JP6977989B2 - Positive electrode and secondary battery - Google Patents

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 large barriers to practical use. This is because zinc constituting the negative electrode forms dendritic crystals called dendrites during charging, and the dendrites break through the separator and cause 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. It also has the advantage that the raw material price is low. Therefore, in a nickel-zinc secondary battery, a technique for preventing a short circuit due to zinc dendrite is strongly desired.

In order to deal with such problems or 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 an LDH separator is provided between a positive electrode and a negative electrode in a nickel-zinc secondary battery for the purpose of preventing a short circuit due to zinc dendrite. Further, Patent Document 2 (International Publication No. 2016/076047) discloses a separator structure including an LDH separator compounded with a porous substrate, and the LDH separator is gas impermeable and / or. It is disclosed that it has a high degree of density enough to have water impermeableness.

On the other hand, Patent Document 3 (Japanese Unexamined Patent Publication No. 61-285661) describes the general formula [M ²⁺ _(1-x) Fe ³⁺ _x (OH) ₂ ] ^{X +} [(x / n) in an alkaline storage battery in a non-charged state. ) X ^n- , yH ₂ O] ^X- (In the formula, x is 0.05 to 0.4, M ²⁺ represents oxidative and reducing cations, and X ^n- is at least complex double hydroxide. It is disclosed to use a positive storage battery electrode which is a double hydroxide (representing any anion that guarantees the charge of a cation of an object).

International Publication No. 2013/118561 International Publication No. 2016/076047 Japanese Unexamined Patent Publication No. 61-285661

The positive electrode of a nickel-zinc battery generally contains nickel hydroxide and / or nickel oxyhydroxide. For example, when the nickel-zinc battery is configured in the discharge end state, the positive electrode contains nickel hydroxide, while when the nickel-zinc battery is configured in the fully charged state, the positive electrode contains nickel oxyhydroxide. 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, which is filled. The discharge reaction reversibly changes to β-Ni (OH) ₂ ⇔ β-NiOOH (average valence 3, density 4.7 g / cm ^3). By the way, when a 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 produced 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), which is a current material that can only be oxidized by one electron, by 1.5 times. However, α-Ni (OH) ₂ generated by reduction due to discharge is unstable and easily returns to β-Ni (OH) _2, which causes volume shrinkage, which causes a problem of deterioration of the electrode. be. Therefore, a positive electrode active material that has the above-mentioned advantages of γ-NiOOH and can avoid the above-mentioned problems is desired.

The present inventors have now stated that a certain layered double hydroxide (LDH) can reversibly change the interlayer distance with charge and discharge, and therefore can be adopted as a positive electrode active material for a secondary battery. I found out. It is also known that by adopting a positive electrode containing LDH as a positive electrode active material (hereinafter, also referred to as LDH positive electrode), it is possible to provide a secondary battery that is expected to increase in capacity and has high stability with charge and discharge. Obtained.

Therefore, an object of the present invention is to provide a positive electrode for a secondary battery, which is expected to increase in capacity and has high stability with charge and discharge.

According to one aspect of the present invention, the positive electrode contains layered double hydroxide (LDH) as a positive electrode active material, and LDH is also in the discharge end state after 30 cycles of charge / discharge cycles including overcharge and discharge. 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 a hydroxide ion conductive solid electrolyte.

It is a figure which conceptually shows the nickel-zinc secondary battery which used the positive electrode by this invention. It is a figure which conceptually shows the LDH secondary battery using the positive electrode by this invention. It is a figure which shows the charge / discharge characteristic of the Ni-V-LDH positive electrode (Ni: V = 4: 1) measured in Example A1. It is a figure which shows the charge / discharge characteristic of the Ni-Fe-LDH positive electrode (Ni: Fe = 4: 1) measured in Example A2. The Ni-V-LDH (Ni: V = 4: 1) measured in Example A1 is fully charged before charge / discharge (as pre), after 10 charge / discharge cycles (after 10 cycle Cha), and charge / discharge. The XRD profile in the discharge end state (after 30 cycle Dis) after 30 cycles is shown. Example A3 (comparison) of β-Ni (OH) ₂ before charge / discharge (as pre), fully charged after 10 charge / discharge cycles (after 10 cycle Cha), and 10 charge / discharge cycles. The XRD profile in the later discharge end state (after 10 cycle Dis) is shown. The Fourier transform diagram of EXAFS before reaction, after charge, and after discharge of Ni-V-LDH (Ni: V = 4: 1) measured in Example A1 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 before the reaction, after charging, and after discharging of _{Ni (OH) 2} measured in Example A3 (comparison) is shown. 6 is an SEM image of the Ni-Fe-LDH powder produced in Example B1 taken at 40,000 times. 6 is a charge / discharge curve measured for a nickel-zinc secondary battery using the Ni-Fe-LDH powder produced in Example B1 as a positive electrode. 6 is an SEM image of the Ni-V-LDH powder produced in Example B4 taken at 40,000 times. 6 is a charge / discharge curve measured for a nickel-zinc secondary battery using the Ni-V-LDH powder produced in Example B4 as a 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 able to maintain the LDH crystal structure even in the end-of-discharge state after repeating 30 cycles of charge / discharge cycles including overcharge and discharge. As described above, the layered double hydroxide (LDH) can reversibly change the interlayer distance with charge and discharge, and therefore can be adopted 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, which is expected to increase in capacity and has high stability with charge and discharge. In particular, the charge / discharge cycle includes overcharging, and it is a surprising finding that the LDH crystal structure can be maintained even if such overcharging is repeated, and it can be said that extremely high stability to charging / discharging can be expected.

