JP4209649B2 - Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery containing the negative electrode material, and lithium ion secondary battery using the negative electrode - Google Patents
Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery containing the negative electrode material, and lithium ion secondary battery using the negative electrode Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、二次電池用の電極材料と該電極材料を含む二次電池用電極、並びに該電極を用いたリチウムイオン二次電池に関するものである。
【０００２】
【従来の技術】
最近、電子機器等の小型化に伴い、電源となる電池も小型化が求められており、特に電池の高容量化の観点からリチウムイオン二次電池が注目されている。リチウムイオン二次電池の中でも、負極に炭素材料を用いたものは、大容量が得られ易く、且つ、安全で高電圧が得られ易いといった点でも有用である。しかし、負極に炭素材料を用いると、サイクル特性が悪くなるという問題があった。
【０００３】
そこで、負極として炭素材料に銅微粒子を均一分散させた電極を用いることにより、負極の利用率を高めてサイクル特性を高める技術が提案されている（例えば、特許文献１参照。）。また、非水電解質二次電池用電極として、負極活物質が塊状黒鉛と鱗片状黒鉛の一種以上とメソフェーズ小球体炭素粉末の混合物からなる電極を用いることにより充・放電サイクル寿命を高める技術が開示されている（例えば、特許文献２参照。）。ところが、本発明者らが検討したところ、これらの電極は、初回の充電時に必要な電気容量が放電に必要な電気容量よりも高く、初回の充電効率（以下、「初期効率」と称する場合がある）に問題がある。
【０００４】
ところで、電池用に用いられる炭素材料としては、天然黒鉛や人造黒鉛があるが、一般に炭素材料の表面には種々の官能基（例えば、カルボキシル基やフェノール基、ラクトン基、カルボニル基など）が存在するので、この官能基が初回の充電時に電解液と反応して副反応を起こし、充電容量を損失させるため初期効率は悪くなる。
【０００５】
こうした問題を解決すべく、炭素材料の原料を真空下または還元性雰囲気下で熱処理することにより、炭素材料の表面に存在する官能基量を低減する技術が提案されている（例えば、特許文献３参照。）。ところがこの技術では、炭素材料として鱗片状の黒鉛粒子を用いていることから、次の様な問題が生じてくる。すなわち、電極を成形する際には、粉砕した天然黒鉛や人造黒鉛と溶媒およびバインダー（結着材）を混合してスラリーとしたものを対象物に塗布するのが一般的であるが、炭素材料として鱗片状の黒鉛粒子を用いるとスラリーの流動性が悪くなって塗布作業性が著しく損なわれる。
【０００６】
本発明者らは、こうした問題の更なる改善を期して研究を重ねた結果、鱗片状の天然黒鉛粒子を粉砕してから再凝集させることにより球状化されると、鱗片状天然黒鉛粒子が本来有している利点を維持しながらスラリー特性が改善されると共に、大きな放電電流値での放電容量の低下も抑えられることをつきとめ、こうした知見を基に先に提案した（例えば、特許文献４参照。）。
【０００７】
しかし、前記特許文献３に指摘されている様に、球状化黒鉛粒子表面に官能基が多量に存在すると、初回充電時に官能基が電解液と反応して初期効率の低下を引き起こす。また、鱗片状の黒鉛を粉砕し再凝集させて球状化すると、スラリー特性は良くなるものの黒鉛粒子表面が平滑になるので、黒鉛粒子同士や黒鉛粒子とバインダー粒子との接点が減少して導電成分同士の密着性が悪くなり、導電性が低下してサイクル特性の低下を引き起こす。
【０００８】
【特許文献１】
特開平9-245798号公報
【特許文献２】
特開2001-236950号公報
【特許文献３】
特開平8-148185号公報
【特許文献４】
特開平11-263612号公報
【０００９】
【発明が解決しようとする課題】
本発明は、この様な問題点に鑑みてなされたものであり、その目的は、二次電池用電極材料として、特に初期効率とサイクル特性に優れた二次電池を実現するために有用な二次電池用電極材料を提供することにある。また、本発明の他の目的は、該電極材料を含む二次電池用電極および該電極を用いたリチウムイオン二次電池を提供することにある。
【００１０】
【課題を解決するための手段】
上記課題を解決することのできた本発明に係る二次電池用電極材料とは、二次電池用電極を製造する際に用いる材料であって、酸性官能基量が１．０ミリ当量／ｋｇ以下である球状化黒鉛粒子と、前記球状化黒鉛粒子より相対的に微細で、且つ、該球状化黒鉛粒子とは異なる導電性炭素質微粒子を混合状態で含有せしめた点に要旨を有する。
【００１１】
上記球状化黒鉛粒子としては、鱗片状黒鉛を粉砕した後、再凝集させたものであり、その平均粒子径が１０μｍ以上のものを用い、このとき前記導電性炭素質微粒子として平均粒子径が１０μｍ未満（０μｍを含まない）のものを用いるのが好ましい。
【００１２】
本発明では、球状化黒鉛粒子と前記導電性炭素質微粒子を混合した混合粉全体の酸性官能基量が２．５ミリ当量／ｋｇ以下であるのが好ましい。
【００１３】
前記導電性炭素質微粒子としては、鱗片状黒鉛微粒子および／またはカーボンブラックを好適に使用でき、鱗片状黒鉛微粒子：１〜２０％（「質量％」の意味。以下同じ。）および／またはカーボンブラック：０．１〜２．０％（「質量％」の意味。以下同じ。）を含むのが望ましい。
【００１４】
この二次電池用電極材料を含む二次電池用電極は、初回の充電時における電解液との反応が少ないと共に、導電率が良く、この二次電池用電極を負極として構成されるリチウムイオン二次電池は、優れた初期効率とサイクル特性を兼ね備えたものとなる。
【００１５】
【発明の実施の形態】
本発明者らは、前述した様な課題を解決すべく、様々な角度から検討してきた。その結果、鱗片状黒鉛を粉砕した後再凝集させることにより球状化した粒子が有する酸性官能基量を所定量以下に抑制すると共に、該球状化黒鉛粒子よりも相対的に微細で、且つ、該球状化黒鉛粒子とは異なる導電性炭素質微粒子を混合してやれば、上記課題が見事解決できることを見出し、上記本発明を完成した。以下、本発明の構成と作用効果について詳細に説明していく。
【００１６】
本発明の二次電池用電極材料は、鱗片状黒鉛を粉砕した後、再凝集させて球状化したものである。上述した様に、鱗片状の黒鉛を粉砕した後、これらを再凝集させて球状化した黒鉛粒子は、溶媒やバインダーと混合するときの流動性が良好となるからである。
【００１７】
ここで、球状とはサッカーボールやテニスボールの様な真球状のみならず、ラグビーボールの様な楕円体のものも含む意味であり、本発明では円形度が0.86程度以上のものを指す。但し、円形度は三次元の黒鉛粒子を二次元平面に投影して算出される指標であるので、例えば一般的に入手できる鱗片状天然黒鉛粒子の円形度を算出すると0.84程度になり、本発明の黒鉛粒子の円形度と近似するが、鱗片状黒鉛粒子（原料）は平面的な粒子であるのに対し、本発明における二次電池用電極材料の実際の形状は立体的であり全く異なる。
【００１８】
球状化黒鉛粒子は、鱗片状黒鉛を粉砕した後、これらを再凝集させることにより得ることができるが、球状化黒鉛粒子を製造する具体的な方法は特に限定されない。例えば、本発明者らが先に提案した方法（特開平11-263612号）やこれに類似する方法で製造できる。以下、製法の一例を図面を参酌しつつ説明する。
【００１９】
図１は、球状化黒鉛粒子の製造に用いられる装置の概略説明図であり、１は槽、２はフィーダー、３は対向ノズル、４は分級機、５は吹き上げノズルを夫々示している。
【００２０】
