JP3673167B2 - Method for producing carbon material for negative electrode - Google Patents

Description

【０１８３】
表１から明らかなように、第２の製造方法により得られた炭素材料を含む負極を備えた実施例１〜４の二次電池は、単位重量当りの負極容量を比較例１〜３に比べて向上できることがわかる。
【０１８４】
これに対し、比較例２のように熱処理雰囲気中の炭酸ガス濃度が１０体積％未満であったり、比較例３のように熱処理雰囲気中の水蒸気濃度を１体積％未満にすると、容量向上の効果があまり見られないことがわかる。
【０１８５】
（実施例５〜１０）
（炭素質物の示差熱分析測定）
市販の高結晶性人工黒鉛についての示差熱分析測定により得られたＴＧ曲線（Ｙ軸）およびＤＴＡ曲線（Ｒ軸）の一例を図４に示す。この示差熱分析測定の条件を説明すると、試料である市販の高結晶性人工黒鉛の重量を５０ｍｇとし、炉内の雰囲気ガスとして炭酸ガスを使用し、ガス流量を１００ｍＬ／ｍｉｎ、測定開始温度を２０℃、測定終了温度を１４００℃、昇温速度を２０℃／ｍｉｎ、基準物質としてα−Ａｌ₂Ｏ₃を使用した。この図４のＤＴＡ曲線における吸熱ピークのピーク温度Ｔ₁は、２８０℃である。一方、発熱ピークのピーク温度Ｔ₂は、１２７０℃である。
【０１８６】
次いで、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍである市販の高結晶性人工黒鉛を用意し、この高結晶性人工黒鉛についての示差熱分析を、炉内の雰囲気ガスとして下記表２に示す４種類の雰囲気（Ｎｏ．１〜Ｎｏ．４のガス雰囲気）を使用して行った。なお、示差熱分析における他の測定条件について説明すると、試料の重量が５０ｍｇ、ガス流量が１００ｍＬ／ｍｉｎ、昇温速度が２０℃／ｍｉｎ、基準物質としてα−Ａｌ₂Ｏ₃を使用した。得られたＤＴＡ曲線から、吸熱ピークのピーク温度Ｔ₁と発熱ピークのピーク温度Ｔ₂を求めると共に、その値から（Ｔ₁＋Ｔ₂）／２を算出した。これらの結果を下記表２に示す。
【０１８７】
【表２】

【０１８８】
なお、表２中で乾燥空気とは８０体積％のＮ₂ガスと２０体積％のＯ₂ガスとの混合ガスを示す。
【０１８９】
（負極炭素材料の製造）
上記の高結晶性人工黒鉛（Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍ）５ｇをアルミナ製るつぼに入れ、前述した表２に示すＮｏ．１の雰囲気ガスの存在下で、かつ下記表３に示す温度で３時間の熱処理を行った。熱処理を行うにあたり大気が残留しないように加熱前に加熱炉の内部を真空にした後ガスを導入し、昇温中、加熱中および放冷中は０．８リットル／分の流量でガスを流した。また、昇温時間は３時間とした。熱処理後は放冷により室温まで試料温度が低下した後、加熱炉より取り出し炭素材料を得た。
【０１９０】
得られた炭素材料を用いて、負極の作製及び試験セルの組み立て、及び充放電試験を実施例１と同様に行った。充放電試験を行った結果を表３に併記する。
【０１９１】
（比較例４）
高結晶性人工黒鉛に熱処理を施さなかったこと以外は、前述した実施例５と同様にして、負極の作製と試験セルの組み立てと充放電試験を行った。充放電試験を行った結果を表３に併記する。
【０１９２】
【表３】

【０１９３】
表３から明らかなように、第１の製造方法で得られた炭素材料を含む負極を備えた実施例５〜１０の二次電池は、単位重量当りの負極容量を比較例４に比べて向上できることがわかる。
【０１９４】
（実施例１１〜１６及び比較例７〜１２）
高結晶性人工黒鉛に施す熱処理における炉内の雰囲気ガスの組成と熱処理温度を下記表４に示すように変更すること以外は、前述した実施例５と同様にして、負極の作製と試験セルの組み立てと充放電試験を行った。充放電試験を行った結果を表４に併記する。
【０１９５】
なお、表４中で乾燥空気とは８０体積％のＮ₂ガスと２０体積％のＯ₂ガスとの混合ガスである。
【０１９６】
【表４】

【０１９７】
表４から明らかなように、第１の製造方法により得られた炭素材料を含む負極を備えた実施例１１〜１６二次電池は、単位重量当りの負極容量を比較例４、７〜１２に比べて向上できることがわかる。
【０１９８】
表１〜表４から明らかなように、本発明の製造方法を用いて処理された炭素材料は、電極容量が大きく増大した。熱処理を含ＣＯ₂または含Ｈ₂Ｏガスフロー中で行った高結晶性人造黒鉛は、大気中での処理の効果をさらに上回る容量増加を示した。黒鉛化処理を含ＣＯ₂または含Ｈ₂Ｏ雰囲気で行った場合も、Ａｒ雰囲気中で黒鉛化した場合よりも大きな容量が得られる。また、ＣＯ₂が５体積％、Ｈ₂Ｏが０．５体積％の比較例においては容量増加の効果がほとんど見られなかった。
【０１９９】
（実施例１７〜２０、比較例１３〜１４）
Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍである市販の高結晶性人工黒鉛５ｇをアルミナ製るつぼに入れ、熱処理を行った。熱処理を行うにあたり大気が残留しないように加熱前に加熱炉の内部を真空にした後、雰囲気ガスを導入した。導入する雰囲気ガスには、Ａｒガスを沸騰した酸水溶液の中に通じ、酸を蒸気として含ませたものを用いた。用いた酸の種類及び酸水溶液の濃度（重量％）を下記表５に示す。昇温中および加熱中は０．４リットル／分の流量でガスを流した。また、熱処理温度は下記表５に示すように設定した。昇温時間は１時間、保持時間は３０分とした。熱処理後は放冷により試料温度が室温まで低下した後、加熱炉より取り出し、炭素材料を得た。
【０２００】
得られた炭素材料を用いて、負極の作製、試験セルの組み立て、及び充放電試験を実施例１と同様に行った。充放電試験を行った結果を表５に併記する。
【０２０１】
【表５】

【０２０２】
表５に見られるとおり、酸蒸気存在下で炭素材を熱処理することで、大きな容量の向上が得られる。この効果は、酸として濃度が６８％の硝酸水溶液を用いた場合が特に大きい。
【０２０３】
（実施例２１〜２３）
Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍである市販の高結晶性人工黒鉛に前述した実施例１１で説明したのと同様な条件、すなわち、雰囲気ガスとして５０体積％の炭酸ガスと５０体積％のアルゴンガスからなる混合ガスを使用し、熱処理温度を１２００℃として熱処理を行った。次いで、この高結晶性人工黒鉛に前述した実施例１７で説明したのと同様な条件、すなわち、濃度が６８％の硝酸水溶液を使用し、熱処理温度を４５０℃として後熱処理を行い、実施例２１に係る炭素材料を得た。
【０２０４】
Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍである市販の高結晶性人工黒鉛に前述した実施例１７で説明したのと同様な条件、すなわち、濃度が６８％の硝酸水溶液を使用し、熱処理温度を４５０℃として熱処理を行った。次いで、この高結晶性人工黒鉛に前述した実施例１１で説明したのと同様な条件、すなわち、雰囲気ガスとして５０体積％の炭酸ガスと５０体積％のアルゴンガスからなる混合ガスを使用し、熱処理温度を１２００℃として後熱処理を行い、実施例２２に係る炭素材料を得た。
【０２０５】
Ｘ線回折による（００２）面の面間隔ｄ₀₀₂が０．３３５４ｎｍである市販の高結晶性人工黒鉛に前述した実施例１７で説明したのと同様な条件、すなわち、濃度が６８％の硝酸水溶液を使用し、熱処理温度を４５０℃として熱処理を行った。次いで、この高結晶性人工黒鉛に前述した実施例１３で説明したのと同様な条件、すなわち、雰囲気ガスとして２０体積％の水蒸気と８０体積％のアルゴンガスからなる混合ガスを使用し、熱処理温度を１２００℃として後熱処理を行い、実施例２３に係る炭素材料を得た。
【０２０６】
得られた３種類の炭素材料について、負極の作製、試験セルの組み立て、及び充放電試験を実施例１と同様に行い、充放電試験結果を下記表６に示す。なお、表６に示す単位重量当りの容量は、比較例４の試験セルの単位重量あたりの容量を１００（％）としたときの値である。表６中で先に行った処理を１段階目の熱処理（Ｉ）、その後に行った処理を２段階目の熱処理（ＩＩ）としている。
【０２０７】
【表６】

