JP4552319B2 - Method for producing positive electrode active material for alkaline storage battery - Google Patents
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- 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
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Description
【０００１】
【発明の属する技術分野】
本発明はアルカリ蓄電池の正極活物質の製造方法に関するものである。
【０００２】
【従来の技術】
ニッケルカドミウム電池、ニッケル水素化物電池、ニッケル亜鉛電池、ニッケル鉄電池等のアルカリ蓄電池に用いる水酸化ニッケルを主成分とするニッケル正極として、ペースト式ニッケル正極が主流となってきている。しかしながらペースト式ニッケル正極においては、活物質を保持する金属多孔基板の孔径が大きく、また水酸化ニッケル自体導電性に乏しいために、極板内で電極反応が円滑に進行せず、容量が得られないという問題点があった。
【０００３】
この問題点を解決する手段として、活物質に炭素や金属などの導電性粉末を添加する方法がある。導電性粉末を用いた場合、活物質と集電体および活物質粒子間の導電性が高まるため、活物質利用率は向上するが、十分な活物質利用率を得るためには多量の導電添加剤を添加する必要がある。しかしながら導電性粉末の添加量を増やすと、その分だけ電極反応に直接寄与する活物質の含有量が低下するため、電極のエネルギー密度が低下することになる。
【０００４】
こうした問題点を解決する手段として、特開昭６１-１３８４５８号公報にはアルカリ水溶液中で2価のイオンを生成するコバルト化合物を添加する方法が提案されている。コバルト化合物はアルカリ電解液中でいったん溶解してＣｏ（ＯＨ）2として正極の活物質粒子表面に再析出する。さらに、再析出したＣｏ（ＯＨ）2が初回の充電操作で電解酸化されて電導性のコバルト酸化物を生成することにより活物質粒子間に電導性パスが形成され、活物質利用率の高いペースト式ニッケル電極となる。
【０００５】
さらに、特公平４−４６９８号公報には、コスト低減(コバルト量の削減)および更なる高エネルギー密度化を図るための方策として、予め水酸化ニッケル粒子表面にコバルト水酸化物層をコーティングした粒子を用いることが提案されている。水酸化ニッケル粒子表面に被覆したコバルト水酸化物は前述のコバルト化合物と同様にして電導性パスで粒子間を接続し、活物質利用率の高いペースト式ニッケル電極を実現するとされる。
【０００６】
また、従来のアルカリ蓄電池は、高温に於いて充電受け入れ特性が不十分であり、その改善が要求されている。高温での充電受けいれ特性を向上する手段の一つとして、特開平９−９２２７９号公報には、Ｃａ化合物や希土類元素の酸化物を正極活物質粒子に添加する方法が提案されている。
【０００７】
これらの添加剤は、充電過程の末期に水酸化ニッケル自身の充電反応と競争して起こる電解液の分解反応、いわゆる酸素ガス発生反応を抑制する機能を持つ。これらの添加剤の使用は、充電受け入れ特性向上には有効であるが、正極の高率放電性能を低下させるという新たな問題を生じた。
【０００８】
前記高率放電性能の低下は、添加された希土類の酸化物がＣｏの再析出を妨げ、正極に電導性パスが形成するのを阻害するためと考えられる。従って、前記添加剤を共存させた系においてＣｏの再析出を促す為、正極活物質に添加するコバルト化合物の比率を増やしたり、あるいは予めＣｏを含む被覆層を形成させておく方式では被覆層の厚みを増やす等して、電導性パスの強化を図ることが試みられた。また、それとは別に初回の充電を電流値の異なる多段式充電にする方法が提案されている。しかし、多段式充電の場合、充電設備が複雑になったり、充電に要する時間が長くなったりするため生産能率が低下する欠点があった。
【０００９】
また、活物質粒子を空気中で放置すると、表面層に含まれるＣｏの水酸化物が空気に含まれる酸素によって容易に酸化され、電気化学的に不活性なＣｏ3Ｏ4に変化する。このため、Ｃｏ（ＯＨ）2で被覆された活物質の保存は酸化が起きないように管理された雰囲気の中で行う必要があった。
【００１０】
【発明が解決しようとする課題】
本発明に係るアルカリ蓄電池用正極活物質の製造方法は、前記従来の問題点に鑑みてなされたものであり、ニッケル正極の高率放電特性を低下させることなく、高温環境下における充電受け入れ性を高めることができ、生産能率が高く、電気的特性の優れたアルカリ蓄電池用正極活物質を提供することができる。
【００１１】
【課題を解決するための手段】
本発明は、水酸化ニッケルを含む芯層と、コバルト（Ｃｏ）および希土類元素の水酸化物を含む表面層とを備えた活物質を、アルカリ水溶液中にて酸化剤を用いて６０℃以上１２０℃以下の温度で酸化処理することにより、前記表面層に含まれるＣｏの酸化数を３価以上にする工程を含むことを特徴とするアルカリ蓄電池用正極活物質の製造方法である。
【００１２】
本発明における前記希土類元素には、Ｈｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、Ｙより選ばれる少なくとも一種の元素を含むことが望ましい。また、本発明においては、表面層の合成過程における酸化剤を用いた酸化処理をＫ、Ｎａ、Ｌｉの少なくとも一種の元素を含む高濃度のアルカリ水溶液中で行うことが望ましい。また、本発明においては、アルカリ水溶液のアルカリ濃度は８Ｎ以上であることが望ましい。
【００１４】
【発明の実施の形態】
本発明に係るアルカリ蓄電池用正極活物質の製造方法においては、芯層が例えばニッケルの一部が２Ａ、３Ａ族元素およびＣｏで置換された水酸化ニッケルを主成分とし、該芯層を被覆する表面層はＣｏと少なくとも一種の希土類元素（以下Ｍと記述する）を含む混合水酸化物から成る正極活物質粒子が用いられる。本発明の製造方法においては、高濃度のアルカリ水溶液中で酸化剤を用いて、前記正極活物質粒子の表面層を酸化して、表面層に含まれるＣｏの酸化数を３価以上とする。
【００１５】
前記正極活物質の表面層の形成に適用される希土類元素Ｍとしては、Ｈｏ、Ｅｒ、Ｔｍ、 Ｙｂ、Ｌｕ、Ｙが望ましく、中でもＹｂを含むことが特に望ましい。これら一群の元素より少なくとも一種が選ばれ、正極活物質粒子表面にＣｏとの混合水酸化物から成る表面層を形成する。
【００１６】
また、前記正極活物質の表面層に含まれるＣｏの水酸化ニッケルを含む芯層に対する比率は、重量比で２．５〜６．３％が望ましく、２．５〜５．１％がさｔらに望ましい。Ｃｏの比率が２．５％未満の場合は、正極活物質の表面に十分な導電性パスが形成されないために、活物質の利用率を高める効果を期待できない虞がある。逆に同比率が６．３％を超えると活物質粒子に含まれる芯層の絶対量が小さくなるので、電池の容量低下を招く。