鱗片状黒鉛（原料）を、槽１に設けられたフィーダー２から槽１内へ供給する。フィーダー２は、ホッパー式のものを槽１の適当箇所に設置することが好ましく、球状化黒鉛粒子の取出口としても利用できる。また、フィーダー２は、スクリュー式のものを槽１の下部に設けてもよい。槽１内への原料供給量は、槽１の容量を考慮して定めれば良い。
【００２１】
槽１の下部側には槽壁を貫通して対向ノズル３を設け、対向ノズル３からジェット気流を吹き込むことにより、槽１内の下部側に衝突域を形成する。衝突域の気流に入った前記鱗片状黒鉛は互いに衝突し、粉砕されながら再凝集して球状化する。
【００２２】
対向ノズル３は、複数個（例えば、三〜四個）設けることが好ましい。対向ノズル３から吹き込むジェット気流の速度、吹き込みガス量、槽圧などは、円滑な衝突と流動が達成できるように設定され、操作時間を適宜に設定することにより鱗片状黒鉛を球状化する。例えば、ノズル吐出圧は0.01〜0.50ＭＰａ程度、吹き込みガス量は0.2〜1.0Ｎｍ3／ｍｉｎ程度、槽圧は−10〜30ｋＰａ程度、操作時間は1〜100分程度とすればよい。なお、対向ノズル３から吹き込むガスとしては空気や窒素、水蒸気などを用いれば良く、また槽１内の温度は0〜60℃程度とすれば良い。
【００２３】
槽１内では気体の対流が起こり、槽１の下部側の衝突域で互いに衝突して球状化した粒子は、槽１内の対流に沿って上部側へ吹き上げられ、その後再び沈降する。すなわち、粒子は槽１の中心部近傍で吹き上げられ、槽１の壁際に沿って降下して、槽１内に循環流動が起こる。
【００２４】
槽１の上部には、分級機４を設けることで分級限界以下の微粉を槽１外に排出できる。分級機４は、公知のものを設ければ良いが、高速回転分級機を用いるのが通常である。このときの排出量は、原料として用いる鱗片状黒鉛の粒度によって異なる。
【００２５】
上記の操作はバッチで行なうことが好ましく、槽１の底部に設けられた吹き上げノズル５から槽１内へ空気を送り込むと球状化黒鉛粒子をフィーダー２から回収できる。
【００２６】
なお、球状化黒鉛粒子の原料としては、鱗片状の天然黒鉛や人造黒鉛を使用することができ、例えば、鱗片状天然黒鉛は、一般に85％から99％を上まわる純度で入手できるのでそのまま用いれば良い。必要に応じて、公知の方法でさらに純度を高めることも好ましい。原料となる鱗片状黒鉛の粒度には種々のものがあるが、後述する様に、鱗片状黒鉛を球状化して得られる球状化黒鉛粒子の平均粒径を10μｍ以上に制御するには、平均粒子径が10〜60μｍ程度の鱗片状黒鉛（原料）を用いるのが良い。
【００２７】
ところで、上記の様な方法によって製造された球状化黒鉛粒子を電極材料として使用した場合、得られる二次電池の初期効率は意外にも低いことが明らかになってきた。そこで、二次電池の初期効率が低下する原因について追求したところ、球状化黒鉛粒子の表面に多量の官能基が存在すると、該官能基が初回充電時に電解液と反応して初期効率の低下を引き起こすことをつきとめた。そして、この官能基と電解液の反応についてさらに検討したところ、官能基の中でも特に酸性官能基が初期効率の低下に大きく影響を及ぼすことが分かった。すなわち、酸性官能基とは、カルボキシル基やフェノール基、ラクトン基、カルボニル基などの官能基を指し、これらの官能基は電解液と特に反応し易いことが判明した。
【００２８】
また、鱗片状黒鉛を粉砕後再凝集させて球状化した黒鉛粒子を電極材料として使用した場合、得られる二次電池のサイクル特性も意外に低い。この原因は、先にも説明した如く、鱗片状黒鉛を凝集させて球状化することにより、粒子同士の接点が減少して密着性が悪くなり、電極自体の導電性が低下するためと思われる。
【００２９】
そこで本発明者らは、球状化黒鉛粒子表面に存在する酸性官能基量を所定量以下に抑制すると共に、黒鉛粒子同士の接点数を増やすことができれば、二次電池としての初期効率やサイクル特性を大幅に改善できるのではないかと考え、その線に沿って研究を進めた。その結果、球状化黒鉛粒子の有する酸性官能基量を所定量以下に抑制して該黒鉛粒子を不活性化すると共に、この球状化黒鉛粒子よりも相対的に微細で、且つ、該球状化黒鉛粒子とは異なる導電性炭素質微粒子を混合してやれば、二次電池の初期効率およびサイクル特性が共に大幅に向上することをつきとめた。
【００３０】
すなわち、球状化黒鉛粒子に、該球状化黒鉛粒子よりも相対的に微細な導電性炭素質微粒子を混合すると、粒子同士の接点が増えるので密着性が良好となり、導電率が高くなりサイクル特性が向上する。しかし、導電性炭素質微粒子自体も後述する如く酸性官能基を有しているので、該導電性炭素質微粒子と球状化黒鉛粒子を混合すると、混合粉全体の酸性官能基量が多くなって初期効率の低下を招く。そこで、球状化黒鉛粒子を導電性炭素質微粒子と混合して使用する際に、混合粉全体としての酸性官能基量を低レベルに抑えるには、併用される導電性炭素質微粒子自身が有する酸性官能基量も考慮に入れて、球状化黒鉛粒子の有する酸性官能基量を少なめに抑えておく必要がある。なお、本発明において「導電性炭素質微粒子」とは、鱗片状黒鉛を粉砕した後、再凝集させた球状化黒鉛粒子以外の粒子を指す（詳細は後述する）。
【００３１】
但し、導電性炭素質微粒子の使用量はその種類にもよるが、後述する如く20％程度までと比較的少なく、且つその中に含まれる酸性官能基量も相対的に少ないので混合粉全体としての酸性官能基量は、主として球状化黒鉛粒子の酸性官能基量に支配される。
【００３２】
この様な理由から本発明では、球状化黒鉛粒子の有する酸性官能基量を1.0ミリ当量／ｋｇ以下に抑制することが重要となる。すなわち、球状化黒鉛粒子の表面に存在する酸性官能基量が1.0ミリ当量／ｋｇ以下であれば、この球状化黒鉛粒子を導電性炭素質微粒子と混合した混合粉を二次電池用電極用の材料として使用しても、混合粉全体としての酸性官能基量は2.5ミリ当量／ｋｇ程度以下の十分な低レベルに抑えられる。その結果、初回の充電時における酸性官能基と電解液の反応が抑制され、初期効率を高めることができる。球状化黒鉛粒子の表面に存在する酸性官能基量は、好ましくは0.5ミリ当量／ｋｇ以下に抑制するのが望ましく、混合粉全体としての酸性官能基量は、好ましくは1.5ミリ当量／ｋｇ以下に抑制するのが望ましい。なお、当量とは、反応したイオンのｍｏｌ数を意味する。
【００３３】
球状化黒鉛粒子の表面に存在する酸性官能基量を所望量以下に抑制する方法は特に限定されないが、例えば、球状化黒鉛粒子を非酸化性雰囲気下で加熱すれば、黒鉛粒子表面に存在する酸性官能基を容易に分解除去できる。非酸化性雰囲気としては、不活性ガスが好ましく、例えば、ＡｒやＮ2、Ｈｅなどのガスを好適に採用できる。球状化黒鉛粒子を加熱するときの温度は、酸性官能基を分解除去できる温度であれば特に限定されないが、例えば600℃以上、好ましくは800℃以上とすれば良い。
【００３４】
酸性官能基量を定量する手段としては、例えば、Boehmらが提案する下記の方法が挙げられる。
【００３５】
＜Boehmらの方法＞
球状化黒鉛粒子10ｇと0.01ｍｏｌ／ＬのＣ2Ｈ5ＯＮａ水溶液50ｇをフラスコ中で2時間攪拌後、22時間室温で静置する。静置後、さらに30分間攪拌してから濾過し、濾液を回収する。回収した濾液25ｍＬを0.01ｍｏｌ／ＬのＨＣｌ水溶液で中和滴定し、ｐＨが4.0に到達するまでに要するＨＣｌ水溶液量（ｍＬ）を測定する。該ＨＣｌ水溶液量と下記（１）式から酸性官能基量（ミリ当量／ｋｇ）を算出する。
酸性官能基量＝（25−ＨＣｌ水溶液量）×2 ・・・（１）
【００３６】
本発明では、上記の様に酸性官能基量を所定量以下に抑えた球状化黒鉛粒子の使用に加えて、該球状化黒鉛粒子より相対的に微細で、且つ、該球状化黒鉛粒子とは異なる導電性炭素質微粒子を混合使用することが重要である。
【００３７】
本発明において導電性炭素質微粒子とは、炭素質の微粒子であって、導電性を示すものを指し、例えば、鱗片状黒鉛微粒子やカーボンブラック、繊維状黒鉛微粒子などが挙げられる。本発明では、特に鱗片状黒鉛微粒子とカーボンブラックを好適に採用でき、夫々単独で使用し得る他、これらを任意の割合で混合したものを用いても良い。但し、本発明における導電性炭素質微粒子には、鱗片状黒鉛を粉砕した後、再凝集させて球状化した球状化黒鉛粒子を含まない。
