JP4089235B2 - Non-aqueous electrolyte and lithium secondary battery using the same - Google Patents

Description

【０００８】
（式中、Ｒ₁〜Ｒ₈はそれぞれ独立して水素原子、炭化水素基又はエステル基を表す。Ｒ₁〜Ｒ₈のうちシクロヘキセン環の異なる炭素原子に結合する２個の基は、互いに結合して環を形成していてもよく、またＲ₁〜Ｒ₈のうちシクロヘキセン環の同じ炭素原子に結合する２個の基は互いに結合して２価の基を形成していてもよい）
また、本発明の他の要旨は、上記非水系電解液を用いたことを特徴とするリチウム二次電池、に存する。
【０００９】
上記の特定のシクロヘキセン型化合物を含有した非水系電解液を用いると保存特性が大幅に向上し、しかも過充電の比較的早期にガスが発生するため、ＣＩＤ等の電池内圧を感知して電流を遮断する方式の安全装置が設けられている電池においては、より安全な段階で過充電を停止することができる。上記のシクロヘキセン型化合物を含有した非水系電解液が保存特性の向上をもたらす要因の詳細は不明であるが、充電の初期にシクロヘキセン環が負極で還元されて良好なＳＥＩ（界面保護被膜）を形成するために、保存時の溶媒等の分解反応を抑制することができ、保存後の容量が低下しにくいものと推定される。後に示すように、シクロヘキセン環の二重結合を構成する炭素原子上に置換基があると初期容量及び保存後の容量が不十分となる。これは置換基の立体障害により密度の低い不安定なＳＥＩを形成してしまうことに由来しているものと思われる。
【００１０】
【発明の実施の形態】
以下、本発明の実施の形態について詳述する。
本発明の非水系電解液は、非水系有機溶媒にリチウム塩が溶解され、さらに特定のシクロヘキセン型化合物が含有されているものである。
本発明では下記一般式（１）で表されるシクロヘキセン型化合物を添加剤として使用する。
【００１１】
【化３】

【００１２】
（式中、Ｒ₁〜Ｒ₈はそれぞれ独立して水素原子、炭化水素基又はエステル基を表す。Ｒ₁〜Ｒ₈のうちシクロヘキセン環の異なる炭素原子に結合する２個の基は、互いに結合して環を形成していてもよく、またＲ₁〜Ｒ₈のうちシクロヘキセン環の同じ炭素原子に結合する２個の基は互いに結合して２価の基を形成していてもよい）
上記一般式（１）において、Ｒ₁〜Ｒ₈はそれぞれ独立して水素原子、炭化水素基又はエステル基を表す。
【００１３】
上記炭化水素基としては、メチル基、エチル基、プロピル基、イソプロピル基、ブチル基等の炭素数１〜４のアルキル基、ビニル基、イソプロペニル基、アリル基等の炭素数２〜４のアルケニル基、フェニル基、トリル基等の炭素数６〜８のアリール基、ベンジル基、フェネチル基等の炭素数７〜８のアラルキル基等が挙げられ、好ましくはメチル基、エチル基、プロピル基等の炭素数１〜３のアルキル基及びビニル基、イソプロペニル基、アリル基等の炭素数２〜３のアルケニル基等が挙げられる。
【００１４】
また上記エステル基としては、メトキシカルボニル基、エトキシカルボニル基、ブトキシカルボニル基等のアルコキシカルボニル基、フェニルオキシカルボニル基、トリルオキシカルボニル基等のアリールオキシカルボニル基等が挙げられる。
またＲ₁〜Ｒ₈のうちシクロヘキセン環の異なる炭素原子に結合する２個の基は、互いに結合して環を形成していてもよく、またＲ₁〜Ｒ₈のうちシクロヘキセン環の同じ炭素原子に結合する２個の基は互いに結合して２価の基を形成していてもよい。かかる２価の基としては、メチレン基、エチリデン基等の、１個の炭素原子が２個の遊離原子価を有する基も含まれる。
【００１５】
上記シクロヘキセン型化合物としては、例えばシクロヘキセン、３−メチルシクロヘキセン、４−メチルシクロヘキセン、４−エチルシクロヘキセン、４−ビニルシクロヘキセン、ｃｉｓ−４−カレン、ｔｒａｎｓ−４−カレン、ｔｒａｎｓ−イソリモネン、ノルボルネン、５−メチレン−２−ノルボルネン、５−エチリデン−２−ノルボルネン、５−ビニル−２−ノルボルネン等の炭化水素化合物、メチル−３−シクロヘキセンカルボキシレート、エチル−３−シクロヘキセンカルボキシレート、ｔ−ブチル−３−シクロヘキセンカルボキシレート、フェニル−３−シクロヘキセンカルボキシレート、ｐ−トリル−３−シクロヘキセンカルボキシレート、ベンジル−３−シクロヘキセンカルボキシレート等のエステル化合物等が挙げられる。これらの中でもシクロヘキセン、４−メチルシクロヘキセン及びメチル−３−シクロヘキセンカルボキシレートは入手が容易であり、かつ保存特性及び過充電防止特性がバランス良く向上するので好適に用いられる。これらの添加剤は２種類以上を混合して使用してもよい。
上記シクロヘキセン型化合物の添加量は特に限定されないが、非水系電解液に対して通常０．０１〜１０重量％、好ましくは０．１〜５重量％である。添加量が多すぎるとイオン伝導度が低下してレート特性などの電池特性が低下する傾向にある。また添加量が少な過ぎる場合は、充分な過充電防止効果が発現しない。
【００１６】
本発明で支持電解質として使用されるリチウム塩としては、特に制限はないが、例えばＬｉＰＦ₆、ＬｉＡｓＦ₆、ＬｉＢＦ₄、ＬｉＳｂＦ₆、ＬｉＡｌＣｌ₄、ＬｉＣｌＯ₄、ＣＦ₃ＳＯ₃Ｌｉ、Ｃ₄Ｆ₉ＳＯ₃Ｌｉ、ＣＦ₃ＣＯＯＬｉ、（ＣＦ₃ＣＯ）₂ＮＬｉ、（ＣＦ₃ＳＯ₂）₂ＮＬｉ、（Ｃ₂Ｆ₅ＳＯ₂）₂ＮＬｉなどのリチウム塩が挙げられる。特に、溶媒に溶けやすくかつ高い解離度を示すＬｉＰＦ₆、ＬｉＢＦ₄、ＣＦ₃ＳＯ₃Ｌｉ及び（ＣＦ₃ＳＯ₂）₂ＮＬｉからなる群から選ばれるリチウム塩は好適に用いられる。また非水系電解液中のリチウム塩の濃度は、非水系電解液に対して通常０．５〜２ｍｏｌ／Ｌの範囲で使用するのが好ましい。
