JP3639376B2

JP3639376B2 - Organic electrolyte secondary battery

Info

Publication number: JP3639376B2
Application number: JP05866396A
Authority: JP
Inventors: 雅治東口; 房次喜多; 章川上
Original assignee: Hitachi Maxell Energy Ltd
Current assignee: Maxell Ltd
Priority date: 1996-03-15
Filing date: 1996-03-15
Publication date: 2005-04-20
Anticipated expiration: 2016-03-15
Also published as: JPH09251862A

Description

【０００１】
【発明の属する技術分野】
この発明は、安全性および貯蔵性の改良された、電池性能にすぐれる有機電解液二次電池に関するものである。
【０００２】
【従来の技術】
有機電解液二次電池は、電解液の溶媒に有機溶媒を用いた二次電池であつて、この種の二次電池は、容量が大きく、かつ高電圧、高エネルギ―密度であることから、その需要がますます増える傾向にある。
【０００３】
このような有機電解液二次電池では、有機溶媒にリチウム塩を溶解させた有機溶媒系の電解液を用い、負極活物質としてリチウムまたはリチウム合金を用いているが、それらの負極活物質による場合、電池特性の低下を引き起こしたり、内部短絡を起こしやすいという、安全性に問題があつた。
【０００４】
このため、特公平４−２４８３１号公報、特公平５−１７６６９号公報などには、上記のリチウムまたはリチウム合金に代えて、活性炭や黒鉛などの炭素材料を負極活物質として用いることが、提案されている。
【０００５】
【発明が解決しようとする課題】
しかるに、上記のような炭素材料では、リテンション（充電容量と放電容量の差）が大きいため、その特性を十分満足できるものとはいえなかつた。とくに、最近の高容量を必要とする二次電池においては、一定単位体積あたりの電池容量が３００Ｗｈ／リツトル以上の高容量であることが望まれており、このような電池では、電極の活物質を最大限に利用することが要求されるため、リテンシヨンのより小さい二次電池が必要とされている。
【０００６】
また、従来の有機電解液二次電池では、電解液の溶媒にプロピレンカ―ボネ―トや１，２−ジメトキシエタンなどを用いてきたが、これらは火災などに対する安全性面で問題があつた。そこで、不燃性か、または難燃性（引火点が１００℃以上）の安全な有機溶媒として、リン酸トリアルキルなどのリン酸トリエステルが注目されている。しかし、リン酸トリエステルを用いた電池では、二酸化マンガンなどの活性な正極活物質に対し、リチウムないしその合金を負極として用いたとき、負極リチウムと溶媒との反応が起こつて、内部抵抗が著しく増大し、電池性能が劣化してくるという、貯蔵性の問題があつた。
【０００７】
この発明は、このような事情に照らし、リテンシヨン（充電容量と放電容量の差）が小さく、高容量であるとともに、安全性および貯蔵性にすぐれた有機電解液二次電池を提供することを目的としている。
【０００８】
【課題を解決するための手段】
この発明者らは、上記の目的を達成するため、鋭意検討した結果、有機電解液の溶媒としてリン酸トリエステルおよび含フツ素系溶媒を用いる一方、負極に、リチウムまたはその合金に代えて、特定の結晶性を有する炭素材料を用いることにより、リテンシヨンが小さく、高容量であり、しかも安全性および貯蔵性にすぐれた有機電解液二次電池が得られること、またその際に電解液中に二酸化炭素を溶解させると、リテンシヨン、放電容量などの電池性能にさらにすぐれた結果が得られることを知り、この発明を完成するに至つた。
【０００９】
すなわち、この発明は、正極、負極および有機電解液を有する有機電解液二次電池において、負極がＸ線回折により測定されるｃ軸方向の結晶子の大きさ〔Ｌｃ〕が２００Å以上の炭素材料からなり、かつ有機電解液の溶媒としてリン酸トリエステルと含フツ素系溶媒を用いたことを特徴とする有機電解液二次電池に係るものであり、とくに、この有機電解液二次電池において上記の有機電解液中に二酸化炭素を溶解させたことを好適な態様としている。
【００１０】
【発明の実施の形態】
この発明において、負極には、炭素材料を構成要素として、これに結着剤などを適宜加えたものを合剤とし、この合剤を銅箔などの集電材料を芯材として成形体に仕上げたものが用いられる。ここで、炭素材料としては、たとえば、熱分解炭素類、コ―クス類、ガラス状炭素類、有機高分子の焼成体、メソカ―ボンマイクロビ―ズ、炭素繊維、活性炭などを用いることができる。
【００１１】
この発明において、このような炭素材料は、Ｘ線回折により測定されるｃ軸方向の結晶子の大きさ〔Ｌｃ〕が２００Å以上、好適には１，０００Å以上（通常、２，０００Å以下）であり、純度が９９．９％以上であることが望ましい。このように結晶性の高い炭素材料によると、リチウムをド―プ、脱ド―プできる格子が多数存在するため、高容量の電池が得られる。この炭素材料の平均粒径は、９μｍ未満では比表面積が大きすぎて安全性に劣るため、９μｍ以上が好ましい。また、Ｘ線回折により測定される（００２）面の面間距離〔ｄ₀₀₂〕が３．３５〜３．５０Å、好ましくは３．３６〜３．３８Åの黒鉛構造を有しているのが電池の高容量化を達成するうえで好ましい。
【００１２】
この発明において、正極には、二酸化マンガン、五酸化バナジウム、クロム酸化物、ＬｉＮｉＯ₂などのリチウムニツケル酸化物、ＬｉＣｏＯ₂などのリチウムコバルト酸化物、ＬｉＭｎ₂Ｏ₄などのリチウムマンガン酸化物などの金属酸化物、または二硫化チタン、二硫化モリブデンなどの金属硫化物、あるいはこれらの正極活物質に導電助剤やポリテトラフルオロエチレンなどの結着剤などを適宜添加した合剤を、ステンレス鋼製網などの集電材料を芯材として、成形体に仕上げたものが用いられる。
【００１３】
上記の負極炭素材料と正極材料に対し、有機溶媒に電解質を溶解した有機電解液を用いることにより、電池性能に好結果が得られるが、その際、通常の有機溶媒を用いると、その可燃性から、電池の安全性に問題がある。この解決のため、この発明では、有機溶媒の一成分として、一般式：（ＲＯ）₃Ｐ＝Ｏ（ただし、Ｒは有機基で、３個の有機基は同一であつても異なつていてもよい）で表わされる、不燃性かまたは難燃性であるリン酸トリエステル、好ましくはＲが炭素数１〜６のアルキル基であるリン酸トリアルキル、たとえば、リン酸トリメチル、リン酸トリメチル、リン酸トリブチルなどを使用する。
【００１４】
このようなリン酸トリエステルを用いた有機電解液二次電池では、１００℃に加熱しても発火しない。しかし、結晶性の高い前記の炭素材料に対し、上記のリン酸トリエステルを有機溶媒としてこれ単独で用いると、電池のリテンシヨンが大きくなり、また電池容量が低下する現象がみられる。そこで、この発明では、上記のリン酸トリエステルとともに、含フツ素系溶媒、たとえば、トリフルオロプロピレンカ―ボネ―トなどを併用して、リテンシヨンおよび電池容量の改善を図つたものである。上記改善を図れる理由は、必ずしも明らかではないが、両溶媒を含む電解液を用いた電池を充放電すると、含フツ素系溶媒が負極炭素材料表面で分解して炭素材料表面に皮膜を形成し、リン酸トリエステルの炭素材料表面での分解を抑制するためではないかと考えられる。
【００１５】
上記のような皮膜の形成は、１２ｋＶ−１０ｍＡの測定条件でのＸＰＳ分析の光電子スペクトルのフツ素の６９０ｅＶ付近のピ―ク強度〔Ｉｆ〕とリンの１３７ｅＶ付近のピ―ク強度〔Ｉｐ〕との比〔Ｉｆ／Ｉｐ〕を測定することにより、確認できる。ＸＰＳ分析によるフツ素のピ―クは炭素材料表面に形成するＣ−Ｆなどの結合によるものであり、リンのピ―クは炭素材料表面のＣ−Ｐなどの結合によるものであると考えられる。本発明者らの検討では、上記〔Ｉｆ／Ｉｐ〕の比が５〜５０の範囲になるように、リン酸トリエステルと含フツ素系溶媒を併用すると、リン酸トリエステルの炭素材料表面の分解が抑制され、リテンシヨンが小さく高容量の電池が得られることが確認されている。
