JP4352622B2 - Non-aqueous electrolyte and lithium secondary battery using the same - Google Patents
Non-aqueous electrolyte and lithium secondary battery using the same Download PDFInfo
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- JP4352622B2 JP4352622B2 JP2001061156A JP2001061156A JP4352622B2 JP 4352622 B2 JP4352622 B2 JP 4352622B2 JP 2001061156 A JP2001061156 A JP 2001061156A JP 2001061156 A JP2001061156 A JP 2001061156A JP 4352622 B2 JP4352622 B2 JP 4352622B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Description
【０００１】
【発明の属する技術分野】
本発明は、電池の高温保存時の回復特性を改善しつつ、大電流（例えば、電流レートが３Ｃ）での過充電時の安全性を確保できる非水電解液およびこの電解液を用いたリチウム二次電池に関する。
【０００２】
【従来の技術】
近年、電子機器の小型軽量化、携帯化が進み、その電源として高エネルギー密度を有する電池の開発が要求されている。このような要求に応える電池としてリチウム二次電池が期待されている。しかしながら、充電器の故障等により、所定の充電電圧以上になった場合、即ち、過充電状態になった場合、正極のリチウムが過剰に放出されてしまい、熱的に不安定になり、負極では、設計容量以上のリチウムが負極表面に析出するため、熱的に不安定になる。このように、正極、負極が熱的に不安定になると、電極表面で電解液の有機溶媒が激しく分解する。この反応は急激な発熱反応であるため、このような状態になると、電池が異常発熱を起こし、熱暴走を引き起こし、最悪の場合、電池が破裂、爆発するというように、非常に危険である。
【０００３】
また、当然のことながら、過充電時の電流が大きくなるほど、電池の安全性を確保することが技術的に益々難しくなってくる。最近になって、リチウム二次電池の主用途の一つである携帯電話では、大電流での過充電時の電池の安全性、例えば、電流レートが３Ｃでの、電池の安全性が要求されている。
【０００４】
このように電池が過充電状態になった時の安全性を確保する方法として、(1)電子回路による方法、(2)過充電時のガス発生を利用した機械的電流遮断による方法、(3)レドックスシャトルによる方法、(4)過充電電位で電解液中の添加剤を重合させる方法、が提案、開示されている。
【０００５】
電子回路による方法や機械的電流遮断による方法では、電池に付加的な構造を付与させるため、電池がコスト高になってしまう。また、電池の小型軽量化に対して、不利になる。
【０００６】
また、特開平９−５０８２２号公報には、レドックスシャトル方法による過充電時の安全性確保が開示されている。この方法では、π電子軌道を有するベンゼン類化合物、例えば２−クロロ−ｐ−キシレンや４−クロロアニソール等を含有した電解液を用いることによって、この化合物が正極と負極で可逆的に酸化還元反応を起こし、過充電電流を消費することで、電池を保護するというものである。しかしながら、この方法では過充電電流が小さい場合は効果を示すもの、過充電電流が大きい場合には、酸化還元反応が可逆的に進まないため、電池の安全性を十分に確保することが難しい。
【０００７】
特開平９−１０６８３５号公報には、ビフェニル、３−クロロチオフェン、フランなどを添加した電解液を用いることで、過充電時にこれらが重合することで電池の内部抵抗を高くし、電池を保護する方法が開示されている。しかしながら、これらの化合物は、過充電時の電流レートが１Ｃ程度であれば、効果を発現するものの、過充電時の電流レートがその３倍（３Ｃ）程度になると、十分に安全性を確保できない。また、これらの化合物の添加量を多くすると、過充電時の電池の安全性は高まるものの、通常の充電状態で高温保存すると、正極で添加剤が一部、酸化重合反応を起こし、正極上に重合被膜が生成するために、電池の分極が大きくなり、電池特性が劣化することが問題である。
【０００８】
【発明が解決しようとする課題】
従来の過充電時の保護方法では、高温保存特性を劣化させずに、過充電時の大電流（例えば、電流レートが３Ｃ）におけるリチウム二次電池の安全性を十分に確保できていない。本発明は、電流レートが３Ｃ程度の大電流での過充電時の安全性を確保しながら、電池の高温保存特性にも優れたリチウム二次電池を提供するものである。
【０００９】
【課題を解決するための手段】
本発明者らは、電解液中に添加する、過充電時に正極表面で酸化重合反応する化合物に関して、化合物の種類とその添加量の最適化によって、過充電時の化合物の酸化重合反応挙動を精密に制御することが可能となり、前記の課題を解決するに至った。
【００１０】
本発明は、非水溶媒に電解質が溶解されている非水電解液において、該非水電解液中に下記一般式（Ｉ）
【００１１】
【化３】
また、本発明は、リチウム含有金属酸化物を含む材料を正極活物質とする正極と、リチウム金属、リチウム合金およびリチウムを吸蔵、放出可能な材料からなる群から選ばれる１種を負極活物質とする負極とを備え、非水溶媒に電解質が溶解されている非水電解液からなるリチウム二次電池において、前記非水電解液中に下記一般式（Ｉ）
【００１２】
【化４】
【００１３】
【発明の実施の形態】
本発明において、酸化重合反応電位の異なる３種類以上の特定の化合物を電解液中に添加することにより、リチウム電池の高温保存時の回復特性と過充電の安全性を制御することができる。本発明における添加物の中で、正極上での酸化重合反応電位は、アルキルビフェニル類が４．３〜４．５Ｖ程度と最も低い。高温保存時の回復特性向上のためには、酸化重合開始電位の低い化合物（アルキルビフェニル類）の添加量が少ない方が好ましいが、逆に過充電時の安全性を確保するためにはできるだけこれらの化合物の添加量を多くする必要がある。特に、電流レートが３Ｃのような大電流での過充電状態での安全性確保には、酸化重合開始電位の低い化合物を大量に添加する必要があるが、そうすると、高温保存特性は大幅に劣化する。即ち、高温保存特性と過充電時の安全性確保はトレードオフの関係にある。従って、単独の化合物や、単に酸化重合開始電位の異なる３種類以上の化合物を電解液中へ添加しただけでは、高温保存特性と過充電時の安全性確保との両方の特性を満足することができない。
