JP2004207140A - Lithium cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium cell reducing chain carbonate in an electrolyte and having excellent safety and low temperature property at the time of overcharging. <P>SOLUTION: This lithium cell comprises a positive electrode including a positive electrode active material capable of occluding and releasing lithium, a negative electrode including a carbon material as a negative electrode active material capable of occluding and releasing lithium, a separator and a nonaqueous electrolyte having nonaqueous solvent. The nonaqueous solvent includes ethylene carbonate and one or more ethylene carbonate derivatives presented by formula 1. R<SB>1</SB>-R<SB>4</SB>in formula 1 are each hydrogen atom, halogen, alkyl or halogenated alkyl, and these can be the same or different. At least two of R<SB>1</SB>-R<SB>4</SB>are the same alkyl or the same halogenated alkyl. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

（式中、Ｒ₁〜Ｒ₄は、水素原子、ハロゲン基、アルキル基、またはハロゲン化アルキル基を表し、これらは同一または異なっていてもよい。ただし、Ｒ₁〜Ｒ₄のうち少なくとも２つは、同一のアルキル基、または同一のハロゲン化アルキル基である。）
【００１１】
前記エチレンカーボネート誘導体では、その基本骨格である環状カーボネート平面(詳しくは、ほぼ平面状)に対して、置換基が上下方向に立体的に配位している。前記一般式（１）では、この立体的な置換基の配位を破線と太線とで表している。
【００１２】
前記エチレンカーボネート誘導体は、充放電に起因して析出するリチウムと緩やかに反応して、電極表面に炭酸リチウムの保護被膜を形成し、電極と電解液との分解反応を緩和する。そのため、前記構成によると、電池が過充電状態になった場合でも発煙、発火、破裂などの重大な異常が起こりにくく、過充電時の安全性（耐過充電特性）に優れたリチウム電池が提供できる。
【００１３】
さらに、前記エチレンカーボネート誘導体は、エチレンカーボネート（ＥＣ）と比べて粘度と凝固点が低いため、エチレンカーボネート（ＥＣ）を含む電解液の粘度を低下させる。このため、前記構成によると、電池が通常使用されうる−２０℃〜０℃の低温環境においても、優れた放電性能（低温特性）を発揮しうるリチウム電池が提供できる。
【００１４】
前記本発明のリチウム電池は、さらに、前記エチレンカーボネート誘導体の前記同一のアルキル基または前記同一のハロゲン化アルキル基が、トランス型に配位している構成とすることができる。
【００１５】
トランス型の配位とは、前記同一のアルキル基または前記同一のハロゲン化アルキル基が、その基本骨格である環状カーボネート平面(詳しくは、ほぼ平面状)に対して、上下方向に互い違いに配位している状態を示す。
【００１６】
トランス型配位のエチレンカーボネート誘導体であると、炭素層間への侵入に際し立体障害の程度が大きくなる。このため、前記構成によると、リチウムイオンと溶媒和したエチレンカーボネート誘導体の炭素層間への侵入が防止されるので、炭素材料の層構造の損傷が抑制され、その結果として充放電特性に優れたリチウム電池を提供できる。
【００１７】
前記本発明のリチウム電池においては、前記非水溶媒がさらにプロピレンカーボネートを含む構成とすることができる。
【００１８】
この構成によると、エチレンカーボネート（ＥＣ）を含む電解液の粘性がさらに低下し電池の低温特性が向上すること、プロピレンカーボネート（ＰＣ）自体が析出リチウムと緩やかに反応する溶媒であるため電解液と析出リチウムとの反応に伴う発熱がさらに分散できること、電極表面に炭酸リチウムの被膜がさらに形成されやすくなるなどの、プロピレンカーボネート（ＰＣ）を配合したことによる利点が、プロピレンカーボネート（ＰＣ）が炭素材料の層構造に損傷を与える欠点を上回るため、優れた耐過充電特性を維持しつつ、さらに低温特性が向上したリチウム電池を提供できる。
【００１９】
前記本発明のリチウム電池においては、さらに、前記エチレンカーボネート誘導体が、４，４’−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、またはｔｒａｎｓ−４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅからなる群から選ばれた１以上の化合物である構成とすることができる。
【００２０】
前記エチレンカーボネート誘導体は、その置換基として、鎖状構造がもっとも短いアルキル基であるメチル基を有するため、他のエチレンカーボネート誘導体に比べ、その混合による電解液の低粘度化が一層図りやすい。したがって、前記構成によると、耐過充電特性を維持しつつ、低温特性が一層優れたリチウム電池を提供できる。
【００２１】
【発明の実施の形態】
以下に、本発明電池を薄型リチウム二次電池に適用した場合について、図面を用いながら説明する。
【００２２】
図１および図２に示すように、本発明に係る薄型リチウム二次電池は、フィルム状外装体の一例であるアルミラミネート外装体３を有している。このアルミラミネート外装体３は、電極体１を収納するための収納空間２を備えた本体部と、アルミラミネート外装体３の開口部を封止する周縁封止部とを有している。さらに、この収納空間２内には、非水電解液が存在している。ここで、この電極体１は、正極と、正極に取り付けられた正極集電タブ４と、負極と、負極に取り付けられた負極集電タブ５と、これら両電極を離間するセパレータとを含んでいる。
【００２３】
上記構造のリチウム二次電池を、以下のようにして作製した。
【００２４】
〔正極の作製〕
正極活物質としてのコバルト酸リチウム（ＬｉＣｏＯ₂）と炭素導電剤としてのグラファイトとを９２：５の質量比で混合して正極合剤粉末とし、混合装置（ホソカワミクロン製メカノフュージョン装置（ＡＭ‐１５Ｆ））内に充填した。この混合装置を回転数１５００ｒｐｍで１０分間作動させて、前記粉末に圧縮・衝撃・せん断力を作用させた混合正極活物質を作製した。この混合正極活物質とフッ素系樹脂結着剤(ポリフッ化ビニリデン：ＰＶＤＦ)とを９７：３の質量比でＮ−メチルピロリドン（ＮＭＰ）溶剤中に混合して正極合剤スラリーとし、このスラリーをアルミニウム箔の両面に塗着し、乾燥後圧延して正極板を作製した。
【００２５】
ここでは正極活物質としてコバルト酸リチウムを用いたが、これに限定されるものではなく、ニッケル酸リチウムに代表されるリチウムニッケル複合酸化物、スピネル型マンガン酸リチウムに代表されるリチウムマンガン複合酸化物やオリビン型リン酸化合物などを用いることができる。
【００２６】
また、メカノフュージョンでの混合は行わなくても良く、そのまま材料をスラリー状態で混合しても良く、また他の方法で混合しても良い。
【００２７】
〔負極の作製〕
負極活物質としての黒鉛と、スチレンブタジエンゴム(ＳＢＲ)とを９８：２の質量比で混合し、銅箔の両面に塗着した後、乾燥圧延して負極板を作製した。
【００２８】
ここでは負極活物質として黒鉛を用いたが、炭素材料としてはグラファイトやコークスであっても、人工の炭素材料であっても良い。さらに、酸化スズ、金属リチウム、珪素を炭素材料と混合させて用いることができる。
【００２９】
〔電解液の作製〕
下記式（２）で示す４，４’−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ（以下、Ｓｏｌ−Ａとする）と、エチレンカーボネート（ＥＣ）と、プロピレンカーボネ−ト（ＰＣ）とを２：４：４の体積比（１０１３ｈＰａ、２５℃における）で混合した混合溶媒に、ＬｉＰＦ₆を１．０ｍｏｌ／ｌの割合で溶解した。さらに、この溶液に、添加剤としてのビニルエチレンカーボネート（ＶＥＣ）とトリオクチルフォスフェート（ＴＯＰ）とを、電解液中の最終濃度がそれぞれ３質量％と１．５質量％となるように添加して電解液を作製した。
【００３０】
【化３】

