JP2004139768A - Porous thin film electrode and lithium secondary battery using this as negative electrode - Google Patents
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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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Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は、多孔質薄膜電極と、これを負極に用いたリチウム二次電池に関する。
【０００２】
【従来の技術】
リチウム二次電池用の負極として、Ｌｉと合金化しない材料からなる集電体上に、Ｌｉと合金化するＳｎ又はＳｎ含有合金からなる薄膜を形成したものがある（特許文献１、特許文献２参照）。特許文献１では、集電体である銅板上に、電気メッキ法によりＳｎ薄膜を形成している。特許文献２では、電気メッキ法により、銅箔上にＳｎ、Ｚｎ、Ｓｂ、或いはそれらを含有する合金を素材とする薄膜を形成している。
【０００３】
本発明は、メソポーラス物質を多孔質薄膜に適用するものであるが、この種のメソポーラス物質として、セラミックス分野において広く知られたメソポーラスシリカがある。一般的にメソポーラス物質の合成は、有機分子集合体を鋳型として、無機構成成分との無機有機メソ構造体の合成と、それに続く鋳型の除去によるメソ孔の生成による。かかるメソポーラス物質は、吸着剤、触媒、フォトニクス材料、エレクトロニクス材料などの多様な分野へ展開されている。
【０００４】
界面活性剤のミセルを鋳型とするメソポーラスＳｎの合成は、非特許文献１や非特許文献２に公知である。非特許文献２には、メソポーラスＳｎ膜がリチウム二次電池の負極材料として動作することが述べられている。
【０００５】
【特許文献１】
特開２００１−６８０９４号公報
【特許文献２】
特開２００１−２５６９６８号公報
【非特許文献１】
Ｓｃｉｅｎｃｅ，２７８巻８３８−４０（１９８７）
【非特許文献２】
Ｃｈｅｍ．Ｃｏｍｍｕｎ．，３３１−２３（１９９９）
【０００６】
【発明が解決しようとする課題】
特許文献１および特許文献２のように電気メッキ法で電極を作成すると、集電体上に目的の金属或いは合金を直接電析させて薄膜を形成できるので、塗布電極に比べて、活物質と集電体の密着性に優れた電極を得ることができる。塗布電極に比べて製造コストが安価に済む利点もある。但し、得られるメッキ層が非常に緻密となるため、電極がＬｉイオンインターカレーションに伴う活物質粒子の体積膨張の影響を受けやすく、結果的に電極の膨張およびクラック発生が顕著に現れる不具合がある。詳しくは、リチウム二次電池用のＳｎ金属負極において、ＬｉＸ Ｓｎの組成式でＸ＝４．４までＬｉ＋ を電気化学的に挿入すると、活物質粒子は、約２６０％の割合で体積膨張する。かかる体積膨張は、リチウム二次電池のサイクル特性の低下を招く。
【０００７】
非特許文献１や非特許文献２に記載された多孔質薄膜であるメソポーラスＳｎ膜をリチウム二次電池の負極に適用すると、充放電に伴う体積膨張をメソ孔により緩和できるので、サイクル特性の向上を図ることができる。しかし、細孔部分を多くして、電極密度を小さくしていくと、集電体とＳｎ膜との界面の密着部分が少なくなるので、薄膜が剥離しやすい。また、電極密度が小さいと、体積膨張時に薄膜内部で導電パスが切断しやすく、その結果容量低下を引き起こす不利がある。さらに、細孔部分の占有率が高くなると、単位体積あたりの放電容量の低下を招くおそれもある。
