JP3807691B2 - Negative electrode for lithium secondary battery and method for producing the same - Google Patents
Negative electrode for lithium secondary battery and method for producing the same 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】
【発明の属する技術分野】
本発明は活物質を炭素質材料から構成したリチウム二次電池用負極及びその製造方法に関する。
【０００２】
【従来の技術】
この種のリチウム二次電池は、負極の炭素質材料にリチウムイオンが出入りして充放電がなされるため、安全性が高いという利点があり、近年、多くの種類のものが開発されている。負極の炭素質材料としては充放電容量の面からグラファイトが最も好ましいとされているが、これはプロピレンカーボネート（ＰＣ）を主体とする電解液に対して反応性があるため、低温特性が悪いエチレンカーボネート（ＥＣ）等を使用せざるを得ず、この点の改良が待たれていた。
【０００３】
グラファイトのＰＣに対する反応性を抑制する技術としては、例えば特開平５−１２１０６６号公報に記載された発明が公知である。これは、グラファイト粒子を石油ピッチ等と混合した後に焼成することにより、グラファイト粒子の表面に無定型炭素の被覆を形成するものである。このような無定形炭素被覆のグラファイトを活物質として使用した負極では、ＰＣとの反応性が抑えられて低温特性に優れたリチウム二次電池を構成することができる。
【０００４】
【発明が解決しようとする課題】
しかしながら、上述の炭素質材料を使用した負極では、電池として構成したときの高温保存特性が悪いという問題があった。すなわち、電池を高温度に保存した場合に、電池容量が著しく低下してしまうのである。これは、次のような原因によると推定される。
【０００５】
すなわち、従来の方法で製造した負極の活物質では、グラファイトと無定形炭素或いは低結晶性炭素からなる皮膜との界面においてグラファイト層と皮膜層との間で十分な化学結合が持たれておらず、グラファイトのエッジ面の炭素原子が不飽和結合の状態で多く残留している。従って、この残留不飽和結合炭素が、高温度になると電解液中のリチウムイオンと反応して充放電に寄与するリチウムイオン量を減らしてしまうのである。
【０００６】
しかも、従来製法では、グラファイトを覆う皮膜の厚さが相当に厚くなってしまうため、充放電容量の少ない皮膜成分が不必要に増えて活物質のエネルギー密度を低下させてしまうという問題もあった。
【０００７】
かかる事情にあるから、無定形炭素や低結晶性炭素などによる耐ＰＣ性皮膜を形成するためには、エッジ面の不飽和炭素のほとんどと化学的に結合し、しかも、できるだけ薄い皮膜を形成することが望ましい。しかし、グラファイト粒子の表面にいわは物理的なコーティングによって低結晶性炭素の被覆を形成するという特開平５−１２１０６６号公報に記載された従来の製造方法では、低結晶性炭素の膜厚を十分に薄くすることは困難である。また、本発明者らは、グラファイトを高温の不活性雰囲気において炭化水素を導入し、炭化水素の熱分解によってグラファイト表面に低結晶性炭素の皮膜を成長させることも試みたが、これでは薄い膜厚の被覆を形成できるものの、電解液との反応性の抑制が十分に行われないという問題が生ずるものであった。
【０００８】
本発明は上記事情に鑑みてなされたもので、その目的は、低温性能及び高温保存特性を改良でき、しかも、容量増大も可能にできるリチウム二次電池用負極及びその製造方法を提供することを目的とする。
【０００９】
【課題を解決するための手段及びその作用・効果】
請求項１の発明は、負極の活物質の製造に際し、グラファイトを非酸化雰囲気で加熱して表面の酸素を取り除く活性化処理を行い、その後、高温不活性雰囲気下で有機物を導入して活性化グラファイトの表面に低結晶性炭素又は炭素化合物の皮膜を形成するところに特徴を有する。
上記発明において、グラファイトを非酸化雰囲気で高温度に加熱すると、表面に結合している酸素がＣＯ又はＣＯ2 の形で除去され、活性化された炭素原子が露出する。その加熱温度については、グラファイト表面の汚れ状態や生産性等を考慮して種々設定することができるが、８００℃〜１２００℃程度とすることが好ましい。
【００１０】
この後、高温不活性雰囲気下で例えば炭化水素等の有機物を導入すると、その有機物の熱分解により生成した炭素がグラファイト粒子の表面に析出して低結晶性炭素又は炭素化合物の薄膜が形成される。この場合の加熱温度及び加熱時間も生産性等を考慮して設定することができるが、６００℃〜１２００℃程度で３０分から９０分程度が好ましく、有機物は例えば窒素ガス等の不活性ガスと共に炉内に導入することが好ましい。
【００１１】
このようにして低結晶性炭素又は炭素化合物により被覆されたグラファイトを活物質とし、例えばこれにバインダを添加して集電体に塗布して乾燥させることによりリチウム二次電池の負極を構成することができる。この負極によれば、グラファイトの表面が低結晶性炭素又は炭素化合物の皮膜により覆われており、グラファイト部分にまでＰＣ電解液が侵入できないため、ＰＣの反応を抑えることができる。また、グラファイト表面の炭素をいったん活性化してから有機物の熱分解によって低結晶性炭素又は炭素化合物をグラファイト表面に結合させるから、化学結合が強固で均一な皮膜を形成することができ、さらにグラファイト表面の未結合の炭素原子（不対電子を有する炭素原子）の数を大幅に減らしてグラファイトと低結晶性炭素又は炭素化合物との間のＣ−Ｃ結合を増大させることができる。このことは、低結晶性炭素又は炭素化合物の被覆の膜厚を薄くしても電解液との反応性を十分に抑えることができることを意味し、充放電容量が少ない部分の膜厚が薄い分、活物質のエネルギー密度を高めて充放電容量を増大させることができる。
しかも、この方法で製造された低結晶性炭素又は炭素化合物の皮膜で覆ったグラファイトでは、上述のようにグラファイト層と低結晶性炭素層又は炭素化合物との間の結合度が高まるから、サイクル特性も大きく向上する。
【００１２】
請求項２の発明は、上記発明の活性化処理に先だってグラファイト表面を酸又はアルカリ液により洗浄する清浄化処理を行うところに特徴を有する。この清浄化処理を行うと、グラファイト表面に付着・吸着或いは結合している不純物が除去されるから、活性化処理がより効率的に行われて一層の好結果が得られる。なお、洗浄液としては、硝酸や硫酸等の強酸が最も好ましい。