【００３１】
ここで、リチウム塩として上記ＬｉＰＦ₆以外に、ＬｉＣｌＯ₄、ＬｉＰＦ₆、ＬｉＮ（ＳＯ₂ＣＦ₃）₂、ＬｉＮ（ＳＯ₂Ｃ₂Ｆ₅）₂、ＬｉＰＦ_6-X(Ｃ_nＦ_2n+1)_X [但し、１＜ｘ≦６， ｎ＝１または２]等の１種、または２種以上を混合して用いることができる。また、電解液中の支持塩の濃度は特に限定されないが０．８〜１．５ｍｏｌ／ｌの割合であることが好ましい。
【００３２】
〔電池体の作製〕
それぞれにリード端子を取り付けた前記正負極を、ポリエチレン製のセパレータを介して対向させ、渦巻状に巻き取ることにより電極体を作製した。この電極体を、有底筒状に封止したアルミラミネート外装体の収納空間内に挿入し、上記電解液を注液した後、開口部を封止して試験用電池(理論容量：７００ｍＡｈ)を作製した。
【００３３】
ここで、前記外装体の材質や形状は特に限定されず、その材質は鉄やアルミニウムであってもよく、その形状は角型や円筒型であってもよい。また、そのサイズについても特に制限するものではない。
【００３４】
つぎに、本発明について、実施例および比較例に基づいてさらに詳細に説明するが、本発明は下記の実施例に限定されるものではない。
（実施例１）
実施例１としては、上記実施の形態に示す方法と同様の方法にて作製したリチウム二次電池を用いた。
【００３５】
（実施例２）
実施例１の電池で用いた混合溶媒中のＳｏｌ−Ａに代えて、４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ（以下、Ｓｏｌ−Ｂとする）を用いたこと以外は実施例１と同様にして電池を作製した。ここで、このＳｏｌ−Ｂは、ｃｉｓ異性体である下記式（３）とｔｒａｎｓ異性体である下記式（４）とで示す化合物の体積比５：５（１０１３ｈＰａ、２５℃における）の混合物である。
【００３６】
【化４】

【００３７】
【化５】

【００３８】
（実施例３）
実施例１の電池で用いた混合溶媒中のＳｏｌ−Ａに代えて、下記式（４）で示す、ｔｒａｎｓ−４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ（以下、Ｓｏｌ−Ｃとする）を用いたこと以外は実施例１と同様にして電池を作製した。
【００３９】
【化６】

【００４０】
（比較例１）
実施例１の電池で用いた混合溶媒に代えて、エチレンカーボネート（ＥＣ）とプロピレンカーボネート（ＰＣ）とを５：５の体積比（１０１３ｈＰａ、２５℃における）で混合した混合溶媒を用いたこと以外は実施例１と同様にして電池を作製した。
【００４１】
（比較例２）
実施例１の電池で用いた混合溶媒に代えて、ＥＣとＰＣとジエチルカーボネート（ＤＥＣ）とを４：４：２の体積比（１０１３ｈＰａ、２５℃における）で混合した混合溶媒を用いたこと以外は実施例１と同様にして電池を作製した。
【００４２】
（比較例３）
実施例１の電池で用いた混合溶媒に代えて、ＥＣとＤＥＣとを５：５の体積比（１０１３ｈＰａ、２５℃における）で混合した混合溶媒を用いたこと以外は実施例１と同様にして電池を作製した。
【００４３】
（比較例４）
実施例１の電池で用いた混合溶媒に代えて、ＥＣとガンマブチロラクトン（ＧＢＬ）とを５：５の体積比（１０１３ｈＰａ、２５℃における）で混合した混合溶媒を用い、電解液中のビニルエチレンカーボネート（ＶＥＣ）の最終濃度が１．５質量％である電解液を用いたこと以外は実施例１と同様にして電池を作製した。
【００４４】
〔実験１〕
電池の過充電に対する安全性と、上記エチレンカーボネート誘導体を含む電解液との関係を調べるため、実施例１〜３および比較例１〜４の電池を用いて以下の耐過充電試験を行った。
【００４５】
〔耐過充電試験〕
保護素子・保護回路等を備えていない、実施例１〜３および比較例１〜４の電池を用意した。７００ｍＡ（１．０Ｉｔ）の定電流で２．７５Ｖになるまで放電した各電池を、１．０Ｉｔ〜５．０Ｉｔの範囲で段階的に設定した定電流値で、電池の電圧が１２Ｖになるまで充電した。この充電中に、煙の発生、破裂、発火などの電池異常が発生した場合の充電電流値を限界充電電流と規定した。なお、実験は室温（２５℃）下にて行った。
【００４６】
この耐過充電試験の結果を下記表１に示す。
【００４７】
【表１】