【０００８】
本発明は、以上のような問題に鑑みてなされたものであり、電池容量とサイクル特性、さらにハイレート特性の向上に寄与し得るリチウム二次電池用の電極、およびこれを負極とするリチウム二次電池を提供することにある。
【０００９】
【課題を解決するための手段】
本発明にかかる電極は、Ｌｉと合金化しない材料からなる集電体と、Ｓｎ又はＳｎ含有合金からなり、電気メッキ法により前記集電体上に形成された薄膜とを有する。そして、前記薄膜が、３ｇ／ｃｃ以上、５ｇ／ｃｃ以下の密度を有する多孔質薄膜であることを特徴とする。
【００１０】
また、本発明にかかる電極は、Ｌｉと合金化しない材料からなる集電体と、Ｓｎ又はＳｎ含有合金からなり、前記集電体上に形成された第１の薄膜と、Ｓｎ又はＳｎ含有合金からなり、電気メッキ法により前記第１の薄膜上に形成された第２の薄膜とを有する。そして、前記第２の薄膜が、３ｇ／ｃｃ以上、５ｇ／ｃｃ以下の密度を有する多孔質薄膜であることを特徴とする。
【００１１】
本発明において、「Ｌｉと合金化しない材料」としては、ＣｕやＮｉ、ステンレスなどを挙げることができる。とくに薄膜との密着性を高めるという観点からは、薄膜と合金化し得る金属からなる銅箔などの金属箔が好ましい。
【００１２】
具体的には、前記多孔質薄膜はメソポーラス物質とする。ここでメソポーラス物質とは、メソ孔（細孔直径の寸法が２ｎｍ以上、５０ｎｍ以下）を有する多孔性物質を言う。かかる多孔質薄膜の膜厚は、２〜２００μｍとする。このメソポーラス物質は、Ｓｎの他に、Ｌｉ、Ｂｉ、Ｃｕ、Ｆｅ、Ｎｉ、Ｚｎ、Ａｇから選ばれる１種類または２種類以上の元素を含むものからなるものとする。詳しくは、その組成は、Ｓｎ元素を１０％〜１００％未満、残部が上記Ｌｉ等の添加金属である。なお、上記添加金属以外の元素が含有された合金であってもよい。
【００１３】
【発明の作用効果】
電気メッキ法により薄膜を形成するようにしてあると、集電体上に目的の金属を直接電析させて薄膜を形成できるので、塗布電極に比べて密着性に優れた電極を得ることができ、従って、当該電極を備える電池のサイクル特性の向上に寄与し得る。塗布電極に比べて製造コストが安価に済む利点もある。薄膜をＳｎ又はＳｎ含有合金からなるものとすることは、電池の高容量化に資する。
【００１４】
そのうえで、薄膜を多孔質薄膜としてあると、緻密な電気メッキ膜に比べて飛躍的に電極反応面積を大きくできるので、電池の高容量化とハイレート特性の向上に寄与できる。
【００１５】
電池容量やハイレート特性等を鑑みると、理論的には多孔質薄膜の密度は小さいほどよい。すなわち、多孔質薄膜の密度を小さくしていくと、電極反応面積が大きくできるので、電池容量およびハイレート特性が向上できる。しかし実際には、多孔質薄膜の密度が３ｇ／ｃｃを下回ると、電池容量は低下傾向に移行する。これは、密度が３ｇ／ｃｃを下回ると、薄膜内部で導電パスの切断が起きること、および単位体積あたりの放電容量が低下することに拠る。また、多孔質薄膜の密度が小さすぎると、集電体と薄膜との界面との密着部分が少なくなって、薄膜の密着性が低下し、その結果薄膜が集電体から剥離しやすくなる不利がある。
【００１６】
一方、多孔質薄膜の密度が５ｇ／ｃｃを上回ると、体積膨張に対する緩和能力が低下し、そうすると、薄膜を構成する活物質と集電体との接着が破壊されて、導電パスが切断されて電池容量の低下を招く。また、５ｇ／ｃｃを上回ると、サイクル特性の低下を招く。以上より、多孔質薄膜の密度は、３ｇ／ｃｃ以上、５ｇ／ｃｃ以下とする。
【００１７】
多孔質薄膜をメソポーラス物質としてあると、メソ孔により充放電に伴う電極の体積変化を抑制できるので、サイクル特性の向上を図ることができる。