【００１３】
ところで、本発明者らの研究によれば、低結晶性炭素被覆を構成する炭素原子の一部を窒素原子で置換した炭素化合物であるＣxＮの皮膜とすると、化学的により安定となって高温保存特性が改善されることが発見された。
【００１４】
このＣxＮの皮膜は、上述した請求項１又は２の発明において、有機物として窒素を含有する有機物を使用すると効率的に形成することができ（請求項３の発明）、特にアセトニトリル蒸気を窒素ガスと共に導入することが最も好ましい。なお、グラファイト粒子の表面に形成されたＣxＮの皮膜は、ＸＰＳ分析（Ｘ線光電子分光法）によって確認することができる。ＸＰＳ分析によっては表面がＣxＯ，ＣyＮＯzという成分として分析されるが、測定深度を深めて行くとＯ，Ｎの相対量が減少してゆくから、ＣxＮの表面皮膜が形成されていることが明らかである。
【００１５】
【実施例】
以下、本発明のいくつかの実施例を比較例とともに説明する。ここで、グラファイト粒子は平均粒径７μｍの天然黒鉛を使用している。
【００１６】
［実施例１］
まず、グラファイト粒子を９４％硝酸によって洗浄してその表面の不純物を除去するとともに、表面を酸化させた（清浄化処理）。次に、そのグラファイトを非酸化雰囲気で加熱して表面の酸素を取り除く活性化処理を行った。これは高温還元炉又は高温真空炉にグラファイトを収容して９５０℃に加熱することにより行い、これにてグラファイト表面の酸素原子がＣＯ又はＣＯ2 の形で取り除かれて活性な炭素原子が表面に露出する。
【００１７】
次に、その活性化グラファイトを外気に触れさせることなく窒素雰囲気炉に導入して９５０℃に加熱しておき、ここにアセトニトリル蒸気を窒素ガスとともに導入した。このときアセトニトリルの分圧は０．９×１０4Ｐａで、全圧は１×１０5Ｐａであり、処理時間は３０分から１時間３０分であった。これにより、活性化グラファイトの表面にＣxＮの皮膜が形成された。
【００１８】
このグラファイト粒子の９２重量部に対して、Ｎ−メチルピロリドンを溶媒とした１０重量％濃度のポリ弗化ビニリデン溶液をポリ弗化ビニリデンが８重量部となるように加えて攪拌し、これを負極ペーストとした。この負極ペーストを厚さ３０μｍの銅箔に厚みが１００μｍとなるように塗布した後、１００℃で２時間真空乾燥した。これを２cm角に切り出して多孔度が２５％となるようにプレスして試験極とした。
相手極には金属リチウム板を用い、これらを例えばポリエチレン製の微孔膜セパレータを挟んで周知の電池構造とし、ここにプロピレンカーボネート（ＰＣ）とエチレンカーボネート（ＥＣ）との体積比１：１の混合物に１モルＬi ＰＦ6を混合した電解液を注液してリチウム二次電池を構成した。
【００１９】
［実施例２］
上述のアセトニトリル蒸気に代えて炭化水素（ベンゼン又はフェノール）を使用したところが相違し、その他は同一条件である。
【００２０】
［比較例］
キノリン１５重量部に３重量部のバインダーピッチを溶解させ、実施例１，２と同一のグラファイト粒子を１重量部浸漬し、次いでキノリンを蒸発させた後に、窒素ガス中で４００℃で焼成した。このようにして製造したグラファイトを使用したところが相違し、その他は実施例１と同一条件である。
【００２１】
［高温保存特性の評価］
上記３種のリチウム二次電池を０．２ｍＡ／cm2 の定電流で負極電位が金属リチウム正極に対して０Ｖになるまで充電した後に、４５℃で３０日間放置した。その後、室温で０．２ｍＡ／cm2 の放電電流によって放電させて測定した場合の残存容量と、初期容量との比率（容量保持率％）を計算した。測定結果は、次の表１のようであり、特にＣxＮ皮膜を形成した実施例１が格段に優れた高温保存性を示した。
【００２２】
【表１】
【００２３】
【表２】
BACKGROUND OF THE INVENTION
The present invention relates to a negative electrode for a lithium secondary battery in which an active material is composed of a carbonaceous material and a method for producing the same.
[0002]
[Prior art]
This type of lithium secondary battery has the advantage of high safety because lithium ions enter and leave the carbonaceous material of the negative electrode and is charged and discharged. In recent years, many types of lithium secondary batteries have been developed. As the carbonaceous material for the negative electrode, graphite is most preferable from the viewpoint of charge / discharge capacity, but this is reactive to electrolytes mainly composed of propylene carbonate (PC), so that ethylene has poor low-temperature characteristics. Carbonate (EC) or the like had to be used, and improvement of this point was awaited.
[0003]