そこで、本発明者らは、鋭意検討した結果、正極上に生成するアルキルビフェニル類の酸化重合被膜が特異的に高い導電性を有することを見出した。このアルキルビフェニル類と、アルキルビフェニル類よりも酸化重合開始電位が４．５〜５．０Ｖと高い添加剤のシクロヘキシルベンゼンと、ｏ−テルフェニル、ビフェニル、ｔｅｒｔ−ブチルベンゼンから選ばれる少なくとも１種とを電解液中へ混合することによって、酸化重合開始電位の低い化合物（アルキルビフェニル類）の添加量を低減して高温保存特性を改善することができる。さらに、過充電時にはアルキルビフェニル類がわずかながらも反応し、正極上に高い導電性を有する重合被膜を形成するので、その後、その導電性の重合被膜上で、アルキルビフェニル類よりも酸化重合開始電位の高い添加剤が連続的に酸化重合反応を引き起こし、正極上に厚い重合被膜を形成する。このため、電流レートが３Ｃのような大電流での過充電時にも安全性を確保できることが分かった。これにより、電池の高温保存時の回復特性と過充電時の安全性確保の両方を満足することが可能となった。なお、後述（比較例３）の通り、シクロヘキシルベンゼンを添加しない場合には、電流レートが３Ｃの場合の過充電時の安全性確保が十分でないことが分かった。
【００１４】
前記一般式（Ｉ）で表されるアルキルビフェニル類の具体例としては、例えば、４−メチルビフェニル、４−エチルビフェニル、４−プロピルビフェニル、４−ｉｓｏ−プロピルビフェニル、４−ブチルビフェニル、４−ｉｓｏ−ブチルビフェニル、４−ｔｅｒｔ−ブチルビフェニル、４−ペンチルビフェニル、４−ｔｅｒｔ−ペンチルビフェニル、４−（１−エチル−１−メチルプロピル）ビフェニルから選ばれる少なくとも１種が挙げられる。特に、４−メチルビフェニル、４−エチルビフェニル、４−ｔｅｒｔ−ブチルビフェニルから選ばれる少なくとも１種が好ましい。
【００１５】
非水電解液中に含有される前記一般式（Ｉ）で表されるアルキルビフェニル類の含有量は、過度に多いと高温保存特性が悪くなり、また、過度に少ないと過充電時の安全性を十分に確保できなくなる。したがって、その含有量は非水電解液の重量に対して０．０１重量％以上１．０重量％未満の範囲とするのがよい。
【００１６】
非水電解液中にシクロヘキシルベンゼンを含有させることにより、電流レートが３Ｃの場合の過充電時の安全性を確保することができる。非水電解液中に含有されるシクロヘキシルベンゼンの含有量は、過度に多いと高温保存特性が悪くなり、また、過度に少ないと過充電時の安全性を確保することができなくなる。したがって、その含有量は非水電解液の重量に対して０．０１重量％以上５重量％以下の範囲とするのがよい。
【００１７】
非水電解液中にｏ−テルフェニルを含有させる場合には、その含有量は、過度に多いと高温保存特性が悪くなり、また、過度に少ないと過充電時の安全性を十分に確保できなくなる。したがって、その含有量は非水電解液の重量に対して０．０１重量％以上５重量％以下の範囲とするのがよい。
また、非水電解液中にビフェニルを含有させる場合には、その含有量は、過度に多いと高温保存特性が悪くなり、また、過度に少ないと過充電時の安全性を十分に確保できなくなる。したがって、その含有量は非水電解液の重量に対して０．０１重量％以上１．０重量％未満の範囲とするのがよい。
さらに、非水電解液中にｔｅｒｔ−ブチルベンゼンを含有させる場合には、その含有量は、過度に多いと高温保存特性が悪くなり、また、過度に少ないと過充電時の安全性を十分に確保できなくなる。したがって、その含有量は非水電解液の重量に対して０．０１重量％以上５重量％以下の範囲とするのがよい。
【００１８】
本発明で使用される非水溶媒としては、例えば、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）、ビニレンカーボネート（ＶＣ）などの環状カーボネート類や、γ−ブチロラクトンなどのラクトン類、ジメチルカーボネート（ＤＭＣ）、メチルエチルカーボネート（ＭＥＣ）、ジエチルカーボネート（ＤＥＣ）などの鎖状カーボネート類、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，４−ジオキサン、１，２−ジメトキシエタン、１，２−ジエトキシエタン、１，２−ジブトキシエタンなどのエーテル類、アセトニトリルなどのニトリル類、プロピオン酸メチル、ピバリン酸メチル、ピバリン酸オクチルなどのエステル類、ジメチルホルムアミドなどのアミド類が挙げられる。
【００１９】
これらの非水溶媒は、１種類で使用してもよく、また２種類以上を組み合わせて使用してもよい。非水溶媒の組み合わせは特に限定されないが、例えば、環状カーボネート類と鎖状カーボネート類との組み合わせ、環状カーボネート類とラクトン類との組み合わせ、環状カーボネート類３種類と鎖状カーボネート類との組み合わせなど種々の組み合わせが挙げられる。
【００２０】
本発明で使用される電解質としては、例えば、ＬｉＰＦ6、ＬｉＢＦ4、ＬｉＣｌＯ4、ＬｉＮ（ＳＯ2ＣＦ3）2、ＬｉＮ（ＳＯ2Ｃ2Ｆ5）2、ＬｉＣ（ＳＯ2ＣＦ3）3、ＬｉＰＦ4（ＣＦ3）2、ＬｉＰＦ3（Ｃ2Ｆ5）3、ＬｉＰＦ3（ＣＦ3）3、ＬｉＰＦ3（ｉｓｏ−Ｃ3Ｆ7）3、ＬｉＰＦ5（ｉｓｏ−Ｃ3Ｆ7）などが挙げられる。これらの電解質は、１種類で使用してもよく、２種類以上組み合わせて使用してもよい。これら電解質は、前記の非水溶媒に通常０．１〜３Ｍ、好ましくは０．５〜１．５Ｍの濃度で溶解されて使用される。
【００２１】
本発明の電解液は、例えば、前記の非水溶媒を混合し、これに前記の電解質を溶解し、前記一般式（Ｉ）で表されるアルキルビフェニル類と、シクロヘキシルベンゼンと、ｏ−テルフェニル、ビフェニル、ｔｅｒｔ−ブチルベンゼンから選ばれる少なくとも１種とを溶解することにより得られる。
【００２２】
本発明の電解液は、二次電池の構成部材、特にリチウム二次電池の構成部材として好適に使用される。二次電池を構成する電解液以外の構成部材については特に限定されず、従来使用されている種々の構成部材を使用できる。
【００２３】
例えば、正極活物質としてはコバルトまたはニッケルを含有するリチウムとの複合金属酸化物が使用される。これらの正極活物質は、１種類だけを選択して使用しても良いし、２種類以上を組み合わせて用いても良い。このような複合金属酸化物としては、例えば、ＬｉＣｏＯ2、ＬｉＮｉＯ2、ＬｉＣｏ1-xＮｉxＯ2（０．０１＜ｘ＜１）などが挙げられる。また、ＬｉＣｏＯ2とＬｉＭｎ2Ｏ4、ＬｉＣｏＯ2とＬｉＮｉＯ2、ＬｉＭｎ2Ｏ4とＬｉＮｉＯ2のように適当に混ぜ合わせて使用しても良い。
【００２４】
正極は、前記の正極活物質をアセチレンブラック、カーボンブラックなどの導電剤、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）などの結着剤および溶剤と混練して正極合剤とした後、この正極材料を集電体としてのアルミニウム箔やステンレス製のラス板に塗布して、乾燥、加圧成型後、５０℃〜２５０℃程度の温度で２時間程度真空下で加熱処理することにより作製される。
【００２５】
負極活物質としては、リチウム金属やリチウム合金、およびリチウムを吸蔵・放出可能な黒鉛型結晶構造を有する炭素材料〔熱分解炭素類、コークス類、グラファイト類（人造黒鉛、天然黒鉛など）、有機高分子化合物燃焼体、炭素繊維〕や複合スズ酸化物などの物質が使用される。特に、格子面（００２）の面間隔（ｄ002）が０．３３５〜０．３４０ｎｍである黒鉛型結晶構造を有する炭素材料を使用することが好ましい。これらの負極活物質は、１種類だけを選択して使用しても良いし、２種類以上を組み合わせて用いても良い。なお、炭素材料のような粉末材料はエチレンプロピレンジエンターポリマー（ＥＰＤＭ）、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）などの結着剤と混練して負極合剤として使用される。負極の製造方法は、特に限定されず、上記の正極の製造方法と同様な方法により製造することができる。
