Iron-carbon composite material, its manufacturing method, anode and nickel-metal hydride battery

Abstract

To provide a material for a negative electrode of a nickel-hydrogen battery containing iron, which is stable under the atmosphere and can suppress the generation of hydrogen gas during charging.SOLUTION: An iron-carbon composite material contains an oxide of iron on at least a part of the surface.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an iron-carbon composite material, a method for producing the same, a negative electrode, and a nickel-metal hydride battery. [Background technology]

[0002] To date, nickel-metal hydride batteries (nickel metal hydride batteries) have been developed that use a hydrogen storage alloy as the negative electrode and nickel hydroxide as the positive electrode (see, for example, Patent Document 1: JP 2018-045942 A). As the hydrogen storage alloy used in the negative electrode of such batteries, M containing a rare earth element M is used. m AB5-type alloys such as Ni5 are used. However, because the anode contains rare earth elements, the anode material is expensive and there is a problem that resources are scarce. In addition, there is a possibility that hydrogen gas will be generated during charging.

[0003] On the other hand, it is theoretically possible to use iron, a relatively inexpensive and abundant resource, for the negative electrode (negative electrode active material) of nickel-metal hydride batteries. Batteries using iron for the negative electrode have the advantage of having a higher charge / discharge voltage and potentially a larger theoretical capacity. However, iron has a higher ionization tendency (lower redox potential) than the hydrogen storage alloys mentioned above, making it more likely that a reaction will occur between iron and H2O to produce iron hydroxide and hydrogen gas. Nickel-metal hydride batteries using iron negative electrodes are therefore more likely to produce hydrogen gas during charging. For this reason, currently, batteries using iron for the negative electrode are rarely put to practical use.

[0004] A method for obtaining Fe-CH compounds (iron-carbon compounds with adsorbed hydrogen) has been proposed, in which graphite is crushed using steel balls in a hydrogen atmosphere (Non-Patent Document 1: H. Miyaoka et al., Anomalous hydrogen absorption on non-stoichiometric iron-carbon compounds, J. Alloys Compd., 507, 547-550, 2010). This Fe-CH compound is thought to be a dispersion of nanoscale iron in carbon due to the hydrogen embrittlement of iron. The Fe-CH compound contains approximately 15% iron by mass and approximately 6% hydrogen by mass, making it a promising material for storing more hydrogen than conventional hydrogen storage alloys. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-045942 [Non-patent literature]

[0006] [Non-Patent Document 1] H. Miyaoka et al., Anomalous hydrogen absorption on non-stoichiometric iron-carbon compound, J. Alloys Compd., 507, 547-550, 2010 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the material (Fe—CH compound) of Non-Patent Document 1 is unstable in the atmosphere and may ignite, making it difficult to apply to the negative electrode of a nickel-metal hydride battery.

[0008] An object of the present disclosure is to provide an iron-containing material for the negative electrode of a nickel-metal hydride battery that is stable in the atmosphere and capable of suppressing the generation of hydrogen gas during charging. [Means for solving the problem]

[0009] [1] An iron-carbon composite material containing iron oxide on at least a portion of its surface.

[0010] [2] A method for producing an iron-carbon composite material containing iron oxide on at least a portion of its surface, comprising: A first step of pulverizing a carbon material in a hydrogen gas atmosphere using a ball mill with iron-containing balls to prepare an Fe—CH compound containing iron, carbon, and hydrogen; a second step of oxidizing the Fe—CH compound to produce an iron-carbon composite material containing an oxide of iron on at least a portion of its surface.

[0011] [3] A method for producing an iron-carbon composite material containing iron oxide on at least a portion of its surface, comprising: A first step of pulverizing a carbon material and iron powder in a ball mill using iron-containing balls in a hydrogen gas atmosphere to prepare an Fe—CH compound containing iron, carbon, and hydrogen; a second step of oxidizing the Fe—CH compound to produce an iron-carbon composite material containing an oxide of iron on at least a portion of its surface.

