JP4159161B2 - Positive electrode active material for alkaline storage battery, method for producing the same, and method for producing positive electrode for alkaline storage battery using the positive electrode active material - Google Patents
Positive electrode active material for alkaline storage battery, method for producing the same, and method for producing positive electrode for alkaline storage battery using the positive electrode active material Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は正極活物質として水酸化ニッケルを用いたニッケル・水素蓄電池、ニッケル・カドミウム蓄電池、ニッケル・亜鉛蓄電池などのアルカリ蓄電池の正極活物質およびその製造方法ならびにこの正極活物質を用いたアルカリ蓄電池用正極の製造方法に関する。
【０００２】
【従来の技術】
近年、携帯用電子・通信機器の急速な普及により従来に増して高性能な蓄電池が要請されている。このような背景にあって、水酸化ニッケルを正極活物質とするアルカリ蓄電池においても、蓄電池の一層の高性能化、高容量化のため、水酸化ニッケル活物質の利用率を改良して高容量化する方法が種々提案されている。例えば、水酸化ニッケル活物質に導電補助剤としてコバルト化合物あるいはニッケル金属粉末を添加する方法、水酸化ニッケル活物質の表面にコバルト化合物あるいはニッケル金属を析出させる方法等が提案されている。
【０００３】
特に、コバルト化合物は、２価の状態では導電性がないが、電池の初回充放電により酸化されて導電性が良好な高次コバルト化合物となる。また、その充放電により、まず、充電により水酸化コバルトが酸化されて水酸化ニッケル活物質の表面にオキシ水酸化コバルトが析出し、放電により一部のオキシ水酸化コバルトが還元されて水酸化コバルトが電解液中に溶解する。このように、充放電により溶解析出反応を伴うため、導電ネットワークが水酸化ニッケル活物質の表面に均一に形成され、電位的に孤立した部分が少なくなるため、活物質利用率が向上することとなって、幅広く採用されるようになった。
【０００４】
【発明が解決しようとする課題】
しかしながら、オキシ水酸化ニッケル活物質の表面にコバルト化合物を析出させる方法においては、十分な容量を取り出すことができないという問題を生じた。これは、酸化還元電位が水酸化ニッケルより卑な２価以下のコバルトが、オキシ水酸化ニッケルの存在により高次化の影響を受け、かつ、極板乾燥時等のアルカリが存在しない状態があるため、導電性の低いコバルト酸化物に変化し、活物質間の導電性を阻害するためと考えられている。
【０００５】
ここで、オキシ水酸化ニッケルと２価以下のコバルト化合物を共存させた場合、アルカリが存在しないと下記の（１）式の反応式に基づく反応が進行する。
【化１】
ＮｉＯＯＨ＋Ｃｏ（ＯＨ）2→ＣｏＨＯ2・・・（１）（Electrochimica Acta 1964 Vol19 P275参照）
この反応は下記の（２）式、（３）式の反応式から成り立っている。
【０００６】
【化２】
ＮｉＯＯＨ＋１／２Ｈ2Ｏ→Ｎｉ（ＯＨ）2＋１／４Ｏ2 ・・・（２）
Ｃｏ（ＯＨ）2＋１／４Ｏ2→ＣｏＨＯ2＋Ｎｉ（ＯＨ）2・・・（３）
上記（２），（３）式は当然、下記の（４）式の半反応式が関与する。
【化３】
Ｈ2Ｏ→１／２Ｏ2 ＋２Ｈ+＋２ｅ- ・・・（４）
【０００７】
上記（１）〜（４）式から言えることは、水酸化コバルトが酸素により酸化された場合、電子の授受が少ないために電子導電性が阻害されるということができる。換言すると、オキシ水酸化ニッケルを正極材料とした場合、２価以下のコバルト化合物を導電補助剤に用いると、導電性が阻害されて容量が低下する結果となる。つまり、オキシ水酸化ニッケルを正極活物質として用いる場合には、酸化による影響を受けない導電補助剤を用いることが必須の条件となる。
【０００８】
また、水酸化コバルトは空気暴露により徐々に変色することから、オキシ水酸化コバルト共存下ではより酸化が加速され、Ｈ2ＯとＯ2の介在と電子の授受により、オキシ水酸化ニッケルの還元と水酸化コバルトの酸化がそれぞれの存在により加速されると考えられる。このことは、水酸化コバルトのみではなく、酸化コバルト、金属コバルトなどの２価以下のコバルトあるいはコバルト化合物を用いた場合も同様である。
以上のことから、オキシ水酸化ニッケルをニッケル正極活物質として用いる場合は、２価以下のコバルト化合物は導電補助剤としては適しないことを意味するということができる。
【０００９】
【課題を解決するための手段およびその作用・効果】
そこで、本発明は上記問題点に鑑みてなされたものであり、オキシ水酸化ニッケルを正極活物質として用いても、導電性に優れたコバルト化合物を得るとともに、機械的強度の高いコバルトの被覆層を設けて高容量のアルカリ蓄電池が得られるようにすることをその目的とする。
【００１０】
このため、本発明のアルカリ蓄電池用正極活物質は、２価より高次な水酸化ニッケルを備えるとともに、この２価より高次な水酸化ニッケルの表面に第１のアルカリカチオンを含有する２価より高次なコバルト化合物を備え、２価より高次な水酸化ニッケルの内部に第２のアルカリカチオンを含有するようにしている。
【００１１】
このように、２価より高次な水酸化ニッケルの表面に第１のアルカリカチオンを含有する２価より高次なコバルト化合物を備えるとともに、２価より高次な水酸化ニッケルの内部に第２のアルカリカチオンを含有すると、２価より高次な水酸化ニッケルの表面に形成された高次コバルト化合物と内部の２価より高次な水酸化ニッケルとの境界がなくなるため、ニッケル−コバルト間の結合が強固になって活物質粒子の機械的強度が増大するとともに、ニッケル−コバルト間の電気抵抗が低下して、高率放電時の容量が高くなる。
【００１２】
そして、２価より高次な水酸化ニッケルの表面に第１のアルカリカチオンを含有する２価より高次なコバルト化合物を備えると、第１のアルカリカチオンはコバルト化合物が酸化剤により酸化されることを防止する作用を有するため、コバルト化合物の安定性を確保できるようになる。また、表面に第１のアルカリカチオンを含有する２価より高次なコバルト化合物を備えた２価より高次な水酸化ニッケルの内部にも第２のアルカリカチオンを存在させると、充放電サイクルに伴い、γ−オキシ水酸化ニッケルが生成した場合でも電解液中のアルカリカチオンの変化を小さくできるため、電池の充放電に伴う電解液濃度の変化を抑制でき、放電電圧の平坦性を増すことができるようになる。
【００１３】
このことは、例えば、Journal of Power Sources 8 1982 p229には「β−オキシ水酸化ニッケル中にはアルカリカチオンは含有されておらず、γ−オキシ水酸化ニッケルにはアルカリカチオンを含有する」なる記載があり、本発明の活物質をＸ線回折による分析結果においても、γ−オキシ水酸化ニッケルが検出されず、アルカリカチオンは過剰な洗浄においても減少しないことから、本発明の活物質は第２のアルカリカチオンを含んだβ−オキシ水酸化ニッケルであると考えられる。
そして、第２のアルカリカチオンとしては、カリウムイオン、ナトリウムイオン、リチウムイオンから選択して用いることができるが、特に、リチウムイオンを用いると、このリチウムイオンが電解液中の水と結びつき、水による酸化が抑制されるとともに、酸素発生電位が向上して放置後の自己放電が抑制されたため、高温での充電放置後の容量を確保できるため好ましい。