【００１６】
この発明において、リン酸トリエステルと含フツ素系溶媒の配合比は、とくに限定されないが、電池の安全性を考慮すれば、より難燃性であるリン酸トリエステルを含フツ素系溶媒よりも多くしたほうが好ましく、リン酸トリエステルと含フツ素系溶媒との体積比で１：１〜５：１の範囲にするのが好ましい。放電性能などを考えると、リン酸トリエステルによる火災に対する安全性が確保できる範囲内で、含フツ素系溶媒をさらに多く使用することもできる。
【００１７】
リン酸トリエステルと含フツ素系溶媒は、有機溶媒の全部を占めてもよいが、場合により、これらの溶媒が有機溶媒の大部分を占め、一部に他の有機溶媒を含んでいてもよい。後者の場合、リン酸トリエステルと含フツ素系溶媒の合計は、有機溶媒全体の４０体積％以上、好ましくは６０体積％以上、より好ましくは９０体積％以上であるのがよい。リン酸トリエステルと含フツ素系溶媒の占める割合が多いと、不燃性ないし難燃性であるという特性に加えて、リン酸トリエステルの分解抑制効果が十分に発揮され、火災に対する安全性が向上するとともに、リテンシヨンをより改善することができる。
【００１８】
他の溶媒としては、誘電率３０以上、とくに５０以上のエステルが好ましい。このような溶媒を用いると、電解質の溶解性の高い難燃性電解液が得られ、また負極の炭素材料表面と電解液との反応活性点がより低減される。すなわち、リン酸トリエステルなどと上記誘電率の高いエステルの併用は、電解質の溶解性が向上し、伝導度も高くなり、容量が著しく向上するため、望ましい。
【００１９】
このような誘電率の高いエステルとしては、炭素数が２〜１０、好ましくは２〜６のアルキレンカ―ボネ―トが用いられる。たとえば、エチレンカ―ボネ―ト、プロピレンカ―ボネ―ト、ブチレンカ―ボネ―ト、γ−ブチロラクトン、エチレングリコ―ルサルファイトなどが挙げられる。この中でも、とくに環状構造のものが好ましく、とりわけ環状のカ―ボネ―トが好ましい。最も好ましいエステルは、エチレンカ―ボネ―トであり、その誘電率は９５である。
【００２０】
これらの誘電率の高いエステルは、可燃性であるので、リン酸トリエステルおよび含フツ素系溶媒との併用にあたり、安全性の面から少ないほうが好ましい。一般には、上記誘電率の高いエステルは、有機溶媒全体の１０体積％以下が好ましく、より好ましくは５体積％以下、さらに好ましくは３体積％以下である。これら誘電率の高いエステルによるリンテンシヨンの向上効果は、上記エステルが有機溶媒全体の１体積％以上になると現れるようになり、２体積％に達すると著しい向上がみられるようになる。
【００２１】
このように、誘電率の高いエステルとリン酸トリエステルおよび含フツ素系溶媒との併用にあたり、誘電率の高いエステルは、有機溶媒全体の１〜１０体積％、とくに２〜５体積％、とりわけ２〜３体積％であるのが好ましい。また、リン酸トリエステルと誘電率の高いエステルの沸点の差は、１５０℃以下であるのが好ましく、より好ましくは１００℃以下、さらに好ましくは５０℃以下、最も好ましくは１０℃以下である。これは、可燃性のエステルと難燃性のリン酸トリエステルとが共沸したほうが、エステルが引火しにくくなるためである。
【００２２】
このような誘電率の高いエステルのほかに、１，２−ジメトキシエタン、１，３−ジオキソラン、テトラヒドロフラン、２−メチル−テトラヒドロフラン、ジエチルエ―テルなども併用できる。また、アミンイミド系有機溶媒や含イオウ系有機溶媒などを併用してもよい。ただし、これらの溶媒も、電解液の有機溶媒全体の１０体積％以下であるのが望ましい。
【００２３】
この発明に用いる有機電解液において、このような有機溶媒に溶解させる電解質としては、たとえば、ＬｉＣｌＯ₄、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＡｓＦ₆、ＬｉＳｂＦ₆、ＬｉＣＦ₃ＳＯ₃、ＬｉＣＦ₃ＣＯ₂などや、その他、Ｌｉ₂Ｃ₂Ｆ₄（ＳＯ₃）₂、ＬｉＮ（ＣＦ₃ＳＯ₂）₂、ＬｉＣ（ＣＦ₃ＳＯ₂）₃、ＬｉＣ_nＦ_2n+1ＳＯ₃（ｎ≧２）などが、単独でまたは２種以上混合して用いられる。これらの中でも、ＬｉＰＦ₆やＬｉＣ_nＦ_2n+1ＳＯ₃は充放電特性が良好なため、好ましく用いられる。これら電解質の電解液中の濃度としては、とくに限定されるものではないが、通常０．１〜２モル／リツトル、好ましくは０．４〜１モル／リツトル程度であるのがよい。
【００２４】
この発明において、このような有機電解液中に二酸化炭素を溶解させると、リテンシヨンをさらに小さくすることができる。二酸化炭素の溶解により上記効果が得られる理由は、必ずしも明らかではない。電解液中に二酸化炭素を溶解させると、負極材料表面と反応して有機または無機の炭素塩などの薄い緻密な皮膜を形成し、この皮膜によりリチウムイオンド―プ時の電解液の有機溶媒と炭素材料との反応がより効率よく抑制されるためではないかと思われる。
【００２５】
また、二酸化炭素の溶解は、正極活物質としてＬｉＮｉＯ₂、ＬｉＣｏＯ₂、ＬｉＭｎ₂Ｏ₄などの充電時の閉路電圧がＬｉ基準で４Ｖ以上を示すリチウム複合酸化物を用いたときに、有効である。これらの正極活物質は高電圧であり、通常の条件では有機電解液が酸化され放電性能が低下するが、有機電解液中に耐酸化性のすぐれた二酸化炭素を溶解させると、二酸化炭素が正極表面での酸化による電解液の分解を抑制する。とくにＬｉＮｉＯ₂は他の金属酸化物より電解液との反応性面から使用できなかつたが、このようなＬｉＮｉＯ₂の場合でも有機電解液との反応が抑制されて、電池の高容量化が達成される。
【００２６】
有機電解液に溶解させる二酸化炭素の量としては、電池内の有機電解液に対して、０．０３モル／リツトル（有機電解液１リツトルに対して二酸化炭素が０．０３モル）以上が好ましく、より好ましくは０．１モル／リツトル以上、さらに好ましくは０．３モル／リツトル以上である。二酸化炭素の量が多いほど、炭素材料の反応活性をより安定して引き出し、また正極活物質の有機電解液への反応性を抑制するが、多くなりすぎると、有機電解液中から蒸発して、電池の内圧を高めて電池の破裂を引き起こす原因になる。したがつて、電池ケ―スや封口部材の耐圧性を考慮すると、２モル／リツトル以下であるのが望ましい。ここで、電池内に入れられて有機電解液中に溶解していない二酸化炭素も、有機電解液中で二酸化炭素が消費された場合や、低温にした場合には、有機電解液中に溶解していくので、実質的に溶解しているものとみなされる。
【００２７】
二酸化炭素を有機電解液に溶解させる方法としては、たとえば、有機電解液に二酸化炭素をバブリングする方法や、液化二酸化炭素を溶解させる方法などを採用できる。バブリングするときの二酸化炭素の圧力は高い方が好ましい。また、有機電解液と二酸化炭素を密閉加圧容器に入れ、圧力をかけて二酸化炭素を有機電解液に溶解させる方法や、電池ケ―スにドライアイスを入れたのち、封口する方法などを採用できるが、必ずしもこれらによらなくてもよい。
【００２８】
二酸化炭素溶解時の二酸化炭素分圧としては、０．５Ｋｇｆ／cm²以上が好ましく、１．０Ｋｇｆ／cm²以上がより好ましい。また、有機電解液の注入も二酸化炭素を含む乾燥雰囲気で行うのが好ましい。さらに、電解液注入時の有機電解液やその注入前の電池の温度は、１０℃以下であるのが好ましく、とくに−２０℃以下であるのが好ましい。ドライアイスや液化二酸化炭素を使用すると、これらを満足しやすいので、好ましい。また、ドライアイスを電池内に投入することも好ましい。この場合、有機電解液中には入れないようにし、セパレ―タの上などに置くのが好ましい。投入後は、１分以内に封口を行うのが好ましく、より好ましくは２０秒以内、さらに好ましくは１０秒以内である。
【００２９】
