JP2022114133A - Negative electrode and nickel hydrogen battery - Google Patents

Abstract

To improve a capacity of a nickel hydrogen battery using a carbon material as a negative electrode.SOLUTION: The present invention provides a negative electrode for a nickel hydrogen battery, containing a carbon material including graphene.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to negative electrodes and nickel-metal hydride batteries.

Hitherto, nickel metal hydride batteries (nickel metal hydride batteries) have been developed using a hydrogen storage alloy as a negative electrode and a nickel hydroxide electrode as a positive electrode. AB ₅ type alloys such as M _m Ni ₅ containing a rare earth element M are used as the hydrogen storage alloys used for the negative electrodes of the above batteries. However, since the negative electrode contains a rare earth element, there is a problem that the negative electrode material is expensive and resources are scarce. Moreover, there is a possibility that hydrogen gas is generated during charging.

On the other hand, a nickel-metal hydride battery using activated carbon as a negative electrode material is also known. For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2006-080335) discloses a nickel-hydrogen battery including a positive electrode containing nickel hydroxide and a negative electrode containing activated carbon. By using activated carbon, which is a carbon material, for the negative electrode, the cost of the negative electrode material can be reduced. In addition, as shown in FIG. 2, since electric charges are accumulated in the electric double layer formed at the interface between the negative electrode, the carbon material, and the electrolyte solution, hydrogen gas is not generated in principle.

JP 2006-080335 A

However, the nickel-metal hydride battery using a carbon material such as activated carbon as a negative electrode material has a problem of low discharge capacity.

An object of the present disclosure is to improve the capacity of a nickel-metal hydride battery using a carbon material as a negative electrode material.

[1] A negative electrode for a nickel-metal hydride battery containing a carbon material containing graphene.

By using a carbon material containing graphene as a negative electrode material, the capacity of a nickel-metal hydride battery can be improved. This is probably because protons can be adsorbed on both surfaces of graphene.
Also, for the same reason, the discharge start voltage of the nickel-metal hydride battery can also be improved.
In addition, the cost of the negative electrode material can be reduced by using the carbon material, which is abundant in resources and does not contain rare earth elements, as the negative electrode material.
In addition, generation of hydrogen gas during charging is suppressed by using a carbon material as the negative electrode material.

[2] The negative electrode according to [1] or [2], wherein the carbon material is formed by cross-linking a plurality of graphenes.

In this case, if the carbon material has a higher-order structure formed by cross-linking a plurality of graphenes, it is considered that the capacity is further improved due to the improvement of the specific surface area.

[3] The negative electrode according to [2], wherein the carbon material is obtained by alkali-activating petroleum coke.

[4] The negative electrode according to [3], wherein the carbon material is obtained by densifying petroleum coke by compressing it after alkali activation.

[5] A nickel-metal hydride battery comprising the negative electrode according to any one of [1] to [4], a positive electrode, and an electrolytic solution.

[6] The nickel-metal hydride battery according to [5], wherein the positive electrode contains nickel hydroxide.

According to the present disclosure, it is possible to improve the capacity of a nickel-metal hydride battery using a carbon material as a negative electrode material.

FIG. 1 is a conceptual diagram showing an example of a “carbon material containing graphene” used for the negative electrode of the present disclosure. FIG. 2 is a conceptual diagram showing reactions in a nickel-hydrogen battery using a carbon material as a negative electrode material. 3 is a graph showing charge-discharge curves of the test cell of Comparative Example 2. FIG. 4 is a graph showing internal pressure changes due to charging of the test cell of Comparative Example 2. FIG. 5 is a graph showing charge-discharge curves of the test cell of Example 1. FIG. 6 is a graph showing internal pressure changes due to charging of the test cell of Example 1. FIG. 7 is a graph showing the relationship between the specific surface area of the negative electrode and the capacity for Example 1, Comparative Examples 1 and 2. FIG. 8 is a graph showing the relationship between the specific surface area of the negative electrode and the average discharge voltage for Example 1, Comparative Examples 1 and 2. FIG. FIG. 9 is a schematic graph showing the difference in X-ray diffraction intensity curves between the carbon material of the embodiment and the conventional carbon material. FIG. 10 is a diagram showing the relationship between the bulk density of the carbon materials of Examples 1, 4 and 5 and the capacity density of the negative electrode produced using each carbon material. FIG. 11 is a schematic diagram showing an example of the configuration of a nickel-metal hydride battery.