【００１７】
さらに、前記混合水酸化物中のＭとＣｏの元素の比率［Ｍ］／［Ｃｏ］が０．０１〜１０であることが望ましい。該比率が０．０１未満では充電時に正極での酸素発生電位を貴な方向にシフトさせる効果が乏しく、充電効率を高める効果が発揮されない。また、比率が１０を超えると、酸化剤を用いた酸化処理をもってしても表面層に含まれるＣｏの酸化反応が進行しにくく、酸化数を３価以上にすることが難しくなる虞があるので好ましくない。
【００１８】
本発明のアルカリ蓄電池用正極活物質の製造方法においては、酸化処理浴には、Ｋ、Ｎａ、Ｌｉのアルカリ金属元素の中少なくとも１種を含ませることが好ましい、これらの元素の存在は酸化処理工程において導電性の低いＣｏＨＯ２等の副生成物の生成を抑制する効果があり、導電性のコバルト化合物からなる強固なネットワークを形成するのに有効である。
【００１９】
本発明に於いては、前記酸化処理過程における酸化剤としてペルオキソ二硫酸塩、次亜塩素酸塩、亜塩素酸塩あるいは塩素酸塩を用いることが望ましい。また酸化処理過程におけるアルカリ処理液のアルカリ濃度は少なくとも８Ｎ以上が望ましく、１０Ｎ以上であることがさらに望ましい。また、酸化処理温度は６０℃以上１２０℃以下であり、８０℃以上１２０℃以下がさらに望ましい。
【００２１】
前記表面層中に含まれる希土類元素は、充電時に電解液中の水の電気分解を抑える作用をするため、電池の充電受け入れ特性が向上する。この効果は特に高温に於いて顕著である。前記希土類元素ＭがＨｏ、Ｅｒ、Ｔｍ、Ｙｂ、Ｌｕ、Ｙの内の少なくても１つを含む場合、中でもＹｂを含む場合に顕著な効果が認められる。
【００２２】
前記表面層の複合酸化物に含まれる酸化数を３価以上としたＣｏは、正極活物質粒子間に良好な電導性ネットワークを形成する。希土類元素が共存すると一旦溶解したＣｏの再析出が阻害されるので、従来の充電による電解酸化では電導性パスを形成し難い。本発明では、アルカリ水溶液中で酸化剤を用いて酸化処理をする。
【００２３】
本発明によると、前記表面層中に希土類元素が存在するにも拘わらず、酸化処理によってＣｏが離散することなく酸化数３価以上に酸化されて、正極活物質粒子間に良好な電導性パスを形成する。従って正極に電導性を付与する添加剤が不要になるので、導電材を混合する設備の削除及び電池製造工程の簡略化が可能となる。さらに、本発明においては電池組立後、電導性ネットワーク形成のための複雑な多段階化成過程を必要としないため、それら設備の削除及び電池製造工程の簡略化、かつ電池の生産性向上が可能となる。
【００２４】
前記酸化処理に適用する、酸化剤としては過硫酸カリウム（Ｋ2Ｓ2Ｏ8）、過硫酸ナトリウム（Ｎａ2Ｓ2Ｏ8）、過硫酸アンモニウム｛（ＮＨ4）2Ｓ2Ｏ8｝、次亜塩素酸カリウム（ＫＣｌＯ）、次亜塩素酸ナトリウム（ＮａＣｌＯ）等であれば同様の効果が得られた。ただし（ＮＨ4）2Ｓ2Ｏ8は酸化処理時に副生成物として刺激臭のあるアンモニアが発生するため、実際の製造には不適である。取り扱いの容易さ、低価格である等の理由によってＫ2Ｓ2Ｏ8、Ｎａ2Ｓ2Ｏ8、ＮａＣｌＯなどが望ましい。
【００２５】
前記酸化処理時の浴中のアルカリ濃度は８Ｎ以上、さらには１０Ｎ以上の高濃度であることが望ましい。アルカリ濃度を前記範囲の高濃度にすることによって導電性に乏しい副生成物の生成を抑制し、導電性を有するコバルト化合物の生成を促して良好な導電性パスを形成することができる。
【００２６】
また、前記酸化処理時の浴の温度は６０〜１２０℃であり、さらには８０〜１２０℃が望ましい。浴の温度をこの範囲に保つことによって、表面層に含まれるＣｏの酸化数を３価以上に高めて、表面層に導電性を付与することができる。
【００２７】
本発明に係るアルカリ蓄電池用正極活物質の製造方法において、芯層の主成分である水酸化ニッケルは、ニッケルの一部を例えばＺｎ、Ｍｇ、Ｃａ等の２Ａ、２Ｂ族から選ばれた元素で置換されていることが好ましい。公知の如く、該置換によって正極の充電受け入れ特性、高率放電特性、サイクル特性が大幅に向上する。特に高密度水酸化ニッケルに適用すると、高容量で且つサイクル特性に優れた正極活物質が得られる。
【００２８】
前記ニッケルの一部がＺｎ、Ｍｇ、Ｃａ等の２Ａ、２Ｂ族から選ばれた元素で置換した高密度水酸化ニッケルを主成分とする芯層に前記表面層を形成させることにより、優れた高率放電特性、サイクル特性と高いエネルギー密度を備え、さらに良好な充電受け入れ特性、高い生産性を併せ持つアルカリ蓄電池が可能となる。
【００２９】
【実施例】
以下に、前記芯層のＮｉの一部を置換する元素をＺｎとした場合を例にとって本発明の実施例を説明するが、本発明はこれに限るものではない。
（実施例１）
(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所定比で溶解した水溶液に硫酸アンモニウムと苛性ソーダ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、反応系のｐＨを１０〜１３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合成した。
【００３０】
（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ｐＨ１０〜１３に制御したのアルカリ水溶液に投入した。該溶液を撹拌しながら、所定濃度の硫酸コバルト、硫酸イッテルビウムおよび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１０〜１３の範囲に維持した。約１時間ｐＨを１０〜１３の範囲に保持し、水酸化ニッケル粒子表面にＣｏとＹｂ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、８．０７ｗｔ％であった。また、表面層中に含まれるＹｂとＣｏ原子比率［Ｙｂ］／［Ｃｏ］は０．６９であった。
【００３１】
また、芯層となる球状高密度水酸化ニッケル粉末を合成するまでは上述と全く同様の操作を行い、その後希土類元素とコバルトとの混合表面層を合成する際に、希土類の化合物として、上述の硫酸イッテルビウムに替えて、硫酸ルテチウム、硫酸ツリウム、硫酸エルビウム、硫酸ホルミウム、および硫酸イットリウムを硫酸コバルトと所定比でそれぞれ溶解した水溶液を用いる以外は全く同様に被覆処理を行い、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を得た。
表面層形成処理に用いる溶液のｐＨおよび液温度は希土類硫酸塩の種類による大きな差異は生じなかった。
【００３２】
前記各正極活物質粒子の表面層の芯層に対する比率および表面層に含まれる希土類元素とＣｏの原子比率［Ｍ］／［Ｃｏ］は表１に示す通りであった。
【表１】
（表面層の酸化処理）
