JPWO2002047192A1 - Non-aqueous electrolyte and secondary battery using the same - Google Patents

Abstract

The present invention protects a rechargeable lithium battery suitable for a portable device, an electric vehicle, and the like from abuse of overcharge, enhances safety, and has a high electric capacity even when charge and discharge are repeatedly performed at a high maximum operating voltage. And a lithium secondary battery using the same. This electrolytic solution is a non-aqueous electrolytic solution obtained by dissolving a lithium salt as an electrolyte in an organic solvent. In the organic solvent, Ar1-R-Ar2 (where Ar1 is a phenyl group which may have a substituent, Ar2 is a bicyclic fused or non-fused aromatic group which may have a substituent, and R represents a direct bond, a methylene group or an alkylidene group). It is contained in an amount of 1 to 20% by weight. Examples of the partial hydride include partial hydrides of terphenyl and benzylbiphenyl.

Description

（但し、式中、Ａｒ_１はフェニル基又は炭素数１〜４のアルキル基で置換されたフェニル基であり、Ａｒ_２は２環の縮合芳香族基、芳香環同士が直接若しくは炭素−原子を介して結合される２環の非縮合芳香族基又は炭素数１〜４のアルキル基で置換された２環の縮合芳香族基若しくは２環の非縮合芳香族基であり、Ｒは単結合又はメチレン基又は炭素数１〜４のアルキル基で置換されたてメチレン基を示す）
３環の芳香族化合物の部分核水素化物としては、ターフェニル類、ベンジルビフェニル類、ジベンジルベンゼン類、フェニルナフタレン類、ベンジルナフタレン類及びこれらの芳香族化合物類の置換可能な水素を有する炭素に炭素数１〜４のアルキル基が置換した族化合物の群れから選択される１種又は２種以上の芳香族化合物の部分核水素化物が好ましく挙げられ、より好ましくはターフェニル又はベンジルビフェニルの部分核水素化物が挙げられる。また、上記部分核水素化物の核水素化率は１０〜６５％であることが好ましい。
更に、本発明は、前記のいずれかに記載の非水系電解液を用いた非水系リチウム二次電池である。
上記一般式（１）において、Ａｒ_１はフェニル基又は炭素数１〜４のアルキル基（以下、低級アルキル基という）が１又は２以上置換した置換フェニル基である。Ｒは単結合又は−Ｃ（Ｒ_３Ｒ_４）−で表されるメチレン基又は１又は２個の低級アルキル基が置換したメチレン基である。Ａｒ_２は低級アルキル基で置換されていてもよい２環の縮合芳香族基又は−Ａｒ_３−Ｒ_２−Ａｒ_４（但し、Ａｒ_３とＡｒ_４は独立に、低級アルキル基で置換されていてもよい単環の芳香族基であり、Ｒ_２は単結合又は−Ｃ（Ｒ_３Ｒ_４）−で表されるメチレン基又は１又は２個の低級アルキル基が置換したメチレン基でを示す）で表される２環の芳香族基である。ここで、Ｒ_３とＲ_４は独立に、水素又は低級アルキル基を示す。低級アルキル基としてはメチル基が好ましく、Ｒ及びＲ_２としては単結合、メチレン基又はエチリデン基が好ましく、２環の縮合芳香族基としてはナフチル基又はメチルナフチル基が好ましく、Ａｒ_１、Ａｒ_４で表される芳香族基としてはフェニル基又はメチルフェニル基が好ましく、Ａｒ_３で表される芳香族基としてはフェニレン基又はメチルフェニレン基が好ましく、−Ａｒ_３−Ｒ_２−Ａｒ_４で表される２環の芳香族基としては、Ｒ_２が単結合であるビフェニリル基又はメチルビフェニリル基や、Ｒ_２がメチレン基又はエチリデン基である芳香族基が好ましく、後者の例としては、フェニルメチルフェニル基、トリルメチルフェニル基、１，１−トリルエチルフェニル基、１，１−トリルエチルトリル基が挙げられる。なお、Ａｒ_１ないしＡｒ_２における置換アルキル基について、個数はＡｒ_１においては２個以下、Ａｒ_２においては６個以下が好ましく、またアルキル基はメチル基又はエチル基が好ましい。また、Ｒが直結合の場合はＡｒ_１−Ａｒ_２となり、メチレン基の場合はＡｒ_１−ＣＨ_２−Ａｒ_２となり、アルキル置換メチレン基の場合はＡｒ_１−Ｃ（Ｒ_３Ｒ_４）−Ａｒ_２となる。ここで、Ｒ_３、Ｒ_４としてはＨ、メチル基又はエチル基（但し、Ｒ_３、Ｒ_４のいずれか一つはＨ以外である）が好ましく挙げられる。
好ましい一般式（１）で表される３環の芳香族化合物を例示すれば、下記式２で表される化合物群がある。

