JP3821495B2 - Non-aqueous electrolyte and non-aqueous electrolyte battery - Google Patents
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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
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Description
【０００１】
【産業上の利用分野】
本発明は新規な非水電解液及びそれを用いた非水電解液電池に関する。
【０００２】
【従来の技術】
非水電解液を用いた電池は、高電圧、高エネルギー密度を有し、かつ貯蔵性などの信頼性に優れているため、広く民生用電子機器の電源に用いられている。非水電解液としては、一般に高誘電率の溶媒である炭酸プロピレン、γ−ブチロラクトン、スルホラン等に、低粘度溶媒であるジメトキシエタン、テトラヒドロフラン、また１，３−ジオキソラン等を混合した溶媒にＬｉＢＦ4、ＬｉＰＦ6、ＬｉＣｌＯ4、ＬｉＡｓＦ6、ＬｉＣＦ3ＳＯ3、ＬｉＮ（ＣＦ3ＳＯ2）2、ＬｉＡｌＣｌ4、ＬｉＳｉＦ6等の電解質を混合したものが用いられている。
【０００３】
しかし、このような非水電解液の溶媒は耐電圧の低いものが多く、耐電圧の低い溶媒を用いた電解液を二次電池に使用した場合、充放電を繰返すと溶媒が電気分解され、そのため生成したガスにより電池の内圧が上昇したり、生成物が重合反応を起こし電極に付着する等の事態が生じてしまう。このため、電池の充放電効率が低下し、電池エネルギー密度の低下により、電池の寿命が短くなる等の問題があった。電解液の耐久性を向上させる試みとしては、従来用いられていたγ−ブチロラクトン、エチルアセテート等のエステル類や、１，３−ジオキソラン、テトラヒドロフラン、ジメトキシエタン等のエーテル類などの耐電圧の低い溶媒の代りに耐電圧の高い炭酸ジエチル等の炭酸エステルを使用し、充放電の反復後の電池エネルギー密度の低下を抑制することが提案されている（例えば特開平２−１０６６６号公報）。
【０００４】
一方、リチウム二次電池の負極には金属リチウム、またはリチウム合金が用いられているが、充放電を繰返すと電解液中のリチウムイオンが負極上に偏って析出し、デンドライトと呼ばれる針状の反応性の高い金属が生成される場合があった。デンドライトが電極から脱落すると、自己消耗し電池のサイクル寿命が短くなってしまう、正極と負極を隔てるセパレータをデンドライトが貫通しショートする可能性がある等の問題点も考えられている。
【０００５】
【発明が解決すべき課題】
ところで、エネルギー密度の高い電池が望まれていることから、高電圧電池について各方面から研究が進められている。例えば、電池の正極にＬｉＣｏＯ2やＬｉＮｉＯ2、Ｌｉ2Ｍｎ2Ｏ4などのリチウムと遷移金属の複合酸化物を使用し、負極に炭素材料を使用した、ロッキングチェア型とよばれる二次電池が研究されてきた。この場合、電池電圧は４Ｖを発生することができ、しかも金属リチウムの析出がないため、過充電、外部ショート、針刺し、押しつぶし等の実験によっても安全性が確認され、民生用として出回るようになっている。しかしながら、今後の大幅な高エネルギー密度化、また大型化がなされた場合には、一層の難燃化などの安全性向上が求められている。現在使用されている電解液溶媒は、必ずしも満足のいく高い引火点を有するものでなく、自己消火性もない。
【０００６】
このため、自己消火性のある化合物として知られているリン酸エステル類を電解液に添加することが提案されている（特開平４−１８４８７０号公報）。しかしながら、この種の化合物を１５重量％以上添加した電解液は、難燃性はクリアーされるが、逆に電池充放電効率、電池のエネルギー密度、電池寿命の点で問題があった。更にリン酸トリメチルやリン酸トリエチルなどのリン酸エステル類は金属リチウムとの反応性があるため、負極に金属リチウム、リチウム合金を用いたリチウム二次電池には適していない。また前述のロッキングチェア型のリチウムイオン二次電池であっても、誤使用によって極度の過充電を行うなど苛酷な試験条件下では、金属リチウムが析出する可能性がある。
【０００７】
本発明は上記の問題点に鑑みてなされたもので、充放電効率の低下や充放電繰返後の電池エネルギー密度の低下を生じることがなく、自己消火性を有し、引火点が高く、しかも金属リチウムとの反応性の低い非水電解液を提供することを目的とする。更に本発明は、耐電圧及び電気伝導性に優れ、負荷特性、低温特性に優れた非水電解液を提供することを目的とする。更に本発明は、高電圧を発生でき、電池性能が優れ、長寿命の非水電解液電池を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者らは、高電圧を発生でき、電池特性に優れた非水電池用電解液を作るため、高い自己消火性作用を持つリン酸エステル化合物について鋭意研究を行った。その結果、リン酸エステルの置換基の少なくとも一つをフッ素原子などのハロゲン原子で置換するとリチウム金属との反応性を低下させることができることを見出し、このリン酸エステルを添加することによって自己消火性を維持し電池性能にも優れた非水電池用電解液が得られることを見出した。
【０００９】
