JP4087138B2

JP4087138B2 - Carbon material and silicon-containing carbon material using the same

Info

Publication number: JP4087138B2
Application number: JP2002099153A
Authority: JP
Inventors: 龍朗佐々木
Original assignee: Sumitomo Bakelite Co Ltd
Current assignee: Sumitomo Bakelite Co Ltd
Priority date: 2002-04-01
Filing date: 2002-04-01
Publication date: 2008-05-21
Anticipated expiration: 2022-04-01
Also published as: JP2003297355A

Description

【０００１】
【発明の属する技術分野】
本発明は、二次電池負極材に用いるケイ素含有炭素材用原料、ケイ素含有炭素材およびそれを用いた二次電池負極材、リチウム二次電池に関する。
【０００２】
【従来の技術】
近年、ビデオカメラやノート型パソコンなどのポータブル機器の普及に伴い、移動用電源として小型高容量の二次電池に対する需要が高まり、リチウム二次電池の使用が拡大されてきた。
上記に示したリチウム二次電池の負極材用炭素材としては、特開平５−７４４５７号公報記載の黒鉛を使用しているものが挙げられる。黒鉛は、サイクル性が非常によいことが特長であるが、理論充放電容量が３７２ｍＡｈ／ｇであるため、これ以上の充放電容量は望めないという欠点がある。また、黒鉛材料以外では、特開平５−２８９９６号公報、特開平７−７３８６８号公報等に示されるピッチコークスを使用した負極材が挙げられる。この材料は易黒鉛化炭素材であるが、焼成温度が２０００℃を超える領域では黒鉛化が進行する。黒鉛になってしまうと充放電容量が決定されてしまう。また黒鉛化される前の温度域（１０００〜１８００℃）においては充放電容量の高い炭素材が得られている。しかしながら、サイクル性が乏しく、ピッチコークスは不純物を多く含んでおり、電池特性に悪影響を及ぼす。
【０００３】
また、熱処理温度が５００℃〜７００℃程度の低温で処理された炭素負極は、次世代の高容量型炭素負極の有力候補の一つである。充電容量で８５０ｍＡｈ／ｇと、重量あたりの容量で黒鉛をこえる。また、低温処理であるため、エネルギーメリットも高い。しかしながら、電位が高く、充放電での電位のヒステリシスが大きいのが難点である。
炭素以外のリチウムイオン負極材として注目されているのが、例えば特開平５−１６６５３６号公報に示される金属酸化物含有炭素材、及び特開平６−２９０７８２号公報に示される窒素含有炭素材である。しかしながら、これらの炭素材では充放電容量８００ｍＡｈ／ｇと非常に大容量ではあるが、瞬間放電量が非常に高いことからその制御が困難であるとされている。
【０００４】
また、リチウムイオンのインターカレーション能が非常に高い材料としてケイ素元素があり、それを用いたケイ素含有炭素材として、特開平０５−１４４７４公報，特開平７−３１５８２２公報，再表９８／０２４１３５公報，特開平０８−２３１２７３公報等がある。これらにおいて、有機ケイ素化合物、無機ケイ素化合物を使用している場合、ケイ素と結合している有機又は無機元素の影響を受けケイ素元素が持っている充放電容量が十分に活かされていない。また、ケイ素元素を使用している場合でも、易黒鉛化炭素前駆体，難黒鉛化炭素前駆体又は炭素材にケイ素元素を混合し炭化処理している。この場合、ケイ素の炭素材への分散性は良い。しかし、炭素材表面へのケイ素元素の露出により充放電容量は高いが、充放電効率が低い。あるいは、ケイ素元素の炭素材表面への露出は少ない場合でも、ケイ素元素へのリチウムイオンのインターカレーションによるケイ素元素の膨張による炭素材の破損を押える事が困難で、充放電効率を低下させる傾向にある。
【０００５】
【発明が解決しようとする課題】
本発明の目的は、高充放電容量を発揮することができる二次電池負極材に用いるケイ素含有炭素材用原料、ケイ素含有炭素材、二次電池負極材およびリチウム二次電池を提供することである。
【０００６】
【課題を解決するための手段】
このような目的は、下記（１）〜（９）の本発明により達成される。
（１）比表面積５０〜1０００ｍ²／ｇである炭素材とケイ素含有炭素前駆体とを含み、溶融混合したことを特徴とする二次電池負極材に用いるケイ素含有炭素材用原料。
（２）前記炭素材は、比表面積８０〜７００ｍ²／ｇである前記（１）に記載の二次電池負極材に用いるケイ素含有炭素材用原料。
（３）前記炭素材は、黒鉛である前記（１）又は（２）に記載の二次電池負極材に用いるケイ素含有炭素材用原料。
（４）前記炭素材は、前記炭素材用原料全体に対して４０〜８０重量％である前記（１）ないし（３）のいずれかに記載の二次電池負極材に用いるケイ素含有炭素材用原料。
（５）前記ケイ素含有炭素前駆体は、ケイ素粉末とピッチとの混合物から構成される前記（１）ないし（４）のいずれかに記載の二次電池負極材に用いるケイ素含有炭素材用原料。
（６）前記ケイ素粉末は、前記ケイ素含有炭素前駆体全体の１０〜６０重量％である前記（５）に記載の二次電池負極材に用いるケイ素含有炭素材用原料。
（７）前記（１）ないし（６）のいずれかに記載の二次電池負極材に用いるケイ素含有炭素材用原料を炭化処理してなるケイ素含有炭素材。
（８）前記（７）に記載のケイ素含有炭素材を含有する二次電池負極材。
（９）前記（８）に記載の二次電池負極材を用いたリチウム二次電池。
【０００７】
【発明の実施の形態】
以下、本発明の二次電池負極材に用いるケイ素含有炭素材用原料、ケイ素含有炭素材、二次電池負極材およびリチウム二次電池について、詳細に説明する。本発明の二次電池負極材に用いるケイ素含有炭素材用原料（以下、炭素材用原料（ｃ）という）は、比表面積が５０〜１０００ｍ²／ｇである炭素材（以下、炭素材（ａ）という）及びケイ素含有炭素前駆体（以下、ケイ素含有炭素前駆体（ｂ）という）を含むものである。また、本発明のケイ素含有炭素材（以下、ケイ素含有炭素材（ｄ）という）は前記炭素材用原料（ｃ）を炭素化処理したものであり、二次電池負極材は前記ケイ素含有炭素材（ｄ）を含むものであり、リチウム二次電池は前記二次電池負極材を用いたものである。
【０００８】
