JP7564941B2 - Zinc secondary battery - Google Patents

Description

The present invention relates to a zinc secondary battery.

It is known that in zinc secondary batteries such as nickel-zinc secondary batteries and air-zinc secondary batteries, metallic zinc precipitates in the form of dendrites from the negative electrode during charging, penetrating the voids in the separator such as nonwoven fabric to reach the positive electrode, resulting in a short circuit. Such short circuits caused by zinc dendrites shorten the repeated charge-discharge life.

To address the above problem, a battery equipped with a layered double hydroxide (LDH) separator that selectively allows hydroxide ions to pass through while preventing the penetration of zinc dendrites has been proposed (see, for example, Patent Document 1 (WO 2016/076047) and Patent Document 2 (WO 2019/124270)). In addition, although they cannot be called LDHs, LDH-like compounds are known as hydroxides and/or oxides with a layered crystal structure similar to LDHs, and exhibit hydroxide ion conductive properties similar enough to be collectively referred to as hydroxide ion conductive layered compounds together with LDHs. For example, Patent Document 3 (WO 2020/255856) discloses a hydroxide ion conductive separator that includes a porous substrate and a layered double hydroxide (LDH)-like compound that blocks the pores of the porous substrate.

By the way, another factor that shortens the life of zinc secondary batteries is the change in shape of zinc, which is the negative electrode active material. That is, as zinc dissolves and precipitates repeatedly due to repeated charging and discharging, the shape of the negative electrode changes, causing high resistance due to pore blockage and a decrease in the charge active material due to the accumulation of isolated zinc, which results in a problem that charging and discharging becomes difficult. In order to address this problem, Patent Document 4 (WO 2020/049902) proposes using ZnO particles in combination with at least two selected from (i) metal Zn particles of a predetermined particle size, (ii) a predetermined metal element, and (iii) a binder resin having a hydroxyl group for use in the negative electrode. According to this negative electrode, it is said that in zinc secondary batteries, the deterioration of the negative electrode due to repeated charging and discharging can be suppressed, durability can be improved, and the cycle life can be extended.

Furthermore, Patent Document 5 (Patent Publication No. 6,190,101) discloses a negative electrode composite material containing a negative electrode active material such as metallic Zn or ZnO, a polymer such as an aromatic group-containing polymer, an ether group-containing polymer, or a hydroxyl group-containing polymer, and a conductive assistant, and describes that the material is suitable for forming a storage battery that exhibits excellent battery performance such as high cycle characteristics, rate characteristics, and coulombic efficiency while suppressing shape changes in the electrode active material, morphological changes in the electrode active material such as dendrites, dissolution, corrosion, and passivation formation.

Furthermore, Patent Document 6 (WO 2020/049901) discloses a zinc secondary battery in which the electrolyte further contains a protein capable of forming a complex with Zn and/or a derivative and/or decomposition product thereof, or a polyphenol capable of forming a complex with Zn and/or a derivative and/or decomposition product thereof, and preferred examples of such substances include casein, leucine, and tannic acid.

International Publication No. 2016/076047 International Publication No. 2019/124270 International Publication No. 2020/255856 International Publication No. 2020/049902 Patent No. 6190101 International Publication No. 2020/049901

However, the charge-discharge cycle performance of existing zinc secondary batteries is not necessarily sufficient, and further improvement in charge-discharge cycle performance is required.

The inventors have now discovered that charge-discharge cycle performance can be improved by constructing a zinc secondary battery so that the results of a specified charge-discharge cycle test fall within the range that satisfies y≧-0.10x+100 and y≦-0.01x+100 in a coordinate system consisting of an x-axis to which the cycle number is assigned and a y-axis to which the remaining area ratio (%) of the negative electrode plate according to the cycle number is assigned.

Therefore, the object of the present invention is to provide a zinc secondary battery exhibiting improved charge-discharge cycle performance.

According to one aspect of the present invention,
a positive electrode plate including a positive electrode active material;
A negative electrode plate including a negative electrode active material including at least one selected from the group consisting of zinc, zinc oxide, a zinc alloy, and a zinc compound;
a hydroxide ion conductive separator that separates the positive electrode plate and the negative electrode plate so as to be capable of conducting hydroxide ions;
An electrolyte;
A zinc secondary battery having a unit cell including the above-mentioned. The zinc secondary battery is subjected to a charge-discharge cycle test under the following conditions:
- Current density: 12.5mA/ ^cm2
- Temperature: 25°C
- Charge/discharge rate: 0.5C
- Depth of discharge (DOD): 70%
- Charge: constant current (CC) charge (cut-off voltage 1.9 V) followed by constant voltage (CV) charge - Discharge: constant current (CC) discharge (cut-off voltage 1.4 V)
- Rest time when switching between charge and discharge: 5 minutes - Number of cycles: When performed 40 times or more,
In a coordinate system having an x-axis to which the number of cycles is assigned and a y-axis to which the remaining area ratio (%) of the negative electrode plate according to the number of cycles is assigned, the results of the charge-discharge cycle test are
y≧−0.10x+100, and y≦−0.01x+100
The zinc secondary battery is provided within the range of the above.

FIG. 1 is a schematic cross-sectional view showing an example of a nickel-zinc secondary battery according to the present invention. FIG. 2 is a diagram showing an example of a cross section of the nickel-zinc secondary battery shown in FIG. 1 taken along line A-A'. 1 is a graph plotting the remaining area ratio of the negative electrode at each cycle number obtained in Examples 1 to 85.

