JP2008282550A - Anode for lithium secondary battery and lithium secondary cell using the same - Google Patents
Anode for lithium secondary battery and lithium secondary cell using the same Download PDFInfo
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- JP2008282550A JP2008282550A JP2007123101A JP2007123101A JP2008282550A JP 2008282550 A JP2008282550 A JP 2008282550A JP 2007123101 A JP2007123101 A JP 2007123101A JP 2007123101 A JP2007123101 A JP 2007123101A JP 2008282550 A JP2008282550 A JP 2008282550A
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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
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
Description
本発明は、リチウム二次電池用負極およびリチウム二次電池に関し、特に容量を改善したリチウム二次電池用負極およびリチウム二次電池に関する。 The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery, and more particularly to a negative electrode for a lithium secondary battery and a lithium secondary battery with improved capacity.
リチウム二次電池などの非水電解液二次電池は、近年、携帯電話、ノート型パソコンなどの電源として広く用いられている。これらの非水電解液二次電池は、従来のアルカリ蓄電池などの二次電池に比べて、体積、あるいは重量容量密度が大きく、しかも高電圧を取り出すことが可能であるので、小型の機器用の電源として広く採用され、今日のモバイル機器の発展に大きく寄与している。 In recent years, non-aqueous electrolyte secondary batteries such as lithium secondary batteries have been widely used as power sources for mobile phones, notebook computers, and the like. These non-aqueous electrolyte secondary batteries are larger in volume or weight capacity density than secondary batteries such as conventional alkaline storage batteries and can take out a high voltage. Widely adopted as a power source, it greatly contributes to the development of today's mobile devices.
このような非水電解液二次電池においては、負極活物質として従来よりリチウムイオンの吸蔵、放出が可能な黒鉛や非晶質炭素などの炭素材料が広く利用されている。ここで、炭素材料を用いて負極を作製する場合には、炭素材料と結着剤とを溶剤中で混合したスラリーを銅箔などの負極集電体上に塗布、乾燥して、負極合剤層を形成していた。 In such a non-aqueous electrolyte secondary battery, a carbon material such as graphite or amorphous carbon that can occlude and release lithium ions has been widely used as a negative electrode active material. Here, when producing a negative electrode using a carbon material, a slurry obtained by mixing a carbon material and a binder in a solvent is applied onto a negative electrode current collector such as a copper foil, and dried to form a negative electrode mixture. A layer was formed.
従来、結着剤としては、ポリビニリデンフルオロライド（ＰＶｄＦ）が広く用いられている。一般にＰＶｄＦなどの結着剤は充放電容量を持たないため、負極合剤中の結着剤の量を減らした方が負極充放電容量は大きくなる。しかしながら、負極合剤中の結着剤の量を減らすと、活物質同士の結着性および集電体と活物質との接着性が弱くなるため、充放電サイクル寿命が低下するなどの問題が生じる。このため、結着剤の量は充放電サイクル寿命の劣化などの問題を引き起こさない必要最小量に設定され、通常は負極合剤中の重量％で２〜２０％の範囲に設定される。 Conventionally, polyvinylidene fluoride (PVdF) has been widely used as a binder. In general, a binder such as PVdF does not have a charge / discharge capacity, and therefore the negative electrode charge / discharge capacity increases when the amount of the binder in the negative electrode mixture is reduced. However, if the amount of the binder in the negative electrode mixture is reduced, the binding property between the active materials and the adhesive property between the current collector and the active material are weakened. Arise. For this reason, the amount of the binder is set to a necessary minimum amount that does not cause problems such as deterioration of the charge / discharge cycle life, and is usually set to a range of 2 to 20% by weight% in the negative electrode mixture.
一方、結着剤としてポリイミドを用いたものが例えば、特許文献１および２などにおいて開示されている。負極活物質として炭素粉末を、結着剤としてポリイミドを用いて一体化した負極が記載されている。一方、特許文献３および４などには、負極活物質としてケイ素またはケイ素合金を含み、結着剤としてポリイミドを使用する提案がなされている。
On the other hand, those using polyimide as a binder are disclosed in
しかしながら、従来技術を記載したこれらの特許文献においては、充放電サイクル寿命を向上させることを目的に、ポリイミドを負極の結着剤として用いることは記載されているものの、初期充放電容量の向上に関する記載はない。従来技術においては、負極合剤における結着剤は充放電容量を示さないため、結着剤の使用による負極充放電容量の低下を招いていた。 However, in these patent documents describing the prior art, although it is described that polyimide is used as a binder for the negative electrode for the purpose of improving the charge / discharge cycle life, it relates to the improvement of the initial charge / discharge capacity. There is no description. In the prior art, since the binder in the negative electrode mixture does not exhibit charge / discharge capacity, the negative electrode charge / discharge capacity is reduced due to the use of the binder.