二酸化炭素を溶解させたのちの有機電解液を電池ケ―ス内に注入する方法としては、たとえば、電池ケ―スおよび有機電解液を−２０℃以下に数時間冷却したのち、その冷却した有機電解液を冷却した電池ケ―スに注入する方法を採用することができる。また、遠心分離機に電池ケ―スをセツトし、有機電解液をすばやく注入する方法や、電池ケ―スを真空にしたのち有機電解液を注入する方法などを採用できるが、必ずしもこれらによらなくてもよい。
【００３０】
この発明の有機電解液二次電池は、電池ケ―ス内に、正極と上記した特定構成の炭素材料を用いた負極とをセパレ―タを介して対向配置させるとともに、リン酸トリエステルと含フツ素系溶媒を主溶媒とした有機電解液を注入し、好ましくはこれに二酸化炭素を溶解させてなるものであり、電池形態としては、筒形、ボタン形、コイン形などの各種の形態が含まれるものである。
【００３１】
【実施例】
つぎに、この発明の実施例を記載して、より具体的に説明する。なお、以下において、部とあるのは重量部を意味するものとする。
【００３２】
実施例１
リン酸トリメチルとトリフルオロピレンカ―ボネ―トを体積比１：１で混合し、この混合溶媒にＬｉＰＦ₆を１．０モル／リツトルの濃度で溶解させ、組成が１．０モル／リツトルＬｉＰＦ₆／〔（ＴＭＰ５０体積％）＋ＴＦＰＣ（５０体積％）〕で示される電解液を調製した。ここで、ＴＭＰはリン酸トリメチル、ＴＦＰＣはトリフルオロピレンカ―ボネ―トの略称である。
【００３３】
負極の炭素材料として、Ｘ線回折により測定されるｃ軸方向の結晶子の大きさ〔Ｌｃ〕が１，３３４Å、（００２）面の面間隔〔ｄ₀₀₂〕が３．３６Å、平均粒径が１０μｍ、純度が９９．９９９％の炭素材料を用いた。この炭素材料９０部に対して、結着剤としてのポリフツ化ビニリデン１０部を混合して、負極合剤とし、これを溶剤で分散させてスラリ―にした。つぎに、この負極合剤スラリ―を、負極集電体としての厚さが２０μｍの帯状の銅箔の両面に均一に塗布して、乾燥した。その後、ロ―ラ―プレス機により圧縮形成したのち、リ―ド体を溶接し、帯状の負極体を作製した。
【００３４】
また、リチウムコバルト酸化物（ＬｉＣｏＯ₂）９１部に、黒鉛６部とポリフツ化ビニリデン３部を加えて混合し、溶剤で分散させてスラリ―にした。この正極合剤スラリ―を、厚さが２０μｍの正極集電体のアルミニウム箔の両面に均一に塗布して、乾燥した。その後、ロ―ラ―プレス機により圧縮成形し、リ―ド体の溶接を行い、帯状の正極体を作製した。
【００３５】
つぎに、上記の帯状正極を、厚さが２５μｍの微孔性ポリプロピレンフイルムからなるセパレ―タを介して、上記のシ―ト状負極と重ね、渦巻状に巻回して、渦巻状電極体とした。この渦巻状電極体を、外径１５mmの有底円筒状の電池ケ―ス内に充てんし、正極および負極のリ―ド体の溶接を行つたのち、前記の有機電解液を電池ケ―ス内に注入した。ついで、電池ケ―スの開口部を封口し、筒形の有機電解液二次電池を作製した。
【００３６】
図１は、このように作製した筒形の有機電解液二次電池の模型図であり、１は正極、２は負極である。この図では、繁雑化をさけるため、正極１や負極２の作製にあたつて使用した集電体などは図示していない。３はセパレ―タ、４は有機電解液である。５はステンレス鋼製の電池ケ―スで、負極端子を兼ねている。電池ケ―ス５の底部にはポリテトラフルオロエチレンシ―トからなる絶縁体６が配置され、また内周部にもポリテトラフルオロエチレンシ―トからなる絶縁体７が配置されている。正極１、負極２およびセパレ―タ３からなる渦巻状電極体や、有機電解液４などは、この電池ケ―ス５内に収納されている。
【００３７】
８はステンレス鋼製の封口板で、中央部にガス通気孔８ａを設けている。９はポリプロピレン製の環状パツキング、１０はチタン製の可撓製薄板である。１１はポリプロピレン製の熱変形部材で、温度により変形して、可撓性薄板１０の破壊圧力を変える作用をする。１２はニツケルメツキを施した圧延鋼製の端子板で、切刃１２ａとガス排出口１２ｂとが設けられ、電池内部にガスが発生して内部圧力が上昇し、この上昇により可撓性薄板１０が変形したときに、切刃１２ａによつて可撓性薄板１０を破壊し、電池内部のガスをガス排出口１２ｂから電池外部に排出して、電池の破壊を防止するように設計してある。１３は絶縁パツキング、１４はリ―ド体で、正極１と封口板８とを電気的に接続しており、端子板１２は封口板８との接触により正極端子として作用する。１５は負極２と電池ケ―ス５とを電気的に接続するリ―ド体である。
【００３８】
このようにして作製し、通常の条件で化成したのち、放電した電池を分解して負極を取り出した。この負極を、ＴＦＰＣで洗浄し真空乾燥後、１２ＫＶ−１０ｍＡの測定条件でのＸＰＳ分析の光電子スペクトルのフツ素の６９０ｅＶ付近のピ―クとリンの１３７ｅＶ付近のピ―クを測定した結果を、図２および図３に示す。この負極炭素材料のフツ素の６９０ｅＶ付近のピ―ク強度〔Ｉｆ〕とリンの１３７ｅＶ付近のピ―ク強度〔Ｉｐ〕との比〔Ｉｆ／Ｉｐ〕は、２１．４であつた。ＸＰＳの測定は、ＥＳＣＡＬＢｍａｒｋ２を用い、真空度７×１０^-7Ｐａ、Ａｒスパツタ処理のない条件で行つた。
【００３９】
実施例２
負極の炭素材料として、Ｘ線回折により測定されるｃ軸方向の結晶子の大きさ〔Ｌｃ〕が３００Å、（００２）面の面間隔〔ｄ₀₀₂〕が３．３８Å、平均粒径が１０μｍ、純度が９９．９９９％の炭素材料を用いた以外は、実施例１と同様にして、筒形の有機電解液二次電池を作製した。この電池について、実施例１と同様に化成処理を行い、放電後の炭素材料表面のフツ素の６９０ｅＶ付近のピ―クとリンの１３７ｅＶ付近のピ―クを実施例１と同様に測定した結果、フツ素の６９０ｅＶ付近のピ―ク強度〔Ｉｆ〕とリンの１３７ｅＶ付近のピ―ク強度〔Ｉｐ〕との比〔Ｉｆ／Ｉｐ〕は、１９．５であつた。
【００４０】
実施例３
実施例１と同様の電極を用い、電解液中に二酸化炭素をバブリングさせ、電解液中に二酸化炭素を分圧１Kg／cm²で溶解させたものを用いた以外は、実施例１と同様にして、筒形の有機電解液二次電池を作製した。この有機電解液二次電池について、実施例１と同様にして測定したフツ素の６９０ｅＶ付近のピ―ク強度〔Ｉｆ〕とリンの１３７ｅＶ付近のピ―ク強度〔Ｉｐ〕との比〔Ｉｆ／Ｉｐ〕は、２１．４であり、実施例１と同じであつた。
【００４１】
比較例１
電解液の有機溶媒として、リン酸トリメチルとトリフルオロプロピレンカ―ボネ―トの代わりに、プロピレンカ―ボネ―トとジメトキシエタンとを体積比１：１で混合したものを用いた以外は、実施例１と同様にして、筒形の有機電解液二次電池を作製した。この電池についてのＸＰＳの測定では、フツ素の６９０ｅＶ付近のピ―クとリンの１３７ｅＶ付近のピ―クはみられなかつた。
【００４２】
比較例２
電解液の有機溶媒として、リン酸トリメチルとトリフルオロプロピレンカ―ボネ―トの代わりに、リン酸トリメチルをこれ単独で用いた以外は、実施例１と同様にして、筒形の有機電解液二次電池を作製した。この電池についてのＸＰＳの測定では、フツ素の６９０ｅＶ付近のピ―クはみられなかつた。
【００４３】
比較例３
負極の炭素材料として、Ｘ線回折により測定されるｃ軸方向の結晶子の大きさ〔Ｌｃ〕が１００Å、（００２）面の面間隔〔ｄ₀₀₂〕が３．６０Å、平均粒径が１０μｍ、純度が９９．９９９％の炭素材料を用いた以外は、実施例１と同様にして、筒形の有機電解液二次電池を作製した。この電池について、実施例１と同様に化成処理を行い、放電後の炭素材料表面のフツ素の６９０ｅＶ付近のピ―クとリンの１３７ｅＶ付近のピ―クを実施例１と同様に測定した結果、フツ素の６９０ｅＶ付近のピ―ク強度〔Ｉｆ〕とリンの１３７ｅＶ付近のピ―ク強度〔Ｉｐ〕との比〔Ｉｆ／Ｉｐ〕は、１０．５であつた。
【００４４】
上記の実施例１〜３および比較例１〜３の各有機電解液二次電池につき、０．２Ｃで、電圧２．７〜４．２Ｖの範囲で充電させ、１サイクル目のリテンシヨンおよび最高放電容量を求めた。この結果を下記の表１に示した。なお、リテンシヨンは、つぎの計算式により求めたものである。
リテンシヨン（％）＝〔（充電容量−放電容量）／充電容量〕×１００
【００４５】

