JP2007056309A - Hydrogen storage alloy, its manufacturing method and nickel hydrogen secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy by which, even in the case where Co content is decreased, pulverization can be suppressed and battery life can be prolonged while securing high electric-discharge capacity and also high rate discharge characteristics can be obtained, to provide its manufacturing method and also to provide a nickel hydrogen secondary battery using the hydrogen storage alloy. <P>SOLUTION: The hydrogen storage alloy is composed of a main phase having CaCu<SB>5</SB>type crystal structure and a secondary phase in which Mg is concentrated. In the hydrogen storage alloy, La constituting a Ca site of the main phase is contained in an amount of 1.5 to <24 mass% and 0.01 to <1 mass% Mg is contained in the alloy. It is preferable that Co constituting a Cu site of the main phase is further contained in an amount of <5 mass%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hydrogen storage alloy, and more particularly to a hydrogen storage alloy used for a negative electrode or the like of a secondary battery, a manufacturing method thereof, and a nickel hydrogen secondary battery using the same.

  In recent years, energy problems such as exhaustion of petroleum resources, as well as environmental problems such as global warming due to carbon dioxide and environmental pollution due to exhaust gas, have been greatly highlighted as issues to be solved by mankind. In order to deal with this problem, energy systems are being developed that convert energy into an energy medium called hydrogen, store it, transport it, and use it.

  However, although hydrogen has a high energy density per unit weight, it is a light gas, so it is not easy to transport and store, and there are many safety problems. Therefore, the development of hydrogen storage alloys that can be safely stored at room temperature and pressure using hydrogen that is approximately 1000 times its volume as an intermetallic compound, some of which are portable electronic devices, hybrid vehicles, It has been put to practical use as a power source (secondary battery) for electric vehicles and the like.

By the way, as a hydrogen storage alloy used for a negative electrode of a secondary battery such as a nickel metal hydride battery, conventionally, an alloy based on LaNi ₅ having a CaCu ₅ type (AB ₅ type) crystal structure is known. Among them, the Ca site of the crystal structure is composed of an inexpensive misch metal (Mm) that is a mixture of rare earth elements such as La, Ce, Pr, and Nd, and a part of Ni constituting the Cu site of the crystal structure. Hydrogen-absorbing alloys made of nickel-based multicomponent alloys (Mm (Ni, Co, Mn, Al) ₅ ) substituted with Co, Mn, Al, etc. are widely used. Each element constituting the hydrogen storage alloy has multiple functions such as improvement of corrosion resistance, suppression of pulverization, improvement of electron conductivity, adjustment of dissociation pressure, and expansion of the surface layer. The amount of the component is adjusted to a suitable range.

For example, in the hydrogen storage alloy composed of Mm (Ni, Co, Mn, Al) ₅ described above, Co increases the capacity of the battery by increasing the hydrogen storage capacity, and is caused by the volume change when storing and releasing hydrogen. It is known that it has an effect of improving the life of the battery by suppressing the pulverization to be performed or suppressing the corrosion and elution of the alloy surface, and about 10 mass% is usually added. However, from the viewpoint of high rate discharge characteristics, it is preferable that Co is less because pulverization is promoted and the surface area is increased. In addition, since Co is expensive, it is preferable that it be less in terms of manufacturing cost.

Therefore, in Patent Document 1, Mg or Ca is contained in the alloy in the range of 0.1 to 1.0 wt% to improve high-rate discharge characteristics while suppressing pulverization, and the La amount is set to 24 to 33 wt%. By increasing the hydrogen storage capacity and increasing the capacity as a relatively high level, even if the Co content in the alloy is reduced to 9 wt% or less, pulverization is suppressed at a high capacity and high discharge characteristics are achieved. It is disclosed that an excellent hydrogen storage alloy can be obtained.
Japanese Patent No. 3603013

  However, although the hydrogen storage alloy of Patent Document 1 is intended to increase the capacity by increasing the hydrogen storage amount, if the La content is increased, the expansion and contraction associated with the storage and release of hydrogen increase, and the alloy In addition to promoting finer powder, pure La is expensive because it requires separation and purification, and there is a problem that raw material costs increase. Further, Co is preferably further reduced because it is expensive and impairs high rate discharge characteristics as described above. Moreover, in the manufacturing method of the hydrogen storage alloy of patent document 1, since much of Mg and Ca evaporate and volatilize at the time of alloy melting, in addition to the problem that a yield falls, there exist problems that workability and safety are impaired. .