本発明で用いる比表面積が５０〜１０００ｍ²／gである炭素材（ａ）としては、例えば、モノマーから細孔形成制御にて前記比表面積を有するフェノール樹脂、メラミン樹脂、ポリイミド樹脂、ピッチ樹脂等を炭化した炭素材、炭化処理条件等で細孔を制御することで比表面積を得た炭素材、水蒸気や薬品等で炭素材を賦活し前記比表面積を有する炭素材、または機械的粉砕にて前記比表面積を有する炭素材等が挙げられる。これらの中でも上記法により得られた黒鉛が好ましい。これにより二次電池に用いた場合に高充放電効率を発揮させることができる。炭素材（ａ）の比表面積が５０ｍ²／g未満であると炭素材（ａ）をケイ素含有炭素前駆体（ｂ）で覆う範囲が多くなるため充放電効率が低くなり、１０００ｍ²／gを越えると炭素材（ａ）とケイ素含有炭素前駆体（ｂ）の密着性が低下するようになり放電容量保持率が十分に発揮することができないようになる。
前記炭素材（ａ）の比表面積は５０〜１０００ｍ²／ｇであるが、特に８０〜７００ｍ²／ｇが好ましい。前記比表面積が前記範囲内であると、炭素材（ａ）の特性を損なうことなく、比表面積を高くしている細孔にケイ素含有炭素前駆体（ｂ）が浸入し炭化処理後、炭素質の楔が生成し、炭素材（ａ）を完全にケイ素含有炭素前駆体（ｂ）で包まなくとも密着性が向上し、二次電池に用いた場合に高充放電容量を維持したまま、高放電容量保持率を発揮することができる。
【０００９】
前記比表面積を有する炭素材（ａ）は、特に限定されないが、炭素材用原料（ｃ）の４０〜８０重量％で有ることが好ましく、特に５０〜７０重量％が好ましい。前記炭素材（ａ）が前記範囲内であると上記効果に加え、二次電池に用いた場合充放電効率を向上させることができる。炭素材（ａ）の割合が前記下限値未満ではケイ素含有炭素前駆体（ｂ）で覆われる量が多くなるため、充放電効率は低くなる傾向にあり、前記上限値を越えると放電容量の低下とともに密着性が低くなる傾向のため放電容量保持率が低下するようになる。
【００１０】
本発明で用いるケイ素含有炭素前駆体（ｂ）としては、例えば、シロキサン，シラザン等の有機ケイ素化合物、有機ケイ素化合物と石油ピッチ，石炭ピッチ等の易黒鉛化炭素前駆体、又はかかる易黒鉛化炭素前駆体とフェノール樹脂，フラン樹脂，エポキシ樹脂等の難黒鉛化炭素前駆体との混合物、又はケイ素、又はケイ素酸化物，ケイ素炭化物等の無機ケイ素化合物と前記易黒鉛化炭素前駆体又は難黒鉛化炭素前駆体との混合物等が挙げられる。
これらの中でも、特に限定されないが、ケイ素粉末と易黒鉛化炭素前駆体又は難黒鉛化炭素前駆体との混合物が好ましい。これにより、二次電池に用いた場合に高充放電容量を発揮することができる。
さらには、前記ケイ素含有炭素前駆体（ｂ）は、ケイ素粉末とピッチとの混合物であることが好ましい。これにより酸素含有量が少なく、炭素化率を上げることができるので、上記の効果に加え、二次電池に用いた場合に放電容量保持率を向上することができる。
また、前記ケイ素粉末の配合量は、ケイ素含有炭素前駆体（ｂ）中１０〜６０重量％が好ましく、特に２０〜５０重量％が好ましい。前記ケイ素粉末が前記範囲内であると、ケイ素の特性を損なうことなく二次電池に用いた場合に高充放電容量を発揮することができる。ケイ素粉末の配合量が前記下限値未満では放電容量が低くなりやすく、前記上限値を越えると充放電効率および放電容量保持率が低下しやすい。
【００１１】
ケイ素含有炭素前駆体（ｂ）は、特に限定されないが、炭素材用原料の２０〜６０重量％で有ることが好ましく、特に３０〜５０重量％が好ましい。前記ケイ素含有炭素前駆体（ｂ）が前記範囲内であると上記効果に加え、二次電池に用いた場合に放電容量保持率を向上することができる。ケイ素含有炭素前駆体（ｂ）の割合が前記下限値未満では放電容量低下とともに密着性が向上せず放電容量保持率が低くなりやすく、前記上限値を越えると充放電効率および放電容量保持率が低くなりやすい。
【００１２】
本発明は、比表面積が５０〜１０００ｍ²／ｇである炭素材（ａ）及びケイ素含有炭素前駆体（ｂ）を含む炭素材用原料（ｃ）を炭化処理して得られるケイ素含有炭素材（ｄ）である。炭化処理は特に限定されないが、例えば、前記炭素材（ａ）とケイ素含有炭素前駆体（ｂ）を溶融混合した後、窒素雰囲気下で５０〜２００℃／時間で昇温し、４００〜６００℃で１〜５時間保持し冷却後、通常１００μｍ以下まで粉砕する。粉砕処理品を更に窒素雰囲気下で１０〜１５０℃／時間で昇温し８００〜１２００℃にて１〜１０時間保持し室温まで冷却し、前記ケイ素含有炭素材を得ることができる。前記ケイ素含有炭素材（ｄ）は、特に限定されないが、平均粒径１〜５０μｍが好ましく、特に５〜３０μｍが好ましい。ケイ素含有炭素材（ｄ）の粒径が前記範囲内であると、負極材作製時の取り扱い性が良く、また、作製後の負極材塗布面が平滑となる。ケイ素含有炭素材（ｄ）の粒径が前記下限値未満では粉体の粉舞が発生するとともに負極材作製の作業性が低下しやすく、前記上限値を越えると負極材塗布面が凹凸となりやすい。
【００１３】
また、本発明は前記ケイ素含有炭素材（ｄ）を含む二次電池負極材である。本発明の二次電池負極材は、例えば、前記ケイ素含有炭素材（ｄ）１００重量部に対しポリエチレン，ポリプロピレン等を含むフッ素系高分子、ブチルゴム，ブタジエンゴム等のゴム状高分子等の有機高分子結着剤１〜３０重量部及び適量のＮ−メチル−２−ピロリドン，ジメチルホルムアミド等の粘度調整用溶剤を添加して混練し、ペースト状にした混合物を圧縮成形，ロール成形等によりシート状、ペレット状等に成形して得ることができる。また、粘度調整用溶剤にてスラリー状にした混合物を銅箔、ニッケル箔等の集電体に塗布成形して得ることもできる。
【００１４】
本発明は前記二次電池用負極材を用いたリチウム二次電池である。本発明のリチウム二次電池に前記二次電池用負極材を適用する場合、例えば、前記二次電池用負極材はセパレータを介して正極材と対向して配置され、電解液を用いリチウム二次電池が得られる。正極材としては特に限定されないが、リチウムコバルト酸化物、リチウムニッケル酸化物，リチウムマンガン酸化物等の複合酸化物やポリアニリン，ポリピロール等の導電性高分子等を用いることができる。セパレータとしては特に限定されないが、ポリエチレン，ポリプロピレン等の微多孔質フィルム、不織布等を用いることができる。電解液としては特に限定されないが、非水系溶媒に電解質となるリチウム塩を溶解したものを用いる。電解質としてはＬｉＣｌＯ₄，ＬｉＰＦ₆等のリチウム金属塩、テトラアルキルアンモニウム塩等を用いることができる。非水系溶媒としては、プロピレンカーボネート、エチレンカーボネート、γ−ブチロラクトン等の環状エステル類、ジエチルカーボネート等の鎖状エステル類、ジメトキシエタン等のエーテル類等の混合物等を用いることができる。また、上記塩類をポリエチレンオキサイド、ポリアクリロニトリル等に混合された固体電解質を用いることもできる。
