JP2004185865A - Lithium ion battery, its manufacturing method, and negative electrode structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion battery provided with an ambient temperature-molten salt electrolyte, and having an improved charge-discharge characteristic (durability or the like), to provide its manufacturing method, and to provide a negative electrode structure suitable for a constituent element or the like of the lithium ion battery. <P>SOLUTION: This lithium ion secondary battery 1 is equipped with a positive electrode 10; a negative electrode 20; and an electrolyte layer 30 using the ambient temperature-molten salt electrolyte 32. On the surface of the negative electrode 20, an interface layer 42 for preventing the ambient temperature-molten salt included in the electrolyte 32 from directly contacting the negative electrode 20 is formed. The interface layer 42 has a lithium ion-conducting property, and substantially shows an electron non-conducting property. A potential gradient is formed between the negative electrode 20 and the electrolyte layer 30 by the interface layer 42 to prevent the ambient temperature-molten salt constituting the electrolyte 32 from being directly exposed to the potential of the negative electrode 20. Thereby, reduction decomposition of the ambient temperature-molten salt is restrained to improve the charge-discharge characteristic of the battery 1. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００２０】
このようなイミダゾリウムカチオンとともに常温溶融塩を構成し得るアニオンとしては、ＢＦ_４ ⁻，ＰＦ_６ ⁻，ＣＦ_３ＳＯ_３ ⁻，（ＣＦ_３ＳＯ_２）_２Ｎ⁻，（Ｃ_２Ｆ_５ＳＯ_２）_２Ｎ⁻等が例示される。イミダゾリウム系常温溶融塩の具体例としては、エチルメチルイミダゾリウムテトラフルオロボレート（ＥＭＩ−ＢＦ_４）、エチルメチルイミダゾリウムトリフルオロメタンスルホニルイミド（ＥＭＩ−ＴＦＳＩ）、プロピルメチルイミダゾリウムテトラフルオロボレート等が挙げられる。
【００２１】
また、アンモニウム系の常温溶融塩を構成するカチオンとしては、下記一般式（２）で表されるもの等を使用することができる。式中のＲ^１〜Ｒ^４は、それぞれ炭素数１〜４（より好ましくは炭素数１〜３）のアルキル基である。
【００２２】
【化２】

【００２３】
このようなアンモニウムカチオンも、上述のアニオンとともに常温溶融塩を構成し得る。アンモニウム系常温溶融塩の具体例としては、ジエチルメチルプロピルアンモニウムトリフルオロメタンスルホニルイミド等が挙げられる。
【００２４】
また、ピリジニウム系の常温溶融塩を構成するカチオンとしては、下記一般式（３）で表されるもの等を使用することができる。式中のＲ^１は、炭素数１〜４（より好ましくは炭素数１〜３）のアルキル基である。
【００２５】
【化３】

【００２６】
このようなピリジニウムカチオンも、上述のアニオンとともに常温溶融塩を構成し得る。ピリジニウム系常温溶融塩の具体例としては、エチルピリジニウムテトラフルオロボレートが挙げられる。
【００２７】
このような常温溶融塩に含有させる溶質化合物としては、各種のリチウム塩を用いることができる。典型的には、常温溶融塩を構成するアニオンと同種のアニオンとリチウムイオンとの塩が溶質化合物として用いられる。特に限定するものではないが、常温溶融塩電解質の組成は、常温溶融塩：溶質化合物（リチウム塩）のモル比が１：１〜１０：１となる範囲とすることができ、２：１〜６：１となる範囲が好ましい。
【００２８】
本発明に用いられる正極および負極は、典型的には、金属製の集電体（アルミニウム箔、ニッケル箔、銅箔等）の表面に、電極活物質を含有する活物質層が設けられた構成を有する。このような活物質層は、例えば、集電体の表面に正極活物質または負極活物質を含む電極ペーストを塗布等により付着（保持）させることにより得られる。
正極活物質としては、リチウムコバルト酸化物（例えばＬｉＣｏＯ_２）、リチウム鉄酸化物、リチウムニッケル酸化物（例えばＬｉＮｉＯ_２）、リチウムニッケルコバルト酸化物、リチウムマンガン酸化物（例えばＬｉＭｎ_２Ｏ_４）等の、リチウムイオン電池（電解質の種類を問わない）の正極活物質として一般的な材料を特に限定なく用いることができる。
【００２９】
また、負極活物質としては、リチウムイオンを吸蔵放出することのできる炭素質材料（アモルファスカーボン、グラファイト等）、周期表ＩＶＢ族元素（スズ（Ｓｎ），珪素（Ｓｉ）等）の単体もしくは合金、チタン酸リチウム（例えばＬｉ_４Ｔｉ_５Ｏ_１２）等を選択し得る。負極活物質として炭素質材料を用いた負極の電位は、通常ほぼ０．５〜０Ｖの範囲にある。また、負極活物質としてスズ合金を用いた負極の電位は、通常ほぼ１．５〜０．２Ｖの範囲にある。負極活物質としてチタン酸リチウムを用いた負極の電位は、通常ほぼ１．８〜１．２Ｖの範囲にある。負極の電位が低く、電池平均電圧の高いリチウムイオン電池を構成しやすいことから、負極活物質として炭素質材料を選択することが好ましい。本発明によると、このような低電位の負極を用いた場合にも常温溶融塩の還元分解を抑制することができる。したがって、電池平均電圧が高くかつ充放電特性に優れたリチウムイオン電池を実現することが可能である。かかる炭素質材料としては、有機溶媒系の電解質（例えばプロピレンカーボネートにＬｉＰＦ_６を溶解させたもの等）を備えたリチウムイオン電池の負極活物質として用いられる材料等を特に限定なく用いることができる。
【００３０】
本発明のリチウムイオン電池の電極を構成する活物質層は、通常は活物質に加えて導電化材および／または結着剤を含有する。特に限定するものではないが、導電化材としてはカーボンブラック（ＣＢ）や黒鉛等を、結着剤としてはポリフッ化ビニリデン（ＰＶＤＦ）等を好ましく選択することができる。
これら活物質層の構成材料を含む電極ペーストを集電体に付着させる方法としては、ロールコーター、バーコーター、ドクターブレード等を用いて塗布する方法、スクリーン印刷による方法、ディッピング、滴下または散布による方法等を特に限定なく採用することができる。集電体に電極ペーストを付着させて活物質層を形成した後に、その活物質層をプレスしてもよい。これにより、（１）．活物質の密度を高めて体積当たりの電池性能を向上させる、（２）．活物質の密度を高めて内部抵抗を低減する、（３）．活物質層の形状維持性を向上させる、（４）．活物質層の集電体への密着性を向上させる、等のうち一または二以上の効果を実現し得る。
【００３１】
次に、本発明のリチウムイオン電池または負極構造体に備えられる界面層について説明する。この界面層は、負極と常温溶融塩電解質との間に介在して、電解質を構成する常温溶融塩が負極の電位に直接曝されることを回避する。これにより常温溶融塩の還元分解が抑制され、電池の充放電特性が改善される。
この界面層は実質的に電子非伝導性であることが好ましい。このことによって負極と界面層の正極側表面との間に電位勾配を効果的に生じさせることができる。ここで「実質的に電子非伝導性」とは、界面層の電子伝導度（導電率）が１０^−６Ｓ／ｃｍ未満であること（好ましくは１０^−７Ｓ／ｃｍ未満であること）をいう。あるいは、界面層の抵抗値が１０Ω以上（好ましくは１００Ω以上）であることをいう。
【００３２】
界面層の正極側表面と負極との間に生じる電位差は、この界面層の材質（電子伝導度）や界面層の厚さ等によって異なる。本発明の好ましい形態では、電池の使用時において、界面層の正極側表面（以下、単に「界面層表面」ともいう。）の電位が負極の電位よりも凡そ０．３Ｖ以上高く、より好ましい形態では凡そ０．５Ｖ以上高く、さらに好ましい形態では１．５Ｖ以上高い。かかる形態によると本発明の適用効果が特によく発揮される。界面層表面と負極との間にこのような電位差を生じ得るように界面層の材質や厚さ等を設定することが好ましい。
【００３３】
常温溶融塩の還元分解開始電位は、その常温溶融塩の種類によって異なる。例えば、典型的なイミダゾリウム系の常温溶融塩の還元分解開始電位は凡そ１．５Ｖ程度である。また、典型的なピリジニウム系の常温溶融塩の還元分解開始電位は凡そ１．７〜１．８Ｖ程度である。典型的なアンモニウム系の常温溶融塩の還元分解開始電位は凡そ０．５Ｖ程度である。このため、電池の使用時における界面層表面の好ましい電位は、その電池に含まれる常温溶融塩の種類（還元分解開始電位）によっても異なる。例えば、イミダゾリウム系の常温溶融塩を主体とする電解質を用いた電池では、電池の使用時における界面層表面の電位が凡そ１．５Ｖ以上であることが好ましく、凡そ２Ｖ以上であることがより好ましい。ピリジニウム系の常温溶融塩を主体とする電解質を用いた電池では凡そ１．８Ｖ以上（より好ましくは凡そ２．３Ｖ以上）であることが好ましく、アンモニウム系の常温溶融塩を主体とする電解質を用いた電池では凡そ０．５Ｖ以上（より好ましくは凡そ１Ｖ以上）であることが好ましい。電池の使用時において界面層表面の電位が上記範囲となるように界面層の材質や厚さ等を設定することが好ましい。
【００３４】
電池の内部抵抗を低減するという観点から、界面層のリチウムイオン伝導率は高いほうが有利である。本発明の電池または負極構造体に備えられる界面層のリチウムイオン伝導率は、１０^−６Ｓ／ｃｍ以上であることが好ましく、１０^−５Ｓ／ｃｍ以上であることがより好ましい。
この界面層にはリチウムを含有させることができる。界面層中に含有させるリチウム（リチウム成分）としては、界面層中のリチウムイオン濃度を向上させ得る化合物が好ましく、典型例としては各種のリチウム塩が挙げられる。かかるリチウム塩としては、常温溶融塩電解質を構成するリチウム塩（溶質化合物）として使用可能な化合物等を用いることができる。その電池に含まれる溶質化合物と同じ種類のリチウム塩を用いてもよく、異なる種類のリチウム塩を用いてもよい。通常は、その電池に含まれる溶質化合物と同じ種類のリチウム塩を用いることが好ましい。界面層にリチウムを含有させることにより、電池の内部抵抗をさらに低減し得る。リチウム（典型的にはリチウム塩）の含有割合は、界面層全体に対して凡そ１〜５０質量％とすることができ、凡そ１０〜３０質量％とすることが好ましい。このほか、界面層には可塑剤、界面活性剤等の公知の添加剤を必要に応じて含有させることができる。
【００３５】
