JP2004303473A - Polyelectrolyte porous membrane - Google Patents
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Abstract
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【０００１】
【発明が属する技術分野】
本発明は、ポリマーリチウム電池、リチウムイオン二次電池などの電気化学素子に用いる高分子電解質多孔質膜に関する。
【０００２】
【従来の技術】
ポリマーリチウム電池、リチウムイオン電池のような二次電池は、動作電圧が高いことから、多種多様な用途に用いられているが、近年、薄型化、高エネルギー密度化などのニーズがますます高まっている。このため、ポリマーリチウム電池に用いられるイオン導電性を有する各種の電解質層、あるいはリチウムイオン電池に用いられるポリオレフィン系のセパレータへの薄膜化の要望が大きくなっている。しかるに、電解質層の場合、薄層化に伴いその製法を問わず電池内部での厚さムラが発生しやすく、また、ポリオレフィン系のセパレータに関しては、主として多孔質化を目的とした延伸の際の応力ムラによる厚さの不均一性により、電池内部での微小短絡が発生しやすくなる欠点がある。
したがって、ポリマーリチウム電池やリチウムイオン電池のような二次電池にはこのように厚さムラや欠陥のない均一で薄い電解質層やセパレータの実現が望まれている。
【０００３】
従来、ポリマーリチウム電池用に幾つかの高分子電解質が提案され、イオン伝導の原理上、電解液によって十分にゲル化するものが多用されている（例えば、特許文献１参照）。従来、このようにゲル化しやすい樹脂を電解質層として用いる場合、電池内部にモノマーを注入し重合する方法が知られている（例えば、特許文献２、特許文献３、特許文献４参照）。しかし、この方法では薄層化に際して、ゲル化した層が部分的に非常に薄くなる場合があり、微小短絡を起こしやすい。一方、これらの不均一性を是正し、薄膜化した場合でも微小短絡の発生を抑制する目的で、電解質層またはセパレータ内に電極間のスペーサとして微小フィラーを混合して用いることが考えられるが、高分子電解質またはセパレータとの分散が不十分な場合は、膜厚が不均一となるほか、高分子電解質やセパレータとの界面での接着が不十分な場合においては、膜の強度を低下し間隙やピンホールが生ずることでかえって微小短絡を発生する原因となっていた。
【０００４】
【特許文献１】
特公昭６１−２３９４７号公報
【特許文献２】
特開平１１−２１９７２８号公報
【特許文献３】
特開２０００−６７８６６号公報
【特許文献４】
特開２０００−６７９１７号公報
【０００５】
【発明が解決しようとする課題】
本発明は、前記従来の高分子電解質多孔質膜の問題に鑑みてなされたものであり、薄膜化しても内部構造が均一性を有し、微小短絡が発生しにくく、耐久性が良好な高分子電解質多孔質膜を提供するものである。
【０００６】
【課題を解決するための手段】
本発明の高分子電解質多孔質膜は、白色度の最大頻度が７０％以上であり、かつ前記白色度の標準偏差値が０．００２５以下であることを特徴とする。
【０００７】
【発明の実施の形態】
本発明者は、目視上では必ずしも明らかとはならないような高分子電解質多孔質膜に存在する内部不均一構造が、微小短絡に結びつく可能性が高いことを見出した。即ち、微小短絡が発生する可能性を評価する尺度として、白色度が非常に重要であることを見出し本発明に至った。
本発明の高分子電解質多孔質膜は、白色度の最大頻度が７０％以上であり、かつ、該白色度の標準偏差値が０．００２５以下である。白色度の最大頻度が７０％未満であり、かつその標準偏差値が０．００２５を超す範囲である場合では、全体に目視では確認しずらい微小なムラが多く、部分的に膜厚が薄いか、または空隙率が不均一な膜部分が存在する。このような不均一な膜部分に過大な電圧や過度の機械的ストレスが負荷された場合に、それらの部分で微小短絡が発生する。
高分子電解質多孔質膜の白色度は、例えば反射濃度計ＴＣ−６Ｄ（東京電色社製）において、操作パネルのレンジをデンシティーとした場合の反射濃度の測定値をいう。本発明における白色度は、高分子電解質多孔質膜をＢ５サイズに裁断した後、マクベス社製の反射濃度計ＲＤ−９１４に用いられる標準板の黒色部分に載置し、高分子電解質多孔質膜表面を上記反射濃度計ＴＣ−６Ｄで測定するものである。
白色度の最大頻度は、上記方法にて高分子電解質多孔質膜の任意の位置４０点を測定し、それらの測定結果から図１に示すようなグラフを作成して得ることができる。図１は、横軸が白色度、縦軸が頻度を表し、２種類の高分子電解質多孔質膜Ａ及びＢにおける４０点の測定値をプロットしたものである。白色度の最大頻度とは、白色度の数値頻度が最も多く得られた部位をいい、図１のＡの場合ではａであり、Ｂの場合ではｂである。この場合Ａの最大頻度は７０％以上であるが、Ｂの最大頻度は７０％未満であることを示している。
また、白色度の標準偏差値は、上記４０点の白色度の測定結果から求めることができる。
【０００８】
また、高分子電解質多孔質膜の白色度の平均値と最小値の差が０．０１５以下であることが好ましい。高分子電解質多孔質膜の全体的な不均一性は、上記の白色度の最大頻度の大きさと、標準偏差値で把握できるものの、白色度の平均値と最小値の差が０．０１５より大きい場合は、機械強度が弱い部分が存在しやすいため、例えば電池を巻回方式で組みたてる場合に、機械強度が弱い部分の膜に亀裂を生じるなどの問題が生じやすく好ましくない。
【０００９】
次に、本発明の高分子電解質多孔質膜の製造方法を下記に述べる。
本発明における高分子電解質多孔質膜は、高分子電解質であるフッ素系樹脂に対して、これと非相溶の溶媒を該フッ素系樹脂に対してミクロ相分離させる製法で得られる。該ミクロ相分離現象を応用した方法としては、湿式法と乾式法が知られている。湿式法は、予め高分子電解質樹脂である溶媒可溶型フッ素系樹脂をＮ−メチルピロリドン、Ｎ，Ｎ−ジメチルアセトアミド、Ｎ，Ｎ−ジメチルホルムアミド、Ｎ，Ｎ−ジメチルスルホキシド等の溶剤に溶解した塗料を、ベースフィルムとなる樹脂フィルム上に、適宜塗工した後に、該フッ素樹脂と相溶しない溶媒中に浸漬した後、乾燥し、ベースフィルムから剥離することで得られる。一方、乾式法では、溶媒可溶型フッ素系樹脂を溶解した塗料中に、該フッ素系樹脂と非相溶であり、かつ高分子電解質に相溶で沸点の高い溶媒を混入した塗料を耐熱性が良好なフィルム上に塗工した後、先に該フッ素系樹脂と相溶の溶媒だけを蒸発した後に、フッ素系樹脂樹脂と非相溶の溶媒を蒸発させる温度にまで更に昇温して非相溶の溶媒を蒸発させ、乾燥することで得られる。このような製造方法により、フッ素系樹脂の単独膜からなる高分子電解質多孔質膜を得ることができる。
