JP3711726B2 - Graphite particles, production method thereof, lithium secondary battery and negative electrode thereof - Google Patents
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明は、黒鉛粒子、その製造法、リチウム二次電池及びその負極に関する。更に詳しくは、ポータブル機器、電気自動車、電力貯蔵等に用いるのに好適な、急速充放電特性、サイクル特性及び安全性等に優れたリチウム二次電池とそれを得るための黒鉛粒子、その製造法、リチウム二次電池用負極に関する。
【０００２】
【従来の技術】
従来黒鉛粒子は、例えば天然黒鉛粒子、コークスを黒鉛化した人造黒鉛粒子、有機系高分子材料、ピッチ等を黒鉛化した人造黒鉛粒子、これらを粉砕した黒鉛粒子などがある。これらの粒子は、有機系結着剤及び有機溶剤と混合して黒鉛ペーストとし、この黒鉛ペーストを銅箔の表面に塗布し、溶剤を乾燥させてリチウムイオン二次電池用負極として使用されている。例えば、特公昭６２−２３４３３号公報に示されるように、負極に黒鉛を使用することでリチウムのデンドライトによる内部短絡の問題を解消し、サイクル特性の改良を図っている。
【０００３】
しかしながら、黒鉛結晶が発達している天然黒鉛粒子及びコークスを黒鉛化した人造黒鉛粒子は、ｃ軸方向の結晶の層間の結合力が、結晶の面方向の結合に比べて弱いため、粉砕により黒鉛層間の結合が切れ、アスペクト比の大きい、いわゆる鱗状の黒鉛粒子となる。この鱗状の黒鉛粒子は、アスペクト比が大きいため、バインダと混練して集電体に塗布して電極を作製した時に、鱗状の黒鉛粒子が集電体の面方向に配向し、その結果、黒鉛粒子へのリチウムの吸蔵・放出の繰り返しによって発生するｃ軸方向の歪みにより電極内部の破壊が生じ、サイクル特性が低下する問題があるばかりでなく、急速充放電特性が悪くなる傾向がある。さらに、アスペクト比の大きな鱗状の黒鉛粒子は、比表面積が大きいため、集電体との密着性が悪く、多くのバインダが必要となる問題点がある。集電体との密着性が悪いと、集電効果が低下し、放電容量、急速充放電特性、サイクル特性等が低下する問題がある。
【０００４】
また、比表面積が大きな鱗状黒鉛粒子は、これを用いたリチウム二次電池の第一回サイクル目の不可逆容量が大きいという問題がある。さらに、比表面積の大きな鱗状黒鉛粒子は、リチウムを吸蔵した状態での熱安定性が低く、リチウム二次電池用負極材料として用いた場合、安全性に問題がある。
そこで、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量に優れ、低比表面積であって、安全性を改善できる黒鉛粒子が要求されている。
また、黒鉛粒子を用いて負極を作製した場合、黒鉛ペーストの粘度が高く集電体への塗工性に問題があったり、電極成形条件によって、電極面内に電極密度が過剰に高くなる部分が発生するような黒鉛粒子を用いると、その結果として電解液に対する濡れ性が悪くなり充放電特性が悪化する等の問題があることが明らかとなった。
【０００５】
【発明が解決しようとする課題】
請求項１記載の発明は、リチウム二次電池の負極材料として用いた際に、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、黒鉛ペーストの粘度が大きく低下し、その結果として集電体への塗布性が大きく改善され、また加圧成形時の過剰な高密度化が抑制され、その結果として電極面内での密度バラツキが抑制され、優れた充放電特性を有するリチウム二次電池が得られる黒鉛粒子を提供するものである。
請求項２記載の発明は、リチウム二次電池の負極材料として用いた際に、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、黒鉛ペーストの粘度が大きく低下し、その結果として集電体への塗布性が大きく改善され、また加圧成形時の過剰な高密度化が抑制され、その結果として電極面内での密度バラツキが抑制され、優れた充放電特性を有するリチウム二次電池が得られる黒鉛粒子が容易に製造できる方法を提供するものである。
【０００６】
請求項３及び４記載の発明は、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、電極面内での密度バラツキが抑制され、優れた充放電特性を有するリチウム二次電池用負極を提供するものである。
請求項５記載の発明は、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、電極面内での密度バラツキが抑制され、優れた充放電特性を有するリチウム二次電池を提供するものである。
【０００７】
【課題を解決するための手段】
本発明は、偏平状の粒子を複数、配向面が非平行となるように集合又は結合させた粒子構造を有し、その粒子表面が非晶質炭素で被覆されてなる黒鉛粒子に関する。
また本発明は、黒鉛化可能な骨材又は黒鉛、黒鉛化可能なバインダ及び黒鉛化触媒を混合する工程、焼成・黒鉛化する工程、粉砕する工程を含む方法により黒鉛粒子を作製し、次いで有機高分子化合物で前記黒鉛粒子表面を被覆し、前記有機高分子化合物を非晶質炭素化することを特徴とする黒鉛粒子の製造法に関する。
また本発明は、前記黒鉛粒子又は前記の製造法により得られる黒鉛粒子を含有してなるリチウム二次電池用負極に関する。
また本発明は、黒鉛粒子と有機系結着剤の混合物を、集電体と一体化してなる前記リチウム二次電池用負極に関する。
さらに本発明は、前記黒鉛粒子又は前記の製造法により得られる黒鉛粒子を負極材として用いてなるリチウム二次電池に関する。
【０００８】
【発明の実施の形態】
本発明の黒鉛粒子は、偏平状の粒子を複数、配向面が非平行となるように集合又は結合させてなる構造の黒鉛粒子であり、その表面の一部または全部が非晶質炭素で被覆されているものである。
本発明において、扁平状の粒子とは、長軸と短軸を有する形状の粒子のことであり、完全な球状でないものをいう。例えば鱗状、鱗片状、一部の塊状等の形状のものがこれに含まれる。
複数の扁平状の粒子において、配向面が非平行とは、それぞれの粒子の形状において有する扁平した面、換言すれば最も平らに近い面を配向面として、複数の粒子がそれぞれの配向面を一定の方向にそろうことなく集合している状態をいう。
【０００９】