本発明は上記課題に鑑みてなされたものであり、負極合剤中の結着剤として用いられるポリイミドが充放電容量を発現する構成とすることにより、充放電容量を向上させたリチウム二次電池用負極およびリチウム二次電池を提供することを目的とする。 The present invention has been made in view of the above problems, and a lithium secondary battery having improved charge / discharge capacity by adopting a configuration in which polyimide used as a binder in a negative electrode mixture exhibits charge / discharge capacity. An object of the present invention is to provide a negative electrode and a lithium secondary battery.
上記課題を解決するため、本発明のリチウム二次電池用負極は、ポリイミドと炭素質粉末からなる導電性付与剤とが重量比で６０：４０〜３０：７０の範囲で混合されたことを特徴とする。 In order to solve the above problems, a negative electrode for a lithium secondary battery according to the present invention is characterized in that polyimide and a conductivity imparting agent made of carbonaceous powder are mixed in a weight ratio of 60:40 to 30:70. And
本発明のリチウム二次電池用負極において前記ポリイミドがリチウムイオンを吸蔵・放出し、４００ｍＡｈ/ｇ以上１５００ｍＡｈ/ｇ以下の放電容量を有することが好ましい。 In the negative electrode for a lithium secondary battery of the present invention, the polyimide preferably occludes / releases lithium ions and has a discharge capacity of 400 mAh / g or more and 1500 mAh / g or less.
本発明のリチウム二次電池用負極は、前記導電性付与剤と、溶媒中に溶解させたポリイミド前駆体とを混合したスラリー溶液を、負極集電体表面に塗工、乾燥後、熱処理により前記ポリイミド前駆体をポリイミド化することを特徴とする。 The negative electrode for a lithium secondary battery of the present invention is prepared by applying a slurry solution obtained by mixing the conductivity-imparting agent and a polyimide precursor dissolved in a solvent onto the surface of the negative electrode current collector, drying, and then performing heat treatment. The polyimide precursor is converted into a polyimide.
本発明のリチウム二次電池用負極は、リチウムイオンを吸蔵・放出する炭素系またはケイ素系の活物質材料とポリイミドとを有し、前記活物質材料と前記ポリイミドの重量比が８０：２０〜９８：２の範囲にあり、且つ前記ポリイミドが４００ｍＡｈ／ｇ以上１５００ｍＡｈ/ｇ以下の放電容量を有することを特徴とする。 The negative electrode for a lithium secondary battery of the present invention has a carbon-based or silicon-based active material that absorbs and releases lithium ions and polyimide, and the weight ratio of the active material to the polyimide is 80:20 to 98. : In the range of 2, and the polyimide has a discharge capacity of 400 mAh / g or more and 1500 mAh / g or less.
また、本発明のリチウム二次電池用負極は、前記リチウムイオンを吸蔵放出する活物質材料と、溶媒中に溶解させたポリイミド前駆体とを混合したスラリー溶液を、負極集電体表面に塗工、乾燥後、熱処理により前記ポリイミド前駆体をポリイミド化することを特徴とする。 Further, the negative electrode for a lithium secondary battery of the present invention is obtained by applying a slurry solution, which is a mixture of the active material that occludes and releases lithium ions, and a polyimide precursor dissolved in a solvent, to the surface of the negative electrode current collector. After drying, the polyimide precursor is converted into a polyimide by heat treatment.
さらに、本発明のリチウム二次電池は前記リチウム二次電池用負極と電解液と正極とを備えたことを特徴とする。 Furthermore, the lithium secondary battery of the present invention includes the above-described negative electrode for a lithium secondary battery, an electrolytic solution, and a positive electrode.
本発明によれば、リチウム二次電池用負極にポリイミドを用いることにより、ポリイミドが負極合剤の結着剤として機能するのみならずポリイミド自身が高い充放電容量を発現するため、リチウム二次電池用負極およびリチウム二次電池の充放電容量を向上させる効果がある。 According to the present invention, by using polyimide as a negative electrode for a lithium secondary battery, not only does the polyimide function as a binder for the negative electrode mixture, but also the polyimide itself exhibits a high charge / discharge capacity. This has the effect of improving the charge / discharge capacity of the negative electrode for the battery and the lithium secondary battery.
次に、本発明の実施の形態について説明する。 Next, an embodiment of the present invention will be described.
（本発明における電池構成）
本発明におけるリチウム二次電池は、正極集電体およびリチウムイオンを吸蔵、放出し得る正極活物質を含有する正極合剤層と、負極集電体およびリチウムイオンを吸蔵、放出する負極活物質を含有する負極合剤層とが、非水電解液、およびこれを含むセパレータを介して対向して配置され、構成されている。
(Battery configuration in the present invention)
The lithium secondary battery according to the present invention includes a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material capable of inserting and extracting lithium ions, and a negative electrode current collector and a negative electrode active material containing and releasing lithium ions. The negative electrode mixture layer to be contained is arranged so as to face the nonaqueous electrolyte solution and a separator including the nonaqueous electrolyte solution.