【００４８】
表１に示すように、混合溶媒に上記一般式（１）で示されるエチレンカーボネート誘導体を含む実施例１〜３の電池では、いずれも限界充電電流が５Ｉｔ（３５００ｍＡ）であり、上記エチレンカーボネート誘導体を混合溶媒中に含まない比較例２〜４の電池とくらべて高い限界充電電流を示した。
【００４９】
このことから、混合溶媒中に上記エチレンカーボネート誘導体を含む本発明電池では、優れた耐過充電特性（高い限界充電電流）が実現することがわかった。
【００５０】
ここで、比較例１の電池ではその限界充電電流が５Ｉｔ（３５００ｍＡ）であり、実施例１〜３の電池と同様に優れた耐過充電特性が認められたが、実施例１〜３の電池は、比較例１の電池よりも低温特性（−２０℃の低温環境における放電容量）に優れることを、後述する実験２の結果により示す。
【００５１】
〔実験２〕
電池の低温特性（−２０℃の低温環境における放電容量）と、上記エチレンカーボネート誘導体とエチレンカーボネート（ＥＣ）とプロピレンカーボネート（ＰＣ）とを有する電解液との関係を調べるため、実施例１〜３および比較例１の電池を用いて以下の低温特性試験を行った。
【００５２】
〔低温特性試験〕
実施例１〜３および比較例１の電池を２個ずつ用意した。各第１の電池と各第２の電池とを、室温（２５℃）下で、７００ｍＡ（１．０Ｉｔ）の充電電流で４．２Ｖになるまで定電流充電し、その後４．２Ｖの定電圧で電流値が３５ｍＡ（０．０５Ｉｔ）になるまで充電した。次に、第１の電池を、７００ｍＡ（１．０Ｉｔ）の定電流で終止電圧が２．７５Ｖになるまで、室温（２５℃）下で放電させ、その放電時間から室温下における放電容量を算出した。また、低温（−２０℃）下で第２の電池を同様に放電させ、低温下における放電容量を算出した。
【００５３】
ここで、各電池の低温特性を表すため、以下の式（５）に従い、それぞれの放電容量比（％）を求めた。この結果と上記限界充電電流値とを表２に示す。
【００５４】
【数１】

