JP4656710B2

JP4656710B2 - Non-aqueous electrolyte secondary battery

Info

Publication number: JP4656710B2
Application number: JP2000298233A
Authority: JP
Inventors: 康文高橋; 良浩小路; 昌利高橋
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 2000-09-29
Filing date: 2000-09-29
Publication date: 2011-03-23
Anticipated expiration: 2020-09-29
Also published as: JP2002110157A

Description

【０００１】
【発明の属する技術分野】
本発明は負極に黒鉛（但し、（００２）面の面間隔（ｄ₀₀₂）が０．３３８０ｎｍ以下であり、ｃ軸方向の結晶子の大きさ（Ｌｃ）が１５ｎｍ以上）を用いるとともに正極にリチウムイオンの吸蔵・放出が可能な材料を用い、かつ有機溶媒にリチウム塩からなる溶質を溶解した非水電解液を備えた非水電解液二次電池に係り、特に、負極の改良に関する。
【０００２】
【従来の技術】
近年、小型軽量でかつ高容量で充放電可能な電池としてリチウム二次電池が実用化されるようになり、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ようになった。この種のリチウム二次電池は、負極活物質としてリチウムイオンを吸蔵・放出し得る炭素系材料を用い、正極活物質として、ＬｉＣｏＯ₂，ＬｉＮｉＯ₂，ＬｉＭｎ₂Ｏ₄，ＬｉＦｅＯ₂等のリチウム含有遷移金属酸化物を用い、有機溶媒にリチウム塩からなる溶質を溶解した非水電解液を用いて構成される二次電池である。
【０００３】
ところで、このようなリチウム二次電池の負極に用いられる炭素系材料は、充放電サイクル中の容量劣化が少なく、優れた耐久性を有することで注目されている。これは炭素系材料は卑な電位でリチウムの吸蔵・放出を可逆的に行うことが可能であるためで、リチウムと炭素系材料との層間化合物が可逆的に形成されることを利用したためである。例えば、十分な量のリチウムを含有する正極と、負極としての炭素系材料とをセパレータを介して対向させて電池缶内に挿入し、これに有機溶媒にリチウム塩からなる溶質を溶解した非水電解液を注入することで、この電池は放電状態で組立が完了することになる。
【０００４】
この電池に対して第１サイクル目の充電を行うと、正極中のリチウムは電気化学的に負極炭素系材料の層間にドープされ、放電を行うと、ドープされていたリチウムは負極炭素材料の層間から脱ドープされて、再び正極中に戻る。
この場合の炭素系材料の単位質量当たりの放電容量（ｍＡｈ／ｇ）は、リチウムの吸蔵・放出可能容量によって決まるため、このような負極ではリチウムの電気化学的な可逆的吸蔵量をできる限り大きくすることが望ましい。そして、電池内で電気化学的にリチウムと炭素の層間化合物を生成させる場合、理論的には炭素原子６個に対してリチウム原子１個の割合で吸蔵された状態が上限で、リチウムと炭素系材料との層間化合物の飽和状態となる。
【０００５】
【発明が解決しようとする課題】
上述のような条件を満たす炭素系材料としては、天然に存在する天然黒鉛、コークスを黒鉛化した人造黒鉛、有機系高分子またはその複合物を黒鉛化した人造黒鉛、ピッチ等を黒鉛化した人造黒鉛などが検討されており、これらの黒鉛材料はリチウムの吸蔵・放出量が大きく、作動電位が全領域にわたって卑で平坦であるため特に注目されている。
【０００６】
ところで、電池容量は限られた電池缶内にいかに多くの活物質を充填するかに係っているので、活物質の単位体積当たりの容量、換言すると、炭素系材料の充填密度をいかに多くするかが重要である。そこで、黒鉛結晶が発達している天然黒鉛やコークスを黒鉛化した人造黒鉛を高密度に充填した炭素電極を得て、この炭素電極を用いてリチウム二次電池を作製したところ、確かに容量が大きな電池が得られるが、反面、高率充放電特性が低下するという問題を生じた。
【０００７】
そこで、高率充放電特性が低下した原因を追求したところ、以下のような知見を得た。即ち、黒鉛結晶が発達している天然黒鉛やコークスを黒鉛化した人造黒鉛は、菱面体構造の結晶子と六方晶構造の結晶子とを有していることが明らかになった。そして、菱面体構造の結晶子が多い黒鉛を用いると放電容量が増加するが、高率放電特性が低下することが分かった。一方、菱面体構造の結晶子が少ない黒鉛を用いると放電容量が低下するが、高率放電特性が向上することが分かった。
本発明は上記知見に基づいてなされたものであって、特定の黒鉛を高密度に充填して、電池容量が大きく、かつ高率充放電特性が低下しない負極を得て、高容量で高率充放電特性に優れた非水電解液二次電池を提供できるようにすることを目的とする。
【０００８】
【課題を解決するための手段およびその作用・効果】
上記目的を達成するため、本発明の非水電解液二次電池は、負極に黒鉛（但し、（００２）面の面間隔（ｄ₀₀₂）が０．３３８０ｎｍ以下であり、ｃ軸方向の結晶子の大きさ（Ｌｃ）が１５ｎｍ以上）を用いるとともに正極にリチウムイオンの挿入・脱離が可能な材料を用い、かつ有機溶媒にリチウム塩からなる溶質を溶解した非水電解液を備えている。
そして、負極に用いられる黒鉛は、少なくとも六方晶構造の結晶子と菱面体構造の結晶子を有するとともに、菱面体構造の結晶子のＣｕ−Ｋα線源を用いたＸ線回折法による（１０１）面のピーク強度（Ｐ１：回折角４３．２°±０．５°）と、六方晶構造の結晶子のＣｕ−Ｋα線源を用いたＸ線回折法による（１０１）面のピーク強度（Ｐ２：回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）が０．２０以上で０．３０以下となるように最適化している。
なお、本発明でいうピーク強度Ｐ１，Ｐ２とは、図１〜図４および図９に示すＸ線回折図におけるバックグランド線（各図の点線）から各回折角でのピークまでの高さを表している。
【０００９】
ここで、六方晶構造の結晶子と菱面体構造の結晶子を有する黒鉛を負極に用いた場合、菱面体構造の結晶子のピーク強度（Ｐ１：回折角４３．２°±０．５°）と、六方晶構造の結晶子のピーク強度（Ｐ２：回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）が０．２０未満であると低負荷であっても放電容量が低下し、また、ピーク強度比（Ｐ１／Ｐ２）が０．３０より大きくなると、高負荷時に容量低下が著しく増大するという結果が得られた。
【００１０】
このような実験結果からすると、上記ピーク強度比が０．２０以上である黒鉛を負極に用いれば高容量が達成でき、逆に上記ピーク強度比が０．３０以下である黒鉛を負極に用いれば高密度充填下であってもリチウムイオンの挿入・脱離反応が円滑に行われて、高負荷であっも容量が低下しないと考えられる。したがって、高容量で高率放電特性に優れた負極とするためには、菱面体構造の結晶子のピーク強度（Ｐ１：回折角４３．２°±０．５°）と、六方晶構造の結晶子のピーク強度（Ｐ２：回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）が０．２０以上で０．３０以下となるように最適化する必要がある。
【００１１】
また、ピーク強度比（Ｐ１／Ｐ２）がこの範囲にない黒鉛でも、二種類以上の結晶性の異なる黒鉛を混合した混合黒鉛とすることにより、ピーク強度比（Ｐ１／Ｐ２）が０．２０以上で０．３０以下となるように混合調製することで、上述と同様な効果を奏するようになる。そして、黒鉛の充填密度が１．６０ｇ／ｃｍ³未満であると、結晶子のピーク強度比（Ｐ１／Ｐ２）を０．２０以上で０．３０以下に最適化しても、高率放電容量および放電容量比を向上させるという効果、即ち高負荷であっても容量が低下しない効果を発揮されにくいので、黒鉛負極に用いる結晶子のピーク強度比（Ｐ１／Ｐ２）を０．２０以上で０．３０以下に最適化する場合には、黒鉛の充填密度を１．６０ｇ／ｃｍ³以上に高密度充填するのが望ましい。
【００１２】
なお、本発明は、正極活物質、非水電解液、セパレータの種類などについては制限することなく使用することができる。例えば、正極活物質としては、マンガン、コバルト、ニッケル、バナジウム、ニオブを少なくとも１種含む金属酸化物、具体的には、組成式がＬｉ_aＭＯ_b（ＭはＭｎ，Ｃｏ，Ｎｉ，Ｖ，Ｎｂなどから選択される１種の金属元素で、０≦ａ≦２で１≦ｂ≦５）で表される金属ＭとＬｉとの複合酸化物を用いることができる。例えば、ＬｉＭｎ₂Ｏ₄，ＬｉＣｏＯ₂，ＬｉＮｉＯ₂，ＬｉＭｎ_1.5Ｎｉ_0.5Ｏ₄などが好ましい。
【００１３】
非水電解液の溶媒としては、ジメチルカーボネート、エチルメチルカーボネート、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ビニレンカーボネート、シクロペンタノン、スルホラン、３−メチルスルホラン、２，４−ジメチルスルホラン、３−メチル−１，３−オキサゾリジン−２−オン、γ−ブチロラクトン、ジエチルカーボネート、ブチルメチルカーボネート、エチルプロピルカーボネート、ブチルエチルカーボネート、ジプロピルカーボネート、１，２−ジメトキシエタン、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，３−ジオキソラン、酢酸メチル、酢酸エチルなどの単体、２成分混合物あるいは３成分混合物等が使用される。また、これらの溶媒に溶解される溶質としては、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣＦ₃ＳＯ₃、ＬｉＡｓＦ₆、ＬｉＮ（ＣＦ₃ＳＯ₂）₂、ＬｉＣ（ＣＦ₃ＳＯ₂）₃、ＬｉＣＦ₃（ＣＦ₂）₃ＳＯ₃等が使用される。
【００１４】
【発明の実施の形態】
以下に、本発明の非水電解液電池の実施の形態を説明する。
１．原料黒鉛粉末
（００２）面の面間隔（ｄ₀₀₂）が０．３３６３ｎｍで、ｃ軸方向の結晶子の大きさ（Ｌｃ）が９０ｎｍで平均粒径が２０μｍの塊状黒鉛（２９５０℃で焼成した人造黒鉛）を用意し、これを黒鉛粉末αとした。また、（００２）面の面間隔（ｄ₀₀₂）が０．３３７０ｎｍで、ｃ軸方向の結晶子の大きさ（Ｌｃ）が６０ｎｍで平均粒径が２０μｍの塊状黒鉛（２８００℃で焼成した人造黒鉛）を用意し、これを黒鉛粉末βとした。
【００１５】
また、（００２）面の面間隔（ｄ₀₀₂）が０．３３８０ｎｍで、ｃ軸方向の結晶子の大きさ（Ｌｃ）が４０ｎｍで平均粒径が２０μｍの塊状黒鉛（２６５０℃で焼成した人造黒鉛）を用意し、これを黒鉛粉末γとした。さらに、（００２）面の面間隔（ｄ₀₀₂）が０．３３５７ｎｍで、ｃ軸方向の結晶子の大きさ（Ｌｃ）が２００ｎｍで平均粒径が２０μｍの塊状黒鉛（天然黒鉛）を用意し、これを黒鉛粉末δとした。
【００１６】
２．単極セルの作製
（１）負極の作製
ついで、上述のように用意した各黒鉛粉末α，β，γ，δをそれぞれ用い、これらの各黒鉛粉末α，β，γ，δとスチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合してそれぞれスラリーを調製した。なお、塊状黒鉛とＳＢＲとＣＭＣとの乾燥後の質量組成比が塊状黒鉛：ＳＢＲ：ＣＭＣ＝９５：３：２となるように調製した。
【００１７】
ついで、銅箔からなる負極集電体を用意し、上述のように調製したそれぞれのスラリーをこの負極集電体の両面にドクターブレード法により塗布し、厚みが８０μｍの黒鉛材料層（なお、表面の面積が８ｃｍ²となるようにした）をそれぞれ形成した。この後、黒鉛材料の充填密度が１．６０ｇ／ｃｍ³になるように圧延し、１００℃で２時間真空乾燥させて、黒鉛負極をそれぞれ作製した。なお、黒鉛粉末αを用いた黒鉛負極を負極ａとし、黒鉛粉末βを用いた黒鉛負極を負極ｂとし、黒鉛粉末γを用いた黒鉛負極を負極ｃとし、黒鉛粉末δを用いた黒鉛負極を負極ｄとした。
【００１８】
（２）ピーク強度比の測定
上述のよう作製した各黒鉛負極ａ，ｂ，ｃ，ｄから黒鉛層をそれぞれ剥離させた後、これらの剥離した各黒鉛層をＣｕ−Ｋα線源を用いたＸ線回折装置でそれぞれＸ線回折した結果、図１（黒鉛負極ａ）、図２（黒鉛負極ｂ）、図３（黒鉛負極ｃ）および図４（黒鉛負極ｄ）に示すようなＸ線回折図を得た。ついで、これらの回折図に基づいて、菱面体構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を求めると、下記の表１に示すような結果となった。なお、各ピーク強度Ｐ１，Ｐ２は、図１〜図４のＸ線回折図におけるバックグランド線（各図の点線）から各回折角でのピークまでの高さを表している。
【００１９】
（３）単極セルの作製
上述のよう作製した各黒鉛負極ａ，ｂ，ｃ，ｄを用い、エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）を体積比が５０対５０となるように混合した混合溶媒にＬｉＰＦ₆を１モル／リットル溶解した溶液を有機電解液とし、ポリプロピレン製の微多孔膜をセパレータとし、各黒鉛負極ａ，ｂ，ｃ，ｄの対極および参照極としてリチウム金属板を用いて三電極式単極セルＡ，Ｂ，Ｃ，Ｄをそれぞれ作製した。なお、黒鉛負極ａを用いた三電極式単極セルを単極セルＡとし、黒鉛負極ｂを用いた三電極式単極セルを単極セルＢとし、黒鉛負極ｃを用いた三電極式単極セルを単極セルＣとし、黒鉛負極ｄを用いた三電極式単極セルを単極セルＤとした。
【００２０】
（４）単極充放電試験
これらの各単極セルＡ，Ｂ，Ｃ，Ｄを用いて、室温（約２５℃）で、０．５ｍＡ／ｃｍ²の電流密度で、電池電圧が２．０ｍＶになるまで定電流充電し、２．０ｍＶで電流密度が０．１０ｍＡ／ｃｍ²に達するまで定電圧充電した後、０．５ｍＡ／ｃｍ²の電流密度で、電池電圧１．０Ｖになるまで放電させるという充放電を１回だけ行い、放電時間から負極の黒鉛材料１ｇ当たりの放電容量（単極放電容量）（ｍＡｈ／ｇ）を求めると下記の表１に示すような結果となった。
【００２１】
【表１】

