JP3954455B2 - Electrode plate for lithium secondary battery and lithium secondary battery - Google Patents

Description

【００２３】

【００２４】
表１および図１に示すように、ＮＵ−１２−２−０６は、６０℃以下の温度領域においてほぼ一定の電気抵抗を有しており、通常使用される温度領域では、極板抵抗の安定した、より効率に優れた電池とすることができる。一方、リチウム二次電池が最終的に発火に至る恐れがある１８０℃以上になると大きく電気抵抗が増加しており、温度が通常の使用温度領域を超えて上昇した場合に、より確実に電気抵抗を増大できることがわかる。
【００２５】
一方、従来のアセチレンブラックでは、温度上昇に伴い電気抵抗が上昇する傾向はみられず、逆に、緩やかに電気抵抗が減少している。
【００２６】
なお、表１、表２および図１に示した体積抵抗率は、以下のように測定した。
【００２７】
上記炭素材料（新日化カーボン製ＮＵ−１２−２−０６およびアセチレンブラック）に１．９６×１０⁴Ｎ（２０００ｋｇｆ）の圧力をかけて加圧充填し、昇温しながら４端子法を用いて、炭素材料の体積抵抗率を測定した。
【００２８】
本発明のリチウム二次電池用極板は、導電性を有し、活物質が機能すれば（即ち、リチウムイオンを可逆的に吸蔵および放出できれば）よい。例えば、活物質と、上記炭素材料を含む導電剤と、結着剤とを混合して得た電極合剤を、集電体に塗布することで作製することができる。
【００２９】
上記炭素材料の形態は、特に制限されないが、電極合剤の作製が容易であることなどから粉末状であることが好ましい。
【００３０】
また、上記炭素材料の含まれる割合が、活物質１００重量部に対して、２重量部以上２０重量部以下であることが好ましい。上記範囲内では、さらに電池容量のサイクル特性を向上させることができる。
【００３１】
極板が正極である場合、正極活物質としては、式Ｌｉ_xＭＯ₂、または、式Ｌｉ_yＭ₂Ｏ₄（ただし、Ｍは遷移金属であり、ｘおよびｙは、０＜ｘ≦１および０＜ｙ≦２を満たす数値である）で示される組成を有するリチウム−遷移金属複合酸化物が好ましい。なかでも遷移金属として、コバルト、ニッケル、マンガン、鉄などを用いることが、電池の起電力の面から好ましい。
【００３２】
このようなリチウム−遷移金属複合酸化物は、例えば、リチウムと遷移金属との炭酸塩または酸化物を、目的とする組成に応じて混合し、酸素存在雰囲気下において焼成（６５０〜１２００℃）することなどにより得ることができる。
【００３３】
正極に用いる集電体としては、アルミニウム、ステンレス鋼、ニッケル、チタン、またはこれらの合金などからなる、フィルム、フォイル、シートなどが好ましい。なかでも、アルミニウム箔が特に好ましい。
【００３４】
極板が負極である場合、負極活物質としては、リチウムイオンを吸蔵および放出できる材料であれば特に限定されない。なかでも、炭素質材料、例えば、熱分解炭素類、コークス類（ピッチコークス、ニードルコークス、石油コークスなど）、黒鉛類、ガラス状炭素類、有機高分子化合物焼成体（フラン樹脂などの樹脂を適当な温度で焼成し炭素化したもの）、炭素繊維、活性炭、メソフェーズ小球体などを用いることが好ましい。炭素質材料を負極活物質として用いれば、充電時にもデンドライトの発生がなく、より安価で信頼性の高いリチウム二次電池とすることができる。
【００３５】
負極に用いる集電体としては、銅、ステンレス鋼、ニッケル、チタン、またはこれらの合金などからなる、フィルム、フォイル、シートなどが好ましい。なかでも、銅箔が特に好ましい。
【００３６】
結着剤としては、ポリアクリル酸エステル系のラテックス、カルボキシメチルセルロース、ポリテトラフルオロエチレン、ポリフッ化ビニリデンなどを用いることが好ましい。これらの材料は、単独で用いても、混合して用いても構わない。
【００３７】
上記炭素材料以外の導電剤成分としては、構成された電池において化学反応を起こさない導電性材料であれば、特に限定されない。なかでも、グラファイトやカーボンブラックが、その良好な導電性から特に好ましい。
【００３８】
上記炭素材料以外の導電剤成分の平均粒子径は、０．０１μｍ以上２０μｍ以下が好ましく、０．０２μｍ以上１０μｍ以下が特に好ましい。複数の導電剤成分を混合することもできる。なかでも、アセチレンブラックなどのカーボンブラック類と、平均粒子径が１μｍ〜１５μｍの範囲である黒鉛粒子とを混合することが好ましい。
【００３９】
また、本発明のリチウム二次電池用極板において、上記炭素材料が、上記炭素材料１００ｇあたり１００ｍｌ以上３００ｍｌ以下のフタル酸ジブチルを含むことができることが好ましい。フタル酸ジブチルの吸油量が上記の範囲内であれば、炭素材料のアグリゲートの凝集状態としてのストラクチャーが発達しており、より優れた電子導電性および分散性を有する炭素材料となる。これにより、リチウム二次電池に組み込まれた際に、上記炭素材料を含む導電剤がより良好に電解質を保持することが可能で、電解質からのリチウムイオンの供給がより容易となるため、より優れたサイクル特性を有するリチウム二次電池を得ることができる。
【００４０】
また、上記炭素材料の平均粒子径は、１０ｎｍ以上１００ｎｍ以下であることが好ましい。上記の範囲内において、炭素材料の導電剤としての電子導電性と、極板製造工程における炭素材料の分散性とを、より最適化することができる。なお、炭素材料の平均粒子径は、レーザー回折式の粒度分布測定装置などにより測定することができる。
【００４１】
次に、本発明のリチウム二次電池について図２を参照しながら説明する。図２は、本発明におけるリチウム二次電池の構造例を示す模式図である。
【００４２】
図２に示すリチウム二次電池は、リチウムイオンを可逆的に吸蔵および放出する負極１と正極２とを、セパレータ３を介して積層し、得られた積層体を捲回した後に、ケース６に収め、リチウムイオン伝導性を有する電解質で全体を満たせば得ることができる。負極１および正極２は、負極リード４および正極リード５によりリチウム二次電池の外部端子に電気的に接続される。
【００４３】
このとき、負極１および正極２から選ばれる少なくとも一方の極板が、通常の使用温度領域（−２０℃〜６０℃）において、十分低抵抗の極板であり、温度が通常の使用温度領域を超えて上昇した場合に、電気抵抗が確実に増大する極板であればよい。
【００４４】
このようなリチウム二次電池は、負極１および正極２から選ばれる少なくとも一方の極板として、上記に記載のリチウム二次電池用極板を用いれば得ることができる。
【００４５】
上記のようなリチウム二次電池とすることにより、電池が押しつぶされたり、釘のような先端が尖った異物が電池に突き刺さったりして正極と負極が短絡した場合においても、電池温度の急激な上昇が抑制され、より安全性、信頼性に優れたリチウムイオン二次電池を得ることができる。
【００４６】
電解質としては、電気的に絶縁性で、水分を含まず、リチウムイオン伝導性を有する材料であれば特に限定されない。電子吸引性の強いリチウム塩、例えば、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＣＦ₃ＳＯ₃、Ｌｉ（ＣＦ₃ＳＯ₂）₂Ｎ、Ｌｉ（Ｃ₂Ｆ₅ＳＯ₂）₂Ｎ、ＬｉＣ₄Ｆ₉ＳＯ₃などを用いることが好ましい。なかでも、リチウム塩を電解質として、これを有機溶媒に溶解させたものが特に好ましい。
