JP4296592B2 - Hydrogen storage alloy powder and manufacturing method thereof, hydrogen storage alloy electrode and nickel metal hydride storage battery - Google Patents
Hydrogen storage alloy powder and manufacturing method thereof, hydrogen storage alloy electrode and nickel metal hydride storage battery Download PDFInfo
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- JP4296592B2 JP4296592B2 JP2003071596A JP2003071596A JP4296592B2 JP 4296592 B2 JP4296592 B2 JP 4296592B2 JP 2003071596 A JP2003071596 A JP 2003071596A JP 2003071596 A JP2003071596 A JP 2003071596A JP 4296592 B2 JP4296592 B2 JP 4296592B2
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- hydrogen storage
- storage alloy
- alloy powder
- hydrogen
- surface layer
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Classifications
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Landscapes
- Secondary Cells (AREA)
- Battery Electrode And Active Subsutance (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、水素吸蔵合金粉末とその製造方法、前記水素吸蔵合金粉末を適用した水素吸蔵合金電極およびニッケル水素蓄電池に関し、さらに詳しくは、高率放電特性に優れた水素吸蔵合金電極およびニッケル水素蓄電池に関するものである。
【０００２】
【従来の技術】
近年、ハイブリッド型電気自動車（ＨＥＶ）、電動工具を始めとする大電流放電が必要な電動機器が急速に増加する傾向にある。これらの機器の電源として、密閉型ニッケル水素蓄電池がニッケルカドミウム蓄電池や鉛蓄電池等よりも単位体積および単位質量当たりのエネルギが高い上、環境にクリーンな電源として最近特に注目されている。また、ニッケル水素蓄電池は、過充電時に正極で発生する酸素を、水素吸蔵合金を含有する負極で吸収することが可能であるため、充電制御方式が容易で、充電回路も簡単になる利点を有している。
【０００３】
前記ＨＥＶや電動工具の駆動電源としての蓄電池には約１０ＩｔＡ放電という高率での放電が求められている。しかしながら、水素吸蔵合金電極がカドミウム電極に比べて活物質表面での電荷移動が遅く、電極反応の活性が低いために、従来のニッケル水素蓄電池は、ニッケルカドミウム蓄電池に比較して高率放電特性が劣り、前記高率放電を要求される用途には不利であった。また、水素吸蔵合金をそのまま電極に適用した場合、充分な放電特性を発揮するまでの初期の活性化に時間がかかり、前記高率放電を要求される用途に適用させようとすると、数十回から場合によっては数百回の活性化のための充放電が必要となっている。
【０００４】
これらの問題点のうち、水素吸蔵合金の活性化の遅い点を解決するため、その表面処理法について多くの提案がなされている。例えば特許文献１には、ｐＨ値が０．５〜３．５の酸性水溶液により表面処理を行う方法が開示されている。該酸処理により、水素吸蔵合金の活性度が向上するが、その効果は大きくない。これは、Ｎｉが酸により溶解するため、生成させたＮｉリッチ層のＮｉが溶出し、酸処理後もＮｉリッチ層を保っておくことが困難なためである。更に、酸処理によって合金表面のＬａを含む希土類元素やマンガン、アルミニウムを溶出させ、表面に水素吸蔵合金粉末内部の母層よりもＮｉの含有比率が大きい層（以下Ｎｉリッチ層と記載する）を生成させ電極反応活性を向上させることができるが、該Ｎｉリッチ層は、水素を吸蔵する性質に乏しく、Ｎｉリッチ層中の水素の拡散も速くはない。そのため、合金の母層に吸蔵された水素が合金粉末表面の電極反応の場に拡散し難い欠点がある。
【０００５】
また、特許文献２には、温度９０℃以上で、水酸化ナトリウムを３０〜８０重量％含む水溶液に浸漬する方法が開示されている。このようなアルカリ水溶液による処理方法を用いると、水素吸蔵合金の電極反応活性が向上するが、その効果は大きくない。これは、アルカリ処理においては電極活性に富んだ表面層の形成速度が極めて遅いためである。通常用いられる粒径が２０〜５０μｍの合金粉末の場合、その比表面積が小さく、電極反応活性の向上には数十ｎｍ以上のＮｉリッチ層を確保する必要がある。ところが、アルカリ処理は一旦極薄いＮｉリッチ層が形成されると形成速度が低下してしまい、電極反応活性の向上に必要な厚さを持った働くＮｉリッチ層を形成することが困難である。また、アルカリ処理を行うと、溶出した希土類元素の水酸化物などが生成し、該生成部が合金表面を覆ってしまうために水素吸蔵合金電極の導電性を低下させ、高率放電特性を低下させる。
【０００６】
さらに、特許文献３や特許文献４には、Ｎｉを主成分とする層の厚さが５０〜２００ｎｍであって、アルカリ溶液に浸漬した後、希薄な酸溶液に浸漬する製造方法や、アルカリ性水溶液処理後、水素吸蔵合金粉末を酸性水溶液で処理する方法が開示されている。このようなアルカリ水溶液による処理の後、酸処理を施す方法を用いると、合金の表面を覆っている前記アルカリ処理による希土類元素の水酸化物等の生成物は除去できるものの、アルカリ処理のみを行った場合と同様に常用される粒径の水素吸蔵合金粉末を用いた場合、その効果は大きくない。
【０００７】
また、特許文献５には、６０℃以上のアルカリ処理溶液にクエン酸、グルコン酸などの錯化剤を加えることが開示されている。しかしながら、特許文献５に記載の方法では、Ｎｉリッチ層が形成しにくく、十分な効果を得ることが出来ない。これは、合金から処理液中に溶出した元素が錯化剤と反応して錯化物を形成するものの、溶液中ではイオン状態で存在するため、処理液中のイオン濃度が高まるにつれてＮｉリッチ層の形成が進みにくくなるためである。
【０００８】
【特許文献１】
特開平７−７３８７８号公報（ページ３、段落００１１）
【特許文献２】
特開２００２−２５６３０１号公報（ページ３、段落０００９）
【特許文献３】
特開平９−７５９１号公報（ページ３、段落００１７〜００１８）
【特許文献４】
特開平９−１７１８２１号公報（ページ２、段落０００７）
【特許文献５】
特開２００１−６８１０４号公報（ページ２および３、段落０００９〜００１７）
【０００９】
【発明が解決しようとする課題】
本発明は、上記問題点を解決するためなされたものであって、その一つの目的は、高率放電性能及び充放電サイクル性能に優れたニッケル水素蓄電池を提供することとニッケル水素蓄電池の化成を大幅に簡略化してニッケル水素蓄電池を容易、かつ安価に製造する方法を提供することである。
【００１０】
【課題を解決するための手段】
上記の課題を達成するために、本発明者らは鋭意検討の結果、Ｌａを含む希土類元素とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分として構成された、水素を吸蔵脱離可能な水素吸蔵合金粉末において、水素吸蔵合金の母相の外側に、母相と組成を異にする特定の組成の合金層を、特定の厚みで配置し、質量飽和磁化の値を特定のものとすることにより、驚くべきことに、優れた高率放電特性とサイクル寿命特性を備えるニッケル水素蓄電池が得られることを見出し、本発明に至った。上記の課題を達成するための本発明の手段は以下に記述する通りである。ただし、作用機構については尚推定を含む部分があり、その作用機構の正否は、本発明を制限するものではない。
【００１１】
本発明に係る水素吸蔵合金粉末は、前記Ｌａを含む希土類元素とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分として構成された、水素を吸蔵脱離可能な水素吸蔵合金粉末を、特定の錯化剤を一定量用いた特定処理液を用いて処理することによって、前記母相の外面に特定の組成および特定の厚さを有する合金の層（以下表面層と記述する）を形成した水素吸蔵合金粉末である。
【００１２】
本発明に係る水素吸蔵合金粉末は、具体的には、Ｌａを含む希土類元素とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分として構成された水素吸蔵合金粉末において、前記水素吸蔵合金粉末の母相の外面に、Ｎｉ含有比率を１００ｍｏｌ％としたときにＬａを５ｍｏｌ％以上含有し、かつ、ＭｎとＡｌの含有比率の和が５ｍｏｌ％以下であって、厚さが５０〜４００ｎｍの表面層を形成させた水素吸蔵合金粉末である。ここで、表面層の厚さは、水素吸蔵合金粉末の断面を透過型電子顕微鏡や収束イオンビーム装置を用いて観察して求められる値である。
【００１３】
前記のように、従来提案されていた水素吸蔵合金粉末の表面に形成させたＮｉリッチ層は、電極反応に対して十分に高い触媒活性を有するものの、Ｎｉリッチ層内を水素が拡散し難く、母相に吸蔵された水素が合金粉末表面の電極反応の場に到達するのが遅れるために十分な高率放電特性が得られにくいものと考えられる。これに対して、本発明の特定組成の表面層は、表面層が水素を吸蔵することができる組成であり、電極反応の触媒活性も高く、表面層中を水素が拡散し易く、母相或いは表面層に吸蔵された水素が電極反応の場である合金粉末の表面にすみやかに到達することができるものと考えられる。
【００１４】
本発明に係る水素吸蔵合金粉末の好ましい態様は、合金粉末の表面に連通する亀裂を有し、該亀裂に面する母相の外面に前記表面層を形成させる。亀裂を有する水素吸蔵合金は、水素吸蔵合金を水素ガス雰囲気に置き、加圧して水素を吸蔵させたのち、水素吸蔵合金を水素雰囲気から取り出し昇温して水素を脱離する（気相での水素の吸蔵脱離反応）ことによって作製することができる。気相反応の他に、アルカリ性水溶液によって合金を部分的に腐食せしめて上記と同様の水素を発生させ、大気圧下に於いて合金に水素を吸蔵させることもできる。水素吸蔵合金に、温度６０℃、平衡水素圧が１メガパスカル（ＭＰａ）の水素雰囲気下における水素吸量の３０％以上の水素を吸蔵させると合金の亀裂が十分に生成して、高率放電特性が大きく改善するのでより好ましい。水素吸蔵合金粉末の態様をこのような態様とすることによって、電極反応の場が拡がり、且つ、母相から電極反応の場に至る水素の拡散速度が速くなるので、高率放電特性を一層高めることが出来たものと考えられる。
【００１５】
また、前記水素吸蔵合金粉末は、質量飽和磁化が１．５〜９ｅｍｕ／ｇであることが好ましい。ここに、質量飽和磁化は、常温において試料である水素吸蔵合金粉末０．３グラムを精秤し、サンプルホルダーに充填して（株）理研電子製、振動試料型磁力計（モデルＢＨＶ−３０）を用いて５ｋエルステッドの磁場をかけて測定した値である。
【００１６】
本発明に係る水素吸蔵合金粉末の製造方法は、特定の錯化剤を含むアルカリ性水溶液中で浸漬処理して表面層を形成させる。本発明に適用する錯化剤は、水素吸蔵合金粉末を表面形成のための処理液に浸漬したときに処理液中に溶出したＭｎおよびＡｌと錯体を形成して、処理液中の両元素のイオン濃度を下げることによって、ＭｎおよびＡｌを優先的に溶出させる作用をする。本発明に係る水素吸蔵合金粉末は電極の活物質としての活性に富み、該水素吸蔵合金を適用することによって、ニッケル水素蓄電池の化成を従来に比べて大幅に簡略化することができ、かつ、高率放電特性に優れた蓄電池とすることができる。
【００１７】
本発明に係る水素吸蔵合金粉末の製造方法は、表面層を形成した後の水素吸蔵合金粉末を、ｐＨが５〜７水溶液に浸漬処理する工程と温度８０℃以上の温水に浸漬処理する工程とを有することが好ましい。
【００１８】
前記表面層の形成過程において、前記溶出した元素の錯体が沈殿物となって合金粉末表面に堆積して絶縁体の被膜を形成し、粉体の電気抵抗が大きくなる虞がある。表面層形成後の水素吸蔵合金粉末を、ｐＨが５〜７水溶液に浸漬処理することによって前記絶縁体皮膜層を溶解させて除去する。
【００１９】
また、前記表面層形成の過程で金属元素を溶出させた際に水素が発生し、水素吸蔵合金内に吸蔵される。水素吸蔵合金粉末内に吸蔵された水素が残存すると、水素吸蔵合金粉末が空気に触れたときに、空気中の酸素と水素吸蔵合金粉末に吸蔵された水素が反応して反応熱を発生させ、該反応熱によって昇温した水素吸蔵合金が空気中の酸素によって酸化され変質する虞がある。本発明に係る水素吸蔵合金粉末の製造方法は、水素吸蔵合金粉末を空気に接触する以前に、温度８０℃以上の温水に浸漬処理することによって水素吸蔵合金粉末に吸蔵された水素を脱離させる工程を有することが好ましい。本発明においては、温水に浸漬処理したあとに、過酸化水素水に浸漬して残存水素を完全に除去することが好ましい。
【００２０】
水素吸蔵合金粉末は、空気に接触して酸化されると活性が低下する。また、合金を含水状態で保管すると合金中の希土類が溶出しアルカリ性を示し、合金の腐食が進行するため、合金を電極にしたときの容量が極端に低下する。真空乾燥を行って合金表面を酸化させていない合金は、空気との反応に対する活性が極めて高く、変質し易い他に大気中で発火する虞がある。本発明に係る水素吸蔵合金粉末の製造方法は、製造の最終の過程で特定の方法で水素吸蔵合金の表面を部分酸化させる工程を有することが好ましい。該酸化処理によって、合金表面に電極反応を阻害しない厚さの薄い酸化被膜を形成させ、高率放電性能を低下させずに長期保管に耐え空気に触れても発火の虞のない水素吸蔵合金粉末を提供することを可能とした。
【００２１】
本発明に係る水素吸蔵合金電極は、前記表面層を形成させた水素吸蔵合金粉末とイッテルビウム（Ｙｂ）、エルビウム（Ｅｒ）、サマリウム（Ｓｍ）、ガドリウム（Ｇｄ）およびイットリウム（Ｙ）から選ばれた少なくとも１種の希土類元素を含有することが好ましい。該希土類元素の存在は、水素吸蔵合金粉末の前記表面層および母相がアルカリ電解液中で腐食して表面層や母相が変質するのを抑制し、表面層を設けた効果を長期に亘って持続させる効果を奏する。
【００２２】
【発明の実施の形態】
