JP3721727B2 - Battery electrode binder - Google Patents
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- JP3721727B2 JP3721727B2 JP19489997A JP19489997A JP3721727B2 JP 3721727 B2 JP3721727 B2 JP 3721727B2 JP 19489997 A JP19489997 A JP 19489997A JP 19489997 A JP19489997 A JP 19489997A JP 3721727 B2 JP3721727 B2 JP 3721727B2
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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
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
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- Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
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Description
【０００１】
【発明の属する技術分野】
本発明は高容量化、放電性能、充放電サイクル性、安全性に優れた２次電池電極用バインダーに関するものである。さらに詳しくは電極活物質が集電材に保持された電池電極用バインダーに関するものである。
【０００２】
【従来の技術】
近年、電子産業における技術進歩は著しく、電池技術においても高エネルギー密度、安全性等の要求が増大している。かかる要求を満足するには従来のニッケル-カドミウム電池では不可能なことから、負極にカドミウムの代わりに水素吸蔵合金を使用するニッケル水素電池や非水系電池であるリチウムイオン電池が注目されている。
リチウムイオン電池はエネルギー密度が高い、保存性が良い、小型軽量という特長を有し、ニッケル水素電池は急速充放電可能、過充電、過放電に強くかつニッケル-カドミウム電池と互換性、類似性があり、現在ニッケル-カドミウム電池が利用されている機器の代替も可能である。
これらの電池にバインダーは、電極活物質を集電材に固定させる目的で使用される。これらの二次電池はリチウムイオンまたは水素イオンの吸放出のし易さが高容量化、長寿命化につながり、これを満たすためにバインダーに要求される性能としては、▲１▼電極活物質と集電材の結着性が良好であること、▲２▼電解液中のイオンをできるだけ抵抗なく自由に移動させること、▲３▼電解液や充放電によって体積変化しないこと、等があげられる。
しかし、従来のバインダーでは電極活物質に対する影響が著しいため、上記の条件をすべてを満たすことは困難であった。
【０００３】
【発明が解決しようとする課題】
本発明では、主に二次電池において、電極活物質の集電性をいかに確保し、その利用効率を向上させ、電極活物質に対する影響が少ないバインダーを用いて長寿命、高容量化を達成することにある。
【０００４】
【課題を解決するための手段】
本発明は、上記の課題を解決するために、（ａ）芳香族ビニル単位、（ｂ）共役ジエン単位、（ｃ）（メタ）アクリル酸エステル単位および（ｄ）エチレン性不飽和カルボン酸単位からなり、（ａ）芳香族ビニル単位が共重合体全体の３０〜８０重量％、（ｂ）共役ジエン単位が共重合体全体の３０重量％未満、（ｃ）（メタ）アクリル酸エステル単位が共重合体全体の１０〜４０重量％、（ｄ）エチレン性不飽和カルボン酸単位が共重合体全体の０．１〜１０重量％であり、ガラス転移点（Ｔｇ）が−１５℃〜１５０℃である共重合体（以下、「特定共重合体」ともいう）の水系分散体を含有することを特徴とする電池電極用バインダーを提供するものである。上記電池電極用バインダーは、コアがガラス転移点が−５０〜５０℃の共重合体（以下、「共重合体Ｘ」ともいう）からなり、かつシェルがガラス転移点が０〜１００℃の共重合体（以下、「共重合体Ｙ」ともいう）からなるコアシェル構造の粒子を含有する電池電極用バインダーであることが好ましい。
【０００５】
【発明の実施の形態】
以下に本発明を詳細に説明する。なお、本発明において「単位」というのは、単量体がラジカル重合した後の各単量体由来の構造を示すものである。
＜特定共重合体＞
本発明に用いられる特定共重合体は、（ａ）芳香族ビニル単位、（ｂ）共役ジエン単位、（ｃ）（メタ）アクリル酸エステル単位および（ｄ）エチレン性不飽和カルボン酸単位からなる共重合体である。
上記（ａ）芳香族ビニル単位としては、例えばスチレン、α−メチルスチレン、ｐ−メチルスチレン、ビニルトルエン、クロルスチレン、ジビニルベンゼンなどの芳香族ビニル化合物がラジカル重合した後の構造が挙げられるが、特に好ましく用いられるのはスチレンである。かかる（ａ）芳香族ビニル単位の割合は、通常、特定共重合体全体の２０〜９０重量％、好ましくは３０〜８０重量％、更に好ましくは４０〜７０重量％である。２０重量％未満では、共重合体がべとつき強度がなくなる場合があり、９０重量％を超えると、共重合体が硬くなり過ぎ、バインダーとしての接着強度、柔軟性が劣るという場合がある。
上記（ｂ）共役ジエン単位は、得られる特定共重合体に適度な柔軟性と伸びを付与するために必須の成分である。具体的には、例えば１，３−ブタジエン、イソプレン、２−クロロ−１、３−ブタジエン、クロロプレンなどの共役ジエン化合物がラジカル重合した後の構造が挙げられ、特に好ましく用いられるのは１，３−ブタジエンである。かかる（ｂ）共役ジエン単位の割合は、特定共重合体全体の３０重量％未満、好ましくは２９重量％以下、さらに好ましくは１０〜２５重量％である。３０重量％以上では、特定共重合体が過度に電極活物質を覆い、電池特性を悪化させるため好ましくない。
上記（ｃ）（メタ）アクリル酸エステル単位としては、例えば（メタ）アクリル酸メチル、（メタ）アクリル酸エチル、（メタ）アクリル酸ｎ−プロピル、（メタ）アクリル酸ｉ−プロピル、（メタ）アクリル酸ｎ−ブチル、（メタ）アクリル酸ｉ−ブチル、（メタ）アクリル酸ｎ−アミル、（メタ）アクリル酸ｉ−アミル、（メタ）アクリル酸ヘキシル、（メタ）アクリル酸２−ヘキシル、（メタ）アクリル酸オクチル、（メタ）アクリル酸ｉ−ノニル、（メタ）アクリル酸デシル、（メタ）アクリル酸ヒドロキシメチル、（メタ）アクリル酸ヒドロキシエチル、（メタ）アクリル酸エチレングリコールジなどの（メタ）アクリル酸エステルがラジカル重合した後の構造が挙げられ、好ましく用いられるのは（メタ）アクリル酸メチル、（メタ）アクリル酸ブチル、特に好ましく用いられるのは（メタ）アクリル酸メチルである。かかる（ｃ）（メタ）アクリル酸エステル単位の割合は、特定共重合体全体の１０〜４０重量％、好ましくは１３〜３５重量％である。１０重量％未満では、得られるバインダーを用いて電池電極用組成物を調製して集電材に塗布した際、塗膜の弾性や強度が劣り、好ましくない。また、４０重量％を超えると特定共重合体の重合系の安定性が劣り、またバインダーとしての密着性なども低下して、好ましくない。
【０００６】
また、上記（ｄ）エチレン性不飽和カルボン酸単位としては、アクリル酸、（メタ）アクリル酸、イタコン酸、フマル酸、マレイン酸などがラジカル重合した後の構造が挙げられる。かかる（ｄ）エチレン性不飽和カルボン酸単位の割合は、好ましくは、特定共重合体の０．１〜１０重量％である。０．１重量％未満では、共重合体のバインダー性能、耐薬品性が劣る場合があり、一方１０重量％を超えると、耐水性、貯蔵安定性が劣るものとなる場合がある。