[0012] [4] The method according to [2] or [3], wherein the second step is carried out by mixing the Fe—CH compound with an alkaline solution in an oxygen-free atmosphere.

[0013] [5] An iron-carbon composite material produced by the production method according to any one of [2] to [4].

[0014] [6] The iron-carbon composite material according to [1] or [5], which is used as a material for the negative electrode of a nickel-metal hydride battery.

[0015] [7] A negative electrode for a nickel-metal hydride battery, comprising the iron-carbon composite material according to [1] or [5].

[0016] [8] A nickel-metal hydride battery comprising the negative electrode according to [7], a positive electrode, and an electrolyte.

[0017] [9] The nickel-metal hydride battery according to [8], wherein the positive electrode contains nickel hydroxide.

[0018] By using an iron-carbon composite material as the negative electrode material for a nickel-metal hydride battery, the carbon can adsorb the hydrogen gas generated by the iron, thereby suppressing the generation of hydrogen gas during charging of the nickel-metal hydride battery. Furthermore, since the iron-carbon composite material contains iron oxide on at least a portion of its surface, the iron-carbon composite material becomes stable in the atmosphere. Furthermore, by using an iron-carbon composite material, which is an abundant resource and does not contain rare earth elements, as the anode material, the cost of the anode material can be reduced. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to provide an iron-containing material for the negative electrode of a nickel-metal hydride battery that is stable in the atmosphere and capable of suppressing the generation of hydrogen gas during charging. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing the X-ray diffraction intensity curve of the iron-carbon composite material produced in Example 1. [Figure 2] FIG. 2 is a graph showing the X-ray diffraction intensity curve of the Fe—CH compound prepared in Reference Example 1. [Figure 3] FIG. 3 is a schematic diagram showing an example of the configuration of a nickel-metal hydride battery. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present disclosure will be described. However, the following description does not limit the scope of the claims.

[0022] <Iron-carbon composite material> In the present disclosure, the iron-carbon composite material contains iron oxide on at least a portion of its surface. For example, the iron-carbon composite material has at least a portion of its surface coated with iron oxide. The present inventors have confirmed that the iron-carbon composite material of the present disclosure is stable under atmospheric conditions, and it is presumed that the iron oxide coating on the surface of the iron-carbon composite material is one factor in its stabilization (passivation).

[0023] The iron-carbon composite material is preferably a porous material. The iron-carbon composite material preferably has a BET specific surface area of ​​100 m 2 / g or more, more preferably 200m 2 / g or more.

[0024] In addition, it is preferable that iron particles are dispersed in the carbon material in the iron-carbon composite material, which allows the carbon to more reliably adsorb hydrogen gas generated by the iron, and more reliably suppresses the generation of hydrogen gas during charging in nickel-metal hydride batteries.

[0025] <Method of manufacturing iron-carbon composite material> The method for producing an iron-carbon composite material according to the present disclosure includes at least the following first and second steps.

[0026] (1st step) In the first step, a carbon material is pulverized in a hydrogen gas atmosphere by a ball mill using iron-containing balls to produce an Fe—CH compound containing iron, carbon, and hydrogen.

[0027] In the first step, the carbon material is not particularly limited as long as it contains carbon, and examples that can be used include graphite powder, activated carbon, carbon nanotubes, and graphite nanofibers.

[0028] The shape of the carbon material is not particularly limited, but is preferably a powder. In this case, the average particle size (D50) of the carbon material is preferably 1 to 100 μm, more preferably 5 to 30 μm. The average particle size (D50) here refers to the cumulative 50% particle size (D50) determined by volume frequency particle size distribution measurement using a laser diffraction / scattering particle size distribution analyzer in accordance with JIS Z 8825.

[0029] As the balls containing iron, for example, steel balls can be used.

[0030] As the ball mill, for example, a Fritsche planetary ball mill, a vibration mill, or the like can be used.

[0031] Specifically, the first step can be carried out by the method disclosed in Non-Patent Document 1, for example.