【００２６】
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、正極、負極および単層又は複層のセパレータを有するコイン型電池、さらに、正極、負極およびロール状のセパレータを有する円筒型電池や角型電池などが一例として挙げられる。なお、セパレータとしては公知のポリオレフィンの微多孔膜、織布、不織布などが使用される。
【００２７】
【実施例】
次に、実施例および比較例を挙げて、本発明を具体的に説明する。
実施例１
〔非水電解液の調製〕
ＥＣ：ＰＣ：ＤＥＣ（容量比）＝３０：５：６５の非水溶媒を調製し、これにＬｉＰＦ6を１Ｍの濃度になるように溶解して非水電解液を調製した後、さらにアルキルビフェニル類として４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．６重量、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して０．６重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．５重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．８重量％、ｔｅｒｔ−ブチルベンゼンを電解液の総量に対して０．５重量％添加した。
【００２８】
〔リチウム二次電池の作製〕
ＬｉＣｏＯ2（正極活物質）を９０重量％、アセチレンブラック（導電剤）を５重量％、ポリフッ化ビニリデン（結着剤）を５重量％の割合で混合し、これに１−メチル−２−ピロリドンを加えてスラリー状にしてアルミニウム箔上に塗布した。その後、これを乾燥し、加圧成型して正極を調製した。人造黒鉛（負極活物質）を９５重量％、ポリフッ化ビニリデン（結着剤）を５重量％の割合で混合し、これに１−メチル−２−ピロリドンを加えてスラリー状にして銅箔上に塗布した。その後、これを乾燥し、加圧成型して負極を調製した。そして、ポリエチレン微多孔性フィルムのセパレータを用い、上記の電解液を注入して１８６５０サイズの円筒型電池（直径１８ｍｍ、高さ６５ｍｍ）を作製した。
【００２９】
実施例２
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．４重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して０．６重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．７重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．８重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して０．５重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３０】
実施例３
電解液の添加剤として、４−メチルビフェニル（ＭＢＰ）を電解液の総量に対して０．６重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して０．７重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．５重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．７重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して０．５重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３１】
実施例４
電解液の添加剤として、ｔｅｒｔ−ブチルビフェニル（ＴＢＢＰ）を電解液の総量に対して０．８重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して０．５重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．４重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．５重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して０．８重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３２】
実施例５
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．２重量％、４−メチルビフェニル（ＭＢＰ）を電解液の総量に対して０．２重量％、ｔｅｒｔ−ブチルビフェニル（ＴＢＢＰ）を電解液の総量に対して０．３重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して０．５重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．６重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．７重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して０．５重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３３】
実施例６
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．４重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して２重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．４重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．６重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して０．６重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３４】
実施例７
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．６重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して１重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して２．４重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３５】
