JP7572056B2 - Hydrogen storage alloy for alkaline batteries - Google Patents

Description

The present invention relates to a hydrogen storage alloy for use in alkaline storage batteries.

Nickel-metal hydride secondary batteries, a typical example of alkaline storage batteries, are characterized by their high capacity compared to nickel-cadmium batteries and by the fact that they do not contain any environmentally hazardous substances. For this reason, in recent years, they have come to be widely used in a variety of applications, from mobile phones and personal computers to power tools and consumer applications as replacements for alkaline primary batteries, to storage batteries for hybrid electric vehicles (HEVs).

Conventionally, hydrogen storage alloys with an _AB5 crystal structure have been used for the negative electrodes of alkaline storage batteries, but this alloy has limitations in terms of reducing the size and weight of batteries, and there has been a demand for the development of new hydrogen storage alloys that can achieve a small size and high capacity. As a solution to this problem, Patent Documents 1 and 2 propose rare earth-Mg transition metal hydrogen storage alloys that contain Mg.

One method for reducing size and weight is to reduce the amount of hydrogen storage alloy used in the negative electrode, but reducing the amount of hydrogen storage alloy creates a new problem of reduced output due to a reduction in nickel active sites. To remedy this, Patent Document 3 proposes a method for increasing the operating voltage by using a hydrogen storage alloy with a high hydrogen equilibrium pressure.

Furthermore, several rare earth-Mg-Ni based alloys have been proposed as hydrogen storage alloys. For example, Patent Document 4 discloses a hydrogen storage alloy having a composition represented by the general formula: (La _a Ce _b Pr _c Nd _d A _e ) _1-x Mg _x (Ni _1-y T _y ) _z (wherein A represents at least one element selected from the group consisting of Pm and the like, T represents at least one element selected from the group consisting of V and the like, a, b, c, d, and e are within the ranges represented by 0≦a≦0.25, 0≦b≦0.2, 0≦c, 0≦d, and 0≦e and satisfy the relationship represented by a+b+c+d+e=1, and x, y, and z are within the ranges represented by 0<x<1, 0≦y≦0.5, and 2.5≦z≦4.5, respectively) for the purpose of providing a secondary battery having a long life suitable for improving the volumetric energy density.

In addition, Patent Document 5 describes a hydrogen storage alloy having the general formula: (La _a Pr _b Nd _c Z _d ) _1-w Mg _w Ni _z-x-y Al _x T _y, which suppresses the increase in the internal pressure of the battery during charging after overdischarge and contributes to improving the cycle life of the battery. (wherein Z represents an element selected from the group consisting of Ce and the like, T represents an element selected from the group consisting of V and the like, the subscripts a, b, c, d are within the ranges indicated by 0≦a≦0.25, 0<b, 0<c, 0≦d≦0.20 and also satisfy the relationships indicated by a+b+c+d=1 and 0.20≦b/c≦0.35, and the subscripts x, y, z, w are within the ranges indicated by 0.15≦x≦0.30, 0≦y≦0.5, 3.3≦z≦3.8, and 0.05≦w≦0.15, respectively) is disclosed.

Patent Document 6 discloses an inexpensive rare earth-Mg-Ni based hydrogen storage alloy having excellent alkali resistance, the hydrogen storage alloy having a composition represented by the general formula: (Ce _a Pr _b Nd _c Y _d A _e ) _1-w Mg _w Ni _x Al _y T _z (wherein A represents at least one element selected from the group consisting of Pm, Sm and the like, T represents at least one element selected from the group consisting of V, Nb and the like, a, b, c, d, and e satisfy the relationships represented by a>0, b≧0, c≧0, d≧0, e≧0, and a+b+c+d+e=1, and w, x, y, and z are within the ranges represented by 0.08≦w≦0.13, 3.2≦x+y+z≦4.2, 0.15≦y≦0.25, and 0≦z≦0.1, respectively).

Furthermore, Patent Document 7 discloses a hydrogen storage _{alloy represented} by the general formula: Ln1 _{- xMgxNiyAz} (wherein Ln is at least one element selected from rare earth elements including Y, Ca, Zr and Ti; A is at least one element selected from Co, Mn, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; and the subscripts x, y and z satisfy the conditions of 0.05≦x0.25, 0<z≦1.5 and 2.8≦y+z≦4.0), in which Ln contains 20 mol % or more of Sm.

Furthermore, Patent Document 8 discloses a hydrogen storage alloy having excellent alkali resistance, which has a composition represented by the general formula: (La _a Sm _b A _c ) _1-w Mg _w Ni _x Al _y T _z (wherein A and T each represent at least one element selected from the group consisting of Pr, Nd, etc. and the group consisting of V, Nb, etc., the subscripts a, b, and c each satisfy the relationships represented by a>0, b>0, 0.1>c≧0, and a+b+c=1, and the subscripts w, x, y, and z each fall within the ranges represented by 0.1<w≦1, 0.05≦y≦0.35, 0≦z≦0.5, and 3.2≦x+y+z≦3.8).