【００２２】
上記表１の結果から、菱面体構造の結晶子の（１０１）面のピーク強度Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を横軸とし、単極放電容量（ｍＡｈ／ｇ）を縦軸としてグラフにすると、図５に示すような結果となった。図５の結果から明らかなように、ピーク強度比（Ｐ１／Ｐ２）が大きくなるほど単極放電容量（ｍＡｈ／ｇ）が大きくなることが分かる。
そして、単極容量が大きい電極を用いた方が容量が大きい電池が得られるので、ピーク強度比（Ｐ１／Ｐ２）ができるだけ大きな黒鉛を用いた方が良いということができるが、実用的には単極容量が３５０ｍＡｈ／ｇ以上であれば、かなり高容量の電池が得られる。したがって、ピーク強度比（Ｐ１／Ｐ２）は０．２０以上にするのが好ましいということができる。
【００２３】
３．非水電解液二次電池
（１）負極の作製
ついで、上述のように用意した各黒鉛粉末α，β，δをそれぞれ用い、これらの各黒鉛粉末α，β，δと、結着剤としてのスチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合してそれぞれスラリーを調製した。なお、塊状黒鉛とＳＢＲとＣＭＣとの乾燥後の質量組成比が塊状黒鉛：ＳＢＲ：ＣＭＣ＝９５：３：２となるように調製した。
【００２４】
なお、結着剤としては、スチレン−ブタジエンゴム（ＳＢＲ）に代えて、メチル（メタ）アクリレート、エチル（メタ）アクリレート、ブチル（メタ）アクリレート、（メタ）アクリロニトリル、ヒドロキシエチル（メタ）アクリレートなどのエチレン性不飽和カルボン酸エステル、あるいはアクリル酸、メタクリル酸、イタコン酸、フマル酸、マレイン酸などのエチレン性不飽和カルボン酸を用いてもよい。また、増粘剤としては、カルボキシメチルセルロース（ＣＭＣ）に代えて、メチルセルロース、ヒドロキシメチルセルロース、エチルセルロース、ポリビニルアルコール、ポリアクリル酸（塩）、酸化スターチ、リン酸化スターチ、カゼインなどを用いてもよい。
【００２５】
ついで、銅箔からなる負極集電体を用意し、上述のように作製したそれぞれのスラリーをこの負極集電体の両面に、ドクターブレード法により、負極集電体の単位面積当たり１００ｇ／ｍ²をそれぞれ塗布して負極黒鉛材料層をそれぞれ形成した。この後、黒鉛材料の充填密度が１．６ｇ／ｃｍ³になるように圧延し、１００℃で２時間真空乾燥させて、黒鉛負極をそれぞれ作製した。なお、黒鉛粉末αを用いた黒鉛負極を負極ｅとし、黒鉛粉末βを用いた黒鉛負極を負極ｆとし、黒鉛粉末δを用いた黒鉛負極を負極ｇとした。
【００２６】
（２）ピーク強度比の測定
上述のよう作製した各黒鉛負極ｅ，ｆ，ｇから黒鉛層をそれぞれ剥離させた後、これらの剥離した各黒鉛層をＣｕ−Ｋα線源を用いたＸ線回折装置でそれぞれＸ線回折して上述と同様にＸ線回折図を得た。ついで、これらの回折図に基づいて、菱面体構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を求め、これらの各ピーク強度Ｐ１，Ｐ２に基づいてピーク強度比（Ｐ１／Ｐ２）を求めると、下記の表２に示すような結果となった。
【００２７】
（３）正極の作製
平均粒径５μｍのコバルト酸リチウム（ＬｉＣｏＯ₂）粉末と導電剤としての人造黒鉛粉末を質量比で９：１の割合で混合して正極合剤を調製した。この正極合剤と、Ｎ−メチル−２−ピロリドン（ＮＭＰ）にポリフッ化ビニリデン（ＰＶｄＦ）を５質量％溶解した結着剤溶液とを固形分の質量比で９５：５となるように混練して、正極スラリーを調製した。
ついで、アルミニウム箔からなる正極集電体を用意し、上述のように作製した正極スラリーを正極集電体の両面に、ドクターブレード法により、正極集電体の単位面積当たり２４０ｇ／ｍ²を塗布して正極合剤層を形成した。この後、正極合剤の充填密度が３．２ｇ／ｃｍ³になるように圧延し、１５０℃で２時間真空乾燥させて正極を作製した。
なお、正極活物質として、ＬｉＣｏＯ₂に代えて、ＬｉＭｎ₂Ｏ₄，ＬｉＮｉＯ₂，ＬｉＭｎ_1.5Ｎｉ_0.5Ｏ₄などの組成式がＬｉ_aＭＯ_b（ＭはＭｎ，Ｃｏ，Ｎｉ，Ｖ，Ｎｂなどから選択される１種の金属元素で、０≦ａ≦２で１≦ｂ≦５）で表される金属ＭとＬｉとの複合酸化物を用いてもよい。
【００２８】
（４）リチウム二次電池の作製
ついで、リチウム二次電池の作製例を図６に基づいて説明する。ここで、図６において、上述のようにして作製した各黒鉛負極ｅ，ｆ，ｇを負極板１０とし、上述のようにして作製した正極を正極板２０として示している。そして、図６に示すように、負極板１０と正極板２０とをこれらの間にポリエチレン製微多孔膜からなるセパレータ３０を介在させて重ね合わせた後、渦巻状に巻回して渦巻状電極体を作製した。この電極体の上下にそれぞれ絶縁板４１を配置した後、１枚板からプレス加工により円筒状に成形した負極端子を兼ねるスチール製の外装缶４０の開口部より、この電極体を挿入した。
【００２９】
ついで、電極体の負極板１０より延出する負極集電タブ１０ａを外装缶４０の内底部に溶接するとともに、電極体の正極板２０より延出する正極集電タブ２０ａを電流遮断封口体５０の底板５４とを溶接した。そして、エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）からなる混合溶媒（ＥＣ：ＤＥＣ＝５０：５０：体積比）にＬｉＰＦ₆を１モル／リットル溶解した有機電解液を外装缶４０内に注入した後、外装缶４０の開口部にポリプロピレン（ＰＰ）製の外装缶用絶縁ガスケット４２を介して電流遮断封口体５０を載置し、外装缶４０の開口部の上端部を電流遮断封口体５０側にかしめて液密に封口して、公称容量が１４５０ｍＡｈのリチウム二次電池Ｅ，Ｆ，Ｇをそれぞれ作製した。なお、黒鉛負極板ｅを用いたリチウム二次電池を電池Ｅとし、黒鉛負極板ｆを用いたリチウム二次電池を電池Ｆとし、黒鉛負極板ｇを用いたリチウム二次電池を電池Ｇとした。
【００３０】
なお、電流遮断封口体５０は、逆皿状（キャップ状）に形成されたステンレス製の正極キャップ５１と、皿状に形成されたステンレス製の底板５４とから構成されている。これらの正極キャップ５１と底板５４との内部には、電池内部のガス圧が上昇して所定の圧力以上になると変形する図示しないアルミニウム箔からなる電力導出板が収容されているとともに、ＰＴＣ（Positive Temperature Coefficient）サーミスタ素子が配設されている。そして、電池内に過電流が流れて異常な発熱現象を生じると、このＰＴＣサーミスタ素子の抵抗値が増大して過電流を減少させる。また、電池内部のガス圧が上昇して所定の圧力以上になると電力導出板が変形するため、電力導出板と正極キャップ５１との接触が遮断されて過電流あるいは短絡電流が遮断されるようになされている。
【００３１】
なお、有機電解液の溶媒としては、エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）からなる混合溶媒に代えて、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ビニレンカーボネート、シクロペンタノン、スルホラン、３−メチルスルホラン、２，４−ジメチルスルホラン、３−メチル−１，３−オキサゾリジン−２−オン、γ−ブチロラクトン、ジメチルカーボネート、エチルメチルカーボネート、ブチルメチルカーボネート、エチルプロピルカーボネート、ブチルエチルカーボネート、ジプロピルカーボネート、１，２−ジメトキシエタン、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，３−ジオキソラン、酢酸メチル、酢酸エチルなどの単体、２成分混合物あるいは３成分混合物を用いてもよい。
また、有機電解液の溶質としては、ＬｉＰＦ₆に代えて、ＬｉＢＦ₄、ＬｉＣＦ₃ＳＯ₃、ＬｉＡｓＦ₆、ＬｉＮ（ＣＦ₃ＳＯ₂）₂、ＬｉＣ（ＣＦ₃ＳＯ₂）₃、ＬｉＣＦ₃（ＣＦ₂）₃ＳＯ₃などを用いてもよい。
【００３２】
（５）リチウム二次電池の充放電試験
これらの各電池Ｅ，Ｆ，Ｇを用いて、室温（約２５℃）で、１４５０ｍＡ（１Ｃ：Ｃは電極容量を表し、Ｉｔともいう）の充電電流で、電池電圧が４．２Ｖになるまで定電流充電し、４．２Ｖの定電圧で電流値が２０ｍＡに達するまで定電圧充電した後、１４５０ｍＡ（１Ｃ）の放電電流で、電池電圧が２．７５Ｖになるまで放電させるという充放電を１回だけ行って、放電時間から１Ｃでの放電容量（ｍＡｈ）を求めた。
【００３３】
また、これらの各電池Ｅ，Ｆ，Ｇを用いて、室温（約２５℃）で、１４５０ｍＡ（１Ｃ）の充電電流で、電池電圧が４．２Ｖになるまで定電流充電し、４．２Ｖの定電圧で電流値が２０ｍＡに達するまで定電圧充電した後、３６２５ｍＡ（２．５Ｃ）の放電電流で、電池電圧が２．７５Ｖになるまで放電させるという充放電を１回だけ行って、放電時間から２．５Ｃでの放電容量（高率放電容量）（ｍＡｈ）を求めた。ついで、２．５Ｃでの放電容量に対する１Ｃでの放電容量の比率を算出して、放電容量比（放電容量比＝（１Ｃでの放電容量／２．５Ｃでの放電容量）×１００％）を求めると下記の表２に示すような結果となった。
【００３４】
【表２】