好ましい３環の芳香族化合物を例示すると、ターフェニル類（類は異性体を含む意味である。以下、同じ）、ベンジルビフェニル類、ジベンジルベンゼン類、フェニルナフタレン類、ベンジルナフタレン類又はこれらの芳香族化合物類の置換可能な水素を有する炭素に低級アルキル基が置換した３環の芳香族化合物がある。より好ましくは、ターフェニル類又はベンジルビフェニル類である。
３環の芳香族化合物の部分核水素化物は、前記芳香族化合物の芳香環の一部が核水素化された構造の部分核水素化物が挙げられる。この部分核水素化物は、３環の芳香族化合物を水素化して得られるものであっても、シクロヘキサン環等の核水素化された環とベンゼン環等の芳香環の両者を最初から有する化合物であっても差し支えないが、前者の方が入手容易である。
３環の芳香族化合物を部分核水素化して得られるものは、通常、核水素化の程度が異なる混合物として得られるが、本発明で使用する部分核水素化物は、核水素化の程度が異なる混合物であっても、これを蒸留等で分離して得られる核水素化の程度が揃った化合物であってもよい。また、未水素化物及び完全核水素化物を蒸留等で分離して得られる部分核水素化物の含有率が７０ｗｔ％以上としたものも好ましい。このようにして得られる部分水素化物は、上記一般式（１）で表される芳香族化合物の芳香環の１つ以上がシクロヘキサン環、シクロヘキセン環となったものなどがある。なお、シクロヘキセン環等の不飽和脂肪族環は少ない方が望ましい場合もあるが、その場合は水素化条件等をそのように制御する。
これらの部分水素化物は、前出のような方法により単離した１種類を単独で用いても、過充電時の電池保護効果と最大作動電圧を高くした場合における電池特性向上の効果を両立しうるが、部分水素化物の混合物を用いることにより、添加剤の粘度をさらに下げ、部分水素化物１種類のみを添加剤として用いる場合よりも電解液の粘度を上昇させにくく、その結果大電流にて充放電する場合の電池特性を良好にすると考えられ好ましい。
ここで、芳香族化合物を部分核水素化する場合の核水素化率は、完全核水素化された場合を１００％としたとき、１０〜５０％、好ましくは１５〜４０％である。５０％を上回ると安全性の付与が困難になり、１０％を下回ると、安全性と電気容量保持が両立する効果が薄くなる。
好ましい部分核水素化物を例示すると、芳香族化合物を部分核水素化する場合は、前記で好ましいとした芳香族化合物の部分核水素化物が挙げられる。例えば、ターフェニルの部分核水素化物、ベンジルビフェニルの部分核水素化物、ジベンジルベンゼンの部分核水素化物、ジベンジルトルエンの部分核水素化物、ジ（α−メチルベンジル）キシレンの部分核水素化物、ベンジルナフタレンの部分核水素化物及びフェニルナフタレンの部分核水素化物等を挙げることができる。
好ましい部分核水素化物の化合物を例示すると下記式（３）で表される化合物の群れが挙げられる。