Zinc secondary battery The zinc secondary battery of the present invention is not particularly limited as long as it is a secondary battery using zinc as the negative electrode and an alkaline electrolyte (typically an aqueous alkali metal hydroxide solution). Therefore, it can be a nickel-zinc secondary battery, a silver oxide zinc secondary battery, a manganese oxide zinc secondary battery, an air zinc secondary battery, or any other type of alkaline zinc secondary battery. For example, it is preferable that the positive electrode plate contains nickel hydroxide and/or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery. Alternatively, the positive electrode plate may be an air electrode layer, thereby making the zinc secondary battery an air zinc secondary battery.

1 and 2 show an embodiment of a zinc secondary battery according to the present invention. The zinc secondary battery 10 shown in Figs. 1 and 2 includes a battery element 11 in a sealed container 20, and the battery element 11 includes a unit cell 10a including a positive electrode plate 12, a negative electrode plate 14, a hydroxide ion conductive separator 16, and an electrolyte 18. The positive electrode plate 12 includes a positive electrode active material. The negative electrode plate 14 includes a negative electrode active material, and the negative electrode active material includes a negative electrode active material including at least one selected from the group consisting of zinc, zinc oxide, zinc alloy, and zinc compound. The hydroxide ion conductive separator 16 separates the positive electrode plate 12 and the negative electrode plate 14 so as to be conductive with hydroxide ions. The zinc secondary battery 10 is then subjected to a charge-discharge cycle test under the following conditions:
- Current density: 12.5mA/ ^cm2
- Temperature: 25°C
- Charge/discharge rate: 0.5C
- Depth of discharge (DOD): 70%
- Charge: constant current (CC) charge (cut-off voltage 1.9 V) followed by constant voltage (CV) charge - Discharge: constant current (CC) discharge (cut-off voltage 1.4 V)
- Rest time when switching between charge and discharge: 5 minutes - Number of cycles: When 40 or more cycles are performed, the result of the charge and discharge cycle test falls within a range that satisfies y≧−0.10x+100 and y≦−0.01x+100 in a coordinate system having an x-axis to which the number of cycles is assigned and a y-axis to which the remaining area rate (%) of the negative electrode plate according to the number of cycles is assigned.

In this way, by constructing a zinc secondary battery so that it falls within the above range, the charge/discharge cycle performance is improved. By using such parameters or indicators, it is possible to conveniently determine a zinc secondary battery with excellent charge/discharge cycle performance. For example, according to the present invention, when constructing a zinc secondary battery with desired specifications, it is possible to relatively accurately predict the long-term performance, such as the cycle life of the cycle performance of the zinc secondary battery, simply by checking whether or not the above range is satisfied by a relatively small number of charge/discharge cycle tests (e.g., 40, 100, 150, 200 times).

y ≧ -0.10x + 100 is a formula that defines the lower limit of the above region, but more preferably y ≧ -0.09x + 100, and even more preferably y ≧ -0.05x + 100. On the other hand, y ≦ -0.01x + 100 is a formula that defines the upper limit of the above region, but the preferred upper limit is y ≦ -0.02x + 100, and more preferably y ≦ -0.03x + 100. That is, the preferred region is a region that satisfies y ≧ -0.09x + 100 and y ≦ -0.02x + 100, and more preferably a region that satisfies y ≧ -0.05x + 100 and y ≦ -0.03x + 100. In these preferred regions, a zinc secondary battery exhibiting even further improved charge-discharge cycle performance can be provided. It goes without saying that the above region satisfies x ≧ 40 (because there is a condition that the number of cycles is 40 or more), but the upper limit is not particularly limited. However, the above range typically satisfies x≦1200, x≦1000, x≦800, x≦600, x≦400, x≦200, x≦150, or x≦100.

In order to construct a zinc secondary battery that falls within the above range, it is preferable to add an additive to the negative electrode plate 14 and/or the electrolyte 18 that has the effect of suppressing negative electrode shape change.

A preferred additive to be added to the negative electrode plate 14 is a nonionic water-absorbing polymer. Preferred examples of the nonionic water-absorbing polymer include polyalkylene oxide-based water-absorbing resin, polyvinyl acetamide-based water-absorbing resin, polyvinyl alcohol (PVA resin), and polyvinyl butyral (PVB resin), and more preferably polyalkylene oxide-based water-absorbing resin. Commercially available polyalkylene oxide-based water-absorbing resins can be used. The nonionic water-absorbing polymer may contain at least one selected from hydrophilic ether groups, hydroxyl groups, amide groups, and acetamide groups. The presence of these functional groups can provide a water-absorbing and releasing function that is more favorable for the battery reaction. The nonionic water-absorbing polymer may remain as particles in the negative electrode, or may cover the active material. When the active material is covered with the nonionic water-absorbing polymer, a method of adding the nonionic water-absorbing polymer in a slurry form, or a method of heating and melting the nonionic water-absorbing polymer during production can be considered. In the latter case, the melting point of the nonionic water-absorbing polymer is preferably 45°C to 350°C, more preferably 45°C to 200°C, and even more preferably 50°C to 100°C. The content of the nonionic water-absorbing polymer in the negative electrode plate 14 is preferably 0.01 to 6.0 parts by weight in terms of solid content, more preferably 0.01 to 5.5 parts by weight, even more preferably 0.05 to 5.0 parts by weight, and particularly preferably 0.07 to 4.0 parts by weight, when the content of the ZnO particles is 100 parts by weight. In addition, the nonionic water-absorbing polymer is preferably particulate. In this case, the particle size of the nonionic water-absorbing polymer is preferably 10 to 200 μm, more preferably 15 to 180 μm, even more preferably 20 to 160 μm, and particularly preferably 30 to 150 μm. It is not necessary that all the particles of the nonionic water-absorbing polymer are within the above numerical range, but it is sufficient that the average particle size D50 falls within the above numerical range.

