JP3911870B2

JP3911870B2 - Electrolyte for lithium secondary battery and lithium secondary battery using the same

Info

Publication number: JP3911870B2
Application number: JP27562498A
Authority: JP
Inventors: 俊一浜本; 明植木; 浩司安部; 勉高井
Original assignee: Ube Industries Ltd
Current assignee: Ube Corp
Priority date: 1998-09-29
Filing date: 1998-09-29
Publication date: 2007-05-09
Anticipated expiration: 2018-09-29
Also published as: JP2000106211A

Description

【０００１】
【産業上の利用分野】
本発明は優れた電池性能を有するリチウム二次電池に関するものである。
【０００２】
【従来の技術】
近年、電子機器の小型軽量化、携帯化が進み、それらの駆動電源として、高エネルギー密度を有する電池の開発が要求されている。このような要求に応える電池としてリチウム二次電池が期待されている。このリチウム二次電池の負極にはリチウムの樹枝状析出物を回避するために従来使用されている金属リチウムに代えてリチウムイオンの挿入・脱離が可能な炭素材料が使用されている。
【０００３】
炭素材料のなかでも、特に結晶性の高い黒鉛は高容量化が達成できるためにリチウム二次電池の負極に多用されている。
【０００４】
しかしながら、エチレンカーボネート（ＥＣ）やプロピレンカーボネート（ＰＣ）などの環状カーボネートを電解液に用いた場合、充電時に負極の黒鉛表面でこれらの電解液が還元分解して、黒鉛の構造を破壊するために十分な電池性能が発現できない。特にＰＣを含有した電解液ではその破壊現象が顕著となり、負極集電体から黒鉛が剥離し、放電容量がゼロになる。
【０００５】
このような充電時の黒鉛表面での電解液の分解を抑制する方法として、電解液中に添加剤を加える方法が提案されている。例えば、Z.X.Shu，R.S.McMillan，and J.J.Murray、J.Electrochem．Soc．，Vol140，No．6，L101(1993)．においてＰＣとＥＣとをベースとした電解液にクラウンエーテル（１２−クラウン−４）を添加することによって電解液の分解が抑制されることが記載されている。しかしながら、クラウンエーテルは高価であり、多量に添加しないと効果が発現できない。
【０００６】
また、特開平８−４５５４５号公報には不飽和結合を含み、かつリチウムよりも１Ｖ高い電位で負極の炭素表面で還元分解する添加剤が炭素負極上に不動態皮膜を形成することにより電解液の分解を抑制することが記載されている。しかしながら、ＥＣ及びＰＣの還元分解電位はそれぞれ０．７０Ｖ及び０．７５Ｖであるのに対して、リチウムよりも１Ｖ以上高い電位で還元分解する添加剤化合物は、充電時に電解液のＥＣやＰＣよりもかなり先に分解するために、炭素負極上に生成する皮膜が過度に厚くなることが分かった。この不動態皮膜の組成は電気絶縁性化合物であるリチウムアルキルカーボネートや炭酸リチウムあるいはフッ化リチウム等であるために、皮膜が厚くなると電池の内部抵抗が増大し、満足する電気容量やサイクル特性が得られないという問題点があった。
【０００７】
また、J.Electrochem.Soc.,Vol.140，No.9,L161(1995)には電解液にクロロエチレンカーボネートを添加することによって、グラファイト電極表面でのＰＣ分解が抑制されることが述べられている。これはクロロエチレンカーボネートの分解生成物がグラファイト表面で不動態皮膜を形成することによると考えられているが、電解液の分解に対する抑制効果が必ずしも良好ではない。
【０００８】
【発明が解決しようとする課題】
従来の方法では炭素負極上に生成する電気絶縁性皮膜が過度に厚くなるために、電気容量、サイクル特性等の電池特性が十分に発現できていない。
【０００９】
【課題を解決するための手段】
本発明者らは、従来の方法では充電時に負極の黒鉛表面に生成する電気絶縁性皮膜が厚くなるために電気容量が低くなっていることに着目し、負極の黒鉛表面に薄い皮膜を形成することを目的として鋭意検討した結果、本発明に至った。
【００１０】
本発明は、リチウムと遷移金属の複合酸化物からなる正極と、結晶面間隔（ｄ₀₀₂）が０．３４ｎｍ以下の黒鉛負極とを備えたリチウム二次電池用電解液において、該電解液は非水溶媒に電解質が溶解されている電解液であって、該電解液中に黒鉛負極においてリチウムよりも０．８Ｖより高く且つ１Ｖより低い電位で還元可能な添加剤が電解液中に０．１〜１０重量％含有され、該添加剤が下記一般式（Ｉ）
【化１】

