JP4501331B2 - Secondary battery and overcharge prevention method - Google Patents

Description

【００１８】
（式中、Ｒ₁〜Ｒ₆はそれぞれ独立して水素原子、ハロゲン原子、又は置換基を有していてもよい炭化水素基、アルコキシ基若しくはアリールオキシ基を表す。また、Ｒ₁とＲ₂とが結合して置換基を有していてもよいフェニレンオキシ基、エチレンオキシ基、トリメチレンオキシ基、プロペニレンオキシ基又はビニレンオキシ基を形成してもよい。）
トリガー物質は、電解質中に通常０．２５重量％以上５重量％以下となるように含有しているのが好ましい。０．２５重量％未満では有効に作用せず、５重量％を超えると電池特性に悪影響を及ぼすことがある。下限としては０．５重量％以上が好ましく、上限としては３重量％以下、特に２重量％以下が好ましい。
【００１９】
正極活物質としてリチウム・遷移金属酸化物、過充電防止剤としてジカルボン酸エステル類、トリガー物質として芳香族化合物を用いた場合、トリガー物質の存在下に過充電状態で固体塩が生成する理由は以下のように考えられる。すなわち、過充電状態になると芳香族化合物の酸化重合反応により水が生成し、ジカルボン酸エステルが加水分解されジカルボン酸が生成する。生成したジカルボン酸と遷移金属イオンとは難溶性の塩を生成し、これが電解液から析出して、セパレーターを閉塞させたり電極を覆うことにより、電池内の抵抗を増大させて過充電を防止する。
【００２０】
本発明に係る二次電池では、上記のようにして過充電を防止することにより、芳香族化合物から発生するガスで電池が破裂したり、正極で生成する電子導電性の皮膜が負極又は負極上に析出した金属リチウムと短絡に至るというような現象を回避することができる。
＜電解質＞
本発明に係る二次電池では、電解質として、リチウム塩と非水系溶媒とを含有する電解液、電解液を高分子化合物で固形化した半固体状電解質、及びリチウム塩と高分子化合物とを有する完全固体電解質等が挙げられる。このうち、リチウム塩等の電解質塩と非水溶媒とを含有するものが好ましい。
【００２１】
電解液に使用する溶媒としては、炭酸エステル、エーテル及びラクトンよりなる群から選ばれる非水系溶媒を主体とするものが好ましい。
炭酸エステルとしては、プロピレンカーボネート及びエチレンカーボネート等の環状炭酸エステル；ジメチルカーボネート、ジエチルカーボネート及びエチルメチルカーボネート等の鎖状炭酸エステルなどが挙げられる。
【００２２】
エーテルとしては、ジメトキシエタン及びジエトキシエタン等が挙げられる。
ラクトンとしては、γ−ブチロラクトン等が挙げられる。
非水系溶媒としては、炭酸エステルを含有するもの、特に環状炭酸エステル又はラクトン類等の高誘電率溶媒と、鎖状炭酸エステルの低粘度溶媒との混合溶媒が好ましい。
【００２３】
電解質塩としてはリチウム塩が好ましい。リチウム塩としては、ＬｉＣｌＯ_４、ＬｉＡｓＦ_６、ＬｉＰＦ_６、ＬｉＢＦ_４、ＬｉＢ（Ｃ_６Ｈ_５）_４、ＬｉＣｌ、ＬｉＢｒ、ＬｉＣＨ_３ＳＯ_３、ＬｉＣＦ_３ＳＯ_３、ＬｉＮ（ＳＯ_２ＣＦ_３）_２、ＬｉＮ（ＳＯ_２Ｃ_２Ｆ_５）_２、ＬｉＣ（ＳＯ_２ＣＦ_３）_３及びＬｉＮ（ＳＯ_３ＣＦ_３）_２等が挙げられる。これらの複数種を併用してもよい。これらのうち、ＬｉＢＦ_４又はＬｉＰＦ_６が好ましい。
【００２４】
リチウム塩は、電解液中に、通常０．５ｍｏｌ／Ｌ以上１．５ｍｏｌ／Ｌ以下となるように含有させる。０．５ｍｏｌ／Ｌ未満でも１．５ｍｏｌ／Ｌを超えても伝導度が低下し、電池特性に悪影響を与えることがある。下限としては０．７５ｍｏｌ／Ｌ以上、上限として１．２５ｍｏｌ／Ｌ以下が好ましい。
電解質中には、必要に応じてさらに他の成分を含有させることができる。他の成分としては、電池の活物質表面に被膜（ＳＥＩ）を形成するための添加剤等が挙げられる。ＳＥＩを形成するための添加剤としては、炭素−炭素不飽和結合を有する環状カーボネート等が挙げられる。炭素−炭素不飽和結合を有する環状カーボネートを使用すると、負極上に良好な皮膜を形成する等の作用があるため、過充電防止剤を負極の影響から保護する効果が考えられることから、一層優れた過充電防止効果が得られるほか、電池の保存安定性やサイクル特性などの他特性も向上させることができる。
【００２５】
炭素−炭素不飽和結合を有する環状カーボネートとしては、ビニレンカーボネート及びビニルエチレンカーボネート等のオレフィン性二重結合を有するもの；フェニルエチレンカーボネート及びカテコールカーボネート等のベンゼン環を有するものなどが挙げられる。このうち、ビニレンカーボネート又はビニルエチレンカーボネートが好ましい。
＜リチウム二次電池＞
本発明に係る二次電池としては、リチウム二次電池等が挙げられる。
【００２６】
二次電池は、正極、負極及び前記電解質を含有する。
正極の活物質としては、高起電力を発生することができるリチウム・遷移金属複合酸化物が好ましい。
