JP2004296237A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having high safety in overcharge, and improved in storage characteristic. <P>SOLUTION: This secondary battery is provided with: a positive electrode containing a positive electrode active material; a negative electrode; and a nonaqueous electrolyte containing a compound expressed by structural formula. In structural formula, R is any of a 1-4C alkyl group, methoxy group, aryl group or a halogen group bonded to at least any position of o-, m- or p- of a benzene ring. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００１３】
但し、Ｒは、ベンゼン環のｏ−、ｍ−、ｐ−の少なくともいずれかの位置に結合した炭素数１〜４のアルキル基、メトキシ基、アリール基、ハロゲン基のいずれかを示す。
【００１４】
また、本発明に係る非水電解質二次電池は、正極活物質を含む正極と、負極と、非水電解質とを具備する非水電解質二次電池において、
前記正極の表面に前述した化３に示す構造式で表わされる化合物が存在することを特徴とするものである。
【００１５】
【発明の実施の形態】
以下、本発明に係る非水電解質二次電池の第一の実施形態について説明する。この非水電解質二次電池は、正極活物質を含む正極と、負極と、前記正極と前記負極の間に配置されるセパレータと、下記化４に示す構造式で表わされる化合物を含む非水電解質とを具備する。
【００１６】
【化４】

【００１７】
但し、Ｒは、ベンゼン環のｏ−、ｍ−、ｐ−の少なくともいずれかの位置に結合した炭素数１〜４のアルキル基、メトキシ基、アリール基、ハロゲン基のいずれかを示す。なお、ベンゼン環の複数の水素原子がＲと置換されている場合、そのうちの少なくともひとつの置換基（Ｒ）がベンゼン環のｏ−、ｍ−、ｐ−のいずれかの位置に結合していれば良い。
【００１８】
以下、非水電解質、正極、負極及びセパレータについて説明する。
【００１９】
１）非水電解質
非水電解質は、非水溶媒と、前記非水溶媒に溶解され、前述した化４に示す構造式で表わされる化合物と、前記非水溶媒に溶解される電解質とを含む。
【００２０】
非水電解質の形態は、液状もしくはゲル状にすることが可能である。
【００２１】
アルキル基の炭素数を１〜４に限定するのは、炭素数が４を超えると、分子量増大のために前記非水溶媒への溶解性が低下して十分な量を添加することができなくなるためである。
【００２２】
また、前述した化合物のうち、メトキシ基あるいはアリール基をＲとして含む化合物は、非水溶媒への溶解性が高いため、好ましい。
【００２３】
前述した化合物としては、ｐ−トリルベンゾエート、アセチルフェニルベンゾエート、ｔｅｒｔ−ブチルフェニルベンゾエート及びｐ−クロロフェニルベンゾエートよりなる群から選択される少なくとも一種類を用いることが望ましく、これらを用いると生成する保護層のインピーダンスをより低くすることができる。
【００２４】
非水電解質中の前記化合物の含有量は１重量％以下にすることが望ましい。これは、含有量が１重量％を超えると、化合物と負極との反応が生じやすくなるため、電池の内部インピーダンスが大きくなり、貯蔵特性が低くなる可能性があるばかりか、初期容量あるいは高率放電特性が損なわれる恐れがあるからである。
含有量のさらに好ましい範囲は、０．５重量％以下である。こうすることにより、正極表面への保護層生成反応を初期充電において完了させることができ、出荷前の温度等の外部条件を最適化した状態で正極表面に保護層を生成させることができるため、保護層の安定性をより高くすることができる。加えて、非水電解質中に残留した前記化合物が、負極等の他の電池構成部材と反応するのを抑制することができる。
【００２５】
また、貯蔵特性と過充電時の安全性の双方を十分に高くするためには、非水電解質中の前記化合物の含有量を０．１重量％以上、１重量％以下にすることが望ましい。
【００２６】
非水電解質中の前記化合物の含有量は、以下に説明する方法で測定される。まず、セルを分解して容器内の内容物（電極群と非水電解質）を取り出し、この内容物を遠心分離機にかけて液体成分を分離する。得られた液体成分にＤＭＳＯ−ｄ_６溶液を添加し、１Ｈ（４００ＭＨｚ）、１３Ｃ（１００ＭＨｚ）核のＮＭＲスペクトル測定を行うことにより、化合物の特定および非水電解質中の化合物の含有量を測定することができる。
【００２７】
非水溶媒としては、非水電解質二次電池用として公知の各種溶媒を使用することができる。特に限定はされないが、非水溶媒としては、環状カーボネート｛例えば、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）｝、鎖状カーボネート｛例えば、ジメチルカーボネート（ＤＭＣ）、メチルエチルカーボネート（ＭＥＣ）、ジエチルカーボネート（ＤＥＣ）｝、γ−ブチロラクトン（ＢＬ）、アセトニトリル（ＡＮ）、酢酸エチル（ＥＡ）、トルエン、キシレン及び酢酸メチル（ＭＡ）よりなる群から選択される少なくとも一種類の溶媒を用いることができる。中でも、環状カーボネート及び鎖状カーボネートのうち少なくとも一方を含む非水溶媒を用いることが望ましい。
【００２８】
電解質としては、例えば、過塩素酸リチウム、六フッ化リン酸リチウム（ＬｉＰＦ_６）、四フッ化ホウ酸リチウム（ＬｉＢＦ_４）、六フッ化砒素リチウム、トリフルオロメタンスルホン酸リチウム、ビストリフルオロメチルスルホニルイミドリチウムなどのリチウム塩を用いることができる。
【００２９】
２）正極
正極は、正極活物質及びバインダーを含む混合物を薄板状もしくはシート状に成形したものが主体となっている。さらに、薄板状もしくはシート状の混合物に集電体を密着させて集電性能を向上させることもできる。
【００３０】
正極活物質としては、例えば、リチウム含有金属酸化物、カルコゲン化合物等を挙げることができる。リチウム含有金属酸化物としては、例えばリチウム含有コバルト複合酸化物、リチウム含有ニッケルコバルト複合酸化物、リチウム含有ニッケル複合酸化物、リチウムマンガン複合酸化物、リチウム含有バナジウム酸化物等を挙げることができる。一方、カルコゲン化合物としては、例えば、二硫化チタン、二硫化モリブデンなどを挙げることができる。なお、使用する正極活物質の種類は、１種類あるいは２種類以上にすることができる。
【００３１】
中でも、充電電位がリチウム金属電位に対して４Ｖ以上であり、かつ前述した化４に示す構造式で表わされる化合物の酸化反応において触媒活性を示す正極活物質が好ましい。このような正極活物質としては、リチウム含有コバルト複合酸化物、リチウム含有ニッケルコバルト複合酸化物などが挙げられる。特に、ＬｉＣｏ_ｘＮｉ_ｙＭｎ_ｚＯ_２（但し、ｘ、ｙ及びｚは、ｘ＋ｙ＋ｚ＝１、０＜ｘ≦１、０≦ｙ＜０．８、０≦ｚ＜０．８を示す）で表わされるリチウム含有金属酸化物が好ましい。このリチウム含有金属酸化物においては、ｙ及びｚがそれぞれ０≦ｙ＜０．５、０≦ｚ＜０．５を満たすことが望ましい。ＬｉＣｏ_ｘＮｉ_ｙＭｎ_ｚＯ_２（但し、ｘ、ｙ及びｚは、ｘ＋ｙ＋ｚ＝１、０＜ｘ≦１、０≦ｙ＜０．５、０≦ｚ＜０．５を示す）で表わされる正極活物質は、リチウム含有コバルト酸化物と類似の結晶構造を有するため、正極表面の触媒活性により、安定性の高い保護層を生成させることができる。なお、ｙの値を０．５以上にすると、正極活物質表面のアルカリ分残留量が多くなるため、保護層の安定性が低下する恐れがある。
