JP5718006B2 - Hydrogen storage alloy and nickel metal hydride secondary battery - Google Patents

Description

  The present invention relates to a hydrogen storage alloy, and more particularly to a Mg, Al-containing rare earth nickel-based superlattice hydrogen storage alloy and a nickel metal hydride secondary battery using the hydrogen storage alloy as an active material for a negative electrode.

  Hydrogen storage alloys are attracting attention as new energy storage materials and energy conversion materials because they can store hydrogen safely and easily at normal temperature and pressure. For example, hydrogen transport and storage, heat storage and transport, heat- Mechanical energy conversion, hydrogen purification / separation, hydrogen isotope separation, hydrogen-based batteries, synthetic chemistry catalysts, temperature sensors, etc. It has been.

  In particular, nickel-metal hydride secondary batteries using hydrogen storage alloys as the negative electrode active material are clean, have higher capacity than nickel-cadmium secondary batteries, are resistant to overcharge / overdischarge, and can be charged and discharged at a high rate. In addition, since it has various features such as compatibility with the nickel-cadmium secondary battery in terms of operating voltage, its application and practical use are actively performed as a consumer secondary battery. Recently, since it has been put into practical use as a battery for hybrid vehicles, further improvement in performance is expected.

Conventionally, rare earth materials such as La, Ce, Pr, Nd, Sm, Gd or mixed elements thereof (Mm (Mish metal)) have been used as the hydrogen storage alloy used for the negative electrode active material of the nickel-hydrogen secondary battery. An AB ₅ type alloy having a CaCu ₅ type crystal structure using a metal at the A site and a transition metal such as Ni, Co, Mn, or Al at the B site has been mainly used (for example, see Patent Document 1). .) Since this type of alloy has an equilibrium pressure of hydrogen absorption / desorption at room temperature of about 0.5 to 5 atm, it has a feature that it can be easily put into practical use as an active material for a negative electrode of a nickel-hydrogen secondary battery.

However, the AB ₅ type rare earth metal-nickel alloy has a problem in the initial hydrogen storage / release characteristics (activation characteristics). For initialization, if charging and discharging are not performed several times, There is a problem that good battery characteristics cannot be obtained.

On the other hand, in order to meet the demand for higher capacity for secondary batteries in recent years, rare earth and Ni have improved battery capacity by using an alloy composition in which the content of transition metals mainly composed of nickel with respect to rare earth metals is reduced. Has been developed (see, for example, Patent Document 2). However, although this hydrogen storage alloy has the effect of making the crystal grains finer, when the atomic ratio of Ni is 4.5 or less, that is, when the composition is much richer in rare earth than AB ₅ , the battery There is a problem that the capacity cannot be increased or the cycle characteristics are deteriorated. The reason for this is that rare earth elements have a strong affinity for hydrogen, so in rare earth-rich compositions, metal hydrides (or hydrogen-induced amorphous phases) are formed when hydrogen is absorbed, and hydrogen is trapped, making it difficult to release hydrogen. This is considered to be because the real capacity decreases.

Therefore, as a technique for solving the above problems, Patent Document 3 discloses (R _1−x L _x ) (Ni _1−y M _y ) _z (where R is La, Ce, Pr, Nd, or these) L represents Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Mg, Ca, or a mixed element thereof, and M represents Co, Al, Mn, Fe , Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B, C, or a mixed element thereof, where x, y, and z are 0.05 ≦ x ≦ 0. 4, 0 ≦ y ≦ 0.5, 3.0 ≦ z <4.5), and the antiphase boundary existing perpendicular to the C axis of the crystal grains in the alloy is C Contains at least 30% and less than 95% by volume of crystals containing 5 or more and less than 25 per 20 nm in the axial direction. There is disclosed a rare earth metal-nickel-based hydrogen storage alloy in which an element to be added is disposed in an antiphase region in an amount of 60% or more and less than 95% of the addition amount.

Japanese Patent Laid-Open No. 09-050807 JP-A-06-145851 Japanese Patent No. 3688716

However, as described above, the alloy having a composition in which the content of transition metal containing nickel as a main component with respect to the rare earth metal described in Patent Document 2 has a hydrogen storage amount of AB type ₅ depending on the chemical composition. In spite of being larger than the alloy, the hydrogen-induced amorphous phase is likely to be formed at the time of hydrogen occlusion, so that the cycle characteristics are deteriorated and the substantial hydrogen occlusion amount is inferior to the AB type ₅ alloy. There is a point.

  In addition, the hydrogen storage alloy described in Patent Document 3 that has improved the above problems has a main phase ratio of 30 vol% or more and less than 95 vol%, and other than the main phase of about 70 vol% at the maximum and 5 vol% at the minimum. Therefore, it has been clarified that there is a problem that the capacity is deteriorated when the phase is pulverized and the hydrogen storage / release cycle is repeated.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a hydrogen storage alloy having a high capacity and excellent activation characteristics as compared with an AB type ₅ alloy and having a battery life (cycle characteristics) equal to or higher than that, and uses the hydrogen storage alloy. The object is to provide a nickel-metal hydride secondary battery with high capacity and excellent cycle characteristics. A specific development target of the present invention is for an anode active material of a nickel-metal hydride secondary battery having an initial discharge capacity of 320 mAh / g or more and a capacity retention ratio of 90% or more after 100 cycles of charge and discharge are repeated. The object is to provide a hydrogen storage alloy.