リチウム・遷移金属複合酸化物としては、ＬｉＣｏＯ_２等のリチウムコバルト複合酸化物、ＬｉＮｉＯ_２等のリチウムニッケル酸化物、ＬｉＭｎ_２Ｏ_４等のリチウムマンガン酸化物などが挙げられる。これらは併用してもよい。このうち、リチウム並びにコバルト及び／又はニッケルを含有するものが好ましい。リチウム・遷移金属複合酸化物は、主体となる遷移金属元素の一部をＡｌ、Ｔｉ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｌｉ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ及びＺｒよりなる群から選ばれる他の金属種で置き換えることにより安定化させるのが好ましい。
【００２７】
負極の活物質としては、リチウムを吸蔵及び放出し得る物質であれば任意のものを用いることができる。このうち、炭素質物が好ましい。
炭素質物としては、様々な熱分解条件での有機物の熱分解物、人造黒鉛及び天然黒鉛等が挙げられる。このうち、種々の原料から得た易黒鉛性ピッチの高温熱処理によって製造された人造黒鉛若しくは黒鉛化メソフェーズ小球体、黒鉛化メソフェーズピッチ系炭素繊維等の他の人造黒鉛若しくは精製天然黒鉛、又はこれらの黒鉛にピッチを含む種々の表面処理を施したものが好ましい。これらの炭素質物は、学振法によるＸ線回折で求めた格子面（００２面）のｄ値（層間距離）が０．３３５〜０．３４ｎｍ、特に０．３３５〜０．３３７ｎｍであるものが好ましい。灰分は通常１重量％以下である。０．５重量％以下、特に０．１重量％以下であるものが好ましい。また、学振法によるＸ線回折で求めた結晶子サイズ（Ｌｃ）は、通常３０ｎｍ以上である。５０ｎｍ以上、特に１００ｎｍ以上であるのが好ましい。
【００２８】
これらの炭素質物にリチウムを吸蔵・放出可能な他の活物質をさらに混合することもできる。他のリチウムを吸蔵・放出可能な活物質としては、酸化錫、酸化珪素等の金属酸化物材料、並びにリチウム金属及び種々のリチウム合金が挙げられる。これらの負極材料は複数種を併用してもよい。
電極に用いる結着剤としては、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、スチレン・ブタジエンゴム、イソプレンゴム、ブダジエンゴム等が挙げられる。
【００２９】
増粘剤としては、カルボキシメチルセルロース、メチルセルロース、ヒドロキシメチルセルロース、エチルセルロース、ポリビニルアルコール、酸化スターチ、リン酸化スターチ及びカゼイン等が挙げられる。
電極中には、銅及びニッケル等の金属材料、並びにグラファイト及びカーボンブラック等の炭素材料等の導電材を含有させてもよい。正極は、導電材を含有させるのが好ましい。
【００３０】
電極は、常法により製造すればよい。例えば、活物質に、結着剤、増粘剤、導電材、及び溶媒等を加えてスラリー状とし、集電体の基板に塗布し、乾燥する方法が挙げられる。また、活物質をそのままロール成形してシート電極としたり、圧縮成形によりペレット電極とすることもできる。
負極集電体としては、銅、ニッケル、ステンレス等の金属又は合金などが挙げられ、このうち銅が好ましい。正極集電体としては、アルミニウム、チタン、タンタル等の金属又は合金等が挙げられ、このうちアルミニウム又はその合金が好ましい。
【００３１】
電池は、通常正極と負極の間にセパレーターが介装される。セパレーターの材質や形状は、任意である。ポリエチレン、ポリプロピレン等のポリオレフィンを原料とする多孔性シート又は不織布等が、電解液に安定で、しかも保液性に優れているので好ましい。
電池の形状は任意であり、シート電極及びセパレーターをスパイラル状にしたシリンダータイプ、ペレット電極及びセパレーターを組み合わせたインサイドアウト構造のシリンダータイプ、ペレット電極及びセパレーターを積層したコインタイプ等が挙げられる。
【００３２】
【実施例】
以下、本発明を実施例に基づき、さらに詳細に説明するが、本発明はその要旨を超えない限り以下の実施例に限定されるものではない。
（正極の作成）
コバルト酸リチウム（ＬｉＣｏＯ_２）９０重量部、アセチレンブラック５重量部、及びポリフッ化ビニリデン５重量部を、Ｎ−メチルピロリドン溶媒中で混合し、スラリー化した。これを２０μｍのアルミ箔の片面に塗布し乾燥し、さらにプレス機で圧延したものを直径１２ｍｍの打ち抜きポンチで打ち抜いて正極とした。
（負極の作成）
黒鉛（面間隔０．３３６ｎｍ）９５重量部、ポリフッ化ビニリデン５重量部を、Ｎ−メチルピロリドン溶媒中で混合し、スラリー化した。これを２０μｍ厚さの銅箔の片面に塗布し乾燥し、さらにプレス機で圧延したものを直径１２ｍｍで打ち抜いて負極とした。
（電池の組立）
アルゴン雰囲気のドライボックス内で、ＣＲ２０３２型コインセルを使用して、リチウム二次電池を作製した。すなわち、正極缶の上に正極を置き、その上にセパレーターとして２５μｍの多孔性ポリエチレンフィルムを置き、ポリプロピレン製ガスケットで押さえた。さらに負極、厚み調整用のスペーサーの順に置いた後、電解液を加え電池内に十分しみこませ、負極缶を載せて電池を封口した。なお、以下の例では、電池の容量を充電上限４．２Ｖ、放電下限３．０Ｖで約４．０ｍＡｈになるように設計した。
【００３３】