本発明者らは、密閉型ニッケル水素蓄電池の高率放電の抵抗成分解析を行うことによって、高率放電時の抵抗の大きな部分を負極が占めることを確認した。そこで、本発明者らは、高率放電時の負極の合金表面における電荷移動速度を向上すべく、水素吸蔵合金の表面処理について検討したが、その組成と構造を特定のものとすることによって、驚くべき高率放電性能が得られることが判った。即ち、Ｌａを含む希土類元素とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分として構成された水素吸蔵合金粉末において、水素吸蔵合金粉末の母相の外面にＬａを５％以上含有し、ＭｎとＡｌの総含有量が５％以下となる表面層を形成したときに、優れた高率放電特性を示すに至った。
【００２３】
中でも、前記表面層の厚さが５０ｎｍ以上、４００ｎｍ以下であって、粉末の質量飽和磁化が１．５〜９ｅｍｕ／ｇの水素吸蔵合金粉末を用いた水素吸蔵合金電極を備えるニッケル水素蓄電池の高率放電特性が大幅に改善することが判った。前記特定組成の層の厚さが５０ｎｍ以下では、高率放電特性の向上効果が小さく、４００ｎｍ以上では高率放電は向上するもののサイクル寿命性能が低下した。
【００２４】
同様に質量飽和磁化が１．５ｅｍｕ／ｇ以下であると高率放電特性の向上効果が認められず、９ｅｍｕ／ｇ以上生成すると合金容量が大きく低下した。高率放電特性は単純に放電反応の場である前記特定組成の層の面積または量で決まるものではなく、その存在する場所にも大きく影響され、合金粉末の表面に連結する亀裂に面した母相の該面に前記特定組成の層があるものが顕著な効果を奏する。
【００２５】
また、特に水素吸蔵合金粉末に防食剤として添加されるＹｂ、Ｅｒ、Ｓｍ、Ｇｄ、およびＹから選択した希土類元素を含有する水素吸蔵合金粉末の場合、合金粉末を単にアルカリや酸に浸漬しただけでは、高率放電特性が優れない。これは、前記希土類元素が、水素吸蔵合金粉末を従来のアルカリ性水溶液や酸性水溶液などに浸漬することによる活性化を妨げるためであると考えられる。
【００２６】
これに対して、前記本発明に係る表面層を有する水素吸蔵合金粉末においては、前記希土類元素から選択した防食剤を添加しても、優れたサイクル寿命特性を失うことなく、優れた高率放電特性を達成することができる。これは、前記表面層が電極反応に対して活性であることおよび層中の水素の拡散が速やかであることによる。該表面層の好ましい特性によって、水素吸蔵合金の母相および表面層に吸蔵された水素が、合金粉末表面の電極反応の場に速やかに移動し、電極反応に寄与することができるようになったためと考えられる。
【００２７】
アルカリ性水溶液によって表面を処理すると、表面の合金中からＬａなどの希土類やＮｉ、Ｃｏ、Ｍｎなどの遷移金属元素およびＡｌが溶出し、希土類イオンや遷移金属イオンがその水酸化物を生成する。通常の塩酸や酢酸を用いた酸処理では、Ｌａなどの希土類が溶出しやすく水素吸蔵可能な特定組成の表面層が形成しにくく、又、ＫＯＨ水溶液などを用いた通常のアルカリ処理においては、Ｌａなどの希土類の溶出を抑制しつつ、Ａｌ、Ｍｎが溶出できるものの、処理液中のこれら溶出元素の濃度が高くなって溶出反応が抑えられるため、十分な層を形成するためには、長時間かつ高温での処理が必要となる。
【００２８】
又、特定のアルカリ金属元素の混合組成と厳密な温度管理とによって、表面組成の制御は可能ではあるものの、温度や処理液の組成比が僅かに変動すると、合金表面の組成が変動するため、特定組成の表面を形成しにくい。これに対して、アルカリ水溶液中に特定の錯化剤を添加したものを用いた場合、錯化剤が溶出成分と錯体沈殿物を形成するので、電解液中の溶出元素イオン濃度を低下させることが出き、驚くべき早い速度で表面層を生成することができる。中でも、錯化剤が単座の配位子しか有さない場合（例えばモノカルボン酸のアルカリ金属塩）、錯体が不安定で反応速度が遅いのに対し、分子内にカルボキシル基を２個を有するカルボン酸のアルカリ金属塩を適用した場合、安定なキレート錯体の沈殿物を形成するためか反応速度が大幅に向上する。酒石酸やクエン酸のような２つのカルボキシル基に挟まれた炭素原子を２個有するカルボン酸のアルカリ金属塩を錯化剤として適用すると、特にＭｎおよびＡｌとの間で安定性の高いキレート錯体を形成する為か、ＭｎおよびＡｌが優先的に溶出し、水素吸蔵合金粉末の表面にＬａの含有比率が高く、ＭｎおよびＡｌの含有比率が小さい層を効果的に形成することができる。
【００２９】
通常のアルカリ水溶液によっては、特定組成の表面層形成の量をコントロールすることが難しい。これは、合金表面に存在する酸化被膜などにより表面層の形成が阻害されるためである。錯化剤の添加量によって、錯体形成物の量をコントロールでき、特定組成の表面層形成の量をコントロールできた。水素吸蔵合金粉末１ｇに対して錯化剤の添加量としては、０．１〜０．９ｍｍｏｌ添加すると、厚さが５０ｎｍ以上、４００ｎｍ以下、質量飽和磁化が１．５から９ｅｍｕ／ｇと、好ましい値の表面層が得られる。
【００３０】
前記表面層形成に適用するアルカリ性水溶液は、特に限定されるものではないが、アルカリ蓄電池の電解液に用いるＫＯＨ、ＮａＯＨ、ＬｉＯＨなどのアルカリ金属水酸化物の水溶液を１種又は２種以上を混合して用いると、その成分比が電解液と類似するため、電池にしたときに合金から電解液中へ新たな元素が溶出することがなく、電池に組み込む前の水素吸蔵合金の組成を維持することができるので好ましい。なかでも、ＮａＯＨ水溶液を適用すると、ＬｉＯＨやＫＯＨ水溶液を適用した場合に比べて優れたサイクル性能が得られるので好ましい。これは、処理液にＮａＯＨ水溶液を適用すると、コバルトの溶出が抑制されて、合金粉末の表面層が母層から剥離しにくいためと考えられる。
【００３１】
ＮａＯＨ水溶液の濃度が低いと、錯体形成するための水素吸蔵合金からの元素の溶出が少ないため、表面層の形成が進まない。適用するＮａＯＨ水溶液は、温度２０℃での密度が１．３ｇ／ｃｍ３以上が好ましいが、同水溶液の密度が１．５ｃｍ３を越えると常温で結晶の析出があり、取り扱いが困難となるため、１．５ｃｍ３以下が好ましい。
【００３２】
表面層を形成させるときの処理温度は、６０℃以上、１００℃以下が好ましく、６０℃以上、８０℃以下がさらに好ましい。処理温度が６０℃未満では表面層形成の反応が進みにくく、１００℃を超えると反応の速度が速すぎるため表面層の厚さを制御することが難しくなる。
【００３３】
また、前記処理温度において、合金から処理液中に溶出した元素が錯化剤と反応して錯体を形成するには処理時間を３０分間以上にすることが好ましいが、長時間になるにつれてアルカリ水溶液による水素吸蔵合金のエッチングが進むので、過度のエッチングを避けるために処理時間は１０時間以下が好ましく、３時間以下がさらに好ましい。
【００３４】
前記のように、錯化剤を添加したアルカリ水溶液に浸漬することによって表面層を形成する過程で、一旦溶出した元素が錯体となって合金粉末に堆積する。該堆積物は、電気絶縁性であり、かつ、合金粉末と電解液の接触を制約して電極反応を阻害するので好ましくない。表面層形成後の合金粉末を酸性水溶液に浸漬して、前記絶縁被膜を除去する必要があるが、ｐＨが低い強酸性の水溶液を用いた場合、表面層からＬａ、Ｎｉ等の成分が溶出し、高率放電特性を低下させてしまう。
【００３５】
前記絶縁皮膜を除去するためにｐＨが５〜７の処理液を適用することが、Ｌａ、Ｎｉ等の成分の溶出を低減させつつ絶縁被膜を除去することができるため好ましい。処理液としては、酢酸等弱酸の水溶液を適用することもできるが、処理液のｐＨを前記範囲内に保つ上でクエン酸ナトリウム−水酸化ナトリウム水溶液や酢酸−酢酸ナトリウム水溶液などのｐＨ緩衝溶液を用いると便利である。中でも取り扱いが簡便で安価な酢酸−酢酸ナトリウム水溶液を用いることが好ましい。
【００３６】
また、表面層形成後の水素吸蔵合金粉末を前記ＰＨ５〜７の水溶液に浸漬して絶縁体被膜層を除去した処理液中に水素吸蔵合金粉末から剥離した絶縁性物質の粒子が浮遊することがある。該絶縁性物質は、水素吸蔵合金粉末に比べて沈降速度が遅いので、両者の沈降速度の差を利用して絶縁性物質を除くことができる。例えば、合金粉末を含む処理液をいれた攪拌タンクの下部から流水をフローさせ、沈降しにくい絶縁性物質の粒子をオーバーフローさせた流水とともに系外に流出させる。
【００３７】
水素を合金から脱離させる方法として、表面層形成処理後の合金粉末をいきなりＨ2Ｏ2の水溶液などのに接触させ酸化することによって脱離する方法があるが、Ｈ2Ｏ2などの薬品は高価であるため大量に用いるのは不利である。水素吸蔵合金から水素を脱離させるのに必要な酸化剤の使用量を低減させるため、温度８０℃以上のｐＨ９以下の温水中に合金を暴露する方法が、効率よく安価に合金含有の水素の多くをガスとして脱離することが出来るため好ましい。温水への浸漬処理後に、酸化剤を用いて酸化処理を行い、前記温水浸漬処理によって脱離仕切れない水素を脱離する。ここで用いる酸化剤は特に限定されないが、過酸化水素は分解後の生成物が合金性能を低下させる不純物を有しないために好ましい。これら酸化剤は、温度４５℃においては合金の表面層に接触すると酸素ガスを放出し自己分解をするので効率が悪く、温度４５℃以下に冷却して用いると効率よく合金中の水素と反応するので好ましい。
【００３８】
本発明においては、前記絶縁性被膜の除去および水素の脱離を終えた水素吸蔵合金粉末の表面を酸化させる。前記のように、過酸化水素水と接触させることによって水素吸蔵合金粉末の表面を酸化する方法も適用できるが、温度６０℃〜９０℃の空気によって乾燥させると、合金表面は酸化されるものの、高率放電特性の低下は限定的であるため好ましい。空気中で乾燥を行った場合、合金表面に生成される酸化物の被膜が薄く、電池組み込み後の電池の充放電による活性化操作で酸化被膜が還元されるか、または酸化物が剥離脱落するためと考えられる。
【００３９】
前記のような合金粉末表面の酸化処理を行えば、長期保管中に品質劣化が抑えられ、保管、作業時に発火の恐れがない、優れた高率放電特性を有する負極用水素吸蔵合金の粉体が得られる。
【００４０】
以下に、本発明の実施の形態を更に詳細に説明する。ただし、本発明は以下の実施の形態に限定されるものではない。
【００４１】
本発明に係る密閉型ニッケル水素電池は、水酸化ニッケルを主要構成成分とする正極と、水素吸蔵合金を主要構成成分とする負極と、アルカリ金属水酸化物の水溶液からなるアルカリ電解液とから構成され、一般的には、正極と負極との間に、セパレータが設けられる。図１に、正極と負極がセパレータを挟んで巻き込まれた典型的な本発明蓄電池の切断面を示す。図において、１は密閉型ニッケル水素蓄電池、２は電池の外装体、３は正極、４は負極、５はセパレータ、６は絶縁ガスケット、７は封口板、８は正極端子、９は集電体である。
【００４２】
正極活物質としては、水酸化ニッケルに水酸化亜鉛、水酸化コバルトを混合したものが用いられるが、これらを共沈法によって均一に分散せしめて得た水酸化ニッケル複合水酸化物の使用が好ましい。前記水酸化ニッケル複合酸化物以外の正極の構成材料には、導電性改質剤として水酸化コバルト、酸化コバルト等を添加するが、前期水酸化ニッケル複合酸化物に水酸化コバルトを被覆したものも適用することができる。さらに、これらの正極材料粉末をアルカリ水溶液の存在下で含酸素気体、Ｋ2Ｓ2Ｏ8や次亜塩素酸などの酸化剤を用いて化学的に酸化したり、電気化学的に酸化して正極材料中に含まれる遷移金属（Ｎｉ、Ｃｏ）の酸化数を２より大きい値に高めることもできる。該酸化処理は、放電リザーブ生成を抑制して放電容量を高めたり、サイクル性能を向上させるのに有効である。
【００４３】
さらに、正極にＨｏ、Ｅｒ、Ｔｍ、Ｙｂ、ＬｕおよびＹの希土類元素から選択した少なくとも１種の元素を酸化物や水酸化物として添加することもできる。これらの元素の化合物を正極に添加することによって、充電時に正極から酸素が発生するのを抑制し、該酸素によって負極の水素吸蔵合金が腐食劣化するのを抑制するのに有効である。
【００４４】
負極の主構成要素である水素吸蔵合金には、Ｌａを含み、Ｌａ以外にセリウム（Ｃｅ）、プラセオヂューム（Ｐｒ）、ネオヂューム（Ｎｄ）等の希土類元素からなる構成要素（以下ミッシュメタルＭｍと記述する）とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分として構成された水素吸蔵合金の粉末を用いることができる。
【００４５】
前記表面層を形成する過程でＬａの溶出を避けることは困難である。本発明の水素吸蔵合金の表面層に含まれるＬａの比率は、Ｎｉの含有比率を１００モル％としたときに、５モル％以上であって高い値であることが好ましい。従って、本発明に適用する水素吸蔵合金の母相は、Ｌａが前記Ｍｍの主構成元素であって、Ｍｍに占めるＬａの比率を５０モル％以上とすることが好ましい。
【００４６】
また、本発明に係る水素吸蔵合金電極においては、防食剤として、Ｙｂ、Ｅｒ、Ｓｍ、ＧｄおよびＹの単体、酸化物あるいは水酸化物を添加したり、あるいは、前記水素吸蔵合金にこれらの元素を合金として含有させることもできる。
【００４７】
正極活物質の粉体及び負極材料の粉体は、平均粒子サイズ５０μｍ以下であることが望ましい。特に、負極活物質である水素吸蔵合金の粉体は、密閉型ニッケル水素電池の高出力特性を向上する目的で粒径は４０μｍ以下の小さいものの方が良いが、高いサイクル寿命を得るためには粒径が２０μｍを下回らないことが望ましい。本発明に係る水素吸蔵合金のうち前記亀裂を設け、粉末の表面および亀裂の両方に面する母相の外面に５０ｎｍ以上４００ｎｍ以下で配置した場合、大きい粒径でも優れた高率放電性能が得られるため、平均粒径としては３０μｍから５０μｍがより好ましい。
【００４８】
所定の粒径、形状を有する粉末を得るためには各種の粉砕機や分級機が用いられる。例えば乳鉢、ボールミル、サンドミル、振動ボールミル、遊星ボールミル、ジェットミル、カウンタージェットミル、旋回気流型ジェットミル等が用いられる。粉砕時には水、あるいはアルカリ金属水酸化物の水溶液を用いて湿式粉砕を用いることもできる。分級方法としては、特に限定はなく、篩や風力分級機などが使用でき、また、乾式、湿式ともに必要に応じて用いられる。
【００４９】
以上、正極及び負極の主要構成成分である正極活物質および負極活物質について詳述したが、前記正極及び負極には、前記主要構成成分の他に、導電剤、結着剤、増粘剤、フィラー等が、他の構成成分として含有されてもよい。
【００５０】
導電剤としては、電池性能に悪影響を及ぼさない電子伝導性材料であれば限定されない。通常、鱗状黒鉛，鱗片状黒鉛，土状黒鉛等の天然黒鉛、人造黒鉛、カーボンブラック、アセチレンブラック、ケッチェンブラック、カーボンウイスカー、炭素繊維、気相成長炭素、金属（銅，ニッケル，金等）粉、金属繊維等の導電性材料を１種またはそれらの混合物として含ませることができる。
【００５１】
これらの導電剤の中では、電子伝導性及び塗工性の観点より黒鉛化度の低い非晶質炭素粉末が望ましい。導電剤の添加量は、正極または負極の総重量に対して０．１重量％〜１０重量％が好ましい。特に黒鉛化度の低い非晶質炭素を０．１〜０．５μｍの超微粒子に粉砕して用いると必要炭素量を削減できるため好ましい。これらの混合方法は、物理的な混合であり、その理想とするところは均一混合である。そのため、Ｖ型混合機、Ｓ型混合機、擂かい機、ボールミル、遊星ボールミルといったような粉体混合機を乾式、あるいは湿式で使用することが可能である。
