JP3851040B2

JP3851040B2 - Lithium secondary battery

Info

Publication number: JP3851040B2
Application number: JP36044099A
Authority: JP
Inventors: 英利本棒; 瀞士武内; 康村中
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1999-12-20
Filing date: 1999-12-20
Publication date: 2006-11-29
Anticipated expiration: 2019-12-20
Also published as: JP2001176512A

Description

【０００１】
【発明の属する技術分野】
本発明は、携帯電話，ノート型パーソナルコンピューター等のポータブル機器用、電気自動車の駆動電源用，電力貯蔵用電源用等に好適なリチウム二次電池に関する。
【０００２】
【従来の技術】
負極に非晶質炭素材料を用いたリチウム二次電池が開発され、ノート型パーソナルコンピューターや携帯電話に広く使用されている。
【０００３】
負極に炭素材料を用いる従来技術として、密度（ρ）が１.７＜ρ＜２.１８（ｇ／ｃｃ）のｎ−ドープ炭素材料を用いることが日本特許第２,６３０,９３９号に開示されている。また、メソフェーズピッチ、メソフェーズピッチグリーンコークスの熱処理物で、密度（ρ）が１.７≦ρ≦２.１（ｇ／ｃｃ）の炭素材料を用いることが特開平７−３０７１６４号公報に開示されている。
【０００４】
さらに、ブタノール法によって測定される密度（ρ_B）がρ_B≦１.７（ｇ／ｃｃ）であって、ヘリウム法によって測定される密度（ρ_H）との比ρ_H／ρ_B≧１.１５である炭素材料を負極に用いることが、特開平８−１１５７２３号公報に開示されている。
【０００５】
【発明が解決しようとする課題】
既述したように、負極に非晶質炭素材料を用いたリチウム二次電池は、広く使用されているが、近年、環境問題への関心の高まりから、リチウム二次電池の電気自動車への搭載が検討されている。
【０００６】
しかし、こうした自動車では、夏期に電池搭載部の温度が５０〜６０℃にまで上昇することが予想され、このような高温でのリチウム二次電池の貯蔵性が不十分であるために、実用化上の課題となっている。
【０００７】
負極に非晶質炭素材料を用いたリチウム二次電池の高温劣化の原因について、６０℃，２０日間、充電状態で放置後の電池を解体分析して調べた。
【０００８】
正極と負極を取り出し、それぞれ別個に電極特性を評価した結果、放置後の正極容量は初期容量に対し、ほとんど変化が見られなかった。しかし、負極容量は初期容量に対して大きく減少していることが分かった。
【０００９】
そこで、負極について詳細に分析したところ、負極に用いている炭素材料の粒子表面に、電解液と負極に吸蔵されたリチウムが反応して生成したと考えられる炭酸リチウム等の堆積物が、多量に形成されていることが分かった。
【００１０】
即ち、高温放置により負極容量が大きく減少した原因は、炭素材料の表面に生成した堆積物がリチウムイオンの吸蔵放出反応を妨げていることが考えられる。
【００１１】
従来技術として日本特許第２,６３０,９３９号、特開平７−３０７１６４号あるいは特開平８−１１５７２３号に開示された炭素材料を、負極として用いた場合も上記と同様に長時間、高温環境下に置かれると、炭素表面に堆積物が生成して変質し、高温環境下での貯蔵時（以下、高温貯蔵と称す）において、負極特性が維持できないことが分かった。
【００１２】
本発明の目的は、携帯電話を始めとするポータブル機器用、並びに、電気自動車の駆動電源等として、特に、貯蔵特性の優れた高信頼性のリチウム二次電池を提供することにある。
【００１３】
【課題を解決するための手段】
上記目的を達成する本発明の要旨は次のとおりである。
【００１４】
〔１〕リチウムイオンを吸蔵放出する正極と負極、前記リチウムイオンを含む電解質を溶解させた有機電解液を有し、前記正極と負極がセパレータを介して配置されているリチウム二次電池において、
前記負極を形成する難黒鉛化炭素が、不活性雰囲気中あるいは該黒鉛化炭素が僅かに酸化する程度の雰囲気中で加熱処理して分解ガスを除去後、加圧下で熱処理し、該難黒鉛化炭素の密度（ブタノール法）が１.６〜１.８ｇ／ｃｃであることを特徴とするリチウム二次電池である。
【００１５】
〔２〕上記リチウム二次電池は、リチウム参照極基準で１.５〜０.０２Ｖの負極の充放電容量が２８０ｍＡｈ／ｇ以上である。
【００１６】
〔３〕前記難黒鉛化炭素のブタノール法による測定密度（ρ_B）と、ヘリウム法による測定密度（ρ_H）との比（ρ_H／ρ_B）が１.０５以下である。
【００１７】
〔４〕また、前記難黒鉛化炭素の六員環層の層間距離が０.３６ｎｍより大きく、０.４１ｎｍ未満、好ましくは、０.３７ｎｍより大きく、０.３９ｎｍ未満である。
【００１８】
【発明の実施の形態】
非晶質炭素の組織構造は、炭素の六員環面（六角網面）が数ｎｍ程度積層した部分（六員環層部分）がランダムにつながり、球殻状の構造を形成した多孔質構造であることが報告されている。前記の日本特許第２,６３０,９３９号、特開平７−３０７１６４号公報、あるいは、特開平８−１１５７２３号公報に開示されている非晶質炭素材料も、六員環が積層した部分と球殻の空洞部分（ミクロポア）とに分けられると考えられる。
【００１９】
一方、これまでに、上記の非晶質炭素材料へのリチウムイオンの吸蔵サイトは六員環層部分とミクロポア部分であることが、第３５回電池討論会要旨集２Ｂ０９に報告されている。
【００２０】
また、六員環層部分とミクロポア部分に格納されたリチウムは、核磁気共鳴法（ＮＭＲ法）によって区別することが可能であり、２本のシグナル（ナイトシフト：２０ｐｐｍ近傍および１２０ｐｐｍ近傍）として観察されることが、第３５回電池討論会要旨集２Ｂ１０に報告されている。さらに、この報告ではミクロポア部分にはリチウムが、クラスターのような形態で高密度格納されることを報告している。
【００２１】
こうした報告を基に、これまでの多くの研究がミクロポアを効率的に活用し、炭素負極の充放電容量を増大させることを目的に行われている。前記特開平８−１１５７２３号公報に開示された技術も、ミクロポアへのリチウム吸蔵量を増加させることを目的に行われたものである。
【００２２】
ところで、本発明者らは、充電反応によって六員環層に吸蔵されたリチウムと、ミクロポアに吸蔵されたリチウムとでは、高温貯蔵での電解液との副反応の起こり易さが異なると考え、その反応性について検討を行った。
【００２３】
非晶質炭素へリチウムイオンが吸蔵される場合、先ず、六員環層に吸蔵され、続いてミクロポアに吸蔵されることが第３５回電池討論会要旨集２Ｂ１０に報告されている。
【００２４】
そこで、充電深度の異なる負極を準備し、６０℃での貯蔵特性を調べた。その結果、ミクロポアにリチウムが吸蔵された充電深度の大きい負極では、容量減少が著しいことが示された。
【００２５】
その原因を調べるため、容量減少の著しい負極の炭素粒子表面を分析したところ、電解液と吸蔵リチウムとが反応して生成したと思われる炭酸リチウム等の堆積物が、多量に存在することが分かった。このことから、ミクロポアに吸蔵されたリチウムは反応性が高く、電解液との副反応が容易に起こることが推測される。
【００２６】
上記の結果から、本発明では、充放電においてミクロポアへのリチウム吸蔵放出反応を、抑制すると云う従来と異なる考えに基づき、主として六員環層にリチウムイオンが吸蔵放出される炭素材料の開発を行った。即ち、ミクロポアをリチウムイオンが侵入できないような閉孔とするか、あるいは、消失させてしまうことを行なった。その具体的手法としては、炭素材料を加圧状態で熱処理する方法で行なった。以下にその具体的な実施例を示す。
【００２７】
原料として、フェノール、フラン、メラミン、エポキシ樹脂等の熱硬化性樹脂、あるいは、石油ピッチ、石炭ピッチから得られた等方性ピッチを用いる。本実施例では２段階の熱処理によって炭素材料を製造する。
【００２８】
１段目では、上記の原料を不活性雰囲気あるいは原料が僅かに酸化する程度の雰囲気中で、１０００℃以下の温度で一旦加熱処理して分解ガスを除去する。
【００２９】
２段目では、不活性雰囲気中、加圧下で１０００℃〜２０００℃の高温で再度処理する。
【００３０】
なお、上記加圧条件は１０気圧以上が望ましく、１００気圧以上が好ましい。本発明の炭素材料は、上記の２段階の熱処理によって得ることができる。
【００３１】
炭素材料には、難黒鉛化炭素と易黒鉛化炭素の２つに大きく分けられるが、本発明の炭素材料は難黒鉛化炭素に属し、上記の加圧，高温加熱処理によっても黒鉛化しない。
【００３２】
前記の日本特許２,６３０,９３９号あるいは特開平７−３０７１６４号公報に開示されている石油ピッチ、石炭ピッチから得られたメソフェーズピッチ、あるいは、石油、石炭コークスを原料に用いた場合は、易黒鉛化炭素となってしまい、本発明での加圧，加熱処理を行った場合も、黒鉛化してしまうために本発明の原料として用いることはできない。
【００３３】
リチウム二次電池の負極として用いるためには、加圧，加熱処理を行った後、粉砕、篩分け（分級）する。負極材料としては、平均粒径が１０〜２５μｍの範囲、粒径５０μｍ以下の粒子が体積分率で９５％以上のものが望ましい。