【００１７】
本発明で用いる非水系有機溶媒としては、リチウム塩を溶解させることができる限り特に限定はされないが、なかでも高いイオン導電性を発現させる溶媒として、通常、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）、メチルプロピルカーボネート、エチルプロピルカーボネート等の鎖状カーボネート類、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）、ブチレンカーボネート（ＢＣ）等の環状カーボネート類、ビニレンカーボネート、ビニルエチレンカーボネート等の不飽和カーボネート類、１，２−ジメトキシエタン、テトラヒドロフランなどのエーテル類、γ−ブチロラクトン、γ−バレロラクトン等の環状エステル類、ギ酸メチル、酢酸メチル、プロピオン酸メチル等の鎖状エステル類が好ましく用いられる。
【００１８】
これらの有機溶媒は、通常、適切な物性を達成するように混合して使用される。例えば一般に上記鎖状カーボネート類と上記環状カーボネート類とを併用するのが好ましい。また上記鎖状カーボネート類の中でも特にエチルメチルカーボネート、メチルプロピルカーボネート、エチルプロピルカーボネート等の非対称カーボネートを混合使用するのは好ましい。そのなかでもエチルメチルカーボネートは粘度が低いためリチウムの移動性を高めるだけでなく、沸点が比較的高いため揮散しにくくて取り扱いやすく、またＬｉとの反応も少ないので好適に用いられる。またビニレンカーボネート、ビニルエチレンカーボネート等の不飽和カーボネート類を混合使用すると、これらの不飽和カーボネート類は初期充電時に還元されやすく、安定な界面保護皮膜（ＳＥＩ）を形成するのに寄与するので好ましい。
【００１９】
本発明の非水系電解液を調製するに際し、非水系電解液の各原料は、予め脱水しておくのが好ましい。水分量は通常５０ｐｐｍ以下、好ましくは３０ｐｐｍ以下とするのがよい。水が多量に存在すると、水の電気分解及びリチウム金属との反応、リチウム塩の加水分解などが起こる可能性があり、電池用の電解質として不適当な場合がある。脱水の手段としては特に制限はないが、溶媒などの液体の場合はモレキュラーシーブ等を用いればよい。またリチウム塩などの固体の場合は分解が起きる温度以下で乾燥すればよい。
【００２０】
本発明における非水系電解液はリチウム二次電池用の電解液として有用である。以下、本発明のリチウム二次電池について説明する。
本発明の非水系電解液を適用しうるリチウム二次電池の基本的構成は、従来公知のリチウム二次電池と同様であり、正極と負極とが多孔膜及び本発明の非水系電解液を介してケースに収納されて構成される。本発明の二次電池に使用される正極及び負極としては、電池の種類に応じて適宜選択すればよいが、少なくとも正極、負極に対応した活物質を含有する。また、活物質を固定するためのバインダーを含有してもよい。
【００２１】
本発明のリチウム二次電池に使用できる正極活物質としては、例えば、Ｆｅ、Ｃｏ、Ｎｉ、Ｍｎ等の遷移金属を有する酸化物、リチウムとの複合酸化物、硫化物等の無機化合物が挙げられる。具体的には、ＭｎＯ、Ｖ₂Ｏ₅、Ｖ₆Ｏ₁₃、ＴｉＯ₂等の遷移金属酸化物、ニッケル酸リチウム、コバルト酸リチウム、マンガン酸リチウムなどのリチウムと遷移金属との複合酸化物、ＴｉＳ₂、ＦｅＳなどの遷移金属硫化物が挙げられる。また、正極活物質として、例えばポリアニリン等の導電性ポリマー等の有機化合物を挙げることもできる。上記の活物質の複数種を混合して用いてもよい。活物質が粒状の場合の粒径は、レ−ト特性、サイクル特性等の電池特性が優れる点で通常１〜３０μｍ、好ましくは１〜１０μｍ程度である。
【００２２】
本発明のリチウム二次電池に使用できる負極活物質としては、リチウム金属、リチウム合金を使用することもできるが、添加剤の使用による皮膜形成の効果が大きいので、リチウムイオンを吸蔵放出可能な化合物としてコークス，アセチレンブラック、メゾフェーズマイクロビーズ、グラファイト等の炭素質物質を使用するのが特に好ましい。粒状の負極活物質の粒径は、初期効率、レ−ト特性、サイクル特性等の電池特性が優れる点で、通常１〜５０μｍ、好ましくは１５〜３０μｍ程度である。
【００２３】
また、上記炭素質物質を有機物等と混合・焼成した材料、あるいはＣＶＤ法等を用いて、少なくとも表面の一部に上記炭素質物に比べて非晶質の炭素を形成した材料もまた、炭素質物質として好適に使用することができる。
上記有機物としては、軟ピッチから硬ピッチまでのコールタールピッチ；乾留液化油等の石炭系重質油；常圧残油、減圧残油等の直留系重質油；原油、ナフサ等の熱分解時に副生する分解系重質油（例えばエチレンヘビーエンド）等の石油系重質油が挙げられる。また、これらの重質油を２００〜４００℃で蒸留して得られた固体状残渣物を、１〜１００μｍに粉砕したものも使用することができる。さらに塩化ビニル樹脂や、焼成によりフェノール樹脂やイミド樹脂となるこれらの樹脂前駆体も使用することができる。
【００２４】
