JP7704614B2 - Anode material for nickel-metal hydride battery and anode composition containing said anode material - Google Patents

Description

The technology disclosed in this specification relates to negative electrode materials and negative electrode compositions for nickel-metal hydride batteries.

Nickel-metal hydride secondary batteries (hereafter simply referred to as nickel-metal hydride batteries) are secondary batteries that use nickel oxide compounds such as nickel hydroxide in the positive electrode and a hydrogen storage alloy in the negative electrode. Nickel-metal hydride batteries use a highly safe alkaline aqueous solution as the electrolyte. For this reason, nickel-metal hydride batteries are widely used as batteries for hybrid vehicles and as a replacement for dry batteries from the standpoints of environmental friendliness, high-speed charging and discharging, cycle life, etc.

The hydrogen storage alloys used as the negative electrode material of nickel-metal hydride batteries absorb protons and electrons from water during charging, storing them as hydrogen atoms, and release the protons and electrons during discharging. Rare earth metal alloys, such as LaNi ₅ alloys, are used as hydrogen storage alloys, but rare earth metals are difficult to secure and expensive to produce. There is also the issue of high equilibrium hydrogen pressure.

On the other hand, activated carbon, which is abundant as a resource, lightweight, and has a large specific surface area, is known as a material for the negative electrode of a capacitor that has the same power storage effect as a nickel-metal hydride battery. For example, a hybrid capacitor that uses nickel hydroxide for the positive electrode and activated carbon for the negative electrode has been disclosed (Patent Document 1).

JP 2006-80335 A

However, carbon materials such as activated carbon generally have a low bulk density, and when the carbon material is filled into a battery container, the charge/discharge capacity per unit volume is insufficient.

The technology disclosed in this specification provides anode materials for nickel-metal hydride batteries that are abundant, lightweight, and have a large charge/discharge capacity per unit volume.

This specification discloses a negative electrode material for a nickel-hydrogen battery, a method for producing the negative electrode material, a composition for the negative electrode of a nickel-hydrogen battery, a negative electrode for a nickel-hydrogen battery, and a nickel-hydrogen battery, etc.

The negative electrode material is a carbon material having a BET specific surface area per unit volume, expressed as BET specific surface area per unit volume [m ² /g] × bulk density [g/cm ³ ], of 700 m ² /cm ³ or more and a bulk density of 0.4 g/cm ³ or more and 1.0 g/cm ³ or less. By using a carbon material having a BET specific surface area per unit volume and a bulk density in the above range as the negative electrode material of a nickel-hydrogen battery, a nickel-hydrogen battery having a large charge/discharge capacity per unit volume can be provided. In addition, carbon materials are abundant, lightweight, and easily available as a resource, so that a compact nickel-hydrogen battery can be provided continuously and at low cost.

This specification discloses a composition for the negative electrode of a nickel-metal hydride battery. The composition for the negative electrode contains the above-mentioned carbon material. With the composition for the negative electrode, it is possible to continuously provide a lightweight and compact nickel-metal hydride battery with a large charge/discharge capacity per unit volume at low cost.

This specification discloses a method for producing an anode material for a nickel-metal hydride battery. The method can include a raw material preparation step of preparing a carbon material having a BET specific surface area per unit mass of 700 m ² /g or more as a raw material, and a pressurizing step of pressurizing the carbon material to make the BET specific surface area per unit volume, expressed as BET specific surface area [m ² /g] × bulk density [g/cm ³ ], 700 m ² /cm ³ or more. By including these steps, it is possible to continuously provide an anode material that has a large charge/discharge capacity per unit volume and can contribute to a lightweight and compact nickel-metal hydride battery at low cost.

This specification discloses a negative electrode for a nickel-metal hydride battery. The negative electrode contains the above-mentioned negative electrode composition. This specification also discloses a nickel-metal hydride battery. The nickel-metal hydride battery includes a positive electrode, an electrolyte, a negative electrode, and a separator. The negative electrode contains the above-mentioned negative electrode composition. A nickel-metal hydride battery including such a negative electrode is lightweight and compact, and can continuously increase the charge/discharge capacity per unit volume at low cost.

This specification provides a vehicle equipped with the above-mentioned nickel-metal hydride battery. With such a vehicle, it is possible to provide a vehicle equipped with a low-cost, lightweight, and compact battery.

1A and 1B are diagrams showing structural models of activated carbon, an example of a carbon material, before and after compression. FIG. 2 is a graph showing the relationship between the BET specific surface area per unit mass and the volumetric charge/discharge capacity of carbon materials in Production Examples and Comparative Examples in the Examples. FIG. 2 is a graph showing the relationship between the specific surface area per unit volume and the volumetric charge/discharge capacity of carbon materials in Production Examples and Comparative Examples in the Examples.

As described above, the negative electrode material of the nickel-hydrogen battery disclosed in this specification is a carbon material, has a BET specific surface area per unit volume of 700 ^m2 / ^cm3 or more, and has a bulk density of 0.4 g/cm3 or ^more and 1.0 g/ ^cm3 or less. By having such characteristics, when the material is used as a negative electrode material to form a negative electrode of a nickel-hydrogen battery, it is possible to increase the charge/discharge capacity per unit volume of the nickel-hydrogen battery.