【００１４】
また、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、水酸化ニッケル化合物の表面に第１のアルカリカチオンを含む２価より高次なコバルト化合物を保持させる保持工程と、表面に第１のアルカリカチオンを含む２価より高次なコバルト化合物を保持させた水酸化ニッケル化合物を第２のアルカリカチオンを含む水溶液中に酸化剤とともに浸漬してこの水酸化ニッケル化合物を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させる高次化含有工程とを備えるようにしている。
【００１５】
保持工程により、水酸化ニッケル化合物の表面に第１のアルカリカチオンを含む２価より高次なコバルト化合物を保持させた後、酸化剤とともに第２のアルカリカチオンを含む水溶液中に浸漬してこの水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化すると、粒子表面のコバルト層と粒子内部のニッケル層との境界がなくなるため、ニッケル層−コバルト層間の結合が強固になって活物質粒子の機械的強度が増大するとともに、ニッケル層−コバルト層間の電気抵抗が低下して、高率放電時の容量が高くなる。
【００１６】
つまり、酸化剤により２価の水酸化ニッケルが３価の水酸化ニッケルに高次化される際に、粒子表面の３価のコバルトと結晶内のニッケル原子間で入れ替えが生じて、コバルト層−ニッケル層の境界が不明瞭になって、機械的強度が向上する。水酸化ニッケルの機械的強度が向上すると、コバルト層−ニッケル層間の電気抵抗が減少するため、大電流放電においても電圧降下が小さくなり、結果として高率放電時の容量が増加する。
【００１７】
そして、保持工程において粒状の水酸化ニッケル化合物をコバルト化合物と混合するかあるいは粒状の水酸化ニッケル化合物をコバルト化合物で被覆した後、アルカリ水溶液と酸素の共存下で加熱処理するようにすると、下記の（５）、（６）の反応式に基づく反応が進行して、粒状の水酸化ニッケル化合物の表面に第１のアルカリカチオンを含む２価より高次なコバルト化合物層が容易に形成できるようになる。
【００１８】
【化４】
Ｃｏ（ＯＨ）2＋ＮａＯＨ→Ｃｏ（Ｎａ）ＯＯＨ＋Ｈ2Ｏ＋ｅ-・・・（５）
１／２Ｏ2＋２Ｈ+＋２ｅ- →Ｈ2Ｏ ・・・（６）
上記（５）、（６）式より明らかなように、電気化学的に酸化（つまり、アルカリの存在の元での酸化）されることにより、電子伝導性が高くなる。
【００１９】
また、本発明のアルカリ蓄電用正極の製造方法においては、水酸化ニッケル化合物をコバルト化合物と混合するかあるいは水酸化ニッケル化合物をコバルト化合物で被覆した後、第１のアルカリカチオンを含有するアルカリ水溶液と酸素の共存下で加熱処理して、水酸化ニッケル化合物の表面に第１のアルカリカチオンを含んだ２価より高次なコバルト化合物を生成させる生成工程と、その表面に第１のアルカリカチオンを含んだ２価より高次なコバルト化合物が生成された水酸化ニッケル化合物を第２のアルカリカチオンを含有するアルカリ水溶液中で酸化剤と共に撹拌して、水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させる高次化含有工程と、水酸化ニッケル化合物に純水を添加してスラリーとし、このスラリーを発泡ニッケルから成る基板に充填する充填工程とを備えるようにしている。
【００２０】
このように、水酸化ニッケル化合物をコバルト化合物で被覆した後、第１のアルカリカチオンを含有するアルカリ水溶液と酸素の共存下で加熱処理することにより、水酸化ニッケル化合物の表面に、導電性に優れた第１のアルカリカチオンを含んだ２価より高次なコバルト化合物層が形成される。この導電性に優れた高次コバルト化合物層が形成された水酸化ニッケル化合物が酸化剤により酸化されると、２価の水酸化ニッケルが３価の水酸化ニッケルに高次化される際に、表面の３価のコバルトと結晶内原子間で入れ替えが生じて、コバルト層−ニッケル層の境界が不明瞭になって、活物質粒子の機械的強度が向上する。活物質粒子の機械的強度が向上すると、後の工程においてスラリーとするための撹拌を行っても、導電性に優れた２価より高次なコバルト化合物が剥離することがないので、導電性に優れた、即ち、活物質利用率が向上したアルカリ蓄電用正極が得られるようになる。
【００２１】
そして、高次化含有工程により水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させた後、充填工程によりこの活物質をスラリーとして基板に充填しても、あるいは充填工程により高次化されていない活物質をスラリーとして基板に充填した後、高次化含有工程により水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させても、第１のアルカリカチオンを含んだ２価より高次なコバルト化合物を表面に備えた水酸化ニッケル化合物の一部が２価より高次な水酸化ニッケルに高次化されるという点で格別相違しないので、高次化含有工程と充填工程との順序が入れ替わってもよい。
【００２２】
【発明の実施の形態】
１．正極材料の作製
（１）実施例１
重量比でニッケル１００に対して亜鉛５重量％、コバルト２重量％となるような硫酸ニッケル、硫酸亜鉛、硫酸コバルトの混合水溶液を攪拌しながら、水酸化ナトリウム水溶液およびアンモニア水溶液を徐々に添加し、反応溶液中のｐＨが１３〜１４になるように維持させて粒状の水酸化ニッケルを析出させる。
【００２３】
次に、粒状の水酸化ニッケルが析出した溶液に、比重１．３０の硫酸コバルト水溶液と２５重量％の水酸化ナトリウム水溶液を添加し、この反応溶液中のｐＨが９〜１０になるように維持させて、水酸化ニッケル析出物を結晶核として、この核の周囲に水酸化コバルトを析出させる。これらの粒状物を採取し、水洗、乾燥して、粒状でその表面に水酸化コバルトを形成した水酸化ニッケル化合物を作製する。なお、このようにして表面に水酸化コバルトを形成させると、水酸化ニッケル化合物全体に対して８重量％（水酸化物換算）の水酸化コバルトが生成される。
【００２４】
このようにして得られたその表面に水酸化コバルトが形成された粒状の水酸化ニッケル化合物を酸素雰囲気の熱気流下でアルカリ水溶液（３５重量％の水酸化ナトリウム）を噴霧する。この場合、その表面に水酸化コバルトが形成された粒状の水酸化ニッケル化合物の温度が６０℃となるように加熱度合いを調整し、コバルト量に対して５倍のアルカリ水溶液（３５重量％の水酸化ナトリウム）を噴霧した後、水酸化ニッケル化合物の温度が９０℃に到達するまで昇温する。
【００２５】
このようなアルカリ熱処理工程により、粒状の水酸化ニッケルの表面に形成された水酸化コバルトの結晶構造が破壊されて結晶構造に乱れを生じると共に、水酸化コバルトの酸化が強力に促進されて、その平均価数が２価より大きい、例えば、２．９価の高次コバルト化合物となる。これにより、導電性のよいアルカリカチオンを含有した２価より高次なコバルト化合物をその表面に偏在形成させた粒状の水酸化ニッケル化合物が形成されることとなる。
【００２６】
ついで、このようにして形成された表面にアルカリカチオンを含有した２価より高次なコバルト化合物を有する水酸化ニッケル化合物を１００ｇを用意する。この水酸化ニッケル化合物を、１０重量％の水酸化ナトリウム水溶液１０００ｍｌに１２重量％の次亜塩素酸ナトリウム（ＮａＣｌＯ）（酸化剤）を１２５ｍｌ溶解させた水溶液中に浸漬して、１０分間撹拌する。これにより、表面にアルカリカチオンを含有した２価より高次なコバルト化合物を有するとともに、その内部にナトリウムイオンを含有した平均価数が２．２価の水酸化ニッケル化合物が得られた。なお、この水酸化ニッケル化合物を組成分析すると、０．２重量％程度のナトリウムイオンを含有していることが分かった。このようにして作製された水酸化ニッケル化合物を実施例１の正極活物質とする。
【００２７】