すなわち、本発明の非水電解液は、負極活物質として金属リチウム、リチウム合金及びリチウムイオンのドープ・脱ドープが可能な炭素材料から選ばれる材料を含む負極を有する電池に用られる非水電解液であって、電解質としてＬｉＰＦ 6 を含み、電解液溶媒は、一般式［１］で表されるリン酸エステル化合物を電解液溶媒全体の７体積％以上２０体積％以下含有し、且つ後述する鎖状エステル化合物及び環状エステル化合物を含有するものである。
【００１０】
【化３】
（Ｒ1、Ｒ2、Ｒ3は同一あるいは異なっていてもよく、それぞれアルキル基またはハロゲン原子置換アルキル基を表し、Ｒ1、Ｒ2、Ｒ3の少なくとも１つはハロゲン原子置換アルキル基を表す。）
また本発明の非水電解液電池は、上記リン酸エステル化合物を含有する電解液を使用するものである。
【００１２】
一般式［１］のリン酸エステル化合物は自己消火性を有し、電解液に添加することにより電解液に自己消火性を付与するとともに、Ｒ1、Ｒ2、Ｒ3のうち少なくとも１つがハロゲン原子で置換されることにより、本発明の非水電解液のリチウム金属との反応性を減少させることができる。Ｒ1、Ｒ2、Ｒ3の少なくとも１つがハロゲン原子置換アルキル基であれば、残りはアルキル基でもハロゲン原子置換アルキル基であってもよい。
【００１３】
ここで、リン酸エステルの自己消火作用は、リン酸エステル中のリン含量が高いほど、すなわち置換基のＲ1、Ｒ2、Ｒ3の分子量が小さいほど大きく、また添加量が多いほど大きい。しかし、分子量の大きいリン酸エステルの添加量を増やすことは、その添加によって電解液の粘度増加が大きくなり電導度が低下するため好ましくない。従って、Ｒ1、Ｒ2、Ｒ3の炭素数はなるべく小さいことが望ましく、Ｒ1、Ｒ2、Ｒ3がアルキル基の場合、好適には１〜４個、ハロゲン原子置換アルキル基の場合に好適には２〜４個である。
【００１４】
アルキル基としては、メチル基、エチル基、ノルマルプロピル基、イソプロピル基、ノルマルブチル基、sec−ブチル基、ｔ−ブチル基、イソブチル基などが挙げられる。またハロゲン原子置換アルキル基としては、フッ素原子置換アルキル基、塩素原子原子置換アルキル基、臭素原子置換アルキル基が挙げられ、また１つの置換基にフッ素、塩素、または臭素が混在しているものでもよい。フッ素原子置換アルキル基の場合を例に挙げると、トリフルオロエチル基、ジフルオロエチル基、モノフルオロエチル基、ペンタフルオロプロピル基、２，２，３，３−テトラフルオロプロピル基、１，１，１−トリフルオロイソプロピル基、１，３−ジフルオロ−２−プロピル基、ヘキサフルオロイソプロピル基、２，２，３，３，４，４，４−ヘプタフルオロブチル基、２，２，３，４，４，４−ヘキサフルオロブチル基、ヘキサフルオロ−２−メチルイソプロピル基、３，３，４，４，４−ペンタフルオロ−２−ブチル基、４，４，４−トリフルオロブチル基、パーフルオロ−ｔ−ブチル基などが挙げられる。その他、上記と同様の構造でフッ素の代りに塩素、臭素で置換したものも例示される。
【００１５】
本発明のリン酸エステル化合物としては、リン酸トリ（トリフルオロエチル）、リン酸メチル（ジトリフルオロエチル）、リン酸ジメチル（トリフルオロエチル）、リン酸エチル（ジトリフルオロエチル）、リン酸ジエチル（トリフルオロエチル）、リン酸プロピル（ジトリフルオロエチル）、リン酸ジプロピル（トリフルオロエチル）、リン酸トリ（ペンタフルオロプロピル）、リン酸メチル（ジペンタフルオロプロピル）、リン酸ジメチル（ペンタフルオロプロピル）、リン酸エチル（ジペンタフルオロプロピル）、リン酸ジエチル（ペンタフルオロプロピル）、リン酸ブチル（ジペンタフルオロプロピル）、リン酸ジブチル（ペンタフルオロプロピル）、そのほか前述のハロゲン原子置換アルキル基とアルキル基とを有するものが挙げられる。
【００１６】
式［１］のハロゲン置換リン酸エステル化合物は前述のように溶媒を燃えにくくする作用があり、しかもリン酸トリメチルなど通常のリン酸エステル化合物に比べリチウム金属との反応性も大きく押さえられているが、それ単独で溶媒として用いた場合には、電解液の伝導度を低下し電池のエネルギー密度を低下する性質を持つ。従って、本発明の非水電解液を実用的な二次電池用電解液として使用するためには、溶媒として上記ハロゲン置換リン酸エステル化合物と他の溶媒との混合溶媒を用いる。この場合、ハロゲン置換リン酸エステル化合物の添加量は、溶媒全体に対し７〜２０体積％、好ましくは７〜１５体積％添加されていればよい。このような範囲とすることにより、電導度の低下や電池のエネルギー密度の低下等、電池性能に影響を及ぼすことなく、しかも十分な自己消化作用を得ることができる。
【００１７】
上記ハロゲン原子置換リン酸エステル化合物が添加される溶媒としては、従来より用いられている、ジメトキシエタンなどの鎖状エーテル類、テトラヒドロフランなどの環状エーテル類、ジメチルホルムアミドなどのアミド類、メチル−Ｎ，Ｎ−ジメチルカーバメートなどのカーバメート類、更に炭酸ジエチル等の鎖状エステル類、炭酸プロピレン等の環状エステル類を１種または２種以上混合して用いることができる。特に高電圧電池として用いる場合には、上記ハロゲン原子置換リン酸エステルは、鎖状エステル及び／または環状エステルとの混合溶媒とすることによって電解液の電導度を高めることができ、電池性能を良好にすることができる。
【００１８】
ここで用いられる鎖状エステルは、一般式［２］で表される１種またはこれらの混合物である。
【００１９】
【化４】
