JP3716818B2 - Method for producing negative electrode material for high performance lithium ion secondary battery using natural graphite - Google Patents
Method for producing negative electrode material for high performance lithium ion secondary battery using natural graphite Download PDFInfo
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Description
【０００１】
本発明は、リチウムイオン二次電池用負極材の製造方法に関し、より詳しくは天然黒鉛をバインダ−を用いて略球形にした黒鉛粒子にバインダ−ピッチを被覆、含浸して得られる高容量でロスが少ない、負極材の製造方法に関する。
【０００２】
【従来の技術】
近年、リチウム二次電池はハイパワ−、高容量の二次電池として携帯電話、パソコン等の可搬型機器類に多く使用され、今後も需要がさらに高まると予想されている。
【０００３】
このような可搬型機器類の小型化への流れを受けて、リチウム二次電池も小型化、軽量化への要請が強まっている。
【０００４】
そのため、リチウム二次電池を構成するパ−ツや材料も高性能化の動きが活発になっており、中でも負極材は電池の性能を左右するものとしてその重要性が高まっている。
【０００５】
この負極材としてカ−ボン系材料が注目されている。カ−ボン系負極材にはまず放電容量が高容量であることが要求されるが、それに加えて容量ロスの低減も重要で、また電池内に多量の負極材を充填できるようにするため高かさ密度であること、さらに急速充電が可能であることも望まれている。
【０００６】
放電容量が高い負極材としては、従来より天然黒鉛が理論値の３７２ｍＡｈ／ｇに近い容量を得られることが知られている。
しかし、天然黒鉛には次のような欠点がある。
【０００７】
まず、天然黒鉛塊を単に粉砕・整粒したものは不純物が１０〜１５％と多く、また産地や鉱床によって不純物の成分や量にばらつきがある。
このため負極材用途に用いるには、天然黒鉛の使用量に対する放電容量の効率や電池に使用する場合の安全性の面で問題がある。
【０００８】
また粉砕した天然黒鉛は、鱗片状あるいは鱗状であり、初期放電容量は大きいが、その扁平な形状のために集電体の銅箔と平行に粒子が配向するので、リチウムイオンの吸蔵．放出による負極の体積膨張・収縮が大きくサイクル特性の劣化が問題となる。
尚、天然黒鉛の一種として土状黒鉛があるが、結晶の発達が悪く、高結晶性の鱗片状や鱗状のものに比べ、容量が低く、負極材用には好ましくない。
【０００９】
上記のような問題点を解決するために、天然黒鉛塊を特殊な方法で略球状になるように粉砕した材料や、鱗片状や鱗状の天然黒鉛粉末を適当なバインダ−を用いて略球形に賦形した負極材が上市されている。
しかしこれらの負極材も天然黒鉛の有する急速充電性が劣るという欠点までは解決し得ない。
【００１０】
このような状況から、天然黒鉛の有する高い放電容量を生かしつつ、その欠点を解消し、市場の要請する高性能のカ−ボン負極材をいかに得るかが技術的課題となっている。
【００１１】
【発明の課題】
上記のようなリチウム二次電池負極材の高性能化への要求に応えるために、本発明者は天然黒鉛を用いて高容量であるとともに、初期銃放電時の容量ロスが小さいリチウムイオン二次電池用負極材を提供する。またかかる負極材を得るために、安価で量産に有利な製造方法を提供する。
【００１２】
【課題解決の手段】
上記のような課題を解決するために、本発明者は鋭意検討した結果、天然黒鉛を樹脂等のバインダ−を用いて略球状に造粒成形して、バインダ−ピッチを被覆、
含浸させることにより、高性能の負極材が得られることを見出した。
【００１３】