Typically, the charge / discharge cycle repeated 30 cycles comprises a positive electrode as an action electrode, a platinum electrode as a counter electrode, an Hg / HgO electrode as a reference electrode, and a 6M KOH aqueous solution as an electrolytic solution. In the electrochemical cell provided, the current density is 50 mA / g, the discharge cutoff voltage is 0.2 V, and the charge cutoff voltage is 0.7 V. Further, in the charge / discharge cycle, it is preferable that the overcharge is performed up to twice the capacity when the positive electrode active material reacts by one electron, 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 the detection of LDH-derived (003) and (006) peaks 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 discharge end state after 30 cycles, respectively. Is preferable.

The positive electrode according to the present invention contains layered double hydroxide (LDH) as a positive electrode active material. LDH is known as a compound in which divalent and trivalent (sometimes tetravalent) ions form a layered hydroxide, and anions such as carbonate ions are inserted between layers in order to balance charges 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 and discharge. As mentioned 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 electric discharge is unstable and easily returns to β-Ni (OH) ₂ , and at that time, volume shrinkage occurs and the electrode deteriorates. Therefore, a positive electrode active material that has the above-mentioned advantages of γ-NiOOH and can avoid the above-mentioned problems is desired. In this respect, according to the positive electrode of the present invention, it can be expected that these requirements are conveniently satisfied.

The positive electrode contains LDH as the 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. 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 basic layers and an intermediate layer interposed between the plurality of hydroxide basic layers. The basic hydroxide layer is mainly composed of metal elements (typically metal ions) and OH groups. Intermediate layer of LDH is composed of anionic and _{H 2} O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anions in LDH contain OH ⁻ and / or CO ₃ ^2- . Therefore, 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 basic layer. Of these, Ni-based LDH containing at least Ni is particularly preferable.

More preferably, 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, LDH contained in the positive electrode preferably contains Ni and V, Ni and Fe, or Ni and Mn as the metal elements constituting the hydroxide basic 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, and even more preferably 0.20 to 0.20. It is 0.35. On the other hand, in the case of Ni-Fe-LDH, the molar ratio of Fe / (Ni + Fe) is preferably 0.15 to 0.50, more preferably 0.20 to 0.40, and even more preferably 0. It is 20 to 0.35. Within these ranges, the charge / discharge capacity can be further improved when used for a positive electrode.

Secondary battery As described above, by adopting the LDH positive electrode of the present invention, it is possible to provide a secondary battery which is expected to increase in capacity and has high stability with charge and discharge. That is, according to a preferred embodiment of the present invention, there is provided a secondary battery including a positive electrode, a negative electrode, and an alkaline electrolyte and / or a 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, and is not particularly limited, and is a zinc negative electrode, an LDH negative electrode, or the like. Various negative electrodes such as various negative electrodes can be adopted. Examples of preferred negative electrodes include zinc negative electrodes and LDH negative electrodes. A nickel-zinc battery can be configured by adopting a zinc negative electrode. Further, the LDH battery can be configured by adopting the LDH negative electrode. Hereinafter, each of these secondary batteries will be specifically described. The alkaline electrolytic solution may be an aqueous alkali metal hydroxide solution such as an aqueous solution of potassium hydroxide. Further, 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 electrolytic solution 16 contains an aqueous alkali metal hydroxide 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 the LDH as the positive electrode of the nickel-zinc battery.