正極または負極に使用できるバインダーとしては、耐候性、耐薬品性、耐熱性、難燃性等の観点から各種の材料が挙げられる。具体的には、シリケート、ガラスのような無機化合物や、ポリエチレン、ポリプロピレン、ポリ−１，１−ジメチルエチレンなどのアルカン系ポリマー；ポリブタジエン、ポリイソプレンなどの不飽和系ポリマー；ポリスチレン、ポリメチルスチレン、ポリビニルピリジン、ポリ−Ｎ−ビニルピロリドンなどの環を有するポリマー；ポリメタクリル酸メチル、ポリメタクリル酸エチル、ポリメタクリル酸ブチル、ポリアクリル酸メチル、ポリアクリル酸エチル、ポリアクリル酸、ポリメタクリル酸、ポリアクリルアミドなどのアクリル誘導体系ポリマー；ポリフッ化ビニル、ポリフッ化ビニリデン、ポリテトラフルオロエチレン等のフッ素系樹脂；ポリアクリロニトリル、ポリビニリデンシアニドなどのＣＮ基含有ポリマー；ポリ酢酸ビニル、ポリビニルアルコールなどのポリビニルアルコール系ポリマー；ポリ塩化ビニル、ポリ塩化ビニリデンなどのハロゲン含有ポリマー；ポリアニリンなどの導電性ポリマーなどが使用できる。また上記のポリマーなどの混合物、変成体、誘導体、ランダム共重合体、交互共重合体、グラフト共重合体、ブロック共重合体などであっても使用できる。これらの樹脂の重量平均分子量は、通常１万〜３００万、好ましくは１０万〜１００万程度である。分子量が低すぎると電極の強度が低下する傾向にある。一方、分子量が高すぎると粘度が高くなり、電極の形成が困難になることがある。好ましいバインダー樹脂は、フッ素系樹脂、ＣＮ基含有ポリマーである。
【００２５】
バインダーの使用量は、活物質１００重量部に対して通常０．１重量部以上、好ましくは１重量部以上であり、また通常３０重量部以下、好ましくは２０重量部以下である。バインダーの量が少なすぎると電極の強度が低下する傾向にあり、バインダーの量が多すぎるとイオン伝導度が低下する傾向にある。電極中には、電極の導電性や機械的強度を向上させるため、導電性材料、補強材など各種の機能を発現する添加剤、粉体、充填材などを含有させてもよい。導電性材料としては、上記活物質に適量混合して導電性を付与できるものであれば特に制限はないが、通常、アセチレンブラック、カーボンブラック、黒鉛などの炭素粉末や、各種の金属のファイバー、箔などが挙げられる。補強材としては各種の無機、有機の球状、繊維状フィラーなどが使用できる。
【００２６】
電極は、活物質やバインダー等の構成成分と溶剤とを含む塗料を塗布・乾燥することによって形成することができる。電極の厚さは、通常１μｍ以上、好ましくは１０μｍ以上、さらに好ましくは２０μｍ以上、最も好ましくは４０μｍ以上であり、また通常２００μｍ以下、好ましくは１５０μｍ以下、さらに好ましくは１００μｍ以下である。薄すぎると塗布が困難になり均一性が確保しにくくなるだけでなく、電池の容量が小さくなりすぎることがある。一方、あまりに厚すぎるとレート特性が低下しすぎることがある。
【００２７】
正極及び負極の少なくとも一方の電極は、通常、集電体上に形成される。集電体としては、各種のものを使用することができるが、通常は金属や合金が用いられる。具体的には、正極の集電体としては、アルミニウムやニッケル、ＳＵＳ等が挙げられ、負極の集電体としては、銅やニッケル、ＳＵＳ等が挙げられる。好ましくは、正極の集電体としてアルミニウムを使用し、負極の集電体として銅を使用する。正負極層との結着効果を向上させるため、これら集電体の表面を予め粗面化処理しておくのが好ましい。表面の粗面化方法としては、ブラスト処理や粗面ロールにより圧延するなどの方法、研磨剤粒子を固着した研磨布紙、砥石、エメリバフ、鋼線などを備えたワイヤ−ブラシなどで集電体表面を研磨する機械的研磨法、電解研磨法、化学研磨法などが挙げられる。
【００２８】
また、電池の重量を低減させる、すなわち重量エネルギー密度を向上させるために、エキスパンドメタルやパンチングメタルのような穴あきタイプの集電体を使用することもできる。この場合、その開口率を変更することで重量も自在に変更可能となる。また、このような穴あけタイプの集電体の両面に活物質を存在させた場合、この穴を通しての塗膜のリベット効果により塗膜の剥離がさらに起こりにくくなる傾向にあるが、開口率があまりに高くなった場合には、塗膜と集電体との接触面積が小さくなるため、かえって接着強度は低くなることがある。
【００２９】
集電体の厚さは、通常１μｍ以上、好ましくは５μｍ以上であり、通常１００μｍ以下、好ましくは５０μｍ以下である。あまりに厚すぎると、電池全体の容量が低下しすぎることになり、逆に薄すぎると取り扱いが困難になることがある。
本発明の非水系電解液は、これを高分子によってゲル化して半固体状にしてもよい。半固体状電解質における上記電解液の使用量は、半固体状電解質の総量に対して、通常３０重量％以上、好ましくは５０重量％以上、さらに好ましくは７５重量％以上であり、また通常９９．９５重量％以下、好ましくは９９重量％以下、さらに好ましくは９８重量％以下とする。使用量が多すぎると、電解液の保持が困難となり液漏れが生じやすくなり、逆に少なすぎると充放電効率や容量の点で不十分となることがある。
【００３０】
正極と負極との間には、短絡を防止する上で、多孔性のスペーサが設けられているのが好ましい。即ち、この場合、非水系電解液は、多孔性のスペーサに含浸されて使用される。スペーサの材料としては、ポリエチレンやポリプロピレン等のポリオレフィンや、ポリテトラフルオロエチレン、ポリエーテルスルホン等を用いることができるが、好ましくはポリオレフィンである。スペーサの厚さは、通常１μｍ以上、好ましくは５μｍ以上、さらに好ましくは１０μｍ以上であり、また通常５０μｍ以下、好ましくは４０μｍ以下、さらに好ましくは３０μｍ以下である。多孔膜が薄すぎると、絶縁性や機械的強度が悪化することがあり、厚すぎるとレート特性等の電池性能が悪化するばかりでなく、電池全体としてのエネルギー密度が低下することがある。スペーサの空孔率としては、通常２０％以上、好ましくは３５％以上、さらに好ましくは４５％以上であり、また通常９０％以下、好ましくは８５％以下、さらに好ましくは７５％以下である。空孔率が小さすぎると膜抵抗が大きくなりレート特性が悪化する傾向にある。また大きすぎると膜の機械的強度が低下し絶縁性が低下する傾向にある。スペーサの平均孔径は、通常０．５μｍ以下、好ましくは０．２μｍ以下であり、また通常０．０５μｍ以上である。あまりに大きいと短絡が生じやすくなり、小さすぎると膜抵抗が大きくなりレート特性が悪化することがある。
