JP4934914B2 - Electrolyte and secondary battery - Google Patents

Description

【００１６】
（式中、Ｒ₁及びＲ₂は炭素数１〜１０のアルキル基またはハロゲン置換アルキル基を表し、ｎは０〜１及び３〜１０の整数である。）
で表される飽和ジカルボン酸ジエステル、及び下記一般式（２）：
【００１７】
【化４】

【００１８】
（式中、Ｒ₃及びＲ₄は炭素数１〜１０のアルキル基またはハロゲン置換アルキル基を表し、ｐ及びｑはそれぞれ０〜５の整数であって、０≦ｐ＋ｑ≦１０である。）
で表される不飽和ジカルボン酸ジエステル、が挙げられる。
上記一般式（１）及び（２）中のＲ₁〜Ｒ₄は炭素数１以上１０以下のアルキル基またはハロゲン置換アルキル基である。具体的には、メチル基、エチル基、プロピル基、イソプロピル基、ブチル基、イソブチル基、ｓ−ブチル基、ｔ−ブチル基、ペンチル基、イソペンチル基、ネオペンチル基、ヘキシル基、ヘプチル基、オクチル基、ノニル基、デシル基、フルオロメチル基、ジフルオロメチル基、トリフルオロメチル基、トリフルオロエチル基、ペンタフルオロプロピル基、トリフルオロエチル基、ヘプタフルオロブチル基、ノナフルオロペンチル基、テトラフルオロプロピル基、ヘキサフルオロブチル基、オクタフルオロペンチル基、プロピルフルオロメチル基、プロピルジフルオロメチル基、プロピルトリフルオロメチル基、ブチルフルオロメチル基、ブチルジフルオロメチル基、ブチルトリフルオロメチル基、ペンタフルオロブチル基、ヘプタフルオロペンチル基等を挙げることができる。また、上記一般式（１）中のｎは０〜１及び３〜１０の整数であり、一般式（２）中のｐ及びｑはそれぞれ０〜５の整数であって、０≦ｐ＋ｑ≦１０の関係式を満足する。上記Ｒ₁ 〜Ｒ₄ の炭素数、ｎ及びｐ＋ｑが１０を超えると、前記溶媒に対する溶解性が低下する傾向にあるため、過充電防止効果が低下する恐れがある。
【００１９】
上記ジカルボン酸ジエステルとしては、ジカルボン酸ジエステルの分子骨格を持つ化合物であれば、コハク酸ジエステルを除き、特に限定されない。
また、これらの誘導体を用いることもできる。誘導体としては、上記ジカルボン酸ジエステルの水素原子の一部を置換基にて置換したもの等、上記ジカルボン酸ジエステル骨格を有する各種の化合物を挙げることができる。上記の置換基としては、例えば、ハロゲン原子、酸素原子、硫黄原子、アミノ基、アルキルアミノ基、アリールアミノ基、カルボンアミノ基、スルホンアミド基、オキシカルボニルアミノ基、オキシスルホニルアミノ基、ウレイド基、ヒドロキシル基、メルカプト基、メトキシル基、炭素数１〜３の低級アルキル基、シクロアルキル基、アルコキシ基、アルケニル基、アルキニル基、アラルキル基、アリール基、シアノ基、ニトロ基、ホルミル基、アリールオキシ基、アルキルチオ基、アクリル基、アリールチオ基、アシルオキシ基、スルホニルオキシ基、アシル基、オキシカルボニル基、カルバモイル基、スルホニル基、スルフィニル基、オキシスルフィニル基、スルファモイル基、カルボン酸基若しくはその塩、スルホン酸基若しくはその塩、ホスホン酸基若しくはその塩、複素環残基、又は水酸基等を挙げることができる。なお、上記置換基の炭素数は通常１０以下、好ましくは５以下である。
【００２０】
使用するジカルボン酸ジエステル及びその誘導体の具体例としては、シュウ酸ジメチル、シュウ酸ジエチル、シュウ酸ジプロピル、シュウ酸ジブチル、シュウ酸ビス（フルオロメチル）、シュウ酸ビス（ジフルオロメチル）、シュウ酸ビス（トリフルオロメチル）等のシュウ酸ジエステル、マロン酸ジメチル、マロン酸ジエチル、マロン酸ジプロピル、マロン酸ジブチル、マロン酸ビス（フルオロメチル）、マロン酸ビス（ジフルオロメチル）、マロン酸ビス（トリフルオロメチル）等のマロン酸ジエステル、マレイン酸ジメチル、マレイン酸ジエチル、マレイン酸ジプロピル、マレイン酸ジブチル、マレイン酸ビス（フルオロメチル）、マレイン酸ビス（ジフルオロメチル）、マレイン酸ビス（トリフルオロメチル）等のマレイン酸ジエステル、フマル酸ジメチル、フマル酸ジエチル、フマル酸ジプロピル、フマル酸ジブチル、フマル酸ビス（フルオロメチル）、フマル酸ビス（ジフルオロメチル）、フマル酸ビス（トリフルオロメチル）等のフマル酸ジエステル、グルタル酸ジメチル、グルタル酸ジエチル、グルタル酸ジプロピル、グルタル酸ジブチル、グルタル酸ビス（フルオロメチル）、グルタル酸ビス（ジフルオロメチル）、グルタル酸ビス（トリフルオロメチル）等のグルタル酸ジエステル、アジピン酸ジメチル、アジピン酸ジエチル、アジピン酸ジプロピル、アジピン酸ジブチル、アジピン酸ビス（フルオロメチル）、アジピン酸ビス（ジフルオロメチル）、アジピン酸ビス（トリフルオロメチル）等のアジピン酸ジエステル、ピメリン酸ジメチル、ピメリン酸ジエチル、ピメリン酸ビス（フルオロメチル）、ピメリン酸ビス（ジフルオロメチル）、ピメリン酸ビス（トリフルオロメチル）等のピメリン酸ジエステル、スベリン酸ジメチル、スベリン酸ジエチル、スベリン酸ジプロピル、スベリン酸ジブチル、スベリン酸ビス（フルオロメチル）、スベリン酸ビス（ジフルオロメチル）、スベリン酸ビス（トリフルオロメチル）等のスベリン酸ジエステル、アゼライン酸ジメチル、アゼライン酸ジエチル、アゼライン酸ジプロピル、アゼライン酸ジブチル、アゼライン酸ビス（フルオロメチル）、アゼライン酸ビス（ジフルオロメチル）、アゼライン酸ビス（トリフルオロメチル）等のアゼライン酸ジエステル、セバシン酸ジメチル、セバシン酸ジエチル、セバシン酸ジプロピル、セバシン酸ジブチル、セバシン酸ビス（フルオロメチル）、セバシン酸ビス（ジフルオロメチル）、セバシン酸ビス（トリフルオロメチル）等のセバシン酸ジエステル、ウンデカン二酸ジメチル、ウンデカン二酸ジエチル、ウンデカン二酸ジプロピル、ウンデカン二酸ジブチル、ウンデカン二酸ビス（フルオロメチル）、ウンデカン二酸ビス（ジフルオロメチル）、ウンデカン二酸ビス（トリフルオロメチル）等のウンデカン二酸ジエステル、ドデカン二酸ジメチル、ドデカン二酸ジエチル、ドデカン二酸ジプロピル、ドデカン二酸ジブチル、ドデカン二酸ビス（フルオロメチル）、ドデカン二酸ビス（ジフルオロメチル）、ドデカン二酸ビス（トリフルオロメチル）等のドデカン二酸ジエステル、アセトンジカルボン酸ジエチル等を挙げることができるが、これらには限定されない。中でも、シュウ酸ジメチル、シュウ酸ジエチル、シュウ酸ジプロピル、シュウ酸ジブチル、シュウ酸ビス（フルオロメチル）、シュウ酸ビス（ジフルオロメチル）、シュウ酸ビス（トリフルオロメチル）等のシュウ酸ジエステル、マロン酸ジメチル、マロン酸ジエチル、マロン酸ジプロピル、マロン酸ジブチル、マロン酸ビス（フルオロメチル）、マロン酸ビス（ジフルオロメチル）、マロン酸ビス（トリフルオロメチル）等のマロン酸ジエステル、マレイン酸ジメチル、マレイン酸ジエチル、マレイン酸ジプロピル、マレイン酸ジブチル、マレイン酸ビス（フルオロメチル）、マレイン酸ビス（ジフルオロメチル）、マレイン酸ビス（トリフルオロメチル）等のマレイン酸ジエステル、フマル酸ジメチル、フマル酸ジエチル、フマル酸ジプロピル、フマル酸ジブチル、フマル酸ビス（フルオロメチル）、フマル酸ビス（ジフルオロメチル）、フマル酸ビス（トリフルオロメチル）等のフマル酸ジエステル、グルタル酸ジメチル、グルタル酸ジエチル、グルタル酸ジプロピル、グルタル酸ジブチル、グルタル酸ビス（フルオロメチル）、グルタル酸ビス（ジフルオロメチル）、グルタル酸ビス（トリフルオロメチル）等のグルタル酸ジエステルが好ましく、さらに好ましくはマロン酸ジメチル、マロン酸ジエチル、マロン酸ジプロピル、マロン酸ジブチル、マロン酸ビス（フルオロメチル）、マロン酸ビス（ジフルオロメチル）、マロン酸ビス（トリフルオロメチル）等のマロン酸ジエステル、マレイン酸ジメチル、マレイン酸ジエチル、マレイン酸ジプロピル、マレイン酸ジブチル、マレイン酸ビス（フルオロメチル）、マレイン酸ビス（ジフルオロメチル）、マレイン酸ビス（トリフルオロメチル）等のマレイン酸ジエステル、フマル酸ジメチル、フマル酸ジエチル、フマル酸ジプロピル、フマル酸ジブチル、フマル酸ビス（フルオロメチル）、フマル酸ビス（ジフルオロメチル）、フマル酸ビス（トリフルオロメチル）等のフマル酸ジエステルであり、最も好ましくはマロン酸ジメチル、マロン酸ジエチル、マロン酸ジプロピル、マロン酸ジブチル、マロン酸ビス（フルオロメチル）、マロン酸ビス（ジフルオロメチル）、マロン酸ビス（トリフルオロメチル）等のマロン酸ジエステルである。無論、これらの具体的化合物の誘導体も同様に好ましく使用することができる。
【００２１】
使用するジカルボン酸ジエステル及びその誘導体は上記非水系溶媒に溶解するものが好ましい。溶解度が低すぎると過充電防止剤として作用するための有効な添加量が得られないという問題が生じることがある。また、その沸点は、通常１００℃以上、好ましくは１２０℃以上である。沸点が低すぎると、電池内部で揮発し、電池使用時に膨れが生じるとか、添加剤として有効に作用しないという問題点が生じることがある。
【００２２】
使用するジカルボン酸ジエステル及びその誘導体は、無論複数種を併用することができる。
ジカルボン酸ジエステル又はその誘導体の含有量は、前記溶媒に対して５重量％以下とするが、好ましくは３重量％以下、さらに好ましくは２重量％以下とする。含有量が多すぎると電池特性に悪影響を及ぼすという問題点が生じることがある。ただし、含有量が少なすぎると過充電防止剤として有効に作用しないことがあるので、通常、０．１重量％以上、好ましくは０．２５重量％以上、さらに好ましくは０．５％重量以上とする。
【００２３】
これら化合物が、少量の添加で過充電防止効果を有する理由については明らかではないが、恐らく過充電時に負極に生成したＬｉ金属とジカルボン酸ジエステルとが電池内で反応し、過充電の進行をくい止めているのであろうと推察している。
さて、本発明においては上記ジカルボン酸ジエステルと、従来公知の過充電電位領域で酸化される分子量５００以下の芳香族化合物とを併用することによって、より優れた過充電防止効果が発揮される。該芳香族化合物を単独で電解液に添加した場合には、過充電時に負極で生じたデンドライト状の析出Ｌｉと、正極上に生じた重合膜とが短絡し、電池内部で大きな発熱につながることがある。この発熱は電池の暴走反応の引き金になり、危険である。これに対し、本発明のようにジカルボン酸ジエステルを併用した場合には、過充電時に負極上でのデンドライト状Ｌｉの析出が抑制されるか、あるいは前記芳香族化合物の酸化生成物との反応で非常に大きな抵抗成分が生じる結果、短絡が生じにくくなり、そのため、より安全な過充電防止効果が発揮されているものと推察される。
【００２４】
上記の従来公知の過充電電位領域で酸化される芳香族化合物として好適な化合物としては、下記一般式（３）：
【００２５】
【化５】