Embodiments of the present disclosure will be described below. However, the following description does not limit the scope of the claims.

<Negative Electrode>
A negative electrode for a nickel-metal hydride battery of the present disclosure includes a carbon material including graphene.

(Carbon material containing graphene)
In the present disclosure, graphene is single-layer graphene having a thickness of one atom of carbon, preferably consisting of a plurality of sp2 ^- bonded carbon atoms. The specific surface area of graphene used in this embodiment is, for example, 2630 m ² /g.

The “carbon material containing graphene” (hereinafter sometimes simply referred to as “carbon material”) used in the present embodiment is preferably a carbon material (crosslinked graphene) formed by cross-linking a plurality of graphenes. . Note that the plurality of graphene crosslinks are preferably formed by sp ³ bonds. As such a carbon material, for example, as shown in FIG. 1, there is a carbon material in which a large number of graphenes (sp ² -bonded carbon) are crosslinked by sp ³ bonds. Note that the length of graphene is, for example, about 3.3 nm.

FIG. 9 is a schematic graph showing the difference in X-ray diffraction intensity curve between the carbon material (crosslinked graphene) of the embodiment and a conventional carbon material. As shown in FIG. 9, it is known that the X-ray diffraction intensity peak (0.337 nm) that appears in graphite, which is a stack of graphenes, disappears in crosslinked graphene.

The BET specific surface area of the carbon material is preferably 2600 m ² /g or more, more preferably 2800 m ² /g or more.

The carbon material is obtained, for example, by alkali-activating petroleum coke.
Alkali activation of petroleum coke can be carried out by, for example, mixing petroleum coke and an alkali agent, dehydrating and activating the mixture. As the alkali agent, for example, a hydroxide having an alkali metal capable of forming an intercalation compound (hydrous potassium hydroxide, etc.) can be used. Specifically, the alkali activation transforms the petroleum coke into a crosslinked body containing multiple graphenes. As a result, the graphene crosslinked structure shown in FIG. 1 is obtained.

Here, the blending amount of the alkaline agent is preferably 2 to 8 times, more preferably 4 to 6 times, the total amount of petroleum coke. In this case, a carbon material having a larger specific surface area can be obtained, and the capacity of the negative electrode can be further increased.
When the alkali agent used for alkali activation is hydrous potassium hydroxide (water content: 15% by mass), the amount of hydrous potassium hydroxide is 2 to 8 times the total amount of petroleum coke. Preferably, 4 to 6 times is more preferable.

The average particle size (D50) of the petroleum coke before activation used as a raw material for the carbon material is preferably 30 to 200 μm, more preferably 60 to 150 μm. The average particle size (D50) is the cumulative 50% particle size (D50) determined by volume frequency particle size distribution measurement using a laser diffraction/scattering particle size distribution analyzer conforming to JIS Z 8825.

In addition to the method of activating petroleum coke with alkali, for example, a method of activating coal, coconut husk, etc. with alkali can be used as the method of producing the carbon material containing graphene. Alternatively, petroleum coke, coal, coconut husk, etc. may be reacted with various acids (such as sulfuric acid) capable of forming intercalation compounds to form intercalation compounds, followed by heat treatment.

Said carbon material is used as a negative electrode active material in a negative electrode. That is, the negative electrode active material forming the negative electrode contains graphene. However, the negative electrode active material may contain an additive such as a conductive agent in addition to the carbon material containing graphene. Since the charge/discharge reaction requires exchange of electrons, reaction activity can be improved by including a conductive agent in the negative electrode active material.

Examples of conductive agents include nickel, copper, cobalt, bismuth, and graphite. Nickel, copper, cobalt, bismuth, etc. must be in a metallic state to function as a conductive agent. Graphite is not particularly limited as long as it has electrical conductivity, and carbon nanotubes, graphene, and the like, for example, are also applicable. These conductive agents not only improve conductivity but also stabilize metallic iron that is reduced and produced during charging.

