JP4296580B2 - Nonaqueous electrolyte lithium secondary battery - Google Patents

Nonaqueous electrolyte lithium secondary battery Download PDF

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Publication number
JP4296580B2
JP4296580B2 JP2000006070A JP2000006070A JP4296580B2 JP 4296580 B2 JP4296580 B2 JP 4296580B2 JP 2000006070 A JP2000006070 A JP 2000006070A JP 2000006070 A JP2000006070 A JP 2000006070A JP 4296580 B2 JP4296580 B2 JP 4296580B2
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lithium
negative electrode
active material
electrode active
battery
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JP2001196061A (en
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隆明 井口
純一 倉富
宏二 桑名
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GS Yuasa Corp
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GS Yuasa Corp
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    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10Energy storage using batteries

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  • Inorganic Compounds Of Heavy Metals (AREA)
  • Battery Electrode And Active Subsutance (AREA)
  • Secondary Cells (AREA)

Description

【0001】
【発明の属する技術分野】
本発明は、非水電解質リチウム二次電池に関し、さらに詳しくはリチウム二次電池に用いる活物質材料に関する。
【0002】
【従来の技術】
非水電解質を用いたリチウム二次電池は、小型、軽量で、エネルギー密度の高い優れた電池であり、これまで、負極活物質としては金属リチウム、リチウム合金あるいはリチウムの吸蔵・放出が可能な炭素材料が使用されてきた。しかし金属リチウムは、充電時に生成するリチウムの樹枝状析出(デンドライト)のため、サイクル寿命の点で問題があり、またこのデンドライトはセパレーターを貫通し内部短絡を引き起こしたり、発火の原因となることもあった。また、上記のような充電時に生成するデンドライトを防止する目的で用いられたリチウム合金も、充電量が大きくなると、負極の微細粉化や負極活物質の脱落などの問題があり、十分なサイクル寿命が得られなかった。
【0003】
一方、炭素材料はリチウムの吸蔵・放出が可能であるため、上記した問題は著しく改善され、リチウム二次電池の長寿命化及び安全性に対して有効である。しかしながら、負極活物質に炭素材料を用いたリチウム二次電池は、金属リチウムやリチウム合金を用いたリチウム二次電池に比べ、自己放電が大きく保存特性が悪いことが知られている。自己放電のメカニズムについての詳細は必ずしも明らかではないが、炭素材料と電解質との副反応等によると考えられ、炭素材料に起因する現象であると考えられる。
【0004】
【発明が解決しようとする課題】
従来のリチウムイオン電池に用いられていた負極活物質としての炭素材料は、リチウムを吸蔵・放出する電位が金属リチウムの電位に近接しているため、電池の充電状態において炭素材料に吸蔵されているリチウムは活性度が高く、電解質等を還元する反応を起こすことが予想される。また、炭素材料はそれ自身が炭素元素のみの骨格から構成されていることから、酸素を含有する化合物である溶媒や支持塩と反応し、炭素−電解質界面に、中間層として酸化物被膜を形成しやすいことが予想される。
【0005】
従って、エネルギー密度が高く、自己放電の少ない保存特性の優れた安全な非水電解質リチウム二次電池を得るためには、リチウムの吸蔵・放出が可能で電解質等との副反応が起こりにくい負極活物質材料の開発が望まれている。
【0006】
本発明は、上記のような課題を解決しようとするもので、リチウムの吸蔵・放出が可能で電解質等との副反応が起こりにくい負極活物質を使用し、高電位でエネルギー密度が高い正極活物質と組み合わせることによって、エネルギー密度が高く、自己放電の少ない保存特性の優れた安全なリチウム二次電池を提供することにある。
【0007】
【課題を解決するための手段】
本発明は、前記問題点に鑑みてなされたものであって、負極活物質の主成分にスピネル構造を有するLixTi5/3-yy4(LはB、Co又はZnを含む元素、4/3≦x≦7/3、0<y≦5/3)で表される酸化物焼成体を用い、正極活物質の主成分にオリビン構造を有するLimMPO4(Mは1種以上の遷移金属で、0≦m≦2.1)で表わせる酸化物焼成体を用いることを特徴とする非水電解質リチウム二次電池である。
【0008】
即ち、本発明者らは、リチウムの吸蔵・放出が可能な酸化物からなる負極活物質を用いることが保存特性向上に対して効果が大きいことを見い出し、本発明に至った。
