JP4682467B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

【００１５】
（ただし、Ｓは測定面積、ρは測定電解液の比抵抗、εはセパレータの体積と重量から計算した空孔率、Ｌはセパレータの膜厚である。）
【００１６】
【数４】

【００１７】
（ただし、εはセパレータの体積と重量から計算した空孔率、πは円周率、ｄは水銀ポロシメータ法で測定した平均孔径である。）
本発明における正極は従来公知の構成であるが、コバルト酸リチウム、ニッケル酸リチウム、マンガンスピネルなどのリチウム含有複合酸化物を活物質とし、導電剤と結着剤を混合した合剤が集電体に塗工されて作製されている。
【００１８】
本発明における負極は、活物質には天然黒鉛や人造黒鉛などの炭素が主に使われるが、その他にアルミニウムやアルミニウムを主体とする種々の合金や、酸化スズなどを初めとする種々の金属酸化物、金属窒化物など従来公知のものがあり、正極と同様に導電剤と結着剤を混合した合剤が集電体に塗工されて作製されている。
【００１９】
非水電解質（以下電解液という）には、非水溶媒としてエチレンカーボネート（ＥＣ）やプロピレンカーボネート（ＰＣ）等の環状カーボネート類と、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）およびエチルメチルカーボネート（ＥＭＣ）などの鎖状カーボネート類の２種以上を混合したものが好ましい。
【００２０】
また、電解質塩は従来公知のリチウム塩が使えるが、ＬｉＰＦ₆やＬｉＢＦ₄などが好ましい。
【００２１】
本発明における添加剤は最大動作電圧を超える電池電圧で重合して、導電性ポリマーを生成することにより、過充電酷使時に内部短絡を発生する芳香族化合物からなる添加剤であり、例えば、ヘキサフェニルベンゼン、ターフェニル、フェニルピペラジン、テトラヒドロイソキノリン、フェニルシクロヘキサン、トリフェニルベンゼン、ドデカヒドロトリフェニレン、ジビニルベンゼン、ジシクロヘキシルベンゼン、ビフェニル、ピロール、Ｎ―メチルピロール、チオフェン、フラン、インドール、クロロチオフェン、ブロモチオフェン、フルオロチオフェン、ジメトキシべンゼンやメチルピリジニウムテトラフルオロボーレート、ジフェニルエーテル、ベンゾフラン、ジアリルエーテル、アリルブチルエーテル、フェノキシトルエン、ターフェニル、メチルターフェニル、ジメチルターフェニルなど、およびこれらの誘導体がある。
【００２２】
この中で、好ましいものとしてジフェニルエーテル、フェニルシクロヘキサン、ターフェニル、ビフェニル、フラン、チオフェン、およびこれらの誘導体がある。具体的には、ビフェニル、ピロール、Ｎ―メチルピロール、チオフェン、フラン、インドール、３−クロロチオフェン、３−ブロモチオフェン、３−フルオロチオフェン、１，２ジメトキシべンゼンや１−メチル−３−ピリジニウムテトラフルオロボーレートなどがある。
【００２３】
これらのうちでも、添加剤の重合電位と電池作動下での安定性の点から、ジフェニルエーテル、フェニルシクロヘキサン、ｏ−ターフェニル、ビフェニル、フラン、インドールおよび３−クロロチオフェンが、特に好ましい。
【００２４】
本発明のセパレータは大きなイオン透過度を持ち、適度な機械的強度がある電子絶縁性の微多孔性薄膜であり、材質としては耐有機溶剤性と疎水性の観点とシャットダウン機能を持つ点から、ポリプロピレン樹脂、ポリエチレン樹脂の単独またはこれらを積層したものや混合・複合したものなどの多孔質ポリオレフィンを用いる。
【００２５】
多孔質ポリオレフィンの製造方法は大きく分けて湿式と乾式の二つのプロセスがあるが、いずれも空孔を持たせた後、一軸あるいは二軸で延伸して多孔質にする。そのため、単純な１本の貫通孔が多数存在しているというわけではないが、孔径や細孔経路長、および孔数を規定するために、図１にしめすモデルが、一般的に使われている。多孔質セパレータ１は、実際にはセパレータ表面２に様々な大きさの孔が空いているが、それらの平均孔径ｄを孔径とする同じ大きさの貫通孔３があると仮定して、細孔経路長や曲路率および単位面積あたりの孔数を規定している。
【００２６】
つまり、細孔経路長は、貫通孔３の長さであり、セパレータ厚み４をＬとすると貫通孔経路長はτＬと表すことができて、この比例係数τを曲路率と呼ぶ。
【００２７】
この曲路率の測定は、多孔質セパレータを完全に電解液に浸し、交流抵抗測定法により行う。この場合、貫通孔３がすべて電解液に満たされた状態となっており、貫通孔３と同じ形状の導電体があると仮定できる。この時、電気抵抗Ｒｍは、貫通孔の経路長τＬと測定面積Ｓ中にある貫通孔の総断面積の比と、電解液の比抵抗ρとの積で表すことが出来る。貫通孔の総断面積は、全空孔体積が測定面積Ｓとセパレータ厚みＬと空孔率εの積（ＳＬε）で表すことができ、これを経路長τＬで割ったものとなる。したがって、これらを整理すると、Ｒｍは、（数５）のように表すことができる。
【００２８】
【数５】

【００２９】
この（数５）を変形して曲路率εについて求めると（数３）になる。
【００３０】
さらに、貫通孔１個の体積Ｖは、円周率をπとして（数６）で表せるから、孔個数をｎとすると全空孔体積（ＳＬε）は、（数７）で表せる。
【００３１】
【数６】