【００２６】
（式中、Ｒ₁〜Ｒ₆は水素原子、ハロゲン原子、炭素数１から１０の鎖状アルキル基、炭素数４から１０の環状アルキル基あるいは置換基を有していてもよいフェニル基を表し、相互に結合して環を形成していてもよい。）
で表される化合物、及び下記一般式（４）：
【００２７】
【化６】

【００２８】
（式中、Ｒ₁は炭素数１から１０の鎖状アルキル基、炭素数４から１０の環状アルキル基あるいは置換基を有していてもよいフェニル基を表し、Ｒ₂〜Ｒ₆は水素原子、ハロゲン原子、炭素数１から１０の鎖状アルキル基、炭素数４から１０の環状アルキル基あるいは置換基を有していてもよいフェニル基を表す。Ｒ₁〜Ｒ₆は相互に結合して環を形成していてもよい。）
で表される化合物が、挙げられる。
【００２９】
さらに、それら芳香族化合物の中で、その酸化電位が４．３〜４．９Ｖの範囲にあるものが好ましい。ここで酸化電位は、下記のサイクリックボルタンメトリー法によって測定することができる。
［酸化電位の測定法］
底面部分のみ露出した１．６ｍｍφの白金を作用極、リチウム金属を対極および参照極とした、ガラスフィルターで作用極側と対極側が区切られたＨ型セルを用いて、ＥＣとＤＥＣとの体積比率７：３の混合溶媒にＬｉＰＦ₆を１ｍｏｌ／Ｌの濃度で溶解した電解液に試料となる芳香族化合物を０．１５ｍｍｏｌ／ｇ添加したものをこのセルに入れる。次いで、作用極の電位を酸化側（貴側に）に２０ｍＶ／秒の掃引速度で掃引する。このとき０．５ｍＡ／ｃｍ²の電流密度が流れ出す電位を酸化開始電位と規定する。測定は便宜上室温（２５℃付近）で行う。
【００３０】
以上の方法によって測定される、芳香族化合物の酸化電位は、４．９Ｖ以下、好ましくは４．７Ｖ以下である。酸化電位が高すぎると過充電防止効果が小さくなる傾向にある。ただし、あまりに酸化電位が低いと通常条件の電池使用時に反応して電池特性を劣化させることがあるので、酸化電位は、４．３Ｖ以上、好ましくは４．４Ｖ以上、さらに好ましくは４．５Ｖ以上とする。
【００３１】
上記条件にかなう好適な化合物として、例えばビフェニル及びその誘導体、シクロヘキシルベンゼン及びその誘導体、ジベンゾフラン及びその誘導体、ターフェニル及びその誘導体、ジフェニルエーテル及びその誘導体等を挙げることができる。
上記分子量５００以下の芳香族化合物の使用量は、溶媒に対して通常０．１〜１０重量％である。
【００３２】
更に、上記ジカルボン酸ジエステル及びその誘導体、及び分子量５００以下の芳香族化合物に加えてビニレンカーボネート又はビニルエチレンカーボネートを併用すると、一層優れた過充電防止効果を発揮させるだけでなく、電池の保存安定性やサイクル特性などの他特性も向上させることが可能となるので好ましい。
ビニレンカーボネート及びビニルエチレンカーボネートの使用量は、溶媒に対して通常０．１〜１０重量％である。
【００３３】
本発明の電解液は、溶媒中にリチウム塩を含有する。リチウム塩としてはＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＢ（Ｃ₆Ｈ₅）₄、ＬｉＣｌ、ＬｉＢｒ、ＬｉＣＨ₃ＳＯ₃、ＬｉＣＦ₃ＳＯ₃、ＬｉＮ（ＳＯ₂ＣＦ₃）₂、ＬｉＮ（ＳＯ₂Ｃ₂Ｆ₅）₂、ＬｉＣ（ＳＯ₂ＣＦ₃）₃、ＬｉＮ（ＳＯ₃ＣＦ₃）₂等を挙げることができる。無論、これらを２種以上混合して用いてもよい。上記の中でも、ＬｉＢＦ₄及びＬｉＰＦ₆を使用するのが好ましい。リチウム塩の濃度は、電解液全体に対して、通常０．５〜１．５モル／ｌ、好ましくは０．７５〜１．２５モル／ｌである。リチウム塩濃度が高すぎても低すぎても電導度の低下が起き、電池特性に悪影響があることがある。
【００３４】
電解液は、必要に応じてさらに他の成分を含有することができる。他の成分としては、例えば、電池の活物質表面に被膜（ＳＥＩ）を形成するための各種の添加剤や界面活性剤を挙げることができる。
本発明の電解液は、リチウム二次電池等の二次電池に用いることができる。
本発明の二次電池は、正極、負極及び前記電解液を含んで構成される。前記電解液は、通常、正極と負極との間の電解質層の成分として用いられるが、過充電時の安全性を向上させることができれば、電池のどこに用いられていてもよい。
【００３５】
本発明の二次電池を構成する正極の活物質としては、好ましくはリチウム遷移金属複合酸化物を使用する。リチウム遷移金属複合酸化物としては、ＬｉＣｏＯ₂等のリチウムコバルト複合酸化物、ＬｉＮｉＯ₂等のリチウムニッケル複合酸化物、ＬｉＭｎ₂Ｏ₄等のリチウムマンガン複合酸化物等を挙げることができるが、本発明は特に、正極の活物質としてリチウム含有量の大きいコバルト系及びニッケル系のリチウム遷移金属複合酸化物、即ちリチウムコバルト複合酸化物及びリチウムニッケル複合酸化物を用いる場合に効果的である。これらリチウム遷移金属複合酸化物は、主体となる遷移金属元素の一部をＡｌ、Ｔｉ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｌｉ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ、Ｚｒ等の他の金属種で置き換えることにより安定化させることもでき、また好ましい。無論、正極の活物質を複数種併用することもできる。
【００３６】
本発明の二次電池を構成する負極の活物質としては、リチウムを吸蔵及び放出し得る物質を用いることができるが、炭素質物が好ましい。該炭素質物の具体例としては、例えば様々な熱分解条件での有機物の熱分解物や、人造黒鉛、天然黒鉛等が挙げられる。好適には種々の原料から得た易黒鉛性ピッチの高温熱処理によって製造された人造黒鉛並びに黒鉛化メソフェーズ小球体、黒鉛化メソフェーズピッチ系炭素繊維等の他の人造黒鉛及び精製天然黒鉛、或いはこれらの黒鉛にピッチを含む種々の表面処理を施した材料が使用される。これらの炭素質物は、学振法によるＸ線回折で求めた格子面（００２面）のｄ値（層間距離）が０．３３５〜０．３４ｎｍであるものが好ましく、０．３３５〜０．３３７ｎｍであるものがより好ましい。灰分は１重量％以下であるのが好ましく、０．５重量％以下であるのがより好ましく、０．１重量％以下であるのが特に好ましい。また、学振法によるＸ線回折で求めた結晶子サイズ（Ｌｃ）が３０ｎｍ以上であるのが好ましく、５０ｎｍ以上であるのがより好ましく、１００ｎｍ以上であるのが特に好ましい。これらの炭素質物にリチウムを吸蔵・放出可能な他の活物質を更に混合して用いることもできる。炭素質物以外のリチウムを吸蔵・放出可能な活物質としては、酸化錫、酸化珪素等の金属酸化物材料、更にはリチウム金属並びに種々のリチウム合金を例示することができる。これらの負極材料は二種類以上混合して用いてもよい。
【００３７】
上記正極及び負極は、それぞれ、通常、上記活物質と結着剤とを含有する。結着剤としては、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、スチレン・ブタジエンゴム、イソプレンゴム、ブタジエンゴム等を挙げることができる。さらに必要に応じて、電極中に、銅やニッケル等の金属材料、グラファイト、カーボンブラック等のような炭素材料等の導電材を含有させることもできる。特に正極については、導電材を含有させるのが好ましい。
【００３８】
電極を製造する方法については、特に限定されない。例えば、活物質に、必要に応じて結着剤、増粘剤、導電材、溶媒等を加えてスラリー状とし、集電体の基板に塗布し、乾燥することにより製造することができるし、また、該活物質をそのままロール成形してシート電極としたり、圧縮成形によりペレット電極とすることもできる。増粘剤としては、カルボキシメチルセルロース、メチルセルロース、ヒドロキシメチルセルロース、エチルセルロース、ポリビニルアルコール、酸化スターチ、リン酸化スターチ、カゼイン等が挙げられる。
