Secondary batteries

Abstract

The object of the present invention is to provide a secondary battery that uses zinc as the negative electrode and copper(II) sulfide as the positive electrode, and has charge-discharge characteristics that enable high-capacity discharge at high potential using an alkaline electrolyte. [Solution] A secondary battery comprising a current collector containing copper, a positive electrode having a positive electrode active material layer, a gel electrolyte holding an alkaline aqueous solution, and a negative electrode having a negative electrode active material layer, wherein the positive electrode active material in the positive electrode active material layer is copper(II) sulfide, and the negative electrode active material in the negative electrode active material layer contains zinc.

Description

[Technical Field]

[0001] This invention relates to a secondary battery using copper(II) sulfide as the positive electrode, zinc as the negative electrode, and a gel electrolyte as the electrolyte. [Background technology]

[0002] In recent years, there has been a great deal of activity in the development of secondary batteries. Among these, secondary batteries with zinc as the negative electrode have been developed that use cobalt-coated nickel hydroxide as the positive electrode active material (see Patent Document 1), or manganese dioxide (see Non-Patent Document 1), among others. In addition, batteries using copper(II) sulfate (CuS) as the positive electrode active material are also being considered. When CuS is used as the positive electrode active material, CuS + Cu is produced in the positive electrode during discharge. 2+ +2e -→The Cu2S reaction occurs, and a two-electron reaction in the presence of Cu ions results in a high theoretical capacity of 561 mAh / g. Secondary batteries using zinc as the negative electrode and CuS as the positive electrode active material have been proposed as described in Non-Patent Documents 2 and 3. Non-Patent Document 2 achieved stable charge / discharge of 500 mAh / g for 250 cycles using a weakly acidic electrolyte, but it was a two-liquid secondary battery that separated the positive and negative electrodes with an anion exchange membrane and used different weakly acidic electrolytes (CuSO4 aqueous solution and ZnSO4 aqueous solution). Therefore, in order to make it a practical cell, measures are required to prevent the two liquids from mixing, in addition to ensuring safety, which makes the cell structure complex. Non-Patent Document 3 describes a secondary battery using an alkaline electrolyte and Cu2S as the positive electrode active material, and a secondary battery using CuS is also described for comparison. However, all of these had small maximum discharge capacities above 1V and small discharge capacity retention rates. Thus, in conventional secondary batteries using zinc as the negative electrode and CuS as the positive electrode active material, two types of electrolytes are required to achieve a discharge potential of 1V or higher with a weakly acidic electrolyte. With an alkaline electrolyte (hereinafter also referred to as alkaline electrolyte), the utilization rate of the active material decreases due to the formation of polysulfide anions, making it impossible to maintain a potential of 1V or higher for a long time during a single discharge. As a result, there were no batteries that could maintain a high potential with an alkaline electrolyte. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2014-29818 [Non-patent literature]

[0004] [Non-Patent Document 1] Aswani Poosapati, Sudharshan Vadnala, Karla Negrete, Yucheng Lan, John Hutchison, Mark Zupan, and Deepa Madan, “Rechargeable Zinc-Electrolytic Manganese Dioxide (EMD) Battery with a Flexible Chitosan-Alkaline Electrolyte”, ACS Appl. Energy Mater., 4, 4248-4258(2021). [Non-Patent Document 2] Liang Li et al., “An Energetic CuS-Cu Battery System Based on CuS Nanosheet Arrays”, ACS Nano, 15, 5420-5427 (2021). [Non-Patent Document 3] Jonathon Duay et al., “Rechargeable Solid-State Copper Sulfide Cathodes for Alkaline Batteries: Importance of the Copper Valence State”, Journal of The Electrochemical Society, 166, A687-A694 (2019). [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The object of the present invention is to provide a secondary battery that uses zinc as the negative electrode and copper(II) sulfide as the positive electrode, and has charge-discharge characteristics that enable high-capacity discharge at high potential using an alkaline electrolyte. [Means for solving the problem]

