【０００１】
【発明の属する技術分野】
本発明は金属リチウムあるいはリチウムカーボン（リチウム−グラファイト）等のインターカレーション化合物を負極活物質とするリチウム二次電池において，負極活物質として使用する珪素と黒鉛を複合化した新負極材料に関するものである。本負極材料は、リチウムイオン二次電池ばかりでなく、リチウムポリマー二次電池、リチウム固体二次電池等、あらゆるリチウム二次電池に関係する。
【０００２】
【従来の技術】
携帯用電子機器の高性能化及び小型軽量化に伴い、高エネルギー密度の二次電池に対する市場のニーズは益々高まりつつある。黒鉛系カーボンとコバルト酸リチウムを用いるリチウムイオン二次電池は現在最も普通に使用されるリチウム二次電池であり、電子機器の更なる高性能化を支援するには更に高エネルギー密度で長寿命の二次電池が求められている。
現在までのリチウムイオン電池の容量増加は、負極カーボンの特性改善に負ってきた。正極材料は最大でも３００ｍＡｈ／ｇを越える材料は見あたらないのに対し、負極材料には現在主に使用されている黒鉛系負極の２倍以上の容量を有する合金系材料も知られている。よく知られている負極材料である金属リチウムは４０００ｍＡｈ／ｇを有するものの、溶解析出型の反応であるためデンドライト状析出がおき、サイクル寿命や安全性の面で実用化にはほど遠いのが現状であろう。
合金系の負極材料は、充電に際して合金形成が進むため、膨張を伴うものの金属リチウム負極のような充電生成物の析出がないため、疑似インターカレーション的であり安全性は金属リチウムよりも圧倒的に高い。しかしながら、従来の技術では充放電時の大きな膨張・収縮に耐えられる電極の作成が難しく、サイクル特性の良好な合金系負極の開発には至っていない。
【０００３】
【発明が解決しようとする課題】
合金系の負極材料にはリチウム錫、リチウムアルミニウム、リチウムアンチモン、リチウムビスマス、リチウム鉛、リチウム珪素等の合金系が着目され、リチウム二次電池負極への応用が試みられてきた。これらの合金系材料は合金自体を負極材料として用いることができるが、多くの場合取扱いの簡便さから金属あるいは半金属の状態で電池を作成し充電により電気化学反応に基づき合金化させる。このようにして作成した合金系負極は黒鉛負極に比べると格段に大きな初期容量を示すものの合金化に伴い活物質の体積が数倍にも増大する。充放電を繰り返すと活物質はこの大きな膨張・収縮に伴い発生する応力に抗しきれず崩壊し、サイクル特性劣化を引き起こす。
本発明は、合金系負極として高容量が期待できる半金属の珪素を主要負極活物質とし、電極活物質構成を工夫することによりサイクル特性およびレート特性の改善をはかった。
本発明の複合化材料では、１μｍ以下に微粉砕された珪素粒子と導電性を有する微粉砕された黒鉛（または黒鉛と銀などの金属粒子）を均一分散させた状態が達成されている。主要活物質である珪素は、表面積が大きく、粒子径が小さいため、実質的な電極反応の生じる界面が増大し、リチウムの拡散パスも短くなるため珪素負極の欠点の一つであるレート特性の悪さが克服できる。また、良好なレート特性は体積膨張の大きなＬｉ４．４Ｓｉの局所的な生成を抑制することにも繋がり、サイクル特性も改善されることとなる。
【０００４】
【問題点を解決するための手段】
珪素系負極のサイクル劣化の原因は、充放電に伴う体積の膨張・収縮により派生する応力による合金粒子の微細化である。珪素負極は充電時、リチウムと合金化し数倍にも体積が膨張する。この膨張が電極全体の膨張につながれば、負極の集電体からの剥離や電池内空隙への負極の逸脱にもつながりかねない。珪素負極の容量は大きいものの、充放電に際し格子の大きな膨張収縮を伴い、サイクル特性が格段に劣るのに加え、導電性が低いためレート特性に劣るという問題点を抱えている。
図１の相図に示すように珪素／リチウム合金組成としては、次の四種が知られている（″Ａｌｌｏｙ Ｐｈａｓｅ Ｄｉａｇｒａｍｓ（ｖｏｌ．３）ｅｄ．ｂｙ Ｈ．Ｂａｋｅｒ，ＡＳＭ Ｉｎｔｅｒｎａｔｉｏｎａｌ社（１９９２），ｐａｇｅ２−２７８）。即ち、Ｌｉ４．４Ｓｉ（Ｌｉ２２Ｓｉ５），Ｌｉ３．２５Ｓｉ（Ｌｉ１３Ｓｉ４），Ｌｉ２．３Ｓｉ（Ｌｉ７Ｓｉ３）およびＬｉ１．７Ｓｉ（Ｌｉ１２Ｓｉ７）である。珪素格子体積の膨張はＬｉ２．３ＳｉあるいはＬｉ１．７Ｓｉまでは大きくなく、約１５％程度の上昇に留まるが、Ｌｉ４．４Ｓｉまでリチウムを挿入すると、４００％の格子体積の上昇となる。従ってＬｉ２ ．３Ｓｉまででリチウムの挿入を止めれば、サイクル特性は向上するはずである。
しかしながら珪素格子内部におけるリチウム（又はリチウムイオン）の拡散は非常に遅く、また拡散速度も均一でないので、珪素負極剤を充電すると、各種組成のＬｉｘＳｉ（０＜ｘ≦４．４）の混合物が粒子内部で生成する。 特にＬｉ４．４Ｓｉが局部的に生成すると、珪素格子の体積膨張率が大きく、珪素粒子が破壊され、サイクル特性を異常に劣化させる。
【０００５】
珪素粒子内部まで均一にＬｉ２．３ＳｉあるいはＬｉ１．７Ｓｉの組成を達成するためには、珪素内部でのリチウムの拡散パスを短くする必要がある。この一つの手段として多孔体である珪素粒子内部までナノサイズの導電性炭素で被覆する方法が開発された。すなわち気相合成法（ＣＶＤ）による炭素で被覆する方法がある。ここでは数μｍオーダーの珪素粒子をＣＶＤ炭素で被覆した負極材料（特許公開公報２０００−２１５８８７）として企業化させている。 第二の方法としては珪素蒸気と炭素源となる化合物を反応させ炭素中に珪素をナノレベルで分散させた負極材料（Ｗｉｌｓｏｎ，Ｊ．Ａｐｐｌ．Ｐｈｙｓ．７７，１５ １９９５）が、黒鉛の理論容量以上の高い負極容量を有するとして報告されている。サイクル特性も比較的良好である。しかし本方法はコスト的に非常に高価であり、工業的生産法としては実現性に乏しい。