【０２０８】
表６から明らかなように、１０体積％以上の炭酸ガスを含む雰囲気での熱処理と気体の酸を含む雰囲気での熱処理の双方が行われた実施例２１，２２の二次電池と、１体積％以上の水蒸気を含む雰囲気での熱処理と気体の酸を含む雰囲気での熱処理の双方が行われた実施例２３の二次電池は、高い容量を得られることがわかる。特に、１０体積％以上の炭酸ガスを含む雰囲気での熱処理の後に気体の酸を含む雰囲気での熱処理を行った実施例２１の二次電池は、容量を大幅に改善することができる。
【０２０９】
（実施例２４〜２７）
平均繊維長が４０μｍで、平均粒径が下記表８に示す値の市販の黒鉛化炭素繊維５ｇをアルミナ製るつぼに入れ、表７に示す組成の雰囲気ガスの存在下で、かつこの表７に併記している温度で３時間の熱処理を行った。熱処理を行うにあたり大気が残留しないように加熱前に加熱炉の内部を真空にした後、ガスを導入し、昇温中、加熱中および放冷中は０．８リットル／分の流量でガスを流した。また、昇温時間は３時間とした。熱処理後は放冷により室温まで試料温度が低下した後、加熱炉より取り出し、繊維状炭素材料を得た。
【０２１０】
得られた炭素材料について、平均粒径と、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂と、ＢＥＴ法による比表面積とを測定し、その結果を下記表８に示す。
【０２１１】
なお、炭素材料のＢＥＴ法による比表面積は、キャリアガスとしてＨｅ−Ｎ₂（７０：３０）の混合ガスを用い、一点式ＢＥＴ法で測定した。また、炭素材料の平均粒径は、レーザー光回折・散乱式粒度分析計（Leeds＆Northrup社製で、型番が９３２０−Ｘ１００）により測定した。すなわち、エチルアルコールからなる分散媒中に炭素材料粒子を懸濁させ、懸濁された炭素材料粒子にレーザー光を照射し、散乱光の強度分布を測定し、コンピュータ解折により粒度分布に変換する。得られた粒度分布における累積平均径（累積５０％径）を求める平均粒径とする。
【０２１２】
さらに、得られた炭素材料について、ｎ−ヘプタンに対する浸漬熱ΔＨ_i ^hと１−ニトロプロパンに対する浸漬熱ΔＨ_i ⁿとを双子型伝熱熱量計（株式会社東京理工製で、型番がＭＭＣ−５１１１）にて測定した。
【０２１３】
すなわち、試料容器に溶媒を注入する。また、ガラスアンプル中に試料を入れて真空脱気をした後、溶封する。前記試料容器の前記溶媒中に前記アンプルを沈めた後、前記双子型伝熱熱量計にセットし、平衡温度に達した後にアンプルを破壊し、攪拌して溶媒と試料を混合し、生じた熱量を測定する。
【０２１４】
前記双子型伝熱熱量計内では、試料容器と恒温体とが感熱体を兼ねた熱良導電体で接続されており、感熱体の両端の温度差の時間経過を測定することで発生熱量と変化速度を求めることができる。
【０２１５】
測定条件を以下に列記する。
【０２１６】
試料である炭素材料の量：１．２ｇ
試料前処理：１５０℃、１０ｍｍＨｇ〜５ｍｍＨｇ、６時間
測定温度：２５℃
浸漬媒はｎ−ヘキサンまたは１−ニトロプロパンで、浸漬媒量：２２ｍＬ
熱量計算：コンピュータによりデータ処理
得られた浸漬熱ΔＨ_i ^hと浸漬熱ΔＨ_i ⁿから浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を算出し、その結果を下記表８に示す。
【０２１７】
（実施例２８）
平均繊維長が４０μｍで、平均粒径が２５μｍである市販の黒鉛化炭素繊維５ｇをアルミナ製るつぼに入れ、１００体積％の炭酸ガスからなる雰囲気ガスの存在下で、１０００℃で３時間の熱処理を行った。熱処理を行うにあたり大気が残留しないように加熱前に加熱炉の内部を真空にした後、ガスを導入し、昇温中、加熱中および放冷中は０．８リットル／分の流量でガスを流した。また、昇温時間は３時間とした。熱処理後は、放冷により室温まで試料温度が低下した後、加熱炉より取り出した。
【０２１８】
次いで、この繊維状炭素材料に再び熱処理を行った。加熱炉に導入する雰囲気ガスには、濃度が６８重量％の硝酸水溶液を沸騰させたものの中にＡｒガスを通じ、硝酸を蒸気として含ませたものを用いた。昇温中および加熱中は０．４リットル／分の流量でガスを流した。また、熱処理温度は４５０℃とし、熱処理時間を３時間とした。昇温時間は１時間、保持時間は３０分とした。熱処理後は放冷により試料温度が室温まで低下した後、加熱炉より取り出し、炭素材料を得た。
【０２１９】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を測定し、その結果を下記表８に示す。
【０２２０】
（実施例２９）
気体の酸を生成させる酸水溶液として、濃度が９８重量％の酢酸水溶液を使用すること以外は、前述した実施例２８と同様にして炭素材料を得た。
【０２２１】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を調べ、その結果を下記表８に示す。
【０２２２】
（実施例３０）
気体の酸を生成させる酸水溶液として、濃度が８５重量％の燐酸水溶液を使用すること以外は、前述した実施例２８と同様にして炭素材料を得た。
【０２２３】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を調べ、その結果を下記表８に示す。
【０２２４】
（実施例３１）
カーボン前駆体である石油ピッチを出発原料とし、これを紡糸後、３００℃で１時間かけて表面の不融化処理を行った。次いで、この炭素前駆体に、１００体積％の炭酸ガスからなる雰囲気ガスの存在下で、９００℃で３時間の熱処理を行い、炭素化材料を得た。この炭素化材料は、無定形炭素もしくはソフトカーボンに属するものであった。次いで、１００体積％の炭酸ガスからなる雰囲気ガスの存在下において、この炭素化材料を２８００℃で３時間熱処理を施すことにより前記炭素化材料を黒鉛化し、繊維状の炭素材料を得た。ガスは加熱前に炉内に導入し完全に置換した後、ガスの流入を停止し、加熱を開始した。
【０２２５】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を調べ、その結果を下記表８に示す。
【０２２６】
（実施例３２）
カーボン前駆体である石油ピッチを出発原料とし、これを紡糸後、３００℃で１時間かけて表面の不融化処理を行った。次いで、この炭素前駆体に、２０体積％のＨ₂Ｏガスと８０体積％のＣＯ₂ガスからなる雰囲気ガスの存在下で、９００℃で３時間の熱処理を行い、炭素化材料を得た。この炭素化材料は、無定形炭素もしくはソフトカーボンに属するものであった。次いで、２０体積％のＨ₂Ｏガスと８０体積％のＣＯ₂ガスからなる雰囲気ガスの存在下において、この炭素化材料を２８００℃で３時間熱処理を施すことにより前記炭素化材料を黒鉛化し、繊維状の炭素材料を得た。ガスは加熱前に炉内に導入し完全に置換した後、ガスの流入を停止し、加熱を開始した。
【０２２７】
次いで、この繊維状炭素材料に熱処理を行った。加熱炉に導入する雰囲気ガスには、沸騰した濃度が６８重量％の硝酸水溶液の中にＡｒガスを通じ、硝酸を蒸気として含ませたものを用いた。昇温中および加熱中は０．４リットル／分の流量でガスを流した。また、熱処理温度は４５０℃とし、熱処理時間を３時間とした。昇温時間は１時間、保持時間は３０分とした。熱処理後は放冷により試料温度が室温まで低下した後、加熱炉より取り出し、炭素材料を得た。
【０２２８】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を調べ、その結果を下記表８に示す。
【０２２９】
（比較例１５〜２３）
平均繊維長が４０μｍで、平均粒径が下記表８に示す値の市販の黒鉛化炭素繊維に下記表７に示す組成の雰囲気ガスの存在下で、この表７に併記する温度で３時間熱処理を行うことにより比較例１６〜２３の炭素材料を得た。また、熱処理を全く行わない黒鉛化炭素繊維を比較例１５の炭素材料として用意した。なお、比較例１６，１７において熱処理雰囲気として使用した空気の組成は、８０体積％のＮ₂ガスと２０体積％のＯ₂ガスとの混合ガスからなる。
【０２３０】
得られた炭素材料について、前述した実施例２４で説明したのと同様な条件で、平均粒径、Ｘ線回折による（００２）面の面間隔ｄ₀₀₂、ＢＥＴ法による比表面積および浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を調べ、その結果を下記表８に示す。
【０２３１】
得られた実施例２４〜３２及び比較例１５〜２３の炭素材料を用いて、以下に説明する方法で試験セルを組立てた。
【０２３２】
まず、各炭素材料にポリテトラフルオロエチレンを加えてシート状とし、これをステンレスメッシュに圧着し、１５０℃で真空乾燥し試験電極とした。また、エチレンカーボネート（ＥＣ）およびメチルエチルカーボネート（ＭＥＣ）を体積比１：２で混合した非水溶媒にＬｉＰＦ₆を１Ｍ溶解させ、液状非水電解質を調製した。得られた試験電極及び液状非水電解質を用いると共に、対極および参照極を金属Ｌｉとしてアルゴン雰囲気中で試験セルを組み立てた。
【０２３３】
得られた実施例２４〜３２及び比較例１５〜２３の試験セルについて、以下の（１）及び（２）に説明する条件で充放電試験を行い、その結果を下記表８に併記する。
【０２３４】
（１）参照極と試験電極間の電位差０．０１Ｖまで１ｍＡ／ｃｍ²の電流密度で充電、さらに０．０１Ｖで５時間の定電圧充電を行った後、１ｍＡ／ｃｍ²の電流密度で２Ｖまで放電して放電容量を測定し、負極炭素材料１ｇ当りの放電容量を下記表８に併記する。
【０２３５】
（２）以下に説明する条件での充放電を繰り返し、放電容量が１サイクル目の容量の８０％に低下するまでに要したサイクル数を測定し、その結果を下記表８に併記する。
【０２３６】
充電：参照極と試験電極間の電位差０．０１Ｖまで１ｍＡ／ｃｍ²の電流密度で充電を行い、さらに０．０１Ｖで５時間の定電圧充電を行う。
【０２３７】
放電：１ｍＡ／ｃｍ²の電流密度で２Ｖまで放電する。
【０２３８】
【表７】