Preferable additives to be added to the electrolyte 18 include proteins capable of forming complexes with Zn and/or derivatives and/or decomposition products thereof, or polyphenols capable of forming complexes with Zn and/or derivatives and/or decomposition products thereof (see Patent Document 6). That is, the electrolyte may contain proteins capable of forming complexes with Zn, but part or all of the proteins may exist as derivatives and/or decomposition products thereof (e.g., peptides or amino acids). Alternatively, the electrolyte may contain polyphenols capable of forming complexes with Zn, but part or all of the proteins may exist as derivatives and/or decomposition products of polyphenols. The protein as an additive is not particularly limited as long as it is capable of forming a complex with Zn. Examples of such proteins include casein, gelatin, etc., and casein is particularly preferred. Proteins such as casein may form micelles in the electrolyte. Derivatives of proteins such as casein are not particularly limited as long as they are compounds that have been modified to an extent that does not significantly change the structure or properties of the protein by the introduction or substitution of functional groups, oxidation, reduction, atomic substitution, etc., and include, for example, sodium salts and potassium salts. Examples of decomposition products of proteins such as casein include polypeptides, tripeptides, dipeptides, and amino acids such as glutamic acid, aspartic acid, leucine, isoleucine, histidine, methionine, glycine, proline, tyrosine, and lysine. From the viewpoint of extending the cycle life, preferred examples of amino acids include aspartic acid, methionine, glycine, leucine, and isoleucine, more preferably glycine, leucine, and isoleucine, particularly preferably leucine and isoleucine, and most preferably leucine. The polyphenol as an additive is not particularly limited as long as it can form a complex with Zn. Examples of such polyphenols include hydrolyzed (pyrogallol type) tannins and condensed (catechol type) tannins, preferably hydrolyzed (pyrogallol type) tannins, and more preferably tannic acid, which is one of the hydrolyzed (pyrogallol type) tannins. The derivatives of polyphenols such as tannic acid are not particularly limited as long as they are modified by the introduction or substitution of functional groups, oxidation, reduction, atomic substitution, etc. to such an extent that the structure or properties of the polyphenols are not significantly changed. Examples of derivatives of tannic acid include compounds having a molecular structure in which at least a portion of the hydroxyl groups contained in the tannic acid molecule is substituted with an alkyl ether group or an alkyl ester group, etc. Examples of decomposition products of tannic acid include compounds produced by hydrolyzing at least a portion of the ester bonds in the tannic acid molecule.

In addition to the additives mentioned above, various additives shown in Table 1 below can be used to configure a zinc secondary battery so that it falls within the range mentioned above. All of these additives are well known and are believed to be effective in suppressing negative electrode shape change.

The positive electrode plate 12 includes a positive electrode active material. The positive electrode active material includes nickel hydroxide and/or nickel oxyhydroxide. Typically, the positive electrode plate 12 further includes a positive electrode collector (not shown), and the positive electrode collector preferably has a positive electrode collector tab 13 extending from an end (e.g., the upper end) of the positive electrode plate 12. A preferred example of the positive electrode collector is a nickel porous substrate such as a foamed nickel plate. In this case, for example, a paste containing an electrode active material such as nickel hydroxide is uniformly applied to a nickel porous substrate and dried to preferably produce a positive electrode plate consisting of a positive electrode/positive electrode collector. At that time, it is also preferable to perform a press process on the dried positive electrode plate (i.e., the positive electrode/positive electrode collector) to prevent the electrode active material from falling off and improve the electrode density. The positive electrode plate 12 shown in FIG. 2 includes a positive electrode collector (e.g., foamed nickel), but is not shown. This is because in the case of a nickel-zinc secondary battery, the positive electrode current collector is integrated with the positive electrode active material, and therefore the positive electrode current collector cannot be depicted separately. The nickel-zinc secondary battery 10 preferably further includes a positive electrode current collector plate connected to the tip of the positive electrode current collector tab 13, and more preferably, a plurality of positive electrode current collector tabs 13 are connected to one positive electrode current collector plate. This allows current collection to be performed with a simple configuration and with good space efficiency, and also makes it easier to connect to the positive electrode terminal 26. The positive electrode current collector plate itself may be used as the positive electrode terminal 26.

The positive electrode plate 12 may contain at least one additive selected from the group consisting of silver compounds, manganese compounds, and titanium compounds, which can promote the positive electrode reaction that absorbs hydrogen gas generated by the self-discharge reaction. The positive electrode plate 12 may further contain cobalt. Cobalt is preferably contained in the positive electrode plate 12 in the form of cobalt oxyhydroxide. In the positive electrode plate 12, cobalt functions as a conductive additive, thereby contributing to improving the charge/discharge capacity.

The negative electrode plate 14 includes a negative electrode active material. The negative electrode active material includes at least one selected from the group consisting of zinc, zinc oxide, zinc alloys, and zinc compounds. Zinc may be included in any form of zinc metal, zinc compounds, and zinc alloys, as long as it has electrochemical activity suitable for the negative electrode. Preferred examples of the negative electrode material include zinc oxide, zinc metal, calcium zincate, and the like, with a mixture of zinc metal and zinc oxide being more preferred. The negative electrode active material may be configured in a gel form, or may be mixed with the electrolyte 18 to form a negative electrode mixture. For example, a gelled negative electrode can be easily obtained by adding an electrolyte and a thickener to the negative electrode active material. Examples of thickeners include polyvinyl alcohol, polyacrylate, CMC, alginic acid, and the like, with polyacrylic acid being preferred because of its excellent chemical resistance to strong alkalis.