【００３４】
本発明において、得られた炭素材料は、加圧，加熱によってミクロポアが閉孔されてその割合が減少するために、加圧しないものに比べて密度が増加することが特徴である。
【００３５】
無加圧の条件で１,０００〜２,０００℃の加熱処理によって得られる炭素材料では、ＪＩＳＲ７２１２に基づくブタノール法によって測定される密度（ρ_B）が１.５（ｇ／ｃｃ）程度である。
【００３６】
これに対して、本発明の加圧下１,０００〜２,０００℃の処理を行ったものでは、密度（ρ_B）は１.６〜１.８（ｇ／ｃｃ）と増加する。さらに、別のヘリウム法によって測定した密度が、前記ブタノール法で測定した場合とほぼ同じ値を示し、ブタノール法による密度（ρ_B）とヘリウム法による密度（ρ_H）の比（ρ_H／ρ_B）が１.０５以下である。
【００３７】
ヘリウム法は、ヘリウムが侵入できるミクロポアが多く存在すると、ブタノール法に比べ密度が増すと考えられる。本発明の炭素材料は加圧によってミクロポアが閉孔となっているために、上記の測定法による両者の密度の差が生じない点が特徴である。
【００３８】
また、本発明において、直接観察される特徴として以下が挙げられる。それは、１段目の加熱処理では分解ガスが発生するため、ガス噴出による気泡の痕跡が生じる。２段目の１０００〜２０００℃での高温加熱処理では加圧するため、上記のようなガス噴出による気泡の痕跡が消失してしまい、炭素粒子表面が滑らかとなる。こうした加圧が無い場合は高温で処理でも気泡の痕跡が見られる。
【００３９】
上記によって得られた本発明の炭素粉末を用い、以下の方法によって負極を作製後、充放電特性と高温貯蔵特性を調べた。
【００４０】
本発明の炭素粉末９０重量％に、バインダとしてポリフッ化ビニリデン（ＰＶｄＦ）を１０重量％加え、さらに溶剤としてｎ−メチル−２−ピロリドン（ＮＭＰ）を適量加えペースト化した。このペーストを集電体である銅箔に塗布した後、ＮＭＰを乾燥後、加圧成形して負極とした。
【００４１】
上記炭素粉末からなる負極２１と、対極２２および参照極２３にリチウム金属を用い、図２に示すような電気化学セルによって、負極の充放電特性を調べた。その結果を図１の曲線１１で、また、比較のため従来炭素負極の充放電特性の結果を図１の曲線１２に示す。
【００４２】
図１から分かるように、従来の炭素負極の充放電特性は、まず、１.５Ｖから０.０２Ｖの範囲になだらかに変化する領域が現れ、次に、０.０２Ｖ以下の平坦領域が現れると云う大きく２つの領域に分かれる。
【００４３】
これに対し、本発明の負極では、１.５Ｖから０.０２Ｖの範囲のなだらかに変化する領域のみが現れ、０.０２Ｖ以下の平坦領域はほとんど見られない。
【００４４】
前記のように、充電反応でリチウムイオンは六員環層に吸蔵され、次に、ミクロポアに吸蔵されると考えられることから、１.５Ｖから０.０２Ｖの範囲のなだらかに変化する領域は前者による反応であり、０.０２Ｖ以下の平坦領域では後者による反応であると考えられる。
【００４５】
本発明の負極では、１.５Ｖから０.０２Ｖの範囲のなだらかに変化する領域のみであることから、充放電での反応が、主としてリチウムイオンが六員環層に吸蔵される反応で、ミクロポアにはほとんどリチウムイオンが吸蔵されていないことが分かる。これは本発明の大きな特徴を示していると云える。
【００４６】
ミクロポアにリチウムイオンが吸蔵されない分、本発明の炭素負極の充放電容量は減少してしまうが、１.５Ｖから０.０２Ｖの範囲のなだらかに電位が変化する領域での充放電容量を比較すると、従来の炭素負極は２００ｍＡｈ／ｇから２４０ｍＡｈ／ｇ程度であるのに対し、本発明の炭素負極では２８０ｍＡｈ／ｇ以上の充放電容量が得られる。これは、加圧，加熱処理を行うことによって、ミクロポアが減少し、六員環層の割合が増加したためと考えられる。
【００４７】
また、リチウム参照極基準で０Ｖまでリチウムイオンを吸蔵させた本発明の負極および従来負極について、ＬｉＣｌを基準（０ｐｐｍ）としたリチウムＮＭＲ（⁷Ｌｉ−ＮＭＲ）分析を行った。
【００４８】
本発明の負極では約２０ｐｐｍに１本のシグナルが確認されたのに対して、従来負極では２０ｐｐｍ付近と１２０ｐｐｍ付近に２本のシグナルが観察された。
【００４９】
ＬｉＣｌを基準（０ｐｐｍ）とした場合のリチウム金属のシグナルは約２７０ｐｐｍであり、従来負極で確認された１２０ｐｐｍのシグナルはイオン性が薄れたクラスター状態のミクロポアに吸蔵されたリチウムを示すものと考えられる。
【００５０】
本発明の炭素粉末では、１２０ｐｐｍのシグナルが現れないことからも、ミクロポアへのリチウムイオンの吸蔵はほとんど無いと解析される。
【００５１】
さらに、０Ｖまでリチウムイオンを吸蔵させた本発明の負極および従来負極を取り出して、電解液に浸漬した状態で６０℃，２０日間放置し、放置後の放電容量を測定して貯蔵特性を調べた。その結果、従来の負極では５０％程度容量の減少が見られたが、本発明の負極では容量減少が２０％以下で、貯蔵特性が向上することが分かった。
【００５２】
ミクロポアに吸蔵されたクラスター状のリチウムは、イオン性が薄れ活性が高くなるため電解液と容易に反応し、その反応生成物が堆積して充放電反応を妨げるため、貯蔵特性が悪くなると考えられる。しかし、六員環層に吸蔵されたリチウムはナイトシフトが小さく、イオン性が大きいため反応性が比較的小さいと考えられる。本発明の負極ではミクロポアへのリチウムの吸蔵が抑制され、主として六員環に吸蔵されているため、高温貯蔵特性が向上するものと考えられる。
【００５３】
また、貯蔵特性が優れる本発明の炭素材料の最適条件を第２段目の加圧条件および熱処理温度を変え、種々の材料を調製して検討した。その結果、ブタノール法によって測定される密度（ρ_B）が１.６〜１.７未満の範囲の場合、Ｘ線回折法によって測定される六員環層の層間距離が０.３６ｎｍより大きく、０.４１ｎｍであり、高温貯蔵での負極容量の減少が小さかった。
【００５４】
また、ブタノール法によって測定される密度（ρ_B）が１.７〜１.８、六員環層の層間距離が０.３７ｎｍより大きく０.３９ｎｍ未満であり、高温貯蔵での負極容量の減少が小さかった。
【００５５】
本発明のリチウム二次電池の正極材料としては、リチウム含有遷移金属酸化物が望ましく、特に、化学式ＬｉＣｏＯ₂、ＬｉＭ_xＣｏ_1-xＯ₂，ＬｉＭｎ₂Ｏ₄、Ｌｉ_1+xＭｎ_2-xＯ₄，Ｌｉ_1+xＭ_yＭｎ_2-x-yＯ₄（ＭはＦｅ，Ｎｉ，Ｃｒ，Ｍｎ，Ａｌ，Ｂ，Ｓｉ，Ｔｉの少なくとも１種、ｘ＞０，ｙ＞０）で表わされる化合物を用いることによって、貯蔵性の優れたリチウム二次電池が実現できる。
【００５６】
本発明で用いる有機電解液の溶媒としては、プロピレンカーボネートとエチレンカーボネートの混合溶媒またはその単独溶媒に、ジメチルカーボネート、ジエチルカーボネート、メチルエチルカーボネート、γ−ブチロラクトン、スルホラン、酢酸メチル、酢酸エチル、プロピオン酸メチル、プロピオン酸エチル、ジメトキシエタン、２−メチルテトラヒドロフランの少なくとも１種を加えた混合溶媒であり、プロピレンカーボネートとエチレンカーボネートの混合溶媒または単独溶媒の体積分率が０.３〜０.６が望ましい。
【００５７】
一方、本発明ではリチウム塩としてＬｉＰＦ₆，ＬｉＢＦ₄，ＬｉＣｌＯ₄，(Ｃ₂Ｆ₅ＳＯ₂)₂ＮＬｉ，(ＣＦ₃ＳＯ₂)₂ＮＬｉの少なくとも１種を用い、その濃度を０.５〜１.５ｍｏｌ／ｌの範囲とすることが望ましい。
【００５８】
また、本発明で用いるセパレータは、ポリエチレン製の厚みが２０〜５０μｍの微孔膜を用いることが望ましい。
【００５９】
本発明のリチウム二次電池は、高温での貯蔵特性が優れ、携帯電話、携帯情報端末機器、パーソナルコンピュータ、または、携帯音響機器，ポータブル機器の他、とりわけ電気自動車の駆動電源、電力貯蔵用電源として用いられる。
【００６０】
以下、本発明を図面を用いて具体的に説明する。図２は本発明で負極の電気化学評価を行うために用いた電気化学セルである。負極２１、Ｌｉ金属対極２２、Ｌｉ金属参照極２３、電解液２４、ガラス容器２５で構成される。Ｌｉ金属を基準電位とした３極式の電気化学セルによって、負極２１へのリチウムイオンの吸蔵放出反応を調べた。
【００６１】
電解液２４には、体積比で１：１のプロピレンカーボネート（ＰＣ）とジエチルカーボネート（ＤＥＣ）の混合溶媒に、ＬｉＰＦ₆を１ｍｏｌ／ｌ溶解させた溶液を使用した。
【００６２】
〔実施例１〕
フェノール樹脂、フラン樹脂、メラミン樹脂、エポキシ樹脂、等方性石炭ピッチ、等方性石油ピッチを原料として、本発明の炭素材料を調製した。
【００６３】
それぞれの原料を用いて、１段目の熱処理として、電気炉にＮ₂ガスを流通した状態で、常圧，８００℃で２時間加熱した。発生ガスはＮ₂と共に排気した。
【００６４】
次に、２段目の熱処理として、内部に加熱部を備えたオートクレーブによって、１段目の熱処理で得られた炭素材料を、１００気圧，１２００℃で２時間加熱した。加熱中、オートクレーブ内にはＮ₂ガスをパージし、１００気圧の状態を維持した。
【００６５】
得られた炭素材料の密度をブタノール法およびヘリウム法によって測定した。また、Ｘ線回折法によって六員環層の層間距離を測定した。これらの結果を表１に示す。
【００６６】
【表１】

