JP4573284B2 - Polyethylene microporous membrane - Google Patents
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- JP4573284B2 JP4573284B2 JP2000281111A JP2000281111A JP4573284B2 JP 4573284 B2 JP4573284 B2 JP 4573284B2 JP 2000281111 A JP2000281111 A JP 2000281111A JP 2000281111 A JP2000281111 A JP 2000281111A JP 4573284 B2 JP4573284 B2 JP 4573284B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明はポリエチレン微多孔膜、およびその電池セパレータへの適用に関する。
【０００２】
【従来の技術】
ポリエチレン微多孔膜は精密濾過膜、電池用セパレータ、コンデンサー用セパレータ等に使用されている。特に近年では厚さ２５μｍ程度のポリエチレン微多孔膜がリチウムイオン二次電池用セパレータに使用されている。
リチウムイオン二次電池用セパレータには電池の高性能化に伴いその要求特性も高レベル化しており、特に最近では透過性の高い微多孔膜が要求されている。
セパレータの透過性は、サイクル性、高温保存特性といった電池の寿命に関する特性と関係深い。寿命に関する特性では電池構造要因が大きいが、長期使用による電解液の分解物の発生とそれに伴うセパレータの目詰まりが性能低下を引き起こすことが知られており、リチウムイオン電池ではこの特性の改善が大きな課題となっている。従って、電池を長寿命化させるために、セパレータには平均孔径と孔径分布が大きく、透過性が優れているものが強く要求されている。
【０００３】
この他にもリチウムイオン二次電池用セパレータには、高強度、高安全性、低電気抵抗であることが望まれている。
強度は、電極とセパレータを高速で巻き上げる捲回工程を有する電池の組立時に電極間の短絡不良などを起こしにくくするために必要な性能であり、突き刺し強度等で表せる性能である。特に近年では電池の高容量化に伴い、電極の活物質の詰め込み密度が上昇した堅い電極でも短絡しない高強度セパレータへの要求が強い。
【０００４】
また、近年のリチウムイオン二次電池の高出力、大容量化にともない、安全性も強く求められるようになった。安全性とは、電池内部が過熱した際にセパレータが溶融して電極を覆う被膜となり、電流を遮断し、それによって電池の安全性を確保する性能である。その際に、十分な被膜量を確保するためのセパレータの目付量を一定の値以上にする必要があり、これには微多孔膜の空孔量（気孔率）を適正な範囲にする必要がある。
【０００５】
さらに、上記物性バランスを保ちつつ、電池の大電流での放電性能や低温での放電性能を向上させるために、セパレータが電解液を保液した状態で流れるイオンの抵抗をできるだけ小さくする必要があり、電解液を含ませた状態での電気抵抗値が低いセパレータが望まれている。
従来、上記の個々の特性については改善する提案がされているが、全ての特性を満たすような電池セパレータ用微多孔膜はなかった。
【０００６】
例えば、透過性能の改善されたポリエチレン微多孔膜は特開平８−１２７９９号公報、特開平５−３１０９８９号公報や特開平６−２４００３６号公報等に開示されていたり、また特開平５−２２２２３６号公報、特開平５−２２２２３７号公報等に開示されている製造方法で生産しうることが知られている。
特開平８−１２７９９号公報の微多孔膜、特開平５−２２２２３６号公報、特開平５−２２２２３７号公報の製法による微多孔膜は透過性能に優れるものの、いずれも造核剤が含まれており、電池内部での副反応による電池性能低下が懸念される。また、この製法による微多孔膜はいずれも気孔率が高く、機械強度の低下をさけられないばかりか、ポリマーの目付量が少ないために、過熱溶融時の電流遮断が不十分である。
【０００７】
特開平５−３１０９８９号公報や特開平６−２４００３６号公報では、機械強度と透過性能を改善した微多孔膜が開示されているが、大きな平均孔径と、広い孔径分布、高強度を同時に達成するには至っていない。
【０００８】
【発明が解決しようとする課題】
本発明は、透過性、機械強度等に優れ、電池用セパレータとして電池の生産性、出力特性、安全性を損ねることなく寿命特性を向上させることができるポリエチレン微多孔膜を提供するものである。
【０００９】
【課題を解決するための手段】
本発明者らは前記課題に対して鋭意研究を重ねた結果、特定の平均孔径、孔径分布、気孔率、機械的強度、電気抵抗を有するポリエチレン微多孔膜が、電池の生産性、安全性、出力特性を損ねることなく寿命特性を向上しうることを見出し、本発明をなすに至った。
すなわち本発明は、
（１）気孔率４０〜６０％、突き刺し強度３００〜１５００ｇ（厚さ２５μｍ換算）、平均孔径０．１〜０．３μｍ、孔径分布指数が１．４０〜２．２、電気抵抗が０．４〜１．０Ωｃｍ2であることを特徴とするポリエチレン微多孔膜。