The conductive agent may be simply mixed with the carbon material particles, but ideally, the conductive agent is introduced in the process of preparing the carbon material, thereby enabling contact with the carbon material at a finer level. Here, nickel, copper, cobalt, bismuth, etc. do not necessarily have to be added in the form of metals. can be reduced to a metallic state.

A negative electrode can be produced by a general method using the above carbon material.
For example, a paste containing a carbon material powder, a conductive agent, and a binder (SBR latex, polyvinylidene fluoride, etc.) is applied to a metal substrate such as a metal foil or a punching metal sheet, or filled in a metal porous body. By doing so, an electrode (negative electrode) can be produced.

Including graphene in the carbon material increases the capacity of the negative electrode and the capacity of the nickel-metal hydride battery. It is considered that this is because protons can be adsorbed on both surfaces of the graphene constituting the negative electrode during charging.
Furthermore, when the carbon material is a carbon material formed by cross-linking a plurality of graphenes (see FIG. 1), the battery capacity increases more reliably. This is probably because the crosslinked structure of graphene further increases the specific surface area of the carbon material (negative electrode active material), thereby increasing the amount of protons adsorbed.

The negative electrode of the present disclosure can be applied to nickel-metal hydride batteries using nickel hydroxide as a positive electrode active material. An example of a nickel-metal hydride battery using the negative electrode of the present disclosure will be described below.

<Nickel metal hydride battery>
The nickel-metal hydride battery (hereinafter sometimes abbreviated as "battery") of the present disclosure can be used, for example, as a battery for mobile devices, a battery for vehicles, a storage battery for renewable energy power generation, and the like. The battery may be a primary battery or a secondary battery. An example of the configuration of a nickel-metal hydride (Ni—H) battery will be described below with reference to the drawings.

FIG. 11 is a schematic diagram showing an example of the configuration of a nickel-metal hydride battery.
Battery 1 is a nickel metal hydride battery. Battery 1 includes housing 2 . The housing 2 is a cylindrical case. The housing 2 is made of metal. However, the housing 2 can have any form. The housing 2 may be, for example, a rectangular case. The housing 2 may be, for example, a pouch made of an aluminum laminate film. The housing 2 may be made of resin, for example.

The housing 2 accommodates the storage element 10 and an electrolytic solution. Storage element 10 includes positive electrode 11 , negative electrode 12 , and separator 13 . The illustrated storage element 10 is of the wound type. The storage element 10 is formed by spirally winding a strip-shaped electrode. The storage element 10 may be of a laminated type, for example. The power storage element 10 may be formed by stacking sheet-shaped electrodes, for example.

《Negative electrode》
The negative electrode 12 is sheet-like. The negative electrode 12 may have a thickness of, for example, 10 μm to 1 mm. The negative electrode 12 has a lower potential than the positive electrode 11 . The negative electrode 12 contains the above-described carbon material containing graphene as a negative electrode active material. The negative electrode 12 may consist essentially of a carbon material (negative electrode active material).

The negative electrode 12 may further contain a current collector, a binder, and the like in addition to the negative electrode active material. The current collector may include, for example, punching metal, metal foil, porous metal sheet, and the like. The current collector may be made of Ni, for example.

For example, it can be produced by coating a current collector with a negative electrode active material and a binder. The binder binds the current collector and the negative electrode active material. The binder can contain optional ingredients. The binder may contain, for example, at least one selected from the group consisting of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE) and acrylic resin. The blending amount of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

《Positive electrode》
The positive electrode 11 is sheet-like. The positive electrode 11 may have a thickness of, for example, 10 μm to 1 mm. The positive electrode 11 has a higher potential than the negative electrode 12 . The positive electrode 11 contains a positive electrode active material. The positive electrode active material can contain any component. Examples of positive electrode active materials include nickel hydroxide, manganese dioxide, and silver oxide. The positive electrode active material is preferably nickel hydroxide.

The positive electrode 11 may consist essentially of the positive electrode active material. The positive electrode 11 may further contain a current collector, a conductive material, a binder, etc. in addition to the positive electrode active material. The current collector may include, for example, a porous metal sheet or the like. The current collector is made of Ni, for example.