（２）実施例２
実施例１と同様にして作製した、導電性のよいアルカリカチオンを含有した２価より高次なコバルト化合物をその表面に偏在形成させた粒状の水酸化ニッケル化合物１００ｇを用意する。この水酸化ニッケル化合物を、１０重量％の水酸化リチウム水溶液１０００ｍｌに１２重量％の次亜塩素酸ナトリウム（ＮａＣｌＯ）（酸化剤）を１２５ｍｌ溶解させた水溶液中に浸漬して、１０分間撹拌する。これにより、表面にアルカリカチオンを含有した２価より高次なコバルト化合物を有するとともに、その内部にリチウムイオンを有する平均価数が２．２価の水酸化ニッケル化合物が得られた。なお、この水酸化ニッケル化合物を組成分析すると、０．７重量％程度のリチウムイオンを含有していることが分かった。このようにして作製された水酸化ニッケル化合物を実施例２の正極活物質とする。
【００２８】
（３）比較例１
重量比でニッケル１００に対して亜鉛５重量％、コバルト２重量％となるような硫酸ニッケル、硫酸亜鉛、硫酸コバルトの混合水溶液を攪拌しながら、水酸化ナトリウム水溶液およびアンモニア水溶液を徐々に添加し、反応溶液中のｐＨが１３〜１４になるように維持させて粒状の水酸化ニッケルを析出させる。
ついで、粒状の水酸化ニッケルが析出した溶液に、２５重量％の水酸化ナトリウム水溶液と次亜塩素酸ナトリウム（ＮａＣｌＯ）（酸化剤）を添加して混合した。これにより、水酸化ニッケルは高次化されてオキシ水酸化ニッケルとなり、その平均価数は２．２価となった。なお、酸化剤（次亜塩素酸ナトリウム（ＮａＣｌＯ））の量を加減することにより、水酸化ニッケル中の平均価数を調整することができる。
【００２９】
次に、このようにして高次化された水酸化ニッケルを採取し、水洗、乾燥して、粒状の水酸化ニッケル化合物とする。この水酸化ニッケル化合物に水酸化コバルトを添加して、水酸化コバルトを含有する水酸化ニッケル化合物を作製する。なお、水酸化コバルトの添加量は水酸化ニッケル化合物全体に対して８重量％（水酸化物換算）になるようにした。このようにして作製された水酸化ニッケル化合物を比較例１の正極活物質とする。
【００３０】
（４）比較例２
比較例１と同様にして作製された高次化された水酸化ニッケルに、比重１．３０の硫酸コバルト水溶液と２５重量％の水酸化ナトリウム水溶液を添加し、この反応溶液中のｐＨが９〜１０になるように維持させて、高次化された水酸化ニッケルを結晶核として、この核の周囲に水酸化コバルトを析出させる。これらの粒状物を採取し、水洗、乾燥して、粒状でその表面に水酸化コバルトを形成した水酸化ニッケル化合物を作製する。なお、このようにして表面に水酸化コバルトを形成させると、水酸化ニッケル化合物全体に対して８重量％（水酸化物換算）の水酸化コバルトが生成される。このようにして作製された水酸化ニッケル化合物を比較例２の正極活物質とする。
【００３１】
（５）比較例３
重量比でニッケル１００に対して亜鉛５重量％、コバルト２重量％となるような硫酸ニッケル、硫酸亜鉛、硫酸コバルトの混合水溶液を攪拌しながら、水酸化ナトリウム水溶液およびアンモニア水溶液を徐々に添加し、反応溶液中のｐＨが１３〜１４になるように維持させて粒状の水酸化ニッケルを析出させる。
ついで、このように析出させた水酸化ニッケルに、比重１．３０の硫酸コバルト水溶液と２５重量％の水酸化ナトリウム水溶液を添加し、この反応溶液中のｐＨが９〜１０になるように維持させて、水酸化ニッケルを結晶核として、この核の周囲に水酸化コバルトを析出させる。
【００３２】
これらの粒状物を採取し、水洗、乾燥して、粒状でその表面に水酸化コバルトを形成した水酸化ニッケル化合物を作製する。なお、このようにして表面に水酸化コバルトを形成させると、水酸化ニッケル化合物全体に対して８重量％（水酸化物換算）の水酸化コバルトが生成される。このようにして表面に水酸化コバルトを形成させた水酸化ニッケル化合物を、１０重量％の水酸化リチウム水溶液１０００ｍｌに１２重量％の次亜塩素酸ナトリウム（ＮａＣｌＯ）（酸化剤）を１２５ｍｌ溶解させた水溶液中に浸漬して、１０分間撹拌して、水酸化ニッケル化合物を高次化し、平均価数が２．２価の水酸化ニッケル化合物を作製する。このようにして作製された水酸化ニッケル化合物を比較例３の正極活物質とする。
【００３３】
（６）比較例４
比較例３と同様にして作製された粒状でその表面に水酸化コバルトを形成した水酸化ニッケル化合物を、コバルトのみを酸化する酸化剤、例えば、過酸化水素水で表面の水酸化コバルトを高次化してオキシ水酸化コバルトとした。このようにして作製された表面に高次コバルト化合物を有する水酸化ニッケル化合物を、１０重量％の水酸化リチウム水溶液１０００ｍｌに１２重量％の次亜塩素酸ナトリウム（ＮａＣｌＯ）（酸化剤）を１２５ｍｌ溶解させた水溶液中に浸漬して、１０分間撹拌して、表面に高次コバルト化合物を有するとともに、その内部にナトリウムイオンを有する平均価数が２．２の水酸化ニッケル化合物を作製する。このようにして作製された水酸化ニッケル化合物を比較例４の正極活物質とする。
【００３４】
２．ニッケル正極の作製
上述のように作製した実施例１，２および比較例１，２，３，４の活物質１００重量部に対して、５重量％のＰＴＦＥ（ポリテトラフルオロエチレン）溶液５０重量部を添加混合してそれぞれ活物質スラリーを作製する。これらの活物質スラリーをそれぞれ多孔度９５％で、厚み１．６ｍｍの発泡ニッケルからなる基板に圧延後の充填密度が７００ｇ／ｍ2となるように充填し、乾燥後、厚みが０．６０ｍｍとなるように圧延を行って非焼結式ニッケル正極をそれぞれ作製した。３．負極の作製
【００３５】
ミッシュメタル（Ｍｍ：希土類元素の混合物）、ニッケル、コバルト、アルミニウム、およびマンガンを１：３．６：０．６：０．２：０．６の比率で混合し、この混合物をアルゴンガス雰囲気の高周波誘導炉で誘導加熱して合金溶湯となす。この合金溶湯を公知の方法で冷却し、組成式Ｍｍ1.0Ｎｉ3.6Ｃｏ0.6Ａｌ0.2Ｍｎ0.6で表される水素吸蔵合金のインゴットを作製する。この水素吸蔵合金インゴットを機械的に粉砕し、平均粒子径が約１００μｍの水素吸蔵合金粉末となし、この水素吸蔵合金粉末にポリエチレンオキサイド等の結着剤と、適量の水を加えて混合して水素吸蔵合金ペーストを作製する。このペーストをパンチングメタルに塗布し、乾燥した後、厚み０．４ｍｍに圧延して水素吸蔵合金負極を作製する。
【００３６】
４．電池の作製
上述のように作製したそれぞれの非焼結式ニッケル正極（水酸化ニッケル活物質が約５ｇとなるように所定寸法に切断したもの）と水素吸蔵合金負極とをポリプロピレン製不織布のセパレータを介して卷回して、渦巻状の電極群を作製した後、この電極群を外装缶に挿入する。その後、外装缶内に電解液として水酸化カリウム水溶液を注入し、更に外装缶を封口して、公称容量１２５０ｍＡＨのＡＡサイズのニッケル−水素蓄電池をそれぞれ組み立てる。
【００３７】
５．試験
（１）単位活物質容量
上述のように作製した各ニッケル−水素蓄電池を雰囲気温度２５℃において、１２５ｍＡ（０．１Ｃ）の充電電流で１６時間充電した後、６２５ｍＡ（０．５Ｃ）の放電電流で電池電圧が１．０Ｖになるまで放電させ、このときの放電時間から水酸化ニッケル活物質１ｇ当たりの放電容量（単位活物質容量）を求めると、下記の表１に示すような結果となった。なお、表１において、実施例１の活物質を用いた電池の単位活物質容量を１００として求めた。
【００３８】
（２）高率放電時容量
上述のように作製したニッケル−水素蓄電池を雰囲気温度２５℃において、１２５ｍＡ（０．１Ｃ）の充電電流で１６時間充電した後、２５００ｍＡ（２Ｃ）の放電電流で電池電圧が１．０Ｖになるまで放電させ、このときの放電時間から放電容量を求め、上述した単位活物質容量に対する容量比を下記の数１の算出式から高率放電時容量として求めると、下記の表１に示すような結果となった。なお、表１において、実施例１の活物質を用いた電池の高率放電時容量を１００として求めた。
【００３９】
【数１】