さらに、本発明において用いられる特定共重合体には、（ｅ）上記（ａ）〜（ｄ）と共重合可能なその他の官能基含有化合物単位が含有されていても良い。上記（ｅ）官能基含有化合物単位としては、（メタ）アクリルアミド、Ｎ−メチロールアクリルアミドなどのエチレン性不飽和カルボン酸のアルキルアミド；酢酸ビニル、プロピオン酸ビニルなどのカルボン酸ビニルエステル；エチレン性不飽和ジカルボン酸の、酸無水物、モノアルキルエステル、モノアミド類；アミノエチルアクリレート、ジメチルアミノエチルアクリレート、ブチルアミノエチルアクリレートなどのエチレン性不飽和カルボン酸のアミノアルキルエステル；アミノエチルアクリルアミド、ジメチルアミノメチルメタクリルアミド、メチルアミノプロピルメタクリルアミドなどのエチレン性不飽和カルボン酸のアミノアルキルアミド；（メタ）アクリロニトリル、α−クロルアクリロニトリルなどのシアン化ビニル系化合物；グリシジル（メタ）アクリレートなどの不飽和脂肪族グリシジルエステルなどがラジカル重合した後の構造が挙げられ、好ましく用いられるのはエチレン性不飽和カルボン酸アミノアルキルアミドである。かかる（ｅ）官能基含有化合物単位の割合は、好ましくは、特定重合体全体の０．１〜１０重量％である。
これら（ａ）〜（ｅ）の共重合単位を構成する単量体は、いずれも１種単独で、または２種以上を併用して用いることが出来る。
本発明において使用される特定共重合体のガラス転移点（Ｔｇ）は、通常−１５℃〜１５０℃であり、好ましくは−５℃〜１００℃、さらに好ましくは５℃〜６０℃である。Ｔｇが−１５℃未満では、共重合体が過度に電極活物質を覆い、インピーダンスが高くなりやすい。また、Ｔｇが１５０℃を超えると、特定共重合体により得られるバインダーは柔軟性、粘着性が乏しくなり、電極活物質の集電材への接着性、電極の成形性が劣る場合がある。
【０００７】
本発明において、特定共重合体がコアシェル構造の粒子である場合には、（ａ−１）芳香族ビニル単位５〜７５重量％、（ｂ−１）共役ジエン単位１５〜６５重量％、（ｃー１）（メタ）アクリル酸エステル単位１１〜４０重量％、（ｄ−１）エチレン性不飽和カルボン酸単位０〜２重量％および（ｅ−１）上記（ａ−１）〜（ｄ−１）のモノマーと共重合可能なその他の官能基含有化合物単位０〜２重量％（ただし、（ａ−１）＋（ｂ−１）＋（ｃ−１）＋（ｄ−１）＋（ｅ−１）＝１００重量％）からなる共重合体Ｘ５〜９０重量部ならびに（ａ−２）芳香族ビニル単位２５〜９０重量％、（ｂ−２）共役ジエン単位３５重量％以下、（ｃ−２）（メタ）アクリル酸エステル単位１１〜４０重量％、（ｄ−２）エチレン性不飽和カルボン酸単位０．５〜３０重量％および（ｅ−２）上記（ａ−２）〜（ｄ−２）のモノマーと共重合可能なその他の官能基含有化合物単位０〜３０重量％（ただし、（ａ−２）＋（ｂ−２）＋（ｃ−２）＋（ｄ−２）＋（ｅ−２）＝１００重量％）からなる共重合体Ｙ１０〜９５重量部（ただし、Ｘ＋Ｙ＝１００重量部）からなることが特に好ましい。ただし、上記構造の特定共重合体を用いる場合、各単量体単位の共重合体全体における割合とは、共重合体Ｘと共重合体Ｙとを合わせた粒子全体における割合を示す。
【０００８】
上記共重合体Ｘ部分における（ａ−１）芳香族ビニル単位としては、上記（ａ）芳香族ビニル単位と同様のものが挙げられる。かかる（ａ−１）芳香族ビニル単位の共重合体Ｘにおける割合は、５〜７５重量％、好ましくは２５〜４５重量％である。５重量％未満では得られるバインダーの強度が不十分になる場合があり、７５重量％を超えると共重合体が硬くなり過ぎ、電極活物質、集電体などへの接着強度や、得られるバインダーの柔軟性が劣る場合がある。
上記（ｂ−１）共役ジエン単量体単位としては、上記（ｂ）共役ジエン単位と同様のものが挙げられる。かかる（ｂ−１）共役ジエン単位の共重合体Ｘにおける割合は、１５〜６５重量％、好ましくは２５〜４５重量％である。１５重量％未満では共重合体が硬くなり過ぎ、電極活物質、集電体などへの接着強度や、得られるバインダーの柔軟性が劣る場合があり、６５重量％を超えるとＴｇが低くなり過ぎ、電極用組成物を調製して集電材に塗布した際、塗膜がタックのあるべとついたものになる場合がある。
上記（ｃ−１）（メタ）アクリル酸エステル単位としては、上記（ｃ）（メタ）アクリル酸エステル単位と同様のものが挙げられる。かかる（ｃ−１）（メタ）アクリル酸エステル単位の共重合体Ｘにおける割合は、１０〜４０重量％、好ましくは２０〜３５重量％である。１０重量％未満では、得られるバインダーを用いて電池電極用組成物を調製して集電材に塗布した際、塗膜の弾性や強度が劣り、好ましくない。また、４０重量％を超えると特定共重合体の重合系の安定性が劣り、またバインダーとしての密着性なども低下して、好ましくない。
上記（ｄ−１）エチレン性不飽和カルボン酸単位としては、上記（ｄ）不飽和カルボン酸単位と同様のものが挙げられ、共重合体Ｘにおける割合は０〜２重量％、好ましくは０〜１重量％の割合で用いられる。２重量％を超えると、得られるバインダーの貯蔵安定性、耐水性、耐アルカリ性が悪化する場合がある。
上記（ｅ−１）官能基含有化合物単位としては、上記（ｅ）官能基含有化合物単位と同様のものが挙げられ、共重合体Ｘにおける割合は０〜２重量％、好ましくは０〜１重量％の割合で用いられる。２重量％を超えると、得られるバインダーの貯蔵安定性、耐水性、耐アルカリ性が悪化する場合がある。
【０００９】
上記共重合体Ｙ部分における（ａ−２）芳香族ビニル単位としては、上記（ａ）芳香族ビニル単位と同様のものが挙げられる。かかる（ａ−２）芳香族ビニル単量体単位の共重合体Ｙにおける割合は、２５〜９０重量％、好ましくは３５〜７５重量％である。２５重量％未満では、共重合体がべとつき強度がなくなる場合があり、９０重量％を超えると、共重合体が硬くなり過ぎ、バインダーとしての接着強度や柔軟性が劣る場合がある。
上記（ｂ−２）共役ジエン単位としては、上記（ｂ）共役ジエン単位と同様のものが挙げられる。かかる（ｂ−２）共役ジエン単位の共重合体Ｙにおける割合は、３５重量％以下、好ましくは３０重量％以下である。３５重量％を超えると、共重合体が過度に電極活物質を覆い、内部抵抗が上がり、電池特性が劣る場合がある。
上記（ｃ−２）（メタ）アクリル酸エステル単位としては、上記（ｃ）（メタ）アクリル酸エステル単位と同様のものが挙げられる。かかる（ｃ−２）（メタ）アクリル酸エステル単位の共重合体Ｙにおける割合は、１１〜４０重量％、好ましくは２０〜３５重量％である。１１重量％未満では、得られるバインダーを用いて電池電極用組成物を調製して集電材に塗布した際、塗膜の弾性や強度が劣り、好ましくない。また、４０重量％を超えると特定共重合体の重合系の安定性が劣り、またバインダーとしての密着性なども低下して、好ましくない。１１重量％未満では塗膜の弾性、強度劣り、また、４０重量％を超えると重合系の安定性が劣り、また密着性なども低下して、好ましくない。
上記（ｄ−２）エチレン性不飽和カルボン酸単位としては、上記（ｄ）不飽和カルボン酸単位と同様のものが挙げられ、共重合体Ｘにおける割合は０．５〜３０重量％、好ましくは２〜１０重量％の割合で用いられる。０．５重量％未満であると、得られるバインダーの安定性が悪くなって凝固物が生成しやすく、また、バインダーの機械的、化学的安定性に劣る。一方、３０重量％を超えると、得られるバインダーの粘度が高くなりすぎ、貯蔵安定性、耐水性および耐アルカリ性に劣る。
上記（ｅ−２）官能基含有化合物単位としては、上記（ｅ）官能基含有化合物単位と同様なものが挙げられ、共重合体Ｙにおける割合は、０〜３０重量％、好ましくは２〜１０重量％である。３０重量％を超えると、得られるバインダーの粘度が高くなりすぎ、貯蔵安定性、耐水性および耐アルカリ性に劣る。
【００１０】
また、特定共重合体（共重合体Ｘ＋共重合体Ｙ）における共重合体Ｘの割合は、５〜９０重量％、好ましくは１０〜８０重量％、さらに好ましくは２０〜７０重量％である。共重合体Ｘの割合が５重量％未満であると、得られるバインダーの電極活物質、集電体などへの接着強度が不足する場合があり、９０重量％を超えると得られるバインダーの強度が劣るものとなる場合がある。
本発明において、特定共重合体は、共重合体Ｘがコア、共重合体Ｙがシェルであるコアシェル構造であることが好ましい。
【００１１】
本発明の電池電極用バインダーは、特定共重合体の水系分散体からなる。この水系分散体中に分散する特定共重合体粒子の平均粒子径は、７０〜３５０ｎｍが好ましく、さらに好ましくは８０〜２５０ｎｍである。
また、特定共重合体の水分散体の固形分濃度は、通常２０〜６５重量％、好ましくは３５〜６０重量％である。