【００３２】
バインダーとしては、例えば、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶｄＦ）、エチレン−プロピレン−ジエン共重合体、スチレン−ブタジエンゴム等を用いることができる。一方、集電体としては、例えば、アルミニウム、ステンレス、チタンなどの金属箔もしくは薄板を用いることができる。
【００３３】
正極には、導電剤が含有されていることが望ましい。導電剤としては、例えば、黒鉛、アセチレンブラック、カーボンブラック等を挙げることができる。
【００３４】
正極活物質と導電剤にバインダーを加えて混練することにより正極活物質を含むシートを得ることができる。あるいは、正極活物質と導電剤とバインダーとをトルエン、Ｎ−メチルピロリドン（ＮＭＰ）等の溶媒に分散させ、得られたスラリーを集電体上に塗布し、乾燥することにより正極を得ることも可能である。
【００３５】
３）負極
負極は、負極活物質及びバインダーを含む混合物を薄板状もしくはシート状に成形したものが主体となっている。さらに、薄板状もしくはシート状の混合物に集電体を密着させて集電性能を向上させることもできる。
【００３６】
負極活物質には、リチウム金属もしくはリチウムを吸蔵放出することが可能な炭素質材料を用いることができる。前記炭素質材料は特に限定されるものではないが、例えば、原料として、石油や石炭などのコークスやピッチ、天然ガスや低級炭化水素などの低分子量有機化合物、ポリアクリロニトリル、フェノール樹脂等の合成高分子などを用い、この原料を７００℃〜３０００℃で焼成することにより炭素化あるいは黒鉛化することにより得られる。また、炭素質材料として天然黒鉛や人造黒鉛を用いても良い。さらに、２種以上の炭素質材料を混合して用いることもできる。炭素質材料の粒子形状としては鱗片状、繊維状、球状など各種形状のものが可能である。特に、人造黒鉛および天然黒鉛のうち少なくとも一方を用いると、前記化４で表される化合物と負極との副反応によるインピーダンス上昇が生じるのを抑制することができるので望ましい。さらに、球状もしくは繊維状のものを用いると炭素質材料の表面積を小さくすることができるので、副反応抑制効果を高くすることができる。
【００３７】
バインダーとしては、例えば、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン（ＰＶｄＦ）、スチレン−ブタジエンゴム、カルボキシメチルセルロース（ＣＭＣ）等を用いることができる。一方、集電体としては、例えば、銅、ステンレス、ニッケル等などの金属箔もしくは薄板を用いることができる。
【００３８】
負極は、例えば、負極活物質と結着剤とを、水、Ｎ−メチルピロリドン（ＮＭＰ）等の溶媒に分散させ、得られたスラリーを集電体上に塗布し、乾燥することにより作製される。
【００３９】
４）セパレータ
このセパレータとしては、例えば、合成樹脂不織布、ポリエチレン多孔質フィルム、ポリプロピレン多孔質フィルム等を用いることができる。
【００４０】
非水電解質二次電池においては、組立て後、初期充電（初充電）が行われる。保護層の生成は、電池組立後の初期充電において、化４で表される化合物が正極で反応することで行われる。そのため、初期充電は小さな電流で時間をかけて行うことが望ましい。具体的には、０．５ＣＡ以下の電流値が望ましい。さらに望ましい初回充電は、保護層生成反応電位で電圧を一定に保ち、０．０２ＣＡまで電流値が収束してから、継続して通常の充電を行うことである。前記充電を行うことにより、初回充電において、添加した前記化合物の大半を安定な保護層に変換せしめることができ、前記化合物を有効に利用することができる。
【００４１】
以上説明した本発明によれば、前述した化４に示す構造式で表わされる化合物を含む非水電解質を備えるため、電池容量等の通常特性を損なうことなく、過充電時の安全性および貯蔵特性に優れた非水電解質二次電池を提供することができる。
【００４２】
すなわち、上記化４の一般式で表される化合物は、非水電解質二次電池に使用される非水溶媒との親和性が高く、非水溶媒のうち特に多用されるエチレンカーボネートやエチルメチルカーボネートなどのカーボネート類への溶解性が高い。
そのため、化合物を予め非水電解質に添加しておき、この非水電解質を用いて通常通りの方法で電池を製造することが可能であると共に、化合物が電極等に析出して電池性能が劣化するのを回避することができる。
【００４３】
さらに、上記保護層は、リチウム電位に対して４．５Ｖ以上の高電圧における安定性が高いため、過充電時の正極活物質と電解液の発熱反応を抑止することができ、過充電時の安全性を向上することができる。なお、上記化合物は、化４におけるＲを持つために、保護層の化学的安定性が向上するのみならず、電解液の液相への溶解も抑制され、効果の増大と、効果持続性を高めることができる。
【００４４】
また、上記化合物によると、非水電解質中の前記化合物の含有量が１重量％以下という少量で貯蔵特性と過充電時の安全性を改善することができる。従って、化合物添加に起因する内部インピーダンス上昇を極めて小さくすることができるため、初期放電容量や高率放電特性などの通常特性に与える影響を最小限に抑えつつ、過充電時の安全性と貯蔵特性を向上することが可能である。
【００４５】
ＬｉＣｏ_ｘＮｉ_ｙＭｎ_ｚＯ_２（但し、ｘ、ｙ及びｚは、ｘ＋ｙ＋ｚ＝１、０＜ｘ≦１、０≦ｙ＜０．５、０≦ｚ＜０．５を満たす）で表わされるリチウム含有金属酸化物を含む正極活物質を用いることにより、リチウム含有コバルト酸化物と類似の結晶構造を有するため、初充電時に正極表面に保護層を安定的かつ速やかに形成することができる。これは、化４で示した化合物が正極活物質の４Ｖ以上のリチウム放出電位付近で反応して保護層を形成するためである。また、得られた保護層は、この正極活物質の１３０〜３００℃における発熱反応を抑制する効果が高いため、過充電時の安全性をより高くすることができる。
【００４６】
次いで、本発明に係る非水電解質二次電池の第２の実施形態について説明する。この非水電解質二次電池は、正極活物質を含む正極と、負極と、前記正極と前記負極との間に介在されるセパレータと、非水電解質とを具備する非水電解質二次電池であって、前記正極の表面に前述した化４に示す構造式で表わされる化合物が存在するものである。
【００４７】
この二次電池は、例えば、前述した化４で表される化合物を含む溶液に正極を浸漬した後、この正極と負極の間にセパレータを介在させて電極群を作製し、この電極群と、非水電解質とを容器内に収納し、封口処理を施した後、初充電を施すことにより得られる。なお、非水電解質には、非水溶媒に電解質が溶解された非水電解液を使用することが望ましいが、少量であれば前述した化４で表される化合物が含有されていても良い。正極、負極、セパレータ、非水溶媒、電解質及び初充電方法については、前述した第１の実施形態で説明したのと同様なものを挙げることができる。
【００４８】
このように、あらかじめ正極に前記化合物を担持させておくことで、前記化合物が直接接触する対象を正極のみに限定することが可能となり、負極等の他の電池内物質との反応を大きく制限することができ、前記化合物を無駄なく保護層生成に用いることができるようになる。
【００４９】
本発明に係る非水電解質二次電池は、円筒形、角形、薄型等の種々の形態に適用することができる。そのうちの円筒形非水電解質二次電池の一例を図１に示す。
【００５０】
鉄やステンレスのような金属製の円筒形電池容器１内には、正極２と負極３とをセパレータ４を介在して渦巻き状に捲回した電極群５が収納されている。絶縁板６は、電池容器１の底部内面と電極群５との間に介装されている。非水電解質は、電池容器１内に収容されている。封口板７と、ガス放出弁及び電流遮断機構８は、電池容器１の開口部に絶縁リング９を介して取り付けられている。
【００５１】
