JP3969551B2

JP3969551B2 - Non-aqueous secondary battery

Info

Publication number: JP3969551B2
Application number: JP08834598A
Authority: JP
Inventors: 房次喜多; 美奈子岩崎; 祐樹石川; 聡北川; 和伸松本
Original assignee: Hitachi Maxell Energy Ltd
Current assignee: Hitachi Maxell Energy Ltd
Priority date: 1998-04-01
Filing date: 1998-04-01
Publication date: 2007-09-05
Anticipated expiration: 2018-04-01
Also published as: JPH11288722A

Description

【０００１】
【発明の属する技術分野】
本発明は、非水二次電池に関し、さらに詳しくは、特に過充電時における信頼性が高く、かつサイクル特性が優れた非水二次電池に関するものである。
【０００２】
【従来の技術】
リチウムイオン電池に代表される非水二次電池は、容量が大きく、かつ高電圧、高エネルギー密度、高出力であることから、ますます需要が増える傾向にある。
【０００３】
上記非水二次電池では、負極は金属製の集電材の少なくとも一方の面に負極活物質やバインダなどを含む負極合剤の層を形成することによって構成されるが、本発明者がこの非水二次電池についてさらに検討を進めている中で、この非水二次電池は、充放電に伴う負極合剤層の体積変化が大きい場合には、誤って過充電されたときに、負極の集電材に亀裂、切断などが生じ、その後のサイクル特性の劣化が大きくなることが判明した。
【０００４】
【発明が解決しようとする課題】
本発明は、上記のような事情に鑑み、負極合剤層の充放電に伴う体積変化が大きい場合においても、過充電時に負極集電材に亀裂、切断などが発生するのを防止して、過充電時における信頼性を高め、かつ上記負極集電材の亀裂、切断などの発生に伴うサイクル特性の劣化を防止して、サイクル特性の優れた非水二次電池を提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明は、正極、負極および電解質を有し、上記負極が集電材の少なくとも一方の面に負極合剤層を形成したものからなり、該負極合剤層の充電後と放電後の最大体積変化率が８％以上である非水二次電池において、負極の集電材として破断伸び率が５％以上８％以下のものを用いることによって、過充電時においても負極の集電材に亀裂、切断などが発生するのを防止し、上記課題を解決したものである。また、負極の集電材として、上記特性に加えて、濡れ性が接触角で４０度未満のものを用いることによって、サイクル特性の劣化をより効率よく防止することができる。
【０００６】
【発明の実施の形態】
本発明において、負極の集電材としては、材質的には、たとえば、銅、ニッケル、ステンレス鋼製で、形態的には、たとえば、箔、網状のものなどが用いられるが、その破断伸び率は５％以上であることが必要である。これは、負極の集電材の破断伸び率が５％以上でなければ充放電に伴う集電材の亀裂、切断などの発生を防止する効果が充分に発現できないからであり、この負極の集電材の破断伸び率としては特に７％以上であることが好ましい。このような破断伸び率を得るには、銅製の集電材を用いることが適している。
【０００７】
本発明において、負極の集電材の破断伸び率とは、電池を２０℃、２．７５Ｖまで１Ｃレートで放電した後、分解し、集電材またはこれを負極合剤層と共に引っ張り試験を行い、集電材が破断するまでの伸び率を言う。集電材の伸びが大きい方が集電材の切断が少ないのは以下の理由による。
【０００８】
負極合剤層の充電後と放電後の最大体積変化率が８％以上である非水二次電池では、負極合剤層の充放電に伴う膨張収縮が大きく、充電するにつれて負極合剤層に引っ張られて集電材も伸びてしまう。このとき、集電材の破断伸び率が小さいと集電材が切断されて一部の負極合剤が利用できなくなり、サイクル特性の劣化が大きくなる。
【０００９】
また、負極合剤層の充電後と放電後の最大体積変化率が１１％以上である非水二次電池では、さらにその影響が大きい。負極合剤層の充電後と放電後の最大体積変化率とは、その電池の標準充電電圧まで１Ｃレートで２時間半充電して分解したときに負極合剤層の厚みを測定し、一方で同様に作製した別の電池を１Ｃレートで２．７５Ｖまで放電して分解したときに負極合剤層の厚みを測定し、その間で最も体積変化率の大きい部分の値である。
【００１０】
集電材の表面粗度も集電材の切断に影響する。集電材の表面が平滑であれば、充電して負極合剤層が膨張する際に集電材との間で滑りが生じ、切断されにくくなる。負極の集電材の表面粗度は粗な面の平均がＲａ（ＩＰＣ−ＭＦ−１５０Ｆ）で０．３μｍ以下が望ましく、さらに０．２５μｍ以下であることが望ましい。
【００１１】
また、負極の集電材の破断伸び率が大きい場合は通常濡れ性が悪く、電池を充放電させた場合のサイクル特性の劣化が大きい傾向にある。そのような場合には接触角で５０度未満にするとサイクル特性の劣化が少なくなる。また、接触角を４０度未満にするとさらに効果が大きくなり、より望ましい。この濡れ性を改善する方法としては、たとえば、集電材にクロメート処理する際にそのクロメート処理をアルカリクロメート処理で行ったり、集電材にクロメート処理する際のクロメート量を低減することが挙げられ、いずれも効果がある。そして、そのクロメート量は０．１５ｍｇ／ｍ²以下が望ましく、０．１ｍｇ／ｍ²以下がより望ましい。
