JP4378962B2 - Electrolyte for lithium secondary battery and lithium secondary battery - Google Patents

Description

ここで、ｂ１は、水素原子、アルキル基、アルケニル基、アルコキシ基を表し、ｂ２はアルキル基、アルケニル基、アルコキシ基を表し、ｂ３は、水素原子又はハロゲン原子を表す。
【００５１】
ｂ１及びｂ２に用いるアルキル基としては、炭素数が、通常１以上、一方、通常２０以下、好ましくは１０以下、より好ましくは５以下のものを挙げることができる。このようなアルキル基としては、例えば、メチル基、エチル基、プロピル基、イソプロピル基、ブチル基、イソブチル基、sec-ブチル基、tert-ブチル基、ペンチル基、イソペンチル基、ネオペンチル基、ヘキシル基、ヘプチル基、オクチル基、ノニル基、デシル基、ウンデシル基、ドデシル基等を挙げることができる。
【００５２】
過充電防止剤として用いられる点から好ましいのは、炭素数１以上５以下のアルキル基である。このようなアルキル基としては、例えば、メチル基、エチル基、プロピル基、イソプロピル基、ブチル基、イソブチル基、sec-ブチル基、tert-ブチル基等を挙げることができる。
ｂ１及びｂ２に用いるアルケニル基としては、炭素数が、通常１以上、一方、通常５以下、好ましくは３以下のものを挙げることができる。このようなアルケニル基としては、例えば、１−プロぺニル基、２−プロぺニル基、イソプロぺニル基、ブチニル基、ペンチニル基、（１−ブチニル）ペンチル基等を挙げることができる。
【００５３】
アルケニル基の中でも、炭素数１以上３以下のアルケニル基を用いることが好ましい。このようなアルケニル基としては、例えば、２−プロぺニル基、イソプロペニル基を挙げることができる。
化合物Ｂの電解液中における安定性を考慮すると、アルケニル基として、特に好ましいのは、＝ＣＨ₂部分を含まないものである。
【００５４】
ｂ１及びｂ２に用いるアルコキシ基としては、炭素数が、通常１以上、一方、通常６以下、好ましくは４以下、より好ましくは３以下のものを挙げることができる。このようなアルコキシ基としては、例えば、メトキシ基、エトキシ基、プロポキシ基、イソプロポキシ基、ブトキシ基、フェノキシ基を挙げることができる。
過充電防止剤として用いられる点から、炭素数１以上３以下のアルコキシ基をい用いることが好ましい。このようなアルコキシ基としては、例えば、メトキシ基、エトキシ基等を挙げることができる。
【００５５】
以上述べたアルキル基、アルケニル基、及びアルコキシ基は、さらに他の基で置換されていてもよい。
ｂ３は、水素原子又はハロゲン原子を表す。用いるハロゲン原子としては、フッ素、塩素、臭素等を挙げることができるが、好ましいのはフッ素又は臭素である。フッ素、臭素を用いることにより、化合物としての安定性を高くすることができる。
【００５６】
また、上記一般式（２）において、ｂ３が水素でｂ１が水素以外の基となる場合、及びｂ３が水素以外の基でｂ１が水素である場合には、化合物Ｂとしては、オルト体、メタ体、パラ体の異性体が存在することとなる。本発明においては、オルト体、メタ体、パラ体のいずれも使用することができる。但し、ｂ１、ｂ２の基を構成する炭素数を多くする観点からは、オルト体、メタ体を用いることが好ましい。オルト体及びメタ体は、パラ体と比較して融点が低くなる傾向にあるため、溶媒として用いるには好ましい。
【００５７】
化合物Ｂとしては、例えば、トルエン、キシレン、エチルベンゼン、ジエチルベンゼン、メチルエチルベンゼン、プロピルベンゼン、ブチルベンゼン、アニソール、フェネトール、プロピルフェニルエーテル、ブチルフェニルエーテル、ジメトキシベンゼン、１ブロモ２，４ジメトキシベンゼン、メトキシエトキシベンゼン、ジエトキシベンゼン、メトキシトルエン、エトキシトルエンを挙げることができる。これら化合物のうち、化合物Ａの溶解度を向上させる点から好ましいのは、アニソール、フェネトール、プロピルフェニルエーテル、ブチルフェニルエーテル、ジメトキシベンゼン、１ブロモ２，４ジメトキシベンゼン、メトキシエトキシベンゼン、ジエトキシベンゼン、メトキシトルエン、エトキシトルエンである。無論、上記一般式（２）で表される化合物は、複数種を併用することができる。
【００５８】
化合物Ａとして上記一般式（１）の化合物を用いる場合、化合物Ｂとしては、下記一般式（３）に示す化合物を用いることもできる。もちろん、化合物Ｂとして、下記一般式（３）に示す化合物と上記一般式（２）に示す化合物とを併用してもよい。
【００５９】
【化６】
ｃ１−Ｙ１−ｃ２ ・・・（３）
（Ｙ１は、周期表第６族元素又は置換されていてもよい脂鎖式炭化水素基を表し、ｃ１及びｃ２はそれぞれ独立に置換されていてもよい芳香族炭化水素基を表し、互いに結合して環を構成してもよい。）
ここで、Ｙ１は、周期表第６族元素を表す。周期表第６族元素としては、酸素、硫黄、セレン、テルル、ポロニウムがある。これら元素のうち、リチウム二次電池の安全性を改良する点及び化合物Ａの溶解度を上昇させる点から、酸素又は硫黄を用いることが好ましい。
【００６０】
また、Ｙ１には、置換されていてもよい脂鎖式炭化水素基を用いることもできる。Ｙ１に用いる脂鎖式炭化水素基としては、化合物Ａを良好に溶解させるために、Ｙ１の炭素数は、通常１以上、好ましくは２以上、一方、通常４以下、好ましくは３以下とするのが好ましい。このような脂鎖式炭化水素基としては、メチレン基、エチレン基、トリメチレン基、テトラメチレン基、プロピレン基等を挙げることができる。
【００６１】
上記脂鎖式炭化水素基の中でも、炭素数が２又は３の脂鎖式炭化水素基を用いるのがより好ましい。このような脂鎖式炭化水素基としては、メチレン基、エチレン基、トリメチレン基を挙げることができるが、特に好ましいのはメチレン基又はエチレン基である。
また、Ｙ１に用いる脂鎖式炭化水素基は、脂鎖式炭化水素に結合する水素の全部又は一部が他の基で置換されていてもよい。このような置換基としては、上記一般式（１）の化合物Ａにおけるａ１、ａ２に用いる置換基と同様のものを用いることができる。
【００６２】
また、ｃ１及びｃ２はそれぞれ独立に置換されていてもよい芳香族炭化水素基を表し、ｃ１とｃ２とは、同一であっても異なっていてもよい。置換されていてもよい芳香族炭化水素基としては、上記一般式（１）で表される化合物Ａにおけるａ１及びａ２と同様のものを用いることができる。
さらに、ｃ１及びｃ２は互いに結合して環を構成していてもよい。即ち、ｃ１及びｃ２は互いに元素Ｙ１を介して結合するが、この結合とは別に、ｃ１とｃ２とを直接又は単数又は複数の原子を介して結合させて、全体として、元素Ｙ１を含む環を構成することができる。
【００６３】
一般式（３）で表される化合物Ｂの具体例としては、フェニルエーテル、ナフチルエーテル、ジフェニルスルフィド、ビス（ｐ−トリル）エーテル、ビス（ｐ−トリル）スルフィド、ビス（ｐ−フルオロフェニル）エーテル、ビス（ｐ−フルオロフェニル）スルフィド、ビス（ｐ−クロロフェニル）エーテル、ジフェノキシベンゼン、ジベンゾフラン、１，４−ジベンゾジオキサン、キサンテン、ジフェニルメタン、ジベンジルなどが例示される。この中でも、化合物Ａの溶解度を上昇させる観点から、フェニルエーテル、ジフェニルスルフィド、ジベンゾフラン、ジフェニルメタン、ジベンジルが特に好ましく、フェニルエーテルが最も好ましい。無論、上記一般式（３）で表される化合物は、複数種を併用することができる。
【００６４】
尚、化合物Ａと化合物Ｂとで同一の化合物を用いても本発明の効果は発揮されないため、化合物Ｂに一般式（３）で表されるものを用いる場合、一般式（１）で表される化合物Ａと同一の構造とはせず、化合物Ａが有する環構造と同一の環構造を有しかつその数を化合物Ａの有する数以下とする必要がある。
［６］化合物Ａと化合物Ｂとの量比
本発明においては、（化合物Ｂの含有量）／（化合物Ａの含有量）を０．７以上とし、好ましくは０．７５以上、更に好ましくは０．８以上、特に好ましくは１以上とする。この範囲とすれば、化合物Ｂを含有させることによってＡの溶解度を上げることができるようになる。一方、通常、化合物Ｂを化合物Ａに対して多く含有させればさせるほど化合物Ａの溶解度が上がるため好ましいが、通常は、（化合物Ｂの含有量）／（化合物Ａの含有量）を１０以下とする。電池容量やレート特性の電池の基本的特性とのバランスを考慮すれば、（化合物Ｂの含有量）／（化合物Ａの含有量）を、好ましくは７以下、より好ましくは５以下、最も好ましくは３以下とする。
【００６５】
電解液中における化合物Ａと化合物Ｂの合計含有量は、通常1重量％以上、好ましくは３重量％以上、より好ましくは５重量％以上とする。上記範囲とすれば安全性の目的が良好に達成されるようになる。一方、化合物Ａと化合物Ｂの電解液中における含有量は、通常５０重量％以下、好ましくは２０重量％以下、より好ましくは１０重量％以下とする。上記範囲とすれば、電解液のイオン伝導度が低下することがなくなり、電池特性への影響を良好に抑制できるようになる。
［７］化合物Ａと化合物Ｂとの性状
化合物Ａは、通常過充電時や高温保存時の安全性を確保するための添加剤として、電解液に含まれる。一方、化合物Ｂは、通常、化合物Ａの電解液中での溶解度を上げるために併用されるものであるが、化合物Ａと同様、リチウム二次電池の安全性を向上させるような機能を有していると一層好ましい。化合物Ａの溶解度向上に加えて、化合物Ｂ自身による安全性の向上効果が相乗的に奏される利点がある。また、化合物Ｂは、リチウム二次電池の容量や繰り返し充放電特性等の電池特性を損なうものでないことが好ましい。
【００６６】
化合物Ａは、一般的に沸点が高いことが好ましい。これは、化合物Ａの沸点が低いと、過充電時又は高温保存時における電池の蓄熱により化合物Ａが気化してしまい、リチウム二次電池の安全性が確保されにくくなるためである。この観点から、化合物Ａの常圧における沸点は、通常１００℃以上、好ましくは１２０℃以上、さらに好ましくは１５０℃以上とする。一方、あまりに沸点の高い化合物は現実的に得にくいので、化合物Ａの沸点は通常３００℃以下である。
【００６７】
ここで、沸点の高い化合物は融点も同時に高くなる傾向があるため、化合物Ａとして沸点の高い化合物を用いると、融点が室温以上となり、化合物Ａは常温常湿（２５℃／５０％ＲＨ）において固体となる場合がある。しかし、化合物Ａが常温常湿で固体であっても、化合物Ｂに低融点の物質を用いて、化合物Ｂ中に化合物Ａを溶解させることにより電解液中で液体として存在することができる。従って、化合物Ａの電解液中での溶解度を上昇させるために用いる化合物Ｂは、その融点を５０℃以下として、常温常湿（２５℃／５０％ＲＨ）において液体とすることが好ましい。このようにすることで、化合物Ａを化合物Ｂ中に容易に溶解させることができるようになる。化合物Ｂの融点は、より好ましくは３０℃以下であり、更に好ましくは０℃以下である。一方、化合物Ｂの融点は、通常−１０℃以上である。
［８］電解液のその他の成分
本発明のリチウム二次電池用の電解液は、化合物Ａ及び化合物Ｂの他、通常、支持電解質であるリチウム塩を含有し、必要に応じて非水系溶媒を含有する。
【００６８】
電解液に含有させる支持電解質であるリチウム塩としては、ＬｉＰＦ₆、ＬｉＡｓＦ₆、ＬｉＳｂＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＩ、ＬｉＢｒ、ＬｉＣｌ、ＬｉＡｌＣｌ、ＬｉＨＦ₂、ＬｉＳＣＮ、ＬｉＳＯ₃ＣＦ₂等を挙げることができる。これらのうちでは特にＬｉＰＦ₆及びＬｉＣｌＯ₄が好適である。これら支持電解質の電解液における含有量は、通常０．５〜２．５ｍｏｌ／ｌである。
【００６９】
必要に応じて含有される非水系溶媒としては、比較的高誘電率の溶媒が好適に用いられる。具体的にはエチレンカーボネート、プロピレンカーボネート等の環状カーボネート類、ジメチルカーボネート、ジエチルカーボネート、エチルメチルカーボネートなどの非環状カーボネート類、テトラヒドロフラン、２−メチルテトラヒドロフラン、ジメトキシエタン等のグライム類、γ−ブチルラクトン等のラクトン類、スルフォラン等の硫黄化合物、アセトニトリル等のニトリル類等を挙げることができる。以上の非水系溶媒は、複数種を併用することができる。
【００７０】
また、電解液中には、必要に応じて、電池の性能向上のために各種の添加剤を添加することができる。
（Ｂ）リチウム二次電池
本発明のリチウム二次電池は、負極、正極及び電解質を有し、電解質中に上記（Ａ）で説明した電解液を含有する。
【００７１】
負極は、通常、活物質層を集電体上に形成してなる。活物質層は、通常負極活物質、バインダー及び必要に応じて導電剤等の添加剤を有する負極材料を含有している。