（集電体）
正極集電体としてはアルミニウム、ステンレス鋼、ニッケル、チタンまたはこれらの合金などを用いることができ、負極集電体としては銅、ステンレス鋼、ニッケル、チタンまたはこれらの合金を用いることができる。
(Current collector)
Aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used as the positive electrode current collector, and copper, stainless steel, nickel, titanium, or an alloy thereof can be used as the negative electrode current collector.
（セパレータ）
セパレータとしては、ポリプロピレン、ポリエチレンなどのポリオレフィン、フッ素樹脂などの多孔性フィルムが用いられる。
(Separator)
As the separator, a polyolefin film such as polypropylene or polyethylene, or a porous film such as a fluororesin is used.
（正極）
正極活物質としては通常リチウム含有複合酸化物が用いられ、具体的にはＬｉＭＯ2（ＭはＭｎ、Ｆｅ、Ｃｏ、Ｎｉより選ばれる１種のみ、または２種以上の混合物であり、一部をＭｇ、Ａｌ、Ｔｉなどその他カチオンで置換してもよい）、ＬｉＭｎ2Ｏ4など汎用の材料を用いることができる。これらから選択された正極活物質を用い、カーボンブラックなどの導電性付与剤、ポリビニリデンフルオライド（ＰＶｄＦ）などの結着剤とともにＮ−メチル−２−ピロリドン（ＮＭＰ）などの溶剤中に分散混練し、これをアルミニウム箔などの正極集電体上に塗布後、溶媒を乾燥させるなどの方法により正極合剤層を得ることができる。
(Positive electrode)
As the positive electrode active material, a lithium-containing composite oxide is usually used. Specifically, LiMO 2 (M is one kind selected from Mn, Fe, Co, Ni, or a mixture of two or more kinds, and a part thereof is used. General-purpose materials such as LiMn 2 O 4 and other cations such as Mg, Al, and Ti may be used. Using a positive electrode active material selected from these, dispersion kneading in a solvent such as N-methyl-2-pyrrolidone (NMP) together with a conductivity imparting agent such as carbon black and a binder such as polyvinylidene fluoride (PVdF) And after apply | coating this on positive electrode electrical power collectors, such as aluminum foil, a positive mix layer can be obtained by methods, such as drying a solvent.
（負極）
本発明の第一の実施の形態のリチウム二次電池用負極は、負極集電体上にポリイミドと炭素質粉末からなる導電性付与剤とを混合して負極合剤層を形成することにより得ることができる。
(Negative electrode)
The negative electrode for a lithium secondary battery according to the first embodiment of the present invention is obtained by mixing a polyimide and a conductivity imparting agent made of carbonaceous powder on a negative electrode current collector to form a negative electrode mixture layer. be able to.
具体的には、導電性付与剤とポリイミド、またはその前駆体を溶解しうるＮ−メチル−２−ピロリドン（ＮＭＰ）などの分散媒で混合したスラリーを、例えば、ホットプレート上にてドクターブレードなどを用いて銅箔などの集電体上に塗布し、溶剤を乾燥した後、プレスなどにより圧縮し、特に前駆体を用いた場合には、加熱処理によりポリイミド化を行い、負極合剤層を形成する。 Specifically, a slurry mixed with a dispersion medium such as N-methyl-2-pyrrolidone (NMP) capable of dissolving a conductivity-imparting agent and polyimide, or a precursor thereof, for example, a doctor blade on a hot plate After applying onto a current collector such as a copper foil using a material, the solvent is dried, and then compressed by a press or the like. In particular, when a precursor is used, a polyimide is formed by heat treatment, and a negative electrode mixture layer is formed. Form.
導電性付与剤としては、カーボンブラック、アセチレンブラックなどの炭素質粉末を用いることができる。 As the conductivity imparting agent, carbonaceous powders such as carbon black and acetylene black can be used.
ポリイミドとしては、直鎖中にイミド結合を含む芳香族ポリイミドであれば特に組成は限定されず、ＮＭＰなどの溶媒に可溶であれば、熱硬化型ポリイミド、熱可塑型ポリイミドのいずれを用いることができる。また、例えば、ポリアミック酸などの前駆体を加熱処理し、ポリイミド化することにより得られるポリイミドなどを用いることができる。製品例としては、宇部興産株式会社製・Ｕ−ワニス、日立化成工業株式会社製・ＨＣＩ、ＯＰＩ、東洋紡績株式会社製・バイロマックス、株式会社アイ、エス、テイ製・Ｐｙｒｅ−Ｍ．Ｌ．、ＳＫＹＢＯＮＤなどが挙げられる。 The composition of the polyimide is not particularly limited as long as it is an aromatic polyimide having an imide bond in the straight chain, and if it is soluble in a solvent such as NMP, either thermosetting polyimide or thermoplastic polyimide should be used. Can do. Further, for example, a polyimide obtained by heat-treating a precursor such as polyamic acid and forming a polyimide can be used. Examples of products include Ube Industries, Ltd., U-Varnish, Hitachi Chemical Co., Ltd., HCI, OPI, Toyobo Co., Ltd., Viromax, I, S, Tei, Pyre-M. L. , SKYBOND and the like.
