JP4184479B2

JP4184479B2 - Nonaqueous electrolyte secondary battery and method for producing the negative electrode

Info

Publication number: JP4184479B2
Application number: JP14566798A
Authority: JP
Inventors: 雅規北川; 良弘樫原; 秀越名; 豊次杉本; 邦夫鶴田; 修二伊藤; 肇西野; 幸治郎石川; 久典杉本; 薫塚本
Original assignee: NIPPON GRAPHITE INDUSTRIES,CO.,LTD.; Panasonic Corp; Matsushita Electric Industrial Co Ltd
Current assignee: NIPPON GRAPHITE INDUSTRIES,CO.,LTD.; Panasonic Corp; Panasonic Holdings Corp
Priority date: 1997-05-30
Filing date: 1998-05-27
Publication date: 2008-11-19
Anticipated expiration: 2018-05-27
Also published as: JPH1145715A

Description

【０００１】
【発明の属する技術分野】
本発明は、非水電解質二次電池に係り、特にリチウムイオン二次電池の負極用炭素材に関する。
【０００２】
【従来の技術】
従来、非水電解質二次電池としては、高電圧、高容量による高エネルギー密度化を志向して、負極活物質として金属リチウム、正極活物質として遷移金属の酸化物や硫化物やセレン化物等のカルコゲン化合物、例えば二酸化マンガンや二硫化モリブデンやセレン化チタンなど、非水電解質としてリチウム塩の有機溶媒溶液からなる有機電解液を用いた、いわゆるリチウム二次電池が検討されている。
【０００３】
しかしながら、このリチウム二次電池は、正極活物質として比較的充放電特性が優れた層間化合物を選択することができるが、負極の金属リチウムの充放電特性は必ずしも優れていない。そのために、充放電を繰り返すサイクル寿命を長くすることが難しく、その上、内部短絡による発熱が起こる恐れがあり、安全性に問題があった。すなわち、負極活物質の金属リチウムは放電により有機電解液中にリチウムイオンとして溶出する。溶出したリチウムイオンは充電により、金属リチウムとして負極表面に析出するが、元のようにすべて平滑に析出せずに、樹枝状または苔状の活性な金属結晶として析出するものがある。活性な金属結晶は電解液中の有機溶媒を分解するとともに、金属結晶自体の表面は不動態被膜で覆われて不活性化し、放電に寄与し難くなる。その結果、充放電サイクルが進むにつれて負極容量が低下するので、セル作製時に、負極容量を正極のそれより著しく大きくする必要があった。また、活性な樹枝状金属リチウム結晶は、セパレータを貫通して正極と接触して、内部短絡する場合がある。内部短絡により、セルは発熱する恐れがある。
【０００４】
そこで、負極材料として充電および放電により、インターカレーションおよびディインターカレーションを可逆的に繰り返すことができる炭素材を用いる、いわゆるリチウムイオン二次電池が提案され、活発に研究開発されて、すでに実用化段階を迎えている。このリチウムイオン二次電池は過充電しない限り、充放電時に、負極表面に活性な樹枝状の金属リチウム結晶が析出しないので、安全性の向上が大いに期待できる。さらに、この電池は金属リチウムを負極活物質に用いるリチウム二次電池よりも高率充放電特性とサイクル寿命が著しく優れているので、近年この電池の需要は急速に伸張している。
【０００５】
４Ｖ級のリチウムイオン二次電池の正極活物質としては、放電状態に相当するＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎＯ₂、ＬｉＭｎ₂Ｏ₄などのリチウムと遷移金属の複合酸化物が採用または検討されている。電解質としては、リチウム二次電池と同様に有機電解液やポリマー固体電解質等の非水電解質が用いられる。
【０００６】
負極材料に黒鉛を用いた場合にリチウムイオンがインターカレーションされて生成する層間化合物のＣ₆Ｌｉを基準にした炭素１ｇ当たりの容量の理論値は３７２ｍＡｈである。従って、種々の炭素材において、この比容量の理論値に近付き、かつ実用電池の負極としては、単位体積当たりの容量値、すなわち、容量密度（ｍＡｈ／ｃｃ）が可及的に高くなるものを選ぶべきである。
【０００７】
各種炭素材のうち、俗にハードカーボンと称される難黒鉛化炭素において、前記した比容量理論値（３７２ｍＡｈ／ｇ）を越える材料が見出されて検討が進められている。しかし、難黒鉛化性の非晶質炭素の真比重は小さく、嵩張るので、単位体積当たりの負極の容量密度を大きくするのは実質的に困難である。その上、充電後の負極電位が金属リチウム電位に近似する程卑とはいえず、放電電位は平坦性も劣る等の課題が多い。
【０００８】
これに対して、結晶性が高い天然黒鉛および人造黒鉛粉末を負極に用いた場合、充電後の電位は金属リチウム電位に近似し、かつ放電電位の平坦性も優れており、実用電池として、充放電特性が向上するので、最近では黒鉛系粉末が負極材料の主流となりつつある。
【０００９】
そのなかにあって、リチウムイオン二次電池の負極用黒鉛粉末の平均粒径が大きければ、高率での充放電特性および低温における放電特性が劣る傾向がある。
【００１０】
そこで、粉末の平均粒径を小さくすれば、高率充放電特性および低温放電特性は向上するが、徒らに平均粒径を小さくし過ぎると、粉末の比表面積が大きくなり過ぎることによって、初充電により粉末中に挿入されたリチウムが第１サイクル以降の放電に寄与できない不可逆容量が大きくなる問題が生ずる。この現象は高エネルギー密度化志向に対して致命的な欠点であるとともに、１００℃を越えるような高温下で電池を放置した場合、有機電解液中の溶媒を分解させて、自己放電するだけでなく、セル内圧を高めて漏液事故を起こす恐れがあり、電池の信頼性を低下させる原因となっていた。
【００１１】
以上のことから、負極用黒鉛粉末には適切な比表面積および平均粒径が重要になることは容易に理解される。そのような観点から提案された発明が例えば、特開平６−２９５７２５号公報において、ＢＥＴ法による比表面積が１〜１０ｍ²／ｇであり、平均粒径が１０〜３０μｍであり、かつ、粒径１０μｍ以下の粉末の含有率および粒径３０μｍ以上の粉末の含有率の少なくとも一方が１０％以下である黒鉛粉末を使用することが開示されている。さらに、特開平７−１３４９８８号公報においては、石油ピッチを低温で熱処理して生成するメソカーボンマイクロビーズを黒鉛化し、広角Ｘ線回折法による（００２）面の面間隔（ｄ００２）が３．３６〜３．４０Åで、ＢＥＴ法による比表面積が０．７〜５．０ｍ²／ｇである球状黒鉛粉末を使用することが開示されている。
【００１２】
【発明が解決しようとする課題】
前述した発明は、リチウムイオン二次電池の高率充放電特性および低温時の放電特性の向上に極めて効果的であるだけでなく、宿命的ともいえる、サイクル初期に決定づけられる不可逆容量の低減に効果的であった。しかし、高温下での放置による保存性や信頼性に対して不十分であり、負極の比容量（ｍＡｈ／ｇ）および容量密度（ｍＡｈ／ｃｃ）の点でも不満が残っていた。
【００１３】
本発明は、リチウム二次電池のさらなる信頼性および高エネルギー密度化の改善をはかることをその目的とする。
【００１４】
【課題を解決するための手段】
前述したリチウムイオン二次電池における課題を解決するために、本発明は、高純度（固定炭素分９８％以上）で、かつ高結晶性の平均粒子径が２０μｍ以上でかつ厚みの平均値が１５μｍ以上の鱗片状あるいは塊状黒鉛粒子を液体中または気体中に分散させ、その液体または気体に圧力をかけてノズルからラセン状に吐出させて微粉砕後篩分けし、タッピング密度、広角Ｘ線回折法による（１１０）／（００４）のＸ線回折ピーク強度比および粒子形状を管理することにより、初期サイクルに認められる不可逆容量を可及的に小さくすると共に、高温下での放置における電池の保存性および信頼性を向上し、優れた高率放電特性および低温における放電特性を確保し、かつ比容量が高い非水電解質二次電池の実現を可能にしたものである。
【００１５】
【発明の実施の形態】
本発明の請求項１に記載の発明は、正極と負極とこれらの間に配されるセパレータを備え、前記負極は、充電および放電によりリチウムイオンがインターカレーションおよびディインターカレーションを可逆的に繰り返すことができる負極材料として、広角Ｘ線回折法による（００２）面の面間隔（ｄ００２）が３．３５０〜３．３６０Åであり、Ｃ軸方向の結晶子の大きさ（Ｌｃ）が少なくとも１０００Å以上で鱗片状あるいは塊状黒鉛粒子をさらに微粉砕する過程で、角取りして、角取りされた鱗片状または塊状黒鉛粒子とし、篩分けにより平均粒径１０〜３０μｍで、一番薄い部分の厚さの平均値が３〜９μｍで、広角Ｘ線回折法による（１１０）／（００４）のＸ線回折ピーク強度比が０．０１５以上の範囲に規制した粉末を用いた非水電解質二次電池にすることにより、リチウム二次電池の諸特性を改善するとともに、高エネルギー密度化を達成し得るものである。
【００１６】
