JP4878690B2

JP4878690B2 - Lithium secondary battery

Info

Publication number: JP4878690B2
Application number: JP2001084751A
Authority: JP
Inventors: 拓也砂川; 吉久三宮本; 昌利高橋
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 2001-03-23
Filing date: 2001-03-23
Publication date: 2012-02-15
Anticipated expiration: 2021-03-23
Also published as: JP2002289175A

Description

【０００１】
【発明の属する技術分野】
本発明はリチウムイオンの吸蔵・放出が可能な正極活物質を含有する正極と、黒鉛を負極活物質として含有する負極と、非水電解質とを備えたリチウム二次電池に係わり、特に、正極活物質と負極活物質の質量割合の最適化に関する。
【０００２】
【従来の技術】
近年、小型ビデオカメラ、携帯電話、ノートパソコン等の携帯用電子・通信機器等に用いられる電池として、リチウムイオンの吸蔵・放出が可能な黒鉛を負極活物質とし、リチウム含有コバルト酸化物（ＬｉＣｏＯ₂）、リチウム含有マンガン酸化物（ＬｉＭｎ₂Ｏ₄）等のリチウム含有遷移金属酸化物を正極活物質とするリチウム二次電池が、小型軽量でかつ高容量な電池として実用化されるようになった。
【０００３】
ところで、正極にリチウム含有コバルト酸化物（ＬｉＣｏＯ₂）を用い、負極に黒鉛を用いたリチウム二次電池においては、充電されるに伴って正極合剤層および負極合剤層が共に膨張し、放電するに伴って正極合剤層および負極合剤層が共に収縮するため、充電時においては正極合剤層および負極合剤層の圧力が増加して、電解液の一部がこれらの合剤層から搾り出されるという現象が生じる。このような現象を生じると、充放電を繰り返すに伴って容量が低下してサイクル特性が劣化するという問題を生じた。また、この正極合剤層および負極合剤層の膨張・収縮は、活物質量が多くなるほど顕著になる。
【０００４】
一方、充電時に正極合剤層および負極合剤層が膨張すると、これらの合剤層と各集電体（活物質保持体）との密着性が低下することとなるが、正極合剤層および負極合剤層が共に膨張するため、正負極間の対向圧力が増大する。これにより、正極合剤層および負極合剤層が各集電体に押し付けられるようになって、各集電体から正極合剤層あるいは負極合剤層が剥離することが抑制されることとなる。このことから、正極活物質量および負極活物質量を必要以上に少なくすると、逆に、充電時において、正極合剤層および負極合剤層が各集電体から剥離することを抑制することができなるため、これらの活物質量を必要以上に減少させることは不適当である。
【０００５】
【発明が解決しようとする課題】
これらのことを考慮すると、正極活物質量と負極活物質量を調整してこれらの活物質量を適度にバランスさせれば、放電容量とサイクル特性が調和した特性を兼ね備えたリチウム二次電池が設計できることとなる。
そこで、充電時に収縮し、放電時に膨張する性質を有するスピネル型結晶構造を有するリチウム含有マンガン酸化物（ＬｉＭｎ₂Ｏ₄：以下ではスピネル型マンガン酸リチウムという）と、充電時に膨張し、放電時に収縮する性質を有するリチウム含有コバルト酸化物（ＬｉＣｏＯ₂：以下ではコバルト酸リチウムという）とを混合した混合正極活物質を用いることが特開平４−１７１６６０号公報にて提案されるようになった。
【０００６】
この特開平４−１７１６６０号公報にて提案された正極においては、スピネル型マンガン酸リチウムとコバルト酸リチウムとを混合して用いることで、コバルト酸リチウムのみを用いた正極よりも充放電による膨張収縮が少なくなる。このため、充電時において、正極合剤層および負極合剤層の膨張に起因する電解液の搾り出し現象を軽減することが可能となる。しかしながら、充電時において、正極合剤の膨張が少なくなると、負極合剤の膨張に起因する集電体からの剥離が抑制できなくなるという問題を生じた。
【０００７】
そこで、正極活物質量および負極活物質量を増やすようにして、充電時に負極合剤層をより膨張させるようにすると、負極合剤層の集電体からの剥離を抑制できるようになる。また、正極活物質量および負極活物質量の増加量を調整することにより、充電時の電解液の搾り出し現象を抑制することが可能になる。しかしながら、負極活物質量が多くなると、充放電に伴う負極合剤層の厚みの変化も大きくなるため、充電状態では正負極に適度の対向圧力を付与することが可能となるが、反面、放電状態では負極の収縮が大きくなるため、正負極に適度の対向圧力を付与することが困難になるという問題を生じた。
【０００８】
本発明は上記問題点を解消するためになされたものであって、コバルト酸リチウムとスピネル型マンガン酸リチウムとが混合された混合正極活物質を用いても、混合正極活物質の配合量を最適化するとともに、この混合正極活物質と負極活物質の質量割合を最適化して、放電容量およびサイクル特性が向上したリチウム二次電池を得られるようにすることを目的とするものである。
【０００９】
【課題を解決するための手段】
上記目的を達成するため、本発明のリチウム二次電池においては、正極はコバルト酸リチウムとスピネル型マンガン酸リチウムとが混合された混合正極活物質を主体とした正極合剤が正極集電体に保持されており、この正極の単位面積当たりの正極活物質の質量をＡとし、負極の単位面積当たりの負極活物質の質量をＢとし、かつ混合正極活物質中のコバルト酸リチウムの質量比をＸとした場合に、正極活物質の質量Ａに対する負極活物質の質量Ｂの質量割合（Ｂ／Ａ）が０．１５Ｘ＋０．２６≦Ｂ／Ａ≦０．１１Ｘ＋０．４１の関係を有するとともに、負極の単位面積当たりの負極活物質の質量Ｂが１６０≦Ｂ≦３９Ｘ＋１８０の関係を有し、かつ、正極合剤層の厚みをａ（μｍ）とし、負極合剤層の厚みをｂ（μｍ）とした場合に、正極合剤層の厚みａに対する負極合剤層の厚みｂの割合（ｂ／ａ）が０．２９Ｘ＋０．５≦ｂ／ａ≦０．２５Ｘ＋０．９５の関係を有するとともに、前記負極合剤層の厚みｂ（μｍ）が８６≦ｂ≦１６３の関係を有するように規制されている。
【００１０】
正極活物質に対する負極活物質の質量割合Ｂ／Ａが０．１５Ｘ＋０．２６より少なくなると、正極活物質量に対して負極活物質量が相対的に少なくなるため、充電時の正負極活物質の膨張の度合いが少なくなる。これは、正極においては、充電時にコバルト酸リチウムは膨張する反面、スピネル型マンガン酸リチウムは収縮して膨張が相殺されるとともに、負極においては、活物質量が少なくなったことで膨張が少なくなったことによる。
この結果、充電時の正極と負極の対向圧力が低下して、負極合剤層と負極集電体との密着性が低下するためにサイクル特性が低下する。また、正極活物質に対する負極活物質の質量比Ｂ／Ａが０．１１Ｘ＋０．４１より多くなると、正極活物質量に対して負極活物質量が相対的に多くなるため、負極合剤層の充放電による体積変化が大きくなって、放電時の正極と負極の対向圧力が必要以上に大きくなり、電解液が搾り出されることによりサイクル特性が低下する。
【００１１】
このため、コバルト酸リチウムの混合比をＸとした場合の正極活物質に対する負極活物質の質量比Ｂ／Ａは、０．１５Ｘ＋０．２６≦Ｂ／Ａ≦０．１１Ｘ＋０．４１の関係を有する範囲に規制することが望ましい。
ここで、正極の単位面積当たりの正極活物質量を４３０ｇ／ｍ²（Ａ＝４３０）として実験を行った結果、０．３７２≦Ｂ／Ａ≦０．０９０７Ｘ＋０．４１９の関係を有する範囲に規制することがさらに望ましいことが分かった。この場合、正極活物質量は４３０ｇ／ｍ²（Ａ＝４３０）であるから、１６０≦Ｂ≦３９Ｘ＋１８０という関係が得られる。
【００１２】
また、正極の単位面積当たりの正極活物質量を４８０ｇ／ｍ²（Ａ＝４８０）として実験を行った結果、０．３３３≦Ｂ／Ａ≦０．０８１３Ｘ＋０．３７５の関係を有する範囲に規制することがさらに望ましいことが分かった。この場合は、正極活物質量は４８０ｇ／ｍ²（Ａ＝４３０）であるから、１６０≦Ｂ≦３９Ｘ＋１８０という関係が得られる。
即ち、負極活物質の単位面積当たりの質量Ｂを１６０≦Ｂ≦３９Ｘ＋１８０という関係が得られるように規制すれば、正極活物質の単位面積当たりの質量Ａを変化させても、正極と負極の対向圧力がより適正な範囲に維持されるということができる。
【００１３】
また、正極合剤層の厚みをａ（μｍ）とし、負極合剤層の厚みをｂ（μｍ）とした場合に、正極合剤層の厚みａに対する負極合剤層の厚みｂの割合（ｂ／ａ）が０．２９Ｘ＋０．５０≦ｂ／ａ≦０．２５Ｘ＋０．９５の関係を有する範囲に規制すると、正極と負極の対向圧力がより適正な範囲に維持されてサイクル特性が向上するので望ましい。
【００１４】
なお、本発明に用いるスピネル型マンガン酸リチウムは、組成式がＬｉ_1+XＭｎ_2-YＭ_ZＯ₄（但し、ＭはＢ，Ｍｇ，Ｃａ，Ｓｒ，Ｂａ，Ｔｉ，Ｖ，Ｃｒ，Ｆｅ，Ｃｏ，Ｎｉ，Ｃｕ，Ａｌ，Ｉｎ，Ｎｂ，Ｍｏ，Ｗ，Ｙ，Ｒｈから選択される少なくとも一種の元素であり、０．５４≦（（１＋Ｘ）＋Ｚ）／（２−Ｙ）≦０．６２で、−０．１５≦Ｘ≦０．１５で、Ｙ≦０．５で、０≦Ｚ≦０．１である）で表される組成のものであれば同様な結果が得られるが、このうち、特に優れた高温特性（高温での充放電サイクル、高温保存性等）を示すためには、Ｍｇ添加系あるいはＡｌ添加系のものを用いるのが望ましい。
【００１５】
また、コバルト酸リチウムとしては、組成式がＬｉＣｏ_1-XＭ_XＯ₂（但し、ＭはＢ，Ｍｇ，Ｃａ，Ｓｒ，Ｂａ，Ｔｉ，Ｖ，Ｃｒ，Ｆｅ，Ｎｉ，Ｃｕ，Ａｌ，Ｉｎ，Ｎｂ，Ｍｏ，Ｗ，Ｙ，Ｒｈから選択される少なくとも一種の元素であり、０≦Ｘ≦０．１である）で表されるコバルト酸リチウムを用いれば、同様な結果が得られるが、このうち、特に優れた放電特性を示すためには、Ｃｒ添加系、Ｍｎ添加系、Ａｌ添加系、Ｔｉ添加系のものを用いるのが望ましい。
【００１６】
【発明の実施の形態】
ついで、本発明の実施の形態を以下に説明する。
１．混合正極活物質の作製
まず、正極活物質として、平均粒径が５μｍのコバルト酸リチウム（ＬｉＣｏＯ₂）粉末と、平均粒径が１０μｍのスピネル型マンガン酸リチウム（Ｌｉ_1.07Ｍｎ_1.89Ｍｇ_0.04Ｏ₄）粉末とをそれぞれ公知の方法で合成した。ついで、これらのコバルト酸リチウム（ＬｉＣｏＯ₂）粉末とマンガン酸リチウム（Ｌｉ_1.07Ｍｎ_1.89Ｍｇ_0.04Ｏ₄）粉末とを下記の表１に示すような質量比となるように混合して、各混合正極活物質α、β、γ、δ、εをそれぞれ作製した。この場合、コバルト酸リチウム（ＬｉＣｏＯ₂）の混合質量比をＸとし、スピネル型マンガン酸リチウム（Ｌｉ_1.07Ｍｎ_1.89Ｍｇ_0.04Ｏ₄）の混合質量比を１−Ｘとした。
【００１７】
【表１】