【００１２】
なお、本発明における濡れ性は接触角で評価するが、その接触角は、スライドガラス上に長さ４ｃｍ、幅３ｃｍの試料をテープで固定し、これに液滴量１μｌの水を滴下して、この画像をコンピュータに取り込み、その画像解析により測定した値の３回の平均値をいい、解析手法は、「コンピュータ画像解析システムによる新しい接触角測定法」〔第４５回コロイドおよび界面化学討論会講演要旨集，ｐ９９（１９９２）〕によるものである。
【００１３】
また、本発明の効果は、電池内部の電極積層体単位体積当たり通常充電での容量が１３０ｍＡｈ／ｃｍ³以上の場合に顕著に発現し、上記容量が１４０ｍＡｈ／ｃｍ³以上の場合により顕著に発現するので、本発明はそのような高容量の電池に適用するのが適している。なお、電極積層体単位体積とは、電池内における正極、負極およびセパレータを積層したものまたは巻回したもののかさ（嵩）体積で、巻軸の体積を含まない正極、負極およびセパレータのかさ体積の合計体積である。
【００１４】
電解質として液状電解質（電解液）を用いる場合、その溶媒成分としてはエステルがよく用いられる。特によく用いられる鎖状エステルは、ジメチルカーボネート、ジエチルカーボネート、メチルエチルカーボネート、プロピオン酸メチルなどの鎖状のＣＯＯ−結合を有する鎖状エステルである。
【００１５】
また、上記鎖状エステルに下記の誘電率の高いエステル（誘電率３０以上）を混合して用いるとさらに望ましい。このような誘電率の高いエステルとしては、たとえばプロピレンカーボネート（ＰＣ）、エチレンカーボネート（ＥＣ）、ブチレンカーボネート（ＢＣ）、ガンマーブチロラクトン（γ−ＢＬ）、エチレングリコールサルファイト（ＥＧＳ）などが挙げられる。特に環状構造のものが望ましく、とりわけ環状のカーボネートが望ましく、エチレンカーボネート（ＥＣ）が最も望ましい。
【００１６】
上記高誘電率エステルは液状電解質の全溶媒中の４０体積％未満が望ましく、より望ましくは３０体積％以下、さらに望ましくは２５体積％以下である。そして、これらの誘電率の高いエステルによる安全性の向上は、上記エステルが液状電解質の全溶媒中で１０体積％以上になると電池特性が良くなり、２０体積％に達するとさらに向上が見られるようになる。
【００１７】
上記エステル以外に併用可能な溶媒としては、たとえば１，２−ジメトキシエタン（ＤＭＥ）、１，３−ジオキソラン（ＤＯ）、テトラヒドロフラン（ＴＨＦ）、２−メチル−テトラヒドロフラン（２Ｍｅ−ＴＨＦ）、ジエチルエーテル（ＤＥＥ）などが挙げられる。そのほか、アミンイミド系有機溶媒や、含イオウまたは含フッ素系有機溶媒なども用いることができる。
【００１８】
液状電解質の溶質としては、たとえばＬｉＣｌＯ₄、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＡｓＦ₆、ＬｉＳｂＦ₆、ＬｉＣＦ₃ＳＯ₃、ＬｉＣ₄Ｆ₉ＳＯ₃、ＬｉＣＦ₃ＣＯ₂、Ｌｉ₂Ｃ₂Ｆ₄（ＳＯ₃）₂、ＬｉＮ（ＣＦ₃ＳＯ₂）₂、ＬｉＣ（ＣＦ₃ＳＯ₂）₃、ＬｉＣ_nＦ_2n+1ＳＯ₃（ｎ≧２）、ＬｉＮ（ＲｆＯＳＯ₂）₂〔ここでＲｆはフルオロアルキル基〕などが単独でまたは２種以上混合して用いられるが、特にＬｉＰＦ₆やＬｉＣ₄Ｆ₉ＳＯ₃などが望ましい。液状電解質中の溶質の濃度は、特に限定されるものではないが、濃度を１ｍｏｌ／ｌ以上の多めにすると安全性がさらに良くなるので望ましく、１．２ｍｏｌ／ｌ以上がさらに望ましい。また、１．７ｍｏｌ／ｌより少ないと電気特性が良くなるので望ましく、１．５ｍｏｌ／ｌより少ないとさらに望ましい。
【００１９】
正極活物質としては、たとえばＬｉＣｏＯ₂などのリチウムコバルト酸化物、ＬｉＭｎ₂Ｏ₄などのリチウムマンガン酸化物、ＬｉＮｉＯ₂などのリチウムニッケル酸化物、二酸化マンガン、五酸化バナジウム、クロム酸化物などの金属酸化物または二硫化チタン、二硫化モリブデンなどの金属硫化物などが用いられ、正極は、たとえば、それらの正極活物質に必要に応じて導電助剤やポリフッ化ビニリデンなどの結着剤などを適宜添加した正極合剤を、アルミニウム箔などの集電材を芯材として成形体に仕上げたものが用いられる。
【００２０】
特にＬｉＮｉＯ₂、ＬｉＣｏＯ₂、ＬｉＭｎ₂Ｏ₄などの充電時の開路電圧がＬｉ基準で４Ｖ以上を示すリチウム複合酸化物を正極活物質として用いる場合には、高エネルギー密度が得られるので望ましい。
【００２１】
負極に用いる活物質はリチウムイオンをドープ、脱ドープできるものであればよく、そのような負極活物質としては、たとえば黒鉛、熱分解炭素類、コークス類、ガラス状炭素類、有機高分子化合物の焼成体、メソカーボンマイクロビーズ、炭素繊維、活性炭などの炭素化合物などを用い得るが、特に２０００℃以上で焼成した炭素化合物は、充放電に伴う体積変化が大きく、本発明はこのような体積変化の大きい活物質を用いる場合に適用すると、特にその効果が顕著に発現する。また、本発明においては、Ｓｉ、Ｓｎ、Ｉｎなどの合金またはＬｉに近い低電位で充放電できるＳｉ、Ｓｎ、Ｉｎなどの酸化物などの化合物も負極活物質として用い得るが、これらを負極活物質として用いた場合、充放電に伴い負極合剤層の最大体積変化率が１０％を超える場合があり、本発明はこのような負極活物質を用いる場合に適用すると、特にその効果が顕著に発現する。