【００１２】
本発明において特定共重合体は、上記構造単位を有する単量体を乳化重合することにより製造することができる。特に、ガラス転移点が−５０〜５０℃である共重合体からなるシード粒子の存在下、その共重合体のガラス転移点が０〜１００℃となる単量体をシード重合する方法が好ましく、具体的には、上記共重合体Ｘからなるシード粒子の存在下、上記共重合体Ｙを与える単量体混合物を重合する方法が好ましい。ここで、シード粒子の製造およびシード重合には共に通常の乳化重合が用いられる。
上記乳化重合に際しては、公知の方法を採用することができ、水性媒体中で乳化剤、重合開始剤、分子量調節剤などを用いて製造することができる。
ここで、乳化剤としては、アニオン性界面活性剤、ノニオン性界面活性剤、両性界面活性剤などが単独で、あるいは２種以上を併用して使用できる。
アニオン性界面活性剤としては、例えば高級アルコールの硫酸エステル、アルキルベンゼンスルホン酸塩、脂肪族スルホン酸塩、ポリエチレングリコールアルキルエーテルの硫酸エステルなどが挙げられる。
また、ノニオン界面活性剤としては、通常のポリエチレングリコールのアルキルエステル型、アルキルエーテル型、アルキルフェニルエーテル型などが用いられる。
両性界面活性剤としては、アニオン部分としてカルボン酸塩、硫酸エステル塩、スルホン酸塩、燐酸エステル塩を、カチオン部分としてはアミン塩、第４級アンモニウム塩を持つものが挙げられ、具体的には、ラウリルベタイン、ステアリルベタインなどのベタイン類；ラウリル−β−アラニン、ステアリル−β−アラニン、ラウリルジ（アミノエチル）グリシン、オクチルジ（アミノエチル）グリシンなどのアミノ酸タイプのものなどが用いられる。
特定共重合体の重合における乳化剤の使用量は、全単量体１００重量部に対して好ましくは０．５〜５重量部である。
【００１３】
上記重合開始剤としては、過硫酸ナトリウム、過硫酸カリウム、過硫酸アンモニウムなどの水溶性重合開始剤；過酸化ベンゾイル、ラウリルパーオキサイド、２，２’−アゾビスイソブチルニトリルなどの油溶性重合開始剤；還元剤との組み合わせによるレドックス系重合開始剤などが、それぞれ単独であるいは組み合わせて使用できる。重合開始剤の使用量は、全単量体１００重量部に対して好ましくは０．５〜３重量部である。
さらに、特定共重合体の重合においては、分子量調節剤、キレート化剤、無機電解質なども公知のものが使用できる。
上記分子量調節剤としては、クロロホルム、四臭化炭素などのハロゲン化炭化水素類；ｎ−ヘキシルメルカプタン、ｎ−オクチルメルカプタン、ｎ−ドデシルメルカプタン、ｔ−ドデシルメルカプタン、チオグリコール酸などのメルカプタン類；ジメチルキサントゲンジサルファイド、ジイソプロピルキサントゲンジサルフィドなどのキサントゲン類；ターピノーレン、α−メチルスチレンダイマーなど通常の乳化重合で使用可能なものを全て使用できる。分子量調節剤の使用量は、全単量体１００重量部に対して通常５重量部以下である。
【００１４】
コアシェル構造を有する特定共重合体の重合方法としては、▲１▼あらかじめ共重合体Ｘを別の重合容器で重合し、この共重合体Ｘをシード粒子として所定量を重合容器に添加した後、共重合体Ｙを与える単量体を重合する方法、あるいは▲２▼共重合体Ｘを重合し、同一重合容器内で共重合体Ｙを与える単量体の重合を行う方法などが挙げられる。なお、いずれの方法においても、共重合体Ｘの重合転化率は５０重量％以上、好ましくは７０重量％以上である。
共重合体Ｙを与える単量体混合物を仕込む方法としては、▲１▼単量体混合物を全量一括で仕込み重合する方法、▲２▼単量体混合物の一部を重合した後、その残りを連続的にあるいは断続的に添加する方法、あるいは▲３▼単量体混合物を重合の始めから連続的に添加する方法などを採ることができる。また、これらの仕込み方法を組み合わせることもできる。
重合温度は、通常共重合体Ｘを重合する場合５〜８０℃、好ましくは５〜５０℃、共重合体部分Ｙを重合する場合は２０〜８０℃、好ましくは２０〜６０℃である。重合時間は、通常１０〜３０時間である。
【００１５】
＜電池電極用組成物＞
本発明の電池電極用バインダーは、特に電極活物質と配合して電池電極用組成物として使用されることが好ましい。この電池電極用組成物を集電材に塗布し、乾燥することにより、電池電極を製造することができる。
上記電極活物質としては、非水系電池に関しては、例えば、ＭｎＯ2 、ＭｏＯ3 、Ｖ2 Ｏ5 、Ｖ6 Ｏ13、Ｆｅ2 Ｏ3 、Ｆｅ3 Ｏ4 、Ｌｉ(1-x) ＣｏＯ2 、Ｌｉ(1-x) ・ＮｉＯ2 、Ｌｉx Ｃｏy Ｓｎz Ｏ2 、ＴｉＳ2 、ＴｉＳ3 、ＭｏＳ3 、ＦｅＳ2 、ＣｕＦ2 、ＮｉＦ2 などの無機化合物；フッ化カーボン、グラファイト、気相成長炭素繊維および／またはその粉砕物、ＰＡＮ系炭素繊維および／またはその粉砕物、ピッチ系炭素繊維および／またはその粉砕物などの炭素材料；ポリアセチレン、ポリ−ｐ−フェニレンなどの導電性高分子などが挙げられる。特にＬｉ(1-x) ＣｏＯ2 、Ｌｉ(1-x) ＮｉＯ2 、Ｌｉx Ｃｏy Ｓｎz Ｏ2 、Ｌｉ(1-X) Ｃｏ(1-x) Ｎｉy Ｏ2 などのリチウムイオン含有複合酸化物を用いた場合、正負極共に放電状態で組み立てることが可能となり好ましい組み合わせとなる。
【００１６】
上記電池電極用組成物において、本発明の電池電極用バインダーは、電極活物質１００重量部に対して固形分で０．１〜２０重量部、好ましくは０．５〜１０重量部配合される。
本発明の電池電極用バインダーの配合量が０．１重量部未満では、集電体などに対する良好な接着力が得られず、２０重量部を超えると過電圧が著しく上昇し電池特性に悪影響をおよぼす。
本発明の電池電極用バインダーを用いる電池電極用組成物には、必要に応じて、水溶性増粘剤が特定共重合体１００重量部に対して１〜２００重量部添加されていてもよい。
上記水溶性増粘剤としては、カルボキシメチルセルロース、メチルセルロース、ヒドロキシメチルセルロース、エチルセルロース、ポリビニルアルコール、ポリアクリル酸（塩）、酸化スターチ、リン酸化スターチ、カゼインなどが挙げられる。
上記電池電極用組成物は、本発明の電池電極用バインダー、電極活物質および必要に応じて水溶性増粘剤からなるが、その他に、ヘキサメタリン酸ソーダ、トリポリリン酸ソーダ、ピロリン酸ソーダ、ポリアクリル酸ソーダなどの分散剤、ラテックスの安定化剤としてのノニオン性、アニオン性界面活性剤などの添加剤、電極の導電性付与の目的でカーボンなどを加えてもよい。
【００１７】
＜電池電極＞
電池電極は、上記電池電極用組成物を、好ましくはスラリー状にして集電材に塗布し、加熱し、乾燥することによって得られる。集電材としては、非水系電池では例えばアルミ箔、銅箔などが挙げられる。
電池電極用組成物の塗布方法としては、リバースロール法、コンマバー法、グラビヤ法、エアーナイフ法など任意のコーターヘッドを用いることができ、乾燥方法としては放置乾燥、送風乾燥機、温風乾燥機、赤外線加熱機、遠赤外線加熱機などが使用できる。乾燥温度は、通常１５０℃前後で行う。
【００１８】
上記のようにして得られた電池電極を用いて非水系電池を組み立てる場合、非水系電解液の電解質としては特に限定されないが、アルカリ二次電池での例を示せば、ＬｉＣｌＯ4 、ＬｉＢＦ4 、ＬｉＡｓＦ6 、ＣＦ3 ＳＯ3 Ｌｉ、ＬｉＰＦ6 、ＬｉＩ、ＬｉＡｌＣｌ4 、ＮａＣｌＯ4 、ＮａＢＦ4 、ＮａＩ、（ｎ−Ｂｕ）4 ＮＣｌＯ4 、（ｎ−Ｂｕ）4 ＮＢＦ4 、ＫＰＦ6 などが挙げられる。また用いられる電解液の有機溶媒としては、例えばエーテル類、ケトン類、ラクトン類、ニトリル類、アミン類、アミド類、硫黄化合物、塩素化炭化水素類、エステル類、カーボネート類、ニトロ化合物、リン酸エステル系化合物、スルホラン系化合物などを用いることができるが、中でもエーテル類、ケトン類、ニトリル類、塩素化炭化水素類、カーボネート類、スルホラン系化合物が好ましい。