【００３１】
リチウム二次電池の構造は特に限定されるものではなく、コイン型電池、円筒型電池、角型電池などが挙げられるが、現行の円筒型電池のようにＣＩＤ等の電池内圧を感知して電流遮断する安全装置が設けられていることが好ましい。
【００３２】
【実施例】
以下、実施例を挙げて本発明の具体的態様を更に説明するが、本発明はその要旨を越えない限りこれらの実施例により限定されるものではない。
実施例１
［正極の製造］
コバルト酸リチウム（ＬｉＣｏＯ₂）９０重量％とポリフッ化ビニリデン（ＰＶｄＦ）５重量％とアセチレンブラック５重量％とを混合し、Ｎ−メチルピロリドンを加えスラリー状にしたものをアルミニウムからなる集電体の片面に塗布・乾燥して正極を得た。
【００３３】
［負極の製造］
グラファイト粉末８７．４重量％とＰＶｄＦ９．７重量％とアセチレンブラック２．９重量％とを混合し、Ｎ−メチルピロリドンを加えスラリー状にしたものを銅からなる集電体の両面に塗布・乾燥して負極を得た。
［電解液の調合］
ＬｉＰＦ₆を１．２５ｍｏｌ／Ｌの割合で含有するエチレンカーボネートとジメチルカーボネートとエチルメチルカーボネートとの混合溶媒（混合体積比２：３：３）１００重量％にビニレンカーボネート２重量％を加えたものをベース電解液とし、これに４−メチルシクロヘキセン４重量％を加えて電解液とした。
【００３４】
［リチウム二次電池の製造］
上記正極、負極、及び膜厚１６μｍ、空孔率４５％、平均孔径０．０５μｍのポリエチレン製２軸延伸多孔膜フィルムに、それぞれ前記電解液を塗布・含浸させた後、負極、セパレータ、正極、セパレータ、負極の順に積層した。こうして得られた電池要素を、まずＰＥＴフィルムで挟んだ後、アルミニウム層の両面を樹脂層で被覆したラミネートフィルムに正極負極の端子を突設させつつ、真空封止してシート状のリチウム二次電池を作製した。さらに電極間の密着性を高めるためにシリコンゴム及びガラス板でシート状電池を挟んだ上で０．３５ｋｇ／ｃｍ²で加圧した。図１に二次電池の概略断面図を示す。
【００３５】
［電池初期特性評価］
コバルト酸リチウムの１時間当たりの放電量を１３８ｍＡｈ／ｇとし、これと評価用リチウム二次電池の正極の活物質量とから放電速度１Ｃを求めてレート設定をした上で、０．２Ｃで４．２Ｖまで充電した後、０．２Ｃで３Ｖまで放電し初期のフォーメーションを行った。ついで０．５Ｃで４．２Ｖまで充電した後、１Ｃで３Ｖまで放電し、１Ｃ放電容量を求めた。結果を表−１に示す。なお充電時のカット電流は何れも０．０５Ｃとした。
【００３６】
［保存特性評価］
初期特性評価の終了した電池を０．５Ｃで４．２Ｖまで充電した後、６０℃の恒温槽に７日間保存した。その後、電池を取り出し、０．５Ｃで４．２Ｖまで充電した後、１Ｃで放電し、保存後の１Ｃ放電容量を求めた。また保存前後の１Ｃ放電容量から下記計算式により容量回復率を求めた。結果を表−１に示す。
【００３７】
【数１】
容量回復率（％）＝保存後１Ｃ放電容量（ｍＡｈ／ｇ）／１Ｃ放電容量（ｍＡｈ／ｇ）
［過充電特性評価］
初期特性評価の終了した電池を０．５Ｃで４．２Ｖまで充電した後、２Ｃの電流値で過充電を開始した。２１分後（ＳＯＣ１７０％に相当）に通電を停止し、ガスの発生量をエタノール浴に電池を漬けて浮力を測定（アルキメデスの原理）して求めた。結果を表−１に示す。
【００３８】
実施例２
添加剤として４−メチルシクロヘキセンの代わりにシクロヘキセンを添加した電解液を使用したこと以外は実施例１と同様にしてリチウム二次電池を作製し、実施例１と同様の電池特性試験（初期特性・保存特性・過充電特性）を実施した。結果を表−１に示す。
【００３９】
実施例３
添加剤として４−メチルシクロヘキセンの代わりにメチル−３−シクロヘキセンカルボキシレートを添加した電解液を使用したこと以外は実施例１と同様にしてリチウム二次電池を作製し、実施例１と同様の電池特性試験（初期特性・保存特性・過充電特性）を実施した。結果を表−１に示す。
【００４０】
比較例１
４−メチルシクロヘキセンを添加しない電解液を使用したこと以外は実施例１と同様にしてリチウム二次電池を作製し、実施例１と同様の電池特性試験（初期特性・保存特性・過充電特性）を実施した。結果を表−１に示す。
比較例２
４−メチルシクロヘキセンの代わりに１−メチルシクロヘキセンを添加した電解液を使用したこと以外は実施例１と同様にしてリチウム二次電池を作製し、実施例１と同様の電池特性試験（初期特性・保存特性）を実施した。結果を表−１に示すが、１Ｃ放電容量が１００ｍＡｈ／ｇを下回った。即ち、シクロヘキセン環の二重結合を構成している炭素原子上に置換基があると抵抗が大きくなり、容量が低下することが判明した。
【００４１】
【表１】

【００４２】
上記表−１から明らかなように、本発明の非水系電解液を用いれば高温保存時の容量劣化が少なく、保存特性が大幅に向上する。また過充電の初期の段階でガスが多量に発生するので、円筒型電池等の、電池内圧を感知して電流遮断するタイプの電池においてより安全な状態で電流遮断することも可能となる。
【００４３】
【発明の効果】
本発明によれば、保存特性及び過充電防止特性に優れた安全な二次電池を得ることができる。
【図面の簡単な説明】
【図１】本発明を実施したリチウム二次電池の構造を示す概略断面図である。
【符号の説明】
１ 正極
２ 負極
３ セパレータ
４ ＰＥＴフィルム
５ シリコンゴム
６ ガラス板
７ ラミネートフィルム
８ 封止材付きリード[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte and a lithium secondary battery using the same.