【００１０】
本発明の高分子電解質多孔質膜の白色度について、その最大頻度が７０％以上に、また標準偏差値を０．００２５以下にするためには、例えば下記に列挙するような製造および材料の選択条件を制御すればよい。
▲１▼湿式法でのフッ素系樹脂に対する非相溶溶媒への浸漬時間（実施例１／比較例１参照）
▲２▼湿式法での高分子電解質型樹脂に対する可溶溶媒の量（実施例１／比較例２参照）
▲３▼多孔質基材の厚み（実施例３／比較例４〜５参照）
▲４▼溶媒可溶型フッ素樹脂の成分組成（実施例１／比較例６参照）
▲５▼有機フィラーの組成（実施例２／比較例７参照）
また、これら諸条件以外に、次に述べるように、塗料の粘度や乾燥条件を適宜選択することによっても、本発明で特定の白色度を得ることが可能である。例えば、塗料粘度が過小である場合は、塗工方法を問わず液ダレが発生しやすく、白色度のバラツキが過大となる場合がある。また、湿式法の場合においては、塗工したフィルムを非相溶の溶媒に浸ける速度にムラがある場合や、塗料中の溶媒と、これと非相溶の溶媒の置換が不十分なまま乾燥した場合に、乾燥後において残存する溶媒で一旦形成された多孔質構造を破壊する場合など、様々である。本発明においては、これらの要因を制御することで、本発明の白色度を有する高分子電解質多孔質膜を得ることができ、その結果、微小短絡の発生を防止することができる。
【００１１】
本発明の高分子電解質多孔質膜には、溶媒可溶型フッ素樹脂を用いることが好ましい。該溶媒可溶型フッ素樹脂として、ポリフッ化ビニリデン樹脂または／およびフッ化ビニリデンの共重合体樹脂がより好適に用いられる。フッ化ビニリデンの共重合体樹脂としては、フッ化ビニリデンと他のモノマーを共重合させた樹脂であり、他のモノマーとして、例えばエチレン、プロピレン等の炭化水素単量体、フッ化ビニル、３フッ化エチレン、３フッ化塩化エチレン、４フッ化エチレン、６フッ化プロピレン、フルオロアルキルビニルエーテル等の含フッ素単量体、マレイン酸モノメチル、シトラコン酸モノメチル等のカルボキシル基含有単量体、またはアリルグリシジエーテル、クロトン酸グリシジルエステル等のエポキシ基含有ビニル単量体、などが挙げられる。
これらの樹脂は、イオン伝導性が良好であるばかりでなく、本発明で特定する白色度を満足できるものであり、多孔質化した場合に適度な柔軟性を持つために、折り曲げても膜の破断が生じにくいなど、電池への組み込みが容易であるなどの効果を有する。また本発明では、溶媒可溶性があれば、これらの樹脂に限定されるものではない。なお、溶媒可溶型フッ素樹脂における溶媒とはアミド系溶媒をいう。
また、溶媒可溶型フッ素樹脂として、ポリフッ化ビニリデン樹脂とその他の溶媒可溶型のフッ素系樹脂を混合して用いた場合では、電解液に対し溶けやすい樹脂を適度に混合することができるので、該高分子電解質多孔質の一部が溶解した電解液が、高分子電解質多孔質膜と電極との隙間に存在することとなる。その結果、電極表面に電解液が常に存在することが可能となり、サイクル特性が向上するため好ましい。
【００１２】
本発明の高分子電解質多孔質膜は、電解溶媒に対する不溶率が９０重量％以上であることが好ましい。特に電解溶媒に対する不溶率が９０重量％以上の溶媒可溶型フッ素樹脂を含むことが好ましい。電解溶媒に対する不溶率が９０重量％未満の場合では、電池にした場合に電解液へ高分子電解質多孔質膜が溶出する量が多くなり、電解液の粘度が過大となることで、イオン伝導を妨げる場合があるため好ましくない。このように電解溶媒に対して溶解しやすい溶媒可溶型フッ素樹脂としては、フッ化ビニリデン−６フッ化プロピレン共重合体樹脂があげられるが、過度に混合すると電解液の粘度が過大となり、むしろ電流密度に対する電池出力の応答性（以下、レート特性という）などの電池としての基本特性を低下せしめることとなり好ましくない。
本発明における電解溶媒に対する不溶率の測定は、高分子電解質多孔質膜を小片に切り出したものをエチレンカーボネート及びプロピレンカーボネートが重量比で等量となるように混合した電解溶媒にガラスサンプル管中で浸漬、密閉し８０℃で１６時間放置する。次に該小片をメタノールで洗浄した後に２５℃で１時間乾燥後、重量を測定する。そして、浸漬後の重量を浸漬前の重量で除した数値をパーセント表示し不溶率とする。
【００１３】
本発明においては、高分子電解質多孔質膜の内部に、有機フィラーを含有することが好ましい。有機フィラーを含有する第一の目的は、該微粒子が電極間のスペーサーとして機能することで、微小短絡の抑制効果を更に向上できることにある。有機フィラーは、少なくともメチルメタアクリレートまたはスチレンの単独または共重合体樹脂からなり、架橋成分を含むものが好適に用いられる。これらの材質からなる有機フィラーは、上記スペーサーの機能のほかに、高分子電解質多孔質膜の機械的強度を向上する作用効果を奏する。特にメチルメタアクリレートはイオン伝導性も良好であり、本発明では好適に使用される。本発明においては、有機フィラーであれば、いずれも好適に使用される。有機フィラーの電解溶媒に対する不溶率は８０℃で９０重量％以上であることが好ましい。該不溶分が９０重量％未満の場合、電解液に該有機フィラーが溶出する量が多くなるため、電解液の粘度が上がり、イオン伝導性に支障をきたす場合があるため好ましくない。また、電解溶媒に対する不溶率が９０重量％以上の有機フィラーは、その一部が電解液に微量に溶解することで、溶解した成分が電解質と有機フィラーとの接着性を高め、微小短絡の発生を防止する効果がある。有機フィラーの電解溶媒に対する不溶率は、前記高分子電解質多孔質膜における電解溶媒に対する不溶率と同様にして測定することができる。
【００１４】
本発明においては、膜厚が３μｍ以上かつ１４μｍ以下であって、透気度が４００（ｓｅｃ／１００ｃｃ）以下の多孔質基材を高分子電解質多孔質膜中に担持させてもよい。多孔質基材の膜厚が３μｍ未満の場合は、多孔質基材の機械的強度が低く、高分子電解質多孔質膜とした場合、部分的に白色度が低く空隙部が不均一な膜となりやすいため好ましくない。１４μｍを超える範囲では、高分子電解質多孔質膜とした場合に膜厚が過大となり、電池の薄型化に貢献できないほか、インピーダンスが過大となり電池性能上好ましくない。
【００１５】