【００５５】
【表２】

【００５６】
表２に示すように、実施例１〜３および比較例１のいずれの電池においても、優れた耐過充電特性（高い限界充電電流）が認められた。しかしながら、比較例１の電池ではその放電容量比が１５％であったのに対して、実施例１〜３の電池では、それぞれ１８％、２４％、３３％と、電池の放電容量比が比較例１の電池よりも向上していることが認められた。
【００５７】
このことから、エチレンカーボネート（ＥＣ）とプロピレンカーボネート（ＰＣ）とを含む混合溶媒に、エチレンカーボネート誘導体を混合させた本発明電池では、優れた耐過充電特性と低温特性が両立して得られることがわかった。
【００５８】
また、Ｓｏｌ−Ａ、Ｓｏｌ−Ｂ、Ｓｏｌ−Ｃの中では、Ｓｏｌ−Ｃを混合した場合に最も優れた低温特性が得られることがわかった。
【００５９】
このように、Ｓｏｌ−Ｃを電解液に含むことにより低温下で優れた放電容量が得られた理由としては、Ｓｏｌ−ＣがＳｏｌ−ＡまたはＳｏｌ−Ｂと比べて、立体障害の度合いが大きいことに加えて、その凝固点（１０１３ｈＰａにおける）が低く（Ｓｏｌ−Ｃ：＜−１０℃、Ｓｏｌ−Ａ：２１℃、Ｓｏｌ−Ｂ：３５℃）、また、その混合によりエチレンカーボネートを含む電解液全体の粘度を下げる効果が大きいことが考えられる。
【００６０】
〔その他の事項〕
（１）本発明のリチウム二次電池は、上記の液系の電池に限らず、ゲル系のポリマー電池に適用した場合にも同様の優れた効果が得られる。ここで、ポリマー電池の固体電解質に使用するポリマー材料としては、ポリエーテル系固体高分子、ポリカーボネート系固体高分子、ポリアクリロニトリル系固体高分子の１種、または２種以上からなる、共重合体や架橋した高分子などが挙げられる。
【００６１】
（２）上記実施例では、エチレンカーボネート誘導体の１種を電解液中に混合させた電池を示しているが、２種以上のエチレンカーボネート誘導体を混合して用いることができる。１種以上のエチレンカーボネート誘導体を電解液中に混合させる場合、エチレンカーボネート（ＥＣ）含有比とプロピレンカーボネート（ＰＣ）含有比とは並行して削減することが好ましい。
【００６２】
（３）電解液中の上記Ｓｏｌ−Ａ、Ｓｏｌ−Ｂ、Ｓｏｌ−Ｃの含有量は、Ｓｏｌ−ＡまたはＳｏｌ−Ｂでは０体積％を超えて３０体積％以下（１０１３ｈＰａ、２５℃において）、Ｓｏｌ−Ｃでは０体積％を超えて７０体積％以下（１０１３ｈＰａ、２５℃において）とすることができるが、電池性能の向上とコスト面とのバランスから、それぞれの含有量は、Ｓｏｌ−ＡまたはＳｏｌ−Ｂでは１０体積％以上２０体積％以下、Ｓｏｌ−Ｃでは１０体積％以上４０体積％以下であることが好ましい。
【００６３】
（４）上記エチレンカーボネート誘導体は、アルキル基の鎖状構造が長くなるほどその粘度が上昇し、比誘電率が下がるため、置換基としてのアルキル基は、低級アルキル基であることが好ましく、特にメチル基またはエチル基が好ましい。さらに、上記置換基としてのハロゲンは、具体的に、フッ素があげられる。
【００６４】
（５）上記ハロゲン化アルキル基として、具体的には、フッ化メチル基（モノフルオロメチル基、ジフルオロメチル基、トリフルオロメチル基）、フッ化エチル基（モノフルオロエチル基、ジフルオロエチル基、トリフルオロエチル基、テトラフルオロエチル基、ペンタフルオロエチル基）があげられる。
【００６５】
（６）前記置換基がメチル基であるエチレンカーボネート誘導体として、具体的には、４，４’−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｃｉｓ−４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｔｒａｎｓ−４，５−Ｄｉｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、または４，４’，５，５’−Ｔｅｔｒａｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００６６】
（７）前記置換基がエチル基であるエチレンカーボネート誘導体として、具体的には、４，４’−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｃｉｓ−４，５−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｔｒａｎｓ−４，５−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、または４，４’，５，５’−Ｔｅｔｒａｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００６７】
（８）前記置換基がメチル基とエチル基であるエチレンカーボネート誘導体として、具体的には、４，４’−Ｄｉｍｅｔｈｙｌ−５，５’−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、４，５−Ｄｉｍｅｔｈｙｌ−４’，５’−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、４，５’−Ｄｉｍｅｔｈｙｌ−４’，５−Ｄｉｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００６８】
（９）前記置換基がフッ化メチル基であるエチレンカーボネート誘導体として、具体的には、４，４’− Ｄｉｆｌｕｏｒｏｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｃｉｓ−４，５−Ｄｉｆｌｕｏｒｏｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｔｒａｎｓ−４，５−Ｄｉｆｌｕｏｒｏｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、または４，４’，５，５’−Ｔｅｔｒａｆｌｕｏｒｏｍｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００６９】
（１０）前記置換基がフッ化エチル基であるエチレンカーボネート誘導体として、具体的には、４，４’− Ｄｉｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｃｉｓ−４，５−Ｄｉｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、ｔｒａｎｓ−４，５−Ｄｉｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、または４，４’，５，５’−Ｔｅｔｒａｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００７０】
（１１）前記置換基がフッ化メチル基とフッ化エチル基であるエチレンカーボネート誘導体として、具体的には、４，４’− Ｄｉｆｌｕｏｒｏｍｅｔｈｙｌ−５，５’− Ｄｉｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅ、４，５’− Ｄｉｆｌｕｏｒｏｍｅｔｈｙｌ−４’，５− Ｄｉｆｌｕｏｒｏｅｔｈｙｌ−１，３−ｄｉｏｘｏｌａｎ−２−ｏｎｅがあげられる。
【００７１】
【発明の効果】
以上説明したように、本発明によると、新規な電解液成分である上記一般式（１）で表されるエチレンカーボネート誘導体が、電極表面に保護被膜を形成し、エチレンカーボネート（ＥＣ）を含む電解液の低粘度化を可能にする。しかも、この物質自体が炭素層間に侵入しにくい立体構造を有するため、充放電がくりかえされても炭素材料の層構造を損傷しない。したがって、本発明によると、耐過充電特性と低温特性とに優れ、サイクル特性にも優れたリチウム電池が提供できる。
【図面の簡単な説明】
【図１】図１は本発明に係る薄型リチウム二次電池の正面図である。
【図２】図２は図１のＡ−Ａ線矢視断面図である。
【符号の説明】
１：電極体
２：収納空間
３：ラミネート外装体
４：正極集電タブ
５：負極集電タブ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a lithium battery, and more particularly, to maintenance of overcharge resistance and improvement of low-temperature characteristics by improving a solvent of an electrolytic solution.
[0002]
[Prior art]
Ethylene carbonate (EC) has a very high relative dielectric constant among the solvents for lithium secondary batteries, and is of great utility in increasing the voltage and energy density of batteries, but ethylene carbonate (EC) Since it is a solid at normal temperature (25 ° C.), it cannot be used alone. For this reason, it is used as a mixed solvent with a chain carbonate represented by diethyl carbonate (DEC) which is liquid at ordinary temperature.
[0003]
However, since the chain carbonate has a low boiling point, the addition thereof decreases the high-temperature storage characteristics of the battery. In particular, since diethyl carbonate (DEC) has high reactivity with lithium, when the battery temperature rises to about 80 ° C. due to overcharging of the battery or the like, it reacts violently with lithium deposited on the electrode surface. Therefore, a battery using a diethyl carbonate (DEC) electrolyte has a problem that the overcharge resistance (safety of the battery in an overcharged state) is poor.
[0004]
This problem in diethyl carbonate (DEC) is attributable to the fact that the decomposition product of diethyl carbonate (DEC) is mainly a metal alcoholate, and thus does not function as a protective coating on the electrode surface that suppresses the reaction between the electrode and the solvent. It is thought that there is.
[0005]
Therefore, in order to improve the overcharge resistance of the lithium secondary battery, instead of chain carbonate, a solvent such as ethylene carbonate (EC) and propylene carbonate (PC), which is a solvent in which a lithium carbonate film is easily formed on the electrode surface, is used. A technique using a mixed solvent of cyclic carbonates or a mixed solvent of a cyclic carbonate and a cyclic lactone such as ethylene carbonate (EC) and gamma-butyrolactone (GBL) has been proposed (for example, see Patent Documents 1 and 2).
[0006]
[Patent Document 1]
JP-A-11-195429 (page 2)
[Patent Document 2]
JP 2000-149992 A (page 2)
[0007]
According to the technology described in Patent Document 1 or Patent Document 2, in a battery using an electrolyte solution in which propylene carbonate (PC) is mixed with ethylene carbonate (EC), a mixed solvent of diethyl carbonate (DEC) and ethylene carbonate (EC) is used. The overcharge resistance is improved as compared with the case where is used. However, propylene carbonate (PC), due to its three-dimensional structure, impairs the graphite structure when entering and exiting between layers of graphite, which is a negative electrode active material, has a cycle characteristic (maintaining a battery capacity accompanying repeated charge / discharge). Rate) is deteriorated. For this reason, the blending amount of propylene carbonate (PC) cannot be increased to a certain level or more, and as a result, the viscosity of the entire electrolyte containing ethylene carbonate (EC) cannot be sufficiently reduced. Therefore, the above technique has a problem that it is difficult to improve the low-temperature characteristics of the battery.
[0008]
When gamma-butyrolactone (GBL) is used instead of propylene carbonate (PC), the reactivity with the graphite negative electrode is not as high as that of propylene carbonate (PC). However, they are susceptible to hydrolysis and the like, and in particular, have a problem that the overcharge resistance is greatly reduced.
[0009]
[Problems to be solved by the invention]
The present invention has been proposed in order to solve the problem of the electrolytic solution containing ethylene carbonate (EC). By mixing a new solvent in the electrolytic solution, the overcharge resistance can be increased to a high level. It is an object of the present invention to provide a lithium battery having further improved low-temperature characteristics while maintaining the same.
[0010]
[Means for Solving the Problems]
The lithium battery of the present invention includes a positive electrode including a positive electrode active material capable of inserting and extracting lithium, a negative electrode including a carbon material capable of inserting and extracting lithium as a negative electrode active material, a separator, and a nonaqueous electrolyte including a nonaqueous solvent. Wherein the nonaqueous solvent contains ethylene carbonate and one or more ethylene carbonate derivatives represented by the following general formula (1).
Embedded image