【００１８】
【実施例】
以下に本発明の代表的な実施例を示し、さらに具体的に説明する。なお、これらは説明のための単なる例示であって、本発明はこれらに制限されるものではない。
【００１９】
（実施例１）
０．２Ｍ ＳｎＳＯ４ と０．３Ｍ Ｈ２ ＳＯ４ の混合溶液に、膨張剤として機能するｎ−ヘプタンと、有機分子集合体の鋳型として機能する界面活性剤であるオクタエチレングリコールモノヘキサデシルエーテルを所定量添加した後、攪拌・混合してメッキ液を得た。このメッキ液を用いて、１０μｍの銅箔上に室温で電流密度５ｍＡ／ｃｍ２ の条件下で電気メッキを行い、約１０μｍの厚さの薄膜を形成した。それらを十分水洗した後、１５０〜２００℃の真空下で熱処理を行い、界面活性剤を除去するとともに、銅箔よりＣｕの拡散、薄膜よりＳｎの拡散を促進させて、銅箔上にＣｕ３ ＳｎとＣｕ６ Ｓｎ５ 組成の金属間化合物を形成させて、メソポーラスＳｎ系合金からなる薄膜を得た。水銀圧入法により細孔径を測定したところ、おおよそ５〜１０ｎｍを持つことがわかった。また、この薄膜を透過型電子顕微鏡等で観察したところ、細孔の形状は主に六角柱で開孔していた。膜密度は、４．０ｇ／ｃｃであった。
【００２０】
次に、評価用の電気化学セルを作製して、薄膜の負極特性を評価した。対極には金属リチウムを用いた。電解液にはエチレンカーボネートとエチルメチルカーボネートを体積比で１：２に混合した非水系有機溶媒に、ＬｉＰＦ６ を１．２Ｍ溶解したリチウム二次電池用電解液を用いた。そして、電流密度０．２ｍＡ／ｃｍ２ で、０〜１．０Ｖの電圧範囲で定電流充放電測定により、負極特性を評価した。その結果、５００ｍＡｈ／ｇ以上の高容量を示し、１００サイクルの充放電後においても、９５％以上の容量を維持し、優れたサイクル特性を示すことがわかった。このような特性は、Ｌｉ＋ の挿入時に伴う薄膜の体積膨張の変化を、メソ孔により三次元方向で体積膨張を緩和できたことと、電気メッキ法により、集電体上に強固な薄膜が形成できたことによる。
【００２１】
（実施例２）
２ＭのＨ２ ＳＯ４ と、０．２ＭのＳｎＳＯ４ と、０．０１ＭのＡｇＮＯ３ と、０．０２ＭのＣｕＳＯ４ ・５Ｈ２ Ｏとの混合溶液に、ｎ−ヘプタンとオクタエチレングリコールモノヘキサデシルエーテルを所定量添加して、メッキ液とした。このメッキ液を用いて、１０μｍの銅箔上に電流密度２０ｍＡ／ｃｍ２ で、電気メッキすることで、約１０μｍの厚さのβ−Ｓｎ，Ａｇ３ Ｓｎ，Ｃｕ６ Ｓｎ５ 組成の多成分系合金および界面活性剤を含む薄膜を形成した。それらを十分水洗した後、１００〜１５０℃で真空乾燥を行なうことで、界面活性剤を除去して、密度３．５ｇ／ｃｃのメソポーラス多成分系合金薄膜を形成した。
【００２２】
実施例２にかかる薄膜に対して、実施例１と同様に評価用の電気化学セルを作製して負極特性を評価した。その結果、実施例１と略同様の性能を示すことがわかった。すなわち、５００ｍＡｈ／ｇ以上の高容量を示し、１００サイクルの充放電後においても、９５％以上の容量を維持し、優れたサイクル特性を示すことがわかった。
【００２３】
（実施例３）
実施例１、２のそれぞれのメッキ液に、ｎ−ヘプタンと界面活性剤のオクタエチレングリコールモノヘキサデシルエーテルを添加しないメッキ液で約１μｍ程度緻密な薄膜を銅箔上に形成した後、ｎ−へプタンと界面活性剤にオクタエチレングリコールモノヘキサデシルエーテルを添加し、メッキ液を攪拌して、再度電気メッキする以外は実施例１、２と同様にした。得られた薄膜は、緻密なメッキ膜上にメソ孔を持つ多孔質膜が形成された多層構造薄膜電極であった。実施例１、２と同様の方法で負極特性を評価したところ、実施例１、２のサイクル特性に遜色ないかそれを上回る傾向にあった。これは、緻密な超薄膜が集電体上に存在するために、集電体のＣｕ箔上に接した部分が多く存在できるので実施例１、２よりも集電体上に強固に薄膜が形成できたためである。容量およびハイレート特性も、実施例１、２と比べて遜色ないことがわかった。