（２）電気抵抗が０．４〜０．９Ωｃｍ2であることを特徴とする（１）に記載のポリエチレン微多孔膜。
（３）バブルポイントが４．５ｋｇ／ｃｍ2以下であることを特徴とする（１）又は（２）に記載のポリエチレン微多孔膜。
（４）重量平均分子量１００万以上の超高分子量ポリエチレンと重量平均分子量５０万以下の高密度ポリエチレンのブレンドであることを特徴とする（１）〜（３）のいずれかに記載のポリエチレン微多孔膜。
（５）膜厚が１０〜３０μｍであることを特徴とする（１）〜（４）のいずれかに記載のポリエチレン微多孔膜。
（６）（１）〜（５）のいずれかに記載のポリエチレン微多孔膜からなる電池用セパレータ。
（７）リチウムイオン電池用である（６）に記載の電池用セパレータ。
（８）（１）〜（５）のいずれかに記載のポリエチレン微多孔膜の製造方法であって、縦延伸速度を１００％／秒以上、横延伸速度を１０％／秒以下で２軸延伸する工程を有する製造方法。
【００１０】
以下、本発明を詳細に説明する。
本発明の微多孔膜はポリエチレンからなる。ここでいうポリエチレンとは、重量平均分子量が好ましくは１０万〜４００万の高密度ポリエチレンである。また、このポリエチレンはエチレン単位に対してプロピレン、ブテン、ペンテン、ヘキセン、オクテン等のα−オレフィンの単位を４モル％以下の割合で含む共重合体（線状共重合ポリエチレン）であってもよい。また、ブレンドや多段重合などの手段によって重量平均分子量を好ましい範囲に調節したものでもかまわなく、好ましくは重量平均分子量１００万以上の超高分子量ポリエチレンと重量平均分子量５０万以下の高密度ポリエチレンのブレンド物である。
【００１１】
さらに、これらに中密度ポリエチレン、線状低密度ポリエチレン、低密度ポリエチレン、ＥＰＲ等のポリオレフィンをブレンドしたものでもかまわない。
本発明の微多孔膜の平均孔径および孔径分布指数は、ハーフドライ法によって測定される値であって、平均孔径は０．１〜０．３μｍであり、好ましくは０．１２〜０．２μｍである。０．１μｍより小さいと電池の長期使用による電解液の分解物の発生と、それに伴うセパレータの目詰まりが性能低下を早く引き起こし、０．３μｍより大きいと短絡不良などを起こす可能性がある。また、孔径分布指数は最大孔径と平均孔径の比であり、１．４０〜２．２であって、好ましくは１．４５〜２．２、さらに好ましくは１．４８〜２．０である。１．４０より小さいと、この場合もセパレータの目詰まりによる性能低下を起こす可能性がある。
【００１２】
このように、微多孔膜の孔径と孔径分布を適度な範囲にすることにより、電池の長期使用による電極や電解液の分解物の発生とそれに伴うセパレータの目詰まりが抑えられ、電池の性能低下が起こりにくくなった。
なお、最大孔径の表し方としてバブルポイント法もよく知られている。本発明のポリエチレン微多孔膜のバブルポイントは４．５ｋｇ／ｃｍ2 以下が好ましく、さらに好ましくは４．２ｋｇ／ｃｍ2以下である。
【００１３】
本発明の微多孔膜の厚さは１０〜３０μｍが好ましく、電極量を多くした高容量電池には１０〜２５μｍがさらに好ましく、特に１０〜２３μｍがさらに好ましい。
気孔率は４０％〜６０％、好ましくは４０％〜５５％の範囲にある。気孔率が４０％未満では電解液の保液量が十分ではなく、一方６０％を超えると十分な機械強度が得られず、また、目付量も減るため電池セパレータとしての安全性も低下する可能性がある。
【００１４】
目付量は微多孔膜１ｍ2当たりの重量で表される数値であって、１０ｇ／ｍ2以上が好ましく、さらに好ましくは１１ｇ／ｍ2以上である。厚さ、気孔率が上記範囲内で目付量が１１ｇ／ｍ2以上あれば、電池内部が過熱した際にセパレータが溶融して電極を覆う被膜量が十分あり、過充電時などの安全性が向上する。
また、本発明の微多孔膜の突き刺し強度は厚さ２５μｍ換算で３００〜１５００ｇであり、好ましくは３５０〜１５００ｇである。３００ｇ未満では、電池用セパレータとして使用した場合に、脱落した活物質等によってセパレータが破れ、短絡を起こす可能性がある。
【００１５】
電気抵抗は１．０Ωｃｍ2以下であり、さらに好ましくは０．４〜１．０Ωｃｍ2である。１．０以下であることによって、電池の高出力放電特性や低温での放電特性が大きく改善される。
次に本発明のポリエチレン微多孔膜の製造例について説明する。
この発明の膜は、例えば下記の（ａ）〜（ｄ）の工程によって作られる。
（ａ）前記載の任意のポリエチレンまたは二種類以上のポリエチレンのブレンド物を有機液状物、無機フィラー及び添加剤とともに造粒する工程。
（ｂ）（ａ）工程で得た混合物を、先端にＴ−ダイを装着した押出機中で溶融混練し、Ｔ−ダイから押出しシート状に成形する工程。
（ｃ）（ｂ）で得たシート状成型物より、有機液状物と無機フィラーを抽出除去する工程。
（ｄ）（ｃ）の成型物を、１枚のまま、或いは数枚重ねて、二軸に延伸処理する工程。２枚重ねることが好ましい。
【００１６】
本発明の製造工程をさらに詳しく説明する。