【００３９】
電極に使用できる集電体としては、負極集電体として、銅、ニッケル、ステンレス等の金属又は合金、好ましくは銅を挙げることができ、また、正極集電体として、アルミニウム、チタン、タンタル等の金属又は合金、好ましくはアルミニウム及びその合金を挙げることができる。
二次電池においては、通常、正極と負極との間にセパレータが介装される。使用するセパレータの材質や形状については、特に限定されないが、電解液に対して安定で、保液性の優れた材料として、ポリエチレン、ポリプロピレン等のポリオレフィンを原料とする多孔性シート又は不織布等を用いるのが好ましい。
【００４０】
少なくとも負極、正極及び非水系電解液を有する本発明に係る非水系二次電池を製造する方法については、特に限定されず、通常採用されている方法の中から適宜選択することができる。
また、電池の形状については特に限定されず、シート電極及びセパレータをスパイラル状にしたシリンダータイプ、ペレット電極及びセパレータを組み合わせたインサイドアウト構造のシリンダータイプ、ペレット電極及びセパレータを積層したコインタイプ等が使用可能である。
【００４１】
【実施例】
以下、本発明の具体的態様を実施例により詳細に説明するが、本発明はその要旨を越えない限り、以下の実施例によって限定されるものではない。
［正極の作製］
正極は、正極活物質としてのコバルト酸リチウム（ＬｉＣｏＯ₂）９０重量％と、導電剤としてのアセチレンブラック５重量％と、結着剤としてのポリフッ化ビニリデン（ＰＶｄＦ）５重量％とを、Ｎ−メチルピロリドン溶媒中で混合して、スラリー化した後、２０μｍのアルミ箔の片面に塗布して乾燥し、さらにプレス機で圧延したものを直径１２ｍｍの打ち抜きポンチで打ち抜いて作製した。
［負極の作製］
負極は、負極活物質としての黒鉛（面間隔０．３３６ｎｍ）９５重量％と結着剤のポリフッ化ビニリデン（ＰＶｄＦ）５重量％とを、Ｎ−メチルピロリドン溶媒中で混合して、スラリー化した後、２０μｍ厚さの銅箔の片面に塗布して乾燥し、さらにプレス機で圧延したものを直径１２ｍｍで打ち抜いて作製した。
［電池の組立]
アルゴン雰囲気のドライボックス内で、ＣＲ２０３２型コインセルを使用して、リチウム二次電池を作製した。即ち、正極缶の上に正極を置き、その上にセパレータとして２５μｍの多孔性ポリエチレンフィルムを置き、ポリプロピレン製ガスケットで押さえた後、負極を置き、厚み調整用のスペーサーを置いた後、電解液を加え、電池内に十分滲みこませた後、負極缶を載せて電池を封口した。なお、実施例および比較例で電池の容量は、充電上限４．２Ｖ、放電下限３．０Ｖで約４．０ｍＡｈになる設計とした。
【００４２】
この際、正極活物質の重量Ｗ（ｃ）と負極活物質の重量Ｗ（ａ）との比率は、二次電池の通常の使用上限電圧において、正極から放出されるリチウムイオンが、対向する負極上でリチウム金属の析出を起こさない範囲が好ましいので、負極と正極との容量比Ｒｑが、１．１≦Ｒｑ≦１．２、となるように、その重量を決定した。なお、容量比Ｒｑは、Ｑ（ａ）×Ｗ（ａ）／｛Ｑ（ｃ）×Ｗ（ｃ）｝、で求めた。ここで、電池の初期充電条件に対応する条件下での、正極活物質の重量当たりの電気容量をＱ（ｃ）ｍＡｈ／ｇ、リチウム金属が析出することなしにリチウムを最大限に吸蔵しうる負極活物質の重量当たりの電気容量をＱ（ａ）ｍＡｈ／ｇとした。Ｑ（ｃ）及びＱ（ａ）は、正極あるいは負極を作用極に、対極にリチウム金属を用い、上記電池を組み立てる際と同じ電解液中でセパレータを介して試験セルを組んで測定した。すなわち目的とする電池系の初期充電条件に対応する正極の上限電位あるいは負極の下限電位まで、可能な限り低い電流密度で、正極が充電（正極からのリチウムイオンの放出）できる容量、負極が放電（負極へのリチウムイオンの吸蔵）できる容量として求めた。
【００４３】
［電池の評価］
電池の評価は、（１）初期充放電（容量確認）、次いで（２）満充電操作、さらに（３）過充電試験の順に行った。
初期充放電（容量確認）においては、１Ｃ（４．０ｍＡ）、４．２Ｖ上限の定電流定電圧法により充電した。充電のカットは、電流値が０．０５ｍＡに到達した時点とした。放電は０．２Ｃで３．０Ｖまで定電流で行った。
【００４４】
満充電操作は、４．２Ｖ上限の定電流定電圧法（０．０５ｍＡカット）により充電した。
過充電試験は、１Ｃで４．９９Ｖカット又は３ｈｒカット（どちらか先に到達した方でカット）とした。
過充電防止効果の優劣を見る指標としては、過充電後のコインセルを解体し、正極中に残存しているＬｉを元素分析で定量した値を、過充電深度として用いた。過充電試験後の正極組成をＬｉ_xＣｏＯ₂と表す時、ｘ（正極Ｌｉ残存量）が大きいほど過充電が進んでおらず、過充電防止効果が高いことになる。
【００４５】
ここで、ｘ（正極Ｌｉ残存量）は元素分析（ＩＣＰ発光分析）により求めた正極中のＣｏと正味のＬｉとのモル数比より求めた。なお、正味のＬｉのモル数は同様の分析で正極中のリン（Ｐ）の定量も行い、これをＬｉＰＦ₆によるものとし、正極中の全Ｌｉモル数からＬｉＰＦ₆に相当するＬｉモル数を差し引いて求めた。
実施例１
電解液として、エチレンカーボネート（ＥＣ）及びジエチルカーボネート（ＤＥＣ）の体積比３：７の混合溶媒に、１モル／リットルの濃度でヘキサフルオロリン酸リチウム（ＬｉＰＦ₆）を溶解させた電解液に添加剤として２重量％のマロン酸ジメチル及び２重量％のジベンゾフランを添加したものを用いた。
【００４６】
前記方法により製造したリチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
実施例２
実施例１において添加した２重量％のマロン酸ジメチル及び２重量％のジベンゾフランの他に２重量％のビニレンカーボネートを更に添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
実施例３
実施例１において添加した２重量％のマロン酸ジメチルの代わりに２重量％のシュウ酸ジエチルを添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
比較例１
実施例１において添加剤を加えなかったこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
比較例２
実施例１において添加剤として２重量％のマロン酸ジメチルのみを添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
比較例３
比較例２において添加剤として２重量％のマロン酸ジメチルの代わりに２重量％のジベンゾフランを添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。なお、過充電時に短絡によると思われる電圧振動が観測され、見かけの過充電電流量は大きくなった。
比較例４
比較例２において添加剤として２重量％のマロン酸ジメチルの代わりに２重量％のビニレンカーボネートを添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。
比較例５
比較例２において添加剤として２重量％のマロン酸ジメチルの代わりに２重量％のジベンゾフラン及び２重量％のビニレンカーボネートを添加したこと以外は同様にして、リチウム二次電池の評価、および過充電後の電池を解体しての電極中のＬｉ分析を行った。結果を表−１に示す。なお比較例３と同様の、過充電時に短絡によると思われる電圧振動が観測され、見かけの過充電電流量は大きくなった。
【００４７】
【表１】