比較例１
電解液の添加剤を加えない以外は、実施例１と同様にして、円筒型電池を作製した。
【００３６】
比較例２
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．７重量％、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して２．３重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３７】
比較例３
電解液の添加剤として、シクロヘキシルベンゼン（ＣＨＢ）を電解液の総量に対して１．２重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して１重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．８重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３８】
比較例４
電解液の添加剤として、４−エチルビフェニル（ＥＢＰ）を電解液の総量に対して０．６重量％、ｏ−テルフェニル（ＯＴＰ）を電解液の総量に対して０．５重量％、ビフェニル（ＢＰ）を電解液の総量に対して０．８重量％、ｔｅｒｔ−ブチルベンゼン（ＴＢＢ）を電解液の総量に対して１．１重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００３９】
比較例５
電解液の添加剤として、ｔｅｒｔ−ブチルビフェニル（ＴＢＢＰ）を電解液の総量に対して４重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００４０】
比較例６
電解液の添加剤として、ビフェニル（ＢＰ）を電解液の総量に対して３重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００４１】
比較例７
電解液の添加剤として、ビフェニル（ＢＰ）を電解液の総量に対して５重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００４２】
比較例８
電解液の添加剤として、４−クロロアニソールを電解液の総量に対して３重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００４３】
比較例９
電解液の添加剤として、フランを電解液の総量に対して３重量％添加した以外は、実施例１と同様にして、円筒型電池を作製した。
【００４４】
次に、本発明の実施例１〜７に示す電池と比較例１〜９に示す電池の過充電試験を実施した。２０℃で充電状態から、さらに３．６Ａ（３Ｃ）で各２０個ずつの電池の過充電を行ない、電池が異常発熱するかどうかを確認した。表１に、試験した電池２０個中で異常発熱した電池の数を示した。
【００４５】
また、高温保存試験として、充電状態の電池を８０℃で４日間放置し、その後の１Ｃでの放電容量を保存前の１Ｃ放電容量と比較し、下記の通り、保存回復率を計算した。
保存回復率＝［保存後の１Ｃ放電容量／保存前の１Ｃ放電容量］×１００
高温保存回復率（％）の結果を表１に示した。
【００４６】
【表１】
なお、本発明は記載の実施例に限定されず、発明の趣旨から容易に類推可能な様々な組み合わせが可能である。特に、上記実施例の溶媒の組み合わせは限定されるものではない。更には、上記実施例は１８６５０サイズの円筒型電池に関するものであるが、本発明は角型、アルミラミネート型、コイン型の電池にも適用される。
【００４８】
【発明の効果】
本発明によって、電池の高温保存時の回復特性を改善しつつ、過充電時の安全性、特にハイレート（３Ｃ）での過充電時の安全性を確保できるリチウム二次電池を提供できる。
【００４９】
このようなリチウム二次電池を用いることによって安全性が高い携帯電話、カムコーダ、パーソナルコンピュータ、ＰＤＡ、電気自動車、ロードレベリング用電源などの機器を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte capable of ensuring safety during overcharge at a large current (for example, a current rate of 3 C) while improving the recovery characteristics of the battery during high-temperature storage, and lithium using the electrolyte The present invention relates to a secondary battery.
[0002]
[Prior art]
In recent years, electronic devices have become smaller and lighter and more portable, and development of a battery having a high energy density as a power source has been demanded. Lithium secondary batteries are expected as batteries that meet these requirements. However, when the charging voltage exceeds the predetermined charging voltage due to a failure of the charger, that is, when the battery is overcharged, the lithium in the positive electrode is excessively released and becomes thermally unstable. Since lithium exceeding the design capacity is deposited on the negative electrode surface, it becomes thermally unstable. As described above, when the positive electrode and the negative electrode are thermally unstable, the organic solvent of the electrolytic solution is violently decomposed on the electrode surface. Since this reaction is an abrupt exothermic reaction, in such a state, the battery will generate abnormal heat, causing thermal runaway, and in the worst case, the battery may burst or explode, which is extremely dangerous.
