JP4141214B2 - Non-aqueous electrolyte and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

【０００８】
（式中、Ｒ¹〜Ｒ¹⁰は、それぞれ独立して、水素原子もしくはメチル基及びアルコキシ基以外の有機基を表すか、又は隣接する２つが互に結合して環を形成している。ただし、Ｒ¹〜Ｒ⁵及びＲ⁶〜Ｒ¹⁰のそれぞれにおいて、その少なくとも１つは、炭素及び水素、もしくは炭素、水素及び酸素からなる分子量２７〜５００でアルコキシ基以外の有機基であるか、又は隣接する２つが互に結合して環を形成している。）
また、本発明は、非水溶媒に電解質を溶解してなる電解液であって、式（２）で表される芳香族炭酸エステルを含有していることを特徴とする二次電池用非水電解液に関する。
【化５】

（式中、Ｒ ¹¹ 〜Ｒ ¹⁵ 及びＲ ¹⁶ 〜Ｒ ²⁰ のそれぞれにおいて、その１〜３個は炭素及び水素、もしくは炭素、水素及び酸素からなる分子量２７〜５００で、アルコキシ基以外の有機基であるか、又は隣接する２つが互に結合して環を形成しており、残余はいずれも水素原子である）
さらに、本発明は、非水溶媒に電解質を溶解してなる電解液であって、式（３）で表される芳香族炭酸エステルを含有していることを特徴とする二次電池用非水電解液に関する。
【化６】

（式中、Ｒ ²¹ 〜Ｒ ²⁵ 及びＲ ²⁶ 〜Ｒ ³⁰ のそれぞれにおいて、そのいずれか１つがシクロヘキシル基、フェニル基又はフェノキシ基であるか又は隣接する２つが互に結合してベンゼン環を形成しており、他は全て水素原子である）
【０００９】
【発明の実施の形態】
本発明によれば、電解液中に式（１）で表される芳香族炭酸エステルを含有させることにより、電池の過充電に対する安全性を高めることができる。式（１）において、Ｒ¹〜Ｒ¹⁰は水素原子もしくは有機基を表すか、又は隣接する２つが互に結合して環を形成している。但し、有機基はメチル基及びアルコキシ基以外のものである。更にＲ¹〜Ｒ⁵及びＲ⁶〜Ｒ¹⁰のそれぞれにおいて、その少なくとも１つは、炭素及び水素、もしくは炭素、水素及び酸素からなる分子量２７〜５００で、アルコキシ基以外の有機基であるか、又は隣接する２つが互に結合して環を形成している。
【００１０】
ベンゼン環に結合している有機基が炭素、水素及び酸素以外の元素を含む場合には、過充電時に式（１）で表される芳香族炭酸エステルが重合するのが阻害され、かつ生成する重合膜の緻密性が低下して、過充電電流を遮断する性能が不十分となり易い。また有機基の分子量が２７以下でも過充電時の電流遮断能が不十分であり、逆に分子量が５００よりも大きくなると電解液中での式（１）の芳香族炭酸エステルの溶解性が悪化する。電解液中への溶解性及び電池性能の安定性の点よりして、有機基の分子量は４００以下、特に３００以下が好ましい。
【００１１】
通常はＲ¹〜Ｒ⁵及びＲ⁶〜Ｒ¹⁰のそれぞれにおいて、その１〜３個が上述の有機基であるか、又は隣接する２つが互に結合して環を形成しており、残りはいずれも水素原子であるのが好ましい。なかでもＲ¹〜Ｒ⁵及びＲ⁶〜Ｒ¹⁰のそれぞれにおいて、その１つが有機基で他は全て水素原子であるか、又は隣接する２つが互に結合して環を形成しており、残余は水素原子であるのが好ましい。なお、炭素及び水素、又は炭素、水素及び酸素からなる有機基は電子供与性であるのが好ましい。
【００１２】
有機基としては、エチル基、プロピル基、イソプロピル基、ブチル基、ｔ−ブチル基、イソブチル基などの炭素数２〜８、好ましくは炭素数２〜６の鎖状アルキル基；シクロブチル基、シクロペンチル基、メチルシクロペンチル基、シクロヘキシル基などの炭素数４〜８、好ましくは炭素数４〜６のシクロアルキル基；置換基を有していてもよいフェニル基、フェノキシ基、ナフチル基、ナフトキシ基などのアリール基やアリールオキシ基などが挙げられる。これらのアリール基やアリールオキシ基に結合する置換基としては、メチル基、エチル基、プロピル基、イソプロピル基、ブチル基、イソブチル基、ｔ−ブチル基などの炭素数１〜４の鎖状アルキル基；シクロブチル基、シクロペンチル基、シクロヘキシル基などのシクロアルキル基；フェニル基、トルイル基、キシリル基などの炭素数６〜１２のアリール基などが挙げられる。
【００１３】