【００４８】
表−１より、ジカルボン酸ジエステル(但し、コハク酸ジエステルを除く)又はその誘導体と分子量５００以下の芳香族化合物とを共に添加することによって、過充電時の安全性が向上することが分かる。
なお、実施例で作成したリチウム二次電池と比較例で作成したリチウム二次電池とでは、初期放電容量、５サイクル後の容量維持率等の電池特性には大きな差は見られなかった。
【００４９】
【発明の効果】
本発明によれば、サイクル特性、レート特性、容量等各種の電池特性を向上させることが可能な電解液を提供することができる。特に、新規な過充電防止剤によって過充電時の安全性を向上させることができる電解液を提供することができる。
【００５０】
また、本発明によれば、サイクル特性、レート特性、容量等各種の電池特性を向上した電池を提供することができる。特に、新規な過充電防止剤によって過充電時の安全性を向上させた電池を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrolytic solution and a secondary battery. Specifically, the present invention relates to a secondary battery having improved safety under overcharge conditions and an electrolyte used for the secondary battery.
[0002]
[Prior art]
For example, as an electrolytic solution for a lithium secondary battery, an electrolytic solution in which a lithium salt is dissolved in a solvent mainly composed of a non-aqueous solvent such as carbonate ester, ether, and lactone is known. These non-aqueous solvents are excellent in battery characteristics such as high dielectric constant and high oxidation potential, and thus excellent stability during battery use.
[0003]
On the other hand, an electrolytic solution using a non-aqueous solvent as described above can be used at a high potential because of the high stability of the non-aqueous solvent. The so-called overcharge phenomenon that becomes a voltage tends to be a problem. When overcharged, not only battery deformation and heat generation, but in severe cases can lead to phenomena such as ignition and rupture, so it is important to improve the safety of secondary batteries during overcharging. .
[0004]
In particular, lithium transition metal composite oxides such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) having a layered structure due to their large capacity per weight as a positive electrode active material of a lithium secondary battery However, these compounds become unstable with the lithium ions almost desorbed in an overcharged state, causing an abrupt exothermic reaction with the electrolyte, or lithium metal on the negative electrode. Since it may be deposited, safety during overcharge is very important.
[0005]
Conventionally, as an attempt to improve the safety at the time of such overcharge, a method of cutting off current by adding an overcharge inhibitor into an electrolyte is known. That is, as an overcharge inhibitor, an aromatic compound such as biphenyl having an oxidation potential equal to or higher than the upper limit voltage value of the battery is added to the electrolyte, and when the overcharge state is reached, the aromatic compound is oxidized and polymerized. In this method, a high resistance film is formed on the surface of the active material to suppress the overcharge current and stop the progress of overcharge (for example, Japanese Patent Laid-Open No. 9-106835, Japanese Patent No. 2939469, Japanese Patent No. 2983205). (Publication etc.)
[0006]
[Problems to be solved by the invention]
However, the current situation is that the above overcharge prevention method is not sufficient. For example, the overcharge preventive agent biphenyl, 3-chlorothiophene, furan, etc. described in JP-A-9-106835 may adversely affect the battery characteristics, and the overcharge described in Japanese Patent No. 2939469. The terphenyl derivative, which is an inhibitor, may cause a decrease in battery performance due to low solubility in the electrolyte solution. Furthermore, the overcharge inhibitor diphenyl ether described in Japanese Patent No. 2983205 has an irritating odor. It had the problem of being difficult to handle strongly.
[0007]
Therefore, a new overcharge preventing agent having a sufficient overcharge preventing effect has been demanded.
[0008]
[Means for Solving the Problems]
The present invention has been made in view of the above-mentioned problems, and its purpose is to prevent overcharge more effectively using an effective overcharge inhibitor and to improve safety during overcharge. is there.
As a result of intensive studies to achieve the above object, the present inventors have found that as an overcharge inhibitor, dicarboxylic acid diesters and derivatives thereof that are not conventionally known aromatic compounds, and conventionally known aromatic compound-based compounds. When used in combination with an overcharge inhibitor, a superior effect of preventing overcharge is exhibited, and a relatively small amount of the above dicarboxylic acid relative to a solvent mainly composed of a non-aqueous solvent such as carbonate ester, ether and lactone. It has been found that sufficient safety during overcharging can be ensured by using a diester and a derivative thereof together with a conventionally known aromatic compound-based overcharge inhibitor, and the present invention has been completed based on this finding.
[0009]
That is, the gist of the present invention is an electrolyte solution for a secondary battery in which a lithium salt is dissolved in a solvent mainly composed of at least one non-aqueous solvent selected from the group consisting of carbonate ester, ether and lactone. , Dimethyl oxalate, diethyl oxalate, dipropyl oxalate, dibutyl oxalate, bis (fluoromethyl) oxalate, bis (difluoromethyl) oxalate, bis (trifluoromethyl) oxalate, dimethyl malonate, diethyl malonate, malonic acid dipropyl, dibutyl malonate, malonate bis (fluoromethyl), malonic acid bis (difluoromethyl), and a dicarboxylic acid ester le or a derivative thereof is selected from the group consisting of malonic acid bis (trifluoromethyl), molecular weight oxidation potential of 4.3~4.9V be 500 or less and an aromatic compound Electrolyte solution for a secondary battery, characterized by having, resides in.
[0010]
Another gist of the present invention resides in a secondary battery comprising the above-described electrolytic solution, a positive electrode, and a negative electrode.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
The solvent used for the electrolytic solution of the present invention is mainly composed of at least one non-aqueous solvent selected from the group consisting of carbonate ester, ether and lactone. The content of these non-aqueous solvents is usually 50% by weight or more, preferably 80% by weight or more, and more preferably 100% by weight with respect to the whole solvent. When the proportion of the non-aqueous solvent is too small, there may be a problem that the electrical conductivity of the electrolytic solution is lowered or the deterioration due to the oxidation-reduction reaction of the electrolytic solution is large.
[0012]
Examples of carbonates that can be used as the non-aqueous solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Examples thereof include chain carbonate esters.
Examples of ethers that can be used as the non-aqueous solvent include dimethoxyethane (DME) and diethoxyethane (DEE).
[0013]
Examples of the lactone that can be used as the non-aqueous solvent include γ-butyrolactone (GBL) and γ-valerolactone.
The non-aqueous solvent may be at least one of carbonate ester, ether and lactone, but preferably contains carbonate ester. Of course, these two or more kinds can be used in combination. Particularly preferred is a mixed solvent of a cyclic carbonate such as PC or EC or a lactone such as GBL which is a high dielectric constant solvent and a chain carbonate such as DMC, DEC or EMC which is a low viscosity solvent.
[0014]
The present invention is characterized in that a dicarboxylic acid diester (excluding succinic acid diester) and derivatives thereof and an aromatic compound having a molecular weight of 500 or less are contained as an overcharge inhibitor in the electrolytic solution. As the dicarboxylic acid diester, a dicarboxylic acid dialkyl ester is preferably used.
As a compound suitable as the dicarboxylic acid diester, the following general formula (1):
[0015]
[Chemical 3]