LDH preferably has a particulate morphology. Since it is in the form of particles, the surface area increases and it becomes possible to cope with a large current discharge. The LDH in the particulate form 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. It is 6 μm, most preferably 0.1 to 0.4 μm. Since LDH generally has poor 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 the present specification, the primary particle size 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 (for example, 40,000 times) at which the primary particles can be observed, and SEM. It can be measured using the length measurement function of the software. Further, the average primary particle size shall be measured for 10 primary particles per field of view, and the average value of the primary particle size for two fields of view (that is, a total of 20 primary particles) shall be calculated. It is desirable to decide.

The method for producing LDH powder is not particularly limited, but it is preferable to adopt the coprecipitation method or the hydrothermal method in that LDH having a small particle size as described above can be produced. For example, Ni-M-LDH powder (M is a transition metal such as V and Fe) can be preferably produced by the following procedure.
1) A metal ion aqueous solution containing Ni and a transition metal M in a desired ratio is added to a mixed solution of ethylene glycol, aqueous ammonia and sodium carbonate, and the mixture is stirred.
2) An aqueous solution of raw materials is placed in an autoclave and crystal-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 suction-filtered to collect the LDH precipitate, washed with water, further washed with ethanol, and then dried in vacuum at 80 ° C. to obtain a 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. Preferred examples of the negative electrode material include zinc oxide, zinc metal, calcium zincate and the like, but a mixture of zinc metal and zinc oxide is more preferable. The negative electrode 14 may be formed in the form of a gel, 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 in the form of powder, which increases the surface area and makes it possible to cope with a large current discharge.

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

The electrolytic solution 16 may be separated into a positive electrode electrolytic solution 16a in which the positive electrode 12 is immersed and a negative electrode electrolytic solution 16b in which the negative electrode 14 is immersed. It is preferable to add a zinc compound such as zinc oxide or zinc hydroxide to the negative electrode electrolytic solution 16b in order to suppress the self-dissolution of zinc and / or zinc oxide. The electrolytic solution 16 (for example, the positive electrode electrolytic solution 16a and the negative electrode electrolytic solution 16b) may be mixed with the positive electrode 12 and / or the negative electrode 14 and exist 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 electrolytic solution 16 is separated into the positive electrode electrolytic solution 16a and the negative electrode electrolytic solution 16b, the positive electrode 12 and the positive electrode electrolytic solution 16a are watered by the negative electrode 14 and the negative electrode electrolytic solution 16b and the layered double hydroxide (LDH) separator 28. It is preferable to be isolated so that the oxide ion can be conducted. Further, 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 electrolytic solution 16 common to the positive electrode chamber and the negative electrode chamber is entangled), the positive electrode is positive. 12 is preferably separated by a negative electrode 14 and an 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, and short-circuits between positive and negative electrodes due to zinc dendrite are caused. Although it can be effectively prevented, a preferred embodiment thereof will be described below.

The LDH separator 28 is a ceramic separator containing a layered double hydroxide (LDH), and separates the positive electrode 12 and the negative electrode 14 so that hydroxide ions can be conducted. The preferred LDH separator 28 has gas impermeable and / or water impermeable. In other words, the LDH separator 28 is preferably densified to have gas impermeableness and / or water impermeableness. In addition, as described in Patent Document 2 (International Publication No. 2016/076047), "having gas impermeable" in the present specification means an object to be measured in water (that is, LDH separator 28 and / or porous). It means that even if the helium gas is brought into contact with one surface side of the quality substrate 30) at a differential pressure of 0.5 atm, no bubbles are generated due to the helium gas from the other surface side. Further, as described in Patent Document 2 (International Publication No. 2016/076047), “having water impermeable” in the present specification means an object to be measured (for example, LDH film and / or a porous substrate). ) Means that water in contact with one side does not permeate the other side. That is, the fact that the LDH separator 28 has gas impermeableness and / or water impermeability means that the LDH separator 28 has a high degree of porosity that does not allow gas or water to pass through, and has water permeability. It means that it is not a porous film or other porous material. By doing so, the LDH separator 28 selectively passes only hydroxide ions due to its hydroxide ion conductivity, and can exhibit a function as a battery separator. Therefore, the configuration is extremely effective in physically preventing the penetration of the separator by the zinc dendrite generated and propagating from the negative electrode 14 during charging to prevent a short circuit between the positive and negative electrodes. However, it goes without saying that the LDH separator 28 may be composited with the porous base material 30 as shown in FIG. In any case, since the LDH separator 28 has hydroxide ion conductivity, the positive electrode 12 and the negative electrode enable efficient transfer of the required hydroxide ion between the positive electrode electrolytic solution 16a and the negative electrode electrolytic solution 16b. The charge / discharge reaction in No. 14 can be realized.

