JP3692965B2 - Lithium secondary battery and method for producing positive electrode thereof - Google Patents

Description

【００２３】
表１より、水系結着剤を用いた電池では、正極活物質の比表面積が大きくなると、電池の初期放電容量が低下していることがわかる。このことは、正極活物質の比表面積が大きくなると、水系結着剤に含まれる水分と反応するためであると考えられる。また、比表面積が０．６５ｍ²／ｇの正極活物質を有する電池は、結着剤に水系結着剤を用いても、有機溶媒系結着剤を用いた場合とほぼ同等の電池初期容量を実現している。
【００２４】
結着剤は、正極活物質を結着する。結着剤は、電解液に対して非膨潤性の水溶性高分子を有し、正極活物質の表面を均一に被覆している。このような結着剤は、耐有機溶媒性に優れているため、リチウム電池の電解液により膨潤、溶解することがなく、かつ電池の充放電における正極活物質の膨張・収縮時に活物質を強固に結着した状態を維持できるため、電池として良好なサイクル特性を実現できる。また、結着剤は、それ自身の粘性により、増粘剤として機能させてもよい。
【００２５】
また、結着剤が電解液に対して非膨潤性の水溶性高分子を有することで、水溶性高分子が正極活物質表面を均一に被覆することができ、リチウム二次電池において正極活物質と電解液との反応を抑制することができる。
【００２６】
すなわち、リチウム二次電池の製造工程中の電極ペースト作製時に、結着剤の水溶性高分子が水に均一に溶解するため、水溶性高分子が均一に溶解した電極ペーストが調製され、正極活物質表面に水溶性高分子が均一に被覆される。
【００２７】
また、正極活物質表面に形成された水溶性高分子の被膜が常に電解液と接するため、電解液に膨潤しない非膨潤性の水溶性高分子が用いられる。また、水溶性高分子は、電解液に溶解しない。すなわち、電解液に溶解しないことで、水溶性高分子により正極活物質表面に形成された被膜が保持される。
【００２８】
このような水溶性高分子としては、メチルセルロース（ＭＣ）、カルボキシメチルセルロース（ＣＭＣ）、エチルセルロース（ＥＣ）、ヒドロキシエチルセルロース（ＨＥＣ）、ヒドロキシプロピルメチルセルロース（ＨＰＭＣ）、ヒドロキシエチルメチルセルロース（ＨＥＭＣ）等のセルロース類、ポリアクリル酸塩等をあげることができる。
【００２９】
水溶性高分子は、水溶性セルロースがより好ましい。
【００３０】
結着剤は、正極に可とう性を付与する樹脂を有することが好ましい。すなわち、結着剤が正極に可とう性を付与する樹脂を有することで、正極が可とう性を有することとなり、製造時等において正極の取り扱いが容易となる。詳しくは、結着剤として水溶性高分子のみを用いて正極活物質、導電剤および集電体を結着することは可能であるが、作製された正極は可とう性が乏しいため、取り扱い時やシート状の正極を捲回させる時に活物質層にひび割れや活物質層の脱落等の正極の損傷が生じやすくなるためである。
【００３１】
また、正極に可とう性を付与する樹脂は、耐電解液性に優れていることが好ましい。すなわち、正極に可とう性を付与する樹脂は、結着剤として用いられるため、電解液に対して膨潤や溶解を生じると、水溶性高分子が正極活物質表面に形成した被膜が破損を生じるようになるためである。
【００３２】
この正極に可とう性を付与する樹脂としては、たとえば、ポリテトラフルオロエチレン（ＰＴＦＥ）、テトラフルオロエチレン−ヘキサフルオロプロピレン共重合体（ＦＥＰ）、テトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体（ＰＦＡ）、テトラフルオロエチレン−エチレン共重合体（ＥＴＦＥ）、テトラフルオロエチレン−ヘキサフルオロプロピレン−パーフルオロアルキルビニルエーテル共重合体（ＥＰＥ）等のフッ素系樹脂、アクリル系樹脂、ポリエチレン、ポリオレフィン等のオレフィン類、ポリイミド樹脂等の樹脂をあげることができる。
【００３３】
正極に可とう性を付与する樹脂は、フッ素系樹脂であることが好ましい。すなわち、フッ素系樹脂は、電解液に対して、膨潤、溶解を生じないため、結着剤として用いた電池の電池特性を向上させることができる。ここで、本発明のリチウム二次電池は、結着剤にフッ素系樹脂を用いることが好ましいが、フッ素系以外の樹脂や高分子材料を結着剤に用いることを排除するものではない。
【００３４】
フッ素系樹脂としては、ポリテトラフルオロエチレンがより好ましい。
【００３５】
ＬｉＮｉＯ₂系化合物は、Ｘ線粉末解析測定結果における（００６）面、（１０２）面および（１０１）面のピーク強度比（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が、０．４２１以下であることが好ましい。すなわち、ＬｉＮｉＯ₂系化合物の結晶性を高め、結晶構造を強化することで、リチウム二次電池のサイクル特性の低下を抑えることができる。これは、以下に示す理由によるためである。
【００３６】
すなわち、理想的なＬｉＮｉＯ₂の構造としては、酸素層の間にＬｉ⁺層、Ｎｉ³⁺層が存在する層状岩塩構造（α−ＮａＦｅＯ₂構造）である。しかし、Ｎｉ³⁺は、Ｎｉ²⁺に還元されやすく、またＮｉ²⁺のイオン半径（０．８３Å）がＬｉ⁺のイオン半径（０．９０Å）とほぼ等しいため、ＬｉＮｉＯ₂合成時に生じるＬｉ⁺欠損サイトにＮｉ²⁺が混入しやすくなっており、［Ｌｉ⁺ _1-xＮｉ²⁺ _x］_3b［Ｎｉ³⁺］_3a［Ｏ^2- ₂］_6Cという構造になりやすくなっている。このＬｉ⁺ _3bサイトにＮｉ²⁺が混入すると、局部的に電気化学的に不活性な構造ができるとともに、Ｌｉ⁺の拡散を妨げるようになる。このため、結晶性の低いＬｉＮｉＯ₂を電極活物質として用いた場合には、電池容量の低下や耐久性の低下といった問題が生じるようになっていた。
【００３７】
そこで、このＬｉＮｉＯ₂の結晶性を定量的に表す方法として、Ｘ線回折を用いた結晶構造解析による（００６）面に起因する回折強度Ｉ₀₀₆と（１０２）面に起因する回折強度Ｉ₁₀₂との和を（１０１）面に起因する回折強度Ｉ₁₀₁で除した値（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁を用いた。この値が小さいほど結晶欠陥が少なく、強固な結合力を持っている。
【００３８】
ここで、ＬｉＮｉＯ₂系化合物のピーク強度比（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁と電池特性の関係について測定し、この測定結果を表２に示した。詳しくは、比表面積が一定でありかつＸＲＤのピーク強度比が異なる正極活物質と、ポリテトラフルオロエチレンを含む結着剤とを用いた正極を有する電池を作製し、充放電試験後の放電容量を測定した。まず、合成時の原材料の配合比、焼成温度、雰囲気（酸素濃度、露点、ＣＯ₂ 含有量等）を変えることにより、ＢＥＴ比表面積を保ったまま、ピーク強度比（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁の異なるＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂のＬｉＮｉＯ₂系化合物よりなる正極活物質サンプルを作製した。この正極活物質にポリテトラフルオロエチレンの水性ディスバージョンと水溶性セルロース樹脂とからなる水系結着剤を用いた正極を有する電池を作製し、初期放電容量および充放電試験後の放電容量を測定した。このとき、この正極活物質のＢＥＴ比表面積は、０．６５ｍ²／ｇであった。また、それぞれの正極活物質を用いて、ＰＶＤＦおよびＮＭＰよりなる有機溶媒系の結着剤を用いたリチウム電池も作製した。なお、初期放電容量およびサイクル特性の測定は、後述の実施例に記載の測定方法を用いた。
【００３９】
【表２】

【００４０】
表２より、水系結着剤を用いた電池は、ピーク強度比によらずに、有機溶剤系結着剤を用いた電池と同等の１５８（ｍＡｈ／ｇ）の初期放電容量が実現されている。また、充放電試験後の放電容量比は、ピーク強度比（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１以下の活物質を用いた電池では、有機溶剤系結着剤を用いた電池よりも優れていることがわかる。
【００４１】
正極活物質は、式Ｌｉ_aＭｎ_bＭｅ_cＯ₄（ＭｅはＣｏ、Ｃｕ、Ｆｅ、Ｎｉ、Ｚｎ、Ａｌ、Ｃｒ、Ｔｉ、ＭｇおよびＶよりなる群から選ばれる少なくとも一種の元素、１≦ａ≦１．１５、０≦ｃ≦０．３５、ａ＋ｂ＋ｃ＝３）で表されるＬｉＭｎＯ₄系化合物を有することが好ましい。このＬｉＭｎＯ₄系化合物を正極活物質に配合することで、リチウム二次電池の充放電曲線を調節することができるようになる。この結果、充放電特性の異なるリチウム二次電池を得ることができ、リチウム二次電池の使用の用途が広がることとなる。詳しくは、ＬｉＭｎ₂Ｏ₄系化合物を配合することで正極活物質の充放電曲線にずれが生じ、ＬｉＭｎ₂Ｏ₄系化合物の配合量を変化させることでずれの量が変化する。充放電曲線が異なると、同一電位からの充放電容量が変化するようになる。この結果、リチウム二次電池の充放電特性が変化することとなる。ここで、ＬｉＭｎ₂Ｏ₄系化合物は、任意の割合で正極活物質に配合することができる。
【００４２】
電解液は、通常のリチウム二次電池に用いられる電解液を用いることができる。この電解液としては、例えば、１、２−ジメトキシエタン、１、２−ジエトキシエタン、プロピレンカーボネート、エチレンカーボネート、γ−ブチロラクトン、テトラヒドラフラン、１、３−ジオキソラン、ジエチレンカーボネート、ジメチルカーボネート、エチルメチルカーボネートなどの単独または２種以上の混合溶媒に、例えば、ＬｉＣＦ₃ ＳＯ₃、ＬｉＣ₄Ｆ₉ＳＯ₃、ＬｉＣｌＯ₄、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＮ（ＣＦ₃ ＳＯ₂）（ＣＦ₃ ＳＯ₂）、ＬｉＮ（Ｃ₄Ｆ₉ＳＯ₂）（ＣＦ₃ＳＯ₂）、ＬｉＮ（Ｃ₂Ｆ₅ＳＯ₂）（Ｃ₂Ｆ₅ＳＯ₂）などの電解質を単独または２種以上を溶解させて調整した有機溶媒系の電解液を用いることができる。
【００４３】
本発明のリチウム二次電池は、正極活物質、結着剤および電解液以外は、通常のリチウム二次電池に用いられる形態とすることができる。また、本発明のリチウム二次電池は、その構造は特に限定されるものでなく、正極および負極をシート状に形成し、セパレータを介して交互に積層させた積層型の電極体を有する積層型電極電池でも、シート状の正極および負極をセパレータを介して巻回させた巻回型の電極体を有する巻回型電極電池としてもよい。
【００４４】
シート状の正極および負極は、活物質、導電材、結着材等を混合して得られた合材が集電体に塗布されて形成されたことが好ましい。
【００４５】
本発明のリチウム二次電池に用いることができる負極活物質、集電体およびセパレータとしては、たとえば、以下に示されるものを用いることができる。
【００４６】
負極活物質としては、鱗片状黒鉛、球状黒鉛、アモルファスカーボンなどのリチウムを吸蔵、放出可能な炭素材料、またはリチウム金属等をあげることができる。
【００４７】
正極の集電体としては、例えば、アルミニウム、ステンレスなど、負極の集電体としては、例えば、銅、ニッケルなどを網、パンチドメタル、フォームメタルや板状に加工した箔などを用いることができる。
【００４８】
セパレ−タとしては、例えば、厚さ１０〜５０（μｍ）で、開孔率３０〜７０％の微多孔性ポリプロピレンフィルムまたは微多孔性ポリエチレンフィルムなどを用いることができる。
【００４９】
本発明のリチウム二次電池は、正極活物質表面の比表面積を低減することで、反応に関与する面積を減少させ、水との反応による電池容量の低下を抑えている。また、結着剤に耐有機溶媒性に優れた水溶性高分子を用い、正極活物質の表面を均一に被覆しているため内部抵抗の増加を抑制し、電解液に対して膨潤、溶解を生じることがないとともに、電池の充放電時に電極活物質が膨張・収縮をしても活物質を強固に結着した状態を維持できる。このため、充放電サイクルに優れた電池となっている。
【００５０】
（リチウム二次電池用正極の製造方法）本発明のリチウム二次電池用正極の製造方法は、式Ｌｉ_xＮｉ_1-yＭ_yＯ₂（ＭはＣｏ、Ｍｎ、Ａｌ、Ｂ、Ｔｉ、Ｍｇ、Ｆｅよりなる群から選ばれる少なくとも一種の元素、０＜ｘ≦１．２、０＜ｙ≦０．２５）で表され、ＢＥＴ比表面積が０．６５ｍ²／ｇ以下であり、かつＸ線粉末解析測定結果における（００６）面、（１０２）面および（１０１）面のピーク強度比（Ｉ ₀₀₆ ＋Ｉ ₁₀₂ ）／Ｉ ₁₀₁ が、０．４２１以下であるＬｉＮｉＯ₂系化合物を有する正極活物質と、リチウム二次電池に用いられる電解液に対して非膨潤性の水溶性高分子と該正極に可とう性を付与する樹脂とを有する結着剤と、水と、を混合してペースト状活物質混合体を調整するペースト調整工程と、ペースト状活物質混合体を集電体に塗布する塗布工程と、を有することを特徴とする。
【００５１】
本発明のリチウム二次電池用正極の製造方法は、正極活物質と、結着剤と、水と、を混合してペースト状活物質混合体を調整するペースト調整工程を有する。このペースト調整工程は、結着剤にリチウム二次電池に用いられる電解液に対して非膨潤性の水溶性高分子を用いているため、水を媒質として用いることができる。この結果、有機溶媒を用いないため、電池の製造に要するコストを低減させることができるようになる。また、ペースト調製工程が正極活物質と、結着剤と、水と、を混合してペースト状活物質混合体を調整することで、正極活物質の表面に結着剤よりなる被膜を形成することができる。
【００５２】
なお、ペースト調整工程は、正極活物質と、結着剤と、水と、が混合したペースト状活物質混合体を調整する工程であり、この混合体を調整できる方法であればどのような方法で調整してもよい。
【００５３】
正極活物質は、式Ｌｉ_xＮｉ_1-yＭ_yＯ₂（ＭはＣｏ、Ｍｎ、Ａｌ、Ｂ、Ｔｉ、Ｍｇ、Ｆｅよりなる群から選ばれる少なくとも一種の元素、０＜ｘ≦１．２、０＜ｙ≦０．２５）で表される化合物を有する。
【００５４】