即ち、本発明者が提案するのは、天然黒鉛をバインダ−を用いて略球状に造粒成形して得た黒鉛粒子にバインダ−ピッチを添加し、加熱混合した後、非酸化性雰囲気下で８００〜１４００℃で焼成することを特徴とするリチウムイオン二次電池用負極材の製造方法である
またかかる製造方法において、黒鉛粒子１００重量部に添加するバインダ−ピッチの割合を３〜３０重量部とするリチウムイオン二次電池負極材の製造方法である。
【００１４】
以下に本発明の構成要件について、さらに詳細に説明する。
【００１５】
本発明における天然黒鉛粉末は、鉱山から採掘された天然黒鉛塊を通常の方法で粉砕し、鱗片状、鱗状にしたものを使用する。
この天然黒鉛粉末は精製処理により、不純物を除去し、灰分を０．２％以下、好ましくは０．１％以下に調整することが好ましい。
精製の方法は、浮遊選鉱、薬品処理、電気化学処理等、適宜な方法でよい。
【００１６】
灰分が０．２％を超えると、負極材として用いた場合、残存した不純物が充放電に寄与しない成分として残るため、負極材の利用率が低下したり、不純物の影響で電池の安全性が損なわれたりするので好ましくない。
【００１７】
また灰分が０．２％を超えたまま、次工程に進むと、最終的な熱処理で、不純物除去のために２８００℃以上で黒鉛化処理が必要なためコスト高となる。
そして黒鉛化処理をすれば、全体が高結晶の材料となるため、電解液にＰＣを含む系を用いると、負極材表面で電解液の分解が生じ易くなる問題もある。
さらに天然黒鉛が元来有する、急速充放電特性が劣るという欠点も解消できず、
好ましくない。
【００１８】
次に上記のように不純物を除去した天然黒鉛を適宜なバインダ−を用いて、略球状に賦形する。
【００１９】
使用するバインダ−は、結合力のある樹脂、ピッチ等であればよく、特に限定されない。
バインダ−の種類としては、フェノ−ル樹脂、セルロ−ス樹脂、エポキシ樹脂等の熱硬化性樹脂や、ポリビニルアルコ−ル、スチレンブタジェンラバ−、ナイロン、ポリエチレン、コ−ルタ−ルピッチ、また前記の樹脂とコ−ルタ−ルの混合物を用いることができる。
【００２０】
バインダ−の使用量は種類により異なるが、天然黒鉛１００重量部に対して、約３〜２０重量部が適当である。
ここでは、バインダ−の使用の目的は天然黒鉛を略球状に賦形することであり、
その目的が達成できれば、それ以上の量を配合する必要はない。
【００２１】
上記のような天然黒鉛とバインダ−を配合して、造粒機を用いて造粒成形を行い略球状の黒鉛粒子を得る。
造粒に用いる造粒機は、一般に市販されているものであればよく、機種は特に指定されない。
使用する造粒機の特徴に従って、配合方法も事前に混合する方式や、流動中の粉に噴霧する方式など適宜な方法を選択すればよい。
【００２２】
造粒した黒鉛粒子の粒径は、負極材として使用可能なものであれば特定されないが、一般に平均粒径は１０〜３０μｍが適当で、最大粒子径は１００μｍ程度、
好ましくは、８０μｍを超えないことが望ましい。
【００２３】
黒鉛粒子の形状は略球形であり、球形度は限定されないが、長径と短径の比が、
２以下程度であれば特に問題はない。
尚、この比が２以上のものは、通常の造粒方法では選択的に製造するのは逆に困難である。
【００２４】
ここで、造粒した黒鉛粒子の形状を保持するため必要に応じて硬化処理を行うのがよい。例えば、バインダ−にフェノ−ル樹脂等の熱硬化性樹脂を使用した場合は、１５０℃程度で硬化処理をすると良い。
【００２５】
次に上記のように略球形に造粒した黒鉛粒子にバインダ−ピッチを添加し、加熱混合する。
バインダ−ピッチは軟化点が８０〜１５０℃程度のものが適当である。
加熱混合は、１００〜２５０℃で行い、溶融したピッチを黒鉛粒子内の空隙に含浸させると同時に、粒子表層を被覆させる。
【００２６】
混合に使用する装置は限定されないが、一般には加熱ニ−ダ−が量産にも適していて望ましい。
【００２７】
混合の割合は、黒鉛粒子１００重量部に対してバインダ−ピッチを３〜３０重量部とすることが好ましい。
３重量部未満では、混合した効果がなく、最終的に得られる負極材の充放電効率、耐ＰＣ性、急速充電性が低下する。