[0002]
[Prior art]
Lithium secondary batteries have the advantages of high energy density and less self-discharge. Therefore, in recent years, it has been widely used as a power source for consumer mobile devices such as mobile phones, notebook computers, and PDAs. The electrolyte for a lithium secondary battery is composed of a lithium salt as a supporting electrolyte and a non-aqueous organic solvent. The non-aqueous organic solvent is required to have a high dielectric constant for dissociating the lithium salt, to exhibit high ionic conductivity in a wide temperature range, and to be stable in the battery. Since it is difficult to achieve these requirements with a single solvent, usually a combination of a high boiling point solvent typified by propylene carbonate and ethylene carbonate and a low boiling point solvent such as dimethyl carbonate and diethyl carbonate is used. Yes.
[0003]
Many methods for adding various additives to the electrolyte have been reported to improve the initial capacity, rate characteristics, cycle characteristics, high temperature storage characteristics, low temperature characteristics, trickle charge characteristics, and the like.
[0004]
[Problems to be solved by the invention]
However, in recent years, the demand for higher performance is increasing, and it is required that the above characteristics be improved in a balanced manner. In recent years, there has been an increasing demand for safety measures such as overcharge prevention. For this reason, additives such as biphenyl exemplified in Japanese Patent No. 30617759 may be added to the electrolyte solution. The characteristics may be greatly deteriorated.
[0005]
Accordingly, there has been a strong demand for an electrolyte solution that provides good storage characteristics while improving overcharge characteristics and the like.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have improved the storage characteristics and overcharge prevention characteristics of the battery at the same time by including a specific compound in the non-aqueous electrolyte. As a result, the present invention has been completed.
That is, the gist of the present invention is a non-aqueous electrolytic solution in which a lithium salt is dissolved in a non-aqueous organic solvent, and the non-aqueous organic solvent converts the cyclohexene type compound represented by the following general formula (1) into the non-aqueous electrolytic solution. The cyclohexene-type compound has at least an effect of enhancing gas generation from the electrolyte solution in an overcharged state, and the non-aqueous solvent is not suitable as an interface protective film forming agent. The present invention resides in a nonaqueous electrolytic solution characterized by containing a saturated carbonate .
[0007]
[Chemical 2]

[0008]
(In the formula, R _{1 to} R ₈ each independently represents a hydrogen atom, a hydrocarbon group or an ester group. Of R _{1 to} R ₈ , two groups bonded to different carbon atoms of the cyclohexene ring are bonded to each other. And two groups bonded to the same carbon atom of the cyclohexene ring among R _{1 to} R ₈ may be bonded to each other to form a divalent group)
Another gist of the present invention resides in a lithium secondary battery using the non-aqueous electrolyte.
[0009]
When a non-aqueous electrolyte containing the above-mentioned specific cyclohexene type compound is used, the storage characteristics are greatly improved, and gas is generated at a relatively early stage of overcharge. Therefore, the current is detected by sensing the internal pressure of the battery such as CID. In a battery provided with a safety device that shuts off, overcharging can be stopped at a safer stage. Although the details of the factors that cause non-aqueous electrolytes containing the above cyclohexene-type compounds to improve storage characteristics are unknown, the cyclohexene ring is reduced at the negative electrode at the beginning of charging to form a good SEI (interface protective coating) Therefore, it is presumed that the decomposition reaction of the solvent or the like during storage can be suppressed, and the capacity after storage is unlikely to decrease. As will be shown later, if there is a substituent on the carbon atom constituting the double bond of the cyclohexene ring, the initial capacity and the capacity after storage will be insufficient. This seems to be derived from the formation of unstable SEI having a low density due to the steric hindrance of the substituent.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
The non-aqueous electrolyte solution of the present invention is a solution in which a lithium salt is dissolved in a non-aqueous organic solvent and a specific cyclohexene type compound is contained.
In the present invention, a cyclohexene type compound represented by the following general formula (1) is used as an additive.
[0011]
[Chemical 3]

[0012]
(In the formula, R _{1 to} R ₈ each independently represents a hydrogen atom, a hydrocarbon group or an ester group. Of R _{1 to} R ₈ , two groups bonded to different carbon atoms of the cyclohexene ring are bonded to each other. And two groups bonded to the same carbon atom of the cyclohexene ring among R _{1 to} R ₈ may be bonded to each other to form a divalent group)
In the general formula (1), R _{1 to} R ₈ each independently represent a hydrogen atom, a hydrocarbon group, or an ester group.
[0013]
Examples of the hydrocarbon group include alkyl groups having 1 to 4 carbon atoms such as methyl group, ethyl group, propyl group, isopropyl group and butyl group, and alkenyl groups having 2 to 4 carbon atoms such as vinyl group, isopropenyl group and allyl group. Group, phenyl group, tolyl group and other aryl groups having 6 to 8 carbon atoms such as benzyl group and phenethyl group, and aralkyl groups having 7 to 8 carbon atoms, preferably methyl group, ethyl group, propyl group and the like. Examples thereof include an alkyl group having 1 to 3 carbon atoms and an alkenyl group having 2 to 3 carbon atoms such as a vinyl group, an isopropenyl group and an allyl group.
[0014]
Examples of the ester group include an alkoxycarbonyl group such as a methoxycarbonyl group, an ethoxycarbonyl group, and a butoxycarbonyl group, and an aryloxycarbonyl group such as a phenyloxycarbonyl group and a tolyloxycarbonyl group.
Further, two groups of R _{1 to} R ₈ that are bonded to different carbon atoms of the cyclohexene ring may be bonded to each other to form a ring, and of R _{1 to} R ₈ , the same carbon atom of the cyclohexene ring Two groups bonded to each other may be bonded to each other to form a divalent group. Examples of such a divalent group include groups having one carbon atom having two free valences such as a methylene group and an ethylidene group.
[0015]
Examples of the cyclohexene type compound include cyclohexene, 3-methylcyclohexene, 4-methylcyclohexene, 4-ethylcyclohexene, 4-vinylcyclohexene, cis-4-carene, trans-4-carene, trans-isolimonene, norbornene, 5- Hydrocarbon compounds such as methylene-2-norbornene, 5-ethylidene-2-norbornene, 5-vinyl-2-norbornene, methyl-3-cyclohexene carboxylate, ethyl-3-cyclohexene carboxylate, t-butyl-3-cyclohexene And ester compounds such as carboxylate, phenyl-3-cyclohexene carboxylate, p-tolyl-3-cyclohexene carboxylate, and benzyl-3-cyclohexene carboxylate. Among these, cyclohexene, 4-methylcyclohexene, and methyl-3-cyclohexene carboxylate are easily used, and are preferably used because their storage characteristics and overcharge prevention characteristics are improved in a well-balanced manner. Two or more kinds of these additives may be mixed and used.