【０００６】
リチウムの拡散パスを短くする方法として、メカニカルアロイイングの手法を用いる。メカニカルアロイイングの手法により珪素を粉砕すれば表面積も増大し、均一なＬｉ／Ｓｉ合金組成の生成も可能となり、充放電容量を制御することによりＬｉ２．３Ｓｉまでの合金組成を維持することに成功した。Ｌｉ３．２５Ｓｉまでの合金組成でも充放電中Ｌｉ４．４Ｓｉを生成することなく均一な組成のＬｉ／Ｓｉ合金を生成する可能性もある。本発明では、黒鉛例えば天然黒鉛、あるいは２５００℃以上で焼成合成した人造黒鉛が珪素の粉砕用助剤として有効であることを見いだした。また、導電性金属を負極中に分散させれば、負極自体の導電性が向上し、レート特性改善、容量増加にも寄与することとなる。
【０００７】
本発明はリチウム珪素合金の有する大きな放電容量と改善されたサイクル特性及びレート特性を有する珪素−黒鉛あるいは珪素−黒鉛−金属複合剤を工業規模で生産する方法を提供するものである。本材料は基本的には、主にサブミクロンオーダーの珪素と炭素粒子が均一に分散した複合体からなる活物質である。粒径は連続的な分布を有し、両者に明瞭に判別できるものではない。しかし、一部粉砕不十分の２−３μｍのシリコン粒子、金属成分も残存している。
【０００８】
以下、本発明の内容を詳細に説明する。
本発明に用いられる珪素は結晶質、非晶質を問わないが、好ましくは非晶質珪素が用いられる。また、珪素の純度も９７％程度で十分である。 原料に用いる珪素粒子径は特に制限はないが、平均粒径２０−３０μｍ以下が良く、数μｍオーダーが望ましい。珪素粒子と黒鉛粒子をメカニカルアロイイングの手法で粉砕する事により、１μｍ以下の複合体の形成が容易となる。
本発明に用いる黒鉛材料の主役割は、珪素の粉砕用助剤である。またその導電性を生かし、負極への導電性の付与と活物質としての機能も有する。黒鉛材料は炭素材料の中でも硬度が高く粉砕助剤として適している。 また導電性が高く、負極活物質としての容量が大きく、レート特性を優れるなど、電池活物質の特性も兼ねているので、本複合体用の炭素材料として最適である。黒鉛材料はメカニカルアロイイングの手法により珪素とともに粉砕され、両者が均一に分散した複合体を形成する必要がある。そのため粉砕助剤としての機能を有しないカーボン（１０００℃前後で焼成した炭素）は望ましくない。従って本発明に用いられる黒鉛剤に適するものとして天然黒鉛、あるいは球状天然黒鉛、２５００℃以上で焼成して得た人造黒鉛などが挙げられる。
【０００９】
複合化に際しての黒鉛剤の使用量は重量ベースで珪素１部に対し、０．４〜２．５部が望ましい。黒鉛の割合が珪素１部に対し０．４部以下になると、粉砕助剤として不十分で、珪素を充分粉砕する事が困難である。 一方、黒鉛の割合が２．５部以上になると、粉砕量としては充分であるが、負極材としての容量の低下を引き起こすことになる。
複合体の導電性がおとる場合は、複合体製造後炭素繊維等の繊維状炭素剤や金属粉を加えることができる。金属粉を加える場合は、複合化の際、加える方が金属粒子の良好な分散がはかれるため優れた電池特性を示すこととなる。
【００１０】
珪素は理論上充電量１６４０ｍＡｈ／ｇ−ＳｉまではＬｉ１．７１Ｓｉが、２２３０ｍＡｈ／ｇ−ＳｉまではＬｉ２．３３Ｓｉが、３１００ｍＡｈ／ｇ−ＳｉまではＬｉ３．２５Ｓｉが生成する。Ｌｉ４．４Ｓｉは充電量４２００ｍＡｈ／ｇ−Ｓｉに相当する。珪素の体積膨張を防止するため、珪素１ｇあたり２２３０ｍＡｈを越えない充電容量とすることが望ましい。このためには定電流定容量充電法、定電流定電圧充電法などを用いて、この充電量を超えないようにする事が望ましい。ただし、１回目の充電においては、製造時に混入したと思われる酸素成分により、２００−３００ｍＡｈ／ｇの不可逆容量があるので、２２３０−ｍＡｈ／ｇ−Ｓｉ以上となっても良い。
しかしサイクル特性を重視せず、大きな充放電容量を利用する場合は、これ以上の容量を用いても良い。
本発明においては１回目の充電量を１２００ｍＡｈ／ｇ−複合体、２回目以降の充電は１００ｍＶまでの定電流定電圧充電法、または２回目以降８００ｍＡｈ／ｇ−複合体の定電流定容量充放電法を採用したが、充放電法は特にこれらの方法に制限される物ではない。一般的には、珪素１ｇあたり２２３０ｍＡｈを越えない充電方法であればよい。
複合体中の黒鉛の重量％が５０ｗｔ％の場合は、充電容量８００ｍＡｈ／ｇ−複合体 は珪素１ｇあたり１６００ｍＡｈ／ｇ−Ｓｉとなる。初回の充電容量１２００ｍＡｈ／ｇ−複合体（即ち、２４００ｍＡｈ／ｇ−Ｓｉ）を採用した理由は、３００ｍＡｈ／ｇ−複合体 程度の不可逆容量があり、これは原料珪素中あるいは合成中に導入された酸化物によるリチウムの消耗、すなわち初回充電時におけるＬｉ２Ｏの生成に起因すると判断している。 従って珪素１ｇあたり２２３０ｍＡｈを越える充電を初回のみ行った。２回目以降の充電容量は、平衡論的にはＬｉ２．３Ｓｉまでの合金生成がなされるよう８００ｍＡｈ／ｇ−複合体とした。
実用的には充電量の制御は電圧制御が容易であり、この場合１００ｍＶ（対金属リチウム）程度とするので充電時に負極へのデンドライト状リチウム金属の析出はなく、安全性の高い電池が製造できる。
【００１１】
上記の珪素−黒鉛複合材料あるいは珪素−黒鉛−金属複合材料を用いてリチウムイオン二次電池の負極を調製する方法には特に限定はなく、例えば該珪素−黒鉛あるいは珪素−黒鉛−金属複合材料にバインダーと溶剤を加え十分に混練して得た負極ペーストを銅箔等の金属集電体に塗布し、その後溶剤を除去して負極を得る。バインダーには公知の材料、ポリビニリデンフルオライド、エチレンプロピレンジエンポリマー、カルボキシメチルセルロース、ＳＢＲラテックスなどが使用できる。