また、ＬｉＮｉＯ₂系化合物は、ＢＥＴ比表面積が０．６５ｍ²／ｇ以下である。ＬｉＮｉＯ₂系化合物のＢＥＴ比表面積を０．６５ｍ²／ｇ以下とすることで、ＬｉＮｉＯ₂系化合物表面での水分によるプロトンとリチウムのイオン交換反応の発生を抑制しているためと考えられる。また、ＢＥＴ比表面積が０．６５ｍ²／ｇを超えると、水と反応を生じ、初期放電容量が低下する。
【００５５】
結着剤は、リチウム二次電池に用いられる電解液に対して非膨潤性の水溶性高分子を有する。結着剤が電解液に対して非膨潤性の水溶性高分子を有することで、ペースト調製工程において水溶性高分子が正極活物質表面を均一に被覆することができ、製造された正極を用いたリチウム二次電池において正極活物質と電解液との反応を抑制することができる。
【００５６】
すなわち、結着剤が水溶性高分子を有することで、ペースト調製工程において、水に均一に溶解し、この溶液中に正極活物質が分散するため、水溶性高分子が均一に被覆した正極活物質が得られるようになる。
【００５７】
また、正極活物質表面に形成された水溶性高分子の被膜は、電池が形成されたときに常に電解液と接するようになるため、水溶性高分子には電解液に膨潤しない非膨潤性の水溶性高分子が用いられる。また、水溶性高分子は、電解液に溶解しない。すなわち、電解液に溶解しないことで、水溶性高分子により正極活物質表面に形成された被膜が保持される。
【００５８】
このような結着剤は、耐有機溶媒性に優れているため、リチウム電池の電解液により膨潤、溶解することがなく、かつ充放電により活物質が膨張、収縮をしても活物質を強固に結着した状態を維持できるため、リチウム電池に良好なサイクル特性を生じさせる。また、結着剤は、それ自身の粘性により、増粘剤として作用させてもよい。
【００５９】
このような水溶性高分子としては、メチルセルロース（ＭＣ）、カルボキシメチルセルロース（ＣＭＣ）、エチルセルロース（ＥＣ）、ヒドロキシエチルセルロース（ＨＥＣ）、ヒドロキシプロピルメチルセルロース（ＨＰＭＣ）、ヒドロキシエチルメチルセルロース（ＨＥＭＣ）等のセルロース類、ポリアクリル酸塩等をあげることができる。
【００６０】
水溶性高分子は、水溶性セルロースがより好ましい。
【００６１】
結着剤は、正極に可とう性を付与する樹脂を有することが好ましい。すなわち、結着剤が正極に可とう性を付与する樹脂を有することで、正極が可とう性を有することとなり、製造時等において正極の取り扱いが容易となる。詳しくは、結着剤として水溶性高分子のみを用いて正極活物質、導電剤および集電体を結着することは可能であるが、作製された正極は可とう性が乏しいため、取り扱い時やシート状の正極を捲回させる時に活物質層にひび割れや活物質層の脱落等の正極の損傷が生じやすくなるためである。
【００６２】
また、正極に可とう性を付与する樹脂は、耐電解液性に優れていることが好ましい。すなわち、正極に可とう性を付与する樹脂は、結着剤として用いられるため、電解液に対して膨潤や溶解を生じると、水溶性高分子が正極活物質表面に形成した被膜が破損を生じるようになるためである。
【００６３】
この正極に可とう性を付与する樹脂としては、たとえば、ポリテトラフルオロエチレン（ＰＴＦＥ）、テトラフルオロエチレン−ヘキサフルオロプロピレン共重合体（ＦＥＰ）、テトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体（ＰＦＡ）、テトラフルオロエチレン−エチレン共重合体（ＥＴＦＥ）、テトラフルオロエチレン−ヘキサフルオロプロピレン−パーフルオロアルキルビニルエーテル共重合体（ＥＰＥ）等のフッ素系樹脂、アクリル系樹脂、ポリエチレン、ポリオレフィン等のオレフィン類、ポリイミド樹脂等の樹脂をあげることができる。
【００６４】
正極に可とう性を付与する樹脂は、フッ素系樹脂であることが好ましい。すなわち、フッ素系樹脂は、電解液に対して、膨潤、溶解を生じないため、結着剤として用いた電池の電池特性を向上させることができる。ここで、本発明においては、結着剤にフッ素系樹脂を用いることが好ましいが、フッ素系以外の樹脂や高分子材料を結着剤に用いることを排除するものではない。
【００６５】
フッ素系樹脂としては、ポリテトラフルオロエチレンがより好ましい。
【００６６】
結着剤が非膨潤性を示すリチウム二次電池に用いられる電解液は、通常のリチウム二次電池に用いられる電解液を用いることができる。この電解液としては、例えば、１、２−ジメトキシエタン、１、２−ジエトキシエタン、プロピレンカーボネート、エチレンカーボネート、γ−ブチロラクトン、テトラヒドラフラン、１、３−ジオキソラン、ジエチレンカーボネート、ジメチルカーボネート、エチルメチルカーボネートなどの単独または２種以上の混合溶媒に、例えば、ＬｉＣＦ₃ ＳＯ₃、ＬｉＣ₄Ｆ₉ＳＯ₃、ＬｉＣｌＯ₄、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＮ（ＣＦ₃ ＳＯ₂）（ＣＦ₃ ＳＯ₂）、ＬｉＮ（Ｃ₄Ｆ₉ＳＯ₂）（ＣＦ₃ＳＯ₂）、ＬｉＮ（Ｃ₂Ｆ₅ＳＯ₂）（Ｃ₂Ｆ₅ＳＯ₂）などの電解質を単独または２種以上を溶解させて調整した有機溶媒系の電解液を用いることができる。
【００６７】
正極活物質は、式Ｌｉ_aＭｎ_bＭｅ_cＯ₄（ＭｅはＣｏ、Ｃｕ、Ｆｅ、Ｎｉ、Ｚｎ、Ａｌ、Ｃｒ、Ｔｉ、ＭｇおよびＶよりなる群から選ばれる少なくとも一種の元素、１≦ａ≦１．１５、０≦ｃ≦０．３５、ａ＋ｂ＋ｃ＝３）で表されるＬｉＭｎ₂Ｏ₄系化合物を有することが好ましい。正極活物質にＬｉＭｎ₂Ｏ₄系化合物を有することでリチウム二次電池の電池特性を変化させることができる。
【００６８】
塗布工程は、ペースト状活物質混合体を集電体に塗布する工程である。このペースト状活物質混合体の集電体への塗布は、通常のペースト状活物質混合体の塗布に用いられている方法を用いることができる。この塗布方法としては、例えば、ダイコータ、コンマコータ、リーバースローラー、ドクターブレード等の塗布方法をあげることができる。
【００６９】
塗布工程は、集電体に塗布されたペースト状活物質混合体を乾燥させる乾燥工程を有することが好ましい。集電体に塗布されたペースト状活物質混合体を乾燥させることで、その後の取り扱いにおいて、ペースト状活物質混合体が集電体の表面を流動してその塗布厚さに変化が生じなくなる。
【００７０】
本発明のリチウム二次電池用正極の製造方法は、ペースト状活物質混合体が塗布された集電体を押圧する圧着工程を有することが好ましい。すなわち、集電体の表面に塗布されたペースト状活物質混合体を押圧することで、集電体に圧着させるとともに、正極の重量エネルギー密度を向上させる。この圧着工程は、乾燥工程が施されたペースト状活物質混合体を押圧することが好ましい。
【００７１】
本発明のリチウム二次電池用正極の製造方法により製造された正極は、通常のリチウム二次電池に用いることができる。すなわち、本発明の製造方法により製造された正極を用いたリチウム二次電池は、正極以外の負極、セパレータ等には、従来のリチウム二次電池に用いられたものと同様のものを用いることができる。また、このときのリチウム二次電池の製造方法は、従来のリチウム二次電池の製造方法と同様の方法を用いることができる。
【００７２】
本発明のリチウム二次電池用正極の製造方法は、比表面積が低減された正極活物質を用いることで、反応に関与する面積を減少させ、水との反応による電池容量の低下を抑えている。また、結着剤に耐有機溶媒性に優れた水溶性高分子を用い、正極活物質の表面を均一に被覆しているため内部抵抗の増加を抑制し、電解液に対して膨潤、溶解を生じることがないとともに、電池の充放電時に電極活物質が膨張・収縮をしても活物質を強固に結着した状態を維持できる。このため、本発明の製造方法により製造された正極を用いたリチウム二次電池は、充放電サイクルに優れた電池となっている。
【００７３】
さらに、正極の結着剤に耐有機溶媒性に優れた水溶性高分子を用いているため、有機溶媒を使用することなく、正極を製造することができる。このため、有機溶媒の使用に要していた、安全管理上のコストや、有機溶媒自身の材料コスト、等を削減することが可能となり、安全かつ安価に正極を製造することができる。
【００７４】
【実施例】
以下、実施例を用いて本発明を説明する。
【００７５】
本発明の実施例として、リチウム二次電池を作製した。
【００７６】
なお、実施例において用いられた正極活物質のＢＥＴ比表面積およびＸＲＤは、以下の測定方法により測定された。
【００７７】
（ＢＥＴ比表面積）
（株）島津製作所製、型式：フローソープＩＩ２３００を用い、窒素吸着法によるＢＥＴ比表面積（以下、窒素ＢＥＴ比表面積と示す）を測定した。
【００７８】
（ＸＲＤ）
理学（株）製、型式：ＲＩＮＴ２０００を用いて、Ｘ線源：ＣｕＫα₁、管電圧：５０（ｋＶ）、管電流：１００（ｍＡ）、発散スリット：１／２（ｄｅｇ）、散乱スリット：１／２（ｄｅｇ）、受光スリット：０．１５（ｍｍ）、走査モ−ド：連続、走査範囲：１５°〜７５°で回折強度の測定を行った。
【００７９】
そのデータをＲＩＳＭ定性分析プログラムを用い、曲率５．０でバックグラウンド除去を行った後、（Ｋα₁／Ｋα₂）の強度比を０．５に設定し、Ｋα₂による影響を除去した。
【００８０】
ここで得られたデータから各指数面に対応するＸ線の回折強度を読み取り、ＬｉＮｉＯ₂正極活物質において結晶性の目安となる００６面、１０２面、および１０１面もピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁を求めた。
【００８１】
（実施例１）
実施例１は、図１に示される円筒形リチウム二次電池１００である。
【００８２】
この円筒形リチウム二次電池１００は、リチウムを含む正極活物質をもち、かつ充電時にはリチウムをリチウムイオンとして放出し、放電時にはリチウムイオンを吸蔵することができる正極１と、炭素材料からなる負極活物質をもち、充電時にはリチウムイオンを吸蔵し放電時にはリチウムイオンを放出することができる負極２と、有機溶媒にリチウムが含まれる支持塩が溶解されてなる非水電解液３とを備えたリチウム二次電池である。
【００８３】
正極１は、正極集電体１１と、正極集電体１１の表面上に形成された正極活物質と結着剤とを有する正極合剤層１２と、正極集電体１１に接合された正極集電リード１３と、からなる電極である。
【００８４】
正極活物質は、ＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂であり、窒素ＢＥＴ比表面積が０．６５ｍ²／ｇ、ＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１であるリチウムニッケル酸化物であり、結着剤は、カルボキシメチルセルロースナトリウム塩水溶液とＰＴＦＥとの混合物である。
【００８５】
負極２は、負極集電体２１と、負極集電体２１の表面上に形成された負極活物質を有する負極合剤層２２と、負極集電体２１に接合された負極集電リード２３と、からなる電極である。
【００８６】
また、正極１と負極２とは、セパレータ４を介して巻回した状態で、ケース７内に保持されている。また、正極１および負極２の集電リード１３、２３は、それぞれケースの正極端子部５および負極端子部６と接続されている。
【００８７】
セパレ−タは、厚さが２５μｍのポリエチレン製多孔質膜を、電解液にはエチレンカーボネート（ＥＣ）とをジエチレンカーボネート（ＤＥＣ）を３０：７０の体積比で混合した溶媒に電解質として１ｍｏｌ／リットルのＬｉＰＦ₆を溶解させたものを用いた。
【００８８】
実施例１のリチウム二次電池を、以下の手順で作製した。
【００８９】
（正極の製造）
まず、正極活物質が８７ｗｔ％、導電剤としてアセチレンブラック（品番：ＨＳ−１００）１０ｗｔ％、増粘剤としても機能する結着剤として２ｗｔ％のカルボキシメチルセルロースナトリウム塩水溶液をカルボキシメチルセルロースナトリウムの固形分が２ｗｔ％となるように混合し、二軸攪拌機にて１時間攪拌する。その後、結着剤として固形分比率約５０％のＰＴＦＥ水性ディスバージョンをＰＴＦＥの固形分が１ｗｔ％となるように添加し、真空乳化攪拌装置を使い３０分間攪拌する。このようにして得られたペーストをコンマコータにてアルミ箔上に片面あたり目付量８．５６（ｍｇ／ｃｍ²）で両面に塗布する。
【００９０】
次に、この電極をロールプレス機に通し、線圧０．７４（ｔｏｎ／ｃｍ）の荷重をかけ、電極密度を２．４１（ｇ／ｃｍ²）にする。その後、この電極を幅５．４（ｃｍ）、長さ８６（ｍｍ）にカットし、電流取り出し用のリードタブ溶接部として長さ２．５（ｃｍ）分の電極合剤を掻き取った。この電極の有効反応面積は５．４（ｃｍ）×８３．５（ｃｍ）×２＝９０１．８（ｃｍ² ）であった。
【００９１】
（負極の製造）
負極活物質として鱗片状グラファイト９２．５ｗｔ％、結着剤としてＰＶＤＦ７．５ｗｔ％を用い、Ｎ−メチル−２−ピロリドン中にＰＶＤＦを溶解した溶液にグラファイトを分散させたペーストを作製し、正極と同様にコンマコータを用いて銅箔上に片面あたりの目付量が５．１（ｍｇ／ｃｍ² ）となるように両面にペーストを塗布した。その後、ロールプレス機を通し、線圧０．２５（ｔｏｎ／ｃｍ）の荷重をかけ、電極密度を１．２５（ｇ／ｃｍ²）にまで上昇させた電極を作製した。次に、この電極を幅５．６（ｃｍ）、長さ９０．５（ｃｍ）にカットし、電流取り出し用のリートタブ溶接部として長さ０．５（ｃｍ）分の電極合剤を掻き取った。この電極の有効反応面積は、５．６（ｃｍ）×９０（ｃｍ）×２＝１００８（ｃｍ² ）であった。
【００９２】
（電池の組立）
以上で得られた正極および負極をセパレータを介した状態で巻回させて、巻回型電極を形成し、この電極をケースの内部に挿入した。このとき、集電リードがケースの正極端子および負極端子に接合された。
【００９３】
その後、ケース内に電解液が注入され、ケースが閉じられた。
【００９４】
以上の手順により、実施例１の円筒形リチウム二次電池が製造された。
【００９５】
（比較例１）
比較例１は、正極の結着剤にポリフッ化ビニリデン（ＰＶＤＦ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。ここで、バインダにＰＶＤＦを用いたときの正極は、Ｎ−メチル−２−ピロリドン中にＰＶＤＦを溶解し、この溶液に正極活物質および導電剤を分散させたペーストを作成し、このペーストをアルミ箔に塗布することで製造した。
【００９６】
（実施例２）
実施例２は、正極活物質の窒素ＢＥＴ比表面積が０．５５（ｍ² ／ｇ）である以外は、実施例１と同様の円筒形リチウム二次電池である。
