JP4436624B2 - Lithium secondary battery - Google Patents

Description

【００１３】
上記のフッ素含有ホウ酸リチウム誘導体は、特許文献２〜５において、リチウム二次電池の非水電解質に溶解させる溶質として開示されている。しかしながら、これらのリチウム二次電池は、いずれも負極活物質に炭素系材料を用いている。また、その作用効果としては、アルミニウム集電体の表面に被膜を形成し、アルミニウム集電体の腐食を防止することが説明されている。従って、リチウムと合金化することによりリチウムを吸蔵するシリコン等の活物質を用いた場合についての作用効果については開示されていない。本発明においては、従来の代表的な非水電解質の溶質であるＬｉＰＦ₆を用いた場合に比べ、充放電サイクル特性が飛躍的に向上するという作用効果が得られる。このような作用効果については、上記特許文献には何ら記載されていない。
【００１４】
本発明において、フッ素含有ホウ酸リチウム誘導体は、非水電解質中に、０．１重量％以上含有されていることが好ましく、さらに好ましくは１重量％以上である。溶解量の上限値は特に限定されるものではなく、使用する溶媒に対する溶解度により異なる。通常、２０重量％以下であることが好ましい。
【００１５】
本発明における活物質の薄膜は、リチウムを吸蔵すると体積が膨張し、吸蔵したリチウムを放出すると体積が収縮する。このような体積の膨張及び収縮により薄膜に切れ目が形成される。特に、集電体表面に凹凸が存在すると、切れ目がより発生し易くなる。
【００１６】
すなわち、表面に凹凸を有する集電体の上に活物質の薄膜を堆積して形成することにより、活物質の薄膜の表面にも、下地層である集電体表面の凹凸に対応した凹凸を形成することができる。このような薄膜の凹凸の谷部と、集電体表面の凹凸の谷部を結ぶ領域に、低密度領域が形成され易い。上記切れ目は、このような領域に沿って形成され、これによって薄膜が柱状に分離される。
【００１７】
本発明において、集電体表面は、上述のように凹凸が形成されていることが好ましい。従って、集電体表面は粗面化されていることが好ましい。集電体表面の算術平均粗さＲａは０．１μｍ以上であることが好ましく、０．１〜１μｍであることがさらに好ましい。算術平均粗さＲａは、日本工業規格（ＪＩＳ Ｂ ０６０１−１９９４）に定められている。算術平均粗さＲａは、例えば、表面粗さ計により測定することができる。
【００１８】
集電体表面を粗面化する方法としては、めっき法、気相成長法、エッチング法、及び研磨法などが挙げられる。めっき法及び気相成長法は、金属箔からなる集電体の上に、表面に凹凸を有する薄膜層を形成することにより、表面を粗面化する方法である。めっき法としては、電解めっき法及び無電解めっき法が挙げられる。また、気相成長法としては、スパッタリング法、ＣＶＤ法、蒸着法等が挙げられる。エッチング法としては、物理的エッチングや化学的エッチングによる方法が挙げられる。また、研磨法としては、サンドペーパーによる研磨やブラスト法による研磨等が挙げられる。
【００１９】
本発明における集電体は、導電性金属箔から形成されていることが好ましい。導電性金属箔としては、例えば、銅、ニッケル、鉄、チタン、コバルト等の金属またはこれらの組み合わせからなる合金のものを挙げることができる。特に、活物質材料中に拡散し易い金属元素を含有するものが好ましい。このようなものとしては、銅元素を含む金属箔、特に銅箔または銅合金箔が挙げられる。銅合金箔としては、耐熱性銅合金箔を用いることが好ましい。耐熱性銅合金とは、２００℃１時間の焼鈍後の引張り強度が３００ＭＰａ以上である銅合金を意味している。このような耐熱性銅合金箔の上に、算術平均粗さＲａを大きくするために、電解法により銅層または銅合金層を設けた集電体が好ましく用いられる。
【００２０】
本発明における活物質は、リチウムと合金化することによりリチウムを吸蔵する活物質である。このような活物質としては、シリコン、ゲルマニウム、錫、鉛、マグネシウム、ナトリウム、アルミニウム、カリウム、インジウム等が挙げられる。これらの中でも、シリコン及びゲルマニウムが、その高い理論容量から好ましく用いられる。特に、本発明における活物質は、シリコンを主成分とすることが好ましい。シリコンを主成分とする薄膜としては、シリコンを５０原子％以上含む薄膜が挙げられる。具体的には、Ｓｉ−Ｃｏ合金薄膜、Ｓｉ−Ｆｅ合金薄膜、Ｓｉ−Ｚｎ合金薄膜、Ｓｉ−Ｚｒ合金薄膜等が挙げられる。
