JP3842339B2 - Nickel metal hydride secondary battery - Google Patents

Description

【００３５】
前記セパレータを構成する全繊維の平均直径Ｘは、１〜２０μｍにする。セパレータにおいて、目付け量及び厚さを一定にして前記平均直径Ｘを小さくすると、繊維同士がより密に絡まり合うために繊維の目開きが小さくなって目開きの電解液保持力が高まり、電解液保持性が高くなる。しかしながら、前記平均直径Ｘを小さくすると、セパレータの強度が低下する。一方、目付け量及び厚さを一定にして前記平均直径Ｘを大きくすると、強度が向上するものの、繊維同士の絡まり合いが疎になるために繊維の目開きが大きくなって目開きの電解液保持力が低下し、電解液保持性が低下する。また、目開きが大きくなると、セパレータの被覆率が低下する。従って、前記平均直径Ｘを１μｍ未満にすると、前記セパレータの機械的強度の低下が顕著になって電池の組み立てが困難になる恐れがある。一方、前記平均直径が２０μｍを越えると、被覆率の低下による正負極間のショートが多発する場合がある。より好ましい平均直径Ｘは、３〜１５μｍである。
【００３６】
セパレータは、前述したように目付け量及び厚さが一定の場合、平均直径Ｘが大きくなるにつれて繊維の目開きが大きくなるため、目開きの電解液保持力が低下し、電解液保持性が低下する。したがって、セパレータに十分な量の電解液を保持させて高温保管時の自己放電特性を改善させるためには、平均直径Ｘの増加に伴ってセパレータ中のイオン交換基量を増加させることにより繊維自体の親水性を高めて大きな目開きでも電解液を十分に保持できるようにする必要がある。すなわち、高温保管時の自己放電特性を改善を図るために最低限必要なカリウムイオン交換量Ｙは、平均直径Ｘを増加させた際に式（０．０５Ｘ＋０．０５）に従って増加させる必要がある。セパレータのカリウムイオン交換量Ｙが式（０．０５Ｘ＋０．０５）を満たすカリウムイオン交換量よりも少ないと、セパレータの電解液保持性が低下し、高温保管時の自己放電特性が劣化する。一方、前記イオン交換量Ｙが２．０ｍｅｑ／ｇを越えると、セパレータのイオン交換能が高くなり、電解液中のアルカリ金属イオンがセパレータに固定化されるため、このセパレータを備えたニッケル水素二次電池は大電流放電の際の作動電圧が低下する。前記イオン交換量Ｙは、下記（３）式を満たすのがより好ましい。
【００３７】
（０．０５Ｘ＋０．１５）≦Ｙ≦１．２………（３）
前記セパレータ３の厚さは、０．１５ｍｍ〜０．３ｍｍの範囲にすることが好ましい。
【００３８】
前記セパレータ３の目付け量は、３０ｇ／ｍ² 〜７０ｇ／ｍ² の範囲にすることが好ましい。前記目付け量を３０ｇ／ｍ² 未満にすると、前記セパレータ３の強度が低下する恐れがある。一方、前記目付け量が７０ｇ／ｍ² を越えると、電池容量が低下する恐れがある。より好ましい目付け量は、４０ｇ／ｍ² 〜６０ｇ／ｍ² の範囲である。
【００３９】
前記セパレータは、例えば、以下に示す（ａ）〜（ｃ）の方法により作製することができる。
（ａ）平均直径が１〜２０μｍのポリオレフィン系合成樹脂繊維製シート状物をイオン交換能を持つビニルモノマーを含む溶液に浸漬して引き上げた後、前記シート状物にエネルギービームを照射して前記ビニルモノマーをグラフト共重合させる。このような方法によりイオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物から形成され、かつ前記（１）式及び前記（２）式を満たすセパレータが作製される。
【００４０】
（ｂ）平均直径が１〜２０μｍのポリオレフィン系合成樹脂繊維製シート状物にエネルギービームを照射した後、イオン交換能を有するビニルモノマーを含む溶液に前記シート状物を浸漬して前記ビニルモノマーをグラフト共重合させる。このような方法によりイオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物から形成され、かつ前記（１）式及び前記（２）式を満たすセパレータが作製される。
【００４１】
（ｃ）イオン交換能を有するビニルモノマーを含む溶液中に平均直径が１〜２０μｍのポリオレフィン系合成樹脂繊維製シート状物を浸漬すると同時に前記シート状物にエネルギービームを照射して前記ビニルモノマーをグラフト共重合させる。このような方法によりイオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物から形成され、かつ前記（１）式及び前記（２）式を満たすセパレータが作製される。
【００４２】
前記ポリオレフィン系合成樹脂繊維製シート状物としては、例えば、ポリオレフィン系合成樹脂繊維からなる不織布、同繊維からなる織布もしくはこれら不織布および織布で複合化された複合シートを挙げることができる。前記不織布は、例えば乾式法、湿式法、スパンボンド法、メルトブロー法等によって作製される。このような方法のうち、スパンボンド法、メルトブロー法は繊維径の細い不織布を作製することができるため、正極と負極の間のショート防止の点で有利である。
【００４３】
前記複合シートとしては、ポリオレフィン系合成樹脂繊維からなる内部層と、前記内部層の両面に形成され、ポリオレフィン系合成樹脂繊維からなる２枚の表面層とから構成され、かつ前記各表面層を構成するポリオレフィン系合成樹脂繊維の平均直径が前記内部層を構成するポリオレフィン系合成樹脂繊維の平均直径に比べて大きい３層積層物を用いることができる。
【００４４】
前記エネルギービームとしては、例えば、例えば紫外線、電子線、Ｘ線のような電離放射線が採用される。
前記セパレータは、第１面が親水性で、かつこの第１面の反対側の第２面が親水部と疎水部を有するようなイオン交換基分布を有していても良い。このようなセパレータは前記正極と前記負極との間に前記第２面が前記負極側に位置するように配置される。前記親水部は前記セパレータの表面のうちイオン交換基が存在する領域であり、また、前記疎水部は前記セパレータ表面のうちイオン交換基が存在しない領域である。
【００４５】
前記セパレータの疎水部は、例えば前記シート状物にエンボス加工を施して部分的にフィルム化するようなイオン交換基を有するビニルモノマーのグラフト共重合が阻害される処理を施すことにより形成することができる。前記エンボス加工は、円形、四角形のような角形状、ストライプ状等任意である。また、前記セパレータの疎水部は前記不織布、織布または複合シートに繊維径の太いポリオレフィン径樹脂繊維の織布を張り合わせることによっても形成される。
【００４６】
前記セパレータの第２面は、前記疎水部の面積比率が２〜２５％であることが好ましい。前記疎水部の面積比率を２％未満にすると、疎水部を設けたことによる過充電時の内圧上昇を抑制する効果が見られない恐れがある。一方、前記疎水部の面積比率が２５％を越えると、このセパレータを備えた二次電池は大電流放電時の作動電圧が低下する恐れがある。より好ましい疎水部の面積比率は、８〜２０％である。
【００４７】
前記セパレータの前記第１面は、疎水部が面積比率で１０％以下存在していても良い。第１面において、疎水部が存在する場合にはその面積比率が前記第２面の疎水部の面積比率より小さくする必要がある。また、前記セパレータの第１面は、全面にイオン交換基が存在していても良い。このような形態のセパレータをペースト式正極と負極の間に前記第２面が負極側、つまり前記第１面が正極側に位置するように配置することによって、負極側に比べて正極側のセパレータ面（第１面）にアルカリ電解液を多く保持することができる。その結果、前記正極表面には電解液膜が存在するため、ニッケル水素二次電池の高温保管時において、前記負極から発生した水素ガスが前記セパレータを透過して正極に到達するのを前記電解液膜により防止することができ、高温保管時の自己放電特性を大幅に向上することができる。
【００４８】
イオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物から形成され、第１面が親水性を示すと共に第２面が親水部と疎水部を有し、かつ前記（１）式及び前記（２）式を満たすセパレータは、例えば、平均直径が１〜２０μｍのポリオレフィン系合成樹脂繊維からなるシート状物の第１面にエンボス加工を施してエンボス面をフィルム化する工程と、前記シート状物をイオン交換能を持つビニルモノマーを含む溶液に浸漬して、その第１面と反対側の第２面のフィルム化された部分を除く前記シート状物両面に前記溶液を付着させる工程と、前記シート状物にエネルギービームを照射して前記溶液の付着面において前記ビニルモノマーをグラフト共重合させる工程とを具備する方法により作製することができる。
【００４９】
具体的には、例えば次の（１）に説明する方法により作製することができる。
（１）まず、図２〜図４に示すように平均直径が１〜２０μｍのポリオレフィン系合成樹脂繊維からなるシート状物（例えば不織布）２１の両面（第１面２２、第２面２３）のうち、第１面２２側からエンボス加工を施す。このエンボス加工により、前記シート状物に複数の円形状凹部２４が形成されると共に前記凹部２４の底面（エンボス面）にフィルム化された部分（フィルム部）２５が形成される。同時に、前記凹部２４に対応する前記シート状物２１の前記第２面２３の部分も、フィルム化された面になる。
【００５０】
次いで、前記シート状物をイオン交換能を持つビニルモノマーを含む溶液に浸漬して引き上げる。この時、前記シート状物２１は前述した図３に示すように前記凹部２４底面にフィルム部２５が形成されているため、前記フィルム部２５の両面には前記溶液が付着されず、前記両面以外の領域、つまり不織布部分に前記溶液を付着される。ただし、前記フィルム部２５の凹部２４側の面２２においては、前記凹部２４内に前記溶液が充満されるために、実質的に前記凹部２４側の面には前記溶液が接触されるようになる。
【００５１】
次いで、前記シート状物にエネルギービームを照射して前記溶液の付着面において前記ビニルモノマーをグラフト共重合させる。このような工程により前記凹部側のシート状物の第１面全体は前記ビニルモノマーがグラフト共重合されて親水化される。前記シート状物の第２面のうち、前記凹部底部のフィルム部では前記溶液が付着さず、これ以外の第２面の領域に前記溶液が付着されている。その結果、前記シート状物の第２面において前記凹部底部のフィルム部の領域が疎水部となり、他の領域が親水部となる。
【００５２】
したがって、前述した工程によってイオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物からなり、前記シート状物の第１面が親水性を示し、この第１面と反対側の第２面が親水部と疎水部を有し、かつ前述した関係式を満たすセパレータが作製される。
【００５３】
前記シート状物の第１面側へのエンボス加工は、例えば互いに反対方向に回転して対向配置された表面が平滑な第１ロールおよび表面に複数のピンポイント形状の突起が形成された第２ロールを用意し、これらロールを例えば１２０℃〜１８０℃に加熱すると共にこれらロール間に前記シート状物を供給して前記第１ロール表面と前記第２ロールの複数の突起との間で前記シート状物のポリオレフィン系合成樹脂繊維を加圧して熱融着することによりなされる。前記第２ロール表面に形成された突起の形状は、ピンポイントの他に例えば先端面の形状が正方形のもの、先端面の形状が菱形のものを挙げることができる。
【００５４】
前記エネルギービームとしては、前述したのと同様なものが用いられる。
また、前記セパレータは次の（２）に示す方法によって作製することができる。
【００５５】
（２）まず、ポリオレフィン系合成樹脂繊維からなる不織布の片面に径の太いポリオレフィン系合成樹脂繊維からなり、かつ目開きの大きな織布を貼着してシート状物とする。このシート状物において、前記織布の未貼着面が第１面、前記織布の貼着面が第２面とする。つづいて、前記シート状物をイオン交換基を有するビニルモノマーを含む溶液に浸漬して引き上げる。この時、前記シート状物の第２面に貼着された織布には、前記溶液が付着されず、前記織布の目開き部に位置する前記不織布と、前記第１面に表出された不織布とに前記溶液が付着される。ひきつづき、前記シート状物に前述した紫外線のようなエネルギービームを照射すると、前記溶液の付着面において前記ビニルモノマーをグラフト共重合させる。このような工程において、前記溶液が全面に付着された前記シート状物の第１面では、前記ビニルモノマーのグラフト共重合により親水化される。一方、前記織布が貼着されたシート状物の第２面では前記織布の目開き部に位置する不織布に前記溶液が付着されているため、その部分のみで前記ビニルモノマーのグラフト共重合がなされる。つまり、前記シート状物の第２面においては前記織布の領域が疎水部となり、他の領域が親水部となる。
【００５６】
したがって、前述した工程にによりポリオレフィン系合成樹脂繊維を含むシート状物から形成され、かつ親水性の第１面と、親水部と疎水部とを持つ第２面とを有し、前記（１）式及び前記（２）式を満たすセパレータが作製される。
【００５７】
２）アルカリ電解液
前記電解液は、水酸化リチウム（ＬｉＯＨ）または水酸化ナトリウム（ＮａＯＨ）の少なくとも一方を含む。このような電解液としては、例えば、水酸化リチウム水溶液、水酸化ナトリウム水溶液、水酸化カリウム（ＫＯＨ）及び水酸化リチウムの混合液、水酸化カリウム及び水酸化ナトリウムの混合液、水酸化カリウム、水酸化リチウム及び水酸化ナトリウムの混合液、水酸化リチウム及び水酸化ナトリウムの混合液を挙げることができる。
【００５８】
前記電解液が少なくとも水酸化リチウムを含む場合、前記電解液中の水酸化リチウムの濃度は０．１Ｎ〜１．５Ｎの範囲が好ましい。前記電解液中の水酸化リチウムの濃度が前記範囲を外れると、ニッケル水素二次電池の高温保管時の自己放電特性が低下する恐れがある。より好ましい濃度は、０．３Ｎ〜１．３Ｎの範囲である。
【００５９】
前記電解液が少なくとも水酸化ナトリウムを含む場合、前記電解液中の水酸化ナトリウムの濃度は０．５Ｎ〜６．０Ｎの範囲が好ましい。前記電解液中の水酸化ナトリウムの濃度が前記範囲を外れると、ニッケル水素二次電池の高温保管時の自己放電特性が低下する恐れがある。より好ましい濃度は、１．０Ｎ〜５．０Ｎの範囲である。
【００６０】
本発明に係るニッケル水素二次電池は、水酸化ニッケルを含む正極と、水素吸蔵合金を含む負極と、前記正極および前記負極の間に介在されたセパレータと、アルカリ電解液を具備し、前記セパレータはイオン交換基を有するポリオレフィン系合成樹脂繊維を含むシート状物から形成され、前記セパレータを構成する全繊維の平均直径をＸμｍとし、前記セパレータのカリウムイオン交換量をＹｍｅｑ／ｇとした際に、これらは下記（１）式及び（２）式を満たし、
（０．０５Ｘ＋０．０５）≦Ｙ≦２．０ （１）
１≦Ｘ≦２０ （２）
前記アルカリ電解液は水酸化リチウムまたは水酸化ナトリウムの少なくとも一方を含む。
【００６１】
前記（１）式及び前記（２）式を満足するセパレータと、水酸化ナトリウムまたは水酸化リチウムの少なくとも１方を含むアルカリ電解液とを備えることによって、ニッケル水素二次電池の高温保管時における自己放電の進行を抑制することができる。同時に、前記二次電池は、大電流放電特性及び充放電サイクル寿命を向上することができる。
【００６２】
更に、前記セパレータのイオン交換基分布を第１面が親水性で、第２面が親水部及び疎水部を有するものにし、このような構成のセパレータを正極および負極の間に前記第１面が前記正極側に接するように配置すると、前記二次電池の高温保管時の自己放電特性を更に改善することができる。
【００６３】
ニッケル水素二次電池を高温環境下において保管すると、負極の水素吸蔵合金の平衡圧に応じて電池内の水素分圧が高くなる場合がある。電池内の水素分圧が上昇すると、水素ガスがセパレータを透過して正極に到達し、前記正極のＮｉＯＯＨを還元する場合がある、つまり自己放電が進行する場合がある。前述した形態及び配置を有するセパレータは正極側の電解液量を負極側に比べて多くすることができるため、前記正極表面に電解液が多く存在する層、つまり電解液膜が存在し、この電解液膜によって前記水素ガスが前記セパレータを透過して前記正極に到達するのを防止できる。その結果、高温保管時の自己放電特性を大幅に向上することができる。
【００６４】
また、前記セパレータは、親水部と疎水部を有する第２面が負極表面と接するため、負極表面に撥水性を付与することができる。このため、前記負極は、この表面と表面上の電解液と酸素ガスとで形成され、過充電時に正極から発生する酸素ガスを消費する三相界面の量を増加させることができる。このような負極は、過充電時の酸素ガス消費速度を増大することができるため、前記二次電池の過充電時の内圧上昇を抑制することができる。また、前記三相界面は逆充電時に前記正極から発生する水素ガスの吸収速度も増加させることができるため、三相界面が増加することによって逆充電時の内圧上昇を抑えることができる。
【００６５】