ここで、ポリイミドと導電性付与剤との重量比は、６０：４０〜３０：７０の範囲とすることが望ましい。通常ポリイミド単独では電気絶縁体であり電気容量は示さないが、このような構成とすることで、導電性付与剤同士の電気的な接触が良好になり、負極合剤層において電子が流れる経路ができ、また、負極合剤は多孔質状であるため電解液が内部に浸透することができる。また、負極合剤内部でポリイミドは分子状態であるいは微小な集合体の状態で存在しているが、これらポリイミドの六員環や窒素、酸素などの原子と作用してリチウムイオンが吸蔵・放出される一方、これらポリイミドと接触している導電性付与剤を通して電子移動がなされ、ポリイミド自身が大きな充放電容量を発現すようになる。即ち、ポリイミドの重量当たり４００ｍＡｈ／ｇ以上、より好ましくは６００ｍＡｈ／ｇ以上の放電容量を示す。なお、上限は明確ではないが１５００ｍＡｈ／ｇ程度が想定される。一方、ポリイミドの重量比が６０％を超えると、負極合剤層の抵抗の増大により充放電容量が低下するため好ましくない。また、３０％未満だと負極合剤中のポリイミドの割合が減少し、負極容量が低下するため好ましくない。 Here, the weight ratio between the polyimide and the conductivity-imparting agent is preferably in the range of 60:40 to 30:70. Normally, polyimide alone is an electrical insulator and does not show electric capacity, but with such a configuration, electrical contact between the conductivity imparting agents becomes good, and there is a path through which electrons flow in the negative electrode mixture layer. In addition, since the negative electrode mixture is porous, the electrolytic solution can penetrate inside. In addition, polyimide exists in the negative electrode mixture in the molecular state or in the form of minute aggregates. Lithium ions are occluded and released by acting on the polyimide's six-membered ring and atoms such as nitrogen and oxygen. On the other hand, electrons are transferred through the conductivity-imparting agent in contact with the polyimide, and the polyimide itself exhibits a large charge / discharge capacity. That is, the discharge capacity is 400 mAh / g or more, more preferably 600 mAh / g or more per weight of polyimide. Although the upper limit is not clear, about 1500 mAh / g is assumed. On the other hand, when the weight ratio of polyimide exceeds 60%, the charge / discharge capacity decreases due to an increase in resistance of the negative electrode mixture layer, which is not preferable. On the other hand, if it is less than 30%, the proportion of polyimide in the negative electrode mixture decreases, and the negative electrode capacity decreases, which is not preferable.
また、本発明の第二の実施の形態のリチウム二次電池用負極は、負極集電体上に、リチウムイオンを吸蔵・放出する活物質材料を上記ポリイミドを結着剤として結合させて負極合剤層を形成することにより得ることができる。 Further, the negative electrode for a lithium secondary battery according to the second embodiment of the present invention has an active material that occludes / releases lithium ions on the negative electrode current collector and binds the above polyimide as a binder to form a negative electrode composite. It can be obtained by forming an agent layer.
この場合、活物質材料とポリイミドとの重量比は、８０：２０〜９８：２の範囲とすることが望ましい。こうすることで、上記したように、ポリイミド自身が４００ｍＡｈ／ｇ以上、より好ましくは６００ｍＡｈ／ｇ以上の充放電容量を発現することが可能となり、負極活物質材料が本来有する充放電容量よりも大きな充放電容量を負極から取り出すことができる。一方、ポリイミドの重量比が２０％を超えると、負極合剤の抵抗の増大により充放電容量が低下するため好ましくない。また、２％未満だと活物質同士の結着性および活物質と負極集電体との接着性が低下するために、負極合剤層の割れや剥離などが生じるために好ましくない。 In this case, the weight ratio between the active material and the polyimide is preferably in the range of 80:20 to 98: 2. By doing so, as described above, the polyimide itself can develop a charge / discharge capacity of 400 mAh / g or more, more preferably 600 mAh / g or more, which is larger than the charge / discharge capacity originally possessed by the negative electrode active material. The charge / discharge capacity can be taken out from the negative electrode. On the other hand, when the weight ratio of polyimide exceeds 20%, the charge / discharge capacity decreases due to the increase in resistance of the negative electrode mixture, which is not preferable. On the other hand, if it is less than 2%, the binding property between the active materials and the adhesion between the active material and the negative electrode current collector are lowered, so that the negative electrode mixture layer is cracked or peeled off.
なお、ポリイミドと活物質材料とからなる負極（Ａ）を得る前に、ポリイミドと導電性付与剤とからなる負極（Ｂ）を作製して、負極（Ｂ）のポリイミドが４００ｍＡｈ／ｇ以上の放電容量を有することを確認して、該ポリイミドを用いて負極（Ａ）を作製することが望ましい。 In addition, before obtaining the negative electrode (A) which consists of a polyimide and an active material, the negative electrode (B) which consists of a polyimide and a electroconductivity imparting agent is produced, and the polyimide of a negative electrode (B) is discharge more than 400 mAh / g. After confirming that it has a capacity, it is desirable to produce the negative electrode (A) using the polyimide.