[0032] The carbon material used as a raw material in the first step may be pulverized together with other materials such as iron, within the scope of the effect of this embodiment. That is, for example, in the first step, the carbon material and iron powder may be pulverized in a ball mill using iron-containing balls in a hydrogen gas atmosphere to produce an Fe—CH compound containing iron, carbon, and hydrogen.

[0033] (2nd process) In the second step, the Fe—CH compound is oxidized to prepare an iron-carbon composite material containing iron oxide on at least a portion of its surface.

[0034] The oxidation of the Fe—CH compound is preferably carried out by a slow oxidation process that does not result in combustion.

[0035] Specifically, the second step is preferably carried out by mixing (contacting) the Fe-CH compound with the alkaline solution in an oxygen-free atmosphere. In this case, an iron-carbon composite material containing iron oxide on at least a portion of its surface can be more reliably produced. The oxygen-free atmosphere is not particularly limited, but examples include an inert gas atmosphere such as Ar gas. It is known that reacting only iron with the alkaline solution can result in combustion.

[0036] The alkaline solution used in this case is not particularly limited, but is preferably an alkaline solution with a pH of 14 or higher, such as an aqueous potassium hydroxide solution, an aqueous sodium hydroxide solution, or an aqueous lithium hydroxide solution. It is believed that the iron in the Fe-CH compound is oxidized by this alkaline solution, thereby producing an iron-carbon composite material containing iron oxide on at least a portion of its surface.

[0037] The amount of alkaline solution used in the second step is not particularly limited as long as it is an amount that can sufficiently oxidize the Fe—CH compound, but for example, it is preferably 1 to 20 times, and more preferably 2 to 10 times, the amount of the Fe—CH compound in mass ratio. More specifically, for example, when the alkaline solution is a potassium hydroxide aqueous solution (concentration: 6 to 8 normal, 27 to 34 mass%), the amount of the potassium hydroxide aqueous solution is preferably 1 to 10 times, and more preferably 3 to 5 times, the amount of the Fe—CH compound in mass ratio. In this case, an iron-carbon composite material containing iron hydroxide on the surface can be obtained more reliably.

[0038] Furthermore, it is more preferable to mix the carbon material with an alkaline solution while milling (performing alkaline milling). In this case, an iron-carbon composite material containing iron oxide on at least a portion of its surface can be more reliably produced. The alkaline milling is also performed in an oxygen-free atmosphere. The alkali treatment milling can be carried out using, for example, a planetary ball milling apparatus. The time for the alkali treatment milling is not particularly limited, but is preferably 1 to 20 hours, more preferably 5 to 15 hours.

[0039] In principle, it is possible to oxidize Fe-CH compounds in an oxygen-containing atmosphere. However, because Fe-CH compounds burn when exposed to the air, oxidation must be performed under conditions that prevent combustion (violent oxidation), such as by using a gas with a sufficiently low oxygen content.

[0040] (Other processes) The iron-carbon composite material obtained in the second step is usually further subjected to a washing step (third step) and a drying step (fourth step) after the washing step.

[0041] The third step (washing step) can be carried out, for example, by removing the iron-carbon composite material obtained in the second step from the milling container in the atmosphere and washing it with ion-exchanged water. For example, the washed iron-carbon composite material may be subjected to suction filtration to remove the ion-exchanged water, and then washed again. This operation may be repeated until the waste liquid reaches a pH of 8, for example.

[0042] The fourth step (drying step) can be carried out, for example, by evacuating the iron-carbon composite material washed in the third step at room temperature.

[0043] In this way, an iron-carbon composite material that can be used as a material for the negative electrode of a nickel-metal hydride battery can be produced.

[0044] <Negative electrode> The negative electrode for a nickel-metal hydride battery of the present disclosure comprises the iron-carbon composite material described above.

[0045] The iron-carbon composite material is used as a negative electrode active material in a negative electrode. That is, the negative electrode active material constituting the negative electrode includes the iron-carbon composite material. However, the negative electrode active material may also include additives such as a conductive agent in addition to the iron-carbon composite material. Because charge-discharge reactions require the exchange of electrons, the reaction activity can be improved by further including a conductive agent in the negative electrode active material.

