JP4476530B2 - Electrolyte, lithium secondary battery, and method for manufacturing lithium secondary battery - Google Patents

Description

【００１５】
【化８】

【００１６】
また、前記ハロゲン化アセトニトリルは、下記［化９］で示されるものが好ましい。
【００１７】
【化９】

【００１８】
また本発明の電解質は、先に記載の電解質であり、ＣＯ₂が溶解されてなることが好ましい。
【００１９】
次に本発明のリチウム二次電池は、リチウムを吸蔵、放出が可能な正極及び負極と、電解質とを具備してなり、前記電解質は、燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を全溶媒中における割合で１０体積％以上含み、更にリチウム塩を含むものであることを特徴とする。
上記電解質は、前記低燃焼性溶媒と前記リチウム塩のみを含むものであってもよく、前記低燃焼性溶媒と前記リチウム塩の他にジエチルカーボネート等の鎖状カーボネートを含んでいても良い。
係るリチウム二次電池によれば、電解質に燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を含むので、過充電や内部短絡等を起こした場合でも、電解質による発火を防止できる。
【００２０】
また本発明のリチウム二次電池は、先に記載のリチウム二次電池であり、前記低燃焼性溶媒が、ＪＩＳ−Ｋ２２６５に規定される引火点試験により引火点を示さないものであることが好ましい。
係るリチウム二次電池によれば、電解質が引火点を示さない低燃焼性溶媒を含むので、電池の安全性をより高めることができる。
【００２１】
また本発明のリチウム二次電池は、先に記載のリチウム二次電池であり、前記電解質に、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種が添加されてなることを特徴とする。尚、ポリアクリレート化合物は、３以上のアクリル基を有するものが好ましい。
【００２２】
係るリチウム二次電池によれば、初充電時の初期にアクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種が重合して負極表面に有機質被膜を形成するため、この有機質被膜によって負極表面上での電解質の分解反応が抑制されるので、電解質の分解によるガス発生や電解質自体の変質が低減され、充放電容量の低下を防止し、サイクル特性を向上できる。
【００２３】
また本発明のリチウム二次電池では、前記電解質中に前記ポリアクリレート化合物が０．０１〜１０質量％の範囲で添加されていることが好ましい。
【００２４】
また本発明のリチウム二次電池は、リチウムを吸蔵、放出が可能な正極及び負極と、電解質とを具備してなり、前記電解質は、３以上のアクリル基を有するポリアクリレート化合物からなる重合体に、１９０００ｋＪ／ｋｇ以下の燃焼熱を示す低燃焼性溶媒が全溶媒中における割合で１０体積％以上含浸されるとともにリチウム塩が含浸されてなり、前記負極の表面に、前記ポリアクリレート化合物からなる有機質被膜が形成されてなることを特徴とする。
また本発明のリチウム二次電池は、リチウムを吸蔵、放出が可能な正極及び負極と、有機電解液を主体とする電解質とを具備してなり、前記電解質には、１９０００ｋＪ／ｋｇ以下の燃焼熱を示す低燃焼性溶媒が全溶媒中における割合で１０体積％以上含まれるとともにリチウム塩が含まれてなり、前記負極の表面に、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種からなる有機質被膜が形成されてなることを特徴とする。
【００２５】
係るリチウム二次電池によれば、電解質に燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を含むので、過充電や内部短絡等を起こした場合でも、電解質による発火を防止できる。
また、負極表面に有機質被膜が形成されているので、この有機質被膜により負極表面上での電解質の分解反応が抑制され、これにより電解質の分解によるガス発生や電解質自体の変質が低減され、充放電容量の低下を防止するとともにサイクル特性を向上できる。
【００２６】
また本発明のリチウム二次電池は、先に記載のリチウム二次電池であり、前記低燃焼性溶媒が、ハロゲン化ベンゼンまたはハロゲン化アセトニトリルであることが好ましい。
ベンゼンまたはアセトニトリルのハロゲン化物は、燃焼熱が低く、自己消火性を示すので、リチウム二次電池の安全性をより高めることができる。
【００２７】
また本発明のリチウム二次電池では、前記ハロゲン化ベンゼンが、下記［化１０］または下記［化１１］で示されるものであることが好ましくい。
【００２８】
【化１０】

【００２９】
【化１１】

【００３０】
また本発明のリチウム二次電池では、前記ハロゲン化アセトニトリルが、下記［化１２］で示されるものであることが好ましい。
【００３１】
【化１２】

【００３２】
また本発明のリチウム二次電池では、前記電解質中にＣＯ₂を溶解させてなることが好ましい。
【００３３】
次に本発明のリチウム二次電池の製造方法は、リチウムを吸蔵、放出が可能な正極及び負極と、電解質とを具備してなるリチウム二次電池の製造方法であり、前記電解質は、１０体積％以上の１９０００ｋＪ／ｋｇ以下の燃焼熱を示す低燃焼性溶媒と、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種と含有してなり、該電解質を少なくとも前記正極及び前記負極の間に配置する組立工程と、金属リチウムを参照極とした場合の前記負極の電位が、０．７Ｖ以上１．５Ｖ以下の範囲に到達するまで定電流充電を行った後に、負極の電位を維持したままで０．１〜８時間の定電圧充電を行う第１充電工程とからなることを特徴とする。尚、ポリアクリレート化合物は、３以上のアクリル基を有するものが好ましい。
【００３４】
また本発明のリチウム二次電池の製造方法は、リチウムを吸蔵、放出が可能な正極及び負極と、電解質とを具備してなるリチウム二次電池の製造方法であり、前記正極の活物質が、コバルト、マンガン、ニッケルから選ばれる少なくとも一種とリチウムとの複合酸化物のいずれか１種以上であり、前記電解質は、１０体積％以上の１９０００ｋＪ／ｋｇ以下の燃焼熱を示す低燃焼性溶媒と、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種と含有してなり、該電解質を少なくとも前記正極及び前記負極の間に配置する組立工程と、電池電圧が２．３Ｖ以上３．１Ｖ以下の範囲に到達するまで定電流充電を行った後に、電池電圧を維持したままで０．１〜８時間の定電圧充電を行う第１充電工程とからなることを特徴とする。尚、ポリアクリレート化合物は、３以上のアクリル基を有するものが好ましい。
【００３５】
係るリチウム二次電池の製造方法によれば、電解質に燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を含むので、過充電や内部短絡等を起こした場合でも電解質による発火を防止でき、安全性に優れたリチウム二次電池を提供できる。
また、第１充電工程により負極表面に吸着したアクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種を重合させて有機質被膜を形成するので、電解質が分解する前に負極の表面上に有機質被膜を形成することができる。この有機質被膜の形成によって、後述の第２充電工程における電解質の分解を抑制することが可能となり、ガス発生及び電解質の変質を防止できる。
また、第１充電工程における定電圧充電が比較的長時間に渡って行われるので、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物等の重合反応が十分に行われ、有機質被膜の反応収率が高くなり、十分な有機質被膜が形成される。
また、第１充電工程を行うことによって電解質の一部が有機質被膜に吸収されるので、有機質被膜と電解質との親和性が向上し、充放電効率を向上させることが可能になる。
【００３６】
また本発明のリチウム二次電池の製造方法は、先に記載のリチウム二次電池の製造方法であり、前記組立工程と前記第１充電工程の間に、少なくとも前記電解質を４０〜１２０℃の範囲で熱処理する熱処理工程を備えることを特徴とする。
係る熱処理により、ポリアクリレート化合物等を熱重合させてポリマー電解質を形成するとともに、負極表面にポリアクリレート化合物等を吸着させて均一な有機質被膜を形成できる。
【００３７】
また本発明のリチウム二次電池の製造方法では、前記電解質中に前記ポリアクリレート化合物を０．０１〜１０質量％の範囲で添加することが好ましい。
【００３８】
また本発明のリチウム二次電池の製造方法では、前記低燃焼性溶媒が、ＪＩＳ−Ｋ２２６５に規定される引火点試験により引火点を示さないものであることが好ましい。
係るリチウム二次電池の製造方法によれば、電解質が引火点を示さない低燃焼性溶媒を含むので、安全性がより優れたリチウム二次電池を提供できる。
【００３９】
また本発明のリチウム二次電池の製造方法では、前記低燃焼性溶媒が、ハロゲン化ベンゼンまたはハロゲン化アセトニトリルであることが好ましい。
ベンゼンまたはアセトニトリルのハロゲン化物は、燃焼熱が低く、自己消火性を示すので、リチウム二次電池の安全性をより高めることができる。
【００４０】
また本発明のリチウム二次電池の製造方法では、前記ハロゲン化ベンゼンが、下記［化１３］または下記［化１４］で示されるものであることが好ましい。
【００４１】
【化１３】

【００４２】
【化１４】

【００４３】
また本発明のリチウム二次電池の製造方法では、前記ハロゲン化アセトニトリルが、下記［化１５］で示されるものであることが好ましい。
【００４４】
【化１５】

【００４５】
また本発明のリチウム二次電池の製造方法では、前記第１充電工程の後に、前記負極の電位が、０Ｖ以上０．１Ｖ以下の範囲に到達するまで定電流充電を行った後に、負極の電位を維持したままで１〜８時間の定電圧充電を行う第２充電工程を行うことが好ましい。
また、前記第１充電工程の後に、電池電圧が４．０Ｖ以上４．３Ｖ以下の範囲に到達するまで定電流充電を行った後に、電池電圧を維持したままで１〜８時間の定電圧充電を行う第２充電工程を行ってもよい。
【００４６】
また本発明のリチウム二次電池の製造方法は、先に記載のリチウム二次電池の製造方法であり、前記組立工程において、前記電解質中にＣＯ₂を溶解させることが好ましい。
係る製造方法によれば、電解質中にＣＯ₂を予め溶解させておくことで、第２充電工程で負極側に移動したリチウムイオンの一部がこのＣＯ₂と反応して炭酸リチウム膜を形成し、この炭酸リチウム膜が負極による電解質の分解を抑制するので、ガス発生及び電解質の変質をより確実に防止できる。
【００４７】
【発明の実施の形態】
以下、本発明の実施の形態を図面を参照して説明する。
本発明のリチウム二次電池は、リチウムを吸蔵、放出が可能な正極及び負極と、電解質とを具備してなり、前記電解質は、燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を全溶媒体積中に１０体積％以上含み、更にリチウム塩を含むものである。また、電解質には、アクリロニトリル、メタクリロニトリル、ポリアクリレート化合物のうちの少なくとも１種が添加されることが好ましい。尚、ポリアクリレート化合物としては、３以上のアクリル基を有するものが好ましい。
【００４８】
正極は、正極活物質粉末にポリフッ化ビニリデン等の結着材とカーボンブラック等の導電助材を混合してシート状、扁平円板状等に成形したものを例示できる。上記の正極活物質としては、コバルト、マンガン、ニッケルから選ばれる少なくとも一種とリチウムとの複合酸化物のいずれか１種以上のものが好ましく、具体的には、ＬｉＭｎ₂Ｏ₄、ＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＦｅＯ₂、Ｖ₂Ｏ₅等が好ましい。また、ＴｉＳ、ＭｏＳ、有機ジスルフィド化合物または有機ポリスルフィド化合物等のリチウムを吸蔵・放出が可能なものを用いても良い。
【００４９】
負極は、リチウムを吸蔵・放出が可能な負極活物質粉末に、ポリフッ化ビニリデン等の結着材と、場合によってカーボンブラック等の導電助材を混合してシート状、扁平円板状等に成形したものを例示できる。負極活物質としては、人造黒鉛、天然黒鉛、黒鉛化炭素繊維、黒鉛化メソカーボンマイクロビーズ、非晶質炭素等の炭素質材料を例示できる。また、リチウムと合金化が可能な金属質物単体やこの金属質物と炭素質材料を含む複合物も負極活物質として例示できる。リチウムと合金化が可能な金属としては、Ａｌ、Ｓｉ、Ｓｎ、Ｐｂ、Ｚｎ、Ｂｉ、Ｉｎ、Ｍｇ、Ｇａ、Ｃｄ等を例示できる。
また負極として金属リチウム箔も使用できる。
【００５０】
本発明に係るポリアクリレート化合物は、負極表面で有機質被膜を形成するほかに、重合体を形成して有機電解液を含むポリマー電解質を形成する場合もある。
この場合、前記電解質は、前記のポリアクリレート化合物からなる重合体に、前記の低燃焼性溶媒が全溶媒における割合で１０体積％以上含浸されるとともにリチウム塩が含浸されてなり、更に負極の表面に、前記ポリアクリレート化合物からなる有機質被膜が形成される。
また本発明に係るポリアクリレート化合物は、負極表面で有機質被膜を形成するのみの場合もある。
この場合、前記電解質は、有機電解液を主体とし、前記の低燃焼性溶媒が全溶媒中における割合で１０体積％以上含まれるとともにリチウム塩が含まれてなり、更に負極の表面に、前記ポリアクリレート化合物からなる有機質被膜が形成される。
尚、上記のいずれの有機被膜には、ポリアクリレート化合物の他に、アクリロニトリル、メタクリロニトリルが含まれることが好ましく、これにより、有機質被膜のリチウムのイオン伝導度が向上し、電池の内部インピーダンスが低減されて充放電効率が向上する。
【００５１】
上記のポリマー電解質は、電解質中のポリアクリレート化合物の含有率が比較的高い場合に、過剰なポリアクリレート化合物によって形成されやすく、また上記の有機電解液を主体とする電解質は、電解質中のポリアクリレート化合物の含有率が比較的低い場合に形成されやすい。
またポリマー電解質は、後述するように組立工程後に熱処理を行うことにより形成される。
【００５２】
本発明に係るポリアクリレート化合物は、例えば、下記［化１６］〜［化１８］に示す構造を有するもので、基炭素-炭素間の二重結合が分子内に３つ以上存在するいわゆる３官能以上のアクリル酸エステル誘導体である。このポリアクリレート化合物は、アニオン重合を行うアニオン付加重合性モノマーであり、加熱するとラジカル重合して重合体を形成し、上述のポリマー電解質が形成される。また充電時に卑な電位を示す負極表面上で有機質被膜を形成する。このポリアクリレート化合物がアニオン重合すると、分子内の３つ以上の二重結合が開裂してそれぞれ別のポリアクリレート化合物と結合する反応が連鎖的に起こり、負極表面上にポリアクリレート化合物が重合してなる有機質被膜が形成される。
【００５３】
また、本発明に係るポリアクリレート化合物は、下記［化１９］で表されるようなジペンタエリスリトール構造を具備してなるものであってもよく、例えば、下記［化２０］で表されるような６つのアクリル基を有するものであってもよい。
【００５４】
【化１６】