【００３５】
上記表２の結果から、菱面体構造の結晶子の（１０１）面のピーク強度Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を横軸とし、放電容量、高率放電容量および放電容量比を縦軸としてグラフにすると、図７に示すような結果となった。図７の結果から明らかなように、１Ｃでの放電容量（図７の黒丸印）はピーク強度比（Ｐ１／Ｐ２）に関わらずほぼ一定であるが、２．５Ｃでの放電容量（図７の白丸印）および放電容量比（図７の×印）はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って低下することが分かる。
【００３６】
図７の結果から高率放電容量および放電容量比はピーク強度比（Ｐ１／Ｐ２）が０．３０まではほぼ一定であるが、特にピーク強度比（Ｐ１／Ｐ２）が０．３０を越えると低下率が増大することから、ピーク強度比（Ｐ１／Ｐ２）は０．３０以下に規制すれば、高率放電容量および放電容量比の優れたリチウム二次電池が得られることが分かる。そして、図５の結果から単極放電容量はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って増加し、ピーク強度比（Ｐ１／Ｐ２）を０．２０以上にすれば容量が大きい電池が得られることを考慮すると、ピーク強度比（Ｐ１／Ｐ２）は０．２０以上で０．３０以下に最適化するのが好ましいということができる。
【００３７】
４．黒鉛負極の充填密度の検討
（１）充填密度が１．５５ｇ／ｃｍ³の黒鉛負極を用いたリチウム二次電池
上述のように用意した各黒鉛粉末α，β，δをそれぞれ用いて、これらの各黒鉛粉末α，β，δとスチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合してそれぞれスラリーを調製した。なお、塊状黒鉛とＳＢＲとＣＭＣとの乾燥後の質量組成比が塊状黒鉛：ＳＢＲ：ＣＭＣ＝９５：３：２となるように調製した。
【００３８】
ついで、銅箔からなる負極集電体を用意し、上述のように作製したそれぞれのスラリーをこの負極集電体の両面に、ドクターブレード法により、負極集電体の単位面積当たり１００ｇ／ｍ²をそれぞれ塗布して負極黒鉛材料層をそれぞれ形成した。この後、黒鉛材料の充填密度が１．５５ｇ／ｃｍ³になるように圧延し、１００℃で２時間真空乾燥させて、黒鉛負極板をそれぞれ作製した。なお、黒鉛粉末αを用いた黒鉛負極板を負極板ｈとし、黒鉛粉末βを用いた黒鉛負極板を負極板ｉとし、黒鉛粉末δを用いた黒鉛負極板を負極板ｊとした。
【００３９】
ついで、上述のよう作製した各黒鉛負極ｈ，ｉ，ｊから黒鉛層をそれぞれ剥離させた後、これらの剥離した各黒鉛層をＣｕ−Ｋα線源を用いたＸ線回折装置でそれぞれＸ線回折して上述と同様にＸ線回折図を得た。ついで、これらの回折図に基づいて、菱面体構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ２（回折角４４．３°±０．５°）を求め、これらの各ピーク強度Ｐ１，Ｐ２に基づいてピーク強度比（Ｐ１／Ｐ２）を求めると、下記の表３に示すような結果となった。
【００４０】
ついで、上述のよう作製した各黒鉛負極板ｈ，ｉ，ｊを用いて、上述と同様に公称容量１４５０ｍＡｈのリチウム二次電池をそれぞれ作製した。なお、黒鉛負極板ｈを用いたリチウム二次電池を電池Ｈとし、黒鉛負極板ｉを用いたリチウム二次電池を電池Ｉとし、黒鉛負極板ｊを用いたリチウム二次電池を電池Ｊとした。ついで、上述と同様に、これらの各電池Ｈ，Ｉ，Ｊを用いて、上述と同様に充放電を行って、１Ｃでの放電容量（ｍＡｈ）および２．５Ｃでの放電容量（高率放電容量）（ｍＡｈ）を求めて、放電容量比（放電容量比＝（１Ｃでの放電容量／２．５Ｃでの放電容量）×１００％）を求めると下記の表３に示すような結果となった。
【００４１】
【表３】

【００４２】
上記表３の結果から、菱面体構造の結晶子の（１０１）面のピーク強度Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を横軸とし、１Ｃでの放電容量、２．５Ｃでの放電容量および放電容量比を縦軸としてグラフにすると、図８に示すような結果となった。図８の結果から明らかなように、１Ｃでの放電容量（図８の黒丸印）はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って若干増大するが、高率放電容量および放電容量比はピーク強度比（Ｐ１／Ｐ２）が大きくなってもほぼ一定であることが分かる。
このことから、黒鉛の充填密度が１．５５ｇ／ｃｍ³で低充填密度の黒鉛負極を用いたリチウム二次電池にあっては、黒鉛負極に用いる結晶子のピーク強度比（Ｐ１／Ｐ２）を０．２０以上で０．３０以下に最適化しても、高率放電容量および放電容量比が向上させるという効果を発揮しにくいということができる。
【００４３】
（２）充填密度が１．７０ｇ／ｃｍ³の黒鉛負極を用いたリチウム二次電池
上述のように用意した各黒鉛粉末α，β，δおよびγとδを混合した混合黒鉛粉末をそれぞれ用いて、これらの各黒鉛粉末α，β，δおよびγとδの混合黒鉛粉末と、スチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合してそれぞれスラリーを調製した。なお、塊状黒鉛とＳＢＲとＣＭＣとの乾燥後の質量組成比が塊状黒鉛：ＳＢＲ：ＣＭＣ＝９５：３：２となるように調製した。
【００４４】
ついで、銅箔からなる負極集電体を用意し、上述のように作製したそれぞれのスラリーをこの負極集電体の両面に、ドクターブレード法により、負極集電体の単位面積当たり１１０ｇ／ｍ²をそれぞれ塗布して負極黒鉛材料層をそれぞれ形成した。この後、黒鉛材料の充填密度が１．７０ｇ／ｃｍ³になるように圧延し、１００℃で２時間真空乾燥させて、黒鉛負極板をそれぞれ作製した。なお、黒鉛粉末αを用いた黒鉛負極板を負極板ｋとし、黒鉛粉末βを用いた黒鉛負極板を負極板ｌとし、黒鉛粉末δを用いた黒鉛負極板を負極板ｍとし、黒鉛粉末γとδを混合した混合黒鉛粉末を用いた黒鉛負極板を負極板ｎとした。
【００４５】
ついで、上述のよう作製した各黒鉛負極ｋ，ｌ，ｍ，ｎから黒鉛層をそれぞれ剥離させた後、これらの剥離した各黒鉛層をＣｕ−Ｋα線源を用いたＸ線回折装置でそれぞれＸ線回折して上述と同様にＸ線回折図を得た。ついで、これらの回折図に基づいて、菱面体構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ２（回折角４４．３°±０．５°）を求め、これらの各ピーク強度Ｐ１，Ｐ２に基づいてピーク強度比（Ｐ１／Ｐ２）を求めると、下記の表４に示すような結果となった。なお、図９は黒鉛負極ｎのＸ線回折図を示している。
【００４６】
ついで、上述のよう作製した各黒鉛負極板ｋ，ｌ，ｍ，ｎを用いて、上述と同様に公称容量１６００ｍＡｈのリチウム二次電池をそれぞれ作製した。なお、この場合は、各黒鉛負極板ｋ，ｌ，ｍ，ｎの容量が大きいため、正極スラリーを正極集電体の単位面積当たり２５０ｇ／ｍ²になるように塗布して正極合剤層を形成し、正極合剤の充填密度が３．２ｇ／ｃｍ³になるように圧延した正極板を用いている。そして、黒鉛負極板ｋを用いたリチウム二次電池を電池Ｋとし、黒鉛負極板ｌを用いたリチウム二次電池を電池Ｌとし、黒鉛負極板ｍを用いたリチウム二次電池を電池Ｍとし、黒鉛負極板ｎを用いたリチウム二次電池を電池Ｎとした。ついで、これらの各電池Ｋ，Ｌ，Ｍ，Ｎを用いて、上述と同様に充放電を行って、１Ｃでの放電容量（ｍＡｈ）および２．５Ｃでの放電容量（高率放電容量）（ｍＡｈ）を求めて、放電容量比（放電容量比＝（１Ｃでの放電容量／２．５Ｃでの放電容量）×１００％）を求めると下記の表４に示すような結果となった。
【００４７】
【表４】