【００１８】
２．正極の作製
ついで、上述のようにして作製された各混合正極活物質α〜εを用い、これらの混合正極活物質α〜εがそれぞれ８５質量部で、導電剤としてのカーボンブラックが１０質量部で、結着剤としてのフッ化ビニリデン系重合体が５質量部となるようにそれぞれ混合して、正極合剤をそれぞれ作製した。ついで、得られた各正極合剤をＮ−メチルピロリドン（ＮＭＰ）と混合して正極スラリーとした後、これらの正極スラリーを厚みが２０μｍの正極集電体（アルミニウム箔またはアルミニウム合金箔）の両面にドクターブレード法により塗布（なお、正極リードを取り付けるために間欠塗布により未塗布部を設けた）して、正極集電体の両面に正極合剤層を形成した。
【００１９】
この場合、各正極合剤層を形成するに際して、正極の単位面積当たりの各混合正極活物質α〜εの質量Ａが４３０ｇ／ｍ²となるように塗布量を調整した。これを乾燥させた後、正極合剤の充填密度が下記の表２に示すような値になるように圧縮ローラを用いて圧延し、所定寸法（例えば幅が４０ｍｍで、長さが２８０ｍｍ）に切断して、正極ａ１〜ｅ１をそれぞれ作製した。
なお、各正極ａ１〜ｅ１の各正極合剤の充填密度を下記の表２に示す範囲に変化させた理由は、正極合剤中の各活物質粒子の電気的接触を充放電中も維持できるようにして、コバルト酸リチウムとスピネル型マンガン酸リチウムの混合質量比が異なっても、各正極のサイクル特性に違いを生じさせないためである。
【００２０】
【表２】

【００２１】
３．負極の作製
天然黒鉛（Ｌｃ値が１５０Å以上で、ｄ値が３．３８Å以下のもの）粉末が９５質量部で、結着剤としてのポリフッ化ビニリデン（ＰＶｄＦ）粉末が５質量部となるように混合して負極合剤を調製した後、これをＮ−メチルピロリドン（ＮＭＰ）と混合して負極スラリーを調製した。この後、得られた負極スラリーを厚みが１８μｍの負極集電体（銅箔）の両面にドクターブレード法により塗布して、負極集電体の両面に負極合剤層を形成した。この場合、負極合剤層を形成するに際して、下記の表３に示すような負極の単位面積当たりの質量Ｂとなるように塗布量を調整した。これを乾燥させた後、充填密度が１．５６ｇ/ｃｍ²になるように圧縮ローラを用いて所定の厚みになるまで圧延し、所定寸法（例えば幅が４２ｍｍで、長さが３００ｍｍ）に切断して負極ｘ１〜ｘ２９を作製した。
【００２２】
【表３】