【０２３９】
【表８】

【０２４０】
表７、表８から明らかなように、前記（１）式で規定される浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）を有する炭素材料を含む負極を備えた実施例２４〜３２の二次電池は、高い放電容量を有し、かつサイクル寿命を長くできることがわかる。
【０２４１】
これに対し、空気もしくはアルゴンガス雰囲気で熱処理が施された炭素材料を含む負極を備えた比較例１６〜１８の二次電池は、浸漬熱比（ΔＨ_i ⁿ／ΔＨ_i ^h）が、熱処理を行っていない炭素材料を含む負極を備えた比較例１５とほとんど変わらず、放電容量及びサイクル寿命が実施例２４〜３２に比べて低いことがわかる。また、比較例１９〜２３の二次電池は、サイクル寿命が実施例２４〜３２に比べて低いことがわかる。
【０２４２】
【発明の効果】
以上詳述したように本発明によれば、非水電解質二次電池の初充放電効率、放電容量、大電流特性及びサイクル寿命を向上することが可能な負極用炭素材料を提供することができる。また、本発明によれば、初充放電効率、放電容量、大電流特性及びサイクル寿命が向上された非水電解質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】本発明に係る非水電解質二次電池の一例である円筒型非水電解質二次電池を示す部分断面図。
【図２】本発明に係る非水電解質二次電池の一例である薄型非水電解質二次電池を示す断面図。
【図３】図２のＡ部を示す断面図。
【図４】実施例における黒鉛についての示差熱分析により得られるＴＧ曲線（Ｙ軸）およびＤＴＡ曲線（Ｒ軸）の一例を示す特性図。
【符号の説明】
１…容器、
３…電極群、
４…正極、
５…セパレータ、
６…負極、
８…封口板、
９…正極端子、
１１…容器、
１２…電極群、
１３…セパレータ、
１４…活物質含有層、
１５…正極集電体、
１６…正極、
１７…負極層、
１８…負極集電体、
１９…負極、
２０…正極端子、
２１…負極端子。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carbon material for a negative electrode used for a negative electrode of a battery such as a non-aqueous electrolyte secondary battery or a non-aqueous electrolyte secondary battery, and a method for producing a carbon material for a negative electrode.
[0002]
[Prior art]
In recent years, various portable electronic devices are becoming widespread due to rapid development of miniaturization technology of electronic devices. Further, miniaturization is also required for batteries that are power sources of these portable electronic devices, and non-aqueous electrolyte secondary batteries having high energy density are attracting attention.
[0003]
Non-aqueous electrolyte secondary batteries using metallic lithium as the negative electrode active material have a very high energy density, but the battery life is short because dendritic crystals called dendrites are deposited on the negative electrode during charging. Since this grows up to reach the positive electrode and causes an internal short circuit, there is a problem in safety.
[0004]
Accordingly, attempts have been made to use lithium alloys, carbon materials, amorphous chalcogen compounds, and the like as negative electrode active materials instead of lithium metal. However, in a negative electrode including a lithium alloy, the lithium alloy is easily pulverized with a charge / discharge cycle, so that the cycle life of the secondary battery is inferior. In addition, the negative electrode containing an amorphous chalcogen compound has a problem in that the initial charge / discharge efficiency is low because an irreversible reaction is likely to occur during the first charge. For these reasons, carbon materials that can ensure the safety and cycle life of secondary batteries are put to practical use exclusively as negative electrode active materials for nonaqueous electrolyte secondary batteries.
[0005]
Conventionally, carbon materials used in non-aqueous electrolyte secondary batteries include carbon precursors such as pitch, coke, and polymers that are carbonized or graphitized by heat treatment in an inert gas atmosphere, natural graphite, artificial carbon Graphite and low-temperature calcined carbon are used.
[0006]
However, since the capacity of the carbon material as the negative electrode active material is smaller than that of lithium metal, lithium alloy, or the like, the nonaqueous electrolyte secondary battery including the negative electrode containing the carbon material cannot obtain a high discharge capacity. Have.
[0007]
By the way, the publication of Japanese Patent Laid-Open No. 5-28996 discloses a secondary battery comprising at least a positive electrode active material, a negative electrode active material, and an organic electrolyte, and 400 to 400 in an inert atmosphere or vacuum before use as a negative electrode active material. An organic electrolyte secondary battery characterized by using natural graphite heat-treated at 800 ° C. alone or in combination is disclosed.
[0008]
On the other hand, JP-A-6-290781 discloses a lithium secondary battery in which natural graphite is used as a negative electrode material capable of occluding and releasing lithium ions, wherein the natural graphite is heated to a temperature of 1800 ° C. or higher in an inert atmosphere. A lithium secondary battery characterized in that it is heat-treated in step 1 is disclosed.
[0009]
Further, JP-A-9-55204 discloses a method for manufacturing a lithium ion secondary battery having an anode containing carbon into which lithium can be inserted reversibly, and is preferably used before assembling the battery. Disclosed is a method of heating the carbon in the presence of oxygen for a sufficient amount of time and temperature to selectively remove not highly reactive carbon atoms from the carbon by gasification and gasification. Yes.
[0010]
In Journal of Power Sources, Vol. 76, pages 180 to 185, 1998, a method for removing impurities by subjecting a carbon material to heat treatment is described.
[0011]
On the other hand, in claim 1 of Japanese Patent Laid-Open No. 10-40914, a negative electrode using graphite particles having amorphous carbon attached to the surface as a negative electrode active material, and a chalcogenide containing lithium are provided. It consists of a positive electrode as a positive electrode active material and a non-aqueous ion conductor, and the negative electrode active material is formed by attaching amorphous carbon to the surface of the graphite particles after oxidizing the graphite particles. A featured non-aqueous secondary battery is disclosed.
[0012]
Further, in Claim 1 of JP-A-10-214615, it is composed of a negative electrode, a positive electrode containing a chalcogenide containing lithium as a positive electrode active material, and a non-aqueous ion conductor, A non-aqueous secondary battery is disclosed in which the negative electrode includes a carbon material in which amorphous carbon is attached to the surface of graphite particles oxidized with potassium permanganate as a negative electrode active material.
[0013]
However, none of the discharge capacities of the secondary batteries disclosed in the above-mentioned six documents are sufficient.
[0014]
[Problems to be solved by the invention]
An object of the present invention is to provide a nonaqueous electrolyte secondary battery having a high capacity and excellent cycle life characteristics.
[0015]
Another object of the present invention is to provide a carbon material for a negative electrode and a method for producing the carbon material for a negative electrode capable of increasing the capacity of a nonaqueous electrolyte secondary battery.
[0016]
[Means for Solving the Problems]
  A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode,
  Immersion heat ratio (ΔH defined by the following formula (1)_i ⁿ/ ΔH_i ^h)Graphitization with an average particle size of 5 to 100 μmA negative electrode including a material;
  With non-aqueous electrolyte
It is characterized by comprising.
[0017]
        1.2 ≦ ΔH_i ⁿ/ ΔH_i ^h≦ 2 (1)
  However, in the formula (1), the ΔH_i ^hSaidGraphitizationHeat of immersion for n-heptane of the material, the ΔH_i ⁿSaidGraphitizationHeat of immersion for the material 1-nitropropane.
[0018]
In the nonaqueous electrolyte secondary battery according to the present invention, the carbon material contains 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂It is preferable to obtain the carbonaceous material containing at least one of the carbonized material and the graphitized material in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O.
[0019]
In the nonaqueous electrolyte secondary battery according to the present invention, the carbon material may be a carbonaceous material containing at least one of a carbonized material and a graphitized material, and 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂Obtained by performing a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O and then performing a heat treatment in an atmosphere containing at least one of an inorganic acid gas and an organic acid gas. It is preferable.
[0020]
In the non-aqueous electrolyte secondary battery according to the present invention, the immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Is preferably in the range of 1.25 to 1.75.
[0021]
In the non-aqueous electrolyte secondary battery according to the present invention, it is preferable that an average particle diameter of the carbon material is in a range of 5 to 100 μm.
[0022]
In the non-aqueous electrolyte secondary battery according to the present invention, the specific surface area of the carbon material according to the BET method is 1 to 50 m.²/ G is desirable.
[0023]
In the non-aqueous electrolyte secondary battery according to the present invention, the surface spacing d of the (002) plane determined by X-ray diffraction of the carbon material.₀₀₂Is preferably in the range of 0.335 nm to 0.34 nm.
[0024]
In the non-aqueous electrolyte secondary battery according to the present invention, the non-aqueous electrolyte is preferably one of a liquid non-aqueous electrolyte, a gel-like non-aqueous electrolyte, and a solid non-aqueous electrolyte.
[0025]
  The carbon material for a negative electrode according to the present invention has an immersion heat ratio (ΔH defined by the following formula (1)_i ⁿ/ ΔH_i ^h)And a graphitized material having an average particle size of 5 to 100 μmIt is characterized by this.
[0026]
        1.2 ≦ ΔH_i ⁿ/ ΔH_i ^h≦ 2 (1)
  However, in the formula (1), the ΔH_i ^hSaidGraphitizationHeat of immersion for n-heptane of the material, the ΔH_i ⁿSaidGraphitizationHeat of immersion for the material 1-nitropropane.
[0027]
In the carbon material for a negative electrode according to the present invention, the carbon material contains 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂It is preferably obtained by subjecting a carbonaceous material containing at least one of a carbonized material and a graphitized material to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O.
[0028]
In the carbon material for a negative electrode according to the present invention, the carbon material contains 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂It is obtained by performing heat treatment in an atmosphere containing at least one of an inorganic acid gas and an organic acid gas after performing the heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O. It is preferable.
[0029]
In the carbon material for a negative electrode according to the present invention, the immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Is preferably in the range of 1.25 to 1.75.
[0030]
In the carbon material for a negative electrode according to the present invention, the average particle size of the carbon material is preferably in the range of 5 to 100 μm.
[0031]
In the carbon material for a negative electrode according to the present invention, the specific surface area of the carbon material according to the BET method is 1 to 50 m.²/ G is desirable.
[0032]
In the carbon material for a negative electrode according to the present invention, the spacing d of (002) planes determined by X-ray diffraction of the carbon material.₀₀₂Is preferably in the range of 0.335 nm to 0.34 nm.
[0033]
The first method for producing a carbon material for a negative electrode according to the present invention comprises 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, the method includes a step of performing a heat treatment on a carbonaceous material containing at least one of a carbonized material and a graphitized material. .
[0034]
In the first manufacturing method according to the present invention, it is preferable that the heat treatment temperature satisfies the following expression (2).
[0035]
(T₁+ T₂) / 2 ≦ T ≦ T₂        (2)
However, in the formula (2), the T is the heat treatment temperature (° C.), and the T₁Is the peak temperature (° C.) of the endothermic peak obtained when differential thermal analysis measurement is performed on the carbonaceous material in the gas atmosphere.₂Is the peak temperature (° C.) of the exothermic peak obtained by the differential thermal analysis measurement.
[0036]
The second method for producing a carbon material for negative electrode according to the present invention comprises 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂It comprises a step of carbonizing or graphitizing the carbon precursor by subjecting the carbon precursor to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O. is there.
[0037]
In the second method for producing a carbon material for a negative electrode according to the present invention, when the heat treatment temperature is in a range of 800 ° C. or more and less than 2000 ° C., the first gas atmosphere and the third gas atmosphere CO₂Concentrations are 50 volume% or more and 100 volume% or less, respectively, and the H in the second gas atmosphere and the third gas atmosphere are each₂The O concentration is preferably 2% by volume or more and 100% by volume or less, respectively.
[0038]
In the second method for producing a carbon material for a negative electrode according to the present invention, when the heat treatment temperature is in a range of 2000 ° C. to 3000 ° C., the CO in the first gas atmosphere and the third gas atmosphere₂The concentration is 10% by volume or more and 60% by volume or less, respectively, and the H in the second gas atmosphere and the third gas atmosphere₂The O concentration is preferably 1.5% by volume or more and 30% by volume or less, respectively.
[0039]
The third method for producing a carbon material for a negative electrode according to the present invention includes a carbon containing at least one of a carbonized material and a graphitized material in an atmosphere containing at least one of an inorganic acid gas and an organic acid gas. It comprises a step of subjecting the material to a heat treatment.
[0040]
The fourth method for producing a carbon material for a negative electrode according to the present invention comprises 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂Applying a heat treatment to a carbonaceous material containing at least one of a carbonized material and a graphitized material in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O;
Contacting the carbonaceous material with a gaseous acid;
It is characterized by comprising.
[0041]
The fifth method for producing a carbon material for negative electrode according to the present invention comprises 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂Carbonizing or graphitizing the carbon precursor by subjecting the carbon precursor to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O;
And a step of bringing a gaseous acid into contact with the carbonized or graphitized carbon precursor.
[0042]
In the third to fifth production methods according to the present invention, the gaseous acid is preferably acid vapor.
[0043]
In the third to fifth manufacturing methods according to the present invention, the gaseous acid is preferably composed of at least one of an inorganic acid vapor and an organic acid vapor.
[0044]
In the third to fifth production methods according to the present invention, the inorganic acid is one or more acids selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, hydrofluoric acid, shelf acid and phosphoric acid, and the organic acid The acid is preferably at least one acid selected from the group consisting of formic acid, acetic acid, propionic acid, phenol and oxalic acid.
[0045]
DETAILED DESCRIPTION OF THE INVENTION
The manufacturing method of the 1st-5th carbon material for negative electrodes which concerns on this invention is demonstrated.
[0046]
1. Method for producing first carbon material for negative electrode
In this production method, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, a step of performing a heat treatment on a carbonaceous material containing at least one of a carbonized material and a graphitized material is provided. Here, the gas atmosphere means a gas introduced into the firing furnace during the heat treatment.
[0047]
The carbonized material can be obtained by subjecting a carbon precursor such as pitch, coke, or polymer to a heat treatment. Specific examples of the carbonized material include amorphous carbon and soft carbon. On the other hand, the graphitized material can be obtained by carbonizing a carbon precursor such as pitch, coke, polymer, etc. by heat treatment, and then subjecting the obtained carbonized material to heat treatment again. The carbonization treatment and the graphitization treatment may each be performed by the method described in the second manufacturing method described later. Further, as the carbonized material, for example, low-temperature calcined carbon can be used. On the other hand, as the graphitized material, for example, natural graphite or artificial graphite may be used.
[0048]
In the present invention, a mixture of a carbonized material and a graphitized material can be used as the carbonaceous material.
[0049]
The carbonized material and the graphitized material each have a (002) plane spacing d determined by X-ray diffraction measurement.₀₀₂Is preferably 0.34 nm or less. A more preferable range is 0.335 nm to 0.34 nm.
[0050]
The carbonized material and the graphitized material may each contain other elements such as boron, phosphorus, and fluorine. In order to increase the reaction rate, it is effective to add an alkali metal or an alkaline earth metal to the carbonaceous material.
[0051]
The heat treatment temperature T (° C.) preferably satisfies the following formula (2).
[0052]
(T₁+ T₂) / 2 ≦ T ≦ T₂        (2)
However, in the formula (2), the T is the temperature (° C.) of the heat treatment, and the T₁Is the peak temperature (° C.) of the endothermic peak obtained when differential thermal analysis measurement is performed on the carbonaceous material in the gas atmosphere.₂Is the peak temperature (° C.) of the exothermic peak obtained by the differential thermal analysis measurement.
[0053]
Differential thermal analysis measurement is performed on the carbonaceous material before heat treatment. The measurement atmosphere in the differential thermal analysis has the same composition as the gas atmosphere for heat treatment. The peak temperature obtained by differential thermal analysis measurement is affected by the amount of sample, the atmospheric gas flow rate, the heating rate, etc.₁And T₂Is preferably set to a sample weight of 50 mg, an atmospheric gas flow rate of 100 mL / min, and a temperature increase rate of about 20 ° C./min.
[0054]
Endothermic peak peak temperature T₁(° C.) is generated by a dehydration reaction of a carbonaceous material or a vaporization reaction of a volatile component from the carbonaceous material. On the other hand, the peak temperature T of the exothermic peak₂(° C.) is generated by an oxidation reaction of the carbonaceous material. By setting the heat treatment temperature T within the range defined by the aforementioned equation (2), the capacity of the secondary battery can be significantly improved. Heat treatment temperature (T₁+ T₂If the temperature is lower than) / 2, the surface modification reaction is difficult to proceed, and a high battery capacity may not be obtained. On the other hand, the heat treatment temperature is T₂When the temperature is higher, the entire carbonaceous material is rapidly oxidized, so that it may be difficult to sufficiently increase the capacity, and the weight loss due to oxidation of the carbonaceous material may increase.
[0055]
Although the optimum heat treatment time tends to vary depending on other manufacturing conditions such as the heat treatment temperature, the preferred heat treatment time is 0.5 hours or more and 48 hours or less, and the more preferred heat treatment time is 1 hour or more, 12 hours or more. Below time.
[0056]
CO₂And H₂Examples of the gas contained in the heat treatment atmosphere other than O include at least one gas selected from the group consisting of oxygen gas, nitrogen gas, and inert gas. Examples of the inert gas include argon gas, helium gas, xenon gas, and krypton gas. Among these, it is preferable to use a non-oxidizing gas typified by nitrogen gas or inert gas in combination. Further, it is better that the oxygen gas is not contained in the heat treatment atmosphere, that is, the gas introduced into the firing furnace. However, when the oxygen gas is contained in the heat treatment atmosphere, the oxygen gas content is 10% by volume or less. It is desirable to make it. If the oxygen gas is contained in the heat treatment atmosphere in an amount exceeding 10% by volume, oxygen may cause a combustion reaction with carbon, which may make it difficult to obtain a preferable surface state. At the same time, the amount of gasification during the surface treatment of the carbonaceous material may increase and the yield may decrease.
[0057]
The first method for producing a carbon material for a negative electrode according to the present invention as described above is obtained by adding 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, a step of performing a heat treatment on a carbonaceous material containing at least one of a carbonized material and a graphitized material is provided. According to such a manufacturing method, the carbon material for negative electrodes which can implement | achieve a high capacity | capacitance nonaqueous electrolyte secondary battery can be provided. I believe this is due to the mechanism described below.
[0058]
That is, O₂The reaction of carbon to carbon dioxide in the presence of carbon is a carbon combustion reaction, which is an exothermic reaction, whereas H₂O or CO₂The reaction in which carbon changes to carbon dioxide in the presence of is an endothermic reaction. For this reason, H₂O or CO₂In the presence of O₂Carbon gasification is less likely to occur than in the presence of. When heat treatment is performed on at least one of the carbonized material and the graphitized material in the first, second, or third gas atmosphere described above, the portion having high conductivity and high degree of graphitization has a gasification rate. A portion having a low degree of graphitization that is easily gasified can be selectively converted into carbon dioxide gas while remaining small. Further, since this gasification reaction proceeds gently, pores can be uniformly formed on the surface and inside of the carbonaceous material by the gasification reaction. Furthermore, the crystallinity of the carbonaceous material can be further increased by the heat treatment. As a result, the conductivity of the carbonaceous material and the utilization rate of the lithium storage site can be improved, and the capacity of the nonaqueous electrolyte secondary battery can be improved because some pores function as lithium storage sites. Can do.
[0059]
By the way, CO in gas atmosphere₂The content is less than 10% by volume and H₂When the O content is less than 1% by volume, the amount of gasification reaction at a portion having a low graphitization degree becomes insufficient, and it becomes difficult to obtain a high discharge capacity in the secondary battery. H₂O and CO₂O instead of₂When carbon dioxide is used, the combustion reaction of carbon occurs rapidly, making it difficult to selectively gasify a portion having a low graphitization degree.
[0060]
In addition, when the heat treatment temperature T is 2000 ° C. or higher, CO in the first gas atmosphere and the third gas atmosphere₂The concentration is preferably 10% by volume or more and 60% by volume or less. This is because when the heat treatment temperature T is 2000 ° C. or higher, CO in the first gas atmosphere₂This is because if the concentration exceeds 60% by volume, the gasification reaction rate increases and gasification may occur up to a portion having a high degree of graphitization. On the other hand, when the heat treatment temperature T is less than 2000 ° C., CO in the first gas atmosphere and the third gas atmosphere₂The concentration is desirably 50% by volume or more and 100% by volume or less. This is because when the heat treatment temperature T is less than 2000 ° C., CO in the first gas atmosphere₂This is because if the concentration is less than 50% by volume, the reaction is difficult to proceed and it may be difficult to obtain a high-capacity carbon material in a short heat treatment time.
[0061]
H in the second gas atmosphere and the third gas atmosphere₂When the heat treatment temperature T is 2000 ° C. or higher, the O concentration is preferably 1.5% by volume or more and 30% by volume or less. This is because when the heat treatment temperature T is 2000 ° C. or higher, the second gas atmosphere H₂This is because if the O concentration exceeds 30% by volume, the gasification reaction rate increases and gasification may occur up to a portion having a high degree of graphitization. Further, H in the second gas atmosphere and the third gas atmosphere₂When the heat treatment temperature T is less than 2000 ° C., the O concentration is desirably 2% by volume or more and 100% by volume or less. This is because when the heat treatment temperature T is less than 2000 ° C., the second gas atmosphere H₂This is because if the O concentration is less than 2% by volume, the reaction is difficult to proceed and it may be difficult to obtain a high-capacity carbon material in a short heat treatment time.
[0062]
In the first method for producing a carbon material for a negative electrode according to the present invention, when the heat treatment temperature T (° C.) satisfies the above-described formula (2), a non-electrode including a negative electrode containing the carbon material produced by this method is provided. The water electrolyte secondary battery can further improve the capacity, and can improve the initial charge / discharge efficiency and large current characteristics.
[0063]
2. Method for producing second carbon material for negative electrode
In this production method, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂A step of carbonizing or graphitizing the carbon precursor by subjecting the carbon precursor to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O.
[0064]
Any material can be used as the carbon precursor as long as it is a carbon-based material, a carbonizable material, or a graphitizable material. Specific examples of the carbon precursor include pitch, coke, and polymer. The carbon precursor may contain other elements such as boron, phosphorus and fluorine. In order to increase the reaction rate, it is effective to add an alkali metal or an alkaline earth metal to the carbon precursor.
[0065]
The heat treatment temperature is preferably in the range of 800 ° C to 3000 ° C. This is due to the following reason. When the heat treatment temperature is less than 800 ° C., it is easy to obtain a carbon material in which the graphite structure is not sufficiently developed and a large amount of volatile components contained in the carbon precursor remain. Since such a carbon material has low conductivity, the large current discharge characteristics and cycle characteristics of the secondary battery are deteriorated. On the other hand, performing the heat treatment at a temperature exceeding 3000 ° C. may cause an increase in the manufacturing cost, or the manufacturing process may be complicated. In particular, by setting the heat treatment temperature within the range of 2000 to 3000 ° C., the discharge capacity and cycle life characteristics of the secondary battery can be improved. Therefore, the heat treatment temperature is more preferably in the range of 2000 to 3000 ° C. The most preferred range is 2500 to 3000 ° C.
[0066]
When the heat treatment temperature is 2000 ° C. or higher, CO in the first gas atmosphere and the third gas atmosphere is used for the same reason as described in the first manufacturing method.₂The concentration is preferably 10% by volume or more and 60% by volume or less, and H in the second gas atmosphere and the third gas atmosphere is preferable.₂The O concentration is preferably 1.5% by volume or more and 30% by volume or less.
[0067]
On the other hand, when the heat treatment temperature T is less than 2000 ° C., the CO in the first gas atmosphere and the third gas atmosphere for the same reason as described in the first manufacturing method described above.₂The concentration is preferably 50% by volume or more and 100% by volume or less, and H in the second gas atmosphere and the third gas atmosphere is preferred.₂The O concentration is desirably 2% by volume or more and 100% by volume or less.
[0068]
Although the optimum heat treatment time tends to vary depending on other manufacturing conditions such as the heat treatment temperature, the preferred heat treatment time is 0.5 hours or more and 48 hours or less, and the more preferred heat treatment time is 1 hour or more, 12 hours or more. Below time.
[0069]
CO₂And H₂Examples of the gas other than O contained in the atmosphere include at least one gas selected from the group consisting of oxygen gas, nitrogen gas, and inert gas. Examples of the inert gas include argon, helium, xenon, krypton, and the like. Among these, it is preferable to use a non-oxidizing gas typified by nitrogen gas or inert gas in combination. In addition, it is better that the gas introduced into the firing furnace, that is, the gas introduced into the firing furnace, does not contain oxygen gas. However, when oxygen gas is contained in the gas atmosphere, the above-described first method has been described. For the same reason as described above, the oxygen gas content is desirably 10% by volume or less.
[0070]
The second method for producing a carbon material for a negative electrode according to the present invention described above is characterized by 10% by volume or more of CO.₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂A step of carbonizing or graphitizing the carbon precursor by subjecting the carbon precursor to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O. According to such a manufacturing method, the carbon material for negative electrodes which can implement | achieve a high capacity | capacitance nonaqueous electrolyte secondary battery can be provided. I believe this is due to the mechanism described below.
[0071]
That is, according to the present invention, the carbon precursor can be carbonized or graphitized, and a region with a low graphitization degree can be selectively gasified, so that the conductivity and utilization rate of the lithium storage site are improved. And a carbon material in which some pores function as lithium storage sites can be produced. As a result, the capacity of the nonaqueous electrolyte secondary battery including a negative electrode including such a carbon material can be improved.
[0072]
3. Method for producing third carbon material for negative electrode
This manufacturing method includes a step of heat-treating a carbonaceous material containing at least one of a carbonized material and a graphitized material in an atmosphere containing at least one of an inorganic acid gas and an organic acid gas.
[0073]
The carbonized material can be obtained by subjecting a carbon precursor such as pitch, coke, or polymer to a heat treatment. Specific examples of the carbonized material include amorphous carbon and soft carbon. On the other hand, the graphitized material can be obtained by carbonizing a carbon precursor such as pitch, coke, polymer, etc. by heat treatment, and then subjecting the obtained carbonized material to heat treatment again. The carbonization treatment and the graphitization treatment are each preferably performed by the method described in the second manufacturing method described above. Further, as the carbonized material, for example, low-temperature calcined carbon can be used. On the other hand, as the graphitized material, for example, natural graphite or artificial graphite may be used.
[0074]
In the present invention, a mixture of a carbonized material and a graphitized material can be used as the carbonaceous material.
[0075]
The carbonized material and the graphitized material each have a (002) plane spacing d determined by X-ray diffraction measurement.₀₀₂Is preferably 0.34 nm or less. A more preferable range is 0.335 nm to 0.34 nm.
[0076]
The carbonized material and the graphitized material may each contain other elements such as boron, phosphorus, and fluorine. In order to increase the reaction rate, it is effective to add an alkali metal or an alkaline earth metal to the at least one carbonaceous material.
[0077]
Examples of the inorganic acid include nitric acid, hydrochloric acid, sulfuric acid, hydrofluoric acid, shelf acid, and phosphoric acid. On the other hand, examples of the organic acid include formic acid, acetic acid, propionic acid, phenol, and oxalic acid.
[0078]
Among the organic acids and the inorganic acids, an acid having a high oxidizing power, specifically, a Lewis acid capable of imparting an oxygen atom is desirable for improving the electrode capacity. In particular, organic acids such as nitric acid and acetic acid having high oxidizing power are desirable for improving electrode characteristics. Most preferred is nitric acid. When nitric acid is used, the reaction temperature is preferably 130 ° C. or higher which is the boiling point of the aqueous nitric acid solution.
[0079]
For the inorganic acid and the organic acid, a functional group having polarity such as a functional group containing a boron atom, a functional group containing a nitrogen atom, a functional group containing an oxygen atom, or a functional group containing a phosphorus atom is introduced into the carbonaceous material. Can do. In particular, functional groups containing an oxygen atom such as a carboxyl group, a carbonyl group, a hydroxyl group, a lactone group, and a ketone group are easily introduced into the carbonaceous material. These functional groups are considered to be introduced into the carbonaceous material during the heat treatment. In addition, these functional groups function as lithium storage sites and improve wettability between the liquid non-aqueous electrolyte, which is a polar solvent, and the carbon material of the negative electrode, so that electrode characteristics can be improved.
[0080]
As the inorganic acid gas, in addition to the inorganic acid vapor, for example, water containing an inorganic acid vapor by boiling an aqueous solution of the inorganic acid can be used. On the other hand, as the organic acid gas, in addition to the organic acid vapor, for example, water vapor containing an organic acid vapor obtained by boiling an organic acid aqueous solution can be used.
[0081]
Examples of the gas contained in the atmosphere other than the inorganic acid gas and the organic acid gas include at least one gas selected from the group consisting of oxygen gas, nitrogen gas, and inert gas. . Examples of the inert gas include argon gas, helium gas, xenon gas, and krypton gas. Among these, it is preferable to use a non-oxidizing gas typified by nitrogen gas or inert gas in combination.
[0082]
The heat treatment temperature is preferably 800 ° C. or lower. When the heat treatment temperature exceeds 800 ° C., the reaction proceeds rapidly, so that it may be difficult to uniformly acid-treat the surface of the carbonaceous material. A more preferable range of the heat treatment temperature is 500 ° C. or less. The lower limit of the heat treatment temperature is set so that an inorganic acid or an organic acid can exist in a gaseous state in the atmosphere in which the heat treatment is performed. Therefore, it is desirable that the heat treatment temperature be higher than the vaporization temperature of the inorganic acid or organic acid and not higher than 800 ° C.
[0083]
Examples of the heat treatment method include the methods described in the following (a) to (c), but are not limited thereto.
[0084]
(A) A method of circulating a gas containing at least one of an inorganic acid gas and an organic acid gas on a carbonaceous material composed of at least one of a carbonized material and a graphitized material, and heating them.
[0085]
(B) A method in which an acid is added to the carbonaceous material to form a dispersion or slurry, followed by heat treatment at a temperature equal to or higher than the boiling point of the acid.
[0086]
(C) A method in which the carbonaceous material is granulated using an acid as a granulating agent and reacted when dried at a high temperature.
[0087]
The above-described third method for producing a carbon material for negative electrode according to the present invention includes at least one of a carbonized material and a graphitized material in an atmosphere containing at least one of an inorganic acid gas and an organic acid gas. The carbonaceous material containing is heat-processed. A nonaqueous electrolyte secondary battery including a negative electrode including a carbon material manufactured by such a method can improve initial charge / discharge efficiency, discharge capacity, and charge / discharge cycle life. I believe this is due to the mechanism described below.
[0088]
As in the third invention, when the carbonaceous material is subjected to a heat treatment in an atmosphere in which at least one of an organic acid gas and an inorganic acid gas is present, pores can be formed on the surface of the carbonaceous material, It is possible to introduce a functional group, particularly a functional group having polarity such as a carboxyl group, a carbonyl group, a hydroxyl group, a lactone group, and a ketone group, on the surface of the carbonaceous material. These functional groups can function as a lithium storage site and can improve the wetting of the liquid non-aqueous electrolyte that is a polar solvent and the carbon material of the negative electrode. As a result, the nonaqueous electrolyte secondary battery including the negative electrode including the carbon material manufactured by the method of the present invention can improve the initial charge / discharge efficiency, the discharge capacity, and the charge / discharge cycle life.
