【０００１】
【発明の属する技術分野】
本発明は、アルカリ蓄電池の負極として使用される水素吸蔵合金電極の製造方法に係わり、特に、低温下での放電容量が大きく、しかも充放電サイクル寿命が長いアルカリ蓄電池を与える水素吸蔵合金電極を提供することを目的とした、電極材料たる水素吸蔵合金粉末の製造方法の改良に関する。
【０００２】
【従来の技術及び発明が解決しようとする課題】
近年、水素吸蔵合金電極を負極として使用したアルカリ蓄電池が、従前のカドミウム電極又は亜鉛電極を負極として使用したアルカリ蓄電池と比較して、エネルギー密度が高いことから、注目されている。
【０００３】
アルカリ蓄電池用の水素吸蔵合金電極には、合金粉末を焼結して作製する焼結式水素吸蔵合金電極と、導電性芯体に合金粉末を含有するペーストを塗布又は充填して作製するペースト式水素吸蔵合金電極とがあるが、いずれの電極も、充放電時の電極の体積変化に伴い合金粉末が微細化して脱落し易く、これが放電容量の低下や充放電サイクル寿命の短命化を招いていた。
【０００４】
上記の問題を解決するために、微細化した合金粉末を使用することが提案されている。例えば、特開昭６０−７０６６５号公報では、粒径２５μｍ以下の水素吸蔵合金粉末（同公報には、水素吸蔵合金として、Ｔｉ２ Ｎｉ、ＬａＮｉ５ 及びＣａＮｉ５ が示されている。）を使用することが提案されている。予め微細化した合金粉末を使用することにより、充放電時の体積変化に伴う微細化の程度を軽減し、合金粉末の脱落を抑制するようにしたものである。
【０００５】
しかしながら、本発明者らが検討した結果、予め微細化した合金粉末を使用した場合は、（１）合金粒子の表面が酸化され易く、反応性が低下する、（２）生成した酸化被膜のために、合金粒子同士の集電性が低下して、低温下での放電容量が減少する、（３）充放電サイクル初期から合金の酸素濃度が高く、劣化し易いために、充放電サイクル寿命もさほど改善されない、などの問題があることが分かった。
【０００６】
したがって、本発明は、放電容量、特に低温下での放電容量が大きく、しかも充放電サイクル寿命が長いアルカリ蓄電池を与える水素吸蔵合金電極の製造方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記目的を達成するための本発明に係るアルカリ蓄電池用水素吸蔵合金電極の製造方法は、組成式Ｍ1 Ｎｉx Ｃｏy Ｍ2 z 〔式中、Ｍ1 はＹ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｓｍ、Ｅｕ、Ｇｄ、Ｔｂ、Ｄｙ、Ｈｏ、Ｅｒ、Ｙｂ及びＬｕよりなる群から選ばれた少なくとも一種の希土類元素、Ｍ2 はＡｌ、Ｍｎ、Ｆｅ、Ｓｎ、Ｓｉ、Ｗ、Ｚｎ、Ｃｒ及びＣｕよりなる群から選ばれた少なくとも一種の元素、３．９≦ｘ≦４．４、０≦ｙ≦０．４、０≦ｚ≦１．０、５．１≦ｘ＋ｙ＋ｚ≦５．４である。〕で表されるＣａＣｕ5 型結晶構造を有する水素吸蔵合金からなり、粒径が２５μｍより大きく、且つ３２μｍ以下の粒子の個数に対する、粒径が２５μｍ以下の粒子の個数が５０％以下であり、且つ粒径が３２μｍより大きい粒子の個数が５％以下である水素吸蔵合金粉末が、水素吸蔵材として使用されたアルカリ蓄電池用水素吸蔵合金電極の製造方法において、前記水素吸蔵合金粉末をグリセリンを含む酸性溶液中で浸漬処理して、当該水素吸蔵合金粉末を構成する粒子の表面から深さ５０Åまでの表面層に存在する全ニッケル原子の１０％以上を単体とすることを特徴とする。
【０００８】
本発明において用いる水素吸蔵合金の組成が、本発明で規制する範囲を外れると、低温下での放電容量が減少するとともに、充放電サイクル寿命が短くなる。また、粒径が２５μｍより大きく、且つ３２μｍ以下の粒子の個数に対する粒径が３２μｍより大きい粒子の個数が５％よりも大きい場合は、充放電時の体積変化に伴う水素吸蔵合金粒子の微細化の程度が大きくなって、合金粉末の脱落が起こり易くなり、低温下での放電容量が減少するとともに、充放電サイクル寿命が短くなる。一方、粒径が２５μｍより大きく、且つ３２μｍ以下の粒子の個数に対する粒径が２５μｍ以下の粒子の個数が５０％よりも大きい場合は、スラリーとしたときの分散性が悪く、極板も作製したときに不均一なものとなり、その不均一な部分の存在により、電極において電気化学反応が起こる部分が偏在するので、充放電サイクル寿命が短くなり、低温下での放電容量が減少する。