【００２３】
４．リチウム二次電池の作製
ついで、上述のように作製した各正極ａ１〜ｅ１と、上述のようにして作製した各負極ｘ１〜ｘ２９とをそれぞれ用いて、これらを下記の表４および表５に示すように組み合わせるとともに、これらの間にポリプロピレン製微多孔膜からなるセパレータを介在させて積層した後、これらを渦巻状にそれぞれ巻回して渦巻状電極群とした。これらをそれぞれ円筒状の金属製外装缶に挿入した後、各集電体から延出する集電タブを各端子に溶接し、エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）との等体積混合溶媒に、ＬｉＰＦ₆を１モル／リットル溶解した非水電解液を注入した。この後、外装缶の開口部に絶縁パッキングを介して正極蓋を取り付けた後、封口してリチウム二次電池Ａ１〜Ａ６、Ｂ１〜Ｂ６、Ｃ１〜Ｃ６、Ｄ１〜Ｄ６およびＥ１〜Ｅ６をそれぞれ作製した。
ここで、正極ａ１を用いたものをリチウム二次電池Ａ１〜Ａ６とし、正極ｂ１を用いたものをリチウム二次電池Ｂ１〜Ｂ６とし、正極ｃ１を用いたものをリチウム二次電池Ｃ１〜Ｃ６とし、正極ｄ１を用いたものをリチウム二次電池Ｄ１〜Ｄ６とし、正極ｅ１を用いたものをリチウム二次電池Ｅ１〜Ｅ６とした。
【００２４】
なお、混合溶媒としては、上述したエチレンカーボネート（ＥＣ）にジエチルカーボネート（ＤＥＣ）を混合したもの以外に、水素イオンを供給する能力のない非プロトン性溶媒を使用し、例えば、プロピレンカーボネート（ＰＣ）、ビニレンカーボネート（ＶＣ）、ブチレンカーボネート（ＢＣ）、γ−ブチロラクトン（ＧＢＬ）等の有機溶媒や、これらとジメチルカーボネート（ＤＭＣ）、メチルエチルカーボネート（ＥＭＣ）、１，２−ジエトキシエタン（ＤＥＥ）、１，２−ジメトキシ工タン（ＤＭＥ）、エトキシメトキシエタン（ＥＭＥ）などの低沸点溶媒との混合溶媒を用いてもよい。また、これらの溶媒に溶解される溶質としては、ＬｉＰＦ₆以外に、ＬｉＢＦ₄、ＬｉＣＦ₃ＳＯ₃、ＬｉＡｓＦ₆、ＬｉＮ（ＣＦ₃ＳＯ₂）₂、ＬｉＣ（ＣＦ₃ＳＯ₂）₃、ＬｉＣＦ₃（ＣＦ₂）₃ＳＯ₃等を用いてもよい。さらに、ポリマー電解質、ポリマーに非水電解液を含浸させたようなゲル状電解質、固体電解質なども使用できる。
【００２５】
５．リチウム二次電池の充放電試験
これらの各電池Ａ１〜Ａ６、Ｂ１〜Ｂ６、Ｃ１〜Ｃ６、Ｄ１〜Ｄ６およびＥ１〜Ｅ６を用いて、室温（約２５℃）で、６０ｍＡの充電電流で、電池電圧が４．２Ｖになるまで定電流充電した後、６００ｍＡの放電電流で電池電圧が３．１Ｖになるまで放電させるという充放電を１サイクルとして、充放電サイクルを繰り返して行い、１サイクル目の放電容量に対する３００サイクル目の放電容量を容量維持率（容量維持率（％）＝（３００サイクル目の放電容量／１サイクル目の放電容量）×１００）として求める、下記の表４および表５に示すような結果となった。
【００２６】
【表４】

【００２７】
【表５】

【００２８】
６．試験結果の検討
上記表４および表５の結果から、コバルト酸リチウムの混合比Ｘを横軸とし、正極活物質に対する負極活物質の質量比（Ｂ／Ａ）を縦軸としてグラフで表すと図１に示すような結果となった。なお、図１において、容量維持率が８４％未満の電池を×印で示し、容量維持率が８４％以上で８８％未満の電池を△印で示し、容量維持率が８８％以上の電池を○印で示している。
【００２９】
ここで、図１において、○印および△印と×印とを区画する下限線（図１の下方の実線）を引くと、Ｂ／Ａ＝０．１５Ｘ＋０．２６という式が得られ、○印および△印と×印とを区画する上限線（図１の上方の実線）を引くと、Ｂ／Ａ＝０．１１Ｘ＋０．４１という式が得られた。このことから、コバルト酸リチウムの混合比をＸとした場合の正極活物質に対する負極活物質の質量比Ｂ／Ａは、０．１５Ｘ＋０．２６≦Ｂ／Ａ≦０．１１Ｘ＋０．４１の関係を有する範囲に規制すると、高容量維持率でサイクル特性に優れたリチウム二次電池が得られることが分かる。
【００３０】
ここで、正極活物質に対する負極活物質の質量比Ｂ／Ａが０．１５Ｘ＋０．２６より少なくなると、正極活物質量に対して負極活物質量が相対的に少なくなるため、充電時の正負極活物質の膨張が少なくなる。これは、正極においては、充電時にコバルト酸リチウムは膨張する反面、スピネル型マンガン酸リチウムは収縮して膨張が相殺されるとともに、負極においては、活物質量が少なくなったことで膨張が少なくなったことによる。この結果、充電時の負極合剤層と負極集電体との密着性が低下して、容量維持率（サイクル特性）が低下したと考えられる。
また、正極活物質に対する負極活物質の質量比Ｂ／Ａが０．１１Ｘ＋０．４１より多くなると、正極活物質量に対して負極活物質量が相対的に多くなるため、負極合剤層の充放電による体積変化が大きくなって、放電時の正極と負極の対向圧力が必要以上に大きくなり、電解液が搾り出されることにより容量維持率（サイクル特性）が低下すると考えられる。
【００３１】
さらに、図１において、○印をできる限り多く含むような下限線（図１の下方の破線）を引くと、Ｂ／Ａ＝０．３７２という式が得られ、上限線（図１の上方の破線）を引くと、Ｂ／Ａ＝０．０９０７Ｘ＋０．４１９という式が得られた。このことから、コバルト酸リチウムの混合比をＸとした場合の正極活物質に対する負極活物質の質量比Ｂ／Ａを、０．３７２≦Ｂ／Ａ≦０．０９０７Ｘ＋０．４１９の関係を有する範囲に規制すると、さらに容量維持率が向上してサイクル特性に優れたリチウム二次電池を得ることが可能となる。この場合、正極の単位面積当たりの正極活物質量は４３０ｇ／ｍ²であるので、上記の範囲は１６０≦Ｂ≦３９Ｘ＋１８０となる。このことは、負極活物質量がこの範囲内であれば、正極と負極の対向圧力がより適正な範囲に維持されるということができる。
【００３２】
７．混合正極活物質量の検討
上述においては、正極の単位面積当たりの質量が４３０ｇ／ｍ²となるように塗布量を調整した例について検討したが、以下においては、正極の単位面積当たりの質量を変化させた場合について検討した。ここで、上述と同様に各混合正極活物質α〜εを用いて、上述と同様に正極合剤および正極スラリーを調製し、これを厚みが２０μｍの正極集電体（アルミニウム箔またはアルミニウム合金箔）の両面にドクターブレード法により塗布して、正極集電体の両面に正極合剤層を形成した。この場合、各正極合剤層を形成するに際して、正極の単位面積当たりの各混合正極活物質α〜εの質量が４８０ｇ／ｍ²となるように塗布量を調整した。これを乾燥させた後、正極合剤の充填密度および厚みが下記の表２に示すような値になるように圧縮ローラを用いて圧延し、所定寸法（例えば幅が４０ｍｍで、長さが２８０ｍｍ）に切断して、正極ａ２〜ｅ２をそれぞれ作製した。
【００３３】
【表６】

【００３４】
ついで、上述のように作製した各正極ａ２〜ｅ２と、上述と同様の各負極ｘ１〜ｘ２９とをそれぞれ用いて、これらを下記の表７および表８に示すように組み合わせるとともに、これらの間にポリプロピレン製微多孔膜からなるセパレータを介在させて積層して、上述と同様に渦巻状電極群とした後に円筒状の金属製外装缶に挿入した。ついで、各集電体から延出する集電タブを各端子に溶接し、上述と同様な非水電解液を注入した後、外装缶の開口部に絶縁パッキングを介して正極蓋を取り付けて、封口してリチウム二次電池Ａ７〜Ａ１３、Ｂ７〜Ｂ１３、Ｃ７〜Ｃ１２、Ｄ７〜Ｄ１１およびＥ７〜Ｅ１２をそれぞれ作製した。
【００３５】
これらの各電池Ａ７〜Ａ１３、Ｂ７〜Ｂ１３、Ｃ７〜Ｃ１２、Ｄ７〜Ｄ１１およびＥ７〜Ｅ１２を用いて、室温（約２５℃）で、６０ｍＡの充電電流で、電池電圧が４．２Ｖになるまで定電流充電した後、６００ｍＡの放電電流で電池電圧が３．１Ｖになるまで放電させるという充放電を１サイクルとして、充放電サイクルを繰り返して行い、１サイクル目の放電容量に対する３００サイクル目の放電容量を容量維持率（容量維持率（％）＝（３００サイクル目の放電容量／１サイクル目の放電容量）×１００）として求める、下記の表７および表８に示すような結果となった。
【００３６】
【表７】