【００９７】
（比較例２）
比較例２は、正極活物質の窒素ＢＥＴ比表面積が０．８３（ｍ²／ｇ）である以外は、実施例１と同様の円筒形リチウム二次電池である。
【００９８】
（比較例３）
比較例３は、正極活物質の窒素ＢＥＴ比表面積が１．０４（ｍ²／ｇ）である以外は、実施例１と同様の円筒形リチウム二次電池である。
【００９９】
（比較例４）
比較例４は、正極の結着剤にＰＶＤＦを用いた以外は、比較例３と同様の円筒形リチウム二次電池である。
【０１００】
（実施例３）
実施例３は、正極活物質のＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４１２である以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１０１】
（比較例５）
比較例５は、正極活物質のＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４４９であるものを用いた以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１０２】
（比較例６）
比較例６は、正極の結着剤にＰＶＤＦを用いた以外は、比較例５と同様の円筒形リチウム二次電池である。
【０１０３】
（実施例４）
実施例４は、正極活物質の窒素ＢＥＴ比表面積が０．５５（ｍ²／ｇ）であり、かつＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４１２である以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１０４】
（実施例５）
実施例５は、正極の結着剤としてＣＭＣにかえてヒドロキシエチルセルロース（ＨＥＣ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。なお、実施例５において、正極の製造は、ＣＭＣのかわりにＨＥＣを用いた以外は、実施例１と同様の方法で行われた。
【０１０５】
（実施例６）
実施例６は、正極の結着剤としてＣＭＣにかえてヒドロキシプロピルメチルセルロース（ＨＰＭＣ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。なお、実施例６において、正極の製造は、ＣＭＣのかわりにＨＰＭＣを用いた以外は、実施例１と同様の方法で行われた。
【０１０６】
（実施例７）
実施例７は、正極の結着剤としてＣＭＣにかえてヒドロキシエチルメチルセルロース（ＨＥＭＣ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。なお、実施例７において、正極の製造は、ＣＭＣのかわりにＨＥＭＣを用いた以外は、実施例１と同様の方法で行われた。
【０１０７】
（実施例８）
実施例８は、正極の結着剤としてＰＴＦＥにかえてテトラフルオロエチレン−ヘキサフルオロプロピレン共重合体（ＦＥＰ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。なお、実施例８において、正極の製造は、ＰＴＦＥ水性ディスパージョンのかわりにＦＥＰ水性ディスパージョンを用いた以外は、実施例１と同様の方法で行われた。
【０１０８】
（実施例９）
実施例９は、正極の結着剤としてＰＴＦＥにかえてテトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体（ＰＦＡ）を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。なお、実施例９において、正極の製造は、ＰＴＦＥ水性ディスパージョンのかわりにＰＦＡ水性ディスパージョンを用いた以外は、実施例１と同様の方法で行われた。
【０１０９】
（実施例１０）
実施例１０は、正極の結着剤としてＰＴＦＥにかえてアクリル系エマルジョン樹脂を用いた以外は、実施例１と同様の円筒形リチウム電池である。なお、実施例１０において、正極の製造は、ＰＴＦＥのかわりにアクリルエマルジョン樹脂を用いた以外は、実施例１と同様の方法で行われた。
【０１１０】
（実施例１１）
実施例１１は、正極活物質がＬｉＮｉ_0.80Ｃｏ_0.15Ａｌ_0.05Ｏ₂であり、かつ窒素ＢＥＴ比表面積が０．６５（ｍ²／ｇ）、ＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１であるものを用いた以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１１１】
（実施例１２）
実施例１２は、正極活物質がＬｉＮｉ_0.90Ｃｏ_0.06Ａｌ_0.04Ｏ₂であり、かつ窒素ＢＥＴ比表面積が０．６５（ｍ²／ｇ）、ＸＲＤによるピーク強度比：（Ｉ_{0 06}＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１であるものを用いた以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１１２】
（実施例１３）
実施例１３は、正極活物質がＬｉＮｉ_0.75Ｃｏ_0.21Ａｌ_0.04Ｏ₂であり、かつ窒素ＢＥＴ比表面積が０．６５（ｍ²／ｇ）、ＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１であるものを用いた以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１１３】
（実施例１４）
実施例１４は、正極活物質を、ＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂とＬｉ_1.12Ｍｎ_1.88Ｏ₄の混合物を用いた以外は、実施例１と同様の円筒形リチウム二次電池である。
【０１１４】
詳しくは、ＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂は、窒素ＢＥＴ比表面積が０．６５（ｍ²／ｇ）、ＸＲＤによるピーク強度比：（Ｉ₀₀₆＋Ｉ₁₀₂）／Ｉ₁₀₁が０．４２１であり、Ｌｉ_1.12Ｍｎ_1.88Ｏ₄は窒素ＢＥＴ比表面積が０．５（ｍ²／ｇ）であった。また、これらの混合割合は、重量比でＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂：Ｌｉ_1.12Ｍｎ_1.88Ｏ₄が６０：２７であった。
【０１１５】
（実施例１５）
実施例１５は、ＬｉＮｉ_0.81Ｃｏ_0.15Ａｌ_0.04Ｏ₂とＬｉ_1.12Ｍｎ_1.88Ｏ₄との混合割合を重量比で２７：６０とした以外は、実施例１４と同様な円筒形リチウム二次電池である。
【０１１６】
（比較例７）
比較例７は、正極の結着剤にＰＶＤＦを用いた以外は、実施例１４と同様の円筒形リチウム二次電池である。
【０１１７】
（比較例８）
比較例８は、正極の結着剤にＰＶＤＦを用いた以外は、実施例１５と同様の円筒形リチウム二次電池である。
【０１１８】
（評価）
実施例および比較例のリチウム二次電池の評価として、初期放電容量、高温サイクル特性および内部抵抗増加率の測定を行った。この測定結果を表３に示した。なお、これらの測定方法は、以下の通りであった。なお、比較例３〜６の電池は、初期放電容量のみを測定した。
【０１１９】
（初期放電容量）
まず、初回は、充電電流２５０（ｍＡ）で４．１（Ｖ）までＣＣ−ＣＶ充電し、放電電流３３３（ｍＡ）で３．０（Ｖ）までＣＣ放電を行った。
【０１２０】
次に、充電電流１０００（ｍＡ）で４．１（Ｖ）までＣＣ−ＣＶ充電、放電電流１０００（ｍＡ）で３．０（Ｖ）までＣＣ放電を４回行った後、充電電流１０００（ｍＡ）で４．１（Ｖ）までＣＣ−ＣＶ充電、放電電流３３３（ｍＡ）で３．０（Ｖ）までＣＣ放電した。
【０１２１】
この時の放電容量を電池初期容量とし、電池内に充填した正極活物質重で規格化した値を用いて比較した。なお、測定は２５℃の雰囲気で行った。
【０１２２】
（高温サイクル特性）
初期容量測定後のリチウム二次電池を、雰囲気温度６０℃の恒温槽内に入れ、充電電流１９８４（ｍＡ）で４．１（Ｖ）までＣＣ充電し、放電電流１９８４（ｍＡ）で３．０（Ｖ）までＣＣ充電を行うサイクルを５００回繰り返し、初回と５００サイクル目の放電容量の比を比較した。
【０１２３】
（内部抵抗）
まず、充電電流１０００（ｍＡ）で３．７５０（Ｖ）まで１．５時間でＣＣ−ＣＶ充電を行った。その後、３００（ｍＡ）で１０秒間放電、３００（ｍＡ）で１０秒間充電、９００（ｍＡ）で１０秒間放電、９００（ｍＡ）で１０秒間充電、２７００（ｍＡ）で１０秒間放電、２７００（ｍＡ）で１０秒間充電、５４００（ｍＡ）で１０秒間放電、５４００（ｍＡ）で１０秒間充電、８１００（ｍＡ）で１０秒間放電、８１００（ｍＡ）で１０秒間充電し、各放電後、充放電後の電圧を縦軸に充放電電流を横軸にとった電流−電圧プロット値の近似一次直線の傾きから内部抵抗値を求めた。
【０１２４】
（抵抗増加率）
内部抵抗増加率は、上記高温サイクル特性において行われる充放電を５００サイクル繰り返した後に、内部抵抗を測定し、充放電前後での内部抵抗の増加量を充放電前の内部抵抗で割ることで得た。
【０１２５】
詳しくは、｛抵抗増加率（％）｝＝｛（高温サイクル試験後の内部抵抗）−（初期の内部抵抗）｝／（初期の内部抵抗）である。
【０１２６】
【表３】

【０１２７】
表３より、実施例１は、初期放電容量が１５８ｍＡｈ／ｇと高く、かつ放電容量比も８２．０％と高い値であり、容量劣化の少ない電池であることがわかる。また、抵抗増加率も１．７％とほとんど内部抵抗の上昇がみられなかった。
【０１２８】
比較例１は、初期放電容量は十分に高いが、放電容量比が７５．０％に低下している。すなわち、結着剤にＰＶＤＦを用いているため、電解液との間での膨潤、溶解等により正極活物質の結着力が低下し、放電容量比が低下している。また、正極活物質表面を結着剤が均一に被覆していないあるいは充放電時に電解液による膨潤により結着剤にすき間が発生し、電解液の分解反応が促進され、フッ化リチウム等の絶縁性被膜が成長し、高温サイクル試験後に抵抗増加率が２４％と大きくなっている。
【０１２９】
実施例２は、正極活物質の比表面積を小さくした電池であり、初期放電容量が１６０ｍＡｈ／ｇ、放電容量比も８２．３％と高い値を示している。また、抵抗増加率も−１．１％とほとんど変化していない。
【０１３０】
比較例３は、窒素ＢＥＴ比表面積が大きくなっているため、初期放電容量が１３８ｍＡｈ／ｇにまで低下している。また、抵抗増加率も−４．７％とほとんど内部抵抗の変化がみられなかった。
【０１３１】
比較例４は、結着剤にＰＶＤＦを用いているため、窒素ＢＥＴ比表面積が大きくなっても、初期容量の低下はみられなかった。しかし、結着剤がＰＶＤＦであるため、放電容量比が７４．７％と大きく低下している。すなわち、結着剤のＰＶＤＦが、電解液との間で膨潤、溶解等を生じ、正極活物質の結着力が低下し、放電容量比を低下させている。また、結着剤がＰＶＤＦのみであるため、比較例１と同様に、抵抗増加率が７５．３％とかなり上昇した。
【０１３２】
実施例３は、初期放電容量が１５８ｍＡｈ／ｇと高く、また、放電容量比も８５．０％とかなり高い値を示している。すなわち、実施例３においては、正極活物質の結晶性を高めているため、サイクル特性の低下が抑えられている。また、抵抗増加率も０．６％とほとんど変化していない。
【０１３３】
比較例５は、初期放電容量は、１５８ｍＡｈ／ｇと高い値を示しているが、サイクル試験後の放電容量比が６５．２％にまで低下している。すなわち、正極活物質の結晶性が低いため、サイクル特性が低下している。また、抵抗増加率も−０．５％とほとんど変化していない。
【０１３４】
比較例６は、正極の結着剤にＰＶＤＦを用いているため、初期放電容量は１６０ｍＡｈ／ｇと高い値を示しているが、サイクル試験後の放電容量比が７２．０％にまで低下している。また、結着剤がＰＶＤＦのみであるため、比較例１と同様に、抵抗増加率が７９．２％とかなり上昇した。
【０１３５】
実施例４は、正極活物質の比表面積を小さくするとともに結晶性を高めているため、初期放電容量が１６０ｍＡｈ／ｇ、放電容量比が８５．１％と、ともに高い値を示している。また、抵抗増加率も１．８％とほとんど変化していない。
【０１３６】
実施例５は、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８１．１％と、ともに高い値を示している。また、正極の結着剤にＨＥＣを用いても、抵抗増加率が８．２％と十分に低く抑えられている。
【０１３７】
実施例６は、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８０．７％と、ともに高い値を示している。また、正極の結着剤にＨＰＭＣを用いても、抵抗増加率が７．９％と十分に低く抑えられている。
【０１３８】
実施例７は、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８０．６％と、ともに高い値を示している。また、正極の結着剤にＨＥＭＣを用いても、抵抗増加率が２．６％と十分に低く抑えられている。
【０１３９】
実施例８は、ＦＥＰ結着剤を用いているが、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８１．５％と、高い値を示している。また、抵抗増加率も＋０．８％とほとんど変化していない。
【０１４０】
実施例９は、ＰＦＡ結着剤を用いているが、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８１．７％と、高い値を示している。また、抵抗増加率も＋２．１％とほとんど変化していない。
【０１４１】
実施例１０は、初期放電容量が１５８ｍＡｈ／ｇ、放電容量比が８０．２％と、ともに高い値を示している。また、正極の結着剤にアクリル系樹脂を用いても、抵抗増加率が１．６％とほとんど変化していない。
【０１４２】
実施例１１〜１３は、正極活物質の組成を変化させた電池であり、初期放電容量および放電容量比が十分に高い値となり、抵抗増加率も＋２．０％以下に抑制されている。
【０１４３】