【００３２】
【数７】

【００３３】
この（数７）を変形して単位面積あたりの孔数（ｎ／Ｓ）について求めると（数４）になる。
【００３４】
この平均孔径ｄの孔がｎ個空いているというモデルは、例えば曲路率τを用いて透気抵抗度Ｔを（数８）のように示せるなどセパレータの物性を理論的に説明するのに都合が良く、一般に広く使われている。
【００３５】
【数８】

【００３６】
ここで、Ｃは比例定数で、例えばポリエチレンセパレータでは概ね０．１３である。
【００３７】
本発明の電池における過充電時の高温環境下での本発明の芳香族添加剤とセパレータによる内部短絡のメカニズムは、概ね次のようである。
【００３８】
過充電時に電池の電圧が最大動作電圧以上になると、芳香族添加剤が正極表面またはセパレータ細孔内で重合して成長し、ポリマー層を形成する。このポリマー層は、さらに電圧が高くなると部分的に酸化されて、正の電荷を持ち、そして正の電荷部に、電解液中の陰イオン、例えばＰＦ^6-などがドープされることにより導電性ポリマーとなる。さらに過充電状態が進むと負極にリチウムが析出する様になる。
【００３９】
正極の表面および正極側のセパレータ表面や細孔内で重合した導電性ポリマーが存在する中、過充電が進行すると負極側に析出したリチウムが針状に正極に向って成長し、ついには正極に達し内部短絡が起こる。次に、この作用において好適なセパレータの単位面積（平方マイクロメートル）あたりの孔数が存在する。この理由は負極側の析出物（以下、デンドライトと言う）がセパレータの細孔径ｄや曲路率τとは無関係に針状に稲妻のように成長していく。過充電の早期段階で内部短絡を起こすにはデンドライトを集中させて細孔内の導電性ポリマーとつなげることが重要であり、セパレータの貫通孔の孔数との相関になる。すなわち単位面積あたりの孔数が技術的に意義のあるパラメータである。
【００４０】
本発明におけるセパレータは、単位面積（平方マイクロメートル）あたりの孔数が１００以下である。この孔数が１０１以上である場合は導電性ポリマーがセパレータを貫通して正極に達するまでの時間がかかり過充電が進行するため危険な状況になる。したがって、前述の単位面積（平方マイクロメートル）あたりの孔数が１００以下であることが好ましい。
【００４１】
この結果、本発明のセパレータを用いた電池では、過充電過程の比較的早い段階から正極と負極の充電深度が必要以上に過度な状態に到達せず、過充電時の危険性を抑制できる。
【００４２】
この作用において、芳香族添加剤が添加されていないか、されていても０．１重量％未満の場合は、導電性ポリマーが十分成長しないうちに過充電状態が危険な状態まで進行してしまう。
【００４３】
また、芳香族添加剤添加量が多い場合、例えば１０重量％より大きい場合は、過充電状態の抑制効果は、発揮できるが、通常の電池特性、特に高温保存時の特性が劣化するため不適合である。
【００４４】
芳香族添加剤とこれら前述の特性パラメータを持つセパレータを組み合わせることにより、過充電時には、重合した導電性ポリマーによってセパレータを貫通して内部短絡が起こり過充電状態を解消する。そのため高容量で信頼性に優れた非水電解質二次電池を提供することが可能になる。
【００４５】
本発明者らは単位面積（平方マイクロメートル）あたりの孔数を確保しつつ通常の電池性能を満足するための透気抵抗度、空孔率、平均孔径、曲路率等から鋭意検討の結果、下記の特性をもつセパレータが、本発明の電池に好適であるという結論を得た。特に、芳香族添加剤が、ジフェニルエーテル、フェニルシクロヘキサン、ｏ−ターフェニル、ビフェニル、フラン、インドールおよび３−クロロチオフェンからなる群から少なくとも１つ選択されるものである場合は、前記セパレータの単位面積当りの孔数が５０以下であるが非常に優れた効果を示す。
【００４６】
セパレータの透気抵抗度は４６０秒／１００ｍｌ以上では電池の高効率放電などの電池特性の点で不利になるため４５０秒／１００ｍｌ以下であることが好ましく空孔率が２９％以下でも同様に電池特性の点で不利なるため３０％以上であることが好ましい。また厚みが３１μｍ以上では電池の高容量化や高率放電などの電池特性の点で不利になるだけでなく、内部短絡が起こりにくくなるため３０μｍ以下が好ましい。また、厚みが７μｍ以下になると過充電時の高温環境下で内部短絡が確実に発生しても、薄さのため電池が危険な状況になる。したがって、セパレータの厚さが８から３０μｍであることが好ましい。しかし通常範囲での電池特性を満足する透気抵抗度や空孔率、厚み等の物性値と過充電時の安全性を確保するための単位面積（平方マイクロメートル）あたりの孔数を確保するための物性値は相反した関係にある。これら両者を満足する共通物性として平均孔径が０．０６μｍ以上であることが好ましい。
【００４７】
ここで、以上述べてきたセパレータ特性のそれぞれの測定法について、以下に説明する。
【００４８】
細孔径については流体透過法を用いて平均孔径を算出する方法、バブルポイント法を用いて最大孔径を検出する方法、水銀圧入法を用いて平均孔径を算出する方法、ＳＥＭ表面写真から表面孔径を観察する方法等さまざまな測定法があるが、これらの値は、測定法によって必ずしも一致せず、測定法の違う値を用い比較することはできない。そこで発明者らは水銀圧入法より測定した平均孔径ｄを採用した。
【００４９】
透気抵抗度Ｔについては、ＪＩＳ Ｐ８１１７−１９９８で規定されている測定法を採用した。
【００５０】
電気抵抗Ｒｍは、セパレータを電解液に完全に浸し、２．５４ｃｍ²の面積を持つステンレスの板で挟んで交流抵抗測定法で測定した。交流抵抗測定に際しては、測定周波数を順次走査しながら測定し、実数軸と交わる値を電気抵抗Ｒｍとした。この値は、理論的には周波数が無限大の値であるが、ポリエチレン単層膜では、１００ｋＨｚで、ほぼこの値と同一になる。
【００５１】
また、空孔率εは、セパレータの体積と重量から見かけの比重を計算し、真比重との違いから求めた。
【００５２】
【実施例】
次に、実施例を用いて本発明の具体例について説明する。
【００５３】
まず、セパレータは多数のポリエチレン単層セパレータを測定して、以下に述べる各種の特性を持つセパレータを抽出した。
【００５４】
＜セパレータの抽出＞
本実施例ではセパレータＡとして、以下の特性を持つ厚さ１６μｍのポリエチレン単層セパレータを選んだ。
【００５５】
水銀圧入法により測定した平均孔径ｄは０．０７５μｍであり、空孔率εは４０％であった。
【００５６】
電気抵抗測定において電解液は、エチレンカーボネート（ＥＣ）とエチルメチルカーボネート（ＥＭＣ）の体積比１：１の混合溶媒にＬｉＰＦ₆を１モル／リットル溶解したものを使用した。この電解液の比抵抗ρは１１６Ωｃｍである。
電気抵抗Ｒｍは、０．７３８Ωであった。
【００５７】
このＲｍ値を用い（数３）から求めた曲路率τは、２．０１であり、さらにこの値を（数４）に代入して求めた単位面積（平方マイクロメートル）あたりの孔数は４５個であった。
【００５８】
最後に２３℃に調温された実験室内でＪＩＳに準拠したＡ型測定装置を用いて透気抵抗度を測定した。透気抵抗度Ｔは３１０秒／１００ｍｌであった。
以下、セパレータＡと同様の方法で、（表１）に示すような物性値のセパレータＡからＮの１３種のセパレータを選びだした。
【００５９】
【表１】