As the zinc alloy, a mercury- and lead-free zinc alloy known as a mercury-free zinc alloy can be used. For example, a zinc alloy containing 0.01 to 0.1 mass% indium, 0.005 to 0.02 mass% bismuth, and 0.0035 to 0.015 mass% aluminum is preferred because it has the effect of suppressing hydrogen gas generation. In particular, indium and bismuth are advantageous in terms of improving discharge performance. The use of a zinc alloy for the negative electrode can improve safety by suppressing hydrogen gas generation by slowing down the self-dissolution rate in an alkaline electrolyte.

The shape of the negative electrode material is not particularly limited, but it is preferably in powder form, which increases the surface area and allows it to withstand large current discharge. The average particle size of the preferred negative electrode material is in the range of 3 to 100 μm in short axis for zinc alloys; within this range, the large surface area makes it suitable for large current discharge, and it is easy to mix uniformly with the electrolyte and gelling agent, and easy to handle when assembling the battery.

Preferably, the negative electrode plate 14 further includes a negative electrode collector 15, which has a negative electrode collector tab 15a extending from an end (e.g., the upper end) of the negative electrode plate 14. The negative electrode collector tab 15a is preferably provided at a position that does not overlap with the positive electrode collector tab 13. The zinc secondary battery 10 preferably further includes a negative electrode collector plate connected to the tip of the negative electrode collector tab 15a, and more preferably, multiple negative electrode collector tabs 15a are connected to one negative electrode collector plate. This allows for efficient space collection with a simple configuration, and also makes it easier to connect to the negative electrode terminal 28. The negative electrode collector plate itself may be used as the negative electrode terminal 28.

Preferred examples of the negative electrode collector 15 include copper foil, copper expanded metal, and copper punched metal, and more preferably copper expanded metal. In this case, for example, a mixture containing zinc oxide powder and/or zinc powder, and optionally a binder (e.g., polytetrafluoroethylene particles) can be applied to the copper expanded metal to preferably prepare a negative electrode plate consisting of a negative electrode/negative electrode collector. In this case, it is also preferable to press the negative electrode plate (i.e., the negative electrode/negative electrode collector) after drying to prevent the electrode active material from falling off and improve the electrode density.

The zinc secondary battery 10 may further include a liquid-retaining member 17 in contact with the positive electrode plate 12 and/or the negative electrode plate 14. For example, it is preferable that not only the hydroxide ion conductive separator 16 but also the liquid-retaining member 17 is interposed between the positive electrode plate 12 and the negative electrode plate 14. As shown in FIG. 2, it is preferable that the positive electrode plate 12 and/or the negative electrode plate 14 is covered or wrapped with the liquid-retaining member 17. However, a simple configuration in which the liquid-retaining member 17 is arranged on one side of the positive electrode plate 12 or the negative electrode plate 14 may also be used. In any case, by interposing the liquid-retaining member 17, the electrolyte 18 can be evenly present between the positive electrode plate 12 and/or the negative electrode plate 14 and the hydroxide ion conductive separator 16, and the exchange of hydroxide ions between the positive electrode plate 12 and/or the negative electrode plate 14 and the hydroxide ion conductive separator 16 can be efficiently performed. The liquid-retaining member 17 is not particularly limited as long as it is a member capable of retaining the electrolyte 18, but is preferably a sheet-like member. Preferred examples of the liquid-retaining member 17 include nonwoven fabric, water-absorbent resin, liquid-retaining resin, porous sheet, and various spacers, but nonwoven fabric is particularly preferred because it can produce a negative electrode structure with good performance at low cost. The liquid-retaining member 17 or nonwoven fabric preferably has a thickness of 10 to 200 μm, more preferably 20 to 200 μm, even more preferably 20 to 150 μm, particularly preferably 20 to 100 μm, and most preferably 20 to 60 μm. If the thickness is within the above range, a sufficient amount of electrolyte 18 can be retained in the liquid-retaining member 17 while keeping the overall size of the positive electrode structure and/or negative electrode structure compact and without waste.

The hydroxide ion conductive separator 16 is provided to isolate the positive electrode plate 12 and the negative electrode plate 14 so as to allow hydroxide ion conductivity. For example, as shown in FIG. 2, the negative electrode plate 14 may be covered or wrapped with the hydroxide ion conductive separator 16. This makes it possible to manufacture a nickel-zinc secondary battery (particularly a laminated battery thereof) capable of preventing zinc dendrite extension extremely easily and with high productivity, without the need for a complicated sealing joint between the hydroxide ion conductive separator 16 and the battery container. However, a simple configuration in which the hydroxide ion conductive separator 16 is arranged on one side of the positive electrode plate 12 or the negative electrode plate 14 may also be used.