また３０重量部を超えると、ピッチの量が過多となり、次工程の焼成において、
黒鉛粒子同士を固着させてしまい、焼成後の粉砕が必要となる。
その結果、黒鉛粒子表面の被覆効果が低下し、負極材において容量ロスが増加したり、天然黒鉛よりも結晶化度の低いピッチ由来の炭素が増加することにより、
放電容量が低下したりする問題が生じる。
【００２８】
加熱混合後は、窒素等の非酸化性ガス雰囲気中で焼成して、ピッチ部分を炭素化して本発明の負極材を得る。
焼成の温度は８００〜１４００℃が好ましく、８００〜１１００℃が最も好ましい。
８００℃未満では揮発分の除去が不十分となり、また保管による容量劣化等の不都合も生じ、１４００℃以上では量産にあたりコスト高になるのでいずれも好ましくない。
【００２９】
以上のようにして、本発明の製造方法によりリチウム二次電池負極材が得られる。
【００３０】
【発明の効果】
本発明によるとリチウム二次電池用カ−ボン負極材につき、基材である天然黒鉛の特性を生かし、高い放電容量の材料とするとともに、ピッチの混合効果により、容量ロスを低減させ、また耐ＰＣ性にも優れた負極材を得ることができる。
また本発明の負極材の製造方法は、最終的な熱処理として、２８００℃以上の黒鉛化を必要とせず、８００〜１４００℃の焼成で足りるため、安価に製造することができ、量産に当りコスト面でのメリットが大である。
【００３１】
【実施例および比較例】
【実施例１】
平均粒径１５μｍ、灰分０．１５％とした中国製鱗片状天然黒鉛１００重量部にバインダ−としてエチルセルロ−ス（ダウケミカル社製）３重量部を用いて造粒成形を行い、平均粒径２３μｍの略球形成形体（長径と短径の比は１．１〜２．０程度に分布）を調整した。
この造粒成形体１００重量部に対し、軟化点１１０℃のコ−ルタ−ルピッチを５重量部配合して、２軸ニ−ダ−中で１５０℃に加熱しながら２時間加熱混合した。
次に、この混合物を窒素雰囲気下で、最高温度１０００℃で６時間熱処理し、焼成品とした後、放冷した。
焼成品は、粒子同士の強固な融着はなく、クイックミル（（株）セイシン企業製）で簡単に解砕することができ、これによりリチウムイオン二次電池用カ−ボン負極材を得た。
得られた負極材は、平均粒径２３μｍ、比表面積４．３ｍ2／ｇ、タップ密度０．９４ｇ／ｃｍ3であった。
【００３２】
次にこの負極材を用いて以下のように電池を作成し、電池特性を評価した。
本来、炭素粉末は負極として用いるが、本発明では対極にリチウム金属を使用したため、正極で電池の特性を評価した。
電極の製造は炭素粉末１００重量部とポリフッ化ビニリデン１０重量部にＮ−メチル−２−ピロリドンを添加してペ−スト化した後、ドクタ−ブレ−ドを用いて銅箔状に塗布し、乾燥させた。
乾燥後、これを１ｃｍ2の面積になるように円形に打ち抜き、更に１ｔｏｎ／ｃｍ2の圧力でプレスし、電極を調整した。
対極及び参照極としてリチウム金属を使用し、電解液として１ＭLiPF6／ＥＣ：ＭＥＣ（体積比１：１）を用いてコインセルを組み立てた。
【００３３】
充電は０．５ｍＡ／ｃｍ2の電流密度で定電流充電後、１０ｍＶで定電圧充電に切り替え、０．０１ｍＡで終止した。
また、放電は、０．５ｍＡ／ｃｍ2の電流密度で定電流放電１．５Ｖまで行った。さらに放電レ−トを変えて放電容量を測定した。測定温度は３０℃である。
測定結果は、放電容量３７１ｍＡｈ／ｇ，電池効率は９０％であった。
また放電レ−ト（Ｃ）を変えての放電容量、放電容量保持率を図１、図２にそれぞれ示す。
【００３４】
【実施例２】
軟化点１１０℃のコ−ルタ−ルピッチを１５重量部用いた他は、実施例１と同様にして、負極材を得た。
得られた負極材は、平均粒径２３μｍ、比表面積３．８ｍ2／ｇ、タップ密度０．９７／ｃｍ3であった。
この負極材を用いて、実施例１と同様にコインセルを組み、電池特性を測定した結果、放電容量は３６１ｍＡｈ／ｇ，電池効率は９２％であった。
また放電レ−ト（Ｃ）を変えての放電容量、放電容量保持率を図１、図２にそれぞれ示す。
尚、ここで放電レ−ト（Ｃ）とは、１Ｃ＝１時間放電、２Ｃ＝０．５時間、０．
２Ｃ＝５時間放電ということで、０．２Ｃ（５時間放電）が通常基準のレ−トとして用いられている。