正極タブ１０は、一端が正極２と電気的に接続され、かつ他端がガス放出弁及び電流遮断機構８と電気的に接続されている。一方、負極タブ１１は、一端が負極３と電気的に接続され、かつ他端が電池容器１の底部内面と電気的に接続されている。ガス抜き孔１２を有する正極端子１３は、ガス放出弁及び電流遮断機構８と電気的に接続され、かつ封口板７上に配置されている。一方、電池容器１の底部が負極端子として機能している。
【００５２】
前述した図１においては、円筒形の電池容器を使用する例を説明したが、電池容器の形状はこれに限らず、例えば、角形、袋状等にすることができる。また、電池容器は、ステンレスや鉄のような金属から形成しても良いが、袋状容器を使用する場合には、ラミネートフィルムから形成することが可能である。
【００５３】
また、前述した図１においては、渦巻形状の電極群を使用する例を説明したが、正極及び負極をセパレータを介在して扁平形状に捲回した扁平型電極群や、正極及び負極をセパレータを介在して積層した積層型電極群などを用いた非水電解質二次電池にも適用することができる。
【００５４】
また、本発明に係る非水電解質二次電池の製造方法は、前述した化４に示す構造式で表わされる化合物を含む電極群を備える未初充電の非水電解質二次電池を組み立てる工程と、
前記未初充電の非水電解質二次電池に初期充電（初充電）を施す工程とを具備するものである。
【００５５】
前述した化４に示す構造式で表わされる化合物は、正極か、非水電解質もしくは正極と非水電解質に含有させることが望ましい。また、初期充電の条件は、前述した第１の実施形態で説明したのと同様な条件にすることが望ましい。
【００５６】
このような製造方法によれば、初充電時に正極に保護層を形成することができるため、過充電時の安全性と貯蔵特性に優れる非水電解質二次電池を提供することができる。
【００５７】
【実施例】
以下、本発明の実施例を前述した図面を参照して詳細に説明する。
【００５８】
（実施例１）
リチウム含有コバルト酸化物（ＬｉＣｏＯ_２）粉末を９０重量％と、アセチレンブラックを２重量％と、グラファイトを３重量％と、バインダーとしてポリフッ化ビニリデン５重量％とを溶媒としてのＮ−メチルピロリドンに分散させ、スラリーを調製した。得られたスラリーを厚さ２０μｍのアルミニウム箔上に塗布し、乾燥することにより正極を得た。この正極の端部に正極タブとしてアルミニウム製リボンを溶接により接続した。
【００５９】
３０００℃焼成のメソフェーズピッチ系繊維状黒鉛粉末を８７重量％と、平均粒径５μｍの人造グラファイトを１０重量％と、カルボキシメチルセルロースを１重量％と、スチレン・ブタジエンゴムを２重量％とを溶媒としての水に分散させ、スラリーを調製した。得られたスラリーを厚さ１２μｍの銅箔上に塗布し、乾燥することにより負極を得た。この負極の端部には、負極タブとしてニッケル製リボンを溶接で接続した。
セパレータにはポリエチレン製多孔質フィルムを用いた。
【００６０】
正極、セパレータ、負極をそれぞれこの順序で積層したのち、スパイラル状に捲回することにより渦巻き状の電極群を得た。
【００６１】
１Ｍの六フッ化リン酸リチウムを、エチレンカーボネートとエチルメチルカーボネートの混合溶媒（体積比率１：１）に溶解させた後、ｐ−トリルベンゾエート（ｐ−Ｔｏｌｙｌ ｂｅｎｚｏａｔｅ）０．４ｗｔ％を添加することにより非水電解液を調製した。
【００６２】
電極群をステンレス製の円筒型缶（直径１８ｍｍ、高さ６５ｍｍ）からなる電池容器内に収納した後、非水電解液を注入し、弁および電流遮断機構を組込み、電池容器の開口部を封止することにより、前述した図１に示す構造を有する円筒形非水電解質二次電池を組み立てた。
【００６３】
得られた非水電解質二次電池に、３００ｍＡで、最大充電電圧を４．２０Ｖとして、４．２０Ｖまで定電流で充電し、４．２０Ｖ到達後は合計時間で１０時間となるまで定電圧で充電を行うことにより初充電を施した。
【００６４】
（実施例２〜５）
非水電解液中のｐ−トリルベンゾエートの含有量を下記表１に示すように変更すること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００６５】
（実施例６）
添加剤の種類を４−（ｔｅｒｔ−ブチル）フェニルベンゾエート（４−（ｔｅｒｔ−ｂｕｔｙｌ）ｐｈｅｎｙｌ ｂｅｎｚｏａｔｅ）に変更すること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００６６】
（実施例７）
添加剤の種類を４−クロロフェニルベンゾエート（４−Ｃｈｌｏｒｏ ｂｅｎｚｏａｔｅ）に変更すること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００６７】
（実施例８）
ジエチルカーボネートとｐ−トリルベンゾエート（ｐ−Ｔｏｌｙｌ ｂｅｎｚｏａｔｅ）を重量比９：１の割合で混合したものに、実施例１と同様の方法で作製した正極を５分間浸漬後、電極表面の液を十分に拭き取った後、実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。なお、非水電解液として、１Ｍの六フッ化リン酸リチウムを、エチレンカーボネートとエチルメチルカーボネートの混合溶媒（体積比率１：１）に溶解させたものを用いた。
【００６８】
（実施例９）
前述した実施例１で説明したのと同様にして組み立てられた非水電解質二次電池に、１００ｍＡで、最大充電電圧を４．２０Ｖとして、４．２０Ｖまで定電流で充電し、４．２０Ｖ到達後に３０ｍＡまで電流が収束するまで定電圧で充電を行うことにより初充電を施した。
【００６９】
（実施例１０）
ＬｉＣｏ_１／３Ｎｉ_１／３Ｍｎ_１／３Ｏ_２で表わされる組成のリチウム含有コバルトニッケルマンガン酸化物粉末を９０重量％と、アセチレンブラックを２重量％と、グラファイトを３重量％と、バインダーとしてポリフッ化ビニリデン５重量％とをＮ−メチルピロリドンに分散させ、スラリーを調製した。得られたスラリーを厚さ２０μｍのアルミニウム箔上に塗布し、乾燥することにより正極を作製した。この正極を用いること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００７０】
（比較例１）
１Ｍの六フッ化リン酸リチウムを、エチレンカーボネートとエチルメチルカーボネートの混合溶媒（体積比率１：１）に溶解させた非水電解液を用いること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００７１】
（比較例２）
添加剤としてフェニルプロピオネート（Ｐｈｅｎｙｌ ｐｒｏｐｉｏｎａｔｅ）を用いること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００７２】
（比較例３）
添加剤として４−クロロアニソール（４−Ｃｈｌｏｒｏａｎｉｓｏｌ）を用いること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００７３】
（比較例４）
添加剤としてフェニルベンゾエート（Ｐｈｅｎｙｌ ｂｅｎｚｏａｔｅ）を用いること以外は、前述した実施例１で説明したのと同様にして非水電解質二次電池の組立てと初充電を行った。
【００７４】
得られた実施例１〜１０および比較例１〜４の非水電解質二次電池について、初期容量、過充電特性、高率放電特性及び高温貯蔵特性を以下に説明する方法で測定し、その結果を下記表１〜２に示す。なお、実施例１〜１０および比較例１〜４の非水電解質二次電池の公称容量は、１５００ｍＡｈである。
【００７５】
＜初期容量＞
１５００ｍＡで３．０Ｖまで放電を行ったときの放電容量を初期容量とした。
【００７６】
＜過充電試験＞
７５０ｍＡ（０．５ＣＡ）の定電流で４．２Ｖまで充電後、４．２Ｖ定電圧で合計充電時間が５時間となるように充電を行った。この後、３０００ｍＡ（２ＣＡ）の充電電流を流して過充電試験を行い、電流遮断機構作動時間と最大温度を測定した。
【００７７】
＜高率放電特性＞
７５０ｍＡ（０．５ＣＡ）の定電流で４．２Ｖまで充電後、４．２Ｖ定電圧で合計充電時間が５時間となるように充電を行った。０．５時間の休止後、４５００ｍＡ（３ＣＡ）の電流で３Ｖとなるまで放電を行い、３ＣＡでの放電容量を測定した。