The inventors have made extensive studies to solve the above-described problems of conventional AB ₅ type hydrogen storage alloys and the hydrogen storage alloys described in Patent Documents 2 and 3.
As a result, the main phase of the hydrogen storage alloy is the A ₂ B ₇ phase, the A ₅ B ₁₉ phase or a mixed phase thereof, and the main phase is increased to a ratio of 95 vol% or higher by increasing the abundance ratio of the main phase. In addition, the AB ₂ phase has an interface that is intergrowth-matched in the direction perpendicular to the C axis of the crystal structure of the main phase, which is not found in conventional hydrogen storage alloys, in an extremely thin layered state, By precipitating 20 vol% or less with respect to the main phase, it has been found that the cycle characteristics, that is, the capacity retention ratio can be improved, and the present invention has been completed.

That is, the present invention provides (RE _1-a Mg _a ) (Ni _1-bc Al _{b Mc} ) _x (wherein RE is La, Ce, Pr, Nd, Sm, Gd, Zr or a mixture thereof) Element, M is Co, Mn or a mixed element thereof, and a, b, c and x are 0.06 <a <0.25, 0.01 ≦ b <0.20, 0.00 ≦ c, respectively. <0.4, 3.2 <x <3.9) A ₂ B ₇ phase having a composition (atomic ratio) represented by Ce ₂ Ni ₇ type or Gd ₂ Co ₇ type crystal structure, The main phase consisting of either the A ₅ B ₁₉ phase having a crystal structure of Pr ₅ Co ₁₉ type or Ce ₅ Co ₁₉ type or a mixed phase thereof has a metal structure that is 95 vol% or more of the total structure, and (RE, Mg) (N) having an interface aligned in a direction perpendicular to the C-axis of the crystal structure of the main phase , _Al) AB ₂ phases ₂ composition, in the range of 1～20Vol% with respect to the main phase, a hydrogen-absorbing alloy characterized by being deposited on the inter-Grouse type.

  The present invention also provides a nickel-metal hydride secondary battery using the hydrogen storage alloy described above as an active material for a negative electrode.

  The nickel metal hydride secondary battery of the present invention is characterized in that the initial discharge capacity is 320 mAh / g or more and the capacity retention rate after repeating 100 cycles of charge and discharge is 90% or more.

According to the present invention, compared with a conventional CaCu ₅ type hydrogen storage alloy, it is possible to provide a hydrogen storage alloy having a high capacity, excellent initial activation characteristics, and excellent long-life characteristics at low cost. It becomes possible.

It is a transmission electron micrograph of the main phase which is the characteristic of this invention alloy. It is a X-ray-diffraction profile of the alloy of the comparative examples 9 and 10 whose ratio of a main phase is less than 95%. It is a scanning electron micrograph of the alloys of Comparative Examples 9 and 10 in which the proportion of the main phase is less than 95%. It is a transmission electron micrograph of the alloy of the comparative example 3 whose value of the atomic ratio b of Al is 0. It is a scanning electron micrograph of the alloy of the comparative example 4 whose value of the atomic ratio b of Al is 0.2 or more. It is a transmission electron micrograph of the alloys of Invention Examples 6 and 7. 4 is a transmission electron micrograph of an alloy of Invention Example 8. 4 is an X-ray diffraction profile of alloys of Invention Examples 1 to 4. It is a scanning electron micrograph of the alloys of Invention Examples 1 to 4. 2 is a transmission electron micrograph of the alloy of Invention Example 1. It is an X-ray diffraction profile of the alloy of the comparative example 1 whose value of the atomic ratio a of Mg is 0.06 or less. It is an X-ray-diffraction profile of the alloy of the comparative example 2 whose value of the atomic ratio a of Mg is 0.25 or more. It is a scanning electron micrograph of the alloy of the comparative example 2 whose value of the atomic ratio a of Mg is 0.25 or more. It is an X-ray-diffraction profile of the alloy of the comparative example 5 whose value of the atomic ratio b of Al is 0.2 or more. It is a scanning electron micrograph of the alloy of the comparative example 5 whose value of the atomic ratio b of Al is 0.2 or more. It is an X-ray-diffraction profile of the alloy of the comparative example 6 whose value of the atomic ratio c of M element is 0.4 or more. It is a scanning electron micrograph of the alloy of the comparative example 6 whose value of the atomic ratio c of M element is 0.4 or more. It is an X-ray-diffraction profile of the alloy of the comparative example 7 whose value of AB ratio x is 3.2 or less. It is a scanning electron micrograph of the alloy of the comparative example 7 whose value of AB ratio x is 3.2 or less. It is an X-ray-diffraction profile of the alloy of the comparative example 8 whose value of AB ratio x is 3.9 or more. It is a scanning electron micrograph of the alloy of the comparative example 8 whose value of AB ratio x is 3.9 or more. 3 is an X-ray diffraction profile of alloys of Invention Examples 9 to 11. It is a scanning electron micrograph of the alloys of Invention Examples 9-11. It is a transmission electron micrograph of the alloy of the comparative example 11 which made the cooling rate 1000 degrees C / second or more. It is an X-ray-diffraction profile of the alloy of the comparative example 12 which made heat processing temperature less than 900 degreeC. It is a scanning electron micrograph of the alloy of the comparative example 12 which made heat processing temperature less than 900 degreeC. It is an X-ray-diffraction profile of the alloy of the comparative example 13 which made heat processing temperature more than melting | fusing point of an intergrowth phase. It is a scanning electron micrograph of the alloy of the comparative example 13 which made heat processing temperature more than melting | fusing point of an intergrowth phase. It is an X-ray-diffraction profile of the alloy of the comparative example 14 whose heat processing time is less than 3 hours. It is a scanning electron micrograph of the alloy of the comparative example 14 whose heat processing time is less than 3 hours.

Hereinafter, embodiments of the present invention will be specifically described.
First, the hydrogen storage alloy of the present invention, the composition (atomic ratio) _is represented by _{(RE 1-a Mg a)} (Ni 1-b-c Al b M c) x. Here, RE in the above formula is La, Ce, Pr, Nd, Sm, Gd, Zr or a mixed element thereof, M is Co, Mn or a mixed element thereof, and a, b, c and x Must be numerical values satisfying 0.06 <a <0.25, 0.01 ≦ b <0.20, 0.00 ≦ c <0.4, and 3.2 <x <3.9, respectively. It is.