（式中、Ｘ ¹ 、Ｘ ² 、Ｘ ³ 、Ｘ ⁴ は炭素原子または窒素原子を示す。また、Ｙは炭素数１〜１２のアシル基、炭素数１〜１２のエステル基または炭素数１〜１２のスルホニル基を示す。また、Ｒ ¹ 、Ｒ ² 、Ｒ ³ 、Ｒ ⁴ はそれぞれ結合しているＸ ¹ 、Ｘ ² 、Ｘ ³ 、Ｘ ⁴ が窒素原子の場合には存在せず、結合しているＸ ¹ 、Ｘ ² 、Ｘ ³ 、Ｘ ⁴ が炭素原子の場合には、水素原子を示す。）で表される複素環アミド、複素環カーバメートおよび複素環スルホンアミド誘導体から選ばれる少なくとも１種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。
【００１１】
また、本発明は、リチウムと遷移金属の複合酸化物からなる正極と、結晶面間隔（ｄ ₀₀₂ ）が０．３４ｎｍ以下の黒鉛負極と、非水溶媒に電解質が溶解されている電解液とを含むリチウム二次電池において、該電解液中に黒鉛負極においてリチウムよりも０．８Ｖより高く且つ１Ｖより低い電位で還元可能な添加剤が電解液中に０．１〜１０重量％含有され、該添加剤が下記一般式（Ｉ）
【化２】

