JP3633269B2

JP3633269B2 - Electrolyte for lithium secondary battery and lithium secondary battery using the same

Info

Publication number: JP3633269B2
Application number: JP07250498A
Authority: JP
Inventors: 俊一浜本; 浩司安部; 勉高井
Original assignee: Ube Industries Ltd
Current assignee: Ube Corp
Priority date: 1998-03-20
Filing date: 1998-03-20
Publication date: 2005-03-30
Anticipated expiration: 2018-03-20
Also published as: JPH11273724A

Description

【０００１】
【発明の属する技術分野】
本発明は、電池のサイクル特性や電気容量、保存特性などの電池特性にも優れたリチウム二次電池を提供することができる新規なリチウム二次電池用電解液、およびそれを用いたリチウム二次電池に関する。
【０００２】
【従来の技術】
近年、リチウム二次電池は小型電子機器などの駆動用電源として広く使用されている。リチウム二次電池は、主に正極、非水電解液および負極から構成されており、特に、ＬｉＣｏＯ_２などのリチウム複合酸化物を正極とし、炭素材料又はリチウム金属を負極としたリチウム二次電池が好適に使用されている。そして、そのリチウム二次電池用の電解液としては、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）などのカーボネート類が好適に使用されている。
【０００３】
【発明が解決しようとする課題】
しかしながら、電池のサイクル特性および電気容量などの電池特性について、さらに優れた特性を有する二次電池が求められている。
負極として例えば天然黒鉛や人造黒鉛などの高結晶化した炭素材料を用いたリチウム二次電池は、炭素材料の剥離が観察され、現象の程度によって容量が不可逆となることがある。この剥離は、電解液中の溶媒が充電時に分解することにより起こるものであり、炭素材料と電解液との界面における溶媒の電気化学的還元に起因するものである。このため、電池のサイクル特性および電気容量などの電池特性は必ずしも満足なものではないのが現状である。
【０００４】
本発明は、前記のようなリチウム二次電池用電解液に関する課題を解決し、電池のサイクル特性に優れ、さらに電気容量や充電状態での保存特性などの電池特性にも優れたリチウム二次電池を構成することができるリチウム二次電池用の電解液、およびそれを用いたリチウム二次電池を提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明は、コバルト、マンガン、ニッケルから選ばれる少なくとも１種の金属とリチウムとの複合酸化物からなる正極、負極および非水溶媒に電解質が溶解されている電解液からなるリチウム二次電池において、該電解液中に下記一般式（Ｉ）
【０００６】
【化７】

【０００７】
（式中、Ｒ^１、Ｒ^２、Ｒ^３およびＲ^４は、それぞれ独立して炭素数1〜4のアルキル基、炭素数2または3のアルケニル基、アリール基、または水素原子を示す。）で表されるカルボン酸ビニルエステル誘導体、下記一般式（ＩＩ）
【０００８】
【化８】

【０００９】
（式中、Ｒ^５、Ｒ^６、Ｒ^７、Ｒ^９、Ｒ^１０、Ｒ^１１は、それぞれ独立して炭素数1〜4のアルキル基、アリール基、または水素原子を示す。また、Ｒ^８は炭素数1〜4の直鎖アルキレン基またはフェニレン基を示す。ただし、ｍは０または１を示す。）で表されるジカルボン酸ジビニルエステル誘導体、および下記一般式（ＩＩＩ）
【００１０】
【化９】

【００１１】
（式中、Ｒ^１２、Ｒ^１３、Ｒ^１４およびＲ^１５は、それぞれ独立して炭素数1〜4のアルキル基、またはアリール基を示す。Ｒ^１３、Ｒ^１４およびＲ^１５は水素原子でもよい。）で表される炭酸ビニルエステル誘導体からなる化合物のうち少なくとも１種が含有されており、前記化合物の含有量が電解液の重量に対して０．０１〜２０重量％であることを特徴とするリチウム二次電池に関する。また、本発明は、前記リチウム二次電池に使用される非水溶媒に電解質が溶解されている電解液において、該電解液中に下記一般式（Ｉ）
【００１２】
【化１０】

【００１３】
（式中、Ｒ^１、Ｒ^２、Ｒ^３およびＲ^４は、それぞれ独立して炭素数1〜4のアルキル基、炭素数2または3のアルケニル基、アリール基、または水素原子を示す。）で表されるカルボン酸ビニルエステル誘導体、下記一般式（ＩＩ）
【００１４】
【化１１】

【００１５】
（式中、Ｒ^５、Ｒ^６、Ｒ^７、Ｒ^９、Ｒ^１０、Ｒ^１１は、それぞれ独立して炭素数1〜4のアルキル基、アリール基、または水素原子を示す。また、Ｒ^８は炭素数1〜4の直鎖アルキレン基またはフェニレン基を示す。ただし、ｍは０または１を示す。）で表されるジカルボン酸ジビニルエステル誘導体、および下記一般式（ＩＩＩ）
【００１６】
【化１２】