これらの代表例としては、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，４−ジオキサン、アニソール、モノグライム、アセトニトリル、プロピオニトリル、４−メチル−２−ペンタノン、ブチロニトリル、バレロニトリル、ベンゾニトリル、１，２−ジクロロエタン、γ−ブチロラクトン、ジメトキシエタン、メチルフオルメイト、プロピレンカーボネート、エチレンカーボネート、ジメチルホルムアミド、ジメチルスルホキシド、ジメチルチオホルムアミド、スルホラン、３−メチル−スルホラン、リン酸トリメチル、リン酸トリエチルおよびこれらの混合溶媒などを挙げることができるが、必ずしもこれらに限定されるものではない。
さらに、要すればセパレーター、集電材、端子、絶縁板などの部品を用いて電池が構成される。また、電池の構造としては、特に限定されるものではないが、正極、負極、さらに要すればセパレーターを単層または複層としたペーパー型電池、または正極、負極、さらに要すればセパレーターをロール状に巻いた円筒状電池などの形態が一例として挙げられる。
本発明の電池電極用バインダーを用いて製造した電池電極は、具体的にＡＶ機器、ＯＡ機器、通信機器などに好適に使用することができる。
【００１９】
【実施例】
以下に実施例にて本発明をさらに詳しく説明する。但し、本発明はこれらの実施例に何ら制約されるものではない。実施例および比較例における各々の評価方法を以下に示す。
（１）電池電極用バインダーの評価
平均粒子径の測定
大塚電子（株）製レーザー粒径解析システムＬＰＡ−３０００ｓ／３１００を用いて粒子径を測定した。
ガラス転移点（Ｔｇ）の測定
０．５Ｎアンモニア水でｐＨ８に調整した水系分散体を、ガラス板に塗布し、１２０℃で１時間乾燥して、重合体フィルムを得た。これを使用し、セイコー電子工業（株）製示差走査熱量計を用いて測定した。
（２）電池電極用組成物の評価
ニードルコークス粉砕品（平均粒径１２μｍ）１００重量部と電池電極用バインダー１重量部、増粘剤としてカルボキシメチルセルロース水溶液を固形分で１重量部、０．５Ｎアンモニア水０．５重量部を加え、よく混合して電池電極用組成物を製造し、下記の評価を行った。
銅箔との結着性
厚さ５０μｍの銅箔を基材として、ロールコーターで得られた電池電極用組成物を２００ｇ／ｍ2 の厚さで塗工し、１５０℃×１０分乾燥後、室温でプレスして厚さ６０μｍの銅箔塗工塗布膜を得た。得られた銅箔塗工塗布膜を用い、テスター産業（株）製のクレメンス型「引っ掻き硬度計」で塗膜強度を測定した。測定方法は、ＪＩＳ Ｋ５４００ ８．４．１に準じて行った。
導電性の測定法
１００μｍのＰＥＴフィルムに電池電極用組成物を４００ｇ／ｍ2 の厚さで塗工し、１５０℃×１０分乾燥し、膜厚１２０μｍの塗布膜を得た。これを用い、４端子法で抵抗を測定した。
耐電解液性
上記銅箔塗工塗布膜を電解液ＬＩＰＡＳＴＥ−ＥＤＥＣ／１（ＬｉＣＬＯ4 ／エチレンカーボネート／ジエチルカーボネート＝８．４％／５２．８％／３８．８％ 富士薬品工業（株）製品）に８０℃×７２ｈｒｓ浸積し、銅箔からの剥離を５点法で観察した。例えば変化のないときを５点、完全に剥離した場合を１点とする。
（３）電池電極の評価
平均粒径２μｍのＬｉ1.03Ｃｏ0.95Ｓｎ0.042 Ｏ2 １００重量部とグラファイト粉７．５重量部、アセトンブラック２．５重量部を混合し、フッ素ゴムのメチルイソブチルケトン溶液（濃度４重量％）を５０重量部加え混合撹拌し塗工液とした。
市販Ａ１箔（厚さ１５μ）を基材としてこの塗工液を２９０ｇ／ｍ2 で塗布乾燥し、厚さ１１０μｍの正極を得た。
次に、上記電池電極用組成物から得られた銅箔塗工塗布膜を負極とし、０．９ｃｍ×５．５ｃｍに切り出してリチウム二次電池を組み立てた。
この電池を４．２Ｖまで充電し、１００ｍＡで２．５Ｖまで放電するサイクルを繰り返し、容量保存率を測定した。また、４．２Ｖに充電したセルを７０℃×３０日間保存し、保存安定性を測定した。
【００２０】
実施例１〜４
（１）共重合体Ｘの重合
撹拌機を備え、温度調節の可能なオートクレーブ中に水２００部、ドデシルベンゼンスルホン酸ナトリウム０．５部、過硫酸カリウム１．０部、重亜硫酸ナトリウム０．５部および下記表１に示した分子量調節剤および共重合体Ｘ部分をあたえる単量体成分を一括して仕込み、４５℃で６時間反応させ、重合転化率が７０％以上であることを確認した。
（２）共重合体Ｙの重合
上記（１）の重合後に引続き、下記表１に示した分子量調節剤および共重合体Ｙを与える単量体成分の混合物を６０℃で７時間にわたって連続的に添加して重合を継続させ、更に連続添加終了後６時間にわたって７０℃で反応させて、特定共重合体の水分散体からなる本発明の電池電極用バインダーを得た。最終的な重合転化率は９８〜９９％であった。
得られた電池電極用バインダーの各々を用い、評価を行った。評価結果を表３および表４に示す。
実施例５
撹拌機を備えたオートクレーブに、イオン交換水７０部および過硫酸カリウム０．３部をそれぞれ仕込み、気相部を１５分間窒素ガスで置換し、８０℃に昇温した。一方、別容器で表１に示す成分を混合し、１５時間かけて上記オートクレーブに滴下した。滴下中は、８０℃で反応を行った。滴下終了後、さらに８５℃で５時間撹拌した後反応を終了させた。２５℃に冷却後、水酸化カリウムでｐＨを７に調整し、その後スチームを導入して残留単量体を除去し、次いで濃縮して特定共重合体の水分散体からなる本発明の電池電極用バインダーを得た。評価結果を、表３および表４に併せて示す。
比較例１〜３
実施例５において、単量体成分の組成を表２のとおりとした以外は、実施例５と同様にして重合体の水分散体からなる電池電極用バインダーを得た。評価結果を表３および表４に併せて示す。
【００２１】
【表１】
【表２】
ＳＴ＝スチレン（（ａ）成分）
ＢＤ＝ブタジエン（（ｂ）成分）
ＭＭＡ＝メタクリル酸メチル（（ｃ）成分）
ＢＡ＝アクリル酸ブチル（（ｃ）成分）
ＡＡ＝アクリル酸（（ｄ）成分）
ＩＡ＝イタコン酸（（ｄ）成分）
Ｎ−ＭＡＭ＝Ｎ−メチロールアクリルアミド（（ｅ）成分）
α-ＭＳＤ＝α-メチルスチレンダイマー（分子量調節剤）
ｔ-ＤＭ＝ｔ−ドデシルメルカプタン（分子量調節剤）
【００２３】
【表３】
【表４】
表１の実施例１〜５は、本発明の範囲の共重合体、表２は本発明の範囲外の共重合体の組成およびＴｇ、平均粒子径である。表３から明らかなように、本発明の共重合体を用いた場合、結着性、導電性、耐電解液性のバランスがとれ、さらに電池特性のサイクル性、保存特性、安全性に優れている。
これに対し、比較例１は、（メタ）アクリル酸エステル単位を含まない共重合体の例であり、結着性、耐電解液性が低く電池特性に劣る。比較例２は、共役ジエン単量体単位が本発明の範囲外である共重合体の例であり、結着性、導電性が低く電池特性に劣る。比較例３は、エチレン性不飽和カルボン酸単位を含まない共重合体の例であり、バインダー性能に劣り、結着性が悪く電池特性に劣る。
【００２６】
【発明の効果】
本発明の電池電極用バインダーは電極活物質とする電池、主に二次電池において、電極活物質の集電性を確保し、その利用効率を向上させ、電極活物質に対する影響が少ないバインダーを用いて長寿命、高容量化を達成することができる。
これまで、電極活物質と集電材との結着性付与の目的で使用されるバインダーは、電解液中のイオンの移動を妨げるものと考えられていた。しかしながら、バインダーに、本発明の共重合体を用い、電解液との親和性を高めることで、ハイレート性能優れた二次電池電極を与えることができる。さらに本発明のガラス転移点の範囲の共重合体を用い、塗工塗布膜の強度を上げることで、高容量化、放電性能、充放電サイクル性、安全性に優れた２次電池電極を与える。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a binder for a secondary battery electrode excellent in high capacity, discharge performance, charge / discharge cycle performance, and safety. More specifically, the present invention relates to a battery electrode binder in which an electrode active material is held by a current collector.