[0003]
Of course, as the current during overcharge increases, it becomes technically more difficult to ensure the safety of the battery. Recently, mobile phones, which are one of the main uses of lithium secondary batteries, require battery safety when overcharged with a large current, for example, battery safety at a current rate of 3C. ing.
[0004]
As a method of ensuring safety when the battery is overcharged in this way, (1) a method using an electronic circuit, (2) a method using a mechanical current interruption utilizing gas generation during overcharge, (3 A method using a redox shuttle, and a method (4) polymerizing an additive in an electrolytic solution at an overcharge potential are proposed and disclosed.
[0005]
In the method using an electronic circuit or the method using a mechanical current interruption, an additional structure is imparted to the battery, which increases the cost of the battery. Moreover, it is disadvantageous for the reduction in size and weight of the battery.
[0006]
Japanese Patent Application Laid-Open No. 9-50822 discloses ensuring safety during overcharge by the redox shuttle method. In this method, by using an electrolytic solution containing a benzene compound having a π electron orbital, such as 2-chloro-p-xylene or 4-chloroanisole, the compound is reversibly oxidized and reduced between the positive electrode and the negative electrode. And protecting the battery by consuming the overcharge current. However, this method is effective when the overcharge current is small, and when the overcharge current is large, the oxidation-reduction reaction does not proceed reversibly, so that it is difficult to ensure sufficient battery safety.
[0007]
Japanese Patent Application Laid-Open No. 9-106835 uses an electrolytic solution to which biphenyl, 3-chlorothiophene, furan, or the like is added, so that they are polymerized at the time of overcharging, thereby increasing the internal resistance of the battery and protecting the battery. A method is disclosed. However, these compounds exhibit an effect if the current rate at the time of overcharge is about 1 C, but cannot sufficiently secure safety if the current rate at the time of overcharge is about 3 times (3 C). . If the amount of these compounds added is increased, the safety of the battery during overcharge increases, but when stored at a high temperature in a normal state of charge, some of the additives in the positive electrode undergo an oxidative polymerization reaction, and are then deposited on the positive electrode. Since the polymerized film is formed, the polarization of the battery is increased, and the battery characteristics are deteriorated.
[0008]
[Problems to be solved by the invention]
The conventional overcharge protection method cannot sufficiently secure the safety of the lithium secondary battery at a large current (for example, a current rate of 3C) during overcharge without deteriorating the high-temperature storage characteristics. The present invention provides a lithium secondary battery excellent in high-temperature storage characteristics of the battery while ensuring safety during overcharge at a large current of about 3 C.
[0009]
[Means for Solving the Problems]
With regard to the compound that is added to the electrolytic solution and undergoes an oxidative polymerization reaction on the positive electrode surface during overcharge, the oxidative polymerization reaction behavior of the compound during overcharge is precisely determined by optimizing the type of compound and the amount added. Thus, the above-described problems have been solved.
[0010]
The present invention relates to a nonaqueous electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent, and the following general formula (I) is contained in the nonaqueous electrolytic solution.
[0011]
[Chemical 3]
In addition, the present invention provides a positive electrode using a material containing a lithium-containing metal oxide as a positive electrode active material, and a negative electrode active material selected from the group consisting of lithium metal, a lithium alloy, and a material capable of occluding and releasing lithium. A lithium secondary battery comprising a non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, wherein the non-aqueous electrolyte includes the following general formula (I):
[0012]
[Formula 4]
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, by adding three or more kinds of specific compounds having different oxidation polymerization reaction potentials to the electrolytic solution, it is possible to control the recovery characteristics of lithium batteries during high temperature storage and the safety of overcharge. Among the additives in the present invention, the oxidative polymerization reaction potential on the positive electrode is the lowest at about 4.3 to 4.5 V for alkylbiphenyls. In order to improve recovery characteristics during high-temperature storage, it is preferable that the amount of the compound (alkylbiphenyl) having a low oxidative polymerization initiation potential is low, but conversely, in order to ensure safety during overcharge, these are as much as possible. It is necessary to increase the amount of the compound added. In particular, in order to ensure safety in an overcharged state with a large current such as a current rate of 3C, it is necessary to add a large amount of a compound having a low oxidative polymerization initiation potential. To do. That is, there is a trade-off between high temperature storage characteristics and ensuring safety during overcharge. Therefore, the addition of a single compound or three or more types of compounds having different oxidative polymerization initiation potentials to the electrolyte may satisfy both the high temperature storage characteristics and the safety during overcharge. Can not.
Therefore, as a result of intensive studies, the present inventors have found that an oxidation polymerized film of alkylbiphenyls formed on the positive electrode has specifically high conductivity. The alkylbiphenyls, cyclohexylbenzene as an additive having a higher oxidative polymerization initiation potential of 4.5 to 5.0 V than alkylbiphenyls, and at least one selected from o-terphenyl, biphenyl, and tert-butylbenzene Can be added to the electrolytic solution to reduce the amount of the compound (alkylbiphenyl) having a low oxidative polymerization initiation potential and improve the high-temperature storage characteristics. In addition, alkylbiphenyls react slightly when overcharged, and form a polymer film with high conductivity on the positive electrode. Then, the oxidation polymerization initiation potential is higher than that of alkylbiphenyls on the polymerized polymer film. Additives that continuously cause an oxidative polymerization reaction, forming a thick polymer film on the positive electrode. For this reason, it turned out that safety | security can be ensured also at the time of the overcharge by a large current like a current rate of 3C. This makes it possible to satisfy both the recovery characteristics of the battery during high-temperature storage and the securing of safety during overcharge. In addition, as described later (Comparative Example 3), when cyclohexylbenzene was not added, it was found that safety at the time of overcharging when the current rate was 3C was not sufficient.