【００１５】
【実施例】
以下、本発明を実施例及び比較例により詳細に説明するが、本発明はこれに限定されるものではない。
【００１６】
実施例１＜ケイ素含有炭素材（ｄ）の製造＞
▲１▼比表面積５０〜１０００ｍ²／gの炭素材（ａ）の作製
平均粒径７０μｍに粉砕された黒鉛２００重量部をローターリーキルン式乾燥炉に入れ４５０℃で５時間１Ｌ／ｍｉｎの空気を通気させながら酸化処理を行った。その後、室温まで冷却し振動ボールミルを用い４５μｍ以下まで粉砕し比表面積２４０ｍ²／ｇの炭素材Ａ１を得た。
▲２▼ケイ素含有炭素前駆体（ｂ）の作製
軟化点１２０℃のピッチ５００重量部を１Ｌのフラスコ入れ、１８０〜２４０℃でピッチを溶解した。溶解したピッチにケイ素粉末１５０重量部を徐々に逐添し、添加終了後、更に１時間攪拌した後、室温まで冷却し、粗砕しケイ素含有炭素前駆体を得た。
▲３▼炭素材用原料（ｃ）の調製
上記にて得られた炭素材Ａ１を全体の６０重量％使用し、前記ケイ素含有炭素前駆体を全体の４０重量％使用し、３つ口フラスコを用いて１時間溶融混合し、釜出し後室温まで冷却し衝撃式粉砕機によりスクリーン１ｍｍφで１００μｍ以下に粉砕した。
▲４▼ケイ素含有炭素材（ｄ）の製造
上記粉砕品を窒素雰囲気下で１００℃／時間で５５０℃まで昇温して１．５時間保持した。その後、冷却して振動ボールミルを用いて４５μｍ以下まで粉砕した。
上記粉砕品を１００℃／時間で１０００℃まで昇温して３時間保持した。その後、冷却して４５μｍ篩で篩い、ケイ素含有炭素材を得た。
【００１７】
実施例２＜ケイ素含有炭素材（ｄ）の製造＞
実施例１で得た前記炭素材Ａ１の使用量を全体の８０重量％とし、前記ケイ素含有炭素前駆体の使用量を全体の２０重量％とした以外は、実施例１と同様にしてケイ素含有炭素材を得た。
【００１８】
実施例３＜ケイ素含有炭素材（ｄ）の製造＞
実施例１で得た前記炭素材Ａ１の使用量を全体の４０重量％とし、前記ケイ素含有炭素前駆体の使用量を全体の６０重量％とした以外は実施例１と同様にしてケイ素含有炭素材を得た。
【００１９】
実施例４＜ケイ素含有炭素材（ｄ）の製造＞
平均粒径１００μｍの球状フェノール樹脂（住友ベークライト製ＰＲ−ＡＣＳ−３）を用い酸化処理し得られた比表面積６１０ｍ²／ｇを有する炭素材Ａ２を用いた以外は、実施例１と同様にしてケイ素含有炭素材を得た。
【００２０】
比較例１
軟化点１２０℃のピッチ５００重量部を１Ｌのフラスコ入れ、１８０〜２４０℃でピッチを溶解した。溶解したピッチにケイ素粉末１５０重量部を徐々に逐添し、添加終了後、更に１時間攪拌した後、室温まで冷却し、粗砕しケイ素含有炭素前駆体を得た。得られた炭素前駆体を窒素雰囲気下１００℃／時間で５００℃まで昇温し５時間その温度で保持した。その後、冷却して振動ボールミルを用いて４５μｍ以下まで粉砕した。粉砕品を１００℃／時間で１０００℃まで昇温して３時間保持した。その後、冷却して４５μｍ篩で篩い、ケイ素含有炭素材を得た。
【００２１】
比較例２
平均粒径７０μｍに粉砕された黒鉛２００重量部をローターリーキルン式乾燥炉に入れ４５０℃で５時間１Ｌ／ｍｉｎの空気を通気させながら酸化処理を行った。その後、室温まで冷却し振動ボールミルを用い４５μｍ以下まで粉砕し比表面積２４０ｍ²／ｇの炭素材を得た。得られた炭素材６０重量％、軟化点１２０℃のピッチ４０重量％を３つ口フラスコに入れ１時間溶融混合し、釜出し後室温まで冷却し衝撃式粉砕機によりスクリーン１ｍｍφで１００μｍ以下に粉砕した。粉砕品を窒素雰囲気下で１００℃／時間で５５０℃まで昇温して１．５時間保持した。その後、冷却して振動ボールミルを用いて４５μｍ以下まで粉砕した。
上記粉砕品を１００℃／時間で１０００℃まで昇温して３時間保持した。その後、冷却して４５μｍ篩で篩い、ケイ素を含まない炭素材を得た。
【００２２】
比較例３
二次電池用負極材用炭素材として、メソカーボンマイクロビーズ（川崎製鉄(株)製「ＫＭＦＣ」、比表面積２ｍ²／ｇ）のみを用いた。
【００２３】
＜二次電池の製造＞
▲１▼各実施例および比較例にて得られたケイ素含有炭素材または炭素材に、これらに対して結合剤としてポリフッ化ビニリデン１０重量％、アセチレンブラック３重量％を添加し、希釈溶媒としてＮ−メチル−２−ピロリドンを適量加え混合し、スラリー状の負極混合物を調製した。調製した負極スラリー状混合物を１０μｍの銅箔の両面に塗布し、その後、１１０℃で１時間真空乾燥した。真空乾燥後、ロールプレスによって電極を加圧成形した。これを幅４０ｍｍで長さ２９０ｍｍの大きさに切り出し負極を作製した。但し、負極両端１０ｍｍの部分は銅箔が露出しており、この一方に負極タブを圧着した。
▲２▼正極は正極活物質としてＬｉＣｏＯ₂３００重量部、アセチレンブラック１５重量部、ポリフッ化ビニリデン１５重量部を配合し、希釈溶媒としてＮ−メチル−２−ピロリドンを適量加え混合し、スラリー状の正極混合物を調製した。調製した正極スラリー状混合物を２５μｍのアルミ箔の両面に塗布し、その後、１１０℃で１時間真空乾燥した。真空乾燥後、ロールプレスによって電極を加圧成形した。これを幅４０ｍｍで長さ２８０ｍｍの大きさに切り出し正極を作製した。但し、正極両端１０ｍｍの部分はアルミ箔が露出しており、この一方に正極タブを圧着した。
▲３▼前記正極、セパレータ（ポリプロピレン製多孔質フィルム：幅４５ｍｍ、厚さ２５μｍ）、前記負極、セパレータ、前記正極…の順で前記負極が外側になるよう渦巻き状に捲回して電極を作製した。作製した電極を単三型の電池缶に挿入し負極タブを缶底と溶接した。電解液として体積比が１：１のエチレンカーボネートとジエチレンカーボネートの混合液に６フッ化リン酸リチウムを１モル／リットル溶解させたものを電池缶に注入した後、正極タブを正極蓋に溶接し、正極蓋をかしめて二次電池を作製した。
【００２４】
上述の実施例および比較例により得られた結果を表１に示す。なお、比表面積は、炭素材製造後にユアサアイオニクス社製ＮＯＶＡ１２００を用いて、窒素ガスＢＥＴ３点法で測定した。
２．５Ｖ放電容量、初回充放電効率および放電容量保持率については、二次電池製造後に測定した。充電条件は、電流２５ｍＡ／ｇの低電流で１ｍＶになるまで保持し、その後、１．２５ｍＡｈ／ｇ以下に電流が減衰するまでとした。また、放電条件のカットオフ電位は２．５Ｖとした。放電容量保持率は初回放電容量に対する３００サイクル後の放電容量の保持率とした。
【００２５】
【表１】