As a technique for suppressing the reactivity of graphite to PC, for example, an invention described in Japanese Patent Laid-Open No. 5-121066 is known. In this method, the graphite particles are mixed with petroleum pitch or the like and then fired to form an amorphous carbon coating on the surface of the graphite particles. A negative electrode using such amorphous carbon-coated graphite as an active material can constitute a lithium secondary battery that has low reactivity with PC and is excellent in low-temperature characteristics.
[0004]
[Problems to be solved by the invention]
However, the negative electrode using the above-mentioned carbonaceous material has a problem that the high-temperature storage characteristics when it is configured as a battery are poor. That is, when the battery is stored at a high temperature, the battery capacity is significantly reduced. This is presumed to be caused by the following causes.
[0005]
That is, the negative electrode active material manufactured by the conventional method does not have a sufficient chemical bond between the graphite layer and the coating layer at the interface between the graphite and the coating made of amorphous carbon or low crystalline carbon. Many carbon atoms on the edge surface of graphite remain in an unsaturated bond state. Therefore, when the residual unsaturated bonded carbon reaches a high temperature, it reacts with the lithium ions in the electrolytic solution to reduce the amount of lithium ions contributing to charge / discharge.
[0006]
Moreover, in the conventional manufacturing method, the thickness of the film covering the graphite is considerably increased, so that there is a problem that the film component having a small charge / discharge capacity is unnecessarily increased and the energy density of the active material is lowered. .
[0007]
Under such circumstances, in order to form a PC-resistant film made of amorphous carbon, low crystalline carbon, etc., it is chemically bonded to most of the unsaturated carbon on the edge surface, and as thin as possible. It is desirable. However, in the conventional manufacturing method described in Japanese Patent Laid-Open No. 5-121066 in which the surface of the graphite particle is formed with a low crystalline carbon coating by physical coating, the film thickness of the low crystalline carbon is sufficiently increased. It is difficult to make it thinner. In addition, the present inventors tried to grow a low crystalline carbon film on the graphite surface by introducing hydrocarbons into graphite in a high-temperature inert atmosphere and thermally decomposing the hydrocarbons. Although a thick coating could be formed, there was a problem that the reactivity with the electrolytic solution was not sufficiently suppressed.
[0008]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode for a lithium secondary battery that can improve low-temperature performance and high-temperature storage characteristics and can also increase capacity, and a method for manufacturing the same. Objective.