かかる界面層は、典型的には負極のうち少なくとも電解質と接触し得る部分を覆うように設けられる。界面層表面の電位を高くする（負極と界面層表面との電位差を大きくする）という観点からは、界面層の厚さが大きいほうが有利である。一方、界面層の厚さが大きくなると電池の内部抵抗は上昇する傾向にある。界面層表面の電位の高さと内部抵抗の低さとを高いレベルでバランスさせるために適当な界面層の厚さは、例えば０．１〜１００μｍであり、好ましくは０．１〜１０μｍであり、より好ましくは０．１〜５μｍである。
【００３６】
このような界面層の好ましい一例として、上記「界面層形成用化合物」を還元分解して形成された界面層が挙げられる。この界面層形成用化合物としては、電解質を構成する常温溶融塩の還元分解開始電位（Ｅ_１）よりも高い還元分解開始電位（Ｅ_２）を有する化合物を用いることができる。また、電解質を構成する常温溶融塩に対して溶解・膨潤しにくい還元分解物を生じる化合物が好ましい。かかる界面層形成用化合物としては、炭酸クロロエチレン、炭酸エチレン、グリコールサルファイト等の、酸素原子を含む環状構造をもつ化合物を用いることができる。また、炭酸ピリジニウム塩、炭酸イミダゾリウム塩等の、常温溶融塩を構成し得るカチオンと炭酸イオン（ＣＯ_３ ^２−およびＨＣＯ_３ ⁻を包含する意味である。）との塩を用いてもよい。このカチオンとしては、本発明のリチウムイオン電池の常温溶融塩電解質を構成する常温溶融塩のカチオンと同種のカチオンが好ましい。また、電解質の主成分たる常温溶融塩（第一の常温溶融塩；例えばイミダゾリウム系の常温溶融塩）の還元分解開始電位に対して、より高い還元分解開始電位を有する常温溶融塩（第二の常温溶融塩；例えばピリジニウム系の常温溶融塩）を界面層形成用化合物として用いてもよい。
【００３７】
このような界面層形成用化合物を用いて界面層を形成する方法の好適例としては、正極と、負極（典型的には炭素質負極）と、界面層形成用化合物を含む常温溶融塩電解質とを用いて電池を組み立てる方法が例示される。かかる電池に対して初期充電を行うと、負極の電位が次第に低下する。このとき、界面層形成用化合物の還元分解開始電位（Ｅ_２）が常温溶融塩の還元分解開始電位（Ｅ_１）よりも高い（Ｅ_１＜Ｅ_２）ことから、常温溶融塩の還元分解が始まる前に界面層形成用化合物が還元分解して界面層が形成される。
なお、負極表面にこのような界面層が形成された負極構造体をあらかじめ作製しておき、かかる負極構造体を用いてリチウムイオン電池を組み立ててもよい。
【００３８】
本発明のリチウムイオン電池または負極構造体に備えられる界面層の他の好ましい一例としては、リチウムイオン伝導性ポリマーを主体とする界面層が挙げられる。かかるリチウムイオン伝導性ポリマーの好適例としては、ポリエチレンオキサイド、ポリプロピレンオキサイド等のポリアルキレンオキサイド類が挙げられる。また、他の好適例としてポリアルキレンオキサイド構造を有する部分を含むポリマーが挙げられる。「ポリアルキレンオキサイド構造」としては、下記構造式（４）で示されるポリエチレンオキサイド構造や、下記構造式（５）で示されるポリプロピレンオキサイド構造等が挙げられる。ポリエチレンオキサイド構造を有する部分を含むリチウムイオン伝導性ポリマーがより好ましい。これらの構造式（４），（５）におけるｎの値は特に限定されない。高いリチウムイオン伝導性を示す界面層を形成しやすいという観点からは、ｎの値が凡そ３以上（より好ましくは５以上）のポリアルキレンオキサイド構造（より好ましくはポリエチレンオキサイド構造）の部分を含むリチウムイオン伝導性ポリマーが好ましく使用される。
【００３９】
−（ＣＨ_２ＣＨ_２Ｏ）_ｎ− ・・・（４）
【００４０】
−（ＣＨ_２ＣＨ（ＣＨ_３）Ｏ）_ｎ− ・・・（５）
【００４１】
かかる「ポリアルキレンオキサイド構造部分を含むポリマー」としては、アルキレンオキサイドと他のモノマーとの共重合体、ポリアルキレンオキサイド構造を分子中に有するモノマーの重合体またはかかるモノマーと他のモノマーとの共重合体、ポリアルキレンオキサイド構造のセグメントをもつブロック共重合体およびグラフト共重合体等が例示される。「ポリアルキレンオキサイド構造を分子中に有するモノマー」としては、重合により導電性ポリマーを構成するモノマーにポリアルキレンオキサイドを反応（結合）させてなるモノマー等を用いることができる。また、あらかじめ重合されたポリマーにポリアルキレンオキサイドを結合させて該ポリマーを変性することによりリチウムイオン伝導性を付与してもよい。このようなポリアルキレンオキサイド構造部分を含むポリマーとポリアルキレンオキサイドとの双方を含む界面層であってもよい。
【００４２】
このようなリチウムイオン伝導性ポリマーを主体とする界面層は、例えば下記（１）．〜（４）．の方法により形成することができる。
（１）．あらかじめ作製された負極にリチウムイオン伝導性ポリマーを付着させて界面層を形成する方法。例えば、適当な溶媒にリチウムイオン伝導性ポリマーを溶解および／または分散させた流動性組成物を負極に塗布し、必要に応じて乾燥や加熱等の処理を施す。あるいは、固体状（粉末状、シート状等）のリチウムイオン伝導性ポリマーを負極表面で加熱溶融させることにより界面層を形成（成膜）してもよい。
【００４３】
（２）．反応によりリチウムイオン伝導性ポリマーを形成するポリマー前駆体（モノマー、オリゴマー等）を、あらかじめ作製された負極上で反応させて界面層を形成する方法。ポリマー前駆体からリチウムイオン伝導性ポリマーを形成する「反応」の典型例としては重合反応が挙げられる。この場合、ポリマー前駆体としては、重合によりリチウムイオン伝導性ポリマーを形成するモノマーおよび／またはオリゴマー等を用いることができる。重合方法としては、ポリマー前駆体またはこれを含む組成物を加熱する方法、紫外線や電子線等の高エネルギー線を照射する方法、電解重合法等を採用することができる。必要に応じて公知の重合開始剤を用いることができる。あるいは、ポリマー前駆体の架橋反応等を利用して界面層を形成してもよい。
【００４４】
（３）．負極活物質を含有するペースト（負極ペースト）にリチウムイオン伝導性ポリマーを含有（溶解および／または分散）させる。このような負極ペーストを負極集電体に付着させた後、必要に応じて乾燥、加熱等の処理を施す。これにより、下部（集電体側）は相対的に負極活物質を多く含み、上部（集電体とは反対側；正極側）は相対的にリチウムイオン伝導性ポリマーを多く含む皮膜を集電体上に形成し得る。この皮膜の上部を界面層として用いることができる。
【００４５】
（４）．前記ポリマー前駆体（重合反応等によりリチウムイオン伝導性ポリマーを形成するモノマー、オリゴマー等）を負極ペーストに含有させる。かかる負極ペーストを集電体に付着させた後、その負極ペースト中のポリマー前駆体を反応（典型的には重合反応）させる。これにより、下部は相対的に負極活物質を多く含み、上部は相対的にリチウムイオン伝導性ポリマーを多く含む皮膜を集電体上に形成し得る。この皮膜の上部を界面層として用いることができる。
【００４６】
なお、上記（１）．〜（４）．のように負極と一体的に界面層を設ける形態の他、負極とは別体として界面層（例えばシート状の界面層）を形成しておき、この界面層が負極表面に配置されるように電池を組み立ててもよい。常温溶融塩が負極表面に到達することをよりよく阻止するという観点からは、負極と一体的に界面層を設けることが好ましい。均一な皮膜が得られやすい等の理由から、上記（１）．または（２）．の界面層形成方法が好ましく採用される。
【００４７】
この発明はまた、下記の形態で実施することができる。
（形態１）
本発明のリチウムイオン電池または負極構造体に備えられる界面層が、実質的に常温溶融塩を含有しない。ここで、常温溶融塩を「実質的に含有しない」とは、界面層の形成時に、その界面層に少なくとも意図的には常温溶融塩を含有させないことをいう。この界面層は、常温溶融塩電解質池を構成する常温溶融塩を透過させにくい性質を有することが好ましい。また、この界面層は、常温溶融塩電解質池を構成する常温溶融塩に溶解したり膨潤したりしない性質を有することが好ましい。かかる界面層を備える電池によれば、常温溶融塩の還元分解がさらによく抑制され得る。
【００４８】
（形態２）
電池の使用時において、界面層表面の電位が負極の電位より凡そ０．５Ｖ以上（好ましくは凡そ１．５Ｖ以上）高い。このような場合には本発明の適用効果が特によく発揮される。この負極が炭素質負極である場合には、界面層表面の電位が負極の電位より凡そ１Ｖ以上（より好ましくは凡そ１．５Ｖ以上、さらに好ましくは凡そ２Ｖ以上）高いことが好ましい。
【００４９】
（形態３）
炭素質負極を用いて構成された電池であって、その使用時において、界面層表面の電位が凡そ１．５Ｖ以上（より好ましい態様では凡そ１．８Ｖ以上）である。かかる形態によると、イミダゾリウム系常温溶融塩（より好ましい態様ではイミダゾリウム系常温溶融塩およびピリジニウム系常温溶融塩）を含む広範囲の材料から選択される常温溶融塩を用いて電池平均電圧が高くかつ耐久性が改善されたリチウムイオン電池を構成することができる。
【００５０】
（形態４）
本発明のリチウムイオン電池または負極構造体を構成する負極の電位が凡そ１．５Ｖ以下（典型的には凡そ０〜１．５Ｖ）であり、好ましくは凡そ０〜１Ｖであり、さらに好ましくは凡そ０〜０．５Ｖである。従来、このように低電位の負極を用いたリチウムイオン電池では常温溶融塩の還元分解が特に引き起こされやすかった。このため、電池の耐久性が低下したり、使用できる常温溶融塩の種類が限定されたりする不都合があった。本発明を適用することにより、そのような不都合を解消または軽減することができる。本発明にとり特に好ましい負極は炭素質負極である。例えば、リチウムイオンを吸蔵放出することのできる炭素質材料を負極活物質とし、かかる負極活物質を含む活物質層を集電体の表面に有する炭素質負極が好ましく用いられる。
【００５１】
（形態５）
本発明のリチウムイオン電池または本発明の負極構造体を用いて構成されたリチウムイオン電池の電池平均電圧が凡そ２．５Ｖ以上（好ましくは凡そ３Ｖ以上、より好ましくは凡そ３．５Ｖ以上）である。本発明によると、かかる電池平均電圧であって、下記条件で測定された１００サイクル後の容量維持率が凡そ５０％以上（より好ましくは凡そ７０％以上）となり得るリチウムイオン電池が提供される。
［容量維持率測定方法］
２５℃の条件下、１．２ｍＡの電流値で４．１Ｖまで充電し、５分間休止した後に１．２ｍＡの電流値で３Ｖまで放電し、５分間休止した後に同電流値で再び４．１Ｖまで充電するという充放電パターンを繰り返す。所定サイクル後の電池容量および正極活物質の比容量から容量維持率（％）を算出する。
【００５２】
【実施例】以下、本発明をリチウムイオン二次電池に適用した実施例を説明するが、本発明をかかる実施例に示すものに限定することを意図したものではない。
【００５３】
＜実施例１：常温溶融塩電解質を用いた電池の作製（１）＞
１−エチル，３−メチルイミダゾリウムトリフルオロスルホニルイミド（ＥＭＩ−ＴＦＳＩ）と、リチウムトリフルオロスルホニルイミド（ＬｉＴＦＳＩ）と、炭酸クロロエチレンとを混合した。これにより、ＬｉＴＦＳＩ（リチウム塩）を１モル／リットルの濃度で含有し、炭酸クロロエチレン（界面層形成用化合物）を凡そ１モル／リットルの濃度で含有する常温溶融塩電解質（常温溶融塩電解質Ａ）を調製した。
【００５４】
ＬｉＣｏＯ_２、カーボンブラック（ＣＢ）およびポリフッ化ビニリデン（ＰＶＤＦ）をＮ−メチルピロリドンとともに混合して正極活物質ペーストを調製した。このペーストは、ＬｉＦｅＰＯ_４：ＣＢ：ＰＶＤＦをほぼ８５：１０：５の質量比で含有する。厚さ約１５μｍのアルミニウム箔（正極集電体）の両面に上記で得られた正極活物質ペーストを塗布した。その塗布物を乾燥させた後、厚み方向にプレスして、集電体の片面に約６ｍｇ／ｃｍ^２の正極活物質層が形成された正極シートを作製した。この正極シートを所定の大きさに打ち抜いて正極とした。
【００５５】
グラファイト（大阪ガス株式会社製の商標「ＮＧ２」を使用した。）およびポリフッ化ビニリデン（ＰＶＤＦ）をＮ−メチルピロリドンとともに混合して負極活物質ペーストを調製した。このペーストは、グラファイト：ＰＶＤＦをほぼ９２．５：７．５の質量比で含有する。厚さ約１５μｍの銅箔（負極集電体）の両面に上記で得られた負極活物質ペーストを塗布した。その塗布物を乾燥させた後、厚み方向にプレスして、集電体の片面に約６ｍｇ／ｃｍ^２の負極活物質層が形成された負極シートを作製した。この負極シートを所定の大きさに打ち抜いて負極とした。