負極活物質としては、通常炭素性物質を用いる。炭素性物質としては、例えば、グラファイト等の黒鉛材料；石炭系コークス、石油系コークス；石炭系ピッチ若しくは石油系ピッチの炭化物、又はこれらピッチを酸化処理したものの炭化物；ニードルコークス、ピッチコークス、フェノール樹脂、結晶セルロース等の炭化物を挙げることができる。さらに上記炭素性物質を一部黒鉛化した炭素材、ファーネスブラック、アセチレンブラック、ピッチ系炭素繊維等を挙げることもできる。
【００７２】
上記炭素性物質のうち、好ましいのは、コークス及びグラファイト等の黒鉛材料であるが、容量が大きい点で、グラファイト等の黒鉛材料が特に好ましい。
黒鉛材料としては、人造黒鉛、天然黒鉛等の黒鉛粉末及びその精製品、アセチレンブラック、ケッチェンブラック等の導電性カーボンブラックの黒鉛化品、気相成長炭素繊維等の炭素繊維が挙げられる。このような黒鉛材料ならどれでもよいが、容量の点から好ましいのは人造黒鉛又は天然黒鉛である。電池性能を制御し易いという観点から特に好ましいのは人造黒鉛である。
【００７３】
尚、黒鉛材料は、表面をアモルファス処理してもよい。
黒鉛材料の平均粒径は、通常１μｍ以上、好ましくは５μｍ以上であり、また通常４５μｍ以下、好ましくは３５μｍ以下、さらに好ましくは２５μｍ以下である。平均粒径が過度に小さいと、黒鉛材料の比表面積が増えることとなり不可逆容量が増え電池容量が低下してしまう。一方、平均粒径が過度に大きいと活物質層の膜厚が制限され均一な活物質層を基材の上に形成させることが難しくなる。
【００７４】
黒鉛材料の比表面積は、通常０．１ｍ²／ｇ以上、好ましくは０．３ｍ²／ｇ以上、より好ましくは０．５ｍ²／ｇ以上とする。比表面積が過度に小さいと電池のレート特性が低下する。一方、黒鉛材料の比表面積は、通常３０ｍ²／ｇ以下、好ましくは２０ｍ²／ｇ以下、より好ましくは１０ｍ²／ｇ以下とする。比表面積が過度に大きいと電池の初期効率が低下する。比表面積の測定はＢＥＴ法に従う。
【００７５】
負極は、上記負極活物質を、バインダーを溶解しうる溶剤を用いて分散塗料化し、その塗料を集電体上に塗布、乾燥することにより製造することができる。
活物質層中の負極活物質の割合は、通常１０重量％以上、好ましくは３０重量％以上、さらに好ましくは５０重量％以上であり、通常９９重量％以下、好ましくは９８重量％以下である。多すぎると電極の機械的強度が劣る傾向にあり、少なすぎると容量等電池性能が劣る傾向にある。
【００７６】
活物質層に使用するバインダーとしては、電解液等に対して安定である必要があり、耐候性、耐薬品性、耐熱性、難燃性等の観点から各種の材料が使用される。具体的には、シリケート、ガラスのような無機化合物や、ポリエチレン、ポリプロピレン、ポリ−１，１−ジメチルエチレンなどのアルカン系ポリマー；ポリブタジエン、ポリイソプレンなどの不飽和系ポリマー；ポリスチレン、ポリメチルスチレン、ポリビニルピリジン、ポリ−Ｎ−ビニルピロリドンなどのポリマー鎖中に環構造を有するポリマー；メチルセルロース等のセルロース類が挙げられる。
【００７７】
他の具体例としては、ポリメタクリル酸メチル、ポリメタクリル酸エチル、ポリメタクリル酸ブチル、ポリアクリル酸メチル、ポリアクリル酸エチル、ポリアクリル酸、ポリメタクリル酸、ポリアクリルアミドなどのアクリル誘導体系ポリマー；ポリフッ化ビニル、ポリフッ化ビニリデン、ポリテトラフルオロエチレン等のフッ素系樹脂；ポリアクリロニトリル、ポリビニリデンシアニドなどのＣＮ基含有ポリマー；ポリ酢酸ビニル、ポリビニルアルコールなどのポリビニルアルコール系ポリマー；ポリ塩化ビニル、ポリ塩化ビニリデンなどのハロゲン含有ポリマー；ポリアニリンなどの導電性ポリマーなどが使用できる。
【００７８】
また上記のポリマーなどの混合物、変成体、誘導体、ランダム共重合体、交互共重合体、グラフト共重合体、ブロック共重合体などであっても使用できる。これらの樹脂の重量平均分子量は、通常１０,０００〜３,０００,０００、好ましくは１００,０００〜１,０００,０００程度である。低すぎると塗膜の強度が低下する傾向にある。一方、高すぎると負極製造用の塗料の粘度が高くなり電極の形成が困難になることがある。好ましいバインダー樹脂としては、フッ素系樹脂、ＣＮ基含有ポリマーが挙げられ、より好ましくはポリフッ化ビニリデンである。
【００７９】
バインダーの使用量は、負極活物質１００重量部に対して通常０．１重量部以上、好ましくは１重量部以上であり、また通常３０重量部以下、好ましくは２０重量部以下、より好ましくは１０重量部以下である。バインダーの量が少なすぎると活物質層の強度が低下する傾向にあり、バインダーの量が多すぎると電池容量が低下する傾向にある。
【００８０】
活物質層中には、必要に応じて、導電性材料、補強材など各種の機能を発現する添加剤、粉体、充填材などを含有させてもよい。
活物質層を形成する際に使用する溶剤としては、例えばＮ−メチルピロリドンや、ジメチルホルムアミドを挙げることができ、好ましくはＮ−メチルピロリドンである。塗料中の溶剤濃度は、少なくとも１０重量％より大きくするが、通常２０重量％以上、好ましくは３０重量％以上、さらに好ましくは３５重量％以上である。また、上限としては、通常９０重量％以下、好ましくは８０重量％以下である。溶剤濃度が低すぎると塗布が困難になることがあり、高すぎると塗布膜厚を上げることが困難になると共に塗料の安定性が悪化することがある。
【００８１】
分散塗料化には通常用いられる分散機が使用でき、プラネタリーミキサー、ボールミル、サンドミル、二軸混練機などが使用できる。
集電体上に塗料を塗布する塗布装置に関しては特に限定されず、スライドコーターやエクストルージョン型のダイコーター、リバースロール、グラビアコーター、ナイフコーター、キスコーター、マイクログラビアコーター、ロッドコーター、ブレードコーターなどが挙げられるが、ダイコーター、ブレードコーター、及びナイフコーターが好ましく、塗料粘度および塗布膜厚等を考慮するとエクストルージョン型のダイコーター、簡便な点からはブレードコーターが最も好ましい。
【００８２】
上記塗料を集電体上に塗布した後、塗膜を例えば１２０℃程度の温度で１０分間程度の時間乾燥させることよって活物質層が形成される。
活物質層の厚さは、通常１０μｍ以上、好ましくは２０μｍ以上であり、通常２００μｍ以下、好ましくは１５０μｍ以下である。活物質層の厚さが過度に薄いと、電池の容量が小さくなりすぎる。一方、過度に厚いとレート特性が低下しることとなる。
【００８３】
負極に使用される集電体としては、電気化学的に溶出等の問題が生じず、電池の集電体として機能しうる各種のものを使用でき、通常は銅、ニッケル、ステンレス等の金属や合金が用いられる。好ましくは、銅を使用する。集電体の形状は特に制限されず、例えば、板状やメッシュ状の形状を挙げることができる。集電体の厚さは、通常０．１μｍ以上、好ましくは１μｍ以上であり、また通常１００μｍ以下、好ましくは３０μｍ以下、さらに好ましくは２０μｍ以下である。薄すぎると機械的強度が弱くなる傾向にあり、生産上問題になる。厚すぎると電池全体としての容量が低下する。
【００８４】
電解質は、上記（Ａ）で説明した電解液を含有し、電解液の保液性を上げて安全性の向上のためにさらにポリマーを含有してもよい。このようなポリマーとしては、例えば、フッ素性樹脂であるＰＶＤＦやアクリル系樹脂であるポリアクリレート等を挙げることができる。
電解質は、正極と負極との内部、及び正極と負極との間に存在するが、正極と負極との間には、正極と負極との短絡防止のために、多孔質フィルムのような支持体を存在させるのが好ましい。多孔質フィルムとしては、高分子樹脂からなるフィルムや、粉体とバインダーからなる薄膜が好ましく使用でき、より好ましくはポリエチレン、ポリプロピレン等からなる多孔質膜である。
【００８５】
リチウム二次電池の正極は、通常集電体上に正極活物質を含有する正極活物質層を有する。使用される正極活物質としては、リチウムと遷移金属との複合酸化物が挙げられ、具体的には、ＬｉＮｉＯ₂、ＬｉＮｉＣｏＯ₂等のリチウムニッケル複合酸化物、ＬｉＣｏＯ₂等のリチウムコバルト複合酸化物、ＬｉＭｎ₂Ｏ₄等のリチウムマンガン複合酸化物が挙げられる。これら複合酸化物の遷移金属サイトの一部は他の元素で置換されていてもよい。遷移金属の一部を他の元素で置換することにより、結晶構造の安定性を向上させることができる。この際の該遷移金属サイトの一部を置換する他元素（以下、置換元素と表記する）としては、Ａｌ、Ｔｉ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｌｉ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ、Ｚｒ等が挙げられ、好ましくはＡｌ、Ｃｒ、Ｆｅ、Ｃｏ、Ｌｉ、Ｎｉ、Ｍｇ、Ｇａ、更に好ましくはＡｌである。なお、遷移金属サイトは２種以上の他元素で置換されていてもよい。置換元素による置換割合は通常ベースとなる遷移金属元素の２．５モル％以上、好ましくはベースとなる遷移金属元素の５モル％以上であり、通常ベースとなる遷移金属元素の３０モル％以下、好ましくはベースとなる遷移金属元素の２０モル％以下である。置換割合が少なすぎると結晶構造の安定化が十分図れない場合があり、多すぎると電池にした場合の容量が低下してしまう場合がある。リチウム遷移金属複合酸化物のうち、より好ましくはリチウムコバルト複合酸化物、リチウムニッケル複合酸化物であり、特に好ましくはＬｉＣｏＯ₂である。正極活物質の粒径は、レート特性、サイクル特性等の電池特性が優れる点で通常１μｍ以上、一方、通常３０μｍ以下、好ましくは１０μｍ以下である。正極は、通常正極活物質とバインダーとを有する活物質層を集電体上に形成してなる。正極に使用されるバインダーの種類や活物質層の形成方法は負極の場合と同様とすればよい。
【００８６】
正極に使用される集電体としては、電気化学的に溶出等の問題が生じず、電池の集電体として機能しうる各種のものを使用でき、通常、アルミニウム、銅、ニッケル、錫、ステンレス鋼等の金属、これら金属の合金等を用いることができる。この場合、正極の集電体としては、通常アルミニウムが用いられる。集電体の形状は特に制限されず、例えば、板状やメッシュ状の形状を挙げることができる。集電体の厚みは通常１以上、一方、通常５０μｍ以下、好ましくは３０μｍ以下である。薄すぎると機械的強度が弱くなるが、厚すぎると電池が大きくなり、電池の中で占めるスペースが大きくなってしまい、電池のエネルギー密度が小さくなる。
【００８７】
正極、負極、及び電解質を有する電池要素はケースに収納される。電池要素としては、例えば、正極と負極とを電解質層を介して積層した積層体を巻回した形態、正極と負極と電解質層を介して平板状に積層した形態、又は前記平板状に積層した電池要素を複数個用意してさらに積層した形態を挙げることができる。
電池要素を収納するケースは、通常、コインセルや乾電池用の金属缶や形状可変性を有するケースを挙げることができる。
【００８８】
リチウム二次電池が電源として使用される電気機器としては、例えば、携帯用パーソナルコンピュータ（本明細書においては、パーソナルコンピュータを単にパソコンという場合がある。）、ペン入力パソコン、モバイルパソコン、電子ブックプレーヤー、携帯電話、コードレスフォン子機、ページャー、ハンディーターミナル、携帯ファックス、携帯コピー、携帯プリンター、ヘッドフォンステレオ、ビデオムービー、液晶テレビ、ハンディークリーナー、ポータブルＣＤ、ミニディスク、電気シェーバー、トランシーバー、電子手帳、電卓、メモリーカード、携帯テープレコーダー、ラジオ、バックアップ電源、モーター、照明器具、玩具、ゲーム機器、ロードコンディショナー、時計、ストロボ、カメラ、医療機器（ペースメーカー、補聴器、肩もみ機など）等を挙げることができる。また、リチウム二次電池は、電気自動車用の電源として用いることもできる。
【００８９】
【実施例】
本発明を実施例により更に詳細に説明するが、本発明はその要旨を越えない限り以下の実施例に限定されるものではない。
（実施例１）
化合物Ａとしてジベンゾフラン（ＤＢＦ）、化合物Ｂとしてフェニルエーテル（ＰｈＥ）を用いた。ＤＢＦ中にはベンゼン環構造が２つあり、ＰｈＥ中にもベンゼン環構造が２つある。ここで、常温常湿（２５℃／５０ＲＨ％）において、ＤＢＦは固体であり、ＰｈＥは液体である。
【００９０】
ＬｉＰＦ₆をプロピレンカーボネート、エチレンカーボネート（１：１）に１ｍｏｌ／Ｌの濃度で溶解した溶液１００重量部に所定量のＰｈＥを添加した電解液に対するＤＢＦの溶解性を調べた。溶解限界を表−１に示す。
尚、溶解限界は、ＰｈＥを０、６、１０、及び１５重量部含有する電解液をそれぞれ用意し、ＤＢＦを０．５重量部刻みで加えていくことにより測定した。そして、電解液が白濁したときのＤＢＦの含有量の一つ手前における含有量を溶解限界（重量部）とした。例えば、ＤＢＦをｚ重量部含有させたときに電解液が白濁したとすれば、（ｚ−０．５）重量部を溶解限界とした。溶解限界の測定方法は、実施例２〜５においても同様とした。
【００９１】
【表１】