【００１７】
（式中、Ｒ^１２、Ｒ^１３、Ｒ^１４およびＲ^１５は、それぞれ独立して炭素数1〜4のアルキル基、またはアリール基を示す。Ｒ^１３、Ｒ^１４およびＲ^１５は水素原子でもよい。）で表される炭酸ビニルエステル誘導体からなる化合物のうち少なくとも１種が含有されており、前記化合物の含有量が電解液の重量に対して０．０１〜２０重量％であることを特徴とするリチウム二次電池用電解液に関する。
【００１８】
電解液中に含有される前記化合物は、炭素材料表面での不働態皮膜形成に寄与して天然黒鉛や人造黒鉛などの活性で高結晶化した炭素材料を不働態皮膜で被覆し、電池の正常な反応を損なうことなく電解液の分解を抑制する効果を有するものと考えられる。
【００１９】
【発明の実施の形態】
非水溶媒に電解質が溶解されている電解液に含有される化合物において、前記式（Ｉ）で表されるカルボン酸ビニルエステル誘導体におけるＲ^１、Ｒ^２、Ｒ^３およびＲ^４は、それぞれ独立してメチル基、エチル基、プロピル基のような炭素数1〜4のアルキル基が好ましい。アルキル基はイソプロピル基、イソブチル基のような分枝アルキル基でもよい。また、ビニル基、アリル基のようなアルケニル基、フェニル基、ベンジル基のような炭素数6〜12のアリール基を含有するものでもよい。さらに、水素原子でもよい。
【００２０】
また、前記式（ＩＩ）で表されるジカルボン酸ジビニルエステル誘導体において、Ｒ^５、Ｒ^６、Ｒ^７、Ｒ^９、Ｒ^１０、Ｒ^１１は、それぞれ独立してメチル基、エチル基、プロピル基のような炭素数1〜4のアルキル基が好ましい。アルキル基はイソプロピル基、イソブチル基のような分枝アルキル基でもよい。また、フェニル基、ベンジル基のような炭素数6〜12のアリール基を含有するものでもよい。さらに、水素原子でもよい。また、ｍが１のとき、Ｒ^８はメチレン基、エチレン基のような炭素数1〜4の直鎖アルキレン基が好ましいが、ｍが０、すなわち直鎖アルキレン基、分枝アルキレン基またはフェニレン基を有しないシュウ酸ジビニルエステル類でもよい。また、分枝アルキレン基、あるいは不飽和結合を含有するものでもよく、さらにアリール基を含有するものでもよい。
【００２１】
また、前記式（ＩＩＩ）で表される炭酸ビニルエステル誘導体において、Ｒ^１２、Ｒ^１３、Ｒ^１４およびＲ^１５は、それぞれ独立してメチル基、エチル基、プロピル基のような炭素数1〜4のアルキル基が好ましい。アルキル基はイソプロピル基、イソブチル基のような分枝アルキル基でもよい。また、フェニル基、ベンジル基のような炭素数6〜12のアリール基を含有するものでもよい。さらに、Ｒ^１３、Ｒ^１４およびＲ^１５は水素原子でもよい。
【００２２】
前記式（Ｉ）で表されるカルボン酸ビニルエステル誘導体の具体例としては、例えば、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕、プロピオン酸ビニル〔Ｒ^１＝エチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕、アクリル酸ビニル〔Ｒ^１＝ビニル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕、クロトン酸ビニル〔Ｒ^１＝２−メチルビニル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕、安息香酸ビニル〔Ｒ^１＝フェニル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕などが挙げられる。
【００２３】
また、前記式（ＩＩ）で表されるジカルボン酸ジビニルエステル誘導体の具体例としては、例えば、シュウ酸ジビニル〔Ｒ^８＝なし（但しｍ＝０）、Ｒ^５＝Ｒ^６＝Ｒ^７＝Ｒ^９＝Ｒ^１０＝Ｒ^１１＝水素原子〕、アジピン酸ジビニル〔Ｒ^８＝テトラメチレン基（但しｍ＝１）、Ｒ^５＝Ｒ^６＝Ｒ^７＝Ｒ^９＝Ｒ^１０＝Ｒ^１１＝水素原子〕、テレフタル酸ジビニル〔Ｒ^８＝−Ｃ_６Ｈ_４−（但しｍ＝１）、Ｒ^５＝Ｒ^６＝Ｒ^７＝Ｒ^９＝Ｒ^１０＝Ｒ^１１＝水素原子〕などが挙げられる。
【００２４】
また、前記式（ＩＩＩ）で表される炭酸ビニルエステル誘導体の具体例としては、例えば、メチルビニルカーボネート〔Ｒ^１２＝メチル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕、エチルビニルカーボネート〔Ｒ^１２＝エチル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕、イソプロピルビニルカーボネート〔Ｒ^１２＝イソプロピル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕、フェニルビニルカーボネート〔Ｒ^１２＝フェニル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕などが挙げられる。
【００２５】
前記化合物を添加する場合において、前記式（Ｉ）で表されるカルボン酸ビニルエステル誘導体、前記式（ＩＩ）で表されるジカルボン酸ジビニルエステル誘導体、または前記式（ＩＩＩ）で表される炭酸ビニルエステル誘導体の含有量は、過度に多いと、電解液の電導度などが変わり電池性能が低下することがあり、また、過度に少ないと、十分な皮膜が形成されず、期待した電池性能が得られないので、電解液の重量に対して０．０１〜２０重量％、特に０．１〜１０重量％の範囲が好ましい。
【００２６】
本発明で使用される非水溶媒としては、高誘電率溶媒と低粘度溶媒とからなるものが好ましい。
高誘電率溶媒としては、例えば、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）、ブチレンカーボネート（ＢＣ）などの環状カーボネート類が好適に挙げられる。これらの高誘電率溶媒は、一種類で使用してもよく、また二種類以上組み合わせて使用してもよい。
【００２７】
低粘度溶媒としては、例えば、ジメチルカーボネート（ＤＭＣ）、メチルエチルカーボネート（ＭＥＣ）、ジエチルカーボネート（ＤＥＣ）などの鎖状カーボネート類、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，４−ジオキサン、１，２−ジメトキシエタン、１，２−ジエトキシエタン、１，２−ジブトキシエタンなどのエーテル類、γ−ブチロラクトンなどのラクトン類、アセトニトリルなどのニトリル類、プロピオン酸メチルなどのエステル類、ジメチルホルムアミドなどのアミド類が挙げられる。これらの低粘度溶媒は一種類で使用してもよく、また二種類以上組み合わせて使用してもよい。
高誘電率溶媒と低粘度溶媒とはそれぞれ任意に選択され組み合わせて使用される。なお、前記の高誘電率溶媒および低粘度溶媒は、容量比（高誘電率溶媒：低粘度溶媒）で通常１：９〜４：１、好ましくは１：４〜７：３の割合で使用される。
【００２８】
本発明で使用される電解質としては、例えば、ＬｉＰＦ_６、ＬｉＢＦ_４、ＬｉＣｌＯ_４、ＬｉＮ（ＳＯ_２ＣＦ_３）_２、ＬｉＮ（ＳＯ_２Ｃ_２Ｆ_５）_２、ＬｉＣ（ＳＯ_２ＣＦ_３）_３などが挙げられる。これらの電解質は、一種類で使用してもよく、二種類以上組み合わせて使用してもよい。これら電解質は、前記の非水溶媒に通常０．１〜３Ｍ、好ましくは０．５〜１．５Ｍの濃度で溶解されて使用される。
【００２９】
本発明の電解液は、例えば、前記の高誘電率溶媒や低粘度溶媒を混合し、これに前記の電解質を溶解し、前記式（Ｉ）で表されるカルボン酸ビニルエステル誘導体、前記式（ＩＩ）で表されるジカルボン酸ジビニルエステル誘導体、または前記式（ＩＩＩ）で表される炭酸ビニルエステル誘導体からなる化合物のうち少なくとも１種を溶解することにより得られる。
【００３０】
本発明の電解液は、リチウム二次電池の構成部材として使用される。二次電池を構成する電解液以外の構成部材については特に限定されず、従来使用されている種々の構成部材を使用できる。
【００３１】
例えば、正極材料（正極活物質）としてはコバルト、マンガン、ニッケル、クロム、鉄およびバナジウムからなる群より選ばれる少なくとも一種類の金属とリチウムとの複合金属酸化物が使用される。このような複合金属酸化物としては、例えば、ＬｉＣｏＯ_２、ＬｉＭｎ_２Ｏ_４、ＬｉＮｉＯ_２などが挙げられる。
【００３２】