[0046] Examples of conductive agents include nickel, copper, cobalt, bismuth, and graphite. Nickel, copper, cobalt, bismuth, and the like must be in a metallic state to function as conductive agents. Graphite is not particularly limited as long as it has electrical conductivity; for example, carbon nanotubes and graphene can also be used. These conductive agents not only improve conductivity but also stabilize the metallic iron that is reduced and generated during charging.

[0047] The conductive agent may simply be mixed with the iron-carbon composite, but ideally, it would be introduced during the preparation process of the iron-carbon composite, allowing for finer contact with the iron-carbon composite. Here, nickel, copper, cobalt, bismuth, and the like do not necessarily need to be added in their metallic state; if they are added in the form of compounds such as salts, oxides, or hydroxides, they can be reduced to their metallic state when the battery is assembled and charged.

[0048] The above iron-carbon composite material can be used to prepare a negative electrode by a general method. For example, an electrode (negative electrode) can be produced by applying a paste containing iron-carbon composite powder, a conductive agent, and a binder (SBR latex, polyvinylidene fluoride, etc.) to a metal substrate such as metal foil or punched metal sheet, or by filling it into a porous metal body.

[0049] The negative electrode of the present disclosure can be applied to a nickel-metal hydride battery that uses nickel hydroxide as a positive electrode active material, etc. An example of a nickel-metal hydride battery that uses the negative electrode of the present disclosure will be described below.

[0050] <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 portable devices, an in-vehicle battery, or a storage battery for renewable energy power generation. 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.

[0051] FIG. 3 is a schematic diagram showing an example of the configuration of a nickel-metal hydride battery. The battery 1 is a nickel-metal hydride battery. The battery 1 includes a housing 2. The housing 2 is a cylindrical case. The housing 2 is made of metal. However, the housing 2 may have any shape. The housing 2 may be, for example, a rectangular case. The housing 2 may be, for example, a pouch made of aluminum laminate film. The housing 2 may be, for example, made of resin.

[0052] The housing 2 houses an electricity storage element 10 and an electrolyte. The electricity storage element 10 includes a positive electrode 11, a negative electrode 12, and a separator 13. The illustrated electricity storage element 10 is a wound type. The electricity storage element 10 is formed by spirally winding strip-shaped electrodes. The electricity storage element 10 may be, for example, a laminated type. The electricity storage element 10 may be, for example, formed by stacking sheet-shaped electrodes.

[0053] 《Negative electrode》 The negative electrode 12 is in the form of a sheet. 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-mentioned iron-carbon composite material as a negative electrode active material. The negative electrode 12 may be made essentially of only the iron-carbon composite material (negative electrode active material).

[0054] In addition to the negative electrode active material, the negative electrode 12 may further include a current collector, a binder, etc. The current collector may include, for example, a punched metal, a metal foil, a porous metal sheet, etc. The current collector may be made of, for example, Ni.

[0055] For example, the negative electrode active material and the binder may be applied to a current collector. The binder binds the current collector and the negative electrode active material. The binder may contain any component. For example, the binder may contain at least one selected from the group consisting of styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE), and acrylic resin. The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of the negative electrode active material.

[0056] 《Positive electrode》 The positive electrode 11 is in the form of a sheet. 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 includes a positive electrode active material. The positive electrode active material may include any component. Examples of the positive electrode active material include nickel hydroxide, manganese dioxide, and silver oxide. The positive electrode active material is preferably nickel hydroxide.

[0057] The positive electrode 11 may be substantially composed of only a positive electrode active material. The positive electrode 11 may further include a current collector, a conductive material, a binder, and the like in addition to the positive electrode active material. The current collector may include, for example, a porous metal sheet. The current collector is made of, for example, Ni.

