JP4081621B2 - Negative electrode carbon material for lithium secondary battery and lithium secondary battery - Google Patents
Negative electrode carbon material for lithium secondary battery and lithium secondary battery Download PDFInfo
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【産業上の利用分野】
本発明は、リチウム二次電池用負極炭素材およびそれを用いた、高い放電容量を有するリチウム二次電池に関する。
【０００２】
【従来の技術】
負極炭素材として黒鉛を用いたリチウム二次電池については、特開昭57-208079号公報（特公昭62-23433号公報）をはじめとして、数多くの公報類に開示されている。
【０００３】
しかしながら、負極炭素材(リチウム担持体)として黒鉛を用いる場合に、担持状態での負極炭素材の組成(LiC6)から求められるリチウム二次電池の理論的最大容量は、372Ah/kg（炭素ベース）であると言われており、放電容量に限界があるという問題点がある。
【０００４】
しかるに、リチウム二次電池を電源として使用するノートパソコンなどの電子機器類の機能向上に伴って、電池自体の性能をさらに一層向上させることが求められており、その負極に対する要求性能も高くなってきている。この様な技術的な状況を考慮すると、負極容量に関して372Ah/kgという数値は、必ずしも満足すべき容量とはいえない。すなわち、従来から提唱されているような黒鉛層間化合物の寄与だけでは、リチウムを貯蔵できる量が充分ではないことが明らかとなってきた。
【０００５】
この問題点を解決するために、特開平8-298114号公報は、黒鉛にハードカーボンをコーティングした負極炭素材を提案している。しかしながら、ハードカーボンには必然的に細孔が多く存在しているために、これをコーティングした負極炭素材の性能は、空気からの吸湿などにより、次第に劣化するという大きな欠点を有している。
【０００６】
また、容量が高くても、初期効率が88％未満という低い負極炭素材を使用する場合には、正極のリチウムが失われるので、リチウム二次電池用負極材料としては不適である。
【０００７】
さらに、炭素材の窒素吸着によるBET比表面積値が3m2/gを超える場合には、電池としての安全性に問題点が発生する確率が急激に高くなるので、リチウム二次電池負極材料としては、不適である。
【０００８】
【発明が解決しようとする問題点】
従って、本発明は、リチウム二次電池の負極材として使用した場合に電池の充放電容量が理論容量である372Ah/kgを超え、かつそれ自身を空気中で放置しても著しい劣化を生じない様な改質された炭素材、この炭素材からなるリチウム二次電池用の負極および負極材料ならびにこの負極材料を使用するリチウム二次電池を提供することを主な目的とする。
【０００９】
【問題点を解決するための手段】
本発明者は、上記の様な技術の現状に鑑みて鋭意研究を重ねた結果、黒鉛とハードカーボンとの複合体の表面を炭素の焼成体でコーティングすることにより、リチウム二次電池の負極材として使用した場合に高容量かつ高初期効率を与える黒鉛系炭素材料が得られることを見出した。
【００１０】
即ち、本発明は、下記の炭素材を提供するものである；
１．黒鉛とハードカーボンとの複合体の表面を炭素の焼成体でコーティングしたリチウム二次電池用負極炭素材であって、
黒鉛とハードカーボンとの混合割合が、重量比で黒鉛：ハードカーボン＝５１：４９〜９０：１０であるリチウム二次電池用負極炭素材。
【００１１】
２．放電容量が372Ah/kgを超えることを特徴とする上記項１に記載のリチウム二次電池用負極炭素材。
【００１２】
３．初期効率が88％以上であることを特徴とする上記項１に記載のリチウム二次電池用負極炭素材。
【００１３】
４．窒素吸着によるBET比表面積の値が、3m2/ｇ以下であることを特徴とする上記項１に記載のリチウム二次電池用負極炭素材。
【００１４】
５．上記項１に記載の炭素材を構成要素として用いるリチウム二次電池用負極材料。
【００１５】
６．上記項５に記載の負極材料を用いるリチウム二次電池用負極。
【００１６】
７．上記項６に記載の負極を用いた非水系リチウム二次電池。
【００１７】
【発明の実施の態様】
本発明において使用する黒鉛としては、天然黒鉛、人造黒鉛、黒鉛化メソカーボンマイクロビーズ、黒鉛化ピッチ系炭素繊維などが挙げられる。黒鉛の平均粒径は、通常1〜100μm程度であり、好ましくは1〜40μm程度である。黒鉛の真密度は、通常2.0g/cc以上であり、好ましくは2.2g/cc以上である。また、Ｘ線広角回折法により得られる(002)面の面間隔は、0.338nm以下であり、より好ましくは0.336nm以下である。
【００１８】
一方、本発明において使用するハードカーボンとしては、炭素繊維の製造過程の中間生成物である不融化糸を1000〜1400℃程度で炭化した炭素繊維、有機化合物を150〜300℃程度で空気酸化した後、1000〜1400℃程度で炭化した炭素材料などが例示できる。炭素材料の製造原料として使用する有機化合物としては、石炭系および石油系の等方性ピッチ、熱硬化性樹脂（フェノール樹脂、フラン樹脂、フルフラール樹脂などが例示される。ハードカーボンを製造するに際し、炭化は、真空下または不活性ガスの流通下に行うことが好ましい。ハードカーボンの平均粒径は、通常1〜100μm程度であり、好ましくは1〜40μm程度である。ハードカーボンの真密度は、通常1.8g/cc以下である。また、Ｘ線広角回折法により得られる(002)面の面間隔は、0.36nm以上である。
【００１９】