【００７８】
＜高温貯蔵特性＞
７５０ｍＡ（０．５ＣＡ）の定電流で４．２Ｖまで充電後、４．２Ｖ定電圧で合計充電時間が５時間となるように充電を行った。次いで、６０℃の恒温槽中に３０日間保存した。これを取り出して室温（２０℃）で、８００ｍＡ（０．５ＣＡ）で２．７Ｖ終止の定電流放電を行って残存放電容量を測定した。この値と初期放電容量の比から保存特性を算出した。
【００７９】
【表１】

【００８０】
【表２】

【００８１】
表１、２から明らかなように、前述した化４で表される化合物を用いた実施例１〜１０の二次電池は、添加剤無添加の比較例１に比較して過充電試験時の電流遮断機構作動時間が短く、最大温度が低く、かつ液漏れがないばかりか、充電状態で高温貯蔵した際の容量保持率が比較例１〜４に比較して高いことが理解できる。また、表１，２から、実施例１〜１０の二次電池は、添加剤無添加の比較例１の場合と比較して遜色のない初期容量および３ＣＡ放電容量を有していることもわかる。実施例１〜１０では、初充電時の高い正極電位において、正極活物質と電解液の反応を阻害する保護層が正極表面に生成されたためであると考えられる。
【００８２】
実施例１〜５を比較することによって、化合物の添加量が１重量％以下である実施例１〜３の二次電池は、初期放電容量、３ＣＡ放電容量及び高温貯蔵時の保存特性が実施例４〜５の二次電池（化合物添加量が２〜５重量％）に比較して高いことがわかる。
【００８３】
これに対し、添加剤無添加の比較例１の二次電池は、過充電試験時に電流遮断機構が作動するまでが遅く、１３５℃の高温に至り、液漏れを生じた。一方、従来の添加剤を使用した比較例２〜４の二次電池では、高温貯蔵時の保存特性が実施例１〜１０に比較して劣っていた。
【００８４】
なお、本発明は上記実施形態そのままに限定されるものではなく、実施段階ではその要旨を逸脱しない範囲で構成要素を変形して具体化できる。また、上記実施形態に開示されている複数の構成要素の適宜な組み合わせにより、種々の発明を形成できる。例えば、実施形態に示される全構成要素から幾つかの構成要素を削除してもよい。さらに、異なる実施形態にわたる構成要素を適宜組み合わせてもよい。
【００８５】
【発明の効果】
以上詳述したように本発明によれば、過充電時の安全性が高く、かつ貯蔵特性に優れた非水電解質二次電池を提供することができる。
【図面の簡単な説明】
【図１】本発明に係る非水電解質二次電池の一実施形態である円筒形非水電解質二次電池を示す断面図。
【符号の説明】
１…容器、２…正極、３…負極、４…セパレータ、５…電極群、６…絶縁板、７…封口板、８…弁及び電流遮断機構、９…絶縁リング、１０…正極タブ、１１…負極タブ、１２…ガス抜き孔、１３…正極端子。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery.
[0002]
[Prior art]
Various secondary batteries have been developed and used as power sources for small electric appliances. 2. Description of the Related Art In recent years, devices have been miniaturized with the development of technology, and various devices have been increasingly portable.
As a result, secondary batteries have come to be used as substitutes for AC power supplies, and applications of secondary batteries are expanding. Also, information devices such as personal computers and mobile phones in which secondary batteries have been used in the past have been required to have improved characteristics. Against this background, secondary batteries have been required to have not only improved capacity as in the past, but also other improved characteristics. Above all, importance is being placed on safety during non-stationary use such as overcharging, long-term storage, and maintaining characteristics at high temperatures.
[0003]
Conventionally, lead storage batteries, nickel cadmium secondary batteries, nickel hydride secondary batteries, and the like have been used as secondary batteries, but these have problems in that they are small and lightweight. On the other hand, non-aqueous electrolyte secondary batteries are small and lightweight and have a large capacity, and thus have been used in the above-mentioned personal computers, mobile phones, digital cameras, video cameras, POS terminals, and the like. . In this type of nonaqueous electrolyte secondary battery, a lithium-containing cobalt composite oxide or a lithium-containing nickel composite oxide is used as a positive electrode active material, and a graphite-based or coke-based carbon-based material is used as a negative electrode active material. LiPF as non-aqueous electrolyte ₆ And LiBF ₄ A non-aqueous electrolyte in which a lithium salt such as described above is dissolved in an organic solvent is used. Each of the positive electrode and the negative electrode has a sheet shape and is impregnated with a non-aqueous electrolyte. The positive electrode and the negative electrode are housed in containers of various shapes while facing each other via a separator that electrically insulates both electrodes.