したがって、前述した構成及び配置のセパレータは、電解液の分布を正極側で多くし、前記負極側では疎水部が出現するように制御することができるため、高温保管時の自己放電特性が向上され、かつ過充電時の内圧上昇が抑制されたニッケル二次電池を提供することができる。
【００６６】
なお、前述した図１では正極２と負極４の間にセパレータ３を介在して渦巻状に捲回し、有底円筒状の容器１内に収納したが、本発明のニッケル水素二次電池はこのような構造に限定されない。例えば、ペースト式正極と負極とをその間に前記セパレータを介在して交互に重ねることによって作製された積層物を有底矩形筒状の容器内に収納して角形ニッケル水素二次電池を構成してもよい。
【００６７】
【実施例】
以下、本発明の好ましい実施例を図面を参照して詳細に説明する。
（実施例１）
＜ペースト式負極の作製＞
市販のランタン富化したミッシュメタルＭｍおよびＮｉ、Ｃｏ、Ｍｎ、Ａｌを用いて高周波炉によって、ＭｍＮｉ_3.6 Ｃｏ_0.8 Ｍｎ_0.4 Ａｌ_0.2 の組成からなる水素吸蔵合金を作製した。前記水素吸蔵合金を機械粉砕し、これを２００メッシュの篩を通過させた。得られた水素吸蔵合金粉末１００重量部に対してポリアクリル酸ナトリウム０．４重量部、カルボキシメチルセルロース（ＣＭＣ）０．１重量部、ポリテトラフルオロエチレンのディスパージョン（比重１．５，固形分６０ｗｔ％）１．５重量部および導電材としてカーボン粉末０．８重量部を水５５重量部と共に混合することによって、ペーストを調製した。このペーストをパンチドメタルに塗布、乾燥した後、加圧成型することによってペースト式負極を作製した。
【００６８】
＜ペースト式正極の作製＞
水酸化ニッケル粉末９０重量部および水酸化コバルト粉末１０重量部からなる混合粉体に、前記水酸化ニッケル粉末に対してカルボキシメチルセルロース０．３重量部、ポリテトラフルオロエチレンの懸濁液（比重１．５，固形分６０重量％）を固形分換算で１．０重量部添加し、これらに純水を３０重量部添加して混練することによりペーストを調製した。つづいて、このペーストをニッケルメッキ繊維基板内に充填した後、更にその両表面に前記ペーストを塗布し、乾燥し、ローラプレスを行って圧延することによりペースト式正極を作製した。
【００６９】
＜セパレータの作製＞
ポリプロピレン樹脂をスパンボンド法を用いて平均直径が１０μｍの長繊維からなり、目付け量が５０ｇ／ｍ² で、厚さが０．２０ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させ、各不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムのイオン交換量が０．８ｍｅｑ／ｇのセパレータを作製した。
【００７０】
前述した（１）式、（０．０５Ｘ＋０．０５）≦Ｙ≦２．０によると、前記セパレータを構成する全繊維の平均直径Ｘが１０μｍであるとき、高温保管時の自己放電特性及び大電流放電特性を優れたものにするのに必要なカリウムイオン交換量Ｙの範囲は０．５５≦Ｙ≦２．０になる。従って、得られたセパレータは平均直径Ｘとカリウムイオン交換量Ｙが前述した式（１）及び式（２）；１≦Ｘ≦２０を同時に満たしている。
【００７１】
次いで、前記セパレータを前記負極と前記正極との間に介装し、渦巻状に捲回して電極群を作製した。このような電極群と０．１ＮのＬｉＯＨおよび７．９ＮのＫＯＨからなる電解液を有底円筒状容器に収納して前述した図１に示す構造を有するＡＡサイズの円筒形ニッケル水素二次電池を組み立てた。
【００７２】
（実施例２）
アルカリ電解液として０．３ＮのＬｉＯＨおよび７．７ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７３】
（実施例３）
アルカリ電解液として０．５ＮのＬｉＯＨおよび７．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７４】
（実施例４）
アルカリ電解液として１．０ＮのＬｉＯＨおよび７．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７５】
（実施例５）
アルカリ電解液として１．３ＮのＬｉＯＨおよび６．７ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７６】
（実施例６）
アルカリ電解液として１．５ＮのＬｉＯＨおよび６．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７７】
（実施例７）
アルカリ電解液として０．５ＮのＮａＯＨおよび７．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７８】
（実施例８）
アルカリ電解液として１．０ＮのＮａＯＨおよび７．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００７９】
（実施例９）
アルカリ電解液として２．０ＮのＮａＯＨおよび６．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００８０】
（実施例１０）
アルカリ電解液として４．０ＮのＮａＯＨおよび４．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００８１】
（実施例１１）
アルカリ電解液として５．０ＮのＮａＯＨおよび３．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００８２】
（実施例１２）
アルカリ電解液として６．０ＮのＮａＯＨおよび２．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００８３】
（比較例１）
アルカリ電解液として８．０ＮのＫＯＨ水溶液を用いること以外は、実施例１と同様なニッケル水素二次電池を組み立てた。
【００８４】
（比較例２）
ポリプロピレン繊維からなる芯材の表面にポリエチレン樹脂が被覆された芯鞘型を有し、平均直径が１５μｍの複合繊維と、ポリプロピレン繊維からなる芯材の表面にエチレン−ビニルアルコール共重合樹脂が被覆された芯鞘型構造を有し、平均直径が１５μｍの複合繊維を混合し、乾式法によって、目付け量が５０ｇ／ｍ² 、厚さが０．２０ｍｍの不織布からなるセパレータを作製した。得られたセパレータのカリウムイオン交換量を前述した滴定法によって測定したところ、０ｍｅｑ／ｇであった。
【００８５】
次いで、前記セパレータを実施例１と同様な負極と正極との間に介装し、渦巻状に捲回して電極群を作製した。このような電極群と８．０ＮのＫＯＨからなる電解液を有底円筒状容器に収納して前述した図１に示す構造を有するＡＡサイズの円筒形ニッケル水素二次電池を組み立てた。
【００８６】
得られた実施例１〜１２および比較例１，２の二次電池について、２０℃の雰囲気において１ＣｍＡで１５０％充電した後、１ＣｍＡで電池電圧が１．０Ｖに達するまで放電する充放電サイクルを３回繰り返した。その後、１ＣｍＡで１５０％充電した状態で２０℃で２時間放置した後、４０℃の恒温槽に１４日間保管した。貯蔵後、２０℃で２時間放置し、１ＣｍＡで電池電圧が１．０Ｖに達するまで放電し、放電容量（残存容量）を測定した。４０℃の恒温槽で１４日間保管する前の１ＣｍＡで１５０％充電し、１ＣｍＡで電池電圧が１．０Ｖまで放電した時の放電容量をＣ´₀ とし、４０℃の恒温槽で１４日間保管した後の放電容量を残存容量Ｃ´_R とした時、容量残存率を下記式から求めた。この容量残存率から自己放電特性を判断した。
【００８７】
容量残存率（％）＝（Ｃ´_R ／Ｃ´₀ ）×１００
実施例１〜６及び比較例１、２の二次電池における電解液中の水酸化リチウム濃度と容量残存率との関係を図５に示す。また、実施例７〜１２及び比較例１、２の二次電池における電解液中の水酸化ナトリウム濃度と容量残存率との関係を図６に示す。
【００８８】
図５及び図６から明らかなように、実施例１〜１２の二次電池は、比較例１、２の二次電池に比べて４０℃の高温保管時の自己放電を抑制できることがわかる。このことから、ニッケル水素二次電池の高温保管時の自己放電特性を改善するには、前述した２つの式を満足するセパレータを用い、同時に水酸化リチウムまたは水酸化ナトリウムの少なくとも１方を含むアルカリ電解液を用いることが重要であることがわかる。
【００８９】
（実施例１３）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが１μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作り、この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。この不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなるカリウムイオン交換量が０．１ｍｅｑ／ｇのセパレータを作製した。なお、カリウムのイオン交換量は、前述した滴定法によって測定した。
【００９０】
次いで、前記セパレータを実施例１と同様な負極と実施例１と同様な正極との間に介装し、渦巻状に捲回して電極群を作製した。このような電極群と７ＮのＫＯＨおよび１ＮのＬｉＯＨからなる電解液を有底円筒状容器に収納して前述した図１に示す構造を有するＡＡサイズの円筒形ニッケル水素二次電池を組み立てた。
【００９１】
（実施例１４）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが１μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作った。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムイオン交換量が１．０ｍｅｑ／ｇのセパレータを作製した。このセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
【００９２】
（実施例１５）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが１μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作った。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムのイオン交換量が２．０ｍｅｑ／ｇのセパレータを作製した。このセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
【００９３】
前述した（１）式によると、セパレータを構成する全繊維の平均直径Ｘが１μｍであるとき、高温保管時の自己放電特性及び大電流放電特性を向上するために必要な前記カリウムイオン交換量Ｙの範囲は０．１≦Ｙ≦２．０になる。従って、得られた実施例１３〜１５の二次電池のセパレータは平均直径Ｘとカリウムイオン交換量Ｙが前述した（１）式及び（２）式を満たしている。
（実施例１６）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２０μｍで、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が１．１ｍｅｑ／ｇセパレータを作製した。
【００９４】
このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（実施例１７）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２０μｍで、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が１．５ｍｅｑ／ｇセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（実施例１８）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２０μｍで、目付け量が５０ｇ／ｍ² で、厚さが０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が２．０ｍｅｑ／ｇセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
【００９５】
前述した（１）式によると、セパレータを構成する全繊維の平均直径Ｘが２０μｍであるとき、高温保管時の自己放電特性及び大電流放電特性を向上するために必要な前記カリウムイオン交換量Ｙの範囲は１．０≦Ｙ≦２．０になる。従って、得られた実施例１６〜１８の二次電池のセパレータは平均直径Ｘとカリウムイオン交換量Ｙが前述した（１）式及び（２）式を満たしている。
（比較例３）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが１μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作り、この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムのイオン交換量が０．０５ｍｅｑ／ｇのセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（比較例４）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが１μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作り、この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムのイオン交換量が２．５ｍｅｑ／ｇのセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（比較例５）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２０μｍで、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が０．６ｍｅｑ／ｇのセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（比較例６）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２０μｍで、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が２．５ｍｅｑ／ｇのセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
（比較例７）
ポリプロピレン樹脂からメルトブロー法により平均直径Ｘが０．５μｍのポリプロピレン繊維からなり、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作り、この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維を主体とする不織布からなり、前述した滴定法によるカリウムのイオン交換量が１．０ｍｅｑ／ｇのセパレータを作製した。このようなセパレータは、強度不足のために正極と負極との間に介装されて渦巻状に捲回される際に破断したため、このセパレータから二次電池を作製できなかった。
（比較例８）
ポリプロピレン樹脂にブテンを共重合させた共重合樹脂からなる繊維を作り、この繊維５０重量％とポリプロピレン樹脂繊維５０重量％とを混合し、湿式法により平均直径Ｘが２５μｍで、目付け量が５０ｇ／ｍ² で、厚さ０．２ｍｍの不織布を作製した。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を有する共重合樹脂繊維を主体とする不織布から形成され、前述した滴定法によるカリウムのイオン交換量が１．０ｍｅｑ／ｇのセパレータを作製した。このようなセパレータを用いること以外は、実施例１３と同様なニッケル水素二次電池を組み立てた。
【００９６】
得られた実施例１３〜１８および比較例３，５の二次電池について、前述した実施例１〜１２の二次電池について行ったのと同様な方法により４０℃での自己放電特性を測定し、その結果を図７〜８に示す。
【００９７】
図７から明らかなように、本発明の実施例１３〜１５の二次電池は比較例３の二次電池に比べて高温保管時の自己放電特性が向上されることがわかる。また、図８から明らかなように、本発明の実施例１６〜１８の二次電池は比較例５の二次電池に比べて高温保管時の自己放電特性が向上されることがわかる。
【００９８】
また、実施例１３〜１８及び比較例８の二次電池について、電極群作製後の絶縁不良発生率を測定した。電極群作製後の絶縁不良の測定は、電極群を１００個作製した直後に絶縁抵抗計によって５００Ｖの電圧を印加した際の絶縁抵抗値を測定することによって行った。その結果を下記表１に示す。
【００９９】
【表１】