置換基を有していてもよいアリール基及びアリールオキシ基の代表的なものを例示すると、フェニル基、１−ナフチル基、２−ナフチル基、２−メチルフェニル基、３−メチルフェニル基、４−メチルフェニル基、２，５−ジメチルフェニル基、２−フェニルフェニル基、３−フェニルフェニル基、４−フェニルフェニル基、フェノキシ基、１−ナフトキシ基、２−ナフトキシ基、２−メチルフェノキシ基、４−メチルフェノキシ基、４−エチルフェノキシ基、２，５−ジメチルフェノキシ基、３−フェニルフェノキシ基、４−フェニルフェノキシ基などが挙げられる。またＲ¹〜Ｒ⁵及びＲ⁶〜Ｒ¹⁰において、その隣接する２つが結合して形成する環は通常はベンゼン環であるが、ナフタレン環やテトラヒドロナフタレン環などであってもよい。またこれらの環に更にメチル基、エチル基などの炭素数１〜４の鎖状アルキル基、シクロブチル基、シクロヘキシル基などの炭素数４〜６のシクロアルキル基、フェニル基などが結合していてもよい。
【００１４】
本発明で用いる好ましい芳香族炭酸エステルとしては、次のようなものが挙げられる。
ビス（２−シクロヘキシルフェニル）カーボネート、ビス（３−シクロヘキシルフェニル）カーボネート、ビス（４−シクロヘキシルフェニル）カーボネート、ビス（２−フェニルフェニル）カーボネート、ビス（３−フェニルフェニル）カーボネート、ビス（４−フェニルフェニル）カーボネート、ビス（２−フェノキシフェニル）カーボネート、ビス（３−フェノキシフェニル）カーボネート、ビス（４−フェノキシフェニル）カーボネート、ビス（１−ナフチル）カーボネート、ビス（２−ナフチル）カーボネート。
【００１５】
これらの芳香族炭酸エステルは、いくつかを併用することもできる。
芳香族炭酸エステルは、電解液中に０．１〜５重量％、特に１〜４重量％となるように含有させるのが好ましい。含有量が少な過ぎると十分な過充電防止効果が得られないし、逆に含有量が多過ぎると電池特性に悪影響を及ぼすようになる。
本発明に係る電解液の主体をなす有機溶媒としては、従来から非水電解液の溶媒として用い得ることが知られているもののなかから、適宜選択して用いることができる。通常はプロピレンカーボネート、エチレンカーボネート、ブチレンカーボネート等の環状炭酸エステル；ジメチルカーボネート、ジエチルカーボネート、エチルメチルカーボネート等の鎖状炭酸エステル；テトラヒドロフラン、２−メチルテトラヒドロフラン、テトラヒドロピラン等の環状エーテル；ジメトキシエタン、ジエトキシエタン等の鎖状エーテル；γ−ブチロラクトン、γ−バレロラクトン等の環状カルボン酸エステル；酢酸メチル、プロピオン酸メチル等の鎖状カルボン酸エステルなどが用いられる。これらは単独で用いることもできるが、通常はいくつかを併用する。その好ましい１例は、環状炭酸エステルと鎖状炭酸エステルとの併用である。
【００１６】
電解液に用いる電解質も、従来から非水電解液の電解質として用い得ることが知られているリチウム塩のなかから、適宜選択して用いることができる。通常は、ＬｉＣｌＯ₄、ＬｉＡｓＦ₆、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＢ（Ｃ₆Ｈ₅）₄、ＬｉＣｌ、ＬｉＢｒ、ＬｉＣＨ₃ＳＯ₃、ＬｉＣＦ₃ＳＯ₃、ＬｉＮ（ＳＯ₂ＣＦ₃）₂、ＬｉＮ（ＳＯ₂Ｃ₂Ｆ₅）₂、ＬｉＣ（ＳＯ₂ＣＦ₃）₃等が用いられる。これらの電解質も、所望ならばいくつかを併用することもできる。好ましくはＬｉＢＦ₄又はＬｉＰＦ₆を用いる。電解質は電解液中に通常は０．５〜２．５モル／Ｌとなるように含有させる。一般に電解質の濃度が低過ぎても高過ぎても電導度が低下して、電池特性に悪影響を及ぼす。電解液中の電解液の好ましい濃度は０．７５〜１．５モル／Ｌである。
【００１７】
本発明に係る電解液には、更に常用の種々の添加剤を含有させることができる。このような添加剤としては、各種の界面活性剤や電池の活物質の表面に被膜を形成する化合物などが挙げられる。
本発明に係る非水電解液二次電池は、上記の電解液を用いる以外は公知のものと同じ構成とすることができる。正極活物質としては、基本組成がＬｉＭｎＯ₂又はＬｉＭｎ₂Ｏ₄で表されるリチウムマンガン複合酸化物、基本組成がＬｉＮｉＯ₂で表されるリチウムニッケル複合酸化物、又は基本組成がＬｉＣｏＯ₂で表されるリチウムコバルト複合酸化物から選ばれるリチウム遷移金属複合酸化物を用いるのが好ましい。これらのリチウム遷移金属複合酸化物は、その遷移金属の一部をＡｌ、Ｔｉ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ、Ｚｒなどの金属で置換することにより、正極活物質としての性能を向上させることができる。リチウム遷移金属複合酸化物としては、主たる遷移金属がニッケル又はコバルトであるものが好ましい。
【００１８】