正極活物質重量Ｗ（ｃ）と負極活物質重量Ｗ（ａ）の比率は、電池の通常使用上限電圧において、正極から放出されるリチウムイオンが、対向する負極上でリチウム金属の析出を起こさない範囲が好ましい。したがって、負極と正極との容量比Ｒｑが１．１≦Ｒｑ≦１．２となるように重量を決定した。なお、容量比Ｒｑは、Ｑ（ａ）×Ｗ（ａ）／｛Ｑ（ｃ）×Ｗ（ｃ）｝で求めた。ここで、電池の初期充電条件に対応する条件下での、正極活物質の重量当たりの電気容量をＱ（ｃ）ｍＡｈ／ｇ、リチウム金属が析出せずにリチウムを最大限に吸蔵しうる負極活物質の重量当たりの電気容量をＱ（ａ）ｍＡｈ／ｇとした。Ｑ（ｃ）及びＱ（ａ）は、正極又は負極を作用極に、対極にリチウム金属を用い、上記電池を組み立てる際と同じ電解液中でセパレーターを介して試験セルを組んで測定した。すなわち、目的とする電池系の初期充電条件に対応する正極の上限電位又は負極の下限電位まで、可能な限り低い電流密度で、正極が充電（正極からのリチウムイオンの放出）できる容量、負極が放電（負極へのリチウムイオンの吸蔵）できる容量として求めた。
（電池の評価）
電池評価は（１）初期充放電（容量確認）、次いで（２）満充電操作、さらに（３）過充電試験の順に行った。
【００３４】
初期充放電（容量確認）は、１Ｃ（４．０ｍＡ）、４．２Ｖ上限の定電流定電圧法により充電した。充電のカットは、電流値が０．０５ｍＡに到達した時点とした。放電は０．２Ｃで３．０Ｖまで定電流で行った。
満充電操作は、４．２Ｖ上限の定電流定電圧法（０．０５ｍＡカット）により充電した。
【００３５】
過充電試験は、１Ｃで４．９９Ｖカット又は３ｈｒカット（どちらか先に到達した方でカット）とした。
過充電防止効果の優劣は、過充電領域における電圧曲線挙動及び見かけの過充電電流量を確認することによって行った。
（実施例１）
エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）の体積比３：７の混合溶媒に、六フッ化リン酸リチウム（ＬｉＰＦ_６）を１ｍｏｌ／Ｌとなるように溶解させ、次いでシュウ酸ジエチル及びジベンゾフランをそれぞれ２重量％となるように溶解させ、電解液とした。
【００３６】
この電解液を用いて、上述の方法によりリチウム二次電池を作製し、評価を行った。
過充電時の電圧は速やかに上昇し、過充電は安定した状態で防止された。また、過充電後の電池を解体してセパレーター上及び電極上に析出した化合物をＩＣＰ、イオンクロマトグラフィー、ＥＳＣＡにより分析したところ、析出物中にシュウ酸コバルトを確認した。以上の結果を表−１に示す。
（実施例２）
実施例１において、シュウ酸ジエチル及びジベンゾフランの他にさらにビニレンカーボネートを２重量％となるように添加したこと以外は実施例１と同様にして、リチウム二次電池を作製し、評価を行った。結果を表−１に示す。なお、実施例１と同様の方法により、析出物中にシュウ酸コバルトを確認した。
（比較例１）
実施例１において、シュウ酸ジエチル及びジベンゾフランを加えなかったこと以外は実施例１と同様にして、リチウム二次電池を作製し、評価を行った。結果を表−１に示す。
（比較例２）
実施例１において、ジベンゾフランのみを２重量％となるように添加したこと以外は実施例１と同様にして、リチウム二次電池を作製し、評価を行った。結果を表−１に示す。なお、過充電時に短絡によると思われる電圧振動が観測され、なかなかカットオフ電圧に至らず、見かけの過充電電流量は大きくなった。
（比較例３）
実施例１において、シュウ酸ジエチルに代えてビニレンカーボネートを２重量％となるように添加したこと以外は実施例１と同様にして、リチウム二次電池を作製し、評価を行った。結果を表−１に示す。なお、過充電時に短絡によると思われる電圧振動が観測され、なかなかカットオフ電圧に至らず、見かけの過充電電流量は大きくなった。
【００３７】
【表１】

【００３８】
表−１より、シュウ酸ジエチルを添加することによって、過充電時に塩が析出し、過充電状態での安全性が向上することが分かる。
なお、実施例で作製したリチウム二次電池と比較例で作製したリチウム二次電池とでは、初期放電容量、５サイクル後の容量維持率等の電池特性に大きな差は見られなかった。
【００３９】
【発明の効果】
本発明に係る二次電池は、従来の方法とは全く異なる方法により、過充電時の安全性を向上させたものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a secondary battery and an electrolyte using the same. Specifically, the present invention relates to a secondary battery having improved safety under overcharge conditions and an electrolyte used for the secondary battery. In the present specification, the electrolyte refers to an electrolytic solution, a semisolid electrolyte, and a complete solid electrolyte that exist between the positive electrode and the negative electrode of the secondary battery.