個々の扁平状の粒子は、材質的には、黒鉛化された骨材または黒鉛であることが好ましい。
この黒鉛粒子において扁平状の粒子は集合又は結合しているが、結合とは互いの粒子がバインダー等を介して接着されている状態をいい、集合とは互いの粒子がバインダー等で接着されてはないが、その形状等に起因して、その集合体としての形状を保っている状態をいう。機械的な強度の面から、結合しているものが好ましい。
該黒鉛粒子を負極に使用すると、集電体上に黒鉛結晶が配向し難く、負極黒鉛にリチウムを吸蔵・放出し易くなるため、得られるリチウム二次電池の急速充放電特性及びサイクル特性を向上させることができる。
【００１０】
本発明では、このような黒鉛粒子の表面の一部または全部を非晶質炭素で被覆することが重要である。非晶質炭素は、一般に有機高分子化合物を炭化して得られる。リチウム二次電池の負極として、非晶質炭素と黒鉛粒子を単に混合したものを用いた場合には、塗布性の改善や電極密度の過剰な上昇を抑制するためにはかなり多量の非晶質炭素を添加する必要が有り、非晶質炭素の充放電容量が一般に小さいことから混合物の充放電容量が大きく低下し、高容量のリチウム二次電池を得ることができなくなる。
【００１１】
黒鉛粒子表面を被覆する非晶質炭素の出発材料となる有機高分子化合物の種類及びこれを炭化して得られる非晶質炭素の被覆量については特に制限はない。
有機高分子化合物としては、フェノール樹脂、フルフリルアルコール樹脂、セルロース樹脂、ポリアクリロニトリル、ポリ塩化ビニル、ポリ塩化ビニリデン、塩素化ポリ塩化ビニルなどのハロゲン化ビニル樹脂、ポリアミドイミド樹脂、ポリアミド樹脂などの固相で炭素化する樹脂、また各種ピッチ類（原油ピッチ、ナフサピッチ、アスファルトピッチ、コールタールピッチ、分解ピッチ等）などのような液相で炭素化する有機高分子化合物が挙げられる。
黒鉛粒子表面を被覆する非晶質炭素の量としては、被覆前の黒鉛粒子重量に対して０．１重量％以上であることが好ましく、その被覆による効果と、充放電容量のバランスから、０．１〜２０重量％であることがより好ましく、１〜１５重量％であることがさらに好ましい。
【００１２】
本発明の黒鉛粒子のアスペクト比は５以下であることが、集電体上で偏平状粒子が配向し難くなり、リチウムイオン二次電池の急速充放電特性及びサイクル特性を一層向上することができるので好ましく、３以下であることがより好ましい。
なお、アスペクト比は、黒鉛粒子の長軸方向の長さをＡ、短軸方向の長さをＢとしたとき、Ａ／Ｂで表される。本発明におけるアスペクト比は、顕微鏡で黒鉛粒子を拡大し、任意に１００個の黒鉛粒子を選択し、Ａ／Ｂを測定し、その平均値をとったものである。
【００１３】
上記本発明の黒鉛粒子は、前記黒鉛化可能な骨材又は黒鉛、黒鉛化可能なバインダ及び黒鉛化触媒を混合する工程、焼成・黒鉛化する工程、粉砕する工程を含む方法により黒鉛粒子を作製し、次いで有機高分子化合物で前記黒鉛粒子表面を被覆し、次いで前記有機高分子化合物を非晶質炭素化することにより得ることができる。
黒鉛化可能な骨材としては、フルードコークス、ニードルコークス等の各種コークス類、樹脂の炭化物などが使用可能である。中でも、ニードルコークス等の黒鉛化しやすいコークス粉末が好ましい。
また黒鉛としては、例えば天然黒鉛粉末、人造黒鉛粉末等が使用できるが粉末状であれば特に制限はない。黒鉛化可能な骨材又は黒鉛の粒径は、本発明で作製する黒鉛粒子の粒径より小さいことが好ましい。
【００１４】
黒鉛化可能なバインダとしては、例えば、石炭系、石油系、人造等の各種ピッチ、タール、熱硬化性樹脂、熱可塑性樹脂などの有機系材料が挙げられる。バインダの配合量は、黒鉛化可能な骨材又は黒鉛に対し、５〜８０重量％添加することが好ましく、１０〜８０重量％添加することがより好ましく、１５〜８０重量％添加することがさらに好ましい。バインダの量が多すぎたり少なすぎると、作製する黒鉛粒子のアスペクト比及び比表面積が大きくなり易い。
黒鉛化可能な骨材又は黒鉛とバインダの混合方法は、特に制限はなく、ニーダー等を用いて行われるが、バインダの軟化点以上の温度で混合することが好ましい。具体的にはバインダがピッチ、タール等の際には、５０〜３００℃が好ましく、熱硬化性樹脂の場合には、２０〜１００℃が好ましい。
【００１５】
黒鉛化触媒としては、例えば鉄、ニッケル、チタン、ケイ素、硼素等の金属、これらの炭化物、酸化物などの黒鉛化触媒が使用できる。これらの中で、ケイ素または硼素の、炭化物または酸化物が好ましい。
黒鉛化可能な骨材又は黒鉛と黒鉛化可能なバインダに対して、黒鉛化触媒は１〜５０重量％添加し混合することが好ましい。１重量％未満であると黒鉛粒子の結晶の発達が悪くなり、充放電容量が低下する傾向にある。一方、５０重量％を超えると、均一に混合することが困難となり、作業性が低下する傾向にある。
【００１６】
混合工程の後に、焼成・黒鉛化工程、粉砕工程をとるが、その順番は特に制限されない。例えば、混合した材料を、焼成・黒鉛化した後、粉砕することができる。また、バインダーとして熱硬化性樹脂を用いたり、焼成・黒鉛化前にバインダーの不融化処理を行う等の場合は、先に粉砕してから焼成・黒鉛化することもできる。
焼成は前記混合物が酸化し難い雰囲気で行うことが好ましく、例えば窒素雰囲気中、アルゴンガス中、真空中で焼成する方法が挙げられる。黒鉛化の温度は２０００℃以上が好ましく、２５００℃以上であることが好ましく、２８００〜３２００℃であることがさらに好ましい。
黒鉛化温度が低いと、黒鉛の結晶の発達が悪くなると共に、黒鉛化触媒が作製した黒鉛粒子に残存し易くなり、いずれの場合も充放電容量が低下する傾向がある。一方、黒鉛化の温度が高すぎると、黒鉛が昇華することがある。
【００１７】
粉砕工程において、粉砕方法については特に制限を設けないが、ジェットミル、振動ミル、ピンミル、ハンマーミル等の既知の方法を用いることができる。粉砕後の平均粒子径は１〜１００μｍが好ましく、１０〜５０μｍがより好ましい。平均粒子径は大きすぎる場合、作製した電極表面に凸凹ができ易くなる。
【００１８】
焼成・黒鉛化する工程及び粉砕する工程を経て得られる黒鉛粒子は、次いで、有機高分子化合物で前記黒鉛粒子表面を被覆し、次いで前記有機高分子化合物を非晶質炭素化する。
有機高分子化合物を被覆する方法としては、有機高分子化合物を溶解した溶液に浸漬し、溶媒を除去し、黒鉛粒子表面に有機高分子化合物皮膜を形成する方法がある。有機高分子化合物を溶解する溶媒については特に制限はなく、使用する有機高分子化合物を溶解する溶媒であればいずれも使用できる。