For example, the positive electrode 11 can be formed by coating a current collector with a positive electrode active material, a conductive material, and a binder. The conductive material has electronic conductivity. The conductive material can contain any component. The conductive material may contain, for example, carbon black, Co, cobalt oxide, and the like. The blending amount of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material. The binder binds the current collector and the positive electrode active material. The binder can contain optional ingredients. The binder may include, for example, ethylene vinyl acetate (EVA) and the like. The blending amount of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material.

《Separator》
The separator 13 is sheet-like. A separator 13 is arranged between the positive electrode 11 and the negative electrode 12 . The separator 13 physically separates the positive electrode 11 and the negative electrode 12 . Separator 13 may, for example, have a thickness of 50-500 μm. Separator 13 is porous. The separator 13 may contain, for example, an extended porous membrane, a non-woven fabric, or the like. Separator 13 is electrically insulating. The separator may be made of, for example, polyolefin, polyamide, or the like.

《Electrolyte》
Although the electrolytic solution is not particularly limited, it is preferably an aqueous electrolytic solution. As the aqueous electrolytic solution, for example, an alkaline aqueous solution or the like can be suitably used. The alkaline aqueous solution contains, for example, water and an alkali metal hydroxide dissolved in water. The alkali metal hydroxide may, for example, have a concentration of 1-20 mol/L. Examples of alkali metal hydroxides include potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide (LiOH) and the like.

Examples of the present disclosure are described below. However, the following description does not limit the scope of the claims.

(Example 1)
In this example, petroleum coke (average particle size (D50): 60 μm) was used as a raw material for the negative electrode material. Hydrous potassium hydroxide (moisture content: about 15% by mass) was added in a weight ratio of 5 times that of the petroleum coke and mixed to obtain a mixture. The resulting mixture was held at 300-500°C for 60 minutes, dehydrated, and then the mixture was activated by holding at 600-800°C for 120 minutes. After the activation, the mixture was cooled, thoroughly washed, and then potassium hydroxide was removed to obtain a “carbon material containing graphene”.

The measured value of the specific surface area of the obtained carbon material was 3000 m ² /g. A high-precision specific surface area measuring device (BELSORP-max) manufactured by Microtrac Bell Co., Ltd. was used to measure the specific surface area (the same applies to the following examples and comparative examples).
The specific surface area of graphene is 2630m ² /g (Yoshitsugu Kojima, Takayuki Ichikawa, Carbon-based Hydrogen Storage Materials, “Development Trends in Adsorbents and Adsorption Processes – For Solving Energy and Environmental Problems,” edited by Hirofumi Kano, CMC Publishing, pp.190-199, 2014), and the specific surface area of the above carbon material is larger than this, so it is thought that graphene forms a higher-order structure in the above carbon material (Fig. 1).
It is also known that the carbon material shrinks when pressurized, and expands and returns to its original volume when the pressure is lowered. From this, it is considered that the carbon material has a crosslinked structure.

A paste was prepared by adding 6% by mass of SBR latex to 84% by mass of the carbon material (crosslinked graphene) obtained as described above and 10% by mass of carbon black powder.
The paste was applied to one side of a Ni porous body punched into a disk shape of 20 mm in diameter, dried, and pressed at a pressure of 27 MPa to prepare an electrode (negative electrode). The amount of negative electrode active material contained in one negative electrode was 0.06 to 0.09 g.

The thus produced negative electrode was combined with a separator and a positive electrode and set in a commercially available battery container (Takumi Giken, flat cell (with pressure sensor)). As the separator, a sulfonated polypropylene non-woven fabric (circular shape, diameter 23 mm) used in ordinary nickel-metal hydride batteries was used. As the positive electrode, a nickel hydroxide electrode (nickel porous body filled with nickel hydroxide, disc-shaped with a diameter of 20 mm) used in a normal nickel-metal hydride battery was used.
0.2 mL of an alkaline electrolyte (6 N (6N) potassium hydroxide aqueous solution) was injected into the battery container. The positive electrode capacity is about 80 mAh, making the positive electrode capacity in excess of the negative electrode capacity. In a practical battery, the positive electrode capacity is made smaller than the negative electrode capacity to control the positive electrode capacity, but in this example, attention is paid to the performance of the negative electrode. .