[0002]
[Prior art]
Unlike primary batteries, secondary batteries need to be charged not only at the factory shipment stage but also in various situations. For example, in a secondary battery used in a digital camera, a mobile phone, or the like, when the secondary battery approaches a discharged state, the secondary battery is installed in the charger and charged. The charger needs to control the speed, voltage, pattern, etc. during charging according to the usage state of the installed battery, but if the charger fails or is installed with the wrong battery type, The secondary battery transitions to an overcharged state.
[0003]
The overcharged state is a state having a voltage that exceeds the voltage (upper limit voltage) that the secondary battery originally has as an upper limit. In an overcharged state, the metal oxide of the positive electrode, which has become unstable due to excessive extraction of lithium ions, reacts with the electrolyte, or the lithium metal deposited on the negative electrode develops in a dendrite shape (dendritic shape). Cause short circuit with the positive electrode. These cause gas generation and heat generation, and may cause deformation, thermal runaway, rupture, etc. due to a sudden increase in internal pressure of the battery.
[0004]
In order to improve the safety of the secondary battery when it is in an overcharged state, a method of adding an overcharge preventing agent to the electrolyte is known.
JP-A-7-302614, JP-A-9-50822, JP-A-9-106835, Japanese Patent No. 2939496, and Japanese Patent No. 2983205 disclose oxidation exceeding the upper limit voltage of the battery in the electrolyte. A method of adding an aromatic compound such as biphenyl having electric potential is described. In these methods, when an overcharged state is reached, the aromatic compound is oxidatively polymerized to form a high-resistance film that inhibits the entry and exit of lithium ions on the active material surface, thereby preventing the progress of overcharge. It is.
[0005]
In addition, JP-A-2000-348759, JP-A-2000-348760, JP-A-2001-52739, and JP-A-2001-52740 added a chain carbamate, resulting in an overcharged state. A mechanical overcharge prevention method is described in which the battery current shut-off valve is actuated by sometimes gasifying and increasing the internal pressure of the battery.
[0006]
[Problems to be solved by the invention]
However, the method of using an aromatic compound as an overcharge inhibitor needs to be further improved in terms of safety because gas is generated by the oxidation reaction of the aromatic compound. In addition, when an aromatic compound is used in an amount sufficient to exhibit an overcharge prevention effect, battery characteristics may be deteriorated during high-temperature storage.
[0007]
In addition, the mechanical overcharge prevention method has various restrictions on battery design because the battery must have an additional structure.
Therefore, an object of the present invention is to provide a secondary battery having an overcharge prevention function that is highly safe and does not require an additional structure, based on an action different from the above-described overcharge prevention method. .
[0008]
[Means for Solving the Problems]
The present inventors have intensively studied in view of the above problems, and found that overcharge can be prevented by using an overcharge inhibitor having the property of precipitating a solid salt in the electrolyte when overcharged. completed. It is considered that the precipitation of the solid salt prevents overcharge due to the solid salt covering the positive electrode surface or blocking the separator to increase the internal resistance of the battery.
[0009]
That is, the gist of the present invention is a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte, and an overcharge inhibitor that reacts with metal ions eluted from the positive electrode in the electrolyte to generate a solid salt. the contain as equal to or less than 5 wt% 0.1 wt%, and is oxidized to become overcharged, constitutes a part of the reaction to produce the solid salt and the product from the overcharge inhibitor A trigger substance composed of any one of a compound that generates water in the overcharge region, a compound that promotes elution of the metal component of the positive electrode active material, and a compound that generates heat and raises the temperature ; A secondary battery comprising 25% by weight or more and 5% by weight or less , and a method for preventing overcharge of a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte. Over Becomes a conductive state overcharge inhibitor resulting solid salt reacts with the metal eluted from the positive electrode ions are oxidized to contain to be 5 wt% or less than 0.1 wt%, and become overcharged, A compound constituting a part of a reaction for generating a solid salt from the product and the overcharge inhibitor , and a compound that generates water in the overcharge region and a compound that promotes elution of the metal component of the positive electrode active material The overcharge prevention method is characterized by containing a trigger substance composed of any one of the compounds that generate heat and raise the temperature so as to be 0.25 wt% or more and 5 wt% or less .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The secondary battery which concerns on this invention contains the overcharge inhibitor which has a property which produces a solid salt in a battery in an overcharge state in electrolyte. This overcharge preventing agent produces a solid salt in an overcharged state, which clogs the separator or coats the electrode, thereby increasing the resistance inside the battery, thereby providing an excellent overcharge preventing effect.
[0011]
As a method for generating a solid salt in the electrolyte in an overcharged state, there is a method in which a salt that decreases in solubility with the temperature rise in the battery and causes precipitation is used as an overcharge inhibitor. It is preferable to use as the overcharge inhibitor a substance that reacts with aldehyde to produce a solid salt. This is because in secondary batteries, metal ions are likely to elute from the positive electrode when overcharged. In particular, in a lithium transition metal composite oxide containing lithium and a transition metal commonly used as a positive electrode active material for a lithium secondary battery, transition metal ions are eluted from the positive electrode when overcharged. Therefore, the overcharge inhibitor is preferably one that reacts with the eluted transition metal ions to form a salt. The overcharge inhibitor may be one that reacts directly with the transition metal ion, or may be one in which a change product of the overcharge inhibitor reacts with the transition metal ion to form a salt. . In addition, in order to make the change of an overcharge inhibitor and a transition metal ion react, it will be oxidized when it will be in an overcharge state, and reaction of the product and an overcharge inhibitor will produce a solid salt from an overcharge inhibitor. It is preferable to contain a compound that constitutes a part of the reaction (hereinafter, a compound that includes such a compound and promotes the formation of a solid salt in an overcharged state is referred to as a “trigger substance”).
