JP7564668B2 - Anode and zinc secondary battery - Google Patents

Description

The present invention relates to a negative electrode and 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 has been proposed that includes a layered double hydroxide (LDH) separator that selectively allows hydroxide ions to pass through while preventing the penetration of zinc dendrites. For example, Patent Document 1 (WO 2013/118561) discloses that an LDH separator is provided between the positive and negative electrodes in a nickel-zinc secondary battery. Patent Document 2 (WO 2016/076047) discloses a separator structure that includes an LDH separator fitted or joined to a resin outer frame, and that the LDH separator has such high density that it is gas impermeable and/or water impermeable. This document also discloses that the LDH separator can be composited with a porous substrate. Patent Document 3 (WO 2016/067884) discloses various methods for forming an LDH dense membrane on the surface of a porous substrate to obtain a composite material. This method involves uniformly attaching a starting substance that can provide a starting point for LDH crystal growth to a porous substrate, and then subjecting the porous substrate to hydrothermal treatment in a raw aqueous solution to form a dense LDH membrane on the surface of the porous substrate.

International Publication No. 2013/118561 International Publication No. 2016/076047 International Publication No. 2016/067884

Another factor that shortens the lifespan of zinc secondary batteries is the change in the morphology of zinc, which is the negative electrode active material. In other words, as zinc dissolves and precipitates repeatedly during repeated charging and discharging, the morphology of the negative electrode changes, causing problems such as high resistance due to blocked pores and a decrease in the charge active material due to the accumulation of isolated zinc, making charging and discharging difficult.

The inventors have now discovered that by using solder as a conductive additive in the negative electrode together with Zn particles and ZnO particles, it is possible to suppress the deterioration of the negative electrode caused by repeated charging and discharging in a zinc secondary battery, thereby improving durability and thereby extending the cycle life.

The object of the present invention is therefore to provide a negative electrode in a zinc secondary battery that suppresses the deterioration of the negative electrode caused by repeated charging and discharging, improves durability, and thereby enables a longer cycle life.

According to one aspect of the present invention, there is provided a negative electrode for use in a zinc secondary battery, comprising:
a negative electrode active material including Zn particles and ZnO particles;
A conductive assistant containing solder;
A negative electrode is provided, comprising:

According to another aspect of the present invention,
A positive electrode and
The negative electrode;
a separator that separates the positive electrode and the negative electrode in a manner capable of conducting hydroxide ions;
An electrolyte;
A zinc secondary battery is provided, comprising:

FIG. 2 is a conceptual diagram for explaining a presumed mechanism of microstructural change during charging and discharging of a negative electrode containing the solder according to the present invention. FIG. 1 is a conceptual diagram for explaining a presumed mechanism of microstructural change in a conventional negative electrode not including solder during charging and discharging.

The negative electrode of the present invention is a negative electrode used in a zinc secondary battery. This negative electrode includes a negative electrode active material and a conductive assistant. The negative electrode active material includes Zn particles and ZnO particles. The conductive assistant includes solder. By using solder as a conductive assistant in the negative electrode together with Zn particles and ZnO particles in this way, deterioration of the negative electrode due to repeated charging and discharging in the zinc secondary battery can be suppressed, improving durability and thereby extending the cycle life.