実施例１４〜１５は、正極活物質にリチウムニッケル酸化物とリチウムマンガン酸化物とを混合させた混合物を用い、正極の結着剤にＰＴＦＥ／ＣＭＣを用いた電池である。このマンガン酸化物が２７／８７の割合で混合された実施例１４は、初期容量および放電容量比が、正極結着剤がＰＶＤＦである比較例７と比較して十分に高い値を示している。また、マンガン酸化物が６０／８７の割合で混合された実施例１５は、初期容量および放電容量比が、正極結着剤がＰＶＤＦである比較例８と比較して十分に高い値を示している。
【０１４４】
実施例および比較例より、正極の結着剤に水溶性高分子を用いるとともに、比表面積を小さくかつ結晶性を高めたリチウムニッケル酸化物よりなる正極活物質を用いることで、初期放電容量、サイクル特性および抵抗増加率に優れたリチウム二次電池が得られる。
【０１４５】
【発明の効果】
本発明のリチウム二次電池は、正極活物質の比表面積を低減することで、反応に関与する面積を減らし、水分との反応による電池容量の低下を抑えている。また、結着剤に耐有機溶媒性に優れた水溶性高分子を有し、この水溶性高分子が正極活物質の表面を均一に被覆しているため、内部抵抗の増加および放電容量の低下が抑制され、充放電サイクルに優れた電池となっている。
【図面の簡単な説明】
【図１】 実施例において作製された電池の構成を示した図である。
【符号の説明】
１００…円筒形リチウム二次電池
１…正極 １１…正極集電体
１２…正極合剤層 １３…正極集電リード
２…負極 ２１…負極集電体
２２…負極合剤層 ２３…負極集電リード
３…非水電解液 ４…セパレータ
５…正極端子部 ６…負極端子部
７…ケース[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having a positive electrode active material and a binder.
[0002]
[Prior art]
In recent years, as a power source for electric devices such as cellular phones and portable video cameras, lithium batteries are becoming mainstream because of their high weight energy density. This lithium battery has a positive electrode active material containing lithium, releases lithium as lithium ions during charging, and can absorb lithium ions during discharging, and has a negative electrode active material, and stores lithium ions during charging. And a negative electrode capable of releasing lithium ions during discharge, and a nonaqueous electrolytic solution in which an electrolyte made of a supporting salt containing lithium in an organic solvent is dissolved.
[0003]
In addition, in order to improve the weight energy density, such a lithium battery has a positive electrode and a negative electrode formed in a sheet shape, and the sheet-like positive electrode and negative electrode are wound or separated through a separator formed in the same sheet shape. In a stacked state, it is housed in a case. The sheet-like positive electrode and negative electrode have a structure in which a mixture layer containing an active material is formed on the surface of a metal foil serving as a current collector.
[0004]
Here, as a positive electrode active material used for a lithium battery, in general, Li_xCoO₂, Li_xNiO₂, Li_xMn₂O_Four, Li_xFeO₂, V₂O_Five, Cr₂O_Five, MnO₂TiS₂, MoS₂Transition metal oxides and chalcogen oxides have been proposed.
[0005]
As a positive electrode active material for this lithium battery, in particular, Li_xCoO₂Or Li_xNiO₂, Li_xMn₂O_FourIs known to be promising as a positive electrode active material for non-aqueous electrolyte lithium batteries of 4V class. Among these Li compounds, Li_xNiO₂Is expected as a positive electrode active material for batteries, which has the largest theoretical capacity and can be supplied at low cost and stably. Furthermore, this Li_xNiO₂Includes Li added with additive element M_xNi_1-yM_yO₂You may use the compound shown by these.
[0006]
As a method for forming an electrode having these positive electrode active materials into a sheet electrode, as disclosed in JP-A-2-15855, an aqueous dispersion of a powdered active material, a conductive agent, a carboxymethyl cellulose aqueous solution, and polytetrafluoroethylene is used. Are uniformly mixed to prepare a paste, which is applied to a film-like conductive foil such as a rolled aluminum foil, dried and rolled.
[0007]
At this time, a resin excellent in organic solvent resistance such as polytetrafluoroethylene and carboxymethylcellulose is used as a binder. These resins do not swell or dissolve in organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC), which are widely used as solvents for electrolytes of lithium batteries. In addition, even when the active material expands and contracts due to the use of the lithium battery, the active material can be kept firmly bound. Here, when the binder swells or dissolves in the organic solvent, the binding force is reduced, and the cycle characteristics are greatly deteriorated. As a result, by using these binders excellent in organic solvent resistance, good cycle characteristics as a battery can be realized. Furthermore, this difference becomes more prominent when the operating temperature of the battery becomes high.
[0008]
Also, from the viewpoint of electrode production, the use of such a water-dispersible or water-soluble resin as the binder eliminates the use of an organic solvent during the production of the electrode, and the cost increases due to the use of the organic solvent. Can be suppressed. In other words, the fact that no organic solvent is used means that the cost of the solvent required for the organic solvent itself in the electrode manufacturing process is reduced, and no organic solvent recovery treatment facility is required in the electrode manufacturing facility. Demonstrate the effect of reducing
[0009]
[Problems to be solved by the invention]
However, Li_xNiO₂The positive electrode active material can increase the battery capacity of a lithium battery. However, when a water-dispersible or water-soluble resin excellent in organic solvent resistance is used as a binder, a lithium battery produced However, problems such as a decrease in battery capacity and a decrease in large current discharge characteristics due to an increase in charge transfer resistance have occurred. This means that Li_xNiO₂Since the positive electrode active material is highly reactive with water, an exchange reaction between protons and lithium ions occurs on the surface of the positive electrode active material in the paste preparation step of preparing an aqueous active material paste at the time of electrode preparation, or Since a phenomenon such as a film formed by reaction with water occurs on the surface of the material, the battery performance decreases.
[0010]
In order to eliminate the influence of moisture on such issues, an attempt has been made to make an electrode by making a paste in which polytetrafluoroethylene is dispersed in a nonaqueous organic solvent. It is not. That is, the dispersibility of polytetrafluoroethylene in a non-aqueous organic solvent is poor, entanglement of polytetrafluoroethylene occurs during paste preparation, gelation occurs, aggregates occur after electrode preparation, and a short circuit occurs during battery assembly. This is because there are problems such as frequent occurrence.