本発明において、「黒鉛とハードカーボンとの複合体」とは、二層構造体（粒径1〜40μm程度の黒鉛を芯材として、ハードカーボンを厚さ0.01〜4μm程度でコーティングした構造体：前者と後者の割合は、複合体重量を100部として、前者：後者＝60:40〜80:20程度（重量比）である）および黒鉛とハードカーボンとの混合物を意味する。二層構造体は、例えば、軟化点以上の等方性ピッチと黒鉛とを混合して、黒鉛表面にピッチをコーティングさせた後、空気酸化し、窒素などの不活性雰囲気中で炭化することにより製造することができる。また、黒鉛とハードカーボンとの混合物は、両者を混合機などにより物理的に混合することにより、調製できる。この混合物において、前者と後者との割合は、前者の量が後者の量よりも大きいことを必要とし、通常前者：後者＝51:49〜90:10程度（重量比）であり、より好ましくは前者：後者＝60:40〜80:20程度（重量比）である。本発明で使用する複合体において、黒鉛の割合が低すぎる場合には、電極特性として、急激な初期効率の低下を引き起こすことがある。
【００２０】
以下に、この様な複合体の表面を炭素の焼成体でコーティングする工程について説明する。
【００２１】
先ず、上記の様にして得られる複合体を下記に示す有機化合物に温度10〜300℃程度で、より好ましくは100〜200℃程度で5〜30分間程度浸漬し、次いで有機化合物から分離した後、有機溶媒を加えて10〜300℃程度、より好ましくは10〜100℃程度で洗浄処理し、次いで非酸化性雰囲気中で炭化することにより、所望の炭素焼成体コーティング層(低結晶性炭素乃至ソフトカーボンからなる被覆層)を備えた炭素材を得ることができる。
【００２２】
この複合体の浸漬工程で用いる有機化合物としては、炭化時に複合体表面にソフトカーボンのコーティング層を形成し得る材料、例えば、ピッチ、タールなどが挙げられる。
【００２３】
洗浄用の有機溶媒としては、特に限定されず、トルエン、メタノール、アセトン、ヘキサン、ベンゼン、キシレン、メチルナフタレン、タール中油などを使用することができる。この洗浄は、複合体表面に付着した有機化合物の厚さを調整し、かつ均一とするために行う。
【００２４】
洗浄処理した複合体の炭化は、例えば、600〜1500℃程度、好ましくは800〜1200℃程度の温度で、1〜20時間程度、好ましくは3〜12時間程度処理することにより実施できる。炭化を真空下で行う場合には、放電容量および初期効率をより一層改善することができる。
【００２５】
負極炭素材表面に形成される炭化層(ソフトカーボン層)が厚い程、負極と非水系リチウム二次電池に用いる電解液の有機溶媒とが反応し難くなる。従って、リチウム二次電池の負極として用いる炭素材のソフトカーボン層が厚い程、電解液の分解および負極の破壊が起こり難い。しかしながら、ソフトカーボン層が厚すぎる場合には、炭素材の負極材料としての充放電特性に悪影響を与える恐れがある。本発明者の研究によれば、ソフトカーボン層の厚さが0.1μm以下、通常0.01〜0.1μm程度である炭素材を負極材料として用いる場合には、安全性と充放電特性とのバランスが良好なリチウム二次電池を製造することができることが判明した。ソフトカーボン層の厚さは、使用する有機化合物の量、浸漬温度、浸漬時間、有機溶媒による洗浄条件などを調節することにより、制御することができる。
【００２６】
このようにして得られた炭素材を粒度調整することにより、リチウム二次電池用負極材料として好適な炭素材を製造できる。粒度調整に際しては、例えば、フェザーミルと風力分級機を用いて、解砕・分級を行うことにより、複数の炭素粒子(一次粒子)が相互に付着して形成されている集合体(二次粒子)を分離させることができ、その結果、粒子径および粒子径分布をリチウム二次電池用負極材料として好適な範囲に制御することができる。炭素材は、数平均粒子径を5〜40μm程度、より好ましくは5〜20μm程度とし、最大粒子径を50μm以下、より好ましくは30μm以下とし、最小粒子径を3μm以上、より好ましくは5μm以上とすることが望ましい。この様に粒度調整を行うことにより、負極を作製する際の取扱が容易となり、負極としたときに効率よく特性を発現し得る負極材料を得ることができる。
【００２７】
かくして、公知のリチウム二次電池におけると同様にして、上記で得られた本発明のリチウム二次電池用負極を正極・電解液と組み合わせることにより、リチウム二次電池を作成することができる。
【００２８】
【発明の効果】
本発明によれば、以下の様な顕著な効果が達成される。
【００２９】
（１）黒鉛とハードカーボンとの複合体の表面を炭素の焼成体でコーティングした炭素材をリチウム二次電池の負極として使用する場合に、初期効率が88％以上でかつ黒鉛の理論容量である372Ah/kgを超える高い放電容量が得られるので、同じ性能ではリチウム二次電池負極の体積および重量を著しく低減できるという効果がある。
【００３０】
（２）リチウム二次電池における上記の高い放電容量は、10サイクル程度の充放電後にも殆ど低下せず、100％あるいはそれに近い放電容量が維持される。
【００３１】
（３）本発明による炭素材を空気中で放置させた場合の性能劣化の度合いは、黒鉛単独の場合と同様に小さいので、本発明炭素材を負極として使用するリチウム二次電池の放電容量維持率および初期効率維持率は、９８％以上である。
【００３２】
【実施例】
以下に実施例を挙げて本発明をさらに詳しく説明する。
【００３３】
実施例１
1.黒鉛とハードカーボンとの複合、および表面コーティング
黒鉛としては、マダガスカル産の天然黒鉛(平均粒径74μm、真密度2.25g/cc、(002)面の面間隔0.335nm以下）を用いた。
【００３４】
ハードカーボンとしては、軟化点280℃の石炭系ピッチを空気雰囲気下250℃で2時間酸化処理を行った後、真空下1100℃で1時間熱処理を行った材料(平均粒径15μm、真密度1.7g/cc、(002)面の面間隔0.37nm以上）を用いた。
【００３５】