[0006] To solve the above problems, the inventors began investigating a secondary battery (Zn-CuS battery) that uses zinc as the negative electrode and CuS as the positive electrode active material, and that can maintain a high potential and high discharge capacity even when using an alkaline electrolyte instead of an acidic electrolyte. In the course of their investigation, the inventors found that by using a copper substrate as the current collector of the positive electrode, a small amount of copper ions eluted from the copper substrate are supplied to the positive electrode, and furthermore, by using a gel electrolyte, the diffusion of the supplied copper ions into the electrolyte can be suppressed. A Zn-CuS battery with this structure can exhibit high capacity at a potential of 1V or higher during the discharge process without using two types of electrolytes, and can maintain a potential of 1V or higher for a long time during a single discharge. This is because the small amount of copper ions eluted from the copper substrate can be retained on the positive electrode side of the gel electrolyte, and even in an alkaline electrolyte, the "CuS + Cu" diffusion reported in weakly acidic electrolytes can be suppressed. 2+ +2e - This is thought to be because the reaction "→Cu2S" can be preferentially and dominantly promoted. As a result, the cycle stability of charge and discharge has also improved. Furthermore, this Zn-CuS battery is safe because it uses a gel electrolyte and there is no risk of leakage. In addition, because this Zn-CuS battery uses CuS as the positive electrode active material, it does not require the use of rare metals such as Mn and Ni that are conventionally used in aqueous batteries, making it easier to secure raw material resources. Moreover, it has a theoretical capacity of 561 mAh / g, making it a high-capacity secondary battery.

[0007] In other words, the present invention is defined by the following: (1) A secondary battery comprising a current collector containing copper, a positive electrode having a positive electrode active material layer, a gel electrolyte holding an alkaline aqueous solution, and a negative electrode having a negative electrode active material layer, wherein the positive electrode active material in the positive electrode active material layer is copper(II) sulfide, and the negative electrode active material in the negative electrode active material layer contains zinc. (2) The secondary battery according to (1) above, characterized in that the current collector containing copper is a copper mesh. (3) The secondary battery according to (1) or (2) above, characterized in that the gel electrolyte is an alkaline aqueous solution held in a conductive polymer with fixed anionic charge. [Effects of the Invention]

[0008] The secondary battery of the present invention uses zinc as the negative electrode and copper(II) sulfide as the positive electrode, and enables high-capacity discharge at high potential using an alkaline electrolyte. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 shows the cell structure of the Zn-CuS battery fabricated in Example 1. [Figure 2] Figure 2 shows the charge and discharge profiles of the Zn-CuS battery fabricated in Example 1. [Figure 3] Figure 3 shows the charge and discharge profiles of the Zn-CuS battery prepared in Comparative Example 1. [Figure 4] Figure 4 shows the charge and discharge profiles of the Zn-CuS battery prepared in Comparative Example 2. [Figure 5] Figure 5 shows the charge and discharge profiles of the Zn-Cu battery prepared in Comparative Example 3. [Modes for carrying out the invention]

[0010] The secondary battery of the present invention comprises a positive electrode having a copper-containing current collector and a positive electrode active material layer, a gel electrolyte holding an alkaline aqueous solution, and a negative electrode having a negative electrode active material layer, wherein the positive electrode active material in the positive electrode active material layer is copper(II) sulfide, and the negative electrode active material in the negative electrode active material layer is zinc. In the present invention, the positive electrode active material in the positive electrode active material layer is copper(II) sulfide (CuS). The positive electrode active material layer in the present invention contains copper(II) sulfide and may consist only of CuS, or it may contain, for example, a conductive additive, a binder, or other positive electrode active material in addition to CuS. The current collector in the present invention contains copper. In the present invention, containing copper means including not only metallic copper but also copper alloys. In the present invention, metallic copper and copper alloys are collectively referred to as copper metal. The current collector in the present invention may consist only of copper metal, or the copper metal may be supported on another substrate. The shape of the current collector in the present invention can be, for example, a plate shape, a mesh shape, a corrugated shape, or a plate shape that has been embossed or debossed with circular, polygonal, or other shapes. As for the method of forming the positive electrode active material layer on the current collector, for example, there is a wet method in which a positive electrode mixture consisting of positive electrode active material, a conductive additive, a binder, a solvent, etc. is applied to the current collector to form the positive electrode active material layer, and there is a dry method in which the positive electrode active material layer is formed using a positive electrode mixture consisting of positive electrode active material, a conductive additive, a binder, etc., and does not contain a solvent, but in the present invention either method can be used. As the current collector in the present invention, from the viewpoint of manufacturing electrodes by a dry method that does not use a solvent, a copper mesh made by processing copper (metallic copper) into a mesh shape is preferred. In the case of copper mesh, it is easy to support the positive electrode active material, and even in the dry method, the positive electrode active material layer can be formed by embedding the positive electrode mixture in the gaps of the copper mesh. The positive electrode in the present invention comprises the positive electrode active material layer and the current collector, wherein the positive electrode active material layer is formed on the current collector. The positive electrode active material layer in the present invention may be formed over the entire surface of the current collector, or it may be formed on a part of the current collector to the extent that it provides the effect of a battery.

