JP4352532B2

JP4352532B2 - Secondary power supply

Info

Publication number: JP4352532B2
Application number: JP30010099A
Authority: JP
Inventors: 学對馬; 剛森本; 勇車
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 1999-10-21
Filing date: 1999-10-21
Publication date: 2009-10-28
Anticipated expiration: 2019-10-21
Also published as: JP2001118576A

Abstract

PROBLEM TO BE SOLVED: To provide a secondary cell having superior characteristics in high withstanding voltage, high capacity or high speed charging and discharging cycles. SOLUTION: A secondary cell has a positive electrode which has active carbon as main body, a negative electrode having carbon material which can adsorb or emit lithium ions whose intensity ratio I1360/I1580 of the peak near 1,360 cm-1 to the peak near 1,580 cm-1 in Lyman spectrum is 0.12&sim;1.0 and carbon material which became graphite, its I1360/I1580 ratio being 0.01&sim;0.115, and organic electrolyte including lithium base.

Description

【０００１】
【発明の属する技術分野】
本発明は、耐電圧が高く、容量が大きく、急速充放電サイクル信頼性の高い二次電源に関する。
【０００２】
【従来の技術】
従来の電気二重層キャパシタの電極には、正極、負極ともに活性炭を主体とする分極性電極が使用されている。電気二重層キャパシタの耐電圧は、水系電解液を使用すると１．２Ｖ、有機系電解液を使用すると２．５〜３．３Ｖである。電気二重層キャパシタのエネルギは電圧の２乗に比例するので、耐電圧の高い有機電解液の方が水系電解液より高エネルギである。しかし、有機電解液を使用した電気二重層キャパシタでもそのエネルギ密度は鉛蓄電池等の二次電池の１／１０以下であり、さらなるエネルギ密度の向上が必要とされている。
【０００３】
これに対し、特開昭６４−１４８８２には、活性炭を主体とする電極を正極とし、Ｘ線回折による［００２］面の面間隔が０．３３８〜０．３５６ｎｍである炭素材料にあらかじめリチウムイオンを吸蔵させた電極を負極とする上限電圧３Ｖの二次電源が、また、特開平８−１０７０４８には、リチウムイオンを吸蔵、脱離しうる炭素材料にあらかじめ化学的方法又は電気化学的方法でリチウムイオンを吸蔵させた炭素材料を負極に用いる電池が、また、特開平９−５５３４２には、リチウムイオンを吸蔵、脱離しうる炭素材料をリチウムと合金を形成しない多孔質集電体に担持させる負極を有する、上限電圧４Ｖの二次電源が、提案されている。しかしこれらの二次電源は、負極の炭素材料にあらかじめリチウムイオンを吸蔵させる工程を必要とする問題があった。
【０００４】
また、電気二重層キャパシタ以外に大電流充放電可能な電源にはリチウムイオン二次電池がある。リチウムイオン二次電池は電気二重層キャパシタに比べて高電圧かつ高容量という性質を有するが、抵抗が高く、急速充放電サイクルによる寿命が電気二重層キャパシタに比べ著しく短い問題があった。
【０００５】
【発明が解決しようとする課題】
そこで本発明は、急速充放電が可能で、高耐電圧かつ高容量でエネルギ密度が高く、充放電サイクル信頼性の高い二次電源を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明は、活性炭を主体とする正極と、ラマンスペクトルにおける１３６０ｃｍ^-1近傍のピーク強度Ｉ₁₃₆₀と１５８０ｃｍ^-1近傍のピーク強度Ｉ₁₅₈₀の比Ｉ₁₃₆₀／Ｉ₁₅₈₀が０．１２〜１．０であるリチウムイオンを吸蔵、脱離しうる炭素材料とＩ₁₃₆₀／Ｉ₁₅₈₀が０．０１〜０．１１５である黒鉛化した炭素材料とを含む負極と、リチウム塩を含む有機電解液と、を有し、前記黒鉛化した炭素材料が、気相成長炭素繊維を黒鉛化したものであることを特徴とする二次電源を提供する。
【０００７】
また、本発明は、活性炭を主体とする正極と、リチウムイオンを吸蔵、脱離しうる炭素材料を含む負極と、リチウム塩を含む有機電解液と、を有する二次電源において、前記負極には、ラマンスペクトルにおける１３６０ｃｍ^-1近傍のピーク強度Ｉ₁₃₆₀と１５８０ｃｍ^-1近傍のピーク強度Ｉ₁₅₈₀の比Ｉ₁₃₆₀／Ｉ₁₅₈₀が０．０１〜０．１１５である黒鉛化した気相成長炭素繊維が５〜２０質量％含まれることを特徴とする二次電源を提供する。