該多孔質基材の透気度が４００（ｓｅｃ／１００ｃｃ）を超える範囲では、高分子電解質多孔質膜とした場合に透気度が過大となり、電池のレート特性を低下するなどの問題を生じる場合がある。従来は、これらの多孔質基材では、膜厚が３μｍ以上かつ１４μｍ以下の範囲に薄膜化した場合、ムラが生じやすく白色度が低くなり、そのままの状態で電池のセパレータとして用いた場合は微小短絡が発生していた。しかしながら、前記フッ素系樹脂と上記多孔質基材を複合化し、高分子電解質多孔質膜中に有することで、均一な白色度が得られ、その結果、薄膜であっても微小短絡を発生しないものを得ることができる。
【００１６】
前記複合化の手段としては、前記フッ素系樹脂の単独膜と多孔質基材とを重ね合わせた上で、加熱または／および加圧することで接合する方法や、このほかに、フッ素系樹脂を塗料化し、ロール塗工、バー塗工、スプレー塗工など既存の塗工方法で多孔質基材の片面又は両面に塗工後、前記の多孔質化の製法である湿式法または乾式法を用いることで多孔質化し、複合化することが可能である。多孔質基材は、上記の膜厚範囲であれば、いずれも好適に用いることができるが、更に好ましくは、少なくともポリオレフィン樹脂を含む多孔質膜またはポリイミド多孔質膜が好ましい。ポリオレフィン樹脂の多孔質膜には、有機フィラーのほか、無機フィラーを添加して、上記のスペーサー効果を高めることも可能である。一方、ポリイミド多孔質膜は、薄膜でも機械的強度が高く、本発明で好適に用いることができる。なお、フッ素系樹脂と多孔質基材とを複合化させた高分子電解質多孔質膜では、フッ素系樹脂を形成させた面の白色度を測定する。
【００１７】
本発明の高分子電解質多孔質膜は多孔質基材を有しない場合は、膜厚として、８〜４０μｍ、好ましくは８〜３５μｍ、更に好ましくは、８〜２０μｍである。８μより薄い場合は、引っ張り強度が低下し電池を組み立てる際に巻回しずらい。一方、４０μｍより厚い場合は、イオン伝導性が劣る場合があり好ましくない。また、多孔質基材を有する高分子電解質多孔質膜では４〜２５μｍ、好ましくは、４〜１８μｍ、更に好ましくは４〜１４μｍである。４μｍより薄い場合は、多孔質基材が有する機械的強度の弱さや空隙の不均一性をフッ素系樹脂で補うことができず、白色度の標準偏差値及び平均値と最小値の差が過多となり好ましくない。一方、２５μｍより厚い場合では、透気度が過多となり、レート特性が低下する場合があるため好ましくない。
【００１８】
【実施例】
以下、本発明の実施例について述べる。
実施例１
ポリフッ化ビニリデン樹脂１５重量部をＮ−メチルピロリドン１３５重量部で加熱溶解した溶液を作製し、該溶液をポリプロピレンフィルム上に塗工した後、該フィルムを水７５重量部とメタノール２５重量部からなる混合溶媒に１５分間浸漬した後、６０℃にて乾燥した後、ポリプロピレンフィルムを剥離、除去して本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚はマイクロメータで測定したところ３１μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９７％であった。
【００１９】
実施例２
実施例１の溶液に、更に有機フィラーとして平均粒子径が０．３μｍのスチレンとメチルメタアクリレートをジビニルベンゼンで架橋した共重合体樹脂（電解溶媒に対する不溶率：９９％）を１５重量部添加した他は、実施例１と同様にして本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ２６μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９８％であった。
【００２０】
実施例３
膜厚８μｍのポリエチレンからなる多孔質基材（透気度：２３０ｓｅｃ／１００ｃｃ）を、粘着剤を塗布したポリプロピレンフィルム上に貼り合わせた後に、該多孔質基材上に実施例１の溶液をスプレー塗工した後、実施例１と同様の方法で該塗工層を多孔質化した上で、ポリプロピレンフィルムを剥離、除去して、本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ１０μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その塗工した面の白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９６％であった。
【００２１】
実施例４
膜厚６μｍのポリエチレンとガラスの微粒子からなる多孔質基材（透気度：７０ｓｅｃ／１００ｃｃ）を、粘着剤を塗布したポリプロピレンフィルム上に貼り合わせた後に、多孔質基材上に実施例１の溶液をスプレー塗工した後、実施例１と同様の方法で該塗工層を多孔質化した上で、ポリプロピレンフィルムを剥離、除去して、本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ９μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その塗工した面の白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９５％であった。
【００２２】
実施例５
膜厚１０μｍのポリイミドからなる多孔質基材（透気度：１００ｓｅｃ／１００ｃｃ）を、粘着剤を塗布したポリプロピレンフィルム上に貼り合わせた後に、多孔質基材上に実施例１の溶液をスプレー塗工した後、実施例１と同様の方法で該塗工層を多孔質化した上で、ポリプロピレンフィルムを剥離、除去して、本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ１３μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その塗工した面の白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９６％であった。
【００２３】
実施例６
実施例１において、ポリフッ化ビニリデン樹脂１５重量部のうち、１．２重量部をフッ化ビニリデン−６フッ化プロピレン共重合体樹脂に置き換えたほかは、実施例１と同様にして、本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ２７μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９３％であった。
【００２４】
実施例７
実施例６で用いた溶液を、実施例３の多孔質基材に対して、実施例３と同様に用い、本発明の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ１１μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その塗工した面の白色度を測定したところ表１の結果を得た。また得られた高分子電解質多孔質膜の不溶率は９８％であった。