[0011] The gel electrolyte in this invention is a gel electrolyte that holds an alkaline aqueous solution. The alkaline aqueous solution in this invention is an electrolyte dissolved in water. The electrolyte is not particularly limited as long as it is an electrolyte that can be used in secondary batteries and the aqueous solution when dissolved in water is alkaline, but examples include hydroxides of alkali metals such as potassium, sodium, and lithium, and hydroxides of alkaline earth metals such as magnesium, calcium, and strontium. The alkaline aqueous solution in this invention may contain one or more of these electrolytes. The concentration of the electrolyte in the alkaline aqueous solution is preferably 1 to 10 M, where M represents mol / L. The gel electrolyte in this invention is a polymer that holds the above alkaline aqueous solution. The polymer is not particularly limited as long as it has a structure that can hold an alkaline aqueous solution, but for example, a polymer having a so-called three-dimensional network structure is preferred, and viscous polysaccharides, PVA, etc. can also be used. As for the polymer in this invention, an anionic charge fixed conductive polymer is more preferred from the viewpoint of suppressing the movement of zinc ions and copper ions dissolved from each electrode. Examples of conductive polymers with fixed anionic charge include chemically crosslinked polymers using one or more monomers selected from acrylamide-methylpropanesulfonic acid, vinyl sulfonic acid, allyl sulfonic acid, ethyl acrylate, butyl acrylate, and isoprene sulfonic acid, as well as crosslinked structures described in International Publication WO2002 / 023663, Japanese Patent No. 6190101, and Japanese Patent Application Publication No. 2024-93026. Specifically, examples include polymers obtained using 2-Acrylamido-2-methylpropanesulfonic Acid (AMPS) and tetraethylene glycol diacrylate, methylenebisacrylamide, etc., as crosslinking agents. Specific examples of crosslinking agents include N-N'-methylenebisacrylamide (MBAA), N,N'-{[(2-acrylamido-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide (FOM-03006), etc.

[0012] The crosslinked structure described in Japanese Patent Laid-Open No. 2024-93026 is a crosslinked structure crosslinked by the interaction between a host group and a guest group. For example, the host group is at least one selected from the group consisting of α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin, and the guest group is at least one selected from the group consisting of an n-butyl group, an n-dodecyl group, a t-butyl group, and an adamantyl group. Examples of such crosslinked structures include crosslinked polymers having a repeating structural unit represented by the following general formula (1a), a repeating structural unit represented by the following general formula (2a), and a repeating structural unit represented by the following general formula (3a). These crosslinked structures may further have crosslinked portions other than the crosslinked portion crosslinked by the interaction between the host group and the guest group.

[0013]

Chemical formula

[0014] In formula (1a), Ra is a hydrogen atom or a methyl group, and R 1 represents a hydroxyl group, a thiol group, an alkoxy group which may have one or more substituents, a thioalkoxy group which may have one or more substituents, an alkyl group which may have one or more substituents, an amino group which may have one substituent, an amide group which may have one or more substituents, an aldehyde group, or a carboxyl group, and represents a divalent group formed by removing one hydrogen atom from a monovalent group selected from the group consisting of these groups. R A represents a host group.