【００２１】
本発明において、活物質は、非晶質または微結晶であることが好ましい。従って、活物質がシリコンである場合には、非晶質シリコンまたは微結晶シリコンであることが好ましい。
【００２２】
本発明において、活物質は、ＣＶＤ法、スパッタリング法、蒸着法またはめっき法により集電体上に堆積して形成された薄膜であることが好ましい。
本発明において、非水電解質の溶質としては、フッ素含有ホウ酸リチウム誘導体以外の溶質が混合して含まれていてもよい。このような溶質としては、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣＦ₃ＳＯ₃、ＬｉＮ（ＣＦ₃ＳＯ₂)₂、ＬｉＮ（Ｃ₂Ｆ₅ＳＯ₂)₂、ＬｉＮ（ＣＦ₃ＳＯ₂)(Ｃ₄Ｆ₉ＳＯ₂)、ＬｉＣ（ＣＦ₃ＳＯ₂)₃、ＬｉＣ（Ｃ₂Ｆ₅ＳＯ₂)₃、ＬｉＡｓＦ₆、ＬｉＣｌＯ₄、Ｌｉ₂Ｂ₁₀Ｃｌ₁₀、Ｌｉ₂Ｂ₁₂Ｃｌ₁₂など及びそれらの混合物が例示される。
【００２３】
本発明のリチウム二次電池に用いる非水電解質の溶媒は、特に限定されるものではなく、リチウム二次電池の溶媒として用いることができるものであればよい。溶媒としては、環状カーボネートあるいは鎖状カーボネートが好ましい。環状カーボネートとしては、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ビニレンカーボネート等が挙げられる。これらの中でも、特にエチレンカーボネートが好ましく用いられる。鎖状カーボネートとしては、ジメチルカーボネート、メチルエチルカーボネート、ジエチルカーボネート等が挙げられる。さらに溶媒としては、２種以上の溶媒を混合した混合溶媒であることが好ましい。特に、環状カーボネートと鎖状カーボネートとを含む混合溶媒であることが好ましい。
【００２４】
また、上記環状カーボネートと、１，２−ジメトキシエタン、１，２−ジエトキシエタン等のエーテル系溶媒との混合溶媒も好ましく用いられる。
また、本発明においては、電解質として、ポリエチレンオキシド、ポリアクリロニトリル等のポリマー電解質に電解液を含浸したゲル状ポリマー電解質や、ＬｉＩ、Ｌｉ₃Ｎなどの無機固体電解質であってもよい。
【００２５】
本発明において、リチウムと合金化することによりリチウムを吸蔵する活物質は、正極または負極のいずれに用いてもよいが、一般には負極活物質として用いることが好ましい。この場合の正極活物質としては、ＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎ₂Ｏ₄、ＬｉＭｎＯ₂、ＬｉＣｏ_0.5Ｎｉ_0.5Ｏ₂、ＬｉＮｉ_0.7Ｃｏ_0.2Ｍｎ_0.1Ｏ₂などのリチウム含有遷移金属酸化物や、ＭｎＯ₂などのリチウムを含有していない金属酸化物が例示される。また、この他にも、リチウムを電気化学的に挿入、脱離する物質であれば、制限なく用いることができる。
【００２６】
【発明の実施の形態】
以下、本発明を実施例に基づいてさらに詳細に説明するが、本発明は以下の実施例に何ら限定されるものではなく、その要旨を変更しない範囲において適宜変更して実施することが可能なものである。
【００２７】
（実験１）
〔負極の作製〕
耐熱性圧延銅合金箔の表面上に、電解法により銅を析出させて表面を粗面化させた銅合金箔（算術平均粗さＲａ：０．２５μｍ、厚み：２６μｍ）を集電体として用いた。この集電体の上に、表１に示す条件で非晶質シリコン薄膜を堆積し、電極を作製した。ここでは、スパッタリング用の電力として直流パルスを供給しているが、直流や高周波でも同様の条件でスパッタリングが可能である。なお、表１において、流量の単位であるｓｃｃｍは、ｓｔａｎｄａｒｄ ｃｕｂｉｃ ｃｅｎｔｉｍｅｔｅｒ ｐｅｒ ｍｉｎｕｔｅｓである。
【００２８】
【表１】

【００２９】
得られた薄膜を、集電体と共に２５ｍｍ×２５ｍｍの大きさに切取り、負極とした。
【００３０】
〔正極の作製〕
出発原料として、Ｌｉ₂ＣＯ₃及びＣｏＣＯ₃を用いて、Ｌｉ：Ｃｏの原子比が１：１となるように秤量して乳鉢で混合し、これを直径１７ｍｍの金型でプレスし、加圧成形した後、空気中において、８００℃で２４時間焼成し、ＬｉＣｏＯ₂の焼成体を得た。これを乳鉢で粉砕し、平均粒子径２０μｍに調製した。