【０１００】
表１から明らかなように、本発明の実施例１３〜１８の二次電池は、電極群作製時に絶縁抵抗不良が発生するのを回避できることがわかる。これは、実施例１３〜１８に使用されているセパレータが緻密であるために捲回時の正負極間のショートを防止できるからである。
【０１０１】
さらに、実施例１３〜１８及び比較例４，６の二次電池について、０．３ＣｍＡで１５０％充電した後、２ＣｍＡで電池電圧が１．０Ｖに達するまで放電した際の放電容量を測定し、大電流放電特性を評価した。放電容量は、２ＣｍＡ放電で電池電圧が１．０Ｖに達するまでの時間から算出した。
【０１０２】
前記大電流放電特性試験の結果を下記表２及び表３に示す。表２においては、実施例１３の放電容量を１００として実施例１４〜１５及び比較例４の放電容量を表した。また、表３においては、実施例１６の放電容量を１００として実施例１７〜１８及び比較例６の放電容量を表した。
【０１０３】
【表２】

【０１０４】
【表３】

【０１０５】
表２から明らかなように、本発明の実施例１３〜１５の二次電池は、比較例４に比べて大電流放電特性が優れていることがわかる。また、表３から明らかなように、本発明の実施例１６〜１８の二次電池は、比較例６に比べて大電流放電特性が優れていることがわかる。これは、比較例４，６の二次電池に用いられるセパレータのイオン交換量が高いため、電解液中のアルカリ金属イオンがセパレータに固定化され、大電流放電時の作動電圧が低下したことによるものである。
（実施例１９）
ポリプロピレン樹脂をスパンボンド法を用いて繊維径が１０μｍの長繊維からなり、目付け量が５０ｇ／ｍ² で、厚さが０．２０ｍｍの不織布を作製した。つづいて、表面が平滑な第１ロールと、表面に複数のピンポイント状の凹凸が形成された第２ロールとを相互に対向して配置し、これらロールを互いに反対方向に回転させると共に１３０℃に加熱した後、これらロール間に前記不織布を通過させて前記第１ロールと前記第２ロールの凸部とで加圧すると共に熱融着させてエンボス加工を施した。このエンボス加工により、前記シート状物に複数の円形状凹部が形成されると共に前記凹部の底面（エンボス面）にフィルム化された部分（フィルム部）が形成される。同時に、前記凹部に対応する前記シート状物の前記第２面の部分も、フィルム化された面になる。前記フィルム部の面積比率は、前記不織布の第２面に対して１６％とした。ひきつづき、前記不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させた。この不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによりセパレータを作製した。
【０１０６】
得られたセパレータは、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維及びＣＯＯＨ基を持たないポリプロピレン繊維を主体とする不織布であって、エンボス加工された第１面が親水化され、かつ前記第１面と反対側の第２面のうちフィルム部以外の領域が親水化され、前記フィルム部が疎水部として残存した。前記第２面の疎水部は面積率１６％であった。また、前記セパレータの親水部について、アクリル酸モノマーのグラフト共重合割合を前述した滴定法により測定した。その結果、カリウムのイオン交換量は０．８ｍｅｑ／ｇであった。前述した（１）式は、セパレータを構成する全繊維の平均直径が１０μｍの時、０．５５≦Ｙ≦２．０となる。従って、前記セパレータは、前記（１）式及び前記（２）式を同時に満足する。
【０１０７】
次いで、前記セパレータを実施例１と同様な負極と正極との間に前記セパレータの親水部と疎水部を有する第２面が前記負極と対向するように介装し、渦巻状に捲回して電極群を作製した。このような電極群と０．１ＮのＬｉＯＨおよび７．９ＮのＫＯＨからなる電解液を有底円筒状容器に収納して前述した図１に示す構造を有するＡＡサイズの円筒形ニッケル水素二次電池を組み立てた。
【０１０８】
（実施例２０）
アルカリ電解液として０．３ＮのＬｉＯＨおよび７．７ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１０９】
（実施例２１）
アルカリ電解液として０．５ＮのＬｉＯＨおよび７．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１０】
（実施例２２）
アルカリ電解液として１．０ＮのＬｉＯＨおよび７．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１１】
（実施例２３）
アルカリ電解液として１．３ＮのＬｉＯＨおよび６．７ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１２】
（実施例２４）
アルカリ電解液として１．５ＮのＬｉＯＨおよび６．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１３】
（実施例２５）
アルカリ電解液として０．５ＮのＮａＯＨおよび７．５ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１４】
（実施例２６）
アルカリ電解液として１．０ＮのＮａＯＨおよび７．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１５】
（実施例２７）
アルカリ電解液として２．０ＮのＮａＯＨおよび６．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１６】
（実施例２８）
アルカリ電解液として４．０ＮのＮａＯＨおよび４．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１７】
（実施例２９）
アルカリ電解液として５．０ＮのＮａＯＨおよび３．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１８】
（実施例３０）
アルカリ電解液として６．０ＮのＮａＯＨおよび２．０ＮのＫＯＨからなる混合液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１１９】
（比較例９）
アルカリ電解液として８．０ＮのＫＯＨ水溶液を用いること以外は、実施例１９と同様なニッケル水素二次電池を組み立てた。
【０１２０】
得られた実施例１９〜３０および比較例９の二次電池について、前述した実施例１〜１２の二次電池について行ったのと同様な方法により４０℃での自己放電特性を測定し、その結果を図９〜１０に示す。なお、図９及び図１０には比較例２の結果を併記する。
【０１２１】
実施例１９〜２４及び比較例２、９の二次電池における電解液中の水酸化リチウム濃度と容量残存率との関係を図９に示す。また、実施例２５〜３０及び比較例２、９の二次電池における電解液中の水酸化ナトリウム濃度と容量残存率との関係を図１０に示す。
【０１２２】
図９から明らかなように、実施例１９〜２４の二次電池は、比較例２、９の二次電池に比べて４０℃の高温保管時の自己放電を抑制できることがわかる。また、図１０から明らかなように、実施例２５〜３０の二次電池は、比較例２、９の二次電池に比べて４０℃の高温保管時の自己放電を抑制できることがわかる。更に、図５と図９、図６と図１０を比較することによって、前記（１）式及び前記（２）式を満たすシート状物であって、かつ第１面が親水性で、第２面が親水部と疎水部を有するセパレータを備え、前記セパレータが正極と負極の間に前記第２面が前記負極と対向するように配置されたニッケル水素二次電池は、両面のイオン交換基量が等しく、かつ前記（１）式及び前記（２）式を満たすセパレータを備えたニッケル水素二次電池に比べて高温保管時の自己放電特性を向上できることがわかる。
【０１２３】
また、得られた実施例１９〜３０及び比較例１、２の二次電池について、電池内圧を測定した。電池内圧の測定は、実施例１９〜３０及び比較例１、２の電池を図１１に示す圧力測定装置の容器内に収納して行った。
【０１２４】
すなわち、各電池内圧測定装置はアクリル樹脂製のケース本体３１とキャップ３２とからなる電池ケースを備える。前記ケース本体３１の中心部には、ＡＡサイズの電池の金属容器と同一の内径および高さを有する空間３３が形成されている。前記空間３３内部には、前記二次電池Ｃが収納されている。前記二次電池Ｃは、有底円筒形容器の上端に封口板が取り付けられずに開放されている。前記ケース本体３１上には、前記キャップ３２がパッキング３４およびＯリング３５を介してボルト３６およびナット３７により気密に固定されている。前記キャップ３２には、圧力検出器３８が取り付けられている。負極からの負極リード３９と正極からの正極リード４０は前記パッキング３４と前記Ｏリング３５との間を通して導出されている。
【０１２５】
このような電池内圧測定装置により実施例１９〜３０及び比較例１、２の二次電池について０．５ＣｍＡの電流で４８０％充電した際の最大電池内圧を測定し、その結果を下記表４、５に示す。
【０１２６】
【表４】