<Overcharge prevention agent>
In the secondary battery according to the present invention, as an overcharge inhibitor, a compound that generates a solid salt with a metal ion or the like in an overcharged state, or a compound that causes a chemical change in an overcharged state to generate a solid salt with a metal ion or the like The solid salt produced has a property of increasing the internal resistance of the battery. The overcharge inhibitor is preferably one that dissolves in the electrolyte because it is easy to prepare the electrolyte.
[0012]
Examples of the overcharge preventing agent include dicarboxylic acid, dicarboxylic acid ester, sulfamic acid, thioacetamide, dimethylglyoxime, 8-quinolinol, 1-nitroso 2-naphthol, lithium carbonate, lithium sulfate, calcium carbonate, manganese sulfate, and chromium. Examples include strontium acid, copper sulfate, and calcium sulfate. Of these, dicarboxylic acid esters, particularly dialkyl oxalates such as dimethyl oxalate or diethyl oxalate are preferred. In addition, multiple overcharge inhibitors may be used in combination.
[0013]
The overcharge inhibitor is usually contained in the electrolyte so as to be 0.1 wt% or more and 5 wt% or less. If it is less than 0.1% by weight, it may not act effectively as an overcharge inhibitor, and if it exceeds 5% by weight, battery characteristics may be adversely affected. The lower limit of the overcharge inhibitor is preferably 0.25% by weight or more, particularly preferably 0.5% by weight or more, and the upper limit is preferably 3% by weight or less, particularly preferably 2% by weight or less.
<Trigger substance>
Examples of the trigger substance include a compound that generates water in the overcharge region, a compound that promotes elution of the metal component of the positive electrode active material, and a compound that generates heat and raises the temperature. Of these, compounds that generate water in an overcharged state are preferred.
[0014]
The oxidation potential of the trigger substance is preferably as low as possible as long as it is equal to or higher than the upper limit potential of the secondary battery to be used.However, if the oxidation potential is low, it tends to react even in an overcharged state. It is preferable to select the trigger substance in consideration.
The specific oxidation potential of the trigger substance is usually about 4.3 to 4.9 V in the case of a lithium secondary battery, for example. If the voltage is less than 4.3V, the battery characteristics may be deteriorated. If the voltage exceeds 4.9V, the overcharge prevention effect is reduced. The lower limit of the oxidation potential of the trigger substance is preferably 4.4 V or higher, particularly 4.5 V or higher, and the upper limit is preferably 4.7 V or lower.
[0015]
The oxidation potential can be measured by a cyclic voltammetry method. That is, LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 7: 3 to prepare an electrolyte solution, and the trigger substance was adjusted to 0.15 mmol / g. Dissolve. This is placed in an H-type cell in which 1.6 mmφ platinum with only the bottom surface exposed is used as a working electrode, lithium metal as a counter electrode and a reference electrode, and the working electrode side and the counter electrode side are separated by a glass filter. Next, the potential of the working electrode is swept to the oxidation side (noble side) at a sweep rate of 20 mV / sec at room temperature (around 25 ° C.), and the potential at which a current density of 0.5 mA / cm ² flows at this time is the oxidation start potential. And
[0016]
Trigger substances include aromatic compounds such as biphenyl compounds, cyclohexylbenzene compounds, dibenzofuran compounds, terphenyl compounds, diphenyl ether compounds; 3-methyl-2-oxazolidone, 3-ethyl-2-oxazolidone, 3-isopropyl-2-oxazolidone , Cyclic carbamate compounds such as 3- (tert-butyl) -2-oxazolidone; alicyclic hydrocarbons such as camphene, pinene, camphor, bornane, fenchan, tetraethoxymethane, and orthoesters such as trimethyl orthoformate It is done. These may use multiple types together. Among these, aromatic compounds, particularly aromatic compounds represented by the following general formula (1) are preferable.
[0017]
[Chemical 3]

[0018]
(Wherein R _{1 to} R ₆ each independently represents a hydrogen atom, a halogen atom, or a hydrocarbon group, alkoxy group or aryloxy group which may have a substituent. R ₁ and R ₂ And may form a phenyleneoxy group, ethyleneoxy group, trimethyleneoxy group, propenyleneoxy group or vinyleneoxy group which may have a substituent.
Trigger material preferably contains such a typically 0.2 5 by weight% to 5% by weight in the electrolyte. If it is less than 0.25% by weight, it does not act effectively, and if it exceeds 5% by weight, battery characteristics may be adversely affected. The lower limit is preferably 0.5% by weight or more, and the upper limit is preferably 3% by weight or less, particularly preferably 2% by weight or less.
[0019]
When lithium / transition metal oxide is used as the positive electrode active material, dicarboxylic acid esters are used as the overcharge inhibitor, and aromatic compounds are used as the trigger material, the reason why a solid salt is generated in the overcharged state in the presence of the trigger material is as follows. It seems like. That is, when the battery is overcharged, water is generated by the oxidative polymerization reaction of the aromatic compound, and the dicarboxylic acid ester is hydrolyzed to generate dicarboxylic acid. The generated dicarboxylic acid and transition metal ion form a sparingly soluble salt, which precipitates from the electrolyte and closes the separator or covers the electrode, thereby increasing the resistance in the battery and preventing overcharging. .