【００４７】
上記有機溶媒としては、特に限定されないが、例えば、プロピレンカーボネート、１，２−ジメトキシエタン、γ−ブチルラクトン、テトラヒドロフラン、ジメチルカーボネート、ジエチルカーボネート、ジプロピルカーボネートなどが好ましい。上記有機溶媒は、単独で用いても、２種以上の溶媒を混合して用いても構わない。上記リチウム塩電解質としては、例えば、ＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＰＦ₆、ＬｉＢＦ₄などが特に好ましい。
【００４８】
その他、セパレータ３など、本発明のリチウム二次電池を構成するその他の部材には、公知である一般的な材料を用いることができる。
【００４９】
次に、本発明におけるリチウム二次電池の別の例について図３を用いて説明する。
【００５０】
図３に示す例は、図２に示す例と同様に、リチウムイオンを可逆的に吸蔵および放出する負極１１と正極１２とを、セパレータ１３を介して積層し、得られた積層体を捲回した後に、ケースに収め、リチウムイオン伝導性を有する電解質で全体を満たせば得ることができる。
【００５１】
図３に示す例では、負極１１とセパレータ１３との間に、以下の式（５）を満たす炭素材料を含む炭素材料層１８が配置されている。
【００５２】
ρ₁≧２．５×ρ₂ （５）
このとき、図２に示す例とは異なり、正極１２および負極１１から選ばれる少なくとも一方の極板が、上記の式（３）を満たす炭素材料を含む導電剤を、含んでいてもいなくてもよい。
【００５３】
炭素材料層１８の存在によって、電池が押しつぶされたり、釘のような先端が尖った異物が電池に突き刺さったりして正極と負極が短絡した場合でも、電池温度の急激な上昇が抑制され、より安全性、信頼性に優れたリチウムイオン二次電池を得ることができる。
【００５４】
また、なかでも、上記炭素材料が、以下の式（６）を満たすことが好ましい。
【００５５】
０＜ρ₁≦１×１０^-1、かつ、ρ₂≧１×１０^-2 （６）
通常の使用温度領域（−２０℃〜６０℃）において、炭素抵抗層は低抵抗であり、より効率のよい安定した電池を得ることができる。また、短絡などで電池の温度が上昇した場合には、高抵抗の層として、より確実に短絡電流を減少させ、電池温度のさらなる上昇を抑制することができる。なお、上記の式（６）における、ρ₁およびρ₂の単位は、「Ω・ｃｍ」である。
【００５６】
負極１１および正極１２は、それぞれ、負極集電体１４および正極集電体１５上に、負極活物質層１６および正極活物質層１７が積層された構造を有している。図３に示す例では、炭素材料層１８は、負極活物質層１６とセパレータ１３との間に配置されているが、その他、両集電体と、両活物質層と、セパレータとの間の任意の位置に配置することができる。例えば、セパレータ１３と正極活物質層１７との間、負極活物質層１６と負極集電体１４との間などである。
【００５７】
また、炭素材料層１８を、負極活物質層１６および正極活物質層１７から選ばれる少なくとも一方の活物質層の内部に配置することもできる。
【００５８】
なお、炭素材料層１８は、一層に限定されず、求める電池の特性に応じて任意の数の層を配置することが可能である。また、炭素材料層１８は、正極および負極から選ばれる少なくとも一方の極板を覆う面積を有していればよい。
【００５９】
炭素材料層に含まれる上記炭素材料の含有量が、炭素材料層の０．１重量％以上５０重量％以下であることが好ましい。この範囲では、炭素材料を構成する炭素粒子間の接触点の数が好ましく、炭素材料層の温度−体積抵抗率特性がより良好となる。
【００６０】
また、炭素材料層の乾燥時の厚さ（乾燥厚さ）が、０．０１μｍ以上２０μｍ以下が好ましい。この範囲では、電池内に少なくとも１層の炭素材料層を配置することによる電池の容量への影響をより小さくすることができる。なお、乾燥時の厚さ、とは、電池に組み込む前の厚さのことである。
【００６１】
なお、図２および図３に示した例において、負極１および１１の活物質には、リチウムイオンを可逆的に吸蔵および放出することができる炭素質材料を含むことが好ましい。
【００６２】
【実施例】
（実施例１）
本実施例では、図２に示した構造を有するリチウム二次電池を作製し、電池特性の評価を行った。
【００６３】
まず、負極を以下のようにして作製した。
【００６４】
負極活物質には、石油ピッチの炭素化過程で生成するメソフェーズ小球体を原料としたメソカーボンマイクロビーズ（以下、ＭＣＭＢ、と表記）を用いた。上記ＭＣＭＢ１００重量部に対し、結着剤としてポリフッ化ビニリデン１０重量部を混合して負極合剤を作製し、この負極合剤を、Ｎ−メチル−２−ピロリドン（ＮＭＰ）に分散させてスラリー状にした。続いて、上記スラリーを負極集電体となる銅箔（厚さ２０μｍ）の両面に塗布し、乾燥後、ローラープレス機で圧縮成型を行ない、厚さ１５０μｍの負極を得た。
【００６５】
次に、正極を以下のようにして作製した。
【００６６】
まず、炭酸コバルトおよび炭酸リチウムを原子組成比でＬｉ／Ｃｏ＝１となるように混合した後に、空気中で焼成し（９００℃、１０時間）、正極活物質として、リチウム−コバルト複合酸化物であるＬｉＣｏＯ₂を得た。これとは別に、導電剤として、ρ₂≧２．５×ρ₁の関係を満たす炭素材料（新日化カーボン製ＮＵ−１２−２−０６）５重量部と、結着剤であるポリフッ化ビニリデン３重量部とを、分散媒であるＮＭＰ１０重量部に混練し、さらにＮＭＰを４０重量部添加して上記導電剤と上記結着剤とを上記分散媒中に均一に分散させた。次に、正極活物質である上記ＬｉＣｏＯ₂１００重量部を上記混練物に添加して全体をスラリー状にした。このスラリーを正極集電体となるアルミニウム箔（厚さ２０μｍ）の両面に塗布し、乾燥後、ローラープレス機で圧縮成型を行ない、厚さ１８０μｍの正極を得た。
【００６７】
その後、上記のように準備した帯状の負極および正極を、微多孔性ポリエチレンフィルムからなるセパレータ（厚さ２５μｍ）を間に挟んで捲回し、得られた渦巻式電極体を電池缶に収納した。
【００６８】
最後に、上記電池缶の中に、プロピレンカーボネート５０体積部とジエチルカーボネート５０体積部との混合溶媒にＬｉＰＦ₆を１モル溶解させた電解液を注入した。これを封口し、直径１４ｍｍ、高さ５０ｍｍの円筒型電池を作製した。
【００６９】
また、比較例（サンプルa0）として、正極用の導電剤を、一般的に用いられているアセチレンブラックに変更した電池を同時に準備した。上記比較例のその他の構成は、上記実施例（サンプルa1）と全く同一である。
【００７０】
以上のように準備した、実施例、比較例それぞれのサンプルに対し、充放電サイクル試験に基づく容量維持率測定と、安全性試験として釘刺し試験を行ない、リチウム二次電池としての特性を評価した。
【００７１】
容量維持率測定は、各サンプルの上限電圧を４．２Ｖに設定し、７００ｍＡの制限電流で１時間充電後、５００ｍＡの放電電流で電池電圧が３．０Ｖになるまで放電する充放電サイクルを繰り返し行ない、各電池における、２サイクル目の初期容量に対する２００サイクル目の容量維持率を求めた。また、釘刺し試験は、電池温度６０℃において行った。上記試験方法は、以下の実施例においても同様である。
【００７２】
以下の表３に、実施例a1および比較例a0の電池特性評価の結果を示す。
【００７３】