【００３８】
なお、導電性微粒子として、例えば金属製の微粒子を使用することも考えられるが、不活性な金属製微粒子では電池容量の低下、活性な金属製微粒子では金属自身のサイクル劣化という新たな問題が生じるので、本発明では導電性炭素質微粒子を採用している。
【００３９】
球状化黒鉛粒子と導電性炭素質微粒子は、例えば、走査型電子顕微鏡を用いて600倍で撮影した電子顕微鏡写真を観察することにより容易に区別できる。
【００４０】
鱗片状黒鉛微粒子を導電性炭素質微粒子として球状化黒鉛粒子に混合する場合は、球状化黒鉛粒子に対して1〜20％混合するのが好ましい。1％以上混合すると、粒子間の接点が増大し、サイクル特性向上の目的が有効に発揮されるからである。より好ましくは3％以上混合するのが良い。しかし、混合量が20％を超えると粒子径の小さい微粒子が多くなるので、表面積が大きくなり過ぎ、安全性の低下という問題を生じたり、この混合粉を電極材料として用いたときに、電極自体が密になり過ぎて粒子間の空隙が減少し、電極内部への電解液の通液性が悪くなってサイクル特性の劣化を招く。また、鱗片状黒鉛微粒子自体も酸性官能基を有しているので、混合量が20％を超えると混合粉全体の酸性官能基量が増え、初期効率の低下原因となる。より好ましくは混合量を15％以下、さらに好ましくは混合量を10％以下にすれば良い。
【００４１】
カーボンブラックを導電性炭素質微粒子として球状化黒鉛粒子に混合する場合は、球状化黒鉛粒子に対して0.1〜2.0％混合するのが好ましい。カーボンブラックは、鱗片状黒鉛微粒子と比べて粒子径が小さいので、鱗片状黒鉛微粒子と較べるとその混合量は少なく0.1％以上混合することで所期の効果を得ることができる。より好ましくは0.3％以上混合するのが良い。但し、混合量が多過ぎると電極の結着性の低下という不具合や酸性官能基量の増加による初期効率の低下を生じるので、混合量は2.0％以下、より好ましくは1.5％以下、さらに好ましくは1.0％以下とするのが望ましい。
【００４２】
前記鱗片状黒鉛微粒子とカーボンブラックは、夫々単独で球状化黒鉛粒子と混合使用し得るほか、任意の割合で混合したものを球状化黒鉛粒子と混ぜて併用してもよい。このとき、導電性炭素質微粒子として、鱗片状黒鉛微粒子とカーボンブラックを併用して用いる場合は、該混合導電性炭素質微粒子と球状化黒鉛粒子を混合した混合粉全体の酸性官能基量が、2.5ミリ当量／ｋｇ以下となる様に前記混合導電性炭素質微粒子を任意の割合で混ぜるのが望ましい。
【００４３】
導電性炭素質微粒子の大きさは、上記球状化黒鉛粒子よりも相対的に小さいものであれば良い。球状化黒鉛粒子よりも相対的に小さい導電性炭素質微粒子を用いる理由は、相対的に大きな球状化黒鉛粒子の間に、相対的に小さな導電性炭素質微粒子を介在させることで粒子同士の接点が増し、導電率が高まるからである。ここで「相対的に」としたのは、球状化黒鉛粒子と導電性炭素質微粒子の平均粒子径を比較したときに、導電性炭素質微粒子の方が小さいものであれば良いという趣旨である。具体的には、球状化黒鉛粒子と導電性炭素質微粒子の平均粒子径の差が1μｍ程度以上、より好ましくは5μｍ程度以上、さらに好ましくは10μｍ程度以上とすれば良い。
【００４４】
本発明では、平均粒子径が10μｍ以上の球状化黒鉛粒子と、平均粒子径が10μｍ未満（0μｍを含まない）の導電性炭素質微粒子を混合使用するのが好ましい。平均粒子径が10μｍ以上の球状化黒鉛粒子を使用することで、該粒子によって構成される電極内部への電解液の通液性が良くなり、電極での反応効率が高まるからである。より好ましくは平均粒子径が15μｍ以上、さらに好ましくは平均粒子径が20μｍ以上の球状化黒鉛粒子を用いるのが望ましい。球状化黒鉛粒子の平均粒子径の上限は特に限定されないが、球状化黒鉛粒子が粗大になり過ぎると導電性炭素質微粒子と混合しても粒子同士の密着性が悪くなりサイクル効率が低下するので、上限は40μｍ程度とするのが良い。一方、導電性炭素質微粒子は、平均粒子径が10μｍ未満のものを用いるのが好ましい。球状化黒鉛粒子間に効率良く侵入し、接点増大による導電率向上効果を有効に発揮させるためである。導電性炭素質微粒子のより好ましい平均粒子径は6μｍ以下である。導電性炭素質微粒子の平均粒子径の下限は特に限定されないが、導電性炭素質微粒子の平均粒子径が小さ過ぎると粒子同士の接点増加に寄与し得なくなるので、下限は0.5μｍ程度にするのが良い。
【００４５】
本発明では、球状化黒鉛粒子と導電性炭素質微粒子を上記要件を満足する様に混合したものを、種々の二次電池用電極材料として用いることができるが、非水系の二次電池用電極材料として用いるのが好適である。非水系の二次電池としては、リチウムイオン二次電池などが挙げられる。
【００４６】
本発明の二次電池用電極材料を用いて電極を作成する際には、バインダーと混合して成形するのが一般的であり、得られた電極は、種々の二次電池用の電極として用いることができる。二次電池としては種々のものがあるが、本発明の二次電池用電極は、非水系の二次電池用電極として好適である。非水系の二次電池にも種々のものがあるが、本発明の二次電池用電極は、黒鉛は多量のリチウムイオンを吸蔵できるので、リチウムイオン二次電池の負極として特に優れた性能を発揮する。
【００４７】
本発明の二次電池用電極を負極として構成されるリチウムイオン二次電池の負極材料としては、本発明の球状化黒鉛粒子の他に、バインダーとして例えばカルボキシメチルセルロースやスチレンブタジエンゴム、ポリフッ化ビニリデン（ＰＶｄＦ）、ポリテトラフルオロエチレンなどを混合し、負極を作成すれば良い。
【００４８】
リチウムイオン二次電池における正極材料としては、例えば、ＬｉＣｏＯ2やＬｉＮｉＯ2、ＬｉＮｉ1-yＣｏyＯ2、ＬｉＭｎＯ2、ＬｉＭｎ2Ｏ4、ＬｉＦｅＯ2などが用いられる。正極のバインダーとしては、例えば、ポリフッ化ビニリデン（ＰＶｄＦ）やポリ四フッ化エチレン（ＰＴＦＥ）などを採用できる。
【００４９】
リチウムイオン二次電池における電解液としては、例えば、エチレンカーボネート（ＥＣ）などの有機溶媒や、該有機溶媒とジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、1，2−ジメトキシエタン、1，2−ジエトキシメタン、エトキシメトキシエタンなどの低沸点溶媒との混合溶媒に、ＬｉＰＦ6やＬｉＢＦ4、ＬｉＣｌＯ4、ＬｉＣＦ3ＳＯ3、ＬｉＡｓＦ6などの電解液溶質（電解質塩）を溶解した溶液が用いられる。
【００５０】
リチウムイオン二次電池におけるセパレータとしては、例えば、ポリエチレンやポリプロピレンなどのポリオレフィンを主成分とした不織布、クロス、微孔フィルム等が用いられる。
【００５１】
【実施例】
以下、本発明を実施例によって更に詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００５２】
下記実験例１〜７で得られた球状化黒鉛粒子または混合粉を用いてリチウムイオン二次電池の負極を作製した。
【００５３】
実験例１
中国産鱗片状天然黒鉛（平均粒子径：25μｍ、純度：99％以上）をホソカワミクロン社製カウンタージェットミル「100ＡＦＧ」を用いて粉砕すると共に再凝集させて球状化した。
【００５４】
図１は、ホソカワミクロン社製カウンタージェットミル「100ＡＦＧ」（球状化黒鉛粒子を製造する装置）の概略説明図である。槽１の内部は円筒状であり、槽１の下部側には三個の対向ノズル３（ノズル内径：2.5ｍｍ）が中心を向く様に対向して配置されている。槽１の頂部には分級機４の一例として高速回転分級機が配置されている。高速回転分級機の回転数は20000ｒｐｍである。フィーダー２は槽１の側壁に設けられており、槽１の底部には吹き上げノズル５が設けられている。なお、図１では、対向ノズルを一個のみ図示した。
【００５５】
前記鱗片状天然黒鉛200ｇをフィーダー２から導入し、次に示す条件で球状化した。球状化条件は、対向ノズル３のノズル吐出空気圧：0.13ＭＰａ、操作時間：10分間、槽１内温度：25℃であり、球状化は大気雰囲気下で行なった。得られた球状化黒鉛粒子の平均粒径は28μｍであった。
【００５６】