Although the disclosure of this specification is not limited, such an action of the negative electrode material disclosed in this specification can be inferred as follows. FIG. 1 shows a structural model of the negative electrode material disclosed in this specification, taking activated carbon as an example of the negative electrode material disclosed in this specification. As shown in FIG. 1(a), it is considered that in conventional activated carbon, graphene is present alone or in a small number of layers, for example, 10 layers or less, and each graphene is present at a distance. As a result, it is considered that a relatively large gap is formed between each graphene. For this reason, the bulk density [g/cm ³ ] is low. In contrast, as shown in FIG. 1(b), it is considered that the activated carbon as the negative electrode material disclosed in this specification has a structure in which the gaps existing between the individual graphenes of conventional activated carbon are compressed. By adopting such a structure in which the gaps are compressed, the graphene present per unit volume is densified, and as a result, the bulk density is increased and the BET specific surface area per unit volume is increased. In addition, it is believed that the close proximity of graphene particles increases the interaction between the molecules (e.g., hydrogen molecules or proton ions) adsorbed between them, thereby improving the electrochemical properties related to charging and discharging, etc.

The following describes the nickel-hydrogen battery anode material disclosed in this specification, the method for producing the anode material, the anode composition for a nickel-hydrogen battery containing the anode material, the anode containing the anode composition and a nickel-hydrogen battery including the anode, and a vehicle including the nickel-hydrogen battery.

In this specification, the terms "more than" and "less than" in relation to numerical values include the meanings of "more than" and "less than," respectively. Therefore, for example, when the term "0.4 g/ ^cm3 or more and 1.0 g/cm3 or ^less" is used in this specification, in addition to the literal meaning, the terms "more than 0.4 g/ ^cm3 and 1.0 g/cm3 or less" are also used, including the embodiments of "more than 0.4 g/ ^cm3 and 1.0 g/ ^cm3 or less,""more than 0.4 g/ ^cm3 and 1.0 g/cm3 or less," and "more than 0.4 g/ ^cm3 and 1.0 g/ ^cm3 or less."

<Negative electrode material for nickel-metal hydride batteries>
The negative electrode material disclosed in this specification is a carbon material. The carbon material is not particularly limited, but may be, for example, a material having at least a part of graphene with a sheet-like structure of SP ² bonded carbon atoms. Examples of such carbon materials include activated carbon, which is a porous material having various forms such as powder, graphite having a single layer or multiple layers of graphene, graphene fibers such as graphene nanofibers having a fiber structure of the graphene, carbon nanotubes having a tube structure of the graphene, carbon nanohorns having a conical structure of the graphene, graphene porous materials such as graphene mesoporous having pore walls of the graphene, carbon nanorods having graphene, graphene nanoribbons, etc. can be mentioned. From the viewpoint of a large BET specific surface area, activated carbon can be used.

The carbon material may have some of the carbon atoms replaced with atoms other than carbon atoms, or other atoms or molecules intercalated between the lattices or between the layers. In addition, functional groups may be introduced into the carbon atoms of the carbon material by known treatment.

In order to promote the adsorption of hydrogen molecules by compressing the above-mentioned carbon material to bring the graphene layers or walls closer together, it is inferred that a binder for maintaining and ensuring the proximity effect caused by compression may be effective. Such a binder may be added in the manufacturing process of the negative electrode material. In this case, the negative electrode material may contain, in advance, a compound such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer, styrene-butadiene rubber, carboxy cellulose, or acrylic resin as a binder for the carbon material.

The binder can be, for example, 0.01% by mass or more relative to the total mass of the carbon material and the binder. The lower limit is, for example, 0.05% by mass or more, for example, 0.1% by mass or more, for example, 0.5% by mass or more, for example, 1% by mass or more, for example, 2% by mass or more, or for example, 3% by mass or more. The upper limit is, for example, 10% by mass or less relative to the total mass. This is because the binder does not have charge/discharge ability, and if the content of the binder exceeds 10% by weight, the charge/discharge amount of the negative electrode material decreases. The upper limit is, for example, 8% by mass or less, for example, 6% by mass or less, or for example, 5% by mass or less. The binder content range can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 0.2% by mass or more and 10% by mass or less, for example, 2% by mass or more and 8% by mass or less, for example, 2% by mass or more and 6% by mass or less, for example, 2% by mass or more and 5% by mass or less, and for example, 2% by mass or more and 4% by mass or less.

The negative electrode material has a BET specific surface area per unit volume of 700 m ² /cm ³ or more. In this specification, the BET specific surface area per unit volume is expressed as the product of the BET specific surface area [m ² /g] of the material and the bulk density [g/cm ³ ] of the material.

In this specification, the BET specific surface area (hereinafter simply referred to as specific surface area) is the specific surface area measured by the BET adsorption method. The BET adsorption method used in this specification is a method in which the amount of nitrogen adsorbed to a material is determined from the pressure change of nitrogen introduced into the measurement cell of a BELSORP-max manufactured by Microtrac-Bell Co., Ltd. using the gas state equation, and the specific surface area of the material is calculated from the adsorption isotherm assumed in the BET theory.

The specific surface area of the negative electrode material is, for example, 700 ^{m 2} /g or more, for example, 800 m ² /g or more, for example, 900 m ² /g or more, for example, 1000 ^{m 2} /g or more, for example, 1100 ^{m 2} /g or more, for example, 1200 m ² /g or more, for example, 1300 m ² /g or more, for example, 1400 m ² /g or more, for example, 1500 m ² /g or more, for example, 1600 ^{m 2} /g or more, for example, 1700 ^{m 2} /g or more, for example, 1800 ^{m 2} /g or more, for example, 1900 ^{m 2} /g or more, for example, 2000 m ² /g or more, for example, 2200 m ^{2 /g or more.} /g or more, for example, 2400 m ² /g or more, for example, 2600 m ² /g or more, and for example, 2800 m ² /g or more. The specific surface area is not particularly limited, but is, for example, 4000 m ² /g or less, for example, 3600 m ² /g or less, for example, 3400 m ² /g or less, for example, 3200 m ² /g or less, and for example, 3000 m ² /g or less. A suitable range of the specific surface area can be set by combining the above-mentioned lower limit value and upper limit value, and can be, for example, 700 m ² /g or more and 3600 ^{m 2} /g or less.