【００４８】
上記表４の結果から、菱面体構造の結晶子の（１０１）面のピーク強度Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を横軸とし、１Ｃでの放電容量、２．５Ｃでの放電容量および放電容量比を縦軸としてグラフにすると、図１０に示すような結果となった。図１０の結果から明らかなように、１Ｃでの放電容量（図１０の黒丸印）はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って容量が多少増大するが、２．５Ｃでの放電容量（図１０の白丸印）および放電容量比（図１０の×印）はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って低下し、特にピーク強度比（Ｐ１／Ｐ２）が０．３を越えると低下率が大きいことが分かる。また、ピーク強度比（Ｐ１／Ｐ２）が０．２より小さい黒鉛粉末（γ）と、ピーク強度比（Ｐ１／Ｐ２）が０．３より大きい黒鉛粉末（δ）を混合して、ピーク強度比（Ｐ１／Ｐ２）が０．２以上０．３以下になるように調製した混合黒鉛粉末を用いて作製した負極板ｎを用いた電池Ｎは、黒鉛粉末を混合していない黒鉛粉末（α，β，δ）を用いて作製した正極板（ｋ，ｌ，ｍ）を用いた電池Ｋ，Ｌ，Ｍと遜色がない性能を有していることが分かる。
【００４９】
（３）充填密度が１．８０ｇ／ｃｍ³の黒鉛負極を用いたリチウム二次電池
上述のように用意した各黒鉛粉末α，β，δをそれぞれ用いて、これらの各黒鉛粉末α，β，δとスチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合してそれぞれスラリーを調製した。なお、塊状黒鉛とＳＢＲとＣＭＣとの乾燥後の質量組成比が塊状黒鉛：ＳＢＲ：ＣＭＣ＝９５：３：２となるように調製した。
【００５０】
ついで、銅箔からなる負極集電体を用意し、上述のように作製したそれぞれのスラリーをこの負極集電体の両面に、ドクターブレード法により、負極集電体の単位面積当たり１１５ｇ／ｍ²をそれぞれ塗布して負極黒鉛材料層をそれぞれ形成した。この後、黒鉛材料の充填密度が１．８０ｇ／ｃｍ³になるように圧延し、１００℃で２時間真空乾燥させて、黒鉛負極板をそれぞれ作製した。なお、黒鉛粉末αを用いた黒鉛負極板を負極板ｏとし、黒鉛粉末βを用いた黒鉛負極板を負極板ｐとし、黒鉛粉末δを用いた黒鉛負極板を負極板ｑとした。
【００５１】
ついで、上述のよう作製した各黒鉛負極ｏ，ｐ，ｑから黒鉛層をそれぞれ剥離させた後、これらの剥離した各黒鉛層をＣｕ−Ｋα線源を用いたＸ線回折装置でそれぞれＸ線回折して上述と同様にＸ線回折図を得た。ついで、これらの回折図に基づいて、菱面体構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度（ｃｐｓ）Ｐ２（回折角４４．３°±０．５°）を求め、これらの各ピーク強度Ｐ１，Ｐ２に基づいてピーク強度比（Ｐ１／Ｐ２）を求めると、下記の表５に示すような結果となった。
【００５２】
ついで、上述のよう作製した各黒鉛負極板ｏ，ｐ，ｑを用いて、上述と同様に公称容量１７００ｍＡｈのリチウム二次電池をそれぞれ作製した。なお、この場合は、各黒鉛負極板ｏ，ｐ，ｑの容量が大きいため、正極スラリーを正極集電体の単位面積当たり２６０ｇ／ｍ²になるように塗布して正極合剤層を形成し、正極合剤の充填密度が３．３ｇ／ｃｍ³になるように圧延した正極板を用いている。なお、黒鉛負極板ｏを用いたリチウム二次電池を電池Ｏとし、黒鉛負極板ｐを用いたリチウム二次電池を電池Ｐとし、黒鉛負極板ｑを用いたリチウム二次電池を電池Ｑとした。ついで、これらの各電池Ｏ，Ｐ，Ｑを用いて、上述と同様に充放電を行って、１Ｃでの放電容量（ｍＡｈ）および２．５Ｃでの放電容量（高率放電容量）（ｍＡｈ）を求めて、放電容量比（放電容量比＝（１Ｃでの放電容量／２．５Ｃでの放電容量）×１００％）を求めると下記の表５に示すような結果となった。
【００５３】
【表５】