実験例２
実験例１で得た球状化黒鉛粒子に、中国産鱗片状黒鉛微粉（平均粒子径：6μｍ）を3％混ぜて混合粉を得た。
【００５７】
実験例３
実験例１で得た球状化黒鉛粒子を、窒素雰囲気下で1200℃、2時間熱処理した。熱処理した球状化黒鉛粒子を、以下「熱処理後の球状化黒鉛粒子」と称する。
【００５８】
実験例４
実験例１で得た球状化黒鉛粒子を、窒素雰囲気下で1200℃、2時間熱処理した。熱処理後の球状化黒鉛粒子に、中国産鱗片状黒鉛微粉（平均粒子径：6μｍ）を3％混ぜて混合粉を得た。
【００５９】
実験例５
実験例１で得た球状化黒鉛粒子を、窒素雰囲気下で1200℃、2時間熱処理した。熱処理後の球状化黒鉛粒子に、中国産鱗片状黒鉛微粉（平均粒子径：6μｍ）を5％混ぜて混合粉を得た。
【００６０】
実験例６
実験例１で得た球状化黒鉛粒子を、窒素雰囲気下で1200℃、2時間熱処理した。熱処理後の球状化黒鉛粒子に、三菱化学社製カーボンブラック（商品名「３０５０Ｂ」、平均粒子径：0.04μｍ）を0.5％混ぜて混合粉を得た。
【００６１】
実験例７
実験例１で得た球状化黒鉛粒子を、窒素雰囲気下で1200℃、2時間熱処理した。熱処理後の球状化黒鉛粒子に、中国産鱗片状黒鉛微粉（平均粒子径：6μｍ）を3％、三菱化学社製カーボンブラック（商品名「３０５０Ｂ」、平均粒子径：0.04μｍ）を0.5％混ぜて混合粉を得た。
【００６２】
上記実験例１〜７で得られた球状化黒鉛粒子（または熱処理後の球状化黒鉛粒子）の酸性官能基量を、前述したBoehmらの方法によって測定した。また、（熱処理後の）球状化黒鉛粒子と、鱗片状黒鉛微粉および／またはカーボンブラックと混合した混合粉の酸性官能基量を同様に測定し、表１に混合粉全体の酸性官能基量を示した。
【００６３】
【表１】
上記実験例１〜７で得られた球状化黒鉛粒子または混合粉を二次電池用電極材料として用いて、コイン型のリチウムイオン二次電池を作製し、負極の性能として初期効率とサイクル特性を評価した。
【００６５】
リチウムイオン二次電池（コイン型）用の負極は、次に示す様に作成した。上記実験例１〜７で得られた球状化黒鉛粒子（または混合粉）100質量部に対して、バインダーとしてカルボキシメチルセルロース（ダイセル化学社製、商品名：ＣＭＣ−１１５０）1質量部およびスチレンブタジエンゴム粉末（日本合成ゴム社製）1質量部を混合し、これに純水100質量部を加えてスラリー状にした。得られたスラリーを厚さ18μｍの銅箔上に塗布し、乾燥機（100℃）で1時間乾燥した。乾燥後の膜を直径1.6ｃｍの円形に打ち抜いたのち、銅箔を除く塗布量を測定すると20ｍｇであった。この膜をローラープレス機で、銅箔上に塗布した塗布物の密度が1.6ｇ／ｃｃとなるようにプレスしてリチウムイオン二次電池用の負極を作製した。
【００６６】
リチウムイオン二次電池（コイン型）用の正極は、初期効率を算出するために作製するリチウムイオン二次電池用の正極としてはリチウム箔を用い、サイクル特性を算出するために作製するリチウムイオン二次電池用の正極としてはＬｉＣｏＯ2を活物質とする電極を用いた。ＬｉＣｏＯ2を活物質とする電極は、次に示す方法で作成した。
【００６７】
ＬｉＣｏＯ290質量部に対して、バインダーとしてポリフッ化ビニリデン12％溶液（ＰＶｄＦ、クレハ化学社製、商品名：ＫＦＰ−１３２０）41.7質量部、導電材としてケッチェンブラック（ケッチェンブラックインターナショナル社製、商品名：ケッチェンブラック）5質量部を夫々混合し、これにＮ−メチル−２−ピロリドン（三菱化学社製）200質量部を加えてスラリー状にする。得られたスラリーを厚さ30μｍのアルミ箔上に塗布し、乾燥機（100℃）で1時間乾燥した。乾燥後の膜を直径1.6ｃｍの円形に打ち抜いたのち、アルミ箔を除く塗布量を測定すると45ｍｇであった。この膜をローラープレス機で、アルミ箔上に塗布した塗布物の密度が2.8ｇ／ｃｃとなるようにプレスしてリチウムイオン二次電池用の正極を作製した。
【００６８】
負極と正極を、セパレータを介して対向させ、ステンレス製セルに組み込み電池を作製した。電解液としては、エチレンカーボネート（ＥＣ）とジメチルカーボネート（ＤＭＣ）を容積比1：1で混合した溶媒に、ＬｉＰＦ6を1Ｍの割合で溶解したもの（三菱化学社製、商品名：ソルライト）を用いた。セパレータはＣｅｌｇａｒｄ社製の「セルガード＃３５０１（商品名）」を用いた。なお、電池の組み立てはアルゴンガス雰囲気下で行なった。
【００６９】
負極の性能を評価するために電池の初期効率を算出した。電池の充電は、電流値0.74ｍＡで0Ｖまで充電した後、0Ｖの定電位で電流値が0.2ｍＡとなるまで行なった。電池の放電は、電流値0.74ｍＡで1.0Ｖになるまで行なった。一回目の充電容量と放電容量から下記（２）式で計算した。算出結果を表１に示す。なお、電池の正極はリチウム箔である。
【００７０】
【数１】
また、負極の性能を評価するために電池のサイクル特性を算出した。電池の充電は、電流値6.4ｍＡで4.2Ｖまで充電した後、4.2Ｖの定電圧で電流値が0.2ｍＡとなるまで行なった。電池の放電は、電流値6.4ｍＡで3.0Ｖとなるまで行なった。サイクル特性は、1サイクル目の放電容量と充放電を200サイクル繰り返したときの放電容量から下記（３）式で算出した。算出結果を表１に併せて示す。なお、電池の正極はＬｉＣｏＯ2を活物質とする電極である。
【００７２】
【数２】
表１から明らかな様に、実験例１〜３は、本発明で規定する何れかの要件を満足しておらず、特にサイクル特性が悪い。一方、実験例４〜７は、本発明の要件を満足しており、初期効率は実験例３と同程度の特性を維持しつつサイクル特性が飛躍的に向上している。
【００７４】
【発明の効果】
本発明によれば、二次電池用電極材料であって、初期効率およびサイクル特性に優れた二次電池を実現するために有用な二次電池用電極材料を提供することができる。また、本発明では、該電極材料を含む二次電池用電極および該電極を用いたリチウムイオン二次電池を提供することができる。
【図面の簡単な説明】
【図１】 球状化黒鉛粒子の製造に使用される装置を例示する概略説明図である。
【符号の説明】
１ 槽 ２ フィーダー
３ 対向ノズル ４ 分級機
５ 吹き上げノズル[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrode material for a secondary battery, an electrode for a secondary battery including the electrode material, and a lithium ion secondary battery using the electrode.
[0002]
[Prior art]
Recently, with the miniaturization of electronic devices and the like, a battery serving as a power source is also required to be miniaturized. In particular, a lithium ion secondary battery has attracted attention from the viewpoint of increasing the capacity of the battery. Among lithium ion secondary batteries, those using a carbon material for the negative electrode are also useful in that a large capacity can be easily obtained and a safe and high voltage can be easily obtained. However, when a carbon material is used for the negative electrode, there is a problem that cycle characteristics are deteriorated.