【００２５】
比較例１
実施例１において、水７５重量部とメタノール２５重量部からなる混合溶媒への浸漬時間を２分とした以外は、実施例１と同様にして比較用の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。この高分子電解質多孔質膜の膜厚は２７μｍであった。
【００２６】
比較例２
実施例１において、Ｎ−メチルピロリドンの量を１８５重量部とした以外は、実施例１と同様にして溶液を調整し、ポリプロピレンフィルム上に塗工した。塗工直後のポリプロピレンフィルムをほぼ直角にして水７５重量部とメタノール２５重量部からなる混合溶媒に約０．１ｍ／ｍｉｎの速度で浸漬した。その後の処理は実施例１と同様にして比較用の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜の膜厚をマイクロメータで測定したところ２９μｍであった。この高分子電解質多孔質膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。
【００２７】
比較例３
実施例３の多孔質基材そのものを比較用の高分子電解質多孔質膜とした。この膜をＢ５サイズに裁断し、その白色度を測定したところ表１の結果を得た。
【００２８】
比較例４
実施例３において、多孔質基材を膜厚２μｍのポリエチレン微多孔膜を用いた以外は、実施例３と同様にして比較用の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜をＢ５サイズに裁断し、塗工した面の白色度を測定した結果を表１に示した。この高分子電解質多孔質膜の膜厚は４μｍであった。
【００２９】
比較例５
実施例３において、多孔質基材を膜厚３０μｍのポリエチレン微多孔膜を用いた以外は、実施例３と同様にして比較用の高分子電解質多孔質膜を得た。得られた高分子電解質多孔質膜をＢ５サイズに裁断し、塗工した面の白色度を測定した結果を表１に示した。この高分子電解質多孔質膜の膜厚は３７μｍであった。
【００３０】
比較例６
実施例１において、ポリフッ化ビニリデン樹脂１５重量部のうち１０重量部をフッ化ビニリデン−６フッ化プロピレン共重合体樹脂に置き換えたほかは、実施例１と同様にして、比較用の高分子電解質多孔質膜を得た。この高分子電解質多孔質膜の膜厚は２５μｍであり、白色度は表１に示す結果であった。この高分子電解質多孔質膜の不溶率は３６％であった。
【００３１】
比較例７
実施例２において、有機フィラーとしてスチレンとブチルアクリレートの非架橋の共重合体樹脂（電解溶媒に対する不溶率：３重量％、平均粒子径：８μｍ）を用いたほかは、実施例２と同様にして比較用の高分子電解質多孔質膜を得た。この高分子電解質多孔質膜の膜厚は２５μｍであり、白色度は表１に示す結果であった。この高分子電解質多孔質膜の不溶率は４６％であった。
【００３２】
【表１】
前記で得られた実施例及び比較例の高分子電解質多孔質膜について、下記のように微小短絡特性を確認した。実施例及び比較例の高分子電解質多孔質膜を真空乾燥後、これらの高分子電解質多孔質膜に対して不活性ガス中で、エチレンカーボネート及びプロピレンカーボネートがそれぞれ等重量となるように調整した混合溶媒に対してＬｉＰＦ６を１モル溶解した電解液を０．０１ｃｃ／ｃｍ２の割合で均等に含浸させた。次にこの電解液を含浸させた高分子電解質多孔質膜を２枚の平坦なステンレス板間に挟み、両ステンレス板を高分子電解質多孔質膜が接触していない外側の面から対向する外力をかけて加圧することで狭持した。次にステンレス板に対して４．２Ｖの定電圧を印加し、電流を徐々に増加することで、微小短絡の発生の有無を確認した。
【００３４】
この結果、実施例１〜７の高分子電解質多孔質膜では、電流値が１０００ｍＡ／ｈまでの範囲でいずれも電圧降下がみられず、微小短絡の発生は認められなかった。一方、比較例４及び５を除く比較例１〜７の高分子電解質多孔質膜では、いずれも電流値が１０００ｍＡ／ｈに至る過程で電圧降下が認められ、微小短絡が発生していることが確認された。比較例４では、ごく薄膜の多孔質基材に更に薄く高分子電解質多孔質膜を複合したものであるが、使用した多孔質基材の機械的強度が弱く、微小短絡試験にかける前の段階で損傷し、同試験を行うことができなかった。一方、比較例５では、微小短絡は認められなかったものの、透気度が無限大となったため、電子顕微鏡にて断面構造を観察したところ、面方向に部分的に一律なリジッドな層が存在し、均一な多孔質となっておらず、実用上問題があることが確認された。
なお、比較例１は、浸漬のための時間が短いために、溶媒が残存し、乾燥過程で部分的に多孔質が融着を起こしたために、その部分の膜厚が減少したことが白色度のバラツキ拡大に繋がったものと推測される。また、比較例２は溶液の粘度が低下したことと、浸漬する際の角度が急峻であるため、塗工面で液ダレがおきて部分的に塗工層の厚みが減少した部分が生じて白色度が低下し、バラツキの拡大に繋がったものと推測される。
【００３５】
比較例３は、多孔質基材が薄く部分的に白色度が低い不均一な膜であった。この多孔質基材は延伸法により作られているが、薄膜化のために破断する限界まで延伸したものであり、その結果、破断寸前の部分が白色度の低下となって現れているものと思われる。この多孔質基材を用いて、高分子電解質多孔質膜を複合した実施例３では白色度のバラツキが少なくなり、微小短絡が発生しなかったことから、本発明の有効性が確認された。比較例６は、白色度のバラツキは、比較例の中では相対的に良好な結果であったものの、電解液に対するフッ化ビニリデン−ヘキサフルオロプロピレン共重合体樹脂の溶解量が過大なために、該樹脂が溶解した部分の電気的な耐性が低下し、微小短絡を起こしたものと推測される。比較例７は、混合した有機フィラーが電解液に対して溶けやすいものであったために、溶け出した部分の電気的な耐性が低下し、微小短絡が発生しやすくなったものと推定される。
【００３６】
【発明の効果】
本発明によれば、薄膜でも非常に均質であり、微小短絡が発生せず、その結果、耐久性が良好な高分子電解質多孔質膜が得られる。
【図面の簡単な説明】
【図１】高分子電解質多孔質膜の白色度の最大頻度について説明する図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a polymer electrolyte porous membrane used for an electrochemical device such as a polymer lithium battery and a lithium ion secondary battery.