（式中、Ｒ4はメチル基、エチル基、プロピル基、メトキシ基あるいはエトキシ基を表し、Ｒ5は炭素数１から３個の鎖状あるいは分枝状アルキル基を表す。）一般式［２］で表される鎖状エステルをハロゲン原子置換リン酸エステルに混合して用いることによって、電解液の粘度を下げることができ、常温から低温での電気伝導性に優れた電解液が得られる。このような鎖状エステルとしては、例えばジメチルカーボネート、メチルエチルカーボネート、ジエチルカーボネート、メチルプロピルカーボネート、メチルイソプロピルカーボネート、メチルブチルカーボネート、エチルブチルカーボネート、蟻酸メチル、蟻酸エチル、蟻酸プロピル、酪酸メチル、酪酸エチル、酢酸メチル、酢酸エチル、酢酸プロピル、プロピオン酸メチル、プロピオン酸エチルなどを例示することができる。特に電池の正極として、４Ｖを発生できるＬｉＣｏＯ2、ＬｉＭｎＯ2、ＬｉＭｎ2Ｏ4、ＬｉＮｉＯ2等を用いた電池の電解液の場合には、耐酸化安定性から、ジメチルカーボネート、ジエチルカーボネート、メチルエチルカーボネートが好ましい。
【００２１】
鎖状エステルの電解液溶媒中の濃度は通常２０〜９０体積％の範囲で用いることができ、好ましくは４０〜８０体積％の範囲で用いることができる。
また環状エステルとしては、プロピレンカーボネート、エチレンカーボネート、ブチレンカーボネート、ビニレンカーボネート、γ−ブチロラクトンまたはスルホランから選ばれる１種またはこれらの混合物が用いられ、好ましくはプロピレンカーボネートとエチレンカーボネートが選ばれる。これら環状エステルは電解液溶媒中の濃度が通常１０〜７０体積％の範囲で用いることができ、好ましくは２０〜６０体積％の範囲で用いることができる。環状エステルをハロゲン置換リン酸エステルに混合して用いることによって電解質の解離性を高めることができ電導度を高めることができる。
【００２２】
また上述の鎖状エステルと環状エステルをともにハロゲン原子置換リン酸エステルに混合して使用すると、粘度低下の効果と電解質の解離の効果が相乗されるため更に好ましい。
尚、電池の正極として、３Ｖ程度の電圧を発生できるＶ2Ｏ5、ポリアニリン等を用いた電池の場合は、これら鎖状エステル及び環状エステルの代りにあるいは併用して、それより耐電圧の低いジメトキシエタンなどの鎖状エーテル類、テトラヒドロフランなどの環状エーテル類、ジメチルホルムアミドなどのアミド類、メチル−Ｎ，Ｎ−ジメチルカーバメートなどのカーバメート類、Ｎ−メチルオキサゾリドン、Ｎ−メチルピロリドンなどのカーバメートやアミド類も使用できる。
【００２３】
本発明の電解液に用いる電解質としては、ＬｉＰＦ6、ＬｉＢＦ4、ＬｉＣｌＯ4、ＬｉＡｓＦ6、ＬｉＣＦ3ＳＯ3、ＬｉＮ（ＣＦ3ＳＯ2）2、ＬｉＡｌＣｌ4、ＬｉＳｉＦ6などのリチウム塩を使用することができるが、特にＬｉＰＦ6が好ましい。電解質としてＬｉＰＦ6を用いた場合、本発明のリン酸エステル化合物の含有量を低くしても高い自己消火性を保持できる。従って、電池の充放電効率やエネルギー密度の低下を防ぐことができる。
【００２４】
電解液中の電解質濃度は通常、０．１〜３モル／リットルの濃度範囲で使用することができ、好ましくは０．５〜２モル／リットルの濃度範囲で用いることができる。
本発明の非水電解液電池は、上記組成の電解液を使用するものであり、少なくとも正極、負極、セパレータから成る電池である。
【００２５】
負極材料としては金属リチウム、リチウム合金、リチウムイオンをドープ・脱ドープが可能な炭素材料を用いることができ、特にリチウムイオンをドープ・脱ドープが可能な炭素材料を用いることが好ましい。このような炭素材料としてはグラファイトでも非晶質炭素でもよく、活性炭、炭素繊維、カーボンブラック、メソカーボンマイクロビーズなどあらゆる炭素材料を用いることができる。
【００２６】
正極材料としては、ＭｏＳ2、ＴｉＳ2、ＭｎＯ2、Ｖ2Ｏ5等の遷移金属酸化物、遷移金属硫化物、ポリアニリン、ポリピロールなどの導電性高分子、ジスルフィド化合物のように可逆的に電解重合、解重合する化合物あるいはＬｉＣｏＯ2、ＬｉＭｎＯ2、ＬｉＭｎ2Ｏ4、ＬｉＮｉＯ2などのリチウムと遷移金属からなる複合酸化物を用いることができ、好ましくはリチウムと遷移金属からなる複合酸化物が用いられる。
【００２７】
本発明の非水電解液電池は電解液として以上説明した非水電解液を含むことにより、高電圧を発生でき、電池充放電効率が高く、充放電の繰返し後でも電池のエネルギー密度の低下のない、実用性に優れた非水二次電池とすることができる。
尚、本発明の非水電解液電池の形状、形態等は特に限定されるものではなく、円筒型、角型、コイン型、カード型、大型など本発明の範囲内で任意に選択することができる。
【００２８】
【実施例】
以下に実施例を挙げて本発明を具体的に説明するが、本発明はこれら実施例に限定されるものではない。
１．リン酸エステル化合物の金属リチウムとの反応性の評価
露点−６０℃のアルゴン雰囲気で、長さ２ｃｍ、幅０．５ｃｍ、厚さ０．５ｍｍの金属リチウム箔と脱水・蒸留精製したリン酸エステル化合物１０ｍｌをガラス容器中に混合し、リチウム箔を金属の清浄面が現れるようにリン酸エステル中でリチウム表面を削った。このガラス容器を種々の温度に加熱し金属リチウムとリン酸エステル化合物の反応を観察した。リン酸エステル化合物としては、フッ素原子置換リン酸エステル化合物であるリン酸トリ（トリフルオロエチル）（以下、ＴＦＥＰＡと略記する）及びリン酸トリス（２−クロロエチル）（以下、ＴＣＥＰＡと略記する）、比較例としてリン酸トリメチル及びリン酸トリエチル （以下、ＴＥＰＡと略記する）を用いた。
【００２９】
その結果、リン酸トリエチルでは１６５℃で激しく発泡して反応し、その反応熱でリチウムが融解し、またリン酸トリメチルでも１８０℃で発泡して反応したのに対し、ＴＦＥＰＡは１８０℃において金属リチウムの融解が見られたが、融解しても表面に灰色の被膜が生成したのみで、反応は継続しなかった。