溶媒の除去方法については、黒鉛粒子・有機高分子化合物溶液を常圧または減圧下で加熱乾燥する方法、有機高分子化合物溶液が付着した黒鉛粒子をろ過して乾燥する方法などが採用できる。
【００１９】
次いで、得られた有機高分子化合物被膜を形成した黒鉛粒子の該被膜を非晶質炭素化するが、これは一般に、非酸化性雰囲気中で加熱して有機高分子化合物を炭化することにより行うことができる。非酸化性雰囲気としては、例えば、窒素雰囲気中、アルゴンガス中、真空中が挙げられる。加熱する温度については、有機高分子化合物が実質的に炭化する温度であればよく、６００℃以上が好ましく、６００〜１２００℃がより好ましい。
【００２０】
前記黒鉛粒子又は前記の製造法により得られる黒鉛粒子は、本発明のリチウム二次電池用負極の材料として使用できる。例えば、黒鉛粒子を有機系結着剤、さらに必要に応じて溶剤と混合し、得られるペーストを集電体と一体化してリチウム二次電池用負極とすることができる。得られるペーストは、シート状、ペレット状等の形状に成形することができる。
有機系結着剤としては、例えばポリエチレン、ポリプロピレン、エチレンプロピレンポリマー、ブタジエンゴム、スチレンブタジエンゴム、ブチルゴム、イオン導電性の大きな高分子化合物が使用できる。
前記イオン導電率の大きな高分子化合物としては、ポリ弗化ビニリデン、ポリエチレンオキサイド、ポリエピクロヒドリン、ポリフォスファゼン、ポリアクリロニトリル等が使用できる。
有機系結着剤の中では、イオン伝導率の大きな高分子化合物が好ましく、ポリフッ化ビニリデンが特に好ましい。
【００２１】
有機系結着剤の含有量は、黒鉛粒子と有機系結着剤との総量に対して３〜２０重量％含有することが好ましい。
溶剤としては特に制限はなく、Ｎ−メチル２−ピロリドン、ジメチルホルムアミド、イソプロパノール等が用いられる。
溶剤の量に特に制限はなく、所望の粘度に調整できればよいが、通常ペーストに対して、３０〜７０重量％用いられることが好ましい。
【００２２】
上記ペーストを集電体と一体化し、リチウム二次電池用負極とするには、粘度を調整したペーストを、例えば集電体に塗布し乾燥する方法がある。
集電体としては、例えばニッケル、銅等の箔、メッシュなどが使用できる。
また一体化は、例えばロール、プレス等の加圧成形法で行うことができる。
【００２３】
このようにして得られたリチウム二次電池用負極は、リチウムイオン二次電池、リチウムポリマ二次電池等のリチウム二次電池に使用できる。リチウムイオン二次電池においては、通常、上記負極を、セパレータを介して正極を対向して配置し、電解液を注入する。またリチウムポリマ二次電池においては、通常、正極と高分子固体電解質を組み合わせて製造される。本発明のリチウム二次電池は、従来の炭素材料を用いたリチウム二次電池と比較して、急速充放電特性、サイクル特性に優れ、不可逆容量が小さく、特に安全性に優れる。
【００２４】
本発明におけるリチウム二次電池の正極に用いられる材料については特に制限はなく、ＬｉＮｉＯ2、ＬｉＣｏＯ2、ＬｉＭｎ2Ｏ4等を単独又は混合して使用することができる。
電解液としては、ＬｉＣｌＯ4、ＬｉＰＦ6、ＬｉＡｓＦ6、ＬｉＢＦ4、ＬｉＳＯ3ＣＦ3等のリチウム塩を、例えばエチレンカーボネート、ジエチルカーボネート、ジメトキシエタン、ジメチルカーボネート、テトラヒドロフラン、プロピレンカーボネート等の非水系溶剤に溶解したいわゆる有機電解液、ポリフッ化ビニリデン等の高分子固体電解質に含ませた固体有機電解液を使用することができる。
【００２５】
セパレータとしては、例えばポリエチレン、ポリプロピレン等のポリオレフィンを主成分とした不織布、クロス、微孔フィルム又はこれらを組み合わせたものを使用することができる。
なお、図１に円筒型リチウムイオン二次電池の一例の一部断面正面図を示す。図１において、１は正極、２は負極、３はセパレータ、４は正極タブ、５は負極タブ、６は正極蓋、７は電池缶及び８はガスケットである。
【００２６】
【実施例】
以下、本発明の実施例を説明する。
実施例１〜５
（１）黒鉛粒子の調製
平均粒径が５μｍのコークス粉末５０重量部、タールピッチ２０重量部、平均粒子径が４８μｍの炭化珪素７重量部及びコールタール１０重量部を混合し、２００℃で１時間混合した。得られた混合物を粉砕し、ペレット状に加圧成形し、次いで窒素雰囲気中、３０００℃で焼成後、ハンマーミルを用いて粉砕し、平均粒径が２０μｍの黒鉛粒子を作製した。この黒鉛粒子のＢＥＴ法による比表面積は３．６m2/g、水銀圧入法による細孔径分布測定を行った結果、１０１〜１０５nmの範囲の細孔体積は０．９cc/gであった。また、得られた黒鉛粒子を１００個任意に選び出し、アスペクト比を測定した結果、２．０であり、黒鉛粒子の、黒鉛粒子のＸ線広角回折による結晶の層間距離ｄ（００２）は３３．６１nm及び結晶子の大きさＬｃ（００２）は１００nm以上であった。さらに、得られた黒鉛粒子の走査型電子顕微鏡（ＳＥＭ）写真によれば、この黒鉛粒子は、偏平状の粒子が複数、配向面が非平行となるように集合又は結合した構造をしていた。
【００２７】
（２）非晶質炭素被覆黒鉛粒子の調製
（１）で調製した黒鉛粒子１００重量部をノボラック型フェノール樹脂メタノール溶液（日立化成工業(株)製、ＶＰ−１３Ｎ、固形分含有量５０重量％）１６０重量部に浸漬、分散して黒鉛粒子・フェノール樹脂混合溶液を作製した。用いたフェノール樹脂の残炭率は４７％（８００℃、窒素雰囲気中焼成で測定）である。この溶液をろ過、乾燥してフェノール樹脂を被覆した黒鉛粒子を得た。フェノール樹脂・メタノール溶液の濃度と得られた黒鉛粒子のフェノール樹脂被覆量との関係を表１に示す。次いで、このフェノール樹脂被覆黒鉛粒子を窒素中８００℃で焼成してフェノール樹脂を炭化し、非晶質炭素で被覆された黒鉛粒子を得た。得られた非晶質炭素被覆黒鉛粒子の被覆炭素量を表１に示す。
【００２８】
（３）黒鉛ペーストの粘度の測定
得られた非晶質炭素被覆黒鉛粒子９０重量％に、Ｎ−メチル−２−ピロリドンに溶解したポリ弗化ビニリデン（ＰＶＤＦ）を固形分で１０重量％加えて混練して黒鉛ペーストを作製した。この黒鉛ペーストについて測定した粘度を表１に示す。フェノール樹脂を炭化して表面を被覆することにより、黒鉛ペーストの粘度が大きく低下することが分かる。
【００２９】
（４）加圧成形時の密度上昇挙動