Patent Document 9 describes a hydrogen storage alloy negative electrode for an alkaline storage battery, which has a general formula (La _x Ln _y ) _1-z Mg _z Ni _t-u T _u (T: selected from Al, Co, Mn, and Zn, Ln is at least one selected from rare earth elements other than La and Y, x>y, 0.09≦z≦0.14, 3.65≦t≦3.80, 0.05≦u≦0.25) in which La is the main rare earth element, and which includes a hexagonal (2H) A ₅ B ₁₉ type structure, a trigonal (3R) A 5 B ₁₉ type structure, and an A ₂ B ₇ type structure, _and the powder X-ray diffraction intensity peak by Cu-Kα radiation of the 2H A ₅ B ₁₉ type structure is smaller than that of the 3R A ₅ B 19 type structure and the A ₂ B ₇ type structure. The use of a larger than ₇ type structure is disclosed.

On the other hand, Non-Patent Document 1 reports a hydrogen storage alloy, La0.8 _{- xCexMg0.2Ni3.5} (x=0 to 0.20) _, in which La is replaced with Ce. The evaluation results of this alloy concluded that the optimal composition is x=0.1 _, taking a comprehensive look at the electrochemical properties.

In addition, Non-Patent Document 2 includes a chapter on the effect of Ce on RE-Mg-Ni hydrogen storage alloys (RE: rare earth elements).
(La _0.5 Nd _0.5 ) _0.85 Mg _0.15 Ni _3.3 Al _0.2
(La _0.45 Nd _0.45 Ce _0.1 ) _0.85 Mg _0.15 Ni _3.3 Al _0.2
(La _0.4 Nd _0.4 Ce _0.2 ) _0.85 Mg _0.15 Ni _3.3 Al _0.2
(La _0.3 Nd _0.3 Ce _0.4 ) _0.85 Mg _0.15 Ni _3.3 Al _0.2
The alloy is disclosed and the results of its evaluation are reported.

Also, Non-Patent Document 3 reports the characteristics of La-Sm-Mg-Ni based hydrogen storage alloys. Specifically, the phase structure and electrochemical characteristics of a La _0.6 Sm _0.15 Mg _0.25 Ni _3.4 alloy are shown.

Japanese Patent Application Publication No. 11-323469 WO 01/48841 JP 2005-32573 A JP 2005-290473 A JP 2007-169724 A JP 2008-84668 A JP 2009-74164 A JP 2009-108379 A JP 2014-229593 A

S. Xiangqian et al. , Intern. J. Hydrogen Energy, 34, 395 (2009) Shigekazu Yasuoka, PhD thesis: Practical application of rare earth-Mg-Ni (superlattice) hydrogen storage alloys and development of high-performance commercial nickel-hydrogen batteries using them (2017, Kyoto University) L. Zhang et al. , Intern. J. Hydrogen Energy 41 1791 (2016)

However, the technologies disclosed in Patent Documents 1 and 2 above did not optimize the alloys and have not yet been put to practical use in a variety of applications.

In addition, the technology disclosed in Patent Document 3 has a new problem in that the charge/discharge cycle life is reduced when a hydrogen storage alloy with a high hydrogen equilibrium pressure is used.

In addition, the technology disclosed in Patent Document 4 keeps the content of La, a relatively inexpensive material, low, and as a result contains a large amount of expensive Pr, Nd, and even Ti, making it impossible to provide an inexpensive hydrogen storage alloy that is both durable and stable.

In addition, the technology disclosed in Patent Document 5, like Patent Document 4, requires Pr and Nd as the alloy, and the La content is low, so it cannot provide an inexpensive hydrogen storage alloy with excellent durability.

In addition, the technology disclosed in Patent Document 6 does not contain La, and although it does contain Ce, the alloy contains relatively large amounts of Pr and Nd, so it is not possible to provide an inexpensive hydrogen storage alloy that is excellent in durability.

In addition, the technology disclosed in Patent Document 8 uses an alloy containing a relatively large amount of Sm, which is a cheaper element than Pr and Nd, but does not provide a hydrogen storage alloy that is cheap and has excellent durability.

In addition, the technology disclosed in Patent Document 8 uses an alloy containing relatively large amounts of La and Sm, which are cheaper elements than Pr and Nd, but does not provide a hydrogen storage alloy that is cheap and has excellent durability. In particular, Zr is required in the examples, and only a B/A ratio of 3.6 is disclosed. In addition, it is said that the hydrogen equilibrium pressure, which is reduced by increasing the La content, can be raised to a level that can be used in batteries, but setting an inexpensive La-rich composition is often insufficient.

Furthermore, the technology disclosed in Patent Document 9 is an alkaline storage battery using a hydrogen storage alloy having an _A5B19 type crystal structure as the main phase _in the negative electrode, which has problems in terms of meeting the demand for further durability and is insufficient for achieving a highly reliable battery.