【０１２７】
【表５】

【０１２８】
表４から明らかなように、実施例１９〜２４の二次電池は、比較例１、２の二次電池に比べて過充電時の最大電池内圧が低いことがわかる。また、表５から明らかなように、実施例２５〜３０の二次電池は、比較例１、２の二次電池に比べて過充電時の最大電池内圧が低いことがわかる。
【０１２９】
従って、実施例１９〜３０の二次電池は、高温保管時の自己放電特性を実施例１〜１２の二次電池に比べて向上できるのみならず、過充電時の内圧上昇を抑制できることがわかる。これは、セパレータのイオン交換基量を表裏で異ならせたこと、つまり、第１面をイオン交換基により親水化し、前記第１面の反対側の第２面がイオン交換基による親水部とイオン交換基非形成領域である疎水部を有する構成にし、このようなセパレータを負極と正極との間に前記第２面が前記負極と対向するように介装したことに起因する。
【０１３０】
（実施例３１）
ポリプロピレン樹脂からメルトブロー法によって、平均直径が３μｍのポリプロピレン繊維からなり、目付け量が１８ｇ／ｍ² の不織布（内部層）を１枚作製した。また、ポリプロピレン繊維からなる芯材の表面にポリエチレン樹脂が被覆された芯鞘形構造を有する平均直径が１５μｍの複合繊維から目付け量が１８ｇ／ｍ² の不織布（表面層）を２枚作製した。ポリプロピレン単繊維製不織布を複合繊維製不織布で挟み、これら不織布を熱融着により一体化した。得られた３層構造の不織布を構成する繊維の平均直径は１１μｍになった。また、前記３層構造の不織布の目付け量は５４ｇ／ｍ² で、厚さは０．１５ｍｍであった。この不織布をアクリル酸水溶液に浸漬した後、紫外線を照射してアクリル酸モノマーをグラフト共重合させ、前記不織布を洗浄して未反応のアクリル酸を除去した後、乾燥することによって、セパレータを作製した。
【０１３１】
得られたセパレータは、イオン交換基としてＣＯＯＨ基を有するポリプロピレン繊維製不織布を主体とする内部層と、前記内部層の両面に積層され、ＣＯＯＨ基を持つ複合繊維を主体とする不織布からなる２枚の表面層とから形成された３層構造を有するものであった。前記各表面層の繊維の平均直径は前記内部層の繊維の平均直径に比べて大きい。また、前記セパレータの前述した滴定法によるカリウムイオン交換量は０．８ｍｅｑ／ｇであった。
【０１３２】
次いで、前記セパレータを実施例１と同様な負極と実施例１と同様な正極との間に介装し、渦巻状に捲回して電極群を作製した。このような電極群と７ＮのＫＯＨおよび１ＮのＬｉＯＨからなる電解液を有底円筒状容器に収納して前述した図１に示す構造を有するＡＡサイズの円筒形ニッケル水素二次電池を組み立てた。 前述した（１）式、（０．０５Ｘ＋０．０５）≦Ｙ≦２．０によると、セパレータを構成する全繊維の平均直径Ｘが１１μｍであるとき、高温保管特性及び大電流放電特性を向上させるのに必要なカリウムイオン交換量Ｙの範囲は０．６≦Ｙ≦２．０になる。従って、この実施例３１の二次電池のセパレータは前述した式（１）及び（２）式（１≦Ｘ≦２０）を満たしている。
【０１３３】
得られた実施例３１の二次電池について、前述した実施例１〜１２の二次電池について行ったのと同様な方法により４０℃での自己放電特性を測定し、その結果を図１２に示す。
【０１３４】
【発明の効果】
以上説明したように本発明によれば、高温保管時の自己放電特性が改善され、大電流放電特性が優れ、かつ長寿命化を達成したニッケル水素二次電池を提供できる。
【図面の簡単な説明】
【図１】本発明に係るニッケル水素二次電池を示す斜視図。
【図２】本発明のセパレータの作製工程を示す平面図。
【図３】図２のIII −III 線に沿う断面図。
【図４】図２の要部拡大平面図。
【図５】本発明の実施例１〜６および比較例１、２のニッケル水素二次電池における容量残存率と電解液中の水酸化リチウム濃度との関係を示す特性図。
【図６】 本発明の実施例７〜１２および比較例１、２のニッケル水素二次電池における容量残存率と電解液中の水酸化ナトリウム濃度との関係を示す特性図。
【図７】本発明の実施例１３〜１５および比較例３のニッケル水素二次電池における容量残存率とセパレータのイオン交換量との関係を示す特性図。
【図８】本発明の実施例１６〜１８および比較例５のニッケル水素二次電池における容量残存率とセパレータのイオン交換量との関係を示す特性図。
【図９】本発明の実施例１９〜２４および比較例２、９のニッケル水素二次電池における容量残存率と電解液中の水酸化リチウム濃度との関係を示す特性図。
【図１０】本発明の実施例２５〜３０および比較例２、９のニッケル水素二次電池における容量残存率と電解液中の水酸化ナトリウム濃度との関係を示す特性図。
【図１１】本発明の実施例１９〜３０のニッケル水素二次電池における電池内圧を測定するための電池内圧測定装置を示す断面図。
【図１２】本発明の実施例３１における高温での保管日数と容量残存率との関係を示す特性図。
【符号の説明】
１…容器、２…ペースト式正極、３…セパレータ、４…負極、７…封口板、８…絶縁ガスケット、１１…安全弁。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nickel metal hydride secondary battery, and more particularly to a nickel metal hydride secondary battery having an improved separator interposed between a positive electrode and a negative electrode.
[0002]
[Prior art]
As a nickel metal hydride secondary battery, an electrode group produced by interposing a separator between a paste type positive electrode containing nickel hydroxide and a paste type negative electrode containing a hydrogen storage alloy was housed in a container together with an alkaline electrolyte. Structures are known. The secondary battery is voltage compatible with a nickel cadmium secondary battery having a negative electrode containing a cadmium compound instead of the hydrogen storage alloy, and has an excellent capacity compared to the nickel cadmium secondary battery. It has the characteristics.
[0003]
However, since the nickel hydride secondary battery has basically the same configuration as the nickel cadmium secondary battery, there is a problem in self-discharge characteristics when stored at a high temperature in a charged state like the nickel cadmium secondary battery.
[0004]
Self-discharge during high-temperature storage of the nickel cadmium secondary battery is (a) oxywater, which is a positive charge product generated due to impurities (for example, nitrate ion, nitrite ion, ammonia) generated by oxidative decomposition of the separator. This occurs due to two reasons: a reduction reaction of nickel oxide (NiOOH) and (b) a self-decomposition reaction of NiOOH. Among these, since the NiOOH self-decomposition reaction (b) is slower than the NiOOH reduction reaction (a), the main cause of the self-discharge reaction during high-temperature storage is the NiOOH reduction reaction (a). It is thought that there is.
[0005]
The above-mentioned oxidative decomposition of the separator appears particularly prominently when a nonwoven fabric made of hydrophilic polyamide fibers widely used in nickel cadmium secondary batteries is used as the separator. This is because impurities such as nitrate ions generated by oxidative decomposition of the polyamide-based synthetic resin fiber reduce the positive electrode NiOOH and promote the self-discharge reaction.
[0006]
For this reason, in the nickel cadmium secondary battery, although it has better oxidation resistance than the polyamide-based fiber, it is attempted to use a separator obtained by hydrophilizing a polyolefin fiber that is basically hydrophobic. .
[0007]
On the other hand, Tanaka et al. (5, 290, 645) discloses a battery separator composed of a sheet material containing polyvinyl alcohol crosslinked with a crosslinking group having a specific structural formula. Further, it is disclosed that the sheet material has a hydrophilic portion and a hydrophobic portion.
[0008]
In addition, in Example 4 of the international publication of the international patent application (publication number: WO 93/01622), it was formed by graft copolymerization of acrylic acid to a nonwoven fabric made of polypropylene single fiber having a fiber diameter of 5 to 10 μm. A nickel metal hydride secondary battery using a separator having an ion exchange capacity of 0.48 (meq / g) and using 30% potassium hydroxide as an electrolyte is disclosed.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide a nickel-metal hydride secondary battery having improved self-discharge characteristics during high-temperature storage.
[0010]
[Means for Solving the Problems]
A nickel metal hydride secondary battery according to the present invention includes a positive electrode containing nickel hydroxide, a negative electrode containing a hydrogen storage alloy, , A separator, and an alkaline electrolyte, wherein the separator is formed from a sheet-like material including polyolefin-based synthetic resin fibers having an ion exchange group, the average diameter of all the fibers constituting the separator is X μm, and the potassium of the separator When the ion exchange amount is Ymeq / g, these satisfy the following formulas (1) and (2),
(0.05X + 0.05) ≦ Y ≦ 2.0 (1)
1 ≦ X ≦ 20 (2)
The alkaline electrolyte contains at least one of lithium hydroxide or sodium hydroxide In addition, the first surface of the separator exhibits hydrophilicity, and the second surface located on the opposite side of the first surface has a hydrophilic portion and a hydrophobic portion, and the separator is interposed between the positive electrode and the negative electrode. It arrange | positions so that the said 2nd surface may be located in the said negative electrode side It is characterized by this.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a nickel metal hydride secondary battery (cylindrical nickel metal hydride secondary battery) according to the present invention will be described with reference to FIG.
In the bottomed cylindrical container 1, an electrode group 5 produced by laminating a paste type positive electrode 2, a separator 3, and a negative electrode 4 and winding them in a spiral shape is housed. The negative electrode 4 is disposed on the outermost periphery of the electrode group 5 and is in electrical contact with the container 1. The alkaline electrolyte is accommodated in the container 1. A circular first sealing plate 7 having a hole 6 in the center is disposed in the upper opening of the container 1. A ring-shaped insulating gasket 8 is disposed between the periphery of the sealing plate 7 and the inner surface of the upper opening of the container 1, and the sealing plate is attached to the container 1 by caulking to reduce the diameter of the upper opening to the inside. 7 is hermetically fixed through the gasket 8. The positive electrode lead 9 has one end connected to the positive electrode 2 and the other end connected to the lower surface of the sealing plate 7. A positive electrode terminal 10 having a hat shape is attached on the sealing plate 7 so as to cover the hole 6. The rubber safety valve 11 is disposed so as to close the hole 6 in a space surrounded by the sealing plate 7 and the positive electrode terminal 10. A circular presser plate 12 made of an insulating material having a hole in the center is arranged on the positive electrode terminal 10 so that the protruding portion of the positive electrode terminal 10 protrudes from the hole of the presser plate 12. The outer tube 13 covers the periphery of the pressing plate 12, the side surface of the container 1, and the bottom periphery of the container 1.
[0013]
Next, the paste type positive electrode 2, the negative electrode 4, the separator 3 and the electrolytic solution will be described.
1) Paste type positive electrode 2
This paste type positive electrode 2 is prepared by adding a conductive material to nickel hydroxide powder as an active material, kneading with a polymer binder and water to prepare a paste, filling the paste on a conductive substrate, and drying. Thereafter, it is produced by molding.
[0014]