【００３１】
得られたＬｉＣｏＯ₂粉末９０重量部と、導電剤としての人工黒鉛粉末５重量部を、結着剤としてのポリフッ化ビニリデン５重量部を含む５重量％のＮ−メチルピロリドン溶液に混合し、正極合剤スラリーとした。
【００３２】
この正極合剤スラリーを、集電体であるアルミニウム箔の上に塗布し、乾燥した後圧延した。得られたものを２０ｍｍ×２０ｍｍに切り抜き、正極とした。
【００３３】
〔電解液Ａの作製〕
エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）を体積比３：７で混合した溶媒に、化学式（１）に示す、セントラル硝子株式会社製フッ素含有ホウ酸リチウム誘電体を１モル／リットル溶解し、電解液Ａを作製した。
【００３４】
〔電解液Ｂの作製〕
エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）を体積比３：７で混合した溶媒に、ＬｉＰＦ₆を１モル／リットル溶解し、電解液Ｂを作製した。
【００３５】
〔電池の作製〕
上記の正極及び負極に、それぞれ正極集電タブ及び負極集電タブを取り付けた後、正極及び負極の間に多孔質ポリエチレンからなるセパレータを挟んで電極群とし、この電極群をアルミニウムラミネートからなる外装体内に挿入した。次に、上記電解液Ａ及びＢを６００μｌ注入し、電池Ａ１及び電池Ｂ１を作製した。電池の設計容量は、１４ｍＡｈである。
【００３６】
〔充放電特性の評価〕
上記の電池Ａ１及びＢ１について、充放電サイクル特性を評価した。各電池を２５℃において、電流値１４ｍＡで４．２Ｖまで充電した後、電流値１４ｍＡで２．７５Ｖまで放電し、これを１サイクルの充放電とした。充放電試験のサイクル特性を図１に示す。また、初期充放電容量と、１００サイクル後及び２００サイクル後の容量維持率を表２に示す。なお、Ｎサイクル後の容量維持率は、以下の計算式により求めた。
【００３７】
Ｎサイクル後の容量維持率（％）＝（Ｎサイクル目の放電容量）／（１サイクル目の放電容量）×１００
【００３８】
【表２】

【００３９】
図１及び表２に示す結果から明らかなように、本発明に従いフッ素含有ホウ酸リチウム誘導体を溶質として含有した電池Ａ１は、電池Ｂ１よりも高い容量維持率を示しており、充放電サイクル特性に優れていることがわかる。これは、非水電解質中の溶質と活物質との反応により、被膜が、活物質薄膜の柱状部分の側部の表面に選択的に形成され、これによって薄膜の柱状構造が安定化されたためと考えられる。このため、柱状部分の劣化や崩壊が抑制され、充放電サイクル特性を向上することができたと考えられる。
【００４０】
〔電極のＳＥＭ観察〕
２００サイクル目の放電後の電池Ａ１から負極Ａ１１を取り出し、走査型電子顕微鏡（ＳＥＭ）で観察した。図４は、負極Ａ１１のＳＥＭ像である。図４から明らかなように、薄膜が厚み方向の切れ目により分離され、柱状構造が形成されていることがわかる。
【００４１】
〔負極の深さ方向の組成分析〕
負極の表面に形成された被膜を調べるため、１サイクルの充電後における電池Ａ１及び電池Ｂ１からそれぞれ負極Ａ１１及び負極Ｂ１１を取り出し、これらの負極をＡｒイオンでエッチングしながらＸ線光電子分光分析装置を用いて、深さ方向の組成を分析した。Ａｒイオンによるスパッタリング速度は１０ｎｍ／分である。
【００４２】
図２は、負極Ａ１１表面の深さ方向の組成を示しており、図３は、負極Ｂ１１表面の深さ方向の組成を示している。組成としては、Ｓｉ、Ｃ、Ｆ及びＢを示している。
【００４３】
また、表面から深さ約５０ｎｍ（スパッタ時間５分）までの酸素（Ｏ）、フッ素（Ｆ）及びホウ素（Ｂ）の最大値を表３に示す。
【００４４】
【表３】

【００４５】
図２及び図３において、電極の表面で炭素濃度が高い値を示しているが、これは電極表面に付着した残留物または汚染によるものと考えられる。
図２、図３及び表３の結果より、本発明に従い溶質としてフッ素含有ホウ酸リチウム誘電体を用いた電極Ａ１１の表面近傍には、電極Ｂ１１に比べ、フッ素及びホウ素が多く含まれていることがわかる。このことから、溶質としてフッ素含有ホウ酸リチウム誘導体を用いることにより、電極の表面にフッ素とホウ素を多く含む安定な被膜が形成されているものと思われる。また、被膜中におけるフッ素とホウ素の組成比はＦ／Ｂ＝３〜１５の範囲で表されるものと考えられる。このような被膜が、充放電によって新たに生じた活性な活物質薄膜の表面に順次形成されることにより、柱状部分の構造が安定化し、充放電サイクル特性が飛躍的に向上したものと考えられる。