【００１０】
本発明に係るアルカリ蓄電池用水素吸蔵合金電極の製造方法においては、上記の水素吸蔵合金粉末を、グリセリンを含む酸性溶液（塩酸、リン酸、硝酸など）中で浸漬処理して、当該水素吸蔵合金粉末を構成する粒子の表面から深さ５０Åまでの表面層に存在する全ニッケル原子の１０％以上を単体とする。これにより、電解液と合金粒子表面との反応性が向上して、低温下での放電容量が増大する。
【００１１】
低温下での放電容量が特に大きい水素吸蔵合金電極を得るためには、水素吸蔵合金粉末として、理由は定かでないが、アトマイズ法により作製した粒子を１０重量％以上含有するものを使用することが好ましい。
【００１２】
【実施例】
以下、本発明を実施例に基づいてさらに詳細に説明するが、本発明は下記実施例に何ら限定されるものではなく、その要旨を変更しない範囲において適宜変更して実施することが可能なものである。
【００２５】
（実験１）
Ｍｍ（ミッシュメタル；Ｌａ２５重量％、Ｃｅ５０重量％、Ｐｒ５重量％、Ｎｄ１５重量％含有）と、Ｎｉ（純度９９．９％）と、Ａｌ（純度９９．９％）と、Ｍｎ（純度９９．９％）とを、原子比１：４．４：０．３：０．６で混合して得た混合物を、アルゴン雰囲気のアーク溶解炉で熔融させた後（溶湯温度１２００°Ｃ）、冷却して、組成式ＭｍＮｉ4.4 Ａｌ0.3 Ｍｎ0.6 で表される水素吸蔵合金の塊（インゴット）を作製した。このようにして得た水素吸蔵合金塊を空気中にて機械的に粉砕し、１００メッシュの篩にかけて粒径１５０μｍ以下の粉末とした後、さらに５００メッシュの篩及び４４０メッシュの篩に順次かけて分級し、粒径が２５μｍ以下の粉末（５００メッシュパスの粉末）、粒径が２５μｍより大きく３２μｍ以下の粉末（４４０メッシュパスの粉末）及び粒径が３２μｍより大きい粉末を得た。このようにして得た３種類の粉末を重量比１．１７：１：０．０８で混合して、水素吸蔵材としての水素吸蔵合金粉末を作製した。この水素吸蔵合金粉末に含まれる上記の３種類の粉末の個数比を、粒度分布より求めたところ、０．４５：１：０．０３であった。この水素吸蔵合金粉末を、グリセリンを含むｐＨ１の塩酸に浸漬し、攪拌しながら浴温４０°Ｃの水浴にて２０分間加熱した後、次亜リン酸ナトリウム水溶液で洗浄し、水洗し、真空乾燥して、粒子の表面から深さ５０Åまでの表面層に存在する全ニッケル原子の５％、１０％、１１％又は２３％が単体化した水素吸蔵合金粉末を作製した。単体化したニッケル原子の割合は、ＸＰＳを用いて測定し、単体化したニッケル原子に帰属されるピーク面積と、単体化していないニッケル原子に帰属されるピークの面積とを比較することにより算出した。単体率は、合金と塩酸の量比、グリセリン濃度、及び塩酸の加熱温度を変えることにより調節した。
上記の各水素吸蔵合金粉末１００重量部と、結着剤としてのポリエチレンオキシド０．５重量部と、適量の水とを、混合してスラリーを調製し、このスラリーを集電体としてのパンチングメタルに塗布し、加熱乾燥して、水素吸蔵合金電極（寸法：縦４０ｍｍ；横８６ｍｍ）を作製した。
負極として上記の各水素吸蔵合金電極を、正極として従来公知の非焼結式ニッケル極を、セパレータとしてポリアミド不織布を、それぞれ使用して、理論容量１０００ｍＡｈのＡＡサイズの密閉型アルカリ蓄電池Ａ１４〜Ａ１７を作製した。いずれの電池も、正極の容量を負極の容量に比べて小さくして、電池の容量が正極の容量により規制されるようにした。ここで、グリセリンは液の粘度を調整するために用いており、これにより、粒子表面のＮｉ単体の生成量を変化させることができる。
【００２６】
〈各電池の低温下での放電容量〉
室温（２５°Ｃ）にて１００ｍＡで１２時間充電した後、室温にて１００ｍＡで１．０Ｖまで放電する予備の充放電を２サイクル行った。次いで、室温にて１００ｍＡで１６時間充電した後、−１０°Ｃにて１０００ｍＡで１．０Ｖまで放電して、各電池の低温下での放電容量を求めた。結果を表１に示す。
【００２７】
〈各電池の充放電サイクル寿命〉
室温にて１２００ｍＡで１時間充電した後、室温にて１２００ｍＡで１．０Ｖまで放電する工程を１サイクルとする充放電サイクル試験を行い、各電池の充放電サイクル寿命を求めた。結果を表１に示す。
【００２８】
【表１】
【００２９】