[0002]
[Prior art]
Secondary batteries, such as polymer lithium batteries and lithium ion batteries, are used in a wide variety of applications due to their high operating voltage.However, in recent years, there has been an increasing need for thinner and higher energy densities. I have. For this reason, there is an increasing demand for thinning various ionic conductive electrolyte layers used in polymer lithium batteries or polyolefin-based separators used in lithium ion batteries. However, in the case of the electrolyte layer, thickness unevenness easily occurs inside the battery regardless of the manufacturing method due to the thinning of the electrolyte layer. There is a disadvantage that a minute short circuit easily occurs inside the battery due to unevenness in thickness due to stress unevenness.
Therefore, a secondary battery such as a polymer lithium battery or a lithium ion battery is desired to have a uniform and thin electrolyte layer and separator free from such unevenness in thickness and defects.
[0003]
Conventionally, several polymer electrolytes have been proposed for polymer lithium batteries, and those which sufficiently gel by an electrolyte solution on the principle of ion conduction are frequently used (for example, see Patent Document 1). Conventionally, when such a gelling resin is used as an electrolyte layer, a method of injecting a monomer into a battery and polymerizing the same is known (for example, see Patent Literature 2, Patent Literature 3, and Patent Literature 4). However, in this method, when the layer is thinned, the gelled layer sometimes becomes extremely thin in some cases, and a minute short circuit is likely to occur. On the other hand, for the purpose of correcting these non-uniformities and suppressing the occurrence of minute short circuits even when the film is thinned, it is conceivable to use a mixture of minute fillers as a spacer between electrodes in an electrolyte layer or a separator. When the dispersion with the polymer electrolyte or the separator is insufficient, the film thickness becomes non-uniform.When the adhesion at the interface with the polymer electrolyte or the separator is insufficient, the strength of the membrane is reduced and the gap is reduced. The occurrence of pinholes or pinholes causes a micro short circuit.
[0004]
[Patent Document 1]
JP-B-61-23947
[Patent Document 2]
JP-A-11-219728
[Patent Document 3]
JP 2000-67866 A
[Patent Document 4]
JP-A-2000-67917
[0005]
[Problems to be solved by the invention]
The present invention has been made in view of the problem of the conventional polymer electrolyte porous membrane, and has a uniform internal structure even when thinned, hardly causes a micro short circuit, and has high durability. It is intended to provide a molecular electrolyte porous membrane.
[0006]
[Means for Solving the Problems]
The porous polymer electrolyte membrane of the present invention is characterized in that the maximum frequency of whiteness is 70% or more and the standard deviation of the whiteness is 0.0025 or less.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventor has found that an internal heterogeneous structure present in the polymer electrolyte porous membrane, which is not always apparent visually, is highly likely to lead to a micro short circuit. That is, the inventors have found that whiteness is very important as a scale for evaluating the possibility of occurrence of a micro short circuit, and have reached the present invention.
In the polymer electrolyte porous membrane of the present invention, the maximum frequency of whiteness is 70% or more, and the standard deviation of the whiteness is 0.0025 or less. When the maximum frequency of whiteness is less than 70% and the standard deviation value is in a range exceeding 0.0025, there are many fine irregularities which are hard to confirm visually, and the film thickness is partially thin. Alternatively, there is a film portion having a non-uniform porosity. When an excessive voltage or excessive mechanical stress is applied to such non-uniform film portions, a micro short circuit occurs in those portions.
The whiteness of the polymer electrolyte porous membrane refers to, for example, a measured value of the reflection density in a reflection densitometer TC-6D (manufactured by Tokyo Denshoku Co., Ltd.) when the range of the operation panel is set to the density. The whiteness in the present invention is determined by cutting the polymer electrolyte porous membrane into B5 size, and then placing the polymer electrolyte porous membrane on a black portion of a standard plate used for a reflection densitometer RD-914 manufactured by Macbeth. The surface is measured by the reflection densitometer TC-6D.
The maximum frequency of the whiteness can be obtained by measuring 40 arbitrary positions of the polymer electrolyte porous membrane by the above-described method and creating a graph as shown in FIG. 1 from the measurement results. In FIG. 1, the horizontal axis represents whiteness, and the vertical axis represents frequency, and plots measured values at 40 points in two types of polymer electrolyte porous membranes A and B. The maximum frequency of the whiteness refers to a portion where the numerical frequency of the whiteness is obtained most, and is a in the case of A in FIG. 1 and b in the case of B in FIG. In this case, the maximum frequency of A is 70% or more, but the maximum frequency of B is less than 70%.
Further, the standard deviation value of the whiteness can be obtained from the measurement results of the whiteness at the 40 points.
[0008]
The difference between the average value and the minimum value of the whiteness of the porous polymer electrolyte membrane is preferably 0.015 or less. The overall nonuniformity of the polymer electrolyte porous membrane can be grasped by the above-mentioned maximum frequency of whiteness and the standard deviation value, but the difference between the average value and the minimum value of whiteness is greater than 0.015. In such a case, a portion having low mechanical strength is likely to be present, and thus, for example, when the battery is assembled by a winding method, a problem such as cracking of the film of the portion having low mechanical strength is likely to occur, which is not preferable.
[0009]
Next, a method for producing the polymer electrolyte porous membrane of the present invention will be described below.
The polymer electrolyte porous membrane in the present invention is obtained by a production method in which a solvent incompatible with a fluororesin which is a polymer electrolyte is microphase-separated from the fluororesin. As a method utilizing the micro phase separation phenomenon, a wet method and a dry method are known. In the wet method, a solvent-soluble fluororesin, which is a polymer electrolyte resin, is previously dissolved in a solvent such as N-methylpyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, N, N-dimethylsulfoxide. The coating can be obtained by appropriately applying a coating on a resin film serving as a base film, immersing the coating in a solvent incompatible with the fluororesin, drying, and peeling the coating from the base film. On the other hand, in the dry method, in a coating material in which a solvent-soluble fluorine-based resin is dissolved, a coating material which is incompatible with the fluorine-based resin and is mixed with a polymer electrolyte and has a high boiling point is mixed with a heat-resistant coating. After coating on a good film, first evaporate only the solvent compatible with the fluororesin, and then further raise the temperature to a temperature at which the solvent incompatible with the fluororesin is evaporated. It is obtained by evaporating the compatible solvent and drying. By such a production method, a polymer electrolyte porous membrane composed of a single membrane of a fluororesin can be obtained.