複合は、黒鉛:ハードカーボン＝6:4（重量比）となる様に両材料を配合し、物理的に混合することにより、行った。
【００３６】
得られた複合体をタールに１５０℃で２０分間浸漬し、次いでタールから分離し、トルエンを加えて約５０℃で洗浄した後、１１００℃で２時間真空下に炭化を行うことにより、複合体表面を炭素の焼成体(ソフトカーボン層)により被覆した。
【００３７】
２．炭素極（作用極）の作成
上記で得られた被覆黒鉛複合体９２重量部とポリフッ化ビニリデン８重量部とを混合し、適量のＮ−メチルピロリドンに加え、撹拌した後、スラリー状物とした。このスラリー状物を電解銅箔上にドクターブレードを用いて塗布した後、１１０℃で３０分間乾燥させ、ロールプレス機によりプレスして、電極材料を得た。この電極材料から１ｃｍ ２ の塗布部だけを残した電極を切り出して炭素極とし、さらに２００℃で６時間真空乾燥した。
【００３８】
３．試験セルの組立
前記の様にして得られた炭素極に対し、対極として十分な量のリチウム金属を使用し、電解液として1mol/lの濃度にLiClO4を溶解させたエチレンカーボネートとジエチルカーボネートとの混合溶媒(体積比1:1)を使用し、セパレータとしてポリプロピレン不織布を使用して、リチウム二次電池(試験セル)を作成した。
【００３９】
４．電極特性の測定
上記で得られたリチウム二次電池の充放電特性を測定した。
【００４０】
充電は、リチウム極に対して1mVまで1mA/cm2の定電流充電した後、1mVで定電位充電をトータルで12時間かけて行った。放電は、1mA/cm2の定電流放電でリチウム極に対して2.0Vまで行った。放電容量は、カット電圧が1.0Ｖの時の容量である。
【００４１】
また、上記の工程で作成した電極を空気中で3日間放置した後、上記と同様にして試験セルを組立て、上記と同様にして電極特性を測定した。
【００４２】
本実施例および実施例２〜３ならびに比較例1〜４の結果を後記の表１に示す。
【００４３】
実施例２
黒鉛とハードカーボンとの複合体の調製に際して、両者の複合割合を黒鉛:ハードカーボン＝7:3（重量比）に設定した以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４４】
実施例３
黒鉛とハードカーボンとの複合体の調製に際して、両者の複合割合を黒鉛：ハードカーボン＝8:2（重量比）に設定した以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４５】
比較例１
黒鉛とハードカーボンとの複合体の調製に際して、両者の複合割合を黒鉛：ハードカーボン＝5:5（重量比）に設定した以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４６】
比較例２
黒鉛とハードカーボンとの複合体に代えて、ハードカーボンのみを使用した以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４７】
比較例３
複合体の表面をタール焼成体によりコーティングしなかった以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４８】
比較例４
黒鉛とハードカーボンとの複合体に代えて、黒鉛のみを使用した以外は実施例１と同様にして、順次所定の操作を行い、試験セルの評価を行った。
【００４９】
【表１】
表１に示す結果から明らかな様に、本発明によれば、初期効率が高く、黒鉛の理論容量を超える放電容量を有するリチウム二次電池が得られる。また、本発明によれば、リチウム二次電池の容量は、10サイクル程度の充放電サイクルでは、殆ど低下しない。しかも、本発明による電極は空気中での安定性に優れているので、空気中に３日間放置した後電池を組み立てても、電池特性には、殆ど変化が生じない。[0001]
[Industrial application fields]
The present invention relates to a negative electrode carbon material for a lithium secondary battery and a lithium secondary battery using the negative carbon material and having a high discharge capacity.
[0002]
[Prior art]
A lithium secondary battery using graphite as a negative electrode carbon material is disclosed in many publications including Japanese Patent Application Laid-Open No. 57-208079 (Japanese Patent Publication No. 62-23433).
[0003]
However, when graphite is used as the negative electrode carbon material (lithium carrier), the theoretical maximum capacity of the lithium secondary battery obtained from the composition of the negative electrode carbon material (LiC 6 ) in the supported state is 372 Ah / kg (carbon base There is a problem that the discharge capacity is limited.
[0004]