【００５５】
【化１７】

【００５６】
【化１８】

【００５７】
【化１９】

【００５８】
【化２０】

【００５９】
なお、上記［化１７］及び［化１８］中のａ、ｂ、ｃは、０≦ａ≦１５、０≦ｂ≦１５、０≦ｃ≦１５、３≦（ａ＋ｂ＋ｃ）≦１５である。
【００６０】
また、上記のポリアクリレート化合物は、上述したように、アクリロニトリルまたはメタクリロニトリルが共存する状態でこれらとともに本発明に係る有機質被膜を形成する場合もある。この場合の皮膜形成機構は、ポリアクリレート化合物がそれぞれ単独の場合と同様で、充電時に卑な電位を示す負極表面上でアニオン重合を行い、本発明に係る有機質被膜を形成する。
この有機質被膜の詳細な構造は不明であるが、おそらくポリアクリレート化合物とアクリロニトリル及び／またはメタクリロニトリルとの共重合体であると考えられる。この有機質被膜は、リチウムのイオン伝導度が高く、４．２Ｖ以上の電圧が印加された状態でも電気分解しない強固な被膜である。
尚、有機質被膜の形成に伴って電解質中に含まれる未反応のポリアクリレート化合物の濃度は著しく減少する。従って残留モノマーが電池特性を劣化させることがない。
【００６１】
有機質被膜の厚さは、数〜数十ｎｍ程度であり、極めて薄い膜である。膜厚が数μｍのオーダーになると、リチウムイオンを透過させることが困難になり、充放電反応が円滑に行えないので好ましくない。また、厚さが例えば１ｎｍ以下程度になると、膜としての形状を維持するのが困難になるので好ましくない。
【００６２】
尚、有機質被膜が負極表面に形成される具体的な形態としては、例えば、前記の負極活物質からなる粒状物の表面に有機質被膜が形成した状態や、金属リチウム箔の表面に有機質被膜が形成した状態が考えられる。
【００６３】
上記の有機質被膜は負極表面上に形成されるので、負極と電解質との直接の接触を防ぐ機能を果たす。これにより、負極表面での電解質の還元分解反応が抑制され、電解質の分解によるガス発生が低減されるととともに電解質自体の変質が防止される。このガス発生の低減によって電池の内圧が上昇せず、電池が変形することがない。更に電解質の変質防止により、電解質量が減少することがなく、充放電反応が円滑に進行して充放電効率が高くなり、サイクル特性が向上する。
更にまた、電解質と負極との反応が抑制されるので、電池を高温で長期間貯蔵した場合でも電解質の変質が起きることがなく、充放電効率やサイクル特性等の電池特性が低下することがない。
特に、後述する低燃焼性溶媒は安全性に優れるものの、負極と反応して分解しやすく充放電容量が低下する傾向になる。そのため、負極表面に有機質被膜を形成することで、低燃焼性溶媒と負極との直接の接触が防止されて充放電容量が向上する。
【００６４】
また、上記の有機質被膜はリチウムのイオン伝導性に優れるので、電解質と負極との間でリチウムイオンを輸送する機能も果たす。
従って、負極表面が有機質被膜で覆われたとしても、リチウムイオンの輸送に何ら障害になることがなく、充放電反応が円滑に進行して充放電効率が高くなり、サイクル特性が向上する。また電池の内部インピーダンスが増加することがなく、充放電容量が大幅に低下することがない。
【００６５】
尚、ポリアクリレート化合物は、有機質被膜の形成前の時点で、電解質中に０．０１〜１０質量％の範囲で添加されていることが好ましい。
ポリアクリレート化合物の添加量が０．０１質量％未満であると、有機質被膜が充分に形成されないので好ましくなく、添加量が１０質量％を越えると、有機質被膜の厚さが増大して内部インピーダンスが増加してしまうので好ましくない。
【００６６】
次に本発明に係る電解質は、燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒が全溶媒中における割合で１０体積％以上含み、更にリチウム塩を含むものである。
低燃焼性溶媒は、燃焼熱が０ｋＪ／ｋｇ以上１９０００ｋＪ／ｋｇ以下のものが好ましく、０ｋＪ／ｋｇ以上１４０００ｋＪ／ｋｇ以下のものがより好ましい。
燃焼熱が１９０００ｋＪ／ｋｇ以下であれば、電池が過充電状態になったり内部短絡等を起こして電解質の分解が起きた場合でも、分解時の発熱が少なくなって発火に至るおそれがなく、電池の安全性を向上できる。
【００６７】
１９０００ｋＪ／ｋｇ以下の燃焼熱を示す低燃焼性溶媒としては、たとえば、上記［化９］に示すハロゲン化ベンゼンまたは上記［化１０］に示すハロゲン化アセトニトリルを例示できる。ベンゼンまたはアセトニトリルのハロゲン化物は、燃焼熱が低く、リチウム二次電池の安全性をより高めることができる。
【００６８】
また低燃焼性溶媒は、ＪＩＳ−Ｋ２２６５に規定される引火点試験により引火点を示さないものであることが好ましい。電解質に引火点を示さない低燃焼性溶媒が含有されていれば、電池の安全性をより高めることができる。
引火性を示さない低燃焼性溶媒の具体例としては、上記［化９］に示すハロゲン化ベンゼンのうち、塩素原子（Ｃｌ）が５個置換し、フッ素原子（Ｆ）が１個置換してなるペンタフルオロクロロベンゼンを例示できる。
【００６９】
上記［化９］に示すハロゲン化ベンゼンは、低粘度で誘電率が低いという性質がある。一方、上記［化１０］に示すハロゲン化アセトニトリルは、高粘度で誘電率が高いという性質がある。
従って、本発明に係る電解質の一例としては、例えば、前記ハロゲン化ベンゼンと前記ハロゲン化アセトニトリルの混合溶媒にリチウム塩を溶解させたものを例示できる。
また、前記ハゲン化ベンゼンと前記ハロゲン化アセトニトリルの他に、従来から知られている非プロトン性溶媒を添加しても良い。
【００７０】
この場合、電解質中の低燃焼性溶媒の含有量は全溶媒の体積分率を１００体積％とした場合に１０体積％以上１００体積％以下の範囲が好ましく、５０体積％以上１００体積％以下の範囲がより好ましい。
低燃焼性溶媒の含有量が１０体積％未満であると、リチウム二次電池の安全性を確保できないので好ましくない。
【００７１】
また、前記ハロゲン化ベンゼンと前記ハロゲン化アセトニトリルの体積比は、ハロゲン化ベンゼン：ハロゲン化アセトニトリル＝４：６〜８：２の範囲が好ましい。
ハロゲン化ベンゼンの体積比が上記の範囲より小さくなると、粘度が高くなってイオン伝導度が低下するのであるので好ましくなく、ハロゲン化ベンゼンの体積比が上記の範囲を越えると、リチウム塩の溶解度が低下するので好ましくない。
【００７２】
尚、ハロゲン化ベンゼンとハロゲン化アセトニトリルは相容性が低く分離しやすいので、分離を防止するためにアセトニトリルを１質量％以上２０質量％以下の範囲で添加すると良い。
【００７３】
また、本発明に係る電解質の別の例としては、例えば、前記ハロゲン化ベンゼンと従来から知られている非プロトン性溶媒との混合溶媒に、リチウム塩を溶解させたものを例示できる。
この場合の非プロトン性溶媒としては、ハロゲン化ベンゼンの低誘電率を補うべく、高誘電率を示すものが好ましく、例えば、エチレンカーボネート、ブチレンカーボネート、ベンゾニトリル、アセトニトリル、γ−ブチロラクトン、４−メチルジオキソラン、Ｎ、Ｎ−ジメチルホルムアミド、ジメチルアセトアミド、ジメチルスルホキシド等を例示できる。
【００７４】
この場合、電解質中のハロゲン化ベンゼンの含有量は１０体積％以上８０体積％以下の範囲が好ましく、２０体積％以上５０体積％以下の範囲がより好ましい。
ハロゲン化ベンゼンの含有量が１０体積％未満であると、リチウム二次電池の安全性を確保できないので好ましくなく、ハロゲン化ベンゼンの含有量が８０体積％を越えると、リチウム塩の溶解性が低下してリチウムイオン伝導度が低下するので好ましくない。
【００７５】
更に、本発明に係る電解質の別の例としては、例えば、前記ハロゲン化アセトニトリルと従来から知られている非プロトン性溶媒との混合溶媒に、リチウム塩を溶解させたものを例示できる。
この場合の非プロトン性溶媒としては、ハロゲン化アセトニトリルの高粘度を補うべく、低粘度を示すものが好ましく、例えば、ジメチルカーボネート、メチルエチルカーボネート、ジエチルカーボネート、メチルプロピルカーボネート、メチルイソプロピルカーボネート、エチルブチルカーボネート、ジプロピルカーボネート、ジイソプロピルカーボネート、ジブチルカーボネート等の鎖状カーボネートや、１，２−ジメトキシエタン、スルホラン、ジクロロエタン、ジオキソラン、ニトロベンゼン、ジエチレングリコール、ジメチルエーテル等を例示できる。
【００７６】
この場合、電解質中のハロゲン化アセトニトリルの含有量は１０体積％以上８０体積％以下の範囲が好ましく、２０体積％以上５０体積％以下の範囲がより好ましい。
ハロゲン化アセトニトリルの含有量が１０体積％未満であると、リチウム二次電池の安全性を確保できないので好ましくなく、ハロゲン化アセトニトリルの含有量が８０体積％を越えると、電解質の粘度が高くなってリチウムイオン伝導度が低下するので好ましくない。
【００７７】
またリチウム塩としては、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＳｂＦ₆、ＬｉＡｓＦ₆、ＬｉＣｌＯ₄、ＬｉＣＦ₃ＳＯ₃、Ｌｉ（ＣＦ₃ＳＯ₂）₂Ｎ、ＬｉＣ₄Ｆ₉ＳＯ₃、ＬｉＳｂＦ₆、ＬｉＡlＯ₄、ＬｉＡlＣl₄、ＬｉＮ（Ｃ_xＦ_2x+1ＳＯ₂）（Ｃ_yＦ_{2y 十 1}ＳＯ₂）（ただしｘ、ｙは自然数）、ＬｉＣｌ、ＬｉＩ等のうちの１種または２種以上のリチウム塩を混合させてなるものを例示できる。
また、ＬｉＢ（ＯＣＯＣＦ₃）₄で示されるものを用いることもできる。
【００７８】
また上記の電解質を、ＰＥＯ、ＰＰＯ、ＰＡＮ、ＰＶＤＦ、ＰＭＡ、ＰＭＭＡ等のポリマーあるいはその重合体に含浸させたものでも良い。
また、上記の電解質中には予めＣＯ₂を溶解させておくことが好ましい。電解質中にＣＯ₂を予め溶解させておくことで、後述する第２充電工程で負極側に移動したリチウムイオンの一部がこのＣＯ₂と反応して炭酸リチウム膜を形成し、この炭酸リチウム膜が負極による電解質の分解を抑制するので、ガス発生及び電解質の変質をより確実に防止できる。
【００７９】
次に本発明のリチウム二次電池の製造方法について説明する。
本発明のリチウム二次電池の製造方法は、ポリアクリレート化合物を含む電解質を前記正極及び前記負極の間に配置する組立工程と、第１充電工程とからなる。
また組立工程と第１充電工程との間に加熱工程を設けても良く、更に第１充電工程の後に第２充電工程を行っても良い。
【００８０】
まず組立工程では、少なくとも、３以上のアクリル基を有するポリアクリレート化合物と、１９０００ｋＪ／ｋｇ以下の燃焼熱を示す上記の低燃焼性溶媒を全溶媒中の割合として１０体積％以上含有してなる電解質を調製する。電解質は、ポリアクリレート化合物と低燃焼性溶媒とリチウム塩とが含有された有機電解液であってもよく、この有機電解液をポリマーに含浸させてポリマー電解質としたものでも良い。
また、従来の非プロトン性溶媒を添加してもよく、相溶性向上のためにアクリロニトリルを添加しても良い。
ポリアクリレート化合物の添加量は、０．０１〜１０質量％の範囲が好ましく、０．１〜５質量％の範囲がより好ましい。また低燃焼性溶媒は１０体積％以上１００体積％以下の範囲で添加することが好ましい。
尚、このとき、電解質中に予めＣＯ₂を溶解させることが好ましい。電解質にＣＯ₂を溶解させるには、有機電解液にＣＯ₂ガスを吹き込む等の手段をとることができる。電解質中にＣＯ₂を予め溶解させておくことで、後述する第２充電工程で負極側に移動したリチウムイオンの一部がこのＣＯ₂と反応して炭酸リチウム膜を形成し、この炭酸リチウム膜が負極による電解質の分解を抑制するので、ガス発生及び電解質の変質をより確実に防止できる。
【００８１】
次に、この電解質を正極と負極の間に配置する。電解質が液状である場合は、正極と負極の間にセパレータを介在させた状態で、これらに電解質を含浸させればよい。また、電解質がポリマー電解質のような固形状若しくは半固形状の場合は、正極と負極の間に電解質を挟めばよい。
【００８２】
次に加熱工程では、少なくともポリアクリレート化合物を含む電解質を正、負極間に配置した状態で、４０〜１２０℃の温度範囲で熱処理を行う。この熱処理により、電解質中のポリアクリレート化合物がラジカル重合して重合体を形成し、この重合体に有機電解液が含浸されて電解質が形成される。また、ポリアクリレート化合物の一部を負極表面に吸着させる。
尚、加熱温度が４０℃未満であると、ポリアクリレート化合物のラジカル重合が十分に進まないので好ましくない。また、加熱温度が１２０℃を越えると、電解質が変質して電池特性を悪化させるので好ましくない。
また、組み立て工程でポリマー電解質を予め正負極間に挟んだ場合は、加熱工程を省略しても良い。更に有機電解液を主体とする電解質を形成する場合も加熱工程を省略しても良い。
更に、ポリマー電解質の形成を望まない場合は、加熱工程を省略して良い。この場合の電解質は有機電解液となる。
【００８３】
次に第１充電工程では、金属リチウムを参照極とした場合の負極の電位が、０．７Ｖ以上１．５Ｖ以下の範囲に到達するまで定電流充電を行った後に、負極の電圧を維持したままで０．１〜８時間の定電圧充電を行う。定電流充電時の電流は、０．０１〜０．３Ｃ程度が好ましい。
この第１充電工程により、電解質の還元分解が起きる前に、ポリアクリレート化合物がアニオン重合し、負極表面上に有機質被膜を形成する。
即ち、ポリアクリレート化合物は、金属リチウムを参照極とした場合の負極の示す電位が０．７〜１．５Ｖの範囲のときにアニオン付加重合を行い、また電位が０．７Ｖ以上では電解質の還元分解が起きないため、充電電圧の下限を０．７Ｖに限定する必要がある。また、この負極表面におけるアニオン重合は反応の進行が比較的遅いことから、重合反応を十分に進行させるべく、上記の充電電圧を維持した状態で１〜８時間の定電圧充電が必要になる。
なお負極の電位が０．７Ｖ未満では、電解質の還元分解反応が併発するので好ましくない。
【００８４】
また、定電流充電における負極の電位が１．５Ｖを越えると、ポリアクリレート化合物の重合反応が開始しないので好ましくない。
次に定電圧充電における充電時間が０．１時間未満では、ポリアクリレート化合物の重合反応が充分に進行せず、有機質被膜に欠陥が発生するおそれがあるので好ましくなく、充電時間が８時間を超えると重合反応がほぼ終了するため、上記の電圧範囲でこれ以上の時間で充電を行う実益がない。
【００８５】
尚、上記の第１充電工程では、正極をＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎ₂Ｏ₄のいずれか１種以上とした場合、電池電圧が２．３Ｖ以上３．１Ｖ以下の範囲に到達するまで定電流充電を行った後に、電池電圧を維持したままで０．１〜８時間の定電圧充電を行うことが好ましい。
【００８６】
また、有機電解液にポリアクリレート化合物と共にアクリロニトリル及び／またはメタクリロニトリルを添加した場合は、ポリアクリレート化合物及びアクリロニトリル及び／またはメタクリロニトリルを含む有機質被膜が形成される。アクリロニトリル及び／またはメタクリロニトリルが含まれると、有機質被膜のリチウムのイオン伝導度が向上し、電池の内部インピーダンスが低減されて充放電効率が向上する。アクリロニトリル及び／またはメタクリロニトリルは、ポリアクリレート化合物と共に重合して有機質被膜中に存在するか、あるいはポリアクリレート化合物のみからなる重合体中に溶解した状態で有機質被膜中に存在するか、のいずれか一方または両方の状態にあると考えられる。
尚、被膜の形成に伴って有機電解液中に含まれるポリアクリレート化合物、アクリロニトリル及び／またはメタクリロニトリルの濃度は著しく減少する。
【００８７】
第２充電工程では、金属リチウムを参照極とした場合の負極の電位が、０．０Ｖ以上０．１Ｖ以下の範囲に到達するまで定電流充電を行った後に、負極電位を０．０Ｖ以上０．１Ｖ以下に維持したままで１〜８時間の定電圧充電を行う。定電流充電時の電流は、０．１〜０．５Ｃ程度が好ましい。
この第２充電工程においては、既に有機質被膜が形成しているため、電解質と負極とが直接に接触することなく、低燃焼性溶媒の還元分解が抑制される。
定電流充電における負極の電位が０．1Ｖを越えると、電池容量が不十分になるので好ましくなく、０．０Ｖ未満であると正極の結晶構造が破壊されるおそれがあるので好ましくない。
また、定電圧充電における充電時間が１時間未満であると、充電が不十分になるので好ましくなく、充電時間が８時間を越えると、過充電状態になって正極が劣化するので好ましくない。
【００８８】
尚、上記の第２充電工程では、正極をＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎ₂Ｏ₄のいずれか１種以上とした場合、電池電圧が４．０Ｖ以上４．３Ｖ以下の範囲に到達するまで定電流充電を行った後に、電池電圧を維持したままで１〜８時間の定電圧充電を行うことが好ましい。
また、第１充電工程と第２充電工程の間に、１〜８時間程度の休止時間を設けることが、第１充電時間が十分長くない場合に重合反応を充分に進行させる点で好ましい。
この第２充電工程では、組立工程で予め電解質中に溶解させたＣＯ₂が、負極側に移動したリチウムイオンの一部と反応して負極表面に炭酸リチウム膜を形成し、この炭酸リチウム膜が負極と電解質との接触を防止して電解質の分解を抑制し、ガス発生及び電解質の変質をより確実に防止できる。
【００８９】
上記のリチウム二次電池の製造方法によれば、電解質に燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を含むので、過充電や内部短絡等を起こした場合でも電解質による発火を防止でき、安全性に優れたリチウム二次電池を提供できる。
また、第１充電工程により負極表面に吸着したポリアクリレート化合物を重合させて有機質被膜を形成するので、電解質が分解する前に負極の表面上に有機質被膜を形成することができる。この有機質被膜の形成によって、後述の第２充電工程における電解質の分解を抑制することが可能となり、ガス発生及び電解質の変質を防止できる。
また、第１充電工程における定電圧充電が比較的長時間に渡って行われるので、ポリアクリレート化合物の重合反応が十分に行われ、有機質被膜の反応収率が高くなり、十分な有機質被膜が形成される。
また、第１充電工程を行うことによって電解質の一部が有機質被膜に吸収されるので、有機質被膜と電解質との親和性が向上し、充放電効率を向上させることが可能になる。
【００９０】
【実施例】
［実施例１、２及び比較例１〜３のリチウム二次電池の評価］
（実施例１のリチウム二次電池の製造）
フッ化アセトニトリル（ＣＨ₂ＦＣＮ）とヘキサフルオロベンゼン（Ｃ₆Ｆ₆）がそれぞれ５０体積％ずつ混合された混合溶媒に対し、１．４モル／Ｌの濃度になるようにＬｉＢ（ＯＣＯＣＦ₃）₄を添加して電解質を調製した。この電解質に、上記［化１６］に示すトリメチロールプロパントリアクリレート（分子量２６９）を０．２質量％添加した。
次に、ＬｉＣｏＯ₂を正極活物質とするシート状の正極と、炭素繊維を負極活物質とするシート状の負極とを重ね合わせて渦巻き状に巻回した状態で電池容器に挿入し、先程の電解質を注入した後に電池容器を封口して、厚さ４ｍｍ、幅３０ｍｍ、高さ６０ｍｍの角型電池を製造した。尚、電解質の注液前に電解質にＣＯ₂を１０分間吹き込んだ。これにより、電解質中にはＣＯ₂が溶解した状態になっている。
【００９１】
得られた角型電池に対し、０．２Ｃの電流で電池電圧が３Ｖ(金属リチウムに対する負極の電位が０．７Ｖ)に達するまで定電流充電を行った後に４時間の定電圧充電を行う第１充電工程により、ポリアクリレート化合物を重合させて有機質被膜を形成した。次に、０．２Ｃの電流で電池電圧が４．２Ｖ(金属リチウムに対する負極の電位が０.1Ｖ)に達するまで定電流充電を行った後に９時間の定電圧充電を行う第２充電工程をすることにより、実施例１のリチウム二次電池を製造した。
尚、第１、第２充電工程の際に負極におけるクーロン効率を測定した。
【００９２】
（実施例２のリチウム二次電池の製造）
アセトニトリル（ＣＨ₃ＣＮ）が２５体積％、クロロペンタフルオロベンゼン（Ｃ₆ＣｌＦ₅）が５０体積％、フッ化アセトニトリル（ＣＨ₂ＦＣＮ）が２５体積％ずつ混合された混合溶媒に対し、１．４モル／Ｌの濃度になるようにＬｉＢ（ＯＣＯＣＦ₃）₄を添加して電解質を調製した。この電解質に、上記［化１６］に示すトリメチロールプロパントリアクリレート（分子量２６９）を０．３質量％添加した。
次に、ＬｉＣｏＯ₂を正極活物質とするシート状の正極と、炭素繊維を負極活物質とするシート状の負極とを重ね合わせて渦巻き状に巻回した状態で電池容器に挿入し、先程の電解質を注入した後に電池容器を封口して、厚さ４ｍｍ、幅３０ｍｍ、高さ６０ｍｍの角型電池を製造した。尚、電解質の注液前に電解質にＣＯ₂を１０分間吹き込んだ。これにより、電解質中にはＣＯ₂が溶解した状態になっている。
【００９３】
得られた角型電池に対し、実施例１と同様にして第１，第２充電工程を行うことにより、実施例２のリチウム二次電池を製造した。
尚、第１、第２充電工程の際に負極におけるクーロン効率を測定した。
【００９４】
（比較例１のリチウム二次電池の製造）
電解質に、上記［化１６］に示すトリメチロールプロパントリアクリレートを添加しないこと以外は実施例１と同様にして、比較例１のリチウム二次電池を製造した。尚、ＣＯ₂雰囲気中における負極の乾燥は行っていない。
尚、実施例１と同様に、第１、第２充電工程の際に負極におけるクーロン効率を測定した。
【００９５】
（比較例２のリチウム二次電池の製造）
エチレンカーボネートが３０体積％、ジエチルカーボネートが７０体積％となるように混合された混合溶媒に対し、１．３モル／Ｌの濃度になるようにＬｉＰＦ₆を添加して電解質を調製したこと以外は上記の実施例１と同様にして比較例２のリチウム二次電池を製造した。尚、ＣＯ₂の吹き込みは行っていない。
尚、実施例１と同様に、第１、第２充電工程の際に負極におけるクーロン効率を測定した。
【００９６】
（比較例３のリチウム二次電池の製造）
アセトニトリル（ＣＨ₃ＣＮ）が２５体積％、クロロペンタフルオロベンゼン（Ｃ₆ＣｌＦ₅）が５０体積％、フッ化アセトニトリル（ＣＨ₂ＦＣＮ）が２５体積％ずつ混合された混合溶媒に対し、１．４モル／Ｌの濃度になるようにＬｉＢ（ＯＣＯＣＦ₃）₄を添加して電解質を調製した。
次に、ＬｉＣｏＯ₂を正極活物質とするシート状の正極と、炭素繊維を負極活物質とするシート状の負極とを重ね合わせて渦巻き状に巻回した状態で電池容器に挿入し、先程の電解質を注入した後に電池容器を封口して、厚さ４ｍｍ、幅３０ｍｍ、高さ６０ｍｍの角型電池を製造した。
【００９７】
得られた角型電池に対し、実施例１と同様にして第１，第２充電工程を行うことにより、比較例３のリチウム二次電池を製造した。尚、ＣＯ₂の吹き込みは行っていない。
尚、第１、第２充電工程の際に負極におけるクーロン効率を測定した。
【００９８】
［実施例１、２及び比較例１〜３のリチウム二次電池のクーロン曲線］
図１及び図２に、実施例１及び比較例１，２の熱処理後の第１、第２充電工程におけるクーロン効率を示す。図１は図２の部分拡大図であって第１充電工程のクーロン効率を示し、図２が第１，第２充電工程のクーロン効率を示す。
また図３及び図４に、実施例２及び比較例３の熱処理後の第１、第２充電工程におけるクーロン効率を示す。図３は図４の部分拡大図であって第１充電工程のクーロン効率を示し、図４が第１，第２充電工程のクーロン効率を示す。
【００９９】
図１に示すように、実施例１の第１充電工程では、充電電圧２．３Ｖ付近にポリアクリレート化合物の重合反応に対応する小さなピークが観察されている。この小さなピークは、負極表面での有機質被膜の形成によるものと考えられる。
次に図２に示す第２充電工程では、充電電圧の向上に伴ってクーロン効率がなだらかに上昇している。これは、第１充電工程で負極表面に有機質被膜が形成されたため、負極と電解質とが直接的に接触せず、負極表面での電解質の分解が抑制されたことによるものと考えられる。
【０１００】
また図２に示すように、実施例２の第１充電工程では、特に目立ったピークは観察されず、ポリアクリレート化合物の重合反応により有機質被膜が形成され、溶媒の分解等は起きていないと考えられる。
【０１０１】
次に、図１に示す比較例１の第１充電工程では、２．７Ｖ付近にヘキサフルオロベンゼンの分解反応によると考えられる大きなピークが観察されている。またこのとき、比較例１の電池の内圧が急激に上昇しており、ヘキサフルオロベンゼンの分解により分解ガスが発生したものと推定される。
次に、図１に示す比較例２の第１充電工程では、特に目立ったピークは観察されず、有機質被膜の形成や溶媒の分解等は起きていないと考えられる。
次に、図２に示す比較例３の第１充電工程では、２．７Ｖ付近にクロロペンタフルオロベンゼン及びアセトニトリルの分解反応によると考えられる大きなピークが観察されている。またこのとき、比較例３の電池の内圧が急激に上昇しており、クロロペンタフルオロベンゼン及びアセトニトリルの分解により分解ガスが発生したものと推定される。
【０１０２】
［実施例１、２及び比較例１〜３のリチウム二次電池の初期放電容量］
第２充電工程後のリチウム二次電池を、０．１Ｃ、０．２Ｃ、０．５Ｃ、１Ｃの電流で放電した際の放電容量を測定した。結果を表１に示す。
【０１０３】
【表１】