工程（ａ）において混合ポリエチレン、有機液状体、無機フィラーの合計重量に対する混合ポリエチレンの割合は１０〜６０重量％、有機液状体と無機フィラーの割合の合計は４０〜９０重量％である。混合ポリエチレンの割合が１０重量％未満では強度が低く、６０重量％を越えると押出成形時の流動性が悪くなり成形加工が困難となる。有機液状体としてはフタル酸エステルやセバシン酸エステル等のエステル類や流動パラフィン等が挙げられ、それらを単独で用いても或いは混合物で用いてもよい。無機フィラーとしては、シリカ、マイカ、タルク等が挙げられ、それらを単独で用いても或いは混合物で用いてもよい。
【００１７】
工程（ｄ）において二軸延伸する場合、延伸温度１１０〜１４０℃の範囲で縦方向に３倍〜１０倍、好ましくは４倍〜８倍延伸し、それに続いて横方向に１．５倍〜５倍、好ましくは１．８倍〜３倍延伸する。このとき、縦延伸倍率／横延伸倍率＝２．０以上となるようにし、かつ、縦延伸速度を１００％／秒以上、横延伸速度を１０％／秒以下とすることが好ましい。
本発明において、膜の強度、気孔率、平均孔径、孔径分布、電気抵抗を特定の範囲にコントロールするための方法としては、上記したような特定の延伸法を用いる方法が最も優れている。
【００１８】
これについて理由は定かではないが、まず、縦方向に高速、高倍率で延伸することにより高強度を実現し、続いて横方向の低速延伸で孔径、孔径分布等を適切な範囲に調節することができるためと考えられる。
さらに、延伸に続いて、または後に、熱固定あるいは熱緩和等の熱処理を行ってもかまわない。
【００１９】
【発明の実施の形態】
次に実施例によって本発明をさらに詳細に説明する。
実施例において示される試験方法は次の通りである。
（１）膜厚
ダイヤルゲージ（尾崎製作所：ＰＥＡＣＯＣＫ Ｎｏ．２５）にて測定した。
（２）気孔率
２０ｃｍ角のサンプルをとり、その体積と重量から次式を用いて計算した。
気孔率（％）＝（体積（ｃｍ3 ）−重量（ｇ）／ポリエチレンの密度）／体積（ｃｍ3 ）×１００
（３）突き刺し強度
カトーテック製ＫＥＳ−Ｇ５ハンディー圧縮試験器を用いて、針先端の曲率半径０．５ｍｍ、突き刺し速度２ｍｍ／ｓｅｃの条件で突き刺し試験を行い、最大突き刺し荷重（ｇ）を測定した。測定値に２５（μｍ）／膜厚（μｍ）を乗じることによって２５μｍ換算突き刺し強度（ｇ）とした。
（４）透気度
ＪＩＳ Ｐ−８１１７準拠のガーレー式透気度計にて測定した。
（５）平均孔径、最大孔径（ハーフドライ法）
ＡＳＴＭ Ｆ−３１６−８６に準拠し、エタノールを使用して測定した。
【００２０】
（６）孔径分布指数
ハーフドライ法で得られた最大孔径と平均孔径から算出した。
孔径分布指数＝最大孔径（μｍ）／平均孔径（μｍ）
（７）電気抵抗
安藤電気製ＬＣＲメーターＡＧ−４３と図１に示したセルを用いて１ｋＨｚの交流にて測定し、次式で算出した。
電気抵抗（Ωｃｍ2）＝（膜が存在するときの抵抗値−膜が存在しないときの抵抗値）×０．７８５
なお、電解液：プロピレンカーボネートとジエトキシエタンの混合溶液（５０／５０容量％）中に過塩素酸リチウム１ｍｏｌ／リットルを溶解した、電極：白金黒電極、極板面積：０．７８５ｃｍ2、極間距離：３ｍｍの条件で測定した。
【００２１】
（８）サイクル試験
ＬｉＣｏＯ2 を正極活物質とし、グラファイトおよびアセチレンブラックを導電剤とし、フッ素ゴムを結着剤とし各々ＬｉＣｏＯ2 ：グラファイト：アセチレンブラック：フッ素ゴム＝８８：７．５：２．５：２の重量比で混合したものをジメチルホルムアミドペーストとしてＡｌ箔に塗布乾燥したシートを正電極として用い、ニードルコークス：フッ素ゴム＝９５：５の重量比で混合したものをジメチルホルムアミドペーストとしてＣｕ箔に塗布乾燥したシートを負電極として用い、電解液としてプロピレンカーボネートとブチロラクトンの混合溶媒（体積比＝１：１）にホウフッ化リチウムを１．０Ｍの濃度で調整した液を用いてリチウムイオン電池を製造した。この電池を温度２５℃の条件の下で、充電電流１Ａで充電終止電圧４．２Ｖまで充電を行い、放電電流１Ａで放電終止電圧３Ｖまで放電を行い、これを１サイクルとして充放電を繰り返し、初期容量に対する５００サイクル後の容量の割合を容量保持率として表した。
（９）安全性試験（過充電試験）
前記サイクル試験と同様の電池を作成し、この電池を４．２Ｖで５時間充電したあと、さらに定電流で過充電を行った。過充電によって電池の内部温度は上昇し、１３０℃付近に達するとセパレータが溶融して孔が閉塞するために電流が遮断されるが、その時に、電流漏れが無い場合を○、電流漏れがある場合を×とした。
【００２２】
【実施例１】
平均分子量２００万の超高分子量ポリエチレン１２重量％、平均分子量２８万の高密度ポリエチレン１２重量％、平均分子量１５万の直鎖状低密度ポリエチレン１６重量％、フタル酸ジオクチル（ＤＯＰ）４２．４重量％、微粉シリカ１７．６重量％を混合造粒した後、Ｔ−ダイを装着した二軸押出機にて混練・押出し厚さ９０μｍのシート状に成形した。該成形物からＤＯＰと微粉シリカを抽出除去し微多孔膜とした。該微多孔膜を２枚重ねて１１８℃に加熱のもと、縦方向に５．３倍（延伸速度１０００％／秒）延伸した後、横方向に１．８倍（延伸速度２％／秒）延伸した。得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２３】
【実施例２】