上記化合物を化合物名で例示すれば、２−シクロヘキシルビフェニル、（２’−フェニル）−シクロヘキシルベンゼン、２−フェニルビシクロヘキシル、１，２−ジシクロヘキシルベンゼン、３−シクロヘキシルビフェニル、（３’−フェニル）−シクロヘキシルベンゼン、３−フェニルビシクロヘキシル、１，３−ジシクロヘキシルベンゼン、２−（シクロヘキシルメチル）ビフェニル、（２’−ベンジル）−シクロヘキシルベンゼン、２−ベンジルシクロヘキシルベンゼン、（２’−シクロヘキシルメチル）−シクロヘキシルベンゼン、２−ベンジルビシクロヘキシル、１−シクロヘキシルメチル−２−シクロヘキシルベンゼン、４−（シクロヘキシルメチル）ビフェニル、（４’−ベンジル）−シクロヘキシルベンゼン、４−ベンジルシクロヘキシルベンゼン、（４’−シクロヘキシルメチル）−シクロヘキシルベンゼン、４−ベンジルビシクロヘキシル、１−シクロヘキシルメチル−４−シクロヘキシルベンゼンなどが挙げられる。これらの化合物は単独で用いてもよく、また二種類以上の化合物を混合して用いてもよい。
以下に、本発明の再充電可能なリチウム電池の実施形態について説明する。
本発明で非水系電解液に含有させる化合物は、過充電の初期領域で酸化反応を起こし、その作用で過充電から電池を保護し、安全性を付与する。また、作動電圧を高くして充放電を繰り返した場合でも電池特性に悪影響を及ぼさないという特徴を有し、高い電気容量の保持を可能にできる過充電防止剤として作用する。これについて、詳しい機構は現在判っていないが、リチウム電池電解液に添加して酸化電位を測定した結果より、ｏ−ターフェニルとほぼ同じ電位で酸化が開始されることと、それより若干低い電位の領域で、ｏ−ターフェニルを添加した電解液には微小な酸化を示す電流が流れているのに対し、本発明の化合物を添加した電解液にはそれが殆どないためと考えられる。
また、非水系電解液に含有させる前記芳香族化合物の部分水素化物の量（二種類以上の化合物を混合して使用する場合はその合計）は、電解液溶媒として用いる有機溶媒に対し０．１〜２０重量％とするが、好ましくは１〜１０重量％、更に好ましくは２〜５重量％とするのがよい。
また、本発明における前記芳香族化合物の部分水素化物は、本発明の効果を阻害しない範囲であれば、既知の過充電防止効果を持つ添加剤との併用を妨げるものではないが、前記芳香族化合物の部分水素化物は上記範囲の含有量が必要である。
本発明の化合物を使用して調製した電解液を用いて作成する二次電池を構成する部材は特に限定されず、従来使用されている種々の構成部材を使用できる。例えば、前記公報に記載されたような構成や部材が使用できる。
例えば正極の材料としては、リチウムを含むもので充放電可能なリチウム電池用に一般的に使用可能なものであれば何れも使用できるがＬｉＭｎ_２Ｏ_４、ＬｉＣｏＯ_２やＬｉＮｉＯ_２等の複合金属酸化物及びリチウムを含む層間化合物等が例示される。これらのリチウム化合物粉末、導電性粉末及び結合剤とを混合したスラリーをアルミニウム箔に塗布後、乾燥し、適宜加工することにより正極箔を作製することができる。
負極の材料としては、リチウムを吸蔵放出可能なものであり充放電可能なリチウム電池用に一般的に使用可能なものであれば何れも使用できるが、炭素の六角網目の層間にリチウムをインターカレートした炭素質系挿入化合物が例示される。炭素質系挿入化合物は炭素材料に電池を組んだ後に電気化学的に電解質のリチウムを挿入することで調製してもよいし、最初から炭素粉と電解質とを予備混合して調製してもよい。このような炭素質系挿入化合物又は炭素と結合剤とを混合したスラリーを銅箔に塗布後、乾燥し、適宜加工することにより負極箔を作製することができる。
セパレーターとしては、充放電可能なリチウム電池用に一般的に使用可能なものであれば何れも使用できるが、微孔性のポリプロピレン、ポリエチレン膜等が例示できる。
非水系電解液は有機溶媒と溶質を適宜組み合わせて使用され、充放電可能なリチウム電池用に一般的に使用可能なものであれば何れも使用できるが、有機溶媒としては、エチレンカーボネート、ジメチルカーボネート、ジエチルカーボネート、プロピレンカーボネート、メチルエチルカーボネート等を含有する溶剤が例示され、液体電解質溶質としては、ヘキサフルオロリン酸リチウム（ＬｉＰＦ_６）、テトラフルオロ硼酸リチウム（ＬｉＢＦ_４）、トリフルオロメタンスルホン酸リチウム（ＬｉＣＦ_３ＳＯ_３）等が例示される。
以上の構成材料を、正極（アルミニウム箔）／セパレーター（非水系電解液含浸）／負極（銅箔）／セパレーター（非水系電解液含浸）と積層して、電池を構成するが、各々の材料の組み合わせについては、例えば黒鉛系の炭素材料を用いた場合は電解液にはプロピレンカーボネートは適さないなどの相性があるので、適宜選択する必要がある。電池の形状としては、充放電可能なリチウム電池用に一般的に製造されている形状であれば、何れにも適用できるが、角柱状電池や小形のコイン形電池等が例示できる。
本発明で電解液中に含有させる化合物は、過充電防止効果と高電圧領域における安定性とを併せ持つという特異的な作用を有する。また、本発明の化合物を添加後であっても電解液の粘度を上昇させにくいので、大電流にて充放電した場合の電池特性も良好になると考えられる。
発明を実施するための最良の形態
次に、実施例及び比較例を挙げて、本発明を具体的に説明するが、これらは、本発明を何ら限定するものではない。
本発明評価方法により評価特性を測定する実験を行った。基本電解液には、エチレンカーボネートとジエチルカーボネートが容量比１：１で構成された有機溶媒に、電解質として六フッ化リン酸リチウム（ＬｉＰＦ_６）を１モル／Ｌ溶解したものを使用した。
実施例１
上記の基本電解液１００ｇに、部分水素化芳香族化合物として２−シクロヘキシルビフェニルを２．０ｇ添加し、電解液Ａを調整した。
また、上記と同様の方法で、基本電解液１００ｇに、部分水素化芳香族化合物として、２’−フェニルシクロヘキシルベンゼン約１．０ｇ、２−シクロヘキシルビフェニル約０．６ｇ、２−フェニルビシクロヘキシル約０．３ｇ、１，２−ジシクロヘキシルベンゼン約０．１ｇで構成される混合物２．０ｇを添加し、電解液Ｂを調整した。
更に、上記と同様の方法で、基本電解液１００ｇに、部分水素化芳香族化合物として、２’−フェニルシクロヘキシルベンゼン約０．９ｇ、２−シクロヘキシルビフェニル約０．５ｇ、２−フェニルビシクロヘキシル約０．２ｇ、１，２−ジシクロヘキシルベンゼン約０．１ｇで構成される混合物１．７ｇと、ｏ−ターフェニル０．３ｇを添加し、電解液Ｃを調整した。
上記と同様の方法で、基本電解液１００ｇに、部分水素化芳香族化合物として、３’−フェニルシクロヘキシルベンゼン約０．５ｇ、３−シクロヘキシルビフェニル約０．４ｇ、３−フェニルビシクロヘキシル約０．６ｇ、１，３−ジシクロヘキシルベンゼン約０．５ｇで構成される混合物２．０ｇを添加し、電解液Ｄを調整した。
また、上記と同様の方法で、基本電解液１００ｇに、部分水素化芳香族化合物として、２−（シクロヘキシルメチル）ビフェニル約０．１ｇ、２’−（シクロヘキシルメチル）シクロヘキシルベンゼン約０．１ｇ、２−ベンジルシクロヘキシルベンゼン約０．２ｇ、２’−ベンジルシクロヘキシルベンゼン約１．１ｇで構成される混合物１．７ｇと、ｏ−ベンジルビフェニル０．３ｇを添加し、電解液Ｅを調整した。
比較のため、上記と同様の方法で、基本電解液１００ｇにｏ−ターフェニルを２．０ｇ添加し、電解液Ｆを調整した。
実施例２
天然黒鉛を平均粒径０．８μｍに粉砕したもの８０重量％、ＬｉＰＦ_６を平均粒径５μｍに粉砕したもの１０重量％に、結合剤としてポリフッ化ビニリデンを１０重量％混合し、Ｎ−メチル−２−ピロリドンでペースト状にしたものを銅箔に塗布し、乾燥した後、ロールプレス機で圧縮成型にて加工し、負極を調製した。
ＬｉＣｏＯ_２粉末８５重量％とポリフッ化ビニリデン７重量％、アセチレンブラック８重量％を混合し、Ｎ−メチル−２−ピロリドンでペースト状にしたものをアルミニウム箔に塗布し、乾燥した後、ロールプレス機で圧縮成型にて加工し、正極を調製した。
所定の大きさに加工した正極と負極の間に、前記の方法で調整した電解液Ａを注入し、同じく電解液Ａを多孔質ポリプロピレンに含浸させたものを挟持して直径２０ｍｍ、厚み５ｍｍのコイン電池を作製した。
実施例３〜６
電解液として前記の方法で調整した電解液Ｂ、Ｃ、Ｄ又はＥを使用した以外は実施例２と同様にコイン電池を作製した。
比較例１
電解液として電解液Ｆを使用した以外は実施例２と同様にコイン電池を作製した。
比較例２
前記基本電解液をそのまま使用した以外は実施例２と同様にコイン電池を作製した。
このようにして作製した電池のサイクル性能を比較するために、上限電圧４．１Ｖとして１Ｃの定電流充電を行い、その後４．１Ｖで３時間充電し満充電状態とした。その後下限電圧を３．０Ｖとして１Ｃで放電を行い、この様な充放電を２０サイクル迄繰り返した。
１サイクル目と２０サイクル目の放電容量を計測して、芳香族炭化水素の部分核水素化物の添加が容量に及ぼす影響を調べた。それぞれ３回試験を行い、表１に試験前（１サイクル目）と試験後（２０サイクル目）の放電容量の比率の平均値を示した。
次いで上限電圧４．２Ｖとして同様の充放電評価を行い、先の場合と同様に１サイクル目と２０サイクル目の放電容量を計測した。同様にそれぞれ３回試験を行い、表１に試験前（１サイクル目）と試験後（２０サイクル目）の放電容量の比率の平均値を示した。
【表１】