【0009】
本発明電池に用いる負極活物質は、金属リチウムの電位に対して約1.5Vの電位を有する。一方、本発明電池に用いる正極活物質は、金属リチウムの電位に対して約4.5〜5.0Vの電位を有する。従って、前記負極活物質と前記正極活物質を組み合わせることにより、負極電位が高いにもかかわらず約3.0〜3.5Vの高い電圧で作動するリチウム二次電池が構成されるので、エネルギー密度が高く保存特性の優れた安全なリチウム二次電池を提供することができる。
【0010】
また、本発明に用いる非水電解質は、高エネルギー密度を達成するためには有機電解液であることが好ましいが、さらに安全性の高いリチウム二次電池を得る為、ゲル電解質や有機固体電解質、あるいは無機固体電解質を用いてもよい。
【0011】
本発明のリチウム二次電池が自己放電の少ない保存特性に優れる理由としては、必ずしも明らかではないが、以下のように考察される。即ち、前記負極活物質の主成分であるLixTi5/3-yy4は、金属リチウムの電位に対して約1.5Vという比較的高い電位でリチウムの吸蔵・放出が起こるため、電池の充電状態において前記負極活物質の分子構造内に吸蔵されているリチウムの活性度は低く、電解質などを還元する作用が非常に小さいと考えられる。また、前記負極活物質が、炭素材料ではなく、酸化物であることから、電解質を構成する溶媒や支持塩が酸素を含有する化合物であっても、これと反応して電解質との界面に中間層として酸化物被膜を形成する作用も非常に小さいことが予想される。従って、電池の充電状態においても電極表面での副反応が起こりにくいので、自己放電の少ない保存特性に優れるリチウム二次電池を得ることができる。
【0012】
本発明電池に用いる負極活物質の主成分であるLixTi5/3-yy4は、スピネル構造を有し、化学式中のLは1種以上のTi以外の元素である。ここで、xは4/3以上7/3以下であり、yは0以上5/3未満が望ましい。
【0013】
異種元素LによってTiの一部を置換することにより、さらに電極性能が向上する。この理由は必ずしも明らかではないが、次のように考えられる。通常、チタンの酸化物は嵩高く、電極として塗布する場合、空隙の大きな電極となりやすい。本発明は、チタンの一部を異種元素で置換することにより、この嵩高さを抑えることができ、粒子間のイオンや電子の授受をスムーズにする働きがあるものと考えられる。置換元素としては、Ti以外の元素が考えられるが、好ましくはBe、B、C、Mg、Al、Si、P、Ca、Sc、V、Cr、Mn、Fe、Co、Ni、Cu、Zn、Ga、Ge、As、Se、Sr、Y、Zr、Nb、Mo、Pd、Ag、Cd、In、Sn、Sb、Te、Ba、La、Ta、W、Au、Hg、Pb等が挙げられる。
【0014】
本発明電池に用いる正極活物質の主成分であるLimMPO4は、オリビン構造を有し、化学式中のMは1種以上の遷移金属元素である。ここで、mは0以上2.1以下であり、遷移金属MとしてはCo、Ni、Fe、Mn、Cu、Zn、Cd等であり、その主となる遷移金属種によって若干異なるが、リチウムの溶解・析出電位に対して4.5〜5.0Vの高電位でリチウムの吸蔵・放出が起こることが特開平9−134724号に記載されている。例えば、遷移金属Mを主としてCoとした場合は5.0V近傍となる。また、例外的にFeのみの単独相の場合は3.5V近傍となることが、Padhi,A.K.;Nanjundaswamy,K.S.;Goodenough,J.B. Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J.Electrochem.Soc. 144,4,1997,1188-1194 に記載されている。
【0015】
【発明の実施の形態】
本発明電池に用いる非水電解質としては、非水電解液や固体電解質等が挙げられ、リチウム二次電池の安全性を向上するための固体電解質としては、例えば無機固体電解質、有機固体電解質、無機有機固体電解質、溶融塩等を用いることができる。非水電解液は、有機溶媒として、プロピレンカーボネート、エチレンカーボネート、ブチレンカーボネート、ジエチルカーボネート、ジメチルカーボネート、メチルエチルカーボネート、γ−ブチロラクトン等のエステル類や、テトラヒドロフラン、2−メチルテトラヒドロフラン等の置換テトラヒドロフラン、ジオキソラン、ジエチルエーテル、ジメトキシエタン、ジエトキシエタン、メトキシエトキシエタン等のエーテル類、ジメチルスルホキシド、スルホラン、メチルスルホラン、アセトニトリル、ギ酸メチル、酢酸メチル、N−メチルピロリドン、ジメチルフォルムアミド等が挙げられ、これらを単独又は混合溶媒として用いることができ、これに支持電解質塩として、LiClO4、LiPF6、LiBF4、LiAsF6、LiCF3SO3、LiN(CF3SO22等を溶解したものが挙げられる。無機固体電解質には、リチウムの窒化物、ハロゲン化物、酸素酸塩、硫化リン化合物などがよく知られており、これらの1種または2種以上を混合して用いることができる。なかでも、Li3N、LiI、Li5NI2、Li3N−LiI−LiOH、Li4SiO4、Li4SiO4−LiI−LiOH、xLi3PO4-(1-x)Li4SiO4、Li2SiS3、LiLaTiO3、LiTi2(PO43等やその類似化合物が有効である。一方有機固体電解質では、ポリエチレンオキサイド誘導体か少なくとも前記誘導体を含むポリマー、ポリプロピレンオキサイド誘導体か少なくとも前記誘導体を含むポリマー、ポリフォスファゼンやポリフォスファゼン誘導体、イオン解離基を含むポリマー、リン酸エステルポリマー誘導体、さらにポリビニルピリジン誘導体、ビスフェノールA誘導体、ポリアクリロニトリル、ポリビニリデンフルオライド、フッ素ゴム等に上記した非水電解液を含有させた高分子マトリックス材料(ゲル電解質)等が有効である。また、これら無機固体電解質と有機固体電解質を併用する方法も有効である。
【0016】