  Therefore, the object of the present invention is to achieve a long battery life by suppressing pulverization even when the Co content is reduced while ensuring a high discharge capacity, and also has a high-rate discharge characteristic and excellent hydrogen storage capacity. It is an object to provide a nickel-metal hydride secondary battery using the alloy, its manufacturing method, and the hydrogen storage alloy.

In order to solve the above-described problems of the prior art, the inventors focused on the component composition and structure of the hydrogen storage alloy and conducted extensive studies. As a result, in a hydrogen storage alloy consisting of a main phase having a CaCu ₅ type crystal structure and a second phase formed along the grain boundary of the main phase, the La of the Ca site of the crystal structure of the main phase While reducing the content to less than 24 mass% and adding an appropriate amount of Mg to concentrate in the second phase, even when the Co content is reduced, the fine powder is suppressed while ensuring a high discharge capacity. As a result, it was found that a hydrogen storage alloy having a long battery life and excellent high-rate discharge characteristics can be obtained, and the present invention has been completed.

That is, the present invention is a hydrogen storage alloy composed of a main phase having a CaCu ₅ type crystal structure and a second phase enriched with Mg, wherein La constituting the Ca site of the main phase is 1.5 mass% or more. It is a hydrogen storage alloy characterized by containing less than 24 mass% and Mg in the alloy of 0.01 mass% or more and less than 1 mass%.

  The hydrogen storage alloy of the present invention is characterized by containing less than 5 mass% of Co constituting the Cu site of the main phase.

  In addition, the hydrogen storage alloy of the present invention is characterized in that the volume fraction of the whole main phase alloy is 95% or more.

  Further, the present invention melts and casts the alloy raw material to produce a hydrogen storage alloy containing La in the range of 1.5 mass% to less than 24 mass% and Mg in the range of 0.01 mass% to less than 1 mass%. Is performed in an inert atmosphere containing 10 vol% or more of helium gas.

  The production method of the present invention is characterized in that the addition of Mg in the melting operation is performed after previously melting an alloy raw material having a melting point higher than that of Mg.

  The present invention also provides a nickel-metal hydride secondary battery using any of the hydrogen storage alloys described above as a negative electrode.

  According to the present invention, even if La is reduced to less than 24 mass% and Co is reduced to less than 5 mass%, a hydrogen storage alloy that is excellent in high rate discharge characteristics while suppressing pulverization while maintaining a high discharge capacity at low cost. Can be provided. Therefore, by using the hydrogen storage alloy of the present invention for the negative electrode, it is possible to provide a nickel-metal hydride secondary battery having high discharge capacity, excellent high rate discharge characteristics and excellent durability at low cost.

The hydrogen storage alloy according to the present invention will be described.
The hydrogen storage alloy of the present invention comprises a main phase having a CaCu ₅ type crystal structure and a second phase enriched in Mg.
Here, the Ca site in the CaCu ₅ type crystal structure of the main phase is composed of misch metal (Mm) which is a mixture of rare earth elements such as La, Ce, Pr, and Nd. In the alloy, the content of La is required to be less than 24 mass% of the entire alloy. If La is 24 mass% or more, the expansion and contraction associated with the occlusion and release of hydrogen increase, and the refinement of the alloy is promoted, which is not preferable for the battery life. Further, since La is expensive, it is preferable to reduce it in order to reduce costs.

  In addition, the minimum of La is accept | permitted to 1.5 mass%. However, from the viewpoint of suppressing an increase in the internal pressure of the battery when used in a nickel metal hydride secondary battery, 15 mass% or more is preferable, and from the viewpoint of maintaining a high discharge capacity, 20 mass% or more is preferable. The component composition constituting the Ca site other than La is not particularly limited, but from the viewpoint of optimizing the equilibrium hydrogen pressure of the hydrogen storage alloy that affects the battery internal pressure, Ce: 5 to 15 mass%, Pr: 0.6 The range of -3 mass% and Nd: 2-8 mass% is preferable.