【００６０】
＜電池の作製＞
本発明の電池の過充電時の温度変化を評価するため、以下に説明する角型電池を作製した。
【００６１】
図２に本発明の実施例１から３４の角型電池の構造図（一部断面図）を示す。
【００６２】
図２において、非水電解質二次電池５は正極６と負極７とセパレータ８が捲回されて、ケース９内に非水溶媒に電解質塩を溶解した電解液（図示せず）とともに内蔵されており、封口板１０で密閉されているおり、封口板には一般の円筒電池においては、安全弁やＰＴＣ素子などの安全素子が組み込まれているが、実施例の電池においては角型のため封口板１０には一切の安全機構は組み込まれていない。
【００６３】
正極６はコバルト酸リチウム粉末８５重量％に対し、導電剤の炭素粉末１０重量％と結着剤のポリ弗化ビニリデン樹脂（ＰＶｄＦ樹脂）５重量％を混合し、これらを脱水ＮＭＰに分散させてスラリーを作製し、アルミ箔からなる正極集電体上に塗布し、乾燥後、圧延して作製した。
【００６４】
負極７は負極活物質として人造黒鉛粉末を用い、これの９５重量％に対して、結着剤のＰＶｄＦ樹脂を５重量％を混合し、これらを脱水ＮＭＰに分散させてスラリーを作製し、銅箔からなる負極集電体上に塗布し、乾燥後、圧延して作製した。
【００６５】
セパレータ８には前述の（表１）に示すセパレータＡからＮの１３種のセパレータを使用した。
【００６６】
また、電解液には、電気抵抗Ｒｍの測定に用いたものと同じ、ＥＣとＥＭＣの体積比１：１の混合溶媒にＬｉＰＦ₆を１モル／リットル溶解したものを使用した。電解液量は、約２．５ｍｌである。
【００６７】
なお、この作製した角型電池は幅３０ｍｍ、高さ４８ｍｍ、厚み５ｍｍである。本サイズで通常市販されているものの設計容量は７００ｍＡｈであり、セパレータ８の厚みも、２５から２７μｍのものが一般的である。本実施例の電池は、それよりも高容量の７５０ｍＡｈを設計容量とした。このためセパレータ８の厚みは２５μｍより大きくなると、捲回した極板群が、ケースに確実に挿入できなかった。
【００６８】
＜実施例１＞
セパレータＡを５重量％のビフェニルを添加した電解液２．５ｍｌとともに電池に組み立てた。この電池を実施例１の電池とする。
【００６９】
＜比較例１＞
実施例１と同様の方法でセパレータＡを用い、芳香族添加剤を一切添加していない電解液とともに電池に組み立てた。
【００７０】
以下、前述の（表１）に示すセパレータＡからＮの１３種のセパレータと、芳香族添加剤およびその添加量を（表２）や（表３）のように組み合わせて、実施例２から１０１と比較例２から９の電池を組み立てた。
【００７１】
＜電池の評価＞
これら作製した電池、計１０５個を以下に述べる方法で評価した。
【００７２】
電池の設計容量は７５０ｍＡである。まず、３８０ｍＡの定電流で４．２Ｖになるまで充電した後、３８０ｍＡの定電流で３．０Ｖになるまで放電する充放電サイクルを１０サイクル繰り返した。この１０サイクル目の放電容量を各電池の初期容量とした。２７個の全ての電池で初期容量が設計容量を満足していた。また、充放電は２０℃の恒温槽の中で行った。その後、各電池を４．２Ｖまで３８０ｍＡの定電流で充電し、１５００ｍＡの定電流で３．０Ｖまで放電した。このときの容量／初期容量×１００で２Ｃ容量比率として算出した。さらに７５０ｍＡの定電流で３時間の過充電試験を行い、この過程での電池の表面温度の測定を行い、電池の最高到達温度を評価した。これらの結果も（表２）に示す。
【００７３】
【表２】