It is preferable that the negative electrode material has a predetermined bulk density. In this specification, the bulk density is calculated by forming the material into a disk of a predetermined diameter at a pressure of 0.1 MPa, measuring the height and diameter of the formed carbon material to determine the volume, and dividing the mass of the material used by the volume.

The bulk density of the negative electrode material is, for example, 0.40 g/cm ³ or more, for example, 0.50 g/cm ³ or more, for example, 0.60 g/cm ³ or more, for example, 0.70 g/cm ³ or more, for example, 0.80 g/cm ³ or more, or for example, 0.90 g/cm ³ or more. In addition, because hydrogen molecules are less likely to penetrate as the interlayer distance of graphene becomes shorter, the bulk density is, for example, 1.00 g/cm ³ or less, for example, 0.95 g/cm ³ or less, for example, 0.90 g/cm ³ or less, for example, 0.85 g/cm ³ or less, for example, 0.80 g/cm ³ or less, for example, 0.75 g/cm ³ or less, for example, 0.70 g/cm ³ or less, for example, 0.65 g/cm ³ or less, for example, 0.60 g/cm ³ or less, for example, 0.65 g/cm ³ or less, for example, 0.60 g/cm ³ or less, for example, 0.55 g/cm ³ or less. A suitable bulk density range can be set by combining the above-mentioned lower limit and upper limit values, and can be, for example, 0.40 g/ ^cm3 or more and 1.00 g/ ^cm3 or less, or, for example, 0.40 g/ ^cm3 or more and 0.95 g/ ^cm3 or less.

The specific surface area per unit volume of the negative electrode material is 700 m ² /cm ³ or more, for example, 800 m ² /cm ³ or more, for example, 900 m ² /cm ³ or more, for example, 1000 m ² /cm ³ or more, for example, 1100 m ² /cm ³ or more. The upper limit is not particularly limited, but is, for example, 3000 m ² /cm ³ or less, for example, 2800 m 2 /cm 3 or less, for example, 2600 m ² /cm ³ or less, for example, 2400 m ² /cm ³ or less, for example, 2200 m ² /cm ³ or less, for example, 2000 m ² /cm ³ or less, for example, 1800 m ² /cm ³ or less, depending on the relationship between the specific surface area and the ^{bulk density} .

<Method of manufacturing negative electrode material>
The negative electrode material disclosed in the present specification can be produced, for example, by the following method, although the method is not particularly limited.

(Raw material preparation process)
The method for producing the negative electrode material may include a raw material preparation step of preparing a carbon material having a specific surface area per unit mass of 700 ^m2 /g or more as a raw material. As the carbon material used as the raw material, various known carbon materials such as activated carbon that constitutes the negative electrode material described above can be used.

The carbon material used as the raw material is not particularly limited, but a carbon material having a specific surface area larger than that of the negative electrode material to be obtained can be selected, because the specific surface area will be reduced by the subsequent pressurizing step. The specific surface area per unit mass of the carbon material as raw material is, for example, 700 m ² /g or more, for example, 750 m ² /g or more, for example, 800 m ² /g or more, for example, 900 m ² /g or more, for example, 1000 m ² /g or more, for example, 1100 ^{m 2} /g or more, for example, 1200 m ² /g or more, for example, 1300 ^{m 2} /g or more, for example, 1400 ^{m 2} /g or more, for example, 1500 m ² /g or more, for example, 1600 m ² /g or more, for example, 1700 m ² /g or more, for example, 1800 ^{m 2} /g or more, for example, 1900 ^{m 2} /g or more, or for example, 2000 m ² /g or more. /g or more, for example, 2200 m ² /g or more, for example, 2400 m ² /g or more, for example, 2600 m ² /g or more, and for example, 2800 m ² /g or more. The specific surface area per unit mass of the carbon material as raw material is not particularly limited, but is, for example, 4000 m ² /g or less, for example, 3600 m ² /g or less, for example, 3400 m ² /g or less, for example, 3200 m ² /g or less, and for example, 3000 m ² /g or less. The suitable range of the specific surface area of the carbon material as raw material can be set by combining the above-mentioned lower limit value and upper limit value, and can be, for example, 750 m ² /g or more and 3600 ^{m 2} /g or less.

The carbon material used as the raw material is not particularly limited, but a carbon material having a bulk density smaller than that of the negative electrode material to be obtained, i.e., a bulky carbon material, can be selected. This is because the bulk density is improved by the subsequent pressurizing step. The bulk density of the carbon material as the raw material is, for example, 0.10 g/cm ³ or more, for example, 0.20 g/cm ³ or more, for example, 0.30 g/cm ³ or more, for example, 0.40 g/cm ³ or more, for example, 0.50 g/cm ³ or more, and for example, 0.60 g/cm ³ or more. The bulk density is, for example, 0.90 g/cm ³ or less, 0.80 g/cm ³ or less, for example, 0.70 g/cm ³ or less, for example, 0.65 g/cm ³ or less, for example, 0.60 g/cm ³ or less, or for example, 0.50 g/cm ³ or less. A suitable range of bulk density of the carbon material as a raw material can be set by combining the above-mentioned lower limit value and upper limit value, and can be, for example, 0.20 g/cm ³ or more and 0.70 g/cm ³ or less, or, for example, 0.20 g/cm ³ or more and 0.65 g/cm ³ or less.