【００２６】
表１に示すように、実施例１〜４により得られたケイ素含有炭素材（ｄ）は、比表面積５０〜１０００ｍ²／gの炭素材（ａ）とケイ素含有炭素前駆体（ｂ）を使用しているため充放電容量及び放電容量保持率に優れる。特に実施例１〜３においては、前記炭素材（ａ）の原料として黒鉛を使用していることから充放電効率にも優れていた。
比較例１はケイ素含有炭素材のみのため、充放電効率および放電容量保持率が不十分であった。比較例２，３は、ケイ素を含まない炭素材あるいは黒鉛のみのため、放電容量が低下していた。
【００２７】
【発明の効果】
本発明の比表面積５０〜1０００ｍ²／ｇである炭素材（ａ）とケイ素含有炭素前駆体（ｂ）とを含むことを特徴とする炭素材用原料（ｃ）は、これを炭化処理して得られた炭素材を二次電池負極材に用いたとき高充放電容量を発揮することができる。また、炭素材（ａ）の原料として黒鉛を使用し、ケイ素含有炭素前駆体（ｂ）としてケイ素粉末とピッチとの混合物を使用し、これらを混合した炭素材用原料（ｃ）は、特に二次電池の充放電効率を維持し、充放電容量，放電容量保持率を向上させることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon-containing carbon material used for a secondary battery negative electrode material, a silicon-containing carbon material, a secondary battery negative electrode material using the same, and a lithium secondary battery.
[0002]
[Prior art]
In recent years, with the widespread use of portable devices such as video cameras and laptop computers, the demand for small, high-capacity secondary batteries as mobile power sources has increased, and the use of lithium secondary batteries has been expanded.
Examples of the carbon material for the negative electrode material of the lithium secondary battery described above include those using graphite described in JP-A-5-74457. Graphite is characterized by extremely good cycleability, but has a disadvantage that a charge / discharge capacity higher than this cannot be expected because the theoretical charge / discharge capacity is 372 mAh / g. In addition to the graphite material, negative electrode materials using pitch coke shown in JP-A-5-28996, JP-A-7-73868 and the like can be mentioned. This material is an easily graphitized carbon material, but graphitization proceeds in a region where the firing temperature exceeds 2000 ° C. If it becomes graphite, the charge / discharge capacity is determined. Moreover, in the temperature range (1000-1800 degreeC) before graphitizing, the carbon material with a high charging / discharging capacity | capacitance is obtained. However, the cycle performance is poor, and pitch coke contains a large amount of impurities, which adversely affects battery characteristics.
[0003]
Further, the carbon negative electrode processed at a low temperature of about 500 ° C. to 700 ° C. is one of the promising candidates for the next generation high capacity carbon negative electrode. Charging capacity exceeds 850 mAh / g, and the capacity per weight exceeds graphite. Moreover, since it is a low-temperature process, energy merit is also high. However, the potential is high and the hysteresis of the potential during charging / discharging is large.
What attracts attention as a lithium ion negative electrode material other than carbon is, for example, a metal oxide-containing carbon material disclosed in JP-A-5-166536 and a nitrogen-containing carbon material disclosed in JP-A-6-290782. . However, although these carbon materials have a very large charge / discharge capacity of 800 mAh / g, the instantaneous discharge amount is very high, so that the control thereof is difficult.
[0004]
Moreover, there is silicon element as a material having a very high lithium ion intercalation ability, and as a silicon-containing carbon material using the element, JP-A-05-14474, JP-A-7-315822, and Table 98/024135 are disclosed. JP-A-08-231273. In these, when the organosilicon compound and the inorganic silicon compound are used, the charge / discharge capacity possessed by the silicon element is not fully utilized under the influence of the organic or inorganic element bonded to silicon. Even when silicon element is used, silicon element is mixed and carbonized with an easily graphitizable carbon precursor, a non-graphitizable carbon precursor or a carbon material. In this case, the dispersibility of silicon in the carbon material is good. However, the charge / discharge capacity is high due to the exposure of silicon element to the carbon material surface, but the charge / discharge efficiency is low. Or, even when there is little exposure of the silicon element to the carbon material surface, it is difficult to suppress the damage of the carbon material due to the expansion of the silicon element due to the lithium ion intercalation into the silicon element, and the tendency to reduce charge and discharge efficiency It is in.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a raw material for a silicon-containing carbon material, a silicon-containing carbon material, a secondary battery negative electrode material, and a lithium secondary battery used for a secondary battery negative electrode material capable of exhibiting a high charge / discharge capacity. is there.
[0006]
[Means for Solving the Problems]
Such an object is achieved by the present inventions (1) to (9) below.
(1) a specific surface area 50~1000m ² / g saw including a carbon material and the silicon-containing carbon precursor is a silicon-containing carbon material for a raw material used in the secondary battery negative electrode material, characterized in that the molten mixture.
(2) The said carbon material is a raw material for silicon containing carbon materials used for the secondary battery negative electrode material as described in said (1) whose specific surface area is 80-700 m < ² > / g.
(3) The raw material for a silicon-containing carbon material used for the secondary battery negative electrode material according to (1) or (2), wherein the carbon material is graphite.
(4) The carbon material is for silicon-containing carbon material used for the secondary battery negative electrode material according to any one of (1) to (3), wherein the carbon material is 40 to 80% by weight with respect to the whole carbon material raw material. material.
(5) The silicon-containing carbon precursor used in the secondary battery negative electrode material according to any one of (1) to (4), wherein the silicon-containing carbon precursor is composed of a mixture of silicon powder and pitch.
(6) The raw material for a silicon-containing carbon material used for the secondary battery negative electrode material according to (5), wherein the silicon powder is 10 to 60% by weight of the entire silicon-containing carbon precursor.
(7) A silicon-containing carbon material obtained by carbonizing a silicon-containing carbon material used in the secondary battery negative electrode material according to any one of (1) to (6).
(8) A secondary battery negative electrode material containing the silicon-containing carbon material according to (7).
(9) A lithium secondary battery using the secondary battery negative electrode material according to (8).
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the raw material for silicon-containing carbon material, silicon-containing carbon material, secondary battery negative electrode material, and lithium secondary battery used for the secondary battery negative electrode material of the present invention will be described in detail. The raw material for silicon-containing carbon material (hereinafter referred to as carbon material (c)) used for the secondary battery negative electrode material of the present invention is a carbon material (hereinafter referred to as carbon material (a) having a specific surface area of 50 to 1000 m ² / g. )) And a silicon-containing carbon precursor (hereinafter referred to as silicon-containing carbon precursor (b)). The silicon-containing carbon material of the present invention (hereinafter referred to as silicon-containing carbon material (d)) is obtained by carbonizing the carbon material (c), and the secondary battery negative electrode material is the silicon-containing carbon material. (D) is included, and a lithium secondary battery uses the said secondary battery negative electrode material.
[0008]
Examples of the carbon material (a) having a specific surface area of 50 to 1000 m ² / g used in the present invention include a phenol resin, a melamine resin, a polyimide resin, and a pitch resin having the specific surface area by controlling pore formation from a monomer. Carbon material obtained by carbonizing carbon, carbon material having a specific surface area obtained by controlling pores under carbonization conditions, carbon material having the specific surface area activated by water vapor or chemicals, or mechanical pulverization Examples thereof include a carbon material having the specific surface area. Among these, graphite obtained by the above method is preferable. Thereby, when it uses for a secondary battery, high charging / discharging efficiency can be exhibited. When the specific surface area of the carbon material (a) is less than 50 m ² / g, the range of covering the carbon material (a) with the silicon-containing carbon precursor (b) increases, so the charge / discharge efficiency decreases, and 1000 m ² / g is reduced. If it exceeds, the adhesion between the carbon material (a) and the silicon-containing carbon precursor (b) will be lowered, and the discharge capacity retention rate cannot be fully exhibited.