[0089]
Note that both the third manufacturing method and the first manufacturing method described above may be applied to the carbonaceous material as the surface modification treatment. Moreover, the carbon material for negative electrodes obtained by the second manufacturing method is subjected to the first manufacturing method, the third manufacturing method, or both the first manufacturing method and the third manufacturing method as the surface modification treatment. Can be applied.
[0090]
Particularly preferred combinations include (I) performing a surface modification treatment on the negative electrode carbon material obtained by the second production method by the third production method, and (II) among the graphitized material and the carbonized material. A carbonaceous material containing at least one may be subjected to a surface modification treatment by the first production method and then a final surface modification treatment by the third production method. According to the methods (I) and (II) described above, a negative electrode carbon material having better characteristics can be obtained by the synergistic effect of each treatment.
[0091]
4). Fourth method for producing carbon material for negative electrode
In this production method, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂Applying a heat treatment to a carbonaceous material containing at least one of a carbonized material and a graphitized material in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O;
Contacting the carbonaceous material with a gaseous acid;
It comprises.
[0092]
The heat treatment step can be performed by the same method as described in the first method for producing a carbon material for negative electrode.
[0093]
For gaseous acids, it is desirable to use acid vapors.
[0094]
As the gaseous acid, one or more kinds of gases selected from the group consisting of inorganic acid gas and organic acid gas can be used. Examples of the inorganic acid include nitric acid, hydrochloric acid, sulfuric acid, hydrofluoric acid, shelf acid, and phosphoric acid. On the other hand, examples of the organic acid include formic acid, acetic acid, propionic acid, phenol, and oxalic acid. Of the organic acid and the inorganic acid, nitric acid having high oxidizing power is preferable.
[0095]
As the inorganic acid gas, in addition to the inorganic acid vapor, for example, water containing an inorganic acid vapor by boiling an aqueous solution of the inorganic acid can be used. On the other hand, as the organic acid gas, in addition to the organic acid vapor, for example, water vapor containing an organic acid vapor obtained by boiling an organic acid aqueous solution can be used.
[0096]
When the gaseous acid is brought into contact with the carbonaceous material that has been subjected to the heat treatment step, the carbonaceous material is preferably subjected to a thermal treatment so that the acid can maintain a gaseous state. The heat treatment temperature is preferably not less than the vaporization temperature of the inorganic acid or organic acid and not more than 800 ° C. When the heat treatment temperature exceeds 800 ° C., the reaction proceeds rapidly, so that it may be difficult to uniformly acid-treat the surface of the carbonaceous material. A more preferable range of the heat treatment temperature is not less than the vaporization temperature of the inorganic acid or organic acid and not more than 500 ° C. For example, when nitric acid is used as the inorganic acid, the heat treatment temperature is desirably in the range of 130 ° C to 500 ° C.
[0097]
Examples of the method of bringing a gaseous acid into contact with the carbonaceous material that has been subjected to the heat treatment step include the methods described in (A) to (C) below, but are not limited thereto.
[0098]
(A) A method in which a gas containing at least one of an inorganic acid gas and an organic acid gas is circulated on a carbonaceous material composed of at least one of a carbonized material and a graphitized material, and these are heated.
[0099]
(B) A method in which an acid is added to the carbonaceous material to form a dispersion or slurry, followed by heat treatment at a temperature equal to or higher than the boiling point of the acid.
[0100]
(C) A method in which the carbonaceous material is granulated using an acid as a granulating agent and reacted when dried at a high temperature.
[0101]
According to the fourth method for producing a carbon material for a negative electrode according to the present invention described above, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, by subjecting the carbonaceous material containing at least one of the carbonized material and the graphitized material to heat treatment, the carbonaceous material is relatively graphitized. Since the gasification rate is lower in the high degree part than in the low graphitization part, the part with the low graphitization degree can be selectively gasified among the carbonaceous substances, and the surface and the inside of the carbonaceous substance are finely divided. The holes can be formed uniformly.
[0102]
Next, by contacting the carbonaceous material with a gaseous acid, polarities such as a functional group, particularly a carboxyl group, a carbonyl group, a hydroxyl group, a lactone group, and a ketone group are formed on the pores present on the surface and inside of the carbonaceous material. A functional group having can be introduced.
[0103]
The carbon material thus obtained has high crystallinity and excellent conductivity. Moreover, this carbon material can function the pores and polar functional groups formed on the surface and inside as lithium storage sites. Furthermore, the carbon material can improve the wetting of the liquid non-aqueous electrolyte, which is a polar solvent, and the carbon material of the negative electrode because the polar functional groups are uniformly present on the surface and inside. As a result, the nonaqueous electrolyte secondary battery including the negative electrode including the carbon material manufactured by the method of the present invention can significantly improve the initial charge / discharge efficiency, the discharge capacity, and the charge / discharge cycle life.
[0104]
5. Method for producing fifth carbon material for negative electrode
In this production method, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂A step of obtaining a carbonaceous material by carbonizing or graphitizing the carbon precursor by subjecting the carbon precursor to a heat treatment in a gas atmosphere selected from the group consisting of a third gas atmosphere containing O;
Contacting the carbonaceous material with a gaseous acid;
It comprises.
[0105]
The carbonization treatment step and the graphitization treatment step can be performed by the same method as described in the second method for producing a carbon material for negative electrode.
[0106]
The acid treatment step can be performed by the same method as described in the above-described fourth method for producing a carbon material for negative electrode.
[0107]
According to the fifth method for producing a carbon material for a negative electrode according to the present invention, 10% by volume or more of CO₂A first gas atmosphere containing 1% by volume or more of H₂Second gas atmosphere containing O and 10% by volume or more of CO₂And 1 vol% or more of H₂In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, the carbon precursor is carbonized or graphitized by subjecting the carbon precursor to a heat treatment to obtain a carbonaceous material on the surface and inside. A carbonaceous material having uniformly formed pores and high conductivity can be produced.
[0108]
Next, by contacting the carbonaceous material with a gaseous acid, polarities such as a functional group, particularly a carboxyl group, a carbonyl group, a hydroxyl group, a lactone group, and a ketone group are formed on the pores present on the surface and inside of the carbonaceous material. A functional group having can be introduced.
[0109]
The carbon material thus obtained has high conductivity. Moreover, this carbon material can function the pores and polar functional groups formed on the surface and inside as lithium storage sites. Furthermore, the carbon material can improve the wetting of the liquid non-aqueous electrolyte, which is a polar solvent, and the carbon material of the negative electrode because the polar functional groups are uniformly present on the surface and inside. As a result, the nonaqueous electrolyte secondary battery including the negative electrode including the carbon material manufactured by the method of the present invention can significantly improve the initial charge / discharge efficiency, the discharge capacity, and the charge / discharge cycle life.
[0110]
Hereinafter, the nonaqueous electrolyte secondary battery according to the present invention will be described.
[0111]
The non-aqueous electrolyte secondary battery according to the present invention includes a container, a positive electrode housed in the container, a soaking heat ratio (ΔH) which is housed in the container and defined by the following equation (1):_i ⁿ/ ΔH_i ^hAnd a non-aqueous electrolyte housed in the container.
[0112]
1.2 ≦ ΔH_i ⁿ/ ΔH_i ^h≦ 2 (1)
However, in the formula (1), the ΔH_i ^hIs the heat of immersion for n-heptane of the carbon material, and the ΔH_i ⁿIs the heat of immersion for the carbon material 1-nitropropane.
[0113]
Hereinafter, the negative electrode, the positive electrode, the nonaqueous electrolyte, and the container will be described.
[0114]
1) Negative electrode
The negative electrode includes a current collector and an active material-containing layer that is supported on one or both surfaces of the current collector and includes a carbon material.
[0115]
First, the carbon material will be described in more detail.
[0116]
Immersion heat (ΔH_i) Is a dispersion force (h_i ^d), Polarization force (h_i ^α), Interaction between the liquid permanent dipole and the electrostatic field on the solid surface (h_i ^μ).
[0117]
ΔH_i= H_i ^d+ H_i ^α+ H_i ^μ        (I)
In the formula (I), h_i ^d+ H_i ^αIs constant, ΔH_iIs h_i ^μRespond to changes. When the strength of the electrostatic field on the solid surface is F, the relationship shown in the following formula (II) is established.
[0118]
h_i ^μ= -NμF (II)
Therefore, the following formula (III) is established.
[0119]
-ΔH_i= NμF + const. (III)
In the formulas (II) and (III), the μ is the dipole efficiency of the immersion liquid, and the n is the number of adsorbed molecules per unit surface area of the solid.
[0120]
If the immersion heat is determined using a series of adsorbates having the same adsorption occupation area of one molecule and different dipole efficiency, the strength of the electrostatic field on the surface can be calculated from the relations of the above formulas (I) to (III). That is, surface polarity is required.
[0121]
n-Heptane and 1-nitropropane have the same adsorption area per molecule. Regarding the dipole efficiency μ, the dipole efficiency μ of n-heptane is 0D, and the dipole efficiency μ of 1-nitropropane is 3.75D. Thus, the difference in immersion heat for these two solvents reflects the surface polarity. Heat of immersion ΔH for n-heptane of carbon material_i ^hHeat of immersion ΔH for 1-nitropropane carbon material_i ⁿRatio (ΔH_i ⁿ/ ΔH_i ^h) Mainly reflects the surface polarity of the carbon material and the magnitude of the dispersion force of the carbon material in the solvent. That is, the immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Increases as the surface polarity of the carbon material increases, and tends to decrease as the particle size of the carbon material decreases and the contribution of the dispersion force of the carbon material to the solvent increases.
[0122]
The polarity of the surface of the carbon material is generated by a hetero atom other than carbon and a functional group having a hetero atom present on the surface of the carbon material. Examples of the hetero atom include a boron atom, a nitrogen atom, an oxygen atom, and a phosphorus atom. Since the hetero atom and the functional group containing the hetero atom present on the surface of the carbon material can function as a lithium occlusion / release site, the lithium occlusion / release site of the carbon material can be increased. In addition, by increasing the polarity of the surface of the carbon material, it is possible to improve the affinity between the carbon material and the liquid nonaqueous electrolyte that is a polar solvent.
[0123]
By the way, the immersion heat about n-hexane of a carbon material is substantially constant irrespective of the magnitude | size of the polarity of the surface of the said carbon material. On the other hand, the immersion heat about 1-nitropropane of a carbon material becomes so large that the polarity of the surface of the said carbon material becomes high. Immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Is less than 1.2, the immersion heat for the carbon material 1-nitropropane is small, and the polarity of the carbon material surface is low. Discharge capacity decreases. On the other hand, the immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Of more than 2 has a large immersion heat with respect to 1-nitropropane of the carbon material and the polarity of the surface of the carbon material is high. Lowering and side reactions such as decomposition of the nonaqueous electrolyte may occur. Immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) Is more preferably 1.25 to 1.75.
[0124]
The average particle size of the carbon material is preferably in the range of 5 to 100 μm. This is due to the following reason. When the average particle size is less than 5 μm, many end faces of the hexagonal mesh face layer constituting the graphite crystallite are exposed on the surface of the carbon material, which may reduce cycle life characteristics and initial charge / discharge efficiency. On the other hand, when the average particle size exceeds 100 μm, the reaction area of the carbon material is insufficient, so that the reaction rate of the lithium occlusion / release reaction is lowered and the discharge capacity of the secondary battery may be lowered. A more preferable range of the average particle diameter is 10 to 80 μm.
[0125]
Since the conventional carbon material has a small surface polarity, the immersion heat ratio (ΔH) when the average particle diameter is in the range of 5 to 100 μm._i ⁿ/ ΔH_i ^h) Is almost 1. 1.2 to 2 immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^hIn addition, the negative electrode containing a carbon material having an average particle size of 5 to 100 μm can improve the lithium storage / release site amount of the negative electrode and improve the affinity between the carbon material and the non-aqueous electrolyte. Therefore, the discharge capacity and cycle characteristics of the secondary battery can be remarkably improved.
[0126]
The particle shape of the carbon material can be, for example, spherical, fibrous, or granular. That is, the negative electrode according to the present invention can contain one or more types of carbon materials selected from the group consisting of fibrous carbon materials, spherical carbon materials, and granular carbon materials.
[0127]
The average fiber length of the fibrous carbon material is preferably in the range of 5 to 100 μm. A more preferable range is 10 to 60 μm.
[0128]
The average fiber diameter of the fibrous carbon material is preferably in the range of 0.1 to 30 μm. A more preferable range is 1 to 15 μm.
[0129]
The average aspect ratio of the fibrous carbon material is preferably in the range of 1-50. A more preferred range is 1.5-20. However, the aspect ratio is the ratio of the fiber length (fiber length / fiber diameter) to the fiber diameter.
[0130]
The ratio of the minor radius to the major radius of the spherical carbonaceous material (minor radius / major axis) is preferably 1/10 or more. A more preferable range is 1/2 or more.
[0131]
The granular carbonaceous material means a carbonaceous material powder having a shape in which a ratio of a minor radius to a major radius (minor radius / major axis) is in a range of 1/100 to 1. A more preferable range of the ratio is 1/10 to 1.
[0132]
The carbon material has an interval d between (002) planes of a graphite structure obtained from X-ray diffraction measurement.₀₀₂Is preferably in the range of 0.335 nm (3.335 mm) or more and 0.34 nm (3.4 mm) or less, which is the theoretical value. Such a carbon material can lower the potential of lithium insertion / release, and thus can improve the energy density of the nonaqueous electrolyte secondary battery.
[0133]
The specific surface area of the carbon material according to the BET method is 1 to 50 m.²/ G is preferable. Specific surface area 1m²If it is less than / g, it may be difficult to sufficiently increase the lithium storage / release sites on the surface of the carbon material. On the other hand, specific surface area is 50m²If it exceeds / g, the decomposition reaction (reductive decomposition) of the nonaqueous electrolyte is promoted, and there is a possibility that excellent cycle characteristics cannot be obtained in the secondary battery. A more preferable range of the specific surface area is 2 to 20 m.²/ G.
[0134]
Immersion heat ratio (ΔH defined by the equation (1)_i ⁿ/ ΔH_i ^h) Can be obtained by, for example, the above-described methods for producing the first to fifth negative electrode carbon materials. That is, according to the first and second negative electrode carbon material manufacturing methods described above, pores can be uniformly formed on the surface and inside of the carbon material, and the pore surface existing inside the carbon material is included. Heterogeneous atoms such as oxygen atoms can be introduced uniformly on the surface of the carbon material. As a result, since the polarity of the surface of the carbon material can be increased, the immersion heat ratio (ΔH defined by the above equation (1) is used._i ⁿ/ ΔH_i ^h) Can be obtained. In particular, according to the fourth and fifth negative electrode carbon material manufacturing methods described above, pores can be uniformly formed on the surface and the inside of the carbon material, and the acid treatment is performed not only on the surface of the carbon material but also on the inside. Can be applied. As a result, the surface of the carbon material including the surface of the pores existing inside the carbon material is uniformly distributed from a functional group containing a boron atom, a functional group containing a nitrogen atom, a functional group containing an oxygen atom, and a functional group containing a phosphorus atom. Since a highly polar functional group consisting of one or more selected from the group can be introduced, the polarity of the surface of the carbon material is higher than that of the first and second methods for producing a carbon material for a negative electrode described above. can do. Therefore, according to the fourth and fifth negative electrode carbon material manufacturing methods described above, the immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And an average particle diameter of 5 to 100 μm.
[0135]
This negative electrode is produced, for example, by adding a binder to a carbon material, suspending these in an appropriate solvent, applying the suspension to a current collector, drying, and pressing. A conductive agent may be further added to the suspension.
[0136]
As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), or the like can be used.
[0137]
As the current collector, a conductive substrate having a porous structure or a nonporous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel. The thickness of the current collector is preferably 5 to 20 μm. This is because within this range, the electrode strength and weight reduction can be balanced.
[0138]
The thickness of the active material containing layer is preferably in the range of 10 to 150 μm. Therefore, when it is carried on both surfaces of the negative electrode current collector, the total thickness of the active material-containing layer is in the range of 20 μm to 300 μm. A more preferable range of the thickness of one side is 30 to 100 μm. Within this range, the large current discharge characteristics and cycle life are greatly improved.
[0139]
2) Positive electrode
The positive electrode has a structure in which an active material-containing layer containing an active material is supported on one side or both sides of a positive electrode current collector.
[0140]
This positive electrode is produced, for example, by suspending a positive electrode active material, a conductive agent and a binder in an appropriate solvent, and applying this suspension to a current collector, drying and pressing to form a strip electrode.
[0141]
Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel cobalt oxide (for example, LiCoO₂), Lithium-containing nickel cobalt oxide (for example, LiNi_0.8Co_0.2O₂), Lithium manganese composite oxide (for example, LiMn)₂O_FourLiMnO₂) Is preferably used. By using such a positive electrode active material, a high voltage can be obtained in the secondary battery.
[0142]
Examples of the conductive agent include acetylene black, carbon black, and graphite.
[0143]
As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), or the like can be used.
[0144]
The blending ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 80 to 95% by weight of the positive electrode active material, 3 to 20% of the conductive agent, and 2 to 7% by weight of the binder.
[0145]
As the current collector, a conductive substrate having a porous structure or a nonporous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel. The thickness of the current collector is preferably 5 to 20 μm. This is because within this range, the electrode strength and weight reduction can be balanced.
[0146]
The thickness of one surface of the active material-containing layer is preferably in the range of 10 to 150 μm. Therefore, when it is carried on both sides of the positive electrode current collector, the total thickness of the active material-containing layer is preferably in the range of 20 to 300 μm. A more preferable range on one side is 30 to 120 μm. Within this range, large current discharge characteristics and cycle life are improved.
[0147]
3) Non-aqueous electrolyte
Examples of the non-aqueous electrolyte include a liquid non-aqueous electrolyte, a gel-like non-aqueous electrolyte, a polymer solid electrolyte, and an inorganic solid electrolyte having lithium ion conductivity. Among these, a liquid nonaqueous electrolyte is preferable.
[0148]
A liquid nonaqueous electrolyte is prepared by, for example, dissolving an electrolyte in a nonaqueous solvent.
[0149]
The gel-like nonaqueous electrolyte includes a polymer material, a nonaqueous solvent combined with the polymer material, and an electrolyte. The gel-like non-aqueous electrolyte is prepared by, for example, mixing the non-aqueous solvent, the electrolyte, and the polymer material, followed by heat treatment to cause gelation. Examples of the polymer material include polyacrylonitrile, polyacrylate, polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), or a copolymer containing acrylonitrile, acrylate, vinylidene fluoride, or ethylene oxide as a monomer. Among these, it is desirable to use a gel electrolyte prepared by the method described below. First, a solution obtained by dissolving a polymer material such as polyvinylidene fluoride hexafluoropropylene copolymer (PVdF-HEP copolymer) in a solvent such as tetrahydroxyfuran (THF), and a liquid non-aqueous electrolyte To prepare a paste. The obtained paste is applied to a substrate and then dried to obtain a thin film. This thin film is interposed between the positive electrode and the negative electrode to produce an electrode group. After the electrode group is impregnated with a liquid nonaqueous electrolyte, the thin film is plasticized by a gelling treatment such as heat treatment to obtain an electrode group in which a gelled nonaqueous electrolyte layer is interposed between the positive electrode and the negative electrode. .
[0150]
The solid electrolyte is obtained by dissolving the electrolyte in a polymer material and solidifying it. Examples of the polymer material include polyacrylonitrile, polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), or a copolymer containing acrylonitrile, vinylidene fluoride, or ethylene oxide as a monomer.
[0151]
Examples of the inorganic solid electrolyte include a ceramic material containing lithium. Above all, Li_ThreeN, Li_ThreePO_Four-Li₂S-SiS₂Glass etc. are mentioned.
[0152]
Hereinafter, the nonaqueous solvent and electrolyte contained in the nonaqueous electrolyte will be described.
[0153]
Examples of the non-aqueous solvent include at least one solvent selected from propylene carbonate (PC) and ethylene carbonate (EC) (hereinafter referred to as a first solvent), and a solvent having a viscosity lower than that of PC or EC (hereinafter referred to as “first solvent”). It is preferable to use mainly a mixed solvent with the second solvent).
[0154]
Examples of the second solvent include chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), methyl propionate, ethyl propionate, γ-butyrolactone (BL), acetonitrile. (AN), ethyl acetate (EA), toluene, xylene, methyl acetate (MA) and the like. As the second solvent, one or a mixture of two or more selected from the types described above can be used. The number of donors in the second solvent is preferably 16.5 or less.
[0155]
The viscosity of the second solvent is preferably 2.8 cmp or less at 25 ° C. The blending amount of the first solvent in the mixed solvent is preferably 10 to 80% by volume ratio. A more preferable amount of the first solvent is 20 to 75% by volume.
[0156]
Examples of the electrolyte include lithium perchlorate (LiClO)._Four), Lithium hexafluorophosphate (LiPF)₆), Lithium borofluoride (LiBF)_Four), Lithium arsenic hexafluoride (LiAsF)₆), Lithium trifluorometasulfonate (LiCF)_ThreeSO_Three), Bistrifluoromethylsulfonylimide lithium [LiN (CF_ThreeSO₂)₂And the like. Among them, LiPF₆, LiBF_FourIs preferably used.
[0157]
The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2 mol / L.
[0158]
4) Separator
The separator functions as an insulating layer between the positive electrode and the negative electrode, and has a structure in which the electrolyte in the nonaqueous electrolyte can move.
[0159]
For the separator, for example, a porous body made of an insulating material can be used. As the insulating material, for example, a porous film containing polyethylene, polypropylene or polyvinylidene fluoride (PVdF), a synthetic resin nonwoven fabric containing polyethylene, polypropylene or polyvinylidene fluoride (PVdF), or the like can be used. Among these, a porous film made of polyethylene, polypropylene, or both is preferable because it can improve the safety of the secondary battery.
[0160]
The thickness of the separator is preferably 30 μm or less. If the thickness exceeds 30 μm, the distance between the positive and negative electrodes may be increased and the internal resistance may be increased. Further, the lower limit value of the thickness is preferably 5 μm. If the thickness is less than 5 μm, the strength of the separator is remarkably lowered and an internal short circuit is likely to occur. The upper limit value of the thickness is more preferably 25 μm, and the lower limit value is more preferably 10 μm.
[0161]
The separator preferably has a heat shrinkage rate of 20% or less when kept at 120 ° C. for 1 hour. If the heat shrinkage rate exceeds 20%, the possibility of a short circuit due to heating increases. The thermal shrinkage rate is more preferably 15% or less.
[0162]
The separator preferably has a porosity in the range of 30 to 70%. This is due to the following reason. If the porosity is less than 30%, it may be difficult to obtain high electrolyte retention in the separator. On the other hand, if the porosity exceeds 60%, sufficient separator strength may not be obtained. A more preferable range of the porosity is 35 to 70%.
[0163]
The separator has an air permeability of 500 seconds / 100 cm.^ThreeThe following is preferable. Air permeability is 500 seconds / 100cm^ThreeIf it exceeds 1, it may be difficult to obtain high lithium ion mobility in the separator. The lower limit of the air permeability is 30 seconds / 100 cm.^ThreeIt is. Air permeability is 30 seconds / 100cm^ThreeIf it is less than this, there is a possibility that sufficient separator strength cannot be obtained. The upper limit of air permeability is 300 seconds / 100cm^ThreeMore preferably, the lower limit is 50 seconds / 100 cm.^ThreeMore preferred.
[0164]
6) Storage container
The storage container stores an electrode group including a positive electrode and a negative electrode and a non-aqueous electrolyte.
[0165]
The shape of the storage container can be, for example, a bottomed cylindrical shape, a bottomed rectangular tube shape, a bag shape, or the like.
[0166]
A storage container can be formed from a film material and a metal plate, for example.
[0167]
Examples of the film material constituting the storage container include a metal film, a resin sheet such as a thermoplastic resin, and a sheet in which a resin layer such as a thermoplastic resin is coated on one or both sides of a flexible metal layer. Etc. can be formed. The resin sheet and the resin layer can be formed from one type of resin or two or more types of resin, respectively. Meanwhile, the metal layer can be formed of one kind of metal or two or more kinds of metals. Moreover, the said metal film can be formed from aluminum, iron, stainless steel, nickel, etc., for example.
[0168]
The thickness of the film material constituting the wall of the storage container is desirably 0.25 mm or less. A particularly desirable thickness range is 0.05 mm to 0.2 mm. This achieves a thinner and lighter battery.
[0169]
In particular, a sheet composed of a flexible metal layer and a resin layer laminated on one or both sides of the metal layer is lightweight, high in strength, and penetrates a substance such as moisture from the outside. Can be prevented, which is desirable. The storage container constituted by the sheet is sealed by, for example, heat sealing. For this reason, it is desirable to arrange a thermoplastic resin on the inner surface of the storage container. The melting point of the thermoplastic resin is preferably 120 ° C or higher, more preferably 140 ° C to 250 ° C. Examples of the thermoplastic resin include polyolefins such as polyethylene and polypropylene. In particular, it is desirable to use polypropylene having a melting point of 150 ° C. or higher because the sealing strength of the heat seal portion is increased. On the other hand, the metal layer is preferably formed of aluminum which prevents water from entering the battery.
[0170]
An example of the non-aqueous electrolyte secondary battery according to the present invention will be described with reference to FIGS.
[0171]
FIG. 1 is a partial cross-sectional view showing a cylindrical nonaqueous electrolyte secondary battery which is an example of a nonaqueous electrolyte secondary battery according to the present invention. FIG. 2 is a thin non-aqueous electrolyte secondary battery which is an example of a nonaqueous electrolyte secondary battery according to the present invention. FIG. 3 is an enlarged cross-sectional view showing a portion A of FIG. 2.
[0172]
As shown in FIG. 1, the bottomed cylindrical container 1 made of stainless steel has an insulator 2 disposed at the bottom. The electrode group 3 is housed in the container 1. The electrode group 3 has a structure in which a belt-like material in which the positive electrode 4, the separator 5, the negative electrode 6, and the separator 5 are laminated is wound in a spiral shape so that the separator 5 is located outside.
[0173]
A non-aqueous electrolyte is accommodated in the container 1. The insulating paper 7 having an open center is disposed above the electrode group 3 in the container 1. The insulating sealing plate 8 is disposed in the upper opening of the container 1, and the sealing plate 8 is fixed to the container 1 by caulking the vicinity of the upper opening. The positive terminal 9 is fitted in the center of the insulating sealing plate 8. One end of the positive electrode lead 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9. The negative electrode 6 is connected to the container 1 which is a negative electrode terminal via a negative electrode lead (not shown).
[0174]
As shown in FIG. 2, the electrode group 12 is accommodated in the storage container 11 which consists of a sheet | seat containing a resin layer, for example. The electrode group 12 has a structure in which a laminate composed of a positive electrode, a separator, and a negative electrode is wound into a flat shape. From the lower side of FIG. 3, the laminate includes the separator 13, the active material containing layer 14, the positive electrode current collector 15, the positive electrode 16 including the active material containing layer 14, the separator 13, the negative electrode layer 17, and the negative electrode current collector 18. And a negative electrode 19 having a negative electrode layer 17, a separator 13, an active material containing layer 14, a positive electrode current collector 15, a positive electrode 16 having an active material containing layer 14, a separator 13, a negative electrode layer 17, and a negative electrode current collector 18. The negative electrode 19 has a structure in which the negative electrodes 19 are laminated in this order. A negative electrode current collector 18 is located on the outermost periphery of the electrode group 12. One end of the strip-like positive electrode lead 20 is connected to the positive electrode current collector 15 of the electrode group 2, and the other end is extended from the storage container 11. On the other hand, one end of the strip-like negative electrode lead 21 is connected to the negative electrode current collector 18 of the electrode group 2, and the other end is extended from the storage container 11.
[0175]
As described above, the nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode and an immersion heat ratio (ΔH defined by the above equation (1)._i ⁿ/ ΔH_i ^h) And a non-aqueous electrolyte. Since such a carbon material has a uniform pore distribution and a moderately high surface polarity, it can increase the storage and release sites of lithium and increase the affinity with the liquid nonaqueous electrolyte. it can. As a result, a nonaqueous electrolyte secondary battery with improved initial initial charge / discharge efficiency, discharge capacity, and charge / discharge cycle life characteristics can be realized.
[0176]
One or more kinds selected from the group consisting of a functional group containing a boron atom, a functional group containing a nitrogen atom, a functional group containing an oxygen atom, and a functional group containing a phosphorus atom on the surface including the inside of the pores in the carbon material The presence of the functional group makes it possible to increase the number of lithium occlusion / release sites of the carbon material and to further increase the affinity between the carbon material and the liquid nonaqueous electrolyte. Therefore, the discharge capacity and charge / discharge cycle life of the nonaqueous electrolyte secondary battery can be further improved. In particular, since the functional group is at least one selected from the group consisting of a carboxyxyl group, a carbonyl group, a hydroxyl group, a lactone group, and a ketone group, the discharge capacity and charge / discharge cycle life of the non-aqueous electrolyte secondary battery can be reduced. It can be greatly improved.
[0177]
【Example】
Hereinafter, specific examples of the present invention will be given and the effects thereof will be described. However, the present invention is not limited to the examples.
[0178]
(Examples 1-4, Comparative Examples 1-3)
(Manufacture of negative electrode carbon materials)
A carbon material was obtained by spinning a petroleum pitch and graphitizing an infusible sample at 350 ° C. in the atmosphere at the heat treatment temperature and atmospheric conditions shown in Table 1. The heating time was 8 hours. The gas was introduced into the furnace before heating and completely replaced, and then the gas flow was stopped and heating was started.
[0179]
(Preparation of negative electrode)
Polytetrafluoroethylene was added to the obtained carbon material to form a sheet, which was pressure-bonded to a stainless steel mesh and vacuum-dried at 150 ° C. to obtain a test electrode.
[0180]
(Assembly of test cell)
LiPF was added to a non-aqueous solvent in which ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed at a volume ratio of 1: 2.₆1M was dissolved to prepare a liquid non-aqueous electrolyte. Using the test electrode and the liquid nonaqueous electrolyte, a cell having a counter electrode and a reference electrode made of metal Li was produced in an argon atmosphere, and a charge / discharge test was performed.
[0181]
The condition of the charge / discharge test is 1 mA / cm up to a potential difference of 0.01 V between the reference electrode and the test electrode.²The battery was charged at a current density of 0.01 V and further charged at a constant voltage of 0.01 V for 5 hours.²The current density was 2 V. Table 1 shows the heat treatment conditions and the capacity per weight of the negative electrode material determined by the charge / discharge test.
[0182]
[Table 1]