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５０ｇを、温度１１０℃の３０ｗｔ％（１０Ｎ）の苛性ソーダ水溶液に投入し、充分に攪拌した。続いて表面層に含まれるコバルトの水酸化物の当量に対して過剰のＫ2Ｓ2Ｏ8を添加し、粒子表面から酸素ガスが発生するのを確認した。活物質粒子をろ過し、水洗、乾燥した。
【００３４】
（正極板の作製）
前記活物質粒子に所定の比率のカルボキシメチルセルローズ（ＣＭＣ）水溶液を添加してペースト状とし、該ペーストをニッケル多孔体に充填した。その後８０℃で乾燥した後、所定の厚みにプレスし、表面にテフロンコーティングを行いニッケル正極板とした。
【００３５】
（特性評価用電池の作製）
ＡＢ5型希土類系の水素吸蔵合金からなる負極とセパレータと前記ニッケル極板とを組み合わせて、比重１．２８の水酸化カリウム水溶液を注液し、開放型試験電池を作製した。表面層の混合水酸化物合成に際してコバルトと共に使用した希土類元素の種類、すなわちＹｂ、Ｌｕ、Ｔｍ、Ｅｒ、Ｈｏ、およびＹのそれぞれの元素に対応する開放型試験電池をＡ１、Ａ２、Ａ３、Ａ４、Ａ５およびＡ６とする。
【００３６】
（実施例２）
（水酸化ニッケル粒子表面への表面層の形成）
芯層となる球状高密度水酸化ニッケル粒子を合成する過程までは実施例1と全く同様の操作を行い、その後硫酸コバルト、硫酸イッテルビウム、硫酸ツリウムおよび硫酸ルテチウムを所定比（硫酸ツリウムおよび硫酸ルテニウムの添加比率を小さくし、表面層に含まれる希土類元素の中、Ｙｂの構成比率を高くした）で溶解した水溶液を用いる以外は実施例1と全く同様に表面層形成処理を行い、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を得た。被覆処理用の液のｐＨおよび液温度は実施例1の場合と大きな差異は生じなかった。表面層の芯層に対する比率は８．０８ｗｔ％であった。
【００３７】
（表面層の酸化処理）
前記多成分混合水酸化物から成る表面層を有する水酸化ニッケル粉末を、温度１１０℃、濃度１０Ｎの苛性ソーダ水溶液に入れ充分に攪拌した。続いて表面層に含まれるＣｏの水酸化物の当量に対して過剰のＫ2Ｓ2Ｏ8を添加し、粒子表面から酸素ガスが発生するのを確認した。活物質粒子をろ過し、水洗、乾燥した。
【００３８】
（正極板の作製）
前記活物質をＣＭＣ水溶液でペースト状とし、該ペーストをニッケル多孔体に充填した。その後、８０℃で乾燥した後所定の厚みにプレスし、表面にテフロンコーティングを行いニッケル極板とした。
【００３９】
（特性評価用電池の作製）
前記ＡＢ5形希土類系の水素吸蔵合金からなる負極とセパレータと前記ニッケル極板とを組み合わせ、比重１．２８の水酸化カリウム水溶液を注液し、開放型試験電池Ａ７を作製した。
【００４０】
（比較例1）
酸化剤を用いた酸化処理を施さないこと以外は実施例1と全く同様にして、ＣｏとＹｂを含む混合水酸化物から成る表面層を持つ高密度水酸化ニッケル粒子を得た。該表面層の芯層に対する比率は、実施例1と同様８．０７ｗｔ％であった。
【００４１】
また前記比較例１記載の活物質を用いて実施例１と同一の条件でニッケル正極板および評価用電池を作製した。該電池をＢ１とする。
【００４２】
(比較例２)
前記水酸化ニッケル粒子表面に、希土類元素を含まずＣｏのみの水酸化物表面層を有する高密度水酸化ニッケル粉末を合成した。希土類元素を含まない以外、表面層合成後の酸化処理等は実施例1と全く同様の操作を行った。表面層の芯層に対する比率は５．０１ｗｔ％であった。次いでこのＣｏの水酸化物表面層を有する水酸化ニッケル粒子１００ｇを、温度１１０℃、濃度１０Ｎの苛性ソーダ水溶液３００ｍｌに投入し、充分に攪拌した。続いて表面層に含まれるコバルトの水酸化物の当量に対して過剰のＫ2Ｓ2Ｏ8を添加し、粒子表面から酸素ガスが発生するのを確認した。活物質粒子をろ過し、水洗、乾燥した。
【００４３】
前記活物質を用いて、実施例１と同一の条件でニッケル正極板を作製した。また、該正極板を用いて評価用電池を作製した。該電池をＣ１とする。
【００４４】
（実施例３）
（表面層の酸化処理）
実施例1と全く同様にして、ＣｏとＹｂを含む混合水酸化物から成る表面層を持つ高密度水酸化ニッケル粒子を得た。該複合水酸化物表面層の比率は実施例1と同様芯層に対し８．０７ｗｔ％であった。該水酸化ニッケル粒子を、温度が１１０℃で濃度がそれぞれ２、４、６、８、および１４Ｎの苛性ソーダ水溶液に投入し、充分に攪拌した。続いて実施例１と同一の条件で酸化処理を行った。処理温度は実施例1と同じ温度となるよう反応槽ヒーターを制御した。次いで活物質粒子をろ過し、水洗、乾燥した。
【００４５】
（比較例３）
実施例1と全く同様にして、ＣｏとＹｂを含む混合水酸化物から成る表面層を持つ高密度水酸化ニッケル粒子を得た。該複合水酸化物表面層の比率は実施例1と同様芯層に対し８．０７ｗｔ％であった。該水酸化ニッケル粒子を、温度が沸騰点の蒸留水に投入し、充分に攪拌した。続いて表面層に含まれるコバルトの水酸化物の当量に対して過剰のＫ2Ｓ2Ｏ8を添加し酸化処理をおこなった。
【００４６】
（正極板および評価用電池の作製）
前記正極活物質を用いて、実施例１と同一の条件ニッケル正極板および評価用電池を作製した。蒸留水および苛性ソーダ濃度２、４、６、８、および１４Ｎに対応する電池を、それぞれ比較例電池Ｄ１および本発明の実施例電池Ｄ２、 電池 Ｄ３、 電池 Ｄ４、 電池Ｄ５、電池Ｄ６とする。
【００４７】
（実施例４）
（表面層の酸化処理）
実施例1と全く同様にして、コバルトとイッテルビウムを含む複合水酸化物から成る表面層を持つ高密度水酸化ニッケル粒子を得た。該複合水酸化物表面層の比率は実施例1と同様に芯層に対して８．０７ｗｔ％であった。該混合水酸化物層で表面層を有する水酸化ニッケル粉末を温度がそれぞれ室温、６０℃、８０℃および１４０℃で濃度が１０Ｎの苛性ソーダ水溶液に入れ充分に攪拌した。次いで実施例１と同一の条件で酸化処理を行った後、ろ過、水洗、乾燥した。
【００４８】
（正極板および評価用電池の作製）
前記正極活物質を用いて、実施例１と同一の条件でニッケル正極板および評価用電池を作製した。酸化処理用の浴温度が室温、６０℃、８０℃および１４０℃に対応した電池を、それぞれ電池Ｅ１、電池Ｅ２、電池 Ｅ３および電池 Ｅ４とする。
【００４９】
（電池性能の評価）
前記試験用電池に標準電極としてＨｇ／ＨｇＯ電極を取り付け、試験温度２０℃において放電試験に供した。充電は電流0.1ＣｍＡで１５時間実施した。放電は電流０．２ＣｍＡとし、正極のＨｇ／ＨｇＯ電極の対する電位が０ｍＶになった時点を放電終止とした。
また、高率放電特性は、放電率１ＣｍＡ、３ＣｍＡ、５ＣｍＡの各率で放電したときの活物質利用率で評価した。正極活物質の利用率を指標として各電池の特性を比較した。正極の放電反応をＮｉ（ＯＨ）2→ＮｉＯＯＨへの１電子反応とし、その時のＮｉ（ＯＨ）21ｇ当たりの理論放電容量２８９ｍＡｈ に対する実際に放電された容量の比を算定して利用率とした。