【００５２】
前記結着剤としては、通常、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリエチレン，ポリプロピレン等の熱可塑性樹脂、エチレン−プロピレン−ジエン系ゴム（ＥＰＤＭ），スルホン化ＥＰＤＭ，スチレンブタジエンゴム（ＳＢＲ）、フッ素ゴム等のゴム弾性を有するポリマーを１種または２種以上の混合物として用いることができる。結着剤の添加量は、正極または負極の総重量に対して０．１〜３重量％が好ましい。
【００５３】
正極および負極には活物質粉末や導電性を高めるための添加剤を水やアルコールなどの液体に分散させてペースト状とし、該ペーストを集電体である多孔性基板に塗布して担持させたものが適用される。前記ペーストを作成するときに通常用いられているカルボキシメチルセルロース（ＣＭＣ）、メチルセルロース（ＭＣ）やキサンタンガム、グアーガム等の多糖類あるいは直鎖状ポリビニルアルコールを単独またはこれらの化合物を２種以上含む混合物からなる増粘剤を適用することができる。増粘剤の添加量は、正極または負極の総重量に対して０．１〜３重量％が好ましい。
【００５４】
正極および負極にフィラーを添加することもできる。フィラーとしては、電池性能に悪影響を及ぼさない材料であれば特に制限はない。通常、ポリプロピレン，ポリエチレン等のオレフィン系ポリマー、炭素等が用いられる。フィラーの添加量は、正極または負極の総重量に対して添加量は、５重量％以下が好ましい。
【００５５】
正極および負極は、それぞれ前記活物質、導電剤および結着剤を水やアルコール等の有機溶媒に混合させた後、得られた混合物を下記に詳述する集電体の上に塗布し、乾燥することによって、好適に作製される。前記塗布方法については、例えば、アプリケーターロールなどのローラーコーティング、スクリーンコーティング、ドクターブレード方式、スピンコーティング、バーコータ等の手段を用いて任意の厚みおよび任意の形状に塗布することが望ましいが、これらに限定されるものではない。
【００５６】
正極の集電体としては、構成された電池に悪影響を及ぼさない電子伝導体であれば特に選ぶところはない。例えば、ニッケルやニッケルメッキを行った鋼板を好適に用いることができ、発泡体、繊維群の形成体、凸凹加工を施した３次元機材の他に、パンチング鋼板等の２次元機材が用いられる。厚さの限定は特にないが、５〜７００μｍのものが用いられる。これら集電体の中で、正極としては、アルカリに対する耐食性と耐酸化性に優れているＮｉを、集電性に優れた構造である多孔体構造の発泡体としたものを使用することが好ましい。
【００５７】
負極の集電体としては、安価で、且つ電導性に優れる鉄または鋼の箔ないし板をパンチング加工し、耐還元性向上のためにＮｉメッキを施した、多孔板を使用することが好ましい。鉄板または鋼板のパンチングの孔径は１．７ｍｍ以下、開口率４０％以上であることが好ましく、これにより少量の結着剤でも負極活物質と集電体との密着性は優れたものとなる。焼成炭素繊維、導電性高分子の他に、接着性、導電性および耐酸化性向上の目的で集電体のニッケルの表面をＮｉ粉末やカーボンや白金等を付着させて処理したものを用いることができる。これらの材料については表面を酸化処理することも可能である。
【００５８】
ニッケル水素電池用セパレータとしては、既知の優れた高率放電特性を示す多孔膜や不織布等を、単独あるいは併用することができる。セパレータを構成する材料としては、例えばポリエチレン，ポリプロピレン等に代表されるポリオレフィン系樹脂や、ナイロンを挙げることができる。セパレータの空孔率は強度、ガス透過性の観点から８０体積％以下が好ましい。また、充放電特性の観点から空孔率は２０体積％以上が好ましい。セパレータは親水化処理を施す事が好ましい。例えば、ポリエチレンなどのポリオレフィン系樹脂繊維の表面に親水基のグラフト重合処理、スルフォン化処理、コロナ処理、ＰＶＡ処理を施したり、これらの処理を既に施された繊維を混合したシートを用いても良い。
【００５９】
電解液としては、一般にアルカリ電池等への使用が提案されているものが使用可能である。電解液には、電解質であるカリウム、ナトリウム、リチウムの水酸化物の単独またはそれら２種以上の混合物を水に溶解したもの等を挙げることができるがこれらに限定されるものではない。電解液には、水酸化カリウムを５〜７ｍｏｌ／ｄｍ3、水酸化リチウムを０．５〜０．８ｍｏｌ／ｄｍ3含む水溶液を適用することが、優れた電池特性を得ることができるので好ましい。
【００６０】
電解液には前記電解質の他に、正極の酸素過電圧の向上、負極の耐食性の向上および自己放電抑制を目的としてＨｏ、Ｅｒ、Ｔｍ、Ｙｂ、ＬｕおよびＹ等の希土類元素の他、カルシウム、硫黄、亜鉛等の化合物を単独またはそれら２種以上混合して添加することができる。
【００６１】
本発明に係る密閉型ニッケル水素蓄電池は、電解液を、例えば、密閉型ニッケル水素蓄電池用セパレータと正極と負極とを積層する前または積層した後に注液し、最終的に、外装材で封止することによって好適に作製される。また、正極と負極とが密閉型ニッケル水素蓄電池用セパレータを介して積層された発電要素を巻回してなる密閉型ニッケル水素蓄電池においては、電解液は、前記発電要素の巻回の前後に発電要素に注液されるのが好ましい。注液法としては、常圧で注液することも可能であるが、真空含浸方法や加圧含浸方法や遠心含浸法も使用可能である。
【００６２】
密閉型ニッケル水素蓄電池の外装体の材料としては、ニッケルメッキした鉄やステンレススチール、ポリオレフィン系樹脂等またはこれらの複合体が挙げられる。
【００６３】
密閉型ニッケル水素蓄電池の構成、形状については特に限定されるものではなく、正極、負極および単層又は複層のセパレータを有するコイン電池やボタン電池、角型電池、扁平型電池、さらに、ロール状の正極、負極およびセパレータを有する円筒型電池等が一例として挙げられる。
【００６４】
【実施例】
以下に、実施例に基づき本発明をさらに詳細に説明するが、本発明は以下の記載により限定されるものではなく、試験方法や構成する電池の正極活物質、負極材料、正極、負極、電解質、セパレータ並びに電池形状等は任意である。
【００６５】
（実施例１〜５）
（水酸化ニッケル粒子の合成）
硫酸ニッケルと硫酸亜鉛および硫酸コバルトを、それぞれの金属の水酸化物が後記の質量比となるように溶解した水溶液に、硫酸アンモニウムとＮａＯＨ水溶液を添加してアンミン錯体を生成させた。反応系を激しく撹拌しながら更に苛性ソーダを滴下し、反応浴の温度を４５±２℃に、ｐＨを１２±０．２に制御して、芯層母材となる球状高密度水酸化ニッケル粒子を、水酸化物の質量比がニッケル：亜鉛：コバルト＝９３：５：２となるように合成した。
【００６６】
（水酸化ニッケル粒子表面への表面層の形成）
前記高密度水酸化ニッケル粒子を、苛性ソーダでｐＨ１２±０．２に制御したアルカリ性水溶液に投入した。該溶液を撹拌しながら、所定濃度の硫酸コバルト、アンモニアを含む水溶液を滴下した。この間、苛性ソーダ水溶液を適宜滴下して反応浴のｐＨを１２±０．２に維持した。約１時間ｐＨを１２±０．２に保持し、水酸化ニッケル粒子表面にＣｏを含む混合水酸化物から成る表面層を形成させた。該混合水酸化物の表面層の比率は芯層母粒子（以下単に芯層と記述する）に対して７ｗｔ％であった。
【００６７】
（正極活物質粉末の部分酸化）
前記混合水酸化物から成る表面層を有する水酸化ニッケル粒子１００ｇを、温度６０℃、濃度１０ｗｔ％の苛性ソーダ水溶液４００ｇ中に投入し、充分に攪拌した。続いて次亜塩素酸ナトリウム溶液（和光純薬株式会社製）４５ｍｌを添加し、３０分間撹拌を続けた。活物質粒子をろ過し、水洗、乾燥した。得られた正極活物質粒子に濃度３０ｗｔ％、温度８０℃のＮａＯＨ水溶液２０ｇを添加して、温度８０℃にて２時間放置した後、水洗、乾燥した。得られた正極活物質粒子（芯層と表面層の両方を含む）に含まれる遷移金属元素（Ｎｉ、Ｃｏ）の平均酸化数を、公知の方法（活物質粒子を硫酸第１鉄アンモニウムと反応させた後、過マンガン酸カリウムを用いて酸化還元滴定を行う）により測定した。その結果、前記平均酸化数は２．１５であった。
【００６８】
（正極板の作製）
前記活物質粒子にカルボキシメチルセルロース（ＣＭＣ）水溶液を添加して前記活物質粒子：ＣＭＣ溶質＝９９．５：０．５のペースト状とし、該ペーストを４５０ｇ／ｍ ２ のニッケル多孔体（発泡ニッケル、住友電工社製、商品名ニッケルセルメット＃８）に充填した。その後８０℃で乾燥した後、所定の厚みにプレスし、表面にテフロン（登録商標）コーティングを行い幅３４ｍｍ（内、無塗工部１ｍｍ）長さ２６０ｍｍの容量３０００ｍＡｈのニッケル正極板とした。
【００６９】
（水素吸蔵合金粉末の作製）
（実施例１〜実施例５）
（表面層の形成）
酒石酸ナトリウム二水和物を水素吸蔵合金１ｇ当たりそれぞれ０．１ｍｍｏｌ／ｇ、０．２ｍｍｏｌ／ｇ、０．４ｍｍｏｌ／ｇ、０．８ｍｍｏｌ／ｇおよび０．９ｍｍｏｌ／ｇ含有した２０℃での密度１．５ｇ／ｃｍ3の水酸化ナトリウム水溶液１００ｍｌを８０℃に加温して得たアルカリ水溶液に、平均粒径３５μｍのＡＢ5型水素吸蔵合金粉末であって、前記ＭｍがＬａを７０％含有する化学式Ｍｍ1.0Ｎｉ3.6Ｃｏ0.6Ａｌ0.3Ｍｎ0.35で表される水素吸蔵合金１００ｇを投入して２時間攪拌を行った。
【００７０】
（絶縁被膜の除去）
表面層形成処理後、内容物（水素吸蔵合金粉末と処理液）を加圧濾過し、濾別した水素吸蔵合金粉末を水洗した後、１．６ｍｏｌ／ｄｍ３の酢酸ナトリウム水溶液３００ｍｌに酢酸を加えたｐＨ５、温度４０℃に加温した酢酸−酢酸ナトリウム水溶液中に投入し、３０分間攪拌を行った。
【００７１】
（水素脱離）
前記絶縁被膜の除去処理後、内容物を加圧濾過し、洗浄液のｐＨが６から７になるまで水洗した後、得られた水素吸蔵合金粉末を純水中に分散し８０℃に加温した。さらに、加圧濾過によって温水を除き、得られた水素吸蔵合金粉末を温度約２５℃の水中に投入し、攪拌下で４％過酸化水素水を合金重量と同量加えて水素脱離処理を行った。
【００７２】
（酸化処理兼乾燥）
前記水素脱理処理後、内容物をろ過して処理液から分離した後、水洗を行い、残留した未反応の過酸化水素水を除去し、空気中において温度８０℃にて２時間乾燥し、電極用水素吸蔵合金粉末を得た。
【００７３】
前記表面層形成工程に適用した処理液に含まれる水素吸蔵合金１ｇ当たりの酒石酸ナトリウム二水和物が０．１ｍｍｏｌ／ｇのものを実施例１、０．２ｍｍｏｌ／ｇのものを実施例２、０．４ｍｍｏｌ／ｇのものを実施例３、０．８ｍｍｏｌ／ｇのものを実施例４、０．９ｍｍｏｌ／ｇのものを実施例５に係る水素吸蔵合金粉末とする。
【００７４】
（実施例６）
前記表面層形成に先だって、予め水素吸蔵合金粉末を温度４５℃において０．４ＭＰａの水素圧雰囲気下に置き、加圧後水素吸蔵合金粉末を水素雰囲気から取り出す処理を行って、水素吸蔵合金粉末に温度６０℃、平衡水素圧１ＭＰａの水素雰囲気下における水素吸蔵量の３０％の水素を吸蔵脱離させた以外は、前記実施例２と同様にして水素吸蔵合金粉末を得た。該水素吸蔵合金を実施例６に係る水素吸蔵合金粉末とする。
【００７５】
（実施例７）
前記表面層形成工程に適用した処理液を構成するＮａＯＨ水溶液の温度２０℃における密度を１．３ｇ／ｃｍ3とした。それ以外は前記実施例１と同様にして水素吸蔵合金粉末を得た。該水素吸蔵合金を実施例７に係る水素吸蔵合金粉末とする。
【００７６】
（比較例１）
前記表面層形成、絶縁被膜除去、水素の脱離、表面酸化の４つの工程を行わなかったこと以外は前記実施例１と同様にして得られた水素吸蔵合金粉末を比較例１に係る水素吸蔵合金粉末とする。
（比較例２）
第１工程に於いて密度が１．５ｇ／ｃｍ3のＮａＯＨ水溶液に、酒石酸ナトリウム二水和物を加えなかったこと以外は、本発明合金１と同様にして得られた水素吸蔵合金粉末を比較例２に係る水素吸蔵合金粉末とする。
（比較例３）
前記表面層形成および絶縁被膜除去工程に代えて、水素吸蔵合金粉末をｐＨ６に保持した温度８０℃の酢酸の水溶液に１時間浸漬処理したこと以外は、実施例１と同様にして得られた水素吸蔵合金粉末を比較例３に係る水素吸蔵合金粉末とする。
（比較例４）
前記表面層形成工程に於いて密度が１．５ｇ／ｃｍ3のＮａＯＨ水溶液に、酒石酸ナトリウム二水和物の量を１．０ｍｍｏｌ添加した以外は実施例１と同様の処理をして得られた水素吸蔵合金を比較例４に係る水素吸蔵合金粉末とする。
【００７７】
（質量飽和磁化の測定）
得られた水素吸蔵合金粉末０．３グラムを精秤し、サンプルホルダーに充填して（株）理研電子製、振動試料型磁力計（モデルＢＨＶ−３０）を用いて５ｋエルステッドの磁場をかけて測定した。実施例１〜実施例７、比較例１〜比較例４それぞれ５回づつ測定しそのメジアン値を各々のサンプルの計測値とした。
（表面層の厚さの測定）
得られた水素吸蔵合金をサンプリングし、サンプルの断面を透過型電子顕微鏡にて観察して合金粉末の表面層の厚さを計測した。表面処理形成層の厚さは、実施例１〜実施例７、比較例１〜比較例４それぞれ１０ケ所の計測値のメジアン値を有効数字２桁とした。
（表面層に含まれるＬａ、Ｍｎ、Ａｌ含有比率の定量）
Ｘ線光電子分光分析装置と分析透過電子顕微鏡を用いて表面層の厚さ方向の中央の組成を分析し、Ｌａ、Ｍｎ、ＡｌおよびＮｉの含有比率を求め、Ｎｉの含有比率を１００モル％とした値に換算した。実施例１〜実施例７、比較例１〜比較例４それぞれ１０ケ所の分析値のメジアン値を各々のサンプルの分析値とした。実施例１〜実施例７、比較例１〜比較例４に係る水素吸蔵合金粉末の計測および分析結果を表１に示す。
【００７８】
【表１】
【００７９】
表１に示す如く、実施例１〜実施例７に係る水素吸蔵合金の表面層中に含まれるＮｉの比率を１００ｍｏｌ％としたときに、同表面層中に含まれるＬａの比率は５ｍｏｌ％以上であって、ＭｎとＡｌの比率の和は、５ｍｏｌ％以下である。これは、処理液に添加した錯化剤である酒石酸ナトリウムの作用によって、ＭｎとＡｌが優先的に溶出して除去されたことによるものである。また、酒石酸の添加量が増すにつれて、表面層の厚さおよび水素吸蔵合金粉末の質量飽和磁化が増大し、かつ表面層に含まれるＬａの比率が高くなる傾向にある。
【００８０】
実施例６に係る水素吸蔵合金粉末は、表面層形成処理に先だって、合金粉末に水素を吸蔵脱離させて亀裂を生じさせたものであるが、表面層の厚さが１８０ｎｍと小さいにも拘わらず、実施例２の表面層の厚さが２２０ｎｍのものと同様に５．０ｅｍｕ／ｇの質量飽和磁化を有している。実施例合金６については亀裂に面する母相の表面にも表面層が形成されていることを電子顕微鏡と集束イオンビーム装置にて確認した。集束イオンビーム装置を用いた実施例合金６の断面写真を図２に示す。合金粉末の内部の母相１２に、亀裂が形成され、該亀裂に面する母相の外面に表面層１１が形成されているのが認められる。この写真は試料を４５度傾けて測定したものであるため、縦の拡大率は横の１／√（ルート）２倍である。なお、図の１４は、断面を観察するために必要上サンプル粉末の表面に蒸着させた白金の層である。