The carbon material (a) has a specific surface area of 50 to 1000 m ² / g, particularly preferably 80 to 700 m ² / g. When the specific surface area is within the above range, the carbon-containing carbon precursor (b) enters the pores having a high specific surface area without impairing the characteristics of the carbon material (a), and after carbonization, The wedge is produced, and the adhesion is improved even if the carbon material (a) is not completely encased in the silicon-containing carbon precursor (b), while maintaining a high charge / discharge capacity when used in a secondary battery. The discharge capacity retention rate can be exhibited.
[0009]
The carbon material (a) having the specific surface area is not particularly limited, but is preferably 40 to 80% by weight, particularly preferably 50 to 70% by weight of the carbon material (c). When the carbon material (a) is within the above range, in addition to the above effects, charge / discharge efficiency can be improved when used in a secondary battery. If the proportion of the carbon material (a) is less than the lower limit, the amount covered with the silicon-containing carbon precursor (b) increases, so the charge / discharge efficiency tends to be low. If the upper limit is exceeded, the discharge capacity decreases. At the same time, the discharge capacity retention rate decreases due to the tendency of the adhesion to be lowered.
[0010]
Examples of the silicon-containing carbon precursor (b) used in the present invention include organosilicon compounds such as siloxane and silazane, graphitizable carbon precursors such as organosilicon compounds and petroleum pitch and coal pitch, or such graphitizable carbon. Mixtures of precursors with non-graphitizable carbon precursors such as phenolic resins, furan resins, and epoxy resins, or silicon or inorganic silicon compounds such as silicon oxide and silicon carbide, and the graphitizable carbon precursors or non-graphitizable Examples thereof include a mixture with a carbon precursor.
Among these, although not particularly limited, a mixture of silicon powder and an easily graphitizable carbon precursor or a non-graphitizable carbon precursor is preferable. Thereby, when used for a secondary battery, a high charge / discharge capacity can be exhibited.
Furthermore, the silicon-containing carbon precursor (b) is preferably a mixture of silicon powder and pitch. As a result, the oxygen content is low and the carbonization rate can be increased. In addition to the above effects, the discharge capacity retention rate can be improved when used in a secondary battery.
Moreover, 10-60 weight% is preferable in a silicon-containing carbon precursor (b), and, as for the compounding quantity of the said silicon powder, 20-50 weight% is especially preferable. When the silicon powder is within the above range, a high charge / discharge capacity can be exhibited when used for a secondary battery without impairing the characteristics of silicon. If the amount of silicon powder is less than the lower limit, the discharge capacity tends to be low, and if the amount exceeds the upper limit, the charge / discharge efficiency and the discharge capacity retention rate tend to decrease.
[0011]
Although a silicon containing carbon precursor (b) is not specifically limited, It is preferable that it is 20 to 60 weight% of the raw material for carbon materials, and 30 to 50 weight% is especially preferable. When the silicon-containing carbon precursor (b) is within the above range, in addition to the above effects, the discharge capacity retention rate can be improved when used in a secondary battery. If the proportion of the silicon-containing carbon precursor (b) is less than the lower limit value, the discharge capacity decreases and the adhesion is not improved and the discharge capacity retention rate tends to be low. If the ratio exceeds the upper limit value, the charge / discharge efficiency and the discharge capacity retention rate are increased. It tends to be low.
[0012]
The present invention provides a silicon-containing carbon material obtained by carbonizing a carbon material (a) having a specific surface area of 50 to 1000 m ² / g and a carbon material raw material (c) containing a silicon-containing carbon precursor (b). d). While carbonization treatment is not particularly limited, for example, after the engagement of the carbon material (a) with the silicon-containing carbon precursor (b) a soluble Toru混, the temperature was raised at 50 to 200 ° C. / time in a nitrogen atmosphere, 400 Hold at ˜600 ° C. for 1 to 5 hours, cool and then pulverize to 100 μm or less. The pulverized product is further heated at 10 to 150 ° C./hour in a nitrogen atmosphere, held at 800 to 1200 ° C. for 1 to 10 hours, and cooled to room temperature to obtain the silicon-containing carbon material. The silicon-containing carbon material (d) is not particularly limited, but an average particle size of 1 to 50 μm is preferable, and 5 to 30 μm is particularly preferable. When the particle size of the silicon-containing carbon material (d) is within the above range, the handleability during the production of the negative electrode material is good, and the negative electrode material application surface after production is smooth. If the particle size of the silicon-containing carbon material (d) is less than the lower limit, powder powder is generated and the workability of the negative electrode material preparation is likely to be reduced. If the upper limit is exceeded, the negative electrode material application surface tends to be uneven. .
[0013]
Moreover, this invention is a secondary battery negative electrode material containing the said silicon containing carbon material (d). The secondary battery negative electrode material of the present invention is, for example, an organic polymer such as a fluorine-containing polymer containing polyethylene, polypropylene, etc., a rubbery polymer such as butyl rubber, butadiene rubber, etc. with respect to 100 parts by weight of the silicon-containing carbon material (d). 1-30 parts by weight of a molecular binder and an appropriate amount of a viscosity adjusting solvent such as N-methyl-2-pyrrolidone and dimethylformamide are added and kneaded, and the paste-like mixture is formed into a sheet by compression molding, roll molding, etc. It can be obtained by molding into a pellet form or the like. Moreover, it can also obtain by apply | coating shaping | molding the mixture made into the slurry form with the solvent for viscosity adjustments on collectors, such as copper foil and nickel foil.
[0014]
The present invention is a lithium secondary battery using the negative electrode material for a secondary battery. When the negative electrode material for a secondary battery is applied to the lithium secondary battery of the present invention, for example, the negative electrode material for the secondary battery is disposed to face the positive electrode material with a separator interposed therebetween, and the lithium secondary battery A battery is obtained. The positive electrode material is not particularly limited, and composite oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, and conductive polymers such as polyaniline and polypyrrole can be used. Although it does not specifically limit as a separator, Microporous films, such as polyethylene and a polypropylene, a nonwoven fabric, etc. can be used. Although it does not specifically limit as electrolyte solution, What melt | dissolved the lithium salt used as electrolyte in a non-aqueous solvent is used. As the electrolyte, lithium metal salts such as LiClO ₄ and LiPF ₆ , tetraalkylammonium salts, and the like can be used. As the non-aqueous solvent, a mixture of cyclic esters such as propylene carbonate, ethylene carbonate and γ-butyrolactone, chain esters such as diethyl carbonate, ethers such as dimethoxyethane, and the like can be used. A solid electrolyte in which the above salts are mixed with polyethylene oxide, polyacrylonitrile, or the like can also be used.