２．電解液の自己消火性評価
電解液溶媒としてプロピレンカーボネート（以下、ＰＣと略記する）、メチルエチルカーボネート（以下、ＭＥＣと略記する）及びリン酸エステル（ＴＦＥＰＡ、ＴＣＥＰＡ）の三者を所定割合で混合した溶媒を用い、電解質として六フッ化リン酸リチウム（ＬｉＰＦ6）を１．０モル／リットル溶解させた電解液を用意した。また比較例として溶媒にＰＣおよびＭＥＣの混合溶媒を用いて電解質として同じ電解質を同量用いた電解液を用意した。
【００３０】
幅１．５ｃｍ、長さ３０ｃｍ、厚さ０．０４ｍｍの短冊状のセパレータ用マニラ紙を、電解液試料の入ったビーカーに１分以上浸した。マニラ紙から滴り落ちる過剰の試料をビーカー壁で拭った後、マニラ紙を２．５ｃｍ間隔で支持針を有するサンプル台の支持針に刺して水平に固定した。マニラ紙とサンプル台を２５ｃｍ×２５ｃｍ×５０ｃｍの金属製の箱に入れ、一端にライターで着火し、燃えた長さ（燃焼長）および最初の針から最後の針までの３０ｃｍの間にマニラ紙が燃えるのに要した時間を各３回測定した。燃焼長と、燃焼に要した時間から算出した燃焼速度とを表１に示した。
【００３１】
【表１】
尚、表中燃焼速度「０」は燃焼しなかったことを示す。
また電解液溶媒としてＰＣ、ＭＥＣ及びＴＦＥＰＡをそれぞれ４０／５０／１０（体積比）で含む混合溶媒を用い、電解質濃度を１．０モル／リットルとし、電解質を種々に変えた場合について上述と同様にセパレータ紙（マニラ紙）の燃えた長さを測定した。その結果を表２に示した。
【００３３】
【表２】
表２からも明らかなように、電解質としてＬｉＰＦ6を用いた場合に特に優れた自己消火性を示した。
３．電解液の耐電圧及び電気伝導度の測定
ＰＣ、ＭＥＣ及びＴＦＥＰＡの混合組成を表３に示すように変えた混合溶媒を用いて、電解質としてＬｉＰＦ6３．８ｇ（２５ｍモル）を各混合溶媒に溶かし、２５ｍｌの電解液（電解質濃度１．０モル／リットル）を調整した。これら電解液の電気伝導度及び耐電圧を測定した。電気伝導度はインピーダンスメータを用い、１０ｋＨｚで測定した。また電解液の耐電圧の測定は、作用極にグラッシーカーボン、対極に白金、参照極にリチウム金属を使用した三極式耐電圧測定セルに上記電解液を入れ、ポテンシオガルバノスタットで１０ｍＶ／ｓｃｃで電位走引し、リチウム金属の電位を基準として酸化分解電流が０．１ｍＡ以上流れなかった範囲を耐電圧とした。結果を表３に示した。
【００３５】
【表３】
表３からも明らかなように、本発明の電解液は高い耐電圧と実用レベルの電気伝導性を示した。
４．電池の充放電効率及びサイクル特性の評価
図１に示すような電池寸法が外径２０ｍｍ、高さ２．５ｍｍのコイン形非水電解液電池を作成した。負極１にはリチウム金属を、正極２にはＬｉＣｏＯ2８５重量部に導電剤としてグラファイト１２重量部、結合剤としてフッ素樹脂３重量部を加えた混合物を加圧成形したものを用いた。これら負極１、正極２を構成する物質は、ポリプロピレンから成る多孔質セパレータ３を介してそれぞれ負極缶４及び正極缶５に圧着されている。このような電池の電解液として、ＰＣ、ＭＥＣ及びＴＦＥＰＡを体積比で４５：４５：１０の割合で混合した溶媒に六フッ化リン酸リチウムを１．０モル／ｌの割合で溶解させたものを用い、封口ガスケット６により封入した。
【００３７】
このように作成した電池（実施例１）について、１．０ｍＡの電流で上限電圧を４．２Ｖとして１０時間充電し、続いて１．０ｍＡの電流で３．０Ｖとなるまで放電した時の充放電効率を測定した。また、このような充放電を所定サイクル繰返し、充放電効率の変化を観察した。図２はその結果を示すもので、充放電効率をサイクル数に対してプロットしたものである。
【００３８】
また実施例２として、電解液溶媒にＰＣ、ＭＥＣ及びＴＣＥＰＡを体積比で４５：４５：１０の混合溶媒を用い、比較例として電解液溶媒にＰＣ、ＭＥＣ及びＴＥＰＡの混合溶媒（体積比で４５：４５：１０）を用い、その他は上述と同様に作成したコイン形電池について、同様の充放電効率を測定した。
図２からも明らかなように、本実施例の電解液溶媒を用いた電池は、優れたサイクル特性を示した。
【００３９】
【発明の効果】
以上の実施例からも明らかなように、本発明によれば電解液溶媒として特定のハロゲン置換リン酸エステル化合物を含む有機溶媒を用いることにより、リチウム金属との反応性が低く自己消火作用を示し、実用レベルの電導度を持つ非水電解液を提供することができる。特に電解液溶媒として特定のハロゲン置換リン酸エステル化合物と特定のエステル化合物との混合溶媒とすることにより、低粘度で低温特性の優れた電解液を得ることができる。また本発明によれば、このような非水電解液を電解液として用いることにより、高電圧を発生することができ、充放電性能等の電池性能が優れた非水二次電池を提供することができる。
【図面の簡単な説明】
【図１】 本発明の非水電解液電池の１実施例を示す概略断面図。
【図２】 本発明の非水電解液を用いた電池の充放電サイクル特性を示す図。
【符号の説明】
１・・・・・・負極
２・・・・・・正極[0001]
[Industrial application fields]
The present invention relates to a novel non-aqueous electrolyte and a non-aqueous electrolyte battery using the same.