[0002]
[Prior art]
In recent years, technological progress in the electronics industry has been remarkable, and demands for high energy density, safety, etc. have been increasing in battery technology. Since the conventional nickel-cadmium battery cannot satisfy this requirement, attention has been drawn to a nickel-metal hydride battery that uses a hydrogen storage alloy instead of cadmium for the negative electrode and a lithium ion battery that is a non-aqueous battery.
Lithium-ion batteries are characterized by high energy density, good storage stability, and small size and light weight. Nickel metal hydride batteries are capable of rapid charge / discharge, are resistant to overcharge and overdischarge, and are compatible and similar to nickel-cadmium batteries. Yes, it is possible to replace equipment that currently uses nickel-cadmium batteries.
In these batteries, the binder is used for the purpose of fixing the electrode active material to the current collector. In these secondary batteries, the ease of absorption and release of lithium ions or hydrogen ions leads to an increase in capacity and life, and the performance required for the binder to satisfy this is as follows: (1) Electrode active material and For example, (2) the ions in the electrolyte solution can be freely moved without resistance as much as possible, and (3) the volume does not change due to the electrolyte solution or charging / discharging.
However, since the conventional binder has a significant influence on the electrode active material, it has been difficult to satisfy all the above conditions.
[0003]
[Problems to be solved by the invention]
In the present invention, mainly in a secondary battery, how to secure the current collecting property of the electrode active material, improve its utilization efficiency, and achieve a long life and high capacity using a binder that has little influence on the electrode active material. There is.
[0004]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides (a) an aromatic vinyl unit, (b) a conjugated diene unit, (c) a (meth) acrylic acid ester unit, and (d) an ethylenically unsaturated carboxylic acid unit. (A) the aromatic vinyl unit is 30 to 80% by weight of the whole copolymer, (b) the conjugated diene unit is less than 30% by weight of the whole copolymer, and (c) the (meth) acrylic acid ester unit is co-polymerized. 10 to 40% by weight of the whole polymer, (d) the ethylenically unsaturated carboxylic acid unit is 0.1 to 10% by weight of the whole copolymer , and the glass transition point (Tg) is −15 ° C. to 150 ° C. A battery electrode binder comprising an aqueous dispersion of a certain copolymer (hereinafter also referred to as “specific copolymer”) is provided. The battery electrode binder is made of a copolymer having a core having a glass transition point of −50 to 50 ° C. (hereinafter also referred to as “copolymer X”), and a shell having a glass transition point of 0 to 100 ° C. polymer (hereinafter, also referred to as "copolymer Y") is preferably a battery electrode binder comprising particles of core-shell structure composed of.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below. In the present invention, the “unit” indicates a structure derived from each monomer after the monomer undergoes radical polymerization.
<Specific copolymer>
The specific copolymer used in the present invention is a copolymer comprising (a) an aromatic vinyl unit, (b) a conjugated diene unit, (c) a (meth) acrylic acid ester unit, and (d) an ethylenically unsaturated carboxylic acid unit. It is a polymer.
Examples of the aromatic vinyl unit (a) include a structure after radical polymerization of an aromatic vinyl compound such as styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, chlorostyrene, and divinylbenzene. Particularly preferably used is styrene. The proportion of the (a) aromatic vinyl unit is usually 20 to 90% by weight, preferably 30 to 80% by weight, more preferably 40 to 70% by weight, based on the entire specific copolymer. If it is less than 20% by weight, the copolymer may lose its sticking strength, and if it exceeds 90% by weight, the copolymer may become too hard and the adhesive strength and flexibility as a binder may be inferior.
The (b) conjugated diene unit is an essential component for imparting appropriate flexibility and elongation to the specific copolymer to be obtained. Specifically, for example, a structure after radical polymerization of a conjugated diene compound such as 1,3-butadiene, isoprene, 2-chloro-1,3-butadiene, chloroprene and the like is used. -Butadiene. The ratio of the (b) conjugated diene unit is less than 30% by weight, preferably 29% by weight or less, more preferably 10 to 25% by weight, based on the entire specific copolymer. If it is 30% by weight or more, the specific copolymer excessively covers the electrode active material and deteriorates battery characteristics.
Examples of the (c) (meth) acrylic acid ester unit include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, (meth) N-butyl acrylate, i-butyl (meth) acrylate, n-amyl (meth) acrylate, i-amyl (meth) acrylate, hexyl (meth) acrylate, 2-hexyl (meth) acrylate, ( (Meth) acrylates such as octyl acrylate, i-nonyl (meth) acrylate, decyl (meth) acrylate, hydroxymethyl (meth) acrylate, hydroxyethyl (meth) acrylate, ethylene glycol dimethacrylate (meth) acrylate ) A structure after the acrylic acid ester is radically polymerized, and preferably used is methyl (meth) acrylate, ( Data) of butyl acrylate, particularly preferably used is a (meth) acrylate. The proportion of the (c) (meth) acrylic acid ester unit is 10 to 40% by weight, preferably 13 to 35% by weight, based on the entire specific copolymer. If it is less than 10% by weight, when the battery electrode composition is prepared using the obtained binder and applied to the current collector, the elasticity and strength of the coating film are inferior, which is not preferable. On the other hand, if it exceeds 40% by weight, the stability of the polymerization system of the specific copolymer is inferior, and the adhesiveness as a binder is lowered, which is not preferable.
[0006]
Examples of the (d) ethylenically unsaturated carboxylic acid unit include structures after radical polymerization of acrylic acid, (meth) acrylic acid, itaconic acid, fumaric acid, maleic acid, and the like. The ratio of the (d) ethylenically unsaturated carboxylic acid unit is preferably 0.1 to 10% by weight of the specific copolymer. If it is less than 0.1% by weight, the binder performance and chemical resistance of the copolymer may be inferior. On the other hand, if it exceeds 10% by weight, the water resistance and storage stability may be inferior.