[0004]
The non-aqueous electrolyte secondary battery as described above has a possibility that a chemical reaction different from that during normal charge / discharge occurs at the positive and negative electrodes at the time of overcharge and becomes thermally unstable, thereby impairing the safety of the battery. There is. Further, if the battery is stored for a long time in a charged state, a side reaction occurs, which causes an increase in internal impedance and a decrease in battery capacity.
[0005]
In order to solve such a problem, it has been studied to change the composition of the non-aqueous electrolyte. However, a significant change in the composition of the non-aqueous electrolyte has the problem of causing a reduction in capacity and a significant increase in internal impedance under the most frequently used normal conditions. In addition, it has been studied to solve the above-mentioned problems without changing the main properties of the electrolytic solution by adding a small amount of a chemical substance as an additive to the electrolytic solution. For example, JP-A-9-50822 discloses that an aromatic compound such as an anisole derivative having a molecular weight of 500 or less and having a reversible oxidation-reduction potential at a potential higher than the positive electrode potential at full charge is added to a non-aqueous electrolyte. It is described. Such an aromatic compound functions to enhance safety during overcharge by consuming the overcharge current by oxidizing the compound. However, when the anisole derivative is added to the non-aqueous electrolyte, the cycle characteristics and the storage characteristics deteriorate. Further, depending on the voltage at the time of overcharging, there is a possibility that the battery container may be broken by gas generation. Further, there is a case where a problem arises in safety when overcharging is continued after consumption of the anisole derivative.
[0006]
In addition, for example, in JP-A-2000-58112, an over-charge is performed by adding an ester derivative having a biphenylyl group to an electrolytic solution and causing a polymer generated by a polymerization reaction of the derivative to act as a resistor during over-charge. It has been proposed to increase safety during times.
[0007]
However, since the derivative has a high decomposition reaction rate, storing the battery in a charged state greatly increases the impedance of the positive and negative electrodes, which causes a problem in that the battery cannot be used at a large current after storage.
[0008]
[Patent Document 1]
JP-A-9-50822 (Claims)
[0009]
[Patent Document 2]
JP-A-2000-58112 (Claims, paragraph [0017])
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a nonaqueous electrolyte secondary battery having high safety during overcharge and excellent storage characteristics.
[0011]
[Means for Solving the Problems]
A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode, and a non-aqueous electrolyte including a compound represented by the following structural formula. is there.
[0012]
Embedded image

[0013]
Here, R represents any of an alkyl group having 1 to 4 carbon atoms, a methoxy group, an aryl group, and a halogen group bonded to at least one of o-, m-, and p-positions of the benzene ring.
[0014]
Further, the non-aqueous electrolyte secondary battery according to the present invention is a positive electrode including a positive electrode active material, a negative electrode, a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte,
A compound represented by the aforementioned structural formula (3) is present on the surface of the positive electrode.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a first embodiment of the nonaqueous electrolyte secondary battery according to the present invention will be described. The non-aqueous electrolyte secondary battery includes a positive electrode including a positive electrode active material, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte including a compound represented by the following structural formula. And
[0016]
Embedded image

[0017]
Here, R represents any of an alkyl group having 1 to 4 carbon atoms, a methoxy group, an aryl group, and a halogen group bonded to at least one of o-, m-, and p-positions of the benzene ring. When a plurality of hydrogen atoms on the benzene ring are substituted with R, at least one of the substituents (R) may be bonded to any one of o-, m- and p- on the benzene ring. Good.
[0018]
Hereinafter, the non-aqueous electrolyte, the positive electrode, the negative electrode, and the separator will be described.
[0019]
1) Non-aqueous electrolyte
The non-aqueous electrolyte includes a non-aqueous solvent, a compound dissolved in the non-aqueous solvent and represented by the structural formula shown in Chemical Formula 4, and an electrolyte dissolved in the non-aqueous solvent.
[0020]
The form of the non-aqueous electrolyte can be liquid or gel.
[0021]
The reason why the number of carbon atoms of the alkyl group is limited to 1 to 4 is that if the number of carbon atoms exceeds 4, the solubility in the non-aqueous solvent decreases due to an increase in molecular weight, and a sufficient amount cannot be added. That's why.
[0022]
Further, among the compounds described above, compounds containing a methoxy group or an aryl group as R are preferable because of their high solubility in nonaqueous solvents.
[0023]
As the above-mentioned compound, it is preferable to use at least one selected from the group consisting of p-tolyl benzoate, acetylphenyl benzoate, tert-butylphenyl benzoate and p-chlorophenyl benzoate. The impedance can be lower.
[0024]
The content of the compound in the non-aqueous electrolyte is desirably 1% by weight or less. This is because if the content exceeds 1% by weight, the reaction between the compound and the negative electrode is liable to occur, so that the internal impedance of the battery is increased and the storage characteristics may be lowered, and the initial capacity or the high rate may be reduced. This is because the discharge characteristics may be impaired.
A more preferable range of the content is 0.5% by weight or less. By doing so, the protective layer generation reaction on the positive electrode surface can be completed in the initial charging, and the protective layer can be generated on the positive electrode surface in a state where the external conditions such as the temperature before shipping are optimized. The stability of the protective layer can be made higher. In addition, the compound remaining in the nonaqueous electrolyte can be suppressed from reacting with other battery components such as the negative electrode.
[0025]
Further, in order to sufficiently increase both the storage characteristics and the safety at the time of overcharging, the content of the compound in the non-aqueous electrolyte is desirably 0.1% by weight or more and 1% by weight or less.
[0026]
The content of the compound in the non-aqueous electrolyte is measured by the method described below. First, the cell is disassembled to take out the contents (electrode group and non-aqueous electrolyte) in the container, and the contents are centrifuged to separate the liquid components. DMSO-d was added to the obtained liquid component. ₆ By adding the solution and performing NMR spectrum measurement of the 1H (400 MHz) and 13C (100 MHz) nuclei, it is possible to specify the compound and measure the content of the compound in the non-aqueous electrolyte.
[0027]
As the non-aqueous solvent, various solvents known for non-aqueous electrolyte secondary batteries can be used. Although not particularly limited, examples of the non-aqueous solvent include cyclic carbonates {eg, ethylene carbonate (EC), propylene carbonate (PC)}, and chain carbonates {eg, dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate. At least one solvent selected from the group consisting of carbonate (DEC)｝, γ-butyrolactone (BL), acetonitrile (AN), ethyl acetate (EA), toluene, xylene and methyl acetate (MA) can be used. . In particular, it is desirable to use a non-aqueous solvent containing at least one of a cyclic carbonate and a chain carbonate.
[0028]
As the electrolyte, for example, lithium perchlorate, lithium hexafluorophosphate (LiPF ₆ ), Lithium tetrafluoroborate (LiBF ₄ ), Lithium salts such as lithium arsenic hexafluoride, lithium trifluoromethanesulfonate, and lithium bistrifluoromethylsulfonylimide.