The specific surface area per unit volume of the carbon material as a raw material is not particularly limited, but is, for example, 400 m ² /cm ³ or more, for example, 450 m ² /cm ³ or more, for example, 500 m ² /cm ³ or more, for example, 600 m ² /cm ³ or more, for example, 650 m ² /cm ³ or more, and for example, 700 m ² /cm ³ or more. The upper limit is not particularly limited, but is, from the relationship between the specific surface area and the bulk density, for example, 800 ^{m 2} /cm ³ or less, for example, 750 m ² /cm ³ or less, for example, 700 m ² /cm ³ or less, for example, 650 m ² /cm ³ or less, and for example, 600 m ² /cm ³ or less. A suitable specific surface area per unit volume of the carbon material as a raw material can be set by combining the above-mentioned lower limit and upper limit, and can be, for example, 450 m ² /cm ³ or more and 800 ^m 2 /cm ³ or less.

These raw materials, such as activated carbon, can be commercially obtained as needed.

In addition to the carbon material, a binder can be used as the raw material. The binder can uniformly pressurize the carbon material to increase its density, and is thought to contribute to maintaining the compacted state of structures such as graphene in the carbon material due to the pressure. There are no particular limitations on the binder as long as it can bind the carbon material, and the binders already described in the negative electrode material can be used. As the binder, polytetrafluoroethylene (PTFE) can be used from the viewpoint that the pressed carbon material can be molded again as it is even after it is crushed after the pressurization process described below.

The content of the binder is not particularly limited as long as it can act as a binder. For example, it can be 0.01% by mass or more relative to the total mass of the carbon material and the binder, and the content of the form already explained in the description of the negative electrode material can be adopted.

(Pressing process)
The pressurizing step is a step in which the carbon material as a raw material is pressurized to make the specific surface area per unit volume, expressed as specific surface area [ ^m2 /g] x bulk density [g/ ^cm3 ], 700 ^m2 / ^cm3 or more. By pressurizing the carbon material as a raw material in the pressurizing step, the distance between the graphene layers or walls is shortened, and as a result, the carbon material is compacted to increase the density, and the bulk density of the raw material is improved. At the same time, the specific surface area of the raw material is thought to decrease.

In the pressurizing step, although there is no particular limitation, for example, a known compression molding machine such as a hydraulic press can be used. The pressurizing step may be performed once, but can also be repeated multiple times on the carbon material as the raw material. By performing the pressurizing step multiple times, sufficient densification can be achieved even with a small pressure. The number of pressurizing steps is not particularly limited and is appropriately set depending on the carbon material used, the pressure at the time of pressurization, and the intended degree of densification, but is, for example, from one time to several tens of times, or from one time to 50 times, or from one time to 20 times, or from one time to 10 times, or from one time to 6 times, or from one time to 5 times.

When the pressurizing step is carried out multiple times, there is no particular limitation, and for example, pressure may be continued to be applied to the raw material after pressurization. Also, for example, the raw material that has been pressurized once may be crushed, crushed, or ground, and the crushed material, etc., may be pressurized again as the raw material. When the pressurizing step is carried out multiple times, the above-mentioned binder may be used as the raw material in addition to the carbon material, and the pressurizing step may be carried out in the presence of the binder, thereby making it possible to more effectively increase the density by carrying out the pressurizing step multiple times.

The crushing, crushing, or grinding of the pressurized raw material is not particularly limited, and any known crushing, crushing, or grinding device can be used. For example, various mills such as planetary ball mills, vibrating ball mills, jet mills, and hammer mills, grinders, blenders, etc. can be used as appropriate. Crushing, etc. can be performed in an air atmosphere at room temperature and normal pressure.

The conditions of the pressurizing step are not particularly limited. The pressure in the pressurizing step is not particularly limited, and is set so as to increase the charge/discharge capacity per volume of the carbon material used as the raw material. The pressure depends on the number of pressurizing steps, but the carbon material as the raw material can be pressurized at, for example, 50 MPa or more, or for example, 100 MPa or more, or for example, 120 MPa or more, or for example, 140 MPa or more, or for example, 200 MPa or more, or for example, 500 MPa or more, or for example, 600 MPa or more, or for example, 700 MPa or more, or for example, 800 MPa or more. If the pressure is less than 50 MPa, the compressing force is small and it is difficult to obtain the desired bulk density. In addition, when activated carbon is used as the raw material, a pressure of 100 MPa or more can be used from the viewpoint of fully expressing the compression effect.

The pressure in the compression process may vary depending on the number of compression processes, but may be, for example, 1200 MPa or less, or, for example, 1000 MPa or less, or, for example, 900 MPa or less, or, for example, 800 MPa or less, or, for example, 750 MPa or less. If the pressure exceeds 1200 MPa, the durability of the processing equipment becomes an issue, making it impractical. In addition, considering the durability of the molding die, it is desirable to perform the high-pressure compression process at a pressure of 500 MPa or less.

The pressure range in the pressure application process can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 50 MPa or more and 1200 MPa or less, or, for example, 100 MPa or more and 1200 MPa or less, or, for example, 700 MPa or more and 1200 MPa or less.

The temperature in the pressurizing step is not particularly limited. For example, the pressurizing step can be performed at a temperature between room temperature and 200°C. The pressurizing time in the pressurizing step is also not particularly limited. For example, the pressurizing time can be a few seconds to about 10 minutes as the time for which the pressure is maintained. The gas atmosphere in the pressurizing step is also not particularly limited, and the pressurizing step can be performed in an air atmosphere under normal pressure.