【００４６】
なお、負極活物質として、黒鉛系材料を用いた場合には、フッ素含有ホウ酸リチウム誘導体を用いることによる充放電サイクル特性の向上の効果は認められなかった。従って、リチウムと合金化することによりリチウムを吸蔵する活物質を用いた場合に、このような充放電サイクル特性の向上が認められるものと思われる。
【００４７】
【発明の効果】
本発明によれば、充放電サイクル特性をさらに向上させることができる。
【図面の簡単な説明】
【図１】本発明に従う実施例のリチウム二次電池の充放電サイクル特性を示す図。
【図２】本発明に従う実施例の負極Ａ１１表面の深さ方向の組成を示す図。
【図３】比較の負極Ｂ１１表面の深さ方向の組成を示す図。
【図４】本発明に従う実施例の負極Ａ１１のＳＥＭ像を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery.
[0002]
[Prior art]
In recent years, lithium secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between a positive electrode and a negative electrode have been used as one of new secondary batteries with high output and high energy density. .
[0003]
As such an electrode for a lithium secondary battery, an electrode using a material alloyed with lithium as a negative electrode active material has been studied. As a material alloyed with lithium, for example, silicon has been studied. However, materials that alloy with lithium, such as silicon, expand and contract the volume of the active material when occluding and releasing lithium, so that the active material is pulverized with charge and discharge, or the active material is a current collector Detach from. For this reason, there existed a problem that the current collection property in an electrode fell and charging / discharging cycling characteristics worsened.
[0004]
The present applicant has shown that an electrode formed by depositing an active material thin film that absorbs and releases lithium, such as an amorphous silicon thin film and a microcrystalline silicon thin film, on a current collector exhibits a high charge / discharge capacity and is excellent. It has been found that it exhibits charge / discharge cycle characteristics (Patent Document 1).