１Ｃ、２Ｃ、３Ｃとレ−トが上がると、放電時間が短くなり、より大きな電流密度で放電することになり、負荷が大きくなる方向にいくことを意味する。
従ってＣレ−トの値が大きくなっても放電容量の低下が少ないことは、急速放電特性が良好であることである。
【００３５】
【比較例１】
平均粒径１５μｍ、灰分０．１５％とした中国製鱗片状天然黒鉛１００重量部にバインダ−として、フェノ−ル樹脂（群栄化学工業（株）製）を５重量部用いて造粒成形を行い、平均粒径２３μｍの略球形成形体（長径と短径の比は１．１〜２．０程度に分布）を調整した。
この成形体を空気中１５０℃で熱硬化した後、窒素雰囲気下で１０００℃で６時間保持して、焼成品とした後、放冷し、負極材を得た。
得られた負極材を用いて実施例１と同様にしてコインセルを組立て、電池特性を測定した結果、 放電容量は３７１ｍＡｈ／ｇ、電池効率は８８％であった。また放電レ−ト（Ｃ）を変えての放電容量、放電容量保持率を図１、図２にそれぞれ示す。
実施例１，２と比較例１とを比較すると、いずれも天然黒鉛を用いているので、
高い放電容量を示しているが、電池効率の面で比較例では９０％未満となっている。（表１参照）
電池効率は９０％未満では容量ロスが大きいと評価されるものなので、実施例のものが電池効率においても優れているとわかる。
図１、図２を見ると、放電レ−トの上昇に従い、明らかに比較例よりも実施例の電池の方が、高い放電容量、放電容量保持率を維持しており、急速放電性に優れていることがわかる。
【００３６】
【実施例３】
実施例１で得られた負極材を用いて、電解液を１Ｍ LiPF6／ＰＣ：ＥＣ：ＭＥＣ（体積比１：１：１）に代えた以外は、実施例１と同様の方法で、コインセルを組立て電池特性を測定した結果、放電容量は３７２ｍＡｈ／ｇ，電池効率は８６％であった。
【００３７】
【実施例４】
実施例２で得られた負極材を用いて、電解液を１Ｍ LiPF6／ＰＣ：ＥＣ：ＭＥＣ（体積比１：１：１）に代えた以外は、実施例１と同様な方法で、コインセルを組立て、電池特性を測定した結果、放電容量は３６４ｍＡ／ｇ，電池効率は８７％であった。
【００３８】
【比較例２】
比較例１で得られた負極材を用いて、電解液を１Ｍ LiPF6／ＰＣ：ＥＣ：ＭＥＣ（体積比１：１：１）に代えた以外は、実施例１と同様の方法で、コインセルを組み電池特性を測定した結果、放電容量は、３６６ｍＡｈ／ｇ，電池効率は７５％であった。（表２参照）
ここで、実施例３、４と比較例２はいずれも電解液にＰＣを使用するものである。
一般に、黒鉛負極材では、放電レ−トが同じでも、電解液にＰＣを添加すると、
その添加量の増加に伴い負極材表面でのＰＣの分解量が多くなり、電池効率が低下するという現象が生じる。
実施例と比較例を比べると、実施例のものがＰＣの分解を抑制し、電池効率の低下を僅少にとどめていることがわかる。
【００３９】
【表１】
【表２】
【図面の簡単な説明】
【図１】 各放電レ−ト（Ｃ）における放電容量を示す図である。
【図２】 各放電レ−ト（Ｃ）における放電容量保持率を示す図である。[0001]
The present invention relates to a method for producing a negative electrode material for a lithium ion secondary battery, and more specifically, high capacity and loss obtained by coating and impregnating a graphite particle obtained by forming natural graphite into a substantially spherical shape using a binder. The present invention relates to a method for producing a negative electrode material with a small amount.