[0014]
Specific examples of the alkylbiphenyls represented by the general formula (I) include, for example, 4-methylbiphenyl, 4-ethylbiphenyl, 4-propylbiphenyl, 4-iso-propylbiphenyl, 4-butylbiphenyl, 4- Examples thereof include at least one selected from iso-butylbiphenyl, 4-tert-butylbiphenyl, 4-pentylbiphenyl, 4-tert-pentylbiphenyl, and 4- (1-ethyl-1-methylpropyl) biphenyl. In particular, at least one selected from 4-methylbiphenyl, 4-ethylbiphenyl, and 4-tert-butylbiphenyl is preferable.
[0015]
If the content of the alkylbiphenyls represented by the general formula (I) contained in the non-aqueous electrolyte is excessively large, the high-temperature storage characteristics deteriorate, and if it is excessively small, the safety during overcharge is deteriorated. Cannot be secured sufficiently. Therefore, the content is preferably in the range of 0.01 wt% or more and less than 1.0 wt% with respect to the weight of the non-aqueous electrolyte.
[0016]
By including cyclohexylbenzene in the non-aqueous electrolyte, it is possible to ensure safety during overcharge when the current rate is 3C. If the content of cyclohexylbenzene contained in the non-aqueous electrolyte is excessively large, the high-temperature storage characteristics deteriorate, and if it is excessively small, safety during overcharge cannot be ensured. Therefore, the content is preferably in the range of 0.01% by weight to 5% by weight with respect to the weight of the non-aqueous electrolyte.
[0017]
When o-terphenyl is contained in the non-aqueous electrolyte, if the content is excessively large, the high-temperature storage characteristics deteriorate, and if it is excessively small, sufficient safety during overcharge can be ensured. Disappear. Therefore, the content is preferably in the range of 0.01% by weight to 5% by weight with respect to the weight of the non-aqueous electrolyte.
In addition, when biphenyl is contained in the non-aqueous electrolyte, if the content is excessively large, the high-temperature storage characteristics deteriorate, and if it is excessively small, sufficient safety during overcharging cannot be ensured. . Therefore, the content is preferably in the range of 0.01 wt% or more and less than 1.0 wt% with respect to the weight of the non-aqueous electrolyte.
Furthermore, when tert-butylbenzene is contained in the non-aqueous electrolyte, if the content is excessively large, the high-temperature storage characteristics deteriorate, and if it is excessively small, sufficient safety during overcharge is obtained. It cannot be secured. Therefore, the content is preferably in the range of 0.01% by weight to 5% by weight with respect to the weight of the non-aqueous electrolyte.
[0018]
Examples of the non-aqueous solvent used in the present invention include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC), and lactones such as γ-butyrolactone. , Chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2 -Ethers such as diethoxyethane and 1,2-dibutoxyethane; nitriles such as acetonitrile; esters such as methyl propionate, methyl pivalate and octyl pivalate; and amides such as dimethylformamide. .
[0019]
These nonaqueous solvents may be used alone or in combination of two or more. The combination of the non-aqueous solvent is not particularly limited. For example, there are various combinations such as a combination of cyclic carbonates and chain carbonates, a combination of cyclic carbonates and lactones, a combination of three types of cyclic carbonates and chain carbonates, and the like. The combination of is mentioned.
[0020]
Examples of the electrolyte used in the present invention include LiPF 6 , LiBF 4 , LiClO 4 , LiN (SO 2 CF 3 ) 2 , LiN (SO 2 C 2 F 5 ) 2 , LiC (SO 2 CF 3 ) 3 , LiPF 4 (CF 3 ) 2 , LiPF 3 (C 2 F 5 ) 3 , LiPF 3 (CF 3 ) 3 , LiPF 3 (iso-C 3 F 7 ) 3 , LiPF 5 (iso-C 3 F 7 ), etc. Can be mentioned. These electrolytes may be used alone or in combination of two or more. These electrolytes are used by being dissolved in the non-aqueous solvent usually at a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.
[0021]
The electrolyte solution of the present invention is prepared by, for example, mixing the non-aqueous solvent, dissolving the electrolyte therein, alkyl biphenyls represented by the general formula (I), cyclohexylbenzene, and o-terphenyl. And at least one selected from biphenyl and tert-butylbenzene.
[0022]
The electrolytic solution of the present invention is suitably used as a constituent member of a secondary battery, particularly as a constituent member of a lithium secondary battery. The constituent members other than the electrolytic solution constituting the secondary battery are not particularly limited, and various conventionally used constituent members can be used.
[0023]
For example, a composite metal oxide with lithium containing cobalt or nickel is used as the positive electrode active material. Only one type of these positive electrode active materials may be selected and used, or two or more types may be used in combination. Examples of such composite metal oxides include LiCoO 2 , LiNiO 2 , LiCo 1-x Ni x O 2 (0.01 <x <1). Further, LiCoO 2 and LiMn 2 O 4 , LiCoO 2 and LiNiO 2 , LiMn 2 O 4 and LiNiO 2 may be appropriately mixed and used.
[0024]
The positive electrode is obtained by kneading the positive electrode active material with a conductive agent such as acetylene black or carbon black, a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), and a solvent to form a positive electrode mixture. By applying this positive electrode material to an aluminum foil or stainless steel lath plate as a current collector, and after drying and pressure molding, heat treatment is performed under vacuum at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. Produced.