正極材料も特に限定されないがＬｉＣｏＯ２，ＬｉＭｎ２Ｏ４，ＬｉＮｉＯ２等のリチウム含有酸化物およびその誘導体の他にリン酸系オリビン類等も使用できる。正極材料はバインダーの他、導電剤、溶剤等を加え、正極ペーストを調製し、集電体に塗布して乾燥後、正極とする。
セパレーター、電解液についても特に限定はなく、公知の材料を用いることができる。
【００１２】
以下、本発明を実施例によりさらに説明するが、本発明はこれらに限定されるものではない。
【実施例１】
粒径１０μｍ程度の珪素２ｇに平均粒径６μｍの黒鉛化メソフェーズカーボンマイクロビーズ（ＭＣＭＢ６−２８、大阪ガス製） ２ｇを遊星ミル試料容器にいれる。 回転数１８０ｒｐｍで２４時間遊星ミルを用いて乾式粉砕を行い複合体を得た。粉砕後の試料を走査型電子顕微鏡（ＳＥＭ）観察すると複合体は、主に微粉際された１μｍ以下の微細粒子と微粉砕されていない数μｍ程度の珪素粒子からなる。大きな粒子はＸ線マイクロアナライザー（ＥＰＭＡ）分析により珪素であることを確認した。又、微粒子は珪素と炭素からなることも明らかとなった。複合化に際して珪素は１／１０以下に粉砕され、ＭＣＭＢはもはや球状の形状は留めず微粉砕されている。図２に微粉砕された部分のＳＥＭ写真を示す。１μｍ程度の比較的大きな粒子のうち塊状粒子が珪素であり、厚み０．１μｍ以下の薄片状粒子が微粉砕されたＭＣＭＢである。両者は全体的にほぼ一様に分散し、均一分散状態になっていることが確認できる。
またＸ線回折（ＸＲＤ）を用いて複合体を測定した結果、回折線は原料の回折線のみからなりＳｉＣ等の合金型化合物は生成していないことが確認できた。
本複合体１．８ｇ，ＰＶＤＦ０．２ｇ，Ｎ−メチルピロリドン０．５ｍｌを用いて負極ペーストを作成し、ドクターブレードを用いて銅箔上に塗布した。溶媒を除去した後、直径１．６ｃｍの電極を切り出し試験用電極とした。乾燥重量から活物質担持量を求めた。担持量は約２．５ｍｇ／ｃｍ２である。この電極は１６０℃で２時間乾燥し、試験極とした。対極には金属リチウムを用いた。電解液には１ＭＬｉＰＦ６ ＥＣ（エチレンカーボネート）−ＤＭＣ（ジメチルカーボネート）（容積比１：２）を用いた。セパレーターにはグラスファイバー濾紙を用い、電解液を含浸させた。セルはアルゴンガス下で組み立て、充放電電流は３ｍＡ（１．５ｍＡ／ｃｍ２）とした。１回目の充電量は定電流定容量充電を用い容量値は１２００ｍＡｈ／ｇ−複合体（２４００ｍＡｈ／ｇ−Ｓｉ）とし、放電は定電流法により１．５Ｖ（対金属リチウム）定電圧終止電圧とした。２回目以降は定電流定電圧充電（３ｍＡ，１００ｍＶ）法で行った。図３に最初の３サイクルの充放電曲線を示す。２サイクル目以降の充電量は９００−９７０ｍＡｈ／ｇであり、放電容量は７８０−８８０ｍＡｈ／ｇ−複合体である。実験した２０サイクルにわたって容量は８００ｍＡｈ／ｇ−複合体（１６００ｍＡｈ／ｇ−Ｓｉ）以上を保持し、良好なサイクル特性を示すことを確認した。
エタノールと水の１：１溶液を加えて湿式粉砕して得た複合体も乾式試料同様に２サイクル目以降８００ｍＡｈ／ｇの容量を示し、サイクル特性も良好であった。
【００１３】
【実施例２】
粒径１０μｍ程度の珪素１．６ｇ、平均粒径６μｍの人造黒鉛（ＭＣＭＢ６−２８）２ｇ、Ｎｉ粉０．４ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。この試料のＳＥＭ像は実施例１の試料と類似し、微粉化した珪素、ＭＣＭＢ，Ｎｉ粉の存在が確認できるのに加え、微粉砕されなかった数μｍの珪素粒子が残存している。Ｘ線回折データも実施例１と同様複合体が珪素、ＭＣＭＢ，Ｎｉ粉の混合物であることを示した。
実施例１に従い電極を作成し、電池性能を評価した。１サイクル目の充電容量は１２００ｍＡｈ／ｇ（以下容量はｇあたり複合体で表示する）とした。１サイクル目の放電容量は９５０ｍＡｈ／ｇであった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。放電容量は２０サイクルに渡って７６０ｍＡｈ／ｇを保持し、サイクル劣化は認められなかった。
【００１４】
【実施例３】
粒径１０μｍ程度の珪素１．９ｇ、平均粒径６μｍの人造黒鉛（ＭＣＭＢ６−２８）２ｇ、Ａｇ粉０．１ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。
実施例１に従い電極を作成し、電池性能を評価した。最初の３サイクルの充放電曲線を図４に示す。１サイクル目の充電容量は１２００ｍＡｈ／ｇとした。１サイクル目の放電容量は９６３ｍＡｈ／ｇであった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。図５に示すように放電容量は１５サイクルに渡って７８０ｍＡｈ／ｇ以上を保持し、サイクル劣化は認められなかった。
【００１５】
【実施例４】
粒径１０μｍ程度の珪素２ｇ、天然黒鉛２ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。
実施例１に従い電極を作成し、電池性能を評価した。１サイクル目の充電容量は１２００ｍＡｈ／ｇに規制した。１サイクル目の放電容量は９１５ｍＡｈ／ｇであった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。放電容量は１５サイクルに渡って７７０ｍＡｈ／ｇ以上を保持し、サイクル劣化は認められなかった。
【００１６】
【実施例５】
粒径１０μｍ程度の珪素１．４ｇ、平均粒径６μｍの人造黒鉛（ＭＣＭＢ６−２８）０．６ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。