【０００８】
本明細書において、リチウムイオンを吸蔵、脱離しうる炭素材料を主体とする負極と集電体とを接合して一体化させたものを負極体という。正極体についても同様の定義とする。また、二次電池も電気二重層キャパシタも二次電源の１種であるが、本明細書では、正極に活性炭を含み、負極にリチウムイオンを吸蔵、脱離しうる炭素材料を含む特定の構成の二次電源を単に二次電源という。
【０００９】
一般に炭素のラマンスペクトルでは、１３６０ｃｍ^-1近傍と１５８０ｃｍ^-1近傍とにピークが現れる。１３６０ｃｍ^-1近傍のピークはＤバンドのピークであり乱層構造に起因する。１５８０ｃｍ^-1近傍のピークはＧバンドのピークであり炭素の黒鉛構造に起因する。したがって黒鉛構造のエッジ面が多いとこのピーク強度比Ｉ₁₃₆₀／Ｉ₁₅₈₀は大きくなり、ベーサル面が多いと強度比は小さくなる。また、Ｉ₁₃₆₀／Ｉ₁₅₈₀は炭素の黒鉛化度を示すパラメータでもあり、この値が小さいほど黒鉛化が進んでいる。本発明では導電剤として黒鉛化炭素を使用するので、黒鉛化が進んでいる炭素の方が好ましい。
【００１０】
そのため、本発明の二次電源においては、Ｉ₁₃₆₀／Ｉ₁₅₈₀が０．０１〜０．１１５の炭素材料を使用する。この値が０．１１５超の炭素材料では黒鉛化が進んでいないため、負極の抵抗低減の効果がほとんど見られない。一方、０．０１未満の炭素材料は得ることが困難である。好ましくは０．０３〜０．１１５であり、さらに好ましくは０．０５〜０．１１である。
【００１１】
なお、本明細書において１５８０ｃｍ^-1近傍のピーク及び１３６０ｃｍ^-1近傍のピークは、それぞれ１５６５〜１５９５ｃｍ^-1に現れるピーク及び１３４５〜１３７５ｃｍ^-1に現れるピークを示すものとする。
【００１２】
本発明における特定のラマンスペクトル特性を有する黒鉛化した炭素材料（以下、本発明における黒鉛化炭素という。）の原料、製法等は特に限定されない。例えば石油系、石炭系のコークス、ピッチ等を原料として最終的に２８００℃以上で熱処理すると黒鉛化した炭素材料が得られる。本発明における黒鉛化炭素としては、原料として気相成長炭素繊維（以下、ＶＧＣＦという。）を用いて、黒鉛化したものであることが好ましい。ＶＧＣＦは、鉄微粒子を触媒としてベンゼン蒸気を１０００℃付近で熱分解させることにより得られる。得られたＶＧＣＦを不活性雰囲気中、２８００℃以上で熱処理すると黒鉛化できる。
【００１３】
本発明では、負極中に本発明における黒鉛化炭素が含まれることにより、負極の抵抗が低減でき、その結果二次電源の抵抗を低減できるため、高出力放電に適する二次電源を得られる。負極の抵抗が低減する機構としては、黒鉛化炭素自体の抵抗が低いことに加え、黒鉛化炭素の存在によりリチウムイオンを吸蔵、脱離しうる炭素材料間の導電性を高められるためと思われる。特に黒鉛化したＶＧＣＦを用いると前記炭素材料間の導電性を高める効果が大きい。黒鉛化したＶＧＣＦを使用する場合、その効果を高めるためには実質的に繊維径が０．１〜２０μｍ、特に１〜１０μｍであることが好ましく、実質的に繊維長が１〜１００μｍ、特に５〜５０μｍであることが好ましい。
【００１４】
負極中の、本発明における黒鉛化炭素の量は、５〜２０質量％が好ましい。５質量％未満では、負極の抵抗を低減できない。一方、２０質量％超では、特に黒鉛化したＶＧＣＦ等の繊維を添加する場合、負極の密度を上げることができず、負極容量が低下する。より好ましくは、８〜１５質量％である。
【００１５】
一般にリチウムイオン二次電池の場合は、正極はリチウム含有遷移金属酸化物を主体とする電極、負極はリチウムイオンを吸蔵、脱離しうる炭素材料を主体とする電極であり、充電によりリチウムイオンが正極のリチウム含有遷移金属酸化物から脱離し、負極のリチウムイオンを吸蔵、脱離しうる炭素材料へ吸蔵され、放電により負極からリチウムイオンが脱離し、正極にリチウムイオンが吸蔵される。したがって、本質的には電解液中のリチウムイオンは電池の充放電に関与しない。
【００１６】
一方、本発明の二次電源は、充電により電解液中のアニオンが正極の活性炭に吸着し、電解液中のリチウムイオンが負極のリチウムイオンを吸蔵、脱離しうる炭素材料へ吸蔵される。そして放電により負極からリチウムイオンが脱離し、正極では前記アニオンが脱着される。すなわち、本発明の二次電源では充放電に電解液の溶質が本質的に関与しており、リチウムイオン電池とは充放電の機構が異なっている。また、リチウムイオン二次電池のように正極活物質自体にリチウムイオンが吸蔵、脱離することがないため、本発明の二次電源は充放電サイクル信頼性に優れている。
【００１７】
正極に活性炭、負極にリチウムイオンを吸蔵、脱離しうる炭素材料を用いた二次電源では、電解液に溶解しているイオンが充放電に関与する。したがって、電解液の溶質濃度が低い場合には十分に充電できなくなるおそれがある。溶質濃度としては０．５〜２．０ｍｏｌ／Ｌ、特に０．７５〜１．５ｍｏｌ／Ｌが好ましい。
【００１８】