請求項２乃至６に記載の発明は、請求項１記載の負極用黒鉛粉末に関し、ＢＥＴ法による比表面積を２．０〜８．０ｍ²／ｇ、タッピング密度を０．６〜１．２ｇ／ｃｃにすることにより、特に、粒径５μｍ未満および粒径５０μｍを越える粉末の含有率を規制することにより、上記した目的を確実に達成せしめたものである。
【００１７】
請求項７に記載の本発明は、請求項１に記載の非水電解質二次電池において、正極活物質にリチウム含有遷移金属酸化物（化学式ＬｉｘＭＯ₂、ただし、ＭはＣｏ、Ｎｉ、Ｍｎ、Ｆｅから選ばれる１種以上の遷移金属、ｘ＝０以上１．２以下）を用いることにより、安全性や高率充放電特性に優れるリチウムイオン二次電池を提供するものである。正極活物質は特に、ＬｉｘＣｏＯ₂、ＬｉｘＮｉＯ₂、ＬｉｘＭｎ₂Ｏ₄および、それらのＣｏ、Ｎｉ、Ｍｎの一部を他の遷移金属などの元素で置換したものが好適である。
【００１８】
請求項８に記載の発明は広角Ｘ線回折法による（００２）面の面間隔（ｄ００２）が３．３５０〜３．３６０Åであり、Ｃ軸方向の結晶子の大きさ（Ｌｃ）が少なくとも１０００Å以上で平均粒子径が２０μｍ以上でかつ一番薄い部分の厚さの平均値が１５μｍ以上の鱗片状あるいは塊状黒鉛粒子を液体中あるいは気体中に分散させ、その液体または気体に圧力をかけてノズルからラセン状に吐出して微粉砕後篩分けし、角取りされ、かつ平均粒径が１０〜３０μ m でかつ一番薄い部分の厚さの平均粒径が３〜９μ m である鱗片状または塊状黒鉛粒子を得て、これを用いて負極を形成することを特徴とする非水電解質二次電池用負極の製造法に係るものである。本発明方法を実施するに当っては、湿式法、乾式法のいずれでもよく、黒鉛粒子を液体中に分散させて微粉砕し、前記角取りされた鱗片状または塊状黒鉛粒子（以下、ディスク状またはタブレット状の黒鉛粒子と称する。）を得る湿式法に関しては、液体中の黒鉛濃度を５〜３０重量％にするのが好ましく、１５〜２５重量％がより好ましい。ノズル径としては、０．３〜３ｍｍにするのが好ましく、０．６〜１．２ｍｍがより好ましい。さらに吐出圧としては、１００〜１０００ｋｇ／ｃｍ²が好ましく、４００〜７００ｋｇ／ｃｍ²がより好ましい。
【００１９】
また、黒鉛粒子を気体中に分散させて微粉砕しディスク状またはタブレット状の黒鉛粒子を得る乾式法に関しては、気体中の黒鉛濃度を１０〜６０ｋｇ／ｍ³するのが好ましい。ノズル径としては、３〜３５ｍｍにするのが好ましく、１５〜２５ｍｍがより好ましい。さらに吐出圧としては、０．３〜１０ｋｇ／ｃｍ²が好ましく、０．５〜３ｋｇ／ｃｍ²がより好ましい。
【００２０】
本発明の湿式法に用いる溶媒としては、水、エタノール、メタノールなどが適当である。また乾式法に用いるガスとしては、空気、窒素、アルゴンなどが適当である。
【００２１】
上記条件下で、液体または気体に圧力をかけてノズルからラセン状に吐出させ、粉砕容器内で渦巻き流をおこし微粉砕することでディスク状またはタブレット状の黒鉛粒子を効率よく得ることができる。
【００２２】
本発明方法の湿式、乾式法とも上記濃度限定範囲以外の、高濃度域では十分に粉砕が進まずディスク状またはタブレット状の黒鉛粒子を得難く、また低濃度領域では生産性に欠ける。
【００２３】
また上記ノズル径範囲以外の、ノズル径の大きい領域では粉砕効率が低下し生産性に欠け、またノズル径の小さい領域では粉砕が進みすぎ、ディスク状またはタブレット状の黒鉛粒子を得難い。
【００２４】
さらに上記吐出圧範囲以外の、吐出圧の小さい領域では粉砕が進まないため生産性に欠け、また大きい領域では逆に粉砕が進みすぎてディスク状またはタブレット状の黒鉛粒子を得難い。
【００２５】
本発明は特に電解質を限定するものではなく、電解液、ポリマー電解質、あるいはそれらの併用などいずれでもよいが、請求項７に記載した４Ｖ級正極と本発明の負極を用いた電池に用いられる電解液の溶媒としては耐酸化性及び低温特性に優れるエチレンカーボネート、プロピレンカーボネート、ブチレンカーボネートなどの環状カーボネート1 種以上と、ジメチルカーボネート、ジエチルカーボネート、エチルメチルカーボネートなどの鎖状カーボネート1 種以上との混合溶媒を主成分とするのが好適である。また、必要に応じて、脂肪族カルボン酸エステルやエーテル類などの他の溶媒を混合できる。混合比率は、体積換算で環状カーボネートが溶媒全体の５〜５０％、特に１５〜４０％、鎖状カーボネートが１０〜９０％、特に２０〜８０％の範囲が好ましい。
【００２６】
また、正極に３Ｖ級などの比較的低電位の材料を使用する場合は、上記溶媒以外の溶媒も使用できる。
【００２７】
これらの溶媒の溶質にはリチウム塩が使用される。一般的に知られているリチウム塩にはＬｉＣｌＯ₄、ＬｉＢＦ₄、ＬｉＰＦ₆、ＬｉＡｌＣｌ₄、ＬｉＳｂＦ₆、ＬｉＳＣＮ、ＬｉＣｌ、ＬｉＣＦ₃ＳＯ₃、ＬｉＣＦ₃ＣＯ₂、Ｌｉ（ＣＦ₃ＳＯ₂）₂、ＬｉＡｓＦ₆、ＬｉＮ（ＣＦ₃ＳＯ₂）₂などがある。
【００２８】
上記以外の電池構成上必要な部材の選択については何ら制約を設けるものではない。
【００２９】
【実施例】
以下、本発明の実施形態について、図表を用いて詳細に説明する。
【００３０】
（基礎実験例）
図１はリチウムイオン二次電池の負極用炭素材料の可逆容量および不可逆容量を測定するためのコイン形セルの断面図である。図１において、ステンレス鋼製セルケース１の内底面にステンレス鋼製のエキスパンドメタルからなるグリッド３を予めスポット溶接しておき、このグリッド３とリチウムイオン二次電池の負極用炭素粉末を主成分とする合剤を缶内成型法により炭素電極５として一体に固定する。炭素電極５の合剤は、供試用炭素粉末とアクリル系結着剤とを重量比で１００：５の比率で混合したものである。ステンレス鋼製のふた２の周縁には、ポリプロピレン製ガスケット７が嵌着されており、かつ、ふた２の内面には金属リチウム４が圧着されている。炭素電極５に非水電解質を注加含浸させた後、微孔性ポリエチレン膜からなるセパレータ６を介してガスケット７付のふた２をセルケース１にカップリングし、セルケース１の上縁開口部を内方向にカールさせて封口する。なお、非水電解質としては、エチレンカーボネートとジエチルカーボネートとの体積比１：１の混合溶媒に六フッ化リン酸リチウムを１ｍｏｌ／ｌの濃度に溶解させた有機電解液を用いた。炭素電極５に２９種類の供試炭素粉末を用いてセルを作製し、炭素電極５を正極、金属リチウム電極４を負極として、２０℃のもとで電流密度０．３ｍＡ／ｃｍ²の定電流で充電および放電する。セル電圧が０Ｖになるまで炭素にリチウムをインターカレートした後、セル電圧が１．０Ｖになるまで炭素からリチウムをディインターカレートして求めた容量を可逆容量とする。インターカレートに要した電気量から可逆容量を除した値を不可逆容量とした。なお、これらテストセルの充放電終止電圧値は、負極炭素／正極ＬｉＣｏＯ₂系の実用電池の充電終止電圧４．２０Ｖおよび放電終止電圧２．７５Ｖにほぼ相当する。
【００３１】
常法により粉砕して得られる鱗片状天然黒鉛（平均粒子径約５０μｍ、一番薄い部分の厚さの平均約２５μｍ）あるいは塊状天然黒鉛（平均粒子径約５０μｍ、一番薄い部分の厚さの平均約３０μｍ）および鱗片状人造黒鉛粒子（平均粒子径約５０μｍ、一番薄い部分の厚さの平均約３０μｍ）を表２あるいは表３に示す本発明の条件で微粉砕した後篩分けすることにより、平均粒子径を規制した黒鉛粉末（試料Ｎｏ．１２〜２９）を負極用供試炭素粉末とし、それら粉末の物性値と前述した可逆容量と不可逆容量とを表２ならびに表３にまとめて示す。また比較試料として、常法により粉砕して得られる鱗片状あるいは塊状天然黒鉛および人造黒鉛粒子をボールミル、ジェットミル、ハンマーミル、ピンミルなどに代表される常法の衝撃粉砕機で微粉砕された（試料Ｎｏ．１〜９）および特開平７−１３４９８８号公報に開示されているメソカーボンマイクロビーズを黒鉛化した球状のメソカーボンマイクロビーズ（ＭＣＭＢ、試料Ｎｏ．１０）および石油ピッチコークス粉末（試料Ｎｏ．１１）を負極用供試炭素粉末とし、それら粉末の物性値ならびに不可逆容量と可逆容量を表１にまとめて示す。
【００３２】
なお、供試炭素粉末のタッピング密度は細川ミクロン社製パウダーテスター（装置名）により測定した。平均粒径は堀場製作所製ＬＡ−９１０（装置名）を用い、レーザー光を照射して光の回折現象（散乱）を解析して求めた。比表面積は島津製作所製ＡＳＡＰ２０１０（装置名）を用い、ＢＥＴ多点法により測定した。炭素粉末の厚さの平均値は、各供試黒鉛粉末を金型を用い加圧成形した後、成型体を加圧方向と平行に切断した面のＳＥＭ像から求めた。すなわち、炭素粉末の一番薄い部分の厚さ方向の値を１００個以上測定し、その平均値を求めた。
【００３３】
（１１０）／（００４）のＸ線ピーク強度比は、金型を用いて炭素粉末を加圧し、密度約１．７ｇ／ｃｃのペレットを成形し、広角Ｘ線回折測定により得られる（１１０）と（００４）面のピーク強度を５点測定し（１１０）／（００４）のピーク強度比を算出し、その平均値を求めた。
【００３４】
（００４）面と（１１０）面の回折線は黒鉛結晶の炭素六員環網状平面ならびにその垂直面での回折線である。鱗片形状が多い場合、ディスク状またはタブレット状の黒鉛粒子が多い場合に比べて、ペレット作製時に加圧面と平行方向に黒鉛粒子が選択的に配向する。従って、ディスク状またはタブレット状の黒鉛粒子に比べて鱗片状粒子の割合が多くなると（１１０）／（００４）のＸ線ピーク強度比は小さくなる。
【００３５】
【表１】