[0002]
[Prior art]
A battery using a non-aqueous electrolyte has a high voltage, a high energy density, and is excellent in reliability such as storability, and thus is widely used as a power source for consumer electronic devices. As a non-aqueous electrolyte, LiBF 4 is generally mixed with a solvent in which propylene carbonate, γ-butyrolactone, sulfolane or the like having a high dielectric constant is mixed with dimethoxyethane, tetrahydrofuran, 1,3-dioxolane or the like having a low viscosity. A mixture of electrolytes such as LiPF 6 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiN (CF 3 SO 2 ) 2 , LiAlCl 4 , LiSiF 6 is used.
[0003]
However, many non-aqueous electrolyte solvents have a low withstand voltage, and when an electrolyte using a low withstand voltage solvent is used in a secondary battery, the solvent is electrolyzed by repeated charge and discharge, As a result, the internal pressure of the battery increases due to the generated gas, or the product causes a polymerization reaction and adheres to the electrode. For this reason, the charge / discharge efficiency of the battery is lowered, and there is a problem that the life of the battery is shortened due to a decrease in battery energy density. As an attempt to improve the durability of the electrolytic solution, conventionally used solvents such as γ-butyrolactone and esters such as ethyl acetate and ethers such as 1,3-dioxolane, tetrahydrofuran and dimethoxyethane have a low withstand voltage. Instead of this, it has been proposed to use a carbonate such as diethyl carbonate having a high withstand voltage to suppress a decrease in battery energy density after repeated charge and discharge (for example, JP-A-2-10666).
[0004]
On the other hand, metallic lithium or a lithium alloy is used for the negative electrode of the lithium secondary battery. However, when charging and discharging are repeated, lithium ions in the electrolyte are unevenly deposited on the negative electrode, resulting in a needle-like reaction called dendrite. In some cases, high-quality metals were produced. When the dendrite falls off from the electrode, problems such as self-consumption and shortening the cycle life of the battery, and the possibility of the dendrite penetrating through the separator separating the positive electrode and the negative electrode are considered.
[0005]
[Problems to be Solved by the Invention]
By the way, since a battery with a high energy density is desired, research on a high voltage battery is being conducted from various directions. For example, a secondary battery called a rocking chair type using a composite oxide of lithium and a transition metal such as LiCoO 2 , LiNiO 2 , or Li 2 Mn 2 O 4 for the positive electrode of the battery and a carbon material for the negative electrode is known. Have been studied. In this case, the battery voltage can generate 4V, and since there is no deposition of lithium metal, safety is confirmed by experiments such as overcharge, external short, needle stick, crush, etc., and it comes out for consumer use. ing. However, when the energy density is greatly increased and the size is increased in the future, further improvement in safety such as flame retardancy is required. Currently used electrolyte solvents do not necessarily have a satisfactory high flash point and are not self-extinguishing.
[0006]
For this reason, it has been proposed to add phosphate esters known as self-extinguishing compounds to the electrolyte (Japanese Patent Laid-Open No. 4-184870). However, an electrolyte solution containing 15% by weight or more of this type of compound has clear flame retardancy, but conversely has problems in terms of battery charge / discharge efficiency, battery energy density, and battery life. Furthermore, phosphate esters such as trimethyl phosphate and triethyl phosphate are not suitable for lithium secondary batteries using metal lithium or a lithium alloy for the negative electrode because of their reactivity with metal lithium. Even in the above-described rocking chair type lithium ion secondary battery, metallic lithium may be deposited under severe test conditions such as extreme overcharge due to misuse.
[0007]
The present invention was made in view of the above-mentioned problems, does not cause a decrease in charge and discharge efficiency and battery energy density after repeated charge and discharge, has self-extinguishing properties, has a high flash point, And it aims at providing the non-aqueous electrolyte with low reactivity with metallic lithium. Furthermore, an object of the present invention is to provide a non-aqueous electrolyte that is excellent in withstand voltage and electrical conductivity, and excellent in load characteristics and low temperature characteristics. A further object of the present invention is to provide a non-aqueous electrolyte battery that can generate a high voltage, has excellent battery performance, and has a long life.
[0008]
[Means for Solving the Problems]
In order to produce an electrolyte for non-aqueous batteries that can generate a high voltage and have excellent battery characteristics, the present inventors have conducted extensive research on phosphate ester compounds having a high self-extinguishing action. As a result, it has been found that the reactivity with lithium metal can be lowered by substituting at least one of the substituents of the phosphate ester with a halogen atom such as a fluorine atom. It was found that an electrolyte solution for a non-aqueous battery with excellent battery performance was obtained.
[0009]
That is, the non-aqueous electrolyte of the present invention is a non-aqueous electrolyte used for a battery having a negative electrode containing a material selected from metallic lithium, a lithium alloy, and a carbon material capable of doping and dedoping lithium ions as a negative electrode active material. The electrolyte solution contains LiPF 6 , and the electrolyte solvent contains a phosphate ester compound represented by the general formula [1] in an amount of 7% by volume to 20% by volume of the total electrolyte solvent, and is a chain described later. Containing a cyclic ester compound and a cyclic ester compound .