[0010]
In order to make the maximum frequency of the whiteness of the polymer electrolyte porous membrane of the present invention not less than 70% and the standard deviation value not more than 0.0025, for example, the production and selection of materials listed below are required. Conditions may be controlled.
{Circle around (1)} Immersion time in a non-compatible solvent for a fluororesin in a wet method (see Example 1 / Comparative Example 1)
{Circle over (2)} Amount of Soluble Solvent for Polymer Electrolyte Resin by Wet Method (See Example 1 / Comparative Example 2)
(3) Thickness of porous substrate (see Example 3 / Comparative Examples 4 and 5)
(4) Component composition of solvent-soluble fluororesin (see Example 1 / Comparative Example 6)
(5) Composition of the organic filler (see Example 2 / Comparative Example 7)
In addition to these various conditions, a specific whiteness can be obtained in the present invention by appropriately selecting the viscosity and drying conditions of the paint as described below. For example, when the paint viscosity is too low, liquid dripping is likely to occur regardless of the coating method, and the variation in whiteness may be excessive. In addition, in the case of the wet method, there is unevenness in the speed at which the coated film is immersed in an incompatible solvent, or when the solvent in the paint and the solvent incompatible with the solvent are insufficiently dried, the drying is performed. In this case, there are various cases, such as when the porous structure once formed is destroyed by the solvent remaining after drying. In the present invention, by controlling these factors, the polymer electrolyte porous membrane having the whiteness of the present invention can be obtained, and as a result, the occurrence of a micro short circuit can be prevented.
[0011]
It is preferable to use a solvent-soluble fluororesin for the polymer electrolyte porous membrane of the present invention. As the solvent-soluble fluororesin, a polyvinylidene fluoride resin or / and a copolymer resin of vinylidene fluoride is more preferably used. The vinylidene fluoride copolymer resin is a resin obtained by copolymerizing vinylidene fluoride and another monomer. Examples of the other monomer include a hydrocarbon monomer such as ethylene and propylene, vinyl fluoride, and 3 fluorocarbon. Fluorinated monomers such as fluorinated ethylene, trifluorinated ethylene, tetrafluoroethylene, propylene hexafluoro, and fluoroalkyl vinyl ether; carboxyl group-containing monomers such as monomethyl maleate and monomethyl citraconic acid; or allyl glycidyl And epoxy group-containing vinyl monomers such as ether and glycidyl crotonate.
These resins not only have good ionic conductivity, but also can satisfy the whiteness specified in the present invention, and have appropriate flexibility when made porous, so that they can be folded to form a film. It has effects such as easy breakage and easy incorporation into batteries. The present invention is not limited to these resins as long as they have solvent solubility. The solvent in the solvent-soluble fluororesin refers to an amide solvent.
Further, when a mixture of a polyvinylidene fluoride resin and another solvent-soluble fluororesin is used as the solvent-soluble fluororesin, a resin that is easily soluble in the electrolyte can be appropriately mixed. The electrolytic solution in which a part of the porous polymer electrolyte is dissolved is present in the gap between the porous polymer electrolyte membrane and the electrode. As a result, the electrolyte can always be present on the electrode surface, and the cycle characteristics are improved, which is preferable.
[0012]
The porous polymer electrolyte membrane of the present invention preferably has an insolubility in an electrolytic solvent of 90% by weight or more. In particular, it is preferable to include a solvent-soluble fluororesin having an insolubility in an electrolytic solvent of 90% by weight or more. When the insolubility in the electrolytic solvent is less than 90% by weight, the amount of the polymer electrolyte porous membrane eluted into the electrolyte when the battery is used is increased, and the viscosity of the electrolyte is excessively increased. It is not preferable because it may interfere. As such a solvent-soluble fluororesin which is easily dissolved in the electrolytic solvent, a vinylidene fluoride-6-propylene copolymer copolymer resin can be cited, but if mixed excessively, the viscosity of the electrolytic solution becomes excessively high. Basic characteristics such as the response of the battery output to the current density (hereinafter, referred to as rate characteristics) are deteriorated, which is not preferable.
The measurement of the insolubility in the electrolytic solvent in the present invention is carried out in a glass sample tube by mixing an electrolytic solvent obtained by cutting a polymer electrolyte porous membrane into small pieces so that ethylene carbonate and propylene carbonate are equal in weight ratio. Immerse, seal, and leave at 80 ° C. for 16 hours. Next, the small pieces are washed with methanol, dried at 25 ° C. for 1 hour, and weighed. Then, the value obtained by dividing the weight after immersion by the weight before immersion is expressed as a percentage and is defined as the insoluble ratio.
[0013]
In the present invention, it is preferable that an organic filler is contained inside the polymer electrolyte porous membrane. The first purpose of containing an organic filler is to allow the fine particles to function as a spacer between electrodes, thereby further improving the effect of suppressing minute short circuits. The organic filler is at least composed of a homo- or copolymer resin of methyl methacrylate or styrene, and one containing a crosslinking component is preferably used. Organic fillers made of these materials have an effect of improving the mechanical strength of the polymer electrolyte porous membrane in addition to the function of the spacer. Particularly, methyl methacrylate has good ion conductivity, and is preferably used in the present invention. In the present invention, any organic filler is suitably used. The insolubility of the organic filler in the electrolytic solvent is preferably 90% by weight or more at 80 ° C. If the insoluble content is less than 90% by weight, the amount of the organic filler eluted into the electrolyte is increased, so that the viscosity of the electrolyte is increased and the ion conductivity may be impaired, which is not preferable. In addition, the organic filler having an insolubility of 90% by weight or more in the electrolytic solvent partially dissolves in the electrolytic solution, so that the dissolved component enhances the adhesiveness between the electrolyte and the organic filler, thereby causing a short circuit. Has the effect of preventing. The insolubility of the organic filler in the electrolytic solvent can be measured in the same manner as the insolubility in the electrolytic solvent of the porous polymer electrolyte membrane.
[0014]
In the present invention, a porous substrate having a film thickness of 3 μm or more and 14 μm or less and an air permeability of 400 (sec / 100 cc) or less may be supported on the polymer electrolyte porous membrane. When the thickness of the porous base material is less than 3 μm, the mechanical strength of the porous base material is low, and when the porous film is a polymer electrolyte, the whiteness is partially low and the voids are nonuniform. It is not preferable because it is easy. If the thickness exceeds 14 μm, the thickness of the polymer electrolyte porous membrane becomes excessively large, which cannot contribute to the thinning of the battery, and the impedance becomes excessively large, which is not preferable in battery performance.