高率放電時容量＝（高率放電時の放電容量／単位活物質容量）×１００
【００４０】
（３）高温保存時容量
上述のように作製したニッケル−水素蓄電池を雰囲気温度６０℃において、２週間放置後、１２５ｍＡ（０．１Ｃ）の充電電流で１６時間充電した後、６２５ｍＡ（０．５Ｃ）の放電電流で電池電圧が１．０Ｖになるまで放電させ、このときの放電時間から放電容量を求め、通常の放電容量に対する容量比を下記の数２の算出式から高温保存時容量として求めると、下記の表１に示すような結果となった。なお、表１において、実施例１の活物質を用いた電池の高温保存時容量を１００として求めた。
【数２】
高温保存時容量＝（高温保存時の放電容量／通常放電容量）×１００
【００４１】
【表１】
上記表１から明らかなように、比較例１〜４の活物質を用いた電池の単位活物質容量および高率放電時容量が低下していることが分かる。これは、以下の原因が考えられる。即ち、比較例１の活物質を用いた電池の場合、酸化剤により水酸化ニッケルの一部をオキシ水酸化ニッケルに変化させて、平均価数を２．２とした水酸化ニッケル化合物に添加した水酸化コバルトが電池が初期充電される前に酸素により酸化され、上述した反応式（１）〜（３）により、導電性の低いコバルト酸化物に変化して、単位活物質容量および高率放電時容量が大幅に低下したと考えられる。
【００４３】
また、比較例２の活物質を用いた電池の場合、酸化剤により水酸化ニッケルの一部をオキシ水酸化ニッケルに変化させて、平均価数を２．２とした水酸化ニッケル化合物に水酸化コバルトを被覆しただけであるため、コバルトは完全には酸化されておらず、比較例１の活物質と同様に導電性の低いコバルト酸化物に変化したためと考えられる。これは、析出反応を制御して水酸化コバルトを被覆しているため、析出速度が遅いと添加したコバルトイオンがオキシ水酸化コバルトのイオンとして所定量の水酸化コバルトが被覆できず、また、析出速度が速いと深緑色の化合物（この化合物の構造式は不明であるが、酸素不足のため、導電性の低い高次コバルト化合物が析出したと考えられる）が生成され、析出条件を調整した場合であっても、容量が低下した結果となった。
そして、活物質に超音波を当てた場合、表面のコバルト化合物は剥がれやすい状態になった。これは、酸化がニッケルとコバルトで同時に進行するため、コバルト層−ニッケル層間での原子交換が生じなかったためと考えられる。
【００４４】
また、比較例３の活物質を用いた電池の場合、酸化剤により水酸化ニッケルの一部をオキシ水酸化ニッケルに変化させて、平均価数を２．２とした水酸化ニッケル化合物に水酸化コバルトを被覆した後、酸化剤により酸化するので、導電性が低いコバルト化合物が生成し、容量が低下したと考えられる。導電性が高いコバルト化合物と導電性が低いコバルト化合物の差は、溶解→酸化→析出のプロセスを経るか否かによることが経験上分かっている。
そして、酸化剤により酸化されたコバルトは溶解→酸化→析出のプロセスを経ていないため、導電性が低く、結果として容量が低下したと考えられる。また、比較例２の活物質と同様に超音波を当てた場合、表面のコバルト化合物は剥がれやすい状態になった。これは、酸化がニッケルとコバルトで同時に進行するため、コバルト層−ニッケル層間での原子交換が生じなかったためと考えられる。
【００４５】
また、比較例４の活物質を用いた電池の場合であっても、オキシ水酸化ニッケルの表面に存在するオキシ水酸化コバルトはアルカリカチオンを含有しないため、酸化剤で酸化の影響を受けて、上述と同様に導電性が低く、結果として容量が低下したと考えられる。
【００４６】
一方、実施例１，２の活物質を用いた電池の場合は、単位活物質容量および高率放電時容量もともに増大した結果となった。これは、アルカリカチオンを含む２価より高次なコバルト化合物を表面に被覆した水酸化ニッケル化合物をアルカリ水溶液中で酸化剤により高次化することで、水酸化ニッケルが高次化される際に、溶解→酸化→析出のプロセスを経ているので、表面のコバルト化合物と結晶構造中で置換が生じたたためと考えられる。
【００４７】
つまり、酸化剤により２価のニッケルが３価のニッケルに高次化される際に、ニッケルの近傍に存在する３価のコバルトと結晶内原子間で入れ替えが生じて、結果として、コバルト層−ニッケル層の境界が不明瞭となり、機械的強度が向上したものと考えられる。水酸化化合物の機械的強度が向上することにより、コバルト層−ニッケル層間の電気抵抗が低下し、大電流放電時に特に顕著な差となって現れる。そして、ニッケル電極の製造時においては、スラリーとするために撹拌処理を行うので、水酸化化合物の機械的強度が向上することは重要な要素となる。
【００４８】
また、実施例１の活物質を用いた電池より実施例２の活物質を用いた電池の方が高温保存時容量が大きい理由は、実施例２の活物質は水酸化ニッケル化合物中にリチウムイオンが存在するため、このリチウムイオンが電解液中の水と結びつき、水による酸化が抑制されるとともに、酸素発生電位が向上して放置後の自己放電が抑制されたためと考えられる。
【００４９】
６．酸化処理時のアルカリ濃度と活物質の嵩密度との関係
ついで、実施例１，２の活物質を製造するに際して、水酸化ニッケル化合物の表面に水酸化コバルトを被覆した後、アルカリ水溶液（水酸化ナトリウム（ＮａＯＨ）水溶液）−酸素共存下で加熱処理して水酸化コバルトを結晶構造の乱れた第１のアルカリカチオン含む２価より高次なコバルトにする。その後水酸化ニッケルを高次化するための酸化処理時のアルカリ水溶液の濃度を５重量％、１０重量％、１５重量％、２０重量％、２５重量％と変化させて、アルカリ水溶液の濃度と活物質の嵩密度との関係について測定すると下記の表２に示すような結果となった。
【００５０】
【表２】
上記表２より明らかなように、水酸化ナトリウム（ＮａＯＨ）水溶液の濃度が２０重量％以下であると、嵩密度の高いオキシ水酸化ニッケル活物質が得られることが分かる。水酸化ナトリウム水溶液の濃度が高い場合の活物質粉体をＸ線回析で分析すると、γ−オキシ水酸化ニッケルの存在が確認できた。このことから、水酸化ニッケル粒子の表面でγ−オキシ水酸化ニッケルが生成し、嵩密度低下したと考えられる。
【００５２】
なお、上述の実施形態においては、水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させた後、充填工程によりこの活物質をスラリーとして基板に充填する例について説明したが、高次化されていない水酸化ニッケル化合物をスラリーとして基板に充填した後、水酸化ニッケル化合物の一部を２価より高次な水酸化ニッケルに高次化するとともに、第２のアルカリカチオンを２価より高次な水酸化ニッケルの内部に含有させても、第１のアルカリカチオンを含んだ２価より高次なコバルト化合物を表面に備えた水酸化ニッケル化合物の一部が２価より高次な水酸化ニッケルに高次化されるという点で格別相違しないので、高次化含有工程と充填工程との順序を入れ替えてもよい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode active material of an alkaline storage battery such as nickel / hydrogen storage battery, nickel / cadmium storage battery, nickel / zinc storage battery using nickel hydroxide as a positive electrode active material, a method for producing the same, and an alkaline storage battery using the positive electrode active material. The present invention relates to a method for manufacturing a positive electrode.