試験前と試験後の放電容量の比率（３．０〜４．１Ｖ）について、本発明の化合物を添加した電解液Ａ、Ｂ、Ｃ、Ｄ、Ｅ及びｏ−ターフェニルを添加した電解液Ｆを使用したセルは、無添加の電解液を使用したセルより若干低下しているが、どれも大差ないレベルである。
また、試験前と試験後の放電容量の比率（３．０〜４．２Ｖ）については、本発明の化合物を添加した電解液Ａ、Ｂ、Ｃ、Ｄ、Ｅを配合したセルは、９０％以上のレベルを維持しているのに対し、ｏ−ターフェニルを添加した電解液Ｆを配合したセルでは８７％と低下している。
これは、動作電圧４．１Ｖと４．２Ｖの間でｏ−ターフェニルがわずかながら反応し、その結果最大作動電圧を高くした際の電池特性に影響を与えるのに対し、本発明の添加剤はその反応がなく、その結果電池特性に影響を与えないからだと考えられる。この推定を補足するため、酸化電位を測定し、電位と添加剤の酸化反応の度合いを調べた。
実施例７
作用極にＳＵＳ３０４（直径　１６．０ｍｍ、厚み　６．０ｍｍ）、対極にリチウム（直径　２０ｍｍ、厚み　０．５５ｍｍ）、ポリプロピレン製セパレータを使用し、各実施例、比較例記載の電解液０．５ｍｌを入れて、評価用のセルを作製した。
このセルに、３．０Ｖから５．０Ｖの電圧（対Ｌｉ／Ｌｉ^＋）を毎秒５ｍＶの速度で印加し、その間に通電した電流を測定すると共に、電流密度値を測定した。測定された最大通電流密度（μＡ／ｃｍ^２）を表２に示す。
【表２】

これらの結果から、いずれの化合物も４．５〜４．７Ｖの比較的高い電圧領域では酸化反応を起こしているが、４．０〜４．２Ｖの比較的低い電圧領域ではｏ−ターフェニルを配合した電解液Ｆを使用すると、本発明の化合物を配合した電解液Ａ、Ｂ、Ｃ、Ｄ、Ｅを使用した場合に比べ酸化電流の値が大きいことが判る。
実施例８
更に、このようにして作製した電池の安全性を比較するために、充放電評価で２０サイクル終了後の電池を再び４．２Ｖで満充電状態とし、その後１Ｃで充電を継続して過充電を起こさしめ、電池が破裂又は発火する前に過充電防止機能が働くかどうか確認した。その結果を表３に示す。本発明の化合物を用いた電解液Ａ〜Ｅの何れも過充電防止機能を有し、安全性を向上する効果があることが示された。
【表３】