The LDH separator 28 contains layered double hydroxides (LDH) and is preferably composed of LDH. As described above, LDH is composed of a plurality of hydroxide basic layers and an intermediate layer interposed between the plurality of hydroxide basic layers. The basic hydroxide layer is mainly composed of metal elements (typically metal ions) and OH groups. Intermediate layer of LDH is composed of anionic and _{H 2} O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anions in LDH contain OH ⁻ and / or CO ₃ ^2- . LDH also has excellent ionic conductivity due to its unique properties. In general, LDH is M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ⁿ⁻ _{x / n} · mH ₂ O (in the formula, 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⁺ can be any divalent cation, but preferred examples include Mg ²⁺ , Ca ²⁺ and Zn ²⁺ , and more preferably Mg ²⁺ . M ³⁺ can be any trivalent cation, with preferred examples being Al ³⁺ or Cr ³⁺ , more preferably 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, but is preferably 1 or 2. x is 0.1 to 0.4, preferably 0.2 to 0.35. m is any number that means the number of moles of water and is a real number greater than or equal to 0, typically greater than or equal to 0 or greater than or equal to 1. However, the above-mentioned basic composition formula is merely a formula of the "basic composition" generally exemplified with respect to LDH, 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 any form of a plate, a film or a layer, and when the LDH separator 28 is in a film or layered form, the film or layered LDH separator 28 is composited with the porous substrate 30. For example, it is preferably formed on or in the porous substrate 30. The plate-like form can secure sufficient hardness and more effectively prevent the penetration of zinc dendrites. On the other hand, the film-like or layered form, which is thinner than the plate-like form, has the advantage that the resistance of the separator can be significantly reduced while ensuring the minimum necessary hardness to prevent the penetration of zinc dendrites. be. The thickness of the plate-shaped LDH separator 28 is preferably 0.01 to 0.5 mm, more preferably 0.02 to 0.2 mm, still more preferably 0.05 to 0.1 mm. Further, the higher the hydroxide ion conductivity of the LDH separator 28 is, the more desirable it is, but it typically ^{has a conductivity of 10 -4} to ^-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. With such a thin thickness, it is possible to realize a low resistance of the LDH separator 28. The lower limit of the thickness is not particularly limited because it varies depending on the application, but it is preferably 1 μm or more, more preferably 2 μm or more in order to secure a certain degree of hardness desired as a separator film or layer. be.

The LDH separator 28 is preferably composited with the porous base material 30. For example, the porous base material 30 may be provided on one side or both sides of the LDH separator 28. When the porous base material 30 is provided on one side of the LDH separator 28, the porous base material 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. May be good. It goes without saying that the porous substrate 30 has water permeability and therefore the electrolytic solution 16 (for example, the positive electrode electrolytic solution 16a and the negative electrode electrolytic solution 16b) can reach the LDH separator 28, but the porous substrate 30 is available. The presence of 30 makes it possible to more stably retain hydroxide ions on the LDH separator 28. Further, since the strength can be imparted by the porous base material 30, the LDH separator 28 can be thinned to reduce the resistance. Further, a dense film or a dense layer of LDH can be formed on or in the porous substrate 30. When a porous base material is provided on one side of the LDH separator 28, a method of preparing a porous base material and forming LDH on the porous base material can be considered. On the other hand, when the porous base material is provided on both sides of the LDH separator 28, it is conceivable to sandwich the LDH raw material powder between the two porous base materials to perform densification. Although the porous substrate 30 is provided over the entire surface of one side of the LDH separator 28 in FIG. 1, it may be provided only on a part of one side of the LDH separator 28 (for example, a region involved in the charge / discharge reaction). .. For example, when LDH is formed in a film or 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 thereof. Is typical. On the other hand, when the LDH body is formed into a self-standing plate shape (which does not require a base material), the porous base material 30 is applied only to 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 base material 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 base material 30. However, it is preferable that the LDH separator 28 is provided on the negative electrode 14 side of the porous base material 30. By doing so, it is possible to more effectively suppress the peeling of the LDH separator 28 from the porous substrate 30. That is, when zinc dendrite grows from the negative electrode 14 and reaches the LDH separator 28, the stress that can be generated with the growth of the zinc dendrite acts in the direction of pressing the LDH separator 28 against the porous substrate 30. As a result, the LDH separator 28 is less likely 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 ceramic materials, metal materials, and polymer materials, more preferably ceramic materials and / or polymer materials, and even more preferably. It is a polymer material. The porous substrate is more preferably composed of a ceramic material. In this case, preferred examples of the ceramic material include alumina, zirconia, titania, magnesia, spinel, calcia, cordylite, zeolite, mullite, ferrite, zinc oxide, silicon carbide, and any combination thereof, more preferably. Is alumina, zirconia, titania, and any combination thereof, particularly preferably alumina and zirconia, and most preferably alumina. When these porous ceramics are used, it is easy to form the LDH separator 28 having excellent denseness. Preferred examples of metallic materials include aluminum and zinc. Preferred examples of the polymer material include polystyrene, polyether sulfone, polypropylene, epoxy resin, polyphenylene sulfide, hydrophilized fluororesin (tetrafluororesin: PTFE, etc.), and any combination thereof. It is more preferable to appropriately select a material having excellent alkali resistance as resistance to the electrolytic solution of the battery 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 the surface of the porous base material 30 (ignoring the fine irregularities due to the porous structure). It is oriented so as to intersect vertically or diagonally with the main surface of the porous substrate when observed macroscopically to the extent possible. The LDH separator 28 may be incorporated at least partially in the pores of the porous substrate 30, and in that case, LDH plate-like particles may also be present in the pores of the porous substrate 30.