【００２２】
負極活物質として炭素材料を用いる場合、該炭素材料は下記の特性を持つものが望ましい。すなわち、その（００２）面の層間距離ｄ_００２に関しては、望ましくは３．４Å以下である。また、ｃ軸方向の結晶子の大きさＬｃは、３０Å以上が望ましく、より望ましくは８０Å以上、さらに望ましくは２５０Å以上である。そして、その平均粒径は８〜１５μｍ、特に１０〜１３μｍが望ましく、純度は９９．９％以上が望ましい。
【００２３】
負極は、たとえば、上記のような負極活物質に、必要に応じ、ポリフッ化ビニリデン、ラテックスゴムなどの結着剤や人造黒鉛などの導電助剤を加えた負極合剤を溶剤に溶解または分散させて調製した負極合剤スラリーを前記の集電材に塗付し、乾燥してスラリー中の溶剤を揮発させて除去し、集電材の少なくとも一方の面に負極合剤層を形成することによって作製される。
【００２４】
【実施例】
つぎに、実施例をあげて本発明をより具体的に説明する。ただし、本発明はそれらの実施例のみに限定されるものではない。
【００２５】
実施例１
メチルエチルカーボネートとエチレンカーボネートとを体積比７５：２５で混合し、この混合溶媒にＬｉＰＦ₆を１．４ｍｏｌ／ｌ溶解させて、組成が１．４ｍｏｌ／ｌＬｉＰＦ₆／ＥＣ：ＭＥＣ（２５：７５体積比）で示される液状電解質を調製した。
【００２６】
上記液状電解質におけるＥＣはエチレンカーボネートの略称であり、ＭＥＣはメチルエチルカーボネートの略称である。従って、上記液状電解質を示す１．４ｍｏｌ／ｌＬｉＰＦ₆／ＥＣ：ＭＥＣ（２５：７５体積比）は、メチルエチルカーボネート７５体積％とエチレンカーボネート２５体積％との混合溶媒にＬｉＰＦ₆を１．４ｍｏｌ／ｌを溶解させたものであることを示している。
【００２７】
上記とは別に、ＬｉＣｏＯ₂に導電助剤として鱗片状黒鉛を重量比１００：７で加えて混合し、この混合物と、ポリフッ化ビニリデンをＮ−メチルピロリドンに溶解させた溶液とを混合してスラリーにした。この正極合剤スラリーを７０メッシュの網を通過させて大きなものを取り除いた後、厚さ２０μｍのアルミニウム箔からなる正極集電材の両面に均一に塗付し、乾燥してスラリー中の溶剤を揮発させて除去し、正極集電材の両面に正極合剤層を形成し、その後、ローラプレス機により圧縮成形し、切断し、リード体を溶接して、帯状の正極を作製した。
【００２８】
また、黒鉛系炭素材料〔ただし、（００２）面の層間距離ｄ₀₀₂＝３．３７Å、ｃ軸方向の結晶子の大きさＬｃ＝９５０Å、平均粒径１０μｍ、純度９９．９％という特性を持つ炭素材料〕を、ポリフッ化ビニリデンをＮ−メチルピロリドンに溶解させた溶液と混合してスラリーにした。この負極合剤スラリーを７０メッシュの網を通過させて大きなものを取り除いた後、厚さ１０μｍの帯状の銅箔からなる負極集電材の両面に均一に塗付し、乾燥してスラリー中の溶剤を揮発させて除去し、負極集電材の両面に負極合剤層を形成し、その後、ローラプレス機により圧縮成形し、切断した後、リード体を溶接して、帯状の負極を作製した。ここで、用いた負極集電材の破断伸び率は８％であり、表面の粗度Ｒａは０．２μｍであった。また、用いた負極集電材の表面の濡れ性を表す接触角は３５度であり、表面のクロメート量は０．０１ｍｇ／ｍ²であった。
【００２９】
前記帯状の正極を厚さ２５μｍの微孔性ポリエチレンフィルムを介して上記帯状の負極に重ね、渦巻状に巻回して渦巻状電極積層体とした後、外径１８ｍｍの有底円筒状の電池ケース内に充填し、正極および負極のリード体の溶接を行った。
【００３０】
つぎに液状電解質を電池ケース内に注入し、液状電解質がセパレータなどに充分に浸透した後、封口し、予備充電、エイジングを行い、図１に示す構造の筒形の非水二次電池を作製した。
【００３１】
図１に示す電池について説明すると、１は前記の正極で、２は前記の負極である。ただし、図１では、繁雑化を避けるため、正極１や負極２の作製にあたって使用された集電材などは図示していない。そして、これらの正極１と負極２はセパレータ３を介して渦巻状に巻回され、渦巻状電極積層体として上記の液状電解質４と共に電池ケース５内に収容されている。
【００３２】
電池ケース５は前記のようにステンレス鋼製で、その底部には上記渦巻状電極積層体の挿入に先立って、ポリプロピレンからなる絶縁体６が配置されている。封口板７はアルミニウム製で、円板状をしていて、中央部に薄肉部７ａを厚み方向の両端面より内部側に設け、かつ上記薄肉部７ａの周囲に電池内圧を防爆弁９に作用させるための圧力導入口７ｂとしての孔が設けられている。そして、この薄肉部７ａの上面に防爆弁９の突出部９ａが溶接され、溶接部分１１を構成している。なお、上記の封口板７に設けた薄肉部７ａや防爆弁９の突出部９ａなどは、図面上での理解がしやすいように、切断面のみを図示しており、切断面後方の輪郭線は図示を省略している。また、封口板７の薄肉部７ａと防爆弁９の突出部９ａとの溶接部分１１も、図面上での理解が容易なように、実際よりは誇張した状態に図示している。
【００３３】