[0009]
[Means for solving the problems and their functions and effects]
According to the first aspect of the present invention, in the production of the negative electrode active material, the graphite is heated in a non-oxidizing atmosphere to carry out an activation treatment to remove surface oxygen, and then activated by introducing an organic substance under a high-temperature inert atmosphere. It is characterized in that a film of low crystalline carbon or a carbon compound is formed on the surface of graphite.
In the above invention, when graphite is heated to a high temperature in a non-oxidizing atmosphere, oxygen bonded to the surface is removed in the form of CO or CO2, and activated carbon atoms are exposed. The heating temperature can be variously set in consideration of the dirt state of the graphite surface, productivity, and the like, but is preferably about 800 ° C to 1200 ° C.
[0010]
Thereafter, when an organic substance such as hydrocarbon is introduced under a high-temperature inert atmosphere, carbon generated by thermal decomposition of the organic substance is deposited on the surface of the graphite particles, and a thin film of low crystalline carbon or carbon compound is formed. . In this case, the heating temperature and the heating time can also be set in consideration of productivity and the like, but it is preferably about 600 to 1200 ° C. and about 30 to 90 minutes, and the organic matter is a furnace together with an inert gas such as nitrogen gas. It is preferable to introduce into the inside.
[0011]
Thus, a negative electrode of a lithium secondary battery is formed by using graphite coated with low crystalline carbon or a carbon compound as an active material, for example, adding a binder to the active material, and applying and drying the current collector. Can do. According to this negative electrode, the surface of the graphite is covered with a film of low crystalline carbon or a carbon compound, and the PC electrolyte cannot penetrate into the graphite portion, so that the PC reaction can be suppressed. In addition, the carbon on the graphite surface is activated once, and the low crystalline carbon or carbon compound is bonded to the graphite surface by pyrolysis of organic matter, so that a uniform film with a strong chemical bond can be formed. The number of unbonded carbon atoms (carbon atoms with unpaired electrons) can be greatly reduced to increase the CC bond between graphite and low crystalline carbon or carbon compounds. This means that the reactivity with the electrolyte can be sufficiently suppressed even if the film thickness of the low crystalline carbon or carbon compound coating is reduced. In addition, the energy density of the active material can be increased to increase the charge / discharge capacity.
Moreover, in the graphite covered with the film of low crystalline carbon or carbon compound produced by this method, since the degree of bonding between the graphite layer and the low crystalline carbon layer or carbon compound is increased as described above, the cycle characteristics are increased. Is greatly improved.
[0012]
The invention of claim 2 is characterized in that, prior to the activation treatment of the above-described invention, a cleaning treatment for washing the graphite surface with an acid or an alkali solution is performed. When this cleaning process is performed, impurities adhering, adsorbing, or bonding to the graphite surface are removed, so that the activation process is performed more efficiently and a better result is obtained. The cleaning liquid is most preferably a strong acid such as nitric acid or sulfuric acid.
[0013]
By the way, according to the study by the present inventors, when a film of CxN, which is a carbon compound in which a part of carbon atoms constituting the low crystalline carbon coating is substituted with nitrogen atoms, is chemically stable and stored at high temperature. It has been discovered that the properties are improved.
[0014]
This CxN film can be formed efficiently when an organic substance containing nitrogen is used as the organic substance in the invention of claim 1 or 2 described above (invention of claim 3), and particularly acetonitrile vapor together with nitrogen gas. Most preferably, it is introduced. The CxN film formed on the surface of the graphite particles can be confirmed by XPS analysis (X-ray photoelectron spectroscopy). Depending on the XPS analysis, the surface is analyzed as CxO, CyNOz, but the relative amount of O and N decreases as the measurement depth increases, so it is clear that a CxN surface film is formed. is there.
[0015]
【Example】
Hereinafter, some examples of the present invention will be described together with comparative examples. Here, natural graphite having an average particle diameter of 7 μm is used as the graphite particles.
[0016]
[Example 1]
First, the graphite particles were washed with 94% nitric acid to remove impurities on the surface, and the surface was oxidized (cleaning treatment). Next, the graphite was heated in a non-oxidizing atmosphere to perform activation treatment for removing oxygen on the surface. This is done by placing graphite in a high-temperature reduction furnace or high-temperature vacuum furnace and heating to 950 ° C., whereby oxygen atoms on the graphite surface are removed in the form of CO or CO 2 and active carbon atoms are exposed on the surface. To do.