上記負極活物質材料としては、黒鉛、非晶質炭素などの炭素材料、あるいはケイ素および／またはケイ素合金と炭素材料との混合物、あるいはケイ素および／またはケイ素合金を用いることができる。 As the negative electrode active material, a carbon material such as graphite or amorphous carbon, a mixture of silicon and / or a silicon alloy and a carbon material, or silicon and / or a silicon alloy can be used.
以上説明したように、本発明によるリチウム二次電池用負極によれば、負極合剤中のポリイミドは結着剤としてだけではなく、ポリイミド自身が充放電容量を発現することにより、負極性能の向上に対して大きな効果が得られる。 As described above, according to the negative electrode for a lithium secondary battery according to the present invention, the polyimide in the negative electrode mixture is not only used as a binder, but also improves the negative electrode performance by developing the charge / discharge capacity of the polyimide itself. A great effect is obtained.
（電解液）
電解液は、電解質が溶解された非水溶媒を用いることができる。電解質は、リチウム二次電池の場合にはリチウム塩を用い、これを非水溶媒中に溶解させる。リチウム塩としては、リチウムイミド塩、ＬｉＰＦ6、ＬｉＡｓＦ6、ＬｉＡｌＣｌ4、ＬｉＣｌＯ4、ＬｉＢＦ4、ＬｉＳｂＦ6などがあげられる。この中でも特にＬｉＰＦ6、ＬｉＢＦ4が好ましい。リチウムイミド塩としてはＬｉＮ（ＣkＦ2k+1ＳＯ2）（ＣmＦ2m+1ＳＯ2）（ｋ、ｍはそれぞれ独立して１または２である）が挙げられる。これらは単独で、または複数種を組み合わせて用いることができる。
(Electrolyte)
As the electrolytic solution, a nonaqueous solvent in which an electrolyte is dissolved can be used. In the case of a lithium secondary battery, the electrolyte uses a lithium salt, which is dissolved in a non-aqueous solvent. The lithium salt, lithium imide salt, LiPF 6, LiAsF 6, LiAlCl 4, LiClO 4, LiBF 4, LiSbF 6 , and the like. Of these, LiPF 6 and LiBF 4 are particularly preferable. Examples of the lithium imide salt include LiN (C k F 2k + 1 SO 2 ) (C m F 2m + 1 SO 2 ) (k and m are each independently 1 or 2). These can be used alone or in combination of two or more.
また非水溶媒としては、環状カーボネート類、鎖状カーボネート類、脂肪族カルボン酸エステル類、γ−ラクトン類、環状エーテル類、鎖状エーテル類およびそれらのフッ化誘導体の有機溶媒から選ばれた少なくとも１種類の有機溶媒を用いる。より具体的には、環状カーボネート類：プロピレンカーボネート（ＰＣ）、エチレンカーボネート（ＥＣ）、ブチレンカーボネート（ＢＣ）、およびこれらの誘導体鎖状カーボネート類：ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）、ジプロピルカーボネート（ＤＰＣ）、およびこれらの誘導体脂肪族カルボン酸エステル類：ギ酸メチル、酢酸メチル、プロピオン酸エチル、およびこれらの誘導体γ−ラクトン類：γ−ブチロラクトン、およびこれらの誘導体環状エーテル類：テトラヒドロフラン、２−メチルテトラヒドロフラン鎖状エーテル類：１、２−エトキシエタン（ＤＥＥ）、エトキシメトキシエタン（ＥＭＥ）、ジエチルエーテル、およびこれらの誘導体その他：ジメチルスルホキシド、１、３−ジオキソラン、ホルムアミド、アセトアミド、ジメチルホルムアミド、ジオキソラン、アセトニトリル、プロピルニトリル、ニトロメタン、エチルモノグライム、リン酸トリエステル、トリメトキシメタン、ジオキソラン誘導体、スルホラン、メチルスルホラン、１、３−ジメチル−２−イミダゾリジノン、３−メチル−２−オキサゾリジノン、プロピレンカーボネート誘導体、テトラヒドロフラン誘導体、エチルエーテル、１、３−プロパンスルトン、アニソール、Ｎ−メチルピロリドン、フッ素化カルボン酸エステルこれらを１種または２種以上を混合して使用することができる。 The non-aqueous solvent is at least selected from organic solvents such as cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, chain ethers and their fluorinated derivatives. One kind of organic solvent is used. More specifically, cyclic carbonates: propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and their derivative chain carbonates: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl Methyl carbonate (EMC), dipropyl carbonate (DPC), and their derivatives Aliphatic carboxylic acid esters: methyl formate, methyl acetate, ethyl propionate, and their derivatives γ-lactones: γ-butyrolactone, and these Derivative cyclic ethers: Tetrahydrofuran, 2-methyltetrahydrofuran chain ethers: 1, 2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, and derivatives thereof Other: Dimethyl Tyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3- Dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid ester Two or more kinds can be mixed and used.