得られた非晶質炭素被覆黒鉛粒子の加圧成形時のカサ密度の変化挙動を測定した。黒鉛粒子を加圧し、カサ密度が１．６g/cm3となる時の加圧圧力を求めた。その結果を表１に示す。フェノール樹脂を炭化して黒鉛粒子表面に炭素を被覆することにより、カサ密度が１．６g/cm3となる圧力が大きく増加し、粒子強度が向上することが分かる。
【００３０】
（５）リチウム二次電池の作製
（３）で作製した黒鉛ペーストを厚さ１０μｍの圧延銅箔に塗布し、さらに乾燥し、面圧４９０Mpa（０．５トン/cm2）の圧力で圧縮成形し、試料電極とした 。黒鉛粒子層の厚さは９０μｍ及び密度は１．６g/cm3とした。
作製した試料電極を３端子法による定電流充放電を行い、リチウム二次電池用負極としての評価を行った。
図２は実験に用いたリチウム二次電池の概略図である。図２に示すようにガラスセル９に、電解液１０としてＬｉＰＦ4をエチレンカーボネート（ＥＣ）及びジメチルカーボネート（ＤＭＣ）（ＥＣとＤＭＣは体積比で１：１）の混合溶媒に１モル／リットルの濃度になるように溶解した溶液を入れ、試料電極（負極）１１、セパレータ１２及び対極（正極）１３を積層して配置し、さらに参照電極１４を上部から吊るしてリチウム二次電池を作製して行った。対極１３及び参照電極１４には金属リチウムを使用し、セパレータ１２にはポリエチレン微孔膜を使用した。０．５mA/cm2の定電流で、５mV（Ｖ ｖｓ Ｌｉ／Ｌｉ+）まで充電し、１Ｖ（Ｖ ｖｓ Ｌｉ／Ｌｉ+）まで放電する試験を繰り返した。
表２に１サイクル目の黒鉛粒子の単位重量当たりの充電容量、放電容量及び不可逆容量を示す。
【００３１】
比較例１
実施例の（１）で作製した黒鉛粒子について、非晶質炭素の被覆を行わずに上記（３）〜（４）を行い、黒鉛ペースト粘度、加圧成形時の密度上昇挙動を測定した。結果を表１に示す。
【００３２】
比較例２
実施例の（１）で作製した黒鉛粒子と、非晶質炭素（平均粒子径２０μｍ）を９０／１０の重量比で秤量し混合した（黒鉛粒子＋非晶質炭素）９０重量部に対し、Ｎ−メチル−２−ピロリドンに溶解したポリ弗化ビニリデン（ＰＶＤＦ）を固形分で１０重量％加えて混練して黒鉛ペーストを作製した。この黒鉛ペーストについて上記（３）〜（５）を行った。測定した粘度、加圧成形時の密度上昇挙動、リチウム二次電池での充放電容量を表１及び表２に示す。
【００３３】
比較例３
比較例２で使用した非晶質炭素（平均粒子径２０μｍ）９０重量部に対し、Ｎ−メチル−２−ピロリドンに溶解したポリ弗化ビニリデン（ＰＶＤＦ）を固形分で１０重量％加えて混練して黒鉛ペーストを作製した。この黒鉛ペーストについて上記（３）〜（５）を行った。測定した粘度、加圧成形時の密度上昇挙動、リチウム二次電池での充放電容量を表１及び表２に示す。
【００３４】
【表１】
【表２】
表１に示されるように、本発明で得られた偏平状の粒子を複数、配向面が非平行となるように集合又は結合させてなる黒鉛粒子で、表面が非晶質炭素で被覆されている黒鉛粒子は、非晶質炭素で被覆されていない黒鉛粒子と比較して黒鉛ペースト粘度が低く、また黒鉛粒子と非晶質炭素を単に混合した場合と比較して少ない炭素量で大きな粘度低下が得られ、結果として塗工性が良好であり、さらに成形時のカサ密度上昇が抑制され、その結果として電極成形時の圧力バラツキによる密度バラツキが少なく、また、少量の非晶質炭素被覆量で効果が得られるため表２に示されるように充分な充放電容量が得られる。
【００３７】
【発明の効果】
請求項１記載の黒鉛粒子は、リチウム二次電池の負極材料として用いた際に、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、黒鉛ペーストの粘度が大きく低下し、その結果として集電体への塗布性が大きく改善され、また加圧成形時の過剰な高密度化が抑制され、その結果として電極面内での密度バラツキが抑制され優れた充放電特性を有するリチウム二次電池が得られるものである。
請求項２記載の黒鉛粒子の製造法によれば、リチウム二次電池の負極材料として用いた際に、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、黒鉛ペーストの粘度が大きく低下し、その結果として集電体への塗布性が大きく改善され、また加圧成形時の過剰な高密度化が抑制され、その結果として電極面内での密度バラツキが抑制され優れた充放電特性を有するリチウム二次電池が得られる黒鉛粒子が容易に製造できる。
【００３８】
請求項３及び４記載のリチウム二次電池用負極は、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、電極面内での密度バラツキが抑制され優れた充放電特性を有する。
請求項５記載のリチウム二次電池は、急速充放電特性、サイクル特性、第一回サイクル目の不可逆容量等に優れるとともに、電極面内での密度バラツキが抑制され優れた充放電特性を有する。
【図面の簡単な説明】
【図１】円筒型リチウム二次電池の一部断面正面図である。
【図２】実施例及び比較例で充放電特性の測定に用いたリチウム二次電池の概略図である。
【符号の説明】
１ 正極
２ 負極
３ セパレータ
４ 正極タブ
５ 負極タブ
６ 正極蓋
７ 電池缶
８ ガスケット
９ ガラスセル
１０ 電解液
１１ 試料電極（負極）
１２ セパレータ
１３ 対極（正極）
１４ 参照極[0001]
BACKGROUND OF THE INVENTION
The present invention relates to graphite particles, a production method thereof, a lithium secondary battery, and a negative electrode thereof. More specifically, a lithium secondary battery excellent in rapid charge / discharge characteristics, cycle characteristics, safety, etc. suitable for use in portable devices, electric vehicles, power storage, etc., and graphite particles for obtaining the same, and a method for producing the same And relates to a negative electrode for a lithium secondary battery.