本発明の非水電解質リチウム二次電池において、電極合剤として正極あるいは負極活物質に導電剤や結着剤やフィラー等を添加することができる。導電剤としては、電池性能に悪影響を及ぼさない電子伝導性材料であれば何でも良い。通常、天然黒鉛(鱗片状黒鉛、土状黒鉛など)、人造黒鉛、カーボンブラック、アセチレンブラック、ケッチェンブラック、カーボンウイスカー、炭素繊維や金属(銅、ニッケル、鉄、銀、金など)粉、金属繊維、金属の蒸着、導電性セラミックス材料等の導電性材料を1種またはそれらの混合物として含ませることができる。その添加量は1〜50重量%が好ましく、特に2〜30重量%が好ましい。
【0017】
結着剤としては、通常、テトラフルオロエチレン、ポリフッ化ビニリデン、ポリエチレン、ポリプロピレン、エチレン−プロピレンジエンターポリマー(EPDM)、スルホン化EPDM、スチレンブタジエンゴム(SBR)、フッ素ゴム、カルボメトキシセルロース等といった熱可塑性樹脂、ゴム弾性を有するポリマー、多糖類等を1種または2種以上の混合物として用いることができる。また、多糖類の様にリチウムと反応する官能基を有する結着剤は、例えばメチル化するなどしてその官能基を失活させておくことが望ましい。その添加量としては、1〜50重量%が好ましく、特に2〜30重量%が好ましい。
【0018】
フィラーとしては、電池性能に悪影響を及ぼさない材料であれば何でも良い。通常、ポリプロピレン、ポリエチレン等のオレフィン系ポリマー、アエロジル、アルミナ、炭素等が用いられる。フィラーの添加量は0〜30重量%が好ましい。
【0019】
固体電解質と併用してセパレーターを用いることができる。セパレーターとしては、イオンの透過度が優れ、機械的強度のある絶縁性薄膜を用いることができる。耐有機溶剤性と疎水性からポリプロピレンやポリエチレンといったオレフィン系のポリマー、ガラス繊維、ポリフッ化ビニリデン、ポリテトラフルオロエチレン等からつくられたシート、微孔膜、不織布が用いられる。セパレーターの孔径は、一般に電池に用いられる範囲のものであり、例えば0.01〜10μmである。またその厚みについても同様で、一般に電池に用いられる範囲のものであり、例えば5〜300μmである。
【0020】
本発明に用いる正・負極活物質は、平均粒子サイズ0.1〜100μmである粉体が望ましい。所定の形状を得る上で、粉体を得るためには粉砕機や分級機や造粒機が用いられる。例えば乳鉢、ボールミル、サンドミル、振動ボールミル、遊星ボールミル、ジェットミル、カウンタージェットミル、旋回気流型ジェットミルや篩等が用いられる。粉砕時には水、あるいはヘキサン等の有機溶剤を共存させた湿式粉砕を用いてもよい。分級方法としては、特に限定はなく、篩や風力分級機などが乾式、湿式ともに必要に応じて用いられる。
【0021】
電極活物質の集電体としては、構成された電池において悪影響を及ぼさない電子伝導体であれば何でもよい。例えば、正極には材料として、アルミニウム、チタン、ステンレス鋼、ニッケル、焼成炭素、導電性高分子、導電性ガラス等の他に、接着性、導電性、耐酸化性向上の目的で、アルミニウムや銅等の表面をカーボン、ニッケル、チタンや銀等で処理した物を用いる事ができる。負極においては、材料として銅、ステンレス鋼、ニッケル、アルミニウム、チタン、焼成炭素、導電性高分子、導電性ガラス、Al−Cd合金等の他に、接着性、導電性、耐還元性向上の目的で、銅、アルミニウム等の表面をカーボン、ニッケル、チタンや銀等で処理した物を用いる事ができる。特に負極活物質がリチウムに対して1.5V程度の電位であるため、軽量化の目的でアルミニウムを用いることができる。これらの材料については表面を酸化処理する事も可能である。これらの形状については、フォイルの他、フィルム、シート、ネット、パンチドメタル、エキスパンドされた物、ラス体、多孔質体、発泡体、繊維群の形成体等が用いられる。厚みは特に限定はないが、1〜500μmのものが用いられる。
【0022】
本発明における非水電解質リチウム二次電池の形状としては、円筒形、角形、コイン形、ボタン形、扁平形、フィルム状等が挙げられる。なかでも、高エネルギー密度を達成するためにはゲル電解質や固体電解質を使用したフィルム状の電池形状とすることが望ましい。
【0023】
【実施例】
図1は、本発明に係るリチウム二次電池の断面図である。正極は、正極活物質、導電剤としてのケッチェンブラック及び結着剤としてのポリテトラフルオロエチレン(PTFE)を含む正極合剤1が、アルミニウム製の正極集電体6上に圧着されている。負極は、負極活物質、導電剤としてのケッチェンブラック及び結着剤としてのポリテトラフルオロエチレン(PTFE)を含む負極合剤2が、銅製の負極集電体7上に圧着されている。セパレータ3は、ポリエチレン製微多孔膜からなり、前記正・負極間に介在している。電解液は、エチレンカーボネート(EC)及びジメチルカーボネート(DMC)を体積比2:1の割合で混合した溶媒に、テトラフルオロホウ酸リチウム(LiBF4)を1モル溶解した非水電解液であり、前記正極、負極及びセパレータに含浸されている。前記正極を収納したアルミニウム製の正極缶4の周縁と、前記負極を収納したステンレス製の負極蓋5の周縁とが、ガスケット8を介して密閉されている。
【0024】
参考例1)水酸化リチウム(LiOH・H2O)と酸化チタン(TiO2)を混合し、これらを酸化雰囲気下において900℃で熱処理して得たチタン酸リチウム(Li4/3Ti5/34)を負極活物質として用いた。一方、四三酸化コバルト(Co34)、リン酸二アンモニウム((NH42HPO4)及び水酸化リチウム(LiOH・H2O)を混合し、窒素気流中において750℃で20時間熱処理して得たコバルト−リン酸リチウム(LiCoPO4)を正極活物質として用いた。
【0025】
正極は、次のようにして得た。正極活物質87重量部、導電剤10重量部及び結着剤3重量部の割合で混合して正極合剤1とした後、成型金型を用いて直径16mmの円板状に打ち抜き、150℃の真空中で10時間乾燥させ、厚さ0.55mmの正極を作製した。