[0016]
(In the formula, R ₁ and R ₂ represent an alkyl group having 1 to 10 carbon atoms or a halogen-substituted alkyl group, and n is an integer of 0 to 1 and 3 to 10).
And a saturated dicarboxylic acid diester represented by the following general formula (2):
[0017]
[Formula 4]

[0018]
(In the formula, R ₃ and R ₄ represent an alkyl group having 1 to 10 carbon atoms or a halogen-substituted alkyl group, and p and q are each an integer of 0 to 5, and 0 ≦ p + q ≦ 10.)
And unsaturated dicarboxylic acid diesters represented by:
R _{1 to} R ₄ in the general formulas (1) and (2) are an alkyl group having 1 to 10 carbon atoms or a halogen-substituted alkyl group. Specifically, methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, s-butyl group, t-butyl group, pentyl group, isopentyl group, neopentyl group, hexyl group, heptyl group, octyl group , Nonyl group, decyl group, fluoromethyl group, difluoromethyl group, trifluoromethyl group, trifluoroethyl group, pentafluoropropyl group, trifluoroethyl group, heptafluorobutyl group, nonafluoropentyl group, tetrafluoropropyl group, Hexafluorobutyl group, octafluoropentyl group, propylfluoromethyl group, propyldifluoromethyl group, propyltrifluoromethyl group, butylfluoromethyl group, butyldifluoromethyl group, butyltrifluoromethyl group, pentafluorobutyl group, heptafluoro Pentyl group, and the like can be mentioned. Moreover, n in the said General formula (1) is an integer of 0-1 and 3-10, p and q in General formula (2) are the integers of 0-5, respectively, and 0 <= p + q <= 10 The following relational expression is satisfied. If the number of carbon atoms, n, and p + q of R _{1 to} R ₄ exceeds 10, the solubility in the solvent tends to be reduced, and the overcharge prevention effect may be reduced.
[0019]
The dicarboxylic acid diester is not particularly limited as long as it is a compound having a molecular skeleton of dicarboxylic acid diester, except for succinic acid diester.
Moreover, these derivatives can also be used. Examples of the derivatives include various compounds having the dicarboxylic acid diester skeleton, such as those obtained by substituting a part of hydrogen atoms of the dicarboxylic acid diester with a substituent. Examples of the substituent include a halogen atom, an oxygen atom, a sulfur atom, an amino group, an alkylamino group, an arylamino group, a carboxylicamino group, a sulfonamide group, an oxycarbonylamino group, an oxysulfonylamino group, a ureido group, Hydroxyl group, mercapto group, methoxyl group, lower alkyl group having 1 to 3 carbon atoms, cycloalkyl group, alkoxy group, alkenyl group, alkynyl group, aralkyl group, aryl group, cyano group, nitro group, formyl group, aryloxy group , Alkylthio group, acrylic group, arylthio group, acyloxy group, sulfonyloxy group, acyl group, oxycarbonyl group, carbamoyl group, sulfonyl group, sulfinyl group, oxysulfinyl group, sulfamoyl group, carboxylic acid group or salt thereof, sulfonic acid group Or Salts, may be mentioned phosphonic acid group or a salt thereof, a heterocyclic residue, or hydroxyl group or the like. In addition, carbon number of the said substituent is 10 or less normally, Preferably it is 5 or less.
[0020]
Specific examples of dicarboxylic acid diesters and derivatives thereof used include dimethyl oxalate, diethyl oxalate, dipropyl oxalate, dibutyl oxalate, bis (fluoromethyl) oxalate, bis (difluoromethyl) oxalate, and bis (oxalate) ( Oxalic acid diesters such as trifluoromethyl), dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, bis (fluoromethyl) malonate, bis (difluoromethyl) malonate, bis (trifluoromethyl) malonate Maleic acid such as malonic acid diester, dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, bis (fluoromethyl) maleate, bis (difluoromethyl) maleate, bis (trifluoromethyl) maleate, etc. Jies , Fumaric acid diesters such as dimethyl fumarate, diethyl fumarate, dipropyl fumarate, dibutyl fumarate, bis (fluoromethyl) fumarate, bis (difluoromethyl) fumarate, bis (trifluoromethyl) fumarate, glutaric acid Dimethyl, diethyl glutarate, dipropyl glutarate, dibutyl glutarate, bis (fluoromethyl) glutarate, bis (difluoromethyl) glutarate, bis (trifluoromethyl) glutarate, dimethyl adipate, dimethyl adipate Adipic acid diesters such as diethyl, dipropyl adipate, dibutyl adipate, bis (fluoromethyl) adipate, bis (difluoromethyl) adipate, bis (trifluoromethyl) adipate, dimethyl pimelate, dieme pimelate Pimelic acid diesters such as bis (fluoromethyl) pimelate, bis (difluoromethyl) pimelate, bis (trifluoromethyl) pimelate, dimethyl suberate, diethyl suberate, dipropyl suberate, dibutyl suberate, suberic acid Suberic acid diesters such as bis (fluoromethyl), bis (difluoromethyl) suberate, bis (trifluoromethyl) suberate, dimethyl azelate, diethyl azelate, dipropyl azelate, dibutyl azelate, bis (fluoromethyl azelate) ), Azelaic acid diesters such as bis (difluoromethyl) azelate, bis (trifluoromethyl) azelate, dimethyl sebacate, diethyl sebacate, dipropyl sebacate, dibutyl sebacate, seba Sebacate diester such as bis (fluoromethyl) citrate, bis (difluoromethyl) sebacate, bis (trifluoromethyl) sebacate, dimethyl undecandioate, diethyl undecandioate, dipropyl undecandioate, dibutyl undecandioate, Undecanedioic acid diesters such as undecanedioic acid bis (fluoromethyl), undecanedioic acid bis (difluoromethyl), undecanedioic acid bis (trifluoromethyl), dodecanedioic acid dimethyl, dodecanedioic acid diethyl, dodecanedioic acid dipropyl, dodecane Examples include dibutyl diacid, dodecanedioic acid bis (fluoromethyl), dodecanedioic acid bis (difluoromethyl), dodecanedioic acid diester such as bis (trifluoromethyl) dodecanedioic acid, diethyl acetonedicarboxylate, and the like. To these But it is not limited. Among them, oxalic acid diesters such as dimethyl oxalate, diethyl oxalate, dipropyl oxalate, dibutyl oxalate, bis (fluoromethyl) oxalate, bis (difluoromethyl) oxalate, and bis (trifluoromethyl) oxalate, malonic acid Dimethyl malonate such as dimethyl, diethyl malonate, dipropyl malonate, dibutyl malonate, bis (fluoromethyl) malonate, bis (difluoromethyl) malonate, bis (trifluoromethyl) malonate, dimethyl maleate, maleic acid Maleic acid diesters such as diethyl, dipropyl maleate, dibutyl maleate, bis (fluoromethyl) maleate, bis (difluoromethyl) maleate, bis (trifluoromethyl) maleate, dimethyl fumarate, diethyl fumarate, fumaric acid Zip Pyr, dibutyl fumarate, bis (fluoromethyl) fumarate, bis (difluoromethyl) fumarate, diester fumarates such as bis (trifluoromethyl) fumarate, dimethyl glutarate, diethyl glutarate, dipropyl glutarate, glutaric acid Glutaric acid diesters such as dibutyl, bis (fluoromethyl) glutarate, bis (difluoromethyl) glutarate, and bis (trifluoromethyl) glutarate are preferred, and dimethyl malonate, diethyl malonate, dipropyl malonate, and malon are more preferred. Dibutyl malate, bis (fluoromethyl) malonate, bis (difluoromethyl) malonate, diester malonate such as bis (trifluoromethyl) malonate, dimethyl maleate, diethyl maleate, dipropyl maleate, dibuty maleate , Maleic acid diesters such as bis (fluoromethyl) maleate, bis (difluoromethyl) maleate, bis (trifluoromethyl) maleate, dimethyl fumarate, diethyl fumarate, dipropyl fumarate, dibutyl fumarate, bis fumarate (Fluoromethyl), bis (difluoromethyl) fumarate, bis (trifluoromethyl) fumarate, and the like, most preferably dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, malonic acid Malonic acid diesters such as bis (fluoromethyl), bis (difluoromethyl) malonate, and bis (trifluoromethyl) malonate. Of course, derivatives of these specific compounds can also be preferably used.
[0021]
The dicarboxylic acid diester and derivatives thereof used are preferably those that are soluble in the non-aqueous solvent. If the solubility is too low, there may be a problem that an effective addition amount for acting as an overcharge inhibitor cannot be obtained. Moreover, the boiling point is 100 degreeC or more normally, Preferably it is 120 degreeC or more. If the boiling point is too low, it may volatilize inside the battery, causing problems such as swelling during use of the battery or ineffective operation as an additive.
[0022]
Of course, a plurality of dicarboxylic acid diesters and derivatives thereof can be used in combination.
The content of dicarboxylic acid diester or a derivative thereof is 5% by weight or less, preferably 3% by weight or less, more preferably 2% by weight or less based on the solvent. If the content is too large, there may be a problem that the battery characteristics are adversely affected. However, if the content is too small, it may not act effectively as an overcharge inhibitor, and is usually 0.1% by weight or more, preferably 0.25% by weight or more, more preferably 0.5% by weight or more. To do.
[0023]
It is not clear why these compounds have the effect of preventing overcharge with a small amount of addition, but the Li metal produced on the negative electrode and the dicarboxylic acid diester at the time of overcharge probably react in the battery, preventing the progress of overcharge. I guess it is.
In the present invention, by using the dicarboxylic acid diester in combination with an aromatic compound having a molecular weight of 500 or less that is oxidized in a conventionally known overcharge potential region, a more excellent overcharge prevention effect is exhibited. When the aromatic compound is added alone to the electrolyte, the dendrite-like deposited Li generated on the negative electrode during overcharge and the polymer film formed on the positive electrode are short-circuited, leading to large heat generation inside the battery. There is. This heat generation is dangerous because it triggers the runaway reaction of the battery. On the other hand, when the dicarboxylic acid diester is used in combination as in the present invention, the precipitation of dendritic Li on the negative electrode during overcharge is suppressed, or the reaction with the oxidation product of the aromatic compound As a result of the generation of a very large resistance component, it is difficult to cause a short circuit. Therefore, it is presumed that a safer overcharge prevention effect is exhibited.
[0024]
As a compound suitable as the aromatic compound that is oxidized in the above-described conventionally known overcharge potential region, the following general formula (3):
[0025]
[Chemical formula 5]

[0026]
(Wherein R _{1 to} R ₆ represent a hydrogen atom, a halogen atom, a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or an optionally substituted phenyl group. And may be bonded to each other to form a ring.)
And the following general formula (4):
[0027]
[Chemical 6]