However, along with improvements in functions of electronic devices such as laptop computers that use lithium secondary batteries as a power source, there is a need to further improve the performance of the batteries themselves, and the required performance for the negative electrode is also increasing. ing. Considering such a technical situation, a value of 372 Ah / kg for the negative electrode capacity is not necessarily a satisfactory capacity. That is, it has been clarified that the amount of lithium that can be stored is not sufficient only by the contribution of a graphite intercalation compound as proposed conventionally.
[0005]
In order to solve this problem, Japanese Patent Application Laid-Open No. 8-298114 proposes a negative electrode carbon material in which hard carbon is coated on graphite. However, since hard carbon inevitably has a large number of pores, the performance of the negative electrode carbon material coated therewith has a great disadvantage that it gradually deteriorates due to moisture absorption from the air and the like.
[0006]
Even when the capacity is high, when a negative electrode carbon material having a low initial efficiency of less than 88% is used, the lithium of the positive electrode is lost, which is not suitable as a negative electrode material for a lithium secondary battery.
[0007]
Furthermore, if the BET specific surface area value due to nitrogen adsorption of the carbon material exceeds 3 m 2 / g, the probability that a problem will occur in the safety of the battery will increase rapidly, so as a lithium secondary battery negative electrode material Is unsuitable.
[0008]
[Problems to be solved by the invention]
Therefore, when the present invention is used as a negative electrode material for a lithium secondary battery, the charge / discharge capacity of the battery exceeds the theoretical capacity of 372 Ah / kg, and no significant deterioration occurs even if the battery is left in the air. The main object is to provide such a modified carbon material, a negative electrode and a negative electrode material for a lithium secondary battery comprising the carbon material, and a lithium secondary battery using the negative electrode material.
[0009]
[Means for solving problems]
As a result of intensive studies in view of the present state of the art as described above, the present inventor has coated a surface of a composite of graphite and hard carbon with a sintered body of carbon, thereby providing a negative electrode material for a lithium secondary battery. It has been found that a graphite-based carbon material that gives a high capacity and a high initial efficiency when used as a catalyst can be obtained.