（実施例２）
化合物Ａとしてビフェニル（ＢＰｈ）、化合物Ｂとしてフェニルエーテル（ＰｈＥ）を用いた。ＢＰｈ中にはベンゼン環構造が２つあり、ＰｈＥ中にもベンゼン環構造が２つある。ここで、常温常湿（２５℃／５０ＲＨ％）において、ＢＰｈは固体である。
【００９２】
ＬｉＰＦ₆をプロピレンカーボネート、エチレンカーボネート（１：１）に１ｍｏｌ／Ｌの濃度で溶解した溶液１００重量部に所定量のＰｈＥを添加した電解液に対するＢＰｈの溶解性を調べた。溶解限界を表−２に示す。
【００９３】
【表２】

（実施例３）
化合物Ａとしてメタ体のテルフェニル（ＴＰｈ：融点８９℃）、化合物Ｂとしてフェニルエーテル（ＰｈＥ）を用いた。ＴＰｈ中にはベンゼン環構造が３つあり、ＰｈＥ中にはベンゼン環構造が２つある。ここで、常温常湿（２５℃／５０ＲＨ％）において、ＴＰｈは固体である。
【００９４】
ＬｉＰＦ₆をプロピレンカーボネート、エチレンカーボネート（１：１）に１ｍｏｌ／Ｌの濃度で溶解した溶液１００重量部に所定量のＰｈＥを添加した電解液に対するＴＰｈの溶解性を調べた。溶解限界を表−３に示す。
【００９５】
【表３】

（実施例４）
化合物Ａとしてジベンゾフラン（ＤＢＦ）、化合物Ｂとして１ブロモ２，４ジメトキシベンゼン（ＢＤＭＢ）を用いた。ＤＢＦ中にはベンゼン環構造が２つあり、ＢＤＭＢ中にはベンゼン環構造が１つある。ここで、常温常湿（２５℃／５０ＲＨ％）において、ＢＤＭＢは液体である。
【００９６】
ＬｉＰＦ₆をプロピレンカーボネート、エチレンカーボネート（１：１）に１ｍｏｌ／Ｌの濃度で溶解した溶液１００重量部に所定量のＢＤＭＢを添加した電解液に対するＤＢＦの溶解性を調べた。溶解限界を表−４に示す。
【００９７】
【表４】

（実施例５）
化合物Ａとしてビフェニル（ＢＰｈ）、化合物Ｂとして１ブロモ２，４ジメトキシベンゼン（ＢＤＭＢ）を用いた。ＢＰｈ中にはベンゼン環構造が２つあり、ＢＤＭＢ中にはベンゼン環構造が１つある。
【００９８】
ＬｉＰＦ₆をプロピレンカーボネート、エチレンカーボネート（１：１）に１ｍｏｌ／Ｌの濃度で溶解した溶液１００重量部に所定量のＢＤＭＢを添加した電解液に対するＢＰｈの溶解性を調べた。溶解限界を表−５に示す。
【００９９】
【表５】

（実施例６〜１２、比較例１〜４）
先ず、以下の表−６の配合組成に従い、各成分を混練機により２時間処理して、正極活物質としてリチウムコバルト複合酸化物を含有する正極を作製するための電極材料層形成用塗料αを調整した。
【０１００】
また、以下の表−７の配合組成に従い、各成分を混練機により２時間処理して、負極の電極材料層形成用塗料βを調製した。
さらに、以下の表−８の配合組成に従い、各成分を混合攪拌して溶解し電解質形成用塗料を調製した。尚、表−８において、化合物Ａ、化合物Ｂ、化合物Ａの含有量、及び化合物Ｂの含有量については、表−９に示すように、様々な組み合わせを用いて実験を行った（実施例６〜１２、比較例１〜４）。
【０１０１】
【表６】

【０１０２】
【表７】

【０１０３】
【表８】

エクストルージョン型のダイコータにより、厚さ２０μｍのアルミニウム製タブ付正極集電体上に電極材料層形成用塗料αを塗布した後、これを乾燥し、多孔質な膜である正極の電極材料層を集電体の片面に結着させた。次いで、ロールプレス（カレンダー）により、上記の正極の電極材料層に圧密処理を施した後、所定のサイズ（有効正極面積５１．５×３１．７ｍｍ）に裁断し、平板状の正極とした。このようにして得られた正極の電極材料層の膜厚は５９．７μｍであった。
【０１０４】
上記正極と同様にして、エクストルージョン型のダイコータにより、厚さ１０μｍの銅製タブ付負極集電体上に負極の電極材料層形成用塗料βを塗布した後、これを乾燥し、多孔質な膜である負極の電極材料層を集電体の片面に結着させた。次いで、ロールプレス（カレンダー）により、上記負極の電極材料層に圧密処理を施した後、所定のサイズに裁断し、負極とした。このようにして得られた負極の電極材料層の膜厚は６２．５μｍであった。
【０１０５】
上記の正極と負極とに上記電解質形成用塗料を塗布し、別途に調製した電解質形成用塗料を含浸させた高分子多孔質フイルム（セパレータ）を間に挟んで積層し、９０℃で１０分間加熱することにより電解質を非流動化し、ポリアクリレート樹脂及び電解液からなるゲル状電解質を形成した。そして、正極、負極及び非流動性の電解質層を有する平板状の単位電池要素を得た。
【０１０６】
得られた単位電池要素を、アルミニウム層とその両面に配置された合成樹脂層から構成されるラミネートフィルムをプリフォーム成形したケースに収納して、ラミネートフィルムの端を真空シールして、リチウム二次電池を作製した。
得られたリチウム二次電池を初期充電し電池特性を確認した後、放電状態から１Ｃ及び３Ｃの電流密度で上限電圧を１０Ｖとした過充電試験をおこない、充電開始から電圧が４．８Ｖに到達するまでの時間を評価した。充電開始から電圧が４．８Ｖに到達するまでの時間が短ければ短いほど、リチウム二次電池の過充電状態での安全性が改良されているといえる。その理由を以下に説明する。
【０１０７】
リチウム二次電池が過充電状態にあるとは、正極活物質から必要以上にリチウムイオンが放出され続ける状態にあることを意味する。そして、この状態においては、リチウムイオンが正極活物質から抜ければ抜けるほどリチウム二次電池は危険な状態となる。本発明のリチウム二次電池においては、化合物Ａが過充電状態において酸化され副次反応を起こして正極活物質表面で被膜を形成し、この被膜がリチウムイオンの正極活物質からの放出を抑制して過充電の進行を停止させ、リチウム二次電池が安全な状態となる。
【０１０８】
そして、正極活物質表面に被膜が形成されたか否かは、リチウム二次電池の正負極間の電圧値から見積もることができる。つまり、被膜が形成されるとリチウム二次電池の抵抗値が上昇することとなる。一方、リチウム二次電池に流れ込む充電電流は、１Ｃ又は３Ｃで一定である。従って、過充電状態での被膜形成によりリチウム二次電池の抵抗値が上がれば、当然、リチウム二次電池の正負極間の電圧値が上昇する。
【０１０９】
このため、充電開始から４．８Ｖに到達する時間が短い場合は、正極活物質表面での被膜形成が迅速に行われ、リチウム二次電池が早い段階で安全な状態となることを意味する。一方で、充電開始から４．８Ｖに到達する時間が長ければ長い程、正極活物質からリチウムイオンが放出され続ける状態が続くこととなるので、リチウム二次電池が危険な状態に近づいていくことになる。
【０１１０】
表−８中の化合物Ａの種類、化合物Ｂの種類、化合物Ａの含有量、及び化合物Ｂの含有量を表−９に示す通りにして作製した電池の過充電試験の結果を表−９に示す。
【０１１１】
【表９】