[0028]
(In the formula, R ₁ represents a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or an optionally substituted phenyl group, and R _{2 to} R ₆ represent hydrogen atoms. , A halogen atom, a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or a phenyl group which may have a substituent, R _{1 to} R ₆ are bonded to each other. A ring may be formed.)
The compound represented by these is mentioned.
[0029]
Further, among these aromatic compounds, those having an oxidation potential in the range of 4.3 to 4.9 V are preferable. Here, the oxidation potential can be measured by the following cyclic voltammetry method.
[Measurement method of oxidation potential]
Volume ratio of EC and DEC using an H-type cell in which the working electrode side and the counter electrode side are separated by a glass filter, with 1.6 mmφ platinum exposed only at the bottom and working electrode of lithium metal as the counter electrode and reference electrode A sample prepared by adding 0.15 mmol / g of an aromatic compound as a sample to an electrolytic solution in which LiPF ₆ is dissolved in a 7: 3 mixed solvent at a concentration of 1 mol / L is placed in this cell. Next, the potential of the working electrode is swept to the oxidation side (to the noble side) at a sweep rate of 20 mV / sec. At this time, a potential at which a current density of 0.5 mA / cm ² starts flowing is defined as an oxidation start potential. The measurement is performed at room temperature (around 25 ° C.) for convenience.
[0030]
The oxidation potential of the aromatic compound measured by the above method is 4.9 V or less, preferably 4.7 V or less. If the oxidation potential is too high, the overcharge prevention effect tends to be small. However, if the oxidation potential is too low, it may react when the battery is used under normal conditions to deteriorate the battery characteristics. Therefore, the oxidation potential is 4.3 V or higher, preferably 4.4 V or higher, more preferably 4.5 V or higher. And
[0031]
Examples of suitable compounds meeting the above conditions include biphenyl and derivatives thereof, cyclohexylbenzene and derivatives thereof, dibenzofuran and derivatives thereof, terphenyl and derivatives thereof, diphenyl ether and derivatives thereof, and the like.
The usage-amount of the said aromatic compound with a molecular weight of 500 or less is 0.1 to 10 weight% normally with respect to a solvent.
[0032]
Furthermore, when vinylene carbonate or vinyl ethylene carbonate is used in combination with the dicarboxylic acid diester and its derivatives, and an aromatic compound having a molecular weight of 500 or less, not only a more excellent overcharge prevention effect is exhibited, but also the storage stability of the battery. And other characteristics such as cycle characteristics can be improved, which is preferable.
The usage-amount of vinylene carbonate and vinyl ethylene carbonate is 0.1 to 10 weight% normally with respect to a solvent.
[0033]
The electrolytic solution of the present invention contains a lithium salt in a solvent. Lithium salts include LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN ( _{SO 2 C 2 F 5) 2} , LiC (SO 2 CF 3) 3, LiN (SO 3 CF 3) may be mentioned _2. Of course, two or more of these may be used in combination. Among the above, it is preferable to use LiBF ₄ and LiPF ₆ . The density | concentration of lithium salt is 0.5-1.5 mol / l normally with respect to the whole electrolyte solution, Preferably it is 0.75-1.25 mol / l. If the lithium salt concentration is too high or too low, the conductivity may be lowered, and the battery characteristics may be adversely affected.
[0034]
The electrolytic solution can further contain other components as necessary. Examples of other components include various additives and surfactants for forming a film (SEI) on the active material surface of the battery.
The electrolytic solution of the present invention can be used for a secondary battery such as a lithium secondary battery.
The secondary battery of the present invention includes a positive electrode, a negative electrode, and the electrolytic solution. The electrolytic solution is usually used as a component of the electrolyte layer between the positive electrode and the negative electrode, but may be used anywhere in the battery as long as the safety during overcharging can be improved.
[0035]
As the positive electrode active material constituting the secondary battery of the present invention, a lithium transition metal composite oxide is preferably used. Examples of the lithium transition metal complex oxide, lithium cobalt composite oxide such as LiCoO _2, lithium-nickel composite oxide such as LiNiO _2, there may be mentioned lithium manganese complex oxides such as LiMn ₂ O _4, the present invention Is particularly effective when a cobalt-based and nickel-based lithium transition metal composite oxide having a large lithium content, that is, a lithium cobalt composite oxide and a lithium nickel composite oxide are used as the positive electrode active material. In these lithium transition metal composite oxides, some of the main transition metal elements are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, etc. It can be stabilized by replacing with a metal species, and is also preferable. Of course, a plurality of positive electrode active materials may be used in combination.
[0036]
As an active material of the negative electrode constituting the secondary battery of the present invention, a material capable of inserting and extracting lithium can be used, but a carbonaceous material is preferable. Specific examples of the carbonaceous material include pyrolysis products of organic matter under various pyrolysis conditions, artificial graphite, natural graphite, and the like. Preferably, artificial graphite produced by high-temperature heat treatment of graphitizable pitch obtained from various raw materials and other artificial graphite such as graphitized mesophase spherules, graphitized mesophase pitch-based carbon fiber, and purified natural graphite, or these A material obtained by subjecting graphite to various surface treatments including pitch is used. These carbonaceous materials preferably have a d-value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method of 0.335 to 0.34 nm, and 0.335 to 0.337 nm. Is more preferable. The ash content is preferably 1% by weight or less, more preferably 0.5% by weight or less, and particularly preferably 0.1% by weight or less. The crystallite size (Lc) determined by X-ray diffraction by the Gakushin method is preferably 30 nm or more, more preferably 50 nm or more, and particularly preferably 100 nm or more. These carbonaceous materials can be further mixed with other active materials capable of inserting and extracting lithium. Examples of the active material capable of inserting and extracting lithium other than the carbonaceous material include metal oxide materials such as tin oxide and silicon oxide, lithium metal, and various lithium alloys. Two or more kinds of these negative electrode materials may be mixed and used.
[0037]
Each of the positive electrode and the negative electrode usually contains the active material and a binder. Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber. Furthermore, a conductive material such as a metal material such as copper or nickel, or a carbon material such as graphite or carbon black can be contained in the electrode as necessary. Particularly for the positive electrode, it is preferable to contain a conductive material.
[0038]
The method for manufacturing the electrode is not particularly limited. For example, it can be produced by adding a binder, a thickener, a conductive material, a solvent, etc. to the active material as necessary to form a slurry, applying it to the substrate of the current collector, and drying, Further, the active material can be roll-formed as it is to obtain a sheet electrode, or a pellet electrode by compression molding. Examples of the thickener include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein.
[0039]
As the current collector that can be used for the electrode, the negative electrode current collector can be a metal or alloy such as copper, nickel, stainless steel, preferably copper, and the positive electrode current collector can be aluminum, titanium, tantalum, etc. And metals and alloys thereof, preferably aluminum and its alloys.
In a secondary battery, a separator is usually interposed between a positive electrode and a negative electrode. The material and shape of the separator to be used are not particularly limited, but a porous sheet or non-woven fabric made of a polyolefin such as polyethylene or polypropylene is used as a material that is stable with respect to the electrolyte and has excellent liquid retention. Is preferred.