[0002]
[Prior art]
Conventional graphite particles include, for example, natural graphite particles, artificial graphite particles obtained by graphitizing coke, organic polymer materials, artificial graphite particles obtained by graphitizing pitch and the like, and graphite particles obtained by pulverizing these. These particles are mixed with an organic binder and an organic solvent to form a graphite paste. The graphite paste is applied to the surface of the copper foil, and the solvent is dried to be used as a negative electrode for a lithium ion secondary battery. . For example, as disclosed in Japanese Examined Patent Publication No. 62-23433, the use of graphite for the negative electrode eliminates the problem of internal short circuit due to lithium dendrite and improves the cycle characteristics.
[0003]
However, natural graphite particles in which graphite crystals are developed and artificial graphite particles graphitized from coke have a weaker bonding force between crystals in the c-axis direction than in the crystal plane direction. The bond between the layers is broken, and so-called scaly graphite particles having a large aspect ratio are obtained. Since the scaly graphite particles have a large aspect ratio, the scaly graphite particles are oriented in the surface direction of the current collector when kneaded with a binder and applied to the current collector to produce an electrode. Distortion in the c-axis direction caused by repeated insertion and extraction of lithium into and from the particles causes destruction of the inside of the electrode, resulting in a problem that cycle characteristics deteriorate, and rapid charge / discharge characteristics tend to deteriorate. Furthermore, since scaly graphite particles having a large aspect ratio have a large specific surface area, there is a problem that adhesion with a current collector is poor and a large amount of binder is required. If the adhesiveness with the current collector is poor, there is a problem that the current collecting effect is lowered and the discharge capacity, rapid charge / discharge characteristics, cycle characteristics, etc. are lowered.
[0004]
In addition, scaly graphite particles having a large specific surface area have a problem that the irreversible capacity of the first cycle of a lithium secondary battery using the particles is large. Furthermore, scaly graphite particles having a large specific surface area have low thermal stability in a state where lithium is occluded, and there is a problem in safety when used as a negative electrode material for a lithium secondary battery.
Therefore, there is a demand for graphite particles that are excellent in rapid charge / discharge characteristics, cycle characteristics, and irreversible capacity in the first cycle, have a low specific surface area, and can improve safety.
In addition, when the negative electrode is prepared using graphite particles, the viscosity of the graphite paste is high, there is a problem in the coating property to the current collector, or the electrode density is excessively increased in the electrode surface depending on the electrode forming conditions. As a result, it has become clear that the use of graphite particles that generate water causes problems such as poor wettability with respect to the electrolyte and deterioration of charge / discharge characteristics.
[0005]
[Problems to be solved by the invention]
The invention according to claim 1 is excellent in rapid charge / discharge characteristics, cycle characteristics, irreversible capacity in the first cycle and the like, and the viscosity of the graphite paste is greatly reduced when used as a negative electrode material for a lithium secondary battery. As a result, applicability to the current collector is greatly improved, and excessive densification during pressure molding is suppressed, resulting in suppression of density variation in the electrode surface and excellent charge / discharge characteristics. The graphite particle | grains from which the lithium secondary battery which has this is obtained are provided.
The invention according to claim 2 is excellent in rapid charge / discharge characteristics, cycle characteristics, irreversible capacity in the first cycle and the like, and the viscosity of the graphite paste is greatly reduced when used as a negative electrode material for a lithium secondary battery. As a result, applicability to the current collector is greatly improved, and excessive densification during pressure molding is suppressed, resulting in suppression of density variation in the electrode surface and excellent charge / discharge characteristics. The present invention provides a method for easily producing graphite particles from which a lithium secondary battery having the above can be obtained.
[0006]
The inventions according to
The invention according to claim 5 is excellent in rapid charge / discharge characteristics, cycle characteristics, irreversible capacity in the first cycle, etc., and density variation in the electrode surface is suppressed, and lithium secondary having excellent charge / discharge characteristics. A battery is provided.
[0007]
[Means for Solving the Problems]
The present invention relates to a graphite particle having a particle structure in which a plurality of flat particles are aggregated or bonded so that their orientation planes are non-parallel, and the particle surface is coated with amorphous carbon.
The present invention also provides graphite particles by a method comprising a step of mixing a graphitizable aggregate or graphite, a graphitizable binder and a graphitization catalyst, a step of firing and graphitizing, and a step of pulverizing, The present invention relates to a method for producing graphite particles, wherein the surface of the graphite particles is coated with a polymer compound, and the organic polymer compound is converted to amorphous carbon.
The present invention also relates to a negative electrode for a lithium secondary battery comprising the graphite particles or the graphite particles obtained by the production method.
The present invention also relates to the negative electrode for a lithium secondary battery in which a mixture of graphite particles and an organic binder is integrated with a current collector.
Furthermore, this invention relates to the lithium secondary battery which uses the graphite particle obtained by the said graphite particle or the said manufacturing method as a negative electrode material.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The graphite particles of the present invention are graphite particles having a structure in which a plurality of flat particles are assembled or bonded so that their orientation planes are non-parallel, and a part or all of the surface thereof is coated with amorphous carbon. It is what has been.