本発明の二次電源では、１度目の充放電サイクルにおける負極のサイクル効率は必ずしも１００％ではなく、吸蔵されたリチウムイオンで脱離しないものもある。その場合、電解液中のリチウムイオン濃度が減少し次の充電から十分に充電できないおそれがあるので、正極にリチウム含有遷移金属酸化物を添加して特性劣化を防ぐことが好ましい。この方法により、負極から脱離できないリチウムイオンを補うことができる。この場合、正極中に含まれるリチウム含有遷移金属酸化物は０．１〜２０質量％、特に３〜１５質量％が好ましい。０．１質量％未満ではその効果が小さく、一方、２０質量％超ではリチウム含有遷移金属酸化物の容量が大きいため、活性炭電極の特徴の高出力かつ高信頼性という二次電源性能が得られなくなる。
【００１９】
上記リチウム含有遷移金属酸化物としては、Ｖ、Ｍｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｚｎ及びＷからなる群から選ばれる１種以上の遷移金属とリチウムとの複合酸化物が好ましい。特に、Ｍｎ、Ｃｏ及びＮｉからなる群から選ばれる１種以上とリチウムとの複合酸化物が好ましく、なかでもＬｉ_xＣｏ_yＮｉ_(1-y)Ｏ₂又はＬｉ_zＭｎ₂Ｏ₄（ただし、０＜ｘ＜２、０≦ｙ≦１、０＜ｚ＜２。）で表される化合物が好ましい。
【００２０】
本発明において、正極に含まれる活性炭は、比表面積が８００〜３０００ｍ²／ｇであることが好ましい。活性炭の原料、賦活条件は限定されないが、例えば原料としてはやしがら、フェノール樹脂、石油コークス等が挙げられ、賦活方法としては水蒸気賦活法、溶融アルカリ賦活法等が挙げられる。本発明では特に、水蒸気賦活したやしがら系活性炭又は水蒸気賦活したフェノール樹脂系活性炭が好ましい。また、正極の抵抗を低くするために、正極中に導電材として導電性のカーボンブラック又は黒鉛を含ませておくのも好ましく、このとき導電材は正極中に０．１〜２０質量％であることが好ましい。
【００２１】
正極体の作製方法としては、例えば活性炭粉末にバインダとしてポリテトラフルオロエチレンを混合し、混練した後シート状に成形して正極とし、これを集電体に導電性接着剤を用いて固定する方法がある。また、バインダとしてポリフッ化ビニリデン、ポリアミドイミド、ポリイミド等を用い、これらを溶解したワニスに活性炭粉末を分散させ、この液をドクターブレード法等によって集電体上に塗工し、乾燥して得てもよい。正極中に含まれるバインダの量は、正極体の強度と容量等の特性とのバランスから質量比で１〜２０％であることが好ましい。
【００２２】
本発明におけるリチウムイオンを吸蔵、脱離しうる炭素材料は、Ｘ線回折の測定による［００２］面の面間隔が０．３３５〜０．４１０ｎｍであることが好ましい。面間隔が０．４１０ｎｍ超の炭素材料は充放電サイクルにおいて劣化しやすい。具体的には石油コークス、メソフェーズピッチ系炭素材料又は気相成長炭素繊維を８００〜３０００℃で熱処理した材料、天然黒鉛、人造黒鉛、難黒鉛性炭素材料等が挙げられる。本発明ではこれらの材料はいずれも好ましく使用できる。このなかでも［００２］面の面間隔が０．３３５〜０．３３７ｎｍの難黒鉛性炭素材料や、天然黒鉛又は易黒鉛性炭素材料を２８００℃以上で熱処理した［００２］面の面間隔が０．３３５〜０．３３７ｎｍの材料は、リチウムイオン吸蔵、脱離の電位が低く好ましい。
【００２３】
なお、本発明において負極に含有されるリチウムイオンを吸蔵、脱離しうる炭素材料は黒鉛系の炭素材料であってもラマンスペクトルにおけるＩ₁₃₆₀／Ｉ₁₅₈₀の値は通常０．０１〜０．１１５の範囲には入らず、通常０．１２〜１．０の範囲である。
【００２４】
本発明における負極体は、正極体同様ポリテトラフルオロエチレンをバインダとしてリチウムイオンを吸蔵、脱離しうる炭素材料と本発明における黒鉛化炭素とを混練してシート状に成形し、導電性接着剤を用いて集電体に接着させて得られる。また、ポリフッ化ビニリデン、ポリアミドイミド又はポリイミドをバインダとし、バインダとなる樹脂又はその前駆体を有機溶媒に溶解させた液に前記炭素材料及び前記黒鉛化炭素を分散させ、集電体に塗工し、乾燥させて得る方法もある。
【００２５】
集電体に液を塗工して負極体を得る方法において、バインダとなる樹脂又はその前駆体を溶解させる溶媒は限定されないが、バインダを構成する樹脂又はその前駆体を容易に溶解でき、入手も容易であることからＮ−メチル−２−ピロリドン（以下、ＮＭＰという）が好ましい。ここで、ポリアミドイミドの前駆体又はポリイミドの前駆体とは、加熱することにより重合してそれぞれポリアミドイミド又はポリイミドとなるものをいう。
【００２６】
上に挙げたバインダは、加熱することにより硬化し、耐薬品性、機械的性質、寸法安定性に優れる。熱処理の温度は２００℃以上であることが好ましい。２００℃以上であれば、ポリアミドイミドの前駆体又はポリイミドの前駆体であっても通常重合して、それぞれポリアミドイミド又はポリイミドとなる。また、熱処理する雰囲気は窒素、アルゴン等の不活性雰囲気又は１３３Ｐａ以下の減圧下が好ましい。ポリアミドイミド又はポリイミドは、本発明で使用される有機電解液に対する耐性があり、また負極から水分を除去するために３００℃程度の高温加熱又は減圧下の加熱をしても充分耐性がある。
【００２７】