【００６７】
また、上記の方法によって調製した本発明のそれぞれの炭素を用いて、以下の方法によって負極を作製し、図２に示す電気化学セルによって室温での充放電試験を行った。
【００６８】
本発明のそれぞれの炭素粉末９０重量％に、バインダとしてポリフッ化ビニリデン（ＰＶｄＦ）を１０重量％加え、さらに溶剤としてｎ−メチル−２−ピロリドン（ＮＭＰ）を適量加えてペースト化した。この合剤ペーストを集電体である銅箔に塗布した後、ＮＭＰを乾燥後、加圧成形して負極を形成した。
【００６９】
充電（リチウム吸蔵）方法を定電流・定電圧充電方式（０.５ｍＡ／ｃｍ²定電流充電０Ｖ定電圧充電）とし、終止条件として充電時間を選択し、その条件を２４時間とした。
【００７０】
室温での原料にフラン樹脂を用いた場合の本発明の炭素負極の充放電カーブを図１の曲線１１に示す。本発明の炭素負極のリチウムイオンの吸蔵放出に伴う電位変化は、１.５Ｖから０.０２Ｖの範囲のなだらかに変化する領域のみが現れ、０.０２Ｖ以下の平坦領域がほとんど見られない。
【００７１】
本実施例の全ての炭素材料が、図１中、１１とほぼ同じ充放電カーブであり、１.５Ｖから０.０２Ｖの範囲のなだらかに変化する領域の充放電容量を表１中にまとめる。
【００７２】
さらに、本実施例の全ての炭素材料について、以下の方法によって⁷Ｌｉ−ＮＭＲ測定を行った。充電状態で負極を取り出し、これをジメチルカーボネート（ＤＭＣ）で十分洗浄し、真空乾燥後、銅箔より炭素粉末の合剤を削り取りＮＭＲの測定サンプルとした。
【００７３】
ＮＭＲ測定は、室温で観測周波数１５５.３７Ｈｚ、ＭＡＳＧＮＮモード、試料回転数４０００Ｈｚ、積算回数１００回の条件で行った。なお、外部標準試料としてはＬｉＣｌを用いた。確認された全てのシグナルのシフト値を表１に示した。
【００７４】
また、本実施例の全ての炭素材料について、充電状態で取り出し、電解液に浸漬したまま大気中の酸素や水分が混入しないように密封して、６０℃，２０日間貯蔵し、貯蔵後、再び、図２に示す電気化学セルを構成して容量維持率を調べた。これらの結果について表１に示す。
【００７５】
〔比較例１〕
実施例１では、１００気圧，１２００℃の条件で２段目の熱処理を行ったが、比較例１では加圧せずに常圧で２段目の熱処理を行い、従来の炭素材料を調製した。なお、本比較例１では、２段目熱処理が常圧である以外、原料、熱処理温度等の条件は全て実施例１と同様に行なった。
【００７６】
本比較例の炭素材料を用い、実施例１と同じ評価を全て実施した。密度、六員環層間距離、１.５Ｖ〜０.０２Ｖの充放電容量、ＭＮＲシフト値、６０℃，２０日貯蔵後の容量維持率の測定結果を表１に示す。
【００７７】
〔比較例２〕
実施例１とは炭素原料を変えメソフェーズピッチを用いて、実施例１と同様にして炭素粉末を調製した。本比較例の炭素材料を用い、実施例１と同様に、密度、六員環層間距離、１.５Ｖ〜０.０２Ｖの充放電容量、ＭＮＲシフト値、６０℃，２０日貯蔵後の容量維持率の測定結果を表１に示す。
【００７８】
実施例１と比較例１，２の測定結果の比較から、本発明の炭素負極では、充電時におけるリチウムの吸蔵状態が１種であるのに対して、比較例１，２では２種であることがＮＭＲの結果より分かる。
【００７９】
前記のように、比較例の炭素負極では、リチウムイオンが六員環層に吸蔵された状態を示す２０ｐｐｍ付近のＮＭＲシグナルと、ミクロポアに吸蔵された状態を示す１２０ｐｐｍ付近のＮＭＲシグナルの両方が観察される。しかし、実施例１の炭素負極では、２０ｐｐｍ付近のＮＭＲシグナルのみが確認された。これは充放電での反応が、主としてリチウムイオンが六員環層に吸蔵される反応であり、ミクロポアにはほとんどリチウムイオンが吸蔵されていないことを示している。
【００８０】
また、１.５Ｖから０.０２Ｖのなだらかに電位が変化する領域での充放電容量を比較すると、比較例の炭素負極は２００〜２４０ｍＡｈ／ｇ程度であるのに対し、実施例１の炭素負極では２８０ｍＡｈ／ｇ以上の充放電容量が得られた。加圧、加熱処理を行うことによって、ミクロポアが減少し六員環層の割合が増加したことを示している。
【００８１】
さらに、実施例１の炭素材料は、加圧によってミクロポアが閉孔となっているため、密度測定法の違いによる差がほとんど無く、ブタノール法により測定される密度（ρＢ）とヘリウム法により測定される密度（ρＨ）の比（ρＨ／ρＢ）が１.０５以下で、特に、この範囲が望ましいことが分かった。
【００８２】
〔実施例２〕
原料にフラン樹脂を用い、２段目の加圧条件を１０〜１２０気圧の範囲で変更して熱処理を行い、実施例１と同様にして炭素粉末を調製した。
【００８３】
本実施例の炭素材料を用い、実施例１と同様に、密度、六員環層間距離、１.５Ｖ〜０.０２Ｖの充放電容量、６０℃，２０日貯蔵後の容量維持率を測定した。これらの結果を表２に示す。
【００８４】
【表２】