【００７４】
表３に示すように、正極の導電剤として、ρ₂≧２．５×ρ₁の関係を満たす炭素材料を含むことで、釘刺し試験の結果が改善していることがわかる。また、容量維持率は比較例a0とほぼ同等であり、極板に上記炭素材料を導電剤として含んだ場合においても、従来の電池と同等の導電性が確保されていると考えられる。
【００７５】
（実施例２）
実施例１と同様の方法を用いて、図２に示したリチウム二次電池を作製した（実施例b1）。ただし、実施例１と異なり、正極の導電剤としてアセチレンブラックを使用した。負極の導電剤には、ρ₂≧２．５×ρ₁の関係を満たす炭素材料（新日化カーボン製ＮＵ−１２−２−０６）を、負極合剤を作製する際にＭＣＭＢ１００重量部に対して５重量部混合した。その他、電池を構成する部材、正極導電剤の配合比率などは実施例１と全く同様である。
【００７６】
以上のように準備した、実施例b1に対し、容量維持率測定と、釘刺し試験を行ない、リチウム二次電池としての特性を評価した。
【００７７】
以下の表４に、電池特性評価の結果を示す。
【００７８】

【００７９】
実施例１における比較例a0と比較すると、負極の導電剤として、ρ₂≧２．５×ρ₁を満たす炭素材料を含んだ場合においても、釘刺し試験の結果が改善していることがわかる。また、実施例１と同様に、容量維持率は比較例a0とほぼ同等である。このことから、上記炭素材料は、正極に含まれる場合でも、負極に含まれる場合でも、リチウム二次電池の信頼性向上に効果的であることがわかる。
【００８０】
（実施例３）
実施例１と同様の方法を用いて、図２に示したリチウム二次電池を作製した。ただし、実施例１と異なり、正極の導電剤である、ρ₂≧２．５×ρ₁の関係を満たす炭素材料の混合比率を、正極活物質１００重量部に対して１重量部から２１重量部の範囲で変化させた。その他、電池を構成する部材などは実施例１と全く同様である。
【００８１】
以上のように準備した、実施例c1〜c8に対し、容量維持率測定と、釘刺し試験を行ない、リチウム二次電池としての特性を評価した。
【００８２】
以下の表５に、正極への上記炭素材料の混合比率と、電池特性の評価結果を示す。
【００８３】

【００８４】
表５に示すように、釘刺し試験の結果は、実施例１における比較例a0に比べて全てのサンプルで改善されており、より安全性、信頼性に優れるリチウム二次電池が得られたことがわかる。また、なかでも、極板が、ρ₂≧２．５×ρ₁を満たす炭素材料を、活物質１００重量部に対して２重量部以上２０重量部以下の範囲で含む場合に、電池の容量維持率が良好な結果を示した。特に、９重量部以上１２重量部以下の範囲では、比較例a0よりも、電池の容量維持率が優れた結果となった。
【００８５】
（実施例４）
本実施例では、図３に示すような炭素材料層を備えたリチウム二次電池を作製し、電池特性の評価を行った。なお、炭素材料層は、正極集電体と、正極活物質層との間に積層させた。
【００８６】
まず、ρ₂≧２．５×ρ₁の関係を満たす炭素材料（新日化カーボン製ＮＵ−１２−２−０６）３０重量部とポリフッ化ビニリデン７０重量部とを、ＮＭＰに混合してスラリー状とした。このスラリーを、正極集電体であるアルミニウム箔（厚さ２０μｍ）の両面に塗布した後に乾燥させ、乾燥時の厚さが３μｍの炭素材料層を正極集電体上に形成した。この炭素材料層には、上記炭素材料が３０重量％含まれることになる。
【００８７】
次に、導電剤としてアセチレンブラック４重量部、結着剤としてポリフッ化ビニリデン３重量部、分散媒としてＮＭＰ４０重量部、正極活物質としてＬｉＣｏＯ₂１００重量部を混合して正極合剤を作製した。この正極合剤を、上記炭素材料層上に塗布し、乾燥後、ローラープレス機で圧縮成型して炭素材料層を含む正極を作製した。
【００８８】
正極として、上記のように準備した正極を用いる以外は、実施例１と同様にしてリチウム二次電池を作製した。正極以外の電池を構成する部材は、全て実施例１と同様である。
【００８９】
上記の方法を用いて、炭素材料層中の上記炭素材料の組成を変更したサンプルを計５種類用意し（サンプルd1〜d5）、容量維持率測定と、釘刺し試験を行ない、リチウム二次電池としての特性を評価した。
【００９０】
以下の表６に、炭素材料中の上記炭素材料の混合比率と、電池特性の評価結果を示す。
【００９１】

【００９２】
表６に示すように、実施例１における比較例a0に比べて、釘刺し試験の結果が全てのサンプルで改善されており、炭素材料層を配置することによっても、より安全性、信頼性に優れるリチウム二次電池が得られたことがわかる。また、なかでも、炭素材料層中に上記炭素材料を５０重量％以下の割合で含む場合に、電池の容量維持率が良好な結果を示した。
【００９３】
（実施例５）
実施例４と同様に、実施例４で用いた炭素材料層の乾燥時の厚さを変更したサンプルを７種類（サンプルe1〜e7）準備し、電池の初期容量測定と、釘刺し試験を行ない、リチウム二次電池としての特性を評価した。なお、炭素材料層中の炭素材料の混合比率は、炭素材料層に対して３０重量％とした。
【００９４】
電池の初期容量は、実施例１で行った容量維持率測定の２サイクル目の電池容量とし、比較例a0の値を１００としてその相対比を算出した。
【００９５】
以下の表７に、炭素材料層の乾燥時の厚さと、電池特性の評価結果を示す。
【００９６】