【0026】
負極は、次のようにして得た。負極活物質87重量部、導電剤10重量部、結着剤3重量部の割合で混合して負極合剤2とした後、成型金型を用いて直径16mmの円板状に打ち抜き、150℃の真空中で10時間乾燥させ、厚さ0.35mmの負極を作製した。
【0027】
前述したセパレータ及び非水電解液を用い、直径20mm、厚さ16mmのコイン型リチウム二次電池を作成した。これを参考例電池1とする。
【0028】
(実施例2)水酸化リチウム(LiOH・H2O)、酸化チタン(TiO2)及び無水ほう酸(B23)を混合し、これらを酸化雰囲気下において900℃で熱処理して得たチタン酸リチウム(Li4/3Ti4/31/34)を負極活物質として用いたこと以外は、参考例1と同様にコイン型リチウム二次電池を作成した。これを本発明電池2とする。
【0029】
(実施例3)水酸化リチウム(LiOH・H2O)、酸化チタン(TiO2)及び四三酸化コバルト(Co34)を混合し、これらを酸化雰囲気下において900℃で熱処理して得たチタン酸リチウム(Li4/3Ti4/3Co1/34)を負極活物質として用いたこと以外は、参考例1と同様にコイン型リチウム二次電池を作成した。これを本発明電池3とする。
【0030】
(実施例4)水酸化リチウム(LiOH・H2O)、酸化チタン(TiO2)及び硝酸亜鉛(Zn(NO32・6H2O)を混合し、これらを酸化雰囲気下において900℃で熱処理して得たチタン酸リチウム(Li4/3Ti4/3Zn1/34)を負極活物質として用いたこと以外は参考例1と同様にコイン型リチウム二次電池を作成した。これを本発明電池4とする。
【0031】
(比較例1)負極活物質としてグラファイトを用い、正極活物質として炭酸マンガン(MnCO3)及び水酸化リチウム(LiOH・H2O)を混合し、これを乾燥空気雰囲気下において750℃で20時間熱処理して得たマンガン酸リチウム(LiMn24)を用いたこと以外は参考例1と同様にコイン型リチウム電池を作成した。これを比較電池1とする。
【0032】
(比較例2)負極活物質としてチタン酸リチウム(Li4/3Ti5/34)を用い、正極活物質としてマンガン酸リチウム(LiMn24)を用いたこと以外は参考例1と同様にコイン型リチウム電池を作成した。これを比較電池2とする。
【0033】
これらの電池を用いて自己放電率を指標とした保存試験を行った。保存試験は室温で実施した。充放電は10時間率で行い、参考電池1及び本発明電池〜4においては、充電終止電圧3.7V、放電終止電圧3.0Vとした。一方、比較例電池1においては、充電終止電圧4.2V、放電終止電圧3.2Vとし、比較電池2においては、充電終止電圧2.8V、放電終止電圧2.0Vとした。以上の充放電条件において、3サイクル目の充電末状態の電池を30日間保存し、保存後の放電容量を測定することにより、以下の数式により自己放電率を算出した。
【0034】
【式】
【0035】
保存試験結果を電池電圧と併せて表1に示す。
【0036】
【表1】
【0037】
(表1)の結果に示されるように、参考例電池1、本発明電池〜4及び比較電池2では、比較電池1に比べて保存時の自己放電率が小さく、保存特性が改善されていることが明確である。しかしながら、この中で比較電池2は、電池の放電電圧は3V以下と低く、保存特性は改善されているものの電池のエネルギー密度が低下する結果となっている。
【0038】
また、本発明電池2〜4の結果より、チタンの一部を他の元素に置換することにより、若干ではあるが、参考例電池1に比べて保存特性が向上することがわかる。この理由としては必ずしも明らかではないが、焼成後の粒子形態に影響し、粒子内部でのイオンや電子の授受を確実に行うことによって、自己放電を抑制できたものと考えられる。
【0039】
上記実施例においては、置換元素としてB、Co、Znを用いたが、Ti以外の他の元素についても同様の効果が確認されている。また、正極活物質についても遷移金属MとしてCoを用いたが、これ以外の遷移金属元素の単体あるいは混合体を用いても同様の効果が得られる。
【0040】
なお、本発明は上記実施例に記載された活物質の出発原料、製造方法、正極、負極、電解質、セパレータ等に限定されるものではない。またコイン型電池はあくまで本発明を説明するためのものであり、電池の形状はこれに限定されるものではない。さらに、主たる負極活物質は、リチウムに対する電位が約1.5Vであるため、集電体として銅の代わりにアルミニウムを用いると、重量エネルギー密度が向上することは当然である。
【0041】
また、負極活物質に、金属リチウムの電位の近傍においてリチウムの吸蔵・放出が起こる材料を使用した従来のリチウム二次電池は、これを複数直並列接合した場合、各電池間の環境温度やインピーダンス等のバラツキによって一部の電池が深い充電状態となる場合があり、負極において金属リチウムの析出を伴うことによって、電池の性能低下や不安全なモードへ誘導される危険性があった。しかしながら、本発明の前記負極活物質は、金属リチウムの電位に対して約1.5Vの電位でリチウムの吸蔵・放出が起こるため、これを使用したリチウム二次電池を複数直並列接合した場合においても、上記の問題を大きく回避することができる。
【0042】
【発明の効果】
本発明は上述したとおりであるので、エネルギー密度が高く、かつ自己放電の少ない保存特性の優れた安全な非水電解質リチウム二次電池を提供できる。
【図面の簡単な説明】
【図1】本発明電池の断面図である。
【符号の説明】
1 正極合剤
2 負極合剤
3 セパレータ
4 正極缶
5 負極蓋
6 正極集電体
7 負極集電体
8 ガスケット
[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte lithium secondary battery, and more particularly to an active material used for a lithium secondary battery.