【００２４】
（比較例１）
メッキ液中に界面活性剤を添加しないこと以外は、実施例１と同様にして電気メッキ法により薄膜電極を作製した。その結果、見かけの膜密度（電極密度）約７ｇ／ｃｃの電極が得られた。電気化学セルを作製して負極特性を評価した。その結果、電流密度０．２ｍＡ／ｃｍ２ での比較的低レートでの充放電における放電容量は、実施例１、２、３とほぼ同程度であったが、サイクル特性やハイレート特性で劣ることがわかった。
【００２５】
（比較例２）
実施例１において、膨張剤の添加量、界面活性剤の添加量等を調整して、密度２．６ｇ／ｃｃの薄膜を有する多孔質薄膜電極を得た。実施例１、２、３と同様に、電気化学セルを作製して負極特性を評価した。その結果、サイクル特性は実施例１、２、３に遜色なかったが、放電容量の低下が見られた。
【００２６】
次に、実施例１、２、３および比較例１、２の薄膜電極を負極とする電池を作製した。正極にはＬｉＣｏＯ２ を用いた。なお、正極集電体には厚さ２０μｍのＡｌ、負極集電体には１０μｍのＣｕ箔、セパレーターには２０μｍのポリエチレンを使用した。正負極の電極面積を約１１ｃｍとして正極／セパレーター／負極の順に多数積層して、高さ４．８ｍｍの角型のステンレス管に挿入し、エチレンカーボネートとエチルメチルカーボネートを体積比で１：２に混合した非水系有機溶媒にＬｉＰＦ６ を１．２Ｍ溶解した電解液を注液して電池を組み立てた。これら電池に対して、初期電池容量値と、１００サイクル経過後の容量維持率を測定した。その結果を表１に示す。
【００２７】
【表１】
表１に示すように、実施例１〜３にかかる電池は、電流密度０．５ｍＡ／ｃｍ２ 時で約１．２Ａｈの容量を示し、１００サイクル経過後も９５％以上の容量を維持していた。また、電流密度２ｍＡ／ｃｍ２ のハイレート放電実験を行ったが、ほとんど容量低下を引き起こさなかった。すなわち、本発明にかかる多孔質薄膜電極を負極とすることにより、高容量であり、サイクル特性とハイレート特性に優れた電池を構成できることがわかった。
【００２９】
比較例１にかかる電極を負極とする電池は、単位重量あたりの容量は、実施例１、２、３に遜色なかったが、５０サイクル経過後には、容量維持率８５％、１００サイクル経過後には、６０％以下に低下した。このような結果は、電極密度が高いために電極の体積膨張を抑制できなかったこと、および電極反応面積が実施例１、２、３の電極と比較して小さかったことによる。
【００３０】
比較例２にかかる電極を負極とする電池は、サイクル特性は実施例１、２、３に遜色なかったが、放電容量の低下が見られた。これは、最適な電極密度（薄膜の密度）が存在することを示している。
【００３１】
実施例１において、メソポーラス物質の細孔形状は、界面活性剤の種類により変化させることが可能であり、六角柱に限るものではない。さらに、見かけの薄膜密度（薄膜重量を薄膜の体積で割った値）は、膨張剤の添加量、界面活性剤の添加量、熱処理温度、熱処理時間等に強く依存する傾向にあり、これら要素を変更することにより、薄膜密度を調整することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a porous thin-film electrode and a lithium secondary battery using the same as a negative electrode.