  As described above, RE can be selected from La, Ce, Pr, Nd, Sm, Gd, Zr or a mixture of these elements. Among these, Pr, Nd, and Sm, which have a large effect of improving the hydrogen storage / release cycle characteristics, can be preferably used. From the viewpoint of reducing costs, it is preferable to use Misch metal (Mm), which is a mixture of La, Ce, and rare earth. Further, when Zr is added to the rare earth element, it has a function of improving the corrosion resistance of the alloy to the electrolytic solution even in a small amount. Therefore, it is preferable to add Zr in a range of 5 at% or less with respect to the total of Mg and RE.

In the hydrogen storage alloy of the present invention, when the RE site is substituted with Mg, the generation of A ₂ B ₇ phase and A ₅ B ₁₉ phase as main phases can be promoted, and the superlattice structure can be stabilized. Both the A ₂ B ₇ phase and the A ₅ B ₁₉ phase have a superlattice structure formed by stacking AB ₂ blocks and AB ₅ blocks. In addition, the generated A ₂ B ₇ phase and A ₅ B ₁₉ phase RE sites are substituted and dissolved by Mg, so that the hydrogen storage / release cycle characteristics (specifically, capacity retention ratio) are dramatically improved. . Therefore, Mg is an essential element in the hydrogen storage alloy of the present invention.

In addition, the hydrogen storage alloy of the present invention has an intergrowth in which the Ni site is substituted with Al so that Al is a constituent element in the main phase and the chemical composition is represented by (RE, Mg) (Ni, Al) _2. Type AB ₂ phase will precipitate. Therefore, the hydrogen storage alloy of the present invention requires the addition of Al. In addition, heat treatment under appropriate conditions described later is indispensable for precipitation of the intergrow type AB _two phase. Furthermore, by replacing the Ni site with M element, that is, Co, Mn, or a mixed element thereof, the mechanical properties of the main phase can be easily caused by plastic deformation, that is, it can be made ductile. As a result, it is possible to suppress the main phase from being pulverized by expansion / contraction during hydrogen storage / release, and it is possible to suppress dry-up and micro short circuit of the electrolyte in the nickel-hydrogen secondary battery, It also improves cycle characteristics.

Further, a, b, c and x representing the atomic ratio of the above elements are 0.06 <a <0.25, 0.01 ≦ b <0.20, 0.00 ≦ c <0.4, 3 .2 <x <3.9 must be satisfied. When a is 0.06 or less, when hydrogen is occluded, the alloy is decomposed into a rare earth hydride and other alloys, making reversible hydrogen occlusion / release difficult. On the other hand, when a is 0.25 or more, problems such as a decrease in the equilibrium hydrogen storage / release pressure to below the practical range and the production of a homogeneous alloy become difficult. On the other hand, when b is less than 0.01, Al necessary for the formation of the intergrowth phase is insufficient, and therefore problems such as generation of the intergrowth phase being suppressed and generation amount being insufficient are not preferable. On the other hand, if b is 0.20 or more, on the contrary, an impurity phase containing Al is easily generated, which is not preferable. Moreover, when c is 0.4 or more, an impurity phase containing Co or Mn occurs, which is not preferable. In addition, when x is 3.20 or less, an AB ₂ phase or an AB ₃ phase as a coarse impurity phase having an interface inconsistent with the main phase is generated in addition to the AB ₂ phase precipitated in the main phase in an intergrow type. However, the proportion of the main phase is reduced. On the other hand, if it is 3.90 or more, the ratio of the AB ₅ phase which is an impurity phase increases, which is not preferable.

In addition, the hydrogen storage alloy of the present invention has an A ₂ B ₇ phase, Pr ₅ Co ₁₉ type or Ce ₅ Co ₁₉ type crystal whose main phase has a crystal structure of Ce ₂ Ni ₇ type or Gd ₂ Co ₇ type. It is necessary that either the A ₅ B ₁₉ phase having a structure or a phase in which they are mixed microscopically and the abundance ratio thereof is 95 vol% or more of the entire structure. When it is less than 95 vol%, the capacity is lowered and the cycle characteristics are deteriorated.

The reason why the main phase is limited to any one of the A ₂ B ₇ phase, the A ₅ B ₁₉ phase or a mixed phase thereof is that these phases have better cycle characteristics than the AB ₂ and AB ₃ phases, This is because a high capacity can be obtained as compared with the AB ₅ phase. In addition, the A ₂ B ₇ phase and the A ₅ B ₁₉ phase are very similar in crystal structure, and even when they become a micro mixed phase, they are mixed with micro consistency in the crystal grains. It can be treated as a single main phase. Furthermore, the reason for limiting the abundance ratio of the main phase to 95 vol% or more of the entire structure is that the capacity can be increased and the impurity phase that becomes the starting point of pulverization when the hydrogen storage / release cycle is repeated. This is because, by reducing the ratio as much as possible, pulverization is suppressed and cycle characteristics are improved.

A slight remainder other than the main phase in the hydrogen storage alloy of the present invention is AB ₂ phase having a crystal structure of MgNi ₂ type, MgCu ₂ type or MgZn ₂ type, AB having a crystal structure of PuNi ₃ type or CeNi ₃ type. It is composed of an impurity phase such as an AB ₅ phase having a _three- phase and CaCu ₅ type crystal structure. These phases have not only unfavorable characteristics such as a small amount of hydrogen occlusion, poor cycle characteristics, and no occlusion of hydrogen compared to the main phase, but may also be a starting point for causing pulverization of the main phase. Therefore, it is desirable that it is as small as possible.