[0017]
Next, the activated graphite was introduced into a nitrogen atmosphere furnace without being exposed to the outside air and heated to 950 ° C., and acetonitrile vapor was introduced into the furnace together with nitrogen gas. At this time, the partial pressure of acetonitrile was 0.9 × 10 4 Pa, the total pressure was 1 × 10 5 Pa, and the treatment time was 30 minutes to 1 hour 30 minutes. As a result, a CxN film was formed on the surface of the activated graphite.
[0018]
To 92 parts by weight of the graphite particles, a polyvinylidene fluoride solution having a concentration of 10% by weight using N-methylpyrrolidone as a solvent was added to 8 parts by weight of polyvinylidene fluoride, and the resulting mixture was stirred. A paste was used. This negative electrode paste was applied to a copper foil having a thickness of 30 μm so as to have a thickness of 100 μm, and then vacuum-dried at 100 ° C. for 2 hours. This was cut into 2 cm squares and pressed to a porosity of 25% to obtain test electrodes.
A metal lithium plate is used for the counter electrode, and these are made into a well-known battery structure with, for example, a polyethylene microporous membrane separator interposed therebetween, and the volume ratio of propylene carbonate (PC) and ethylene carbonate (EC) is 1: 1. An electrolytic solution in which 1 mol LiPF 6 was mixed with the mixture was injected to constitute a lithium secondary battery.
[0019]
[Example 2]
The difference is that a hydrocarbon (benzene or phenol) is used instead of the above-mentioned acetonitrile vapor, and the other conditions are the same.
[0020]
[Comparative example]
3 parts by weight of binder pitch was dissolved in 15 parts by weight of quinoline, 1 part by weight of the same graphite particles as in Examples 1 and 2 was immersed, and then the quinoline was evaporated, followed by firing at 400 ° C. in nitrogen gas. The use of the graphite thus produced is different, and the other conditions are the same as in Example 1.
[0021]
[Evaluation of high-temperature storage characteristics]
The three lithium secondary batteries were charged at a constant current of 0.2 mA / cm @ 2 until the negative electrode potential was 0 V with respect to the metal lithium positive electrode, and then allowed to stand at 45 DEG C. for 30 days. Thereafter, the ratio (capacity retention%) between the residual capacity and the initial capacity when measured by discharging with a discharge current of 0.2 mA / cm @ 2 at room temperature was calculated. The measurement results are as shown in Table 1 below, and in particular, Example 1 in which the CxN film was formed showed remarkably excellent high-temperature storage stability.
[0022]
[Table 1]
[0023]
[Table 2]
Claims (5)
負極の活物質は、グラファイトを非酸化雰囲気で加熱して表面の酸素を取り除く活性化処理を行い、その後、高温の不活性雰囲気下で有機物を導入して活性化グラファイトの表面に炭素又は炭素化合物からなる皮膜を形成することを特徴とするリチウム二次電池用負極の製造方法。A method for producing a negative electrode for a lithium secondary battery used together with a non-aqueous electrolyte of a lithium secondary battery,
Active material of the negative electrode, performs activation processing for removing oxygen surface by heating the graphite in a non-oxidizing atmosphere, then charcoal Motomata the surface of activated graphite to introduce organics under high-temperature inert atmosphere carbon The manufacturing method of the negative electrode for lithium secondary batteries characterized by forming the membrane | film | coat consisting of a compound.
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| JP14430197A JP3807691B2 (en) | 1997-06-02 | 1997-06-02 | Negative electrode for lithium secondary battery and method for producing the same |
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| JP14430197A JP3807691B2 (en) | 1997-06-02 | 1997-06-02 | Negative electrode for lithium secondary battery and method for producing the same |
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| JPH10334916A JPH10334916A (en) | 1998-12-18 |
| JP3807691B2 true JP3807691B2 (en) | 2006-08-09 |
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| CN104276566A (en) * | 2014-09-09 | 2015-01-14 | 刘剑洪 | Graphene and preparation method thereof |
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| JP5731732B2 (en) * | 2007-10-17 | 2015-06-10 | 日立化成株式会社 | Carbon-coated graphite negative electrode material for lithium ion secondary battery, production method thereof, negative electrode for lithium ion secondary battery using the negative electrode material, and lithium ion secondary battery |
| WO2018070423A1 (en) | 2016-10-13 | 2018-04-19 | 国立研究開発法人産業技術総合研究所 | Lithium ion secondary battery and electric device using same |
| CN116081615A (en) * | 2021-11-08 | 2023-05-09 | 湖南中科星城石墨有限公司 | Artificial graphite negative electrode material, preparation method and application |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN104276566A (en) * | 2014-09-09 | 2015-01-14 | 刘剑洪 | Graphene and preparation method thereof |
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| JPH10334916A (en) | 1998-12-18 |
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