実施例１に従い電極を作成し、電池性能を評価した。１サイクル目の充電容量は１２００ｍＡｈ／ｇとした。１サイクル目の放電容量は７８０ｍＡｈ／ｇであった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。放電容量は１０サイクルに渡って７４０ｍＡｈ／ｇ以上を保持し、サイクル劣化は認められなかった。
【００１７】
【実施例６】
実施例３で合成した複合体１ｇに化学蒸着処理法により、カーボン被覆を施した。即ち、炭素源にはトルエンを用い、アルゴンガスにより反応管に導入し９００℃で３０分間炭化処理を行った。複合体の重量変化から求めたカーボン被覆量は４．２％であった。カーボン被覆によりＢＥＴ比表面積は６５ｍ２／ｇから５ｍ２／ｇへと減少する。この原因は微細粒子がカーボン層に覆われたためである。
実施例１に従い電極を作成し、電池性能を評価した。１サイクル目の充電容量は１２００ｍＡｈ／ｇとした。１サイクル目の放電容量は９４１ｍＡｈ／ｇであった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。放電容量は１５サイクルに渡って７７０ｍＡｈ／ｇ以上を保持し、実施例３の試料同様良好なサイクル特性を示した。
カーボン被覆は処理温度７００℃以上で可能であるが、８００℃−１１００℃が望ましい。低温では電気伝導性の低い炭素皮膜が生成し、充放電速度が低下し放電容量の低下を引き起こす。高温になると繊維状炭素が析出し、被覆効果が薄れ、サイクル特性が低下する。
処理時間が長くなると被覆カーボンの重量がまし珪素の含量が減るため容量減少につながる。被覆量としては、良好なレート特性が保持可能な５％以内が望ましい。
【００１８】
【比較例１】
複合体の代わりに平均粒径８μｍの珪素０．９ｇと平均粒径６μｍの人造黒鉛（ＭＣＭＢ６−２８）、０．９ｇを用いて実施例１に従い電極を作成した。
１サイクル目の充電容量は１２００ｍＡｈ／ｇとした。１サイクル目の放電容量は７２０ｍＡｈ／ｇであり、実施例１と比べると２００ｍＡｈ／ｇ程低い。２サイクル目以降８００ｍＡｈ／ｇの充電を行った。放電容量は２サイクル目には２００ｍＡｈ／ｇ程度に、３サイクル目は１００ｍＡｈ／ｇ以下と著しいサイクル劣化を示した。従って単に混合しただけでは、良好な負極剤となり得なかった。
【００１９】
【比較例２】
粒径１０μｍ程度の珪素１．９ｇ、平均粒径６μｍのカーボン（１０００℃で合成されたメソフェーズカーボン、ＭＣＭＢ６−１０， 大阪ガス製）２ｇ、Ａｇ粉０．１ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。
実施例１に従い電極を作成し、電池性能を評価した。最初の３サイクルの充放電曲線を図６に示す。１サイクル目の充電容量は１２００ｍＡｈ／ｇとした。充電容量１００ｍＡｈ／ｇ以内で既に電位は１００ｍＶ（ｖｓ．Ｌｉ）まで低下し、リチウムの挿入は０Ｖ付近で進行する。１サイクル目の放電容量は僅か２３９ｍＡｈ／ｇに過ぎなかった。以後充電電気量を８００ｍＡｈ／ｇに制限し、サイクル特性評価を行った。容量低下は認められないものの、放電容量は２５０ｍＡｈ／ｇ前後に留まり、クーロン効率が低く実用材料には向かない。徐々に増加するものの１０サイクル後でも５００ｍＡｈ／ｇ以下であった。
【００２０】
【比較例３】
粒径１０μｍ程度の珪素１．８ｇ、平均粒径６μｍの人造黒鉛（ＭＣＭＢ６−２８）０．２ｇ、Ａｇ粉０．２ｇを遊星ミル試料容器にいれる。実施例１と同様に遊星ミルで粉砕し複合体をえた。
実施例１に従い電極を作成し、電池性能を評価した。最初の２サイクルの充放電曲線を図７に示す。放電挙動は比較例２と類似し、容量が低いことが難点である。放電容量はサイクルとともに増加する傾向が認められるものの１０サイクル後でも５００ｍＡｈ／ｇ以下であった。
【００２１】
【図面の簡単な説明】
【図１】リチウム珪素合金の相図
【図２】実施例１の複合体のＳＥＭ写真
【図３】実施例１の充放電曲線
１ｃ，２ｃ，３ｃ：１，２，３サイクル目の充電、１ｄ，２ｄ，３ｄ：１，２，３サイクル目の放電
【図４】実施例３の充放電曲線
１ｃ，２ｃ，３ｃ：１，２，３サイクル目の充電、１ｄ，２ｄ，３ｄ：１，２，３サイクル目の放電
【図５】実施例３のサイクル特性
【図６】比較例２の充放電曲線
１ｃ，２ｃ，３ｃ：１，２，３サイクル目の充電、１ｄ，２ｄ，３ｄ：１，２，３サイクル目の放電
【図７】比較例３の充放電曲線
１ｃ：１サイクル目の充電、１ｄ，２ｄ：１，２サイクル目の放電[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a new negative electrode material in which silicon and graphite are used as a negative electrode active material in a lithium secondary battery using an intercalation compound such as metallic lithium or lithium carbon (lithium-graphite) as a negative electrode active material. is there. The present negative electrode material relates to not only lithium ion secondary batteries but also all lithium secondary batteries such as lithium polymer secondary batteries and lithium solid secondary batteries.