【００９７】
表７に示すように、釘刺し試験の結果は、実施例１における比較例a0に比べて全てのサンプルで改善されており、より安全性、信頼性に優れるリチウム二次電池が得られたことがわかる。
【００９８】
また、容量相対比についてみてみると、炭素材料層の乾燥時の厚みが２０μｍ以上の場合に、炭素材料層を設けていない比較例a0と比較して、初期容量が７％減少する結果となった。このことは、極板内部で炭素抵抗層が占める割合が大きくなり、結果的に電池容量が減少することを示している。
【００９９】
以上のことから、炭素材料層の効果を保ちつつ、電池の容量を維持するためには、炭素材料層の乾燥時の厚さが２０μｍ以下であることが好ましいと言える。
【０１００】
なお、実施例１〜５では、円筒形のリチウム二次電池を作製したが、これに限定されるものではなく、例えば、角型、コイン型、ボタン型などのリチウム二次電池であっても同様の効果を得ることができる。
【０１０１】
【発明の効果】
以上説明したように、本発明によれば、電池が押しつぶされた場合や、先端が尖った異物が電池に突き刺ささるなどで正極と負極が短絡した場合でも、電池の急激な温度上昇を抑制することができる、より安全性、信頼性に優れるリチウム二次電池を得ることができる。また、上記リチウム二次電池を実現するリチウム二次電池用極板を得ることができる。
【図面の簡単な説明】
【図１】 本発明に用いられる炭素材料および従来の炭素材料における、温度と体積抵抗率との関係を示す図。
【図２】 本発明におけるリチウム二次電池の構造例を示す断面図。
【図３】 本発明におけるリチウム二次電池の別の構造例を示す断面図。
【符号の説明】
１、１１ 負極
２、１２ 正極
３、１３ セパレータ
４ 負極リード
５ 正極リード
６ ケース
１４ 負極集電体
１５ 正極集電体
１６ 負極活物質層
１７ 正極活物質層
１８ 炭素材料層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrode plate for a lithium secondary battery and a lithium secondary battery.
[0002]
[Prior art]
In recent years, as mobile communication devices and portable electronic terminals are rapidly becoming smaller and lighter, lithium secondary batteries featuring high energy density have been actively researched and developed as power sources for driving these devices. There are various types of lithium secondary batteries depending on the material constituting the battery.
[0003]
In the case of a lithium secondary battery using lithium metal as a negative electrode material, the energy density is superior to other types, but there is a problem that dendrites are deposited on the negative electrode during charging. Depending on the degree of deposition, dendrites may break through the separator in the battery and reach the positive electrode side, causing an internal short circuit, which may cause accidents such as heat generation, explosion, or ignition of the battery. In addition, the deposited dendrites have a higher specific activity than the ordinary lithium metal constituting the electrode, and thus have a high reaction activity, and tend to react with the solvent in the electrolytic solution to form a solid electrolyte-like interface film. . For this reason, the internal resistance of the battery is increased, or particles isolated from the network of electron conduction are present, which can be a factor for reducing the charge / discharge efficiency of the battery. Thus, in the case of a lithium secondary battery using lithium metal as a negative electrode material, securing durability and safety is a problem.
[0004]
Currently, batteries using carbon materials capable of occluding and releasing lithium ions instead of lithium metal have been put into practical use as negative electrode materials. When the above carbon material is used as the negative electrode material, lithium ions are occluded between the layers of the carbon material during charging, so that metallic lithium does not deposit on the negative electrode and avoids the problem of internal short circuit due to dendrites. Can do. In addition, for example, the theoretical capacity of graphite, which is one of the above carbon materials, is 372 mAh / g, which is as low as about one-tenth of the theoretical capacity of a single lithium metal, but lithium 2 having very excellent charge / discharge cycle characteristics. A secondary battery can be obtained.
[0005]
[Problems to be solved by the invention]
However, even in a battery using the above carbon material as the negative electrode material, when the battery is crushed or a foreign object with a sharp tip such as a nail pierces the battery, the positive electrode and the negative electrode break through the separator and contact in the battery. In some cases, an internal short circuit may occur. When an internal short circuit occurs, a large current concentrates at the portion where the positive electrode and the negative electrode are in contact with each other, and heat is generated due to Joule heat, which may cause the battery temperature to rise rapidly and lead to the worst ignition.
[0006]
In addition, as with the above carbon materials, other negative electrode materials that can avoid the problem of internal short circuit by suppressing the formation of dendrites have been reported, but it is safer under actual battery usage conditions. There is a need for lithium secondary batteries that are excellent in reliability and reliability.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, an electrode plate for a lithium secondary battery according to the present invention is an electrode plate for a lithium secondary battery including an active material capable of reversibly inserting and extracting lithium ions and a conductive agent. And the said electrically conductive agent is characterized by including the carbon material which satisfy | fills the following formula | equation (1).
[0008]
ρ ₂ ≧ 2.5 × ρ ₁ (1)
However, in the above formula, ρ ₁ Is the volume resistivity of the carbon material at −20 ° C. to 60 ° C., and ρ ₂ Is the volume resistivity at 180 ° C. of the carbon material. The following ρ ₁ And ρ ₂ The same is true for.
[0009]
The lithium secondary battery of the present invention includes a positive electrode that reversibly absorbs and releases lithium ions, a negative electrode that reversibly absorbs and releases lithium ions, and an electrolyte having lithium ion conductivity. In the battery, at least one electrode plate selected from the positive electrode and the negative electrode is the electrode plate for a lithium secondary battery described above.
[0010]
The lithium secondary battery of the present invention has a characteristic that the electrical resistance of the electrode plate increases at a high temperature. This is because the electrode plate for a lithium secondary battery described above contains a carbon material whose electrical resistance increases with increasing temperature in the conductive agent in the electrode plate. Therefore, even if the positive and negative electrodes of the battery are short-circuited, a large current is temporarily concentrated on the short-circuited part, but the electrical resistance of the electrode plate increases due to the generated Joule heat, and the short-circuit current is limited. Thus, a rapid increase in battery temperature can be suppressed.
[0011]
Therefore, by using the lithium secondary battery as described above, even when the battery is crushed or a foreign object with a sharp tip such as a nail pierces the battery, the positive and negative electrodes are short-circuited. A rapid rise is suppressed, and a lithium ion secondary battery with higher safety and reliability can be obtained.
[0012]
Note that −20 ° C. to 60 ° C. reflects the temperature range in which lithium secondary batteries are normally used, and 180 ° C. refers to the temperature range in which lithium secondary batteries may eventually ignite. Reflects.
[0013]
The lithium secondary battery of the present invention is also a lithium secondary battery including an electrode plate group and an electrolyte having lithium ion conductivity, and the electrode plate group reversibly absorbs and releases lithium ions. It may be characterized by including a positive electrode, a negative electrode that reversibly occludes and releases lithium ions, a separator, and a first layer containing a carbon material that satisfies the following formula (2).
[0014]
ρ ₂ ≧ 2.5 × ρ ₁ (2)
In addition to the case where a carbon material whose electric resistance increases with temperature rise is included in the conductive agent in the electrode plate, the battery is crushed by arranging the first layer containing the carbon material inside the battery. When the positive electrode and the negative electrode are short-circuited, for example, when a foreign object with a sharp tip such as a nail sticks into the battery, the rapid rise in battery temperature is suppressed, and lithium ion secondary that is more safe and reliable. A secondary battery can be obtained.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
First, the electrode plate for a lithium secondary battery in the present invention will be described.
[0016]
The electrode plate for a lithium secondary battery of the present invention includes an active material capable of reversibly occluding and releasing lithium ions and a conductive agent, and the conductive agent satisfies the following formula (3). Contains carbon material.
[0017]
ρ ₂ ≧ 2.5 × ρ ₁ (3)
When the conductive agent contains the carbon material, the electrical resistance of the electrode plate can be increased when the temperature rises. Therefore, in the lithium secondary battery provided with the above electrode plate, even when the battery temperature rises due to a short circuit between the positive electrode and the negative electrode, the short circuit current decreases as the temperature rises, so further increase in the battery temperature is suppressed, A lithium ion secondary battery with better safety and reliability can be obtained.
[0018]
Especially, it is preferable that the said carbon material satisfy | fills the following formula | equation (4).
[0019]
0 <ρ ₁ ≦ 1 × 10 ^-1 And ρ ₂ ≧ 1 × 10 ^-2 (4)
In a normal operating temperature range (−20 ° C. to 60 ° C.), a more efficient and stable battery can be obtained as a low-resistance electrode plate. Moreover, when the temperature of the battery rises due to a short circuit or the like, the short-circuit current can be more reliably reduced as a high-resistance electrode plate, and further increase in the battery temperature can be suppressed. In the above equation (4), ρ ₁ And ρ ₂ The unit of is “Ω · cm”.
[0020]
As the carbon material, ρ ₁ And ρ ₂ As long as the above satisfies the above relationship, there is no particular limitation. The reason why such characteristics occur is not clear at this stage, but the resistance is sufficiently low in the normal operating temperature range, and the electrical resistance increases more reliably when the temperature rises beyond the normal operating temperature range. Any material can be used.
[0021]
As such a carbon material, for example, NU-12-2-06 made by Nippon Nissha Carbon, which is a kind of carbon black, can be used. As an example, the relationship between temperature and volume resistivity in the above NU-12-2-06 is shown in Table 1 and FIG. For comparison, Table 2 and FIG. 1 also show the relationship between temperature and volume resistivity of acetylene black that has been conventionally used as a conductive agent.
[0022]