In the present invention, flat particles are particles having a major axis and a minor axis, and are not completely spherical. For example, those having a shape such as a scale shape, a scale shape, or a part of a lump shape are included.
In a plurality of flat particles, the orientation plane is non-parallel means that the flat surface in the shape of each particle, in other words, the plane that is closest to the plane is the orientation plane, and the plurality of particles have a constant orientation plane. A state of gathering without aligning in the direction of.
[0009]
The individual flat particles are preferably graphitized aggregate or graphite in terms of material.
In these graphite particles, the flat particles are aggregated or bonded, but the bond means a state in which the particles are bonded via a binder or the like, and the aggregate is that the particles are bonded by a binder or the like. Although there is no, it refers to the state of maintaining the shape as an aggregate due to its shape and the like. From the viewpoint of mechanical strength, those bonded are preferable.
When the graphite particles are used for the negative electrode, the graphite crystals are difficult to orient on the current collector, and it becomes easier to occlude and release lithium into the negative electrode graphite, improving the rapid charge / discharge characteristics and cycle characteristics of the resulting lithium secondary battery. Can be made.
[0010]
In the present invention, it is important to cover part or all of the surface of such graphite particles with amorphous carbon. Amorphous carbon is generally obtained by carbonizing an organic polymer compound. When a negative electrode of a lithium secondary battery is simply a mixture of amorphous carbon and graphite particles, a considerably large amount of amorphous is used to improve the coatability and suppress an excessive increase in electrode density. Since it is necessary to add carbon and the charge / discharge capacity of amorphous carbon is generally small, the charge / discharge capacity of the mixture is greatly reduced, making it impossible to obtain a high-capacity lithium secondary battery.
[0011]
There are no particular restrictions on the type of organic polymer compound used as the starting material for the amorphous carbon coating the surface of the graphite particles and the coating amount of the amorphous carbon obtained by carbonizing this.
Examples of organic polymer compounds include solid resins such as phenolic resins, furfuryl alcohol resins, cellulose resins, polyacrylonitrile, polyvinyl chloride, polyvinylidene chloride, chlorinated polyvinyl chloride, and polyamideimide resins and polyamide resins. Examples thereof include resins that are carbonized in a phase, and organic polymer compounds that are carbonized in a liquid phase such as various pitches (crude oil pitch, naphtha pitch, asphalt pitch, coal tar pitch, cracked pitch, etc.).
The amount of the amorphous carbon coating the surface of the graphite particles is preferably 0.1% by weight or more based on the weight of the graphite particles before coating. From the balance between the effect of the coating and the charge / discharge capacity, 0 More preferably, it is 1 to 20% by weight, and further preferably 1 to 15% by weight.
[0012]
When the aspect ratio of the graphite particles of the present invention is 5 or less, the flat particles are hardly oriented on the current collector, and the rapid charge / discharge characteristics and cycle characteristics of the lithium ion secondary battery can be further improved. Therefore, it is preferably 3 or less.
The aspect ratio is represented by A / B, where A is the length in the major axis direction of the graphite particles and B is the length in the minor axis direction. The aspect ratio in the present invention is obtained by enlarging graphite particles with a microscope, arbitrarily selecting 100 graphite particles, measuring A / B, and taking the average value.
[0013]
The graphite particles of the present invention are produced by a method including the step of mixing the graphitizable aggregate or graphite, the graphitizable binder and the graphitization catalyst, the step of firing and graphitizing, and the step of pulverizing. Then, the surface of the graphite particles can be coated with an organic polymer compound, and then the organic polymer compound can be obtained by amorphous carbonization.
As aggregates that can be graphitized, various cokes such as fluid coke and needle coke, resin carbide, and the like can be used. Among these, coke powder that is easily graphitized such as needle coke is preferable.
Moreover, as graphite, natural graphite powder, artificial graphite powder, etc. can be used, for example, but there is no restriction | limiting in particular if it is a powder form. The particle size of the graphitizable aggregate or graphite is preferably smaller than the particle size of the graphite particles produced in the present invention.
[0014]
Examples of the graphitizable binder include organic materials such as coal-based, petroleum-based, artificial pitches, tars, thermosetting resins, and thermoplastic resins. The blending amount of the binder is preferably 5 to 80% by weight, more preferably 10 to 80% by weight, more preferably 15 to 80% by weight based on the graphitizable aggregate or graphite. preferable. If the amount of the binder is too much or too little, the aspect ratio and specific surface area of the graphite particles to be produced tend to increase.
The method for mixing the graphitizable aggregate or graphite and the binder is not particularly limited and is performed using a kneader or the like, but it is preferable to mix at a temperature equal to or higher than the softening point of the binder. Specifically, when the binder is pitch, tar or the like, 50 to 300 ° C is preferable, and when the binder is a thermosetting resin, 20 to 100 ° C is preferable.
[0015]
As the graphitization catalyst, for example, a graphitization catalyst such as a metal such as iron, nickel, titanium, silicon, or boron, or a carbide or oxide thereof can be used. Of these, carbides or oxides of silicon or boron are preferred.
The graphitization catalyst is preferably added in an amount of 1 to 50% by weight with respect to the graphitizable aggregate or graphite and the graphitizable binder. If it is less than 1% by weight, the development of the graphite particle crystals tends to deteriorate, and the charge / discharge capacity tends to decrease. On the other hand, when it exceeds 50% by weight, it becomes difficult to mix uniformly, and workability tends to be lowered.
[0016]
After the mixing step, a firing / graphitization step and a pulverization step are taken, but the order is not particularly limited. For example, the mixed material can be pulverized after firing and graphitization. Further, when a thermosetting resin is used as the binder or the binder is infusibilized before firing and graphitization, the binder can be pulverized before firing and graphitization.
Firing is preferably performed in an atmosphere in which the mixture is not easily oxidized, and examples thereof include a method of firing in a nitrogen atmosphere, argon gas, and vacuum. The graphitization temperature is preferably 2000 ° C. or higher, preferably 2500 ° C. or higher, more preferably 2800 to 3200 ° C.
When the graphitization temperature is low, the development of graphite crystals deteriorates and the graphitization catalyst tends to remain in the produced graphite particles, and in either case, the charge / discharge capacity tends to decrease. On the other hand, if the graphitization temperature is too high, the graphite may sublime.