[0002]
[Prior art]
Lithium secondary batteries using non-aqueous electrolytes are small, lightweight, and excellent batteries with high energy density. To date, negative electrode active materials include metallic lithium, lithium alloys, or carbon that can store and release lithium. Material has been used. However, metallic lithium has a problem in terms of cycle life due to dendritic precipitation of lithium (dendrite) produced during charging, and this dendrite penetrates the separator and may cause internal short circuit or cause ignition. there were. In addition, the lithium alloy used for the purpose of preventing the dendrite generated during charging as described above has problems such as fine powdering of the negative electrode and falling off of the negative electrode active material when the charge amount is large, and a sufficient cycle life. Was not obtained.
[0003]
On the other hand, since the carbon material can occlude and release lithium, the above-described problems are remarkably improved, and it is effective for extending the life and safety of the lithium secondary battery. However, it is known that a lithium secondary battery using a carbon material as a negative electrode active material has a large self-discharge and poor storage characteristics compared to a lithium secondary battery using metallic lithium or a lithium alloy. Although details about the self-discharge mechanism are not necessarily clear, it is thought to be due to a side reaction between the carbon material and the electrolyte, and is considered to be a phenomenon caused by the carbon material.
[0004]
[Problems to be solved by the invention]
Carbon materials as negative electrode active materials used in conventional lithium ion batteries are occluded by the carbon material in the charged state of the battery because the potential for occlusion / release of lithium is close to that of metallic lithium. Lithium is highly active and is expected to cause a reaction that reduces electrolytes and the like. In addition, since the carbon material itself is composed of a skeleton consisting of only carbon elements, it reacts with a solvent or supporting salt, which is a compound containing oxygen, to form an oxide film as an intermediate layer at the carbon-electrolyte interface. It is expected to be easy to do.
[0005]
Therefore, in order to obtain a safe non-aqueous electrolyte lithium secondary battery with high energy density and low self-discharge storage characteristics, it is possible to occlude and release lithium and to prevent negative reactions with electrolytes and the like. Development of material materials is desired.
[0006]
The present invention is intended to solve the above-described problems, and uses a negative electrode active material that can occlude and release lithium and hardly causes a side reaction with an electrolyte, and has a high potential and high energy density. By combining with a substance, an object is to provide a safe lithium secondary battery having high energy density and excellent self-discharge and storage characteristics.
[0007]
[Means for Solving the Problems]
The present invention was made in view of the above problems, the Li x Ti 5/3-y L y O 4 (L having a spinel structure as a main component of the anode active material including B, and Co or Zn Li m MPO 4 (M is 1) having an olivine structure as a main component of the positive electrode active material using an oxide fired body represented by the elements 4/3 ≦ x ≦ 7/3, 0 <y ≦ 5/3) A non-aqueous electrolyte lithium secondary battery using an oxide fired body expressed by 0 ≦ m ≦ 2.1) with at least one kind of transition metal.
[0008]
That is, the present inventors have found that the use of a negative electrode active material made of an oxide capable of occluding and releasing lithium has a great effect on improving the storage characteristics, and have led to the present invention.
[0009]
The negative electrode active material used in the battery of the present invention has a potential of about 1.5 V with respect to the potential of metallic lithium. On the other hand, the positive electrode active material used in the battery of the present invention has a potential of about 4.5 to 5.0 V with respect to the potential of metallic lithium. Therefore, by combining the negative electrode active material and the positive electrode active material, a lithium secondary battery that operates at a high voltage of about 3.0 to 3.5 V despite the high negative electrode potential is formed. Therefore, it is possible to provide a safe lithium secondary battery having high storage characteristics and excellent storage characteristics.
[0010]
The non-aqueous electrolyte used in the present invention is preferably an organic electrolyte in order to achieve a high energy density, but in order to obtain a lithium secondary battery with higher safety, a gel electrolyte, an organic solid electrolyte, Alternatively, an inorganic solid electrolyte may be used.
[0011]
The reason why the lithium secondary battery of the present invention is excellent in storage characteristics with less self-discharge is not necessarily clear, but is considered as follows. That is, Li x Ti 5 / 3-y L y O 4 , which is the main component of the negative electrode active material, absorbs and releases lithium at a relatively high potential of about 1.5 V with respect to the potential of metallic lithium. In the charged state of the battery, the activity of lithium occluded in the molecular structure of the negative electrode active material is low, and the action of reducing the electrolyte and the like is considered to be very small. In addition, since the negative electrode active material is not a carbon material but an oxide, even if the solvent or the supporting salt constituting the electrolyte is a compound containing oxygen, it reacts with the intermediate and enters the interface with the electrolyte. The effect of forming an oxide film as a layer is also expected to be very small. Therefore, a side reaction on the electrode surface hardly occurs even in a charged state of the battery, so that a lithium secondary battery excellent in storage characteristics with little self-discharge can be obtained.