端子板８は、圧延鋼製で表面にニッケルメッキが施され、周縁部が鍔状になった帽子状をしており、この端子板８にはガス排出孔８ａが設けられている。防爆弁９は、アルミニウム製で、円板状をしており、その中央部には発電要素側（図１では、下側）に先端部を有する突出部９ａが設けられ、かつ薄肉部９ｂが設けられ、上記突出部９ａの下面が、前記したように、封口板７の薄肉部７ａの上面に溶接され、溶接部分１１を構成している。絶縁パッキング１０は、ポリプロピレン製で、環状をしており、封口板７の周縁部の上部に配置され、その上部に防爆弁９が配置していて、封口板７と防爆弁９とを絶縁するとともに、両者の間から液状電解質が漏れないように両者の間隙を封止している。環状ガスケット１２はポリプロピレン製で、リード体１３はアルミニウム製で、前記封口板７と正極１とを接続し、渦巻状電極積層体の上部には絶縁体１４が配置され、負極２と電池ケース５の底部とはニッケル製のリード体１５で接続されている。
【００３４】
実施例２
負極集電材として、破断伸び率が７％、表面の粗度Ｒａが０．３μｍ、表面の濡れ性を表す接触角が６５度で、表面のクロメート量が０．０２ｍｇ／ｍ²のものを用いた以外は、実施例１と同様にして筒形の非水二次電池を作製した。
【００３５】
比較例１
負極集電材として、破断伸び率が４％、表面の粗度Ｒａが０．３μｍ、表面の濡れ性を表す接触角が７９度のものを用いた以外は、実施例１と同様にして筒形の非水二次電池を作製した。
【００３６】
比較例２
実施例１の黒鉛系炭素材料の代わりに（００２）面の層間距離ｄ₀₀₂が３．４４Å、ｃ軸方向の結晶子の大きさＬｃが３２Å、平均粒径が１０μｍで純度が９９．９％のコークス系炭素材料を用いたほかは、実施例１と同様に負極合剤層を形成した。ただし、この比較例２の炭素材料は、充放電に伴う体積変化は少ないものの、負極の利用率が約３割低下し、かつ電極密度も低下するため、正極活物質を約２０％少なくしなければならず、そのため、約２０％の容量減となってしまった。また、負極集電材としては、破断伸び率が４％、表面の粗度Ｒａが０．３μｍ、表面の濡れ性を表す接触角が７９度のものを用いた。そして、それら以外は、実施例１と同様にして筒形の非水二次電池を作製した。
【００３７】
上記実施例１〜２および比較例１〜２の電池を、１５５０ｍＡ（１Ｃ）で２．７５Ｖまで放電した後、１５５０ｍＡで充電し、４．３Ｖに達した後は、４．３Ｖの定電圧に保つ条件で３時間半の過充電を行った。その後、電池を１５５０ｍＡで２．７５Ｖまで放電した後、一部の電池を分解し、負極集電材の銅箔にヒビや亀裂、切断などの異常が発生しているかどうかを調べた。その結果を表１に示す。また、表１には、４．２Ｖ充電時と２．７５Ｖまで放電した時の負極合剤層の最大体積変化率も併せて示す。
【００３８】
また、残りの電池を１５５０ｍＡで充電し、４．２Ｖに達した後は４．２Ｖの定電圧に保つ条件で２時間半の充電を行い、さらに１５５０ｍＡで２．７５Ｖまで放電する充放電サイクルを繰り返し行った。そして、５０サイクル時の１サイクル目に対する容量保持率〔（５０サイクル目の放電容量）／（１サクイル目の放電容量）×１００〕を測定した。その結果を表２に示す。また、初度サイクルにおいて、４．２Ｖまで充電した後、２．７５Ｖまで放電して放電容量を測定し、それに基づいて電極積層体単位体積当たりの容量を求めた。その結果も表２に示す。
【００３９】
【表１】

【００４０】
【表２】

【００４１】
表１に示すように、実施例１〜２は、負極合剤層の最大体積変化率が１１％あるにもかかわらず、負極集電材にヒビ、亀裂、切断などの異常発生がまったくなかった。これを詳しく説明すると、実施例１や実施例２では、負極集電材として破断伸び率が５％以上のものを用いている関係で、負極合剤層の最大体積変化率が１１％と非常に大きいにもかかわらず、負極集電材にヒビ、亀裂、切断などの異常発生がまったくなかった。
【００４２】
これに対して、負極集電材として破断伸び率が４％と伸びの小さいものを用いた比較例１では、充放電に伴って集電材に亀裂が発生した。従って、この比較例１ではサイクル特性の劣化が大きくなることが予測される。なお、比較例２は負極合剤層の最大体積変化率が１％以下と小さいため、充放電に伴う負極集電材のヒビ、亀裂、切断などの異常発生はなかったが、この比較例２では、負極の密度が低くなり、電池容量が実施例１〜２に比べて約８０％になっていて、容量が低いという問題を有していた。
【００４３】
また、表２に示すように、比較例１では５０サイクル時に容量保持率が５０％以下にまで低下したのに対し、実施例１〜２では８５〜９１％と高い容量保持率を有していた。特に集電材の表面の濡れ性を表す接触角が３５度という濡れ性の高い負極集電材を用いた実施例１の電池では、容量保持率が９１％と最も優れていた。上記のように、比較例１の５０サイクル時の容量保持率が５０％以下と低くなったのは、充放電に伴って負極集電体に亀裂、切断などが発生したことによるものと考えられる。
【００４４】
【発明の効果】
以上説明したように、本発明では、負極合剤層の充放電に伴う最大体積変化率が８％以上という体積変化の大きい負極合剤を用いる場合においても、負極集電材の亀裂、切断などの発生を防止して、過充電時における信頼性を高め、かつ上記負極集電材の亀裂、切断などに伴うサイクル特性の劣化を防止して、サイクル特性の優れた非水二次電池を提供することができた。
【図面の簡単な説明】
【図１】本発明に係る非水二次電池の一例を示す縦断面図である。
【符号の説明】
１正極
２負極
３セパレータ
４液状電解質[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery having high reliability particularly during overcharge and excellent cycle characteristics.