【００８１】
実施例７の場合は、処理に用いたＮａＯＨ水溶液の密度が１．３ｇ／ｃｍ３と低かった（ＮａＯＨの濃度が低い）ために表面層形成が進みにくかったものと考えられる。質量飽和磁化の値は、表面形成層の厚みと相関が見られるが、これらの値は、密度が１．３ｇ／ｃｍ３未満では殆ど上昇しないことを確認した。又、密度が１．３〜１．４ｇ／ｃｍ３の間では上昇が非常に遅くなることを確認した。このことから、処理に用いるＮａＯＨ水溶液の密度は１．３ｇ／ｃｍ３以上が好ましく、より好ましくは１．４ｇ／ｃｍ３以上である。又、常温でＮａＯＨの結晶が析出しない１．５ｇ／ｃｍ３以下が好ましい。
【００８２】
他方、比較例２のように錯化剤を適用しなかったものは表面層の形成が極めてわずかであって、表面層の組成分析ができなかった。さらに、処理液に酢酸の水溶液を適用した比較例３の場合は表面層の形成が認められたが、表面層に含まれるＬａの比率は１ｍｏｌと小さい値であった。また、本発明の狙いとする表面層中に含まれるＬａの比率が５ｍｏｌ％以上であって、ＭｎとＡｌの比率の和が５ｍｏｌ％以下、且つ、厚さが５０〜４００ｎｍの表面層を形成するためには、表面層を形成するための処理液に添加する錯化剤の量を水素吸蔵合金１ｇ当たり０．１〜０．９ｍｍｏｌとするのが良いことが分かった。
【００８３】
（負極板の作成）
前記の実施例１〜実施例７、比較例１〜比較例４に係る水素吸蔵合金粉末とスチレンブタジエン共重合体エマルジョンとを乾量基準の質量比で９９．３５：０．６５の割合で混合し、水で分散してペースト状にし、ブレードコーターを用いて、鉄にニッケルメッキを施したパンチング鋼板に塗布した後、８０℃で乾燥し、所定の厚みにプレスして幅３４ｍｍ（内、無塗工部１ｍｍ）長さ２６０ｍｍの容量４８００ｍＡｈの水素吸蔵負極板とした。
【００８４】
（評価電池の作製）
前記の実施例１〜実施例７、比較例１〜比較例４に係る水素吸蔵合金を適用した水素吸蔵合金負極板と、スルフォン化処理を施した厚み１２０μｍのポリプロピレンの不織布状セパレータと、前記ニッケル極板とを組み合わせてロール状に巻回し、６．８Ｎの水酸化カリウム水溶液に０．８ｍｏｌ／ｌの水酸化リチウムを溶解したアルカリ電解液を注液し、開弁圧２．５ＭＰａの逆止弁を具備するｓｕｂＣ形の密閉型ニッケル水素蓄電池を作製した。該電池を実施例１〜実施例７、比較例１〜比較例４に係るニッケル水素蓄電池とする。
【００８５】
（電池の特性評価）
前記実施例１〜実施例７、比較例１〜比較例４に係るニッケル水素蓄電池を４０℃１２時間の保管処理の後、２０℃に於いて０．０２ＩｔＡにて６００ｍＡｈ充電し、さらに０．１ＩｔＡで１２時間充電した。さらに０．２ＩｔＡで１Ｖまで放電した後、０．１ＩｔＡで１２時間充電、０．２ＩｔＡで１Ｖまで放電する操作を４回繰り返した。４回目の放電容量を２０℃での初期放電容量とする。該放電終了後、この電池を０．１ＩｔＡで１２時間充電した後、温度５℃に５時間放置したのち、同温度において０．８Ｖまで１０ＩｔＡで放電を行った。該放電で得られた放電容量の前記の２０℃での初期放電量に対する比率（％）を求めその結果を表２に記載した。その後さらに、温度２５℃において２ＩｔＡで−ΔＶ５ｍＶまで（充電電圧がピークを過ぎてピークから５ｍＶ低下する時点まで）充電し、充電終了後１０分間休止し、２ＩｔＡで１．０Ｖまで放電し、放電終了後１０分間休止した。該充電・放電を１サイクルとしてサイクルを繰り返し行い、該サイクルにおける放電容量が前記初期放電容量の８０％に到達したサイクル数をサイクル寿命として表２に記載した。表２に示した電池の試験結果は、５個の電池を試験して得られた値の平均値である。
【００８６】
【表２】
【００８７】
本発明に係る前記表面層がＬａを５ｍｏｌ％以上含有し、ＭｎとＡｌの含有比率の和が５ｍｏｌ％以下であって、厚さが５０ｎｍ以上、質量飽和磁化が１．５ｅｍｕ／ｇ以上の水素吸蔵合金粉末を適用した実施例１〜実施例７に係るニッケル水素蓄電池においては、前記表面層を形成してないもの（比較例１）あるいは表面層の形成が不十分なもの（比較例２）さらには表面層が形成されていても表面層中のＬａの含有比率が小さいもの（比較例３）に比べて優れた高率放電特性を示すことが判った。表面層の厚さが１００ｎｍ、質量飽和磁化が２．５の実施例１においては比較例３に比べて高率放電において２倍以上の放電容量が得られた。さらに、表面層が１８０〜２２０ｎｍ以上、質量飽和磁化５．０ｅｍｕ／ｇ以上の場合、高率放電特性が飛躍的に向上した。実施例６の表面層に含まれるＬａの含有比率およびＭｎとＡｌの含有比率の和が同じであり、かつ質量飽和磁化も実施例２と同じであるが、実施例６の高率放電特性は実施例２を上回っている。これは、実施例６においては水素吸蔵合金粉末に亀裂を設けた効果によるものと考えられる。なお、前記のように実施例１〜実施例７においては、化成のための充放電の繰り返しを４回行ったのみであり、本発明に係るニッケル水素蓄電池は、このように少ない充放電回数を行っただけで極めて優れた高率放電特性を示す蓄電池である。
【００８８】
表面層の厚みが５５０ｎｍ、質量飽和磁化１０ｅｍｕ／ｇの場合（比較例４）においては高率放電特性は良いもののサイクル寿命が劣っていた。比較例４の場合は、形成された表面層の量が大き過ぎ、負極の容量と正極の容量の比（Ｎ／Ｐ）が小さくなったために負極の酸素吸収能力が低下し、繰り返し充電することによって電解液が減少したために、サイクルの経過に伴い急速に容量が低下したものと考えられる。
【００８９】
以上に示した結果から、水素吸蔵合金粉末の母相の外面にＬａの比率が５ｍｏｌ％以上ＭｎとＡｌの含有比率の和が５ｍｏｌ以下であって厚さが５０〜４００ｎｍの表面層を形成させることによって、化成のための充放電回数が少ない状態において優れた高率放電特性を有する水素吸蔵電極並びにニッケル水素蓄電池が得られることが分かった。これは、水素吸蔵合金に設けた表面層が電極反応の活性に富み、かつ、表面層中を水素が拡散し易いためと考えられる。なお、表２に示した結果から表面層の厚さは、５０ｎｍ以上、４００ｎｍ以下が良く、１００〜４００ｎｍが好ましく、１８０〜４００ｎｍがさらに好ましい。また、水素吸蔵合金粉末の質量飽和磁化は１．５〜９ｅｍｕ／ｇが好ましく、２．５〜９ｅｍｕ／ｇがさらに好ましく、５．０〜９ｅｍｕ／ｇが特に好ましい。
【００９０】
（比較例５）
前記絶縁被膜除去処理を行わなかったこと以外は、実施例２と同様にして得られた水素吸蔵合金を比較例５に係る水素吸蔵合金粉末とする。該水素吸蔵合金粉末を適用して前記実施例と同様にして水素吸蔵負極板を作成し、さらに前記実施例と同様にしてニッケル水素電池を作成した。該電池を用いて、０．１ＩｔＡで１２時間充電した後、５℃に５時間放置して十分冷却したのち、０．８Ｖまで１０ＩｔＡ放電を行った時の放電容量は０％であった。比較例５に係る水素吸蔵合金を適用した水素吸蔵負極板においては表面層形成の過程で溶出した元素と酒石酸との錯体からなる被膜が合金粉末の周囲に残留し、導電性を低下せしめたため高率放電特性を低下させたものと考えられる。
【００９１】
集束イオンビーム装置を用いた比較例５に係る水素吸蔵合金粉末５の断面写真を図３に示す。母相１２の外面に形成させた表面層１１の外側に絶縁性被膜１３が生成しているのが認められる。合金表面に明らかに合金の表面処理形成層と異なる被膜が観察される。この写真は試料を４５度傾けて測定したものであるため、縦の拡大率は横の１／√（ルート）２倍である。
【００９２】
（比較例６）
前記水素脱離の工程に於いて、過酸化水素による水素の脱離を行わず、８０℃にて乾燥して得た合金は、空気に触れた際、酸化によるものと考えられる発熱を示した。この合金を用いて前記方法にて電池を作製して得られた電池を比較電池６とする。比較例６においては活性化時の４サイクル目の容量が、１０００ｍＡｈしか得られなかった。これは、真空にして水素を脱離させたものの、水素が残留しており空気中の酸素に触れた際に酸化し、この発熱によって合金粉末が酸化され、活物質としての活性が低下したものと考えられる。
【００９３】
（比較例７）
表面層形成の工程に於いて、錯化剤として酒石酸ナトリウムの代わりに、グルコン酸ナトリウムを用いたこと以外は、前記実施例２と同様にして得られた水素吸蔵合金を比較例７に係る水素吸蔵合金粉末とする。該合金の表面層の厚さおよび質量飽和磁化の値、該合金を適用して作製したニッケル水素蓄電池の温度５℃、１０ＩｔＡ放電における放電容量を表３に示す。表３に示した電池の試験結果は、５個の電池を試験して得られた値の平均値である。
【００９４】
【表３】
【００９５】
グルコン酸は、酒石酸やクエン酸と異なりモノカルボン酸であるために水素吸蔵合金を表面層形成用の処理液に浸漬したときに溶出した金属元素（特にＭｎ、Ａｌ）とキレート構造を有する錯体を形成しにくく、本発明に係る水素吸蔵合金粉末のようにＬａの含有比率が高く、Ｍｎ、Ａｌの含有比率が低い表面層の形成が進みにくいと考えられる。これに対して、酒石酸やクエン酸のようにトリまたはジカルボン酸であって、隣合うカルボキシル基の間に２個の炭素原子を有するカルボン酸のアルカリ金属塩は、溶出した金属元素とキレート構造を有する錯体を形成し易いために前記表面層の形成が進むと考えられる。
【００９６】
（実施例８、９、１０）
水素吸蔵合金のＭｍ（ミッシュメタル）組成中にＹを５ｍｏｌ％添加したこと以外は、実施例２と同様にして得られた合金粉末を実施例８に係る水素吸蔵合金粉末、Ｅｒを５ｍｏｌ％添加したこと以外は、実施例２と同様にして得られた合金粉末を実施例９に係る水素吸蔵合金粉末、Ｙｂを５ｍｏｌ％添加したこと以外は、実施例２と同様にして得られた合金粉末を実施例１０に係る水素吸蔵合金粉末とし、これらの合金粉末を使用して作製した前記同様にしてニッケル水素蓄電池を作製した。該電池の評価結果を表４に示す。表４に示した電池の試験結果は、５個の電池を試験して得られた値の平均値である。
【００９７】
【表４】
【００９８】
合金にＹ、Ｅｒ、Ｙｂを組成中に添加すると、従来のアルカリ処理による活性化では水素吸蔵合金の活性化が進み難く、高率放電特性は優れたものではない。しかし、本発明に係る製造法を適用することによって、５℃における１０ＩｔＡの高率放電で２０℃での初期容量の８０％以上の容量を達成することが出来た。さらにＹ、Ｅｒ、Ｙｂの添加によって合金の耐蝕性が向上したために、表４の結果に示したように、実施例８〜実施例１０において、サイクル寿命が大幅に向上した。特にＹを添加した実施例８は，８００サイクルという優れたサイクル寿命特性を得られた。なお、詳細な結果は省略するがＳｍ、Ｇｄを添加した場合も前記Ｙ、Ｅｒ、Ｙｂを添加した場合と同様サイクル性能向上に有効であることが確かめられた。ここで添加した希土類元素がＭｍ中に占める比率は３〜１０ｍｏｌ％が好ましい。添加比率が３ｍｏｌ％未満の場合は、サイクル特性向上の効果が得られず、添加比率が１０ｍｏｌ％を超えると電極反応の活性および容量が低下する虞がある。また、前記比較例４と同様Ｎ／Ｐ比が小さくなり、サイクル性能が劣る虞がある。
【００９９】
尚、前記合金中の水素を脱離する工程に於いては、処理液の酸性度が高い場合、微量でも処理液が残存すると合金の溶出が生じ、乾燥工程において溶出物が合金粉末表面に電気絶縁性の物質として再析出することがあると考えられる。
【０１００】
また、合金を空気によって乾燥する第４工程において、乾燥温度が６０℃以下では合金を乾燥するのに数時間を要した。又、９０℃より高い温度で乾燥した場合、厚さの大きい合金の酸化被膜が形成されるためか合金の活性が低下して、高率放電出来なくなる現象が認められた。従って、処理後の水素吸蔵合金粉末を空気中で乾燥する場合、乾燥温度は、６０〜９０℃に設定することが好ましい。
【０１０１】
なお、本発明は、上記実施例に記載された活物質の出発原料、製造方法、正極、負極、電解質、セパレータ及び電池形状などに限定されるものではない。特に水素吸蔵合金粉末は、Ｌａを含む希土類元素とＮｉ、Ｃｏ、Ｍｎからなる遷移金属元素およびＡｌを主成分とするものであればよく、Ｎｉ、Ｃｏ、Ｍｎ以外の遷移金属元素を少量含むものにも適用できる。
【発明の効果】
【０１０２】
本発明に係る水素吸蔵合金粉末は、電極反応の活性が高く、且つ、母相ならびに表面層に吸蔵された水素が合金粉末の表面に拡散し易い水素吸蔵合金粉末である。
【０１０３】
本発明に係る水素吸蔵合金粉末の製造方法によれば、水素吸蔵合金粉末の表面に電極反応の活性が高く、水素が拡散し易い表面層を形成することができる。
【０１０４】
本発明に係る水素吸蔵合金粉末の製造方法では、水素吸蔵合金粉末を、錯化剤を含むアルカリ水溶液に浸漬したのちにｐＨが５〜７の水溶液に浸漬する工程を行うことによって、表面層形成操作によって水素吸蔵合金粉末の表面に生成した絶縁性物質を除去することができる。
【０１０５】
本発明に係る水素吸蔵合金粉末の製造方法では、水素吸蔵合金粉末を、錯化剤を含むアルカリ水溶液に浸漬したのちに温度８０℃以上、ｐＨ６〜７の温水に浸漬して水素を除去する工程を行うことによって、表面層形成操作によって水素吸蔵合金内に吸蔵された水素を容易且つ安価に除去し、水素吸蔵合金が、空気等の酸化剤に触れたときに水素吸蔵合金に吸蔵された水素が酸化されると同時に発熱して変質したり発火したりするのを防ぐことができる。
【０１０６】
本発明に係る水素吸蔵合金粉末の製造方法では、水素吸蔵合金粉末を錯化剤を含むアルカリ水溶液に浸漬し、水素吸蔵合金粉末をｐＨが５〜７の水溶液に浸漬する工程および水素を除去した後に水素吸蔵合金粉末の表面を酸化する工程を行うことによって、水素吸蔵合金粉末の化学的な安定性を向上させ、保管中の変質を防ぐ効果がある。
【０１０９】
本発明に係るニッケル水素蓄電池は、化成のための蓄電池の充放電操作を大幅に簡略化でき、高率放電特性およびサイクル特性に優れたニッケル水素蓄電池である。
【図面の簡単な説明】
【図１】図１は、本発明の一実施例を示す蓄電池の内部の構造を示す説明図である。
【図２】本発明に係る水素吸蔵合金の拡大断面写真である。
【図３】水素吸蔵合金粉末の表面における絶縁性生成物の存在を示す拡大断面写真である。
【符号の説明】
１ 密閉型ニッケル水素蓄電池
３ 正極
４ 負極
５ セパレータ
１１ 表面層
１２ 母相
１３ 絶縁被膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hydrogen storage alloy powder, a method for producing the same, a hydrogen storage alloy electrode and a nickel hydride storage battery to which the hydrogen storage alloy powder is applied, and more particularly, a hydrogen storage alloy electrode and a nickel hydride storage battery excellent in high rate discharge characteristics. It is about.