On the other hand, the Cu site in the CaCu ₅ type crystal structure of the main phase is mainly composed of Ni, and a part thereof is substituted with Co, Mn, Al, etc. In the hydrogen storage alloy, the Co content needs to be less than 5 mass%. This is because Co is conventionally added about 10 mass% in order to suppress the miniaturization of the alloy. However, as described later, in the present invention, the addition of Mg can suppress the miniaturization, and Co This is because it is not preferable for improving the high-rate discharge characteristics, and it is preferable that it is less because it is the most expensive metal among the elements constituting the hydrogen storage alloy of the present invention. The lower limit of Co is not particularly defined, but is preferably 1 mass% or more from the viewpoint of maintaining a CaCu ₅ type crystal structure.

  The hydrogen storage alloy of this invention needs to contain Mg 0.01 mass% or more and less than 1 mass% other than the said component. Mg is a unique element that suppresses refinement of the alloy and improves high-rate discharge characteristics by containing 0.01 mass%. However, addition exceeding 1 mass% is not preferable because it causes a decrease in discharge capacity and a large amount of Mg evaporates during dissolution. From the viewpoint of securing a high discharge capacity, the amount of Mg added is preferably in the range of 0.1 to 0.5 mass%.

The reason why the above effect can be obtained by the addition of Mg has not been clarified yet, but Mg is considered to be an element constituting the Ca site in the CaCu ₅ type crystal structure of the main phase. Rather, there is a tendency to concentrate in the second phase formed along the grain boundary between the main phase and the main phase, and this Mg concentration is caused by the volume change accompanying the occlusion and release of hydrogen. It is considered that miniaturization is suppressed and that high-rate discharge characteristics are improved.

  In addition, in order to improve the charge / discharge characteristics of the battery, the hydrogen storage alloy of the present invention has Sm, Eu, Gd, Tb, Dy, Ho in addition to La, Ce, Pr, Nd in the Ca phase of the main phase. , Er, Tm, Yb, Lu, and Y may be included within a range of 3 mass% or less.

As described above, the hydrogen storage alloy of the present invention comprises a main phase having a CaCu ₅ type crystal structure and a second phase formed along a grain boundary between the main phase and the main phase. However, the proportion of the main phase in the total alloy is preferably 95% or more and 99.9% or less in terms of volume fraction. If it is less than 95%, it is difficult to maintain a high discharge capacity, and if it exceeds 99.9%, the effect of suppressing miniaturization cannot be obtained. The main phase and the second phase can be easily discriminated by polishing the cross section of the alloy and observing it with an SEM or the like.

Next, the manufacturing method of the hydrogen storage alloy of this invention is demonstrated.
In the hydrogen storage alloy of the present invention, raw materials for each alloy component are prepared so that the alloy component composition falls within the above range, and this is melted in a high-frequency melting furnace, an arc melting furnace, etc., and then cast into a mold. It can be manufactured by casting and solidifying by a known method such as a mold casting method to make a lump (ingot), a table casting method in which a molten metal is poured onto a turntable or roll to solidify, a roll quenching method, or an atomizing method. it can.

  At this time, in the production method of the present invention, it is necessary to perform melting and casting of the alloy in an inert atmosphere in which helium gas is mixed at 10 vol% or more. This is because conventionally, melting and casting of active hydrogen storage alloys have been performed under an inert atmosphere such as argon gas. However, Mg is an element having a low melting point and a high vapor pressure. Therefore, when melted in an inert atmosphere such as argon gas, a large amount of Mg evaporates and volatilizes during melting, making it difficult to control the composition, There is a risk of fire or explosion when attached to walls, etc., hindering observation in the furnace or touching the atmosphere.

