaction, and the discharge is performed at the same current density as the overcharge.   The positive electrode according to any one of claims 1 to 3, wherein the LDH crystal structure is identified by detecting (003) peak and (006) peak derived from LDH in X-ray diffraction.   The (003) peak and the (006) peak derived from the LDH are detected at positions within 2θ = ± 1 ° with respect to the peak position before the charge / discharge cycle, respectively, in the final discharge state after the 30 cycles. The positive electrode according to claim 4.   The positive electrode according to any one of claims 1 to 5, wherein the LDH contains at least one selected from the group consisting of Ni, Fe and Mn as a constituent element.   The positive electrode according to claim 6, wherein the LDH contains a combination of Ni and V, a combination of Ni and Fe, or a combination of Ni and Mn as constituent elements.   The positive electrode according to claim 7, wherein the LDH contains a combination of Ni and V as constituent elements, and a molar ratio of V / (Ni + V) is 0.15 to 0.50.   The positive electrode according to claim 7, wherein the LDH contains a combination of Ni and Fe as constituent elements, and a molar ratio of Fe / (Ni + Fe) is 0.15 to 0.50. A secondary battery comprising the positive electrode according to any one of claims 1 to 9, a negative electrode, and an alkaline electrolyte and / or a hydroxide ion conductive solid electrolyte.