On the other hand, Non-Patent Document 1 shows a rare earth-Mg-Ni alloy in which part of the La is replaced with Ce, and in a prototype evaluation of this alloy, it is concluded that an alloy with a Ce replacement amount of x = 0.1 is the optimal composition after considering the overall electrochemical properties, but it has not yet been put to practical use.

In addition, Non-Patent Document 2 concludes that it has become clear that rare earth-Mg-Ni alloys containing Ce have a low hydrogen absorption/release capacity and are prone to pulverization when hydrogen absorption/release is repeated, resulting in significant degradation in batteries.

Furthermore, Non-Patent Document 3 concludes by reporting the electrochemical characteristics of a La _0.60 Sm _0.15 Mg _0.25 Ni _3.4 alloy, but the cycle characteristics are not sufficient, decreasing to 80% of the initial capacity after 140 charge/discharge cycles.

In other words, in rare earth-Mg-Ni hydrogen storage alloys, repeated hydrogen absorption and release causes cracks in the alloy, accelerating pulverization, and creating new surfaces. If the alloy has low corrosion resistance, the alloy surface will react to produce rare earth hydroxides, consuming the electrolyte, and as a result, the internal resistance of the battery will increase and the discharge capacity will decrease, shortening the battery life.

The present invention was made in consideration of these problems with the conventional technology, and aims to provide a hydrogen storage alloy that contributes to the realization of highly durable alkaline storage batteries.

In order to achieve the above objective, we conducted extensive research into hydrogen storage alloys for use in the negative electrodes of alkaline storage batteries that combine durability, high battery discharge capacity, and low cost. As a result, we discovered that an alloy containing at least rare earth elements, magnesium, nickel, and aluminum can achieve durability by controlling the diffraction intensity based on a specific crystal structure, leading to the development of the present invention.

That is, the hydrogen storage alloy for alkaline storage batteries according to the present invention is a hydrogen storage alloy whose main phase has an _A2B7 type crystal structure, contains rare earth elements, magnesium, _nickel , and at least one selected from aluminum, zinc, tin, and silicon, and is characterized in that in X-ray diffraction measurement using Cu-Kα radiation as an X-ray source, the ratio Ib/Ia of the diffraction line intensity Ib of the (109) plane of _{the A5B19 2H structure appearing at 2θ=31.5 to 32.5° to the diffraction line intensity Ia of the (107) plane of the A2B7 2H structure} appearing at 2θ=32.5 to 33.5° is 0.05 or more and 1.0 or less.

In addition, the hydrogen storage alloy for an alkaline storage battery according to the present invention is preferably represented by the following general formula (1).
(La _1-a-b Ce _a Sm _b ) _1-c Mg _c Ni _d M _e T _f ...(1)
Here, M, T and the subscripts a, b, c, d, e and f in the above formula (1) are
M: at least one selected from Al, Zn, Sn, and Si;
T: at least one selected from Cr, Mo, and V;
0<a≦0.10,
0≦b≦0.15,
0<a+b≦0.16
0.225≦c≦0.32,
0.03≦e≦0.16,
0≦f≦0.03,
3.25≦d+e+f<3.50
Meet the conditions.

The hydrogen storage alloy for alkaline storage batteries of the present invention has excellent durability and discharge capacity, and nickel-hydrogen secondary batteries using it have high power density and excellent charge/discharge cycle life, so they have excellent discharge capacity characteristics and can be used for a variety of purposes, such as consumer applications, industrial applications, and vehicle applications.

1 is a partially cutaway perspective view illustrating an alkaline storage battery using a hydrogen storage alloy according to the present invention. 1 is a graph showing an example of the results of X-ray diffraction measurement of a hydrogen storage alloy according to the present invention.

An alkaline storage battery using the hydrogen storage alloy of the present invention will be described with reference to Fig. 1, which is a partially cutaway perspective view showing one example of the battery. The alkaline storage battery 10 is a storage battery having an electrode group consisting of a nickel positive electrode 1 having nickel hydroxide (Ni(OH) ₂ ) as the main positive electrode active material, a hydrogen storage alloy negative electrode 2 having the hydrogen storage alloy (MH) of the present invention as the negative electrode active material, and a separator 3, in a housing 4 together with an electrolyte layer (not shown) filled with an alkaline electrolyte.

This battery 10 is a so-called nickel-metal hydride battery (Ni-MH battery), and the following reaction occurs:

Positive electrode: NiOOH+H ₂ O+e ^- = Ni(OH) ₂ +OH ^-
Negative electrode: MH+OH ^- =M+H ₂ O+e ^-

[Hydrogen storage alloy]
The hydrogen storage alloy according to the present invention for use in the negative electrode of an alkaline storage battery will now be described.