（式中、Ｘ¹、Ｘ²、Ｘ³、Ｘ⁴は炭素原子または窒素原子を示す。また、Ｙは炭素数１〜１２のアシル基、炭素数１〜１２のエステル基または炭素数１〜１２のスルホニル基を示す。また、Ｒ¹、Ｒ²、Ｒ³、Ｒ⁴はそれぞれ結合しているＸ¹、Ｘ²、Ｘ³、Ｘ⁴が窒素原子の場合には存在せず、結合しているＸ¹、Ｘ²、Ｘ³、Ｘ⁴が炭素原子の場合には、水素原子を示す。）で表される複素環アミド、複素環カーバメートおよび複素環スルホンアミド誘導体から選ばれる少なくとも１種であることを特徴とするリチウム二次電池に関する。
【００１２】
本発明に用いられる添加剤の還元電位は電解液の還元電位よりもやや高いことが好ましい。リチウム二次電池用電解液として通常好適に使用される非水溶媒であるＥＣ、ＰＣの還元電位はそれぞれ０．７０Ｖ、０．７５Ｖであるので、添加剤としては、０．８Ｖより高い還元電位を有するものがよい。また、添加剤としては、過度に還元されない電位、即ち１Ｖ（対リチウム電位）より低い電位で還元分解されるものがよい。
【００１３】
本発明の添加剤は、還元分解電位が電解液よりも高いので、電解液よりも時間的に先に還元分解され、しかも、１Ｖよりも低い電位で還元可能であるので、添加剤は過度に還元分解されることがない。このため、黒鉛負極表面には非常に薄い電気絶縁性皮膜が形成されることとなり、充電時に黒鉛負極表面でのＥＣ、ＰＣ等の電解液の分解が抑制されるものと考えられる。
【００１４】
本発明で使用される黒鉛負極においてリチウムよりも０．８Ｖより高くかつ１Ｖより低い電位で還元可能な添加剤としては、前記一般式（Ｉ）で表される添加剤が挙げられ、具体的には例えば、１−プロピオニルピラゾール［Ｙ＝プロピオニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子〕、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］、１−メタンスルホニルピラゾール［Ｙ＝メタンスルホニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］、１−メトキシカルボニル−１，２，４−トリアゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝Ｒ³＝なし、Ｒ²＝Ｒ⁴＝水素原子、Ｘ¹＝Ｘ³＝窒素原子、Ｘ²＝Ｘ⁴＝炭素原子］等の複素環アミド、複素環カーバメート、複素環スルホンアミド誘導体が挙げられる。
前記添加剤の含有量は、過度に多いと炭素負極表面に生成する皮膜が厚くなるのでサイクル特性が悪くなり、過度に少ないと電解液の分解を十分に抑制できないので、添加剤は電解液中に０．１〜１０重量％含有されるのが好ましい。
【００１５】
【発明の実施の形態】
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適である。
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ましい。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。
【００１６】
低粘度溶媒としては、例えばジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、メチルエチルカーボネート（ＭＥＣ）などの鎖状カーボネート類、γ−ブチロラクトンなどのラクトン類、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、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。
【００１７】
本発明で使用される電解質としては、例えばＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＮ（ＳＯ₂ＣＦ₃）₂、ＬｉＮ（ＳＯ₂Ｃ₂Ｆ₅）₂、ＬｉＣ（ＳＯ₂ＣＦ₃）₃、ＬｉＣＦ₃ＳＯ₃、ＬｉＡｓＦ₆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。これらの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。
【００１８】
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）で表される複素環アミド、複素環カーバメイトあるいは複素環スルホンアミド誘導体を溶解することにより得られる。
【００１９】
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。
【００２０】
例えば、正極活物質としてはコバルト、マンガン、ニッケル、クロム、鉄またはバナジウムのような遷移元素から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ＬｉＣｏＯ₂、ＬｉＭｎ₂Ｏ₄、ＬｉＮｉＯ₂などが挙げられる。
【００２１】
正極は、前記の正極活物質をアセチレンブラック、カーボンブラックなどの導電材、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）などの結着材および溶剤と混練して正極合材とした後、この正極合材を集電体としてのアルミニウム箔やステンレス製のラス板に塗布して、乾燥、加圧成型後、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。
【００２２】
負極活物質としては、格子面（００２）の面間隔（ｄ₀₀₂）が０．３４ｎｍ以下、特に０．３３５〜０．３３７ｎｍ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。この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ターポリマー（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）などの結着材と混練して負極合材として使用される。
【００２３】
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。
【００２４】
【実施例】
［還元電位の測定］
天然黒鉛粉末を試験極にして、対極と参照極にＬｉメタルを使用した三極式セルにて、リチウム二次電池用電解液および添加剤化合物の還元電位を測定した。電解質としてＬｉＰＦ₆を使用して、これを１Ｍの濃度になるように非水溶媒に溶解させて電解液を調製した。添加剤はＰＣ電解液に溶解させて還元電位を測定した。還元電位の測定結果を表１に示す。
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。
【００２５】
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ＬｉＰＦ₆を１Ｍの濃度になるように溶解して電解液を調製した後、さらに複素環アミド誘導体（添加剤）として、１−プロピオニルピラゾール［Ｙ＝プロピオニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子〕を電解液に対して２重量％となるように加えた。
【００２６】
［リチウム二次電池の作製および電池特性の測定］
ＬｉＣｏＯ₂（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、これに１−メチル−２−ピロリドン溶剤を加えて混合したものをアルミ箔上に塗布し、乾燥、加圧成型、加熱処理して正極を調製した。結晶面間隔（ｄ₀₀₂）が０．３３５ｎｍの天然黒鉛（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、これに１−メチル−２−ピロリドン溶剤を加えて混合したものを銅箔上に塗布し、乾燥、加圧成型、加熱処理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させてコイン電池（直径２０ｍｍ、厚さ３．２ｍｍ）を作製した。なお、正極活物質と負極活物質との重量比率は、ほぼ同じ電気容量になるようにした。