【００８５】
〔実施例３〕
原料として等方性石油ピッチを用い、２段目の熱処理温度を１０００℃〜２０００℃の範囲で変更し、実施例１と同様にして炭素粉末を調製した。
【００８６】
本実施例の炭素材料を用い、実施例１と同様に、密度、六員環層間距離、１.５Ｖ〜０.０２Ｖの充放電容量、６０℃，２０日貯蔵後の容量維持率を測定した。これらの結果を表３に示す。
【００８７】
【表３】

【００８８】
実施例２，３の結果より、ブタノール法によって測定される密度（ρＢ）が１.６〜１.７未満の範囲では、六員環層の層間距離が０.３６ｎｍより大きく、０.４１ｎｍ未満の条件を満たす炭素材料が、高温貯蔵での容量維持率が大きい。また、ブタノール法によって測定される密度（ρＢ）が１.７〜１.８の範囲である場合、六員環層の層間距離が０.３７ｎｍより大きく、０.３９ｎｍ未満の条件を満たす炭素材料が高温貯蔵での容量維持率が大きく好ましい。
【００８９】
〔実施例４〕
図３は、本発明のリチウム二次電池の一実施例を示す模式断面図である。正極３０、セパレータ３１、負極３２、セパレータ３１の順で積層し、捲回して電池缶３３に収められている。正極３０には正極タブ３４、負極３２には負極タブ３５が取付けられており、正極タブ３４は電池内蓋３６、負極タブ３５は電池缶３３に接続されている。
【００９０】
また、電池内蓋３６には安全弁（電流遮断弁）３７が接続され、１０気圧以上の内圧上昇によって安全弁（電流遮断弁）３７が変形し、両者の電気的接続が絶たれる。以下に、本実施例のリチウム二次電池の作製方法について説明する。
【００９１】
ＬｉＣｏＯ₂で表される正極活物質に、結着剤としてポリフッ化ビニリデン（ＰＶｄＦ）、導電助材として黒鉛粉末を用い、これらをそれぞれ重量で８８％、７％、５％の割合で配合し、溶剤としてＮ−メチル−２−ピロリドン（ＮＭＰ）を加えて正極合剤ペーストを調製した。
【００９２】
これを厚み２０μｍのＡｌ箔を用いて、該Ａｌ箔の一方の面に、一定間隔で塗布部分と未塗布部分を設ける間欠塗布を行なった。この後、塗布した正極合剤ペースト中のＮＭＰを乾燥して正極合剤膜を形成した。
【００９３】
さらに、Ａｌ箔のもう一方の面も同様にして正極合剤膜を形成し、塗布電極を得た。このとき、塗布部分と未塗布部分がＡｌ箔の両面で丁度重なり合うようにした。その後、上記の塗布電極をロールプレスによって加圧成形し正極シートを作製した。
【００９４】
さらに、加圧成型した正極シートを、片側に未塗布部、即ち、Ａｌ箔が露出した部分を残した状態で短冊状に切り出し、集電体として正極タブを片側のＡｌ箔部分にスポット溶接して取付けた。
【００９５】
一方、負極活物質として、実施例１の表１に示した原料の等方性石炭ピッチを用いて作製した本発明の難黒鉛化炭素を用いた。この難黒鉛化炭素とポリフッ化ビニリデン（ＰＶｄＦ）をそれぞれ重量比９０％、１０％の割合で配合し、これに溶剤としてＮ−メチル−２−ピロリドン（ＮＭＰ）を加え、負極合剤を調製した。
【００９６】
この負極合剤を１８μｍのＣｕ箔に、前記の正極と同様にして塗布部分と未塗布部分を設け、塗布電極を作製し、加圧成型して負極シートを得た。さらに、片側に未塗布部分、即ち、Ｃｕ箔の部分を残した状態で短冊状に切り出し、集電体として負極タブを片側のＣｕ箔部分にスポット溶接して取付けた。
【００９７】
上述の正極および負極と、ポリエチレン製微孔膜のセパレータを用い、正極、セパレータ、負極、セパレータの順序で積層し、これを渦巻き状に捲回して電極群を形成した。正極タブと負極タブは互いに捲回群の上下になるように構成した。この電極群を電池缶に納め、電池内蓋に正極タブを、電池缶に負極タブをスポット溶接により接続した。
【００９８】
電解液として、１：１のプロピレンカーボネート（ＰＣ）とジエチルカーボネート（ＤＥＣ）との混合溶媒に、ＬｉＰＦ₆を１ｍｏｌ／ｌ溶解させた溶液を調製し、これらの電解液を注液した後、電池蓋を電池缶に取付け、円筒型電池を作製した。
【００９９】
〔比較例３〕
負極活物質として、比較例１の表１に示した原料、等方性石炭ピッチを用いて作製した従来の難黒鉛化炭素を用い、実施例４と同様にしてリチウム二次電池を作製した。
【０１００】
実施例４および比較例３で作製したリチウム二次電池をそれぞれ５本づつ用い、電池容量が安定するまでのエージング充放電として、２５℃で１０サイクル行った。
【０１０１】
充電は、先ず電流０.５Ａで定電流充電して、電池電圧が４.２Ｖに達した時点で、次に４.２Ｖ定電圧充電を行い、充電開始から５時間経過した時点で充電を終了した（以下、０.５Ａ、４.２Ｖ、５時間終止定電流定電圧充電と略す）。
【０１０２】
放電は、電流０.５Ａで定電流放電して、電池電圧が２.８Ｖに達した時点で終了した（以下、０.５Ａ、２.８Ｖ終止定電流放電と略す）。
【０１０３】
１１サイクル目に、再度、２５℃で０.５Ａ、４.２Ｖ、５時間終止定電流定電圧充電を行い、充電電池を６０℃の雰囲気で２０日間保存した。保存後、電池温度を室温まで冷却して、２５℃で０.５Ａ、２.８Ｖ終止定電流放電を行って、貯蔵前後の電池容量の変化を調べた。
【０１０４】
実施例４および比較例３においてそれぞれ作製した５本の電池の６０℃貯蔵前の１０サイクル目の充放電容量、貯蔵後の１１サイクル目の充放電容量、および１１サイクル目の充電容量に対する放電容量の維持率を表４に示す。
【０１０５】
【表４】