【００３６】
【表２】

【００３７】
【表３】

【００３８】
表１、２、３の結果からＬｃが１０００Å未満である比較試料の球状黒鉛粉末（試料Ｎｏ．１０）およびコークス粉末（試料Ｎｏ．１１）は、不可逆容量は比較的小さいが、エネルギー密度に大きく影響する可逆容量はどちらも３００ｍＡｈ／ｇ未満と小さかった。これらに対して、原材料が天然黒鉛および人造黒鉛粉末の試料Ｎｏ．１〜９ならびに１２〜２９の可逆容量はすべて少なくとも３５０ｍＡｈ／ｇで、比容量の理論値（３７２ｍＡｈ／ｇ）に近似した値となった。これらのなかで、試料Ｎｏ．１２〜２９の黒鉛粉末の不可逆容量は１７〜３０ｍＡｈ／ｇで、他の黒鉛粉末（試料Ｎｏ．１〜９）のそれより同レベルかさらに小さいレベルにあることが注目される。
【００３９】
本発明の前提条件として広角Ｘ線回折による（００２）面の面間隔（ｄ００２）が３．３５０〜３．３６０Åであり、Ｃ軸方向の結晶子の大きさ（Ｌｃ）が少なくとも１０００Åである結晶化度および純度が高い天然黒鉛または人造黒鉛をリチウムイオン二次電池の負極材に用いることより高水準の可逆容量が得られることが理解される。
【００４０】
（実施例及び比較例）
基礎実験例１で、可逆容量および不可逆容量を求めた負極用炭素粉末（試料Ｎｏ．１〜２９）を用いて、円筒形セルを作製し、低温における高率放電特性および充電状態で高温放置した場合の漏液性を測定した。
【００４１】
図２は渦巻状電極群構成の円筒形セルの断面図である。図２において、各１枚の帯状正極１０と負極１１とを微孔性ポリエチレン膜からなるセパレータ１２を介して渦巻状に捲回して電極群が構成される。正極１０は活物質材料のリチウムとコバルトとの複合酸化物であるＬｉＣｏＯ₂と導電材のカーボンブラックと結着剤のポリ四フッ化エチレン（ＰＴＦＥ）とを重量比で１００：３：１０の割合で混合したペーストを集電体であるアルミニウム箔の両面に塗着、乾燥後ロールプレスし、所定寸法に裁断したものである。なお、結着剤のＰＴＦＥはディスパージョン溶液のものを用いた。正極１０のアルミニウム箔には、正極リード片１３がスポット溶接されている。負極１１は供試炭素粉末にアクリル系結着剤溶液を加えて混合したペーストを集電体である銅箔の両面に塗着、乾燥後ロールプレスし、所定の寸法に裁断したものである。負極１１の銅箔には負極リード片１４がスポット溶接されている。捲回した電極群の下面に底部絶縁板１５を装着して、ニッケル鍍鋼板製のセルケース１６内に収容した後、負極リード片１４をセルケース１６の内底面にスポット溶接する。その後電極群上に上部絶縁板１７を載置してからセルケース１６の開口部の所定位置に溝入れし、所定量の有機電解液を注入含浸させる。有機電解液としては基礎実験例と同じ有機電解液を用いた。
【００４２】
その後、周縁にガスケット１８が嵌着された封口板１９の内底面に正極リード片１３をスポット溶接する。封口板１９をセルケース１６の開口部にガスケット１８を介して嵌め込んで、セルケース１６の上縁を内方向にカールして封口すればセルは完成する。
【００４３】
各セルの放電容量は負極容量で規制されるようにし、種類にかかわらず各セルの負極用炭素粉末重量を同じにした。他の部品材料の使用量、作製方法は全く同じにして負極用炭素粉末の比較ができるようにした。
【００４４】
２９種類の負極用炭素粉末を用いたセルａ〜ｋおよびＡ〜Ｒ各５セルについて、２０℃ですべてのセルを１００ｍＡ（１／５Ｃ）定電流で各セルの端子電圧が４．２Ｖになるまで充電した後、１００ｍＡ（１／５Ｃ）定電流で２．７５Ｖまで放電して、１／５Ｃ放電容量を求めた。その後、同様に充電した後５００ｍＡ（１Ｃ）定電流で２．７５Ｖまで放電して、１Ｃ放電容量を求めた。次いで、２０℃で充電した後、−２０℃で２４時間放置し、同じ−２０℃で１Ｃ放電容量を求めた。各セルを２０℃に静置し、セルの温度が２０℃に復してから同じ方法で充電した後、１００℃で１日放置し、セルの温度が２０℃になってから漏液の有無を全セルについて観察した。
【００４５】
供試炭素粉末の物性値に対比して、前述した電池性能（５セルの平均値）をまとめて表４に示す。
【００４６】
【表４】

