Positive electrode layer and nickel-metal hydride battery

Abstract

The primary objective of this disclosure is to provide a cathode layer in which performance degradation due to corrosion of carbon materials is suppressed. [Solution] The present disclosure provides a positive electrode layer for use in a nickel-metal hydride battery, wherein the positive electrode layer contains a positive electrode active material, a conductive agent, an additive, and a binder, the conductive agent being a carbon material, and the additive having an oxygen evolution potential in an alkaline aqueous solution that is the same as or higher than the oxygen evolution potential of the carbon material, thereby solving the above problem.

Description

Technical Field

[0005]

[0001] The present disclosure relates to a positive electrode layer and a nickel-hydrogen battery.

Background Art

[0002] Nickel-hydrogen batteries are known as alkaline batteries. Nickel-hydrogen batteries are expected to be used as a power source for vehicles such as hybrid vehicles (HEVs), and various studies have been conducted.

[0003] For example, Patent Document 1 discloses a positive electrode for an alkaline secondary battery containing nickel hydroxide as an active material and a cobalt compound as an additive. Further, Patent Document 2 discloses a positive electrode for an alkaline storage battery including a positive electrode mixture containing nickel oxide as a positive electrode active material, a first additive, and a second additive different from the first additive. Further, Patent Document 3 discloses a nickel-hydrogen battery using a positive electrode composite material containing nickel hydroxide and graphite.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] From the perspective of providing good electronic conductivity, the use of carbon materials as conductive agents in the positive electrode layer is being considered. Furthermore, in nickel-metal hydride batteries, alkaline aqueous solutions are generally used as the electrolyte. However, during charging and discharging of the battery, when water in the alkaline aqueous solution is electrolyzed and oxygen is generated, the intermediate radicals react with carbon, potentially causing corrosion of the carbon material. As a result, the performance of the positive electrode layer may deteriorate.

[0006] This disclosure is made in view of the above circumstances and primarily aims to provide a positive electrode layer in which performance degradation due to corrosion of carbon materials is suppressed. [Means for solving the problem]

[0007] [1] A positive electrode layer used in nickel-metal hydride batteries, The above positive electrode layer contains a positive electrode active material, a conductive agent, additives, and a binder. The above conductive agent is a carbon material, The above additive is a positive electrode layer whose oxygen evolution potential in an alkaline aqueous solution is the same as or higher than that of the above carbon material.

[0008] [2] The positive electrode layer according to [1], wherein the additive contains at least one of the elements Ni, Fe, and Pb.

[0009] [3] The cathode layer according to [1] or [2], wherein the proportion of the additive in the cathode layer is 1% by weight or more and 10% by weight or less.

[0010] [4] A positive electrode layer according to any one of [1] to [3], wherein the oxygen evolution potential of the carbon material is 1.55V (vs. RHE) or higher and 1.60V (vs. RHE) or lower.

[0011] [5] A nickel-metal hydride battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, A nickel-hydrogen battery in which the above positive electrode layer is the positive electrode layer described in any one of [1] to [4].

Advantages of the Invention

[0012] In the present disclosure, there is an effect that a positive electrode layer in which performance degradation due to corrosion of the carbon material is suppressed can be provided.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram for explaining the additive in the present disclosure. [Figure 2] It is a diagram for explaining the additive in the present disclosure. [Figure 3] It is a schematic cross-sectional view illustrating the nickel-hydrogen battery in the present disclosure. [Figure 4] It is a graph showing the results of the examples and comparative examples.

Modes for Carrying Out the Invention

[0014] Hereinafter, the positive electrode layer and the nickel-hydrogen battery in the present disclosure will be described in detail. Each of the drawings shown below is schematically shown, and the size and shape of each part are exaggerated as appropriate for easy understanding.

[0015] A. Positive electrode layer The positive electrode layer in the present disclosure is a positive electrode layer used in a nickel-hydrogen battery, and contains a positive electrode active material, a conductive agent, an additive, and a binder. Further, the conductive agent is a carbon material, and the additive has an oxygen evolution potential in an alkaline aqueous solution that is the same as or higher than the oxygen evolution potential of the carbon material.