[0058] For example, the positive electrode 11 can be formed by applying a positive electrode active material, a conductive material, and a binder to a current collector. 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, etc. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass per 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 any component. The binder may contain, for example, ethylene vinyl acetate (EVA), etc. The amount of the binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of the positive electrode active material.

[0059] <Separator> The separator 13 is in the form of a sheet. The separator 13 is disposed between the positive electrode 11 and the negative electrode 12. The separator 13 physically separates the positive electrode 11 and the negative electrode 12. The separator 13 may have a thickness of, for example, 50 to 500 μm. The separator 13 is porous. The separator 13 may include, for example, a stretched porous film, a nonwoven fabric, or the like. The separator 13 is electrically insulating. The separator may be made of, for example, polyolefin, polyamide, or the like.

[0060] 《Electrolyte》 The electrolyte is not particularly limited, but is preferably an aqueous electrolyte. For example, an alkaline aqueous solution can be suitably used as the aqueous electrolyte. The alkaline aqueous solution contains, for example, water and an alkali metal hydroxide dissolved in the water. The alkali metal hydroxide may have a concentration of, for example, 1 to 20 mol / L. Examples of the alkali metal hydroxide include potassium hydroxide (KOH), sodium hydroxide (NaOH), and lithium hydroxide (LiOH). [Example]

[0061] Hereinafter, embodiments of the present disclosure will be described. However, the following description does not limit the scope of the claims.

[0062] Example 1 In this example, a test cell of a nickel-metal hydride battery was fabricated using the above iron-carbon composite material as the negative electrode material.

[0063] First, an Fe-CH compound containing iron, carbon, and hydrogen was prepared according to the method described in Non-Patent Document 1 (H. Miyaoka et al., Anomalous hydrogen absorption on non-stoichiometric iron-carbon compounds, J. Alloys Compd., 507, 547-550, 2010). Specifically, 300 mg of graphite powder (99.9995%, Alfa Aesor) and 20 steel balls were placed in a milling container, which was then evacuated and filled with 1 MPa of hydrogen gas. The graphite powder was then milled in a Fritsche planetary ball mill for 80 hours to prepare the Fe-CH compound. Next, in a glove box filled with Ar gas, 1 mL of 8N KOH solution was added to the milling container containing the Fe-CH compound, and the Fe-CH compound in the container was milled for 12 hours using a planetary ball milling machine. The sample (Fe-CH compound) was removed from the milling container in the atmosphere. The sample was repeatedly washed with ion-exchanged water and then removed by suction filtration. These washing procedures were repeated until the pH of the waste ion-exchanged water reached 8. The sample was then dried overnight under vacuum at room temperature. This resulted in an iron-carbon composite material (BET specific surface area: 500 m) stabilized in the atmosphere. 2 / g) was produced.

[0064] A paste was prepared by mixing 84% by mass of the obtained iron-carbon composite material, 10% by mass of carbon black powder, and 6% by mass of SBR latex. The paste was applied to one side of a nickel 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 an electrode (negative electrode). The amount of negative electrode active material contained in one negative electrode was approximately 0.1 g.

[0065] The negative electrode thus prepared was combined with a separator and a positive electrode and placed in a commercially available battery container (Takumi Giken, Flat Cell (with pressure sensor)). The separator was a sulfonated polypropylene nonwoven fabric (circular, 23 mm diameter) used in ordinary nickel-hydrogen batteries. The positive electrode was a nickel hydroxide electrode (nickel porous material filled with nickel hydroxide, disc-shaped, 20 mm diameter) used in ordinary nickel-hydrogen batteries. 0.2 mL of alkaline electrolyte (6N potassium hydroxide aqueous solution) was poured into the battery container. The positive electrode capacity was approximately 70 mAh, and the positive electrode capacity was in excess of the negative electrode capacity. In practical batteries, the positive electrode capacity is made smaller than the negative electrode capacity, and the positive electrode capacity is dominant. However, in this example, since the focus is on the performance of the negative electrode, the positive electrode capacity was made in excess of the negative electrode capacity, and the negative electrode capacity was dominant.