【００５０】
（1）酸化処理の効果
本発明電池Ａ１、比較例電池Ｂ１の高率放電特性を図1に示す。本発明電池Ａ１の利用率が高く、比較例電池Ｂ１に比べて遥かに優れた特性を示す。この結果は、正極活物質を酸化剤を用いて酸化処理することによって、特性が顕著に向上したことを示すものである。正極活物質を酸化剤を用いて酸化処理を施すことによって、希土類元素が存在するにも拘わらず表面層に含まれるＣｏの水酸化物が電導性の化合物に変換されて、正極活物質粒子間に強固な電導性パスが形成されるために、高率放電時においても高い利用率を示すと考えられる。一方、比較例電池Ｂ1ではコバルト化合物の電導性パスの形成が表面層に存在する希土類元素により阻害され、高率放電時、十分な放電容量が得られないと考えられる。
【００５１】
（2）被覆層希土類元素の種類
本発明電池Ａ１、Ａ２、Ａ３、Ａ４、Ａ５、Ａ６、Ａ７および比較例電池Ｃ１を各温度で充電および放電を行った時の、活物質利用率と温度の関係を図２に示す。本発明に係る電池Ａ１〜Ａ７は、比較例電池Ｃ１に比べ高温において充電、放電を行った場合でも良好な利用率を示すことが判る。本発明においては、前記表面層への希土類元素添加により、ニッケル電極充電末期に競争的に起こる酸素ガス発生反応が抑制され、充電受け入れ性が改善されている。そのため表面層に希土類元素を含まない比較例電池Ｃ１に比べて顕著に利用率が高くなったと考えられる。希土類元素のうちＹｂ、Ｔｍ、Ｌｕ添加による改善効果が大きく、中でもＹｂを添加した時の改善効果が特に大きい。
【００５２】
（3）アルカリ水溶液濃度の効果
Ｃｏと希土類元素を含む混合水酸化物から成る表面層の酸化処理浴中のアルカリ濃度を変えた電池Ａ１、Ｄ１、Ｄ４およびＤ５の初回充電曲線を図３に示す。図には記載していないが電池Ｄ２およびＤ３は、Ｄ４と同様の曲線を示した。また、電池Ｄ６についても図への記載を省いたが、Ａ１と同様の曲線を示した。初回充電においては、芯層の含まれるＮｉに比べて酸化反応電位が卑なＣｏ(II)の酸化が優先して起こり、引き続いて同電位が貴なＮｉ(II)の酸化が起こる。
【００５３】
図３に示したように比較例電池Ｄ１では、充電初期に於いて電位が一旦卑な方向に移行する現象が認められ、充電に伴う正極の電位の立ち上がりが遅い。これは充電初期によって、表面層に含まれるＣｏ(II)からＣｏ(III)への酸化が生じているためである。この結果は、電池Ｄ１においては、酸化剤を用いた酸化処理による酸化が進行していないことを示すものである。また、酸化剤にＫ2Ｓ2Ｏ8を、酸化処理浴に水を使用すると浴が弱酸性を呈することとなり、Ｎｉが溶出する虞があるので好ましくない。
【００５４】
一方、本発明電池Ｄ５、Ｄ６および Ａ１においては初期から電位が４００ｍＶ以上に立ち上がっており、充電前に十分に表面層のＣｏ(II)からＣｏ(III)への酸化が進んでいることが判る。電池Ｄ２、Ｄ３およびＤ４は電池Ｄ１と電池Ｄ５、Ｄ６およびＡ１の中間の電位挙動を示すことが判った。このことから、酸化処理浴のアルカリ濃度が高い方が表面層に含まれるＣｏ(II)からＣｏ(III)への酸化反応が速やかに起きることが判る。以上の結果から、酸化処理浴のアルカリ濃度は８Ｎ以上が望ましく、１０Ｎ以上がさらに望ましい。また、アルカリ濃度１４Ｎは溶解度の上限の値であり、さらに高濃度の領域を実施することは困難である。以上のことから、酸化処理浴のアルカリ濃度は８Ｎ以上、さらに望ましくは１０Ｎ以上が良く、その上限値は溶解度の上限である約１４Ｎである。
【００５５】
（4）アルカリ水溶液温度の効果
Ｃｏと希土類元素を含む混合水酸化物から成る表面層の酸化処理浴の温度を変えた。電池Ａ１、Ｅ１、Ｅ２、Ｅ３およびＥ４の初期サイクル放電容量を図４に示す。本発明電池Ａ１では初期サイクルから充分な容量が得られる。本発明電池の中では、酸化処理温度が室温の電池Ｅ１の場合、容量が安定するまでに５回の充放電サイクルを必要とする。初期充放電サイクルにおいて電池Ｅ１の活物質利用率が低いのは、酸化処理浴温度が低いと表面層に含まれるＣｏ(II)からＣｏ(III)への酸化の進行が遅いため、電導性パスが十分に形成されず、性能が発揮しきれないことによる。電池Ｅ１の場合、充電によってＣｏ(II)からＣｏ(III)への酸化が進行するのに、約５サイクルの充放電の繰り返しを要したと考えられる。
【００５６】
これに対して、本発明に係る電池Ａ１、Ｅ４および電池Ｅ３が初回から高い放電容量を示すのは、表面層に含まれるＣｏ(II)からＣｏ(III)への酸化が酸化処理によって予め完結しており、導電性パスが形成されるているためであると考えられる。種々検討した結果、酸化処理の温度は６０℃以上が望ましく、８０℃以上がさらに望ましいことが判った。このように、酸化処理の温度は、酸化反応を促進する上からはできるだけ高温であることが望ましい。しかしながら、実際の操作でアルカリ処理液の温度を沸点近くに長時間保つことは困難であり、また、高温領域では酸化剤自身の自己分解反応が競争して起こるため、逆に酸化反応効率が低下する。このため酸化処理浴の温度は６０℃〜１２０℃が望ましく、８０℃〜１２０℃がさらに望ましい。
【００５７】
尚、上記実施例において酸化剤としてＫ2Ｓ2Ｏ8を用いたが、他の酸化剤、例えばＮａ2Ｓ2Ｏ8、（ＮＨ4）2Ｓ2Ｏ8、ＮａＣｌＯ等であれば同様の効果が得られた。ただし（ＮＨ4）2Ｓ2Ｏ8は酸化処理時に副生成物として刺激臭のあるアンモニアが発生するため、実際の製造ではＫ2Ｓ2Ｏ8、Ｎａ2Ｓ2Ｏ8、次亜塩素酸ソーダ、亜塩素酸ソーダ、塩素酸ソーダなど取り扱いが容易な酸化剤の方が望ましい。
【００５８】
【発明の効果】
上述した如く、本発明に係るアルカリ蓄電池用正極活物質の製造方法は、ニッケル正極の高率放電特性を低下させることなく高温環境下における充電受け入れ性を高めることができる正極活物質を提供するものである。さらに、本発明に係るアルカリ蓄電池用正極活物質の製造方法は、化成が不要であって電池生産効率の高い安価なニッケル極板およびアルカリ蓄電池を得ることができる正極活物質を提供するものである。そのため、本発明の工業的価値は大である。
【００５９】
【図面の簡単な説明】
【図１】本発明に係る実施例電池および比較例電池の高率放電特性を示すグラフである。
【図２】本発明に係る実施例電池および比較例電池の温度特性を示すグラフである。
【図３】本発明に係る実施例電池および比較例電池の初回充電曲線の一部を示すグラフである。
【図４】本発明に係る実施例電池の初期充放電サイクル時の放電容量の推移を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode active material for an alkaline storage battery.Manufacturing methodIt is about.