【００４６】
つぎに、上記の実施例１と比較例１の各電池について、安全弁が作動した状態（すなわち、図１に示す電池において、電解液中からの溶媒の蒸発などにより、電池内部にガスが発生し、電池内圧が上昇し、可撓性薄板１０が端子板１２側に膨脹し、切刃１２ａに接触して、可撓性薄板１０が破壊され、電池内部のガスがガス排出孔１２ｂから電池外部に排出される状態）になつたことを想定し、あらかじめ可撓性薄板１０を破壊しておき、その状態で電池を１００℃まで加熱し、電池のガス排出孔１２ｂに火を近づけて、引火するかどうか調べた。結果は、下記の表２に示されるとおりであつた。
【００４７】

【００４８】
上記の表２の結果から明らかなように、有機電解液の溶媒として通常の有機溶媒を用いた比較例１の二次電池では、約４０℃に加熱した時点で引火し燃え出したが、リン酸トリアルキルおよびトリフルオロプロピレンカ―ボネ―トを溶媒として用いた実施例１の二次電池では、１００℃まで加熱しても引火せず、火災に対して高い安全性を有していた。
【００４９】
【発明の効果】
以上のように、この発明では、リン酸トリアルキルおよび含フツ素系溶媒を有機電解液の溶媒として用いる一方、負極に特定構成の炭素材料を用いたことにより、またその際とくに有機電解液中に二酸化炭素を溶解させたことにより、安全性および貯蔵性にすぐれ、かつリテンシヨン（充電容量と放電容量との差）、放電容量などの電池性能にすぐれた有機電解液二次電池を提供できる。
【図面の簡単な説明】
【図１】この発明の有機電解液二次電池の構成例を示す縦断面図である。
【図２】この発明の実施例１の負極炭素材料のＸＰＳ測定による６９０ｅＶ付近のスペクトルを概略的に示す特性図である。
【図３】この発明の実施例１の負極炭素材料のＸＰＳ測定による１３７ｅＶ付近のスペクトルを概略的に示す特性図である。
【符号の説明】
１正極
２負極
３セパレ―タ
４有機電解液
５電池ケ―ス[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an organic electrolyte secondary battery with improved battery performance and improved safety and storability.
[0002]
[Prior art]
An organic electrolyte secondary battery is a secondary battery using an organic solvent as a solvent for the electrolyte, and this type of secondary battery has a large capacity, high voltage, and high energy density. The demand tends to increase more and more.
[0003]
In such an organic electrolyte secondary battery, an organic solvent based electrolyte in which a lithium salt is dissolved in an organic solvent is used, and lithium or a lithium alloy is used as a negative electrode active material. There are problems with safety, such as the deterioration of battery characteristics and the possibility of internal short circuit.
[0004]
For this reason, Japanese Patent Publication No. 4-24831 and Japanese Patent Publication No. 5-17669 propose use of a carbon material such as activated carbon or graphite as the negative electrode active material instead of the above lithium or lithium alloy. ing.
[0005]
[Problems to be solved by the invention]
However, the carbon material as described above has a large retention (difference between the charge capacity and the discharge capacity), so that the characteristics cannot be sufficiently satisfied. In particular, in a secondary battery that requires a high capacity in recent years, it is desired that the battery capacity per unit volume is a high capacity of 300 Wh / liter or more. Therefore, a secondary battery having a smaller retention is required.
[0006]
In addition, in conventional organic electrolyte secondary batteries, propylene carbonate, 1,2-dimethoxyethane, etc. have been used as the solvent of the electrolyte, but these have problems in terms of safety against fires and the like. . Thus, phosphate triesters such as trialkyl phosphates have attracted attention as safe organic solvents that are nonflammable or flame retardant (flash point of 100 ° C. or higher). However, in batteries using phosphoric acid triesters, when lithium or an alloy thereof is used as the negative electrode for an active positive electrode active material such as manganese dioxide, the reaction between the negative electrode lithium and the solvent occurs and the internal resistance is remarkably increased. There was a problem of storability that increased and battery performance deteriorated.
[0007]
In light of such circumstances, an object of the present invention is to provide an organic electrolyte secondary battery that has a small retention (difference between charge capacity and discharge capacity), high capacity, and excellent safety and storability. It is said.
[0008]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors use phosphoric triester and fluorine-containing solvent as the solvent of the organic electrolyte solution, while replacing the lithium or its alloy with the negative electrode, By using a carbon material having a specific crystallinity, it is possible to obtain an organic electrolyte secondary battery that has a small retention, a high capacity, and excellent safety and storability. When carbon dioxide was dissolved, it was found that better results in battery performance such as retention and discharge capacity were obtained, and the present invention was completed.
[0009]
That is, the present invention relates to an organic electrolyte secondary battery having a positive electrode, a negative electrode, and an organic electrolyte, wherein the negative electrode is a carbon material having a crystallite size [Lc] in the c-axis direction measured by X-ray diffraction of 200 L or more. And an organic electrolyte secondary battery characterized by using a phosphoric acid triester and a fluorine-containing solvent as a solvent for the organic electrolyte. In a preferred embodiment, carbon dioxide is dissolved in the organic electrolyte.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In this invention, the negative electrode is composed of a carbon material as a constituent element and a binder appropriately added thereto, and this mixture is finished into a molded body using a current collector material such as copper foil as a core material. Is used. Here, as the carbon material, for example, pyrolytic carbons, cokes, glassy carbons, organic polymer fired bodies, mesocarbon micro beads, carbon fibers, activated carbon, and the like can be used. .
[0011]
In the present invention, such a carbon material has a crystallite size [Lc] in the c-axis direction measured by X-ray diffraction of 200 Å or more, preferably 1,000 Å or more (usually 2,000 Å or less). And the purity is preferably 99.9% or more. Thus, according to the carbon material having high crystallinity, a large capacity battery can be obtained because there are many lattices capable of doping and dedoping lithium. The average particle size of the carbon material is preferably 9 μm or more because the specific surface area is too large and the safety is inferior if it is less than 9 μm. Further, the distance between planes of (002) plane [d ₀₀₂ ] Has a graphite structure of 3.35 to 3.50Å, preferably 3.36 to 3.38Å, in order to achieve high battery capacity.
[0012]
In the present invention, the positive electrode includes manganese dioxide, vanadium pentoxide, chromium oxide, LiNiO. ₂ Lithium nickel oxide such as LiCoO ₂ Lithium cobalt oxide such as LiMn ₂ O _Four As appropriate, metal oxides such as lithium manganese oxide, metal sulfides such as titanium disulfide and molybdenum disulfide, or binders such as polytetrafluoroethylene are added to these positive electrode active materials. The resulting mixture is finished into a molded body using a current collecting material such as a stainless steel net as a core material.
[0013]
By using an organic electrolytic solution in which an electrolyte is dissolved in an organic solvent with respect to the negative electrode carbon material and the positive electrode material described above, good results can be obtained in battery performance. Therefore, there is a problem with the safety of the battery. In order to solve this problem, in the present invention, as a component of the organic solvent, the general formula: (RO) _Three A non-combustible or flame-retardant phosphate triester represented by P = O (wherein R is an organic group and the three organic groups may be the same or different), preferably Trialkyl phosphates in which R is an alkyl group having 1 to 6 carbon atoms, for example, trimethyl phosphate, trimethyl phosphate, tributyl phosphate and the like are used.
[0014]
An organic electrolyte secondary battery using such a phosphoric acid triester does not ignite even when heated to 100 ° C. However, when the above-described phosphoric acid triester is used alone as an organic solvent for the carbon material having high crystallinity, there is a phenomenon in which the battery retention increases and the battery capacity decreases. Therefore, in the present invention, a fluorine-containing solvent such as trifluoropropylene carbonate is used in combination with the above phosphoric triester to improve the retention and battery capacity. The reason for the improvement is not necessarily clear, but when a battery using an electrolyte containing both solvents is charged and discharged, the fluorine-containing solvent decomposes on the surface of the negative electrode carbon material and forms a film on the surface of the carbon material. This is thought to be for suppressing the decomposition of the phosphoric acid triester on the carbon material surface.
[0015]
Formation of the film as described above is based on the peak intensity [If] of fluorine near 690 eV and the peak intensity [Ip] near 137 eV of phosphorus in the photoelectron spectrum of XPS analysis under the measurement condition of 12 kV-10 mA. This can be confirmed by measuring the ratio [If / Ip]. The peak of fluorine by XPS analysis is considered to be due to bonds such as C—F formed on the surface of the carbon material, and the peak of phosphorus is considered to be due to bonds such as C—P on the surface of the carbon material. . In the study by the present inventors, when the phosphate triester and the fluorine-containing solvent are used in combination so that the ratio [If / Ip] is in the range of 5 to 50, the surface of the carbon material surface of the phosphate triester It has been confirmed that decomposition is suppressed and a battery with a small retention and a high capacity can be obtained.
[0016]