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。初回の充電容量は４１０ｍＡｈ／ｇ炭素で、初回放電容量は３１０ｍＡｈ／ｇ炭素であった。また、充放電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８０．３％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２に示す。
【００２７】
実施例２
添加剤として、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して２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８３．５％であった。コイン電池の作製条件および電池特性を表２に示す。
【００２８】
実施例３
添加剤として、１−メタンスルホニルピラゾール［Ｙ＝メタンスルホニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して２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８１．６％であった。コイン電池の作製条件および電池特性を表２に示す。
【００２９】
実施例４
添加剤として、１−メトキシカルボニル−１，２，４−トリアゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝Ｒ³＝なし、Ｒ²＝Ｒ⁴＝水素原子、Ｘ¹＝Ｘ³＝窒素原子、Ｘ²＝Ｘ⁴＝炭素原子］を電解液に対して２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７９．３％であった。コイン電池の作製条件および電池特性を表２に示す。
【００３０】
実施例５
添加剤として、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して０．２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８０．７％であった。コイン電池の作製条件および電池特性を表２に示す。
【００３１】
実施例６
添加剤として、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して５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８１．２％であった。コイン電池の作製条件および電池特性を表２に示す。
【００３２】
比較例１
添加剤として、クロロエチレンカーボネートを電解液に対して２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７４．３％であった。コイン電池の作製条件および電池特性を表２に示す。
【００３３】
比較例２
添加剤として、ペンタフルオロベンゾニトリルを電解液に対して２重量％使用たほかは実施例１と同様に電解液を調製してコイン電池を作製し、電池特性を測定したところ、初回放電容量が５０ｍＡｈ／ｇ炭素で、５０サイクル後の放電容量は０ｍＡｈ／ｇ炭素であった。コイン電池の作製条件および電池特性を表２に示す。
【００３４】
比較例３
ＰＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ₆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２に示す。
【００３５】
実施例７
ＥＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ₆を１Ｍの濃度になるように溶解して電解液を調製した後、さらに複素環アミド誘導体（添加剤）として、１−プロピオニルピラゾール［Ｙ＝プロピオニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して２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、初回充電容量は４３０ｍＡｈ／ｇ炭素で、初回放電容量は３３０ｍＡｈ／ｇ炭素であった。また、５０サイクル後の電池特性を測定したところ、初回放電容量を１００％としたときの放電容量維持率は９０．５％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３に示す。
【００３６】
実施例８
添加剤として、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して２重量％使用したほかは実施例７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９１．２％であった。コイン電池の作製条件および電池特性を表３に示す。
【００３７】
実施例９
添加剤として、１−メタンスルホニルピラゾール［Ｙ＝メタンスルホニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して２重量％使用したほかは実施例７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９１．０％であった。コイン電池の作製条件および電池特性を表３に示す。
【００３８】
実施例１０
添加剤として、１−メトキシカルボニル−１，２，４−トリアゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝Ｒ³＝なし、Ｒ²＝Ｒ⁴＝水素原子、Ｘ¹＝Ｘ³＝窒素原子、Ｘ²＝Ｘ⁴＝炭素原子］を電解液に対して２重量％使用したほかは実施例７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８８．８％であった。コイン電池の作製条件および電池特性を表３に示す。
【００３９】
実施例１１
正極活物質として、ＬｉＣｏＯ₂に代えて、ＬｉＭｎ₂Ｏ₄を使用し、添加剤として、１−メトキシカルボニルピラゾール［Ｙ＝メトキシカルボニル基、Ｒ¹＝なし、Ｒ²＝Ｒ³＝Ｒ⁴＝水素原子、Ｘ¹＝窒素原子、Ｘ²＝Ｘ³＝Ｘ⁴＝炭素原子］を電解液に対して３重量％使用したほかは実施例７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９０．３％であった。コイン電池の作製条件および電池特性を表３に示す。
【００４０】
比較例４
添加剤として、クロロエチレンカーボネートを電解液に対して２重量％使用したほかは実施例７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８３．４％であった。コイン電池の作製条件および電池特性を表３に示す。
【００４１】
比較例５
添加剤として、ペンタフルオロベンゾニトリルを電解液に対して２重量％使用したほかは実施例７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５８．２％であった。コイン電池の作製条件および電池特性を表３に示す。
【００４２】
比較例６
ＥＣ：ＤＭＣ（容量比）＝１：２の非水溶媒を調製し、これにＬｉＰＦ₆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、初回充電容量は４３０ｍＡｈ／ｇ炭素で、初回放電容量は３１０ｍＡｈ／ｇ炭素であった。また、５０サイクル後の電池特性を測定したところ、初回放電容量を１００％としたとき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３に示す。
【００４３】
【表１】