(In the formula, R _{1 to} R ₄ represent a hydrogen atom, a halogen group, an alkyl group, or a halogenated alkyl group, which may be the same or different. However, at least two of R _{1 to} R ₄ Are the same alkyl group or the same halogenated alkyl group.)
[0011]
In the ethylene carbonate derivative, the substituent is three-dimensionally coordinated in the vertical direction with respect to the cyclic carbonate plane (specifically, substantially planar) which is the basic skeleton. In the general formula (1), the steric substituent coordination is represented by a broken line and a thick line.
[0012]
The ethylene carbonate derivative reacts slowly with lithium precipitated due to charge and discharge to form a protective film of lithium carbonate on the surface of the electrode, thereby alleviating the decomposition reaction between the electrode and the electrolytic solution. Therefore, according to the above configuration, even when the battery is in an overcharged state, serious abnormalities such as smoke, ignition, and rupture are unlikely to occur, and a lithium battery with excellent overcharge safety (overcharge resistance) is provided. it can.
[0013]
Further, since the ethylene carbonate derivative has a lower viscosity and a lower freezing point than ethylene carbonate (EC), it lowers the viscosity of the electrolyte solution containing ethylene carbonate (EC). For this reason, according to the said structure, the lithium battery which can exhibit the outstanding discharge performance (low temperature characteristic) even in the low temperature environment of -20 degreeC-0 degreeC where a battery can be normally used can be provided.
[0014]
The lithium battery of the present invention may have a configuration in which the same alkyl group or the same halogenated alkyl group of the ethylene carbonate derivative is coordinated in a trans form.
[0015]
The trans-type coordination means that the same alkyl group or the same halogenated alkyl group is staggered up and down with respect to a cyclic carbonate plane (specifically, substantially planar) which is a basic skeleton thereof. Shows the status of
[0016]
In the case of a trans-coordinated ethylene carbonate derivative, the degree of steric hindrance at the time of intrusion between carbon layers increases. For this reason, according to the above configuration, the penetration of the ethylene carbonate derivative solvated with lithium ions into the carbon layer is prevented, so that damage to the layer structure of the carbon material is suppressed, and as a result, lithium having excellent charge / discharge characteristics is obtained. Battery can be provided.
[0017]
In the lithium battery of the present invention, the non-aqueous solvent may further include propylene carbonate.
[0018]
According to this configuration, the viscosity of the electrolyte containing ethylene carbonate (EC) is further reduced, and the low-temperature characteristics of the battery are improved. Also, since propylene carbonate (PC) itself is a solvent that slowly reacts with the deposited lithium, the electrolyte is The advantages of blending propylene carbonate (PC), such as that the heat generated by the reaction with the precipitated lithium can be further dispersed and that a lithium carbonate coating is more easily formed on the electrode surface, are the advantages of propylene carbonate (PC) being a carbon material. Thus, it is possible to provide a lithium battery with improved low-temperature characteristics while maintaining excellent overcharge resistance characteristics, because it overcomes the disadvantage of damaging the layer structure of the above.
[0019]
In the lithium battery of the present invention, the ethylene carbonate derivative may further include 4,4′-dimethyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, Alternatively, the composition may be one or more compounds selected from the group consisting of trans-4,5-dimethyl-1,3-dioxolan-2-one.
[0020]
Since the ethylene carbonate derivative has, as a substituent, a methyl group which is an alkyl group having the shortest chain structure, it is easier to lower the viscosity of the electrolyte by mixing the ethylene carbonate derivative than other ethylene carbonate derivatives. Therefore, according to the above configuration, it is possible to provide a lithium battery having excellent low-temperature characteristics while maintaining overcharge resistance.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a case where the battery of the present invention is applied to a thin lithium secondary battery will be described with reference to the drawings.
[0022]
As shown in FIGS. 1 and 2, the thin lithium secondary battery according to the present invention has an aluminum laminate exterior body 3 which is an example of a film exterior body. The aluminum laminate exterior body 3 has a main body having a storage space 2 for accommodating the electrode body 1, and a peripheral sealing portion for sealing an opening of the aluminum laminate exterior body 3. Further, a non-aqueous electrolyte is present in the storage space 2. Here, the electrode body 1 includes a positive electrode, a positive electrode current collecting tab 4 attached to the positive electrode, a negative electrode, a negative electrode current collecting tab 5 attached to the negative electrode, and a separator separating these two electrodes. I have.
[0023]
The lithium secondary battery having the above structure was manufactured as follows.
[0024]
(Preparation of positive electrode)
Lithium cobaltate (LiCoO ₂ ) as a positive electrode active material and graphite as a carbon conductive agent were mixed at a mass ratio of 92: 5 to form a positive electrode mixture powder, and a mixing device (Mesofusion device (AM-15F manufactured by Hosokawa Micron)) ). This mixing device was operated at a rotation speed of 1500 rpm for 10 minutes to produce a mixed positive electrode active material in which the powder was subjected to compression, impact and shear forces. This mixed positive electrode active material and a fluorine-based resin binder (polyvinylidene fluoride: PVDF) were mixed in an N-methylpyrrolidone (NMP) solvent at a mass ratio of 97: 3 to form a positive electrode mixture slurry. Coating was performed on both sides of the aluminum foil, followed by drying and rolling to prepare a positive electrode plate.
[0025]
Here, lithium cobalt oxide was used as the positive electrode active material. However, the present invention is not limited to this. A lithium nickel composite oxide represented by lithium nickel oxide and a lithium manganese composite oxide represented by spinel lithium manganate And an olivine-type phosphate compound.
[0026]
Further, the mixing in the mechanofusion need not be performed, and the materials may be mixed as they are in a slurry state, or may be mixed by another method.
[0027]
(Preparation of negative electrode)
Graphite as a negative electrode active material and styrene butadiene rubber (SBR) were mixed at a mass ratio of 98: 2, applied to both surfaces of a copper foil, and dried and rolled to prepare a negative electrode plate.
[0028]
Here, graphite was used as the negative electrode active material, but the carbon material may be graphite or coke, or an artificial carbon material. Further, tin oxide, metallic lithium, and silicon can be used by being mixed with a carbon material.
[0029]
(Preparation of electrolyte solution)
4,4′-Dimethyl-1,3-dioxolan-2-one (hereinafter referred to as Sol-A) represented by the following formula (2), ethylene carbonate (EC), and propylene carbonate (PC) Was mixed at a volume ratio of 2: 4: 4 (1013 hPa, at 25 ° C.), and LiPF ₆ was dissolved at a ratio of 1.0 mol / l. Further, to this solution, vinylethylene carbonate (VEC) and trioctyl phosphate (TOP) as additives were added so that the final concentrations in the electrolytic solution were 3% by mass and 1.5% by mass, respectively. To prepare an electrolytic solution.
[0030]
Embedded image