[0002]
[Prior art]
Non-aqueous secondary batteries represented by lithium-ion batteries have a large capacity, high voltage, high energy density, and high output, and therefore demand is increasing.
[0003]
In the non-aqueous secondary battery, the negative electrode is formed by forming a layer of a negative electrode mixture containing a negative electrode active material or a binder on at least one surface of a metal current collector. In the course of further study of water secondary batteries, this non-aqueous secondary battery has a negative electrode mixture layer with a large volume change due to charge / discharge. It was found that the current collector was cracked, cut, etc., and the subsequent deterioration of the cycle characteristics increased.
[0004]
[Problems to be solved by the invention]
In view of the circumstances as described above, the present invention prevents the negative electrode current collector from cracking, cutting, etc. during overcharge even when the volume change accompanying charging / discharging of the negative electrode mixture layer is large. An object of the present invention is to provide a non-aqueous secondary battery having excellent cycle characteristics by improving reliability during charging and preventing deterioration of cycle characteristics due to occurrence of cracks, cuts, etc. of the negative electrode current collector.
[0005]
[Means for Solving the Problems]
The present invention comprises a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is formed by forming a negative electrode mixture layer on at least one surface of a current collector, and the maximum volume change after charging and discharging of the negative electrode mixture layer In non-aqueous secondary batteries with a rate of 8% or more, by using a negative electrode current collector having a breaking elongation of 5% or more and 8% or less, the negative electrode current collector is cracked or cut even during overcharge. Is a solution to the above problem. Further, in addition to the above characteristics, the use of a negative electrode current collector having a wettability of less than 40 degrees in contact angle can more effectively prevent deterioration of cycle characteristics.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the current collector of the negative electrode is made of, for example, copper, nickel, stainless steel, and, for example, a foil or a net-like material is used. It is necessary to be 5% or more. This is because if the elongation at break of the current collector of the negative electrode is not more than 5%, the effect of preventing the occurrence of cracking, cutting, etc. of the current collector due to charge / discharge cannot be sufficiently exhibited. The elongation at break is particularly preferably 7% or more. In order to obtain such elongation at break, it is suitable to use a current collector made of copper.
[0007]
In the present invention, the elongation at break of the current collector of the negative electrode means that the battery is discharged at a rate of 1C up to 2.75 V at 20 ° C., then decomposed and subjected to a tensile test together with the current collector or the negative electrode mixture layer. The elongation until the electric material breaks. The reason why the current collector is less cut when the current collector is larger is as follows.
[0008]
In a non-aqueous secondary battery having a maximum volume change rate of 8% or more after charging and discharging of the negative electrode mixture layer, the expansion and contraction associated with charging and discharging of the negative electrode mixture layer is large. The current collector is stretched by being pulled. At this time, if the elongation at break of the current collector is small, the current collector is cut and some of the negative electrode mixture cannot be used, and the deterioration of the cycle characteristics increases.
[0009]
In addition, the influence is even greater in a non-aqueous secondary battery in which the maximum volume change after charging and discharging of the negative electrode mixture layer is 11% or more. The maximum volume change rate after charging and discharging of the negative electrode mixture layer is determined by measuring the thickness of the negative electrode mixture layer when it is decomposed by charging at a 1C rate for 2 hours and a half to the standard charging voltage of the battery. The thickness of the negative electrode mixture layer was measured when another battery produced in the same manner was discharged at a 1C rate to 2.75 V and decomposed, and the value of the portion with the largest volume change rate was obtained.
[0010]
The surface roughness of the current collector also affects the cutting of the current collector. If the surface of the current collector is smooth, slipping occurs between the current collector and the negative electrode mixture layer when charged and expands, making it difficult to cut. The surface roughness of the negative electrode current collector is preferably 0.3 μm or less, more preferably 0.25 μm or less, in terms of Ra (IPC-MF-150F).
[0011]
Further, when the breaking elongation of the current collector of the negative electrode is large, the wettability is usually poor, and the cycle characteristics tend to be greatly deteriorated when the battery is charged and discharged. In such a case, if the contact angle is less than 50 degrees, the deterioration of cycle characteristics is reduced. Further, if the contact angle is less than 40 degrees, the effect is further increased, which is more desirable. As a method for improving the wettability, for example, when chromating the current collector, the chromate treatment is performed by alkali chromate treatment, or reducing the chromate amount when chromating the current collector. Is also effective. Then, the chromate amount 0.15 mg / m ² or less is preferable, 0.1 mg / m ² or less is more preferable.
[0012]
Note that the wettability in the present invention is evaluated by a contact angle. The contact angle is determined by fixing a sample having a length of 4 cm and a width of 3 cm on a slide glass with a tape, and dropping 1 μl of water in a droplet amount onto the sample. This image is taken into a computer and means the average of three values measured by image analysis. The analysis method is "a new contact angle measurement method using computer image analysis system" [The 45th Colloid and Interface Chemistry Conference Summary of Lecture, p99 (1992)].
[0013]
In addition, the effect of the present invention is remarkably exhibited when the capacity at the normal charge per unit volume of the electrode laminated body inside the battery is 130 mAh / cm ³ or more, and more remarkably when the capacity is 140 mAh / cm ³ or more. Therefore, the present invention is suitable for application to such a high capacity battery. The electrode laminate unit volume is the bulk volume of the positive electrode, negative electrode and separator laminated or wound in the battery, and does not include the volume of the winding shaft. The total volume of
[0014]
When a liquid electrolyte (electrolytic solution) is used as the electrolyte, an ester is often used as the solvent component. A chain ester particularly often used is a chain ester having a chain COO- bond such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, or methyl propionate.