[0066] In this manner, a nickel-metal hydride battery (test cell) of Example 2 was assembled, which was provided with the above negative electrode.

[0067] (Reference example 1: Fe-CH compound) An Fe-CH compound was produced in the same manner as in Example 1. Specifically, 300 mg of graphite powder (99.9995%, Alfa Aesor) and 20 steel balls were placed in a milling container, which was then evacuated and filled with hydrogen gas at 1 MPa. The graphite powder was then milled in a Fritsche planetary ball mill for 80 hours to produce the Fe-CH compound. This Fe-CH compound was found to be unstable and ignite in the atmosphere, so it could not be used as a battery anode.

[0068] (Comparative Example 1) In Comparative Example 1, a test cell of a nickel-metal hydride battery was fabricated using a hydrogen storage alloy as the negative electrode material.

[0069] First, a mixture containing La, Ce, Pr, Nd, Ni, Co, Mn, and Al in a predetermined ratio was melted in an arc melting furnace under an argon gas atmosphere to obtain an AB5-type hydrogen storage alloy (La 0.24 Ce 0.54 Pr 0.054 Nd 0.16 Ni 3.99 Co 0.60 Mn 0.36 Al 0.053 ) was produced. Next, a negative electrode was produced in the same manner as in Example 1, using the obtained hydrogen storage alloy in place of the iron-carbon composite material, and a nickel-metal hydride battery (test cell) was produced.

[0070] Example 2 A mixture of graphite and iron powder (particle size 3-5 μm, iron powder blending ratio: 50 mass%) was milled with steel balls under a hydrogen atmosphere. The milled mixture was then inactivated by treating it with an aqueous potassium hydroxide solution, washing and filtering (pH: 8), and drying, to obtain the iron-carbon composite material of Example 2. The iron content of the obtained iron-carbon composite material was 58 mass%, and the BET specific surface area was 330 m. 2 / g. Using the obtained iron-carbon composite material, a negative electrode was produced in the same manner as in Example 1, and a nickel-metal hydride battery (test cell) was produced.

[0071] Example 3 The iron-carbon composite material of Example 1 was obtained in the same manner as in Example 2, except that the blending ratio of iron powder in the mixture of graphite and iron powder was 80 mass %. The iron content of the obtained iron-carbon composite material was 83 mass %, and the BET specific surface area was 220 m 2 / g. Using the obtained iron-carbon composite material, a negative electrode was produced in the same manner as in Example 1, and a nickel-metal hydride battery (test cell) was produced.

[0072] 〔evaluation〕 For the test cells of Examples 1 to 3 and Comparative Example 1, 1.6 mA / cm was measured in a closed container. 2A constant current of 0.1C was applied to charge and discharge the battery. "C" is the unit of current rate. "1C" indicates the current rate at which the SOC (State of Charge) reaches 100% from 0% after 1 hour of charging. SOC is the ratio of the charge amount to the battery's charge capacity. The discharge end voltage was set to 0.8V.

[0073] During the above charge and discharge cycles, the discharge capacity (mAh / g) of the test cell (battery) was measured. The discharge capacity is the capacity of the battery (capacity per 1 g of the iron-carbon composite material constituting the negative electrode) at the end of discharge (voltage: 0.8 V).

[0074] Furthermore, when the charging voltage reached 1.5 to 1.6 V during charging, the internal pressure of the battery was measured with a pressure sensor to evaluate whether or not hydrogen gas had been generated. If the internal pressure measured by the pressure sensor was less than 0.005 MPa, it was evaluated as "no hydrogen gas generation," and if it was 0.005 MPa or higher, it was evaluated as "present" hydrogen gas generation.

[0075] Table 1 shows the evaluation results of "presence or absence of hydrogen generation during charging" and "discharge capacity" in the above evaluation.

[0076] [Table 1]

[0077] The results shown in Table 1 (whether or not hydrogen is generated during charging) show that hydrogen is generated during charging in the test cell of Comparative Example 1, which uses a hydrogen storage alloy in the negative electrode, whereas hydrogen gas is not generated during charging in the test cells of Examples 1 to 3, which use an iron-carbon composite material in the negative electrode.