【００３７】
【表８】

【００３８】
上記表７および表８の結果から、コバルト酸リチウムの混合比Ｘを横軸とし、正極活物質に対する負極活物質の質量比（Ｂ／Ａ）を縦軸としてグラフで表すと図２に示すような結果となった。なお、図２において、容量維持率が８４％未満の電池を×印で示し、容量維持率が８４％以上で８８％未満の電池を△印で示し、容量維持率が８８％以上の電池を○印で示している。
【００３９】
ここで、図２において、○印および△印と×印とを区画する下限線（図２の下方の実線）を引くと、Ｂ／Ａ＝０．１５Ｘ＋０．２６という式が得られ、○印および△印と×印とを区画する上限線（図２の上方の実線）を引くと、Ｂ／Ａ＝０．１１Ｘ＋０．４１という式が得られた。このことから、コバルト酸リチウムの混合比をＸとした場合の正極活物質に対する負極活物質の質量比Ｂ／Ａは、０．１５Ｘ＋０．２６≦Ｂ／Ａ≦０．１１Ｘ＋０．４１の関係を有する範囲に規制すると、高容量維持率でサイクル特性に優れたリチウム二次電池が得られることが分かる。なお、この範囲は図１で求めた範囲と一致する。このことは、正極の単位面積当たりの質量を変化させても、正極活物質に対する負極活物質の質量比（Ｂ／Ａ）の好ましい範囲は等しいことを意味している。
【００４０】
さらに、図２において、○印のみを含むように下限線（図２の下方の破線）を引くと、Ｂ／Ａ＝０．３３３という式が得られ、上限線（図２の上方の破線）を引くと、Ｂ／Ａ＝０．０８１３Ｘ＋０．３７５という式が得られた。このことから、コバルト酸リチウムの混合比をＸとした場合の正極活物質に対する負極活物質の質量比Ｂ／Ａを、０．３３３≦Ｂ／Ａ≦０．０８１３Ｘ＋０．３７５の関係を有する範囲に規制すると、さらに高容量維持率でサイクル特性に優れたリチウム二次電池を得ることが可能となる。この場合、正極の単位面積当たりの正極活物質は４８０ｇ／ｍ²であるので、上記の範囲は１６０≦Ｂ≦３９Ｘ＋１８０となり、図１で求めた範囲と一致する。
【００４１】
９．負極の厚みの検討
上述と同様な天然黒鉛（Ｌｃ値が１５０Å以上で、ｄ値が３．３８Å以下のもの）粉末を用いて、上述と同様に負極スラリーを調製した後、これを厚みが１８μｍの負極集電体（銅箔）の両面にドクターブレード法により塗布して、負極集電体の両面に負極合剤層を形成した。この場合、負極合剤層を形成するに際して、下記の表９に示すような負極の単位面積当たりの質量となるように塗布量を調整した。これを乾燥させた後、圧縮ローラを用いて負極合剤層が表９に示すような厚みになるまで圧延し、所定寸法（例えば幅が４２ｍｍで、長さが３００ｍｍ）に切断して負極ｘ３０〜ｘ４７を作製した。なお、下記の表９には、上述した負極ｘ１０，ｘ１２，ｘ１４，ｘ１６の結果も併せて示している。
【００４２】
【表９】

【００４３】
ついで、上述のように作製した各正極ａ２〜ｅ２と、上述のようにして作製した各負極ｘ３０〜ｘ４７とをそれぞれ用いて、これらを下記の表１０に示すように組み合わせるとともに、これらの間にポリプロピレン製微多孔膜からなるセパレータを介在させて積層して、上述と同様に渦巻状電極群とした後に円筒状の金属製外装缶に挿入した。ついで、各集電体から延出する集電タブを各端子に溶接し、上述と同様な非水電解液を注入した後、外装缶の開口部に絶縁パッキングを介して正極蓋を取り付けて、封口してリチウム二次電池Ａ１４〜Ａ１７、Ｂ１４〜Ｂ１７、Ｃ１３〜Ｃ１６、Ｄ１２〜Ｄ１５およびＥ１３〜Ｅ１６をそれぞれ作製した。
【００４４】
これらの各電池Ａ１４〜Ａ１７、Ｂ１４〜Ｂ１７、Ｃ１３〜Ｃ１６、Ｄ１２〜Ｄ１５およびＥ１３〜Ｅ１６を用いて、室温（約２５℃）で、６０ｍＡの充電電流で、電池電圧が４．２Ｖになるまで定電流充電した後、６００ｍＡの放電電流で電池電圧が３．１Ｖになるまで放電させるという充放電を１サイクルとして、充放電サイクルを繰り返して行い、１サイクル目の放電容量に対する３００サイクル目の放電容量を容量維持率（容量維持率（％）＝（３００サイクル目の放電容量／１サイクル目の放電容量）×１００）として求める、下記の表１０および表１１に示すような結果となった。
【００４５】
【表１０】