The method for producing the LDH separator 28, for example, the LDH separator 28 composited with the porous substrate 30, is not particularly limited, and the LDH separator 28 is produced by referring to already known methods for producing LDH separators (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 base material 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 electrolytic solution and / or a solid electrolyte 26. The positive electrode 22 is the LDH positive electrode of the present invention, and preferably contains a layered double hydroxide (LDH) containing at least one selected from the group consisting of Ni, Fe and Mn as a constituent element as the positive electrode active material. .. The negative electrode 24 contains a layered double hydroxide (LDH) containing at least one selected from the group consisting of Cu, Al and Zn as a constituent element as the negative electrode active material. The electrolytic solution and / or the solid electrolyte 26 (hereinafter, both may be collectively referred to as the electrolyte 26) is an alkaline electrolytic solution and / or a hydroxide ion conductive solid electrolyte. As described above, by adopting the LDH positive electrode 22 of the present invention, the interlayer distance of LDH can be reversibly changed with charge and discharge. Further, by adopting a layered double hydroxide (LDH) containing a predetermined constituent element such as Cu and 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 in which an increase in capacity can be expected, stability due to charging and discharging is high, and the problem of short circuit due to zinc dendrite does not occur.

The negative electrode 14 contains LDH as a negative electrode active material. That is, in the secondary battery 20 of the present invention, LDH itself is involved 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 basic layers and an intermediate layer interposed between the plurality of hydroxide basic layers. The basic hydroxide layer is mainly composed of metal elements (typically metal ions) and OH groups. Intermediate layer of LDH is composed of anionic and _{H 2} O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anions in LDH contain OH ⁻ and / or CO ₃ ^2- . Therefore, 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 basic layer. Of these, Al-based LDH containing Al is preferable.

More preferably, LDH contained in the negative electrode 24 contains a combination of Cu and Al or a combination of Zn and Al as a constituent element. 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 basic layer. In other words, Cu-Al-LDH and Zn-Al-LDH are preferred examples of LDH contained in the negative electrode 24.

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-based battery, an all-solid-state battery, or 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, but 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-shaped member that separates the positive electrode 22 and the negative electrode 24 so that the positive electrode 22 and the negative electrode 24 can conduct hydroxide ions and do not allow electron conduction. It is preferable to have (that is, in the form of a separator). This solid electrolyte may be dense or porous, and its 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 pressurized, or it may be integrated by a method such as heating or hydrothermal treatment. May be good. In particular, since the secondary battery 20 of the present invention does not need to use an electrolytic solution, no particular problem (for example, deterioration or collapse due to permeation of the electrolytic solution) occurs even if the green compact layer is used. Further, a hydroxide ion conductive solid electrolyte molded into a film may be arranged 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 zinc-air secondary batteries (see Patent Documents 1 and 2), and the LDH separator can be used as the secondary battery 20 of the present invention. Can also be preferably used. This LDH separator may be composited with a porous substrate as disclosed in Patent Documents 1 and 2, but in that case, pores are formed throughout the thickness direction in the porous substrate. It is desirable that LDH is filled inside. By doing so, smooth transfer of hydroxide ions to and from the positive electrode 22 and the negative electrode 24 in contact with the LDH separator becomes possible. Therefore, if there is a portion of the porous substrate that is not filled with LDH, it is desirable to remove such portion by cutting, polishing, or the like and use it as a separator.