[0016] According to the present disclosure, since the oxygen evolution potential in an alkaline aqueous solution contains an additive having the same or higher oxygen evolution potential as that of the carbon material, corrosion of the carbon material can be suppressed. When radicals generated due to electrolysis of water react with the carbon material, it is considered that CO or CO2 is generated from the C element in the carbon material, and the carbon material decreases (corrodes). On the other hand, an additive having a higher oxygen evolution potential than that of the carbon material is presumed to be able to absorb radicals prior to the carbon material, and thus suppress the reaction between the radicals and the carbon material. In addition, an additive having the same oxygen evolution potential as that of the carbon material is presumed to compete with the carbon material for radicals, and as a result, reduce the number of radicals reacting with the carbon material. Thereby, it is presumed that corrosion of the carbon material can be suppressed and deterioration of the performance of the positive electrode layer can be suppressed. Further, since the carbon material is a relatively inexpensive conductive agent, the positive electrode layer containing the carbon material as a conductive agent has an advantage in terms of cost.

[0017] 1. Additive The additive in the present disclosure is an additive having the same or higher oxygen evolution potential in an alkaline aqueous solution as that of the carbon material.

[0018] When the oxygen evolution potential of the additive is higher than that of the carbon material, the difference between the oxygen evolution potential of the carbon material and the oxygen evolution potential of the additive is, for example, 0.1 V (vs. RHE) or more, and may be 0.5 V (vs. RHE) or more. On the other hand, the difference in oxygen evolution potential is, for example, 1.0 V (vs. RHE) or less.

[0019] Here, the oxygen evolution potentials of the carbon material and the additive, and examples of the additive will be described using FIGS. 1 and 2. Note that FIG. 1 is a graph obtained by adding the range of the oxygen evolution potential on the carbon surface to the graph published in the academic journal (Shunsuke Yagi: Materia, Vol. 59, No. 10 (2020), 521-526). Further, FIG. 2 is a graph obtained by adding the range of the oxygen evolution potential on the carbon surface to the graph published in the academic journal (Jin Suntivich et al., Science, 2011, 334, 1383-1385).

[0020] As shown in Figures 1 and 2, the oxygen evolution potential of carbon materials is, for example, 1.55V (vs. RHE) or higher and 1.60V (vs. RHE) or lower. It should be noted that the oxygen evolution potential is thought to vary depending on environmental factors such as the temperature and pH of the alkaline aqueous solution. Therefore, the oxygen evolution potential of the additive in this disclosure is, for example, 1.55V (vs. RHE) or higher, but may also be 1.57V (vs. RHE) or higher, 1.60V (vs. RHE) or higher, or 1.62V (vs. RHE) or higher. On the other hand, the oxygen evolution potential of the additive is, for example, 1.90V (vs. RHE) or lower, but may also be 1.85V (vs. RHE) or lower, 1.80V (vs. RHE) or lower, 1.75V (vs. RHE) or lower, or 1.70V (vs. RHE) or lower.

[0021] The oxygen evolution potential of carbon materials and additives can be measured by conventionally known methods. The alkaline solution is not particularly limited, but is typically the electrolyte used in nickel-metal hydride batteries, as described later.

[0022] The type of additive is not particularly limited as long as the above-mentioned relationship of oxygen evolution potential is satisfied, but examples include additives containing Fe, Pb, La, and Ni. Examples of additives containing Fe include elemental Fe, Fe alloys, and Fe compounds such as Fe3O4 shown in Figure 1 (e.g., Fe oxides). Examples of additives containing Pb include elemental Pb, Pb alloys, and Pb compounds such as PbO2 shown in Figure 1 (e.g., Pb oxides). Examples of additives containing La include elemental La, La alloys, and La compounds such as LaMnO3, LaFeO3, and LaCrO3 shown in Figure 2 (e.g., La oxides, La composite oxides). Examples of additives containing Ni include elemental Ni, Ni alloys, and Ni compounds such as Ni oxides, as described in the examples below. The additive may contain only one of the above elements, or it may contain two or more, as in LaFeO3.

[0023] The additives in this disclosure may be one type or two or more types.

[0024] The proportion of additives in the positive electrode layer is not particularly limited, but may be, for example, 1% by weight or more, 3% by weight or more, or 5% by weight or more. Alternatively, the proportion of additives may be, for example, 12% by weight or less, 10% by weight or less, 8% by weight or less, or 6% by weight or less.

[0025] In the positive electrode layer, the ratio of additives to the total amount of carbon material and additives is not particularly limited, but may be, for example, 10% by weight or more, 20% by weight or more, 30% by weight or more, or 40% by weight or more. On the other hand, the above ratio may be, for example, 65% by weight or less, 60% by weight or less, 55% by weight or less, or 50% by weight or less.

[0026] 2.Cathode active material Examples of positive electrode active materials include conventionally known active materials used in nickel-metal hydride batteries. Examples of positive electrode active materials include nickel compounds such as nickel hydroxide and hydrates of nickel compounds.

[0027] The proportion of positive electrode active material in the positive electrode layer is not particularly limited, but for example, it is between 50% by weight and 90% by weight.