さらに電解液添加剤として、一般的な、例えば、ビニレンカーボネート（ＶＣ）などを用いることも可能である。 Furthermore, it is also possible to use common, for example, vinylene carbonate (VC) as the electrolyte solution additive.
本発明に係るリチウム二次電池の形状としては、特に制限はないが、例えば、円筒型、角型、コイン型、ラミネート型などがあげられる。この中でラミネート型とは合成樹脂と金属箔との積層体からなる可撓性フィルムなどよりなる外装体によって封口された形状を有するものであり、軽量化が可能であることより、電池エネルギー密度の向上を図る上でラミネート外装を用いることが、より好ましい。 The shape of the lithium secondary battery according to the present invention is not particularly limited, and examples thereof include a cylindrical shape, a square shape, a coin shape, and a laminate shape. Among them, the laminate type has a shape sealed by an exterior body made of a flexible film made of a laminate of a synthetic resin and a metal foil, and the battery energy density can be reduced because it can be reduced in weight. In order to improve this, it is more preferable to use a laminate sheath.
本発明に係るリチウム二次電池は、乾燥空気または不活性ガス雰囲気において、負極層および正極層を、セパレータを介して積層、あるいは積層したものを捲回した後に、外装体に挿入し、電解液を含浸させた後、電池外装体を封止することで得られる。 The lithium secondary battery according to the present invention includes a negative electrode layer and a positive electrode layer laminated via a separator in a dry air or inert gas atmosphere. After impregnating, the battery outer package is sealed.
以下に本発明の実施例について詳細に説明するが、本発明は以下の実施例のみに限定されるものではない。 Examples of the present invention will be described in detail below, but the present invention is not limited to the following examples.
（実施例１）
（試料極の作製）
導電性付与剤を、ポリイミド前駆体を溶解させたＮＭＰ中に均一に分散させてスラリーを作製した。導電性与剤としては、カーボンブラックを用い、ポリイミド前駆体としては、宇部興産株式会社製・Ｕ−ワニス−Ａを用いた。そのスラリーを負極集電体となる厚み１５μｍの銅箔上に塗布後、１２５℃にて１０分間ＮＭＰを蒸発させることにより負極合剤層を形成し、プレス後、窒素ガス雰囲気下、３５０℃にて１時間の加熱処理を行い、負極を作製した。負極合剤層のポリイミドと導電性付与剤との重量比は５０：５０とし、上記のように作製した負極を切り出して試料極とした。
Example 1
(Production of sample electrode)
The conductivity imparting agent was uniformly dispersed in NMP in which the polyimide precursor was dissolved to prepare a slurry. Carbon black was used as the conductive additive, and U-Varnish-A manufactured by Ube Industries, Ltd. was used as the polyimide precursor. The slurry was applied on a 15 μm thick copper foil serving as a negative electrode current collector, and NMP was evaporated at 125 ° C. for 10 minutes to form a negative electrode mixture layer. After pressing, the temperature was increased to 350 ° C. in a nitrogen gas atmosphere. Then, a heat treatment was performed for 1 hour to prepare a negative electrode. The weight ratio between the polyimide in the negative electrode mixture layer and the conductivity-imparting agent was 50:50, and the negative electrode produced as described above was cut out to obtain a sample electrode.
（実施例２、３）
負極合剤層のポリイミドと導電性付与剤との重量比を実施例２では６０：４０、実施例３では３０：７０とした以外は実施例１と同様に試料極を作製した。
(Examples 2 and 3)
A sample electrode was prepared in the same manner as in Example 1 except that the weight ratio between the polyimide in the negative electrode mixture layer and the conductivity-imparting agent was 60:40 in Example 2 and 30:70 in Example 3.
（電解液）
電解液は、溶媒としてＥＣ：ＤＥＣ＝３０：７０（体積％）に、電解質として１ｍｏｌ／ＬのＬｉＰＦ6を溶解したものを用いた。
(Electrolyte)
As the electrolytic solution, EC: DEC = 30: 70 (volume%) as a solvent and 1 mol / L LiPF 6 as an electrolyte were dissolved.
（コイン型電池の作製）
円形に打ち抜いた試料極を、ステンレス製のコイン型のケースの一方に入れ、ポリエチレンおよびポリプロピレンからなるセパレータを負極合剤層に置き、電解液を含浸させた後、金属Ｌｉをセパレータ上に置き、ガスケットを介して他方のコイン型ケースをかぶせて、かしめ、コイン型電池を作製した。コイン型電池を定電流充放電試験法により負極としての性能評価を行った。充電時の終止電圧を０Ｖ、放電時の終止電圧を２．５Ｖとし、電流値は負極合剤量に対して４０ｍＡ／ｇに設定した。
(Production of coin-type battery)
Put the sample electrode punched into a circle into one of the coin-shaped case made of stainless steel, place a separator made of polyethylene and polypropylene on the negative electrode mixture layer, impregnate the electrolyte, and then place the metal Li on the separator, The other coin type case was covered with a gasket and caulked to produce a coin type battery. The performance of the coin-type battery as a negative electrode was evaluated by a constant current charge / discharge test method. The final voltage during charging was 0 V, the final voltage during discharging was 2.5 V, and the current value was set to 40 mA / g with respect to the negative electrode mixture amount.