As mentioned above, in conventional negative electrodes, as zinc dissolves and precipitates repeatedly during charging and discharging, the morphology of the negative electrode changes, causing high resistance due to blockage of pores and a decrease in the charge active material due to accumulation of isolated zinc, which results in problems such as difficulty in charging and discharging. The estimated mechanism for this is as follows. In the negative electrode, the charge and discharge reaction formula is:
ZnO+H ₂ O+2OH ^- +2e ^- ⇔ Zn+4OH ^- ... Formula (1)
According to the above, ZnO changes to Zn during charging, while Zn changes to ZnO during discharging. However, not all Zn particles generated by charging can be returned to ZnO particles during discharging, and some of the Zn remains and accumulates. Then, as charging and discharging are repeated, the required ZnO particles gradually decrease and the capacity decreases. This series of estimated mechanisms is conceptually shown in FIG. 2. In FIG. 2, a conventional negative electrode 10' (not including solder) including a negative electrode active material 12 containing Zn particles (discharge active material) and ZnO particles (charge active material and electronic conductive material) and a current collector 16 is shown together with an electrolyte 18 and a separator 20. First, before the start of the charge and discharge cycle, as shown in FIG. 2(a), the negative electrode 10' contains more ZnO particles than Zn particles. Next, when charging, the reaction of formula (1) proceeds to the right, and as a result, the ZnO particles change to Zn particles as shown in FIG. 2(b), and the Zn particles increase. Since Zn particles function not only as a charging active material but also as an electronic conductive material, the increase in Zn particles increases the electronic conductive path. On the other hand, during discharge, the reaction of formula (1) proceeds to the left, and as a result, the Zn particles change to ZnO particles as shown in FIG. 2(c), and the Zn particles decrease. However, at that time, the Zn particles near the surface of the negative electrode 10' far from the current collector 16 (area marked A in the figure) are electrically isolated, and cannot be discharged and accumulate as Zn particles, resulting in a decrease in the amount of ZnO. Then, when charging and discharging are repeated, the amount of Zn accumulation gradually increases as shown in FIG. 2(d), while the amount of ZnO required for charging gradually decreases, making it impossible to charge to the rated capacity.

In contrast, as conceptually shown in FIG. 1, the negative electrode 10 of the present invention contains solder as a conductive assistant 14 together with Zn particles and ZnO particles, and it is believed that this conductive assistant 14, i.e., solder, suppresses the increase in the amount of Zn accumulation (i.e., the decrease in the amount of ZnO). First, before the start of the charge-discharge cycle, as shown in FIG. 1(a), the negative electrode 10 contains more ZnO particles than Zn particles. Next, when charging, the reaction of formula (1) proceeds to the right, and as shown in FIG. 1(b), the ZnO particles change to Zn particles and the Zn particles increase. Since the Zn particles function not only as a charging active material but also as an electronic conductive material, the increase in the Zn particles increases the electronic conductive path. On the other hand, during discharge, the reaction of formula (1) proceeds to the left, and as shown in FIG. 2(c), the Zn particles change to ZnO particles and the Zn particles decrease. At this time, even if the Zn particles are reduced, the conductive assistant 14 (solder) ensures an electronic conduction path, so that electrical conduction can be established with the Zn particles near the surface of the negative electrode 10 far from the current collector 16 (area marked A in the figure), and therefore a larger portion (ideally almost all) of the Zn particles generated by charging can be involved in the discharge, and as a result, the reduction in the ZnO particles is effectively suppressed. That is, the electrical isolation and accumulation of Zn particles near the surface of the negative electrode 10 far from the current collector 16, and the resulting reduction in the amount of ZnO can be effectively suppressed. Thus, even if charging and discharging are repeated, the amount of Zn accumulation does not gradually increase as shown in FIG. 1(d), and therefore the amount of ZnO required for charging can be sufficiently secured, and charging to the rated capacity can be performed. In this way, the deterioration of the negative electrode 10 due to repeated charging and discharging can be suppressed, durability can be improved, and the cycle life can be extended.

In FIG. 1, the area of conductive additive 14 (solder) is depicted as not being connected between the upper and lower surfaces of the negative electrode 10, but this is because it is depicted two-dimensionally as a cross section. Taking the depth into account, in three dimensions, the area of conductive additive 14 is preferably connected between the upper and lower surfaces of the negative electrode 10, thereby ensuring an electronic conduction path for conductive additive 14.