In this invention, the mixing ratio of the phosphoric acid triester and the fluorine-containing solvent is not particularly limited, but considering the safety of the battery, the phosphoric acid triester that is more flame retardant than the fluorine-containing solvent. The volume ratio of phosphoric acid triester and fluorine-containing solvent is preferably in the range of 1: 1 to 5: 1. Considering the discharge performance and the like, more fluorine-containing solvent can be used as long as the safety against fire caused by the phosphoric acid triester can be secured.
[0017]
The phosphoric acid triester and fluorine-containing solvent may occupy all of the organic solvent, but in some cases, these solvents may occupy most of the organic solvent, and some of them may contain other organic solvents. Good. In the latter case, the total of the phosphoric acid triester and the fluorine-containing solvent should be 40% by volume or more, preferably 60% by volume or more, more preferably 90% by volume or more of the entire organic solvent. If the proportion of phosphoric acid triester and fluorine-containing solvent is large, in addition to the property of being non-flammable or flame retardant, the phosphoric acid triester's decomposition suppression effect is sufficiently exerted, and fire safety is improved. As well as improving, the retention can be further improved.
[0018]
Other solvents are preferably esters having a dielectric constant of 30 or more, particularly 50 or more. When such a solvent is used, a flame-retardant electrolyte solution having high electrolyte solubility is obtained, and the reaction active point between the carbon material surface of the negative electrode and the electrolyte solution is further reduced. That is, the combined use of a phosphoric acid triester or the like and an ester having a high dielectric constant is desirable because the solubility of the electrolyte is improved, the conductivity is increased, and the capacity is significantly improved.
[0019]
As such an ester having a high dielectric constant, an alkylene carbonate having 2 to 10 carbon atoms, preferably 2 to 6 carbon atoms, is used. Examples thereof include ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite and the like. Of these, those having a cyclic structure are preferred, and cyclic carbonates are particularly preferred. The most preferred ester is ethylene carbonate with a dielectric constant of 95.
[0020]
Since these esters having a high dielectric constant are flammable, it is preferable to use fewer esters from the viewpoint of safety when used in combination with a phosphoric acid triester and a fluorine-containing solvent. Generally, the ester having a high dielectric constant is preferably 10% by volume or less of the total organic solvent, more preferably 5% by volume or less, and further preferably 3% by volume or less. The effect of improving the phosphorescence by the ester having a high dielectric constant appears when the amount of the ester becomes 1% by volume or more of the whole organic solvent, and when the amount reaches 2% by volume, a remarkable improvement is observed.
[0021]
Thus, in the combined use of an ester having a high dielectric constant, a phosphoric acid triester and a fluorine-containing solvent, the ester having a high dielectric constant is 1 to 10% by volume, particularly 2 to 5% by volume of the total organic solvent, It is preferably 2 to 3% by volume. Further, the difference in boiling point between the phosphate triester and the ester having a high dielectric constant is preferably 150 ° C. or less, more preferably 100 ° C. or less, still more preferably 50 ° C. or less, and most preferably 10 ° C. or less. This is because when the flammable ester and the flame retardant phosphoric acid triester are azeotroped, the ester is less flammable.
[0022]
In addition to such an ester having a high dielectric constant, 1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether and the like can be used in combination. Further, an amine imide organic solvent or a sulfur-containing organic solvent may be used in combination. However, these solvents are also preferably 10% by volume or less of the entire organic solvent of the electrolytic solution.
[0023]
In the organic electrolyte used in the present invention, examples of the electrolyte dissolved in such an organic solvent include LiClO. _Four , LiPF ₆ , LiBF _Four , LiAsF ₆ , LiSbF ₆ , LiCF _Three SO _Three , LiCF _Three CO ₂ And others, Li ₂ C ₂ F _Four (SO _Three ) ₂ , LiN (CF _Three SO ₂ ) ₂ , LiC (CF _Three SO ₂ ) _Three , LiC _n F _{2n + 1} SO _Three (N ≧ 2) and the like are used alone or in combination of two or more. Among these, LiPF ₆ And LiC _n F _{2n + 1} SO _Three Is preferably used because of its good charge / discharge characteristics. The concentration of these electrolytes in the electrolytic solution is not particularly limited, but is usually 0.1 to 2 mol / liter, preferably about 0.4 to 1 mol / liter.
[0024]
In the present invention, when carbon dioxide is dissolved in such an organic electrolytic solution, the retention can be further reduced. The reason why the above effect is obtained by dissolving carbon dioxide is not always clear. When carbon dioxide is dissolved in the electrolytic solution, it reacts with the surface of the negative electrode material to form a thin dense film such as an organic or inorganic carbon salt, and this film forms an organic solvent and carbon in the electrolytic solution during lithium ion doping. It seems that the reaction with the material is more efficiently suppressed.
[0025]
Also, carbon dioxide is dissolved in LiNiO as a positive electrode active material. ₂ LiCoO ₂ , LiMn ₂ O _Four This is effective when a lithium composite oxide having a closed-circuit voltage of 4 V or more on the basis of Li is used. These positive electrode active materials are high voltage, and under normal conditions, the organic electrolyte is oxidized and the discharge performance is reduced. However, if carbon dioxide with excellent oxidation resistance is dissolved in the organic electrolyte, carbon dioxide is converted into the positive electrode. Suppresses the decomposition of the electrolyte due to oxidation on the surface. Especially LiNiO ₂ Can not be used from the viewpoint of reactivity with the electrolytic solution than other metal oxides, but such LiNiO ₂ Even in this case, the reaction with the organic electrolyte is suppressed, and the capacity of the battery is increased.
[0026]
The amount of carbon dioxide dissolved in the organic electrolyte is preferably 0.03 mol / liter (0.03 mol of carbon dioxide relative to 1 liter of organic electrolyte) or more with respect to the organic electrolyte in the battery. More preferably, it is 0.1 mol / liter or more, and still more preferably 0.3 mol / liter or more. As the amount of carbon dioxide increases, the reaction activity of the carbon material is more stably extracted, and the reactivity of the positive electrode active material to the organic electrolyte is suppressed. , Increase the internal pressure of the battery, causing the battery to burst. Therefore, in consideration of the pressure resistance of the battery case and the sealing member, it is desirable that it is 2 mol / liter or less. Here, carbon dioxide that is put in the battery and not dissolved in the organic electrolyte also dissolves in the organic electrolyte when carbon dioxide is consumed in the organic electrolyte or when the temperature is lowered. Is considered to be substantially dissolved.
[0027]
As a method for dissolving carbon dioxide in the organic electrolyte, for example, a method of bubbling carbon dioxide in the organic electrolyte or a method of dissolving liquefied carbon dioxide can be employed. The pressure of carbon dioxide when bubbling is preferably higher. In addition, the method of putting the organic electrolyte and carbon dioxide into a sealed pressurized container and applying pressure to dissolve the carbon dioxide in the organic electrolyte, and the method of sealing after putting dry ice in the battery case are adopted. Although it is possible, it does not necessarily have to be based on these.
[0028]
The partial pressure of carbon dioxide when dissolving carbon dioxide is 0.5 kgf / cm. ² Or more, 1.0 kgf / cm ² The above is more preferable. In addition, the injection of the organic electrolyte is preferably performed in a dry atmosphere containing carbon dioxide. Furthermore, the temperature of the organic electrolyte at the time of electrolyte injection and the battery before the injection is preferably 10 ° C. or less, and particularly preferably −20 ° C. or less. Use of dry ice or liquefied carbon dioxide is preferable because these are easily satisfied. It is also preferable to put dry ice into the battery. In this case, it is preferable not to put it in the organic electrolyte but to put it on a separator or the like. After the addition, it is preferable to perform sealing within 1 minute, more preferably within 20 seconds, and even more preferably within 10 seconds.
[0029]
As a method for injecting the organic electrolyte after dissolving carbon dioxide into the battery case, for example, after cooling the battery case and the organic electrolyte to −20 ° C. or lower for several hours, the cooled organic A method of injecting the electrolyte into a cooled battery case can be employed. In addition, the battery case is set in a centrifuge, and a method of quickly injecting an organic electrolyte solution or a method of injecting an organic electrolyte solution after evacuating the battery case can be adopted. It does not have to be.