[0183]
As is clear from Table 1, the secondary batteries of Examples 1 to 4 provided with the negative electrode containing the carbon material obtained by the second production method had a negative electrode capacity per unit weight compared to Comparative Examples 1 to 3. It can be seen that it can be improved.
[0184]
In contrast, when the carbon dioxide gas concentration in the heat treatment atmosphere is less than 10% by volume as in Comparative Example 2 or when the water vapor concentration in the heat treatment atmosphere is less than 1% by volume as in Comparative Example 3, the capacity improvement effect is achieved. It turns out that is not seen so much.
[0185]
(Examples 5 to 10)
(Differential thermal analysis measurement of carbonaceous materials)
An example of a TG curve (Y axis) and a DTA curve (R axis) obtained by differential thermal analysis measurement for commercially available highly crystalline artificial graphite is shown in FIG. The conditions for this differential thermal analysis measurement will be explained. The weight of commercially available high crystalline artificial graphite as a sample is 50 mg, carbon dioxide is used as the atmosphere gas in the furnace, the gas flow rate is 100 mL / min, and the measurement start temperature is 20 ° C., measurement end temperature 1400 ° C., heating rate 20 ° C./min, α-Al as reference material₂O_ThreeIt was used. The peak temperature T of the endothermic peak in the DTA curve of FIG.₁Is 280 ° C. On the other hand, the peak temperature T of the exothermic peak₂Is 1270 ° C.
[0186]
Next, the (002) plane spacing d by X-ray diffraction₀₀₂A commercially available high crystalline artificial graphite having a thickness of 0.3354 nm is prepared, and the differential thermal analysis of the high crystalline artificial graphite is performed using four types of atmospheres (No. 1 to No. 1) shown in Table 2 below as atmospheric gases in the furnace. No. 4 gas atmosphere). The other measurement conditions in the differential thermal analysis will be described. The weight of the sample is 50 mg, the gas flow rate is 100 mL / min, the heating rate is 20 ° C./min, and α-Al is used as a reference substance.₂O_ThreeIt was used. From the obtained DTA curve, the peak temperature T of the endothermic peak₁And peak temperature T of exothermic peak₂From the value (T₁+ T₂) / 2 was calculated. These results are shown in Table 2 below.
[0187]
[Table 2]