[0025]
Examples of the negative electrode active material include lithium metal and lithium alloy, and carbon materials having a graphite-type crystal structure capable of occluding and releasing lithium (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), Materials such as molecular compound combustor, carbon fiber] and composite tin oxide are used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which a lattice spacing ( 002 ) (d 002 ) is 0.335 to 0.340 nm. Only one kind of these negative electrode active materials may be selected and used, or two or more kinds may be used in combination. A powder material such as a carbon material is kneaded with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) and used as a negative electrode mixture. The manufacturing method of a negative electrode is not specifically limited, It can manufacture with the method similar to the manufacturing method of said positive electrode.
[0026]
The structure of the lithium secondary battery is not particularly limited, and a coin-type battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and a cylindrical battery or a square type having a positive electrode, a negative electrode, and a roll separator. An example is a battery. A known polyolefin microporous film, woven fabric, non-woven fabric or the like is used as the separator.
[0027]
【Example】
Next, an Example and a comparative example are given and this invention is demonstrated concretely.
Example 1
(Preparation of non-aqueous electrolyte)
A non-aqueous solvent of EC: PC: DEC (volume ratio) = 30: 5: 65 was prepared, and LiPF 6 was dissolved therein to a concentration of 1M to prepare a non-aqueous electrolyte solution. As an example, 4-ethylbiphenyl (EBP) is 0.6% by weight based on the total amount of the electrolytic solution, cyclohexylbenzene (CHB) is 0.6% by weight based on the total amount of the electrolytic solution, and o-terphenyl (OTP) is electrolyzed. 0.5% by weight with respect to the total amount of the liquid, 0.8% by weight of biphenyl (BP) with respect to the total amount of the electrolytic solution, and 0.5% by weight of tert-butylbenzene with respect to the total amount of the electrolytic solution.
[0028]
[Production of lithium secondary battery]
90% by weight of LiCoO 2 (positive electrode active material), 5% by weight of acetylene black (conductive agent), and 5% by weight of polyvinylidene fluoride (binder) are mixed, and this is mixed with 1-methyl-2-pyrrolidone. Was added to form a slurry and coated on an aluminum foil. Then, this was dried and pressure-molded to prepare a positive electrode. 95% by weight of artificial graphite (negative electrode active material) and 5% by weight of polyvinylidene fluoride (binder) are mixed, and 1-methyl-2-pyrrolidone is added to this to form a slurry on a copper foil. Applied. Then, this was dried and pressure-molded to prepare a negative electrode. And using the separator of a polyethylene microporous film, said electrolyte solution was inject | poured and the cylindrical battery (diameter 18mm, height 65mm) of 18650 size was produced.
[0029]
Example 2
As an additive for the electrolytic solution, 4-ethylbiphenyl (EBP) is 0.4% by weight with respect to the total amount of the electrolytic solution, cyclohexylbenzene (CHB) is 0.6% by weight with respect to the total amount of the electrolytic solution, o-tel Phenyl (OTP) is 0.7% by weight based on the total amount of the electrolytic solution, biphenyl (BP) is 0.8% by weight based on the total amount of the electrolytic solution, and tert-butylbenzene (TBB) is based on the total amount of the electrolytic solution. A cylindrical battery was produced in the same manner as in Example 1 except that 0.5% by weight was added.
[0030]
Example 3
As an additive of the electrolytic solution, 4-methylbiphenyl (MBP) is 0.6% by weight with respect to the total amount of the electrolytic solution, cyclohexylbenzene (CHB) is 0.7% by weight with respect to the total amount of the electrolytic solution, o-tel Phenyl (OTP) is 0.5% by weight based on the total amount of the electrolytic solution, Biphenyl (BP) is 0.7% by weight based on the total amount of the electrolytic solution, and tert-butylbenzene (TBB) is based on the total amount of the electrolytic solution. A cylindrical battery was produced in the same manner as in Example 1 except that 0.5% by weight was added.
[0031]
Example 4
As an electrolyte additive, tert-butylbiphenyl (TBBP) is 0.8% by weight with respect to the total amount of the electrolyte, cyclohexylbenzene (CHB) is 0.5% by weight with respect to the total amount of the electrolyte, Phenyl (OTP) is 0.4% by weight based on the total amount of the electrolytic solution, biphenyl (BP) is 0.5% by weight based on the total amount of the electrolytic solution, and tert-butylbenzene (TBB) is based on the total amount of the electrolytic solution. A cylindrical battery was produced in the same manner as in Example 1 except that 0.8% by weight was added.
[0032]
Example 5
As an additive of the electrolytic solution, 4-ethylbiphenyl (EBP) is 0.2% by weight based on the total amount of the electrolytic solution, 4-methylbiphenyl (MBP) is 0.2% by weight based on the total amount of the electrolytic solution, tert -0.3% by weight of butyl biphenyl (TBBP) with respect to the total amount of the electrolytic solution, 0.5% by weight of cyclohexylbenzene (CHB) with respect to the total amount of the electrolytic solution, and o-terphenyl (OTP) with respect to the total amount of the electrolytic solution. 0.6% by weight with respect to the total amount, 0.7% by weight of biphenyl (BP) with respect to the total amount of the electrolytic solution, and 0.5% by weight of tert-butylbenzene (TBB) with respect to the total amount of the electrolytic solution. Except for the above, a cylindrical battery was produced in the same manner as in Example 1.