The addition amount of the cyclohexene type compound is not particularly limited, but is usually 0.01 to 10% by weight, preferably 0.1 to 5% by weight with respect to the nonaqueous electrolytic solution. If the amount added is too large, the ionic conductivity tends to decrease and battery characteristics such as rate characteristics tend to deteriorate. Moreover, when there is too little addition amount, sufficient overcharge prevention effect is not expressed.
[0016]
The lithium salt used as a supporting electrolyte in the present invention is not particularly limited, for example _{LiPF 6, LiAsF 6, LiBF 4} , LiSbF 6, LiAlCl 4, LiClO 4, CF 3 SO 3 Li, C 4 F 9 SO Examples include lithium salts such as ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, and (C ₂ F ₅ SO ₂ ) ₂ NLi. In particular, a lithium salt selected from the group consisting of LiPF ₆ , LiBF ₄ , CF ₃ SO ₃ Li and (CF ₃ SO ₂ ) ₂ NLi that is easily soluble in a solvent and exhibits a high degree of dissociation is preferably used. Moreover, it is preferable to use the density | concentration of the lithium salt in a non-aqueous electrolyte solution normally in the range of 0.5-2 mol / L with respect to a non-aqueous electrolyte solution.
[0017]
The non-aqueous organic solvent used in the present invention is not particularly limited as long as the lithium salt can be dissolved, but among them, as a solvent for expressing high ionic conductivity, dimethyl carbonate (DMC), diethyl carbonate (DEC) are usually used. ), Chain carbonates such as ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), vinylene carbonate, vinyl Unsaturated carbonates such as ethylene carbonate, ethers such as 1,2-dimethoxyethane and tetrahydrofuran, cyclic esters such as γ-butyrolactone and γ-valerolactone, methyl formate, methyl acetate, propylene A chain ester such as methyl onate is preferably used.
[0018]
These organic solvents are usually used by mixing so as to achieve appropriate physical properties. For example, it is generally preferable to use the chain carbonates and the cyclic carbonates in combination. Among the chain carbonates, it is particularly preferable to use a mixture of asymmetric carbonates such as ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. Among them, ethyl methyl carbonate is preferably used because it not only enhances lithium mobility because of its low viscosity, but also has a relatively high boiling point, so that it is difficult to volatilize and is easy to handle, and has little reaction with Li. In addition, it is preferable to use a mixture of unsaturated carbonates such as vinylene carbonate and vinyl ethylene carbonate because these unsaturated carbonates are easily reduced during initial charging and contribute to the formation of a stable interface protective film (SEI).
[0019]
When preparing the non-aqueous electrolyte solution of the present invention, each raw material of the non-aqueous electrolyte solution is preferably dehydrated in advance. The water content is usually 50 ppm or less, preferably 30 ppm or less. When a large amount of water is present, there is a possibility that electrolysis of water, reaction with lithium metal, hydrolysis of lithium salt, and the like may occur, which may be inappropriate as an electrolyte for a battery. The means for dehydration is not particularly limited, but in the case of a liquid such as a solvent, a molecular sieve or the like may be used. In the case of a solid such as a lithium salt, it may be dried at a temperature lower than the temperature at which decomposition occurs.
[0020]
The non-aqueous electrolyte in the present invention is useful as an electrolyte for a lithium secondary battery. Hereinafter, the lithium secondary battery of the present invention will be described.
The basic configuration of a lithium secondary battery to which the non-aqueous electrolyte of the present invention can be applied is the same as that of a conventionally known lithium secondary battery, and the positive electrode and the negative electrode are interposed via the porous membrane and the non-aqueous electrolyte of the present invention. And housed in a case. The positive electrode and the negative electrode used in the secondary battery of the present invention may be appropriately selected according to the type of the battery, but contain at least active materials corresponding to the positive electrode and the negative electrode. Moreover, you may contain the binder for fixing an active material.
[0021]
Examples of the positive electrode active material that can be used in the lithium secondary battery of the present invention include oxides having transition metals such as Fe, Co, Ni, and Mn, complex oxides with lithium, and inorganic compounds such as sulfides. . Specifically, transition metal oxides such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ , composite oxides of lithium and transition metals such as lithium nickelate, lithium cobaltate, and lithium manganate, TiS ₂ and transition metal sulfides such as FeS. Moreover, as a positive electrode active material, organic compounds, such as conductive polymers, such as polyaniline, can also be mentioned, for example. You may mix and use multiple types of said active material. When the active material is granular, the particle size is usually about 1 to 30 μm, preferably about 1 to 10 μm, from the viewpoint of excellent battery characteristics such as rate characteristics and cycle characteristics.
[0022]
As the negative electrode active material that can be used in the lithium secondary battery of the present invention, lithium metal and lithium alloy can be used, but since the effect of forming a film by the use of an additive is great, a compound capable of occluding and releasing lithium ions. It is particularly preferable to use carbonaceous materials such as coke, acetylene black, mesophase microbeads, and graphite. The particle size of the granular negative electrode active material is usually about 1 to 50 μm, preferably about 15 to 30 μm, from the viewpoint of excellent battery characteristics such as initial efficiency, rate characteristics, and cycle characteristics.
[0023]
In addition, a material obtained by mixing and baking the carbonaceous substance with an organic substance or the like, or a material in which amorphous carbon is formed on at least a part of the surface by using a CVD method or the like compared to the carbonaceous substance is also carbonaceous. It can be suitably used as a substance.
Examples of the organic matter include coal tar pitch from soft pitch to hard pitch; heavy coal oil such as dry distillation liquefied oil; straight heavy oil such as atmospheric residual oil and vacuum residual oil; heat of crude oil, naphtha, etc. Examples include petroleum heavy oils such as cracking heavy oils (for example, ethylene heavy end) that are by-produced during cracking. Moreover, what grind | pulverized the solid residue obtained by distilling these heavy oils at 200-400 degreeC to 1-100 micrometers can also be used. Furthermore, a vinyl chloride resin and these resin precursors which become a phenol resin or an imide resin by firing can also be used.