【００４７】
表４から、表１で示した可逆容量が小さかった試料Ｎｏ．１０および１１の２０℃での１／５Ｃおよび１Ｃ放電容量は低いが、試料Ｎｏ．１〜９の黒鉛粉末のそれらは相対的に大きい。しかしながら、低温における高率放電容量（−２０℃、１Ｃ）が４１５ｍＡｈ以上を示したのは、試料Ｎｏ．１、２、６、７、８、１０および１２〜２９の黒鉛粉末によるセルａ、ｂ、ｆ、ｇ、ｈ、ｊおよびＡ〜Ｒだけであった。さらに、高温放置後に漏液が全く認められなかったのは、試料Ｎｏ．４および１０〜２９の炭素粉末によるセルｄ、ｊ、ｋおよびＡ〜Ｒであった。これらの結果からすべての電池性能にわたって優れていたのは本発明の試料Ｎｏ．１２〜２９の黒鉛粉末によるセルＡ〜Ｒであった。
【００４８】
試料Ｎｏ．１２〜２９の黒鉛粉末の物性値で、他の黒鉛粉末（試料Ｎｏ．１〜９）と大きく相違する点はタッピング密度が０．２１〜０．４６ｇ／ｃｃに対して、０．６０〜１．１５ｇ／ｃｃと高いこと、また広角Ｘ線回折法による（１１０）／（００４）のＸ線回折ピーク強度比が０．０１５以上である点である。これらはＮｏ．１２〜２９の黒鉛粉末の平均粒径が１０．３〜２９．５μｍであり、かつ、ＢＥＴ法による比表面積が２．１〜７．４ｍ²／ｇで、平均粒径の割に、比表面積が徒らに大き過ぎないことに関連し、さらに、試料Ｎｏ．１２〜２９の黒鉛粉末の平均の厚さが３．１〜８．９μｍで、他の黒鉛粉末（試料Ｎｏ．１〜９）の１．１〜２．５μｍよりも大きい点、すなわち、本発明による黒鉛粉末は、結晶化度および純度が高い鱗片状あるいは塊状黒鉛粒子を液体中または気体中に分散させ、その液体または気体に圧力をかけてノズルからラセン状に吐出させて渦巻き流をおこすことにより微粉砕する過程で角とりしてディスク状またはタブレット状粒子とし、その後所望の粒径に篩分けすることで、タッピング密度が高く、また広角Ｘ線回折法による（１１０）／（００４）のＸ線回折ピーク強度比が大きいものとなっているわけである。従って、適切に微粉砕し、かつ篩分けすることで黒鉛粉末を所望の平均粒径分布とすることにより、高率充放電性能および低温での高率放電性能を向上している。その上、微粉砕化されても、粉末の厚さが大きく、鱗片状粒子のなかでも球形に近いものを集めているので、徒らに比表面積を増大させず、高温下でも電解液中の有機溶媒を分解させ難く、セル内圧の上昇をさせ難くなったことが漏液事故を皆無にし得たものと考えられる。
【００４９】
従来、リチウムイオン二次電池の負極用炭素、特に黒鉛粉末を平均粒径および比表面積だけで管理していたが、タッピング密度で規制することの重要性が理解されよう。なお、上記以外の数多くの実験の結果、本発明による黒鉛粉末の平均粒径が１０〜３０μｍで、一番薄い部分の厚さの平均値が３〜９μｍで、広角Ｘ線回折法による（１１０）／（００４）のＸ線回折ピーク強度比が０．０１５以上の範囲が適切であることが判明している。また、その場合のＢＥＴ法による比表面積が２．０〜８．０ｍ²／ｇ、またタッピング密度の規制の範囲は０．６〜１．２ｇ／ｃｃになることも確認されている。さらに、粒径５μｍ未満の高温放置において信頼性を低下させる微細に過ぎる粉末含有量は１５％以下にすること、および、高率放電性能を阻害する５０μｍを越える粉末の含有率は３０％以下にすべきことも確認されている。
【００５０】
【発明の効果】
以上のように本発明による負極用黒鉛粉末を使用することにより、比容量の理論値（３７２ｍＡｈ／ｇ）の少なくとも９４％の３５１〜３６０ｍＡｈ／ｇ（９４．４〜９６．８％）であり、不可逆容量は１７〜３０ｍＡｈ／ｇと極めて小さく、エネルギー密度の向上に資するものである。さらに、高率充放電および低温高率放電性能が優れるだけでなく、高温放置によっても漏液事故が発生せず信頼性の高いリチウム二次電池を提供できるという効果を奏し得るものである。
【図面の簡単な説明】
【図１】本発明の効果を検討すべく可逆容量および不可逆容量を測定するためのコイン形セルの断面図。
【図２】本発明の実施形態による渦巻状電極群構成の円筒形セルの断面図。
【符号の説明】
１：セルケース
２：ふた
３：グリッド
４：金属リチウム電極
５：炭素電極
６：セパレータ
７：ガスケット
１０：正極
１１：負極
１２：セパレータ
１３：正極リード片
１４：負極リード片
１５：底部絶縁板
１６：セルケース
１７：上部絶縁板
１８：ガスケット
１９：封口板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to a carbon material for a negative electrode of a lithium ion secondary battery.
[0002]
[Prior art]
Conventionally, as non-aqueous electrolyte secondary batteries, aiming at high energy density by high voltage and high capacity, metal lithium as a negative electrode active material, transition metal oxides, sulfides, selenides, etc. as a positive electrode active material A so-called lithium secondary battery using an organic electrolytic solution composed of an organic solvent solution of a lithium salt as a nonaqueous electrolyte, such as a chalcogen compound such as manganese dioxide, molybdenum disulfide, or titanium selenide, has been studied.
[0003]
However, although this lithium secondary battery can select an intercalation compound having relatively excellent charge / discharge characteristics as the positive electrode active material, the charge / discharge characteristics of metallic lithium of the negative electrode are not necessarily excellent. For this reason, it is difficult to extend the cycle life of repeated charge and discharge, and there is a risk of heat generation due to an internal short circuit, which causes a safety problem. That is, the lithium metal of the negative electrode active material is eluted as lithium ions in the organic electrolyte by discharge. The eluted lithium ions are deposited on the surface of the negative electrode as metallic lithium by charging, but some of them are deposited as dendritic or mossy active metal crystals without being deposited as smooth as the original. The active metal crystal decomposes the organic solvent in the electrolytic solution, and the surface of the metal crystal itself is covered with a passive film to be inactivated, making it difficult to contribute to the discharge. As a result, since the negative electrode capacity decreases as the charge / discharge cycle proceeds, it is necessary to make the negative electrode capacity significantly larger than that of the positive electrode at the time of cell preparation. In addition, active dendritic metal lithium crystals may penetrate the separator and contact the positive electrode, causing an internal short circuit. The cell may generate heat due to an internal short circuit.
[0004]
Therefore, a so-called lithium ion secondary battery using a carbon material capable of reversibly repeating intercalation and deintercalation by charging and discharging as a negative electrode material has been proposed, actively researched and developed, and already in practical use. It is in the transition stage. As long as this lithium ion secondary battery is not overcharged, active dendritic metal lithium crystals are not deposited on the negative electrode surface during charge and discharge, and thus safety can be greatly improved. Furthermore, since this battery has significantly higher charge / discharge characteristics and cycle life than lithium secondary batteries using metallic lithium as a negative electrode active material, the demand for this battery has been rapidly increasing in recent years.
[0005]
As a positive electrode active material of a 4V class lithium ion secondary battery, LiCoO corresponding to a discharged state is used.₂, LiNiO₂LiMnO₂, LiMn₂O_FourLithium and transition metal composite oxides such as these have been adopted or studied. As the electrolyte, a nonaqueous electrolyte such as an organic electrolyte or a polymer solid electrolyte is used as in the case of the lithium secondary battery.
[0006]
C of intercalation compound produced by intercalation of lithium ions when graphite is used as the negative electrode material₆The theoretical value of the capacity per gram of carbon based on Li is 372 mAh. Accordingly, in various carbon materials, the specific value of the specific capacity approaches the theoretical value, and the negative electrode of the practical battery has a capacity value per unit volume, that is, a capacity density (mAh / cc) as high as possible. Should choose.
[0007]
Among various carbon materials, in the non-graphitizable carbon commonly called hard carbon, a material exceeding the above-described theoretical specific capacity (372 mAh / g) has been found and studied. However, since the true specific gravity of non-graphitizable amorphous carbon is small and bulky, it is practically difficult to increase the capacity density of the negative electrode per unit volume. Moreover, it cannot be said that the negative electrode potential after charging approximates to the metallic lithium potential, and the discharge potential has many problems such as poor flatness.
[0008]
On the other hand, when natural graphite and artificial graphite powder with high crystallinity are used for the negative electrode, the potential after charging is close to the lithium metal potential, and the flatness of the discharge potential is excellent. Since the discharge characteristics are improved, graphite-based powders are becoming the mainstream of negative electrode materials recently.
[0009]
Among them, if the average particle diameter of the negative electrode graphite powder of the lithium ion secondary battery is large, the charge / discharge characteristics at a high rate and the discharge characteristics at a low temperature tend to be inferior.
[0010]
Therefore, if the average particle size of the powder is reduced, the high rate charge / discharge characteristics and the low-temperature discharge property are improved. However, if the average particle size is too small, the specific surface area of the powder becomes too large. There arises a problem that the irreversible capacity, in which lithium inserted into the powder by charging cannot contribute to the discharge after the first cycle, becomes large. This phenomenon is a fatal defect for high energy density orientation, and when the battery is left at a high temperature exceeding 100 ° C, the solvent in the organic electrolyte is decomposed and self-discharge occurs. However, there was a risk of increasing the internal pressure of the cell and causing a liquid leakage accident, which was a cause of lowering the reliability of the battery.
[0011]
From the above, it is easily understood that an appropriate specific surface area and an average particle size are important for the negative electrode graphite powder. The invention proposed from such a viewpoint is disclosed in, for example, Japanese Patent Laid-Open No. 6-295725, in which the specific surface area by the BET method is 1 to 10 m.²Graphite powder having an average particle diameter of 10 to 30 μm and at least one of the content ratio of the powder having a particle diameter of 10 μm or less and the content ratio of the powder having a particle diameter of 30 μm or more is 10% or less. It is disclosed. Furthermore, in Japanese Patent Laid-Open No. 7-134888, mesocarbon microbeads produced by heat treatment of petroleum pitch at low temperature are graphitized, and the surface spacing (d002) of (002) plane by wide angle X-ray diffraction method is 3.36. ~ 3.40mm, specific surface area by BET method is 0.7 ~ 5.0m²It is disclosed to use spheroidal graphite powder that is / g.
[0012]
[Problems to be solved by the invention]
The above-described invention is not only extremely effective in improving the high-rate charge / discharge characteristics and discharge characteristics at low temperatures of lithium ion secondary batteries, but also effective in reducing the irreversible capacity determined at the beginning of the cycle, which can be said to be fatal. It was the target. However, it is insufficient for storage stability and reliability due to standing at high temperatures, and dissatisfaction remained in terms of specific capacity (mAh / g) and capacity density (mAh / cc) of the negative electrode.
[0013]
An object of the present invention is to further improve the reliability and increase in energy density of a lithium secondary battery.
[0014]
[Means for Solving the Problems]
In order to solve the above-described problems in the lithium ion secondary battery, the present invention has a high purity (fixed carbon content of 98% or more), a high crystallinity average particle diameter of 20 μm or more, and an average thickness of 15 μm. Disperse the above scaly or massive graphite particles in a liquid or gas, apply pressure to the liquid or gas, discharge it in a spiral form from a nozzle, finely pulverize and sieve, tapping density, wide-angle X-ray diffraction method By controlling the (110) / (004) X-ray diffraction peak intensity ratio and particle shape according to the above, the irreversible capacity observed in the initial cycle is made as small as possible, and the storage stability of the battery when left at high temperature In addition, it is possible to improve the reliability, ensure excellent high rate discharge characteristics and discharge characteristics at low temperature, and realize a non-aqueous electrolyte secondary battery having a high specific capacity.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The invention according to claim 1 of the present invention includes a positive electrode, a negative electrode, and a separator disposed between them, and the negative electrode reversibly intercalates and deintercalates when lithium ions are charged and discharged. As a negative electrode material that can be repeated, the surface spacing (d002) of (002) planes by wide-angle X-ray diffraction method is 3.350 to 3.360 mm, and the crystallite size (Lc) in the C-axis direction is at least 1000 mm. In the process of further pulverizing the scaly or massive graphite particles,, Chamfered scaly or massive graphite particlesThe average particle size is 10-30 μm by sieving, the average thickness of the thinnest part is 3-9 μm, and the X-ray diffraction peak intensity ratio of (110) / (004) by the wide-angle X-ray diffraction method is By making a non-aqueous electrolyte secondary battery using powder regulated to a range of 0.015 or more, various characteristics of the lithium secondary battery can be improved and a high energy density can be achieved.