[0030]
In the organic electrolyte secondary battery of the present invention, the positive electrode and the negative electrode using the carbon material having the specific configuration described above are disposed opposite to each other through a separator in the battery case, and the phosphoric acid triester and the negative electrode are included. An organic electrolytic solution containing a fluorine-based solvent as a main solvent is injected, and carbon dioxide is preferably dissolved therein, and various battery forms such as a cylindrical shape, a button shape, and a coin shape are available. It is included.
[0031]
【Example】
Next, an embodiment of the present invention will be described in more detail. In the following, “parts” means parts by weight.
[0032]
Example 1
Trimethyl phosphate and trifluoropyrene carbonate are mixed at a volume ratio of 1: 1, and this mixed solvent is mixed with LiPF. ₆ Is dissolved at a concentration of 1.0 mol / liter and the composition is 1.0 mol / liter LiPF. ₆ / [(TMP50 volume%) + TFPC (50 volume%)] was prepared. Here, TMP is an abbreviation for trimethyl phosphate, and TFPC is an abbreviation for trifluoropyrene carbonate.
[0033]
As the carbon material of the negative electrode, the crystallite size [Lc] in the c-axis direction measured by X-ray diffraction is 1,334 mm, and the surface spacing of the (002) plane [d ₀₀₂ ] Was 3.36 mm, the average particle size was 10 μm, and the purity was 99.999%. 90 parts of this carbon material was mixed with 10 parts of polyvinylidene fluoride as a binder to form a negative electrode mixture, which was dispersed with a solvent to form a slurry. Next, this negative electrode mixture slurry was uniformly applied to both sides of a strip-shaped copper foil having a thickness of 20 μm as a negative electrode current collector and dried. Then, after compression forming with a roller press, the lead body was welded to produce a strip-like negative electrode body.
[0034]
Also, lithium cobalt oxide (LiCoO ₂ ) In 91 parts, 6 parts of graphite and 3 parts of polyvinylidene fluoride were added and mixed, and dispersed with a solvent to form a slurry. This positive electrode mixture slurry was uniformly applied to both surfaces of an aluminum foil of a positive electrode current collector having a thickness of 20 μm and dried. Then, it was compression-molded with a roller press and the lead body was welded to produce a strip-like positive electrode body.
[0035]
Next, the belt-like positive electrode is overlapped with the sheet-like negative electrode via a separator made of a microporous polypropylene film having a thickness of 25 μm and wound in a spiral shape to form a spiral electrode body. did. The spiral electrode body is filled in a cylindrical battery case having a bottomed outer diameter of 15 mm, and the lead body of the positive electrode and the negative electrode is welded. Then, the organic electrolyte is added to the battery case. Injected into. Next, the opening of the battery case was sealed to produce a cylindrical organic electrolyte secondary battery.
[0036]
FIG. 1 is a model diagram of a cylindrical organic electrolyte secondary battery produced in this manner, where 1 is a positive electrode and 2 is a negative electrode. In this figure, in order to avoid complication, the current collector used for manufacturing the positive electrode 1 and the negative electrode 2 is not shown. 3 is a separator and 4 is an organic electrolyte. 5 is a battery case made of stainless steel, which also serves as a negative electrode terminal. An insulator 6 made of polytetrafluoroethylene sheet is disposed at the bottom of the battery case 5, and an insulator 7 made of polytetrafluoroethylene sheet is also disposed at the inner periphery. A spiral electrode body including the positive electrode 1, the negative electrode 2, and the separator 3, the organic electrolyte solution 4, and the like are accommodated in the battery case 5.
[0037]
A stainless steel sealing plate 8 is provided with a gas vent 8a in the center. 9 is an annular packing made of polypropylene, and 10 is a flexible thin plate made of titanium. 11 is a heat deformation member made of polypropylene, which is deformed by temperature and acts to change the breaking pressure of the flexible thin plate 10. Reference numeral 12 is a rolled steel terminal plate having a nickel seal, which is provided with a cutting edge 12a and a gas discharge port 12b. Gas is generated inside the battery to increase the internal pressure. When deformed, the flexible thin plate 10 is broken by the cutting blade 12a, and the gas inside the battery is discharged from the gas discharge port 12b to the outside of the battery to prevent the battery from being broken. 13 is an insulating packing, 14 is a lead body, and electrically connects the positive electrode 1 and the sealing plate 8, and the terminal plate 12 acts as a positive electrode terminal by contact with the sealing plate 8. A lead body 15 electrically connects the negative electrode 2 and the battery case 5.
[0038]
Thus produced and formed under normal conditions, the discharged battery was disassembled and the negative electrode was taken out. The negative electrode was washed with TFPC and vacuum-dried, and the result of measuring the peak near 690 eV of fluorine and the peak near 137 eV of phosphorus in the photoelectron spectrum of XPS analysis under the measurement condition of 12 KV-10 mA It is shown in FIG. 2 and FIG. The ratio [If / Ip] of the peak strength [If] of the negative electrode carbon material near 690 eV to the peak strength [Ip] of phosphorus near 137 eV was 21.4. For XPS measurement, ESCALB mark2 was used, and the degree of vacuum was 7 × 10. ^-7 The test was carried out without Pa and Ar spatter treatment.
[0039]
Example 2
As the carbon material of the negative electrode, the size [Lc] of crystallites in the c-axis direction measured by X-ray diffraction is 300 mm, and the spacing between (002) planes [d ₀₀₂ ] Was 3.38 mm, the average particle size was 10 μm, and a carbon material having a purity of 99.999% was used to produce a cylindrical organic electrolyte secondary battery in the same manner as in Example 1. This battery was subjected to a chemical conversion treatment in the same manner as in Example 1, and the peak near 690 eV of fluorine on the surface of the carbon material after discharge and the peak near 137 eV of phosphorus were measured in the same manner as in Example 1. The ratio [If / Ip] between the peak intensity [If] near 690 eV of fluorine and the peak intensity [Ip] near 137 eV of phosphorus was 19.5.
[0040]
Example 3
Using the same electrode as in Example 1, carbon dioxide was bubbled into the electrolyte, and the partial pressure of carbon dioxide was 1 kg / cm in the electrolyte. ² A cylindrical organic electrolyte secondary battery was produced in the same manner as in Example 1 except that the material dissolved in (1) was used. For the organic electrolyte secondary battery, the ratio of the peak strength [If] of fluorine near 690 eV measured in the same manner as in Example 1 to the peak strength [Ip] of phosphorus near 137 eV [If / Ip] was 21.4, which was the same as Example 1.
[0041]
Comparative Example 1
As an organic solvent for the electrolytic solution, except that trimethyl phosphate and trifluoropropylene carbonate were mixed with propylene carbonate and dimethoxyethane in a volume ratio of 1: 1 instead of trimethyl phosphate and trifluoropropylene carbonate. In the same manner as in Example 1, a cylindrical organic electrolyte secondary battery was produced. In the XPS measurement for this battery, no peak was observed near 690 eV for fluorine and 137 eV for phosphorus.
[0042]
Comparative Example 2
In the same manner as in Example 1, except that trimethyl phosphate was used alone instead of trimethyl phosphate and trifluoropropylene carbonate as an organic solvent for the electrolyte, a cylindrical organic electrolyte 2 A secondary battery was produced. In the XPS measurement of this battery, no peak of fluorine around 690 eV was found.
[0043]
Comparative Example 3
As the carbon material of the negative electrode, the crystallite size [Lc] in the c-axis direction measured by X-ray diffraction is 100 mm, and the spacing between (002) planes [d ₀₀₂ ] Was 3.60 mm, the average particle size was 10 μm, and a carbon material having a purity of 99.999% was used to produce a cylindrical organic electrolyte secondary battery in the same manner as in Example 1. This battery was subjected to a chemical conversion treatment in the same manner as in Example 1, and the peak near 690 eV of fluorine on the surface of the carbon material after discharge and the peak near 137 eV of phosphorus were measured in the same manner as in Example 1. The ratio [If / Ip] of the peak intensity [If] near 690 eV of fluorine and the peak intensity [Ip] near 137 eV of phosphorus was 10.5.
[0044]
Each of the organic electrolyte secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 was charged at 0.2 C in a voltage range of 2.7 to 4.2 V, and the first cycle retention and maximum discharge The capacity was determined. The results are shown in Table 1 below. The retention is determined by the following calculation formula.
Retention (%) = [(charge capacity−discharge capacity) / charge capacity] × 100
[0045]