[0002]
[Prior art]
In recent years, lithium secondary batteries are often used as high-power, high-capacity secondary batteries in portable devices such as mobile phones and personal computers, and demand is expected to increase further in the future.
[0003]
In response to the trend toward miniaturization of portable devices, there is an increasing demand for miniaturization and weight reduction of lithium secondary batteries.
[0004]
For this reason, parts and materials constituting lithium secondary batteries are also becoming increasingly active, and among them, the importance of the negative electrode material is increasing as it affects the performance of the battery.
[0005]
Carbon-based materials are attracting attention as this negative electrode material. Carbon-based negative electrode materials are required to have a high discharge capacity, but in addition to that, reduction of capacity loss is also important, and high capacity is required so that a large amount of negative electrode material can be filled in the battery. It is also desired to have a bulk density and to be capable of rapid charging.
[0006]
As a negative electrode material having a high discharge capacity, it has been known that natural graphite can obtain a capacity close to the theoretical value of 372 mAh / g.
However, natural graphite has the following drawbacks.
[0007]
First, natural graphite lumps are simply crushed and sized so that the impurities are as large as 10 to 15%, and the components and amounts of impurities vary depending on the production area and the ore deposit.
For this reason, when it uses for a negative electrode material use, there exists a problem in terms of the efficiency of the discharge capacity with respect to the usage-amount of natural graphite, and the safety | security at the time of using for a battery.
[0008]
The pulverized natural graphite is scaly or scaly and has a large initial discharge capacity, but because of its flat shape, the particles are oriented parallel to the copper foil of the current collector. The volume expansion / contraction of the negative electrode due to the release is large, and deterioration of cycle characteristics becomes a problem.
In addition, there is earthy graphite as a kind of natural graphite, but the development of crystals is poor, and the capacity is lower than that of highly crystalline scaly or scaly ones, which is not preferable for a negative electrode material.
[0009]
In order to solve the problems as described above, a material obtained by pulverizing natural graphite lumps into a substantially spherical shape by a special method, and scaly or scaly natural graphite powder into a substantially spherical shape using an appropriate binder. Shaped negative electrode materials are on the market.
However, these negative electrode materials also cannot solve the disadvantage that the quick chargeability of natural graphite is inferior.
[0010]
Under such circumstances, it has become a technical problem how to obtain the high-performance carbon negative electrode material required by the market while taking advantage of the high discharge capacity of natural graphite while eliminating its drawbacks.
[0011]
[Problems of the Invention]
In order to meet the demand for higher performance of the negative electrode material for lithium secondary batteries as described above, the present inventor has a high capacity using natural graphite and a lithium ion secondary that has a small capacity loss during initial gun discharge. A negative electrode material for a battery is provided. Moreover, in order to obtain such a negative electrode material, an inexpensive and advantageous production method is provided.
[0012]
[Means for solving problems]
In order to solve the above-mentioned problems, the present inventors have intensively studied, and as a result, natural graphite is granulated and formed into a substantially spherical shape using a binder such as a resin, and the binder pitch is covered.
It has been found that a high-performance negative electrode material can be obtained by impregnation.
[0013]
That is, the inventor proposes to add a binder pitch to graphite particles obtained by granulating and forming natural graphite into a substantially spherical shape using a binder, heat-mix, and then in a non-oxidizing atmosphere. It is a manufacturing method of the negative electrode material for lithium ion secondary batteries characterized by baking at 800-1400 degreeC. Moreover, in this manufacturing method, the ratio of the binder pitch added to 100 weight part of graphite particles is 3-30 weight part And a method for producing a negative electrode material for a lithium ion secondary battery.
[0014]
Hereinafter, the constituent requirements of the present invention will be described in more detail.