[0078] Furthermore, from the results (discharge capacity) shown in Table 1, it can be seen that in the batteries (test cells) of Examples 1 to 3, which correspond to the nickel-metal hydride batteries of the present disclosure, the use of the iron-carbon composite material as the negative electrode material results in a larger discharge capacity than the battery of Comparative Example 1, which used a conventional hydrogen storage alloy for the negative electrode. Note that the discharge capacity of the battery of Comparative Example 1 is 0 if no hydrogen is generated.

[0079] The discharge voltages shown in Table 1 mean, for example, that in Example 1, discharging was performed until the battery voltage dropped from 1.4 V to 0.8 V, and in Comparative Example 1, discharging was not possible without hydrogen generation.

[0080] [Surface composition analysis by X-ray diffraction intensity measurement] The iron-carbon composite material produced in Example 1 and the Fe—CH compound produced in Reference Example 1 were subjected to surface composition analysis by X-ray diffraction intensity measurement. For the measurement of X-ray diffraction intensity, an X-ray diffraction intensity curve was obtained using an X-ray diffractometer (XRD) "RINT2500V" manufactured by Rigaku Corporation. Note that for Reference Example 1, the X-ray diffraction intensity was measured in a state where the sample was covered with a Kapton film, since the sample would burn in the air. 1 shows the X-ray diffraction intensity curve of the iron-carbon composite material prepared in Example 1. FIG. 2 shows the X-ray diffraction intensity curve of the Fe—CH compound prepared in Reference Example 1.

[0081] The results shown in Figure 1 indicate that trace amounts of iron oxides (FeO3 and Fe3O4) are present on the surface of the iron-carbon composite material produced in Example 1. This is because the 2θ values ​​at which peaks appear in the X-ray diffraction of the iron-carbon composite material correspond to the 2θ values ​​at which peaks appear in the X-ray diffraction of FeO3 and Fe3O4 (powder diffraction database).

[0082] In contrast, in the Fe—CH compound of Reference Example 1, the graphite became amorphous after milling for 80 hours, and the presence of Fe could not be confirmed from the X-ray diffraction intensity curve (see FIG. 2). However, the presence of Fe was confirmed by separate energy dispersive X-ray analysis (EDS).

[0083] From these results, it is believed that the iron-carbon composite material used in the negative electrode in Example 1 suppresses the generation of hydrogen during charging due to the formation of iron oxide on its surface.

[0084] The embodiments and examples disclosed herein are illustrative in all respects and are not limiting. The technical scope defined by the claims encompasses all modifications within the meaning equivalent to the claims. The technical scope defined by the claims encompasses all modifications within the scope equivalent to the claims. [Explanation of symbols]

[0085] 1 battery, 10 storage element, 11 positive electrode, 12 negative electrode, 13 separator, 2 housing.

Claims

1. A method for producing an iron-carbon composite material containing iron oxide on at least a portion of its surface, comprising the steps of: A first step of pulverizing a carbon material in a hydrogen gas atmosphere using a ball mill with iron-containing balls to prepare an Fe—C—H compound containing iron, carbon, and hydrogen; a second step of oxidizing a portion of the iron in the Fe—C—H compound to form an iron oxide that coats at least a portion of the surface of the Fe—C—H compound.

2. A method for producing an iron-carbon composite material containing iron oxide on at least a portion of its surface, comprising the steps of: A first step of pulverizing a carbon material and iron powder in a ball mill using iron-containing balls under a hydrogen gas atmosphere to prepare an Fe—C—H compound containing iron, carbon, and hydrogen; a second step of oxidizing a portion of the iron in the Fe—C—H compound to form an iron oxide that coats at least a portion of the surface of the Fe—C—H compound.

3. 3. The method according to claim 1, wherein the second step is carried out by mixing the Fe—C—H compound with an alkaline solution in an oxygen-free atmosphere.

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