[0015]
When the air permeability of the porous substrate exceeds 400 (sec / 100 cc), the air permeability becomes excessive when a polymer electrolyte porous membrane is used, and problems such as a decrease in the rate characteristics of the battery occur. There are cases. Conventionally, when the thickness of these porous substrates is reduced to a range of 3 μm or more and 14 μm or less, unevenness is likely to occur and the whiteness is reduced. A short circuit had occurred. However, by combining the fluorine-based resin and the porous substrate and having the composite in the polymer electrolyte porous membrane, uniform whiteness can be obtained, and as a result, even if it is a thin film, a micro short circuit does not occur. Can be obtained.
[0016]
Examples of the means for the composite include a method in which a single film of the fluororesin is overlapped with a porous base material and then bonded by heating or / and pressurizing. After coating on one or both sides of the porous substrate by an existing coating method such as roll coating, bar coating, spray coating, using the wet method or dry method which is the method of making porous described above It can be made porous and composite. Any porous substrate can be suitably used as long as it is in the above-mentioned thickness range, but more preferably a porous film containing at least a polyolefin resin or a polyimide porous film. The spacer effect can be enhanced by adding an inorganic filler to the polyolefin resin porous film in addition to the organic filler. On the other hand, a polyimide porous film has high mechanical strength even in a thin film, and can be suitably used in the present invention. In the case of a polymer electrolyte porous membrane in which a fluorine-based resin and a porous substrate are combined, the whiteness of the surface on which the fluorine-based resin is formed is measured.
[0017]
When the porous polymer electrolyte membrane of the present invention does not have a porous substrate, the thickness is 8 to 40 μm, preferably 8 to 35 μm, and more preferably 8 to 20 μm. If the thickness is less than 8 μm, the tensile strength decreases, and it is difficult to wind the battery when assembling it. On the other hand, if the thickness is more than 40 μm, the ion conductivity may be poor, which is not preferable. In the case of a polymer electrolyte porous membrane having a porous substrate, the thickness is 4 to 25 μm, preferably 4 to 18 μm, and more preferably 4 to 14 μm. If the thickness is less than 4 μm, the weakness of the mechanical strength and the non-uniformity of the voids of the porous substrate cannot be compensated for by the fluorine-based resin, and the standard deviation value of the whiteness and the difference between the average value and the minimum value are excessive. Is not preferred. On the other hand, when the thickness is larger than 25 μm, the air permeability becomes excessive and the rate characteristics may be undesirably reduced.
[0018]
【Example】
Hereinafter, examples of the present invention will be described.
Example 1
A solution is prepared by heating and dissolving 15 parts by weight of polyvinylidene fluoride resin with 135 parts by weight of N-methylpyrrolidone, and the solution is applied on a polypropylene film. The film is composed of 75 parts by weight of water and 25 parts by weight of methanol. After immersion in a mixed solvent for 15 minutes and drying at 60 ° C., the polypropylene film was peeled off and removed to obtain a polymer electrolyte porous membrane of the present invention. The thickness of the obtained polymer electrolyte porous membrane was 31 μm as measured by a micrometer. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness was measured. The results shown in Table 1 were obtained. The insolubility of the obtained polymer electrolyte porous membrane was 97%.
[0019]
Example 2
To the solution of Example 1, 15 parts by weight of a copolymer resin obtained by crosslinking styrene having an average particle diameter of 0.3 μm and methyl methacrylate with divinylbenzene (insoluble ratio in an electrolytic solvent: 99%) was further added as an organic filler. Otherwise in the same manner as in Example 1, a polymer electrolyte porous membrane of the present invention was obtained. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 26 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness was measured. The results shown in Table 1 were obtained. The insolubility of the obtained porous polymer electrolyte membrane was 98%.
[0020]
Example 3
After bonding a porous substrate (air permeability: 230 sec / 100 cc) made of polyethylene having a thickness of 8 μm onto a polypropylene film coated with an adhesive, the solution of Example 1 is sprayed on the porous substrate. After coating, the coating layer was made porous in the same manner as in Example 1, and then the polypropylene film was peeled off and removed to obtain a polymer electrolyte porous membrane of the present invention. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 10 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness of the coated surface was measured. The results shown in Table 1 were obtained. The insolubility of the obtained polymer electrolyte porous membrane was 96%.
[0021]
Example 4
After laminating a porous substrate (air permeability: 70 sec / 100 cc) made of polyethylene and glass microparticles having a thickness of 6 μm on a polypropylene film coated with an adhesive, the porous substrate of Example 1 was applied. After spray-coating the solution, the coating layer was made porous by the same method as in Example 1, and then the polypropylene film was peeled off and removed to obtain a polymer electrolyte porous membrane of the present invention. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 9 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness of the coated surface was measured. The results shown in Table 1 were obtained. The insolubility of the obtained polymer electrolyte porous membrane was 95%.
[0022]
Example 5
After laminating a 10 μm thick polyimide porous substrate (air permeability: 100 sec / 100 cc) on a polypropylene film coated with an adhesive, the solution of Example 1 was spray-coated on the porous substrate. After working, the coating layer was made porous by the same method as in Example 1, and then the polypropylene film was peeled off and removed to obtain a polymer electrolyte porous membrane of the present invention. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 13 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness of the coated surface was measured. The results shown in Table 1 were obtained. The insolubility of the obtained polymer electrolyte porous membrane was 96%.
[0023]
Example 6
In the same manner as in Example 1, except that 1.2 parts by weight of 15 parts by weight of the polyvinylidene fluoride resin was replaced with a vinylidene fluoride-6-propylene fluorinated copolymer resin in the same manner as in Example 1, A polymer electrolyte porous membrane was obtained. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 27 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness was measured. The results shown in Table 1 were obtained. The insolubility of the obtained porous polymer electrolyte membrane was 93%.
[0024]
Example 7
The solution used in Example 6 was used for the porous substrate of Example 3 in the same manner as in Example 3 to obtain a polymer electrolyte porous membrane of the present invention. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 11 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness of the coated surface was measured. The results shown in Table 1 were obtained. The insolubility of the obtained porous polymer electrolyte membrane was 98%.