[0012]
Li x Ti 5 / 3-y L y O 4 , which is the main component of the negative electrode active material used in the battery of the present invention, has a spinel structure, and L in the chemical formula is one or more elements other than Ti. Here, x is preferably 4/3 or more and 7/3 or less, and y is preferably 0 or more and less than 5/3.
[0013]
By substituting a part of Ti with the different element L, the electrode performance is further improved. The reason for this is not necessarily clear, but is considered as follows. Usually, titanium oxide is bulky, and when applied as an electrode, it tends to be an electrode having a large gap. In the present invention, this bulkiness can be suppressed by substituting a part of titanium with a different element, and it is considered that there is a function of smoothly transferring ions and electrons between particles. Substitution elements other than Ti are conceivable, but preferably, Be, B, C, Mg, Al, Si, P, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Examples include Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, Te, Ba, La, Ta, W, Au, Hg, and Pb.
[0014]
Li m MPO 4 which is a main component of the positive electrode active material used in the battery of the present invention has an olivine structure, and M in the chemical formula is one or more transition metal elements. Here, m is 0 or more and 2.1 or less, and the transition metal M is Co, Ni, Fe, Mn, Cu, Zn, Cd, etc., and varies slightly depending on the main transition metal species. Japanese Patent Laid-Open No. 9-134724 describes that lithium is absorbed and released at a high potential of 4.5 to 5.0 V with respect to the dissolution / precipitation potential. For example, when the transition metal M is mainly Co, it is around 5.0V. In addition, in the case of a single phase containing only Fe, it may be around 3.5 V. Padhi, AK; Nanjundaswamy, KS; Goodenough, JB Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 144, 4, 1997, 1188-1194.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Examples of the non-aqueous electrolyte used in the battery of the present invention include a non-aqueous electrolyte and a solid electrolyte. Examples of the solid electrolyte for improving the safety of the lithium secondary battery include an inorganic solid electrolyte, an organic solid electrolyte, and an inorganic electrolyte. Organic solid electrolytes, molten salts, and the like can be used. Non-aqueous electrolytes include, as organic solvents, esters such as propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and γ-butyrolactone, substituted tetrahydrofuran such as tetrahydrofuran and 2-methyltetrahydrofuran, and dioxolane. , Ethers such as diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide, etc. can be used alone or as a mixed solvent, a supporting electrolyte salt thereto, LiClO 4, LiPF 6, LiBF 4, LiAsF 6, Li It obtained by dissolving a F 3 SO 3, LiN (CF 3 SO 2) 2 and the like. As the inorganic solid electrolyte, lithium nitride, halide, oxyacid salt, phosphorus sulfide compound, and the like are well known, and one or more of these can be used in combination. Among them, Li 3 N, LiI, Li 5 NI 2, Li 3 N-LiI-LiOH, Li 4 SiO 4, Li 4 SiO 4 -LiI-LiOH, xLi 3 PO 4- (1-x) Li 4 SiO 4 , Li 2 SiS 3 , LiLaTiO 3 , LiTi 2 (PO 4 ) 3 and the like, and similar compounds are effective. On the other hand, in the organic solid electrolyte, a polyethylene oxide derivative or a polymer containing at least the above derivative, a polypropylene oxide derivative or a polymer containing at least the above derivative, a polyphosphazene or a polyphosphazene derivative, a polymer containing an ion dissociation group, a phosphate ester polymer derivative, Furthermore, a polymer matrix material (gel electrolyte) in which the above non-aqueous electrolyte is contained in a polyvinylpyridine derivative, a bisphenol A derivative, polyacrylonitrile, polyvinylidene fluoride, fluorine rubber or the like is effective. Further, a method of using these inorganic solid electrolyte and organic solid electrolyte in combination is also effective.
[0016]
In the nonaqueous electrolyte lithium secondary battery of the present invention, a conductive agent, a binder, a filler, or the like can be added to the positive electrode or the negative electrode active material as an electrode mixture. As the conductive agent, any electronic conductive material that does not adversely affect battery performance may be used. Usually, natural graphite (flaky graphite, earthy graphite, etc.), artificial graphite, carbon black, acetylene black, ketjen black, carbon whisker, carbon fiber and metal (copper, nickel, iron, silver, gold, etc.) powder, metal Conductive materials such as fibers, metal vapor deposition, and conductive ceramic materials can be included as one type or a mixture thereof. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.
[0017]
As the binder, heat such as tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, carbomethoxy cellulose, etc. A plastic resin, a polymer having rubber elasticity, a polysaccharide, or the like can be used as one kind or a mixture of two or more kinds. In addition, it is desirable that a binder having a functional group that reacts with lithium, such as a polysaccharide, be deactivated by, for example, methylation. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.
[0018]
As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, aerosil, alumina, carbon and the like are used. The amount of filler added is preferably 0 to 30% by weight.
[0019]
A separator can be used in combination with a solid electrolyte. As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Sheets, microporous membranes, and nonwoven fabrics made from olefin polymers such as polypropylene and polyethylene, glass fibers, polyvinylidene fluoride, polytetrafluoroethylene, etc. are used because of their organic solvent resistance and hydrophobicity. The pore diameter of the separator is in a range generally used for batteries, and is, for example, 0.01 to 10 μm. The thickness is also the same, generally in the range used for batteries, for example, 5 to 300 μm.