[0011]
For this reason, a method of adding a compound that is soluble in a non-aqueous organic solvent has been proposed for the purpose of improving the dispersibility of polytetrafluoroethylene in the non-aqueous organic solvent. For example, JP-A-8-106887 adds polyvinylidene fluoride (PVDF) and polyvinylidene chloride soluble in a non-aqueous organic solvent, and JP-A-10-12243 discloses a method of adding polyvinylbutyral. It is disclosed.
[0012]
However, these added compounds are inferior to polytetrafluoroethylene in resistance to electrolytic solution, so that they swell and dissolve in the electrolytic solution, and also have binding force due to expansion / contraction of the electrode active material due to charge / discharge. Has been reduced, resulting in a problem of increased cycle deterioration.
[0013]
In addition, when a resin that easily swells and dissolves in the electrolyte is used as a binder, the coverage of the positive electrode active material by the binder decreases, and the contact area between the electrolyte and the positive electrode active material increases during charge and discharge. In addition, the reactivity between the electrolytic solution and the positive electrode active material increases. When the reaction between the electrolytic solution and the positive electrode active material occurs, an insulating film such as lithium fluoride is formed on the surface of the positive electrode active material by a decomposition reaction of the electrolytic solution or the like. When the insulating film is formed, there arises a problem that the internal resistance of the battery is increased.
[0014]
The present invention has been made in view of the above situation, and uses a water-soluble polymer excellent in organic solvent resistance as a binder, and has a high capacity, excellent charge / discharge cycle, and an increase in internal resistance is suppressed. It is an object to provide a secondary battery.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, as a result of repeated studies on the positive electrode active material and the binder, the area involved in the reaction with water is reduced by limiting the specific surface area of the positive electrode active material, and the electrolytic solution resistance The present inventors have found that the above-mentioned problems can be solved by uniformly forming a water-soluble polymer film on the surface of the positive electrode active material by suppressing an increase in internal resistance associated with a charge / discharge cycle.
[0016]
[Means for Solving the Problems]
  That is, the lithium secondary battery of the present invention has the formula Li_xNi_1-yM_yO₂(M is at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, and Fe, 0 <x ≦ 1.2, 0 <y ≦ 0.25) In a lithium secondary battery comprising a positive electrode active material having a positive electrode active material, a binder containing the positive electrode active material, and an electrolyte, LiNiO₂System BET specific surface area is 0.65m²/ G or less,The LiNiO ₂ The compound is a peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ Is 0.421 or less,The binder has a water-soluble polymer that is non-swelling with respect to the electrolyte solution, and the surface of the positive electrode active material is covered with the water-soluble polymer.
[0017]
The lithium secondary battery of the present invention has a positive electrode active material, LiNiO.₂By reducing the specific surface area of the surface of the system compound, the area involved in the reaction is reduced, and the decrease in battery capacity due to the reaction with water during battery production is suppressed. In addition, the binder has a water-soluble polymer excellent in organic solvent resistance, and this water-soluble polymer uniformly coats the surface of the positive electrode active material, so that the internal resistance increases and the discharge capacity decreases. The battery is suppressed and has excellent charge / discharge cycle characteristics.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
(Lithium secondary battery)
The lithium secondary battery of the present invention is a lithium secondary battery having a positive electrode active material, a positive electrode containing a binder, and an electrolytic solution.
[0019]
The positive electrode active material has the formula Li_xNi_1-yM_yO₂(M is at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, and Fe, 0 <x ≦ 1.2, 0 <y ≦ 0.25) LiNiO₂System compounds. Here, when a plurality of elements are used for M, y represents the sum of the plurality of elements.
[0020]
LiNiO₂-Based compounds have a BET specific surface area of 0.65 m²/ G or less. LiNiO₂-Based BET specific surface area of 0.65m²/ G or less suppresses the ion exchange reaction between protons and lithium caused by the water content of the dispersion medium on the active material surface. Here, the BET specific surface area is 0.65 m²When it exceeds / g, the discharge capacity of the lithium secondary battery is reduced.
[0021]
Here, LiNiO having different specific surface areas₂A battery having a positive electrode using a positive electrode active material made of a base compound and a binder containing polytetrafluoroethylene was prepared, and the initial battery capacity was measured. Adjustment of the specific surface area of the positive electrode active material is carried out by adjusting the mixing ratio of raw materials during the synthesis of the active material, the firing temperature, and the atmosphere (oxygen concentration, dew point, CO₂The content was changed). A positive electrode was produced by using an aqueous polyversion of polytetrafluoroethylene and an aqueous binder composed of a water-soluble cellulose resin for an active material sample having a different specific surface area, and a battery having this positive electrode was produced. The initial discharge capacity of this battery was measured. For comparison, a battery using an organic solvent-based binder composed of PVDF and N-methyl-2-pyrrolidone (NMP) as a binder was also prepared, and the discharge capacity was measured. This initial discharge capacity measurement method was the same as the measurement method performed in the evaluation of the examples. The measurement results are shown in Table 1.
[0022]
[Table 1]

[0023]
From Table 1, it can be seen that in the battery using the aqueous binder, the initial discharge capacity of the battery decreases as the specific surface area of the positive electrode active material increases. This is considered to be because when the specific surface area of the positive electrode active material is increased, it reacts with moisture contained in the aqueous binder. The specific surface area is 0.65m²A battery having a positive electrode active material of / g realizes a battery initial capacity almost equal to that when an organic solvent-based binder is used, even when an aqueous binder is used as the binder.
[0024]
The binder binds the positive electrode active material. The binder has a water-soluble polymer that is non-swellable with respect to the electrolytic solution, and uniformly covers the surface of the positive electrode active material. Since such a binder has excellent organic solvent resistance, it does not swell or dissolve in the electrolyte solution of a lithium battery, and the active material is strong when the positive electrode active material expands or contracts during charge / discharge of the battery. As a battery, good cycle characteristics can be realized. The binder may function as a thickener due to its own viscosity.
[0025]
Further, since the binder has a water-soluble polymer that is non-swellable with respect to the electrolytic solution, the water-soluble polymer can uniformly coat the surface of the positive electrode active material, and the positive electrode active material in the lithium secondary battery And the electrolyte solution can be suppressed.
[0026]
That is, when the electrode paste is manufactured during the manufacturing process of the lithium secondary battery, the water-soluble polymer of the binder is uniformly dissolved in water. Therefore, an electrode paste in which the water-soluble polymer is uniformly dissolved is prepared. A water-soluble polymer is uniformly coated on the material surface.
[0027]
In addition, since the water-soluble polymer film formed on the surface of the positive electrode active material is always in contact with the electrolytic solution, a non-swellable water-soluble polymer that does not swell in the electrolytic solution is used. Further, the water-soluble polymer does not dissolve in the electrolytic solution. That is, the film formed on the surface of the positive electrode active material by the water-soluble polymer is retained by not being dissolved in the electrolytic solution.
[0028]
Such water-soluble polymers include celluloses such as methylcellulose (MC), carboxymethylcellulose (CMC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), hydroxyethylmethylcellulose (HEMC), Examples thereof include polyacrylate.
[0029]
The water-soluble polymer is more preferably water-soluble cellulose.
[0030]
The binder preferably has a resin that imparts flexibility to the positive electrode. That is, when the binder includes a resin that imparts flexibility to the positive electrode, the positive electrode has flexibility, so that the positive electrode can be easily handled during production. Specifically, it is possible to bind a positive electrode active material, a conductive agent and a current collector using only a water-soluble polymer as a binder, but the produced positive electrode has poor flexibility, so This is because when the sheet-like positive electrode is wound, the active material layer is likely to be damaged, such as cracking or dropping off of the active material layer.
[0031]
Moreover, it is preferable that the resin which imparts flexibility to the positive electrode is excellent in resistance to electrolytic solution. That is, since the resin that imparts flexibility to the positive electrode is used as a binder, if the electrolyte swells or dissolves, the coating formed by the water-soluble polymer on the surface of the positive electrode active material is damaged. This is because
[0032]
Examples of the resin imparting flexibility to the positive electrode include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). ), Fluoropolymers such as tetrafluoroethylene-ethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), olefins such as acrylic resin, polyethylene, and polyolefin, A resin such as a polyimide resin can be used.
[0033]
The resin imparting flexibility to the positive electrode is preferably a fluororesin. That is, since the fluororesin does not swell or dissolve in the electrolytic solution, the battery characteristics of the battery used as the binder can be improved. Here, in the lithium secondary battery of the present invention, it is preferable to use a fluorine-based resin as the binder, but it does not exclude the use of a resin or a polymer material other than the fluorine-based resin as the binder.
[0034]
As the fluororesin, polytetrafluoroethylene is more preferable.
[0035]
LiNiO₂The compound based on the peak intensity ratio (I) of the (006) plane, (102) plane and (101) plane in the X-ray powder analysis measurement results₀₀₆+ I₁₀₂) / I₁₀₁Is preferably 0.421 or less. That is, LiNiO₂By improving the crystallinity of the system compound and strengthening the crystal structure, it is possible to suppress a decrease in cycle characteristics of the lithium secondary battery. This is because of the following reasons.
[0036]
That is, ideal LiNiO₂The structure of Li is between the oxygen layers⁺Layer, Ni³⁺Layered rock salt structure (α-NaFeO₂Structure). However, Ni³⁺Ni²⁺Easily reduced to Ni, and Ni²⁺The ion radius (0.83Å) of Li is⁺LiNiO is almost equal to the ionic radius (0.90０．９) of₂Li generated during synthesis⁺Ni on the defect site²⁺Is easy to mix, [Li⁺ _1-xNi²⁺ _x]_3b[Ni³⁺]_3a[O^2- ₂]_6CIt becomes easy to become the structure. This Li⁺ _3bNi on the site²⁺When mixed, a locally electrochemically inactive structure is formed, and Li⁺Will prevent the spread of. For this reason, LiNiO with low crystallinity₂When is used as an electrode active material, problems such as a decrease in battery capacity and a decrease in durability have occurred.
[0037]
So this LiNiO₂As a method for quantitatively expressing the crystallinity of the crystal, the diffraction intensity I attributed to the (006) plane by crystal structure analysis using X-ray diffraction₀₀₆And diffraction intensity I due to the (102) plane₁₀₂Is the diffraction intensity I due to the (101) plane.₁₀₁Value divided by (I₀₀₆+ I₁₀₂) / I₁₀₁Was used. The smaller this value, the fewer crystal defects and the stronger the bonding strength.
[0038]
Where LiNiO₂Peak intensity ratio (I₀₀₆+ I₁₀₂) / I₁₀₁Table 2 shows the measurement results. Specifically, a battery having a positive electrode using a positive electrode active material having a constant specific surface area and a different XRD peak intensity ratio and a binder containing polytetrafluoroethylene was prepared, and a discharge capacity after a charge / discharge test. Was measured. First, the mixing ratio of raw materials at the time of synthesis, firing temperature, atmosphere (oxygen concentration, dew point, CO₂By changing the content, etc., the peak intensity ratio (I₀₀₆+ I₁₀₂) / I₁₀₁LiNi of different_0.81Co_0.15Al_0.04O₂LiNiO₂A positive electrode active material sample made of a compound was prepared. A battery having a positive electrode using an aqueous binder composed of an aqueous dispersion of polytetrafluoroethylene and a water-soluble cellulose resin as the positive electrode active material was produced, and the initial discharge capacity and the discharge capacity after the charge / discharge test were measured. . At this time, the positive electrode active material has a BET specific surface area of 0.65 m.²/ G. In addition, a lithium battery using an organic solvent-based binder composed of PVDF and NMP was also produced using each positive electrode active material. In addition, the measurement method as described in the below-mentioned Example was used for the measurement of initial stage discharge capacity and cycling characteristics.
[0039]
[Table 2]

[0040]
From Table 2, the battery using the water-based binder has an initial discharge capacity of 158 (mAh / g) equivalent to the battery using the organic solvent-based binder regardless of the peak intensity ratio. . The discharge capacity ratio after the charge / discharge test is the peak intensity ratio (I₀₀₆+ I₁₀₂) / I₁₀₁It can be seen that a battery using an active material having an A of 0.421 or less is superior to a battery using an organic solvent binder.