Examples of the conductive material include cobalt oxide and cobalt hydroxide.
Examples of the polymer binder include carboxymethyl cellulose, methyl cellulose, sodium polyacrylate, polytetrafluoroethylene, and the like.
[0015]
Examples of the conductive substrate include a net-like, sponge-like, fiber-like, or felt-like metal porous body made of nickel, stainless steel, or nickel-plated metal.
[0016]
2) Negative electrode 4
For example, the negative electrode 4 is prepared by adding a conductive material to a hydrogen storage alloy powder, kneading with a polymer binder and water to prepare a paste, filling the conductive substrate with the paste, drying, and then molding. It is manufactured by.
[0017]
The hydrogen storage alloy is not particularly limited as long as it can store hydrogen generated electrochemically in the electrolyte and can easily release the stored hydrogen during discharge. As this hydrogen storage alloy, for example, LaNi _Five , MmNi _Five (Mm; Misch metal), LmNi _Five (Lm: lanthanum-rich misch metal), or a multi-element type in which a part of these Ni is substituted with an element such as Al, Mn, Co, Ti, Cu, Zn, Zr, Cr, B, or TiNi type and TiFe type can be mentioned. Among them, the general formula LmNi _x Mn _y A _z (However, A is at least one metal selected from Al and Co, and the atomic ratio x, y, z is preferably a total value of 4.8 ≦ x + y + z ≦ 5.4). . Since the negative electrode containing such a hydrogen storage alloy can suppress pulverization accompanying the progress of the charge / discharge cycle, the charge / discharge cycle life of the secondary battery can be improved.
[0018]
Examples of the polymer binder include those similar to those used in the positive electrode 2.
For example, carbon black or the like can be used as the conductive material.
[0019]
Examples of the conductive substrate include two-dimensional substrates such as punched metal, expanded metal, perforated rigid plate, and nickel net, and three-dimensional substrates such as felt-like metal porous bodies and sponge-like metal substrates. .
[0020]
3) Separator 3
This separator 3 is formed from a sheet-like material containing polyolefin synthetic resin fibers having ion exchange groups. Moreover, when the average diameter of all the fibers of the separator 3 is X μm and the potassium ion exchange amount of the separator 3 is Ymeq / g, these satisfy the following expressions (1) and (2).
[0021]
(0.05X + 0.05) ≦ Y ≦ 2.0 (1)
1 ≦ X ≦ 20 (2)
Examples of the sheet-like material include a non-woven fabric containing a polyolefin-based synthetic resin fiber having an ion exchange group, a woven fabric containing the fiber, or a composite sheet composed of these non-woven fabric and woven fabric.
[0022]
The sheet-like material may be formed only from a polyolefin-based synthetic resin fiber having an ion-exchange group, but may include a polyolefin-based synthetic resin fiber having no ion-exchange group.
[0023]
Polyolefin-based synthetic resin fibers include polyolefin single fibers, core-sheathed composite fibers in which the surface of a core made of polyolefin fibers is coated with polyolefin fibers different from the polyolefin fibers, and different polyolefin fibers are joined in a circular shape. Examples thereof include composite fibers having a split structure, fibers made of a copolymer resin of polyolefin and butene, and the like. Examples of the polyolefin include polyethylene and polypropylene.
[0024]
When the sheet-like material is formed from the composite sheet, the average diameters of the polyolefin-based synthetic resin fibers of the respective layers constituting the composite sheet may be the same or different from each other.
[0025]
The sheet-like material includes an inner layer including a polyolefin-based synthetic resin fiber having an ion-exchange group, and two surface layers formed on both surfaces of the inner layer and including the polyolefin-based synthetic resin fiber having an ion-exchange group. It is preferable that the composite sheet has a three-layer structure in which the average diameter of all the fibers constituting each surface layer is larger than the average diameter of all the fibers constituting the inner layer. A separator that is a sheet-like material having such a three-layer structure and that satisfies the expressions (1) and (2) can provide a large amount of electrolyte solution by the inner layer while ensuring sufficient strength by the surface layer. Since it can hold | maintain, high intensity | strength and the outstanding electrolyte solution retainability can be satisfied simultaneously. The nickel metal hydride secondary battery including this separator and the alkaline electrolyte having the specific composition described above can greatly improve the self-discharge characteristics during high-temperature storage.
[0026]
In the three-layered sheet-like material, the average diameter of all the fibers constituting the inner layer is in the range of 0.5 μm to 5 μm, and the average diameter of all the fibers constituting each of the surface layers is in the range of 7 μm to 20 μm. It is preferable to make it. The surface layers may have the same average diameter or may be different from each other.
[0027]
The reason why the average diameter in the inner layer is limited to the above range is as follows. When the average diameter is less than 0.5 μm, the strength of the separator is lowered, and there is a possibility that it cannot be used as a separator. On the other hand, if the average diameter exceeds 5 μm, the electrolyte retention of the separator may be lowered. A more preferred average diameter in the inner layer is in the range of 1 μm to 3 μm.
[0028]
The reason why the average diameter in the surface layer is limited to the above range is as follows. If the average diameter is less than 7 μm, it becomes difficult to maintain the mechanical strength of the separator, and there is a possibility that it cannot be used as a separator. On the other hand, if the average diameter exceeds 20 μm, the electrolyte retention of the separator may be lowered. A more preferable average diameter in the surface layer is in the range of 8 μm to 15 μm.
[0029]
Examples of the ion exchange group include a COOH group and SO. _Three H group, OH group, etc. can be mentioned. The OH group preferably exhibits strong acidity when attached to the polyolefin-based synthetic resin fiber. Of the ion exchange groups, a COOH group is preferred.
[0030]
The ion exchange group of the polyolefin-based synthetic resin fiber can be formed, for example, by graft copolymerization of a vinyl monomer having ion exchange ability.
Examples of the vinyl monomer include an acrylic acid monomer, a methacrylic acid monomer, an acrylic acid or methacrylic acid ester monomer, a vinyl pyridine monomer, a vinyl pyrrolidone monomer, a styrene sulfonic acid monomer, and a styrene monomer, which directly react with an acid or a base. And vinyl monomers having a functional group capable of forming a salt, or vinyl monomers having a functional group capable of forming a salt upon hydrolysis. Among the vinyl monomers, acrylic acid monomers are preferred.
[0031]
When the separator indicates the amount of ion exchange groups retained, the potassium ion exchange amount is Ymeq / g (milli-equivalent per gramme), and the average diameter of all the fibers constituting the separator is X μm, these are as follows: The expressions (1) and (2) are satisfied.
[0032]
(0.05X + 0.05) ≦ Y ≦ 2.0 (1)
1 ≦ X ≦ 20 (2)
The potassium ion exchange amount Y (meq / g) of the separator is determined by a titration method described below.
[0033]
(Titration method)
First, when 0.5 to 1 g of a sample (for example, a non-woven fabric made of polyolefin fibers is graft-copolymerized with acrylic acid) is taken into a 100 ml polyethylene jar, 100 ml of 1N HCl solution is added, and the sample is lifted Is completely submerged and stored in a constant temperature bath at 60 ° C. for 1 hour. Subsequently, the sample is transferred to a beaker containing 200 ml of ion-exchanged water, stirred with a glass rod, and washed while changing the ion-exchanged water until the pH of the washing solution becomes 6-7. The sample is drained, spread on a stainless steel tray, and dried in a dryer at 100 ° C. for 1 hour. After cooling, the sample is weighed to 0.1 mg, transferred to a 100 ml polyethylene jar, and 110 g ± 0.01 g of the 0.01 N KOH solution is added. On the other hand, 110 g ± 0.01 g of 0.01N-KOH solution is collected in a 100 ml polyethylene jar as a blank sample. Next, put these jars in a constant temperature bath at 60 ° C., shake lightly every 30 minutes, and store for 2 hours. After each of the jars is shaken lightly, each sample is taken out and allowed to cool to room temperature. About 100 g of the test solution after being allowed to cool is weighed to 0.01 g in a 200 ml conical beaker, neutralized with 0.1 N HCl solution using phenolphthalein as an indicator. The blank test solution is titrated by operating in the same manner. By such titration, the potassium ion exchange amount is calculated by the equation shown in the following formula 1.
[0034]
[Expression 1]