[0003]
Therefore, a technique has been proposed in which an electrode in which copper fine particles are uniformly dispersed in a carbon material is used as the negative electrode, thereby increasing the utilization factor of the negative electrode and improving the cycle characteristics (see, for example, Patent Document 1). Also disclosed is a technique for increasing the charge / discharge cycle life by using an electrode made of a mixture of at least one of massive graphite and flake graphite and mesophase microsphere carbon powder as a nonaqueous electrolyte secondary battery electrode. (For example, see Patent Document 2). However, as a result of studies by the present inventors, these electrodes have a higher electric capacity required for the first charge than the electric capacity required for the discharge, and the initial charge efficiency (hereinafter referred to as “initial efficiency”). There is a problem.
[0004]
By the way, as carbon materials used for batteries, there are natural graphite and artificial graphite, but generally there are various functional groups (for example, carboxyl group, phenol group, lactone group, carbonyl group, etc.) on the surface of the carbon material. As a result, this functional group reacts with the electrolytic solution during the first charge to cause a side reaction and the charge capacity is lost, so the initial efficiency is deteriorated.
[0005]
In order to solve such a problem, a technique for reducing the amount of functional groups present on the surface of the carbon material by heat-treating the raw material of the carbon material in a vacuum or a reducing atmosphere has been proposed (for example, Patent Document 3). reference.). However, in this technique, the following problems arise because scaly graphite particles are used as the carbon material. That is, when forming an electrode, it is common to apply a slurry obtained by mixing pulverized natural graphite or artificial graphite, a solvent, and a binder (binder) to a target. When scaly graphite particles are used, the fluidity of the slurry is deteriorated and the coating workability is significantly impaired.
[0006]
As a result of repeated studies aimed at further improvement of these problems, the present inventors have found that when flaky natural graphite particles are pulverized and then re-agglomerated, the flaky natural graphite particles are inherently It was found that the slurry characteristics were improved while maintaining the advantages possessed, and that the decrease in the discharge capacity at a large discharge current value was also suppressed, and was previously proposed based on these findings (for example, see Patent Document 4). .)
[0007]
However, as pointed out in Patent Document 3, when a large amount of functional groups are present on the surface of the spheroidized graphite particles, the functional groups react with the electrolytic solution during the initial charge, causing a decrease in initial efficiency. In addition, when the flaky graphite is pulverized and re-agglomerated to make it spherical, the surface of the graphite particles becomes smooth although the slurry characteristics are improved, so that the contact between the graphite particles and between the graphite particles and the binder particles is reduced, and the conductive component Adhesiveness between the two deteriorates, and the conductivity is lowered to cause deterioration of cycle characteristics.
[0008]
[Patent Document 1]
JP-A-9-245798
[Patent Document 2]
JP 2001-236950 A
[Patent Document 3]
Japanese Unexamined Patent Publication No. 8-148185
[Patent Document 4]
Japanese Patent Laid-Open No. 11-263612
[0009]
[Problems to be solved by the invention]
The present invention has been made in view of such problems, and its object is to provide a secondary battery particularly useful for realizing a secondary battery excellent in initial efficiency and cycle characteristics as an electrode material for a secondary battery. It is providing the electrode material for secondary batteries. Another object of the present invention is to provide a secondary battery electrode including the electrode material and a lithium ion secondary battery using the electrode.
[0010]
[Means for Solving the Problems]
The electrode material for a secondary battery according to the present invention that has solved the above-mentioned problems is a material used when producing an electrode for a secondary battery, and the amount of acidic functional groups is 1.0 meq / kg or less. The spheroidized graphite particles and the conductive carbonaceous fine particles that are relatively finer than the spheroidized graphite particles and different from the spheroidized graphite particles are included in a mixed state.
[0011]
The spheroidized graphite particles are those obtained by pulverizing and re-aggregating scaly graphite, and those having an average particle size of 10 μm or more are used. At this time, the conductive carbonaceous fine particles have an average particle size of 10 μm. It is preferable to use less than (not including 0 μm).
[0012]
In the present invention, the total amount of acidic functional groups in the mixed powder obtained by mixing the spheroidized graphite particles and the conductive carbonaceous fine particles is preferably 2.5 meq / kg or less.
[0013]
As the conductive carbonaceous fine particles, flaky graphite fine particles and / or carbon black can be suitably used, and flaky graphite fine particles: 1 to 20% (meaning “mass%”; the same shall apply hereinafter) and / or carbon black. : 0.1 to 2.0% (meaning “mass%”; the same shall apply hereinafter) is desirable.
[0014]
The secondary battery electrode including the secondary battery electrode material has little reaction with the electrolytic solution at the time of the first charge, and has good conductivity, and the lithium ion secondary battery having the secondary battery electrode as a negative electrode. The secondary battery has excellent initial efficiency and cycle characteristics.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have studied from various angles in order to solve the problems as described above. As a result, the amount of acidic functional groups contained in the spheroidized particles by pulverizing and re-aggregating the scaly graphite is suppressed to a predetermined amount or less, and the particles are relatively finer than the spheroidized graphite particles, and The inventors have found that the above problems can be solved by mixing conductive carbonaceous fine particles different from the spheroidized graphite particles, and the present invention has been completed. Hereinafter, the configuration and operational effects of the present invention will be described in detail.
[0016]
The electrode material for a secondary battery of the present invention is obtained by pulverizing scaly graphite and then reaggregating it to make it spherical. This is because, as described above, graphite particles obtained by pulverizing scale-like graphite and then re-aggregating and spheroidizing them have good fluidity when mixed with a solvent or a binder.
[0017]
Here, the spherical shape means not only a true spherical shape such as a soccer ball or a tennis ball but also an elliptical shape such as a rugby ball. In the present invention, it means a circularity of about 0.86 or more. However, since the circularity is an index calculated by projecting three-dimensional graphite particles onto a two-dimensional plane, for example, when calculating the circularity of scaly natural graphite particles that are generally available, the circularity is about 0.84. The graphite particles (raw material) are two-dimensional particles, whereas the actual shape of the electrode material for a secondary battery in the present invention is three-dimensional and completely different.
[0018]
The spheroidized graphite particles can be obtained by pulverizing scaly graphite and then re-aggregating them, but the specific method for producing the spheroidized graphite particles is not particularly limited. For example, it can be produced by a method previously proposed by the present inventors (Japanese Patent Laid-Open No. 11-263612) or a method similar thereto. Hereinafter, an example of the manufacturing method will be described with reference to the drawings.
[0019]
FIG. 1 is a schematic explanatory view of an apparatus used for producing spheroidized graphite particles, wherein 1 is a tank, 2 is a feeder, 3 is a counter nozzle, 4 is a classifier, and 5 is a blowing nozzle.
[0020]
Scaly graphite (raw material) is fed into the tank 1 from a
[0021]
A counter nozzle 3 is provided on the lower side of the tank 1 through the tank wall, and a jet stream is blown from the counter nozzle 3 to form a collision area on the lower side of the tank 1. The scaly graphites that have entered the air current in the collision zone collide with each other, re-aggregate and spheroidize while being crushed.
[0022]
It is preferable to provide a plurality of counter nozzles 3 (for example, three to four). The speed of the jet stream blown from the counter nozzle 3, the amount of blown gas, the tank pressure, etc. are set so that smooth collision and flow can be achieved, and the flake graphite is spheroidized by appropriately setting the operation time. For example, the nozzle discharge pressure is about 0.01 to 0.50 MPa, and the amount of blown gas is 0.2 to 1.0 Nm.Three/ Min, the tank pressure may be about −10 to 30 kPa, and the operation time may be about 1 to 100 minutes. In addition, what is necessary is just to use air, nitrogen, water vapor | steam etc. as a gas blown from the opposing nozzle 3, and the temperature in the tank 1 should just be about 0-60 degreeC.
[0023]
Gas convection occurs in the tank 1, and the particles that collide with each other in the collision area on the lower side of the tank 1 and spheroidize are blown upward along the convection in the tank 1, and then settle again. That is, the particles are blown up in the vicinity of the center of the tank 1 and descend along the wall of the tank 1 to cause a circulating flow in the tank 1.
[0024]
By providing a classifier 4 at the top of the tank 1, fine powder below the classification limit can be discharged out of the tank 1. The classifier 4 may be a known one, but a high-speed rotation classifier is usually used. The discharge amount at this time varies depending on the particle size of the flake graphite used as a raw material.
[0025]
The above operation is preferably performed in a batch. When air is fed into the tank 1 from the blowing
[0026]
As a raw material for the spheroidized graphite particles, scaly natural graphite or artificial graphite can be used. For example, scaly natural graphite is generally used as it is available with a purity exceeding 85% to 99%. It ’s fine. If necessary, it is also preferred to further increase the purity by a known method. There are various particle sizes of the scaly graphite as a raw material. As will be described later, in order to control the average particle size of the spheroidized graphite particles obtained by spheroidizing the scaly graphite to 10 μm or more, the average particle It is preferable to use flaky graphite (raw material) having a diameter of about 10 to 60 μm.