[0023]

[0024]
As shown in Table 1 and FIG. 1, NU-12-2-06 has a substantially constant electric resistance in a temperature range of 60 ° C. or lower, and in a normally used temperature range, stable plate resistance. Thus, a more efficient battery can be obtained. On the other hand, when the temperature of the lithium secondary battery reaches 180 ° C., which may eventually cause ignition, the electrical resistance increases greatly. When the temperature rises beyond the normal operating temperature range, the electrical resistance is more reliably determined. It can be seen that can be increased.
[0025]
On the other hand, in the conventional acetylene black, there is no tendency for the electrical resistance to increase with increasing temperature, and conversely, the electrical resistance gradually decreases.
[0026]
In addition, the volume resistivity shown in Table 1, Table 2, and FIG. 1 was measured as follows.
[0027]
1.96 × 10 in the above carbon material (NU-12-2-06 and acetylene black manufactured by Nisshin Carbon) ^Four The volume resistivity of the carbon material was measured using a four-terminal method while increasing the temperature and applying a pressure of N (2000 kgf) while heating.
[0028]
The electrode plate for a lithium secondary battery of the present invention only needs to have conductivity and function as an active material (that is, reversibly occlude and release lithium ions). For example, it can be produced by applying an electrode mixture obtained by mixing an active material, a conductive agent containing the carbon material, and a binder to a current collector.
[0029]
The form of the carbon material is not particularly limited, but is preferably powdery because the electrode mixture can be easily prepared.
[0030]
Moreover, it is preferable that the ratio in which the said carbon material is contained is 2 to 20 weight part with respect to 100 weight part of active materials. Within the above range, the cycle characteristics of the battery capacity can be further improved.
[0031]
When the electrode plate is a positive electrode, the positive electrode active material includes the formula Li _x MO ₂ Or the formula Li _y M ₂ O _Four (Wherein M is a transition metal and x and y are numerical values satisfying 0 <x ≦ 1 and 0 <y ≦ 2), a lithium-transition metal composite oxide having a composition represented by Among these, cobalt, nickel, manganese, iron, etc. are preferably used as the transition metal from the viewpoint of the electromotive force of the battery.
[0032]
Such a lithium-transition metal composite oxide is, for example, mixed with a carbonate or oxide of lithium and a transition metal according to the target composition, and fired (650-1200 ° C.) in an oxygen-existing atmosphere. Can be obtained.
[0033]
The current collector used for the positive electrode is preferably a film, foil, sheet, or the like made of aluminum, stainless steel, nickel, titanium, or an alloy thereof. Of these, aluminum foil is particularly preferable.
[0034]
When the electrode plate is a negative electrode, the negative electrode active material is not particularly limited as long as it is a material that can occlude and release lithium ions. Among them, carbonaceous materials such as pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic polymer compound fired bodies (furan resin, etc.) are suitable. It is preferable to use carbon fiber, activated carbon, mesophase spherules, etc. If a carbonaceous material is used as a negative electrode active material, dendrites are not generated even during charging, and a more inexpensive and reliable lithium secondary battery can be obtained.
[0035]
The current collector used for the negative electrode is preferably a film, foil, sheet, or the like made of copper, stainless steel, nickel, titanium, or an alloy thereof. Among these, copper foil is particularly preferable.
[0036]
As the binder, it is preferable to use polyacrylate latex, carboxymethyl cellulose, polytetrafluoroethylene, polyvinylidene fluoride, or the like. These materials may be used alone or in combination.
[0037]
The conductive agent component other than the carbon material is not particularly limited as long as it is a conductive material that does not cause a chemical reaction in the constituted battery. Of these, graphite and carbon black are particularly preferable because of their good conductivity.
[0038]
The average particle diameter of the conductive agent component other than the carbon material is preferably 0.01 μm or more and 20 μm or less, and particularly preferably 0.02 μm or more and 10 μm or less. A plurality of conductive agent components can also be mixed. Especially, it is preferable to mix carbon blacks, such as acetylene black, and the graphite particle whose average particle diameter is the range of 1 micrometer-15 micrometers.
[0039]
In the electrode plate for a lithium secondary battery according to the present invention, it is preferable that the carbon material can contain 100 ml or more and 300 ml or less of dibutyl phthalate per 100 g of the carbon material. If the oil absorption of dibutyl phthalate is within the above range, the structure as an aggregated state of the aggregate of the carbon material is developed, and the carbon material has more excellent electronic conductivity and dispersibility. As a result, when incorporated in a lithium secondary battery, the conductive agent containing the carbon material can hold the electrolyte better, and the supply of lithium ions from the electrolyte becomes easier. A lithium secondary battery having excellent cycle characteristics can be obtained.
[0040]
The average particle size of the carbon material is preferably 10 nm or more and 100 nm or less. Within the above range, the electronic conductivity of the carbon material as a conductive agent and the dispersibility of the carbon material in the electrode plate manufacturing process can be further optimized. The average particle size of the carbon material can be measured by a laser diffraction type particle size distribution measuring device or the like.
[0041]
Next, the lithium secondary battery of the present invention will be described with reference to FIG. FIG. 2 is a schematic diagram showing a structural example of the lithium secondary battery in the present invention.
[0042]
In the lithium secondary battery shown in FIG. 2, a negative electrode 1 and a positive electrode 2 that reversibly occlude and release lithium ions are laminated via a separator 3, and the obtained laminate is wound. It can be obtained by filling the whole with an electrolyte having lithium ion conductivity. The negative electrode 1 and the positive electrode 2 are electrically connected to the external terminal of the lithium secondary battery by the negative electrode lead 4 and the positive electrode lead 5.
[0043]
At this time, at least one electrode plate selected from the negative electrode 1 and the positive electrode 2 is a sufficiently low resistance electrode plate in a normal use temperature region (−20 ° C. to 60 ° C.), and the temperature is within a normal use temperature region. What is necessary is just an electrode plate which an electrical resistance increases reliably when it raises exceeding it.
[0044]
Such a lithium secondary battery can be obtained by using the electrode plate for a lithium secondary battery described above as at least one electrode plate selected from the negative electrode 1 and the positive electrode 2.
[0045]
By using the lithium secondary battery as described above, even when the battery is crushed or a foreign object with a sharp tip such as a nail pierces the battery, the positive electrode and the negative electrode are short-circuited. The rise is suppressed, and a lithium ion secondary battery with better safety and reliability can be obtained.
[0046]
The electrolyte is not particularly limited as long as it is electrically insulating, does not contain moisture, and has lithium ion conductivity. Lithium salt with strong electron withdrawing property, for example, LiPF ₆ , LiBF _Four LiClO _Four , LiAsF ₆ , LiCF _Three SO _Three , Li (CF _Three SO ₂ ) ₂ N, Li (C ₂ F _Five SO ₂ ) ₂ N, LiC _Four F ₉ SO _Three Etc. are preferably used. Of these, a lithium salt as an electrolyte and dissolved in an organic solvent is particularly preferable.
[0047]
The organic solvent is not particularly limited, but for example, propylene carbonate, 1,2-dimethoxyethane, γ-butyllactone, tetrahydrofuran, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and the like are preferable. The above organic solvents may be used alone or in combination of two or more solvents. Examples of the lithium salt electrolyte include LiClO. _Four , LiAsF ₆ , LiPF ₆ , LiBF _Four Etc. are particularly preferred.