[0020]
The positive / negative electrode active material used in the present invention is preferably a powder having an average particle size of 0.1 to 100 μm. In obtaining a predetermined shape, a pulverizer, a classifier, or a granulator is used to obtain a powder. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like is used. At the time of pulverization, water or wet pulverization in the presence of an organic solvent such as hexane may be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like is used as necessary for both dry and wet methods.
[0021]
The current collector for the electrode active material may be any electronic conductor as long as it does not adversely affect the constructed battery. For example, in addition to aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, etc., the positive electrode is made of aluminum or copper for the purpose of improving adhesiveness, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. In the negative electrode, in addition to copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the purpose of improving adhesiveness, conductivity, reduction resistance Thus, a material obtained by treating the surface of copper, aluminum or the like with carbon, nickel, titanium, silver or the like can be used. In particular, since the negative electrode active material has a potential of about 1.5 V with respect to lithium, aluminum can be used for the purpose of weight reduction. The surface of these materials can be oxidized. Regarding these shapes, films, sheets, nets, punched metals, expanded products, lath bodies, porous bodies, foams, formed bodies of fiber groups, and the like are used in addition to foils. The thickness is not particularly limited, but a thickness of 1 to 500 μm is used.
[0022]
Examples of the shape of the nonaqueous electrolyte lithium secondary battery in the present invention include a cylindrical shape, a square shape, a coin shape, a button shape, a flat shape, and a film shape. Especially, in order to achieve a high energy density, it is desirable to use a film-like battery shape using a gel electrolyte or a solid electrolyte.
[0023]
【Example】
FIG. 1 is a cross-sectional view of a lithium secondary battery according to the present invention. In the positive electrode, a positive electrode mixture 1 containing a positive electrode active material, ketjen black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder is pressure-bonded onto a positive electrode current collector 6 made of aluminum. In the negative electrode, a negative electrode mixture 2 containing a negative electrode active material, ketjen black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder is pressure-bonded onto a negative electrode current collector 7 made of copper. The separator 3 is made of a polyethylene microporous film and is interposed between the positive and negative electrodes. The electrolytic solution is a non-aqueous electrolytic solution in which 1 mol of lithium tetrafluoroborate (LiBF 4 ) is dissolved in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 2: 1. The positive electrode, the negative electrode, and the separator are impregnated. The peripheral edge of the aluminum positive electrode can 4 containing the positive electrode and the peripheral edge of the stainless steel negative electrode lid 5 containing the negative electrode are sealed with a gasket 8.
[0024]
Reference Example 1 Lithium titanate (Li 4/3 Ti 5 ) obtained by mixing lithium hydroxide (LiOH · H 2 O) and titanium oxide (TiO 2 ) and heat-treating them at 900 ° C. in an oxidizing atmosphere. / 3 O 4 ) was used as the negative electrode active material. On the other hand, tribasic cobalt oxide (Co 3 O 4 ), diammonium phosphate ((NH 4 ) 2 HPO 4 ), and lithium hydroxide (LiOH · H 2 O) were mixed, and the mixture was mixed at 750 ° C. for 20 hours in a nitrogen stream. Cobalt-lithium phosphate (LiCoPO 4 ) obtained by heat treatment was used as the positive electrode active material.
[0025]
The positive electrode was obtained as follows. After mixing in a ratio of 87 parts by weight of the positive electrode active material, 10 parts by weight of the conductive agent and 3 parts by weight of the binder to make the positive electrode mixture 1, it was punched into a disk shape having a diameter of 16 mm using a molding die, and 150 ° C. Were dried in a vacuum for 10 hours to produce a positive electrode having a thickness of 0.55 mm.
[0026]
The negative electrode was obtained as follows. The mixture was mixed at a ratio of 87 parts by weight of the negative electrode active material, 10 parts by weight of the conductive agent, and 3 parts by weight of the binder to form the negative electrode mixture 2, and then punched into a disk shape having a diameter of 16 mm using a molding die. Were dried in a vacuum for 10 hours to produce a negative electrode having a thickness of 0.35 mm.
[0027]
A coin-type lithium secondary battery having a diameter of 20 mm and a thickness of 16 mm was prepared using the separator and non-aqueous electrolyte described above. This is referred to as Reference Example Battery 1.
[0028]
(Example 2) Titanium obtained by mixing lithium hydroxide (LiOH.H 2 O), titanium oxide (TiO 2 ) and boric anhydride (B 2 O 3 ) and heat-treating them at 900 ° C. in an oxidizing atmosphere. A coin-type lithium secondary battery was produced in the same manner as in Reference Example 1 except that lithium acid (Li 4/3 Ti 4/3 B 1/3 O 4 ) was used as the negative electrode active material. This is referred to as the battery 2 of the present invention.
[0029]
(Example 3) Lithium hydroxide (LiOH.H 2 O), titanium oxide (TiO 2 ), and tribasic cobalt oxide (Co 3 O 4 ) were mixed and heat-treated at 900 ° C. in an oxidizing atmosphere. was except that lithium titanate (Li 4/3 Ti 4/3 Co 1/3 O 4) was used as a negative electrode active material to prepare a coin-type lithium secondary batteries in the same manner as in reference example 1. This is referred to as the present invention battery 3.