According to a preferred embodiment of the present invention, the secondary battery 20 contains an alkaline electrolytic solution. That is, the secondary battery 20 can be configured as a liquid-based battery. The preferable alkaline electrolytic solution is an aqueous alkali metal hydroxide solution as described above, and more preferably an aqueous solution of potassium hydroxide. It is preferable that the secondary battery 20 further includes a separator that separately 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 non-woven fabric that allows the passage of an electrolytic solution or hydroxide ions may be used. The advantages of the secondary battery 20 in the case of a liquid battery are that (i) zinc dissolution precipitation and shape deterioration that are problematic with the zinc negative electrode do not occur, and (ii) the volume change of the positive electrode and the negative electrode is small. , And (iii) no short circuit due to zinc dendrite occurs. In addition, the advantages of (i) and (ii) above lead to good cycle characteristics.

According to another preferred embodiment of the present invention, the secondary battery 20 comprises a hydroxide ion conductive solid electrolyte, thereby forming an all-solid secondary battery. The preferred hydroxide ion conductive solid electrolyte in this embodiment is a layered double hydroxide (Mg-Al-LDH) containing Mg and Al as components. Again, as is generally known, LDH is composed of a plurality of hydroxide basic layers and an intermediate layer interposed between the plurality of hydroxide basic layers. The basic hydroxide layer is mainly composed of metal elements (typically metal ions) and OH groups. Intermediate layer of LDH is composed of anionic and _{H 2} O. The anion is a monovalent or higher anion, preferably a monovalent or divalent ion. Preferably, the anions in LDH contain OH ⁻ and / or CO ₃ ^2- . Therefore, Mg-Al-LDH contains Mg and Al as metal elements constituting the hydroxide basic layer. Particularly preferably, the hydroxide ion conductive solid electrolyte is a green compact of Mg-Al-LDH. The advantages of the secondary battery 20 in the case of an all-solid-state battery are (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-based batteries.

According to yet another preferred embodiment of the present invention, the secondary battery 20 contains 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-based battery and an all-solid-state battery. The preferred hydroxide ion conductive solid electrolyte in this embodiment is a layered double hydroxide (Mg-Al-LDH) containing Mg and Al as components, and Mg-Al-LDH is as described above. .. Specifically, the hydroxide ion conductive solid electrolyte is an Mg-Al-LDH green compact, and it is preferable that the green compact is impregnated with an alkaline electrolytic solution. The advantages of the secondary battery 20 in the case of a battery having the elements of a liquid-based battery and an all-solid-state battery are (i) high energy density, (ii) less likely to cause liquid leakage, and (iii) liquid. Gas generation that causes deterioration of the system battery is less likely to occur, it is safer than the water system battery, and (iv) the interface resistance is lower than that of the all-solid-state battery.

The present invention will be described in more detail with reference to 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 as positive electrode active materials (1) KFL- # 9130, manufactured by Kuraha Co., Ltd.) was mixed in a weight ratio of 7: 3: 35: 4. The obtained mixture was applied to carbon paper (Trading Card TGP-H-120, manufactured by Toray Industries, Inc.) and then vacuum dried at 80 ° C. to scatter the solvent to obtain an LDH positive electrode. The crystal structure of the positive electrode in the initial state was XRD analyzed to obtain an XRD profile. In this XRD analysis, the LDH crystal phase of the positive electrode active material was confirmed 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 Co., Ltd.). ..

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

(3) Charge / discharge test For the obtained electrochemical cell, twice the capacity when the active material reacts with one electron (strictly speaking, with respect to the capacity when Ni in LDH reacts with one electron). It was overcharged with a current density of 50 mA / g to a capacity of twice (500 mAh / g) and discharged to 0.2 V at the same current density. This charge / discharge test was repeated for 10 cycles or 30 cycles with this as one cycle. These charge / discharge cycles were performed with a discharge cutoff voltage of 0.2 V and a charge cutoff voltage of 0.7 V. The positive electrode was taken out from the beaker cell, XRD analysis was performed in the same manner as in (1) above, and the 10th cycle was performed. The XRD profiles were obtained in the end-of-charge state and the end-of-discharge state after 30 cycles, respectively. It was confirmed whether or not the (003) peak and (006) peak remained even after 30 cycles of discharge. If the (003) peak and the (006) peak remained, the position was within 2θ = ± 1 ° from the corresponding peak position in the initial state. It was confirmed whether the peak had returned to.

Example A2
An electrochemical cell was prepared and a charge / discharge test was carried out 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 carried out 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, overcharging 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). gone.