本発明における有機電解液に含まれるリチウム塩は、ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＮ（ＳＯ₂ＣＦ₃）₂、ＣＦ₃ＳＯ₃Ｌｉ、ＬｉＣ（ＳＯ₂ＣＦ₃）₃、ＬｉＡｓＦ₆及びＬｉＳｂＦ₆からなる群から選ばれる１種以上が好ましい。溶媒はエチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ジメチルカーボネート、エチルメチルカーボネート、ジエチルカーボネート、スルホラン及びジメトキシエタンからなる群から選ばれる１種以上を含むことが好ましい。これらのリチウム塩と溶媒とからなる電解液は耐電圧が高く、電気伝導度も高い。
【００２８】
上記溶媒のなかでも、特にプロピレンカーボネートを含む溶媒が好ましい。活性炭は上記の溶媒のなかで特にプロピレンカーボネートに対して安定であり、電解液溶媒中にプロピレンカーボネートは５０％以上含まれることが好ましい。しかし、プロピレンカーボネート以外の溶媒系ではリチウムイオンを吸蔵、脱離しうる炭素材料である黒鉛系の炭素材料を、プロピレンカーボネートを含む溶媒系で使用するとリチウムイオンを吸蔵できない。この場合、電解液中に１２−クラウン−４等のクラウンエーテルを添加すると黒鉛系の炭素材料でもリチウムイオンの吸蔵が可能になる。このときのクラウンエーテルの濃度は電解液に対し０．１〜１０質量％が好ましい。０．１質量％未満ではクラウンエーテル添加の効果は見られず、１０質量％超では正極の劣化が大きい。
【００２９】
また、特に好ましい電解液は、正極の活性炭と組み合わせたときに高性能が得られるＬｉＢＦ₄を含むプロピレンカーボネート溶液であり、充放電サイクル特性及び電圧印加特性に優れる。
【００３０】
【実施例】
次に、実施例（例１、２）及び比較例（例３）により本発明をさらに具体的に説明するが、本発明はこれらにより限定されない。なお、例１〜３におけるセルの作製及び測定はすべて露点が−６０℃以下のアルゴングローブボックス中で行った。
【００３１】
［例１］
フェノール樹脂を原料として水蒸気賦活法によって得られた比表面積２０００ｍ²／ｇの活性炭８０質量％、導電性カーボンブラック１０質量％、及びバインダとしてのポリテトラフルオロエチレン１０質量％からなる混合物をエタノールを加えて混練し、圧延した後、２００℃で２時間真空乾燥して電極シートを得た。この電極シートから大きさ６ｃｍ×３ｃｍ、厚さ１５０μｍの電極を得て、ポリアミドイミドをバインダとする導電性接着剤を用いてアルミニウム箔に接合し、減圧下で３００℃で２時間熱処理し、正極体とした。
【００３２】
次に、リチウムイオンを吸蔵、脱離しうる炭素材料として、Ｘ線回折による［００２］面の面間隔０．３３６ｎｍ、粒径６μｍの黒鉛化メソカーボンマイクロビーズ（大阪ガス社製、ラマンスペクトルの強度比Ｉ₁₃₆₀／Ｉ₁₅₈₀は０．１４）と、ラマンスペクトルの強度比Ｉ₁₃₆₀／Ｉ₁₅₈₀が０．０６の黒鉛化したＶＧＣＦとを、ポリフッ化ビニリデンをＮＭＰに溶解した溶液に分散させて、銅からなる集電体に塗布し乾燥して負極体を得た。負極中の黒鉛化メソカーボンマイクロビーズ：黒鉛化したＶＧＣＦ：ポリフッ化ビニリデンは、質量比で８：１：１であった。この負極体をさらにロールプレス機でプレスし、大きさ６ｃｍ×３ｃｍ、厚さ１５μｍの負極が形成された負極体とした。
【００３３】
上記のように得られた正極体と負極体をポリプロピレン製のセパレータを介して対向させ、１ｍｏｌ／ＬのＬｉＢＦ₄をエチレンカーボネートとジエチルカーボネートの混合溶媒（体積比で１：１）に溶解した溶液に充分な時間含浸させて、二次電源を得た。この二次電源の初期容量を測定後、充放電電流１８０ｍＡで４．２Ｖから２．７５Ｖまでの範囲で充放電サイクル試験を行い、１０００サイクル後の容量を測定し、容量変化率を算出した。結果を表１に示す。
【００３４】
［例２］
リチウムイオンを吸蔵、脱離しうる炭素材料を、Ｘ線回折による［００２］面の面間隔０．３７３ｎｍ、粒径１９μｍ、ラマンスペクトルの強度比Ｉ₁₃₆₀／Ｉ₁₅₈₀が０．７６の難黒鉛性炭素材料に変更し、負極の厚さを２６μｍとした以外は例１と同様にして負極体を得た。上記負極体を用い、電解液として１ｍｏｌ／ＬのＬｉＢＦ₄をエチレンカーボネートとプロピレンカーボネートとの混合溶媒（体積比で１：１）に溶解した溶液を用いた以外は例１と同様にして二次電源を作製し、例１と同様に評価した。結果を表１に示す。
【００３５】
［例３］
黒鉛化したＶＧＣＦを加えず、負極中の難黒鉛性炭素材料とポリフッ化ビニリデンとの含有割合を質量比で９：１とした以外は例２と同様にして負極体を得た。この負極体を用いた以外は例１と同様にして二次電源を作製し、例１と同様に評価した。結果を表１に示す。
【００３６】
【表１】

【００３７】
【発明の効果】
本発明によれば、耐電圧が高く、容量が大きく、かつ急速充放電サイクル信頼性の高い二次電源を提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a secondary power source with high withstand voltage, large capacity, and high rapid charge / discharge cycle reliability.