[0015]

Chemical formula

[0016] In formula (2a), Ra is a hydrogen atom or a methyl group, and R 2represents a divalent group formed by removing one hydrogen atom from a monovalent group selected from the group consisting of a hydroxyl group, a thiol group, an alkoxy group which may have one or more substituents, a thioalkoxy group which may have one or more substituents, an alkyl group which may have one or more substituents, an amino group which may have one substituent, an amide group which may have one or more substituents, an aldehyde group, and a carboxyl group, and R B represents a guest group.

[0017] [Chemical formula]

[0018] In formula (3a), Ra is a hydrogen atom or a methyl group, and R 3 represents a halogen atom, a hydroxyl group, a thiol group, an amino group which may have one or more substituents, a carboxyl group which may have one substituent, or an amide group which may have one or more substituents.

[0019] In formula (3a), the substituent in R 3 may have an ionic group or may be an ionic group. Examples of the ionic group include an anionic group, a cationic group, or an amphoteric ionic group. The ionic group is not particularly limited, and examples of the anionic group include a sulfo group, a phosphoric acid group, a carboxyl group, a boronic acid group, a sulfonylimide group, etc., and examples of the cationic group include an ammonium group, etc. Examples of the substituent having an ionic group include an alkyl group (e.g., methyl group, ethyl group, propyl group, butyl group) substituted by an ionic group, a hydroxyalkyl group (e.g., hydroxymethyl group, hydroxyethyl group, hydroxypropyl group, hydroxybutyl group), etc. The anionic group is preferred as the ionic group, and the sulfo group is more preferred.

[0020] The polymer content in the gel electrolyte in the present invention is not particularly limited as long as it can maintain a gel state that can withstand use as a gel electrolyte. However, from the viewpoint of better maintaining the gel state and maintaining its function as an electrolyte, 20 to 90% by mass, more preferably 30 to 80% by mass, and even more preferably 40 to 70% by mass are preferred based on the total mass of the gel electrolyte. The electrolyte (electrolyte salt) content in the gel electrolyte in the present invention is not particularly limited as long as it is within the range that can be used as a gel electrolyte. However, from the viewpoint of maintaining its properties as a gel electrolyte, 3 to 30% by mass, and more preferably 5 to 15% by mass are preferred based on the total mass of the hydrogel electrolyte.

[0021] There are no particular limitations on the method for making a polymer retain an alkaline aqueous solution, but examples include impregnating a dried polymer with an alkaline aqueous solution by immersing it in the solution. Another example is to form a polymer (crosslinked structure), then immerse the formed crosslinked structure in an electrolyte, and replace the aqueous medium contained in the crosslinked structure with the electrolyte by utilizing the difference in the degree of penetration of the aqueous medium contained in the crosslinked structure and the electrolyte. In this way, a gel electrolyte according to the present invention can be created by making a polymer retain an alkaline aqueous solution.

[0022] The negative electrode in this invention has a negative electrode active material layer, and the negative electrode active material in the negative electrode active material layer contains zinc. In this invention, "containing zinc as the negative electrode active material" means not only containing metallic zinc, but also containing zinc compounds such as zinc alloys and zinc oxides. The negative electrode active material layer in this invention may contain other metals in addition to zinc, may contain conductive additives, binders, etc., and may contain other negative electrode active materials. The negative electrode in this invention may consist only of a negative electrode active material layer, or it may be a negative electrode active material layer formed on a current collector. The shape of the negative electrode in this invention can be, for example, a plate shape, a mesh shape, a corrugated shape, or a plate shape that has been embossed or debossed into a circular or polygonal shape. In this invention, the negative electrode is preferably made of a metallic zinc plate, in which case the metallic zinc plate can serve as both the negative electrode active material layer and the current collector. Furthermore, metallic zinc, zinc alloys, zinc compounds, etc., may be supported on the current collector or substrate. For example, these negative electrode active materials may be supported by mixing them with a binder, conductive additive, solvent, etc., and applying the mixture to the current collector or substrate. When using a current collector, there are no particular restrictions on the current collector as long as it is made of a metal with good conductivity, and when using a substrate, there are no particular restrictions on the material, shape, etc., as long as it can support the negative electrode active material. When using a current collector or substrate, the positive electrode active material layer may be formed over the entire surface of the current collector or substrate, or it may be formed on a part of the current collector or substrate to the extent that it provides the effect of a battery.