[0010]
[Chemical 3]
(R 1 , R 2 and R 3 may be the same or different and each represents an alkyl group or a halogen atom-substituted alkyl group, and at least one of R 1 , R 2 and R 3 represents a halogen atom-substituted alkyl group. .)
The non-aqueous electrolyte battery of the present invention uses an electrolytic solution containing the above phosphoric ester compound.
[0012]
The phosphoric ester compound of the general formula [1] has self-extinguishing properties, and when added to the electrolytic solution, imparts self-extinguishing properties to the electrolytic solution, and at least one of R 1 , R 2 , and R 3 is halogenated. Substitution with atoms can reduce the reactivity of the non-aqueous electrolyte of the present invention with lithium metal. If at least one of R 1 , R 2 and R 3 is a halogen atom-substituted alkyl group, the rest may be an alkyl group or a halogen atom-substituted alkyl group.
[0013]
Here, the self-extinguishing action of the phosphate ester increases as the phosphorus content in the phosphate ester increases, that is, as the molecular weight of the substituents R 1 , R 2 and R 3 decreases, and as the amount added increases. However, it is not preferable to increase the addition amount of a phosphoric ester having a large molecular weight because the increase in the viscosity of the electrolytic solution increases due to the addition and the electrical conductivity decreases. Therefore, R 1 , R 2 and R 3 preferably have as few carbon atoms as possible. When R 1 , R 2 and R 3 are alkyl groups, preferably 1 to 4 carbon atoms and halogen atom-substituted alkyl groups. The number is preferably 2-4.
[0014]
Examples of the alkyl group include a methyl group, an ethyl group, a normal propyl group, an isopropyl group, a normal butyl group, a sec-butyl group, a t-butyl group, and an isobutyl group. In addition, examples of the halogen atom-substituted alkyl group include a fluorine atom-substituted alkyl group, a chlorine atom-substituted alkyl group, and a bromine atom-substituted alkyl group, and those in which fluorine, chlorine, or bromine are mixed in one substituent. Good. Examples of fluorine atom-substituted alkyl groups include trifluoroethyl group, difluoroethyl group, monofluoroethyl group, pentafluoropropyl group, 2,2,3,3-tetrafluoropropyl group, 1,1,1. -Trifluoroisopropyl group, 1,3-difluoro-2-propyl group, hexafluoroisopropyl group, 2,2,3,3,4,4,4-heptafluorobutyl group, 2,2,3,4,4 , 4-hexafluorobutyl group, hexafluoro-2-methylisopropyl group, 3,3,4,4,4-pentafluoro-2-butyl group, 4,4,4-trifluorobutyl group, perfluoro-t -A butyl group etc. are mentioned. In addition, those having the same structure as described above and substituted with chlorine or bromine instead of fluorine are also exemplified.
[0015]
Examples of the phosphate ester compound of the present invention include tri (trifluoroethyl) phosphate, methyl phosphate (ditrifluoroethyl), dimethyl phosphate (trifluoroethyl), ethyl phosphate (ditrifluoroethyl), diethyl phosphate ( Trifluoroethyl), propyl phosphate (ditrifluoroethyl), dipropyl phosphate (trifluoroethyl), tri (pentafluoropropyl) phosphate, methyl phosphate (dipentafluoropropyl), dimethyl phosphate (pentafluoropropyl) , Ethyl phosphate (dipentafluoropropyl), diethyl phosphate (pentafluoropropyl), butyl phosphate (dipentafluoropropyl), dibutyl phosphate (pentafluoropropyl), and other halogen atom-substituted alkyl groups and alkyl groups described above And those with It is.
[0016]
The halogen-substituted phosphate ester compound of the formula [1] has the effect of making the solvent difficult to burn as described above, and the reactivity with lithium metal is greatly suppressed as compared with ordinary phosphate ester compounds such as trimethyl phosphate. However, when used alone as a solvent, it has the property of lowering the conductivity of the electrolyte and lowering the energy density of the battery. Therefore, in order to use the nonaqueous electrolytic solution of the present invention as a practical secondary battery electrolytic solution, a mixed solvent of the halogen-substituted phosphate compound and another solvent is used as a solvent. In this case, the addition amount of the halogen-substituted phosphate compound may be 7 to 20% by volume, preferably 7 to 15% by volume, based on the entire solvent . By setting it as such a range, a sufficient self-digesting action can be obtained without affecting the battery performance, such as a decrease in conductivity and a decrease in battery energy density.
[0017]
As the solvent to which the halogen atom-substituted phosphate compound is added, conventionally used chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, amides such as dimethylformamide, methyl-N, Carbamates such as N-dimethylcarbamate, chain esters such as diethyl carbonate, and cyclic esters such as propylene carbonate can be used singly or in combination. Particularly when used as a high-voltage battery, the halogen atom-substituted phosphate ester can increase the conductivity of the electrolyte by using a mixed solvent with a chain ester and / or a cyclic ester, and the battery performance is good. Can be.
[0018]
The chain ester used here is one kind represented by the general formula [2] or a mixture thereof.
[0019]
[Formula 4]
(In the formula, R 4 represents a methyl group, an ethyl group, a propyl group, a methoxy group or an ethoxy group, and R 5 represents a chain or branched alkyl group having 1 to 3 carbon atoms.) General formula [2 ] Is mixed with a halogen atom-substituted phosphate ester, the viscosity of the electrolytic solution can be lowered, and an electrolytic solution excellent in electrical conductivity from room temperature to low temperature can be obtained. Examples of such chain esters include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, methyl formate, ethyl formate, propyl formate, methyl butyrate, and ethyl butyrate. , Methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and the like. In particular, in the case of a battery electrolyte using LiCoO 2 , LiMnO 2 , LiMn 2 O 4 , LiNiO 2 or the like that can generate 4 V as a positive electrode of a battery, dimethyl carbonate, diethyl carbonate, methyl ethyl are used because of oxidation resistance stability. Carbonate is preferred.