負極活物質としては、酸化錫、酸化珪素等の金属酸化物をはじめリチウムを吸蔵及び放出し得るものとして提案されている任意のものを用いることができ、所望ならばリチウム金属やリチウム合金を用いることもできるが、黒鉛その他の炭素質物を用いるのが好ましい。なかでも好ましいのは、易黒鉛性ピッチを熱処理して得られた人造黒鉛、メソフェーズ小球体やメソフェーズピッチ系炭素繊維の黒鉛化物などである。また黒鉛にピッチなどを被覆したのち炭化して、黒鉛の表面に黒鉛化度の低い炭素質物の被覆を形成したものも好ましい。
【００１９】
炭素質物として特に好ましいのは、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のものである。ｄ値が０．３３５〜０．３３７ｎｍであれば更に好ましい。灰分は少ない方が好ましく、通常は１重量％以下のものを用いる。好ましくは灰分が０．５重量％以下、特に０．１重量％以下のものを用いる。更に学振法によるＸ線回折で求めた結晶子サイズ（ＬＣ）は３０ｎｍ以上であるのが好ましい。結晶子サイズ（ＬＣ）が５０ｎｍ以上、特に１００ｎｍ以上であれば更に好ましい。負極活物質も所望ならばいくつかを併用することもできる。
【００２０】
正極及び負極は、上記の活物質に、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、ブタジエンゴム等の結着剤を加えたものを溶媒に分散させてスラリーとし、集電体に塗布したのち乾燥することにより製造することができる。集電体としては、正極にはアルミニウム、チタン、タンタル等の金属やその合金を用いればよく、負極には銅、ニッケルやその合金、ステンレス鋼などを用いればよい。なお、スラリー、特に正極用のスラリー中には、導電材として黒鉛、カーボンブラック等の炭素質材料や銅、ニッケル等の金属を含有させるのが好ましい。またスラリー中には、所望により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、カゼイン等の増粘剤を含有させることもできる。なお、集電体を用いる代りに、活物質に結着剤及び導電材などを配合したものを圧縮成形して電極とすることもできる。
【００２１】
正極と負極との間に介装させるセパレーターも、常用の任意のものを用いることができるが、ポリオレフィンの多孔性シート又は不織布を用いるのが好ましい。特に好ましいのは超高分子量ポリエチレンの多孔性シートである。
【００２２】
【実施例】
以下に実施例により本発明を更に具体的に説明するが、本発明は以下の実施例に限定されるものではない。
正極の作製；コバルト酸リチウム（ＬｉＣｏＯ₂）９０重量部、アセチレンブラック５重量部、及びポリフッ化ビニリデン５重量部の混合物に、Ｎ−メチルピロリドンを加えてスラリーとした。このスラリーを厚さ２０μｍのアルミニウム箔の片面に塗布して乾燥したのち、プレス機で圧延した。これからポンチで直径１２ｍｍの円板を打抜き正極とした。
【００２３】
負極の作製；黒鉛（ｄ＝０．３３６ｎｍ）９５重量部とポリフッ化ビニリデン５重量部の混合物を、Ｎ−メチルピロリドンでスラリーとした。これを厚さ２０μｍの銅箔の片面に塗布して乾燥したのち、プレス機で圧延した。これからポンチで直径１２ｍｍの円板を打抜き負極とした。
電解液の調製；エチレンカーボネートとジエチルカーボネートとを体積比で３：７で混合したものに、６−フッ化リン酸リチウム（ＬｉＰＦ₆）を１モル／Ｌとなるように溶解させた。これに更に芳香族カーボネートを２重量％の濃度となるように添加して電解液とした。
【００２４】
電池の作製；アルゴン雰囲気のドライボックス内で、ＣＲ２０３２型コインセルを用いてリチウム二次電池を作製した。正極缶の中に正極を入れ、その上に厚さ２５μｍの多孔性ポリエチレンフィルム（セパレーター）を載置した。これをポリプロピレン製ガスケットで押えたのち、更に負極を載置した。厚み調整用のスペーサーを載置したのち電解液を加えて内部に電解液を十分に行きわたらせた。次いで負極缶を載置したのち封口して電池とした。
【００２５】
電池の容量は充電４．２Ｖ、放電３．０Ｖで約４．０ｍＡｈである。また、正極活物質重量（Ｗｃ）と負極活物質重量（Ｗａ）との比率は、負極と正極との容量比Ｒｑが１．１≦Ｒｑ≦１．２となるようにした。なお容量比Ｒｑは下記により算出した。
【００２６】
【数１】
Ｒｑ＝Ｑａ×Ｗａ／Ｑｃ×Ｗｃ
【００２７】
Ｑｃ：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りの電気容量（ｍＡｈ／ｇ）
Ｑａ：電池の初期放電条件に対応する条件下で、リチウム金属を析出することなしに、リチウムを最大限に吸蔵し得る負極活物質の重量当りの電気容量（ｍＡｈ／ｇ）