[0024]
Examples of the binder that can be used for the positive electrode or the negative electrode include various materials from the viewpoints of weather resistance, chemical resistance, heat resistance, flame retardancy, and the like. Specifically, inorganic compounds such as silicate and glass, alkane polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene; unsaturated polymers such as polybutadiene and polyisoprene; polystyrene, polymethylstyrene, Polymers having rings such as polyvinyl pyridine and poly-N-vinyl pyrrolidone; polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, poly Acrylic derivative polymers such as acrylamide; Fluorine resins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide; Polyvinyl alcohol polymers such as polyvinyl alcohol; polyvinyl chloride, halogen-containing polymers such as polyvinylidene chloride; and conductive polymers such as polyaniline can be used. Further, a mixture such as the above-mentioned polymer, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer, and the like can be used. The weight average molecular weight of these resins is usually 10,000 to 3,000,000, preferably about 100,000 to 1,000,000. If the molecular weight is too low, the strength of the electrode tends to decrease. On the other hand, if the molecular weight is too high, the viscosity becomes high and it may be difficult to form an electrode. Preferred binder resins are fluororesins and CN group-containing polymers.
[0025]
The usage-amount of a binder is 0.1 weight part or more normally with respect to 100 weight part of active materials, Preferably it is 1 weight part or more, and is 30 weight part or less normally, Preferably it is 20 weight part or less. If the amount of the binder is too small, the strength of the electrode tends to decrease, and if the amount of the binder is too large, the ionic conductivity tends to decrease. In order to improve the electroconductivity and mechanical strength of an electrode, you may contain the additive, powder, filler, etc. which express various functions, such as an electroconductive material and a reinforcing material. The conductive material is not particularly limited as long as it can impart conductivity by mixing an appropriate amount of the above active material. Usually, carbon powder such as acetylene black, carbon black, graphite, various metal fibers, Examples include foil. As the reinforcing material, various inorganic, organic spherical and fibrous fillers can be used.
[0026]
The electrode can be formed by applying and drying a paint containing a component such as an active material or a binder and a solvent. The thickness of the electrode is usually 1 μm or more, preferably 10 μm or more, more preferably 20 μm or more, most preferably 40 μm or more, and usually 200 μm or less, preferably 150 μm or less, more preferably 100 μm or less. If it is too thin, the coating becomes difficult and it becomes difficult to ensure uniformity, and the battery capacity may be too small. On the other hand, if it is too thick, the rate characteristics may be deteriorated too much.
[0027]
At least one of the positive electrode and the negative electrode is usually formed on a current collector. Various types of current collectors can be used, but usually metals and alloys are used. Specifically, examples of the positive electrode current collector include aluminum, nickel, and SUS, and examples of the negative electrode current collector include copper, nickel, and SUS. Preferably, aluminum is used as the positive electrode current collector, and copper is used as the negative electrode current collector. In order to improve the binding effect with the positive and negative electrode layers, it is preferable that the surfaces of these current collectors are roughened in advance. Current roughening methods include a method of rolling with a blasting process or a rough roll, a polishing cloth with a fixed abrasive particle, a grinding wheel, emery buff, a wire brush with a steel wire, etc. Examples thereof include a mechanical polishing method for polishing the surface, an electrolytic polishing method, and a chemical polishing method.
[0028]
In addition, in order to reduce the weight of the battery, that is, to improve the weight energy density, a perforated current collector such as an expanded metal or a punching metal can be used. In this case, the weight can be freely changed by changing the aperture ratio. In addition, when an active material is present on both sides of such a hole-type current collector, the rivet effect of the coating film through this hole tends to make the coating film more difficult to peel off, but the aperture ratio is too high. When it becomes high, the contact area between the coating film and the current collector becomes small, so that the adhesive strength may be lowered.
[0029]
The thickness of the current collector is usually 1 μm or more, preferably 5 μm or more, and is usually 100 μm or less, preferably 50 μm or less. If it is too thick, the capacity of the entire battery will be too low. Conversely, if it is too thin, handling may be difficult.
The non-aqueous electrolyte solution of the present invention may be gelled with a polymer to form a semi-solid. The amount of the electrolytic solution used in the semisolid electrolyte is usually 30% by weight or more, preferably 50% by weight or more, more preferably 75% by weight or more, based on the total amount of the semisolid electrolyte. 95% by weight or less, preferably 99% by weight or less, more preferably 98% by weight or less. If the amount used is too large, it is difficult to retain the electrolyte solution and liquid leakage tends to occur. Conversely, if the amount is too small, the charge / discharge efficiency and capacity may be insufficient.
[0030]
A porous spacer is preferably provided between the positive electrode and the negative electrode to prevent a short circuit. That is, in this case, the nonaqueous electrolytic solution is used by being impregnated into a porous spacer. As a material for the spacer, polyolefin such as polyethylene or polypropylene, polytetrafluoroethylene, polyethersulfone, or the like can be used, and polyolefin is preferable. The thickness of the spacer is usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and usually 50 μm or less, preferably 40 μm or less, more preferably 30 μm or less. If the porous film is too thin, the insulation and mechanical strength may be deteriorated. If the porous film is too thick, not only the battery performance such as the rate characteristic is deteriorated, but also the energy density of the whole battery may be lowered. The porosity of the spacer is usually 20% or more, preferably 35% or more, more preferably 45% or more, and usually 90% or less, preferably 85% or less, more preferably 75% or less. If the porosity is too small, the membrane resistance increases and the rate characteristics tend to deteriorate. On the other hand, if it is too large, the mechanical strength of the film tends to decrease and the insulating property tends to decrease. The average pore diameter of the spacer is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If it is too large, a short circuit tends to occur, and if it is too small, the film resistance increases and the rate characteristics may deteriorate.
[0031]
The structure of the lithium secondary battery is not particularly limited, and examples thereof include a coin-type battery, a cylindrical battery, and a square battery. However, as in the case of the current cylindrical battery, the current is detected by detecting the battery internal pressure such as CID. It is preferable that a safety device for blocking is provided.
[0032]
【Example】
EXAMPLES Hereinafter, although an Example is given and the specific aspect of this invention is further demonstrated, this invention is not limited by these Examples, unless the summary is exceeded.
Example 1
[Production of positive electrode]
A current collector made of aluminum was prepared by mixing 90% by weight of lithium cobaltate (LiCoO ₂ ), 5% by weight of polyvinylidene fluoride (PVdF), and 5% by weight of acetylene black and adding N-methylpyrrolidone to form a slurry. One side was coated and dried to obtain a positive electrode.
[0033]
[Manufacture of negative electrode]
A mixture of 87.4% by weight of graphite powder, 9.7% by weight of PVdF and 2.9% by weight of acetylene black, and added to a slurry by adding N-methylpyrrolidone is applied to both sides of a copper current collector and dried. Thus, a negative electrode was obtained.