[0002]
[Prior art]
In recent years, due to the rapid spread of portable electronic / communication equipment, a battery with higher performance than before has been demanded. Against this background, even in alkaline storage batteries using nickel hydroxide as the positive electrode active material, the utilization rate of the nickel hydroxide active material has been improved for higher performance and higher capacity of the storage battery. Various methods have been proposed. For example, a method of adding a cobalt compound or nickel metal powder as a conductive auxiliary agent to a nickel hydroxide active material, a method of depositing a cobalt compound or nickel metal on the surface of the nickel hydroxide active material, and the like have been proposed.
[0003]
In particular, the cobalt compound is not conductive in a divalent state, but is oxidized by the initial charge / discharge of the battery to become a higher cobalt compound having good conductivity. In addition, by the charge / discharge, cobalt hydroxide is first oxidized by charging to deposit cobalt oxyhydroxide on the surface of the nickel hydroxide active material, and a portion of the cobalt oxyhydroxide is reduced by the discharge, resulting in cobalt hydroxide. Dissolves in the electrolyte. As described above, since the dissolution and precipitation reaction is accompanied by charging and discharging, the conductive network is uniformly formed on the surface of the nickel hydroxide active material, and the number of potential isolated portions is reduced. It became widely adopted.
[0004]
[Problems to be solved by the invention]
However, in the method of depositing a cobalt compound on the surface of the nickel oxyhydroxide active material, there arises a problem that a sufficient capacity cannot be taken out. This is because there is a state in which cobalt having a redox potential lower than that of nickel hydroxide is affected by higher order oxidation due to the presence of nickel oxyhydroxide, and there is no alkali during electrode plate drying. Therefore, it is considered to change to a cobalt oxide with low conductivity and to inhibit conductivity between active materials.
[0005]
Here, when nickel oxyhydroxide and a cobalt compound having a valence of 2 or less coexist, if there is no alkali, a reaction based on the following reaction formula (1) proceeds.
[Chemical 1]
NiOOH + Co (OH)2→ CoHO2... (1) (Refer to Electrochimica Acta 1964 Vol19 P275)
This reaction is composed of the following reaction formulas (2) and (3).
[0006]
[Chemical 2]
NiOOH + 1 / 2H2O → Ni (OH)2+ 1 / 4O2... (2)
Co (OH)2+ 1 / 4O2→ CoHO2+ Ni (OH)2... (3)
The above equations (2) and (3) naturally involve the half reaction equation of the following equation (4).
[Chemical 3]
H2O → 1 / 2O2+ 2H++ 2e-... (4)
[0007]
What can be said from the above formulas (1) to (4) can be said that when cobalt hydroxide is oxidized by oxygen, the electron conductivity is hindered due to a small amount of electron transfer. In other words, when nickel oxyhydroxide is used as the positive electrode material, if a divalent or lower valent cobalt compound is used as the conductive auxiliary agent, the conductivity is hindered, resulting in a decrease in capacity. That is, when nickel oxyhydroxide is used as the positive electrode active material, it is an essential condition to use a conductive additive that is not affected by oxidation.
[0008]
In addition, since cobalt hydroxide gradually changes color when exposed to air, oxidation is accelerated in the presence of cobalt oxyhydroxide,2O and O2It is considered that the reduction of nickel oxyhydroxide and the oxidation of cobalt hydroxide are accelerated by the presence of each other and the exchange of electrons. This is the same when not only cobalt hydroxide but also divalent or less-valent cobalt such as cobalt oxide or metallic cobalt or a cobalt compound is used.
From the above, when nickel oxyhydroxide is used as the nickel positive electrode active material, it can be said that a cobalt compound having a valence of 2 or less is not suitable as a conductive additive.
[0009]
[Means for solving the problems and their functions and effects]
Therefore, the present invention has been made in view of the above problems, and even when nickel oxyhydroxide is used as a positive electrode active material, a cobalt compound having excellent conductivity is obtained, and a cobalt coating layer having high mechanical strength is obtained. To provide a high capacity alkaline storage battery.
[0010]
For this reason, the positive electrode active material for an alkaline storage battery of the present invention includes nickel hydroxide having a higher order than divalent and contains a first alkali cation on the surface of the nickel hydroxide higher than the divalent.Higher than bivalentA cobalt compound is provided, and a second alkali cation is contained in nickel hydroxide higher than divalent.
[0011]
As described above, the first alkali cation is contained on the surface of nickel hydroxide higher than divalent.Higher than bivalentWhen the cobalt compound is provided and the second alkali cation is contained in the nickel hydroxide higher than divalent, the higher cobalt compound formed on the surface of the nickel hydroxide higher than divalent and the internal 2 Since there is no boundary with higher-order nickel hydroxide, the bond between nickel and cobalt becomes stronger, the mechanical strength of the active material particles increases, and the electrical resistance between nickel and cobalt decreases, The capacity during high rate discharge increases.
[0012]
And it contains the first alkali cation on the surface of nickel hydroxide higher than divalentHigher than bivalentWhen the cobalt compound is provided, the first alkali cation has an action of preventing the cobalt compound from being oxidized by the oxidizing agent, so that the stability of the cobalt compound can be secured. Further, the surface contains a first alkali cation.Higher than bivalentWhen the second alkali cation is also present in the nickel hydroxide higher than divalent nickel-containing cobalt compound, the alkali in the electrolytic solution is generated even when γ-nickel oxyhydroxide is generated along with the charge / discharge cycle. Since the change of the cation can be reduced, the change in the electrolyte concentration accompanying the charging / discharging of the battery can be suppressed, and the flatness of the discharge voltage can be increased.
[0013]
This is described, for example, in Journal of Power Sources 8 1982 p229, "No alkaline cation is contained in β-nickel oxyhydroxide, and alkali cation is contained in γ-nickel oxyhydroxide". In the analysis result of the active material of the present invention by X-ray diffraction, γ-nickel oxyhydroxide is not detected, and the alkali cation is not reduced by excessive washing. It is thought that it is (beta) -nickel oxyhydroxide containing the alkali cation of this.
The second alkali cation can be selected from potassium ion, sodium ion, and lithium ion. In particular, when lithium ion is used, the lithium ion is combined with water in the electrolytic solution, and the water depends on water. This is preferable because the oxidation is suppressed and the oxygen generation potential is improved and the self-discharge after being left is suppressed, so that the capacity after being left at a high temperature can be secured.
[0014]
Moreover, in the manufacturing method of the positive electrode active material for alkaline storage batteries of this invention, a 1st alkali cation is included in the surface of a nickel hydroxide compound.Higher than bivalentA holding step for holding a cobalt compound and a first alkali cation on the surfaceHigher than bivalentA nickel hydroxide compound holding a cobalt compound is immersed in an aqueous solution containing a second alkali cation together with an oxidizing agent to make this nickel hydroxide compound higher than bivalent nickel hydroxide. And a higher-order containing step of incorporating the alkali cation of 2 into the inside of the nickel hydroxide higher than divalent.
[0015]
The holding step includes the first alkali cation on the surface of the nickel hydroxide compound.Higher than bivalentAfter retaining the cobalt compound, when immersed in an aqueous solution containing a second alkali cation together with an oxidant to make a part of this nickel hydroxide compound higher than nickel hydride, the particle surface Since the boundary between the cobalt layer and the nickel layer inside the particle is eliminated, the bond between the nickel layer and the cobalt layer is strengthened, the mechanical strength of the active material particles is increased, and the electrical resistance between the nickel layer and the cobalt layer is reduced. As a result, the capacity during high rate discharge increases.
[0016]
That is, when the divalent nickel hydroxide is made higher by the oxidizing agent into the trivalent nickel hydroxide, the exchange between the trivalent cobalt on the particle surface and the nickel atoms in the crystal occurs. The boundary of the nickel layer becomes unclear and the mechanical strength is improved. When the mechanical strength of nickel hydroxide is improved, the electrical resistance between the cobalt layer and the nickel layer is reduced, so that the voltage drop is reduced even in a large current discharge, and as a result, the capacity during high rate discharge is increased.