[0002]
[Prior art]
As portable electronic devices have been improved in performance and reduced in size and weight, market needs for secondary batteries having a high energy density have been increasing. Lithium-ion rechargeable batteries using graphite-based carbon and lithium cobalt oxide are currently the most commonly used rechargeable lithium batteries. To support higher performance of electronic devices, higher energy density and longer life are required. There is a need for secondary batteries.
Up to now, the capacity increase of lithium ion batteries has been attributed to the improvement of the characteristics of the negative electrode carbon. Although no positive electrode material exceeding 300 mAh / g has been found at the maximum, an alloy-based material having twice or more the capacity of a graphite-based negative electrode currently mainly used is also known as a negative electrode material. Although lithium metal, which is a well-known negative electrode material, has 4000 mAh / g, since it is a solution precipitation type reaction, dendrite-like precipitation occurs, and it is far from practical use in terms of cycle life and safety. There will be.
The alloy-based negative electrode material is quasi-intercalated and safer than lithium metal because the formation of the alloy proceeds during charging, but the expansion is accompanied by no precipitation of the charge product as in the case of lithium metal anode. High. However, it is difficult to produce an electrode that can withstand large expansion and contraction during charge and discharge with the conventional technology, and an alloy-based negative electrode having good cycle characteristics has not been developed.
[0003]
[Problems to be solved by the invention]
Attention has been paid to alloys such as lithium tin, lithium aluminum, lithium antimony, lithium bismuth, lithium lead, and lithium silicon as alloy-based negative electrode materials, and application to lithium secondary battery negative electrodes has been attempted. These alloy-based materials can use the alloy itself as a negative electrode material. However, in many cases, a battery is prepared in a metal or semimetal state for ease of handling, and the battery is alloyed based on an electrochemical reaction by charging. Although the alloy-based negative electrode thus produced has a much larger initial capacity than the graphite negative electrode, the volume of the active material increases several times with alloying. When charge and discharge are repeated, the active material collapses without being able to withstand the stress generated due to the large expansion and contraction, and causes deterioration in cycle characteristics.
According to the present invention, cycle characteristics and rate characteristics are improved by using semimetal silicon, which can be expected to have a high capacity, as a main negative electrode active material as an alloy-based negative electrode, and devising a configuration of the electrode active material.
In the composite material of the present invention, a state in which silicon particles finely ground to 1 μm or less and finely ground graphite (or metal particles such as graphite and silver) having conductivity are uniformly dispersed is achieved. Silicon, which is a main active material, has a large surface area and a small particle size, which increases the interface where substantial electrode reaction occurs, and shortens the lithium diffusion path. The evil can be overcome. In addition, good rate characteristics lead to suppression of local generation of Li 4.4 Si having large volume expansion, and cycle characteristics are also improved.
[0004]
[Means for solving the problem]
The cause of cycle deterioration of the silicon-based negative electrode is miniaturization of alloy particles due to stress derived from expansion and contraction of volume due to charge and discharge. When charged, the silicon negative electrode alloys with lithium and expands in volume several times. If this expansion leads to expansion of the entire electrode, the negative electrode may peel off from the current collector, or may deviate to the voids in the battery. Although the capacity of the silicon negative electrode is large, there is a problem that the charge / discharge is accompanied by a large expansion and contraction of the lattice, the cycle characteristics are remarkably inferior, and the rate characteristics are inferior due to low conductivity.
As shown in the phase diagram of FIG. 1, the following four types of silicon / lithium alloy compositions are known ("Alloy Phase Diagrams (vol. 3) ed. By H. Baker, ASM International (1992), page 2-278), that is, Li 4.4 Si (Li 22 Si 5 ), Li 3.25 Si (Li 13 Si 4 ), Li 2.3 Si (Li 7 Si 3 ) and Li 1.7 Si (Li 12 Si 7) a. expansion of silicon lattice volume is not large until Li 2.3 Si or Li 1.7 Si, but remain in the increase of about 15%, inserting a lithium to Li 4.4 Si, the increase of 400% of the cell volume. Accordingly be stopped a lithium insertion up to Li 2 .3 Si, cycle characteristics should be improved.
However, the diffusion of lithium (or lithium ions) inside the silicon lattice is extremely slow, and the diffusion rate is not uniform. Therefore, when the silicon negative electrode agent is charged, a mixture of LixSi (0 <x ≦ 4.4) having various compositions becomes particles. Generate internally. In particular, when Li 4.4 Si is locally generated, the volume expansion coefficient of the silicon lattice is large, the silicon particles are broken, and the cycle characteristics are abnormally deteriorated.