正極は、前記の正極材料をアセチレンブラック、カーボンブラックなどの導電剤およびポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）などの結着剤と混練して正極合剤とした後、この正極材料を集電体としてのアルミニウムやステンレス製の箔やラス板に圧延して、５０℃〜２５０℃程度の温度で２時間程度真空下で加熱処理することにより作製される。
【００３３】
負極（負極活物質）としては、リチウム金属やリチウム合金、およびリチウムを吸蔵・放出可能な黒鉛型結晶構造を有する炭素材料〔熱分解炭素類、コークス類、グラファイト類（人造黒鉛、天然黒鉛など）、ガラス状炭素類、有機高分子化合物燃焼体、炭素繊維、活性炭〕や複合スズ酸化物などの物質が使用される。特に、格子面（００２）の面間隔（ｄ_００２）が３．３５〜３．４０Åである黒鉛型結晶構造を有する炭素材料を使用することが好ましい。なお、炭素材料のような粉末材料はエチレンプロピレンジエンモノマー（ＥＰＤＭ）、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）などの結着剤と混練して負極合剤として使用される。
【００３４】
リチウム二次電池の構造は特に限定されるものではなく、正極、負極および単層又は複層のセパレータを有するコイン型電池、さらに、正極、負極およびロール状のセパレータを有する円筒型電池や角型電池などが一例として挙げられる。なお、セパレータとしては公知のポリオレフィンの微多孔膜、織布、不織布などが使用される。
【００３５】
【実施例】
次に、実施例および比較例を挙げて、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。
実施例１
〔電解液の調製〕
ＰＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ_６を１Ｍの濃度になるように溶解して電解液を調製した後、さらに添加剤として、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕を電解液に対して１．０重量％となるように加えた。
【００３６】
〔リチウム二次電池の作製および電池特性の測定〕
ＬｉＣｏＯ_２（正極活物質）を８０重量％、アセチレンブラック（導電剤）を１０重量％、ポリフッ化ビニリデン（結着剤）を１０重量％の割合で混合し、これを圧縮成型して正極を調製した。天然黒鉛（負極活物質）を９０重量％、ポリフッ化ビニリデン（結着剤）を１０重量％の割合で混合し、これを圧縮成型して負極を調製した。そして、ポリプロピレン微多孔性フィルムのセパレータを用い、上記の電解液を注入してコイン電池（直径２０ｍｍ、厚さ３．２ｍｍ）を作製した。
このコイン電池を用いて、室温（２０℃）下、０．８ｍＡの定電流及び定電圧で、終止電圧４．２Ｖまで充電し、次に０．８ｍＡの定電流下、終止電圧２．７Ｖまで放電し、この充放電を繰り返した。初期充放電容量は、ＥＣ−ＤＭＣ（１／２）を電解液として用いた場合（比較例２）とほぼ同等であり、５０サイクル後の電池特性を測定したところ、初期放電容量を１００％としたときの放電容量維持率は８５．７％であった。また、低温特性も良好であった。コイン電池の作製条件および電池特性を表１に示す。
【００３７】
実施例２
添加剤として、アジピン酸ジビニル〔Ｒ^８＝テトラメチレン（但しｍ＝１）、Ｒ^５＝Ｒ^６＝Ｒ^７＝Ｒ^９＝Ｒ^１０＝Ｒ^１１＝水素原子〕を電解液に対して１．０重量％使用したほかは実施例１と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は８４．２％であった。コイン電池の作製条件および電池特性を表１に示す。
【００３８】
実施例３
添加剤として、メチルビニルカーボネート〔Ｒ^１２＝メチル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕を電解液に対して１．０重量％使用したほかは実施例１と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は８０．２％であった。コイン電池の作製条件および電池特性を表１に示す。
【００３９】
実施例４
添加剤として、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕を電解液に対して０．１重量％使用したほかは実施例１と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は８０．６％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４０】
実施例５
添加剤として、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕を電解液に対して５．０重量％使用したほかは実施例１と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は８１．１％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４１】
比較例１
ＰＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ_６を１Ｍの濃度になるように溶解した。このとき添加剤は全く添加しなかった。この電解液を使用して実施例１と同様にコイン電池を作製し、電池特性を測定したところ、初回充電時にＰＣの分解が起こり全く放電できなかった。初回充電後の電池を解体して観察した結果、黒鉛負極に剥離が認められた。コイン電池の作製条件および電池特性を表１に示す。
【００４２】
実施例６
ＥＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ_６を１Ｍの濃度になるように溶解して電解液を調製した後、さらに添加剤として、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕を電解液に対して１．０重量％となるように加えた。この電解液を使用して実施例１と同様にコイン電池を作製し、電池特性を測定したところ、ＥＣ−ＤＭＣ（１／２）のみを電解液として用いた場合（比較例２）とほぼ同等であり、５０サイクル後の電池特性を測定したところ、初期放電容量を１００％としたときの放電容量維持率は９１．８％であった。また、低温特性も良好であった。コイン電池の作製条件および電池特性を表１に示す。
【００４３】
実施例７
添加剤として、アジピン酸ジビニル〔Ｒ^８＝テトラメチレン（但しｍ＝１、Ｒ^５＝Ｒ^６＝Ｒ^７＝Ｒ^９＝Ｒ^１０＝Ｒ^１１＝水素原子〕を電解液に対して１．０重量％使用したほかは実施例６と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は９１．１％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４４】
実施例８
添加剤として、メチルビニルカーボネート〔Ｒ^１２＝メチル基、Ｒ^１３＝Ｒ^１４＝Ｒ^１５＝水素原子〕を電解液に対して１．０重量％使用したほかは実施例６と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は９０．６％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４５】
実施例９
正極活物質として、ＬｉＣｏＯ_２に代えてＬｉＭｎ_２Ｏ_４を使用し、添加剤として、酢酸ビニル〔Ｒ^１＝メチル基、Ｒ^２＝Ｒ^３＝Ｒ^４＝水素原子〕を電解液に対して３．０重量％使用したほかは実施例６と同様に電解液を調製してコイン電池を作製し、５０サイクル後の電池特性を測定したところ、放電容量維持率は９０．２％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４６】
比較例２
ＥＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ_６を１Ｍの濃度になるように溶解した。このとき添加剤は全く添加しなかった。（下線部分は削除しても良い）この電解液を使用して実施例１と同様にコイン電池を作製し、電池特性を測定した。初期放電容量に対し、５０サイクル後の放電容量維持率は８３．８％であった。コイン電池の作製条件および電池特性を表１に示す。
【００４７】
【表１】