The hydrogen storage alloy of the present invention is a hydrogen storage alloy whose main phase has an _{A2B7 crystal} structure, contains rare earth elements, magnesium, nickel, and at least one selected from aluminum, zinc, tin, and silicon, and in an X-ray diffraction (XRD) measurement using Cu-Kα radiation as an X-ray source, the ratio (Ib/Ia) of the diffraction line intensity (Ia) of the (107) plane of _{the A2B7} crystal structure (2H type) appearing at 2θ=32.5 to 33.5° and the diffraction line intensity (Ib) of the (109) plane of the _{A5B19 crystal} structure (2H type) appearing at 2θ=31.5 to 32.5° must be in the range of 0.05 to 1.0. Here, if the ratio (Ib _{/ Ia} ) of the diffraction intensity (Ib) of the (109) face of the _A5B19 type crystal structure having a hexagonal (2H) crystal to the diffraction intensity (Ia) of the (107) face of the _A2B7 type crystal structure having a hexagonal (2H) crystal is less than 0.05 or exceeds 1.0, cracking (pulverization) due to hydrogen absorption and release is promoted, and durability becomes insufficient. Preferably, Ib/Ia is in the range of 0.08 to 0.95.

The hydrogen storage alloy of the present invention preferably has a component composition represented by the following general formula (1).
(La _1-a-b Ce _a Sm _b ) _1-c Mg _c Ni _d M _e T _f ...(1)
Here, M, T and the subscripts a, b, c, d, e and f in the above formula (1) are
M: at least one selected from Al, Zn, Sn, and Si;
T: at least one selected from Cr, Mo, and V;
0<a≦0.10,
0≦b≦0.15,
0<a+b≦0.16
0.225≦c≦0.32,
0.03≦e≦0.16,
0≦f≦0.03,
3.25≦d+e+f<3.50
Meet the conditions.

When a hydrogen storage alloy having the above-mentioned X-ray diffraction intensity ratio between _{the A2B7} type phase (2H) and _{the A5B19 type} phase (2H) and preferably satisfying the component composition represented by the above general formula (1) is used as the negative electrode of an alkaline storage battery, it imparts high discharge capacity and cycle life characteristics to the battery, thereby contributing to the achievement of smaller, lighter alkaline storage batteries and higher durability.

The reasons for limiting the composition of the hydrogen storage alloy of the present invention will be explained below.
Rare earth element: La _1-a-b Ce _a Sm _b (where 0<a≦0.10, 0≦b≦0.15, 0<a+b≦0.16)
The hydrogen storage alloy of the present invention contains rare earth elements as the A component elements of the A ₂ B ₇ type structure of the main phase, and the A ₅ B ₁₉ type structure, AB ₃ type structure, AB ₂ type structure, and AB ₅ type structure of the subphase. As the rare earth elements, two elements La and Ce are indispensable as basic components that bring about hydrogen storage capacity. In addition, since La and Ce have different atomic radii, the hydrogen equilibrium pressure can be controlled by this component ratio, and the hydrogen equilibrium pressure required for the battery can be set arbitrarily. The atomic ratio a value of Ce in the rare earth elements must be in the range of more than 0 and not more than 0.10. If the a value exceeds 0.10, cracking associated with hydrogen absorption and release is promoted, leading to a decrease in cycle life. On the other hand, if the a value is 0, that is, if Ce is not included, it becomes difficult to sufficiently control the hydrogen equilibrium pressure, which adversely affects the battery characteristics. If it is within this range, it is easy to set the hydrogen equilibrium pressure suitable for the battery. Preferably, the atomic ratio a value of Ce is in the range of 0.005 to 0.09.

Sm can be optionally contained as a rare earth element other than La and Ce. Like La and Ce, Sm is an element that occupies a rare earth site as an element of the A component of the A ₂ B ₇ type structure of the main phase, the A ₅ B ₁₉ type structure, AB ₃ type structure, AB ₂ type structure, and AB ₅ type structure of the subphase, and is a component that brings about hydrogen storage capacity like these elements. Sm has a lower effect of increasing the equilibrium pressure compared to Ce, but durability is improved by substituting La together with Ce. The upper limit of the b value representing the atomic ratio of Sm in the rare earth elements is 0.15, and if it exceeds that, the cycle life characteristics will decrease due to the balance with the amount of Ce. Preferably, b≦0.14. Moreover, the substitution amount of Ce and Sm for La is in the range of 0<a+b≦0.16. If a+b exceeds 0.16, crack suppression will not be sufficient. Preferably, 0.005≦a+b≦0.15.

A composition with a high La content has a high discharge capacity, and when combined with other elements, the discharge capacity characteristics are further improved. In addition, Pr and Nd are not actively used as rare earth elements, but may be included at the unavoidable impurity level.