[0016]
The invention according to any one of claims 2 to 6 relates to the graphite powder for negative electrode according to claim 1, wherein the specific surface area by the BET method is 2.0 to 8.0 m.²/ G and a tapping density of 0.6 to 1.2 g / cc, in particular, by regulating the content of powders having a particle size of less than 5 μm and a particle size of more than 50 μm, the above-mentioned object can be reliably achieved. It is a thing.
[0017]
According to a seventh aspect of the present invention, in the nonaqueous electrolyte secondary battery according to the first aspect, the lithium-containing transition metal oxide (chemical formula LixMO) is used as the positive electrode active material.₂However, when M is one or more transition metals selected from Co, Ni, Mn, and Fe, x = 0 to 1.2, lithium ion secondary excellent in safety and high rate charge / discharge characteristics A battery is provided. The positive electrode active material is particularly LixCoO.₂LixNiO₂LixMn₂O_FourAnd what substituted a part of those Co, Ni, and Mn with elements, such as another transition metal, is suitable.
[0018]
In the invention described in claim 8, the (002) plane spacing (d002) by wide-angle X-ray diffraction method is 3.350 to 3.360 mm, and the crystallite size (Lc) in the C-axis direction is at least 1000 mm. As described above, the scaly or massive graphite particles having an average particle diameter of 20 μm or more and an average thickness of the thinnest part of 15 μm or more are dispersed in a liquid or gas, and a nozzle is applied by applying pressure to the liquid or gas. Discharged into a spiral shape, finely pulverized and sieved,Chamfered and average particle size is 10-30μ m And the average particle size of the thickness of the thinnest part is 3-9μ m Scale-like or massive graphite particlesAnd a negative electrode is formed using the same, and relates to a method for producing a negative electrode for a non-aqueous electrolyte secondary battery. In carrying out the method of the present invention, either a wet method or a dry method may be used, and graphite particles are dispersed in a liquid and finely pulverized.The chamfered scale-like or massive graphite particles (hereinafter referred to as disk-like or tablet-like graphite particles).As for the wet method for obtaining the above, the graphite concentration in the liquid is preferably 5 to 30% by weight, more preferably 15 to 25% by weight. The nozzle diameter is preferably 0.3 to 3 mm, and more preferably 0.6 to 1.2 mm. Furthermore, the discharge pressure is 100 to 1000 kg / cm.²Is preferred, 400-700 kg / cm²Is more preferable.
[0019]
Further, regarding a dry method in which graphite particles are dispersed in a gas and finely pulverized to obtain disk-like or tablet-like graphite particles, the graphite concentration in the gas is 10 to 60 kg / m 2.^ThreeIt is preferable to do this. The nozzle diameter is preferably 3 to 35 mm, more preferably 15 to 25 mm. Furthermore, the discharge pressure is 0.3 to 10 kg / cm.²Is preferred, 0.5-3 kg / cm²Is more preferable.
[0020]
As the solvent used in the wet method of the present invention, water, ethanol, methanol and the like are suitable. As the gas used in the dry method, air, nitrogen, argon or the like is appropriate.
[0021]
Under the above conditions, a liquid or gas is pressurized and discharged in a spiral shape from a nozzle, and a swirl flow is generated in a pulverization container, followed by fine pulverization, whereby disk-shaped or tablet-shaped graphite particles can be efficiently obtained.
[0022]
In both the wet and dry methods of the present invention, pulverization does not proceed sufficiently in a high concentration region other than the above-mentioned concentration limited range, and it is difficult to obtain disc-shaped or tablet-like graphite particles, and productivity is poor in a low concentration region.
[0023]
In addition, in a region where the nozzle diameter is large other than the above-mentioned nozzle diameter range, the pulverization efficiency is lowered and the productivity is poor, and in the region where the nozzle diameter is small, the pulverization proceeds so much that it is difficult to obtain disc-shaped or tablet-shaped graphite particles.
[0024]
Further, since the pulverization does not proceed in a region where the discharge pressure is low, other than the above discharge pressure range, the productivity is insufficient, and in the large region, conversely, the pulverization proceeds excessively, making it difficult to obtain disc-shaped or tablet-like graphite particles.
[0025]
The present invention is not particularly limited to the electrolyte, and may be any one of an electrolytic solution, a polymer electrolyte, or a combination thereof. Electrolysis used for a battery using the 4V class positive electrode according to claim 7 and the negative electrode of the present invention. As a liquid solvent, a mixture of one or more cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, which have excellent oxidation resistance and low temperature characteristics, and one or more chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. It is preferable to use a solvent as a main component. Moreover, other solvents, such as aliphatic carboxylic acid ester and ethers, can be mixed as needed. The mixing ratio is preferably in the range of 5 to 50%, particularly 15 to 40% of the total amount of cyclic carbonate, and 10 to 90%, and particularly preferably 20 to 80% of chain carbonate in terms of volume.
[0026]
When a relatively low potential material such as 3V class is used for the positive electrode, a solvent other than the above solvents can also be used.
[0027]
Lithium salts are used as solutes for these solvents. Commonly known lithium salts include LiClO_Four, LiBF_Four, LiPF₆LiAlCl_Four, LiSbF₆, LiSCN, LiCl, LiCF_ThreeSO_Three, LiCF_ThreeCO₂, Li (CF_ThreeSO₂)₂, LiAsF₆, LiN (CF_ThreeSO₂)₂and so on.
[0028]
There are no restrictions on the selection of members necessary for the battery configuration other than those described above.
[0029]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0030]
(Example of basic experiment)
FIG. 1 is a cross-sectional view of a coin-shaped cell for measuring the reversible capacity and irreversible capacity of a carbon material for a negative electrode of a lithium ion secondary battery. In FIG. 1, a grid 3 made of stainless steel expanded metal is spot-welded in advance to the inner bottom surface of a stainless steel cell case 1, and the grid 3 and carbon powder for a negative electrode of a lithium ion secondary battery are used as main components. The mixture to be fixed is integrally fixed as the carbon electrode 5 by an in-can molding method. The mixture of the carbon electrode 5 is a mixture of the test carbon powder and the acrylic binder in a weight ratio of 100: 5. A polypropylene gasket 7 is fitted to the periphery of the stainless steel lid 2, and metal lithium 4 is pressure-bonded to the inner surface of the lid 2. After the carbon electrode 5 is poured and impregnated with a nonaqueous electrolyte, the lid 2 with the gasket 7 is coupled to the cell case 1 through the separator 6 made of a microporous polyethylene film, and the upper edge opening of the cell case 1 is coupled. Curl inward to seal. As the non-aqueous electrolyte, an organic electrolytic solution in which lithium hexafluorophosphate was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1 was used. A cell was prepared by using 29 kinds of test carbon powders for the carbon electrode 5, and the current density was 0.3 mA / cm at 20 ° C. using the carbon electrode 5 as a positive electrode and the metal lithium electrode 4 as a negative electrode.²The battery is charged and discharged at a constant current. The capacity obtained by intercalating lithium into carbon until the cell voltage reaches 0 V and then deintercalating lithium from carbon until the cell voltage reaches 1.0 V is defined as the reversible capacity. The value obtained by dividing the amount of electricity required for intercalation by the reversible capacity was defined as the irreversible capacity. The charge / discharge end voltage values of these test cells are negative carbon / positive electrode LiCoO.₂This corresponds approximately to the end-of-charge voltage of 4.20 V and the end-of-discharge voltage of 2.75 V of the practical battery of the system.
[0031]
Scale-like natural graphite (average particle diameter of about 50 μm, average thickness of the thinnest part is about 25 μm) or bulk natural graphite (average particle diameter of about 50 μm, thickness of the thinnest part) obtained by grinding by a conventional method (Average 30 μm) and flaky artificial graphite particles (average particle diameter of about 50 μm, average thickness of the thinnest part about 30 μm) are finely pulverized under the conditions of the present invention shown in Table 2 or 3 and then sieved. As a result, graphite powders (sample Nos. 12 to 29) whose average particle diameter is regulated are used as test carbon powders for negative electrodes. Show. As a comparative sample, scale-like or massive natural graphite and artificial graphite particles obtained by pulverization by a conventional method were finely pulverized by a conventional impact pulverizer represented by a ball mill, jet mill, hammer mill, pin mill, etc. ( Spherical mesocarbon microbeads (MCMB, sample No.10) obtained by graphitizing mesocarbon microbeads disclosed in Sample No. 1 to 9) and JP-A-7-134988 and petroleum pitch coke powder (sample No. .11) is a test carbon powder for a negative electrode, and the physical properties, irreversible capacity and reversible capacity of the powder are summarized in Table 1.
[0032]
The tapping density of the test carbon powder was measured with a powder tester (device name) manufactured by Hosokawa Micron. The average particle diameter was obtained by analyzing the diffraction phenomenon (scattering) of light by irradiating with laser light using LA-910 (device name) manufactured by Horiba. The specific surface area was measured by BET multipoint method using ASAP2010 (device name) manufactured by Shimadzu Corporation. The average value of the thickness of the carbon powder was obtained from the SEM image of the surface obtained by press-molding each test graphite powder using a mold and cutting the molded body parallel to the pressurizing direction. That is, 100 or more values in the thickness direction of the thinnest part of the carbon powder were measured, and the average value was obtained.
[0033]
The X-ray peak intensity ratio of (110) / (004) is obtained by pressurizing carbon powder using a mold, forming pellets with a density of about 1.7 g / cc, and performing wide-angle X-ray diffraction measurement (110) The (004) plane peak intensity was measured at five points, the peak intensity ratio of (110) / (004) was calculated, and the average value was obtained.
[0034]
The diffraction lines on the (004) plane and the (110) plane are diffraction lines on the carbon six-membered ring network plane of the graphite crystal and its vertical plane. When the scale shape is large, the graphite particles are selectively oriented in the direction parallel to the pressing surface during pellet production, as compared with the case where there are many disk-shaped or tablet-shaped graphite particles. Therefore, the X-ray peak intensity ratio of (110) / (004) decreases as the proportion of scale-like particles increases as compared to disk-shaped or tablet-shaped graphite particles.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
[Table 3]