【００４８】
なお、本発明は記載の実施例に限定されず、発明の趣旨から容易に類推可能な様々な組み合わせが可能である。特に、上記実施例の溶媒の組み合わせは限定されるものではない。更には、上記実施例はコイン電池に関するものであるが、本発明は円筒形、角柱形の電池にも適用される。
【００４９】
【発明の効果】
本発明によれば、電池のサイクル特性、電気容量、保存特性などの電池特性に優れたリチウム二次電池を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a novel lithium secondary battery electrolyte solution that is excellent in battery characteristics such as battery cycle characteristics, electric capacity, and storage characteristics, and a lithium secondary battery using the same. It relates to batteries.
[0002]
[Prior art]
In recent years, lithium secondary batteries have been widely used as driving power sources for small electronic devices and the like. A lithium secondary battery is mainly composed of a positive electrode, a non-aqueous electrolyte, and a negative electrode. In particular, a lithium secondary battery using a lithium composite oxide such as LiCoO ₂ as a positive electrode and a carbon material or lithium metal as a negative electrode is used. It is preferably used. As the electrolyte for the lithium secondary battery, carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are preferably used.
[0003]
[Problems to be solved by the invention]
However, there is a demand for a secondary battery having more excellent battery characteristics such as battery cycle characteristics and electric capacity.
In a lithium secondary battery using a highly crystallized carbon material such as natural graphite or artificial graphite as the negative electrode, peeling of the carbon material is observed, and the capacity may be irreversible depending on the degree of the phenomenon. This peeling is caused by decomposition of the solvent in the electrolytic solution during charging, and is caused by electrochemical reduction of the solvent at the interface between the carbon material and the electrolytic solution. For this reason, at present, battery characteristics such as battery cycle characteristics and electric capacity are not always satisfactory.
[0004]
The present invention solves the above-described problems relating to the electrolyte for a lithium secondary battery, is excellent in cycle characteristics of the battery, and further excellent in battery characteristics such as electric capacity and storage characteristics in a charged state. It is an object of the present invention to provide an electrolytic solution for a lithium secondary battery that can constitute the battery, and a lithium secondary battery using the same.
[0005]
[Means for Solving the Problems]
The present invention relates to a lithium secondary battery comprising a positive electrode comprising a composite oxide of at least one metal selected from cobalt, manganese and nickel and lithium , a negative electrode and an electrolyte in which an electrolyte is dissolved in a nonaqueous solvent. In the electrolyte, the following general formula (I)
[0006]
[Chemical 7]