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

(1) Measurement of charge / discharge characteristics When the electrochemical cells were charged / discharged in Examples A1 and A2, a capacity exceeding that of 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 change in the valence of Ni in the Ni-V-LDH positive electrode and the Ni-Fe-LDH positive electrode charged using X-ray absorption fine structure analysis (XAFS) was investigated, it changed from divalent to 3.5 valence due to charging. It was confirmed that

(2) XRD analysis of structural changes due to charge / discharge In Example A1, ex-situ XRD analysis is performed to investigate the detailed mechanism of the charge / discharge reaction in the Ni-V-LDH positive electrode (Ni: V = 4: 1). Was done. FIG. 5 shows the Ni-V-LDH in a fully charged state before charging / discharging (as pre), in a fully charged state after 10 charge / discharge cycles (after 10 cycle Cha), and in a discharge end state after 30 charge / discharge cycles (after 30). The XRD profile in the cycle Dis) is shown. As can be seen from FIG. 5, the peaks caused by the (003) and (006) planes 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 caused by the (003) plane and the (006) plane have returned to the same positions as before the charge / discharge, that is, have returned to the original structure. From this, it can be seen that Ni-V-LDH exhibits extremely high reversibility in the charge / discharge cycle. That is, in Example A1, the (003) and (006) peaks, which are the crystal phase peaks derived from LDH, remain even in the discharge end state after 30 cycles, and their 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 (comparison), 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 overcharging (caused by the (001) and (002) surfaces of γ-NiOOH. It was found that the structure did not return to the original structure even when discharged, and the structure collapsed. It was also found that the crystals became amorphous by repeated charging and discharging. No phase of α-Ni (OH) ₂ was observed. On the other hand, the Ni-V-LDH positive electrode of Example A1 can be said to be a stable positive electrode material even under strong alkali because it maintains its structure even after discharge, in contrast to _{β-Ni (OH) 2.} ..

(3) Estimating structural changes during charging / discharging from EXAFS spectra 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, the measurement was performed on the BL11S2 line of the Aichi Science and Technology Foundation Aichi Synchrotron Optical Center. FIG. 7 shows a Fourier transform diagram of EXAFS before reaction, after charging, and after discharging Ni-V-LDH (eg A1), and FIG. 8 shows a Fourier transform diagram of Ni-Fe-LDH (eg A2) before reaction. The Fourier transform diagram of EXAFS after charging and discharging 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 of the Fourier transform diagram of EXAFS indicates an ion located at a distance on the horizontal axis from the nickel ion existing in the center. In this regard, in Ni-V-LDH and Ni-Fe-LDH shown in FIGS. 7 and 8, the distance between the oxygen ion existing in the first coordination sphere and the adjacent nickel ion existing in the second coordination sphere will be any longer. Has 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 positions of adjacent nickel ions have changed significantly due to charging and have not returned to their original positions even after discharging. Therefore, it was found that LDH, in contrast to Ni (OH) ₂ , reversibly repeats expansion and contraction and expansion both in layers and in-plane with charge and discharge.

(4) Consideration of reaction mechanism associated with charge / discharge Nickel hydroxide desorbs protons with charge and recombines with protons with discharge, but LDH and nickel hydroxide are significantly different from each other in terms of structural changes. Therefore, the reaction mechanism of LDH is considered to be different from the desorption / recombination of protons, which is the main mechanism of nickel hydroxide. In particular, the volume change during charging and discharging of LDH is about 9%, which is significantly different from 66% of nickel hydroxide, which is rather close to 4% of _{LiCoO 2, which is a typical positive electrode material of lithium ion batteries.} From this, it is considered that the charging / discharging mechanism of LDH is not the desorption / recombination of protons but the insertion / desorption of ions as in a lithium ion battery. More specifically, it seems that anions are inserted between the layers in order to balance the charges with charging (oxidation), and the inserted anions are desorbed with discharging (reduction). 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 (for example, Cu-Al-LDH) from which anions are eliminated by reduction 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 ⁻ ⇔ Negative electrode + A ⁻
(A ^- is an anion, Ni-V-LDH · A is a Ni-V-LDH has an anion inserted)
Etc. are conceivable. An example of a typical A ^{− is OH −} . LDH (for example, Cu-Al-LDH) can be used for the negative electrode ^{as a material from which OH − is desorbed by reduction.} Then, 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-state LDH battery can be constructed. Since the all-solid-state LDH battery can be easily stacked, the voltage can be increased, and the volume of the electrolytic solution and the packaging material can be saved, so that the energy density also increases. Therefore, this type of battery is expected to be most suitable for battery applications that require high energy density, such as stationary type and car-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 but 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. To the resulting mixture, nickel nitrate (II) 6 hydrate _{(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 solution of raw materials was placed in an autoclave and crystal-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 recovered LDH precipitate was washed with water, further washed with ethanol, and then dried in vacuum at 80 ° C. to obtain Ni-Fe-LDH powder.