[0017]
In the holding step, the particulate nickel hydroxide compound is mixed with the cobalt compound or the particulate nickel hydroxide compound is coated with the cobalt compound, and then heat-treated in the presence of an alkaline aqueous solution and oxygen. The reaction based on the reaction formulas (5) and (6) proceeds, and the surface of the granular nickel hydroxide compound contains the first alkali cation.Higher than bivalentA cobalt compound layer can be formed easily.
[0018]
[Formula 4]
Co (OH)2+ NaOH → Co (Na) OOH + H2O + e-... (5)
1 / 2O2+ 2H++ 2e-→ H2O ... (6)
As is clear from the above formulas (5) and (6), the electron conductivity is enhanced by electrochemical oxidation (that is, oxidation in the presence of alkali).
[0019]
In the method for producing a positive electrode for alkaline storage of the present invention, after mixing a nickel hydroxide compound with a cobalt compound or coating the nickel hydroxide compound with a cobalt compound, an alkaline aqueous solution containing a first alkali cation and Heat treatment was performed in the presence of oxygen, and the first alkali cation was included on the surface of the nickel hydroxide compound.Higher than bivalentA production step for producing a cobalt compound and a first alkali cation on the surface thereofHigher than bivalentThe nickel hydroxide compound in which the cobalt compound is formed is stirred with an oxidizing agent in an alkaline aqueous solution containing a second alkali cation, and a part of the nickel hydroxide compound is converted into higher-order nickel hydroxide. And adding a second alkali cation to the inside of the nickel hydroxide having a higher order than the bivalent nickel hydroxide, and adding pure water to the nickel hydroxide compound to form a slurry. And a filling step for filling the substrate.
[0020]
As described above, after coating the nickel hydroxide compound with the cobalt compound, the surface of the nickel hydroxide compound is excellent in conductivity by heat treatment in the presence of an aqueous alkali solution containing the first alkali cation and oxygen. Containing the first alkali cationHigher than bivalentA cobalt compound layer is formed. When the nickel hydroxide compound in which the high-order cobalt compound layer excellent in conductivity is formed is oxidized by the oxidizing agent, when the divalent nickel hydroxide is converted into trivalent nickel hydroxide, Interchange occurs between trivalent cobalt on the surface and atoms in the crystal, the boundary between the cobalt layer and the nickel layer becomes unclear, and the mechanical strength of the active material particles is improved. When the mechanical strength of the active material particles is improved, the conductivity is excellent even if stirring is performed to form a slurry in a later stepHigher than bivalentSince the cobalt compound does not peel off, it is possible to obtain an alkaline storage positive electrode having excellent conductivity, that is, having an improved active material utilization rate.
[0021]
Then, a part of the nickel hydroxide compound is higher-ordered to higher-order nickel hydroxide by the higher-order containing step, and the second alkali cation is placed inside the higher-valent nickel hydroxide. After the inclusion, even if the substrate is filled with this active material as a slurry in the filling step, or the substrate is filled with the active material that has not been advanced in the filling step as a slurry, and then is hydroxylated in the higher-order containing step. Even if a part of the nickel compound is higher-ordered to nickel hydroxide higher than divalent, and the second alkali cation is contained in the nickel hydroxide higher than divalent, the first alkali cation IncludedHigher than bivalentSince there is no particular difference in that a part of the nickel hydroxide compound having a cobalt compound on its surface is higher-ordered to nickel hydroxide higher than bivalent, the order of the higher-order containing step and the filling step is It may be replaced.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
1. Production of cathode material
(1) Example 1
While stirring a mixed aqueous solution of nickel sulfate, zinc sulfate, and cobalt sulfate such that zinc is 5% by weight and cobalt is 2% by weight with respect to nickel 100, sodium hydroxide aqueous solution and aqueous ammonia solution are gradually added, Particulate nickel hydroxide is precipitated while maintaining the pH in the reaction solution at 13-14.
[0023]
Next, a cobalt sulfate aqueous solution having a specific gravity of 1.30 and a 25% by weight sodium hydroxide aqueous solution are added to the solution in which the particulate nickel hydroxide is deposited, and the pH in the reaction solution is maintained at 9 to 10. Then, nickel hydroxide precipitates are used as crystal nuclei, and cobalt hydroxide is precipitated around the nuclei. These granular materials are collected, washed with water and dried to produce a nickel hydroxide compound which is granular and has cobalt hydroxide formed on the surface thereof. In addition, when cobalt hydroxide is formed on the surface in this way, 8% by weight (in terms of hydroxide) of cobalt hydroxide is generated with respect to the entire nickel hydroxide compound.
[0024]
The granular nickel hydroxide compound having cobalt hydroxide formed on the surface thus obtained is sprayed with an alkaline aqueous solution (35 wt% sodium hydroxide) under a hot air stream in an oxygen atmosphere. In this case, the degree of heating was adjusted so that the temperature of the granular nickel hydroxide compound having cobalt hydroxide formed on its surface was 60 ° C., and an alkaline aqueous solution (35% by weight of water) that was 5 times the amount of cobalt. After spraying (sodium oxide), the temperature is increased until the temperature of the nickel hydroxide compound reaches 90 ° C.
[0025]
By such an alkali heat treatment step, the crystal structure of cobalt hydroxide formed on the surface of the granular nickel hydroxide is destroyed and the crystal structure is disturbed, and the oxidation of cobalt hydroxide is strongly promoted. The average valence is higher than the divalent, for example, a 2.9-valent higher cobalt compound. This contained a highly conductive alkali cation.Higher than bivalentA granular nickel hydroxide compound in which a cobalt compound is unevenly distributed on the surface is formed.
[0026]
The surface thus formed contained alkali cations.Higher than bivalent100 g of a nickel hydroxide compound having a cobalt compound is prepared. This nickel hydroxide compound is immersed in an aqueous solution in which 125 ml of 12% by weight sodium hypochlorite (NaClO) (oxidant) is dissolved in 1000 ml of a 10% by weight aqueous sodium hydroxide solution and stirred for 10 minutes. This contained alkali cations on the surfaceHigher than bivalentA nickel hydroxide compound having a cobalt compound and an average valence of 2.2 containing sodium ions therein was obtained. The composition analysis of this nickel hydroxide compound revealed that it contained about 0.2% by weight of sodium ions. The nickel hydroxide compound thus produced is used as the positive electrode active material of Example 1.
[0027]
(2) Example 2
It was produced in the same manner as in Example 1 and contained an alkali cation with good conductivity.Higher than bivalentA granular nickel hydroxide compound 100 g in which a cobalt compound is unevenly distributed on the surface is prepared. The nickel hydroxide compound is immersed in an aqueous solution in which 125 ml of 12 wt% sodium hypochlorite (NaClO) (oxidant) is dissolved in 1000 ml of a 10 wt% lithium hydroxide aqueous solution and stirred for 10 minutes. This contained alkali cations on the surfaceHigher than bivalentA nickel hydroxide compound having a cobalt compound and having an average valence of 2.2 having lithium ions therein was obtained. The composition of this nickel hydroxide compound was found to contain about 0.7% by weight of lithium ions. The nickel hydroxide compound thus produced is used as the positive electrode active material of Example 2.
[0028]
(3) Comparative Example 1
While stirring a mixed aqueous solution of nickel sulfate, zinc sulfate, and cobalt sulfate such that zinc is 5% by weight and cobalt is 2% by weight with respect to nickel 100, sodium hydroxide aqueous solution and aqueous ammonia solution are gradually added, Particulate nickel hydroxide is precipitated while maintaining the pH in the reaction solution at 13-14.