平均分子量２００万の超高分子量ポリエチレン９重量％、平均分子量２８万の高密度ポリエチレン９重量％、平均分子量１５万の直鎖状低密度ポリエチレン１２重量％、フタル酸ジオクチル（ＤＯＰ）４９．５重量％、微粉シリカ２０．５重量％を混合造粒した後、Ｔ−ダイを装着した二軸押出機にて混練・押出し厚さ９０μｍのシート状に成形した。該成形物からＤＯＰと微粉シリカを抽出除去し微多孔膜とした。該微多孔膜を２枚重ねて１１８℃に加熱のもと、縦方向に４．５倍（延伸速度１０００％／秒）延伸した後、横方向に１．８倍（延伸速度２％／秒）延伸した。
得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２４】
【実施例３】
実施例１と同様に微多孔シートを作製し、該微多孔膜を２枚重ねて１１５℃に加熱のもと、縦方向に４．５倍（延伸速度１０００％／秒）延伸した後、横方向に２．２倍（延伸速度２％／秒）延伸した。
得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２５】
【実施例４】
実施例２と同様に微多孔膜を作製し、該微多孔膜を２枚重ねて１２８℃に加熱のもと、縦方向に４．５倍（延伸速度１０００％／秒）延伸した後、横方向に１．８倍（延伸速度２％／秒）延伸した。
得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２６】
【比較例１】
実施例１と同様に微多孔膜を作製し、該微多孔膜を２枚重ねて１１８℃に加熱のもと、縦方向に５．３倍（延伸速度１０００％／秒）延伸した後、横方向に１．８倍（延伸速度２０％／秒）延伸した。
得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２７】
【比較例２】
実施例１と同様に微多孔膜を作製し、該微多孔膜を２枚重ねて１１８℃に加熱のもと、縦方向に４倍（延伸速度１０００％／秒）延伸した後、横方向に４倍（延伸速度２０％／秒）延伸した。
得られた膜の物性、またこれをセパレータとして用いた電池の特性を表１に記載した。
【００２８】
【表１】
【発明の効果】
本発明のポリエチレン微多孔膜は、良好な透過性能と高い強度を併せ持ち、特にリチウムイオン二次電池用セパレータに好適である。
【図面の簡単な説明】
【図１】電気抵抗測定における組立の概略図
【符号の説明】
１ 電極
２ 外径２ｃｍ、内径１ｃｍ、厚み１ｍｍのテフロンパッキン
３ 膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polyethylene microporous membrane and its application to a battery separator.
[0002]
[Prior art]
Polyethylene microporous membranes are used in microfiltration membranes, battery separators, capacitor separators, and the like. Particularly in recent years, a polyethylene microporous film having a thickness of about 25 μm has been used for a separator for a lithium ion secondary battery.
The required characteristics of lithium ion secondary battery separators have increased with the performance of batteries, and recently, a microporous membrane with high permeability has been required.
The permeability of the separator is closely related to battery life characteristics such as cycle characteristics and high temperature storage characteristics. Battery characteristics are significant in terms of life characteristics, but it is known that the degradation of electrolytes due to long-term use and the resulting clogging of the separator cause performance degradation. It has become a challenge. Therefore, in order to extend the life of the battery, a separator having a large average pore size and a large pore size distribution and excellent permeability is strongly required.