[0002]
[Prior art]
As a negative electrode for a lithium secondary battery, there is a negative electrode in which a thin film made of Sn or a Sn-containing alloy which is alloyed with Li is formed on a current collector made of a material not alloyed with Li (Patent Documents 1 and 2). reference). In Patent Document 1, a Sn thin film is formed on a copper plate as a current collector by an electroplating method. In Patent Document 2, a thin film made of Sn, Zn, Sb, or an alloy containing them is formed on a copper foil by an electroplating method.
[0003]
In the present invention, a mesoporous material is applied to a porous thin film. As such a mesoporous material, there is mesoporous silica widely known in the ceramics field. In general, the synthesis of a mesoporous substance is based on the synthesis of an inorganic organic mesostructure with inorganic constituents using an organic molecular assembly as a template, and the subsequent formation of a mesopore by removing the template. Such mesoporous substances have been developed in various fields such as adsorbents, catalysts, photonic materials, and electronic materials.
[0004]
The synthesis of mesoporous Sn using surfactant micelles as a template is known from Non-Patent Documents 1 and 2. Non-Patent Document 2 describes that a mesoporous Sn film operates as a negative electrode material of a lithium secondary battery.
[0005]
[Patent Document 1]
JP 2001-68094 A [Patent Document 2]
JP 2001-256968 A [Non-Patent Document 1]
Science, vol. 278, 838-40 (1987)
[Non-patent document 2]
Chem. Commun. , 331-23 (1999).
[0006]
[Problems to be solved by the invention]
When an electrode is formed by an electroplating method as in Patent Literature 1 and Patent Literature 2, a thin film can be formed by directly depositing a target metal or alloy on a current collector. An electrode having excellent current collector adhesion can be obtained. There is also an advantage that the manufacturing cost is lower than that of the coated electrode. However, since the obtained plating layer becomes very dense, the electrode is easily affected by the volume expansion of the active material particles accompanying the Li ion intercalation, and consequently the electrode is notably expanded and cracks appear remarkably. is there. More specifically, in a Sn metal negative electrode for a lithium secondary battery, when Li + is electrochemically inserted up to X = 4.4 in the composition formula of Li X Sn, the active material particles expand at a rate of about 260%. I do. Such volume expansion causes deterioration of the cycle characteristics of the lithium secondary battery.
[0007]
When the mesoporous Sn film, which is a porous thin film described in Non-Patent Document 1 or Non-Patent Document 2, is applied to a negative electrode of a lithium secondary battery, volume expansion due to charging and discharging can be reduced by the mesopores, thereby improving cycle characteristics. Can be achieved. However, when the electrode density is reduced by increasing the number of the pores, the adhesion portion at the interface between the current collector and the Sn film is reduced, so that the thin film is easily peeled. In addition, when the electrode density is low, the conductive path is easily cut inside the thin film at the time of volume expansion, and there is a disadvantage that the capacity is reduced. Further, when the occupation ratio of the pore portion is increased, the discharge capacity per unit volume may be reduced.
[0008]
The present invention has been made in view of the above problems, and has an electrode for a lithium secondary battery capable of contributing to improvement of battery capacity and cycle characteristics, and further improvement of high-rate characteristics, and a lithium secondary battery using the same as a negative electrode. It is to provide a battery.
[0009]
[Means for Solving the Problems]
The electrode according to the present invention has a current collector made of a material that is not alloyed with Li, and a thin film made of Sn or a Sn-containing alloy and formed on the current collector by an electroplating method. Further, the thin film is a porous thin film having a density of 3 g / cc or more and 5 g / cc or less.
[0010]
In addition, the electrode according to the present invention includes a current collector made of a material that does not alloy with Li, a first thin film formed on the current collector made of Sn or a Sn-containing alloy, and a Sn or Sn-containing alloy. And a second thin film formed on the first thin film by an electroplating method. Further, the second thin film is a porous thin film having a density of 3 g / cc or more and 5 g / cc or less.