[0038]
From the results of Tables 1, 2 and 3, the comparative graphite spheroidal graphite powder (sample No. 10) and coke powder (sample No. 11) having a Lc of less than 1000 kg have a relatively small irreversible capacity but a large energy density. Both influencing reversible capacities were as low as less than 300 mAh / g. On the other hand, sample Nos. Made of natural graphite and artificial graphite powder were used as raw materials. The reversible capacities of 1 to 9 and 12 to 29 were all at least 350 mAh / g, which approximated the theoretical specific capacity (372 mAh / g). Among these, sample no. It is noted that the irreversible capacity of the graphite powders 12 to 29 is 17 to 30 mAh / g, which is the same level or smaller than that of the other graphite powders (sample Nos. 1 to 9).
[0039]
As a precondition of the present invention, a crystal having a (002) plane spacing (d002) of 3.350 to 3.360 mm and a crystallite size (Lc) in the C-axis direction of at least 1000 mm by wide-angle X-ray diffraction. It is understood that a high level of reversible capacity can be obtained by using natural graphite or artificial graphite having a high degree of conversion and purity for the negative electrode material of a lithium ion secondary battery.
[0040]
(Examples and Comparative Examples)
A cylindrical cell was prepared using the carbon powder for negative electrode (sample Nos. 1 to 29) whose reversible capacity and irreversible capacity were obtained in basic experimental example 1, and left at high temperature with high rate discharge characteristics at low temperature and in a charged state. The liquid leakage of the case was measured.
[0041]
FIG. 2 is a sectional view of a cylindrical cell having a spiral electrode group configuration. In FIG. 2, an electrode group is formed by winding each strip-like positive electrode 10 and negative electrode 11 in a spiral shape via a separator 12 made of a microporous polyethylene film. The positive electrode 10 is an active material, LiCoO, which is a composite oxide of lithium and cobalt.₂A paste prepared by mixing a conductive material carbon black and a binder polytetrafluoroethylene (PTFE) in a weight ratio of 100: 3: 10 was applied to both sides of an aluminum foil as a current collector and dried. It is a post-roll press and cut into a predetermined size. The binder PTFE used was a dispersion solution. A positive electrode lead piece 13 is spot welded to the aluminum foil of the positive electrode 10. The negative electrode 11 is obtained by applying a paste obtained by adding an acrylic binder solution to a test carbon powder and mixing it on both surfaces of a copper foil as a current collector, followed by roll pressing and cutting into a predetermined size. A negative electrode lead piece 14 is spot-welded to the copper foil of the negative electrode 11. The bottom insulating plate 15 is attached to the lower surface of the wound electrode group and accommodated in a cell case 16 made of nickel-plated steel, and then the negative electrode lead piece 14 is spot welded to the inner bottom surface of the cell case 16. Thereafter, the upper insulating plate 17 is placed on the electrode group, and then grooved in a predetermined position of the opening of the cell case 16 to inject and impregnate a predetermined amount of organic electrolyte. As the organic electrolyte, the same organic electrolyte as in the basic experiment example was used.
[0042]
Thereafter, the positive electrode lead piece 13 is spot welded to the inner bottom surface of the sealing plate 19 having the gasket 18 fitted to the periphery. The cell is completed by fitting the sealing plate 19 into the opening of the cell case 16 via the gasket 18 and curling the upper edge of the cell case 16 inward.
[0043]
The discharge capacity of each cell was regulated by the negative electrode capacity, and the weight of the carbon powder for negative electrode of each cell was the same regardless of the type. The amount of other component materials used and the production method were completely the same so that the carbon powder for negative electrode could be compared.
[0044]
With respect to 5 cells each of cells a to k and A to R using 29 kinds of negative electrode carbon powders, the terminal voltage of each cell becomes 4.2 V at a constant current of 100 mA (1/5 C) at 20 ° C. Then, the battery was discharged to 2.75 V at a constant current of 100 mA (1/5 C), and the 1/5 C discharge capacity was determined. Thereafter, the battery was charged in the same manner, and then discharged to 2.75 V at a constant current of 500 mA (1C) to obtain a 1C discharge capacity. Next, after charging at 20 ° C., it was allowed to stand at −20 ° C. for 24 hours, and the 1C discharge capacity was determined at the same −20 ° C. Leave each cell at 20 ° C, charge it in the same way after the cell temperature returns to 20 ° C, leave it at 100 ° C for 1 day, and check if there is any leakage after the cell temperature reaches 20 ° C Were observed for all cells.
[0045]
Table 4 shows the above-described battery performance (average value of 5 cells) in comparison with the physical property values of the test carbon powder.
[0046]
[Table 4]