[0041]
The positive electrode active material has the formula Li_aMn_bMe_cO_Four(Me is at least one element selected from the group consisting of Co, Cu, Fe, Ni, Zn, Al, Cr, Ti, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.35, a + b + c = LiMnO represented by 3)_FourIt is preferable to have a system compound. This LiMnO_FourThe charge / discharge curve of the lithium secondary battery can be adjusted by blending the system compound into the positive electrode active material. As a result, lithium secondary batteries having different charge / discharge characteristics can be obtained, and the use of the lithium secondary battery can be expanded. Specifically, LiMn₂O_FourDispersion in the charge / discharge curve of the positive electrode active material is caused by blending a compound based on LiMn.₂O_FourThe amount of deviation changes by changing the compounding amount of the system compound. When the charge / discharge curves are different, the charge / discharge capacity from the same potential changes. As a result, the charge / discharge characteristics of the lithium secondary battery change. Where LiMn₂O_FourA system compound can be mix | blended with a positive electrode active material in arbitrary ratios.
[0042]
As the electrolytic solution, an electrolytic solution used for a normal lithium secondary battery can be used. Examples of the electrolyte include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrafuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate, and ethyl. For example, LiCF can be used alone or in a mixed solvent of two or more kinds such as methyl carbonate._ThreeSO_Three, LiC_FourF₉SO_ThreeLiClO_Four, LiPF₆, LiBF_Four, LiN (CF_ThreeSO₂) (CF_ThreeSO₂), LiN (C_FourF₉SO₂) (CF_ThreeSO₂), LiN (C₂F_FiveSO₂) (C₂F_FiveSO₂An organic solvent-based electrolyte prepared by dissolving an electrolyte such as) alone or by dissolving two or more of them can be used.
[0043]
The lithium secondary battery of the present invention can be in a form used for a normal lithium secondary battery except for the positive electrode active material, the binder, and the electrolytic solution. Further, the structure of the lithium secondary battery of the present invention is not particularly limited, and a laminated type having a laminated electrode body in which positive and negative electrodes are formed in a sheet shape and are alternately laminated via separators. The electrode battery may be a wound electrode battery having a wound electrode body in which a sheet-like positive electrode and a negative electrode are wound through a separator.
[0044]
The sheet-like positive electrode and negative electrode are preferably formed by applying a mixture obtained by mixing an active material, a conductive material, a binder and the like to a current collector.
[0045]
As a negative electrode active material, a current collector, and a separator that can be used in the lithium secondary battery of the present invention, for example, those shown below can be used.
[0046]
Examples of the negative electrode active material include carbon materials capable of occluding and releasing lithium, such as flaky graphite, spherical graphite, and amorphous carbon, or lithium metal.
[0047]
As the positive electrode current collector, for example, aluminum or stainless steel, and as the negative electrode current collector, for example, copper, nickel, or the like is used as a net, punched metal, foam metal, or foil processed into a plate shape. it can.
[0048]
As the separator, for example, a microporous polypropylene film or a microporous polyethylene film having a thickness of 10 to 50 (μm) and a porosity of 30 to 70% can be used.
[0049]
The lithium secondary battery of the present invention reduces the area involved in the reaction by reducing the specific surface area of the surface of the positive electrode active material, and suppresses the decrease in battery capacity due to the reaction with water. In addition, a water-soluble polymer with excellent organic solvent resistance is used as the binder, and the surface of the positive electrode active material is uniformly coated, so that an increase in internal resistance is suppressed, and swelling and dissolution in the electrolyte solution are prevented. It does not occur, and even when the electrode active material expands / shrinks during charging / discharging of the battery, the active material can be kept firmly bound. For this reason, it is a battery excellent in the charge / discharge cycle.
[0050]
  (Manufacturing method of positive electrode for lithium secondary battery) The manufacturing method of the positive electrode for lithium secondary battery of the present invention has the formula Li_xNi_1-yM_yO₂(M is at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, Fe, 0 <x ≦ 1.2, 0 <y ≦ 0.25), and BET specific surface area Is 0.65m²/ G or lessAnd peak intensity ratio (I) of (006) plane, (102) plane and (101) plane in the X-ray powder analysis measurement result ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ Is 0.421 or lessLiNiO₂A positive electrode active material having an organic compound, a binder having a non-swelling water-soluble polymer with respect to an electrolyte used in a lithium secondary battery, and a resin that imparts flexibility to the positive electrode, water, And a paste adjusting step of adjusting the paste-like active material mixture, and an application step of applying the paste-like active material mixture to the current collector.
[0051]
The manufacturing method of the positive electrode for lithium secondary batteries of this invention has a paste adjustment process which mixes a positive electrode active material, a binder, and water, and adjusts a paste-form active material mixture. In this paste adjusting step, water is used as a medium because a water-soluble polymer that is non-swelling with respect to the electrolyte used in the lithium secondary battery is used as the binder. As a result, since an organic solvent is not used, the cost required for manufacturing the battery can be reduced. Further, the paste preparation step mixes the positive electrode active material, the binder, and water to adjust the paste-like active material mixture, thereby forming a film made of the binder on the surface of the positive electrode active material. be able to.
[0052]
The paste adjustment step is a step of adjusting a paste-like active material mixture in which a positive electrode active material, a binder, and water are mixed, and any method can be used as long as the mixture can be adjusted. You may adjust with.
[0053]
The positive electrode active material has the formula Li_xNi_1-yM_yO₂(M is a compound represented by at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, Fe, 0 <x ≦ 1.2, 0 <y ≦ 0.25) .
[0054]
LiNiO₂-Based compounds have a BET specific surface area of 0.65 m²/ G or less. LiNiO₂-Based BET specific surface area of 0.65m²/ NigO or less, LiNiO₂This is probably because the ion exchange reaction between protons and lithium due to moisture on the surface of the compound is suppressed. Also, BET specific surface area is 0.65m²When it exceeds / g, it reacts with water and the initial discharge capacity decreases.
[0055]
The binder has a water-soluble polymer that is non-swellable with respect to the electrolyte used in the lithium secondary battery. Since the binder has a water-soluble polymer that is non-swellable with respect to the electrolyte, the water-soluble polymer can uniformly coat the surface of the positive electrode active material in the paste preparation process, and the manufactured positive electrode can be used. In the lithium secondary battery, the reaction between the positive electrode active material and the electrolytic solution can be suppressed.
[0056]
That is, since the binder has a water-soluble polymer, it is uniformly dissolved in water and the positive electrode active material is dispersed in this solution in the paste preparation process, so that the positive electrode active material uniformly coated with the water-soluble polymer is dispersed. The substance becomes available.
[0057]
In addition, since the water-soluble polymer film formed on the surface of the positive electrode active material always comes into contact with the electrolyte when the battery is formed, the water-soluble polymer does not swell in the electrolyte. A water-soluble polymer is used. Further, the water-soluble polymer does not dissolve in the electrolytic solution. That is, the film formed on the surface of the positive electrode active material by the water-soluble polymer is retained by not being dissolved in the electrolytic solution.
[0058]
Since such a binder has excellent organic solvent resistance, it does not swell or dissolve due to the electrolyte of the lithium battery, and the active material is strong even if the active material expands or contracts due to charge / discharge. Therefore, good cycle characteristics can be produced in the lithium battery. Also, the binder may act as a thickener due to its own viscosity.
[0059]
Such water-soluble polymers include celluloses such as methylcellulose (MC), carboxymethylcellulose (CMC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), hydroxyethylmethylcellulose (HEMC), Examples thereof include polyacrylate.
[0060]
The water-soluble polymer is more preferably water-soluble cellulose.
[0061]
The binder preferably has a resin that imparts flexibility to the positive electrode. That is, when the binder includes a resin that imparts flexibility to the positive electrode, the positive electrode has flexibility, so that the positive electrode can be easily handled during production. Specifically, it is possible to bind a positive electrode active material, a conductive agent and a current collector using only a water-soluble polymer as a binder, but the produced positive electrode has poor flexibility, so This is because when the sheet-like positive electrode is wound, the active material layer is likely to be damaged, such as cracking or dropping off of the active material layer.
[0062]
Moreover, it is preferable that the resin which imparts flexibility to the positive electrode is excellent in resistance to electrolytic solution. That is, since the resin that imparts flexibility to the positive electrode is used as a binder, if the electrolyte swells or dissolves, the coating formed by the water-soluble polymer on the surface of the positive electrode active material is damaged. This is because
[0063]
Examples of the resin imparting flexibility to the positive electrode include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). ), Fluoropolymers such as tetrafluoroethylene-ethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), olefins such as acrylic resin, polyethylene, and polyolefin, A resin such as a polyimide resin can be used.
[0064]
The resin imparting flexibility to the positive electrode is preferably a fluororesin. That is, since the fluororesin does not swell or dissolve in the electrolytic solution, the battery characteristics of the battery used as the binder can be improved. Here, in the present invention, it is preferable to use a fluorine-based resin for the binder, but it does not exclude the use of a resin or a polymer material other than the fluorine-based resin for the binder.
[0065]
As the fluororesin, polytetrafluoroethylene is more preferable.
[0066]
As the electrolytic solution used for the lithium secondary battery in which the binder exhibits non-swellability, an electrolytic solution used for a normal lithium secondary battery can be used. Examples of the electrolyte include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrafuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate, and ethyl. For example, LiCF can be used alone or in a mixed solvent of two or more kinds such as methyl carbonate._ThreeSO_Three, LiC_FourF₉SO_ThreeLiClO_Four, LiPF₆, LiBF_Four, LiN (CF_ThreeSO₂) (CF_ThreeSO₂), LiN (C_FourF₉SO₂) (CF_ThreeSO₂), LiN (C₂F_FiveSO₂) (C₂F_FiveSO₂An organic solvent-based electrolyte prepared by dissolving an electrolyte such as) alone or by dissolving two or more of them can be used.
[0067]
The positive electrode active material has the formula Li_aMn_bMe_cO_Four(Me is at least one element selected from the group consisting of Co, Cu, Fe, Ni, Zn, Al, Cr, Ti, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.35, a + b + c = LiMn represented by 3)₂O_FourIt is preferable to have a system compound. LiMn as positive electrode active material₂O_FourThe battery characteristic of a lithium secondary battery can be changed by having a system compound.
[0068]
The application step is a step of applying the paste-like active material mixture to the current collector. The pasty active material mixture can be applied to the current collector by a method used for applying a normal pasty active material mixture. Examples of the coating method include coating methods such as a die coater, a comma coater, a reverse roller, and a doctor blade.
[0069]
The coating step preferably includes a drying step of drying the pasty active material mixture applied to the current collector. By drying the paste-like active material mixture applied to the current collector, the paste-like active material mixture flows on the surface of the current collector in subsequent handling, so that the coating thickness does not change.
[0070]
It is preferable that the manufacturing method of the positive electrode for lithium secondary batteries of this invention has a crimping | compression-bonding process which presses the electrical power collector with which the paste-form active material mixture was apply | coated. That is, by pressing the paste-like active material mixture applied to the surface of the current collector, it is pressed against the current collector and the weight energy density of the positive electrode is improved. In this crimping step, it is preferable to press the paste-like active material mixture that has been subjected to the drying step.
[0071]
The positive electrode manufactured by the manufacturing method of the positive electrode for lithium secondary batteries of this invention can be used for a normal lithium secondary battery. That is, in the lithium secondary battery using the positive electrode manufactured by the manufacturing method of the present invention, the same negative electrode, separator, etc. as those used in the conventional lithium secondary battery can be used. it can. Moreover, the manufacturing method of the lithium secondary battery at this time can use the same method as the manufacturing method of the conventional lithium secondary battery.
[0072]
The method for producing a positive electrode for a lithium secondary battery of the present invention uses a positive electrode active material having a reduced specific surface area, thereby reducing the area involved in the reaction and suppressing a decrease in battery capacity due to a reaction with water. . In addition, a water-soluble polymer with excellent organic solvent resistance is used as the binder, and the surface of the positive electrode active material is uniformly coated, so that an increase in internal resistance is suppressed, and swelling and dissolution in the electrolyte solution are prevented. It does not occur, and even when the electrode active material expands / shrinks during charging / discharging of the battery, the active material can be kept firmly bound. For this reason, the lithium secondary battery using the positive electrode manufactured by the manufacturing method of this invention is a battery excellent in the charge / discharge cycle.
[0073]
Furthermore, since a water-soluble polymer having excellent organic solvent resistance is used as the binder for the positive electrode, the positive electrode can be produced without using an organic solvent. For this reason, it becomes possible to reduce the cost in safety management, the material cost of the organic solvent itself, etc. required for the use of the organic solvent, and the positive electrode can be manufactured safely and inexpensively.
[0074]
【Example】
Hereinafter, the present invention will be described using examples.
[0075]
As an example of the present invention, a lithium secondary battery was produced.