[0011]
In the present invention, examples of the “material that does not alloy with Li” include Cu, Ni, and stainless steel. In particular, from the viewpoint of improving the adhesion to the thin film, a metal foil such as a copper foil made of a metal that can be alloyed with the thin film is preferable.
[0012]
Specifically, the porous thin film is a mesoporous material. Here, the mesoporous substance refers to a porous substance having mesopores (pore diameter is 2 nm or more and 50 nm or less). The thickness of such a porous thin film is 2 to 200 μm. The mesoporous material is made of a material containing one or more elements selected from Li, Bi, Cu, Fe, Ni, Zn, and Ag in addition to Sn. More specifically, the composition is such that the Sn element is 10% to less than 100%, and the remainder is an additive metal such as Li. Note that an alloy containing an element other than the above-mentioned additional metal may be used.
[0013]
Operation and Effect of the Invention
When a thin film is formed by an electroplating method, the target metal can be directly deposited on the current collector to form a thin film, so that an electrode having better adhesion than a coated electrode can be obtained. Therefore, it can contribute to improvement of the cycle characteristics of the battery including the electrode. There is also an advantage that the manufacturing cost is lower than that of the coated electrode. Making the thin film made of Sn or an alloy containing Sn contributes to increasing the capacity of the battery.
[0014]
In addition, if the thin film is a porous thin film, the electrode reaction area can be significantly increased as compared with a dense electroplated film, which can contribute to an increase in battery capacity and an improvement in high rate characteristics.
[0015]
In consideration of battery capacity, high-rate characteristics, and the like, the theoretically, the smaller the density of the porous thin film, the better. That is, as the density of the porous thin film is reduced, the electrode reaction area can be increased, so that the battery capacity and high-rate characteristics can be improved. However, in practice, when the density of the porous thin film falls below 3 g / cc, the battery capacity shifts to a decreasing tendency. This is based on the fact that when the density is less than 3 g / cc, disconnection of the conductive path occurs inside the thin film, and the discharge capacity per unit volume decreases. On the other hand, if the density of the porous thin film is too low, the contact portion between the current collector and the thin film is reduced, and the adhesion of the thin film is reduced. As a result, the thin film is easily peeled from the current collector. There is.
[0016]
On the other hand, if the density of the porous thin film exceeds 5 g / cc, the ability to alleviate volume expansion is reduced, and the adhesion between the active material constituting the thin film and the current collector is broken, and the conductive path is cut. This leads to a decrease in battery capacity. On the other hand, if it exceeds 5 g / cc, the cycle characteristics deteriorate. From the above, the density of the porous thin film is set to 3 g / cc or more and 5 g / cc or less.
[0017]
When the porous thin film is made of a mesoporous material, the mesopores can suppress a change in the volume of the electrode due to charging and discharging, and thus the cycle characteristics can be improved.
[0018]
【Example】
Hereinafter, typical examples of the present invention will be described, and the present invention will be described more specifically. These are merely examples for explanation, and the present invention is not limited to these.
[0019]
(Example 1)
In a mixed solution of 0.2 M SnSO 4 and 0.3 MH 2 SO 4 , n-heptane functioning as a swelling agent and octaethylene glycol monohexadecyl ether as a surfactant functioning as a template for an organic molecular assembly are added. After adding a predetermined amount, the mixture was stirred and mixed to obtain a plating solution. Using this plating solution, electroplating was performed on a 10-μm copper foil at room temperature under the condition of a current density of 5 mA / cm 2 to form a thin film of about 10 μm. After sufficiently washing them with water, a heat treatment is performed under a vacuum of 150 to 200 ° C. to remove the surfactant, promote the diffusion of Cu from the copper foil and the diffusion of Sn from the thin film, and form Cu 3 on the copper foil. An intermetallic compound having a composition of Sn and Cu 6 Sn 5 was formed to obtain a thin film made of a mesoporous Sn-based alloy. When the pore diameter was measured by the mercury intrusion method, it was found that the pore diameter was approximately 5 to 10 nm. Further, when this thin film was observed with a transmission electron microscope or the like, the shape of the pores was mainly formed by hexagonal columns. The film density was 4.0 g / cc.