Ｑａ及びＱｃは、正極又は負極を作用極、リチウム金属を対極とし、電解液としてエチレンカーボネートとジエチルカーボネートを体積比７：３で混合したものに六フッ化リン酸リチウムを１モル／Ｌとなるように溶解したものを用いて調製した試験セルを用い、初期充電条件に対応する正極の上限電位（４．２Ｖ）、又は負極の下限電位（３．０Ｖ）まで、可能な限り低い電流密度で、正極が充電できる容量（正極からのリチウムイオンの放出）、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。
【００２８】
電池の評価；
電池の評価としては、（１）２０℃過充電試験、及び（２）６０℃過充電試験の２種類を行った。
（１）と（２）の過充電試験は、環境温度が異なる以外は同一であり、▲１▼初期充放電（容量確認）、▲２▼満充電操作及び▲３▼過充電操作の順に行った。
【００２９】
初期充放電はＩＣ（４．０ｍＡｈ）、４．２Ｖ上限の定電流定電圧法により充電した。終点は電流値が０．０５ｍＡｈ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（なお、ＩＣとは１時間で満充電できる電流値を表わす。）
満充電操作は、ＩＣ、４．２Ｖ上限の定電流定電圧法で充電し、電流値が０．０５ｍＡｈに到達した時点を終点とした。
過充電試験は、ＩＣで４．９９Ｖ又は充電時間３時間のいずれか早く到達した時点を終点とした。
【００３０】
実施例１〜６及び比較例１、２
電解液として表１に示す芳香族炭酸エステルを含むものを用いて、電池を作成し、その特性を評価した。結果を表１に示す。
【００３１】
【表１】

【００３２】
過充電率＝（過充電操作における充電量／満充電操作における充電量）×１００（％）
過充電率が小さいほど過充電電流を速かに遮断できるので、ジナフチルカーボネートやビス（フェノキシメチル）カーボネートは過充電防止剤として特に優れている。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte and a highly safe non-aqueous electrolyte secondary battery during overcharge using the same.
[0002]
[Prior art]
In recent years, non-aqueous lithium secondary batteries having a high energy density have been put into practical use. Various types of electrolytes have been proposed for this battery, but one of them is preferably a high dielectric constant such as a cyclic carbonate such as ethylene carbonate or propylene carbonate, or a cyclic carboxylic acid ester such as γ-butyrolactone. And a low-viscosity solvent such as a chain carbonate such as diethyl carbonate or dimethyl carbonate or an ether such as dimethoxyethane. Since this mixed solvent has a high dielectric constant and a high oxidation potential, it has excellent stability.
[0003]
One of the problems with lithium secondary batteries is that they may cause so-called overcharge, which exceeds a predetermined upper limit voltage during charging. When overcharging occurs, the battery not only generates heat and deforms, but in severe cases it ignites and explodes, which is extremely dangerous.