[0002]
[Prior art]
As a nickel positive electrode mainly composed of nickel hydroxide used for alkaline storage batteries such as a nickel cadmium battery, a nickel hydride battery, a nickel zinc battery, and a nickel iron battery, a paste type nickel positive electrode has become mainstream. However, in the paste type nickel positive electrode, the pore diameter of the metal porous substrate holding the active material is large, and the nickel hydroxide itself is poor in conductivity, so the electrode reaction does not proceed smoothly in the electrode plate, and the capacity is obtained. There was no problem.
[0003]
As a means for solving this problem, there is a method of adding conductive powder such as carbon or metal to the active material. When the conductive powder is used, the conductivity between the active material and the current collector and the active material particles is increased, so that the active material utilization rate is improved. However, in order to obtain a sufficient active material utilization rate, a large amount of conductive additive is added. It is necessary to add an agent. However, when the amount of the conductive powder added is increased, the content of the active material that directly contributes to the electrode reaction is reduced by that amount, and thus the energy density of the electrode is reduced.
[0004]
As means for solving these problems, Japanese Patent Application Laid-Open No. 61-138458 proposes a method of adding a cobalt compound that generates divalent ions in an alkaline aqueous solution. Cobalt compound is once dissolved in alkaline electrolyte and Co (OH)2As redeposited on the surface of the active material particles of the positive electrode. Furthermore, re-deposited Co (OH)2Is subjected to electrolytic oxidation in the first charging operation to produce conductive cobalt oxide, whereby a conductive path is formed between the active material particles, resulting in a paste-type nickel electrode having a high active material utilization rate.
[0005]
Furthermore, in Japanese Patent Publication No. 4-4698, as a measure for cost reduction (reduction of cobalt amount) and further increase in energy density, particles in which a nickel hydroxide particle surface is coated in advance with a cobalt hydroxide layer are disclosed. It has been proposed to use The cobalt hydroxide coated on the surface of nickel hydroxide particles is assumed to realize a paste-type nickel electrode having a high active material utilization rate by connecting the particles with a conductive path in the same manner as the above-described cobalt compound.
[0006]
In addition, conventional alkaline storage batteries have insufficient charge acceptance characteristics at high temperatures, and there is a need for improvement. As one means for improving the charge acceptance characteristics at high temperatures, Japanese Patent Laid-Open No. 9-92279 proposes a method of adding a Ca compound or a rare earth element oxide to the positive electrode active material particles.
[0007]
These additives have a function of suppressing the decomposition reaction of the electrolytic solution that occurs in competition with the charging reaction of nickel hydroxide itself at the end of the charging process, so-called oxygen gas generation reaction. The use of these additives is effective in improving the charge acceptance characteristics, but has caused a new problem of reducing the high rate discharge performance of the positive electrode.
[0008]
The decrease in the high rate discharge performance is considered to be because the added rare earth oxide prevents Co reprecipitation and prevents the formation of a conductive path in the positive electrode. Therefore, in order to promote the reprecipitation of Co in the system in which the additive coexists, the ratio of the cobalt compound to be added to the positive electrode active material is increased or the coating layer containing Co is formed in advance. Attempts have been made to strengthen the conductive path by increasing the thickness. In addition, a method has been proposed in which the first charge is a multistage charge having a different current value. However, in the case of multistage charging, there is a drawback that the production efficiency is lowered because the charging equipment becomes complicated or the time required for charging becomes long.
[0009]
Further, when the active material particles are left in the air, the Co hydroxide contained in the surface layer is easily oxidized by the oxygen contained in the air, and the electrochemically inactive CoThreeOFourTo change. For this reason, Co (OH)2It was necessary to preserve the active material coated with the solution in an atmosphere controlled so as not to cause oxidation.
[0010]
[Problems to be solved by the invention]
The present inventionThe method for producing a positive electrode active material for an alkaline storage battery according toThe present invention has been made in view of the above-mentioned conventional problems, and enhances charge acceptance in a high temperature environment without deteriorating the high rate discharge characteristics of the nickel positive electrode.It is possibleProvides positive electrode active materials for alkaline storage batteries with high production efficiency and excellent electrical characteristicsbe able to.
[0011]
[Means for Solving the Problems]
Main departureMing uses an active material comprising a core layer containing nickel hydroxide and a surface layer containing cobalt (Co) and a rare earth element hydroxide in an alkaline aqueous solution at 60 ° C. to 120 ° C. using an oxidizing agent. A method for producing a positive electrode active material for an alkaline storage battery comprising a step of oxidizing the Co contained in the surface layer to a trivalent or higher by oxidizing at the following temperatureIt is.
[0012]
The rare earth element in the present invention preferably contains at least one element selected from Ho, Er, Tm, Yb, Lu, and Y. Also,In the present invention,It is desirable that the oxidation treatment using an oxidizing agent in the process of synthesizing the surface layer is performed in a high-concentration alkaline aqueous solution containing at least one element of K, Na, and Li.In the present invention, the alkali concentration of the alkaline aqueous solution is desirably 8N or more.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Positive electrode active material for alkaline storage battery according to the present inventionIn the manufacturing methodThe core layer is mainly composed of nickel hydroxide in which a part of nickel is substituted with 2A, 3A group element and Co, and the surface layer covering the core layer is made of Co and at least one rare earth element (hereinafter referred to as M and Consisting of mixed hydroxide containingPositive electrode active material particles are used. In the production method of the present invention,The surface layer of the positive electrode active material particles is oxidized using an oxidizing agent in a high-concentration alkaline aqueous solution, so that the oxidation number of Co contained in the surface layer is 3 or more.
[0015]
As the rare earth element M applied to the formation of the surface layer of the positive electrode active material, Ho, Er, Tm, Yb, Lu, and Y are desirable, and it is particularly desirable that Yb is included. At least one element selected from the group of elements is formed, and a surface layer made of a mixed hydroxide with Co is formed on the surface of the positive electrode active material particles.
[0016]
Further, the ratio of Co contained in the surface layer of the positive electrode active material to the core layer containing nickel hydroxide is preferably 2.5 to 6.3% by weight, and 2.5 to 5.1%. More desirable. When the ratio of Co is less than 2.5%, a sufficient conductive path is not formed on the surface of the positive electrode active material, and thus there is a possibility that the effect of increasing the utilization factor of the active material cannot be expected. On the other hand, if the ratio exceeds 6.3%, the absolute amount of the core layer contained in the active material particles becomes small, resulting in a decrease in battery capacity.