[0033]
Example 6
As an additive for the electrolytic solution, 4-ethylbiphenyl (EBP) is 0.4% by weight based on the total amount of the electrolytic solution, cyclohexylbenzene (CHB) is 2% by weight based on the total amount of the electrolytic solution, o-terphenyl ( OTP) is 0.4% by weight with respect to the total amount of the electrolytic solution, biphenyl (BP) is 0.6% by weight with respect to the total amount of the electrolytic solution, and tert-butylbenzene (TBB) is 0% with respect to the total amount of the electrolytic solution. A cylindrical battery was produced in the same manner as in Example 1 except that 6% by weight was added.
[0034]
Example 7
As an additive for the electrolytic solution, 4-ethylbiphenyl (EBP) is 0.6% by weight based on the total amount of the electrolytic solution, cyclohexylbenzene (CHB) is 1% by weight based on the total amount of the electrolytic solution, and tert-butylbenzene ( A cylindrical battery was fabricated in the same manner as in Example 1 except that 2.4% by weight of TBB) was added to the total amount of the electrolytic solution.
[0035]
Comparative Example 1
A cylindrical battery was produced in the same manner as in Example 1 except that the electrolyte additive was not added.
[0036]
Comparative Example 2
As an additive of the electrolytic solution, 4-ethylbiphenyl (EBP) was added by 0.7% by weight with respect to the total amount of the electrolytic solution, and cyclohexylbenzene (CHB) was added by 2.3% by weight with respect to the total amount of the electrolytic solution. In the same manner as in Example 1, a cylindrical battery was produced.
[0037]
Comparative Example 3
As an additive for the electrolyte, cyclohexylbenzene (CHB) is 1.2% by weight with respect to the total amount of the electrolyte, o-terphenyl (OTP) is 1% by weight with respect to the total amount of the electrolyte, and biphenyl (BP) is added. A cylindrical battery was fabricated in the same manner as in Example 1 except that 0.8% by weight was added to the total amount of the electrolytic solution.
[0038]
Comparative Example 4
As an additive for the electrolytic solution, 4-ethylbiphenyl (EBP) is 0.6% by weight with respect to the total amount of the electrolytic solution, o-terphenyl (OTP) is 0.5% by weight with respect to the total amount of the electrolytic solution, biphenyl. Except for adding 0.8 wt% of (BP) to the total amount of the electrolyte and 1.1 wt% of tert-butylbenzene (TBB) based on the total amount of the electrolyte, A cylindrical battery was produced.
[0039]
Comparative Example 5
A cylindrical battery was produced in the same manner as in Example 1 except that 4 wt% of tert-butylbiphenyl (TBBP) was added as an additive of the electrolytic solution based on the total amount of the electrolytic solution.
[0040]
Comparative Example 6
A cylindrical battery was fabricated in the same manner as in Example 1, except that 3 wt% of biphenyl (BP) was added as an additive of the electrolytic solution with respect to the total amount of the electrolytic solution.
[0041]
Comparative Example 7
A cylindrical battery was fabricated in the same manner as in Example 1 except that biphenyl (BP) was added in an amount of 5% by weight based on the total amount of the electrolytic solution as an additive for the electrolytic solution.
[0042]
Comparative Example 8
A cylindrical battery was fabricated in the same manner as in Example 1 except that 4-chloroanisole was added as an additive for the electrolytic solution in an amount of 3% by weight based on the total amount of the electrolytic solution.
[0043]
Comparative Example 9
A cylindrical battery was fabricated in the same manner as in Example 1 except that 3% by weight of furan was added to the total amount of the electrolytic solution as an additive for the electrolytic solution.
[0044]
Next, overcharge tests of the batteries shown in Examples 1 to 7 and the batteries shown in Comparative Examples 1 to 9 were performed. From the charged state at 20 ° C., over 20 batteries were further overcharged at 3.6A (3C), and it was confirmed whether or not the batteries generated abnormal heat. Table 1 shows the number of abnormally heated batteries among the 20 batteries tested.
[0045]
Further, as a high temperature storage test, a charged battery was left at 80 ° C. for 4 days, and then the discharge capacity at 1 C was compared with the 1 C discharge capacity before storage, and the storage recovery rate was calculated as follows.
Storage recovery rate = [1C discharge capacity after storage / 1C discharge capacity before storage] × 100
The results of the high temperature storage recovery rate (%) are shown in Table 1.
[0046]
[Table 1]
In addition, this invention is not limited to the Example described, The various combination which can be easily guessed from the meaning of invention is possible. In particular, the combination of solvents in the above examples is not limited. Furthermore, although the said Example is related with a cylindrical battery of 18650 size, this invention is applied also to a square-type, an aluminum laminate type, and a coin-type battery.
[0048]
【The invention's effect】
According to the present invention, it is possible to provide a lithium secondary battery that can ensure the safety at the time of overcharging, in particular, the safety at the time of overcharging at a high rate (3C) while improving the recovery characteristics of the battery during high temperature storage.
[0049]
By using such a lithium secondary battery, highly safe devices such as a mobile phone, a camcorder, a personal computer, a PDA, an electric vehicle, and a power supply for load leveling can be provided.
Claims (4)
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| JP2001061156A JP4352622B2 (en) | 2001-03-06 | 2001-03-06 | Non-aqueous electrolyte and lithium secondary battery using the same |
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| JP2001061156A JP4352622B2 (en) | 2001-03-06 | 2001-03-06 | Non-aqueous electrolyte and lithium secondary battery using the same |
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