[0020]
Next, an electrochemical cell for evaluation was prepared, and the negative electrode characteristics of the thin film were evaluated. Metal lithium was used for the counter electrode. As the electrolytic solution, a lithium secondary battery electrolytic solution in which 1.2 M of LiPF 6 was dissolved in a nonaqueous organic solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2 was used. The negative electrode characteristics were evaluated by constant current charge / discharge measurement at a current density of 0.2 mA / cm 2 and a voltage range of 0 to 1.0 V. As a result, it was found that the battery exhibited a high capacity of 500 mAh / g or more, maintained a capacity of 95% or more even after 100 cycles of charge and discharge, and exhibited excellent cycle characteristics. These characteristics are due to the fact that the volume expansion of the thin film due to the insertion of Li + can be reduced by the mesopores in three-dimensional direction, and that a strong thin film can be formed on the current collector by electroplating. Due to the formation.
[0021]
(Example 2)
And H 2 SO 4 of 2M, and SnSO 4 of 0.2 M, and AgNO 3 of 0.01 M, the mixed solution of CuSO 4 · 5H 2 O of 0.02 M, n-heptane and octaethylene glycol monohexadecyl A predetermined amount of ether was added to obtain a plating solution. Using this plating solution, electroplating is performed on a 10 μm copper foil at a current density of 20 mA / cm 2 , thereby obtaining a multi-component system having a composition of β-Sn, Ag 3 Sn, and Cu 6 Sn 5 having a thickness of about 10 μm. A thin film containing the alloy and the surfactant was formed. After sufficient washing with water, the surfactant was removed by vacuum drying at 100 to 150 ° C. to form a mesoporous multi-component alloy thin film having a density of 3.5 g / cc.
[0022]
An electrochemical cell for evaluation was prepared for the thin film according to Example 2 in the same manner as in Example 1, and the negative electrode characteristics were evaluated. As a result, it was found that the performance was substantially the same as that of Example 1. That is, it was found that the battery exhibited a high capacity of 500 mAh / g or more, maintained a capacity of 95% or more even after 100 cycles of charge and discharge, and exhibited excellent cycle characteristics.
[0023]
(Example 3)
A thin film of about 1 μm was formed on a copper foil with a plating solution in which n-heptane and a surfactant, octaethylene glycol monohexadecyl ether, were not added to the respective plating solutions of Examples 1 and 2, and then n-heptane was added. Octaethylene glycol monohexadecyl ether was added to heptane and a surfactant, the plating solution was stirred, and electroplating was performed again. The obtained thin film was a multilayer structure thin film electrode in which a porous film having mesopores was formed on a dense plating film. When the negative electrode characteristics were evaluated in the same manner as in Examples 1 and 2, the cycle characteristics of Examples 1 and 2 were inferior to or exceeded the cycle characteristics. This is because the dense ultra-thin film is present on the current collector, so that a large portion of the current collector in contact with the Cu foil can be present. This is because they could be formed. It was also found that the capacity and high rate characteristics were comparable to those of Examples 1 and 2.
[0024]
(Comparative Example 1)
Except that no surfactant was added to the plating solution, a thin-film electrode was produced by an electroplating method in the same manner as in Example 1. As a result, an electrode having an apparent film density (electrode density) of about 7 g / cc was obtained. An electrochemical cell was prepared and the negative electrode characteristics were evaluated. As a result, the discharge capacity in charging and discharging at a relatively low rate at a current density of 0.2 mA / cm 2 was almost the same as in Examples 1, 2, and 3, but was inferior in cycle characteristics and high rate characteristics. I understood.
[0025]
(Comparative Example 2)
In Example 1, the amount of the expanding agent and the amount of the surfactant were adjusted to obtain a porous thin film electrode having a thin film having a density of 2.6 g / cc. In the same manner as in Examples 1, 2, and 3, an electrochemical cell was manufactured and the negative electrode characteristics were evaluated. As a result, the cycle characteristics were comparable to those of Examples 1, 2, and 3, but a decrease in discharge capacity was observed.