Note that the ratio of the main phase and the impurity phase such as AB ₂ phase is in good agreement between the value calculated by observation with a scanning electron microscope and the value obtained by Rietveld analysis of the X-ray diffraction measurement result. Therefore, any method may be used for measurement. Moreover, when observing a backscattered electron image, the difference in the chemical composition of each phase can be identified more clearly by corroding the alloy with an acid.

Furthermore, in the hydrogen storage alloy of the present invention, the AB ₂ phase whose chemical composition is represented by (RE, Mg) (Ni, Al) ₂ is in an intergrous type in the direction perpendicular to the C axis of the crystal structure of the main phase. That is, it is characterized in that it has an interface that is consistent with the main phase and is a very thin layer with a thickness of 10 nm or less, preferably 10 nm or less, and is deposited in an amount of 20 vol% or less with respect to the main phase. It is. Inter Grouse type precipitates of the AB ₂ composition, not found in conventional hydrogen storage alloy, it is specific to the hydrogen storage alloy of the present invention, finely precipitated in the special form in the main phase Therefore, it becomes possible to greatly improve the cycle characteristics of the nickel hydride secondary battery, that is, the capacity retention rate.

The reason why the above effect can be obtained is that the intergrowth type AB ₂ phase is deposited in a very thin layer with a consistent interface in the main phase, and the mechanical properties of the main phase change, resulting in hydrogen The resistance to expansion / contraction due to occlusion / release is increased, and as a result, pulverization is suppressed, and even if pulverized, grain damage does not become as small as several μm, which improves cycle characteristics. It is done.

In addition, the AB ₂ phase is dispersed and deposited in an extremely thin layer in an intergrow type in the main phase, thereby suppressing the precipitation of the AB ₅ phase, the AB ₃ phase and the AB ₂ phase as the _second phase. This is considered to be the reason for improving the cycle characteristics. That is, there is an effect of widening the range of chemical compositions in which the main phase can be a single phase. For example, without considering the heat treatment conditions and the addition amounts of Al and Mg, simply producing an alloy having a composition of AB _3.7 at the time of casting is theoretically 87% of AB _3.5 which is the main phase. Phase (= A ₂ B ₇ phase) and an impurity phase of 13% AB ₅ phase, resulting in a two-phase metal structure, resulting in a decrease in capacity due to a shortage of the main phase amount in the present invention, and a large amount of precipitation with the main phase. The cycle characteristics are deteriorated by the strain generated due to the difference in the amount of expansion / contraction at the boundary with the AB ₅ phase. However, when the heat treatment conditions and the addition amounts of Al and Mg are adjusted in consideration of the formation of the intergrowth type AB ₂ phase to produce an alloy of AB _3.7 , the main phase has a very similar crystal structure. Because the intergrowth type AB ₂ phase is precipitated in the micro mixed phase of AB _3.5 phase (= A ₂ B ₇ phase) and AB _3.8 phase (= A ₅ B ₁₉ phase)恰 also becomes a single-phase metal structure, and it is possible to suppress capacity reduction and cycle deterioration due to the impurity phase. Note that the intergrowth phase cannot be confirmed by ordinary X-ray analysis or scanning electron microscope. This is because the intergrowth type precipitate is present in a very thin layer form of several tens nm or less, preferably 10 nm or less.

Note that the above-described effect can be obtained only when the AB ₂ phase is deposited in a coherent manner in an intergrow type in a direction perpendicular to the C axis of the main phase. Therefore, such an effect cannot be obtained in the AB ₂ phase having a large size of several μm or more deposited with an interface inconsistent with the main phase, that is, the so-called second phase or AB ₂ phase as an impurity phase. On the contrary, the AB ₂ phase decomposes or becomes amorphous when hydrogen is occluded, making reversible hydrogen occlusion / release reaction difficult, resulting in a decrease in cycle characteristics. In addition, the AB ₂ phase has a bad influence on the cycle characteristics even if it is the starting point for the pulverization of the main phase.

In addition, as a result of examining the optimum amount of the intergrowth phase for improving the cycle characteristics, the present invention has found that the alloy has excellent cycle characteristics when it is in the range of 1 to 20 vol% with respect to the main phase. If it is less than 1 vol%, the above effect cannot be sufficiently obtained. On the other hand, if it exceeds 20 vol%, an independent AB ₂ composition impurity phase having an interface inconsistent with the main phase is generated. It is. Preferably, it is the range of 5-15 vol%.

Next, the manufacturing method of the hydrogen storage alloy of this invention is demonstrated.
Since the hydrogen storage alloy of the present invention contains Mg having a low melting point and a high vapor pressure as a main component, when the raw materials of all the alloy components are dissolved at once, Mg evaporates and an alloy having a target chemical composition is formed. Can't get. Therefore, in producing the hydrogen storage alloy of the present invention by the melting method, first, other alloy components except for Mg are melted, and then Mg raw material made of metal Mg and / or Mg alloy is put into the molten metal. Is preferred. Moreover, it is desirable to perform this dissolution step in an inert gas atmosphere such as argon or helium. Specifically, the inert gas containing 80 vol% or more of argon gas was adjusted to 0.05 to 0.2 MPa. It is preferable to carry out under an atmosphere.

  The alloy melted under the above conditions is then preferably cast into a water-cooled mold and solidified to form a hydrogen storage alloy ingot. The cooling rate at this time is preferably 1000 ° C./second or less. This is because if the cooling rate exceeds 1000 ° C., the intergrowth phase will not precipitate due to the effect of solution treatment by rapid cooling.