[0035]
The average diameter X of all fibers constituting the separator is 1 to 20 μm. In the separator, when the basis weight and thickness are kept constant and the average diameter X is reduced, the fibers are more closely entangled with each other, so that the opening of the fibers is reduced and the electrolyte holding power of the openings is increased. Retention is increased. However, when the average diameter X is reduced, the strength of the separator is reduced. On the other hand, when the average diameter X is increased with the basis weight and the thickness being constant, the strength is improved, but the entanglement between the fibers becomes sparse, so that the opening of the fibers is increased and the electrolyte is retained. The force is reduced, and the electrolyte retention is reduced. Moreover, when the mesh opening is increased, the coverage of the separator is lowered. Therefore, if the average diameter X is less than 1 μm, the mechanical strength of the separator is significantly reduced, and it may be difficult to assemble the battery. On the other hand, if the average diameter exceeds 20 μm, shorts between the positive and negative electrodes may occur frequently due to a decrease in coverage. A more preferable average diameter X is 3 to 15 μm.
[0036]
As described above, when the basis weight and thickness are constant, the separator has a larger opening of the fiber as the average diameter X increases, so that the electrolyte holding power of the opening decreases and the electrolyte holding ability decreases. To do. Therefore, in order to improve the self-discharge characteristics during high-temperature storage by holding a sufficient amount of electrolyte in the separator, the fiber itself can be obtained by increasing the amount of ion-exchange groups in the separator as the average diameter X increases. It is necessary to increase the hydrophilicity of the electrolyte so that the electrolyte can be sufficiently retained even with large openings. That is, the minimum amount of potassium ion exchange Y required to improve the self-discharge characteristics during high-temperature storage must be increased according to the formula (0.05X + 0.05) when the average diameter X is increased. When the potassium ion exchange amount Y of the separator is less than the potassium ion exchange amount satisfying the formula (0.05X + 0.05), the electrolytic solution retention of the separator is lowered, and the self-discharge characteristics during high temperature storage are deteriorated. On the other hand, if the ion exchange amount Y exceeds 2.0 meq / g, the ion exchange capacity of the separator increases, and alkali metal ions in the electrolyte are immobilized on the separator. In the secondary battery, the operating voltage at the time of large current discharge decreases. The ion exchange amount Y preferably satisfies the following formula (3).
[0037]
(0.05X + 0.15) ≦ Y ≦ 1.2 (3)
The thickness of the separator 3 is preferably in the range of 0.15 mm to 0.3 mm.
[0038]
The basis weight of the separator 3 is 30 g / m. ² ~ 70g / m ² It is preferable to be in the range. The basis weight is 30 g / m ² If it is less, the strength of the separator 3 may be reduced. On the other hand, the basis weight is 70 g / m. ² If it exceeds, the battery capacity may be reduced. A more preferred basis weight is 40 g / m. ² ~ 60g / m ² Range.
[0039]
The separator can be produced by, for example, the following methods (a) to (c).
(A) After immersing and pulling up a sheet of polyolefin-based synthetic resin fiber having an average diameter of 1 to 20 μm in a solution containing a vinyl monomer having ion exchange capacity, the sheet is irradiated with an energy beam, and Graft copolymerize vinyl monomers. By such a method, a separator that is formed from a sheet-like material including a polyolefin-based synthetic resin fiber having an ion exchange group and that satisfies the above formulas (1) and (2) is produced.
[0040]
(B) After irradiating an energy beam to a polyolefin synthetic resin fiber sheet having an average diameter of 1 to 20 μm, the sheet monomer is immersed in a solution containing a vinyl monomer having ion exchange ability to Graft copolymerize. By such a method, a separator that is formed from a sheet-like material including a polyolefin-based synthetic resin fiber having an ion exchange group and that satisfies the above formulas (1) and (2) is produced.
[0041]
(C) A polyolefin synthetic resin fiber sheet having an average diameter of 1 to 20 μm is immersed in a solution containing a vinyl monomer having ion exchange capacity, and at the same time, the sheet monomer is irradiated with an energy beam to form the vinyl monomer. Graft copolymerize. By such a method, a separator that is formed from a sheet-like material including a polyolefin-based synthetic resin fiber having an ion exchange group and that satisfies the above formulas (1) and (2) is produced.
[0042]
Examples of the sheet made of polyolefin-based synthetic resin fiber include a nonwoven fabric made of polyolefin-based synthetic resin fiber, a woven fabric made of the same fiber, or a composite sheet composed of these nonwoven fabric and woven fabric. The nonwoven fabric is produced by, for example, a dry method, a wet method, a spun bond method, a melt blow method, or the like. Among such methods, the spunbond method and the melt blow method are advantageous in terms of prevention of a short circuit between the positive electrode and the negative electrode because a nonwoven fabric having a thin fiber diameter can be produced.
[0043]
The composite sheet is composed of an inner layer made of a polyolefin-based synthetic resin fiber, and two surface layers formed on both surfaces of the inner layer and made of a polyolefin-based synthetic resin fiber, and each surface layer is constituted It is possible to use a three-layer laminate in which the average diameter of the polyolefin-based synthetic resin fibers is larger than the average diameter of the polyolefin-based synthetic resin fibers constituting the inner layer.
[0044]
As the energy beam, for example, ionizing radiation such as ultraviolet rays, electron beams, and X-rays is employed.
The separator may have an ion exchange group distribution such that the first surface is hydrophilic and the second surface opposite to the first surface has a hydrophilic portion and a hydrophobic portion. Such a separator is disposed between the positive electrode and the negative electrode so that the second surface is located on the negative electrode side. The hydrophilic portion is a region where ion exchange groups are present on the surface of the separator, and the hydrophobic portion is a region where ion exchange groups are not present on the separator surface.
[0045]
The hydrophobic portion of the separator may be formed, for example, by subjecting the sheet-like material to a treatment that inhibits graft copolymerization of a vinyl monomer having an ion exchange group that is partially embossed to form a film. it can. The embossing is optional such as a circle, a square shape such as a quadrangle, and a stripe shape. The hydrophobic portion of the separator can also be formed by laminating a woven fabric of polyolefin resin resin fibers having a large fiber diameter to the nonwoven fabric, woven fabric or composite sheet.
[0046]
The second surface of the separator preferably has an area ratio of the hydrophobic portion of 2 to 25%. If the area ratio of the hydrophobic portion is less than 2%, there is a possibility that the effect of suppressing the increase in internal pressure during overcharging due to the provision of the hydrophobic portion may not be seen. On the other hand, if the area ratio of the hydrophobic portion exceeds 25%, the operating voltage at the time of large current discharge may be reduced in the secondary battery including the separator. A more preferable area ratio of the hydrophobic portion is 8 to 20%.
[0047]
The first surface of the separator may have a hydrophobic portion having an area ratio of 10% or less. In the first surface, when there is a hydrophobic portion, the area ratio needs to be smaller than the area ratio of the hydrophobic portion of the second surface. The first surface of the separator may have ion exchange groups on the entire surface. By disposing the separator in such a form between the paste-type positive electrode and the negative electrode so that the second surface is on the negative electrode side, that is, the first surface is on the positive electrode side, the separator on the positive electrode side compared to the negative electrode side. A large amount of alkaline electrolyte can be retained on the surface (first surface). As a result, since an electrolyte film is present on the surface of the positive electrode, when the nickel hydride secondary battery is stored at a high temperature, the hydrogen gas generated from the negative electrode permeates the separator and reaches the positive electrode. This can be prevented by the film, and the self-discharge characteristics during high temperature storage can be greatly improved.
[0048]
It is formed from a sheet-like material including a polyolefin-based synthetic resin fiber having an ion exchange group, the first surface has hydrophilicity, the second surface has a hydrophilic portion and a hydrophobic portion, and the formula (1) and the above ( The separator satisfying the formula (2) is, for example, a step of embossing a first surface of a sheet-like material made of polyolefin synthetic resin fibers having an average diameter of 1 to 20 μm to form an embossed surface into a film, and the sheet-like material In a solution containing a vinyl monomer having ion exchange capacity, and attaching the solution to both sides of the sheet-like material excluding the film-formed portion of the second surface opposite to the first surface; A sheet-like material is irradiated with an energy beam and the vinyl monomer is graft-copolymerized on the surface to which the solution is attached.
[0049]
Specifically, for example, it can be produced by the method described in the following (1).
(1) First, as shown in FIGS. 2 to 4, both surfaces (first surface 22, second surface 23) of a sheet-like material (for example, non-woven fabric) 21 made of polyolefin synthetic resin fibers having an average diameter of 1 to 20 μm. Of these, embossing is performed from the first surface 22 side. By this embossing, a plurality of circular concave portions 24 are formed in the sheet-like material, and a filmed portion (film portion) 25 is formed on the bottom surface (embossed surface) of the concave portion 24. At the same time, the portion of the second surface 23 of the sheet-like material 21 corresponding to the recess 24 also becomes a filmed surface.
[0050]
Next, the sheet-like material is dipped in a solution containing a vinyl monomer having ion exchange ability and pulled up. At this time, since the sheet-like material 21 has the film portion 25 formed on the bottom surface of the concave portion 24 as shown in FIG. 3 described above, the solution does not adhere to both surfaces of the film portion 25, and other than the both surfaces. The solution is applied to the region of the non-woven fabric. However, since the solution is filled in the concave portion 24 on the surface 22 on the concave portion 24 side of the film portion 25, the solution is substantially brought into contact with the surface on the concave portion 24 side. .
[0051]
Next, the vinyl monomer is graft copolymerized on the surface to which the solution is adhered by irradiating the sheet-like material with an energy beam. Through such a process, the entire first surface of the sheet-like material on the concave side is made hydrophilic by graft copolymerization of the vinyl monomer. Of the second surface of the sheet-like material, the solution does not adhere to the film portion at the bottom of the recess, and the solution adheres to the region of the other second surface. As a result, on the second surface of the sheet-like material, the film portion region at the bottom of the recess becomes a hydrophobic portion, and the other region becomes a hydrophilic portion.
[0052]
Therefore, it consists of the sheet-like material containing the polyolefin synthetic resin fiber which has an ion exchange group by the process mentioned above, the 1st surface of the said sheet-like material shows hydrophilicity, and the 2nd surface on the opposite side to this 1st surface is A separator having a hydrophilic part and a hydrophobic part and satisfying the above-described relational expression is produced.
[0053]
The embossing of the sheet-like material on the first surface side is, for example, a first roll having a smooth surface and a plurality of pinpoint-shaped protrusions formed on the surface, which are opposed to each other by rotating in opposite directions. Rolls are prepared, and the rolls are heated to, for example, 120 ° C. to 180 ° C., and the sheet-like material is supplied between the rolls so that the sheet is between the first roll surface and the plurality of protrusions of the second roll. This is done by pressurizing and heat-sealing the polyolefin-based synthetic resin fibers. As for the shape of the protrusion formed on the surface of the second roll, in addition to the pinpoint, for example, the shape of the tip surface can be a square shape, and the shape of the tip surface can be a diamond shape.
[0054]
As the energy beam, the same one as described above is used.
The separator can be produced by the method shown in the following (2).
[0055]
(2) First, a woven fabric made of a polyolefin synthetic resin fiber having a large diameter and having a large opening is stuck on one side of a nonwoven fabric made of a polyolefin synthetic resin fiber to obtain a sheet-like material. In this sheet-like material, the non-sticking surface of the woven fabric is the first surface, and the sticking surface of the woven fabric is the second surface. Subsequently, the sheet-like material is dipped in a solution containing a vinyl monomer having an ion exchange group and pulled up. At this time, the solution is not attached to the woven fabric adhered to the second surface of the sheet-like material, and is exposed to the first surface and the non-woven fabric located in the opening portion of the woven fabric. The solution adheres to the nonwoven fabric. Subsequently, when the sheet-like material is irradiated with the above-described energy beam such as ultraviolet rays, the vinyl monomer is graft-copolymerized on the adhesion surface of the solution. In such a process, the first surface of the sheet-like material on which the solution is adhered to the entire surface is hydrophilized by graft copolymerization of the vinyl monomer. On the other hand, on the second surface of the sheet-like material to which the woven fabric is adhered, since the solution is attached to the non-woven fabric located at the opening of the woven fabric, the vinyl monomer graft copolymer only at that portion. Is made. That is, on the second surface of the sheet-like material, the region of the woven fabric becomes a hydrophobic portion, and the other region becomes a hydrophilic portion.
[0056]
Therefore, it has a hydrophilic first surface and a second surface having a hydrophilic portion and a hydrophobic portion formed from a sheet-like material containing polyolefin-based synthetic resin fibers by the above-described process, A separator satisfying the formula and the formula (2) is produced.
[0057]
2) Alkaline electrolyte
The electrolytic solution contains at least one of lithium hydroxide (LiOH) or sodium hydroxide (NaOH). Examples of such an electrolytic solution include a lithium hydroxide aqueous solution, a sodium hydroxide aqueous solution, a mixed solution of potassium hydroxide (KOH) and lithium hydroxide, a mixed solution of potassium hydroxide and sodium hydroxide, potassium hydroxide, water Examples thereof include a mixed solution of lithium oxide and sodium hydroxide, and a mixed solution of lithium hydroxide and sodium hydroxide.
[0058]
When the electrolytic solution contains at least lithium hydroxide, the concentration of lithium hydroxide in the electrolytic solution is preferably in the range of 0.1N to 1.5N. If the concentration of lithium hydroxide in the electrolytic solution is out of the above range, the self-discharge characteristics of the nickel-hydrogen secondary battery during high-temperature storage may be reduced. A more preferred concentration is in the range of 0.3N to 1.3N.
[0059]
When the electrolytic solution contains at least sodium hydroxide, the concentration of sodium hydroxide in the electrolytic solution is preferably in the range of 0.5N to 6.0N. If the concentration of sodium hydroxide in the electrolytic solution is out of the above range, the self-discharge characteristic during high-temperature storage of the nickel-hydrogen secondary battery may be reduced. A more preferred concentration is in the range of 1.0N to 5.0N.
[0060]
A nickel metal hydride secondary battery according to the present invention includes a positive electrode including nickel hydroxide, a negative electrode including a hydrogen storage alloy, a separator interposed between the positive electrode and the negative electrode, and an alkaline electrolyte. Is formed from a sheet-like material containing a polyolefin-based synthetic resin fiber having an ion exchange group, the average diameter of all the fibers constituting the separator is X μm, and the potassium ion exchange amount of the separator is Ymeq / g, These satisfy the following formulas (1) and (2),
(0.05X + 0.05) ≦ Y ≦ 2.0 (1)
1 ≦ X ≦ 20 (2)
The alkaline electrolyte contains at least one of lithium hydroxide or sodium hydroxide.
[0061]
By providing a separator that satisfies the formula (1) and the formula (2), and an alkaline electrolyte containing at least one of sodium hydroxide and lithium hydroxide, self-retention during high-temperature storage of the nickel-hydrogen secondary battery The progress of discharge can be suppressed. At the same time, the secondary battery can improve large current discharge characteristics and charge / discharge cycle life.
[0062]
Further, the ion exchange group distribution of the separator is such that the first surface is hydrophilic and the second surface has a hydrophilic portion and a hydrophobic portion, and the separator having such a configuration has the first surface between the positive electrode and the negative electrode. If it arrange | positions so that the said positive electrode side may be contacted, the self-discharge characteristic at the time of the high temperature storage of the said secondary battery can further be improved.
[0063]
When the nickel metal hydride secondary battery is stored in a high temperature environment, the hydrogen partial pressure in the battery may increase depending on the equilibrium pressure of the hydrogen storage alloy of the negative electrode. When the hydrogen partial pressure in the battery rises, hydrogen gas may pass through the separator and reach the positive electrode to reduce NiOOH of the positive electrode, that is, self-discharge may proceed. Since the separator having the above-described form and arrangement can increase the amount of electrolyte on the positive electrode side as compared with the negative electrode side, a layer containing a large amount of electrolyte on the surface of the positive electrode, that is, an electrolyte film, exists. The liquid film can prevent the hydrogen gas from passing through the separator and reaching the positive electrode. As a result, the self-discharge characteristics during high temperature storage can be greatly improved.