【０１０４】
表１に示すように、実施例１及び２は、０．１Ｃ〜１Ｃの範囲で、比較例２の放電容量とほぼ同等の放電容量を示すことが分かる。また、比較例１では、０．１Ｃのような比較的低い電流で放電した場合でも、実施例１または比較例２に対して１０％程度放電容量が減少している。
尚、実施例１では、有機質被膜の形成に加えて、電解質中にＣＯ₂が溶解した状態になっているため、第２充電工程の末期に負極表面に炭酸リチウム膜が形成されていると考えられ、この炭酸リチウム膜が有機質被膜とともにヘキサフルオロベンゼンの分解を抑制していると考えられる。
比較例１及び３では、ポリアクリレート化合物が無添加であるために負極表面に有機質被膜が形成されず、このためハロゲン化ベンゼンの分解反応が生じ、電解質が分解されたために放電容量が低下したものと考えられる。
【０１０５】
［実施例１、２及び比較例３のリチウム二次電池の加熱試験］
上記実施例１、２及び比較例３のリチウム二次電池について、加熱試験を行った。加熱試験は、リチウム二次電池を加熱オーブン中で昇温速度５℃／分で１６０℃まで昇温して６０分間保持する条件で行った。
その結果、実施例１及び２のリチウム二次電池は、特に異常は見られなかったが、比較例３に電池については、自己発熱が生じて電池温度が上昇し、安全弁が作動して破裂するに至った。
【０１０６】
【発明の効果】
以上、詳細に説明したように、本発明のリチウム二次電池によれば、電解質に燃焼熱が１９０００ｋＪ／ｋｇ以下の低燃焼性溶媒を含むので、過充電や内部短絡等を起こした場合でも、電解質による発火を防止できる。
【図面の簡単な説明】
【図１】 実施例１及び比較例１，２の第１充電工程における充電電圧に対するクーロン効率を示す図である。
【図２】 実施例１及び比較例１，２の第１，第２充電工程における充電電圧に対するクーロン効率を示す図である。
【図３】 実施例２及び比較例３の第１充電工程における充電電圧に対するクーロン効率を示す図である。
【図４】 実施例２及び比較例３の第１，第２充電工程における充電電圧に対するクーロン効率を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrolyte and a lithium secondary battery using the electrolyte, and particularly relates to a lithium secondary battery having high safety and excellent discharge characteristics.
[0002]
[Prior art]
A lithium secondary battery using an electrolyte such as an organic electrolyte has a high voltage and a high energy density, is excellent in storage performance and low-temperature operability, and is widely used in portable consumer electronic products. In addition, research and development for increasing the size of this battery and using it as a night-time power storage device for electric vehicles and homes has been actively conducted.
[0003]
By the way, since many of the solvents used in the conventional organic electrolyte have a low flash point and high combustibility, there is a risk of ignition due to overcharge or heating. Therefore, recently, as organic electrolytes, a mixture of a halogenated formate that can be expected to suppress combustion, mixed with a cyclic carbonate, a mixture of a halogenated ester, and a phosphate that is expected to have a self-extinguishing action. The thing etc. which contained are proposed.
[0004]
[Problems to be solved by the invention]
However, the lithium secondary battery using the above-described organic electrolyte has problems that the discharge capacity is not sufficient and the cycle characteristics are inferior.
During charging, halogenated esters, phosphate esters, etc. are decomposed on the negative electrode surface, and with the generation of gas, decomposition products remain in the organic electrolyte, causing the organic electrolyte to change, and thereby discharge capacity and cycle characteristics. It is probable that the deterioration occurred.
[0005]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium secondary battery and an electrolyte thereof that have excellent safety, high discharge capacity, and excellent cycle characteristics.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention employs the following configuration.
The electrolyte of the present invention is characterized by containing a low-flammability solvent having a combustion heat of 19000 kJ / kg or less in a proportion of 10% by volume or more, and further containing a lithium salt.
The electrolyte may contain only the low flammability solvent and the lithium salt, and may contain a chain carbonate such as diethyl carbonate in addition to the low flammability solvent and the lithium salt.
[0007]
According to such an electrolyte, since it contains a low-flammability solvent exhibiting a combustion heat of 19000 kJ / kg or less, it is possible to prevent ignition by the electrolyte even when a battery equipped with this electrolyte causes an overcharge or an internal short circuit. it can.
[0008]
In particular, it is preferable that the low-flammability solvent does not show a flash point by a flash point test specified in JIS-K2265.
If it is an electrolyte containing the low combustibility solvent which does not show a flash point, the safety | security of a battery can be improved more.
[0009]
The electrolyte of the present invention is the electrolyte described above, and is characterized in that at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound is added. The polyacrylate compound preferably has three or more acrylic groups.
[0010]
According to such an electrolyte, when this electrolyte is used in a lithium secondary battery, at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound is polymerized to form an organic coating on the negative electrode surface at the initial stage of initial charging. Therefore, the organic coating suppresses the decomposition reaction of the electrolyte on the negative electrode surface, thereby reducing gas generation due to the decomposition of the electrolyte and alteration of the electrolyte itself, and preventing a decrease in charge / discharge capacity of the lithium secondary battery. In addition, cycle characteristics can be improved.
[0011]
The polyacrylate compound is preferably added in the range of 0.01 to 10% by mass.
[0012]
In the electrolyte of the present invention, the low-flammability solvent is preferably halogenated benzene or halogenated acetonitrile.
The halide of benzene or acetonitrile has low combustion heat and exhibits self-extinguishing properties, so that the safety of the lithium secondary battery can be further increased.
[0013]
In particular, the halogenated benzene is preferably represented by the following [Chemical Formula 7], more preferably the following [Chemical Formula 8]. If it is shown by the following [Chemical Formula 8], combustion heat can be made smaller.
[0014]
[Chemical 7]