[0020]
In the secondary battery according to the present invention, by preventing overcharge as described above, the battery is ruptured by the gas generated from the aromatic compound, or the electronic conductive film generated at the positive electrode is on the negative electrode or the negative electrode. It is possible to avoid a phenomenon such as a short circuit with metallic lithium deposited on the metal.
<Electrolyte>
The secondary battery according to the present invention includes an electrolyte containing a lithium salt and a non-aqueous solvent, a semisolid electrolyte obtained by solidifying the electrolyte with a polymer compound, and a lithium salt and a polymer compound as an electrolyte. Examples include a completely solid electrolyte. Of these, those containing an electrolyte salt such as a lithium salt and a nonaqueous solvent are preferred.
[0021]
As the solvent used for the electrolytic solution, a solvent mainly composed of a non-aqueous solvent selected from the group consisting of carbonate ester, ether and lactone is preferable.
Examples of the carbonate include cyclic carbonates such as propylene carbonate and ethylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.
[0022]
Examples of ethers include dimethoxyethane and diethoxyethane.
Examples of the lactone include γ-butyrolactone.
As the non-aqueous solvent, a solvent containing a carbonate, in particular, a mixed solvent of a high dielectric constant solvent such as a cyclic carbonate or lactone and a low viscosity solvent of a chain carbonate is preferable.
[0023]
The electrolyte salt is preferably a lithium salt. Lithium salts include LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN _{(SO 2 C 2 F 5)} 2, LiC (SO 2 CF 3) 3 and _LiN (SO 3 _{CF 3) 2} and the like. You may use these multiple types together. Of these, LiBF ₄ or LiPF ₆ is preferred.
[0024]
The lithium salt is usually contained in the electrolyte so as to be 0.5 mol / L or more and 1.5 mol / L or less. Even if it is less than 0.5 mol / L or more than 1.5 mol / L, the conductivity may be lowered, and the battery characteristics may be adversely affected. The lower limit is preferably 0.75 mol / L or more and the upper limit is 1.25 mol / L or less.
The electrolyte can further contain other components as required. Examples of other components include additives for forming a coating (SEI) on the active material surface of the battery. Examples of the additive for forming SEI include a cyclic carbonate having a carbon-carbon unsaturated bond. Use of a cyclic carbonate having a carbon-carbon unsaturated bond has an effect such as forming a good film on the negative electrode, so that the effect of protecting the overcharge inhibitor from the influence of the negative electrode can be considered. In addition to the effect of preventing overcharge, other characteristics such as storage stability and cycle characteristics of the battery can be improved.
[0025]
Examples of the cyclic carbonate having a carbon-carbon unsaturated bond include those having an olefinic double bond such as vinylene carbonate and vinylethylene carbonate; those having a benzene ring such as phenylethylene carbonate and catechol carbonate. Among these, vinylene carbonate or vinyl ethylene carbonate is preferable.
<Lithium secondary battery>
Examples of the secondary battery according to the present invention include a lithium secondary battery.
[0026]
The secondary battery contains a positive electrode, a negative electrode, and the electrolyte.
As the positive electrode active material, a lithium / transition metal composite oxide capable of generating a high electromotive force is preferable.
Examples of the lithium-transition metal composite oxide, lithium-cobalt composite oxide such as LiCoO _2, lithium nickel oxides such as LiNiO _2, lithium-manganese oxide such as LiMn ₂ O _4. These may be used in combination. Among these, what contains lithium and cobalt and / or nickel is preferable. Lithium transition metal Shokufuku if oxide becomes a part of the transition metal elements consisting mainly Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, than Ga and Zr It is preferable to stabilize by replacing with another metal species selected from the group.
[0027]
As the negative electrode active material, any material can be used as long as it can occlude and release lithium. Of these, carbonaceous materials are preferred.
Examples of the carbonaceous material include pyrolysates of organic matter under various pyrolysis conditions, artificial graphite, natural graphite, and the like. Among these, artificial graphite or graphitized mesophase microspheres produced by high-temperature heat treatment of graphitizable pitch obtained from various raw materials, other artificial graphite or purified natural graphite such as graphitized mesophase pitch-based carbon fiber, or these It is preferable that graphite is subjected to various surface treatments including pitch. These carbonaceous materials have a lattice plane (002 plane) d value (interlayer distance) of 0.335 to 0.34 nm, particularly 0.335 to 0.337 nm, determined by X-ray diffraction by the Gakushin method. preferable. The ash content is usually 1% by weight or less. It is preferably 0.5% by weight or less, particularly preferably 0.1% by weight or less. The crystallite size (Lc) determined by X-ray diffraction by the Gakushin method is usually 30 nm or more. It is preferably 50 nm or more, particularly 100 nm or more.
[0028]
These carbonaceous materials can be further mixed with other active materials capable of inserting and extracting lithium. Examples of other active materials capable of inserting and extracting lithium include metal oxide materials such as tin oxide and silicon oxide, and lithium metal and various lithium alloys. A plurality of these negative electrode materials may be used in combination.
Examples of the binder used for the electrode include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber.
[0029]
Examples of the thickener include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein.