The hydroxide ion conductive separator 16 is not particularly limited as long as it is a separator capable of isolating the positive electrode plate 12 and the negative electrode plate 14 in a hydroxide ion conductive manner, but is typically a separator that contains a hydroxide ion conductive solid electrolyte and selectively passes hydroxide ions solely by utilizing hydroxide ion conductivity. A preferred hydroxide ion conductive solid electrolyte is a layered double hydroxide (LDH) and/or an LDH-like compound. Therefore, the hydroxide ion conductive separator 16 is preferably an LDH separator. In this specification, an "LDH separator" is defined as a separator that contains LDH and/or an LDH-like compound and selectively passes hydroxide ions solely by utilizing the hydroxide ion conductivity of the LDH and/or the LDH-like compound. In this specification, an "LDH-like compound" is a hydroxide and/or oxide of a layered crystal structure similar to LDH that may not be called LDH, and can be said to be an equivalent of LDH. However, in a broad definition, "LDH" can be interpreted as including not only LDH but also LDH-like compounds. The LDH separator is preferably composited with a porous substrate. Therefore, the LDH separator preferably further comprises a porous substrate, and is composited with the porous substrate in a form in which the pores of the porous substrate are filled with LDH and/or LDH-like compounds. That is, in a preferred LDH separator, the pores of the porous substrate are blocked with LDH and/or LDH-like compounds so as to exhibit hydroxide ion conductivity and gas impermeability (and therefore function as an LDH separator exhibiting hydroxide ion conductivity). The porous substrate is preferably made of a polymeric material, and it is particularly preferred that the LDH is incorporated throughout the entire thickness of the porous substrate made of a polymeric material. For example, known LDH separators such as those disclosed in Patent Documents 1 to 4 can be used. The thickness of the LDH separator is preferably 5 to 100 μm, more preferably 5 to 80 μm, further preferably 5 to 60 μm, and particularly preferably 5 to 40 μm.

When the positive electrode plate 12 and/or the negative electrode plate 14 are covered or wrapped with the liquid-retaining member 17 and/or the separator 16, it is preferable that the outer edges of the plates are closed (except for the edges from which the positive electrode current collector tab 13 and the negative electrode current collector tab 15a extend). In this case, it is preferable that the closed edges of the outer edges of the liquid-retaining member 17 and/or the separator 16 are realized by folding the liquid-retaining member 17 and/or the separator 16, or by sealing the liquid-retaining members 17 together and/or the separators 16 together. Preferred examples of sealing methods include adhesives, heat welding, ultrasonic welding, adhesive tape, sealing tape, and combinations thereof. In particular, since the LDH separator including a porous substrate made of a polymer material has the advantage of being flexible and therefore easy to bend, it is preferable to form the LDH separator into a long shape and fold it to form a state in which one side of the outer edge is closed. Thermal welding and ultrasonic welding may be performed using a commercially available heat sealer or the like, but in the case of sealing between LDH separators, it is preferable to perform thermal welding and ultrasonic welding by sandwiching the outer peripheral portion of the liquid retaining member 17 between the LDH separators constituting the outer peripheral portion, in order to perform more effective sealing. On the other hand, the adhesive, adhesive tape, and sealing tape may be commercially available products, but it is preferable to use those containing a resin having alkali resistance in order to prevent deterioration in an alkaline electrolyte. From this viewpoint, examples of preferable adhesives include epoxy resin adhesives, natural resin adhesives, modified olefin resin adhesives, and modified silicone resin adhesives, and among them, epoxy resin adhesives are more preferable because they are particularly excellent in alkali resistance. An example of a product of an epoxy resin adhesive is the epoxy adhesive Hysol (registered trademark) (manufactured by Henkel).

It is preferable that the outer edge of one side that is the upper end of the separator 16 is open. This open-top type configuration makes it possible to deal with problems that occur when a nickel-zinc battery or the like is overcharged. That is, when a nickel-zinc battery or the like is overcharged, oxygen (O ₂ ) may be generated at the positive electrode plate 12, but the LDH separator has such a high density that it allows only hydroxide ions to pass through, and therefore does not allow O ₂ to pass through. In this regard, according to the open-top type configuration, in the sealed container 20, O ₂ can be released above the positive electrode plate 12 and sent to the negative electrode plate 14 side through the open-top part, thereby oxidizing Zn of the negative electrode active material with O ₂ and returning it to ZnO. By going through such an oxygen reaction cycle, the open-top type battery element 11 can be used in a sealed nickel-zinc secondary battery to improve overcharge resistance. Note that even if the outer edge of one side that is the upper end of the separator 16 or the liquid-retaining member 17 is closed, the same effect as the open-top type configuration can be expected by providing a vent in a part of the closed outer edge. For example, the outer edge of one side of the LDH separator that will be the upper end may be sealed and then an air hole may be opened, or during sealing, part of the outer edge may be left unsealed so that an air hole can be formed.

The electrolyte 18 preferably contains an aqueous solution of an alkali metal hydroxide. In FIG. 2, the electrolyte 18 is only shown locally because it is distributed throughout the positive and negative plates 12 and 14. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, lithium hydroxide, and ammonium hydroxide, with potassium hydroxide being more preferred. In order to suppress the self-dissolution of zinc and/or zinc oxide, a zinc compound such as zinc oxide or zinc hydroxide may be added to the electrolyte. As described above, the electrolyte may be mixed with the positive electrode active material and/or the negative electrode active material to be present in the form of a positive electrode composite and/or a negative electrode composite. The electrolyte may also be gelled to prevent leakage of the electrolyte. As the gelling agent, it is preferable to use a polymer that absorbs the solvent of the electrolyte and swells, and polymers such as polyethylene oxide, polyvinyl alcohol, and polyacrylamide, or starch are used.

As shown in Figure 2, the battery element 11 preferably has a positive and negative electrode laminate having multiple positive plates 12, multiple negative plates 14, and multiple separators 16, and is stacked so that the unit of positive plate 12/separator 16/negative plate 14 is repeated. In other words, the zinc secondary battery 10 preferably has multiple unit cells 10a, and the multiple unit cells 10a as a whole form a multi-layer cell. This is the so-called assembled battery or stacked battery configuration, and is advantageous in that it can obtain high voltage and large current.