The carbon material obtained through the pressurizing process and the mixed material of the carbon material and the binder are provided with graphene in which the carbon material has been densified or compacted by the pressurization. Such mixed materials are supplied as negative electrode materials for nickel-metal hydride batteries either as molded bodies or in powder form after crushing, pulverizing, or grinding the molded bodies. When supplied as molded bodies, they are crushed or the like before preparing the negative electrode composition.

<Negative electrode composition for nickel-metal hydride battery>
The negative electrode composition for a nickel-metal hydride battery disclosed in this specification contains the above-mentioned negative electrode material. The negative electrode composition can effectively provide a negative electrode for a nickel-metal hydride battery with an increased charge capacity per volume.

The negative electrode composition may contain a binder for the carbon material as the negative electrode material. As already explained in relation to the negative electrode material, various known binders such as polytetrafluoroethylene may be used as the binder. The binder contained in the negative electrode composition may be only the binder used in the production of the negative electrode material, may be only the binder newly added to the negative electrode material when preparing the negative electrode composition, or may contain both of these binders. Either type of binder can contribute to an increase in the charging capacity per unit volume due to the densified carbon material.

The content of the binder in the negative electrode composition is not particularly limited, but can be in the same range as in the negative electrode material described above, for example, 0.01% by mass or more and 10% by mass or less relative to the total mass of the carbon material and the binder.

The negative electrode composition may contain components contained in known compositions for the negative electrode of nickel-hydrogen batteries, such as metal powders such as Ni powder, oxides such as cobalt oxide, and conductive assistants such as graphite and carbon materials such as carbon nanotubes, in addition to the negative electrode material. The amount of conductive assistant added is not particularly limited, but is preferably in the range of 0.1 parts by mass to 50 parts by mass, and may be in the range of 0.1 parts by mass to 30 parts by mass, per 100 parts by mass of the negative electrode material. Such a negative electrode composition may also be used as, for example, a negative electrode paste. A person skilled in the art can prepare a negative electrode composition by appropriately mixing the negative electrode material and such known components.

<Nickel-metal hydride battery and its negative electrode>
The nickel-metal hydride battery disclosed in this specification includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the negative electrode contains the above-mentioned negative electrode composition. These battery elements of the nickel-metal hydride battery are housed in a housing (battery case). The battery elements of the positive electrode, the negative electrode, the electrolyte, and the separator can have a known form, such as a wound type or a laminated type. The shape of the battery can be, for example, a coin type, a laminated type, a cylindrical type, a square type, or the like.

(positive electrode)
The positive electrode is usually composed of a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer contains at least a positive electrode active material. The positive electrode active material layer may further contain at least one of a conductive assistant, a binder, and a thickener. The positive electrode active material is not particularly limited as long as it functions as a battery when combined with a negative electrode material, and examples of the positive electrode active material include a metal element, an alloy, and a hydroxide. The positive electrode active material may include a material that contains nickel oxide and is mainly composed of nickel oxyhydroxide and/or nickel hydroxide. The amount of nickel oxide in the positive electrode active material is, for example, 90% by mass or more and 100% by mass or less, and 95% by mass or more and 100% by mass or less.

The conductive assistant is not particularly limited as long as it is a material that can impart electronic conductivity, and examples of the conductive assistant include metal powders such as Ni powder, oxides such as cobalt oxide, graphite, and carbon materials such as carbon nanotubes. The amount of conductive assistant added is not particularly limited, but is, for example, 0.1 parts by mass to 50 parts by mass, or, for example, 0.1 parts by mass to 30 parts by mass, relative to 100 parts by mass of the positive electrode active material. Examples of the binder include synthetic rubbers such as styrene-butadiene rubber (SBR), celluloses such as carboxymethyl cellulose (CMC), polyols such as polyvinyl alcohol (PVA), fluororesins such as polyvinylidene fluoride (PVDF), and acrylic resins. The amount of binder may be, for example, 7 parts by mass or less, 0.01 parts by mass to 5 parts by mass, or, for example, 0.05 parts by mass to 2 parts by mass, relative to 100 parts by mass of the positive electrode active material.

Examples of thickeners include carboxymethylcellulose and its modified products (including salts such as Na salts), cellulose derivatives such as methylcellulose, saponified polymers having vinyl acetate units such as polyvinyl alcohol, and polyalkylene oxides such as polyethylene oxide. These thickeners can be used alone or in combination of two or more. The amount of thickener is, for example, 5 parts by mass or less, for example, 0.01 parts by mass or more and 3 parts by mass or less, or for example, 0.05 parts by mass or more and 1.5 parts by mass or less, per 100 parts by mass of the positive electrode active material.

Examples of the material for the positive electrode current collector include stainless steel, aluminum, nickel, iron, and titanium. The shape of the positive electrode current collector can be, for example, foil, mesh, or porous.

The positive electrode can be formed by attaching a positive electrode composition containing a positive electrode active material to a support (positive electrode current collector). The positive electrode composition is usually prepared by forming a paste with the positive electrode active material described above, a conductive assistant, and a binder. The dispersion medium can be water, an organic medium, or a mixed medium of two or more media selected from these.

The positive electrode may be formed by applying the positive electrode mixture paste to a support or filling the pores of the support depending on the shape of the support. The positive electrode may be formed by applying or filling the positive electrode mixture paste to a support, drying to remove the dispersion medium, and compressing the resulting dried material in the thickness direction (e.g., rolling between a pair of rolls).