Furthermore, the specific copolymer used in the present invention may contain (e) other functional group-containing compound units copolymerizable with the above (a) to (d). Examples of the above (e) functional group-containing compound unit include alkyl amides of ethylenically unsaturated carboxylic acids such as (meth) acrylamide and N-methylol acrylamide; vinyl carboxylic acid esters such as vinyl acetate and vinyl propionate; ethylenically unsaturated Acid anhydrides, monoalkyl esters, monoamides of dicarboxylic acids; aminoalkyl esters of ethylenically unsaturated carboxylic acids such as aminoethyl acrylate, dimethylaminoethyl acrylate, butylaminoethyl acrylate; aminoethylacrylamide, dimethylaminomethylmethacrylamide Aminoalkylamides of ethylenically unsaturated carboxylic acids such as methylaminopropyl methacrylamide; vinyl cyanide compounds such as (meth) acrylonitrile and α-chloroacrylonitrile ; Glycidyl (meth) an unsaturated aliphatic glycidyl esters such as acrylates include structure after radical polymerization, preferably used are ethylenically unsaturated carboxylic acid aminoalkyl amides. The ratio of the (e) functional group-containing compound unit is preferably 0.1 to 10% by weight of the entire specific polymer.
These monomers constituting the copolymer units (a) to (e) can be used alone or in combination of two or more.
The glass transition point (Tg) of the specific copolymer used in the present invention is usually -15 ° C to 150 ° C, preferably -5 ° C to 100 ° C, more preferably 5 ° C to 60 ° C. When Tg is less than −15 ° C., the copolymer excessively covers the electrode active material, and the impedance tends to increase. On the other hand, when Tg exceeds 150 ° C., the binder obtained from the specific copolymer is poor in flexibility and tackiness, and the adhesion of the electrode active material to the current collector and the moldability of the electrode may be inferior.
[0007]
In the present invention, when the specific copolymer is a particle having a core-shell structure, (a-1) 5-75% by weight of aromatic vinyl units, (b-1) 15-65% by weight of conjugated diene units, (c -1) 11 to 40% by weight of (meth) acrylic acid ester unit, (d-1) 0 to 2% by weight of ethylenically unsaturated carboxylic acid unit, and (e-1) above (a-1) to (d-1) ) Other functional group-containing compound units copolymerizable with monomers of 0 to 2% by weight (provided that (a-1) + (b-1) + (c-1) + (d-1) + (e- 1) = 100% by weight of a copolymer X consisting of 5 to 90 parts by weight and (a-2) 25 to 90% by weight of an aromatic vinyl unit, (b-2) 35% by weight or less of a conjugated diene unit, (c-2) ) (Meth) acrylic acid ester unit 11 to 40% by weight, (d-2) ethylenically unsaturated carboxylic acid 0.5 to 30% by weight of units and (e-2) 0 to 30% by weight of other functional group-containing compound units copolymerizable with the monomers (a-2) to (d-2) above (however, (a -2) + (b-2) + (c-2) + (d-2) + (e-2) = 100% by weight) copolymer Y10 to 95 parts by weight (where X + Y = 100 parts by weight) It is particularly preferable to consist of However, when using the specific copolymer of the said structure, the ratio in the whole copolymer of each monomer unit shows the ratio in the whole particle | grains which put the copolymer X and the copolymer Y together.
[0008]
Examples of the (a-1) aromatic vinyl unit in the copolymer X moiety include the same as the above (a) aromatic vinyl unit. The proportion of the (a-1) aromatic vinyl unit in the copolymer X is 5 to 75% by weight, preferably 25 to 45% by weight. If it is less than 5% by weight, the strength of the obtained binder may be insufficient. If it exceeds 75% by weight, the copolymer becomes too hard, and the adhesive strength to the electrode active material, current collector, etc. May be less flexible.
As said (b-1) conjugated diene monomer unit, the same thing as said (b) conjugated diene unit is mentioned. The proportion of the (b-1) conjugated diene unit in the copolymer X is 15 to 65% by weight, preferably 25 to 45% by weight. If it is less than 15% by weight, the copolymer becomes too hard, and the adhesive strength to the electrode active material, current collector, etc. and the flexibility of the resulting binder may be inferior. If it exceeds 65% by weight, the Tg becomes too low. When the electrode composition is prepared and applied to the current collector, the coating film may become tacky.
As said (c-1) (meth) acrylic acid ester unit, the same thing as said (c) (meth) acrylic acid ester unit is mentioned. The ratio of the (c-1) (meth) acrylic acid ester unit in the copolymer X is 10 to 40% by weight, preferably 20 to 35% by weight. If it is less than 10% by weight, when the battery electrode composition is prepared using the obtained binder and applied to the current collector, the elasticity and strength of the coating film are inferior, which is not preferable. On the other hand, if it exceeds 40% by weight, the stability of the polymerization system of the specific copolymer is inferior, and the adhesiveness as a binder is lowered, which is not preferable.
As said (d-1) ethylenically unsaturated carboxylic acid unit, the thing similar to said (d) unsaturated carboxylic acid unit is mentioned, The ratio in the copolymer X is 0 to 2 weight%, Preferably 0 to Used in a proportion of 1% by weight. If it exceeds 2% by weight, the storage stability, water resistance and alkali resistance of the resulting binder may be deteriorated.
As said (e-1) functional group containing compound unit, the same thing as said (e) functional group containing compound unit is mentioned, The ratio in the copolymer X is 0 to 2 weight%, Preferably it is 0 to 1 weight. % Is used. If it exceeds 2% by weight, the storage stability, water resistance and alkali resistance of the resulting binder may be deteriorated.
[0009]
Examples of the (a-2) aromatic vinyl unit in the copolymer Y moiety include the same as the above (a) aromatic vinyl unit. The proportion of the (a-2) aromatic vinyl monomer unit in the copolymer Y is 25 to 90% by weight, preferably 35 to 75% by weight. If it is less than 25% by weight, the copolymer may lose its sticking strength, and if it exceeds 90% by weight, the copolymer may become too hard and the adhesive strength and flexibility as a binder may be inferior.
As said (b-2) conjugated diene unit, the thing similar to said (b) conjugated diene unit is mentioned. The proportion of the (b-2) conjugated diene unit in the copolymer Y is 35% by weight or less, preferably 30% by weight or less. If it exceeds 35% by weight, the copolymer may excessively cover the electrode active material, resulting in an increase in internal resistance and battery characteristics.
As said (c-2) (meth) acrylic acid ester unit, the same thing as said (c) (meth) acrylic acid ester unit is mentioned. The proportion of the (c-2) (meth) acrylic acid ester unit in the copolymer Y is 11 to 40% by weight, preferably 20 to 35% by weight. If it is less than 11 weight%, when the composition for battery electrodes is prepared using the obtained binder and it apply | coats to a current collection material, the elasticity and intensity | strength of a coating film are inferior, and it is not preferable. On the other hand, if it exceeds 40% by weight, the stability of the polymerization system of the specific copolymer is inferior, and the adhesiveness as a binder is lowered, which is not preferable. If it is less than 11% by weight, the elasticity and strength of the coating film are inferior, and if it exceeds 40% by weight, the stability of the polymerization system is inferior and the adhesiveness is also lowered, which is not preferable.
Examples of the (d-2) ethylenically unsaturated carboxylic acid unit include the same as the above (d) unsaturated carboxylic acid unit, and the proportion in the copolymer X is 0.5 to 30% by weight, preferably Used in a proportion of 2 to 10% by weight. If it is less than 0.5% by weight, the stability of the resulting binder is deteriorated and a coagulated product is easily formed, and the mechanical and chemical stability of the binder is inferior. On the other hand, when it exceeds 30% by weight, the viscosity of the resulting binder becomes too high, and the storage stability, water resistance and alkali resistance are poor.
As said (e-2) functional group containing compound unit, the same thing as the said (e) functional group containing compound unit is mentioned, The ratio in the copolymer Y is 0-30 weight%, Preferably it is 2-10. % By weight. When it exceeds 30% by weight, the viscosity of the resulting binder becomes too high, and the storage stability, water resistance and alkali resistance are poor.
[0010]
Moreover, the ratio of the copolymer X in a specific copolymer (copolymer X + copolymer Y) is 5-90 weight%, Preferably it is 10-80 weight%, More preferably, it is 20-70 weight%. When the proportion of the copolymer X is less than 5% by weight, the adhesive strength of the obtained binder to the electrode active material, current collector, etc. may be insufficient, and when it exceeds 90% by weight, the strength of the obtained binder is low. It may be inferior.
In the present invention, the specific copolymer preferably has a core-shell structure in which the copolymer X is a core and the copolymer Y is a shell.