注）ただし、ＥＣの還元電位は４０℃で測定した。その他の還元電位は２０℃で測定した。
【００４４】
【表２】

【００４５】
【表３】

【００４６】
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。
【００４７】
【発明の効果】
本発明によれば、電池のサイクル特性、電気容量や充電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]
[Industrial application fields]
The present invention relates to a lithium secondary battery having excellent battery performance.
[0002]
[Prior art]
In recent years, electronic devices have become smaller and lighter and more portable, and as a driving power source for them, development of batteries having high energy density is required. Lithium secondary batteries are expected as batteries that meet these requirements. In order to avoid lithium dendritic precipitates, a carbon material capable of inserting / extracting lithium ions is used for the negative electrode of the lithium secondary battery in place of the metal lithium conventionally used.
[0003]
Among carbon materials, graphite with particularly high crystallinity is frequently used for the negative electrode of lithium secondary batteries because it can achieve high capacity.
[0004]
However, when cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are used in the electrolyte solution, these electrolyte solutions are reduced and decomposed on the graphite surface of the negative electrode during charging to destroy the graphite structure. Sufficient battery performance cannot be achieved. In particular, in an electrolyte containing PC, the destruction phenomenon becomes remarkable, and the graphite peels from the negative electrode current collector, and the discharge capacity becomes zero.
[0005]
As a method for suppressing the decomposition of the electrolytic solution on the graphite surface during charging, a method of adding an additive to the electrolytic solution has been proposed. For example, ZXShu, RSMcMillan, and JJ Murray, J. Electrochem. Soc. , Vol140, No. 6, L101 (1993). Describes that the decomposition of the electrolyte is suppressed by adding crown ether (12-crown-4) to the electrolyte based on PC and EC. However, crown ether is expensive, and the effect cannot be exhibited unless it is added in a large amount.
[0006]
JP-A-8-45545 discloses that an electrolyte containing an unsaturated bond and reductively decomposed on the carbon surface of the negative electrode at a potential 1V higher than lithium forms a passive film on the carbon negative electrode. It is described that the decomposition of is suppressed. However, the reductive decomposition potentials of EC and PC are 0.70 V and 0.75 V, respectively, whereas the additive compound that undergoes reductive decomposition at a potential higher by 1 V or more than lithium is higher than the EC or PC of the electrolyte during charging. However, it was found that the film formed on the carbon negative electrode becomes excessively thick because it decomposes much earlier. Since the composition of this passive film is an electrically insulating compound such as lithium alkyl carbonate, lithium carbonate, or lithium fluoride, as the film becomes thicker, the internal resistance of the battery increases, and satisfactory electric capacity and cycle characteristics are obtained. There was a problem that it was not possible.
[0007]
In addition, J. Electrochem. Soc., Vol. 140, No. 9, L161 (1995) states that the PC decomposition on the surface of the graphite electrode is suppressed by adding chloroethylene carbonate to the electrolyte. ing. This is thought to be due to the fact that the decomposition product of chloroethylene carbonate forms a passive film on the graphite surface, but the inhibitory effect on the decomposition of the electrolytic solution is not necessarily good.
[0008]
[Problems to be solved by the invention]
In the conventional method, since the electrically insulating film formed on the carbon negative electrode becomes excessively thick, battery characteristics such as electric capacity and cycle characteristics cannot be sufficiently exhibited.
[0009]
[Means for Solving the Problems]
The present inventors pay attention to the fact that in the conventional method, the electric insulating film formed on the graphite surface of the negative electrode during charging becomes thick and the electric capacity is low, and a thin film is formed on the graphite surface of the negative electrode. As a result of intensive studies aimed at this, the present invention has been achieved.
[0010]
The present invention relates to an electrolyte for a lithium secondary battery comprising a positive electrode made of a composite oxide of lithium and a transition metal, and a graphite negative electrode having a crystal plane distance (d ₀₀₂ ) of 0.34 nm or less. An electrolyte in which an electrolyte is dissolved in an aqueous solvent, and an additive that can be reduced at a potential higher than 0.8V and lower than 1V than lithium in the graphite negative electrode is 0.1% in the electrolyte. Is contained in an amount of 10% by weight , and the additive is represented by the following general formula (I)
[Chemical 1]

(Wherein X ¹ , X ² , X ³ and X ⁴ represent a carbon atom or a nitrogen atom. Y represents an acyl group having 1 to 12 carbon atoms, an ester group having 1 to 12 carbon atoms, or 1 to 1 carbon atoms. And R ¹ , R ² , R ³ , and R ⁴ are not present when X ¹ , X ² , X ³ , and X ⁴ bonded to each other are nitrogen atoms, and bonded to each other. Wherein X ¹ , X ² , X ³ , and X ⁴ are carbon atoms, it represents a hydrogen atom.) At least one selected from heterocyclic amides, heterocyclic carbamates and heterocyclic sulfonamide derivatives It is related with the electrolyte solution for lithium secondary batteries characterized by these.
[0011]
The present invention also includes a positive electrode made of a composite oxide of lithium and a transition metal, a graphite negative electrode having a crystal plane distance (d ₀₀₂ ) of 0.34 nm or less, and an electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent. In the lithium secondary battery comprising, the electrolyte contains 0.1 to 10% by weight of an additive capable of being reduced at a potential higher than 0.8 V and lower than 1 V than lithium in the graphite negative electrode, The additive is represented by the following general formula (I)
[Chemical 2]