【００５４】
上記表５の結果から、菱面体構造の結晶子の（１０１）面のピーク強度Ｐ１（回折角４３．２°±０．５°）と、六方晶構造の結晶子の（１０１）面のピーク強度Ｐ２（回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）を横軸とし、１Ｃでの放電容量、２．５Ｃでの放電容量および放電容量比を縦軸としてグラフにすると、図１１に示すような結果となった。図１１の結果から明らかなように、１Ｃでの放電容量はピーク強度比（Ｐ１／Ｐ２）が大きくなるに伴って若干大きくなるが、２．５Ｃでの放電容量および放電容量比はピーク強度比（Ｐ１／Ｐ２）が０．３０を越えると急激に低下することが分かる。
【００５５】
そして、以上の図７，図８，図１０，図１１の各結果から次のことが明らかとなった。即ち、黒鉛の充填密度が１．５５ｇ／ｃｍ³で低充填密度の黒鉛負極を用いたリチウム二次電池にあっては、黒鉛負極に用いる結晶子のピーク強度比（Ｐ１／Ｐ２）を０．２０以上で０．３０以下に最適化しても、高率放電容量および放電容量比を向上させるという効果を発揮しにくいので、黒鉛負極に用いる結晶子のピーク強度比（Ｐ１／Ｐ２）を０．２０以上で０．３０以下に最適化する場合には、黒鉛の充填密度を１．６０ｇ／ｃｍ³以上に高密度充填するのが好ましい。
【００５６】
また、黒鉛の充填密度を１．６０ｇ／ｃｍ³以上に高密度充填する場合は、図５の結果から、ピーク強度比（Ｐ１／Ｐ２）を０．２０以上にすれば容量が大きい電池が得られ、図８、図１０および図１１の結果から、ピーク強度比（Ｐ１／Ｐ２）が０．３０を越えると高率放電容量および放電容量比が低下することを考慮すると、ピーク強度比（Ｐ１／Ｐ２）は０．２０以上で０．３０以下に最適化するのが好ましいということができる。また、結晶性の異なる二種類以上の黒鉛を混合することにより、ピーク強度比（Ｐ１／Ｐ２）を０．２以上で０．３以下となるように調製しても同等の効果を奏する。
【００５７】
上述したように、本発明においては、菱面体構造の結晶子のＣｕ−Ｋα線源を用いたＸ線回折法による（１０１）面のピーク強度（Ｐ１：回折角４３．２°±０．５°）と、六方晶構造の結晶子のＣｕ−Ｋα線源を用いたＸ線回折法による（１０１）面のピーク強度（Ｐ２：回折角４４．３°±０．５°）とのピーク強度比（Ｐ１／Ｐ２）が０．２０以上で０．３０以下となるように最適化している。このため、高容量で、かつ高密度充填下であってもリチウムイオンの挿入・脱離反応が円滑に行われて、高負荷であっても容量が低下しないリチウム二次電池が得られるようになる。
【図面の簡単な説明】
【図１】黒鉛負極ａのＸ線回折角（２θ）に対する強度の関係を示すＸ線回折図である。
【図２】黒鉛負極ｂのＸ線回折角（２θ）に対する強度の関係を示すＸ線回折図である。
【図３】黒鉛負極ｃのＸ線回折角（２θ）に対する強度の関係を示すＸ線回折図である。
【図４】黒鉛負極ｄのＸ線回折角（２θ）に対する強度の関係を示すＸ線回折図である。
【図５】ピーク強度比（Ｐ１／Ｐ２）と単極放電容量（ｍＡｈ／ｇ）との関係を示す図である。
【図６】本発明の一実施形態のリチウム二次電池の断面を示す図である。
【図７】黒鉛の充填密度が１．６０ｇ／ｃｍ³の場合のピーク強度比（Ｐ１／Ｐ２）と、１Ｃでの放電容量および２．５Ｃでの放電容量ならびに放電容量比との関係を示す図である。
【図８】黒鉛の充填密度が１．５５ｇ／ｃｍ³の場合のピーク強度比（Ｐ１／Ｐ２）と、１Ｃでの放電容量および２．５Ｃでの放電容量ならびに放電容量比との関係を示す図である。
【図９】黒鉛負極ｎのＸ線回折角（２θ）に対する強度の関係を示すＸ線回折図である。
【図１０】黒鉛の充填密度が１．７０ｇ／ｃｍ³の場合のピーク強度比（Ｐ１／Ｐ２）と、１Ｃでの放電容量および２．５Ｃでの放電容量ならびに放電容量比との関係を示す図である。
【図１１】黒鉛の充填密度が１．８０ｇ／ｃｍ³の場合のピーク強度比（Ｐ１／Ｐ２）と、１Ｃでの放電容量および２．５Ｃでの放電容量ならびに放電容量比との関係を示す図である。
【符号の説明】
１０…黒鉛負極板、１０ａ…負極集電タブ、２０…正極板、２０ａ…正極集電タブ、３０…セパレータ、４０…外装缶、４１…スペーサ、４２…外装缶用絶縁ガスケット、５０…電流遮断封口体[0001]
BACKGROUND OF THE INVENTION
In the present invention, the negative electrode has a graphite (however, (002) plane spacing (d ₀₀₂ ) Is 0.3380 nm or less, the crystallite size in the c-axis direction (Lc) is 15 nm or more), a material capable of occluding and releasing lithium ions is used for the positive electrode, and a lithium salt is used for the organic solvent. The present invention relates to a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte solution in which a solute is dissolved, and particularly to improvement of a negative electrode.
[0002]
[Prior art]
In recent years, lithium secondary batteries have come into practical use as compact, lightweight, high-capacity chargeable / dischargeable batteries, and are used in portable electronic and communication devices such as small video cameras, mobile phones, and notebook computers. Became. This type of lithium secondary battery uses a carbon-based material capable of occluding and releasing lithium ions as a negative electrode active material, and LiCoO as a positive electrode active material. ₂ , LiNiO ₂ , LiMn ₂ O _Four , LiFeO ₂ And a non-aqueous electrolyte solution in which a solute composed of a lithium salt is dissolved in an organic solvent.
[0003]
By the way, the carbonaceous material used for the negative electrode of such a lithium secondary battery attracts attention because it has little capacity deterioration during the charge / discharge cycle and has excellent durability. This is because carbon-based materials can reversibly store and release lithium at a low potential, making use of the reversible formation of intercalation compounds between lithium and carbon-based materials. . For example, a non-aqueous solution in which a positive electrode containing a sufficient amount of lithium and a carbon-based material as a negative electrode are opposed to each other through a separator and inserted into a battery can, and a solute composed of a lithium salt is dissolved in an organic solvent. By injecting the electrolyte, the battery is assembled in a discharged state.
[0004]
When the battery is charged in the first cycle, lithium in the positive electrode is electrochemically doped between the layers of the negative electrode carbon-based material, and when discharged, the doped lithium is added between the layers of the negative electrode carbon material. Is dedoped and returns to the positive electrode again.
In this case, since the discharge capacity per unit mass (mAh / g) of the carbon-based material is determined by the capacity capable of occluding and releasing lithium, the electrochemical reversible storage capacity of lithium is increased as much as possible in such a negative electrode. It is desirable to do. When electrochemically generating an intercalation compound of lithium and carbon in the battery, the upper limit is the state in which it is occluded at a ratio of one lithium atom to six carbon atoms. The intercalation compound with the material becomes saturated.
[0005]
[Problems to be solved by the invention]
Examples of carbon-based materials that satisfy the above conditions include naturally occurring natural graphite, artificial graphite obtained by graphitizing coke, artificial graphite obtained by graphitizing organic polymers or composites thereof, and artificial graphite obtained by graphitizing pitch, etc. Graphite and the like have been studied, and these graphite materials are particularly attracting attention because they have a large amount of lithium occlusion and release, and the operating potential is flat and flat over the entire region.
[0006]
By the way, since the battery capacity depends on how many active materials are filled in a limited battery can, how much the capacity per unit volume of the active material, in other words, the packing density of the carbon-based material is increased. Is important. Therefore, we obtained a carbon electrode filled with natural graphite with graphite crystals and artificial graphite graphitized from coke, and produced a lithium secondary battery using this carbon electrode. A large battery can be obtained, but on the other hand, there is a problem in that the high rate charge / discharge characteristics deteriorate.
[0007]
Then, the following knowledge was acquired when the cause which the high rate charge-and-discharge characteristic fell was pursued. That is, it has been clarified that natural graphite in which graphite crystals are developed and artificial graphite obtained by graphitizing coke have crystallites having a rhombohedral structure and crystallites having a hexagonal structure. Then, it was found that when graphite having a large number of rhombohedral crystallites is used, the discharge capacity increases, but the high-rate discharge characteristics deteriorate. On the other hand, it was found that when graphite having a small rhombohedral structure crystallite is used, the discharge capacity is lowered, but the high rate discharge characteristics are improved.
The present invention has been made on the basis of the above knowledge, and it is possible to obtain a negative electrode that is filled with specific graphite at a high density, has a large battery capacity, and does not deteriorate high rate charge / discharge characteristics. An object of the present invention is to provide a non-aqueous electrolyte secondary battery excellent in charge / discharge characteristics.
[0008]
[Means for solving the problems and their functions and effects]
In order to achieve the above object, the non-aqueous electrolyte secondary battery of the present invention has a negative electrode with a spacing (d) of graphite (however, (002) plane) ₀₀₂ ) Is 0.3380 nm or less, the crystallite size in the c-axis direction (Lc) is 15 nm or more), a material capable of inserting / extracting lithium ions in the positive electrode, and a lithium salt in the organic solvent A non-aqueous electrolyte in which a solute consisting of
The graphite used for the negative electrode has at least a hexagonal crystallite and a rhombohedral crystallite, and is obtained by an X-ray diffraction method using a rhombohedral crystallite Cu-Kα ray source (101). The peak intensity (P1: diffraction angle 43.2 ° ± 0.5 °) of the plane and the peak intensity (P2) of the (101) plane by X-ray diffraction using a Cu—Kα radiation source of hexagonal crystallites : Optimization is made so that the peak intensity ratio (P1 / P2) with respect to the diffraction angle 44.3 ° ± 0.5 ° is 0.20 or more and 0.30 or less.
The peak intensities P1 and P2 referred to in the present invention represent the height from the background line (dotted line in each figure) to the peak at each diffraction angle in the X-ray diffraction diagrams shown in FIGS. ing.
[0009]
Here, when graphite having hexagonal crystallites and rhombohedral crystallites is used for the negative electrode, the peak intensity of the rhombohedral crystallites (P1: diffraction angle 43.2 ° ± 0.5 °). When the peak intensity ratio (P1 / P2) to the peak intensity (P2: diffraction angle 44.3 ° ± 0.5 °) of the hexagonal crystallite is less than 0.20, even when the load is low When the discharge capacity is reduced and the peak intensity ratio (P1 / P2) is larger than 0.30, the result is that the capacity reduction is remarkably increased at high load.
[0010]
From these experimental results, high capacity can be achieved if graphite having a peak intensity ratio of 0.20 or more is used for the negative electrode, and conversely if graphite having the peak intensity ratio of 0.30 or less is used for the negative electrode. Even under high-density filling, lithium ion insertion / extraction reactions are carried out smoothly, and it is considered that the capacity does not decrease even at high loads. Therefore, in order to obtain a negative electrode having a high capacity and an excellent high rate discharge characteristic, the peak intensity (P1: diffraction angle 43.2 ° ± 0.5 °) of the rhombohedral crystallite and the hexagonal crystal It is necessary to optimize so that the peak intensity ratio (P1 / P2) to the child peak intensity (P2: diffraction angle 44.3 ° ± 0.5 °) is 0.20 or more and 0.30 or less.
[0011]
Further, even if graphite whose peak intensity ratio (P1 / P2) is not within this range, a peak intensity ratio (P1 / P2) of 0.20 or more can be obtained by mixing two or more types of graphite having different crystallinity. By mixing and preparing so that it may become 0.30 or less, there exists an effect similar to the above-mentioned. The graphite packing density is 1.60 g / cm. ^Three If the ratio is less than 0.30, even if the peak intensity ratio (P1 / P2) of the crystallite is optimized to 0.20 or more and 0.30 or less, the effect of improving the high rate discharge capacity and the discharge capacity ratio, that is, at high load. Since the effect of not reducing the capacity is hardly exhibited even if it is, when the peak intensity ratio (P1 / P2) of the crystallite used for the graphite negative electrode is optimized to 0.20 or more and 0.30 or less, the graphite filling Density 1.60 g / cm ^Three It is desirable to perform high-density filling as described above.
[0012]
In addition, this invention can be used without restrict | limiting about a positive electrode active material, a non-aqueous electrolyte, the kind of separator. For example, as the positive electrode active material, a metal oxide containing at least one kind of manganese, cobalt, nickel, vanadium, and niobium, specifically, a composition formula of Li _a MO _b (M is a metal element selected from Mn, Co, Ni, V, Nb, etc., and a composite oxide of metal M and Li represented by 0 ≦ a ≦ 2 and 1 ≦ b ≦ 5) Can be used. For example, LiMn ₂ O _Four , LiCoO ₂ , LiNiO ₂ , LiMn _1.5 Ni _0.5 O _Four Etc. are preferable.
[0013]
As a solvent for the non-aqueous electrolyte, dimethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl- 1,3-oxazolidine-2-one, γ-butyrolactone, diethyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3 -A simple substance such as dioxolane, methyl acetate or ethyl acetate, a two-component mixture or a three-component mixture is used. Moreover, as a solute dissolved in these solvents, LiPF ₆ , 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. are used.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the non-aqueous electrolyte battery of the present invention will be described.
1. Raw material graphite powder
(002) Surface spacing (d ₀₀₂ ) Is 0.3363 nm, the c-axis direction crystallite size (Lc) is 90 nm, and the average particle size is 20 μm, which is a lump graphite (artificial graphite fired at 2950 ° C.), which is used as graphite powder α. . In addition, the distance between the (002) planes (d ₀₀₂ ) Is 0.3370 nm, and the c-axis direction crystallite size (Lc) is 60 nm and the average particle diameter is 20 μm, which is a lump graphite (artificial graphite fired at 2800 ° C.). .
[0015]
In addition, the distance between the (002) planes (d ₀₀₂ ) Is 0.3380 nm, a massive graphite (artificial graphite fired at 2650 ° C.) having a crystallite size (Lc) in the c-axis direction of 40 nm and an average particle diameter of 20 μm is prepared, and this is used as graphite powder γ. . Further, the (002) plane spacing (d ₀₀₂ ) Is 0.3357 nm, a lump graphite (natural graphite) having a crystallite size (Lc) in the c-axis direction of 200 nm and an average particle diameter of 20 μm is prepared, and this is used as graphite powder δ.
[0016]
2. Monopolar cell fabrication
(1) Production of negative electrode
Next, using each of the graphite powders α, β, γ, and δ prepared as described above, dispersion of these graphite powders α, β, γ, and δ and styrene-butadiene rubber (SBR) (solid content is 48 mass%) was dispersed in water, and then carboxymethyl cellulose (CMC) as a thickener was added and mixed to prepare respective slurries. In addition, it prepared so that the mass composition ratio after drying of massive graphite, SBR, and CMC might be massive graphite: SBR: CMC = 95: 3: 2.
[0017]
Next, a negative electrode current collector made of copper foil was prepared, and each slurry prepared as described above was applied to both surfaces of the negative electrode current collector by a doctor blade method, and a graphite material layer having a thickness of 80 μm (note that the surface Area is 8cm ² Were formed). Thereafter, the packing density of the graphite material is 1.60 g / cm. ^Three And vacuum dried at 100 ° C. for 2 hours to prepare graphite negative electrodes. A graphite negative electrode using graphite powder α is a negative electrode a, a graphite negative electrode using graphite powder β is a negative electrode b, a graphite negative electrode using graphite powder γ is a negative electrode c, and a graphite negative electrode using graphite powder δ is a negative electrode. A negative electrode d was obtained.
[0018]
(2) Measurement of peak intensity ratio
After the graphite layers were peeled off from the graphite negative electrodes a, b, c and d produced as described above, the peeled graphite layers were each subjected to X-ray diffraction by an X-ray diffractometer using a Cu-Kα radiation source. As a result, X-ray diffraction patterns as shown in FIG. 1 (graphite negative electrode a), FIG. 2 (graphite negative electrode b), FIG. 3 (graphite negative electrode c) and FIG. 4 (graphite negative electrode d) were obtained. Then, based on these diffraction diagrams, the peak intensity (cps) P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the crystallite of the hexagonal crystal structure When the peak intensity ratio (P1 / P2) with the peak intensity (cps) P2 (diffraction angle 44.3 ° ± 0.5 °) of the (101) plane was obtained, the results shown in Table 1 below were obtained. . Each of the peak intensities P1 and P2 represents the height from the background line (dotted line in each figure) to the peak at each diffraction angle in the X-ray diffraction diagrams of FIGS.
[0019]
(3) Fabrication of monopolar cell
Using each of the graphite negative electrodes a, b, c, d produced as described above, LiPF was mixed with a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed so that the volume ratio was 50:50. ₆ A solution in which 1 mol / liter is dissolved is used as an organic electrolytic solution, a polypropylene microporous membrane is used as a separator, and a lithium metal plate is used as a counter electrode and a reference electrode of each graphite negative electrode a, b, c, d. Polar cells A, B, C, and D were produced, respectively. A three-electrode single electrode cell using a graphite negative electrode a is a single electrode cell A, a three-electrode single electrode cell using a graphite negative electrode b is a single electrode cell B, and a three-electrode single cell using a graphite negative electrode c. The electrode cell was a single electrode cell C, and the three electrode type single electrode cell using the graphite negative electrode d was a single electrode cell D.
[0020]
(4) Single electrode charge / discharge test
Using each of these monopolar cells A, B, C, D, 0.5 mA / cm at room temperature (about 25 ° C.) ² Constant current charging until the battery voltage reaches 2.0 mV, and at 2.0 mV, the current density is 0.10 mA / cm. ² After charging at a constant voltage until reaching 0.5 mA / cm ² When the discharge capacity (single electrode discharge capacity) per 1 g of the graphite material of the negative electrode (mAh / g) is obtained from the discharge time, the battery is charged and discharged only once until the battery voltage reaches 1.0V. The results shown in Table 1 below were obtained.
[0021]
[Table 1]