[0046]
Next, for each of the batteries of Example 1 and Comparative Example 1 described above, the safety valve was activated (that is, in the battery shown in FIG. 1, gas was generated inside the battery due to evaporation of the solvent from the electrolyte, etc. The internal pressure of the battery rises, the flexible thin plate 10 expands to the terminal plate 12 side, contacts the cutting edge 12a, the flexible thin plate 10 is destroyed, and the gas inside the battery is discharged from the gas discharge hole 12b to the outside of the battery. In this state, the flexible thin plate 10 is destroyed in advance, and the battery is heated to 100 ° C. in that state, and a fire is brought close to the gas discharge hole 12b of the battery to ignite. Investigate whether to do. The results were as shown in Table 2 below.
[0047]

[0048]
As is clear from the results in Table 2 above, the secondary battery of Comparative Example 1 using a normal organic solvent as the solvent of the organic electrolyte ignited and burned out when heated to about 40 ° C. The secondary battery of Example 1 using trialkyl acid and trifluoropropylene carbonate as a solvent did not ignite even when heated to 100 ° C., and had high safety against fire.
[0049]
【The invention's effect】
As described above, in the present invention, a trialkyl phosphate and a fluorine-containing solvent are used as a solvent for an organic electrolytic solution, while a carbon material having a specific configuration is used for the negative electrode, and in that case, particularly in the organic electrolytic solution. By dissolving carbon dioxide in the organic electrolyte secondary battery, it is possible to provide an organic electrolyte secondary battery that is excellent in safety and storability, and excellent in battery performance such as retention (difference between charge capacity and discharge capacity) and discharge capacity.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing a configuration example of an organic electrolyte secondary battery according to the present invention.
FIG. 2 is a characteristic diagram schematically showing a spectrum around 690 eV by XPS measurement of the negative electrode carbon material of Example 1 of the present invention.
FIG. 3 is a characteristic diagram schematically showing a spectrum around 137 eV by XPS measurement of the negative electrode carbon material of Example 1 of the present invention.
[Explanation of symbols]
1 Positive electrode
2 Negative electrode
3 Separator
4 Organic electrolyte
5 Battery case