[0002]
[Prior art]
As an electrode of a conventional electric double layer capacitor, a polarizable electrode mainly composed of activated carbon is used for both the positive electrode and the negative electrode. The withstand voltage of the electric double layer capacitor is 1.2 V when an aqueous electrolyte is used, and 2.5 to 3.3 V when an organic electrolyte is used. Since the energy of the electric double layer capacitor is proportional to the square of the voltage, the organic electrolyte having a higher withstand voltage has a higher energy than the aqueous electrolyte. However, even in an electric double layer capacitor using an organic electrolyte, its energy density is 1/10 or less that of a secondary battery such as a lead storage battery, and further improvement in energy density is required.
[0003]
On the other hand, Japanese Patent Laid-Open No. 64-14882 discloses that a lithium ion is previously applied to a carbon material having an electrode mainly composed of activated carbon as a positive electrode and having a [002] plane spacing of 0.338 to 0.356 nm by X-ray diffraction. A secondary power supply with an upper limit voltage of 3 V using an electrode that occludes lithium as a negative electrode is disclosed in Japanese Patent Laid-Open No. Hei 8-107048 by a chemical method or an electrochemical method in advance on a carbon material capable of occluding and desorbing lithium ions. A battery using a carbon material in which ions are occluded as a negative electrode is disclosed in Japanese Patent Application Laid-Open No. 9-55342 in which a carbon material capable of occluding and desorbing lithium ions is supported on a porous current collector that does not form an alloy with lithium. A secondary power supply with an upper limit voltage of 4V having However, these secondary power sources have a problem of requiring a step of previously storing lithium ions in the carbon material of the negative electrode.
[0004]
In addition to the electric double layer capacitor, there is a lithium ion secondary battery as a power source capable of charging and discharging a large current. Lithium ion secondary batteries have the properties of higher voltage and higher capacity than electric double layer capacitors, but they have high resistance and have a problem that their life due to rapid charge / discharge cycles is significantly shorter than electric double layer capacitors.
[0005]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION An object of the present invention is to provide a secondary power source that can be rapidly charged / discharged, has high withstand voltage, high capacity, high energy density, and high charge / discharge cycle reliability.
[0006]
[Means for Solving the Problems]
The present invention includes a positive electrode made mainly of activated carbon, the ratio I ₁₃₆₀ / I ₁₅₈₀ of the peak intensity of 1360 cm ^-1 vicinity of the Raman spectrum I ₁₃₆₀ and 1580 cm ^-1 vicinity of the peak intensity I ₁₅₈₀ is at 0.12 to 1.0 certain lithium ion occlusion, possess a negative electrode including a carbon material graphitized carbon material and I ₁₃₆₀ / I ₁₅₈₀ capable desorbed is from 0.01 to 0.115, an organic electrolyte containing a lithium salt, a The secondary power source is characterized in that the graphitized carbon material is a graphitized vapor-grown carbon fiber .
[0007]
Further, the present invention provides a secondary power source having a positive electrode mainly composed of activated carbon, a negative electrode containing a carbon material capable of inserting and extracting lithium ions, and an organic electrolyte containing a lithium salt. vapor-grown carbon fiber ratio I ₁₃₆₀ / I ₁₅₈₀ is obtained by graphitization is from 0.01 to 0.115 of the peak intensity of 1360 cm ^-1 vicinity of the Raman spectrum I ₁₃₆₀ and 1580 cm ^-1 vicinity of the peak intensity I ₁₅₈₀ is 5 A secondary power source comprising 20% by mass is provided.
[0008]
In this specification, a negative electrode body is formed by joining and integrating a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium ions and a current collector. The same definition applies to the positive electrode body. A secondary battery and an electric double layer capacitor are both types of secondary power sources. In this specification, the positive electrode includes activated carbon, and the negative electrode includes a carbon material capable of inserting and extracting lithium ions. The secondary power source is simply called a secondary power source.
[0009]
The Raman spectra of general carbon peak appears in the vicinity of 1360 cm ^-1 vicinity and 1580 cm ^-1. The peak in the vicinity of 1360 cm ⁻¹ is a D band peak and is caused by the disordered layer structure. The peak in the vicinity of 1580 cm ⁻¹ is a G band peak and is attributed to the graphite structure of carbon. Therefore, the peak intensity ratio I ₁₃₆₀ / I ₁₅₈₀ increases when the edge surface of the graphite structure is large, and the intensity ratio decreases when the basal surface is large. I ₁₃₆₀ / I ₁₅₈₀ is also a parameter indicating the degree of graphitization of carbon. The smaller this value, the more graphitization proceeds. In the present invention, graphitized carbon is used as a conductive agent, and therefore, graphitized carbon is preferred.
[0010]
Therefore, in the secondary power source of the present invention, a carbon material having I ₁₃₆₀ / I ₁₅₈₀ of 0.01 to 0.115 is used. In the carbon material having this value exceeding 0.115, graphitization has not progressed, so that the resistance reduction effect of the negative electrode is hardly seen. On the other hand, it is difficult to obtain a carbon material of less than 0.01. Preferably it is 0.03-0.115, More preferably, it is 0.05-0.11.