(Negative electrode)
The negative electrode is usually composed of a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer contains, as a negative electrode active material, a negative electrode composition containing at least the above-described negative electrode material.

Examples of materials for the negative electrode current collector include steel, stainless steel, aluminum, nickel, iron, titanium, and carbon. The shape of the negative electrode current collector can be, for example, foil, mesh, or porous.

The negative electrode can be obtained by applying a paste-like negative electrode composition onto a negative electrode current collector and drying it to form a negative electrode active material layer on the negative electrode current collector. It can also be obtained by forming the negative electrode composition into a predetermined shape and supporting the formed negative electrode paste with the negative electrode current collector.

(Electrolyte layer: electrolyte and separator)
The electrolyte layer is a layer containing an electrolyte solution formed between the positive electrode and the negative electrode. In this specification, the electrolyte solution is an electrolyte solution that mainly uses water as a solvent, and the solvent may contain something other than water. The ratio of water to the entire solvent of the electrolyte solution is, for example, 50 mol% or more, for example, 70 mol% or more, for example, 90 mol% or more, and for example, 100 mol%.

The electrolyte is preferably an alkaline aqueous solution. Examples of solutes in the alkaline aqueous solution include potassium hydroxide (KOH) and sodium hydroxide (NaOH), which may also contain LiOH. The higher the concentration of the solute in the electrolyte, the more preferable, for example, 3 mol/l or more, and for example, 5 mol/l or more.

The electrolyte layer has a separator. Examples of the separator include a nonwoven fabric or a porous membrane containing a resin such as sulfonated polyethylene or polypropylene.

(Housing)
The case is a battery case (cell container) that houses the positive electrode, the negative electrode, and the separator and is filled with an electrolyte. The case may be any case that is stable and not corroded by the electrolyte, and can retain the gas (oxygen or hydrogen) temporarily generated during charging and the electrolyte without leaking to the outside. For example, a metal case, a resin case, or the like is generally used.

The nickel-metal hydride battery and its negative electrode disclosed in this specification are suitable, for example, for use as a secondary battery for an automobile. The battery for an automobile may be a battery for a hybrid automobile that supplies power to a motor for driving the vehicle, or a battery that supplies power to a starter motor. Note that the term "secondary battery" also includes a secondary battery used as a primary battery (used only for one discharge after charging).

<Vehicles>
The vehicle disclosed in this specification is equipped with a nickel-metal hydride battery, such as a nickel-metal hydride battery, using the above-mentioned negative electrode material composition in the negative electrode, as a power supply source for a motor, etc. Because the nickel-metal hydride battery is lightweight, low-cost, and compact, it can contribute to the fuel efficiency, etc., of the vehicle.

This specification includes the following aspects.
[1] A negative electrode material for a nickel-metal hydride battery, which is a carbon material having a BET specific surface area per unit volume, expressed as BET specific surface area [ ^m2 /g] x bulk density [g/cm3 ^] , of 700 ^m2 /cm3 or ^more and a bulk density of 0.4 g/ ^cm3 or more and 1.0 g/cm3 or ^less .
[2] The negative electrode material according to [1], which has a BET specific surface area per unit volume of 900 m ² /cm ³ or more.
[3] The negative electrode material according to [1], which has a BET specific surface area per unit volume of 1000 m ² /cm ³ or more.
[4] A composition for a negative electrode of a nickel-metal hydride battery, comprising the negative electrode material according to any one of [1] to [3].
[5] The negative electrode composition according to [4], further comprising a binder for the carbon material.
[6] The negative electrode composition according to [5], containing 10% by mass or less of the binder.
[7] The negative electrode composition according to [5] or [6], wherein the binder is polytetrafluoroethylene.
[8] A method for producing a negative electrode material for a nickel-metal hydride battery, comprising the steps of:
A raw material preparation step of preparing a carbon material having a BET specific surface area per unit mass of 700 m ² /g or more as a raw material;
a pressurizing step of pressing the carbon material so that the BET specific surface area per unit volume, expressed as BET specific surface area [m ² /g]×bulk density [g/cm ³ ], is 700 ^{m 2} /cm ³ or more;
A manufacturing method comprising:
[9] The method according to [8], wherein the carbon material is activated carbon.
[10] The manufacturing method according to [8] or [9], wherein the pressurizing step is a step of pressurizing the carbon material at a pressure of 50 MPa or more and 1200 MPa or less.
[11] The method according to any one of [8] to [10], wherein the pressurizing step is a step of pressurizing the carbon material in the presence of a binder.
[12] A nickel-metal hydride battery,
A nickel-metal hydride battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the negative electrode contains the negative electrode composition according to any one of [4] to [7].
[13] A vehicle,
A vehicle equipped with the nickel-metal hydride battery according to [12].

Specific examples that embody the disclosure of this specification are described below, but the disclosure of this specification is not limited to the following specific examples.

(Production of negative electrode material)
In this example, carbon materials having various specific surface areas and a binder were mixed to prepare a mixed raw material, and the mixed raw material was then compressed at various pressures to increase the density, thereby producing a carbon material as a negative electrode material.

As the carbon material, commercially available activated carbon (activated carbons 1 to 5) with the specific surface area and bulk density shown in Table 1 were procured and used to manufacture the negative electrode materials of Production Examples 1 to 5. In Production Examples 1 to 5, PTFE was used as a binder. Furthermore, in addition to activated carbons 1 to 5, activated carbon 6 shown in Table 1 was also used as a comparative production example (hereinafter simply referred to as the comparative example). The specific surface area and bulk density per unit mass of activated carbons 1 to 6 shown in Table 1 were measured by the following method. In the following explanation, the specific surface area and bulk density were measured by the following method.