[0076]
In addition, the BET specific surface area and XRD of the positive electrode active material used in the examples were measured by the following measuring methods.
[0077]
(BET specific surface area)
A BET specific surface area (hereinafter referred to as a nitrogen BET specific surface area) was measured by a nitrogen adsorption method using Shimadzu Corporation model: Flow Soap II2300.
[0078]
(XRD)
Made by Rigaku Corporation, model: RINT2000, X-ray source: CuKα₁Tube voltage: 50 (kV), tube current: 100 (mA), divergence slit: 1/2 (deg), scattering slit: 1/2 (deg), light receiving slit: 0.15 (mm), scanning mode D: Continuous, scanning range: The diffraction intensity was measured at 15 ° to 75 °.
[0079]
The data was subjected to background removal with a curvature of 5.0 using a RISM qualitative analysis program, and (Kα₁/ Kα₂) Intensity ratio to 0.5 and Kα₂Removed the effect of.
[0080]
The X-ray diffraction intensity corresponding to each index plane is read from the data obtained here, and LiNiO₂The 006 plane, 102 plane, and 101 plane, which serve as a measure of crystallinity in the positive electrode active material, also have peak intensity ratios: (I₀₀₆+ I₁₀₂) / I₁₀₁Asked.
[0081]
Example 1
Example 1 is the cylindrical lithium secondary battery 100 shown in FIG.
[0082]
The cylindrical lithium secondary battery 100 includes a positive electrode 1 having a positive electrode active material containing lithium, releasing lithium as lithium ions at the time of charging, and occluding lithium ions at the time of discharging, and a negative electrode active material made of a carbon material. A negative electrode 2 having a substance, capable of occluding lithium ions at the time of charging and releasing lithium ions at the time of discharging, and a nonaqueous electrolytic solution 3 in which a supporting salt containing lithium in an organic solvent is dissolved is provided. Next battery.
[0083]
The positive electrode 1 includes a positive electrode current collector 11, a positive electrode mixture layer 12 having a positive electrode active material and a binder formed on the surface of the positive electrode current collector 11, and a positive electrode bonded to the positive electrode current collector 11. An electrode comprising a current collecting lead 13.
[0084]
The positive electrode active material is LiNi_0.81Co_0.15Al_0.04O₂Nitrogen BET specific surface area is 0.65m²/ G, peak intensity ratio by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁Is a lithium nickel oxide having a ratio of 0.421, and the binder is a mixture of an aqueous carboxymethylcellulose sodium salt solution and PTFE.
[0085]
The negative electrode 2 includes a negative electrode current collector 21, a negative electrode mixture layer 22 having a negative electrode active material formed on the surface of the negative electrode current collector 21, and a negative electrode current collector lead 23 bonded to the negative electrode current collector 21. It is an electrode consisting of.
[0086]
Further, the positive electrode 1 and the negative electrode 2 are held in the case 7 while being wound around the separator 4. The current collecting leads 13 and 23 of the positive electrode 1 and the negative electrode 2 are connected to the positive electrode terminal portion 5 and the negative electrode terminal portion 6 of the case, respectively.
[0087]
The separator is a porous film made of polyethylene having a thickness of 25 μm, and 1 mol / liter as an electrolyte in a solvent obtained by mixing ethylene carbonate (EC) and diethylene carbonate (DEC) in a volume ratio of 30:70 as an electrolyte. LiPF₆What was dissolved was used.
[0088]
The lithium secondary battery of Example 1 was produced by the following procedure.
[0089]
(Manufacture of positive electrode)
First, 87 wt% of the positive electrode active material, 10 wt% of acetylene black (product number: HS-100) as a conductive agent, and 2 wt% of a carboxymethyl cellulose sodium salt aqueous solution as a binder that also functions as a thickener are solid content of sodium carboxymethyl cellulose. Are mixed so as to be 2 wt%, and stirred for 1 hour with a biaxial stirrer. Thereafter, an aqueous PTFE dispersion having a solid content ratio of about 50% is added as a binder so that the solid content of PTFE is 1 wt%, and the mixture is stirred for 30 minutes using a vacuum emulsification stirrer. The paste thus obtained was applied on an aluminum foil with a comma coater to a basis weight of 8.56 (mg / cm per side).²) On both sides.
[0090]
Next, this electrode was passed through a roll press machine, a load of linear pressure 0.74 (ton / cm) was applied, and the electrode density was 2.41 (g / cm²). Then, this electrode was cut into a width of 5.4 (cm) and a length of 86 (mm), and an electrode mixture for a length of 2.5 (cm) was scraped off as a lead tab weld for extracting current. The effective reaction area of this electrode is 5.4 (cm) × 83.5 (cm) × 2 = 901.8 (cm²)Met.
[0091]
(Manufacture of negative electrode)
Using 92.5 wt% of scaly graphite as a negative electrode active material and 7.5 wt% of PVDF as a binder, a paste was prepared by dispersing graphite in a solution of PVDF dissolved in N-methyl-2-pyrrolidone. Similarly, using a comma coater, the basis weight per side on the copper foil is 5.1 (mg / cm²The paste was applied on both sides so that Then, a roll press machine is used to apply a load of linear pressure 0.25 (ton / cm), and the electrode density is 1.25 (g / cm).²The electrode was raised to). Next, this electrode was cut into a width of 5.6 (cm) and a length of 90.5 (cm), and an electrode mixture for a length of 0.5 (cm) was scraped off as a Riettab weld for extracting current. It was. The effective reaction area of this electrode is 5.6 (cm) × 90 (cm) × 2 = 1008 (cm²)Met.
[0092]
(Battery assembly)
The positive electrode and the negative electrode obtained above were wound with a separator interposed therebetween to form a wound electrode, and this electrode was inserted into the case. At this time, the current collecting lead was joined to the positive terminal and the negative terminal of the case.
[0093]
Thereafter, an electrolytic solution was injected into the case, and the case was closed.
[0094]
The cylindrical lithium secondary battery of Example 1 was manufactured by the above procedure.
[0095]
(Comparative Example 1)
Comparative Example 1 is a cylindrical lithium secondary battery similar to Example 1 except that polyvinylidene fluoride (PVDF) is used as the positive electrode binder. Here, when using PVDF as the binder, the positive electrode was prepared by dissolving PVDF in N-methyl-2-pyrrolidone and dispersing a positive electrode active material and a conductive agent in this solution. Manufactured by applying to foil.
[0096]
(Example 2)
In Example 2, the positive electrode active material had a nitrogen BET specific surface area of 0.55 (m²/ G) is the same cylindrical lithium secondary battery as in Example 1.
[0097]
(Comparative Example 2)
In Comparative Example 2, the positive electrode active material had a nitrogen BET specific surface area of 0.83 (m²/ G) is the same cylindrical lithium secondary battery as in Example 1.
[0098]
(Comparative Example 3)
In Comparative Example 3, the positive electrode active material had a nitrogen BET specific surface area of 1.04 (m²/ G) is the same cylindrical lithium secondary battery as in Example 1.
[0099]
(Comparative Example 4)
Comparative Example 4 is a cylindrical lithium secondary battery similar to Comparative Example 3 except that PVDF is used as the positive electrode binder.
[0100]
(Example 3)
In Example 3, the peak intensity ratio of the positive electrode active material by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁Is a cylindrical lithium secondary battery similar to Example 1, except that is 0.412.
[0101]
(Comparative Example 5)
In Comparative Example 5, the positive electrode active material XRD peak intensity ratio: (I₀₀₆+ I₁₀₂) / I₁₀₁This is a cylindrical lithium secondary battery similar to that of Example 1 except that a battery having a value of 0.449 is used.
[0102]
(Comparative Example 6)
Comparative Example 6 is a cylindrical lithium secondary battery similar to Comparative Example 5 except that PVDF is used as the positive electrode binder.
[0103]
Example 4
In Example 4, the positive electrode active material had a nitrogen BET specific surface area of 0.55 (m²/ G) and the peak intensity ratio by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁Is a cylindrical lithium secondary battery similar to Example 1, except that is 0.412.
[0104]
(Example 5)
Example 5 is a cylindrical lithium secondary battery similar to Example 1 except that hydroxyethyl cellulose (HEC) was used instead of CMC as the binder for the positive electrode. In Example 5, the positive electrode was produced in the same manner as in Example 1 except that HEC was used instead of CMC.
[0105]
(Example 6)
Example 6 is a cylindrical lithium secondary battery similar to Example 1 except that hydroxypropylmethylcellulose (HPMC) is used instead of CMC as the binder for the positive electrode. In Example 6, the positive electrode was manufactured in the same manner as Example 1 except that HPMC was used instead of CMC.
[0106]
(Example 7)
Example 7 is a cylindrical lithium secondary battery similar to Example 1 except that hydroxyethyl methylcellulose (HEMC) is used in place of CMC as the binder for the positive electrode. In Example 7, the positive electrode was produced in the same manner as in Example 1 except that HEMC was used instead of CMC.
[0107]
(Example 8)
Example 8 is a cylindrical lithium secondary battery similar to Example 1 except that tetrafluoroethylene-hexafluoropropylene copolymer (FEP) was used as the positive electrode binder instead of PTFE. In Example 8, the production of the positive electrode was performed in the same manner as in Example 1 except that the FEP aqueous dispersion was used instead of the PTFE aqueous dispersion.
[0108]
Example 9
Example 9 is a cylindrical lithium secondary battery similar to Example 1 except that tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) is used as the positive electrode binder instead of PTFE. In Example 9, the positive electrode was produced in the same manner as in Example 1 except that a PFA aqueous dispersion was used instead of the PTFE aqueous dispersion.
[0109]
(Example 10)
Example 10 is a cylindrical lithium battery similar to Example 1 except that an acrylic emulsion resin was used instead of PTFE as the positive electrode binder. In Example 10, the positive electrode was produced in the same manner as in Example 1 except that an acrylic emulsion resin was used instead of PTFE.
[0110]
(Example 11)
In Example 11, the positive electrode active material was LiNi_0.80Co_0.15Al_0.05O₂And the nitrogen BET specific surface area is 0.65 (m²/ G), peak intensity ratio by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁This is a cylindrical lithium secondary battery similar to that of Example 1 except that a battery having a value of 0.421 is used.
[0111]
(Example 12)
In Example 12, the positive electrode active material was LiNi_0.90Co_0.06Al_0.04O₂And the nitrogen BET specific surface area is 0.65 (m²/ G), peak intensity ratio by XRD: (I_{0 06}+ I₁₀₂) / I₁₀₁This is a cylindrical lithium secondary battery similar to that of Example 1 except that a battery having a value of 0.421 is used.
[0112]
(Example 13)
In Example 13, the positive electrode active material was LiNi_0.75Co_0.21Al_0.04O₂And the nitrogen BET specific surface area is 0.65 (m²/ G), peak intensity ratio by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁This is a cylindrical lithium secondary battery similar to that of Example 1 except that a battery having a value of 0.421 is used.
[0113]
(Example 14)
In Example 14, the positive electrode active material was LiNi._0.81Co_0.15Al_0.04O₂And Li_1.12Mn_1.88O_FourThis is a cylindrical lithium secondary battery similar to that in Example 1 except that the above mixture was used.
[0114]
For details, see LiNi_0.81Co_0.15Al_0.04O₂Has a nitrogen BET specific surface area of 0.65 (m²/ G), peak intensity ratio by XRD: (I₀₀₆+ I₁₀₂) / I₁₀₁Is 0.421 and Li_1.12Mn_1.88O_FourHas a nitrogen BET specific surface area of 0.5 (m²/ G). Moreover, these mixing ratios are LiNi by weight ratio._0.81Co_0.15Al_0.04O₂: Li_1.12Mn_1.88O_FourWas 60:27.
[0115]
(Example 15)
Example 15 is LiNi_0.81Co_0.15Al_0.04O₂And Li_1.12Mn_1.88O_FourThis is a cylindrical lithium secondary battery similar to that of Example 14 except that the mixing ratio is 27:60 by weight.
[0116]
(Comparative Example 7)
Comparative Example 7 is a cylindrical lithium secondary battery similar to Example 14 except that PVDF is used as the positive electrode binder.
[0117]
(Comparative Example 8)
Comparative Example 8 is a cylindrical lithium secondary battery similar to Example 15 except that PVDF is used as the positive electrode binder.
[0118]
(Evaluation)
As an evaluation of the lithium secondary batteries of Examples and Comparative Examples, initial discharge capacity, high temperature cycle characteristics, and internal resistance increase rate were measured. The measurement results are shown in Table 3. These measurement methods were as follows. In addition, the batteries of Comparative Examples 3 to 6 measured only the initial discharge capacity.