Various methods for preventing overcharge have also been proposed. One of them is that an aromatic compound or other compound that is polymerized by oxidation is contained in the electrolytic solution, and polymerization is caused on the electrode surface during overcharge. As a result, the surface of the electrode is covered with the polymer film, the current is interrupted, and further overcharge is prevented from proceeding. Various compounds have been proposed as candidates (see, for example, Patent Document 1).
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-329497
[Problems to be solved by the invention]
However, to the best of our knowledge, none of the previously proposed compounds is yet satisfactory. Accordingly, the present invention is intended to provide a non-aqueous electrolyte containing an overcharge inhibitor that can quickly prevent the progression of overcharge when it occurs.
[0006]
[Means for Solving the Problems]
According to the present invention, in a non-aqueous electrolyte secondary battery, by using an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent and containing an aromatic carbonate represented by the formula (1) , Safety against overcharging can be enhanced.
[0007]
[Formula 4]

[0008]
(Wherein R ^{1 to} R ¹⁰ each independently represents a hydrogen atom or an organic group other than a methyl group and an alkoxy group, or two adjacent groups are bonded to each other to form a ring. , R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , at least one of them is carbon and hydrogen, or an organic group other than an alkoxy group having a molecular weight of 27 to 500 consisting of carbon, hydrogen and oxygen, or Two adjacent groups are bonded to each other to form a ring.)
The present invention also provides an electrolytic solution obtained by dissolving an electrolyte in a nonaqueous solvent, which contains an aromatic carbonate represented by the formula (2), and is nonaqueous for a secondary battery It relates to an electrolytic solution.
[Chemical formula 5]

(In the formula, in each of R ^{11 to} R ¹⁵ and R ^{16 to} R ²⁰ , 1 to 3 are carbon and hydrogen, or a molecular weight of 27 to 500 consisting of carbon, hydrogen and oxygen, and an organic group other than an alkoxy group. Or two adjacent groups are bonded to each other to form a ring, and the rest are all hydrogen atoms)
Furthermore, the present invention provides an electrolyte solution obtained by dissolving an electrolyte in a nonaqueous solvent, which contains an aromatic carbonate represented by the formula (3), and is nonaqueous for a secondary battery It relates to an electrolytic solution.
[Chemical 6]

(In the formula, in each of R ^{21 to} R ²⁵ and R ^{26 to} R ³⁰ , any one of them is a cyclohexyl group, a phenyl group or a phenoxy group, or two adjacent ones are bonded to each other to form a benzene ring. All others are hydrogen atoms)
[0009]
DETAILED DESCRIPTION OF THE INVENTION
According to this invention, the safety | security with respect to the overcharge of a battery can be improved by containing the aromatic carbonate represented by Formula (1) in electrolyte solution. In formula (1), R ^{1 to} R ¹⁰ represent a hydrogen atom or an organic group, or two adjacent groups are bonded to each other to form a ring. However, the organic group is other than a methyl group and an alkoxy group. Further, in each of R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , at least one of them is carbon and hydrogen, or an organic group other than an alkoxy group having a molecular weight of 27 to 500 consisting of carbon, hydrogen and oxygen, Or two adjacent ones are bonded to each other to form a ring.
[0010]
When the organic group bonded to the benzene ring contains an element other than carbon, hydrogen, and oxygen, polymerization of the aromatic carbonate represented by the formula (1) is inhibited and generated during overcharge. The denseness of the polymerized film is lowered, and the performance for interrupting the overcharge current tends to be insufficient. Moreover, even when the molecular weight of the organic group is 27 or less, the current blocking ability at the time of overcharge is insufficient, and conversely, when the molecular weight is larger than 500, the solubility of the aromatic carbonate of formula (1) in the electrolytic solution deteriorates. To do. From the viewpoint of solubility in the electrolytic solution and stability of battery performance, the molecular weight of the organic group is preferably 400 or less, particularly preferably 300 or less.
[0011]
Usually, in each of R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , 1 to 3 thereof are the above-mentioned organic groups, or two adjacent groups are bonded to each other to form a ring, and the rest are Any of them is preferably a hydrogen atom. In particular, in each of R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , one of them is an organic group and the others are all hydrogen atoms, or two adjacent groups are bonded to each other to form a ring, and the rest Is preferably a hydrogen atom. Note that an organic group composed of carbon and hydrogen, or carbon, hydrogen and oxygen is preferably electron-donating.