[0027]
By the way, when the spheroidized graphite particles produced by the method as described above are used as an electrode material, it has been revealed that the initial efficiency of the obtained secondary battery is unexpectedly low. Therefore, when the cause of the decrease in the initial efficiency of the secondary battery was pursued, if a large amount of functional groups exist on the surface of the spheroidized graphite particles, the functional groups react with the electrolyte during the initial charge, and the initial efficiency is decreased. I have found out what causes it. Further examination of the reaction between the functional group and the electrolytic solution revealed that the acidic functional group among the functional groups has a great influence on the decrease in initial efficiency. That is, the acidic functional group refers to a functional group such as a carboxyl group, a phenol group, a lactone group, or a carbonyl group, and it has been found that these functional groups are particularly easy to react with the electrolytic solution.
[0028]
In addition, when graphite particles obtained by pulverizing and re-aggregating scaly graphite and spheroidizing are used as electrode materials, the cycle characteristics of the obtained secondary battery are also surprisingly low. The reason for this is that, as explained above, the flake graphite is agglomerated and spheroidized to reduce the contact between the particles, resulting in poor adhesion and lowering the conductivity of the electrode itself. .
[0029]
Accordingly, the present inventors can suppress the amount of acidic functional groups present on the surface of the spheroidized graphite particles to a predetermined amount or less and increase the number of contacts between the graphite particles, so that the initial efficiency and cycle characteristics as a secondary battery can be increased. We thought that it could be improved significantly, and proceeded with research along that line. As a result, the amount of acidic functional groups contained in the spheroidized graphite particles is suppressed to a predetermined amount or less to inactivate the graphite particles, and the spheroidized graphite particles are relatively finer than the spheroidized graphite particles. It has been found that if conductive carbonaceous fine particles different from the particles are mixed, both the initial efficiency and cycle characteristics of the secondary battery are greatly improved.
[0030]
That is, when conductive carbonaceous fine particles that are relatively finer than the spheroidized graphite particles are mixed with the spheroidized graphite particles, the number of contact points between the particles increases, so that adhesion is improved, conductivity is increased, and cycle characteristics are improved. improves. However, since the conductive carbonaceous fine particles themselves also have acidic functional groups as will be described later, when the conductive carbonaceous fine particles and the spheroidized graphite particles are mixed, the amount of acidic functional groups in the entire mixed powder increases and the initial value is increased. It causes a decrease in efficiency. Therefore, when mixing the spheroidized graphite particles with the conductive carbonaceous fine particles, to reduce the amount of the acidic functional group as a whole of the mixed powder to a low level, the combined conductive carbonaceous fine particles themselves have an acidic property. In consideration of the functional group amount, it is necessary to suppress the acidic functional group amount of the spheroidized graphite particles to a small amount. In the present invention, “conductive carbonaceous fine particles” refer to particles other than spheroidized graphite particles that have been pulverized and then re-agglomerated after flaky graphite (details will be described later).
[0031]
However, although the amount of conductive carbonaceous fine particles used depends on the type, the total amount of the mixed powder is relatively small as about 20% as described later, and the amount of acidic functional groups contained therein is relatively small. The amount of acidic functional groups is mainly governed by the amount of acidic functional groups of the spheroidized graphite particles.
[0032]
For these reasons, in the present invention, it is important to suppress the amount of acidic functional groups possessed by the spheroidized graphite particles to 1.0 meq / kg or less. That is, if the amount of acidic functional groups present on the surface of the spheroidized graphite particles is 1.0 meq / kg or less, the mixed powder obtained by mixing the spheroidized graphite particles with the conductive carbonaceous fine particles is used for a secondary battery electrode. Even when used as a material, the amount of acidic functional groups as a whole of the mixed powder can be suppressed to a sufficiently low level of about 2.5 meq / kg or less. As a result, the reaction between the acidic functional group and the electrolytic solution during the first charge is suppressed, and the initial efficiency can be increased. The amount of acidic functional groups present on the surface of the spheroidized graphite particles is preferably suppressed to 0.5 meq / kg or less, and the amount of acidic functional groups as a whole of the mixed powder is preferably 1.5 meq / kg or less. It is desirable to suppress it. The equivalent means the number of moles of reacted ions.
[0033]
The method for suppressing the amount of acidic functional groups present on the surface of the spheroidized graphite particles to a desired amount or less is not particularly limited. For example, if the spheroidized graphite particles are heated in a non-oxidizing atmosphere, they exist on the surface of the graphite particles. Acidic functional groups can be easily decomposed and removed. As the non-oxidizing atmosphere, an inert gas is preferable, for example, Ar or N2A gas such as He can be preferably used. The temperature at which the spheroidized graphite particles are heated is not particularly limited as long as the acidic functional group can be decomposed and removed, but may be 600 ° C. or higher, preferably 800 ° C. or higher, for example.
[0034]
Examples of means for quantifying the amount of acidic functional groups include the following method proposed by Boehm et al.
[0035]
<Boehm's method>
10 g of spheroidized graphite particles and 0.01 mol / L C2HFive50 g of ONa aqueous solution is stirred in the flask for 2 hours and then allowed to stand at room temperature for 22 hours. After standing, the mixture is further stirred for 30 minutes and then filtered to collect the filtrate. 25 mL of the collected filtrate is neutralized and titrated with 0.01 mol / L HCl aqueous solution, and the amount of HCl aqueous solution (mL) required until pH reaches 4.0 is measured. The amount of acidic functional group (milli equivalent / kg) is calculated from the amount of the aqueous HCl and the following formula (1).
Acidic functional group amount = (25-HCl aqueous solution amount) × 2 (1)
[0036]
In the present invention, in addition to the use of spheroidized graphite particles in which the amount of acidic functional groups is suppressed to a predetermined amount or less as described above, the spheroidized graphite particles are relatively finer than the spheroidized graphite particles, and It is important to use a mixture of different conductive carbonaceous fine particles.
[0037]
In the present invention, the conductive carbonaceous fine particles are carbonaceous fine particles that exhibit conductivity, and examples thereof include flaky graphite fine particles, carbon black, and fibrous graphite fine particles. In the present invention, in particular, scaly graphite fine particles and carbon black can be suitably used, and each can be used alone, or a mixture of these in an arbitrary ratio may be used. However, the conductive carbonaceous fine particles in the present invention do not include spheroidized graphite particles obtained by pulverizing and re-aggregating flaky graphite.
[0038]
For example, metallic fine particles may be used as the conductive fine particles. However, inactive metal fine particles cause a new problem such as a decrease in battery capacity, and active metal fine particles cause cycle deterioration of the metal itself. Therefore, conductive carbonaceous fine particles are employed in the present invention.
[0039]
The spheroidized graphite particles and the conductive carbonaceous fine particles can be easily distinguished from each other by observing, for example, an electron micrograph taken at 600 times using a scanning electron microscope.
[0040]
When the flaky graphite fine particles are mixed with the spheroidized graphite particles as conductive carbonaceous fine particles, it is preferable to mix 1 to 20% with respect to the spheroidized graphite particles. This is because when the content is 1% or more, the contact between the particles increases, and the purpose of improving the cycle characteristics is effectively exhibited. More preferably, 3% or more is mixed. However, if the mixing amount exceeds 20%, the number of fine particles having a small particle diameter increases, so that the surface area becomes too large, resulting in a problem of reduced safety, or when this mixed powder is used as an electrode material, the electrode itself Becomes too dense and voids between the particles are reduced, and the electrolyte permeability to the inside of the electrode is deteriorated, resulting in deterioration of cycle characteristics. In addition, since the scaly graphite fine particles themselves have acidic functional groups, when the mixing amount exceeds 20%, the amount of acidic functional groups in the whole mixed powder increases, which causes a decrease in initial efficiency. The mixing amount is more preferably 15% or less, and further preferably the mixing amount is 10% or less.
[0041]
When carbon black is mixed with the spheroidized graphite particles as conductive carbonaceous fine particles, it is preferable to mix 0.1 to 2.0% with respect to the spheroidized graphite particles. Since carbon black has a smaller particle size than the flaky graphite fine particles, the mixing amount is smaller than that of the flaky graphite fine particles, and the desired effect can be obtained by mixing 0.1% or more. More preferably, 0.3% or more is mixed. However, if the mixing amount is too large, the electrode efficiency decreases due to the problem of reduced electrode binding and the amount of acidic functional groups, so the mixing amount is 2.0% or less, more preferably 1.5% or less, more preferably 1.0% or less is desirable.