[0002]
[Prior art]
In recent years, there has been a rapid increase in the number of electric devices that require a large current discharge, such as hybrid electric vehicles (HEV) and electric tools. As a power source for these devices, a sealed nickel-metal hydride storage battery has been attracting attention as a clean power source for the environment in addition to higher energy per unit volume and unit mass than a nickel cadmium storage battery or a lead storage battery. In addition, the nickel metal hydride storage battery can absorb oxygen generated at the positive electrode during overcharge by the negative electrode containing the hydrogen storage alloy, and thus has an advantage that the charge control system is easy and the charging circuit is simplified. is doing.
[0003]
The storage battery as a driving power source for the HEV or electric tool is required to discharge at a high rate of about 10 ItA discharge. However, since the hydrogen storage alloy electrode has a slower charge transfer on the active material surface than the cadmium electrode and the activity of the electrode reaction is low, the conventional nickel-metal hydride storage battery has a high rate discharge characteristic compared to the nickel cadmium storage battery. Inferior, it was disadvantageous for applications requiring the high rate discharge. In addition, when the hydrogen storage alloy is applied to the electrode as it is, it takes time for the initial activation until sufficient discharge characteristics are exhibited, and it is several tens of times if the high-rate discharge is applied to the required application. In some cases, charging / discharging for activation several hundred times is required.
[0004]
In order to solve the slow activation of the hydrogen storage alloy among these problems, many proposals have been made on the surface treatment method. For example, Patent Document 1 discloses a method of performing surface treatment with an acidic aqueous solution having a pH value of 0.5 to 3.5. The acid treatment improves the activity of the hydrogen storage alloy, but the effect is not great. This is because Ni is dissolved by the acid, so that Ni in the generated Ni-rich layer is eluted and it is difficult to keep the Ni-rich layer even after the acid treatment. Furthermore, a rare earth element containing manganese on the alloy surface, manganese, and aluminum are eluted by acid treatment, and a layer having a higher Ni content ratio than the mother layer inside the hydrogen storage alloy powder on the surface (hereinafter referred to as a Ni-rich layer). The electrode reaction activity can be improved, but the Ni-rich layer is poor in the ability to absorb hydrogen and the diffusion of hydrogen in the Ni-rich layer is not fast. Therefore, there is a defect that hydrogen occluded in the alloy mother layer is difficult to diffuse into the electrode reaction field on the surface of the alloy powder.
[0005]
Patent Document 2 discloses a method of dipping in an aqueous solution containing 30 to 80% by weight of sodium hydroxide at a temperature of 90 ° C. or higher. When such a treatment method using an alkaline aqueous solution is used, the electrode reaction activity of the hydrogen storage alloy is improved, but the effect is not great. This is because the formation rate of the surface layer rich in electrode activity is extremely low in the alkali treatment. In the case of a normally used alloy powder having a particle size of 20 to 50 μm, the specific surface area is small, and it is necessary to secure a Ni-rich layer of several tens of nm or more in order to improve electrode reaction activity. However, in the alkali treatment, once an extremely thin Ni-rich layer is formed, the formation speed is reduced, and it is difficult to form a working Ni-rich layer having a thickness necessary for improving electrode reaction activity. In addition, when alkali treatment is performed, the eluted rare earth element hydroxide and the like are generated, and the generated part covers the alloy surface, so that the conductivity of the hydrogen storage alloy electrode is lowered and the high rate discharge characteristics are lowered. Let
[0006]
Furthermore, in Patent Document 3 and Patent Document 4, the thickness of the layer containing Ni as a main component is 50 to 200 nm, and the manufacturing method in which the layer is immersed in a dilute acid solution after being immersed in an alkaline solution or an alkaline aqueous solution is disclosed. A method of treating the hydrogen storage alloy powder with an acidic aqueous solution after the treatment is disclosed. If a method of performing acid treatment after such treatment with an aqueous alkali solution is used, products such as hydroxides of rare earth elements by the alkali treatment covering the surface of the alloy can be removed, but only alkali treatment is performed. In the same way as in the case of using the hydrogen storage alloy powder having a particle size that is commonly used, the effect is not great.
[0007]
Patent Document 5 discloses that a complexing agent such as citric acid or gluconic acid is added to an alkali treatment solution at 60 ° C. or higher. However, with the method described in Patent Document 5, it is difficult to form a Ni-rich layer, and a sufficient effect cannot be obtained. This is because the element eluted from the alloy into the treatment liquid reacts with the complexing agent to form a complex, but it exists in an ionic state in the solution. Therefore, as the ion concentration in the treatment liquid increases, the Ni-rich layer This is because formation becomes difficult to proceed.
[0008]
[Patent Document 1]
JP-A-7-73878 (page 3, paragraph 0011)
[Patent Document 2]
JP 2002-256301 A (page 3, paragraph 0009)
[Patent Document 3]
JP-A-9-7591 (Page 3, paragraphs 0017 to 0018)
[Patent Document 4]
Japanese Patent Laid-Open No. 9-171821 (page 2, paragraph 0007)
[Patent Document 5]
JP 2001-68104 A (pages 2 and 3, paragraphs 0009 to 0017)
[0009]
[Problems to be solved by the invention]
The present invention has been made to solve the above-mentioned problems, and one object thereof is to provide a nickel-metal hydride storage battery excellent in high-rate discharge performance and charge / discharge cycle performance and to form a nickel-metal hydride storage battery. It is intended to provide a method for manufacturing nickel-metal hydride storage batteries easily and inexpensively by greatly simplifying.
[0010]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have intensively studied, and as a result, occluded and desorbed hydrogen, which is composed mainly of rare earth elements including La, transition metal elements composed of Ni, Co, and Mn, and Al. Among possible hydrogen storage alloy powders, hydrogen storage alloy motherphaseOutside, motherphaseSurprisingly, excellent high-rate discharge characteristics and cycle life characteristics are obtained by arranging alloy layers with specific compositions that differ in composition from each other with specific thicknesses and specific values of mass saturation magnetization. It has been found that a nickel-metal hydride storage battery having the above can be obtained, and has led to the present invention. Means of the present invention for achieving the above object are as described below. However, the action mechanism still includes a part including estimation, and the correctness of the action mechanism does not limit the present invention.
[0011]
The hydrogen storage alloy powder according to the present invention is a hydrogen storage alloy powder capable of occluding and desorbing hydrogen, comprising a rare earth element containing La, a transition metal element composed of Ni, Co, and Mn and Al as main components. By treating with a specific treatment liquid using a specific amount of a specific complexing agent, the motherphaseThis is a hydrogen storage alloy powder in which a layer of an alloy having a specific composition and a specific thickness (hereinafter referred to as a surface layer) is formed on the outer surface.
[0012]
Specifically, the hydrogen storage alloy powder according to the present invention is a hydrogen storage alloy powder composed mainly of a rare earth element containing La, a transition metal element composed of Ni, Co, and Mn and Al, and the hydrogen storage alloy powder. Powder motherphaseA surface layer having a thickness of 50 to 400 nm containing La at 5 mol% or more when the Ni content ratio is 100 mol% and the sum of the Mn and Al content ratios is 5 mol% or less. It is the formed hydrogen storage alloy powder. Here, the thickness of the surface layer is a value obtained by observing the cross section of the hydrogen storage alloy powder using a transmission electron microscope or a focused ion beam apparatus.
[0013]
As described above, the Ni-rich layer formed on the surface of the conventionally proposed hydrogen storage alloy powder has a sufficiently high catalytic activity for the electrode reaction, but hydrogen hardly diffuses in the Ni-rich layer. motherphaseIt is considered that sufficient high rate discharge characteristics are difficult to obtain because the hydrogen occluded in the metal is delayed in reaching the electrode reaction field on the alloy powder surface. On the other hand, the surface layer of the specific composition of the present invention is a composition in which the surface layer can occlude hydrogen, has a high catalytic activity for electrode reaction, and hydrogen easily diffuses in the surface layer.phaseAlternatively, it is considered that hydrogen occluded in the surface layer can quickly reach the surface of the alloy powder, which is an electrode reaction field.
[0014]
A preferred embodiment of the hydrogen storage alloy powder according to the present invention has a crack communicating with the surface of the alloy powder, and the mother facing the crack.phaseThe surface layer is formed on the outer surface of the substrate. For a hydrogen storage alloy having cracks, after placing the hydrogen storage alloy in a hydrogen gas atmosphere and pressurizing it to store hydrogen, the hydrogen storage alloy is taken out of the hydrogen atmosphere and heated to desorb hydrogen (in the gas phase). (Occlusion and desorption reaction of hydrogen). In addition to the gas phase reaction, the alloy can be partially corroded with an alkaline aqueous solution to generate hydrogen similar to the above, and the alloy can store hydrogen under atmospheric pressure. If a hydrogen storage alloy is stored with hydrogen of 30% or more of the hydrogen absorption in a hydrogen atmosphere at a temperature of 60 ° C. and an equilibrium hydrogen pressure of 1 megapascal (MPa), sufficient cracks will be generated in the alloy, resulting in high rate discharge. It is more preferable because the characteristics are greatly improved. By setting the aspect of the hydrogen storage alloy powder to such an aspect, the field of electrode reaction is expanded, and the motherphaseIt is considered that the high rate discharge characteristics could be further improved because the diffusion rate of hydrogen from to the electrode reaction field is increased.
[0015]
The hydrogen storage alloy powder preferably has a mass saturation magnetization of 1.5 to 9 emu / g. Here, the mass saturation magnetization is obtained by accurately weighing 0.3 g of a hydrogen storage alloy powder, which is a sample at room temperature, and filling the sample holder with a vibrating sample magnetometer (model BHV-30) manufactured by Riken Electronics Co., Ltd. Is a value measured by applying a magnetic field of 5 k Oersted using
[0016]
The method for producing a hydrogen storage alloy powder according to the present invention comprises:specificA surface layer is formed by dipping in an alkaline aqueous solution containing a complexing agent. The complexing agent applied to the present invention forms a complex with Mn and Al eluted in the treatment liquid when the hydrogen storage alloy powder is immersed in the treatment liquid for surface formation, so that both elements in the treatment liquid By lowering the ion concentration, Mn and Al are preferentially eluted. The hydrogen storage alloy powder according to the present invention is rich in activity as an active material of an electrode, and by applying the hydrogen storage alloy, the formation of a nickel-metal hydride storage battery can be greatly simplified as compared with the prior art, and It can be set as the storage battery excellent in the high rate discharge characteristic.
[0017]
The method for producing a hydrogen storage alloy powder according to the present invention includes a step of immersing the hydrogen storage alloy powder after forming the surface layer in an aqueous solution having a pH of 5 to 7 and a step of immersing it in warm water having a temperature of 80 ° C. or higher. HaveIs preferable.
[0018]
In the process of forming the surface layer, the eluted element complex may be deposited as a precipitate on the surface of the alloy powder to form an insulating film, which may increase the electrical resistance of the powder. The insulator film layer is dissolved and removed by immersing the hydrogen storage alloy powder after forming the surface layer in an aqueous solution having a pH of 5 to 7.
[0019]
Further, when the metal element is eluted in the process of forming the surface layer, hydrogen is generated and stored in the hydrogen storage alloy. When the hydrogen stored in the hydrogen storage alloy powder remains, when the hydrogen storage alloy powder touches the air, oxygen in the air reacts with the hydrogen stored in the hydrogen storage alloy powder to generate reaction heat, There is a possibility that the hydrogen storage alloy heated by the reaction heat is oxidized and deteriorated by oxygen in the air. In the method for producing a hydrogen storage alloy powder according to the present invention, before the hydrogen storage alloy powder is brought into contact with air, the hydrogen stored in the hydrogen storage alloy powder is desorbed by immersing in hot water at a temperature of 80 ° C. or higher.Preferably having a step. In the present invention, it is preferable to completely remove residual hydrogen by immersing in warm water and then immersing in hydrogen peroxide.
[0020]
The activity of the hydrogen storage alloy powder decreases when it is oxidized by contact with air. Further, when the alloy is stored in a water-containing state, the rare earth in the alloy is eluted to show alkalinity, and the corrosion of the alloy proceeds, so that the capacity when the alloy is used as an electrode is extremely reduced. An alloy in which the surface of the alloy is not oxidized by performing vacuum drying has extremely high activity with respect to the reaction with air, and is likely to be ignited in the atmosphere in addition to being easily altered. In the method for producing a hydrogen storage alloy powder according to the present invention, the surface of the hydrogen storage alloy is partially oxidized by a specific method in the final process of manufacture.Preferably having a step. By this oxidation treatment, a thin oxide film that does not inhibit the electrode reaction is formed on the alloy surface, and hydrogen storage alloy powder that can withstand long-term storage and does not ignite even when exposed to air without deteriorating high-rate discharge performance. Made it possible to provide.
[0021]
The hydrogen storage alloy electrode according to the present invention was selected from the hydrogen storage alloy powder having the surface layer formed thereon and ytterbium (Yb), erbium (Er), samarium (Sm), gadolinium (Gd), and yttrium (Y). Contains at least one rare earth elementIs preferable. The presence of the rare earth element is caused by the surface layer of the hydrogen storage alloy powder and the matrix.phaseCorroded in alkaline electrolyte and surface layer and motherphaseEffect of providing a surface layer that suppresses deteriorationTheThere is an effect of sustaining for a long time.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have confirmed that the negative electrode occupies a large portion of resistance during high rate discharge by performing a resistance component analysis of high rate discharge of the sealed nickel-metal hydride battery. Therefore, the present inventors examined the surface treatment of the hydrogen storage alloy in order to improve the charge transfer rate on the alloy surface of the negative electrode during high rate discharge, but by making the composition and structure specific, It has been found that a surprisingly high rate discharge performance can be obtained. That is, in a hydrogen storage alloy powder composed mainly of a rare earth element containing La, a transition metal element composed of Ni, Co, Mn, and Al, the mother of the hydrogen storage alloy powder.phaseWhen a surface layer containing 5% or more of La and 5% or less of the total content of Mn and Al was formed on the outer surface of the film, excellent high rate discharge characteristics were exhibited.
[0023]
Among these, a high-performance nickel-metal hydride storage battery including a hydrogen storage alloy electrode using a hydrogen storage alloy powder having a thickness of the surface layer of 50 nm to 400 nm and a powder mass saturation magnetization of 1.5 to 9 emu / g. It has been found that the rate discharge characteristics are greatly improved. When the thickness of the layer having the specific composition is 50 nm or less, the effect of improving the high-rate discharge characteristics is small. When the thickness is 400 nm or more, the high-rate discharge is improved, but the cycle life performance is lowered.