[0029]
2) Positive electrode
The positive electrode is mainly formed by forming a mixture containing a positive electrode active material and a binder into a thin plate or sheet shape. Furthermore, the current collector can be improved by bringing the current collector into close contact with the thin plate or sheet mixture.
[0030]
Examples of the positive electrode active material include a lithium-containing metal oxide and a chalcogen compound. Examples of the lithium-containing metal oxide include a lithium-containing cobalt composite oxide, a lithium-containing nickel-cobalt composite oxide, a lithium-containing nickel composite oxide, a lithium manganese composite oxide, and a lithium-containing vanadium oxide. On the other hand, examples of the chalcogen compound include titanium disulfide and molybdenum disulfide. In addition, the kind of the positive electrode active material to be used can be one kind or two or more kinds.
[0031]
Among them, a positive electrode active material having a charge potential of 4 V or more with respect to the lithium metal potential and exhibiting catalytic activity in the oxidation reaction of the compound represented by the structural formula shown in Chemical Formula 4 is preferable. Examples of such a positive electrode active material include a lithium-containing cobalt composite oxide and a lithium-containing nickel-cobalt composite oxide. In particular, LiCo _x Ni _y Mn _z O ₂ (However, x, y, and z represent x + y + z = 1, 0 <x ≦ 1, 0 ≦ y <0.8, and 0 ≦ z <0.8), and are preferably lithium-containing metal oxides. In this lithium-containing metal oxide, y and z preferably satisfy 0 ≦ y <0.5 and 0 ≦ z <0.5, respectively. LiCo _x Ni _y Mn _z O ₂ (Where x, y, and z represent x + y + z = 1, 0 <x ≦ 1, 0 ≦ y <0.5, and 0 ≦ z <0.5). Since it has a crystal structure similar to that of the positive electrode, a highly stable protective layer can be generated by the catalytic activity of the positive electrode surface. If the value of y is 0.5 or more, the amount of alkali remaining on the surface of the positive electrode active material increases, so that the stability of the protective layer may decrease.
[0032]
As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), an ethylene-propylene-diene copolymer, styrene-butadiene rubber, or the like can be used. On the other hand, as the current collector, for example, a metal foil or a thin plate of aluminum, stainless steel, titanium, or the like can be used.
[0033]
It is desirable that the positive electrode contains a conductive agent. Examples of the conductive agent include graphite, acetylene black, carbon black, and the like.
[0034]
A sheet containing the positive electrode active material can be obtained by adding and kneading a binder to the positive electrode active material and the conductive agent. Alternatively, a positive electrode may be obtained by dispersing a positive electrode active material, a conductive agent, and a binder in a solvent such as toluene or N-methylpyrrolidone (NMP), applying the obtained slurry on a current collector, and drying. It is possible.
[0035]
3) Negative electrode
The negative electrode is mainly formed by molding a mixture containing a negative electrode active material and a binder into a thin plate or sheet shape. Furthermore, the current collector can be improved by bringing the current collector into close contact with the thin plate or sheet mixture.
[0036]
As the negative electrode active material, lithium metal or a carbonaceous material capable of inserting and extracting lithium can be used. Although the carbonaceous material is not particularly limited, for example, as a raw material, it is possible to obtain a synthetic material such as coke or pitch such as petroleum or coal, a low molecular weight organic compound such as natural gas or a lower hydrocarbon, polyacrylonitrile, or a phenol resin. The raw material is obtained by baking this raw material at 700 ° C. to 3000 ° C. using a molecule or the like to carbonize or graphitize. Further, natural graphite or artificial graphite may be used as the carbonaceous material. Further, two or more kinds of carbonaceous materials can be used in combination. As the particle shape of the carbonaceous material, various shapes such as a scale, a fiber, and a sphere can be used. In particular, it is desirable to use at least one of artificial graphite and natural graphite, since it is possible to suppress an increase in impedance due to a side reaction between the compound represented by Chemical Formula 4 and the negative electrode. Further, when a spherical or fibrous material is used, the surface area of the carbonaceous material can be reduced, so that the side reaction suppressing effect can be enhanced.
[0037]
As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), styrene-butadiene rubber, carboxymethyl cellulose (CMC), or the like can be used. On the other hand, as the current collector, for example, a metal foil or a thin plate of copper, stainless steel, nickel, or the like can be used.
[0038]
The negative electrode is produced, for example, by dispersing a negative electrode active material and a binder in a solvent such as water or N-methylpyrrolidone (NMP), applying the obtained slurry on a current collector, and drying. You.
[0039]
4) Separator
As the separator, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like can be used.
[0040]
In a non-aqueous electrolyte secondary battery, initial charging (initial charging) is performed after assembly. The formation of the protective layer is performed by the reaction of the compound represented by Chemical Formula 4 at the positive electrode in the initial charge after the battery is assembled. Therefore, it is desirable that the initial charging be performed with a small current for a long time. Specifically, a current value of 0.5 CA or less is desirable. A more desirable initial charge is to keep the voltage constant at the protective layer generation reaction potential and continue normal charging after the current value converges to 0.02 CA. By performing the charging, most of the added compound can be converted to a stable protective layer in the initial charging, and the compound can be effectively used.
[0041]
According to the present invention described above, since a non-aqueous electrolyte containing the compound represented by the structural formula shown in Chemical Formula 4 is provided, safety and storage characteristics at the time of overcharge are maintained without impairing normal characteristics such as battery capacity. It is possible to provide a non-aqueous electrolyte secondary battery having excellent characteristics.
[0042]
That is, the compound represented by the general formula (4) has a high affinity for a non-aqueous solvent used for a non-aqueous electrolyte secondary battery, and ethylene carbonate and ethyl methyl carbonate, which are particularly frequently used among the non-aqueous solvents, are used. High solubility in carbonates such as
Therefore, the compound is added to the non-aqueous electrolyte in advance, and a battery can be manufactured by a normal method using the non-aqueous electrolyte. At the same time, the compound is deposited on an electrode or the like, and the battery performance deteriorates. Can be avoided.
[0043]
Further, since the protective layer has high stability at a high voltage of 4.5 V or more with respect to the lithium potential, it is possible to suppress an exothermic reaction between the positive electrode active material and the electrolytic solution at the time of overcharging, and Safety can be improved. In addition, since the above compound has R in Chemical Formula 4, not only the chemical stability of the protective layer is improved, but also the dissolution of the electrolytic solution in the liquid phase is suppressed, and the effect is increased and the effect persistence is improved. Can be enhanced.
[0044]
Further, according to the above compound, the storage characteristics and the safety at the time of overcharge can be improved with a small content of the compound in the non-aqueous electrolyte of 1% by weight or less. Therefore, since the rise in internal impedance due to the addition of the compound can be extremely reduced, the effect on normal characteristics such as initial discharge capacity and high-rate discharge characteristics is minimized, while safety and storage characteristics during overcharge are minimized. Can be improved.