[0015]
As the natural graphite powder in the present invention, a natural graphite lump mined from a mine is crushed by a usual method to obtain a scaly or scaly shape.
The natural graphite powder is preferably refined to remove impurities and adjust the ash content to 0.2% or less, preferably 0.1% or less.
The purification method may be an appropriate method such as flotation, chemical treatment, or electrochemical treatment.
[0016]
If the ash content exceeds 0.2%, when used as a negative electrode material, the remaining impurities remain as components that do not contribute to charge / discharge, so the utilization factor of the negative electrode material is reduced or the safety of the battery is affected by the influence of impurities. It is not preferable because it is damaged.
[0017]
If the ash content exceeds 0.2% and the process proceeds to the next step, the final heat treatment requires graphitization at 2800 ° C. or higher to remove impurities, resulting in high costs.
If graphitization is performed, the entire material becomes a highly crystalline material. Therefore, when a system containing PC is used as the electrolytic solution, there is a problem that the electrolytic solution is easily decomposed on the surface of the negative electrode material.
Furthermore, the natural graphite inherently has the disadvantage of inferior rapid charge / discharge characteristics.
It is not preferable.
[0018]
Next, natural graphite from which impurities have been removed as described above is shaped into a substantially spherical shape using an appropriate binder.
[0019]
The binder to be used is not particularly limited as long as it is a resin having a bonding force, a pitch, or the like.
Examples of the binder include thermosetting resins such as phenol resin, cellulose resin, and epoxy resin, polyvinyl alcohol, styrene butadiene rubber, nylon, polyethylene, cold pitch, A mixture of a resin and a coal tar can be used.
[0020]
The amount of binder used varies depending on the type, but about 3 to 20 parts by weight is appropriate for 100 parts by weight of natural graphite.
Here, the purpose of using the binder is to shape natural graphite into a substantially spherical shape,
If the purpose can be achieved, it is not necessary to add more amount.
[0021]
Natural graphite and a binder as described above are blended and granulated using a granulator to obtain substantially spherical graphite particles.
The granulator used for granulation should just be generally marketed, and a model in particular is not designated.
According to the characteristics of the granulator to be used, an appropriate method such as a method of mixing in advance or a method of spraying on the powder that is flowing may be selected.
[0022]
The particle size of the granulated graphite particles is not specified as long as it can be used as a negative electrode material. Generally, the average particle size is suitably 10 to 30 μm, and the maximum particle size is about 100 μm.
Preferably, it does not exceed 80 μm.
[0023]
The shape of the graphite particles is substantially spherical, and the sphericity is not limited, but the ratio of the major axis to the minor axis is
If it is about 2 or less, there is no problem.
In contrast, it is difficult to selectively produce those having a ratio of 2 or more by an ordinary granulation method.
[0024]
Here, in order to maintain the shape of the granulated graphite particles, it is preferable to perform a curing treatment as necessary. For example, when a thermosetting resin such as phenol resin is used for the binder, it is preferable to perform the curing process at about 150 ° C.
[0025]
Next, a binder pitch is added to the graphite particles granulated into a substantially spherical shape as described above, and the mixture is heated and mixed.
A binder pitch having a softening point of about 80 to 150 ° C. is appropriate.
The heating and mixing are performed at 100 to 250 ° C., and the molten pitch is impregnated into the voids in the graphite particles, and at the same time, the particle surface layer is coated.
[0026]
The apparatus used for mixing is not limited, but generally a heating kneader is suitable for mass production and is desirable.
[0027]
The mixing ratio is preferably 3 to 30 parts by weight of the binder pitch with respect to 100 parts by weight of the graphite particles.
If it is less than 3 parts by weight, there is no mixed effect, and the charge / discharge efficiency, PC resistance and rapid chargeability of the finally obtained negative electrode material are lowered.
On the other hand, if it exceeds 30 parts by weight, the amount of pitch becomes excessive,
The graphite particles are fixed to each other, and pulverization after firing is required.
As a result, the coating effect on the surface of the graphite particles decreases, the capacity loss increases in the negative electrode material, or the pitch-derived carbon having a lower crystallinity than natural graphite increases.
There arises a problem that the discharge capacity is lowered.