【００４６】
【表１１】

【００４７】
上記表１０および表１１の結果から、コバルト酸リチウムの混合比Ｘを横軸とし、正極合剤層の厚みに対する負極合剤層の厚みの比（ｂ／ａ）を縦軸としてグラフで表すと図３に示すような結果となった。なお、図３において、容量維持率が８９％未満の電池を×印で示し、容量維持率が８９％以上の電池を○印で示している。
【００４８】
ここで、図３において、○印と×印を区画する下限線（図３の下方の実線）を引くと、ｂ／ａ＝０．２９Ｘ＋０．５という式が得られ、○印と×印を区画する上限線（図３の上方の実線）を引くと、ｂ／ａ＝０．２５Ｘ＋０．９５という式が得られる。このことから、コバルト酸リチウムの混合比をＸとした場合の正極合剤層の厚みに対する負極合剤層の厚みの比（ｂ／ａ）は、０．２９Ｘ＋０．５≦ｂ／ａ≦０．２５Ｘ＋０．９５の関係を有する範囲に規制すると、高容量維持率でサイクル特性に優れたリチウム二次電池を得ることが可能となる。
【００４９】
上述したように、本発明においては、正極の単位面積当たりの正極活物質の質量をＡとし、負極の単位面積当たりの負極活物質の質量をＢとし、かつ混合正極活物質中のコバルト酸リチウムの質量比をＸとした場合に、正極活物質の質量Ａに対する負極活物質の質量Ｂの質量割合（Ｂ／Ａ）が０．１５Ｘ＋０．２６≦Ｂ／Ａ≦０．１１Ｘ＋０．４１の範囲になるように規制している。
このため、充電時に正負極活物質が膨張し、また正極活物質のスピネル型マンガン酸リチウムとコバルト酸リチウムの混合比が変化して、正極活物質の膨張の幅が変化しても、正極と負極の対向圧力は適度に調整されるため、負極合剤と負極集電体との密着性が低下することが抑制できるようになる。
また、放電時に、正負極活物質が収縮しても、負極の活物質量が調整されているため、放電時の正負極の対向圧力が必要以上に小さくなることが抑制できるようになる。この結果、高容量維持率でサイクル特性に優れたリチウム二次電池が得られるようになる。
【００５０】
なお、上述した実施の形態においては、スピネル型マンガン酸リチウムとしてＬｉ_1.07Ｍｎ_1.89Ｍｇ_0.04Ｏ₄を用いる例について説明したが、スピネル型マンガン酸リチウムとしては、組成式がＬｉ_1+XＭｎ_2-YＭ_ZＯ₄（但し、ＭはＢ，Ｍｇ，Ｃａ，Ｓｒ，Ｂａ，Ｔｉ，Ｖ，Ｃｒ，Ｆｅ，Ｃｏ，Ｎｉ，Ｃｕ，Ａｌ，Ｉｎ，Ｎｂ，Ｍｏ，Ｗ，Ｙ，Ｒｈから選択される少なくとも一種の元素であり、０．５４≦（（１＋Ｘ）＋Ｚ）／（２−Ｙ）≦０．６２で、−０．１５≦Ｘ≦０．１５で、Ｙ≦０．５で、０≦Ｚ≦０．１である）で表される組成のものも同様な結果が得られる。このうち、特に優れた高温特性（高温での充放電サイクル、高温保存性等）を示すためには、Ｍｇ添加系あるいはＡｌ添加系のものを用いるのが望ましい。
【００５１】
また、上述した実施の形態においては、コバルト酸リチウムとしてＬｉＣｏＯ₂を用いる例について説明したが、コバルト酸リチウムとしては、組成式がＬｉＣｏ_1-XＭ_XＯ₂（但し、ＭはＢ，Ｍｇ，Ｃａ，Ｓｒ，Ｂａ，Ｔｉ，Ｖ，Ｃｒ，Ｆｅ，Ｎｉ，Ｃｕ，Ａｌ，Ｉｎ，Ｎｂ，Ｍｏ，Ｗ，Ｙ，Ｒｈから選択される少なくとも一種の元素であり、０≦Ｘ≦０．１である）で表される組成のものも同様な結果が得られる。このうち、特に優れた放電特性を示すためには、Ｃｒ添加系、Ｍｎ添加系、Ａｌ添加系、Ｔｉ添加系のものを用いるのが望ましい。
【図面の簡単な説明】
【図１】正極の単位面積当たりの質量を４３０ｇ／ｍ²とした場合の正極活物質のコバルト酸リチウムの混合比と正負極活物質の質量比の関係を示す図である。
【図２】正極の単位面積当たりの質量を４８０ｇ／ｍ²とした場合の正極活物質のコバルト酸リチウムの混合比と正負極活物質の質量比の関係を示す図である。
【図３】コバルト酸リチウムの混合比と正負極活物質層の厚み比との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery comprising a positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing graphite as a negative electrode active material, and a non-aqueous electrolyte, and in particular, a positive electrode active material. The present invention relates to optimization of the mass ratio of a material and a negative electrode active material.
[0002]
[Prior art]
In recent years, as a battery used for portable electronic / communication equipment such as a small video camera, a mobile phone, and a notebook computer, graphite capable of occluding and releasing lithium ions is used as a negative electrode active material, and lithium-containing cobalt oxide (LiCoO). ₂ ), Lithium-containing manganese oxide (LiMn) ₂ O _Four Lithium secondary batteries using a lithium-containing transition metal oxide such as) as a positive electrode active material have come into practical use as small, light and high capacity batteries.
[0003]
By the way, lithium-containing cobalt oxide (LiCoO ₂ In the lithium secondary battery using graphite as the negative electrode, both the positive electrode mixture layer and the negative electrode mixture layer expand as they are charged, and the positive electrode mixture layer and the negative electrode mixture are discharged as they are discharged. Since both layers contract, the pressure of the positive electrode mixture layer and the negative electrode mixture layer increases during charging, and a phenomenon occurs in which part of the electrolyte solution is squeezed out from these mixture layers. When such a phenomenon occurs, there is a problem that the capacity is reduced and the cycle characteristics are deteriorated as charging and discharging are repeated. Further, the expansion / contraction of the positive electrode mixture layer and the negative electrode mixture layer becomes more significant as the amount of the active material increases.
[0004]
On the other hand, when the positive electrode mixture layer and the negative electrode mixture layer expand at the time of charging, the adhesion between these mixture layers and each current collector (active material holding body) decreases. Since the negative electrode mixture layer expands together, the opposing pressure between the positive and negative electrodes increases. As a result, the positive electrode mixture layer and the negative electrode mixture layer are pressed against each current collector, and the positive electrode mixture layer or the negative electrode mixture layer is prevented from being peeled off from each current collector. . From this, if the amount of the positive electrode active material and the amount of the negative electrode active material are reduced more than necessary, it is possible to suppress the separation of the positive electrode mixture layer and the negative electrode mixture layer from each current collector during charging. Therefore, it is inappropriate to reduce the amount of these active materials more than necessary.
[0005]
[Problems to be solved by the invention]
Considering these things, if the amount of the positive electrode active material and the amount of the negative electrode active material are adjusted and the amount of these active materials is appropriately balanced, a lithium secondary battery having characteristics in which discharge capacity and cycle characteristics are harmonized is obtained. It can be designed.
Therefore, a lithium-containing manganese oxide (LiMn) having a spinel crystal structure that contracts during charging and expands during discharging. ₂ O _Four : Spinel type lithium manganate below) and lithium-containing cobalt oxide (LiCoO) having the property of expanding during charging and contracting during discharging ₂ : Hereinafter referred to as JP-A-4-171660 has been proposed to use a mixed positive electrode active material mixed with lithium cobaltate).
[0006]
In the positive electrode proposed in Japanese Patent Laid-Open No. 4-171660, by using a mixture of spinel type lithium manganate and lithium cobaltate, the expansion and contraction due to charge / discharge is higher than that of the positive electrode using only lithium cobaltate. Less. For this reason, at the time of charge, it becomes possible to reduce the squeezing phenomenon of the electrolytic solution due to the expansion of the positive electrode mixture layer and the negative electrode mixture layer. However, when the positive electrode mixture expands less during charging, there is a problem that peeling from the current collector due to the expansion of the negative electrode mixture cannot be suppressed.
[0007]
Therefore, when the amount of the positive electrode active material and the amount of the negative electrode active material are increased so that the negative electrode mixture layer is further expanded during charging, the peeling of the negative electrode mixture layer from the current collector can be suppressed. In addition, by adjusting the amount of increase in the amount of the positive electrode active material and the amount of the negative electrode active material, it is possible to suppress the squeezing phenomenon of the electrolyte during charging. However, as the amount of the negative electrode active material increases, the change in the thickness of the negative electrode mixture layer accompanying charging / discharging also increases, so that it is possible to apply an appropriate counter pressure to the positive and negative electrodes in the charged state. In this state, since the negative electrode contracts greatly, there is a problem that it is difficult to apply an appropriate counter pressure to the positive and negative electrodes.
[0008]
The present invention has been made to solve the above problems, and even when a mixed positive electrode active material in which lithium cobaltate and spinel type lithium manganate are mixed is used, the mixing amount of the mixed positive electrode active material is optimized. In addition, the object is to optimize the mass ratio of the mixed positive electrode active material and the negative electrode active material to obtain a lithium secondary battery with improved discharge capacity and cycle characteristics.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, in the lithium secondary battery of the present invention, the positive electrode is composed of a positive electrode mixture mainly composed of a mixed positive electrode active material in which lithium cobaltate and spinel type lithium manganate are mixed. The mass of the positive electrode active material per unit area of the positive electrode is A, the mass of the negative electrode active material per unit area of the negative electrode is B, and the mass ratio of lithium cobaltate in the mixed positive electrode active material is In the case of X, the mass ratio (B / A) of the mass B of the negative electrode active material to the mass A of the positive electrode active material is 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41. And the mass B of the negative electrode active material per unit area of the negative electrode has a relationship of 160 ≦ B ≦ 39X + 180, and the thickness of the positive electrode mixture layer is a (μm), and the thickness of the negative electrode mixture layer The ratio of the thickness b of the negative electrode mixture layer to the thickness a of the positive electrode mixture layer (b / a) is 0.29X + 0.5 ≦ b / a ≦ 0.25X + 0.95 And the thickness b (μm) of the negative electrode mixture layer has a relationship of 86 ≦ b ≦ 163. It is regulated to
[0010]
When the mass ratio B / A of the negative electrode active material to the positive electrode active material is less than 0.15X + 0.26, the amount of the negative electrode active material is relatively small with respect to the amount of the positive electrode active material. The degree of expansion is reduced. This is because the lithium cobaltate expands during charging in the positive electrode, while the spinel type lithium manganate contracts to cancel the expansion, and in the negative electrode, the expansion decreases due to the reduced amount of active material. It depends.
As a result, the opposing pressure between the positive electrode and the negative electrode during charging decreases, and the adhesion between the negative electrode mixture layer and the negative electrode current collector decreases, so the cycle characteristics deteriorate. Further, when the mass ratio B / A of the negative electrode active material to the positive electrode active material is larger than 0.11X + 0.41, the amount of the negative electrode active material is relatively larger than the amount of the positive electrode active material. The volume change due to the discharge increases, the opposing pressure between the positive electrode and the negative electrode during discharge becomes larger than necessary, and the cycle characteristics are deteriorated by squeezing out the electrolyte.
[0011]
Therefore, the mass ratio B / A of the negative electrode active material to the positive electrode active material when the mixing ratio of lithium cobaltate is X is a range having a relationship of 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41 It is desirable to regulate to
Here, the amount of the positive electrode active material per unit area of the positive electrode is 430 g / m. ² As a result of conducting the experiment as (A = 430), it was found that it is more desirable to regulate the range to have a relationship of 0.372 ≦ B / A ≦ 0.0907X + 0.419. In this case, the positive electrode active material amount is 430 g / m. ² Since (A = 430), a relationship of 160 ≦ B ≦ 39X + 180 is obtained.
[0012]
Further, the positive electrode active material amount per unit area of the positive electrode is 480 g / m. ² As a result of conducting an experiment as (A = 480), it was found that it is more desirable to regulate the range to have a relationship of 0.333 ≦ B / A ≦ 0.0813X + 0.375. In this case, the positive electrode active material amount is 480 g / m. ² Since (A = 430), a relationship of 160 ≦ B ≦ 39X + 180 is obtained.
That is, if the mass B per unit area of the negative electrode active material is regulated so that the relationship of 160 ≦ B ≦ 39X + 180 is obtained, even if the mass A per unit area of the positive electrode active material is changed, It can be said that the pressure is maintained in a more appropriate range.
[0013]
Further, when the thickness of the positive electrode mixture layer is a (μm) and the thickness of the negative electrode mixture layer is b (μm), the ratio of the thickness b of the negative electrode mixture layer to the thickness a of the positive electrode mixture layer (b / A) is restricted to a range having a relationship of 0.29X + 0.50 ≦ b / a ≦ 0.25X + 0.95, because the counter pressure between the positive electrode and the negative electrode is maintained in a more appropriate range, and cycle characteristics are improved. .
[0014]
The spinel type lithium manganate used in the present invention has a composition formula of Li _{1 + X} Mn _2-Y M _Z O _Four (However, M is at least one element selected from B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh. 0.54 ≦ ((1 + X) + Z) / (2-Y) ≦ 0.62, −0.15 ≦ X ≦ 0.15, Y ≦ 0.5, and 0 ≦ Z ≦ 0. 1), a similar result can be obtained. Among these, in order to exhibit particularly excellent high-temperature properties (high-temperature charge / discharge cycle, high-temperature storage stability, etc.), Mg It is desirable to use an additive system or an Al additive system.
[0015]
Further, as lithium cobaltate, the composition formula is LiCo _1-X M _X O ₂ (However, M is at least one element selected from B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh. , 0 ≦ X ≦ 0.1), a similar result can be obtained. Among these, in order to exhibit particularly excellent discharge characteristics, a Cr addition system, Mn addition It is desirable to use a system, an Al addition system, or a Ti addition system.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described below.
1. Preparation of mixed cathode active material
First, as a positive electrode active material, lithium cobaltate (LiCoO) having an average particle diameter of 5 μm. ₂ ) Powder and spinel type lithium manganate (Li _1.07 Mn _1.89 Mg _0.04 O _Four ) Powders were synthesized by known methods. Next, these lithium cobalt oxides (LiCoO ₂ ) Powder and lithium manganate (Li _1.07 Mn _1.89 Mg _0.04 O _Four ) The powder was mixed so as to have a mass ratio as shown in Table 1 below to prepare each of the mixed positive electrode active materials α, β, γ, δ, and ε. In this case, lithium cobalt oxide (LiCoO ₂ ) Is the mixing mass ratio of X, and spinel type lithium manganate (Li _1.07 Mn _1.89 Mg _0.04 O _Four ) Was set to 1-X.
[0017]
[Table 1]