【００７４】
【表３】

【００７５】
（表２）からわかる通り、実施例１から５８の電池ではセパレータが薄くなっているのもかかわらず、異常昇温が抑えられた。すべての実施例の電池で、電圧がかかった状態で電流が流れており、セパレータ内で微小な内部短絡が起こっていた。それに対し比較例１から９の電池は全て異常昇温が起こった。
【００７６】
使用した全ての極板寸法、セパレータの常温の寸法は同じであるのに、実施例と比較例の電池のように異常昇温の起こる電池と起こらない電池があったのは、前述のとおり単位面積当りの孔数が違うためである。
【００７７】
比較例１は、芳香族添加剤が添加されたいないため、異常昇温が起こったと考えられる。
【００７８】
そして、セパレータＫ、ＭおよびＬを使用した比較例の電池（比較例２、３、４、６、７、８）は単位面積当りの孔数が多いため成長が分散し内部短絡に至らず過充電状態が危険な状態まで進んだため異常昇温が起こったと考えられる。
【００７９】
さらに、セパレータＮを使用した比較例の電池（比較例９）は、内部短絡はするものの厚みが小さいため強度が極端に弱く内部短絡はするものの破膜が起こっために内部短絡があまりにも多く発生し短絡電流による温度上昇が大きいため、異常昇温が起こったと考えられる。
【００８０】
なお、実施例１から１０１の全ての添加剤で添加量が２．５重量％のものと５重量％のもので、効果に違いが無かった。
【００８１】
【発明の効果】
以上述べた通り、本発明によれば、薄いセパレータを用いたにもかかわらず非水電解質二次電池の高温状況下での安全性を高めることができる。
【図面の簡単な説明】
【図１】セパレータの貫通孔モデルを表す概念図
【図２】本発明の実施例で用いた角型電池の概略図（一部断面図）
【符号の説明】
１ 多孔質セパレータ
２ セパレータ表面
３ 貫通孔
４ セパレータ厚み
５ 非水電解質二次電池
６ 正極
７ 負極
８ セパレータ
９ ケース
１０ 封口板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an additive and a separator for a non-aqueous electrolyte secondary battery having high performance and high safety in a high temperature environment during overcharge.
[0002]
[Prior art]
There are two methods for maintaining the safety of non-aqueous electrolyte secondary batteries in a high temperature environment during overcharging: a method using a built-in safety device and a method of giving the power generating element itself anti-overcharge characteristics. .
[0003]
Among these, examples of a method for imparting overcharge resistance to the power generation element itself include a separator and an overcharge additive.
[0004]
The role of the separator in terms of battery safety is to prevent short circuit between the positive electrode and negative electrode during normal operation. However, as a function unique to the separator of non-aqueous electrolyte secondary batteries, excess is caused by external short circuit in porous polyolefin separators, etc. When the battery temperature rises significantly due to an electric current or the like, there is a so-called shutdown function in which the porous separator becomes soft and becomes substantially non-porous so that no current flows.
[0005]
When the temperature of the battery rises even after shutdown, the separator melts and opens a large hole, causing a short circuit between the positive electrode and the negative electrode (hereinafter referred to as meltdown). It can be said that the higher the temperature, the higher the safety. In order to reinforce the shutdown function, there is a conflicting phenomenon that when the heat melting property is increased, the meltdown temperature is lowered and the safety is lowered.
[0006]
Next, there are various roles played by the anti-overcharge additive. For example, a method for protecting the battery from overcharge by increasing the internal resistance of the battery by polymerizing the additive during overcharge or overcharge. There are a method in which gas is generated during charging and the internal electrical cutting device is reliably operated at a predetermined internal pressure, a method in which a conductive polymer is generated during overcharge abuse, a short circuit is generated inside the battery, and automatic discharge is performed.
[0007]
Due to development competition in recent years, non-aqueous electrolyte secondary batteries have an increasingly higher capacity. Although this increase in capacity has improved performance by improving the active material of the electrode, the volume of members other than the active material is reduced to increase the amount of substantial active material in the battery. Has been done. Accordingly, positive and negative current collectors and separators are becoming increasingly thinner. As the separator becomes thinner, the safety against short circuits and the like is worsened, and the amount of the active material is substantially increased, so the demand for safety is further increased.
[0008]
Therefore, in a high temperature environment at the time of overcharging when using a thin separator, not only a method of stopping overcharging but also a method of eliminating the overcharged state is effective. Specifically, in the above method, the additive is polymerized when the separator is shut down or the additive is overcharged, so that the internal resistance of the battery is increased, and gas is generated at the time of overcharge by a method of protecting the battery from overcharge. Rather than a method of operating the internal electrical cutting device with a predetermined internal pressure, a method of generating a short circuit inside the battery and automatically discharging it is preferable.
[0009]
However, when an aromatic additive is used as an additive, even if the same additive is used from time to time, as described in Japanese Patent No. 30617756, Japanese Patent No. 30671759, and Japanese Patent Laid-Open No. 10-32258, during overcharge, A method of increasing the internal resistance of the battery by polymerizing the additive and protecting the battery from overcharging, a method of operating an internal electric cutting device that generates gas during overcharging and operates at a predetermined internal pressure, and overcharge abuse Occasionally, there is a mixed effect of generating a conductive polymer and automatically discharging. For this reason, before the overcharged state is eliminated by automatic discharge, the internal resistance increases or the generation of gas hinders the generation of the conductive polymer, and it is difficult to reliably ensure safety by internal short circuit It is.
[0010]
[Problems to be solved by the invention]
The present invention solves the above-mentioned problems and uses a combination of an additive and a suitable separator to ensure safety by an internal short circuit and to ensure high-capacity non-aqueous electrolyte that is excellent in safety in a high-temperature environment. An object is to provide a secondary battery.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, the battery of the present invention is polymerized at a battery voltage exceeding the maximum operating voltage during overcharge abuse, and becomes a polymer, and when the voltage is further increased, a part of the polymer is oxidized to have a positive charge. And an anionic additive in the positive charge portion is doped with the anion in the non-aqueous electrolyte to produce a conductive polymer, and an internal additive is added to the non-aqueous electrolyte. The number of holes per unit area (square micrometer) of the separator is 100 or less.
[0012]
The non-aqueous electrolyte secondary battery of the present invention combines an aromatic additive and a separator having the above-mentioned characteristic parameters, so that when overcharged, the polymerized conductive polymer penetrates the separator and causes an internal short circuit. Cancel the condition. Therefore, it is possible to provide a non-aqueous electrolyte secondary battery with high capacity and excellent reliability.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The nonaqueous electrolyte secondary battery of the present invention is a nonaqueous electrolyte secondary battery comprising a separator made of a porous polyolefin and a nonaqueous electrolyte in which an electrolyte salt is dissolved in a positive electrode, a negative electrode, and a nonaqueous solvent. Polymerization at a battery voltage exceeding the maximum operating voltage becomes a polymerizable polymer, and the conductive polymer is taken in by the conductive polymer taking in lithium deposited by overcharging, and an aromatic addition has the effect of generating an internal short circuit. The separator is mixed with a non-aqueous electrolyte, and the separator calculates the curvature τ from (Equation 3) using the electrical resistance Rm measured using the AC resistance measurement method, and further calculates the calculated value τ. Furthermore, the number of holes per unit area (square micrometer) calculated from (Equation 4) is 100 or less.
[0014]
[Equation 3]