[0012]
Examples of the organic group include a chain alkyl group having 2 to 8 carbon atoms, preferably 2 to 6 carbon atoms such as an ethyl group, a propyl group, an isopropyl group, a butyl group, a t-butyl group, and an isobutyl group; a cyclobutyl group and a cyclopentyl group. A cycloalkyl group having 4 to 8 carbon atoms, preferably 4 to 6 carbon atoms such as a methylcyclopentyl group and a cyclohexyl group; an aryl such as a phenyl group, a phenoxy group, a naphthyl group and a naphthoxy group which may have a substituent. Group and aryloxy group. Examples of the substituent bonded to the aryl group or aryloxy group include a chain alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, and a t-butyl group. A cycloalkyl group such as a cyclobutyl group, a cyclopentyl group or a cyclohexyl group; an aryl group having 6 to 12 carbon atoms such as a phenyl group, a toluyl group or a xylyl group;
[0013]
Typical examples of the aryl group and aryloxy group which may have a substituent are phenyl group, 1-naphthyl group, 2-naphthyl group, 2-methylphenyl group, 3-methylphenyl group, 4 -Methylphenyl group, 2,5-dimethylphenyl group, 2-phenylphenyl group, 3-phenylphenyl group, 4-phenylphenyl group, phenoxy group, 1-naphthoxy group, 2-naphthoxy group, 2-methylphenoxy group, 4-methylphenoxy group, 4-ethylphenoxy group, 2,5-dimethylphenoxy group, 3-phenylphenoxy group, 4-phenylphenoxy group and the like can be mentioned. In R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , the ring formed by combining two adjacent ones is usually a benzene ring, but may be a naphthalene ring or a tetrahydronaphthalene ring. In addition, a chain alkyl group having 1 to 4 carbon atoms such as a methyl group or an ethyl group, a cycloalkyl group having 4 to 6 carbon atoms such as a cyclobutyl group or a cyclohexyl group, or a phenyl group may be bonded to these rings. Good.
[0014]
Preferred aromatic carbonates used in the present invention include the following.
Bis (2-cyclohexylphenyl) carbonate, bis (3-cyclohexylphenyl) carbonate, bis (4-cyclohexylphenyl) carbonate, bis (2-phenylphenyl) carbonate, bis (3-phenylphenyl) carbonate, bis (4-phenyl) Phenyl) carbonate, bis (2-phenoxyphenyl) carbonate, bis (3-phenoxyphenyl) carbonate, bis (4-phenoxyphenyl) carbonate, bis (1-naphthyl) carbonate, bis (2-naphthyl) carbonate.
[0015]
Some of these aromatic carbonates can be used in combination.
The aromatic carbonate is preferably contained in the electrolyte so as to be 0.1 to 5% by weight, particularly 1 to 4% by weight. If the content is too small, a sufficient overcharge preventing effect cannot be obtained. Conversely, if the content is too large, the battery characteristics are adversely affected.
As the organic solvent which is the main component of the electrolytic solution according to the present invention, it can be appropriately selected and used from those conventionally known as solvents for non-aqueous electrolytic solutions. Usually, cyclic carbonates such as propylene carbonate, ethylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, and tetrahydropyran; Chain ethers such as ethoxyethane; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; chain carboxylic acid esters such as methyl acetate and methyl propionate are used. These can be used alone, but several are usually used together. A preferred example is the combined use of a cyclic carbonate and a chain carbonate.
[0016]
The electrolyte used for the electrolytic solution can also be appropriately selected from lithium salts that are conventionally known to be usable as an electrolyte for nonaqueous electrolytic solutions. Usually, 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 2 C 2 F 5) 2,} LiC (SO 2 CF 3) 3 or the like is used. Some of these electrolytes can be used in combination if desired. Preferably, LiBF ₄ or LiPF ₆ is used. The electrolyte is usually contained in the electrolyte so as to be 0.5 to 2.5 mol / L. In general, if the concentration of the electrolyte is too low or too high, the conductivity is lowered, which adversely affects the battery characteristics. The preferable density | concentration of the electrolyte solution in electrolyte solution is 0.75-1.5 mol / L.
[0017]
The electrolyte solution according to the present invention can further contain various commonly used additives. Examples of such additives include various surfactants and compounds that form a film on the surface of the active material of the battery.
The non-aqueous electrolyte secondary battery according to the present invention can have the same configuration as a known one except that the above electrolyte is used. As the positive electrode active material, a lithium manganese composite oxide whose basic composition is represented by LiMnO ₂ or LiMn ₂ O ₄ , a lithium nickel composite oxide whose basic composition is represented by LiNiO ₂ , or a basic composition represented by LiCoO ₂ It is preferable to use a lithium transition metal composite oxide selected from lithium cobalt composite oxides. These lithium transition metal composite oxides are obtained by substituting a part of the transition metal with a metal such as Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Ga, and Zr. The performance as a positive electrode active material can be improved. The lithium transition metal composite oxide is preferably one in which the main transition metal is nickel or cobalt.