[0024]
Similarly, when the mass saturation magnetization is 1.5 emu / g or less, the effect of improving the high-rate discharge characteristics is not recognized, and when the mass saturation magnetization is generated at 9 emu / g or more, the alloy capacity is greatly reduced. The high-rate discharge characteristics are not simply determined by the area or amount of the layer having the specific composition, which is the place of the discharge reaction, but are also greatly affected by the location of the layer, and the matrix facing the crack connected to the surface of the alloy powder.phaseThose having the layer of the specific composition on the surface have a remarkable effect.
[0025]
In particular, in the case of a hydrogen storage alloy powder containing a rare earth element selected from Yb, Er, Sm, Gd, and Y added as an anticorrosive to the hydrogen storage alloy powder, the alloy powder is simply immersed in an alkali or acid. Then, the high rate discharge characteristic is not excellent. This is considered to be because the rare earth element hinders activation by immersing the hydrogen storage alloy powder in a conventional alkaline aqueous solution or acidic aqueous solution.
[0026]
On the other hand, in the hydrogen storage alloy powder having a surface layer according to the present invention, even when an anticorrosive agent selected from the rare earth elements is added, excellent high rate discharge without losing excellent cycle life characteristics Properties can be achieved. This is due to the fact that the surface layer is active against electrode reactions and the diffusion of hydrogen in the layer is rapid. Depending on the preferred properties of the surface layer, the mother of the hydrogen storage alloyphaseIt is considered that the hydrogen occluded in the surface layer quickly moved to the electrode reaction field on the surface of the alloy powder and contributed to the electrode reaction.
[0027]
When the surface is treated with an alkaline aqueous solution, rare earth ions such as La, transition metal elements such as Ni, Co, and Mn, and Al are eluted from the surface alloy, and the rare earth ions and transition metal ions generate their hydroxides. In ordinary acid treatment using hydrochloric acid or acetic acid, a surface layer having a specific composition capable of leaching rare earth such as La and capable of absorbing hydrogen is difficult to form. In ordinary alkali treatment using an aqueous KOH solution, La is used. Al and Mn can be eluted while suppressing the elution of rare earths such as, but the concentration of these elution elements in the treatment liquid becomes high and the elution reaction is suppressed, so in order to form a sufficient layer, it takes a long time. In addition, processing at a high temperature is required.
[0028]
Although the surface composition can be controlled by mixing the specific alkali metal element and strict temperature control, the composition of the alloy surface changes if the temperature or composition ratio of the treatment liquid changes slightly. It is difficult to form a surface with a specific composition. On the other hand, when using a solution containing a specific complexing agent in an alkaline aqueous solution, the complexing agent forms a complex precipitate with the elution component, thus reducing the concentration of eluted element ions in the electrolyte. And the surface layer can be generated at a surprisingly fast rate. In particular, when the complexing agent has only a monodentate ligand (for example, an alkali metal salt of a monocarboxylic acid), the complex is unstable and the reaction rate is slow, whereas the molecule has two carboxyl groups. When an alkali metal salt of carboxylic acid is applied, the reaction rate is greatly improved to form a stable chelate complex precipitate. When an alkali metal salt of a carboxylic acid having two carbon atoms sandwiched between two carboxyl groups such as tartaric acid and citric acid is applied as a complexing agent, a chelate complex having high stability particularly with Mn and Al. For this reason, Mn and Al are preferentially eluted, and a layer having a high La content ratio and a low Mn and Al content ratio can be effectively formed on the surface of the hydrogen storage alloy powder.
[0029]
Depending on the ordinary alkaline aqueous solution, it is difficult to control the amount of surface layer formation of a specific composition. This is because the formation of the surface layer is inhibited by an oxide film or the like existing on the alloy surface. The amount of complex-forming product could be controlled by the amount of complexing agent added, and the amount of surface layer formation with a specific composition could be controlled. As the addition amount of the complexing agent with respect to 1 g of the hydrogen storage alloy powder, when 0.1 to 0.9 mmol is added, the thickness is preferably 50 nm or more and 400 nm or less, and the mass saturation magnetization is preferably 1.5 to 9 emu / g. A value surface layer is obtained.
[0030]
The alkaline aqueous solution applied to the surface layer formation is not particularly limited, but one or more aqueous solutions of alkali metal hydroxides such as KOH, NaOH and LiOH used for the electrolyte of alkaline storage batteries are mixed. When used as a battery, the component ratio is similar to that of the electrolytic solution, so that when a battery is formed, no new elements are eluted from the alloy into the electrolytic solution, and the composition of the hydrogen storage alloy before being incorporated into the battery is maintained. This is preferable. Among these, it is preferable to use an aqueous NaOH solution because excellent cycle performance can be obtained as compared with the case of using an aqueous LiOH or KOH solution. This is considered to be because when a NaOH aqueous solution is applied to the treatment liquid, the elution of cobalt is suppressed and the surface layer of the alloy powder is difficult to peel from the mother layer.
[0031]
When the concentration of the NaOH aqueous solution is low, the elution of elements from the hydrogen storage alloy for forming a complex is small, and the formation of the surface layer does not proceed. The NaOH aqueous solution to be applied has a density of 1.3 g / cm at a temperature of 20 ° C.3Although the above is preferable, the density of the aqueous solution is 1.5 cm.3Exceeding 1.5 cm, crystal precipitation occurs at room temperature, making it difficult to handle.3The following is preferred.
[0032]
The treatment temperature when forming the surface layer is preferably 60 ° C. or higher and 100 ° C. or lower, more preferably 60 ° C. or higher and 80 ° C. or lower. When the treatment temperature is less than 60 ° C., the reaction for forming the surface layer is difficult to proceed, and when it exceeds 100 ° C., the reaction rate is too fast, and it becomes difficult to control the thickness of the surface layer.
[0033]
Further, at the treatment temperature, it is preferable that the treatment time is 30 minutes or longer for the element eluted from the alloy into the treatment liquid to react with the complexing agent to form a complex. In order to avoid excessive etching, the treatment time is preferably 10 hours or less, and more preferably 3 hours or less.
[0034]
As described above, in the process of forming the surface layer by immersing in an alkaline aqueous solution to which a complexing agent is added, the element once eluted becomes a complex and deposits on the alloy powder. The deposit is not preferable because it is electrically insulating and restricts the contact between the alloy powder and the electrolytic solution to inhibit the electrode reaction. It is necessary to immerse the alloy powder after forming the surface layer in an acidic aqueous solution to remove the insulating coating, but when a strongly acidic aqueous solution having a low pH is used, components such as La and Ni are eluted from the surface layer. The high rate discharge characteristics are deteriorated.
[0035]
In order to remove the insulating film, it is preferable to apply a treatment solution having a pH of 5 to 7 because the insulating film can be removed while reducing elution of components such as La and Ni. As the treatment liquid, an aqueous solution of a weak acid such as acetic acid can be applied. However, a pH buffer solution such as a sodium citrate-sodium hydroxide aqueous solution or an acetic acid-sodium acetate aqueous solution is used to keep the pH of the treatment liquid within the above range. It is convenient to use. Among them, it is preferable to use an acetic acid-sodium acetate aqueous solution that is easy to handle and inexpensive.
[0036]
Further, the particles of the insulating material separated from the hydrogen storage alloy powder may float in the treatment solution in which the hydrogen storage alloy powder after the surface layer formation is immersed in the aqueous solution of PH5-7 to remove the insulator coating layer. is there. Since the insulating material has a lower sedimentation rate than the hydrogen storage alloy powder, the insulating material can be removed by utilizing the difference between the two sedimentation rates. For example, flowing water is allowed to flow from the lower part of the stirring tank containing the treatment liquid containing the alloy powder, and the particles of the insulating substance that is difficult to settle are discharged out of the system together with the overflowing flowing water.
[0037]
As a method for desorbing hydrogen from the alloy, the alloy powder after the surface layer formation treatment is suddenly treated with H.2O2There is a method of desorbing by contacting with an aqueous solution or the like and oxidizing,2O2Since such chemicals are expensive, it is disadvantageous to use them in large quantities. In order to reduce the amount of oxidant used to desorb hydrogen from the hydrogen storage alloy, the method of exposing the alloy to warm water having a temperature of 80 ° C. or higher and a pH of 9 or lower is an efficient and inexpensive way to reduce the amount of hydrogen contained in the alloy. It is preferable because much can be desorbed as a gas. After the immersion treatment in warm water, an oxidation treatment is performed using an oxidizing agent, and hydrogen that is not desorbed by the warm water immersion treatment is desorbed. The oxidizing agent used here is not particularly limited, but hydrogen peroxide is preferable because the product after decomposition does not have impurities that degrade the alloy performance. These oxidants are inefficient because they release oxygen gas and self-decompose when they come into contact with the surface layer of the alloy at a temperature of 45 ° C., and react efficiently with hydrogen in the alloy when used at a temperature of 45 ° C. or lower. Therefore, it is preferable.
[0038]
In the present invention, the surface of the hydrogen storage alloy powder after the removal of the insulating coating and the desorption of hydrogen is oxidized. As described above, a method of oxidizing the surface of the hydrogen storage alloy powder by contacting with hydrogen peroxide solution can also be applied. However, when dried by air at a temperature of 60 ° C. to 90 ° C., the alloy surface is oxidized, The reduction in high rate discharge characteristics is preferable because it is limited. When drying in the air, the oxide film formed on the alloy surface is thin, and the oxide film is reduced by the activation operation by charging / discharging the battery after the battery is installed, or the oxide is peeled off. This is probably because of this.
[0039]
By performing oxidation treatment on the surface of the alloy powder as described above, quality deterioration is suppressed during long-term storage, and there is no risk of ignition during storage and operation, and the powder of hydrogen storage alloy for negative electrode has excellent high rate discharge characteristics. Is obtained.
[0040]
Hereinafter, embodiments of the present invention will be described in more detail. However, the present invention is not limited to the following embodiments.
[0041]
A sealed nickel-metal hydride battery according to the present invention includes a positive electrode mainly composed of nickel hydroxide, a negative electrode mainly composed of a hydrogen storage alloy, and an alkaline electrolyte composed of an aqueous solution of an alkali metal hydroxide. In general, a separator is provided between the positive electrode and the negative electrode. FIG. 1 shows a cut surface of a typical storage battery of the present invention in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. In the figure, 1 is a sealed nickel-metal hydride battery, 2 is a battery casing, 3 is a positive electrode, 4 is a negative electrode, 5 is a separator, 6 is an insulating gasket, 7 is a sealing plate, 8 is a positive electrode terminal, and 9 is a current collector. It is.
[0042]
As the positive electrode active material, a mixture of nickel hydroxide with zinc hydroxide and cobalt hydroxide is used, but it is preferable to use a nickel hydroxide composite hydroxide obtained by uniformly dispersing these by a coprecipitation method. . The constituent materials of the positive electrode other than the nickel hydroxide composite oxide include cobalt hydroxide, cobalt oxide and the like as a conductive modifier. Can be applied. Further, these positive electrode material powders are mixed with oxygen-containing gas, K in the presence of an alkaline aqueous solution.2S2O8The oxidation number of transition metals (Ni, Co) contained in the positive electrode material is increased to a value larger than 2 by chemical oxidation using an oxidizing agent such as chlorous acid or hypochlorous acid, or electrochemical oxidation. You can also. The oxidation treatment is effective for suppressing discharge reserve generation to increase discharge capacity and improving cycle performance.
[0043]
Further, at least one element selected from Ho, Er, Tm, Yb, Lu, and Y rare earth elements can be added to the positive electrode as an oxide or a hydroxide. By adding a compound of these elements to the positive electrode, it is effective to suppress the generation of oxygen from the positive electrode during charging and to suppress the deterioration of the hydrogen storage alloy of the negative electrode due to the oxygen.
[0044]
The hydrogen storage alloy, which is the main component of the negative electrode, contains La, and is composed of rare earth elements such as cerium (Ce), praseodymium (Pr), and neodymium (Nd) in addition to La (hereinafter referred to as Misch metal Mm). ), A transition metal element composed of Ni, Co, and Mn, and a hydrogen storage alloy powder composed mainly of Al can be used.
[0045]
It is difficult to avoid elution of La in the process of forming the surface layer. The ratio of La contained in the surface layer of the hydrogen storage alloy of the present invention is preferably 5 mol% or more and a high value when the Ni content ratio is 100 mol%. Therefore, the mother of the hydrogen storage alloy applied to the present inventionphaseLa is the main constituent element of Mm, and the ratio of La in Mm is preferably 50 mol% or more.
[0046]
Further, in the hydrogen storage alloy electrode according to the present invention, as a corrosion inhibitor, Yb, Er, Sm, Gd and Y alone, oxides or hydroxides are added, or these elements are added to the hydrogen storage alloy. Can also be included as an alloy.
[0047]
It is desirable that the positive electrode active material powder and the negative electrode material powder have an average particle size of 50 μm or less. In particular, the powder of the hydrogen storage alloy, which is the negative electrode active material, should have a small particle size of 40 μm or less for the purpose of improving the high output characteristics of the sealed nickel-metal hydride battery. It is desirable that the particle size not be less than 20 μm. BookInventionOf the hydrogen storage alloy, the crack is provided, and the mother faces both the powder surface and the crack.phaseWhen the outer surface is arranged at 50 nm or more and 400 nm or less, an excellent high rate discharge performance can be obtained even with a large particle size, and therefore the average particle size is more preferably 30 μm to 50 μm.
[0048]
Various pulverizers and classifiers are used to obtain a powder having a predetermined particle size and shape. For example, a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow type jet mill, or the like is used. At the time of pulverization, wet pulverization may be used using water or an aqueous solution of an alkali metal hydroxide. The classification method is not particularly limited, and a sieve, an air classifier, or the like can be used. Both dry and wet methods are used as necessary.
[0049]
As described above, the positive electrode active material and the negative electrode active material which are main components of the positive electrode and the negative electrode have been described in detail. In addition to the main component, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, A filler etc. may be contained as another structural component.
[0050]
The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect battery performance. Usually, natural graphite such as scaly graphite, scaly graphite, earthy graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon whisker, carbon fiber, vapor grown carbon, metal (copper, nickel, gold, etc.) Conductive materials such as powder and metal fibers can be included as one kind or a mixture thereof.
[0051]
Among these conductive agents, amorphous carbon powder having a low graphitization degree is desirable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by weight to 10% by weight with respect to the total weight of the positive electrode or the negative electrode. In particular, it is preferable to use amorphous carbon having a low graphitization degree after being pulverized into ultrafine particles of 0.1 to 0.5 μm because the necessary carbon amount can be reduced. These mixing methods are physical mixing, and the ideal is uniform mixing. Therefore, a powder mixer such as a V-type mixer, an S-type mixer, a grinder, a ball mill, or a planetary ball mill can be used in a dry or wet manner.
[0052]
As the binder, usually, thermoplastic resins such as polytetrafluoroethylene (PTFE), polyethylene, polypropylene, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber Such polymers having rubber elasticity can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 0.1 to 3% by weight with respect to the total weight of the positive electrode or the negative electrode.
[0053]
In the positive electrode and the negative electrode, active material powder and additives for enhancing conductivity are dispersed in a liquid such as water or alcohol to form a paste, and the paste is applied to and supported on a porous substrate as a current collector. Things apply. It consists of carboxymethylcellulose (CMC), methylcellulose (MC), xanthan gum, guar gum and other polysaccharides or linear polyvinyl alcohol that are usually used when making the paste, or a mixture containing two or more of these compounds. A thickener can be applied. The addition amount of the thickener is preferably 0.1 to 3% by weight with respect to the total weight of the positive electrode or the negative electrode.