産業上の利用可能性
本発明によれば、充放電可能なリチウム電池において、過充電から電池が保護され、発火、破裂等の危険が回避できる。また、最大作動電圧を高くした場合でも充放電サイクルに伴う電池容量低下が少ないので、有効に電気容量をとりだし、且つ長期の使用を可能とする。TECHNICAL FIELD The present invention relates to an aromatic additive to be added to an electrolyte for preventing overcharge of a chargeable / dischargeable lithium battery using a nonaqueous electrolyte, and a lithium secondary battery using the same.
BACKGROUND ART Lithium has an extremely low potential and an extremely large charge per weight, and thus is suitable as a high-voltage, high-capacity rechargeable battery material. On the other hand, lithium is an extremely reactive and unstable substance, so there is a high risk of fire, etc.Also, in a battery capable of high capacity, the stored energy is large, and the electrochemical reaction has run away. The danger in the case is even greater. In order to use rechargeable lithium batteries (lithium rechargeable batteries) as batteries for mobile devices and electric vehicles, securing safety is an important issue, and research and development are being actively pursued. .
Therefore, it is possible to charge and discharge using a carbon-based material capable of inserting and extracting lithium ions as the negative electrode active material, using a lithium-containing transition metal oxide as the positive electrode active material, and using a non-aqueous solvent in which a lithium salt is dissolved as the electrolyte. In such batteries, excessive extraction and insertion of lithium occurs at the electrode during overcharge, and as a result, the organic solvent electrolyte is decomposed and the battery eventually generates abnormal heat, There is a problem that the battery ignites and explodes.
In order to ensure the safety of chargeable and dischargeable lithium batteries, research and development are being actively conducted, and safety valves, current cutoff valves, protection circuits, etc. that have been provided with overcharge countermeasures have been proposed. In order to ensure the performance, it is required to take simple and triple safety measures that are reliable and do not sacrifice the battery characteristics. As one of such measures, for example, in JP-A-2000-58116, by using an electrolytic solution containing a terphenyl compound such as o-terphenyl which may be substituted with an alkyl group, the electrolytic solution is reduced. It is disclosed that safety can be ensured even when the provided non-aqueous secondary battery is placed in an overcharged state, and that there is little adverse effect on battery characteristics such as low-temperature characteristics and storage characteristics. According to this, it is confirmed that the terphenyl compound has an overcharge protection effect and has a reduced adverse effect on low-temperature characteristics and storage characteristics, as compared with the additives previously proposed. In the above publication, the prior art is also described as follows.
In those proposed in JP-A-7-302614 and US Pat. No. 5,709,968, the anisole derivative effectively acts on overcharging, but adversely affects cycle characteristics and storage characteristics. In addition to being oxidized and decomposed at a potential of about 4.5 V to generate gas and form a polymer, which consumes overcharge and protects the battery. May dissolve and overcharge cannot be consumed. After all, aromatic compounds such as anisole derivatives having a π-electron orbit cannot always be said to suppress overcharge. In the method proposed in US Pat. No. 5,879,834, biphenyl used as an additive for the electrolyte has low polarity and low solubility in the electrolyte, so that the additive partially precipitates at the time of low-temperature operation. Causes deterioration of battery characteristics. Further, 3-chloro-thiophene is irritating, has a strong odor, is difficult to handle, and is liable to be oxidatively decomposed. Furan is also liable to be oxidatively decomposed, and any of the compounds adversely affects battery characteristics. There is a problem.
In addition, Japanese Patent Application Laid-Open No. 10-74537 proposes to add various compounds in order to improve charge / discharge characteristics. Among them, aromatic compounds and partially hydrogenated aromatic compounds are included. I have.
On the other hand, in recent years, as the stability of the electrolyte in a high voltage region has been improved, it has been required to increase the maximum operating voltage. When the maximum operating voltage increases, the electric capacity inherent in the battery system as an assembly of the members can be more effectively used, and the charge / discharge capacity of the battery can be substantially improved.
However, when the maximum operating voltage is increased as described above, when the above-mentioned terphenyl compound is used as an electrolyte additive, the battery characteristics, particularly, the electric capacity during repeated charge and discharge are significantly deteriorated. Be recognized. This is probably because the terphenyl compound gradually causes undesirable reactions such as oxidative decomposition and polymerization in the high voltage region.
An object of the present invention is to provide a chargeable / dischargeable lithium battery having an overcharge prevention effect equal to or higher than that of a conventional electrolyte additive, and in a high voltage region as compared with a conventional electrolyte additive. It is to provide a stable additive.
DISCLOSURE OF THE INVENTION As a result of intensive studies to solve these problems, the present inventors have found that a specific three-ring partially hydrogenated aromatic compound shows more excellent performance and completed the present invention. did.
That is, the present invention relates to a non-aqueous electrolyte obtained by dissolving a lithium salt as an electrolyte in an organic solvent, wherein the partially-nuclear hydride of a tricyclic aromatic compound represented by the following general formula (1) is 0.1 to 0.1%. It is a non-aqueous electrolyte solution containing 20% by weight.

(Wherein, Ar ₁ is a phenyl group or a phenyl group substituted with an alkyl group having 1 to 4 carbon atoms, and Ar ₂ is a condensed bicyclic aromatic group, wherein the aromatic rings are directly bonded to each other or a carbon atom is A two-ring non-fused aromatic group or a two-ring fused aromatic group or a two-ring non-fused aromatic group substituted with an alkyl group having 1 to 4 carbon atoms, wherein R is a single bond or Represents a methylene group or a methylene group substituted with an alkyl group having 1 to 4 carbon atoms)
Examples of partial hydrides of tricyclic aromatic compounds include terphenyls, benzylbiphenyls, dibenzylbenzenes, phenylnaphthalenes, benzylnaphthalenes, and carbons having hydrogen which can be substituted in these aromatic compounds. A partial hydride of one or more aromatic compounds selected from the group of group compounds substituted with an alkyl group having 1 to 4 carbon atoms is preferable, and a partial nucleus of terphenyl or benzylbiphenyl is more preferable. Hydrides. Further, the nuclear hydrogenation rate of the partial nuclear hydride is preferably 10 to 65%.
Further, the present invention is a non-aqueous lithium secondary battery using the non-aqueous electrolyte described in any of the above.
In the general formula (1), Ar ₁ is a phenyl group or a substituted phenyl group in which one or more alkyl groups having 1 to 4 carbon atoms (hereinafter referred to as lower alkyl groups) are substituted. R is a single bond, a methylene group represented by —C (R ₃ R ₄ ) —, or a methylene group substituted by one or two lower alkyl groups. Ar ₂ is a bicyclic fused aromatic group which may be substituted with a lower alkyl group or —Ar ₃ —R ₂ —Ar ₄ (provided that Ar ₃ and Ar ₄ are independently substituted with a lower alkyl group; R ₂ is a single bond or a methylene group represented by —C (R ₃ R ₄ ) — or a methylene group substituted by one or two lower alkyl groups. Is a bicyclic aromatic group represented by Here, R ₃ and R ₄ independently represent hydrogen or a lower alkyl group. The lower alkyl group is preferably a methyl group, R and R ₂ are preferably a single bond, a methylene group or an ethylidene group, and the bicyclic fused aromatic group is preferably a naphthyl group or a methylnaphthyl group, and Ar ₁ and Ar ₄ preferably a phenyl group or a methylphenyl group as in represented by an aromatic group, the aromatic group represented by Ar ₃ preferably is a phenylene group or a methyl phenylene group represented by -Ar ₃ -R ₂ -Ar ₄ The bicyclic aromatic group is preferably a biphenylyl group or a methylbiphenylyl group in which R ₂ is a single bond, or an aromatic group in which R ₂ is a methylene group or an ethylidene group. Examples of the latter are phenylmethyl Examples include a phenyl group, a tolylmethylphenyl group, a 1,1-tolylethylphenyl group, and a 1,1-tolylethyltolyl group. Incidentally, the absence Ar ₁ for substituted alkyl groups in Ar _2, number is 2 or less in Ar _1, preferably 6 or less in Ar _2, the alkyl group is preferably a methyl group or an ethyl group. When R is a direct bond, it is Ar ₁ -Ar ₂ , when it is a methylene group, it is Ar ₁ -CH ₂ -Ar ₂ , and when it is an alkyl-substituted methylene group, it is Ar ₁ -C (R ₃ R ₄ ) -Ar It becomes ₂ . Here, R ₃ and R ₄ are preferably H, a methyl group or an ethyl group (provided that one of R ₃ and R ₄ is other than H).
An example of a preferred three-ring aromatic compound represented by the general formula (1) is a compound group represented by the following formula 2.