実施例６〜８及び実施例９〜１１の結果から、化合物ＢとしてＰｈＥ（ベンゼン環構造２つ）を含有させることにより、化合物ＡであるＤＢＦ（ベンゼン環構造２つ）の含有量を増加させることができるようになり、化合物Ａの増量と共に１Ｃ及び３Ｃ過充電試験における４．８Ｖ到達時間を短くすることができる。
【０１１２】
一方、比較例１の結果から、化合物Ａを用いない場合には、１Ｃ及び３Ｃ過充電試験における４．８Ｖ到達時間が長くなり、正極活物質からリチウムイオンが引く抜かれ続けることによってリチウム二次電池が危険な状態に近づいていくことが分かる。
また、比較例２、３の結果から、ＤＢＦ及びＰｈＥを化合物Ａとして含有させれば、含有させない場合（比較例１）と比較して、１Ｃ及び３Ｃ過充電試験における４．８Ｖ到達時間が幾分短くなる。しかし、これら化合物Ａは、電解液中での溶解度が低く電解液中での含有量が制限される。このため、リチウム二次電池の過充電時の安全性改良効果は不十分となる。
【０１１３】
さらに、実施例１２の結果と比較例４の結果との比較から、化合物ＢとしてＰｈＥ（ベンゼン環構造２つ）を用いることにより、化合物ＡであるＢＰｈ（ベンゼン環構造２つ）の電解液に対する溶解度を上げることができるようになり、その結果、過充電時の安全性が良好となることがわかる。
【０１１４】
【発明の効果】
本発明によれば、リチウム二次電池の過充電時又は高温保存時の安全性を向上させる添加剤である化合物Ａ（例えば、ビフェニル、テルフェニル、ジベンゾフラン）と類似構造を有する化合物Ｂとを併用することにより、化合物Ａの電解液中への溶解度を上げることができるようになるので、リチウム二次電池の安全性が飛躍的に向上するようになる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrolytic solution used for a lithium secondary battery and a lithium secondary battery using the electrolytic solution.
[0002]
[Prior art]
A lithium secondary battery having an electrolyte containing a positive electrode, a negative electrode, and an electrolytic solution needs to ensure safety against overcharge and high-temperature storage. Conventionally, the safety of lithium secondary batteries is based on lithium secondary batteries that include a current interrupting device that mechanically interrupts overcurrent during overcharging and a protective circuit such as a thermal fuse that opens the battery at high temperatures. Has been secured by mounting.
[0003]
However, such a current interrupting device and a protection circuit not only increase the cost of the lithium secondary battery, but also prevent downsizing of the lithium secondary battery, so that an additive is dissolved in the electrolyte in the electrolyte. Research and development to ensure safety during high temperature storage and overcharging are also being actively conducted. For example, it is known that a small amount of biphenyl or the like is dissolved in an electrolytic solution in order to ensure the safety of a lithium secondary battery during overcharge (see Patent Document 1).
[0004]
[Patent Document 1]
JP-A-9-106835
[0005]
[Problems to be solved by the invention]
An additive for ensuring the safety of these batteries has a benzene ring structure, which is a kind of hydrocarbon ring, in the molecule in the case of an aromatic additive such as a hydrocarbon ring or a heterocyclic ring (biphenyl or the like). However, in order to further increase safety during overcharge, the content needs to be more than a certain level.
[0006]
However, an additive having a hydrocarbon ring or a heterocyclic ring in the molecule generally has low solubility in an electrolyte having a high polarity. Therefore, even if it is going to improve the safety | security at the time of an overcharge, there exists a subject that content of the said additive in electrolyte solution is restrict | limited.
[0007]
[Means for Solving the Problems]
In view of the above problems, the present inventor electrolyzed an additive having a hydrocarbon ring or a heterocyclic ring (in this specification, the hydrocarbon ring and the heterocyclic ring may be collectively referred to as a ring structure) in the molecule. In order to improve the solubility in the liquid, even when the protection circuit such as the current interrupting device or the thermal fuse is removed, we will intensively study to ensure sufficient safety when the lithium secondary battery is overcharged or stored at high temperature. It was. As a result, the inventors have found that the solubility of the additive having the ring structure can be improved by coexisting a predetermined compound having a structure similar to the additive having the ring structure.
[0008]
That is, the first gist of the present invention is an electrolytic solution for a lithium secondary battery, in which the electrolytic solution has two or more ring structures of at least one of a hydrocarbon ring or a heterocyclic ring in the molecule. And Compound B having the number of ring structures equal to or less than the number of Compound A, and (the content of Compound B) / (the content of Compound A) in the electrolytic solution is 0.7 or more. It exists in the electrolyte solution characterized by this.
[0009]
The second gist of the present invention resides in a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte is contained in the electrolyte.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
(A) Electrolytic solution for lithium secondary battery
The electrolyte solution for a lithium secondary battery of the present invention includes a compound A having at least two ring structures of at least one of a hydrocarbon ring or a heterocyclic ring in the molecule, and the number of the ring structures is equal to or less than the number of the compound A. A certain compound B is contained, and (content of compound B) / (content of compound A) in the electrolytic solution is 0.7 or more.
[0011]
First, in the present invention, Compound A and Compound B having the same hydrocarbon ring or heterocyclic ring in the molecule are used. And the number of the said ring structures in the compound A shall be 2 or more, and the number of the said ring structures in the compound B shall be below the number of the said ring structures in the compound A. For example, when compound A has three hydrocarbon rings in the molecule, compound B will have 1 to 3 identical hydrocarbon rings. Thus, by using the compound B having a similar structure to the compound A, the solubility of the compound A in the electrolytic solution can be increased.
[0012]
Next, in the present invention, (content of compound B) / (content of compound A) is set to 0.7 or more. That is, by incorporating the compound B in a predetermined amount or more with respect to the compound A, the solubility of the compound A in the electrolytic solution can be increased.
In addition, the hydrocarbon ring in this invention means the structure where the carbon atom couple | bonded cyclically, and is classified into an alicyclic hydrocarbon ring and an aromatic hydrocarbon ring. The heterocyclic ring in the present invention refers to a cyclic structure in which the elements constituting the ring include not only carbon atoms but also heteroatoms such as oxygen, sulfur, nitrogen, phosphorus, and arsenic.
[0013]
In the present invention, the relationship between the number of hydrocarbon rings or heterocyclic rings that compound A and compound B have is important. In the present invention, the hydrocarbon ring or heterocyclic ring is counted by the following method.
That is, in the hydrocarbon ring, a single ring structure in which carbon atoms are bonded in a cyclic manner is counted as one hydrocarbon ring. For example, naphthalene and biphenyl are considered to have two benzene rings which are aromatic hydrocarbon rings among the hydrocarbon rings. From the same idea, tetralin and cyclohexylbenzene are considered to have one benzene ring that is an aromatic hydrocarbon ring among hydrocarbon rings and one cyclohexane ring that is an alicyclic hydrocarbon ring among hydrocarbon rings. .
[0014]
On the other hand, regarding a heterocyclic ring, a single ring structure composed of a heteroatom and a carbon atom is considered as one heterocyclic ring. For example, a purine is considered to have one pyrimidine ring and one pyrrole ring.
Regarding the ring structure formed by condensing a heterocycle and a hydrocarbon ring, the number of hydrocarbon rings or heterocycles is counted in the same way. For example, indole is considered to have one benzene ring, which is a kind of hydrocarbon ring, and one pyrrole ring, which is a kind of heterocyclic ring. Similarly, quinoline is considered to have one benzene ring and one pyridine ring.
[0015]
Hereinafter, regarding the present invention, [1] Significance of using Compound A and Compound B together, [2] Hydrocarbon ring or heterocyclic ring common to Compound A and Compound B [3] Compound A, [4] Compound B, [5] Specific examples of combinations of compound A and compound B, [6] quantitative ratio of compound A and compound B, and [7] properties of compound A and compound B will be described in detail.
[1] Significance of using Compound A and Compound B
As described above, in the present invention, Compound A and Compound B having the same hydrocarbon ring or heterocyclic ring are used in combination. That is, Compound A and Compound B have the same ring structure. The reason why the compound A and the compound B having such a similar structure are used in the present invention will be described below.
[0016]
As the compound A, a compound having an aromatic ring or the like (for example, biphenyl) which is known as an additive for improving the safety against overcharge of a lithium secondary battery is generally used. Such a compound A is generally nonpolar.
On the other hand, the solvent of the electrolytic solution typified by propylene carbonate and γ-butyrolactone is generally high in polarity. Accordingly, the affinity between the nonpolar compound A and the solvent of the electrolytic solution is lowered, and the solubility of the compound A in the electrolytic solution cannot be increased. For this reason, in the present invention, by adding a secondary substance (compound B) having an affinity for both the nonpolar compound A and the solvent of the electrolyte solution with high polarity to the electrolyte solution, Many can be dissolved in the electrolyte.
[2] Hydrocarbon ring or heterocyclic ring common to compound A and compound B
The hydrocarbon ring or heterocyclic ring common to Compound A and Compound B will be described.
[0017]
A hydrocarbon ring refers to a structure in which carbon atoms are bonded in a cyclic manner. The hydrocarbon ring is classified into an alicyclic hydrocarbon ring and an aromatic hydrocarbon ring.
Among the hydrocarbon rings, examples of the alicyclic hydrocarbon ring include those usually having 3 to 20 carbon atoms. From the viewpoint of being easily obtained industrially, an alicyclic hydrocarbon ring having 3 to 10 carbon atoms is preferably used, and an alicyclic hydrocarbon ring having 5 or 6 carbon atoms is particularly preferably used.
[0018]
Examples of the alicyclic hydrocarbon ring having 5 or 6 carbon atoms include cyclopentane, cyclohexane, cyclohexene, 1,3-cyclohexadiene, and the like. Of these alicyclic hydrocarbon rings, cyclohexane is preferred from the viewpoint of easy industrial availability.
Among the hydrocarbon rings, examples of the aromatic hydrocarbon ring include benzene having 6 carbon atoms.
[0019]
On the other hand, a heterocyclic ring refers to a cyclic structure in which the elements constituting the ring include not only carbon atoms but also heteroatoms such as oxygen, sulfur, nitrogen, phosphorus, and arsenic. In the present invention, among these heteroatoms, it is preferable to use at least one of oxygen, sulfur, and nitrogen from the viewpoint of ensuring the safety of the lithium secondary battery. Particular preference is given to using oxygen or sulfur. Furthermore, the heterocyclic ring used for the heterocyclic group may contain two or more heteroatoms. Sulfur and nitrogen are preferred as a combination of two or more heteroatoms from the viewpoint of easy industrial availability.
[0020]
The number of heteroatoms in the heterocycle is usually 1 or more and 5 or less. Considering the safety of the lithium secondary battery and the ease of synthesis, the number of heteroatoms is preferably 1 or more, 3 or less, particularly preferably 1 or 2, and most preferably 1.
As the size of the heterocyclic ring, various kinds of rings ranging from a triatomic ring to a macrocyclic compound can be used. However, considering the safety of lithium secondary batteries and the ease of synthesis, the triatomic ring to the hexaatomic ring Are preferably used, and it is particularly preferable to use either a 5-atom ring or a 6-atom ring.
[0021]
Specific examples of the heterocyclic ring include a heterocyclic ring containing oxygen, a heterocyclic ring containing sulfur, a heterocyclic ring containing nitrogen, and a heterocyclic ring containing sulfur and nitrogen.
Examples of the heterocyclic ring containing oxygen include ethylene oxide, dioxane, furan, and γ-pyran, but it is preferable to use furan from the viewpoint of ensuring the safety of the lithium secondary battery.
[0022]
Examples of the heterocyclic ring containing sulfur include thiophene and γ-thiopyran. However, thiophene is preferably used from the viewpoint of ensuring safety in the lithium secondary battery.
Examples of the nitrogen-containing heterocyclic ring include pyrrole, pyridine, imidazole, pyrimidine, and triazine. From the viewpoint of ensuring safety in the lithium secondary battery, pyrrole, pyridine, imidazole, pyrimidine, and triazine. It is preferable to use either one.
[0023]
Examples of the heterocyclic ring containing sulfur and nitrogen include thiazole.
In the present invention, a particularly preferred ring structure is to use cyclohexane, benzene, or furan, and the most preferred ring structure is benzene. Since many of the overcharge inhibitors have a benzene ring structure in the molecule, it is preferable to use a compound A having two or more benzene ring structures in the molecule. As the compound A uses a benzene ring structure, the effect of the present invention is remarkably exhibited by using a compound having a benzene ring structure in the molecule as the compound B.
[0024]
Examples of the state of the benzene ring structure contained in the molecules of Compound A and Compound B include phenyl group, o-tolyl group, m-tolyl group, p-tolyl group, o-phenylene group, m-phenylene group, p -Phenylene group, o-cumenyl group, m-cumenyl group, p-cumenyl group, 1-naphthyl group (in this case, two benzene ring structures are considered to exist), 2-naphthyl group (in this case, benzene ring structure) And benzyl group, phenethyl group, styryl group, and cinnamyl group. Of these benzene ring structures, a phenyl group, an m-phenylene group, a p-phenylene group, a 1-naphthyl group, and a 2-naphthyl group are preferably used from the viewpoint of ensuring the safety of the lithium secondary battery.
[3] Compound A
Next, Compound A will be described. As described in [2] above, various hydrocarbon rings or heterocycles that the compound A and the compound B have in the molecule may be used for the purpose of improving the safety of the lithium secondary battery. Although it is possible, it is most preferred to use benzene. For this reason, the case where the benzene ring structure which is the most preferable aspect is used as an example of the compound A is demonstrated below.
[0025]
When benzene is used as the ring structure, the compound A has two or more benzene ring structures. As the compound A having such a structure, for example, when two or more benzene ring structures are condensed by sharing two or more atoms (for example, when having a structure such as naphthalene), two or more Benzene ring structures bonded to each other by sharing one atom (for example, when having a structure like biphenyl), and two or more benzene ring structures are connected to each other through other atoms or other ring structures Can be mentioned.
[0026]
First, as compound A, a compound having a structure in which two or more benzene ring structures are condensed by sharing two or more atoms (hereinafter sometimes simply referred to as a condensed polycyclic hydrocarbon) is used. Will be described.
Examples of the condensed polycyclic hydrocarbon include naphthalene (having two benzene ring structures in the molecule), anthracene (having three benzene ring structures in the molecule), and phenanthrene (having a benzene ring structure in the molecule). 3), pyrene (having 4 benzene ring structures in the molecule), perylene (having 5 benzene ring structures in the molecule), and fullerenes (for example, C ₆₀ In the case of having 20 benzene ring structures in the molecule, C ₇₀ In this case, the molecule has 25 benzene ring structures. ). When a condensed polycyclic hydrocarbon is used as the compound A, it is preferable to use fullerenes. Because fullerenes have a radical scavenging function, it is considered that the use of fullerenes can improve the safety of lithium secondary batteries during overcharge and high-temperature storage.