[0010]
That is, the present invention provides the following carbon material;
1. A negative electrode carbon material for a lithium secondary battery in which the surface of a composite of graphite and hard carbon is coated with a carbon fired body ,
A negative electrode carbon material for a lithium secondary battery in which a mixing ratio of graphite and hard carbon is graphite: hard carbon = 51: 49 to 90:10 by weight ratio .
[0011]
2. Item 2. The negative electrode carbon material for a lithium secondary battery according to Item 1, wherein the discharge capacity exceeds 372 Ah / kg.
[0012]
3. Item 2. The negative electrode carbon material for a lithium secondary battery according to Item 1, wherein the initial efficiency is 88% or more.
[0013]
4). Item 2. The negative electrode carbon material for a lithium secondary battery according to Item 1, wherein the value of the BET specific surface area by nitrogen adsorption is 3 m 2 / g or less.
[0014]
5. A negative electrode material for a lithium secondary battery using the carbon material according to item 1 as a constituent element.
[0015]
6). A negative electrode for a lithium secondary battery using the negative electrode material according to Item 5.
[0016]
7). A non-aqueous lithium secondary battery using the negative electrode according to Item 6.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Examples of the graphite used in the present invention include natural graphite, artificial graphite, graphitized mesocarbon microbeads, and graphitized pitch-based carbon fiber. The average particle diameter of graphite is usually about 1 to 100 μm, preferably about 1 to 40 μm. The true density of graphite is usually 2.0 g / cc or more, preferably 2.2 g / cc or more. Further, the (002) plane spacing obtained by the X-ray wide angle diffraction method is 0.338 nm or less, more preferably 0.336 nm or less.
[0018]
On the other hand, as the hard carbon used in the present invention, carbon fiber obtained by carbonizing an infusible yarn, which is an intermediate product in the production process of carbon fiber, at about 1000 to 1400 ° C., and an organic compound are air oxidized at about 150 to 300 ° C. Thereafter, a carbon material carbonized at about 1000 to 1400 ° C. can be exemplified. Examples of organic compounds used as raw materials for producing carbon materials include coal-based and petroleum-based isotropic pitches, thermosetting resins (phenolic resins, furan resins, furfural resins, etc. In producing hard carbon, The carbonization is preferably performed under vacuum or in the flow of an inert gas, and the average particle size of the hard carbon is usually about 1 to 100 μm, preferably about 1 to 40 μm. Usually, it is 1.8 g / cc or less, and the plane spacing of the (002) plane obtained by the X-ray wide angle diffraction method is 0.36 nm or more.
[0019]
In the present invention, the “composite of graphite and hard carbon” refers to a two-layer structure (a structure in which hard carbon is coated with a thickness of about 0.01 to 4 μm using graphite having a particle size of about 1 to 40 μm as a core material: The ratio of the former and the latter means a mixture of graphite and hard carbon, with the composite weight being 100 parts, the former: the latter = about 60:40 to 80:20 (weight ratio)). The two-layer structure is obtained by, for example, mixing isotropic pitch above the softening point and graphite, coating the pitch on the graphite surface, air oxidizing, and carbonizing in an inert atmosphere such as nitrogen. Can be manufactured. Moreover, the mixture of graphite and hard carbon can be prepared by physically mixing both with a mixer or the like. In this mixture, the ratio of the former to the latter requires that the amount of the former is larger than the amount of the latter, and usually the former: the latter = about 51:49 to 90:10 (weight ratio), more preferably The former: the latter = about 60:40 to 80:20 (weight ratio). In the composite used in the present invention, when the ratio of graphite is too low, the initial efficiency may be rapidly lowered as electrode characteristics.
[0020]
Hereinafter, a process of coating the surface of such a composite with a carbon fired body will be described.
[0021]
First, the composite obtained as described above is immersed in the organic compound shown below at a temperature of about 10 to 300 ° C., more preferably about 100 to 200 ° C. for about 5 to 30 minutes, and then separated from the organic compound. Then, an organic solvent is added and washed at about 10 to 300 ° C., more preferably about 10 to 100 ° C., and then carbonized in a non-oxidizing atmosphere, thereby obtaining a desired carbon fired body coating layer (low crystalline carbon to A carbon material provided with a coating layer made of soft carbon) can be obtained.
[0022]
Examples of the organic compound used in the step of immersing the composite include materials that can form a soft carbon coating layer on the surface of the composite during carbonization, such as pitch and tar.