【００５６】
得られた負極の負極活物質層が形成された側に、上述の常温溶融塩電解質Ａを滴下した。その上からセパレータ（ここでは多孔質ポリプロピレンフィルム）を積層配置した。セパレータの上からさらに常温溶融塩電解質Ａを滴下した後に正極を被せた。このようにして、電極面積約２ｃｍ^２、厚さ約３．２ｍｍのコイン型リチウムイオン二次電池を作製した。
【００５７】
＜実施例２：常温溶融塩電解質を用いた電池の作製（２）＞
ＥＭＩ−ＴＦＳＩとＬｉＴＦＳＩとを混合して、ＬｉＴＦＳＩ（リチウム塩）を凡そ１モル／リットルの濃度で含有する常温溶融塩電解質Ｂを調製した。
【００５８】
テトラヒドロフラン（ＴＨＦ）とポリエチレンオキサイド（ＰＥＯ；平均分子量約１０万）とを凡そ４０：６０の質量比で含有するとともに、そのＰＥＯに対して凡そ１モル／リットルの割合でＬｉＴＦＳＩを含有するリチウムイオン伝導性ポリマー溶液（流動性組成物）を調製した。実施例１と同様にして作製した負極をこの溶液中に投入し、引き上げて約８０℃でＴＨＦを揮発させた。この操作を５回繰り返すことにより、負極表面に厚さ約５μｍの界面層を形成させた。このようにして負極構造体を作製した。
【００５９】
得られた負極構造体の界面層が形成された側に、上述の常温溶融塩電解質Ｂを滴下した。その上から実施例１と同様のセパレータを積層配置し、さらに常温溶融塩電解質Ｂを滴下した後に正極を被せた。このようにして、電極面積約２ｃｍ^２、厚さ約３．２ｍｍのコイン型リチウムイオン二次電池を作製した。
【００６０】
このようにして作製された電池の概略構造を図１に模式的に示す。正極１０は、正極集電体１２の片面に正極活物質層１４を形成してなる。負極２０は、負極集電体２２の片面に負極活物質層２４を形成してなる。負極活物質層２４の表面には界面層４２が形成されている。この界面層４２および負極２０によって負極構造体４０が構成されている。正極１０と負極構造体４０とは、正極活物質層１４と界面層４２とが対向するように配置されている。それらの間に、常温溶融塩電解質３２とセパレータ３４とを備える電解質層３０が設けられている。常温溶融塩電解質３２を構成する常温溶融塩と負極２０（負極活物質層２４）との直接接触は、負極活物質層２４の表面に形成された界面層４２によって妨げられている。本実施例のリチウムイオン二次電池１はこのような構成を有する。
【００６１】
＜実施例３：常温溶融塩電解質を用いた電池の作製（３）＞
３−チオフェンカルボン酸のカルボキシル基とＰＥＯ（平均分子量約５００）の末端水酸基との脱水反応によりＰＥＯ変性チオフェンモノマーを合成した。その反応の概略を下記反応式（６）に示す。
【００６２】
【化４】

【００６３】
このＰＥＯ変性チオフェンモノマーを用いて、モノマー含有割合が５質量％であり、そのモノマーに対して凡そ１モル／リットルの割合でＬｉＴＦＳＩを含有するＴＨＦ溶液を調製した。このＴＨＦ溶液を電解液として用いた。実施例１と同様にして作製した負極を上記電解液に浸漬し、その負極に凡そ４．５Ｖの電位（貴な電位）を約１時間印加した。これにより、電解液中に含まれるモノマーが電解重合し、負極表面を覆って厚さ約２０μｍの界面層が形成された。このようにして負極構造体を作製した。電解重合により生成したポリマーは、典型的には、下記構造式（７）に示すように、ポリチオフェン構造を主鎖とし、ＰＥＯ構造の側鎖を有する分子構造をもつ。このＰＥＯ側鎖を有することにより、生成したポリマーはリチウムイオン伝導性を示す。
【００６４】
【化５】

【００６５】
得られた負極構造体と、実施例１と同様にして作製した正極と、実施例２と同様の常温溶融塩電解質Ｂとを用いて、実施例２と同様にしてコイン型リチウムイオン二次電池を作製した。
【００６６】
＜比較例１：界面層の形成されない電池の作製（１）＞
負極にリチウムイオン伝導性ポリマー溶液を塗布しない（界面層を設けない）点以外は実施例２と同様にしてコイン型リチウムイオン二次電池を作製した。
【００６７】
＜実施例４：性能評価（１）−充放電時の電圧プロファイル＞
上記実施例１〜３および比較例１で作製したリチウムイオン二次電池に対し、２５℃の条件下、０．５ｍＡの電流値で４．１Ｖまで充電し、５分間休止した後に０．５ｍＡの電流値で３Ｖまで放電し、５分間休止した後に再び４．１Ｖまで充電するという充放電パターンを繰り返した。このときの各電池の電圧プロファイルを図２に示す。
【００６８】
図２に示すように、炭素質負極の表面に界面層が設けられていない比較例１の電池の電圧プロファイルには、３〜３．５Ｖ付近に平坦な部分（電圧平坦部）がみられる。特に初期充電時（１サイクル目の充電時）に顕著である。この電圧平坦部は、電解質を構成する常温溶融塩（ここではＥＭＩ−ＴＦＳＩ）が炭素質負極の表面で還元分解されていることを表している。このように常温溶融塩が還元分解されるため、比較例１の電池ではリチウムイオンの炭素質負極への挿入・脱離を十分に行うことができなかった。すなわち、電池の充放電を適切に行うことができなかった。
【００６９】
一方、電解質に界面層形成用化合物（炭酸クロロエチレン）を含有させた実施例１の電圧プロファイルでは、初期充電時に、３Ｖ付近に一段目の電圧平坦部がみられる。この電圧平坦部は、炭酸クロロエチレンが還元分解されていることを表している。これにより負極表面に界面層が形成される。初期充電時の二段目の電圧平坦部は３．７Ｖ付近にある。これはリチウムイオンが炭素質負極に挿入される電圧に対応する。このことは、実施例１の電池が充放電に適した状態になったことを示している。１サイクル目の放電時以降は３．７Ｖ付近のみに電圧平坦部が見られるようになり、この電池が安定して充放電可能な状態になったことが判る。また、あらかじめ負極表面に界面層が設けられている実施例２および実施例３の電池では、初期充電時から３．７Ｖ付近にのみ電圧平坦部がみられる。このことは、これらの電池では１サイクル目から充放電が良好に行われていることを示唆している。なお、これら実施例１〜３の電池の電池平均電圧はいずれも約３．７〜３．８Ｖであった。
【００７０】
＜実施例５：性能評価（２）−充放電サイクルに対する容量維持率＞
上記実施例１〜３および比較例１で作製したリチウムイオン二次電池に対し、２５℃の条件下、１．２ｍＡの電流値で４．１Ｖまで充電し、５分間休止した後に１．２ｍＡの電流値で３Ｖまで放電し、５分間休止した後に同電流値で再び４．１Ｖまで充電するという充放電パターンを繰り返した。この充放電パターンを１００サイクル行った後の電池容量および正極活物質（ここではＬｉＣｏＯ_２）の比容量（ここでは１４０ｍＡｈとして計算）から容量維持率（％）を算出した。その結果を表１に示す。なお、実施例４の結果から判るように、比較例１の電池は充放電を適切に行うことができなかったため、表１には容量維持率を０％として示している。
【００７１】
【表１】

【００７２】
表１に示すように、実施例１〜３の電池はいずれも１００サイクル後の容量維持率が５０％以上と良好であった。実施例２および３の電池では、実施例１の電池よりもさらに良好な結果が得られた。これは、実施例２および３では実施例１に比べて界面層が負極表面を緻密に覆っている等の要因によるものと推察される。このため、常温溶融塩の分解がよりよく抑制されたものと考えられる。
【００７３】
＜参考例１＞
負極活物質として、グラファイト（電位約０Ｖ）に代えてＬｉ_４Ｔｉ_５Ｏ_１２（電位約１．４Ｖ）を用いた。その他の点については実施例１と同様にして負極を作製した。この負極および常温溶融塩電解質Ｂを用いて、実施例２と同様にしてコイン型リチウムイオン二次電池を作製した。この電池につき、実施例４と同様にして充放電試験を行った。
かかる電池では、負極と常温溶融塩電解質との間に界面層は設けられていないが、比較例１とは異なり、電解質を構成する常温溶融塩（ここではＥＭＩ−ＴＦＳＩ）の還元分解開始電位よりも負極の電位が高いため、常温溶融塩がが還元分解される現象はみられなかった。このリチウムイオン電池の電池平均電圧は約２．２〜２．４Ｖであった。
【００７４】
以上、本発明の具体例を詳細に説明したが、これらは例示にすぎず、特許請求の範囲を限定するものではない。特許請求の範囲に記載の技術には、以上に例示した具体例を様々に変形、変更したものが含まれる。
また、本明細書または図面に説明した技術要素は、単独であるいは各種の組み合わせによって技術的有用性を発揮するものであり、出願時請求項記載の組み合わせに限定されるものではない。また、本明細書または図面に例示した技術は複数目的を同時に達成するものであり、そのうちの一つの目的を達成すること自体で技術的有用性を持つものである。
【図面の簡単な説明】
【図１】実施例のリチウムイオン電池の概略構成を示す模式的断面図である。
【図２】実施例および比較例のリチウムイオン二次電池につき、充放電サイクルに対する電圧プロファイルを示す特性図である。
【符号の説明】
１：リチウムイオン二次電池
１０：正極
２０：負極
３２：常温溶融塩電解質
４０：負極構造体
４２：界面層[0001]
The present invention relates to a lithium ion battery using an electrolyte containing a room temperature molten salt and a method for producing the same. The present invention also relates to a negative electrode structure suitable as a component of such a lithium battery.
[0002]
2. Description of the Related Art An organic solvent-based electrolyte obtained by dissolving a lithium salt in an organic solvent (for example, LiPF in propylene carbonate)₆And the like are known). Many lithium ion batteries of this type use lithium cobaltate (LiCoO₂) Is used. As the negative electrode material, a carbonaceous material capable of occluding and releasing lithium ions is used. Usually, the potential of a negative electrode mainly composed of a carbonaceous material (hereinafter, also referred to as “carbonaceous negative electrode”) is approximately 0 V with respect to a lithium electrode (hereinafter, unless otherwise specified, the potential value is lithium electrode). The value can be converted to a potential value based on a standard hydrogen electrode by subtracting about 3 V from the potential value based on the lithium electrode.) According to the lithium ion battery configured using the negative electrode having a low potential (low potential), it is possible to obtain a battery average voltage of about 3.5 to 4 V.