[0027]
Examples of fullerenes include fullerenes, fullerene derivatives, fullerenes, and mixtures of fullerene derivatives.
Fullerene is a spherical shell-like carbon molecule and is not limited as long as the object of the present invention is satisfied. ₆₀ , C ₇₀ , C ₇₄ , C ₇₆ , C ₇₈ , C ₈₀ , C ₈₂ , C ₈₄ , C ₈₆ , C ₈₈ , C ₉₀ , C ₉₂ , C ₉₄ , C ₉₆ , C ₉₈ , C ₁₀₀ Or a dimer or trimer of these compounds. Among these fullerenes, C is preferable. ₆₀ , C ₇₀ Or a dimer or a trimer thereof. C ₆₀ , C ₇₀ Is particularly preferred because it is industrially easy to obtain and has high solubility in solvents. A plurality of these fullerenes may be used in combination. However, when a plurality of these fullerenes are used in combination, it is preferable to use C from the viewpoint of increasing solubility. ₆₀ And C ₇₀ Is to use.
[0028]
The fullerene derivative refers to a compound in which an atomic group forming a part of an organic compound or an atomic group composed of an inorganic element is bonded to at least one carbon constituting the fullerene. The fullerene used for obtaining the fullerene derivative is not limited as long as the object of the present invention is satisfied, and any of the fullerenes specifically shown above may be used. As the fullerene derivative, for example, hydrogenated fullerene, oxidized fullerene, fullerene hydroxide, halogenated (F, Cl, Br, I) fullerene, or the like can be used. A plurality of these fullerene derivatives may be used in combination.
[0029]
Fullerenes can be obtained, for example, by extraction and separation from fullerene-containing soot obtained by a resistance heating method, a laser heating method, an arc discharge method, a combustion method, or the like. At this time, it is not always necessary to completely separate, and the content of fullerene can be adjusted within a range that does not impair the performance. Moreover, a fullerene derivative is compoundable using a conventionally well-known method with respect to fullerene. For example, a desired fullerene derivative can be obtained by utilizing a reaction with a nucleophile (nucleophilic addition reaction), a cycloaddition reaction, a photoaddition (cyclization) reaction, an oxidation reaction, or the like.
[0030]
The fullerenes thus obtained usually have powdery properties at room temperature (25 ° C.) and normal humidity (50% RH), and their secondary particle size is usually 10 nm or more, preferably 50 nm or more. More preferably, it is 100 nm or more, usually 1 mm or less, preferably 500 μm or less, more preferably 100 μm or less.
On the other hand, as the compound A, a compound in which two or more benzene ring structures are bonded to each other by sharing one atom can also be used.
[0031]
Examples of such compound A include biphenyl (having two benzene rings in the molecule) and terphenyl (having three benzene rings in the molecule). Biphenyl and terphenyl are favorably used as additives for improving the safety of lithium secondary batteries.
Furthermore, as the compound A, a compound in which two or more benzene ring structures are condensed via other atoms or other ring structures can also be used.
[0032]
Examples of the compound A in which two or more benzene ring structures are bonded through other atoms include compounds in which two benzene ring structures are bonded through Group 6A elements of the periodic table, and two benzene ring structures. Mention may be made of compounds which are condensed via a heterocycle.
Examples of the compound in which two benzene ring structures are bonded via the Group 6A element of the periodic table include, for example, phenyl ether (two benzene ring structures are bonded through oxygen atoms), diphenyl sulfide (sulfur atoms are bonded). Through which two benzene ring structures are bonded.). Phenyl ether and diphenyl sulfide are favorably used as additives for improving the safety of lithium secondary batteries.
[0033]
Examples of the compound in which two benzene ring structures are condensed via a heterocyclic ring include dibenzofuran (two benzene ring structures are condensed via a furan ring) and xanthene (two benzene rings via a pyran ring). Carbazole (two benzene ring structures are condensed via a pyrrole ring), acridine (two benzene ring structures are condensed via a pyridine ring). be able to. Of these compounds, dibenzofuran is preferred. Dibenzofuran is favorably used as an additive for improving the safety of lithium secondary batteries.
[0034]
As described above, various compounds can be used as the compound A. Of course, multiple types of compound A may be used together.
Compound A has 2 or more ring structures in the molecule, preferably 3 or more, and more preferably 4 or more. Thus, the larger the number of ring structures in the molecule of Compound A, the more preferable it is because it becomes easier to ensure safety during overcharging of the lithium secondary battery.
[0035]
That is, when the lithium secondary battery is overcharged, the compound A is oxidized to cause a side reaction and binds to each other. As a result, a film is formed on the electrode surface, and this film blocks the charging current. Although it is estimated that the safety of lithium secondary batteries during overcharging is ensured, ring structures having a conjugated system such as a benzene ring structure are susceptible to oxidation, and secondary reactions in the above overcharged state are easy to proceed. For this purpose, the greater the number of ring structures that compound A has, the better.
[0036]
On the other hand, the larger the number of ring structures in Compound A, the better, but the number of ring structures is usually 40 or less, preferably 30 or less, more preferably 25 or less, and most preferably 20 or less.
Moreover, in order to ensure the safety at the time of overcharge of a lithium secondary battery, it exists in the molecule | numerator of the compound A as a means to make film formation easy by the side reaction of the compound A at the time of overcharge. It is also preferable that two or more ring structures exist in the same plane or can exist. If two or more ring structures exist on the same plane, the compound A can be oxidized in a state of covering and adsorbing the surface of the active material, and bonds within the same molecule or between adjacent molecules can be oxidized. Formation is more efficient and a film is easily formed.
[0037]
Examples of compounds in which two or more ring structures existing in the molecule of compound A are present on the same plane include compounds such as naphthalene, anthracene, phenanthrene, and dibenzofuran. In addition, two or more ring structures existing in the molecule of compound A can exist in the same plane as biphenyl, terphenyl, diphenyl ether, diphenyl having a structure in which the benzene ring structure in the molecule rotates freely. Mention may be made of compounds such as sulfides.
[4] Compound B
Compound B may have the same hydrocarbon ring or heterocyclic ring as compound A, and the number thereof may be less than or equal to the number of compound A. The compound B needs to be a compound having an affinity for the compound A and depends on what is used for the compound A. Although a single compound may be used as the compound B, it goes without saying that a plurality of compounds may be used in combination as long as the above condition is satisfied in relation to the compound A.
[0038]
Specific examples of preferable combinations of Compound A and Compound B will be described below.
[5] Specific examples of combinations of compound A and compound B
[5-1] Compound A
Preferred as the compound A having two or more ring structures in the molecule is a compound represented by the following general formula (1).
[0039]
[Formula 4]
a1- (X1) _n -A2 (1)
(N is 0 or 1,
When n = 0, a1 and a2 are each independently an aromatic hydrocarbon group that may be substituted.
[0040]
When n = 1, X1 is any one of Group 6 elements of the periodic table, an optionally substituted aromatic hydrocarbon group, and an optionally substituted alicyclic hydrocarbon group, and a1 and a2 is an aromatic hydrocarbon group which may be independently substituted, and a1 and a2 may be bonded to each other to form a ring. )
When n = 0, in order to ensure that the ring structure in compound A is 2 or more, a1 and a2 are each independently an aromatic hydrocarbon group that may be substituted. Also when n = 1, a1 and a2 are each independently an aromatic hydrocarbon group that may be substituted.
[0041]
Examples of the aromatic hydrocarbon group used as a1 and a2 include aromatic hydrocarbon groups having 6 to 20 carbon atoms. The number of carbons constituting the aromatic hydrocarbon group used for a1 and a2 is usually 6 or more, and is usually 20 or less, preferably 16 or less, more preferably 10 or less. By using the aromatic hydrocarbon group having the above carbon number, the melting point of the compound A can be controlled well.
Examples of the aromatic hydrocarbon group having 6 to 20 carbon atoms include a phenyl group, a biphenylyl group, a terphenylyl group, a 1-naphthyl group, a 2-naphthyl group, a 1-anthryl group, a 2-anthryl group, and a 9-anthryl group. Group, 1-phenanthryl group, 2-phenanthryl group, 3-phenanthryl group, 4-phenanthryl group, 9-phenanthryl group, 1-pyrenyl group, 2-pyrenyl group, 4-pyrenyl group, 1-fluorenyl group, 2-fluorenyl group Group, 3-fluorenyl group, 4-fluorenyl group, 9-fluorenyl group, 1-indenyl group, 2-indenyl group, 3-indenyl group, 4-indenyl group, 5-indenyl group, 6-indenyl group, 7-indenyl group Groups and the like.
[0042]
An aromatic hydrocarbon group having 6 to 10 carbon atoms is preferred from the viewpoint of easy industrial synthesis. As such a thing, a phenyl group, a biphenylyl group, and a benzyl group can be mentioned, for example.
Of course, all or part of the hydrogen atoms of the aromatic hydrocarbon group may be substituted with other substituents. Such substituents include chain, branched or cyclic alkyl groups, chain, branched or cyclic alkenyl groups, aryl groups, heterocyclic groups, alkoxy groups, aryloxy groups, heterocyclic oxy groups, halogens. Examples include substituents such as atoms. Here, in a chain, branched or cyclic alkyl group, a chain, branched or cyclic alkenyl group, aryl group, heterocyclic group, alkoxy group, aryloxy group or heterocyclic oxy group that can be used as a substituent. The number of carbon atoms is preferably 15 or less, more preferably 10 or less, and most preferably 5 or less. Further, hydrogen bonded to the chain, branched or cyclic alkyl group, chain, branched or cyclic alkenyl group, aryl group, heterocyclic group, alkoxy group, aryloxy group and heterocyclic oxy group. The atom may be further substituted with a halogen atom or the like.
[0043]
Specific examples of the substituent include a chain, branched or cyclic alkyl group, methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, sec-butyl group, t-butyl group. , N-pentyl group, n-hexyl group, cyclopropyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group and the like. Examples of the chain, branched or cyclic alkenyl group that can be used as a substituent include a vinyl group, a propenyl group, a butenyl group, and a hexenyl group. Furthermore, examples of the aryl group that can be used as a substituent include a phenyl group and a naphthyl group. Furthermore, examples of the heterocyclic group that can be used as a substituent include a pyridyl group, a thiazolyl group, a benzothiazolyl group, an oxazolyl group, a benzoxazolyl group, and a benzofuranyl group. Furthermore, examples of the alkoxy group that can be used as a substituent include a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, and an n-butoxy group. Furthermore, examples of the aryloxy group that can be used as a substituent include a phenoxy group and a naphthyloxy group. Furthermore, examples of the heterocyclic oxy group that can be used as a substituent include a pyridyloxy group, a furyloxy group, a thiazolyloxy group, and the like. Furthermore, examples of the halogen atom that can be used as a substituent include fluorine, chlorine, bromine and the like.
[0044]
On the other hand, when n = 1, X1 is any one of Group 6 elements of the periodic table, an optionally substituted aromatic hydrocarbon group, and an optionally substituted alicyclic hydrocarbon group.
Examples of Group 6 elements in the periodic table used for X1 include oxygen, sulfur, selenium, tellurium, and polonium. From the viewpoint of improving safety during high-temperature storage and overcharge, it is preferable to use oxygen or sulfur among the above elements.
[0045]
Examples of the aromatic hydrocarbon group used for X1 include aromatic hydrocarbon groups having 6 to 20 carbon atoms. The number of carbon atoms constituting the aromatic hydrocarbon group used for X1 is usually 6 or more, and is usually 20 or less, preferably 16 or less, more preferably 12 or less. By using the aromatic hydrocarbon group having the above carbon number, the safety of the lithium secondary battery can be secured satisfactorily.
Examples of the aromatic hydrocarbon group having 6 to 20 carbon atoms include m-phenylene group, p-phenylene group, biphenylene group, naphthylene group, and triphenylene group.
[0046]
Among the aromatic hydrocarbon groups, an aromatic hydrocarbon group having 6 to 12 carbon atoms is preferable. As such an aromatic hydrocarbon group, it is preferable to use an m-phenylene group, a p-phenylene group, or a biphenylene group.
In the aromatic hydrocarbon group used for X1, all or part of the hydrogen bonded to the aromatic ring may be substituted with another group. As such a substituent, the same substituents as those described for a1 and a2 can be used.
[0047]
The alicyclic hydrocarbon group used for X1 will be described. From the viewpoint of compatibility with Compound B, the carbon number of X1 is usually 1 or more, preferably 2 or more, and usually 4 or less, preferably 3 or less. Examples of such alicyclic hydrocarbon groups include methylene group, ethylene group, trimethylene group, tetramethylene group, propylene group and the like.
Among the above alicyclic hydrocarbon groups, it is more preferable to use an alicyclic hydrocarbon group having 2 or 3 carbon atoms. Examples of the alicyclic hydrocarbon group include a methylene group, an ethylene group, and a trimethylene group, and a methylene group or an ethylene group is particularly preferable.
[0048]
In the alicyclic hydrocarbon group used for X1, all or part of the hydrogen bonded to the alicyclic hydrocarbon may be substituted with another group. As such a substituent, the same substituents as those described for a1 and a2 can be used.
When n = 1, a1 and a2 may be bonded to each other to form a ring. That is, a1 and a2 are bonded to each other via X1, but apart from this bond, a1 and a2 are bonded directly or via one or more atoms to form a ring containing X1 as a whole. be able to.
[0049]
Specific examples of the compound A represented by the general formula (1) include phenyl ether, naphthyl ether, diphenyl sulfide, bis (p-tolyl) ether, bis (p-tolyl) sulfide, and bis (p-fluorophenyl) ether. Bis (p-fluorophenyl) sulfide, bis (p-chlorophenyl) ether, diphenoxybenzene, dibenzofuran, 1,4-dibenzodioxane, xanthene, biphenyl, terphenyl, diphenylmethane, dibenzyl and the like. Among these, from the viewpoint of ensuring the safety of the lithium secondary battery, it is more preferable to use phenyl ether, diphenyl sulfide, dibenzofuran, biphenyl, terphenyl, diphenylmethane, and dibenzyl. Most preferably, terphenyl is used. Of course, the compound represented by the general formula (1) may be used in combination of two or more.
[5-2] Compound B
When the compound A having the above general formula (1) is used as the compound A, the compound B having the same ring structure as the compound A and the number of which is not more than the number of the compound A is represented by the following general formula ( It is preferable to use the compound represented by 2).
[0050]
[Chemical formula 5]