[0015]
(Where S is the measurement area, ρ is the specific resistance of the measured electrolyte, ε is the porosity calculated from the volume and weight of the separator, and L is the thickness of the separator.)
[0016]
[Expression 4]

[0017]
(Where ε is the porosity calculated from the volume and weight of the separator, π is the circumference, and d is the average pore diameter measured by the mercury porosimeter method.)
The positive electrode in the present invention has a conventionally known structure, but a mixture comprising a lithium-containing composite oxide such as lithium cobaltate, lithium nickelate, and manganese spinel as an active material and a mixture of a conductive agent and a binder is a current collector. It is made by coating.
[0018]
In the negative electrode of the present invention, carbon such as natural graphite and artificial graphite is mainly used as an active material, but various other metal oxides such as aluminum, various alloys mainly composed of aluminum, tin oxide, and the like are also used. Conventionally known materials such as metal nitrides and metal nitrides are prepared by coating a current collector with a mixture obtained by mixing a conductive agent and a binder as in the case of the positive electrode.
[0019]
Nonaqueous electrolytes (hereinafter referred to as electrolyte solutions) include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) as nonaqueous solvents, dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate ( What mixed 2 or more types of chain carbonates, such as EMC), is preferable.
[0020]
As the electrolyte salt, a conventionally known lithium salt can be used, and LiPF ₆ and LiBF ₄ are preferable.
[0021]
The additive in the present invention is an additive composed of an aromatic compound that polymerizes at a battery voltage exceeding the maximum operating voltage to generate a conductive polymer, thereby generating an internal short circuit during overcharge abuse. For example, hexaphenyl Benzene, terphenyl, phenylpiperazine, tetrahydroisoquinoline, phenylcyclohexane, triphenylbenzene, dodecahydrotriphenylene, divinylbenzene, dicyclohexylbenzene, biphenyl, pyrrole, N-methylpyrrole, thiophene, furan, indole, chlorothiophene, bromothiophene, fluoro Thiophene, dimethoxybenzene, methylpyridinium tetrafluoroborate, diphenyl ether, benzofuran, diallyl ether, allyl butyl ether, phenoxytoluene, tar Eniru, methyl terphenyl, dimethyl terphenyl etc., and derivatives thereof.
[0022]
Among these, preferred are diphenyl ether, phenylcyclohexane, terphenyl, biphenyl, furan, thiophene, and derivatives thereof. Specifically, biphenyl, pyrrole, N-methylpyrrole, thiophene, furan, indole, 3-chlorothiophene, 3-bromothiophene, 3-fluorothiophene, 1,2 dimethoxybenzene, 1-methyl-3-pyridinium tetra There is a fluorobaud rate.
[0023]
Among these, diphenyl ether, phenylcyclohexane, o-terphenyl, biphenyl, furan, indole and 3-chlorothiophene are particularly preferable from the viewpoint of the polymerization potential of the additive and the stability under battery operation.
[0024]
The separator of the present invention is an electronically insulating microporous thin film having a large ion permeability and moderate mechanical strength, and has a shutdown function as a material from the viewpoint of organic solvent resistance and hydrophobicity. A porous polyolefin such as a polypropylene resin, a polyethylene resin alone, or a laminate or a mixture / combination of these is used.
[0025]
The production method of porous polyolefin is roughly divided into two processes, wet and dry. In either case, after forming pores, the porous polyolefin is stretched uniaxially or biaxially to make it porous. Therefore, although there are not many simple one through-holes, the model shown in FIG. 1 is generally used to define the pore diameter, the pore path length, and the number of pores. Yes. The porous separator 1 actually has pores of various sizes on the separator surface 2, but assuming that there are through-holes 3 of the same size with the average pore diameter d as the pore diameter, The path length, curvature, and number of holes per unit area are specified.
[0026]
That is, the pore path length is the length of the through-hole 3, and when the separator thickness 4 is L, the through-hole path length can be expressed as τL, and this proportionality coefficient τ is called a curvature.
[0027]
The curvature is measured by immersing the porous separator completely in the electrolytic solution and using the AC resistance measurement method. In this case, it can be assumed that all the through holes 3 are filled with the electrolytic solution, and there is a conductor having the same shape as the through holes 3. At this time, the electric resistance Rm can be expressed by the product of the ratio of the path length τL of the through-hole and the total cross-sectional area of the through-hole in the measurement area S and the specific resistance ρ of the electrolytic solution. The total cross-sectional area of the through-hole can be expressed by the total pore volume represented by the product (SLε) of the measurement area S, the separator thickness L, and the porosity ε, which is divided by the path length τL. Therefore, when these are arranged, Rm can be expressed as (Equation 5).
[0028]
[Equation 5]