[0005]
In order to achieve a uniform composition of Li 2.3 Si or Li 1.7 Si even inside the silicon particles, it is necessary to shorten the lithium diffusion path inside the silicon. As one of the means, a method has been developed in which the inside of silicon particles, which is a porous body, is coated with nano-sized conductive carbon. That is, there is a method of coating with carbon by a vapor phase synthesis method (CVD). Here, it is commercialized as a negative electrode material (patent publication 2000-215887) in which silicon particles on the order of several μm are coated with CVD carbon. As a second method, a negative electrode material (Wilson, J. Appl. Phys. 77, 15 1995) in which silicon vapor is reacted with a compound serving as a carbon source to disperse silicon at a nano level in carbon has a theoretical capacity of graphite. It is reported that it has the above-mentioned high negative electrode capacity. The cycle characteristics are also relatively good. However, this method is very expensive in terms of cost and is not feasible as an industrial production method.
[0006]
As a method of shortening the lithium diffusion path, a technique of mechanical alloying is used. If silicon is pulverized by a mechanical alloying method, the surface area increases, and a uniform Li / Si alloy composition can be generated. By controlling the charge / discharge capacity, the alloy composition up to Li 2.3 Si is maintained. succeeded in. Even with an alloy composition up to Li 3.25 Si, there is a possibility that a Li / Si alloy having a uniform composition may be produced without producing Li 4.4 Si during charge and discharge. In the present invention, it has been found that graphite such as natural graphite or artificial graphite fired and synthesized at 2500 ° C. or higher is effective as an auxiliary agent for pulverizing silicon. Further, when the conductive metal is dispersed in the negative electrode, the conductivity of the negative electrode itself is improved, which contributes to an improvement in rate characteristics and an increase in capacity.
[0007]
The present invention provides a method for producing silicon-graphite or a silicon-graphite-metal composite having the large discharge capacity and improved cycle characteristics and rate characteristics of a lithium silicon alloy on an industrial scale. This material is basically an active material mainly composed of a composite in which silicon and carbon particles on the order of submicrons are uniformly dispersed. The particle size has a continuous distribution and cannot be clearly distinguished from each other. However, 2-3 μm silicon particles and metal components which are partially insufficiently pulverized also remain.
[0008]
Hereinafter, the contents of the present invention will be described in detail.
Silicon used in the present invention may be crystalline or amorphous, but amorphous silicon is preferably used. A silicon purity of about 97% is sufficient. The silicon particle diameter used as a raw material is not particularly limited, but is preferably 20 to 30 μm or less in average particle diameter, and is preferably on the order of several μm. By pulverizing silicon particles and graphite particles by a mechanical alloying method, it is easy to form a composite having a size of 1 μm or less.
The main role of the graphite material used in the present invention is an auxiliary agent for grinding silicon. Utilizing the conductivity, it also has conductivity to the negative electrode and functions as an active material. Graphite material has high hardness among carbon materials and is suitable as a grinding aid. In addition, since it also has characteristics of a battery active material such as high conductivity, large capacity as a negative electrode active material, and excellent rate characteristics, it is most suitable as a carbon material for the present composite. The graphite material needs to be ground together with silicon by a mechanical alloying technique to form a composite in which both are uniformly dispersed. Therefore, carbon having no function as a grinding aid (carbon fired at around 1000 ° C.) is not desirable. Accordingly, suitable examples of the graphite agent used in the present invention include natural graphite and spherical natural graphite, and artificial graphite obtained by firing at 2500 ° C. or higher.
[0009]
The amount of the graphite agent used in the composite is preferably 0.4 to 2.5 parts by weight based on 1 part of silicon. If the ratio of graphite is less than 0.4 part per 1 part of silicon, it is insufficient as a grinding aid and it is difficult to pulverize silicon sufficiently. On the other hand, when the proportion of graphite is 2.5 parts or more, although the amount of pulverization is sufficient, the capacity as a negative electrode material is reduced.
When the conductivity of the composite is low, a fibrous carbon agent such as carbon fiber or a metal powder can be added after the composite is manufactured. In the case of adding a metal powder, when the composite is formed, the addition of the metal powder results in a good dispersion of the metal particles, thereby exhibiting excellent battery characteristics.
[0010]
Silicon theoretically produces Li 1.71 Si up to a charge amount of 1640 mAh / g-Si, Li 2.33 Si up to 2230 mAh / g-Si, and Li 3.25 Si up to 3100 mAh / g-Si. Li 4.4 Si corresponds to a charge amount of 4200 mAh / g-Si. In order to prevent volume expansion of silicon, it is desirable to set the charging capacity to not exceed 2230 mAh per gram of silicon. For this purpose, it is desirable to use a constant-current / constant-capacity charging method, a constant-current / constant-voltage charging method, or the like so as not to exceed the charged amount. However, in the first charging, there is an irreversible capacity of 200 to 300 mAh / g due to an oxygen component considered to have been mixed during the production, so that it may be 2230-mAh / g-Si or more.
However, when a large charge / discharge capacity is used without emphasizing the cycle characteristics, a capacity larger than this may be used.
In the present invention, the first charge amount is 1200 mAh / g-composite, the second and subsequent charges are constant current / constant voltage charging method up to 100 mV, or the second and subsequent charges are 800 mAh / g-complex constant current constant capacity charge / discharge. However, the charge / discharge method is not particularly limited to these methods. Generally, any charging method that does not exceed 2230 mAh per gram of silicon may be used.