[0028] 3. Conductive agent In this disclosure, the conductive agent in the positive electrode layer is a carbon material. Examples of carbon materials include Ketjenblack (KB), vapor-processed carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, and graphite. The positive electrode layer may contain one type of carbon material as the conductive agent, or it may contain two or more types of carbon materials.

[0029] Furthermore, the positive electrode layer may contain only carbon material as a conductive agent, or it may also contain conductive agents other than carbon material. In the latter case, the proportion of carbon material to all conductive agents is, for example, 70% by weight or more and 99% by weight or less. Examples of conductive agents other than carbon material include metallic materials such as manganese.

[0030] In this disclosure, it is preferable that the positive electrode layer does not contain cobalt-based conductive agents such as cobalt oxide as a conductive agent. Since cobalt is a relatively expensive material, a positive electrode layer that does not contain cobalt has advantages in terms of cost. It also has advantages in terms of avoiding resource risks. Furthermore, for example, when the positive electrode layer is manufactured by a coating method using a slurry, as described in the examples below, it is presumed that the viscosity stability over time will be improved in a slurry that does not contain cobalt compounds. This is because cobalt compounds may become polyvalent cations in the slurry, which may form cross-linked structures with the binder in the slurry, potentially causing the slurry to thicken and gel. In addition, cobalt compounds need to be handled relatively delicately. Thus, by not using cobalt-based conductive agents as the material for the positive electrode layer, there is an advantage in that the positive electrode layer can be manufactured more easily.

[0031] The proportion of conductive agent in the positive electrode layer is not particularly limited, but for example, it is between 1% by weight and 15% by weight.

[0032] 4. Binder Examples of binders used in the positive electrode layer include conventionally known binders used in nickel-metal hydride batteries. Examples of binders include styrene-butadiene rubber (SBR), polyethylene (PE), carboxymethylcellulose (CMC), polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). There may be only one type of binder, or two or more types. In addition, a portion of the binder contained in the positive electrode layer may be binder additives such as thickeners. Examples of thickeners include natural polysaccharides such as dieutan gum.

[0033] The proportion of the binder in the positive electrode layer is not particularly limited, but for example, it is between 1% by weight and 10% by weight.

[0034] 5. Positive electrode layer The positive electrode layer in this disclosure is used in nickel-metal hydride batteries. Nickel-metal hydride batteries will be described later. The thickness of the positive electrode layer is not particularly limited, but for example, it is between 1 μm and 1000 μm.

[0035] B. Nickel-metal hydride batteries Figure 3 is a schematic cross-sectional view illustrating a nickel-metal hydride battery in this disclosure. The nickel-metal hydride battery 10 shown in Figure 3 has a positive electrode layer 1, a negative electrode layer 2, and an electrolyte layer 3 disposed between the positive electrode layer 1 and the negative electrode layer 2. In the nickel-metal hydride battery 10, the positive electrode layer 1 is the positive electrode layer in this disclosure described above.

[0036] The nickel-metal hydride battery in this disclosure has the above-described positive electrode layer, thus suppressing the degradation of battery performance.

[0037] 1. Positive electrode layer The positive electrode layer is the same as described in "A. Positive Electrode Layer".

[0038] 2. Negative electrode layer The negative electrode layer contains at least a negative electrode active material. Examples of the negative electrode active material include conventionally known hydrogen storage alloys used in nickel-metal hydride batteries. The negative electrode layer may also contain, if necessary, at least one of a conductive agent and a binder. The binder is the same as described in "A. Positive Electrode Layer". Examples of the conductive agent include those described in "A. Positive Electrode Layer".

[0039] The thickness of the negative electrode layer is not particularly limited, and can be, for example, 1 μm or more and 1000 μm or less.

[0040] 3. Electrolyte layer The electrolyte layer typically contains an electrolyte solution. Examples of electrolyte solutions include conventionally known electrolytes used in nickel-metal hydride batteries. Examples of electrolyte solutions include aqueous solutions containing dissolved electrolytes such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and lithium hydroxide (LiOH). The electrolyte solution may contain one type of electrolyte or two or more types. The concentration of the electrolyte in the electrolyte solution is, for example, between 1 M and 10 M.

[0041] Furthermore, the electrolyte layer may be a layer in which the electrolyte solution is impregnated into the separator. The material of the separator may be an organic material or an inorganic material. Specifically, examples include porous membranes such as polyethylene (PE), polypropylene (PP), cellulose, polyvinylidene fluoride, polyamide, and polyimide, nonwoven fabrics such as resin nonwoven fabrics and glass fiber nonwoven fabrics, and porous ceramic membranes. The separator may also have a single-layer structure or a laminated structure.