[0015]
[Chemical 8]

[0016]
The halogenated acetonitrile is preferably one represented by the following [Chemical 9].
[0017]
[Chemical 9]

[0018]
The electrolyte of the present invention is the electrolyte described above, and CO.₂Is preferably dissolved.
[0019]
Next, the lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of occluding and releasing lithium, and an electrolyte. The electrolyte contains a low-flammability solvent having a combustion heat of 19000 kJ / kg or less. It is characterized by containing 10% by volume or more in a ratio in a solvent and further containing a lithium salt.
The electrolyte may contain only the low flammability solvent and the lithium salt, and may contain a chain carbonate such as diethyl carbonate in addition to the low flammability solvent and the lithium salt.
According to the lithium secondary battery, since the electrolyte includes a low-flammability solvent having a combustion heat of 19000 kJ / kg or less, ignition due to the electrolyte can be prevented even when an overcharge or an internal short circuit occurs.
[0020]
Moreover, the lithium secondary battery of the present invention is the lithium secondary battery described above, and the low-flammability solvent preferably does not show a flash point by a flash point test defined in JIS-K2265. .
According to such a lithium secondary battery, since the electrolyte contains a low-flammability solvent that does not exhibit a flash point, the safety of the battery can be further improved.
[0021]
The lithium secondary battery of the present invention is the lithium secondary battery described above, wherein at least one of acrylonitrile, methacrylonitrile, and a polyacrylate compound is added to the electrolyte. . The polyacrylate compound preferably has three or more acrylic groups.
[0022]
According to the lithium secondary battery, since at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound is polymerized to form an organic film on the negative electrode surface in the initial stage of the initial charge, the organic film forms the negative electrode surface. Since the decomposition reaction of the electrolyte is suppressed, gas generation due to the decomposition of the electrolyte and alteration of the electrolyte itself are reduced, and a decrease in charge / discharge capacity can be prevented and cycle characteristics can be improved.
[0023]
Moreover, in the lithium secondary battery of this invention, it is preferable that the said polyacrylate compound is added in 0.01-10 mass% in the said electrolyte.
[0024]
The lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, and the electrolyte is a polymer composed of a polyacrylate compound having three or more acrylic groups. , An organic material comprising the polyacrylate compound on the surface of the negative electrode, impregnated with 10% by volume or more of a low flammability solvent exhibiting combustion heat of 19000 kJ / kg or less in a proportion of the total solvent and impregnated with a lithium salt. A film is formed.
The lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte mainly composed of an organic electrolyte, and the electrolyte has a combustion heat of 19000 kJ / kg or less. A low flammability solvent is contained in an amount of 10% by volume or more in a proportion of the total solvent and a lithium salt is contained. The surface of the negative electrode is made of at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound. An organic film is formed.
[0025]
According to the lithium secondary battery, since the electrolyte includes a low-flammability solvent having a combustion heat of 19000 kJ / kg or less, ignition due to the electrolyte can be prevented even when an overcharge or an internal short circuit occurs.
In addition, since the organic film is formed on the negative electrode surface, the organic film suppresses the decomposition reaction of the electrolyte on the negative electrode surface, thereby reducing the gas generation due to the decomposition of the electrolyte and the alteration of the electrolyte itself. It is possible to prevent a decrease in capacity and improve cycle characteristics.
[0026]
The lithium secondary battery of the present invention is the lithium secondary battery described above, and the low-flammability solvent is preferably halogenated benzene or halogenated acetonitrile.
The halide of benzene or acetonitrile has low combustion heat and exhibits self-extinguishing properties, so that the safety of the lithium secondary battery can be further increased.
[0027]
In the lithium secondary battery of the present invention, the halogenated benzene is preferably represented by the following [Chemical Formula 10] or the following [Chemical Formula 11].
[0028]
[Chemical Formula 10]