[0028]
After heating and mixing, firing is performed in a non-oxidizing gas atmosphere such as nitrogen, and the pitch portion is carbonized to obtain the negative electrode material of the present invention.
The firing temperature is preferably 800 to 1400 ° C, and most preferably 800 to 1100 ° C.
If it is less than 800 ° C., removal of volatile components becomes insufficient, and inconveniences such as capacity deterioration due to storage occur, and if it is 1400 ° C. or more, the cost is high for mass production.
[0029]
As described above, the lithium secondary battery negative electrode material is obtained by the production method of the present invention.
[0030]
【The invention's effect】
According to the present invention, the carbon negative electrode material for a lithium secondary battery makes use of the characteristics of natural graphite as a base material to make a material with a high discharge capacity. A negative electrode material excellent in PC property can be obtained.
Further, the method for producing a negative electrode material of the present invention does not require graphitization at 2800 ° C. or higher as final heat treatment, and baking at 800 to 1400 ° C. is sufficient, so that it can be produced at low cost and cost per mass production. The advantage in terms is great.
[0031]
Examples and Comparative Examples
[Example 1]
Granule molding is carried out using 3 parts by weight of ethyl cellulose (manufactured by Dow Chemical Co.) as a binder to 100 parts by weight of Chinese scale-like natural graphite having an average particle diameter of 15 μm and an ash content of 0.15%, and an average particle diameter of 23 μm The substantially spherical forming body (the ratio of the major axis to the minor axis is distributed in the range of about 1.1 to 2.0).
5 parts by weight of a coal pitch having a softening point of 110 ° C. was blended with 100 parts by weight of the granulated molded body, and the mixture was heated and mixed for 2 hours while heating to 150 ° C. in a biaxial kneader.
Next, this mixture was heat-treated at a maximum temperature of 1000 ° C. for 6 hours under a nitrogen atmosphere to obtain a fired product, and then allowed to cool.
The fired product has no strong fusion between particles, and can be easily crushed with a quick mill (manufactured by Seishin Enterprise Co., Ltd.), thereby obtaining a carbon negative electrode material for a lithium ion secondary battery. .
The obtained negative electrode material had an average particle diameter of 23 μm, a specific surface area of 4.3 m 2 / g, and a tap density of 0.94 g / cm 3 .
[0032]
Next, using this negative electrode material, a battery was prepared as follows, and the battery characteristics were evaluated.
Originally, carbon powder is used as a negative electrode, but in the present invention, lithium metal was used for the counter electrode, and thus the characteristics of the battery were evaluated using the positive electrode.
The electrode was produced by adding N-methyl-2-pyrrolidone to 100 parts by weight of carbon powder and 10 parts by weight of polyvinylidene fluoride to form a paste, and then applying it to a copper foil using a doctor blade. Dried.
After drying, this was punched out in a circular shape so as to have an area of 1 cm 2 , and further pressed with a pressure of 1 ton / cm 2 to adjust the electrode.
A coin cell was assembled using lithium metal as a counter electrode and a reference electrode, and using 1M LiPF6 / EC: MEC (volume ratio 1: 1) as an electrolyte.
[0033]
Charging was performed at a current density of 0.5 mA / cm 2 and then switched to constant voltage charging at 10 mV and terminated at 0.01 mA.
The discharge was performed at a current density of 0.5 mA / cm 2 up to a constant current discharge of 1.5V. Further, the discharge capacity was measured by changing the discharge rate. The measurement temperature is 30 ° C.
The measurement results were a discharge capacity of 371 mAh / g and a battery efficiency of 90%.
Further, the discharge capacity and the discharge capacity retention rate with different discharge rates (C) are shown in FIGS. 1 and 2, respectively.
[0034]
[Example 2]
A negative electrode material was obtained in the same manner as in Example 1 except that 15 parts by weight of a caltar pitch having a softening point of 110 ° C. was used.
The obtained negative electrode material had an average particle diameter of 23 μm, a specific surface area of 3.8 m 2 / g, and a tap density of 0.97 / cm 3 .