（実施例４〜６）
導電性付与剤の代わりに負極活物質として平均粒径２０μｍの天然黒鉛粉末を用い、負極活物質とポリイミドとの重量比を実施例４では９５：５、実施例５では８０：２０、実施例６では９８：２、とした以外は、実施例１同様の方法にてコイン型電池を作製し、負極の性能評価を行った。ただし電流値は１０ｍＡ／ｇに設定した。
(Examples 4 to 6)
Natural graphite powder having an average particle diameter of 20 μm was used as the negative electrode active material instead of the conductivity-imparting agent, and the weight ratio of the negative electrode active material and polyimide was 95: 5 in Example 4, 80:20 in Example 5, and Example. A coin-type battery was produced in the same manner as in Example 1 except that 98: 2 was set for 6 and the performance of the negative electrode was evaluated. However, the current value was set to 10 mA / g.
（比較例１〜３）
ポリイミドと導電性付与剤との重量比を、比較例１では７０：３０、比較例２では８５：１５とした以外は実施例１と同様の方法にてコイン型電池を作製して負極の性能評価を行った。また、比較例３においてはポリイミドの代わりにＰＶｄＦ用い、プレス後の加熱処理を行わないこと以外は、実施例１と同様の方法にてコイン型電池を作製して負極の性能評価を行った。
(Comparative Examples 1-3)
A coin-type battery was produced in the same manner as in Example 1 except that the weight ratio of polyimide to the conductivity-imparting agent was 70:30 in Comparative Example 1 and 85:15 in Comparative Example 2, and the performance of the negative electrode was prepared. Evaluation was performed. In Comparative Example 3, a coin-type battery was prepared in the same manner as in Example 1 except that PVdF was used instead of polyimide and heat treatment after pressing was not performed, and the performance of the negative electrode was evaluated.
（比較例４）
ポリイミドの代わりにＰＶｄＦ用い、プレス後の加熱処理を行わないこと以外は、実施例２と同様の方法にてコイン型電池を作製して負極の性能評価を行った。
(Comparative Example 4)
A coin-type battery was prepared in the same manner as in Example 2 except that PVdF was used instead of polyimide and heat treatment after pressing was not performed, and the performance of the negative electrode was evaluated.
（性能試験）
表１に、実施例および比較例の放電容量（ｍＡｈ／ｇ）の測定値を示す。実施例１〜３および比較例１〜３では、負極合剤成分は導電性付与材と結着材とからなり、放電容量（ｍＡｈ／ｇ）は負極合剤中の樹脂（結着剤）量に対する値で示した。また、電極容量は（ｍＡｈ／ｇ）は負極合剤量に対する値であって電極として得られる実効的な容量（放電容量×結着剤比率）を表している。ポリイミドの重量比率が５０％である実施例１においては、放電容量は９４０ｍＡｈ／ｇ、電極容量は４７０ｍＡｈ／ｇと大きな値が得られた。ポリイミドの重量比率が６０％である実施例２においては、ポリイミドの比率は増大したものの放電容量が４５０ｍＡｈ／ｇに低下したため、電極容量は２７０ｍＡｈ／ｇであった。ポリイミドの重量比率が３０％である実施例３においては、放電容量は９４０ｍＡｈ／ｇと大きいもののポリイミドの比率が減少したため電極容量は２８２ｍＡｈ／ｇであった。一方、比較例１および比較例２においてポリイミドの重量比率を７０％、８５％と増やすと放電容量は顕著に低下した。また、比較例３においてはＰＶｄＦの重量比率が５０％でも、ほとんど放電容量は得られなかった。
(performance test)
Table 1 shows measured values of discharge capacity (mAh / g) of the examples and comparative examples. In Examples 1 to 3 and Comparative Examples 1 to 3, the negative electrode mixture component is composed of a conductivity imparting material and a binder, and the discharge capacity (mAh / g) is the amount of resin (binder) in the negative electrode mixture. It is shown by the value for. The electrode capacity (mAh / g) is a value with respect to the amount of the negative electrode mixture and represents an effective capacity (discharge capacity × binder ratio) obtained as an electrode. In Example 1 in which the weight ratio of polyimide was 50%, the discharge capacity was 940 mAh / g, and the electrode capacity was as large as 470 mAh / g. In Example 2 in which the weight ratio of polyimide was 60%, the discharge capacity decreased to 450 mAh / g although the ratio of polyimide increased, so the electrode capacity was 270 mAh / g. In Example 3 where the weight ratio of polyimide was 30%, the discharge capacity was as large as 940 mAh / g, but the ratio of polyimide was decreased, so the electrode capacity was 282 mAh / g. On the other hand, in Comparative Example 1 and Comparative Example 2, when the weight ratio of polyimide was increased to 70% and 85%, the discharge capacity was significantly reduced. In Comparative Example 3, even when the PVdF weight ratio was 50%, almost no discharge capacity was obtained.