[0017]
Furthermore, it is desirable that the ratio [M] / [Co] of the elements of M and Co in the mixed hydroxide is 0.01-10. When the ratio is less than 0.01, the effect of shifting the oxygen generation potential at the positive electrode in the noble direction during charging is poor, and the effect of increasing the charging efficiency is not exhibited. Further, if the ratio exceeds 10, the oxidation reaction of Co contained in the surface layer is difficult to proceed even with an oxidation treatment using an oxidizing agent, and it may be difficult to make the oxidation number trivalent or higher. It is not preferable.
[0018]
The present inventionIn the manufacturing method of the positive electrode active material for alkaline storage batteries,The oxidation treatment bath preferably contains at least one of alkali metal elements such as K, Na, and Li. The presence of these elements is a low-conductivity CoHO in the oxidation treatment step.2And the like, and is effective in forming a strong network composed of a conductive cobalt compound.
[0019]
In the present invention, it is desirable to use peroxodisulfate, hypochlorite, chlorite or chlorate as the oxidizing agent in the oxidation treatment process. Further, the alkali concentration of the alkali treatment liquid in the oxidation treatment process is preferably at least 8N or more, more preferably 10N or more. The oxidation treatment temperature is 60 ° C or higher and 120 ° C or lower.And80 ° C. or higher and 120 ° C. or lower is more desirable.
[0021]
Since the rare earth element contained in the surface layer acts to suppress the electrolysis of water in the electrolyte during charging, the charge acceptance characteristics of the battery are improved. This effect is particularly remarkable at high temperatures. When the rare earth element M includes at least one of Ho, Er, Tm, Yb, Lu, and Y, and particularly includes Yb, a remarkable effect is recognized.
[0022]
Co having an oxidation number of 3 or more contained in the composite oxide of the surface layer forms a good conductive network between the positive electrode active material particles. Coexistence of rare earth elements inhibits reprecipitation of Co once dissolved, so that it is difficult to form a conductive path by electrolytic oxidation by conventional charging. In the present invention, oxidation treatment is performed using an oxidizing agent in an alkaline aqueous solution.
[0023]
According to the present invention, despite the presence of rare earth elements in the surface layer, Co is oxidized to an oxidation number of 3 or more without being dispersed by oxidation treatment, and a good conductive path is obtained between the positive electrode active material particles. Form. Accordingly, since an additive for imparting conductivity to the positive electrode is not necessary, the facility for mixing the conductive material can be eliminated and the battery manufacturing process can be simplified. Furthermore, in the present invention, after the battery is assembled, a complicated multi-step formation process for forming a conductive network is not required, so that it is possible to delete these facilities, simplify the battery manufacturing process, and improve battery productivity. Become.
[0024]
As an oxidizing agent applied to the oxidation treatment, potassium persulfate (K2S2O8), Sodium persulfate (Na2S2O8), Ammonium persulfate {(NHFour)2S2O8}, Potassium hypochlorite (KClO), sodium hypochlorite (NaClO), etc., the same effect was obtained. However, (NHFour)2S2O8Is unsuitable for actual production because ammonia with an irritating odor is generated as a by-product during the oxidation treatment. K for reasons such as ease of handling and low price2S2O8, Na2S2O8NaClO or the like is desirable.
[0025]
The alkali concentration in the bath during the oxidation treatment is preferably 8N or higher, more preferably 10N or higher. By setting the alkali concentration to a high concentration within the above range, it is possible to suppress the formation of by-products having poor conductivity, promote the formation of a cobalt compound having conductivity, and form a favorable conductive path.
[0026]
The bath temperature during the oxidation treatment is 60 to 120 ° C.AndFurthermore, 80-120 degreeC is desirable. By keeping the bath temperature in this range, the oxidation number of Co contained in the surface layer can be increased to 3 or more and conductivity can be imparted to the surface layer.
[0027]
According to the present inventionFor alkaline storage batteryCathode active materialIn the manufacturing method,In the nickel hydroxide as the main component of the core layer, a part of nickel is preferably substituted with an element selected from 2A and 2B groups such as Zn, Mg, and Ca. As is well known, the substitution significantly improves the charge acceptance characteristics, high rate discharge characteristics, and cycle characteristics of the positive electrode. In particular, when applied to high-density nickel hydroxide, a positive electrode active material having a high capacity and excellent cycle characteristics can be obtained.
[0028]
By forming the surface layer on a core layer mainly composed of high-density nickel hydroxide in which a part of the nickel is substituted with an element selected from 2A and 2B groups such as Zn, Mg, and Ca, an excellent high An alkaline storage battery having rate discharge characteristics, cycle characteristics, and high energy density, and further having good charge acceptance characteristics and high productivity is possible.
[0029]
【Example】
Examples of the present invention will be described below by taking as an example the case where Zn is used as an element for substituting a part of Ni in the core layer, but the present invention is not limited thereto.
Example 1
(Synthesis of nickel hydroxide particles)
An ammonium complex and an aqueous sodium hydroxide solution were added to an aqueous solution in which nickel sulfate, zinc sulfate and cobalt sulfate were dissolved at a predetermined ratio to form an ammine complex. Caustic soda was further added dropwise with vigorous stirring of the reaction system, and the pH of the reaction system was controlled to 10 to 13 to synthesize spherical high density nickel hydroxide particles serving as a core layer base material.
[0030]
(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 10-13 with caustic soda. While stirring the solution, an aqueous solution containing cobalt sulfate, ytterbium 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 in the range of 10-13. The pH was maintained in the range of 10 to 13 for about 1 hour, and a surface layer made of a mixed hydroxide containing Co and Yb was formed on the surface of the nickel hydroxide particles. The ratio of the surface layer of the mixed hydroxide was 8.07 wt% with respect to the core layer mother particles (hereinafter simply referred to as the core layer). Moreover, Yb and Co atomic ratio [Yb] / [Co] contained in the surface layer was 0.69.
[0031]
In addition, the same operation as described above is performed until the spherical high-density nickel hydroxide powder to be the core layer is synthesized, and then when the mixed surface layer of rare earth element and cobalt is synthesized, Instead of ytterbium sulfate, the coating treatment is exactly the same except that an aqueous solution in which lutetium sulfate, thulium sulfate, erbium sulfate, holmium sulfate, and yttrium sulfate are dissolved in a predetermined ratio with cobalt sulfate is used. Nickel hydroxide particles having a surface layer were obtained.
The pH and solution temperature of the solution used for the surface layer formation treatment did not differ greatly depending on the type of rare earth sulfate.
[0032]
The ratio of the surface layer to the core layer of each positive electrode active material particle and the atomic ratio [M] / [Co] of the rare earth element and Co contained in the surface layer are as shown in Table 1.
[Table 1]
(Oxidation treatment of surface layer)
50 g of nickel hydroxide particles having a surface layer made of the mixed hydroxide was put into a 30 wt% (10N) aqueous sodium hydroxide solution at a temperature of 110 ° C. and sufficiently stirred. Subsequently, an excess of K with respect to the equivalent amount of cobalt hydroxide contained in the surface layer.2S2O8And oxygen gas was confirmed to be generated from the particle surface. The active material particles were filtered, washed with water and dried.