[0048]
In addition, well-known general materials can be used for other members constituting the lithium secondary battery of the present invention such as the separator 3.
[0049]
Next, another example of the lithium secondary battery in the present invention will be described with reference to FIG.
[0050]
In the example shown in FIG. 3, similarly to the example shown in FIG. 2, a negative electrode 11 and a positive electrode 12 that reversibly occlude and release lithium ions are laminated via a separator 13, and the obtained laminated body is wound. After that, it can be obtained by placing it in a case and filling the whole with an electrolyte having lithium ion conductivity.
[0051]
In the example shown in FIG. 3, a carbon material layer 18 containing a carbon material that satisfies the following formula (5) is disposed between the negative electrode 11 and the separator 13.
[0052]
ρ ₁ ≧ 2.5 × ρ ₂ (5)
At this time, unlike the example shown in FIG. 2, at least one electrode plate selected from the positive electrode 12 and the negative electrode 11 may or may not contain a conductive agent containing a carbon material that satisfies the above formula (3). Good.
[0053]
Due to the presence of the carbon material layer 18, even when the battery is crushed or a foreign object with a sharp tip such as a nail pierces the battery and the positive electrode and the negative electrode are short-circuited, the rapid increase in the battery temperature is suppressed. A lithium ion secondary battery excellent in safety and reliability can be obtained.
[0054]
In particular, the carbon material preferably satisfies the following formula (6).
[0055]
0 <ρ ₁ ≦ 1 × 10 ^-1 And ρ ₂ ≧ 1 × 10 ^-2 (6)
In the normal use temperature range (-20 ° C to 60 ° C), the carbon resistance layer has a low resistance, and a more efficient and stable battery can be obtained. Further, when the temperature of the battery rises due to a short circuit or the like, the short circuit current can be more reliably reduced as a high resistance layer, and further increase in the battery temperature can be suppressed. In the above equation (6), ρ ₁ And ρ ₂ The unit of is “Ω · cm”.
[0056]
The negative electrode 11 and the positive electrode 12 have a structure in which a negative electrode active material layer 16 and a positive electrode active material layer 17 are laminated on a negative electrode current collector 14 and a positive electrode current collector 15, respectively. In the example shown in FIG. 3, the carbon material layer 18 is disposed between the negative electrode active material layer 16 and the separator 13, but in addition, between the both current collectors, both active material layers, and the separator. It can be placed at any position. For example, between the separator 13 and the positive electrode active material layer 17 and between the negative electrode active material layer 16 and the negative electrode current collector 14.
[0057]
In addition, the carbon material layer 18 may be disposed inside at least one active material layer selected from the negative electrode active material layer 16 and the positive electrode active material layer 17.
[0058]
The carbon material layer 18 is not limited to a single layer, and an arbitrary number of layers can be arranged according to the required battery characteristics. The carbon material layer 18 only needs to have an area covering at least one electrode plate selected from a positive electrode and a negative electrode.
[0059]
It is preferable that content of the said carbon material contained in a carbon material layer is 0.1 to 50 weight% of a carbon material layer. In this range, the number of contact points between the carbon particles constituting the carbon material is preferable, and the temperature-volume resistivity characteristic of the carbon material layer becomes better.
[0060]
Moreover, the thickness (dry thickness) at the time of drying of the carbon material layer is preferably 0.01 μm or more and 20 μm or less. In this range, the influence on the capacity of the battery due to the arrangement of at least one carbon material layer in the battery can be further reduced. In addition, the thickness at the time of drying is the thickness before incorporating in a battery.
[0061]
In the example shown in FIGS. 2 and 3, the active material of the negative electrodes 1 and 11 preferably contains a carbonaceous material capable of reversibly occluding and releasing lithium ions.
[0062]
【Example】
Example 1
In this example, a lithium secondary battery having the structure shown in FIG. 2 was produced and the battery characteristics were evaluated.
[0063]
First, the negative electrode was produced as follows.
[0064]
As the negative electrode active material, mesocarbon microbeads (hereinafter referred to as MCMB) made from mesophase microspheres produced during the carbonization process of petroleum pitch were used. A negative electrode mixture is prepared by mixing 10 parts by weight of polyvinylidene fluoride as a binder with respect to 100 parts by weight of the MCMB, and the negative electrode mixture is dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry. I made it. Subsequently, the slurry was applied to both surfaces of a copper foil (thickness 20 μm) serving as a negative electrode current collector, and after drying, compression molding was performed with a roller press to obtain a negative electrode having a thickness of 150 μm.
[0065]
Next, the positive electrode was produced as follows.
[0066]
First, cobalt carbonate and lithium carbonate are mixed so that the atomic composition ratio is Li / Co = 1, and then fired in air (900 ° C., 10 hours). As a positive electrode active material, a lithium-cobalt composite oxide is used. LiCoO ₂ Got. Apart from this, ρ ₂ ≧ 2.5 × ρ ₁ 5 parts by weight of a carbon material satisfying the relationship (NU-12-2-06 made by Nisshin Carbon) and 3 parts by weight of polyvinylidene fluoride as a binder are kneaded in 10 parts by weight of NMP as a dispersion medium, Further, 40 parts by weight of NMP was added to uniformly disperse the conductive agent and the binder in the dispersion medium. Next, the LiCoO which is a positive electrode active material ₂ 100 parts by weight was added to the kneaded product to make a slurry. This slurry was applied to both surfaces of an aluminum foil (thickness 20 μm) serving as a positive electrode current collector, and after drying, compression molding was performed with a roller press to obtain a positive electrode having a thickness of 180 μm.
[0067]
Thereafter, the strip-shaped negative electrode and positive electrode prepared as described above were wound with a separator (thickness 25 μm) made of a microporous polyethylene film interposed therebetween, and the obtained spiral electrode body was housed in a battery can.
[0068]
Finally, in the battery can, LiPF is mixed in a mixed solvent of 50 parts by volume of propylene carbonate and 50 parts by volume of diethyl carbonate. ₆ An electrolyte solution in which 1 mol was dissolved was injected. This was sealed to produce a cylindrical battery having a diameter of 14 mm and a height of 50 mm.
[0069]
Further, as a comparative example (sample a0), a battery in which the conductive agent for the positive electrode was changed to acetylene black generally used was prepared at the same time. Other configurations of the comparative example are exactly the same as those of the example (sample a1).
[0070]
The capacity maintenance rate measurement based on the charge / discharge cycle test and the nail penetration test as a safety test were performed on the samples of the examples and comparative examples prepared as described above, and the characteristics as lithium secondary batteries were evaluated. .
[0071]
The capacity maintenance ratio measurement is performed by repeating the charge / discharge cycle in which the upper limit voltage of each sample is set to 4.2 V, the battery is charged with a limit current of 700 mA for 1 hour, and then discharged with a discharge current of 500 mA until the battery voltage becomes 3.0 V. In each battery, the capacity retention rate at the 200th cycle relative to the initial capacity at the second cycle was determined. The nail penetration test was conducted at a battery temperature of 60 ° C. The test method is the same in the following examples.
[0072]
Table 3 below shows the results of battery characteristic evaluation of Example a1 and Comparative Example a0.
[0073]