[0188]
In Table 2, dry air is 80% by volume N₂Gas and 20 vol% O₂The mixed gas with gas is shown.
[0189]
(Manufacture of negative electrode carbon materials)
The above highly crystalline artificial graphite (surface spacing d of (002) plane by X-ray diffraction d₀₀₂No. 0.3354 nm) was placed in an alumina crucible and No. 2 shown in Table 2 above. Heat treatment was performed in the presence of 1 atmosphere gas for 3 hours at the temperature shown in Table 3 below. Before performing heating, the inside of the heating furnace is evacuated before heating so that the atmosphere does not remain, and then gas is introduced, and the gas is flowed at a flow rate of 0.8 liter / min during heating, heating, and cooling. did. The temperature raising time was 3 hours. After the heat treatment, the sample temperature was lowered to room temperature by allowing to cool, and then taken out from the heating furnace to obtain a carbon material.
[0190]
Using the obtained carbon material, production of a negative electrode, assembly of a test cell, and a charge / discharge test were performed in the same manner as in Example 1. The results of the charge / discharge test are also shown in Table 3.
[0191]
(Comparative Example 4)
Except that the high crystalline artificial graphite was not heat-treated, a negative electrode was prepared, a test cell was assembled, and a charge / discharge test was conducted in the same manner as in Example 5 described above. The results of the charge / discharge test are also shown in Table 3.
[0192]
[Table 3]