[0025]
Comparative Example 1
A comparative polymer electrolyte porous membrane was obtained in the same manner as in Example 1 except that the immersion time in a mixed solvent consisting of 75 parts by weight of water and 25 parts by weight of methanol was changed to 2 minutes. The obtained polymer electrolyte porous membrane was cut into B5 size and its whiteness was measured. The results shown in Table 1 were obtained. The thickness of the polymer electrolyte porous membrane was 27 μm.
[0026]
Comparative Example 2
A solution was prepared and coated on a polypropylene film in the same manner as in Example 1 except that the amount of N-methylpyrrolidone was changed to 185 parts by weight. The polypropylene film immediately after coating was almost perpendicularly immersed in a mixed solvent consisting of 75 parts by weight of water and 25 parts by weight of methanol at a rate of about 0.1 m / min. The subsequent treatment was performed in the same manner as in Example 1 to obtain a polymer electrolyte porous membrane for comparison. The thickness of the obtained porous polymer electrolyte membrane was measured by a micrometer and found to be 29 μm. The polymer electrolyte porous membrane was cut into a B5 size, and the whiteness was measured. The results shown in Table 1 were obtained.
[0027]
Comparative Example 3
The porous substrate itself of Example 3 was used as a comparative polymer electrolyte porous membrane. This film was cut into B5 size, and its whiteness was measured. The results shown in Table 1 were obtained.
[0028]
Comparative Example 4
A comparative polymer electrolyte porous membrane was obtained in the same manner as in Example 3 except that the porous substrate was a polyethylene microporous membrane having a thickness of 2 μm. The obtained polymer electrolyte porous membrane was cut into B5 size, and the whiteness of the coated surface was measured. The results are shown in Table 1. The thickness of the porous polymer electrolyte membrane was 4 μm.
[0029]
Comparative Example 5
A comparative polymer electrolyte porous membrane was obtained in the same manner as in Example 3 except that a microporous polyethylene membrane having a thickness of 30 μm was used as the porous substrate. The obtained polymer electrolyte porous membrane was cut into B5 size, and the whiteness of the coated surface was measured. The results are shown in Table 1. The thickness of the porous polymer electrolyte membrane was 37 μm.
[0030]
Comparative Example 6
A polymer electrolyte for comparison was prepared in the same manner as in Example 1 except that 10 parts by weight of 15 parts by weight of polyvinylidene fluoride resin was replaced with a vinylidene fluoride-6-propylene copolymer resin. A porous membrane was obtained. The thickness of this polymer electrolyte porous membrane was 25 μm, and the whiteness was the result shown in Table 1. The insolubility of this porous polymer electrolyte membrane was 36%.
[0031]
Comparative Example 7
In the same manner as in Example 2, except that a non-crosslinked copolymer resin of styrene and butyl acrylate (insoluble ratio in an electrolytic solvent: 3% by weight, average particle diameter: 8 μm) was used as an organic filler. A comparative polymer electrolyte porous membrane was obtained. The thickness of this polymer electrolyte porous membrane was 25 μm, and the whiteness was the result shown in Table 1. The insolubility of this porous polymer electrolyte membrane was 46%.
[0032]
[Table 1]
With respect to the polymer electrolyte porous membranes of the examples and comparative examples obtained above, the micro short-circuit characteristics were confirmed as follows. After vacuum drying the polymer electrolyte porous membranes of the examples and the comparative examples, in an inert gas with respect to these polymer electrolyte porous membranes, ethylene carbonate and propylene carbonate were adjusted to be equal weights, respectively, and mixed. An electrolytic solution in which 1 mol of LiPF6 is dissolved in a solvent is 0.01 cc / cm. 2 At the same rate. Next, the polymer electrolyte porous membrane impregnated with this electrolyte is sandwiched between two flat stainless steel plates, and both stainless steel plates are subjected to an external force opposing from the outer surface where the polymer electrolyte porous membrane is not in contact. It was held by applying pressure. Next, by applying a constant voltage of 4.2 V to the stainless steel plate and gradually increasing the current, it was confirmed whether or not a short circuit occurred.
[0034]
As a result, in the polymer electrolyte porous membranes of Examples 1 to 7, no voltage drop was observed in any of the current values in the range up to 1000 mA / h, and no generation of a micro short circuit was observed. On the other hand, in the polymer electrolyte porous membranes of Comparative Examples 1 to 7 excluding Comparative Examples 4 and 5, a voltage drop was observed in the process where the current value reached 1000 mA / h, and a micro short circuit occurred. confirmed. In Comparative Example 4, a very thin porous substrate was combined with a thinner polymer electrolyte porous membrane. However, the mechanical strength of the porous substrate used was weak, and the stage before the micro-short circuit test was performed. And the test could not be performed. On the other hand, in Comparative Example 5, although a micro short circuit was not observed, the air permeability became infinite, so when the cross-sectional structure was observed with an electron microscope, a uniform rigid layer was found to be partially present in the plane direction. However, it was not uniform porous, and it was confirmed that there was a problem in practical use.
In Comparative Example 1, since the time for immersion was short, the solvent remained, and the porous material partially fused during the drying process. It is presumed that this led to the spread of the variation. In Comparative Example 2, since the viscosity of the solution was lowered and the angle at the time of immersion was steep, liquid dripping occurred on the coating surface and a portion where the thickness of the coating layer was partially reduced was generated, and white color was obtained. It is presumed that the degree decreased, leading to an increase in variation.
[0035]
Comparative Example 3 was a non-uniform film having a thin porous base material and partially low whiteness. Although this porous substrate is made by a stretching method, it has been stretched to the limit of breaking for thinning, and as a result, the portion just before the breaking appears as a decrease in whiteness. Seem. In Example 3 in which a porous polymer electrolyte membrane was composited using this porous substrate, the variation in whiteness was reduced and no micro short circuit occurred, confirming the effectiveness of the present invention. In Comparative Example 6, although the variation in whiteness was a relatively good result in Comparative Example, the amount of vinylidene fluoride-hexafluoropropylene copolymer resin dissolved in the electrolytic solution was excessive, It is presumed that the electrical resistance of the portion in which the resin was dissolved was reduced and a micro short circuit occurred. In Comparative Example 7, since the mixed organic filler was easily soluble in the electrolytic solution, it is presumed that the electrical resistance of the melted-out portion was reduced, and a micro short circuit was likely to occur.
[0036]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, even if it is a thin film, it is very homogeneous and a micro short circuit does not generate | occur | produce, As a result, the polymer electrolyte porous film with favorable durability is obtained.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating the maximum frequency of whiteness of a polymer electrolyte porous membrane.
Claims (9)
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