[0031]
Here, in addition to the above-mentioned LiPF ₆ as a lithium salt, LiClO ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-X (C _n F _{2n + 1} ) _X [where 1 <x ≦ 6, n = 1 or 2] and the like, or a mixture of two or more thereof can be used. The concentration of the supporting salt in the electrolyte is not particularly limited, but is preferably 0.8 to 1.5 mol / l.
[0032]
(Preparation of battery body)
The positive and negative electrodes, each having a lead terminal attached thereto, were opposed to each other with a polyethylene separator interposed therebetween and spirally wound to produce an electrode body. This electrode body was inserted into a storage space of an aluminum laminate exterior body sealed in a bottomed cylindrical shape, and after the above-mentioned electrolyte was injected, the opening was sealed and a test battery (theoretical capacity: 700 mAh). Was prepared.
[0033]
Here, the material and shape of the exterior body are not particularly limited, and the material may be iron or aluminum, and the shape may be square or cylindrical. Also, the size is not particularly limited.
[0034]
Next, the present invention will be described in more detail based on examples and comparative examples, but the present invention is not limited to the following examples.
(Example 1)
In Example 1, a lithium secondary battery manufactured by a method similar to the method described in the above embodiment mode was used.
[0035]
(Example 2)
The procedure was performed except that 4,5-dimethyl-1,3-dioxolan-2-one (hereinafter referred to as Sol-B) was used instead of Sol-A in the mixed solvent used in the battery of Example 1. A battery was produced in the same manner as in Example 1. Here, this Sol-B is a mixture of compounds represented by the following formula (3) which is a cis isomer and the following formula (4) which is a trans isomer in a volume ratio of 5: 5 (at 1013 hPa at 25 ° C.). is there.
[0036]
Embedded image

[0037]
Embedded image

[0038]
(Example 3)
In place of Sol-A in the mixed solvent used in the battery of Example 1, trans-4,5-dimethyl-1,3-dioxolan-2-one (hereinafter referred to as Sol-C) represented by the following formula (4): ) Was used in the same manner as in Example 1 except that the above-described method was used.
[0039]
Embedded image