【０１０６】
本発明のリチウム二次電池の６０℃貯蔵後の容量維持率は８０％以上であり、比較例の容量維持率が５０％程度であることから、本発明の二次電池が優れた高温貯蔵特性を有することが分かる。
【０１０７】
【発明の効果】
本発明に用いた炭素材料は、充放電反応でリチウムイオンが主として六員層に吸蔵放出される。六員環層に吸蔵されたリチウムイオンは、電解液との反応性が小さいため、負極の副反応による容量減少が低減でき、高温貯蔵特性の優れたリチウム二次電池を実現することができる。
【図面の簡単な説明】
【図１】炭素材料のリチウムイオンの吸蔵放出に伴なう電位変化のグラフである。
【図２】負極のリチウムイオン吸蔵放出特性の測定に用いた電気化学セルの模式構成図である。
【図３】本発明のリチウム二次電池の模式断面図である。
【符号の説明】
２１…負極、２２…Ｌｉ金属対極、２３…Ｌｉ金属参照極、２４…電解液、２５…ガラス容器、３０…正極、３１…セパレータ、３２…負極、３３…電池缶、３４…正極タブ、３５…負極タブ、３６…電池内蓋、３７…安全弁（電流遮断弁）、３８…ガスケット、３９…絶縁板、４０…電池外蓋。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery suitable for portable equipment such as a mobile phone and a notebook personal computer, a drive power source for an electric vehicle, and a power storage power source.
[0002]
[Prior art]
Lithium secondary batteries using an amorphous carbon material for the negative electrode have been developed and are widely used in notebook personal computers and mobile phones.
[0003]
Japanese Patent No. 2,630,939 discloses the use of an n-doped carbon material having a density (ρ) of 1.7 <ρ <2.18 (g / cc) as a conventional technique using a carbon material for the negative electrode. Has been. JP-A-7-307164 discloses the use of a carbon material having a density (ρ) of 1.7 ≦ ρ ≦ 2.1 (g / cc) as a heat-treated product of mesophase pitch and mesophase pitch green coke. ing.
[0004]
Furthermore, the density measured by the butanol method (ρ _B ) Is ρ _B ≦ 1.7 (g / cc) and density measured by helium method (ρ _H ) _H / Ρ _B Japanese Patent Laid-Open No. 8-115723 discloses that a carbon material satisfying ≧ 1.15 is used for the negative electrode.
[0005]
[Problems to be solved by the invention]
As described above, lithium secondary batteries using an amorphous carbon material for the negative electrode are widely used. However, in recent years, due to the growing interest in environmental issues, lithium secondary batteries have been installed in electric vehicles. Is being considered.
[0006]
However, in such automobiles, the temperature of the battery mounting portion is expected to rise to 50 to 60 ° C. in the summer, and the storage property of the lithium secondary battery at such a high temperature is insufficient. It is an upper issue.
[0007]
The cause of the high temperature deterioration of the lithium secondary battery using an amorphous carbon material for the negative electrode was examined by disassembling the battery after being left in a charged state at 60 ° C. for 20 days.
[0008]
As a result of taking out the positive electrode and the negative electrode and separately evaluating the electrode characteristics, the positive electrode capacity after being left was hardly changed with respect to the initial capacity. However, it was found that the negative electrode capacity greatly decreased with respect to the initial capacity.
[0009]
Thus, when the negative electrode was analyzed in detail, a large amount of deposits such as lithium carbonate, which was thought to be generated by the reaction between the electrolyte and lithium occluded in the negative electrode, were formed on the surface of the carbon material particles used in the negative electrode. It was found that it was formed.
[0010]
That is, the cause of the large decrease in the negative electrode capacity due to standing at high temperature is considered that the deposit generated on the surface of the carbon material hinders the lithium ion occlusion and release reaction.
[0011]
When the carbon material disclosed in Japanese Patent No. 2,630,939, Japanese Patent Laid-Open No. 7-307164 or Japanese Patent Laid-Open No. 8-115723 is used as a negative electrode as a prior art, it is kept in a high temperature environment for a long time as described above. It was found that deposits were generated on the carbon surface and changed in quality, and the negative electrode characteristics could not be maintained during storage in a high temperature environment (hereinafter referred to as high temperature storage).
[0012]
An object of the present invention is to provide a highly reliable lithium secondary battery excellent in storage characteristics, particularly for portable equipment such as a mobile phone and as a driving power source for an electric vehicle.
[0013]
[Means for Solving the Problems]
The gist of the present invention that achieves the above object is as follows.
[0014]
[1] A lithium secondary battery having a positive electrode and a negative electrode that occlude and release lithium ions, an organic electrolytic solution in which an electrolyte containing the lithium ions is dissolved, and the positive electrode and the negative electrode are disposed via a separator.
The non-graphitizable carbon forming the negative electrode is heat-treated in an inert atmosphere or an atmosphere in which the graphitized carbon is slightly oxidized to remove decomposition gas, and then heat-treated under pressure to form the non-graphitizable carbon. It is a lithium secondary battery characterized by having a carbon density (butanol method) of 1.6 to 1.8 g / cc.
[0015]
[2] In the lithium secondary battery, the charge / discharge capacity of the negative electrode of 1.5 to 0.02 V is 280 mAh / g or more based on the lithium reference electrode.
[0016]
[3] Density measured by the butanol method (ρ _B ) And density measured by helium method (ρ _H )) (Rho _H / Ρ _B ) Is 1.05 or less.
[0017]
[4] The interlayer distance of the six-membered ring layer of non-graphitizable carbon is greater than 0.36 nm and less than 0.41 nm, preferably greater than 0.37 nm and less than 0.39 nm.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The structure of amorphous carbon is a porous structure in which a portion (six-membered ring layer portion) in which a carbon six-membered ring surface (hexagonal network surface) is stacked several nanometers is randomly connected to form a spherical shell-like structure. It has been reported that. The amorphous carbon material disclosed in Japanese Patent No. 2,630,939, Japanese Patent Application Laid-Open No. 7-307164, or Japanese Patent Application Laid-Open No. 8-115723 also includes a portion in which a six-membered ring is laminated and a sphere. It can be divided into hollow parts (micropores) of the shell.
[0019]
On the other hand, it has been reported in the 35th Battery Discussion Meeting 2B09 that the lithium ion storage sites in the amorphous carbon material are the six-membered ring layer portion and the micropore portion.
[0020]
Further, lithium stored in the six-membered ring layer portion and the micropore portion can be distinguished by a nuclear magnetic resonance method (NMR method), and is observed as two signals (night shift: around 20 ppm and around 120 ppm). This is reported in the 35th Battery Symposium Abstracts 2B10. Furthermore, this report reports that lithium is stored in the micropore part in a cluster-like form at high density.
[0021]
Based on these reports, many previous studies have been conducted with the aim of efficiently utilizing micropores and increasing the charge / discharge capacity of the carbon anode. The technique disclosed in Japanese Patent Laid-Open No. 8-115723 is also performed for the purpose of increasing the amount of lithium stored in the micropores.
[0022]
By the way, the present inventors consider that the lithium occluded in the six-membered ring layer by the charging reaction and the lithium occluded in the micropore are different in the likelihood of a side reaction with the electrolyte in high-temperature storage, The reactivity was examined.
[0023]
When lithium ions are occluded into amorphous carbon, it is first reported in the 35th Battery Discussion Meeting 2B10 that occlusion in the six-membered ring layer and subsequent occlusion in the micropores.
[0024]
Therefore, negative electrodes with different charging depths were prepared, and the storage characteristics at 60 ° C. were examined. As a result, it was shown that the capacity reduction was remarkable in the negative electrode having a large charging depth in which lithium was occluded in the micropore.
[0025]
In order to investigate the cause, an analysis of the carbon particle surface of the negative electrode with a significant decrease in capacity revealed that there was a large amount of deposits such as lithium carbonate that appeared to have been formed by the reaction between the electrolyte and lithium occlusion. It was. From this, it is surmised that lithium occluded in the micropores has high reactivity, and side reactions with the electrolyte easily occur.
[0026]
Based on the above results, the present invention has developed a carbon material that mainly stores and releases lithium ions in the six-membered ring layer, based on a different idea of suppressing the lithium occlusion / release reaction to the micropores during charge and discharge. It was. That is, the micropores were closed so that lithium ions could not enter, or disappeared. As a specific method, a carbon material was heat-treated in a pressurized state. Specific examples are shown below.
[0027]
As a raw material, thermosetting resins such as phenol, furan, melamine and epoxy resin, or isotropic pitch obtained from petroleum pitch and coal pitch are used. In this embodiment, the carbon material is manufactured by two-stage heat treatment.
[0028]
In the first stage, the above raw material is once heat-treated at a temperature of 1000 ° C. or lower in an inert atmosphere or an atmosphere in which the raw material is slightly oxidized to remove cracked gas.
[0029]
In the second stage, the treatment is again performed at a high temperature of 1000 ° C. to 2000 ° C. under pressure in an inert atmosphere.
[0030]
The pressurizing condition is preferably 10 atm or more, and preferably 100 atm or more. The carbon material of the present invention can be obtained by the above-described two-stage heat treatment.
[0031]
Carbon materials can be broadly divided into two types: non-graphitizable carbon and graphitizable carbon. The carbon material of the present invention belongs to non-graphitizable carbon and is not graphitized even by the above-described pressure and high-temperature heat treatment.
[0032]
When the oil pitch disclosed in Japanese Patent No. 2,630,939 or JP-A-7-307164, mesophase pitch obtained from coal pitch, or oil or coal coke is used as a raw material, Even if it becomes graphitized carbon and is subjected to pressurization and heat treatment in the present invention, it is graphitized and cannot be used as a raw material of the present invention.
[0033]
In order to use it as a negative electrode of a lithium secondary battery, it is subjected to pressure and heat treatment, and then pulverized and sieved (classified). As the negative electrode material, particles having an average particle diameter in the range of 10 to 25 μm and particles having a particle diameter of 50 μm or less are desirably 95% or more in volume fraction.
[0034]
In the present invention, the obtained carbon material is characterized in that the density is increased as compared with a case where the carbon material is not pressurized because the micropores are closed by pressurization and heating and the ratio thereof is reduced.