[0021]
The concentration of the chain ester in the electrolyte solvent can be usually used in the range of 20 to 90% by volume, and preferably in the range of 40 to 80% by volume.
As the cyclic ester, one kind selected from propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone or sulfolane or a mixture thereof is used, and propylene carbonate and ethylene carbonate are preferably selected. These cyclic esters can be used in a concentration of usually 10 to 70% by volume in the electrolyte solvent, preferably 20 to 60% by volume. By using the cyclic ester mixed with the halogen-substituted phosphate ester, the dissociation property of the electrolyte can be increased, and the electrical conductivity can be increased.
[0022]
Further, it is more preferable to use both the chain ester and the cyclic ester mixed with a halogen atom-substituted phosphate ester because the effect of decreasing the viscosity and the effect of dissociating the electrolyte are synergistic.
In the case of a battery using V 2 O 5 , polyaniline or the like that can generate a voltage of about 3 V as the positive electrode of the battery, the withstand voltage is lower than that in combination with or in combination with these chain esters and cyclic esters. Chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, amides such as dimethylformamide, carbamates such as methyl-N, N-dimethylcarbamate, carbamates and amides such as N-methyloxazolidone and N-methylpyrrolidone Can also be used.
[0023]
As an electrolyte used for the electrolytic solution of the present invention, a lithium salt such as LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiN (CF 3 SO 2 ) 2 , LiAlCl 4 , LiSiF 6 is used. LiPF 6 is particularly preferable. When LiPF 6 is used as the electrolyte, high self-extinguishing properties can be maintained even if the content of the phosphate ester compound of the present invention is lowered. Therefore, it is possible to prevent the charge / discharge efficiency and energy density of the battery from decreasing.
[0024]
The electrolyte concentration in the electrolytic solution can be generally used in a concentration range of 0.1 to 3 mol / liter, and preferably in a concentration range of 0.5 to 2 mol / liter.
The non-aqueous electrolyte battery of the present invention uses an electrolyte solution having the above composition, and is a battery comprising at least a positive electrode, a negative electrode, and a separator.
[0025]
As the negative electrode material, metallic lithium, a lithium alloy, or a carbon material that can be doped / undoped with lithium ions can be used, and it is particularly preferable to use a carbon material that can be doped / undoped with lithium ions. Such a carbon material may be graphite or amorphous carbon, and any carbon material such as activated carbon, carbon fiber, carbon black, and mesocarbon microbeads can be used.
[0026]
Positive electrode materials include reversible electrolytic polymerization such as transition metal oxides such as MoS 2 , TiS 2 , MnO 2 and V 2 O 5 , conductive polymers such as transition metal sulfides, polyaniline and polypyrrole, and disulfide compounds. , A compound to be depolymerized or a composite oxide composed of lithium and a transition metal such as LiCoO 2 , LiMnO 2 , LiMn 2 O 4 , LiNiO 2 can be used, and preferably a composite oxide composed of lithium and a transition metal is used. .
[0027]
The non-aqueous electrolyte battery of the present invention includes the non-aqueous electrolyte described above as an electrolyte, so that a high voltage can be generated, the battery charge / discharge efficiency is high, and the energy density of the battery is reduced even after repeated charge / discharge. No non-aqueous secondary battery with excellent practicality can be obtained.
The shape, form, etc. of the nonaqueous electrolyte battery of the present invention are not particularly limited, and can be arbitrarily selected within the scope of the present invention, such as a cylindrical shape, a square shape, a coin shape, a card shape, and a large size. it can.
[0028]
【Example】
EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.
1. Evaluation of Reactivity of Phosphate Ester Compound with Metallic Lithium Phosphate Ester Compound Dehydrated and Distilled and Purified with 2cm Length, 0.5cm Width, and 0.5mm Thickness in Argon At a Dew Point of -60 ° C 10 ml was mixed in a glass container, and the lithium surface was shaved in a phosphate ester so that a clean surface of the metal appeared. The glass container was heated to various temperatures, and the reaction between the lithium metal and the phosphate ester compound was observed. Examples of the phosphoric acid ester compound include tri (trifluoroethyl) phosphate (hereinafter abbreviated as TFEPA) and tris (2-chloroethyl) phosphate (hereinafter abbreviated as TCEPA), which are fluorine atom-substituted phosphoric acid ester compounds. As comparative examples, trimethyl phosphate and triethyl phosphate (hereinafter abbreviated as TEPA) were used.
[0029]
As a result, triethyl phosphate reacted by foaming vigorously at 165 ° C., and lithium was melted by the heat of reaction. Trimethyl phosphate also reacted by foaming at 180 ° C., whereas TFEPA reacted with metallic lithium at 180 ° C. However, even after melting, only a gray film was formed on the surface, and the reaction did not continue.
2. Evaluation of self-extinguishing properties of electrolyte Propylene carbonate (hereinafter abbreviated as PC), methyl ethyl carbonate (hereinafter abbreviated as MEC) and phosphate ester (TFEPA, TCEPA) as a solvent for the electrolyte solution are mixed at a predetermined ratio. An electrolyte solution in which 1.0 mol / liter of lithium hexafluorophosphate (LiPF 6 ) was dissolved as an electrolyte was prepared. In addition, as a comparative example, an electrolytic solution using a mixed solvent of PC and MEC as a solvent and the same amount of the same electrolyte as an electrolyte was prepared.
[0030]
Strip-shaped Manila paper for separators having a width of 1.5 cm, a length of 30 cm, and a thickness of 0.04 mm was immersed in a beaker containing an electrolyte sample for 1 minute or more. After the excess sample dripping from the Manila paper was wiped with a beaker wall, the Manila paper was fixed horizontally by piercing the support needles of the sample stage having support needles at intervals of 2.5 cm. Place manila paper and sample table in a metal box of 25cm x 25cm x 50cm, ignite with lighter at one end, burnt length (burning length) and manila paper between 30cm from first needle to last needle The time required for burning was measured 3 times each. Table 1 shows the combustion length and the combustion speed calculated from the time required for combustion.