[0005]
In such an electrode, the active material thin film is separated in a columnar shape by a cut formed in the thickness direction thereof, and has a structure in which the bottom of the columnar portion is in close contact with the current collector. In the electrode having such a structure, a gap is formed around the columnar portion, and the stress due to expansion and contraction of the thin film accompanying the charge / discharge cycle is relieved by this gap, and the active material thin film is peeled off from the current collector. Since generation | occurrence | production of such a stress can be suppressed, the outstanding charge / discharge cycle characteristic is acquired.
[0006]
[Patent Document 1]
International Publication No. 01/29913 Pamphlet [Patent Document 2]
JP 2002-110235 A [Patent Document 3]
JP 2002-184460 A [Patent Document 4]
JP 2002-184465 A [Patent Document 5]
Japanese Patent Laid-Open No. 2002-373703
[Problems to be solved by the invention]
However, in a lithium secondary battery using such an electrode, there has been a demand for further improving charge / discharge cycle characteristics.
[0008]
An object of the present invention is to provide a lithium secondary battery using an electrode formed by depositing an active material that occludes lithium on a current collector by alloying with lithium, and further improving the charge / discharge cycle characteristics. The next battery is to provide.
[0009]
[Means for Solving the Problems]
The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode or the negative electrode is formed by depositing an active material that occludes lithium on the current collector by alloying with lithium. In the lithium secondary battery, the active material is separated into a columnar shape by a cut formed in the thickness direction, and the bottom of the columnar portion is in contact with the current collector. As a solute of the water electrolyte, a fluorine-containing lithium borate derivative is contained , and a film is formed on the side surface of the columnar part .
[0010]
In the present invention, an electrode formed by depositing an active material that occludes lithium on a current collector by alloying with lithium is used as the positive electrode or the negative electrode. In this electrode, a cut is formed in the thickness direction of the thin film of the active material, and the thin film is separated into columns. For this reason, even if the volume of the thin film expands and contracts due to charge and discharge, the expansion and contraction of the volume is absorbed by the voids around the columnar portion, and the generation of stress in the thin film is suppressed. be able to. For this reason, pulverization of a thin film and peeling from a current collector can be prevented, and charge / discharge cycle characteristics can be improved. In the present invention, a fluorine-containing lithium borate derivative is contained as a solute of the nonaqueous electrolyte, and this fluorine-containing lithium borate derivative acts on the side surface of the columnar portion formed in this way. It seems to form a film. Since a stable film is formed by the fluorine-containing lithium borate derivative, the structure of the columnar part is stabilized, the fine powdering of the active material thin film is further suppressed, and the charge / discharge cycle characteristics are expected to be dramatically improved. It is.
[0011]
Examples of the fluorine-containing lithium borate derivative used in the present invention include compounds represented by the following chemical formula (1).
[0012]
[Chemical formula 2]

[0013]
Said fluorine-containing lithium borate derivative is disclosed in Patent Documents 2 to 5 as a solute to be dissolved in the non-aqueous electrolyte of a lithium secondary battery. However, all of these lithium secondary batteries use a carbon-based material as the negative electrode active material. Further, as its effect, it is described that a film is formed on the surface of the aluminum current collector to prevent corrosion of the aluminum current collector. Therefore, there is no disclosure about the effect of using an active material such as silicon that occludes lithium by alloying with lithium. In the present invention, the effect of dramatically improving the charge / discharge cycle characteristics can be obtained as compared with the case of using LiPF ₆ which is a typical representative nonaqueous electrolyte solute. Such an effect is not described in the above patent document.
[0014]
In the present invention, the fluorine-containing lithium borate derivative is preferably contained in the nonaqueous electrolyte in an amount of 0.1% by weight or more, more preferably 1% by weight or more. The upper limit of the amount of dissolution is not particularly limited, and varies depending on the solubility in the solvent used. Usually, it is preferably 20% by weight or less.
[0015]
The active material thin film of the present invention expands in volume when lithium is occluded, and shrinks in volume when occluded lithium is released. A cut is formed in the thin film by such expansion and contraction of the volume. In particular, when unevenness is present on the surface of the current collector, breaks are more likely to occur.