[0018]
2. Fabrication of positive electrode
Next, using each of the mixed positive electrode active materials α to ε prepared as described above, the mixed positive electrode active materials α to ε were 85 parts by mass, and carbon black as a conductive agent was 10 parts by mass. The positive electrode mixture was prepared by mixing the vinylidene fluoride polymer as the adhering agent so as to be 5 parts by mass. Next, each positive electrode mixture obtained was mixed with N-methylpyrrolidone (NMP) to form a positive electrode slurry, and then these positive electrode slurries were mixed on both surfaces of a positive electrode current collector (aluminum foil or aluminum alloy foil) having a thickness of 20 μm. The positive electrode mixture layer was formed on both surfaces of the positive electrode current collector by applying a doctor blade method (with an uncoated portion provided by intermittent application to attach the positive electrode lead).
[0019]
In this case, when each positive electrode mixture layer is formed, the mass A of each mixed positive electrode active material α to ε per unit area of the positive electrode is 430 g / m. ² The coating amount was adjusted so that After drying this, it is rolled using a compression roller so that the packing density of the positive electrode mixture becomes a value as shown in Table 2 below, to a predetermined dimension (for example, the width is 40 mm and the length is 280 mm). It cut | disconnected and produced each of positive electrode a1-e1.
In addition, the reason why the packing density of each positive electrode mixture of each positive electrode a1 to e1 was changed to the range shown in Table 2 below can maintain the electrical contact of each active material particle in the positive electrode mixture even during charge and discharge. In this way, even if the mixing mass ratio of lithium cobaltate and spinel type lithium manganate is different, it does not cause a difference in cycle characteristics of each positive electrode.
[0020]
[Table 2]

[0021]
3. Production of negative electrode
Natural graphite (Lc value is 150 Å or more and d value is 3.38 Å or less) powder is 95 parts by mass, and polyvinylidene fluoride (PVdF) powder as a binder is mixed so that 5 parts by mass. After preparing a negative electrode mixture, this was mixed with N-methylpyrrolidone (NMP) to prepare a negative electrode slurry. Thereafter, the obtained negative electrode slurry was applied to both surfaces of a negative electrode current collector (copper foil) having a thickness of 18 μm by a doctor blade method to form a negative electrode mixture layer on both surfaces of the negative electrode current collector. In this case, when forming the negative electrode mixture layer, the coating amount was adjusted so that the mass B per unit area of the negative electrode as shown in Table 3 below was obtained. After drying this, the packing density is 1.56 g / cm. ² Were rolled to a predetermined thickness using a compression roller, and cut into predetermined dimensions (for example, a width of 42 mm and a length of 300 mm) to produce negative electrodes x1 to x29.
[0022]
[Table 3]

[0023]
4). Fabrication of lithium secondary battery
Then, using each of the positive electrodes a1 to e1 prepared as described above and each of the negative electrodes x1 to x29 prepared as described above, these are combined as shown in Table 4 and Table 5 below, After being laminated with a separator made of a polypropylene microporous film interposed therebetween, these were wound in a spiral shape to form a spiral electrode group. After these are inserted into cylindrical metal outer cans, current collecting tabs extending from each current collector are welded to each terminal, and an equal volume mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) And LiPF ₆ A non-aqueous electrolyte solution in which 1 mol / liter was dissolved was injected. Then, after attaching a positive electrode lid to the opening of the outer can through an insulating packing, sealing is performed to produce lithium secondary batteries A1 to A6, B1 to B6, C1 to C6, D1 to D6, and E1 to E6, respectively. did.
Here, the lithium secondary batteries A1 to A6 using the positive electrode a1, the lithium secondary batteries B1 to B6 using the positive electrode b1, and the lithium secondary batteries C1 to C6 using the positive electrode c1. Those using the positive electrode d1 were designated as lithium secondary batteries D1 to D6, and those using the positive electrode e1 were designated as lithium secondary batteries E1 to E6.
[0024]
As the mixed solvent, an aprotic solvent that does not have the ability to supply hydrogen ions is used in addition to the above-mentioned mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). For example, propylene carbonate (PC) Organic solvents such as vinylene carbonate (VC), butylene carbonate (BC), and γ-butyrolactone (GBL), and dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and 1,2-diethoxyethane (DEE) A mixed solvent with a low-boiling solvent such as 1,2-dimethoxytechtane (DME) or ethoxymethoxyethane (EME) may be used. Moreover, as a solute dissolved in these solvents, LiPF ₆ In addition to LiBF _Four , LiCF _Three SO _Three , LiAsF ₆ , LiN (CF _Three SO ₂ ) ₂ , LiC (CF _Three SO ₂ ) _Three , LiCF _Three (CF ₂ ) _Three SO _Three Etc. may be used. Furthermore, a polymer electrolyte, a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolyte, a solid electrolyte, and the like can also be used.
[0025]
5. Charge and discharge test of lithium secondary battery
Using each of these batteries A1 to A6, B1 to B6, C1 to C6, D1 to D6 and E1 to E6, until the battery voltage reaches 4.2 V at a charging current of 60 mA at room temperature (about 25 ° C.) After charging at a constant current, charging / discharging at a discharge current of 600 mA until the battery voltage reaches 3.1 V is taken as one cycle, and the charging / discharging cycle is repeated, and the discharging at the 300th cycle with respect to the discharging capacity at the first cycle. The capacity was calculated as a capacity maintenance ratio (capacity maintenance ratio (%) = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100), and the results were as shown in Tables 4 and 5 below.
[0026]
[Table 4]