Mg: _Mgc (where 0.225≦c≦0.32)
Mg is an essential element in the present invention as an element of the A component of the A ₂ B ₇ type structure of the main phase, the A ₅ B ₁₉ type structure, AB ₃ type structure, AB ₂ type structure, and AB ₅ type structure of the subphase, and contributes to improving the discharge capacity and the cycle life characteristics. The c value, which represents the atomic ratio of Mg in the A component, is in the range of 0.225 to 0.32. If the c value is less than 0.225, the X-ray diffraction intensity ratio of the A ₂ B ₇ phase and the A ₅ B ₁₉ phase cannot be controlled, and as a result, sufficient cycle life characteristics are not achieved, and the hydrogen release ability gradually decreases, resulting in a decrease in the discharge capacity. On the other hand, if the c value exceeds 0.32, cracking associated with hydrogen absorption and release is promoted, and the cycle life characteristics, i.e., durability, decrease. Preferably, the c value is in the range of 0.225 to 0.30.

Ni: Ni _d
Ni is a main element of the B component in the A ₂ B ₇ type structure of the main phase and the A ₅ B ₁₉ type structure, AB ₃ type structure, AB ₂ type structure, and AB ₅ type structure of the subphase. The atomic ratio d value will be described later.

M: _Me (where 0.03≦e≦0.16)
M is at least one selected from Al, Sn, Zn, and Si, and is an element contained as an element of the B component of the A ₂ B ₇ type structure of the main phase, and the A ₅ B ₁₉ type structure, AB ₃ type structure, AB ₂ type structure, and AB ₅ type structure of the subphase. It is effective in adjusting the hydrogen equilibrium pressure related to the battery voltage, and can improve corrosion resistance, and is effective in improving the durability of the fine hydrogen storage alloy, that is, the cycle life characteristics. In order to reliably express the above effect, the e value representing the atomic ratio of M to the A component is set to a range of 0.03 to 0.16. If the e value is less than 0.03, the corrosion resistance becomes insufficient, resulting in an increase in saturation magnetization and an insufficient cycle life. On the other hand, if the e value exceeds 0.16, the discharge capacity decreases. The preferred e value is in the range of 0.035 to 0.15. In addition, it is preferable that Al is present among the M elements, and the atomic ratio of Al is preferably 0.03 or more within the range of e.

T: _Tf (where 0≦f≦0.03)
T is at least one selected from Cr, Mo, and V, and is an element contained as an element of the _B component of the _A2B7 type structure of the main phase, and _{the A5B19} type structure, _AB3 type structure, _AB2 type structure, and _AB5 type structure of the subphase, like M. It is effective in adjusting the hydrogen equilibrium pressure related to the battery voltage, and the synergistic effect with the M element increases corrosion resistance and improves durability. In particular, it is effective in improving the durability of fine hydrogen storage alloys, that is, cycle life characteristics. In order to reliably express the above effect, the f value, which represents the atomic ratio of T to the A component, is 0.03 or less. If the f value exceeds 0.03, excessive T induces cracks associated with the absorption and release of hydrogen, resulting in a decrease in durability and an insufficient cycle life. The preferred f value is in the range of 0.025 or less, and it is preferable that the T element, especially Cr, is present within the scope of the invention in which the amount of the M element is particularly small. In addition, a combination of Cr and Mo or V is preferred.

Ratio of component A to component B: 3.25≦d+e+f<3.50
The _{stoichiometric} ratio, which is the molar ratio of the B component (Ni, M and T) to the A component consisting of the _A2B7 type structure of the main phase and the _A5B19 type structure, _AB3 type structure, _{AB2 type} structure, and _AB5 type structure of the subphase, i.e., the value of d+e+f expressed by the general formula, is preferably in the range of 3.25 to less than 3.50. If it is less than 3.25, the subphase, i.e., the _AB2 phase, gradually increases, and in particular the discharge capacity decreases, and _{the A5B19} phase is not formed, resulting in a decrease in cycle life. On the other hand, if it is 3.50 or more, the _AB5 phase or _{the A5B19 phase} increases too much, which promotes cracking due to hydrogen absorption and release, resulting in a decrease in durability, i.e., a decrease in cycle life. It is preferably in the range of 3.30 to 3.49.

[Method of manufacturing hydrogen storage alloy]
Next, a method for producing the hydrogen storage alloy of the present invention will be described.
The hydrogen storage alloy of the present invention is prepared by weighing out rare earth elements (Ce, Sm, La, etc.), magnesium (Mg), nickel (Ni), and other elements such as aluminum (Al) and chromium (Cr) to substitute for Ni in a predetermined molar ratio, and then putting these raw materials into an alumina crucible placed in a high-frequency induction furnace and melting them under an inert gas atmosphere such as argon gas, and then casting the melt into a mold to prepare an ingot of the hydrogen storage alloy. Alternatively, a flake-shaped sample having a thickness of about 200 to 500 μm may be directly prepared by strip casting.