表１に示すように、電池Ａ１５〜１７は、電池Ａ１４に比べて、低温下での放電容量が大きい。この結果から、水素吸蔵合金粉末を酸性水溶液に浸漬して、合金粒子の表面に存在する全ニッケル原子の１０％以上を単体化することにより、低温下での放電容量が増大することが分かる。
【００３０】
（実験２）
アトマイズ法（溶湯温度１２００°Ｃ、アルゴン雰囲気）により作製した、組成式ＭｍＮｉ4.4 Ａｌ0.3 Ｍｎ0.6 で表される水素吸蔵合金を、１００メッシュの篩にかけて粒径１５０μｍ以下の粉末とした後、さらに５００メッシュの篩及び４４０メッシュの篩に順次かけて分級し、粒径が２５μｍ以下の粉末（５００メッシュパスの粉末）、粒径が２５μｍより大きく３２μｍ以下の粉末（４４０メッシュパスの粉末）及び粒径が３２μｍより大きい粉末を得た。このようにして得た３種類の粉末を重量比１．１７：１：０．０８で混合して、水素吸蔵材としての水素吸蔵合金粉末を作製した。この水素吸蔵合金粉末に含まれる上記の３種類の粉末の個数比を、粒度分布より求めたところ、０．４５：１：０．０３であった。この水素吸蔵合金粉末を、グリセリンを含むｐＨ１の塩酸に浸漬し、攪拌しながら浴温４０°Ｃの水浴にて２０分間加熱した後、次亜リン酸ナトリウム水溶液で洗浄し、水洗し、真空乾燥して、粒子の表面から深さ５０Åまでの表面層に存在する全ニッケル原子の１２％が単体化した水素吸蔵合金粉末を作製した。また、Ｍｍ（ミッシュメタル；Ｌａ２５重量％、Ｃｅ５０重量％、Ｐｒ５重量％、Ｎｄ１５重量％含有）と、Ｎｉ（純度９９．９％）と、Ａｌ（純度９９．９％）と、Ｍｎ（純度９９．９％）とを、原子比１：４．４：０．３：０．６で混合して得た混合物を、アルゴン雰囲気のアーク溶解炉で熔融させた後（溶湯温度１２００°Ｃ）、冷却して、組成式ＭｍＮｉ 4.4 Ａｌ 0.3 Ｍｎ 0.6 で表される水素吸蔵合金の塊（インゴット）を作製した。このようにして得た水素吸蔵合金塊をアルゴン中にて機械的に粉砕し、１００メッシュの篩にかけて粒径１５０μｍ以下の粉末とした後、さらに５００メッシュの篩及び４４０メッシュの篩に順次かけて分級し、粒径が２５μｍ以下の粉末（５００メッシュパスの粉末）、粒径が２５μｍより大きく３２μｍ以下の粉末（４４０メッシュパスの粉末）及び粒径が３２μｍより大きい粉末を得た。このようにして得た３種類の粉末を重量比１．１７：１：０．０８で混合して、水素吸蔵材としての水素吸蔵合金粉末を作製した。この水素吸蔵合金粉末に含まれる上記の３種類の粉末の個数比を、粒度分布より求めたところ、０．４５：１：０．０３であった。
上記の２種の水素吸蔵合金粉末、又は、これら２種の水素吸蔵合金粉末の種々の割合の混合粉末を使用したこと以外は実験１と同様にして、密閉型アルカリ蓄電池Ａ１８〜Ａ２３を作製した。
【００３１】
〈各電池の低温下での放電容量〉
室温（２５°Ｃ）にて１００ｍＡで１２時間充電した後、室温にて１００ｍＡで１．０Ｖまで放電する予備の充放電を２サイクル行った。次いで、室温にて１００ｍＡで１６時間充電した後、−１５°Ｃにて１０００ｍＡで１．０Ｖまで放電して、各電池の低温下での放電容量を求めた。結果を表２に示す。
【００３２】
〈各電池の充放電サイクル寿命〉
室温にて１０００ｍＡで１時間充電した後、室温にて１０００ｍＡで１．０Ｖまで放電する工程を１サイクルとする充放電サイクル試験を行い、各電池の充放電サイクル寿命を求めた。結果を表２に示す。
【００３３】
【表２】
【００３４】
表２に示すように、電池Ａ２０〜２３は、電池Ａ１８，１９に比べて、低温下での放電容量が大きい。この結果から、水素吸蔵合金粉末として、アトマイズ法により作製した粒子を１０重量％以上含有するものを使用することにより、低温下での放電容量が増大することが分かる。
【００３５】
上記の実施例では、本発明に係る製造方法をペースト式水素吸蔵合金電極の製造に適用する場合について述べたが、本発明に係る製造方法は焼結式水素吸蔵合金電極の製造にも適用可能である。
【００３６】
【発明の効果】