[0031]
[Table 1]
In the table, the burning rate “0” indicates that no combustion occurred.
In addition, the same applies to the case where a mixed solvent containing PC, MEC and TFEPA at 40/50/10 (volume ratio) is used as the electrolyte solvent, the electrolyte concentration is 1.0 mol / liter, and the electrolyte is variously changed. The burned length of separator paper (Manila paper) was measured. The results are shown in Table 2.
[0033]
[Table 2]
As is clear from Table 2, particularly excellent self-extinguishing properties were exhibited when LiPF 6 was used as the electrolyte.
3. Measurement of the withstand voltage and electric conductivity of the electrolytic solution Using a mixed solvent in which the mixed composition of PC, MEC and TFEPA was changed as shown in Table 3, 3.8 g (25 mmol) of LiPF 6 as an electrolyte was added to each mixed solvent. After dissolution, 25 ml of electrolyte (electrolyte concentration: 1.0 mol / liter) was prepared. The electric conductivity and withstand voltage of these electrolytes were measured. The electrical conductivity was measured at 10 kHz using an impedance meter. In addition, the withstand voltage of the electrolyte is measured by placing the electrolyte in a triode withstand voltage measuring cell using glassy carbon as a working electrode, platinum as a counter electrode, and lithium metal as a reference electrode, and 10 mV / scc with a potentio galvanostat. The potential withstand voltage was defined as the range in which the oxidative decomposition current did not flow more than 0.1 mA with reference to the potential of the lithium metal. The results are shown in Table 3.
[0035]
[Table 3]
As is apparent from Table 3, the electrolytic solution of the present invention exhibited a high withstand voltage and a practical level of electrical conductivity.
4). Evaluation of Battery Charging / Discharging Efficiency and Cycle Characteristics A coin-type non-aqueous electrolyte battery having an outer diameter of 20 mm and a height of 2.5 mm as shown in FIG. 1 was prepared. Lithium metal was used for the negative electrode 1, and the
[0037]
The battery thus prepared (Example 1) was charged at a current of 1.0 mA with an upper limit voltage of 4.2 V for 10 hours, and then charged at a current of 1.0 mA until 3.0 V was discharged. The discharge efficiency was measured. Moreover, such charging / discharging was repeated a predetermined cycle, and the change of charging / discharging efficiency was observed. FIG. 2 shows the result, and the charge / discharge efficiency is plotted against the number of cycles.
[0038]
In Example 2, a mixed solvent of PC, MEC, and TCEPA in a volume ratio of 45:45:10 was used as the electrolyte solvent, and as a comparative example, a mixed solvent of PC, MEC, and TEPA (in a volume ratio of 45 : 45: 10), and the other charge-discharge efficiency was measured with respect to a coin-shaped battery prepared in the same manner as described above.
As is clear from FIG. 2, the battery using the electrolyte solvent of this example showed excellent cycle characteristics.
[0039]
【The invention's effect】
As is clear from the above examples, according to the present invention, by using an organic solvent containing a specific halogen-substituted phosphate compound as an electrolyte solvent, the reactivity with lithium metal is low and self-extinguishing action is exhibited. It is possible to provide a nonaqueous electrolytic solution having a practical level of conductivity. In particular, by using a mixed solvent of a specific halogen-substituted phosphate compound and a specific ester compound as an electrolyte solution solvent, an electrolyte solution having low viscosity and excellent low-temperature characteristics can be obtained. In addition, according to the present invention, by using such a non-aqueous electrolyte as an electrolyte, a high voltage can be generated and a non-aqueous secondary battery excellent in battery performance such as charge / discharge performance is provided. Can do.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing one embodiment of a non-aqueous electrolyte battery according to the present invention.
FIG. 2 is a graph showing charge / discharge cycle characteristics of a battery using the nonaqueous electrolytic solution of the present invention.
[Explanation of symbols]
1 ....
Claims (6)
且つ前記電解液溶媒として、一般式［２］で表される鎖状エステル化合物及び環状エステル化合物を含有することを特徴とする非水電解液。
A non-aqueous electrolytic solution comprising a chain ester compound and a cyclic ester compound represented by the general formula [2] as the electrolytic solution solvent.
Priority Applications (7)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP22166394A JP3821495B2 (en) | 1994-09-16 | 1994-09-16 | Non-aqueous electrolyte and non-aqueous electrolyte battery |
| EP97119010A EP0825664B1 (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolytic solutions and non-aqueous electrolyte cells comprising the same |
| EP95304775A EP0696077B1 (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolyte solutions and secondary cells comprising the same |
| DE69531901T DE69531901T2 (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolyte solutions and cells containing these electrolyte solutions |
| CA002153478A CA2153478C (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolytic solutions and non-aqueous electrolyte cells comprising the same |
| US08/499,393 US5580684A (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolytic solutions and non-aqueous electrolyte cells comprising the same |
| DE69508671T DE69508671T2 (en) | 1994-07-07 | 1995-07-07 | Non-aqueous electrolyte solutions and secondary cells containing them |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP22166394A JP3821495B2 (en) | 1994-09-16 | 1994-09-16 | Non-aqueous electrolyte and non-aqueous electrolyte battery |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH0888023A JPH0888023A (en) | 1996-04-02 |
| JP3821495B2 true JP3821495B2 (en) | 2006-09-13 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP22166394A Expired - Lifetime JP3821495B2 (en) | 1994-07-07 | 1994-09-16 | Non-aqueous electrolyte and non-aqueous electrolyte battery |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP3821495B2 (en) |
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