[0064]
Moreover, since the 2nd surface which has a hydrophilic part and a hydrophobic part contacts the negative electrode surface, the said separator can provide water repellency to the negative electrode surface. For this reason, the said negative electrode is formed with this surface, the electrolyte solution on the surface, and oxygen gas, and can increase the quantity of the three-phase interface which consumes the oxygen gas generated from a positive electrode at the time of overcharge. Such a negative electrode can increase the oxygen gas consumption rate at the time of overcharging, and therefore can suppress an increase in internal pressure at the time of overcharging of the secondary battery. In addition, since the three-phase interface can increase the absorption rate of hydrogen gas generated from the positive electrode during reverse charging, an increase in internal pressure during reverse charging can be suppressed by increasing the three-phase interface.
[0065]
Therefore, the separator having the above-described configuration and arrangement can increase the distribution of the electrolyte on the positive electrode side and can control the hydrophobic portion to appear on the negative electrode side, thereby improving the self-discharge characteristics during high-temperature storage. And the nickel secondary battery by which the internal pressure rise at the time of overcharge was suppressed can be provided.
[0066]
In FIG. 1 described above, the separator 3 is interposed between the positive electrode 2 and the negative electrode 4 and wound in a spiral shape and stored in a bottomed cylindrical container 1. It is not limited to such a structure. For example, a prismatic nickel metal hydride secondary battery is constructed by storing a laminate prepared by alternately stacking a paste type positive electrode and a negative electrode with the separator interposed therebetween in a bottomed rectangular cylindrical container. Also good.
[0067]
【Example】
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
Example 1
<Preparation of paste type negative electrode>
Commercially available lanthanum-rich misch metal Mm and Ni, Co, Mn, Al using a high frequency furnace, MmNi _3.6 Co _0.8 Mn _0.4 Al _0.2 A hydrogen storage alloy having the following composition was prepared. The hydrogen storage alloy was mechanically pulverized and passed through a 200 mesh sieve. 0.4 parts by weight of sodium polyacrylate, 0.1 parts by weight of carboxymethyl cellulose (CMC), polytetrafluoroethylene dispersion (specific gravity 1.5, solid content 60 wt) with respect to 100 parts by weight of the obtained hydrogen storage alloy powder %) 1.5 parts by weight and 0.8 parts by weight of carbon powder as a conductive material were mixed with 55 parts by weight of water to prepare a paste. The paste was applied to a punched metal, dried, and then subjected to pressure molding to produce a paste-type negative electrode.
[0068]
<Preparation of paste type positive electrode>
To a mixed powder composed of 90 parts by weight of nickel hydroxide powder and 10 parts by weight of cobalt hydroxide powder, 0.3 parts by weight of carboxymethylcellulose and a suspension of polytetrafluoroethylene (specific gravity 1. 5, solid content 60% by weight) was added in an amount of 1.0 part by weight in terms of solid content, and 30 parts by weight of pure water was added thereto and kneaded to prepare a paste. Subsequently, after filling this paste into a nickel-plated fiber substrate, the paste was further applied to both surfaces thereof, dried, and subjected to roller press to roll to produce a paste type positive electrode.
[0069]
<Preparation of separator>
Polypropylene resin is made of long fibers with an average diameter of 10 μm using the spunbond method, and the basis weight is 50 g / m. ² Thus, a nonwoven fabric having a thickness of 0.20 mm was produced. After immersing this nonwoven fabric in an acrylic acid aqueous solution, ultraviolet rays are irradiated to graft copolymerize acrylic acid monomers, each nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form ion exchange groups. A separator made of a nonwoven fabric mainly composed of a polypropylene fiber having a COOH group and having a potassium ion exchange amount of 0.8 meq / g by the titration method described above was produced.
[0070]
According to the above-mentioned formula (1), (0.05X + 0.05) ≦ Y ≦ 2.0, when the average diameter X of all the fibers constituting the separator is 10 μm, self-discharge characteristics and large current during high-temperature storage The range of the potassium ion exchange amount Y necessary for making the discharge characteristics excellent is 0.55 ≦ Y ≦ 2.0. Therefore, in the obtained separator, the average diameter X and the potassium ion exchange amount Y simultaneously satisfy the above-described formulas (1) and (2); 1 ≦ X ≦ 20.
[0071]
Next, the separator was interposed between the negative electrode and the positive electrode and wound in a spiral shape to produce an electrode group. An AA-sized cylindrical nickel-hydrogen secondary battery having the structure shown in FIG. 1 described above, in which an electrolytic solution composed of such an electrode group and 0.1 N LiOH and 7.9 N KOH is housed in a bottomed cylindrical container. Assembled.
[0072]
(Example 2)
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 0.3 N LiOH and 7.7 N KOH was used as the alkaline electrolyte.
[0073]
Example 3
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 0.5 N LiOH and 7.5 N KOH was used as the alkaline electrolyte.
[0074]
Example 4
A nickel-metal hydride secondary battery similar to that in Example 1 was assembled except that a mixed liquid composed of 1.0 N LiOH and 7.0 N KOH was used as the alkaline electrolyte.
[0075]
(Example 5)
A nickel-metal hydride secondary battery similar to that in Example 1 was assembled except that a mixed liquid composed of 1.3N LiOH and 6.7N KOH was used as the alkaline electrolyte.
[0076]
(Example 6)
A nickel-hydrogen secondary battery similar to that in Example 1 was assembled except that a mixed liquid composed of 1.5N LiOH and 6.5N KOH was used as the alkaline electrolyte.
[0077]
(Example 7)
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 0.5N NaOH and 7.5N KOH was used as the alkaline electrolyte.
[0078]
(Example 8)
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 1.0 N NaOH and 7.0 N KOH was used as the alkaline electrolyte.
[0079]
Example 9
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 2.0 N NaOH and 6.0 N KOH was used as the alkaline electrolyte.
[0080]
(Example 10)
A nickel-metal hydride secondary battery similar to that in Example 1 was assembled except that a mixed liquid composed of 4.0 N NaOH and 4.0 N KOH was used as the alkaline electrolyte.
[0081]
(Example 11)
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 5.0 N NaOH and 3.0 N KOH was used as the alkaline electrolyte.
[0082]
(Example 12)
A nickel-metal hydride secondary battery similar to that of Example 1 was assembled except that a mixed liquid composed of 6.0 N NaOH and 2.0 N KOH was used as the alkaline electrolyte.
[0083]
(Comparative Example 1)
A nickel-hydrogen secondary battery similar to that of Example 1 was assembled except that an 8.0 N aqueous KOH solution was used as the alkaline electrolyte.
[0084]
(Comparative Example 2)
It has a core-sheath type in which the surface of the core material made of polypropylene fiber is coated with polyethylene resin, the composite fiber having an average diameter of 15 μm, and the surface of the core material made of polypropylene fiber is coated with ethylene-vinyl alcohol copolymer resin. A composite fiber having a core-sheath structure and an average diameter of 15 μm is mixed, and the basis weight is 50 g / m by a dry method. ² A separator made of a nonwoven fabric having a thickness of 0.20 mm was prepared. It was 0 meq / g when the potassium ion exchange amount of the obtained separator was measured by the titration method mentioned above.
[0085]
Next, the separator was interposed between a negative electrode and a positive electrode similar to those in Example 1, and wound in a spiral shape to produce an electrode group. An AA-sized cylindrical nickel-metal hydride secondary battery having the structure shown in FIG. 1 was assembled by storing such an electrode group and an electrolyte composed of 8.0 N KOH in a bottomed cylindrical container.
[0086]
About the obtained secondary batteries of Examples 1 to 12 and Comparative Examples 1 and 2, after charging 150% at 1 CmA in an atmosphere of 20 ° C., a charge / discharge cycle was performed until the battery voltage reached 1.0 V at 1 CmA. Repeated 3 times. Thereafter, the battery was left at 20 ° C. for 2 hours while being charged with 1 CmA at 150%, and then stored in a constant temperature bath at 40 ° C. for 14 days. After storage, it was left at 20 ° C. for 2 hours, discharged at 1 CmA until the battery voltage reached 1.0 V, and the discharge capacity (remaining capacity) was measured. 150% charged at 1 CmA before being stored in a constant temperature bath at 40 ° C. for 14 days, and the discharge capacity when the battery voltage was discharged to 1.0 V at 1 CmA was C ′ ₀ The discharge capacity after storage for 14 days in a constant temperature bath at 40 ° C. is the remaining capacity C ′. _R The residual capacity rate was determined from the following formula. The self-discharge characteristics were judged from the capacity remaining rate.
[0087]
Capacity remaining rate (%) = (C ′ _R / C ' ₀ ) × 100
FIG. 5 shows the relationship between the lithium hydroxide concentration in the electrolyte solution and the capacity remaining rate in the secondary batteries of Examples 1 to 6 and Comparative Examples 1 and 2. Moreover, the relationship between the sodium hydroxide density | concentration in electrolyte solution and the capacity | capacitance residual rate in the secondary battery of Examples 7-12 and Comparative Examples 1 and 2 is shown in FIG.
[0088]
As is apparent from FIGS. 5 and 6, the secondary batteries of Examples 1 to 12 can suppress self-discharge during storage at a high temperature of 40 ° C. as compared with the secondary batteries of Comparative Examples 1 and 2. From this, in order to improve the self-discharge characteristics during high-temperature storage of the nickel-metal hydride secondary battery, an alkali containing at least one of lithium hydroxide or sodium hydroxide is used at the same time, using a separator that satisfies the above two formulas. It can be seen that it is important to use an electrolytic solution.
[0089]
(Example 13)
Polypropylene resin is made of polypropylene fiber with an average diameter X of 1 μm by a melt blow method, and the basis weight is 50 g / m. ² Then, a non-woven fabric having a thickness of 0.2 mm was prepared, and the non-woven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize the acrylic acid monomer. A separator having a potassium ion exchange amount of 0.1 meq / g made of a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups by washing the nonwoven fabric to remove unreacted acrylic acid and then drying the nonwoven fabric. Was made. The ion exchange amount of potassium was measured by the titration method described above.
[0090]
Next, the separator was interposed between a negative electrode similar to Example 1 and a positive electrode similar to Example 1, and wound into a spiral shape to produce an electrode group. An AA-sized cylindrical nickel-metal hydride secondary battery having the structure shown in FIG. 1 was assembled by storing such an electrode group and an electrolytic solution composed of 7N KOH and 1N LiOH in a bottomed cylindrical container.
[0091]
(Example 14)
Polypropylene resin is made of polypropylene fiber with an average diameter X of 1 μm by a melt blow method, and the basis weight is 50 g / m. ² Then, a non-woven fabric having a thickness of 0.2 mm was made. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups. The amount of potassium ion exchange by the titration method described above is 1 A separator of 0.0 meq / g was produced. A nickel-hydrogen secondary battery similar to that in Example 13 was assembled except that this separator was used.
[0092]
(Example 15)
Polypropylene resin is made of polypropylene fiber with an average diameter X of 1 μm by a melt blow method, and the basis weight is 50 g / m. ² Then, a non-woven fabric having a thickness of 0.2 mm was made. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups. The ion exchange amount of potassium by the titration method described above is A 2.0 meq / g separator was produced. A nickel-hydrogen secondary battery similar to that in Example 13 was assembled except that this separator was used.
[0093]
According to the above-mentioned formula (1), when the average diameter X of all the fibers constituting the separator is 1 μm, the potassium ion exchange amount Y required for improving the self-discharge characteristics and the high-current discharge characteristics during high-temperature storage. The range is 0.1 ≦ Y ≦ 2.0. Therefore, the separators of the obtained secondary batteries of Examples 13 to 15 have the average diameter X and the potassium ion exchange amount Y satisfying the expressions (1) and (2) described above.
(Example 16)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, and 50% by weight of the fiber and 50% by weight of the polypropylene resin fiber are mixed. m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 1.1 meq / g by a titration method was prepared.
[0094]
A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Example 17)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, and 50% by weight of the fiber and 50% by weight of the polypropylene resin fiber are mixed. m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 1.5 meq / g by the titration method was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Example 18)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, and 50% by weight of the fiber and 50% by weight of the polypropylene resin fiber are mixed. m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 2.0 meq / g by a titration method was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
[0095]
According to the above-mentioned formula (1), when the average diameter X of all the fibers constituting the separator is 20 μm, the potassium ion exchange amount Y required for improving the self-discharge characteristics and the large-current discharge characteristics during high-temperature storage. The range is 1.0 ≦ Y ≦ 2.0. Therefore, the obtained separators of the secondary batteries of Examples 16 to 18 have the average diameter X and the potassium ion exchange amount Y satisfying the expressions (1) and (2) described above.
(Comparative Example 3)
Polypropylene resin is made of polypropylene fiber with an average diameter X of 1 μm by a melt blow method, and the basis weight is 50 g / m. ² Then, a non-woven fabric having a thickness of 0.2 mm was prepared, and the non-woven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize the acrylic acid monomer. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups. The ion exchange amount of potassium by the titration method described above is A 0.05 meq / g separator was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Comparative Example 4)
Polypropylene resin is made of polypropylene fiber with an average diameter X of 1 μm by a melt blow method, and the basis weight is 50 g / m. ² Then, a non-woven fabric having a thickness of 0.2 mm was prepared, and the non-woven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize the acrylic acid monomer. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups. The ion exchange amount of potassium by the titration method described above is A 2.5 meq / g separator was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Comparative Example 5)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, and 50% by weight of the fiber and 50% by weight of the polypropylene resin fiber are mixed. m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 0.6 meq / g by a titration method was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Comparative Example 6)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, 50% by weight of this fiber and 50% by weight of the polypropylene resin fiber are mixed, and an average diameter X is 20 μm by a wet method, and the basis weight is 50 g / m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 2.5 meq / g by a titration method was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
(Comparative Example 7)
Made from polypropylene resin, polypropylene fiber with an average diameter X of 0.5 μm by melt-blowing, with a basis weight of 50 g / m ² Then, a non-woven fabric having a thickness of 0.2 mm was prepared, and the non-woven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize the acrylic acid monomer. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups. The ion exchange amount of potassium by the titration method described above is A 1.0 meq / g separator was produced. Since such a separator was interposed between the positive electrode and the negative electrode due to insufficient strength and broke when wound in a spiral shape, a secondary battery could not be produced from this separator.
(Comparative Example 8)
A fiber made of a copolymer resin obtained by copolymerizing butene with polypropylene resin is prepared, 50% by weight of this fiber and 50% by weight of the polypropylene resin fiber are mixed, and an average diameter X is 25 μm by a wet method. m ² Thus, a nonwoven fabric having a thickness of 0.2 mm was produced. This nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize acrylic acid monomers. The nonwoven fabric is washed to remove unreacted acrylic acid, and then dried to form a nonwoven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and copolymer resin fibers having COOH groups. A separator having a potassium ion exchange amount of 1.0 meq / g by a titration method was prepared. A nickel-metal hydride secondary battery similar to that in Example 13 was assembled except that such a separator was used.
[0096]
For the obtained secondary batteries of Examples 13 to 18 and Comparative Examples 3 and 5, the self-discharge characteristics at 40 ° C. were measured by the same method as that for the secondary batteries of Examples 1 to 12 described above. The results are shown in FIGS.
[0097]
As is clear from FIG. 7, it can be seen that the secondary batteries of Examples 13 to 15 of the present invention have improved self-discharge characteristics during high-temperature storage as compared with the secondary battery of Comparative Example 3. Further, as is apparent from FIG. 8, it can be seen that the secondary batteries of Examples 16 to 18 of the present invention have improved self-discharge characteristics during high-temperature storage as compared with the secondary battery of Comparative Example 5.
[0098]
Moreover, about the secondary battery of Examples 13-18 and the comparative example 8, the insulation defective incidence after electrode group preparation was measured. The insulation failure after the electrode group was produced was measured by measuring the insulation resistance value when a voltage of 500 V was applied by an insulation resistance meter immediately after 100 electrode groups were produced. The results are shown in Table 1 below.
[0099]
[Table 1]