On the other hand, as one form of a lithium ion battery, there is a battery using an electrolyte in which a room temperature molten salt contains a lithium salt (room temperature molten salt electrolyte). Prior art documents relating to a lithium ion battery using a room temperature molten salt electrolyte include Patent Document 1 and Patent Document 2 below.
[0003]
[Patent Document 1]
JP-A-11-86905
[Patent Document 2]
JP-A-4-349365
[0004]
However, lithium ion batteries provided with a room temperature molten salt electrolyte tend to have low durability (for example, capacity retention performance) with respect to charge / discharge cycles. In particular, when a low-potential negative electrode such as the above-mentioned carbonaceous negative electrode was used, such a tendency was remarkable. For this reason, in the above-mentioned patent document 1, the kind of the room temperature molten salt used for the electrolyte is limited. It would be useful to have a solution that could be applied to lithium ion batteries using a wider range of room temperature molten salts.
[0005]
The present invention relates to a lithium ion battery provided with an electrolyte containing a room temperature molten salt, and an object of the present invention is to provide a lithium ion battery with improved charge / discharge characteristics (durability and the like) and a method for manufacturing the same. Another object of the present invention is to provide a negative electrode structure suitable as a component of such a lithium ion battery.
[0006]
Means for Solving the Problems, Functions and Effects The present inventors have found that the above-mentioned problems can be solved by providing a predetermined interface layer between the negative electrode and the room temperature molten salt, and have completed the present invention. .
[0007]
The present invention relates to a lithium ion battery provided with a positive electrode, a negative electrode, and an electrolyte containing a normal temperature molten salt. On the surface of the negative electrode, an interface layer that prevents direct contact between the room temperature molten salt and the negative electrode is provided. The interface layer has lithium ion conductivity. The interface layer is substantially non-conductive at the negative electrode potential when the battery is used.
In this specification, “when using a battery” refers to a state in which the battery can generate electromotive force. Further, “electron non-conductivity” or “substantially electron non-conductivity” means that the battery is substantially electron non-conductive (electrically insulating) at the negative electrode potential when the battery is used.
[0008]
Generally, a room temperature molten salt has a property that it is easily decomposed by decomposition when exposed to a potential lower than a predetermined potential (hereinafter, the predetermined potential, that is, the upper limit potential at which the room temperature molten salt can be reductively decomposed, is referred to as the room temperature molten salt. Also referred to as the "reduction decomposition onset potential" of the salt.) When the room-temperature molten salt constituting the electrolyte is decomposed, battery performance (charge / discharge characteristics and the like) deteriorates.
According to the configuration of the present invention, since the interface layer provided on the negative electrode surface is non-electroconductive, the negative electrode surface and the positive electrode side surface of the interface layer (the side exposed to the room temperature molten salt electrolyte; ), And the potential on the surface of the interface layer becomes higher than the potential on the surface of the negative electrode. Therefore, the phenomenon that the room temperature molten salt is exposed to a low potential (potential lower than the reductive decomposition onset potential) and reductively decomposed can be suppressed as compared with the case where such an interface layer is not provided. This improves the charge / discharge characteristics (capacity maintenance performance and the like) of the battery.
[0009]
In a preferred embodiment of the present invention, the negative electrode is mainly composed of a carbonaceous material. As described above, such a carbonaceous negative electrode typically has a low potential of about 0 V (about -3 V with respect to a standard hydrogen electrode). For this reason, the problem of reductive decomposition of a room temperature molten salt is likely to occur in a lithium ion battery configured using a carbonaceous negative electrode. Therefore, the benefits of applying the present invention are great. Such a lithium ion battery is preferable because the battery average voltage can be high and the charge / discharge characteristics can be improved. Here, the “battery average voltage” refers to a time average of voltage values when the battery is charged and discharged at a constant current at an upper limit voltage and a lower limit voltage without cycle deterioration.
[0010]
The interface layer may contain lithium (typically a lithium salt). Thereby, the performance of the lithium ion battery of the present invention can be further improved. For example, the internal resistance of the battery can be reduced.