Furthermore, in the hydrogen storage alloy of the present invention, the ingot after casting is made at 900 ° C. or higher and in an intergrowth phase in an inert gas such as argon or helium, nitrogen gas, or a mixed gas atmosphere thereof. It is necessary to perform a heat treatment for 3 to 50 hours at a temperature below the melting point. By this heat treatment, _{A 5} having in the hydrogen-absorbing alloy, _A 2 _{B 7} phase having a _Ce 2 Ni ₇ type or _Gd 2 Co ₇ type crystal _structure, the Pr 5 _{Co 19} type or _Ce 5 _{Co 19} type crystal structure The proportion of the main phase consisting of any one of the B ₁₉ phases or their micro mixed phase is grown to 95 vol% or more, and at the same time, the AB ₂ phase, AB ₃ phase, and AB ₅ phase, which are impurity phases, are reduced or eliminated. Can be made. In addition, by performing heat treatment under the above conditions, a layered AB ₂ phase in a direction perpendicular to the C axis with a consistent interface in the main phase is formed, and the intergrow type is 1 to 20 vol% with respect to the main phase. It can be deposited in an amount in the range of. Furthermore, homogenization of the main phase chemical components and removal of defects such as vacancies and dislocations proceed sufficiently.

If the heat treatment temperature is less than 900 ° C., the diffusion of the elements is insufficient, so that the impurity phase remains and the ratio of the main phase cannot be stably increased to 95 vol% or more. It will cause deterioration of the characteristics. On the other hand, when the heat treatment temperature exceeds the melting point of the inter Growth phase decomposition of inter Growth phase diffusion takes place, the generation of AB ₅ phase which is a refractory phase of impurities than the main phase from being promoted. Note that, at a temperature slightly below the melting point, excessive intergrowth phase growth and enlargement may occur, and the effect of improving the mechanical properties of the main phase due to fine dispersion may not be obtained. Furthermore, as a result of enlargement of crystal grains of the main phase and evaporation of the Mg component, there is a possibility that the occlusion amount may decrease due to pulverization or chemical composition change. Therefore, the heat treatment temperature is preferably in the range of 900 ° C. to (the melting point of the intergrous phase−10 ° C.).

In addition, when the heat treatment holding time is 3 hours or less, the ratio of the main phase cannot be stably set to 95 vol% or more, and moreover, the generation of intergrowth phase in the main phase, vacancies in the main phase, Removal of defects such as dislocations is also insufficient. In addition, since the homogenization of the chemical components of the main phase is insufficient, expansion / contraction during hydrogen storage / release occurs non-uniformly, increasing the amount of generated strain and defects and adversely affecting cycle characteristics. The heat treatment holding time is preferably 5 hours or more from the viewpoint of the formation of the intergrowth phase, and is preferably 8 hours or more from the viewpoint of homogenization of the main phase and improvement of crystallinity. . However, if the holding time exceeds 50 hours, the amount of Mg evaporated increases and the chemical composition changes, resulting in the generation of an AB ₅ type impurity phase. Further, it is not preferable because it may cause an increase in manufacturing cost and dust explosion due to evaporated Mg fine powder.

  The other components excluding Mg from the components constituting the hydrogen storage alloy were melted in a reduced-pressure atmosphere filled with argon gas using a high-frequency induction melting furnace, and then the metal Mg raw material was added into the molten metal. After adjusting to the component composition shown in Table 1, the hydrogen storage alloy melt was cast into a water-cooled mold and solidified by changing the cooling rate. Next, the hydrogen storage alloy was subjected to heat treatment for 2 to 75 hours at various temperatures between 900 ° C. and the melting point of the intergrowth phase in an argon gas atmosphere, and then pulverized to obtain a predetermined particle size (average particle size: 50 to 500 μm) of hydrogen storage alloy powder was obtained.

The hydrogen storage alloy powder obtained as described above was subjected to the following measurement.
<Chemical component analysis>
The chemical components of the ingot after melting and the alloy after heat treatment were measured by the ICP method, and changes in the chemical components due to melting and changes in the component composition due to Mg evaporation during heat treatment were confirmed.
<X-ray diffraction>
The crystal structure and the ratio of phases constituting the hydrogen storage alloy were measured using a powder X-ray diffraction method. The X-ray diffraction measurement was carried out under the conditions of 40 kV and 40 mA (X-ray tube: Cu) after the obtained alloy was pulverized to 75 μm or less in an argon gas using a mortar. The structure was obtained by analyzing the profile obtained by X-ray diffraction by the Rietveld method.
The ratio of the main phase and impurity phase was also calculated by observing the reflected electron image with a scanning electron microscope. As a result, the results consistent with the Rietveld analysis of the X-ray diffraction results were obtained. In the example, the identification of the main phase and the impurity phase and the measurement of the ratio were performed mainly by Rietveld analysis of the X-ray diffraction results.
<Observation by high-angle scattering annular dark field method with scanning transmission electron microscope>
The above pulverized powder is observed by a high-angle scattering annular dark field method using a scanning transmission electron microscope (STEM), and Z-contrast images are taken at 20 locations. It was confirmed whether or not the AB ₂ phase was dispersed and precipitated. Further, the thickness in the plane direction perpendicular to the C-axis of the precipitated phase (intergrowth phase) in the obtained image was calculated based on the value of the lattice constant of the main phase obtained from the X-ray diffraction result.