本発明により、低温下での放電容量が大きく、しかも充放電サイクル寿命が長いアルカリ蓄電池を与える水素吸蔵合金電極の製造方法が提供される。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hydrogen storage alloy electrode used as a negative electrode of an alkaline storage battery , and in particular, to provide a hydrogen storage alloy electrode which provides an alkaline storage battery having a large discharge capacity at a low temperature and a long charge / discharge cycle life. The present invention relates to an improvement in a method for producing a hydrogen storage alloy powder, which is an electrode material, for the purpose of carrying out the method .
[0002]
Problems to be solved by the prior art and the invention
In recent years, an alkaline storage battery using a hydrogen storage alloy electrode as a negative electrode has attracted attention because it has a higher energy density than a conventional alkaline storage battery using a cadmium electrode or a zinc electrode as a negative electrode.
[0003]
A hydrogen storage alloy electrode for an alkaline storage battery includes a sintered hydrogen storage alloy electrode produced by sintering alloy powder, and a paste-type electrode produced by applying or filling a paste containing the alloy powder on a conductive core. There is a hydrogen storage alloy electrode, but in each case, the alloy powder becomes finer and easily falls off as the volume of the electrode changes during charging and discharging, which leads to a decrease in discharge capacity and a shortened life of the charge and discharge cycle life. Was.
[0004]
In order to solve the above-mentioned problem, it has been proposed to use a finer alloy powder. For example, in Japanese Patent Application Laid-Open No. 60-70665, a hydrogen storage alloy powder having a particle size of 25 μm or less (Ti 2 Ni, LaNi 5 and CaNi 5 are shown as hydrogen storage alloys in the same publication). It has been proposed to. By using an alloy powder that has been fined in advance, the degree of the fineness due to a change in volume during charging and discharging is reduced, and the alloy powder is prevented from falling off.