[0011]
The peak of the peak and 1360 cm ^-1 vicinity of 1580 cm ^-1 vicinity herein denote the peak appearing in the peak and 1345～1375Cm ^-1 appears at 1565～1595Cm ^-1 respectively.
[0012]
There are no particular limitations on the raw material, production method, etc. of the graphitized carbon material (hereinafter referred to as graphitized carbon in the present invention) having specific Raman spectral characteristics in the present invention. For example, a graphitized carbon material can be obtained by finally heat-treating petroleum or coal-based coke, pitch or the like at 2800 ° C. or higher. The graphitized carbon in the present invention is preferably graphitized using a vapor-grown carbon fiber (hereinafter referred to as VGCF) as a raw material. VGCF is obtained by thermally decomposing benzene vapor at around 1000 ° C. using iron fine particles as a catalyst. The obtained VGCF can be graphitized by heat treatment at 2800 ° C. or higher in an inert atmosphere.
[0013]
In the present invention, since the graphitized carbon in the present invention is contained in the negative electrode, the resistance of the negative electrode can be reduced, and as a result, the resistance of the secondary power supply can be reduced. Therefore, a secondary power supply suitable for high output discharge can be obtained. The mechanism for reducing the resistance of the negative electrode seems to be that, in addition to the low resistance of graphitized carbon itself, the presence of graphitized carbon increases the conductivity between carbon materials that can occlude and desorb lithium ions. In particular, when graphitized VGCF is used, the effect of increasing the conductivity between the carbon materials is great. When using graphitized VGCF, in order to enhance the effect, the fiber diameter is preferably 0.1 to 20 μm, particularly preferably 1 to 10 μm, and the fiber length is substantially 1 to 100 μm, particularly 5 It is preferably ˜50 μm.
[0014]
The amount of graphitized carbon in the present invention in the negative electrode is preferably 5 to 20% by mass. If it is less than 5% by mass, the resistance of the negative electrode cannot be reduced. On the other hand, if it exceeds 20% by mass, particularly when a graphitized fiber such as VGCF is added, the density of the negative electrode cannot be increased, and the negative electrode capacity decreases. More preferably, it is 8-15 mass%.
[0015]
In general, in the case of a lithium ion secondary battery, the positive electrode is an electrode mainly composed of a lithium-containing transition metal oxide, the negative electrode is an electrode mainly composed of a carbon material capable of inserting and extracting lithium ions, and the lithium ions are positively charged by charging. The lithium-containing transition metal oxide is desorbed, the lithium ion of the negative electrode is occluded and occluded in a carbon material that can be desorbed, and the lithium ion is desorbed from the negative electrode by discharge, and the lithium ion is occluded in the positive electrode. Therefore, essentially, lithium ions in the electrolytic solution are not involved in charging / discharging of the battery.
[0016]
On the other hand, in the secondary power source of the present invention, the anion in the electrolytic solution is adsorbed on the activated carbon of the positive electrode by charging, and the lithium ion in the electrolytic solution is occluded in the carbon material that can occlude and desorb the lithium ion of the negative electrode. Then, lithium ions are desorbed from the negative electrode by discharge, and the anions are desorbed from the positive electrode. That is, in the secondary power source of the present invention, the solute of the electrolytic solution is essentially involved in charging / discharging, and the charging / discharging mechanism is different from that of the lithium ion battery. Further, unlike the lithium ion secondary battery, lithium ions are not occluded and desorbed in the positive electrode active material itself, and therefore the secondary power source of the present invention is excellent in charge / discharge cycle reliability.
[0017]
In a secondary power source using activated carbon for the positive electrode and a carbon material capable of inserting and extracting lithium ions in the negative electrode, ions dissolved in the electrolytic solution are involved in charging and discharging. Therefore, when the solute concentration of the electrolytic solution is low, there is a possibility that sufficient charging cannot be performed. The solute concentration is preferably 0.5 to 2.0 mol / L, particularly preferably 0.75 to 1.5 mol / L.
[0018]
In the secondary power source of the present invention, the cycle efficiency of the negative electrode in the first charge / discharge cycle is not necessarily 100%, and there are some that do not desorb due to occluded lithium ions. In this case, since the lithium ion concentration in the electrolytic solution may decrease and charging may not be sufficiently performed from the next charging, it is preferable to add lithium-containing transition metal oxide to the positive electrode to prevent characteristic deterioration. By this method, lithium ions that cannot be desorbed from the negative electrode can be supplemented. In this case, the lithium-containing transition metal oxide contained in the positive electrode is preferably 0.1 to 20% by mass, particularly 3 to 15% by mass. If the content is less than 0.1% by mass, the effect is small. On the other hand, if the content exceeds 20% by mass, the capacity of the lithium-containing transition metal oxide is large. Disappear.
[0019]
The lithium-containing transition metal oxide is preferably a composite oxide of lithium and at least one transition metal selected from the group consisting of V, Mn, Fe, Co, Ni, Zn, and W. In particular, a composite oxide of lithium and at least one selected from the group consisting of Mn, Co and Ni is preferable, and Li _x Co _y Ni _(1-y) O ₂ or Li _z Mn ₂ O ₄ (however, A compound represented by 0 <x <2, 0 ≦ y ≦ 1, 0 <z <2.) Is preferable.