[0007]
(Wherein R ¹ , R ² , R ³ and R ⁴ each independently represents an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 or 3 carbon atoms, an aryl group, or a hydrogen atom). Carboxylic acid vinyl ester derivatives represented by the following general formula (II)
[0008]
[Chemical 8]

[0009]
^{^{^{^{(Wherein, R 5, R 6, R}}}} 7, R 9, R 10, R 11 represents independently an alkyl group having 1 to 4 carbon atoms, an aryl group or a hydrogen atom. In addition, ^{R 8} is A straight-chain alkylene group having 1 to 4 carbon atoms or a phenylene group, where m represents 0 or 1.) and a dicarboxylic acid divinyl ester derivative represented by the following general formula (III):
[0010]
[Chemical 9]

[0011]
(Wherein R ¹² , R ¹³ , R ¹⁴ and R ¹⁵ each independently represents an alkyl group having 1 to 4 carbon atoms or an aryl group. R ¹³ , R ¹⁴ and R ¹⁵ may be hydrogen atoms. And at least one compound composed of a vinyl carbonate derivative represented by formula (1), wherein the content of the compound is 0.01 to 20% by weight with respect to the weight of the electrolytic solution. The present invention relates to a lithium secondary battery. The present invention also provides an electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent used in the lithium secondary battery, wherein the electrolyte has the following general formula (I):
[0012]
[Chemical Formula 10]

[0013]
(Wherein R ¹ , R ² , R ³ and R ⁴ each independently represents an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 or 3 carbon atoms, an aryl group, or a hydrogen atom). Carboxylic acid vinyl ester derivatives represented by the following general formula (II)
[0014]
Embedded image

[0015]
^{^{^{^{(Wherein, R 5, R 6, R}}}} 7, R 9, R 10, R 11 represents independently an alkyl group having 1 to 4 carbon atoms, an aryl group or a hydrogen atom. In addition, ^{R 8} is A straight-chain alkylene group having 1 to 4 carbon atoms or a phenylene group, where m represents 0 or 1.) and a dicarboxylic acid divinyl ester derivative represented by the following general formula (III):
[0016]
Embedded image