[0005]
However, as a result of investigations by the present inventors, when alloy powder that has been previously refined is used, (1) the surface of the alloy particles is easily oxidized and the reactivity decreases, and (2) the oxide film formed In addition, the current collecting property between the alloy particles is reduced, and the discharge capacity at a low temperature is reduced. (3) The oxygen concentration of the alloy is high from the beginning of the charge / discharge cycle, and the charge / discharge cycle life is increased because the alloy is easily deteriorated. It turned out that there was a problem that it was not much improved.
[0006]
Therefore, an object of the present invention is to provide a method for producing a hydrogen storage alloy electrode that provides an alkaline storage battery having a large discharge capacity, particularly a discharge capacity at a low temperature, and a long charge-discharge cycle life.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a method for producing a hydrogen storage alloy electrode for an alkaline storage battery according to the present invention comprises a composition formula M 1 Ni x Co y M 2 z [where M 1 is Y, La, Ce, Pr, Nd , Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and at least one rare earth element selected from the group consisting of Lu; M 2 is Al, Mn, Fe, Sn, Si, W, Zn, Cr; At least one element selected from the group consisting of Cu, 3.9 ≦ x ≦ 4.4, 0 ≦ y ≦ 0.4, 0 ≦ z ≦ 1.0, 5.1 ≦ x + y + z ≦ 5.4. . A hydrogen storage alloy having a CaCu 5 type crystal structure represented by the formula: wherein the number of particles having a particle size of 25 μm or less is 50% or less with respect to the number of particles having a particle size of more than 25 μm and 32 μm or less; A method for producing a hydrogen storage alloy electrode for an alkaline storage battery, wherein the number of particles having a particle size of more than 32 μm is 5% or less is used as a hydrogen storage material , wherein the hydrogen storage alloy powder contains glycerin. By immersing in an acidic solution, at least 10% of all nickel atoms present in a surface layer from the surface of the particles constituting the hydrogen storage alloy powder to a depth of 50 ° are made into a simple substance.
[0008]
When the composition of the hydrogen storage alloy used in the present invention is out of the range regulated by the present invention, the discharge capacity at a low temperature is reduced and the charge / discharge cycle life is shortened. If the number of particles having a particle size larger than 32 μm is larger than 5% with respect to the number of particles having a particle size larger than 25 μm and smaller than 32 μm, the hydrogen storage alloy particles are reduced in size due to a change in volume during charging and discharging. , The alloy powder is apt to fall off, the discharge capacity at low temperatures is reduced, and the charge / discharge cycle life is shortened. On the other hand, when the number of particles having a particle size of 25 μm or less was larger than 50% of the number of particles having a particle size of more than 25 μm and 32 μm or less, the dispersibility of the slurry was poor, and an electrode plate was also prepared. Occasionally, the electrode becomes non-uniform, and due to the non-uniform part, the part where the electrochemical reaction occurs in the electrode is unevenly distributed, so that the charge / discharge cycle life is shortened and the discharge capacity at a low temperature is reduced.
[0010]
In the method for producing a hydrogen storage alloy electrode for an alkaline storage battery according to the present invention, the hydrogen storage alloy powder is immersed in an acidic solution containing glycerin (eg, hydrochloric acid, phosphoric acid, nitric acid, etc.), At least 10% of all nickel atoms existing in the surface layer from the surface of the particles constituting the powder to a depth of 50 ° are defined as a simple substance. Thereby, the reactivity between the electrolytic solution and the surface of the alloy particles is improved, and the discharge capacity at a low temperature is increased.
[0011]
To obtain a particularly large hydrogen storage alloy electrode discharge capacity under low temperature, as the hydrogen-absorbing alloy powder, although the reason is not clear, the use of those containing particles produced by an atomizing method 10 wt% or more Is preferred.
[0012]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and can be implemented by appropriately changing the scope without changing the gist thereof. It is.