[0003]
In addition, a lithium ion secondary battery separator is desired to have high strength, high safety, and low electrical resistance.
The strength is a performance required to make it difficult for a short circuit failure between the electrodes to occur at the time of assembling a battery having a winding process for winding the electrode and the separator at a high speed, and can be expressed by a puncture strength or the like. Particularly in recent years, with the increase in capacity of batteries, there is a strong demand for a high-strength separator that does not short-circuit even a stiff electrode whose packing density of the active material of the electrode has increased.
[0004]
In addition, with the recent increase in output and capacity of lithium ion secondary batteries, safety has been strongly demanded. The safety is a performance in which when the inside of the battery is overheated, the separator melts to become a film covering the electrode, interrupting the current, thereby ensuring the safety of the battery. At that time, it is necessary to make the basis weight of the separator to ensure a sufficient coating amount above a certain value, and for this, it is necessary to set the pore volume (porosity) of the microporous membrane within an appropriate range. is there.
[0005]
Furthermore, in order to improve the discharge performance at high currents and low temperature discharge performance of the battery while maintaining the above physical property balance, it is necessary to reduce the resistance of ions flowing while the separator holds the electrolyte solution as much as possible. In addition, a separator having a low electric resistance value in a state where an electrolytic solution is contained is desired.
Conventionally, proposals have been made to improve the above individual characteristics, but there has been no microporous membrane for battery separators that satisfies all the characteristics.
[0006]
For example, polyethylene microporous membranes having improved permeation performance are disclosed in JP-A-8-12799, JP-A-5-310989, JP-A-6-240036, and the like, and JP-A-5-222236. It is known that it can be produced by the production method disclosed in Japanese Patent Laid-Open No. 5-222237.
The microporous membrane disclosed in JP-A-8-12799 and the microporous membrane produced by the methods described in JP-A-5-222236 and JP-A-5-222237 are excellent in permeation performance, but all contain a nucleating agent. There is a concern about battery performance degradation due to side reactions inside the battery. In addition, all of the microporous membranes produced by this production method have a high porosity and cannot avoid a decrease in mechanical strength. In addition, since the amount of polymer is small, current interruption during overheating melting is insufficient.
[0007]
JP-A-5-310989 and JP-A-6-240036 disclose microporous membranes with improved mechanical strength and permeation performance, but simultaneously achieve a large average pore size, a wide pore size distribution, and high strength. It has not reached.
[0008]
[Problems to be solved by the invention]
The present invention provides a polyethylene microporous membrane that is excellent in permeability, mechanical strength, etc., and that can improve life characteristics without impairing battery productivity, output characteristics, and safety as a battery separator.
[0009]
[Means for Solving the Problems]
As a result of intensive research on the above problems, the present inventors have obtained a polyethylene microporous membrane having a specific average pore size, pore size distribution, porosity, mechanical strength, and electrical resistance, resulting in battery productivity, safety, It has been found that the life characteristics can be improved without impairing the output characteristics, and the present invention has been made.
That is, the present invention
(1) Porosity 40 to 60%, puncture strength 300 to 1500 g (thickness 25 μm conversion), average pore diameter 0.1 to 0.3 μm, pore diameter distribution index 1.40 to 2.2, electric resistance 0.4 A polyethylene microporous membrane characterized by having a viscosity of ˜1.0 Ωcm 2 .
(2) The polyethylene microporous membrane according to (1), wherein the electrical resistance is 0.4 to 0.9 Ωcm 2 .
(3) The polyethylene microporous membrane according to (1) or (2), wherein the bubble point is 4.5 kg / cm 2 or less.
(4) The polyethylene microporous material according to any one of (1) to (3), which is a blend of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more and high density polyethylene having a weight average molecular weight of 500,000 or less. film.
(5) The polyethylene microporous film according to any one of (1) to (4), wherein the film thickness is 10 to 30 μm.
( 6 ) A battery separator comprising the polyethylene microporous membrane according to any one of (1) to ( 5 ).
(7) The battery separator according to (6), which is for a lithium ion battery.
(8) The method for producing a polyethylene microporous membrane according to any one of (1) to (5), wherein biaxial stretching is performed at a longitudinal stretching speed of 100% / second or more and a transverse stretching speed of 10% / second or less. The manufacturing method which has a process to do.
[0010]
Hereinafter, the present invention will be described in detail.
The microporous membrane of the present invention is made of polyethylene. The polyethylene here is a high density polyethylene having a weight average molecular weight of preferably 100,000 to 4,000,000. Further, this polyethylene may be a copolymer (linear copolymer polyethylene) containing an α-olefin unit such as propylene, butene, pentene, hexene, octene or the like in an amount of 4 mol% or less with respect to the ethylene unit. . Also, the weight average molecular weight may be adjusted to a preferred range by means of blending or multistage polymerization, preferably a blend of ultra high molecular weight polyethylene having a weight average molecular weight of 1 million or more and high density polyethylene having a weight average molecular weight of 500,000 or less. It is a thing.