The sealed container 20 is preferably made of resin. The resin constituting the sealed container 20 is preferably a resin resistant to alkali metal hydroxides such as potassium hydroxide, more preferably a polyolefin resin, ABS resin, or modified polyphenylene ether, and even more preferably ABS resin or modified polyphenylene ether. The sealed container 20 has a top lid 20a. The sealed container 20 (e.g., top lid 20a) may have a pressure relief valve for releasing gas. In addition, a group of cases in which two or more sealed containers 20 are arranged may be housed within an outer frame to form a battery module.

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

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

The LDH-like compound can be identified by X-ray diffraction. Specifically, when X-ray diffraction is performed on the surface of the LDH separator, a peak derived from the LDH-like compound is typically detected in the range of 5°≦2θ≦10°, more typically in the range of 7°≦2θ≦10°. As described above, LDH is a substance having an alternating laminate structure in which exchangeable anions and H ₂ O exist as intermediate layers between stacked hydroxide basic layers. In this regard, when LDH is measured by X-ray diffraction, a peak due to the crystal structure of LDH (i.e., the (003) peak of LDH) is essentially detected at a position of 2θ=11 to 12°. In contrast, when an LDH-like compound is measured by X-ray diffraction, a peak is typically detected in the above-mentioned range shifted to the lower angle side from the above-mentioned peak position of LDH. In addition, the interlayer distance of the layered crystal structure can be determined by the Bragg formula using 2θ corresponding to the peak derived from the LDH-like compound in X-ray diffraction. The interlayer distance of the layered crystal structure constituting the LDH-like compound determined in this manner is typically 0.883 to 1.8 nm, and more typically 0.883 to 1.3 nm.

In the LDH separator according to the above aspect (a), the atomic ratio of Mg/(Mg+Ti+Y+Al) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.03 to 0.25, more preferably 0.05 to 0.2. The atomic ratio of Ti/(Mg+Ti+Y+Al) in the LDH-like compound is preferably 0.40 to 0.97, more preferably 0.47 to 0.94. The atomic ratio of Y/(Mg+Ti+Y+Al) in the LDH-like compound is preferably 0 to 0.45, more preferably 0 to 0.37. The atomic ratio of Al/(Mg+Ti+Y+Al) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.03. Within the above range, the alkali resistance is more excellent, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively realized. Meanwhile, LDH conventionally known for LDH separators can be expressed by a basic composition of the general formula: M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ^n- _x/ n.mH ₂ O (wherein M ²⁺ is a divalent cation, M ³⁺ is a trivalent cation, A ^n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In contrast, the above atomic ratio in the LDH-like compound generally deviates from the above general formula of LDH. For this reason, it can be said that the LDH-like compound in this embodiment generally has a composition ratio (atomic ratio) different from that of conventional LDH. The EDS analysis is preferably performed using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing an image at an accelerating voltage of 20 kV and a magnification of 5,000 times, 2) performing three-point analysis in a point analysis mode with an interval of about 5 μm, 3) repeating the above 1) and 2) once more, and 4) calculating the average value of the total six points.

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

In the LDH separator according to the above aspect (b), the atomic ratio of Ti/(Mg+Al+Ti+Y+M) in the LDH-like compound, as determined by energy dispersive X-ray analysis (EDS), is preferably 0.50 to 0.85, more preferably 0.56 to 0.81. The atomic ratio of Y/(Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.20, more preferably 0.07 to 0.15. The atomic ratio of M/(Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0.03 to 0.35, more preferably 0.03 to 0.32. The atomic ratio of Mg/(Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.10, more preferably 0 to 0.02. The atomic ratio of Al/(Mg+Al+Ti+Y+M) in the LDH-like compound is preferably 0 to 0.05, more preferably 0 to 0.04. Within the above range, the alkali resistance is more excellent, and the effect of suppressing short circuits caused by zinc dendrites (i.e., dendrite resistance) can be more effectively realized. Meanwhile, LDH, which has been conventionally known for LDH separators, can be expressed by a basic composition of the general formula: M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ^n- _x/ n.mH ₂ O (wherein M ²⁺ is a divalent cation, M ³⁺ is a trivalent cation, A ^n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In contrast, the above atomic ratio in the LDH-like compound generally deviates from the above general formula of LDH. Therefore, it can be said that the LDH-like compound in this embodiment generally has a composition ratio (atomic ratio) different from that of conventional LDH. It is preferable to perform the EDS analysis by using an EDS analyzer (e.g., X-act, manufactured by Oxford Instruments) by 1) capturing an image at an accelerating voltage of 20 kV and a magnification of 5,000 times, 2) performing three-point analysis with an interval of about 5 μm in a point analysis mode, 3) repeating the above 1) and 2) once more, and 4) calculating the average value of the total six points.