When the weight percent of graphite in the composite is 50 wt%, the charge capacity of 800 mAh / g-composite is 1600 mAh / g-Si per gram of silicon. The reason why the initial charge capacity of 1200 mAh / g-composite (that is, 2400 mAh / g-Si) was adopted is that there was an irreversible capacity of about 300 mAh / g-composite, which was introduced into the raw material silicon or during the synthesis. It is determined that the loss is caused by consumption of lithium by the oxide, that is, generation of Li 2 O at the time of the first charge. Therefore, charging exceeding 2230 mAh per gram of silicon was performed only for the first time. The charge capacity after the second time was set to 800 mAh / g-composite so that an alloy was formed up to Li 2.3 Si in terms of equilibrium theory.
In practice, voltage control is easy to control the amount of charge, and in this case, it is about 100 mV (with respect to metal lithium). Therefore, there is no precipitation of dendritic lithium metal on the negative electrode during charging, and a highly safe battery can be manufactured. .
[0011]
The method for preparing a negative electrode of a lithium ion secondary battery using the above-mentioned silicon-graphite composite material or silicon-graphite-metal composite material is not particularly limited. For example, the method for preparing the silicon-graphite or silicon-graphite-metal composite material A negative electrode paste obtained by adding a binder and a solvent and kneading sufficiently is applied to a metal current collector such as a copper foil, and then the solvent is removed to obtain a negative electrode. As the binder, a known material, polyvinylidene fluoride, ethylene propylene diene polymer, carboxymethyl cellulose, SBR latex, or the like can be used.
The cathode material is not particularly limited, but phosphate-based olivines and the like can be used in addition to lithium-containing oxides such as LiCoO 2 , LiMn 2 O 4 , and LiNiO 2 and derivatives thereof. The positive electrode material is prepared by adding a conductive agent, a solvent, and the like, in addition to a binder, to prepare a positive electrode paste, applying the paste to a current collector, and then drying the paste to form a positive electrode.
There is no particular limitation on the separator and the electrolytic solution, and known materials can be used.
[0012]
Hereinafter, the present invention will be further described with reference to Examples, but the present invention is not limited thereto.
Embodiment 1
2 g of silicon having a particle diameter of about 10 μm and 2 g of graphitized mesophase carbon microbeads (MCMB6-28, manufactured by Osaka Gas) having an average particle diameter of 6 μm are put in a planetary mill sample container. Dry grinding was performed using a planetary mill at a rotation speed of 180 rpm for 24 hours to obtain a composite. When the sample after pulverization is observed with a scanning electron microscope (SEM), the composite is mainly composed of fine particles of 1 μm or less that have been pulverized and silicon particles of about several μm that have not been pulverized. X-ray microanalyzer (EPMA) analysis confirmed that the large particles were silicon. Further, it was also found that the fine particles consisted of silicon and carbon. At the time of compounding, silicon is pulverized to 1/10 or less, and MCMB is no longer spherical but remains finely pulverized. FIG. 2 shows an SEM photograph of the pulverized portion. Among the relatively large particles of about 1 μm, the bulk particles are silicon, and the flaky particles having a thickness of 0.1 μm or less are finely ground MCMB. It can be confirmed that both are substantially uniformly dispersed as a whole and are in a uniformly dispersed state.
In addition, as a result of measuring the composite using X-ray diffraction (XRD), it was confirmed that the diffraction lines consisted only of the diffraction lines of the raw material and that no alloy type compound such as SiC was generated.
A negative electrode paste was prepared using 1.8 g of the present composite, 0.2 g of PVDF, and 0.5 ml of N-methylpyrrolidone, and applied to a copper foil using a doctor blade. After removing the solvent, an electrode having a diameter of 1.6 cm was cut out and used as a test electrode. The amount of the active material carried was determined from the dry weight. The loading amount is about 2.5 mg / cm 2 . This electrode was dried at 160 ° C. for 2 hours to obtain a test electrode. Metal lithium was used for the counter electrode. 1 M LiPF 6 EC (ethylene carbonate) -DMC (dimethyl carbonate) (volume ratio 1: 2) was used as the electrolytic solution. Glass fiber filter paper was used for the separator, and the electrolyte was impregnated. The cell was assembled under argon gas, and the charge / discharge current was 3 mA (1.5 mA / cm 2 ). The first charge amount was a constant current constant capacity charge, the capacity value was 1200 mAh / g-composite (2400 mAh / g-Si), and the discharge was 1.5 V (to lithium metal) constant voltage end voltage by the constant current method. did. The second and subsequent times were performed by a constant current and constant voltage charging (3 mA, 100 mV) method. FIG. 3 shows charge / discharge curves of the first three cycles. The charge amount after the second cycle is 900-970 mAh / g, and the discharge capacity is 780-880 mAh / g-composite. It was confirmed that the capacity was maintained at 800 mAh / g-composite (1600 mAh / g-Si) or more over the 20 cycles of the experiment, and good cycle characteristics were exhibited.
The composite obtained by adding and mixing a 1: 1 solution of ethanol and water and wet-pulverizing showed a capacity of 800 mAh / g after the second cycle similarly to the dry-type sample, and also showed good cycle characteristics.
[0013]
Embodiment 2
1.6 g of silicon having a particle size of about 10 μm, 2 g of artificial graphite (MCMB6-28) having an average particle size of 6 μm, and 0.4 g of Ni powder are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite. The SEM image of this sample is similar to that of the sample of Example 1, and in addition to confirming the presence of finely divided silicon, MCMB, and Ni powder, silicon particles of several μm that have not been pulverized remain. The X-ray diffraction data also showed that the composite was a mixture of silicon, MCMB, and Ni powder as in Example 1.
Electrodes were prepared according to Example 1, and battery performance was evaluated. The charge capacity in the first cycle was set to 1200 mAh / g (the capacity is indicated by a complex per g). The discharge capacity at the first cycle was 950 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. The discharge capacity maintained 760 mAh / g for 20 cycles, and no cycle deterioration was observed.
[0014]
Embodiment 3
1.9 g of silicon having a particle diameter of about 10 μm, 2 g of artificial graphite (MCMB6-28) having an average particle diameter of 6 μm, and 0.1 g of Ag powder are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite.