[0029]
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[0030]
In the lithium secondary battery of the present invention, the halogenated acetonitrile is preferably one represented by the following [Chemical Formula 12].
[0031]
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[0032]
In the lithium secondary battery of the present invention, the electrolyte contains CO.₂Is preferably dissolved.
[0033]
Next, a method for producing a lithium secondary battery according to the present invention is a method for producing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte. % Of a low flammability solvent exhibiting combustion heat of 19000 kJ / kg or less and at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound, and the electrolyte is at least between the positive electrode and the negative electrode. And the negative electrode potential was maintained after constant current charging until the potential of the negative electrode in the case of using metallic lithium as a reference electrode reached a range of 0.7 V or more and 1.5 V or less. It consists of the 1st charge process which performs the constant voltage charge for 0.1 to 8 hours as it is. The polyacrylate compound preferably has three or more acrylic groups.
[0034]
The method for producing a lithium secondary battery of the present invention is a method for producing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, and the active material of the positive electrode is At least one selected from cobalt, manganese, and nickel, and a composite oxide of lithium and at least one selected from the group consisting of lithium and a low-flammability solvent that exhibits a combustion heat of 10% by volume or more and 19000 kJ / kg or less; An assembling step comprising at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound, and disposing the electrolyte between at least the positive electrode and the negative electrode; and a battery voltage of 2.3 V to 3.1 V From the first charging step of performing constant voltage charging for 0.1 to 8 hours while maintaining the battery voltage after performing constant current charging until reaching the range of And wherein the Rukoto. The polyacrylate compound preferably has three or more acrylic groups.
[0035]
According to the method for producing a lithium secondary battery, since the electrolyte contains a low-flammability solvent with a combustion heat of 19000 kJ / kg or less, ignition can be prevented by the electrolyte even when overcharging or internal short circuit occurs, and safety is ensured. Can provide a lithium secondary battery excellent in performance.
In addition, since an organic coating is formed by polymerizing at least one of acrylonitrile, methacrylonitrile, and polyacrylate compound adsorbed on the negative electrode surface in the first charging step, an organic substance is formed on the negative electrode surface before the electrolyte is decomposed. A film can be formed. Formation of this organic coating makes it possible to suppress decomposition of the electrolyte in the second charging step described later, and to prevent gas generation and alteration of the electrolyte.
In addition, since the constant voltage charging in the first charging process is performed for a relatively long time, the polymerization reaction of acrylonitrile, methacrylonitrile, polyacrylate compound, etc. is sufficiently performed, and the reaction yield of the organic coating is increased. A sufficient organic film is formed.
Moreover, since a part of electrolyte is absorbed by an organic film by performing a 1st charge process, the affinity of an organic film and an electrolyte improves, and it becomes possible to improve charging / discharging efficiency.
[0036]
Moreover, the manufacturing method of the lithium secondary battery of the present invention is the manufacturing method of the lithium secondary battery described above, and at least the electrolyte is in a range of 40 to 120 ° C. between the assembly step and the first charging step. It is characterized by comprising a heat treatment step of heat-treating.
By such heat treatment, a polyacrylate compound or the like is thermally polymerized to form a polymer electrolyte, and a polyacrylate compound or the like is adsorbed on the negative electrode surface to form a uniform organic film.
[0037]
Moreover, in the manufacturing method of the lithium secondary battery of this invention, it is preferable to add the said polyacrylate compound in 0.01-10 mass% in the said electrolyte.
[0038]
Moreover, in the manufacturing method of the lithium secondary battery of this invention, it is preferable that the said low-flammability solvent does not show a flash point by the flash point test prescribed | regulated to JIS-K2265.
According to the method for producing a lithium secondary battery, since the electrolyte includes a low-flammability solvent that does not exhibit a flash point, a lithium secondary battery with higher safety can be provided.
[0039]
In the method for producing a lithium secondary battery of the present invention, the low-flammability solvent is preferably halogenated benzene or halogenated acetonitrile.
The halide of benzene or acetonitrile has low combustion heat and exhibits self-extinguishing properties, so that the safety of the lithium secondary battery can be further increased.
[0040]
In the method for producing a lithium secondary battery of the present invention, the halogenated benzene is preferably represented by the following [Chemical 13] or [Chemical 14] below.
[0041]
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[0042]
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[0043]
Moreover, in the manufacturing method of the lithium secondary battery of this invention, it is preferable that the said halogenated acetonitrile is what is shown by following [Chemical 15].
[0044]
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[0045]
In the method for producing a lithium secondary battery of the present invention, after the first charging step, the negative electrode potential is obtained after constant current charging is performed until the negative electrode potential reaches a range of 0 V to 0.1 V. It is preferable to perform the 2nd charge process which performs the constant voltage charge for 1 to 8 hours, maintaining this.
In addition, after the first charging step, constant current charging is performed until the battery voltage reaches a range of 4.0 V or more and 4.3 V or less, and then the constant voltage charging is performed for 1 to 8 hours while maintaining the battery voltage. You may perform the 2nd charge process which performs.
[0046]
The method for producing a lithium secondary battery according to the present invention is the method for producing a lithium secondary battery described above, and in the assembly step, CO is contained in the electrolyte.₂Is preferably dissolved.
According to this manufacturing method, CO is contained in the electrolyte.₂Is dissolved in advance, so that a part of the lithium ions that have moved to the negative electrode side in the second charging step can be₂To form a lithium carbonate film, and this lithium carbonate film suppresses the decomposition of the electrolyte by the negative electrode, so that the generation of gas and the alteration of the electrolyte can be prevented more reliably.
[0047]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The lithium secondary battery of the present invention includes a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte. The electrolyte includes a low combustible solvent having a combustion heat of 19000 kJ / kg or less in a total solvent volume. It contains 10% by volume or more, and further contains a lithium salt. Moreover, it is preferable that at least one of acrylonitrile, methacrylonitrile, and polyacrylate compounds is added to the electrolyte. In addition, as a polyacrylate compound, what has a 3 or more acryl group is preferable.
[0048]
Examples of the positive electrode include a positive electrode active material powder mixed with a binder such as polyvinylidene fluoride and a conductive additive such as carbon black and formed into a sheet shape, a flat disk shape, or the like. The positive electrode active material is preferably one or more of complex oxides of lithium and at least one selected from cobalt, manganese, and nickel. Specifically, LiMn₂O_FourLiCoO₂LiNiO₂LiFeO₂, V₂O_FiveEtc. are preferred. Moreover, you may use what can occlude / release lithium, such as TiS, MoS, an organic disulfide compound, or an organic polysulfide compound.
[0049]
The negative electrode is formed into a sheet shape, flat disk shape, etc. by mixing a negative electrode active material powder capable of occluding and releasing lithium with a binder such as polyvinylidene fluoride and, optionally, a conductive additive such as carbon black. Can be illustrated. Examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, and amorphous carbon. Moreover, the metal substance simple substance which can be alloyed with lithium, and the composite containing this metal substance and carbonaceous material can be illustrated as a negative electrode active material. Examples of metals that can be alloyed with lithium include Al, Si, Sn, Pb, Zn, Bi, In, Mg, Ga, and Cd.
A metal lithium foil can also be used as the negative electrode.
[0050]
In addition to forming an organic film on the negative electrode surface, the polyacrylate compound according to the present invention may form a polymer to form a polymer electrolyte containing an organic electrolyte.
In this case, the electrolyte is formed by impregnating the polymer composed of the polyacrylate compound with 10% by volume or more of the low-flammability solvent in a ratio of the total solvent, and further impregnating with a lithium salt. In addition, an organic film made of the polyacrylate compound is formed.
The polyacrylate compound according to the present invention may only form an organic film on the negative electrode surface.
In this case, the electrolyte is mainly composed of an organic electrolytic solution, and the low-flammability solvent is contained in an amount of 10% by volume or more in a ratio of the total solvent, and a lithium salt is further contained. An organic film made of an acrylate compound is formed.
In addition to the polyacrylate compound, any of the organic coatings described above preferably contains acrylonitrile and methacrylonitrile. This improves the ionic conductivity of lithium in the organic coating and improves the internal impedance of the battery. The charge / discharge efficiency is improved.
[0051]
The polymer electrolyte is easily formed by excess polyacrylate compound when the content of the polyacrylate compound in the electrolyte is relatively high, and the electrolyte mainly composed of the organic electrolyte is polyacrylate in the electrolyte. It tends to be formed when the content of the compound is relatively low.
The polymer electrolyte is formed by performing a heat treatment after the assembly process as will be described later.
[0052]
The polyacrylate compound according to the present invention has, for example, a structure represented by the following [Chemical Formula 16] to [Chemical Formula 18], and is a so-called trifunctional compound in which three or more double bonds between the base carbon and carbon are present in the molecule. These are the above acrylate derivatives. This polyacrylate compound is an anionic addition polymerizable monomer that performs anionic polymerization. When heated, the polyacrylate compound forms a polymer by radical polymerization to form the above-described polymer electrolyte. In addition, an organic film is formed on the negative electrode surface showing a base potential during charging. When this polyacrylate compound is anionically polymerized, a reaction in which three or more double bonds in the molecule are cleaved and bonded to different polyacrylate compounds occurs in a chain, and the polyacrylate compound is polymerized on the negative electrode surface. An organic film is formed.
[0053]
Further, the polyacrylate compound according to the present invention may have a dipentaerythritol structure represented by the following [Chemical Formula 19], for example, as represented by the following [Chemical Formula 20]. It may have six acrylic groups.
[0054]
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[0055]
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[0056]
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[0057]
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[0058]
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[0059]
In the above [Chemical Formula 17] and [Chemical Formula 18], a, b, and c are 0 ≦ a ≦ 15, 0 ≦ b ≦ 15, 0 ≦ c ≦ 15, and 3 ≦ (a + b + c) ≦ 15.
[0060]
In addition, as described above, the polyacrylate compound may form the organic film according to the present invention together with acrylonitrile or methacrylonitrile in the coexistence state. The film formation mechanism in this case is the same as that in the case where each of the polyacrylate compounds is alone, and an anionic polymerization is performed on the negative electrode surface showing a base potential at the time of charging to form the organic film according to the present invention.
The detailed structure of this organic coating is unknown, but is probably a copolymer of a polyacrylate compound and acrylonitrile and / or methacrylonitrile. This organic coating is a strong coating that has high ionic conductivity of lithium and does not electrolyze even when a voltage of 4.2 V or higher is applied.
Note that the concentration of the unreacted polyacrylate compound contained in the electrolyte is significantly reduced with the formation of the organic coating. Therefore, the residual monomer does not deteriorate the battery characteristics.
[0061]
The thickness of the organic film is about several to several tens of nanometers, and is an extremely thin film. When the film thickness is on the order of several μm, it is difficult to transmit lithium ions, and the charge / discharge reaction cannot be performed smoothly. Further, if the thickness is about 1 nm or less, for example, it is difficult to maintain the shape as a film, which is not preferable.
[0062]
In addition, as a specific form in which the organic film is formed on the surface of the negative electrode, for example, the state in which the organic film is formed on the surface of the granular material made of the negative electrode active material, or the organic film is formed on the surface of the metal lithium foil It is possible that
[0063]
Since the organic coating is formed on the negative electrode surface, it functions to prevent direct contact between the negative electrode and the electrolyte. Thereby, reductive decomposition reaction of the electrolyte on the negative electrode surface is suppressed, gas generation due to decomposition of the electrolyte is reduced, and alteration of the electrolyte itself is prevented. This reduction in gas generation does not increase the internal pressure of the battery and prevents the battery from being deformed. Furthermore, by preventing the electrolyte from changing, the electrolytic mass does not decrease, the charge / discharge reaction proceeds smoothly, the charge / discharge efficiency increases, and the cycle characteristics improve.
Furthermore, since the reaction between the electrolyte and the negative electrode is suppressed, even when the battery is stored at a high temperature for a long time, the electrolyte does not deteriorate, and the battery characteristics such as charge / discharge efficiency and cycle characteristics do not deteriorate. .
In particular, although the low-flammability solvent described later is excellent in safety, it tends to decompose by reacting with the negative electrode, and the charge / discharge capacity tends to decrease. Therefore, by forming an organic coating on the negative electrode surface, direct contact between the low-flammability solvent and the negative electrode is prevented, and charge / discharge capacity is improved.
[0064]
Moreover, since the above-mentioned organic coating is excellent in lithium ion conductivity, it also functions to transport lithium ions between the electrolyte and the negative electrode.
Therefore, even if the negative electrode surface is covered with an organic film, there is no obstacle to the transport of lithium ions, the charge / discharge reaction proceeds smoothly, the charge / discharge efficiency is increased, and the cycle characteristics are improved. Further, the internal impedance of the battery does not increase, and the charge / discharge capacity does not significantly decrease.
[0065]
In addition, it is preferable that the polyacrylate compound is added in the range of 0.01-10 mass% in electrolyte before the formation of an organic film.
If the addition amount of the polyacrylate compound is less than 0.01% by mass, an organic film is not sufficiently formed, which is not preferable. If the addition amount exceeds 10% by mass, the thickness of the organic film increases and the internal impedance increases. Since it increases, it is not preferable.
[0066]
Next, the electrolyte according to the present invention contains a low-flammability solvent having a combustion heat of 19000 kJ / kg or less in a ratio of 10% by volume or more in the total solvent, and further contains a lithium salt.
The low-flammability solvent preferably has a combustion heat of 0 kJ / kg or more and 19000 kJ / kg or less, and more preferably 0 kJ / kg or more and 14000 kJ / kg or less.
If the combustion heat is 19000 kJ / kg or less, even if the battery is overcharged or an internal short circuit or the like causes decomposition of the electrolyte, there is no risk of ignition due to less heat generation during decomposition. Can improve safety.
[0067]
Examples of the low flammability solvent exhibiting heat of combustion of 19000 kJ / kg or less include halogenated benzene represented by the above [Chemical 9] or halogenated acetonitrile represented by the above [Chemical 10]. Benzene or acetonitrile halide has low combustion heat and can further enhance the safety of the lithium secondary battery.
[0068]
Moreover, it is preferable that a low combustibility solvent is a thing which does not show a flash point by the flash point test prescribed | regulated to JIS-K2265. If the electrolyte contains a low-flammability solvent that does not exhibit a flash point, the safety of the battery can be further improved.
As a specific example of the low flammability solvent that does not exhibit flammability, among the halogenated benzenes shown in the above [Chemical 9], 5 chlorine atoms (Cl) are substituted and 1 fluorine atom (F) is substituted. The following pentafluorochlorobenzene can be illustrated.
[0069]
The halogenated benzene shown in the above [Chemical 9] has properties of low viscosity and low dielectric constant. On the other hand, the halogenated acetonitrile shown in the above [Chemical Formula 10] has properties of high viscosity and high dielectric constant.
Therefore, as an example of the electrolyte according to the present invention, for example, a lithium salt dissolved in a mixed solvent of the halogenated benzene and the halogenated acetonitrile can be exemplified.
In addition to the hagenated benzene and the halogenated acetonitrile, a conventionally known aprotic solvent may be added.
[0070]
In this case, the content of the low flammability solvent in the electrolyte is preferably in the range of 10% by volume to 100% by volume when the volume fraction of the total solvent is 100% by volume, and is 50% by volume to 100% by volume. A range is more preferred.
If the content of the low-flammability solvent is less than 10% by volume, it is not preferable because the safety of the lithium secondary battery cannot be ensured.
[0071]
The volume ratio of the halogenated benzene and the halogenated acetonitrile is preferably in the range of halogenated benzene: halogenated acetonitrile = 4: 6 to 8: 2.
When the volume ratio of the halogenated benzene is smaller than the above range, the viscosity is increased and the ionic conductivity is lowered, which is not preferable. When the volume ratio of the halogenated benzene exceeds the above range, the solubility of the lithium salt is decreased. Since it falls, it is not preferable.
[0072]
Since halogenated benzene and halogenated acetonitrile have low compatibility and are easily separated, acetonitrile is preferably added in the range of 1 to 20% by mass in order to prevent separation.
[0073]
Moreover, as another example of the electrolyte according to the present invention, for example, a lithium salt dissolved in a mixed solvent of the halogenated benzene and a conventionally known aprotic solvent can be exemplified.
As the aprotic solvent in this case, those showing a high dielectric constant are preferable in order to compensate for the low dielectric constant of the halogenated benzene. For example, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, γ-butyrolactone, 4-methyl Examples include dioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide and the like.
[0074]
In this case, the content of halogenated benzene in the electrolyte is preferably in the range of 10% by volume to 80% by volume, and more preferably in the range of 20% by volume to 50% by volume.
If the halogenated benzene content is less than 10% by volume, it is not preferable because the safety of the lithium secondary battery cannot be ensured. If the halogenated benzene content exceeds 80% by volume, the solubility of the lithium salt decreases. This is not preferable because the lithium ion conductivity is lowered.