[0042] 4. Other configurations A nickel-metal hydride battery may include components such as a positive electrode current collector, a negative electrode current collector, terminals, and a battery case. These components are not particularly limited, and conventionally known components used in nickel-metal hydride batteries can be used. A nickel-metal hydride battery may be a stacked battery or a wound battery. Furthermore, a nickel-metal hydride battery may be a monopolar battery or a bipolar battery.

[0043] While the applications of nickel-metal hydride batteries are not particularly limited, they can be used as power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. They are particularly preferred for use as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Nickel-metal hydride batteries may also be used as power sources for other mobile devices (e.g., trains, ships, aircraft), and as power sources for electrical products such as information processing devices.

[0044] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure. [Examples]

[0045] [Example 1] A three-electrode cell was prepared as the evaluation battery. The working electrode was prepared by coating a Ni-coated Fe foil with a conductive slurry containing a binder, conductive agent, and additives, drying it, and pressing it. Carboxymethylcellulose (CMC) and polypropylene (PP) were used as binders. Graphene was used as the conductive agent. Ni was used as the additive. The oxygen evolution potential of Ni is presumed to be similar to that of carbon. The weight ratio of the binder, conductive agent, and additives in the conductive slurry was 10:89:1. That is, the weight percentage (amount added) of the additive was 1% by weight.

[0046] The counter electrode used was an iron-coated Fe foil.

[0047] An aqueous solution containing 5.35 M KOH and 0.8 M NaOH was used as the electrolyte. An Hg|HgO electrode was used as the reference electrode.

[0048] [Examples 2-6] An evaluation battery was prepared in the same manner as in Example 1, except that the type and amount of additives in the conductive layer (conductive slurry) were changed as shown in Table 1. The proportion of conductive agent was also decreased as the proportion of additives increased.

[0049] [Comparative Example 1] An evaluation battery was prepared in the same manner as in Example 1, except that no additives were used in the conductive layer (conductive slurry).

[0050] [Comparative Examples 2-4] Except for using MnO2, which has a lower oxygen evolution potential than carbon materials, as an additive in the conductive layer (conductive slurry), evaluation batteries were prepared in the same manner as in Examples 1 to 3.

[0051] [evaluation] <Durability Test> When carbon material reacts with oxygen radicals to produce CO or CO2, the weight of the carbon material in the conductive layer decreases, leading to a decrease in the weight of the working electrode. Therefore, a durability test was conducted as described below, and the weight loss rate (%) was calculated from the weight of the working electrode before and after the durability test to compare the degree of carbon corrosion in the conductive layer. The results are shown in Table 1 and Figure 4. Durability test: At 65°C, a potential of 0.6V (vs Hg|HgO) was applied to the working electrode for 65 hours.

[0052] [Table 1]

[0053] As shown in Table 1 and Figure 4, in Comparative Example 2, where MnO2, which has a lower oxygen evolution potential than carbon material, was used as an additive, the decrease in working electrode weight was not suppressed, confirming that corrosion of the carbon material was not inhibited. On the other hand, in Examples 1-6, where Ni, which has the same oxygen evolution potential as carbon, or Fe3O4, which is nobler than carbon, was used as an additive, the weight loss rate of the working electrode was smaller compared to Comparative Examples 1-4, confirming that corrosion of the carbon material was suppressed. It was also confirmed that the weight loss rate decreased as the proportion of the additive increased. [Explanation of Symbols]

[0054] 1 ... Positive electrode layer 2 ... Negative electrode layer 3...electrolyte layer 10… Nickel-metal hydride battery

Claims

1. A positive electrode layer used in nickel-metal hydride batteries, The positive electrode layer contains a positive electrode active material, a conductive agent, an additive, and a binder. The conductive agent is a carbon material, The additive is a positive electrode layer whose oxygen evolution potential in an alkaline aqueous solution is the same as or higher than that of the carbon material.

2. The positive electrode layer according to claim 1, wherein the additive contains at least one of Ni, Fe, and Pb elements.

3. The positive electrode layer according to claim 1, wherein the proportion of the additive in the positive electrode layer is 1% by weight or more and 10% by weight or less.

4. The positive electrode layer according to claim 1, wherein the oxygen evolution potential of the carbon material is 1.55 V (vs. RHE) or more and 1.60 V (vs. RHE) or less.

5. A nickel-metal hydride battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, A nickel-metal hydride battery, wherein the positive electrode layer is the positive electrode layer described in any one of claims 1 to 4.

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