<Evaluation of battery characteristics>
Various hydrogen storage alloy powders obtained as described above were used as active materials to prepare nickel-metal hydride secondary batteries, and the battery characteristics (capacity, high rate discharge capacity, capacity retention rate) were investigated as follows.
Each alloy powder obtained according to the component composition and production conditions shown in Table 1 is used as an active material for the negative electrode, and nickel powder is used as a conductive auxiliary agent, and 2 mass% polyvinyl alcohol (PVA) aqueous solution is used as a binder. : Conductive auxiliary agent: Binder = 80: 5: 15 was added to form a slurry of the active material, filled in a foamed nickel base material, and used as a negative electrode. The weight of the filled slurry was about 1.4 g. In addition, the size of the substrate was 25 mm × 30 mm, and the 30 mm side was crushed and masked by 5 mm, and the current collector nickel lead wire was used as a spot welded portion. Therefore, the area filled with the active material is 25 mm × 25 mm. After filling, the negative electrode is dried at 80 ° C. for 2 hours. After the moisture of the PVA aqueous solution is blown off, the active material and the conductive auxiliary agent are combined, and then the masking tape attached for spot welding the nickel lead is peeled off. After that, it was rolled using a roll press, and a nickel lead having a width of 5 mm was spot-welded to the rolled electrode to obtain a current collecting lead.

  The structure of the cell is a polypropylene nonwoven fabric separator with a negative electrode in the center and a sintered nickel hydroxide electrode of 30 mm x 40 mm size (having sufficient electric capacity for the negative electrode) on both sides as a counter electrode (positive electrode). It was set as the structure arranged through. The sintered nickel hydroxide electrode was activated in advance by charging 120% using a nickel plate as a counter electrode in a 30 mass% potassium hydroxide solution (including 30 g / L lithium hydroxide), and then adding distilled water. The alkaline solution was washed with water and dried with a heater at 60 ° C. for 1 hour, and a nickel lead having a width of 5 mm was spot welded in the same manner as the negative electrode to obtain a current collecting lead. Next, the negative electrode was sandwiched between polypropylene nonwoven fabric separators, sandwiched from both sides with a counter electrode sintered nickel hydroxide electrode, and further sandwiched between acrylic plates was used as the final electrode. As the electrolytic solution, a 30 mass% potassium hydroxide solution (containing 30 g / L lithium hydroxide) was used, and this was filled in a sufficient amount into the acrylic container to obtain an open cell.

  The battery characteristics were evaluated by charging for 6 hours up to a voltage of 2 V at a current value of 0.2 C (calculated assuming the electric capacity of the filled alloy was 350 mAh / g) for the activation and capacity evaluation of the negative electrode. (120% charge) was performed, and after a pause of 15 min, discharge was performed at 0.3 C to 1 V for 4 hr (120% discharge). This was repeated 5 times to activate, and the initial battery capacity was determined. In addition, the test was done in a 25 degreeC thermostat. Thereafter, charge and discharge at 0.5 C were repeated 100 cycles, and (discharge capacity at 100 cycles / initial discharge capacity) × 100 (%) was calculated as a capacity retention rate.

Table 2 shows the measurement results of the hydrogen storage alloy and the evaluation results of the battery characteristics. From Table 1 and Table 2, the hydrogen storage alloy of the present invention example having a component composition suitable for the present invention and manufactured under appropriate conditions (cooling rate, heat treatment condition) has a main phase of A ₂ B ₇ and And / or A ₅ B ₁₉ phase, the total proportion of which is 95 vol% or more, and the AB ₂ phase has an intergrowth-type interface in the direction perpendicular to the C-axis of the main phase crystal structure It was confirmed that an appropriate amount was deposited. The nickel metal hydride secondary battery produced using these alloys according to the examples of the present invention has target characteristics (discharge capacity: 320 mAh / g or more, capacity maintenance ratio) in both initial discharge capacity and capacity maintenance ratio after 100 cycles. : 90% or more).
On the other hand, the hydrogen storage alloy of the comparative example that does not satisfy all of the conditions of the present invention is inferior to the target characteristics in either the initial discharge capacity or the capacity retention rate after 100 cycles. I understand.

First, the main phase that is a feature of the hydrogen storage alloy of the present invention will be described.
FIG. 1 shows that the main phases of the alloy, the A ₂ B ₇ phase and the A ₅ B ₁₉ phase, form a single main phase with a perfectly matched interface. It is the result (henceforth "a transmission electron micrograph") observed with the electron microscope. Further, among this photo, it can also be confirmed irregularly _{portion A} 2 _{B 7} phase and _A 5 _{B 19} phase are mixed. As can be seen, the A ₂ B ₇ phase and the A ₅ B ₁₉ phase are very similar in crystal structure and can be adjacent with a consistent interface. This is precisely why the present invention treats these phases as a single main phase. It is also a feature of the alloy of the present invention that there is an intergrowth type precipitation phase having an interface aligned in a direction perpendicular to the C axis of the crystal structure of the main phase.

Hereinafter, the alloys of the inventive examples shown in Table 1 and Table 2 will be compared with the alloys of the comparative examples, and will be described individually and specifically.
FIG. 2 shows the results of X-ray diffraction (hereinafter referred to as “X-ray diffraction profile”) of the alloys of Comparative Examples 9 and 10 in which the value of the AB ratio x is outside the range of the present invention, and FIG. Similarly, the results of observation of the alloys of Comparative Examples 9 and 10 with a scanning electron microscope (hereinafter referred to as “scanning electron micrographs”) are shown, and both alloys are the main phase A ₂ B ₇ phase and The sum of the proportions of the A ₅ B ₁₉ phase is 70, 80 vol%, which is lower than the range of the present invention. As a result, as shown in Table 2, compared with the alloy of Invention Example 5, the initial discharge capacity is the same, but the cycle characteristics are inferior and cannot be put to practical use.