[0035]
In the carbon material obtained by heat treatment at 1,000 to 2,000 ° C. under no pressure condition, the density (ρ) measured by the butanol method based on JIS R7212. _B ) Is about 1.5 (g / cc).
[0036]
On the other hand, in the case where the treatment at 1,000 to 2,000 ° C. is performed under the pressure of the present invention, the density (ρ _B ) Increases from 1.6 to 1.8 (g / cc). Further, the density measured by another helium method shows almost the same value as that measured by the butanol method, and the density by the butanol method (ρ _B ) And helium density (ρ _H ) Ratio (ρ _H / Ρ _B ) Is 1.05 or less.
[0037]
The helium method is thought to have a higher density than the butanol method if there are many micropores into which helium can penetrate. The carbon material of the present invention is characterized in that since the micropores are closed by pressurization, a difference in density between the two is not caused by the above measurement method.
[0038]
Further, in the present invention, the following features are directly observed. This is because cracked gas is generated in the first stage heat treatment, so that traces of bubbles are generated due to gas ejection. Since pressure is applied in the second stage of high-temperature heat treatment at 1000 to 2000 ° C., the traces of bubbles due to gas ejection as described above disappear, and the carbon particle surface becomes smooth. In the absence of such pressurization, traces of bubbles can be seen even at high temperatures.
[0039]
Using the carbon powder of the present invention obtained as described above, a negative electrode was produced by the following method, and then charge / discharge characteristics and high-temperature storage characteristics were examined.
[0040]
10% by weight of polyvinylidene fluoride (PVdF) as a binder was added to 90% by weight of the carbon powder of the present invention, and an appropriate amount of n-methyl-2-pyrrolidone (NMP) as a solvent was added to form a paste. After applying this paste to a copper foil as a current collector, NMP was dried and then pressure-molded to obtain a negative electrode.
[0041]
The negative electrode 21 made of the above carbon powder, the counter electrode 22 and the reference electrode 23 were made of lithium metal, and the charge / discharge characteristics of the negative electrode were examined using an electrochemical cell as shown in FIG. The result is shown by a curve 11 in FIG. 1, and the result of charge / discharge characteristics of a conventional carbon negative electrode is shown by a curve 12 in FIG. 1 for comparison.
[0042]
As can be seen from FIG. 1, in the charge / discharge characteristics of the conventional carbon negative electrode, first, a region that gently changes in the range of 1.5V to 0.02V appears, and then a flat region of 0.02V or less appears. It is divided into two areas.
[0043]
On the other hand, in the negative electrode of the present invention, only a gently changing region in the range of 1.5V to 0.02V appears, and a flat region of 0.02V or less is hardly seen.
[0044]
As described above, it is considered that lithium ions are occluded in the six-membered ring layer in the charging reaction, and then occluded in the micropores. Therefore, the region in which the voltage gradually changes from 1.5 V to 0.02 V is the former. It is considered that the reaction is due to the latter in a flat region of 0.02 V or less.
[0045]
In the negative electrode of the present invention, since it is only a region that changes gently in the range of 1.5 V to 0.02 V, the charge / discharge reaction is mainly a reaction in which lithium ions are occluded in the six-membered ring layer. It can be seen that almost no lithium ions are occluded. This can be said to show a great feature of the present invention.
[0046]
When the lithium ion is not occluded in the micropore, the charge / discharge capacity of the carbon negative electrode of the present invention is reduced, but when the charge / discharge capacity in the region where the potential changes gently in the range of 1.5V to 0.02V is compared. The conventional carbon negative electrode is about 200 mAh / g to 240 mAh / g, whereas the carbon negative electrode of the present invention has a charge / discharge capacity of 280 mAh / g or more. This is presumably because the micropores decreased and the proportion of the six-membered ring layer increased by applying pressure and heat treatment.
[0047]
Further, for the negative electrode of the present invention in which lithium ions were occluded up to 0 V based on the lithium reference electrode standard and the conventional negative electrode, lithium NMR (based on LiCl (0 ppm)) ( ⁷ Li-NMR) analysis was performed.
[0048]
In the negative electrode of the present invention, one signal was confirmed at about 20 ppm, whereas in the conventional negative electrode, two signals were observed near 20 ppm and 120 ppm.
[0049]
The signal of lithium metal when LiCl is the reference (0 ppm) is about 270 ppm, and the signal of 120 ppm confirmed in the conventional negative electrode is considered to indicate lithium occluded in the clustered micropores with reduced ionicity. .
[0050]
In the carbon powder of the present invention, since a signal of 120 ppm does not appear, it is analyzed that there is almost no occlusion of lithium ions into the micropore.
[0051]
Further, the negative electrode of the present invention in which lithium ions were occluded to 0 V and the conventional negative electrode were taken out, left in an electrolytic solution in a state of 60 ° C. for 20 days, and the storage capacity was examined by measuring the discharge capacity after being left. . As a result, in the conventional negative electrode, the capacity was reduced by about 50%, but in the negative electrode of the present invention, the capacity reduction was 20% or less, and the storage characteristics were improved.
[0052]
The clustered lithium occluded in the micropores is thought to deteriorate storage characteristics because it reacts easily with the electrolyte because the ionicity is low and the activity is high, and the reaction product accumulates to hinder the charge / discharge reaction. . However, it is considered that lithium occluded in the six-membered ring layer has a small night shift and a large ionicity, so that the reactivity is relatively small. In the negative electrode of the present invention, the occlusion of lithium in the micropores is suppressed and mainly occluded in the six-membered ring, so that the high temperature storage characteristics are considered to be improved.
[0053]
In addition, the optimum conditions for the carbon material of the present invention having excellent storage characteristics were studied by preparing various materials by changing the pressure condition and heat treatment temperature in the second stage. As a result, the density measured by the butanol method (ρ _B ) Is in the range of 1.6 to less than 1.7, the interlayer distance of the six-membered ring layer measured by the X-ray diffraction method is greater than 0.36 nm and 0.41 nm, and the negative electrode capacity during high-temperature storage is The decrease was small.
[0054]
The density measured by the butanol method (ρ _B ) Is 1.7 to 1.8, and the distance between the six-membered ring layers is more than 0.37 nm and less than 0.39 nm, and the decrease in the negative electrode capacity during high-temperature storage is small.
[0055]
As the positive electrode material of the lithium secondary battery of the present invention, a lithium-containing transition metal oxide is desirable, and in particular, the chemical formula LiCoO ₂ , LiM _x Co _1-x O ₂ , LiMn ₂ O _Four , Li _{1 + x} Mn _2-x O _Four , Li _{1 + x} M _y Mn _2-xy O _Four By using a compound represented by (M is at least one of Fe, Ni, Cr, Mn, Al, B, Si, and Ti, x> 0, y> 0), a lithium secondary battery having excellent storability can be obtained. realizable.
[0056]
As a solvent for the organic electrolyte used in the present invention, a mixed solvent of propylene carbonate and ethylene carbonate or a single solvent thereof, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, sulfolane, methyl acetate, ethyl acetate, propionic acid A mixed solvent in which at least one of methyl, ethyl propionate, dimethoxyethane, and 2-methyltetrahydrofuran is added, and the volume fraction of a mixed solvent of propylene carbonate and ethylene carbonate or a single solvent is preferably 0.3 to 0.6. .
[0057]
On the other hand, in the present invention, LiPF is used as the lithium salt. ₆ , LiBF _Four , LiClO _Four , (C ₂ F _Five SO ₂ ) ₂ NLi, (CF _Three SO ₂ ) ₂ It is desirable to use at least one kind of NLi and set the concentration in the range of 0.5 to 1.5 mol / l.
[0058]
The separator used in the present invention is preferably a polyethylene microporous film having a thickness of 20 to 50 μm.
[0059]
The lithium secondary battery of the present invention has excellent storage characteristics at a high temperature, and in addition to a mobile phone, a portable information terminal device, a personal computer, or a portable audio device and a portable device, in particular, a driving power source for an electric vehicle and a power storage power source. Used as
[0060]
Hereinafter, the present invention will be specifically described with reference to the drawings. FIG. 2 shows an electrochemical cell used for electrochemical evaluation of the negative electrode in the present invention. A negative electrode 21, a Li metal counter electrode 22, a Li metal reference electrode 23, an electrolyte solution 24, and a glass container 25 are included. The lithium ion storage / release reaction on the negative electrode 21 was examined by a tripolar electrochemical cell using Li metal as a reference potential.
[0061]
The electrolyte solution 24 includes a mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) at a volume ratio of 1: 1, and LiPF. ₆ A solution in which 1 mol / l was dissolved was used.
[0062]
[Example 1]
The carbon material of the present invention was prepared using phenol resin, furan resin, melamine resin, epoxy resin, isotropic coal pitch, and isotropic petroleum pitch as raw materials.
[0063]
Using each raw material, N in the electric furnace as the first stage heat treatment ₂ Heating was performed at normal pressure and 800 ° C. for 2 hours while the gas was circulated. Generated gas is N ₂ And exhausted.
[0064]
Next, as a second stage heat treatment, the carbon material obtained by the first stage heat treatment was heated at 100 atm and 1200 ° C. for 2 hours by an autoclave provided with a heating unit therein. During heating, N in the autoclave ₂ The gas was purged and maintained at 100 atm.
[0065]
The density of the obtained carbon material was measured by a butanol method and a helium method. Moreover, the interlayer distance of the six-membered ring layer was measured by the X-ray diffraction method. These results are shown in Table 1.
[0066]
[Table 1]