Preferred examples of the three-ring aromatic compound include terphenyls (the meanings include isomers; the same applies hereinafter), benzylbiphenyls, dibenzylbenzenes, phenylnaphthalenes, benzylnaphthalenes, and aromatic compounds thereof. There is a three-ring aromatic compound in which a carbon having a substitutable hydrogen of a group III compound is substituted by a lower alkyl group. More preferred are terphenyls and benzylbiphenyls.
Examples of the partial hydride of a three-ring aromatic compound include partial hydrides having a structure in which a part of the aromatic ring of the aromatic compound is nuclei-hydrogenated. This partially nuclear hydride is a compound having both a nuclear hydrogenated ring such as a cyclohexane ring and an aromatic ring such as a benzene ring from the beginning, even if it is obtained by hydrogenating a three-ring aromatic compound. Although there is no problem, the former is easier to obtain.
The product obtained by partial nuclear hydrogenation of a three-ring aromatic compound is usually obtained as a mixture having different degrees of nuclear hydrogenation, but the partial nuclear hydride used in the present invention has a different degree of nuclear hydrogenation. It may be a mixture or a compound having a uniform degree of nuclear hydrogenation obtained by separating it by distillation or the like. It is also preferable that the content of the partial hydride obtained by separating the unhydride and the complete hydride by distillation or the like is 70 wt% or more. Examples of the partially hydride thus obtained include those in which one or more of the aromatic rings of the aromatic compound represented by the general formula (1) is a cyclohexane ring or a cyclohexene ring. In some cases, it is desirable to reduce the number of unsaturated aliphatic rings such as cyclohexene rings. In such a case, the hydrogenation conditions and the like are controlled as such.
Even when these partial hydrides are used alone by one kind isolated by the method described above, they can achieve both the effect of protecting the battery during overcharge and the effect of improving the battery characteristics when the maximum operating voltage is increased. However, by using a mixture of partial hydrides, the viscosity of the additive is further reduced, and the viscosity of the electrolyte is less likely to increase than when only one type of partial hydride is used as an additive. It is considered that battery characteristics when charging and discharging are improved, which is preferable.
Here, the nuclear hydrogenation rate in the case of partial nuclear hydrogenation of an aromatic compound is 10 to 50%, preferably 15 to 40%, when 100% is taken for complete nuclear hydrogenation. If it is more than 50%, it is difficult to provide safety, and if it is less than 10%, the effect of achieving both the safety and the retention of the electric capacity is reduced.
When a partial nuclear hydride of an aromatic compound is exemplified as a preferred partial nuclear hydride, the partial nuclear hydride of the aromatic compound which has been preferred above may be mentioned. For example, a partial hydride of terphenyl, a partial hydride of benzylbiphenyl, a partial hydride of dibenzylbenzene, a partial hydride of dibenzyltoluene, a partial hydride of di (α-methylbenzyl) xylene, Partial hydrides of benzylnaphthalene and partially hydrides of phenylnaphthalene can be exemplified.
Examples of preferred partial hydride compounds include a group of compounds represented by the following formula (3).