[0040]
(Comparative Example 1)
Instead of using a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) at a volume ratio of 5: 5 (1013 hPa at 25 ° C.) instead of the mixed solvent used in the battery of Example 1. A battery was produced in the same manner as in Example 1.
[0041]
(Comparative Example 2)
Except that a mixed solvent of EC, PC and diethyl carbonate (DEC) at a volume ratio of 4: 4: 2 (at 1013 hPa at 25 ° C.) was used instead of the mixed solvent used in the battery of Example 1. A battery was produced in the same manner as in Example 1.
[0042]
(Comparative Example 3)
In the same manner as in Example 1 except that a mixed solvent in which EC and DEC were mixed at a volume ratio of 5: 5 (at 1013 hPa at 25 ° C.) was used instead of the mixed solvent used in the battery of Example 1, A battery was manufactured.
[0043]
(Comparative Example 4)
Instead of the mixed solvent used in the battery of Example 1, a mixed solvent of EC and gamma-butyrolactone (GBL) mixed at a volume ratio of 5: 5 (at 1013 hPa at 25 ° C.) was used. A battery was fabricated in the same manner as in Example 1, except that an electrolyte having a final concentration of carbonate (VEC) of 1.5% by mass was used.
[0044]
[Experiment 1]
The following overcharge resistance test was performed using the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 in order to investigate the relationship between the safety of the battery against overcharge and the electrolyte solution containing the ethylene carbonate derivative.
[0045]
(Overcharge test)
Batteries of Examples 1 to 3 and Comparative Examples 1 to 4, which were not provided with a protection element and a protection circuit, were prepared. Each battery discharged at a constant current of 700 mA (1.0 It) until it reaches 2.75 V is subjected to a constant current value set stepwise within a range of 1.0 It to 5.0 It until the battery voltage reaches 12 V. Charged. During this charging, the charging current value when a battery abnormality such as smoke, rupture, or ignition occurred was defined as the limit charging current. The experiment was performed at room temperature (25 ° C.).
[0046]
The results of this overcharge resistance test are shown in Table 1 below.
[0047]
[Table 1]

[0048]
As shown in Table 1, in the batteries of Examples 1 to 3 including the ethylene carbonate derivative represented by the above general formula (1) in the mixed solvent, the limit charging current was 5 It (3500 mA), and the above ethylene carbonate derivative was used. Showed a higher limit charging current than those of the batteries of Comparative Examples 2 to 4 containing no in the mixed solvent.
[0049]
From this, it was found that the battery of the present invention containing the above-mentioned ethylene carbonate derivative in the mixed solvent achieves excellent overcharge resistance (high limit charging current).
[0050]
Here, in the battery of Comparative Example 1, the limit charging current was 5 It (3500 mA), and excellent overcharge resistance was recognized as in the batteries of Examples 1 to 3. Shows that the battery of Comparative Example 1 is superior to the battery of Comparative Example 1 in low-temperature characteristics (discharge capacity in a low-temperature environment of −20 ° C.) by the results of Experiment 2 described below.
[0051]
[Experiment 2]
To examine the relationship between the low temperature characteristics of the battery (discharge capacity in a low temperature environment of −20 ° C.) and the electrolytic solution having the above ethylene carbonate derivative, ethylene carbonate (EC) and propylene carbonate (PC), Examples 1 to 3 were used. The following low-temperature characteristic test was performed using the battery of Comparative Example 1.
[0052]
(Low temperature characteristic test)
Two batteries each of Examples 1 to 3 and Comparative Example 1 were prepared. Each of the first batteries and each of the second batteries were charged at a constant current of 700 mA (1.0 It) at room temperature (25 ° C.) with a charging current of 700 mA (1.0 It) until the voltage reached 4.2 V, and then a constant voltage of 4.2 V was applied. And charged until the current value became 35 mA (0.05 It). Next, the first battery is discharged at room temperature (25 ° C.) at a constant current of 700 mA (1.0 It) until the final voltage becomes 2.75 V, and the discharge capacity at room temperature is calculated from the discharge time. did. Further, the second battery was similarly discharged at a low temperature (−20 ° C.), and the discharge capacity at a low temperature was calculated.
[0053]
Here, in order to represent the low-temperature characteristics of each battery, the respective discharge capacity ratios (%) were determined according to the following equation (5). Table 2 shows the result and the limit charging current value.
[0054]
(Equation 1)

[0055]
[Table 2]