Table 3 also shows alloys of Invention Examples 6 to 8 manufactured under appropriate conditions in which the composition of the alloy complies with the present invention, and the intergrowth phase is observed using a scanning transmission electron microscope. It shows the result of analyzing the component composition of the intergrowth phase by measuring the amount and analyzing with an energy dispersive spectroscope EDS (Energy Dispersive X-ray Spectrometer). From this result, in the alloy of the present invention example, the chemical composition is (RE, Mg) (Ni, Al) ₂ in the main phase and the AB ₂ type intergrowth phase is in the range of 1 to 20 vol% with respect to the main phase. It can be seen that it is generated within. Table 3 also shows the results of analysis of the amount and composition of the intergrowth phase of the alloys of comparative examples 3 and 4 in which the atomic ratio of Al is outside the present invention, but is necessary for the generation of the intergrowth phase. As can be seen from the transmission electron micrograph shown in FIG. 4, the alloy of Comparative Example 3 that does not contain any Al does not produce an intergrowth phase, while the Al atomic ratio b is 0.22 and Al is excessive. In the alloy of Comparative Example 4 included in FIG. 5, the intergrowous phase is produced in a large amount exceeding the proper range of the present invention and becomes enlarged, and as can be seen from the scanning electron micrograph shown in FIG. Impurity AB having an interface is changed to a _two- phase.

6 shows a transmission electron micrograph of the intergrowth phase observed in the alloys of Invention Examples 6 and 7, and FIG. 7 shows the EDS analysis of the alloy of Invention Example 8 observed with a scanning transmission electron microscope. The result of having analyzed the component composition of the intergrowth phase by is shown. As can be seen from these results, in the hydrogen storage alloy of the present invention, the boundary between the AB ₂ phase and the main phase precipitated in the intergrowth type is a consistent interface, and the phases on both sides sandwiching the interface are completely different. The crystal structure is characterized in that the arrangement pattern is different. On the other hand, the antiphase boundary of the crystal described in Patent Document 3 is that the phases on both sides are the same and have the same regular arrangement across the boundary. Therefore, it can be seen that the boundary between the intergrowth type precipitation phase and the main phase in the present invention is completely different from the antiphase boundary.

Next, the influence of the component composition on the characteristics of the hydrogen storage alloy of the present invention will be described.
FIGS. 8 and 9 show the X-ray diffraction profiles and scanning electron micrographs of the alloys of Invention Examples 1 to 4 having the composition of the alloy suitable for the present invention and manufactured under appropriate conditions, respectively. Moreover, the result of having observed the intergrowth phase of the alloy of invention example 1 with the scanning transmission electron microscope was shown in FIG. From these results, by adjusting the component composition of the hydrogen storage alloy in accordance with the present invention and controlling the manufacturing conditions within an appropriate range, the ratio of the main phase is 95 vol% or more and there is almost no impurity phase. Furthermore, it can be seen that a predetermined amount of intergrowth phase can be precipitated, and as a result, as shown in Table 2, a hydrogen storage alloy having excellent initial capacity and cycle characteristics can be obtained.

  On the other hand, Comparative Example 1 is an alloy example in which the atomic ratio a of Mg is 0.04 outside the scope of the present invention, and the nickel metal hydride secondary battery using this alloy is as shown in Table 2, The initial discharge capacity is low, and the capacity retention rate at 100 cycles is only 28%. FIG. 11 shows a comparison of X-ray diffraction profiles before and after hydrogen storage of this alloy. In an alloy with a small amount of Mg, the superlattice structure can be easily decomposed by storing hydrogen. Recognize. This result shows that a predetermined amount of Mg is required to reversibly store and release hydrogen.

Comparative Example 2 is an alloy example in which the atomic ratio a of Mg is 0.30 outside the scope of the present invention, and although the initial discharge capacity is high, the capacity retention rate at 100 cycles is only 64%. This is because, as shown in Table 2, since the AB ₂ phase value obtained by Rietveld analysis of the X-ray diffraction profile (FIG. 12) of this alloy is 10%, it cannot be replaced with RE excessively. It is presumed that Mg becomes an AB ₂ phase, which is an impurity phase, and precipitates in the alloy, resulting in deterioration of cycle characteristics. FIG. 13 is a scanning electron micrograph of the alloy of Comparative Example 2, and it can be seen that many AB ₂ phases are precipitated.

  Comparative Example 3 is an alloy example that does not contain Al, and although the initial discharge capacity is high, the capacity retention rate at 100 cycles is only 58%. It is surmised that this is because Al is not necessary for the generation of the intergrowth phase, so that the intergrowth phase is not generated and the pulverization is promoted (see Table 3 and FIG. 4 described above). .

Comparative example 4 is an alloy example in which the atomic ratio b of Al is 0.22 which is 0.2 or more, the initial discharge capacity is low, and the capacity retention rate at 100 cycles is only 84%. This is thought to be due to the fact that an excessive amount of intergrowth phase is generated due to excessively contained Al, and an independent AB ₂ composition impurity phase having an interface inconsistent with the main phase is generated (see Table 3 and FIG. 4). Further, Comparative Example 5 is an alloy example of 0.27 in which the atomic ratio b of Al is further higher. As can be seen from the X-ray diffraction profile (FIG. 14) and the scanning electron micrograph (FIG. 15), the intergrowth phase As a result of abnormal growth and formation of the impurity phase AB ₂ having an interface inconsistent with the main phase, the characteristics are further deteriorated as compared with the alloy of Comparative Example 4.

Comparative Example 6 is an alloy example in which the atomic ratio c of M atoms (Co, Mn) is 0.42 which is 0.4 or more, the initial discharge capacity is slightly lower, and the capacity retention rate at 100 cycles. Is only 74%. As can be seen from the X-ray diffraction profile (FIG. 16) and scanning electron micrograph (FIG. 17) of this alloy, the AB ₅ phase, which is an impurity phase, is precipitated excessively as a result of adding a large amount of M atoms. It is thought that the ratio of the main phase has decreased.