[0011]
In the lithium ion battery of the present invention, not only the room temperature molten salt having a relatively low reductive decomposition onset potential (which is less susceptible to reductive decomposition), but also a relatively high reductive decomposition onset potential (which is easily reductively decomposed) is suitable. Can be used. According to the present invention, for example, even when a room temperature molten salt electrolyte mainly composed of a room temperature molten salt having a reductive decomposition onset potential of about 1 V or more (about −2 V or more based on a standard hydrogen electrode) is used, charge and discharge are performed. A lithium ion battery with good characteristics can be constructed. As described above, since the restriction on the onset potential for reductive decomposition can be relaxed, the room-temperature molten salt used in the lithium ion battery of the present invention can be selected from a wide range of materials.
[0012]
Further, according to the present invention, a lithium ion battery before initial charging is provided. The battery includes a positive electrode, a negative electrode, and an electrolyte containing a room-temperature molten salt and a compound for forming an interface layer. The compound for forming an interface layer is reduced and decomposed to form an interface layer on the surface of the negative electrode that prevents direct contact between the room-temperature molten salt and the negative electrode. The interface layer has lithium ion conductivity. Also, the interface layer is substantially non-conductive. The lower limit potential that the negative electrode can take when using this battery is E₀The reductive decomposition onset potential of the room-temperature molten salt is represented by E₁, The reductive decomposition onset potential of the interfacial layer forming compound₂, These potentials become E₀<E₁<E₂Satisfy the relationship.
When initial charging is performed on such a lithium ion battery (lithium ion battery before initial charging), E₀<E₂Therefore, the compound for forming the interface layer contained in the electrolyte is reductively decomposed on the surface of the negative electrode and / or in the vicinity thereof. The reduced decomposition products can form an interface layer on the negative electrode surface that prevents direct contact between the room-temperature molten salt and the negative electrode. Thereby, any of the above-described lithium ion batteries of the present invention can be formed. Also, E₁<E₂Therefore, when the potential of the negative electrode is lowered by the initial charge, the compound for forming the interface layer is reduced and decomposed before the reductive decomposition of the room-temperature molten salt is started, and an interface layer is formed on the surface of the negative electrode. Therefore, reductive decomposition of the room temperature molten salt can be prevented beforehand.
[0013]
According to the present invention, there is provided a method of manufacturing a lithium ion battery including a positive electrode, a negative electrode, and an electrolyte containing a room temperature molten salt. In the manufacturing method, an interface layer is formed on the negative electrode surface. The interface layer has lithium ion conductivity. The interface layer is substantially non-conductive. And the interface layer does not substantially contain a room temperature molten salt. A battery is formed using the negative electrode on which such an interface layer is formed. Such a manufacturing method is suitable as a method for manufacturing the lithium ion battery of the present invention.
[0014]
Further, according to the present invention, there is provided a negative electrode structure including a negative electrode and an interface layer formed on a surface of the negative electrode. The interface layer has lithium ion conductivity. Also, the interface layer is substantially non-conductive. The interface layer does not substantially contain the room temperature molten salt. This negative electrode structure is particularly suitable as a component of a lithium ion battery provided with an electrolyte containing a room temperature molten salt. In a preferred embodiment of the negative electrode structure, the negative electrode is mainly composed of a carbonaceous material. Such a negative electrode structure is typically manufactured by forming the above-mentioned interface layer on the negative electrode surface.
The negative electrode structure of the present invention is not limited to use as a constituent element of a lithium ion battery using a room temperature molten salt electrolyte, but may be used for other types of lithium ion batteries (for example, lithium with an organic solvent-based electrolyte). (Ion battery).
[0015]
In manufacturing such a lithium ion battery or a negative electrode structure, a preferable example of a method of forming an interface layer is a method of applying a composition containing a lithium ion conductive polymer to a negative electrode surface. Alternatively, a composition containing a polymer precursor (typically, a monomer) that forms a lithium ion conductive polymer by polymerization may be brought into contact with the surface of the negative electrode, and the polymer precursor may be polymerized to form an interface layer.
In another preferred example of the method for forming an interface layer on the surface of the negative electrode, an electrolytic solution containing a polymer precursor (typically, a monomer) for forming a conductive polymer by polymerization is used. The interface layer can be formed by bringing a negative electrode into contact with such an electrolytic solution, applying a noble potential to the negative electrode, and subjecting the precursor to electrolytic polymerization. The conductive polymer produced by the electrolytic polymerization preferably exhibits substantially non-electroconductivity (electrical insulation) at the negative electrode potential when the battery is used. The conductive polymer preferably has a lithium ion conductive side chain.
[0016]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. It should be noted that technical matters other than those specifically mentioned in the present specification and necessary for implementing the present invention can be grasped as design matters of those skilled in the art based on the conventional technology. The present invention can be carried out based on the technical contents disclosed in the present specification and common technical knowledge in the relevant field.
[0017]
The lithium ion battery of the present invention includes an electrolyte containing a room temperature molten salt. Typically, there is provided an electrolyte (room temperature molten salt electrolyte) mainly composed of a room temperature molten salt, to which a solute compound (a compound contributing to a battery reaction: a lithium salt in a lithium ion battery) is added. In the following description, a “lithium ion battery” means a lithium ion battery provided with an electrolyte containing a room temperature molten salt (room temperature molten salt electrolyte) unless otherwise specified.
[0018]
As the room temperature molten salt, an appropriate one can be selected from ordinary imidazolium, ammonium, pyridinium and other room temperature molten salts.
As the cation constituting the imidazolium-based room-temperature molten salt, those represented by the following general formula (1) can be used. R in the formula¹And R²Is an alkyl group having 1 to 4 carbon atoms (more preferably 1 to 3 carbon atoms).
[0019]
Embedded image

[0020]
Anions that can form a room-temperature molten salt together with such imidazolium cations include BF.₄ ⁻, PF₆ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, (C₂F₅SO₂)₂N⁻Etc. are exemplified. Specific examples of the imidazolium-based room-temperature molten salt include ethyl methyl imidazolium tetrafluoroborate (EMI-BF₄), Ethylmethylimidazolium trifluoromethanesulfonylimide (EMI-TFSI), propylmethylimidazolium tetrafluoroborate, and the like.
[0021]
In addition, as the cation constituting the ammonium-based room-temperature molten salt, a cation represented by the following general formula (2) can be used. R in the formula¹~ R⁴Is an alkyl group having 1 to 4 carbon atoms (more preferably 1 to 3 carbon atoms).
[0022]
Embedded image

[0023]
Such an ammonium cation can also form a room-temperature molten salt together with the anion described above. Specific examples of the ammonium-based room temperature molten salt include diethylmethylpropylammonium trifluoromethanesulfonylimide.
[0024]
Further, as the cation constituting the pyridinium-based room-temperature molten salt, a cation represented by the following general formula (3) can be used. R in the formula¹Is an alkyl group having 1 to 4 carbon atoms (more preferably 1 to 3 carbon atoms).
[0025]
Embedded image

[0026]
Such a pyridinium cation can also form a room-temperature molten salt together with the anion described above. Specific examples of the pyridinium-based room temperature molten salt include ethylpyridinium tetrafluoroborate.
[0027]
As a solute compound to be contained in such a room temperature molten salt, various lithium salts can be used. Typically, a salt of an anion of the same kind as the anion constituting the room-temperature molten salt and a lithium ion is used as the solute compound. Although not particularly limited, the composition of the room temperature molten salt electrolyte can be in a range where the molar ratio of room temperature molten salt: solute compound (lithium salt) is 1: 1 to 10: 1, and 2: 1 to 1: 1. A range of 6: 1 is preferable.
[0028]
The positive electrode and the negative electrode used in the present invention typically have a structure in which an active material layer containing an electrode active material is provided on a surface of a metal current collector (aluminum foil, nickel foil, copper foil, or the like). Having. Such an active material layer is obtained, for example, by attaching (holding) an electrode paste containing a positive electrode active material or a negative electrode active material to the surface of the current collector by coating or the like.
As the positive electrode active material, lithium cobalt oxide (for example, LiCoO₂), Lithium iron oxide, lithium nickel oxide (eg, LiNiO₂), Lithium nickel cobalt oxide, lithium manganese oxide (eg, LiMn₂O₄) Can be used without particular limitation as a positive electrode active material for a lithium ion battery (regardless of the type of electrolyte).
[0029]
Examples of the negative electrode active material include a carbonaceous material (amorphous carbon, graphite, etc.) capable of inserting and extracting lithium ions, a simple substance or an alloy of a Group IVB element of the periodic table (tin (Sn), silicon (Si), etc.), Lithium titanate (eg, Li₄Ti₅O₁₂) Etc. can be selected. The potential of a negative electrode using a carbonaceous material as the negative electrode active material is generally in the range of approximately 0.5 to 0V. The potential of a negative electrode using a tin alloy as the negative electrode active material is generally in the range of approximately 1.5 to 0.2V. The potential of the negative electrode using lithium titanate as the negative electrode active material is generally in the range of about 1.8 to 1.2 V. It is preferable to select a carbonaceous material as the negative electrode active material because a lithium ion battery having a low negative electrode potential and a high average battery voltage is easily formed. According to the present invention, even when such a low-potential negative electrode is used, reductive decomposition of a room-temperature molten salt can be suppressed. Therefore, it is possible to realize a lithium ion battery having a high battery average voltage and excellent charge / discharge characteristics. Such carbonaceous materials include organic solvent-based electrolytes (for example, propylene carbonate and LiPF).₆Can be used without particular limitation as the material used as the negative electrode active material of the lithium ion battery provided with the above.
[0030]
The active material layer constituting the electrode of the lithium ion battery of the present invention usually contains a conductive material and / or a binder in addition to the active material. Although not particularly limited, carbon black (CB), graphite or the like can be preferably selected as the conductive material, and polyvinylidene fluoride (PVDF) or the like can be preferably selected as the binder.