[0020]
In the present invention, the activated carbon contained in the positive electrode preferably has a specific surface area of 800 to 3000 m ² / g. Although the raw material of activated carbon and activation conditions are not limited, For example, as a raw material, a phenol resin, petroleum coke, etc. are mentioned, As a activation method, a steam activation method, a molten alkali activation method, etc. are mentioned. In the present invention, steam activated activated coconut palm activated carbon or steam activated activated phenol resin activated carbon is particularly preferable. In order to reduce the resistance of the positive electrode, it is also preferable to include conductive carbon black or graphite as a conductive material in the positive electrode. At this time, the conductive material is 0.1 to 20% by mass in the positive electrode. It is preferable.
[0021]
As a method for producing the positive electrode, for example, activated carbon powder is mixed with polytetrafluoroethylene as a binder, kneaded and then molded into a sheet shape to form a positive electrode, which is fixed to the current collector using a conductive adhesive There is. Also, using polyvinylidene fluoride, polyamideimide, polyimide, etc. as the binder, disperse the activated carbon powder in the varnish in which these are dissolved, and apply this solution on the current collector by the doctor blade method, etc., and dry it. Also good. The amount of the binder contained in the positive electrode is preferably 1 to 20% in terms of mass ratio from the balance between the strength of the positive electrode body and characteristics such as capacity.
[0022]
The carbon material capable of inserting and extracting lithium ions in the present invention preferably has a [002] plane spacing of 0.335 to 0.410 nm as measured by X-ray diffraction. A carbon material having an interplanar spacing of more than 0.410 nm is likely to deteriorate in a charge / discharge cycle. Specifically, petroleum coke, mesophase pitch-based carbon material or vapor-grown carbon fiber is heat-treated at 800 to 3000 ° C., natural graphite, artificial graphite, non-graphitizable carbon material, and the like. In the present invention, any of these materials can be preferably used. Among these, the [002] plane spacing between the [002] planes of 0.335 to 0.337 nm, natural graphite or graphitizable carbon material heat-treated at 2800 ° C. or higher is 0. A material having a thickness of .335 to 0.337 nm is preferable because it has a low potential for occlusion and desorption of lithium ions.
[0023]
In the present invention, even if the carbon material capable of inserting and extracting lithium ions contained in the negative electrode is a graphite-based carbon material, the value of I ₁₃₆₀ / I ₁₅₈₀ in the Raman spectrum is usually 0.01 to 0.115. It is not in the range and is usually in the range of 0.12 to 1.0.
[0024]
The negative electrode body in the present invention is formed into a sheet by kneading a carbon material capable of occluding and desorbing lithium ions with polytetrafluoroethylene as a binder, and graphitized carbon in the present invention, and forming a conductive adhesive. And obtained by adhering to the current collector. In addition, polyvinylidene fluoride, polyamideimide or polyimide is used as a binder, and the carbon material and the graphitized carbon are dispersed in a solution in which a binder resin or a precursor thereof is dissolved in an organic solvent, and then applied to a current collector. There is also a method obtained by drying.
[0025]
In the method of obtaining a negative electrode body by applying a liquid to the current collector, the solvent for dissolving the resin to be the binder or its precursor is not limited, but the resin constituting the binder or its precursor can be easily dissolved and obtained. N-methyl-2-pyrrolidone (hereinafter referred to as NMP) is preferable. Here, the precursor of polyamideimide or the precursor of polyimide means one that is polymerized by heating to become polyamideimide or polyimide, respectively.
[0026]
The binders listed above are cured by heating and are excellent in chemical resistance, mechanical properties, and dimensional stability. It is preferable that the temperature of heat processing is 200 degreeC or more. If it is 200 degreeC or more, even if it is a polyamideimide precursor or a polyimide precursor, it will superpose | polymerize normally and it will become a polyamideimide or a polyimide, respectively. The atmosphere for the heat treatment is preferably an inert atmosphere such as nitrogen or argon or a reduced pressure of 133 Pa or less. Polyamideimide or polyimide is resistant to the organic electrolyte used in the present invention, and is sufficiently resistant to high temperature heating at about 300 ° C. or heating under reduced pressure in order to remove moisture from the negative electrode.
[0027]
The lithium salt contained in the organic electrolyte in the present invention includes LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , CF ₃ SO ₃ Li, LiC (SO ₂ CF ₃ ) ₃ , LiAsF ₆ and LiSbF. One or more selected from the group consisting of ₆ are preferred. The solvent preferably contains one or more selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, and dimethoxyethane. Electrolytic solutions composed of these lithium salts and solvents have high withstand voltage and high electrical conductivity.
[0028]
Among the above solvents, a solvent containing propylene carbonate is particularly preferable. Among the above solvents, activated carbon is particularly stable against propylene carbonate, and it is preferable that 50% or more of propylene carbonate is contained in the electrolyte solvent. However, in a solvent system other than propylene carbonate, if a graphite-based carbon material, which is a carbon material capable of inserting and extracting lithium ions, is used in a solvent system containing propylene carbonate, lithium ions cannot be stored. In this case, when crown ether such as 12-crown-4 is added to the electrolytic solution, it is possible to occlude lithium ions even in a graphite-based carbon material. The concentration of the crown ether at this time is preferably 0.1 to 10% by mass with respect to the electrolytic solution. If it is less than 0.1% by mass, the effect of adding crown ether is not observed, and if it exceeds 10% by mass, the cathode is greatly deteriorated.