Using this negative electrode material, coin cells were assembled in the same manner as in Example 1, and the battery characteristics were measured. As a result, the discharge capacity was 361 mAh / g, and the battery efficiency was 92%.
Further, the discharge capacity and the discharge capacity retention rate with different discharge rates (C) are shown in FIGS. 1 and 2, respectively.
Here, the discharge rate (C) means 1C = 1 hour discharge, 2C = 0.5 hour, 0.
Since 2C = 5 hours of discharge, 0.2C (5 hours of discharge) is usually used as the standard rate.
When the rate is increased to 1C, 2C, 3C, the discharge time is shortened, and the discharge is performed at a higher current density, which means that the load is increased.
Therefore, even if the C-rate value increases, the decrease in the discharge capacity is small, which means that the rapid discharge characteristics are good.
[0035]
[Comparative Example 1]
Using 100 parts by weight of Chinese scale-like natural graphite with an average particle size of 15 μm and an ash content of 0.15% as a binder, 5 parts by weight of phenol resin (manufactured by Gunei Chemical Industry Co., Ltd.) is used for granulation molding. This was done to adjust a substantially spherical formation having an average particle size of 23 μm (the ratio of the major axis to the minor axis was distributed in the range of about 1.1 to 2.0).
The molded body was heat-cured at 150 ° C. in air, held at 1000 ° C. for 6 hours in a nitrogen atmosphere to obtain a fired product, and then allowed to cool to obtain a negative electrode material.
As a result of assembling a coin cell using the obtained negative electrode material and measuring the battery characteristics in the same manner as in Example 1, the discharge capacity was 371 mAh / g, and the battery efficiency was 88%. Further, the discharge capacity and the discharge capacity retention rate with different discharge rates (C) are shown in FIGS. 1 and 2, respectively.
When Examples 1 and 2 are compared with Comparative Example 1, both use natural graphite.
Although high discharge capacity is shown, it is less than 90% in the comparative example in terms of battery efficiency. (See Table 1)
Since the battery efficiency is evaluated as having a large capacity loss when the battery efficiency is less than 90%, it can be seen that the battery efficiency of the example is also excellent in the battery efficiency.
As can be seen from FIG. 1 and FIG. 2, the battery of the example clearly maintains a higher discharge capacity and discharge capacity retention rate than the comparative example as the discharge rate increases, and is excellent in rapid discharge performance. You can see that
[0036]
[Example 3]
A coin cell was prepared in the same manner as in Example 1 except that the electrolyte was changed to 1M LiPF6 / PC: EC: MEC (volume ratio 1: 1: 1) using the negative electrode material obtained in Example 1. As a result of measuring the assembled battery characteristics, the discharge capacity was 372 mAh / g, and the battery efficiency was 86%.
[0037]
[Example 4]
Using the negative electrode material obtained in Example 2, the coin cell was changed in the same manner as in Example 1 except that the electrolyte was changed to 1M LiPF6 / PC: EC: MEC (volume ratio 1: 1: 1). As a result of assembling and measuring battery characteristics, the discharge capacity was 364 mA / g, and the battery efficiency was 87%.
[0038]
[Comparative Example 2]
A coin cell was prepared in the same manner as in Example 1 except that the electrolyte was changed to 1M LiPF6 / PC: EC: MEC (volume ratio 1: 1: 1) using the negative electrode material obtained in Comparative Example 1. As a result of measuring the assembled battery characteristics, the discharge capacity was 366 mAh / g, and the battery efficiency was 75%. (See Table 2)
Here, Examples 3 and 4 and Comparative Example 2 all use PC as the electrolyte.
In general, in the graphite negative electrode material, even if the discharge rate is the same, when PC is added to the electrolyte,
As the amount added increases, the amount of PC decomposed on the surface of the negative electrode material increases, resulting in a phenomenon that the battery efficiency decreases.
Comparing the example and the comparative example, it can be seen that the example suppresses the decomposition of the PC and only slightly reduces the battery efficiency.
[0039]
[Table 1]
[Table 2]
[Brief description of the drawings]
FIG. 1 is a diagram showing a discharge capacity at each discharge rate (C).
FIG. 2 is a diagram showing a discharge capacity retention rate at each discharge rate (C).
Claims (2)
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