The negative electrode active material 12 includes Zn particles and ZnO particles. The Zn particles are typically metal Zn particles, but particles of Zn alloys or Zn compounds may also be used. Metal Zn particles commonly used in zinc secondary batteries can be used as the metal Zn particles, but the use of smaller metal Zn particles is more preferable from the viewpoint of extending the cycle life of the battery. Specifically, the D50 particle size of the metal Zn particles is preferably 5 to 200 μm, more preferably 8 to 100 μm, and even more preferably 10 to 50 μm. The preferred content of the Zn particles in the negative electrode 10 is preferably 1 to 50 parts by weight, more preferably 2 to 45 parts by weight, and even more preferably 5 to 35 parts by weight, when the content of the ZnO particles is 100 parts by weight. Alternatively, the preferred content of Zn particles in the negative electrode 10 is preferably 1 to 40 parts by volume, more preferably 2 to 35 parts by volume, and even more preferably 4 to 30 parts by volume, when the content of ZnO particles is 100 parts by volume. As described later, the metal Zn particles may be doped with a dopant such as In or Bi. The ZnO particles are not particularly limited and may be a commercially available zinc oxide powder used in zinc secondary batteries, or a zinc oxide powder obtained by growing particles by a solid-phase reaction using the commercially available zinc oxide powder as a starting material. The D50 particle size of the ZnO particles is preferably 0.1 to 20 μm, more preferably 0.1 to 10 μm, and even more preferably 0.1 to 5 μm.

The content of the negative electrode active material 12 relative to the total volume of the negative electrode active material 12, the conductive assistant 14, and (if the negative electrode 10 contains a binder resin) the binder resin is preferably 40 to 95% by volume, more preferably 50 to 90% by volume, and even more preferably 60 to 80% by volume. In calculating the solder content, the amount of metal components in the solder (converted value) is used as the solder content, while the amount of solids in the binder resin (converted value) is used as the binder resin content.

The conductive additive 14 includes solder. As long as it does not impair the function of the zinc secondary battery, any type of solder can be used. A preferred solder contains at least one selected from the group consisting of Sn, Pb, Bi, In, and Zn, and more preferably contains at least Sn. By using a solder of such a composition, undesirable reactions (e.g., hydrogen generation) are less likely to occur in a strong alkaline electrolyte (e.g., potassium hydroxide aqueous solution). Specific examples of such preferred solders include Sn-Pb solder, Sn-Bi solder, Sn-In solder, Sn-Zn solder, and combinations thereof. The compositions of Sn-Pb solder, Sn-Bi solder, Sn-In solder, and Sn-Zn solder can be generally known compositions and are not particularly limited, but typically contain 40 to 95% by weight of Sn, with the remainder being other elements such as Pb, Bi, In, and Zn. A particularly preferred composition is a eutectic composition in that it can be melted at the lowest temperature. The eutectic compositions (weight %) of Sn-Pb solder, Sn-Bi solder, Sn-In solder, and Sn-Zn solder are Sn63%-Pb37%, Sn42%-Bi58%, Sn48%-In52%, and Sn91%-Zn9%, respectively. However, the composition of the solder does not have to be a eutectic composition as long as the desired characteristics are obtained. The solder is preferably in the form of particles with a particle size of 0.1 to 100 μm, more preferably 3 to 100 μm, even more preferably 3 to 50 μm, and particularly preferably 5 to 30 μm. In particular, the solder forms a three-dimensional network, and Zn particles and ZnO particles are incorporated into the three-dimensional network. In this way, undesirable movement of the conductive assistant 14 due to repeated charging and discharging can be effectively suppressed, and the conductive assistant 14 can be effectively held in the desired position. The three-dimensional solder network can be formed by melting the solder particles and bonding them together. It is preferable that the conductive additive 14 is composed of solder alone, but the negative electrode 10 may contain other conductive additives to the extent that the above-mentioned effects are not impaired.

The solder content relative to the total volume of the negative electrode active material 12, the conductive assistant 14, and (if the negative electrode 10 contains a binder resin) the binder resin is preferably 5 to 60% by volume, more preferably 10 to 50% by volume, and even more preferably 20 to 40% by volume. In calculating the solder content, the amount of metal components in the solder (converted value) is used as the solder content, while the amount of solids in the binder resin (converted value) is used as the binder resin content.

The negative electrode 10 may further include a binder resin (not shown). When the negative electrode 10 includes a binder, the shape of the negative electrode is easily maintained. Various known binders can be used as the binder resin, but preferred examples include polyvinyl alcohol (PVA) and polytetrafluoroethylene (PTFE). It is particularly preferred to use a combination of both PVA and PTFE as the binder.