Next, a 25% by weight sodium hydroxide aqueous solution and sodium hypochlorite (NaClO) (oxidant) were added to and mixed with the solution in which granular nickel hydroxide was precipitated. As a result, nickel hydroxide was highly ordered to become nickel oxyhydroxide, and its average valence was 2.2. The average valence in nickel hydroxide can be adjusted by adjusting the amount of oxidizing agent (sodium hypochlorite (NaClO)).
[0029]
Next, the highly ordered nickel hydroxide is collected, washed with water and dried to obtain a granular nickel hydroxide compound. Cobalt hydroxide is added to this nickel hydroxide compound to produce a nickel hydroxide compound containing cobalt hydroxide. The addition amount of cobalt hydroxide was set to 8% by weight (in terms of hydroxide) with respect to the entire nickel hydroxide compound. The nickel hydroxide compound thus produced is used as the positive electrode active material of Comparative Example 1.
[0030]
(4) Comparative Example 2
A cobalt sulfate aqueous solution having a specific gravity of 1.30 and a 25% by weight sodium hydroxide aqueous solution were added to nickel hydroxide having a higher order produced in the same manner as in Comparative Example 1, and the pH of the reaction solution was 9 to 9%. The nickel hydroxide having a higher order is used as a crystal nucleus, and cobalt hydroxide is precipitated around the nucleus. These granular materials are collected, washed with water and dried to produce a nickel hydroxide compound which is granular and has cobalt hydroxide formed on the surface thereof. In addition, when cobalt hydroxide is formed on the surface in this way, 8% by weight (in terms of hydroxide) of cobalt hydroxide is generated with respect to the entire nickel hydroxide compound. The nickel hydroxide compound thus produced is used as the positive electrode active material of Comparative Example 2.
[0031]
(5) Comparative Example 3
While stirring a mixed aqueous solution of nickel sulfate, zinc sulfate, and cobalt sulfate such that zinc is 5% by weight and cobalt is 2% by weight with respect to nickel 100, sodium hydroxide aqueous solution and aqueous ammonia solution are gradually added, Particulate nickel hydroxide is precipitated while maintaining the pH in the reaction solution at 13-14.
Next, a cobalt sulfate aqueous solution having a specific gravity of 1.30 and a 25 wt% sodium hydroxide aqueous solution are added to the nickel hydroxide thus precipitated, and the pH in the reaction solution is maintained at 9-10. Then, using nickel hydroxide as a crystal nucleus, cobalt hydroxide is precipitated around the nucleus.
[0032]
These granular materials are collected, washed with water and dried to produce a nickel hydroxide compound which is granular and has cobalt hydroxide formed on the surface thereof. In addition, when cobalt hydroxide is formed on the surface in this way, 8% by weight (in terms of hydroxide) of cobalt hydroxide is generated with respect to the entire nickel hydroxide compound. In this manner, 125 ml of 12 wt% sodium hypochlorite (NaClO) (oxidant) was dissolved in 1000 ml of 10 wt% lithium hydroxide aqueous solution. It is immersed in an aqueous solution and stirred for 10 minutes to increase the order of the nickel hydroxide compound to produce a nickel hydroxide compound having an average valence of 2.2. The nickel hydroxide compound thus produced is used as the positive electrode active material of Comparative Example 3.
[0033]
(6) Comparative Example 4
A nickel hydroxide compound produced in the same manner as in Comparative Example 3 and having cobalt hydroxide formed on the surface thereof, an oxidizing agent that oxidizes only cobalt, for example, hydrogen peroxide solution is used to increase the surface cobalt hydroxide. Into cobalt oxyhydroxide. 125 ml of a nickel hydroxide compound having a high-order cobalt compound on the surface thus prepared was dissolved in 1000 ml of a 10 wt% lithium hydroxide aqueous solution and 12 wt% sodium hypochlorite (NaClO) (oxidant). The nickel hydroxide compound having an average valence of 2.2 having a high-order cobalt compound on the surface and having sodium ions therein is prepared by dipping in the aqueous solution and stirring for 10 minutes. The nickel hydroxide compound thus produced is used as the positive electrode active material of Comparative Example 4.
[0034]
2. Preparation of nickel positive electrode
50 parts by weight of a 5 wt% PTFE (polytetrafluoroethylene) solution was added to and mixed with 100 parts by weight of the active materials of Examples 1 and 2 and Comparative Examples 1, 2, 3 and 4 prepared as described above. Active material slurry is prepared respectively. Each of these active material slurries has a porosity of 95% and a packing density after rolling of 700 g / m on a substrate made of nickel foam having a thickness of 1.6 mm.2Then, after drying, rolling was performed so that the thickness became 0.60 mm, and non-sintered nickel positive electrodes were respectively produced. 3. Production of negative electrode
[0035]
Mish metal (Mm: mixture of rare earth elements), nickel, cobalt, aluminum, and manganese were mixed at a ratio of 1: 3.6: 0.6: 0.2: 0.6. Inductively heated in a high frequency induction furnace to make a molten alloy. This molten alloy is cooled by a known method, and the composition formula Mm1.0Ni3.6Co0.6Al0.2Mn0.6An ingot of a hydrogen storage alloy represented by The hydrogen storage alloy ingot is mechanically pulverized to form a hydrogen storage alloy powder having an average particle size of about 100 μm. A binder such as polyethylene oxide and an appropriate amount of water are added to the hydrogen storage alloy powder and mixed. A hydrogen storage alloy paste is prepared. This paste is applied to a punching metal, dried, and then rolled to a thickness of 0.4 mm to produce a hydrogen storage alloy negative electrode.
[0036]
4). Battery fabrication
Each non-sintered nickel positive electrode produced as described above (cut to a predetermined size so that the nickel hydroxide active material is about 5 g) and the hydrogen storage alloy negative electrode were placed through a polypropylene nonwoven fabric separator. After rotating to produce a spiral electrode group, this electrode group is inserted into an outer can. Thereafter, an aqueous potassium hydroxide solution is injected as an electrolyte into the outer can, and the outer can is further sealed to assemble AA size nickel-hydrogen storage batteries having a nominal capacity of 1250 mAH.
[0037]
5. test
(1) Unit active material capacity
Each nickel-hydrogen storage battery produced as described above was charged for 16 hours at a charging current of 125 mA (0.1 C) at an ambient temperature of 25 ° C., and then the battery voltage was 1.0 V with a discharging current of 625 mA (0.5 C). When the discharge capacity per unit of nickel hydroxide active material (unit active material capacity) was determined from the discharge time at this time, the results shown in Table 1 below were obtained. In Table 1, the unit active material capacity of the battery using the active material of Example 1 was determined as 100.
[0038]
(2) Capacity at high rate discharge
The nickel-hydrogen storage battery manufactured as described above was charged at an ambient temperature of 25 ° C. with a charging current of 125 mA (0.1 C) for 16 hours, and then the battery voltage reached 1.0 V with a discharging current of 2500 mA (2 C). When discharging, the discharge capacity is obtained from the discharge time at this time, and the capacity ratio with respect to the unit active material capacity described above is obtained as a high-rate discharge capacity from the following formula 1, the results shown in Table 1 below are obtained. It became. In Table 1, the high-rate discharge capacity of the battery using the active material of Example 1 was determined as 100.
[0039]
[Expression 1]
Capacity at high rate discharge = (discharge capacity at high rate discharge / unit active material capacity) × 100
[0040]
(3) High temperature storage capacity
The nickel-hydrogen storage battery produced as described above was allowed to stand for 2 weeks at an ambient temperature of 60 ° C., charged with a charging current of 125 mA (0.1 C) for 16 hours, and then charged with a discharging current of 625 mA (0.5 C). The discharge capacity is obtained from the discharge time at this time, and the capacity ratio to the normal discharge capacity is obtained as the high-temperature storage capacity from the following equation (2). The result was as shown. In Table 1, the capacity of the battery using the active material of Example 1 was determined as 100 when stored at high temperature.