[0022]
From the results of Table 1 above, the peak intensity P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the peak of the (101) plane of the hexagonal crystallite are shown. When the peak intensity ratio (P1 / P2) with the intensity P2 (diffraction angle 44.3 ° ± 0.5 °) is plotted on the horizontal axis and the unipolar discharge capacity (mAh / g) is plotted on the vertical axis, the graph is shown in FIG. The result was as shown. As is clear from the results of FIG. 5, it can be seen that the unipolar discharge capacity (mAh / g) increases as the peak intensity ratio (P1 / P2) increases.
Since a battery having a large capacity can be obtained by using an electrode having a large single electrode capacity, it is better to use graphite having a peak intensity ratio (P1 / P2) as large as possible. If the single electrode capacity is 350 mAh / g or more, a battery with a considerably high capacity can be obtained. Therefore, it can be said that the peak intensity ratio (P1 / P2) is preferably 0.20 or more.
[0023]
3. Non-aqueous electrolyte secondary battery
(1) Production of negative electrode
Next, using each of the graphite powders α, β, and δ prepared as described above, a dispersion (solid) of the graphite powders α, β, and δ and a styrene-butadiene rubber (SBR) as a binder is used. 48% by mass was dispersed in water, and carboxymethyl cellulose (CMC) as a thickener was added and mixed to prepare slurries. In addition, it prepared so that the mass composition ratio after drying of massive graphite, SBR, and CMC might be massive graphite: SBR: CMC = 95: 3: 2.
[0024]
As the binder, instead of styrene-butadiene rubber (SBR), methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) acrylate, etc. Ethylenically unsaturated carboxylic acid esters or ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid may be used. As the thickener, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like may be used instead of carboxymethylcellulose (CMC).
[0025]
Next, a negative electrode current collector made of copper foil was prepared, and each slurry prepared as described above was applied to both sides of the negative electrode current collector by a doctor blade method to 100 g / m per unit area of the negative electrode current collector. ² Were respectively applied to form negative electrode graphite material layers. Thereafter, the packing density of the graphite material is 1.6 g / cm. ^Three And vacuum dried at 100 ° C. for 2 hours to prepare graphite negative electrodes. A graphite negative electrode using graphite powder α was used as negative electrode e, a graphite negative electrode using graphite powder β was used as negative electrode f, and a graphite negative electrode using graphite powder δ was used as negative electrode g.
[0026]
(2) Measurement of peak intensity ratio
After the graphite layers were peeled off from the graphite negative electrodes e, f, and g produced as described above, the peeled graphite layers were X-ray diffracted by an X-ray diffractometer using a Cu-Kα radiation source. An X-ray diffraction pattern was obtained in the same manner as described above. Then, based on these diffraction diagrams, the peak intensity (cps) P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the crystallite of the hexagonal crystal structure The peak intensity ratio (P1 / P2) with the peak intensity (cps) P2 (diffraction angle 44.3 ° ± 0.5 °) of the (101) plane is obtained, and the peak intensity is based on these peak intensities P1 and P2. When the ratio (P1 / P2) was determined, the results shown in Table 2 below were obtained.
[0027]
(3) Fabrication of positive electrode
Lithium cobalt oxide (LiCoO) with an average particle size of 5 μm ₂ ) Powder and artificial graphite powder as a conductive agent were mixed at a mass ratio of 9: 1 to prepare a positive electrode mixture. This positive electrode mixture and a binder solution in which 5% by mass of polyvinylidene fluoride (PVdF) is dissolved in N-methyl-2-pyrrolidone (NMP) are kneaded so that the mass ratio of the solid content is 95: 5. Thus, a positive electrode slurry was prepared.
Next, a positive electrode current collector made of aluminum foil was prepared, and the positive electrode slurry produced as described above was applied to both surfaces of the positive electrode current collector by 240 g / m per unit area of the positive electrode current collector by the doctor blade method. ² Was applied to form a positive electrode mixture layer. Thereafter, the packing density of the positive electrode mixture is 3.2 g / cm. ^Three And then vacuum-dried at 150 ° C. for 2 hours to produce a positive electrode.
As the positive electrode active material, LiCoO ₂ Instead of LiMn ₂ O _Four , LiNiO ₂ , LiMn _1.5 Ni _0.5 O _Four And the composition formula is Li _a MO _b (M is a metal element selected from Mn, Co, Ni, V, Nb, etc., and a composite oxide of metal M and Li represented by 0 ≦ a ≦ 2 and 1 ≦ b ≦ 5) It may be used.
[0028]
(4) Preparation of lithium secondary battery
Next, an example of manufacturing a lithium secondary battery will be described with reference to FIGS. Here, in FIG. 6, the graphite negative electrodes e, f, and g produced as described above are shown as the negative electrode plate 10, and the positive electrode produced as described above is shown as the positive electrode plate 20. Then, as shown in FIG. 6, the negative electrode plate 10 and the positive electrode plate 20 are overlapped with a separator 30 made of a polyethylene microporous film interposed therebetween, and then wound in a spiral shape to form a spiral electrode body. Was made. After the insulating plates 41 were respectively arranged above and below the electrode body, the electrode body was inserted from an opening of a steel outer can 40 that also served as a negative electrode terminal formed into a cylindrical shape by pressing from a single plate.
[0029]
Next, the negative electrode current collecting tab 10a extending from the negative electrode plate 10 of the electrode body is welded to the inner bottom portion of the outer can 40, and the positive electrode current collecting tab 20a extending from the positive electrode plate 20 of the electrode body is welded to the current blocking sealing body 50. The bottom plate 54 was welded. Then, LiPF is added to a mixed solvent (EC: DEC = 50: 50: volume ratio) composed of ethylene carbonate (EC) and diethyl carbonate (DEC). ₆ 1 mol / liter of an organic electrolyte solution is injected into the outer can 40, and then a current blocking sealing body 50 is placed in the opening of the outer can 40 through an insulating gasket 42 for outer can made of polypropylene (PP). And the upper end part of the opening part of the armored can 40 was crimped to the electric current interruption sealing body 50 side, and it sealed liquid-tightly, and produced the lithium secondary batteries E, F, and G whose nominal capacity is 1450 mAh, respectively. The lithium secondary battery using the graphite negative electrode plate e is referred to as battery E, the lithium secondary battery using the graphite negative electrode plate f is referred to as battery F, and the lithium secondary battery using the graphite negative electrode plate g is referred to as battery G. .
[0030]
In addition, the electric current interruption sealing body 50 is comprised from the stainless steel positive electrode cap 51 formed in the reverse dish shape (cap shape), and the stainless steel baseplate 54 formed in the dish shape. The positive electrode cap 51 and the bottom plate 54 accommodate a power lead plate made of an aluminum foil (not shown) that deforms when the gas pressure inside the battery rises to a predetermined pressure or higher, and is PTC (Positive). Temperature Coefficient) thermistor element is provided. When an overcurrent flows in the battery and an abnormal heat generation phenomenon occurs, the resistance value of the PTC thermistor element increases to reduce the overcurrent. Further, when the gas pressure inside the battery rises and exceeds a predetermined pressure, the power lead plate is deformed, so that the contact between the power lead plate and the positive electrode cap 51 is cut off so that overcurrent or short circuit current is cut off. Has been made.
[0031]
In addition, as a solvent for the organic electrolyte, instead of a mixed solvent composed of ethylene carbonate (EC) and diethyl carbonate (DEC), ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, 3-methyl Sulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, ethyl methyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, etc. Things may be used.
Moreover, as a solute of the organic electrolyte, LiPF ₆ Instead of 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.
[0032]
(5) Lithium secondary battery charge / discharge test
Using each of these batteries E, F, and G, at room temperature (about 25 ° C.), with a charging current of 1450 mA (1C: C represents electrode capacity, also referred to as It), until the battery voltage becomes 4.2V Charging / discharging is performed with constant current charging, constant voltage charging at a constant voltage of 4.2 V until the current value reaches 20 mA, and then discharging with a discharge current of 1450 mA (1 C) until the battery voltage reaches 2.75 V. The discharge capacity (mAh) at 1 C was determined from the discharge time.
[0033]
Further, using each of these batteries E, F, and G, the battery was charged at a constant current at a room temperature (about 25 ° C.) with a charging current of 1450 mA (1C) until the battery voltage reached 4.2 V, and 4.2 V After charging at a constant voltage until the current value reaches 20 mA, charging and discharging are performed only once with a discharge current of 3625 mA (2.5 C) until the battery voltage reaches 2.75 V, and the discharge time The discharge capacity (high rate discharge capacity) at 2.5 C (mAh) was determined. Next, the ratio of the discharge capacity at 1 C to the discharge capacity at 2.5 C is calculated, and the discharge capacity ratio (discharge capacity ratio = (discharge capacity at 1 C / discharge capacity at 2.5 C) × 100%) is calculated. As a result, the results shown in Table 2 below were obtained.
[0034]
[Table 2]