Electrodes were prepared according to Example 1, and battery performance was evaluated. FIG. 4 shows charge / discharge curves of the first three cycles. The charge capacity in the first cycle was 1200 mAh / g. The discharge capacity at the first cycle was 963 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. As shown in FIG. 5, the discharge capacity was maintained at 780 mAh / g or more over 15 cycles, and no cycle deterioration was observed.
[0015]
Embodiment 4
2 g of silicon having a particle size of about 10 μm and 2 g of natural graphite are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite.
Electrodes were prepared according to Example 1, and battery performance was evaluated. The charge capacity in the first cycle was regulated to 1200 mAh / g. The discharge capacity at the first cycle was 915 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. The discharge capacity maintained 770 mAh / g or more over 15 cycles, and no cycle deterioration was observed.
[0016]
Embodiment 5
1.4 g of silicon having a particle diameter of about 10 μm and 0.6 g of artificial graphite (MCMB6-28) having an average particle diameter of 6 μm are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite.
Electrodes were prepared according to Example 1, and battery performance was evaluated. The charge capacity in the first cycle was 1200 mAh / g. The discharge capacity at the first cycle was 780 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. The discharge capacity was maintained at 740 mAh / g or more over 10 cycles, and no cycle deterioration was observed.
[0017]
Embodiment 6
1 g of the composite synthesized in Example 3 was coated with carbon by a chemical vapor deposition method. That is, toluene was used as a carbon source, introduced into a reaction tube with argon gas, and carbonized at 900 ° C. for 30 minutes. The carbon coating amount determined from the weight change of the composite was 4.2%. The carbon coating reduces the BET specific surface area from 65 m 2 / g to 5 m 2 / g. This is because the fine particles were covered by the carbon layer.
Electrodes were prepared according to Example 1, and battery performance was evaluated. The charge capacity in the first cycle was 1200 mAh / g. The discharge capacity at the first cycle was 941 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. The discharge capacity was maintained at 770 mAh / g or more over 15 cycles, and showed good cycle characteristics as in the sample of Example 3.
Although carbon coating is possible at a processing temperature of 700 ° C. or higher, 800 ° C. to 1100 ° C. is desirable. At a low temperature, a carbon film having low electric conductivity is generated, and the charge / discharge rate is reduced, which causes a decrease in the discharge capacity. At a high temperature, fibrous carbon is deposited, the coating effect is weakened, and the cycle characteristics are reduced.
When the treatment time is prolonged, the weight of the coated carbon is increased and the content of silicon is reduced, which leads to a decrease in capacity. The coating amount is desirably 5% or less at which good rate characteristics can be maintained.
[0018]
[Comparative Example 1]
An electrode was prepared according to Example 1 using 0.9 g of silicon having an average particle size of 8 μm and 0.9 g of artificial graphite (MCMB6-28) having an average particle size of 6 μm instead of the composite.
The charge capacity in the first cycle was 1200 mAh / g. The discharge capacity in the first cycle was 720 mAh / g, which was lower than that of Example 1 by about 200 mAh / g. After the second cycle, charging was performed at 800 mAh / g. The discharge capacity showed a remarkable cycle deterioration of about 200 mAh / g in the second cycle, and 100 mAh / g or less in the third cycle. Therefore, it was not possible to obtain a good negative electrode agent simply by mixing.
[0019]
[Comparative Example 2]
1.9 g of silicon having a particle diameter of about 10 μm, 2 g of carbon having an average particle diameter of 6 μm (mesophase carbon synthesized at 1000 ° C., MCMB6-10, manufactured by Osaka Gas), and 0.1 g of Ag powder are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite.
Electrodes were prepared according to Example 1, and battery performance was evaluated. FIG. 6 shows charge / discharge curves of the first three cycles. The charge capacity in the first cycle was 1200 mAh / g. The potential already drops to 100 mV (vs. Li) within a charging capacity of 100 mAh / g, and the insertion of lithium proceeds at around 0 V. The discharge capacity in the first cycle was only 239 mAh / g. Thereafter, the charge amount was limited to 800 mAh / g, and the cycle characteristics were evaluated. Although no decrease in capacity is observed, the discharge capacity remains around 250 mAh / g, and the Coulomb efficiency is low, making it unsuitable for practical materials. Although it gradually increased, it was 500 mAh / g or less even after 10 cycles.
[0020]
[Comparative Example 3]
1.8 g of silicon having a particle diameter of about 10 μm, 0.2 g of artificial graphite (MCMB6-28) having an average particle diameter of 6 μm, and 0.2 g of Ag powder are put in a planetary mill sample container. It was pulverized with a planetary mill in the same manner as in Example 1 to obtain a composite.
Electrodes were prepared according to Example 1, and battery performance was evaluated. FIG. 7 shows charge / discharge curves of the first two cycles. The discharge behavior is similar to that of Comparative Example 2, and the drawback is that the capacity is low. Although the discharge capacity tended to increase with the cycle, it was 500 mAh / g or less even after 10 cycles.
[0021]
[Brief description of the drawings]
FIG. 1 is a phase diagram of a lithium silicon alloy. FIG. 2 is an SEM photograph of the composite of Example 1. FIG. 3 is a charge / discharge curve 1c, 2c, 3c of Example 1. 1d, 2d, 3d: discharge at 1, 2, 3rd cycle. FIG. 4 Charge / discharge curves 1c, 2c, 3c of Example 3: charge at 1, 2, 3rd cycle, 1d, 2d, 3d: 1, Discharge at the second and third cycles. FIG. 5: Cycle characteristics of Example 3. FIG. 6: Charge / discharge curves 1c, 2c, 3c of Comparative Example 2: Charges at first, second, and third cycles, 1d, 2d, 3d: FIG. 7: Charge / discharge curve 1c of Comparative Example 3: Charge of 1st cycle, 1d, 2d: Discharge of 1st and 2nd cycles