[0074]
As shown in Table 3, as a conductive agent for the positive electrode, ρ ₂ ≧ 2.5 × ρ ₁ It can be seen that the result of the nail penetration test is improved by including a carbon material satisfying the above relationship. Further, the capacity retention rate is almost the same as that of Comparative Example a0, and it is considered that the same conductivity as that of the conventional battery is ensured even when the electrode plate contains the carbon material as a conductive agent.
[0075]
(Example 2)
The lithium secondary battery shown in FIG. 2 was produced using the same method as in Example 1 (Example b1). However, unlike Example 1, acetylene black was used as a conductive agent for the positive electrode. For the negative electrode conductive agent, ρ ₂ ≧ 2.5 × ρ ₁ 5 parts by weight of carbon material (NU-12-2-06 made by Nisshin Carbon Co., Ltd.) satisfying the above relationship was mixed with respect to 100 parts by weight of MCMB. In addition, the components constituting the battery, the blending ratio of the positive electrode conductive agent, and the like are exactly the same as in Example 1.
[0076]
The capacity maintenance rate measurement and the nail penetration test were performed on Example b1 prepared as described above, and the characteristics as a lithium secondary battery were evaluated.
[0077]
Table 4 below shows the results of battery characteristic evaluation.
[0078]

[0079]
Compared with Comparative Example a0 in Example 1, as a negative electrode conductive agent, ρ ₂ ≧ 2.5 × ρ ₁ It can be seen that the result of the nail penetration test is improved even when the carbon material satisfying the above condition is included. Further, similarly to Example 1, the capacity retention rate is substantially the same as that of Comparative Example a0. From this, it can be seen that the carbon material is effective in improving the reliability of the lithium secondary battery regardless of whether it is included in the positive electrode or the negative electrode.
[0080]
(Example 3)
The lithium secondary battery shown in FIG. 2 was produced using the same method as in Example 1. However, unlike Example 1, the positive electrode conductive agent, ρ ₂ ≧ 2.5 × ρ ₁ The mixing ratio of the carbon material satisfying the above relationship was changed in the range of 1 to 21 parts by weight with respect to 100 parts by weight of the positive electrode active material. In addition, members constituting the battery are exactly the same as those in the first embodiment.
[0081]
The capacity maintenance rate measurement and the nail penetration test were performed on Examples c1 to c8 prepared as described above, and the characteristics as the lithium secondary battery were evaluated.
[0082]
Table 5 below shows the mixing ratio of the carbon material to the positive electrode and the evaluation results of the battery characteristics.
[0083]

[0084]
As shown in Table 5, the results of the nail penetration test were improved in all the samples as compared with Comparative Example a0 in Example 1, and a lithium secondary battery having higher safety and reliability was obtained. I understand. Among them, the electrode plate is ρ ₂ ≧ 2.5 × ρ ₁ When the carbon material satisfying the above condition is included in the range of 2 parts by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the active material, the battery has a good capacity retention rate. In particular, in the range of 9 parts by weight to 12 parts by weight, the battery capacity retention rate was superior to that of Comparative Example a0.
[0085]
Example 4
In this example, a lithium secondary battery having a carbon material layer as shown in FIG. 3 was produced, and the battery characteristics were evaluated. Note that the carbon material layer was stacked between the positive electrode current collector and the positive electrode active material layer.
[0086]
First, ρ ₂ ≧ 2.5 × ρ ₁ 30 parts by weight of a carbon material satisfying the above relationship (NU-12-2-06 made by Nisshin Carbon) and 70 parts by weight of polyvinylidene fluoride were mixed with NMP to form a slurry. This slurry was applied to both sides of an aluminum foil (thickness: 20 μm) as a positive electrode current collector and then dried to form a carbon material layer having a dry thickness of 3 μm on the positive electrode current collector. The carbon material layer contains 30% by weight of the carbon material.
[0087]
Next, 4 parts by weight of acetylene black as a conductive agent, 3 parts by weight of polyvinylidene fluoride as a binder, 40 parts by weight of NMP as a dispersion medium, and LiCoO as a positive electrode active material ₂ A positive electrode mixture was prepared by mixing 100 parts by weight. This positive electrode mixture was applied onto the carbon material layer, dried, and then compression molded with a roller press to produce a positive electrode including the carbon material layer.
[0088]
A lithium secondary battery was produced in the same manner as in Example 1 except that the positive electrode prepared as described above was used as the positive electrode. All members constituting the battery other than the positive electrode are the same as those in the first embodiment.
[0089]
Using the above method, prepare a total of 5 samples (samples d1 to d5) in which the composition of the carbon material in the carbon material layer is changed, measure the capacity retention rate, and perform the nail penetration test. As the characteristics were evaluated.
[0090]
Table 6 below shows the mixing ratio of the carbon material in the carbon material and the evaluation results of the battery characteristics.
[0091]