[0017]
(Wherein R ¹² , R ¹³ , R ¹⁴ and R ¹⁵ each independently represents an alkyl group having 1 to 4 carbon atoms or an aryl group. R ¹³ , R ¹⁴ and R ¹⁵ may be hydrogen atoms. And at least one compound composed of a vinyl carbonate derivative represented by formula (1), wherein the content of the compound is 0.01 to 20% by weight with respect to the weight of the electrolytic solution. The present invention relates to an electrolyte for a lithium secondary battery.
[0018]
The compound contained in the electrolyte solution contributes to the formation of a passive film on the surface of the carbon material, and the active and highly crystallized carbon material such as natural graphite or artificial graphite is coated with the passive film, so that the normality of the battery This is considered to have an effect of suppressing the decomposition of the electrolytic solution without impairing the reaction.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
In the compound contained in the electrolytic solution in which the electrolyte is dissolved in the nonaqueous solvent, R ¹ , R ² , R ³ and R ⁴ in the carboxylic acid vinyl ester derivative represented by the formula (I) are each independently An alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group or a propyl group is preferred. The alkyl group may be a branched alkyl group such as isopropyl group or isobutyl group. Further, it may contain an alkenyl group such as a vinyl group or an allyl group, an aryl group having 6 to 12 carbon atoms such as a phenyl group or a benzyl group. Furthermore, a hydrogen atom may be sufficient.
[0020]
In the dicarboxylic acid divinyl ester derivative represented by the formula (II), R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , and R ¹¹ are each independently a methyl group, an ethyl group, or a propyl group. Such an alkyl group having 1 to 4 carbon atoms is preferred. The alkyl group may be a branched alkyl group such as isopropyl group or isobutyl group. Moreover, you may contain a C6-C12 aryl group like a phenyl group and a benzyl group. Furthermore, a hydrogen atom may be sufficient. When m is 1, R ⁸ is preferably a linear alkylene group having 1 to 4 carbon atoms such as a methylene group or an ethylene group, but m is 0, that is, a linear alkylene group, a branched alkylene group or a phenylene group. Oxalic acid divinyl esters that do not contain Further, it may contain a branched alkylene group or an unsaturated bond, and may further contain an aryl group.
[0021]
In the vinyl carbonate derivative represented by the formula (III), R ¹² , R ¹³ , R ¹⁴ and R ¹⁵ are each independently 1-4 carbon atoms such as a methyl group, an ethyl group and a propyl group. Are preferred. The alkyl group may be a branched alkyl group such as isopropyl group or isobutyl group. Moreover, you may contain a C6-C12 aryl group like a phenyl group and a benzyl group. Further, R ¹³ , R ¹⁴ and R ¹⁵ may be a hydrogen atom.
[0022]
Specific examples of the carboxylic acid vinyl ester derivative represented by the formula (I) include, for example, vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom], vinyl propionate [R ¹ = Ethyl group, R ² = R ³ = R ⁴ = hydrogen atom], vinyl acrylate [R ¹ = vinyl group, R ² = R ³ = R ⁴ = hydrogen atom], vinyl crotonate [R ¹ = 2-methyl Vinyl group, R ² = R ³ = R ⁴ = hydrogen atom], vinyl benzoate [R ¹ = phenyl group, R ² = R ³ = R ⁴ = hydrogen atom] and the like.
[0023]
Specific examples of the dicarboxylic acid divinyl ester derivative represented by the formula (II) include, for example, divinyl oxalate [R ⁸ = none (provided that m = 0), R ⁵ = R ⁶ = R ⁷ = R ^9]. = R ¹⁰ = R ¹¹ = hydrogen atom], divinyl adipate [R ⁸ = tetramethylene group (where m = 1), R ⁵ = R ⁶ = R ⁷ = R ⁹ = R ¹⁰ = R ¹¹ = hydrogen atom], And divinyl terephthalate [R ⁸ = —C ₆ H ₄ — (where m = 1), R ⁵ = R ⁶ = R ⁷ = R ⁹ = R ¹⁰ = R ¹¹ = hydrogen atom].
[0024]
Specific examples of the vinyl carbonate derivative represented by the formula (III) include, for example, methyl vinyl carbonate [R ¹² = methyl group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom], ethyl vinyl carbonate [ R ¹² = ethyl group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom], isopropyl vinyl carbonate [R ¹² = isopropyl group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom], phenyl vinyl carbonate [R ¹² = phenyl Group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom] and the like.
[0025]
When the compound is added, the carboxylic acid vinyl ester derivative represented by the formula (I), the dicarboxylic acid divinyl ester derivative represented by the formula (II), or the vinyl carbonate represented by the formula (III) If the ester derivative content is excessively large, the conductivity of the electrolyte may change and the battery performance may decrease.If it is excessively small, a sufficient film will not be formed, and the expected battery performance will be obtained. Therefore, the range of 0.01 to 20% by weight, particularly 0.1 to 10% by weight with respect to the weight of the electrolytic solution is preferable.
[0026]
As the non-aqueous solvent used in the present invention, a solvent composed of a high dielectric constant solvent and a low viscosity solvent is preferable.
Suitable examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric constant solvents may be used alone or in combination of two or more.
[0027]
Examples of the low viscosity solvent include chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2- Ethers such as dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl propionate, amides such as dimethylformamide Kind. These low viscosity solvents may be used alone or in combination of two or more.
The high dielectric constant solvent and the low viscosity solvent are arbitrarily selected and used in combination. The high dielectric constant solvent and the low viscosity solvent are usually used in a volume ratio (high dielectric constant solvent: low viscosity solvent) of 1: 9 to 4: 1, preferably 1: 4 to 7: 3. The
[0028]
Examples of the electrolyte used in the present invention include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) _{3 and the} like. Is mentioned. These electrolytes may be used alone or in combination of two or more. These electrolytes are used by being dissolved in the non-aqueous solvent usually at a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.
[0029]
The electrolytic solution of the present invention is prepared by, for example, mixing the above-mentioned high dielectric constant solvent or low-viscosity solvent, dissolving the above-described electrolyte therein, and the carboxylic acid vinyl ester derivative represented by the above formula (I), the above formula ( It can be obtained by dissolving at least one of a dicarboxylic acid divinyl ester derivative represented by II) or a compound comprising a vinyl carbonate ester derivative represented by the formula (III).
[0030]
The electrolytic solution of the present invention is used as a constituent member of a lithium secondary battery. The constituent members other than the electrolytic solution constituting the secondary battery are not particularly limited, and various conventionally used constituent members can be used.
[0031]
For example, as the positive electrode material (positive electrode active material), a composite metal oxide of at least one metal selected from the group consisting of cobalt, manganese, nickel, chromium, iron and vanadium and lithium is used. Examples of such composite metal oxides include LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ .
[0032]
The positive electrode is prepared by kneading the positive electrode material with a conductive agent such as acetylene black or carbon black and a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) to form a positive electrode mixture. It is produced by rolling the material into a foil or lath plate made of aluminum or stainless steel as a current collector and heat-treating it under vacuum at a temperature of about 50 ° C. to 250 ° C. for about 2 hours.
[0033]