(2) SEM observation and particle size measurement The obtained Ni-Fe-LDH powder was observed with a scanning electron microscope (SEM) (JSM-6610LV, manufactured by JEOL). FIG. 10 shows an SEM image of LDH powder taken at a magnification (40,000 times) suitable for observing primary particles. 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 particles in the SEM image. The magnification of the SEM image used for this measurement is 40,000 times, and the primary particle size is measured for 10 primary particles per field, and the primary particle size for 2 fields (that is, a total of 20 primary particles). The average value was calculated and used as the average primary particle size. The length measurement function of the SEM software was used for the 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 (Rigaku Co., Ltd., RINT TTR III), an XRD profile of the above powder is obtained under measurement conditions of voltage: 50 kV, current value: 300 mA, and measurement range: 10 to 70 °. rice field. From the obtained XRD profile, it was confirmed that the powder was LDH.

(4) Fabrication and evaluation of nickel-zinc secondary battery (4a) Electrode fabrication 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 on nickel foam to prepare an LDH electrode.

(4b) Preparation of Nickel-Zinc Secondary Battery An LDH electrode as a positive electrode, a Zn-ZnO electrode (including a mixture of zinc metal and zinc oxide) as a negative electrode, and a 6M KOH aqueous solution saturated with zinc oxide as an electrolytic solution. It was used to construct a nickel-zinc secondary battery.

(4c) Charge / discharge test A charge / discharge test was performed on the obtained nickel-zinc secondary battery. The charge / discharge test was performed at a 10-hour rate using a 580-inch Battery Test System manufactured by Toyo Corporation. The obtained charge / discharge curve is shown in FIG. The measured discharge capacity is shown in Table 1 as a relative value with respect to the case where _{a Ni (OH) 2 positive electrode is used (Example B7 described later).}

Examples B2 and B3
The same as in Example B1 except that the Ni—Fe-LDH composition was changed so that the molar ratio of Ni: Fe shown in Table 1 was 3: 1 (Example B2) or 2: 1 (Example B3). Nickel-zinc secondary batteries were manufactured and evaluated. The results are 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) Preparation of Ni-V-LDH powder Ni-V-LDH (Ni ₄ V-LDH (Ni ₈ V ₂ (OH) ₂₀ (CO ₃ ) ・ mH ₂ O) with a molar ratio of Ni: V of 4: 1 However, 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) 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 10 ml of metal ion aqueous solution Was added and stirred. The obtained raw material aqueous solution was placed in an autoclave, and crystals were grown by hydrothermal synthesis at 170 ° C. for 18 hours to precipitate an LDH precipitate. The precipitate-containing solution thus obtained was suction-filtered. The precipitate was recovered. The recovered LDH precipitate was washed with water, further washed with ethanol, and then dried in vacuum at 80 ° C. to obtain a Ni-V-LDH powder.

(2) SEM observation and particle size measurement The obtained Ni-V-LDH powder was SEM observed in the same manner as in Example B1. FIG. 12 shows an SEM image of LDH powder taken at a magnification (40,000 times) suitable for observing primary particles. 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 The obtained Ni-V-LDH powder was subjected to X-ray diffraction measurement 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) Preparation and Evaluation of Nickel-Zinc Secondary Battery A nickel-zinc secondary battery was prepared 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. The measured discharge capacity is shown in Table 1 as a relative value with respect to the case where _{a Ni (OH) 2 positive electrode is used (Example B7 described later).}

Examples B5 and B6
The same as in Example 4 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). Nickel-zinc secondary batteries were manufactured and evaluated. The results are 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 prepared 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 are as shown in Table 1.

Claims (7)

A positive electrode containing layered double hydroxide (LDH) as a positive electrode active material, wherein the LDH contains a combination of Ni and V as a constituent element, and after repeating a charge / discharge cycle including overcharging and discharging for 30 cycles. A positive electrode that can maintain the LDH crystal structure even in the state of the end of discharge. The charge / discharge cycle is a current in an electrochemical cell comprising the positive electrode as an working electrode, a platinum electrode as a counter electrode, an Hg / HgO electrode as a reference electrode, and a 6M KOH aqueous solution as an electrolytic solution. The positive electrode according to claim 1, wherein the positive electrode is performed under the 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 undergoes a 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 detection of (003) peak and (006) peak derived from LDH in X-ray diffraction. 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 discharge end state after the 30 cycles, respectively. The positive electrode according to claim 4. The positive electrode according to any one of claims 1 to 5, wherein the molar ratio of V / (Ni + V) is 0.15 to 0.50. A secondary battery comprising the positive electrode according to any one of claims 1 to 6 , a negative electrode, and an alkaline electrolyte and / or a hydroxide ion conductive solid electrolyte.

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