[0027]
[Table 5]

[0028]
6). Examination of test results
From the results of Table 4 and Table 5 above, when the mixing ratio X of lithium cobaltate is plotted on the horizontal axis and the mass ratio (B / A) of the negative electrode active material to the positive electrode active material is plotted on the vertical axis, the graph is shown in FIG. It became a result. In FIG. 1, a battery having a capacity maintenance ratio of less than 84% is indicated by a cross, a battery having a capacity maintenance ratio of 84% or more and less than 88% is indicated by a triangle, and a battery having a capacity maintenance ratio of 88% or more. Shown with a circle.
[0029]
Here, in FIG. 1, by drawing a lower limit line (solid line in the lower part of FIG. 1) dividing the circle mark, the triangle mark, and the mark x, an equation B / A = 0.15X + 0.26 is obtained. When the upper limit line (the upper solid line in FIG. 1) dividing the Δ mark and the X mark is drawn, the equation B / A = 0.11X + 0.41 was obtained. From this, the mass ratio B / A of the negative electrode active material to the positive electrode active material when the mixing ratio of lithium cobaltate is X has a relationship of 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41. It can be seen that when the range is regulated, a lithium secondary battery having a high capacity retention rate and excellent cycle characteristics can be obtained.
[0030]
Here, when the mass ratio B / A of the negative electrode active material to the positive electrode active material is less than 0.15X + 0.26, the amount of the negative electrode active material is relatively small with respect to the amount of the positive electrode active material. The expansion of the active material is reduced. This is because the lithium cobaltate expands during charging in the positive electrode, while the spinel type lithium manganate contracts to cancel the expansion, and in the negative electrode, the expansion decreases due to the reduced amount of active material. It depends. As a result, it is considered that the adhesion between the negative electrode mixture layer and the negative electrode current collector during charging was reduced, and the capacity retention rate (cycle characteristics) was reduced.
Further, when the mass ratio B / A of the negative electrode active material to the positive electrode active material is larger than 0.11X + 0.41, the amount of the negative electrode active material is relatively larger than the amount of the positive electrode active material. It is considered that the capacity change rate (cycle characteristics) is reduced by increasing the volume change due to discharge, increasing the opposing pressure between the positive electrode and the negative electrode during discharge more than necessary, and squeezing out the electrolyte.
[0031]
Further, in FIG. 1, when a lower limit line (broken line in the lower part of FIG. 1) including as many circles as possible is drawn, an expression of B / A = 0.372 is obtained, and an upper limit line (upper part of FIG. 1) is obtained. When the broken line) was drawn, the equation B / A = 0.0907X + 0.419 was obtained. From this, the mass ratio B / A of the negative electrode active material to the positive electrode active material when the mixing ratio of lithium cobaltate is X is in a range having a relationship of 0.372 ≦ B / A ≦ 0.0907X + 0.419. When regulated, the capacity retention rate is further improved, and a lithium secondary battery having excellent cycle characteristics can be obtained. In this case, the positive electrode active material amount per unit area of the positive electrode is 430 g / m. ² Therefore, the above range is 160 ≦ B ≦ 39X + 180. This means that if the amount of the negative electrode active material is within this range, the opposing pressure between the positive electrode and the negative electrode is maintained in a more appropriate range.
[0032]
7). Examination of amount of mixed cathode active material
In the above, the mass per unit area of the positive electrode is 430 g / m. ² In the following, the case where the mass per unit area of the positive electrode was changed was examined. Here, using the mixed positive electrode active materials α to ε in the same manner as described above, a positive electrode mixture and a positive electrode slurry were prepared in the same manner as described above, and the positive electrode current collector (aluminum foil or aluminum alloy foil) having a thickness of 20 μm was prepared. ) Was applied by a doctor blade method to form a positive electrode mixture layer on both surfaces of the positive electrode current collector. In this case, when each positive electrode mixture layer is formed, the mass of each of the mixed positive electrode active materials α to ε per unit area of the positive electrode is 480 g / m. ² The coating amount was adjusted so that After drying this, it is rolled using a compression roller so that the packing density and thickness of the positive electrode mixture become the values shown in Table 2 below, and predetermined dimensions (for example, the width is 40 mm and the length is 280 mm). ) To produce positive electrodes a2 to e2.
[0033]
[Table 6]

[0034]
Then, using each of the positive electrodes a2 to e2 prepared as described above and the negative electrodes x1 to x29 similar to those described above, these are combined as shown in Table 7 and Table 8 below, and between these, After laminating with a separator made of a polypropylene microporous membrane, a spiral electrode group was formed in the same manner as described above, and then inserted into a cylindrical metal outer can. Next, a current collecting tab extending from each current collector is welded to each terminal, and after injecting a non-aqueous electrolyte similar to that described above, a positive electrode lid is attached to the opening of the outer can via an insulating packing, Sealed to prepare lithium secondary batteries A7 to A13, B7 to B13, C7 to C12, D7 to D11, and E7 to E12, respectively.
[0035]
Using each of these batteries A7 to A13, B7 to B13, C7 to C12, D7 to D11, and E7 to E12, at a room temperature (about 25 ° C.), with a charging current of 60 mA, until the battery voltage becomes 4.2V After charging at a constant current, charging / discharging at a discharge current of 600 mA until the battery voltage reaches 3.1 V is taken as one cycle, and the charging / discharging cycle is repeated, and the discharging at the 300th cycle with respect to the discharging capacity at the first cycle. The capacity was calculated as a capacity maintenance ratio (capacity maintenance ratio (%) = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100), and the results were as shown in Table 7 and Table 8 below.
[0036]
[Table 7]

[0037]
[Table 8]

[0038]
From the results of Tables 7 and 8 above, the mixing ratio X of lithium cobaltate is plotted on the horizontal axis, and the mass ratio (B / A) of the negative electrode active material to the positive electrode active material is plotted on the vertical axis, as shown in FIG. It became a result. In FIG. 2, a battery having a capacity maintenance ratio of less than 84% is indicated by a cross, a battery having a capacity maintenance ratio of 84% or more and less than 88% is indicated by a triangle, and a battery having a capacity maintenance ratio of 88% or more. Shown with a circle.
[0039]
Here, in FIG. 2, when a lower limit line (solid line in the lower part of FIG. 2) dividing the circle mark, the triangle mark, and the x mark is drawn, an equation B / A = 0.15X + 0.26 is obtained. When the upper limit line (the solid line in the upper part of FIG. 2) dividing the Δ mark and the X mark is drawn, the equation B / A = 0.11X + 0.41 was obtained. From this, the mass ratio B / A of the negative electrode active material to the positive electrode active material when the mixing ratio of lithium cobaltate is X has a relationship of 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41. It can be seen that when the range is regulated, a lithium secondary battery having a high capacity retention rate and excellent cycle characteristics can be obtained. This range matches the range obtained in FIG. This means that even if the mass per unit area of the positive electrode is changed, the preferable range of the mass ratio (B / A) of the negative electrode active material to the positive electrode active material is equal.
[0040]
Furthermore, in FIG. 2, when a lower limit line (broken line in the lower part of FIG. 2) is drawn so as to include only the circle mark, an expression of B / A = 0.333 is obtained, and an upper limit line (upper broken line in FIG. 2) is obtained. When subtracting, the equation B / A = 0.0813X + 0.375 was obtained. From this, the mass ratio B / A of the negative electrode active material to the positive electrode active material when the mixing ratio of lithium cobaltate is X is in a range having a relationship of 0.333 ≦ B / A ≦ 0.0813X + 0.375. When regulated, it becomes possible to obtain a lithium secondary battery having a higher capacity retention rate and excellent cycle characteristics. In this case, the positive electrode active material per unit area of the positive electrode is 480 g / m. ² Therefore, the above range is 160 ≦ B ≦ 39X + 180, which matches the range obtained in FIG.
[0041]
9. Examination of negative electrode thickness
A negative electrode current collector having a thickness of 18 μm was prepared after preparing a negative electrode slurry in the same manner as described above using a natural graphite powder having an Lc value of 150% or more and a d value of 3.38% or less as described above. The negative electrode mixture layer was formed on both surfaces of the negative electrode current collector by applying to both surfaces of (copper foil) by the doctor blade method. In this case, when forming the negative electrode mixture layer, the coating amount was adjusted so that the mass per unit area of the negative electrode as shown in Table 9 below was obtained. After drying this, it is rolled using a compression roller until the negative electrode mixture layer has a thickness as shown in Table 9, cut into predetermined dimensions (for example, a width of 42 mm and a length of 300 mm), and negative electrode x30 -X47 were produced. Table 9 below also shows the results of the negative electrodes x10, x12, x14, and x16 described above.
[0042]
[Table 9]