[0040]
The method for producing the non-aqueous secondary battery according to the present invention having at least a negative electrode, a positive electrode, and a non-aqueous electrolyte solution is not particularly limited, and can be appropriately selected from commonly employed methods.
In addition, the shape of the battery is not particularly limited, and a cylinder type in which a sheet electrode and a separator are spiraled, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked, and the like are used. Is possible.
[0041]
【Example】
Hereinafter, specific embodiments of the present invention will be described in detail by way of examples. However, the present invention is not limited by the following examples unless it exceeds the gist.
[Production of positive electrode]
The positive electrode comprises 90% by weight of lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, 5% by weight of acetylene black as a conductive agent, and 5% by weight of polyvinylidene fluoride (PVdF) as a binder. After mixing in a methylpyrrolidone solvent to make a slurry, it was applied to one side of a 20 μm aluminum foil, dried, and then rolled with a press machine and punched with a punch having a diameter of 12 mm.
[Production of negative electrode]
The negative electrode was made into a slurry by mixing 95% by weight of graphite (surface spacing: 0.336 nm) as a negative electrode active material and 5% by weight of polyvinylidene fluoride (PVdF) as a binder in an N-methylpyrrolidone solvent. Thereafter, a copper foil having a thickness of 20 μm was coated on one side, dried, and rolled with a press to punch out with a diameter of 12 mm.
[Battery assembly]
A lithium secondary battery was manufactured using a CR2032-type coin cell in a dry box in an argon atmosphere. That is, a positive electrode is placed on a positive electrode can, a 25 μm porous polyethylene film is placed thereon as a separator, pressed with a polypropylene gasket, a negative electrode is placed, a spacer for adjusting the thickness is placed, and the electrolyte is then added. In addition, after the battery was sufficiently infiltrated, the negative electrode can was placed and the battery was sealed. In the examples and comparative examples, the battery capacity was designed to be about 4.0 mAh with a charging upper limit of 4.2 V and a discharging lower limit of 3.0 V.
[0042]
At this time, the ratio of the weight W (c) of the positive electrode active material to the weight W (a) of the negative electrode active material is such that the lithium ions released from the positive electrode face each other at the normal upper limit voltage of the secondary battery. Since the range which does not raise | generate precipitation of a lithium metal is preferable above, the weight was determined so that the capacity ratio Rq of a negative electrode and a positive electrode might be set to 1.1 <= Rq <= 1.2. The capacity ratio Rq was obtained by Q (a) × W (a) / {Q (c) × W (c)}. Here, the electrical capacity per weight of the positive electrode active material under conditions corresponding to the initial charging conditions of the battery is Q (c) mAh / g, and lithium can be occluded to the maximum without precipitation of lithium metal. The electric capacity per weight of the negative electrode active material was Q (a) mAh / g. Q (c) and Q (a) were measured by using a positive electrode or a negative electrode as a working electrode and lithium metal as a counter electrode, and assembling a test cell through a separator in the same electrolytic solution as that for assembling the battery. That is, the capacity at which the positive electrode can be charged (release lithium ions from the positive electrode) and discharged at the lowest possible current density up to the upper limit potential of the positive electrode or the lower limit potential of the negative electrode corresponding to the initial charging conditions of the target battery system, It calculated | required as a capacity | capacitance which can occlude the lithium ion to a negative electrode.
[0043]
[Battery evaluation]
The battery was evaluated in the order of (1) initial charge / discharge (capacity check), (2) full charge operation, and (3) overcharge test.
In the initial charge / discharge (capacity check), the battery was charged by a constant current constant voltage method with an upper limit of 1C (4.0 mA) and 4.2V. Charging was cut when the current value reached 0.05 mA. Discharging was performed at a constant current up to 3.0 V at 0.2C.
[0044]
The full charge operation was performed by a constant current / constant voltage method (0.05 mA cut) with an upper limit of 4.2 V.
The overcharge test was 4.99V cut or 3hr cut at 1C (cut whichever comes first).
As an index for determining the superiority or inferiority of the overcharge prevention effect, a value obtained by disassembling a coin cell after overcharge and quantifying Li remaining in the positive electrode by elemental analysis was used as the overcharge depth. When the positive electrode composition after the overcharge test is expressed as Li _x CoO ₂ , the larger the x (positive electrode Li residual amount), the more the overcharge does not progress and the higher the overcharge prevention effect.
[0045]
Here, x (residual amount of positive electrode Li) was obtained from the molar ratio of Co in the positive electrode and net Li obtained by elemental analysis (ICP emission analysis). The net number of moles of Li is also determined by the same analysis of phosphorus (P) in the positive electrode, which is based on LiPF ₆ , and the number of Li moles corresponding to LiPF ₆ is calculated from the total number of moles of Li in the positive electrode. Calculated by subtracting.
Example 1
As an electrolytic solution, added to an electrolytic solution obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7. As the agent, 2% by weight of dimethyl malonate and 2% by weight of dibenzofuran were used.
[0046]
Evaluation of the lithium secondary battery produced by the above method and analysis of Li in the electrode after disassembling the battery after overcharging were performed. The results are shown in Table-1.
Example 2
In the same manner as in Example 1, except that 2% by weight of vinylene carbonate was further added in addition to 2% by weight of dimethyl malonate and 2% by weight of dibenzofuran. The Li in the electrode after disassembling the battery was analyzed. The results are shown in Table-1.
Example 3
In the same manner as in Example 1, except that 2% by weight of diethyl oxalate was added instead of 2% by weight of dimethyl malonate, the evaluation of the lithium secondary battery and the battery after overcharging were disassembled. Li analysis in the electrode was performed. The results are shown in Table-1.
Comparative Example 1
Except that the additive was not added in Example 1, the evaluation of the lithium secondary battery and the analysis of Li in the electrode after disassembling the battery after overcharging were performed. The results are shown in Table-1.
Comparative Example 2
In the same manner as in Example 1, except that only 2% by weight of dimethyl malonate was added as an additive, evaluation of the lithium secondary battery and analysis of Li in the electrode after disassembling the battery after overcharging were performed. It was. The results are shown in Table-1.
Comparative Example 3
In the same manner as in Comparative Example 2, except that 2% by weight of dibenzofuran was added instead of 2% by weight of dimethyl malonate, evaluation of the lithium secondary battery and disassembly of the battery after overcharging were performed. Li analysis in the electrode was performed. The results are shown in Table-1. In addition, voltage oscillation that seems to be caused by short circuit during overcharge was observed, and the apparent overcharge current increased.
Comparative Example 4
In the same manner as in Comparative Example 2, except that 2% by weight of vinylene carbonate was added instead of 2% by weight of dimethyl malonate, the evaluation of the lithium secondary battery and the battery after overcharging were disassembled. Li analysis in the electrode was performed. The results are shown in Table-1.
Comparative Example 5
In the same manner as in Comparative Example 2, except that 2% by weight of dibenzofuran and 2% by weight of vinylene carbonate were added instead of 2% by weight of dimethyl malonate, evaluation of the lithium secondary battery, and after overcharging The Li in the electrode after disassembling the battery was analyzed. The results are shown in Table-1. Similar to Comparative Example 3, voltage oscillation that seems to be caused by a short circuit during overcharge was observed, and the apparent overcharge current amount increased.
[0047]
[Table 1]