[0193]
As is clear from Table 3, the secondary batteries of Examples 5 to 10 having the negative electrode containing the carbon material obtained by the first manufacturing method improved the negative electrode capacity per unit weight as compared with Comparative Example 4. I understand that I can do it.
[0194]
(Examples 11-16 and Comparative Examples 7-12)
Except that the composition of the atmospheric gas in the furnace and the heat treatment temperature in the heat treatment applied to the highly crystalline artificial graphite were changed as shown in Table 4 below, the production of the negative electrode and the test cell were performed in the same manner as in Example 5 described above. Assembly and charge / discharge tests were performed. The results of the charge / discharge test are also shown in Table 4.
[0195]
In Table 4, dry air is 80% by volume N₂Gas and 20 vol% O₂It is a mixed gas with gas.
[0196]
[Table 4]

[0197]
As is clear from Table 4, Examples 11 to 16 secondary batteries provided with the negative electrode containing the carbon material obtained by the first production method had negative electrode capacities per unit weight of Comparative Examples 4 and 7 to 12. It can be seen that it can be improved.
[0198]
As is apparent from Tables 1 to 4, the carbon capacity of the carbon material treated using the production method of the present invention greatly increased. CO including heat treatment₂Or H₂The highly crystalline artificial graphite performed in the O gas flow showed an increase in capacity that exceeded the effect of treatment in air. CO containing graphitization₂Or H₂When performed in an O atmosphere, a larger capacity can be obtained than when graphitized in an Ar atmosphere. CO₂Is 5% by volume, H₂In the comparative example in which O was 0.5% by volume, the effect of increasing the capacity was hardly observed.
[0199]
(Examples 17-20, Comparative Examples 13-14)
(002) plane spacing d by X-ray diffraction₀₀₂5 g of commercially available high crystalline artificial graphite having a thickness of 0.3354 nm was placed in an alumina crucible and subjected to heat treatment. Before performing the heat treatment, an atmosphere gas was introduced after evacuating the inside of the heating furnace so that the atmosphere did not remain. As the atmospheric gas to be introduced, an Ar gas was passed through a boiling acid aqueous solution and an acid was contained as a vapor. The type of acid used and the concentration (% by weight) of the aqueous acid solution are shown in Table 5 below. Gas was flowed at a flow rate of 0.4 liter / min during the temperature rising and heating. The heat treatment temperature was set as shown in Table 5 below. The temperature raising time was 1 hour, and the holding time was 30 minutes. After the heat treatment, the sample temperature was lowered to room temperature by cooling, and then taken out from the heating furnace to obtain a carbon material.
[0200]
Using the obtained carbon material, production of a negative electrode, assembly of a test cell, and a charge / discharge test were performed in the same manner as in Example 1. The results of the charge / discharge test are also shown in Table 5.
[0201]
[Table 5]

[0202]
As seen in Table 5, a large improvement in capacity can be obtained by heat treating the carbon material in the presence of acid vapor. This effect is particularly great when a nitric acid aqueous solution having a concentration of 68% is used as the acid.
[0203]
(Examples 21 to 23)
(002) plane spacing d by X-ray diffraction₀₀₂The same conditions as described in Example 11 above for commercially available high crystalline artificial graphite having a thickness of 0.3354 nm, that is, a mixed gas comprising 50% by volume of carbon dioxide gas and 50% by volume of argon gas as the atmospheric gas And the heat treatment was performed at a heat treatment temperature of 1200 ° C. Next, post-heat treatment was performed on the highly crystalline artificial graphite under the same conditions as described in Example 17 above, that is, using a nitric acid aqueous solution having a concentration of 68% and a heat treatment temperature of 450 ° C. The carbon material which concerns on was obtained.
[0204]
(002) plane spacing d by X-ray diffraction₀₀₂A commercially available high crystalline artificial graphite having a thickness of 0.3354 nm was subjected to heat treatment using the same conditions as described in Example 17 above, that is, using a nitric acid aqueous solution having a concentration of 68% and a heat treatment temperature of 450 ° C. It was. Next, the same conditions as described in Example 11 were used for this highly crystalline artificial graphite, that is, a mixed gas composed of 50 volume% carbon dioxide gas and 50 volume% argon gas was used as the atmosphere gas, and heat treatment was performed. A post-heat treatment was performed at a temperature of 1200 ° C. to obtain a carbon material according to Example 22.
[0205]
(002) plane spacing d by X-ray diffraction₀₀₂A commercially available high crystalline artificial graphite having a thickness of 0.3354 nm was subjected to heat treatment using the same conditions as described in Example 17 above, that is, using a nitric acid aqueous solution having a concentration of 68% and a heat treatment temperature of 450 ° C. It was. Next, the same conditions as described in Example 13 were used for this highly crystalline artificial graphite, that is, a mixed gas composed of 20% by volume of water vapor and 80% by volume of argon gas was used as the atmosphere gas, and the heat treatment temperature was Was post-heat-treated at 1200 ° C. to obtain a carbon material according to Example 23.
[0206]
About the obtained three types of carbon materials, preparation of a negative electrode, assembly of a test cell, and a charge / discharge test were performed in the same manner as in Example 1, and the results of the charge / discharge test are shown in Table 6 below. In addition, the capacity | capacitance per unit weight shown in Table 6 is a value when the capacity | capacitance per unit weight of the test cell of the comparative example 4 is set to 100 (%). In Table 6, the treatment performed first is the first-stage heat treatment (I), and the treatment performed thereafter is the second-stage heat treatment (II).
[0207]
[Table 6]