Examples of the negative electrode (negative electrode active material) include lithium metal and lithium alloy, and carbon materials having a graphite-type crystal structure capable of inserting and extracting lithium (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.)) , Glassy carbons, organic polymer compound combustion bodies, carbon fibers, activated carbon] and composite tin oxides are used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which the lattice spacing ( ₀₀₂ ) has an interval (d ₀₀₂ ) of 3.35 to 3.40 mm. A powder material such as a carbon material is kneaded with a binder such as ethylene propylene diene monomer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) and used as a negative electrode mixture.
[0034]
The structure of the lithium secondary battery is not particularly limited, and a coin-type battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and a cylindrical battery or a square type having a positive electrode, a negative electrode, and a roll separator. An example is a battery. A known polyolefin microporous film, woven fabric, non-woven fabric or the like is used as the separator.
[0035]
【Example】
Next, although an Example and a comparative example are given and this invention is demonstrated concretely, these do not limit this invention at all.
Example 1
(Preparation of electrolyte)
A non-aqueous solvent of PC: DMC (volume ratio) = 1: 2 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1M to prepare an electrolytic solution. Further, vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom] was added at 1.0 wt% with respect to the electrolyte.
[0036]
[Production of lithium secondary battery and measurement of battery characteristics]
80% by weight of LiCoO ₂ (positive electrode active material), 10% by weight of acetylene black (conductive agent), and 10% by weight of polyvinylidene fluoride (binder) are mixed, and this is compression molded to prepare a positive electrode. did. 90% by weight of natural graphite (negative electrode active material) and 10% by weight of polyvinylidene fluoride (binder) were mixed and compression molded to prepare a negative electrode. And using the separator of a polypropylene microporous film, said electrolyte solution was inject | poured and the coin battery (diameter 20mm, thickness 3.2mm) was produced.
Using this coin battery, it is charged to a final voltage of 4.2 V at a constant current and constant voltage of 0.8 mA at room temperature (20 ° C.), and then to a final voltage of 2.7 V under a constant current of 0.8 mA. The battery was discharged and this charge / discharge was repeated. The initial charge / discharge capacity is almost the same as that when EC-DMC (1/2) is used as the electrolyte (Comparative Example 2), and the battery characteristics after 50 cycles were measured. The initial discharge capacity was 100%. The discharge capacity retention rate was 85.7%. Also, the low temperature characteristics were good. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0037]
Example 2
As an additive, divinyl adipate [R ⁸ = tetramethylene (where m = 1), R ⁵ = R ⁶ = R ⁷ = R ⁹ = R ¹⁰ = R ¹¹ = hydrogen atom] is 1.0 with respect to the electrolytic solution. A coin battery was prepared by preparing an electrolyte solution in the same manner as in Example 1 except that the weight percentage was used, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 84.2%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0038]
Example 3
The electrolyte solution was the same as in Example 1 except that methyl vinyl carbonate [R ¹² = methyl group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom] was used in an amount of 1.0% by weight based on the electrolyte solution. A coin battery was prepared to measure the battery characteristics after 50 cycles, and the discharge capacity retention rate was 80.2%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0039]
Example 4
An electrolytic solution was prepared in the same manner as in Example 1 except that 0.1 wt% of vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom] was used as an additive. Then, a coin battery was produced, and the battery characteristics after 50 cycles were measured. As a result, the discharge capacity retention rate was 80.6%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0040]
Example 5
An electrolytic solution was prepared in the same manner as in Example 1, except that 5.0 wt% of vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom] was used as an additive. Then, a coin battery was produced, and the battery characteristics after 50 cycles were measured. As a result, the discharge capacity retention rate was 81.1%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0041]
Comparative Example 1
A non-aqueous solvent of PC: DMC (volume ratio) = 1: 2 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1M. At this time, no additive was added. Using this electrolytic solution, a coin battery was produced in the same manner as in Example 1, and the battery characteristics were measured. As a result, the PC was decomposed during the first charge and could not be discharged at all. As a result of disassembling and observing the battery after the first charge, peeling was observed on the graphite negative electrode. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0042]
Example 6
A non-aqueous solvent having an EC: DMC (volume ratio) = 1: 2 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1 M to prepare an electrolyte solution. Further, vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom] was added at 1.0 wt% with respect to the electrolyte. Using this electrolytic solution, a coin battery was prepared in the same manner as in Example 1 and the battery characteristics were measured. As a result, almost the same as in the case where only EC-DMC (1/2) was used as the electrolytic solution (Comparative Example 2). The battery characteristics after 50 cycles were measured, and the discharge capacity retention rate was 91.8% when the initial discharge capacity was 100%. Also, the low temperature characteristics were good. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0043]
Example 7
As an additive, divinyl adipate [R ⁸ = tetramethylene (where m = 1, R ⁵ = R ⁶ = R ⁷ = R ⁹ = R ¹⁰ = R ¹¹ = hydrogen atom) is 1.0 wt. A coin battery was prepared by preparing an electrolyte solution in the same manner as in Example 6, except that the battery capacity after 50 cycles was measured, and the discharge capacity retention rate was 91.1%. The production conditions and battery characteristics are shown in Table 1.
[0044]
Example 8
As an additive, methyl vinyl carbonate [R ¹² = methyl group, R ¹³ = R ¹⁴ = R ¹⁵ = hydrogen atom] was used in the same manner as in Example 6 except that 1.0 wt% of the electrolyte was used. The coin battery was prepared to measure the battery characteristics after 50 cycles, and the discharge capacity retention rate was 90.6%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0045]
Example 9
LiMn ₂ O ₄ is used instead of LiCoO ₂ as the positive electrode active material, and vinyl acetate [R ¹ = methyl group, R ² = R ³ = R ⁴ = hydrogen atom] is used as the additive for the electrolyte 3 A coin battery was prepared by preparing an electrolyte solution in the same manner as in Example 6 except that 0.0% by weight was used. The battery characteristics after 50 cycles were measured, and the discharge capacity retention rate was 90.2%. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0046]
Comparative Example 2
A non-aqueous solvent of EC: DMC (volume ratio) = 1: 2 was prepared, and LiPF ₆ was dissolved in this to a concentration of 1M. At this time, no additive was added. (The underlined portion may be deleted) Using this electrolytic solution, a coin battery was produced in the same manner as in Example 1, and the battery characteristics were measured. The discharge capacity retention rate after 50 cycles was 83.8% with respect to the initial discharge capacity. The production conditions and battery characteristics of the coin battery are shown in Table 1.
[0047]
[Table 1]