[0018]
As the negative electrode active material, any metal oxide such as tin oxide, silicon oxide and the like proposed to be capable of occluding and releasing lithium can be used. If desired, lithium metal or lithium alloy is used. However, it is preferable to use graphite or other carbonaceous materials. Among these, artificial graphite obtained by heat-treating graphitizable pitch, mesophase spherules, graphitized mesophase pitch carbon fibers, and the like are preferable. In addition, it is also preferable that graphite is coated with pitch and then carbonized to form a coating of a carbonaceous material having a low degree of graphitization on the surface of graphite.
[0019]
Particularly preferable carbonaceous materials are those having 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. More preferably, the d value is 0.335 to 0.337 nm. It is preferable that the ash content is low, and usually 1% by weight or less is used. Preferably, an ash content of 0.5% by weight or less, particularly 0.1% by weight or less is used. Further, the crystallite size (LC) determined by X-ray diffraction by the Gakushin method is preferably 30 nm or more. More preferably, the crystallite size (LC) is 50 nm or more, particularly 100 nm or more. Some negative electrode active materials can be used in combination if desired.
[0020]
The positive electrode and the negative electrode are prepared by dispersing a mixture of the above active material and a binder such as polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, isoprene rubber, butadiene rubber, and the like into a slurry. It can be manufactured by applying to the body and then drying. As the current collector, a metal such as aluminum, titanium, or tantalum or an alloy thereof may be used for the positive electrode, and copper, nickel, an alloy thereof, stainless steel, or the like may be used for the negative electrode. In addition, it is preferable to contain carbonaceous materials, such as graphite and carbon black, and metals, such as copper and nickel, in a slurry, especially the slurry for positive electrodes. Further, the slurry may contain a thickener such as carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein as desired. Instead of using the current collector, an electrode obtained by compression-molding an active material containing a binder and a conductive material can be used.
[0021]
As the separator interposed between the positive electrode and the negative electrode, any conventional one can be used, but it is preferable to use a porous sheet or non-woven fabric of polyolefin. Particularly preferred is a porous sheet of ultra high molecular weight polyethylene.
[0022]
【Example】
The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to the following examples.
Preparation of positive electrode: N-methylpyrrolidone was added to a mixture of 90 parts by weight of lithium cobaltate (LiCoO ₂ ), 5 parts by weight of acetylene black, and 5 parts by weight of polyvinylidene fluoride to prepare a slurry. This slurry was applied to one side of an aluminum foil having a thickness of 20 μm, dried, and then rolled with a press. From this, a disk with a diameter of 12 mm was punched out with a punch and used as a positive electrode.
[0023]
Production of negative electrode: A mixture of 95 parts by weight of graphite (d = 0.336 nm) and 5 parts by weight of polyvinylidene fluoride was slurried with N-methylpyrrolidone. This was coated on one side of a 20 μm thick copper foil, dried, and then rolled with a press. From this, a circular plate having a diameter of 12 mm was punched out with a punch and used as a negative electrode.
Preparation of electrolyte solution; Lithium _6- fluorophosphate (LiPF ₆ ) was dissolved in a mixture of ethylene carbonate and diethyl carbonate at a volume ratio of 3: 7 so as to be 1 mol / L. To this was further added aromatic carbonate to a concentration of 2% by weight to obtain an electrolytic solution.
[0024]
Production of battery: A lithium secondary battery was produced using a CR2032-type coin cell in a dry box in an argon atmosphere. A positive electrode was placed in a positive electrode can, and a 25 μm thick porous polyethylene film (separator) was placed thereon. After pressing this with a polypropylene gasket, a negative electrode was further placed. After placing a spacer for adjusting the thickness, an electrolytic solution was added to sufficiently distribute the electrolytic solution inside. Then, after placing the negative electrode can, it was sealed to obtain a battery.
[0025]
The capacity of the battery is about 4.0 mAh at a charge of 4.2V and a discharge of 3.0V. The ratio of the positive electrode active material weight (Wc) to the negative electrode active material weight (Wa) was such that the capacity ratio Rq between the negative electrode and the positive electrode was 1.1 ≦ Rq ≦ 1.2. The capacity ratio Rq was calculated as follows.