[0011]
Further, they may be blended with polyolefins such as medium density polyethylene, linear low density polyethylene, low density polyethylene and EPR.
The average pore size and pore size distribution index of the microporous membrane of the present invention are values measured by a half-dry method, and the average pore size is 0.1 to 0.3 μm, preferably 0.12 to 0.2 μm. is there. If it is smaller than 0.1 μm, the decomposition of the electrolytic solution due to the long-term use of the battery and the clogging of the separator accompanying it may cause a decrease in performance quickly, and if it is larger than 0.3 μm, a short circuit failure may occur. The pore size distribution index is a ratio between the maximum pore size and the average pore size, and is 1.40 to 2.2, preferably 1.45 to 2.2, and more preferably 1.48 to 2.0. If it is smaller than 1.40, there is a possibility that the performance is deteriorated due to the clogging of the separator.
[0012]
In this way, by setting the pore size and pore size distribution of the microporous membrane within an appropriate range, the generation of electrodes and electrolyte decomposition products due to long-term use of the battery and the accompanying clogging of the separator can be suppressed, and the performance of the battery is reduced. Is less likely to occur.
The bubble point method is also well known as a method for expressing the maximum pore diameter. The bubble point of the polyethylene microporous membrane of the present invention is preferably 4.5 kg / cm 2 or less, more preferably 4.2 kg / cm 2 or less.
[0013]
The thickness of the microporous membrane of the present invention is preferably 10 to 30 μm, more preferably 10 to 25 μm, and particularly preferably 10 to 23 μm for a high capacity battery with an increased amount of electrodes.
The porosity is in the range of 40% to 60%, preferably 40% to 55%. If the porosity is less than 40%, the amount of electrolyte retained is not sufficient. On the other hand, if the porosity exceeds 60%, sufficient mechanical strength cannot be obtained. There is sex.
[0014]
The basis weight is a numerical value expressed by the weight per 1 m 2 of the microporous membrane, and is preferably 10 g / m 2 or more, more preferably 11 g / m 2 or more. If the thickness and porosity are within the above ranges and the basis weight is 11 g / m 2 or more, the separator melts when the battery is overheated, and there is a sufficient amount of coating to cover the electrode. improves.
Further, the piercing strength of the microporous membrane of the present invention is 300 to 1500 g, preferably 350 to 1500 g, in terms of thickness of 25 μm. If it is less than 300 g, when used as a battery separator, the separator may be broken by a fallen active material or the like, causing a short circuit.
[0015]
The electric resistance is 1.0 Ωcm 2 or less, more preferably 0.4 to 1.0 Ωcm 2 . By being 1.0 or less, the high output discharge characteristics and low temperature discharge characteristics of the battery are greatly improved.
Next, production examples of the polyethylene microporous membrane of the present invention will be described.
The film of the present invention is produced, for example, by the following steps (a) to (d).
(A) A step of granulating an arbitrary polyethylene described above or a blend of two or more kinds of polyethylene together with an organic liquid, an inorganic filler, and an additive.
(B) A step in which the mixture obtained in the step (a) is melt-kneaded in an extruder equipped with a T-die at the tip and formed into an extruded sheet from the T-die.
(C) A step of extracting and removing the organic liquid material and the inorganic filler from the sheet-like molded product obtained in (b).
(D) A step of biaxially stretching the molded product of (c) as it is or several layers. It is preferable to stack two sheets.
[0016]
The production process of the present invention will be described in more detail.
In the step (a), the ratio of the mixed polyethylene to the total weight of the mixed polyethylene, organic liquid, and inorganic filler is 10 to 60% by weight, and the total ratio of the organic liquid and inorganic filler is 40 to 90% by weight. If the proportion of the mixed polyethylene is less than 10% by weight, the strength is low. Examples of the organic liquid include esters such as phthalic acid ester and sebacic acid ester, liquid paraffin, and the like, and these may be used alone or in a mixture. Examples of the inorganic filler include silica, mica, talc and the like, and these may be used alone or in a mixture.
[0017]
In the case of biaxial stretching in the step (d), stretching in the longitudinal direction is 3 to 10 times, preferably 4 to 8 times in the range of 110 to 140 ° C., followed by 1.5 to The film is stretched 5 times, preferably 1.8 times to 3 times. At this time, it is preferable that the longitudinal stretching ratio / lateral stretching ratio = 2.0 or more, the longitudinal stretching speed is 100% / second or more, and the lateral stretching speed is 10% / second or less.
In the present invention, as a method for controlling the strength, porosity, average pore size, pore size distribution, and electrical resistance of the membrane within a specific range, the method using the specific stretching method as described above is most excellent.
[0018]
The reason for this is not clear, but first, high strength is achieved by stretching in the longitudinal direction at a high speed and high magnification, and then the pore diameter, pore size distribution, etc. are adjusted to an appropriate range by stretching in the transverse direction at a low speed. This is thought to be possible.