[0023]
The organic solvent for cleaning is not particularly limited, and toluene, methanol, acetone, hexane, benzene, xylene, methyl naphthalene, tar oil, and the like can be used. This cleaning is performed in order to adjust the thickness of the organic compound attached to the composite surface and make it uniform.
[0024]
Carbonization of the washed composite can be carried out, for example, by treating at a temperature of about 600 to 1500 ° C., preferably about 800 to 1200 ° C., for about 1 to 20 hours, preferably about 3 to 12 hours. When carbonization is performed under vacuum, the discharge capacity and initial efficiency can be further improved.
[0025]
The thicker the carbonized layer (soft carbon layer) formed on the surface of the negative electrode carbon material, the less the reaction between the negative electrode and the organic solvent of the electrolytic solution used in the non-aqueous lithium secondary battery. Therefore, as the soft carbon layer of the carbon material used as the negative electrode of the lithium secondary battery is thicker, the decomposition of the electrolyte and the breakdown of the negative electrode are less likely to occur. However, when the soft carbon layer is too thick, the charge / discharge characteristics of the carbon material as a negative electrode material may be adversely affected. According to the research of the present inventor, when a carbon material having a soft carbon layer thickness of 0.1 μm or less, usually about 0.01 to 0.1 μm, is used as a negative electrode material, the balance between safety and charge / discharge characteristics is good. It has been found that a lithium secondary battery can be manufactured. The thickness of the soft carbon layer can be controlled by adjusting the amount of the organic compound to be used, the immersion temperature, the immersion time, the cleaning conditions with an organic solvent, and the like.
[0026]
By adjusting the particle size of the carbon material thus obtained, a carbon material suitable as a negative electrode material for a lithium secondary battery can be produced. When adjusting the particle size, for example, by using a feather mill and an air classifier, crushing and classification, an aggregate (secondary particles) formed by adhering a plurality of carbon particles (primary particles) to each other ) Can be separated, and as a result, the particle size and particle size distribution can be controlled within a range suitable as a negative electrode material for a lithium secondary battery. The carbon material has a number average particle size of about 5 to 40 μm, more preferably about 5 to 20 μm, a maximum particle size of 50 μm or less, more preferably 30 μm or less, and a minimum particle size of 3 μm or more, more preferably 5 μm or more. It is desirable to do. By adjusting the particle size in this way, handling during production of the negative electrode is facilitated, and a negative electrode material that can efficiently exhibit characteristics when used as a negative electrode can be obtained.
[0027]
Thus, in the same manner as in known lithium secondary batteries, a lithium secondary battery can be prepared by combining the negative electrode for a lithium secondary battery of the present invention obtained above with a positive electrode / electrolyte.
[0028]
【The invention's effect】
According to the present invention, the following remarkable effects are achieved.
[0029]
(1) When a carbon material in which the surface of a composite of graphite and hard carbon is coated with a carbon fired body is used as a negative electrode of a lithium secondary battery, the initial efficiency is 88% or more and the theoretical capacity of graphite. Since a high discharge capacity exceeding 372 Ah / kg is obtained, the same performance has the effect that the volume and weight of the lithium secondary battery negative electrode can be significantly reduced.
[0030]
(2) The high discharge capacity in the lithium secondary battery hardly decreases even after charging and discharging for about 10 cycles, and the discharge capacity is maintained at 100% or close to it.
[0031]
(3) Since the degree of performance deterioration when the carbon material according to the present invention is allowed to stand in the air is small as in the case of graphite alone, the discharge capacity of the lithium secondary battery using the carbon material of the present invention as a negative electrode is maintained. The rate and the initial efficiency maintenance rate are 98% or more.
[0032]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0033]
Example 1
1. Natural graphite (average particle size 74 μm, true density 2.25 g / cc, (002) plane spacing 0.335 nm or less) was used as a composite of graphite and hard carbon, and as surface coating graphite.
[0034]
As hard carbon, a material obtained by subjecting a coal pitch with a softening point of 280 ° C to an oxidation treatment at 250 ° C in an air atmosphere for 2 hours and then heat treatment at 1100 ° C in a vacuum for 1 hour (average particle size 15μm, true density 1.7 g / cc, (002) plane spacing of 0.37 nm or more) was used.
[0035]
Composite was performed by blending both materials so that graphite: hard carbon = 6: 4 (weight ratio) and physically mixing them.
[0036]
The resulting composite was immersed for 20 minutes at 0.99 ° C. to tar, then separated from the tar, washed with approximately 50 ° C. by adding toluene, by intends line carbonization under vacuum for 2 hours at 1100 ° C., the composite The body surface was covered with a carbon fired body (soft carbon layer).