As a method of attaching the electrode paste containing the constituent material of these active material layers to the current collector, a method of applying using a roll coater, a bar coater, a doctor blade, or the like, a method by screen printing, a method by dipping, dropping or spraying Etc. can be adopted without particular limitation. After the electrode paste is attached to the current collector to form an active material layer, the active material layer may be pressed. Thereby, (1). (2) increasing the density of the active material to improve the battery performance per volume; (3) to increase the density of the active material and reduce the internal resistance. (4) improving the shape retention of the active material layer; One or more effects of improving the adhesion of the active material layer to the current collector and the like can be realized.
[0031]
Next, the interface layer provided in the lithium ion battery or the negative electrode structure of the present invention will be described. This interface layer is interposed between the negative electrode and the room temperature molten salt electrolyte to prevent the room temperature molten salt constituting the electrolyte from being directly exposed to the potential of the negative electrode. Thereby, the reductive decomposition of the room temperature molten salt is suppressed, and the charge / discharge characteristics of the battery are improved.
Preferably, the interface layer is substantially non-conductive. Thereby, a potential gradient can be effectively generated between the negative electrode and the positive electrode side surface of the interface layer. Here, “substantially non-conductive” means that the electronic conductivity (conductivity) of the interface layer is 10%.^-6S / cm (preferably 10^-7S / cm). Alternatively, it means that the resistance value of the interface layer is 10Ω or more (preferably 100Ω or more).
[0032]
The potential difference between the positive electrode side surface of the interface layer and the negative electrode varies depending on the material (electron conductivity) of the interface layer, the thickness of the interface layer, and the like. In a preferred embodiment of the present invention, when the battery is used, the potential of the positive electrode side surface of the interface layer (hereinafter, also simply referred to as “interface layer surface”) is higher than the potential of the negative electrode by about 0.3 V or more. In this case, the voltage is about 0.5 V or more, and in a more preferred embodiment, it is 1.5 V or more. According to this mode, the application effects of the present invention are particularly well exhibited. It is preferable to set the material and thickness of the interface layer so that such a potential difference can be generated between the surface of the interface layer and the negative electrode.
[0033]
The reductive decomposition onset potential of the room temperature molten salt varies depending on the type of the room temperature molten salt. For example, the reductive decomposition onset potential of a typical imidazolium-based room temperature molten salt is about 1.5 V. In addition, the reductive decomposition onset potential of a typical pyridinium-based room temperature molten salt is about 1.7 to 1.8 V. A typical ammonium-based room temperature molten salt has an onset potential for reductive decomposition of about 0.5V. For this reason, the preferable potential on the surface of the interface layer when the battery is used also differs depending on the type of room-temperature molten salt (reduction decomposition starting potential) contained in the battery. For example, in a battery using an electrolyte mainly composed of an imidazolium-based room-temperature molten salt, the potential of the interface layer surface during use of the battery is preferably about 1.5 V or more, and more preferably about 2 V or more. preferable. In a battery using an electrolyte mainly composed of a pyridinium-based room temperature molten salt, the voltage is preferably about 1.8 V or more (more preferably, about 2.3 V or more). It is preferable that the battery has a voltage of about 0.5 V or more (more preferably, about 1 V or more). It is preferable to set the material and thickness of the interface layer so that the potential of the interface layer surface is in the above range when the battery is used.
[0034]
From the viewpoint of reducing the internal resistance of the battery, it is advantageous that the lithium ion conductivity of the interface layer is higher. The lithium ion conductivity of the interface layer provided in the battery or the negative electrode structure of the present invention is 10^-6S / cm or more, preferably 10^-5More preferably, it is S / cm or more.
This interface layer can contain lithium. As the lithium (lithium component) contained in the interface layer, a compound capable of improving the lithium ion concentration in the interface layer is preferable, and typical examples include various lithium salts. As such a lithium salt, a compound or the like which can be used as a lithium salt (solute compound) constituting a room-temperature molten salt electrolyte can be used. The same kind of lithium salt as the solute compound contained in the battery may be used, or a different kind of lithium salt may be used. Usually, it is preferable to use the same kind of lithium salt as the solute compound contained in the battery. By including lithium in the interface layer, the internal resistance of the battery can be further reduced. The content ratio of lithium (typically, lithium salt) can be approximately 1 to 50% by mass, and preferably approximately 10 to 30% by mass, based on the entire interface layer. In addition, known additives such as a plasticizer and a surfactant can be contained in the interface layer as needed.
[0035]
Such an interface layer is typically provided so as to cover at least a portion of the negative electrode that can come into contact with the electrolyte. From the viewpoint of increasing the potential on the surface of the interface layer (increase the potential difference between the negative electrode and the surface of the interface layer), it is advantageous to increase the thickness of the interface layer. On the other hand, as the thickness of the interface layer increases, the internal resistance of the battery tends to increase. The appropriate thickness of the interface layer for balancing the high potential of the interface layer surface and the low internal resistance at a high level is, for example, 0.1 to 100 μm, preferably 0.1 to 10 μm, Preferably it is 0.1-5 μm.
[0036]
A preferred example of such an interface layer is an interface layer formed by reducing and decomposing the above “compound for forming an interface layer”. As the compound for forming the interface layer, the reductive decomposition onset potential (E₁) Than the reductive decomposition onset potential (E₂) Can be used. Further, a compound which produces a reduced decomposition product which is hardly dissolved and swelled in a room temperature molten salt constituting an electrolyte is preferable. As the compound for forming the interface layer, a compound having a cyclic structure containing an oxygen atom, such as chloroethylene carbonate, ethylene carbonate, and glycol sulfite, can be used. Further, a cation capable of forming a room-temperature molten salt such as a pyridinium carbonate salt or an imidazolium carbonate salt and a carbonate ion (CO₃ ^2-And HCO₃ ⁻It is meant to include. ) May be used. The cation is preferably the same cation as the cation of the room temperature molten salt constituting the room temperature molten salt electrolyte of the lithium ion battery of the present invention. Further, a room temperature molten salt (second room temperature molten salt having a higher reductive decomposition onset potential than a room temperature molten salt (first room temperature molten salt; for example, an imidazolium-based room temperature molten salt)) as a main component of the electrolyte is used. Room temperature molten salt; for example, a pyridinium-based room temperature molten salt) may be used as the compound for forming the interface layer.
[0037]
Preferred examples of the method for forming an interface layer using such an interface layer forming compound include a positive electrode, a negative electrode (typically, a carbonaceous negative electrode), and a room temperature molten salt electrolyte containing the interface layer forming compound. A method of assembling a battery using the above is exemplified. When the battery is initially charged, the potential of the negative electrode gradually decreases. At this time, the reductive decomposition onset potential (E₂) Is the reductive decomposition onset potential (E₁) Higher than (E₁<E₂Therefore, before the reductive decomposition of the room temperature molten salt starts, the interfacial layer forming compound undergoes reductive decomposition to form an interfacial layer.
Note that a negative electrode structure having such an interface layer formed on the negative electrode surface may be prepared in advance, and a lithium ion battery may be assembled using the negative electrode structure.
[0038]
Another preferable example of the interface layer provided in the lithium ion battery or the negative electrode structure of the present invention is an interface layer mainly composed of a lithium ion conductive polymer. Preferred examples of the lithium ion conductive polymer include polyalkylene oxides such as polyethylene oxide and polypropylene oxide. Another preferred example is a polymer containing a portion having a polyalkylene oxide structure. Examples of the “polyalkylene oxide structure” include a polyethylene oxide structure represented by the following structural formula (4) and a polypropylene oxide structure represented by the following structural formula (5). A lithium ion conductive polymer containing a portion having a polyethylene oxide structure is more preferred. The value of n in these structural formulas (4) and (5) is not particularly limited. From the viewpoint that an interface layer exhibiting high lithium ion conductivity is easily formed, lithium containing a portion of a polyalkylene oxide structure (more preferably, a polyethylene oxide structure) having an n value of about 3 or more (more preferably 5 or more) is used. Ion conductive polymers are preferably used.
[0039]
− (CH₂CH₂O)_n-... (4)
[0040]
− (CH₂CH (CH₃) O)_n-... (5)
[0041]
Examples of the “polymer containing a polyalkylene oxide structure” include a copolymer of an alkylene oxide and another monomer, a polymer of a monomer having a polyalkylene oxide structure in a molecule, or a copolymer of such a monomer and another monomer. Examples thereof include a copolymer, a block copolymer having a segment of a polyalkylene oxide structure, and a graft copolymer. As the “monomer having a polyalkylene oxide structure in the molecule”, a monomer obtained by reacting (bonding) a polyalkylene oxide with a monomer constituting a conductive polymer by polymerization can be used. Alternatively, lithium ion conductivity may be imparted by bonding a polyalkylene oxide to a polymer that has been polymerized in advance to modify the polymer. An interface layer containing both a polymer containing such a polyalkylene oxide structural portion and a polyalkylene oxide may be used.
[0042]
Such an interface layer mainly composed of a lithium ion conductive polymer is described in, for example, the following (1). -(4). The method can be used.
(1). A method of forming an interface layer by attaching a lithium ion conductive polymer to a previously prepared negative electrode. For example, a fluid composition obtained by dissolving and / or dispersing a lithium ion conductive polymer in an appropriate solvent is applied to the negative electrode, and subjected to a treatment such as drying and heating as necessary. Alternatively, the interface layer may be formed (formed) by heating and melting a solid (powder, sheet, or the like) lithium ion conductive polymer on the surface of the negative electrode.