  In order to suppress the above phenomenon, it is effective to mix helium gas in the dissolution atmosphere. The reason why Mg evaporation is suppressed by mixing helium gas is that helium gas has higher thermal conductivity, lower density, and longer mean free path than other inert gases. Conceivable. The mixing amount of the helium gas needs to be mixed at least 10 vol%, more preferably 30 vol% or more, and further preferably 50 vol% or more. Moreover, it is preferable that the pressure of the helium gas mixed atmosphere in melt | dissolution and casting operation | work is 0.01-1MPa. If it is less than 0.01 MPa, the evaporation temperature of Mg is lowered and evaporation is promoted, and if it exceeds 1 MPa, the melting point rises and dissolution becomes difficult.

  Argon gas is most preferable as another gas (inert gas) mixed with helium gas. This is because argon gas is less expensive than helium gas and does not react with Mg, Mm, Ni, etc. even at high temperatures.

  Further, in the production method of the present invention, it is preferable that the addition of Mg in the melting operation of the hydrogen storage alloy is performed by previously melting an alloy raw material having a melting point higher than that of metal Mg and then adding it. This is because, as described above, Mg is an element having a low melting point and a high vapor pressure. Therefore, when it is dissolved together with other alloy raw materials, Mg is first dissolved and evaporated. This is because Mg evaporation cannot be completely prevented only by controlling the atmosphere. Therefore, it is preferable to add Mg as late as possible in the melting operation to suppress evaporation. The raw material of Mg is not limited to metal Mg, and it is of course possible to use, for example, an Mg—Ni alloy that has been alloyed with other alloy constituent elements to increase the melting point.

  The alloy obtained as described above is then subjected to heat treatment at 600 to 1200 ° C. for 0.5 to 50 hours in an inert atmosphere (vacuum to 1 MPa) as necessary, The powder is pulverized by a generally known method such as a roll mill, a jaw crusher, or a ball mill to obtain an alloy powder having an average particle size of 10 to 150 μm.

The hydrogen storage alloy of the present invention obtained as described above can be used for a negative electrode of a nickel metal hydride secondary battery by a generally known method. For example, an appropriate amount of a binder such as PVA, PTFE, CMC, or SBR is mixed with the above alloy powder, and carbon graphite, Ni, or Cu powder is added as necessary, and kneaded to obtain an alloy paste. Can be made into a negative electrode plate by filling a three-dimensional conductive support such as a nickel foam or nickel fiber body, or by applying to a two-dimensional conductive support such as a punching plate, drying, and pressing with a roll. .
The nickel-metal hydride secondary battery using the hydrogen storage alloy of the present invention for the negative electrode has excellent characteristics such as high discharge capacity, high rate discharge characteristics, and long life even at low Co and low La. Have.

Hereinafter, the present invention will be described while comparing the inventive examples with the comparative examples.
La, Ce, Pr, Nd, which constitute the Ca site of hydrogen storage alloys with a CaCu _5- type crystal structure, Ni, Co, Mn, Al, which occupy the Cu site, and Mg added to the alloy Each raw material (purity 99.9 mass% or more) is prepared, each raw material is weighed in consideration of the yield, etc. so that the component composition as a product is shown in Table 1, and these raw materials are used in a high frequency melting furnace. Is melted in an argon gas atmosphere mixed with 70 vol% helium gas, cast into an iron mold to form ingots, and then the ingots are heated at a temperature of about 1000 ° C. for 20 hours under an Ar gas atmosphere. after subjected to heat treatment, the particle size D ₅₀ with a hammer mill has a powdery hydrogen-absorbing alloy 35～37Myuemu. Here, the particle size D ₅₀ means the particle size when the cumulative mass of particles smaller than a certain particle size occupies 50% of the total powder mass when the particle size distribution of the powder is measured. In the present invention, the particle size distribution of the hydrogen storage alloy was measured using a laser diffraction type particle size distribution measuring device.

  The hydrogen storage alloy powder obtained as described above is mixed with an aqueous solution containing CMC, PTFE or the like as a binder, carbon powder is mixed as a conductive agent, kneaded to obtain an alloy paste, and the alloy paste is punched. It apply | coated to the plate, it dried, and it pressurized with the roll, and produced the negative electrode plate whose thickness is 0.6-0.8 mm. Moreover, the thing which apply | coated nickel hydroxide as a positive electrode plate was prepared, the lead wire was welded to each electrode plate, and it immersed in 6N KOH electrolyte solution, and produced the nickel-hydrogen secondary battery for evaluation.