[0048]
Note that the present invention is not limited to the described embodiments, and various combinations that can be easily inferred from the gist of the invention are possible. In particular, the combination of solvents in the above examples is not limited. Furthermore, although the said Example is related with a coin battery, this invention is applied also to a cylindrical and prismatic battery.
[0049]
【The invention's effect】
According to the present invention, it is possible to provide a lithium secondary battery excellent in battery characteristics such as battery cycle characteristics, electric capacity, and storage characteristics.

Claims

In a lithium secondary battery comprising a positive electrode comprising a composite oxide of at least one metal selected from cobalt, manganese and nickel and lithium , a negative electrode, and an electrolyte dissolved in a non-aqueous solvent, The following general formula (I)

(Wherein R ¹ , R ² , R ³ and R ⁴ each independently represents an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 or 3 carbon atoms, an aryl group, or a hydrogen atom). Carboxylic acid vinyl ester derivatives represented by the following general formula (II)

^{^{^{^{(Wherein, R 5, R 6, R}}}} 7, R 9, R 10, R 11 represents independently an alkyl group having 1 to 4 carbon atoms, an aryl group or a hydrogen atom. In addition, ^{R 8} is A straight-chain alkylene group having 1 to 4 carbon atoms or a phenylene group, where m represents 0 or 1.) and a dicarboxylic acid divinyl ester derivative represented by the following general formula (III):

(Wherein R ¹² , R ¹³ , R ¹⁴ and R ¹⁵ each independently represents an alkyl group having 1 to 4 carbon atoms or an aryl group. R ¹³ , R ¹⁴ and R ¹⁵ may be hydrogen atoms. And at least one compound composed of a vinyl carbonate derivative represented by formula (1), wherein the content of the compound is 0.01 to 20% by weight with respect to the weight of the electrolytic solution. Lithium secondary battery.

The lithium secondary battery according to claim 1, wherein the negative electrode is a carbon material having a graphite-type crystal structure capable of inserting and extracting lithium.

The compound is selected from vinyl acetate, vinyl propionate, vinyl acrylate, vinyl crotonate, vinyl benzoate, divinyl oxalate, divinyl adipate, divinyl terephthalate, methyl vinyl carbonate, ethyl vinyl carbonate, isopropyl vinyl carbonate, and phenyl vinyl carbonate. The lithium secondary battery according to claim 1 or 2, which is at least one selected.

The lithium secondary battery according to any one of claims 1 to 3, wherein a content of the compound is 0.1 to 10% by weight with respect to a weight of the electrolytic solution.

An electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent used in the lithium secondary battery according to claim 1, wherein the electrolyte has the following general formula (I):

(Wherein R ¹² , R ¹³ , R ¹⁴ and R ¹⁵ each independently represents an alkyl group having 1 to 4 carbon atoms or an aryl group. R ¹³ , R ¹⁴ and R ¹⁵ may be hydrogen atoms. And at least one compound composed of a vinyl carbonate derivative represented by formula (1), wherein the content of the compound is 0.01 to 20% by weight with respect to the weight of the electrolytic solution. Electrolyte for lithium secondary battery.

The compound is selected from vinyl acetate, vinyl propionate, vinyl acrylate, vinyl crotonate, vinyl benzoate, divinyl oxalate, divinyl adipate, divinyl terephthalate, methyl vinyl carbonate, ethyl vinyl carbonate, isopropyl vinyl carbonate, and phenyl vinyl carbonate. The electrolyte solution for a lithium secondary battery according to claim 5, which is at least one selected.

The electrolyte solution for a lithium secondary battery according to claim 5 or 6, wherein the content of the compound is 0.1 to 10% by weight with respect to the weight of the electrolyte solution.