[0035]
From the results of Table 2, the peak intensity P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the peak of the (101) plane of the hexagonal crystallite are shown. When the peak intensity ratio (P1 / P2) with the intensity P2 (diffraction angle 44.3 ° ± 0.5 °) is plotted on the horizontal axis and the discharge capacity, high-rate discharge capacity, and discharge capacity ratio are plotted on the vertical axis, Results were as shown in FIG. As is apparent from the results of FIG. 7, the discharge capacity at 1C (black circles in FIG. 7) is substantially constant regardless of the peak intensity ratio (P1 / P2), but the discharge capacity at 2.5C (FIG. 7). It can be seen that the white circle mark) and the discharge capacity ratio (x mark in FIG. 7) decrease as the peak intensity ratio (P1 / P2) increases.
[0036]
From the results of FIG. 7, the high rate discharge capacity and the discharge capacity ratio are almost constant until the peak intensity ratio (P1 / P2) is 0.30, but particularly when the peak intensity ratio (P1 / P2) exceeds 0.30. Since the decrease rate increases, it can be seen that a lithium secondary battery having an excellent high rate discharge capacity and discharge capacity ratio can be obtained by limiting the peak intensity ratio (P1 / P2) to 0.30 or less. From the results of FIG. 5, the unipolar discharge capacity increases as the peak intensity ratio (P1 / P2) increases, and if the peak intensity ratio (P1 / P2) is 0.20 or more, a battery having a large capacity is obtained. Considering that it is obtained, it can be said that the peak intensity ratio (P1 / P2) is preferably optimized to 0.20 or more and 0.30 or less.
[0037]
4). Examination of packing density of graphite negative electrode
(1) Packing density is 1.55 g / cm ^Three Lithium secondary battery using graphite negative electrode
Using each of the graphite powders α, β, and δ prepared as described above, a dispersion of each of these graphite powders α, β, and δ and styrene-butadiene rubber (SBR) (solid content is 48% by mass). After dispersing in water, carboxymethyl cellulose (CMC) as a thickener was added and mixed to prepare respective slurries. In addition, it prepared so that the mass composition ratio after drying of massive graphite, SBR, and CMC might be massive graphite: SBR: CMC = 95: 3: 2.
[0038]
Next, a negative electrode current collector made of copper foil was prepared, and each slurry prepared as described above was applied to both sides of the negative electrode current collector by a doctor blade method to 100 g / m per unit area of the negative electrode current collector. ² Were respectively applied to form negative electrode graphite material layers. Thereafter, the packing density of the graphite material is 1.55 g / cm. ^Three And vacuum-dried at 100 ° C. for 2 hours to prepare graphite negative electrode plates, respectively. A graphite negative electrode plate using graphite powder α was used as a negative electrode plate h, a graphite negative electrode plate using graphite powder β was used as a negative electrode plate i, and a graphite negative electrode plate using graphite powder δ was used as a negative electrode plate j.
[0039]
Next, after peeling the graphite layer from each of the graphite negative electrodes h, i, j produced as described above, each of the peeled graphite layers was subjected to X-ray diffraction by an X-ray diffractometer using a Cu-Kα radiation source. As described above, an X-ray diffraction pattern was obtained. Then, based on these diffraction diagrams, the peak intensity (cps) P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the crystallite of the hexagonal crystal structure When the peak intensity (cps) P2 (diffraction angle 44.3 ° ± 0.5 °) of the (101) plane is obtained and the peak intensity ratio (P1 / P2) is obtained based on these peak intensities P1 and P2, The results shown in Table 3 below were obtained.
[0040]
Next, lithium secondary batteries having a nominal capacity of 1450 mAh were respectively produced using the graphite negative electrode plates h, i, j produced as described above. The lithium secondary battery using the graphite negative electrode plate h is referred to as battery H, the lithium secondary battery using the graphite negative electrode plate i is referred to as battery I, and the lithium secondary battery using the graphite negative electrode plate j is referred to as battery J. . Then, similarly to the above, using each of these batteries H, I, and J, charging and discharging was performed in the same manner as described above, and the discharge capacity (mAh) at 1C and the discharge capacity (high rate discharge) at 2.5C. When the capacity (mAh) is obtained and the discharge capacity ratio (discharge capacity ratio = (discharge capacity at 1 C / discharge capacity at 2.5 C) × 100%) is obtained, the results shown in Table 3 below are obtained. It was.
[0041]
[Table 3]

[0042]
From the results of Table 3 above, the peak intensity P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the peak of the (101) plane of the hexagonal crystallite are shown. The horizontal axis represents the peak intensity ratio (P1 / P2) with the intensity P2 (diffraction angle 44.3 ° ± 0.5 °), and the vertical axis represents the discharge capacity at 1C, the discharge capacity at 2.5C, and the discharge capacity ratio. As a graph, the result shown in FIG. 8 was obtained. As is clear from the results of FIG. 8, the discharge capacity at 1C (black circles in FIG. 8) slightly increases as the peak intensity ratio (P1 / P2) increases, but the high-rate discharge capacity and discharge capacity ratio It can be seen that even if the peak intensity ratio (P1 / P2) increases, it is substantially constant.
From this, the packing density of graphite is 1.55 g / cm. ^Three In a lithium secondary battery using a graphite negative electrode with a low packing density, even if the peak intensity ratio (P1 / P2) of the crystallite used in the graphite negative electrode is optimized from 0.20 to 0.30, It can be said that the effect of improving the high rate discharge capacity and the discharge capacity ratio is hardly exhibited.
[0043]
(2) Packing density is 1.70 g / cm ^Three Lithium secondary battery using graphite negative electrode
Using each of the graphite powders α, β, δ, and γ and δ mixed as prepared above, the graphite powders α, β, δ, γ and δ mixed graphite powder, and styrene -Dispersion with butadiene rubber (SBR) (solid content is 48% by mass) was dispersed in water, and then carboxymethyl cellulose (CMC) as a thickener was added and mixed to prepare respective slurries. In addition, it prepared so that the mass composition ratio after drying of massive graphite, SBR, and CMC might be massive graphite: SBR: CMC = 95: 3: 2.
[0044]
Next, a negative electrode current collector made of copper foil was prepared, and each slurry prepared as described above was applied to both sides of the negative electrode current collector by a doctor blade method to 110 g / m per unit area of the negative electrode current collector. ² Were respectively applied to form negative electrode graphite material layers. After this, the packing density of the graphite material is 1.70 g / cm. ^Three And vacuum-dried at 100 ° C. for 2 hours to prepare graphite negative electrode plates, respectively. A graphite negative electrode plate using graphite powder α is a negative electrode plate k, a graphite negative electrode plate using graphite powder β is a negative electrode plate l, a graphite negative electrode plate using graphite powder δ is a negative electrode plate m, and a graphite powder γ. A negative electrode plate made of graphite mixed with δ and δ was used as negative electrode plate n.
[0045]
Next, after peeling the graphite layer from each of the graphite negative electrodes k, l, m, and n produced as described above, each of the peeled graphite layers was X-diffracted with an X-ray diffractometer using a Cu-Kα radiation source. X-ray diffraction patterns were obtained by line diffraction. Then, based on these diffraction diagrams, the peak intensity (cps) P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the crystallite of the hexagonal crystal structure When the peak intensity (cps) P2 (diffraction angle 44.3 ° ± 0.5 °) of the (101) plane is obtained and the peak intensity ratio (P1 / P2) is obtained based on these peak intensities P1 and P2, The results shown in Table 4 below were obtained. FIG. 9 shows an X-ray diffraction pattern of the graphite negative electrode n.
[0046]
Next, lithium secondary batteries having a nominal capacity of 1600 mAh were produced in the same manner as described above using the graphite negative electrode plates k, l, m, and n produced as described above. In this case, since the capacity of each graphite negative electrode plate k, l, m, n is large, the positive electrode slurry is 250 g / m per unit area of the positive electrode current collector. ² To form a positive electrode mixture layer, and the packing density of the positive electrode mixture is 3.2 g / cm ^Three The positive electrode plate rolled to become is used. A lithium secondary battery using the graphite negative electrode plate k is referred to as a battery K, a lithium secondary battery using the graphite negative electrode plate l is referred to as a battery L, and a lithium secondary battery using the graphite negative electrode plate m is referred to as a battery M. A lithium secondary battery using the graphite negative electrode plate n was designated as battery N. Then, using each of these batteries K, L, M, and N, charging / discharging was performed in the same manner as described above, and the discharge capacity at 1 C (mAh) and the discharge capacity at 2.5 C (high rate discharge capacity) ( mAh) was obtained, and the discharge capacity ratio (discharge capacity ratio = (discharge capacity at 1 C / discharge capacity at 2.5 C) × 100%) was obtained, and the results shown in Table 4 below were obtained.
[0047]
[Table 4]