In this manner, a nickel-metal hydride battery (test cell) of Example 1 was assembled including the negative electrode described above.

(Example 2)
In this example, the added amount of hydrous potassium hydroxide (water content of about 15%) was changed to three times the weight ratio of petroleum coke. A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.
The measured value of the specific surface area of the carbon material used in Example 2 was 2700 m ² /g.

(Example 3)
The amount of hydrated potassium hydroxide (moisture content: about 15%) added was changed to 9 times the weight of petroleum coke. A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.
The measured value of the specific surface area of the carbon material used in Example 3 was 2500 m ² /g.

(Example 4)
The carbon material (crosslinked graphene, bulk density: 0.26 g/cm ³ ) obtained in the same manner as in Example 1 was subjected to pressure compression treatment, resulting in a bulk density of 0.3 g/cm ³ . A densified graphene crosslinked body was obtained. In addition, the pressurization pressure was set to 300 MPa in the pressurization compression process. A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.

(Example 5)
In this example, a carbon material (crosslinked graphene) obtained in the same manner as in Example 1 and Teflon (registered trademark) powder (3% by mass with respect to the total amount of crosslinked graphene) were mixed in a mortar. As a result, a dispersion was obtained in which fibrillar Teflon (registered trademark) was dispersed in the crosslinked graphene. By subjecting this dispersion to the same pressure compression treatment (130 MPa) as in Example 4, a densified graphene crosslinked body having a bulk density of 0.4 g/cm ³ was obtained. A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.

(Comparative example 1)
In Comparative Example 1, Osaka Gas Chemical's activated carbon "GTSX" was used instead of the carbon material. The measured specific surface area of this activated carbon was 630 m ² /g. A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.

(Comparative example 2)
In Comparative Example 2, activated carbon made from pulverized graphite (milled graphite) was used as the negative electrode material.
First, 300 mg of graphite powder (99.9995%, Alfa Aesor) and 20 ZrO ₂ balls were placed in a milling container, and after the inside of the container was evacuated, the inside of the container was filled with hydrogen gas of 1 MPa. Milling graphite was prepared by grinding graphite powder in a container for 80 hours using a Fritsche planetary ball mill. In a glove box filled with Ar gas, 1 mL of 8N KOH solution was added to the container containing the milled graphite, and the milled graphite in the container was milled for 12 hours with a planetary ball milling apparatus. A sample (milled graphite) was removed from the milling vessel in air. Washing with ion-exchanged water and removal of ion-exchanged water by suction filtration were repeated for the sample, and these washing operations were repeated until the pH of the waste liquid of ion-exchanged water reached pH8. After that, the sample was dried overnight by vacuuming at room temperature to prepare activated carbon (carbon material) used in Comparative Example 2. The measured specific surface area of the obtained activated carbon was 130 m ² /g.
A negative electrode and a nickel-metal hydride battery (test cell) were produced in the same manner as in Example 1 except for the above.

〔evaluation〕
The test cells of Examples 1-3 and Comparative Examples 1-2 were subjected to constant current charge/discharge at 0.1 C with a current of 1.6 mA/cm ² in a closed container. Note that "C" is the unit of current rate. "1C" indicates the current rate at which the SOC (state of charge) reaches from 0% to 100% by charging for 1 hour. SOC is the ratio of the amount of charge to the charge capacity of the battery. The final discharge voltage was set to 0.8V.

During the charging and discharging, the charge capacity and discharge capacity (mAh/g) of the test cell (battery) were measured. The discharge capacity is the capacity of the battery (capacity per 1 g of carbon constituting the negative electrode: capacity density) at the end of discharge (voltage: 0.8 V). The average discharge voltage (V) is the arithmetic mean of the discharge start voltage and the discharge end voltage (0.8V). The charge capacity is the capacity of the battery (capacity per 1 g of carbon constituting the negative electrode) when the charge voltage reaches 1.6 V during charging.

In addition, when the charging voltage reached 1.6 V during charging, the internal pressure of the battery was measured with a pressure sensor to evaluate the presence or absence of hydrogen gas generation. When the internal pressure measured by the pressure sensor was less than 0.005 MPa, it was evaluated as "no hydrogen gas generation", and when it was 0.005 MPa or more, it was evaluated as "yes" hydrogen gas generation.