[0043]
(2). A method in which a polymer precursor (monomer, oligomer, or the like) that forms a lithium ion conductive polymer by a reaction is reacted on a previously prepared negative electrode to form an interface layer. A typical example of the “reaction” for forming a lithium ion conductive polymer from a polymer precursor is a polymerization reaction. In this case, a monomer and / or oligomer that forms a lithium ion conductive polymer by polymerization can be used as the polymer precursor. As a polymerization method, a method of heating a polymer precursor or a composition containing the same, a method of irradiating a high energy beam such as an ultraviolet ray or an electron beam, an electrolytic polymerization method, and the like can be employed. Known polymerization initiators can be used if necessary. Alternatively, the interface layer may be formed by utilizing a crosslinking reaction of a polymer precursor or the like.
[0044]
(3). A lithium ion conductive polymer is contained (dissolved and / or dispersed) in a paste containing the negative electrode active material (negative electrode paste). After such a negative electrode paste is adhered to the negative electrode current collector, processing such as drying and heating is performed as necessary. As a result, the lower portion (current collector side) contains a relatively large amount of the negative electrode active material, and the upper portion (the opposite side to the current collector; the positive electrode side) forms a film containing a relatively large amount of lithium ion conductive polymer. Can be formed on. The top of this film can be used as an interface layer.
[0045]
(4). The polymer precursor (a monomer, an oligomer, or the like that forms a lithium ion conductive polymer by a polymerization reaction or the like) is contained in a negative electrode paste. After attaching the negative electrode paste to the current collector, the polymer precursor in the negative electrode paste is reacted (typically, a polymerization reaction). Accordingly, a film containing a relatively large amount of the negative electrode active material at the lower portion and a film containing a relatively large amount of the lithium ion conductive polymer at the upper portion can be formed on the current collector. The top of this film can be used as an interface layer.
[0046]
The above (1). -(4). In addition to the embodiment in which the interface layer is provided integrally with the negative electrode, an interface layer (for example, a sheet-like interface layer) is formed separately from the negative electrode, and the interface layer is disposed on the negative electrode surface. The battery may be assembled. From the viewpoint of better preventing the room temperature molten salt from reaching the negative electrode surface, it is preferable to provide an interface layer integrally with the negative electrode. For reasons such as easy formation of a uniform film, the above (1). Or (2). Is preferably employed.
[0047]
The present invention can also be carried out in the following modes.
(Form 1)
The interface layer provided in the lithium ion battery or the negative electrode structure of the present invention does not substantially contain a room-temperature molten salt. Here, "substantially not containing a room temperature molten salt" means that the room temperature molten salt is not at least intentionally contained in the interface layer when the interface layer is formed. It is preferable that the interface layer has a property that the room temperature molten salt constituting the room temperature molten salt electrolyte pond is hardly permeated. The interface layer preferably has a property of not dissolving or swelling in the room temperature molten salt constituting the room temperature molten salt electrolyte pond. According to the battery including such an interface layer, the reductive decomposition of the room-temperature molten salt can be further suppressed.
[0048]
(Form 2)
During use of the battery, the potential of the interface layer surface is higher than the potential of the negative electrode by about 0.5 V or more (preferably, about 1.5 V or more). In such a case, the application effects of the present invention are particularly well exhibited. When the negative electrode is a carbonaceous negative electrode, the potential of the interface layer surface is preferably higher than the potential of the negative electrode by about 1 V or more (more preferably about 1.5 V or more, and still more preferably about 2 V or more).
[0049]
(Form 3)
A battery constituted by using a carbonaceous negative electrode, and in use, the potential of the interface layer surface is about 1.5 V or more (more preferably, about 1.8 V or more in a more preferable embodiment). According to this embodiment, the average battery voltage is high using a room temperature molten salt selected from a wide range of materials including imidazolium room temperature molten salt (in a more preferred embodiment, an imidazolium room temperature molten salt and a pyridinium room temperature molten salt), and A lithium ion battery with improved durability can be configured.
[0050]
(Form 4)
The potential of the negative electrode constituting the lithium ion battery or the negative electrode structure of the present invention is about 1.5 V or less (typically about 0 to 1.5 V), preferably about 0 to 1 V, and more preferably about 0 V. 0 to 0.5V. Conventionally, in a lithium ion battery using such a low-potential negative electrode, reductive decomposition of a room-temperature molten salt was particularly easily caused. For this reason, there were inconveniences that the durability of the battery was lowered and the types of room temperature molten salts that could be used were limited. By applying the present invention, such inconveniences can be eliminated or reduced. A particularly preferred anode for the present invention is a carbonaceous anode. For example, a carbonaceous negative electrode having a carbonaceous material capable of inserting and extracting lithium ions as a negative electrode active material and having an active material layer containing such a negative electrode active material on the surface of a current collector is preferably used.
[0051]
(Form 5)
The battery average voltage of the lithium ion battery of the present invention or the lithium ion battery formed using the negative electrode structure of the present invention is about 2.5 V or more (preferably about 3 V or more, more preferably about 3.5 V or more). . According to the present invention, there is provided a lithium ion battery capable of maintaining the battery average voltage at a capacity retention rate after 100 cycles measured under the following conditions of about 50% or more (more preferably about 70% or more).
[Capacity maintenance rate measurement method]
Under a condition of 25 ° C., the battery was charged to 4.1 V at a current of 1.2 mA, paused for 5 minutes, discharged to 3 V at a current of 1.2 mA, paused for 5 minutes, and then reactivated at 4.1 V at the same current value. The charge / discharge pattern of charging until is repeated. The capacity retention rate (%) is calculated from the battery capacity after a predetermined cycle and the specific capacity of the positive electrode active material.
[0052]
EXAMPLES Hereinafter, examples in which the present invention is applied to a lithium ion secondary battery will be described, but the present invention is not intended to be limited to those shown in the examples.
[0053]
<Example 1: Fabrication of battery using room temperature molten salt electrolyte (1)>
1-ethyl, 3-methylimidazolium trifluorosulfonylimide (EMI-TFSI), lithium trifluorosulfonylimide (LiTFSI), and chloroethylene carbonate were mixed. Thereby, a room temperature molten salt electrolyte (room temperature molten salt electrolyte A) containing LiTFSI (lithium salt) at a concentration of 1 mol / l and chloroethylene carbonate (compound for forming an interface layer) at a concentration of approximately 1 mol / l. ) Was prepared.
[0054]
LiCoO₂, Carbon black (CB) and polyvinylidene fluoride (PVDF) were mixed with N-methylpyrrolidone to prepare a positive electrode active material paste. This paste is LiFePO₄: CB: PVDF in a mass ratio of approximately 85: 10: 5. The positive electrode active material paste obtained above was applied to both surfaces of an aluminum foil (positive electrode current collector) having a thickness of about 15 μm. After the coated material was dried, it was pressed in the thickness direction, and about 6 mg / cm²The positive electrode sheet on which the positive electrode active material layer was formed was produced. This positive electrode sheet was punched into a predetermined size to obtain a positive electrode.
[0055]
Graphite (trade name "NG2" manufactured by Osaka Gas Co., Ltd. was used) and polyvinylidene fluoride (PVDF) were mixed with N-methylpyrrolidone to prepare a negative electrode active material paste. This paste contains graphite: PVDF in a mass ratio of approximately 92.5: 7.5. The negative electrode active material paste obtained above was applied to both surfaces of a copper foil (negative electrode current collector) having a thickness of about 15 μm. After the coated material was dried, it was pressed in the thickness direction, and about 6 mg / cm²The negative electrode sheet on which the negative electrode active material layer was formed was produced. This negative electrode sheet was punched into a predetermined size to obtain a negative electrode.
[0056]
The above-mentioned room temperature molten salt electrolyte A was dropped on the side of the obtained negative electrode on which the negative electrode active material layer was formed. A separator (here, a porous polypropylene film) was laminated and arranged thereon. After the room temperature molten salt electrolyte A was further dropped from above the separator, the positive electrode was covered. Thus, the electrode area is about 2 cm²And a coin-type lithium ion secondary battery having a thickness of about 3.2 mm.
[0057]
<Example 2: Production of battery using room temperature molten salt electrolyte (2)>
EMI-TFSI and LiTFSI were mixed to prepare a room temperature molten salt electrolyte B containing LiTFSI (lithium salt) at a concentration of about 1 mol / liter.
[0058]
Lithium ion conductor containing tetrahydrofuran (THF) and polyethylene oxide (PEO; average molecular weight of about 100,000) at a mass ratio of about 40:60 and LiTFSI at a ratio of about 1 mol / l to the PEO. A water-soluble polymer solution (flowable composition) was prepared. A negative electrode produced in the same manner as in Example 1 was put into this solution, pulled up, and THF was volatilized at about 80 ° C. By repeating this operation five times, an interface layer having a thickness of about 5 μm was formed on the negative electrode surface. Thus, a negative electrode structure was produced.
[0059]
The above-mentioned room temperature molten salt electrolyte B was dropped on the side of the obtained negative electrode structure on which the interface layer was formed. A separator similar to that of Example 1 was stacked and disposed thereon, and a room temperature molten salt electrolyte B was further dropped, and then a positive electrode was covered. Thus, the electrode area is about 2 cm²And a coin-type lithium ion secondary battery having a thickness of about 3.2 mm.
[0060]
FIG. 1 schematically shows a schematic structure of the battery thus manufactured. The positive electrode 10 is formed by forming a positive electrode active material layer 14 on one surface of a positive electrode current collector 12. The negative electrode 20 is formed by forming a negative electrode active material layer 24 on one surface of a negative electrode current collector 22. An interface layer 42 is formed on the surface of the negative electrode active material layer 24. The negative electrode structure 40 is configured by the interface layer 42 and the negative electrode 20. The positive electrode 10 and the negative electrode structure 40 are arranged such that the positive electrode active material layer 14 and the interface layer 42 face each other. An electrolyte layer 30 including a room temperature molten salt electrolyte 32 and a separator 34 is provided between them. The direct contact between the room temperature molten salt constituting the room temperature molten salt electrolyte 32 and the negative electrode 20 (negative electrode active material layer 24) is prevented by the interface layer 42 formed on the surface of the negative electrode active material layer 24. The lithium ion secondary battery 1 of the present embodiment has such a configuration.