[0029]
By transforming (Equation 5) and obtaining the curvature ε, (Equation 3) is obtained.
[0030]
Furthermore, since the volume V of one through-hole can be expressed by (Equation 6) with the circumference being π, the total pore volume (SLε) can be expressed by (Equation 7) where n is the number of holes.
[0031]
[Formula 6]

[0032]
[Expression 7]

[0033]
By transforming (Equation 7) and obtaining the number of holes (n / S) per unit area, (Equation 4) is obtained.
[0034]
This model of n holes having an average hole diameter d is used to theoretically explain the physical properties of the separator, for example, by using the curvature τ to indicate the air resistance T as shown in (Equation 8). Convenient and generally used widely.
[0035]
[Equation 8]

[0036]
Here, C is a proportional constant, for example, about 0.13 for a polyethylene separator.
[0037]
The internal short-circuit mechanism by the aromatic additive and separator of the present invention in a high temperature environment during overcharging in the battery of the present invention is generally as follows.
[0038]
When the battery voltage exceeds the maximum operating voltage during overcharge, the aromatic additive polymerizes and grows on the positive electrode surface or in the separator pores, forming a polymer layer. This polymer layer is partially oxidized and has a positive charge when the voltage is further increased, and the positive charge part is electrically conductive by being doped with an anion in the electrolyte, such as PF ⁶⁻ . It becomes a polymer. When the overcharge state further proceeds, lithium is deposited on the negative electrode.
[0039]
In the presence of a conductive polymer polymerized on the surface of the positive electrode and the separator surface and pores on the positive electrode side, when overcharge proceeds, lithium deposited on the negative electrode side grows in a needle shape toward the positive electrode, and finally on the positive electrode An internal short circuit occurs. Next, there is a number of pores per unit area (square micrometer) of the separator that is suitable for this action. This is because the deposit on the negative electrode side (hereinafter referred to as dendrite) grows like a lightning bolt in a needle-like manner regardless of the pore diameter d and the curvature τ of the separator. In order to cause an internal short circuit at an early stage of overcharging, it is important to concentrate the dendrite and connect it to the conductive polymer in the pores, which is correlated with the number of through holes in the separator. That is, the number of holes per unit area is a technically significant parameter.
[0040]
The separator in the present invention has 100 or less holes per unit area (square micrometer). When the number of holes is 101 or more, it takes a long time for the conductive polymer to pass through the separator and reach the positive electrode, and overcharge proceeds. Therefore, the number of holes per unit area (square micrometer) is preferably 100 or less.
[0041]
As a result, in the battery using the separator of the present invention, the charging depth of the positive electrode and the negative electrode does not reach an excessive state more than necessary from a relatively early stage of the overcharge process, and the risk during overcharge can be suppressed.
[0042]
In this operation, if the aromatic additive is not added or is less than 0.1% by weight, the overcharged state proceeds to a dangerous state before the conductive polymer is sufficiently grown. .
[0043]
In addition, when the amount of aromatic additive added is large, for example, when it is larger than 10% by weight, the effect of suppressing the overcharge state can be exhibited, but it is not suitable because the normal battery characteristics, particularly the characteristics during high temperature storage deteriorate. is there.
[0044]
By combining the aromatic additive and the separator having the above-mentioned characteristic parameters, at the time of overcharging, the polymerized conductive polymer penetrates the separator and causes an internal short circuit to eliminate the overcharged state. Therefore, it is possible to provide a non-aqueous electrolyte secondary battery with high capacity and excellent reliability.
[0045]
As a result of intensive studies from the air resistance, porosity, average pore diameter, curvature, etc. to satisfy normal battery performance while ensuring the number of holes per unit area (square micrometer), the present inventors. The present inventors have concluded that a separator having the following characteristics is suitable for the battery of the present invention. In particular, when the aromatic additive is at least one selected from the group consisting of diphenyl ether, phenylcyclohexane, o-terphenyl, biphenyl, furan, indole, and 3-chlorothiophene, per unit area of the separator. Although the number of pores is 50 or less, a very excellent effect is exhibited.
[0046]
When the air permeability resistance of the separator is 460 seconds / 100 ml or more, it is disadvantageous in terms of battery characteristics such as high-efficiency discharge of the battery. Therefore, the separator is preferably 450 seconds / 100 ml or less, and even if the porosity is 29% or less, the battery Since it is disadvantageous in terms of characteristics, it is preferably 30% or more. Further, if the thickness is 31 μm or more, it is not only disadvantageous in terms of battery characteristics such as an increase in capacity of the battery and high rate discharge, but also an internal short circuit is less likely to occur. On the other hand, when the thickness is 7 μm or less, even if an internal short circuit occurs reliably in a high temperature environment during overcharging, the battery becomes dangerous due to its thinness. Therefore, the thickness of the separator is preferably 8 to 30 μm. However, physical properties that satisfy battery characteristics in the normal range, such as air resistance, porosity, and thickness, and the number of holes per unit area (square micrometer) to ensure safety during overcharge are secured. Therefore, the physical property values are in a contradictory relationship. It is preferable that the average pore diameter is 0.06 μm or more as a common physical property satisfying both of these.
[0047]
Here, each measuring method of the separator characteristic described above will be described below.
[0048]
For the pore size, the method of calculating the average pore size using the fluid permeation method, the method of detecting the maximum pore size using the bubble point method, the method of calculating the average pore size using the mercury intrusion method, and the surface pore size from the SEM surface photograph There are various measurement methods such as an observation method, but these values do not necessarily match depending on the measurement method, and comparison using values of different measurement methods is not possible. Therefore, the inventors adopted the average pore diameter d measured by the mercury intrusion method.
[0049]
For the air resistance T, the measurement method defined in JIS P8117-1998 was adopted.
[0050]
The electrical resistance Rm was measured by an AC resistance measurement method by completely immersing the separator in an electrolytic solution and sandwiching the separator with a stainless steel plate having an area of 2.54 cm ² . When measuring the AC resistance, the measurement frequency was measured while being sequentially scanned, and the value intersecting the real axis was defined as the electric resistance Rm. This value is theoretically an infinite value, but in the case of a polyethylene single layer film, it is almost the same as this value at 100 kHz.
[0051]
The porosity ε was calculated from the difference from the true specific gravity by calculating the apparent specific gravity from the volume and weight of the separator.
[0052]
【Example】
Next, specific examples of the present invention will be described using examples.
[0053]
First, the separator measured many polyethylene single layer separators, and extracted the separator with the various characteristics described below.
[0054]
<Extraction of separator>
In this example, a 16 μm thick polyethylene single layer separator having the following characteristics was selected as the separator A.
[0055]
The average pore diameter d measured by mercury porosimetry was 0.075 μm, and the porosity ε was 40%.
[0056]
In the electrical resistance measurement, the electrolytic solution used was a solution of 1 mol / liter of LiPF _{6 in} a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1. The specific resistance ρ of this electrolytic solution is 116 Ωcm.
The electric resistance Rm was 0.738Ω.
[0057]
The curvature ratio τ obtained from (Equation 3) using this Rm value is 2.01, and the number of holes per unit area (square micrometer) obtained by substituting this value into (Equation 4) is There were 45.
[0058]
Finally, the air resistance was measured using an A-type measuring device conforming to JIS in a laboratory adjusted to 23 ° C. The air resistance T was 310 seconds / 100 ml.
In the same manner as in the separator A, 13 types of separators A to N having physical properties as shown in (Table 1) were selected.
[0059]
[Table 1]