[0015]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to this.
[0016]
Example 1 <Production of silicon-containing carbon material (d)>
(1) Preparation of carbon material (a) having a specific surface area of 50 to 1000 m ² / g 200 parts by weight of graphite pulverized to an average particle size of 70 μm was placed in a rotary kiln type drying furnace at 450 ° C. for 5 hours at 1 L / min air. Oxidation treatment was performed while aeration was conducted. Then, it cooled to room temperature and grind | pulverized to 45 micrometers or less using the vibration ball mill, and obtained carbon material A1 with a specific surface area of 240 m < ² > / g.
(2) Production of silicon-containing carbon precursor (b) 500 parts by weight of a pitch having a softening point of 120 ° C was placed in a 1 L flask, and the pitch was dissolved at 180 to 240 ° C. 150 parts by weight of silicon powder was gradually added to the dissolved pitch. After the addition was completed, the mixture was further stirred for 1 hour, cooled to room temperature, and roughly crushed to obtain a silicon-containing carbon precursor.
(3) Preparation of carbon material (c) 60% by weight of the carbon material A1 obtained above was used, 40% by weight of the silicon-containing carbon precursor was used. The mixture was melted and mixed for 1 hour, taken out from the kettle, cooled to room temperature, and pulverized to 100 μm or less with a screen 1 mmφ by an impact pulverizer.
(4) Production of silicon-containing carbon material (d) The pulverized product was heated to 550 ° C. at 100 ° C./hour in a nitrogen atmosphere and held for 1.5 hours. Then, it cooled and grind | pulverized to 45 micrometers or less using the vibration ball mill.
The pulverized product was heated to 1000 ° C. at 100 ° C./hour and held for 3 hours. Then, it cooled and sieved with a 45 micrometer sieve, and the silicon containing carbon material was obtained.
[0017]
Example 2 <Production of silicon-containing carbon material (d)>
The silicon-containing material was the same as in Example 1 except that the amount of the carbon material A1 obtained in Example 1 was 80% by weight of the whole and the amount of the silicon-containing carbon precursor was 20% by weight of the whole. Carbon material was obtained.
[0018]
Example 3 <Production of silicon-containing carbon material (d)>
Silicon-containing charcoal in the same manner as in Example 1 except that the amount of the carbon material A1 obtained in Example 1 is 40% by weight and the amount of the silicon-containing carbon precursor is 60% by weight. I got the material.
[0019]
Example 4 <Production of silicon-containing carbon material (d)>
Except for using a carbon material A2 having a specific surface area of 610 m ² / g obtained by oxidation treatment using a spherical phenol resin (PR-ACS-3 manufactured by Sumitomo Bakelite) having an average particle size of 100 μm, the same procedure as in Example 1 was performed. A silicon-containing carbon material was obtained.
[0020]
Comparative Example 1
500 parts by weight of a pitch having a softening point of 120 ° C. was placed in a 1 L flask, and the pitch was dissolved at 180 to 240 ° C. 150 parts by weight of silicon powder was gradually added to the dissolved pitch. After the addition was completed, the mixture was further stirred for 1 hour, cooled to room temperature, and roughly crushed to obtain a silicon-containing carbon precursor. The obtained carbon precursor was heated up to 500 ° C. at 100 ° C./hour in a nitrogen atmosphere and held at that temperature for 5 hours. Then, it cooled and grind | pulverized to 45 micrometers or less using the vibration ball mill. The pulverized product was heated to 1000 ° C. at 100 ° C./hour and held for 3 hours. Then, it cooled and sieved with a 45 micrometer sieve, and the silicon containing carbon material was obtained.
[0021]
Comparative Example 2
200 parts by weight of graphite pulverized to an average particle size of 70 μm was placed in a rotary kiln-type drying furnace and oxidized at 450 ° C. for 5 hours with aeration of 1 L / min air. Then, it cooled to room temperature and grind | pulverized to 45 micrometers or less using the vibration ball mill, and obtained the carbon material with a specific surface area of 240 m < ² > / g. 60% by weight of the carbon material obtained and 40% by weight of a pitch with a softening point of 120 ° C. are placed in a three-necked flask, melted and mixed for 1 hour, cooled to room temperature, crushed to 100 μm or less with a screen 1 mmφ by an impact pulverizer did. The pulverized product was heated to 550 ° C. at 100 ° C./hour under a nitrogen atmosphere and held for 1.5 hours. Then, it cooled and grind | pulverized to 45 micrometers or less using the vibration ball mill.
The pulverized product was heated to 1000 ° C. at 100 ° C./hour and held for 3 hours. Then, it cooled and sieved with a 45 micrometer sieve, and the carbon material which does not contain silicon was obtained.
[0022]
Comparative Example 3
Only mesocarbon micro beads (“KMFC” manufactured by Kawasaki Steel Corporation, specific surface area 2 m ² / g) were used as the carbon material for the negative electrode material for the secondary battery.
[0023]
<Manufacture of secondary batteries>
(1) 10% by weight of polyvinylidene fluoride and 3% by weight of acetylene black are added as binders to the silicon-containing carbon materials or carbon materials obtained in the examples and comparative examples, and N is used as a diluent solvent. -A suitable amount of methyl-2-pyrrolidone was added and mixed to prepare a slurry-like negative electrode mixture. The prepared negative electrode slurry mixture was applied to both sides of a 10 μm copper foil, and then vacuum-dried at 110 ° C. for 1 hour. After vacuum drying, the electrode was pressure-formed by a roll press. This was cut into a size of 40 mm in width and 290 mm in length to produce a negative electrode. However, the copper foil was exposed at the 10 mm both ends of the negative electrode, and a negative electrode tab was pressure-bonded to this one.
(2) The positive electrode was mixed with 300 parts by weight of LiCoO ₂ as a positive electrode active material, 15 parts by weight of acetylene black and 15 parts by weight of polyvinylidene fluoride, mixed with an appropriate amount of N-methyl-2-pyrrolidone as a diluent solvent, A positive electrode mixture was prepared. The prepared positive electrode slurry mixture was applied to both sides of a 25 μm aluminum foil, and then vacuum dried at 110 ° C. for 1 hour. After vacuum drying, the electrode was pressure-formed by a roll press. This was cut into a size of 40 mm in width and 280 mm in length to produce a positive electrode. However, the aluminum foil was exposed at the 10 mm both ends of the positive electrode, and the positive electrode tab was pressure-bonded to this one.
(3) A positive electrode, a separator (polypropylene porous film: width 45 mm, thickness 25 μm), a negative electrode, a separator, a positive electrode, and the like were wound in a spiral shape in this order to produce an electrode. . The produced electrode was inserted into an AA battery can and the negative electrode tab was welded to the bottom of the can. After pouring lithium hexafluorophosphate 1 mol / liter in a mixed solution of ethylene carbonate and diethylene carbonate having a volume ratio of 1: 1 as an electrolyte into a battery can, the positive electrode tab was welded to the positive electrode lid. A secondary battery was fabricated by crimping the positive electrode lid.
[0024]
Table 1 shows the results obtained by the above-described examples and comparative examples. The specific surface area was measured by a nitrogen gas BET three-point method using NOVA1200 manufactured by Yuasa Ionics Co., Ltd. after the carbon material was manufactured.
The 2.5 V discharge capacity, initial charge / discharge efficiency, and discharge capacity retention were measured after the secondary battery was manufactured. The charging condition was maintained at 1 mA at a low current of 25 mA / g until the current attenuated to 1.25 mAh / g or less. The cut-off potential under discharge conditions was 2.5V. The discharge capacity retention rate was the discharge capacity retention rate after 300 cycles with respect to the initial discharge capacity.
[0025]
[Table 1]