Using the evaluation battery, the standard capacity, high rate discharge capacity, pulverization characteristics, and battery life of each hydrogen storage alloy were evaluated in the following manner.
Charging / discharging each battery at 120 ° C with 120% charge at 0.2C relative to the negative electrode capacity, resting for 30 minutes, and then discharging until the battery voltage reaches 0.8V at 0.2C The test was repeated 15 cycles, and the maximum discharge capacity obtained during that period was taken as the standard capacity.
Thereafter, similarly, at 20 ° C., after 120% charge at 0.2 C, a test for discharging at 2.0 C was conducted five times, and the maximum capacity obtained at this time was defined as a high rate discharge capacity (high rate characteristic). .
Thereafter, remove the hydrogen storage alloy negative electrode plate disassembled battery after the total of 20 cycle test, after removing the binder and conductive agent, the particle size D ₅₀ of the alloy, a laser diffraction type particle size distribution measurement described above It measured using the apparatus and evaluated the pulverization characteristic (easiness of pulverization) of each hydrogen storage alloy.
The battery life was evaluated by the number of cycles until the discharge capacity of the evaluation nickel-hydrogen secondary battery described above was reduced to 80% of the standard capacity. The battery life was evaluated relatively by a life index (%), in which the battery life of a No. 19 hydrogen storage alloy containing no Co and 5.60 mass% Co was 100%.

  The results of the above measurements are shown together in Table 1. From Table 1, by adding 1.5 mass% or more and less than 24 mass% of La in the alloy, adding Mg in the range of 0.01 to less than 1 mass%, and further setting the volume fraction of the main phase to 95% or more, It can be seen that even if the Co content is reduced to less than 5 mass%, it is possible to obtain a hydrogen storage alloy that suppresses pulverization while maintaining the standard capacity and is excellent in high rate discharge characteristics.

In the same manner as in Example 1, the hydrogen storage alloys Nos. 21 to 25 having the composition shown in Table 2 and the volume fraction of the main phase having a CaCu ₅ type crystal structure were changed were dissolved. And pulverized to produce powders of these alloys. Next, the battery characteristics (standard capacity) of the hydrogen storage alloy were evaluated in the same manner as in Example 1, and the results are also shown in Table 2. Table 2 shows that a high discharge capacity can be obtained in a hydrogen storage alloy in which the main phase having a CaCu ₅ type crystal structure has a volume fraction of 95% or more and 99.9% or less of the whole alloy.

The hydrogen storage alloy of the present invention can be suitably used not only as a negative electrode material of a secondary battery such as a nickel metal hydride battery, but also as a fuel cell or a fuel cell accessory component.

Claims (6)

A hydrogen storage alloy consisting of a main phase with a CaCu ₅ type crystal structure and a second phase enriched with Mg, and the La constituting the Ca site of the main phase is 1.5 mass% or more and less than 24 mass%, in the alloy A hydrogen storage alloy characterized by containing Mg in an amount of 0.01 mass% to less than 1 mass%. The hydrogen storage alloy according to claim 1, comprising less than 5 mass% of Co constituting the Cu site of the main phase. 3. The hydrogen storage alloy according to claim 1, wherein a volume fraction of the main phase in the entire alloy is 95% or more. When the alloy material is melted and cast to produce a hydrogen storage alloy containing La in the range of 1.5 mass% to less than 24 mass% and Mg in the range of 0.01 mass% to less than 1 mass%, the above melting and casting operations are performed using 10 vol of helium gas. % Of hydrogen storage alloy manufacturing method characterized by performing in inert atmosphere containing more than%. The method for producing a hydrogen storage alloy according to claim 4, wherein the addition of Mg in the melting operation is performed after previously melting an alloy material having a melting point higher than that of Mg. A nickel metal hydride secondary battery using the hydrogen storage alloy according to claim 1 for a negative electrode.