[0037]
2. Preparation of carbon electrode (working electrode) 92 parts by weight of the coated graphite composite obtained above and 8 parts by weight of polyvinylidene fluoride were mixed, added to an appropriate amount of N-methylpyrrolidone, stirred, and then made into a slurry. . The slurry was applied on an electrolytic copper foil using a doctor blade, dried at 110 ° C. for 30 minutes, and pressed with a roll press to obtain an electrode material. An electrode with only a 1 cm 2 coating portion left was cut out from this electrode material to form a carbon electrode, and further vacuum dried at 200 ° C. for 6 hours.
[0038]
3. Assembling of the test cell With respect to the carbon electrode obtained as described above, a sufficient amount of lithium metal was used as a counter electrode, and ethylene carbonate and diethyl carbonate in which LiClO 4 was dissolved at a concentration of 1 mol / l as an electrolyte solution A lithium secondary battery (test cell) was prepared using the above mixed solvent (volume ratio 1: 1) and a polypropylene nonwoven fabric as a separator.
[0039]
4). Measurement of electrode characteristics The charge / discharge characteristics of the lithium secondary battery obtained above were measured.
[0040]
Charging was performed at a constant current of 1 mA / cm 2 up to 1 mV with respect to the lithium electrode, followed by constant potential charging at 1 mV over a total of 12 hours. The discharge was performed at a constant current of 1 mA / cm 2 up to 2.0 V with respect to the lithium electrode. The discharge capacity is a capacity when the cut voltage is 1.0V.
[0041]
Further, after the electrode prepared in the above process was left in the air for 3 days, a test cell was assembled in the same manner as described above, and the electrode characteristics were measured in the same manner as described above.
[0042]
The results of this example, Examples 2-3 and Comparative Examples 1-4 are shown in Table 1 below.
[0043]
Example 2
When preparing a composite of graphite and hard carbon, a predetermined operation was sequentially performed in the same manner as in Example 1 except that the composite ratio of both was set to graphite: hard carbon = 7: 3 (weight ratio). The cell was evaluated.
[0044]
Example 3
When preparing a composite of graphite and hard carbon, a predetermined operation was sequentially performed in the same manner as in Example 1 except that the composite ratio of both was set to graphite: hard carbon = 8: 2 (weight ratio). The cell was evaluated.
[0045]
Comparative Example 1
When preparing a composite of graphite and hard carbon, a predetermined operation was sequentially performed in the same manner as in Example 1 except that the composite ratio of both was set to graphite: hard carbon = 5: 5 (weight ratio). The cell was evaluated.
[0046]
Comparative Example 2
Instead of using a composite of graphite and hard carbon, a predetermined operation was sequentially performed in the same manner as in Example 1 except that only hard carbon was used, and the test cells were evaluated.
[0047]
Comparative Example 3
The test cells were evaluated in order by sequentially performing predetermined operations in the same manner as in Example 1 except that the surface of the composite was not coated with the tar fired body.
[0048]
Comparative Example 4
The test cell was evaluated in order as in Example 1 except that only graphite was used instead of the composite of graphite and hard carbon.
[0049]
[Table 1]
As is apparent from the results shown in Table 1, according to the present invention, a lithium secondary battery having a high initial efficiency and a discharge capacity exceeding the theoretical capacity of graphite can be obtained. Further, according to the present invention, the capacity of the lithium secondary battery hardly decreases in a charge / discharge cycle of about 10 cycles. Moreover, since the electrode according to the present invention is excellent in stability in the air, even if the battery is assembled after being left in the air for 3 days, the battery characteristics hardly change.
Claims (7)
黒鉛とハードカーボンとの混合割合が、重量比で黒鉛：ハードカーボン＝５１：４９〜９０：１０であるリチウム二次電池用負極炭素材。A negative electrode carbon material for a lithium secondary battery in which the surface of a composite of graphite and hard carbon is coated with a carbon fired body ,
A negative electrode carbon material for a lithium secondary battery in which a mixing ratio of graphite and hard carbon is graphite: hard carbon = 51: 49 to 90:10 by weight .