[0026]
As shown in Table 1, the silicon-containing carbon material (d) obtained in Examples 1 to 4 uses a carbon material (a) having a specific surface area of 50 to 1000 m ² / g and a silicon-containing carbon precursor (b). Therefore, the charge / discharge capacity and the discharge capacity retention rate are excellent. Especially in Examples 1-3, since the graphite was used as a raw material of the said carbon material (a), it was excellent also in charging / discharging efficiency.
Since Comparative Example 1 was only a silicon-containing carbon material, the charge / discharge efficiency and the discharge capacity retention rate were insufficient. In Comparative Examples 2 and 3, the discharge capacity was reduced because only the carbon material or graphite not containing silicon was used.
[0027]
【The invention's effect】
The carbon material raw material (c) comprising the carbon material (a) having a specific surface area of 50 to 1,000 m ² / g and the silicon-containing carbon precursor (b) according to the present invention is carbonized. When the obtained carbon material is used for a secondary battery negative electrode material, a high charge / discharge capacity can be exhibited. Further, graphite is used as a raw material for the carbon material (a), and a mixture of silicon powder and pitch is used as the silicon-containing carbon precursor (b). The charge / discharge efficiency of the secondary battery can be maintained, and the charge / discharge capacity and the discharge capacity retention rate can be improved.