Comparative example 7 is an alloy example in which the value of AB ratio x is 3.0 which is 3.2 or less. Although the initial discharge capacity is high, the capacity retention rate at 100 cycles is only 69%. As can be seen from the X-ray diffraction profile (FIG. 18) and scanning electron micrograph (FIG. 19) of this alloy, a large amount of a phase having an AB ₃ type crystal structure, which is an impurity phase, is precipitated. It is inferred that the ratio of phases decreased and the cycle characteristics deteriorated.

Comparative Example 8 is an alloy example of 4.1 with an AB ratio x value of 3.9 or more. The initial discharge capacity is slightly lower, and the capacity retention rate at 100 cycles is only 76%. As can be seen from the X-ray diffraction profile (FIG. 20) and scanning electron micrograph (FIG. 21) of this alloy, the AB ₅ phase precipitates in large quantities, the ratio of the main phase decreases, and the capacity decreases. It was inferred that

Next, the influence of manufacturing conditions (casting conditions, heat treatment conditions) on the characteristics of the hydrogen storage alloy of the present invention will be described.
Invention Example 9 casts an alloy having a component composition of Pr _0.85 Mg _0.15 Ni _3.45 Al _0.25 suitable for the present invention in terms of the composition at the time of melting under conditions suitable for the present invention, This is an example of heat-treated alloy. As shown in Table 2, the initial discharge capacity is 347 mAh / g, and the capacity retention rate at 100 cycles is 93%. Similarly, the alloys of Inventive Examples 10 and 11 having component compositions suitable for the present invention different from Inventive Example 9 described above show similar results when manufactured under conditions suitable for the present invention. . 22 and 23 show scanning electron micrographs and X-ray diffraction profiles of the alloys of Invention Examples 9 to 11.

  On the other hand, Comparative Example 11 is an example of an alloy produced by rapidly cooling an alloy having the same composition as that of Invention Example 9 at a cooling rate of 2000 ° C./second exceeding 1000 ° C./second at the time of casting. The rate is 77% and the cycle characteristics are inferior. As can be seen from the transmission electron micrograph (FIG. 24) of this alloy, the cause is considered to be that the intergrowth phase cannot be formed in the main phase in the case of rapid solidification.

  Comparative Example 12 is an alloy example in which the heat treatment of the alloy of Invention Example 9 was performed at a temperature of 850 ° C. below 900 ° C. The initial discharge capacity was slightly lower, and the capacity retention rate at 100 cycles was 82 It is only%. This is because a broad peak or a peak derived from the impurity phase has been confirmed in the X-ray diffraction profile of this alloy (FIG. 25). This is probably because it was insufficient. FIG. 26 shows a scanning electron micrograph of the same alloy.

Comparative Example 13 is an alloy example in which the heat treatment of the alloy of Invention Example 9 was performed at a temperature equal to or higher than the melting point of the intergrowth phase. The initial discharge capacity was slightly lower, and the capacity retention rate at 100 cycles was 79. It is only%. The cause of this is that the X-ray diffraction profile and scanning electron micrograph of this alloy are shown in FIGS. 27 and 28, respectively. This is presumably because it changed to an AB ₂ type impurity phase having an interface inconsistent with the main phase.

  Comparative Example 14 is an alloy example in which the heat treatment time of the alloy of Invention Example 9 is less than 3 hours, the initial discharge capacity is slightly lower, and the capacity retention rate at 100 cycles is only 79%. In the case of this alloy, since a broad peak, a peak derived from the impurity phase, and a decrease in the ratio of the main phase can be confirmed in the X-ray diffraction profile (FIG. 29), removal of distortion of the main phase and removal of the impurity phase are not sufficient. This is thought to be the cause. In addition, in FIG. 30, the scanning electron micrograph of the same alloy was shown.

  On the contrary, Comparative Example 15 is an alloy example in which the alloy of Invention Example 9 was heat-treated for 50 hours or more, and the capacity retention rate at 100 cycles was only 81%. The cause of this is that Mg evaporates by long-time heat treatment, and the composition of the alloy changes from “Pr0.85Mg0.15Ni3.45Al0.25” after casting to “Pr0.95Mg0.05Ni3.68Al0.21”, This is probably because the ratio of the main phase has greatly decreased.

  The hydrogen storage alloy of the present invention is not limited to the use as a negative electrode active material of a secondary battery, and can be applied and used for hydrogen transport and storage.

Claims (3)

_{(RE 1-a Mg a)} (Ni 1-b-c Al b M c) x
(Wherein RE is La, Ce, Pr, Nd, Sm, Gd, Zr or a mixed element thereof, M is Co, Mn or a mixed element thereof, and a, b, c and x are respectively The composition (atomic ratio) represented by 0.06 <a <0.25, 0.01 ≦ b <0.20, 0.00 ≦ c <0.4, 3.2 <x <3.9) Have
A ₂ B ₇ phase having a Ce ₂ Ni ₇ type or Gd ₂ Co ₇ type crystal structure, an A ₅ B ₁₉ phase having a Pr ₅ Co ₁₉ type or Ce ₅ Co ₁₉ type crystal structure, or a mixed phase thereof The main phase composed of any one has a metal structure that is 95 vol% or more of the entire structure, and
The AB ₂ phase of (RE, Mg) (Ni, Al) ₂ composition having an interface aligned in the direction perpendicular to the C axis of the crystal structure of the main phase is in the range of 1 to 20 vol% with respect to the main phase, Hydrogen storage alloy characterized by being deposited in an intergrowth type. A nickel metal hydride secondary battery comprising the hydrogen storage alloy according to claim 1 as an active material for a negative electrode. The nickel metal hydride secondary battery according to claim 2, wherein the nickel metal hydride secondary battery has an initial discharge capacity of 320 mAh / g or more and a capacity retention ratio of 90% or more after 100 cycles of charge and discharge are repeated. Secondary battery.

Images