[0023] The secondary battery of the present invention can be manufactured by stacking the positive electrode, gel electrolyte, and negative electrode according to the present invention. In the secondary battery of the present invention, since a gel-like electrolyte is used, there is no need to use a separator as when a liquid electrolyte (electrolyte solution) is used, and the battery size can be reduced and the electrical resistance can be reduced. For components other than the positive electrode, gel electrolyte, and negative electrode, components that are normally used in secondary batteries can be used. [Examples]

[0024] The present invention will be described in detail below with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0025] [Example 1] (Polymer synthesis) The main raw material monomer is 4.00 g (1.93 mmol) of 2-Acrylamido-2-methylpropanesulfonic Acid (AMPS), the crosslinking agent is 0.196 g (0.385 mmol) of N,N'-{[(2-acrylamido-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide (FOM-03006), the initiator is 0.0216 g (0.0965 mol) of 2,2'-Azobis(2-methylpropionamidine) Dihydrochloride, and the initiators are 2-Hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone An aqueous solution consisting of 26.1 mg (0.00965 mol) was prepared, and a hydrogel with a diameter of 20 mm and a thickness of 2 mm was fabricated by UV irradiation (λ=365 nm) and heating at 70°C. Subsequently, the hydrogel was neutralized with an aqueous KOH solution, and washed and dried with a large amount of water and methanol to obtain an AMPS network of single-ion conductive polymer with fixed anionic charge.

[0026] (Preparation of gel electrolytes) A gel electrolyte was obtained by impregnating a dried AMPS network with a 4.2 M KOH aqueous solution (electrolyte). The AMPS network content in the obtained gel electrolyte was 50% by mass.

[0027] (Fabrication of CuS cathode) A positive electrode was fabricated by kneading CuS, Ketjenbrak (KB) as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder in a mortar in a mass ratio of 6:3:1, placing the mixture on a Cu mesh (CU-118100, Φ0.11, 100 mesh, approximately 0.2g: manufactured by Niraco Co., Ltd.) and pressing (compressing) it.

[0028] (Fabrication of Zn-CuS batteries) As shown in Figure 1, a zinc plate (negative electrode), a gel electrolyte (15 mm in diameter, 2 mm thick), the CuS positive electrode prepared above, and another polyolefin plate were stacked on a polyolefin plate in that order. In addition, to prevent the gel electrolyte from being compressed unnecessarily, a silicon plate with a Φ20 mm hole in the center was used, and the gel electrolyte was placed inside it. In this way, a Zn-CuS battery cell was fabricated.

[0029] [Comparative Example 1] A Zn-CuS battery cell was fabricated using the same electrolyte as in Example 1, and a cellulose filter paper separator (circular qualitative filter paper No. 2: manufactured by Advantec Toyo Co., Ltd.) moistened with the same amount of electrolyte as the gel electrolyte in Example 1, instead of the gel electrolyte. A charge-discharge test was performed on the fabricated Zn-CuS battery cell. At this time, the filter paper showed some leakage of the electrolyte.

[0030] [Comparative Example 2] Except for changing the current collector from Cu mesh to Ni mesh (NI-318100, Φ0.10, 100 mesh, approximately 0.2g: manufactured by Niraco Co., Ltd.), the positive electrode was prepared in the same manner as the CuS positive electrode in Example 1, and a Zn-CuS battery cell was prepared using this electrode in the same manner as in Example 1.

[0031] [Comparative Example 3] A Zn-Cu battery cell was fabricated in the same manner as in Example 1, except that a Cu mesh without supporting CuS was used as the positive electrode, i.e., the Cu mesh was used as both the positive electrode active material and the current collector.