[0056]
As shown in Table 2, in all of the batteries of Examples 1 to 3 and Comparative Example 1, excellent overcharge resistance (high limit charging current) was observed. However, the discharge capacity ratio of the battery of Comparative Example 1 was 15%, whereas the discharge capacity ratios of the batteries of Examples 1 to 3 were 18%, 24%, and 33%, respectively. It was observed that the battery was improved over the battery of Example 1.
[0057]
From this, it can be seen that in the battery of the present invention in which an ethylene carbonate derivative is mixed with a mixed solvent containing ethylene carbonate (EC) and propylene carbonate (PC), both excellent overcharge resistance and low-temperature characteristics can be obtained. I understood.
[0058]
It was also found that among Sol-A, Sol-B, and Sol-C, the best low-temperature characteristics were obtained when Sol-C was mixed.
[0059]
As described above, the reason why the excellent discharge capacity was obtained at a low temperature by including Sol-C in the electrolytic solution is that Sol-C has a higher degree of steric hindrance than Sol-A or Sol-B. In addition, its freezing point (at 1013 hPa) is low (Sol-C: <−10 ° C., Sol-A: 21 ° C., Sol-B: 35 ° C.), and the entire electrolyte containing ethylene carbonate by mixing. It is thought that the effect of lowering the viscosity of the polymer is great.
[0060]
[Other matters]
(1) The lithium secondary battery of the present invention is not limited to the above-mentioned liquid battery, and the same excellent effects can be obtained when applied to a gel polymer battery. Here, as the polymer material used for the solid electrolyte of the polymer battery, a polymer or a copolymer of one or more of a polyether-based solid polymer, a polycarbonate-based solid polymer, and a polyacrylonitrile-based solid polymer is used. Crosslinked polymers and the like can be mentioned.
[0061]
(2) In the above embodiment, a battery in which one kind of ethylene carbonate derivative is mixed in the electrolytic solution is shown, but two or more kinds of ethylene carbonate derivatives can be used in combination. When one or more ethylene carbonate derivatives are mixed in the electrolytic solution, it is preferable to reduce the ethylene carbonate (EC) content ratio and the propylene carbonate (PC) content ratio in parallel.
[0062]
(3) The content of Sol-A, Sol-B, and Sol-C in the electrolytic solution is more than 0% by volume and 30% by volume or less (at 1013 hPa and 25 ° C.) in Sol-A or Sol-B. In Sol-C, the content can be set to more than 0% by volume and 70% by volume or less (at 1013 hPa and 25 ° C.). However, from the viewpoint of improvement in battery performance and cost, each content is Sol-A or For Sol-B, the content is preferably 10% by volume or more and 20% by volume or less, and for Sol-C it is preferably 10% by volume or more and 40% by volume or less.
[0063]
(4) The longer the chain structure of the alkyl group, the higher the viscosity of the ethylene carbonate derivative and the lower the relative permittivity. Therefore, the alkyl group as a substituent is preferably a lower alkyl group, and in particular, methyl Groups or ethyl groups are preferred. Further, the halogen as the substituent specifically includes fluorine.
[0064]
(5) As the halogenated alkyl group, specifically, a methyl fluoride group (monofluoromethyl group, difluoromethyl group, trifluoromethyl group), an ethyl fluoride group (monofluoroethyl group, difluoroethyl group, Fluoroethyl group, tetrafluoroethyl group, and pentafluoroethyl group).
[0065]
(6) Specific examples of the ethylene carbonate derivative in which the substituent is a methyl group include 4,4′-dimethyl-1,3-dioxolan-2-one, cis-4,5-dimethyl-1,3- Dioxolan-2-one, trans-4,5-Dimethyl-1,3-dioxolan-2-one, or 4,4 ', 5,5'-Tetramethyl-1,3-dioxolan-2-one.
[0066]
(7) As the ethylene carbonate derivative wherein the substituent is an ethyl group, specifically, 4,4′-Diethyl-1,3-dioxolan-2-one, cis-4,5-Diethyl-1,3- Dioxolan-2-one, trans-4,5-Diethyl-1,3-dioxolan-2-one, or 4,4 ′, 5,5′-Tetraethyl-1,3-dioxolan-2-one.
[0067]
(8) As the ethylene carbonate derivative in which the substituent is a methyl group or an ethyl group, specifically, 4,4′-dimethyl-5,5′-diethyl-1,3-dioxolan-2-one, 4, 5-Dimethyl-4 ', 5'-Diethyl-1,3-dioxolan-2-one, 4,5'-Dimethyl-4', 5-Diethyl-1,3-dioxolan-2-one.
[0068]
(9) Specific examples of the ethylene carbonate derivative in which the substituent is a fluoromethyl group include, specifically, 4,4′-difluoromethyl-1,3-dioxolan-2-one, cis-4,5-difluoromethyl-1, 3-dioxolan-2-one, trans-4,5-difluoromethyl-1,3-dioxolan-2-one, or 4,4 ', 5,5'-tetrafluoromethyl-1,3-dioxolan-2-one. Can be
[0069]
(10) Specific examples of the ethylene carbonate derivative in which the substituent is a fluoroethyl group include, specifically, 4,4′-difluoroethyl-1,3-dioxolan-2-one, cis-4,5-difluoroethyl-1, 3-dioxolan-2-one, trans-4,5-difluoroethyl-1,3-dioxolan-2-one, or 4,4 ', 5,5'-tetrafluoroethyl-1,3-dioxolan-2-one. Can be
[0070]
(11) Specific examples of the ethylene carbonate derivatives in which the substituents are a methyl fluoride group and an ethyl fluoride group include, specifically, 4,4′-difluoromethyl-5,5′-difluoroethyl-1,3-dioxolan-2-. one, 4,5'-Difluoromethyl-4 ', 5-Difluoroethyl-1,3-dioxolan-2-one.
[0071]
【The invention's effect】
As described above, according to the present invention, the ethylene carbonate derivative represented by the above general formula (1), which is a novel electrolytic solution component, forms a protective film on the electrode surface, and contains an electrolytic solution containing ethylene carbonate (EC). Enables low viscosity liquids. In addition, since the substance itself has a three-dimensional structure that does not easily penetrate between carbon layers, the layer structure of the carbon material is not damaged even if charge and discharge are repeated. Therefore, according to the present invention, it is possible to provide a lithium battery having excellent overcharge resistance and low-temperature characteristics, and also excellent cycle characteristics.
[Brief description of the drawings]
FIG. 1 is a front view of a thin lithium secondary battery according to the present invention.
FIG. 2 is a sectional view taken along line AA of FIG. 1;
[Explanation of symbols]
1: electrode body 2: storage space 3: laminate exterior body 4: positive electrode current collecting tab 5: negative electrode current collecting tab

Claims (4)

In a lithium battery including a positive electrode including a positive electrode active material capable of inserting and extracting lithium, a negative electrode including a carbon material capable of inserting and extracting lithium as a negative electrode active material, a separator, and a nonaqueous electrolyte having a nonaqueous solvent,
This non-aqueous solvent is
Ethylene carbonate;
One or more ethylene carbonate derivatives represented by the following general formula (1),
A lithium battery comprising:

(In the formula, R _{1 to} R ₄ represent a hydrogen atom, a halogen group, an alkyl group, or a halogenated alkyl group, which may be the same or different. However, at least two of R _{1 to} R ₄ Are the same alkyl group or the same halogenated alkyl group.) The lithium battery according to claim 1, wherein the same alkyl group or the same halogenated alkyl group of the ethylene carbonate derivative is coordinated in a trans form. The lithium battery according to claim 1, wherein the non-aqueous solvent further includes propylene carbonate. The ethylene carbonate derivative is 4,4'-dimethyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, or trans-4,5-dimethyl-1, The lithium battery according to claim 1, wherein the lithium battery is one or more compounds selected from the group consisting of 3-dioxolan-2-one.

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