[0011]
The binder for battery electrodes of the present invention comprises an aqueous dispersion of a specific copolymer. The average particle size of the specific copolymer particles dispersed in the aqueous dispersion is preferably 70 to 350 nm, more preferably 80 to 250 nm.
The solid content concentration of the aqueous dispersion of the specific copolymer is usually 20 to 65% by weight, preferably 35 to 60% by weight.
[0012]
In the present invention, the specific copolymer can be produced by emulsion polymerization of a monomer having the above structural unit. In particular, in the presence of seed particles made of a copolymer having a glass transition point of −50 to 50 ° C., a method of seed polymerizing a monomer having a glass transition point of 0 to 100 ° C. of the copolymer is preferable, Specifically, a method of polymerizing a monomer mixture that gives the copolymer Y in the presence of seed particles made of the copolymer X is preferable. Here, both the production of seed particles and the seed polymerization use ordinary emulsion polymerization.
In the above emulsion polymerization, a known method can be employed, and the emulsion polymerization can be carried out using an emulsifier, a polymerization initiator, a molecular weight regulator and the like in an aqueous medium.
Here, as the emulsifier, anionic surfactants, nonionic surfactants, amphoteric surfactants and the like can be used alone or in combination of two or more.
Examples of the anionic surfactant include sulfates of higher alcohols, alkylbenzene sulfonates, aliphatic sulfonates, and sulfates of polyethylene glycol alkyl ethers.
As the nonionic surfactant, a normal polyethylene glycol alkyl ester type, alkyl ether type, alkylphenyl ether type, or the like is used.
Examples of amphoteric surfactants include those having a carboxylate, sulfate, sulfonate, and phosphate ester salt as the anion moiety, and an amine salt and quaternary ammonium salt as the cation moiety. And betaines such as lauryl betaine and stearyl betaine; amino acid types such as lauryl-β-alanine, stearyl-β-alanine, lauryl di (aminoethyl) glycine, and octyldi (aminoethyl) glycine are used.
The amount of the emulsifier used in the polymerization of the specific copolymer is preferably 0.5 to 5 parts by weight with respect to 100 parts by weight of all monomers.
[0013]
Examples of the polymerization initiator include water-soluble polymerization initiators such as sodium persulfate, potassium persulfate, and ammonium persulfate; oil-soluble polymerization initiators such as benzoyl peroxide, lauryl peroxide, and 2,2′-azobisisobutylnitrile; Redox polymerization initiators in combination with a reducing agent can be used alone or in combination. The amount of the polymerization initiator used is preferably 0.5 to 3 parts by weight with respect to 100 parts by weight of the total monomers.
Furthermore, in the polymerization of the specific copolymer, known molecular weight regulators, chelating agents, inorganic electrolytes and the like can be used.
Examples of the molecular weight regulator include halogenated hydrocarbons such as chloroform and carbon tetrabromide; mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan and thioglycolic acid; dimethyl Xanthogens such as xanthogen disulfide and diisopropylxanthogen disulfide; all those that can be used in usual emulsion polymerization such as terpinolene and α-methylstyrene dimer can be used. The usage-amount of a molecular weight regulator is 5 parts weight or less normally with respect to 100 weight part of all monomers.
[0014]
As a method for polymerizing a specific copolymer having a core-shell structure, (1) polymerizing copolymer X in a separate polymerization vessel in advance, and adding a predetermined amount to the polymerization vessel using this copolymer X as seed particles, Examples thereof include a method of polymerizing a monomer that gives the copolymer Y, and a method (2) of polymerizing the copolymer X and polymerizing the monomer that gives the copolymer Y in the same polymerization vessel. In any method, the polymerization conversion rate of the copolymer X is 50% by weight or more, preferably 70% by weight or more.
As a method of charging the monomer mixture that gives the copolymer Y, (1) a method in which the monomer mixture is charged all at once, and (2) after polymerizing a part of the monomer mixture, the remainder is A method of continuously or intermittently adding or (3) a method of continuously adding the monomer mixture from the beginning of the polymerization can be employed. Moreover, these preparation methods can also be combined.
The polymerization temperature is usually 5 to 80 ° C., preferably 5 to 50 ° C. when the copolymer X is polymerized, and 20 to 80 ° C., preferably 20 to 60 ° C. when the copolymer part Y is polymerized. The polymerization time is usually 10 to 30 hours.
[0015]
<Composition for battery electrode>
The battery electrode binder of the present invention is particularly preferably blended with an electrode active material and used as a battery electrode composition. A battery electrode can be produced by applying the battery electrode composition to a current collector and drying it.
As the electrode active material, for non-aqueous batteries, for example, MnO 2 , MoO 3 , V 2 O 5 , V 6 O 13 , Fe 2 O 3 , Fe 3 O 4 , Li (1-x) CoO 2 , Li (1-x) · NiO 2, Li x Co y Sn z O 2, TiS 2, TiS 3, MoS 3, FeS 2, CuF 2, inorganic compounds such as NiF 2; carbon fluoride, graphite, vapor Carbon materials such as carbon fibers and / or pulverized products thereof, PAN-based carbon fibers and / or pulverized products thereof, pitch-based carbon fibers and / or pulverized products thereof; and conductive polymers such as polyacetylene and poly-p-phenylene Can be mentioned. In particular Li (1-x) CoO 2 , Li (1-x) NiO 2, Li x Co y Sn z O 2, Li (1-X) Co (1-x) Li-ion containing complex such as Ni y O 2 When an oxide is used, both positive and negative electrodes can be assembled in a discharged state, which is a preferable combination.
[0016]
In the battery electrode composition, the battery electrode binder of the present invention is blended in an amount of 0.1 to 20 parts by weight, preferably 0.5 to 10 parts by weight, based on 100 parts by weight of the electrode active material.
When the blending amount of the binder for battery electrodes of the present invention is less than 0.1 parts by weight, good adhesion to a current collector or the like cannot be obtained, and when it exceeds 20 parts by weight, the overvoltage is remarkably increased and adversely affects battery characteristics. .
In the battery electrode composition using the battery electrode binder of the present invention, 1 to 200 parts by weight of a water-soluble thickener may be added to 100 parts by weight of the specific copolymer as necessary.
Examples of the water-soluble thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, and casein.
The battery electrode composition comprises the battery electrode binder of the present invention, an electrode active material, and, if necessary, a water-soluble thickener. In addition, sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, polyacrylic Dispersants such as acid soda, nonionic as a latex stabilizer, additives such as anionic surfactants, and carbon may be added for the purpose of imparting conductivity to the electrode.
[0017]
<Battery electrode>
The battery electrode is obtained by applying the battery electrode composition, preferably in a slurry form, to a current collector, heating, and drying. Examples of the current collector include aluminum foil and copper foil for non-aqueous batteries.
As a coating method for the battery electrode composition, any coater head such as reverse roll method, comma bar method, gravure method, air knife method, etc. can be used. Infrared heaters, far-infrared heaters, etc. can be used. The drying temperature is usually about 150 ° C.
[0018]
When assembling a non-aqueous battery using the battery electrode obtained as described above, the electrolyte of the non-aqueous electrolyte is not particularly limited, but examples of alkaline secondary batteries include LiClO 4 , LiBF 4 , Examples include LiAsF 6 , CF 3 SO 3 Li, LiPF 6 , LiI, LiAlCl 4 , NaClO 4 , NaBF 4 , NaI, (n-Bu) 4 NClO 4 , (n-Bu) 4 NBF 4 , and KPF 6 . Examples of the organic solvent for the electrolytic solution used include ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates, nitro compounds, and phosphoric acid. Ester compounds, sulfolane compounds, and the like can be used, and ethers, ketones, nitriles, chlorinated hydrocarbons, carbonates, and sulfolane compounds are particularly preferable.
Typical examples of these include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, acetonitrile, propionitrile, 4-methyl-2-pentanone, butyronitrile, valeronitrile, benzonitrile, 1,2- Dichloroethane, γ-butyrolactone, dimethoxyethane, methyl formate, propylene carbonate, ethylene carbonate, dimethylformamide, dimethyl sulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate and mixed solvents thereof However, it is not necessarily limited to these.
Further, if necessary, a battery is configured using components such as a separator, a current collector, a terminal, and an insulating plate. Further, the structure of the battery is not particularly limited, but a positive electrode, a negative electrode, and if necessary, a paper type battery having a separator as a single layer or multiple layers, or a positive electrode, a negative electrode, and further, a separator is rolled. An example is a form of a cylindrical battery wound in a shape.