[0025]
(Experiment 1)
Mm (Misch metal; La 25% by weight, Ce 50% by weight, Pr 5% by weight, Nd 15% by weight), Ni (purity 99.9%), Al (purity 99.9%), Mn (purity 99.9%) %) With an atomic ratio of 1: 4.4: 0.3: 0.6, and the mixture was melted in an arc melting furnace in an argon atmosphere (melt temperature: 1200 ° C.), and then cooled. Thus, a lump (ingot) of a hydrogen storage alloy represented by a composition formula of MmNi 4.4 Al 0.3 Mn 0.6 was produced. The hydrogen storage alloy lump thus obtained was mechanically pulverized in the air, sieved with a 100-mesh sieve to obtain a powder having a particle size of 150 μm or less, and then sequentially sieved with a 500-mesh sieve and a 440-mesh sieve. After classification, a powder having a particle size of 25 μm or less (powder of 500 mesh pass), a powder having a particle size of more than 25 μm and 32 μm or less (powder of 440 mesh pass), and a powder having a particle size of more than 32 μm were obtained. The three kinds of powders thus obtained were mixed at a weight ratio of 1.17: 1: 0.08 to prepare a hydrogen storage alloy powder as a hydrogen storage material. The number ratio of the above three kinds of powders contained in the hydrogen storage alloy powder was 0.45: 1: 0.03 as determined from the particle size distribution. This hydrogen storage alloy powder is immersed in hydrochloric acid of pH 1 containing glycerin, heated with stirring in a water bath at a bath temperature of 40 ° C. for 20 minutes, washed with an aqueous solution of sodium hypophosphite, washed with water, and dried under vacuum. Then, a hydrogen storage alloy powder in which 5%, 10%, 11% or 23% of all nickel atoms existing in the surface layer from the surface of the particle to a depth of 50 ° were singulated was produced. The proportion of singulated nickel atoms was measured using XPS, and was calculated by comparing the peak area attributed to singulated nickel atoms with the peak area attributed to unsingulated nickel atoms. . The simplex ratio was adjusted by changing the amount ratio of the alloy and hydrochloric acid, the glycerin concentration, and the heating temperature of hydrochloric acid.
100 parts by weight of each of the above hydrogen storage alloy powders, 0.5 part by weight of polyethylene oxide as a binder, and an appropriate amount of water were mixed to prepare a slurry, and this slurry was used as a punching metal as a current collector. And dried by heating to prepare a hydrogen storage alloy electrode (dimensions: 40 mm long ; 86 mm wide ).
Each of the above-mentioned hydrogen storage alloy electrodes as a negative electrode, a conventionally known non-sintered nickel electrode as a positive electrode, and a polyamide non-woven fabric as a separator were used, and AA-size sealed alkaline storage batteries A14 to A17 having a theoretical capacity of 1000 mAh were used. Produced. In each of the batteries, the capacity of the positive electrode was made smaller than the capacity of the negative electrode, so that the capacity of the battery was regulated by the capacity of the positive electrode. Here, glycerin is used to adjust the viscosity of the liquid, whereby the amount of Ni alone generated on the particle surface can be changed.
[0026]
<Discharge capacity of each battery at low temperature>
After charging for 12 hours at 100 mA at room temperature (25 ° C.), two preliminary charge / discharge cycles of discharging to 1.0 V at 100 mA at room temperature were performed. Next, the battery was charged at room temperature for 16 hours at 100 mA, and then discharged at -10 ° C. at 1000 mA to 1.0 V to determine the discharge capacity of each battery at a low temperature. Table 1 shows the results.
[0027]
<Charge / discharge cycle life of each battery>
After charging at 1200 mA for 1 hour at room temperature, a charge / discharge cycle test was performed in which a step of discharging to 1.0 V at 1200 mA at room temperature was performed as one cycle, and the charge / discharge cycle life of each battery was determined. Table 1 shows the results.
[0028]
[Table 1]
[0029]
As shown in Table 1 , batteries A15 to A17 have a larger discharge capacity at low temperatures than battery A14. From these results, it can be seen that the discharge capacity at low temperature is increased by immersing the hydrogen storage alloy powder in an acidic aqueous solution to singulate 10% or more of all nickel atoms present on the surface of the alloy particles.