The hydrogen storage alloy of the present invention contains Mg as its main component, which has a low melting point and high vapor pressure. If all the raw materials of the alloy components are melted at once, the Mg will evaporate, and it may be difficult to obtain an alloy with the desired chemical composition. Therefore, when manufacturing the hydrogen storage alloy of the present invention by a melting method, it is preferable to first melt the other alloy components except Mg, and then add Mg raw materials such as metallic Mg and Mg alloys to the molten metal. This melting process is preferably performed in an inert gas atmosphere such as argon or helium. Specifically, it is preferable to perform the melting process in a reduced pressure or pressurized atmosphere in which an inert gas containing 80 vol% or more of argon gas is adjusted to 0.05 to 0.2 MPa.

The alloy melted under the above conditions is then preferably cast into a water-cooled mold and solidified to form an ingot of a hydrogen storage alloy. Next, the melting point (T _m ) of each of the obtained ingots of the hydrogen storage alloy is measured using a DSC (differential scanning calorimeter). This is because the hydrogen storage alloy of the present invention is preferably subjected to a heat treatment in which the ingot after the casting is held at a temperature of 700° C. or higher and the melting point (T _m ) of the alloy or lower for 3 to 50 hours in an atmosphere of an inert gas such as argon or helium, or nitrogen gas, or a mixed gas thereof. This heat treatment makes it possible to have the A ₂ B ₇ phase as the main phase and at least one or more crystal phases selected from the A ₅ B ₁₉ phase, AB ₃ phase, AB ₂ phase, and AB ₅ phase as the subphase. The ratio of the A ₂ B ₇ phase in the hydrogen storage alloy is the main phase if it is the largest ratio among the constituent phases, and it is sufficient that it is at least 40 mass% or more. As is clear from the X-ray diffraction measurement results (FIG. 2), the _A2B7 phase is a _{hexagonal (2H) Ce2Ni7 type, but may contain some rhombohedral (3R) Gd2Co7 type. Subphases A5B19 type structure ( hexagonal Gd5Co19 phase} or _{rhombohedral Pr5Co19} phase) and _AB3 type structure (hexagonal _CeNi3 phase or _{rhombohedral PuNi3 phase} ) may be present, and small amounts of _AB2 phase ( _MgZn2 phase) and _AB5 phase ( _CaCu5 phase) may be present, but it is preferable to reduce them as much as possible because the _AB2 phase reduces the discharge capacity and the _AB5 phase reduces the cycle life. The crystal structure of the main phase of the obtained hydrogen storage alloy can be confirmed to be an _{A2B7 type} structure by X-ray diffraction measurement using Cu-Kα radiation and by metal structure observation using a SEM. It can also be determined from the X-ray diffraction results by Rietveld analysis.

If the heat treatment temperature is less than 700°C, the diffusion of elements is insufficient, and subphases remain, which may lead to a decrease in the discharge capacity of the battery and a deterioration in the cycle life characteristics. On the other hand, if the heat treatment temperature is −20°C or more lower than the melting point _Tm of the alloy ( _Tm −20°C or more), the crystal grains of the main phase may become coarse and the Mg component may evaporate, resulting in pulverization and a decrease in the hydrogen storage capacity due to changes in the chemical composition. Therefore, the heat treatment temperature is preferably in the range of 750°C to ( _Tm −30°C). More preferably, it is in the range of 770°C to ( _Tm −50°C).

In addition, if the holding time of the heat treatment is 3 hours or less, the ratio of the main phase may not be stably 40 mass% or more. In addition, since the homogenization of the chemical components of the main phase is insufficient, the expansion and contraction during hydrogen absorption and release may become nonuniform, and the amount of distortion and defects generated may increase, which may adversely affect the cycle life characteristics. The holding time of the heat treatment is preferably 4 hours or more, and from the viewpoint of homogenization of the main phase and improvement of crystallinity, it is more preferable to set it to 5 hours or more. However, if the holding time exceeds 50 hours, the amount of evaporation of Mg increases, changing the chemical composition, and as a result, there is a risk of the _AB5 type subphase being generated. Furthermore, it is not preferable because it may cause an increase in manufacturing cost and a dust explosion due to evaporated Mg fine powder.

The heat-treated alloy is pulverized by a dry or wet method. When pulverizing by a dry method, for example, a hammer mill or ACM pulverizer can be used to obtain a powder with an average particle size of 20 to 100 μm. On the other hand, when pulverizing by a wet method, a bead mill or attritor can be used. In particular, when obtaining a fine powder with an average particle size of 20 μm or less, wet pulverization is preferable because it can be produced safely. The particle size may be set to an appropriate range depending on the application, for example, D50 = 8 to 100 μm.
Here, the average particle size D50 of the alloy particles is a value measured using a laser diffraction/scattering type particle size distribution measuring device, and as the measuring device, for example, the MT3300EXII type manufactured by Microtrack Bell, etc. can be used.