Table 1 shows the evaluation results of "presence or absence of hydrogen generation during charging", "discharge capacity" and "average discharge voltage" in the above evaluation.
3 shows charge-discharge curves of the test cell of Comparative Example 2. As shown in FIG. FIG. 5 shows charge-discharge curves of the test cell of Example 1. As shown in FIG.
Further, FIG. 4 shows changes in internal pressure due to charging of the test cell of Comparative Example 2. As shown in FIG. FIG. 6 shows changes in internal pressure due to charging of the test cell of Example 1. As shown in FIG.
7 shows the relationship between the specific surface area of the negative electrode and the capacity for Example 1, Comparative Examples 1 and 2. As shown in FIG.
8 shows the relationship between the specific surface area of the negative electrode and the average discharge voltage for Example 1, Comparative Examples 1 and 2. In FIG.

From the results shown in Table 1 (whether or not hydrogen is generated during charging), it can be seen that hydrogen gas is not generated during charging by using a carbon material as the negative electrode material as in the above examples and comparative examples ( 4 and 6).

Further, from the results (discharge capacity and average discharge voltage) shown in Table 1, in the batteries (test cells) of Examples 1 to 3 corresponding to the nickel-metal hydride batteries of the present disclosure, the graphene-containing carbon material (alkali-activated The discharge capacity is 50 mAh/g or more, and the average discharge voltage is 1.1 V or more (see FIG. 5 for Example 1). On the other hand, in the battery using conventional activated carbon as the negative electrode material (Comparative Example 1) and the battery using milled graphite as the negative electrode material (Comparative Example 2), the discharge capacity was smaller than that of the example, and the average discharge voltage was 1. 0.1 V or less, which is lower than that of the example (see FIG. 3 for comparative example 2).

This is probably because, in the example, protons adsorbed on both surfaces of the graphene contained in the carbon material constituting the negative electrode during charging, thereby increasing the capacity of the negative electrode and improving the discharge starting voltage.

Furthermore, from the results shown in FIG. 7, it can be seen that the larger the specific surface area of the negative electrode (the carbon material constituting the negative electrode), the larger the battery capacity (charge capacity and discharge capacity).

Also, from the results shown in FIG. 8, it can be seen that the average discharge voltage of the battery increases as the specific surface area of the negative electrode (carbon material constituting the negative electrode) increases.

Further, for the test cells of Examples 1, 4 and 5, the discharge capacity (capacity per 1 g of carbon constituting the negative electrode: capacity density) was measured in the same manner as described above.
FIG. 10 shows the relationship between the bulk density of the carbon materials (graphene crosslinked bodies) of Examples 1, 4 and 5 and the discharge capacity (capacity density) of the negative electrode produced using each carbon material.
As shown in FIG. 10, it can be seen that the discharge capacity increases for Examples 3 and 4 in which the carbon material (crosslinked graphene) is densified by compression. In addition, in the test cells of Examples 1, 4 and 5, the discharge voltage was about 1.5 V regardless of the bulk density of the carbon material.

The embodiments and examples disclosed this time are illustrative in all respects and are not restrictive. The technical scope defined by the description of the claims includes all changes in the meaning of equivalents to the claims. The technical scope determined by the description of the claims includes all modifications within the scope of equivalents of the description of the claims.

1 battery, 10 storage element, 11 positive electrode, 12 negative electrode, 13 separator, 2 housing.

Claims (6)

A negative electrode for a nickel-metal hydride battery, comprising a carbon material containing graphene. 2. The negative electrode according to claim 1, wherein the carbon material is formed by cross-linking a plurality of graphenes. 3. The negative electrode according to claim 1, wherein the carbon material is obtained by alkali-activating petroleum coke. 4. The negative electrode according to claim 3, wherein the carbon material is obtained by densifying petroleum coke by compressing it after alkali activation. A nickel-metal hydride battery comprising the negative electrode according to any one of claims 1 to 4, a positive electrode, and an electrolytic solution. 6. The nickel-metal hydride battery of claim 5, wherein said positive electrode comprises nickel hydroxide.

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