<Specific surface area>
The activated carbon to be measured was placed in a measurement cell of a BELSORP-max manufactured by Microtrac-Bell Corporation, and the amount of nitrogen adsorbed onto the activated carbon was determined from the pressure change of the nitrogen introduced into the measurement cell using the gas state equation, and the specific surface area of the activated carbon was calculated from the adsorption isotherm assumed by the BET theory.

<Bulk density>
The material was molded into a disk of a specified diameter at a pressure of 0.1 MPa, the height and diameter of the molded carbon material were measured to determine the volume, and the mass of the material used was divided by the volume to calculate the volume.

Below, the negative electrode materials of Production Examples 1 to 5 are explained with reference to Table 2, and the negative electrode materials of Comparative Examples 1 to 6 are explained with reference to Table 3.

<Production Example 1>
A mixture (PTFE content 3% by mass) was prepared by mixing 0.485 g of activated carbon 1 and 0.015 g of PTFE. This mixture was placed in a 16 mm diameter mold of a hydraulic compression molding machine and molded under pressure at room temperature at the pressure shown in Table 2 to obtain a cylindrical molded body. The molded body was crushed using an agate mortar and pestle, and the crushed product was again pressed at the pressure shown in Table 2. This pressing operation was repeated a total of four times, and the final molded body was crushed using an agate mortar and pestle to measure its specific surface area and bulk density. The specific surface area per unit volume was also calculated. These results are shown in Table 2.

<Production Example 2>
A negative electrode material of Production Example 2 was prepared in the same manner as Production Example 1, except that activated carbon 2 was used and pressure was applied at the pressure and number of times shown in Table 2. The specific surface area and bulk density were measured, and the specific surface area per unit volume was calculated. These results are shown in Table 2.

<Production Examples 3 to 5>
0.294 g of activated carbon 3 to 5 and 0.016 g of PTFE were mixed to prepare each mixture (PTFE content: 2% by mass). The mixture was placed in a 10 mm diameter mold of a hydraulic compression molding machine, and compressed at the pressure and number of times shown in Table 2. The negative electrode materials of Production Examples 3 to 5 were prepared in the same manner as Production Example 1, and the specific surface area and bulk density were measured to calculate the specific surface area per unit volume. The results are shown in Table 2.

<Comparative Examples 1 to 7>
As shown in Table 3, the negative electrode materials of Comparative Examples 1 to 6 were obtained by using activated carbons 1 to 6 as they were. In Comparative Example 7, 0.294 g of activated carbon 6 and 0.016 g of PTFE were mixed to prepare a mixture (PTFE content: 2% by mass). The negative electrode material of Comparative Example 7 was prepared in the same manner as in Production Example 1, except that it was placed in a 10 mm diameter mold of a hydraulic compression molding machine and pressed at the pressure and number of times shown in Table 3, and the specific surface area and bulk density were measured, and the specific surface area per unit volume was calculated. These results are shown in Table 3.

As shown in Tables 2 and 3, the untreated activated carbons 1 to 5 of Comparative Examples 1 to 5 had a bulk density of 0.20 to 0.62 g/cm ^3. The negative electrode materials of Production Examples 1 to 5 obtained by pressurizing the activated carbons 1 to 5 all had a bulk density of 0.40 to 1.0 g/cm ³ , which was larger than that of the activated carbons used as raw materials. That is, the bulk density was increased by about 1.5 to 2.2 times due to pressurization. The untreated activated carbons 1 to 5 of Comparative Examples 1 to 5 had a specific surface area per unit mass of 750 to 3250 ^{m 2} /g, but the negative electrode materials of Production Examples 1 to 5 that were pressurized had a specific surface area of 750 to 2840 ^{m 2} /g, which was the same or smaller for each activated carbon. That is, there was a tendency for the specific surface area to be the same or slightly smaller due to pressurization.

(Manufacture of nickel-metal hydride batteries and evaluation of charge/discharge capacity)
In this example, negative electrodes were produced using the negative electrode materials of Production Examples 1 to 5 produced in Example 1 and Comparative Examples 1 to 7, and further, nickel-metal hydride batteries were produced and their charge/discharge capacities were evaluated.

A paste was prepared by adding 6% by mass of SBR latex to 84% by mass of the negative electrode material of the manufacturing example, 10% by mass of carbon black, and 20% by mass of the carbon black to prepare a negative electrode composition. This paste was applied to one side of a Ni porous body punched into a disk shape with a diameter of 20 mm, dried, and then pressed at a pressure of 27 MPa to prepare a negative electrode. In the same manner, negative electrodes were prepared using Comparative Examples 1 to 7. The amount of negative electrode material contained in this negative electrode was 0.06 to 0.09 g.

The prepared negative electrode was combined with a separator and a positive electrode and placed in a commercially available battery container (Takumi Giken, flat cell (with pressure sensor)). A nickel hydroxide electrode (nickel porous body filled with nickel hydroxide, disk-shaped with a diameter of 20 mm) was used as the positive electrode. A sulfonated polypropylene nonwoven fabric (circular, diameter 23 mm) was used as the separator. 0.3 ml of alkaline electrolyte (7N potassium hydroxide aqueous solution) was poured into the battery container.