[0208]
As is clear from Table 6, the secondary batteries of Examples 21 and 22 in which both the heat treatment in an atmosphere containing 10% by volume or more of carbon dioxide gas and the heat treatment in an atmosphere containing gaseous acid were performed, and 1 volume It can be seen that the secondary battery of Example 23 in which both the heat treatment in an atmosphere containing% or more of water vapor and the heat treatment in an atmosphere containing gaseous acid were performed can obtain a high capacity. In particular, the capacity of the secondary battery of Example 21 in which the heat treatment in an atmosphere containing a gaseous acid after the heat treatment in an atmosphere containing 10% by volume or more of carbon dioxide gas can be greatly improved.
[0209]
(Examples 24-27)
5 g of commercially available graphitized carbon fiber having an average fiber length of 40 μm and an average particle diameter of the values shown in Table 8 below was placed in an alumina crucible, and in the presence of atmospheric gas having the composition shown in Table 7, Heat treatment was performed for 3 hours at the indicated temperature. Before heating, the inside of the heating furnace is evacuated so that the atmosphere does not remain in the heat treatment, and then the gas is introduced, and the gas is supplied at a flow rate of 0.8 liters / minute during heating, heating and cooling. Washed away. The temperature raising time was 3 hours. After the heat treatment, the sample temperature was lowered to room temperature by cooling, and then taken out from the heating furnace to obtain a fibrous carbon material.
[0210]
With respect to the obtained carbon material, the average particle diameter and the interplanar spacing d of the (002) plane by X-ray diffraction₀₀₂And the specific surface area by BET method was measured, and the result is shown in Table 8 below.
[0211]
The specific surface area of the carbon material according to the BET method is He—N as a carrier gas.₂Using a (70:30) mixed gas, measurement was performed by a one-point BET method. The average particle size of the carbon material was measured by a laser diffraction / scattering particle size analyzer (manufactured by Leeds & Northrup, model number 9320-X100). That is, carbon material particles are suspended in a dispersion medium composed of ethyl alcohol, the suspended carbon material particles are irradiated with laser light, the intensity distribution of scattered light is measured, and the particle size distribution is converted by computer analysis. . The cumulative average diameter (cumulative 50% diameter) in the obtained particle size distribution is taken as the average particle diameter.
[0212]
Furthermore, about the obtained carbon material, immersion heat (DELTA) H with respect to n-heptane_i ^hAnd immersion heat ΔH for 1-nitropropane_i ⁿAnd a twin-type heat transfer calorimeter (manufactured by Tokyo Riko Co., Ltd., model number: MMC-5111).
[0213]
That is, a solvent is injected into the sample container. Moreover, after putting a sample in a glass ampule and carrying out vacuum deaeration, it seals. After submerging the ampoule in the solvent of the sample container, set it in the twin heat transfer calorimeter, break the ampoule after reaching the equilibrium temperature, and stir to mix the sample with the solvent. Measure.
[0214]
In the twin-type heat transfer calorimeter, the sample container and the constant temperature body are connected by a good thermal conductor that also serves as a heat sensitive body, and the amount of generated heat is measured by measuring the time difference of the temperature difference between both ends of the heat sensitive body. The rate of change can be determined.
[0215]
The measurement conditions are listed below.
[0216]
Amount of sample carbon material: 1.2 g
Sample pretreatment: 150 ° C., 10 mmHg to 5 mmHg, 6 hours
Measurement temperature: 25 ° C
The immersion medium is n-hexane or 1-nitropropane, and the immersion medium amount: 22 mL
Calorie calculation: Data processing by computer
Obtained immersion heat ΔH_i ^hAnd immersion heat ΔH_i ⁿTo immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0217]
(Example 28)
5 g of a commercially available graphitized carbon fiber having an average fiber length of 40 μm and an average particle diameter of 25 μm is placed in an alumina crucible and heat-treated at 1000 ° C. for 3 hours in the presence of an atmospheric gas composed of 100% by volume of carbon dioxide. Went. Before heating, the inside of the heating furnace is evacuated so that the atmosphere does not remain in the heat treatment, and then the gas is introduced, and the gas is supplied at a flow rate of 0.8 liters / minute during heating, heating, and cooling. Washed away. The temperature raising time was 3 hours. After the heat treatment, the sample temperature was lowered to room temperature by cooling, and then taken out from the heating furnace.
[0218]
Next, the fibrous carbon material was again heat-treated. As the atmospheric gas introduced into the heating furnace, a gas in which nitric acid was contained as a vapor through Ar gas in a boiled nitric acid aqueous solution having a concentration of 68% by weight was used. Gas was flowed at a flow rate of 0.4 liter / min during the temperature rising and heating. The heat treatment temperature was 450 ° C., and the heat treatment time was 3 hours. The temperature raising time was 1 hour, and the holding time was 30 minutes. After the heat treatment, the sample temperature was lowered to room temperature by cooling, and then taken out from the heating furnace to obtain a carbon material.
[0219]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0220]
(Example 29)
A carbon material was obtained in the same manner as in Example 28 described above except that an aqueous acetic acid solution having a concentration of 98% by weight was used as the aqueous acid solution for generating gaseous acid.
[0221]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0222]
(Example 30)
A carbon material was obtained in the same manner as in Example 28 described above, except that an aqueous phosphoric acid solution having a concentration of 85% by weight was used as the aqueous acid solution for generating gaseous acid.
[0223]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0224]
(Example 31)
Petroleum pitch, which is a carbon precursor, was used as a starting material. After spinning, the surface was infusibilized at 300 ° C. for 1 hour. Next, the carbon precursor was subjected to a heat treatment at 900 ° C. for 3 hours in the presence of an atmospheric gas composed of 100% by volume of carbon dioxide gas to obtain a carbonized material. This carbonized material belonged to amorphous carbon or soft carbon. Next, the carbonized material was graphitized by subjecting the carbonized material to heat treatment at 2800 ° C. for 3 hours in the presence of an atmospheric gas composed of 100% by volume of carbon dioxide gas to obtain a fibrous carbon material. The gas was introduced into the furnace before heating and completely replaced, and then the gas flow was stopped and heating was started.
[0225]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0226]
(Example 32)
Petroleum pitch, which is a carbon precursor, was used as a starting material. After spinning, the surface was infusibilized at 300 ° C. for 1 hour. The carbon precursor was then added to 20 volume% H₂O gas and 80 vol% CO₂A heat treatment was performed at 900 ° C. for 3 hours in the presence of an atmospheric gas composed of a gas to obtain a carbonized material. This carbonized material belonged to amorphous carbon or soft carbon. Then 20% by volume H₂O gas and 80 vol% CO₂The carbonized material was graphitized by subjecting the carbonized material to a heat treatment at 2800 ° C. for 3 hours in the presence of atmospheric gas, thereby obtaining a fibrous carbon material. The gas was introduced into the furnace before heating and completely replaced, and then the gas flow was stopped and heating was started.
[0227]
Next, the fibrous carbon material was subjected to heat treatment. As the atmospheric gas introduced into the heating furnace, an aqueous nitric acid solution having a boiling concentration of 68 wt. Gas was flowed at a flow rate of 0.4 liter / min during the temperature rising and heating. The heat treatment temperature was 450 ° C., and the heat treatment time was 3 hours. The temperature raising time was 1 hour, and the holding time was 30 minutes. After the heat treatment, the sample temperature was lowered to room temperature by cooling, and then taken out from the heating furnace to obtain a carbon material.
[0228]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0229]
(Comparative Examples 15-23)
A commercially available graphitized carbon fiber having an average fiber length of 40 μm and an average particle size shown in the following Table 8 was heat treated for 3 hours at the temperature shown in Table 7 in the presence of an atmospheric gas having the composition shown in Table 7 below. The carbon materials of Comparative Examples 16 to 23 were obtained. In addition, a graphitized carbon fiber that was not subjected to any heat treatment was prepared as a carbon material of Comparative Example 15. In Comparative Examples 16 and 17, the composition of air used as the heat treatment atmosphere was 80% by volume N₂Gas and 20 vol% O₂It consists of a gas mixture with gas.
[0230]
With respect to the obtained carbon material, under the same conditions as described in Example 24 described above, the average particle diameter, (002) plane spacing d by X-ray diffraction, d₀₀₂, BET specific surface area and immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^h) And the results are shown in Table 8 below.
[0231]
Using the obtained carbon materials of Examples 24-32 and Comparative Examples 15-23, test cells were assembled by the method described below.
[0232]
First, polytetrafluoroethylene was added to each carbon material to form a sheet, which was pressure-bonded to a stainless steel mesh and vacuum-dried at 150 ° C. to obtain a test electrode. In addition, LiPF was added to a non-aqueous solvent in which ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed at a volume ratio of 1: 2.₆1M was dissolved to prepare a liquid non-aqueous electrolyte. While using the obtained test electrode and liquid nonaqueous electrolyte, the test cell was assembled in argon atmosphere by using the counter electrode and the reference electrode as metal Li.
[0233]
The obtained test cells of Examples 24-32 and Comparative Examples 15-23 were subjected to charge / discharge tests under the conditions described in (1) and (2) below, and the results are also shown in Table 8 below.
[0234]
(1) 1 mA / cm up to a potential difference of 0.01 V between the reference electrode and the test electrode²1 mA / cm after charging at a current density of 0.01 V and further at a constant voltage of 0.01 V for 5 hours.²The discharge capacity was measured at a current density of 2 V and the discharge capacity was measured. The discharge capacity per gram of the negative electrode carbon material is also shown in Table 8 below.
[0235]
(2) The charge / discharge under the conditions described below is repeated, the number of cycles required until the discharge capacity is reduced to 80% of the capacity at the first cycle is measured, and the results are also shown in Table 8 below.
[0236]
Charging: 1 mA / cm up to a potential difference of 0.01 V between the reference electrode and the test electrode²And a constant voltage charge at 0.01V for 5 hours.
[0237]
Discharge: 1 mA / cm²Discharge to 2V at a current density of.
[0238]
[Table 7]

[0239]
[Table 8]

[0240]
As is apparent from Tables 7 and 8, the immersion heat ratio (ΔH defined by the above equation (1))._i ⁿ/ ΔH_i ^hIt can be seen that the secondary batteries of Examples 24-32 provided with a negative electrode including a carbon material having a high discharge capacity have a high discharge capacity and can extend the cycle life.
[0241]
On the other hand, the secondary batteries of Comparative Examples 16 to 18 including the negative electrode including the carbon material that was heat-treated in an air or argon gas atmosphere had an immersion heat ratio (ΔH_i ⁿ/ ΔH_i ^hHowever, it is found that the discharge capacity and the cycle life are lower than those of Examples 24-32, which is almost the same as that of Comparative Example 15 including a negative electrode containing a carbon material that has not been heat-treated. Moreover, it turns out that the secondary battery of Comparative Examples 19-23 has a low cycle life compared with Examples 24-32.
[0242]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to provide a carbon material for a negative electrode that can improve initial charge / discharge efficiency, discharge capacity, large current characteristics, and cycle life of a nonaqueous electrolyte secondary battery. . In addition, according to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery with improved initial charge / discharge efficiency, discharge capacity, large current characteristics, and cycle life.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view showing a cylindrical nonaqueous electrolyte secondary battery which is an example of a nonaqueous electrolyte secondary battery according to the present invention.
FIG. 2 is a cross-sectional view showing a thin nonaqueous electrolyte secondary battery which is an example of a nonaqueous electrolyte secondary battery according to the present invention.
3 is a cross-sectional view showing a part A of FIG. 2;
FIG. 4 is a characteristic diagram showing an example of a TG curve (Y axis) and a DTA curve (R axis) obtained by differential thermal analysis for graphite in Examples.
[Explanation of symbols]
1 ... container,
3 ... Electrode group,
4 ... positive electrode,
5 ... separator,
6 ... negative electrode,
8 ... sealing plate,
9: Positive terminal,
11 ... container,
12 ... Electrode group,
13 ... Separator,
14 ... Active material-containing layer,
15 ... positive electrode current collector,
16 ... positive electrode,
17 ... negative electrode layer,
18 ... negative electrode current collector,
19 ... negative electrode,
20: Positive terminal,
21: Negative terminal.

Claims (4)

Immersion heat ratio (ΔH defined by the following formula (1) _i ⁿ / ΔH _i ^h ) And a carbon material for a negative electrode which is a graphitized material having an average particle diameter of 5 to 100 μm,
        1.2 ≦ ΔH _i ⁿ / ΔH _i ^h ≦ 2 (1)
  However, in the formula (1), the ΔH _i ^h Is the heat of immersion for n-heptane of the graphitized material, and the ΔH _i ⁿ Is the heat of immersion for 1-nitropropane of the graphitized material,
CO of 10 volume% or more ₂ A first gas atmosphere containing 1% by volume or more of H ₂ Second gas atmosphere containing O and 10% by volume or more of CO ₂ And 1 vol% or more of H ₂ In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, a heat treatment at a heat treatment temperature T (° C.) defined by the following formula (2) is applied to a graphitized material having an average particle diameter of 5 to 100 μm. For 0.5 to 48 hours, a method for producing a carbon material for a negative electrode.
(T ₁ + T ₂ ) / 2 ≦ T ≦ T ₂         (2)
However, in the equation (2), the T ₁ Is the peak temperature (° C.) of the endothermic peak obtained when differential thermal analysis measurement is performed on the graphitized material in the gas atmosphere. ₂ Is the peak temperature (° C.) of the exothermic peak obtained by the differential thermal analysis measurement. The method for producing a carbon material for a negative electrode according to claim 1 , further comprising a step of bringing a gaseous acid into contact with the graphitized material subjected to the heat treatment . Immersion heat ratio (ΔH defined by the following formula (1) _i ⁿ / ΔH _i ^h ) And a carbon material for a negative electrode which is a graphitized material having an average particle diameter of 5 to 100 μm,
        1.2 ≦ ΔH _i ⁿ / ΔH _i ^h ≦ 2 (1)
  However, in the formula (1), the ΔH _i ^h Is the heat of immersion for n-heptane of the graphitized material, and the ΔH _i ⁿ Is the heat of immersion for 1-nitropropane of the graphitized material,
CO of 10 volume% or more ₂ A first gas atmosphere containing 1% by volume or more of H ₂ Second gas atmosphere containing O and 10% by volume or more of CO ₂ And 1 vol% or more of H ₂ In a gas atmosphere selected from the group consisting of a third gas atmosphere containing O, a carbon precursor having an average particle size of 5 to 100 μm is subjected to heat treatment at 2000 to 3000 ° C. for 0.5 to 48 hours. The manufacturing method of the carbon material for negative electrodes characterized by comprising the process of graphitizing a carbon precursor. The method for producing a carbon material for a negative electrode according to claim 3 , further comprising a step of bringing a gaseous acid into contact with the carbon precursor subjected to the graphitization treatment .

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