[0029]
Further, a particularly preferable electrolytic solution is a propylene carbonate solution containing LiBF ₄ that provides high performance when combined with the activated carbon of the positive electrode, and is excellent in charge / discharge cycle characteristics and voltage application characteristics.
[0030]
【Example】
Next, although an Example (Examples 1 and 2) and a comparative example (Example 3) demonstrate this invention further more concretely, this invention is not limited by these. The production and measurement of the cells in Examples 1 to 3 were all performed in an argon glove box having a dew point of −60 ° C. or less.
[0031]
[Example 1]
Ethanol was added to a mixture of 80% by mass of activated carbon having a specific surface area of 2000 m ² / g obtained from a phenol resin as a raw material by a steam activation method, 10% by mass of conductive carbon black, and 10% by mass of polytetrafluoroethylene as a binder. After kneading, rolling, and vacuum drying at 200 ° C. for 2 hours, an electrode sheet was obtained. An electrode having a size of 6 cm × 3 cm and a thickness of 150 μm is obtained from this electrode sheet, bonded to an aluminum foil using a conductive adhesive having polyamideimide as a binder, and heat-treated at 300 ° C. for 2 hours under reduced pressure. The body.
[0032]
Next, as a carbon material capable of inserting and extracting lithium ions, graphitized mesocarbon microbeads having an [002] plane spacing of 0.336 nm and a particle size of 6 μm by X-ray diffraction (manufactured by Osaka Gas Co., Ltd., intensity of Raman spectrum) The ratio I ₁₃₆₀ / I ₁₅₈₀ is 0.14) and graphitized VGCF having a Raman spectrum intensity ratio I ₁₃₆₀ / I ₁₅₈₀ of 0.06 are dispersed in a solution of polyvinylidene fluoride in NMP to obtain copper. It applied to the collector which consists of and dried, and the negative electrode body was obtained. The weight ratio of graphitized mesocarbon microbeads: graphitized VGCF: polyvinylidene fluoride in the negative electrode was 8: 1: 1. This negative electrode body was further pressed by a roll press machine to obtain a negative electrode body in which a negative electrode having a size of 6 cm × 3 cm and a thickness of 15 μm was formed.
[0033]
A solution in which 1 mol / L LiBF ₄ is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 by volume) with the positive electrode body and the negative electrode body obtained as described above facing each other through a polypropylene separator. For a sufficient time to obtain a secondary power source. After measuring the initial capacity of the secondary power source, a charge / discharge cycle test was performed at a charge / discharge current of 180 mA in the range from 4.2 V to 2.75 V, the capacity after 1000 cycles was measured, and the capacity change rate was calculated. The results are shown in Table 1.
[0034]
[Example 2]
A carbon material capable of absorbing and desorbing lithium ions is a non-graphitizable carbon having a [002] plane spacing of 0.373 nm, a particle size of 19 μm, and a Raman spectrum intensity ratio I ₁₃₆₀ / I ₁₅₈₀ of 0.76 by X-ray diffraction. A negative electrode body was obtained in the same manner as in Example 1 except that the material was changed and the thickness of the negative electrode was changed to 26 μm. A secondary battery was prepared in the same manner as in Example 1 except that the above negative electrode body was used and a solution of 1 mol / L LiBF ₄ dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (1: 1 by volume) was used as the electrolyte. A power source was prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0035]
[Example 3]
A negative electrode body was obtained in the same manner as in Example 2 except that the graphitized VGCF was not added and the content ratio of the non-graphitizable carbon material and polyvinylidene fluoride in the negative electrode was 9: 1 by mass ratio. A secondary power source was prepared in the same manner as in Example 1 except that this negative electrode was used, and evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0036]
[Table 1]

[0037]
【The invention's effect】
According to the present invention, it is possible to provide a secondary power source having a high withstand voltage, a large capacity, and high rapid charge / discharge cycle reliability.

Claims

A positive electrode made mainly of activated carbon, the ratio _I 1360 _{/ I 1580} of the peak intensity of 1360 cm ^-1 vicinity of the Raman spectrum _{I 1360} and 1580 cm ^-1 vicinity of the peak intensity _{I 1580} is a lithium ion is from 0.12 to 1.0 storage, carbon material and _I 1360 _{/ I 1580} capable desorbed possess a negative electrode containing a graphitized carbon material is 0.01 to 0.115, an organic electrolyte containing a lithium salt, wherein the graphitized A secondary power source characterized in that the carbon material obtained is graphitized vapor phase grown carbon fiber .

The secondary power supply according to claim 1, wherein the negative electrode contains 5 to 20 mass% of the graphitized carbon material.

In a secondary power source having a positive electrode mainly composed of activated carbon, a negative electrode including a carbon material capable of inserting and extracting lithium ions, and an organic electrolyte including a lithium salt, the negative electrode includes 1360 cm ⁻¹ in a Raman spectrum. 5-20% by mass of graphitized vapor-grown carbon fiber having a ratio I ₁₃₆₀ / I ₁₅₈₀ of the peak intensity I _{1360 in the} vicinity to ₁₅₈₀ cm ⁻¹ of the peak intensity I _{1580 in} the vicinity of ₁₅₈₀ cm ⁻¹ of 0.01 to 0.115 is included. Secondary power supply characterized by.

The secondary power supply according to claim 1, 2 or 3 , wherein the carbon material capable of inserting and extracting lithium ions has a [002] plane spacing of 0.335 to 0.410 nm.