[0032] (Charge / discharge measurement) The battery cells prepared in the examples and comparative examples were evaluated using a battery charge / discharge device HJ1001SD8 (manufactured by Meiden Hokuto Co., Ltd.), with the measurement temperature controlled to room temperature (25°C), the measurement rate to 0.5C (discharge / charge time 2 hours each), and the cutoff potential to 0.5~1.6V. The evaluation results of the battery cells obtained in Example 1 and Comparative Examples 1-3 are shown in Figures 2-5, respectively. Figure 2 shows the evaluation results of the Zn-CuS battery cell obtained in Example 1 (positive electrode combination of CuS and Cu mesh, using gel electrolyte), and Figure 3 shows the evaluation results of the Zn-CuS battery cell obtained in Comparative Example 1 (positive electrode combination of CuS and Cu mesh, using liquid electrolyte). Figure 4 shows the evaluation results of the Zn-CuS battery cell obtained in Comparative Example 2 (positive electrode combination of CuS and Ni mesh, using gel electrolyte), and Figure 5 shows the evaluation results of the Zn-Cu battery cell obtained in Comparative Example 3 (positive electrode of Cu mesh, using gel electrolyte). Figures 2-5 show the results for cycles 1, 10, 20, and 30.

[0033] Figure 2 shows that in the Zn-CuS battery fabricated in Example 1, the discharge capacity above 1V increased with each cycle, reaching 450 mAh / g in terms of CuS active material after 30 cycles. Thus, the Zn-CuS battery of Example 1 had a large maximum discharge capacity above 1V, and was able to maintain a potential of 1V or higher for a long time during a single discharge, resulting in a high discharge capacity retention rate. Furthermore, the discharge capacity increased with increasing cycle count. On the other hand, Figure 3 shows that in the Zn-CuS battery using a liquid electrolyte without polymer fabricated in Comparative Example 1, the maximum discharge capacity was 350 mAh / g in terms of CuS active material over 30 charge-discharge cycles, and its discharge potential was also low. Furthermore, significant overcharging was observed during the charging process, indicating unstable charging behavior. It is thought that zinc ions inhibited the reaction at the positive electrode, and side reactions occurred due to the unstable charging behavior. In addition, the discharge capacity decreased with increasing cycle count. As can be seen from the results of Example 1 and Comparative Example 1, the Zn-CuS battery of the present invention exhibited excellent charge-discharge characteristics not seen in Zn-CuS batteries using liquid electrolytes. Furthermore, as shown in Figure 4, the Zn-CuS battery using Ni mesh as the current collector in Comparative Example 2 showed a total discharge capacity of 450 mAh / g or more, but the discharge capacity at potentials of 1V or higher was 250 mAh / g or less, exhibiting lower discharge capacity and potential behavior compared to the Zn-CuS battery using Cu mesh as the current collector in Example 1. This is thought to be because in Example 1, Cu ions are supplied to CuS by the leaching of a small amount of Cu ions from the Cu mesh. As shown in Figure 5, the Zn-Cu battery of Comparative Example 3, which used Cu as the positive electrode active material, did not achieve almost any discharge capacity over 30 cycles. From this, it was shown that Cu alone does not function as a positive electrode active material, and that CuS is indispensable as an active material for high-capacity discharge at potentials of 1V or higher. [Industrial applicability]

[0034] The secondary battery of the present invention can be suitably used as a secondary battery for various applications, such as for automotive use, stationary use, and small electronic devices.

Claims

1. A secondary battery comprising a current collector containing copper, a positive electrode having a positive electrode active material layer, a gel electrolyte holding an alkaline aqueous solution, and a negative electrode having a negative electrode active material layer, wherein the positive electrode active material in the positive electrode active material layer is copper(II) sulfide, and the negative electrode active material in the negative electrode active material layer contains zinc.

2. The secondary battery according to claim 1, characterized in that the current collector containing copper is a copper mesh.

3. The secondary battery according to claim 1 or 2, characterized in that the gel electrolyte is an alkaline aqueous solution held in a conductive polymer with fixed anionic charge.

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