The finely pulverized alloy particles may then be subjected to a surface treatment, such as an alkali treatment using an alkaline aqueous solution of KOH or NaOH, or an acid treatment using an aqueous solution of nitric acid, sulfuric acid, or hydrochloric acid. By performing these surface treatments, a layer made of Ni (alkali-treated layer or acid-treated layer) is formed on at least a part of the alloy particle surface, which can suppress the progress of alloy corrosion and increase durability, thereby improving the cycle life characteristics of the battery and the discharge characteristics over a wide temperature range. In particular, in the case of acid treatment, it is preferable to use hydrochloric acid, since it is possible to precipitate Ni while minimizing damage to the alloy surface. In addition, when the alloy is pulverized by a wet method, the surface treatment can be performed at the same time.

The present invention will be described below with reference to examples.
Evaluation cells using hydrogen storage alloys No. 1 to 50 having the composition shown in Tables 1-1 to 1-3 below as negative electrode active materials were prepared as described below, and experiments were conducted to evaluate the characteristics of the evaluation cells. Note that alloys No. 1 to 21 and 34 to 50 shown in Tables 1-1 to 1-3 are alloy examples (invention examples) that meet the conditions of the present invention, and No. 22 to 33 are alloy examples (comparative examples) that do not meet the conditions of the present invention. Furthermore, alloy No. 22 of the comparative example was used as a reference alloy for evaluating the characteristics of the cells.

(Preparation of negative electrode active material)
The raw materials of the alloys No. 1 to 50 shown in Tables 1-1 to 1-3 (Sm, La, Ce, Mg, Ni, Al, Cr, Zn, Sn, Si, V and Mo each have a purity of 99% or more) were melted in an argon atmosphere (Ar: 100 vol%, 0.1 MPa) using a high-frequency induction heating furnace, and cast into ingots. Next, these alloy ingots were subjected to a heat treatment in an argon atmosphere (Ar: 90 vol%, 0.1 MPa) at a temperature (940 to 1130 ° C.) equal to the melting point T _m of each alloy for 10 hours, and then coarsely crushed and finely crushed to 25 μm in mass-based D50 with a hammer mill to obtain samples (negative electrode active materials) for cell evaluation.

For these alloys, X-ray diffraction (XRD) measurements were performed using Cu-Kα radiation as an X-ray source, and the ratio Ib/Ia of the diffraction line intensity Ib of the (109) plane of the A ₅ B ₁₉ 2H structure to the diffraction line intensity Ia of the (107) plane of the A ₂ B ₇ 2H structure was obtained and listed in Tables 1-1 to 1-3. The XRD measurement conditions were a scan speed of 0.5°/min, a tube voltage of 40 kV, a tube current of 40 mA, and a scan step of 0.02°. An example of the XRD measurement results is shown in FIG. 2. In FIG. 2, A is the 2θ diffraction line of the (107) plane of the A ₂ B ₇ phase (2H type), and B is the 2θ diffraction line of the (109) plane of the A ₅ B ₁₉ phase (2H type). No. 22 has a CaCu ₅ type crystal structure, and these cannot be evaluated.

(Preparation of evaluation cells)
<Negative Electrode>
The negative electrode active material prepared above, Ni powder as a conductive assistant, and two types of binders (styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC)) were mixed and kneaded to obtain a paste-like composition in a weight ratio of negative electrode active material:Ni powder:SBR:CMC=95.5:3.0:1.0:0.5. This paste-like composition was applied to a punching metal, dried at 80°C, and then roll-pressed with a load of 15kN to obtain a negative electrode.

<Positive electrode>
Nickel hydroxide (Ni(OH) ₂ ), metal cobalt (Co) as a conductive additive, and two types of binders (styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC)) were mixed in a mass ratio of Ni(OH) ₂ :Co:SBR:CMC=95.5:2.0:2.0:0.5, and kneaded to form a paste-like composition. This paste-like composition was applied to porous nickel, dried at 80°C, and then roll-pressed with a load of 15 kN to obtain a positive electrode.

<Electrolyte>
The electrolyte used was an alkaline aqueous solution prepared by mixing pure water with potassium hydroxide (KOH) to give a concentration of 6 mol/L.

<Evaluation cell>
The positive electrode and the negative electrode were placed in an acrylic case as a counter electrode and a working electrode, respectively, and the electrolyte was poured into the cell to prepare a cell using a Hg/HgO electrode as a reference electrode, which was then used for an evaluation test. The capacity ratio of the working electrode to the counter electrode was adjusted to be working electrode:counter electrode=1:3. A polyethylene nonwoven fabric was placed between the positive electrode and the negative electrode as a separator.

(Cell Characterization)
The evaluation tests of the evaluation cells for the alloys No. 1 to 50 obtained as described above were carried out in the following manner.