[0015]
Further, it is more desirable to use the above-mentioned chain ester with the following ester having a high dielectric constant (dielectric constant 30 or more). Examples of such high dielectric constant esters include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), gamma-butyrolactone (γ-BL), ethylene glycol sulfite (EGS), and the like. In particular, those having a cyclic structure are desirable, cyclic carbonates are particularly desirable, and ethylene carbonate (EC) is most desirable.
[0016]
The high dielectric constant ester is desirably less than 40% by volume in the total solvent of the liquid electrolyte, more desirably 30% by volume or less, and even more desirably 25% by volume or less. And the improvement of safety by these esters having a high dielectric constant is such that the battery characteristics are improved when the ester is 10% by volume or more in the total solvent of the liquid electrolyte, and further improvement is seen when the ester reaches 20% by volume. become.
[0017]
Examples of solvents that can be used in addition to the above esters include 1,2-dimethoxyethane (DME), 1,3-dioxolane (DO), tetrahydrofuran (THF), 2-methyl-tetrahydrofuran (2Me-THF), diethyl ether ( DEE). In addition, amine imide organic solvents, sulfur-containing or fluorine-containing organic solvents, and the like can also be used.
[0018]
The solute of a liquid electrolyte, for example _{_{_{LiClO 4, LiPF 6, LiBF 4}}} , LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group], etc. Are used alone or in admixture of two or more. LiPF ₆ and LiC ₄ F ₉ SO ₃ are particularly desirable. The concentration of the solute in the liquid electrolyte is not particularly limited, but it is desirable that the concentration be 1 mol / l or more because safety is further improved, and 1.2 mol / l or more is more desirable. Further, if it is less than 1.7 mol / l, it is desirable because electrical characteristics are improved, and if it is less than 1.5 mol / l, it is more desirable.
[0019]
Examples of positive electrode active materials include lithium cobalt oxides such as LiCoO ₂ , lithium manganese oxides such as LiMn ₂ O ₄ , lithium nickel oxides such as LiNiO ₂ , manganese dioxide, vanadium pentoxide, and chromium oxides. Or metal sulfides such as titanium disulfide and molybdenum disulfide are used. For the positive electrode, for example, a conductive additive or a binder such as polyvinylidene fluoride is appropriately added to the positive electrode active material as necessary. The finished positive electrode mixture is finished into a molded body using a current collector such as an aluminum foil as a core material.
[0020]
In particular, when a lithium composite oxide, such as LiNiO ₂ , LiCoO ₂ , or LiMn ₂ O _4, whose open circuit voltage during charging is 4 V or more on the basis of Li is used as the positive electrode active material, it is desirable because a high energy density is obtained.
[0021]
The active material used for the negative electrode may be any material that can be doped and dedoped with lithium ions. Examples of such a negative electrode active material include graphite, pyrolytic carbons, cokes, glassy carbons, and organic polymer compounds. Carbon compounds such as calcined bodies, mesocarbon microbeads, carbon fibers, activated carbon and the like can be used. Particularly, carbon compounds calcined at 2000 ° C. or more have a large volume change due to charge / discharge, and the present invention has such volume changes. When applied to a case where an active material having a large size is used, the effect is particularly prominent. In the present invention, an alloy such as Si, Sn, In, or a compound such as an oxide such as Si, Sn, In that can be charged and discharged at a low potential close to Li can be used as the negative electrode active material. When used as a material, the maximum volume change rate of the negative electrode mixture layer may exceed 10% with charge and discharge, and the present invention is particularly effective when applied to the case of using such a negative electrode active material. To express.
[0022]
When a carbon material is used as the negative electrode active material, the carbon material preferably has the following characteristics. That is, the interlayer distance d ₀₀₂ of the (002) plane is preferably 3.4 mm or less. The crystallite size Lc in the c-axis direction is preferably 30 mm or more, more preferably 80 mm or more, and further preferably 250 mm or more. The average particle size is preferably 8 to 15 μm, particularly preferably 10 to 13 μm, and the purity is preferably 99.9% or more.
[0023]
The negative electrode is prepared by, for example, dissolving or dispersing, in a solvent, a negative electrode mixture obtained by adding a binder such as polyvinylidene fluoride or latex rubber or a conductive assistant such as artificial graphite to the negative electrode active material as described above. The negative electrode mixture slurry prepared in this manner is applied to the current collector, dried to volatilize and remove the solvent in the slurry, and a negative electrode mixture layer is formed on at least one surface of the current collector. The
[0024]
【Example】
Next, the present invention will be described more specifically with reference to examples. However, this invention is not limited only to those Examples.
[0025]
Example 1
Methyl ethyl carbonate and ethylene carbonate are mixed at a volume ratio of 75:25, LiPF ₆ is dissolved in 1.4 mol / l in this mixed solvent, and the composition is 1.4 mol / l LiPF ₆ / EC: MEC (25:75 A liquid electrolyte represented by a volume ratio) was prepared.
[0026]
EC in the liquid electrolyte is an abbreviation for ethylene carbonate, and MEC is an abbreviation for methyl ethyl carbonate. Therefore, 1.4 mol / l LiPF ₆ / EC: MEC (25:75 volume ratio) indicating the above liquid electrolyte is obtained by adding 1.4 mol of LiPF ₆ to a mixed solvent of 75% by volume of methyl ethyl carbonate and 25% by volume of ethylene carbonate. / L is dissolved.