The negative electrode 10 is preferably a sheet-shaped press-molded body. This can prevent the negative electrode active material 12 from falling off and improve the electrode density, and can more effectively suppress changes in the shape of the negative electrode 10. Such a sheet-shaped press-molded body can be produced by adding a binder to the negative electrode material and kneading the mixture, and then applying press molding such as roll pressing to the resulting kneaded mixture to form it into a sheet.

The negative electrode 10 is preferably provided with a current collector 16. Preferred examples of the current collector 16 include copper punched metal and copper expanded metal. In this case, for example, a mixture containing Zn particles, ZnO particles, solder, and optionally a binder resin (e.g., polytetrafluoroethylene particles) can be applied to the copper punched metal or copper expanded metal to preferably prepare a negative electrode plate consisting of the negative electrode 10/current collector 16. In this case, it is also preferable to press the negative electrode plate (i.e., the negative electrode 10/current collector 16) after drying to prevent the negative electrode active material 12 from falling off and improve the electrode density. Alternatively, the above-mentioned sheet-shaped press molded body may be pressed onto the current collector 16 such as copper expanded metal.

Zinc secondary battery The negative electrode 10 of the present invention is preferably applied to a zinc secondary battery. Therefore, according to a preferred embodiment of the present invention, a zinc secondary battery is provided, which includes a positive electrode (not shown), a negative electrode 10, a separator 20 that separates the positive electrode and the negative electrode 10 so as to be capable of conducting hydroxide ions, and an electrolyte 18. The zinc secondary battery of the present invention is not particularly limited as long as it is a secondary battery that uses the above-mentioned negative electrode 10 and uses the electrolyte 18 (typically an aqueous alkali metal hydroxide solution). Therefore, it can be a nickel-zinc secondary battery, a silver oxide zinc secondary battery, a manganese oxide zinc secondary battery, a zinc-air secondary battery, or any other type of alkaline zinc secondary battery. For example, it is preferable that the positive electrode contains nickel hydroxide and/or nickel oxyhydroxide, thereby forming a nickel-zinc secondary battery. Alternatively, the positive electrode may be an air electrode, thereby forming a zinc secondary battery.

The separator 20 is preferably a layered double hydroxide (LDH) separator. That is, as mentioned above, LDH separators are known in the fields of nickel-zinc secondary batteries and air-zinc secondary batteries (see Patent Documents 1 to 3), and this LDH separator can be preferably used in the zinc secondary battery of the present invention. The LDH separator can selectively allow hydroxide ions to pass through while preventing the penetration of zinc dendrites. Combined with the effect of adopting the negative electrode of the present invention, the durability of the zinc secondary battery can be further improved.

The LDH separator may be a composite with a porous substrate as disclosed in Patent Documents 1 to 3. The porous substrate may be made of any of ceramic materials, metal materials, and polymer materials, but is particularly preferably made of a polymer material. The polymer porous substrate has the following advantages: 1) flexibility (so it is difficult to break even if it is made thin); 2) it is easy to increase the porosity; 3) it is easy to increase the conductivity (because the thickness can be reduced while increasing the porosity); and 4) it is easy to manufacture and handle. Particularly preferred polymer materials are polyolefins such as polypropylene and polyethylene, which are excellent in hot water resistance, acid resistance, and alkali resistance, and are low cost, and most preferably polypropylene. When the porous substrate is made of a polymer material, it is particularly preferred that the functional layer is incorporated throughout the entire thickness of the porous substrate (for example, most or almost all of the pores inside the porous substrate are filled with LDH). In this case, the preferred thickness of the polymer porous substrate is 5 to 200 μm, more preferably 5 to 100 μm, and even more preferably 5 to 30 μm. As such a polymeric porous substrate, a microporous membrane such as one commercially available as a lithium battery separator can be preferably used.

The electrolyte 18 preferably contains an aqueous solution of an alkali metal hydroxide. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, lithium hydroxide, and ammonium hydroxide, with potassium hydroxide being more preferred. Zinc oxide, zinc hydroxide, etc. may be added to the electrolyte to suppress self-dissolution of zinc-containing materials.