[0054]
A filler can also be added to the positive electrode and the negative electrode. The filler is not particularly limited as long as it does not adversely affect battery performance. Usually, olefinic polymers such as polypropylene and polyethylene, carbon and the like are used. The addition amount of the filler is preferably 5% by weight or less with respect to the total weight of the positive electrode or the negative electrode.
[0055]
The positive electrode and the negative electrode were mixed with the active material, the conductive agent and the binder in an organic solvent such as water and alcohol, respectively, and the resulting mixture was applied onto a current collector described in detail below and dried. By doing so, it is suitably manufactured. About the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. Is not to be done.
[0056]
As the positive electrode current collector, there is no particular choice as long as it is an electronic conductor that does not adversely affect the constructed battery. For example, nickel or a nickel-plated steel plate can be suitably used, and a two-dimensional device such as a punched steel plate can be used in addition to a foam, a formed fiber group, and a three-dimensional device subjected to uneven processing. The thickness is not particularly limited, but a thickness of 5 to 700 μm is used. Among these current collectors, it is preferable to use, as the positive electrode, a porous structure foam made of Ni, which has excellent corrosion resistance and oxidation resistance against alkali, and has a structure excellent in current collection. .
[0057]
As the current collector for the negative electrode, it is preferable to use a porous plate which is punched with an iron or steel foil or plate which is inexpensive and excellent in electrical conductivity and plated with Ni for improving reduction resistance. The punching hole diameter of the iron plate or the steel plate is preferably 1.7 mm or less and the opening ratio is 40% or more. Thereby, the adhesion between the negative electrode active material and the current collector is excellent even with a small amount of binder. In addition to the calcined carbon fiber and conductive polymer, the nickel surface of the current collector should be treated with Ni powder, carbon, platinum, etc. attached for the purpose of improving adhesion, conductivity and oxidation resistance. Can do. The surface of these materials can be oxidized.
[0058]
As a separator for a nickel metal hydride battery, a known porous film or nonwoven fabric exhibiting excellent high rate discharge characteristics can be used alone or in combination. Examples of the material constituting the separator include polyolefin resins typified by polyethylene and polypropylene, and nylon. The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength and gas permeability. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics. The separator is preferably subjected to a hydrophilic treatment. For example, a hydrophilic group graft polymerization treatment, sulfonation treatment, corona treatment, PVA treatment may be applied to the surface of a polyolefin resin fiber such as polyethylene, or a sheet obtained by mixing fibers that have already undergone these treatments may be used. .
[0059]
As the electrolytic solution, those generally proposed for use in alkaline batteries and the like can be used. Examples of the electrolytic solution include, but are not limited to, those obtained by dissolving potassium, sodium, and lithium hydroxides, which are electrolytes, in water or a mixture of two or more thereof in water. In the electrolyte, potassium hydroxide is 5 to 7 mol / dm.ThreeAnd 0.5 to 0.8 mol / dm of lithium hydroxideThreeIt is preferable to apply an aqueous solution containing the same because excellent battery characteristics can be obtained.
[0060]
In addition to the electrolyte described above, the electrolyte solution includes rare earth elements such as Ho, Er, Tm, Yb, Lu, and Y, as well as calcium, sulfur, for the purpose of improving the oxygen overvoltage of the positive electrode, improving the corrosion resistance of the negative electrode, and suppressing self-discharge. A compound such as zinc can be added singly or as a mixture of two or more thereof.
[0061]
In the sealed nickel-metal hydride storage battery according to the present invention, the electrolyte is injected before or after the separator for the sealed nickel-metal hydride battery, the positive electrode, and the negative electrode, for example, and finally sealed with an exterior material. It is suitably manufactured by doing. Further, in a sealed nickel-metal hydride battery in which a power generation element in which a positive electrode and a negative electrode are stacked via a separator for a sealed nickel-metal hydride battery is wound, the electrolytic solution is generated before and after the winding of the power generation element. It is preferable that the liquid is injected. As the injection method, it is possible to inject at normal pressure, but a vacuum impregnation method, a pressure impregnation method, and a centrifugal impregnation method can also be used.
[0062]
Examples of the material for the outer package of the sealed nickel-metal hydride battery include nickel-plated iron, stainless steel, polyolefin resin, and the like, or composites thereof.
[0063]
The configuration and shape of the sealed nickel-metal hydride storage battery are not particularly limited, and a coin battery or button battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, a square battery, a flat battery, and a roll shape A cylindrical battery having a positive electrode, a negative electrode, and a separator is an example.
[0064]
【Example】
Hereinafter, the present invention will be described in more detail on the basis of examples. However, the present invention is not limited to the following description, and the positive electrode active material, the negative electrode material, the positive electrode, the negative electrode, and the electrolyte of the test method and constituting battery are not limited thereto. The separator and battery shape are arbitrary.
[0065]
(Examples 1-5)
(Synthesis of nickel hydroxide particles)
An ammonium complex and an aqueous NaOH solution were added to an aqueous solution in which nickel sulfate, zinc sulfate, and cobalt sulfate were dissolved so that each metal hydroxide had a mass ratio described later, thereby forming an ammine complex. While vigorously stirring the reaction system, caustic soda is further added dropwise, the temperature of the reaction bath is controlled to 45 ± 2 ° C., the pH is controlled to 12 ± 0.2, and spherical high density nickel hydroxide particles serving as the core layer base material are formed. The hydroxide was synthesized so that the mass ratio of hydroxide was nickel: zinc: cobalt = 93: 5: 2.
[0066]
(Formation of surface layer on nickel hydroxide particle surface)
The high-density nickel hydroxide particles were put into an alkaline aqueous solution controlled to pH 12 ± 0.2 with caustic soda. While stirring the solution, an aqueous solution containing cobalt sulfate and ammonia at predetermined concentrations was added dropwise. During this time, an aqueous caustic soda solution was appropriately added dropwise to maintain the pH of the reaction bath at 12 ± 0.2. The pH was maintained at 12 ± 0.2 for about 1 hour, and a surface layer made of a mixed hydroxide containing Co was formed on the surface of the nickel hydroxide particles. The ratio of the surface layer of the mixed hydroxide was 7 wt% with respect to the core layer mother particles (hereinafter simply referred to as the core layer).
[0067]
(Partial oxidation of positive electrode active material powder)
100 g of nickel hydroxide particles having a surface layer made of the mixed hydroxide was put into 400 g of an aqueous caustic soda solution having a temperature of 60 ° C. and a concentration of 10 wt%, and sufficiently stirred. Subsequently, 45 ml of a sodium hypochlorite solution (manufactured by Wako Pure Chemical Industries, Ltd.) was added, and stirring was continued for 30 minutes. The active material particles were filtered, washed with water and dried. To the obtained positive electrode active material particles, 20 g of an aqueous NaOH solution having a concentration of 30 wt% and a temperature of 80 ° C. was added, left at a temperature of 80 ° C. for 2 hours, washed with water and dried. The average oxidation number of transition metal elements (Ni, Co) contained in the obtained positive electrode active material particles (including both the core layer and the surface layer) is determined by a known method (reactive material particles are reacted with ferrous ammonium sulfate. Then, redox titration is performed using potassium permanganate). As a result, the average oxidation number was 2.15.
[0068]
(Preparation of positive electrode plate)
In the active material particlesCarboxymethylcellulose(CMC) Aqueous solution was added to make a paste of the active material particles: CMC solute = 99.5: 0.5, and the paste was 450 g / m. 2 Of nickel porous material (foam nickel, manufactured by Sumitomo Electric Co., Ltd., trade name: Nickel Celmet # 8). Then, after drying at 80 ° C., it was pressed to a predetermined thickness, the surface was coated with Teflon (registered trademark), and a nickel positive electrode plate with a capacity of 3000 mAh having a width of 34 mm (inside, uncoated part 1 mm) and a length of 260 mm was obtained.
[0069]
(Preparation of hydrogen storage alloy powder)
(Example 1 to Example 5)
(Formation of surface layer)
Density at 20 ° C. containing sodium tartrate dihydrate at 0.1 mmol / g, 0.2 mmol / g, 0.4 mmol / g, 0.8 mmol / g and 0.9 mmol / g, respectively, per 1 g of hydrogen storage alloy .5g / cmThreeTo an alkaline aqueous solution obtained by heating 100 ml of an aqueous sodium hydroxide solution to 80 ° C., AB having an average particle diameter of 35 μm was added.FiveType hydrogen storage alloy powder, wherein Mm contains 70% La1.0Ni3.6Co0.6Al0.3Mn0.35100 g of a hydrogen storage alloy represented by the following formula was added and stirred for 2 hours.
[0070]
(Removal of insulation coating)
After the surface layer forming treatment, the contents (hydrogen storage alloy powder and treatment liquid) are filtered under pressure, and the hydrogen storage alloy powder separated by filtration is washed with water, and then 1.6 mol / dm.3Acetic acid was added to 300 ml of sodium acetate aqueous solution at pH 5 and the temperature was raised to 40 ° C.VinegarThe solution was put into an acid-sodium acetate aqueous solution and stirred for 30 minutes.
[0071]
(Hydrogen desorption)
After the treatment for removing the insulating coating, the content was filtered under pressure and washed with water until the pH of the cleaning solution reached 6 to 7, and then the obtained hydrogen storage alloy powder was dispersed in pure water and heated to 80 ° C. . Furthermore, hot water is removed by pressure filtration, and the obtained hydrogen storage alloy powder is put into water at a temperature of about 25 ° C., and 4% hydrogen peroxide water is added in an amount equal to the alloy weight under stirring to perform hydrogen desorption treatment. went.
[0072]
(Oxidation treatment and drying)
After the hydrogen removal treatment, the content is filtered and separated from the treatment liquid, followed by washing with water to remove the remaining unreacted hydrogen peroxide solution and drying in air at a temperature of 80 ° C. for 2 hours. A hydrogen storage alloy powder for an electrode was obtained.
[0073]
Example 1 in which sodium tartrate dihydrate per 1 g of hydrogen storage alloy contained in the treatment liquid applied to the surface layer forming step was 0.1 mmol / g, Example 2, 0.2 mmol / g. The hydrogen storage alloy powder according to Example 3, the one with 0.8 mmol / g, and the one with 0.8 mmol / g as Example 4, and the one with 0.9 mmol / g as the Example 5 are used.
[0074]
(Example 6)
Prior to the formation of the surface layer, the hydrogen storage alloy powder is previously placed in a hydrogen pressure atmosphere of 0.4 MPa at a temperature of 45 ° C., and after the pressurization, the hydrogen storage alloy powder is removed from the hydrogen atmosphere to obtain a hydrogen storage alloy powder. A hydrogen storage alloy powder was obtained in the same manner as in Example 2 except that 30% of the hydrogen stored in a hydrogen atmosphere at a temperature of 60 ° C. and an equilibrium hydrogen pressure of 1 MPa was stored and released. Let this hydrogen storage alloy be the hydrogen storage alloy powder according to Example 6.
[0075]
(Example 7)
The density at a temperature of 20 ° C. of the NaOH aqueous solution constituting the treatment liquid applied to the surface layer forming step is 1.3 g / cm.ThreeIt was. Otherwise, a hydrogen storage alloy powder was obtained in the same manner as in Example 1. Let this hydrogen storage alloy be the hydrogen storage alloy powder according to Example 7.
[0076]
(Comparative Example 1)
The hydrogen storage alloy powder obtained in the same manner as in Example 1 except that the four steps of surface layer formation, insulation film removal, hydrogen desorption, and surface oxidation were not performed was used as a hydrogen storage alloy according to Comparative Example 1. Alloy powder.
(Comparative Example 2)
In the first step, the density is 1.5 g / cmThreeA hydrogen storage alloy powder according to Comparative Example 2 is obtained as the hydrogen storage alloy powder obtained in the same manner as the alloy 1 of the present invention except that sodium tartrate dihydrate was not added to the NaOH aqueous solution.
(Comparative Example 3)
Hydrogen obtained in the same manner as in Example 1, except that the hydrogen storage alloy powder was immersed in an aqueous solution of acetic acid at a temperature of 80 ° C. maintained at pH 6 for 1 hour instead of the surface layer formation and insulating film removal step. The storage alloy powder is a hydrogen storage alloy powder according to Comparative Example 3.
(Comparative Example 4)
In the surface layer forming step, the density is 1.5 g / cm.ThreeA hydrogen storage alloy powder obtained by performing the same treatment as in Example 1 except that 1.0 mmol of sodium tartrate dihydrate was added to an aqueous NaOH solution was used as a hydrogen storage alloy powder according to Comparative Example 4.
[0077]
(Measurement of mass saturation magnetization)
0.3 g of the obtained hydrogen storage alloy powder was precisely weighed, filled in a sample holder, and subjected to a magnetic field of 5 k Oersted using a vibration sample type magnetometer (model BHV-30) manufactured by Riken Denshi Co., Ltd. It was measured. Each of Examples 1 to 7 and Comparative Examples 1 to 4 was measured five times, and the median value was taken as the measured value of each sample.
(Measurement of surface layer thickness)
The obtained hydrogen storage alloy was sampled, the cross section of the sample was observed with a transmission electron microscope, and the thickness of the surface layer of the alloy powder was measured. As for the thickness of the surface treatment forming layer, the median value of the measured values at 10 locations in each of Examples 1 to 7 and Comparative Examples 1 to 4 was two significant figures.
(Quantification of La, Mn, Al content ratio in surface layer)
The composition in the center of the surface layer in the thickness direction is analyzed using an X-ray photoelectron spectrometer and an analytical transmission electron microscope, the content ratio of La, Mn, Al and Ni is determined, and the Ni content ratio is 100 mol%. It converted into the value. In each of Examples 1 to 7 and Comparative Examples 1 to 4, the median value of 10 analysis values was used as the analysis value of each sample. Table 1 shows the measurement and analysis results of the hydrogen storage alloy powders according to Examples 1 to 7 and Comparative Examples 1 to 4.
[0078]
[Table 1]
[0079]
As shown in Table 1, when the ratio of Ni contained in the surface layer of the hydrogen storage alloys according to Examples 1 to 7 is 100 mol%, the ratio of La contained in the surface layer is 5 mol% or more. And the sum of the ratio of Mn and Al is 5 mol% or less. This is due to the preferential elution and removal of Mn and Al by the action of sodium tartrate, which is a complexing agent added to the treatment liquid. Moreover, as the amount of tartaric acid increases, the thickness of the surface layer and the mass saturation magnetization of the hydrogen storage alloy powder tend to increase, and the ratio of La contained in the surface layer tends to increase.
[0080]
The hydrogen storage alloy powder according to Example 6 was formed by cracking the alloy powder by absorbing and desorbing hydrogen prior to the surface layer formation treatment, although the thickness of the surface layer was as small as 180 nm. The surface layer thickness of Example 2 is similar to that of 220 nm.InIt has a mass saturation magnetization of 5.0 emu / g. For Example Alloy 6, the mother facing the crackphaseIt was confirmed with an electron microscope and a focused ion beam apparatus that a surface layer was also formed on the surface of the film. A cross-sectional photograph of Example Alloy 6 using a focused ion beam device is shown in FIG. Mother of alloy powder insidephase12, a crack is formed, and the mother facing the crackphaseIt can be seen that the surface layer 11 is formed on the outer surface. Since this photograph was measured by tilting the sample by 45 degrees, the vertical enlargement ratio is 1 / √ (root) twice the horizontal. In addition, 14 of the figure is a platinum layer vapor-deposited on the surface of the sample powder as necessary to observe the cross section.
[0081]
In the case of Example 7, the density of the NaOH aqueous solution used for the treatment was 1.3 g / cm.3It was thought that the surface layer formation was difficult to proceed because it was low (NaOH concentration was low)ThisThe The value of mass saturation magnetization correlates with the thickness of the surface forming layer, but these values have a density of 1.3 g / cm.3It was confirmed that there was almost no increase below this value. The density is 1.3 to 1.4 g / cm.3In the meantime, the rise was confirmed to be very slow. From this, the density of the NaOH aqueous solution used for processing is 1.3 g / cm.3Or more, more preferably 1.4 g / cm3That's it. In addition, NaOH crystal does not precipitate at room temperature 1.5g / cm3The following is preferred.