[0067]
Moreover, using each carbon of this invention prepared by said method, the negative electrode was produced with the following method, and the charging / discharging test at room temperature was done with the electrochemical cell shown in FIG.
[0068]
10% by weight of polyvinylidene fluoride (PVdF) as a binder was added to 90% by weight of each carbon powder of the present invention, and an appropriate amount of n-methyl-2-pyrrolidone (NMP) was added as a solvent to make a paste. After this mixture paste was applied to a copper foil as a current collector, NMP was dried and then pressure molded to form a negative electrode.
[0069]
Charging (lithium storage) method is constant current / constant voltage charging method (0.5 mA / cm ² Constant current charge 0V constant voltage charge), a charge time was selected as the termination condition, and the condition was 24 hours.
[0070]
A charge / discharge curve of the carbon negative electrode of the present invention when a furan resin is used as a raw material at room temperature is shown by a curve 11 in FIG. In the carbon negative electrode of the present invention, the potential change associated with the occlusion and release of lithium ions shows only a gently changing region in the range of 1.5 V to 0.02 V, and hardly shows a flat region of 0.02 V or less.
[0071]
All the carbon materials of the present example have substantially the same charge / discharge curves as 11 in FIG. 1, and the charge / discharge capacities in a region that changes gently in the range from 1.5 V to 0.02 V are summarized in Table 1.
[0072]
Furthermore, for all the carbon materials of this example, the following method is used. ⁷ Li-NMR measurement was performed. The negative electrode was taken out in a charged state, sufficiently washed with dimethyl carbonate (DMC), dried in vacuum, and then the carbon powder mixture was scraped from the copper foil to obtain an NMR measurement sample.
[0073]
The NMR measurement was performed at room temperature under the conditions of an observation frequency of 155.37 Hz, a MASGNN mode, a sample rotation speed of 4000 Hz, and an integration count of 100 times. Note that LiCl was used as an external standard sample. The shift values of all confirmed signals are shown in Table 1.
[0074]
Moreover, about all the carbon materials of a present Example, it takes out in a charging state, it seals so that oxygen and a water | moisture content in air | atmosphere may not mix with being immersed in electrolyte solution, It stores at 60 degreeC for 20 days, After storage, it again The electrochemical cell shown in FIG. 2 was constructed and the capacity retention rate was examined. These results are shown in Table 1.
[0075]
[Comparative Example 1]
In Example 1, the second stage heat treatment was performed under the conditions of 100 atm and 1200 ° C., but in Comparative Example 1, the second stage heat treatment was performed at normal pressure without applying pressure to prepare a conventional carbon material. . In Comparative Example 1, all the conditions such as the raw material and the heat treatment temperature were the same as in Example 1 except that the second-stage heat treatment was at normal pressure.
[0076]
All the same evaluation as Example 1 was implemented using the carbon material of this comparative example. Table 1 shows the measurement results of density, interlayer distance between six-membered rings, charge / discharge capacity of 1.5 V to 0.02 V, MNR shift value, capacity retention rate after storage at 60 ° C. for 20 days.
[0077]
[Comparative Example 2]
A carbon powder was prepared in the same manner as in Example 1, except that the carbon raw material was changed and mesophase pitch was used. Using the carbon material of this comparative example, as in Example 1, the density, the distance between the six-membered rings, the charge / discharge capacity of 1.5V to 0.02V, the MNR shift value, 60 ° C., capacity maintenance after 20 days storage The rate measurement results are shown in Table 1.
[0078]
From the comparison of the measurement results of Example 1 and Comparative Examples 1 and 2, in the carbon negative electrode of the present invention, the lithium occlusion state at the time of charging is one type, whereas in Comparative Examples 1 and 2, there are two types. This can be seen from the NMR results.
[0079]
As described above, in the carbon negative electrode of the comparative example, both the NMR signal around 20 ppm indicating the state where lithium ions are occluded in the six-membered ring layer and the NMR signal around 120 ppm indicating the state occluded in the micropores are observed. Is done. However, in the carbon negative electrode of Example 1, only an NMR signal near 20 ppm was confirmed. This indicates that the charge / discharge reaction is mainly a reaction in which lithium ions are occluded in the six-membered ring layer, and almost no lithium ions are occluded in the micropores.
[0080]
Further, when comparing the charge and discharge capacity in a region where the potential changes gradually from 1.5 V to 0.02 V, the carbon negative electrode of the comparative example is about 200 to 240 mAh / g, whereas the carbon negative electrode of Example 1 Then, a charge / discharge capacity of 280 mAh / g or more was obtained. This shows that the micropores decreased and the proportion of the six-membered ring layer increased by the pressurization and heat treatment.
[0081]
Furthermore, the carbon material of Example 1 has micropores closed by pressurization, so there is almost no difference due to the difference in density measurement method, and the density (ρB) measured by the butanol method and the helium method are used. It was found that the ratio (ρH / ρB) of the density (ρH) to be obtained is 1.05 or less, and this range is particularly desirable.
[0082]
Example 2
A furan resin was used as a raw material, and the heat treatment was performed by changing the second-stage pressurization condition in the range of 10 to 120 atmospheres, and carbon powder was prepared in the same manner as in Example 1.
[0083]
Using the carbon material of this example, the density, the distance between the six-membered rings, the charge / discharge capacity of 1.5 V to 0.02 V, and the capacity retention rate after storage at 60 ° C. for 20 days were measured as in Example 1. . These results are shown in Table 2.
[0084]
[Table 2]