[0060]
<Production of battery>
In order to evaluate the temperature change at the time of overcharging of the battery of the present invention, a square battery described below was produced.
[0061]
FIG. 2 shows a structural diagram (partial cross-sectional view) of a prismatic battery according to Examples 1 to 34 of the present invention.
[0062]
In FIG. 2, a nonaqueous electrolyte secondary battery 5 is wound with a positive electrode 6, a negative electrode 7, and a separator 8, and is built in a case 9 together with an electrolyte solution (not shown) in which an electrolyte salt is dissolved in a nonaqueous solvent. In a general cylindrical battery, a safety element such as a safety valve or a PTC element is incorporated in the sealing plate, but in the battery of the embodiment, the sealing plate is a square shape. 10 does not incorporate any safety mechanism.
[0063]
In the positive electrode 6, 10% by weight of carbon powder as a conductive agent and 5% by weight of polyvinylidene fluoride resin (PVdF resin) as a binder are mixed with 85% by weight of lithium cobalt oxide powder, and these are dispersed in dehydrated NMP. A slurry was prepared, applied on a positive electrode current collector made of aluminum foil, dried and rolled.
[0064]
The negative electrode 7 uses artificial graphite powder as a negative electrode active material, 95% by weight of this is mixed with 5% by weight of a PVdF resin as a binder, and these are dispersed in dehydrated NMP to produce a slurry. It was coated on a negative electrode current collector made of foil, dried and rolled.
[0065]
As the separator 8, 13 types of separators A to N shown in the above (Table 1) were used.
[0066]
The electrolyte used was the same as that used for measurement of the electrical resistance Rm, in which 1 mol / liter of LiPF ₆ was dissolved in a mixed solvent of EC and EMC at a volume ratio of 1: 1. The amount of the electrolytic solution is about 2.5 ml.
[0067]
The produced square battery has a width of 30 mm, a height of 48 mm, and a thickness of 5 mm. The design capacity of this size, which is usually commercially available, is 700 mAh, and the thickness of the separator 8 is generally 25 to 27 μm. The battery of this example had a design capacity of 750 mAh having a higher capacity. Therefore, when the thickness of the separator 8 is greater than 25 μm, the wound electrode plate group cannot be reliably inserted into the case.
[0068]
<Example 1>
Separator A was assembled into a battery along with 2.5 ml of electrolyte with 5 wt% biphenyl added. This battery is referred to as the battery of Example 1.
[0069]
<Comparative Example 1>
The separator A was used in the same manner as in Example 1, and the battery was assembled together with an electrolyte solution to which no aromatic additive was added.
[0070]
Hereinafter, the separators A to N shown in the above (Table 1), the aromatic additives and the addition amount thereof were combined as shown in (Table 2) and (Table 3), and Examples 2 to 101 were combined. The batteries of Comparative Examples 2 to 9 were assembled.
[0071]
<Battery evaluation>
A total of 105 batteries prepared were evaluated by the method described below.
[0072]
The design capacity of the battery is 750 mA. First, after charging to 4.2 V at a constant current of 380 mA, a charge / discharge cycle of discharging to 3.0 V at a constant current of 380 mA was repeated 10 cycles. The discharge capacity at the 10th cycle was defined as the initial capacity of each battery. The initial capacity of all 27 batteries satisfied the design capacity. Moreover, charging / discharging was performed in a 20 degreeC thermostat. Then, each battery was charged with a constant current of 380 mA up to 4.2 V, and discharged to 3.0 V with a constant current of 1500 mA. The capacity at this time / initial capacity × 100 was calculated as a 2C capacity ratio. Further, an overcharge test was conducted for 3 hours at a constant current of 750 mA, the surface temperature of the battery in this process was measured, and the maximum temperature reached of the battery was evaluated. These results are also shown in (Table 2).
[0073]
[Table 2]