(Wherein X ¹ , X ² , X ³ and X ⁴ represent a carbon atom or a nitrogen atom. Y represents an acyl group having 1 to 12 carbon atoms, an ester group having 1 to 12 carbon atoms, or 1 to 1 carbon atoms. And R ¹ , R ² , R ³ , and R ⁴ are not present when X ¹ , X ² , X ³ , and X ⁴ bonded to each other are nitrogen atoms, and bonded to each other. Wherein X ¹ , X ² , X ³ , and X ⁴ are carbon atoms, it represents a hydrogen atom.) At least one selected from heterocyclic amides, heterocyclic carbamates and heterocyclic sulfonamide derivatives The present invention relates to a lithium secondary battery.
[0012]
The reduction potential of the additive used in the present invention is preferably slightly higher than the reduction potential of the electrolytic solution. Since the reduction potentials of EC and PC, which are non-aqueous solvents that are usually suitably used as electrolytes for lithium secondary batteries, are 0.70 V and 0.75 V, respectively, the additive has a reduction potential higher than 0.8 V. It is good to have. The additive is preferably one that undergoes reductive decomposition at a potential that is not excessively reduced, that is, a potential lower than 1 V (vs. lithium potential).
[0013]
Since the additive of the present invention has a reductive decomposition potential higher than that of the electrolytic solution, it is reduced and decomposed earlier than the electrolytic solution and can be reduced at a potential lower than 1 V. There is no reductive decomposition. For this reason, a very thin electrically insulating film is formed on the surface of the graphite negative electrode, and it is considered that the decomposition of the electrolytic solution such as EC and PC on the surface of the graphite negative electrode is suppressed during charging.
[0014]
Examples of the additive that can be reduced at a potential higher than 0.8 V and lower than 1 V than lithium in the graphite negative electrode used in the present invention include the additive represented by the general formula (I). Is, for example, 1-propionylpyrazole [Y = propionyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom], 1 -Methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom], 1-methane Sulfonylpyrazole [Y = methanesulfonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom], 1-methoxycarbonyl- 1,2,4-triazole [Y = methoxycarbonyl Group, R ¹ = R ³ = none, R ² = R ⁴ = hydrogen atom, X ¹ = X ³ = nitrogen atom, X ² = X ⁴ = heterocyclic amides such as carbon atoms, a heterocyclic carbamate, heterocyclic sulfone Amide derivatives are mentioned.
If the content of the additive is excessively large, the film formed on the surface of the carbon negative electrode becomes thick, resulting in poor cycle characteristics. If the content is excessively small, decomposition of the electrolytic solution cannot be sufficiently suppressed. Is preferably contained in an amount of 0.1 to 10% by weight.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
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 suitable.
As the high dielectric constant solvent, for example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC) are preferable. These high dielectric constant solvents may be used alone or in combination of two or more.
[0016]
Examples of the low viscosity solvent include chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC), lactones such as γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4 -Ethers such as dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, nitriles such as acetonitrile, esters such as methyl propionate, amides such as dimethylformamide Is mentioned. 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
[0017]
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 , LiCF. ₃ SO ₃ , LiAsF _{6 and the} like. These electrolytes may be used alone or in combination of two or more. These electrolytes are used by dissolving in the above non-aqueous solvent at a concentration of usually 0.1 to 3M, preferably 0.5 to 1.5M.
[0018]
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-mentioned electrolyte in this, and heterocyclic amide, heterocyclic carbamate or heterocyclic represented by the above formula (I). It can be obtained by dissolving the sulfonamide derivative.
[0019]
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.
[0020]
For example, as the positive electrode active material, a composite metal oxide of at least one metal selected from transition elements such as cobalt, manganese, nickel, chromium, iron, or vanadium and lithium is used. Examples of such composite metal oxides include LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ .
[0021]
The positive electrode is obtained by kneading the positive electrode active material with a conductive material such as acetylene black or carbon black, a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), and a solvent to form a positive electrode mixture. By applying this positive electrode mixture to an aluminum foil or stainless steel lath plate as a current collector, and after drying and pressure molding, heat treatment is performed under vacuum at a temperature of about 50 to 250 ° C. for about 2 hours. Produced.
[0022]
As the negative electrode active material, it is preferable to use a carbon material having a graphite-type crystal structure in which the lattice spacing ( ₀₀₂ ) (d ₀₀₂ ) is 0.34 nm or less, particularly 0.335 to 0.337 nm. A powder material such as this carbon material is kneaded with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) and used as a negative electrode mixture.
[0023]
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 and a square battery having a positive electrode, a negative electrode, and a roll separator. An example is given. A known polyolefin microporous film, woven fabric, non-woven fabric or the like is used as the separator.
[0024]
【Example】
[Measurement of reduction potential]
Using natural graphite powder as a test electrode, the reduction potential of the electrolyte solution and additive compound for a lithium secondary battery was measured in a three-electrode cell using Li metal as a counter electrode and a reference electrode. LiPF ₆ was used as an electrolyte, and this was dissolved in a non-aqueous solvent to a concentration of 1M to prepare an electrolytic solution. The additive was dissolved in the PC electrolyte and the reduction potential was measured. Table 1 shows the measurement results of the reduction potential.
Next, although an Example and a comparative example are given and this invention is demonstrated concretely, these do not limit this invention at all.
[0025]
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, a heterocyclic amide derivative (additive) 1-propionylpyrazole [Y = propionyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom] It added so that it might become 2 weight% with respect to.
[0026]