(1) Discharge Capacity of Electrode The discharge capacity of the working electrode was confirmed by the following procedure. After constant current charging at a current value of 80 mA/g per active material of the working electrode for 10 hours, constant current discharging was performed at a current value of 40 mA/g per active material of the working electrode. The discharge termination condition was a working electrode potential of -0.5 V. The above charge/discharge was repeated 10 times, and the maximum value of the discharge capacity was taken as the discharge capacity of the working electrode. It was confirmed that the discharge capacity of the working electrode was saturated and stabilized after 10 charge/discharge cycles. The evaluation temperature was 25°C.
The measured discharge capacity was calculated by the following formula (2) using the discharge capacity of alloy No. 22 shown in Table 1-2 as the reference capacity, and the ratio to this was calculated. Those with this ratio of more than 1.15 were evaluated as having a larger discharge capacity than alloy No. 22 and being superior. The evaluation temperature was 25°C.
Discharge capacity=(discharge capacity of evaluated alloy)/(discharge capacity of alloy No. 22) (2)

(2) Cycle Life Characteristics Using the cells in which the discharge capacity of the working electrode was confirmed in the above (1) Electrode Discharge Capacity, the cycle life characteristics of the working electrode were determined by the following procedure. The evaluation temperature was 45°C.
When the current value required to complete charging or discharging the discharge capacity of the working electrode confirmed in the above (1) Discharge capacity of electrode in 1 hour was defined as 1C, constant current charging and constant current discharging were performed at a current value of C/2 within a charging rate of the working electrode in the range of 30 to 70%, and this cycle was repeated 300 times. The discharge capacity after 300 cycles was measured, and the capacity retention rate was calculated by the following formula (3).
Capacity retention rate=(discharge capacity at 300th cycle)/(discharge capacity at 1st cycle) (3)
The cycle life characteristics were evaluated by setting the capacity retention rate after 300 cycles of Alloy No. 22 shown in Table 1-2 as the reference capacity retention rate, and calculating the ratio to this using the following formula (4). Alloys with a ratio of more than 1.20 were evaluated as having a larger cycle life characteristic than Alloy No. 22 and being superior.
Cycle life characteristic=(Capacity retention rate of measured alloy after 300 cycles)/(Capacity retention rate of alloy No. 22 after 300 cycles) (4)

(3) Cost The alloy cost was calculated from the unit price per mass and mass ratio of each raw material of the alloys having the composition shown in Tables 1-1 to 1-3. The alloy cost was evaluated relative to the reference alloy No. 22. In the cost evaluation shown in Tables 1-1 to 1-3, 1 or less (cheaper than the reference alloy) was evaluated as ◯, 1.2 times or less than the same was evaluated as △, and 1.2 times or more was evaluated as ×.

As is clear from Tables 1-1 to 1-3, the alloys No. 1 to 21 and 34 to 50 of the invention examples have better characteristics in various parameters than alloy No. 22, and as a result, it is clear that the evaluation values of the discharge capacity and cycle life characteristics are improved in a well-balanced manner in the battery evaluation. Specifically, the discharge capacity is 1.15 times or more than that of the reference alloy (No. 22), and the cycle life characteristics are 1.20 times or more. In contrast, it can be seen that the evaluation values of any of the electrode characteristics of the alloys No. 23 to 33 of the comparative examples do not satisfy the above criteria. In terms of cost, the alloys of the present invention are also kept low.

The hydrogen storage alloy of the present invention is superior to the conventionally used _AB5 type hydrogen storage alloys in both discharge capacity and cycle life characteristics, and is therefore suitable as an alloy for the negative electrodes of a wide range of alkaline storage batteries, from consumer applications as a replacement for alkaline primary batteries to various industrial and automotive applications.

1: Positive electrode 2: Negative electrode 3: Separator 4: Housing (battery case)
10. Alkaline storage battery

Claims (1)

A _hydrogen storage alloy having a crystal structure including an _A2B7 phase, _{an A5B19} phase, and optionally at least one selected from an _AB3 phase, an _AB2 phase, and an _AB5 phase, _{the A2B7} phase accounting for the largest proportion of the constituent phases on a mass basis,
The hydrogen storage alloy is represented by the following general formula (1):
A hydrogen storage alloy for alkaline storage batteries, characterized in that in X-ray diffraction measurement using Cu-Kα radiation as an X-ray source, the ratio Ib/Ia of the diffraction line intensity Ib of the (109) plane of the A ₅ B ₁₉ 2H structure appearing at 2θ=31.5 to 32.5° to the diffraction line intensity Ia of the (107) plane of the A ₂ B ₇ 2H structure appearing at 2θ=32.5 to 33.5° is 0.05 or more and 1.0 or less.
(La _1-a-b Ce _a Sm _b ) _1-c Mg _c Ni _d M _e T _f ...(1)
Here, M, T and the subscripts a, b, c, d, e and f in the above formula (1) are
M: Al, or Al and at least one selected from Zn, Sn, and Si;
T: at least one selected from Cr, Mo, and V;
0<a≦0.10,
0≦b≦0.15,
0<a+b≦0.16
0.225≦c≦0.32,
0.03≦e≦0.16,
0≦f≦0.03,
3.25≦d+e+f<3.50
Meet the conditions.

Images