When the above compound is exemplified by a compound name, 2-cyclohexylbiphenyl, (2′-phenyl) -cyclohexylbenzene, 2-phenylbicyclohexyl, 1,2-dicyclohexylbenzene, 3-cyclohexylbiphenyl, (3′-phenyl)- Cyclohexylbenzene, 3-phenylbicyclohexyl, 1,3-dicyclohexylbenzene, 2- (cyclohexylmethyl) biphenyl, (2′-benzyl) -cyclohexylbenzene, 2-benzylcyclohexylbenzene, (2′-cyclohexylmethyl) -cyclohexylbenzene , 2-benzylbicyclohexyl, 1-cyclohexylmethyl-2-cyclohexylbenzene, 4- (cyclohexylmethyl) biphenyl, (4'-benzyl) -cyclohexylbenzene, 4-benzylcyclo Hexyl benzene, (4'-cyclohexylmethyl) - cyclohexyl benzene, 4-benzyl bicyclohexyl, 1-cyclohexylmethyl-4-cyclohexyl benzene. These compounds may be used alone or as a mixture of two or more compounds.
Hereinafter, embodiments of the rechargeable lithium battery of the present invention will be described.
The compound contained in the non-aqueous electrolytic solution in the present invention causes an oxidation reaction in the initial region of overcharge, and protects the battery from overcharge by its action and provides safety. Further, it has a feature that the battery characteristics are not adversely affected even when charge and discharge are repeated with a high operating voltage, and acts as an overcharge prevention agent that can maintain a high electric capacity. Although the detailed mechanism of this is not known at present, the oxidation potential was measured at the same potential as that of o-terphenyl, and the potential was slightly lower than that of o-terphenyl. It is considered that in the region of the above, a current showing minute oxidation flows in the electrolytic solution to which o-terphenyl is added, whereas almost no current flows in the electrolytic solution to which the compound of the present invention is added.
The amount of the partially hydrided aromatic compound to be contained in the non-aqueous electrolyte (the sum of two or more compounds when used as a mixture) is 0.1% with respect to the organic solvent used as the electrolyte solvent. It is preferably 20 to 20% by weight, preferably 1 to 10% by weight, and more preferably 2 to 5% by weight.
Further, the partially hydride of the aromatic compound in the present invention does not hinder the combined use with a known additive having an overcharge preventing effect as long as the effect of the present invention is not impaired. The partial hydride of the compound must have a content in the above range.
The members constituting the secondary battery prepared using the electrolytic solution prepared using the compound of the present invention are not particularly limited, and various conventionally used members can be used. For example, the configurations and members described in the above publication can be used.
For example, as the material of the positive electrode, any material can be used as long as it contains lithium and can be generally used for a rechargeable lithium battery. However, a composite metal oxide such as LiMn ₂ O ₄ , LiCoO ₂ or LiNiO ₂ can be used. And intercalation compounds containing lithium. A positive electrode foil can be prepared by applying a slurry obtained by mixing the lithium compound powder, the conductive powder and the binder to an aluminum foil, followed by drying and processing as appropriate.
As the material of the negative electrode, any material can be used as long as it can absorb and release lithium and can be generally used for rechargeable lithium batteries, but lithium is intercalated between carbon hexagonal mesh layers. And carbonaceous insertion compounds. The carbonaceous insertion compound may be prepared by electrochemically inserting lithium of the electrolyte after assembling the battery with the carbon material, or may be prepared by premixing the carbon powder and the electrolyte from the beginning. . A negative electrode foil can be prepared by applying such a slurry containing a carbonaceous insertion compound or a mixture of carbon and a binder to a copper foil, drying and appropriately processing the copper foil.
Any separator can be used as long as it can be generally used for a rechargeable lithium battery, and examples thereof include microporous polypropylene and polyethylene membranes.
The non-aqueous electrolyte is used by appropriately combining an organic solvent and a solute, and any of those generally usable for a rechargeable lithium battery can be used. Examples of the organic solvent include ethylene carbonate and dimethyl carbonate. , Diethyl carbonate, propylene carbonate, methyl ethyl carbonate and the like. Examples of the liquid electrolyte solute include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and lithium trifluoromethane sulfonate ( LiCF ₃ SO ₃ ) and the like.
The above constituent materials are laminated with a positive electrode (aluminum foil) / separator (impregnated with non-aqueous electrolyte) / negative electrode (copper foil) / separator (impregnated with non-aqueous electrolyte) to form a battery. As for the combination, for example, when a graphite-based carbon material is used, propylene carbonate is not suitable for the electrolytic solution. The shape of the battery can be applied to any shape that is generally manufactured for a chargeable / dischargeable lithium battery, and examples thereof include a prismatic battery and a small coin battery.
The compound contained in the electrolytic solution in the present invention has a specific action of having both an overcharge prevention effect and stability in a high voltage region. Further, even after the addition of the compound of the present invention, it is difficult to increase the viscosity of the electrolytic solution, so that it is considered that the battery characteristics when charging and discharging with a large current are improved.
BEST MODE FOR CARRYING OUT THE INVENTION Next, the present invention will be described specifically with reference to examples and comparative examples, but these do not limit the present invention at all.
An experiment for measuring evaluation characteristics by the evaluation method of the present invention was performed. As the basic electrolytic solution, a solution prepared by dissolving 1 mol / L of lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in an organic solvent composed of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used.
Example 1
To 100 g of the above basic electrolyte, 2.0 g of 2-cyclohexylbiphenyl was added as a partially hydrogenated aromatic compound to prepare an electrolyte A.
In the same manner as described above, about 1.0 g of 2'-phenylcyclohexylbenzene, about 0.6 g of 2-cyclohexylbiphenyl, and about 0 g of 2-phenylbicyclohexyl were used as partially hydrogenated aromatic compounds in 100 g of the basic electrolyte. Then, 2.0 g of a mixture composed of 0.3 g and about 0.1 g of 1,2-dicyclohexylbenzene was added to prepare an electrolyte B.
Further, in the same manner as described above, about 0.9 g of 2'-phenylcyclohexylbenzene, about 0.5 g of 2-cyclohexylbiphenyl, and about 0 g of 2-phenylbicyclohexyl in 100 g of the basic electrolyte as partially hydrogenated aromatic compounds. Electrolyte solution C was prepared by adding 1.7 g of a mixture composed of 0.2 g and about 0.1 g of 1,2-dicyclohexylbenzene, and 0.3 g of o-terphenyl.
In the same manner as described above, about 0.5 g of 3′-phenylcyclohexylbenzene, about 0.4 g of 3-cyclohexylbiphenyl, and about 0.6 g of 3-phenylbicyclohexyl as a partially hydrogenated aromatic compound in 100 g of the basic electrolyte. And 2.0 g of a mixture composed of about 0.5 g of 1,3-dicyclohexylbenzene was added to prepare an electrolyte D.
In the same manner as described above, about 0.1 g of 2- (cyclohexylmethyl) biphenyl and about 0.1 g of 2 ′-(cyclohexylmethyl) cyclohexylbenzene as a partially hydrogenated aromatic compound were added to 100 g of the basic electrolyte. The electrolyte solution E was prepared by adding 1.7 g of a mixture composed of about 0.2 g of -benzylcyclohexylbenzene and about 1.1 g of 2'-benzylcyclohexylbenzene and 0.3 g of o-benzylbiphenyl.
For comparison, 2.0 g of o-terphenyl was added to 100 g of the basic electrolyte to prepare an electrolyte F in the same manner as described above.
Example 2
80% by weight of natural graphite pulverized to an average particle size of 0.8 μm, 10% by weight of LiPF ₆ pulverized to an average particle size of 5 μm, and 10% by weight of polyvinylidene fluoride as a binder were mixed. A paste made with 2-pyrrolidone was applied to a copper foil, dried, and then processed by compression molding using a roll press to prepare a negative electrode.
A mixture of 85% by weight of LiCoO ₂ powder, 7% by weight of polyvinylidene fluoride, and 8% by weight of acetylene black, made into a paste with N-methyl-2-pyrrolidone, applied to an aluminum foil, dried, and then roll-pressed And processed by compression molding to prepare a positive electrode.
The electrolyte solution A prepared by the above method is injected between the positive electrode and the negative electrode processed into a predetermined size, and the electrolyte solution A impregnated with porous polypropylene is sandwiched between the positive electrode and the negative electrode to have a diameter of 20 mm and a thickness of 5 mm. A coin battery was manufactured.
Examples 3 to 6
A coin battery was produced in the same manner as in Example 2, except that the electrolytic solution B, C, D or E prepared by the above method was used as the electrolytic solution.
Comparative Example 1
A coin battery was produced in the same manner as in Example 2 except that the electrolytic solution F was used as the electrolytic solution.
Comparative Example 2
A coin battery was produced in the same manner as in Example 2 except that the basic electrolyte was used as it was.
In order to compare the cycle performance of the battery thus manufactured, the battery was charged at a constant current of 1 C at an upper limit voltage of 4.1 V, and then charged at 4.1 V for 3 hours to obtain a fully charged state. Thereafter, discharge was performed at 1 C with the lower limit voltage set to 3.0 V, and such charge / discharge was repeated up to 20 cycles.
The discharge capacity at the 1st cycle and the 20th cycle was measured, and the effect of the addition of the partial hydride of the aromatic hydrocarbon on the capacity was examined. Each test was performed three times, and Table 1 shows the average value of the discharge capacity ratio before the test (first cycle) and after the test (20th cycle).
Next, the same charge / discharge evaluation was performed at an upper limit voltage of 4.2 V, and the discharge capacities at the first cycle and the 20th cycle were measured in the same manner as in the previous case. Similarly, the test was performed three times, and Table 1 shows the average value of the discharge capacity ratio before the test (first cycle) and after the test (20th cycle).
[Table 1]