[0030]
(Experiment 2)
A hydrogen storage alloy represented by a composition formula of MmNi 4.4 Al 0.3 Mn 0.6 produced by an atomizing method (melt temperature: 1200 ° C., argon atmosphere) was sieved with a 100-mesh sieve to obtain a powder having a particle size of 150 μm or less. The particles are classified by sequentially passing through a mesh sieve and a 440 mesh sieve, and powder having a particle size of 25 μm or less (powder of 500 mesh pass), powder having a particle size of more than 25 μm and 32 μm or less (powder of 440 mesh pass) and particle size Was larger than 32 μm. The three kinds of powders thus obtained were mixed at a weight ratio of 1.17: 1: 0.08 to prepare a hydrogen storage alloy powder as a hydrogen storage material. The number ratio of the above three kinds of powders contained in the hydrogen storage alloy powder was 0.45: 1: 0.03 as determined from the particle size distribution. This hydrogen storage alloy powder is immersed in hydrochloric acid of pH 1 containing glycerin, heated with stirring in a water bath at a bath temperature of 40 ° C. for 20 minutes, washed with an aqueous solution of sodium hypophosphite, washed with water, and dried under vacuum. Thus, a hydrogen storage alloy powder was prepared in which 12% of all nickel atoms present in the surface layer from the surface of the particle to a depth of 50 ° were singulated. Further, Mm (Misch metal; containing 25% by weight of La, 50% by weight of Ce, 5% by weight of Pr, and 15% by weight of Nd), Ni (purity 99.9%), Al (purity 99.9%), and Mn (purity 99%) 9.9%) and a mixture obtained by mixing at an atomic ratio of 1: 4.4: 0.3: 0.6 in an arc melting furnace in an argon atmosphere (melt temperature: 1200 ° C.) By cooling, a lump (ingot) of a hydrogen storage alloy represented by the composition formula MmNi 4.4 Al 0.3 Mn 0.6 was produced. The hydrogen storage alloy lump thus obtained is mechanically pulverized in argon, sieved with a 100-mesh sieve to obtain a powder having a particle size of 150 μm or less, and then sequentially sieved with a 500-mesh sieve and a 440-mesh sieve. After classification, a powder having a particle size of 25 μm or less (powder of 500 mesh pass), a powder having a particle size of more than 25 μm and 32 μm or less (powder of 440 mesh pass), and a powder having a particle size of more than 32 μm were obtained. The three kinds of powders thus obtained were mixed at a weight ratio of 1.17: 1: 0.08 to prepare a hydrogen storage alloy powder as a hydrogen storage material. The number ratio of the above three kinds of powders contained in the hydrogen storage alloy powder was 0.45: 1: 0.03 as determined from the particle size distribution.
The sealed alkaline storage batteries A18 to A23 were produced in the same manner as in Experiment 1, except that the above two kinds of hydrogen storage alloy powders or the mixed powders of various ratios of these two kinds of hydrogen storage alloy powders were used. .
[0031]
<Discharge capacity of each battery at low temperature>
After charging at 100 mA for 12 hours at room temperature (25 ° C.), two preliminary charge / discharge cycles of discharging to 1.0 V at 100 mA at room temperature were performed. Next, the battery was charged at room temperature for 16 hours at 100 mA, and then discharged at -15 ° C. at 1000 mA to 1.0 V to determine the discharge capacity of each battery at a low temperature. Table 2 shows the results.
[0032]
<Charge / discharge cycle life of each battery>
After charging at 1000 mA for 1 hour at room temperature, a charge / discharge cycle test was performed in which the process of discharging to 1.0 V at 1000 mA at room temperature was one cycle, and the charge / discharge cycle life of each battery was determined. Table 2 shows the results.
[0033]
[Table 2]
[0034]
As shown in Table 2 , batteries A20 to A23 have a larger discharge capacity at low temperatures than batteries A18 and A19. From these results, it can be seen that the use of the hydrogen storage alloy powder containing 10% by weight or more of particles produced by the atomization method increases the discharge capacity at low temperatures.
[0035]
In the above embodiment, the case where the production method according to the present invention is applied to the production of a paste-type hydrogen storage alloy electrode has been described. However, the production method according to the present invention is also applicable to the production of a sintered hydrogen storage alloy electrode. It is.
[0036]
【The invention's effect】
According to the present invention, there is provided a method for producing a hydrogen storage alloy electrode which provides an alkaline storage battery having a large discharge capacity at a low temperature and a long charge / discharge cycle life.