Claims

In an organic electrolyte secondary battery having a positive electrode, a negative electrode, and an organic electrolyte, the negative electrode is made of a carbon material having a crystallite size [Lc] in the c-axis direction measured by X-ray diffraction of 200 L or more, and organic electrolysis An organic electrolyte secondary battery using phosphoric acid triester and a fluorine-containing solvent as a solvent for the liquid.

The organic electrolyte secondary battery according to claim 1, wherein an ester having a dielectric constant of 30 or more is used as a solvent for the organic electrolyte together with a phosphoric acid triester and a fluorine-containing solvent.

(RO) _{3 Three} 3. The organic electrolyte secondary battery according to claim 1, wherein in the phosphoric acid triester represented by P═O, R is the same or different alkyl group having 1 to 6 carbon atoms.

The organic electrolyte secondary battery according to any one of claims 1 to 3, wherein the total of the phosphoric acid triester and the fluorine-containing solvent is 40% by volume or more of the whole organic solvent.

The organic electrolyte secondary battery according to any one of claims 1 to 4, wherein a mixing ratio of the phosphoric acid triester and the fluorine-containing solvent is in a range of 1: 1 to 5: 1 by volume ratio.

The organic electrolyte secondary battery according to claim 2, wherein the ester having a dielectric constant of 30 or more is a cyclic carbonate having 2 to 10 carbon atoms, and is 10% by volume or less of the whole organic solvent.

The organic electrolyte secondary battery according to any one of claims 1 to 6, wherein a lithium-containing composite oxide such as lithium nickel oxide, lithium cobalt oxide, or lithium manganese oxide is used as the positive electrode active material.

In the negative electrode carbon material, the distance d between (002) planes measured by X-ray diffraction. _{002 002} Is an organic electrolyte secondary battery according to any one of claims 1 to 7.

The organic electrolyte secondary battery according to any one of claims 1 to 8 , wherein carbon dioxide is dissolved in the organic electrolyte.

The organic electrolyte secondary battery according to claim 9 , wherein the amount of carbon dioxide dissolved is 0.03 mol / liter or more.

The partial pressure of carbon dioxide when carbon dioxide is dissolved in the organic electrolyte is 0.5 kgf / cm ^{2 2} It is the above, The organic electrolyte secondary battery of Claim 9 or 10.

In the carbon material of the negative electrode, the ratio [If / Ip] of the peak intensity [If] near 690 eV of fluorine and the peak intensity [Ip] near 137 eV of phosphorus in the XPS photoelectron spectrum under the measurement condition 12 kV-10 mA is 5 to 5. It is the range of 50, The organic electrolyte secondary battery in any one of Claims 1-11.