[0075]
Furthermore, as another example of the electrolyte according to the present invention, for example, a lithium salt dissolved in a mixed solvent of the halogenated acetonitrile and a conventionally known aprotic solvent can be exemplified.
In this case, the aprotic solvent preferably has a low viscosity so as to compensate for the high viscosity of the halogenated acetonitrile. For example, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl Examples include chain carbonates such as carbonate, dipropyl carbonate, diisopropyl carbonate, and dibutyl carbonate, 1,2-dimethoxyethane, sulfolane, dichloroethane, dioxolane, nitrobenzene, diethylene glycol, dimethyl ether, and the like.
[0076]
In this case, the content of halogenated acetonitrile in the electrolyte is preferably in the range of 10% by volume to 80% by volume, and more preferably in the range of 20% by volume to 50% by volume.
If the halogenated acetonitrile content is less than 10% by volume, it is not preferable because the safety of the lithium secondary battery cannot be ensured. If the halogenated acetonitrile content exceeds 80% by volume, the viscosity of the electrolyte increases. Since lithium ion conductivity falls, it is not preferable.
[0077]
Moreover, as a lithium salt, LiPF₆, LiBF_Four, LiSbF₆, LiAsF₆LiClO_Four, LiCF_ThreeSO_Three, Li (CF_ThreeSO₂)₂N, LiC_FourF₉SO_Three, LiSbF₆, LiAlO_Four, LiAlCl_Four, LiN (C_xF_{2x + 1}SO₂) (C_yF_{2y Ten 1}SO₂(Where x and y are natural numbers), one obtained by mixing one or more lithium salts of LiCl, LiI and the like.
LiB (OCOCF_Three)_FourThe thing shown by can also be used.
[0078]
Further, the electrolyte may be impregnated with a polymer such as PEO, PPO, PAN, PVDF, PMA, PMMA, or a polymer thereof.
In addition, in the above electrolyte, CO₂Is preferably dissolved. CO in the electrolyte₂Is dissolved in advance, so that some of the lithium ions that have moved to the negative electrode side in the second charging step, which will be described later, are part of this CO.₂To form a lithium carbonate film, and this lithium carbonate film suppresses the decomposition of the electrolyte by the negative electrode, so that the generation of gas and the alteration of the electrolyte can be prevented more reliably.
[0079]
Next, the manufacturing method of the lithium secondary battery of this invention is demonstrated.
The manufacturing method of the lithium secondary battery of this invention consists of the assembly process which arrange | positions the electrolyte containing a polyacrylate compound between the said positive electrode and the said negative electrode, and a 1st charge process.
Moreover, a heating process may be provided between the assembly process and the first charging process, and a second charging process may be performed after the first charging process.
[0080]
First, in the assembly process, an electrolyte comprising at least 10% by volume of a polyacrylate compound having 3 or more acrylic groups and the above low-flammability solvent exhibiting combustion heat of 19000 kJ / kg or less as a ratio in the total solvent. To prepare. The electrolyte may be an organic electrolytic solution containing a polyacrylate compound, a low flammability solvent, and a lithium salt, or may be a polymer electrolyte obtained by impregnating a polymer with the organic electrolytic solution.
Further, a conventional aprotic solvent may be added, and acrylonitrile may be added to improve compatibility.
The addition amount of the polyacrylate compound is preferably in the range of 0.01 to 10% by mass, and more preferably in the range of 0.1 to 5% by mass. Moreover, it is preferable to add the low flammability solvent in the range of 10 volume% or more and 100 volume% or less.
At this time, in the electrolyte, CO₂Is preferably dissolved. CO for electrolyte₂Is dissolved in the organic electrolyte.₂Means such as blowing gas can be taken. CO in the electrolyte₂Is dissolved in advance, so that some of the lithium ions that have moved to the negative electrode side in the second charging step, which will be described later, are part of this CO.₂To form a lithium carbonate film, and this lithium carbonate film suppresses the decomposition of the electrolyte by the negative electrode, so that the generation of gas and the alteration of the electrolyte can be prevented more reliably.
[0081]
Next, this electrolyte is disposed between the positive electrode and the negative electrode. When the electrolyte is in a liquid state, it may be impregnated with the electrolyte in a state where a separator is interposed between the positive electrode and the negative electrode. When the electrolyte is solid or semi-solid like a polymer electrolyte, the electrolyte may be sandwiched between the positive electrode and the negative electrode.
[0082]
Next, in the heating step, heat treatment is performed in a temperature range of 40 to 120 ° C. with an electrolyte containing at least a polyacrylate compound disposed between the positive and negative electrodes. By this heat treatment, the polyacrylate compound in the electrolyte undergoes radical polymerization to form a polymer, and this polymer is impregnated with an organic electrolyte to form an electrolyte. Further, a part of the polyacrylate compound is adsorbed on the negative electrode surface.
In addition, it is not preferable that the heating temperature is less than 40 ° C. because radical polymerization of the polyacrylate compound does not proceed sufficiently. On the other hand, when the heating temperature exceeds 120 ° C., the electrolyte is deteriorated and battery characteristics are deteriorated.
Moreover, when the polymer electrolyte is previously sandwiched between the positive and negative electrodes in the assembly process, the heating process may be omitted. Furthermore, the heating step may be omitted when an electrolyte mainly composed of an organic electrolyte is formed.
Furthermore, when it is not desired to form a polymer electrolyte, the heating step may be omitted. In this case, the electrolyte is an organic electrolyte.
[0083]
Next, in the first charging step, constant voltage charging was performed until the potential of the negative electrode when metallic lithium was used as a reference electrode reached a range of 0.7 V or more and 1.5 V or less, and then the voltage of the negative electrode was maintained. A constant voltage charge is performed for 0.1 to 8 hours. The current during constant current charging is preferably about 0.01 to 0.3C.
By this first charging step, the polyacrylate compound is anionically polymerized to form an organic coating on the negative electrode surface before reductive decomposition of the electrolyte occurs.
That is, the polyacrylate compound undergoes anion addition polymerization when the potential of the negative electrode in the case of using metallic lithium as a reference electrode is in the range of 0.7 to 1.5 V, and when the potential is 0.7 V or more, the electrolyte is reduced. Since decomposition does not occur, it is necessary to limit the lower limit of the charging voltage to 0.7V. In addition, since the anionic polymerization on the negative electrode surface has a relatively slow reaction, constant voltage charging for 1 to 8 hours is required in the state where the above charging voltage is maintained in order to sufficiently advance the polymerization reaction.
A negative electrode potential of less than 0.7 V is not preferable because reductive decomposition reaction of the electrolyte occurs simultaneously.
[0084]
Moreover, it is not preferable that the potential of the negative electrode exceeds 1.5 V in constant current charging because the polymerization reaction of the polyacrylate compound does not start.
Next, if the charging time in constant voltage charging is less than 0.1 hour, the polymerization reaction of the polyacrylate compound does not proceed sufficiently, and defects may occur in the organic coating, which is not preferable, and the charging time exceeds 8 hours. Since the polymerization reaction is almost completed, there is no practical benefit of charging in the above voltage range in a longer time.
[0085]
In the first charging step, the positive electrode is LiCoO.₂LiNiO₂, LiMn₂O_FourIn the case of any one or more of the above, after performing constant current charging until the battery voltage reaches the range of 2.3 V or more and 3.1 V or less, the battery voltage is maintained for 0.1 to 8 hours. It is preferable to perform voltage charging.
[0086]
When acrylonitrile and / or methacrylonitrile is added to the organic electrolyte together with the polyacrylate compound, an organic film containing the polyacrylate compound and acrylonitrile and / or methacrylonitrile is formed. When acrylonitrile and / or methacrylonitrile is included, the ion conductivity of lithium in the organic coating is improved, the internal impedance of the battery is reduced, and the charge / discharge efficiency is improved. Either acrylonitrile and / or methacrylonitrile is polymerized together with the polyacrylate compound and is present in the organic film, or is present in the organic film in a dissolved state in the polymer composed only of the polyacrylate compound. It is considered to be in one or both states.
In addition, the concentration of the polyacrylate compound, acrylonitrile and / or methacrylonitrile contained in the organic electrolyte is remarkably reduced with the formation of the film.
[0087]
In the second charging step, constant current charging is performed until the negative electrode potential reaches a range of 0.0 V or more and 0.1 V or less when metallic lithium is used as a reference electrode, and then the negative electrode potential is set to 0.0 V or more and 0. .Constant voltage charging for 1 to 8 hours while maintaining 1V or less. The current during constant current charging is preferably about 0.1 to 0.5C.
In the second charging step, since the organic coating has already been formed, reductive decomposition of the low-flammability solvent is suppressed without the electrolyte and the negative electrode being in direct contact.
If the potential of the negative electrode in constant current charging exceeds 0.1V, the battery capacity becomes insufficient, which is not preferable, and if it is less than 0.0V, the crystal structure of the positive electrode may be destroyed.
Further, if the charging time in constant voltage charging is less than 1 hour, it is not preferable because charging becomes insufficient, and if the charging time exceeds 8 hours, it is not preferable because an overcharged state occurs and the positive electrode deteriorates.
[0088]
In the second charging step, the positive electrode is LiCoO.₂LiNiO₂, LiMn₂O_FourWhen one or more of the above is selected, constant current charging is performed until the battery voltage reaches a range of 4.0 V or more and 4.3 V or less, and then the constant voltage charging is performed for 1 to 8 hours while maintaining the battery voltage. It is preferable to carry out.
In addition, it is preferable to provide a pause time of about 1 to 8 hours between the first charging step and the second charging step because the polymerization reaction proceeds sufficiently when the first charging time is not sufficiently long.
In this second charging step, CO dissolved in the electrolyte in the assembly step in advance.₂However, it reacts with some of the lithium ions that have moved to the negative electrode side to form a lithium carbonate film on the negative electrode surface. This lithium carbonate film prevents contact between the negative electrode and the electrolyte and suppresses decomposition of the electrolyte, generating gas. In addition, it is possible to prevent the alteration of the electrolyte more reliably.
[0089]
According to the above method for producing a lithium secondary battery, the electrolyte contains a low-flammability solvent having a combustion heat of 19000 kJ / kg or less, so that it is possible to prevent ignition by the electrolyte even when overcharging or internal short-circuiting occurs. A lithium secondary battery excellent in performance can be provided.
In addition, since the organic film is formed by polymerizing the polyacrylate compound adsorbed on the negative electrode surface in the first charging step, the organic film can be formed on the surface of the negative electrode before the electrolyte is decomposed. Formation of this organic coating makes it possible to suppress decomposition of the electrolyte in the second charging step described later, and to prevent gas generation and alteration of the electrolyte.
In addition, since the constant voltage charging in the first charging step is performed for a relatively long time, the polymerization reaction of the polyacrylate compound is sufficiently performed, the reaction yield of the organic coating is increased, and a sufficient organic coating is formed. Is done.
Moreover, since a part of electrolyte is absorbed by an organic film by performing a 1st charge process, the affinity of an organic film and an electrolyte improves, and it becomes possible to improve charging / discharging efficiency.
[0090]
【Example】
[Evaluation of lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3]
(Production of lithium secondary battery of Example 1)
Acetonitrile fluoride (CH₂FCN) and hexafluorobenzene (C₆F₆LiB (OCOCF) so that the concentration becomes 1.4 mol / L with respect to the mixed solvent in which 50% by volume is mixed._Three)_FourWas added to prepare an electrolyte. To this electrolyte, 0.2% by mass of trimethylolpropane triacrylate (molecular weight 269) shown in the above [Chemical Formula 16] was added.
Next, LiCoO₂After inserting a sheet-like positive electrode having a positive electrode active material and a sheet-like negative electrode having carbon fiber as a negative electrode active material into a spirally wound state and injecting the above electrolyte The battery container was sealed to manufacture a square battery having a thickness of 4 mm, a width of 30 mm, and a height of 60 mm. In addition, before injecting the electrolyte,₂Was blown in for 10 minutes. As a result, the electrolyte contains CO.₂Is in a dissolved state.
[0091]
The obtained prismatic battery is charged with a constant current until the battery voltage reaches 3V (the potential of the negative electrode with respect to metallic lithium is 0.7V) at a current of 0.2C, and then is charged with a constant voltage for 4 hours. In one charging step, the polyacrylate compound was polymerized to form an organic film. Next, a second charging step is performed in which constant current charging is performed until the battery voltage reaches 4.2V (the potential of the negative electrode with respect to metallic lithium is 0.1V) at a current of 0.2 C, and then constant voltage charging is performed for 9 hours. As a result, the lithium secondary battery of Example 1 was manufactured.
In addition, the coulomb efficiency in a negative electrode was measured in the case of the 1st, 2nd charge process.
[0092]
(Production of lithium secondary battery of Example 2)
Acetonitrile (CH_ThreeCN) is 25% by volume, chloropentafluorobenzene (C₆ClF_Five) 50% by volume, acetonitrile fluoride (CH₂FCB) is mixed with 25% by volume of LiB (OCOCF) so that the concentration becomes 1.4 mol / L._Three)_FourWas added to prepare an electrolyte. To this electrolyte, 0.3% by mass of trimethylolpropane triacrylate (molecular weight 269) shown in the above [Chemical Formula 16] was added.
Next, LiCoO₂After inserting a sheet-like positive electrode having a positive electrode active material and a sheet-like negative electrode having carbon fiber as a negative electrode active material into a spirally wound state and injecting the above electrolyte The battery container was sealed to manufacture a square battery having a thickness of 4 mm, a width of 30 mm, and a height of 60 mm. In addition, before injecting the electrolyte,₂Was blown in for 10 minutes. As a result, the electrolyte contains CO.₂Is in a dissolved state.
[0093]
The lithium secondary battery of Example 2 was manufactured by performing the 1st, 2nd charging process like Example 1 with respect to the obtained square battery.
In addition, the Coulomb efficiency in the negative electrode was measured during the first and second charging steps.
[0094]
(Production of lithium secondary battery of Comparative Example 1)
A lithium secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the trimethylolpropane triacrylate shown in the above [Chemical Formula 16] was not added to the electrolyte. CO₂The negative electrode is not dried in the atmosphere.
As in Example 1, the Coulomb efficiency at the negative electrode was measured during the first and second charging steps.
[0095]
(Production of lithium secondary battery of Comparative Example 2)
LiPF was adjusted to a concentration of 1.3 mol / L with respect to a mixed solvent in which ethylene carbonate was mixed at 30% by volume and diethyl carbonate at 70% by volume.₆A lithium secondary battery of Comparative Example 2 was produced in the same manner as in Example 1 except that the electrolyte was prepared by adding. CO₂There is no blowing.
As in Example 1, the Coulomb efficiency at the negative electrode was measured during the first and second charging steps.
[0096]
(Production of lithium secondary battery of Comparative Example 3)
Acetonitrile (CH_ThreeCN) is 25% by volume, chloropentafluorobenzene (C₆ClF_Five) 50% by volume, acetonitrile fluoride (CH₂FCB) is mixed with 25% by volume of LiB (OCOCF) so that the concentration becomes 1.4 mol / L._Three)_FourWas added to prepare an electrolyte.
Next, LiCoO₂After inserting a sheet-like positive electrode having a positive electrode active material and a sheet-like negative electrode having carbon fiber as a negative electrode active material into a spirally wound state and injecting the above electrolyte The battery container was sealed to manufacture a square battery having a thickness of 4 mm, a width of 30 mm, and a height of 60 mm.
[0097]
The lithium secondary battery of Comparative Example 3 was manufactured by performing the first and second charging steps on the obtained prismatic battery in the same manner as in Example 1. CO₂There is no blowing.
In addition, the Coulomb efficiency in the negative electrode was measured during the first and second charging steps.
[0098]
[Coulomb curves of lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3]
1 and 2 show the Coulomb efficiency in the first and second charging steps after the heat treatment of Example 1 and Comparative Examples 1 and 2. FIG. FIG. 1 is a partially enlarged view of FIG. 2, showing the coulomb efficiency of the first charging step, and FIG. 2 showing the coulomb efficiency of the first and second charging steps.
3 and 4 show the Coulomb efficiency in the first and second charging steps after the heat treatment of Example 2 and Comparative Example 3. FIG. FIG. 3 is a partially enlarged view of FIG. 4 showing the coulomb efficiency of the first charging process, and FIG. 4 shows the coulomb efficiency of the first and second charging processes.
[0099]
As shown in FIG. 1, in the 1st charge process of Example 1, the small peak corresponding to the polymerization reaction of a polyacrylate compound is observed by charging voltage 2.3V vicinity. This small peak is thought to be due to the formation of an organic film on the negative electrode surface.
Next, in the second charging step shown in FIG. 2, the coulomb efficiency is gently increased as the charging voltage is improved. This is presumably because the organic coating was formed on the negative electrode surface in the first charging step, so that the negative electrode and the electrolyte were not in direct contact, and the decomposition of the electrolyte on the negative electrode surface was suppressed.
[0100]
Further, as shown in FIG. 2, in the first charging step of Example 2, no particularly conspicuous peak was observed, an organic film was formed by the polymerization reaction of the polyacrylate compound, and decomposition of the solvent did not occur. It is done.
[0101]
Next, in the 1st charge process of the comparative example 1 shown in FIG. 1, the big peak considered to be due to the decomposition reaction of hexafluorobenzene is observed by 2.7V vicinity. Further, at this time, the internal pressure of the battery of Comparative Example 1 is rapidly increased, and it is presumed that decomposition gas is generated due to decomposition of hexafluorobenzene.
Next, in the 1st charge process of the comparative example 2 shown in FIG. 1, the conspicuous peak is not observed and it is thought that formation of an organic film, decomposition | disassembly of a solvent, etc. have not occurred.
Next, in the 1st charge process of the comparative example 3 shown in FIG. 2, the big peak considered to be due to the decomposition reaction of chloropentafluorobenzene and acetonitrile is observed by 2.7V vicinity. Further, at this time, the internal pressure of the battery of Comparative Example 3 is rapidly increased, and it is presumed that decomposition gas is generated due to decomposition of chloropentafluorobenzene and acetonitrile.
[0102]
[Initial discharge capacities of lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3]
The discharge capacity when the lithium secondary battery after the second charging step was discharged with a current of 0.1 C, 0.2 C, 0.5 C, and 1 C was measured. The results are shown in Table 1.
[0103]
[Table 1]