[0043]
Then, using each of the positive electrodes a2 to e2 manufactured as described above and each of the negative electrodes x30 to x47 manufactured as described above, these are combined as shown in Table 10 below, and between them, After laminating with a separator made of a polypropylene microporous membrane, a spiral electrode group was formed in the same manner as described above, and then inserted into a cylindrical metal outer can. Next, a current collecting tab extending from each current collector is welded to each terminal, and after injecting a non-aqueous electrolyte similar to that described above, a positive electrode lid is attached to the opening of the outer can via an insulating packing, Sealed to prepare lithium secondary batteries A14 to A17, B14 to B17, C13 to C16, D12 to D15, and E13 to E16, respectively.
[0044]
Using each of these batteries A14 to A17, B14 to B17, C13 to C16, D12 to D15, and E13 to E16, until the battery voltage reaches 4.2 V at a charging current of 60 mA at room temperature (about 25 ° C.) After charging at a constant current, charging / discharging at a discharge current of 600 mA until the battery voltage reaches 3.1 V is taken as one cycle, and the charging / discharging cycle is repeated, and the discharging at the 300th cycle with respect to the discharging capacity at the first cycle. The capacity was obtained as a capacity maintenance ratio (capacity maintenance ratio (%) = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100), and the results were as shown in Table 10 and Table 11 below.
[0045]
[Table 10]

[0046]
[Table 11]

[0047]
From the results of Table 10 and Table 11 above, when the mixing ratio X of lithium cobaltate is the horizontal axis, the ratio of the thickness of the negative electrode mixture layer to the thickness of the positive electrode mixture layer (b / a) is expressed as a graph. The result was as shown in FIG. In FIG. 3, a battery having a capacity maintenance ratio of less than 89% is indicated by a cross, and a battery having a capacity maintenance ratio of 89% or more is indicated by a circle.
[0048]
Here, in FIG. 3, when a lower limit line (solid line below FIG. 3) dividing the circle mark and the x mark is drawn, the formula b / a = 0.29X + 0.5 is obtained. When the upper limit line (the solid line in the upper part of FIG. 3) is drawn, the equation b / a = 0.25X + 0.95 is obtained. From this, the ratio (b / a) of the thickness of the negative electrode mixture layer to the thickness of the positive electrode mixture layer when the mixing ratio of lithium cobaltate is X is 0.29X + 0.5 ≦ b / a ≦ 0. By restricting to a range having a relationship of 25X + 0.95, it is possible to obtain a lithium secondary battery having a high capacity retention rate and excellent cycle characteristics.
[0049]
As described above, in the present invention, the mass of the positive electrode active material per unit area of the positive electrode is A, the mass of the negative electrode active material per unit area of the negative electrode is B, and lithium cobalt oxide in the mixed positive electrode active material When the mass ratio of X is X, the mass ratio (B / A) of the mass B of the negative electrode active material to the mass A of the positive electrode active material is in the range of 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41. It regulates to become.
For this reason, even if the positive and negative active materials expand during charging, the mixing ratio of the spinel type lithium manganate and lithium cobaltate of the positive active material changes, and the expansion width of the positive active material changes, Since the opposing pressure of the negative electrode is appropriately adjusted, it is possible to suppress a decrease in the adhesion between the negative electrode mixture and the negative electrode current collector.
Further, even if the positive and negative electrode active materials contract during discharge, the amount of active material of the negative electrode is adjusted, so that the opposing pressure between the positive and negative electrodes during discharge can be suppressed from becoming unnecessarily small. As a result, a lithium secondary battery having a high capacity retention rate and excellent cycle characteristics can be obtained.
[0050]
In the above-described embodiment, the spinel type lithium manganate is Li. _1.07 Mn _1.89 Mg _0.04 O _Four As an example of using spinel type lithium manganate, the composition formula is Li. _{1 + X} Mn _2-Y M _Z O _Four (However, M is at least one element selected from B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh. 0.54 ≦ ((1 + X) + Z) / (2-Y) ≦ 0.62, −0.15 ≦ X ≦ 0.15, Y ≦ 0.5, and 0 ≦ Z ≦ 0. The same result can be obtained with the composition represented by 1). Among these, in order to exhibit particularly excellent high-temperature characteristics (charge / discharge cycle at high temperature, high-temperature storage stability, etc.), it is desirable to use an Mg-added or Al-added system.
[0051]
In the embodiment described above, LiCoO is used as lithium cobalt oxide. ₂ As an example of lithium cobaltate, the composition formula is LiCo. _1-X M _X O ₂ (However, M is at least one element selected from B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh. And 0 ≦ X ≦ 0.1), a similar result can be obtained. Among these, in order to exhibit particularly excellent discharge characteristics, it is desirable to use a Cr addition system, a Mn addition system, an Al addition system, or a Ti addition system.
[Brief description of the drawings]
FIG. 1 shows a mass per unit area of a positive electrode of 430 g / m. ² It is a figure which shows the relationship between the mixing ratio of the lithium cobaltate of a positive electrode active material in the case of having taken, and the mass ratio of a positive / negative electrode active material.
[Fig. 2] The mass per unit area of the positive electrode is 480 g / m. ² It is a figure which shows the relationship between the mixing ratio of the lithium cobaltate of a positive electrode active material in the case of having taken, and the mass ratio of a positive / negative electrode active material.
FIG. 3 is a diagram showing the relationship between the mixing ratio of lithium cobaltate and the thickness ratio of the positive and negative electrode active material layers.

Claims

A lithium secondary battery comprising a positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing graphite as a negative electrode active material, and a non-aqueous electrolyte,
A positive electrode mixture mainly composed of a mixed positive electrode active material in which lithium cobaltate and spinel type lithium manganate are mixed is held in the positive electrode current collector,
The mass of the mixed positive electrode active material per unit area of the positive electrode is A, the mass of the negative electrode active material per unit area of the negative electrode is B, and the mass ratio of the lithium cobalt oxide in the mixed positive electrode active material is X. If
The mass ratio (B / A) of the mass B of the negative electrode active material to the mass A of the positive electrode active material has a relationship of 0.15X + 0.26 ≦ B / A ≦ 0.11X + 0.41, and the unit area of the negative electrode The mass B of the negative active material per contact has a relationship of 160 ≦ B ≦ 39X + 180,
And when the thickness of the positive electrode mixture layer is a (μm) and the thickness of the negative electrode mixture layer is b (μm), the ratio of the thickness b of the negative electrode mixture layer to the thickness a of the positive electrode mixture layer (b / a) along with having a relationship 0.29X + 0.5 ≦ b / a ≦ 0.25X + 0.95, that have a relationship of the thickness of the negative electrode mixture layer b ([mu] m) is 86 ≦ b ≦ 163 A lithium secondary battery characterized by being regulated as follows.

The spinel type lithium manganate has a composition formula of Li _{1 + X} Mn _{2 -Y} M _Z O ₄ (where M is B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu) , Al, In, Nb, Mo, W, Y, Rh, 0.54 ≦ ((1 + X) + Z) / (2-Y) ≦ 0.62, and −0. The lithium secondary battery according to claim 1, wherein 15 ≦ X ≦ 0.15, Y ≦ 0.5, and 0 ≦ Z ≦ 0.1.

3. The lithium secondary battery according to claim 2 , wherein M of the spinel type lithium manganate represented by Li _{1 + X} Mn _{2 -Y} M _Z O ₄ is Al or Mg.

The _{_{Li 1 + X Mn 2-Y}} M Z O 4 spinel-type lithium manganate represented by Li _1.07 Mn _1.89 Mg _0.04 lithium secondary according to claim 2 or claim 3, characterized in that O is ₄ Next battery.

The lithium cobalt oxide has a composition formula of LiCo _1-X M _X O ₂ (where M is B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Ni, Cu, Al, In, Nb, Mo) , W, Y, and at least one element selected from Rh, lithium according to any one of claims 1 to 4, characterized by being represented by a 0 ≦ X ≦ 0.1) Secondary battery.