Further, heat treatment such as heat fixation or heat relaxation may be performed following or after stretching.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in more detail with reference to examples.
The test methods shown in the examples are as follows.
(1) Measured with a film thickness dial gauge (Ozaki Seisakusho: PEACOCK No. 25).
(2) A sample having a porosity of 20 cm square was taken and calculated from the volume and weight using the following equation.
Porosity (%) = (volume (cm 3 ) −weight (g) / density of polyethylene) / volume (cm 3 ) × 100
(3) Puncture strength Using a KES-G5 handy compression tester manufactured by Kato Tech, a puncture test was performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec, and the maximum puncture load (g) was measured. . The measured value was multiplied by 25 (μm) / film thickness (μm) to obtain a puncture strength (g) in terms of 25 μm.
(4) Air permeability Measured with a Gurley air permeability meter in accordance with JIS P-8117.
(5) Average pore size, maximum pore size (half dry method)
Measurement was performed using ethanol in accordance with ASTM F-316-86.
[0020]
(6) Pore size distribution index It was calculated from the maximum pore size and average pore size obtained by the half dry method.
Pore size distribution index = maximum pore size (μm) / average pore size (μm)
(7) Electric resistance Measured with an alternating current of 1 kHz using an Ando Electric LCR meter AG-43 and the cell shown in FIG.
Electrical resistance (Ωcm 2 ) = (resistance value when film is present−resistance value when film is not present) × 0.785
Electrolytic solution: 1 mol / liter of lithium perchlorate dissolved in a mixed solution of propylene carbonate and diethoxyethane (50/50% by volume), electrode: platinum black electrode, electrode plate area: 0.785 cm 2 , electrode Distance was measured under the condition of 3 mm.
[0021]
(8) Cycle test LiCoO 2 was used as the positive electrode active material, graphite and acetylene black were used as the conductive agent, and fluororubber was used as the binder, and LiCoO 2 : graphite: acetylene black: fluororubber = 88: 7.5: 2.5 A mixture of 2 weight ratios as a dimethylformamide paste was applied to an Al foil and dried as a positive electrode, and a mixture of needle coke: fluororubber = 95: 5 weight ratio was used as a dimethylformamide paste Cu foil. A lithium ion battery using a sheet prepared by coating and drying as a negative electrode and a solution prepared by adjusting lithium borofluoride at a concentration of 1.0 M to a mixed solvent of propylene carbonate and butyrolactone (volume ratio = 1: 1) as an electrolytic solution. Manufactured. This battery was charged to a charge end voltage of 4.2 V with a charge current of 1 A under the condition of a temperature of 25 ° C., discharged to a discharge end voltage of 3 V with a discharge current of 1 A, and this was repeated as a cycle, The ratio of the capacity after 500 cycles to the initial capacity was expressed as capacity retention.
(9) Safety test (overcharge test)
A battery similar to that in the cycle test was prepared, and the battery was charged with 4.2 V for 5 hours, and then overcharged with a constant current. The internal temperature of the battery rises due to overcharging, and when the temperature reaches around 130 ° C, the separator melts and the hole is blocked, so that the current is interrupted. At that time, there is no current leakage. The case was marked with x.
[0022]
[Example 1]
12% by weight of ultra high molecular weight polyethylene with an average molecular weight of 2 million, 12% by weight of high density polyethylene with an average molecular weight of 280,000, 16% by weight of linear low density polyethylene with an average molecular weight of 150,000, 42.4% by weight of dioctyl phthalate (DOP) % And 17.6% by weight of fine silica were mixed and granulated, and then kneaded and extruded by a twin-screw extruder equipped with a T-die to form a sheet having a thickness of 90 μm. DOP and fine silica were extracted and removed from the molded product to form a microporous membrane. Two microporous membranes are stacked and heated to 118 ° C., stretched 5.3 times in the machine direction (stretching speed 1000% / second), and then 1.8 times in the transverse direction (stretching
[0023]
[Example 2]
9% by weight of ultra high molecular weight polyethylene with an average molecular weight of 2 million, 9% by weight of high density polyethylene with an average molecular weight of 280,000, 12% by weight of linear low density polyethylene with an average molecular weight of 150,000, 49.5% by weight of dioctyl phthalate (DOP) % And 20.5% by weight of fine silica were mixed and granulated, and then kneaded and extruded by a twin screw extruder equipped with a T-die to form a sheet having a thickness of 90 μm. DOP and fine silica were extracted and removed from the molded product to form a microporous membrane. Two microporous membranes are stacked and heated to 118 ° C., stretched 4.5 times in the longitudinal direction (stretching speed 1000% / second), and then 1.8 times in the lateral direction (stretching
Table 1 shows the physical properties of the obtained film and the characteristics of the battery using this as a separator.
[0024]
[Example 3]
A microporous sheet was prepared in the same manner as in Example 1, and two microporous films were stacked and heated to 115 ° C. and stretched 4.5 times in the longitudinal direction (stretching speed 1000% / second), The film was stretched 2.2 times in the direction (stretching
Table 1 shows the physical properties of the obtained film and the characteristics of the battery using this as a separator.