[0016]
That is, by forming a thin film of an active material on a current collector having irregularities on the surface, the surface of the thin film of active material also has irregularities corresponding to the irregularities of the current collector surface that is the underlayer. Can be formed. A low density region is likely to be formed in a region connecting the uneven valley of the thin film and the uneven valley of the current collector surface. The cut is formed along such a region, whereby the thin film is separated into columns.
[0017]
In the present invention, the surface of the current collector is preferably provided with irregularities as described above. Therefore, the current collector surface is preferably roughened. The arithmetic average roughness Ra of the current collector surface is preferably 0.1 μm or more, and more preferably 0.1 to 1 μm. The arithmetic average roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994). The arithmetic average roughness Ra can be measured by, for example, a surface roughness meter.
[0018]
Examples of the method for roughening the current collector surface include a plating method, a vapor phase growth method, an etching method, and a polishing method. The plating method and the vapor phase growth method are methods for roughening the surface by forming a thin film layer having irregularities on the surface of a current collector made of a metal foil. Examples of the plating method include an electrolytic plating method and an electroless plating method. Examples of the vapor phase growth method include a sputtering method, a CVD method, and a vapor deposition method. Examples of the etching method include a physical etching method and a chemical etching method. Examples of the polishing method include sandpaper polishing and blasting.
[0019]
The current collector in the present invention is preferably formed from a conductive metal foil. Examples of the conductive metal foil include metals such as copper, nickel, iron, titanium, cobalt, and alloys made of combinations thereof. In particular, a material containing a metal element that easily diffuses into the active material is preferable. As such a thing, the metal foil containing a copper element, especially copper foil or copper alloy foil is mentioned. It is preferable to use a heat resistant copper alloy foil as the copper alloy foil. The heat resistant copper alloy means a copper alloy having a tensile strength of 300 MPa or more after annealing at 200 ° C. for 1 hour. In order to increase the arithmetic average roughness Ra on such a heat-resistant copper alloy foil, a current collector provided with a copper layer or a copper alloy layer by an electrolytic method is preferably used.
[0020]
The active material in the present invention is an active material that occludes lithium by alloying with lithium. Examples of such an active material include silicon, germanium, tin, lead, magnesium, sodium, aluminum, potassium, and indium. Among these, silicon and germanium are preferably used because of their high theoretical capacity. In particular, the active material in the present invention is preferably composed mainly of silicon. Examples of the thin film containing silicon as a main component include a thin film containing 50 atomic% or more of silicon. Specific examples include Si—Co alloy thin films, Si—Fe alloy thin films, Si—Zn alloy thin films, Si—Zr alloy thin films, and the like.
[0021]
In the present invention, the active material is preferably amorphous or microcrystalline. Therefore, when the active material is silicon, amorphous silicon or microcrystalline silicon is preferable.
[0022]
In the present invention, the active material is preferably a thin film formed by being deposited on a current collector by a CVD method, a sputtering method, a vapor deposition method or a plating method.
In the present invention, as the solute of the nonaqueous electrolyte, a solute other than the fluorine-containing lithium borate derivative may be mixed and contained. Such solutes include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F _{9 SO 2), LiC (CF 3} SO 2) 3, LiC (C 2 F 5 SO 2) 3, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and the like and mixtures thereof examples Is done.
[0023]
The non-aqueous electrolyte solvent used in the lithium secondary battery of the present invention is not particularly limited as long as it can be used as a solvent for the lithium secondary battery. As the solvent, cyclic carbonate or chain carbonate is preferable. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate and the like. Among these, ethylene carbonate is particularly preferably used. Examples of the chain carbonate include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and the like. Further, the solvent is preferably a mixed solvent obtained by mixing two or more solvents. In particular, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferable.
[0024]
A mixed solvent of the cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane is also preferably used.
In the present invention, the electrolyte may be a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide or polyacrylonitrile with an electrolytic solution, or an inorganic solid electrolyte such as LiI or Li ₃ N.
[0025]
In the present invention, the active material that occludes lithium by alloying with lithium may be used for either the positive electrode or the negative electrode, but is generally preferably used as the negative electrode active material. Examples of the positive electrode active material in this case include lithium-containing transition metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ , and MnO _2. Examples thereof include metal oxides not containing lithium. In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range not changing the gist thereof. Is.