[0104]
As shown in Table 1, it can be seen that Examples 1 and 2 show a discharge capacity substantially equal to the discharge capacity of Comparative Example 2 in the range of 0.1 C to 1 C. Further, in Comparative Example 1, even when discharged at a relatively low current such as 0.1 C, the discharge capacity is reduced by about 10% compared to Example 1 or Comparative Example 2.
In Example 1, in addition to the formation of the organic film, the electrolyte contains CO 2.₂It is considered that a lithium carbonate film is formed on the negative electrode surface at the end of the second charging step, and this lithium carbonate film suppresses the decomposition of hexafluorobenzene together with the organic film. it is conceivable that.
In Comparative Examples 1 and 3, an organic coating was not formed on the negative electrode surface because the polyacrylate compound was not added. This caused a decomposition reaction of the halogenated benzene, resulting in a decrease in discharge capacity due to decomposition of the electrolyte. it is conceivable that.
[0105]
[Heating test of lithium secondary batteries of Examples 1 and 2 and Comparative Example 3]
The lithium secondary batteries of Examples 1 and 2 and Comparative Example 3 were subjected to a heating test. The heating test was performed under the condition that the lithium secondary battery was heated to 160 ° C. in a heating oven at a temperature rising rate of 5 ° C./min and held for 60 minutes.
As a result, the lithium secondary batteries of Examples 1 and 2 were not abnormal, but the battery of Comparative Example 3 was self-heated, the battery temperature increased, and the safety valve was activated and burst. It came to.
[0106]
【The invention's effect】
As described above in detail, according to the lithium secondary battery of the present invention, the electrolyte contains a low-flammability solvent having a combustion heat of 19000 kJ / kg or less, so even when overcharging or internal short circuit occurs, It can prevent ignition by electrolyte.
[Brief description of the drawings]
FIG. 1 is a diagram showing coulomb efficiency with respect to a charging voltage in a first charging process of Example 1 and Comparative Examples 1 and 2. FIG.
FIG. 2 is a graph showing the coulomb efficiency with respect to the charging voltage in the first and second charging steps of Example 1 and Comparative Examples 1 and 2;
3 is a graph showing coulomb efficiency with respect to a charging voltage in the first charging step of Example 2 and Comparative Example 3. FIG.
4 is a graph showing coulomb efficiency with respect to charging voltage in the first and second charging steps of Example 2 and Comparative Example 3. FIG.