[0047]
From Table 4, Sample No. with the small reversible capacity shown in Table 1 was used. 10 and 11 have low 1 / 5C and 1C discharge capacities at 20 ° C. Those of graphite powders 1-9 are relatively large. However, the high-rate discharge capacity (−20 ° C., 1 C) at low temperature was 415 mAh or more. Only cells a, b, f, g, h, j and A to R with graphite powders of 1, 2, 6, 7, 8, 10 and 12 to 29. Furthermore, no liquid leakage was observed after standing at high temperature. Cells d, j, k and A-R with 4 and 10-29 carbon powder. From these results, the sample No. of the present invention was excellent over all battery performances. It was cells A to R with 12 to 29 graphite powder.
[0048]
Sample No. The physical properties of graphite powders 12 to 29, which are greatly different from other graphite powders (sample Nos. 1 to 9), are 0.60 to 1 for tapping densities of 0.21 to 0.46 g / cc. It is as high as .15 g / cc, and the X-ray diffraction peak intensity ratio of (110) / (004) by the wide-angle X-ray diffraction method is 0.015 or more. These are No. The average particle size of the graphite powder of 12 to 29 is 10.3 to 29.5 μm, and the specific surface area by the BET method is 2.1 to 7.4 m.²/ G in relation to the specific surface area not being too large for the average particle size. The average thickness of 12 to 29 graphite powder is 3.1 to 8.9 μm, which is larger than 1.1 to 2.5 μm of other graphite powder (sample No. 1 to 9), that is, the present invention. Graphite powder produced by the above method is a method in which scaly or massive graphite particles having a high degree of crystallinity and purity are dispersed in a liquid or gas, and the liquid or gas is pressurized and discharged in a spiral form from a nozzle to cause a vortex flow. In the process of fine pulverization, the particles are rounded to form disk-like or tablet-like particles, and then sieved to a desired particle size, so that the tapping density is high and (110) / (004) by the wide-angle X-ray diffraction method. The X-ray diffraction peak intensity ratio is large. Therefore, high rate charge / discharge performance and high rate discharge performance at low temperature are improved by appropriately pulverizing and sieving the graphite powder to have a desired average particle size distribution. In addition, even if finely pulverized, the powder is large in thickness, and since nearly spherical particles are collected, they do not increase the specific surface area. It is considered that it was difficult to decompose the organic solvent and to increase the internal pressure of the cell, and it was possible to eliminate the liquid leakage accident.
[0049]
Conventionally, carbon for negative electrodes of lithium ion secondary batteries, particularly graphite powder, has been managed only by the average particle diameter and specific surface area, but it will be understood the importance of regulating with tapping density. As a result of many experiments other than the above, the average particle diameter of the graphite powder according to the present invention is 10 to 30 μm, the average value of the thickness of the thinnest part is 3 to 9 μm, and it is determined by the wide angle X-ray diffraction method (110 ) / (004) X-ray diffraction peak intensity ratio has been found to be appropriate in the range of 0.015 or more. In that case, the specific surface area according to the BET method is 2.0 to 8.0 m.²It has also been confirmed that the tapping density regulation range is 0.6 to 1.2 g / cc. Furthermore, the content of powder that is too fine to reduce reliability when left at a high temperature of less than 5 μm is set to 15% or less, and the content of powder exceeding 50 μm that inhibits high rate discharge performance is set to 30% or less. It is also confirmed what to do.
[0050]
【The invention's effect】
As described above, by using the graphite powder for negative electrode according to the present invention, at least 94% of the theoretical value of specific capacity (372 mAh / g) is 351 to 360 mAh / g (94.4 to 96.8%), The irreversible capacity is as extremely small as 17 to 30 mAh / g, which contributes to an improvement in energy density. Furthermore, not only are high-rate charge / discharge and low-temperature / high-rate discharge performance excellent, but a liquid secondary battery can be provided without causing a liquid leakage accident even when left at a high temperature.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a coin-shaped cell for measuring a reversible capacity and an irreversible capacity in order to examine the effect of the present invention.
FIG. 2 is a cross-sectional view of a cylindrical cell having a spiral electrode group configuration according to an embodiment of the present invention.
[Explanation of symbols]
1: Cell case
2: Lid
3: Grid
4: Metallic lithium electrode
5: Carbon electrode
6: Separator
7: Gasket
10: Positive electrode
11: Negative electrode
12: Separator
13: Positive electrode lead piece
14: Negative electrode lead piece
15: Bottom insulating plate
16: Cell case
17: Upper insulating plate
18: Gasket
19: Sealing plate