[0017]
In the pulverization step, the pulverization method is not particularly limited, but a known method such as a jet mill, a vibration mill, a pin mill, or a hammer mill can be used. The average particle size after pulverization is preferably 1 to 100 μm, and more preferably 10 to 50 μm. When the average particle diameter is too large, unevenness is easily formed on the produced electrode surface.
[0018]
The graphite particles obtained through the firing and graphitizing step and the pulverizing step are then coated on the surface of the graphite particles with an organic polymer compound, and then the organic polymer compound is amorphous carbonized.
As a method for coating the organic polymer compound, there is a method of immersing in a solution in which the organic polymer compound is dissolved, removing the solvent, and forming an organic polymer compound film on the surface of the graphite particles. There is no restriction | limiting in particular about the solvent which melt | dissolves an organic polymer compound, Any can be used if it is a solvent which melt | dissolves the organic polymer compound to be used.
As a method for removing the solvent, a method of heating and drying the graphite particle / organic polymer compound solution under normal pressure or reduced pressure, a method of filtering and drying the graphite particles to which the organic polymer compound solution is adhered, and the like can be employed.
[0019]
Next, the graphite particles on which the obtained organic polymer compound film is formed are amorphous carbonized, and this is generally performed by heating in a non-oxidizing atmosphere to carbonize the organic polymer compound. be able to. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon gas, and a vacuum. About the temperature to heat, what is necessary is just the temperature which an organic polymer compound carbonizes substantially, 600 degreeC or more is preferable and 600-1200 degreeC is more preferable.
[0020]
The graphite particles or the graphite particles obtained by the production method can be used as a material for the negative electrode for a lithium secondary battery of the present invention. For example, graphite particles can be mixed with an organic binder and, if necessary, a solvent, and the resulting paste can be integrated with a current collector to form a negative electrode for a lithium secondary battery. The obtained paste can be formed into a sheet shape, a pellet shape or the like.
As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having a large ion conductivity can be used.
As the polymer compound having a high ionic conductivity, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile and the like can be used.
Among the organic binders, a polymer compound having a high ionic conductivity is preferable, and polyvinylidene fluoride is particularly preferable.
[0021]
The content of the organic binder is preferably 3 to 20% by weight with respect to the total amount of the graphite particles and the organic binder.
There is no restriction | limiting in particular as a solvent, N-methyl 2-pyrrolidone, a dimethylformamide, isopropanol etc. are used.
There is no restriction | limiting in particular in the quantity of a solvent, Although it should just be able to adjust to a desired viscosity, It is preferable to use 30 to 70 weight% normally with respect to a paste.
[0022]
In order to integrate the paste with a current collector to form a negative electrode for a lithium secondary battery, there is a method in which a paste with adjusted viscosity is applied to a current collector and dried, for example.
As the current collector, for example, a foil such as nickel or copper, a mesh, or the like can be used.
The integration can be performed by a pressure forming method such as a roll or a press.
[0023]
The negative electrode for a lithium secondary battery thus obtained can be used for lithium secondary batteries such as lithium ion secondary batteries and lithium polymer secondary batteries. In a lithium ion secondary battery, the negative electrode is usually placed with the positive electrode facing the separator through a separator, and an electrolyte solution is injected. A lithium polymer secondary battery is usually manufactured by combining a positive electrode and a polymer solid electrolyte. The lithium secondary battery of the present invention is excellent in rapid charge / discharge characteristics and cycle characteristics, has a small irreversible capacity, and is particularly excellent in safety as compared with a lithium secondary battery using a conventional carbon material.
[0024]
There is no particular limitation on the material used for a cathode of a lithium secondary battery of the present invention may be used alone or as a mixture of LiNiO 2, LiCoO 2, LiMn 2 O 4 or the like.
As the electrolyte, lithium salts such as LiClO 4 , LiPF 6 , LiAsF 6 , LiBF 4 , LiSO 3 CF 3 are used in non-aqueous solvents such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran, and propylene carbonate. A so-called dissolved organic electrolyte solution or a solid organic electrolyte solution contained in a polymer solid electrolyte such as polyvinylidene fluoride can be used.
[0025]
As the separator, for example, a nonwoven fabric, a cloth, a microporous film, or a combination of these having a polyolefin such as polyethylene or polypropylene as a main component can be used.
FIG. 1 shows a partial cross-sectional front view of an example of a cylindrical lithium ion secondary battery. In FIG. 1, 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is a positive electrode tab, 5 is a negative electrode tab, 6 is a positive electrode lid, 7 is a battery can, and 8 is a gasket.
[0026]
【Example】
Examples of the present invention will be described below.
Examples 1-5
(1) Preparation of graphite particles 50 parts by weight of coke powder having an average particle diameter of 5 μm, 20 parts by weight of tar pitch, 7 parts by weight of silicon carbide having an average particle diameter of 48 μm, and 10 parts by weight of coal tar are mixed at 200 ° C. Mixed for hours. The obtained mixture was pulverized, pressed into a pellet, fired at 3000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill to produce graphite particles having an average particle diameter of 20 μm. The specific surface area of this graphite particle by BET method was 3.6 m 2 / g, and pore diameter distribution measurement by mercury porosimetry was performed. As a result, the pore volume in the range of 101 to 105 nm was 0.9 cc / g. Further, 100 pieces of the obtained graphite particles were arbitrarily selected and the aspect ratio was measured. As a result, it was 2.0, and the interlayer distance d (002) of the graphite by X-ray wide angle diffraction of the graphite particles was 33. The crystallite size Lc (002) was 61 nm or more at 61 nm. Furthermore, according to the scanning electron microscope (SEM) photograph of the obtained graphite particles, the graphite particles had a structure in which a plurality of flat particles were assembled or combined so that the orientation planes were non-parallel. .
[0027]
(2) Preparation of amorphous carbon-coated graphite particles 100 parts by weight of the graphite particles prepared in (1) were added to a novolak type phenol resin methanol solution (manufactured by Hitachi Chemical Co., Ltd., VP-13N, solid content: 50% by weight). ) It was immersed and dispersed in 160 parts by weight to prepare a graphite particle / phenol resin mixed solution. The residual carbon ratio of the used phenol resin is 47% (measured by firing in a nitrogen atmosphere at 800 ° C.). This solution was filtered and dried to obtain graphite particles coated with a phenol resin. Table 1 shows the relationship between the concentration of the phenol resin / methanol solution and the phenol resin coating amount of the obtained graphite particles. Next, the phenol resin-coated graphite particles were calcined in nitrogen at 800 ° C. to carbonize the phenol resin, thereby obtaining graphite particles coated with amorphous carbon. Table 1 shows the coating carbon amount of the obtained amorphous carbon-coated graphite particles.