[0074]
[Table 3]

[0075]
As can be seen from (Table 2), in the batteries of Examples 1 to 58, although the separator was thin, abnormal temperature increase was suppressed. In all the batteries of the examples, current was flowing in a state where voltage was applied, and a minute internal short circuit occurred in the separator. On the other hand, the batteries of Comparative Examples 1 to 9 all had an abnormal temperature rise.
[0076]
Although all the electrode plate dimensions used and the room temperature dimensions of the separators were the same, there were some batteries that caused abnormal temperature rise and others that did not occur like the batteries in the examples and comparative examples. This is because the number of holes per area is different.
[0077]
In Comparative Example 1, since the aromatic additive was not added, it is considered that an abnormal temperature increase occurred.
[0078]
The batteries of the comparative examples using the separators K, M, and L (Comparative Examples 2, 3, 4, 6, 7, 8) have a large number of holes per unit area, so that the growth is dispersed and the internal short circuit is not excessive. It is probable that abnormal temperature rise occurred because the state of charge progressed to a dangerous state.
[0079]
In addition, the battery of the comparative example using the separator N (Comparative Example 9) has an internal short circuit, but the thickness is small, so the strength is extremely weak and the internal short circuit is caused, but a film breakage occurs, resulting in too many internal short circuits. Since the temperature rise due to the short-circuit current is large, it is considered that an abnormal temperature rise occurred.
[0080]
In all the additives of Examples 1 to 101, the addition amount was 2.5% by weight and 5% by weight, and there was no difference in effect.
[0081]
【The invention's effect】
As described above, according to the present invention, it is possible to improve the safety of the nonaqueous electrolyte secondary battery under high temperature conditions despite the use of a thin separator.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a through-hole model of a separator. FIG. 2 is a schematic diagram (partially sectional view) of a prismatic battery used in an example of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Porous separator 2 Separator surface 3 Through-hole 4 Separator thickness 5 Nonaqueous electrolyte secondary battery 6 Positive electrode 7 Negative electrode 8 Separator 9 Case 10 Sealing plate

Claims (7)

In a nonaqueous electrolyte secondary battery comprising a separator made of a porous polyolefin and a nonaqueous electrolyte in which an electrolyte salt is dissolved in a positive electrode, a negative electrode, and a nonaqueous solvent,
When overcharged, the polymer is polymerized at a battery voltage exceeding the maximum operating voltage, and when the voltage is further increased, a part of the polymer is oxidized to have a positive charge, and the non-water is charged in the positive charge portion. An aromatic additive having an effect of generating a conductive polymer by being doped with an anion in the electrolyte and generating an internal short circuit is mixed with the non-aqueous electrolyte, and the separator has an AC resistance measurement method. The curvature τ is calculated from (Equation 1) using the electrical resistance Rm measured using the equation (1), and the number of holes per unit area (square micrometer) calculated from (Equation 2) using the calculated value τ (n / S) is 100 or less, the nonaqueous electrolyte secondary battery characterized by the above-mentioned.

(Where S is the measurement area of the separator, ρ is the specific resistance of the measured electrolyte, ε is the porosity calculated from the volume and weight of the separator, and L is the thickness of the separator.)

(Where ε is the porosity calculated from the volume and weight of the separator, π is the circumference, and d is the average pore diameter measured by the mercury porosimeter method.) The non-aqueous electrolyte according to claim 1, wherein the aromatic additive is at least one selected from the group consisting of diphenyl ether, phenylcyclohexane, terphenyl, biphenyl, furan, thiophene, and derivatives thereof. Secondary battery. The aromatic additive is at least one selected from the group consisting of diphenyl ether, phenylcyclohexane, o-terphenyl, biphenyl, furan, indole and 3-chlorothiophene, and the number of pores per unit area of the separator is The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the nonaqueous electrolyte secondary battery is 50 or less. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has an air resistance specified by JIS P8117-1998 of 450 seconds / 100 ml or less. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has an average pore diameter of 0.06 μm or more. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has a porosity of 30% or more. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has a thickness of 8 μm to 30 μm.

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