According to yet another preferred embodiment (c) of the present invention, the LDH-like compound is a hydroxide and/or oxide of a layered crystal structure containing Mg, Ti, Y, and optionally Al and/or In, and the LDH-like compound may be present in the form of a mixture with In(OH) _3. The LDH-like compound of this embodiment is a hydroxide and/or oxide of a layered crystal structure containing Mg, Ti, Y, and optionally Al and/or In. Thus, a typical LDH-like compound is a composite hydroxide and/or composite oxide of Mg, Ti, Y, optionally Al, and optionally In. The In that may be contained in the LDH-like compound may not only be that which is intentionally added to the LDH-like compound, but may also be that which is inevitably mixed into the LDH-like compound due to the formation of In(OH) ₃ , etc. The above elements may be replaced with other elements or ions to the extent that the basic properties of the LDH-like compound are not impaired, but it is preferable that the LDH-like compound does not contain Ni. Incidentally, conventionally known LDHs for LDH separators can be expressed by a basic composition represented by the general formula: M ²⁺ _1-x M ³⁺ _x (OH) ₂ A ^n- _x/ n.mH ₂ O (wherein M ²⁺ is a divalent cation, M ³⁺ is a trivalent cation, A ^n- is an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In contrast, the atomic ratios in LDH-like compounds generally deviate from the above general formula of LDH. For this reason, it can be said that the LDH-like compound in this embodiment generally has a composition ratio (atomic ratio) different from that of conventional LDHs.

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

The present invention will be further illustrated by the following examples.

Examples 1 to 75
Simple nickel-zinc secondary battery cells of various specifications were fabricated using the following procedure, and charge-discharge cycle tests were performed.

(1) Preparation of Positive Electrode A paste-type nickel hydroxide positive electrode (capacity density: about 700 mAh/cm ³ ) was prepared.

(2) Preparation of Negative Electrode The following raw materials were prepared.
<Negative Electrode Active Material>
ZnO powder (manufactured by Seido Chemical Industry Co., Ltd., JIS standard type 1 grade, average particle size D50: 0.2 μm)
Metal Zn powder (manufactured by DOWA Electronics Co., Ltd., doped with Bi and In, Bi: 70 ppm by weight, In: 200 ppm by weight, average particle size D50: 120 μm)
<Additive 1>
・Milled carbon fiber (product name: DonaCarbo Milt, manufactured by Osaka Gas Chemicals Co., Ltd.)
・Acetylene black (product name: Denka Black, manufactured by Denka Co., Ltd.)
・Carbon nanotubes (product name: VGCF, manufactured by Showa Denko K.K.)
S-100/g (product name: ChemiQ S-100, manufactured by Chemic Co., Ltd.)
・Powdered PTFE (product name: Lubron, manufactured by Daikin Industries, Ltd.)
_{In2O3 powder} (product name: indium oxide, manufactured by Takanami Inorganic Co., Ltd.)
<Additive 2>
Propylene glycol (PG) (product name: propylene glycol, manufactured by Kanto Chemical Co., Ltd.)
<Additive 3>
Polyvinyl alcohol (PVA) (product name: polyvinyl alcohol, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
- Polyolefin aqueous dispersion (product name: Chemipearl S100, manufactured by Mitsui Chemicals, Inc.)
- Polyolefin aqueous dispersion (product name: Chemipearl S650, manufactured by Mitsui Chemicals, Inc.)
<Additive 4>
- PTFE (product name: Polyflon PTFE F Series, manufactured by Daikin Industries, Ltd.)

According to the compounding ratios shown in Tables 2A and 2B, ZnO powder was mixed with metal Zn powder and various additives. The resulting mixture was rolled with a roll press to obtain a negative electrode active material sheet. The negative electrode active material sheet was pressed onto a tin-plated copper expand metal to obtain a negative electrode. In this way, the 21 types of negative electrode plates shown in Tables 2A and 2B were produced.

(3) Preparation of Electrolyte Solution After adding ion-exchanged water to a 48% aqueous potassium hydroxide solution (special grade, manufactured by Kanto Chemical Co., Ltd.) to adjust the KOH concentration to 5.4 mol%, zinc oxide was dissolved at 0.42 mol/L by heating and stirring to obtain an electrolyte solution. In Examples 30 to 32, casein was added to this electrolyte solution in an amount of 2.0/50 CC.

(4) Preparation of Evaluation Cells Cells were prepared using various negative electrodes as shown in Tables 3 to 8 by the following procedure. First, each of the positive and negative electrodes was wrapped in nonwoven fabric, and a current extraction terminal was welded. The positive and negative electrodes thus prepared were opposed to each other via an Mg-Al-Ti-Y-LDH separator, sandwiched between a laminate film provided with a current extraction port, and three sides of the laminate film were heat-sealed. An electrolyte was added to the cell container thus obtained with the top open, and the electrolyte was sufficiently permeated into the positive and negative electrodes by evacuation or the like. Thereafter, the remaining side of the laminate film was also heat-sealed to form a simple sealed cell.

(5) Charge-discharge cycle test A charge-discharge cycle test was performed on the simple sealed cell at 25° C. using a charge-discharge device (TOSCAT3100, manufactured by Toyo Systems Co., Ltd.). This cycle test consisted of constant current (CC) charging at a discharge rate of 0.5C (current density 12.5mA/cm ² ) until the battery voltage reached 1.9V, followed by constant voltage (CV) charging at a voltage of 1.9V until the battery voltage reached 63% of the positive electrode's installed capacity, followed by a 5-minute charge rest period, followed by constant current (CC) discharging at a charge rate of 0.5C (current density 12.5mA/cm ² ) until the depth of discharge (DOD) reached 70% at a cutoff voltage of 1.4V, followed by a 5-minute discharge rest period. The charge-discharge cycle was performed the number of times shown in Tables 3 to 8.

A planar image of the negative plate in the battery after the charge-discharge cycle test was taken, and the image was analyzed to calculate the remaining area ratio (%) of the negative plate. Specifically, the remaining area ratio of the negative plate is the area of the part (visually recognized as black) in which the negative electrode active material (ZnO) remains in the negative plate after the cycle test divided by the area (visually recognized as black) covered with the negative electrode active material (ZnO) in the negative plate before the cycle test, multiplied by 100. The results are shown in Tables 3 to 8 and FIG. 3.