[0028]
(3) Measurement of Viscosity of Graphite Paste 10% by weight of solid content of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added to 90% by weight of the obtained amorphous carbon-coated graphite particles. The graphite paste was prepared by kneading. Table 1 shows the viscosity measured for this graphite paste. It can be seen that the viscosity of the graphite paste is greatly reduced by carbonizing the phenol resin to coat the surface.
[0029]
(4) Density increase behavior during pressure molding The change behavior of the bulk density during pressure molding of the obtained amorphous carbon-coated graphite particles was measured. The graphite particles were pressurized, and the pressure applied when the bulk density was 1.6 g / cm 3 was determined. The results are shown in Table 1. It can be seen that by carbonizing the phenol resin and coating the surface of the graphite particles with carbon, the pressure at which the bulk density becomes 1.6 g / cm 3 is greatly increased, and the particle strength is improved.
[0030]
(5) Production of lithium secondary battery The graphite paste produced in (3) was applied to a rolled copper foil having a thickness of 10 μm, dried, and compression molded at a surface pressure of 490 Mpa (0.5 ton / cm 2 ). The sample electrode was used. The graphite particle layer had a thickness of 90 μm and a density of 1.6 g / cm 3 .
The prepared sample electrode was subjected to constant current charge / discharge by the three-terminal method, and evaluated as a negative electrode for a lithium secondary battery.
FIG. 2 is a schematic view of a lithium secondary battery used in the experiment. As shown in FIG. 2, in a glass cell 9, LiPF 4 as an electrolytic solution 10 was mixed with 1 mol / liter of a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC and DMC are 1: 1 by volume). A solution dissolved so as to have a concentration is put, a sample electrode (negative electrode) 11, a separator 12 and a counter electrode (positive electrode) 13 are stacked and arranged, and a reference electrode 14 is suspended from the top to produce a lithium secondary battery. went. Metallic lithium was used for the counter electrode 13 and the reference electrode 14, and a polyethylene microporous film was used for the separator 12. The test of charging to 5 mV (V vs Li / Li + ) and discharging to 1 V (V vs Li / Li + ) at a constant current of 0.5 mA / cm 2 was repeated.
Table 2 shows the charge capacity, discharge capacity, and irreversible capacity per unit weight of the graphite particles in the first cycle.
[0031]
Comparative Example 1
The graphite particles produced in Example (1) were subjected to the above (3) to (4) without coating with amorphous carbon, and the graphite paste viscosity and the behavior of increasing density during pressure molding were measured. The results are shown in Table 1.
[0032]
Comparative Example 2
With respect to 90 parts by weight of graphite particles prepared in Example (1) and amorphous carbon (average particle diameter 20 μm) in a weight ratio of 90/10 and mixed (graphite particles + amorphous carbon), Polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added in a solid content of 10% by weight and kneaded to prepare a graphite paste. The graphite paste was subjected to the above (3) to (5). Tables 1 and 2 show the measured viscosity, the behavior of increasing the density during pressure molding, and the charge / discharge capacity of the lithium secondary battery.
[0033]
Comparative Example 3
Polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added at a solid content of 10% by weight to 90 parts by weight of amorphous carbon (average particle size 20 μm) used in Comparative Example 2, and kneaded. Thus, a graphite paste was prepared. The graphite paste was subjected to the above (3) to (5). Tables 1 and 2 show the measured viscosity, the behavior of increasing the density during pressure molding, and the charge / discharge capacity of the lithium secondary battery.
[0034]
[Table 1]
[Table 2]
As shown in Table 1, graphite particles obtained by assembling or bonding a plurality of flat particles obtained in the present invention so that their orientation planes are non-parallel, and the surface is coated with amorphous carbon. Graphite particles have lower graphite paste viscosity than graphite particles that are not coated with amorphous carbon, and a large decrease in viscosity with a small amount of carbon compared to simply mixing graphite particles and amorphous carbon. As a result, the coatability is good, and further, the density increase during molding is suppressed. As a result, there is little density variation due to pressure variation during electrode molding, and there is a small amount of amorphous carbon coating. As shown in Table 2, a sufficient charge / discharge capacity can be obtained.
[0037]
【The invention's effect】
The graphite particles according to claim 1 have excellent rapid charge / discharge characteristics, cycle characteristics, irreversible capacity at the first cycle, and the like, and the viscosity of the graphite paste is greatly reduced when used as a negative electrode material for a lithium secondary battery. As a result, the applicability to the current collector is greatly improved, and excessive densification during pressing is suppressed, resulting in reduced density variation in the electrode surface and excellent charge / discharge characteristics. A lithium secondary battery having the following can be obtained.
According to the method for producing graphite particles according to claim 2, when used as a negative electrode material for a lithium secondary battery, it has excellent rapid charge / discharge characteristics, cycle characteristics, irreversible capacity at the first cycle, and the like, and graphite paste As a result, the applicability to the current collector is greatly improved, and excessive densification during pressure molding is suppressed, resulting in suppression of density variation within the electrode surface. Graphite particles from which a lithium secondary battery having excellent charge / discharge characteristics can be easily produced.
[0038]
The negative electrode for a lithium secondary battery according to
The lithium secondary battery according to claim 5 is excellent in rapid charge / discharge characteristics, cycle characteristics, irreversible capacity in the first cycle, and the like, and has excellent charge / discharge characteristics in which density variation in the electrode surface is suppressed.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional front view of a cylindrical lithium secondary battery.
FIG. 2 is a schematic view of a lithium secondary battery used for measurement of charge / discharge characteristics in Examples and Comparative Examples.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Positive electrode 2
12 Separator 13 Counter electrode (positive electrode)
14 Reference pole
Claims (5)
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| JP00815198A JP3711726B2 (en) | 1998-01-20 | 1998-01-20 | Graphite particles, production method thereof, lithium secondary battery and negative electrode thereof |
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| JP00815198A JP3711726B2 (en) | 1998-01-20 | 1998-01-20 | Graphite particles, production method thereof, lithium secondary battery and negative electrode thereof |
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| WO2010084906A1 (en) | 2009-01-23 | 2010-07-29 | Ｎｅｃエナジーデバイス株式会社 | Lithium ion cell |
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- 1998-01-20 JP JP00815198A patent/JP3711726B2/en not_active Expired - Lifetime
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