[0034]
(Preparation of positive electrode plate)
A predetermined ratio of an aqueous solution of carboxymethyl cellulose (CMC) was added to the active material particles to form a paste, and the paste was filled in a nickel porous body. Then, after drying at 80 ° C., it was pressed to a predetermined thickness, and the surface was coated with Teflon to obtain a nickel positive electrode plate.
[0035]
(Production of battery for characteristic evaluation)
ABFiveA negative electrode made of a type III rare earth-based hydrogen storage alloy, a separator, and the nickel electrode plate were combined, and an aqueous potassium hydroxide solution having a specific gravity of 1.28 was injected to prepare an open type test battery. A1, A2, A3, A4 are open-type test batteries corresponding to the types of rare earth elements used together with cobalt in the synthesis of the surface layer mixed hydroxide, ie, Yb, Lu, Tm, Er, Ho, and Y. , A5 and A6.
[0036]
(Example 2)
(Formation of surface layer on nickel hydroxide particle surface)
The same operation as in Example 1 was performed until the process of synthesizing spherical high-density nickel hydroxide particles serving as the core layer, and then cobalt sulfate, ytterbium sulfate, thulium sulfate, and lutetium sulfate were mixed at a predetermined ratio (thulium sulfate and ruthenium sulfate). The surface layer formation treatment was carried out in the same manner as in Example 1 except that the aqueous solution dissolved in the rare earth elements contained in the surface layer was reduced and the constituent ratio of Yb was increased) was used. Nickel hydroxide particles having a surface layer consisting of The pH and temperature of the coating treatment liquid were not significantly different from those in Example 1. The ratio of the surface layer to the core layer was 8.08 wt%.
[0037]
(Oxidation treatment of surface layer)
The nickel hydroxide powder having a surface layer made of the multi-component mixed hydroxide was placed in a caustic soda aqueous solution having a temperature of 110 ° C. and a concentration of 10 N and sufficiently stirred. Subsequently, an excess of K with respect to the equivalent amount of Co hydroxide contained in the surface layer.2S2O8And oxygen gas was confirmed to be generated from the particle surface. The active material particles were filtered, washed with water and dried.
[0038]
(Preparation of positive electrode plate)
The active material was made into a paste with an aqueous CMC solution, and the paste was filled in a nickel porous body. Then, after drying at 80 degreeC, it pressed to predetermined thickness, the surface was coated with Teflon, and it was set as the nickel electrode plate.
[0039]
(Production of battery for characteristic evaluation)
ABFiveA negative electrode made of a rare earth-type hydrogen storage alloy, a separator, and the nickel electrode plate were combined, and an aqueous potassium hydroxide solution having a specific gravity of 1.28 was injected to prepare an open test battery A7.
[0040]
(Comparative Example 1)
High density nickel hydroxide particles having a surface layer composed of a mixed hydroxide containing Co and Yb were obtained in exactly the same manner as in Example 1 except that the oxidation treatment using an oxidizing agent was not performed. The ratio of the surface layer to the core layer was 8.07 wt% as in Example 1.
[0041]
A nickel positive electrode plate and an evaluation battery were produced using the active material described in Comparative Example 1 under the same conditions as in Example 1. This battery is designated as B1.
[0042]
(Comparative Example 2)
A high-density nickel hydroxide powder having a hydroxide surface layer containing only rare earth elements and containing only Co was synthesized on the surface of the nickel hydroxide particles. Except for not containing rare earth elements, the oxidation treatment after the surface layer synthesis was performed in exactly the same manner as in Example 1. The ratio of the surface layer to the core layer was 5.01 wt%. Next, 100 g of nickel hydroxide particles having a Co hydroxide surface layer were put into 300 ml of an aqueous caustic soda solution having a temperature of 110 ° C. and a concentration of 10 N, and sufficiently stirred. Subsequently, an excess of K with respect to the equivalent amount of cobalt hydroxide contained in the surface layer.2S2O8And oxygen gas was confirmed to be generated from the particle surface. The active material particles were filtered, washed with water and dried.
[0043]
Using the active material, a nickel positive electrode plate was produced under the same conditions as in Example 1. Moreover, the battery for evaluation was produced using this positive electrode plate. The battery is designated C1.
[0044]
(Example 3)
(Oxidation treatment of surface layer)
In the same manner as in Example 1, high-density nickel hydroxide particles having a surface layer made of a mixed hydroxide containing Co and Yb were obtained. The ratio of the composite hydroxide surface layer was 8.07 wt% with respect to the core layer as in Example 1. The nickel hydroxide particles were put into an aqueous caustic soda solution having a temperature of 110 ° C. and concentrations of 2, 4, 6, 8, and 14 N, respectively, and sufficiently stirred. Subsequently, an oxidation treatment was performed under the same conditions as in Example 1. The reaction vessel heater was controlled so that the treatment temperature was the same as in Example 1. Next, the active material particles were filtered, washed with water and dried.
[0045]
(Comparative Example 3)
In the same manner as in Example 1, high-density nickel hydroxide particles having a surface layer made of a mixed hydroxide containing Co and Yb were obtained. The ratio of the composite hydroxide surface layer was 8.07 wt% with respect to the core layer as in Example 1. The nickel hydroxide particles were put into distilled water having a boiling point and sufficiently stirred. Subsequently, an excess of K with respect to the equivalent amount of cobalt hydroxide contained in the surface layer.2S2O8Was added for oxidation treatment.
[0046]
(Preparation of positive electrode plate and battery for evaluation)
Using the positive electrode active material, the same condition nickel positive electrode plate and evaluation battery as those of Example 1 were produced. The batteries corresponding to the distilled water and
[0047]
Example 4
(Oxidation treatment of surface layer)
In the same manner as in Example 1, high-density nickel hydroxide particles having a surface layer made of a composite hydroxide containing cobalt and ytterbium were obtained. The ratio of the composite hydroxide surface layer was 8.07 wt% with respect to the core layer as in Example 1. The nickel hydroxide powder having a surface layer with the mixed hydroxide layer was placed in an aqueous caustic soda solution having a temperature of room temperature, 60 ° C., 80 ° C. and 140 ° C. and a concentration of 10N, and was sufficiently stirred. Next, after oxidation treatment under the same conditions as in Example 1, filtration, washing and drying were performed.
[0048]
(Preparation of positive electrode plate and battery for evaluation)
Using the positive electrode active material, a nickel positive electrode plate and an evaluation battery were produced under the same conditions as in Example 1. The batteries corresponding to oxidation bath temperatures of room temperature, 60 ° C., 80 ° C., and 140 ° C. are referred to as a battery E1, a battery E2, a battery E3, and a battery E4, respectively.
[0049]
(Evaluation of battery performance)
An Hg / HgO electrode was attached as a standard electrode to the test battery and subjected to a discharge test at a test temperature of 20 ° C. Charging was performed at a current of 0.1 CmA for 15 hours. The discharge was performed at a current of 0.2 CmA, and the discharge was terminated when the potential of the positive electrode Hg / HgO electrode was 0 mV.
Moreover, the high rate discharge characteristic was evaluated by the active material utilization rate when discharged at each rate of 1 CmA, 3 CmA, and 5 CmA. The characteristics of the batteries were compared using the utilization rate of the positive electrode active material as an index. The discharge reaction of the positive electrode is Ni (OH)2→ 1 electron reaction to NiOOH, Ni (OH) at that time2The ratio of the actually discharged capacity to the theoretical discharge capacity of 289 mAh per gram was calculated and used as the utilization factor.