The electrode may contain a metal material such as copper and nickel, and a conductive material such as a carbon material such as graphite and carbon black. The positive electrode preferably contains a conductive material.
[0030]
What is necessary is just to manufacture an electrode by a conventional method. For example, a method in which a binder, a thickener, a conductive material, a solvent, and the like are added to the active material to form a slurry, which is applied to a current collector substrate and dried. Further, the active material can be roll-formed as it is to form a sheet electrode, or it can be formed into a pellet electrode by compression molding.
Examples of the negative electrode current collector include metals or alloys such as copper, nickel, and stainless steel, among which copper is preferable. Examples of the positive electrode current collector include metals or alloys such as aluminum, titanium, and tantalum, and among these, aluminum or an alloy thereof is preferable.
[0031]
A battery usually has a separator interposed between a positive electrode and a negative electrode. The material and shape of the separator are arbitrary. A porous sheet or non-woven fabric made of a polyolefin such as polyethylene and polypropylene is preferable because it is stable in the electrolyte and excellent in liquid retention.
The shape of the battery is arbitrary, and examples include a cylinder type in which a sheet electrode and a separator are spiral, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are stacked.
[0032]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to a following example, unless the summary is exceeded.
(Creation of positive electrode)
90 parts by weight of lithium cobaltate (LiCoO ₂ ), 5 parts by weight of acetylene black, and 5 parts by weight of polyvinylidene fluoride were mixed in an N-methylpyrrolidone solvent to form a slurry. This was coated on one side of a 20 μm aluminum foil, dried, and then rolled with a press to be punched with a punch having a diameter of 12 mm to obtain a positive electrode.
(Creation of negative electrode)
Graphite (plane spacing 0.336 nm) 95 parts by weight and polyvinylidene fluoride 5 parts by weight were mixed in an N-methylpyrrolidone solvent to form a slurry. This was coated on one side of a 20 μm-thick copper foil, dried, and then rolled with a press to punch out with a diameter of 12 mm to obtain a negative electrode.
(Battery assembly)
A lithium secondary battery was manufactured using a CR2032-type coin cell in a dry box in an argon atmosphere. That is, a positive electrode was placed on a positive electrode can, a 25 μm porous polyethylene film was placed thereon as a separator, and pressed with a polypropylene gasket. Further, after placing the negative electrode and the spacer for adjusting the thickness in this order, the electrolyte was added to sufficiently infiltrate the battery, and the negative electrode can was placed to seal the battery. In the following example, the battery capacity was designed to be about 4.0 mAh with a charge upper limit of 4.2 V and a discharge lower limit of 3.0 V.
[0033]
The ratio of the weight of the positive electrode active material W (c) to the weight of the negative electrode active material W (a) is such that lithium ions released from the positive electrode do not cause lithium metal precipitation on the opposite negative electrode at the normal use upper limit voltage of the battery. A range is preferred. Therefore, the weight was determined so that the capacity ratio Rq between the negative electrode and the positive electrode was 1.1 ≦ Rq ≦ 1.2. The capacity ratio Rq was determined by Q (a) × W (a) / {Q (c) × W (c)}. Here, the electric capacity per weight of the positive electrode active material under the condition corresponding to the initial charge condition of the battery is Q (c) mAh / g, and the negative electrode capable of occluding lithium to the maximum without depositing lithium metal The electric capacity per weight of the active material was Q (a) mAh / g. Q (c) and Q (a) were measured using a positive electrode or a negative electrode as a working electrode and a lithium metal as a counter electrode, and using a test cell in a same electrolytic solution as that for assembling the battery via a separator. That is, the capacity at which the positive electrode can be charged (release lithium ions from the positive electrode) at the lowest possible current density up to the upper limit potential of the positive electrode or the lower limit potential of the negative electrode corresponding to the initial charging conditions of the target battery system, It calculated | required as a capacity | capacitance which can be discharged (occlusion of the lithium ion to a negative electrode).
(Battery evaluation)
Battery evaluation was performed in the order of (1) initial charge / discharge (capacity check), (2) full charge operation, and (3) overcharge test.
[0034]
Initial charge / discharge (capacity check) was charged by a constant current constant voltage method with an upper limit of 1C (4.0 mA) and 4.2V. Charging was cut when the current value reached 0.05 mA. Discharging was performed at a constant current up to 3.0 V at 0.2C.
The full charge operation was performed by a constant current / constant voltage method (0.05 mA cut) with an upper limit of 4.2 V.
[0035]
The overcharge test was 4.99V cut or 3hr cut at 1C (cut whichever comes first).
The superiority or inferiority of the overcharge prevention effect was performed by confirming the voltage curve behavior and the apparent overcharge current amount in the overcharge region.
Example 1
In a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) is dissolved to 1 mol / L, and then diethyl oxalate and dibenzofuran are dissolved. Each was dissolved so that it might become 2 weight%, and it was set as the electrolyte solution.
[0036]
Using this electrolytic solution, a lithium secondary battery was produced by the method described above and evaluated.
The voltage at the time of overcharge increased rapidly, and overcharge was prevented in a stable state. Moreover, when the battery after the overcharge was disassembled and the compound deposited on the separator and the electrode was analyzed by ICP, ion chromatography, and ESCA, cobalt oxalate was confirmed in the precipitate. The results are shown in Table-1.
(Example 2)
In Example 1, a lithium secondary battery was produced and evaluated in the same manner as in Example 1 except that vinylene carbonate was further added to 2 wt% in addition to diethyl oxalate and dibenzofuran. The results are shown in Table-1. In addition, cobalt oxalate was confirmed in the precipitate by the same method as in Example 1.