Claims

Look containing a carbon material and the silicon-containing carbon precursor is a specific surface area 50~1000m ² / g, a silicon-containing carbon material for a raw material used in the secondary battery negative electrode material, characterized in that the molten mixture.

The raw material for a silicon-containing carbon material used for the secondary battery negative electrode material according to claim 1, wherein the carbon material has a specific surface area of 80 to 700 m ² / g.

The raw material for a silicon-containing carbon material used for the secondary battery negative electrode material according to claim 1 or 2, wherein the carbon material is graphite.

The said carbon material is 40 to 80 weight% with respect to the said raw material for carbon materials whole, The raw material for silicon-containing carbon materials used for the secondary battery negative electrode material in any one of Claim 1 thru | or 3.

The said silicon containing carbon precursor is a raw material for silicon containing carbon materials used for the secondary battery negative electrode material in any one of Claim 1 thru | or 4 comprised from the mixture of a silicon powder and a pitch.

The said silicon powder is 10-60 weight% of the whole said silicon containing carbon precursor, The raw material for silicon containing carbon materials used for the secondary battery negative electrode material of Claim 5.

A silicon-containing carbon material obtained by carbonizing a raw material for a silicon-containing carbon material used for the secondary battery negative electrode material according to claim 1.

A secondary battery negative electrode material comprising the silicon-containing carbon material according to claim 7.

A lithium secondary battery using the secondary battery negative electrode material according to claim 8.