Examples 76 to 85
Simple nickel-zinc secondary battery cells of various specifications were fabricated using the following procedure, and charge-discharge cycle tests were performed.

(1) Preparation of Positive Electrode A paste-type nickel hydroxide positive electrode (capacity density: about 700 mAh/cm ³ ) was prepared.

(2) Preparation of Negative Electrode The following raw materials were prepared.
<Negative Electrode Active Material>
ZnO powder (manufactured by Seido Chemical Industry Co., Ltd., JIS standard type 1 grade, average particle size D50: 0.2 μm)
Metal Zn powder (manufactured by DOWA Electronics Co., Ltd., doped with Bi and In, Bi: 70 ppm by weight, In: 200 ppm by weight, average particle size D50: 120 μm)
<Additives>
- Nonionic water-absorbing polymer (sodium polyacrylate, product name: Aquacork C-PF, manufactured by Sumitomo Seika Chemicals Co., Ltd.)
_{In2O3 powder} (product name: indium oxide, manufactured by Takanami Inorganic Co., Ltd.)
Propylene glycol (PG) (product name: propylene glycol, manufactured by Kanto Chemical Co., Ltd.)
- PTFE (product name: Polyflon PTFE F Series, manufactured by Daikin Industries, Ltd.)

According to the compounding ratios shown in Tables 9 and 10, ZnO powder was mixed with metal Zn powder, polytetrafluoroethylene (PTFE), and other additives, and kneaded with propylene glycol. The resulting mixture was rolled with a roll press to obtain a negative electrode active material sheet. The negative electrode active material sheet was pressed onto a tin-plated copper expand metal to obtain a negative electrode. Various negative electrode plates were produced in this manner.

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

(4) Preparation of Evaluation Cells Cells were prepared using various negative electrodes as shown in Tables 9 and 10 by the following procedure. First, two positive electrodes and one negative electrode were wrapped in nonwoven fabric, and a current extraction terminal was welded. The two positive electrodes and one negative electrode thus prepared were laminated in the order of positive electrode/negative electrode/positive electrode with an Mg-Al-Ti-Y-LDH separator interposed therebetween, sandwiched between laminate films provided with current extraction ports, and three sides of the laminate film were heat-sealed. An electrolyte was added to the cell container thus obtained with the top open, and the electrolyte was sufficiently permeated into the positive and negative electrodes by evacuation or the like. Thereafter, the remaining side of the laminate film was also heat-sealed to form a simple sealed cell.

(5) Charge-discharge cycle test A charge-discharge cycle test was performed in the same manner as in Examples 1 to 75, and the remaining area ratio (%) of the negative electrode plate was measured. The results are shown in Tables 9 and 10 and FIG.

3 is a graph plotting the results of each of Examples 1 to 85 in a coordinate system consisting of an x-axis to which the cycle number is assigned, and a y-axis to which the remaining area ratio (%) of the negative electrode plate according to the cycle number is assigned. From FIG. 3, it can be seen that a zinc secondary battery configured to be within the range of a region satisfying y≧−0.10x+100 (preferably y≧−0.09x+100, more preferably y≧−0.05x+100) and y≦−0.01x+100 (preferably y≦−0.02x+100, more preferably y≦−0.03x+100) when evaluated in a predetermined charge-discharge cycle test shows improved cycle characteristics (longer cycle life).

Claims (7)

a positive electrode plate including a positive electrode active material;
A negative electrode plate including a negative electrode active material including at least one selected from the group consisting of zinc, zinc oxide, a zinc alloy, and a zinc compound;
a hydroxide ion conductive separator that separates the positive electrode plate and the negative electrode plate so as to be capable of conducting hydroxide ions;
An electrolyte;
A zinc secondary battery having a unit cell including the above-mentioned. The zinc secondary battery is subjected to a charge-discharge cycle test under the following conditions:
- Current density: 12.5mA/ ^cm2
- Temperature: 25°C
- Charge/discharge rate: 0.5C
- Depth of discharge (DOD): 70%
- Charge: constant current (CC) charge (cut-off voltage 1.9 V) followed by constant voltage (CV) charge - Discharge: constant current (CC) discharge (cut-off voltage 1.4 V)
- Rest time when switching between charge and discharge: 5 minutes - Number of cycles: When performed 40 times or more,
In a coordinate system having an x-axis to which the number of cycles is assigned and a y-axis to which the remaining area ratio (%) of the negative electrode plate according to the number of cycles is assigned, the results of the charge-discharge cycle test are
y≧ -0.05x +100, and y≦ -0.03x +100
Zinc secondary batteries fall within the range of the above. 2. The zinc secondary battery of claim 1 , wherein the hydroxide ion conducting separator is a layered double hydroxide (LDH) separator comprising an LDH and/or an LDH-like compound. The zinc secondary battery described in claim 2, wherein the LDH separator further comprises a porous substrate, and the LDH and/ or LDH-like compound is composited with the porous substrate in a form filled in the pores of the porous substrate. The zinc secondary battery according to claim 3 , wherein the porous substrate is made of a polymeric material. The zinc secondary battery according to any one of claims 1 to 4 , wherein the positive electrode plate contains nickel hydroxide and/or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery. The zinc secondary battery according to any one of claims 1 to 5 , wherein the positive electrode plate is an air electrode, thereby making the zinc secondary battery an air-zinc secondary battery. 7. The zinc secondary battery according to claim 1, comprising a plurality of said unit cells, the plurality of said unit cells forming a multi-layer cell as a whole.

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