[0027]
In addition to the above, scale-like graphite as a conductive additive is added to LiCoO ₂ at a weight ratio of 100: 7 and mixed, and this mixture is mixed with a solution in which polyvinylidene fluoride is dissolved in N-methylpyrrolidone to obtain a slurry. I made it. This positive electrode mixture slurry is passed through a 70 mesh net to remove large particles, and then uniformly applied to both surfaces of a positive electrode current collector made of aluminum foil having a thickness of 20 μm and dried to volatilize the solvent in the slurry. Then, a positive electrode mixture layer was formed on both surfaces of the positive electrode current collector, and then compression-molded by a roller press, cut, and the lead body was welded to produce a belt-like positive electrode.
[0028]
Further, a graphite-based carbon material [where (002) plane interlayer distance d ₀₀₂ = 3.37 Å, c-axis direction crystallite size Lc = 950 Å, average particle size 10 μm, purity 99.9%. The carbon material] was mixed with a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone to form a slurry. This negative electrode mixture slurry was passed through a 70 mesh net to remove large particles, and then uniformly applied to both sides of a negative electrode current collector made of a strip-shaped copper foil having a thickness of 10 μm and dried to obtain a solvent in the slurry. Was removed by volatilization, a negative electrode mixture layer was formed on both surfaces of the negative electrode current collector, and then compression molded by a roller press machine and cut, and then the lead body was welded to produce a strip-shaped negative electrode. Here, the elongation at break of the negative electrode current collector used was 8%, and the surface roughness Ra was 0.2 μm. Further, the contact angle representing the wettability of the surface of the used negative electrode current collector was 35 degrees, and the amount of chromate on the surface was 0.01 mg / m ² .
[0029]
The belt-like positive electrode is laminated on the belt-like negative electrode through a microporous polyethylene film having a thickness of 25 μm and wound in a spiral shape to form a spiral electrode laminated body, and then a bottomed cylindrical battery case having an outer diameter of 18 mm The lead body of the positive electrode and the negative electrode was welded.
[0030]
Next, a liquid electrolyte is poured into the battery case, and after the liquid electrolyte has sufficiently penetrated into the separator, etc., it is sealed, precharged, and aged to produce a cylindrical non-aqueous secondary battery having the structure shown in FIG. did.
[0031]
Referring to the battery shown in FIG. 1, 1 is the positive electrode and 2 is the negative electrode. However, in FIG. 1, in order to avoid complication, the current collecting material used in manufacturing the positive electrode 1 and the negative electrode 2 is not shown. The positive electrode 1 and the negative electrode 2 are spirally wound through a separator 3 and are housed in a battery case 5 together with the liquid electrolyte 4 as a spiral electrode laminate.
[0032]
The battery case 5 is made of stainless steel as described above, and an insulator 6 made of polypropylene is disposed at the bottom of the battery case 5 prior to the insertion of the spiral electrode laminate. The sealing plate 7 is made of aluminum and has a disk shape. A thin portion 7a is provided in the center portion on the inner side from both end faces in the thickness direction, and the internal pressure of the battery acts on the explosion-proof valve 9 around the thin portion 7a. A hole is provided as a pressure introduction port 7b. And the protrusion part 9a of the explosion-proof valve 9 is welded to the upper surface of this thin part 7a, and the welding part 11 is comprised. Note that the thin-walled portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are shown only on the cut surface for easy understanding on the drawing, and the contour line behind the cut surface is shown. Is not shown. In addition, the welded portion 11 between the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is also shown in an exaggerated state so as to facilitate understanding on the drawing.
[0033]
The terminal board 8 is made of rolled steel, has a nickel plating on the surface, and has a hat shape with a peripheral edge portion, and the terminal board 8 is provided with a gas discharge hole 8a. The explosion-proof valve 9 is made of aluminum and has a disk shape, and a central portion is provided with a protruding portion 9a having a tip portion on the power generation element side (lower side in FIG. 1), and a thin-walled portion 9b. As described above, the lower surface of the protruding portion 9a is welded to the upper surface of the thin portion 7a of the sealing plate 7 to constitute the welded portion 11. The insulating packing 10 is made of polypropylene and has an annular shape. The insulating packing 10 is arranged at the upper part of the peripheral portion of the sealing plate 7, and the explosion-proof valve 9 is arranged at the upper portion thereof, so that the sealing plate 7 and the explosion-proof valve 9 are insulated. At the same time, the gap between the two is sealed so that the liquid electrolyte does not leak between the two. The annular gasket 12 is made of polypropylene, the lead body 13 is made of aluminum, the sealing plate 7 and the positive electrode 1 are connected, an insulator 14 is disposed on the upper part of the spiral electrode laminate, and the negative electrode 2 and the battery case 5 are connected. Are connected by a nickel lead body 15.
[0034]
Example 2
As the negative electrode current collector, a material having an elongation at break of 7%, a surface roughness Ra of 0.3 μm, a contact angle representing surface wettability of 65 degrees, and a surface chromate amount of 0.02 mg / m ² is used. A cylindrical non-aqueous secondary battery was produced in the same manner as in Example 1 except that.
[0035]
Comparative Example 1
As the negative electrode current collector, a cylindrical shape was used in the same manner as in Example 1 except that a material having a breaking elongation of 4%, a surface roughness Ra of 0.3 μm, and a contact angle representing surface wettability of 79 degrees was used. A non-aqueous secondary battery was prepared.
[0036]
Comparative Example 2
Example 1 of graphite interlayer distance d ₀₀₂ in place in the (002) plane of the carbon material 3.44A, the size Lc in the c-axis direction of the crystallite 32 Å, purity an average particle diameter of 10μm is 99.9% A negative electrode mixture layer was formed in the same manner as in Example 1 except that the coke-based carbon material was used. However, in the carbon material of Comparative Example 2, although the volume change due to charging / discharging is small, the utilization factor of the negative electrode is reduced by about 30% and the electrode density is also reduced, so that the positive electrode active material must be reduced by about 20%. Therefore, the capacity was reduced by about 20%. As the negative electrode current collector, one having a breaking elongation of 4%, a surface roughness Ra of 0.3 μm, and a contact angle representing surface wettability of 79 degrees was used. Except for these, a cylindrical nonaqueous secondary battery was produced in the same manner as in Example 1.