[Expression 2]
Capacity at high temperature storage = (discharge capacity at high temperature storage / normal discharge capacity) × 100
[0041]
[Table 1]
As is clear from Table 1 above, it can be seen that the unit active material capacity and the high rate discharge capacity of the batteries using the active materials of Comparative Examples 1 to 4 are reduced. This can be caused by the following causes. That is, in the case of the battery using the active material of Comparative Example 1, a part of nickel hydroxide was changed to nickel oxyhydroxide by an oxidizing agent and added to the nickel hydroxide compound having an average valence of 2.2. Cobalt hydroxide is oxidized by oxygen before the battery is initially charged, and converted into cobalt oxide having low conductivity according to the above-described reaction formulas (1) to (3). It is thought that the hourly capacity has decreased significantly.
[0043]
Further, in the case of a battery using the active material of Comparative Example 2, a part of nickel hydroxide was changed to nickel oxyhydroxide by an oxidizing agent, and the nickel hydroxide compound having an average valence of 2.2 was hydroxylated. This is probably because cobalt was not completely oxidized because it was only coated with cobalt, and it was changed to a cobalt oxide having low conductivity like the active material of Comparative Example 1. This is because the deposition reaction is controlled to coat cobalt hydroxide, so if the deposition rate is slow, the added cobalt ions cannot be coated with a predetermined amount of cobalt hydroxide as cobalt oxyhydroxide ions. When the speed is high, a dark green compound (the structural formula of this compound is unknown, but it is thought that a high-order cobalt compound with low conductivity has precipitated due to lack of oxygen) is produced, and the deposition conditions are adjusted Even so, the capacity decreased.
When ultrasonic waves were applied to the active material, the cobalt compound on the surface was in a state where it was easily peeled off. This is considered because the atomic exchange between the cobalt layer and the nickel layer did not occur because the oxidation proceeded simultaneously with nickel and cobalt.
[0044]
Further, in the case of the battery using the active material of Comparative Example 3, a part of nickel hydroxide was changed to nickel oxyhydroxide by an oxidizing agent, and the nickel hydroxide compound having an average valence of 2.2 was hydroxylated. Since cobalt is coated and then oxidized by an oxidizing agent, a cobalt compound with low conductivity is generated, and the capacity is considered to be reduced. Experience has shown that the difference between a cobalt compound with high conductivity and a cobalt compound with low conductivity is due to whether or not it goes through a process of dissolution → oxidation → precipitation.
And since the cobalt oxidized by the oxidizing agent has not gone through the process of dissolution → oxidation → precipitation, it is considered that the conductivity is low and as a result, the capacity is reduced. Moreover, when the ultrasonic wave was applied like the active material of the comparative example 2, the surface cobalt compound was in the state which peeled easily. This is considered because the atomic exchange between the cobalt layer and the nickel layer did not occur because the oxidation proceeded simultaneously with nickel and cobalt.
[0045]
Moreover, even in the case of the battery using the active material of Comparative Example 4, since the cobalt oxyhydroxide present on the surface of the nickel oxyhydroxide does not contain an alkali cation, it is affected by oxidation with an oxidizing agent, As described above, the conductivity is low, and it is considered that the capacity is reduced as a result.
[0046]
On the other hand, in the case of the batteries using the active materials of Examples 1 and 2, both the unit active material capacity and the high rate discharge capacity were increased. This includes alkali cationsHigher than bivalentSince the nickel hydroxide compound with the cobalt compound coated on the surface is made higher by an oxidizing agent in an alkaline aqueous solution, when nickel hydroxide is made higher, it goes through a process of dissolution → oxidation → precipitation. This is probably because substitution occurred in the crystal structure with the cobalt compound on the surface.
[0047]
In other words, when divalent nickel is made higher by trioxidative nickel by an oxidizing agent, exchange occurs between trivalent cobalt existing in the vicinity of nickel and atoms in the crystal, and as a result, cobalt layer- It is thought that the boundary of the nickel layer became unclear and the mechanical strength was improved. As the mechanical strength of the hydroxide compound is improved, the electrical resistance between the cobalt layer and the nickel layer is lowered, and this becomes a particularly significant difference during large current discharge. Further, when the nickel electrode is manufactured, the stirring treatment is performed to obtain a slurry, so that it is an important factor to improve the mechanical strength of the hydroxide compound.
[0048]
Moreover, the reason why the battery using the active material of Example 2 has a higher capacity at high temperature storage than the battery using the active material of Example 1 is that the active material of Example 2 contains lithium ions in the nickel hydroxide compound. Therefore, it is considered that this lithium ion is combined with water in the electrolytic solution, and the oxidation by water is suppressed, and the oxygen generation potential is improved and the self-discharge after being left is suppressed.
[0049]
6). Relationship between alkali concentration during oxidation treatment and bulk density of active material
Next, when manufacturing the active materials of Examples 1 and 2, the surface of the nickel hydroxide compound was coated with cobalt hydroxide, and then heat-treated in the presence of an alkaline aqueous solution (sodium hydroxide (NaOH) aqueous solution) -oxygen. Cobalt hydroxide containing a first alkali cation having a disordered crystal structureHigher than bivalentMake cobalt. Thereafter, the concentration and activity of the alkaline aqueous solution are changed by changing the concentration of the alkaline aqueous solution during oxidation treatment for higher order of nickel hydroxide to 5% by weight, 10% by weight, 15% by weight, 20% by weight, and 25% by weight. When the relationship with the bulk density of the substance was measured, the results shown in Table 2 below were obtained.
[0050]
[Table 2]
As is clear from Table 2 above, it is understood that a nickel oxyhydroxide active material having a high bulk density can be obtained when the concentration of the sodium hydroxide (NaOH) aqueous solution is 20% by weight or less. When the active material powder in the case where the concentration of the aqueous sodium hydroxide solution was high was analyzed by X-ray diffraction, the presence of γ-nickel oxyhydroxide could be confirmed. From this, it is considered that γ-nickel oxyhydroxide was generated on the surface of the nickel hydroxide particles and the bulk density was lowered.
[0052]
In the above-described embodiment, a part of the nickel hydroxide compound is made higher-ordered into nickel hydroxide having a higher order than divalent, and the second alkali cation is contained inside the nickel hydroxide having a higher order than divalent. In the above example, the active material is filled into the substrate as a slurry in the filling step. However, after filling the substrate with a non-ordered nickel hydroxide compound as a slurry, The first alkali cation was included even when the second alkali cation was included in the nickel hydroxide higher than divalent, while the part was made higher in the nickel hydroxide higher than divalent.Higher than bivalentSince there is no particular difference in that a part of the nickel hydroxide compound having a cobalt compound on its surface is higher-ordered to nickel hydroxide higher than divalent, the order of the higher-order containing step and the filling step is changed. It may be replaced.
Claims (11)
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| JP4701500B2 (en) * | 2000-12-27 | 2011-06-15 | ソニー株式会社 | Nickel zinc battery |
| JP4765195B2 (en) * | 2001-05-10 | 2011-09-07 | ソニー株式会社 | Positive electrode active material, method for producing positive electrode active material, positive electrode for battery, and battery |
| JP2003045422A (en) * | 2001-07-30 | 2003-02-14 | Hitachi Maxell Ltd | Alkaline storage battery |
| JP2003077469A (en) * | 2001-09-03 | 2003-03-14 | Yuasa Corp | Nickel electrode material, its manufacturing method, nickel electrode and alkaline battery |
| EP1424741A4 (en) * | 2001-09-03 | 2006-10-04 | Yuasa Battery Co Ltd | Nickel electrode material and production method therefor, and nickel electrode and alkaline battery |
| JP4569068B2 (en) * | 2003-02-25 | 2010-10-27 | 株式会社Ｇｓユアサ | Thickener for nickel electrode, nickel electrode and method for producing the same, and alkaline storage battery |
| CN101080831B (en) * | 2004-12-17 | 2011-06-15 | 株式会社杰士汤浅国际 | Alkaline secondary battery-use nickel electrode and production method therefor and alkaline secondary battery |
| EP2665113B1 (en) | 2011-01-11 | 2018-03-14 | GS Yuasa International Ltd. | Positive electrode active material for alkaline storage battery, manufacturing method for positive electrode active material, and alkaline storage battery |
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