[0026]
Next, batteries using the thin-film electrodes of Examples 1, 2, and 3 and Comparative Examples 1 and 2 as negative electrodes were manufactured. LiCoO 2 was used for the positive electrode. In addition, Al of 20 μm thickness was used for the positive electrode current collector, Cu foil of 10 μm was used for the negative electrode current collector, and polyethylene of 20 μm was used for the separator. A large number of positive / negative electrode areas are laminated in the order of positive electrode / separator / negative electrode with an electrode area of about 11 cm, and inserted into a square stainless steel tube having a height of 4.8 mm. A battery was assembled by injecting an electrolytic solution in which 1.2 M of LiPF 6 was dissolved in the mixed nonaqueous organic solvent. For these batteries, the initial battery capacity value and the capacity retention after 100 cycles were measured. Table 1 shows the results.
[0027]
[Table 1]
As shown in Table 1, the batteries according to Examples 1 to 3 exhibited a capacity of about 1.2 Ah at a current density of 0.5 mA / cm 2 and maintained a capacity of 95% or more even after 100 cycles. Was. A high-rate discharge experiment at a current density of 2 mA / cm 2 was performed, but hardly caused a decrease in capacity. That is, it was found that by using the porous thin film electrode according to the present invention as a negative electrode, a battery having high capacity and excellent in cycle characteristics and high rate characteristics can be formed.
[0029]
In the battery using the electrode according to Comparative Example 1 as the negative electrode, the capacity per unit weight was comparable to Examples 1, 2 and 3, but after 50 cycles, the capacity retention rate was 85%, and after 100 cycles, , To 60% or less. Such a result is due to the fact that the electrode density could not be suppressed due to the high electrode density, and the electrode reaction area was smaller than those of the electrodes of Examples 1, 2, and 3.
[0030]
In the battery using the electrode according to Comparative Example 2 as a negative electrode, the cycle characteristics were comparable to those of Examples 1, 2, and 3, but a decrease in discharge capacity was observed. This indicates that an optimum electrode density (density of the thin film) exists.
[0031]
In Example 1, the pore shape of the mesoporous substance can be changed depending on the type of the surfactant, and is not limited to a hexagonal prism. Furthermore, the apparent thin film density (the value obtained by dividing the thin film weight by the thin film volume) tends to strongly depend on the amount of the expanding agent added, the amount of the surfactant added, the heat treatment temperature, the heat treatment time, and the like. By changing, the density of the thin film can be adjusted.
Claims (5)
Ｓｎ又はＳｎ含有合金からなり、電気メッキ法により前記集電体上に形成された薄膜とを有し、
前記薄膜が、３ｇ／ｃｃ以上、５ｇ／ｃｃ以下の密度を有する多孔質薄膜であることを特徴とする多孔質薄膜電極。A current collector made of a material that does not alloy with Li;
Comprising a thin film formed of Sn or an Sn-containing alloy on the current collector by an electroplating method,
The porous thin film electrode, wherein the thin film is a porous thin film having a density of 3 g / cc or more and 5 g / cc or less.
Ｓｎ又はＳｎ含有合金からなり、前記集電体上に形成された第１の薄膜と、
Ｓｎ又はＳｎ含有合金からなり、電気メッキ法により前記第１の薄膜上に形成された第２の薄膜とを有し、
前記第２の薄膜が、３ｇ／ｃｃ以上、５ｇ／ｃｃ以下の密度を有する多孔質薄膜であることを特徴とする多孔質薄膜電極。A current collector made of a material that does not alloy with Li;
A first thin film made of Sn or an Sn-containing alloy and formed on the current collector;
A second thin film made of Sn or an Sn-containing alloy and formed on the first thin film by an electroplating method,
A porous thin-film electrode, wherein the second thin film is a porous thin film having a density of 3 g / cc or more and 5 g / cc or less.
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