[0092]
As shown in Table 6, the results of the nail penetration test are improved in all the samples as compared with Comparative Example a0 in Example 1, and the safety and reliability can be improved by arranging the carbon material layer. It can be seen that an excellent lithium secondary battery was obtained. Moreover, especially when the said carbon material was included in the carbon material layer in the ratio of 50 weight% or less, the capacity | capacitance maintenance factor of the battery showed the favorable result.
[0093]
(Example 5)
In the same manner as in Example 4, seven types of samples (samples e1 to e7) in which the thickness of the carbon material layer used in Example 4 was changed were prepared, and the initial capacity measurement of the battery and the nail penetration test were performed. The characteristics as a lithium secondary battery were evaluated. The mixing ratio of the carbon material in the carbon material layer was 30% by weight with respect to the carbon material layer.
[0094]
The initial capacity of the battery was the battery capacity in the second cycle of the capacity maintenance rate measurement performed in Example 1, and the relative ratio was calculated with the value of Comparative Example a0 being 100.
[0095]
Table 7 below shows the dry thickness of the carbon material layer and the evaluation results of the battery characteristics.
[0096]

[0097]
As shown in Table 7, the results of the nail penetration test were improved in all the samples as compared with Comparative Example a0 in Example 1, and a lithium secondary battery superior in safety and reliability was obtained. I understand.
[0098]
Further, regarding the relative capacity ratio, when the thickness of the carbon material layer when dried is 20 μm or more, the initial capacity is reduced by 7% compared to Comparative Example a0 in which no carbon material layer is provided. It was. This indicates that the proportion of the carbon resistance layer in the electrode plate increases, resulting in a decrease in battery capacity.
[0099]
From the above, it can be said that the thickness of the carbon material layer when dried is preferably 20 μm or less in order to maintain the capacity of the battery while maintaining the effect of the carbon material layer.
[0100]
In Examples 1 to 5, a cylindrical lithium secondary battery was manufactured. However, the present invention is not limited to this, and for example, a lithium secondary battery such as a square type, a coin type, or a button type may be used. Similar effects can be obtained.
[0101]
【The invention's effect】
As described above, according to the present invention, even when the battery is crushed or when the positive electrode and the negative electrode are short-circuited due to a foreign object with a sharp tip piercing the battery, the rapid temperature rise of the battery is suppressed. It is possible to obtain a lithium secondary battery that is superior in safety and reliability. Moreover, the electrode plate for lithium secondary batteries which implement | achieves the said lithium secondary battery can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between temperature and volume resistivity in a carbon material used in the present invention and a conventional carbon material.
FIG. 2 is a cross-sectional view showing a structural example of a lithium secondary battery according to the present invention.
FIG. 3 is a cross-sectional view showing another structural example of the lithium secondary battery in the present invention.
[Explanation of symbols]
1,11 Negative electrode
2,12 Positive electrode
3, 13 Separator
4 Negative lead
5 Positive lead
6 cases
14 Negative electrode current collector
15 Positive current collector
16 Negative electrode active material layer
17 Positive electrode active material layer
18 Carbon material layer

Claims (15)

An electrode plate for a lithium secondary battery comprising an active material capable of reversibly occluding and releasing lithium ions, and a conductive agent,
The electrode plate for a lithium secondary battery, wherein the conductive agent includes a carbon material satisfying the following formula.
ρ ₂ ≧ 2.5 × ρ ₁
In the above formula, ρ ₁ is the volume resistivity of the carbon material at −20 ° C. to 60 ° C., and ρ ₂ is the volume resistivity of the carbon material at 180 ° C. The electrode plate for a lithium secondary battery according to claim 1, wherein the carbon material satisfies the following formula.
0 <ρ ₁ ≦ 1 × 10 ⁻¹ and ρ ₂ ≧ 1 × 10 ⁻²
However, the unit of ρ ₁ and ρ ₂ in the above formula is Ω · cm.   2. The electrode plate for a lithium secondary battery according to claim 1, wherein the carbon material is contained at a ratio of 2 parts by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the active material. The carbon material, oil absorption of dibutyl phthalate, lithium secondary battery plate of claim 1 wherein at most carbon material 100g per 100ml or 300 ml.   The electrode plate for a lithium secondary battery according to claim 1, wherein an average particle diameter of the carbon material is 10 nm or more and 100 nm or less. A lithium secondary battery comprising a positive electrode that reversibly occludes and releases lithium ions, a negative electrode that reversibly occludes and releases lithium ions, and an electrolyte having lithium ion conductivity,
The lithium secondary battery, wherein at least one electrode plate selected from the positive electrode and the negative electrode is the electrode plate for a lithium secondary battery according to any one of claims 1 to 5. A lithium secondary battery comprising an electrode plate group and an electrolyte having lithium ion conductivity,
The electrode group includes a positive electrode that reversibly stores and releases lithium ions, a negative electrode that reversibly stores and releases lithium ions, a separator, and a conductive carbon material that satisfies the following formula: And a lithium secondary battery.
ρ ₂ ≧ 2.5 × ρ ₁
In the above formula, ρ ₁ is the volume resistivity of the carbon material at −20 ° C. to 60 ° C., and ρ ₂ is the volume resistivity of the carbon material at 180 ° C. The lithium secondary battery according to claim 7, wherein the carbon material satisfies the following formula.
0 <ρ ₁ ≦ 1 × 10 ⁻¹ and ρ ₂ ≧ 1 × 10 ⁻²
However, the unit of ρ ₁ and ρ ₂ in the above formula is Ω · cm. The positive electrode includes a first current collector and a positive electrode active material layer supported by the first current collector;
The negative electrode includes a second current collector and a negative electrode active material layer supported by the second current collector;
The lithium secondary battery according to claim 7 or 8, wherein the first layer is disposed between at least one active material layer selected from the positive electrode active material layer and the negative electrode active material layer and the separator. . The positive electrode includes a first current collector and a positive electrode active material layer supported by the first current collector;
The negative electrode includes a second current collector and a negative electrode active material layer supported by the second current collector;
The lithium secondary battery according to claim 7 or 8, wherein the first layer is disposed inside at least one active material layer selected from the positive electrode active material layer and the negative electrode active material layer. The positive electrode includes a first current collector and a positive electrode active material layer supported by the first current collector;
The negative electrode includes a second current collector and a negative electrode active material layer supported by the second current collector;
The lithium secondary battery according to claim 7 or 8, wherein the first layer is disposed between the positive electrode active material layer and the first current collector. The positive electrode includes a first current collector and a positive electrode active material layer supported by the first current collector;
The negative electrode includes a second current collector and a negative electrode active material layer supported by the second current collector;
The lithium secondary battery according to claim 7 or 8, wherein the first layer is disposed between the negative electrode active material layer and the second current collector.   The lithium secondary battery according to any one of claims 7 to 12, wherein the first layer contains the carbon material in an amount of 0.1 wt% to 50 wt%.   The lithium secondary battery according to claim 7, wherein a dry thickness of the first layer is 0.01 μm or more and 20 μm or less.   The lithium secondary battery according to any one of claims 6 to 14, wherein the negative electrode includes a carbonaceous material capable of reversibly occluding and releasing lithium ions as an active material.

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