Claims

Provided with a positive electrode, a negative electrode, and a separator disposed between them, the negative electrode is a wide-angle X-ray diffraction as a negative electrode material capable of reversibly repeating lithium ion intercalation and deintercalation by charging and discharging. The scaly or massive graphite particles having a (002) plane spacing (d002) of 3.350 to 3.360 mm and a crystallite size (Lc) in the C-axis direction of at least 1000 mm or more are further refined. In the process of pulverization, chamfered scaled or lump graphite particles are obtained, and the average particle diameter is 10 to 30 μm by sieving, and the average thickness of the thinnest part is 3 to 9 μm. A non-aqueous electrolyte secondary battery using a powder in which an X-ray diffraction peak intensity ratio of (110) / (004) by a wide-angle X-ray diffraction method is regulated to 0.015 or more.

The nonaqueous electrolyte secondary battery according to claim 1, wherein the specific surface area of the graphite powder for negative electrode by the BET method is 2.0 to 8.0 m ² / g.

The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the powder having a particle diameter of less than 5 µm in the negative electrode graphite powder is 15% or less.

The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the powder having a particle size exceeding 50 µm in the negative electrode graphite powder is 30% or less.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the graphite powder for negative electrode, the content of the powder having a particle size of less than 5 μm and a particle size exceeding 50 μm is 15% and 30% or less, respectively.

The nonaqueous electrolyte secondary battery according to claim 1, wherein the graphite powder for negative electrode has a tapping density of 0.6 to 1.2 g / cc.

A positive electrode, a negative electrode, and a separator disposed between the positive electrode, the positive electrode is a lithium-containing transition metal oxide (chemical formula LixMO ₂ , where M is one or more transition metals selected from Co, Ni, Mn, Fe; x = 0 to 1.2) as an active material, and the negative electrode is a wide-angle X-ray diffraction method as a negative electrode material in which lithium ions can reversibly repeat intercalation and deintercalation by charging and discharging. The flaky or massive graphite particles having a (002) plane spacing (d002) of 3.350-3.360 mm and a crystallite size (Lc) in the C-axis direction of at least 1000 mm or more are further pulverized in the process of, and chamfering, the chamfering is flaky or massive graphite particles, the thickness average particle size of a and thinnest portions 10~30μm by sieving Mean non-aqueous electrolyte secondary battery using the powder X-ray diffraction peak intensity ratio is regulated to 0.015 or more (110) / (004) by and wide angle X-ray diffraction method at 3~9μm of.

The (002) plane spacing (d002) by wide-angle X-ray diffraction method is 3.350 to 3.360 mm, the crystallite size (Lc) in the C-axis direction is at least 1000 mm and the average particle diameter is 20 μm or more. In addition, flaky or massive graphite particles having an average thickness of 15 μm or more at the thinnest part are dispersed in a liquid or gas, and the liquid or gas is pressurized and discharged from a nozzle in a spiral shape. sieved after grinding, is chamfered, and the average particle size of the average particle size 10～30Myu m a and the thickness of the thinnest portion to obtain a scaly or massive graphite particles is 3～9Myu m, which A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, characterized in that a negative electrode is formed by using a material.