[0100]
As is clear from Table 1, it can be seen that the secondary batteries of Examples 13 to 18 of the present invention can avoid the occurrence of defective insulation resistance when the electrode group is produced. This is because the separators used in Examples 13 to 18 are dense and can prevent a short circuit between the positive and negative electrodes during winding.
[0101]
Further, for the secondary batteries of Examples 13 to 18 and Comparative Examples 4 and 6, after charging 150% at 0.3 CmA, the discharge capacity was measured until the battery voltage reached 1.0 V at 2 CmA, The large current discharge characteristics were evaluated. The discharge capacity was calculated from the time until the battery voltage reached 1.0 V with 2 CmA discharge.
[0102]
The results of the large current discharge characteristic test are shown in Tables 2 and 3 below. In Table 2, the discharge capacities of Examples 14 to 15 and Comparative Example 4 are shown with the discharge capacity of Example 13 being 100. In Table 3, the discharge capacities of Examples 17 to 18 and Comparative Example 6 are shown with the discharge capacity of Example 16 being 100.
[0103]
[Table 2]

[0104]
[Table 3]

[0105]
As is clear from Table 2, it can be seen that the secondary batteries of Examples 13 to 15 of the present invention are superior in large current discharge characteristics as compared with Comparative Example 4. Further, as is clear from Table 3, it can be seen that the secondary batteries of Examples 16 to 18 of the present invention are superior in large current discharge characteristics as compared with Comparative Example 6. This is because the ion exchange amount of the separator used in the secondary batteries of Comparative Examples 4 and 6 is high, so that alkali metal ions in the electrolytic solution are immobilized on the separator, and the operating voltage during large current discharge is reduced. Is.
(Example 19)
Polypropylene resin is made of long fibers with a fiber diameter of 10 μm using the spunbond method, and the basis weight is 50 g / m ² Thus, a nonwoven fabric having a thickness of 0.20 mm was produced. Subsequently, a first roll having a smooth surface and a second roll having a plurality of pinpoint-like irregularities formed on the surface are arranged to face each other, and these rolls are rotated in opposite directions to each other and at 130 ° C. Then, the nonwoven fabric was passed between these rolls, pressed with the convex portions of the first roll and the second roll, and heat-fused to emboss. By this embossing, a plurality of circular concave portions are formed in the sheet-like material, and a filmed portion (film portion) is formed on the bottom surface (embossed surface) of the concave portion. At the same time, the portion of the second surface of the sheet-like material corresponding to the concave portion also becomes a filmed surface. The area ratio of the film part was 16% with respect to the second surface of the nonwoven fabric. Subsequently, the nonwoven fabric was immersed in an aqueous acrylic acid solution, and then irradiated with ultraviolet rays to graft copolymerize the acrylic acid monomer. The nonwoven fabric was washed to remove unreacted acrylic acid, and then dried to prepare a separator.
[0106]
The obtained separator is a non-woven fabric mainly composed of polypropylene fibers having COOH groups as ion exchange groups and polypropylene fibers not having COOH groups, and the embossed first surface is hydrophilized, and the first surface Regions other than the film portion of the second surface on the opposite side were made hydrophilic, and the film portion remained as a hydrophobic portion. The hydrophobic part of the second surface had an area ratio of 16%. Further, the graft copolymerization ratio of the acrylic acid monomer was measured by the titration method described above for the hydrophilic portion of the separator. As a result, the ion exchange amount of potassium was 0.8 meq / g. Formula (1) described above satisfies 0.55 ≦ Y ≦ 2.0 when the average diameter of all the fibers constituting the separator is 10 μm. Therefore, the separator simultaneously satisfies the formula (1) and the formula (2).
[0107]
Next, the separator is interposed between the negative electrode and the positive electrode similar to those in Example 1 so that the second surface having the hydrophilic portion and the hydrophobic portion of the separator faces the negative electrode, and is wound in a spiral shape to form an electrode Groups were made. An AA-sized cylindrical nickel-hydrogen secondary battery having the structure shown in FIG. 1 described above, in which an electrolytic solution composed of such an electrode group and 0.1 N LiOH and 7.9 N KOH is housed in a bottomed cylindrical container. Assembled.
[0108]
(Example 20)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 0.3 N LiOH and 7.7 N KOH was used as the alkaline electrolyte.
[0109]
(Example 21)
A nickel-hydrogen secondary battery similar to that in Example 19 was assembled, except that a mixed liquid composed of 0.5 N LiOH and 7.5 N KOH was used as the alkaline electrolyte.
[0110]
(Example 22)
A nickel-hydrogen secondary battery similar to that in Example 19 was assembled, except that a mixed liquid composed of 1.0 N LiOH and 7.0 N KOH was used as the alkaline electrolyte.
[0111]
(Example 23)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 1.3N LiOH and 6.7N KOH was used as the alkaline electrolyte.
[0112]
(Example 24)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 1.5N LiOH and 6.5N KOH was used as the alkaline electrolyte.
[0113]
(Example 25)
A nickel-hydrogen secondary battery similar to that in Example 19 was assembled, except that a mixed liquid composed of 0.5N NaOH and 7.5N KOH was used as the alkaline electrolyte.
[0114]
(Example 26)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 1.0 N NaOH and 7.0 N KOH was used as the alkaline electrolyte.
[0115]
(Example 27)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 2.0 N NaOH and 6.0 N KOH was used as the alkaline electrolyte.
[0116]
(Example 28)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 4.0 N NaOH and 4.0 N KOH was used as the alkaline electrolyte.
[0117]
(Example 29)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled, except that a mixed liquid composed of 5.0 N NaOH and 3.0 N KOH was used as the alkaline electrolyte.
[0118]
(Example 30)
A nickel-metal hydride secondary battery similar to that in Example 19 was assembled except that a mixed liquid composed of 6.0 N NaOH and 2.0 N KOH was used as the alkaline electrolyte.
[0119]
(Comparative Example 9)
A nickel-hydrogen secondary battery similar to that in Example 19 was assembled except that an 8.0 N aqueous KOH solution was used as the alkaline electrolyte.
[0120]
For the obtained secondary batteries of Examples 19 to 30 and Comparative Example 9, the self-discharge characteristics at 40 ° C. were measured in the same manner as performed for the secondary batteries of Examples 1 to 12 described above. The results are shown in FIGS. In addition, the result of the comparative example 2 is written together in FIG.9 and FIG.10.
[0121]
FIG. 9 shows the relationship between the lithium hydroxide concentration in the electrolyte solution and the capacity remaining rate in the secondary batteries of Examples 19 to 24 and Comparative Examples 2 and 9. FIG. 10 shows the relationship between the sodium hydroxide concentration in the electrolyte solution and the capacity remaining rate in the secondary batteries of Examples 25 to 30 and Comparative Examples 2 and 9.
[0122]
As is clear from FIG. 9, the secondary batteries of Examples 19 to 24 can suppress self-discharge during storage at a high temperature of 40 ° C. as compared with the secondary batteries of Comparative Examples 2 and 9. Further, as is clear from FIG. 10, it can be seen that the secondary batteries of Examples 25 to 30 can suppress self-discharge during high-temperature storage at 40 ° C. as compared with the secondary batteries of Comparative Examples 2 and 9. Further, by comparing FIG. 5 and FIG. 9 and FIG. 6 and FIG. 10, it is a sheet-like material satisfying the above formula (1) and the above formula (2), and the first surface is hydrophilic. A nickel-metal hydride secondary battery having a separator having a hydrophilic part and a hydrophobic part on the surface, the separator being disposed between the positive electrode and the negative electrode so that the second surface faces the negative electrode, It can be seen that the self-discharge characteristics during high-temperature storage can be improved as compared with a nickel-metal hydride secondary battery provided with separators that are equal to each other and satisfy the expressions (1) and (2).
[0123]
Moreover, about the obtained secondary battery of Examples 19-30 and Comparative Examples 1 and 2, battery internal pressure was measured. The battery internal pressure was measured by storing the batteries of Examples 19 to 30 and Comparative Examples 1 and 2 in the container of the pressure measuring device shown in FIG.
[0124]
That is, each battery internal pressure measuring device includes a battery case including an acrylic resin case main body 31 and a cap 32. A space 33 having the same inner diameter and height as the metal container of the AA size battery is formed at the center of the case body 31. The secondary battery C is accommodated in the space 33. The secondary battery C is opened without a sealing plate attached to the upper end of the bottomed cylindrical container. On the case body 31, the cap 32 is airtightly fixed by a bolt 36 and a nut 37 via a packing 34 and an O-ring 35. A pressure detector 38 is attached to the cap 32. A negative electrode lead 39 from the negative electrode and a positive electrode lead 40 from the positive electrode are led out between the packing 34 and the O-ring 35.
[0125]
With such a battery internal pressure measuring device, the maximum battery internal pressure when the secondary batteries of Examples 19 to 30 and Comparative Examples 1 and 2 were charged 480% at a current of 0.5 CmA was measured, and the results are shown in Table 4 below. As shown in FIG.
[0126]
[Table 4]

[0127]
[Table 5]

[0128]
As is apparent from Table 4, the secondary batteries of Examples 19 to 24 have a lower maximum battery internal pressure during overcharging than the secondary batteries of Comparative Examples 1 and 2. Further, as is clear from Table 5, it can be seen that the secondary batteries of Examples 25 to 30 have a lower maximum battery internal pressure during overcharge than the secondary batteries of Comparative Examples 1 and 2.
[0129]
Therefore, it can be seen that the secondary batteries of Examples 19 to 30 can not only improve the self-discharge characteristics during high-temperature storage compared to the secondary batteries of Examples 1 to 12, but also can suppress an increase in internal pressure during overcharging. . This is because the amount of ion exchange groups of the separators is different between the front and back, that is, the first surface is hydrophilized by ion exchange groups, and the second surface opposite to the first surface is formed by the ion exchange group and the hydrophilic portion. This is because a structure having a hydrophobic part which is an exchange group non-forming region is provided, and such a separator is interposed between the negative electrode and the positive electrode so that the second surface faces the negative electrode.
[0130]
(Example 31)
Made from polypropylene resin, polypropylene fiber with an average diameter of 3 μm by a melt blow method, with a basis weight of 18 g / m ² One non-woven fabric (inner layer) was prepared. Further, the weight per unit area is 18 g / m from a composite fiber having a core-sheath structure in which the surface of the core material made of polypropylene fiber is coated with polyethylene resin and having an average diameter of 15 μm. ² Two nonwoven fabrics (surface layer) were prepared. Polypropylene monofilament nonwoven fabrics were sandwiched between composite fiber nonwoven fabrics, and these nonwoven fabrics were integrated by heat fusion. The average diameter of the fibers constituting the resulting three-layer nonwoven fabric was 11 μm. Further, the basis weight of the three-layered nonwoven fabric is 54 g / m. ² The thickness was 0.15 mm. After immersing this nonwoven fabric in an aqueous acrylic acid solution, ultraviolet rays were applied to graft copolymerize the acrylic acid monomer, the nonwoven fabric was washed to remove unreacted acrylic acid, and then dried to produce a separator. .
[0131]
The obtained separator is composed of two layers comprising an inner layer mainly composed of a polypropylene fiber non-woven fabric having COOH groups as ion exchange groups, and a non-woven fabric mainly composed of composite fibers having COOH groups laminated on both sides of the inner layer. And a three-layer structure formed from the surface layer. The average diameter of the fibers of each surface layer is larger than the average diameter of the fibers of the inner layer. Moreover, the potassium ion exchange amount by the titration method mentioned above of the separator was 0.8 meq / g.
[0132]
Next, the separator was interposed between a negative electrode similar to Example 1 and a positive electrode similar to Example 1, and wound into a spiral shape to produce an electrode group. An AA-sized cylindrical nickel-metal hydride secondary battery having the structure shown in FIG. 1 was assembled by storing such an electrode group and an electrolytic solution composed of 7N KOH and 1N LiOH in a bottomed cylindrical container. According to the formula (1), (0.05X + 0.05) ≦ Y ≦ 2.0, when the average diameter X of all the fibers constituting the separator is 11 μm, the high temperature storage characteristics and the large current discharge characteristics are improved. The range of potassium ion exchange amount Y required for this is 0.6 ≦ Y ≦ 2.0. Therefore, the separator of the secondary battery of Example 31 satisfies the above-described formulas (1) and (2) (1 ≦ X ≦ 20).
[0133]
With respect to the obtained secondary battery of Example 31, self-discharge characteristics at 40 ° C. were measured by the same method as that for the secondary batteries of Examples 1 to 12 described above, and the results are shown in FIG. .
[0134]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a nickel-metal hydride secondary battery with improved self-discharge characteristics during high-temperature storage, excellent high-current discharge characteristics, and long life.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a nickel metal hydride secondary battery according to the present invention.
FIG. 2 is a plan view showing a manufacturing process of the separator of the present invention.
3 is a cross-sectional view taken along line III-III in FIG.
4 is an enlarged plan view of a main part of FIG. 2;
FIG. 5 is a characteristic diagram showing the relationship between the capacity remaining rate and the lithium hydroxide concentration in the electrolyte in the nickel metal hydride secondary batteries of Examples 1 to 6 and Comparative Examples 1 and 2 of the present invention.
6 is a characteristic diagram showing the relationship between the capacity remaining rate and the sodium hydroxide concentration in the electrolytic solution in the nickel-hydrogen secondary batteries of Examples 7 to 12 and Comparative Examples 1 and 2 of the present invention. FIG.
7 is a characteristic diagram showing the relationship between the capacity remaining rate and the ion exchange amount of the separator in the nickel-metal hydride secondary batteries of Examples 13 to 15 and Comparative Example 3 of the present invention. FIG.
FIG. 8 is a characteristic diagram showing the relationship between the capacity remaining rate and the ion exchange amount of the separator in the nickel metal hydride secondary batteries of Examples 16 to 18 and Comparative Example 5 of the present invention.
FIG. 9 is a characteristic diagram showing the relationship between the capacity remaining rate and the lithium hydroxide concentration in the electrolyte in the nickel-metal hydride secondary batteries of Examples 19 to 24 and Comparative Examples 2 and 9 of the present invention.
FIG. 10 is a characteristic diagram showing the relationship between the capacity remaining rate and the sodium hydroxide concentration in the electrolytic solution in the nickel hydride secondary batteries of Examples 25 to 30 and Comparative Examples 2 and 9 of the present invention.
FIG. 11 is a cross-sectional view showing a battery internal pressure measuring device for measuring battery internal pressure in nickel-metal hydride secondary batteries of Examples 19 to 30 of the present invention.
FIG. 12 is a characteristic diagram showing the relationship between the storage days at high temperature and the capacity remaining rate in Example 31 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Container, 2 ... Paste type positive electrode, 3 ... Separator, 4 ... Negative electrode, 7 ... Sealing plate, 8 ... Insulating gasket, 11 ... Safety valve.

Claims (2)

Comprising a positive electrode including a nickel hydroxide, a negative electrode containing a hydrogen storage alloy, and separators, an alkaline electrolyte,
The separator is formed from a sheet-like material containing polyolefin synthetic resin fibers having ion exchange groups, the average diameter of all the fibers constituting the separator is X μm, and the potassium ion exchange amount of the separator is Ymeq / g In addition, these satisfy the following formulas (1) and (2),
(0.05X + 0.05) ≦ Y ≦ 2.0 (1)
1 ≦ X ≦ 20 (2)
The alkaline electrolyte contains at least one of lithium hydroxide or sodium hydroxide , the first surface of the separator exhibits hydrophilicity, and the second surface located on the opposite side of the first surface has a hydrophilic portion and a hydrophobic surface. And the separator is arranged between the positive electrode and the negative electrode so that the second surface is located on the negative electrode side . The sheet-like material includes an inner layer including a polyolefin-based synthetic resin fiber having the ion-exchange group, and two surface layers formed on both surfaces of the inner layer and including the polyolefin-based synthetic resin fiber having the ion-exchange group. The nickel hydride secondary battery according to claim 1, wherein an average diameter of all the fibers constituting each of the surface layers is larger than an average diameter of all the fibers constituting the inner layer .

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