[0085]
[Example 3]
An isotropic petroleum pitch was used as a raw material, and the second heat treatment temperature was changed in the range of 1000 ° C. to 2000 ° C., and carbon powder was prepared in the same manner as in Example 1.
[0086]
Using the carbon material of this example, the density, the distance between the six-membered rings, the charge / discharge capacity of 1.5 V to 0.02 V, and the capacity retention rate after storage at 60 ° C. for 20 days were measured as in Example 1. . These results are shown in Table 3.
[0087]
[Table 3]

[0088]
From the results of Examples 2 and 3, when the density (ρB) measured by the butanol method is in the range of 1.6 to less than 1.7, the interlayer distance of the six-membered ring layer is greater than 0.36 nm and less than 0.41 nm. A carbon material that satisfies the above conditions has a large capacity retention rate during high-temperature storage. Further, when the density (ρB) measured by the butanol method is in the range of 1.7 to 1.8, the carbon material satisfying the condition that the interlayer distance of the six-membered ring layer is greater than 0.37 nm and less than 0.39 nm. However, it is preferable because of its large capacity retention rate during high-temperature storage.
[0089]
[Example 4]
FIG. 3 is a schematic cross-sectional view showing an embodiment of the lithium secondary battery of the present invention. The positive electrode 30, the separator 31, the negative electrode 32, and the separator 31 are stacked in this order, wound and stored in the battery can 33. A positive electrode tab 34 is attached to the positive electrode 30, and a negative electrode tab 35 is attached to the negative electrode 32. The positive electrode tab 34 is connected to the battery inner lid 36, and the negative electrode tab 35 is connected to the battery can 33.
[0090]
In addition, a safety valve (current cutoff valve) 37 is connected to the battery inner lid 36, and the safety valve (current cutoff valve) 37 is deformed by an increase in internal pressure of 10 atmospheres or more, and the electrical connection between them is cut off. A method for manufacturing the lithium secondary battery of this example will be described below.
[0091]
LiCoO ₂ In the positive electrode active material represented by the formula, polyvinylidene fluoride (PVdF) is used as a binder, graphite powder is used as a conductive additive, and these are blended in proportions of 88%, 7%, and 5%, respectively, as a solvent. N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture paste.
[0092]
Using an Al foil having a thickness of 20 μm, intermittent coating was performed in which a coated portion and an uncoated portion were provided at regular intervals on one surface of the Al foil. Thereafter, NMP in the applied positive electrode mixture paste was dried to form a positive electrode mixture film.
[0093]
Further, a positive electrode mixture film was formed on the other surface of the Al foil in the same manner to obtain a coated electrode. At this time, the coated portion and the uncoated portion were just overlapped on both sides of the Al foil. Thereafter, the coated electrode was pressure-formed by a roll press to produce a positive electrode sheet.
[0094]
Further, the positive electrode sheet formed by pressure molding was cut into a strip shape with the uncoated part, that is, the exposed part of the Al foil remaining on one side, and the positive electrode tab as a current collector was spot welded to the Al foil part on one side. Installed.
[0095]
On the other hand, as the negative electrode active material, the non-graphitizable carbon of the present invention produced using the raw material isotropic coal pitch shown in Table 1 of Example 1 was used. The non-graphitizable carbon and polyvinylidene fluoride (PVdF) were blended in a ratio of 90% by weight and 10%, respectively, and N-methyl-2-pyrrolidone (NMP) was added as a solvent to prepare a negative electrode mixture. .
[0096]
The negative electrode mixture was applied to an 18 μm Cu foil in the same manner as the positive electrode, and a coated portion and an uncoated portion were provided. A coated electrode was prepared and pressure-molded to obtain a negative electrode sheet. Furthermore, it cut out in strip shape in the state which left the uncoated part, ie, Cu foil part, in one side, and attached the negative electrode tab as a collector to the Cu foil part on one side by spot welding.
[0097]
Using the above-described positive electrode and negative electrode and a polyethylene microporous membrane separator, the positive electrode, the separator, the negative electrode, and the separator were laminated in this order, and this was wound spirally to form an electrode group. The positive electrode tab and the negative electrode tab were configured to be above and below the wound group. The electrode group was placed in a battery can, and a positive electrode tab was connected to the battery inner lid, and a negative electrode tab was connected to the battery can by spot welding.
[0098]
As an electrolytic solution, a mixed solvent of 1: 1 propylene carbonate (PC) and diethyl carbonate (DEC) was added to LiPF. ₆ 1 mol / l of a solution was prepared, and these electrolytes were injected, and then a battery lid was attached to the battery can to produce a cylindrical battery.
[0099]
[Comparative Example 3]
A lithium secondary battery was produced in the same manner as in Example 4 using the conventional non-graphitizable carbon produced using the raw materials shown in Table 1 of Comparative Example 1 and an isotropic coal pitch as the negative electrode active material.
[0100]
Five lithium secondary batteries prepared in Example 4 and Comparative Example 3 were used, and 10 cycles were performed at 25 ° C. as aging charge / discharge until the battery capacity was stabilized.
[0101]
Charging is carried out at a constant current of 0.5 A, and when the battery voltage reaches 4.2 V, then 4.2 V constant voltage charging is performed, and charging ends when 5 hours have elapsed from the start of charging. (Hereinafter, abbreviated as 0.5A, 4.2V, 5 hours stop constant current constant voltage charging).
[0102]
The discharge was terminated at a constant current of 0.5 A and finished when the battery voltage reached 2.8 V (hereinafter abbreviated as 0.5 A, 2.8 V final constant current discharge).
[0103]
In the 11th cycle, 0.5 A, 4.2 V, and 5 hour constant current constant voltage charging were performed again at 25 ° C., and the rechargeable battery was stored in an atmosphere at 60 ° C. for 20 days. After storage, the battery temperature was cooled to room temperature, 0.5 A, 2.8 V constant current discharge was performed at 25 ° C., and the change in battery capacity before and after storage was examined.
[0104]
The charge / discharge capacity at the 10th cycle before storage at 60 ° C., the charge / discharge capacity at the 11th cycle after storage, and the discharge capacity relative to the charge capacity at the 11th cycle of the five batteries prepared in Example 4 and Comparative Example 3, respectively. Table 4 shows the maintenance rate.
[0105]
[Table 4]

[0106]
The capacity retention rate after storage at 60 ° C. of the lithium secondary battery of the present invention is 80% or more, and the capacity retention ratio of the comparative example is about 50%. Therefore, the secondary battery of the present invention has excellent high-temperature storage characteristics. It can be seen that
[0107]
【The invention's effect】
In the carbon material used in the present invention, lithium ions are mainly occluded and released in the six-membered layer by a charge / discharge reaction. Since the lithium ions occluded in the six-membered ring layer have a low reactivity with the electrolytic solution, a reduction in capacity due to a side reaction of the negative electrode can be reduced, and a lithium secondary battery excellent in high-temperature storage characteristics can be realized.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph of potential change associated with insertion and extraction of lithium ions of a carbon material.
FIG. 2 is a schematic configuration diagram of an electrochemical cell used for measurement of lithium ion storage / release characteristics of a negative electrode.
FIG. 3 is a schematic cross-sectional view of a lithium secondary battery of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 21 ... Negative electrode, 22 ... Li metal counter electrode, 23 ... Li metal reference electrode, 24 ... Electrolyte, 25 ... Glass container, 30 ... Positive electrode, 31 ... Separator, 32 ... Negative electrode, 33 ... Battery can, 34 ... Positive electrode tab, 35 ... Negative electrode tab, 36 ... Battery inner lid, 37 ... Safety valve (current cutoff valve), 38 ... Gasket, 39 ... Insulating plate, 40 ... Battery outer lid.

Claims

In a lithium secondary battery having a positive electrode and a negative electrode that occlude and release lithium ions, an organic electrolyte solution in which an electrolyte containing lithium ions is dissolved, and the positive electrode and the negative electrode are disposed via a separator, the negative electrode is formed. The ratio (ρ _H / ρ _B ) of the measured density (ρ _B ) of the non-graphitizable carbon by the butanol method and the measured density (ρ _H ) by the helium method is 1.05 or less, and the butanol of the non-graphitizable carbon A lithium secondary battery characterized by having a density measured by a method (ρ _B ) of 1.6 to 1.8 g / cc .

The lithium secondary battery according to claim 1, wherein an interlayer distance of the six-membered ring layer of the non-graphitizable carbon is larger than 0.36 nm and smaller than 0.41 nm.

The lithium secondary battery according to claim 1, wherein an interlayer distance of the six-membered ring layer of the non-graphitizable carbon is greater than 0.37 nm and smaller than 0.39 nm.

2. The lithium secondary battery according to claim 1, wherein a charge / discharge capacity of a negative electrode of 1.5 to 0.02 V based on a lithium reference electrode is 280 mAh / g or more.

In the ⁷ Li-NMR spectrum of the non-graphitizable carbon in a negative electrode that was charged with a constant voltage of 0 V on the basis of a lithium reference electrode, one signal is present in the range of NMR shift value of 20 to 22 ppm. The lithium secondary battery according to claim 1.

Graphitizable carbon, in an inert atmosphere, after removing the cracked gas by heat treatment, heat-treated under a pressure of 10 to 120 atm, measured density by butanol method of the flame-graphitizable carbon ([rho _B) 1.6 to 1.8 g / cc, and the ratio (ρ _H / ρ _B ) between the measured density (ρ _B ) by the butanol method and the measured density (ρ _H ) by the helium method is 1.05 or less. A method for producing a negative electrode material used for a lithium secondary battery.

The temperature of the said heat processing is 1000 degrees C or less, The temperature to heat-process under the said pressurization is 1000-2000 degreeC, The manufacturing method of the negative electrode material used for the lithium secondary battery of Claim 6 characterized by the above-mentioned.

A negative electrode material for a lithium secondary battery in which non-graphitizable carbon is used, wherein the measurement density (ρ _B ) of the non- graphitizable carbon by a butanol method is 1.6 to 1.8 g / cc, depending on the butanol method A negative electrode material for a lithium secondary battery, wherein a ratio (ρ _H / ρ _B ) between a measured density (ρ _B ) and a measured density (ρ _H ) by a helium method is 1.05 or less.

In a lithium secondary battery having a positive electrode and a negative electrode that occlude and release lithium ions, an organic electrolyte solution in which an electrolyte containing lithium ions is dissolved, and the positive electrode and the negative electrode are disposed via a separator, the negative electrode is formed. The ratio (ρ _H / ρ _B ) between the measured density (ρ _B ) of the non-graphitizable carbon using isotropic pitch as a raw material by the butanol method and the measured density (ρ _H ) by the helium method is 1.05 or less. A lithium secondary battery having a measured density (ρ _B ) by a butanol method of 1.6 to 1.8 g / cc.

In the ⁷ Li-NMR spectrum of the non-graphitizable carbon in the negative electrode that was charged with a constant voltage of 0 V on the basis of the lithium reference electrode, one signal exists in the range of the NMR shift value of 20 to 22 ppm, and 120 to 124 ppm. The lithium secondary battery according to claim 1 or 9, wherein the lithium secondary battery does not exist in the range of.