Specifically, the battery electrode produced using the battery electrode binder of the present invention can be suitably used for AV equipment, OA equipment, communication equipment, and the like.
[0019]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples. Each evaluation method in an Example and a comparative example is shown below.
(1) Evaluation of battery electrode binder
Measurement of average particle size The particle size was measured using a laser particle size analysis system LPA-3000s / 3100 manufactured by Otsuka Electronics Co., Ltd.
Measurement of Glass Transition Point (Tg) An aqueous dispersion adjusted to pH 8 with 0.5N aqueous ammonia was applied to a glass plate and dried at 120 ° C. for 1 hour to obtain a polymer film. Using this, it measured using the differential scanning calorimeter by Seiko Electronic Industry Co., Ltd.
(2) Evaluation of battery electrode composition 100 parts by weight of needle coke pulverized product (average particle size 12 μm), 1 part by weight of battery electrode binder, 1 part by weight of carboxymethyl cellulose aqueous solution as a thickener, 0.5 N A battery electrode composition was prepared by adding 0.5 parts by weight of aqueous ammonia and mixing well, and the following evaluation was performed.
Binding property with copper foil Using a copper foil having a thickness of 50 μm as a base material, a composition for a battery electrode obtained by a roll coater was applied at a thickness of 200 g / m 2 , and 150 ° C. × 10 After partial drying, it was pressed at room temperature to obtain a 60 μm thick copper foil coating film. Using the obtained copper foil coating film, the film strength was measured with a Clemens type “scratch hardness meter” manufactured by Tester Sangyo Co., Ltd. The measuring method was performed according to JIS K5400 8.4.1.
Conductivity Measurement Method A battery electrode composition was applied to a PET film having a thickness of 100 μm at a thickness of 400 g / m 2 and dried at 150 ° C. for 10 minutes to obtain a coating film having a thickness of 120 μm. Using this, resistance was measured by a four-terminal method.
Electrolytic solution resistance The above copper foil coating film was applied to the electrolytic solution LIPASTE-EDEC / 1 (LiCLO 4 / ethylene carbonate / diethyl carbonate = 8.4% / 52.8% / 38.8% Fuji Pharmaceutical Co., Ltd.) (Product Co., Ltd.) was immersed at 80 ° C. for 72 hrs, and peeling from the copper foil was observed by a five-point method. For example, 5 points are given when there is no change, and 1 point is given when peeling completely.
(3) Evaluation of battery electrode Mixing 100 parts by weight of Li 1.03 Co 0.95 Sn 0.042 O 2 with an average particle diameter of 2 μm, 7.5 parts by weight of graphite powder, and 2.5 parts by weight of acetone black, a methyl isobutyl ketone solution of fluororubber 50 parts by weight of (concentration 4% by weight) was added and mixed and stirred to obtain a coating solution.
This coating solution was applied and dried at 290 g / m 2 using a commercially available A1 foil (thickness: 15 μm) as a base material to obtain a positive electrode having a thickness of 110 μm.
Next, the copper foil coating film obtained from the battery electrode composition was used as a negative electrode, cut into 0.9 cm × 5.5 cm, and a lithium secondary battery was assembled.
The battery was charged to 4.2 V and the cycle of discharging to 100 V at 2.5 V was repeated, and the capacity storage rate was measured. The cell charged to 4.2 V was stored at 70 ° C. for 30 days, and the storage stability was measured.
[0020]
Examples 1-4
(1) A copolymer stirrer for the copolymer X is provided, and 200 parts of water, 0.5 part of sodium dodecylbenzenesulfonate, 1.0 part of potassium persulfate, 0.5 part of sodium bisulfite in an autoclave capable of adjusting temperature. And the monomer component giving the molecular weight regulator and the copolymer X part shown in Table 1 below were charged together and reacted at 45 ° C. for 6 hours to confirm that the polymerization conversion was 70% or more. .
(2) Polymerization of copolymer Y Subsequently to the polymerization in (1) above, a mixture of monomer components giving the molecular weight regulator and copolymer Y shown in Table 1 below was continuously added at 60 ° C. for 7 hours. The polymerization was continued by addition, and the reaction was continued at 70 ° C. for 6 hours after the completion of the continuous addition to obtain a battery electrode binder of the present invention comprising an aqueous dispersion of the specific copolymer. The final polymerization conversion was 98-99%.
Each battery electrode binder obtained was evaluated. The evaluation results are shown in Table 3 and Table 4.
Example 5
An autoclave equipped with a stirrer was charged with 70 parts of ion-exchanged water and 0.3 part of potassium persulfate, the gas phase part was replaced with nitrogen gas for 15 minutes, and the temperature was raised to 80 ° C. On the other hand, the components shown in Table 1 were mixed in a separate container and dropped into the autoclave over 15 hours. During the dropping, the reaction was carried out at 80 ° C. After completion of the dropwise addition, the reaction was terminated after further stirring at 85 ° C. for 5 hours. After cooling to 25 ° C., the pH is adjusted to 7 with potassium hydroxide, and then steam is introduced to remove residual monomers, followed by concentration, and the battery electrode of the present invention comprising an aqueous dispersion of a specific copolymer A binder was obtained. An evaluation result is combined with Table 3 and Table 4, and is shown.
Comparative Examples 1-3
In Example 5, except that the composition of the monomer component was as shown in Table 2, a battery electrode binder comprising a polymer aqueous dispersion was obtained in the same manner as in Example 5. The evaluation results are also shown in Table 3 and Table 4.
[0021]
[Table 1]
[Table 2]
ST = styrene (component (a))
BD = butadiene (component (b))
MMA = methyl methacrylate (component (c))
BA = butyl acrylate (component (c))
AA = acrylic acid (component (d))
IA = Itaconic acid (component (d))
N-MAM = N-methylolacrylamide (component (e))
α-MSD = α-methylstyrene dimer (molecular weight regulator)
t-DM = t-dodecyl mercaptan (molecular weight regulator)
[0023]
[Table 3]
[Table 4]
Examples 1 to 5 in Table 1 are copolymers within the scope of the present invention, and Table 2 is the composition, Tg, and average particle diameter of the copolymer outside the scope of the present invention. As is apparent from Table 3, when the copolymer of the present invention is used, the binding property, conductivity, and electrolyte resistance are balanced, and the battery properties are excellent in cycle performance, storage properties, and safety. Yes.
On the other hand, Comparative Example 1 is an example of a copolymer that does not contain a (meth) acrylic acid ester unit, and has low binding properties and electrolytic solution resistance and is inferior in battery characteristics. Comparative Example 2 is an example of a copolymer in which the conjugated diene monomer unit is outside the scope of the present invention, and the binding property and conductivity are low and the battery characteristics are poor. Comparative Example 3 is an example of a copolymer that does not contain an ethylenically unsaturated carboxylic acid unit, which is inferior in binder performance, poor in binding properties and inferior in battery characteristics.
[0026]
【The invention's effect】
The binder for the battery electrode of the present invention uses a binder that ensures the current collecting property of the electrode active material, improves its utilization efficiency, and has little influence on the electrode active material in a battery as an electrode active material, mainly a secondary battery. Long life and high capacity can be achieved.
Until now, it was thought that the binder used for the purpose of imparting the binding property between the electrode active material and the current collector hinders the movement of ions in the electrolytic solution. However, the secondary battery electrode excellent in high rate performance can be provided by using the copolymer of the present invention as the binder and increasing the affinity with the electrolytic solution. Further, by using the copolymer in the range of the glass transition point of the present invention and increasing the strength of the coating film, a secondary battery electrode having excellent capacity, discharge performance, charge / discharge cycle performance, and safety is provided. .
Claims (2)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP19489997A JP3721727B2 (en) | 1997-07-04 | 1997-07-04 | Battery electrode binder |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP19489997A JP3721727B2 (en) | 1997-07-04 | 1997-07-04 | Battery electrode binder |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH1125989A JPH1125989A (en) | 1999-01-29 |
| JP3721727B2 true JP3721727B2 (en) | 2005-11-30 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP19489997A Expired - Lifetime JP3721727B2 (en) | 1997-07-04 | 1997-07-04 | Battery electrode binder |
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| JP (1) | JP3721727B2 (en) |
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