[0119]
(Initial discharge capacity)
First, CC-CV charge was performed to 4.1 (V) with the charging current 250 (mA), and CC discharge was performed to 3.0 (V) with the discharge current 333 (mA).
[0120]
Next, CC-CV charge is performed up to 4.1 (V) at a charge current of 1000 (mA), and CC discharge is performed four times up to 3.0 (V) at a discharge current of 1000 (mA). ) To CC (CV) up to 4.1 (V), and CC discharge to 3.0 (V) at a discharge current of 333 (mA).
[0121]
The discharge capacity at this time was defined as the initial capacity of the battery, and comparison was made using a value normalized by the weight of the positive electrode active material filled in the battery. The measurement was performed in an atmosphere at 25 ° C.
[0122]
(High temperature cycle characteristics)
The lithium secondary battery after the initial capacity measurement is placed in a thermostatic chamber having an atmospheric temperature of 60 ° C., CC charged to 4.1 (V) with a charging current 1984 (mA), and 3.0 with a discharging current 1984 (mA). The cycle of performing CC charging up to (V) was repeated 500 times, and the ratio between the discharge capacity at the first time and the 500th cycle was compared.
[0123]
(Internal resistance)
First, CC-CV charging was performed in 1.5 hours to 3.750 (V) at a charging current of 1000 (mA). Then, discharge at 300 (mA) for 10 seconds, charge at 300 (mA) for 10 seconds, discharge at 900 (mA) for 10 seconds, charge at 900 (mA) for 10 seconds, discharge at 2700 (mA) for 10 seconds, 2700 (mA ) For 10 seconds, 5400 (mA) for 10 seconds, 5400 (mA) for 10 seconds, 8100 (mA) for 10 seconds, 8100 (mA) for 10 seconds, after each discharge, after charge and discharge The internal resistance value was obtained from the slope of the approximate linear line of the current-voltage plot value with the voltage of γ as the vertical axis and the charge / discharge current as the horizontal axis.
[0124]
(Resistance increase rate)
The internal resistance increase rate is obtained by measuring the internal resistance after repeating 500 cycles of charge / discharge performed in the above high-temperature cycle characteristics, and dividing the increase in internal resistance before and after charge / discharge by the internal resistance before charge / discharge. It was.
[0125]
Specifically, {resistance increase rate (%)} = {(internal resistance after high-temperature cycle test) − (initial internal resistance)} / (initial internal resistance).
[0126]
[Table 3]

[0127]
From Table 3, it can be seen that Example 1 has a high initial discharge capacity of 158 mAh / g and a high discharge capacity ratio of 82.0%, and is a battery with little capacity deterioration. The resistance increase rate was 1.7%, showing little increase in internal resistance.
[0128]
In Comparative Example 1, the initial discharge capacity is sufficiently high, but the discharge capacity ratio is reduced to 75.0%. That is, since PVDF is used as the binder, the binding force of the positive electrode active material is reduced due to swelling and dissolution with the electrolytic solution, and the discharge capacity ratio is reduced. In addition, the binder is not uniformly coated on the surface of the positive electrode active material, or a gap is generated in the binder due to swelling by the electrolyte during charging and discharging, and the decomposition reaction of the electrolyte is promoted, so that insulation such as lithium fluoride can be achieved. As a result, the rate of increase in resistance is as high as 24% after the high-temperature cycle test.
[0129]
Example 2 is a battery in which the specific surface area of the positive electrode active material is reduced, and the initial discharge capacity is 160 mAh / g and the discharge capacity ratio is 82.3%. Also, the rate of increase in resistance is almost unchanged at -1.1%.
[0130]
In Comparative Example 3, since the nitrogen BET specific surface area is large, the initial discharge capacity is reduced to 138 mAh / g. Also, the resistance increase rate was -4.7%, showing almost no change in internal resistance.
[0131]
In Comparative Example 4, since PVDF was used as the binder, even when the nitrogen BET specific surface area was increased, the initial capacity was not reduced. However, since the binder is PVDF, the discharge capacity ratio is greatly reduced to 74.7%. That is, PVDF as a binder causes swelling, dissolution, and the like with the electrolytic solution, the binding force of the positive electrode active material is reduced, and the discharge capacity ratio is reduced. Further, since the binder is only PVDF, the resistance increase rate was considerably increased to 75.3% as in Comparative Example 1.
[0132]
In Example 3, the initial discharge capacity is as high as 158 mAh / g, and the discharge capacity ratio is as high as 85.0%. That is, in Example 3, since the crystallinity of the positive electrode active material is increased, the deterioration of the cycle characteristics is suppressed. Also, the resistance increase rate is almost unchanged at 0.6%.
[0133]
In Comparative Example 5, the initial discharge capacity is as high as 158 mAh / g, but the discharge capacity ratio after the cycle test is reduced to 65.2%. That is, since the crystallinity of the positive electrode active material is low, the cycle characteristics are degraded. Further, the resistance increase rate is hardly changed to -0.5%.
[0134]
Since Comparative Example 6 uses PVDF as the binder for the positive electrode, the initial discharge capacity is as high as 160 mAh / g, but the discharge capacity ratio after the cycle test is reduced to 72.0%. ing. Further, since the binder was only PVDF, the resistance increase rate was considerably increased to 79.2% as in Comparative Example 1.
[0135]
In Example 4, since the specific surface area of the positive electrode active material was reduced and the crystallinity was increased, the initial discharge capacity was 160 mAh / g and the discharge capacity ratio was 85.1%, both showing high values. Also, the rate of increase in resistance is almost unchanged at 1.8%.
[0136]
In Example 5, the initial discharge capacity is 158 mAh / g and the discharge capacity ratio is 81.1%, both of which are high values. Further, even when HEC is used as the binder for the positive electrode, the rate of increase in resistance is suppressed to a sufficiently low 8.2%.
[0137]
In Example 6, the initial discharge capacity was 158 mAh / g and the discharge capacity ratio was 80.7%, both showing high values. Further, even when HPMC is used as the binder for the positive electrode, the rate of increase in resistance is suppressed to a sufficiently low value of 7.9%.
[0138]
In Example 7, the initial discharge capacity was 158 mAh / g, and the discharge capacity ratio was 80.6%, both showing high values. Moreover, even when HEMC is used as the binder for the positive electrode, the rate of increase in resistance is sufficiently low at 2.6%.
[0139]
In Example 8, the FEP binder was used, but the initial discharge capacity was 158 mAh / g, and the discharge capacity ratio was 81.5%, showing high values. Also, the rate of increase in resistance is almost unchanged at + 0.8%.
[0140]
In Example 9, a PFA binder was used, but the initial discharge capacity was 158 mAh / g, and the discharge capacity ratio was 81.7%, showing high values. Also, the rate of increase in resistance is almost unchanged at + 2.1%.
[0141]
In Example 10, the initial discharge capacity was 158 mAh / g and the discharge capacity ratio was 80.2%, both showing high values. Further, even when an acrylic resin is used as the binder for the positive electrode, the resistance increase rate is almost unchanged at 1.6%.
[0142]
Examples 11 to 13 are batteries in which the composition of the positive electrode active material was changed. The initial discharge capacity and the discharge capacity ratio were sufficiently high, and the resistance increase rate was suppressed to + 2.0% or less.
[0143]
Examples 14 to 15 are batteries using a mixture in which lithium nickel oxide and lithium manganese oxide are mixed as a positive electrode active material, and PTFE / CMC as a binder for the positive electrode. In Example 14 in which this manganese oxide was mixed at a ratio of 27/87, the initial capacity and discharge capacity ratio showed sufficiently high values compared with Comparative Example 7 in which the positive electrode binder was PVDF. . Further, in Example 15 in which manganese oxide was mixed at a ratio of 60/87, the initial capacity and the discharge capacity ratio showed sufficiently high values as compared with Comparative Example 8 in which the positive electrode binder was PVDF. Yes.
[0144]
From the examples and comparative examples, by using a water-soluble polymer as the binder for the positive electrode and using a positive electrode active material made of lithium nickel oxide having a small specific surface area and high crystallinity, the initial discharge capacity, cycle A lithium secondary battery excellent in characteristics and resistance increase rate can be obtained.
[0145]
【The invention's effect】
The lithium secondary battery of the present invention reduces the specific surface area of the positive electrode active material, thereby reducing the area involved in the reaction and suppressing the decrease in battery capacity due to the reaction with moisture. In addition, the binder has a water-soluble polymer with excellent organic solvent resistance, and the water-soluble polymer uniformly covers the surface of the positive electrode active material, thereby increasing the internal resistance and decreasing the discharge capacity. Is suppressed, and the battery has an excellent charge / discharge cycle.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a battery manufactured in an example.
[Explanation of symbols]
100 ... Cylindrical lithium secondary battery
DESCRIPTION OF SYMBOLS 1 ... Positive electrode 11 ... Positive electrode electrical power collector
12 ... Positive electrode mixture layer 13 ... Positive electrode current collector lead
2 ... Negative electrode 21 ... Negative electrode current collector
22 ... Negative electrode mixture layer 23 ... Negative electrode current collector lead
3 ... Non-aqueous electrolyte 4 ... Separator
5 ... Positive terminal 6 ... Negative terminal
7 ... Case

Claims (10)

At least one element has the formula _{Li x Ni 1-y M y} O 2 (M Co, Mn, Al, B, Ti, Mg, selected from the group consisting of Fe, 0 <x ≦ 1.2,0 < y ≦ In a lithium secondary battery having a positive electrode active material having a LiNiO ₂ -based compound represented by 0.25), a positive electrode including a binder that binds the positive electrode active material, and an electrolyte solution,
The LiNiO ₂ compound has a BET specific surface area of 0.65 m ² / g or less, and the LiNiO ₂ compound has peaks on the (006) plane, the (102) plane, and the (101) plane in the X-ray powder analysis measurement results. The strength ratio (I ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ is 0.421 or less, and the binder imparts flexibility to the electrolyte and the non-swellable water-soluble polymer and the positive electrode. A lithium secondary battery, wherein the water-soluble polymer covers the surface of the positive electrode active material.   The lithium secondary battery according to claim 1, wherein the water-soluble polymer is water-soluble cellulose.   The lithium secondary battery according to claim 1, wherein the resin imparting flexibility to the positive electrode is a fluororesin.   The lithium secondary battery according to claim 3, wherein the fluororesin is polytetrafluoroethylene. The positive electrode active material has the formula Li _a Mn _b Me _c O ₄ (Me is at least one element selected from the group consisting of Co, Cu, Fe, Ni, Zn, Al, Cr, Ti, Mg and V, 1 ≦ 5. The lithium secondary battery according to claim 1, comprising a LiMn 2 O 4 -based compound represented by a ≦ 1.15, 0 ≦ c ≦ 0.35, a + b + c = 3). At least one element has the formula _{Li x Ni 1-y M y} O 2 (M Co, Mn, Al, B, Ti, Mg, selected from the group consisting of Fe, 0 <x ≦ 1.2,0 < y ≦ 0.25), the BET specific surface area is 0.65 m ² / g or less, and the peak intensity ratio (I) of the (006) plane, (102) plane and (101) plane in the X-ray powder analysis measurement results ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ is 0.421 or less, a positive electrode active material having a LiNiO ₂ -based compound, a non-swelling water-soluble polymer with respect to an electrolyte used in a lithium secondary battery, and the positive electrode A paste adjusting step of adjusting a paste-like active material mixture by mixing a binder having a resin imparting flexibility and water;
And a coating step of applying the paste-like active material mixture to a current collector.   The method for producing a positive electrode of a lithium secondary battery according to claim 6, wherein the water-soluble polymer is water-soluble cellulose.   The method for producing a positive electrode of a lithium secondary battery according to claim 6, wherein the resin imparting flexibility to the positive electrode is a fluorine-based resin. The method for producing a positive electrode of a lithium secondary battery according to claim 8 , wherein the fluororesin is polytetrafluoroethylene. The positive electrode active material has the formula Li _a Mn _b Me _c O ₄ (Me is at least one element selected from the group consisting of Co, Cu, Fe, Ni, Zn, Al, Cr, Ti, Mg and V, 1 ≦ a ≦ 1.15,0 ≦ c ≦ 0.35, a + b + c = 3) in the manufacturing method of the positive electrode of a lithium secondary battery according to claim 6-9, further comprising a LiMn ₂ O ₄ compound represented.

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