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

Examples 1 to 16
(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 Various raw material powders shown below 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)
Metallic Zn powder (manufactured by Mitsui Mining & Smelting Co., Ltd., doped with Bi and In, Bi: 1000 ppm by weight, In: 1000 ppm by weight, average particle size D50: 50 μm)
<Conductive assistant>
Solder (powder or paste) of various compositions and particle sizes shown in Table 1
<Resin binder>
Polytetrafluoroethylene (PTFE) aqueous dispersion (manufactured by Daikin Industries, Ltd., solid content 60%)

For Examples 1 to 15, according to the mixing ratios shown in Table 1 (solder converted into metal components, PTFE converted into solids), metal Zn powder and solder were added to the ZnO powder, PTFE was further added, and the mixture was kneaded with propylene glycol. On the other hand, for Example 16, according to the mixing ratios shown in Table 1 (solder converted into metal components, PTFE converted into solids), metal Zn powder was added to the ZnO powder, PTFE was further added, and the mixture was kneaded with propylene glycol.

The resulting kneaded material was rolled using a roll press to obtain a sheet of negative electrode active material. The sheet of negative electrode active material was then pressed onto a tin-plated copper expand metal. The samples with added solder (Examples 1 to 15) were heat-treated under conditions suitable for the solder components and used as negative electrodes, while the sample without added solder (Example 16) was used as a negative electrode without heat treatment.

(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 cell 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 placed opposite each other via an 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) Evaluation Using a charge/discharge device (TOSCAT3100, manufactured by Toyo Systems Co., Ltd.), the simple sealed cell was subjected to formation with 0.1C charge and 0.2C discharge. Then, a 1C charge/discharge cycle was performed. The charge/discharge cycle was repeatedly performed under the same conditions, and the number of charge/discharge cycles until the discharge capacity of the prototype battery decreased to 70% of the discharge capacity of the first cycle was recorded. The number of charge/discharge cycles of each example is shown in Table 1 as a relative value when the number of charge/discharge cycles in Example 16 is set to 1.0.

Reference Signs List 10, 10' Negative electrode 12 Negative electrode active material 14 Conductive assistant 16 Current collector 18 Electrolyte 20 Separator

Claims (12)

A negative electrode for use in a zinc secondary battery,
a negative electrode active material including Zn particles and ZnO particles;
A conductive assistant containing solder;
Including,
The solder contains one or more elements selected from the group consisting of Sn, Pb, Bi, In, and Zn, including at least Sn;
A negative electrode, wherein the solder forms a three-dimensional network, and Zn particles and ZnO particles are embedded in the three-dimensional network . 2. The negative electrode according to claim 1 , wherein the solder is at least one selected from the group consisting of Sn-Pb solder, Sn-Bi solder, Sn-In solder, and Sn-Zn solder. 3. The negative electrode according to claim 1, wherein the solder is in the form of particles having a particle size of 0.1 to 100 μm. 4. The negative electrode according to claim 3 , wherein the solder has a particle size of 3 to 100 μm. 4. The negative electrode according to claim 3 , wherein the solder has a particle size of 3 to 50 μm. The negative electrode according to claim 1 , further comprising a binder resin. The negative electrode according to any one of claims 1 to 6, wherein a content ratio of the solder to a total volume of the negative electrode active material, the conductive assistant, and (when the negative electrode contains a binder resin) the binder resin is 5 to 60 volume %. A positive electrode and
The negative electrode according to any one of claims 1 to 7 ,
a separator that separates the positive electrode and the negative electrode in a manner capable of conducting hydroxide ions;
An electrolyte;
A zinc secondary battery comprising: 9. The zinc secondary battery of claim 8 , wherein the separator is a layered double hydroxide (LDH) separator. The zinc secondary battery according to claim 9 , wherein the LDH separator is composited with a porous substrate. The zinc secondary battery according to any one of claims 8 to 10 , wherein the positive electrode contains nickel hydroxide and/or nickel oxyhydroxide, thereby making the zinc secondary battery a nickel-zinc secondary battery. The zinc secondary battery according to any one of claims 8 to 10 , wherein the positive electrode is an air electrode, thereby making the zinc secondary battery a zinc-air secondary battery.

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