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| US6737191B2 (en) * | 2000-11-17 | 2004-05-18 | Wilson Greatbatch Ltd. | Double current collector negative electrode design for alkali metal ion electrochemical cells |
| TW574764B (en) | 2002-01-25 | 2004-02-01 | Toyo Tanso Co | Negative electrode material for lithium ion secondary battery |
| US20030160215A1 (en) | 2002-01-31 | 2003-08-28 | Zhenhua Mao | Coated carbonaceous particles particularly useful as electrode materials in electrical storage cells, and methods of making the same |
| US7618678B2 (en) | 2003-12-19 | 2009-11-17 | Conocophillips Company | Carbon-coated silicon particle powders as the anode material for lithium ion batteries and the method of making the same |
| US7785661B2 (en) | 2003-12-19 | 2010-08-31 | Conocophillips Company | Methods of preparing composite carbon-graphite-silicon particles and using same |
| CN100341173C (en) * | 2004-04-29 | 2007-10-03 | 宁波杉杉新材料科技有限公司 | Method for preparing natural graphite cathode material of lithium ion battery |
| CN1331253C (en) * | 2004-04-29 | 2007-08-08 | 宁波杉杉新材料科技有限公司 | Method for preparing artificial graphite cathode material of lithium ion battery |
| JP5105765B2 (en) * | 2006-04-20 | 2012-12-26 | Ｎｅｃエナジーデバイス株式会社 | Lithium ion secondary battery |
| JP5636149B2 (en) * | 2007-04-27 | 2014-12-03 | 日立化成株式会社 | A negative electrode material for a non-aqueous electrolyte secondary battery, a manufacturing method thereof, a negative electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. |
| JP5623686B2 (en) | 2007-06-01 | 2014-11-12 | パナソニック株式会社 | Composite negative electrode active material and non-aqueous electrolyte secondary battery |
| JP2013218856A (en) * | 2012-04-06 | 2013-10-24 | Sumitomo Bakelite Co Ltd | Negative electrode material, negative electrode and lithium ion secondary battery |
| US9960427B2 (en) * | 2013-05-30 | 2018-05-01 | Lg Chem, Ltd. | Conductive material for lithium secondary battery and electrode for lithium secondary battery including the same |
| JP6647211B2 (en) * | 2014-11-11 | 2020-02-14 | 株式会社Adeka | Non-aqueous electrolyte secondary battery |
| JP6759583B2 (en) * | 2015-02-06 | 2020-09-23 | 東ソー株式会社 | Composite active material for lithium secondary battery and its manufacturing method, lithium secondary battery |
| KR101833615B1 (en) | 2015-04-29 | 2018-02-28 | 주식회사 엘지화학 | Negative electrode active material and negative electrode comprising the same |
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| CN111682177B (en) * | 2020-06-18 | 2023-01-10 | 贝特瑞新材料集团股份有限公司 | Graphite composite material, preparation method and application thereof |
| CN114852989A (en) * | 2022-05-05 | 2022-08-05 | 常州烯源谷新材料科技有限公司 | Preparation method of soft carbon-hard carbon composite material with high first efficiency |
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| JPH05121066A (en) * | 1991-10-29 | 1993-05-18 | Asahi Chem Ind Co Ltd | Negative electrode for nonaqueous battery |
| JP3061337B2 (en) * | 1993-05-20 | 2000-07-10 | 松下電器産業株式会社 | Non-aqueous electrolyte secondary battery |
| JP3430614B2 (en) * | 1993-06-03 | 2003-07-28 | ソニー株式会社 | Non-aqueous electrolyte secondary battery |
| JPH07326343A (en) * | 1994-05-30 | 1995-12-12 | Matsushita Electric Ind Co Ltd | Negative electrode material for nonaqueous electrolytic secondary battery and its manufacture |
| JPH0869819A (en) * | 1994-08-29 | 1996-03-12 | Murata Mfg Co Ltd | Nonaqueous electrolytic secondary battery |
| JP3666032B2 (en) * | 1994-09-30 | 2005-06-29 | 三菱化学株式会社 | Method for producing carbon-based composite material |
| JPH08227714A (en) * | 1995-02-21 | 1996-09-03 | Mitsubishi Pencil Co Ltd | Carbon material for negative electrode of lithium ion secondary battery and manufacture thereof |
| JP3292792B2 (en) * | 1995-07-13 | 2002-06-17 | エフ・ディ−・ケイ株式会社 | Lithium secondary battery |
| CN1091072C (en) * | 1995-11-14 | 2002-09-18 | 大阪瓦斯株式会社 | Cathode material for lithium secondary battery, process for manufacturing the same, and secondary battery using the same |
| JP3873325B2 (en) * | 1996-07-25 | 2007-01-24 | 大阪瓦斯株式会社 | Carbon material for lithium secondary battery negative electrode and method for producing the same |
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