[0048]
From Table 1, it can be seen that the addition of a dicarboxylic acid diester (excluding succinic acid diester) or a derivative thereof and an aromatic compound having a molecular weight of 500 or less improves the safety during overcharge.
In addition, the lithium secondary battery created in the example and the lithium secondary battery created in the comparative example did not show a great difference in battery characteristics such as initial discharge capacity and capacity retention after 5 cycles.
[0049]
【Effect of the invention】
According to the present invention, it is possible to provide an electrolytic solution that can improve various battery characteristics such as cycle characteristics, rate characteristics, and capacity. In particular, it is possible to provide an electrolytic solution that can improve safety during overcharging by a novel overcharge inhibitor.
[0050]
Further, according to the present invention, it is possible to provide a battery having various battery characteristics such as cycle characteristics, rate characteristics, and capacity. In particular, a battery with improved safety during overcharging can be provided by a novel overcharge inhibitor.

Claims (9)

An electrolyte solution for a secondary battery in which a lithium salt is dissolved in a solvent mainly composed of at least one non-aqueous solvent selected from the group consisting of carbonate ester, ether and lactone, wherein the solvent is dimethyl oxalate or oxalic acid Diethyl, dipropyl oxalate, dibutyl oxalate, bis (fluoromethyl) oxalate, bis (difluoromethyl) oxalate, bis (trifluoromethyl) oxalate, dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate , malonic acid bis (fluoromethyl), malonic acid bis (difluoromethyl), and malonic acid bis (trifluoromethyl) also dicarboxylic acid ester Le selected from the group consisting of and a derivative thereof, comprising a molecular weight of 500 or less oxide and wherein the potential comprises an aromatic compound 4.3~4.9V Electrolytic solution for a secondary battery that. The electrolyte solution for secondary batteries of Claim 1 in which the said dicarboxylic acid diester and its derivative are contained in the ratio of 0.1-5 weight% with respect to the solvent. The electrolyte solution for secondary batteries according to claim 1 or 2 , wherein the aromatic compound has a skeleton of a compound represented by the following general formula (3).

(Wherein R _{1 to} R ₆ represent a hydrogen atom, a halogen atom, a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or an optionally substituted phenyl group. These may be bonded to each other to form a ring.) The electrolyte solution for secondary batteries according to claim 1 or 2, wherein the aromatic compound has a skeleton of a compound represented by the following general formula (4).

(In the formula, R ₁ represents a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or a phenyl group which may have a substituent, and R _{2 to} R ₆ are hydrogen atoms. , A halogen atom, a chain alkyl group having 1 to 10 carbon atoms, a cyclic alkyl group having 4 to 10 carbon atoms, or a phenyl group which may have a substituent, R _{1 to} R ₆ are bonded to each other. A ring may be formed.) The electrolyte solution for secondary batteries in any one of Claims 1-4 in which the said aromatic compound is added in the ratio of 0.1-10 weight% with respect to the solvent. Furthermore, the electrolyte solution for secondary batteries in any one of Claims 1-5 in which vinylene carbonate or / and vinyl ethylene carbonate are added in the ratio of 0.1 to 10 weight% with respect to the solvent. Secondary battery comprising: the electrolyte solution for a secondary battery according to any one of claims 1 to 6 and a positive electrode, a negative electrode. The secondary battery according to claim 7 , wherein the positive electrode contains a lithium transition metal composite oxide. The secondary battery according to claim 7 or 8 , wherein the negative electrode contains a carbonaceous material.