Here, b1 represents a hydrogen atom, an alkyl group, an alkenyl group, or an alkoxy group, b2 represents an alkyl group, an alkenyl group, or an alkoxy group, and b3 represents a hydrogen atom or a halogen atom.
[0051]
Examples of the alkyl group used for b1 and b2 include those having usually 1 or more carbon atoms, usually 20 or less, preferably 10 or less, more preferably 5 or less. Examples of such alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, hexyl, A heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group etc. can be mentioned.
[0052]
From the viewpoint of being used as an overcharge inhibitor, an alkyl group having 1 to 5 carbon atoms is preferred. Examples of such an alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group.
Examples of the alkenyl group used for b1 and b2 include those having usually 1 or more carbon atoms and usually 5 or less, preferably 3 or less. Examples of such an alkenyl group include a 1-propenyl group, a 2-propenyl group, an isopropenyl group, a butynyl group, a pentynyl group, and a (1-butynyl) pentyl group.
[0053]
Among the alkenyl groups, it is preferable to use an alkenyl group having 1 to 3 carbon atoms. Examples of such an alkenyl group include a 2-propenyl group and an isopropenyl group.
Considering the stability of the compound B in the electrolyte, the alkenyl group is particularly preferably ═CH ₂ The part is not included.
[0054]
Examples of the alkoxy group used for b1 and b2 include those having usually 1 or more carbon atoms, usually 6 or less, preferably 4 or less, more preferably 3 or less. Examples of such an alkoxy group include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, and a phenoxy group.
From the viewpoint of being used as an overcharge inhibitor, it is preferable to use an alkoxy group having 1 to 3 carbon atoms. Examples of such an alkoxy group include a methoxy group and an ethoxy group.
[0055]
The alkyl group, alkenyl group, and alkoxy group described above may be further substituted with other groups.
b3 represents a hydrogen atom or a halogen atom. Examples of the halogen atom to be used include fluorine, chlorine, bromine and the like, and preferred is fluorine or bromine. By using fluorine or bromine, the stability as a compound can be increased.
[0056]
In the above general formula (2), when b3 is hydrogen and b1 is a group other than hydrogen, and when b3 is a group other than hydrogen and b1 is hydrogen, the compound B is an ortho, meta Body and para isomers exist. In the present invention, any of ortho, meta, and para isomers can be used. However, from the viewpoint of increasing the number of carbon atoms constituting the groups b1 and b2, it is preferable to use an ortho form or a meta form. Ortho and meta isomers are preferred for use as solvents because melting points tend to be lower than para isomers.
[0057]
Examples of the compound B include toluene, xylene, ethylbenzene, diethylbenzene, methylethylbenzene, propylbenzene, butylbenzene, anisole, phenetole, propylphenyl ether, butylphenyl ether, dimethoxybenzene, 1bromo2,4dimethoxybenzene, methoxyethoxybenzene. , Diethoxybenzene, methoxytoluene, and ethoxytoluene. Of these compounds, preferred from the viewpoint of improving the solubility of Compound A is anisole, phenetole, propylphenyl ether, butylphenyl ether, dimethoxybenzene, 1bromo2,4dimethoxybenzene, methoxyethoxybenzene, diethoxybenzene, methoxy. Toluene and ethoxytoluene. Of course, the compound represented by the said General formula (2) can use multiple types together.
[0058]
When using the compound of the said General formula (1) as the compound A, the compound shown to the following general formula (3) can also be used as the compound B. Of course, as the compound B, a compound represented by the following general formula (3) and a compound represented by the above general formula (2) may be used in combination.
[0059]
[Chemical 6]
c1-Y1-c2 (3)
(Y1 represents a group 6 element of the periodic table or an optionally substituted alicyclic hydrocarbon group, c1 and c2 each independently represent an aromatic hydrocarbon group that may be substituted, and are bonded to each other. To form a ring.)
Here, Y1 represents a periodic table group 6 element. Examples of Group 6 elements of the periodic table include oxygen, sulfur, selenium, tellurium, and polonium. Of these elements, oxygen or sulfur is preferably used from the viewpoint of improving the safety of the lithium secondary battery and increasing the solubility of the compound A.
[0060]
Moreover, the alicyclic hydrocarbon group which may be substituted can also be used for Y1. As the alicyclic hydrocarbon group used for Y1, in order to dissolve compound A well, the carbon number of Y1 is usually 1 or more, preferably 2 or more, and usually 4 or less, preferably 3 or less. Is preferred. Examples of such alicyclic hydrocarbon groups include methylene group, ethylene group, trimethylene group, tetramethylene group, propylene group and the like.
[0061]
Among the above alicyclic hydrocarbon groups, it is more preferable to use an alicyclic hydrocarbon group having 2 or 3 carbon atoms. Examples of the alicyclic hydrocarbon group include a methylene group, an ethylene group, and a trimethylene group, and a methylene group or an ethylene group is particularly preferable.
In the alicyclic hydrocarbon group used for Y1, all or part of the hydrogen bonded to the alicyclic hydrocarbon may be substituted with another group. As such a substituent, the same substituents as those used for a1 and a2 in the compound A of the general formula (1) can be used.
[0062]
C1 and c2 each independently represent an optionally substituted aromatic hydrocarbon group, and c1 and c2 may be the same or different. As the optionally substituted aromatic hydrocarbon group, those similar to a1 and a2 in the compound A represented by the general formula (1) can be used.
Furthermore, c1 and c2 may combine with each other to form a ring. That is, c1 and c2 are bonded to each other through the element Y1, but separately from this bond, c1 and c2 are bonded directly or through one or more atoms to form a ring containing the element Y1 as a whole. Can be configured.
[0063]
Specific examples of the compound B represented by the general formula (3) include phenyl ether, naphthyl ether, diphenyl sulfide, bis (p-tolyl) ether, bis (p-tolyl) sulfide, bis (p-fluorophenyl) ether. Bis (p-fluorophenyl) sulfide, bis (p-chlorophenyl) ether, diphenoxybenzene, dibenzofuran, 1,4-dibenzodioxane, xanthene, diphenylmethane, dibenzyl and the like. Among these, from the viewpoint of increasing the solubility of Compound A, phenyl ether, diphenyl sulfide, dibenzofuran, diphenylmethane, and dibenzyl are particularly preferable, and phenyl ether is most preferable. Of course, the compound represented by the general formula (3) may be used in combination of two or more.
[0064]
In addition, since the effect of this invention is not exhibited even if it uses the same compound by the compound A and the compound B, when using what is represented by General formula (3) for Compound B, it is represented by General formula (1). It is not necessary to have the same structure as that of Compound A, but to have the same ring structure as that of Compound A and to have the number of compounds A or less.
[6] Quantity ratio of compound A and compound B
In the present invention, (compound B content) / (compound A content) is 0.7 or more, preferably 0.75 or more, more preferably 0.8 or more, and particularly preferably 1 or more. Within this range, the solubility of A can be increased by containing compound B. On the other hand, usually, the more compound B is contained in compound A, the more preferable it is because the solubility of compound A increases. Usually, (content of compound B) / (content of compound A) is 10 or less. And Considering the balance between the battery capacity and the rate characteristics with the basic characteristics of the battery, (compound B content) / (compound A content) is preferably 7 or less, more preferably 5 or less, most preferably 3 or less.
[0065]
The total content of Compound A and Compound B in the electrolytic solution is usually 1% by weight or more, preferably 3% by weight or more, more preferably 5% by weight or more. If it is within the above range, the purpose of safety can be satisfactorily achieved. On the other hand, the content of Compound A and Compound B in the electrolytic solution is usually 50% by weight or less, preferably 20% by weight or less, more preferably 10% by weight or less. If it is the said range, the ionic conductivity of electrolyte solution will not fall, and the influence on a battery characteristic can be suppressed favorably.
[7] Properties of Compound A and Compound B
Compound A is usually contained in the electrolyte as an additive for ensuring safety during overcharge or storage at high temperatures. On the other hand, the compound B is usually used in combination to increase the solubility of the compound A in the electrolyte solution, and like the compound A, it has a function of improving the safety of the lithium secondary battery. More preferably. In addition to improving the solubility of Compound A, there is an advantage that the effect of improving safety by Compound B itself is synergistically exhibited. Moreover, it is preferable that the compound B does not impair battery characteristics, such as a capacity | capacitance of lithium secondary battery, and a repeated charging / discharging characteristic.
[0066]
In general, the compound A preferably has a high boiling point. This is because if the boiling point of the compound A is low, the compound A is vaporized due to the heat storage of the battery during overcharge or high temperature storage, and it becomes difficult to ensure the safety of the lithium secondary battery. From this viewpoint, the boiling point of compound A at normal pressure is usually 100 ° C. or higher, preferably 120 ° C. or higher, more preferably 150 ° C. or higher. On the other hand, since a compound having a very high boiling point is difficult to obtain in practice, the boiling point of compound A is usually 300 ° C. or lower.
[0067]
Here, since a compound having a high boiling point tends to have a high melting point at the same time, when a compound having a high boiling point is used as the compound A, the melting point becomes room temperature or higher, and the compound A is at room temperature and normal humidity (25 ° C./50% RH). May become solid. However, even if the compound A is solid at normal temperature and humidity, it can exist as a liquid in the electrolytic solution by dissolving the compound A in the compound B by using a low melting point substance in the compound B. Therefore, it is preferable that the compound B used for increasing the solubility of the compound A in the electrolyte solution be a liquid at room temperature and normal humidity (25 ° C./50% RH) with a melting point of 50 ° C. or lower. By doing so, the compound A can be easily dissolved in the compound B. The melting point of Compound B is more preferably 30 ° C. or lower, and further preferably 0 ° C. or lower. On the other hand, the melting point of Compound B is usually −10 ° C. or higher.
[8] Other components of electrolyte
The electrolyte solution for a lithium secondary battery of the present invention usually contains a lithium salt that is a supporting electrolyte in addition to Compound A and Compound B, and optionally contains a non-aqueous solvent.
[0068]
As a lithium salt that is a supporting electrolyte to be contained in the electrolytic solution, LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF _Four LiClO _Four , LiI, LiBr, LiCl, LiAlCl, LiHF ₂ , LiSCN, LiSO _Three CF ₂ Etc. Of these, LiPF in particular ₆ And LiClO _Four Is preferred. The content of these supporting electrolytes in the electrolytic solution is usually 0.5 to 2.5 mol / l.
[0069]
As the non-aqueous solvent contained as necessary, a solvent having a relatively high dielectric constant is preferably used. Specifically, cyclic carbonates such as ethylene carbonate and propylene carbonate, acyclic carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, glymes such as tetrahydrofuran, 2-methyltetrahydrofuran and dimethoxyethane, γ-butyllactone, etc. Lactones, sulfur compounds such as sulfolane, nitriles such as acetonitrile, and the like. The above non-aqueous solvent can use multiple types together.
[0070]
In addition, various additives can be added to the electrolytic solution as necessary in order to improve battery performance.
(B) Lithium secondary battery
The lithium secondary battery of the present invention has a negative electrode, a positive electrode, and an electrolyte, and contains the electrolytic solution described in (A) above in the electrolyte.
[0071]
The negative electrode is usually formed by forming an active material layer on a current collector. The active material layer usually contains a negative electrode material having a negative electrode active material, a binder, and optionally an additive such as a conductive agent.
As the negative electrode active material, a carbonaceous material is usually used. Examples of carbonaceous materials include graphite materials such as graphite; coal-based coke, petroleum-based coke; carbides of coal-based pitch or petroleum-based pitch, or carbides obtained by oxidizing these pitches; needle coke, pitch coke, and phenol resin. And carbides such as crystalline cellulose. Further, carbon materials partially graphitized from the above carbonaceous materials, furnace black, acetylene black, pitch-based carbon fibers and the like can also be mentioned.
[0072]
Among the carbonaceous substances, graphite materials such as coke and graphite are preferable, but graphite materials such as graphite are particularly preferable because of their large capacity.
Examples of the graphite material include graphite powder such as artificial graphite and natural graphite and purified products thereof, graphitized products of conductive carbon black such as acetylene black and ketjen black, and carbon fibers such as vapor grown carbon fiber. Any graphite material may be used, but artificial graphite or natural graphite is preferable from the viewpoint of capacity. Artificial graphite is particularly preferable from the viewpoint of easy control of battery performance.
[0073]
The surface of the graphite material may be subjected to an amorphous treatment.
The average particle size of the graphite material is usually 1 μm or more, preferably 5 μm or more, and is usually 45 μm or less, preferably 35 μm or less, more preferably 25 μm or less. If the average particle size is excessively small, the specific surface area of the graphite material increases and the irreversible capacity increases and the battery capacity decreases. On the other hand, if the average particle size is excessively large, the thickness of the active material layer is limited, and it becomes difficult to form a uniform active material layer on the substrate.
[0074]
The specific surface area of graphite material is usually 0.1m ² / G or more, preferably 0.3 m ² / G or more, more preferably 0.5 m ² / G or more. When the specific surface area is excessively small, the rate characteristics of the battery are lowered. On the other hand, the specific surface area of the graphite material is usually 30 m. ² / G or less, preferably 20 m ² / G or less, more preferably 10 m ² / G or less. If the specific surface area is excessively large, the initial efficiency of the battery is lowered. The specific surface area is measured according to the BET method.
[0075]
The negative electrode can be produced by converting the negative electrode active material into a dispersion paint using a solvent capable of dissolving the binder, and applying and drying the paint on a current collector.
The ratio of the negative electrode active material in the active material layer is usually 10% by weight or more, preferably 30% by weight or more, more preferably 50% by weight or more, and usually 99% by weight or less, preferably 98% by weight or less. If the amount is too large, the mechanical strength of the electrode tends to be inferior. If the amount is too small, the battery performance such as capacity tends to be inferior.
[0076]
The binder used for the active material layer needs to be stable with respect to the electrolytic solution and the like, and various materials are used from the viewpoint of weather resistance, chemical resistance, heat resistance, flame retardancy, and the like. Specifically, inorganic compounds such as silicate and glass, alkane polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene; unsaturated polymers such as polybutadiene and polyisoprene; polystyrene, polymethylstyrene, Polymers having a ring structure in a polymer chain such as polyvinyl pyridine and poly-N-vinyl pyrrolidone; and celluloses such as methyl cellulose.
[0077]
Other specific examples include acrylic derivative polymers such as polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, and polyacrylamide; Fluorine resins such as vinyl chloride, polyvinylidene fluoride and polytetrafluoroethylene; CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide; Polyvinyl alcohol polymers such as polyvinyl acetate and polyvinyl alcohol; Halogen-containing polymers such as vinylidene; conductive polymers such as polyaniline can be used.
[0078]
Further, a mixture such as the above-mentioned polymer, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer, and the like can be used. The weight average molecular weight of these resins is usually about 10,000 to 3,000,000, preferably about 100,000 to 1,000,000. If it is too low, the strength of the coating film tends to decrease. On the other hand, if it is too high, the viscosity of the coating material for producing the negative electrode becomes high, and it may be difficult to form the electrode. Preferable binder resins include fluorine resins and CN group-containing polymers, and more preferably polyvinylidene fluoride.
[0079]
The amount of the binder used is usually 0.1 parts by weight or more, preferably 1 part by weight or more, and usually 30 parts by weight or less, preferably 20 parts by weight or less, more preferably 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. Less than parts by weight. If the amount of the binder is too small, the strength of the active material layer tends to decrease, and if the amount of the binder is too large, the battery capacity tends to decrease.
[0080]
If necessary, the active material layer may contain additives, powders, fillers, and the like that exhibit various functions such as a conductive material and a reinforcing material.
Examples of the solvent used when forming the active material layer include N-methylpyrrolidone and dimethylformamide, and N-methylpyrrolidone is preferable. The solvent concentration in the coating is at least 10% by weight, but is usually 20% by weight or more, preferably 30% by weight or more, and more preferably 35% by weight or more. Moreover, as an upper limit, it is 90 weight% or less normally, Preferably it is 80 weight% or less. If the solvent concentration is too low, coating may be difficult. If the solvent concentration is too high, it may be difficult to increase the coating thickness and the stability of the coating may be deteriorated.
[0081]
For dispersion coating, a commonly used disperser can be used, and a planetary mixer, a ball mill, a sand mill, a biaxial kneader, or the like can be used.
There are no particular restrictions on the coating device for applying the paint on the current collector, and slide coaters, extrusion type die coaters, reverse rolls, gravure coaters, knife coaters, kiss coaters, micro gravure coaters, rod coaters, blade coaters, etc. Among them, a die coater, a blade coater, and a knife coater are preferable, and an extrusion type die coater and a blade coater are most preferable from the viewpoint of simplicity in consideration of paint viscosity and coating film thickness.
[0082]
After the coating material is applied on the current collector, the active material layer is formed by drying the coating film at a temperature of about 120 ° C. for about 10 minutes, for example.
The thickness of the active material layer is usually 10 μm or more, preferably 20 μm or more, and is usually 200 μm or less, preferably 150 μm or less. If the thickness of the active material layer is too thin, the battery capacity becomes too small. On the other hand, if the thickness is excessively high, the rate characteristics will deteriorate.
[0083]
As the current collector used for the negative electrode, various materials that can function as a battery current collector can be used without causing problems such as electrochemical elution, and usually metals such as copper, nickel, stainless steel, etc. An alloy is used. Preferably, copper is used. The shape of the current collector is not particularly limited, and examples thereof include a plate shape and a mesh shape. The thickness of the current collector is usually 0.1 μm or more, preferably 1 μm or more, and is usually 100 μm or less, preferably 30 μm or less, more preferably 20 μm or less. If it is too thin, the mechanical strength tends to be weak, which causes a problem in production. If it is too thick, the capacity of the battery as a whole decreases.
[0084]
The electrolyte contains the electrolytic solution described in (A) above, and may further contain a polymer in order to improve the liquid retention of the electrolytic solution and improve safety. Examples of such a polymer include PVDF, which is a fluororesin, and polyacrylate, which is an acrylic resin.
The electrolyte is present inside the positive electrode and the negative electrode and between the positive electrode and the negative electrode, but between the positive electrode and the negative electrode, a support such as a porous film is used to prevent a short circuit between the positive electrode and the negative electrode. Is preferably present. As the porous film, a film made of a polymer resin or a thin film made of a powder and a binder can be preferably used, and more preferably a porous film made of polyethylene, polypropylene or the like.
[0085]
The positive electrode of a lithium secondary battery usually has a positive electrode active material layer containing a positive electrode active material on a current collector. Examples of the positive electrode active material to be used include a composite oxide of lithium and a transition metal, specifically, LiNiO. ₂ LiNiCoO ₂ Lithium nickel composite oxide such as LiCoO ₂ Lithium cobalt composite oxide such as LiMn ₂ O _Four And lithium manganese composite oxide. Some of the transition metal sites of these composite oxides may be substituted with other elements. By substituting a part of the transition metal with another element, the stability of the crystal structure can be improved. As other elements for substituting a part of the transition metal site at this time (hereinafter referred to as substitution elements), Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg , Ga, Zr, etc., preferably Al, Cr, Fe, Co, Li, Ni, Mg, Ga, and more preferably Al. The transition metal site may be substituted with two or more other elements. The substitution ratio by the substitution element is usually 2.5 mol% or more of the base transition metal element, preferably 5 mol% or more of the base transition metal element, and usually 30 mol% or less of the base transition metal element, Preferably it is 20 mol% or less of the transition metal element used as a base. If the substitution ratio is too small, the crystal structure may not be sufficiently stabilized. If the substitution ratio is too large, the capacity of the battery may be reduced. Among lithium transition metal complex oxides, more preferred are lithium cobalt complex oxide and lithium nickel complex oxide, and particularly preferred is LiCoO. ₂ It is. The particle size of the positive electrode active material is usually 1 μm or more, and usually 30 μm or less, preferably 10 μm or less in terms of excellent battery characteristics such as rate characteristics and cycle characteristics. The positive electrode is usually formed by forming an active material layer having a positive electrode active material and a binder on a current collector. The type of binder used for the positive electrode and the method for forming the active material layer may be the same as those for the negative electrode.
[0086]
As the current collector used for the positive electrode, various materials that can function as a battery current collector can be used without causing problems such as electrochemical dissolution, and are usually aluminum, copper, nickel, tin, and stainless steel. A metal such as steel or an alloy of these metals can be used. In this case, aluminum is usually used as the positive electrode current collector. The shape of the current collector is not particularly limited, and examples thereof include a plate shape and a mesh shape. The thickness of the current collector is usually 1 or more, on the other hand, usually 50 μm or less, preferably 30 μm or less. If it is too thin, the mechanical strength will be weak, but if it is too thick, the battery will be large, the space occupied in the battery will be large, and the energy density of the battery will be small.
[0087]
A battery element having a positive electrode, a negative electrode, and an electrolyte is housed in a case. As the battery element, for example, a form in which a laminate in which a positive electrode and a negative electrode are laminated via an electrolyte layer is wound, a form in which a positive electrode, a negative electrode, and an electrolyte layer are laminated in a flat plate shape, or the flat plate shape is laminated. A form in which a plurality of battery elements are prepared and further laminated can be exemplified.
Examples of the case for storing the battery element include a coin cell, a metal can for a dry battery, and a case having shape variability.
[0088]
Examples of the electric device in which the lithium secondary battery is used as a power source include a portable personal computer (in this specification, the personal computer may be simply referred to as a personal computer), a pen input personal computer, a mobile personal computer, and an electronic book player. , Mobile phone, cordless phone, pager, handy terminal, mobile fax, mobile copy, mobile printer, headphone stereo, video movie, LCD TV, handy cleaner, portable CD, minidisc, electric shaver, transceiver, electronic notebook, calculator , Memory card, portable tape recorder, radio, backup power supply, motor, lighting equipment, toys, game equipment, road conditioner, clock, strobe, camera, medical equipment (pacemaker, Hearing aid, such as massaging machines), and the like. The lithium secondary battery can also be used as a power source for electric vehicles.
[0089]
【Example】
Examples The present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist.
Example 1
Dibenzofuran (DBF) was used as Compound A, and phenyl ether (PhE) was used as Compound B. There are two benzene ring structures in DBF, and there are also two benzene ring structures in PhE. Here, at normal temperature and humidity (25 ° C./50 RH%), DBF is a solid and PhE is a liquid.
[0090]
LiPF ₆ The solubility of DBF in an electrolytic solution in which a predetermined amount of PhE was added to 100 parts by weight of a solution prepared by dissolving 1 mol / L in propylene carbonate and ethylene carbonate (1: 1) was examined. The solubility limit is shown in Table-1.
The solubility limit was measured by preparing electrolytic solutions containing 0, 6, 10, and 15 parts by weight of PhE, and adding DBF in increments of 0.5 parts by weight. Then, the content immediately before the content of DBF when the electrolytic solution became cloudy was defined as the solubility limit (parts by weight). For example, if the electrolyte solution becomes clouded when z parts by weight of DBF is contained, (z-0.5) parts by weight is taken as the solubility limit. The method for measuring the solubility limit was the same in Examples 2 to 5.
[0091]
[Table 1]