[0042]
The scaly graphite fine particles and the carbon black can be used alone and mixed with the spheroidized graphite particles, respectively, or those mixed at an arbitrary ratio may be used in combination with the spheroidized graphite particles. At this time, when the scaly graphite fine particles and carbon black are used in combination as the conductive carbonaceous fine particles, the amount of acidic functional groups in the whole mixed powder obtained by mixing the mixed conductive carbonaceous fine particles and the spheroidized graphite particles is It is desirable to mix the mixed conductive carbonaceous fine particles at an arbitrary ratio so as to be 2.5 meq / kg or less.
[0043]
The size of the conductive carbonaceous fine particles may be any size that is relatively smaller than the spheroidized graphite particles. The reason for using conductive carbonaceous fine particles that are relatively smaller than the spheroidized graphite particles is to contact the particles by interposing relatively small conductive carbonaceous fine particles between the relatively large spheroidized graphite particles. This is because the conductivity increases. Here, “relatively” means that when the average particle size of the spheroidized graphite particles and the conductive carbonaceous fine particles are compared, it is sufficient that the conductive carbonaceous fine particles are smaller. . Specifically, the difference in average particle size between the spheroidized graphite particles and the conductive carbonaceous fine particles may be about 1 μm or more, more preferably about 5 μm or more, and even more preferably about 10 μm or more.
[0044]
In the present invention, it is preferable to mix and use spheroidized graphite particles having an average particle diameter of 10 μm or more and conductive carbonaceous fine particles having an average particle diameter of less than 10 μm (not including 0 μm). This is because by using spheroidized graphite particles having an average particle size of 10 μm or more, the liquid permeability of the electrolytic solution into the electrode constituted by the particles is improved, and the reaction efficiency at the electrode is increased. More preferably, it is desirable to use spheroidized graphite particles having an average particle diameter of 15 μm or more, more preferably 20 μm or more. The upper limit of the average particle diameter of the spheroidized graphite particles is not particularly limited, but if the spheroidized graphite particles become too coarse, even if they are mixed with conductive carbonaceous fine particles, the adhesion between the particles deteriorates and the cycle efficiency decreases. The upper limit is preferably about 40 μm. On the other hand, it is preferable to use conductive carbonaceous fine particles having an average particle size of less than 10 μm. This is for efficiently infiltrating between the spheroidized graphite particles and effectively exhibiting the effect of improving the conductivity by increasing the contact. A more preferable average particle diameter of the conductive carbonaceous fine particles is 6 μm or less. The lower limit of the average particle diameter of the conductive carbonaceous fine particles is not particularly limited, but if the average particle diameter of the conductive carbonaceous fine particles is too small, it cannot contribute to the increase in contact between the particles, so the lower limit is about 0.5 μm. Is good.
[0045]
In the present invention, a mixture of spheroidized graphite particles and conductive carbonaceous fine particles so as to satisfy the above requirements can be used as various secondary battery electrode materials. It is preferable to use it as a material. Examples of non-aqueous secondary batteries include lithium ion secondary batteries.
[0046]
When preparing an electrode using the electrode material for secondary battery of the present invention, it is generally mixed with a binder and molded, and the obtained electrode is used as an electrode for various secondary batteries. be able to. Although there are various types of secondary batteries, the secondary battery electrode of the present invention is suitable as a non-aqueous secondary battery electrode. There are various types of non-aqueous secondary batteries, but the secondary battery electrode of the present invention exhibits particularly excellent performance as a negative electrode for lithium ion secondary batteries because graphite can occlude a large amount of lithium ions. To do.
[0047]
In addition to the spheroidized graphite particles of the present invention, the negative electrode material of the lithium ion secondary battery configured using the secondary battery electrode of the present invention as a negative electrode includes, for example, carboxymethyl cellulose, styrene butadiene rubber, polyvinylidene fluoride ( PVdF), polytetrafluoroethylene, or the like may be mixed to produce the negative electrode.
[0048]
As a positive electrode material in a lithium ion secondary battery, for example, LiCoO2And LiNiO2, LiNi1-yCoyO2LiMnO2, LiMn2OFourLiFeO2Etc. are used. For example, polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE) can be used as the positive electrode binder.
[0049]
Examples of the electrolyte solution in the lithium ion secondary battery include an organic solvent such as ethylene carbonate (EC), the organic solvent and dimethyl carbonate (DMC), diethyl carbonate (DEC), 1,2-dimethoxyethane, and 1,2. -LiPF in a mixed solvent with a low boiling point solvent such as diethoxymethane, ethoxymethoxyethane, etc.6And LiBFFourLiClOFour, LiCFThreeSOThree, LiAsF6A solution in which an electrolyte solution solute (electrolyte salt) such as the above is dissolved is used.
[0050]
As the separator in the lithium ion secondary battery, for example, a nonwoven fabric, a cloth, a microporous film, or the like whose main component is a polyolefin such as polyethylene or polypropylene is used.
[0051]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.
[0052]
A negative electrode of a lithium ion secondary battery was produced using the spheroidized graphite particles or mixed powder obtained in Experimental Examples 1 to 7 below.
[0053]
Experimental example 1
Chinese scaly natural graphite (average particle size: 25 μm, purity: 99% or more) was pulverized and re-agglomerated using a counter jet mill “100AFG” manufactured by Hosokawa Micron.
[0054]
FIG. 1 is a schematic explanatory diagram of a counter jet mill “100AFG” (apparatus for producing spheroidized graphite particles) manufactured by Hosokawa Micron. The inside of the tank 1 has a cylindrical shape, and three opposed nozzles 3 (nozzle inner diameter: 2.5 mm) are arranged on the lower side of the tank 1 so as to face each other. As an example of the classifier 4, a high-speed rotary classifier is disposed at the top of the tank 1. The rotation speed of the high-speed rotation classifier is 20000 rpm. The
[0055]
The scaly natural graphite 200g was introduced from the
[0056]
Experimental example 2
The spheroidized graphite particles obtained in Experimental Example 1 were mixed with 3% of Chinese flake graphite powder (average particle size: 6 μm) to obtain a mixed powder.
[0057]
Experimental example 3
The spheroidized graphite particles obtained in Experimental Example 1 were heat-treated at 1200 ° C. for 2 hours in a nitrogen atmosphere. The heat-treated spheroidized graphite particles are hereinafter referred to as “spheroidized graphite particles after heat treatment”.
[0058]
Experimental Example 4
The spheroidized graphite particles obtained in Experimental Example 1 were heat-treated at 1200 ° C. for 2 hours in a nitrogen atmosphere. The spheroidized graphite particles after heat treatment were mixed with 3% of Chinese flake graphite powder (average particle size: 6 μm) to obtain a mixed powder.
[0059]
Experimental Example 5
The spheroidized graphite particles obtained in Experimental Example 1 were heat-treated at 1200 ° C. for 2 hours in a nitrogen atmosphere. The spheroidized graphite particles after heat treatment were mixed with 5% of Chinese flake graphite powder (average particle size: 6 μm) to obtain a mixed powder.
[0060]
Experimental Example 6
The spheroidized graphite particles obtained in Experimental Example 1 were heat-treated at 1200 ° C. for 2 hours in a nitrogen atmosphere. 0.5% of carbon black (trade name “3050B”, average particle size: 0.04 μm) manufactured by Mitsubishi Chemical Corporation was mixed with the spheroidized graphite particles after the heat treatment to obtain a mixed powder.
[0061]
Experimental Example 7
The spheroidized graphite particles obtained in Experimental Example 1 were heat-treated at 1200 ° C. for 2 hours in a nitrogen atmosphere. Spherical graphite particles after heat treatment are mixed with 3% Chinese flake graphite powder (average particle size: 6μm) and 0.5% carbon black (trade name “3050B”, average particle size: 0.04μm) manufactured by Mitsubishi Chemical Corporation. To obtain a mixed powder.
[0062]
The amount of acidic functional groups of the spheroidized graphite particles (or the spheroidized graphite particles after heat treatment) obtained in the above Experimental Examples 1 to 7 was measured by the method of Boehm et al. Also, the amount of acidic functional groups in the mixed powder mixed with the spheroidized graphite particles (after heat treatment) and the flaky graphite fine powder and / or carbon black was measured in the same manner. Indicated.
[0063]
[Table 1]
Using the spheroidized graphite particles or mixed powder obtained in Experimental Examples 1 to 7 as an electrode material for a secondary battery, a coin-type lithium ion secondary battery was produced, and the initial efficiency and cycle characteristics were obtained as the negative electrode performance. evaluated.