[Preparation of electrolyte]
A mixed solvent of ethylene carbonate and dimethyl carbonate and ethyl methyl carbonate containing LiPF ₆ at a rate of 1.25 mol / L (mixing volume ratio of 2: 3: 3) to 100 wt% plus 2 wt% vinylene carbonate A base electrolyte was used, and 4% by weight of 4-methylcyclohexene was added thereto to prepare an electrolyte.
[0034]
[Manufacture of lithium secondary batteries]
After applying and impregnating the electrolyte solution to the positive electrode, the negative electrode, and a biaxially stretched porous film made of polyethylene having a film thickness of 16 μm, a porosity of 45%, and an average pore diameter of 0.05 μm, respectively, the negative electrode, separator, positive electrode, The separator and the negative electrode were laminated in this order. The battery element thus obtained was first sandwiched between PET films, and then vacuum sealed while projecting positive and negative terminals on a laminate film in which both surfaces of an aluminum layer were covered with a resin layer, and sheet-like lithium secondary A battery was produced. Further, in order to enhance the adhesion between the electrodes, the sheet-like battery was sandwiched between silicon rubber and a glass plate and then pressurized at 0.35 kg / cm ² . FIG. 1 shows a schematic cross-sectional view of a secondary battery.
[0035]
[Evaluation of initial battery characteristics]
The discharge rate per hour of lithium cobaltate was set to 138 mAh / g, and the discharge rate was determined from this and the active material amount of the positive electrode of the lithium secondary battery for evaluation, and the rate was set. After charging to .2V, it was discharged to 3V at 0.2C to perform the initial formation. Subsequently, the battery was charged to 4.2 V at 0.5 C, discharged to 3 V at 1 C, and the 1 C discharge capacity was obtained. The results are shown in Table-1. The cut current during charging was set to 0.05C.
[0036]
[Evaluation of storage characteristics]
After the initial characteristic evaluation was completed, the battery was charged to 4.2 V at 0.5 C, and then stored in a thermostat at 60 ° C. for 7 days. Thereafter, the battery was taken out, charged to 4.2 V at 0.5 C, discharged at 1 C, and the 1 C discharge capacity after storage was determined. Further, the capacity recovery rate was obtained from the 1C discharge capacity before and after storage by the following formula. The results are shown in Table-1.
[0037]
[Expression 1]
Capacity recovery rate (%) = 1C discharge capacity after storage (mAh / g) / 1C discharge capacity (mAh / g)
[Overcharge characteristics evaluation]
After the initial characteristics evaluation was completed, the battery was charged to 4.2 V at 0.5 C, and overcharging was started at a current value of 2 C. After 21 minutes (corresponding to SOC 170%), the energization was stopped, and the amount of gas generated was determined by immersing the battery in an ethanol bath and measuring the buoyancy (Archimedes principle). The results are shown in Table-1.
[0038]
Example 2
A lithium secondary battery was prepared in the same manner as in Example 1 except that an electrolyte solution in which cyclohexene was added instead of 4-methylcyclohexene was used as an additive, and a battery characteristic test (initial characteristics / Storage characteristics and overcharge characteristics) were carried out. The results are shown in Table-1.
[0039]
Example 3
A lithium secondary battery was produced in the same manner as in Example 1 except that an electrolytic solution in which methyl-3-cyclohexenecarboxylate was added instead of 4-methylcyclohexene was used as an additive. Characteristic tests (initial characteristics, storage characteristics, overcharge characteristics) were conducted. The results are shown in Table-1.
[0040]
Comparative Example 1
A lithium secondary battery was produced in the same manner as in Example 1 except that an electrolytic solution to which 4-methylcyclohexene was not added was used, and the same battery characteristics test as in Example 1 (initial characteristics, storage characteristics, overcharge characteristics) Carried out. The results are shown in Table-1.
Comparative Example 2
A lithium secondary battery was produced in the same manner as in Example 1 except that an electrolytic solution to which 1-methylcyclohexene was added instead of 4-methylcyclohexene was used, and a battery characteristic test (initial characteristics / Storage characteristics). The results are shown in Table 1. The 1C discharge capacity was less than 100 mAh / g. That is, it has been found that if there is a substituent on the carbon atom constituting the double bond of the cyclohexene ring, the resistance increases and the capacity decreases.
[0041]
[Table 1]

[0042]
As is apparent from Table 1 above, when the non-aqueous electrolyte solution of the present invention is used, there is little capacity deterioration during high temperature storage, and storage characteristics are greatly improved. Further, since a large amount of gas is generated at the initial stage of overcharging, it is possible to cut off the current in a safer state in a battery such as a cylindrical battery that senses the internal pressure of the battery and cuts off the current.
[0043]
【The invention's effect】
According to the present invention, a safe secondary battery having excellent storage characteristics and overcharge prevention characteristics can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing the structure of a lithium secondary battery embodying the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 PET film 5 Silicon rubber 6 Glass plate 7 Laminate film 8 Lead with sealing material

Claims (6)

A non-aqueous electrolyte lithium salt dissolved in a nonaqueous organic solvent, the nonaqueous organic solvent, a cyclohexene type compound represented by the following general formula (1) with respect to the non-aqueous electrolyte 0.01 containing 10 wt%, the cyclohexene-type compound has at least a potentiation of gas generation from the electrolyte in the overcharged state, and non-aqueous organic solvent containing an unsaturated carbonate as surface protective film-forming agent A nonaqueous electrolytic solution for a lithium secondary battery .

(In the formula, R _{1 to} R ₈ are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms, an aryl group having 6 to 8 carbon atoms, or an alkyl group having 7 to 8 carbon atoms. Represents an aralkyl group, an alkoxycarbonyl group, or an aryloxycarbonyl group .)   The non-aqueous electrolyte solution according to claim 1, wherein the non-aqueous organic solvent contains an asymmetric chain carbonate.   The non-aqueous electrolyte solution according to claim 2, wherein the asymmetric chain carbonate is selected from the group consisting of ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.   The non-aqueous electrolyte solution according to any one of claims 1 to 3, wherein the unsaturated carbonate is vinylene carbonate or vinyl ethylene carbonate.   The electrolyte solution in any one of Claims 1-4 whose nonaqueous electrolyte solution is for lithium secondary batteries which use a carbonaceous material as a negative electrode active material.   A lithium secondary battery using the non-aqueous electrolyte solution according to claim 1.

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