(Comparative Example 1)
In Example 1, a lithium secondary battery was produced and evaluated in the same manner as in Example 1 except that diethyl oxalate and dibenzofuran were not added. The results are shown in Table-1.
(Comparative Example 2)
In Example 1, a lithium secondary battery was produced and evaluated in the same manner as in Example 1 except that only dibenzofuran was added to 2 wt%. The results are shown in Table-1. In addition, the voltage oscillation which seems to be due to the short circuit at the time of overcharge was observed, the cut-off voltage was not easily reached, and the apparent amount of overcharge current increased.
(Comparative Example 3)
In Example 1, a lithium secondary battery was produced and evaluated in the same manner as in Example 1 except that vinylene carbonate was added in an amount of 2% by weight instead of diethyl oxalate. The results are shown in Table-1. In addition, the voltage oscillation which seems to be due to the short circuit at the time of overcharge was observed, the cut-off voltage was not easily reached, and the apparent amount of overcharge current increased.
[0037]
[Table 1]

[0038]
From Table 1, it can be seen that by adding diethyl oxalate, salt precipitates during overcharge, and the safety in the overcharge state is improved.
In addition, the lithium secondary battery produced by the Example and the lithium secondary battery produced by the comparative example did not have a big difference in battery characteristics, such as initial stage discharge capacity and the capacity maintenance rate after 5 cycles.
[0039]
【The invention's effect】
The secondary battery according to the present invention has improved safety during overcharge by a method completely different from the conventional method.

Claims (15)

A lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte, and an overcharge inhibitor that reacts with metal ions eluted from the positive electrode when in an overcharged state to produce a solid salt , 0.1 wt% or more 5 wt% contained as to become less, and is oxidized to become overcharged, a compound which constitutes a part of the reaction to produce the solid salt and the product from the overcharge inhibitor, overcharge region In addition, a trigger substance composed of any one of a compound that generates water, a compound that promotes elution of the metal component of the positive electrode active material, and a compound that generates heat and raises the temperature is 0.25 wt% to 5 wt% Lithium secondary battery characterized by containing so that it may become .   The secondary battery according to claim 1, wherein the positive electrode contains a lithium / transition metal composite oxide and elutes metal ions into the electrolyte when overcharged. Overcharge inhibitor, secondary battery according to claim 1 or 2, wherein the a dicarboxylic acid ester. The secondary battery according to any one of claims 1 to 3 , wherein the solid salt is generated from a metal ion eluted from the positive electrode and an overcharge inhibitor or a change thereof. The oxidation potential of the trigger substance is a secondary battery according to any one of claims 1 to 4, characterized in that at 4.3V or 4.9V or less. Trigger substance, rechargeable battery according to any one of claims 1 to 5, characterized in that an aromatic compound. The secondary battery according to claim 6 , wherein the aromatic compound is represented by the following general formula (1).

(Wherein R _{1 to} R ₆ each independently represents a hydrogen atom, a halogen atom, or a hydrocarbon group, alkoxy group or aryloxy group which may have a substituent. R ₁ and R ₂ And may form a phenyleneoxy group, ethyleneoxy group, trimethyleneoxy group, propenyleneoxy group or vinyleneoxy group which may have a substituent. Cathode, an overcharge prevention method of a lithium secondary battery having a negative electrode and an electrolyte, in the electrolyte, the overcharge inhibitor causing overcharge state becomes the solid salt reacts with the metal eluted from the positive electrode ion, 0 5 wt% .1% by weight or more is contained as to become less, and is oxidized to become overcharged, there in compounds forming part of the reaction to produce the solid salt from its product the overcharge inhibitor In the overcharge region , 0.25% by weight of a trigger substance composed of any one of a compound that generates water, a compound that promotes elution of the metal component of the positive electrode active material, and a compound that generates heat and raises the temperature An overcharge prevention method characterized by containing 5% by weight or less . The method according to claim 8 , wherein the positive electrode contains a lithium / transition metal composite oxide and elutes metal ions into the electrolyte when overcharged. The method according to claim 8 or 9 , wherein the overcharge inhibitor is a dicarboxylic acid ester. The method according to any one of claims 8 to 10 , wherein the solid salt is generated from a metal ion eluted from the positive electrode and an overcharge inhibitor or a variation thereof. The method according to any one of claims 8 to 11 , wherein the oxidation potential of the trigger substance is 4.3 V or more and 4.9 V or less. The method according to any one of claims 8 to 12 , wherein the trigger substance is an aromatic compound. The method according to claim 13 , wherein the aromatic compound is represented by the following general formula (1).

(Wherein R _{1 to} R ₆ each independently represents a hydrogen atom, a halogen atom, or a hydrocarbon group, alkoxy group or aryloxy group which may have a substituent. R ₁ and R ₂ And may form a phenyleneoxy group, ethyleneoxy group, trimethyleneoxy group, propenyleneoxy group or vinyleneoxy group which may have a substituent. A method for preventing overcharge of a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein an oxalic acid dialkyl ester is contained in the electrolyte in an amount of 0.1 wt% to 5 wt% , and the oxidation potential is By containing an aromatic compound of 4.3 V or more and 4.9 V or less in an amount of 0.25 wt% or more and 5 wt% or less , a solid salt is generated in the electrolyte when overcharged. Overcharge prevention method.