[0027]
(Experiment 1)
(Production of negative electrode)
A copper alloy foil (arithmetic mean roughness Ra: 0.25 μm, thickness: 26 μm) obtained by precipitating copper by an electrolytic method on the surface of a heat resistant rolled copper alloy foil is used as a current collector. It was. An amorphous silicon thin film was deposited on the current collector under the conditions shown in Table 1 to produce an electrode. Here, a DC pulse is supplied as power for sputtering, but sputtering can be performed under the same conditions even at DC or high frequency. In Table 1, sccm, which is a unit of flow rate, is standard cubic centimeter per minutes.
[0028]
[Table 1]

[0029]
The obtained thin film was cut into a size of 25 mm × 25 mm together with the current collector to obtain a negative electrode.
[0030]
[Production of positive electrode]
Using Li ₂ CO ₃ and CoCO ₃ as starting materials, weigh them so that the atomic ratio of Li: Co is 1: 1, mix them in a mortar, press this with a 17 mm diameter mold, pressurize After the molding, it was fired at 800 ° C. for 24 hours in the air to obtain a LiCoO ₂ fired body. This was pulverized in a mortar to prepare an average particle size of 20 μm.
[0031]
90 parts by weight of the obtained LiCoO ₂ powder and 5 parts by weight of artificial graphite powder as a conductive agent were mixed in a 5% by weight N-methylpyrrolidone solution containing 5 parts by weight of polyvinylidene fluoride as a binder. A mixture slurry was obtained.
[0032]
This positive electrode mixture slurry was applied onto an aluminum foil as a current collector, dried and then rolled. The obtained product was cut out to 20 mm × 20 mm to obtain a positive electrode.
[0033]
[Preparation of Electrolytic Solution A]
In a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7, 1 mol / liter of a fluorine-containing lithium borate dielectric made by Central Glass Co., Ltd., represented by chemical formula (1), is dissolved. Electrolyte A was produced.
[0034]
[Preparation of Electrolytic Solution B]
LiPF ₆ was dissolved by 1 mol / liter in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 to prepare an electrolytic solution B.
[0035]
[Production of battery]
After attaching a positive electrode current collecting tab and a negative electrode current collecting tab to the positive electrode and the negative electrode, respectively, an electrode group is formed by sandwiching a separator made of porous polyethylene between the positive electrode and the negative electrode, and this electrode group is made of an aluminum laminate. Inserted into the body. Next, 600 μl of the electrolytic solutions A and B were injected to prepare a battery A1 and a battery B1. The design capacity of the battery is 14 mAh.
[0036]
[Evaluation of charge / discharge characteristics]
Charge / discharge cycle characteristics of the batteries A1 and B1 were evaluated. Each battery was charged to 4.2 V at a current value of 14 mA at 25 ° C., and then discharged to 2.75 V at a current value of 14 mA. The cycle characteristics of the charge / discharge test are shown in FIG. Table 2 shows the initial charge / discharge capacity and the capacity retention rate after 100 cycles and 200 cycles. The capacity retention rate after N cycles was determined by the following calculation formula.
[0037]
Capacity retention ratio after N cycles (%) = (discharge capacity at N cycle) / (discharge capacity at first cycle) × 100
[0038]
[Table 2]

[0039]
As is clear from the results shown in FIG. 1 and Table 2, the battery A1 containing the fluorine-containing lithium borate derivative as a solute according to the present invention shows a higher capacity retention rate than the battery B1, and has a charge / discharge cycle characteristic. It turns out that it is excellent. This is because the film is selectively formed on the side surface of the columnar portion of the active material thin film due to the reaction between the solute in the nonaqueous electrolyte and the active material, thereby stabilizing the columnar structure of the thin film. Conceivable. For this reason, it is thought that deterioration and collapse of the columnar part were suppressed, and charge / discharge cycle characteristics could be improved.