[0045]
LiCo _x Ni _y Mn _z O ₂ (Where x, y and z satisfy x + y + z = 1, 0 <x ≦ 1, 0 ≦ y <0.5, and 0 ≦ z <0.5). The use of the substance has a crystal structure similar to that of the lithium-containing cobalt oxide, so that a protective layer can be stably and promptly formed on the positive electrode surface at the time of initial charging. This is because the compound represented by Chemical formula 4 reacts near the lithium emission potential of 4 V or more of the positive electrode active material to form a protective layer. Further, the obtained protective layer has a high effect of suppressing the exothermic reaction of the positive electrode active material at 130 to 300 ° C., so that the safety at the time of overcharge can be further increased.
[0046]
Next, a second embodiment of the nonaqueous electrolyte secondary battery according to the present invention will be described. This non-aqueous electrolyte secondary battery is a non-aqueous electrolyte secondary battery including a positive electrode including a positive electrode active material, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. Wherein the compound represented by the structural formula shown in Chemical Formula 4 exists on the surface of the positive electrode.
[0047]
For this secondary battery, for example, after immersing the positive electrode in a solution containing the compound represented by Chemical Formula 4, a separator is interposed between the positive electrode and the negative electrode to form an electrode group, It is obtained by storing the nonaqueous electrolyte in a container, performing a sealing treatment, and then performing an initial charge. It is desirable to use a non-aqueous electrolyte in which the electrolyte is dissolved in a non-aqueous solvent, but the non-aqueous electrolyte may contain a compound represented by Chemical Formula 4 described above in a small amount. The positive electrode, the negative electrode, the separator, the non-aqueous solvent, the electrolyte, and the initial charging method may be the same as those described in the first embodiment.
[0048]
In this way, by pre-loading the compound on the positive electrode, it is possible to limit the contact of the compound directly to only the positive electrode, which greatly limits the reaction with other materials in the battery such as the negative electrode. Thus, the compound can be used for forming the protective layer without waste.
[0049]
The nonaqueous electrolyte secondary battery according to the present invention can be applied to various forms such as a cylindrical shape, a rectangular shape, and a thin shape. One example of the cylindrical non-aqueous electrolyte secondary battery is shown in FIG.
[0050]
An electrode group 5 in which a positive electrode 2 and a negative electrode 3 are spirally wound with a separator 4 interposed therebetween is housed in a cylindrical battery case 1 made of metal such as iron or stainless steel. The insulating plate 6 is interposed between the inner surface of the bottom of the battery container 1 and the electrode group 5. The non-aqueous electrolyte is accommodated in the battery container 1. The sealing plate 7, the gas release valve and the current cutoff mechanism 8 are attached to the opening of the battery container 1 via an insulating ring 9.
[0051]
The positive electrode tab 10 has one end electrically connected to the positive electrode 2 and the other end electrically connected to the gas release valve and the current cutoff mechanism 8. On the other hand, one end of the negative electrode tab 11 is electrically connected to the negative electrode 3, and the other end is electrically connected to the bottom inner surface of the battery container 1. The positive electrode terminal 13 having the gas vent hole 12 is electrically connected to the gas release valve and the current cutoff mechanism 8 and is disposed on the sealing plate 7. On the other hand, the bottom of the battery case 1 functions as a negative electrode terminal.
[0052]
In FIG. 1 described above, an example in which a cylindrical battery container is used has been described. However, the shape of the battery container is not limited to this, and may be, for example, a square shape, a bag shape, or the like. Further, the battery container may be formed from a metal such as stainless steel or iron, but when a bag-shaped container is used, it can be formed from a laminate film.
[0053]
Further, in FIG. 1 described above, an example in which a spiral electrode group is used has been described, but a flat electrode group in which a positive electrode and a negative electrode are wound in a flat shape with a separator interposed therebetween, and a positive electrode and a negative electrode in which a separator is used. The present invention can also be applied to a non-aqueous electrolyte secondary battery using a stacked electrode group and the like interposed therebetween.
[0054]
Further, the method for producing a non-aqueous electrolyte secondary battery according to the present invention includes a step of assembling an unprimed non-aqueous electrolyte secondary battery including an electrode group including a compound represented by the above-described structural formula shown in Chemical Formula 4,
Performing an initial charge (initial charge) on the non-aqueous non-aqueous electrolyte secondary battery.
[0055]
It is desirable that the compound represented by the structural formula shown in Chemical Formula 4 be contained in the positive electrode, the nonaqueous electrolyte, or the positive electrode and the nonaqueous electrolyte. Further, it is desirable that the conditions for the initial charging be the same as those described in the first embodiment.
[0056]
According to such a manufacturing method, since the protective layer can be formed on the positive electrode at the time of initial charging, it is possible to provide a nonaqueous electrolyte secondary battery having excellent safety and storage characteristics at the time of overcharging.
[0057]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings described above.
[0058]
(Example 1)
Lithium-containing cobalt oxide (LiCoO ₂ ) A slurry was prepared by dispersing 90% by weight of powder, 2% by weight of acetylene black, 3% by weight of graphite, and 5% by weight of polyvinylidene fluoride as a binder in N-methylpyrrolidone as a solvent. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, and dried to obtain a positive electrode. An aluminum ribbon as a positive electrode tab was connected to the end of the positive electrode by welding.
[0059]
87% by weight of mesophase pitch-based fibrous graphite powder fired at 3000 ° C., 10% by weight of artificial graphite having an average particle size of 5 μm, 1% by weight of carboxymethylcellulose, and 2% by weight of styrene-butadiene rubber as a solvent. In water to prepare a slurry. The obtained slurry was applied on a copper foil having a thickness of 12 μm and dried to obtain a negative electrode. A nickel ribbon as a negative electrode tab was connected to the end of the negative electrode by welding.
A polyethylene porous film was used for the separator.
[0060]
After laminating a positive electrode, a separator, and a negative electrode in this order, the spirally wound electrode group was obtained by spirally winding.
[0061]
After dissolving 1 M lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 1), 0.4 wt% of p-tolyl benzoate is added. To prepare a non-aqueous electrolyte.
[0062]
After storing the electrode group in a battery container made of a stainless steel cylindrical can (diameter 18 mm, height 65 mm), a non-aqueous electrolyte is injected, a valve and a current cutoff mechanism are incorporated, and the opening of the battery container is sealed. By stopping, the cylindrical non-aqueous electrolyte secondary battery having the structure shown in FIG. 1 was assembled.
[0063]
The obtained non-aqueous electrolyte secondary battery was charged at 300 mA at a maximum charging voltage of 4.20 V with a constant current up to 4.20 V, and after reaching 4.20 V, at a constant voltage until the total time reached 10 hours. The battery was charged for the first time.
[0064]
(Examples 2 to 5)
Except that the content of p-tolylbenzoate in the non-aqueous electrolyte was changed as shown in Table 1 below, the assembly of the non-aqueous electrolyte secondary battery and the initial Charged.
[0065]
(Example 6)
Except that the type of the additive was changed to 4- (tert-butyl) phenylbenzoate (4- (tert-butyl) phenyl benzoate), the non-aqueous electrolyte secondary The battery was assembled and first charged.
[0066]
(Example 7)
The non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1 except that the type of the additive was changed to 4-chlorophenyl benzoate.