[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 mixed with 1-methyl-2-pyrrolidone. A mixture obtained by adding a solvent was applied onto an aluminum foil, dried, pressure-molded, and heat-treated to prepare a positive electrode. 90% by weight of natural graphite (negative electrode active material) having a crystal plane spacing (d ₀₀₂ ) of 0.335 nm and 10% by weight of polyvinylidene fluoride (binder) are mixed with 1-methyl-2- What mixed the pyrrolidone solvent and mixed was apply | coated on copper foil, and it dried, pressure-molded, and heat-processed, and prepared the 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. The weight ratio between the positive electrode active material and the negative electrode active material was set to be approximately the same electric capacity.
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 capacity was 410 mAh / g carbon and the initial discharge capacity was 310 mAh / g carbon. Moreover, when the battery characteristics after 50 cycles of charge and discharge were measured, the discharge capacity retention rate was 80.3% when the initial discharge capacity was 100%. Also, the low temperature characteristics were good. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0027]
Example 2
As an additive, 1-methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 1 to prepare a coin battery, and the battery characteristics after 50 cycles were measured. As a result, the discharge capacity retention rate was 83. It was 5%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0028]
Example 3
As an additive, 1-methanesulfonylpyrazole [Y = methanesulfonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 1 to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 81. It was 6%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0029]
Example 4
As an additive, 1-methoxycarbonyl-1,2,4-triazole [Y = methoxycarbonyl group, R ¹ = R ³ = none, R ² = R ⁴ = hydrogen atom, X ¹ = X ³ = nitrogen atom, X ² = X ⁴ = carbon atom] was used in an amount of 2% by weight with respect to the electrolyte solution, and an electrolyte solution was prepared in the same manner as in Example 1 to produce a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 79.3%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0030]
Example 5
As an additive, 1-methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 1 except that 0.2% by weight was used with respect to the electrolytic solution to prepare a coin battery, and the battery characteristics after 50 cycles were measured. It was 80.7%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0031]
Example 6
As an additive, 1-methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 1 except that 5% by weight of the electrolyte solution was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured. 2%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0032]
Comparative Example 1
As an additive, except that 2% by weight of chloroethylene carbonate was used with respect to the electrolytic solution, an electrolytic solution was prepared in the same manner as in Example 1 to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The capacity retention rate was 74.3%. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0033]
Comparative Example 2
As an additive, except that pentafluorobenzonitrile was used in an amount of 2% by weight based on the electrolytic solution, an electrolytic solution was prepared in the same manner as in Example 1 to prepare a coin battery, and the battery characteristics were measured. The discharge capacity after 50 cycles was 0 mAh / g carbon at 50 mAh / g carbon. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0034]
Comparative Example 3
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. Table 2 shows the production conditions and battery characteristics of the coin battery.
[0035]
Example 7
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 electrolytic solution, and then a heterocyclic amide derivative (additive). 1-propionylpyrazole [Y = propionyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom] It added so that it might become 2 weight% with respect to. Using this electrolytic solution, a coin battery was prepared in the same manner as in Example 1, and the battery characteristics were measured. The initial charge capacity was 430 mAh / g carbon, and the initial discharge capacity was 330 mAh / g carbon. Moreover, when the battery characteristics after 50 cycles were measured, the discharge capacity retention rate was 90.5% when the initial discharge capacity was 100%. Also, the low temperature characteristics were good. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0036]
Example 8
As an additive, 1-methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 7 except that 2% by weight was used for the electrolytic solution to prepare a coin battery, and the battery characteristics after 50 cycles were measured. 2%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0037]
Example 9
As an additive, 1-methanesulfonylpyrazole [Y = methanesulfonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen atom, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom ] Was used in the same manner as in Example 7 except that 2% by weight was used for the electrolytic solution to prepare a coin battery, and the battery characteristics after 50 cycles were measured. 0%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0038]
Example 10
As an additive, 1-methoxycarbonyl-1,2,4-triazole [Y = methoxycarbonyl group, R ¹ = R ³ = none, R ² = R ⁴ = hydrogen atom, X ¹ = X ³ = nitrogen atom, X ² = X ⁴ = carbon atom] was used in the same manner as in Example 7 except that 2% by weight of the electrolyte was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 88.8%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0039]
Example 11
LiMn ₂ O ₄ is used instead of LiCoO ₂ as a positive electrode active material, and 1-methoxycarbonylpyrazole [Y = methoxycarbonyl group, R ¹ = none, R ² = R ³ = R ⁴ = hydrogen as an additive A coin battery was prepared by preparing an electrolyte solution in the same manner as in Example 7 except that 3 wt% of atoms, X ¹ = nitrogen atom, X ² = X ³ = X ⁴ = carbon atom] was used with respect to the electrolyte solution. When the battery characteristics after 50 cycles were measured, the discharge capacity retention rate was 90.3%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0040]
Comparative Example 4
A coin battery was prepared by preparing an electrolyte solution in the same manner as in Example 7 except that 2% by weight of chloroethylene carbonate was used as an additive with respect to the electrolyte solution, and the battery characteristics after 50 cycles were measured. The capacity retention rate was 83.4%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0041]
Comparative Example 5
As an additive, except that pentafluorobenzonitrile was used in an amount of 2% by weight based on the electrolytic solution, an electrolytic solution was prepared in the same manner as in Example 7 to produce a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 58.2%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0042]
Comparative Example 6
A non-aqueous solvent with EC: 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. The initial charge capacity was 430 mAh / g carbon, and the initial discharge capacity was 310 mAh / g carbon. When the battery characteristics after 50 cycles were measured, the discharge capacity retention rate was 83.8% when the initial discharge capacity was 100%. Table 3 shows the production conditions and battery characteristics of the coin battery.
[0043]
[Table 1]