[0061]
<Example 3: Production of battery using room temperature molten salt electrolyte (3)>
A PEO-modified thiophene monomer was synthesized by a dehydration reaction between a carboxyl group of 3-thiophene carboxylic acid and a terminal hydroxyl group of PEO (average molecular weight: about 500). The outline of the reaction is shown in the following reaction formula (6).
[0062]
Embedded image

[0063]
Using this PEO-modified thiophene monomer, a THF solution containing a monomer content of 5% by mass and containing LiTFSI at a rate of about 1 mol / liter with respect to the monomer was prepared. This THF solution was used as an electrolyte. A negative electrode produced in the same manner as in Example 1 was immersed in the above-mentioned electrolytic solution, and a potential (noble potential) of about 4.5 V was applied to the negative electrode for about 1 hour. As a result, the monomer contained in the electrolytic solution was electrolytically polymerized, and an interface layer having a thickness of about 20 μm was formed covering the surface of the negative electrode. Thus, a negative electrode structure was produced. The polymer produced by the electrolytic polymerization typically has a molecular structure having a polythiophene structure as a main chain and side chains of a PEO structure, as shown in the following structural formula (7). By having this PEO side chain, the produced polymer exhibits lithium ion conductivity.
[0064]
Embedded image

[0065]
Using the obtained negative electrode structure, the positive electrode produced in the same manner as in Example 1, and the room-temperature molten salt electrolyte B in the same manner as in Example 2, a coin-type lithium ion secondary battery in the same manner as in Example 2 Was prepared.
[0066]
<Comparative Example 1: Production of battery without formation of interface layer (1)>
A coin-type lithium ion secondary battery was produced in the same manner as in Example 2, except that the lithium ion conductive polymer solution was not applied to the negative electrode (the interface layer was not provided).
[0067]
<Example 4: Performance evaluation (1)-Voltage profile during charging and discharging>
The lithium ion secondary batteries prepared in Examples 1 to 3 and Comparative Example 1 were charged to 4.1 V at a current value of 0.5 mA under a condition of 25 ° C., and after a pause of 5 minutes, a current of 0.5 mA was applied. A charge / discharge pattern of discharging to a current value of 3 V, suspending for 5 minutes, and then charging again to 4.1 V was repeated. FIG. 2 shows the voltage profile of each battery at this time.
[0068]
As shown in FIG. 2, in the voltage profile of the battery of Comparative Example 1 in which the interface layer was not provided on the surface of the carbonaceous negative electrode, a flat portion (voltage flat portion) was observed around 3 to 3.5 V. This is particularly noticeable at the time of initial charging (at the time of charging in the first cycle). This voltage flat portion indicates that the room temperature molten salt (here, EMI-TFSI) constituting the electrolyte is reductively decomposed on the surface of the carbonaceous negative electrode. As described above, since the room temperature molten salt is reductively decomposed, in the battery of Comparative Example 1, lithium ions could not be sufficiently inserted and removed from the carbonaceous negative electrode. That is, charging and discharging of the battery could not be properly performed.
[0069]
On the other hand, in the voltage profile of Example 1 in which the compound for forming an interface layer (chloroethylene carbonate) was contained in the electrolyte, a first-stage voltage flat portion was observed at around 3 V during the initial charging. This voltage flat portion indicates that chloroethylene carbonate is reductively decomposed. Thereby, an interface layer is formed on the negative electrode surface. The voltage flat portion of the second stage at the time of the initial charge is around 3.7V. This corresponds to the voltage at which lithium ions are inserted into the carbonaceous anode. This indicates that the battery of Example 1 was in a state suitable for charging and discharging. After the discharge in the first cycle, a voltage flat portion is observed only at around 3.7 V, which indicates that the battery is stably charged and discharged. Further, in the batteries of Examples 2 and 3 in which an interface layer was previously provided on the surface of the negative electrode, a voltage flat portion was observed only around 3.7 V from the time of the initial charge. This suggests that these batteries have been well charged and discharged since the first cycle. The average battery voltage of each of the batteries of Examples 1 to 3 was about 3.7 to 3.8 V.
[0070]
<Example 5: Performance evaluation (2)-Capacity retention rate for charge / discharge cycle>
The lithium ion secondary batteries prepared in Examples 1 to 3 and Comparative Example 1 were charged to 4.1 V at a current value of 1.2 mA under a condition of 25 ° C., and after a pause of 5 minutes, a charge of 1.2 mA was obtained. A charge / discharge pattern in which the battery was discharged to a current value of 3 V, paused for 5 minutes, and then charged again to the same current value to 4.1 V was repeated. The battery capacity and the positive electrode active material (here, LiCoO₂) Was calculated from the specific capacity (calculated here as 140 mAh). Table 1 shows the results. As can be seen from the results of Example 4, since the battery of Comparative Example 1 could not be properly charged and discharged, Table 1 shows the capacity retention rate as 0%.
[0071]
[Table 1]

[0072]
As shown in Table 1, the batteries of Examples 1 to 3 all had good capacity retention rates of 50% or more after 100 cycles. In the batteries of Examples 2 and 3, even better results were obtained than in the battery of Example 1. This is presumed to be due to factors such as the interface layer covering the negative electrode surface more densely in Examples 2 and 3 than in Example 1. For this reason, it is considered that the decomposition of the room temperature molten salt was better suppressed.
[0073]
<Reference Example 1>
Lithium is used as the negative electrode active material instead of graphite (potential about 0 V).₄Ti₅O₁₂(Potential: about 1.4 V). In other respects, a negative electrode was manufactured in the same manner as in Example 1. Using this negative electrode and the room temperature molten salt electrolyte B, a coin-type lithium ion secondary battery was produced in the same manner as in Example 2. The battery was subjected to a charge / discharge test in the same manner as in Example 4.
In such a battery, an interface layer was not provided between the negative electrode and the room temperature molten salt electrolyte, but unlike Comparative Example 1, the reductive decomposition onset potential of the room temperature molten salt (here, EMI-TFSI) constituting the electrolyte was determined. Also, since the potential of the negative electrode was high, the phenomenon that the room temperature molten salt was reductively decomposed was not observed. The average battery voltage of this lithium ion battery was about 2.2 to 2.4 V.
[0074]
As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and alterations of the specific examples illustrated above.
In addition, the technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. The technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing a schematic configuration of a lithium ion battery according to an embodiment.
FIG. 2 is a characteristic diagram showing a voltage profile with respect to a charge / discharge cycle for lithium ion secondary batteries of Examples and Comparative Examples.
[Explanation of symbols]
1: Lithium ion secondary battery
10: positive electrode
20: negative electrode
32: room temperature molten salt electrolyte
40: negative electrode structure
42: interface layer

Claims (11)

A positive electrode, a negative electrode, and an electrolyte containing a room-temperature molten salt,
On the surface of the negative electrode, there is provided an interface layer that has lithium ion conductivity, is substantially non-conductive at the negative electrode potential when the battery is used, and prevents direct contact between the room temperature molten salt and the negative electrode. A lithium ion battery. The lithium ion battery according to claim 1, wherein the negative electrode is mainly composed of a carbonaceous material. The lithium ion battery according to claim 1, wherein the interface layer contains lithium. 4. The lithium ion battery according to claim 1, wherein the electrolyte is mainly composed of a room-temperature molten salt having an upper limit potential of reductive decomposition that is about 1 V or more with respect to a lithium electrode. 5. Lithium-ion battery before initial charge,
A positive electrode, a negative electrode, and an electrolyte containing a room-temperature molten salt and a compound for forming an interface layer,
Here, the compound for forming an interface layer is reductively decomposed, has lithium ion conductivity, is substantially non-conductive at the negative electrode potential when the battery is used, and has the room temperature molten salt and the negative electrode. Forming an interface layer on the negative electrode surface that prevents direct contact of
When the lower limit potential that the negative electrode can take when the battery is used is E ₀ , the upper limit potential at which the room temperature molten salt can be reductively decomposed is E ₁ , and the upper limit potential at which the interfacial layer forming compound can be reductively decomposed is E ₂ And a lithium ion battery before charging in which these potentials satisfy the relationship of E ₀ <E ₁ <E ₂ . A method for manufacturing a lithium ion battery according to claim 1,
A method for manufacturing a lithium-ion battery, wherein the battery according to claim 5 is initially charged. A method for manufacturing a lithium ion battery including a positive electrode, a negative electrode, and an electrolyte containing a room temperature molten salt,
A negative electrode having an interface layer having lithium ion conductivity and being substantially non-conductive at the negative electrode potential when the battery is used and substantially not containing a room-temperature molten salt on the negative electrode surface, and the interface layer is formed. A method for manufacturing a lithium ion battery, comprising forming a battery by using the method. The method for manufacturing a lithium ion battery according to claim 7, wherein the interface layer is formed by applying a composition containing a lithium ion conductive polymer to the surface of the negative electrode. The interface layer is formed by bringing a negative electrode into contact with an electrolytic solution containing a polymer precursor that forms a conductive polymer by polymerization, and applying a noble potential to the negative electrode to subject the precursor to electrolytic polymerization,
The method for producing a lithium ion battery according to claim 7, wherein the conductive polymer has a side chain of lithium ion conductivity and exhibits substantially electron non-conductivity at a negative electrode potential when the battery is used. A negative electrode structure for constituting a lithium ion battery provided with an electrolyte containing a room temperature molten salt,
A negative electrode and an interface layer formed on the surface of the negative electrode,
The negative electrode structure, wherein the interface layer has lithium ion conductivity, is substantially non-conductive at the negative electrode potential when the battery is used, and does not substantially contain the room-temperature molten salt. The negative electrode structure according to claim 10, wherein the negative electrode is mainly composed of a carbonaceous material.

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