[0067]
(Example 8)
A mixture of diethyl carbonate and p-tolyl benzoate (p-tolyl benzoate) at a weight ratio of 9: 1 was immersed for 5 minutes in the positive electrode produced in the same manner as in Example 1, and then the liquid on the electrode surface was sufficiently immersed. After that, the non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1. As the non-aqueous electrolyte, a solution prepared by dissolving 1 M lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 1) was used.
[0068]
(Example 9)
A non-aqueous electrolyte secondary battery assembled in the same manner as described in Example 1 was charged at a constant current up to 4.20 V at 100 mA at a maximum charging voltage of 4.20 V, and reached 4.20 V. After that, initial charging was performed by charging at a constant voltage until the current converged to 30 mA.
[0069]
(Example 10)
LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ 90% by weight of a lithium-containing cobalt nickel manganese oxide powder having a composition represented by the formula, 2% by weight of acetylene black, 3% by weight of graphite, and 5% by weight of polyvinylidene fluoride as a binder are dispersed in N-methylpyrrolidone. To prepare a slurry. The obtained slurry was applied on an aluminum foil having a thickness of 20 μm and dried to prepare a positive electrode. A non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1 except that this positive electrode was used.
[0070]
(Comparative Example 1)
Except for using a non-aqueous electrolyte obtained by dissolving 1M lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 1), the same as described in Example 1 described above was used. Similarly, the non-aqueous electrolyte secondary battery was assembled and initially charged.
[0071]
(Comparative Example 2)
A non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1 except that phenyl propionate was used as an additive.
[0072]
(Comparative Example 3)
A non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1 except that 4-chloroanisole was used as an additive.
[0073]
(Comparative Example 4)
A non-aqueous electrolyte secondary battery was assembled and initially charged in the same manner as described in Example 1 except that phenyl benzoate was used as an additive.
[0074]
For the obtained non-aqueous electrolyte secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 4, the initial capacity, overcharge characteristics, high-rate discharge characteristics, and high-temperature storage characteristics were measured by the methods described below, and the results were obtained. Are shown in Tables 1 and 2 below. The nominal capacity of the non-aqueous electrolyte secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 4 was 1500 mAh.
[0075]
<Initial capacity>
The discharge capacity at the time of discharging to 3.0 V at 1500 mA was defined as the initial capacity.
[0076]
<Overcharge test>
After charging to 4.2 V at a constant current of 750 mA (0.5 CA), charging was performed at a constant voltage of 4.2 V so that the total charging time was 5 hours. Thereafter, a charge current of 3000 mA (2 CA) was passed to perform an overcharge test, and the current cutoff mechanism operating time and the maximum temperature were measured.
[0077]
<High-rate discharge characteristics>
After charging at a constant current of 750 mA (0.5 CA) to 4.2 V, charging was performed at a constant voltage of 4.2 V so that the total charging time was 5 hours. After resting for 0.5 hour, discharging was performed at a current of 4500 mA (3 CA) until the voltage became 3 V, and the discharge capacity at 3 CA was measured.
[0078]
<High temperature storage characteristics>
After charging at a constant current of 750 mA (0.5 CA) to 4.2 V, charging was performed at a constant voltage of 4.2 V so that the total charging time was 5 hours. Then, it was stored in a thermostat at 60 ° C. for 30 days. This was taken out and subjected to a constant current discharge of 2.7 V termination at 800 mA (0.5 CA) at room temperature (20 ° C.) to measure the remaining discharge capacity. Storage characteristics were calculated from the ratio of this value to the initial discharge capacity.
[0079]
[Table 1]

[0080]
[Table 2]

[0081]
As is evident from Tables 1 and 2, the secondary batteries of Examples 1 to 10 using the compound represented by Chemical Formula 4 described above exhibited an overcharge test compared to Comparative Example 1 in which no additive was added. It can be understood that not only the operation time of the current cutoff mechanism is short, the maximum temperature is low, and there is no liquid leakage, but also the capacity retention ratio when stored at a high temperature in a charged state is higher than Comparative Examples 1 to 4. Tables 1 and 2 also show that the secondary batteries of Examples 1 to 10 have an initial capacity and a 3CA discharge capacity that are comparable to those of Comparative Example 1 in which no additive is added. . It is considered that in Examples 1 to 10, the protective layer that inhibits the reaction between the positive electrode active material and the electrolytic solution was formed on the positive electrode surface at the high positive electrode potential at the time of the initial charge.
[0082]
By comparing Examples 1 to 5, the secondary batteries of Examples 1 to 3 in which the addition amount of the compound is 1% by weight or less show that the initial discharge capacity, the 3CA discharge capacity, and the storage characteristics during high-temperature storage are the same. It turns out that it is higher than 4 to 5 secondary batteries (compound addition amount is 2 to 5% by weight).
[0083]
On the other hand, in the secondary battery of Comparative Example 1 in which no additive was added, the time until the current cutoff mechanism was activated during the overcharge test was delayed, and the temperature reached a high temperature of 135 ° C., and liquid leakage occurred. On the other hand, in the secondary batteries of Comparative Examples 2 to 4 using the conventional additives, the storage characteristics during high-temperature storage were inferior to those of Examples 1 to 10.
[0084]
Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements in an implementation stage without departing from the scope of the invention. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiments. For example, some components may be deleted from all the components shown in the embodiment. Further, components of different embodiments may be appropriately combined.
[0085]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having high safety during overcharge and excellent storage characteristics.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a cylindrical non-aqueous electrolyte secondary battery which is one embodiment of a non-aqueous electrolyte secondary battery according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Container, 2 ... Positive electrode, 3 ... Negative electrode, 4 ... Separator, 5 ... Electrode group, 6 ... Insulating plate, 7 ... Sealing plate, 8 ... Valve and current cutoff mechanism, 9 ... Insulating ring, 10 ... Positive electrode tab, 11 ... negative electrode tab, 12 ... gas vent hole, 13 ... positive electrode terminal.

Claims (4)

A non-aqueous electrolyte secondary battery comprising: a positive electrode including a positive electrode active material; a negative electrode; and a non-aqueous electrolyte including a compound represented by the following structural formula.

Here, R represents any of an alkyl group having 1 to 4 carbon atoms, a methoxy group, an aryl group, and a halogen group bonded to at least one of o-, m-, and p-positions of the benzene ring. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the compound in the non-aqueous electrolyte is 1% by weight or less. In a non-aqueous electrolyte secondary battery including a positive electrode including a positive electrode active material, a negative electrode, and a non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery, wherein a compound represented by a structural formula shown below is present on the surface of the positive electrode.

Here, R represents any of an alkyl group having 1 to 4 carbon atoms, a methoxy group, an aryl group, and a halogen group bonded to at least one of o-, m-, and p-positions of the benzene ring. The positive electrode active material is LiCo _x Ni _y Mn _z O ₂ (where x, y and z are x + y + z = 1, 0 <x ≦ 1, 0 ≦ y <0.5, 0 ≦ z <0.5 The non-aqueous electrolyte secondary battery according to claim 1, further comprising a lithium-containing metal oxide represented by the following formula:

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