Note) However, the reduction potential of EC was measured at 40 ° C. Other reduction potentials were measured at 20 ° C.
[0044]
[Table 2]

[0045]
[Table 3]

[0046]
In addition, this invention is not limited to the Example described, The various combination which can be easily guessed from the meaning of invention is 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.
[0047]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery excellent in battery characteristics, such as a cycling characteristic of a battery, an electrical capacity, and a charge storage characteristic, can be provided.

Claims

An electrolyte for a lithium secondary battery comprising a positive electrode made of a composite oxide of lithium and a transition metal and a graphite negative electrode having a crystal plane distance (d ₀₀₂ ) of 0.34 nm or less. The electrolyte is an electrolyte in a non-aqueous solvent. In the electrolyte, 0.1 to 10% by weight of an additive capable of being reduced at a potential higher than 0.8V and lower than 1V than lithium in the graphite negative electrode in the electrolytic solution. And the additive is represented by the following general formula (I)

(Wherein X ¹ , X ² , X ³ and X ⁴ represent a carbon atom or a nitrogen atom. Y represents an acyl group having 1 to 12 carbon atoms, an ester group having 1 to 12 carbon atoms, or 1 to 1 carbon atoms. And R ¹ , R ² , R ³ , and R ⁴ are not present when X ¹ , X ² , X ³ , and X ⁴ bonded to each other are nitrogen atoms, and bonded to each other. Wherein X ¹ , X ² , X ³ , and X ⁴ are carbon atoms, it represents a hydrogen atom.) At least one selected from heterocyclic amides, heterocyclic carbamates and heterocyclic sulfonamide derivatives electrolyte for lithium secondary batteries, characterized in that it.

In a lithium secondary battery including a positive electrode made of a composite oxide of lithium and a transition metal, a graphite negative electrode having a crystal plane distance (d ₀₀₂ ) of 0.34 nm or less, and an electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent , additive reducible by higher than and lower than 1V potential 0.8V than lithium in the graphite negative electrode is contained 0.1 to 10 wt% in the electrolyte solution in the electrolytic solution, the additive is represented by the following general formula (I)

(Wherein X ¹ , X ² , X ³ and X ⁴ represent a carbon atom or a nitrogen atom. Y represents an acyl group having 1 to 12 carbon atoms, an ester group having 1 to 12 carbon atoms, or 1 to 1 carbon atoms. And R ¹ , R ² , R ³ , and R ⁴ are not present when X ¹ , X ² , X ³ , and X ⁴ bonded to each other are nitrogen atoms, and bonded to each other. Wherein X ¹ , X ² , X ³ , and X ⁴ are carbon atoms, it represents a hydrogen atom.) At least one selected from heterocyclic amides, heterocyclic carbamates and heterocyclic sulfonamide derivatives lithium secondary battery, characterized by at.