[0037]
The batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were discharged at 1550 mA (1C) to 2.75 V, charged at 1550 mA, and after reaching 4.3 V, the voltage was adjusted to 4.3 V. The battery was overcharged for 3 and a half hours under the condition of keeping. Thereafter, the batteries were discharged at 1550 mA to 2.75 V, and then a part of the batteries was disassembled, and it was examined whether abnormalities such as cracks, cracks, and cutting occurred in the copper foil of the negative electrode current collector. The results are shown in Table 1. Table 1 also shows the maximum volume change rate of the negative electrode mixture layer when charged to 4.2 V and discharged to 2.75 V.
[0038]
In addition, the remaining battery is charged at 1550 mA, and after reaching 4.2 V, charging is performed for 2.5 hours under the condition of maintaining a constant voltage of 4.2 V, and further, a charging / discharging cycle of discharging to 2.75 V at 1550 mA is performed. Repeatedly. Then, the capacity retention ratio [(discharge capacity at the 50th cycle) / (discharge capacity at the 1st cycle) × 100] with respect to the first cycle at 50 cycles was measured. The results are shown in Table 2. In the initial cycle, the battery was charged to 4.2 V, then discharged to 2.75 V, the discharge capacity was measured, and the capacity per unit volume of the electrode laminate was determined based on the discharge capacity. The results are also shown in Table 2.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
As shown in Table 1, in Examples 1 and 2, although the maximum volume change rate of the negative electrode mixture layer was 11%, the negative electrode current collector had no abnormality such as cracks, cracks, and cuts. Explaining this in detail, in Example 1 and Example 2, the maximum volume change rate of the negative electrode mixture layer was extremely 11%, because the negative electrode current collector had a breaking elongation of 5% or more. Despite being large, the negative electrode current collector had no abnormalities such as cracks, cracks and cuts.
[0042]
On the other hand, in Comparative Example 1 using a material having a small elongation at break of 4% as the negative electrode current collector, cracks occurred in the current collector as a result of charging and discharging. Therefore, in this comparative example 1, it is predicted that the deterioration of the cycle characteristics will increase. In Comparative Example 2, since the maximum volume change rate of the negative electrode mixture layer was as small as 1% or less, there was no abnormality such as cracking, cracking or cutting of the negative electrode current collector due to charging / discharging. The density of the negative electrode was lowered, the battery capacity was about 80% compared to Examples 1-2, and the capacity was low.
[0043]
Further, as shown in Table 2, in Comparative Example 1, the capacity retention ratio decreased to 50% or less at 50 cycles, whereas in Examples 1 and 2, the capacity retention ratio was as high as 85 to 91%. It was. In particular, the battery of Example 1 using the negative electrode current collector having a high wettability with a contact angle of 35 degrees representing the wettability of the surface of the current collector had the most excellent capacity retention rate of 91%. As described above, the reason why the capacity retention rate at 50 cycles in Comparative Example 1 was as low as 50% or less is considered to be that the negative electrode current collector was cracked, cut, or the like accompanying charging / discharging. .
[0044]
【The invention's effect】
As described above, in the present invention, even when a negative electrode mixture having a large volume change with a maximum volume change rate of 8% or more accompanying charge / discharge of the negative electrode mixture layer is used, cracking, cutting, etc. of the negative electrode current collector are performed. Providing a non-aqueous secondary battery with excellent cycle characteristics by preventing occurrence, improving reliability during overcharge, and preventing deterioration of cycle characteristics due to cracking, cutting, etc. of the negative electrode current collector I was able to.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an example of a nonaqueous secondary battery according to the present invention.
[Explanation of symbols]
1 Positive electrode 2 Negative electrode 3 Separator 4 Liquid electrolyte

Claims

A non-aqueous secondary battery having a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode has a negative electrode mixture layer formed on at least one surface of a current collector ;
Another non-aqueous secondary battery having the same configuration as the non-aqueous secondary battery and the thickness of the negative electrode mixture layer when the non-aqueous secondary battery was charged to 4.3 V at a 1C rate for 2 and a half hours and then decomposed After being charged to 4.3 V at 1C rate for 2 and a half hours, after discharging to 2.75 V and then the thickness of the negative electrode mixture layer when disassembled , charging and discharging of the negative electrode mixture layer obtained by the following formula nonaqueous secondary, wherein the maximum volume change rate is the maximum value of the volume change rate after the Ri der least 8%, elongation at break of the current collector of the negative electrode is 5% or less than 8% Next battery.
Volume change rate = (thickness of negative electrode mixture layer after charging−thickness of negative electrode mixture layer after discharge)
÷ (Thickness of negative electrode mixture layer after discharge)

The non-aqueous secondary battery according to claim 1, wherein the wettability of the negative electrode current collector is less than 40 degrees in terms of contact angle.

The nonaqueous secondary battery according to claim 1 or 2, wherein the capacity of the electrode stack per unit volume in normal charging is 130 mAh / cm ³ or more.

Liquid electrolyte chain ester and dielectric constant in is included and a 30 or more cyclic structures ester, a cyclic structure according to claim amounts Ru der less than 40% by volume of the ester 1-3 in the total solvent in the liquid electrolyte The nonaqueous secondary battery in any one of.

As the negative electrode active material, the (002) plane interlayer distance d ₀₀₂ is 3. The non-aqueous secondary battery according to any one of claims 1 to 4, wherein a carbon material having a size of 4 Å or less is used.