[0048]
From the results in Table 4 above, the peak intensity P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the peak of the (101) plane of the hexagonal crystallite are shown. The horizontal axis represents the peak intensity ratio (P1 / P2) with the intensity P2 (diffraction angle 44.3 ° ± 0.5 °), and the vertical axis represents the discharge capacity at 1C, the discharge capacity at 2.5C, and the discharge capacity ratio. As a graph, the result shown in FIG. 10 was obtained. As is apparent from the results of FIG. 10, the discharge capacity at 1C (black circles in FIG. 10) increases slightly as the peak intensity ratio (P1 / P2) increases, but the discharge at 2.5C The capacity (white circle in FIG. 10) and the discharge capacity ratio (x in FIG. 10) decrease as the peak intensity ratio (P1 / P2) increases, and in particular, the peak intensity ratio (P1 / P2) is 0.3. It can be seen that the decrease rate is large when the value exceeds. Further, a graphite powder (γ) having a peak intensity ratio (P1 / P2) of less than 0.2 and a graphite powder (δ) having a peak intensity ratio (P1 / P2) of more than 0.3 are mixed to obtain a peak intensity ratio. Battery N using negative electrode plate n produced using mixed graphite powder prepared so that (P1 / P2) is 0.2 or more and 0.3 or less is graphite powder (α, It can be seen that the battery has the same performance as the batteries K, L, M using the positive plates (k, l, m) produced using β, δ).
[0049]
(3) Packing density is 1.80 g / cm ^Three Lithium secondary battery using graphite negative electrode
Using each of the graphite powders α, β, and δ prepared as described above, a dispersion of each of these graphite powders α, β, and δ and styrene-butadiene rubber (SBR) (solid content is 48% by mass). After dispersing in water, carboxymethyl cellulose (CMC) as a thickener was added and mixed to prepare respective slurries. In addition, it prepared so that the mass composition ratio after drying of massive graphite, SBR, and CMC might be massive graphite: SBR: CMC = 95: 3: 2.
[0050]
Next, a negative electrode current collector made of copper foil was prepared, and each slurry prepared as described above was applied to both surfaces of the negative electrode current collector by 115 g / m per unit area of the negative electrode current collector by a doctor blade method. ² Were respectively applied to form negative electrode graphite material layers. After this, the packing density of the graphite material is 1.80 g / cm. ^Three And vacuum-dried at 100 ° C. for 2 hours to prepare graphite negative electrode plates, respectively. A graphite negative electrode plate using graphite powder α was used as a negative electrode plate o, a graphite negative electrode plate using graphite powder β was used as a negative electrode plate p, and a graphite negative electrode plate using graphite powder δ was used as a negative electrode plate q.
[0051]
Next, the graphite layers were peeled off from the graphite negative electrodes o, p, q prepared as described above, and then the peeled graphite layers were each subjected to X-ray diffraction by an X-ray diffractometer using a Cu-Kα radiation source. As described above, an X-ray diffraction pattern was obtained. Then, based on these diffraction diagrams, the peak intensity (cps) P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the crystallite of the hexagonal crystal structure When the peak intensity (cps) P2 (diffraction angle 44.3 ° ± 0.5 °) of the (101) plane is obtained and the peak intensity ratio (P1 / P2) is obtained based on these peak intensities P1 and P2, The results shown in Table 5 below were obtained.
[0052]
Next, lithium secondary batteries having a nominal capacity of 1700 mAh were prepared using the graphite negative electrode plates o, p, and q produced as described above in the same manner as described above. In this case, since the capacity of each graphite negative electrode plate o, p, q is large, the positive electrode slurry is 260 g / m per unit area of the positive electrode current collector. ² To form a positive electrode mixture layer, and the packing density of the positive electrode mixture is 3.3 g / cm ^Three The positive electrode plate rolled to become is used. A lithium secondary battery using the graphite negative electrode plate o is referred to as a battery O, a lithium secondary battery using the graphite negative electrode plate p is referred to as a battery P, and a lithium secondary battery using the graphite negative electrode plate q is referred to as a battery Q. . Then, using each of these batteries O, P, and Q, charging and discharging were performed in the same manner as described above, and the discharge capacity at 1 C (mAh) and the discharge capacity at 2.5 C (high rate discharge capacity) (mAh) And the discharge capacity ratio (discharge capacity ratio = (discharge capacity at 1 C / discharge capacity at 2.5 C) × 100%) was obtained as shown in Table 5 below.
[0053]
[Table 5]

[0054]
From the results of Table 5, the peak intensity P1 (diffraction angle 43.2 ° ± 0.5 °) of the (101) plane of the rhombohedral crystallite and the peak of the (101) plane of the hexagonal crystallite are shown. The horizontal axis represents the peak intensity ratio (P1 / P2) with the intensity P2 (diffraction angle 44.3 ° ± 0.5 °), and the vertical axis represents the discharge capacity at 1C, the discharge capacity at 2.5C, and the discharge capacity ratio. As a graph, the result shown in FIG. 11 was obtained. As is clear from the results of FIG. 11, the discharge capacity at 1C increases slightly as the peak intensity ratio (P1 / P2) increases, but the discharge capacity and discharge capacity ratio at 2.5C increase with the peak intensity ratio. It can be seen that when (P1 / P2) exceeds 0.30, it rapidly decreases.
[0055]
And the following became clear from each result of FIG. 7, FIG. 8, FIG. 10, and FIG. That is, the packing density of graphite is 1.55 g / cm. ^Three In a lithium secondary battery using a graphite negative electrode with a low packing density, even if the peak intensity ratio (P1 / P2) of the crystallite used in the graphite negative electrode is optimized from 0.20 to 0.30, Since the effect of improving the high rate discharge capacity and the discharge capacity ratio is hardly exhibited, the peak intensity ratio (P1 / P2) of the crystallite used for the graphite negative electrode is optimized to 0.20 or more and 0.30 or less. The graphite packing density is 1.60 g / cm ^Three It is preferable to perform high-density filling as described above.
[0056]
Moreover, the packing density of graphite is 1.60 g / cm. ^Three In the case of high-density filling as described above, from the results of FIG. 5, if the peak intensity ratio (P1 / P2) is 0.20 or more, a battery having a large capacity can be obtained. From the results of FIGS. Considering that the high rate discharge capacity and the discharge capacity ratio decrease when the peak intensity ratio (P1 / P2) exceeds 0.30, the peak intensity ratio (P1 / P2) is 0.20 or more and 0.30 or less. It can be said that it is preferable to optimize. Moreover, even if it adjusts so that peak intensity ratio (P1 / P2) may be 0.2 or more and 0.3 or less by mixing 2 or more types of graphite from which crystallinity differs, there exists an equivalent effect.
[0057]
As described above, in the present invention, the peak intensity of the (101) plane (P1: diffraction angle 43.2 ° ± 0.5 by the X-ray diffraction method using a rhombohedral crystallite Cu—Kα ray source is used. And the peak intensity of the (101) plane (P2: diffraction angle 44.3 ° ± 0.5 °) by X-ray diffraction using a Cu—Kα ray source of hexagonal crystallites. Optimization is performed so that the ratio (P1 / P2) is 0.20 or more and 0.30 or less. Therefore, a lithium secondary battery can be obtained that has a high capacity and can smoothly insert and desorb lithium ions even under high-density filling, and that does not decrease in capacity even under high loads. Become.
[Brief description of the drawings]
FIG. 1 is an X-ray diffraction diagram showing the relationship of strength with respect to an X-ray diffraction angle (2θ) of a graphite negative electrode a.
FIG. 2 is an X-ray diffraction diagram showing a relationship of intensity with respect to an X-ray diffraction angle (2θ) of a graphite negative electrode b.
FIG. 3 is an X-ray diffraction diagram showing the relationship between the strength of a graphite negative electrode c and the X-ray diffraction angle (2θ).
FIG. 4 is an X-ray diffraction diagram showing the relationship of strength with respect to an X-ray diffraction angle (2θ) of a graphite negative electrode d.
FIG. 5 is a graph showing a relationship between a peak intensity ratio (P1 / P2) and a single electrode discharge capacity (mAh / g).
FIG. 6 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present invention.
FIG. 7 is a graphite packing density of 1.60 g / cm. ^Three It is a figure which shows the relationship between the peak intensity ratio in the case of (P1 / P2), the discharge capacity in 1C, the discharge capacity in 2.5C, and the discharge capacity ratio.
FIG. 8 is a graphite packing density of 1.55 g / cm. ^Three It is a figure which shows the relationship between the peak intensity ratio in the case of (P1 / P2), the discharge capacity in 1C, the discharge capacity in 2.5C, and the discharge capacity ratio.
FIG. 9 is an X-ray diffraction diagram showing the relationship of strength with respect to the X-ray diffraction angle (2θ) of graphite negative electrode n.
FIG. 10 is a graphite packing density of 1.70 g / cm. ^Three It is a figure which shows the relationship between the peak intensity ratio in the case of (P1 / P2), the discharge capacity in 1C, the discharge capacity in 2.5C, and the discharge capacity ratio.
FIG. 11 is a graphite packing density of 1.80 g / cm. ^Three It is a figure which shows the relationship between the peak intensity ratio in the case of (P1 / P2), the discharge capacity in 1C, the discharge capacity in 2.5C, and the discharge capacity ratio.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Graphite negative electrode plate, 10a ... Negative electrode current collection tab, 20 ... Positive electrode plate, 20a ... Positive electrode current collection tab, 30 ... Separator, 40 ... Exterior can, 41 ... Spacer, 42 ... Insulation gasket for exterior can, 50 ... Current interruption Sealing body

Claims

Graphite (however, the (002) plane spacing (d ₀₀₂ ) is 0.3380 nm or less and the c-axis crystallite size (Lc) is 15 nm or more) is used for the negative electrode, and lithium ions are occluded in the positive electrode. A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte using a releasable material and having a solute composed of a lithium salt dissolved in an organic solvent,
The graphite has at least rhombohedral crystallites and hexagonal crystallites,
The peak intensity (P1) of the (101) plane of the rhombohedral crystallite by X-ray diffraction and the peak intensity (P2) of the (101) plane by X-ray diffraction of the hexagonal crystallite. A non-aqueous electrolyte secondary battery having a peak intensity ratio (P1 / P2) of 0.20 or more and 0.30 or less.

Graphite (however, the (002) plane spacing (d ₀₀₂ ) is 0.3380 nm or less and the c-axis crystallite size (Lc) is 15 nm or more) is used for the negative electrode, and lithium ions are occluded in the positive electrode. A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte using a releasable material and having a solute composed of a lithium salt dissolved in an organic solvent,
The graphite includes a mixed graphite in which two or more kinds of graphite having different crystal structures, each having at least a rhombohedral crystallite and a hexagonal crystallite, are mixed.
The peak intensity (P1) of the (101) plane of the rhombohedral crystallite by X-ray diffraction and the peak intensity (P2) of the (101) plane by X-ray diffraction of the hexagonal crystallite. A non-aqueous electrolyte secondary battery prepared so that a peak intensity ratio (P1 / P2) is 0.20 or more and 0.30 or less.

The non-aqueous electrolyte secondary battery according to claim 1, wherein the graphite has a packing density of 1.60 g / cm ³ or more.