The battery's positive electrode capacity was approximately 80 mAh, and the positive electrode capacity was in excess of the negative electrode capacity. In practical batteries, the positive electrode capacity is less than the negative electrode capacity, and the positive electrode capacity dominates. However, in this example, since the focus is on the performance of the negative electrode, the positive electrode capacity was in excess of the negative electrode capacity, and the negative electrode capacity dominates. Charge and discharge evaluation was performed using the following method.

<Charge/Discharge Evaluation Method>
The fabricated battery was charged and discharged at a constant current of 60 mA per 1 g of the mass of the negative electrode material in a thermostatic chamber at 25° C. The charge cut-off voltage was set to 1.6 V, and the discharge cut-off voltage was set to 1.0 V or 0.8 V, and charge and discharge evaluation was performed.

The results of the charge/discharge evaluation are shown in Tables 2 and 3. Figure 2 shows the relationship between the gravimetric charge/discharge capacity and the specific surface area per unit mass when the end-of-charge voltage is 1.6 V and the nominal discharge voltage is 1.0 V, and Figure 3 shows the relationship between the volumetric charge/discharge capacity and the specific surface area per unit volume under the same conditions.

Regarding hydrogen gas generation, the internal pressure of the battery was measured with a pressure sensor when the charging voltage reached 1.6 V during charging to evaluate whether or not hydrogen gas was generated. If the pressure measured by the pressure sensor was less than 0.005 MPa, it was evaluated as "no hydrogen gas generation," and if the measured value was 0.005 MPa or higher, it was evaluated as "hydrogen gas generation." The results are shown in Tables 2 and 3.

2 and 3, the activated carbons not subjected to pressure in Comparative Examples 1 to 6 (258 to 780 ^{m 2} /g, 0.20 to 0.63 g/ ^{cm 3} ) and the activated carbons subjected to pressure but with small specific surface areas and large bulk densities (400 m ² /g, 1.2 g/cm ³ ) had volumetric charge/discharge capacities of 6 to 16 mAh/cm ³ . The specific surface areas per unit volume in Comparative Examples 1 to 7 were 258 to 700 m ² /cm ³ , and none of the Comparative Examples satisfied the requirements of a specific surface area per unit volume of 700 m ² /cm ³ or more and a bulk density of 0.40 to 1.00 g/cm ³ or less.

On the other hand, as shown in Table 2 and Figures 2 and 3, the volumetric charge-discharge capacities of the negative electrode materials of Production Examples 1 to 5 were 17 to 35 mAh/ ^cm3 , which was 1.4 to 2.2 times higher than the volumetric charge-discharge capacities of the corresponding Comparative Examples 1 to 5. From these results, it was considered that the interaction between protons and carbon during charging was increased due to the higher density. In addition, the specific surface area per unit volume of Production Examples 1 to 5 was 700 ^m2 / ^cm3 to 1136 ^m2 / ^cm3 , and the bulk density was in the range of 0.40 to 1.0 g/ ^cm3 or less.

Based on the above, it is believed that by compressing carbon materials such as activated carbon that have a specific surface area per unit mass of a certain amount or more, densifying them and increasing their bulk density, it is possible to increase the interaction between protons and the carbon during charging, thereby increasing the charge/discharge capacity per unit volume of the nickel-metal hydride battery.

In addition, as shown in Tables 2 and 3, it was found that hydrogen gas was not generated by using a carbon material as the negative electrode material. In other words, it was thought that hydrogen gas was sufficiently adsorbed by the negative electrode material.

From the above, it was found that the negative electrode materials prepared in the Production Examples are useful as negative electrode materials for nickel-metal hydride batteries.

Claims (9)

The BET specific surface area [m ² /g] is 2600 [m ² /g] or more and 2840 [m ² /g] or less,
A carbon material having a bulk density of 0.40 g/cm ³ or more and 0.43 g/cm ³ or less, and a BET specific surface area per unit volume expressed by BET specific surface area [m ² /g] × bulk density [g/cm ³ ] of 1100 m ² /cm ³ or more;
A negative electrode material for nickel-metal hydride batteries comprising: A composition for the negative electrode of a nickel-metal hydride battery, comprising the negative electrode material according to claim 1 . The negative electrode composition according to claim 2 , further comprising a binder for the carbon material. The negative electrode composition according to claim 3 , wherein the binder is polytetrafluoroethylene. A method for producing a negative electrode material for a nickel-metal hydride battery, comprising the steps of:
a raw material preparation step of preparing a carbon material having a BET specific surface area per unit mass of 3000 m ² /g or more as a raw material;
a pressurizing step of pressurizing the mixture containing the carbon material to a BET specific surface area [m2 ^/ g] of 2600 [m2 ^/ g] or more and 2840 [m2 ^/ g] or less, a bulk density of 0.40 g/cm3 ^or more and 0.43 g/cm3 or ^less , and a BET specific surface area per unit volume expressed by BET specific surface area [ ^m2 /g] x bulk density [g/ ^cm3 ] of 1100 ^m2 /cm3 or ^more ;
A manufacturing method comprising: The manufacturing method according to claim 5, wherein the carbon material in the raw material preparation step is activated carbon having a BET specific surface area [m2 ^/ g] of 3000 [m2 ^/ g] or more and 3250 [m2 ^/ g] or less and a bulk density of 0.20 g/ ^cm3 or more and 0.26 g/cm3 ^or less . The method according to claim 5 or 6 , wherein the pressurizing step is a step of pressing the carbon material in the presence of a binder. A nickel-metal hydride battery,
A nickel-metal hydride battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, the negative electrode containing the negative electrode composition according to any one of claims 2 to 4 . A vehicle equipped with the nickel-metal hydride battery according to claim 8 .

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