Claims (24)

A low-flammability solvent which is a halogenated benzene or a halogenated acetonitrile, in a proportion of 10% by volume or more in the total solvent, and further contains a lithium salt;
Furthermore, at least 1 sort (s) of acrylonitrile, methacrylonitrile, trifunctional or more than trifunctional acrylate derivative is added, The electrolyte characterized by the above-mentioned.   2. The electrolyte according to claim 1, wherein the low-flammability solvent does not exhibit a flash point by a flash point test defined in JIS-K2265.   2. The electrolyte according to claim 1, wherein the trifunctional or higher functional acrylate derivative is added in an amount of 0.01 to 10% by mass. The electrolyte according to any one of claims 1 to 3, wherein the halogenated benzene is represented by the following [Chemical Formula 1].

The electrolyte according to any one of claims 1 to 3, wherein the halogenated acetonitrile is represented by the following [Chemical Formula 2].

Electrolyte according to any one of claims 1 to 5 CO ₂ is characterized by comprising dissolved. A positive electrode and a negative electrode capable of occluding and releasing lithium; and an electrolyte, wherein the electrolyte includes a low-flammability solvent which is a halogenated benzene or a halogenated acetonitrile in a proportion of 10% by volume or more in the total solvent, Further containing lithium salt,
Furthermore, at least 1 sort (s) of acrylonitrile, a methacrylonitrile, a trifunctional or more than trifunctional acrylic acid ester derivative is added, The lithium secondary battery characterized by the above-mentioned. The lithium secondary battery according to claim 7 , wherein the low-flammability solvent does not exhibit a flash point by a flash point test specified in JIS-K2265. The lithium secondary battery according to claim 7 , wherein the trifunctional or higher functional acrylate derivative is added in the range of 0.01 to 10% by mass in the electrolyte. A positive and negative electrode capable of occluding and releasing lithium, and an electrolyte. The electrolyte is a polymer composed of a trifunctional or higher functional acrylate derivative, and is a halogenated benzene or halogenated acetonitrile. The organic solvent is impregnated with 10% by volume or more in a proportion of the total solvent and impregnated with a lithium salt,
A lithium secondary battery, wherein an organic film made of a trifunctional or higher functional acrylate derivative is formed on the surface of the negative electrode. Comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte mainly composed of an organic electrolyte,
The electrolyte contains 10% by volume or more of a low flammability solvent, which is halogenated benzene or halogenated acetonitrile , in addition to lithium salt, and further contains acrylonitrile, methacrylonitrile, trifunctional or higher acrylic. At least one of acid ester derivatives is added,
A lithium secondary battery, wherein an organic film made of at least one of acrylonitrile, methacrylonitrile, trifunctional or higher acrylate derivatives is formed on a surface of the negative electrode. 12. The lithium secondary battery according to claim 7 , wherein the halogenated benzene is represented by the following [Chemical Formula 3].

12. The lithium secondary battery according to claim 7 , wherein the halogenated acetonitrile is represented by the following [Chemical Formula 4].

The lithium secondary battery according to any one of claims 7 to 13, characterized in that by dissolving the CO ₂ in the electrolyte. A method for producing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte,
The electrolyte contains 10% by volume or more of a low flammability solvent that is halogenated benzene or halogenated acetonitrile, and at least one of acrylonitrile, methacrylonitrile, trifunctional or higher acrylate derivatives,
An assembly step of disposing the electrolyte between at least the positive electrode and the negative electrode;
After performing constant current charging until the potential of the negative electrode in the case of using metal lithium as a reference electrode reaches a range of 0.7 V or more and 1.5 V or less, the potential of the negative electrode is maintained while maintaining the potential of the negative electrode. A method for producing a lithium secondary battery, comprising: a first charging step for performing constant voltage charging for a period of time. A method for producing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte,
The active material of the positive electrode is at least one of complex oxides of lithium and at least one selected from cobalt, manganese, and nickel,
The electrolyte contains 10% by volume or more of a low flammability solvent that is halogenated benzene or halogenated acetonitrile, and at least one of acrylonitrile, methacrylonitrile, trifunctional or higher acrylate derivatives,
An assembly step of disposing the electrolyte between at least the positive electrode and the negative electrode;
From the first charging step of performing constant voltage charging for 0.1 to 8 hours while maintaining the battery voltage after performing constant current charging until the battery voltage reaches a range of 2.3 V or more and 3.1 V or less. A method for producing a lithium secondary battery, comprising: The lithium secondary battery according to claim 15 or 16 , further comprising a heat treatment step of heat-treating at least the electrolyte in a range of 40 to 120 ° C between the assembly step and the first charging step. Production method. The lithium secondary battery according to any one of claims 15 to 17 , wherein the trifunctional or higher functional acrylate derivative is added in an amount of 0.01 to 10% by mass in the electrolyte. Method. The lithium secondary battery according to any one of claims 15 to 18 , wherein the low-flammability solvent does not exhibit a flash point by a flash point test specified in JIS-K2265. Method. The method for producing a lithium secondary battery according to any one of claims 15 to 19, wherein the halogenated benzene is represented by the following [Chemical Formula 5].

The method for manufacturing a lithium secondary battery according to any one of claims 15 to 19, wherein the halogenated acetonitrile is represented by the following [Chemical Formula 6].

After the first charging step, constant current charging is performed until the potential of the negative electrode reaches a range of 0 V to 0.1 V, and then constant voltage charging is performed for 1 to 8 hours while maintaining the potential of the negative electrode. method for producing a lithium secondary battery according to any one of claims 15 or claim 17 to claim 21, characterized in that performing the second charging step of performing. After the first charging step, constant current charging is performed until the battery voltage reaches a range of 4.0 V to 4.3 V, and then constant voltage charging is performed for 1 to 8 hours while maintaining the battery voltage. The method for manufacturing a lithium secondary battery according to any one of claims 16 to 21, wherein a second charging step is performed. In the assembly step, the manufacturing method of a lithium secondary battery according to any one of claims 15 to claim 23, characterized in that dissolving of CO ₂ in the electrolyte.

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