Regarding the ratio of the discharge capacity before and after the test (3.0 to 4.1 V), the electrolytic solutions A, B, C, D, E containing the compound of the present invention and the electrolytic solution F containing o-terphenyl were added. In the cell using, there was slightly lower than the cell using the electrolyte solution without any additive, but none of them was at a level that was much different.
Also, regarding the ratio of the discharge capacity before the test and after the test (3.0 to 4.2 V), the cells containing the electrolytic solutions A, B, C, D and E to which the compound of the present invention was added were 90% While the above level is maintained, the cell containing the electrolytic solution F to which o-terphenyl is added is reduced to 87%.
This means that the o-terphenyl reacts slightly between operating voltages of 4.1 V and 4.2 V, thus affecting the battery properties when the maximum operating voltage is increased, whereas the additive of the present invention It is considered that there is no reaction, and as a result, the battery characteristics are not affected. To supplement this estimation, the oxidation potential was measured, and the potential and the degree of the oxidation reaction of the additive were examined.
Example 7
Using SUS304 (diameter 16.0 mm, thickness 6.0 mm) for the working electrode, lithium (diameter 20 mm, thickness 0.55 mm) for the counter electrode and a polypropylene separator, 0.5 ml of the electrolyte described in each of the examples and comparative examples was used. Then, a cell for evaluation was prepared.
A voltage of 3.0 V to 5.0 V (vs. Li / Li ⁺ ) was applied to this cell at a rate of 5 mV per second, and a current flowing during that time was measured, and a current density value was measured. Table 2 shows the measured maximum current density (μA / cm ² ).
[Table 2]

From these results, all of the compounds undergo an oxidation reaction in a relatively high voltage range of 4.5 to 4.7 V, but o-terphenyl is generated in a relatively low voltage range of 4.0 to 4.2 V. It can be seen that the use of the blended electrolytic solution F results in a larger value of the oxidation current than the case of using the electrolytic solutions A, B, C, D, and E blended with the compound of the present invention.
Example 8
Further, in order to compare the safety of the battery manufactured in this way, the battery after the completion of 20 cycles in charge / discharge evaluation was again set to a full charge state at 4.2 V, and thereafter was continuously charged at 1 C to overcharge. It was woken up, and it was checked whether the overcharge prevention function worked before the battery exploded or ignited. Table 3 shows the results. It was shown that each of the electrolytes A to E using the compound of the present invention has an overcharge preventing function and has an effect of improving safety.
[Table 3]

INDUSTRIAL APPLICABILITY According to the present invention, in a chargeable / dischargeable lithium battery, the battery is protected from overcharge, and the danger of ignition, explosion, and the like can be avoided. Further, even when the maximum operating voltage is increased, the battery capacity is not significantly reduced due to the charge / discharge cycle, so that the electric capacity can be effectively taken out and long-term use is possible.

Claims (5)

In a non-aqueous electrolyte obtained by dissolving a lithium salt as an electrolyte in an organic solvent, 0.1 to 20% by weight of a partial hydride of a tricyclic aromatic compound represented by the following general formula (1) is contained. A non-aqueous electrolyte solution.

(However, in the formula, Ar ₁ is a phenyl group or a phenyl group substituted with an alkyl group having 1 to 4 carbon atoms, and Ar ₂ is a condensed two-ring aromatic group, wherein the aromatic rings are directly bonded to each other or have one carbon atom. Is a two-ring non-fused aromatic group or a two-ring fused aromatic group or a two-ring non-fused aromatic group substituted with an alkyl group having 1 to 4 carbon atoms, wherein R is a single bond or Represents a methylene group or a methylene group substituted by an alkyl group having 1 to 4 carbon atoms) Partially hydrides include terphenyls, benzylbiphenyls, dibenzylbenzenes, phenylnaphthalenes, benzylnaphthalenes, and alkyl groups having 1 to 4 carbon atoms on carbons having hydrogen which can be substituted in these aromatic compounds. 2. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte is at least one member selected from the group consisting of partially hydrides of substituted three-ring aromatic compounds. 2. The non-aqueous electrolyte according to claim 1, wherein the partially hydride is a partially hydride of terphenyl or benzylbiphenyl. The non-aqueous electrolyte according to claim 1, wherein the nuclear hydrogenation rate of the partially nuclear hydride is 10 to 65%. A non-aqueous lithium secondary battery using the non-aqueous electrolyte according to claim 1.