[0082]
On the other hand, in the case where no complexing agent was applied as in Comparative Example 2, the formation of the surface layer was very slight, and the composition analysis of the surface layer could not be performed. Furthermore, in the case of Comparative Example 3 in which an aqueous solution of acetic acid was applied as the treatment liquid, formation of a surface layer was observed, but the ratio of La contained in the surface layer was a small value of 1 mol. In addition, a surface layer having a La ratio of 5 mol% or more, a sum of Mn and Al ratios of 5 mol% or less, and a thickness of 50 to 400 nm is formed in the target surface layer of the present invention. In order to achieve this, it has been found that the amount of the complexing agent added to the treatment liquid for forming the surface layer should be 0.1 to 0.9 mmol per 1 g of the hydrogen storage alloy.
[0083]
(Creation of negative electrode plate)
The hydrogen storage alloy powders according to Examples 1 to 7 and Comparative Examples 1 to 4 and the styrene butadiene copolymer emulsion were mixed at a mass ratio of 99.35: 0.65 on a dry basis. Then, it is dispersed in water to form a paste, applied to a punched steel sheet obtained by applying nickel plating to iron using a blade coater, dried at 80 ° C., pressed to a predetermined thickness, and a width of 34 mm (inside, none The coated portion was 1 mm), and a hydrogen storage negative electrode plate having a length of 260 mm and a capacity of 4800 mAh was obtained.
[0084]
(Production of evaluation battery)
A hydrogen storage alloy negative electrode plate to which the hydrogen storage alloys according to Examples 1 to 7 and Comparative Examples 1 to 4 are applied, a polypropylene non-woven separator with a thickness of 120 μm, and the nickel Combined with an electrode plate, it was wound into a roll shape, and an alkaline electrolyte in which 0.8 mol / l lithium hydroxide was dissolved in a 6.8N aqueous potassium hydroxide solution was injected, and a check at a valve opening pressure of 2.5 MPa was made. A subC-type sealed nickel-metal hydride storage battery having a valve was produced. Let this battery be the nickel hydride storage battery which concerns on Example 1- Example 7 and Comparative Example 1- Comparative Example 4. FIG.
[0085]
(Characteristic evaluation of battery)
The nickel metal hydride storage batteries according to Examples 1 to 7 and Comparative Examples 1 to 4 were stored at 40 ° C. for 12 hours, charged at 600 ° C. at 0.02 ItA at 20 ° C., and further 0.1 ItA. And charged for 12 hours. Further, after discharging to 0.2V at 1 It, charging for 12 hours at 0.1 ItA and discharging to 0.2V at 0.2 ItA were repeated 4 times. The fourth discharge capacity is defined as the initial discharge capacity at 20 ° C. After completion of the discharge, the battery was charged with 0.1 ItA for 12 hours, left at 5 ° C. for 5 hours, and then discharged at 10 ItA to 0.8 V at the same temperature. The ratio (%) of the discharge capacity obtained by the discharge to the initial discharge amount at 20 ° C. was determined and the result is shown in Table 2. After that, the battery is further charged at 2 ItA to −ΔV5 mV at a temperature of 25 ° C. (until the charging voltage passes the peak and decreases to 5 mV from the peak), pauses for 10 minutes after the charge is completed, is discharged to 1.0 V at 2 ItA, and is discharged. After a 10 minute rest. The cycle was repeated by setting the charge / discharge as one cycle, and the number of cycles in which the discharge capacity in the cycle reached 80% of the initial discharge capacity is shown in Table 2 as the cycle life. The battery test results shown in Table 2 are average values of values obtained by testing five batteries.
[0086]
[Table 2]
[0087]
The surface layer according to the present invention contains 5 mol% or more of La, the sum of the content ratio of Mn and Al is 5 mol% or less, the thickness is 50 nm or more, and the mass saturation magnetization is 1.5 emu / g or more. In the nickel metal hydride storage batteries according to Examples 1 to 7 to which the storage alloy powder is applied, the surface layer is not formed (Comparative Example 1) or the surface layer is not sufficiently formed (Comparative Example 2). Furthermore, it was found that even when the surface layer was formed, excellent high rate discharge characteristics were exhibited as compared with one having a small La content ratio in the surface layer (Comparative Example 3). In Example 1 in which the thickness of the surface layer was 100 nm and the mass saturation magnetization was 2.5, the discharge capacity more than doubled was obtained in the high rate discharge as compared with Comparative Example 3. Furthermore, when the surface layer is 180 to 220 nm or more and the mass saturation magnetization is 5.0 emu / g or more, the high-rate discharge characteristics are dramatically improved. The content ratio of La contained in the surface layer of Example 6 and the sum of the content ratios of Mn and Al are the same, and the mass saturation magnetization is the same as in Example 2. However, the high rate discharge characteristics of Example 6 are as follows. Exceeds Example 2. This is considered to be due to the effect of providing cracks in the hydrogen storage alloy powder in Example 6. As described above, in Examples 1 to 7, the charging / discharging for chemical conversion was only repeated four times, and the nickel hydride storage battery according to the present invention has such a small number of times of charging / discharging. It is a storage battery that exhibits extremely high rate discharge characteristics simply by being performed.
[0088]
In the case where the thickness of the surface layer was 550 nm and the mass saturation magnetization was 10 emu / g (Comparative Example 4), the high-rate discharge characteristics were good, but the cycle life was poor. In the case of Comparative Example 4, the amount of the surface layer formed was too large, and the ratio of the negative electrode capacity to the positive electrode capacity (N / P) was small, so the oxygen absorption capacity of the negative electrode was reduced, and charging was repeated. It is considered that the capacity rapidly decreased with the progress of the cycle because the electrolyte decreased due to the above.
[0089]
From the results shown above, the hydrogen storage alloy powder motherphaseIn a state where the number of charge and discharge for chemical formation is small by forming a surface layer having a La ratio of 5 mol% or more and a sum of the content ratio of Mn and Al of 5 mol or less and a thickness of 50 to 400 nm on the outer surface of It has been found that a hydrogen storage electrode and a nickel hydride storage battery having excellent high rate discharge characteristics can be obtained. This is presumably because the surface layer provided on the hydrogen storage alloy is rich in electrode reaction activity and hydrogen easily diffuses in the surface layer. From the results shown in Table 2, the thickness of the surface layer is preferably 50 nm or more and 400 nm or less, preferably 100 to 400 nm, and more preferably 180 to 400 nm. The mass saturation magnetization of the hydrogen storage alloy powder is preferably 1.5 to 9 emu / g, more preferably 2.5 to 9 emu / g, and particularly preferably 5.0 to 9 emu / g.
[0090]
(Comparative Example 5)
Let the hydrogen storage alloy powder obtained in the same manner as in Example 2 be the hydrogen storage alloy powder according to Comparative Example 5 except that the insulating film removal treatment was not performed. The hydrogen storage alloy powder was applied to prepare a hydrogen storage negative electrode plate in the same manner as in the above example, and a nickel metal hydride battery was prepared in the same manner as in the above example. The battery was charged with 0.1 ItA for 12 hours, then left at 5 ° C. for 5 hours to cool sufficiently, and then the discharge capacity when discharged with 10 ItA to 0.8 V was 0%.. ratioIn the hydrogen storage negative electrode plate to which the hydrogen storage alloy according to Comparative Example 5 was applied, a film composed of a complex of an element eluted in the process of forming the surface layer and tartaric acid remained around the alloy powder, and the conductivity was lowered. It is thought that the rate discharge characteristic was lowered.
[0091]
A cross-sectional photograph of the hydrogen storage alloy powder 5 according to Comparative Example 5 using a focused ion beam apparatus is shown in FIG. motherphaseIt can be seen that an insulating coating 13 is formed on the outer side of the surface layer 11 formed on the outer surface 12. A film clearly different from the surface treatment forming layer of the alloy is observed on the alloy surface. Since this photograph was measured by tilting the sample by 45 degrees, the vertical enlargement ratio is 1 / √ (root) twice the horizontal.
[0092]
(Comparative Example 6)
In the hydrogen desorption process, the alloy obtained by drying at 80 ° C. without desorption of hydrogen by hydrogen peroxide showed an exotherm considered to be due to oxidation when exposed to air. . A battery obtained by producing a battery by the above method using this alloy is referred to as a comparative battery 6. In Comparative Example 6, the capacity of the fourth cycle upon activation was only 1000 mAh. This is because hydrogen is desorbed in a vacuum, but hydrogen remains and it oxidizes when it comes into contact with oxygen in the air, and this heat generation oxidizes the alloy powder, reducing the activity as an active material. it is conceivable that.
[0093]
(Comparative Example 7)
The hydrogen storage alloy obtained in the same manner as in Example 2 except that sodium gluconate was used as a complexing agent in place of sodium tartrate as a complexing agent in the surface layer forming step. Use occluded alloy powder. Table 3 shows the value of the surface layer thickness and mass saturation magnetization of the alloy, and the discharge capacity at a temperature of 5 ° C. and 10 ItA discharge of a nickel-metal hydride battery produced by applying the alloy. The battery test results shown in Table 3 are average values of values obtained by testing five batteries.
[0094]
[Table 3]
[0095]
Gluconic acid is a monocarboxylic acid, unlike tartaric acid and citric acid, and therefore has a chelate structure complexed with metal elements (especially Mn and Al) eluted when the hydrogen storage alloy is immersed in the treatment solution for forming the surface layer. It is difficult to form, and it is considered that formation of a surface layer having a high La content ratio and a low Mn and Al content ratio is unlikely to proceed as in the hydrogen storage alloy powder according to the present invention. In contrast, an alkali metal salt of a carboxylic acid that is a tri- or dicarboxylic acid such as tartaric acid or citric acid and has two carbon atoms between adjacent carboxyl groups has a chelate structure with the eluted metal element. It is considered that the formation of the surface layer proceeds because it is easy to form a complex having the same.
[0096]
(Examples 8, 9, and 10)
The alloy powder obtained in the same manner as in Example 2 except that 5 mol% of Y was added to the Mm (Misch metal) composition of the hydrogen storage alloy was added 5 mol% of the hydrogen storage alloy powder according to Example 8 and Er. The alloy powder obtained in the same manner as in Example 2 except that the alloy powder obtained in the same manner as in Example 2 was used, and the alloy powder obtained in the same manner as in Example 2 except that 5 mol% of Yb was added. The hydrogen storage alloy powder according to Example 10 was used, and a nickel-metal hydride storage battery was manufactured in the same manner as described above using these alloy powders. The evaluation results of the battery are shown in Table 4. The test results of the batteries shown in Table 4 are average values of values obtained by testing five batteries.
[0097]
[Table 4]
[0098]
When Y, Er, and Yb are added to the alloy in the composition, activation by the conventional alkali treatment makes it difficult for the hydrogen storage alloy to be activated, and the high rate discharge characteristics are not excellent. However, by applying the manufacturing method according to the present invention, it was possible to achieve a capacity of 80% or more of the initial capacity at 20 ° C. with a high rate discharge of 10 ItA at 5 ° C. Furthermore, since the corrosion resistance of the alloy was improved by the addition of Y, Er, and Yb, the cycle life was significantly improved in Examples 8 to 10, as shown in the results of Table 4. In particular, in Example 8 to which Y was added, an excellent cycle life characteristic of 800 cycles was obtained. Although detailed results are omitted, it was confirmed that the addition of Sm and Gd is effective for improving the cycle performance as in the case of adding Y, Er, and Yb. The rare earth element added here occupies Mm.Ratio is 3-10 mol% is preferable. When the addition ratio is less than 3 mol%, the effect of improving the cycle characteristics cannot be obtained, and when the addition ratio exceeds 10 mol%, the activity and capacity of the electrode reaction may decrease. Further, the N / P ratio becomes small as in Comparative Example 4, and the cycle performance may be inferior.
[0099]
In the process of desorbing hydrogen in the alloyIsWhen the acidity of the chemical liquid is high, the elution of the alloy occurs when the treatment liquid remains even in a small amount, and the eluate is reprecipitated as an electrically insulating substance on the surface of the alloy powder in the drying process.Sometimesit is conceivable that.
[0100]
Further, in the fourth step of drying the alloy with air, it took several hours to dry the alloy when the drying temperature was 60 ° C. or lower. In addition, when dried at a temperature higher than 90 ° C., a phenomenon was observed in which the oxide activity of the alloy was reduced due to the formation of an oxide film with a large thickness, making it impossible to discharge at a high rate. Accordingly, when the treated hydrogen storage alloy powder is dried in air, the drying temperature is preferably set to 60 to 90 ° C.
[0101]
In addition, this invention is not limited to the starting material of the active material described in the said Example, the manufacturing method, a positive electrode, a negative electrode, an electrolyte, a separator, a battery shape, etc. In particular, the hydrogen storage alloy powder is sufficient if it contains a rare earth element containing La, a transition metal element composed of Ni, Co, and Mn and Al as a main component, and contains a small amount of a transition metal element other than Ni, Co, and Mn. It can also be applied to.
【The invention's effect】
[0102]
Main departureClearlySuch a hydrogen storage alloy powder has high electrode reaction activity,phaseIn addition, hydrogen occluded in the surface layer is a hydrogen occluded alloy powder that easily diffuses to the surface of the alloy powder.
[0103]
The present inventionFor producing hydrogen storage alloy powder according to claim 1According to the above, it is possible to form a surface layer on the surface of the hydrogen storage alloy powder that has a high electrode reaction activity and that facilitates hydrogen diffusion.
[0104]
The present inventionIn the method for producing a hydrogen storage alloy powder according to the present invention, the hydrogen storage alloy powder is immersed in an aqueous solution having a pH of 5 to 7 after being immersed in an alkaline aqueous solution containing a complexing agent.The insulating material generated on the surface of the hydrogen storage alloy powder by the surface layer forming operation can be removed.
[0105]
The present inventionIn the method for producing a hydrogen storage alloy powder according to the present invention, the hydrogen storage alloy powder is immersed in an aqueous alkaline solution containing a complexing agent, and then immersed in warm water at a temperature of 80 ° C. or higher and pH 6 to 7 to remove hydrogen. ByThe hydrogen stored in the hydrogen storage alloy is easily and inexpensively removed by the surface layer forming operation, and the hydrogen stored in the hydrogen storage alloy is oxidized when the hydrogen storage alloy touches an oxidant such as air.WhenAt the same time, it is possible to prevent heat generation from altering or igniting.
[0106]
The present inventionIn the method for producing a hydrogen storage alloy powder according to the present invention, the hydrogen storage alloy powder is immersed in an alkaline aqueous solution containing a complexing agent, the hydrogen storage alloy powder is immersed in an aqueous solution having a pH of 5 to 7, and the hydrogen is removed after removing the hydrogen. By performing the process of oxidizing the surface of the storage alloy powderIt has the effect of improving the chemical stability of the hydrogen storage alloy powder and preventing alteration during storage.
[0109]
The present inventionInSuch a nickel-metal hydride storage battery is a nickel-metal hydride storage battery that can greatly simplify the charge / discharge operation of the storage battery for chemical conversion and is excellent in high-rate discharge characteristics and cycle characteristics.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an internal structure of a storage battery according to an embodiment of the present invention.
FIG. 2 is an enlarged cross-sectional photograph of a hydrogen storage alloy according to the present invention.
FIG. 3 is an enlarged cross-sectional photograph showing the presence of an insulating product on the surface of the hydrogen storage alloy powder.
[Explanation of symbols]
1 Sealed nickel metal hydride storage battery
3 Positive electrode
4 Negative electrode
5 Separator
11 Surface layer
12 Motherphase
13 Insulation coating
Claims (6)
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| JP4296592B2 true JP4296592B2 (en) | 2009-07-15 |
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