[0050]
(1) Effect of oxidation treatment
The high rate discharge characteristics of the present invention battery A1 and comparative battery B1 are shown in FIG. The utilization rate of the battery A1 of the present invention is high, and the characteristics are far superior to those of the comparative battery B1. This result shows that the characteristics were remarkably improved by oxidizing the positive electrode active material with an oxidizing agent. By subjecting the positive electrode active material to an oxidation treatment using an oxidizing agent, the Co hydroxide contained in the surface layer is converted into a conductive compound in spite of the presence of rare earth elements. It is considered that a highly conductive path is formed, so that a high utilization rate is exhibited even during high rate discharge. On the other hand, in the comparative battery B1, the formation of the conductive path of the cobalt compound is hindered by the rare earth elements present in the surface layer, and it is considered that sufficient discharge capacity cannot be obtained during high rate discharge.
[0051]
(2) Types of rare earth elements in the coating layer
FIG. 2 shows the relationship between the active material utilization rate and the temperature when the present invention batteries A1, A2, A3, A4, A5, A6, A7 and the comparative example battery C1 are charged and discharged at each temperature. It turns out that battery A1-A7 which concerns on this invention shows a favorable utilization rate, even when it charges and discharges at high temperature compared with comparative example battery C1. In the present invention, the addition of rare earth elements to the surface layer suppresses the oxygen gas generation reaction that occurs competitively at the end of the nickel electrode charging, thereby improving the charge acceptance. For this reason, it is considered that the utilization rate is remarkably increased as compared with Comparative Battery C1 in which the surface layer does not contain a rare earth element. Among the rare earth elements, the improvement effect by adding Yb, Tm, and Lu is large, and the improvement effect when Yb is added is particularly large.
[0052]
(3) Effect of alkaline aqueous solution concentration
FIG. 3 shows initial charge curves of the batteries A1, D1, D4 and D5 in which the alkali concentration in the oxidation treatment bath of the surface layer made of a mixed hydroxide containing Co and rare earth elements was changed. Although not shown in the figure, the batteries D2 and D3 showed the same curve as D4. Further, the battery D6 was omitted from the drawing, but showed the same curve as that of A1. In the initial charge, the oxidation of Co (II) having a low oxidation reaction potential takes precedence over the Ni contained in the core layer, followed by the oxidation of Ni (II) having the same potential.
[0053]
As shown in FIG. 3, in the comparative battery D1, a phenomenon in which the potential once shifts in a base direction at the initial stage of charging is recognized, and the rising of the potential of the positive electrode accompanying charging is slow. This is because oxidation from Co (II) contained in the surface layer to Co (III) occurs in the initial charging stage. This result indicates that the oxidation by the oxidation treatment using the oxidizing agent does not proceed in the battery D1. In addition, oxidizer is K2S2O8If water is used for the oxidation treatment bath, the bath exhibits weak acidity, and Ni may be eluted, which is not preferable.
[0054]
On the other hand, in the batteries D5, D6 and A1 of the present invention, the potential has risen to 400 mV or more from the beginning, and it can be seen that the oxidation of the surface layer from Co (II) to Co (III) is sufficiently advanced before charging. . It has been found that batteries D2, D3 and D4 exhibit potential behavior intermediate between battery D1 and batteries D5, D6 and A1. From this, it can be seen that the oxidation reaction from Co (II) contained in the surface layer to Co (III) occurs more rapidly when the alkali concentration of the oxidation treatment bath is higher. From the above results, the alkali concentration of the oxidation treatment bath is preferably 8N or more, and more preferably 10N or more. Further, the alkali concentration 14N is the upper limit value of the solubility, and it is difficult to carry out a higher concentration region. From the above, the alkali concentration of the oxidation treatment bath is 8N or higher, more preferably 10N or higher, and the upper limit is about 14N which is the upper limit of solubility.
[0055]
(4) Effect of alkaline aqueous solution temperature
The temperature of the oxidation treatment bath of the surface layer made of a mixed hydroxide containing Co and rare earth elements was changed. The initial cycle discharge capacities of batteries A1, E1, E2, E3 and E4 are shown in FIG. In the present invention battery A1, a sufficient capacity can be obtained from the initial cycle. Among the batteries of the present invention, in the case of the battery E1 having an oxidation temperature of room temperature, five charge / discharge cycles are required until the capacity is stabilized. In the initial charge / discharge cycle, the active material utilization rate of the battery E1 is low because the oxidation process from Co (II) contained in the surface layer to Co (III) proceeds slowly when the oxidation bath temperature is low. Is not formed sufficiently, and the performance cannot be fully exhibited. In the case of the battery E1, it is considered that about 5 cycles of charging / discharging were required for the oxidation from Co (II) to Co (III) to proceed by charging.
[0056]
In contrast, the batteries A1, E4 and the battery E3 according to the present invention exhibit a high discharge capacity from the first time because the oxidation from Co (II) to Co (III) contained in the surface layer is completed in advance by the oxidation treatment. This is probably because a conductive path is formed. As a result of various studies, it has been found that the temperature of the oxidation treatment is preferably 60 ° C. or higher, and more preferably 80 ° C. or higher. Thus, it is desirable that the temperature of the oxidation treatment is as high as possible in order to promote the oxidation reaction. However, it is difficult to keep the temperature of the alkali treatment liquid close to the boiling point for a long time in actual operation, and the oxidation reaction efficiency decreases because the self-decomposition reaction of the oxidant itself occurs at high temperatures. To do. For this reason, the temperature of the oxidation treatment bath is desirably 60 ° C to 120 ° C, and more desirably 80 ° C to 120 ° C.
[0057]
In the above embodiment, K is used as the oxidizing agent.2S2O8But other oxidizing agents such as Na2S2O8, (NHFour)2S2O8In the case of NaClO or the like, the same effect was obtained. However, (NHFour)2S2O8In the actual manufacturing process, ammonia with an irritating odor is generated as a by-product during oxidation treatment.2S2O8, Na2S2O8It is preferable to use an oxidizing agent that is easy to handle, such as sodium hypochlorite, sodium chlorite, and sodium chlorate.
[0058]
【The invention's effect】
As described above, the present inventionThe manufacturing method of the positive electrode active material for alkaline storage batteries according toImproves charge acceptance in high-temperature environments without degrading the high rate discharge characteristics of nickel positive electrodesThe positive electrode active material which can be manufactured is provided.further,The method for producing a positive electrode active material for an alkaline storage battery according to the present invention comprises:An inexpensive nickel electrode plate and alkaline storage battery that do not require chemical conversion and have high battery production efficiencyA positive electrode active material that can be obtained is provided. Therefore, the present inventionIndustrial value is great.
[0059]
[Brief description of the drawings]
FIG. 1 is a graph showing high rate discharge characteristics of an example battery and a comparative battery according to the present invention.
FIG. 2 is a graph showing temperature characteristics of an example battery and a comparative battery according to the present invention.
FIG. 3 is a graph showing a part of initial charge curves of an example battery and a comparative battery according to the present invention.
FIG. 4 is a graph showing changes in discharge capacity during an initial charge / discharge cycle of an example battery according to the present invention.
Claims (6)
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