[0030]
(Example 4) Lithium hydroxide (LiOH · H 2 O), titanium oxide (TiO 2 ) and zinc nitrate (Zn (NO 3 ) 2 · 6H 2 O) were mixed, and these were mixed at 900 ° C. in an oxidizing atmosphere. except for using lithium titanate obtained by heat treatment (Li 4/3 Ti 4/3 Zn 1/3 O 4) as a negative electrode active material was prepared a coin-type lithium secondary batteries in the same manner as in reference example 1 . This is the battery 4 of the present invention.
[0031]
Comparative Example 1 Graphite is used as the negative electrode active material, and manganese carbonate (MnCO 3 ) and lithium hydroxide (LiOH · H 2 O) are mixed as the positive electrode active material, and this is mixed at 750 ° C. for 20 hours in a dry air atmosphere. except for using lithium manganate obtained by heat treatment (LiMn 2 O 4) were prepared in the same manner as in a coin-type lithium batteries as in reference example 1. This is referred to as comparative battery 1.
[0032]
Comparative Example 2 Reference Example 1 except that lithium titanate (Li 4/3 Ti 5/3 O 4 ) was used as the negative electrode active material and lithium manganate (LiMn 2 O 4 ) was used as the positive electrode active material. We have created a coin-type lithium batteries in the same way. This is referred to as comparative battery 2.
[0033]
Using these batteries, a storage test was conducted using the self-discharge rate as an index. The storage test was performed at room temperature. Charging / discharging was performed at a rate of 10 hours, and in the reference battery 1 and the present invention batteries 2 to 4, the charge end voltage was 3.7V and the discharge end voltage was 3.0V. On the other hand, in the comparative battery 1, the charge end voltage was 4.2V and the discharge end voltage was 3.2V, and in the comparative battery 2, the charge end voltage was 2.8V and the discharge end voltage was 2.0V. Under the above charging / discharging conditions, the battery in the final charge state in the third cycle was stored for 30 days, and the discharge capacity after storage was measured to calculate the self-discharge rate according to the following formula.
[0034]
【formula】
[0035]
The storage test results are shown in Table 1 together with the battery voltage.
[0036]
[Table 1]
[0037]
As shown in the results of (Table 1), the reference battery 1, the inventive batteries 2 to 4 and the comparative battery 2 have a lower self-discharge rate during storage than the comparative battery 1, and improved storage characteristics. It is clear that However, the comparative battery 2 has a low battery discharge voltage of 3 V or less, and the storage characteristics are improved, but the energy density of the battery is lowered.
[0038]
In addition, it can be seen from the results of the batteries 2 to 4 of the present invention that the storage characteristics are slightly improved as compared with the reference battery 1 by substituting a part of titanium with another element. The reason for this is not necessarily clear, but it is considered that self-discharge can be suppressed by affecting the particle morphology after firing and reliably transferring ions and electrons inside the particles.
[0039]
In the above embodiment, B, Co, and Zn were used as the substitution elements, but the same effect has been confirmed for elements other than Ti. In addition, although Co is used as the transition metal M for the positive electrode active material, the same effect can be obtained by using a single element or a mixture of other transition metal elements.
[0040]
In addition, this invention is not limited to the starting material of the active material, the manufacturing method, the positive electrode, the negative electrode, the electrolyte, the separator etc. which were described in the said Example. The coin type battery is only for explaining the present invention, and the shape of the battery is not limited to this. Further, since the main negative electrode active material has a potential with respect to lithium of about 1.5 V, it is natural that the weight energy density is improved when aluminum is used instead of copper as a current collector.
[0041]
In addition, the conventional lithium secondary battery using a negative electrode active material in which lithium occlusion / release occurs in the vicinity of the potential of metallic lithium, when multiple series-parallel junctions are used, the environmental temperature and impedance between the batteries. Some of the batteries may be in a deeply charged state due to variations such as the above, and there is a risk of being led to a battery performance degradation or an unsafe mode due to the deposition of metallic lithium in the negative electrode. However, since the negative electrode active material of the present invention causes lithium insertion / release at a potential of about 1.5 V with respect to the potential of metallic lithium, when a plurality of lithium secondary batteries using the lithium secondary battery are connected in series and parallel, However, the above problem can be largely avoided.
[0042]
【The invention's effect】
Since the present invention is as described above, it is possible to provide a safe non-aqueous electrolyte lithium secondary battery having high energy density and excellent self-discharge and storage characteristics.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a battery of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Positive electrode mixture 2 Negative electrode mixture 3 Separator 4 Positive electrode can 5 Negative electrode lid 6 Positive electrode collector 7 Negative electrode collector 8 Gasket

Claims (1)

負極活物質の主成分にスピネル構造を有するLixTi5/3-yy4(LはB、Co又はZnを含む元素、4/3≦x≦7/3、0<y≦5/3)で表される酸化物焼成体を用い、正極活物質の主成分にオリビン構造を有するLimMPO4(Mは1種以上の遷移金属で、0≦m≦2.1)で表わせる酸化物焼成体を用いることを特徴とする非水電解質リチウム二次電池。Li x Ti 5 / 3-y L y O 4 having a spinel structure as a main component of the negative electrode active material (L is an element containing B, Co or Zn, 4/3 ≦ x ≦ 7/3, 0 <y ≦ 5 / 3 using an oxide sintered body represented by), the Li m MPO 4 (M having the olivine structure as a main component of the positive electrode active material in one or more transition metals, expressed by 0 ≦ m ≦ 2.1) A non-aqueous electrolyte lithium secondary battery characterized by using an oxide fired body.
JP2000006070A 2000-01-11 2000-01-11 Nonaqueous electrolyte lithium secondary battery Expired - Fee Related JP4296580B2 (en)

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