[0040]
[SEM observation of electrodes]
The negative electrode A11 was taken out from the battery A1 after discharge at the 200th cycle and observed with a scanning electron microscope (SEM). FIG. 4 is an SEM image of the negative electrode A11. As is apparent from FIG. 4, the thin films are separated by the cuts in the thickness direction, and a columnar structure is formed.
[0041]
[Analytical composition in the depth direction of the negative electrode]
In order to examine the film formed on the surface of the negative electrode, the negative electrode A11 and the negative electrode B11 were taken out from the battery A1 and the battery B1 after one cycle of charging, respectively, and an X-ray photoelectron spectrometer was used while etching these negative electrodes with Ar ions. Used to analyze the composition in the depth direction. The sputtering rate by Ar ions is 10 nm / min.
[0042]
FIG. 2 shows the composition in the depth direction on the surface of the negative electrode A11, and FIG. 3 shows the composition in the depth direction on the surface of the negative electrode B11. As the composition, Si, C, F and B are shown.
[0043]
Table 3 shows the maximum values of oxygen (O), fluorine (F), and boron (B) from the surface to a depth of about 50 nm (sputtering time 5 minutes).
[0044]
[Table 3]

[0045]
2 and 3, the carbon concentration at the surface of the electrode shows a high value, which is considered to be due to residue or contamination attached to the electrode surface.
2, 3 and Table 3 show that the vicinity of the surface of the electrode A11 using the fluorine-containing lithium borate dielectric as a solute according to the present invention contains more fluorine and boron than the electrode B11. I understand. From this, it is considered that a stable film containing a large amount of fluorine and boron is formed on the surface of the electrode by using a fluorine-containing lithium borate derivative as a solute. Moreover, it is thought that the composition ratio of fluorine and boron in the film is expressed in the range of F / B = 3-15. It is considered that such a film is formed sequentially on the surface of the active active material thin film newly generated by charging / discharging, so that the structure of the columnar portion is stabilized and the charge / discharge cycle characteristics are dramatically improved. .
[0046]
When a graphite-based material was used as the negative electrode active material, no effect of improving charge / discharge cycle characteristics by using a fluorine-containing lithium borate derivative was observed. Therefore, when an active material that occludes lithium by alloying with lithium is used, such an improvement in charge / discharge cycle characteristics is recognized.
[0047]
【The invention's effect】
According to the present invention, charge / discharge cycle characteristics can be further improved.
[Brief description of the drawings]
FIG. 1 is a graph showing charge / discharge cycle characteristics of a lithium secondary battery according to an embodiment of the present invention.
FIG. 2 is a diagram showing a composition in the depth direction of the surface of a negative electrode A11 of an example according to the present invention.
FIG. 3 is a view showing the composition in the depth direction of the surface of a comparative negative electrode B11.
FIG. 4 is a view showing an SEM image of a negative electrode A11 of an example according to the present invention.

Claims (9)

A positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode or the negative electrode is an electrode formed by depositing an active material that occludes lithium by alloying with lithium on the current collector, and In the lithium secondary battery that is an electrode in which the active material is separated into columns by the cuts formed in the thickness direction, and the bottom of the columnar part is in close contact with the current collector,
As the solute of the non-aqueous electrolyte, a fluorine-containing lithium borate derivative is contained ,
A lithium secondary battery, wherein a film is formed on a surface of a side portion of the columnar portion . The lithium secondary battery according to claim 1, wherein the current collector surface has an arithmetic average roughness Ra of 0.1 to 1 μm. The lithium secondary battery according to claim 1, wherein the active material is a thin film formed by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method. The lithium secondary battery according to claim 1, wherein the active material contains silicon as a main component. The lithium secondary battery according to claim 1, wherein the active material is amorphous silicon or microcrystalline silicon. The lithium secondary battery according to any one of claims 1 to 5, wherein the fluorine-containing lithium borate derivative is represented by the following chemical formula (1).

The lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte includes a mixed solvent composed of two or more kinds of solvents. The lithium secondary battery according to claim 7, wherein the mixed solvent is a mixed solvent containing a cyclic carbonate and a chain carbonate. The lithium secondary battery according to claim 8, wherein ethylene carbonate is included as the cyclic carbonate.

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