また、図１に実施例１のコイン型電池の充放電曲線を示す。通常の黒鉛と比較して電位の平坦性や充放電効率は劣るものの、高い充放電容量が得られていることがわかる。 FIG. 1 shows a charge / discharge curve of the coin-type battery of Example 1. Although the potential flatness and charge / discharge efficiency are inferior to those of normal graphite, it is understood that a high charge / discharge capacity is obtained.
電極容量は大きいほど好ましく、少なくとも２５０ｍＡｈ／ｇ以上の容量を有することが望ましい。したがって、導電性付与剤とポリイミドからなる負極合剤において、ポリイミドの重量比率は３０〜６０％の範囲にあることが望ましい。 The larger the electrode capacity, the better, and it is desirable to have a capacity of at least 250 mAh / g. Therefore, in the negative electrode mixture composed of the conductivity-imparting agent and polyimide, it is desirable that the weight ratio of polyimide is in the range of 30 to 60%.
実施例４〜６および比較例４では、負極合剤成分は負極活物質（黒鉛）と結着材とからなり、放電容量（ｍＡｈ／ｇ）は負極合剤中の活物質量に対する値で示した。電極容量は（ｍＡｈ／ｇ）は負極合剤量に対する値であって電極として得られる実効的な容量（放電容量×活物質比率）を表している。ポリイミドを５重量％用いた実施例４の電極容量は、ＰＶｄＦを用いた比較例４よりも約５０ｍＡｈ／ｇ大きい値を示した。また、ポリイミドの重量比率を２０％とした実施例５の電極容量は比較例４よりも約３５ｍＡｈ／ｇ大きい値を示した。ただし、これ以上ポリイミドを増やすと抵抗増大による容量低下が大きくなるため、好ましくない。また、ポリイミドの重量比率を２％とした実施例６の電極容量は比較例４よりも約３０ｍＡｈ／ｇ大きい値を示した。しかし、これ以上ポリイミドを減らすと結着性の低下のため負極合剤が集電体から剥離して電極を作製することが困難となるため、好ましくない。実施例４〜６において放電容量および電極容量の値が黒鉛の理論容量である３７２ｍＡｈ／ｇを上回っているが、これは上記したようにポリイミド自身が放電容量に寄与したためである。なお、ポリイミドの比率が２重量％、５重量％、２０重量％の各場合についてポリイミド重量当たりの放電容量を求めると、それぞれ約９９０、９９０、４５０ｍＡｈ／ｇと推定とされた。以上の結果から、活物質とポリイミドからなる負極合剤において、ポリイミドの重量比率は２０〜２％の範囲にあることが望ましい。こうすることで、結着材として用いたポリイミドが容量を発現し、電極容量の向上に寄与することが可能となる。 In Examples 4 to 6 and Comparative Example 4, the negative electrode mixture component is composed of a negative electrode active material (graphite) and a binder, and the discharge capacity (mAh / g) is expressed as a value relative to the amount of active material in the negative electrode mixture. It was. The electrode capacity (mAh / g) is a value with respect to the amount of the negative electrode mixture, and represents an effective capacity (discharge capacity × active material ratio) obtained as an electrode. The electrode capacity of Example 4 using 5% by weight of polyimide showed a value about 50 mAh / g larger than that of Comparative Example 4 using PVdF. Further, the electrode capacity of Example 5 in which the weight ratio of polyimide was 20% was about 35 mAh / g larger than that of Comparative Example 4. However, it is not preferable to increase the polyimide more than this, because the capacity drop due to the increase in resistance becomes large. Further, the electrode capacity of Example 6 in which the weight ratio of polyimide was 2% showed a value about 30 mAh / g larger than that of Comparative Example 4. However, if the polyimide is further reduced, it is not preferable because the negative electrode mixture is peeled off from the current collector due to a decrease in binding properties, and it becomes difficult to produce an electrode. In Examples 4 to 6, the values of the discharge capacity and the electrode capacity exceeded 372 mAh / g, which is the theoretical capacity of graphite. This is because polyimide itself contributed to the discharge capacity as described above. In addition, when the discharge capacity per polyimide weight was determined for each of the polyimide ratios of 2 wt%, 5 wt%, and 20 wt%, it was estimated to be about 990, 990, and 450 mAh / g, respectively. From the above results, it is desirable that the weight ratio of polyimide is in the range of 20 to 2% in the negative electrode mixture composed of the active material and polyimide. By doing so, the polyimide used as the binder exhibits a capacity and can contribute to an improvement in the electrode capacity.
また、実施例４および比較例４のコイン型電池の充放電曲線を図２、図３に示す。実施例４は比較例４と比較して大きい放電容量が得られていることがわかる。 Moreover, the charging / discharging curve of the coin-type battery of Example 4 and Comparative Example 4 is shown in FIGS. It can be seen that the discharge capacity of Example 4 is larger than that of Comparative Example 4.
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