[0025]
[Example 4]
A microporous membrane was prepared in the same manner as in Example 2, and two microporous membranes were stacked and heated to 128 ° C. and stretched 4.5 times in the longitudinal direction (stretching speed 1000% / second), The film was stretched 1.8 times in the direction (stretching
Table 1 shows the physical properties of the obtained film and the characteristics of the battery using this as a separator.
[0026]
[Comparative Example 1]
A microporous membrane was prepared in the same manner as in Example 1, and two microporous membranes were stacked and heated to 118 ° C. and stretched 5.3 times in the longitudinal direction (stretching speed 1000% / second), The film was stretched 1.8 times in the direction (stretching speed 20% / second).
Table 1 shows the physical properties of the obtained film and the characteristics of the battery using this as a separator.
[0027]
[Comparative Example 2]
A microporous membrane was prepared in the same manner as in Example 1, and the two microporous membranes were stacked and heated to 118 ° C. and stretched 4 times in the longitudinal direction (stretching speed 1000% / second), and then in the lateral direction. The film was stretched 4 times (stretching speed 20% / second).
Table 1 shows the physical properties of the obtained film and the characteristics of the battery using this as a separator.
[0028]
[Table 1]
【The invention's effect】
The polyethylene microporous membrane of the present invention has both good permeation performance and high strength, and is particularly suitable for a separator for a lithium ion secondary battery.
[Brief description of the drawings]
Fig. 1 Schematic diagram of assembly in electrical resistance measurement
1
Claims (8)
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| JP2000281111A JP4573284B2 (en) | 2000-09-18 | 2000-09-18 | Polyethylene microporous membrane |
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| JP4563008B2 (en) * | 2002-08-14 | 2010-10-13 | 三菱化学株式会社 | Lithium secondary battery separator and lithium secondary battery using the same |
| WO2005015660A1 (en) | 2003-08-06 | 2005-02-17 | Mitsubishi Chemical Corporation | Separator for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery including the same |
| JP4984372B2 (en) * | 2003-08-06 | 2012-07-25 | 三菱化学株式会社 | Nonaqueous electrolyte secondary battery separator and nonaqueous electrolyte secondary battery using the same |
| EP2472638A3 (en) * | 2003-12-15 | 2013-09-11 | Mitsubishi Chemical Corporation | Nonaqueous-Electrolyte Secondary Battery |
| WO2005061599A1 (en) * | 2003-12-24 | 2005-07-07 | Asahi Kasei Chemicals Corporation | Microporous membrane made from polyolefin |
| JP2005293950A (en) * | 2004-03-31 | 2005-10-20 | Tdk Corp | Lithium ion secondary battery and charging method of lithium ion secondary battery |
| JP4628764B2 (en) * | 2004-07-06 | 2011-02-09 | 旭化成株式会社 | Storage device separator |
| JP5417686B2 (en) * | 2006-01-24 | 2014-02-19 | ソニー株式会社 | Separator and battery |
| WO2008093572A1 (en) | 2007-01-30 | 2008-08-07 | Asahi Kasei E-Materials Corporation | Polyolefin microporous membrane |
| JP4979432B2 (en) * | 2007-03-28 | 2012-07-18 | 三洋電機株式会社 | Cylindrical lithium secondary battery |
| JP5572334B2 (en) * | 2008-05-30 | 2014-08-13 | 旭化成イーマテリアルズ株式会社 | Polyolefin microporous membrane |
| JP5546144B2 (en) * | 2009-03-05 | 2014-07-09 | 旭化成イーマテリアルズ株式会社 | Polyolefin microporous membrane |
| KR101336593B1 (en) | 2010-04-20 | 2013-12-05 | 에스케이이노베이션 주식회사 | Method with good productivity for preparing microporous polyolefin film with various properties |
| JP5577964B2 (en) * | 2010-09-01 | 2014-08-27 | 日産自動車株式会社 | Lithium ion secondary battery |
| JP2018147688A (en) | 2017-03-03 | 2018-09-20 | 住友化学株式会社 | Separator for nonaqueous electrolyte secondary battery |
| JP7169149B2 (en) | 2017-10-20 | 2022-11-10 | 住友化学株式会社 | Separator for non-aqueous electrolyte secondary battery |
| US20240055722A1 (en) | 2020-10-30 | 2024-02-15 | Asahi Kasei Kabushiki Kaisha | Polyolefin microporous membrane |
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| JPH06212006A (en) * | 1993-01-19 | 1994-08-02 | Asahi Chem Ind Co Ltd | Polyethylene millipore membrane |
| JPH09169867A (en) * | 1995-10-17 | 1997-06-30 | Asahi Chem Ind Co Ltd | Microporous film and its production |
| JPH10258462A (en) * | 1996-11-19 | 1998-09-29 | Mitsui Chem Inc | High molecular weight polyolefin porous film and its manufacture |
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