[0026]
[Expression 1]
Rq = Qa × Wa / Qc × Wc
[0027]
Qc: Electric capacity per unit weight (mAh / g) of the positive electrode active material under conditions corresponding to the initial charging conditions of the battery
Qa: Electric capacity per unit weight (mAh / g) of the negative electrode active material capable of occluding lithium to the maximum without depositing lithium metal under conditions corresponding to the initial discharge conditions of the battery
Qa and Qc have a positive electrode or a negative electrode as a working electrode, a lithium metal as a counter electrode, and a mixture of ethylene carbonate and diethyl carbonate as an electrolytic solution in a volume ratio of 7: 3 to 1 mol / L of lithium hexafluorophosphate. Using a test cell prepared using a so-dissolved sample, the current density is as low as possible up to the upper limit potential of the positive electrode (4.2 V) or the lower limit potential of the negative electrode (3.0 V) corresponding to the initial charging conditions. The capacity that the positive electrode can be charged (release of lithium ions from the positive electrode) and the capacity that the negative electrode can discharge (occlude lithium ions into the negative electrode) were obtained.
[0028]
Battery evaluation;
As the evaluation of the battery, two types of (1) 20 ° C. overcharge test and (2) 60 ° C. overcharge test were performed.
The overcharge tests in (1) and (2) are the same except for the environmental temperature, and are performed in the order of (1) initial charge / discharge (capacity check), (2) full charge operation, and (3) overcharge operation. It was.
[0029]
Initial charging / discharging was performed by a constant current / constant voltage method with an IC (4.0 mAh) and 4.2 V upper limit. The end point was the time when the current value reached 0.05 mAh. Discharging was performed at a constant current up to 3.0 V at 0.2 C (Note that IC represents a current value that can be fully charged in one hour).
The full charge operation was performed by charging with an IC, a constant current / constant voltage method with an upper limit of 4.2 V, and the end point was when the current value reached 0.05 mAh.
In the overcharge test, the end point was reached when the IC reached 4.99 V or the charge time of 3 hours, whichever comes first.
[0030]
Examples 1 to 6 and Comparative Examples 1 and 2
A battery was prepared by using an electrolyte containing an aromatic carbonate shown in Table 1 and its characteristics were evaluated. The results are shown in Table 1.
[0031]
[Table 1]

[0032]
Overcharge rate = (charge amount in overcharge operation / charge amount in full charge operation) × 100 (%)
The smaller the overcharge rate, the faster the overcharge current can be cut off, so dinaphthyl carbonate and bis (phenoxymethyl) carbonate are particularly excellent as overcharge inhibitors.

Claims (6)

A nonaqueous electrolytic solution for a secondary battery, which is an electrolytic solution obtained by dissolving an electrolyte in a nonaqueous solvent and containing an aromatic carbonate represented by the formula (1).

(Wherein R ^{1 to} R ¹⁰ each independently represents a hydrogen atom or an organic group other than a methyl group and an alkoxy group, or two adjacent groups are bonded to each other to form a ring. In each of R ^{1 to} R ⁵ and R ^{6 to} R ¹⁰ , at least one of them is an organic group other than an alkoxy group having a molecular weight of 27 to 500 consisting of carbon and hydrogen or carbon, hydrogen and oxygen, or adjacent thereto. Two are bonded to each other to form a ring.) A nonaqueous electrolytic solution for a secondary battery, which is an electrolytic solution obtained by dissolving an electrolyte in a nonaqueous solvent and containing an aromatic carbonate represented by the formula (2).

(In the formula, in each of R ^{11 to} R ¹⁵ and R ^{16 to} R ²⁰ , 1 to 3 are carbon and hydrogen, or a molecular weight of 27 to 500 consisting of carbon, hydrogen and oxygen, and an organic group other than an alkoxy group. Or two adjacent groups are bonded to each other to form a ring, and the rest are all hydrogen atoms) A non-aqueous electrolyte for a secondary battery, which is an electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent and containing an aromatic carbonate represented by the formula (3).

(In the formula, in each of R ^{21 to} R ²⁵ and R ^{26 to} R ³⁰ , any one of them is a cyclohexyl group, a phenyl group or a phenoxy group, or two adjacent ones are bonded to each other to form a benzene ring. All others are hydrogen atoms)   The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 3, wherein the content of the aromatic carbonate is 0.1 to 5% by weight.   A non-aqueous electrolyte secondary battery comprising: a positive electrode and a negative electrode capable of inserting and extracting lithium; and the electrolytic solution according to claim 1.   The positive electrode contains a lithium transition metal composite oxide, and the negative electrode contains a carbon material having a (002 plane) d value of 0.335 to 0.340 nm in X-ray diffraction. 5. The nonaqueous electrolyte secondary battery according to 5.