(Example 2)
Biphenyl (BPh) was used as Compound A, and phenyl ether (PhE) was used as Compound B. There are two benzene ring structures in BPh, and there are also two benzene ring structures in PhE. Here, at normal temperature and normal humidity (25 ° C./50 RH%), BPh is a solid.
[0092]
LiPF ₆ The solubility of BPh in an electrolytic solution in which a predetermined amount of PhE was added to 100 parts by weight of a solution obtained by dissolving the solution in propylene carbonate and ethylene carbonate (1: 1) at a concentration of 1 mol / L was examined. The solubility limit is shown in Table-2.
[0093]
[Table 2]

(Example 3)
Meta-terphenyl (TPh: melting point 89 ° C.) was used as Compound A, and phenyl ether (PhE) was used as Compound B. There are three benzene ring structures in TPh and two benzene ring structures in PhE. Here, at normal temperature and normal humidity (25 ° C./50 RH%), TPh is a solid.
[0094]
LiPF ₆ The solubility of TPh in an electrolyte solution in which a predetermined amount of PhE was added to 100 parts by weight of a solution obtained by dissolving the solution in propylene carbonate and ethylene carbonate (1: 1) at a concentration of 1 mol / L was examined. The solubility limit is shown in Table-3.
[0095]
[Table 3]