[0065]
A negative electrode for a lithium ion secondary battery (coin type) was prepared as follows. 1 part by mass of carboxymethyl cellulose (manufactured by Daicel Chemical Industries, trade name: CMC-1150) as a binder and styrene-butadiene rubber with respect to 100 parts by mass of the spheroidized graphite particles (or mixed powder) obtained in Experimental Examples 1 to 7 above. 1 part by mass of powder (manufactured by Nippon Synthetic Rubber) was mixed, and 100 parts by mass of pure water was added thereto to form a slurry. The obtained slurry was applied onto a copper foil having a thickness of 18 μm and dried for 1 hour with a dryer (100 ° C.). After the dried film was punched into a circle having a diameter of 1.6 cm, the coating amount excluding the copper foil was 20 mg. This film was pressed with a roller press so that the density of the coating applied on the copper foil was 1.6 g / cc to prepare a negative electrode for a lithium ion secondary battery.
[0066]
The positive electrode for a lithium ion secondary battery (coin type) uses a lithium foil as the positive electrode for a lithium ion secondary battery produced to calculate initial efficiency, and the lithium ion secondary produced to calculate cycle characteristics. LiCoO as a positive electrode for secondary batteries2Was used as an active material. LiCoO2The electrode using the active material was prepared by the following method.
[0067]
LiCoO290 parts by mass, 41.7 parts by mass of a 12% polyvinylidene fluoride solution (PVdF, manufactured by Kureha Chemical Co., Ltd., trade name: KFP-1320) as a binder, and Ketjen Black (made by Ketjen Black International Co., Ltd., trade name) as a conductive material : Ketjen Black) 5 parts by mass are mixed, and 200 parts by mass of N-methyl-2-pyrrolidone (Mitsubishi Chemical Corporation) is added to form a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 30 μm, and dried for 1 hour with a dryer (100 ° C.). After the dried film was punched into a circle having a diameter of 1.6 cm, the coating amount excluding the aluminum foil was 45 mg. This film was pressed with a roller press so that the density of the coating applied on the aluminum foil was 2.8 g / cc to produce a positive electrode for a lithium ion secondary battery.
[0068]
The negative electrode and the positive electrode were opposed to each other through a separator, and the battery was assembled in a stainless steel cell. As an electrolyte, LiPF was added to a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 1.6(Mitsubishi Chemical Co., Ltd., trade name: Sollite) was used. The separator used was “Celguard # 3501 (trade name)” manufactured by Celgard. The battery was assembled in an argon gas atmosphere.
[0069]
In order to evaluate the performance of the negative electrode, the initial efficiency of the battery was calculated. The battery was charged to 0 V at a current value of 0.74 mA and then until the current value reached 0.2 mA at a constant potential of 0 V. The battery was discharged until it reached 1.0 V at a current value of 0.74 mA. It calculated by the following formula (2) from the first charge capacity and discharge capacity. The calculation results are shown in Table 1. The positive electrode of the battery is a lithium foil.
[0070]
[Expression 1]
Moreover, in order to evaluate the performance of a negative electrode, the cycle characteristic of the battery was computed. The battery was charged to 4.2 V at a current value of 6.4 mA, and then charged at a constant voltage of 4.2 V until the current value reached 0.2 mA. The battery was discharged until it reached 3.0 V at a current value of 6.4 mA. The cycle characteristics were calculated by the following formula (3) from the discharge capacity at the first cycle and the discharge capacity when charging and discharging were repeated 200 cycles. The calculation results are also shown in Table 1. The positive electrode of the battery is LiCoO2It is an electrode which uses as active material.
[0072]
[Expression 2]
As is apparent from Table 1, Experimental Examples 1 to 3 do not satisfy any of the requirements defined in the present invention, and the cycle characteristics are particularly poor. On the other hand, Experimental Examples 4 to 7 satisfy the requirements of the present invention, and the cycle efficiency is dramatically improved while maintaining the same initial efficiency as that of Experimental Example 3.
[0074]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it is a secondary battery electrode material, Comprising: The secondary battery electrode material useful in order to implement | achieve the secondary battery excellent in initial efficiency and cycling characteristics can be provided. Moreover, in this invention, the electrode for secondary batteries containing this electrode material and the lithium ion secondary battery using this electrode can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view illustrating an apparatus used for producing spheroidized graphite particles.
[Explanation of symbols]
1
3 Counter nozzle 4 Classifier
5 Blow-up nozzle
Claims (6)
酸性官能基量が１．０ミリ当量／ｋｇ以下である球状化黒鉛粒子と、
前記球状化黒鉛粒子より相対的に微細で、且つ、該球状化黒鉛粒子とは異なる導電性炭素質微粒子を混合状態で含有し、
前記導電性炭素質微粒子として、球状化黒鉛粒子に対して鱗片状黒鉛微粒子：１〜１０％（「質量％」の意味。以下同じ。）および／またはカーボンブラック：０．１〜２．０％（「質量％」の意味。以下同じ。）を含むことを特徴とするリチウムイオン二次電池用負極材料。A material used when manufacturing a secondary battery electrode,
Spheroidized graphite particles having an acidic functional group amount of 1.0 meq / kg or less;
Containing conductive carbonaceous fine particles that are relatively finer than the spheroidized graphite particles and different from the spheroidized graphite particles in a mixed state;
As the conductive carbonaceous fine particles, scaly graphite fine particles: 1 to 10% (meaning “mass%”; the same applies hereinafter) and / or carbon black: 0.1 to 2.0% with respect to the spheroidized graphite particles. (The meaning of “mass%”. The same shall apply hereinafter.) A negative electrode material for a lithium ion secondary battery.
前記導電性炭素質微粒子の平均粒子径が１０μｍ未満である請求項１に記載のリチウムイオン二次電池用負極材料。The spheroidized graphite particles are obtained by pulverizing and re-aggregating scaly graphite, the average particle diameter thereof is 10 μm or more, and the average particle diameter of the conductive carbonaceous fine particles is less than 10 μm. The negative electrode material for lithium ion secondary batteries as described in 2.
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| JP4812391B2 (en) * | 2005-10-14 | 2011-11-09 | 昭和電工株式会社 | Thermally conductive resin composition, structure thereof and use thereof |
| JP4933092B2 (en) * | 2005-12-26 | 2012-05-16 | Ｊｆｅケミカル株式会社 | Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery |
| KR100817977B1 (en) * | 2007-05-03 | 2008-03-31 | 엘에스전선 주식회사 | Anode material of secondary battery and secondary battery using the same |
| JP2013016353A (en) * | 2011-07-04 | 2013-01-24 | Gs Yuasa Corp | Nonaqueous electrolyte secondary battery |
| WO2013008524A1 (en) * | 2011-07-14 | 2013-01-17 | Ｎｅｃエナジーデバイス株式会社 | Lithium-ion-battery negative electrode and lithium-ion battery |
| US9923239B2 (en) | 2011-07-14 | 2018-03-20 | Nec Energy Devices, Ltd. | Lithium ion battery |
| JP5736049B2 (en) * | 2011-08-08 | 2015-06-17 | 日立オートモティブシステムズ株式会社 | Non-aqueous electrolyte secondary battery |
| CN104169215B (en) * | 2012-03-22 | 2016-12-14 | 新日铁住金株式会社 | Composite graphite matter granule and manufacture method thereof |
| CN104919629B (en) * | 2013-01-11 | 2018-02-02 | 日本电气株式会社 | Lithium rechargeable battery |
| WO2014157318A1 (en) | 2013-03-26 | 2014-10-02 | 三菱化学株式会社 | Carbonaceous material and nonaqueous secondary cell in which same is used |
| JP6100681B2 (en) * | 2013-12-19 | 2017-03-22 | トヨタ自動車株式会社 | Non-aqueous electrolyte secondary battery electrode manufacturing method and non-aqueous electrolyte secondary battery manufacturing method |
| JP6877916B2 (en) * | 2016-08-17 | 2021-05-26 | 日鉄ケミカル＆マテリアル株式会社 | A method for producing a conductive resin composition, a conductive resin molded product, and a carbon material for the conductive resin composition. |
| KR102034013B1 (en) * | 2017-12-26 | 2019-10-18 | 주식회사 포스코 | Negative electrode for lithium secondary battery, method of preparing the same, and lithium secondaey battery using the same |
| JP7288474B2 (en) * | 2021-03-10 | 2023-06-07 | プライムプラネットエナジー＆ソリューションズ株式会社 | Method for producing non-aqueous electrolyte secondary battery, and negative electrode active material |
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