(Example 4)
Dibenzofuran (DBF) was used as Compound A, and 1 bromo 2,4 dimethoxybenzene (BDMB) was used as Compound B. There are two benzene ring structures in DBF, and one benzene ring structure in BDMB. Here, at normal temperature and normal humidity (25 ° C./50 RH%), BDMB is a liquid.
[0096]
LiPF ₆ The solubility of DBF in an electrolytic solution in which a predetermined amount of BDMB was added to 100 parts by weight of a solution obtained by dissolving the solution in propylene carbonate and ethylene carbonate (1: 1) at a concentration of 1 mol / L was examined. The solubility limit is shown in Table-4.
[0097]
[Table 4]

(Example 5)
Biphenyl (BPh) was used as compound A, and 1 bromo 2,4 dimethoxybenzene (BDMB) was used as compound B. BPh has two benzene ring structures, and BDMB has one benzene ring structure.
[0098]
LiPF ₆ The solubility of BPh in an electrolytic solution in which a predetermined amount of BDMB was added to 100 parts by weight of a solution obtained by dissolving the solution in propylene carbonate and ethylene carbonate (1: 1) at a concentration of 1 mol / L was examined. The solubility limit is shown in Table-5.
[0099]
[Table 5]

(Examples 6-12, Comparative Examples 1-4)
First, according to the composition of Table-6 below, each component was treated for 2 hours with a kneader to prepare a coating material α for forming an electrode material layer for producing a positive electrode containing a lithium cobalt composite oxide as a positive electrode active material. It was adjusted.
[0100]
Moreover, according to the compounding composition of the following Table-7, each component was processed with the kneader for 2 hours, and the coating material (beta) for electrode material layer formation of a negative electrode was prepared.
Furthermore, according to the composition of the following Table-8, each component was mixed and stirred and dissolved to prepare a coating material for forming an electrolyte. In Table-8, the contents of Compound A, Compound B, Compound A, and the content of Compound B were tested using various combinations as shown in Table-9 (Example 6). To 12, Comparative Examples 1 to 4).
[0101]
[Table 6]

[0102]
[Table 7]

[0103]
[Table 8]

After applying the coating material α for forming an electrode material layer on a positive electrode current collector with an aluminum tab having a thickness of 20 μm by an extrusion type die coater, this is dried, and the positive electrode material layer which is a porous film is formed. It was bound to one side of the current collector. Next, the electrode material layer of the positive electrode was subjected to consolidation treatment by a roll press (calendar), and then cut into a predetermined size (effective positive electrode area 51.5 × 31.7 mm) to obtain a flat positive electrode. The film thickness of the positive electrode material layer thus obtained was 59.7 μm.
[0104]
In the same manner as the above positive electrode, the negative electrode material layer-forming coating material β was applied on a negative electrode current collector with a copper tab having a thickness of 10 μm by an extrusion type die coater, and then dried to obtain a porous film. The negative electrode material layer was bound to one side of the current collector. Next, the electrode material layer of the negative electrode was subjected to consolidation treatment by a roll press (calendar), and then cut into a predetermined size to obtain a negative electrode. The film thickness of the negative electrode material layer thus obtained was 62.5 μm.
[0105]
Apply the above electrolyte forming paint on the above positive electrode and negative electrode, laminate the polymer porous film (separator) impregnated with the separately prepared electrolyte forming paint, and heat at 90 ° C. for 10 minutes Thus, the electrolyte was made non-fluidized to form a gel electrolyte composed of a polyacrylate resin and an electrolytic solution. And the flat unit battery element which has a positive electrode, a negative electrode, and a non-fluid electrolyte layer was obtained.
[0106]
The obtained unit battery element is housed in a case in which a laminate film composed of an aluminum layer and a synthetic resin layer disposed on both sides thereof is preformed, and the end of the laminate film is vacuum-sealed to form a lithium secondary A battery was produced.
After the initial charge of the obtained lithium secondary battery and confirmation of the battery characteristics, an overcharge test was performed with a current density of 1C and 3C and an upper limit voltage of 10 V from the discharged state, and the voltage reached 4.8 V from the start of charging. The time to do was evaluated. It can be said that the shorter the time from the start of charging until the voltage reaches 4.8 V, the more improved the safety of the lithium secondary battery in the overcharged state. The reason will be described below.
[0107]
That the lithium secondary battery is in an overcharged state means that lithium ions are continuously released from the positive electrode active material more than necessary. In this state, the more lithium ions are released from the positive electrode active material, the more dangerous the lithium secondary battery is. In the lithium secondary battery of the present invention, compound A is oxidized in an overcharged state to cause a side reaction to form a film on the surface of the positive electrode active material, and this film suppresses release of lithium ions from the positive electrode active material. Thus, the progress of overcharge is stopped, and the lithium secondary battery is in a safe state.
[0108]
And whether the film was formed in the positive electrode active material surface can be estimated from the voltage value between the positive electrode and the negative electrode of the lithium secondary battery. That is, when the film is formed, the resistance value of the lithium secondary battery is increased. On the other hand, the charging current flowing into the lithium secondary battery is constant at 1C or 3C. Therefore, if the resistance value of the lithium secondary battery increases due to the formation of a film in an overcharged state, the voltage value between the positive and negative electrodes of the lithium secondary battery naturally increases.
[0109]
For this reason, when the time to reach 4.8 V from the start of charging is short, it means that the film formation on the surface of the positive electrode active material is performed quickly, and the lithium secondary battery is in a safe state at an early stage. On the other hand, the longer it takes to reach 4.8 V from the start of charging, the longer the lithium ion is released from the positive electrode active material, so the lithium secondary battery approaches a dangerous state. become.
[0110]
Table-9 shows the result of the overcharge test of the battery prepared in Table-8 with the type of Compound A, the type of Compound B, the content of Compound A, and the content of Compound B shown in Table-9. Show.
[0111]
[Table 9]

From the results of Examples 6 to 8 and Examples 9 to 11, the inclusion of PhE (two benzene ring structures) as compound B increases the content of DBF (two benzene ring structures) as compound A. As a result, the 4.8 V arrival time in the 1C and 3C overcharge tests can be shortened with the increase in the amount of Compound A.
[0112]
On the other hand, from the result of Comparative Example 1, when the compound A is not used, the time to reach 4.8 V in the 1C and 3C overcharge tests becomes long, and lithium ions are continuously pulled out from the positive electrode active material. It can be seen that is approaching a dangerous state.
In addition, from the results of Comparative Examples 2 and 3, if DBF and PhE were included as Compound A, the 4.8 V arrival time in the 1C and 3C overcharge tests was somewhat different from the case where they were not included (Comparative Example 1). Shorten minutes. However, these compounds A have low solubility in the electrolytic solution, and the content in the electrolytic solution is limited. For this reason, the safety improvement effect at the time of overcharge of a lithium secondary battery becomes inadequate.
[0113]
Further, from the comparison between the result of Example 12 and the result of Comparative Example 4, by using PhE (two benzene ring structures) as the compound B, the electrolyte solution of BPh (two benzene ring structures) as the compound A was used. It can be seen that the solubility can be increased, and as a result, the safety during overcharge is improved.
[0114]
【The invention's effect】
According to the present invention, a compound A (for example, biphenyl, terphenyl, dibenzofuran), which is an additive that improves the safety of lithium secondary batteries when overcharged or stored at high temperatures, and a compound B having a similar structure are used in combination. As a result, the solubility of the compound A in the electrolytic solution can be increased, and the safety of the lithium secondary battery is dramatically improved.

Claims (6)

An electrolyte for a lithium secondary battery,
The electrolytic solution contains a compound A having at least two ring structures of at least one of a hydrocarbon ring or a heterocyclic ring in the molecule, and a compound B in which the number of the ring structures is less than or equal to the number of the compound A,
The total content of Compound A and Compound B in the electrolytic solution is 1 to 50% by weight,
In the electrolytic solution, (content of compound B) / (content of compound A) is 0.7 or more, compound A is represented by the following general formula (1), and compound B is represented by the following general formula (3). The electrolyte solution characterized by these.
a1- (X1) _n -a2 (1)
(N is 0 or 1,
When n = 0, a1 and a2 are each independently an optionally substituted aromatic hydrocarbon group or an optionally substituted alicyclic hydrocarbon group.
When n = 1, X1 is any one of Group 6 elements of the periodic table, an optionally substituted aromatic hydrocarbon group, and an optionally substituted alicyclic hydrocarbon group, and a1 and a2 is each independently an optionally substituted aromatic hydrocarbon group or an optionally substituted alicyclic hydrocarbon group, and a1 and a2 may be bonded to each other to form a ring. However, when X1 is a Group 6 element of the periodic table, a1 and a2 are bonded to each other to form a ring. )
c1-Y1-c2 (3)
(Y1 represents a Group 6 element of the periodic table, c1 and c2 each independently represent an aromatic hydrocarbon group that may be substituted, and may be bonded to each other to form a ring.)   The electrolytic solution according to claim 1, wherein n in the general formula (1) is 1.   The electrolyte solution according to claim 1, wherein the melting point of compound B is 50 ° C. or less.   The electrolytic solution according to claim 1, wherein the ring structure in Compound A and Compound B is a benzene ring structure.   The electrolyte solution according to any one of claims 1 to 4, wherein the compound A is dibenzofuran or terphenyl, and the compound B is phenyl ether.   A lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the electrolytic solution according to any one of claims 1 to 5 is contained in the electrolyte.